Lanthanide Hydroxide Cluster Complexes via Ligand-Controlled Hydrolysis of the Lanthanide Ions

Abstract

We survey in this chapter the lanthanide hydroxide cluster complexes since the publication of the comprehensive review on the same subject (Handbook of physics and chemistry of rare earths 40:109–240, 2010). Specifically, polynuclear complexes with carboxylate, diketonate, phosphate, sulfonate, and polyoxometalate (POM) ligands featuring polyhedral cluster-type lanthanide-hydroxo (Ln-OH) core motifs are summarized. The synthetic procedures leading to the production of the cluster species and the unique cluster core motifs are the focus of the discussion. Within each ligand family, we organize the cluster complexes according to their nuclearity with the intention to demonstrate the formal assembly of higher-nuclearity complexes using smaller and recognizable motifs as secondary building units. It is clear that a number of such motifs are prevalent and are shared by cluster complexes with ligands that are structurally and functionally distinct. With the work reviewed previously and the rapidly increasing number of polynuclear lanthanide hydroxide complexes, we hope to validate that once a synthetic serendipity, the chemistry of lanthanide hydroxide complexes is now a legitimate new paradigm of lanthanide coordination chemistry that is of fundamental interest and potential useful applications.

1 Introduction

Polynuclear lanthanide hydroxide clusters are a class of fundamentally interesting and practically significant substances. They are attracting widespread current interest because of their appealing structures, synthetic challenges, and, most importantly, their potential applications. Continuous development of this burgeoning class of lanthanide complexes will help define a new paradigm of coordination chemistry of these unique metal elements. These fundamental efforts will also lead to the development of advanced materials of practical applications. For example, some lanthanide hydroxide clusters have been used as precursors for oxide-based electrical and optical materials [1], while others have been incorporated into polymers to prepare hybrid materials with enhanced mechanical properties [2]. In addition, intriguing molecule-based magnetic phenomena have been observed in lanthanide hydroxide clusters, potentially useful for quantum computing, magnetic information storage [3], and environmentally friendly magnetic refrigeration [4]. Some lanthanide hydroxide cluster complexes have been found to catalyze chemical transformations including hydrolytic cleavage of nucleic acids [5]. Some cluster complexes have also been proposed as potentially more efficient contrast-enhancing agents in biomedical imaging [6]. Indeed, molar relaxivities greatly surpassing those of current working force of contrast agents in magnetic resonance imaging (MRI) have been demonstrated in the laboratories. Moreover, fixation of atmospheric CO₂ by lanthanide hydroxide complexes has recently been reported, which bears significant environmental ramifications [7]. These exciting and useful applications of lanthanide hydroxide cluster complexes are probably the main driving force for the presently widespread interest in this special class of lanthanide-containing substance, and the extensive research activities, some of which being reviewed below, are consistent with this assessment.

In order to put the materials reviewed here in the developmental context and to help the readers who are interested in this research topic but not necessarily working in the field, the following explanatory notes are warranted:

• 1. Should Cotton’s original definition [8] of a metal cluster be strictly followed, few of these polynuclear lanthanide hydroxide complexes may be qualified as “clusters” simply because metal–metal bonding or electronic/magnetic interactions between individual metal centers are insignificant in these species. The use of the term is thus for the description of an assembly of metal atoms bridged by ligands from a mere structural perspective.

• 2. Lanthanide cluster compounds have been obtained by two major routes, one involving organometallic syntheses that typically generate moisture and/or air-sensitive species and the other under hydrolytic conditions but not necessarily in aqueous solutions. We limit our discussion to the cluster-type polynuclear lanthanide hydroxide complexes prepared by the latter means with a note that similar products have also been isolated but generally unexpectedly from some organometallic procedures.

• 3. As the chemistry of lanthanide hydroxide clusters has been enjoying a rapid development, there are now a large number of such compounds in the literature, with the number still growing at a fast pace. If species containing both lanthanides and other metal elements are included, this number is even bigger. We therefore limit our discussion to only new lanthanide-exclusive species since the publication of the 2010 review on a similar topic in the Handbook of Physical and Chemistry of the Rare Earth Elements [9].

2 Ligand-Controlled Lnathanide Hydrolysis

The lanthanide ions, prevalently trivalent, are hard Lewis acids that prefer O-based ligands with aqua coordination being most revealing. Lewis acid-activation of the coordinated water molecule renders the complex susceptible to hydrolysis, and if the pH condition is not carefully controlled, intractable product mixture consisting of lanthanide hydroxides and/or oxides are typically obtained. In fact, except for certain multiply charged chelating ligands such as ethylenediaminetetraacetate (EDTA), lanthanide complexes are generally prepared under highly acidic conditions. However, adventitious hydrolysis does occur with the production of lanthanide complexes characterized by the unexpected presence of hydroxo and/or, much less frequently, oxo groups in the cluster-type core structures. Though interesting, reports of such species were sporadic and reproducibility was problematic prior to the systematic work by Zheng and coworkers [10].

Attracted by the structural aesthetics and tempted by the potential of developing rational synthesis of such otherwise synthetically elusive species, we set out almost two decades ago to explore a systematic approach in which deliberate hydrolysis of the lanthanide ions is carried out in the presence of ligands capable of limiting the degree of hydrolysis of the lanthanide ions [11, 12]. Three considerations went into our hypothesis. First, adventitious hydrolysis was commonly accepted as being responsible for the unexpected production and isolation of the hydroxo/oxo complexes. But can such unintended hydrolysis be exploited in a deliberate and, more importantly, reproducible manner? Second, the presence of the primary ligand, with respect to the “secondary” hydroxo/oxo ligand, is probably critical in arresting or limiting the otherwise extensive hydrolysis of the lanthanide ion to prevent the formation of the eventual precipitate products. If so, are there any specific structural and functional features required for such ligands? Third, despite the different lanthanide ions and/or ligands used, a number of these unexpected hydroxo cluster complexes share some prevalent Ln-OH core motifs. This suggests that a common reaction pathway may exist for the assembly of the cluster core. In other words, a systematic synthetic approach to these new lanthanide hydroxide complexes may be developed. Then, what is the scope of such a new paradigm of lanthanide coordination chemistry in terms of the nature of the lanthanide ions and any applicable supporting ligands?

These thoughts are reflected in the approach of “ligand-controlled hydrolysis” schematically shown in Fig. 1. Key to the success of this approach is the pre-occupation of part of the lanthanide coordination sphere by the supporting ligands, leaving only a limited number of sites available for aqua coordination. Deprotonation of the lanthanide-activated aqua ligands upon base addition is thus limited, and so is olation – the aggregation of the resulting lanthanide-hydroxo (Ln-OH) species via sharing of the hydroxo groups – leading eventually to cluster species rather than intractable precipitates of lanthanide oxides/hydroxides.

Fig. 1

Ligand-controlled hydrolytic approach to the assembly of lanthanide hydroxide clusters

Significant progress in this new chemistry of lanthanide coordination has been made through our own efforts and those of others [9] since our first report of a pentadecanuclear europium cluster complex assembled by using tyrosine as the hydrolysis-limiting ligand [11]. With almost two decades of development, ligand-controlled hydrolysis has become a time-honored approach to the synthesis of lanthanide hydroxide/oxide clusters [9, 10].

The survey of the new cluster species and related discussion in this chapter are organized according to the type of ligands used for hydrolysis control (Table 1). Within each type of ligands, clusters are presented and discussed in ascending order of the cluster’s nuclearity. A brief summary will be provided at the conclusion of the chapter in which the authors’ personal perspective of what future directions this research may head toward is offered.

Table 1 Abbreviations of ligands

2.1 Carboxylates

Carboxylates are time-honored ligands for lanthanide coordination. These include simple carboxylates such as formate and acetate [13–16], (poly)amino(poly)carboxylates [10], and those that are structurally and functionally more sophisticated [17]. It should be noted that lanthanide carboxylate complexes have traditionally been prepared under highly acidic conditions (pH 3–4) due exactly to the hydrolysis concern alluded to above. It was Zheng et al. who explored the otherwise well-established lanthanide coordination chemistry with α-amino acids under pH conditions that are 2–3 orders of magnitude higher than the commonly accepted acidic conditions that uncovered the wealth of the “high-pH” coordination chemistry of the lanthanides [10–12]. Polynuclear lanthanide complexes characterized by the presence of polyhedral lanthanide-oxo/hydroxo core motifs have been obtained with the amino acid ligands serving as hydrolysis-limiting and structure-supporting ligands. It is believed that the presence of amino group and other hydrophilic functional group(s) helps enhance the water solubility of the complexes formed at a lower pH, allowing subsequent deprotonation of any available aqua ligand(s) or hydrolysis to occur upon addition of a base. It is understandable that not all carboxylate ligands can be used to support the hydroxide complexes due to the fact that many lanthanide complexes with such ligands are insoluble and precipitate out before the pH of the reaction mixture may be enhanced. Equally possible is that researchers, wary of the formation of intractable lanthanide oxide/hydroxide precipitates, were simply trying to avoid any high-pH conditions.

This ligand-controlled hydrolytic approach has since become a standard method for the preparation of lanthanide hydroxide cluster complexes. Understandably, drastically different cluster species have been obtained depending sensitively on the supporting ligands used. The structure of the resulting cluster is also dependent on other reaction conditions such as the presence of any additional ligands or reactants, although these species may not eventually be incorporated into the final cluster products.

2.1.1 Tetra-, Penta-, and Heptanuclear Clusters

Long et al. reported two tetranuclear lanthanide hydroxide cluster complexes [Ln₄(μ ₃-OH)₄(Acc)₆(H₂O)₇(ClO₄)]·(ClO₄)₇·11H₂O·(Ln = Dy, Yb)·(HAcc = 1-amino-cyclohexanel-carboxylic acid) by using amino acid-like ligand HAcc to control the lanthanide hydrolysis [18]. The cluster core, now a well-established motif in the literature, consists of four Ln³⁺ ions and four triply bridging hydroxo groups occupying the alternating vertices of a distorted cubane. Each edge of the Ln₄ tetrahedron is bridged by a carboxylate group of the organic ligand. The coordination spheres are completed by aqua ligands and for one of them, a monodentate perchlorate (Fig. 2a). It is of note that the reactions using two lighter lanthanide ions La³⁺ and Nd³⁺ under otherwise identical conditions produced trinulcear complexes of the common formula [Ln₃(Acc)₁₀(H₂O)₆]·(ClO₄)₉·4H₂O·(Ln = La, Nd) in which three lanthanide ions are in a linear arrangement with neighboring Ln³⁺ ions being bridged by four carboxylate groups from different Acc⁻ ligands. Each of the terminal Ln³⁺ ions is further coordinated by three aqua ligands and a carboxylate group, one being monodentate and the other, chelating (Fig. 2b). Formation of different products probably reflects the influences of the size and/or Lewis acidity of the lanthanide ions: The lighter and larger lanthanide ions (La³⁺ and Nd³⁺) may not be as adequately Lewis acidic to be hydrolyzed as the heavier and smaller, and therefore more acidic Dy³⁺ and Yb³⁺.

Fig. 2

Structure of: (a) [Dy₄(μ ₃-OH)₄(Acc)₆(H₂O)₇(ClO₄)]⁷⁺ and (b) [La₃(Acc)₁₀(H₂O)₆]⁹⁺. Reprinted with the permission from [18] Copyright 2011 Royal Society of Chemistry

Using nicotinic acid in a similar capacity, Zheng et al. obtained and structurally characterized isostructural tetranuclear complexes of the formula [Ln₄ (μ ₃-OH)₄(Hnic)₅(H₂O)₁₂](ClO₄)₈·(Ln = Eu, Gd; Hnic = pyridinium nicotinate) [19]. The cluster core is the same as the aforementioned distorted cubane. However, only five of the six edges of the Ln₄ tetrahedron are bridged by the carboxylate group of the zwitterionic ligand; the coordination of the two unique lanthanide ions is made up for by using additional aqua ligands (Fig. 3).

Fig. 3

Structure of the [Eu₄(μ ₃-OH)₄(Hnic)₅(H₂O)₁₂]⁸⁺. Reprinted with the permission from [19] Copyright 2009 American Chemical Society

When hydrolysis was carried out with the use of isonicotinate (ina) as supporting ligand, a tetranuclear complex formulated as [Dy₄(μ ₃-OH)₄(ina)₆(py)(CH₃OH)₇](ClO₄)₂·py·4CH₃OH (py = pyridine) was obtained [20]. Its core structure is the same as the one when nicotinic acid was used [19]. In addition to the bridging by in a carboxylate group, seven methanol molecules and one pyridine molecule help complete the metal coordination (Fig. 4). This cluster complex was shown to display properties characteristic of a single-molecule magnet.

Fig. 4

Structure of [Dy₄(μ ₃-OH)₄(ina)₆(py)(CH₃OH)₇]²⁺. Reprinted with the permission from [20] Copyright 2009 American Chemical Society

In ligand-supported assembly of hydroxide clusters, the use of organic co-ligand(s) other than coordinating solvent(s) is a common practice. For example, Zhao et al. reported a tetranuclear complex [Dy₄(μ ₃-OH)₂(L)₁₀(bipy)₂(H₂O)₂] (HL = 3-fluoro-4-(trifluoromethyl)benzoic acid; bipy = 2,2′-bipyridine) in which the metal coordination is achieved by both L and the chelating bipy, in addition to the hydroxo and aqua ligands [21]. The parallelogram-shaped cluster core consists of four coplanar lanthanide atoms connected by two μ ₃-OH groups, one on each opposite sides of the plane. This motif is also frequently encountered in lanthanide hydroxide complexes. Two of the four edges of the parallelogram are each bridged by two carboxylate groups from different L ligands, while the other two are each bridged by one carboxylate group and one μ ₂-H₂O molecule. The coordination sphere is completed by either a bipy or a monodentate L ligand (Fig. 5).

Fig. 5

Structure of [Dy₄(μ ₃-OH)₂(L)₁₀(bipy)₂(H₂O)₂]. Reprinted with the permission from [21] Copyright 2014 Royal Society of Chemistry

Two additional series of tetranuclear hydroxide clusters featuring the same core motif were reported. Murray et al. reported the isostructural complexes [Ln₄(μ ₃-OH)₂(o-van)₄(O₂C^tBu)₄(NO₃)₂]·CH₂Cl₂·1.5H₂O·(Ln = Gd, Dy; o-van = 3-methoxysalicylaldehydato anion; O₂C^tBu = pivalate or (CH₃)₃CCO₂ ⁻) [22], while Powell et al. reported five isostructural complexes of the common formula [Ln₄(μ ₃-OH)₂(mdeaH)₂(O₂C^tBu)₈] (mdeaH₂ = N-methyldiethanolamine; Ln = Tb, Dy, Ho, Er, Tm) [23]. Together with o-van in the former and mdeaH in the latter, pivalate serves in both series to stabilize the cluster core. Crystal structures of the complexes representing the two series are shown in Fig. 6.

Fig. 6

Structures of [Dy₄(μ ₃-OH)₂(o-van)₄(O₂C^tBu)₄(NO₃)₂] (left) and [Dy₄(μ ₃-OH)₂(mdeaH)₂(O₂C^tBu)₈] (right). Reprinted with the permission from [22] Copyright 2011 Royal Society of Chemistry and [23] Copyright 2010 American Chemical Society

With the use of 1-amino-cyclohexanel-carboxylic acid (Acc), Long et al. isolated [Dy₅(μ ₃-OH)₆(Acc)₆(H₂O)₁₀]·Cl₉·24H₂O [24] when DyCl₃ was used, which differs sharply from the tetranuclear species when Dy(ClO₄)₃ was used as the starting lanthanide salt [18]. The profound anion-template effects on the cluster nuclearity have previously been established [25], but we note that the anions do not participate in the metal coordination in either of these two complexes. Thus, the exact roles played by the anions in dictating the outcome of the reactions carried out under otherwise identical conditions remain to be understood.

In the cluster core, the five Dy³⁺ ions are organized into a trigonal bipyramidal geometry. Alternatively, it may be viewed as two distorted cubanes joined together by sharing a trimetallic face. Each triangular metal face is capped by a μ ₃-OH group, while each non-equatorial metal edge is bridged by an Acc carboxylate group. The coordination sphere of each Dy³⁺ ion is completed by two aqua ligands (Fig. 7).

Fig. 7

Structure of [Dy₅(μ ₃-OH)₆(Acc)₆(H₂O)₁₀]⁹⁺. Reprinted with the permission from [24] Copyright 2012 American Chemical Society

Collison et al. reported two isostructural heptanuclear complexes [Ln₇(OH)₆(thmeH₂)₅(thmeH)(tpa)₆(MeCN)₂](NO₃)₂·(Ln = Gd, Dy; thmeH₃ = tris(hydroxymethyl)ethane; tpaH = triphenylacetic acid) [26]. The synthesis was carried out under solvothermal conditions using a mixture of lanthanide nitrate hydrates, thmeH₃, tpaH, and triethylamine in acetonitrile. The cluster core consists of seven coplanar Ln³⁺ ions organized into a disc-like hexagon with six peripheral Ln³⁺ ions occupying the vertices of the hexagon and the remaining Ln³⁺ ion sitting at the center of hexagon and connecting the peripheral metal ions through six μ ₃-OH groups. Alternatively this cluster core can be viewed as two of the coplanar tetranulcear units, such as those shown in Figs. 5 and 6, joined together by two μ ₃-OH groups. In effect, the six μ ₃-OH groups are alternatingly above and below the disc plane. In addition to the coordination by these OH groups, the central lanthanide ion is further coordinated with two trans-disposed acetonitrile molecules. Each edge of the lanthanide hexagon is bridged by one tpa carboxylate group and one thmeH₂ ⁻ or thmeH₂ ⁻ ligand (Fig. 8).

Fig. 8

Structure of [Ln₇((μ ₃-OH)₆(thmeH₂)₅(thmeH)(tpa)₆(MeCN)₂]²⁺ (color code: purple, Ln; yellow, O; blue, N; and skeletal, C). Reprinted with the permission from [26] Copyright 2011 Royal Society of Chemistry

2.1.2 Decanuclear and Higher-Nuclearity Clusters

An increasing number of lanthanide hydroxide complexes of even higher nuclearities have also appeared in the literature, although their assembly generally cannot be predicted. A number of factors may be responsible for the formation of such giant cluster species. These include the nature of the ligands, the lanthanide ions, available anionic templates, as well as pH condition. For example, in the aforementioned work by Zhao and coworkers in which tetranuclear cluster complexes were obtained, a decanuclear complex [Dy₁₀(μ ₃-OH)₈(L)₂₂(bipy)₂(H₂O)₂]·5H₂O·(L = 3-fluoro-4-(trifluoromethyl)benzoate) was also isolated when the reaction pH was adjusted to 10 with NaOH prior to the hydrothermal treatment [21]. The complex structure as shown in Fig. 9 has a formal crystallographic center symmetry. The Dy³⁺ ions are connected by eight μ ₃-OH groups and the L carboxylate groups. The coordination spheres are further fulfilled by either chelating bipy or aqua ligands.

Fig. 9

Structure of [Dy₁₀(μ ₃-OH)₈(L)₂₂(bipy)₂(H₂O)₂](L = 3-fluoro-4-(trifluoromethyl)benzoate. Reprinted with the permission from [21] Copyright 2014 Royal Society of Chemistry

It should be noted that a gadolinium complex [Gd₁₀(μ ₃-OH)₈(3-TCA)₂₂(H₂O)₄]·(3-TCAH = thiophene-3-carboxylic acid) with a similar decanuclear core (Fig. 10) had been reported by Bu and his coworkers, but the primarily supporting ligand is different [27]. In addition, no co-ligand was utilized.

Fig. 10

Structure of [Gd₁₀(μ ₃-OH)₈(3-TCA)₂₂(H₂O)₄]. Reprinted with the permission from [27] Copyright 2013 American Chemical Society

Distinctly different from the above compounds, two lanthanide complexes of the common formula [Co₃(μ ₃-O)(O₂C^tBu)₆(py)₃][Ln₁₀(O₂C^tBu)₁₈(O₃P^tBu)₆(OH)(H₂O)₄] (Ln = Dy, Gd) reported by Winpenny et al. possess a decanuclear cluster core that features a nine-metal ring surrounding a central metal atom in the complex anion [28]. The lanthanide ions are essentially coplanar with those in the ring occupying at the vertices of a nearly regular nonagon, each connecting the central lanthanide ion via an O of the O₃P^tBu ligand. Connection between neighboring metal atoms in the ring is achieved by O₃P^tBu, O₂C^tBu, and bridging aqua and/or OH ligands (Fig. 11).

Fig. 11

Structure of [Dy₁₀(O₂C^tBu)₁₈(O₃P^tBu)₆(OH)(H₂O)₄]⁻. Reprinted with the permission from [28] Copyright 2014 Royal Society of Chemistry

With isonicotinic acid (Hina) and o-vanillin as protecting ligands, Murray et al. obtained a decanuclear complex [Dy₁₀(μ ₄-O)₂(μ ₃-OH)₆(o-van)₆(ina)₁₃(H₂O)₂](NO₃) that can be viewed as two pentanuclear complex units bridged by one ina carboxylate group [29]. This pentanuclear cluster core has the structure of a distorted trigonal bipyramid similar to the one discussed above [24]. Within each pentanuclear unit, the metal ions are bridged by one μ ₄-O group, three μ ₃-OH groups, the ina carboxylate, and the O atom of the deprotonated phenol groups of the o-van ligands. The coordination sphere is completed by the OH and MeO groups of the o-van ligand and aqua ligands (Fig. 12).

Fig. 12

Structure of [Dy₁₀(μ ₄-O)₂(μ ₃-OH)₆(o-van)₆(ina)₁₃(H₂O)₂]⁺ cluster. Reprinted with the permission from [29] Copyright 2013 Elsevier

With the combined use of structurally or functionally more sophisticated ligands, lanthanide hydroxide complexes of even higher nuclearities can be obtained. For example, Ogden et al. reported the use of a tetrazole-functionalized calixarene (1) in combination with acetic acid or phenylcarboxylic acid in the controlled assembly of lanthanide cluster complexes [30]. The synthesis was carried out by using a mixture of ligand 1, Dy(NO₃)₃(DMSO)₃, and ammonium acetate or ammonium benzoate in H₂O/ethanol. With the sterically more hindered phenylcarboxylate, they obtained a dodecanuclear complex [Dy₁₂(1-3H)₃(1-2H)₃(PhCO₂)₅(μ ₃-OH)₁₆(H₂O)₂₁] (1-3H and 1-2H represent, respectively, triply and doubly deprotonated ligand 1), whereas with acetate, they isolated a nonadecanuclear complex [Dy₁₉(1-3H)(1-2H)₁₁(CH₃CO₂)₆(μ ₃-OH)₂₆(H₂O)₃₀]. The cluster core of [Dy₁₂(OH)₁₆] in the smaller complex can be viewed as two trigonal bipyramids and one distorted tetrahedron joined together by sharing vertices (Fig. 13a, top) with each of the triangular Dy3 faces being capped by a μ ₃-OH group. The cluster core is encapsulated by the organic protecting sphere formed by both the carboxylate and the calixarene ligands (Fig. 13a, bottom). In the larger complex, the core of [Dy₁₉(μ ₃-OH)₂₆] can be conveniently viewed as being elongated by adding one trigonal bipyramid and one distorted tetrahedron to the dodecanuclear core. Alternatively the core may be more straightforwardly viewed as three trigonal bipyramids being sandwiched by two distorted tetrahedra with neighboring polyhedra being joined together by sharing a Dy-vertex (Fig. 13b, top). Corresponding to the larger and elongated cluster core, there are 26 μ ₃-OH groups, each capping a triangular metal face. This hydroxide cluster core is protected in an organic sphere composed of acetate ligands and the tetrazole groups of the calixarene ligand (Fig. 13b, bottom). In both complexes, aqua ligands fulfill the rest of the metal coordination sphere.

Fig. 13

Structures of the cluster core (top) and the complex (bottom) of: (a) [Dy₁₂(1-3H)₃(1-2H)₃(PhCO₂)₅(OH)₁₆(H₂O)₂₁] and (b) [Dy₁₉(1-3H)(1-2H)₁₁−(CH₃CO₂)₆(OH)₂₆(H₂O)₃₀]. Reprinted with the permission from [30] Copyright 2014 American Chemical Society

Even larger lanthanide hydroxide clusters have also been reported, generally as unintended outcome of reactions originally aiming at different synthetic targets. As an example, three 26-metal lanthanide hydroxide cluster complexes were reported by two different groups. They are [Er₂₆I(μ ₃-OH)₂₀(μ ₃-O)₆(NO₃)₉(ina)₃₃(OH)₃(H₂O)₃₃] reported by Xue et al. [31] and [Ho₂₆(ina)₂₈(CH₃COO)₄(CO₃)₁₀(OH)₂₆(H₂O)₁₈]·20H₂O and [Er₂₆(ina)₂₉(CH₃COO)₃(CO₃)₁₀(OH)₂₆(H₂O)₁₉]·26H₂O by Xu and his coworkers [32]. The first member of the three was isolated from a hydrothermal reaction using a mixture of Er₂O₃, AgI, isonicotinic acid, and HNO₃, whereas the others were obtained, also under hydrothermal conditions, using a mixture of Ln₂O₃, Mn(OAc)₂·4H₂O, isonicotinic acid, and formic acid.

Surprisingly, despite the different synthetic procedures and the compositions of the final product, the polyhedral arrangement of the core metal atoms is actually the same with the difference being only in the type and the number of bridging ligands. As such, only the representative structure of [Er₂₆I(μ ₃-OH)₂₀ (μ ₃-O)₆(NO₃)₉(ina)₃₃(OH)₃(H₂O)₃₃] is shown in Fig. 14. All three complexes have the same number (26) of triply bridging oxo/hydroxo groups. Furthermore, in each of the lanthanide-oxo/hydroxo cores there are a total of 42 bridging ligands that connect adjacent lanthanide atoms. These bridging ligands are all O-based with ina being common in all three clusters. The remaining bridging ligands are either inorganic (NO₃ ⁻, CO₃ ²⁻) or acetate ion. Coordination of the lanthanide ions is completed by aqua and other small-entity ligands that do not alter the overall complex structures.

Fig. 14

Structure of [Er₂₆I(μ ₃-OH)₂₀(μ ₃-O)₆(NO₃)₉(ina)₃₃(OH)₃(H₂O)₃₃]. Reprinted with the permission from [31] Copyright 2008 Elsevier

In addition to the essential presence of bridging hydroxo group, anion species including O²⁻, N₃ ⁻, NO₃ ⁻, halides, CO₃ ²⁻, and ClO₄ ⁻ have frequently been observed in high-nuclearity lanthanide hydroxide clusters wherein such anions serve presumably to template the assembly of the giant clusters. We note that the genesis of these anions (if they are not from the starting materials) and/or their role(s) constitute an active research topic for which definitive answers remain unclear. In the work reported by Hong et al., two 36-metal cluster complexes formulated as [Ln₃₆(nic)₃₆(OH)₄₉(O)₆(NO₃)₆(N₃)₃(H₂O)₂₀]·Cl₂·28H₂O·(Ln = Gd, Dy) were obtained from a hydrothermal reaction using a mixture of lanthanide chloride, NaN₃, nicotinic acid, and HNO₃ [33]. Structural studies by single-crystal X-ray diffraction revealed 36 Ln³⁺ ions organized into a cage-like structure featuring coordination by bridging OH⁻, O²⁻, N₃ ⁻, and NO₃ ⁻ groups (Fig. 15a). The distorted cubane units of [Ln₄(μ ₃-OH)₄]⁸⁺ are easily recognizable in the core structure, together with other types of Ln-OH motifs that link these cubane units. The Ln³⁺ coordination sphere is completed by aqua ligands, carboxylate O atoms, as well as the nic chelating carboxylate groups (Fig. 15b).

Fig. 15

Structure of: (a) [Ln₃₆(nic)₃₆(OH)₄₉(O)₆(NO₃)₆(N₃)₃(H₂O)₂₀]²⁺ and (b) the hydroxide cluster core. Reprinted with the permission from [33] Copyright 2013 Royal Society of Chemistry

Tong et al. reported two high-nuclearity complexes [Gd₃₈(μ-O)(μ ₈-ClO₄)₆ (μ ₃-OH)₄₂(caa)₃₇(H₂O)₃₆(EtOH)₆](ClO₄)₁₀·(OH)₁₇·14DMSO·13H₂O and [Gd₄₈(μ ₄-O)₆ (μ ₃-OH)₈₄(caa)₃₆(NO₃)₆(H₂O)₂₄(EtOH)₁₂(NO₃)Cl₂]Cl₃·6DMF·5EtOH·20H₂O (Hcaa = chloroacetic acid) [34]. The profound influence of the nature of the anions on the structure of the resulting clusters is clearly shown here. While the reaction of a mixture containing chloroacetic acid, gadolinium perchlorate, and NaOH in a water/ethanol/DMSO mixed solvent produced the 38-metal cluster complex, the use of gadolinium nitrate or chloride hydrate in place of gadolinium perchlorate afforded the 48-metal complex under nearly identical reaction conditions.

The 38 Gd³⁺ ions in the smaller complex are organized into a cage-like structure featuring twelve vertex-sharing {Gd₄} tetrahedra with the metals joined together by one μ-O, μ ₈-ClO₄ ⁻, μ ₃-OH, and caa carboxylate groups. The coordination sphere is completed by aqua and ethanol ligands (Fig. 16a). In comparison, the 48 metal atoms in the larger complex are connected by six μ ₄-O anions, 84 μ ₃-OH groups_, and caa carboxylate groups into a barrel-like structure. One NO₃ ⁻ anion and two Cl⁻ anions are imbedded inside the void of the barrel through hydrogen bonding. Aqua and ethanol ligands as well as NO₃ ⁻ anions complete the lanthanide coordination (Fig. 16b).

Fig. 16

Structure of: (a) {Gd₃₈(O)(ClO₄)₆(OH)₄₂}⁶⁴⁺ and (b) {Gd₄₈(μ ₄-O)₆(μ ₃-OH)₈₄Cl₂(NO₃)}. Reprinted with the permission from [34] Copyright 2013 Wiley-VCH Verlag GmbH & Co

Although nicotinic acid was used, a very similar hydroxide cluster core as in the work by Tong et al. [34] with an identical arrangement of 48 lanthanide ions was reported by Hong and coworkers [35]. Specifically, using a mixture of NaN₃, nicotinic acid, NaNO₃, and erbium chloride, a hydrothermal reaction produced {[Cl₂&(NO₃)]@[Er₄₈(nic)₄₄(OH)₉₀(N₃)(H₂O)₂₄]}·6Cl·35H₂O wherein the occlusion of two Cl⁻ and one NO₃ ⁻ ions (Fig. 17b) is also observed. The barrel-like cluster core may alternatively be viewed as an {Er₁₂} ring being sandwiched between two {Er₁₈} wheels (Fig. 17a). The assembly of the two almost identical cluster cores suggests that this cluster motif, though obtained in different reactions, may be a common one in the family of lanthanide hydroxide complexes.

Fig. 17

Structure of: (a) the cluster core of {[Cl₂&(NO₃)]@[Er₄₈(nic)₄₄(OH)₉₀(N₃)-(H₂O)₂₄]}6Cl·35H₂O with the encapsulated anions and bridging atoms displayed and (b) the metallic backbone of the cluster core. Reprinted with the permission from [35] Copyright 2013 Royal Society of Chemistry

The significant templating roles played by small anions are further exemplified in the assembly of [Er₆₀(l-thre)₃₄(μ ₆-CO₃)₈(μ ₃-OH)₉₆(μ ₂-O)₂(H₂O)₁₈]Br₁₂(ClO₄)₁₈(H₂O)₄₀ (l-thre = l-threonine) by Zheng and his coworkers. This giant complex was prepared by the hydrolysis of Er³⁺ using l-threonine as supporting ligand [36]. The 60 Er³⁺ ions are arranged into a discrete sodalite cage (Fig. 18a) with each of its 24 vertices being occupied by an [Er₄(μ ₃-OH)₄]⁸⁺ cubane unit (Fig. 18b). Alternatively, the cage structure can be viewed as being built by using two different yet related cubane-wheel second-building units (SBUs), one being dodecanuclear (composed of four vertex-sharing cubanes) and the other octadecanuclear (consisting of six vertex-sharing cubanes) with the latter being templated by a μ ₆-CO₃ ²⁻ ion. The lanthanide hydroxide cluster core is encapsulated by l-thre⁻ ligands (Fig. 18c).

Fig. 18

(a) Structure of the 24-cubane cluster core in [Er₆₀(l-thre)₃₄(μ ₆-CO₃)₈(μ ₃-OH)₉₆ (μ ₂-O)₂(H₂O)₁₈]³⁰⁺; (b) formal assembly of a discrete sodalite cage by using cluster cubane units as SBUs; and (c) structure of the cationic 60-metal complex. Reprinted with the permission from [36] Copyright 2009 American Chemical Society

More recently, three isostructural cluster complexes with a record-high 104 lanthanide atoms were reported by Long and his coworkers [37]. These complexes, formulated as [Nd₁₀₄(ClO₄)₆(CH₃COO)₆₀(μ ₃-OH)₁₆₈(μ ₄-O)₃₀(H₂O)₁₁₂](ClO₄)₁₈·(CH₃CH₂OH)₈·xH₂O and [Ln₁₀₄(ClO₄)₆(CH₃COO)₅₆(μ ₃-OH)₁₆₈ (μ ₄-O)₃₀(H₂O)₁₁₂]·(ClO₄)₂₂·(CH₃CH₂OH)₂·xH₂O (Ln = Nd, Gd), were obtained from the reaction of N-acetyl-d-glucosamine, Co(CH₃COO)₂·4H₂O, and lanthanide perchlorate in ethanol under either solvothermal or ambient-pressure conditions. It is of note that Co²⁺ was not incorporated into the product, but replacing the transition metal acetate for sodium acetate did not lead to the same cluster species; the role of the transition metal ion remains unclear. The 104 metal ions are organized into an aesthetically pleasing four-shell cage structure with an ideal cubic symmetry (Fig. 19a). An alternative way of looking at the structure is that it can be built by 24 square pyramidal [Ln₅(μ ₄-O)(μ ₃-OH)₄]⁹⁺ and 8 [Ln(μ ₃-OH)₆]³⁻ units. Every four adjacent units of [Ln₅(μ ₄-O)(μ ₃-OH)₄]⁹⁺ are joined together by centering around one μ ₄-O²⁻ anion to form an [Ln₁₆(μ ₄-O)₅(μ ₃-OH)₂₀]¹⁸⁺ wheel that occupies one vertex of a perfect octahedron (Fig. 19b). The Ln³⁺ ions are further connected by acetate ligands (Fig. 19c). Water molecules and 6 ClO₄ ⁻ anions are encapsulated within the void of the nanoscopic cluster. The 104-Gd complex has been shown to possess one of the largest magnetocaloric effects measured for all lanthanide-exclusive clusters reported. These magnetic lanthanide clusters are of interest in developing energetically more efficient and more environmentally friendly cooling technologies.

Fig. 19

(a) Four-shell organization of the 104 lanthanide atoms; (b) building the cluster core by using square pyramidal SBUs; and (c) structure of the cationic complex [Ln₁₀₄(μ ₃-OH)₁₆₈ (μ ₄-O)₃₀]⁸⁴⁺ showing the coordination/passivation of the cluster core by acetate ligands. Reprinted with the permission from [37] Copyright 2014 American Chemical Society

2.2 Diketonates

Equally extensively utilized in lanthanide coordination chemistry are diketonate-based ligands [38–43]. Recent years have seen their increasing use in supporting the assembly of lanthanide hydroxide cluster complexes [44, 45]. A number of cluster core motifs have been reported and are collected in Fig. 20 with the planar tetranuclear and square pyramidal pentanuclear motifs being more frequently observed than the rest. We note that lanthanide hydroxide clusters supported by carboxylate-based ligands exhibit a much greater structural variety than those with diketonate ligands.

Fig. 20

Structure of representative core motifs in lanthanide-oxo/hydroxo cluster complexes supported by diketonate-based ligands

2.2.1 Clusters with Nuclearity Smaller Than Nine

Trinuclear lanthanide hydroxide cluster complexes are not common. [46] One recent example is [Dy₃(OH)(teaH₂)₃(paa)₃]Cl₂·MeCN·4H₂O (teaH₃ = triethanolamine; paaH = N-(2-pyridyl)-acetoacetamide) wherein deprotonated teaH₃ and paaH were used together to support a cuboidal or incomplete cubane core of [Dy₃(μ ₃-OH)] [29]. The three Dy³⁺ ions together with the μ ₃-OH group form a pyramid that is encapsulated by the diketonate ligands. Each of the three Dy…Dy edges is bridged by one ethanoxide O atom of one teaH₂ ligand that also uses its N atom and the remaining two ethanol OH groups to coordinate the same lanthanide ion. Each lanthanide ion is also chelated by two ketonate O atoms of a paa ligand (Fig. 21).

Fig. 21

Side view (a) and top view (b) of [Dy₃(OH)(teaH₂)₃(paa)₃]²⁺. Reprinted with the permission from [29] Copyright 2013 Elsevier

Tetranuclear cluster motifs are either a distorted cubane or a planar arrangement of 4 lanthanide ions with two μ ₃-OH groups. Both motifs, already discussed above, have seen frequent occurrence in the literature.

Zheng et al. conducted a systematic study on utilizing acetylacetonate (acac) as protecting ligand to support the assembly of hydroxide cluster complexes in organic solution. A structure representing the isostructural tetranuclear complexes Ln₄(μ ₃-OH)₂(μ ₃-OCH₃)₂(CH₃OH)₂(acac)₈ (Ln = Nd, Sm) is shown in Fig. 22 [47]. The cubane cluster core of [Ln₄(μ ₃-OH)₂(μ ₃-OCH₃)₂]⁸⁺ features coordination by two μ ₃-OH and two μ ₃-OCH₃ ⁻ groups (Fig. 22a). Each Ln³⁺ ion is chelated by two acac ligands with two of the lanthanide ions being also coordinated with one methanol molecule (Fig. 22b).

Fig. 22

Structures of: (a) the cubane cluster core and (b) [Ln₄(μ ₃-OH)₂(μ ₃-OCH₃)₂(CH₃OH)₂(acac)₈] [47]. Reprinted with the permission from [47] Copyright 2011 Royal Society of Chemistry

Using ortho ring-functionalized 1-phenylbutane-1,3-dione ligands bearing nitro (Hnpd and Hnmc), methoxy (Hmmc), or fluoro (Hfpp) groups, MacLellan et al. reported a series of tetranuclear hydroxide cluster complexes [Er₄(μ ₃-OH)₄(H₂O)₂(npd)₈], [Ln₄(μ ₃-OH)₄(nmc)₈]·(Ln = Gd, Tb, Dy and Er), [Er₄(μ ₃-OH)₄(mmc)₈], and [Er₄(μ ₃-OH)₄(H₂O)₂(fpp)₈] [48]. These complexes were prepared in methanol using a reaction mixture containing lanthanide chloride hydrates, one of the diketone ligands, and trimethylamine; the organic base is responsible for promoting the hydrolysis of the lanthanide hydrates.

All of these complexes contain the same cubane cluster core, and the structures of [Er₄(μ ₃-OH)₄(H₂O)₂(npd)₈] and [Er₄(μ ₃-OH)₄(nmc)₈] are shown in Fig. 23. In both complexes, each of the Er³⁺ ions is chelated by two diketonate ligands (either npd or nmc). In [Er₄(μ ₃-OH)₄(H₂O)₂(npd)₈], two of the four Er³⁺ ions are also each coordinated by one aqua ligand in addition to two chelating diketonate ligands (Fig. 23a).

Fig. 23

Structure of: (a) [Er₄(μ ₃-OH)₄(H₂O)₂(npd)₈] and (b) [Er₄(μ ₃-OH)₄(nmc)₈]. Reprinted with the permission from [48] Copyright 2011 Royal Society of Chemistry

A number of tetranuclear complexes with different diketonate ligands but bearing the same planar cluster motif have been reported. Shown in Fig. 24 are the structures of [Er₄(dbm)₆(O-btd)₄(μ ₃-OH)₂] and [Er₄(dbm)₄(O-btd)₆(μ ₃-OH)₂] (dbm = dibenzoylmethanide; O-btd = 4-hydroxo-2,1,3-benzothiadiazolate) [49]. In both clusters, the planar rhomboid cluster core is encapsulated by a combination of the bridging–chelating O-btd ligands and chelating-only dbm ligands.

Fig. 24

Structure of: (a) [Er₄(dbm)₆(O-btd)₄(OH)₂] and (b) [Er₄(dbm)₄(O-btd)₆(OH)₂]. Reprinted with the permission from [49] Copyright 2015 Royal Society of Chemistry

Using a combination of Hacac and H₂L₆ = N,N′-bis(salicylidene)-1,2-cyclohexanediamine, Sun et al. obtained four isostructural complexes of the common formula [Ln₄(μ ₃-OH)₂(L₆)₂(acac)₆]·xH₂L₆·yCH₃CN·zH₂O·(Ln = Sm, Gd, Tb, and Dy) [50]. The synthesis was carried out by slowly adding a methanolic solution of a lanthanide acetylacetonate hydrate to an acetonitrile solution of H₂L₆, followed by reflux of the resulting solution mixture. Two opposite edges of the rhomboid cluster core are each bridged by one L₆phenol O, while the other two edges are each bridged by one L₆phenol O as well as one O atom of the chelating–bridging acac ligand. Each of the lanthanide ions is further coordinated by one chelating-only acac ligand (Fig. 25).

Fig. 25

Structure of [Dy₄(μ ₃-OH)₂(L₆)₂(acac)₆]. Reprinted with the permission from [50] Copyright 2011 American Chemical Society

Lastly, Urbatsch et al. reported two tetranuclear complexes formulated as [Ln₄(μ ₃-OH)₂{(μ-O)-k²-htp}₂{(μ-O)₂-k²-htp}₂(k²-htp)₆] (Ln = Nd, Eu) by using (Z)-3-hydroxy-3-phenyl-1-(thiophen-2-yl)prop-2-en-1-one (Hhtp; Fig. 26a) – a thiophene-containing β-diketone – as supporting ligand [51]. The rhomboid cluster core is coordinated with the diketonate ligands in three different modes: Two opposite edges are each uniquely bridged by the two O atoms from the same htp ligand, while the other two are each bridged by one htpO atom; these latter two htp ligands each chelate one lanthanide ion. The remaining 6 htp ligands are of the chelating-only type to complete the octacoordinate sphere for each lanthanide ion (Fig. 26b).

Fig. 26

(a) Schematic illustration of the Hhtp ligand and (b) structure of [Eu₄(μ ₃-OH)₂{(μ-O)-k²-htp}₂{(μ-O)₂-k²-htp}₂(k²-htp)₆]. Reprinted with the permission from [51] Copyright 2012 Wiley-VCH Verlag GmbH & Co

Together with the above tetranuclear complexes, a pentanuclear complex Er₅(μ ₃-OH)₄(μ ₄-OH)(μ-η ²-htp}₄(η ²-htp)₆ was also isolated [51]. Though not entirely clear, the formation of a larger cluster under otherwise identical reaction conditions may be due to the difference in the lanthanide ion size (Er³⁺ versus Nd³⁺/Eu³⁺). The five Er³⁺ ions are organized into a square pyramid with a μ ₄-OH group situated at the center of its basal plane and coordinating all four basal Er³⁺ ions. Each of the four triangular faces of the square pyramid is capped by a μ ₃-OH group. This [Er₅(μ ₃-OH)₄(μ ₄-OH)]¹⁰⁺ core is encapsulated in the coordination sphere formed by 10 htp ligands, of which four are bridging–chelating that uses one of its two ketonate O atoms to bridge one basal Er…Er linkage while using the very same O atom together with the other ketonate O to chelate one of the Er³⁺ ions. Each basal Er³⁺ is also coordinated by a second chelating-only htp ligand. The Er³⁺ ion at the axial position is unique; it is coordinated by two chelating-only htp ligands (Fig. 27).

Fig. 27

Structure of [Er₅(μ ₃-OH)₄(μ ₄-OH)(μ-η ²-htp}₄(η ²-htp)₆). Reprinted with the permission from [51] Copyright 2012 Wiley-VCH Verlag GmbH & Co

It appears that such coordinating modes are prevalent. Despite the different diketonate ligands used, the same coordination modes have been observed in the six isostructural pentanuclear complexes Ln₅(dbm)₁₀(μ ₃-OH)₄(μ ₄-OH)·n(solvent) (Ln = Nd, Eu, Gd, Tb, Er, Yb; solvent = acetonitrile or toluene) independently reported by Holiday, Luneau, and their respective coworkers [52, 53], as well as in Ln₅(μ ₃-OH)₄(μ ₄-OH)(Iphacac)₁₀ (Ln = Tb, Dy, Yb; bis(para-iododibenzoyl)-methanide = Iphacac) by Thielemann et al. [54], and in Ln₅(μ ₃-OH)₄(μ ₄-OH)(L₇)₁₀ (Ln = Eu, Ho; 1,3-bis(4-ethoxyphenyl)propane-1,3-dione = HL₇) by Silberstein and his coworkers [55].

Interestingly, some of the diketonate ligands can be replaced by other types of ligands that are capable of both bridging and chelating metal ions. Roesky et al. reported four pentanuclear yttrium complexes of the general formula [Y₅(μ ₃-OH)₄(μ ₄-OH)(α-AA)₄(dbm)₆] (α-AA = d-phenyl glycine; l-proline; l-valine; and l-tryptophan) in which the amino acids can be viewed as a diketonate surrogate [56]. Each of the amino acid ligands uses one of its carboxylate O atom to bridge one basal Ln…Ln linkage while using this very same O atom together with the amino N atom to chelate one of the four basal lanthanide ions; the other carboxylate O remains uncoordinated (Fig. 28).

Fig. 28

Structure of [Y₅(μ ₃-OH)₄(μ ₄-OH)(d-PhGly)₄(dbm)₆]. Reprinted with the permission from [56] Copyright 2010 Royal Society of Chemistry

Using o-hydroxydibenzoylmethane (HO-Hdbm) as supporting ligand, Baskar et al. were able to obtain a hexanuclear complex [Y₆(O-dbm)₆(HO-dbm)₄(μ ₃-OH)₂(MeOH)₄] with a rare core motif (Fig. 29) [57]. The six Y³⁺ ions are nearly coplanar and can be viewed as being constructed by adding one Y³⁺ ion on each side of the planar tetranulcear motif. This is made possible by the coordination of the phenoxide O of 6 doubly negatively charged O-dbm ligands. The four Y³⁺ ions in the central rhomboid are bridged by two μ ₃-OH groups with each of its edges being bridged by one O-dbm O atom. This very O atom, together with the phenoxide O of the same ligand, chelates one of the four Y³⁺ ions, while the same phenoxide O also bridges the added terminal Y³⁺ ion. Each of these two added Y³⁺ ions is additionally coordinated by two chelating HO-dbm whose phenol moiety remains neutral and uncoordinated (Fig. 29).

Fig. 29

Structure of [Y₆(O-dbm)₆(HO-dbm)₄(μ ₃-OH)₂(MeOH)₄]. Reprinted with the permission from [57] Copyright 2009 Elsevier

When in assessing the influence of ligand sterics on the structure of the resulting hydroxide clusters, Luneau et al. obtained two octanuclear complexes [Ln₈(thd)₁₀(μ ₄-O)(μ ₃-OH)₁₂] (Ln = Eu, Y) with 2,2,6,6-tetramethylheptane-3,5-dione (Hthd), while pentanuclear clusters were isolated when dbm was the hydrolysis-limiting ligand under otherwise identical conditions [52]. The eight Ln³⁺ ions in the cluster core are bridged by one μ ₄-O group and twelve μ ₃-OH groups (Fig. 30a). The structure may be viewed as two distorted cubanes joined together by the μ ₄-O group with the Ln…Ln linkages (Y1…Y2) at the juncture disposed orthogonally with each other. Each lanthanide ion is chelated by one thd ligand. There are two additional ligands, each bridging a pair of “external” lanthanide ions that are not associated with the μ ₄-O group (Fig. 30b).

Fig. 30

Structure of: (a) the octanuclear cluster core with bridging oxo/hydroxo groups displayed and (b) [Y₈(thd)₁₀(μ ₄-O)(μ ₃-OH)₁₂]. Reprinted with the permission from [52] Copyright 2009 Royal Society of Chemistry

2.2.2 Nonanuclear or Higher-Nuclearity Clusters

From the hydroxide complexes presented above, it becomes clear that higher-nuclearity cluster core motifs can be formally constructed by using smaller polyhedral units as SBUs. If two pentanuclear square pyramids are joined by sharing the non-basal vertex lanthanide ion, an hourglass-shaped nonanuclear motif is produced (Fig. 31a). Such a core motif is present in [Ln₉(acac)₁₆(μ ₃-OH)₈(μ ₄-O)(μ ₄-OH)]·H₂O (Ln = Eu, Gd, Tb, Dy, Er, Yb, Y), prepared independently by Luneau, Zheng, and their respective coworkers [47, 52, 58]. The two recognizable pentanuclear units are disposed 90° with respect to each other. One of the basal planes is capped by a μ ₄-O group, while the other one by a μ ₄-OH group. Each of the triangular faces is capped by a μ ₃-OH to give the core formula of [Ln₉(μ ₃-OH)₈(μ ₄-O)(μ ₄-OH)]¹⁶⁺. The acac ligands provide the coordination sheath with 8 being chelating only for each of the basal lanthanide ions and the remaining 8 being both chelating and bridging (Fig. 31b). It should be noted that the ligand coordination mode for the basal lanthanide ions is the same as exhibited by ketonate ligands in the pentanuclear cluster complexes discussed above.

Fig. 31

Structure of: (a) the nonanuclear cluster core with bridging oxo/hydroxo groups displayed and (b) [Ln₉(acac)₁₆(μ ₃-OH)₈(μ ₄-O)(μ ₄-OH)]. Reprinted with the permission from [47] Copyright 2011 Royal Society of Chemistry

The vertex-sharing can occur between other types of SBUs that are of the same or different kinds. Shown in Fig. 32 is the structure of a tetradecanuclear complex [Dy₁₄(μ ₄-OH)₂(μ ₃-OH)₁₆(μ-η ²-acac)₈(η ²-acac)₁₆] [59]. The cluster core can be formally built by sandwiching a hexanuclear octahedral hydroxide unit between two pentanuclear square pyramids via vertex-sharing. The two terminal square basal faces are each capped by a μ ₄-OH group. The coordination modes of the acac ligands for the basal lanthanide ions are exactly the same as seen above in the nonanuclear complex. The remaining eight acac ligands are chelating only, two on each of the four equatorial lanthanide ions of the central octahedral unit.

Fig. 32

Structure of [Dy₁₄(μ ₄-OH)₂(μ ₃-OH)₁₆(μ-η ²-acac)₈(η ²-acac)₁₆]. Reprinted with the permission from [59] Copyright 2011 Royal Society of Chemistry

Pentadecanuclear hydroxide complexes with tyrosinate as supporting ligand are arguably one of the more notable high-nuclearity lanthanide clusters whose core motif features five cubane units joined together by sharing two vertex lanthanide ions with the assistance of a templating halide ion [11]. More recently, Roesky et al. reported a series of isostructural cluster complexes of the general formula [Ln₁₅(μ ₃-OH)₂₀(PepCO₂)₁₀(dbm)₁₀Cl]·Cl₄·(PepCO₂ = 2-[{3-(((tert-butoxycarbonyl)amino)methyl)benzyl}-amino]acetate; Ln = Eu, Tb, Dy, Y) that contain the same pentadecanuclear core (Fig. 33) [60, 61]. The Eu³⁺ and Tb³⁺ complexes have been shown to luminesce in cellular structures and are therefore potentially useful for biological imaging and immunoassay.

Fig. 33

Structure of: (a) the pentadecanuclear core with a templating chloride ion and (b) [Tb₁₅(μ ₃-OH)₂₀(PepCO₂)₁₀(dbm)₁₀Cl]⁴⁺. Reprinted with the permission from [60] Copyright 2013 American Chemical Society

2.3 Phosphonates and Sulfonates

Inspired by the great success of utilizing carboxylate and diketonate ligands as supporting ligands for the assembly of high-nuclearity lanthanide hydroxide clusters, chemists have also turned to other O-based ligands such as phosphonates and sulfonates with the hopes of creating cluster complexes with novel structures and properties. Such ligands can be used alone or in combination with other type(s) of ligands.

Treating lanthanide nitrate hydrates with pyridine in the presence of pivalic acid and t-butyl phosphonic acid in isobutanol under reflux led to the production of a series of tetranuclear lanthanide hydroxide complexes of the formula [pyH]₄[Ln₄(μ ₃-OH)(O₃P^tBu)₃(HO₃P^tBu)(O₂C^tBu)₂(NO₃)₆]·(Ln = Gd, Tb, Dy, Ho, Er) [62]. The representative structure of the Gd³⁺ complex is shown in Fig. 34. The tetranuclear core may be viewed as a μ ₃-OH-bridged trinuclear cuboidal unit being connected to the fourth metal through the bridging of three μ ₃-O₃P^tBu²⁻ ligands; each of the phosphate ligands uses one of its O atom to coordinate this fourth metal, the second to bridge two adjacent lanthanide ions within the cuboidal units, and the third, together with the second one, to chelate one of the three metals in the cuboidal unit. In addition, there are one μ ₂-HO₃P^tBu⁻ and two μ ₂-O₂C^tBu⁻ ligands along the edge of the cuboidal unit, each bridging a pair of adjacent lanthanide ions. Each lanthanide ion within the cuboidal unit is also chelated by an NO₃ ⁻ anion, whereas the coordination sphere of the fourth metal atom is completed with three chelating NO₃ ⁻ anions (Fig. 34).

Fig. 34

Structure of the anionic cluster complex [Gd₄(μ ₃-OH)(O₃P^tBu)₃(HO₃P^tBu)(O₂C^tBu)₂(NO₃)₆]⁴⁻. Reprinted with the permission from [62] Copyright 2014 Royal Society of Chemistry

With the same ligand set but using isopropylamine in place of pyridine to promote hydrolysis, Winpenny et al. were able to obtain three isostructural octanuclear complexes of the formula [Ln₈(O₃P^tBu)₆(μ ₃-OH)₂(H₂O)₂(HOⁱBu)(O₂C^tBu)₁₂](NH₃ ⁱPr)₂·(Ln = Gd, Dy, Tb; ⁱPrNH₂ = isopropylamine; HOⁱBu = isobutyl alcohol) [63]. The eight Ln³⁺ ions are arranged into a horseshoe-like structure (Fig. 35a) with the component lanthanide ions bridged together by six O₃P^tBu²⁻ ligands, two μ ₂-O₂C^tBu⁻, and four μ-η ²-O₂C^tBu⁻ ligands (Fig. 35b). The coordination spheres are completed by bidentate chelating O₂C^tBu⁻ ligands, HOⁱBu molecules, and aqua ligands. Alternatively, the cluster motif can be viewed as two μ ₃-OH-containing cuboidal units joined together by two O₃P^tBu²⁻ and one μ-η ²-O₂C^tBu⁻ ligands with an add-on lanthanide ion on each side of the double-cuboidal arrangement.

Fig. 35

Structure of: (a) the octanuclear core with bridging atoms displayed and (b) [Ln₈(O₃P^tBu)₆(μ ₃-OH)₂(H₂O)₂(HOⁱBu)(O₂C^tBu)₁₂](NH₃ ⁱPr)₂. Reprinted with the permission from [63] Copyright 2013 Royal Society of Chemistry

Cao et al. reported two isostructural nonanuclear complexes [Ln₉(μ ₂-OH)(Hpmp)₁₂(ClO₄)(H₂O)₂₆](ClO₄)₁₃·18H₂O (Ln = Nd, Pr) by using N-pipe-ridinomethane-1-phosphonic acid (H₂pmp) as supporting ligand [64]. The synthesis was carried out by adding NaOH into an aqueous solution containing H₂pmp·HCl and lanthanide perchlorate salt until pH reached about 6.2. The nine Ln³⁺ ions are organized into a unique lotus-leaf-shaped arrangement with one μ ₂-OH group and 12 phosphonate bridging ligands. The coordination spheres are completed with ClO₄ ⁻ anion and aqua ligands (Fig. 36).

Fig. 36

Structure of cationic complex of [Nd₉(μ ₂-OH)(Hpmp)₁₂(ClO₄)(H₂O)₂₆]¹³⁺. Reprinted with the permission from [64] Copyright 2009 Elsevier

Phosphate can also be used as a second or ancillary ligand to support the assembly of polynuclear lanthanide hydroxide complexes. As an example, Hong et al. reported two isostructural decanuclear complexes [Ln₁₀(TBC8A)₂(PhPO₃)₄(OH)₂(HCO₃)(HCOO)(DMF)₁₄]·(H₆TBC8A)·xDMF·yCH₃OH·(Ln = Pr, Nd; H₈TBC8A = p-tert-butylcalix[8]arene; H₂PhPO₃ = phenylphosphonic acid) using a mixture of lanthanide nitrate hydrate, H₈TBC8A, and H₂PhPO₃ in DMF/methanol [65]. The ten Ln³⁺ ions are encapsulated by two TBC8A⁸⁻ ligands that are in their cup-conformation with their lower-rim phenoxide O atoms coordinating the lanthanide ions (Fig. 37). Four PhPO₃ ²⁻ ligands, two OH⁻ groups, one HCO₃ ⁻ anion, and one HCOO⁻ ligand help further stabilize the multinuclear arrangement of the core. In the crystal lattice, the [Ln₁₀(TBC8A)₂(PhPO₃)₄(OH)₂(HCO₃)(HCOO)(DMF)₁₄]²⁺ cations and (H₆TBC8A)²⁻ anions are arranged alternatively to the stable crystalline bulk phase.

Fig. 37

Structure of [Ln₁₀(TBC8A)₂(PhPO₃)₄(OH)₂(HCO₃)(HCOO)(DMF)₁₄](H₆TBC8A). Reprinted with the permission from [65] Copyright 2015 Elsevier

There is one report of lanthanide hydroxide cluster complex featuring a sulfonate supporting ligand. Zhang et al. isolated a hexanuclear complex [Yb₆(μ ₆-O)(μ ₃-OH)₈(mds)₄(H₂O)₆] by reacting Yb₂O₃ with methylenedisulfonic acid (H₂mds) under hydrothermal conditions [66]. As shown in Fig. 38, the six Yb³⁺ ions are organized into a μ ₆-O-centered regular octahedron with each of the triangular faces being capped by one μ ₃-OH group. The complex unit is not a discrete one, however. They are instead connected into a one-dimensional column structure via two opposite Yb³⁺ ions by way of mds coordination. Specifically, the disulfonate ligand is bridging with each of its two sulfonate groups contributing one O for the coordination of one Yb³⁺ from different cluster unit. This unit-connecting Yb³⁺ is also coordinated by two aqua ligands. The remaining four Yb³⁺ ions are of two different types in terms of their coordination spheres: Both are coordinated by one chelating mds ligand with one of them also coordinated by an aqua ligand but not the other.

Fig. 38

Structure of Yb₆(μ ₆-O)(μ ₃-OH)₈(mds)₄(H₂O)₆. Reprinted with the permission from [66] Copyright 2008 IUCr Journals

2.4 Polyoxometalates

Another emerging class of ligands to support the assembly of lanthanide hydroxide complexes are POMs. POMs are known for their facile synthesis and tunable chemical composition [67–69], and have been found to be valuable for promoting many organic transformations [70, 71], catalyzing water-splitting process [72], and making novel memory devices [73, 74].

The increasing use of POMs in lanthanide coordination is due presumably to two reasons. First, POMs are anions with a large number of O atoms on the surface. The electrostatic attractions between lanthanide ions and a POM, together with the desirable hard Lewis acid/base match, make POMs an attractive class of protecting ligands and/or templating anions for the assembly of lanthanide clusters. We must note that: (1) not all the examples shown below have the polyhedral Ln-O/OH motif to be qualified as “lanthanide oxide or hydroxide clusters”; the hydroxo groups in some cases are in fact associated with the metal ion in the POM ligands rather than a lanthanide ion and (2) many of the species actually contain lanthanide atoms that are separated by a distance beyond what is anticipated for a conventional cluster motif. Second, as POMs are generally weakly coordinating, the Lewis acidity of lanthanide ions is enhanced in an Ln-POM combination with respect to the complexes with more strongly coordinating ligands. This feature may help enhance the catalytic efficiency when Ln-POM complexes are used in Lewis acid-promoted reactions.

Zhang et al. reported three isostructural lanthanide tungstobismuthate complexes Na _x H_22-x {(BiW₉O₃₃)₄(WO₃)[Bi₆(μ ₃-O)₄(μ ₂-OH)₃][Ln₃(H₂O)₆(CO₃)]}·nH₂O·(Ln = Pr, Nd, La) from an aqueous reaction involving Na₁₂[Bi₂W₂₂O₇₄(OH)₂]·44H₂O, a lanthanide chloride hydrate, Na₉[BiW₉O₃₃]·16H₂O, and Na₂CO₃ [75]. As shown in Fig. 39, the three Ln³⁺ ions are organized into a trigonal planar arrangement around a μ ₃-CO₃ ²⁻ that uses each of its O atoms to link two lanthanide ions along the edge of the triangle. Each Ln³⁺ ion is further coordinated with two aqua ligands. This [Ln₃(H₂O)₆(CO₃)]⁷⁺ motif is then encapsulated by four [BiW₉O₃₃]⁹⁻ anions, three of which being directly connected to the [Ln₃(H₂O)₆(CO₃)]⁷⁺ core with the fourth one through one [Bi₆(μ ₃-O)₄(μ ₂-OH)₃]⁷⁺ unit.

Fig. 39

(a) Structure of {(BiW₉O₃₃)₄(WO₃)[Bi₆(μ ₃-O)₄(μ ₂-OH)₃][Pr₃(H₂O)₆(CO₃)]}²²⁻ and (b) building blocks for the cluster (H atoms and lattice solvent molecules were omitted for clarity). Reprinted with the permission from [75] Copyright 2012 Royal Society of Chemistry

By reacting Na₂WO₄·2H₂O, oxalic acid, and lanthanide chloride, without or with the presence KCl in an aqueous solution at pH 7.5, Chen et al. obtained Na₁₀[Ln₂(C₂O₄)(H₂O)₄(μ ₂-OH)(W₄O₁₆)]₂·30H₂O and K₄Na₁₆[Ln(C₂O₄)(W₅O₁₈)]₄·60H₂O·(Ln = Eu, Ho, Er, Tb), respectively [76]. As shown in Fig. 40a, the core of the former consists of a rectangular arrangement of four lanthanide ions with its two longer sides each being bridged by a C₂O₄ ²⁻ ligand and the shorter side by a μ ₂-OH group. In addition, there are two aqua ligands on each of the lanthanide ions. This core motif is then sandwiched along the direction of the longer side by two W₄O₁₆ ⁸⁻ units via O-Ln coordination to bridge the two lanthanide ions along the shorter side.

Fig. 40

Structure of: (a) {[Eu₂(C₂O₄)(H₂O)₄(μ ₂-OH)(W₄O₁₆)]₂}¹⁰⁻ (top) and cluster core (bottom) and (b) {[Eu(C₂O₄)(W₅O₁₈)]₄}²⁰⁻ (top) and the cluster core (bottom). Reprinted with the permission from [76] Copyright 2014 American Chemical Society

In the latter cluster complex, the four lanthanide ions are arranged into a square with each of its sides being bridged by a C₂O₄ ²⁻ ligand; there are no aqua ligands or hydroxo groups (Fig. 40b). Each of the lanthanide ions is then coordinated via O-coordination to one W₅O₁₈ ⁶⁻ capping ligand.

Another series of tetranuclear Ln-POM complexes, formulated as [PMo^V ₈Mo^VI ₄O₃₆[Ln(H₂O)₄(OH)]₄]·Cl₅·xH₂O·(Ln = La, Ce, Nd, Sm), were reported by Dolbecq and coworkers [77]. The [PMo^V ₈Mo^VI ₄O₃₆]¹¹⁻ anion in these compounds serves as a support or platform for the attachment of four [Ln(H₂O)₄(OH)]²⁺ units into a tetrahedral arrangement (Fig. 41).

Fig. 41

(a) Structure of [PMo^V ₈Mo^VI ₄O₃₆]¹¹⁻ illustrating the triangular face used for the coordination of the [Ln(H₂O)₄(OH)]²⁺ unit and (b) structure of {PMo^V ₈Mo^VI ₄O₃₆[Ln(H₂O)₄(OH)]₄}⁵⁺. Reprinted with the permission from [77] Copyright 2011 Wiley-VCH Verlag GmbH & Co

It is clear from the above examples, POM ligands can be used to protect lanthanide ions by encapsulation and to support the attachment of lanthanide ions onto their surface. In a rare example provided by Wang et al., these two coordination modes of the POM ligands are demonstrated. Shown in Fig. 42 is the chain structure of the anionic complex in Na₁₀[Ln₆(H₂O)_x{As₄W₄₄(OH)₂(proline)₂O₁₅₁}]·nH₂O·(Ln = Tb, Dy, Nd) linked through hydrated Ln³⁺ ions [78].

Fig. 42

Structure of: (a) the [Ln₄(H₂O)₁₆{As₄W₄₄(OH)₂(proline)₂O₁₅₁}]¹⁶⁻ building block and (b) [Ln₆(H₂O) _x {As₄W₄₄(OH)₂(proline)₂O₁₅₁}]¹⁰⁻. Reprinted with the permission from [78] Copyright 2013 Royal Society of Chemistry

Linkages can also be provided by organic ligands that serve to coordinate metal ions from different Ln-POM SBUs. Shown in Fig. 43 is the structure of the anionic complex unit in K₂₀Li₂[Ln₃(μ ₃-OH)(H₂O)₈(AsW₉O₃₃)(AsW₁₀O₃₅)(mal)]₂·17H₂O·(Ln = Dy, Tb, Gd, Eu, Sm, mal = malate) [79]. It can be viewed as two μ ₃-OH group-bridged cuboidal building blocks connected by two mal ligands. This complex motif is then sandwiched in between one {AsW₉O₃₃} and one {AsW₁₀O₃₅} unit with the latter being also coordinated by one mal ligand. The coordination sphere of the Ln³⁺ ion chelated by the mal ligand is completed by two aqua ligands while that of the other two Ln³⁺ ions is each completed by three aqua ligands.

Fig. 43

Structure of {[Ln₃(μ ₃-OH)(H₂O)₈(AsW₉O₃₃)(AsW₁₀O₃₅)(mal)]₂}²²⁻. Reprinted with the permission from [79] Copyright 2015 Royal Society of Chemistry

Using an aqueous mixture of samarium chloride, Na₂CO₃, KCl, and Na₁₀[A-α-SiW₉O₃₄]·xH₂O, Davoodi et al. obtained [(A-α-SiW₉O₃₄)₂(H₂OSm)₃CO₃]¹³⁻ which decomposed slowly in a concentrated solution to afford an anionic octanuclear complex [(SiW₁₀Sm₂O₃₈)₄(W₃O₈)(OH)₄(H₂O)₂)]²⁶⁻ as the final product [80]. The complex unit can be viewed as eight Sm³⁺ ions wrapping around one [W₃O₈(OH)₄(H₂O)₂]²⁻ template anion. This arrangement is then encapsulated by four [SiW₁₀O₃₈]¹²⁻ ligands (Fig. 44).

Fig. 44

Structure of the [(SiW₁₀Sm₂O₃₈)₄(W₃O₈)(OH)₄(H₂O)₂)]²⁶⁻ cluster core. Reprinted with the permission from [80] Copyright 2012 Elsevier

Lastly, Patzke et al. reported a series of hexadecanuclear lanthanide polyoxotungstate complexes with the core formulated as [Ln₁₆As₁₆W₁₆₄O₅₇₆(OH)₈(H₂O)₄₂]⁸⁰⁻·(Ln = Eu, Gd, Tb, Dy, Ho) [81]. The product was obtained from an aqueous reaction of K₁₄[As₂W₁₉O₆₇(H₂O)], lanthanide nitrate hydrate, NaCl, and CsCl. Each of the 16 Ln³⁺ ions is capped by one {AsW₉O₃₃} unit, and the 16 {LnAsW₉O₃₃} units are connected by 20 tungstate anions, eight OH⁻ groups, and four Cs⁺ cations. The coordination spheres of Ln³⁺ ions are completed by aqua ligands (Fig. 45).

Fig. 45

Structure of the [Ln₁₆As₁₆W₁₆₄O₅₇₆(OH)₈(H₂O)₄₂]⁸⁰⁻ cluster core (color code: blue, Ln; yellow, As; green, W; red, O; orange, Cs; and purple triangular planes, W₃O₁₃ triads of the {AsW₉O₃₃} units). Reprinted with the permission from [81] Copyright 2011 Royal Society of Chemistry

2.5 Miscellaneous Ligands

There are also some structurally interesting lanthanide hydroxide cluster complexes supported by ligands that do not belong to the types discussed above. For example, Alikberova et al. reported two hexanuclear complexes [Ln₆(H₂O)₂₃(OH)₁₀]I₈·8H₂O·(Ln = La, Nd) by directly reacting La₂(CO₃)₃·6H₂O and Nd₂O₃ with an aqueous solution of HI [82]. We note that a very similar octahedral hexanuclear cluster complex with exclusively H₂O-based ligands was previously reported by direct hydrolysis of simple lanthanide salts [83]. The cluster core is essentially the same as in Yb₆(μ ₆-O)(μ ₃-OH)₈(mds)₄(H₂O)₆ discussed above [66] with six Ln³⁺ ions arranged into an octahedron centering around a μ ₆-OH rather than a μ ₆-O group. Each face of the octahedron was capped by one μ ₃-OH group. The coordination spheres of five of the six Ln³⁺ ions are each completed by four aqua ligands, while that of the sixth one is fulfilled by three aqua ligands and one OH⁻ group (Fig. 46).

Fig. 46

Structure of [La₆(H₂O)₂₃(OH)₁₀]⁸⁺. Reprinted with the permission from [82] Copyright 2009 Springer

Complexes with similar hexanuclear core have also been reported with the use of a triazole ligand 4-amino-3,5-dimethyl-1,2,4-triazole (L₈) [84]. Cheng et al. reported a series of hexanuclear complexes of the common formula [Ln₆(μ ₆-O)(μ ₃-OH)₈(L₈)₄(H₂O)₁₄]Cl₈·2 L₈·6H₂O·(Ln = Er, Ho, Dy). As shown in Fig. 47, ligand L₈ bridges four equatorial Ln…Ln edges of the octahedral core. The rest of the coordination sphere is fulfilled by either aqua ligands or an aqua/chloro ligand combination.

Fig. 47

Structure of: (a) the hexanuclear [Ln₆(μ ₆-O)(μ ₃-OH)₈]⁸⁺ cluster core and (b) [Er₆(μ ₆-O)(μ ₃-OH)₈(L₈)₄(H₂O)₁₄]⁸⁺. Reprinted with the permission from [84] Copyright 2009 Royal Society of Chemistry

Batten et al. reported two tetradecanuclear lanthanide hydroxide complexes, [Gd₁₄(CO₃)₁₃(ccnm)₉(OH)(H₂O)₆(phen)₁₃(NO₃)](CO₃)_2.5·(phen)_0.5 and [Dy₁₄(CO₃)₁₃(ccnm)₁₀(OH)(H₂O)₆(phen)₁₃](CO₃)_2.5·(phen)_0.5 by using a combination of 1,10-phenanthroline (phen) and carbamoylcyanonitrosomethanide (ccnm) as the protecting ligands [85]. They share the same cluster core, differing only slightly in the peripheral coordination ligands and anions. The structures of the core and the cationic Gd³⁺ complex are shown in Fig. 48. The 14 Gd³⁺ ions are bridged by one μ ₃-OH group and 13 CO₃ ²⁻ anions (Fig. 48a). The coordination spheres of Gd³⁺ ions are completed by ccnm, phen, aqua, and chelating NO₃ ⁻ ligands (Fig. 48b).

Fig. 48

Structure of: (a) the cluster core [Gd₁₄(CO₃)₁₃(OH)(NO₃)]¹⁴⁺ and (b) [Gd₁₄(CO₃)₁₃(ccnm)₉(OH)(H₂O)₆(phen)₁₃(NO₃)]⁵⁺. Reprinted with the permission from [85] Copyright 2012 Royal Society of Chemistry

3 Summary and Perspectives

In this chapter, we survey the lanthanide hydroxide cluster complexes that are supported by carboxylate, diketonate, phosphate, sulfonate, and POM ligands. The synthetic procedures leading to the production of the cluster species and the unique cluster core motifs are the focus of the discussion. The extensive scope in terms of the variety of ligands to control the lanthanide hydrolysis as well as the diverse structures of the cluster motifs indicates that this sub-area of lanthanide coordination chemistry is full of potential for further synthetic development and materials discovery. The following are the key conclusions drawn from the work summarized in this chapter and that prior to this review:

• 1. It is the high pH at which a reaction is conducted that makes the critical difference in the complex products when compared with lanthanide coordination with the same types of ligands at a lower pH. The assembly of the cluster species hinges upon the formation of the hydroxo intermediate produced upon deprotonation of aqua ligand(s).

• 2. Hydrolysis can occur in either aqueous or organic media, using hydrated lanthanide complexes or salts, or oxides, promoted by using an inorganic or organic base, and under ambient-pressure or hydro/solvothermal conditions.

• 3. The supporting ligands, most of which being organic with O-based functional groups, should support a reasonable degree of water solubility of their complexes if the hydrolysis is to be executed in an aqueous solution. This is exemplified by the results obtained by using amino acids as supporting ligands for lanthanide hydrolysis. In comparison, hydrated complexes with simple carboxylic acids are generally insoluble, and therefore precipitate out before any hydrolysis may occur.

• 4. Supporting ligands can be used alone or in combination with other ligands. The accompanying use of an O-based ligand, inorganic ones included, is generally required if the other supporting ligand does not carry any O-based functional groups. The nature (nuclearity and structure) of the lanthanide hydroxide clusters is critically dependent on the ancillary ligands used.

• 5. A number of factors other than the supporting ligand are also significant in determining the reaction outcome. These include the nature of the lanthanide ions (contrary to the common perception of lanthanide chemistry being similar among different lanthanide ions due to lanthanide contraction), the template effects of certain small anions, and the participation of transition metal ions (not discussed herein).

• 6. Highly sophisticated lanthanide-containing clusters can be formally constructed by using smaller and recognizable cluster units as formal secondary building units (SBUs).

One should not be surprised that the research activities of lanthanide hydroxide clusters will continue to grow with high possibility of finding interesting materials with useful applications that have not yet been realized or even contemplated. Rapid progress notwithstanding, questions such as the scope of the chemistry, the robustness of the synthetic approach, and effects of experimental conditions, and any structure–property relationship remain to be answered. Thus, the primary goal of any future efforts is to systematically assess the effects of factors such as the nature of supporting ligands and metal ions, and experimental conditions on the reaction outcome, with the hopes of developing a robust and generally applicable approach to these unique lanthanide-containing substances. The ultimate goal is to discover lanthanide-containing materials for catalysis, magnetic, optical, biomedical, and other advanced technological applications.