Hydrogen Activation in Water by Organometallic Complexes

Abstract

Hydrogen has found important applications as reducing agent for chemical transformations and is nowadays considered as one of the most promising energy vectors able to fuel devices to produce electricity on demand (direct hydrogen fuel cells). Crucial to its application is the understanding at the molecular level of how hydrogen interacts with (transition) metals which are commonly used as catalysts to lower the energy barrier to split the H₂ molecule into its components and allow transfer and reactivity. In this chapter, selected examples of hydrogen activation by water-soluble organometallic complexes are summarized.

14.1 Introduction

Hydrogen is one of the most widely used chemicals and has found various applications, the most common for the reduction of unsaturated bonds in molecules (hydrogenation) and as energy vector, a field which has recently received large attention from academia and industry in the quest for novel processes and technologies towards the development of a hydrogen economy [1].

In order to react with other molecules, hydrogen needs to be activated, i.e. the H-H covalent bond must be cleaved efficiently under mild conditions, either in a homolytic or heterolytic way. Over the last 80 years, many different ways to achieve this target have been discovered, often based on catalyzed processes in solid–gas phases (heterogeneous) or in liquid–gas phase (homogeneous). The presence of metals, often including noble and non-noble transition ones, was shown to be crucial to lower the activation barriers associated with hydrogenation reactions. It was also discovered that some natural enzymes such as hydrogenases, containing Fe only or Fe-Ni moieties, are nature’s reply to hydrogen activation, and many research groups around the world have been inspired by these observations trying to mimic the active sites of such enzymes to achieve efficient catalysts for such a process.

Already in 1973, James’ monography Homogeneous Hydrogenation [2] cited ca. 2,000 references related to research of the past 20 years. Among these, only a few recognized the role of metals as active sites for hydrogen splitting, for example, Calvin’s report (1938) on quinone hydrogenation based on Cu(I) acetate [3], although it was already known at that time that hydrogen can bind to transition metals such as Fe to give H₂Fe(CO)₄ [4]. However, the first fully characterized transition metal complex showing the presence of M-H bonds, namely, [Cp₂Re(H)₂](BF₄), was reported later on 1955 by Wilkinson et al. [5].

Setting the ground for metal hydride chemistry, many authors contributed to the development of this research area, which grew to a mature field when it was discovered that transition metal hydrides were pivotal for homogeneously catalyzed hydrogenations and olefin hydroformylation [6]. Another important step in the understanding of the nature of hydrogen activation by metals was to establish that this process can occur both in homolytic and heterolytic ways. The next section of this chapter will summarize the fundamental aspects related to these two mechanisms.

Only after some decades, when the use of water as a benign, cheap, abundant and harmless solvent for chemical reactions was proposed to tackle the problems related to the use of toxic organic solvents, chemists considered of interest to run homogenous catalytic processes such as hydrogenation and hydroformylation in this medium. It was soon discovered that the general understanding that water and transition metal hydrides were poorly compatible had significant exceptions and in fact water-soluble complexes could be synthesized, isolated and characterized or clearly identified in water-phase catalytic reactions. The role of water in hydrogen activation mechanism was later identified as non-innocent, e.g. in some cases this solvent can take part in the splitting of dihydrogen by assisting its heterolytic cleavage in the pathway to metal coordination. The understanding of this role has in turn deep consequences in the design of catalysts and evaluation of catalytic processes run in water. In this chapter, selected examples of water-soluble hydrides will be discussed in dedicated sections.

14.2 Hydrogen Coordination and Activation by Metals: The Principles

Already in the 1950s, a considerable debate existed on how hydrogen molecules interact with metal in order to cleave their covalent bond giving initially a M(η²-H₂) nonclassical dihydrogen complex, where H₂ behaves as a ligand and weak Lewis base [7, 8]. The first identified example of this class of complexes is Kubas’ tungsten carbonyl [W(CO₃)(PⁱPr₃)₂(η²-H₂)] complex [9]. The commonly accepted orbital interaction considers the Dewar-Chatt-Duncanson model, similar to olefin π-coordination, with electron donation from the σ-bond of the ligand to an empty metal orbital accompanied by backdonation from a filled d-orbital into the σ*-orbital of the ligand (Scheme 14.1). Theoretical studies clarified further important details of these interactions, namely, the possibility of a nonclassical, 3-centre bonding, the role of backdonation and the effect of trans ligands on binding and activation, the rotation of H₂ ligand. A recent review article by Kubas has recently appeared in the literature covering these and other issues related to dihydrogen coordination to metals [10], following on considerations by the same author on the role of such a class of compounds for hydrogen storage and production [11].

Scheme 14.1

Reproduced from Ref. [10], Copyright 2014, with permission from Elsevier

In the 1990s, further studies based on X-ray crystallography and NMR data highlighted that intermediate bonding situations between nonclassical dihydrogen and dihydride complexes could be possible, the so-called elongated H₂ complexes. Scheme 14.2 summarizes the commonly accepted range of H-H bond lengths corresponding to the various cases, from intact H₂ molecule to fully activated (dihydride situation). It was observed that in passing from M(η²-H₂) to M(H)₂, d _π basicity increased with the H-H bond distance [10, 12].

Scheme 14.2

Reproduced from Ref. [11], Copyright 2014, with permission from Elsevier

In general, first-row transition metals, electron-withdrawing ancillary ligands and cationic complexes favour binding of H₂ to the metal centre and shorten H-H distance. A strong influence on d_H-H, hence on the nature of the resulting complex upon H₂ activation, is also dictated by the nature of the ligand trans to H₂. Strong π-acceptors such as CO reduce backdonation giving rather short d _H-H values, at ca. 0.9 Å.

14.2.1 Homolytic Versus Heterolytic Activation

It is worth here summarizing the two main mechanisms underlying hydrogen activation by metals, i.e. homolytic versus heterolytic cleavage of H₂. As a general observation, while homolytic pathway is favoured by highly basic transition metal centres, the heterolytic counterpart is preferred in the case of electrophilic metal complexes or complexes with highly π-acidic trans ligands. A simplified view of the dual pathways for H₂ bond cleavage is shown in Scheme 14.3 [10].

Scheme 14.3

Reproduced from Ref. [10], Copyright 2014, with permission from Elsevier

In the gas phase, homolytic cleavage of molecular hydrogen is thermodynamically more favoured than the heterolytic one (ΔH° = 436 vs. 1,675 kJ mol⁻¹, respectively), but this can be compensated in water phase by the high proton hydration energy (ΔH° = −1,084 kJ mol⁻¹) together with the binding energy of H⁻ to the metal centres.

The homolytic mechanism is ubiquitous in transition metal-catalyzed processes such as hydrogenation and hydroformylation [13–15], as early demonstrated by Vaska and Wilkinson for Ir(I) and Rh(I) complexes, respectively [16, 17]. After an initial transient state that was later understood to be an σ-complex of H₂, both H atoms are transferred to the metal centre which increases its oxidation state by two (oxidative addition), yielding a cis-dihydride metal complex.

In the case of heterolytic splitting, polarization of the coordinated H-H molecule occurs giving a hydride (H⁻) and proton (H⁺) which is captured either intermolecularly by an external base (B⁻, intermolecular process) or by a basic site on the ancillary ligands bound to the metal centre or counter-anion (intramolecular process), the latter exploited, for example, in Noyori-type asymmetric ketone hydrogenation [18, 19]. A general scheme for heterolytic H-H activation is shown in Scheme 14.4 [10]. The relevance to catalytic hydrogenation has been reviewed [20]. The heterolytic pathway is generally preferred in polar media, including water, and is considered as energetically more facile, as all reactions are in principle reversible, no change in the oxidation state of the metal is required, and generally little or no ligand rearrangement must occur to give the final metal hydride product [21–23]. One of the most widely found cases of heterolytic hydrogen activation is H/D exchange which is a powerful tool for mechanistic investigations. It is of great importance also to other fields of research other than catalysis as it is related to living organisms and enzymatic activation of H₂ in dehydrogenases.

Scheme 14.4

Reproduced from Ref. [10], Copyright 2014, with permission from Elsevier

Using these two activation pathways, hydrogen can behave as an amphoteric ligand, both as a Lewis base by σ-donation and as Lewis acid by electron donation to σ*-orbitals. In turn, this involves that coordinated dihydrogen can span a rather broad range of pK _a values, usually in the range 5–16, but including examples of superacidic behaviour (−6 in pseudo-aqueous scale) as for complex [Os(dppe)₂(CO)(η²-H₂)]²⁺ [24].

14.3 Hydrogen Activation by Ru Water-Soluble Complexes

In this chapter, an overview of selected examples of Ru complexes and organometallic compounds which were shown to activate hydrogen in water will be given. The first paragraph will include the reactivity of a narrow class of complexes bearing water as ligands (aquo complexes). In the following two paragraphs, emphasis will be put on phosphine-stabilized complexes, with particular attention to anionic sulfonated phosphines (the most widely used) and a class of cagelike neutral aminophosphines based on 1,3,5-triaza-7-phosphaadamantane (PTA).

14.3.1 Aquo Complexes

The old prejudice that haunted early organometallic chemists against the compatibility of water phase and this class of complexes was ruled out as early as 1972, when Zelonka and Baird [25] published on the reactivity of [Ru(η⁶-C₆H₆)Cl₂]₂ (1) with D₂O to give a mixture of hydrolysis products which was later confirmed to be [Ru(η⁶-C₆H₆)Cl₂(D₂O)] and [Ru(η⁶-C₆H₆)Cl(D₂O)₂]⁺.

The tris-aquo analogue [Ru(η⁶-C₆H₆)(H₂O)₃](SO₄)₂ (2) was later isolated and fully characterized by Merbach and colleagues [26], by reaction of solid [Ru(H₂O)₆](tos)₂ (3; tos = p-toluenesulfonate) with either 1,3- or 1,4-cyclohexadiene in EtOH followed by addition of Ag(SO₄)₂. The silver salt was needed to remove any trace of chloride anions and shift the aquation equilibrium towards the trisubstituted derivative.

All these complexes were shown to be able to activate H₂ in water. Süss-Fink and coworkers studied the reactivity of water solutions of 1 (given as described mixtures of variously substituted aquo derivatives) with hydrogen under different conditions [27]. At 20 °C under 1.5 bar of hydrogen pressure, the reaction gave the electron-deficient (58e) brown tetrametallic tetrahydrido cluster [H₄Ru₄(η⁶-C₆H₆)₄]²⁺ (4), according to the formal stoichiometry given in Scheme 14.5, which was fully characterized both in solution by NMR and in the solid state by single crystal X-ray diffraction (Fig. 14.1) [28].

Scheme 14.5

Synthesis of 4 by reaction of 1 with H₂

Fig. 14.1

X-ray crystal structure of cluster cation [H₄Ru₄(η⁶-C₆H₆)₄]²⁺ (4)

By increasing the temperature to 55 °C and the hydrogen pressure to 60 bar, the 60-e hexahydrido analogue of 4, namely, [H₆Ru₄(η⁶-C₆H₆)₄]²⁺ (5), was obtained (Scheme 14.6). This violet complex is rather unstable towards oxygen, reverting back to 4 with elimination of water, according to Scheme 14.7.

Scheme 14.6

Synthesis of 5 by reaction of 1 with H₂

Scheme 14.7

Reproduced from Ref. [27], Copyright 2014, with permission from Elsevier

¹H NMR spectra of complexes 4 and 5 show a single hydride resonance at room temperature, suggesting four equivalent μ³-H hydrides for 4 and six equivalent μ²-H hydrides for 6, respectively. However, ¹H NMR spectra and T₁ measurements of 5 at −120 °C in THF-d ₄ /MeOH-d ₄ (1:1) established that this complex should exist in solution at low temperature and in the solid state as the tetrahydro-dihydrogen cluster [H₄Ru₄(η⁶-C₆H₆)₄(H₂)]²⁺, a proposal which was further supported by DFT calculations [29].

The same authors explored the reactivity of other water-soluble Ru-arene dimers with hydrogen, in particular using the more bulky analogue of 1, i.e. [Ru(η⁶-C₆Me₆)Cl₂]₂ (6). Interestingly, even under forcing conditions (pH₂ = 60 bar, 55 °C), a lower nuclearity cluster was obtained, e.g. the 30-e bimetallic complex [H₃Ru₂(η⁶-C₆Me₆)₂]⁺ (7) [30], whose X-ray crystal structure is shown in Fig. 14.2.

Fig. 14.2

X-ray crystal structure of bimetallic cation [H₃Ru₂(η⁶-C₆Me₆)₂]⁺ (7)

Although the Ru-Ru bond distance obtained from the solid-state structure is as short as 2.468 Å, compatible with a (formally expected) metal-metal triple bond (Scheme 14.8, left), DFT calculations showed that the bonding situation is better described as two RuHRu three-centre, two-electron interaction (Scheme 14.8, right).

Scheme 14.8

Reproduced from Ref. [27], Copyright 2014, with permission from Elsevier

For intermediate steric bulkiness of the arene ring, as for [Ru(η⁶-C₆Me₄H₂)Cl₂]₂ (8), reaction with hydrogen in water gave trinuclear tris-hydrido clusters as in Scheme 14.9. Interestingly, the intermediate chloro-capped hydrogen activation product [H₃Ru₃(η⁶-C₆Me₄H₂)₃Cl]²⁺ (9), which was isolated as chloride salt, hydrolyzes to the oxo-capped monocation analogue [H₃Ru₃(η⁶-C₆Me₄H₂)₃(H₃O)]⁺ (10) in water in the presence of NaBF₄ (Scheme 14.10) [31, 32].

Scheme 14.9

Synthesis of 9 by reaction of 8 with H₂

Scheme 14.10

Reproduced from Ref. [27], Copyright 2014, with permission from Elsevier

It is worth mentioning here the reactivity with hydrogen of the simplest Ru-aquo complex, namely, [Ru(H₂O)₆](tos)₂ (3), as it paved the way for a rich chemistry and mechanistic observations and is considered as a milestone achievement for researchers working in the field of aqueous-phase Ru chemistry and catalysis.

In 1998, Merbach and colleagues described the reactivity of 3 with H₂ gas under pressure [33]. In detail, a 0.1 M solution of 3 in D₂O was pressurized to 40 bar in a HPNMR tube, and the corresponding ¹H NMR spectrum displayed a signal at −7.68 ppm, in the chemical shift range expected upon formation of hydride species. As Ru(0) was observed to form under these conditions, the authors repeated the experiment in the presence of p-toluenesulfonic acid (0.13 M). After 19 h, the spectrum showed singlets at −7.65 ppm, attributed to [Ru(H₂O)₅(η²-H₂)]²⁺ (11), 4.62 ppm (free H₂), and triplets at 4.59 (free HD, ¹ J _HD = 42.8 Hz) and −7.68 ppm due to the H/D exchange product [Ru(H₂O)₅(η²-HD)]²⁺ (12, ¹ J _HD = 31.2 Hz). From ¹ J _HD values, it was possible to calculate [34] the Ru-bound H-H distance as 0.889 Å, only slightly longer than the bond length of free hydrogen (0.740 Å), indicative of little backdonation from the metal d-orbitals to the σ*-antibonding orbital of the (η²-H₂) ligand, which should then have a Lewis acidity character. This also explains why the presence of an acid stabilized solutions of 11 for more than three days under H₂ pressure. The authors proposed that a pH-dependent equilibrium was active in the H/D exchange for this system in D₂O, involving as intermediate the monohydrido complex [Ru(H₂O)₅H]⁺ (13) (Scheme 14.11).

Scheme 14.11

H/D exchange pathways from 11 to 13 via intermediate 12

Further studies were carried out on this system [35], including ¹H, ²H and ¹⁷O NMR spectroscopies and DFT calculations (Fig. 14.3). Kinetic studies gave for 11 a formation rate and equilibrium constants of k _f  = (1.7 ± 0.2) × 10⁻³ kg mol⁻¹ s⁻¹ and K _eq = 4.0 ± 0.5 mol kg⁻¹, suggesting that the reaction of 3 with H₂ to give 11 follows an I _d mechanism as for related small ligands [36].

Fig. 14.3

Calculated structures of [Ru(H₂O)₅(η²-H₂)]²⁺ (11) and [Ru(H₂O)₅H]⁺ (13) (Reprinted with permission from Ref. [36]. Copyright 1993 American Chemical Society)

14.3.2 Hydroxylated and Methoxylated Phosphines

The use of water-soluble ancillary ligands is a common strategy to convey the properties of transition metal complexes in water. Among the most widely used ligands, water-soluble phosphines have received particular attention, and especially sulfonated phosphines (i.e. bearing a SO₃ ⁻ substituent, generally on a phenyl group linked to the P donor atom) are a well-developed class of such ligands. Examples of hydrogen activation in water brought about by such complexes will be given in the next paragraph.

An interesting although less represented class of water-soluble diphosphines bearing either hydroxy- or methoxy-ending groups was developed by Tyler and coworkers. Ligand 1,2-bis(bis(methoxypropylphosphino)ethane) (DMeOPrPE) was reacted with [Ru(cod)Cl₂]_n to yield trans-[Ru(DMeOPrPE)₂Cl₂] [37]. This species reacted smoothly (Scheme 14.12) under a pressure of H₂ (25 bar) in water (buffered, pH = 7) at 85 °C in ca. 2 h to give trans-[Ru(DMeOPrPE)₂H(η²-H₂)]⁺ (14).

Scheme 14.12

Hydrogen activation by trans-[Ru(DMeOPrPE)₂Cl₂] yielding 14

³¹P{¹H}NMR gave a single resonance at 63.4 ppm in MeOH-d ₄ , whereas the corresponding ¹H NMR resonances were observed as a broad singlet at −6.6 and a quintet at −11.4 ppm. T ₁ measurements (500 MHz, −20 °C) gave a value of 21.1 ms corresponding to an H-H distance of 0.86 and 1.06 Å for slow and fast rotation, respectively. This complex was used to probe the existence of DHHB (dihydrogen hydrogen bonding) due to the inertness of the η²-H₂ to water substitution, using a combination of NMR-based texts including rotational dynamics.

The study was extended to other trans-[Ru(diphosphine)₂Cl₂] complexes obtained with DMeOPrPE analogues (Scheme 14.13), differing for the length of alkyl chain bound to P and the kind of terminal group (OH vs. OMe). Their behaviour towards hydrogen activation and the stability of the product trans-[Ru(diphosphine)₂H(η²-H₂)]⁺ obtained by stepwise H₂ addition/heterolysis pathway were compared to water-insoluble analogues [38]. In fact, water solubility was facilitated in all cases by the cationic nature of the complexes. The general conclusion was that with less electron-donating ligands (DHMPE, DPPE), substitution reactions exchanging η²-H₂ with H₂O occurred, whereas with more electron-donating ligands (DHPrPE, DMeOPrPE, DEPE, DMPE), no substitution was observed even at concentrations of 55 M after 1 week at 75 °C. This is in line with the higher d _π-σ* backbonding interaction ruling the strength of the M-(η²-H₂) bond.

Scheme 14.13

Syntheses of water-soluble trans-[Ru(WSDP)₂Cl₂] (WSDP = water-soluble diphosphines)

14.3.3 Sulfonated Phosphines

Sulfonated phosphines, commonly obtained by reaction of the water-insoluble parent compounds with oleum/sulfuric acid under controlled conditions, constitute without doubts the most represented class of water-soluble phosphines which have received attention from academia and industry for use in aqueous-phase catalyzed processes. The well-known Ruhrchemie/Rhône-Poulenc rhodium-catalyzed olefin hydroformylation process, which makes use of TPPTS [TPPTS = tris(m-sulfonatophenyl)phosphine trisodium salt], is considered as a paradigm in this field and has inspired further research efforts in ligand design and optimization by many authors over the years [39].

Although it is not the purpose of this chapter to describe the use of transition metal complexes bearing sulfonated ligands in catalysis, as this topic has already been thoroughly reviewed in the past [40–43], it is worth mentioning a few examples where clearly identified Ru-hydrido complexes found specific applications.

The water-soluble complex ruthenium complex [H₂Ru(CO)(TPPMS)₃] [15, TPPMS = (m-sulfonatophenyl)phosphine sodium salt] was shown to be an efficient catalyst precursor for the aqueous-biphasic hydroformylation of terminal, substituted and cyclic alkenes [44]. Ionic strength of the reaction media and temperature were often observed to determine the selectivity towards the final product. For example, hydroformylation of 4-penten-1-ol in water using [HRu(CO)(TPPTS)₃] (16) gave preferentially the linear product 6-hydroxy-hexan-1-al, whereas 2-hydroxy-3-methyltetrahydropyran derived from the corresponding branched aldehyde with an increase of the ionic strength [45].

Complexes [HRu(CO)(CH₃CN)(L)₃][BF₄] (17, L = TPPTS; TPPMS) were used as catalyst precursors for the hydroformylation of eugenol, estragole, safrole and trans-anethole under moderate conditions in biphasic media and their activities. Higher activities were observed using TPPTS, due to the higher water solubility of such ligand compared to TPPMS [46].

Joó and colleagues established some of the key effects related to the formation and molecular distribution of water-soluble Ru(II) hydrido complexes stabilized by TPPMS and derived from hydrogen activation in water. By a combination of pH-potentiometric and NMR studies, it was observed that the conversion of dimeric [RuCl₂(TPPMS)₂]₂ to [HRuCl(TPPMS)₃] (18) was favoured at pH ≤ 3.3, whereas at pH ≥ 7, the dihydride cis-[H₂Ru(TPPMS)₄] (19), characterized by a ¹H NMR signal at δ_H = −10.4 ppm, formed preferentially [47, 48]. Figure 14.4 shows the distribution curves of such species at various pH values. The observed pH-dependent hydrogen activation has a profound influence of the catalytic properties of [RuCl₂(TPPMS)₂]₂, for example, in the chemoselective hydrogenation of cinnamaldehyde. Complex 18 catalyzed the selective hydrogenation of the C=C bond, while the C=O group could be reduced efficiently in the presence of 19 giving cinnamol as main product. Further studies showed that high selectivity to C=O reduction could also be achieved at constant (buffered) pH = 3.04 by increasing the hydrogen pressure to 8 bar. Higher pressures would promote the conversion of 18 to 19 even at acidic pH values (Scheme 14.14) [49].

Fig. 14.4

Distribution of water-soluble ruthenium(II)-hydrides as a function of pH, based on the average of ¹H and ³¹P NMR integrated intensities. ■ [HRuCl(TPPMS)₃], ♦ [H₂Ru(TPPMS)₄], ● [HRuCl(TPPMS)₂]₂. [Ru] = 2.4 × 10⁻² M, [TPPMS] = 7.2 × 10⁻² M, [KCl] = 0.2 M, 50 °C, H₂, P_total = 1 bar (Reprinted from Ref. [48], Copyright 1988, with permission from Elsevier)

Scheme 14.14

Reproduced from Ref. [49] with permission of John Wiley & Sons Ltd

The active role of water in the selective C=O bond hydrogenation by 19 was studied in detail by DFT calculations with the inclusion of a (H₂O)₃ cluster in addition to a continuum model [50]. The catalytic cycle, modelled using PH₃ in place of the full phosphine ligand and acrolein instead of cinnamaldehyde, involves at first dissociation of a TPPMS ligand from 19 to form the active penta-coordinate species [H₂Ru(TPPTS)₃] (20), followed by substrate coordination and hydrogen transfer from the Ru centre. The second hydrogenation step comes from metal-coordinated H₂O. H₂ coordination regenerates the initial active species 20 by eliminating the unsaturated alcohol. The authors concluded that in the proposed mechanism the role of the protonating agent (H₂O) is adequate in basic media to promote C=O reduction, whereas the stronger H₃O⁺ is needed (acidic conditions) to bring about C=C bond hydrogenation, justifying the experimentally observed selectivity at different pH values.

The formation of different classical and nonclassical Ru hydrides derived from water-phase hydrogen activation starting from [RuCl₂(TPPMS)₂]₂ was recently reinvestigated by Laurenczy and Joó [51]. In detail, under 1 bar H₂ at 333 K in the absence of added phosphine or halide, the monohydride dimer [RuHCl(TPPMS)₂]₂ (21) is formed as stable compound, without any sign of Cl bridge cleavage up to 100 bar and further heating. In the presence of NaCl (0.102 M), complex 18 was observed to form. Under acidic conditions (pH = 3.01), raising the H₂ pressure to > 5 bar, and in the presence of added TPPMS, the trans-isomer of complex 19, namely, trans-[H₂Ru(TPPMS)₄] (22), was formed. At variance with 19, this complex is characterized by a ¹H NMR quintet signal at δ_H = −7.7 ppm and a ³¹P {¹H}NMR singlet at δ_P = −57.2 ppm. Under basic or neutral conditions, even in the presence of a Ru-TPPMS ratio of 1:4 or 1:5, the Ru-containing species was cis-fac-[H₂Ru(H₂O)(TPPMS)₃] (23). Interestingly, under moderate H₂ pressure (ca. 5 bar), at pH = 10.0, solutions containing [RuCl₂(TPPMS)₂]₂ and TPPMS (total Ru-P = 1:4) gave a broad singlet at δ_H = −7.2 ppm, which remained unchanged up to 100 bar pressure. T ₁(min) measurements gave a value of 18 ms, indicative of a coordinated dihydrogen ligand, and four hydrogen atoms in fast exchange on NMR timescale. The unprecedented [RuH₂(η²-H₂)(TPPMS)₃] (24) was proposed for this species, formally arising from 23 by replacement of H₂O with H₂ at elevated pressures. The study was complemented by reactivity studies of [RuCl₂(TPPMS)₂]₂ with sodium formate (transfer hydrogenation conditions). A summary of the various situations observed under different conditions is shown in Scheme 14.15.

Scheme 14.15

Reproduced from Ref. [51] with permission from the Royal Society of Chemistry

The combination of Ru and sulfonated phosphines such as TPPMS and TPPTS have found important applications in the field of hydrogen storage and production from organic compounds in water phase, namely, the reversible activation of formic acid/formate to hydrogen and carbon dioxide mixtures [52, 53]. This system allows for efficient storage of hydrogen using CO₂ as feedstock for hydrogenation to formate, while the reverse reaction, formic acid dehydrogenation, gives fuel cell-grade hydrogen generation on demand. The process runs efficiently at moderate pressures and temperature and constitutes a zero carbon footprint cycle for energy storage. Laurenczy et al. reported [54, 55] that catalytic systems generated either in situ from [Ru(H₂O)₆](tos)₂ (tos = toluene-4-sulfonate) or commercial hydrated RuCl₃ in the presence of m-TPPTS catalyzed the dehydrogenation reaction already at 25 °C, reaching a TOF of 460 h⁻¹ at 120 °C. Noteworthy, this system showed to be active in a wide temperature range between 25 and 170 °C, giving in all cases conversions of 90–95 %. Furthermore, the hydrogen produced was of high purity and no CO formation was observed by FTIR (detection limit 2 ppm) even at high temperatures, making this catalytic system suitable for fuel cell applications. The influence of different hydrophilic ligands on the FA dehydrogenation in the presence of RuCl₃ × 3H₂O has also been investigated in detail. These studies showed that ligand basicity, its hydrophilic properties as well as steric effects were the main parameters which influenced the catalytic activity [56].

Joó and Laurenczy [57] showed that stirring aqueous solutions of HCO₂Na with [RuCl₂(TPPMS)₂]₂ in an atmospheric gas burette at 40–80 °C yielded substantial amounts of virtually CO-free gas (<10 ppm). The turnover number [TON = mol reacted substrate (mol catalyst)⁻¹] achieved at 80 °C in 1 h was 120, and the maximum amount of the gas evolved corresponded to 47 % of the theoretical yield. In closed pressure tubes, at 80 °C the final pressure was measured as 6.2 bar, equivalent to a 37 % yield of bicarbonate. The formate/bicarbonate equilibrium could be shifted to the desired direction (i.e. hydrogen release or storage) by simple pressure change. This was demonstrated by experiments using a medium pressure sapphire NMR tube, where an aqueous solution of H¹³CO₃Na was pressurized with 100 bar H₂ at 83 °C in the presence of [RuCl₂(TPPMS)₂]₂ and TPPMS, and the reaction was monitored by recording ¹³C NMR spectra of the solution. In 200 min, 90 % of substrate was hydrogenated to H¹³CO₂Na. At this point the pressure was released, and after closing the tube, the reaction mixture was left to equilibrate at 83 °C, leading to decomposition of formate. The hydrogenation/decomposition cycle was repeated for three consecutive cycles showing good and steady efficiency.

These studies follow on earlier reports by the same authors on homogeneous hydrogenation of aqueous hydrogen carbonate to formate under mild conditions [58], where a series of Ru, Rh, Ir and Pd complexes bearing water-soluble phosphines such as TPPMS and 1,3,5-triaza-7-phosphaadamantane (PTA, see next chapter) were tested. Density functional theory (DFT) studies later allowed to propose a reaction mechanism for catalytic bicarbonate hydrogenation in aqueous phase (Scheme 14.16), highlighting the role of Ru-hydride, Ru-dihydrogen and Ru-aquo complexes, respectively [59].

Scheme 14.16

Reprinted from Ref. [59], Copyright 2006, with permission from Elsevier

14.3.4 Amphiphilic Neutral Phosphines

Among neutral water-soluble phosphines, the cagelike aminophosphine 1,3,5-triaza-7-phosphaadamantane (PTA) has received special attention for synthetic coordination chemistry and catalytic applications, as it combines high Lewis basicity and small Tolman cone angle (103°) with easier functionalization compared to sulfonated phosphines [60–62]. A large library of PTA derivatives is nowadays available, due to the contributions of many research groups worldwide. Due to the interest in medicinal and catalytic applications, many Ru complexes bearing PTA and derivatives were obtained during the years, some of which were used for hydrogenation reactions and hydrogen activation in water.

Complex cis-[RuCl₂(PTA)₄], prepared by reduction of RuCl₃ in ethanol in the presence of PTA, was used as efficient catalyst for the regioselective C=O bond hydrogenation of unsaturated aldehydes to the corresponding alcohols under transfer hydrogenation conditions and to convert CO₂ and bicarbonate to formate in the absence of amine or other additives under mild conditions and pressure of H₂ [63]. Also in this case, the pH of the aqueous media was important to determine the nature of Ru-hydrido species formed. At pH = 12.0, complex cis-[H₂Ru(PTA)₄] (25) was observed to form, whereas complex cis-[HRu(PTA)₄X]ⁿ (26, X = Cl⁻ or H₂O; n = 0, +1) formed at pH = 2.0. In the presence of an excess of PTA, complex [HRu(PTA)₅]⁺ (27) instead appeared as determined by NMR measurements. The kind of complex formed at different pH values had strong influence on the rate of CO₂ hydrogenation, which gave the highest initial reaction rate (TOF = 807.3 h⁻¹) at pH = 5.86 and decreased at lower or higher pH. On the base of these results, a mechanism based on 26 as catalytically active species was proposed (Scheme 14.17).

Scheme 14.17

Water-phase sodium bicarbonate hydrogenation mechanism catalysed by 26

Piano-stool Ru(II) complexes bearing PTA, namely, [CpRuCl(PTA)₂] and [Cp*RuCl(PTA)₂] (Cp = η⁵-cyclopentadienyl; Cp* = η⁵-pentamethyl cyclopentadienyl), were prepared in our laboratories [64] and showed good activity in the regioselective C = C bond reduction of α, β-unsaturated ketones (benzylidene acetone) under water/octane biphasic conditions under a pressure of hydrogen. Using high-pressure ³¹P NMR, [CpRuCl(PTA)₂] was observed to activate hydrogen at 50 °C under 30 bar pressure in H₂O/THF-d ₈ to form the monohydride species [CpRuH(PTA)₂] (28) which remained stable upon heating to 80 °C. In contrast, [Cp*RuCl(PTA)₂] gave the dihydride complex [Cp*RuH₂(PTA)₂]Cl (29) which converted to the monohydride [Cp*RuH(PTA)₂] (30) after heating to 80 °C (Scheme 14.18). Complex 29 was also identified as the active species generated in water during NaHCO₃ hydrogenation under 100 bar H₂ pressure at 80 bar [65].

Scheme 14.18

Hydrogen activation in water by piano-stool complexes [Cp^RRu(PTA)₂Cl] (Cp^R = C₅H₅, C₅Me₅)

Frost et al. reported that 28 is also active for the same reaction at room temperature and low pressures of H₂ (10–150 psi), with modest TOFs [66]. The study includes the evaluation of pH effects. At pH > 7 and pH < 3.6, poor catalytic performances were observed, with the highest activity at pH = 4.7. At pH 2.1, the selectivity to products changed with the type of buffer used (HBF₄/NaH₂PO₄, 99 % of 4-phenylbutan-2-one; HCl/NaH₂PO₄, 77.5 % of 4-phenylbut-3-en-2-ol). By NMR methods, the authors proposed that the active catalytic species was [CpRuH(PTA)(PTAH)]⁺ (31) resulting from protonation of PTA.

A more detailed view of the mechanism of hydrogen activation by these complexes was obtained by theoretical DFT calculations [67, 68], highlighting the non-innocent role of water in facilitating heterolytic H-H bond cleavage in solution by extensive hydrogen bonding networking, which helps in lowering the energy barriers of the overall process. PTA is also involved in H-H activation and in particular through quaternization of one of the basic N atoms, giving 31 as the most stable product (deepest energy minimum). In Fig. 14.5 are shown the optimized structures of the transition state for heterolytic hydrogen splitting and the water-stabilized monohydride [CpRuH(PTA)(PTAH)]⁺.(H₂O)₃ [31.(H ₂ O) ₃ ]. Calculated pK _a values established the highly acidic nature of Ru-dihydrogen complexes in water, while the lower acidity for 30 (4.7) compared to 29 (0.3) agrees with the experimental observation of the former as a stable intermediate at 50 °C.

Fig. 14.5

Optimized structures of the transition state for heterolytic hydrogen splitting and the water-stabilized monohydride [CpRuH(PTA)(PTAH)]⁺.(H₂O)₃ [31.(H ₂ O) ₃ ] (Reprinted with permission from Ref. [67]. Copyright 2007 American Chemical Society)

14.4 Hydrogen Activation by Other Transition Metals in Water

Rhodium water-soluble complexes bearing either sulfonated phosphines such as TPPMS or PTA were studied for their catalytic properties and found to be able to bring about H₂ activation. Besides catalyzing the amine-free CO₂ hydrogenation in sodium formate solutions [69], complex [RhCl(TPPMS)₃] was studied together with its TPPTS and PTA analogues for isotope exchange reactions in H₂-D₂O and D₂-H₂O systems, and the results were compared with the activities of their Ru counterparts in the temperature range 20–70 °C [70]. The specific rates of H-D exchange were observed to be pH dependent. For [RhCl(PTA)₃], [RhCl(TPPMS)₃] and [RhCl(TPPTS)₃], the obtained values of TOF (h⁻¹) were 908, 806 and 989, respectively. Theoretical DFT calculations showed that the protonation of hydride ligands in [Rh(H)₂Cl(PR₃)₃]⁺ (PR₃ = TPPMS, TPPTS, PTA) by H₃O⁺ occurs via dihydrogen-bonded adducts, giving the cationic hydrido-dihydrogen complexes [RhH(η²-H₂)Cl(PR₃)₃]⁺. Two possible mechanisms were proposed [71].

The water-soluble analogue of Wilkinson’s catalyst [RhCl(TPPMS)₃] by reaction with H₂ under atmospheric pressure gave the monohydride product [HRh(TPPMS)₃] (32) instead of the oxidative addition product [Rh(H)₂Cl(TPPMS)₃] (33) as for the original water-insoluble analogue, accompanied by a pH drop close to 2.0 [72]. The authors proposed a monohydride-based mechanism to explain the change in selectivity for maleic and fumaric acid hydrogenation using this system, although they could not exclude hydrodechlorination from 33 (however not detected) to 32 in water phase.

In contrast to the Rh/TPPMS system, hydrogenation reactions in the presence of [RhCl(PTA)₃] in water gave a more complicated situation, and PTA oxide was formed extensively, accompanied by formation of colloidal metal. In H₂O/EtOH, hydrogenation of cinnamaldehyde gave TOF = 250 h⁻¹ at 78 °C. In the presence of an excess of free ligand, metal dihydrides were proposed to form, as shown in Scheme 14.19, based on NMR analysis [73].

Scheme 14.19

Reprinted from Ref. [73], Copyright 1995, with permission from Elsevier

A more detailed study involving the role of pH effects on the nature and distribution of complexes derived from hydrogen activation by [RhCl(PTA)₃] was then carried out by the same authors [74]. Starting from catalytic tests on the reduction of olefins and oxo acids, a detailed kinetic study was carried out with crotonic acid as substrate. It was observed that the highest rate of the reactions was obtained at pH = 4.7 and that proton liberation was associated with the reaction of the catalyst with H₂ in aqueous solutions. These results, confirmed by H/D exchange experiments in D₂O, suggested that, similarly to [RhCl(TPPMS)₃], a reductive dehydrochlorination of an intermediate Rh(III)-dihydride must take place leading to a mechanism involving [HRh(PTA)₃] (34) as the active species. From kinetic measurements, the hydrogenation of crotonic acid to butyric acid was proposed to follow the mechanism shown in Scheme 14.20.

Scheme 14.20

Reprinted from Ref. [74], Copyright 1996, with permission from Elsevier

The water-soluble analogues of Vaska’s complex trans-[IrCl(CO)(PPh₃)₂] were obtained using TPPMS and PTA instead of PPh₃. Hydrogen activation by trans-[IrCl(CO)(TPPMS)₂] gave different products, again depending on pH of the aqueous phase. The total amount of protons produced per moles of Ir is ca. zero at pH <3 and is ca. 1 at pH = 11. Under acidic conditions, oxidative addition products [(H)₂IrCl(CO)(TPPMS)₂] (35a) or [(H)₂Ir(CO)(TPPMS)₂]⁺ (35b), generated upon chloride dissociation, may form. Conversely, in less acidic or basic solutions, heterolytic hydrogen splitting occurs, leading to [(H)₃Ir(CO)(TPPMS)₂] (36) or [HIr(CO)(TPPMS)₃] (37) in the presence of excess of TPPMS [75].

The iridium(III) complexes [Cp*IrCl₂(PTA)] (38) and [Cp*IrCl(PTA)₂]Cl (39) were used as catalysts for the hydrogenation of hydrogen carbonate in water. By reaction with H₂ (100 bar), the former gave quantitatively the dihydride derivative [Cp*Ir(H)₂(PTA)] (40) as shown by multinuclear NMR spectroscopy [76]. The presence of the two hydride ligands was indicated by the ¹H and ³¹P{¹H} NMR analysis showing a doublet at δ _H = −18.4 ppm (¹ J _PH = 30 Hz) in the hydride region of the corresponding ¹H NMR spectrum, which was simplified into a singlet in the ¹H{³¹P} NMR spectrum and a triplet in the ³¹P NMR spectrum at δ _P = −68.5 ppm (² J _PH = 30 Hz). Under the same conditions, the latter gave the cationic monohydride [Cp*IrH(PTA)₂]⁺ (41), characterized by a triplet at δ _H = −17.9 ppm (² J _PH = 30 Hz) in the ¹H NMR spectrum and by a doublet at δ _P = −76.3 ppm (² J _PH = 30 Hz) in the ³¹P{¹H} NMR spectrum.

The Fe(II) analogue of trans-[Ru(DMeOPrPE)₂Cl₂] reacted with H₂ in water to give trans-[Fe(DMeOPrPE)₂H(η²-H₂)]⁺ (42), characterized by a doublet in the ³¹P NMR spectrum at δ _P = 88.9 ppm (² J _PH = 45 Hz) and a quintet at δ _H = −15.1 ppm (² J _PH = 45 Hz) and a broad singlet at δ _H = −10.9 ppm in the ¹H NMR spectrum measured at 233 K. T ₁(min) measurements (500 MHz) gave a value of 19.5 ms corresponding to an H-H distance of 0.85 Å (slow rotation) and of 1.07 Å (fast rotation) [77]. Proton release was observed to be associated with hydrogen activation, giving solutions with pH = 2.6, also due to the contribution of the buffering capacity of the free ligand. In absence of proton sponge (PS), which was used to obtain quantitative formation of 42, the overall stoichiometry implies ligand decoordination and protonation, as shown in Scheme 14.21.

Scheme 14.21

Hydrogen activation by trans-[Fe(DMeOPrPE)₂Cl₂] yielding 42

More detailed mechanistic experiments were then carried out, showing that in water, trans-[Fe(DMeOPrPE)₂Cl₂] reacts instantaneously to give trans-[Fe(DMeOPrPE)₂(H₂O)Cl]⁺, which then activates hydrogen to yield complex 42, with the intermediate formation of trans-[Fe(DMeOPrPE)(H)Cl] (43) resulting from heterolytic H-H splitting. In contrast, hydrogen activation by the same Fe(II) initial complex proceeds in nonaqueous solvents such as toluene-d ₈ with initial formation of trans-[Fe(DMeOPrPE)(η²-H₂)Cl]⁺ (43), requiring a chloride scavenger (TlPF₆) and a base (PS) to reach complete conversion to 42 [78].

Pd(II) complexes of sulfonated tetrahydrosalen (sulfosalan, HSS) were obtained by Joó et al. and applied in hydrogenation and redox isomerization of allylic alcohols in neat water or water/organic solvent biphasic systems. DFT calculations allowed to establish that hydrogen is activated heterolytically, giving the Pd(II)-hydride complex [HPd(HSS-Hphen)] (44), where one of the phenolate oxygens acts as a base for the proton resulting from H-H splitting (Scheme 14.22) [79].

Scheme 14.22

Synthesis of complex 44 involving heterolytic hydrogen splitting