Introduction

Coordination compounds of lanthanides have attracted considerable attention because of possessing a number of useful properties: from the creation of up-lighting to usage as biomedical sensors for biological objects imaging [1,2,3,4,5,6,7,8,9]. Among the most widely investigated groups of lanthanide complexes there are β-diketonates due to easy synthesis of the ligands and unique luminescence properties of their complexes [10,11,12,13,14]. Carbacylamidophosphates (CAPh) are the best known structural analogues of β-diketones having similar coordination ability [15]. The presence of peptide and phosphoramide groups combined together in the fragment C(O)N(H)P(O) determines their interaction with biological molecules and cell membranes. Variation of substituents near phosphoryl and carbonyl groups gives possibility to modulate CAPh stereochemical and pharmacological properties. In particular, different CAPh representatives were shown to possess antineoplastic [16, 17], antibacterial activity [18, 19] and enzyme inhibitor properties [20, 21]. The lanthanide chelates of CAPhs exhibit biological activities and in vitro tests show their strong anticancer properties [22]. The presence of the phosphoryl group provides a high affinity towards majority of metal ions, including lanthanides and actinides [21, 23, 24]. CAPh compounds may be regarded as powerful chelating systems and for this reason they are used as extractants, namely those of them containing the long alkyl chains (n-C5H11–n-C10H21) near the carbonyl carbon atom [25, 26].

The possibility of inclusion into the coordination sphere both in the molecular and acidic forms makes the coordination chemistry of CAPh extremely diverse and interesting and allows to use these ligands both for the 4f and 3d-metals bonding [27,28,29,30,31,32,33,34,35].

As a part of our study of the trichloro-bis[2,2,2-trichloro-N-(dimorpholinophosphoryl)acetamide coordination ability we described herein the synthesis, structural features and Hirshfeld surface analysis of a new aqua-trichloro-bis[2,2,2-trichloro-N-(dimorpholinophosphoryl)acetamide]holmium(III)] [Ho(HL)2(H2O)Cl3] (1). This work is a part of the project dedicated to explore the coordination behavior of the molecular forms of CAPh ligands with respect to Ln(III) ions and to determine the structure of the CAPh-complexes with octahedral geometry of Ln(III) center.

Experimental

Elemental analyses (Ln, C, H, N) were performed using standard titrimetric method for lanthanide ions [36] and the EL III Universal CHNOS elemental analyzer.

Fourier transform infrared spectroscopy (FT-IR) spectra were recorded on a Perkin–Elmer Spectrum BX spectrometer using KBr pellets with resolution of 1 cm−1.

X-ray Crystallography

Crystal data for [Ho(HL)2(H2O)Cl3] (1) was measured at 294 K on a “Xcalibur-3” diffractometer (graphite monochromated MoKα radiation with λ = 0.71073 Å, CCD detector, ω-scanning).

Refinement

The structure was solved by the direct method using SHELXTL package [37]. Full-matrix least-squares refinement against F2 in anisotropic approximation was used for non-hydrogen atoms. Positions of hydrogen atoms were determined from electron density difference maps and refined by “riding” model with Uiso = nUeq of the carrier atom (n = 1.5 for hydrogens of water and n = 1.2 for other hydrogen atoms). Crystallographic data (excluding structure factors) for the structure 1 have been deposited with the Cambridge Crystallographic Data Center as supplementary publication number CCDC 1943482. Copies of the data can be obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK, (fax: + 44-(0)1223–336,033 or e-mail: deposit@ccdc.cam.ac.uk, website: https://www.ccdc.cam.ac.uk).

Structural and refinement parameters are given in Table 1. Bond lengths are given in Supplementary Information, angles and atomic displacement parameters—in the.cif file.

Table 1 Crystal data and structure refinement for 1
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Chemistry General Information

All chemicals were purchased from commercial sources and used as received unless otherwise stated. Basic solvents for synthesis were dried using literature methods. Solvents for spectroscopic investigations were of the highest purity available.

Carbacylamidophosphate ligand 2,2,2-trichloro-N-(dimorpholinophosphoryl)acetamide (HL) was synthesized via the three-step procedure based on the Kirsanov reaction [38, 39], and its structure was identified using FT-IR and NMR spectroscopy.

Synthesis and Crystallization of Compound 1

Aqua-trichloro-bis[2,2,2-trichloro-N-(dimorpholinophosphoryl)acetamide]holmium(III)] [Ho(HL)2(H2O)Cl3] (1) (Fig. 1a) was synthesized as follows: hydrated holmium chloride HoCl3·6H2O (0.1 mmol) was dissolved in 15 mL of acetone/isopropanol mixture (1:1, v/v), then heated to a boiling temperature for 2 min after what the mixture was added to the solution of HL (0.2 mmol) in isopropanol (15 mL). The obtained solution was left under vacuum in a desiccator over CaCl2. Crystals of the complexes were formed in 1–2 days, filtered, washed with cooled isopropanol, and air-dried (yield 76%). The complexes, as prepared, are soluble in non-polar aprotic solvents, acetone, acetonitrile, alcohols, and toluene; and insoluble in water, hexane, and cyclohexane. Yellowish white SC-XRD quality prismatic crystals were obtained by slow evaporation of a solution of 1 in acetone/isopropanol mixture (1:1, v/v) over 2 days.

Fig. 1
figure 1

a Representation of the CAPh-ligand coordination mode in [Ho(HL)2(H2O)Cl3]. b FT-IR spectra of HL and [Ho(HL)2(H2O)Cl3] (KBr pellet)

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IR (KBr, cm−1): 3390 vs, b (νO–H), 3013 m, b (νN–H), 1725 vs, sp (νC=O), 1635 m, sp (δHOH), 1450 s, sp (Amide II), 1201 s, sp (νP=O), 975 s, sp (νP–N), 674 s, sp(νC–Cl), 507 m, sp (ρPNC), where s is strong, vs is very strong, m is medium, b is broad, sp is sharp (Fig. 1b).

For C20H36Cl9HoN6O9P2 the elemental composition was determined, %: C, 22.10; H, 3.57; N, 8.12; Ho, 15.56, and calculated, %: C, 22.87; H, 3.45; N, 8.00; Ho, 15.70. Excessive content of Hydrogen and Oxygen in the real analysis may indicate the presence of extrasphere water in complex composition (in this case the contents of elements are, %: C, 22.48; H, 3.58; N, 7.87; Ho 15.44.

Structural Analysis

The three-dimensional Hirshfeld surfaces (HSs) and twodimensional fingerprint plots for 1 were generated using Crystal Explorer 3.0 [40, 41]. The dnorm plots were mapped with color scale in between − 0.649 au (blue) and 1.554 au (red). The fingerprint plots were displayed by using the expanded 0.4–2.4–3.0 Å view with the de and di distance scales displayed on the graph axes. When the.cif files of 1 was uploaded into the CrystalExplorer software, all bond lengths to hydrogen were automatically modified to typical standard neutron values, i.e., d(C–H) = 1.083 and d(O–H) = 0.983 Å.

Results and Discussion

Molecular and Crystal Structure of [Ho(HL)2(H2O)Cl3]

In the asymmetric part of the unit cell is one molecule of [Ho(HL)2(H2O)Cl3] (1) (Fig. 1, Table 1). The complex 1 contains one Ho (III) center, two CAPh ligands, one water ligand and three chloride ligands. The structure of [Ho(HL)2(H2O)Cl3] (1) are rather similar to the previously studied [Er(HL1)3Cl3] (triclinic symmetry, P \(\overline{1}\), CCDC: 774162), [Pr(HL2)3Cl3] (trigonal, R3, CCDC: 1316528) and [Sm(HL3)3Cl3] (trigonal, R3, CCDC: 869525) complexes [42,43,44] with the different CAPh ligands CCl3C(O)N(H)P(O)[NC4H8]2 (HL1, CCDC: 240149), CCl3C(O)N(H)P(O)[NEt2]2 (HL2, CCDC: 1276961) and CCl3C(O)N(H)P(O)[N(CH2)5]2 (HL3, CCDC: 871746) respectively. In all complexes the molecules of CAPh ligand are coordinated to the metal center monodentately via the O atom of the phosphorylic group. The next common feature of all the complexes is disposition of the phosphoryl group in an anti-position to the carbonyl one similarly to the crystal structure of the free ligands HL, CCDC: 1254024 [38], HL1 [45, 46], HL2 [47] and HL3 [32] described recently.

The central Ho atom of [Ho(HL)2(H2O)Cl3] has a distorted octahedral environment (mer isomer) coordinated by three chloride ions, two O atoms of two 2,2,2-trichloro-N-(dimorpholinophosphoryl)acetamide molecules and one O atom of water molecule (Fig. 2). Similar slightly distorted octahedral LnO3Cl3 geometry with a meridional arrangement of the donor atoms was fixed earlier for the complex [Pr(HMPA)3Cl3], CCDC: 1177132 (HMPA: is phosphorylic ligand hexamethylphosphoramide) [48]. However, Ln (III) environment in most known LnO3Cl3-core complexes realized in the form of a facial isomer, in contrast to these mentioned two structures (for example, in the complexes [Er(HL1)3Cl3], [Pr(HL2)3Cl3] and [Sm(HL3)3Cl3] [41,42,43]). The Ho–Cl distances are in the range 2.57–2.61 Å, the Ho–OP distances are shorter than Ho–Ow (Table 2). Such ranges of bond lengths are typical for complexes of lanthanides with neutral form of CAPh ligands [23]. The valence angles in coordination polyhedron deviate from ideal values 90° and 180° for octahedron (see Table 2), but the dihedral angles between the faces clearly demonstrate the distortion of the octahedron. In contrast to regular value 109.47° real ones are in the range of 56.0°–72.3° (Table 2).

Fig. 2
figure 2

a Representation of the molecular structure of 1. Thermal ellipsoids are shown at 50% probability level. Hydrogen atoms are omitted for clarity. b Coordination polyhedron around the Ho atom

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Table 2 Selected geometric parameters of the coordination polyhedron of Ho(HL)2(H2O)Cl3 (1) (Å, °)
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In the crystal phase the molecules of 1 are linked via Owater–H⋯Omorpholine hydrogen bonds (Table 3) and form chains along [100] crystallographic direction (Figs. 3, 4). These chains are bound into a three-dimensional framework due to the Cmorpholine–H⋯Cl intermolecular hydrogen bonds (Table 3). The H atoms of the amino group ligand participate in the intramolecular hydrogen bonding with Cl− ligands (Table 3), similarly to the chlorido-complexes [Er(HL1)3Cl3], [Pr(HL2)3Cl3] and [Sm(HL3)3Cl3] described earlier [42,43,44].

Table 3 Hydrogen-bond geometry for [Ho(HL)2(H2O)Cl3] (symmetry codes: 1− x, − y + 1, − z; 2− x + 1, − y + 1, − z; 3x, y + 1, z + 1; 4− x + 1, − y, − z; 5− x, − y, − z)
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Fig. 3
figure 3

The fragment of hydrogen bonded chains with hydrogen bonds highlighted in bold blue line (Color figure online)

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Fig. 4
figure 4

General view of a three-dimensional framework; view along the crystallographic axis a

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Hirshfeld Surface Analysis and Fingerprint Plots

Hirshfeld surface analysis is an effective tool for exploring packing modes and intermolecular interactions in molecular crystals [48], as they provide a visual picture of intermolecular interactions and of molecular shapes in a crystalline environment. Surface features characteristic of different types of intermolecular interactions can be identified, and these features can be revealed by color coding distances from the surface to the nearest atom exterior (de plots) or interior (di plots) to the surface. This gives a visual picture of different types of interactions present, and also reflects their relative contributions from molecule to molecule. Further, 2D fingerprint plots (FP), in particular the breakdown of FP into specific atom⋯atom contacts in a crystal, provide a quantitative idea of the types of intermolecular contacts experienced by molecules in the bulk and presents this information in a convenient color plot. Hirshfeld surfaces comprising dnorm surface plots and FP were generated and analyzed for the crystal of 1 in order to explore their packing modes and intermolecular interactions. The dnorm surfaces of molecules 1 are shown in Fig. 5, and the FP for the overall contacts and individual atom⋯atom contacts in 1 are shown in Fig. 6. The dark-red spots on the dnorm surface arise as a result of the short interatomic contacts, i.e., strong hydrogen bonds, while the intermolecular interactions appear as light-red spots. The analysis of the dnorm surface and FP gives a pictorial conformation (both qualitatively as well as quantitatively) to the nature and geometries of the hydrogen bonds and intermolecular interactions described in the crystal structure of 1. The dnorm and FP of 1 are consistent with the observed intermolecular interactions.

Fig. 5
figure 5

dnorm mapped on the Hirshfeld surface to visualize the intermolecular interactions in 1

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Fig. 6
figure 6

Hirshfeld fingerprint plots for a all intermolecular interactions. b H⋯O/O⋯H interactions. c Cl⋯H/H⋯Cl interactions. d H⋯H interactions

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The analysis of the FP of 1 (Fig. 6) shows that the major contribution to the Hirshfeld surface is from Cl⋯H contact, with a 44.1% contribution, the closest Cl⋯H contact occurs at di + de = 2.6 Å. The light red spots on the dnorm surface of 1 located near the coordinated chlorine atoms and CCl3 group chlorine atoms (Fig. 6) can be attributed to the Cl⋯H contacts displayed in 1. The smallest of which are 2.68 and 2.78 Å for Cl1⋯H6A and Cl1⋯H15B respectively.

The second largest contribution comes from the closest H⋯H contact, with a 28.4% contribution occurs at di + de = 2.4 Å.

H⋯O/O⋯H contacts (15.9%), the closest one observed at di + de = 1.8 Å, which are closely to the interatomic distances of 1.77 and 1.95 Å (Table 3) observed for the H10B⋯O4 and H10A⋯O3 contacts in the crystal structure of 1 respectively.

The four red bright spots in the dnorm surface around the H10B, H10A of coordinated water molecule, O4 and O3 oxygen atoms of morpholine groups (Fig. 5) gives conclusive evidence for the participation of the lone pair of electrons of oxygen atom of morpholine rings as bifurcated hydrogen bond acceptors in the crystal structure of 1.

Thus, the Hirshfeld surface and FP analysis of 1 give conclusive evidence, both qualitatively and quantitatively, of the various hydrogen bonds/intermolecular interactions displayed in the crystal structure of 1.

Conclusions

Here, we have presented the solid-state structural analysis of a new hexacoordinate holmium(III) complex with the carbacylamidophosphate (CAPh) ligand 2,2,2-trichloro-N-(dimorpholinophosphoryl)acetamide. The central Ho atom of [Ho(HL)2(H2O)Cl3] has a distorted octahedral environment (mer isomer) coordinated by three chloride ions, two O atoms of two CAPh ligand molecules and one O atom of water molecule. Intramolecular H-bonding is present in the structure, and Hirshfeld surface analysis has been used to reveal intermolecular interactions that would otherwise have been difficult to elucidate.