Synthesis,spectral and thermal studies on dicarboxylate-bridged palladium(II) coordination polymers. Part II

The synthesis and thermal behavior of the new [Pd(fum)(bipy)] _n ·2nH₂O (1), [Pd(fum)(bpe)] _n ·nH₂O (2) and [Pd(fum)(pz)] _n ·3nH₂O (3) {bipy = 4,4′-bipyridine, bpe = 1,2-bis(4-pyridyl)ethene and pz = pyrazine} fumarate complexes are described in this work as well their characterization by IR and ¹³C CPMAS NMR spectroscopies. TG curves showed that the compounds released organic ligands and lattice water molecules in the temperature range of 46–491 °C. In all the cases, metallic palladium was identified as the final residue.     

Synthesis,Crystal Structure and Catalytic Property of a New Cadmium Coordination Polymer

A new cadmium coordination polymer [Cd(L)(Cl)(H₂O)]_n (1) (L = 5-(imidazol-1-yl)-2-pyridine carboxylic anion) constructed from dinuclear cadmium clusters has been synthesized under hydrothermal condition and structurally characterized by single-crystal X-ray diffraction analysis. The compound crystallizes in triclinic system, space group P-1, with a = 6.8747(6), b = 9.7434(8), c = 9.9119(7) Å, α = 118.615(8)°, β = 104.445(7)°, γ = 94.815(7)°, V = 548.01(9) ų, Z = 2. Compound 1 is a one-dimensional (1D) double chain structure based on dinuclear [Cd₂(Cl)₂] clusters, and is further extended to a 3D supramolecular framework by hydrogen bonds. As a Lewis acid catalyst, compound 1 exhibits excellent catalytic performance for the acetalization reaction under mild conditions and can be reused several times without a significant decrease of the catalytic activity.     

Synthesis,crystal structures,and photocatalytic properties of two 2D supramolecular silver(I) coordination polymers derived from bis(1,2,4-triazole) and aromatic dicarboxylate ligands

Two silver(I)-mixed ligand coordination polymers (CPs), {[Ag(L)(Hmip)]·H₂O} _n (1) and {[Ag(L)]·0.5(DCTP)·H₂O]} _n (2) (H₂mip = 5-methylisophthalic acid, H₂DCTP = 2,5-dichloroterephthalic acid, and L = 4,4′-bis(1,2,4-triazol-1-ylmethyl)biphenyl) have been hydrothermally synthesized and structurally characterized. Two CPs display 1D zigzag chain and linear chain structures, respectively. Furthermore, these 1D chains are extended into 2D supramolecular networks through hydrogen bonding interactions. The luminescence properties and photocatalytic behaviors of both CPs are reported.     

Synthesis and structural characterization of Palladium(II) complex bearing <Emphasis Type="Italic">N</Emphasis>-Heterocyclic Carbene

Trans-Bis[1,3-bis(2,4-dimethylphenylimidazolidin-2-ylidene)]dichloropalladium(II), 4, was prepared from 1,3-bis(2,4-dimetilphenyl)imidazoliniumchloride. The crystal and molecular structure of 4 have been determined by single crystal X-ray diffraction. The title compound, C₃₈H₄₄N₄PdCl₂, crystalizes in the monoclinic space group P 21/n with a = 13.8713(9) Å, b = 12.1365(6) Å, c = 21.5499(15) Å. The Pd atom has a slightly distorted square planar coordination geometry. The molecules of the title compound are linked by C–H···Cl weak hydrogen bonds into two-dimensional sheets parallel to the (001) plane. In addition, the title compound was characterized by elemental analyses and NMR spectroscopy.     

Synthesis,structure and electrochemical properties of some thiosemicarbazone complexes of iridium

Reaction of thiosemicarbazones of salicylaldehyde and 2-hydroxyacetophenone (H₂L¹ and H₂L²) with [Ir(PPh₃)₃Cl] affords complexes of type [Ir(PPh₃)₂(L)(H)] (L = L¹ or L²) in ethanol. A similar reaction carried out in toluene affords the [Ir(PPh₃)₂(L)(H)] complexes along with complexes of type [Ir(PPh₃)₂(L)Cl], where a chloride is coordinated to iridium instead of the hydride. The structure of the [Ir(PPh₃)₂(L²)(H)] and [Ir(PPh₃)₂(L²)Cl] complexes has been determined by X-ray crystallography. Crystal data for [Ir(PPh₃)₂(L²)(H)]: space group, P2₁/c; crystal system, monoclinic; a=12.110(2) Å, b=17.983(4) Å, c=18.437(4) Å, β=103.42(3)°, Z=4; R ₁=0.0591, wR ₂=0.1107. Crystal data for [Ir(PPh₃)₂(L²)Cl]: space group, P2₁/c; crystal system, monoclinic; a=17.9374(11) Å, b=19.2570(10) Å, c=24.9135(16) Å, β=108.145(5)°, Z=4; R ₁=0.0463, wR ₂=0.0901. In all the complexes the thiosemicarbazones are coordinated to the metal center as dianionic tridentate O, N, S-donors and the two triphenylphosphines are trans. The complexes are diamagnetic (low-spin d? ⁶, S=0) and show intense MLCT transitions in the visible region. Cyclic voltammetry on all the [Ir(PPh₃)₂(L)(H)] and [Ir(PPh₃)₂(L)Cl] complexes shows a quasi-reversible Ir(III)–Ir(IV) oxidation within 0.55–0.78 V vs. SCE followed by an irreversible oxidation of the thiosemicarbazone within 0.91–1.27 V vs. SCE. An irreversible reduction of the thiosemicarbazone is also observed within ?1.10 to ?1.23 V vs. SCE.     

Silver-based coordination complexes of carboxylate ligands: crystal structures,luminescence and photocatalytic properties

The complexes [Ag₄(dpe)₄]·(btec) (1) and [Ag₄(bpy)₄]·(btec)·12H₂O (2) (dpe = 1,2-di(4-pyridyl)ethylene, bpy = 4,4′-bipyridine, H₄btec = 1,2,4,5-benzenetetracarboxylic acid) have been synthesized in aqueous alcohol/ammonia by slow evaporation at room temperature and characterized by elemental analysis, single-crystal X-ray diffraction, FTIR, UV–Vis and luminescence spectroscopies. Both complexes are composed of 1D infinite cationic [Ag/dpe(bpy)] _n ⁿ⁺ chains and discrete btec^4? anions. Their three-dimensional supramolecular structures are built up of cationic sheets formed from [Ag/dpe(bpy)] _n ⁿ⁺ units via weak Ag…Ag and Ag…N interactions, plus anionic btec^4? sheets featuring electrostatic, π–π and hydrogen bonding interactions. Both complexes exhibited photocatalytic activity for the decomposition of methyl orange under UV light irradiation.     

Molecular and crystal structure of 15-acetylsongorine and songoramine isolated from <Emphasis Type="Italic">Aconitum szechenyianum</Emphasis> Gay

Two napelline skeletal diterpenoid alkaloids 15-acetylsongorine, C₂₄H₃₃NO₄ I, and songoramine, C₂₂H₂₉NO₃ II, were first isolated from the roots of Aconitum Szechenyianum Gay. The crystal structures were determined by X-ray single-crystal diffraction analysis. The crystal I is the triclinic system with space group P1 having unit cell parameters of a = 9.360(8) Å, b = 11.593(9) Å, c = 11.830(16) Å, α = 113.223(15)°, β = 105.950(16)°, γ = 101.296(12)°, and Z = 2. Hydrogen bonds O–H···O and O–H···N joint the molecules into dimer. The crystal II belongs to the orthorhombic system with space group P2₁2₁2₁ having unit cell parameters of a = 8.950(2) Å, b = 13.272(3) Å, c = 15.454(4) Å and Z = 4. The O–H···O hydrogen bonding interaction links the molecule into linear chains. The distortion of rings of compound I and II were evaluated by calculation of the Cremer and Pople puckering parameters. The presence of the C–O–C bond in the compound II results in the changes of ring conformations compared with that of the compound I.     

Synthesis,crystal structures and photocatalytic properties of four silver(I) coordination polymers based on semirigid bis(pyrazole) and carboxylic acid co-ligands

Four Ag(I) coordination polymers, formulated as [Ag(L1)(tpa)_0.5] _n (1), {[Ag(L2)(ndc)_0.5]·0.5H₂ndc} _n (2), [Ag(L3)_0.5(ndc)_0.5] _n (3) and {[Ag(L3)]·H₃bptc} _n (4) (L1 = 4,4′-bis(pyrazole-1-ylmethyl)-biphenyl, L2 = 4,4′-bis(3,5-dimethylpyrazol-1-ylmethyl)-biphenyl, L3 = 1,4-bis(3,5-dimethylpyrazol-1-ylmethyl)benzene, H₂tpa = terephthalic acid, H₂ndc = 2,6-naphthalenedicarboxylic acid, H₄bptc = 3,3′,4,4′-biphenyltetracarboxylic acid), have been hydrothermally synthesized and structurally characterized. Complex 1 features the rare binodal (4,4)-connected 2D 4,4L10 topological network with a point symbol of {3²·4.6²·7}₂{3²·6²·7²}. Complex 2 has a folded ladder-like chain structure, which is further extended into a 3D supramolecular network via O–H···O hydrogen bonding and π···π stacking interactions. Complexes 3 and 4 both possess 1D zigzag chain structures. Complex 3 is further extended into a binodal (3,4)-connected network with the point symbol of {4.8⁴·10}{6²·8²}₂ by Ag···O weak interactions, while complex 4 is further connected through O–H···O hydrogen bonding and π···π interactions to afford a 2D supramolecular structure. The photoluminescence spectra and photocatalytic properties of these complexes for degradation of methylene blue and methyl orange are reported.     

Structural aspects of some organoselenium compounds

The crystal structures of four organoselenium compounds, viz. bis(2-formylphenyl)diselenide (5), bis(2-methylnaphthyl)diselenide (6), organoselenenyl sulfide (7), and spiroselenurane (8) are described. Crystal data for 5: space group Pca2₁, crystal system orthorhombic, a=7.9969(4) Å, b=20.8794(12) Å, c=15.8307(13) Å, Z=8, R=0.0292. Owing to the presence of a strong Se···O interaction in compound 5 the geometry around the selenium atom may be considered as T-shaped. Crystal data for 6: space group Pna2₁, crystal system orthorhombic, a=18.2253(12) Å, b=13.0714(8) Å, c=7.7355(5) Å, Z=4, R=0.0570. The molecule has a cisoid conformation. Crystal data for 7: space group Pbcn, crystal system orthorhombic, a=22.2144(13) Å, b=8.0255(4) Å, c=15.4496(9) Å, Z=8, R=0.0292. Due to intramolecular Se···N interaction in 7 the geometry around selenium is T- shaped. Crystal data for 8: space group P2₁/c, crystal system monoclinic, a=7.4585(5) Å, b=19.5634(13) Å, c=8.0428(5) Å, β=97.1320(10)°, Z=4, R=0.0254. The O?Se?O angle is 172.86(6)°.     

Synthesis,characterization, crystal and molecular structure analysis of a novel 1-benzhydryl piperazine derivative: 1-benzhydryl-4-(2-nitro-benzenesulfonyl)-piperazine

A novel 1-benzhydryl piperazine derivative 1-benzhydryl-4-(2-nitro-benzenesulfonyl)-piperazine was synthesized by the nucleophilic substitution of 1-benzhydryl piperazine with 2-nitro-benzenesulfonyl chloride. The product obtained was characterized spectroscopically and finally confirmed by X-ray diffraction study. The title compound, C₂₃H₂₃N₃O₄S crystallizes in the monoclinic space group C2/c with cell parameters a = 13.1120(9) Å, b = 21.4990(9) Å, c = 16.655(1) Å, β = 111.352(2)°, Z = 8, and V = 4372.7(4) Å. The structure reveals that the piperazine ring is in a chair conformation. The geometry around the S atom is distorted tetrahedral. There is a large discrepancy in the bond angles around the piperazine N atoms. The structure is stablized by C–H···O type intermolecular hydrogen bonding interactions.     

A Novel 3D Platinum–Thallium Coordination Polymer having Strong Metal–Metal Interactions

A new three-dimensional platinum(II)–thallium(I) coordination polymer [{Pt(pda)(NHCOtBu)₂}₄Tl₄][Pt(CN)₄]₂·2H ₂ O (pda = 1,2-propyldiamine) has been prepared from the direct reaction of [Tl₂Pt(CN)₄] and [Pt(pda)(NHCOtBu)₂] in water, and its structure was characterized by X-ray diffraction analysis. The compound crystallizes in monoclinic, space group Pn, a = 11.567(2) Å, b = 11.570(2) Å, c = 37.677(8)Å, β = 94.64(3)°, V = 5025.8(17) Å³, Z = 2, R1 = 0.0679 and wR2 = 0.1574 [I >  2σ (I)], Goodness-of-fit on F ² = 1.055. The compound exhibits a novel 3D network structure consisting of [Pt(CN)₄]^2? connected 1D infinite Pt–Tl–Pt–Tl chains via strong Pt–Tl bonds.     

Synthesis,crystal, and molecular structure of coordination polymers constructed by self-assembly of NiN<Subscript>4</Subscript> cores with 2,2′-iminodibenzoate and nitroprusside ions

Two coordination compounds of compositions [Ni(L¹)(idba)(H₂O)]·1.5 H₂O (1) and [Ni(L²)Fe(CN)₅NO]·C₂H₅OH (2) where L¹ is N, N′-bis(3-aminopropyl)ethylenediamine, L² is 2,12-dimethyl-3,7,11,17-tetraazabicyclo[11.3.1]heptadeca-1(17),2,11,13,15-pentaene, and idba^2? is 2,2′-iminodibenzoate have been synthesized and characterized by elemental analysis, IR spectroscopy, thermal analysis, and single-crystal X-ray diffraction. Complex 1 crystallizes in the monoclinic space group P2₁/n (No. 14) with a=9.810(2) Å, b=10.230(2) Å, c=25.350(5) Å, V=2543.6(9) Å³, Z=4, and R=0.0727. The nickel atom is six-coordinated by four N atoms of amine and two O atoms of water and idba^2?. The molecular packing of the complex comprises of an infinite one-dimensional layered network in which the molecules in the crystal are held together by a system of hydrogen bonding. Complex 2, however, crystallizes in the space group C2/c (No. 15) of the monoclinic system with a=19.7990(4) Å, b=14.9440(3) Å, c=19.8800(3) Å, V=5115.90(17) Å³, Z=4, and R=0.0540. The Ni ion in compound 2 has a slightly distorted octahedral arrangement of the N₄ donor atoms of primary ligand L² and two N-donor atoms of the secondary nitroprusside ligand. The structure of 2 displays an extended one-dimensional network formed by linear [—Ni—NC—Fe—CN—] units. A cyclic voltammetric study shows that compound 1 undergoes a quasireversible oxidation attributable to Ni²⁺ → Ni³⁺ in the range 300–420 mV vs SCE.     

Synthesis,crystal structure,and hydrogen-bonded displacive-type structural phase transition of guanidine dichloroacetate

Guanidine dichloroacetate was synthesized and separated as crystals. Differential scanning calorimetry (DSC) measurement shows that this compound undergoes a reversible phase transition at about 275 K with a heat hysteresis of 28 K. Step-like dielectric anomaly observed at 274 K further confirms the phase transition. The single-crystal X-ray diffraction data suggested that these was a transition from a room-temperature phase with the space group of P2₁/n (a = 8.030(5), b = 12.014(9), c = 8.124(6) Å, β = 96.089(1)°, V = 779.3(1) Å³, and Z = 4) to a low-temperature one with the space group of P2₁/c (a = 7.941(2), b = 11.828(3), c = 10.614(2) Å, β = 130.985(1)°, V = 752.6(3) Å³, and Z = 4). The displacements of hydrogen bonds induce the structure phase transition.     

A novel layered supramolecular compound containing 2D network of eight-member water clusters and terephthalato extended-bridged

A novel layered supramolecular compound [Ni(L)(TPHA)]·8H₂O, containing two-dimensional (2D) water clusters and terephthalato-bridged ligand, where L = 1,3,6,9,11,14-hexaazatricyclo[12.2.1.1^6,9] octadecane, TPHA = terephthalate dianion, has been synthesized and structurally characterized by spectroscopy and X-ray crystallography. The complex is neutral, in which the nickel(II) ions are bridged by the TPHA to form a one-dimensional (1D) infinite chain structure, and containing the eight-member water cluster. The presence of octameric water clusters have effectively increased the 1D coordination polymer to a three-dimensional layered structure. Every water cluster is connected strongly by a O–H···O hydrogen bond (range of the bonds between 2.724 and 3.056 Å). The complex crystallizes in monoclinic, space group P21/c with a = 9.788(3), b = 13.030(4), c = 11.646(4) Å, and β = 104.551(5)°.     

Syntheses,crystal structures and in vitro anticancer activities of oxovanadium(IV) complexes of amino acid Schiff base and 1,10-phenanthroline ligands

Four oxovanadium(IV) complexes, namely [VO(desa-met)(phen)]·MeOH·2H₂O (1) (desa-met = Schiff base derived from 4-(diethylamino)salicylaldehyde and dl-methionine, phen = 1,10-phenanthroline), [VO(o-van-met) (phen)]·MeOH·CH₂Cl₂·3H₂O (2) (o-van-met = Schiff base derived from o-vanillin and dl-methionine), [VO(dtbs-napa)(phen)]·2H₂O (3) (dtbs-napa = Schiff base derived from 3,5-di-tert-butyl salicylaldehyde and 3-(1-naphthyl)-l-alanine) and [VO(hyna-napa)(phen)]·1.5H₂O (4) (hyna-napa = Schiff base derived from 2-hydroxy-1-naphthaldehyde and 3-(1-naphthyl)-l-alanine), were synthesized and characterized by IR, HRMS, UV–vis spectra, molar conductance and single-crystal X-ray diffraction (XRD). X-ray structural analysis showed that the V(IV) atoms in all four complexes are six-coordinated in a distorted octahedral environment. In the crystals of complexes 1 and 2, π–π stacking interactions together with hydrogen bonds connect the molecular units into 2D networks. Meanwhile, CH–π stacking interactions are observed between the aromatic rings in the crystals of 1 and 4, while the π–π stacking interactions between aromatic rings in the crystals of 2 and 3 are arranged with a face-to-face mode. The in vitro anticancer activities of these complexes against A-549 and HeGp2 cells were tested by MTT assay.     

Diverse topologies of three cobalt(II) coordination polymers constructed from flexible bis(benzimidazole) and dicarboxylate ligands

Three cobalt(II) coordination polymers {[Co(L1)(nda)(H₂O)₂]·2H₂O} _n (1), [Co(L2)(tbi)(H₂O)] _n (2) and [Co(L2)(bpdc)(H₂O)] _n (3) (L1 = 1,3-bis(5,6-dimethylbenzimidazol-1-yl)-2-propanol, L2 = 1,3-bis(benzimidazol-1-yl)-2-propanol, H₂nda = 2,6-naphthalenedicarboxylic acid, H₂tbi = 5-tert-butyl isophthalic acid and H₂bpdc = 4,4′-biphenyldicarboxylic acid) were synthesized and characterized by physicochemical and spectroscopic methods. Complex 1 exhibits a 1D loop-like structure, which is further extended into a 3D 3,3,4T31 network through two O–H···O hydrogen bonding interactions. Complex 2 displays a 1D ladder-like chain, arranged into a 2D supramolecular network with 3,3,4L34 topology via classical O–H···O hydrogen bonding interactions, whereas complex 3 features a 2D 3,4L13 layer structure and further assembles into a 3D framework with a twofold interpenetrating sqc65 topology through O–H···O hydrogen bonding interactions. The fluorescence and catalytic properties of these complexes for the degradation of Congo red in a Fenton-like process have been investigated.     

Synthesis,crystal structure,and thermodynamics of a high-nitrogen copper complex with <Emphasis Type="Italic">N</Emphasis>, <Emphasis Type="Italic">N</Emphasis>-bis-(1(2)H-tetrazol-5-yl) amine

A new high-nitrogen complex [Cu(Hbta)₂]·4H₂O (H₂bta = N,N-bis-(1(2)H-tetrazol-5-yl) amine) was synthesized and characterized by elemental analysis, single crystal X-ray diffraction and thermogravimetric analyses. X-ray structural analyses revealed that the crystal was monoclinic, space group P2(1)/c with lattice parameters a = 14.695(3) Å, b = 6.975(2) Å, c = 18.807(3) Å, β = 126.603(1)°, Z = 4, D _c = 1.888 g cm^?3, and F(000) = 892. The complex exhibits a 3D supermolecular structure which is built up from 1D zigzag chains. The enthalpy change of the reaction of formation for the complex was determined by an RD496–III microcalorimeter at 25 °C with the value of ?47.905 ± 0.021 kJ mol^?1. In addition, the thermodynamics of the reaction of formation of the complex was investigated and the fundamental parameters k, E, n, \( \Updelta S_{ \ne }^{{{\uptheta}}} \), \( \Updelta H_{ \ne }^{{{\uptheta}}} \), and \( \Updelta G_{ \ne }^{{{\uptheta}}} \) were obtained. The effects of the complex on the thermal decomposition behaviors of the main component of solid propellant (HMX and RDX) indicated that the complex possessed good performance for HMX and RDX.     

Blue Photoluminescent Dimeric Cu(I) Dithiophosphates Bridged by a Flexible Ligand 1,2-Bis(4-Pyridyl)Ethane

The title compound {Cu[S₂P(OC₂H₅)₂](bpe)} _n (1) is constructed from flexible ligand bpe (bpe = 1,2-bis(4-pyridyl)ethane) and the Cu[S₂P(OC₂H₅)₂], which was characterized by single crystal structure determination, elemental analysis, XRD, and IR spectra. X-ray diffraction studies revealed that polymeric compound 1 consists of dinuclear module to form 1-D chains with the intramolecular Cu···Cu interactions (ca. 2.63 Å). This interaction may show a profound influence on the observed blue luminescence emission spectrum for 1. Crystal data for 1 at 293(2) K: Space group P ? 1, a = 9.277(3), b = 10.504 (4), c = 31.801(1) Å, α = 92.849(3), β = 90.401(2), γ = 114.547(7)°, V = 2813.9(2) ų, Z = 2, R ₁ = 0.041.     

Synthesis,crystal structures,luminescence, and photocatalytic properties of two 1D Co(II) coordination polymers constructed with semirigid bis(benzimidazole) and dicarboxylate ligands

Two ternary cobalt(II) coordination polymers (CPs), namely [Co(L1)(npht)] _n (1) and {[Co₂(L2)₂(npht)₂(H₂O)]·H₂O} _n (2) (L1 = 4,4′-bis(benzimidazol-1-ylmethyl)biphenyl, L2 = 1,2-bis(5,6-dimethylbenzimidazol-1-ylmethyl)benzene, and H₂npht = 4-nitrophthalic acid) have been synthesized and structurally characterized by X-ray crystallography. Both CPs feature similar 1D infinite chains containing two distinct loops. CP 1 further forms a 3D supramolecular network via weak C–H···O hydrogen bond interactions. CP 2 shows a 1D two-layer chain structure, assembled through π–π stacking interactions. The electrochemical, luminescence, and photocatalytic activities of the two CPs for the removal of methylene blue under visible or UV light were investigated. Possible photocatalytic mechanisms are discussed.     

Self-Assembly,Crystal Structure and Analysis of Intermolecular Interactions of the Supramolecular Compound Based on Hexamolybdochromate(III), Sulfate and Piperazine

A new supramolecular compound based on Anderson-B hexamolybdochromate, (H₂Pz)₃[Cr(OH)₆Mo₆O₁₈H](SO₄)₂·12H₂O (1) (Pz = piperazine) was synthesized and characterized by elemental analysis, IR spectroscopy, and single-crystal X-ray diffraction (Mo Kα). The compound crystallizes in monoclinic system, P21/c space group with a = 13.5708(6) Å, b = 17.3711(8) Å, c = 22.2387(9) Å, β = 110.631(2)°; V = 4906.3(4) Å³, Z = 4, D _c  = 2.290 g/cm³, F(000) = 3364.0; μ = 1.905, S = 1.033. The final R = 0.0398 and wR = 0.0971. The H₂pz²⁺ ions and sulfate anions in 1 are arranged through hydrogen bonds into a hexagonal network in [202] plane and hexamolybdochromates anions (CrMo₆) fill in the hexagonal vacancies. The networks stack in such a way that each anion links two sulfate ions from adjacent networks via hydrogen bonds with short (CrMo₆)O···OSO₃ distances of 2.637–2.697 Å. A lot of hydrogen bonds are formed between water molecules, sulfate, H₂pz²⁺ ions and CrMo₆ anions, which are the dominating force constructing the supramolecular structure. Hirshfeld surface analysis of 1 gives us the details of intermolecular interactions in the crystals of 1 in a visual manner and shows that the CrMo₆ anion acts as a stronger hydrogen bond donor than as an acceptor.