Journal of Molecular Structure 1051 (2013) 354–360

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Journal of Molecular Structure

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Molybdenum(V) complexes with formate: Geometric isomerismof the [Mo2O4Cl2(Py)2(HCOO)]� ion

0022-2860/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.molstruc.2013.08.009

⇑ Corresponding author. Tel.: +386 (0)1 2419 108; fax: +386 (0)1 2419 220.E-mail address: barbara.modec@fkkt.uni-lj.si (B. Modec).

Barbara Modec ⇑, Darko DolencDepartment of Chemistry and Chemical Technology, University of Ljubljana, Aškerceva 5, 1000 Ljubljana, Slovenia

h i g h l i g h t s

� Formate binds to the fMo2O4g2þ coreto form dinuclear anionic complexes.� The [Mo2O4Cl2(Py)2(HCOO)]� ion

exists as trans and cis geometricisomers.� DFT calculations show the trans

isomer to be energetically morefavored.� H-bonds and CAH� � �Cl interactions

between PyH+ and complex ionsstabilize the cis isomer.

g r a p h i c a l a b s t r a c t

The PyH+� � �cis-[Mo2O4Cl2(Py)2(HCOO)]� synthon.

a r t i c l e i n f o

Article history:Received 21 May 2013Received in revised form 23 July 2013Accepted 8 August 2013Available online 20 August 2013

Keywords:Molybdenum(V) coordination chemistry{Mo2O4}2+ compoundsCarboxylato ligandGeometric isomerismDFT calculations

a b s t r a c t

Reactions of (PyH)5[MoOCl4(H2O)]3Cl2 with formate resulted in trans-{(Py)2H}[Mo2O4Cl2(Py)2(HCOO)] (1)and cis-(PyH)[Mo2O4Cl2(Py)2(HCOO)] (2), whereas the bromide starting material (PyH)[MoOBr4], yielded(PyH)3[Mo2O4Br4(HCOO)]�2CH3CN (3) and cis-(PyH)[Mo2O4Br2(Py)2(HCOO)] (4) (where Py stands for pyr-idine, PyH+ for pyridinium cation and (Py)2H+ for a hydrogen-bonded PyH+� � �Py ion). In all, the dinuclearmetal–metal bonded fMo2O4g2þ core may be recognized with its six coordination sites distributedamong halides, pyridine ligands and formate. The latter is coordinated via both oxygen atoms, with eachto a different metal ion. The [Mo2O4Cl2(Py)2(HCOO)]� ion exhibits geometric isomerism: the pyridineligands, on each metal ion one, are either trans or cis to each other. The trans isomer crystallized with(Py)2H+ countercations, whereas the cis isomer as a PyH+ salt. In the crystal lattice of cis-(PyH)[Mo2O4Cl2

(Py)2(HCOO)] (2), as confirmed by the X-ray structure analysis, pyridinium cation forms a hydrogen bondwith the doubly-bridging oxide of the cis-[Mo2O4Cl2(Py)2(HCOO)]� ion. The countercations oftrans-{(Py)2H}[Mo2O4Cl2(Py)2(HCOO)] (1) cannot participate in hydrogen-bonding. The DFT calculationson the isomers of the [Mo2O4Cl2(Py)2(HCOO)]� ion show the trans isomer to be by ca. 15 kJ/mol morestable than the cis isomer. The calculations on the hydrogen-bonded PyH+� � �[Mo2O4Cl2(Py)2(HCOO)]�

ion-pairs show a reversed order of stability. Hydrogen-bonding and weak CAH� � �Cl interactions betweenPyH+ cations and the cis-[Mo2O4Cl2(Py)2(HCOO)]� ion increase the stability of the cis compound.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction in this oxidation state [1]. Its structure was undoubtedly confirmed

The dinuclear singly metal–metal bonded fMo2O4g2þ core(illustrated in Scheme 1) pervades the chemistry of molybdenum

with the X-ray structure determination of Ba[Mo2O4(g2-C2O4)2

(H2O)2]�3H2O in 1965 [2]. Recent reports on the {Mo2O4}2+-containing compounds appear in very diverse contexts ofmolybdenum coordination chemistry. Examples include discretedinuclear species with ligands such as sulfite [3], tris(1-pyrazolyl)

Scheme 1. The fMo2O4g2þ core: terminal oxides are cis to each other, the{Mo(l2-O)2Mo} rhombus is not planar and the intermetal distance of 2.5–2.6 Åsignifies a single metal–metal bond [1].

B. Modec, D. Dolenc / Journal of Molecular Structure 1051 (2013) 354–360 355

methanesulfonate [4], imidazole [5], pyrazolate [6] or diketiminate[7] and self-assemblies of two or more {Mo2O4}2+ cores [8–13]. Thelinkage of dinuclear cores can be achieved also with the agency ofmultidentate oxygen- and, rarely, nitrogen-donor ligands. Some ofthe ligands serving this role are sulfite [3], biphosphonate [14],oxalate [15], molybdate(VI) [16] and pyrazine [17]. Of the latter,only the oxalate and molybdate(VI) covalently linked the{Mo2O4}2+ cores into infinite chain structures. The carboxylatespresent themselves as another type of suitable ligands. Owing tothe variety of the coordination modes that the carboxylate functioncan employ, even the ligand with one functional group can link two{Mo2O4}2+ cores. The latter was demonstrated by the acetate in[Mo4O8(CH3COO)3(OH)(Py)4] with the carboxylate serving eitheras a monodentate ligand or as bidentate bridging ones to a pairof metal–metal bonded molybdenum atoms or to molybdenumatoms from different {Mo2O4}2+ cores [18]. The number of coordi-nation patterns increases with the number of carboxylate groups inthe ligand. Malonate dianion (mal2�), a ligand with two carboxyl-ate groups, coordinated in [Mo2O4Cl(g2-mal)(l2-Hmal)(Py)]2�

(Hmal� = hydrogen malonate) in a chelating manner engagingthe oxygens from different carboxylate groups [19]. On the otherhand, one of the malonate ions of [{Mo2O4(g2-mal)2}2(l4-mal)]6�

bonded via all four oxygen atoms and thereby linked two{Mo2O4}2+ cores into a tetranuclear anion [20]. Similarly, the hep-tanedioate (hda2�) fully exploited its coordination abilities and assuch linked two dinuclear cores into [(Mo2O4Cl4)2(l4-hda)]6� [21].

In continuation of our research on the carboxylato complexes ofmolybdenum(V) the interaction of (PyH)5[MoOCl4(H2O)]3Cl2 or(PyH)[MoOBr4] with formate was investigated. Herein we reporton the preparation and structural characterization of a series ofproducts of these reactions, trans-{(Py)2H}[Mo2O4Cl2(Py)2(HCOO)](1), cis-(PyH)[Mo2O4Cl2(Py)2(HCOO)] (2), (PyH)3[Mo2O4Br4

(HCOO)]�2CH3CN (3) and cis-(PyH)[Mo2O4Br2(Py)2(HCOO)] (4).Surprisingly, a dinuclear complex anion with the [Mo2O4Cl2(Py)2

(HCOO)]� composition was isolated as two geometric isomers,one with the trans and the other with the cis disposition of pyridineligands. Previously prepared complexes with the [Mo2O4Cl2(Py)2

(RCOO)]� (RCOO� = acetate, pivalate, hydrogen succinate) compo-sition are with no exception cis isomers [19,22]. This motivatedus to see if we could understand the preference for the cis isomericform.

2. Experimental section

2.1. General procedures

Chemicals were obtained from Aldrich Chemical Co. They wereused without further purification. Acetonitrile solution of pyridiniumformate was prepared by dissolving the appropriate amounts ofpyridine and formic acid in a 1:1 M ratio. (PyH)5[MoOCl4(H2O)]3Cl2and (PyH)[MoOBr4] were prepared by published procedures [23].Elemental analyses were performed by the ChemistryDepartment service at the University of Ljubljana. The infraredspectra were measured on solid samples as Nujol or poly(chlorotri-fluoroethylene) mulls using a Perkin Elmer 2000 Series FT-IRspectrometer.

2.2. Preparation of trans-{(Py)2H}[Mo2O4Cl2(Py)2(HCOO)] (1)

Procedure A. (PyH)5[MoOCl4(H2O)]3Cl2 (400 mg, 0.93 mmol ofMo) was dissolved in the mixture of pyridine (4 mL) andformic acid (0.211 g, 4.50 mmol). The solution of deep red colorwas left to stand in a closed vessel at ambient conditions. Withintwo days, red crystals of 1 deposited from the solution. Yield:90 mg; 28%. Procedure B. A glass tube was charged with(PyH)5[MoOCl4(H2O)]3Cl2 (400 mg, 0.93 mmol of Mo), formic acid(0.211 g, 4.50 mmol) and pyridine (4 mL). The tube was sealedand heated in an electric oven maintained at 115 �C for 120 h. Aftercooling to room temperature, the resulting red solution was con-centrated under vacuum to ca. one third of the initial volume.Methanol (1 mL), acetonitrile (1 mL) and diethyl ether (4 mL) wereadded to the residue. The resulting solution was left to stand atambient conditions in a closed flask. Within two days a copiousamount of red crystals of 1 deposited from the solution. Yield:232 mg; 72%. Found C, 36.52; H, 3.26; N, 7.93%. C21H22Cl2Mo2N4O6

requires C, 36.60; H, 3.22; N, 8.13%. IR (cm�1): 1635m, 1603s,1571m, 1558vs [masym(OCO)], 1447vvs, 1345vs [msym(OCO)],1253w, 1239w, 1217s, 1205w, 1160m, 1147m, 1075vs, 1067s,1044vs, 1039vs, 1017m, 1006s, 956vvs, 938vvs, 897m, 879m,873m, 762vs, 752vvs, 735vvs, 720vs, 711vvs, 693vs, 683vvs,650w, 640m, 630s, 572m, 506w, 479vs, 436m.

2.3. Preparation of cis-(PyH)[Mo2O4Cl2(Py)2(HCOO)] (2)

(PyH)5[MoOCl4(H2O)]3Cl2 (150 mg, 0.35 mmol of Mo) wasdissolved in acetonitrile (20 mL). Upon the addition of pyridiniumformate (2.0 mmol, 4.0 mL of 0.5 M solution in acetonitrile) thesolution changed color from green to orange. The solution wasconcentrated under vacuum to ca. one half of the initial volumeand then left to stand at 8 �C. Within two days red crystals of 2deposited from the solution. The crystals were filtered off andwashed with the hexanes. Yield: 56 mg; 52%. Found C, 31.07; H,2.88; N, 6.61%. C16H17Cl2Mo2N3O6 requires C, 31.50; H, 2.81; N,6.89%. IR (cm�1): 1607s, 1557vvs [masym(OCO)], 1449vvs, 1346vvs[msym(OCO)], 1219s, 1199w, 1158m, 1070s, 1046s, 1017m, 1000w,960vvs, 941vs, 881w, 755vs, 729vvs, 693vs, 681vs, 643m, 604m,489m, 439m.

2.4. Preparation of (PyH)3[Mo2O4Br4(HCOO)]�2CH3CN (3) andcis-(PyH)[Mo2O4Br2(Py)2(HCOO)] (4)

(PyH)[MoOBr4] (512 mg, 1.0 mmol) was dissolved in acetoni-trile (20 mL). Upon the addition of pyridinium formate (2.70 mmol,5.4 ml of 0.5 M solution in acetonitrile) the solution acquired adeep red color. The mixture was divided into two aliquots. Onewas concentrated under vacuum to ca. one half of the initialvolume and then left to stand at ambient conditions. Orangecrystals of 3 deposited from the solution overnight. Yield: 95 mg;40%. Found C, 20.31; H, 2.15; N, 3.39%. C14H16Br4Mo2N2O6 (driedsample) requires C, 20.51; H, 1.97; N, 3.42%. IR (cm�1): 1634m,1608m, 1567vs [masym(OCO)], 1531vs, 1505w, 1338vvs [msym(OCO)],1248w, 1196s, 1164w, 1052m, 1002m, 957vvs, 943vs, 778m,745vvs, 720w, 699vs, 673vvs, 604vs, 481s. The second aliquotwas left to stand at ambient conditions for two weeks. Red crystalsof 4 were collected by filtration and washed with the hexanes.Yield: 48 mg; 27%. Found C, 27.61; H, 2.88; N, 6.61%. C16H17Br2Mo2

N3O6 requires C, 27.49; H, 2.45; N, 6.01%. IR (cm�1): 1660vs,1632w, 1618w, 1606vs, 1548vs [masym(OCO)], 1445vvs, 1346vvs[msym(OCO)], 1277w, 1251m, 1214s, 1201m, 1162w, 1148m,1067vvs, 1046s, 1014s, 998m, 956vvs, 935vvs, 874m, 790w,766vs, 750vvs, 731vvs, 704vvs, 681vvs, 643m, 604vs, 502w, 481s.

356 B. Modec, D. Dolenc / Journal of Molecular Structure 1051 (2013) 354–360

2.5. X-ray structure determinations

Data were collected on Nonius Kappa CCD diffractometer usinggraphite monochromated Mo Ka radiation (k = 0.71073 Å). Datareduction and integration were performed with the software pack-age DENZO-SMN [24]. Specific absorption corrections were notapplied since the averaging of the symmetry-equivalent reflectionslargely compensated for any absorption effects. The coordinates ofthe majority of non-hydrogen atoms were found via direct meth-ods using the structure solution program SHELXS [25]. The posi-tions of the remaining non-hydrogen atoms were located by useof a combination of least-squares refinement and difference Fou-rier maps in the SHELXL-97 program [25]. Hydrogen atoms wereincluded in the structure factor calculations at idealized positions.In 1, the cationic part consists of two pyridines sharing a proton.The protonation site could not be located from the electron densitymap. To maintain the charge balance, the proton was placed onN(4). The structures of 1 and 4 were solved and successfully refinedin non-centrosymmetric space groups, P bc21 and P 212121, respec-tively. The Flack parameter was 0.47(4) for 1, and 0.029(8) for 4[26]. In the case of 1, the value suggests that the crystal was aracemic twin. Figures depicting the structures were prepared byORTEP3 [27] and Mercury [28]. Cell parameters and refinementresults are summarized in Table 1.

2.6. Computational details

All calculations were performed with Jaguar package of pro-grams [29]. Geometry optimizations were carried out initially by

Table 1Crystallographic data for compounds 1–4.

Compound 1 Compoun

Empirical formula C21H22Cl2Mo2N4O6 C16H17Cl2MFormula mass 689.21 610.11Crystal system Orthorhombic TriclinicSpace group P bc21 P �1T (K) 150(2) 150(2)a (Å) 7.9138(1) 8.0427(2)b (Å) 16.7747(3) 9.5438(3)c (Å) 19.0363(4) 15.2704(5a (�) 90 102.5990(b (�) 90 98.8810(1c (�) 90 96.4090(1V (Å3) 2527.10(8) 1117.28(6Z 4 2k (Å) 0.71073 0.71073qcalc (g cm–3) 1.811 1.814l (mm–1) 1.248 1.397Collected reflections 9854 9024Unique reflections, Rint 4300, 0.041 5043, 0.02Observed reflections 4129 4450R1a (I > 2r(I)) 0.0289 0.0330wR2b (all data) 0.0661 0.0889

a R1 =P

||Fo|–|Fc||/P

|Fo|.

b wR2 =P½wðF2

o � F2c Þ

2�=P½wðF2

oÞ2�

n o1=2.

Table 2Comparison of the experimentally determined and calculated bond lengths (Å) and angles

X-ray M

MoAMo 2.5483(6) 2.Mo@O 1.685(3), 1.686(3) 1.MoAO(l2) 1.921(3)–1.937(3) 1.O(l2)AMoAO(l2) 95.83(13), 95.92(13) 95MoAO(l2)AMo 82.48(12), 82.78(12) 82MoAO(formate) 2.290(3), 2.305(3) 2.MoAN(Py) 2.238(4), 2.253(4) 2.MoACl 2.454(1), 2.461(1) 2.

B3LYP, MPW1K and M06 [30] density functionals with 6-311+G��

basis set and LACV3P��+ pseudopotential for molybdenum. Bycomparison of the calculated geometries with the experimentalones for the trans-[Mo2O4Cl2(Py)2(HCOO)]– ion, it was found thatMPW1K functional behaves the best and all subsequent calcula-tions were made at MPW1K/6-311+G�� level of theory. The opti-mized molecular structures for the trans/cis geometric isomers ofthe [Mo2O4Cl2(Py)2(HCOO)]– ion and the hydrogen-bondedPyH+� � �[Mo2O4Cl2(Py)2(HCOO)]– ion-pairs at the MPW1K/6-311+G�� level are shown in Supporting information. An overallagreement has been found between the calculated and experimen-tal geometric parameters (Tables 2 and 3).

3. Results and discussion

3.1. Structural studies

trans-{(Py)2H}[Mo2O4Cl2(Py)2(HCOO)] (1) crystallizes in anorthorhombic space group P bc21. Its crystal structure is built ofdinuclear complex anions with the trans-[Mo2O4Cl2(Py)2(HCOO)]–

composition and PyH+� � �Py as the cationic part, hereafter desig-nated in short as (Py)2H+. The N(3)� � �N(4) distance in the cation,2.716(6) Å, is significantly shorter than the sum of the van derWaals radii, 3.10 Å [31,32]. The ORTEP drawing of the cation isshown in Fig. 1 and that of the anion in Fig. 2. The complex anionconsists of a central {Mo2O4}2+ core. The coordination environmentof the two metal ions is the same: a terminal oxide, two doubly-bridging oxides, a pyridine ligand, a chloride and a formate oxygendefine a highly distorted octahedron. The origin of the distortion

d 2 Compound 3 Compound 4

o2N3O6 C20H25Br4Mo2N5O6 C16H17Br2Mo2N3O6

942.97 699.03Monoclinic OrthorhombicP 21/m P 212121

150(2) 150(2)8.0655(1) 10.03160(10)16.6510(3) 14.4172(2)

) 11.1006(2) 14.8396(2)10) 90 900) 94.2535(7) 900) 90 90) 1486.69(4) 2146.22(5)

2 40.71073 0.710732.106 2.1636.261 4.9296713 13741

01 3510, 0.0210 4807, 0.0433211 47180.0303 0.02670.0775 0.0721

(�) for the trans-[Mo2O4Cl2(Py)2(HCOO)]– ion.

PW1K M06 B3LYP

564 2.601 2.613679 1.698 1.709921–1.953 1.937–1.979 1.945–1.984.60, 95.62 95.38 95.18, 95.21.88 83.22 83.36

320 2.360, 2.361 2.363, 2.365276, 2.277 2.307, 2.309 2.327452 2.469, 2.470 2.494

Fig. 1. (Py)2H+ cation in 1.

Fig. 2. ORTEP drawing of trans-[Mo2O4Cl2(Py)2(HCOO)]–, an anion of 1, with thedisplacement ellipsoids drawn at the 30% probability level.

Fig. 3. A view along the Mo@O bonds of trans-[Mo2O4Cl2(Py)2(HCOO)]–, highlight-ing a trans disposition of pyridine ligands.

Fig. 4. ORTEP drawing of cis-[Mo2O4Cl2(Py)2(HCOO)]–, an anion of 2, with thedisplacement ellipsoids drawn at the 30% probability level.

Table 3Comparison of the experimentally determined and calculated bond lengths (Å) andangles (�) for the hydrogen-bonded PyH+� � �cis-[Mo2O4Cl2(Py)2(HCOO)]– ion-pair.

X-ray MPW1K

MoAMo 2.5639(4) 2.577Mo@O 1.683(2) 1.669, 1.670MoAO(l2) 1.931(2)–1.949(2) 1.936–1.964O(l2)AMoAO(l2) 95.79(9), 96.20(9) 95.98, 96.11MoAO(l2)AMo 82.32(8), 82.93(8) 82.01, 83.30MoAO(formate) 2.273(2), 2.289(2) 2.293, 2.308MoAN(Py) 2.233(3), 2.242(3) 2.239, 2.241MoACl 2.4419(8), 2.4517(9) 2.443N(PyH+)� � �O(l2) 2.658(4) 2.481CAH� � �Cl 3.494(4), 3.569(4) 3.490, 3.494

B. Modec, D. Dolenc / Journal of Molecular Structure 1051 (2013) 354–360 357

lies mainly in the trans influence of the molybdenyl moiety [1]. Thesites trans to the Mo@O moieties are occupied with formate oxy-gens. The formate has engaged in coordination both oxygen atoms,it binded with each to a different metal atom. The formate therebyserves as a third bridging ligand between the two molybdenumatoms. Its presence is reflected in the value of the dihedral anglebetween the two Mo(l2-O)2 planes. In 1, the dihedral angle is160.4(2)�, a value which is by ca. 10� larger than in {Mo2O4}2+ spe-cies without a third bridging ligand [23]. The formate-to-molybde-num bond lengths do not differ significantly, i.e., 2.290(3) vs.2.305(3) Å. The distribution of pyridine or chloride ligands on thetwo metal atoms is trans (Fig. 3). The overall symmetry of the anionapproximates that of C2 point group. The cis isomer of the[Mo2O4Cl2(Py)2(HCOO)]– complex ion crystallizes as a pyridiniumsalt in triclinic space group P �1. The ORTEP drawing of the complexanion of cis-(PyH)[Mo2O4Cl2(Py)2(HCOO)] (2) is shown in Fig. 4,whereas Fig. 5 gives its projection along the Mo@O bonds. Withthe distribution of pyridine ligands being cis, the anion has a virtualsymmetry Cs. Relevant geometric parameters of the anions in 1 and2 are given in Table 4. As for trans-[Mo2O4Cl2(Py)2(HCOO)]– ion, nopronounced asymmetry in the formate-to-molybdenum bonding

pattern could be observed for cis-[Mo2O4Cl2(Py)2(HCOO)]– ion,i.e., 2.273(2) vs. 2.289(2) Å. However, some of the structuralparameters of the cis-[Mo2O4Cl2(Py)2(HCOO)]– ion are significantlydifferent from the ones determined for the trans isomer. Themetal–metal bond is longer for the cis isomer, i.e., 2.5639(4) Å vs.2.5483(6) Å. The strength of the latter bond is influenced mostlyby the nature of the ligands which apart from the oxides completethe metal’s coordination sphere. With both isomers containing thesame ligands, a difference of this magnitude is rather surprising.The distances between the bridging oxides and molybdenumatoms in the cis isomer are also longer than in the trans isomer.

The structures of trans-{(Py)2H}[Mo2O4Cl2(Py)2(HCOO)] (1)and cis-(PyH)[Mo2O4Cl2(Py)2(HCOO)] (2) differ also in theintermolecular interactions. In 2, an interaction of the formatehydrogen with the molybdenyl oxide from an adjacent anion isobserved, C(1)� � �O(2) [x + 1, y, z] = 3.301(4) Å [32]. As a result,the cis-[Mo2O4Cl2(Py)2(HCOO)]– anions are linked into chainswhich propagate along the a-axis (Fig. 6). Surprisingly, no suchinteraction exists in the solid state structure of trans-{(Py)2H}[Mo2O4Cl2(Py)2(HCOO)] (1). Another apparent difference is thatpyridinium cations of 2 are hydrogen-bond donors whereas the

Table 4Relevant geometric parameters (Å, �) of the geometric isomers of the [Mo2O4Cl2(Py)2

(HCOO)]– anion.

Compound 1 Compound 2

Isomer trans cisMoAMo 2.5483(6) 2.5639(4)Fold anglea 160.4(2) 161.2(1)Mo@O 1.685(3), 1.686(3) 1.683(2), 1.683(2)MoAO(l2) 1.921(3)–1.937(3) 1.931(2)–1.949(2)O(l2)AMoAO(l2) 95.83(13), 95.92(13) 95.79(9), 96.20(9)MoAO(l2)AMo 82.48(12), 82.78(12) 82.32(8), 82.93(8)MoACl 2.454(1), 2.461(1) 2.4419(8), 2.4517(9)MoAN(Py) 2.238(4), 2.253(4) 2.233(3), 2.242(3)MoAO(formate) 2.290(3), 2.305(3) 2.273(2), 2.289(2)

a Defined as a dihedral angle between the two Mo(l2-O)2 planes.

Fig. 6. (a) A linkage of cis-[Mo2O4Cl2(Py)2(HCOO)]– anions in 2 via CAH(formate)� � �O(molybdenyl) interactions. (b) Pyridinium cations are H-bonded tothese chains. (c) Weak intermolecular interactions link the chains with theappended cations into a three-dimensional structure with channels running parallelto the a-axis.

Fig. 5. A view along the Mo@O bonds of cis-[Mo2O4Cl2(Py)2(HCOO)]–, highlighting acis disposition of pyridine ligands.

358 B. Modec, D. Dolenc / Journal of Molecular Structure 1051 (2013) 354–360

cations of 1, i.e., the (Py)2H+ ions, are not. Consequently, thepyridinium cations in 2 are H-bonded to the anionic chains. Weakintermolecular interactions which include also the p� � �p stackinginteractions between the coordinated pyridine rings link thechains into a three-dimensional structure which possesseschannels running parallel to the a-axis. The solid state structuresof 1 is governed solely by the weak intermolecular interactionswhich produce the following connectivity patterns: (i) the trans-[Mo2O4Cl2(Py)2(HCOO)]– anions are linked with 6 adjacent onesto form layers which are coplanar with the bc plane, and (ii) eachlayer is covered on both sides with the (Py)2H+ cations (Fig. 7). Itis of interest to note that the interaction of the formate hydrogenwith the molybdenyl oxide was observed also in the solid statestructures of both bromide compounds, (PyH)3[Mo2O4Br4(HCOO)]�2CH3CN (3) and cis-(PyH)[Mo2O4Br2(Py)2(HCOO)] (4). This interac-tion is particularly strong in 4, C(1)� � �O(2) [�x + 0.5, �y + 1,z–0.5] = 3.150(5) Å [33]. Similarly to the structure of 2, the anionsof (PyH)3[Mo2O4Br4(HCOO)]�2CH3CN (3) or cis-(PyH)[Mo2O4Br2

(Py)2(HCOO)] (4) are linked into infinite chains with pyridiniumcations attached with hydrogen bonds to the bridging oxides(compounds 3 and 4) and to the formate oxygens (3) (Figs. S1and S2).

(PyH)3[Mo2O4Br4(HCOO)]�2CH3CN (3) crystallizes in a mono-clinic space group P 21/m with the asymmetric unit containing

two protonated pyridine molecules, an acetonitrile solvent mole-cule and a half of the [Mo2O4Br4(HCOO)]3– complex atom. Theanion is located with some of its atoms on the mirror plane.cis-(PyH)[Mo2O4Br2(Py)2(HCOO)] (4) crystallizes in an orthorhom-bic space group P 212121 with both the pyridinium cations and thecis-[Mo2O4Br2(Py)2(HCOO)]– complex anions occupying the gen-eral positions. Relevant geometric parameters of the anions in 3and 4 are given in Table 5, whereas their ORTEP drawings are givenin Figs. S3 and S4. The metal–metal bond lengths are 2.5728(5) Åfor [Mo2O4Br4(HCOO)]3� and 2.5662(5) Å for cis-[Mo2O4Br2(Py)2

(HCOO)]�. The anion of 3 displays the longest metal–metal bondlength. The explanation lies in the absence of pyridine ligandswhich are electron-donating. On the other hand, in species withpyridine ligands, as exemplified by cis-[Mo2O4Br2(Py)2(HCOO)]–,the metal–metal bond lengths are shorter. The anion of 3 alsodisplays the longest formate-to-molybdenum bond lengths of allfour compounds.

It is to be noted that the structures of the two compounds con-taining the cis isomers, cis-(PyH)[Mo2O4Cl2(Py)2(HCOO)] (2) and

Fig. 7. (a) Section of a layer of trans-[Mo2O4Cl2(Py)2(HCOO)]– anions in 1, viewedalong a-axis. (b) A layer with both of its sides covered with (Py)2H+ cations, a viewalong c-axis.

Table 5Relevant geometric parameters (Å, �) of the bromide compounds, (PyH)3

[Mo2O4Br4(HCOO)]�2CH3CN (3) and cis-(PyH)[Mo2O4Br2(Py)2(HCOO)] (4).

Compound 3 Compound 4

MoAMo 2.5728(5) 2.5662(5)Fold anglea 157.18(9) 161.6(2)Mo@O 1.674(3), 1.685(3) 1.679(3), 1.682(3)MoAO(l2) 1.945(2), 1.951(2) 1.928(3)–1.966(3)O(l2)AMoAO(l2) 95.11(14), 95.47(14) 95.87(12), 96.27(12)MoAO(l2)AMo 82.66(10) 81.98(11), 83.23(11)MoABr 2.6030(5), 2.6064(5) 2.6048(6), 2.6113(6)MoAN(Py) – 2.237(3), 2.243(3)MoAO(formate) 2.306(3) 2.262(3), 2.276(3)

a Defined as a dihedral angle between the two Mo(l2-O)2 planes.

Table 6Total energies of the geometric isomers of the [Mo2O4Cl2(Py)2(HCOO)]– ion.

Method Etot (Hartree) Etot(cis)–Etot(trans)(kJ/mol)

cis trans

B3LYP/LACV3P��+ �2042.864622 �2042.870183 +14.6M06/LACV3P��+ �2042.105075 �2042.110534 +14.3MPW1K/LACV3P��+ �2042.425918 �2042.431756 +15.3

Fig. 8. The PyH+� � �cis-[Mo2O4Cl2(Py)2(HCOO)]– synthon in 2, viewed along theMo@O bond vectors.

B. Modec, D. Dolenc / Journal of Molecular Structure 1051 (2013) 354–360 359

cis-(PyH)[Mo2O4Br2(Py)2(HCOO)] (4), are not isotypical. A closerinspection reveals that the complex anions of 2 and 4 differ inthe mutual orientation of coordinated pyridine ligands.

3.2. DFT calculations and theoretical considerations

With the exception of trans-{(Py)2H}[Mo2O4Cl2(Py)2(HCOO)] (1)all previously isolated compounds containing the analogous anions

with other carboxylates are cis isomers [19,22]. The prevalence ofthe cis isomeric form over the trans could speak of its greater sta-bility. We have employed theory to get insight into this question.The density functional theory calculations were carried out onthe trans/cis geometric isomers of the [Mo2O4Cl2(Py)2(HCOO)]–

ion. Contrary to our expectations, all the calculation methods showthe trans isomer to be energetically more favored than the cis iso-mer (Table 6). With such a small difference, ca. 15 kJ/mol, both iso-mers are likely to persist in the solution. A certain isomeric form inthe gas phase and a crystalline compound containing the same spe-cies can be differentiated by the intermolecular interactions whichoccur not only among ions of opposite charges but also among ionsof the same type. When the differences in the stabilities of the twofree isomers are small, the determining factor for the crystalliza-tion of either isomer are the crystal packing forces. With thechange of the countercation, the intermolecular interactionschange as well. All cis isomers, including the title compounds 2and 4, are pyridinium salts [19,22]. Pyridinium cation as a goodhydrogen-bond donor forms in all compounds a hydrogen-bondinginteraction with a doubly-bridging oxide (Fig. 8). The resultingN� � �O contact is typically around 2.70 Å and the orientation ofpyridinium cation such that its ortho hydrogens participate inweak interactions with coordinated halides. The CAH� � �Cl contactsin 2 are 3.494(4) and 3.569(4) Å, and the CAH� � �Br contacts in 4 are3.478(5) and 3.989(7) Å. Contacts of approximately the samelength were observed for other cis compounds [19,22]. It appearsthat the formation of the ion-pair between the pyridinium cationand the [Mo2O4Cl2(Py)2(HCOO)]– ion is possible only if the disposi-tion of pyridine ligands is cis, whereas in the trans isomer thepyridine ligand could present a steric hindrance and thereforemake such an interaction less likely to take place. In order toconfirm the latter, the DFT calculations on the hydrogen-bondedPyH+� � �[Mo2O4Cl2(Py)2(HCOO)]– ion-pairs were also performed.The results show the ion-pair with the cis disposition of pyridinerings in the [Mo2O4Cl2(Py)2(HCOO)]– ion to be by 24 kJ/mol morestable than the one with the trans disposition. According to the

360 B. Modec, D. Dolenc / Journal of Molecular Structure 1051 (2013) 354–360

calculations, the interaction of pyridinium cation with the bridgingoxide of the trans-[Mo2O4Cl2(Py)2(HCOO)]– ion no longer producesthe ion-pair. Namely, the PyH+ proton has migrated to the bridgingoxide and the resulting pair is Py� � �trans-[Mo2O3(OH)Cl2(Py)2

(HCOO)] with one CAH� � �Cl interaction (see Supporting informa-tion). However, the solid state structure of a PyH+ salt of a complexwith a single pyridine ligand, i.e., [Mo2O4Cl3(Py)(Piv)]2– (Piv– =pivalate) shows that pyridine ligand does not prevent theH-bonding interaction of the l2-oxide with PyH+ ion [22]. Namely,in the latter compound both l2-oxides are engaged in H-bondingwith the N� � �O contacts of about 2.68 Å. Nevertheless, trans-[Mo2O4Cl2(Py)2(HCOO)]– ion does not crystallize with pyridiniumcations: it crystallizes with the (Py)2H+ countercations which can-not form hydrogen bonds. Another question to be addressed is inwhat way the reaction conditions employed in the preparation ofthe only trans isomer, i.e., compound 1, differ from all others. It isto be noted that our reaction mixtures contain PyH+ cations inexcessive amounts. When the reaction is run in pure pyridine, asin the preparation of trans-{(Py)2H}[Mo2O4Cl2(Py)2(HCOO)] (1),the PyH+ ions interact with pyridine molecules to form the(Py)2H+ cations. On the other hand, the acetate compound,cis-(PyH)[Mo2O4Cl2(Py)2(CH3COO)]�Py, whose composition ishighly reminiscent to that of 1 and which was also isolated frompure pyridine, shows that this is not always the case [19]. Thisexample also speaks in favor of the fact that the presence ofPyH+ cations in the compound imposes a restriction on thedisposition of pyridine rings in the anion. cis-(PyH)[Mo2O4Cl2(Py)2

(CH3COO)]�Py, as confirmed by its X-ray analysis, is a proper PyH+

salt with the cation H-bonded to the doubly-bridging oxide of thecis-[Mo2O4Cl2(Py)2(CH3COO)]– ion and pyridine molecule of crys-tallization trapped into the crystal lattice via weak interactionsonly.

The metal–metal bond length in cis-(PyH)[Mo2O4Cl2(Py)2

(HCOO)] (2) merits further comment. It is noticeably longer thanin trans-{(Py)2H}[Mo2O4Cl2(Py)2(HCOO)] (1). It is known thatbonding between the molybdenum(V) centers of the {Mo2O4}2+

core occurs through r overlap of the dxy orbitals [2]. In addition,the theoretical calculations have shown a significant contributionfrom the bridging oxides [8,34]. The involvement of a bridgingoxide in hydrogen-bonding, as in the case of cis-(PyH)[Mo2O4Cl2

(Py)2(HCOO)] (2), probably diminishes the strength of the overallMoAMo bonding.

3.3. Infrared spectroscopy

The masym(OCO) and msym(OCO) bands of the coordinated for-mate ions appear in the spectra of compounds 1–4 at ca. 1560and 1340 cm–1, respectively. Their positions are in agreement withthe literature data [35]. The spectra of trans-{(Py)2H}[Mo2O4Cl2

(Py)2(HCOO)] (1) and cis-(PyH)[Mo2O4Cl2(Py)2(HCOO)] (2) are notthe same: many pyridine-associated bands are either split or havedifferent intensities.

4. Conclusions

Reactions of molybdenum(V) starting materials with formateafforded discrete dinuclear [Mo2O4Br4(HCOO)]3– and [Mo2O4X2(Py)2

(HCOO)]– (X = Cl, Br) anionic species. In all, the carboxylate servedas a third bridging ligand in the {Mo2O4}2+ core. The [Mo2O4Cl2(Py)2

(HCOO)]– ion exhibited the trans/cis geometric isomerism. Thetheoretical calculations on both geometric isomers in the gas phasehave shown the trans isomer to be more stable. Hydrogen bondingand CAH� � �Cl interactions with pyridinium cations in the solid statestabilize the cis isomer. The higher thermodynamic stability of the

PyH+� � �cis-[Mo2O4Cl2(Py)2(HCOO)]– ion-pair is in agreement withthe cis arrangement observed for all PyH+ salts of the homologous[Mo2O4Cl2(Py)2(RCOO)]– (RCOO– = acetate, pivalate, hydrogen succi-nate) ions.

Acknowledgment

The work was supported by the Slovenian Ministry of Educa-tion, Science and Sport (Grant P1-0134).

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.molstruc.2013.08.009.

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