Hydrogen-Bonding in Two Pyridinium Salts of [Mo2O4Cl4(μ2-dmsH)] 3 − Complex (dmsH − = a Half-Neutralized Form of 2,2-Dimethylsuccinic Acid)

Reactions of a mononuclear molybdenum(V) starting material, (PyH)5[MoOCl4(H2O)]3Cl2, with 2,2-dimethylsuccinic acid in the presence of base afforded two products, (PyH)3[Mo2O4Cl4(μ2-dmsH)]·1/2CH3CN (1) and (PyH)4[Mo2O4Cl4(μ2-dmsH)]Cl (2). As revealed by the X-ray structure analysis, the half-neutralized form of the dicarboxylic acid, the dmsH− ion, coordinated to the well-known {Mo2O4}2+ core in the syn-syn bidentate bridging manner. In both compounds, the non-ionized terminus of the ligand, the COOH function, participated in hydrogen-bonding interactions. The incorporation of the chloride counteranion in 2, prevented the formation of the common “carboxylic acid dimer” which was observed for 1. Instead, a hydrogen-bonded linkage of the COOH function with the chloride occurred.


Introduction
The singly metal−metal bonded {Mo 2 (μ 2 -O) 2 O 2 } 2+ structural fragment, shortly denoted as {Mo 2 O 4 } 2+ , pervades the chemistry of molybdenum(V) [1][2][3][4].Its ability to retain its structural integrity upon coordination of various ligands to its peripheral sites makes it a suitable candidate for the construction of metal-organic frameworks.The designed synthesis of such compounds has been an area of active research over the past two decades [5].For example, the appropriately designed Mo 2 4+ dimetal units served as preformed molecular building blocks in the formation of higher order structures [6][7][8].

OPEN ACCESS
We hoped that the combination of the {Mo 2 O 4 } 2+ building blocks with multidentate ligands such as di-or tricarboxylates would under favourable conditions result either in discrete clusters or polymeric materials.Our goal was realized by the reaction system of a mononuclear molybdenum(V) starting material, (PyH) 5 [MoOCl 4 (H 2 O)] 3 Cl 2 , and 1,3,5-benzenetricarboxylic acid.In the presence of a weak base, pyridine, a dinuclear anionic complex (PyH) 4 [Mo 2 O 4 Cl 4 (μ 2 -btcH 2 )]Cl and a tetranuclear neutral complex [{Mo 2 O 4 (Py) 3 } 2 (μ 3 -btcH) 2 ]•6Py (where PyH + is pyridinium cation, whereas btcH 2 − and btcH 2− stand for anionic forms of the acid with one or two ionized functions, respectively) were prepared [9].Only the ionized functions of the acid were seen to participate in bonding interactions with the metal atoms.In both compounds, a carboxylate function coordinated to the {Mo 2 O 4 } 2+ core in a syn-syn bidentate bridging manner with the carboxylate oxygens occupying the positions which were trans to the terminal oxides (Figure 1).

Solid State Structures
The X-ray structure analysis revealed that compounds  The geometric properties of the [Mo 2 O 4 Cl 4 (μ 2 -dmsH)] 3− anions of 1 and 2 do not show significant differences (see Table 1).The overall features of the {Mo 2 O 4 } 2+ cores in the [Mo 2 O 4 Cl 4 (μ 2 -dmsH)] 3− anions are non-exceptional: (i) a short distance between a pair of molybdenum atoms, 2.5713(4) and 2.5827(4) Å in 1 and 2.5929(7) Å in 2, and (ii) a non-planar Mo(μ 2 -O) 2 Mo rhombus whose deviation from planarity is given by a dihedral angle between two Mo(μ 2 -O) 2 planes.The latter angle is known also as a fold angle.The larger the fold angle, the more planar moiety.The fold angles in compounds 1 and 2 fall in the interval of values observed for other {Mo 2 O 4 } 2+ complexes with the carboxylate serving as a third bridging ligand.Geometric parameters of a series of such complexes are summarized in Table 2.The series displays with no exception larger fold angles than the {Mo 2 O 4 } 2+ complexes without a third bridging ligand.For example, in a dinuclear [Mo 2 O 4 Cl 4 (MeOH) 2 ] 2− anion an angle of 146.0(1)° was observed [10].Nevertheless, the puckering of the Mo(μ 2 -O) 2 Mo moiety is a means of allowing a close approach of the metal atoms [11].Therefore a somewhat more flattened Mo(μ 2 -O) 2 Mo moiety in 2 is accompanied by a longer metal−metal bond length.A distorted octahedral coordination of each metal center of the {Mo 2 O 4 } 2+ core in the complex anions of 1 and 2 is completed by a pair of chlorides and an oxygen from the carboxylate group.In both compounds, the dmsH − ion, an anionic form of the parent acid with one ionized COOH function, is bound to the metal ions of the {Mo 2 O 4 } 2+ core in a syn-syn bidentate bridging manner.The carboxylate group occupies a pair of trans positions relative to the Mo=O bonds and is, therefore, subjected to their well-documented trans influence [12].The molybdenum-to-the-carboxylate bond lengths listed in Table 2 are seen to span a wide range.Furthermore, in some complexes a non-equivalence in the pair of molybdenum-to-the-carboxylate bond lengths is displayed.In the case of [Mo 2 O 4 Cl 4 (μ 2 -dmsH)] 3− anions, a more pronounced asymmetry in the carboxylate coordination occurs only for one of the two complex anions in the asymmetric unit of 1.The corresponding bond distances are 2.286(2) vs. 2.344(2) Å.In this case, the asymmetry can be traced also to the involvement of the particular carboxylate oxygen in another bonding interaction, i.e., a hydrogen bond with pyridinium cation.It is to be noted that the main difference in the two complex anions of the asymmetric unit in 1 is in the relative orientation of the dmsH − ligand [13].The similarities and the differences can be seen in Figure 4 which shows an overlay of the two complex anions.3).The corresponding O•••O distances, 2.609(3) and 2.630(3) Å, are significantly shorter than the sum of the van der Waals radii, 3.04 Å [17].Such a linkage, known also as a "carboxylic acid dimer" and is illustrated in Figure 9, is a dominant recognition motif in the structures of carboxylic acids [18].As will be shown presently, the molybdenum(V) complexes with multicarboxylate ligands which possess at least one non-ionized COOH function reveal several other connectivity patterns.The explanation for a displayed diversity lies in the presence of structural entities which can participate in interactions with the COOH entity.Typically, these are pyridine solvent molecules and chloride counteranions.The outcome of the unavoidable competition between the several hydrogen bond donors and acceptors is unpredictable.
The distances may be compared to the sums of the corresponding van der Waals radii: 3.04 Å for O+O, and 3.07 Å for N+O [17]; c COO − , the carboxylate group coordinated to the {Mo 2 O 4 } 2+ core.

Infrared Spectroscopy
The positions of the asymmetric and symmetric ν(COO) vibrations can be used to diagnose the carboxylate binding mode [20,21].Due to the presence of two functions in dmsH − ligands in 1 and 2, no unambiguous assignation can be made.A group of bands in the region of 1550−1350 cm −1 finds its origin in the ν asym (COO) and ν sym (COO) of the coordinated carboxylate moiety.Strong bands at 1698 cm −1 for 1 and at 1710 cm −1 for 2 may be assigned to the C=O stretching frequency for the acid end of the dmsH − ligand [22].The shift to lower frequency for 1 is in accordance with the involvement of the COOH function in a strong hydrogen-bonding to form a dimer.

General
All procedures were conducted in air.Most chemicals were purchased from Aldrich Chemical Co., and triethylamine from Fluka.They were used without further purification.(PyH) 5 [MoOCl 4 (H 2 O)] 3 Cl 2 was prepared following the published procedure [10].The infrared spectra were measured on solid samples as nujol mulls using a Perkin Elmer 2000 series FT-IR spectrometer.Elemental analyses were carried out by the Chemistry Department service at the University of Ljubljana.

Preparation of (PyH
2,2-dimethylsuccinc acid (353 mg, 2.415 mmol) was added to the solution of triethylamine (490 mg, 4.84 mmol) in acetonitrile (25 mL).To this solution, (PyH) 5 [MoOCl 4 (H 2 O)] 3 Cl 2 (562 mg, 1.31 mmol of molybdenum) was added.The red solution was left to stand in a closed Erlenmeyer flask at ambient conditions overnight.On the following day, the solution was placed in the refrigerator.Orange crystals of 1 that formed after two days were filtered off.Yield: 248 mg (47%

Preparation of (PyH
2,2-dimethylsuccinc acid (730 mg, 5.0 mmol) was dissolved in methanol (5 mL), followed by the addition of a methanol solution (1.0 M) of tetrabutylammonium hydroxide (10 mL, 10.0 mmol).Methanol was removed by careful pumping on the vacuum line.Acetonitrile (20 mL) was added to the oily residue.Each gram of thus obtained solution contained 0.319 mmol of (n-Bu 4 N) 2 dms.This solution (2.19 g, 0.70 mmol of (n-Bu 4 N) 2 dms) was added to the acetonitrile (20 mL) solution of (PyH) 5 [MoOCl 4 (H 2 O)] 3 Cl 2 (450 mg, 1.05 mmol of molybdenum).The solution of deep yellow colour was left to stand in a closed Erlenmeyer flask at ambient conditions overnight.On the following day, the volume of the solution was reduced to ca. one half by pumping on the vacuum line.The resulting solution was placed in the refrigerator.The orange crystals of 2 which formed after two days were filtered off and washed with the hexanes.Yield: 160 mg (34%

X-ray Crystallography
Crystals were mounted on the tip of a glass fibre with a small amount of silicon grease and transferred to a goniometer head.Data were collected on a Nonius Kappa CCD diffractometer.Data reduction and integration were performed with the software package DENZO-SMN [23].Averaging of the symmetry-equivalent reflections largely compensated for the absorption effects.For both compounds, the coordinates of some or all of the non-hydrogen atoms were found via direct methods using the structure solution program SHELXS [24].The positions of the remaining non-hydrogen atoms were located by use of a combination of least-squares refinement and difference Fourier maps in the SHELXL-97 program.Hydrogen atoms, including the carboxylic hydrogen atoms, were added in calculated positions.All calculations were performed using WinGX System Version 1.80.05 [25].Figures depicting the structures were prepared using ORTEP3 [26], SHELXTL [27], Mercury [28], and CrystalMaker [29].Cell parameters and refinement results are summarized in Table 5.Further details on the crystal structure investigations may be obtained free of charge from The Cambridge Crystallographic Data Centre [30].Cambridge Structural Database deposition numbers: CCDC-917918 (1) and -917919 (2).

Conclusions
Reactions of (PyH) 5 [MoOCl 4 (H 2 O)] 3 Cl 2 with 2,2-dimethylsuccinic acid afforded a dinuclear anionic molybdenum(V) complex which crystallized as two pyridinium salts, (PyH) 3 [Mo 2 O 4 Cl 4 (μ 2 -dmsH)]•1/2CH 3 CN (1) and (PyH) 4 [Mo 2 O 4 Cl 4 (μ 2 -dmsH)]Cl (2).In order to deprotonate the acid, triethylamine and tetrabutylammonium hydroxide were used.In either case, products with a dmsH − ion, a half-neutralized acid, were isolated.The ligand, the dmsH − ion, coordinated to the {Mo 2 O 4 } 2+ core in the already observed manner: with both oxygen atoms of the carboxylate to trans sites within the {Mo 2 O 4 } 2+ core.The non-ionized COOH function of the ligand got engaged in hydrogen bonds.In 1, the COOH groups of two ligands associated to form the well-known "carboxylic acid dimer".Due to the presence of chloride counteranions in 2, a different intermolecular connectivity was observed.The COOH group formed a hydrogen bond with the chloride.The [Mo 2 O 4 Cl 4 (μ 2 -dmsH)] 3− complex represents a new addition to the growing family of the {Mo 2 O 4 } 2+ -containing complexes with multicarboxylate ligands.In cases when the ligands possess non-ionized functions, diverse hydrogen-bonding motifs which involve the COOH functions are displayed.
The carboxylate ligand of [{Mo 2 O 4 (Py) 3 } 2 (μ 3 -btcH) 2 ]•6Py possessed another ionized function which employed a different binding mode: it coordinated in a monodentate manner to a metal ion in an adjacent dinuclear unit and a discrete tetranuclear cluster formed.Since the reaction outcome depends upon the number of the ionized functions in the acid, other bases apart from pyridine were used.At first, reactions of an acid with two carboxylic groups, 2,2-dimethylsuccinic acid, hereafter designated as dmsH 2 , were investigated.The structural formula of the acid is shown in Figure2.Following similar procedures to those employed with 1,3,5-benzentricarboxylic acid, the reactions with dmsH 2 in the presence of triethylamine or tetrabutylammonium hydroxide afforded two products, (PyH) 3 [Mo 2 O 4 Cl 4 (μ 2 -dmsH)]•1/2CH 3 CN (1) and (PyH) 4 [Mo 2 O 4 Cl 4 (μ 2 -dmsH)]Cl (2).Herein, their solid state structures are presented with the emphasis on the differences in the intermolecular interactions.

Figure 1 .
Figure 1.A typical coordination of the carboxylate ligand to the {Mo 2 O 4 } 2+ structural core.

Figure 4 .
Figure 4.An overlay of the crystallographically independent complex anions in 1.
Compound 2 exemplifies the case when the incorporation of the chloride counteranion into the structure interferes with the formation of the carboxylic acid dimer.In 2, the COOH function is engaged in a hydrogen bond with chloride, O(31)•••Cl(5) iii = 3.044(4) Å [(iii) symmetry code: x + 0.5, −y + 0.5, z + 0.5] which forms another hydrogen bond of a comparable length with protonated pyridine, i.e., N(3) iii •••Cl(5) iii = 3.047(5) Å (Table4).The other three pyridinium cations in 2 interact with the μ 2 -oxides of the {Mo 2 O 4 } 2+ core.The resulting N•••O contacts are in the 2.620(8)−2.807(7)Å range.This type of hydrogen-bonding appears as a recurrent structural motif among the anionic {Mo 2 O 4 } 2+ complexes with protonated pyridine molecules as countercations[10].Similarly, four out of six pyridinium cations in the asymmetric unit of 1, participate in the same type of the interaction, whereas the remaining two cations form hydrogen bonds with the carboxylate oxygens.
Figure ions in atoms sphere the dim

Figure 9 .
Figure 9. Pair-wise association of two carboxylic acids, a common hydrogen bond synthon.

Table 2 .
Dimensions (Å, deg) of {Mo 2 O 4 } 2+ units in carboxylate complexes.In all, the carboxylate ligand L is coordinated in a bidentate bridging manner to a pair of trans sites in the {Mo 2 O 4 } 2+ a