Syntheses and a Solid State Structure of a Dinuclear Molybdenum(V) Complex with Pyridine

A mononuclear complex [MoOCl4(H2O)]− readily forms a metal−metal bonded {Mo2O4}2+ core. A high content of pyridine in the reaction mixture prevents further aggregation of dinuclear cores into larger clusters and a neutral, dinuclear complex with the [Mo2O4Cl2(Py)4] composition is isolated as a product. Solid state structures of two compounds containing this complex, [Mo2O4Cl2(Py)4]·2.25Py (1) and [Mo2O4Cl2(Py)4]·1.5PyHCl (2), were investigated by X-ray crystallography.


Introduction
The {Mo 2 O 4 } 2+ core (shown in Scheme 1) with d electrons localized in a Mo(V)−Mo(V) single bond appears as a recurrent structural motif in many contexts of molybdenum(V) coordination chemistry [1][2][3][4][5][6][7]. A group of high-nuclearity oxomolybdenum(V) complexes whose structures may be seen as the assemblies of two or more {Mo 2 O 4 } 2+ units has emerged in the past two decades [8]. The aggregation of dinuclear units is made possible by the ability of oxide and alkoxide ions to participate in μ 2 -, μ 3 -or even μ 4 -bridging interactions. The oxomolybdenum(V) clusters are of interest because of their intermediacy between the oligomeric alkoxides and the polymeric oxides on the other side. The expectation is that their chemistry may provide some insight into the catalytic processes taking place on the surface of the solid metal oxides [9].

[MoOCl 4 (H 2 O)] 3 Cl 2 with a mononuclear [MoOCl 4 (H 2 O)]
− ion has been shown to serve as a suitable starting material in the preparation of {Mo 2 O 4 } 2+ -containing clusters [10]. Changes in the contents of reaction mixtures or reaction conditions resulted in clusters with different nuclearities or different connectivities among the dinuclear building blocks.  8 ] compositions [11][12][13]. The subtle nature of the aggregation is illustrated by the assembly of four {Mo 2 O 4 } 2+ units which was seen to produce three different architectures: (i) a cyclic {Mo 8 8 } core, and two cores with more condensed structures, (ii) {Mo 8 [12] and (iii) {Mo 8 [14]. The presence of methanol in the reaction mixture was found to be crucial for the aggregation process. The possible role that methanol or alcohols in general play in the formation of dinuclear units and their further aggregation will be discussed presently. In the absence of alcohol, (PyH) 5

Results and Discussion
The metal−metal bonded, dinuclear {Mo 2 O 4 } 2+ core is a result of the facile substitution chemistry on the mononuclear [MoOCl 4 (H 2 O)] − ion. In spite of the many known compounds possessing the dinuclear structural core, the details of its formation remain unknown. Undoubtedly, one of the key prerequisites is the presence of water. Results of our previous work show that methanol actively participates in substitution and/or dimerization reactions through the coordination of the methoxide ions formed in situ. The coordinated methoxide can in turn react with water to form an oxido ligand. A direct evidence of the latter process is the observed transformation of [Mo 2 O 3 (OMe)(OOCCH 3 )Cl 4 ] 2− , a complex with a bridging methoxide, to the {Mo 2 O 4 } 2+ -containing products [15]. The formation of {Mo 2 O 4 } 2+ species was found to be faster and more reproducible when methanol was present in the reaction mixture. Furthermore, it has also been observed that the reaction mixtures containing large amounts of methanol or other alcohols tend to produce higher-nuclearity clusters with either oxido or alkoxido ligands shared between the constituent dinuclear units. In such clusters, the relative content of the chlorido ligands was found to be rather small. Conversely, the reaction systems lacking methanol do not favour further association of dinuclear cores and as such provide a convenient environment for the isolation of the {Mo 2 O 4 } 2+ prototypes. The reaction system consisting solely of molybdenum(V) starting material and pyridine exemplifies such a medium. When pyridine occupies  4 ] crystallizes only with the incorporation of either a huge amount of pyridine solvent molecules (compound 1) or pyridinium chloride (compound 2). None of the two compounds is stable when removed from the mother liquor. Compound 1 dissolves almost instantaneously in its own solvent molecules.
As shown by the X-ray structure analysis, 1 and 2 are both [Mo 2 O 4 Cl 2 (Py) 4 ]-containing compounds ( Figure 1) which differ in the content of the solvent molecules of crystallization or counter ions. For both, the asymmetric unit contains two crystallographically independent complex molecules with very similar structural parameters (Table 1).   The overall features of the basic {Mo 2 O 4 } 2+ core are as determined previously in related compounds [8]: (i) a short distance between molybdenum atoms, 2.5502(8) and 2.5517(8) Å for 1 and 2.5427(6) and 2.5441(6) Å for 2, typical for a single metal−metal bond; (ii) a puckered Mo(μ 2 -O) 2 Mo ring with the metal atoms above and the bridging oxygen atoms below the mean plane. The rather short metal−metal bond lengths are due to the high content of pyridine ligands whose nature is electron-donating. The dihedral angle between the two Mo(μ 2 -O) 2 planes is by ca. 10° smaller than the angles observed in a series of oxalato complexes of the [Mo 2 O 4 (η 2 -C 2 O 4 ) 2 (R-Py) 2 ] 2− (R-Py = alkylsubstituted pyridine) type [16]. A distorted octahedral coordination of each metal centre of the {Mo 2 O 4 } 2+ core is completed by a chloride at 2.452(2)−2.477(2) Å for 1 and at 2.453(1)−2.466(1) Å for 2 and a pair of pyridine ligands with two distinctly different bond lengths. A short one occupies the 2.256(5)−2.276(5) Å range, whereas a long one the 2.381(4)−2.407(5) Å range. The reason lies in one of the two pyridine ligands being bound to a position trans relative to the Mo=O group and being, therefore, subject to its well-documented trans influence [17]. The lengthening of the bonds trans to the terminal oxide is a general phenomenon found throughout the {MoO} 3+ compounds. The molybdenum-to-pyridine bond lengths in 1 and 2 are shorter than the ones found in [Mo 2 O 2 (μ 2 -S) 2 {S 2 P(OEt) 2 } 2 ]·2Py [18]. The latter compound exemplifies an extreme case of trans influence of the molybdenyl group: because of the extreme bond length, 2.545(7) and 2.569(7) Å, the compound is considered as an adduct with pyridine. The extent to which the trans influence is expressed within a certain complex differs from one case to another.

X-ray crystallography
Each crystal was 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 Bruker SMART CCD (compound 1) or Nonius Kappa CCD (compound 2) diffractometer. SHELXL-97 was employed for the structure solution and refinement [22]. The high content of pyridine solvent molecules in 1 resulted in large R1 and wR2 residuals. One of the pyridine solvent molecules was located with its centre of gravity on the inversion centre. Although the disorder was resolved using a PART −1 instruction, the displacement parameters of its atoms remained large. The structure of 2 was solved and successfully refined in a non-centrosymmetric P 2 1 space group. When refined in a centrosymmetric P 2 1 /c space group, a small fragment of the structure, a pyridinium cation with a hydrogen-bonded chloride counter anion, was located on the inversion centre. Its disorder could not be resolved. Figures depicting the structures were prepared by Ortep-3 [23] and CrystalMaker [24]. Cell parameters and refinement results are summarized in Table 2. Further details on the crystal structure investigations may be obtained from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif by quoting the deposition numbers CCDC-755860 (1) and 755861 (2).