Pyridinium Pentachloridooxomolybdate(V), (PyH) 2 [MoOCl 5 ], Revisited: Single-Crystal X-ray Structure Determination

: (PyH) 2 [MoOCl 5 ] was obtained in the form of emerald green crystals unintentionally from (PyH) 5 [MoOCl 4 (H 2 O)] 3 Cl 2 in acetonitrile. (PyH) 2 [MoOCl 5 ] has been used as a starting material in molybdenum(V) coordination chemistry for decades, yet its true identity has not been known until now. The X-ray structure analysis has undoubtedly conﬁrmed the existence of this compound. The [MoOCl 5 ] 2 − ion displays the usual structural characteristics of the mononuclear MoO 3+ -containing compounds.


Introduction
The main motivation for pursuing research on the coordination chemistry of molybdenum has been its biological importance. As of today, molybdenum has been found in over 50 enzymes [1]. Two groups may be recognised: nitrogenases with an iron-molybdenum cluster as a cofactor and a larger group of enzymes with a pterin-based cofactor [1]. These enzymes catalyse oxidation-reduction reactions that form part of nitrogen, sulphur, and carbon metabolism [2]. Versatile redox behaviour and oxophilic nature make molybdenum suitable for this role. Molybdenum has a strong tendency to bind oxygen and, at the same time, the capacity to lose it. In the course of catalysis, molybdenum shuttles between oxidation states +6, +5, and +4 with some reactions involving oxygen atom transfer (OAT) processes.
We have been interested in the coordination chemistry of the intermediate oxidation state, +5 [3,4]. The MoO 3+ structural entity pervades this oxidation state. Owing to its multiple bond character, it lies at the origin of the geometric distortions of molybdenum(V) complex species which are eventually manifested also in their reactivity. Herein, a crystal structure of (PyH) 2 [MoOCl 5 ], another MoO 3+ -containing species, will be presented. The compounds containing [MoOCl 5 ] 2− ions were prepared as early as 1927 [5]. (PyH) 2 [MoOCl 5 ] formed upon the molybdate(VI) reduction with hydrazine in 11 M hydrochloric acid. After the addition of pyridine, needle-shaped crystals of emerald green colour formed [6]. The compound was of interest as it provided, in spite of its inherent air sensitivity, a suitable entry into the molybdenum(V) coordination chemistry. A drawback of using this compound was that its true identity was not known until 2005 [7]. Its composition was proposed on the basis of the elemental analyses on Cl and Mo. On the other hand, a comparison of the reflectance spectrum of the solid ammonium pentachloridooxomolybdate(V) with the spectrum of its HCl solution has suggested the presence of either the [MoOCl 5 ] 2− or the [MoOCl 4 (H 2 O)] − ions [8]. The X-ray structure analysis of crystals obtained by a modified procedure, a reduction of MoO 3 with hydroiodic acid in concentrated HCl(aq), followed by the addition of pyridine, has disclosed the [MoOCl 4 (H 2 O)] − ions as the only molybdenum(V) species in the product. The correct composition of the emerald green crystals, unequivocally established by the X-ray structure analysis, turned out to be more complex than initially assumed. The [MoOCl 4 (H 2 O)] − ions co-crystallised with chloride and pyridinium cations resulting in the (PyH) 5 5 ], has been obtained just recently in our laboratory. It was isolated in the course of the (PyH) 5 [MoOCl 4 (H 2 O)] 3 Cl 2 reactions with pyrazinecarboxylic acid in acetonitrile. With this reaction aiming towards the coordination of pyrazinecarboxylic acid to molybdenum(V), the formation of (PyH) 2 [MoOCl 5 ] was surprising. Although the reaction was found to be reproducible, no rational explanation can be provided for the transformation of [MoOCl 4 3 Cl 2 , which is the source of chloride, provides only two-thirds of the amount necessary for a quantitative transformation. Even more puzzling is the role of pyrazinecarboxylic acid. In its absence, no (PyH) 2 [MoOCl 5 ] could be isolated. Although the synthetic conditions necessary for the formation of (PyH) 2 [MoOCl 5 ] remain as elusive as ever, our study undoubtedly confirms its existence. We may also conclude that with the available literature data, it remains unclear whether the early reports were on (PyH) 5

Results
The solid-state structure of pyridinium pentachloridooxomolybdate(V) consists of the mononuclear [MoOCl 5 ] 2− anions and protonated pyridine molecules as countercations.
Although part of the [MoOCl 5 ] 2− ion resides on the twofold rotation axis, its symmetry is that of the C 4v point group. The drawing of the formula unit is shown in Figure 1, and relevant geometric parameters of the [MoOCl 5 ] 2− ion are listed in Table 1.  Parameter  (3), which occupies the position trans to the multiply bonded oxide, is at a significantly longer distance, 2.6038(10) Å. The highly distorted octahedral environment of molybdenum(V) is a result of an operating trans influence of the multiply bonded oxide [9]. With the metal ion being located 0.2333(7) Å above the best plane of four equatorial chlorides, the shape of the [MoOCl 5 ] 2− ion may be described as umbrella-like.
It should be noted that the lengthening of the Mo(1)-Cl (3) bond is a joint result of the operating trans influence of the molybdenyl moiety and the engagement of the Cl(3) chloride in hydrogen bonding with pyridinium cations. Cl(3) forms two hydrogen bonds with two pyridinium cations, with the N(1)···Cl(3) contacts being 3.092(3) Å, a significantly shorter distance than the sum of the N and Cl van der Waals radii, 3.3 Å [10]. Such connectivity is known as a bifurcated hydrogen bond. The pattern, the [MoOCl 5 ] 2− ion with two hydrogen-bonded pyridinium cations, is shown in Figure 1. These hydrogen-bonded clusters interact with adjacent ones via π···π stacking interactions occurring between pairs of pyridinium cations ( Table 2, Figures 2 and 3).     [16], features a slightly longer bond between molybdenum and chloride that is trans to the terminal oxide, 2.6910(6) Å. An even larger discrepancy in the molybdenum-to-the-chloride bonding pattern was observed for (C 5 H 10 NO) 2 [MoOCl 5 ] in which the bonds to equatorial chlorides are in the 2.3741(7)-2.3820(7) Å range, whereas the bond to the apical chloride is as long as 2.7582(7) Å [14]. The elongation of this bond was explained with the engagement of the trans-positioned chloride in hydrogen-bonding interactions with the countercations.

General
All reagents but (PyH) 5 [MoOCl 4 (H 2 O)] 3 Cl 2 were purchased from commercial sources and used without further purification. Molybdenum(V) starting material was prepared following the published procedure [7]. IR spectrum of the Nujol suspension was recorded in the 4000-600 cm −1 spectral region using an FTIR instrument PerkinElmer Spectrum 100 (PerkinElmer, Shelton, CT, USA). Owing to the decomposition of crystalline (PyH) 2 [MoOCl 5 ] when exposed to the air atmosphere, no elemental CHN analysis was performed. Single crystal X-ray diffraction data were collected on an Agilent SuperNova diffractometer (Agilent Technologies XRD Products, Oxfordshire, UK) with copper (Cu-K α , λ = 1.54184 Å) X-ray source at 150 K. CrysAlis PRO [18] was used for data processing and Olex 2 software [19] for data analysis. The structure was solved by ShelXT [20] and refined by the least-squares method in ShelXL [21]. Anisotropic displacement parameters were determined for all nonhydrogen atoms. Platon [22] and Mercury [23] were used for the analysis of the crystal structure and the preparation of figures. The crystal structure was deposited to the CCDC and assigned the deposition number 2088930. These data can be obtained free of charge via http://www.ccdc. cam.ac.uk/conts/retrieving.html (or from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44 1223 336033; E-mail: deposit@ccdc.cam.ac.uk). The crystallographic data are summarised in Table 3.  Figure S1: IR spectrum of (PyH) 2