The reactions of MoOCl4 with neutral group 15 and 16 ligands and a re-investigation of some N-donor ligand complexes of MoOCl3
Graphical abstract
Reaction of MoOCl4 with a range of neutral donor ligands invariably cause reduction to Mo(V) complexes, representative examples of which are authenticated crystallographically; the green product formed by reaction of MoOCl4 with diimines is proposed to be formed of [MoOCl3(diamine)] co-crystallised with an O-free Mo(V) or (IV) complex.
Introduction
The chemistries of molybdenum and tungsten are in general very similar [1], [2], but significant differences are seen in their highest oxidation state (+6), where the tungsten compounds are often much more stable. For example, WBr6, WOBr4 and WSCl4 have been known since the mid-twentieth century [3], but the corresponding MoBr6, MoOBr4 and MoSCl4 have never been obtained. Also, while WCl6 is commercially available as a precursor for other tungsten compounds, MoCl6 was only characterised in 2013 and decomposes below room temperature [4]. The lower stability of the analogous molybdenum complexes also extends to their coordination complexes. The coordination chemistry of the molybdenum oxide chlorides, MoVIO2Cl2 and MoVOCl3 received significant effort in the period 1960–1990, much of it driven by catalytic applications and attempts to model the active sites of molybdenum enzymes [1], [2]. The third oxide chloride, MoVIOCl4 was very little studied and it was usually reported to be reduced by neutral organic ligands [5], [6], [7], although detailed descriptions were rare. It is commercially available and can be made readily from reaction of MoO3⋅nH2O with SOCl2 followed by vacuum sublimation [6].
We have recently examined various series of coordination complexes with WOCl4 and WSCl4, including with nitrogen and oxygen donor ligands [8], phosphines and arsines [9], and thio-, seleno- and telluro-ethers [10] (of which some had been described in early literature) and we demonstrated that some thioether complexes of WSCl4 can function as single source precursors for low pressure CVD (chemical vapour deposition) growth of semi-conducting WS2 thin films [10]. We have also established three series of complexes of MoOCl3 with thio-, seleno- and telluro-ethers, [MoOCl3(L-L)] (L-L = RS(CH2)2SR, R = iPr, Ph; MeS(CH2)3SMe; MeSe(CH2)nSeMe, n = 2, 3), [{MoOCl2(EMe2)}2(µ-Cl)2] (E = S, Se, Te) and [(MoOCl3)2(L′-L′)] (L′-L′ = o-C6H4(SeMe)2, o-C6H4(TeMe)2, MeTe(CH2)3TeMe) [11].
In terms of Mo(VI) complexes, the first neutral organic ligand complexes of molybdenum(VI) oxide tetrafluoride, MoOF4, [MoOF4(L)] (L = MeCN, thf, OPPh3, OPMe3, dmso, dmf, bipy) have been obtained very recently [12], but in contrast to WOF4, which formed [WOF4(L-L)] (L-L = Me2P(CH2)2PMe2, o-C6H4(PMe2)2) and [WOF4(PMe3)] [13], MoOF4 is reduced by phosphines, arsines or thioethers [12].
Here we describe the reactions of MoOCl4 with a variety of donor ligands from Groups 15 and 16, including re-investigation of some examples described previously, and the characterisation of MoOCl3 complexes with some neutral nitrogen donor ligands. The molybdenum (III, IV and V) complexes of the nitrogen ligands 2,2′-bipyridyl and 1,10-phenanthroline (L-L) were much studied in 1960–1980′s, but the literature is complicated, confused and often contradictory. For example, describing green, pink-purple and sometimes khaki “isomers” of [MoOCl3(L-L)], in addition to dimers and protonated imine salts of oxochloromolybdate(V) anions [1], [7]. The reports are mostly based upon wet analyses and limited spectroscopy and no structures were reported. In this paper we have also attempted to clarify some of this work.
Section snippets
Experimental
Syntheses were performed using standard Schlenk and glovebox techniques under a dry N2 atmosphere. MoOCl4 was obtained from Climax Molybdenum and MoOCl3 made by the literature route from MoCl5 and O(SiMe3)2 [14]. Solvents were dried by distillation from CaH2 (MeCN and CH2Cl2) or Na/benzophenone ketyl (toluene, n-hexane). Ligands were dried by heating in vacuo (bipy, phen, Ph3PO) or over molecular sieves (liquid ligands). Infrared spectra were recorded on a PerkinElmer Spectrum 100 spectrometer
Reaction of MoOCl4 with neutral ligands
The reactions of MoOCl4 with the neutral ligands were carried out in either anhydrous CH2Cl2 or toluene at ambient temperatures, unless indicated otherwise (Scheme 1).
Reaction of MoOCl4 with dry MeCN in CH2Cl2 solution at ambient temperature gave a paramagnetic pale green solid, which on closer examination was seen to contain both green and yellow powder. Repeating this reaction varying the MoOCl4:MeCN ratio, in an ice bath, under reflux, in toluene solution, or using neat MeCN, also gave
Conclusions
The reactions of MoOCl4 with representative neutral donor ligands from Groups 15 and 16, even under mild conditions (CH2Cl2 solution at room temperature or below), result in reduction to molybdenum(V), mostly producing complexes of MoOCl3. Even with [NEt4]Cl the product is the Mo(V) anion [Mo2O2Cl6]2-, the X-ray structure of which shows it to be a weakly associated dimer, and in contrast to some other examples, there are no cation-anion interactions. However, the salts, [SeMe3][MoOCl4(H2O)],
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
We thank EPSRC for support via EP/P025137/1 and EP/N509747/1.
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