High oxidation state imido metallasiloxanes: synthesis and structural characterisation of novel bis-imido chromium(VI) and molybdenum(VI) compounds †

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For reactions between [M(NBut)2Cl2] (M = Cr or Mo) or [Mo(NAr)2Cl2]·dme, Ar = 2,6-Pri2C6H3, dme = 1,2-dimethoxyethane, and tetraphenyldisiloxanediol, [O(Ph2SiOH)2] (2∶1), in the presence of a nitrogen donor base, changing the nitrogen donor from pyridine, py, to triethylamine, NEt3, inhibited the protonation of imido groups and facilitated the formation of bis-imido compounds. Thus the compounds [Cr(NBut)2(O(Ph2SiO)2)]21, [Mo(NBut)2(O(Ph2SiO)2)2] 2 and [Mo(NAr)2(O(Ph2SiO)2)]23 were isolated when NEt3 was used as base. Compound 3 was also formed when the dilithium reagent [O(Ph2SiOLi)2] was used. The pyridine adduct [Mo(NAr)2Cl2]·2py 4 was isolated from the reaction between [Mo(NAr)2Cl2] and O(Ph2SiOH)2 and py (1∶1∶2 molar ratio). Compounds 2, 3·C6H6 and 4·0.5C6H5Me have been characterised by X-ray crystallography. The dimeric composition of 1 in the gas phase and in benzene was confirmed by mass spectroscopy and cryoscopic molecular weight determination respectively. All compounds were characterised by NMR spectroscopy.