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

For reactions between [M(NBut)(2)Cl-2] (M = Cr or Mo) or [Mo(NAr)(2)Cl-2]. dme, Ar = 2,6-(Pr2C6H3)-C-i, dme = 1,2-dimethoxyethane, and tetraphenyldisi loxanediol, [O(Ph2SiOH)(2)] (2 : 1), in the presence of a nitrogen donor ba se, 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))](2) 1, [ Mo(NBut)(2)(O(Ph2SiO)(2))(2)] 2 and [Mo(NAr)(2)(O(Ph2SiO)(2))](2) 3 were is olated when NEt3 was used as base. Compound 3 was also formed when the dili thium reagent [O(Ph2SiOLi)(2)] was used. The pyridine adduct [Mo(NAr)(2)Cl- 2]. 2py 4 was isolated from the reaction between [Mo(NAr)(2)Cl-2] and O(Ph2 SiOH)(2) and py (1 : 1 : 2 molar ratio). Compounds 2, 3 . C6H6 and 4 . 0.5C (6)H(5)Me have been characterised by X-ray crystallography. The dimeric com position of 1 in the gas phase and in benzene was confirmed by mass spectro scopy and cryoscopic molecular weight determination respectively. All compo unds were characterised by NMR spectroscopy.