Formation of η2-aroyl, η1-halogenocarbyne or sterically crowded aryldiazenide ligands in the reactions of ring-substituted tricarbonyl[hydrotris(pyrazolyl)borato]-molybdate and -tungstate anions with arenediazonium cations and related oxidants

Although the hydrotris(pyrazolyl)borato complex [Mo{HB(pz)3}(CO)3]– reacted with 3- or 4-substituted arenediazonium cations [R′N2]+ yielding carbonyl-substitution (i.e. aryldiazenido) products [Mo{HB(pz)3}(CO)2(N2R′)]. reaction of the methylsubstituted analogue [ML*(CO)3]–[L*= tris(3,5-dimethylpyrazolyl)hydroborate; M = Mo or W] led, via oxidative formation of aryl radicals and [ML*(CO)3]˙, to η2-aroyl complexes [ML*(CO)2(η2-COR′)][R′= C6H4X-4 (X = NO2, CN, COMe, CF3, H, Me, OMe or NMe2) or C6H4X-3 (X = NO2 or OMe)] in acetonitrile or to the halogenocarbyne complexes [ML*(CO)2(CX)](X = Cl, Br or I) in the presence of the halogenoalkanes CH2Cl2 or CHX3(X = Br or I). The complex [MoL*(CO)3]– reacted similarly with diphenyliodonium or triphenylsulfonium cations, but in the latter case anion–cation redox is very slow in the dark but rapid upon irradiation with sunlight. Comparison of these results with those obtained for [ML*(CO)3]– analogues with different substituents in the pyrazolyl rings demonstrates that oxidation of the former by arenediazonium cations occurs in response to the steric rather than the electronic effect of the 3-methyl substituents. However further steric crowding in either the hydrotris(pyrazolyl)borate ligand or the diazonium cation promotes a reversion to the carbonyl-substitution pathway. A mechanism to account for these observations is proposed. Attempts to extend the chlorocarbyne synthesis to systems other than [ML*(CO)3]– met with only limited success. Spectroscopic data for the new complexes are reported and discussed. Two aryldiazenido complexes, [MoL*(CO)2(N2R′)](R′= 2,6-Me2C6H3 or 2,3-dimethyl-5-oxo-1-phenyl-3-pyrazolin-4-yl) have been characterised by single-crystal X-ray diffraction studies and are found to differ in the manner in which the aryldiazenide ligand accommodates to steric crowding in the molybdenum co-ordination sphere.