'Pincer' dicarbene complexes of some early transition metals and uranium

The complexes [(C-N-C)MXn(thf)(m)] with the 'pincer' 2,6-bis(imidazolylidene)pyridine, (C-N-C) = 2,6-bis(arylimidazol-2-ylidene)pyriditie, aryl = 2,6-(Pr2C6H3)-C-1, M = V, X = Cl, n = 2, to = I 1a; M = Cr, X = Cl, n = 2, m = 0, 2a, X = Br, 2b; M = Mn, X = Br, n = 2, m = 0, 3; M = Nb, X = Cl, n = 3, m = 0, 4; and M = U, X = Cl, n = 4, m = 0, 5, were synthesised by (a) substitution of labile tined (1a), thf (2a, 3, 5) or dine (4) by free (C-N-C) or by (b) reaction of the bisimidazolium salt (CH-N-CH)Br-2 with {Cr[N(SiMe3)(2)](2)(thf)(2)} followed by amine elimination (2b). Attempted alkylation of 1a, 2, 3a and 4 with Grignard or alkyl lithiums gave intractable mixtures, and in one case [reaction of 1a with (mesityl)MgBr] resulted in exchange of Cl by Br (1b). Oxidation of 1a or [(C-N-C)VCl3] with 4-methylmorpholine N-oxide afforded the trans-V(C-N-C)(=O)Cl-2, 6, which by reaction with AgBF4 in MeCN gave trans-[V(C-N-C)(=O)(MeCN)(2)][BF4](2), 7. Reaction of 1a with p-tolyl azide gave trans-V(C-N-C)(=N-p-tolyl)Cl-2 8. The complex trans-Ti(C-N-C)(=NBu1)Cl-2, 9, was prepared by substitution of the pyridine ligands in Ti(NBu1)Cl-2(py)(3) by C-N-C.