‘Pincer’ dicarbene complexes of some early transition metals and uranium

Abstract

The complexes [(C–N–C)MX_n(thf)_m] with the ‘pincer’ 2,6-bis(imidazolylidene)pyridine, (C–N–C) = 2,6-bis(arylimidazol-2-ylidene)pyridine, aryl = 2,6-Prⁱ₂C₆H₃, M = V, X = Cl, n = 2, m = 1 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 tmed (1a), thf (2a, 3, 5) or dme (4) by free (C–N–C) or by (b) reaction of the bisimidazolium salt (CH–N–CH)Br₂ 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)Cl₂, 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)Cl₂8. The complex trans-Ti(C–N–C)(NBu^t)Cl₂, 9, was prepared by substitution of the pyridine ligands in Ti(NBu^t)Cl₂(py)₃ by C–N–C.