Stable coordination complexes of α-diimines with Nb(v) and Ta(v) halides

Uncommon examples of coordination compounds of high valent transition metals with α-diimines were obtained and characterized by IR and NMR spectroscopy, X-ray diffraction and/or DFT analysis. The 1 : 1 molar reactions of NbF₅ with a selection of α-diimines afforded the ionic compounds [NbF₄(DAD^R)₂][NbF₆] (DAD^R = DAD^Dip, 1a; DAD^Xyl, 1b; DAD^Mes, 1c) in 80-90% yields. The addition of DAD^Dip to NbOCl₃ gave NbOCl₃(DAD^Dip), 7 (62% yield). [MBr₄(DAD^Dip)][MBr₆] (M = Nb, 7a; M = Ta, 7b) were afforded in moderate yields from MBr₅ and DAD^Dip. The reactions of NbCl₅ with DAD^Dip and DAD^Xyl proceeded with non-selective activation of the organic substrate affording complex mixtures of products, including the quinoxalinium salt [{2,6-C₆H₃(CHMe₂)₂}N(CH)₂NCC(CHMe₂)(CH)₃C][NbCl₆], 2, the Nb(iv) coordination complex NbCl₄(DAD^Dip), 3, the N-(monoprotonated) α-diimine salts [DAD^Dip(H)][MCl₆], 4a-b, and the iminomethyl-imidazolium [(2,6-C₆H₃Me₂)NCHCHN(2,6-C₆H₃Me₂)CCHN(2,6-C₆H₃Me₂)][NbCl₆], 5.