HIGH-VALENCY DERIVATIVES OF D-METAL ACIDS .10. IMIDO COMPLEXES OF PENTAVALENT AND TETRAVALENT NIOBIUM AND TANTALUM WITH HALF-SANDWICH AND METALLOCENE STRUCTURE

The halfsandwich complexes [(eta5-C5R5)Nb(NtBu)Cl2] (R = H (1), Me (2)) and [(eta5-C5R5)Ta(NtBu)Cl2] (R = H (4), Me (5)) are synthezised by reacting the precursors (eta5-C5R5)MCI, with tBuNH2 in the presence of bases such as tBuNH2, LiNH(t)Bu and LiNtBu(SiMe3). These complexes are starting material for the preparation of the corresponding methyl complexes [(eta5-C5Me5)M(NtBu)(CH3)2] (M = Nb (3), Ta (6)), and of d0-niobocene imido complexes [(eta5-C5H5)2Nb(NtBu)Cl] (7) and [(eta5-C5H5)(eta5-C5Me5)Nb(NtBu)Cl] (8), or d0-tantalocene imido complexes [(eta5-C5H5)2Ta(NtBu)Cl] (9) and [(eta5-C5H5)(eta5-C5Me5)Ta(NtBu)Cl] (10), respectively. The most efficient synthesis for 7 and 9 makes use of the reaction of readily available pyridin imido complexes [M(NtBU)Cl3(Py)2] (M = Nb, Ta) with NaC5H5. With methyllithium 7 and 9 are converted into methyl complexes [eta5-C5H5)2M(NtBu)CH3] (M = Nb (11), Ta (12)). The one-electron reduction of 1 and 4 leads to diamagnetic dinuclear complexes with bridging imido ligands [(eta5-C5H5)Nb(mu-NtBu)Cl]2 (13) and [(eta5-C5H5)Ta(mu-NtBu)Cl]2 (14). Similarly, the reduction of d0-niobocene imido complex 7 results in the formation of a dinuclear complex [(eta5-C5H5)2Nb(NtBu)]2 (15) with spin coupling of both d1-metal centres.