Isocyanide–Group 5 complexes and metal-centred C–C coupling

Abstract

The Nb I complexes [XNb(CO) 2(CNR) 4] (X=I, Br; R= tBu, cHex, 2,6-Me 2C 6H 3) ( 4) have been identified as intermediates in the system [Nb(CO) 6] −/CNR/X 2, which generates the Nb III complexes [I 2Nb(CNR) 6]I ( 1) and [X 3Nb(CNR) 3] ( 2) or, on addition of tolane, [I 2Nb(CN tBu) 3tolane]I ( 3). In the presence of small amounts of water, the aminoalkyne–Nb III complexes [X 2Nb(CNR) 4RHNCCNHR]X (R= tBu; X=I: 5a, X=Br: 5b) are formed. The complex 4a (X=I, R= tBu) converts to 5a in the presence of water, suggesting successive protonation and intramolecular reductive isocyanide coupling mediated by the Nb I centre. The V I complexes [XV(CO) 2(CNR) 2dppe] (X=I, R= tBu, cHex; X=Br, R= tBu) ( 6) and the V III complex [I 2V(CN tBu) 6]I ( 7) have been obtained from the system [V(CO) 6] −/CNR/X 2 in dry THF, while the coupling product [I 2V(CN tBu) 4 tBuHNCCNH tBu]I ( 8) is recovered as water is added. In contrast to the Nb system, V II complexes of composition [I 2V(CNR) 4] ( 9) and [I 2V(CN tBu) 2(PR 3) 2] ( 10) can also be synthesised. The complex 10b (PR 3=PPhMe 2) has also been prepared from cis-[V(CO) 4(PPhMe 2) 2] − ( 11). The system [Ta(CO) 6] −/CN tBu/I 2 yielded the complexes [ITa(CO) 3(CN tBu) 3] ( 13a), [ITa(CO) 3(CN tBu)dppe] ( 13b) and [I 2Ta(CN tBu) 6]I ( 14). The chloro–isocyanide complexes [Cl 2V(CCPh)(CN tBu) 2] ( 12) and [Cl 2Ta(CN cHex) 4] ( 15) have been generated from the metal chlorides as starting products. [INb(CO) 2(CNtBu) 4]·2toluene ( 4a·2toluene), trans-[I 2V{CN(2,6-Me 2C 6H 3} 4] ( 9a) and cis-[Et 4N][V(CO) 4(PPhMe 2) 2] ([Et 4N]- 11) have been characterised by single-crystal X-ray diffraction.