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

The Nb-I complexes [XNb(CO)(2)(CNR)(4)] (X = I, Br; R = tBu, cHex, 2,6-Me2C 6H3) (4) have been identified as intermediates in the system [Nb(CO)(6)](-) /CNR/X-2, which generates the Nb-III complexes [I2Nb(CNR)(6)]I (1) and [X3N b(CNR)(3)] (2) or, on addition of tolane, [I2Nb(CNtBu)(3)tolane]I (3). In t he presence of small amounts of water, the aminoalkyne-Nb-III complexes [X2 Nb(CNR)(4)RHNC=CNHR]X (R = tBu; X = I: 5a, X = Br: 5b) are formed. The comp lex 4a (X = I, R = tBu) converts to 5a in the presence of water, suggesting successive protonation and intramolecular reductive isocyanide coupling me diated by the Nb-I centre. The V-I complexes [XV(CO)(2)(CNR)(2)dppe] (X = I , R = tBu, cHex; X = Br, R = tBu) (6) and the V-III complex [I2V(CNtBu)(6)] I (7) have been obtained from the system [V(CO)(6)](-)/CNR/X-2 in dry THF, while the coupling product [I2V(CNtBu)(4)tBuHNC=CNHtBu]I (8) is recovered a s water is added. In contrast to the Nb system, V-II complexes of compositi on [I2V(CNR)(4)] (9) and [I2V(CNtBu)(2)(PR3)(2)] (10) can also be synthesis ed. The complex 10b (PR3 = PPhMe2) has also been prepared from cis-[V(CO)(4 )(PPhMe2)(2)](-) (11). The system [Ta(CO)(6)](-)/CNtBu/I-2 yielded the comp lexes [ITa(CO)(3)(CNtBu)(3)] (13a), [ITa(CO)(3)(CNtBu)dppe] (13b) and [I2Ta (CNtBu)(6)]I (14). The chloro-isocyanide complexes [Cl2V(CCPh)(CNtBu)(2)] ( 12) and [Cl2Ta(CNcHex)(4)] (15) have been generated from the metal chloride s as starting products. [INb(CO)(2)(CNtBu)(4)]. 2toluene (4a.2toluene), tra ns-[I2V{CN(2,6-Me2C6H3}(4)] (9a) and cis-[Et4N][V(CO)(4)(PPhMe2)(2)] ([Et4N ]-11) have been characterised by single-crystal X-ray diffraction. (C) 1999 Elsevier Science S.A. All rights reserved.