Isocyanide insertion reactivity of dinuclear niobium and tantalum imido complexes: X-ray crystal structure of [{Nb(η5-C5H4SiMe3)(CH2Ph)2}2(μ-1,4-NC6H4N)]

The reactivity of dinuclear niobium and tantalum imido complexes with the isocyanide compound 2,6-Me2C6H3NC has been studied. The trialkyl complexes [{NbR3(CH3CN)}2(μ-1,3-NC6H4N)], [{NbR3(CH3CN)}2(μ-1,4-NC6H4N)] and [{TaR3(CH3CN)}2(μ-1,4-NC6H4N)] (R=CH2SiMe3) gave [{Nb(η2-RCNAr)2R}2(μ-1,3-NC6H4N)] (1), [{Nb(η2-RCNAr)2R}2(μ-1,4-NC6H4N)] (2) and [{Ta(η2-RCNAr)2R}2(μ-1,4-NC6H4N)] (3) (R=CH2SiMe3; Ar=2,6-Me2C6H3), from the isocyanide insertion in two of the metal alkyl carbon bonds. The reaction of the isocyanide reagent with the di-alkyl mono-cyclopentadienyl derivatives [{Nb(η5-C5H4SiMe3)R2}2(μ-1,3-NC6H4N)] (R=Me, CH2Ph, CH2SiMe3), [{Nb(η5-C5H4SiMe3)R2}2(μ-1,4-NC6H4N)] (R=Me, CH2Ph (4), CH2SiMe3) and [{Ta(η5-C5Me5)(CH2SiMe3)2}2(μ-1,4-NC6H4N)] yielded [{Nb(η5-C5H4SiMe3)(η2-RCNAr)R}2(μ-1,3-NC6H4N)] (R=Me (5), CH2Ph (6), CH2SiMe3 (7)), [{Nb(η5-C5H4SiMe3)(η2-RCNAr)R}2(μ-1,4-NC6H4N)] (R=Me (8), CH2Ph (9), CH2SiMe3 (10)) and [{Ta(η5-C5Me5)(η2-Me3SiCH2CNAr)CH2SiMe3}2(μ-1,4-NC6H4N)] (11) (Ar=2,6-Me2C6H3), respectively, from a single insertion process. The reaction with the mono-alkyl complex [{Nb(η5-C5H4SiMe3)(Me)Cl}2(μ-1,4-NC6H4N)] gave [{Nb(η5-C5H4SiMe3)(η2-MeCNAr)Cl}2(μ-1,4-NC6H4N)] (12), produced from the isocyanide insertion in the metal-alkyl carbon bond. The alkyl-amido complex [{Nb(η5-C5H4SiMe3)(Me)NMe2}2(μ-1,4-NC6H4N)] gave, from the preferential isocyanide insertion in the metal-amide nitrogen bond, [{Nb(η5-C5H4SiMe3)(η2-Me2NCNAr)Me}2(μ-1,4-NC6H4N)] (13). The molecular structure of one of the alkyl precursors, [{Nb(η5-C5H4SiMe3)(CH2Ph)2}2(μ-1,4-NC6H4N)] (4), has been determined.