FORMATION OF HIGHLY FUNCTIONALIZED METAL-BOUND ACETYLENES BY REDUCTIVE COUPLING OF CARBON-MONOXIDE AND METHYL ISOCYANIDE LIGANDS

Reduction of [M(CNMe)(CO)(dmpe)2Cl] (M = Nb, Ta; dmpe = 1,2-bis(dimethylphosphino)ethane) with 40% Na/Hg followed by addition of Me3SiCl achieved the reductive coupling of the CO and CNMe ligands to form [M-{(Me3Si)(Me)NC = CO(SiMe3)}(dmpe)2Cl]. The niobium complex was characterized in a single-crystal X-ray diffraction study, which revealed the presence of the unprecedented RR'NC = COR acetylene ligand. The reductive coupling reaction proceeds by a mechanism analogous to that previously reported for the symmetric coupling of two carbon monoxide or two alkyl isocyanide ligands. The first step in the reaction is the formation of an aminocarbyne intermediate, a triphenylsilyl derivative of which was crystallographically characterized. Addition of Me3SiCl to [Nb{CN(Me)(SiMe2Bu(t))}(CO)(dmpe)2] generated the asymmetric coupled product, [Nb{((t)BuMe2Si)(Me)NC = CO(SiMe3)}(dmpe)2Cl], demonstrating that aminocarbyne complexes are on the mechanistic pathway to reductive coupling. Addition of aqueous acid to [Ta{(Me3Si)(Me)NC = CO(SiMe3)}(dmpe)2Cl] provided the first stabilized hydroxy(alkylamino)acetylene complex, [TaH(MeHNC = COH)(dmpe)2Cl]Cl, a compound that was also crystallographically characterized. Finally, reductive coupling of two isocyanide ligands in [Ta(CNMe)2(dmpe)2Cl] has been accomplished, demonstrating that all three possible combinations of CO and CNR ligands can be reductively coupled in reactions that use the same metal framework and generalized mechanism.