Reactions of alkynes with phosphido niobocenes: A combined experimental and theoretical study

The reactions of phosphido complexes [Nb(η⁵-C₅H₄SiMe₃)₂(L)(PPh₂)] [L = CO (1), CNxylyl (2), CNCy (3)] with alkynes have been carried out. The new diphenylphosphinoalkenyl niobocene complexes [Nb(η⁵-C₅H₄SiMe₃)₂(η¹-C-C(CO₂CH₃)=C(R)PPh₂)(CO)] [R = H (4), CH₃ (5)] and [Nb(η⁵-C₅H₄SiMe₃)₂(η¹-C-C(CO₂R)=C(CO₂R)PPh₂)(CO)] [R = CH₃, (6), R = tBu, (7)] were successfully synthesized by the reaction of 1 with methyl propiolate (HC≡CCO₂CH₃) or methyl 2-butynoate (CH₃C≡CCO₂CH₃) and dimethyl 2-butynedioate [(CH₃≡O₂C)C≡C(CO₂CH₃)] or di(tert-butyl) 2-butynedioate [(tBuO₂C)C≡C(CO₂tBu)], respectively. However, reaction was not observed with more electron-rich alkynes. Complex 2 reacted with methyl propiolate, methyl 2-butynoate (MeC≡CCO₂Me) or di(tert-butyl) 2-butynedioate to give surprising new heteroniobacycle complexes [Nb(η⁵-C₅H₄SiMe₃)₂(η¹-C-C(=NXylyl)C(R¹)=C(R₂)PPh₂-κ¹-P)] [R¹ = H, R² = CO₂Me (8); R¹ = Me, R² = CO₂Me (9); R¹ = CO₂tBu, R² = CO₂tBu (10)]. Finally, the phosphido complexes 2 and 3 reacted with phenylacetylene (PhC≡CH) to give new diphenylphosphinoalkenyl niobocene derivatives [Nb(η⁵-C₅H₄SiMe₃)₂(η¹-C-C(C₆H₅)=C(H)PPh₂)(CNR)] [R = xylyl (11), Cy (12)]. All of these compounds were characterized by NMR spectroscopy and the molecular structure of 8 was determined by single-crystal X-ray diffraction studies. Theoretical studies were also carried out by means of density functional theory (DFT) calculations on the insertions of alkynes into the Nb-P bond in the phosphido niobocenes.