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

Abstract

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 (HCCCO₂CH₃) or methyl 2-butynoate (CH₃CCCO₂CH₃) and dimethyl 2-butynedioate [(CH₃ O₂C)CC(CO₂CH₃)] or di(tert-butyl) 2-butynedioate [(^tBuO₂C)CC(CO₂^tBu)], respectively. However, reaction was not observed with more electron-rich alkynes. Complex 2 reacted with methyl propiolate, methyl 2-butynoate (MeCCCO₂Me) 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 = CO₂^tBu, R² = CO₂^tBu (10)]. Finally, the phosphido complexes 2 and 3 reacted with phenylacetylene (PhCCH) 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.