Fischer aminocarbene conformers containing a 2-thienyl or 2-furyl ring: a crystallographic, NMR, and DFT study

Fischer aminocarbene complexes [(CO)₅M=C(NHR)Y] (M=Cr or W; R=H, Cy or C₂H₄NH₂; Y=2-thienyl or 2-furyl) containing an amino group exist as two isomers in solution, the E and Z isomers. The two isomers arise from restricted rotation about the N–C_carbene bond, that exhibits double bond character due to π-donation from nitrogen to the carbene carbon. Each isomer exists as two conformers in fast equilibrium with each other. The conformers arise from the rotation of the aryl ring around the C_carbene–C_aryl single bond with a DFT calculated rotation barrier of 0.1–0.5 eV. The main isomer, isolated in the solid state, generally exhibits a syn orientation of the aryl ring relative to the amino substituent and a Z configuration of the amino substituent relative to the metal.