Effect of metal complexation on the equilibrium between methylphosphepine and methylphosphanorcaradiene and their benzo analogues

Abstract

Theoretical studies are reported on methylphosphepine, methylbenzophosphepine, their norcaradiene isomers, and their metal complexes with Fe(CO)₃ and CpCo (Cp = η⁵-C₅H₅) units. Both methylphosphepine and methylphosphanorcaradiene are C₆H₆PCH₃ species existing as anti/syn stereoisomer pairs with the methylphosphepine structures at slightly higher energies. The transition states for the interconversion of these isomers and their benzo derivatives lie ∼20 kcal mol⁻¹ in energy above the methylphosphanorcaradiene isomers. Complexation of either C₆H₆PCH₃ ligand with the transition metal units Fe(CO)₃ and CoCp leads to energetically closely spaced η⁴-tetrahapto and η³-trihapto isomers of the methylphosphepine complexes and η⁴-tetrahapto isomers of the methylphosphanorcaradiene complexes. However, the bis(dihapto) (η^2,2-C₆H₆PCH₃)Fe(CO)₃ and (η^2,2-C₆H₆PCH₃)CoCp complexes involving coordination of non-adjacent CC double bonds lie at significantly higher energies. Fusion of a benzene ring to the C₆H₆PCH₃ rings in methylphosphepine and methylphosphanorcaradiene leads to significantly different structures of their lowest energy metal complexes. Thus the lowest energy (C₁₀H₈PCH₃)Fe(CO)₃ and (C₁₀H₈PCH₃)CoCp structures are η^2,2 and η⁴ methylbenzophosphepine complexes, which avoid using any carbon atoms of the benzene ring in the ligand for metal complexation. Higher energy (C₁₀H₈PCH₃)Fe(CO)₃ and (C₁₀H₈PCH₃)CoCp structures have tetrahapto ligands with one or both CC double bonds of the benzene ring complexed with the metal atom.