Solution thermochemical study of ligand steric influences on substitution enthalpies in the L2Fe(CO)3 system

Article abstract of DOI:10.1021/om00008a028

The enthalpies of reaction of (BDA)Fe(CO)₃ (BDA = (C₆H₅)CH=CHO(CH₃), benzylidene-acetone) with a series of sterically demanding phosphine ligands, leading to the formation of (L)₂Fe(CO)₃ complexes (L = phosphine) have been measured by solution calorimetry in THF at 50°C. The range of reaction enthalpies spans some 7 kcal/mol. The overall relative order of stability established is as follows (PR₃; - ΔH, kcal/mol): PPh₃ < PCy₂Ph < PCyPh₂ < PCy₃ < PⁱPr₃ < PPh₂Et < PBz₃. A quantitative analysis of ligand effect of the present and previously obtained data for L₂Fe(CO)₃ complexes helps clarify the exact steric versus electronic ligand contributions to the enthalpy of reaction in this system. Results of a single-crystal diffraction study for the complex diaxal-(PPh₂Cy)₂Fe(CO)₃ (2) show the molecule to be monoclinic, P2₁/n, with a = 12.393(5) A?, b = 15.811(6) A?, c = 18.029(7) A?, α = γ = 90°, β = 108.00(2)°, V = 43360(5) A?³, Z = 4, d_calcd = 1.337 g cm^-1, n_obsd = 3130, and R = 0.052. Electronic effects are overwhelmingly important in this system, yet a steric threshold of 135° can be extracted from the QALE treatment which shows at which point steric factors begin to influence and contribute to the measured enthalpy of ligand substitution.