Organoiron thermochemistry. Solution thermochemical investigation of tertiary phosphine ligand electronic effects in trans-(P(p-XC6H4)3)2Fe(CO) 3 complexes

Article abstract of DOI:10.1021/om00003a038

The enthalpies of reaction of (BDA)Fe(CO)₃ (BDA = (C₆H₅)CH=CHC(O)(CH₃), benzylidene-acetone) with a series of para-substituted triphenylphosphine ligands, leading to the formation of trans-(P(p-XC₆H₄)₃)₂Fe(CO) ₃ complexes (X = H, CH₃O, CH₃, Cl, F, CF₃), have been measured by solution calorimetry in THF at 50°C. The range of reaction enthalpies spans some 7.7 kcal/mol and helps establish a relative order of complex stability for 14 compounds in the iron tricarbonyl system. The relative enthalpy scale for tertiary phosphine complexes, trans-(P(p-XC₆H₄)₃)₂Fe(CO) ₃, is as follows (X, ΔH_reacn in kcal/mol): CF₃, 22.4(0.3); Cl, 25.0(0.1); F, 25.8(0.2); H, 26.9(0.2); CH₃, 28.2(0.3); CH₃O, 30.1(0.2). The thermodynamic investigation of these isosteric tertiary phosphine ligands helps to independently determine the importance of phosphine electronic contribution to the enthalpy of reaction in this system. Correlations of various factors gauging the electron-donating properties of the phosphine ligands clearly show the electronic factor to be the overwhelming contributor to the enthalpy of reaction in the trans-(PR₃)₂Fe(CO)₃ system.