Low-Temperature Photochemistry of (η5-C5R5)Fe(CO)2Mn(CO)5 (R = H, Me): Substitution by P-Donor Ligands and Kinaetics of Thermal Fe-Mn Bond Homolysis

Article abstract of DOI:10.1021/ja00249a017

Low-temperature irradiation of (η⁵ C5R5)Fe(CO)2Mn(CO)5 (R = H, Me) results in loss of CO as the only detectable photoprocess (Φ_app for CO = 10^-3 at 313 nm, and Φ₃₁₃/Φ₃₆₆ = 20 at 93 K) and yields a coordinatively unsaturated dinuclear photoproduct, (η⁵C5R5)FeMn(CO)6.It should be appreciated that CO loss and Mn-Fe bond cleavage are competitive processes with CO loss far more dominant at low temperature in a rigid glass.Warming of a glass containing (η⁵-C5R5)FeMn(CO)6 in the presence of PR3 (R = Ph, OPh) results in formation of (η⁵-C5R5)Fe(CO)2Mn(CO)4PR3.The substitution product could be generated by an independent route from irradiation of a room temperature solution of <(η⁵-C5R5)Fe(CO)2>2 and 2.Spectroscopic evidence, including IR, UV-vis, NMR, and MS, supports the conclusion that substitution of CO by PR3 in (η⁵-C5R5)Fe(CO)2Mn(CO)5 occurs exclusively on the Mn atom.It was further determined that Φ_diss for (η⁵-C5H5)Fe(CO)2Mn(CO)5 at room temperature in the presence of P(OPh)3 in CCl4 is 0.89 +/- 0.08 and 0.89 +/- 0.1 at 313 and 366 nm, respectively, and Φ_app for (η⁵-C5H5)Fe(CO)2Mn(CO)4P(OPh)3 is 0.26 +/- 0.03 and 0.29 +/- 0.04 at 313 and 366 nm, respectively, in the same solution.The products (η⁵-C5R5)Fe(CO)2Mn(CO)4PPh3 are labile with respect to Fe-Mn bond cleavage and a kinetic analysis yielded activation parameters for this thermal reaction ΔH^<*> = 106 +/- 6 and 84.3 +/- 4 kJ mol^-1 for R = H, Me, respectively, and ΔS^<*> = 41.5 +/- 40 and -21.4 +/- 23 J mol^-1 K^-1 for R = H, Me, respectively.