Low-temperature Photochemistry of (η⁵ -c₅r₅)fe(co)₂mn(co)₅(r = H, Me): Substitution By P-donor Ligands and Kinetics of Thermal Fe-mn Bond Homolysis

Low-temperature irradiation of (i)5-C5R5)Fe(CO)2Mn(CO)5 (R = H, Me) results in loss of CO as the only detectable photoprocess (app for CO = 10~3 at 313 nm, and 3 3/ 366 = 20 at 93 K) and yields a coordinatively unsaturated dinuclear photoproduct, (jji-C5R5)FeMn(CO)6. It should be appreciated that CO loss and -Fe bond cleavage are competitive processes with CO loss far more dominant at low temperature in a rigid glass. Warming of a glass containing (7j5-C5R5)FeMn(CO)6 in the presence of PR3 (R = Ph, OPh) results in formation of (775-C5R5)Fe(CO)2Mn(CO)4PR3. The substitution product could be generated by an independent route from irradiation of a room temperature solution of [(7!5-C5R5)Fe(CO)2]2 and [Mn(CO)4PR3]2. Spectroscopic evidence, including IR, UV-vis, NMR, and MS, supports the conclusion that substitution of CO by PR3 in (i)5-C5R5)Fe(CO)2Mn(CO)5 occurs exclusively on the Mn atom. It was further determined that ^ for (7)5-C5H5)Fe-(CO)2Mn(CO)5 at room temperature in the presence of P(OPh)3 in CC14 is 0.89 ± 0.08 and 0.89 ±0.1 at 313 and 366 nm, respectively, and 3 for (775-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 (i)5-C5R5)Fe(CO)2Mn(CO)4PPh3 are labile with respect to Fe- bond cleavage and a kinetic analysis yielded activation parameters for this thermal reaction: AH* = 106 ± 6 and 84.3 ± 4 kJ mol1 for R =H, Me, respectively, and AS* = 41.5 ± 40 and -21.4 ± 23 J mol1 K1 for R = H and Me, respectively.