Intermediates Relevant to the Carbonylation of Manganese Alkyl Complexes Interrogated by Time Resolved Infrared and Optical Spectroscopy

Abstract

Intermediates relevant to the carbonylation of metal alkyl complexes have been generated by laser flash photolysis of the manganese acyl complexes RC(0)Mn(C0)5 (R = CH3, CD3, CH2CH3, CH2F, CF3). This results in immediate CO photodissociation to give intermediate acyl complexes which were observed by time resolved infrared (TRIR) and time resolved optical (TRO) spectroscopy. In the presence of added ligands, such intermediates are trapped to form stable as-substituted octahedral complexes in competition with alkyl migration from the acyl group to give the alkyl pentacarbonyl complexes RMn(CO)s. The spectra and reactivity of the intermediate (I) derived from CH3C(0)Mn(C0)s (A) indicate that this exists as the chelated acyl complex (?/2 3-CH3CO)Mn(CO)4 in weakly coordinating solvents such as cyclohexane but as the solvento species as-CH3C(0)Mn(C0)4(THF) in tetrahydrofuran. Comparisons with thermal reaction kinetics support the assertion that the intermediates generated photochemically are indeed relevant to understanding the mechanism for CH3Mn(CO)s carbonylation. The CF3 and CH2F analogs of I are much more reactive than I in cyclohexane solution, and this has been interpreted in terms of the ?y2-acyl configuration being destabilized by these electron-withdrawing groups. Solvent effects on the rates of methyl migration and ligand trapping reactions of the intermediate species are described and analyzed in terms of their relevance to the migratory insertion mechanism.

From curiosity to applications. A personal perspective on inorganic photochemistry

“…Comparisons of rates measured for I in this manner to the competitive reactivities deduced for intermediates in the thermal migratory insertion kinetics provide guidance relevant to mechanisms of these reactions. We studied such migratory insertion pathways by starting with Mn(CO) 5 (C(O)R), 65 , 66 (η 5 -C 5 H 5 )Fe(CO)(C(O)R), 67 CH 3 C(O)Co(CO) 3 PPh 3 ( ref. 68–70 ) and [CH 3 Ir(CO) 2 I 3 ] – .…”

Transition Metal Alkane Complexes†

“…For instance, C s -HMn(CO) 4 displayed a visible absorption band at 445 nm in argon shifting to 400 nm in methane (a shift of 2528 cm -1 compared to 1688 cm -1 for Cr(CO) 5 in argon and methane 6 ). Belt et al used TRIR to detect MeMn(CO) 4 (alkane) complexes as intermediates in methyl migration reactions in hydrocarbon solution …”

Reactive Intermediates in the Photodecarbonylation of the Cyclopentadienyl and Indenyl Complexes CpFe(CO)2(C(O)CH3) and IndFe(CO)2(C(O)CH3) (Cp = η5-C5H5; Ind = η5-C9H7)

“…The mechanism has been extensively studied for prototypes such as Mn(CO) 5 R and CpFe(CO) 2 R (R = alkyl, Cp = η 5 -C 5 H 5 ), e.g., using thermal chemical techniques; , however, such methods face difficulties in elucidating the structures and reactivities of reactive intermediates formed in low steady-state concentrations . In this context, recent studies at Santa Barbara have focused on the application of flash photolysis methodologies to generate possibly relevant intermediates from suitable precursors and to interrogate these short-lived species using time-resolved optical (TRO) and time-resolved infrared (TRIR) spectral methods. − The TRIR technique is especially useful since ν co bands often allow resolution of individual species and serve as probes of electronic properties at the metal center …”