PHOTOCHEMISTRY OF MONOSUBSTITUTED MANGANESE CARBONYL COMPOUNDS, MN-2(CO)(9)L, IN 3-METHYLPENTANE AT LOW TEMPERATURE - FLASH PHOTOLYSIS STUDIES IN HEXANE AT ROOM TEMPERATURE

摘要

The photochemical behavior of monosubstituted manganese carbonyl dimers, Mn-2(CO)(9)L (L =PMe(3), P(n-Bu)(3), PPh(3), P(CH(2)Ph)(3), and P(i-Pr)(3)), in 3-methylpentane at 93 K has been studied. Loss of CO leads to formation of the semibridging structure Mn-2(CO)(7)L(mu-eta(1),eta(2)CO). Because loss of CO might occur from either of the two nonequivalent metals, and because the phosphine exhibits positional isomerism, several isomeric semibridging structures are possible, depending on the steric requirements of the phosphine. Each gives rise to a unique semibridging CO stretching frequency. Upon warming of the low-temperature glass, recombination with CO and intramolecular rearrangements of the semibridging forms occur in competition. Recombination with CO occurs most readily at the least sterically hindered sites. Thus, recombination to form the parent carbonyl, Mn-2(CO)(9)L, occurs most readily with Mn-2(CO)(9)PMe(3) and most slowly with Mn-2(CO)(9)P(i-Pr)(3). Studies of the PMe(3) and PPh(3) complexes show that recombination with CO leads to substantial quantities of the equatorial form, which isomerizes to the axial form as the solution is warmed. At room temperature the thermodynamic equilibrium between the equatorial and axial isomers of Mn-2(CO)(9)L lies heavily toward the axial form. Flash photolysis of hexane solutions of Mn-2(CO)(9)L compounds in hexane solutions at room temperature reveal a complex decay of absorbances due to CO loss products. Analysis of the transient behavior leads to. three pseudo-first-order rate constants: one is due to recombination of CO with Mn-2(CO)(9), formed via photochemical loss of L. The other two correspond to recombination of CO with Mn-2(CO)(8)L intermediates in which the CO has been lost from either Mn center. [References: 33]