Alkyl-dependent photochemistry of Mn(R)(CO)(3)(R'-DAB) (R=Me, Bz; R'=iPr, pTol): Homolysis of the Mn-R bond for R=Bz and release of CO for R=Me

The spectroscopic properties and photochemistry of the complexes Mn(R)(CO)(3)(R'-DAB)(R = Me, Bz; R' = iPr, pTol) are reported. The UV-photoelectron spectrum of Mn(Me)(CO)(3)(iPr-DAB) shows that the sigma(Mn-Me) orbital has a higher ionization potential than the d(pi)(Mn) orbitals, which is of importance for the photochemical behavior of this complex. The R group bound to the metal determines the photochemistry of these complexes. The Me complexes lose CO upon irradiation into their MLCT bands. The CO-loss products react with Lewis bases, the final product being cis(CO,CO),trans(Me,L)-Mn(Me)(L)(CO)(2)(R'-DAB). The structure of one of these products, viz. cis,trans-Mn(Me)(P(OMe)(3))(CO)(2)(iPr-DAB)(C14H28N2O5PMn) has been determined by a single-crystal X-ray diffraction study (T = 200 K). The crystal is monoclinic, space group P2(1)/n with unit cell dimensions a = 15.435(3), b = 15.200(3), c = 18.192(4) Angstrom, V = 3915(2) Angstrom(3), Z = 8. The structure refinement converged to R = 0.077 for 2829 observed reflections (total number of parameters: 319). Transient absorption spectroscopy shows that an equatorial CO ligand is lost upon excitation and that the final product is formed via different cis,cis-isomers. For R = Bz, visible excitation leads to efficient homolysis (Phi = 0.4 for R' = iPr) of the Mn-Bz bond, resulting in the formation of radicals which are characterized by ESR spectroscopy. The different behavior of the Bz and Me complexes is attributed to a difference in relative energies of two reactive excited states. For R = Me, the complexes lose CO from the lowest MLCT state; for R = Bz they undergo homolysis of the Mn-Bz bond from the lowest sigma pi* state.