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

Article abstract of DOI:10.1016/0020-1693(95)04962-2

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 σ(Mn-Me) orbital has a higher ionization potential than the d_π(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 P2₁/n with unit cell dimensions a = 15.435(3), b = 15.200(3), c = 18.192(4) A, V = 3915(2) A³, 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 (Φ=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 σπ* state.