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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(MnMe) orbital has a higher ionization potential than the Dπ(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 MnBz 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 MnBz bond from the lowest σπ∗ state. |
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