A Mechanistic Study of the Photochemistry of Fe(CO)3(R-DAB) (R-DAB = 1,4-Diaza-1,3-butadiene), a Unique Group of Complexes with Two Close-Lying Reactive Excited States

Article abstract of DOI:10.1021/ja00184a022

The results are reported of a mechanistic study of the photosubstitution and low-temperature photolysis reactions of four complexes of the type Fe(CO)3(R-DAB) (Ia-d: R=cHex (a), R=pTol (b), R=iPr2Ph (c), R=tBu (d)).Although analogous photoproducts are obtained for these complexes at room temperature, flash photolysis shows that these reactions can proceed via one of two mechanisms.For complexes Ia-c loss of CO is the primary photoprocess, and complex Id reacts via breaking of a metal-nitrogen bond.For several phosphine ligands an intermediate in the reaction with Id could be stabilized and identified in low-temperature solution.For a photosubstitution reaction of the representative complex Ia the quantum yield was found to increase from 0.05 to 0.20 mol/einstein with the energy of irradiation.Furthermore, an activation energy ΔE_a of 525 cm^-1 was established for this reaction from the temperature dependence of φ upon low-energy excitation.Photolysis of Ia or Ib in n-pentane at 150 K in the absence of a substituting ligand affords two photoproducts, a binuclear CO-bridged complex IV and an isomeric form V of complex I in which the R-DAB ligand is η⁴-CN,C'N' coordinated to the metal.In agreement with the wavelength dependence of the photosubstitution quantum yield, the concentration of IVa increases at the expense of Va upon going to higher energy excitation.These results are rationalized in terms of an energy diagram with close-lying metal-ligand (ML) and ligand-field (LF) excited states.According to this model, the photosubstitution products as well as complexes IV are formed by reaction from the lowest ³LF state, while complexes V are formed by reaction from the lowest ³ML state.