Photochemistry of tricarbonyl(α-di-imine)iron complexes. New mechanistic aspects for CO photosubstitution in solution and evidence for π,π-co-ordination of 1,4-diaza-1,3-butadiene ligands in matrices at 10 K

Abstract

The metal-to-ligand charge-transfer photochemistry of [Fe(CO)₃(α-di-imine)] has been studied in detail. For [Fe(CO)₃L](L = 1,4-diaza-1,3-butadiene, RNCH–CHNR) CO photosubstitution takes place with high quantum yield (Φ≃ 0.2) in solution via a partial dissociation of L. In matrices at 10 K, a changeover from σ,σ-N,N′- to π,π-co-ordination is found as primary step in the photolysis of [Fe(CO)₃L] complexes. The resulting [Fe(CO)₃(π,π-L)] decomposes thermally to a [Fe(CO)₂(σ,σ-N,N′-L)] fragment with simultaneous release of CO. For [Fe(CO)₃(dpipy)][dpipy = 2-(2′,6′-di-isopropylphenylimino)pyridine] no π,π-co-ordination is observed. A ¹³CO-labelling force-field calculation has been performed for a [Fe(CO)₃L] complex and its photolysis product [Fe(CO)₂(σ,σ,-N,N′-L)], showing two structurally different conformers of the latter product. The different photochemical behaviour in solution and in a matrix is discussed and a mechanism is proposed for the photosubstitutional reactions in solution.