Griffith model bonding in dioxygen complexes of manganese porphyrins

Abstract

The binding of O/sub 2/ to Mn(II) porphyrins greatly alters their optical and EPR spectra. The optical spectra of these pentacoordinate complexes are transformed from the normal to hyper type with a split Soret band typical of Mn(III) porphyrins. EPR spectra indicate a spin change from S = 5/2 to S = 3/2, and /sup 17/O substitution reveals little unpaired spin density on the O/sub 2/. Contradictory interpretations of the electronic and geometric structures of these complexes have recently been advanced: analysis of the EPR data supports a d/sup 3/ Mn/sup IV/O/sub 2//sup 2 -/ with O/sub 2/ bound in the Griffith mode (edge-on, parallel to the porphyrin plane) whereas ab initio calculations favor a d/sup 2/..pi..* Mn/sup III/O/sub 2//sup -/ with O/sub 2/ bound in the Pauling mode (end-on, bent). Charge iterative extended Hueckel calculations on both Griffith and Pauling models of oxymanganese porphine are reported here in which the O-O, Mn-O, and out-of-plane Mn distances and the O-O orientation above the porphinato plane were varied. For all the calculations, the porphine ring as well as the metal transfers considerable charge density onto the O/sub 2/, suggesting that a formal charge description of these complexes solely in terms ofmore » the O/sub 2/Mn core is an oversimplification. The resultant wave functions were used to calculate /sup 55/Mn and /sup 17/O hyperfine splittings. Griffith models with three unpaired electrons occupying orbitals of predominantly d character give excellent agreement with the observed /sup 55/Mn and /sup 17/O values. The optical hyperspectra can be ascribed to porphine ..pi.. ..-->.. d/sub x/,O/sub 2/..pi..* charge-transfer transitions that mix with the porphine ..pi.. ..-->.. ..pi..* Soret transitions. All the experimental results can be explained in terms of a d/sup 3/ configuration and Griffith binding of the dioxygen but not Pauling binding.« less