Griffith Model Bonding in Dioxygen Complexes of Manganese Porphyrins

The binding of 02 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 17O substitution reveals little unpaired spin density on the 02. Contradictory interpretations of the electronic and geometric structures of these complexes have recently been advanced: analysis of the EPR data supports a “d3” MnIVO22- with 02 bound in the Griffith mode (edge-on, parallel to the porphyrin plane) whereas ab initio calculations favor a “d2π*” MnIVO2- with O2 bound in the Pauling mode (end-on, bent). We report here charge iterative extended Hückel calculations on both Griffith and Pauling models of oxymanganese porphine, 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 O21 suggesting that a formal charge description of these complexes solely in terms of the 02Mn core is an oversimplification. The resultant wave functions were used to calculate 55Mn and 17O hyperfine splittings. Griffith models with three unpaired electrons occupying orbitals of predominantly d character give excellent agreement with the observed 55Mn and 17O values. Models with an unpaired electron in a predominantly dioxygen orbital are ruled out. The optical hyperspectra can be ascribed to porphine π -π* dπ,O2π* charge-transfer transitions that mix with the porphine π→π* Soret transitions. Only the Griffith models place these charge-transfer transitions at favorable energies. Thus all the experimental results can be explained in terms of a d3 configuration and Griffith binding of the dioxygen but not Pauling binding. The Griffith geometry with molecular orbital energies most consistent with the optical spectra and which best fits the observed 55Mn and l70 hyperfine splittings has the Mn ~0.5 Å out-of-plane toward the O2, a long, “peroxo”, O-O bond, and the O-O staggered with respect to the pyrrole nitrogens.