Synthesis, Characterization, and Laser Flash Photolysis Reactivity of a Carbonmonoxy Heme Complex

The unsaturated homoleptic manganese carbonyls Mn2(CO)n (n = 7, 8, 9) are characterized by their equilibrium geometries, thermochemistry, and vibrational frequencies using methods from density functional theory (DFT). The computed metal-metal distances for global minima range from 3.01 (angstrom) for the unbridged Mn2(CO)10 with a Mn-Mn single bond to 2.14 (angstrom) for a monobridged Mn2(CO)7 formulated with a metal-metal quadruple bond. The global minimum for Mn2(CO)9 has a four-electron bridging (mu)(eta)^2-CO group and a 2.96 (angstrom) Mn-Mn distance suggestive of the single bond required for 18-electron configurations for both metal atoms. This structure is closely related to an experimentally realized structure for the isolated and structurally characterized stable phosphine complex [R2PCH2PR2]2Mn2(CO)4((mu)(eta)^2-CO). An unbridged (OC)4Mn-Mn(CO)5 structure for Mn2(CO)9 has only slightly (<6 kcal/mol) higher energy with a somewhat shorter metal-metal distance of 2.77 (angstrom). For Mn2(CO)8 the lowest energy structure is a D2d unbridged structure with a 2.36 (angstrom) metal-metal distance suggesting the triple bond required for the favored 18-electron configuration for both metal atoms. However, the unbridged unsymmetrical (CO)3Mn-Mn(CO)5 structure with a metal-metal bond distance of 2.40 (angstrom) lies only 1 to 3 kcal/mol above this global minimum. The lowest energy structure of Mn2(CO)7 is an unbridged Cs structure with a short metal-metal distance of 2.26 (angstrom). This is followed energetically by another Cs unbridged Mn2(CO)7 structure with a somewhat longer metal-metal distance of 2.38 (angstrom).