METAL TO LIGAND CHARGE-TRANSFER PHOTOCHEMISTRY OF METAL METAL-BONDED COMPLEXES .8. PHOTOCHEMISTRY OF (CO)5MNMN(CO)3(ALPHA-DIIMINE) COMPLEXES - COUPLING REACTIONS OF THE RADICALS FORMED AND X-RAY STRUCTURE OF THE PHOTOPRODUCT (CO)4MN(SIGMA-N,SIGMA-N',ETA-2-CN-IPR-PYCA)MN(CO)3

This article describes the photochemistry between 133 and 298 K of five metal-metal-bonded carbonyls (CO)5MnMn(CO)3(alpha-diimine) (1a-e) (alpha-diimine = 4,4'-dimethyl-2,2'-bipyridine (bpy' (1a)), pyridine-2-carbaldehyde N-isopropylimine (iPr-pyca (1b)), 1,4-diisopropyl-1,4-diaza-1,3-butadiene (iPr-DAB (1c)), 1,4-di-p-tolyl-1,4-diaza-1,3-butadiene (pTol-DAB (1d)), 1,4-di-panisyl-1,4-diaza-1,3-butadiene (pAn-DAB (1e))) by irradiation into their metal to alpha-diimine charge-transfer (MLCT) band. At room temperature these complexes undergo homolysis of the metal-metal bond and the radicals formed dimerize to give Mn2(CO)10 and Mn2(CO)6(alpha-diimine)2 (2a-e). Of these dimers, 2d,e were thermally unstable at room temperature. They decomposed into their radicals, which were characterized with ESR in the case of 2d. Complexes 1b-e showed a side reaction at room temperature, giving rise to the formation of (CO)4Mn(sigma-N,sigma-N',eta-2-CN-iPr-pyca)Mn(CO)3 (3b) and (CO)3Mn(sigma-N,sigma-N',eta-2-CN,eta-2-C'N'-R-DAB) (4c-e), respectively. The crystal structure of 3b was determined, and the data are as follows: monoclinic, P2(1)/a, with a = 15.425 (1) angstrom, b = 9.867 (1) angstrom, c = 12.988 (1) angstrom, beta = 111.310 (9)-degrees, and Z = 4; R = 0.041. Both Mn atoms possess a distorted octahedral geometry, and the Mn-Mn distance is shorter than that in Mn2(CO)10. The formation of these complexes 3b, 4c-e was quenched by radical scavengers and favored in viscous solvents such as paraffin. At lower temperatures, the quantum yields for the photoproduction of 2-4 decreased, and in the case of 1c, a novel complex, 5c, was formed at T congruent-to 180 K by reaction of the Mn(CO)3(iPr-DAB) radicals. 5c was identified as Mn2(CO)4(sigma-N,sigma-N',eta-2-CN-iPr-DAB)2. Raising the temperature above 180 K caused a thermal conversion of 5c into Mn2(CO)5(sigma-N,sigma-N'-iPr-DAB)(sigma-N,sigma-N',eta-2-CN-iPr-DAB) (6c). A further increase of temperature above 200 K caused the formation of 2c out of 6c. At temperatures below 183 K, homolysis products were no longer formed, but instead the CO-loss complexes (CO)4Mn(mu-CO)Mn(CO)2(alpha-diimine) (7) were produced. The thermal and photochemical reactions of the CO-bridged complex 7a were studied. For both primary photoprocesses, homolysis and release of CO, the quantum yields were high and wavelength independent throughout the MLCT band. They are therefore proposed to occur from the same 3-sigma-b-sigma* state of the complex after intersystem crossing/internal conversion from the MLCT state(s). The relative quantum yields of homolysis and CO-loss reactions resemble the ones that were derived for Mn2(CO)10, which points to a similar mechanism for the photochemistry of both types of complexes.