COORDINATION OF THE POTENTIALLY TRIDENTATE LIGANDS 2,6-DIACETYLPYRIDINE-BIS(ANIL) (DAPA) AND 2-(2-(2'-METHYLIDENEPYRIDYL)AMINOETHYL)PYRIDINE (MAP) IN THE COMPLEXES FAC-BRMN(CO)(3)L, FAC-(CO)(5)MM'(CO)(3)L (M, M'=MN, RE, L=DAPA, MAP) AND THEIR PHOTOPRODUCTS - THE CRYSTAL-STRUCTURE OF BRMN(CO)(2)(N,N,N-DAPA)

In complexes of the type fac-(CO)(5)MM'(CO)(3)(N,N-L) (M, M' = Mn, Re; L = dapa, map) the potentially tridentate ligand L is bidentately (N,N) coordinated to the metal centre M'. Irradiation of these complexes into their lowest-energy absorption band resulted in metal-metal bond homolysis and/or loss of CO from the M'(CO)(3)(N,N-L) fragment. Upon irradiation of fac-(CO)(5)MRe(CO)(3)(N,N-dapa) (M = Mn, Re) only the metal-metal bond homolysis reaction was observed. The 16-electron Re+(CO)(3)(N,N-dapa(+)) species thus formed was readily converted into the 18-electron radical complex Re+(CO)(3)(N,N,N-dapa-radical anion) by coordination of the third nitrogen donor atom of dapa to the Re centre, as shown by ESR spectroscopy. Addition of excess PR(3) (R = nBu, Ph) gave rise to the immediate conversion of Re+(CO)(3)(N,N,N-dapa-radical anion) into the Re+(CO)(3)(N,N-dapa-radical anion)(PR(3)) adduct. The tendency of the dapa ligand to adopt a tridentate coordination mode was also reflected in the ready formation of the complexes BrMn(CO)(2)(N,N,N-dapa) and (CO)(5)ReMn(CO)(2)(N,N,N-dapa). The X-ray structure of the former complex was determined; the dapa ligand is coordinated in-plane and the CO ligands are in a mutually cis-disposition. Irradiation of fac-(CO)(5)MMn(CO)(3)(N,N-map) (M = Mn, Re) at low temperature gave the CO-bridged complex (CO)(4)M(mu-CO)Mn(CO)(2)(N,N-map). In the absence of CO, the latter complex reacted thermally to give (CO)(5)MMn(CO)(2)(N,N,N-map). For M = Mn this complex was stable only below 263 K, whereas for M = Re it was obtained as a stable photoproduct at room temperature. Metal-metal bond homolysis of fac-(CO)(5)ReRe(CO)(3)(N,N-map) at temperatures below 203 K was followed by a recombination of the resulting .Re(CO)(5) and Re+(CO)(3)(N,N-map-radical anion) radicals, which was accompanied by a change in coordination of the map ligand.