Light-Induced Insertion of a CO Ligand into an Os−N Bond of the Clusters Os3(CO)10(L), Where L Represents a Potentially Terdentate N,N‘-Chelating α-Diimine

Abstract

Irradiation of the clusters Os3(CO)10(L) (L = pyridine-2-carbaldehyde-N‘-R imine (R−PyCa), R = Me2N(CH2)2 (1), Me2N(CH2)3 (5), (2-pyridyl)(CH2)2 (3); L = 2-acetylpyridine-N‘-R imine (R−AcPy), R = Me2N(CH2)2 (2), (2-pyridyl)(CH2)2 (4)) with visible light leads to the formation of the novel CO-bridged (Os1C(O)N‘Os3; N‘ = imine nitrogen) photoproducts Os3(CO)9(μ-1κC(CO):1κC(imine):3κ3N,N‘,N‘‘(R)-L-N‘-C(O)) (2 Os−Os), 1b−5b, demonstrating the profound influence of the pendant Lewis base on the course of the photoreactions. Formation of the products 1b−5b occurs via the initial formation of a zwitterion. The efficiency of this step determines the quantum yield of the photoreaction. In strongly coordinating solvents (e.g., S = pyridine, acetonitrile) this is the solvent-stabilized zwitterion (CO)4Os-Os(CO)4Os+(CO)2(S)(κ2N,N‘-L), whereas in noncoordinating solvents and in THF at ambient temperatures this is the intramolecularly stabilized zwitterion (CO)4Os-Os(CO)4Os+(CO)2(κ3N,N‘,N‘‘(R)-L). The latter species is formed from the corresponding biradical (the primary photoproduct in these solvents) via attack of the pendant Lewis base on the {Os+(CO)2(κ2N,N‘-L•-)} moiety, inducing an electron-transfer reaction. The biradical also undergoes a radical coupling side reaction, affording an unstable η2-imine-bridged product with the coordinatively unsaturated Os(CO)2(L) moiety, similar to an intermediate previously observed in the photoisomerization of Os3(CO)10(R−PyCa) (R = alkyl). This path does not contribute significantly to the net product formation. The novel compounds 1b−5b have been characterized by IR and (with the exception of thermally unstable 4b) NMR spectroscopy. The structure of 2b has been determined by an X-ray diffraction study.