Photoinduced and Chemical Oxidation of Coordinated Imine to Amide in Isomeric Osmium(II) Complexes of N-Arylpyridine-2-carboxaldimines. Synthesis, Characterization, Electron Transfer Properties, and Structural Studies

The reaction of N-arylpyridine-2-carboxaldimine [C5H4NC(H)NC6H4R] (HL) with ammonium hexabromoosmate (NH4)2[OsBr6] in boiling 2-methoxyethanol afforded a violet solution from which two geometrical isomers of [OsBr2(HL)2] (1 and
2) were isolated. These are characterized by analytical and spectroscopic data. 1H NMR spectral data were used for the
identification of the isomers. The blue-violet isomer, 1 (designated as ctc), has a 2-fold symmetry axis and gave rise to
resonances for only one coordinated HL. The geometry of the ctc-isomer was, however, revealed from the X-ray structure
determination of a representative example. The red-violet isomer (2, designated as ccc), on the other hand, is unsymmetrical
and gave rise to a large number of proton resonances. The isomeric complexes, [OsBr2(HL)2], showed intense MLCT
transitions in the visible region. This transition, in the ccc-isomer, is slightly (10 nm) red shifted in comparison to the
ctc-isomer. These diimine complexes showed one metal based reversible oxidation assignable to the Os(III)/Os(II) process
followed by two irreversible oxidations at more anodic potentials (>1.4 V). In addition to these, the complexes also
showed two irreversible ligand reductions at high cathodic potentials (<−1.4 V). An unusual type of photochemical
transformation of the azomethine function of coordinated HL in osmium compounds 1 is studied. When an air equilibrated
acetonitrile solution of 1 was exposed to a xenon lamp, it underwent oxidation affording the mixed ligand, amido complexes
of general formula [OsBr2(HL)(LO)], 3 (LO = C5H4NC(O)−N−C6H4R), in an excellent yield (>95%). This transformation
(1 → 3) was achieved chemically when H2O2 was used as an oxidant. Notably, the chemical oxidation with H2O2 also
led to the formation of a tetravalent complex, [OsBr2(LO)2], 4, as a minor product. Compound 3 was characterized by
various spectroscopic and analytical techniques. The room temperature magnetic moment of 3 corresponds to a t25
configuration for the osmium(III) center. EPR spectra of the amido complexes were recorded at 77 K in 1:1 dichloromethane−toluene glass, and they were anisotropic in nature. FAB mass spectra of 3 displayed intense peaks due to parent
molecular ions. For example, the complex [OsBr2(HL1)(L1O)], 3a, showed a strong peak at m/z 729 amu. The electronic
spectrum of compound 3 consisted of a broad LMCT transition (ca. 525 nm; ε, 3000 M-1 cm-1). The cyclic voltammogram
of compound 3 consisted of two responses, one each on the positive and negative side of SCE, corresponding to
Os(IV)/Os(III) (ca. 0.8V) and Os(III)/Os(II) (ca. −0.3V) couples, respectively. There has been a large cathodic shift of
potential for the Os(III)/Os(II) couple in 3 in comparison to that in the parent complex, 1. The diamido compound [OsBr2(LO)2], 4, is diamagnetic and insoluble in common solvents. The X-ray structure determination of a representative sample,
4a, is reported. The molecule contains a C2-symmetry axis with bromide ions in relative cis positions. The Os−N(amide)
bond lengths are considerably shorter than the Os−N(pyridine) lengths. All other bond lengths and angles fall within the
expected range.