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

Abstract

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.