Osmium(II) complexes of 2-[(arylamido)phenylazo]pyridines. New examples of deamination reactions—X-ray structure and redox properties

Abstract

Reaction of [NH₄]₂[OsBr₆] with 2-[(arylamino)phenylazo]pyridine, NH₄C₅NNC₆H₄N(H)C₆H₄(R) [R = H (HL^a) or CH₃ (HL^b)], in the presence of dilute NEt₃ affords multiple products. Five compounds of molecular formulas [Os(HL)(L)Br], 1, [Os(L)(pap)Br], 2, two isomers of [Os(pap)₂Br₂], 3 and 5, and [Os(HL)(pap)Br₂], 4, where L and pap stand for the conjugate base of HL and 2-(phenylazo)pyridine, respectively, have been separated on a preparative TLC plate. The X-ray structures of the new and representative complexes 1a, 2a and 4a have been solved to characterise them. The complexes 3 and 5 were characterised by comparing their spectral properties with those of the known and analysed samples of isomeric [Os(pap)₂Br₂]. Except for complex 1, the rest are formed due to cleavage of an otherwise unreactive C(phenyl)–N(amine) bond which is promoted by the metal ion. The bidentate-tridentate combination of HL and L in 1 is due to electronic factors. Structural data of the compounds have revealed very strong metal-ligand interactions. Osmium(II)-ligand interactions with the neutral azo ligands, viz. HL or pap, are stronger than those with the anionic L ligand. All of the complexes display resolved ¹H NMR spectra. However, the spectral pattern is complex due to serious overlap of the resonances. There are multiple electronic transitions in the range 1200–220 nm. The lowest energy transition (HOMO → LUMO) is presumably due to metal-to-ligand charge transfer (MLCT). These complexes undergo multiple and successive one-electron redox processes. The lowest potential anodic response, in each case, has been assigned to the Os^II/Os^III couple. E_1/2 of this response in 1 and 2 is similar and occurs near 0.45 V. Low oxidation potentials of the above couples allowed the generation of [1a]⁺ and [2a]⁺ in solution by exhaustive constant potential coulometry. The trivalent osmium complexes showed rhombic EPR spectra at 77 K. Distortion parameters using the observed g values have been computed.