Halide Control of N,N-Coordination versus N,C-Cyclometalation and Stereospecific Phenyl Ring Deuteration of Osmium(II) p-Cymene Phenylazobenzothiazole Complexes

We report the synthesis of halido Os(II) p -cymene complexes bearing bidentate chelating phenylazobenzothiazole (AZBTZ) ligands. Unlike the analogous phenylazopyridine (AZPY) complexes, AZBTZ-NMe 2 is capable of both N,N -coordination to Os(II) and cyclometalation to form N,C -coordinated species. N,C -Coordination occurs via an azo nitrogen and an ortho carbon on the aniline ring, as identified by 1 H NMR and X-ray crystallography of [Os( p -cym)( N,N -AZBTZ-NMe 2 )Cl]PF 6 ( 1a ), [Os( p -cym)( N,N -AZBTZ-NMe 2 )Br]PF 6 ( 2a ), [Os( p -cym)( N,C -AZBTZ-NMe 2 )Br] ( 2b ), and [Os( p -cym)( N,C -AZBTZ-NMe 2 )I] ( 3b ). The N,C -coordinated species is more stable and is not readily converted to the N,N -coordinated complex. Analysis of the crystal structures suggests that their formation is influenced by steric interactions between the p -cym and AZBTZ-NMe 2 ligands: in particular, larger monodentate halide ligands favor N,C -coordination. The complexes [Os( p -cym)( N,N -Me 2 -AZBTZ-NH 2 )Cl]PF 6 ( 4 ) and [Os( p -cym)( N,N -Me 2 -AZBTZ-NH 2 )I]PF 6 ( 5 ) were synthesized with methyl groups blocking the ortho positions on the aniline ring, forcing an N,N -coordination geometry. 1 H NMR NOE experiments confirmed hindered rotation of the arene ligand and steric crowding around the metal center. Complex 2b exhibited unexpected behavior under acidic conditions, involving regiospecific deuteration of the aniline ring at the meta position, as observed by 1 H NMR and high-resolution ESI-MS. Deuterium exchange occurs only under acidic conditions, suggesting an associative mechanism. The calculated partial charges on 2b show that the meta carbon is significantly more negatively charged, which may account for the regiospecificity of deuterium exchange.