Abstract
We report the synthesis of halido Os(II) p-cymene complexes bearing bidentate chelating phenylazobenzothiazole (AZBTZ) ligands. Unlike the analogous phenylazopyridine (AZPY) complexes, AZBTZ-NMe2 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 1H NMR and X-ray crystallography of [Os(p-cym)(N,N-AZBTZ-NMe2)Cl]PF6 (1a), [Os(p-cym)(N,N-AZBTZ-NMe2)Br]PF6 (2a), [Os(p-cym)(N,C-AZBTZ-NMe2)Br] (2b), and [Os(p-cym)(N,C-AZBTZ-NMe2)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-NMe2 ligands: in particular, larger monodentate halide ligands favor N,C-coordination. The complexes [Os(p-cym)(N,N-Me2-AZBTZ-NH2)Cl]PF6 (4) and [Os(p-cym)(N,N-Me2-AZBTZ-NH2)I]PF6 (5) were synthesized with methyl groups blocking the ortho positions on the aniline ring, forcing an N,N-coordination geometry. 1H 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 1H 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.
Highlights
There is growing interest in the chemistry of organometallic osmium complexes, with potential applications in a range of areas, including catalysis[1−4] and anticancer activity.[5−9] For example, osmium(II) arene complexes containing N,Nchelating phenylazopyridine (AZPY) ligands exhibit promising anticancer properties and novel mechanisms of action.[10−14] Here we explore Os(II) complexes with phenylazobenzothiazole (AZBTZ) ligands
AZBTZs have established applications in the field of dyes and pigments owing to their intense red coloration.[15]
AZBTZs are analogous to AZPY ligands, the pyridine being substituted by a benzothiazole unit
Summary
There is growing interest in the chemistry of organometallic osmium complexes, with potential applications in a range of areas, including catalysis[1−4] and anticancer activity.[5−9] For example, osmium(II) arene complexes containing N,Nchelating phenylazopyridine (AZPY) ligands exhibit promising anticancer properties and novel mechanisms of action.[10−14] Here we explore Os(II) complexes with phenylazobenzothiazole (AZBTZ) ligands. Benzothiazole is a bicyclic ring system consisting of a benzene ring fused to a fivemembered 1,3-thiazole ring. Because of their pronounced biological and pharmacological activities, AZBTZ derivatives are of great interest for medicinal applications. Numerous organic benzothiazole compounds have been reported to have promising anticancer activity,[17−20] suggesting that Os(II) complexes containing benzothiazole groups may possess potential as anticancer agents. Most notable are Os(II) and Ru(II) complexes reported by Keppler et al as benzothiazole and benzimidazole pharmacophoric inhibitors of protein kinases.[22]
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