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

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.

Chart 1. Os(II) Complexes Formed from Ligands L and L*d

Scheme 1. Synthetic Route to Charged N,N-Coordinated Complexes, A, and Neutral N,C-Coordinated Complexes, B

Figure 1

(A) 600 MHz ¹H NMR spectrum of 5 in chloroform-d₁. (B) The corresponding ¹H-selective NOE spectrum. The deshielded proton at 6.84 ppm, i₁, was irradiated. Protons close (ca. <4 Å) are labeled. The residual solvent peak (S), residual water peak (W), and the impurity in the chloroform-d₁ solvent (*) are highlighted. The iodido monodentate ligand is pointing into the plane of the page for clarity.

Figure 2

ORTEP diagrams of complexes 1a, 2a, 2b·0.5C₃H₆O, and 3b·0.5CHCl₃. Ellipsoids are shown at the 50% probability level, and all hydrogens, counterions, and solvent molecules have been omitted for clarity.

Figure 3

400 MHz ¹H NMR spectra of complex 2b (aromatic region) with 3 mol equiv of HBr in methanol-d₄. The disappearance of proton H_a (7.58 ppm) is accompanied by loss of long-range coupling between protons H_b and H_a (⁴J = 2.5 Hz).

Figure 4

Diagram illustrating the possible influence of steric effects on the ratios of products formed when [Os(η⁶-p-cym)X₂]₂ reacts with L. N,N-Coordination results in steric clashes between p-cym and L and is less favored when the monodentate ligand is large. N,C-Coordination is preferred, relieves steric tension, and involves spontaneous deprotonation of the aromatic ring.