Protonation and H/D exchange reactions promoted by a sulfur-rich osmium hydride complex: identification of a labile dihydrogen complex

Abstract

The sulfur-rich osmium nitrosyl complexes Bu₄N[Os(NO)(^buS₂)₂] (1) [^buS₂²⁻ = 3,5-tert-butyl-1,2-benzenedithiolate(2−)] and [Os(NO)(py^buS₄)]Br (2) [py^buS₄²⁻ = 2,6-bis(2-sulfanyl-3,5-di-tert-butylphenylthio)dimethylpyridine(2−)] have been synthesized. The molecular structure of 1 exhibits a square-pyramidal geometry with the NO group at the apical position. A pseudo-octahedral geometry with two thiolate and two thioether donors in trans configuration is found for 2. Compound 2 shows two quasi-reversible one-electron redox waves at E_1/2 = 0.51 and −0.46 V vs. NHE for the redox couples [Os(NO)(py^buS₄)]^+1/0 and [Os(NO)(py^buS₄)]^0/−1, respectively. 2 reacts with NaAlH₄ to produce Na[Os(H)(py^buS₄)] (3), which exhibits a typical hydride resonance in the ¹H NMR spectrum at δ = −15.03 ppm. Protonation of 3 with HBF₄/CD₃OD at 20 °C rapidly releases H₂/HD to afford the dinuclear complex [Os(py^buS₄)]₂ (4). Low temperature ¹H and ²H NMR spectra of 3 in [D₈]THF with CH₃OH or CD₃OD at −80 °C allow the observation of the formation of [Os(H₂/HD)(py^buS₄)]. A 1 ∶ 1 ∶ 1 triplet at δ = −7.84 ppm [J(HD) = 31.2 Hz] and a relaxation time of T₁(min) = 6 ms (−65 °C, 270 MHz) firmly establish the presence of η²-H₂/HD ligand. At room temperature, 3 interacts with D₂ (1 atm) and undergoes heterolytic D₂ cleavage followed by H/D exchange to form Na[Os(D)(py^buS₄)] (3a). A plausible cyclic mechanism has been proposed.