Quantifying variation in cooperative B–H bond activations using Os(ii) and Os(iii) κ2-N,S-chelated complexes: same, but different

Abstract

In an effort to investigate small molecule activation by heavier transition metal (TM) based κ²-N,S-chelated species, we have synthesised a series of bis-κ²-1,3-N,S-chelated complexes of osmium, [Os(PPh₃)₂(κ²-N,S-L^a/L^b)₂], 2a–b, and [Os(PPh₃)(L^a/L^b)(κ²-N,S-L^a/L^b)₂], 3a–b (2a and 3a: L^a = C₇H₄NS₂; 2b and 3b: L^b = C₅H₄NS), from the thermolysis of [Os(PPh₃)₃Cl₂], 1, with a potassium salt of heterocyclic ligands L^a and L^b. The former complexes are diamagnetic in nature, while the EPR spectra, XPS study and density functional theory (DFT) calculations have substantiated the paramagnetic behaviour of 3a–b with a significant spin contribution from non-innocent ligands. These species were engaged in B–H activation of boranes utilizing the combined effect of hemilability and metal–ligand cooperativity (MLC), where 2a–b upon treatment with BH₃·SMe₂ yielded Os(σ-borate)hydride complexes, [Os(PPh₃)₂(H){κ³-H,S,S′-BH₂(L^a/L^b)₂}], 4a–b (4a: L^a = C₇H₄NS₂; 4b: L^b = C₅H₄NS). The formation of 4a–b was emphasized on the basis of possible dual B–H activation that involved a few concerted steps, i.e., (i) cleavage of one hemilabile Os–N bond and cooperative B–H bond activation, (ii) cleavage of another hemilabile Os–N bond and formation of a B–N bond and (iii) activation of another B–H bond. In stark contrast, the paramagnetic bis-κ²-1,3-N,S-chelated species 3a–b manifested diverse activation patterns in the light of the different electronic nature of non-innocent ligands. While the reaction of 3b with borane generated dihydridoborate species, [Os(PPh₃)(κ²-N,S-L^b)(κ³-H,H,S′-BH₂(OH)C₅H₄NS)], 5, complex 3a led to the formation of an Os(σ-borate) complex, [Os(PPh₃)(κ²-N,S-L^a)(κ³-H,S,S′-BH₂L₂^a)], trans-6. As the σ-borate entity shows a tendency to adapt to different spatial arrangements around the metal center, we have established a modified synthetic strategy to isolate the cis isomer of 6 that involved the reaction of [Os(PPh₃)₃Cl₂], 1, with NaBH₂L^a₂. In a similar fashion, the treatment of [Os(PPh₃)₃Cl₂], 1, with NaBH₂L^b₂ yielded [Os(PPh₃)(κ²-N,S-L^b)(κ³-H,S,S′-BH₂L₂^b)], cis-7. The kinetic and thermodynamic stability of these isomeric species were investigated on the basis of extensive density functional theory (DFT) calculations. Theoretical calculations also provided insightful information on the electronic nature of the species, generated from B–H activations of boranes.