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-La/Lb)2], 2a-b, and [Os(PPh3)(La/Lb)(κ2-N,S-La/Lb)2], 3a-b (2a and 3a: La[double bond, length as m-dash] = C7H4NS2; 2b and 3b: Lb = C5H4NS), from the thermolysis of [Os(PPh3)3Cl2], 1, with a potassium salt of heterocyclic ligands La and Lb. 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(La/Lb)2}], 4a-b (4a: La = C7H4NS2; 4b: Lb = 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-Lb)(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-La)(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 NaBH2La 2. In a similar fashion, the treatment of [Os(PPh3)3Cl2], 1, with NaBH2Lb 2 yielded [Os(PPh3)(κ2-N,S-Lb)(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.