Experimental and computational preference for phosphine regioselectivity and stereoselective tripodal rotation in...

The site preference for ligand substitution in the benzothiazolate-bridged cluster HOs₃(CO)₁₀(μ-1,2-N,C-η¹,κ¹-C₇H₄NS) (1) has been investigated using PPh₃. 1 reacts with PPh₃ in the presence of Me₃NO to afford the mono- and bisphosphine substituted clusters HOs₃(CO)₉(PPh₃)(μ-1,2-N,C-η¹,κ¹-C₇H₄NS) (2) and HOs₃(CO)₈(PPh₃)₂(μ-1,2-N,C-η¹,κ¹-C₇H₄NS) (3), respectively. 2 exists as a pair of non-interconverting isomers where the PPh₃ ligand is situated at one of the equatorial sites syn to the edge-bridging hydride that shares a common Os-Os bond with the metalated heterocycle. The solid-state structure of the major isomer establishes the PPh₃ regiochemistry at the N-substituted osmium center. DFT calculations confirm the thermodynamic preference for this particular isomer relative to the minor isomer whose phosphine ligand is located at the adjacent C-metalated osmium center. 2 also reacts with PPh₃ to give 3. The locus of the second substitution occurs at one of the two equatorial sites at the Os(CO)₄ moiety in 2 and gives rise to a pair of fluxional stereoisomers where the new phosphine ligand is scrambled between the two equatorial sites at the Os(CO)₃P moiety. The molecular structure of the major isomer has been determined by X-ray diffraction analysis and found to represent the lowest energy structure of the different stereoisomers computed for HOs₃(CO)₈(PPh₃)₂(μ-1,2-N,C-η¹,κ¹-C₇H₄NS). The fluxional behavior displayed by 3 has been examined by VT NMR spectroscopy, and DFT calculations provide evidence for stereoselective tripodal rotation at the Os(CO)₃P moiety that serves to equilibrate the second phosphine between the two available equatorial sites.