Elongated σ-Borane versus σ-Borane in Pincer–POP–Osmium Complexes

Square pyramidal metal fragments OsHX{κ 3 - P, O, P -[xant(P i Pr 2 ) 2 ]} (X = Cl, H; xant(P i Pr 2 ) 2 = 9,9-dimethyl-4,5-bis(diisopropylphosphine)xanthene) coordinate the B–H bond of boranes trans to the ligand X. As a consequence, elongated σ-borane and σ-borane pincer–POP–osmium complexes have been isolated and fully characterized. The interaction between the metal fragment and the coordinated B–H has been analyzed as a function of X, from spectroscopic, X-ray diffraction, and theoretical points of view. The dinuclear complex [(Os(H···H){κ 3 - P, O, P -[xant(P i Pr 2 ) 2 ]}) 2 (μ-Cl) 2 ][BF 4 ] 2 ( 3 ) reacts with catecholborane (HBcat) and pinacolborane (HBpin) to give the elongated σ-borane derivative OsHCl(η 2 -H-BR 2 ){κ 3 - P, O, P -[xant(P i Pr 2 ) 2 ]} (BR 2 = Bcat ( 4 ) and Bpin ( 5 )), as well as H 2, FBR 2, and BF 3 . The elongated σ-borane character of 4 and 5 is supported by X-ray diffraction analysis and DFT-optimized structures of both compounds, which show distances between the coordinated B and H atoms of the borane in the range of 1.6–1.7 Å. AIM analysis of 4 reveals a triangular topology for the OsHB unit involving Os–B, Os–H, and B–H bond critical points and a ring critical point. In contrast to 3, the reaction of tetrahydride complex OsH 4 {κ 3 - P, O, P -[xant(P i Pr 2 ) 2 ]} ( 6 ) with HBcat leads to σ-borane derivative OsH 2 (η 2 -H-Bcat){κ 3 - P, O, P -[xant(P i Pr 2 ) 2 ]} ( 7 ), which shows a distance between the atoms of the coordinated B–H bond in the range of 1.4–1.5 Å. AIM analysis for the OsHB unit of 7 only displays Os–B and B–H bond critical points; therefore, it lacks a similar topology.