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

Square pyramidal metal fragments OsHX­{κ3-P,O,P-[xant­(PiPr2)2]} (X = Cl, H; xant­(PiPr2)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)­{κ3-P,O,P-[xant­(PiPr2)2]})2(μ-Cl)2]­[BF4]2 (3) reacts with catecholborane (HBcat) and pinacolborane (HBpin) to give the elongated σ-borane derivative OsHCl­(η2-H-BR2)­{κ3-P,O,P-[xant­(PiPr2)2]} (BR2 = Bcat (4) and Bpin (5)), as well as H2, FBR2, and BF3. 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 OsH4{κ3-P,O,P-[xant­(PiPr2)2]} (6) with HBcat leads to σ-borane derivative OsH2(η2-H-Bcat)­{κ3-P,O,P-[xant­(PiPr2)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.