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

Square pyramidal metal fragments OsHX­{κ³-<i>P</i>,<i>O</i>,<i>P</i>-[xant­(P^<i>i</i>Pr₂)₂]} (X = Cl, H; xant­(P^<i>i</i>Pr₂)₂ = 9,9-dimethyl-4,5-bis­(diisopropylphosphine)­xanthene) coordinate the B–H bond of boranes <i>trans</i> 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-<i>P</i>,<i>O</i>,<i>P</i>-[xant­(P^<i>i</i>Pr₂)₂]})₂(μ-Cl)₂]­[BF₄]₂ (<b>3</b>) reacts with catecholborane (HBcat) and pinacolborane (HBpin) to give the elongated σ-borane derivative OsHCl­(η²-H-BR₂)­{κ³-<i>P</i>,<i>O</i>,<i>P</i>-[xant­(P^<i>i</i>Pr₂)₂]} (BR₂ = Bcat (<b>4</b>) and Bpin (<b>5</b>)), as well as H₂, FBR₂, and BF₃. The elongated σ-borane character of <b>4</b> and <b>5</b> 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 <b>4</b> 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 <b>3</b>, the reaction of tetrahydride complex OsH₄{κ³-<i>P</i>,<i>O</i>,<i>P</i>-[xant­(P^<i>i</i>Pr₂)₂]} (<b>6</b>) with HBcat leads to σ-borane derivative OsH₂(η²-H-Bcat)­{κ³-<i>P</i>,<i>O</i>,<i>P</i>-[xant­(P^<i>i</i>Pr₂)₂]} (<b>7</b>), 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 <b>7</b> only displays Os–B and B–H bond critical points; therefore, it lacks a similar topology