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

Square pyramidal metal fragments OsHX{kappa(3)-P,O,P-[xant((PPr2)-Pr-i)(2)]} (X = Cl, H; xant((PPr2)-Pr-i)(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 sigma-borane and sigma-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(HH){kappa(3)-P,O,P-[xant((PPr2)-Pr-i)(2)]})(2)(mu-Cl)(2)][BF4](2) (3) reacts with catecholborane (HBcat) and pinacolborane (HBpin) to give the elongated sigma-borane derivative OsHCl(eta(2)-H-BR2){kappa(3)-P,O,P-[xant((PPr2)-Pr-i)(2)]} (BR2 = Bcat (4) and Bpin (5)), as well as H-2, FBR2, and BF3. The elongated s-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 1.61.7 angstrom. 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{kappa(3)-P,O,P-[xant((PPr2)-Pr-i)(2)]} (6) with HBcat leads to sigma-borane derivative OsH2(eta(2)-H-Bcat){kappa(3)-P,O,P-[xant((PPr2)-Pr-i)(2)]} (7), which shows a distance between the atoms of the coordinated BH bond in the range of 1.41.5 angstrom. AIM analysis for the OsHB unit of 7 only displays Os-B and B-H bond critical points; therefore, it lacks a similar topology.