Coordination Chemistry of the (η6-p-Cymene)ruthenium(II) Fragment with Bis-, Tris-, and Tetrakis(pyrazol-1-yl)borate Ligands: Synthesis, Structural, Electrochemical, and Catalytic Diastereoselective Nitroaldol Reaction Studies

Novel [Ru(η6-p-cymene)(κ2-L)X] and [Ru(η6-p-cymene)(κ3-L)]X·nH2O complexes (L = bis-, tris-, or tetrakis-pyrazolylborate; X = Cl, N3, PF6, or CF3SO3) are prepared by treatment of [Ru(η6-p-cymene)Cl2]2 with poly(pyrazolyl)borate derivatives [M(L)] (L in general; in detail L = Ph2Bp = diphenylbis(pyrazol-1-yl)borate; L = Tp = hydrotris(pyrazol-1-yl)borate; L = pzTp = tetrakis(pyrazol-1-yl)borate; L = Tp4Bo = hydrotris(indazol-1-yl)borate, L = Tp4Bo,5Me = hydrotris(5-methylindazol-1-yl)borate; L = TpBn,4Ph = hydrotris(3-benzyl-4-phenylpyrazol-1-yl)borate; M = Na, K, or Tl) and characterized by analytical and spectral data (IR, ESIMS, 1H and 13C NMR). The structures of [Ru(η6-p-cymene)(Ph2Bp)Cl] (1) and [Ru(η6-p-cymene)(Tp)Cl] (3) have been established by single-crystal X-ray diffraction analysis. Electrochemical studies allowed comparing the electron-donor characters of Tp and related ligands and estimating the corresponding values of the Lever EL ligand parameter. The complexes [Ru(η6-p-cymene)(κ2-L)X] and [Ru(η6-p-cymene)(κ3-L)]X·nH2O act as catalyst precursors for the diastereoselective nitroaldol reaction of benzaldehyde and nitroethane to the corresponding β-nitroalkanol (up to 82% yield, at room temperature) with diastereoselectivity toward the formation of the threo isomer.