Toward Multifunctional Mo(VI−IV) Complexes:  cis-Dioxomolybdenum(VI) Complexes Containing Hydrogen-Bond Acceptors or Donors

The complexes cis-TpiPrMoVIO2(OAr-R) (TpiPr = hydrotris(3-isopropylpyrazol-1-yl)borate, -OAr-R = hydrogen-bonding phenolate derivative) are formed upon reaction of TpiPrMoO2Cl, HOAr-R, and NEt3 in dichloromethane. The orange, diamagnetic, dioxo−Mo(VI) complexes exhibit strong ν(MoO2) IR bands at ca. 935 and 900 cm-1 and NMR spectra indicative of Cs symmetry. They undergo electrochemically reversible, one-electron reductions at potentials in the range −0.836 to −0.598 V vs SCE; the only exception is the 2-CO2Ph derivative, which exhibits an irreversible reduction at −0.924 V. The complexes display distorted octahedral geometries, with a cis arrangement of terminal oxo ligands and with d(MoO)av = 1.695 Å and ∠(MoO2)av = 103.2°. The R groups of the 2-CHO and 2-NHCOMe derivatives are directed away from the oxo groups and into a cleft in the TpiPr ligand; these derivatives are characterized by Mo−O−Cipso angles of ca. 131° (conformation 1). The R group(s) in the 2-CO2Me and 2,3-(OMe)2 derivatives lie above the face of the three O-donor atoms (directed away from the TpiPr ligand) and the complexes display Mo−O−Cipso angles of 153.1(2) and 149.7(2)°, respectively (conformation 2). Conformations 1 and 2 are both observed in the positionally disordered 2-COMe and 2-COEt derivatives, the two conformers having Mo−O−Cipso angles of 130−140 and >150°, respectively. The 3-COMe and 3-NEt2 derivatives have substituents that project away from the TpiPr ligand and Mo−O−Cipso angles of 134.2(2) and 147.7(2)°, respectively. Many of the complexes exhibit fluxional behavior on the NMR time scale, consistent with the rapid interconversion of two conformers in solution.