cis-Dioxomolybdenum(VI) and Oxo(phosphine oxide)molybdenum(IV) Complexes: Steric and Electronic Fine-Tuning of cis-[MoOS]2+ Precursors

The complexes cis-TpiPrMoVIO2(OAr) (TpiPr = hydrotris(3-isopropylpyrazol-1-yl)borate, -OAr = phenolate or naphtholate derivative) are formed upon metathesis of TpiPrMoO2Cl and HOAr/NEt3 in dichloromethane. The orange, diamagnetic dioxo-Mo(VI) complexes exhibit strong (upsilon)(MoO2) IR bands at ca. 930 and 905 cm^-1 and NMR spectra indicative of Cs symmetry. They undergo electrochemically reversible, one-electron reductions at potentials in the range -0.714 to -0.855 V vs SCE (in MeCN) and react with PEt3 to produce TpiPrMoIVO(OAr)(OPEt3). The green, diamagnetic oxo-Mo(IV) complexes display a single (upsilon)(MoO) IR band at ca. 950 cm^-1 and exhibit NMR spectra indicative of C1 symmetry. The crystal structures of eight dioxo-Mo(VI) complexes have been determined to assess the degree of frontal (O3-donor face) steric congestion at the Mo center, to identify complexes amenable to conversion into monomeric oxosulfido-Mo(VI) derivatives. The complexes display distorted octahedral geometries, with a cis arrangement of terminal oxo ligands, with d(Mo=O)av = 1.694 (angstrom) and (angle)(MoO2)av = 103.4°. Maximal frontal steric congestion is observed in the 2-phenolate derivatives, and these are identified as precursors for strictly monomeric (solid and solution state) oxosulfido-Mo(VI) counterparts.