π-Acid/π-Base Carbonyloxomolybdenum(IV) Complexes and Their Oxomolybdenum(VI/IV) Precursors

Brown TpiPrMoO(SR)(CO) (TpiPr = hydrotris(3-isopropylpyrazol-1-yl)borate; R = Et, iPr, Ph, p-tol, Bz) are formed
when TpiPrMoO(SR)(NCMe) react with CO gas in toluene. The carbonyloxomolybdenum(IV) complexes exhibit ν(CO) and ν(MoO) IR bands at ca. 2025 and 935 cm-1, respectively, and NMR spectra indicative of C1 symmetry,
with δC(CO) ca. 250. The crystal structure of TpiPrMoO(SiPr)(CO), the first for a mononuclear carbonyloxomolybdenum
complex, revealed a distorted octahedral geometry, with d(MoO) = 1.683(3) Å, d(Mo−C) = 2.043(5) Å, and
∠(OMo−C) = 90.87(16)°. The blue-green acetonitrile precursors are generated by reacting cis-TpiPrMoO2(SR)
with PPh3; they are unstable, display a single ν(MoO) IR band at ca. 950 cm-1, and exhibit NMR spectra
consistent with C1 symmetry. Red-brown cis-TpiPrMoO2(SR) (R = as above and tBu) are formed by metathesis of
TpiPrMoO2Cl and HSR/NEt3 in dichloromethane. The complexes exhibit strong ν(MoO2) IR bands at ca. 925 and
895 cm-1, and NMR spectra indicative of Cs symmetry; the isopropyl, p-tolyl, and benzyl derivatives possess
distorted octahedral geometries, with d(MoO)av = 1.698 Å and ∠(MoO2)av = 103.5°.