Hydrocarbon solutions of Mo2(NMe2)6 and 2,2′-methylenebis(6-tert-butyl-4-methylphenol) (≥2 equiv), HOunknown signCH2unknown signOH, yield Mo2(NMe2)2(Ounknown signCH2unknown signO)2, I, which exists in bridged Ib and chelated Ic isomers. These are formed under kinetic control, and recrystallization allows the separation of Ib (orange cubes) from Ic (yellow cubes) both of which have been crystallographically characterized. In each there is an ethane-like O2NMo≡MoO2N core with Mo-Mo = 2.2 Å (average). In Ib the two Ounknown signCH2unknown signO ligands span the Mo≡Mo bond yielding a molecule of C2 symmetry. In Ic the molecule has near-C2 symmetry in the solid state, but in solution there is either rapid rotation about the M≡M bond or the anti-rotamer is preferred. In benzene-d6, Ib and Ic do not interconvert at 110 °C over a period of days. However, the addition of pyridine or acetonitrile causes the isomerization of Ib to Ic, thereby establishing that Ic is the thermodynamic isomer. The rate of conversion of Ib to Ic has been shown to be dependent on the square of the concentration of added pyridine: kobs = k[py]2. From the temperature dependence of kobs, we determine ΔH‡ = 19 (±1) kcal/mol and ΔS‡ = -25 (±3) eu for the pyridine-promoted isomerization of Ib to Ic. The related reaction involving W2(NMe2)6 and HOunknown signCH2unknown signOH (≥2 equiv) in hydrocarbon solvents at room temperature and below yields a dark brown crystalline compound, wherein C-H activation has occurred at one of the Ounknown signCH2unknown signO diolate ligands, W2(μ-H)(μ-NMe2)(NMe2)(η 2-Ounknown signCH2unknown signO)-(η3Ounknown signCHunknown signO)(HNMe2), 2. The W-W distance in 2 is 2.495(1) Å, consistent with a (W=W)8+ core. Heating 2 in the solid-state under a dynamic vacuum leads to the elimination of HNMe2 and the formation of 3, W2(NMe2)2(η2-Ounknown signCH2unknown signO)2, an analog of Ic. In benzene-d6 the equilibrium involving 2 and 3 + HNMe2 has been observed by 1H NMR spectroscopy. The addition of pyridine to hydrocarbon solutions of 3 yields W2(μ-H)(μ-NMe2)(η2-Ounknown signCH2unknown signO)(μ3-Ounknown signCHunknown signO)(NMe2)(py), 4, which has been shown by single-crystal X-ray crystallography to be an analogue of 2. Studies of the addition of PMe3 to toluene-d8 solutions of 3 at low temperatures reveal that adduct formation occurs prior to C-H oxidative addition. For the equilibrium involving 4 and 3 + py in benzene-d6, ΔH° = 14 (±1) kcal/mol and ΔS° = 22 (±3) eu.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry
- Inorganic Chemistry