The enthalpies of oxygen atom transfer (OAT) from mesityl nitrile oxide (MesCNO) to Me 3P, Cy 3P, Ph 3P, and the complex (Ar[ tBu]N) 3MoP (Ar = 3,5-C 6H 3Me 2) have been measured by solution calorimetry yielding the following P-O bond dissociation enthalpy estimates in toluene solution (±3 kcal mol -1): Me 3PO [138.5], Cy 3PO [137.6], Ph 3PO [132.2], (Ar[ tBu]N) 3MoPO [108.9]. The data for (Ar[ tBu]N) 3MoPO yield an estimate of 60.2 kcal mol -1 for dissociation of PO from (Ar[ tBu]N) 3MoPO. The mechanism of OAT from MesCNO to R 3P and (Ar[ tBu]N) 3MoP has been investigated by UV-vis and FTIR kinetic studies as well as computationally. Reactivity of R 3P and (Ar[ tBu]N) 3MoP with MesCNO is proposed to occur by nucleophilic attack by the lone pair of electrons on the phosphine or phosphide to the electrophilic C atom of MesCNO forming an adduct rather than direct attack at the terminal O. This mechanism is supported by computational studies. In addition, reaction of the N-heterocyclic carbene SIPr (SIPr = 1,3-bis(diisopropyl)phenylimidazolin-2-ylidene) with MesCNO results in formation of a stable adduct in which the lone pair of the carbene attacks the C atom of MesCNO. The crystal structure of the blue SIPr·MesCNO adduct is reported, and resembles one of the computed structures for attack of the lone pair of electrons of Me 3P on the C atom of MesCNO. Furthermore, this adduct in which the electrophilic C atom of MesCNO is blocked by coordination to the NHC does not undergo OAT with R 3P. However, it does undergo rapid OAT with coordinatively unsaturated metal complexes such as (Ar[ tBu]N) 3V since these proceed by attack of the unblocked terminal O site of the SIPr·MesCNO adduct rather than at the blocked C site. OAT from MesCNO to pyridine, tetrahydrothiophene, and (Ar[ tBu]N) 3MoN was found not to proceed in spite of thermochemical favorability.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry