Thermodynamic and Kinetic Study of Cleavage of the N-O Bond of N-Oxides by a Vanadium(Ⅲ) Complex: Enhanced Oxygen Atom Transfer Reaction Rates for Adducts of Nitrous Oxide and Mesityl Nitrile Oxide

摘要

Thermodynamic, kinetic, and computational studies are reported for oxygen atom transfer (OAT) to the complex V(N[t-Bu]Ar)_3 (Ar = 3,5-C_6H_3Me_2, 1) from compounds containing N-O bonds with a range of BDEs spanning nearly 100 kcal mol~(-1): PhNO (108) ＞ SIPr/MesCNO (75) ＞ PyO (63) ＞ IPr/N_2O (62) ＞ MesCNO (53) ＞ N_2O (40) ＞ dbabhNO (10) (Mes = mesityl; SIPr = 1,3-bis(diisopropyl)- phenylimidazolin-2-ylidene; Py = pyridine; IPr = 1,3-bis(diisopropyl)phenyIimidazol-2- ylidene; dbabh = 2,3:5,6-dibenzo-7-azabicydo[2.2.l]hepta-2,5-diene). Stopped flow kinetic studies of the OAT reactions show a range of kinetic behavior influenced by both the mode and strength of coordination of the O donor and its ease of atom transfer. Four categories of kinetic behavior are observed depending upon the magnitudes of the rate constants involved: (Ⅰ) dinuclear OAT following an overall third order rate law (N_2O); (Ⅱ) formation of stable oxidant-bound complexes followed by OAT in a separate step (PyO and PhNO); (Ⅲ) transient formation and decay of metastable oxidant-bound intermediates on the same time scale as OAT (SIPr/MesCNO and IPr/N_2O); (Ⅳ) steady-state kinetics in which no detectable intermediates are observed (dbabhNO and MesCNO). Thermochemical studies of OAT to 1 show that the V-O bond in O≡V(N[f-Bu]Ar)_3 is strong (BDE = 154 ± 3 kcal mol~(-1)) compared with all the N-O bonds cleaved. In contrast, measurement of the N-O bond in dbabhNO show it to be especially weak (BDE = 10 ± 3 kcal mol~(-1)) and that dissociation of dbabhNO to anthracene, N_2, and a ~3O atom is thermodynamically favorable at room temperature. Comparison of the OAT of adducts of N_2O and MesCNO to the bulky complex 1 show a faster rate than in the case of free N_2O or MesCNO despite increased steric hindrance of the adducts.