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
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Abstract
Thermodynamic,
kinetic, and computational studies are reported for oxygen atom transfer
(OAT) to the complex V(N[<i>t</i>-Bu]Ar)<sub>3</sub> (Ar
= 3,5-C<sub>6</sub>H<sub>3</sub>Me<sub>2</sub>, <b>1</b>) from
compounds containing N–O bonds with a range of BDEs spanning
nearly 100 kcal mol<sup>–1</sup>: PhNO (108) > SIPr/MesCNO
(75) > PyO (63) > IPr/N<sub>2</sub>O (62) > MesCNO (53) >
N<sub>2</sub>O (40) > dbabhNO (10) (Mes = mesityl; SIPr = 1,3-bis(diisopropyl)phenylimidazolin-2-ylidene;
Py = pyridine; IPr = 1,3-bis(diisopropyl)phenylimidazol-2-ylidene;
dbabh = 2,3:5,6-dibenzo-7-azabicyclo[2.2.1]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<sub>2</sub>O); (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<sub>2</sub>O); (IV)
steady-state kinetics in which no detectable intermediates are observed
(dbabhNO and MesCNO). Thermochemical studies of OAT to <b>1</b> show that the V–O bond in OV(N[<i>t</i>-Bu]Ar)<sub>3</sub> is strong (BDE = 154 ± 3 kcal mol<sup>–1</sup>) 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<sup>–1</sup>) and that dissociation
of dbabhNO to anthracene, N<sub>2</sub>, and a <sup>3</sup>O atom
is thermodynamically favorable at room temperature. Comparison of
the OAT of adducts of N<sub>2</sub>O and MesCNO to the bulky complex <b>1</b> show a faster rate than in the case of free N<sub>2</sub>O or MesCNO despite increased steric hindrance of the adducts
