Reaction
of a Bridged Frustrated Lewis Pair with Nitric
Oxide: A Kinetics Study
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Abstract
Described
is a kinetics and computational study of the reaction
of NO with the intramolecular bridged P/B frustrated Lewis pair (FLP) <i>endo</i>-2-(dimesitylphosphino)-<i>exo</i>-3-bis(pentafluorophenyl)boryl-norbornane
to give a persistent FLP-NO aminoxyl radical. This reaction follows
a second-order rate law, first-order in [FLP] and first-order in [NO],
and is markedly faster in toluene than in dichloromethane. By contrast,
the NO oxidation of the phosphine base 2-(dimesitylphosphino)norbornene
to the corresponding phosphine oxide follows a third-order rate law,
first-order in [phosphine] and second-order in [NO]. Formation of
the FLP-NO radical in toluene occurs with a Δ<i>H</i><sup>⧧</sup> of 13 kcal mol<sup>–1</sup>, a feature
that conflicts with the computation-based conclusion that NO addition
to a properly oriented B/P pair should be nearly barrierless. Since
the calculations show the B/P pair in the most stable solution structure
of this FLP to have an unfavorable orientation for concerted reaction,
the observed barrier is rationalized in terms of the reversible formation
of a [B]-NO complex intermediate followed by a slower isomerization–ring
closure step to the cyclic aminoxyl radical. This combined kinetics/theoretical
study for the first time provides insight into mechanistic details
for the activation of a diatomic molecule by a prototypical FLP