posted on 2015-02-04, 00:00authored byJustin
T. Henthorn, Sibo Lin, Theodor Agapie
A series
of π-bound Mo−quinonoid complexes supported
by pendant phosphines have been synthesized. Structural characterization
revealed strong metal–arene interactions between Mo and the
π system of the quinonoid fragment. The Mo–catechol complex
(2a) was found to react within minutes with 0.5 equiv
of O2 to yield a Mo–quinone complex (3), H2O, and CO. Si- and B-protected Mo–catecholate
complexes also react with O2 to yield 3 along
with (R2SiO)n and (ArBO)3 byproducts, respectively. Formally, the Mo–catecholate
fragment provides two electrons, while the elements bound to the catecholate
moiety act as acceptors for the O2 oxygens. Unreactive
by itself, the Mo–dimethyl catecholate analogue reduces O2 in the presence of added Lewis acid, B(C6F5)3, to generate a MoI species and a
bis(borane)-supported peroxide dianion, [[(F5C6)3B]2O22–], demonstrating
single-electron-transfer chemistry from Mo to the O2 moiety.
The intramolecular combination of a molybdenum center, redox-active
ligand, and Lewis acid reduces O2 with pendant acids weaker
than B(C6F5)3. Overall, the π-bound
catecholate moiety acts as a two-electron donor. A mechanism is proposed
in which O2 is reduced through an initial one-electron
transfer, coupled with transfer of the Lewis acidic moiety bound to
the quinonoid oxygen atoms to the reduced O2 species.