S. Hikichi et al., ALIPHATIC C-H BOND OXYGENATION BY THE (COOOX)-O-II SPECIES WITH THE HINDERED HYDROTRIS(PYRAZOLYL)BORATE LIGAND (X = CO(II), ALKYL, H), Journal of the American Chemical Society, 120(19), 1998, pp. 4699-4710
Aliphatic C-H bond oxygenation is achieved by Co(II)-peroxo species. T
he dinuclear Co(II)- mu-peroxo complex, {Co[HB(3,5-Pr(2)(i)pz)(3)]}(2)
(mu-O-2) (2), is yielded by reaction of the bis(mu-hydroxo)-Co(II) com
plex, {Co(OH)[HB (3,5-Pr(2)(i)pz)(3)]}(2) (1), with an equimolar amoun
t of H2O2. Spontaneous decomposition of the mu-peroxo complex 2 yields
a mono-oxygenated mu-alkoxo-mu-hydroxo complex, CO2(mu-OH) [HB(mu-3-O
CMe2-5-Pr(i)pz)(3,5-Pr(2)(i)pz)(2)] [HB(3,5-Pr(2)(i)pz)(3)] (4), In wh
ich one of the six 3-isopropyl methine carbon atoms is oxygenated and
the resulting alkoxo ligand bridges the two Co(II) centers. In contras
t, decomposition of 2 in the presence of an excess amount of H2O2 resu
lts in further oxygenation to give a mixture of the dinuclear Co(II)-b
is(mu-alkoxo) complex, o[HB(mu-3-OCMe2-5-Pr(i)pz)(3,5-Pr(2)(i)pz)(2)]}
(2) (5), and the mononuclear Co(II)-hydroxo alcohol complex, Co(OH)[HB
(3-Me2COH-5-Pr(i)pz)(3-Me2COH-5-Pr(i)pz)(2)] (6). In the bis(mu-alkox
o) complex 5, one of the three 3-isopropyl groups in each ligand is fu
nctionalized to give the alkoxide group, which bridges the two metal c
enters. In the mononuclear hydroxo alcohol complex 6, all of the three
3-isopropyl groups are oxygenated. The Co(II) center is coordinated b
y the resulting alcohol ligand as well as an hydroxide. Reaction of 1
with 2 equiv of POOH (R = Bu-t and PhMe2C) at low temperature yields t
he blue thermally unstable alkylperoxo compound 3. The HB(3-Bu-t-5-Pr(
i)pz)(3) derivative, Co(OOCMe2Ph) [HB (3-Bu-t-5-Pr(i)pz)(3)] (9), char
acterized successfully by X-ray crystallography contains the tetrahedr
al Co center. The monomeric alkylperoxo complexes 3 also decomposes to
give the bis(mu-alkoxo) complex 5, but neither mono-oxygenated 4 nor
fully oxygenated 6 is obtained. These observations suggest that the pr
esent aliphatic C-H bond oxygenations may proceed via homolysis of the
O-O bond of the Co(II)-peroxo species.