A. Bharath et al., COBALT-MEDIATED SELECTIVE C-H BOND ACTIVATION - DIRECT AROMATIC HYDROXYLATION IN THE COMPLEXES [CO-III(O-OC6H3(R)N=NC5H4N)(2)]CLO4-CENTER-DOT-H2O (R = H, O-ME CL, M-ME/CL OR P-ME/CL) - SYNTHESIS, SPECTROSCOPICAND REDOX PROPERTIES/, Journal of the Chemical Society. Dalton transactions, (16), 1998, pp. 2643-2650
The reactions of low-spin complexes [(CoL3)-L-II][ClO4](2). H2O 1 [L =
2-(arylazo)pyridine, (R)C6H4N=NC5H4N (R = H, o-Me/Cl, m-Me/Cl or p-Me
/Cl] with m-chloroperbenzoic acid (m-ClC6H4CO3H) in acetonitrile solve
nt at room temperature resulted in low-spin [(CoL)-L-III'(2)]ClO4. H2O
2 [L' = o-OC6H3(R)N=NC5H4N]. In complexes 2 the o-carbon-hydrogen bon
d of the pendant phenyl ring of both parent ligands L has been selecti
vely and spontaneously hydroxylated. During the transformation of 1 to
2 the metal ion is oxidised from the starting Co-II to Co-III. The me
ridional configuration (cis-trans-cis with respect to the oxygen, azo
and pyridine nitrogens respectively) of complexes 2 has been establish
ed by H-1 and C-13 NMR spectroscopy. When one methyl or chloro group w
as present at the meta position of the pendant phenyl ring of L the re
action resulted in two isomeric complexes due to free rotation of the
singly bonded meta-substituted phenyl ring with respect to the azo gro
up. In acetonitrile solvent, complexes 2 systematically display one d-
d transition ((1)A(1g) --> T-1(1g) near 850 nm, two metal to ligand ch
arge-transfer transitions in the visible region and intraligand transi
tions in the UV region. In acetonitrile solution all complexes 2 exhib
it irreversible Co-III --> Co-IV oxidation near 2 V and reversible Co-
III reversible arrow Co-II reduction near 0.0 V versus Ag-AgCl. The li
gand-based expected four azo (N=N) reductions are observed sequentiall
y for all the complexes at the negative side of the reference Ag-AgCl.
Complexes 2 can be quantitatively and stereoretentively reduced to th
e low-spin cobalt(II) congeners, [(CoL)-L-II'(2)] 2(-) electrochemical
ly as well as chemically by using hydrazine hydrate. These complexes d
isplay eight-line EPR spectra in acetonitrile solution at 77 K. Comple
x 2a(-) exhibits a ligand to metal charge-transfer transition at 534 n
m and intraligand transition at 345 nm. Two possible d-d transitions,
E-2 --> T-2(1) and E-2 --> T-2(2) are observed at 700 and 800 nm respe
ctively.