Mononuclear manganese(II) and iron(III) flavonolates were synthesized as synthetic enzyme-substrate complexes, and their oxygenation reactions as biomimetic functional models with relevance to flavonol 2,4-dioxygenases are briefly described.
Stoichiometric quantities of 3,5-di-tert-butylcatechol
and aqueous ammonia react in pyridine solution to form
2-amino-4,6-di-tert-butylphenol. Under an atmosphere of
dioxygen the aminophenol is oxidized to either the
corresponding iminosemiquinone or iminobenzoquinone. In the
presence of Cu(II) iminosemiquinone condensation
with the aminophenol gives the Cat-N-SQ radical ligand obtained as the
Cu(py)2(Cat-N-SQ) complex.
Metal
and ligand magnetic orbitals are orthogonal and couple
ferromagnetically to give a S = 1 molecular spin state
at
temperatures up to 300 K. In nonpolar solvents the complex
undergoes ligand oxidation and disproportionation
to give Cu(Cat-N-BQ)2. Crystallographic
characterization on crystals obtained as the i-propanol
solvate
[orthorhombic, C2221, a = 19.548(3) Å,
b = 24.536(5) Å, c = 23.655(5) Å,
V = 11346(4) Å3, Z = 8,
R = 0.068]
show that the expected Jahn−Teller distortion appears in the
trans Cu−O lengths of the equatorial plane
rather
than for the axial Cu−N lengths. Reactions carried out with both
Cu(I) and Cu metal require metal oxidation to
give the Cu(II) products obtained. With metallic Cu this
occurs by a reaction with iminoquinone to give bis(iminosemiquinone)copper(II). Further reaction of this
product with aminophenol gives Cu(py)2(Cat-N-SQ)
by
condensation, and, with O2, oxidation gives a coordinated
azophenolate ligand in Cu(py)(azophenolate) by a
unique N−N bond-forming reaction. Cu(py)(azophenolate)
has been characterized crystallographically [monoclinic,
P21/n, a =
10.990(2) Å, b = 10.736(3) Å, c =
26.848(4) Å, β = 98.09(1)°, V =
3136(1) Å3, Z = 4, R =
0.048].
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