Preparation and Dioxygenation of Copper(II) Nitrocatecholates
Scheme 1
systems suggest that the extradiol-cleaving catechol dioxy-
genases selectively favor the monoanionic state of catechol,
in contrast to the intradiol enzymes which bind catechol as
a dianion.40 Here we describe several new copper(II) cat-
echolate complexes as model compounds for catechol 1,2-
dioxygenases with the chromophoric probe 4-nitrocatechol
and various bidentate ligands. The steric and electronic
influence of the nitrogen-donor ligand set and the effect of
the binding mode of the 4-nitrocatechol on the biomimetic
catechol 1,2-dioxygenase activity is probed with spectro-
scopic and functional investigations on the prepared copper-
(II) compounds.
outside the two ortho-hydroxo groups.14 The intradiol-
cleaving catechol dioxygenases (b in Scheme 1) contain an
Fe(III)15-18 or Cu(II),19 while in contrast, the extradiol-
cleaving catechol dioxygenases (c in Scheme 1) typically
contain an Fe(II) active site.3-5,20-22 However, Mn(II)-
dependent extradiol-cleaving enzymes have also been
characterized,23-25 and in one case, a Mg(II)-dependent
extradiol-cleaving catechol dioxygenase has been reported.26
In previous studies, several functional models have been
synthesized to gain insight into the mechanism of catechol
cleavage.27-30 Complexes of copper have appeared promi-
nently in these investigations.31-34 Questions regarding the
mechanism of dioxygen activation, stereoselectivity, and
intra- versus extradiol ring cleavage remain as subjects of
investigation. Earlier models have shown that the dioxyge-
nase activity strongly depends on the nature of the ligand
set and the coordination mode of the catecholate ligand.35-39
4-Nitrocatechol binding studies on iron and manganese model
Experimental Section
General Remarks. Solvents used for the reactions were purified
using literature methods41 and stored under argon. The ligands
N,N,N′,N′-tetramethylethylenediamine and 4-nitrocatechol (Aldrich)
were used as provided. All air-sensitive compounds were handled
under argon using standard Schlenk techniques.42 Caution: Per-
chlorate salts of metal complexes with organic ligands are
potentially explosiVe. Only small amounts of material should be
prepared, and these should be handled with great care! Infrared
spectra were recorded on a Specord 75 IR (Carl Zeiss) spectro-
photometer using samples mulled in Nujol between KBr plates or
in KBr pellets. UV-vis spectra were recorded on a Shimadzu UV-
160 spectrophotometer using quartz cells. The EPR spectra were
recorded at room temperature by an X-band Bruker Elexsys 500
spectrometer. GC analyses were performed on a HP 5830A gas
chromatograph equipped with a flame ionization detector. Analyses
were performed on a HP 5890II/5971 GC/MSD apparatus equipped
with a column identical to that used for GC analyses. Cyclic
voltammetry experiments for compounds 1 and 2 in the absence
and also in the presence of HClO4 were carried out with a Pt
working electrode on a BAS CV-1B cyclic voltammeter in DMF
(∼10-4 M solution) at room temperature with NBu4ClO4 as
supporting electrolyte and a scan rate 60 mV s-1; the potential
values are relative to the saturated calomel electrode (SCE) using
an Ag-AgCl reference electrode (Figure S1). Microanalyses were
done by the Microanalytical Service of the Pannon University.
N,N,N′,N′-Tetrabenzylethylenediamine (tbeda). Benzyl bro-
mide (15.39 g, 0.09 mol) and then benzylethylenediamine (4.5 g,
0.03 mol) were added slowly to 90 mL of water under cooling. A
KOH (15 mL, 6 M) solution was then added to the stirred solution,
along with an additional 2-3 mL of acetonitrile. The mixture was
left overnight, and the pink crystals were filtered, dissolved in ether,
dried over MgSO4, and concentrated under vacuum to give colorless
crystals (6.3 g, 50%). Mp: 91 °C. IR (KBr): ν 3081, 3061, 3023,
2946, 2791, 2708, 1493, 1451, 1371, 1363, 1234, 1122, 1088, 1066,
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