CHEMICAL MODELS OF ENZYMES - New Trends in Enzyme Catalysis and Biomimetic Chemical Reactions - page 174

including water, turned out to be homogeneous (Nikonova and Shilov., 1977; Shilov,
1997). In the presence of this complex the following reactions take place:

Recently, a detailed analysis was performed of the EPR spectra of the V(II)-
pyrocatechol complex active in the reduction of dinitrogen in solution. (Shestakov and
Shilov, 2001). The hyperfine structure of the EPR spectrum of the test complex was
explained as a consequence of strong exchange interactions. Conclusions were reached
on the tetranuclear character of the active complex of the centrosymme ric structure with

the romboid disposition of the vanadium atoms. These authors suggested that the
vanadium four-nuclear complex reduces dinotrogen by thr four-electron mechanism. A
hydrazine derivative and hydrogen are shown to be formed in the coordination sphere of
tetramers, while ammonia is formed in the coordination sphere of an octamer (Dzabiev
et al., 1999). Redaction of nitrogen to ammonia is accompanied by the evolution of
diydrogen.
The systems involving Nb(III), Ta(III), Ti(II) and Cr(II) also reduced

t

in protic

media. Mo(III) complexes catalyzed reduction of by and sodium amalgam

(Volpin and Shilov, 1995).
Successful attempts have been made to model different steps of the nitrogenase
reactions (Henderson and Leigh 1999; Sellmann, 1995; Sellmann et al., 1999; Leigh,
1995; 1998; Lee, 2002; Helleren et al., 1999, 2000; Hauser et al., 2002). Dinitrogen can
displace a variety of ligands in metal complexes, including ammonia, water, chloride and
dihydrogen (Leigh, 1995; Helleren et al., 1999). The substitution of dihydrogen in
complexes or
for ligands L (MeCN, PhCN, or Cl - ) has been shown
in both acetone and THF. The stepwise protonation of dinirogen bond to molybdenum
(0) has been established (Henderson et al., 1983). The [(L) cuban
clusters have been found effective in the catalytic reduction of hydrazine to ammonia in
the presence of cobaltocene as a source of electrons and lutidine hydrocloride as a source
of protons (Coucouvanis , 2000;Coucouvanis et al., 1995).
A large new series of mono- and double cubane-type and
clusters (z = 1+, 2+, 3+) with ligands and other ligands has been
prepared as a possible precursor species for clusters related to those present in
vanadium-containing nitrogenase. Structure and redox conversions of the molybdenum-
iron sulfide-bridged double cubanes have been characterized by X-ray crystallographic
analysis, magnetic measurements, M

ö ssbauer and ESR spectroscopies (Hauser et al.,

2002 and references therin).
A study was conducted to elucidate the mechanism of FeMo-cofactor catalytic
activity in non-enzymic conditions and to compare its catalytic behavior with that of the
nitrogenase (Bazhenova et al., 2000). The kinetics were investigated of redaction
by multielectron donors, Zn and Eu amalgams, catalyzed by isolated FeMo-cofactor and