transition in equation (10). The obtained value, 30 420 cmϪ1,
compares well with that calculated from equation (12).
The remaining electronic transition, dz ← dxy, could not
be assigned, as only two d–d bands are observed in these
spectra and our model has identified them as the other d–d
transitions. Furthermore, as its energy does not explicitly
appear in the expression for g and A(M), EPR data do not
provide any information concerning its energy. However, as in
the framework of the results of Nishida et al.28,29 this transition
must be associated with any of the two d–d bands.
EPR signals have been identified for the active forms of MCR,
MCR-red1(axial) and MCR-red2 (rhombic), that can be inter-
converted by the action of substrates. Both the characterization
of these EPR signals and the co-ordination chemistry of the
nickel cofactor are still not settled, thus making the use of
models for F430 an area of current interest.
2
Spectroscopic characterization of [NiI(bphen)]Ϫ shows that
the tetradentate ligand induces two sets of Ni᎐N bonds
(Ni᎐Namide and Ni᎐Npyridyl) and behaves as a flexible ligand
that allows some π-electronic delocalization. Furthermore, the
g values of [NiI(bphen)]Ϫ compare well with those of the
12,13 diepimer of the reduced factor F430 pentacarboxylic acid
(aqueous solution),5f and are similar to those of the reduced
factor F430 pentacarboxylic acid (aqueous solution)5f and
of the pentamethyl ester F430M.5d,e These observations may
suggest that the electronic structure of the NiI of our complex
is similar to that of the nickel() in F430, and led us to compare
their electronic spectra.
Structural implications from the spectroscopic data
The combination of EPR and UV/VIS studies of isoelectronic
square-planar complexes of 63CuII and 61NiI with the ligand
bphen2Ϫ has allowed what we believe is the first assignment of
visible electronic bands of a nickel() complex, and provided
insights into its structural and bonding properties.
The EPR spectra of [NiI(bphen)]Ϫ exhibit higher g values
than does the homologous copper() complex, as is usually
observed for complexes of NiI and CuII which have ligands in
common. This general observation cannot be rationalized in
terms of the relative metal spin–orbit couplings, as it can be
seen from equations (6) and (7) and considering that λCu(II) is
larger. Instead, this behaviour is thought to be a consequence of
the lower charge/lower electronegativity of the nickel() ion,
and the data obtained for our complexes provide direct evi-
dence that NiI: (i) has lower σ and π covalency in M᎐N bonding
(higher values for α2, δ2) and (ii) lower d–d orbital separation
Analysis of the electronic spectra of reduced factor F430
pentacarboxylic acid and its 12,13 diepimer and of the penta-
methyl ester F430M shows that the electronic bands can be
grouped based on their absorption coefficient: (i) one band in
the range 13 300–14 100 cmϪ1 with the medium ε values, and
(ii) two bands in the range 26 300–37 700 cmϪ1 with high ε
values. The band at low energy compares well with the low-
energy medium-intensity electronic band (at 13 510 cmϪ1) of
our complex which has been assigned to the dxz, dyz ← dxy
electronic transition, thus allowing us to propose the assign-
ment of the electronic band at 13 300–14 410 cmϪ1 in the elec-
tronic spectra of F430 pentacarboxylic acid, of its 12,13
diepimer and of the pentamethyl ester F430M to the dxz,
dyz ← dxy electronic transitions. The other d–d transition
observed in our spectra and which is not detected in the spectra
2 Ϫ y2
[smaller values for the energy difference E (dx
and E(dxz, dyz ← dxy)].
← dxy)
A striking difference in EPR parameters of these complexes
lies in the values of ∆xy: nickel() complexes exhibit values that
are ten times larger than those of the copper() complex (Table
1). This difference must be related to structural modifications
concomitant with the reduction NiII → NiI. In the absence of
crystallographic data for the nickel() complex the explanation
cannot be ascertained directly, but some clues can be provided
by spectroscopic data.
of F430 pentacarboxylic acid and of pentamethyl ester F430
M
may be masked by the high-energy charge-transfer bands.
Support for this statement can be gained by noting that in
the spectra of the 12,13 diepimer of the reduced factor F430
pentacarboxylic acid (aqueous solution)5f a medium-intensity
shoulder is observed at 20 830 cmϪ1 and must be associated
As has been referred to above, both the nickel() and cop-
per() complexes reveal two sets of M᎐N bond distances, M᎐N
(amide) and M᎐N (pyridyl), which are larger for the copper()
2 Ϫ y2
with the dx
← dxy transition.
Finally, our nickel() complex may act as a potential model
for F430, and reactions of [NiI(bphen)]Ϫ with several Lewis bases
and substrates are now under investigation.
complex. However, their differences,
∆bond = (M᎐Namide) Ϫ
(M᎐Npyridyl), are slightly larger for the nickel() complex, (CuII,
0.090; NiIIA, 0.101 or NiIIB, 0.095), but this difference is
insufficient to account for the difference in ∆xy of the copper()
and nickel() EPR spectra, unless we propose that reduction of
[NiII(bphen)] to [NiI(bphen)]Ϫ induces an alteration of the
nickel core: either its distortion and/or unequal expansion of
the Ni᎐N bond distances. Such changes are known to occur
with macrocyclic ligands:14,17–20,44 flexible saturated systems
show expansion of Ni᎐N bond lengths, whereas rigid unsatur-
ated ligands impose mainly distortions on the core without an
expansion of the hole. The ligand used in the present work
when bound to the metal ion allows some π-electron delocaliz-
ation, but forms a non-closed ring that can easily accommodate
changes in metal-ion size without necessarily imposing a distor-
tion on the nickel core.
Acknowledgements
This work was partially supported by Praxis XXI through
project Praxis/2/2.1/QUI/316/94.
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in the two sets of bond lengths increases on reducing NiII to NiI.
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difference between the two sets of bonds increased.44
Biological relevance of the [NiI(bphen)]؊ spectroscopic properties
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J. Chem. Soc., Dalton Trans., 1998, Pages 1557–1562
1561