One-Electron Reduction of Acenaphthene-1,2-Diimine Nickel(II) Complexes

Article abstract of DOI:10.1002/asia.201900677

New nickel-based complexes of 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene (dpp-bian) with BF₄ ^? counterion or halide co-ligands were synthesized in THF and MeCN. The nickel(I) complexes were obtained by using two approaches: 1) electrochemical reduction of the corresponding nickel(II) precursors; and 2) a chemical comproportionation reaction. The structural features and redox properties of these complexes were investigated by using single-crystal X-ray diffraction (XRD), cyclic voltammetry (CV), and electron paramagnetic resonance (EPR) and UV/Vis spectroscopy. The influence of temperature and solvent on the structure of the nickel(I) complexes was studied in detail, and an uncommon reversible solvent-induced monomer/dimer transformation was observed. In the case of the fluoride complex, the unpaired electron was found to be localized on the dpp-bian ligand, whereas all of the other nickel complexes contained neutral dpp-bian moieties.

Full text of DOI:10.1002/asia.201900677

CHEMISTRY
AN ASIAN JOURNAL
www.chemasianj.org
Accepted Article
Title: Regularities of One-Electron reduction of Acenaphthene-1,2-
Diimine Nickel(II) Complexes
Authors: Vera Vasilevna Khrizanforova, Robert R. Fayzullin, Vladimir
I. Morozov, Ildar Gilmutdinov, Anton N. Lukoyanov, Olga N.
Kataeva, Tatiana P. Gerasimova, Sergey A. Katsyuba, Igor L.
Fedushkin, Konstantin A. Lyssenko, and Yulia H. Budnikova
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10.1002/asia.201900677
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Regularities of One-Electron reduction of Acenaphthene-1,2-
Diimine Nickel(II) Complexes
Vera V. Khrizanforova,a* Robert R. Fayzullin,a Vladimir I. Morozov,a Ildar F. Gilmutdinov,b Anton N.
Lukoyanov,c Olga N. Kataeva,a Tatiana P. Gerasimova,a Sergey A. Katsyuba,a Igor L. Fedushkin,c
Konstantin A. Lyssenko,d Yulia H. Budnikovaa,f
Abstract:
New
Ni-based
complexes
of
1,2-bis[(2,6-
olefins into high molecular weight polymers.12-19 The mechanistic
data available to date assumed that cationic divalent LNiII-alkyl
complexes can be considered as active species in the catalytic
systems.20-22 According to the known common mechanism, the
main role of diimine ligands is providing for a steric effect.13,15,16
However, it was shown that electron transfer may take place
during catalysis, which results in the formation of diimine radical
anions that dramatically influence on the polyethylene branching
system.14 Some previous reports23,24 also demonstrate that real
catalyst systems considerably differ from the model system
described by Brookhart and coworkers.13 Namely, cationic
complexes of NiII formed in real catalytic systems LNiBr2/MAO or
diisopropylphenyl)imino]acenaphtene (dpp-bian) with halides as co-
–
ligands and BF4 counterion were synthesized in THF and MeCN.
The NiI complexes were obtained using two approaches:
electrochemical reduction of NiII precursors or
a
chemical
comproportionation reaction. Their structural features and redox
properties were determined by means of single crystal X-ray
diffraction,
cyclic
voltammetry,
EPR,
and
UV/Vis
spectroelectrochemistry. The influence of temperature and solvent
nature on the structure of the NiI complexes was studied in detail,
namely an uncommon reversible solvent-induced monomer–dimer
transformation of the NiI complexes was observed. In the case of
fluoride complex, the unpaired electron was found to be localized on
the dpp-bian ligand, whilst all the other nickel complexes consist of
neutral dpp-bian.
LNiBr2/MMAO (where
dimethyl-1,4-diazabuta-1,3-diene, MMAO
L
=
1,4-bis-2,4,6-dimethylphenyl-2,3-
AliBu3-modified
=
methylalumoxane) were studied in situ by 1H NMR spectroscopy
at temperatures below –20°C.23 At the same time, the
conversion of these complexes to EPR active species at
warming and predominating of paramagnetic compounds in the
reaction solution at room temperature were found.24 This once
again emphasizes the importance of establishing the location of
an unpaired electron in reduced nickel complexes for the best
understanding of the catalyst mechanism. Furthermore, despite
the widespread use of Ni-based catalysts with different bians,
only a few examples of structurally characterized complexes
with a formal nickel oxidation state of +1 are known.12,25
Introduction
The great interest to metal complexes with redox non-innocent
ligands, such as α-diimines, is caused by their ability to be a
donor or an acceptor of one or more electrons.1-11 Nickel
complexes with bis-iminoacenaphtenes (bians) are known as
utterly active catalysts for the conversion of ethylene and α-
The importance of low-valent nickel for catalysis and redox
diversity of bian complexes induced us to investigate one-
electron reduction products of NiI complexes with 1,2-bis[(2,6-
diisopropylphenyl)imino]acenaphtene (dpp-bian) with different
suitable techniques, such as electrochemical approaches, EPR,
UV/Vis spectroscopy, which could facilitate the establishment of
the nature of metal complexes in solutions. As a result,
complexes with unpaired electron localization on a metal or a
ligand were found depending on halide type. Moreover, an
[a]
Dr. V. V. Khrizanforova, Dr. R. R. Fayzullin, Dr. V. I. Morozov, Prof.,
Dr. O. N. Kataeva, Dr. T. P. Gerasimova, Prof., Dr. S. A. Katsyuba,
Prof., Dr. Yu. H. Budnikova
Arbuzov Institute of Organic and Physical Chemistry, FRC Kazan
Scientific Center, Russian Academy of Sciences, Arbuzov Street, 8,
420088 Kazan, Russian Federation
khrizanforovavera@yandex.ru
Dr I. F. Gilmutdinov
[b]
[c]
Institute of Physics, Kazan Federal University, Kremlevskaya Street,
18, 420008 Kazan, Russian Federation
Dr. Anton N. Lukoyanov, Prof., Dr. Igor L. Fedushkin
G.A. Razuvaev Institute of Organometallic Chemistry of Russian
Academy of Sciences, Tropinin Street, 49, 603137 Nizhny
Novgorod, Russian Federation
uncommon
reversible
solvent-induced
monomer–dimer
transformation of the NiI complexes was observed.
Results and Discussion
[d]
Prof., Dr. Konstantin A. Lyssenko
A.N. Nesmeyanov Institute of Organoelement Compounds of
Russian Academy of Sciences, Vavilov Street, 28, 119334 Moscow,
Russian Federation
Synthesis and structure of nickel(II) complexes with dpp-
bian
[f] ]
Prof., Dr. Yu. H. Budnikova
The College of Chemistry and Molecular Engineering, Zhengzhou
University, Zhengzhou 450001, PR China
In this report nickel(II) complexes with dpp-bian were obtained
with yields up to 90% by the treatment of the corresponding
anhydrous nickel(II) salts with one equivalent of dpp-bian in THF
or MeCN by analogy with previously reported methods.15-17 The
ratio of dpp-bian to Ni in all the complexes was 1 : 1. In this work
the complexes were named as I–IV, where I is the complex with
Electronic Supplementary Information (ESI) available: The
crystallographic data for the investigated structures have been
deposited in the Cambridge Crystallographic Data Centre as
supplementary publication numbers CCDC 1846515, 1846519,
1846520, 1846521, 1846522 and 1846523.
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–
a BF4 counter ion; II, III, and IV are the complexes with I, Br,
and Cl anionic co-ligands, respectively (Scheme 1).
Figure 1. ORTEP projections of complexes
I (a) and III (b) showing
anisotropic displacement ellipsoids at the 50 % probability level. Hydrogen
atoms, minor disordered components, and solvent molecules are omitted for
clarity. Selected interatomic distances for I [Å]: Ni1–N1 2.1300(12), Ni1–N2
2.1184(12), Ni1–N3 2.0633(14), Ni1–N4 2.0710(14), Ni1–N5 2.0570(14), Ni1–
N6 2.0870(14), N1–C1 1.2807(19), N2–C2 1.2830(18), C1–C2 1.5161(19).
Selected interatomic distances for III [Å]: Br1–Ni1 2.3301(6), Br2–Ni1
2.3373(6), Ni1–N1 2.027(3), Ni1–N2 2.020(3), N1–C1 1.286(4), N2–C2
1.279(4), C1–C2 1.501(5).
Scheme 1 Synthesis of nickel(II) complexes with dpp-bian
The molecular structures of I and III were determined by single
crystal X-ray diffraction. According to these data, compound I
proved to be octahedral nickel(II) cationic complex [(dpp-
–
bian)Ni(NCMe)4]2+ with two isolated BF4 anions (Figure 1a). As
Electrochemical properties of Ni(II) complexes with dpp-
bian
shown in Figure 1b, compound III represents a four-coordinate
neutral nickel(II) complex: two dpp-bian nitrogen atoms and two
bromide ligands form a tetrahedral coordination environment
around the metal center. Selected bond distances in I and III are
listed in the caption and close to those values found in the other
NiII complexes with neutral diimine ligands.19
At the same time, dpp-bian nickel(II) complex with fluoride ligand
cannot be prepared using NiF2 directly due to poor solubility of
the latter in organic solvents. Although desired complex (dpp-
bian)NiF2 was able to be prepared by treating of complex I with
Me4NF, low solubility of the product prevented its investigation.
The redox properties of nickel(II) complexes I–IV were studied
by cyclic voltammetry (CV) in MeCN solutions using Bu4NBF4 as
a supporting electrolyte. Complexes I–IV demonstrated similar
behaviour. Four reversible or quasi-reversible one-electron
reduction peaks from –0.77 to –0.97, from –1.45 to –1.52, from –
1.72 to –1.76, and at –2.50 V were observed on the CVs of NiII
complexes in MeCN (Figure 2). The first two peaks are related
to the couples NiII/I and NiI/0 reductions, which is confirmed by
further EPR studies. The other reduction peaks correspond to
dpp-bian0/1– and dpp-bian1–/2– couples, respectively.26,27
Reduction potentials referenced vs. Fc+/Fc couple for all the
complexes in MeCN were summarized in Table 1. In general,
the potentials of the NiII/I/0 couples are shifted toward more
–
negative values in the series BF4 , I, Br, Cl.
Table 1. Electrochemical data for NiII complexes I–IV in MeCN (ΔE = Ea–Ec,
mV, is shown in brackets). Potentials were measured vs. Fc+/Fc, Bu4NBF4
was used as a supporting electrolyte
Epred(NiII/I),
V
Epred(BIAN0/1–),
V
Epred(BIAN1–/2–),
V
Complex
Epred(NiI/0), V
–1.45 (80)
–1.46 (75)
–1.47 (70)
–1.52 (130)
–0.77
(200)
–0.80
(240)
–0.81
(160)
–0.97
(270)
I
–1.74 (90)
–1.76 (90)
–1.72 (70)
–1.74 (90)
–2.50 (200)
–2.50 (190)
–2.50 (200)
–2.50 (210)
II
III
IV
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Scheme 2. Schematic representation of two synthetic approaches
Reversible or quasi-reversible peaks corresponding to the
oxidation of the NiI/II couple at the peak potentials from –0.42 to
–0.76
V
(vs. Fc+/Fc) were observed for electrochemically
reduced nickel complexes 1-4 on CVs in MeCN and THF
solutions (Figure 4, See ESI). In the case of transition from the
iodide through bromide to chloride co-ligand, the oxidation
potentials of the NiI/II pair are also shifted to the cathodic region
resembling the precursor NiII complexes (see ESI). The
reversible reduction peak for the NiI/0 is observed at potentials
from –0.58 to –0.78 V (See ESI) in both solutions. In general,
the reduced nickel complexes in THF are oxidized more difficult
than corresponding complexes in MeCN.
Figure 2. Cyclic voltammogramms for a 1.5 mM solution of complex I in MeCN.
Conditions: a working electrode was glassy carbon; an auxiliary electrode was
Pt; Bu4NBF4 was used as a supporting electrolyte (SE).
Synthesis of reduced nickel complexes and their
spectroelectrochemical investigation
Two pathways for the synthesis of the one-electron reduced
nickel complexes, namely complexes 1-4 with a BF4– anion and I,
Br, and Cl co-ligands, respectively, were performed (Scheme 2).
First, following the cyclic voltammetry data, bulk electrolysis of a
1.5 mM solution of NiII complexes I-IV in anhydrous acetonitrile
or THF solutions was carried out at the NiII/I couple potentials.
Under spectroelectrochemical conditions, appearance and
increase of intensity of new absorption bands for one-electron
reduced nickel complexes 1-4 were detected in both solutions in
the UV/Vis spectra. Interestingly, the UV/Vis spectra of the 1-4 in
MeCN and THF were found to be notably different. Spectra of
MeCN solutions of 1-4 demonstrated strong bands at 800 and
550 nm with a shoulder at 480 nm (Figure 3a). In the spectra of
THF solutions of 1-4, two bands at 750 and 850 nm were
observed in the long-wave region, while only one band was
registered at 550 nm (Figure 3b). The mentioned differences in
the UV/Vis spectra are most probably caused by the structural
differences of the complexes in the two different solvents,
namely in the NiI environment since the longwave bands can be
assigned to d-d NiI transitions.28
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investigation for all the synthesized complexes are discussed
below.
Figure 3. Absorption spectra recorded in the course of electrochemical
reduction of III in MeCN (a) and THF (b) (SE is Bu4NBF4). In both cases black
curves correspond to the starting complex III prior electrochemical reduction.
Figure 4. Cyclic voltammogramms for a 1.5 mM solution of 3 in MeCN.
Conditions: a working electrode was glassy carbon; an auxiliary electrode was
Pt; Bu4NBF4 was used as supporting electrolyte (SE).
Chemical pathway to the one-electron reduction products
In order to isolate the one-electron reduction products, the
chemical pathway was performed. Thus,
a comproportion
reaction between 1 equiv. of NiII salt, 1 equiv. of Ni(COD)2 and 2
equiv. of dpp-bian in MeCN and THF led to the one-electron
reduced nickel complexes with the simplified formula [(dpp-
bian)Ni(X)]n {where n = 1, 2; X = BF4 (1), I (2), Br (3), Cl (4)}. The
complexes obtained in MeCN and in THF were named as 1a-4a
and 2b-4b, respectively. The single crystals of the one-electron
reduced nickel complexes 2a-3a and 2b-3b were grown by slow
diffusion of diethyl ether into acetonitrile or THF solutions at –
33 °C, respectively. It turned out that compounds 2a and 3a
(Figure 5) represent monomeric molecular complexes with the
nickel atom being tetrahedrally coordinated. It should be
mentioned that one MeCN molecule was coordinated by the
nickel center. The complexes 2b and 3b are halogen-bridged
binuclear complexes (Figure 6).12 Both metallocycles in 2b and
3b are approximately planar with the nickel atoms having a
distorted tetrahedral geometry. Selected interatomic distances
for the complexes in the solid state are listed in the caption of
Figures 5 and 6. The lengths of bonds N1–C1, N2–C2, and C1–
C2 of dpp-bian in the complexes should have indicated the
presence of monoanionic radical in complexes 2a, 3a, 2b, and
3b.29 Similar regularity was also observed earlier for complex
dpp-bianNi-Cp.25 However, the EPR spectrum of the complexes
indicated the presence of NiI state and showed no signal
corresponding to a dpp-bian radical-anion. Details of our EPR
Figure 5. ORTEP projections of complexes 2a (a) and 3a (b) showing
anisotropic displacement ellipsoids at the 50 % probability level. Hydrogen
atoms and solvent molecules are omitted for clarity. Selected interatomic
distances for 2a [Å]: I1–Ni1 2.5507(5), Ni1–N1 1.955(3), Ni1–N2 1.952(3),
Ni1–N3 1.965(3), N1–C1 1.309(4), N2–C2 1.309(4), C1–C2 1.448(4). Selected
interatomic distances for 3a [Å]: Br1–Ni1 2.3651(6), Ni1–N1 1.958(3), Ni1–N2
1.964(3), Ni1–N3 1.970(3), N1–C1 1.317(4), N2–C2 1.307(4), C1–C2 1.440(5).
Interesting, stepwise re-dissolution of the solid samples, which
were obtained after solvent removal, in THF, MeCN, and again
in THF led to predictable changing in the UV/Vis spectra. Initially,
the spectra with bands at 480, 550, and 800 nm were observed
in MeCN. After removing MeCN and dissolution of the solid
residue in THF, the new bands at 550, 750, and 800 nm
appeared. Then, removing THF and dissolution of the solid in
MeCN led to returning the initial spectra (Figure 7). These
experiments indicate reversible solvent-induced monomer to
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dimer conversions for NiI complexes. Further EPR experiments
also confirmed this observation.
Figure 7 UV/Vis spectra of 3 in MeCN (black), 3 resolved in THF (red), 3 in
MeCN (after previous dissolution in THF) (blue). See details in the text.
Electronic paramagnetic resonance study
The signals in the EPR spectra corresponding to the NiI state,
namely complexes with the metal-centered unpaired electron
localization10 were observed for 1-4 in MeCN and THF (Table 2).
At 295 K the isotropic spectra of 1 (both in MeCN and THF) are
characterized with g-value of 2.20 (Figure 8).30 The anisotropic
spectra with three lines were observed at 215 K (g||=2.13 and g┴
= 2.27; gaverage=2.20) in both solvents.
Figure 6. ORTEP projections of complexes 2b (a) and 3b (b) showing
anisotropic displacement ellipsoids at the 50 % probability level. Hydrogen
atoms and solvent molecule are omitted for clarity. Selected interatomic
distances for 2b [Å]: Ni1–Ni1i 3.4282(5), I1–Ni1 2.5842(3), I1–Ni1i 2.5985(3),
Ni1–N1 1.9527(10), Ni1–N1ii 1.9526(10), N1–C1 1.3103(14), C1–C1ii 1.454(2);
equivalent atoms are labeled by the superscripts i (–x+1, –y, –z+1) and ii (x, y,
–z+1). Selected interatomic distances for 3b [Å]: Ni1–Ni1i 3.2996(6), Br1–Ni1
2.4091(4), Br1i–Ni1 2.4118(4), Ni1–N2 1.9427(18), Ni1–N1 1.9500(18), N1–
C1 1.308(3), N2–C2 1.318(3), C1–C2 1.449(3); equivalent atoms are labeled
by the superscript i (–x+1, –y+1, –z+1).
Figure 8. Temperature dependence of EPR spectra for 1 in MeCN.
The halogen-bridged dimers 2b-4b in THF at room temperature
were characterized with a single EPR signal with the g-factor of
2.26 and ΔH of 30-50 G (see ESI). The mentioned value is also
typical and corresponds to NiI state.30-34 At the temperature of
the frozen solution, the intensity of EPR was decreased, that is
consistent with the dimeric form of complexes. The anisotropic
spectra were complicated by additional signals with gaverage of
2.36.34
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In MeCN complexes with the halogenide co-ligands were
characterized by two signals with g-factors of 2.20 and 2.26 and
ΔH of 40-50 G at room temperature (Figure 9) Thus, it can be
assumed that two types of complexes with the monomeric (g-
factor = 2.20) and halogen-bridged dimeric (g-factor = 2.26)
structures are in equilibrium in MeCN solution at room
temperature. The intensity of the signal for dimer complex
decreases at lowering temperature, whereas the signal for
monomer complex raises. In case of the iodide co-ligand the
monomeric form in MeCN is predominant (See ESI). The EPR
spectra of halogenide complexes in MeCN measured in the solid
state (195 K and lower) gave a single broad line (ΔH = 150 G)
with g = 2.189 (See ESI).35
Interesting features for fluoride complexes were observed in
EPR spectra. So, expected two signals in EPR spectra were
observed at room temperature for the MeCN solutions (Table 2,
Figure 10). The first signal with g-factor of 2.20 obviously
corresponds to monomer complex with metal-centered unpaired
electron localization similarly to other halide complexes with
dpp-bian. The second signal is a multiline pattern with g-factor of
2.003 and ΔH of 10 G corresponding to the dpp-bian anion
radical in nickel complexes (See ESI).11,12 Two forms are also in
equilibrium at room temperature. But no shift of equilibrium from
one form to another was registered during changing the
temperature. At low temperatures, anisotropic spectra with
gaverage=2.20 and anion radical signal were found. In the EPR
spectra of fluoride nickel complex in THF only one multiline
signal with g-factor of 2.003 and ΔH of 10 G (Figure 10)
corresponding to the complex with ligand-centered unpaired
electron localization was observed.11,12
Similarly to the above-mentioned UV/Vis experiments, one EPR
signal with g-factor of 2.26 transformed into two signals with g-
factors of 2.20 and 2.26 and vice versa after stepwise re-
dissolution of the solid samples, which were obtained after
removing a solvent, in MeCN and then in THF.
Figure 9. EPR spectra of 3 in MeCN at room temperature.
Figure 10. EPR spectra of 5 in MeCN (red) and THF (blue) at room
temperature
Table 2. . Magnetic resonance parameters for 1-5 in MeCN and THF
The values of the effective moment for the solid reduced nickel
complexes 2-5 were measured. Supposing the presence of two
non-interacting Ni–Hal (Hal = I, Br, Cl, F) ions, then to calculate
295 K
175 K
Complex
g value
ΔH (G)
g value
2.27, 2.19, 2.13
2.26
effective moments per dimer we have to divide µeff by 2. Thus,
√
1
2.200
2.200;
2.260
2.260
2.200;
2.003
2.003
30-40
30-40;
60
70-80
35-40;
10
the values µeff for 2-4 were found to be in the range from 2.1 to
2-4 in MeCN
2-4 in THF
5 in MeCN
5 in THF
2.3 at 2 K that close to the effective moment of spin S=1/2 (µeff
=
2.36, 2.30, 2.15
2.27, 2.19, 2.13;
2.003
1.7) and support by EPR data. In case of complex 5 with the
fluoride co-ligand, the value µeff was found 4.6 at 2 K and 4.4 at
300 K. Both Ni ions and the ligand are supposed to have a non-
zero magnetic moment. NiII ions have S = 1 and the ligand S =
1/2. Therefore it is difficult to estimate their particular
contribution to the total effective magnetic moment. Thus, it is
clearly established that the electronic structures of paramagnetic
fluoride nickel complexes and other halide nickel complexes are
different. Thereby the detailed investigation of magnetic
properties of obtained complexes is the focus of our further
investigations.
10
2.003
One-electron reduced nickel complex with the fluoride co-
ligand
Complex 5 with the fluoride co-ligand was prepared by a
comproportion reaction between 1 equiv. of NiF2, 1 equiv. of
Ni(COD)2 and 2 equiv. of dpp-bian in both THF and MeCN.
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Computations
density distribution and C–N bond lengths of 1.32-1.33 Å (Figure
12) clearly indicate that SBIAN=1/2 and SNi=1.
For complexes 2a-4a the structures optimized with m = 2 are
energetically more stable than in the case of m = 4. The energy
difference between the low-spin and high-spin species increases
with the increase of the co-ligand size (Cl: 2.3 kcal mol-1, Br: 2.5
kcal mol-1, I: 3.0 kcal mol-1). In contrast, for 5a the computations
predict the form with higher multiplicity to be more stable by 8
kcal mol-1.
For deeper understanding of experimental results quantum
chemical computations were carried out. For all the complexes
three variants of spin and electron density distribution can be
envisaged: (i) BIAN0, Ni+ with SNi = 1/2, and multiplicity for the
whole complex m = 2; (ii) BIAN- with SBIAN = 1/2, Ni2+ with S = 0,
m = 2; and (iii) BIAN- with SBIAN = 1/2, Ni2+ with SNi = 1, m = 4. To
model these cases, monomeric complexes 2a-5a were
optimized with two different multiplicities: m = 2 (S = 1/2) and m
= 4 (S = 3/2).
Thus, according to quantum chemical computations, in the case
of complex 5a variant (iii) of spin and electron density
distribution is realized with one unpaired electron at bian1- and
high-spin state of Ni2+ (SNi=1). This corresponds to EPR
experiments, demonstrating that unpaired electron in 5a is
delocalized at the ligand. High-spin Ni2+ ion is very difficult to
observe in the spectra as it appears as a very broadened signal.
In the case of complexes 2a-4a computations predicted spin
density mostly localized on metal center, however partly shifted
to bian, which is corroborated by EPR experiments. Moreover,
this explains why C–N bond lengths found in X-ray diffraction
analysis are intermediate between values typical for bian0 and
for bian1-. The difference in electronic structures of 5a compared
to the other complexes in the series is also confirmed by
analysis of IR spectra of bian and complexes 1-5 in THF (see
ESI). Unfortunately, THF has its own spectrum which masks
region below 1500 cm-1. Nevertheless, in the THF transmission
window IR spectra of complexes 1-4 contain the band at 1533
cm-1, which is absent in the spectra of the ligand and complex 5.
Computations show that this band is associated with
asymmetrical C–N stretching vibrations and predicted at 1553
cm-1 for the most stable form of complex 3a with m=2, whereas
in the spectrum of the most stable form of complex 5a with m=4
it is shifted to low-frequency range (1510 cm-1) and mixed with
CH deformations, and in the spectra of bian it is predicted at
1770 cm-1. Thus the corresponding band in the spectrum of 5 in
THF is apparently masked by THF absorbance.
Optimization of the structures of complexes 2a-4a with m = 2 led
to geometries with a tetrahedral environment of the Ni center,
whereas the optimized structure of 5a had an almost square-
planar geometry (Figure 9). Attempts to optimize a square-
planar structure for 2a-4a (corresponding to Ni+ with SNi = 1/2 or
Ni2+ with S = 0) as well as a tetrahedral structure for 5a were
unsuccessful. According to literature data C–N bond lengths can
be used as indicators of bian charge: in a neutral ligand they
amount to 1.28-1.29 Å, whereas in a monoanionic form they
increase to 1.34 Å.29
The computed C–N distances in complexes 2a-4a are equal to
1.31-1.32 Å, whereas in 5a they slightly decrease to 1.29-1.30 Å.
For complexes 2a-4a spin density at Ni ions is predicted to be
1.5, and at the NCCN fragments it amounts to –0.5 (Figure 11).
The corresponding values computed for complex 5a equal to 0.8
and 0.2, respectively. Thus, when m = 2 is defined, both the spin
density distribution and C–N bond lengths suggest that in 5a the
ligand is neutral and one unpaired electron is localized at Ni+ ion,
i.e. variant (i) (vide supra) is realized. On the same ground, we
can conclude that in the case of 2a-4a the computations suggest
a variant intermediate between cases (i) and (ii), in other words
in these complexes partial electron density transfer from nickel
to bian is expected.
Figure 11. Optimized structures of the monomeric and dimeric complexes with
m=2 and m=3, respectively: 2a - 4a,b (a) and 5a,b (b). X is Cl, Br or I; R is 2,6
–di-isopropylphenyl.
Figure 12. Spin density distribution in the most stable structures of 3a (m = 2)
and 5a (m = 4).
In the case of m = 4, optimization of the structures of complexes
2a-5a led to a tetrahedral Ni environment, similar to that shown
in Figure 11a. For all the complexes spin density is close to 1.7
at Ni centers and to 0.9 at the NCCN part of bian. Both the spin
Similar tendencies are predicted also for dimeric forms,
calculated only for species with bromide and fluoride ligands (3b
and 5b, respectively). For these complexes, multiplicities m = 3
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(S = 1) and m = 7 (S = 2) were considered. Again optimization of
complex 5b structure at m = 3 resulted in the almost square-
planar environment of the metal center, whereas in 3b it
remained in a tetrahedral form. Relative energies of various
forms are predicted to be also similar to the monomeric case: for
3b the state with m = 3 is more stable than the form with m = 7
by 12 kcal mole-1, whereas for 5b the high-spin state is more
energetically advantageous by 12 kcal mole-1. Thus, according
to computations for both monomeric and dimeric complexes,
fluoride ligand stabilizes forms with higher multiplicity and with
spin density being localized not only at the Ni ion but also at the
bian ligand. For all the other co-ligands, the forms with lower
multiplicity and spin density localized mostly on the Ni ions are
more energetically stable.
unpaired electron at bian1- and high-spin Ni2+ (SNi = 1) is realized.
The discovered features about structural and electronic diversity
of reduced nickel complexes may be useful tool for analysis and
planning the catalytic systems based on them.
Acknowledgements
The authors gratefully acknowledge the RFBR (grant № 19-03-
00084) for financial support and the CSF-SAC FRC KSC RAS
for providing necessary facilities to carry out this work.
For the most energetically stable high-spin form of dimeric
complex 5b with spin density localized both on Ni and bian
fragments the following broken-symmetry (BS) states were also
considered: (a) antiparallel spins at bians and parallel spins at Ni
ions, m = 5; (b) antiparallel spins at Ni ions and parallel spins at
bians, m=3; and (c) antiparallel spins both at Ni ions and bians,
m=1. All these cases are energetically very slightly less stable
compared to m=7 state, indicating very weak ferromagnetic
interactions of less than 2 cm-1.
Keywords: nickel • dpp-bian • reduction • electrochemistry •
cyclic voltammetry
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The one-electron reduction reaction of
nickel(II) complexes with dpp-bian has
been studied. Monomeric and dimeric
NiI complexes have been obtained
with a BF4– anion, iodide, bromide and
chloride anionic co-ligands. For
fluoride nickel complexes, unpaired
electron localization on dpp-bian
moiety was found.
Vera V. Khrizanforova,* Robert R.
Fayzullin, Vladimir I. Morozov, Ildar F.
Gilmutdinov, Anton N. Lukoyanov, Olga
N. Kataeva, Tatiana P. Gerasimova,
Sergey A. Katsyuba, Igor L. Fedushkin,
Konstantin A. Lyssenko, Yulia H.
Budnikova
Page No. – Page No.
Regularities of One-Electron
reduction of Acenaphthene-1,2-
Diimine Nickel (II) Complexes
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