Mono- and binuclear arylnickel complexes of the α-diimine bridging ligand 2,2' -bipyrimidine (bpym)

Article abstract of DOI:10.1002/ejic.200900945

The mono- and binuclear organometallic Ni^II complexes [(μ-bpym){Ni(Mes)Br}_n] (bpym = 2,2'-bipyrimidine; n = 1 or 2; Mes = mesityl = 2,4,6-trimethylphenyl) were prepared, and characterised electrochemically and spectroscopically (NMR, UV/Vis/NIR) in detail. The long-wavelength absorptions for the binuclear complex reveal, a marked electronic coupling of the two metal centres over the ligand bridge via their lowlying π*-orbitals. While the mononuclear complex, undergoes rapid dissociation of the bromido ligand after one-electron reduction the binuclear derivative exhibits reversible reductive electrochemistry and both of them yield stable radi-cal anionic complexes with mainly bpym ligand centred spin density as shown by EPR spectroscopy of the free ligand bpym and the nickel complexes. The molecular structure of the binuclear bpym complex [(μ-bpym)(Ni(Mes)Br} ₂] was studied by EXAFS in comparison to the mononuclear analogue [(bpym)Ni(Mes)Br] revealing markedly increased Ni-C/N distance of the first coordination, shell for the binuclear derivative suggesting an optimum overlap for the mononuclear complex, while two nickel complex fragments (Ni-(Mes)Br} are seemingly too large to fit into the bis-chelate coordination site.

Full text of DOI:10.1002/ejic.200900945

FULL PAPER
DOI: 10.1002/ejic.200900945
Mono- and Binuclear Arylnickel Complexes of the α-Diimine Bridging Ligand
2,2Ј-Bipyrimidine (bpym)
Axel Klein,*[a] Ann-Kathrin Schmieder,[a] Natascha Hurkes,[a] Claudia Hamacher,[a]
Andreas O. Schüren,[a] Martin P. Feth,*[b] and Helmut Bertagnolli[b]
Keywords: Nickel / Electrochemistry / EPR spectroscopy / N ligands / Radical complexes / Spectroelectrochemistry
The mono- and binuclear organometallic NiII complexes [(µ-
bpym){Ni(Mes)Br}n] (bpym = 2,2Ј-bipyrimidine; n = 1 or 2;
Mes = mesityl = 2,4,6-trimethylphenyl) were prepared and
characterised electrochemically and spectroscopically (NMR,
UV/Vis/NIR) in detail. The long-wavelength absorptions for
the binuclear complex reveal a marked electronic coupling
of the two metal centres over the ligand bridge via their low-
lying π*-orbitals. While the mononuclear complex undergoes
rapid dissociation of the bromido ligand after one-electron
cal anionic complexes with mainly bpym ligand centred spin
density as shown by EPR spectroscopy of the free ligand
bpym and the nickel complexes. The molecular structure of
the binuclear bpym complex [(µ-bpym){Ni(Mes)Br}2] was
studied by EXAFS in comparison to the mononuclear ana-
logue [(bpym)Ni(Mes)Br] revealing markedly increased Ni–
C/N distance of the first coordination shell for the binuclear
derivative suggesting an optimum overlap for the mononu-
clear complex, while two nickel complex fragments {Ni-
reduction the binuclear derivative exhibits reversible re- (Mes)Br} are seemingly too large to fit into the bis-chelate
ductive electrochemistry and both of them yield stable radi-
coordination site.
plexes with α-diimines have been carried out.[9–17] We have
Introduction
contributed to this with the investigation of a number of
∧
∧
Organometallic nickel complexes with α-diimine (diaz- organonickel complexes [(N N)Ni(Mes)X] (N N = α-di-
ene) ligands like 2,2Ј-bipyridine (bpy), 1,10-phenanthroline imine ligands, Mes = mesityl = 2,4,6-trimethylphenyl, X =
(phen) or diazabutadienes (R-DAB) have gained an enor- halides) with various diimine ligands. Their strucures and
mous interest in the last decade. This is mainly due to a reactivity towards ligand exchange reactions have been
number of important catalytic processes like olefin oligo- studied using X-ray diffraction and absorption spec-
or polymerization, olefin/CO co-polymerization,[1,2] and troscopy,[13,14] their photophysics and photochemistry were
various (electro)catalytic C–C coupling reactions.[3–10] explored by a combination of multiple spectroscopy and
Paralleling their use in catalysis, fundamental studies on quantum chemical calculations,[13,15,16] and finally, their re-
structures and electronic properties of organonickel com- dox chemistry was investigated,[16,17] with respect to the ap-
Scheme 1. Schematical drawing of the mononuclear complex [(bpym)Ni(Mes)Br] (left) and the two possible stereoisomers of the binuclear
complex [(µ-bpym){Ni(Mes)Br}2] (right) with partial numbering.
[a] Department für Chemie, Institut für Anorganische Chemie,
Universität zu Köln,
plication of such systems in electrocatalytical C–C coupling
reactions.[6] From theses studies we have a clear picture of
the crucial role of the diimine ligand in these complexes.
The lowest unoccupied MOs are mainly ligand(π*) centred,
with the consequences, that both the intense colours (long-
wavelength absorption bands) and the relatively low first
Greinstraße 6, 50939 Köln, Germany
E-mail: axel.klein@uni-koeln.de
[b] Institut für Physikalische Chemie, Universität Stuttgart,
Pfaffenwaldring 55, 70569 Stuttgart, Germany
E-mail: martinphilipp.feth@sanofi-aventis.com
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.200900945.
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Arylnickel Complexes of a α-Diimine Bridging Ligand
and second reduction potentials are strongly dependent on peak, which occurs at about 8337 eV, is due to a 1sǞ4pz
the nature of the diimine ligand.
transition with shakedown contributions.[21] Between both
These findings have promoted a study on the mono and complexes clear differences in the Ni-XANES region can
binuclear complexes of the α-diimine (bis)chelate ligand be found, indicating structural differences in the local envi-
2,2Ј-bipyrimidine [(bpym){Ni(Mes)Br}n] (n = 1 or 2; see ronment of the absorbing nickel atoms of the complexes.
Scheme 1), since we expected that the electronic coupling of
two metal centres might greatly enhance these interesting
properties.[18,19] In view of catalytical applications of such
complexes binuclear derivatives are supposed to addition-
ally offer higher thermal stability, as has been concluded in
recent work e.g. by Jin and Huang.[2] The two metal atoms
in complexes of bpym are oriented face to face with metal–
metal distances of about 5.5–6 Å. This is too long for a
direct metal–metal interaction, however the metals could
interact via the low-lying lowest unoccupied molecular or-
bitals (LUMOs) of bpym.[19]
Results and Discussion
Synthesis and General Properties
Figure 1. Comparison of the Ni-K-edge XANES spectra of [(bpym)-
Ni(Mes)Br] and [(µ-bpym){Ni(Mes)Br}2].
The two neutral complexes [(bpym){Ni(Mes)Br}n] (n = 1
or 2) were synthesised from the precursor complex trans-
[(PPh3)2Ni(Mes)Br] by ligand exchange reactions in toluene
The fitting of the Ni-K-edge EXAFS spectra (Figure 2)
of both investigated complexes was performed using a 3
shell model, in which the first coordination shell at about
1.9–2.0 Å consists of the two coordinating nitrogen atoms
of the diimine ligand and the carbon atom of the mesityl
group, the second shell of the bromine backscatterer at
about 2.3 Å and the third coordination shell at about 2.8 Å
of the further backbone-carbon-backscatterers of the
mesityl and diimine ligand. Because of the nearly equal dis-
tances and backscatter behaviour of the nitrogen and car-
bon backscatterers in the first coordination shell, only one
shell was fitted with nitrogen amplitude- and phase-func-
tions. The Br-K-edge EXAFS spectra (Figure 3) were fitted
with one coordination shell of nickel at ca. 2.3 Å.
1
solution and were analysed by H NMR, 13C NMR and
elemental analysis (see Exp. Sect.). The stability of the two
complexes was studied by recording the absorption spectra
in various solvents to establish the solvatochromism of the
long-wavelength absorption bands (see below). The two
compounds are stable in solvents like CH2Cl2, THF, tolu-
ene, acetone and DMF towards ligand exchange reactions
(dissociation of the bromido ligand).[14,16] In nitriles,
DMSO and alcohols the complexes undergo the same de-
composition reaction as reported for a number of monomo-
∧
∧
lecular diimine complexes [(N N)NiMesX] (N N = vari-
ous diimine ligands, X = halogenido).[14–16] This will be im-
portant for the further measurements in THF or DMF
solutions (see below).
1H NMR spectra revealed that the binuclear complex
was obtained as 1.45 trans/1 cis mixtures of the two stereo-
isomers depicted in Scheme 1. Recrystallisation has no ef-
fect on the isomeric ratio. The complete spectral assignment
of the isomers is given in the Experimental Section.
EXAFS Spectroscopy
Unfortunately, none of the two complexes could be ob-
tained in form of single crystals. A suitable sample of the
binuclear complex [(µ-bpym){Ni(Mes)Br}2] was thus sub-
mitted to an EXAFS study. The data can be compared to
the recently reported data of the mononuclear complex.[15]
Figure 1 illustrates the Ni-K-edge XANES spectra for
[(bpym)Ni(Mes)Br] (complex 2d in ref.[15]
) and [(µ-
bpym){Ni(Mes)Br}2]. In both spectra the typical two pre-
edge features, identifying square planar NiII complexes, can
be observed.[20] The pre-peak at about 8333 eV can be as-
Figure 2. Experimental (solid line) and calculated (dotted line) k3
χ(k) functions (a,c) and their Fourier transforms (b,d) of solid
[(bpym)Ni(Mes)Br] (a,b) and [(µ-bpym){Ni(Mes)Br}2] (c,d) at the
signed to a 1sǞ3d electron transition, while the second Ni-K-edge.
Eur. J. Inorg. Chem. 2010, 934–941
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935

A. Klein, M. P. Feth et al.
Table 1. Structural parameter of the solid complexes determined from the Ni-K- and Br-K-edge EXAFS spectrum.
FULL PAPER
[a]
r /Å
N
σ /Å
∆E0 /eV
k range /Å–1
(fit index, R)
[(bpym)Ni(Mes)Br]
Ni–C/N
Ni–Br
Ni–C
1.91Ϯ0.02
2.29Ϯ0.02
2.82Ϯ0.03
2.29Ϯ0.02
2.8Ϯ0.3
1.2Ϯ0.2
2.8Ϯ0.8
0.9Ϯ0.1
0.056Ϯ0.006
0.054Ϯ0.008
0.063Ϯ0.019
0.051Ϯ0.007
27.6
3.70–12.50
(23.7)
Complex 2d in ref.[15]
Br–Ni
13.5
22.3
4.00–12.50
(30.6)
3.65–15.80
(26.2)
[(µ-bpym){Ni(Mes)Br}2]
Ni–C/N
Ni–Br
Ni–C
2.03Ϯ0.02
2.30Ϯ0.02
2.82Ϯ0.03
2.30Ϯ0.02
2.6Ϯ0.3
0.8Ϯ0.1
1.7Ϯ1.4
1.0Ϯ0.1
0.097Ϯ0.009
0.063Ϯ0.009
0.092Ϯ0.027
0.077Ϯ0.008
This work
Br–Ni
15.7
3.70–15.00
(47.0)
[a] Absorber–backscatterer distance r, coordination number N, Debye–Waller factor σ with their calculated deviation, shift of the threshold
energy ∆E0 and the fit index R.
shell of the binuclear complex compared to the mononu-
clear derivative is probably due to the difficulties of the
bpym bridging ligand to coordinate two metal atoms in an
optimal way. For mononuclear complexes of bpym the bite
angle N–Ni–N can be optimised for the metal and the li-
gand (preferably about 80°) by distorting the bpym ligand
(enlargement of the N–C–C–N dihedral angle on the side
of the metal) or by tilting the pyrimidine planes towards
the coordination plane. For a binuclear complex only the
latter option is principally available. However, for electronic
reasons completely planar complexes were expected and in-
deed found.[19] Thus, as a last option to accommodate two
metal atoms, the M–ligand distances increase. Such results
were also reported earlier for similar binuclear com-
plexes.[19]
Figure 3. Experimental (solid line) and calculated (dotted line) k3
χ(k) functions (a,c) and their Fourier transforms (b,d) of solid
[(bpym)Ni(Mes)Br] (a,b) and [(µ-bpym){Ni(Mes)Br}2] (c,d) at the
Br-K-edge.
The Br–Ni distance in both complexes remains constant
in the order of magnitude, however its noteworthy to men-
tion that the Debye-Waller factor of the coordination shell
of bromine is increased for [(µ-bpym){Ni(Mes)Br}2] com-
pared with [(bpym)Ni(Mes)Br]. This might be due to two
slightly different distances of Ni–Br in the binuclear com-
pound in comparison with one Ni–Br distance in [(bpym)-
Ni(Mes)Br]. The Ni–C-distance of the third coordination
shell remains constant in both complexes, whereas the coor-
dination numbers decreases from ca. 3 in [(bpym)Ni(Mes)-
Br] to ca. 2 in [(µ-bpym){Ni(Mes)Br}2].
Structural parameters of the investigated complexes de-
termined by curve fitting analyses of the EXAFS spectra at
the Ni-K- and Br-K-edge are summarised in Table 1.
The results of the EXAFS analysis (distances, coordina-
tion numbers and Debye-Waller factors) at both edges of
the binuclear complex are the average values for two nickel
atoms present in the compound.
Significant structural differences between the mononu-
clear complex [(bpym)Ni(Mes)Br] and the binuclear deriva-
tive [(µ-bpym){Ni(Mes)Br}2] were found from the analyses
of the Ni-K-edge EXAFS spectra. In the binuclear complex
the Ni–C/N distance of the first coordination shell is 0.12 Å
longer than in the mononuclear species. This can be seen
clearly in the Fourier transformed Ni-K-edge EXAFS spec-
trum of [(µ-bpym){Ni(Mes)Br}2] (Figure 2), where the sig-
nals of the first (carbon and nitrogen backscatterers) and
second (bromine backscatterer) coordination shells are no
longer separated like in the Fourier transformed spectra of
[(bpym)Ni(Mes)Br]. The Debye–Waller factor σ is also in-
Electrochemistry
The electrochemistry of the mononuclear complex has
been described before.[17] The complex behaviour of the re-
duction waves can be explained with the rapid dissociation
of the bromido ligand from the mono-reduced radical com-
plex; see Equation (1).[17]
[(bpym)Ni(Mes)Br] + e– Ǟ [(bpym)Ni(Mes)Br]·–
[(bpym)Ni(Mes)]· + Br– (E + C)
Ǟ
(1)
The subsequently formed solvent complex [(bpym)Ni-
creased from 0.056 Å (N = 2.8) in the mononuclear com- (Mes)(Solv)]· undergoes further reductions. The presence of
pound to 0.097 Å (N = 2.6) in the binuclear complex. The a solvent complex can be inferred from the marked depen-
increase in the Ni–C/N distance of the first coordination dence of the reduction potentials on the solvent (Table 2),
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Arylnickel Complexes of a α-Diimine Bridging Ligand
Table 2. Electrochemical data of the nickel complexes [(bpym){NiMesBr}n] (n = 1 or 2).[a]
n
Epa
Ox2
Epa
Ox1
E1/2 (∆Epp) or Epc
Red1
E1/2 (∆Epp) or Epc
Red2
Epc
Red3
Solvent / T [°C]
1
1
1
–
–
–
0.53
0.50
0.67
0.31
0.31
0.41
0.32
0.30
0.38
–1.72 irr.
–1.58 irr.
–1.53 (81)
–1.12 (75)
–1.10 (84)
–1.11 (69)
–2.10 irr.
–1.89 irr.
–2.26 irr.
–1.88 (92)[c]
–1.85 irr.
–1.77 (78)
–2.58
–2.32
–
–3.07
–3.05
–3.00
THF / 25
DMF / 25
DMF / –60
THF / 25
DMF / 25
DMF / –50
2[b]
2[b]
2[b]
[a] Potentials from cyclic or square wave voltammetry in 0.1  nBu4NPF6/solvent solutions (in V) vs. ferrocene/ferrocinium. Half wave
potentials E1/2 given with peak-to-peak separation ∆Epp in parentheses (in mV) for reversible processes; cathodic Epc, or anodic Epa peak
potentials for irreversible processes. [b] The measurements were performed on a 1.45:1 trans/cis isomeric mixture. [c] The wave is only
partly reversible with a current ratio Ipa/Ipc of 0.3.
however due to the irreversible nature of the waves these Absorption Spectroscopy
values are somewhat arbitrarily, since they also depend on
The absorption spectra of both complexes (Figure 4) are
the analyte concentration. At low temperature the first re-
dominated by two broad bands of medium intensity in the
duction occurs reversibly indicating, that the cleavage of the
visible region and intense bands in the UV region. The two
bromido ligand is retarded. The underlying mechanism of
broad bands show negative solvatochromism, while the UV
the bromide dissociation [Equation (1)] is the primarily di-
bands are solvent invariant. This was established be mea-
imine-centred reduction which is supported by spectroelec-
surements in various solvent and correlation of the band
trochemistry[17] and in line with findings for related transi-
energies to the E*MLCT solvent parameter established by
tions metal complexes of bpym.[18,19] The second step is a
Manuta and Lees for metal-to-ligand charge-transfer bands
more or less rapid cleavage of the bromido ligand as a result
(for details see Supporting Information).[22]
of partial transfer of the additional electron density to the
metal. The latter idea is supported by findings on a series
∧
of mononuclear complexes [(N N)Ni(Mes)Br], with di-
∧
imine ligands (N N) ranging from the rather basic but only
weakly accepting ligand 3,4,7,8-tetramethyl-1,10-phen-
anthroline (tmphen) to the excellent acceptor 2,2Ј-bipyr-
azine (bpz). In this series the first reduction gained some
reversibility with increasing acceptor capability of the di-
imine ligand. Indeed, E1 for the bpym complex is partly
reversible, while e.g. for the tmphen derivative the first re-
duction is completely irreversible.[17]
The electrochemistry of the binuclear complex exhibits a
first reversible reduction wave and depending on the solvent
Figure 4. Absorption spectra of [(bpym)Ni(Mes)Br] (-----) and [(µ-
bpym){Ni(Mes)Br}2] (____) in CH2Cl2 solution.
a second irreversible (DMF) or partly reversible (THF)
wave and all these waves occur at far higher (less negative)
potentials compared to the mononuclear derivative. Follow-
ing the explanations for the mononuclear complex,[17] the
observed reversibility implies that the bromide dissociation
is largely diminished for the binuclear complex if not com-
pletely inhibited. The reason lies in the coordination of a
second metal ion (for the binuclear complex), which leads
to the stabilisation of the ligand’s LUMO,[19] thus leading
to higher reduction potentials and to a decreased transfer
of electron density from the reduced ligand (in the reduced
complex) to the nickel atoms (which is the driving force for
the Br dissociation).
On anodic scans broad irreversible oxidation waves were
observed, which account for one electron for the mononu-
clear and two electrons for the binuclear complex. Based on
similar results from mononuclear derivatives, we assign
them to metal-centred oxidations NiII/NiIII for the two
nickel atoms.[16,17] From the wave of the binuclear complex
it is not discernible if both nickel atoms are oxidised at the
Based on the band intensities, the observed solvatochro-
mism and studies of related mononuclear complexes[14] we
can assign the bands at high energy (band system I in
Table 3) to ligand-centred (π-π*) transitions, while the two
long-wavelength band systems (II and III) result from
charge transfer transitions (mainly MLCT).
Table 3. Absorption maxima of parent nickel complexes
[(bpym){Ni(Mes)Br}n] (n = 1 or 2).[a]
λ /nm
II
n
I
III
1
2
252 (28), 265 sh
246 (42), 279 sh
358 (3.8), 411 sh
391 sh, 414 (7.6)
494 sh, 511 (3.0)
483 sh, 567 sh, 624
(3.9), 690 sh
[a] Absorption maxima λ in nm as measured in CH2Cl2 solution,
main maxima are in italics, extinction coefficients ε in 1000 –1 cm–1
are given in parentheses.
Comparison of the two complexes reveals that the two
same potential (no electronic coupling), or if a mixed-valent central bands shift from 358 and 511 nm (mononuclear) to
state occurs (coupling of the metal centres). 414 and 624 nm (binuclear) with markedly higher intensities
Eur. J. Inorg. Chem. 2010, 934–941
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A. Klein, M. P. Feth et al.
FULL PAPER
for the binuclear complex. This red-shift is commonly ob-
served for related mono- and binuclear complexes of bpym
and can be assigned to metal–metal coupling via the
LUMO of the bridging ligand.[18,19]
EPR Spectroscopy
EPR spectra were recorded at ambient temperature from
electrochemically generated radical anions of the two com-
plexes. From the electrochemistry it can be expected that
the reduced mononuclear complex [(bpym)Ni(Mes)Br]·–
will readily cleave the bromido ligand and the neutral spe-
cies [(bpym)Ni(Mes)(Solv)]· (Solv = THF or DMF) will be
Figure 5. X-band EPR spectra of radical complexes generated from
[(µ-bpym){Ni(Mes)Br}2] in THF/nBu4NPF6 in situ electrolysis at
298 K and measured at 298 K (left, together with spectral simula-
tion) and in glassy frozen solution at 110 K (right).
detected. In both solvents well resolved spectra were ob- strongly indicate that the singly occupied molecular orbitals
served with hyperfine splitting (HFS) due to coupling to (SOMOs) of the binuclear and the mononuclear species are
the ligands N and H atoms. The simulation of the spectra mainly centred on the organic ligand. The observed in-
yielded spectral parameters (see Table 4), which are typical crease of the coupling of the unpaired electron with the N
for transition metal complexes with a reduced bpym li- atoms and the H4/H6 atoms of the ligand in the coordi-
gand.[19e,23,24] The spectral parameters of the species in nated case are due to metal–ligand interaction. In contrast
DMF or THF solution are almost identical underlining to this, in DMF solution only an ill-resolved spectrum was
that the influence of the solvent ligand in the detected spe- obtained, which could not be simulated. The two binuclear
cies [(bpym–)NiII(Mes)(Solv)]· is marginal. The binuclear species exhibit markedly different g values and we can con-
complex [(µ-bpym){Ni(Mes)Br}2] was also reduced in THF clude different radical complexes to be responsible for the
and DMF solution in which the first reductions occurs re- signals. It is reasonable to assume, that while in THF solu-
versibly on the time scale of the CV experiment. Neverthe- tion the complex remains coordinated by bromido co-
less, due to the long electrolysis time, partial bromide disso- ligands, in DMF we obtain solvated species under the elec-
ciation leading to [Br(Mes)Ni(µ-bpym)Ni(Mes)(Solv)]· or trolysis conditions.
[(µ-bpym){Ni(Mes)(Solv)2}]·+ cannot be excluded. In THF
The g values of all radicals are quite close to those of
solution a well-resolved EPR spectrum (Figure 5), which (bpym)·– (see Table 4), which strongly points to the bpym
was simulated with the values collected in Table 4 was ob- centred character of the radical complexes. A very suitable
served (more figures are provided in the Supporting Infor- parameter to estimate the metal contribution is the g an-
mation).
isotropy (∆g), as measured from spectra at low temperature
The coupling constants in the complexes are significantly (in glassy frozen solutions).[17,18c] For the mononuclear and
increased compared to the free ligand. In the binuclear binuclear complexes ∆g values ranging from 0.020 to 0.025
complex the values of the coupling are symmetrically dis- were obtained from unresolved spectra of rhombic sym-
tributed, whereas the mononuclear complex shows asym- metry (Figure 5 and Supporting Information), while the
metric distributed coupling constants. The same behaviour averaged gav values are identical to the isotropic g values,
has been reported for related Cr0 complexes,[19d,25] and testifying their identity. The absolute values of ∆g indicate
Table 4. EPR data of radical anionic ligand (bpym)·– and reduced (radical) complexes.[a]
Parent species
giso
aN1
aN3
aH4
aH5
aH6
solvent
[(µ-bpym){Ni(Mes)Br}2]
[(µ-bpym){Ni(Mes)Br}2]
[(bpym)Ni(Mes)Br]
[(bpym)Ni(Mes)Br]
bpym[c,d]
2.0044
2.0025
2.0038
2.0038
2.0030
2.0022
2.0022
2.0026
0.211
–
0.211
–
0.047
–
0.454
–
0.047
–
THF
DMF
THF
DMF
THF
DMF
DMF
THF
[b]
[b]
[b]
[b]
[b]
0.239
0.239
0.143
0.234
0.239
0.239
0.192
0.192
0.143
0.234
0.152
0.152
0.038
0.038
0.015
0.052
0.024
0.024
0.446
0.452
0.492
0.503
0.500
0.500
0.045
0.045
0.015
0.052
0.057
0.057
[(µ-bpym){Cr(CO)4}2][e]
[(bpym)Cr(CO)4][e]
[(bpym)Cr(CO)4][d]
[f]
[f]
gav
g1
g2
g3
∆g
[(µ-bpym){Ni(Mes)Br}2]
[(µ-bpym){Ni(Mes)Br}2]
[(bpym)Ni(Mes)Br]
2.0044
2.0025
2.0037
2.0158
2.0119
2.0151
2.0066
2.0041
2.0049
1.9908
1.9915
1.9911
0.025
0.020
0.024
THF
DMF
THF
[a] Generated in situ by electrolysis in THF or DMF with nBu4NBF6 as electrolyte and measured at 298 K; Isotropic g values giso and
coupling constants aX from spectral simulation (ax in mT; 1 T = 104 G). [b] Simulation not possible due to bad spectral resolution (see
text). [c] From ref.[24] [d] From ref.[25] [e] From ref.[19e] [f] Averaged g values gav = (g1 + g2 + g3)/3 and g anisotropy ∆g = g1 – g3 obtained
from measurements at 110 K in glassy frozen solutions.
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Arylnickel Complexes of a α-Diimine Bridging Ligand
spectra were recorded on Varian Cary 05E or Cary50 Scan spectro-
photometers. Electrochemical experiments were carried out in 0.1 
nBu4NPF6 solutions using a three-electrode configuration (glassy
carbon electrode, Pt counter electrode, Ag/AgCl reference) and an
Autolab PGSTAT30 potentiostat and function generator. The fer-
rocene/ferrocenium couple served as internal reference. EPR spec-
tra were recorded in the X band on the Bruker system ELEXSYS
500E, with a Bruker variable-temperature unit ER4131VT. g values
were calibrated by using a dpph sample. Spectral simulation was
performed using Bruker Simphonia or PEST WinSim free software
rather low nickel contributions. Comparable values of
0.0165 and 0.018 have been obtained recently for the related
species [(bpm)Ni(Mes)(Solv)]· (bpm = 4,4Ј-bipyrimidine)
and [(bpy)Ni(Mes)2]·, both of which are also clearly NiII
complexes of reduced α-diimine ligands.[17] It is remarkable,
that the binuclear species (measured in THF solution) exhi-
bits an only slightly higher ∆g than the mononuclear, al-
though from simple considerations one would expect an ap-
proximately doubled value (two metal contributions instead
of one). However, our EXAFS results have clearly shown Version 0.96 (National Institute of Environmental Health Sci-
ences).
that the ligand cannot accommodate two nickel atoms with-
out lengthening the N–Ni bonds. This seems to be true also
for the radical complexes. Thus, the individual contri-
butions of the nickel atoms in the binuclear radical complex
are far lower than the contribution by one nickel atom in
the mononuclear complex and in summation they only
EXAFS Measurements: The XANES and EXAFS measurements
were performed at the beamlines X1.1 (RÖMO II) and E4 at the
Hamburger Synchrotronstrahlungslabor des Deutschen Elek-
tronen-Synchrotrons (HASYLAB at DESY, Hamburg, Germany),
at beamline 2–3 at the Stanford Synchrotron Radiation Laboratory
slightly exceed the latter. Very similar results have been re- (SSRL) at SLAC (Stanford, USA) and at the newly designed EX-
AFS-beamline KMC-2 at the Berliner Elektronenspeicherring-Ge-
sellschaft für Synchrotronstrahlung m.b.H (BESSY II, Berlin, Ger-
many).
ported for reduced mono- and binuclear platinum(II) com-
plexes of bpym.[19a,26]
For the measurements at the Ni-K-edge (8332.8 eV) a Si(111)
double crystal monochromator was used at the SSRL and at
DESY. For the measurements at the Br-K-edge (13474.0 eV) at be-
amline X1.1 a Si(311) double crystal monochromator was used. At
BESSY a SiGe(220) double graded-crystal (0.5% Ge/cm) mono-
chromator was used for the nickel and the bromine edge. The syn-
chrotron beam current was between 80–100 mA at HASYLAB
(positron energy 4.45 GeV), between 100–250 mA at BESSY (elec-
tron energy 1.7 GeV) and between 80–100 mA at the SSRL (elec-
tron energy 1.7 GeV).
Conclusions
Comparison of the mono and binuclear organometallic
nickel complexes [(µ-bpym){Ni(Mes)Br}n] (bpym = 2,2Ј-bi-
pyrimidine; n = 1 or 2; Mes = mesityl = 2,4,6-trimeth-
ylphenyl) reveal a marked electronic coupling of the two
metal centres over the ligand bridge via their low-lying π*-
orbitals allowing the formation of one-electron reduced bi-
nuclear species [(µ-bpym–){NiII(Mes)Br}2]·–, while the mo-
nonuclear derivative undergoes rapid dissociation of the
bromido ligand yielding [(bpym–)NiII(Mes)(Solv)]· Both
radicals contain a mainly ligand-centred unpaired electron
as evidenced from EPR spectroscopy. From the same
method we can conclude, that the metal contribution is
more or less the same for both radical complexes indicating
a weaker metal-to-ligand interaction for the binuclear com-
plex. This is not only true for the reduced species but also
for the parent complexes as could be shown by EXAFS.
Short Ni–N bonds providing an optimum orbital overlap,
were found for the mononuclear complex, while two nickel
complex fragments {Ni(Mes)Br} are seemingly too large to
fit into the bis-chelate coordination site, which leads to pro-
longed Ni–N bonds and thus decreased metal-to-ligand
overlap.
All experiments were carried out under ambient conditions at
25 °C. The tilt of the second monochromator crystal was set to
30% harmonic rejection. Energy resolution was estimated to be
about 0.7–2.0 eV for the Ni-K-edge and 4.0 eV for the Br-K-edge.
The spectra were collected in transmission mode with ion cham-
bers. All ion chambers were filled with nitrogen in the case of the
measurements at the Ni-K-edge, and the second and third chamber
with argon in the case of the measurements at the Br-K-edge. En-
ergy calibration was performed with the corresponding metal foils
in the case of nickel and lead metal foil (Pb-LIII-edge) in the case
of bromine. The solid complexes were embedded in a polyethylene
matrix and pressed to pellets. The concentration of all samples was
adjusted to yield an absorption jump of ∆µd ≈ 1.5.
Data evaluation started with background absorption removal from
the experimental absorption spectrum by subtraction of a Victo-
reen-type polynomial. Then the background subtracted spectrum
was convoluted with a series of increasingly broader Gauss func-
tions and the common intersection point of the convoluted spectra
was taken as energy E0.[27] To determine the smooth part of the
spectrum, corrected for pre-edge absorption, a piecewise polyno-
mial was used. It was adjusted in such a way that the low-R compo-
nents of the resulting Fourier transform were minimal. After divi-
sion of the background subtracted spectrum by its smooth part,
the photon energy was converted to photoelectron wave numbers
k. The resulting EXAFS function was weighted with k3 Data analy-
Experimental Section
Instrumentation: Elemental analysis was obtained using a HEKA-
tech CHNS EuroEA 3000 analyzer. NMR spectra were recorded
on Bruker Avance II 300 MHz (1H: 300.13 MHz, 13C: 75.47 MHz)
or Bruker Avance 400 MHz (1H: 400.13 MHz, 13C: 100.61 MHz)
spectrometers, using a triple resonance 1H,19F,BB inverse probe
head. The unambiguous assignment of the H and 13C resonances
1
was obtained from 1H TOCSY, 1H COSY, gradient selected 1H, sis in k space was performed according to the curved wave multiple
13C HSQC and HMBC experiments. All 2D NMR experiments
were performed using standard pulse sequences from the Bruker
pulse program library. Chemical shifts were relative to TMS. Spec-
tra were evaluated with Bruker TopSpin2. UV/Vis/NIR absorption
scattering formalism of the program EXCURV92 with XALPHA
phase and amplitude functions.[28] The mean free path of the scat-
tered electrons was calculated from the imaginary part of the po-
tential (VPI was set to –4.00) and an overall energy shift (∆E0) was
Eur. J. Inorg. Chem. 2010, 934–941
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
939

A. Klein, M. P. Feth et al.
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3
δ = 9.50 (d, JH6ЈH5Ј = JH4ЈH5Ј = 5.45 Hz, 2 H, H6Ј,4Ј), 8.24
(t, 1 H, H5Ј), 7.60 (t, 3JH5H6 = 3JH4H5 = 5.68 Hz, 1 H, H5), 7.30
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H, p-CH3); trans isomer: δ = 9.47 (dd, 3JH6H5 = 3JH4ЈH5Ј = 5.39,
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4JH4ЈH6Ј = 1.81 Hz, 2 H, H4Ј6), 7.92 (t, JH6ЈH5Ј = 5.72 Hz, 2
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Acknowledgments
We wish to thank HASYLAB at DESY (Hamburg, Germany), and
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Received: September 22, 2009
Published Online: January 12, 2010
Eur. J. Inorg. Chem. 2010, 934–941
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
941