Binuclear arylnickel complexes with bridging bis(arylimino)-1,4-pyrazine ligands

Article abstract of DOI:10.1002/ejic.201101436

Binuclear organometallic nickel complexes of the type [(μ-NN){Ni(Mes)Br} ₂] {NN = α-diimine chelate ligand of the type 2,5-bis[1-(aryl)iminoethyl]pyrazine; Mes = mesityl = 2,4,6-trimethylphenyl} have been prepared and characterised electrochemically and spectroscopically in detail. A combination of NMR spectroscopy and quantum chemical calculations allowed the assignment of stereoisomers and their relative stability. The long-wavelength absorptions (600-1000 nm) assignable to charge-transfer transitions reveal a marked electronic coupling of the two metal centres over the ligand bridge via their low-lying π* orbitals. The reversible reductive electrochemistry yields stable radical anionic complexes with mainly ligand-centred spin density as shown by electron paramagnetic resonance (EPR) spectroscopy and UV/Vis spectroelectrochemistry of the free ligands and their nickel complexes in combination with DFT calculations. Preliminary investigations of the complexes as catalysts in Negishi cross-coupling reactions gave promising results. Copyright

Full text of DOI:10.1002/ejic.201101436

FULL PAPER
DOI: 10.1002/ejic.201101436
Binuclear Arylnickel Complexes with Bridging Bis(arylimino)-1,4-pyrazine
Ligands
Axel Klein,*[a] Christian Biewer,[a] Claudia Hamacher,[a] Natascha Hurkes,[a]
Jessica Perez Outeiral,[a] Erik Mora Paniagua,[a] Ann-Kathrin Schmieder,[a]
Andreas O. Schüren,[a] Prabhakara Rao Burma,[b] James T. Ciszewski,[b] and
David A. Vicic[b]
Keywords: Nickel / Organometallics / Electrochemistry / Spectroelectrochemistry / EPR spectroscopy
Binuclear organometallic nickel complexes of the type [(μ- tronic coupling of the two metal centres over the ligand
∧
∧
N N){Ni(Mes)Br}2] {N N = α-diimine chelate ligand of the bridge via their low-lying π* orbitals. The reversible reductive
type 2,5-bis[1-(aryl)iminoethyl]pyrazine; Mes = mesityl = electrochemistry yields stable radical anionic complexes with
2,4,6-trimethylphenyl} have been prepared and characterised mainly ligand-centred spin density as shown by electron
electrochemically and spectroscopically in detail. A combina- paramagnetic resonance (EPR) spectroscopy and UV/Vis
tion of NMR spectroscopy and quantum chemical calculations spectroelectrochemistry of the free ligands and their nickel
allowed the assignment of stereoisomers and their relative complexes in combination with DFT calculations. Preliminary
stability. The long-wavelength absorptions (600–1000 nm) as- investigations of the complexes as catalysts in Negishi cross-
signable to charge-transfer transitions reveal a marked elec- coupling reactions gave promising results.
From these studies we have a clear picture of the crucial
Introduction
role of the diimine ligand in these complexes. The lowest
Organometallic nickel complexes with α-diimine ligands
unoccupied MOs are mainly ligand(π*)-centred, with the
such as 2,2Ј-bipyridine (bpy), 1,10-phenanthroline (phen)
consequences, that both the intense colours (long-wave-
or diazabutadienes (R-DAB) have gained enormous interest
length absorption bands) and the relatively low first and
in the last decade. This is mainly because of a number of
second reduction potentials are strongly dependent on the
important catalytic processes like olefin oligo- or polymeri-
nature of the diimine ligand.
sation, olefin/CO co-polymerisation,[1,2] and various (elec-
This promoted the idea of studying analogous binuclear
tro)catalytic C–C coupling reactions.[3–11] Paralleling their
∧
complexes [(μ-N N){Ni(Mes)Br}2] with bridging diimine
use in catalysis, fundamental studies on structures and elec-
∧
ligands (N N) (see Scheme 1), since we expected that the
tronic properties of organonickel complexes with α-diimines
use of bridging ligands, which were able to electronically
have been carried out.[9–18] We have contributed to this with
couple the two metal centres, might greatly enhance these
the investigation of a number of organonickel complexes
interesting properties.[19–21] In view of catalytical applica-
∧
∧
[(N N)Ni(Mes)X] (N N = α-diimine ligands, Mes = mes-
ityl = 2,4,6-trimethylphenyl, X = halides) with various di-
imine ligands. Their structures and reactivity towards li-
gand exchange reactions have been studied using X-ray dif-
fraction and absorption spectroscopy,[14–16] their pho-
tophysics and photochemistry were explored by a combina-
tion of multiple spectroscopy and quantum chemical calcu-
lations,[14,16,17] and finally, their redox chemistry was inves-
tigated,[17,18] with respect to the application of such systems
in electrocatalytical C–C coupling reactions.[6]
tions of such complexes binuclear derivatives are supposed
to additionally offer higher thermal stability, as has been
concluded in recent work.[2]
Since the geometrical orientation of the two metal atoms
might be important, we used two different types of bridging
ligands. Recently we used the established ligand 2,2Ј-bipyr-
idine (bpym) that allows one to direct the two metal atoms
face to face, with distances of 5.5–6 Å.[16,20] In contrast to
the bpym ligand, the five ligands bpip, bdip, btip, bxip and
bmip bridge the two metal atoms by a central 1,4-pyrazine
unit, resulting in much larger metal–metal distances. In
both cases the metal–metal interaction is not direct but sup-
ported by the bridging ligand and also herein bpym differs
from the so-called S-frame ligands bpip–bmip,[21] where the
bridging ligand holds the two metal atoms in the two
clamps of an S. Importantly, from the viewpoint of metal–
[a] Department für Chemie, Institut für Anorganische Chemie,
Universität zu Köln,
Greinstraße 6, 50939 Köln, Germany
E-mail: axel.klein@uni-koeln.de
[b] Department of Chemistry, University of Hawaii,
2545 McCarthy Mall, Honolulu, HI 96822, USA
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201101436.
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Binuclear Ni Complexes with Bridging 1,4-Pyrazine Ligands
in acetone or toluene solution (Scheme 2) and were ana-
1
lysed by H NMR, 13C NMR and elemental analysis (see
Experimental Section).
In contrast to the recently reported mononuclear com-
∧
plexes [(N N)Ni(Mes)Br] with rather basic diimine ligands
such as 2,2Ј-bipyridine (bpy)[14,15] or the related binuclear
bpym complex [(μ-bpym){Ni(Mes)Br}2][16] for which the
preparation reaction depicted in Scheme 2 proceeds very
smoothly with high yield, for the less basic ligands bpip–
bmip the preparation required thorough recrystallisation
due to the presence of starting complex [(PPh3)2Ni(Mes)
Br]. From the very weakly basic but excellent acceptors
2,2Ј-azopyridine[23]
or
bis(2-pyridyl)-3,4,5,6-tetrazine
Scheme 1. Bridging diimine ligands used in this study. bpip = 2,5-
bis(1-phenyliminoethyl)pyrazine, bdip = 2,5-bis[1-(3,5-dimethyl-
(bptz),[24] no products were formed upon reaction with
[(PPh3)2Ni(Mes)Br]. Once formed and in the absence of
PPh3 or other strong ligands the new compounds are stable
in solvents like CH2Cl2, THF, toluene, acetone or even
DMF towards ligand exchange reactions.[15] The complexes
undergo the same decomposition reaction as reported for
the monomolecular analogues only in nitriles and
alcohols.[15,16,17] Examination of their thermal stability in
the solid state by dynamic DSC (Differential Scanning Cal-
orimetry) showed that the bxip and bmip derivatives are
completely stable up to 205 °C, at which point they melt.
Above 220 °C the complexes slowly decompose. In DMF
solution the bmip derivative decomposed only upon pro-
longed (more than 8 h) heating above 152 °C (boiling) as
investigated by NMR spectroscopy.
phenyl)iminoethyl]pyrazine, btip
= 2,5-bis[1-(2-methylphenyl)-
iminoethyl]pyrazine, bxip = 2,5-bis[1-(2,6-dimethylphenyl)imino-
ethyl]pyrazine and bmip = 2,5-bis[1-(2,4,6-trimethylphenyl)imino-
ethyl]pyrazine (including numbering of the pyrazine core). Indi-
cated by [M] are the potential binding sites of metals.
ligand–metal interaction bpip and its derivatives are (bis-
chelate) derivatives of 1,4-pyrazine and should thus com-
bine high binding stability (chelate) with the excellent
“bridging abilities” of 1,4-pyrazine, well established in the
so-called “Creutz–Taube” ion.[22] The variation of bpip to
bdip, btip, bxip and bmip was motivated from the expected
steric effects,[21a] which play a very important role for exam-
ple in the Brookhart-type ligands and is established as a
pre-requisite for effective catalysts.[1]
1
Complex H NMR spectra were observed after short re-
action times and rapid workup. Closer inspection, in part
with the help of correlation methods, revealed that mixtures
of isomers were present in these solutions. Scheme 3 depicts
the three possible isomers for the bpip complex. However,
the NMR experiments only show that there were two dif-
Results and Discussion
Synthesis and General Properties
∧
∧
The neutral complexes [(μ-N N){Ni(Mes)Br}2] (N N = ferent orientations of the mesityl co-ligand, either trans to
diimine ligands) were synthesised from the precursor com- the aniline N atom (called the cis isomer) or trans to the
plex trans-[(PPh3)2Ni(Mes)Br] by ligand exchange reactions pyrazine N atom (called the trans isomer). The depicted
∧
Scheme 2. Preparation of complexes [(μ-N N){Ni(Mes)Br}2] from trans-[(PPh3)2Ni(Mes)Br] and the bridging ligands.
Scheme 3. Possible isomers for binuclear bromo mesityl nickel complexes.
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2445

A. Klein et al.
FULL PAPER
meso isomer cannot be observed, but the signals might be Table 2. Essential DFT-calculated structural data of the ligand rad-
ical anions at the (RI-)BP86/SV(P) level of theory.
coincident with those of the cis and trans isomers, thus the
[bpip]·– [bdip]·– [btip]·– [bxip]·– [bmip]·–
meso isomer cannot be ruled out. Leaving the reaction mix-
tures for a longer period of time or recrystallising the prod-
ucts several times yields pure spectra corresponding to the
cis isomers. The complete spectral assignment of the iso-
mers is given in the Experimental Section.
Distances [Å]
C(1)–N(1)
C(4)–N(1)
C(4)–C(5)
C(5)–N(2)
C(1)–C(2)
1.3703 1.3719 1.3708 1.3786 1.3787
1.3200 1.3199 1.3202 1.3179 1.3175
1.4576 1.4582 1.4569 1.4559 1.4557
1.3790 1.3798 1.3792 1.3818 1.3820
1.4324 1.4282 1.4297 1.4364 1.4329
Structures
Angles [°]
C(2)–C(1)–N(1)
N(1)–C(4)–C(5)
N(2)–C(5)–C(4)
126.4 123.9 127.2
119.3 119.4 119.2
119.1 119.0 119.1
126.1
119.9
118.8
126.7
119.9
118.9
Unfortunately, none of the binuclear complexes could be
obtained in the form of single crystals. A crystal structure
was obtained from the new ligand bmip (details in the Sup-
porting Information). Additionally, the structures of the
free ligands, the one-electron reduced ligands and the binu-
clear [Ni(Mes)Br] complexes were calculated by DFT meth-
ods. The calculated geometries for the ligands are almost
perfectly in line with the experimentally obtained (XRD)
Dihedral angle [°]
C(4)–N(1)–C(1)–C(2) 49.0
48.7
46.5
77.9
76.2
Both the free ligands and the binuclear complexes reveal
structures of the two ligands bmip and bpip.[21b] We calcu- an essentially co-planar arrangement of the central pyrazine
lated the molecular structures of the binuclear nickel com- and the 2,5-substituted iminoethyl group (the largest tilt an-
plexes in their cis isomeric form, and an example is shown gle is 6.2° for bmip). In the complexes the nickel atoms were
in Figure 1. Tables 1, 2 and 3 list relevant structural param- located quite perfectly in this plane thus showing perfect
eters (full data in the Supporting Information).
square-planar coordination (see Table 3). Compared to this
Figure 1. DFT-calculated molecular structure of cis-[(bmip){Ni(Mes)Br}2] at the (RI-)BP86/SV(P) level of theory. Only one half of the
centro-symmetric molecule was calculated.
Table 1. Essential DFT-calculated structural data of the free ligands at the (RI-)BP86/SV(P) level of theory.
bpip[a]
bpip
bdip
btip
bxip
bmip
bmip[b]
Distances [Å]
C(1)–N(1)
C(4)–N(1)
C(4)–C(5)
C(5)–N(2)
C(1)–C(2)
1.419(2)
1.279(2)
1.489(2)
1.336(2)
1.407(2)
1.3937
1.2912
1.5017
1.3497
1.4200
1.3952
1.2912
1.5006
1.3497
1.4292
1.3982
1.2911
1.5004
1.3498
1.4193
1.4024
1.2885
1.5032
1.3492
1.4248
1.4024
1.2881
1.5030
1.3494
1.4221
1.427(2)
1.273(2)
1.495(2)
1.338(2)
1.401(2)
Angles [°]
C(2)–C(1)–N(1)
N(1)–C(4)–C(5)
N(2)–C(5)–C(4)
122.6(1)
116.0(1)
117.3(1)
122.6
117.0
118.3
122.4
118.6
118.3
122.3
116.9
118.3
120.3
117.4
118.2
120.9
117.4
118.2
119.4(1)
116.6(1)
117.7(1)
Dihedral angles [°]
C(4)–N(1)–C(1)–C(2)
C(6)–C(5)–C(4)–N(1)
55.2(2)
1.6(1)
60.0
0.5
58.9
3.7
55.9
0.6
81.5
2
79.9
3.1
82.5(1)
6.2(1)
[a] X-ray diffraction data from ref.[21b] [b] Data from X-ray diffraction (for details see Supporting Information).
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Binuclear Ni Complexes with Bridging 1,4-Pyrazine Ligands
Table 3. Essential DFT-calculated structural data of [(μ-bmip){Ni(Mes)Br}2] and [(μ-bpip){Ni(Mes)Br}2] at the (RI-)BP86/SV(P) level of
theory.
cis-[(μ-L){Ni(Mes)Br}2]
trans-[(μ-L){Ni(Mes)Br}2]
Distances [Å]
(bmip)
(bpip)
(bmip)
(bpip)
Ni(1)–N(1)
Ni(1)–N(2)
Ni(1)–C(10)
Ni(1)–Br(1)
2.009
1.897
1.901
2.290
2.016
1.888
1.901
2.287
Ni(1)–N(1)
Ni(1)–N(2)
Ni(1)–C(10)
Ni(1)–Br(1)
1.921
1.975
1.911
2.301
1.906
1.983
1.902
2.297
Angles [°]
N(1)–Ni(1)–N(2)
N(2)–Ni(1)–C(10)
C(10)–Ni(1)–Br(1)
Br(1)–Ni(1)–N(1)
N(1)–Ni(1)–C(10)
N(2)–Ni(1)–Br(1)
Sum of angles [°]
82.4
93.3
87.4
96.9
175.7
179.2
360.0
82.1
93.1
86.5
98.1
175.1
179.6
359.8
N(1)–Ni(1)–N(2)
N(2)–Ni(1)–Br(1)
Br(1)–Ni(1)–C(10)
C(10)–Ni(1)–N(1)
N(1)–Ni(1)–Br(1)
N(2)–Ni(1)–C(10)
sum of angles [°]
82.1
96.2
87.6
97.7
167.9
161.5
363.6
82.2
95.6
87.1
95.4
175.0
173.8
360.3
Dihedral angles [°]
C(4)–N(1)–C(1)–C(2)
N(1)–Ni(1)–C(10)–C(11)
C(6)–C(5)–C(4)–N(1)
91.5
82.9
179.9
66.2
99.3
179.3
C(4)–N(1)–C(1)–C(2)
N(1)–Ni(1)–C(10)–C(11)
C(6)–C(5)–C(4)–N(1)
82.4
83.9
173.2
71.9
90.9
177.5
plane the “aniline group” of the ligand scaffold and the aryl group, thus supporting the assumed interaction (extension
co-ligand at the nickel atom are markedly tilted. The latter of the pyrazine π-system is favourable for the reduced spe-
tilt angles are about 83° for the bmip derivative, while the cies). Additionally, the calculated geometrical changes in
bpip complex shows 81 or 90°. Related mononuclear dimes- the ligand scaffold upon one-electron addition (presumably
ityl nickel or dimesityl platinum complexes revealed rather to the π*-orbital) are in line with the expected change of
invariant tilt angles of about 70°.[15] The tilt angle of the the character of the central pyrazine core from aromatic
aniline group in the two complexes depends strongly on the 1,4-pyrazine towards a p-benzoquinone diimine structure.
substituents and it has been proposed that the electron den- Importantly, these changes extend into the aniline groups
sity on the imino N atom (and thus a number of properties) [e.g. C(1)–N(1) and C(1)–C(2) in Table 2]. Coordination of
is determined by this tilt angle.[21] Thus, while for steric two [Ni(Mes)Br] fragments leads to an almost perpendicu-
reasons a tilt angle of 90° would be preferable, deviations lar arrangement of both the aniline group and the mesityl
from 90° indicate an electronic interaction of the aromatic co-ligands towards the coordination plane in the binuclear
substituent with the aromatic scaffold of the ligand. Struc- bmip complex (exhibiting higher steric strain), while the tilt
tural and theoretical studies on the ligand bpip (tilt angle angle of the aniline group is markedly lower for the bpip
54.9°) and the CuI complex [(μ-bpip){Cu(PPh3)2}2](BF4)2 derivative. In this case it is obvious that lower steric strain
(tilt angle 89.3°) have revealed an interaction between the allows an electronically favourable orientation of the aniline
phenyl orbitals and π-orbitals of the binding plane.[21b] group. Support for the assumed electronic interaction be-
While the tilt angle in the free ligand bpip provides a perfect tween the binding plane and the aniline groups has to come
angle for this interaction, steric restraints from the (phos- from electrochemistry and spectroscopy in solution.
phane) co-ligands force the angle to almost 90° in the cop-
In addition to the structural parameters, we also calcu-
per bpip complex, thus minimising the interaction. Further- lated the energy difference between the cis and trans isomers
more, it can be expected that ortho-substitution on the ani- for the binuclear complexes of bpip and the highest substi-
line groups will increase the steric restraints (repulsion of tuted ligand bmip. In the case of bpip the cis isomer was
the ortho groups and the iminoethyl group of the ligand found to be 4.41 kJmol–1 (ΔG) lower in energy, hence more
and co-ligands on the metal) and might also lead to tilt stable, than the trans isomer. As expected, the energy differ-
angles around 90° thus minimising the steric repulsion but ence between both isomers of the sterically bulky ligand
at the same time the aniline-binding plane interaction.
bmip is significantly higher. The cis isomer is 25.4 kJmol–1
Our calculations show that the dihedral angles between lower in energy than the trans isomer.
the binding plane and the aniline group fall into two groups
for the ligands (Table 1). The first group exhibits angles
roughly around 60° and contains all ligands with non-bulky
aniline groups. Interestingly, this includes the 2-tolyl deriva-
Electrochemistry
tive btip. The second group showing an angle of about 80°
The electrochemistry of the free ligands (see data in the
is formed by bxip and bmip, both with a 2,6-dimethyl sub- Supporting Information) and nickel complexes was studied
stitution pattern. The same groups are observed for the using cyclic and square-wave voltammetry. The cyclic vol-
structures of the one-electron reduced ligands (Table 2). For tammograms of all complexes in THF solution, as exem-
these species the tilt angle is markedly decreased for the first plified in Figure 2 (top), show two reversible one-electron
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2447

A. Klein et al.
FULL PAPER
reductions. Comparison with other transition-metal com- apparently completely different reduction behaviour in
plexes of these ligands suggest ligand-centred re- DMF solution (Figure 2, bottom). Instead of two reduction
ductions.[19–21] Remarkably, the anodic shift for the first re- waves (as in THF) we can see two packages of several
duction wave for the free ligands and the binuclear nickel waves. Each of these packages contains one electron and all
complexes accounts for more than one Volt. Further re- these processes are completely reversible. Using square-
duction waves are irreversible. In the anodic area broad wave voltammetry (Figure 3), the complex features can be
irreversible oxidation waves were observed, which account resolved into three waves [entries (1) to (3)]. The thus-ob-
for two electrons. On the basis of similar results from mo- tained 2ϫ3 potentials are listed in Table 2 for the bmip
nonuclear derivatives, we assign them to metal-centred oxi- complex. At higher scan rates (up to 5 Vs–1) and/or at lower
dations NiII/NiIII for the two nickel atoms.[17,18] From the temperatures the situation remains largely the same, only
voltammetry there is no evidence that the two oxidation the third wave [entry (3)] of each package gains some inten-
events are separated due to electronic coupling of the metal sity, while the other two decrease slightly. The oxidation
centres, which would allow generation of an intermediate waves in DMF solution look essentially the same as for the
mixed-valent state. Thus, the highest occupied molecular measurements in THF. Importantly, this behaviour is ob-
orbital in these complexes (HOMO) is rather localised on served for both isomeric mixtures as well as for materials
the NiII (d8) centres, in line with similar observations for containing the pure cis isomers.
RuII, RhIII or IrIII bpip complexes (Table 4).[21,22]
Figure 3. Square-wave voltammetry of [(μ-bmip){Ni(Mes)Br}2] in
Figure 2. Cyclic voltammograms of [(bmip){Ni(Mes)Br}2] in THF/
DMF/nBu4NPF6 solution, measurements at 298 K and 20 mVs–1
scan rate showing both an anodic and a cathodic scan of the first
reduction. The three bars represent the three obtained potentials.
nBu4NPF6 (top), and in DMF/nBu4NPF6 solution (bottom),
measurements at 298 K and 100 mVs–1 scan rate.
The peculiar behaviour of these complexes in DMF solu-
tion might be explained by the partial splitting of the brom-
ido ligand from the parent compounds or by the presence
of isomers. However, for the detailed electrochemical mea-
surements carried out on these two complexes we used the
pure cis isomers and from 1H NMR measurements in DMF
we have no indication that isomerisation in this solvent oc-
curs to a large extent. A comparison of the reduction poten-
tials of the bpip complex in THF or DMF solution reveals
no marked dependence of the potentials on the solvent po-
larity. If this holds also for the other derivatives, the third
wave (3) of each package should correspond to the re-
Table 4. Selected electrochemical data of binuclear complexes [(μ-
∧
N N){Ni(Mes)Br}2].[a,b]
∧
N N
Epa Ox1 E1/2 Red1 E1/2 Red2 Epc Red3 Solvent/T
(ΔEpp (ΔEpp
)
)
bpip
bdip
btip
bxip
bmip
0.30
0.34
0.33
0.34
0.41
–0.78 (75) –1.60 (78) –1.97
–0.84 (68) –1.66 (90) –2.05
–0.78 (78) –1.66 (92) –2.02
–0.82 (60) –1.73 (66) –3.07
–0.82 (70) –1.68 (90) –2.11
THF/r.t.
THF/r.t.
THF/r.t.
THF/r.t.
THF/r.t.
bpip
bpip
bmip
0.29
0.40
(1) 0.29
(2)
–0.76 (75) –1.58 (78) –1.93
DMF/r.t.
DMF/–60 °C
DMF/r.t.
–0.77 (70) –1.48 (71)
–
–0.55 (75) –1.26 (91) –1.93
–0.65 (75) –1.39 (78)
–0.76 (75) –1.57 (75)
–
–
∧
duction of the parent complexes cis-[(μ-N N){Ni(Mes)-
(3)
Br}2], which is supported by fast-scan and low-temperature
experiments. From this we can conclude that the species
responsible for the complicated plots exhibit slightly less
negative reduction potentials than the parent complexes (or
isomers). For the trans isomer and the possible meso isomer
(if present – see NMR) we would, however, expect more
negative values, because of the electron-donating character
of the mesityl co-ligand in trans position to the main re-
duction target, which is most probably the pyrazine core
bmip
(1) 0.41
(2)
(3)
–0.55 (74) –1.32 (71) –1.90
DMF/–60 °C
–0.69 (74) –1.48 (81)
–0.78 (74)
–
–
–
[a] The measurements were performed on the pure cis isomers of
the complexes. [b] Potentials from cyclic or square wave voltamme-
try in 0.1-m nBu4NPF6/solvent solutions (in V) vs. ferrocene/ferro-
cenium. Half-wave potentials E1/2 given with peak-to-peak separa-
tion ΔEpp in parentheses (in mV) for reversible processes; cathodic
Epc, or anodic Epa peak potentials for irreversible processes.
While the cyclic voltammograms of the bpip complex in (see EPR spectroscopy below). Additionally, for the related
DMF solution differ only marginally from those in THF complex [(μ-bpym){Ni(Mes)Br}2] no differences in the re-
solution, the bdip, btip, bxip, and bmip derivatives exhibit duction behaviour for the two isomers (cis and trans) was
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Binuclear Ni Complexes with Bridging 1,4-Pyrazine Ligands
Scheme 4. Schematic representation of the assumed electron transfer-induced solvolysis reaction.
observed.[16] Therefore, we can very probably discard the overall electronic structure of the complexes is essentially
possibility of isomerisation being responsible for the pecu- the same. The observed lability in DMF solution is ascribed
liar behaviour. Conversely, it was previously established, to a loss of Br– from the reduced complexes, a reaction
that the one-electron reduced mononuclear complexes which seems to be slow for the bpip complex and is ac-
·–
∧
[(N N)Ni(Mes)Br] readily cleave the Br co-ligand (as celerated for derivatives with methyl-substituted aniline
bromide),[6,18] and the resulting complexes [(N N)Ni(Mes)- groups. Since a straight correlation with observed structural
∧
(Solv)] (Solv = solvent molecule) exhibit re-oxidation po- parameters, especially the tilt angle between the coordina-
tentials anodically shifted by approx. 0.3 V compared to the tion plane and the aniline group and the electrochemical
neutral precursor complexes. We thus carried out corre- data cannot be drawn, we might also assume, that the elec-
sponding experiments and could show that upon addition tron-donating nature of the methyl substituents generally
of nBu4NBr or upon applying low temperatures the split- favours the cleavage of the Br– ligand, although a clear cor-
ting process can be slowed down (see Supporting Infor- relation of the number of substituents with the reduction
mation).[18] Therefore, we assume partially dehalogenated potentials also fails. In any case a marked difference be-
+
∧
species [{Ni(Mes)Br}(μ-N N){Ni(Mes)(Solv)}] or [(μ- tween the bpip complex and the other derivatives has been
2+
∧
N N){Ni(Mes)(Solv)}2] to account for the minor waves found.
at slightly higher potentials [packages (1) and (2)]. Since
from H NMR spectroscopy of the parent complexes we
1
have no evidence for such solvolysis, we might assume, that
they are formed by an electron transfer-induced solvolysis
reaction (Scheme 4).[25]
Absorption Spectroscopy
The binuclear complexes all exhibit a bright green colour
in the solid as well as in solution. The absorption spectra
(Figure 4) are dominated by two broad bands of medium
intensity in the visible region and intense bands in the UV
region. The two broad bands in the visible show negative
Provided the solvolysis reaction C occurs very quickly
[see Equations (1) and (2) in Scheme 4] the resulting radical
·n+
∧
complexes [{Ni(Mes)Br}(μ-N N){Ni(Mes)(Solv)}]
(n =
0, 1) might reduce the parent complex [see parts a of Equa-
tions (3) and (4)], while forming the cationic solvent com-
plexes [{Ni(Mes)Br}(μ-N N){Ni(Mes)(Solv)}]m+ (m = 1 or
∧
2), which have less negative reduction potentials (E1Ј or
E1ЈЈ) than the parent complex (E1) [see parts b of Equa-
tions (3) and (4)]. Hence, although only a very small frac-
tion of the parent complex is reduced at potentials E Ͼ
E1, an appreciable amount of cationic complexes could be
formed. The assumed mechanism is strongly supported by
the square-wave voltammogram shown in Figure 3. The an-
odic scan starting from –0.3 V exhibits a far larger amount
of the species responsible for the highest potential (presum-
ably the dicationic solvent complex), than observed for the
cathodic scan (starting from 0.3 V). Interestingly, the elec-
trochemical behaviour of bpip is thus different to the rest
of the series, the bdip, btip, bxip derivatives all strongly re-
semble the bmip complex. Since the electrochemical poten-
tials for the first and second reduction do not differ mark-
Figure 4. Absorption spectra of [(μ-bmip){Ni(Mes)Br}2] (____) and
[(μ-bpip){Ni(Mes)Br}2] (-----) in CH2Cl2 solution (the insert shows
the spectra of the free ligands bmip and bpip measured in CH2Cl2
edly within our series of complexes we can state that the solution).
Eur. J. Inorg. Chem. 2012, 2444–2455
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2449

A. Klein et al.
FULL PAPER
∧
Table 5. Absorption maxima of binuclear nickel complexes [(N N){Ni(Mes)Br}2].[a]
∧
N N
λ [nm]
bpip
bdip
btip
bxip
bmip
292 (32), 327 sh (18)
256 (30), 307 (21)
284 (27), 323 sh (17)
237 (32), 291 (19)
239 (37), 304 (20)
401 sh (15.0)
674 (5.5)
912 sh (3.1)
424 (7.6), 481 sh (4.1)
435 (7.3), 491 sh (6.4)
432 sh (3.8), 494 (4.1)
430 sh (4.0), 494 (4.8)
771 (10.8)
782 (17.4)
784 (10.5)
783 (11.0)
923 sh (3.5)
928 (9.4)
936 (5.8)
930 (6.1)
[a] Absorption maxima λ in nm as measured in CH2Cl2 solution, main maxima are underlined, extinction coefficients ε in 1000 m–1 cm–1
are given in parentheses.
solvatochromism (spectra in the Supporting Information), sorption at 750 nm. In the visible, two intense and partially
while the UV bands are solvent invariant.
structured bands rise with maxima at 381 and 589 nm
On the basis of the band intensities, the observed solva- (Table 6).
tochromism and studies of related mononuclear complex-
es[15b] we can assign the bands at high energy to ligand-
Table 6. Spectroelectrochemical data of btip and [(μ-btip){Ni(Mes)-
Br}2].[a]
centred (π-π*) transitions, while the two long-wavelength
band systems result from metal-to-ligand charge-transfer
(MLCT) transitions (data in Table 5). The very low energy
of the latter can be attributed to a marked electronic cou-
pling of the two metal centres over the ligand bridge via
their low-lying π* orbitals.[21]
A closer look reveals that the dominant MLCT bands
are around 780 nm for the bmip, btip and bxip complexes.
Slightly higher energies are found for the bdip and mark-
edly higher energies for the bpip complex. The same is true
for the very low energy maxima at around 920 nm and the
high-energy MLCT bands at around 420 nm. Also in the
λ [nm]
[(btip){Ni(Mes)Br}2]
272 420
480 sh 767
902 sh
887 sh
[(btip){Ni(Mes)Br}2]·– 246 381
589
740
[(btip){Ni(Mes)Br}2]2– 246 320 sh
360 sh 549
btip
291 294
280 460 sh
276 328 sh sh 360 sh 479
330 sh 365 sh
[btip]·–
[btip]2–
480
720
885 sh
[a] Electrolysed in situ in an OTTLE cell in THF/nBu4PF6; absorp-
tion maxima λ in nm, main maxima are underlined.
The second reduction leads essentially to a blue-shift of
UV region the spectrum for the bpip complex reveals the two visible bands and an increase in intensity. For the
marked differences to the other derivatives. Remarkably free ligand essentially the same spectroscopic changes were
also the spectra of the uncoordinated ligands exhibit quali- observed during reductive electrolysis (Table 6) and similar
tatively the same differences (Figure 4 inset), which thus can absorptions have been observed for reduced bpip complexes
be traced to the different aniline groups (full spectroscopic of [Re(CO)3Cl] or [M(C5Me5)Cl] (M = Ir or Rh).[19b,20a]
data of the ligands in the Supporting Information).
Thus, the two reductions of the nickel complexes [(μ-
∧
The free ligand btip and the corresponding nickel com- N N){Ni(Mes)Br}2] can be assigned to essentially ligand-
plex [(btip){Ni(Mes)Br}2] have been submitted to spectro- based processes; the stepwise reduction leads to blue-shift-
electrochemical experiments examining the two reversible ing and bleaching of the MLCT bands and the appearance
reduction waves (Figure 5). During the first reduction step of additional π-π* absorption in the reduced ligands
·–
2–
∧
∧
the broad MLCT band vanishes leaving some residual ab- (N N) and (N N) .
Figure 5. Absorption spectra of [(μ-btip){Ni(Mes)Br}2] recorded during in situ electrolysis in THF/nBu4NPF6 at 298 K; left: during first
reduction, right: during second reduction.
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Binuclear Ni Complexes with Bridging 1,4-Pyrazine Ligands
∧
EPR Spectroscopy
Table 7. EPR data of radical anionic ligands (N N)·–, (dapz)·– and
reduced nickel complexes.[a]
EPR spectra have been recorded on electrochemically
generated radical anions of the binuclear nickel complexes
at 298 K in fluid solution and in glassy frozen solutions at
110 K. Furthermore, the reduced ligand bxip·– and the radi-
cal anion generated from the precursor molecule 2,5-diace-
tyl-1,4-pyrazine (dapz) were investigated. Figure 6 shows
representative examples and Table 7 summarises the data.
From Figure 6 we can see, that the signal for the free ligand
radical anion bxip·– exhibits hyperfine splitting (hfs), the
same is true for dapz and the previously reported bpip·–.[26]
From spectral simulation of all three species we can con-
clude that the hfs is due to the coupling to the pyrazine N
Radical
giso
aN–pyz aN–imin aH–methyl aH–pyz
(dapz)·–
(bpip)·–[b]
(bxip)·–
2.005
2.003
2.003
0.173
0.159
0.323
–
0.173
0.186
0.200
0.173
0.018
0.170
0.159
0.323
∧
[(N N){Ni(Mes)Br}2]·– giso
gav
g1
g2
g3
Δg
bpip
bdip
btip
bxip
bmip
2.0180 2.0180 2.0353 2.0183
2.0186 2.0186 2.0359 2.0146
2.0180 2.0180 2.0456 2.0120
2.0187 2.0187 2.0503 2.0117
2.0182 2.0183 2.0515 2.0143
2.0004 0.0349
2.0053 0.0306
1.9965 0.0491
1.9942 0.0561
1.9891 0.0622
[a] Radical species generated in situ by electrolysis in THF/
nBu4NBF6 at ambient temperature. Coupling constants aX and iso-
atoms, the imino N atom, the iminoethyl H and the pyr- tropic g values giso from spectral simulation in mT (1 T = 104 G)
from measurements at 298 K. Averaged g values gav = (g1 + g2 +
azine H atoms. Comparison of the coupling constants re-
g3)/3 and g anisotropy Δg = g1 – g3 from measurements at 110 K.
veals a re-distribution of the spin density within the series
[b] From ref.[26]
dapz·–, bpip·– and bxip·–. The observed differences between
bpip and bxip are completely in line with the DFT-calcu-
lated spin densities (Supporting Information). Increasing
and the rather small g anisotropy Δg we can conclude that
substitution on the aniline group decreases the (re)distribu-
the singly occupied molecular orbitals (SOMOs) in these
tion of spin density over the entire molecule (up to the ani-
complexes are mainly centred on the organic bridging li-
line group), thus increasing the density in the central pyr-
gand with only very small contributions from the nickel
azine-imine entity (with the consequence of increased cou-
atoms.
pling constants).
Thus, the radical complexes can be described as [NiII-
·–
∧
(Mes)Br] complex fragments coordinated to (μ-N N)
radical bridging ligands, in line with the UV/Vis spectro-
electrochemical experiments. Compared to the above-men-
tioned complexes [(μ-bpym){Ni(Mes)Br}2]·– and [(bpym)-
Ni(Mes)Br]·– both the giso (ca. 2.004) and Δg (ca. 0.025)
values are slightly increased pointing to slightly increased
nickel contributions, however, for radical complexes with a
marked nickel contribution to the SOMO (in the sense of
NiI complexes) far higher g values (Ͼ2.1) and Δg (Ͼ0.1)
were observed.[27]
Catalytic Negishi Cross-Coupling Reactions
With the binuclear complexes in hand, we wanted to
demonstrate the proof-in-principle that such frameworks
can support catalytic reactions of current interest. Nickel
has been especially useful in the cross-coupling of aryl hal-
ides with alkylzinc nucleophiles to generate new C(sp2)–
C(sp3) bonds.[8c] Table 8 describes our initial results using
Figure 6. X-band EPR spectra of (bxip)·– (a) with spectral simula-
tion (b) and [(μ-bxip){Ni(Mes)Br}2]·– (c) recorded during in situ
electrolysis in THF/nBu4NPF6 at 298 K.
∧
For the binuclear nickel complex [(μ-N N){Ni(Mes)- the binuclear complexes based on the bxip and btip ligands
Br}2]·– (generated by electrolysis) we obtained unresolved in comparison with the recently reported binuclear complex
isotropic EPR signals at 298 K in fluid solution centred at [(bpym){Ni(Mes)Br}2].[16]
around g = 2.02 (Figure 6). In contrast to this, the radical
anions [(μ-bpym){Ni(Mes)Br}2]·– and [(bpym)Ni(Mes)Br]·– mediate the Negishi reaction described in Table 8 under
gave well-resolved EPR spectra.[16]
mild room temperature conditions, with the bpym complex
Gratifyingly, all three complexes were able to catalytically
In glassy frozen solutions at 110 K anisotropic spectra of affording the highest yield of product. No side products re-
rhombic symmetry were observed with averaged gav values sulting from β-hydride eliminations were detected in the
identical to the isotropic signals (giso) at 298 K thus con- GC–MS.
firming that the same species were observed (Figures in the
Interestingly, the complexes displayed higher reactivity
Supporting Information). Unfortunately the signals were with the functionalised 2-(1,3-dioxan-2-yl)ethylzinc brom-
completely unresolved. Nevertheless, from the giso = gav val- ide than the non-functionalised pentylzinc bromide. While
ues not far above the value of the free electron ge = 2.0023 the yields are moderate and not yet competitive with the
Eur. J. Inorg. Chem. 2012, 2444–2455
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2451

A. Klein et al.
FULL PAPER
Table 8. Activity of selected binuclear complexes for aryl–alkyl cross-coupling reactions.[a]
[a] All yields based on GC analyses relative to a calibrated internal standard.
use of bipyridine nickel catalysts,[8c] the results do show that singly occupied molecular orbital (SOMO) is delocalised
the binuclear complexes can indeed carry out synthetic
transformations that have been historically quite difficult.
over the entire π-system of the ligand, including the N- and
α-C atom of the aniline substituent. Since these substituents
are involved in repulsive interaction with the mesityl co-
ligands on the nickel atom the tilt angle of the aniline sub-
stituent does not allow further delocalisation. As inferred
from the g values and the small g anisotropy of the EPR
spectra at low temperature, the contribution of nickel to the
SOMO is small thus, the complexes are best described as
[NiII(Mes)Br] fragments coordinating to radical anionic
Conclusions
The binuclear organometallic nickel complexes [(μ-
∧
∧
N N){Ni(Mes)Br}2] {N N
= 2,5-bis[1-(aryl)iminoeth-
·–
∧
bridging ligands (μ-N N) . Interestingly, while the aryl =
phenyl derivative shows two well-defined reduction waves
in cyclovoltammetric measurements, the complexes carrying
more sterically demanding substituents such as tolyl, xylyl
or mesityl exhibit a complex behaviour in DMF solution.
It is assumed that this polar solvent is able to partly replace
the bromido ligand thus leading to minor amounts of sol-
vent complexes. The latter are probably interesting candi-
dates for nickel-based (electro)catalytical C–C coupling re-
actions. Preliminary Negishi cross-coupling reactions show
that the complexes are moderately active. Future work will
focus on whether the binuclear systems are inherently more
stable than mononuclear derivatives (from our preliminary
experiments we cannot draw an unequivocal conclusion on
this), on whether their reactivity can be increased by ad-
dition of polar solvents (Br-splitting), and on determination
of the background of the observed higher reactivity with
the functionalised 2-(1,3-dioxan-2-yl)ethylzinc bromide.
yl]pyrazine with aryl = phenyl, 2-tolyl, 2,6-xylyl, 3,5-xylyl
and mesityl; Mes = mesityl = 2,4,6-trimethylphenyl} were
prepared and characterised electrochemically and spectro-
scopically in detail. A combination of NMR spectroscopy
and quantum chemical calculations gave a reasonable pic-
ture of the molecular structures and also showed that the
cis configuration of the complexes is more stable. The com-
plexes exhibit very long wavelength absorptions (600–
1000 nm) which can be assigned to metal-to-ligand charge-
transfer transitions. The very low energy of them (1.2–2 eV)
is ascribed to a marked electronic coupling of the two metal
centres over the ligand bridge via the low-lying π* orbitals.
In contrast to this, in voltammetric experiments no cou-
pling of the two nickel-centred irreversible oxidations NiII/
NiIII was observed. The reversible reductive electrochemis-
try yields stable radical anionic complexes with mainly li-
gand-centred spin density as shown by EPR spectroscopy,
UV/Vis spectroelectrochemistry and DFT calculations. The
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Binuclear Ni Complexes with Bridging 1,4-Pyrazine Ligands
NMR ([D6]acetone): δ = 9.53 (s, 2 H, pz-H), 6.92 (s, 4 H, m-H),
2.26 (s, 6 H, CH3imine), 2.19 (s, 6 H, p-CH3), 2.00 (s, 12 H, o-
CH3) ppm. Other reagents were commercially available and used
without further purification. All preparations and physical mea-
surements were carried out in dried solvents under argon, using
Schlenk techniques.
Experimental Section
Instrumentation: Elemental analyses were obtained with a HEKA-
tech CHNS EuroEA 3000 analyser. NMR spectra were recorded
with 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 in-
1
General Procedure for the Negishi Cross-Coupling Reactions: A
solution of aryl iodide (0.5 mmol) in THF (1 mL) was added to the
alkylzinc bromide reagent (0.5 mmol) in THF (1 mL) at ambient
temperature. To this mixture was added the nickel catalyst (2.5 mol-
%) and the hexamethyl benzene (0.5 mmol) internal standard. The
resulting mixture was stirred at room temperature and aliquots
were checked periodically by GC–MS until all aryl iodide was con-
sumed.
verse probe head. The unambiguous assignment of the H and 13
C
resonances was obtained from 1H TOCSY, 1H COSY, gradient-
selected 1H, 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
Tetramethylsilane (TMS). Spectra were evaluated with Bruker Top-
Spin2. UV/Vis/NIR absorption spectra were recorded with Varian
Cary 05E or Cary50 Scan spectrophotometers. Electrochemical ex-
periments were carried out in 0.1-m nBu4NPF6 solutions using a
three-electrode configuration (glassy carbon electrode, Pt counter
electrode, Ag/AgCl reference) and an Autolab PGSTAT30 po-
tentiostat and function generator. The ferrocene/ferrocenium cou-
ple served as the internal reference. UV/Vis spectroelectrochemical
measurements were performed with an optical transparent thin-
layer electrochemical (OTTLE) cell.[28] EPR spectra were recorded
in the X-band with a Bruker System ELEXSYS 500E, using a
Bruker Variable Temperature Unit ER 4131VT. g values were cal-
ibrated using a dpph sample. Spectral simulation was performed
using Bruker SimFonia V1.26 or P.E.S.T. WinSim free software
v. 0.96 (National Institute of Environmental Health Sciences). Dy-
namic DSC was measured with a Netzsch DTA404 PC under ar-
gon.
Synthesis of the Complexes
General Procedure for the Preparation of the Binuclear Complexes
∧
∧
[μ-(N N){Ni(Mes)Br}2] (N N = bpip, bdip, btip, bxip, bmip): In
∧
each reaction 0.45 mmol of the N N ligand [141 mg of bpip,
154 mg of btip, 167 mg of bxip (or bdip), or 180 mg of bmip] were
suspended in toluene (100 mL) together with [(PPh3)2Ni(Mes)Br]
(707 mg, 0.90 mmol) and stirred for three days at ambient tempera-
ture giving dark green reaction mixtures. The solvent was evapo-
rated to dryness and the residue was washed three times with 10-
mL n-heptane and pentane. The dark green products had to be
recrystallised from CH2Cl2/n-heptane (1:1) to remove unreacted
[(PPh3)2Ni(Mes)Br].
Thus, we obtained [(μ-bpip){Ni(Mes)Br}2] (224 mg, 0.27 mmol,
60%) as a dark green powder. C38H40Br2N4Ni2 (829.99): calcd. C
54.99, H 4.86, N 6.75; found C 54.91, H 4.82, N 6.77. 1H NMR
(CD2Cl2): trans isomer: δ = 9.63 (s, 2 H, Hpz), 7.7–7.2 (m, 10 H,
Ph), 5.67 (s, 4 H, m-H), 2.71 (s, 6 H, CH3imine), 2.25 (s, 12 H, o-
CH3), 2.19 (s, 6 H, p-CH3) ppm; cis isomer: δ = 7.68–7.45 (m, 10
H, HPh), 6.91 (s, 2 H, Hpz), 6.78 (s, 4 H, m-H), 2.36 (s, 6 H,
CH3imine), 2.25 (s, 12 H, o-CH3), 2.23 (s, 6 H, p-CH3) ppm.
Crystal Structure Analysis for bmip: Data collection was performed
at T = 173(2) K on a Siemens P4 diffractometer with Mo-Kα radia-
tion (λ = 0.71073 Å). The structure was solved by direct methods
using the SHELXTL-PLUS package[29] and refinement was carried
out with SHELXL 97 employing full-matrix least-squares methods
2
2
on F2 with F0 Ն –2σ(F0 ). Empirical absorption correction was
performed using Ψ-scans. All non-hydrogen atoms were treated an-
isotropically; hydrogen atoms were found from the Fourier map
and refined.
[(μ-bdip){Ni(Mes)Br}2] (284 mg, 0.32 mmol, 72%) was obtained as
an emerald green microcrystalline powder. C42H48Br2N4Ni2
(886.05): calcd. C 56.93, H 5.46, N 6.32; found C 56.88, H 5.42, N
CCDC-853102 contains the crystallographic data for bmip. These
data can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
1
6.27. H NMR: cis isomer (CD2Cl2): δ = 7.35 (s, 2 H, Hpz), 7.19
(s, 4 H, m-HMesNi), 6.90 (s, 2 H, p-HXyl), 6.75 (s, 4 H, m-HXyl),
2.82 (s, 12 H, m-CH3Xyl), 2.30 (s, 12 H, o-CH3MesNi), 2.28 (s, 6
H, p-CH3MesNi), 1.47 (s, 6 H, CH3imine) ppm.
Quantum Chemical Calculation: All calculations were performed
using density functional theory as implemented in the Turbomole
6.3[30] package using the resolution of identity (RI) approxi-
mation.[31–33] The geometry optimisations were performed at the
(RI-)BP86/SV(P) level. All minima were confirmed as such by the
absence of imaginary frequencies. The agreement between experi-
mentally determined and calculated structures of the ligands bpip
and bmip was very good. For the geometry and energy of the binu-
clear complexes only half of the centro-symmetric cis and trans
isomers were calculated.
[(μ-btip){Ni(Mes)Br}2] (257 mg, 0.30 mmol, 67%) was obtained as
an emerald green microcrystalline powder. C40H44Br2N4Ni2
(858.00): calcd. C 55.99, H 5.17, N 6.53; found C 56.03, H 5.20, N
1
6.54. H NMR: cis isomer (CD2Cl2): δ = 7.37 (s, 2 H, Hpz), 7.35–
7.25 (m, 6 H, HTol), 6.94 (t, 2 H, p-HTol), 6.64 (s, 4 H, m-
HMesNi), 2.75 (dd, 6 H, o-CH3MesNi), 2.68 (dd, 6 H, o-
CH3MesNi), 2.37 (s, 6 H, CH3Tol), 2.22 (s, 6 H, p-CH3MesNi),
1.51 (s, 6 H, CH3imine) ppm.
Materials and Procedures: The precursor complex [(PPh3)2Ni(Mes)-
Br][13,14] and the ligand bpip[26] were obtained following literature
procedures. The ligands bpip, bdip, btip, bxip and bmip were pre-
pared in an analogous way as the xylyl derivative.[21a] bdip: 1H
[(μ-bxip){Ni(Mes)Br}2] (370 mg, 0.42 mmol, 93%) was obtained as
an emerald green microcrystalline powder. C42H48Br2N4Ni2
(886.09): calcd. C 56.93, H 5.46, N 6.32; found C 56.85, H 5.43, N
1
6.29. H NMR: trans isomer ([D6]acetone): δ = 9.19 (s, 2 H, Hpz);
NMR ([D6]acetone): δ = 9.41 (s, 2 H, pz-H), 6.80 (s, 2 H, p-H), 6.85 (s, 4 H, m-HMesN); 6.55 (s, 4 H, m-HMesNi); 2.23 (s, 6 H, p-
6.52 (s, 4 H, o-H), 2.36 (s, 6 H, CH3imin), 2.32 (s, 12 H, CH3) ppm. CH3MesNi); 2.21 (s, 12 H, o-CH3MesNi); 2.17 (s, 6 H, p-
1
btip: H NMR ([D1]chloroform): δ = 9.56 (s, 2 H, pz-H), 7.25 (m,
4 H, m-H), 7.09 (t, 2 H, p-H), 6.73 (dd, 2 H, o-H), 2.35 (s, 6 H,
CH3MesN); 1.96 (s, 12 H, o-CH3MesN); 1.60 (s, 6 H, CH3imine)
ppm; cis isomer (CD2Cl2): δ = 7.27 (s, 2 H, Hpz); 7.19 (s, 4 H, m-
HMesNi); 6.79 (s, 2 H, p-HXylN); 6.65 (s, 4 H, m-HXylN); 2.72
1
CH3imine), 2.17 (s, 6 H, TolCH3) ppm. bxip: H NMR ([D6]acet-
one): δ = 9.17 (s, 2 H, pz-H), 6.82 (d, 4 H, m-H), 6.45 (t, 2 H, p- (s, 12 H, o-CH3XylN); 2.26 (s, 12 H, o-CH3MesNi); 2.22 (s, 6 H,
H), 2.68 (s, 6 H, CH3imine), 2.12 (s, 12 H, CH3) ppm. bmip: 1H
p-CH3MesNi); 1.46 (s, 6 H, CH3imine) ppm.
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A. Klein et al.
FULL PAPER
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[(μ-bmip){Ni(Mes)Br}2] (366 mg, 0.40 mmol, 89%) was obtained
as an emerald green microcrystalline powder. C44H52Br2N4Ni2
(914.15): calcd. C 57.81, H 5.73, N 6.13; found C 57.85, H 5.73, N
1
6.13. H NMR ([D6]acetone): trans isomer: δ = 9.19 (s, 2 H, Hpz);
6.85 (s, 4 H, m-HMesN); 6.55 (s, 4 H, m-HMesNi); 2.23 (s, 6 H, p-
CH3MesNi); 2.21 (s, 12 H, o-CH3MesNi); 2.17 (s, 6 H, p-
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ppm; cis isomer: δ = 7.30 (s, 2 H, Hpz); 6.93 (s, 4 H, m-HMesNi);
6.44 (s, 4 H, m-HMesN); 2.90 (s, 6 H, p-CH3MesN); 2.78 (s, 6
H, p-CH3MesNi); 2.73 (s, 12 H, o-CH3MesN); 2.29 (s, 12 H, o-
CH3MesNi); 2.28 (s, 6 H, CH3imine) ppm.
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Supporting Information (see footnote on the first page of this arti-
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∧
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Acknowledgments
Stefan Brandholt and Peter Kliesen, University of Cologne, are
acknowledged for contributions in preparative work and DSC
measurements. C. H. and A. K. are grateful for financial support
by the Deutsche Forschungsgemeinschaft (DFG) (KL 1194/4-1 and
KL 1194/5-1). D. A. V. thanks the Office of Basic Energy Sciences
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Received: December 23, 2011
Published Online: March 30, 2012
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© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
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