Back-bonding in organonickel complexes with terpyridine ligands - A structural approach

Article abstract of DOI:10.1002/zaac.200700250

A series of organonickel complexes [(R′terpy)Ni(aryl)]X (R′terpy = derivatives of 2,2′;6′,6″-terpyridine; R′ = 4-H, 4-Cl, 4-Tol and 4,4′,4″-^tBu₃; aryl = 2,6-dimethylphenyl = Xyl or 2,4,6-trimethylphenyl = Mes; X = Br or PF₆) have been prepared and characterized. The crystal structures exhibit a number of intermolecular H bond type interactions, but the structure determining force seems to be the packing of the aryl co-ligands. The molecules reveal rather undistorted square planar coordination with a N₃C ligand set, the central Ni-N bond being remarkably short, despite the expected strong trans influence of the aryl co-ligands. The longwavelength absorptions were assigned to charge transfer transitions. No emission is observed at ambient temperature in the solid and in solution and at low temperature in glasses.

Full text of DOI:10.1002/zaac.200700250

DOI: 10.1002/zaac.200700250
Back-bonding in Organonickel Complexes with Terpyridine Ligands ؊
A Structural Approach
´
Claudia Hamacher, Natascha Hurkes, Andre Kaiser, and Axel Klein*
Köln/Germany, Universität zu Köln, Institut für Anorganische Chemie
Received May 16th, 2007.
Abstract. A series of organonickel complexes [(RЈterpy)Ni(aryl)]X
(RЈterpy ϭ derivatives of 2,2Ј;6Ј,6Љ-terpyridine; RЈ ϭ 4-H, 4-Cl,
4-Tol and 4,4Ј,4Љ-tBu3; aryl ϭ 2,6-dimethylphenyl ϭ Xyl or 2,4,6-
trimethylphenyl ϭ Mes; X ϭ Br or PF6) have been prepared and
characterized. The crystal structures exhibit a number of intermo-
lecular H bond type interactions, but the structure determining
force seems to be the packing of the aryl co-ligands. The molecules
reveal rather undistorted square planar coordination with a N3C
ligand set, the central NiϪN bond being remarkably short, despite
the expected strong trans influence of the aryl co-ligands. The long-
wavelength absorptions were assigned to charge transfer tran-
sitions. No emission is observed at ambient temperature in the solid
and in solution and at low temperature in glasses.
Keywords: Terpyridine; Nickel; Crystal structure; Aryl ligands;
Absorption spectroscopy
1 Introduction
ous platinum(II) complexes with interesting photophysical
properties e.g. long-lived emission [17Ϫ19], photocatalysis
[20], or photoinduced electron transfer [21]. In contrast to
this there are no reports on related photophysical or photo-
chemical properties of corresponding nickel(II) complexes.
The second important property of the terpy ligand, its
rigidity, has been frequently used in nickel terpy coordi-
nation chemistry e.g. for the build-up of supramolecular as-
semblies [15a, 22], for crystal engineering [23], and for the
preparation of five-coordinated nickel(II) complexes, which
are suitable for materials with interesting magnetic proper-
ties [22b, 24], to mimic the binuclear nickel/iron sites in
[FeNi] or [FeNiSe] hydrogenases [25], or together with
MAO as polymerization catalyst in vinyl-type polymeriz-
ation of norbornene [26].
Very recently Vicic et al. have reported the use of methyl-
nickel complexes of terpy in alkyl-alkyl cross-coupling
catalysis [6]. Such reactions can be carried out using
[(tmeda)Ni(Me)2] and RЈterpy ligands as pre-catalysts and
the species involved are supposed to be [(RЈterpy)Ni(Me)2],
[(RЈterpy)Ni(Me)] and [(RЈterpy)Ni(Me)]ϩ. For the for-
mally reduced complex [(RЈterpy)Ni(Me)] the two canoni-
cal forms [(RЈterpyϪ)NiII(Me)] ↔ [(RЈterpy)NiI(Me)] with
either reduced terpy ligand and nickel(II) or neutral terpy
and a nickel(I) atom have to be considered. EPR experi-
ments and DFT calculations have proved a markedly higher
contribution for the further [6a].
Organometallic nickel complexes with α-diimine ligands
like 2,2Ј-bipyridine (bpy), 1,10-phenanthroline (phen) or di-
azabutadienes (R-DAB) have gained an enormous interest
in the last decade. This is mainly due to their success as
effective catalysts in olefin polymerisation or olefin/CO
co-polymerisation [1Ϫ4], as well as their role in catalytic
electrochemically [4, 5] or chemically driven [6] CϪC cross-
coupling reactions. Associated with this development an
increasing interest in the fundamental investigation of
structures, spectroscopic and electrochemical properties of
such organonickel complexes can be stated [7Ϫ13].
The potentially tridentate diimine ligand 2,2Ј;6Ј,6Љ-terpy-
ridine (terpy) combines an orbital pattern with several
rather close lying π*-orbitals with a very rigid backbone
(when tri-coordinated) and easy chemical variability, mainly
of the central pyridine unit [14], which makes terpy a very
attractive ligand in coordination chemistry [15]. As a first
consequence of the above described properties terpy usually
enables corresponding transition metal complexes to exhibit
intense and relatively long-wavelength emissions from cor-
3
responding π-π* states [16]. In the field of group 10 or-
ganometallic complexes with terpy ligands there are numer-
* Prof. Dr. Axel Klein
Institut für Anorganische Chemie
der Universität zu Köln
Greinstraße 6
D-50939 Köln (Germany)
Phone: ϩ49 211 4704006; FAX: ϩ49 221 4704899
e-mail: axel.klein@uni-koeln.de
In this paper we want to report on a series of novel or-
ganometallic terpy nickel complexes of the type [(RЈterpy)-
Ni(aryl)]X (RЈterpy ϭ terpy, 4Ј-Cl-terpy (Clterpy), 4Ј-p-to-
lyl-terpy (Tolterpy) or 4,4Ј,4Љ-tBu3terpy (Bu3terpy); aryl ϭ
2,6-dimethylphenyl (Xyl) or 2,4,6-trimethylphenyl (Mes);
X ϭ Br or PF6). The use or aryl co-ligand instead of methyl
is intended to enhance the stability of such organonickel
complexes and thus facilitate thorough investigations of the
Supporting information for this article is available on the
WWW under http://www.wiley-vch.de/home/zaac or from the
author
Z. Anorg. Allg. Chem. 2007, 633, 2711Ϫ2718
 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
2711

C. Hamacher, N. Hurkes, A. Kaiser, A. Klein
a Bruker AC 200 or Bruker Avance 400 spectrometer (1H:
400,13 MHz, 13C: 100,61 MHz) using a triple resonance 1H,19F,BB
inverse probe head. The unambiguous assignment of the 1H was
obtained from 1H NOESY and 1H COSY experiments. All 2D
NMR experiments were performed using standard pulse sequences
from the Bruker pulse program library. Chemical shifts were rela-
tive to TMS. UV/vis/NIR absorption spectra were recorded on
Varian Cary 05E or Cary50 Scan spectrophotometers. Lumi-
nescence measurements were performed on solid samples (powders)
or solution at ambient temperature on a SPEX Fluorolog photo-
spectrometer. For [(terpy)Ni(Mes)Br emission experiments were
carried out on samples in glassy frozen MeCN or PrCN at 77 K
using an Oxford Instruments cryostat, a Spectra Physics GCR3
Nd:YAG laser as excitation source and an OMA detection system
described elsewhere [27].
molecular and electronic structure. The variation of the
terpy ligand will help to shed more light on its role in theses
complexes, which seem to have a great potential in or-
ganometallic catalysis. We have synthesized and charac-
terized these complexes by NMR and UV/vis absorption
spectroscopy and obtained a number of crystal and molecu-
lar structures from single crystal XRD experiments, al-
lowing for the first time to probe for these properties in a
sizable series of organonickel complexes with terpy ligands.
2 Experimental Part
General Information. Commercially available reagents (solvents and
ligands) from Sigma-Aldrich or Acros were used without further
purification. Solvents were dried by standard procedures. All reac-
tions involving metal complexes were conducted under argon by
standard Schlenk techniques. The complexes trans-[(PPh3)2Ni-
(aryl)Br] (aryl ϭ Mes or Xyl) were obtained as described for the
Mes derivative [10].
Crystal structure analysis. The measurement were performed using
˚
graphite-monochromatized Mo-Kα radiation (λ ϭ 7.1073 A) on
IPDS I (at 293 K), IPDS II (173 K) (STOE and Cie.), or Bruker
P4 (153 K) diffractometers. The structures were solved by direct
methods (SHELXS-97) [28] and refined by full-matrix least-squares
techniques against F2 (SHELXL-97) [29]. The non-hydrogen atoms
were refined with anisotropic displacement parameters without any
constraints. The hydrogen atoms were included by using appropri-
ate riding models. Further details are listed in Table 3 and outlined
in the text.
Synthesis of the complexes [(RЈterpy)Ni(aryl)]Br. In a typical reac-
tion 0.35 mmol of arylnickel precursor trans-[(PPh3)2Ni(aryl)Br]
and 0.36 mmol of the RЈterpy ligand were mixed in 100 mL of tolu-
ene and stirred at ambient temperature for at least 8 h, during
which the products slowly precipitate. After completion of the reac-
tion the volume of the solvent was reduced to half and the crude
products isolated by filtration. After recrystallization from dichlor-
methane and heptane (3:1) the complexes were obtained as air-
stable orange to red microcrystalline materials. Elemental analysis
and selected NMR data is summarized in Table 1.
Full structural information has been deposited with the Cambridge
Crystallographic Data Centre, No. CCDC-646981 for [(terpy)Ni-
(Mes)]Br, -646982 for [(Clterpy)Ni(Mes)]Br, -646983 for [(Clterpy)-
NiXyl]Br, -646984 for [(terpy)Ni(Xyl)](PF6) and -646985 for
[(Clterpy)Ni(Xyl)](PF6). Copies of the data can be obtained,
free of charge, on application to CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK (fax: ϩ44-1223 336033 or e-mail:
deposit@ccdc.cam.ac.uk)
Synthesis of the complexes [(RЈterpy)Ni(aryl)]PF6. In a typical reac-
tion 0.1 mmol of the complex [(RЈterpy)Ni(aryl)]Br and 0.1 mmol
KPF6 were mixed in 20 mL of acetone and stirred for three days,
during which KBr slowly precipitates. After careful filtration, the
filtrate was evacuated to dryness and the residue recrystallized from
dichloromethane and heptane (1:1). The complexes were obtained
as microcrystalline materials. Elemental analyses are summarized
in Table 1. The NMR data is identical to the bromide derivatives.
3 Results and Discussion
3.1 Preparation
The complexes [(terpy)Ni(aryl)]Br were prepared from the
Instrumentation. Elemental analysis was obtained using a Henatech
CHNS EuroEA 3000 analyzer. NMR spectra were recorded on
precursor compounds trans-[(PPh3)2Ni(aryl)]Br and the
Table 1 Analytical data of complexes [(RЈterpy)Ni(aryl)]X.
elemental analyses / %
found
C
(calcd.)
H
yield/ %
formula (MW / g·molϪ1
)
N
Terpy / Mes / Br
95
98
88
97
86
94
88
89
83
92
91
97
83
95
81
90
C24H22N3Ni1Br1 (491.07)
C24H22N3Ni1P1F6 (560.17)
C31H28N3Ni1Br1 (581.20)
C31H28N3Ni1P1F6 (646.27)
C36H46N3Ni1Br1 (659.40)
C36H46N3Ni1P1F6 (724.47)
C24H21N3Cl1Ni1Br1 (525.51)
C24H21N3Cl1Ni1P1F6 (724.47)
C23H20N3Ni1Br1 (477.04)
C23H20N3Ni1P1F6 (542.11)
C30H26N3Ni1Br1 (567.17)
C30H26N3Ni1P1F6 (632.24)
C35H44N3Ni1Br1 (645.37)
C35H44N3Ni1P1F6 (710.44)
C23H19N3Cl1Ni1Br1 (511.49)
C23H19N3Cl1Ni1P1F6 (567.56)
58.73 (58.70)
52.73 (51.83)
64.03 (64.06)
58.73 (58.70)
65.61 (65.57)
60.16 (59.68)
54.82 (54.85)
49.01 (48.81)
57.88 (57.91)
51.01 (50.96)
63.58 (63.53)
56.89 (56.99)
65.12 (65.14)
59.16 (59.17)
54.02 (54.01)
47.96 (47.91)
4.58 (4.52)
3.94 (3.99)
4.60 (4.86)
4.58 (4.52)
7.08 (7.03)
6.35 (6.40)
3.98 (4.03)
3.62 (3.58)
4.31 (4.23)
3.80 (3.72)
4.71 (4.62)
4.10 (4.15)
6.94 (6.87)
6.35 (6.24)
3.78 (3.74)
3.38 (3.32)
8.59 (8.56)
7.59 (7.56)
7.36 (7.23)
8.59 (8.56)
6.38 (6.37)
5.79 (5.80)
8.03 (8.00)
7.08 (7.12)
8.78 (8.81)
7.73 (7.75)
7.43 (7.41)
6.60 (6.65)
6.66 (6.51)
5.83 (5.91)
8.12 (8.22)
7.30 (7.29)
Terpy / Mes / PF6
Tolterpy / Mes / Br
Tolterpy / Mes / PF6
tBu3terpy / Mes / Br
tBu3terpy / Mes / PF6
Clterpy / Mes / Br
Clterpy / Mes / PF6
Terpy /Xyl / Br
Terpy /Xyl / PF6
Tolterpy / Xyl / Br
Tolterpy / Xyl / PF6
tBu3terpy / Xyl / Br
tBu3terpy / Xyl / PF6
Clterpy / Xyl / Br
Clterpy / Xyl / PF6
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 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2007, 2711Ϫ2718

Back-bonding in Organonickel Complexes with Terpyridine Ligands
a)
Table 2 1H NMR data of complexes [(RЈterpy)Ni(aryl)]X.
1H NMR δ/ppmb)
3Ј,5Ј
6,6Љ
5,5Љ
4,4Љ
3,3Љ
4Ј
o-CH3
m-H
p-CH3
Terpy / Mes
7.48 d
7.48 d
7.34 d
7.46 d
6,6Љ
7.48 d
7.47 d
7.34 d
7.46 d
7.61 dd
7.64 dd
7.57 d
7.64 dd
5,5Љ
7.66 dd
7.63 dd
7.55 dd
7.66 dd
8.35 dd
8.36 dd
Ϫ
8.34 dd
4,4Љ
8.40 dd
8.36 dd
Ϫ
8.69 d
8.93 d
8.92 s
8.94 d
3,3Љ
8.85 d
8.92 d
8.90 d
8.87 d
8.72 s
9.05 s
8.95 s
9.06 s
3Ј,5Ј;
8.85 s
9.03 s
8.92 s
8.98 s
8.52 dd
Ϫ
Ϫ
Ϫ
4Ј
8.60 dd
Ϫ
Ϫ
Ϫ
2.96 s
2.99 s
2.95 s
2.93 s
o-CH3
3.05 s
3.04 s
3.01 s
3.00 s
6.72 s
6.74 s
6.72 s
6.70 s
m-H
6.92 m
6.90 m
6.88 m
6.87 m
2.29 s
2.30 s
2.29 s
2.27 s
p-H
6.92 m
6.90 m
6.88 m
6.87 m
c)
Tolterpy /Mes
tBu3terpy /Mes
Clterpy / Mes
d)
Terpy / Xyl
e)
Tolterpy /Xyl
tBu3terpy /Xyl
Clterpy / Xyl
d)
8.37 dd
a) The nature of X has no impact on the 1H NMR data. b) Measured in acetone-d6. c) Further signals at 7.48 (m, 2H, 3,5Tol); 8.20 (d, 2H, 2,6Tol); 2.45 (s, 3H,
pTolCH3) ppm. d) Further signal at 1.43 (s, 27H,tBu) ppm. e) Further signals at 7.47 (m, 2H, 3,5Tol); 8.18 (d, 2H, 2,6Tol); 2.45 (s, 3H, pTolCH3) ppm.
corresponding RЈterpy in an alternative way to what
has been reported for [(terpy)Ni(Mes)]Br [11]. To enhance
the solubility of the cationic complex species
[(RЈterpy)Ni(aryl)]ϩ in unpolar solvents the bromide was
Ϫ
exchanged by PF6 by a typical anion metathesis reaction.
Indeed, the solubility of the PF6-complexes is greatly
enhanced in all kind of organic solvents (e.g. up to 100
times in CH2Cl2). Analytical data (Table 1) and NMR spec-
troscopy (Table 2) confirmed the expected constitution of
the complexes. The compounds are air-stable and remark-
ably moisture-stable. It was impossible to determine the
melting points of the compounds, since at temperatures
above 250-280 °C they decompose.
3.2 Crystal and molecular structures
Figure 1 Crystal structure of [(terpy)Ni(Mes)]Br, viewed along the
b axis.
From five of the complexes single crystals were obtained
and submitted to X-ray diffraction experiments. The com-
plexes [(terpy)Ni(Mes)]Br and [(Clterpy)Ni(Xyl)]Br were
¯
found to crystallize in the triclinic space group P1 whereas
for [(Clterpy)Ni(Mes)]Br, [(Clterpy)Ni(Mes)](PF6) and
[(Clterpy)Ni(Xyl)](PF6) the structures were solved and re-
¯
fined in trigonal R3. Details are listed in Table 3. The over-
all quality of the two triclinic structures and the structure
of [(Clterpy)Ni(Xyl)]Br is rather poor as can be seen from
the R values. However, alternative structure solutions have
been tried, but failed and the observed residual electron
density and holes for the resolved structures are in an
acceptable range. Moreover, the molecular structures show
a very high degree of conformity (see e.g. Table 4), therefore
we are confident that the structure solutions and refine-
ments were carried out correctly and only poor crystal
quality has led to the observed deficiencies.
Figure 1 and 2 exhibit the packing of the molecules in
two selected samples. The three bromide compounds exhibit
a number of intermolecular Br···H interaction (Figure 1),
whereas the PF6 salts show close F···H contacts. Regardless
of such XϪH interactions, all three trigonal structures exhi-
bit two rosette motifs formed by the aryl co-ligands (Fig-
ure 2), whereas the triclinic structures can be characterized
by a layer-type packing of the aryl co-ligands.
Figure 2 Crystal structure of [(Clterpy)Ni(Xyl)](PF6), viewed
along the c axis.
Figure 1 shows the packing of the molecules in the unit
cell of [(terpy)Ni(Mes)]Br indicating some of the observed
H···Br contacts. For the other complexes similar intermo-
lecular H···Br contacts were found ranging from 2.34 to
˚
˚
3.00 A. Most of them (values > 2.5 A) are not significant
Z. Anorg. Allg. Chem. 2007, 2711Ϫ2718
 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
2713

C. Hamacher, N. Hurkes, A. Kaiser, A. Klein
for effective hydrogen bridges [30], only the short contact
of Br···H(4)ϪC(4) of 2.345(6) in [(terpy)Ni(Mes)Br] lies in
the range of significant or strong H bridges. Also the
Br···H-C angle of 176(1)° is in line with strong H binding
interaction. However, for the isostructural compound
[(Clterpy)Ni(Xyl)]Br the analogous contact is far longer
giving rise to dimers in the crystal structure [6b]. Stacking
is also very frequent in square planar terpy complexes of
palladium(II) [31], platinum(II) [32], or copper(II) [33], the
number of examples with no such interaction is marginal.
They contain either a sterically highly demanding co-ligand
like e.g. [(terpy)Pt(aryl)]3ϩ (aryl ϭ 2,6-di(N-piperidinium-meth-
yl)phenyl) [34], or the terpy ligand is substituted with bulky
groups as e.g. in [(tBu3terpy)Pt(CϵCC6H4CϵCH](OTf) [tBu3-
terpy ϭ 4,4Ј4Љ-tri-tbutyl(terpyridine)] [35]. It is therefore
very likely that the bulky xylyl or mesityl co-ligands prevent
effective π-stacking in our complexes.
˚
(2.926(4) A) and the aromatic CϪH unit is not an ideal
H donor function [30]. For the compounds containing
PF6Ϫ anions short contacts were found between F atoms of
˚
the anions and HϪC(2) units (2.328(3) A) in [(Clterpy)-
Ni(Xyl)](PF6). In [(terpy)Ni(Xyl)](PF6) short contacts be-
˚
tween F atoms and HϪC(2) of 2.552(3) A, HϪC(3) of
˚
˚
2.544(4) A and HϪC(3) units of 2.481(4) A respectively are
observed. Also here the F···HϪC angles agree with a strong
H bonding interaction (170-180°). In contrast to this, the
thermal ellipsoids of the corresponding F atoms do not re-
veal any sign of orientation towards the HϪC unit.
The rosette motif found in the three trigonal structures is
formed by two threefold packing of aryl co-ligands in two
layers connected by a S3 axis. In contrast to this, the aryl
co-ligands in the triclinic structures are packed in ex-
tended layers.
Figure 3 Molecular structures of [(terpy)Ni(Xyl)](PF6) (left) and
[(Clterpy)Ni(Mes)]Br (right) with full numbering. Shown are 30 %
thermal ellipsoids, H atoms and Br or PF6 counter ions are omitted
for clarity.
Whether the intermolecular X···H interaction or the co-
ligand packing is the dominant force for the formation of
the crystal structures cannot be answered unequivocally. In
¯
¯
view of the found selectivity P1 vs. R3 the packing of the
co-ligand being responsible for the crystal structure seems
to be more plausible.
Two examples of the molecular structures are displayed
in Figure 3. The coordination around the nickel atom is
almost planar. The C(20) atom lies slightly above the plane
defined by Ni(1), N(1), N(2) and N(3) with a maximum
Furthermore in none of the structures we observe π-
stacking interactions of the planar aromatic terpy ligand.
Such stacking has been reported for [(tBu3terpy)Ni(Me)]
˚
value of about 0.23 A found for [(Clterpy)Ni(Mes)]Br (see
Figure 3 right). In the same complex also the dihedral angle
Table 3 Crystallographic and structure refinement data of terpy nickel complexes.a)
compound
[(terpy)Ni
(Mes)]Br
[(Clterpy)Ni
(Mes)]Br
[(Clterpy)Ni
(Xyl)]Br
[(terpy)Ni
(Xyl)](PF6)
[(Clterpy)Ni
(Xyl)](PF6)
Formula
C24H22N3NiBr
491.07
C24H21ClN3NiBr
525.51
C23H19ClN3NiBr
511.48
C23H20N3NiPF6
542.10
C23H19ClN3NiPF6
576.54
Weight /g·molϪ1
Crystal system
Space group
triclinic
trigonal
triclinic
trigonal
trigonal
¯
¯
¯
¯
¯
P1
R3
P1
R3
R3
Temperature /K
153(2)
293(2)
41.324(6)
41.324(6)
7.4671(15)
90
90
120
11043(3) / 18
1.422
2.542 / 4788
Ϫ53 < h < 52,
Ϫ53 < k < 52,
Ϫ8 < l < 9
53722 / 5515
0.1353
293(2)
7.955(2)
11.106(3)
14.907(4)
68.16(3)
83.46(3)
72.60(3)
1166.5(5) / 2
1.456
2.672 / 516
Ϫ10 < h < 10,
Ϫ14 < k < 14,
Ϫ19 < l < 19
14107 / 5261
0.2227
293(2)
36.991(5)
36.991(5)
8.8017(18)
90
173(2)
38.207(5)
38.207(5)
8.5164(17)
90
90
120
10766(3) / 18
1.601
1.054 / 5256
Ϫ49 < h < 49,
Ϫ49 < k < 48,
Ϫ10 < l < 10
42237 / 5366
0.1000
˚
cell
a /A
8.1620(16)
8.5790(17)
17.136(3)
76.81(3)
86.28(3)
70.14(3)
1098.7(4) / 2
1.484
2.716 / 500
Ϫ10 < h < 10,
Ϫ11 < k < 10,
Ϫ22 < l < 22
31507 / 5045
0.1195
˚
b /A
˚
c /A
α /°
β /°
γ /°
90
120
3
3
˚
˚
V /A / Z
10430(3) A , 18
1.553
ρcalc /g cmϪ3
µ /mmϪ1 / F(000)
Limiting indices
0.971 / 4968
Ϫ47 < h < 47,
Ϫ47 < k < 47,
Ϫ10 < l < 11
22171 / 4938
0.0718
Refl. collect. / unique
Rint
Data / restr. / param.
5045 / 0 / 275
1.133
5515 / 0 / 275
0.882
5261 / 0 / 274
0.813
4938 / 0 / 309
0.866
5366 / 0 / 393
0.911
Goof. on F2
Final R1, wR2 indices
[I>2σ(I)]
R1, wR2 (all data)
R1 ϭ 0.0790,
wR2 ϭ 0.2171
R1 ϭ 0.0993,
wR2 ϭ 0.2282
0.993 and Ϫ0.656
R1 ϭ 0.0667,
wR2 ϭ 0.1739
R1 ϭ 0.1488,
wR2 ϭ 0.2183
0.904 and Ϫ0.432
R1 ϭ 0.1036,
wR2 ϭ 0.2170
R1 ϭ 0.2839,
wR2 ϭ 0.2889
0.975 and Ϫ0.737
R1 ϭ 0.0403,
wR2 ϭ 0.0940
R1 ϭ 0.0862,
wR2 ϭ 0.1092
0.266 and Ϫ0.571
R1 ϭ 0.0505,
wR2 ϭ 0.1274
R1 ϭ 0.0808,
wR2 ϭ 0.1434
0.481 and Ϫ0.507
Ϫ3
˚
Largest diff. peak and hole /e·A
a)
2
˚
Radiation wavelength λ ϭ 0.71073 A; Refinement method: Full-matrix least-squares on F
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Z. Anorg. Allg. Chem. 2007, 2711Ϫ2718

Back-bonding in Organonickel Complexes with Terpyridine Ligands
˚
˚
of the pyridine moieties has its maximum with 10.9(4)°. The
values for the whole series range down to 1° but do not
correlate with the composition changes along the series.
The aryl co-ligands are oriented almost perpendicular to
the coordination plane. The corresponding dihedral angles
range between 90.40(11) and 99.9(2)°. The ortho-methyl
groups effectively shield the nickel atom, the distances
(1.880(7) A) and NiϪC bond (1.921(9) A) are longer in this
latter system [6a, b], but still the NiϪN(2) distance is small
˚
compared to non-organometallic derivatives (ca. 1.98 A).
Upon reduction of the complex [(tButerpy)Ni(Me)]ϩ to
[(tButerpy)Ni(Me)] the distance is reduced from 1.880(7)
˚
to 1.845(3) A, DFT calculation for the terpy derivative
[(terpy)Ni(Me)] gave a value of 1.867 A [6a].
˚
˚
(3.10Ϫ3.21 A) lie in the range observed for related com-
plexes [10, 11, 13].
It seems that the strong carbanionic ligand provokes a
more effective Ni-terpy back-bonding thus shortening the
NiϪN(2) bond. To judge the extend of back-bonding to the
terpy ligand, the contraction of the C(5)ϪC(6) and
C(10)ϪC(11) distances in comparison to the free ligand is a
good measure. Going from the analogous methyl complexes
[(tButerpy)Ni(Me)]ϩ to the reduced species [(tButerpy)Ni(Me)]
the distances decrease from 1.489(11) and 1.465(12) A to
1.450(4) and 1.455(4) A, respectively. DFT calculation for
the terpy derivative [(terpy)Ni(Me)] gave almost the same
values [6a]. The C(5)ϪC(6) and C(10)ϪC(11) distances ob-
served for our complexes lie markedly below the values for
The most striking structural feature is the shortness of
the Ni(1)ϪN(2) bond. The fact that this central NiϪN
bond is shorter than the two peripheral ones might be attri-
buted to the relatively rigid ligand geometry allowing only
an optimum overlap for the central N(2) atom with the
nickel orbitals. E.g. the central NiϪN(2) bond is also
shorter compared to the peripheral NiϪN bonds in com-
plexes like [(terpy)Ni(ER)2] (E ϭ Se or S, R ϭ mesityl or
xylyl) with a trigonal bipyramidal geometry [25b, 36]. How-
˚
˚
˚
ever, the observed values of about 1.86 A are markedly
˚
shorter than those for these non-organometallic derivatives
the free ligand of 1.488(8) and 1.493(8) A [37] and slightly
˚
(ca. 1.98 A), which stands in contrast to the expected strong
below the values found for the methyl derivative
[(tButerpy)Ni(Me)]ϩ. This is a strong indication that effec-
tive back-bonding takes place in the organometallic com-
plexes, with a slightly higher efficiency for the aryl deriva-
tives. For the chloro substituted derivatives the back-bond-
ing should be enhanced compared to the terpy analogues,
due to the electron-withdrawing nature of chlorine, but in
our series no such effect was observed.
trans influence of the aryl co-ligands. They are also shorter
than the NiϪN bonds in the comparable bipyridine com-
plex [(bpy)Ni(Mes)Br] [10a]. In this complex the NiϪN
bond trans to the mesityl co-ligand was found to be far
˚
longer with 1.980(6) or 1.987(6) A respectively (two inde-
pendent molecules) and the NiϪN distances trans to the
˚
weak ligand Br are much the same (1.895(6) or 1.897(6) A
respectively) as the NiϪN(1) and NiϪN(3) distances in the
terpy complexes. The NiϪC distances in both systems are
also quite similar (1.904(8) or 1.887(8) A for the bpy com-
Interestingly the coordination number of four seems to
be restricted to organonickel derivatives, since the numerous
so far crystallographically characterized non-organometal-
lic terpy nickel complexes contain two or three co-ligands.
The reason for this probably lies in the fact that the terpy
˚
plex). Comparison with the recently reported methyl deriva-
tive [(tButerpy)Ni(Me)]I reveals that both NiϪN(2)
Table 4 Selected structural data of [(terpy)NiMes]Br.
[(terpy)Ni
(Mes)]Br
[(Clterpy)Ni
(Mes)]Br
[(Clterpy)Ni
(Xyl)]Br
[(terpy)Ni
(Xyl)](PF6)
[(Clterpy)Ni
(Xyl)](PF6)
˚
distances/ A
Ni(1)ϪN(1)
Ni(1)ϪN(2)
Ni(1)ϪN(3)
Ni(1)ϪC(20)
C(5)ϪC(6)
C(10)ϪC(11)
Ni(1)ϪC(26)
Ni(1)ϪC(28)
C20 above the terpyNi plane
1.906(5)
1.864(5)
1.907(5)
1.899(5)
1.478(9)
1.472(8)
3.215(6)
3.102(6)
0.129(6)
1.908(6)
1.865(5)
1.906(5)
1.916(6)
1.471(9)
1.451(9)
3.210(7)
3.177(7)
0.234(7)
1.921(10)
1.845(11)
1.917(11)
1.864(14)
1.50(2)
1.899(2)
1.868(2)
1.917(3)
1.898(3)
1.464(5)
1.478(5)
3.128(2)
3.154(2)
0.006(3)
1.919(3)
1.860(3)
1.906(3)
1.890(3)
1.473(5)
1.475(5)
3.153(3)
3.106(3)
0.048(3)
1.44(2)
3.125(10)
3.132(10)
0.014(10)
angles / °
N(2)ϪNi(1)ϪC(20)
N(2)ϪNi(1)ϪN(1)
N(1)ϪNi(1)ϪN(3)
N(2)ϪNi(1)ϪN(3)
C(20)ϪNi(1)ϪN(1)
C(20)ϪNi(1)ϪN(3)
175.8(2)
82.9(2)
165.3(2)
82.4(2)
97.7(2)
96.8(2)
175.6(3)
82.5(2)
164.5(2)
82.6(2)
96.7(3)
98.4(3)
179.0(6)
81.6(5)
164.6(5)
83.1(6)
97.5(5)
97.9(6)
178.94(11)
82.87(11)
165.72(10)
82.86(11)
98.20(10)
96.08(11)
177.79(14)
82.76(12)
165.86(12)
83.12(12)
96.13(13)
97.95(13)
dihedral angles / °
terpyNi / aryl
pyN(1) / pyN(3)
pyN(1) / pyN(2)
pyN(3) / py N(2)
93.6(2)
1.0(4)
2.4(4)
1.8(5)
99.9(2)
10.9(4)
5.3(5)
94.7(5)
4.1(7)
4.6(7)
5.3(7)
90.4(1)
2.2(2)
3.5(2)
4.4(2)
93.6(1)
1.7(2)
6.5(2)
6.0(2)
6.2(5)
Z. Anorg. Allg. Chem. 2007, 2711Ϫ2718
 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
2715

C. Hamacher, N. Hurkes, A. Kaiser, A. Klein
ligand represents three coordinating donor atom but usu-
ally cannot cover three fourths of the coordination sphere.
The bite angle of the two peripheral pyridine N atoms with
the nickel atom [N(1)ϪNiϪN(3)] lies in most cases about
156° or below. We suppose that strong carbanionic aryl or
alkyl co-ligands render the ligand field high enough to shift
the preference to four-fold square planar coordination. Fur-
thermore the bite angle in the organometallic systems is in-
creased to about 165° which points to a slight enlargement
of the nickel atom resulting from the electron donation by
the carbanionic co-ligands. For analogous palladium or
platinum complexes the (four-fold) square planar coordi-
nation sphere for terpy complexes is the normal case due to
the intrinsic higher ligand field and the larger size of the
atoms [31, 32, 34, 35].
[(terpy)Ni(Me)] by Vicic et al. revealed that the SOMO has
b1 character with a small but appreciable contribution from
the 4, 4Ј and 4Љ positions [6a]. This explains well our finding
t
that the Bu group exhibits the highest and the Tol group
the lowest absorption energies in the series, since the first
destabilizes the b1 LUMO, whereas the latter extends the π-
systems thus stabilizing the LUMO. The strong electron-
donating power of the co-ligands, destabilizing the HOMO,
is manifested in the relatively low-lying charge transfer ab-
sorptions. Variation of the co-ligand has almost no impact
on the absorption energy, as expected for the very similar
groups Mes vs. Xyl. Table 5 lists the observed absorption
maxima for the complexes of this study.
Table 5 Absorption spectra of complexes [(RЈterpy)Ni(aryl)]Br in
a)
MeCN solution.
Compound
λ4 (ε)
λ3 (ε)
λ2 (ε)
λ1 (ε)
3.3 Absorption and emission spectroscopy
[(terpy)Ni(Xyl)]Br
264 (22.8) 337 (16.8) 451 (2.1) 491sh (1.1)
262 (28.3) 336 (19.4) 454 (2.8) 487sh (1.5)
269 (29.0) 339 (22.0) 454 (2.3) 497sh (1.0)
263 (25.1) 334 (20.2) 443 (2.9) 484sh (1.5)
264 (26.9) 337 (20.3) 452 (2.6) 481sh (1.6)
262 (28.9) 336 (20.5) 455 (3.0) 487sh (1.5)
268 (21.9) 339 (15.8) 455 (2.5) 497sh (0.9)
The absorption spectra of [(terpy)Ni(Mes)]Br have been re-
ported recently in the frame of a number of organonickel
complexes [(N∧N)Ni(Mes)Br] [11]. The long-wavelengths
band systems at around 450 nm (maxima at 487 nm in tolu-
ene and 452 nm in MeCN respectively; see Figure 4) are
attributed to mixed M(Ni)-L(terpy)/L(Mes)-L(terpy) charge
transfer transitions (MLCT/LЈLCT), on the basis of their
similarity to the complexes [(N∧N)Ni(Mes)Br], for which
the assignment is supported by a combination of spec-
troscopy (absorption, resonance Raman) and quantum
chemical calculations [11]. The more intense structured
bands at around 340 nm (maximum at 341 in toluene and
337 nm in MeCN) are assigned to intraligand(terpy) (π-π*)
transitions. The variation of the RЈ substituent does not
lead to marked shifts of the absorption energy. There is a
slight red-shift for the intra-ligand band around 340 nm
along the series tBu > Cl > H > Tol, however, for the charge
[(Clterpy)Ni(Xyl)]Br
[(Tolterpy)Ni(Xyl)]Br
[(tBu3terpy)Ni(Xyl)]Br
[(terpy)Ni(Mes)]Br
[(Clterpy)Ni(Mes)]Br
[(Tolterpy)Ni(Mes)]Br
[(tBu3terpy)Ni(Mes)]Br 262 (24.7) 334 (19.7) 444 (2.8) 481sh (1.5)
a) Wavelengths λ in nm (extinction coefficients ε in 1000 MϪ1cmϪ1).
Scheme 1 Schematic representation of the a2 LUMO in the free
ligand terpy [38] and of the b1 SOMO in [(terpy)Ni(Me)] [6a].
t
transfer band at around 450 nm the sequence is Bu > H >
Cl > Tol. In the free ligand the a2 LUMO exhibits only
marginal electron density at the 4, 4Ј and 4Љ positions
(Scheme 1) [38]. Calculation for the reduced complex
The extinction coefficients ε for the charge transfer bands
lie in the range of 2000 to 3000 MϪ1cmϪ1. They are com-
parable to those observed for the diimine complexes
[(N∧N)Ni(Mes)Br] (N∧N ϭ α-diimine ligands such as bpy)
[11], but are considerably lower than the values obtained
for iron(II) complexes; e.g. for [Fe(terpy)2]2ϩ the charge
transfer band at 550 nm (in MeOH) has an ε of about
10000 MϪ1cmϪ1 [39]. From this side, the assumption of
very effective back-bonding strengthening the Ni-terpy
bonds is not supported, however, the intensity of absorp-
tion bands is ruled by many factors [16]. Quantum chemical
calculations might provide a better insight in these systems
and are thus sought for in the future.
For the recently investigated complexes [(N∧N)Ni(Mes)Br]
(N∧N ϭ α-diimine ligands such as bpy) no luminescence
was observed and we assume rapid radiationless deacti-
vation to be responsible for this [11]. Since the rigidity of
the terpy ligands should hamper some of the deactivation
pathways (vibrations) we anticipated luminescence for the
Figure 4 Absorption spectrum of [(terpy)Ni(Mes)]Br in MeCN
solution.
2716
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 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2007, 2711Ϫ2718

Back-bonding in Organonickel Complexes with Terpyridine Ligands
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compounds were examined in the solid (microcrystalline
powders) and in MeCN solution at ambient temperature,
additionally the compound [(terpy)Ni(Mes)]Br was meas-
ured at in at 110 K in glassy frozen MeCN solution). How-
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4 Summary and Outlook
The present series of arylnickel complexes with various
RЈterpy ligands has allowed for the first time an extended
investigation of the molecular structures and their absorp-
tion spectroscopy. The most striking feature in the molecu-
lar structures is the extremely short central NiϪN(2) dis-
tance compared to the distances to the peripheral N(1) and
N(3) atoms, but also compared to NiϪN(2) distances found
in non-organometallic nickel complexes of terpy. The short-
ening of the NiϪN(2) bond compared to NiϪN(1) and
NiϪN(3) is primarily due to the ligand geometry. The
shortening of the NiϪN(2) bond in comparison with non-
organometallic derivatives can be explained by an enhanced
back-donation from nickel to the terpy π* orbital. Whether
arylnickel derivatives are advantageous over methyl
derivatives due to additional co-ligand-nickel interactions
(potentially π-accepting) can only be assumed from the ob-
tained data. Since the RЈ substituent on the terpy has a
marked impact on the corresponding π* LUMO and the
only so far structurally characterized methyl derivative
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t
[(tButerpy)Ni(Me)]I contains the Buterpy ligand, which is
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missing in our series, an unambiguous conclusion in that
respect is not feasible. The results from optical spectroscopy
agree qualitatively well with the assumptions derived from
the molecular structures. Future investigation will seek to
close three gaps. First, the crystal and molecular structures
of tButerpy derivatives need to be studied, secondly, electro-
chemical and spectroelectrochemical measurement might
provide even better insight into the electronic structure.
And finally quantum chemical calculation will probably
amend the understanding of the optical spectra of such
complexes.
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Supporting Information. 25 tables containing full structural infor-
mation and 7 supplementary figures illustrating the crystal and mo-
lecular structures are provided.
Acknowledgments. We thank Dr. Ingo Pantenburg (University
of Cologne) for crystal data collection and helpful discussions.
Dr. Frantisek Hartl and Dr. Ron Jukes (University of Amsterdam)
are acknowledged for LT luminescence experiments on [(terpy)Ni-
(Mes)]Br.
´ ˇ
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