Nickel(II) complexes of the type [RM(oxam)MR] (oxam: Oxalamidinate, R=n-butyl, n-hexyl): The first binuclear n-alkyl nickel complexes

Article abstract of DOI:10.1016/j.jorganchem.2003.07.021

The reaction between one equivalent of [(acac)Ni(A)Ni(acac)] (A: N ¹, N ²-bis(2-pyridylmethyl)- N ³, N ⁴-bis-(2,4,6-trimethylphenyl)oxalamidinate) and two equivalents of R-Li (R= n -butyl; n -hexyl) results in the formation of the binuclear complexes [(R-Ni)(A)(Ni-R)] (1: R= n -butyl: 2= n -hexyl). Both compounds were characterized by ¹H- and ¹³C-NMR spectroscopy, elemental analysis, and mass spectroscopy. X-ray single diffraction studies of single crystals of 1 and 2 show that symmetrical binuclear complexes are formed in which the two Ni(II) centers are connected by the oxalamidinato bridging ligand A in a planar-square environment. No agostic interactions between the β-hydrogens of the n -alkyl groups and the metal centers were observed. DTA- and DTG-investigations show, that 1 and 2 are surprisingly thermally stable (decomposition temperature of 1: 188 °C under formation of butenes). Heating up a 1:1 mixture of 1 and 2 in toluene results in the formation of octane, decane and dodecane indicating an intermolecular transfer reaction of the n -alkyl-groups in solution. CV measurements display that the oxam complexes [(R-M)(A)(M-R)] (M=Ni, R=CH₃ (3), Ph (4), CCH (6), CCPh (7); M=Pd, R=CH₃ (5) are reversibly reduced in two steps indicating electronic interactions between the two metal centers.

Full text of DOI:10.1016/j.jorganchem.2003.07.021

Journal of Organometallic Chemistry 687 (2003) 153ꢁ160
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www.elsevier.com/locate/jorganchem
Nickel(II) complexes of the type [RM(oxam)MR] (oxam:
oxalamidinate, Rꢀ
/
n-butyl, n-hexyl): the first binuclear n-alkyl nickel
complexes
Michael Stollenz, Manfred Rudolph, Helmar Gorls, Dirk Walther *
¨
Institut fur Anorganische und Analytische Chemie der Friedrich-Schiller-Universitat Jena, August-Bebel-Straße 2, 07743 Jena, Germany
¨ ¨
Received in revised form 14 July 2003; accepted 14 July 2003
Abstract
The reaction between one equivalent of [(acac)Ni(A)Ni(acac)] (A: N1,N2-bis(2-pyridylmethyl)-N3,N4-bis-(2,4,6-trimethylpheny-
l)oxalamidinate) and two equivalents of R-Li (Rꢀn-butyl; n-hexyl) results in the formation of the binuclear complexes [(Rꢁ
Ni)(A)(NiꢁR)] (1: Rꢀn-butyl: 2ꢀ
n-hexyl). Both compounds were characterized by H- and 13C-NMR spectroscopy, elemental
/
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1
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/
analysis, and mass spectroscopy. X-ray single diffraction studies of single crystals of 1 and 2 show that symmetrical binuclear
complexes are formed in which the two Ni(II) centers are connected by the oxalamidinato bridging ligand A in a planar-square
environment. No agostic interactions between the b-hydrogens of the n-alkyl groups and the metal centers were observed. DTA- and
DTG-investigations show, that 1 and 2 are surprisingly thermally stable (decomposition temperature of 1: 188 8C under formation
of butenes). Heating up a 1:1 mixture of 1 and 2 in toluene results in the formation of octane, decane and dodecane indicating an
intermolecular transfer reaction of the n-alkyl-groups in solution. CV measurements display that the oxam complexes [(Rꢁ
/
M)(A)(MꢁR)] (MꢀNi, RꢀCH3 (3), Ph (4), CCH (6), CCPh (7); MꢀPd, RꢀCH3 (5) are reversibly reduced in two steps
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indicating electronic interactions between the two metal centers.
# 2003 Elsevier B.V. All rights reserved.
Keywords: Nickel; Palladium; Bridging ligand; Cyclic voltammetry; Electronic communication
1. Introduction
catalytically active in the Kumada cross coupling,
Sonogashira reaction and in the Heck reaction [2].
Moreover, a variety of Ni complexes of this type is
capable of oligomerizing/polymerizing ethylene [1,2]. In
this catalytic reaction two Ni centers, situated at the
periphery of the complexes, are found to be catalytically
active. Therefore, in the catalytic cycle both metals
should react to form reactive binuclear n-alkyl Ni
Bi- and oligonuclear complexes containing transition
metals bridged by chelating ligands which allow electro-
nic interactions between the metals are of growing
interest in catalysis [1ꢁ3]. Interesting and very variable
/
building block ligands are the oxalic amidines and their
dianions (‘oxam-type ligands’). These bridging ligands
can coordinate very different metals in different coordi-
nation modes [4,5].
Recently we described the first organometallics with
these ligands containing late metals (Ni, Pd) [2,6,7].
Furthermore, we constructed the first tri- and tetra-
nuclear complexes [1] in which two different metals are
linked by oxam bridges. Some of these complexes are
species containing R?(CH2)n ꢁ
/
Ni(oxam)Niꢁ(CH2)nR?
/
fragments which can undergo insertion of further
ethylene molecules to form two growing carbon chains
on the opposite sides of the oxam bridge. In this
connection the question arises wether it is possible to
isolate such types of binuclear n-alkyl Ni complexes
which could act as model compounds for the above
catalytic species.
Here we describe the synthesis, NMR spectra and
* Corresponding author. Tel.: ꢂ
948-102.
E-mail address: cdw@rz.uni-jena.de (D. Walther).
/
49-3641-948-119; fax: ꢂ49-3641-
/
solid state structure of the complexes (Rꢁ
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Ni)(A)(Niꢁ
/
R)
n-butyl; 2: n-hexyl; A: N1,N2-bis(2-pyridyl-
(1: Rꢀ
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0022-328X/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2003.07.021

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M. Stollenz et al. / Journal of Organometallic Chemistry 687 (2003) 153ꢁ160
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methyl)-N3,N4-bis(2,4,6-trimethylphenyl)oxalamidinat).
To the best of our knowledge these complexes are the
first binuclear n-alkyl-Ni compounds which could
synthesized and structurally elucidated.
Complexes 1 and 2 are remarkable thermally stable.
Furthermore, electrochemical measurements confirm
electronic interaction between the both Ni centers.
Comparison with other s-organo nickel complexes of
the above type shows that the electronic communication
between the metals strongly depends on the nature of
the group R.
found. HH-COSY experiments show that the second
multiplet belongs to the methyl groups of the butyl
groups. Three singlets for the methyl mesityl groups and
the CH2 groups were detected at dꢀ
ppm. The protons of the aromatic rings were found
between dꢀ5.64 and 7.90 ppm. These results indicate
that the structure of 1 is highly symmetrical in solution.
The corresponding 50 MHzꢁ13
C-NMR spectrum is in
agreement with this conclusion. The signals of the n-
butyl-chains appear at dꢀ14.0, 14.6, 27.0 and 33.0
/
2.23, 2.75 and 3.76
/
/
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ppm. Furthermore, the resonances of the methyl mesityl
groups were observed at 19.6 and 21.2 ppm. The CH2
groups of the oxam ligand were detected at 52.8 ppm. In
the region of the aromatic carbons the expecteted
number of 10 signals were found between 120.4 and
168.2 ppm.
2. Results and discussion
2.1. Synthesis and structures of the complexes 1 and 2
The molecular structure of 1 was determined by an X-
ray analysis of single crystals isolated from toluene (Fig.
Reaction of one equivalent [(acac)Ni(A)Ni(acac)] in
THF with two equivalents n-butyllithium in hexane at
1). Similar to other oxam complexes [1,2,4ꢁ7], 1 consists
/
of a bimetallic centrosymmetric unit in which the oxam
ligand acts in a bis-chelating fashion to bridge two
nickel centers. Each nickel atom is surrounded by three
N donor atoms of the oxam ligand and the a-carbon of
ꢃ
/
78 8C resulted in a dark red solution from which
orangeꢁbrown crystals of 1 of the composition [(Rꢁ
Ni)(A)(NiꢁR)] (Rꢀn-butyl) were obtained in 42%
yield. Using n-hexyllithium instead of n-butyllithium
yielded the analogous complex 2 (Rꢀn-hexyl) which
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the n-butyl chain. The central N2Cꢁ
essentially planar. Typically for bimetallic centrosym-
metric complexes of the oxam type the CꢁN bonds of
this framework are almost equivalent within the experi-
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CN2-framework is
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was isolated as a golden brown crystalline solid in 49%
yield. Single crystals of 1 and 2 were grown from toluene
at ꢃ18 8C. Both compounds are readily soluble in
/
/
˚
mental error (C1ꢁ
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N1 1.333(4) and C2ꢁN2 1.304(4) A)
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THF, toluene and benzene. It is noteworthy that
complex 2 is also soluble in pentane. DTA measure-
ments under isochoric conditions showed the remark-
able high thermal stability of the complexes. Complex 1
started to decompose at 188 8C in the solid state and 2
decomposes at 186 8C (Scheme 1).
Elemental analysis and mass spectra of 1 and 2
confirmed their compositions. Both complexes 1 and 2
are diamagnetic, so their NMR spectra showed signals
in the expected range for a diamagnetic binuclear
complex, wherein the oxam acts as a bischelating
anionic ligand. In the 1H-NMR spectrum of 1, recorded
at 400 MHz in benzene-d6 at room temperature, three
indicating a high double bond character due to the
electron delocalization over the two CN2 units. Conse-
quently, the Cꢁ
single bond (C1ꢁ
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C bond between these units is that of a
˚
C2 1.507(5) A).
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The two n-butyl-groups are coordinated at the metal
centers situated on opposite sides of the oxam bridge
and occupy trans positions to each other. The Ni1ꢁ
/
C33
bond distance of 1 is relatively short (1.948(4) A), but it
is in the same range as the NiꢁC bonds in the
corresponding methyl compound 3 (1.950(3) A [6]. The
C33ꢁC34 and C34ꢁC35 bond lengths of 1.507(6) and
1.516(6) A, respectively, and the corresponding bond
angles C34ꢁC33ꢁNi1 and C33ꢁC34ꢁC35 of 112.6(3)
and 115.7(4)8 confirm that the carbons in the n-butyl
˚
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signals of the n-butyl-groups between dꢀ
0.01), 0.86ꢁ0.89 and 1.25ꢁ1.49 ppm as multiplets were
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ꢃ
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0.03ꢁ
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(ꢃ
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Scheme 1. Formation of the complexes 1 and 2.

M. Stollenz et al. / Journal of Organometallic Chemistry 687 (2003) 153ꢁ
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160
155
ppm. The singlets at dꢀ
associated with the g-, d- or the o-CH2 groups.
Furthermore, at dꢀ30.6 ppm a singlet of the b-CH2
/23.2 and 32.0 and 34.1 ppm are
/
groups was observed. As expected for the oxam ligand,
its aliphatic resonances were found at 19.6, 21.2 and 52.8
ppm. Their aromatic signals, very close to complex 1,
appear between 120.4 and 168.2 ppm.
Single crystals of 2 suitable for an X-ray analysis were
grown from toluene at ꢃ
is shown in Fig. 2.
Similar to complex 1, both alkyl chains coordinating
to the Ni atoms lie trans to each other. The NiꢁC bond
lengths are typical for a s-bonded alkyl group (Ni1ꢁ
/
18 8C. The molecular structure
/
/
˚
˚
C33: 1.931(8) A, Ni2ꢁ
lengths C33ꢁC34 and C34ꢁ
and C40ꢁ41) (between 1.511(13) and 1.556(12) A) are
those of CC-single bonds. The bond angles C33ꢁC34ꢁ
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C39: 1.952(8) A). The bond
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C35 (respectively C39ꢁC40
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˚
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C40ꢁC41 (114.4(8) and 113.8(8) A) are
C35 and C39ꢁ
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much closer to sp3- than to sp2-hybridization.
˚
Fig. 1. Molecular structure of complex 1. Selected bond distances (A)
and bond angles (8): Ni1ꢁN1 1.912(3), Ni1ꢁN2 1.901(3), NiꢁN3
1.907(3), Ni1ꢁC33 1.948(4), Ni2ꢁN4 1.908(3), Ni2ꢁN5 1.915(3), Ni2ꢁ
N6 1.897(3), Ni2ꢁC37 1.922(4), C1ꢁN1 1.333(4), C1ꢁN4 1.309(4),
C2ꢁN2 1.304(4), C2ꢁN5 1.333(4), C1ꢁC2 1.507(5), C33ꢁC34
1.507(6), C34ꢁC35 1.516(6), C35ꢁC36 1.552(8), C37ꢁC38 1.480(7),
C38ꢁC39 1.593(7), C39ꢁC40 1.399(19), N1ꢁNi1ꢁN2 83.3(1), N1ꢁ
Ni1ꢁN3 167.6(1), N1ꢁNi1ꢁC33 96.0(1), C34ꢁC33ꢁNi1 112.6(3),
C33ꢁC34ꢁC35 115.7(4), N2ꢁNi1ꢁN3 84.3(1), N2ꢁNi1ꢁC33
178.1(2), N3ꢁNi1ꢁC33 96.4(2), N4ꢁNi2ꢁN5 83.3(1), N4ꢁNi2ꢁN6
84.5(1), N4ꢁNi2ꢁC37 178.3(2), N5ꢁNi2ꢁN6 167.9(1), N5ꢁNi2ꢁC37
95.0(1), N6ꢁNi2ꢁC37 97.1(2), N1ꢁC1ꢁC2 114.2(3), N1ꢁC1ꢁ
134.0(3), N4ꢁC1ꢁC2 111.8(3), C1ꢁC2ꢁN2 111.6(3), C1ꢁC2ꢁ
114.3(3), N2ꢁC2ꢁN5 134.1(3).
Recently, Okuda et al. reported about rare earth
metal complexes which contain bridging n-alkyl ligands
with agostic interactions and their role in styrene
polymerization [8].
These complexes are extremely air- and moisture-
sensitive and decompose under argon at room tempera-
ture over a period of days. In contrast, the nickelorgano
compounds 1 and 2 are stable under these conditions.
For a few minutes no change of colour is observed,
when compound 2 is handled under air. Contrary to the
n-hexyl complex, compound 1 changes its colour from
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/N4
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/N5
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orangeꢁbrown to dark-brown. Hydrolysis of both
/
chain are sp3 hybridized. The molecular structure state
gives no evidences for agostic interactions with the b- or
g-hydrogen of the n-butyl groups.
species 1 and 2 results in the formation of n-butane
and n-hexane, respectively. However, the corresponding
binuclear oxam NiꢁOH species could not be isolated.
/
In general, n-butyl and n-hexyl transition metal
transition metal complexes are rare. Only a few com-
pounds are known which have been determined by X-
ray diffraction studies, namely Rh-, Ir- and Pt-contain-
In the 1H-NMR spectrum (benzene-d6) of 2 the
resonances of the n-hexyl chains were observed in four
separated signals. The CH2 groups were found as a
separated multiplet between ꢃ
/
0.05 and ꢃ0.01 ppm.
/
ing n-butyl compounds have been investigated [9ꢁ12].
/
HH-COSY experiments allow to assign the other signals
with the resonances of the protons of the n-hexyl
groups. The CH3 groups of the n-Ni alkyl chains of 2
appear as a multiplet between 0.90 and 0.93 ppm. The g-
, d-, and o-CH2 groups were also observed as a multiplet
between 1.21 and 1.33 ppm. Furthermore, the b-CH2
groups were found between 1.41 and 1.47 ppm. Addi-
tionally, the three singlets of the methyl mesityl groups
and the bridging CH2 groups of the oxam bridge were
observed at 2.25, 2.74 and 3.75 ppm with the expected
intensity ratio of 3:6:2.
The 13C-NMR spectrum of 2 in benzene-d6 also shows
a simple pattern corresponding to a highly symmetrical
complex. The n-hexyl carbon resonances appear as six
separated signals. CH-COSY experiments show that the
signal at 14.5 ppm belongs to the methyl group of the n-
Girolami et al. described chromium compounds which
coordinate two n-butyl chains at one metal center
[13,14]. Recently, Cr complexes with triamidoamine
ligands stabilizing methyl and n-butyl groups have
been investigated [15]. Mononuclear nickel complexes
which are able to stabilize ethyl- and n-butyl-groups
have been described by Liang et al.; however, no X-ray
diffraction studies of these compounds have been
published [16]. Very little is known about such transition
compounds containing n-hexyl groups and even less of
them have been characterized completely [17ꢁ
/
20].
2.2. Thermal reactions of 1 and 2
Measurements of the thermal decomposition of 1 and
2 in the solid state by DTA and DTG measurements
showed that compounds are surprisingly stable. Com-
hexyl chain. The a-CH2 groups were found at dꢀ
/
15.2

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M. Stollenz et al. / Journal of Organometallic Chemistry 687 (2003) 153ꢁ160
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˚
Fig. 2. Molecular structure of complex 2. Selected bond distances (A) and bond angles (8): Ni1ꢁ
Ni1ꢁC33 1.931(8), Ni2ꢁN4 1.911(7), Ni2ꢁN5 1.907(7), Ni2ꢁN6 1.910(7), Ni2ꢁC39 1.952(8), C1ꢁ
1.308(10), C2ꢁN4 1.336(10), C33ꢁC34 1.518(13), C34ꢁC35 1.556(12), C35ꢁC36 1.527(14), C36ꢁC37 1.533(14), C37ꢁ
1.511(13), C40ꢁC41 1.529(12), C41ꢁC42 1.564(15), C42ꢁC43 1.472(14), C43ꢁC44 1.494(16), N1ꢁNi1ꢁN2 83.5(3), N1ꢁ
Ni1ꢁC33 96.1(3), N2ꢁNi1ꢁN3 84.8(3), N2ꢁNi1ꢁC33 175.8(4), N3ꢁNi1ꢁC33 95.8(3), C33ꢁC34ꢁC35 114.4(8), N4ꢁNi2ꢁN5 84.0(3), N4ꢁ
167.1(3), N4ꢁNi2ꢁC39 95.0(3), N5ꢁNi2ꢁN6 83.9(3), N5ꢁNi2ꢁC39 174.1(4), N6ꢁNi2ꢁC39 97.5(3), C39ꢁC40ꢁC41 113.8(8), N1ꢁC1ꢁ
N1ꢁC1ꢁN5 133.6(8), N5ꢁC1ꢁC2 112.4(7), C1ꢁC2ꢁN2 112.8(7), C1ꢁC2ꢁN4 114.4(7), N2ꢁC2ꢁN4 132.7(8).
/
N1 1.913(7), Ni1ꢁ
N1 1.344(10), C1ꢁ
C38 1.518(18), C39ꢁ
Ni1ꢁ
/
N2 1.928(7), Niꢁ
N5 1.305(10), C2ꢁ
C40
/
N3 1.916(7),
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/N2
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N3 167.8(3), N1ꢁ
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/Ni2ꢁ
/N6
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/C2 113.9(7),
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plex 1 starts to decompose at 188 8C, 2 starts to
decompose at 186 8C (DTA). In both cases of the
DTG investigations three main stages of decomposition
were observed. According to the mass lost, the first
DTG peak (at 210 8C) corresponds to the eliminiation of
the n-butyl groups in 1. Integration of this peak area
gives a loss of 14 wt.% (calculated 16 wt.%).
that the formation of the alkanes occurs mainly via an
intermolecular reaction.
2.3. Electrochemical behaviour of oxam complexes
Binuclear complexes with oxam ligands are useful
catalysts for some CꢁC linking reactions in which both
/
To investigate the reaction products given by the first
decomposition stage, samples of 1 and 2 were heated in
separate Schlenk tubes up to 200 8C. Under these
conditions only the a-olefins 1-butene and 1-hexene,
respectively were detected.
Heating solutions of both 1 and 2 in toluene under
reflux gives a mixture of a-olefins and the homocoupling
products n-octane and n-dodecane. This suggests that in
solution the n-alkyl chains are eliminated not only by b-
hydrogen elimination but also by homogeneous splitting
metal centers can be involved in the catalytic steps. Of
special interest are such reactions where the oxidation
states of the metal centers change in the catalytic cycle
(e.g. between ꢂ2 and 0) because the change of the
/
oxidation states of one metal center should strongly
effect the catalytic reactivity of the other metal on the
opposite site of the oxam bridge.
The extent of the electronic interaction between the
metals mediated by the bridging ligand is reflected in the
redox potentials of the metal centers and has been
measured by cyclic voltammetry. However, due to the
relatively small energetic effect observed for some of the
complexes the evaluation of the redox potentials of both
processes from the overlapping waves could be accom-
plished only by fitting simulated CVs to experimental
ones using the capabilities of the DIGISIM 3.0 simulation
software [21]. A detailed study of the scan rate
dependence of the current revealed that the experimental
data are in agreement with the assumption of a simple
successive two-electron reduction only when working
with sufficiently high scan rates. At lower scan rates the
experimental current curves associated with the second
charge transfer step become significantly higher than
of the NiꢁC bonds.
/
Thermal decomposition in solution of a 1:1 mixture of
compounds 1 and 2 should result in the formation of n-
octane and n-dodecane in the ratio of about 1:1, if only
an intramolecular pathway will be favoured. In case of
an intermolecular reaction n-octane, n-dodecane and, in
addition, n-decane in the ratio of about 1:1:4 should be
observed.
Heating a solution of a mixture of 1 and 2 in a molar
ratio of 1.0:1.1 in toluene under reflux yields a-olefins
and, as expected, n-octane and n-dodecane, but also n-
decane in a ratio of 1.0:1.2:2.1. Therefore, we suggest

M. Stollenz et al. / Journal of Organometallic Chemistry 687 (2003) 153ꢁ
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160
157
expected for a simple diffusion controlled process. A
satisfying agreement between theory and experiment
could be obtained on the basis of the ECE mechanism
shown in (Scheme 2) based on the assumption that a
new species ‘P’ is formed by the twofold reduced
complex in a chemical follow-up reaction. It can be
excluded from the scan rate dependence of the current
signal that the chemical follow-up reaction results in a
simple catalytic regeneration of complex a or b even
though the species ‘P’ must also be reducible in the
potential range where the reduction from b to c takes
place.
A comparison of experimental CVs (*/) and simu-
lated ones (kꢁkꢁk) based on the ECE mechanism in
/
/
(Scheme 2) is shown Fig. 3 for complex 3. The cyclic
voltammograms of the other complexes are very similar.
It should be emphasized at this point that the
chemical characterization of the side product ‘P’ formed
after the second reduction step was beyond the scope of
the present paper. Nevertheless, it was necessary to
examine the kinetic of the underlying decomposition
reaction carefully to get the value of the energetic
interaction of both metal centres (reflected in the
separation of the redox potentials) we were actually
interested in.
Fig. 3. Experimental (*) and simulated (k) cyclic voltammograms of
/
complex 3 using scan rates between 10 (bottom) and 200 V sꢃ1 (top).
theoretical value of 35.6 mV [22,23] reflecting the pure
entropy effect. Obviously in the palladium complex 5 the
electronic interaction between both redox sides is
weaker than in the nickel compounds resulting in a
potential difference of only 80 mV. When replacing the
alkyl- or aryl ligands by acetylide and phenylacetylide
(complexes 6 and 7), the first reduction step proceeds at
much more positive potentials as in the case of
Table 1 shows the results of a systematic study
executed not only for 1, and 2, but also for some other
complexes of the type [(Rꢁ
the rate constant kf2 was always in the range between
1ꢄ
105 and 2ꢄ105 Mꢃ1 sꢃ1 for each complex and has
/
M)(A)(MꢁR)]: The value of
/
/
/
not been reported in Table 1.
compounds 1ꢁ5, while the potential of the second
/
It is likely that one M(II) center is reduced first giving
Ni(I) or Pd(I). However, attempts to generate and
isolate such M(I) species by reducing of the oxam
complexes with Na/Hg in separate Schlenk flasks were
not successful.
The reduced metal species effects the electronic
properties of the second one thereby shifting the
reduction potential for the second charge transfer
process to more negative potentials. The differences
between the standard potentials of both reduction steps
vary from 115 to 135 mV indicating a strong electronic
communication between both redox centers. In the
absence of such a mutual communication, the difference
between the standard potentials should be close to the
reduction step is almost unaffected. Consequently, the
difference between both potentials is much larger as in
the case of 1ꢁ5 (6: 380 and 7: 375 mV). This is a clear
/
indication for a much stronger electronic communica-
tion between both redox centres. It can also be
concluded that the properties of the twofold reduced
species of 6 and 7 are very similar to those of 1ꢁ5,
/
because the reduction potentials of the latter are closer
together than the ones of the corresponding species
formed after the first reduction step. In contrast to the
alkyl- and aryl oxam complexes, the formation of the
product ‘P’ is less preferred by 6 and 7, as indicated by
the smaller values of the rate constant kf1 compared to
those of 1ꢁ
reduction potentials associated with the first reduction
of 6 and 7 compared to those of 1ꢁ5 may be due to the
/5. The much more positive values of the
/
unsaturated character of the acetylide group.
Electronic communication between two redox centers
in binuclear complexes continues to attract considerable
attention [24ꢁ27]. For example, Garc´ıa-Herbosa re-
/
ported about binuclear Pd(II) complexes containing
1,3-di-p-tolyltriazenido bridging ligands and the depen-
dence of their redox properties on cis and trans
isomerization of the two equivalent redox sides [28]. In
Scheme 2. Postulated mechanism for the electrochemical reduction of
1ꢁ/7.

158
M. Stollenz et al. / Journal of Organometallic Chemistry 687 (2003) 153ꢁ160
/
Table 1
Halfwave potentials and kinetic constants for the electrochemical reduction of 1ꢁ
/
7 in THFꢂ(n-Bu)4NPF6
/
No.
R
M
/
E(1 (V) a
/
E(2 (V) a
/
D(E(1 ꢃE(2 ) (mV)
kf1 (sꢃ1
)
1
2
3
4
5
6
7
ꢁ
ꢁ/
ꢁ/
ꢁ/
ꢁ/
ꢁ/
ꢁ/
/
n-Bu
n-Hex
CH3
Ni
Ni
Ni
Ni
Pd
Ni
Ni
ꢃ
/
1.930
1.950
1.925
1.865
1.970
1.455
1.400
ꢃ
/
2.055
2.070
2.040
2.000
2.050
1.835
1.775
125
120
115
135
80
230
35
ꢃ
/
ꢃ
/
ꢃ
/
ꢃ
/
40
75
Ph
ꢃ
/
ꢃ
/
CH3
ꢃ
/
ꢃ
/
230
10
CCH
ꢃ
/
ꢃ
/
380
375
CCPh
ꢃ/
ꢃ/
10
All potentials are referenced with respect to a Ag j AgCl electrode in acetonitrile containing 0.25 M N(n-Bu)4Cl.
a
Rounded to 95 mV.
/
all these cases no particular attention has been paid to
evaluate the electrochemistry of the complexes quanti-
tatively using the sophisticated simulation tools avail-
able nowadays.
Bruker Avance 200 and 400 spectrometers. Mass spectra
were recorded on a Finnigan MAT SSQ 710. Values for
m/z are for the most intense peak of isotope envelope.
The measured isotopic patterns for the nickel containing
species are in good agreement with the calculated
isotopic pattern. Elemental analyses were performed
with Leco CHNS-932. Ni analyses were carried out by
titration of a solution in dilute hydrochloric acid with
3. Conclusions
0.01
M
EDTA using murexid as indicator.
To the best of our knowledge, the planar oxam
complexes 1 and 2 are the first binuclear Ni(II)
organometallic compounds containing s-bounded n-
butyl- and n-hexyl groups, which have been character-
ized by mass spectrometry, elemental analysis, NMR
and X-ray diffraction analysis. The molecular structures
of 1 and 2 give no evidences for agostic interactions
between the b- nor the g-hydrogen atoms of the n-alkyl
chains and the Ni center atoms. Both complexes are
unusually thermally stable and only start to decompose
at above 180 8C in the solid state. The thermal reaction
of a mixture of 1 and 2 in toluene gives clear evidences of
an intermolecular transfer mechanism of the n-alkyl
chains.
[(acac)Ni(A)Ni(acac)] was prepared according to de-
scribed methods [4]. DTA and DTG measurements were
performed with selfmade devices.
4.1.1. Complex 1
A solution of [(acac)Ni(A)Ni(acac)] (1.227 g, 1.50
mmol) in THF (120 ml) was treated with n-butyllithium
(2.0 ml, 3.2 mmol), dissolved in hexane, dropwise at ꢃ
/
78 8C. After stirring the solution at room temperature
(r.t.) for 18 h, the solvent was removed by oil pump
vacuum. The residue was extracted with toluene (60 ml).
The extract was filtered and the precipitate was washed
with toluene (10 ml). The unified clear solution was
concentrated by oil pump vacuum (ca. 20 ml) and kept
Electrochemical studies of a number of n-alkyl-, aryl-
and acetylideꢁoxam Ni(II) and Pd(II) complexes in-
/
at ꢃ18 8C for 3 days. Brown crystals of 1 crystallized.
/
dicate that the two metal centers in the binuclear
complexes communicate to each other mediated by the
bridging oxam ligand. This is of general importance for
those catalytic reactions of the complexes in which the
two metals are catalytically active.
After removing the mother liquid the crystals were
washed with cold toluene (20 ml), pentane (20 ml) and
were dried by oil pump vacuum (15 min) at r.t. Yield:
0.459 g (42%). Elemental Anal. Calc. for C40H52N6Ni2
(Mwꢀ
15.98. Found: C, 65.30; H, 7.21; N, 11.07; Ni, 15.97%.
1H-NMR (400 MHz, benzene-d6) d ꢃ
0.03ꢁ(ꢃ0.01)
(m, 4H, CH2, n-butyl a-CH2), 0.86ꢁ0.89 (m, 6H, CH3,
n-butyl CH3), 1.25ꢁ
/
734.27 g molꢃ1): C, 65.43; H, 7.14; N, 11.45; Ni,
/
/
/
4. Experimental
/
/
1.49 (m, 8H, CH2, n-butyl b-, g-
4.1. General procedures
CH2), 2.23 (s, 6H, CH3, mes para), 2.75 (s, 12H, CH,
mes ortho), 3.76 (s, 4H, CH2, picolyl CH2), 5.64 (d, 2H,
3JHH
3
All manipulations were carried out by using modified
Schlenk techniques under an atmosphere of argon. Prior
to use, tetrahydrofurane and toluene were dried over
ꢀ
/
7.8 Hz, CH, 3-pyridyl), 6.07 (t, 2H, JHH
ꢀ6.4
/
3
Hz, CH, 5-pyridyl), 6.42 (t, 2H, JHH
pyridyl), 6.80 (s, 4H, CH, mes meta), 7.90 (d, 2H,
3JHH 5.7 Hz, CH, 6-pyridyl). 13C-NMR (50 MHz,
ꢀ7.6 Hz, CH, 4-
/
potassium hydroxide and distilled over Naꢁ
/
benzophe-
ꢀ
/
none. Hexane solutions of n-butyllithium (1.6 M,
Aldrich) and n-hexyllithium (2.5 M, Fluka) were used
as received. 1H- and 13C-NMR spectra were recorded on
benzene-d6) d 14.0 (s, CH3, n-butyl), 14.6 (s, CH2, n-
butyl), 19.6 (s, CH3, mes ortho), 21.2 (CH3, mes para),
27.0 (s, CH2, n-butyl), 33.0 (s, CH2, butyl), 52.8 (s, CH2,

M. Stollenz et al. / Journal of Organometallic Chemistry 687 (2003) 153ꢁ
/
160
159
picolyl CH2), 120.4 (s, CH, 3-pyridyl), 121.9 (CH, 5-
pyridyl), 128.3 (C, CH, mes meta), 133.1 (s, C), 134.6 (s,
CH, 4-pyridyl), 135.6 (s, C), 143.9 (s, C), 144.1 (s, C),
30 min). After cooling the Schlenk flasks with liquid
nitrogen the residues were extracted with toluene (1 ml).
The solutions were analyzed by gas chromatography
with the standards decane (for thermal decomposition
experiments of 1 and 2 in the solid state), and
tetradecane (for thermal decomposition in solution).
1
148.3 (s, CH, 6-pyridyl), 162.6, 168.2 (s, C). H-NMR
(400 MHz, THF-d8) d ꢃ
butyl a-CH2), 0.50ꢁ0.53 (m, 6H, n-butyl CH3), 0.81ꢁ
1.02 (m, 4H,
/
0.60ꢁ
/
(ꢃ0.55) (m, 4H, CH2, n-
/
/
/
0.89 (m, 4H, CH2, n-butyl g-CH2), 0,98ꢁ
/
CH2, n-butyl b-CH2), 2.23 (s, 6H, CH3, mes para), 2.45
(s, 12H, CH, mes ortho), 3.60 (s, 4H, CH2, picolyl CH2),
4.3. Cyclic voltammetry
3
6.64 (d, 2H, JHH
ꢀ
/
7.8 Hz, CH, 3-pyridyl), 6.73 (s, 4H,
Cyclic voltammetric measurements have been con-
ducted in 3-electrode technique using an home-built
computer controlled instrument based on the DAP-
3200a data acquisition board (DATALOG Systems).
The experiments were performed in THF containing
0.5M tetra-n-butylammonium-hexafluorophosphate un-
der a blanket of solvent-saturated argon. The ohmic
resistance, which had to be compensated for, was
determined by measuring the impedance of the system
at potentials where the faradaic current was negligibly
small. Background correction was accomplished by
subtracting the current curves of the blank electrolyte
(containing the same concentration of supporting elec-
trolyte) from the experimental CVs. The reference
electrode was an Ag j AgCl electrode in acetonitrile
containing 0.25M tetra-n-butylammonium chloride.
The potential of this reference system was calibrated
by measuring the potential of the ferrocenium/ferrocene
couple at the end of each experiment. The latter redox
3
CH, mes meta), 6.98 (t, 2H, JHH
ꢀ/6.1 Hz, CH, 5-
pyridyl), 7.53 (t, 2H, 3JHH
ꢀ/7.6 Hz, CH, 4-pyridyl), 7.83
3
(d, 2H, JHH
ꢀ
5.6 Hz, CH, 6-pyridyl). 13C-NMR (100
/
MHz, THF-d8) d 13.8 (CH3, n-butyl CH3), 14.1 (CH2,
n-butyl a-CH2), 19.4 (CH3, mes ortho), 21.1 (CH3, mes
para), 27.1 (CH2, n-butyl g-CH2), 33.1 (CH2, n-butyl b-
CH2), 53.2 (CH2, picolyl CH2), 121.4 (CH, 3-pyridyl),
123.1 (CH, 5-pyridyl), 127.7 (CH, mes meta), 133.6,
135.9 (C), 136.1 (CH, 4-pyridyl), 144.2 (C), 149.1 (CH,
6-pyridyl), 162.8, 169.1 (C). MS (neg. DCI/H2O) m/z:
774, [Mꢂ
C3H6]ꢂ (100%); 732, [M]ꢂ (44%).
/
4.1.2. Complex 2
Analogously to 1, [(acac)Ni(A)Ni(acac)] (1.227 g, 1.50
mmol) was reacted with a solution of n-hexyllithium (1.3
ml, 3.3 mmol). A golden brown crystalline solid was
obtained. Yield: 0.576 g (49%). Elemental Anal. Calc.
for C44H60N6Ni2 (Mwꢀ
790.38 g molꢃ1): C, 66.86; H,
7.65; N, 10.63; Ni, 14.85. Found: C, 66.55; H, 7.52; N,
/
couple was found to be at ꢂ
The working electrode was an hanging mercury drop
(mHg-drop 3.95ꢁ4 mg) produced by the CGME (Bioa-
/0.875 V.
1
10.41; Ni, 14.96%. H-NMR (400 MHz, benzene-d6) d
ꢃ
/
0.05ꢁ
0.93 (m, 6H, CH3, n-hexyl CH3), 1.21ꢁ
CH2, n-hexyl g-, d-, o-CH2), 1.41ꢁ1.47 (m, 4H, CH2, n-
/
(ꢃ
/
0.01) (m, 4H, CH2, n-hexyl a-CH2), 0.90ꢁ
/
ꢀ
/
/
/
1.33 (m, 12H,
nalytical Systems, Inc., West Lafayette, USA).
/
hexyl b-CH2), 2.25 (s, 6H, CH3, mes para), 2.74 (s, 12H,
CH, mes ortho), 3.75 (s, 4H, CH2, picolyl CH2), 5.65 (d,
4.4. Crystal structure determination
3
2H, JHH
3
7.9, CH, 3-pyridyl), 6.09 (t, 2H, JHH
ꢀ
/
ꢀ/6.2,
The intensity data for the compound 1 were collected
on a Nonius CAD4 diffractometer and for the com-
pound 2 on a Nonius Kappa CCD diffractometer, using
3
CH, 5-pyridyl), 6.43 (t, 2H, JHH
ꢀ7.7, CH, 4-pyridyl),
/
3
6.80 (s, 4H, CH, mes meta), 7.90 (d, 2H, JHH
ꢀ5.6,
/
CH, 6-pyridyl). 13C-NMR (100 MHz, benzene-d6): d
14.5 (s, CH3, n-hexyl-CH3), 15.2 (s, CH2, n-hexyl a-
CH2), 19.6 (s, CH3, mes ortho), 21.2 (s, CH3, mes para),
23.2 (s, CH2, n-hexyl g-, d-, o-CH2), 30.6 (s, CH2, n-
hexyl b-CH2), 32.0, 34.1 (s, CH2, n-hexyl g-, d-, o-CH2),
52.8 (s, CH2, picolyl CH2), 120.4 (s, CH, 3-pyridyl),
121.9 (s, CH, 5-pyridyl), 127.6 (s, CH, mes meta), 133.1
(s, C), 134.1 (s, CH, 4-pyridyl), 135.6, 143.9 (s, C), 148.4
(s, CH, 6-pyridyl), 162.6, 168.2 (s, C). MS (DEI) m/z:
graphite-monochromated Moꢁ
corrected for Lorentz and polarization effects, but not
for absorption [29ꢁ31].
/
Ka radiation. Data were
/
The structures were solved by direct methods (SHELXS
[32]) and refined by full-matrix least-squares techniques
against F2o (SHELXL-97 [33]). The hydrogen atoms of the
structures were included at calculated positions with
fixed thermal parameters. All non-hydrogen atoms were
refined anisotropically [33]. XP (Siemens Analytical X-
ray Instruments Inc.) was used for structure representa-
tions.
788, [M]ꢂ (B
[Mꢃ2C6H13ꢂ
/
0.5%); 704, [Mꢃ
/
C6H13ꢂ
/
1]ꢂ (0.5%); 620,
/
/
2]ꢂ (2.3%); 618, [Mꢃ
/
2C6H13]ꢂ (1.6%).
4.2. Thermal investigations
In a typical run 0.1ꢁ0.2 mmol of 1 and 2, respectively
were heated at 200 8C for 10 min in vacuo (investiga-
tions in the liquid phase: Equimolar amounts of 1 and 2
were dissolved in toluene (2 ml) and heated at 110 8C for
4.4.1. Crystal data for 1
C49H52N6Ni2×
/
5/3C7H8, Mrꢀ
887.85 g molꢃ1, brown
/
/
prism, size 0.18ꢄ
/
0.16ꢄ
11.518(3), bꢀ
98.57(3), bꢀ91.67(3), gꢀ
Tꢀ 90 8C, Zꢀ3, rcalc
1.245 g cmꢃ3, m(Moꢁ
/
0.10 mm3, triclinic, space group
˚
¯
P1, aꢀ
/
/
16.283(3), cꢀ
/
19.753(6) A, aꢀ
/
3
˚
/
/
103.68(4)8, Vꢀ
/
3551(1) A ,
Ka)ꢀ
/
ꢃ
/
/
ꢀ
/
/
/

160
M. Stollenz et al. / Journal of Organometallic Chemistry 687 (2003) 153ꢁ160
/
8.36 cmꢃ1, F(000)ꢀ
14), k(0/21), l(ꢃ25/25), measured in the range 2.495
U 527.438, completeness Umax 99.8%, 16 156 inde-
pendent reflections, Rint 0.018, 10540 reflections with
Foꢀ4s(Fo), 732 parameters, two restraints, R1 obs
0.061, wR2 obs 0.179, R1 all 0.116, wR2 all 0.209,
/
1420, 16727 reflections in h(ꢃ14/
/
[6] D. Walther, M. Stollenz, H. Gorls, Organometallics 20 (2001)
¨
4221.
[7] K. Lamm, M. Stollenz, M. Meier, H. Gorls, D. Walther, J.
¨
/
/
/
ꢀ
/
Organomet. Chem. 681 (2003) 24.
[8] P. Voth, S. Arndt, T.P. Spaniol, J. Okuda, L.J. Ackerman,
M.L.H. Green, Organometallics 22 (2003) 65.
ꢀ
/
/
ꢀ
/
[9] D.J. Anderson, R. Eisenberg, Inorg. Chem. 33 (1994) 5378.
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Carmona, J. Am. Chem. Soc. 114 (1992) 7288.
ꢀ
/
ꢀ
/
ꢀ
/
GOFꢀ
/
1.053, largest difference peak and hole: 1.055/
ꢃ3
˚
1.112 e A
ꢃ
/
.
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4.4.2. Crystal data for 2
C44H60N6Ni2×1/4C4H8O, Mrꢀ
brown prism, size 0.10ꢄ0.10ꢄ
0.03 mm3, orthorhom-
bic, space group Pbcn, aꢀ47.967(3), bꢀ11.2398(6),
90 8C, Zꢀ8,
8.89 cmꢃ1
3456, 28 307 reflections in h(ꢃ45/57), k(ꢃ
13/8), l(ꢃ10/19), measured in the range 2.215U 5
25.358, completeness Umax 97.6%, 7902 independent
reflections, Rint 0.158, 3806 reflections with Foꢀ
4s(Fo), 479 parameters, zero restraints, R1 obs 0.103,
wR2 obs 0.220, R1 all 0.2212, wR2 all 0.274, GOFꢀ
1.044, largest difference peak and hole: 0.970/ꢃ0.717 e
/
/808.43
g
molꢃ1
,
[13] T.G. Gardner, G.S. Girolami, J. Chem. Soc. Chem. Commun.
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/
/
/
/
[14] E.G. Thaler, K. Folting, J.C. Huffman, K.G. Caulton, J.
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3
˚
˚
cꢀ
rcalc
F(000)ꢀ
/
16.3960(8) A, Vꢀ
/
8839.7(8) A , Tꢀ
/
ꢃ
/
/
ꢀ
/
1.215
g
cmꢃ3
,
m(MoꢁKa)ꢀ
/
/
,
[15] A.C. Filippou, S. Schneider, Organometallics 22 (2003) 3010.
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/
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/
/
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ꢀ
/
ꢀ
/
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ꢀ
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ꢀ
/
ꢀ
/
/
/
J. Re´tey, Angew. Chem. 107 (1995) 2580.
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A
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5. Supplementary material
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC nos. 214426 and 214427. Copies of
this information may be obtained free of charge from
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