Redox-active p-quinone-based bis(pyrazol-l-yl)methane ligands: Synthesis and coordination behaviour

Article abstract of DOI:10.1002/chem.200701615

The synthesis, structural characterisation and coordination behaviour of mono- and ditopic p-hydroquinone-based bis(pyrazol-l -yl)methane ligands is described (i.e., 2-(Pz₂CH)C₆H₃(OH)₂ (2a), 2-(pz₂CH)

Full text of DOI:10.1002/chem.200701615

DOI: 10.1002/chem.200701615
Redox-Active p-Quinone-Based Bis(pyrazol-1-yl)methane Ligands:
Synthesis and Coordination Behaviour
Sebastian Scheuermann,[a] Tonia Kretz,[a] Hannes Vitze,[a] Jan W. Bats,[b] Michael Bolte,[a]
Hans-Wolfram Lerner,[a] and Matthias Wagner*[a]
Dedicated to Professor Dr. Wolfgang A. Herrmann on the occasion of his 60th birthday
Abstract: The synthesis, structural
characterisation and coordination be-
haviour of mono- and ditopic p-hydro-
square-planar
PdII
complexes
ent are fully reversible. In [Pd2boxCl2],
the methine proton is highly acidic and
can be abstracted with bases as weak
as NEt3. The resulting anion dimerises
to give a dinuclear macrocyclic PdII
complex, which has been structurally
characterised. The methylated ligand 2-
(pz2CMe)C6H3O2 (11ox) and its PdII
complex [Pd11oxCl2] are base-stable. A
new class of redox-active ligands is
now available with the potential for ap-
plications both in catalysis and in mate-
rials science.
[Pd2bCl2] and [Pd2boxCl2] have been
prepared. In the two PdII species, the li-
gands are coordinated only through
their pyrazolyl rings. The fact that
[Pd2bCl2] and [Pd2boxCl2] are isolable
compounds proves that redox transi-
tions involving the p-quinone substitu-
quinone-based
bis(pyrazol-1-yl)me-
thane ligands is described (i.e., 2-
(pz2CH)C6H3(OH)2 (2a), 2-(pz2CH)-6-
(tBu)C6H2(OH)2 (2b), 2-(pz2CH)-6-
(tBu)C6H2(OSiiPr3)(OH) (2c), 2,5-
C
(pz2CH)2C6H2(OH)2 (4)). Ligands 2a,
2b and 4 can be oxidised to their p-
benzoquinone state on a preparative
scale (2aox, 2box, 4ox). An octahedral
Keywords: ligand design · N,O li-
gands · palladium · pyrazolylme-
thanes · quinones
NiII complex [trans-Ni
(2c)2] and
A
Introduction
multi-electron transfers that are the sum of the oxidation-
state changes of the metal centre plus the redox changes of
the ligand framework.[1]
Particularly impressive examples of the effectiveness of
electroactive ligands in catalysis are provided by metalloen-
zymes like the Cyt-P450 systems in which a one-electron-
oxidised porphyrinato ligand is present in the highly oxi-
dised intermediate forms of the enzyme.[2]
In materials science, great attention is currently being
paid to oligonuclear transition-metal aggregates with redox-
active bridging units that are designed to have useful coop-
erative magnetic and electronic properties.[1,3,4]
The importance of transition-metal ions in homogeneous
catalysis and materials science rests to a large extent on
their ability to adopt different oxidation states. This redox
capacity can be further enhanced by coordinating the metal
ion with an electroactive ligand. Provided that the redox po-
tential of the non-metal component matches that of the
metal ion, the resulting complex is capable of undergoing
[a] S. Scheuermann, Dr. T. Kretz, H. Vitze, Dr. M. Bolte,
Dr. H.-W. Lerner, Prof. Dr. M. Wagner
Institut für Anorganische und Analytische Chemie
Johann Wolfgang Goethe-Universität Frankfurt am Main
Max-von-Laue-Straße 7, 60438 Frankfurt (Germany)
Fax : (+49)69-798-29260
Our group has a long-standing interest in the develop-
ment of novel electroactive ligands both for use in homoge-
neous catalysis and in the assembly of coordination poly-
mers and networks.[5] Initially we relied on ferrocene as a
E-mail: Matthias.Wagner@chemie.uni-frankfurt.de
redox-active unit and prepared mono- and ditopic ferro-
[b] Dr. J. W. Bats
ꢀ
cene-based poly(pyrazol-1-yl)borates [FcB(Me)npz3ꢀn
]
and
Institut für Organische Chemie und Chemische Biologie
Johann Wolfgang Goethe-Universität Frankfurt am Main
Max-von-Laue-Straße 7, 60438 Frankfurt (Germany)
[fc{B(Me)npz3ꢀn}2]2ꢀ (Fc=(C5H5)Fe
A
N
pz=pyrazol-1-yl). Treatment of these ligands with metal
ions (Mn+) gives discrete oligonuclear complexes, coordina-
tion polymers and multiple-decker sandwich complexes.[6–13]
In some cases, cyclic voltammetric measurements indicate a
dependence of the Fc/Fc+ or fc/fc+ redox potential on the
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author. It contains crys-
tallographic characterisation of 2a, 2box, 2c, 4·DMF2 and 9; synthesis,
spectroscopic and crystallographic characterisation of 13 and cyclic
voltammograms of 11, 12 and 13.
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FULL PAPER
oxidation state of Mn+ (and
vice versa), however, the
degree of electronic communi-
cation is generally not very
high.[7,9]
In the search for more
strongly coupled systems, o-
and p-quinone derivatives ap-
peared to be particularly prom-
ising candidates as they are
known to exist not only as
closed-shell species, but also as
reasonably stable radicals (cf.
the p-hydroquinone/p-semiqui-
none/p-benzoquinone
series;
note: in the following, we will
use the general term p-quinone
when we refer to both the p-
hydro- and the p-benzoquinone
state).[1] As a consequence, the
assignment of a formal oxida-
tion number to the metal centre
in hydroquinonate complexes
often becomes ambiguous such
that the more meaningful physi-
cal oxidation number has to be
established experimentally.[14]
In our own studies, we initial-
ly employed the ditopic p-hy-
droquinone chelate ligand A[15]
(Figure 1). Treatment of A with
CuII ions leads to the purple-
coloured coordination polymer
D, which consists of magnetical-
ly isolated linear chains with
Figure 2. Heisenberg (S=1= ) spin chains D from ligand A and polymeric
2
S=1= spin dimer system E from ligand B.
2
generation of derivatives B (Figure 1) equipped with biden-
tate tethers to occupy three coordination sites of the com-
plexed metal ion. Depending on the steric demand of the R
substituents, the new B ligands provided access to stable di-
nuclear CuII complexes (R=NiPr2) and to zigzag polymers
Figure 1. Redox-active, p-hy-
droquinone-based ditopic li-
gands: bidentate pyrazolyl
ligand A, tridentate meridio-
nally coordinating imine ligand
B and tridentate facially coor-
dinating bis(pyrazol-1-yl)me-
thane ligand C.
E
(R=2-pyridyl,
DMF=N,N-dimethylformamide;
Figure 2).[22] Compound E, which behaves as an S=1= spin
2
dimer system with weak three-dimensional magnetic cou-
plings, turned out to be particularly interesting. At very low
temperatures, the material undergoes a magnetic-field-in-
duced phase transition that can be interpreted as a Bose–
Einstein condensation of magnons.[22–25]
All structurally characterised complexes of B are either
square planar or octahedral and show a meridional coordi-
nation mode of the tridentate chelator. To broaden the
scope of the ligand system further, we thus decided to devel-
op a third generation of facially coordinating p-hydroqui-
none derivatives. Given the rich chemistry of ferrocene-
based poly(pyrazol-1-yl)borates, an appealing approach
would be to introduce bis(pyrazol-1-yl)borate substituents at
the 2- and 5-positions of the p-hydroquinone core. However,
the preparation of these new target molecules would require
extensive protective group chemistry to avoid problems that
arise from the simultaneous presence of OH substituents
and three-coordinate boryl groups in the key intermediates
of the synthesis sequence. To circumvent these problems, we
decided to switch from bis(pyrazol-1-yl)borates to bis(pyra-
CuII ions in a square-planar coordination environment
(Figure 2).
The compound behaves as a homogeneous antiferromag-
netic S=1= Heisenberg spin chain with a moderate antifer-
2
romagnetic
exchange-coupling
constant
jJj/kB
of
21.5 K.[16–20] Moreover, it was found that subtle changes in
the substitution pattern of ligand A led to redox reactions
with the formation of CuI ions and p-benzoquinones rather
than to polymer formation.[21] These results not only prove
the promising potential of A as a mediator of magnetic
CuII–CuII interactions, but also suggest extensive ligand-to-
metal charge transfer in its CuII complexes. In spite of such
desirable properties, ligand A also suffers from certain dis-
advantages. Most importantly, the synthesis of discrete dinu-
clear CuII complexes of A turned out to be difficult mainly
because of a pronounced tendency for the components to
form insoluble microcrystalline precipitates.[18] Discrete com-
plexes of p-hydroquinonate ligands not only serve as impor-
tant soluble model compounds for notoriously insoluble co-
ordination polymers, but they are also interesting in their
own right. With the aim of having more versatile p-hydro-
quinone ligands at hand, we therefore prepared a second
zol-1-yl)methanes[26] and to prepare ditopic ligand
C
(Figure 1), which has the advantage of being accessible from
the same starting material (2,5-diformyl-1,4-dihydroxyben-
zene[27]) as imine ligand B. Moreover, deprotonation of the
OH groups during metal complexation renders each of the
metal binding sites monoanionic so that they have the same
charge as common poly(pyrazol-1-yl)borates.[13] Similar to
boron-based scorpionate ligands, variation of the R’ sub-
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2591

M. Wagner et al.
stituents at the 3-positions of the pyrazolyl rings allows ex-
tensive control over the steric demand of ligand system C.
The range of potential applications of p-quinone-based
bis(pyrazol-1-yl)methane ligands goes far beyond materials
science. In 1997, Higgs and Carrano established (2-hydroxy-
phenyl)bis(pyrazol-1-yl)methanes as a new class of biomi-
metically relevant scorpionate ligands.[28] Because p-qui-
nones have not only been recognised as ubiquitous electron-
transfer agents in biology, but also as important redox cofac-
tors (“quinoenzymes”),[29] replacement of the 2-hydroxy-
phenyl substituent in (2-hydroxyphenyl)bis(pyrazol-1-yl)me-
thanes by a p-hydroquinone moiety as in C could provide
interesting perspectives for modelling the active sites of p-
hydroquinone-containing metalloenzymes, such as copper-
dependent amine oxidases (cofactor: topaquinone=2,4,5-tri-
hydroxyphenylalaninequinone).[30–32] Further evidence for
the potential of C-type ligands in homogeneous catalysis
stems from Bäckvall et al. who employed p-hydroquinone as
a co-catalyst in PdII-catalysed aerobic oxidations of mono-
olefins and conjugated dienes.[33] Moreover, PdII complexes
of simple bidentate nitrogen ligands are active catalysts for
styrene/CO copolymerisation provided that p-benzoquinone
is added to the methanolic reaction mixture.[34] The role of
p-benzoquinone lies in its ability to stabilise Pd0 intermedi-
ates and, in the presence of acid, to reoxidise them back to
PdII.[35] The PdII/p-benzoquinone system can also be applied
ꢀ
to electrophilic C H activations that can be regarded as
combined Murai and Heck reactions.[36] Recently, homocou-
pling reactions of aryl halides have been achieved with the
help of PdII catalysts that contain 1,4-diazabutadiene ligands
in the presence of stoichiometric amounts of p-hydroqui-
none.[37] For a related reaction, a mechanistic model that
provides an insight into the role of the p-hydroquinone re-
agent has been proposed.[38]
Thus, for applications in catalysis and materials science,
mono- (e.g., 2a–2c; Scheme 1) and ditopic (4; Scheme 1) p-
hydroquinone-based bis(pyrazol-1-yl)methane ligands have
been included in our investigations and their synthesis, reac-
tivity and coordination behaviour will be described herein.
Scheme 1. Synthesis of mono- and ditopic p-hydroquinone-based bis(pyr-
azol-1-yl)methane ligands 2a–c, 4 and their oxidised p-benzoquinone
forms 2aox, 2box and 4ox. Reagents and conditions: a) pz2CO (1 equiv),
cat. CoCl2, THF, reflux; b) pz2CO (2 equiv), cat. CoCl2, THF, reflux;
c) [Ce
A
E
(one-electron transition) rather than between the p-hydro-
quinone and the p-benzoquinone forms (two-electron transi-
tion).
Results and Discussion
Synthesis and spectroscopy: In addition to parent compound
2a (Scheme 1), we also prepared its tert-butyl-substituted
derivative 2b for the following reasons: 1) A tBu group in-
creases the solubility of the free ligand and its complexes.
2) Steric shielding of the metal binding site provides extra
kinetic stability for the complexes and decreases the propen-
sity of the adjacent hydroxy group to adopt a bridging posi-
tion between two metal ions.[39] 3) It is important to prevent
the formation of Michael adducts between the substrate
amine and the ligand in its p-benzoquinone state by steric
protection, with regard to amine oxidase model systems in
particular.[40] Triisopropylsilyl derivative 2c (Scheme 1) was
synthesised to gain access to a ligand that shuttles between
the p-hydroquinone and the p-semiquinone radical state
For the preparation of 2a to 2c, a synthetic protocol
based on that of Peterson et al.[41–43] was employed. Treat-
ment of aldehydes 1a to 1c (Scheme 1) with bis(pyrazol-1-
yl)methanone (1.2 equiv) in the presence of a catalytic
amount of anhydrous CoCl2 gave 2a to 2c in yields of be-
tween 23 and 36%. In a similar reaction, compound 4 was
obtained from 2,5-diformyl-1,4-dihydroxybenzene (3) and
bis(pyrazol-1-yl)methanone (2.4 equiv; Scheme 1).
Oxidation of 2a, 2b and 4 to 2aox, 2box and 4ox was con-
veniently achieved by using [Ce
N
N
CH3CN/H2O (Scheme 1). In the case of 2box we have also
successfully tested FeCl3 and NaIO4 as oxidising agents.
We started our assessment of the coordination properties
of ligands 2 with the d8 metal ions NiII and PdII. In the case
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Bis(pyrazol-1-yl)methane Ligands
FULL PAPER
of NiII, octahedral complex 5 was obtained from [Ni-
of the p-benzoquinone substituent. To test this hypothesis,
NEt3 was added to a small quantity of the PdII complex 6ox
in CH3CN. An NMR spectroscopic investigation of the solu-
tion revealed a mixture of several products from which the
remarkable dinuclear PdII macrocycle 7 could be isolated as
orange single crystals (Scheme 2).
(OAc)2·4H2O] and 2c (2 equiv) in MeOH with NaOMe as
the base (Scheme 2). Square-planar PdII complexes 6 and
From this result it became apparent that more base-stable
ligands are required for some of our further investigations
and we therefore decided to replace the reactive methine
proton by an inert methyl group (Scheme 3). To this end, we
started from O-protected precursor 8 and transformed it
into bis(pyrazol-1-yl)methane derivative 9 by the reaction
sequence already employed for the synthesis of 2a to 2c.
Deprotonation of 9 with nBuLi and subsequent methylation
with MeI led to compound 10 in high yields. Deprotection
Scheme 2. Synthesis of the NiII and PdII complexes 5, 6 and 6ox. Forma-
tion of macrocyclic dinuclear PdII complex 7 by deprotonation of 6ox. Re-
agents and conditions: a) NaOMe, MeOH, RT; b) CH3CN, RT; c) NEt3,
CH3CN, RT
6ox, with their redox-active substituents in the p-hydroqui-
none and p-benzoquinone state, respectively, were readily
formed from [PdCl2(cod)] and 2b/2box in CH3CN
U
(Scheme 2; cod: cyclooctadiene).
The electron-withdrawing effect of the two sp2 nitrogen
substituents in bis(pyrazol-1-yl)methane leads to an increase
in the acidity of the methylene protons so that they can be
easily abstracted by suitable bases. The considerable syn-
thetic potential of this reaction was first exploited by Otero
et al. who prepared the N,N,O-scorpionate ligand bis(3,5-di-
methylpyrazol-1-yl)acetate by treatment of bis(3,5-dimethyl-
pyrazol-1-yl)methane with nBuLi followed by the addition
of CO2.[44] We expected the methine protons in 2aox and
2box to be even more acidic than the methylene protons in
the parent H2Cpz2 because of the negative mesomeric effect
Scheme 3. Synthesis of methylated p-hydroquinone-based bis(pyrazol-1-
yl)methane ligand 11, its oxidised p-benzoquinone form 11ox and the cor-
responding PdCl2 complexes 12 and 12ox
a) pz2CO (1 equiv), cat. CoCl2, THF, reflux; b) i) nBuLi (1 equiv), THF,
08C; ii) MeI (1 equiv), RT; c) excess [Ce(NH4)2(NO3)6], THF/H2O, 0 8C;
. Reagents and conditions:
A
ACHTREUNG
d) excess Na2S2O4, THF/H2O, RT; e) CH3CN, reflux; f) CH3CN, RT
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M. Wagner et al.
of 10 was less straightforward. In the past, we successfully
used a mixture of hydrobromic and acetic acids to generate
various p-hydroquinone derivatives from their methoxy pre-
cursors.[27] In the special case of 10, however, we encoun-
tered problems with the stability of the bis(pyrazol-1-yl)me-
thane moiety in strongly acidic media. We finally developed
a convenient oxidative procedure that leads to 11ox upon
presence of two CHpz2 substituents in this molecule. More-
over, 4 and 4ox give rise to fewer NMR signals than 2a and
2aox, in accordance with the higher molecular symmetry of
the ditopic ligands.
Complex 5 contains a high-spin NiII centre and is there-
fore paramagnetic. As a result, NMR spectra obtained were
not interpretable.
treatment of 10 with excess [Ce
The corresponding p-hydroquinone ligand 11 is readily ac-
cessible from 11ox and Na2S2O4 as the reducing agent. Reac-
A
G
The NMR spectra of 6 and 6ox are rather similar to those
of free ligands 2b and 2box. However, as a characteristic dif-
ference we note a downfield shift of the pzC-3,5 resonances
upon complexation (2b: d=141.1, 131.8 ppm; 6: d=145.0,
136.5 ppm; 2box: d=142.2, 131.7 ppm; 6ox: d=145.7,
137.0 ppm).
tion of 11/11ox with [PdCl2
G
spectively, provided the PdCl2 complexes 12/12ox. For rea-
sons not yet understood, the synthesis of 12 from 11 and
[PdCl
A
As can be expected, the NMR spectra of 11 and 11ox
1
was successfully employed in the preparation of the closely
closely resemble those of 2a and 2aox. In the H NMR spec-
related complex 6 (Scheme 2).
The H NMR spectrum of 2a in MeOD is characterised
tra of 11 and 11ox the characteristic low-field resonances of
the methine protons are missing and signals for the
C(Me)pz2 methyl groups are found instead (11, 11ox: d=
2.67 ppm). In the 13C NMR spectra, the resonances of the
methyl carbon atoms C(Me)pz2 appear at d=25.4 ppm in 11
and at d=25.1 ppm in 11ox.
All relevant NMR data for 12 and 12ox are consistent with
those of 6 and 6ox and therefore do not merit further discus-
sion.
1
by a singlet (integrating to 1H) at d=7.91 ppm for the me-
thine proton CHpz2, a set of three signals (d=7.61, 7.47 and
6.36 ppm; each integrating to 2H) for the two chemically
equivalent pyrazol-1-yl substituents and two signals (d=6.73
(2H) and 6.27 ppm (1H)) for the p-hydroquinone core.
Upon oxidation to 2aox, the resonance of the methine
proton is shifted by 0.07 ppm to a higher field (2aox: d-
(CHpz2)=7.84; MeOD). Minor differences between 2a and
2aox are also observed in the chemical shifts of the pyrazol-
1-yl signals (d=7.78, 7.63 and 6.41 ppm) and in the resonan-
ces of the six-membered ring (d=6.84 (2H) and 6.19 ppm
(1H)). Two of the three protons of the p-quinone fragments
in 2a and 2aox are fortuitously isochronous.
X-ray crystallography: Details of the X-ray crystal structure
analyses of 5, 6, 7 and 12ox are summarised in Table 1. The
X-ray crystal structure analyses of the free ligands 2a, 2box,
2c, 4, the intermediate 9 and complex 13 (Scheme 3) are de-
scribed in the Supporting Information.
The 13C NMR resonances of the parent p-hydroquinone
appear at d=150.6 (COH) and 117.0 ppm (CH) and those
of p-benzoquinone at d=187.0 (CO) and 136.4 ppm
(CH).[45] Thus, the two resonances at 151.6 and 149.4 ppm in
the spectrum of 2a have to be assigned to the hydroxylated
carbon atoms. The corresponding CO signals in 2aox possess
chemical shifts of d=188.3 and 186.3 ppm. As can be ex-
pected from the comparison of the 13C NMR spectra of the
parent species, the remaining four resonances of the six-
membered ring of 2a are grouped around d=117 ppm (i.e.,
d=123.3, 118.8, 117.7 and 115.5 ppm), whereas the corre-
sponding chemical shifts of 2aox are close to d=136 ppm
(i.e., d=143.7, 138.2, 137.9 and 135.5 ppm). The signals of
the methine carbon atoms of 2a and 2aox are found at d=
75.0 and 72.8 ppm, respectively. Similar to the 1H NMR
spectra, the 13C NMR spectra of 2a and 2aox reveal only
negligible differences in the chemical shifts of their pyrazol-
1-yl signals.
In complex 5, two ligands 2c bind to a NiII ion in a trans
configuration (Figure 3). The molecule contains a centre of
inversion at Ni1 together with a mirror plane that is spanned
by the two p-hydroquinonyl moieties and the NiII ion.
The octahedral coordination sphere of NiII is largely un-
distorted. The angles between the trans positions possess a
crystallographically imposed value of precisely 180.08 and
the angles between the cis positions lie in the range of
86.5(1) (N-Ni-N chelate bite angle) to 93.5(1)8 (N-Ni-N
ꢀ
ꢀ
non-chelate angle). The Ni N and Ni O bond lengths of
2.079(1) and 2.022(1) Š, respectively, are comparable to the
corresponding values in the related complex trans-[NiL2],[46]
which features a NiII ion and two deprotonated (2-hydroxy-
phenyl)bis(pyrazol-1-yl)methane ligands Lꢀ (two crystallo-
graphically independent molecules in the solid state; aver-
ꢀ
ꢀ
age values Ni N/Ni O: 2.088(7)/2.011(7) Š). It is thus ap-
parent that the presence of the bulky tert-butyl group in 5
ꢀ
does not enforce significant stretching of the Ni
A
ꢀ
In the NMR spectra of 2b and 2box, the signals of the tBu
bonds. Upon coordination, the O Cipso bond shortens from
groups are visible at d(1H)=1.41 (2b) and 1.21 ppm (2box)
1.368(1) Š in 2c (cf. the Supporting Information) to
ꢀ
and d
A
1.304(2) Š in 5. In contrast, the iPr3SiO Cipso bond lengths
NMR spectra of 2c are characterised by additional resonan-
in 2c (1.372(1) Š) and 5 (1.385(2) Š) are roughly the same.
Differences similar to those noted for 2c and 5 have also
been observed between the free ligand (2-hydroxyphenyl)-
ces at d(1H)=1.19, 1.08 ppm and d
the SiiPr3 fragment.
A
bis(pyrazol-1-yl)methane (LH: HO Cipso 1.372(7) Š[28]) and
ꢀ
The NMR spectra of 4 and 4ox contain the same general
features as those of 2a and 2aox. However, the integral
ratios of the proton resonances of 4 clearly indicate the
its corresponding complex [NiL2] (NiO Cipso 1.309(13) Š[46]).
ꢀ
ꢀ
We attribute the shorter NiO Cipso bonds in 5 and [NiL2] to
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Bis(pyrazol-1-yl)methane Ligands
FULL PAPER
Table 1. Crystal data and structure refinement details for 5, 6, 7 and 12ox
.
Compound
5
6
7
12ox
formula
Mr
C52H78N8NiO4Si2
994.11
C17H20Cl2N4O2Pd·0.5C2H3N
510.20
C34H34Cl2N8O4Pd2·4C2H3N
1066.61
C14H12Cl2N4O2Pd·C2H3N
486.63
colour, shape
T [K]
brown, plate
175(2)
yellow, plate
165(2)
orange, block
173(2)
orange-brown, plate
173(2)
radiation, l [Š]
crystal system
space group
a [Š]
b [Š]
c [Š]
MoKa, 0.71073
monoclinic
C2/m
19.046(3)
12.5401(15)
14.1605(16)
90
90.796(11)
90
3381.7(7)
2
MoKa, 0.71073
triclinic
P1
MoKa, 0.71073
triclinic
P1
MoKa, 0.71073
orthorhombic
Pbca
14.1130(5)
14.7185(6)
18.1331(6)
90
¯
¯
11.262(2)
12.6863(16)
15.9035(15)
111.725(9)
93.102(16)
97.949(11)
2076.8(5)
4
9.4514(9)
10.8218(10)
12.7740(12)
70.762(7)
75.746(8)
73.388(7)
1165.29(19)
1
a [8]
b [8]
90
90
g [8]
V [Š3]
3766.6(2)
8
1.716
Z
Dcalcd [gcmꢀ3
]
0.976
1.632
1.520
F
A
1068
1028
540
1936
]
0.362
1.173
0.940
1.290
crystal size [mm]
rflns collected
0.44”0.42”0.14
21104
0.70”0.50”0.06
32005
0.16”0.14”0.08
19315
0.28”0.12”0.03
56934
independent rflns (Rint
data/parameters
GOF on F2
)
4973 (0.0276)
4973/186
1.057
11532 (0.0771)
11532/501
1.009
4374 (0.0821)
4374/283
1.032
4343 (0.0529)
4343/238
1.037
R1, wR2 [I>2s(I)]
R1, wR2 (all data)
largest diff peak and hole [eŠ
0.038, 0.099
0.048, 0.104
0.38, ꢀ0.29
0.041 , 0.095
0.067, 0.110
0.86, ꢀ1.43
0.065, 0.168
0.070, 0.174
1.17, ꢀ1.88
0.023, 0.057
0.028, 0.058
0.36, ꢀ0.56
ꢀ3
]
Figure 3. X-ray crystal structure of 5 (50% probability ellipsoids). Select-
ꢀ
ed bond lengths [Š], bond angles [8] and torsion angles [8]: Ni1 N1
ꢀ
ꢀ
ꢀ
ꢀ
2.079(1), Ni1 O1 2.022(1), O1 C10 1.304(2), O2 C7 1.385(2), N2 C4
Figure 4. X-ray crystal structure of 6A (50% probability ellipsoids). Se-
lected bond lengths [Š], bond angles [8], torsion angles [8] and dihedral
ꢀ
1.463(1), C4 C5 1.511(2); O1-Ni1-O1* 180.0, N2-C4-N2# 110.5(1), C4-
C5-C6 114.5(1), C4-C5-C10 123.1(1); N1-N2-C4-C5 77.8(1). Symmetry
transformations used to generate equivalent atoms: ꢀx,ꢀy+1,ꢀz+1 (*);
x,ꢀy+1,z (#).
ꢀ
ꢀ
ꢀ
ꢀ
angles [8]: Pd1 Cl1 2.283(1), Pd1 Cl2 2.295(1), Pd1 N2 2.024(3), Pd1
ꢀ
ꢀ
ꢀ
ꢀ
N4 2.020(3), O1 C10 1.368(4), O2 C13 1.396(3), N1 C7 1.456(4), N3
C7 1.461(4); Cl1-Pd1-Cl2 90.2(1), Cl1-Pd1-N2 90.5(1), Cl1-Pd1-N4
179.5(1), N2-Pd1-N4 89.3(1), N2-Pd1-Cl2 175.6(1), N4-Pd1-Cl2 90.0(1);
N2-N1-C7-C8 ꢀ69.6(4), N4-N3-C7-C8 76.8(4); Cl1-Pd1-Cl2//N2-Pd1-N4
4.4, N1pz//N3pz 49.0.
a larger degree of O!Aryl p bonding, which leads to more
ꢀ
O C double bond character compared with the protonated
free ligands.
Compound 6 crystallises from CH3CN with two crystallo-
graphically independent molecules in the asymmetric unit
(6A and 6B). Both molecules adopt a chiral conformation.
Differences in the structural parameters of 6A and 6B are
close to the margins of experimental error and therefore do
not merit further discussion. Thus, only the molecular struc-
ture of 6A is considered herein (Figure 4).
The square-planar complex of 6A features a PdCl2 moiety
coordinated to the two pyrazol-1-yl rings of the ligand. All
of the bond lengths and bond angles agree nicely with the
values of the related compound [PdCl
2{Ph2C(pz)2}][47] on the
A
one hand and 2a (cf. the Supporting Information) on the
other. Somewhat surprisingly, the complex fragment is
turned away from the HO2 hydroxy group even though a
Chem. Eur. J. 2008, 14, 2590 – 2601
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2595

M. Wagner et al.
closer approach between Pd1 and the O2 atom might have
been anticipated on electrostatic grounds.
data lead to the conclusion that treatment of 6ox with NEt3
causes deprotonation of the methine carbon atom (C1 in
Figure 5) with subsequent delocalisation of the resulting
electron lone-pair into the p-benzoquinonyl substituent. In
turn, the electron density is shifted to the carbon atom ortho
to the bis(pyrazol-1-yl)methyl fragment (C36 in Figure 5),
which thereby gains sufficient nucleophilic character to bind
to the Lewis acidic PdII centre.
Single crystals of 12ox were grown from Et2O/CH3CN
(Figure 6); the compound crystallises with CH3CN (1 equiv).
The molecule contains one PdII ion coordinated by two chlo-
ride ions and the bis(pyrazol-1-yl)methane fragment in a
square-planar fashion.
Centrosymmetric macrocycle 7 contains two PdII ions,
each of them surrounded by one chloride ligand, two pyra-
zolyl rings and one carbon atom in a square-planar fashion
(Figure 5). For the bis(pyrazol-1-yl)methyl moieties, a bridg-
ing rather than a chelating coordination mode is observed,
ꢀ
which results in significantly different Pd N bond lengths of
ꢀ
ꢀ
2.053(4) (Pd1 N12) and 2.139(4) Š (Pd1 N22A).
Figure 5. X-ray crystal structure of 7 (50% probability ellipsoids). Select-
ꢀ
ed bond lengths [Š], bond angles [8] and torsion angles [8]: Pd1 Cl1
ꢀ
ꢀ
ꢀ
2.309(1), Pd1 N12 2.053(4), Pd1 N22A 2.139(4), Pd1 C36 2.082(5),
ꢀ
ꢀ
ꢀ
ꢀ
N11 C1 1.417(6), N21 C1 1.414(6), C1 C31 1.351(7), O32 C32 1.225(6),
ꢀ
ꢀ
ꢀ
ꢀ
O35 C35 1.225(6), C31 C32 1.504(6), C31 C36 1.469(7), C32 C33
Figure 6. X-ray crystal structure of 12ox (50% probability ellipsoids). Se-
lected bond lengths [Š], bond angles [8], torsion angles [8] and dihedral
ꢀ
ꢀ
ꢀ
1.513(7), C33 C34 1.336(8), C34 C35 1.489(7), C35 C36 1.490(7); Cl1-
Pd1-N12 176.2(1), Cl1-Pd1-N22A 90.6(1), Cl1-Pd1-C36 94.6(1), N12-Pd1-
N22A 92.5(2), N22A-Pd1-C36 173.6 (2), N12-Pd1-C36 82.4(2), Pd1-C36-
C31 101.8(3), N11-C1-N21 112.9(4), N11-C1-C31 121.0(4), N21-C1-C31
126.1(4); N11-C1-C31-C32 ꢀ163.5(4), N21-C1-C31-C32 18.2(7). Symme-
try transformation used to generate equivalent atoms: ꢀx+1,ꢀy+1,ꢀz+
1 (A).
ꢀ
ꢀ
ꢀ
ꢀ
angles [8]: Pd1 Cl1 2.290(1), Pd1 Cl2 2.294(1), Pd1 N12 2.024(1), Pd1
ꢀ
ꢀ
ꢀ
N22 2.034(2), O32 C32 1.223(2), O35 C35 1.231(2), C31 C32 1.498(2),
ꢀ
ꢀ
ꢀ
ꢀ
C32 C33 1.487(3), C33 C34 1.335(3), C34 C35 1.472(3), C35 C36
ꢀ
ꢀ
ꢀ
1.487(2), C31 C36 1.340(2), N11 C1 1.474(2), N21 C1 1.479(2); Cl1-
Pd1-Cl2 92.49(2), Cl1-Pd1-N12 177.2(1), Cl1-Pd1-N22 90.1(1), N12-Pd1-
N22 87.1(1), N12-Pd1-Cl2 90.3(1), N22-Pd1-Cl2 177.3(1); N12-N11-C1-
C31 53.1(2), N22-N21-C1-C31 ꢀ63.3(2); Cl1-Pd1-Cl2//N12-Pd1-N22 0.9,
N11pz//N21pz 57.9.
ꢀ
The shorter Pd N bond is part of a six-membered PdN2C3
ꢀ
palladacycle. We note that the Pd1 C36 (2.082(5) Š) bond
(sp2) bond in
Similar to the molecular structure of 6, there is no short
contact between the O32 atom and the PdII centre in 12ox.
ꢀ
of
7
is 0.112 Š longer than the Pd C
[PdCl(Ph)(2,2’-bipy)] (1.970(7) Š)[48] and still 0.062 Š longer
_
ꢀ
ꢀ
than the Pd C
(sp3) bond in [PdCl
U
G
The redox-active substituent shows two short C O bonds
_
(2.020(11) Š;[49] NN: ArN=C(H)C(H)=NAr, Ar: 2,6-
(average value=1.227(2) Š) together with strongly alternat-
ꢀ
ꢀ
ꢀ
ꢀ
(iPr)2C6H3). However, it agrees nicely with the Pd C bond
ing C C bond lengths (e.g., C32 C33 1.487(3) Š, C33 C34
1.335(3) Š), which clearly proves that it is in its p-benzoqui-
none state. All of the relevant bond lengths and angles of
12ox are close to the corresponding values in the free p-ben-
zoquinone-based ligand 2box (cf. the Supporting Informa-
length in [PdCl
ACHTREUNG
which the electron lone-pair at the donor carbon atom is
conjugated with a carbonyl group. This situation is closely
ꢀ
ꢀ
related to our macrocycle. The C31 C36 and C1 C31 dis-
tances in 7 are indicative of a shortened single bond
(1.469(7) Š) and a double bond (1.351(7) Š), respectively.
Moreover, the Pd1-C36-C31 bond angle of 101.8(3)8 is close
to the ideal tetrahedral angle (109.58) and both sums of
angles with vertices at C1 and C31 exactly match 3608. The
tion) and in the related PdII complex [PdCl2{Ph2C(pz)2}].[47]
A
Electrochemical investigations: Compounds 11, 11ox, 12 and
12ox were investigated by cyclic voltammetry (CH3CN,
[NBu4]PF6 (0.1m); vs. FcH/FcH+). For comparison, PdII
complex 13 (Scheme 3), which bears a redox-inactive phenyl
substituent, was synthesised (cf. the Supporting Information)
and included in our investigation.
ꢀ
CO bond lengths of 1.225(6) Š and the C33 C34 bond
length of 1.336(8) Š are almost identical to the correspond-
ing values in 2box (cf. the Supporting Information). These
2596
www.chemeurj.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 2590 – 2601

Bis(pyrazol-1-yl)methane Ligands
FULL PAPER
The cyclic voltammograms (CV) of 11 and 12 reveal irre-
versible redox transitions with peak potentials of Epa ꢁ0.55,
0.68 (11) and 0.63, 0.79 V (12), which were assigned to the
p-hydroquinone!p-semiquinone!p-benzoquinone redox
transitions (cf. Figure S7 in the Supporting Information).
The chemical irreversibility most likely results from the loss
of HO protons during oxidation.[51] We note that the voltam-
mogram of 11 is qualitatively similar to that of the related
p-hydroquinone-containing ditopic imine ligand B (R=
NiPr2; Figure 1).[22] Compound 12 features an irreversible
reduction wave at Epc =ꢀ1.23 V that is also present in the
CV of 13 (Epc ꢁꢀ1.30 V; Figure S7) but absent in the vol-
tammogram of 11. We therefore assign this redox event to a
palladium-centred electron transition.
sible one-electron step: DE=59 mV). The first one-electron
Cꢀ
1
reduction of 11ox to [11ox] occurs at a potential of E = =
2
ꢀ0.79 V (DE=85 mV); for the second reduction step to
2ꢀ
1
[11ox] , a potential of E = =ꢀ1.53 V (DE=110 mV) was de-
2
termined. The peak current associated with the second elec-
tron transfer is somewhat smaller than the peak current of
the first transition. This phenomenon has already been de-
scribed in the literature for other p-benzoquinone deriva-
tives.[52,53] The CV of the PdII complex 12ox recorded up to a
potential of E=ꢀ1.2 V (bold line in Figure 7b) exhibits a
1
chemically reversible redox wave at a potential of E = =
2
ꢀ0.69 V (DE=79 mV) that agrees nicely with the first
redox event of 11ox. When the sweep is continued into the
cathodic regime, a broad and irreversible wave is detected
at Epc ꢁꢀ1.58 V. Moreover, the anodic current of the first
redox event decreases significantly in the reverse scan and
new, ill-defined features appear in the potential range be-
tween 0 and 0.5 V (Figure 7). Similar features are apparent
in the CV of complex 13, together with an irreversible wave
at Epc ꢁꢀ1.30 V. The CVs of 11ox and 13 indicate that p-
semiquinonate and PdII reduction require comparable elec-
trode potentials. As a consequence, it cannot be safely con-
cluded whether the broad cathodic wave of 12ox can be as-
signed to a ligand- or metal-centred redox transition or
whether both occur simultaneously.
p-Benzoquinone species 11ox and 12ox are electrochemical-
ly better behaved than their p-hydroquinone congeners.
CVs of 11ox, 12ox and 13 are juxtaposed in Figure 7.
Exploratory reactivity studies: As already mentioned in the
Introduction, PdII/1,4-diazabutadiene complexes in the pres-
ence of p-hydroquinone have been successfully used in the
Ullmann-type homocoupling of aryl halides (typical reaction
conditions: K2CO3, [NBu4]Br, DMF, 1358C, 24 h).[37] Thus,
we chose this conversion to assess the reactivity of our new
complexes. As preliminary tests on the base stability of 6/6ox
have revealed the ready abstraction of the methine proton
with formation of macrocyclic dimer 7 (Scheme 2), we em-
ployed methyl derivative 12 (Scheme 3) in our investigation.
Treatment of 12 with a stoichiometric amount of iodoben-
zene (Cs2CO3, DMF, 1508C, 5 h) led to the quantitative con-
sumption of the substrate with the concomitant formation of
biphenyl and benzene (NMR spectroscopic and HPLC con-
trol). Although biphenyl is the expected product, the mecha-
nism of benzene formation is not yet understood. Neverthe-
less, our results can best be explained if we assume an oxida-
tive addition of iodobenzene at the palladium centre preced-
ed by the intramolecular electron-transfer reaction p-hydro-
quinone/PdII!p-benzoquinone/Pd0.
Figure 7. Cyclic voltammograms of a) 11ox, b) 12ox and c) 13. CH3CN solu-
tions, [NBu4]PF6 (0.1m), scan rates: a) 100 mVsꢀ1, b and c) 200 mVsꢀ1
,
vs. FcH/FcH+ (*).
Conclusion
Free ligand 11ox gives rise to two redox waves with fea-
tures that indicate electrochemical reversibility: the current
Our group is interested in the development of novel redox-
active ligands both for the generation of metal-containing
polymers and for use in homogeneous catalysis. To this end,
we have prepared a series of p-hydroquinone-substituted
bis(pyrazol-1-yl)methane ligands (2a, 2b, 2c and 11) and
shown that the PdII complex of even the sterically least
shielded derivative (11) is perfectly stable as a monomeric
ratios (ipc/ipa) are constantly equal to one, the current func-
1
tions ipa/v = remain constant and the peak-to-peak separa-
2
tions (DE) do not deviate appreciably from the value found
for the internal ferrocene standard (DE
A
expected value for a chemically and electrochemically rever-
Chem. Eur. J. 2008, 14, 2590 – 2601
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2597

M. Wagner et al.
C13H12N4O2·0.5H2O (256.27+9.01): C 58.86, H 4.94, N 21.12; found: C
species in solution. This is in contrast, for example, to the re-
lated chelate ligand 2-(pz)C6H3(OH)2, which tends to give
insoluble coordination polymers with various metal ions.[18]
Electrochemical investigations and chemical oxidations on a
preparative scale clearly reveal that free p-hydroquinone li-
gands 2a and 2b as well as the PdII complex [Pd2bCl2] un-
dergo reversible electron transfer to their p-benzoquinone
state (2aox, 2box, [Pd2boxCl2]). This is an important proof-of-
principle of our concept given the fact that we want the p-
quinone substituent to aid the redox chemistry of the com-
plexed metal ion by acting as a two-electron reservoir. Prob-
lems, however, can result from the high acidity of the me-
thine proton especially when the ligands are in their p-ben-
zoquinone state. Thus, we also prepared a derivative of 2a/
2aox in which the methine proton is replaced by a methyl
group and which turned out to be perfectly base-stable. The
corresponding PdCl2 complex of the methylated ligand is
active in the transformation of iodobenzene into biphenyl
and benzene. The chemistry described in this paper is not
restricted to monotopic ligands. The ditopic p-quinone-
bridged bis(pyrazol-1-yl)methane ligands 4 and 4ox are also
accessible in good yields and we are currently exploring the
scope of 4 and 4ox as facially coordinating linkers for coordi-
nation polymer synthesis.
58.75, H 4.56, N 20.71.
1
2b: Rf =0.52 (silica gel; Et2O/hexane 1:1); H NMR (250.1 MHz, CDCl3):
d=10.87 (s, 1H; OH), 7.79, 7.57 (2”m, 2”2H; pzH-3,5), 7.21 (s, 1H;
CHpz2), 6.92, 6.68 (2”d, 4J
ACHTREUNG
AHCTREUNG
CD3CN): d=150.6, 148.0, 142.6 (HQC-1,4,6), 141.1, 131.8 (pzC-3,5),
124.2 (HQC-2), 117.5, 115.9 (HQC-3,5), 106.8 (pzC-4), 78.7 (CHpz2), 35.7
(C
N
N
+H], 245 (100) [M+ꢀpz]; elemental analysis calcd (%) for
C17H20N4O2·0.5H2O (312.37+9.01): C 63.53, H 6.59, N 17.42; found: C
63.00, H 6.91, N 17.57.
1
2c: Rf =0.61 (silica gel; Et2O/hexane 1:1); H NMR (250.1 MHz, CDCl3):
d=10.88 (s, 1H; OH), 7.78, 7.58 (2”m, 2”2H; pzH-3,5), 7.22 (s, 1H;
CHpz2), 6.99, 6.69 (2”d, 4J
ACHTREUNG
2H; pzH-4), 1.44 (s, 9H; C
A
ACHTREUNG
(m, 18H; CH
AHCTREUNG
148.3, 141.6, 140.5 (HQC-1,4,6; pzC-3 or pzC-5), 130.7 (pzC-5 or pzC-3),
121.8, 121.2, 119.6 (HQC-2,3,5), 106.1 (pzC-4), 79.3 (CHpz2), 35.2 (C-
A
G
G
G
(ESI+): m/z (%): 469 (17) [M+ +H], 401 (100) [M+ꢀpz]; elemental anal-
ysis calcd (%) for C26H40N4O2Si (468.71): C 66.63, H 8.60, N 11.95;
found: C 65.65, H 8.50, N 11.76.
Synthesis of 4: A solution of 3 (0.498 g, 3.00 mmol) in THF (80 mL) was
treated with bis(pyrazol-1-yl)methanone (1.151 g, 7.10 mmol) and anhy-
drous CoCl2 (0.156 g, 1.20 mmol). The resulting suspension was heated at
reflux for 24 h and then allowed to cool to room temperature. H2O
(50 mL) was added and stirring was continued for another 30 min. The
product was extracted into CH2Cl2 (3”50 mL), the combined organic
phases were dried (MgSO4), filtered and the filtrate evaporated to dry-
ness in vacuo to yield 4 as a pale yellow solid (0.340 g, 28%). Single crys-
tals of 4·DMF2 formed upon gas-phase diffusion of Et2O into a saturated
solution of 4 in DMF. 1H NMR (250.1 MHz, [D7]DMF): d=9.82 (s, 2H;
OH), 8.00 (s, 2H; CHpz2), 7.75, 7.58 (2”m, 2”4H; pzH-3,5), 6.69 (s, 2H;
HQH-3,3’), 6.32 ppm (m, 4H; pzH-4); 13C NMR (62.9 MHz, [D7]DMF):
d=147.8 (HQC-1,1’), 140.0, 129.9 (pzC-3,5), 124.6 (HQC-2,2’), 115.2
(HQC-3,3’), 105.7 (pzC-4), 72.9 ppm (CHpz2); LR-MS (ESI+): m/z (%):
403 (21) [M+ +H], 335 (62) [M+ꢀpz]; elemental analysis calcd (%) for
C20H18N8O2·2C3H7NO (402.48+146.14): C 56.92, H 5.88, N 25.52; found:
C 56.17, H 5.85, N 25.12.
Experimental Section
General: All reactions and manipulations of air-sensitive compounds
were carried out in dry, oxygen-free nitrogen by using standard Schlenk
ware. THF was freshly distilled under argon from Na/benzophenone and
CH3CN was stored over 4 Š molecular sieves prior to use. NMR spectra
were recorded with Bruker AM-250, Bruker Avance-300 and Bruker
Avance-400 spectrometers. All NMR spectra were recorded at 278C. Ab-
breviations: s=singlet; d=doublet; t=triplet; vt=virtual triplet; dd=
doublet of doublets; br=signal broadened; m=multiplet; n.r.=multiplet
Synthesis of 2aox, 2box and 4ox: A solution of the appropriate p-hydroqui-
none derivative (2a (0.051 g, 0.20 mmol), 2b (0.051 g, 0.16 mmol), 4
1
(0.081 g, 0.20 mmol)) in CH3CN (20 mL) was treated with [Ce
A
expected in the H NMR spectrum, but not resolved; n.o.=signal not ob-
AHCTREUNG
served; pz=pyrazol-1-yl; HQ=p-hydroquinone core; BQ=p-benzoqui-
none core. Elemental analyses were performed by the Microanalytical
Laboratory of the University of Frankfurt and by Quantitative Technolo-
gies Inc. (QTI). Mass spectra were recorded with a VG PLATFORM II-
mass spectrometer. Compounds 1a and 8 are commercially available.
Compounds 1b,[54] 1c,[54] 3[27] and bis(pyrazol-1-yl)methanone[42] were syn-
thesised according to literature procedures.
warmed to room temperature and stirred for 3 h. The respective product
was extracted into EtOAc (3”20 mL) and the combined organic phases
dried (MgSO4) and filtered. The filtrate was evaporated to dryness in
vacuo to give 2aox as an orange solid (0.048 g, 95%), 2box as a yellow
solid (0.047 g, 95%) and 4ox as a yellow oil (0.076 g, 95%). Single crystals
of 2box were grown from CH2Cl2/Et2O (2:1) at 58C.
2aox:
1H NMR (250.1 MHz, MeOD): d=7.84 (s, 1H; CHpz2), 7.78, 7.63
Synthesis of 2a to 2c:
A solution of the corresponding aldehyde
(m, n.r., 2”2H; pzH-3,5), 6.84 (m, 2H; BQH-5,6), 6.41 (n.r., 2H; pzH-
4), 6.19 ppm (s, 1H; BQH-3); 13C NMR (62.9 MHz, MeOD): d=188.3,
186.3 (BQC-1,4), 143.7 (BQC-2), 142.4 (pzC-3 or pzC-5), 138.2, 137.9,
135.5 (BQC-3,5,6), 132.1 (pzC-5 or pzC-3), 108.3 (pzC-4), 72.8 ppm
(CHpz2); elemental analysis calcd (%) for C13H10N4O2 (254.25): C 61.41,
H 3.96, N 22.04; found: C 60.94, H 3.90, N 21.88.
(7.80 mmol) in THF (100 mL) was treated with bis(pyrazol-1-yl)metha-
none (1.508 g, 9.30 mmol) and anhydrous CoCl2 (0.325 g, 2.50 mmol).
The resulting suspension was heated at reflux for 24 h and then allowed
to cool to room temperature. H2O (50 mL) was added and the mixture
stirred for an additional 30 min. The product was extracted into CH2Cl2
(3”50 mL), the combined organic phases were dried (MgSO4), filtered
and the filtrate was evaporated to dryness in vacuo. Flash column chro-
matography yielded pure products 2a (0.720 g, 36%), 2b (0.682 g, 28%)
and 2c (0.841 g, 23%). Single crystals of 2a were grown from hexane/
MeOH (1:1) at 58C.
2box
pzH-5), 7.56 (m, 2H; pzH-5 or pzH-3), 6.62 (d, 4J
BQH-5), 6.35 (vt, 2H; pzH-4), 6.09 (dd, 4J
(H,H)=1.6 Hz, 2.4 Hz, 1H;
BQH-3), 1.21 ppm (s, 9H; C
(CH3)3); 13C NMR (100.6 MHz, CD3CN):
:
1H NMR (400.1 MHz, CD3CN): d=7.71 (m, 3H; CHpz2, pzH-3 or
AHCTREUNG
AHCTREUNG
AHCTREUNG
2a: Rf =0.52 (silica gel; EtOAc/hexane/MeOH 1:1:1); 1H NMR
(250.1 MHz, MeOD): d=7.91 (s, 1H; CHpz2), 7.61, 7.47 (2”m, 2”2H;
pzH-3,5), 6.73 (m, 2H; HQH-5,6), 6.36 (vt, 2H; pzH-4), 6.27 ppm (m,
1H; HQH-3); 13C NMR (62.9 MHz, MeOD): d=151.6, 149.4 (HQC-1,4),
141.7, 131.3 (pzC-3,5), 123.3 (HQC-2), 118.8, 117.7 (HQC-5,6), 115.5
(HQC-3), 107.3 (pzC-4), 75.0 ppm (CHpz2); LR-MS (ESI+): m/z (%):
257 (25) [M+ +H], 189 (100) [M+ꢀpz]; elemental analysis calcd (%) for
d=188.9, 187.0 (BQC-1,4), 157.7, 145.9 (BQC-2,6), 142.2 (pzC-3 or pzC-
5), 134.3, 133.3 (BQC-3,5), 131.7 (pzC-5 or pzC-3), 108.3 (pzC-4), 72.3
(CHpz2), 36.5 (C
C
G
(310.35): C 65.79, H 5.85, N 18.05; found: C 65.35, H 5.91, N 17.79.
4ox
8.01 (s, 2H; CHpz2), 7.64 (n.r., 4H; pzH-5 or pzH-3), 6.41 ppm (n.r., 6H;
:
1H NMR (250.1 MHz, [D7]DMF): d=8.06 (m, 4H; pzH-3 or pzH-5),
2598
www.chemeurj.org
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 2590 – 2601

Bis(pyrazol-1-yl)methane Ligands
FULL PAPER
BQH-3,3’, pzH-4); 13C NMR (62.9 MHz, [D7]DMF): d=185.1 (BQC-
1,1’), 143.3 (BQC-2,2’), 141.1 (pzC-3 or pzC-5), 134.8 (BQC-3,3’), 131.2
(pzC-5 or pzC-3), 107.0 (pzC-4), 71.2 ppm (CHpz2); LR-MS (ESI+): m/z
(%): 401 (100) [M++H], 333 (17) [M+ꢀpz]; elemental analysis calcd
(%) for C20H16N8O2 (400.39): C 59.99, H 4.03, N 27.99; found: C 59.48, H
4.03, N 27.50.
calcd (%) for C15H16N4O2 (284.32): C 63.37, H 5.67, N 19.70; found: C
63.35, H 5.81, N 19.84.
Synthesis of 10:
A solution of nBuLi in hexane (2.1m, 1.3 mL,
2.80 mmol) was added dropwise with stirring to a solution of 9 (0.800 g,
2.81 mmol) in THF (40 mL) at 08C. After 30 min, the resulting dark-
yellow solution was allowed to warm to room temperature. Neat MeI
(0.397 g, 2.80 mmol) was added and the reaction mixture was stirred for
another 30 min. Water (100 mL) was added and 10 was extracted into
CH2Cl2 (3”50 mL). The combined organic phases were dried over
MgSO4. Filtration and evaporation of the solvent in vacuo gave a yellow
oil. The pure product of 10 (0.620 g, 74%) was obtained as a colourless
oil after flash column chromatography (Rf =0.23; silica gel; EtOAc/
hexane 1:2). 1H NMR (250.1 MHz, CDCl3): d=7.63, 7.25 (2”m, 2”2H;
pzH-3,5), 6.85 (m, 2H; HQH-5,6), 6.29 (m, 2H; pzH-4), 5.42 (m, 1H;
HQH-3), 3.59, 3.58 (2”s, 2”3H; OMe), 2.78 ppm (s, 3H; Me); 13C NMR
(75.5 MHz, CDCl3): d=153.7, 150.7 (HQC-1,4), 139.9 (pzC-3 or pzC-5),
131.8 (HQC-2), 129.3 (pzC-5 or pzC-3), 114.7, 114.0, 113.3 (HQC-3,5,6),
106.0 (pzC-4), 81.9 (CMepz2), 56.2, 55.5 (OMe), 25.2 ppm (Me); LR-MS
(ESI+): m/z (%): 231 (100) [M+ꢀpz]; elemental analysis calcd (%) for
C16H18N4O2 (298.34): C 64.41, H 6.08, N 18.77; found: C 64.22, H 6.27, N
18.87.
Synthesis of 5: A solution of 2c (0.189 g, 0.40 mmol) in MeOH (15 mL)
was treated with a solution of NaOMe in MeOH (1 mL, 0.5m). After the
addition of [Ni(OAc)2·4H2O] (0.055 g, 0.22 mmol), the reaction mixture
G
was stirred for 12 h at room temperature. All of the volatiles were re-
moved in vacuo to give 5 as a pale-red solid (0.302 g, 76%). Single crys-
tals of 5 grew upon storage of a solution in CH2Cl2 at ꢀ208C. LR-MS
(ESI+): m/z (%): 994 (100) [M++H]; elemental analysis calcd (%) for
C52H78N8NiO4Si2 (994.11): C 62.83, H 7.91, N 11.27; found: C 62.33, H
7.74, N 11.28.
Synthesis of 6: Neat [PdCl2(cod)] (0.137 g, 0.48 mmol) was added to a so-
E
lution of 2b (0.150 g, 0.48 mmol) in CH3CN (30 mL). The resulting
yellow solution was stirred for 12 h at room temperature. All of the vola-
tiles were removed in vacuo and the solid residue was redissolved in a
minimum amount of CH2Cl2. Hexane was added to the solution to pre-
cipitate 6 as a yellow solid (0.20 g, 85%). Single crystals of 6 were ob-
a
solution in CH3CN. 1H NMR
Synthesis of 11ox: A solution of [Ce
N
N
tained upon slow evaporation of
(250.1 MHz, CD3CN): d=8.13 (m, 2H; pzH-3 or pzH-5), 8.10 (s, 1H;
CHpz2), 8.02 (m, 2H; pzH-5 or pzH-3), 6.99 (d, 4J
(H,H)=3.0 Hz, 1H;
HQH-3 or HQH-5), 6.90 (s, 1H; OH), 6.80 (d, 4J
(H,H)=3.0 Hz, 1H;
HQH-5 or HQH-3), 6.50 (m, 2H; pzH-4), 5.60 (s, 1H; OH), 1.36 ppm (s,
9H; C
(CH3)3); 13C NMR (75.5 MHz, CD3CN): d=152.2, 146.1 (HQC-
in H2O (50 mL) was added with stirring at 08C to a solution of 10
(1.800 g, 6.03 mmol) in THF (100 mL). The reaction mixture was allowed
to warm to room temperature and stirred for 3 h. Compound 11ox was ex-
tracted into CH2Cl2 (3”50 mL), the extract was dried (MgSO4), filtered
and the filtrate evaporated to dryness in vacuo. The crude product of 11ox
(1.050 g, 65%) was purified by flash column chromatography (Rf =0.68;
silica gel; EtOAc/hexane 2:1; still contaminated with ca. 15% of 10).
1H NMR (250.1 MHz, CDCl3): d=7.59, 7.36 (2”m, 2”2H; pzH-3,5),
6.71 (m, 2H; BQH-5,6), 6.35 (m, 2H; pzH-4), 5.45 (m, 1H; BQH-3),
2.67 ppm (s, 3H; Me); 13C NMR (62.9 MHz, CDCl3): d=187.2, 184.8
(BQC-1,4), 148.1 (BQC-2), 140.3 (pzC-3 or pzC-5), 137.6, 136.0, 132.6
(BQC-3,5,6), 128.4 (pzC-5 or pzC-3), 107.4 (pzC-4), 79.7 (CMepz2),
25.1 ppm (Me); LR-MS (ESI+): m/z (%): 269 (82) [M++H], 201 (84)
[M+ꢀpz]; elemental analysis was not obtained because even after HPLC
purification 11ox was still contaminated by small amounts of 10.
AHCTREUNG
AHCTREUNG
ACHTREUNG
1,4), 145.0 (pzC-3 or pzC-5), 142.6 (HQC-6), 136.5 (pzC-5 or pzC-3),
125.8 (HQC-2), 117.8, 113.4 (HQC-3,5), 107.9 (pzC-4), 73.4 (CHpz2), 35.2
(C
N
N
[MꢀꢀH], 525 (100) [Mꢀ+Cl]; elemental analysis calcd (%) for
C17H20Cl2N4O2Pd·H2O (489.69+18.02): C 40.22, H 4.37, N 11.03; found:
C 40.32, H 4.50, N 10.74.
Synthesis of 6ox: Neat [PdCl2(cod)] (0.048 g, 0.17 mmol) was added to a
R
solution of 2box (0.053 g, 0.17 mmol) in CH3CN (15 mL). The resulting
yellow solution was stirred for 12 h at room temperature, the solvent was
removed in vacuo and the residue dissolved in a minimum amount of
CH2Cl2. Hexane was added to precipitate a yellow solid of 6ox that was
isolated by filtration (0.060 g, 71%). 1H NMR (250.1 MHz, CD3CN): d=
8.15, 8.10 (2”m, 2”2H; pzH-3,5), 8.00 (n.r., 1H; CHpz2), 6.74, 6.65 (d,
Synthesis of 11:
A solution of technical grade Na2S2O4 (4.000 g,
22.97 mmol) in H2O (40 mL) was added dropwise with stirring at room
temperature to a solution of 11ox (1.050 g, 3.91 mmol) in THF (60 mL).
After 2 h of vigorous stirring, the reaction mixture was extracted with
CH2Cl2 (3”50 mL) and the combined organic phases were dried
(MgSO4) and filtered. The filtrate was evaporated to dryness in vacuo
and the solid residue was dissolved in a minimum amount of hexane/
EtOAc (1:2). The solution was stored at 58C for several hours where-
upon a colourless precipitate of 11 formed that was isolated by filtration
and washed with a sparse amount of CHCl3 (0.549 g, 52%). 1H NMR
(300.0 MHz, CD3CN): d=7.57, 7.28 (2”m, 2”2H; pzH-3,5), 7.03 (br,
1H; OH), 6.70 (m, 2H; HQH-5,6), 6.50 (br, 1H; OH), 6.31 (m, 2H;
m, 4J
(s, 9H; C
ACHTREUNG
ACHTREUNG
1,4), 157.4 (BQC-2 or BQC-6), 145.7 (pzC-3 or pzC-5), 142.7 (BQC-6 or
BQC-2), 137.0 (pzC-5 or pzC-3), 135.0, 133.0 (BQC-3,5), 108.6 (pzC-4),
71.0 (CHpz2), 36.1 (C
N
N
(%): 487 (100) [MꢀꢀH]; elemental analysis calcd (%) for
C17H18Cl2N4O2Pd·H2O (487.66+18.02): C 40.38, H 3.99, N 11.07; found:
C 40.64, H 3.69, N 11.11.
pzH-4), 5.33 (dd, 4J
N
N
Synthesis of 7: NEt3 (0.010 g, 0.10 mmol) was added to a solution of 6ox
(0.020 g, 0.04 mmol) in CH3CN (5 mL). After stirring for 12 h at room
temperature, the mixture was treated with EtOAc (5 mL), washed with
brine and the organic phase was separated and dried (MgSO4). All of the
volatiles were removed in vacuo and the greenish-brown residue was dis-
solved in CH3CN (1 mL). After several days, a few orange crystals
formed that were suitable for X-ray structure analysis.
(s, 3H; Me); 13C NMR (75.5 MHz, CD3CN): d=151.0, 148.0 (HQC-1,4),
140.5, 130.5 (pzC-3,5), 130.0 (HQC-2), 118.8, 117.6, 115.2 (HQC-3,5,6),
106.7 (pzC-4), 82.6 (CMepz2), 25.4 (Me); LR-MS (ESI+): m/z (%): 293
(100) [M++Na]; elemental analysis calcd (%) for C14H14N4O2 (270.29): C
62.21, H 5.22, N 20.72; found: C 61.92, H 5.41, N 20.60.
Synthesis of 12: Neat [PdCl2(CH3CN)2] (0.144 g, 0.56 mmol) was added
N
Synthesis of 9: Compound 9 was synthesised in a similar manner to 2a–
2c from 8 (3.390 g, 20.40 mmol) and bis(pyrazol-1-yl)methanone (3.308 g,
20.40 mmol) by using a catalytic amount of anhydrous CoCl2 (0.010 g,
0.08 mmol). Yield: 4.524 g (78%); Rf =0.43 (silica gel, EtOAc/hexane,
1:1). Single crystals of 9 formed from a concentrated solution in CHCl3
at room temperature. 1H NMR (300.0 MHz, CDCl3): d=7.97 (s, 1H;
CHpz2), 7.62, 7.41 (2”m, 2”2H; pzH-3,5), 6.88 (m, 2H; HQH-5,6), 6.49
(m, 1H; HQH-3), 6.30 (m, 2H; pzH-4), 3.71, 3.68 ppm (2”s, 2”3H;
OMe); 13C NMR (75.5 MHz, CDCl3): d=153.8, 151.0 (HQC-1,4), 140.8,
129.6 (pzC-3,5), 125.3 (HQC-2), 115.4 (HQC-5 or HQC-6), 114.4 (HQC-
3), 112.3 (HQC-6 or HQC-5), 106.2 (pzC-4), 73.2 (CHpz2), 56.4, 55.8 ppm
(OMe); LR-MS (ESI+): m/z (%): 217 (100) [M+ꢀpz]; elemental analysis
at room temperature to a solution of 11 (0.150 g, 0.56 mmmol) in CH3CN
(25 mL). The resulting mixture was heated at reflux for 6 h. After cooling
to room temperature, the solvent was removed in vacuo. The yellow resi-
due of 12 was first washed with a mixture of CH3CN (2 mL)/CHCl3
(5 mL) and subsequently with hexane (10 mL) to give the final product
(0.205 g, 82%). 1H NMR (300.0 MHz, CD3CN): d=8.19, 8.02 (2”br, 2”
2H; pzH-3,5), 6.82, 6.76 (2”m, 2”1H; HQH-5,6), 6.75 (s, 1H; OH), 6.63
(s, 1H; OH), 6.52 (br, 2H; pzH-4), 5.49 (n.r., 1H; HQH-3), 2.66 ppm (s,
3H; Me); 13C NMR (75.5 MHz, CD3CN/[D6]DMSO, 50:1): d=151.2,
148.3 (HQC-1,4), 144.9, 134.3 (2”br, pzC-3,5), 127.5 (HQC-2), 118.9,
118.8 (HQC-5,6), 115.1 (HQC-3), 107.4 (br, pzC-4), 82.0 (CMepz2),
24.8 ppm (Me); LR-MS (ESI+): m/z (%): 375 (100) [M+ꢀ2ClꢀH]; ele-
Chem. Eur. J. 2008, 14, 2590 – 2601
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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2599

M. Wagner et al.
mental analysis calcd (%) for C14H14Cl2N4O2Pd (447.61): C 37.57, H 3.15,
N 12.52; found: C 37.51, H 3.29, N 12.64.
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Inorg. Chem. 1997, 36, 2103–2111.
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2005, 127, 10656–10666.
Synthesis of 12ox: Neat [PdCl2(cod)] (0.031 g, 0.11 mmol) was added to a
A
solution of 11ox (0.030 g, 0.11 mmol) in CH3CN (5 mL) at room tempera-
ture. After the resulting mixture had been stirred for 48 h at room tem-
perature, all of the volatiles were removed in vacuo. The yellow solid res-
idue of 12ox was first washed with CH2Cl2 (1 mL) and then with hexane
(10 mL) to give the final product (0.029 g, 59%). Single crystals of 12ox
formed upon gas-phase diffusion of Et2O into a saturated solution of 12ox
in CH3CN. 1H NMR (250.1 MHz, CD3CN): d=8.23, 7.96 (m, br, 2”2H;
pzH-3,5), 6.97 (dd, 3J
N
G
3
4
6.83 (d, J
C
C
[13] Ditopic neutral phenylene-bridged bis(pyrazol-1-yl)methane ligands
have been reported by Reger et al., see: a) D. L. Reger, R. P.
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1544–1555; b) D. L. Reger, R. P. Watson, M. D. Smith Inorg. Chem.
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(62.9 MHz, CD3CN): d=187.3, 185.1 (BQC-1,4), 145.7 (br, pzC-3 or pzC-
5), 145.4 (BQC-2), 138.7 (BQC-6), 137.4 (BQC-5), 137.0 (BQC-3), 134.7
(br, pzC-5 or pzC-3), 108.5 (pzC-4), 79.9 (CMepz2), 23.8 ppm (Me); LR-
MS (ESI+): m/z (%): 450 (100) [M+ꢀCl+CH3CN]; elemental analysis
calcd (%) for C14H12Cl2N4O2Pd (445.60): C 37.74, H 2.71, N 12.57; found:
C 37.65, H 2.95, N 12.40.
Crystal structure determinations of 5, 6, 7 and 12ox: Crystals of 5 and 6
were measured by using a Siemens SMART 1K-CCD diffractometer with
graphite-monochromated MoKa radiation. An empirical absorption cor-
rection was performed for 5 by using the SADABS[55] program and a nu-
merical absorption correction was performed for 6. Data for 7 and 12ox
were collected by using a Stoe-IPDS-II two-circle diffractometer with
graphite-monochromated MoKa radiation. Empirical absorption correc-
tions were performed by using the MULABS option[56] in the
PLATON[57] program. All of the structures were solved by direct meth-
ods[58] and refined by full-matrix least-squares methods on F2 by using
the SHELXL97[59] program. Hydrogen atoms were placed at ideal posi-
tions and refined with fixed isotropic displacement parameters by using a
riding model. The unit cell of 5 contains two symmetry-related solvent
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3
accessible areas of about 479 Š each which contain completely disor-
dered solvate. The PLATON/SQUEEZE[57] program was used to account
for the solvent electron density. Complex 7 crystallises with four mole-
cules of CH3CN in the asymmetric unit whereas complex 12ox crystallises
with one molecule of CH3CN in the asymmetric unit.
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CCDC 663883 (2a), 663884 (2box), 663885 (2c), 663886 (4·DMF2), 663887
(5), 663888 (6), 662352 (7), 662354 (9), 662353 (12ox) and 662355 (13)
contain the supplementary crystallographic data for this paper. These
data can be obtained free of charge from the Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif
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Bis(pyrazol-1-yl)methane Ligands
FULL PAPER
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Received: October 12, 2007
Published online: January 10, 2008
Chem. Eur. J. 2008, 14, 2590 – 2601
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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