Synthesis and 1H NMR studies of paramagnetic nickel(ii) complexes containing bis(pyrazolyl)methane ligands with dendritic substituents

Article abstract of DOI:10.1039/b607090f

The substituted bis(pyrazolyl)methane ligands RCH(3,5-Me ₂pz)₂ (R = SiMe₃ (1), CH₂Ph (2), G1 (3), G2 (4), and G3 (5); Gn = Frechet-type dendritic wedges of generation n) have been prepared starting from H_2<

Full text of DOI:10.1039/b607090f

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/dalton | Dalton Transactions
Synthesis and 1H NMR studies of paramagnetic nickel(II) complexes
containing bis(pyrazolyl)methane ligands with dendritic substituents†
Alberto Sa´nchez-Me´ndez, Jose Mar´ıa Benito, Ernesto de Jesu´s,* F. Javier de la Mata, Juan C. Flores,*
Rafael Go´mez and Pilar Go´mez-Sal
Received 18th May 2006, Accepted 29th June 2006
First published as an Advance Article on the web 18th July 2006
DOI: 10.1039/b607090f
The substituted bis(pyrazolyl)methane ligands RCH(3,5-Me2pz)2 (R = SiMe3 (1), CH2Ph (2), G1 (3),
G2 (4), and G3 (5); Gn = Fre´chet-type dendritic wedges of generation n) have been prepared starting
from H2C(3,5-Me2pz)2. Reaction of these didentate ligands with [NiBr2(DME)] is a straightforward
procedure that allows the synthesis of the nickel(II) complexes [NiBr2{RCH(3,5-Me2pz)2}] (6–10). The
molecular structure of compound 7 (R = CH2Ph) has been determined by X-ray diffraction studies.
The nickel centre coordinates two bromine and two nitrogen atoms in a tetrahedral environment, and
the metallacycle Ni(NN)2C adopts a boat conformation with the benzyl group in an axial position. 1H
NMR studies have been carried out to characterize these paramagnetic nickel compounds in solution.
Valuable information about the disposition of the ligands and dendritic wedges in solution has been
obtained thanks to the influence of the paramagnetic centre on the proton resonances.
where we reported the synthesis of a tetranuclear compound
Introduction
with a carbosilane core along with four tris(pyrazolyl)methane
Since Trofimenko’s pioneering work introducing poly(pyrazol-1-
molybdenum complexes, we concluded that a broader utilization
yl)borate ligands,1 also known as scorpionates, the coordination
of this type of N-heterocyclic donors in dendrimer chemistry
chemistry of this type of polydentate N-donor ligands has
requires the modification of the ligand in order to reduce steric
crowding and to increase solubility.14
been extensively developed. The versatility of their substitution
allows the controlled modification of their steric and electronic
properties and, therefore, of their coordination capability. In
fact, scorpionates have been used to synthesize complexes of
most metals of the periodic table, some of them with useful
applications in a wide range of areas.2 The neutral and isoelec-
tronic poly(pyrazolyl)alkane ligands, however, received much less
attention until a significant improvement of their synthesis was
published.3 This area of chemistry has subsequently generated new
modified ligands and complexes, including those functionalized
at the methine or methylene bridging carbon atom in tris-4–6
or bis-(pyrazolyl)methanes,7–9 respectively. The substitution of
the reactive methine proton by other functional groups has led,
for instance, to multitopic ligands and polynuclear compounds
linked to a molecular core,4 or to the preparation of ligands
and complexes that are soluble and stable in water;5 on the
other hand, functionalization at the bridging CH2 has offered the
opportunity to design tailored heteroscorpionate systems such as
hybrid scorpionate/cyclopentadienyl complexes.8b
In light of this, we have undertaken new studies with the
alkylated bis(pyrazolyl)methane [H2C(3,5-Me2pz)2] as anchoring
ligand, and have synthesized discrete models and Fre´chet-type
dendritic wedges by functionalizing the bridging CH2 group.
We describe here the preparation and characterization of these
didentate ligands, and the synthesis of monometallic nickel(II)
complexes by metalation at the focal point of these wedges.
Results and discussion
Synthesis of ligands
The deprotonation of bis(3,5-dimethylpyrazol-1-yl)methane with
nBuLi was carried out as described by Otero et al. (Scheme 1).8 In-
termediate I was subsequently treated in situ with chlorotrimethyl-
silane or with the appropriate dendritic benzyl bromides Gn–Br, at
low temperature, to form ligands 1–5. Alternatively, intermediate
I can be isolated and stored in a dry-box, since it precipitates as
an off-white solid when its preparation is carried out in diethyl
ether and the mixture is allowed to warm to room temperature.
This method for the isolation of I improves the purity of the
crude compounds 1–5 and therefore raises the yields after the
appropriate work up. These ligands were isolated as pale-yellow
(1) or white (2–5), air-stable solids. Silane 1 and benzylic derivative
2 are soluble in all common organic solvents, whereas dendritic
benzyl ethers 3–5 are scarcely soluble in alkanes.
Research on metallodendrimers10 is now a prominent area of
nanoscience with multiple applications, including catalysis as a
main objective.11 We have used N-donor ligands bonded at the
focal point or at the periphery of carbosilane dendrimers to
coordinate early12 or late13,14 transition metals. In a previous paper,
Departamento de Qu´ımica Inorga´nica, Universidad de Alcala´, Campus Uni-
versitario, 28871, Alcala´ de Henares, Madrid, Spain. E-mail: ernesto.
dejesus@uah.es, juanc.flores@uah.es; Fax: +34 91 885 4683; Tel: +34 91
885 4607
† Electronic supplementary information (ESI) available: Fig. S1 and S2:
MS spectral information of dendrimers 5 and 10, respectively; Fig. S3 and
S4: Curie and polynomial plots, respectively, for protons in compound 7.
See DOI: 10.1039/b607090f
1
1
The H and 13C{ H} NMR spectra of compounds 1–5 show
only one set of resonances for both pyrazolyl rings. Thus, these
rings give rise to three singlets at about d = 2.0, 2.2, and 5.7 ppm, in
a 3 : 3 : 1 ratio, corresponding to the Me3, Me5, and H4 pyrazolyl
This journal is
The Royal Society of Chemistry 2006
Dalton Trans., 2006, 5379–5389 | 5379
©

View Article Online
in the normal range for free pyrazolyl rings, which appears together
with two absorptions at about 1600 and 1450 cm−1 for compounds
=
2–5 due to the C C bonds of the pyrazolyl and the aromatic rings,
and with intense bands assigned to the C–O–C vibrations in the
case of ligands 3–5.
All ligands gave satisfactory elemental analyses, and their
positive-ion electrospray mass spectra (ESI+/MS), carried out
in 0.1% formic acid solution in MeOH, exhibit a peak assignable
to the protonated molecular ion [M + H]+, with peak isotope
distributions that match the calculated patterns (see, for example,
Fig. S1 in the ESI†). In addition, fragments corresponding to
the loss of one or two pyrazolyl rings ([M − Me2pz]+ or [M −
2Me2pz]+) are also observed for compounds 1–4.
Synthesis of nickel complexes
The addition of a dichloromethane solution of chelates 1–5 to
one equivalent of [NiBr2(DME)] (DME = dimethoxyethane) in
the same solvent readily leads to the formation of compounds
6–10 by displacement of the labile DME ligand (Scheme 1 and
Fig. 1). In all the cases the initial orange suspension of the
nickel compound quickly turned deep blue. Complexes 6–10 were
isolated as purple paramagnetic solids that are stable to air in
the solid and in solution and are insoluble in alkanes but soluble
in polar, chlorinated, and aromatic solvents. Metalation at the
periphery of the dendrimers usually results in systems with general
properties that are determined by their polymetallic nature as the
generation increases, whereas they approximate to those of the
Scheme 1
protons, respectively (see Scheme 1 for the numbering scheme).
The resonance for the proton of the bridging CH group (RCHpz2)
occurs at around d = 6.0 ppm, and is observed as a singlet for 1
together with another singlet for the SiMe3 group (d = 0.2 ppm),
and as a triplet for 2–5 due to coupling with the benzylic CH2
protons of the R group, which appear as a doublet at around d =
1
3.8 ppm. The 13C{ H} spectra show singlets at about d = 11 (Me3),
14 (Me5), 106 (C4), 139 (C5), 147 (C3), and 72 ppm (bridging CH
group, d = 68 ppm for 1) for the bis(pyrazolyl)methane moieties
in 1–5, at d = −1.2 ppm for the SiMe3 group in 1, and at d =
40 ppm for the benzylic carbon attached directly to the chelate
ligand in 2–5. Two multiplets due to the phenyl group (d = 7.0–
7.2 ppm) are the remaining resonances observed in the 1H NMR
spectrum of 2. As described previously for this type of structure,15
the polyether protons of dendrimers 3–5 give sets of resonances
in three main regions, namely multiplets at d = 7.3–7.4 ppm for
the terminal phenyl groups (Ph), a doublet and a triplet at d =
6.2–6.7 ppm for each layer of internal aryl groups (Ar), and a
singlet at d = 4.8–5.0 ppm for each generation of –CH2O-groups.
Individual sets of resonances for each layer, which indicates slightly
different chemical environments amongst them, are also observed
1
in 13C{ H} NMR spectroscopy. Thus, their corresponding spectra
show one (3), two (4) or three (5) sets of singlets for the CH2O,
p-Ar, o-Ar, ipso-Ar, and m-Ar carbon atoms, at around d = 70,
101, 107, 139, and 160 ppm, respectively. The relative intensity of
these sets of singlets increases on going from the focal point to the
periphery of the dendrimer.
The IR spectra of ligands 1–5 show an absorption assigned to
−1
=
the asymmetric stretching of the C N bond at around 1555 cm ,
Fig. 1 Metallodendrimer [NiBr2{G3-CH(3,5-Me2pz)2}2] (10).
5380 | Dalton Trans., 2006, 5379–5389
This journal is
The Royal Society of Chemistry 2006
©

View Article Online
free ligand in monometallic dendrimers functionalized at their
focal point, like the nickel compounds described here.
The nickel complexes were characterized by 1H NMR and
IR spectroscopy, mass spectrometry, and elemental analysis. The
molecular structure of compound 7 was also determined by X-ray
diffraction studies. The crystal structure and NMR spectroscopic
data are discussed below.
with earlier findings using the bis(pyrazolyl)nickel(II) compounds
with independent pyrazolyl rings described by Mapolie et al.,18
or related complexes containing planar-conjugated chelates,13
and is similar to the behavior reported by Jordan et al. for
bis(pyrazolyl)methane-palladium(II) compounds.19
Structure of [NiBr2{G0-CH(3,5-Me2pz)2}] (7)
The IR spectra of compounds 6–10 are fairly similar to those
Fig. 2 shows an ORTEP representation of the molecular structure
of compound 7 in the solid state as determined by single-crystal
X-ray diffraction studies; the relevant structural data are given in
Table 1.
=
of the free ligands, with the masym(C N) absorption at around
1560 cm−1 being the most significant. Intense bands assigned to
=
the C C and C–O–C vibrations are also clearly visible.
The nickel compounds gave satisfactory elemental analyses, and
several fragmentation patterns could be detected when analyzing
their ESI+/MS data obtained in acetonitrile. The molecular ion
[M]+ was only observed, as a low intensity peak, in the spectrum
of complex 6. Peaks corresponding to the loss of one, or even
two, bromine atoms ([M − Br]+ or [M − 2Br]+; low intensity), to
dissociation and protonation of the free ligands ([LH]+, L = 1–5),
and even to the fragmentation of the latter by the loss of a pyrazolyl
ring ([L − Me2pz]+) were all commonly detected for complexes 6–
10. Interestingly, a peak corresponding to loss of the bromo ligands
and coordination of a third pyrazolyl ring and H2O is clearly
observed in all the spectra (m/z = 447, 465, 677, 1101, and 1952,
for 6–10, respectively). These observations are compatible with
the formation of hydrated tris(pyrazolyl)methane nickel cations
of general formula [Ni(H3O+){RC(Me2pz)3}] under these exper-
imental conditions. Additionally, a fragment corresponding to a
dicationic-bischelate [Ni{RCH(Me2pz)2}2]2+ species was detected
for compounds 9 and 10 (m/z = 960 and 1809, respectively).
The MALDI-TOF mass spectrum for 10, recorded in dithranol
(1,8,9-anthracenethiol, C14H10O3) as the matrix, shows peaks for
the cation after the loss of one or two bromine atoms (m/z =
1918 and 1829, respectively; Fig. S2 in the ESI†), and peaks
due to dimetallic species containing halide bridges (i.e., [M2 −
Br]+ and [(M − Br)2]+), with isotope distributions that match the
calculated patterns. A fragmentation pattern with halide loss has
been shown to occur for similar nickel compounds,16 and a similar
recombination leading to dimetallic structures is also known.13,17
Activation of 6–10 by MAO for ethylene polymerization was
briefly evaluated. None of them show activity (room temp.,
2 bar, toluene, 24 h, Al/Ni = 1000). This result contrasts
Fig. 2 ORTEP diagram of the structure of compound 7 with thermal
ellipsoids at 50% probability.
The molecular structure of 7 consists of a discrete molecule
where the central nickel atom binds to two bromine atoms and
two nitrogen atoms of a bis(pyrazolyl) chelate ligand, and exhibits
a distorted tetrahedral geometry. Thus, the bond angles around
the metal centre vary from 118.17(5)◦ for the bromine ligands
to 93.9(2)◦ for the donor atoms of the pyrazolyl ligands, while
˚
˚
the average Ni–Br (2.3648(14) A) and Ni–N (1.996(6) A) bond
◦
◦
˚
Table 1 Bond lengths [A], angles [ ], and torsion angles [ ] for compound 7
Ni(1)–Br(1)
Ni(1)–N(1)
N(1)–N(2)
N(2)–C(11)
2.3549(14)
1.996(6)
1.385(7)
1.453(10)
1.541(10)
118.17(5)
112.81(17)
108.85(19)
122.5(5)
119.9(6)
111.0(6)
110.6(6)
130.0(5)
106.2(6)
102.5(5)
−124.3(5)
−11.2(5)
59.6(8)
Ni(1)–Br(3)
Ni(1)–N(3)
N(3)–N(4)
N(4)–C(11)
2.3747(14)
1.996(6)
1.374(7)
1.446(9)
1.512(10)
93.9(2)
110.12(19)
110.37(17)
120.7(5)
120.4(6)
112.3(6)
112.4(6)
129.2(5)
106.0(6)
−109.1(5)
118.6(5)
6.8(5)
C(11)–C(12)
C(12)–C(13)
Br(1)–Ni(1)–Br(3)
Br(1)–Ni(1)–N(1)
Br(3)–Ni(1)–N(1)
Ni(1)–N(1)–N(2)
N(1)–N(2)–C(11)
N(2)–C(11)–N(4)
N(2)–C(11)–C(12)
Ni(1)–N(1)–C(1)
N(2)–N(1)–C(1)
Br(1)–Ni(1)–N(1)–N(2)
Br(3)–Ni(1)–N(1)–N(2)
N(3)–Ni(1)–N(1)–N(2)
N(1)–N(2)–C(11)–N(4)
N(1)–Ni(1)–N(3)
Br(1)–Ni(1)–N(3)
Br(3)–Ni(1)–N(3)
Ni(1)–N(3)–N(4)
N(3)–N(4)–C(11)
C(11)–C(12)–C(13)
N(4)–C(11)–C(12)
Ni(1)–N(3)–C(6)
N(4)–N(3)–C(6)
Br(1)–Ni(1)–N(3)–N(4)
Br(3)–Ni(1)–N(3)–N(4)
N(1)–Ni(1)–N(3)–N(4)
N(3)–N(4)–C(11)–N(2)
−65.8(8)
This journal is
The Royal Society of Chemistry 2006
Dalton Trans., 2006, 5379–5389 | 5381
©

View Article Online
distances fall in the usual range.18 The value of these angles are
about 120◦ and 99◦, respectively, in the analogous compound with
two independent monodentate pyrazolyl rings. The distortions in
compound 7 are the result of both the steric repulsion between
the large halide atoms and the presence of a chelate ligand with
an acute N–Ni–N bite angle. The dihedral angle defined by
the N(1)–Ni(1)–N(3) and Br(1)–Ni(1)–Br(3) planes is 88.4(1)◦,
almost a right angle. It is worth mentioning at this point that
a related nickel(II) compound containing the CH2(3,5-Me2pz)2
ligand has been described and structurally characterized. In the
solid state the structure of this complex consists of a dimeric
molecule formed by bridging halides, each unit of which contains
a five-coordinate metal centre in a geometry intermediate between
trigonal bipyramidal and square pyramidal.20
The Ni(1)(NN)2C(11) nickellacycle formed by coordination of
the didentate ligand presents a boat conformation in which the
benzyl group attached to the methine carbon C(11) occupies
an axial position, thus avoiding the steric hindrance that would
arise between the benzyl group in the equatorial location and
the adjacent methyl groups (Me5). A similar result has been
reported for some palladium(II) complexes containing RCH(3,5-
Me2pz)2 ligands (R = phenyl or pyridyl), where the steric
repulsion between the R and pz-methyl groups confers rigidity
on the metallacycle as no boat-to-boat conformation exchange
is observed in solution.21 It is noteworthy that no matter what
the Group 10 metal environment is—tetrahedral (Ni) or square-
planar (Pd)—the bridging substituent is always positioned axially
in the solid state. The boat conformation is less pronounced on
the side of the metal centre than on that of the methine bridge, as
evidenced by the angles between the N(1)–N(2)–N(3)–N(4) plane
and the planes N(1)–Ni(1)–N(3) and N(2)–C(11)–N(4) (7.8(3)◦
and 51.5(4)◦, respectively).
1H NMR studies of paramagnetic complexes 6–10
High-resolution NMR spectra can be helpful for the charac-
terization of paramagnetic complexes if the relaxation of the
unpaired electron is fast enough with respect to the NMR
timescale. In this case, the electronic magnetic field shortens the
nuclear longitudinal relaxation times (T1) of the observed nuclei
less effectively, thereby sharpening the broad resonances usually
associated with paramagnetic species. First-row transition metal
complexes with triply degenerate (T) ground states often have short
electron-spin lifetimes that allow well-resolved NMR signals from
which structural information can be retrieved.22 Thus, nickel(II)
complexes with tetrahedral or pseudo-tetrahedral, and high-spin
pentacoordinate geometries typically produce sharper resonances
than monomeric octahedral complexes, although the resonances
of the latter are also sharpened because of zero-field splitting.23
The 1H NMR spectra of compounds 6–10 show fairly well
resolved and sharp signals (Dm1/2 < 120 Hz) with hyperfine shifted
resonances spread over a relatively narrow spectral window (about
80 ppm; Table 2). The presence of an effective plane of symmetry
in the molecule, as defined by the metal centre and both bromine
atoms, is confirmed by the chemical equivalence of both pyrazolyl
groups. All the protons of [{Me3SiCH(pz)2}NiBr2] (6) have shorter
T1 values (from 4 to 125 ms) and are strongly down- or upfield
shifted from d = −18 to 59 ppm with respect to their position
in the diamagnetic free ligand 1. For complexes 7–10, comparably
short T1 values and large hyperfine shifts are found for the protons
located at the [CH2CH(pz)2NiBr2] moiety and at the ortho position
of the aromatic ring adjacent to the methine bridge. The remaining
protons of the dendritic substituents appear in ranges close to
those found in the diamagnetic free ligands (d = 7.8–7.2 vs. 7.4–
7.2 (Ph), 6.7–6.2 vs. 7.0–6.5 (Ar), and 5.6–5.1 vs. 5.0–4.8 ppm
(CH2)), with line widths (<8 Hz) and longitudinal relaxation rates
progressively approaching those of diamagnetic compounds as the
distance from the metal increases. For instance, the three CH2O
methylene protons of the G3 complex 10 show Dm1/2 values of 7.7,
4.9, and 3.9 Hz, and T1 times of 84, 211, and 380 ms, going from
the dendritic focal point to the periphery.
The benzyl group is orientated asymmetrically towards the
hemi-space that contains the N(3)–N(4) pyrazolyl ring, with
a dihedral angle defined by the C(12)–C(13) bond and the
Ni(1) · · · C(11) molecular axis of 114.6◦. The two coordinated
rings have different inclinations with respect to the coordination
planes of the metal, probably as a result of the steric interaction of
the phenyl group with one of the coordinated pyrazolyl rings.
Thus, the dihedral angle between the N(1)–Ni(1)–N(3) plane
and the pyrazolyl rings N(3)–N(4) and N(1)–N(2) are 25.0(2)◦
and 19.9(2)◦, respectively. The asymmetric orientation of the
benzyl group is accompanied by the presence of a short contact
between one of the benzylic protons and the closest bromine atom
The full assignment of signals has been based on their position in
the different spectra, on their relative areas, and on measurements
of T1 values. Thus, for complex 7 (Table 3), the single resonance at
d = −8.2 ppm can be unequivocally assigned to the bridging CH
group due to its integrated intensity, whereas the two resonances
integrating for six protons are assigned to each of the methyl
pyrazolyl groups (Me3 and Me5) due to their measured T1 values.
˚
[Br(1) · · · H12B = 2.94 A].
Table 2 d and T1 values for the nearest protons to the metal atom in 6–10 in CDCl3 at 293 Ka
Assignment
6
7
8
9
10
d (ppm)
T1/ms
d (ppm)
T1/ms
d (ppm)
T1/ms
d (ppm)
T1/ms
d (ppm)
T1/ms
pz-H4
pz-Me3
pz-Me5
CH
CH2
SiMe3
64.7
38.5
15.6
−12.0
—
36
4
125
26
—
22
69.3
21.4
4.1
−8.2
−9.1
—
33
5.2
110
23
4.6
—
69.6
21.4
4.0
−8.2
−9.3
—
26
5
96
69.7
21.4
4.0
−8.5
−9.5
—
37
5
93
17
5
69.9
21.2
4.0
−8.4
−9.4
—
37
5
92
26
23
b
b
−4.4
—
—
—
a T1 values were obtained at 300 MHz. b Fitting was unreliable.
5382 | Dalton Trans., 2006, 5379–5389
This journal is
The Royal Society of Chemistry 2006
©

View Article Online
Table 3 1H NMR parameters in CDCl3 at 293 K for 7
Relative
area
c
d
Dm1/2a/Hz
T1b/ms
Ni · · · H (exp) /A
Ni · · · H (calc) /A
˚
˚
Assign.
d (ppm)
pz-H4
2 H
6 H
6 H
1 H
2 H
2 H
69.3
21.4
4.1
−8.2
−9.1
5.0
18
84
4
18
67
33e
5.2
110
23
5.02
3.66
5.98
4.20
3.51
5.08
7.00
7.80
5.02e
3.7
6.1
4.7
3.6
4.7
—
pz-Me3
pz-Me5
CH
CH2
4.6
Ph, ortho
Ph, meta
Ph, para
19
21
f
f
3 H
7.2
f
f
—
a Line width at half-maximum. b T1 values were obtained at 300 MHz. c Crystallographic distances determined by X-ray diffraction. In the case of
equivalent protons, the arithmetic average distance is given. d Average distances in solution obtained on the basis of a model taking into account a
predominant dipolar relaxation mechanism. The Ni · · · H distances (dNi · · · H) were calculated from the measured proton relaxation times (T1) with respect
1/6
to that of a reference resonance (T1 ref), according to the equation: dNi · · · H = dNi · · · H ref (T1/T1 ref
reference. e Taken as reference. f m-Ph and p-Ph resonances overlap.
)
(see ref. 24). The pyrazolyl proton pz-H4 was used as
A COSY experiment was performed in order to assign the three
resonances corresponding to the pz-H4, benzylic methylene, and o-
Ph protons. A clear cross-peak is observed between the resonances
at d = 7.2 (m-Ph) and 5.0 ppm and thus the latter can be assigned
to the o-Ph protons. The pz-H4 and the CH2 protons can clearly be
assigned to the resonances at d = 69.3 and −9.1 ppm, respectively,
shows the Ni · · · H distances obtained for complex 7 in the solid
state by X-ray diffraction and those estimated in solution on
the basis of the measured T1 values. The pz-H4 proton was
chosen as reference because the distance to the nickel atom
is essentially conformationally insensitive. The good qualitative
agreement between the experimental and calculated distance
values, with differences not exceeding 12%, supports the resonance
assignements and the assumptions made for these calculations. As
discussed previously, the boat conformation found in the solid
state for 7 (Fig. 2) arranges the benzyl group in an apical position
with the phenyl ring facing one of the pz-Me5 groups. Since the
experimental distances are derived from crystal data, it can be
concluded that a similar conformation to that found in the solid
state predominates in solution. Conformational calculations at
the MM2 level gave a minimum energy for a disposition that
closely resembles that found in the solid state. Thus, the dihedral
angle, h, defined by Ni · · · CH–CH2–Ph is equal to 121.0◦ in the
optimized MM2 structure versus 118.9◦ in the crystal structure.
The maximal energy barriers for rotation of the phenyl group
around the CHCH2 axe are estimated to be 5–6 kcal mol−1.
As pointed out above, the conformational characteristics of the
corresponding metallacycle in 8–10 should be very similar to those
described here for 7. The latter is also supported by the similarities
of the T1 values found in solution for the protons nearest to the
nickel atom in the full series of complexes 7–10 (see Table 2).
Examination of the 1H NMR spectra of 7 over the temperature
range 224–323 K reveals that the resonances move toward the
diamagnetic region upon increasing the temperature, showing a
typical Curie law behavior with linear fits of reasonably good
quality and correlation coefficients greater than 0.99 (Fig. 3 and
Table 4, full plot and Curie fits provided as ESI,† Fig. S3). The only
exception is for the pz-Me5 methyl protons and will be discussed
later. However, deviations to the Curie law are suggested by the fact
that intercepts at 0 K−1 for the resonances corresponding to the
CH2CH(3,5-Me2pz)2 moiety differ significantly from the expected
diamagnetic shifts (ddia in the free ligand, Table 4). The fit quality
improved when using a second-order polynomial procedure with
fixed intercepts at the corresponding ddia in the free ligand (Table 4;
full plot in Fig. S4 in the ESI†). The chemical shift assigned to
the pz-Me5 protons of complex 7 shows a temperature dependence
that does not fit into the range of temperatures studied in any of the
1
by comparison with the H NMR spectrum of 6, where a SiMe3
group has replaced the benzyl group. Moreover, the large hyperfine
shift to lower field of the former resonance is attributable to
a contact contribution transmitted through the bonds from the
unpaired electron density to the given nucleus, and therefore is
more likely assignable to the pz-H4 than to the methylene protons.
1H NMR parameters in paramagnetic complexes are particu-
larly sensitive to the conformation adopted by the molecule in
solution. At this point, it should be noted that data collected for
the pyrazolyl, methine, and methylene protons of the G0 complex
7 virtually match those found for analogous nuclei in G1 to G3
dendrimers 8–10, with maximal differences of 0.5 ppm in the case
of the chemical shifts (see Table 2). Larger differences are found
between the chemical shifts of 6 and 7–10 and it is remarkable
that the resonance for the distal methyl groups (pz-Me5 protons) is
significantly shifted in the case of compound 6 (d = 15.6 ppm), and
only slightly for 7–10 (d = 4.0–4.1 ppm, see below). As mentioned
for pz-H4, the hyperfine shifts for the protons closest to the
Ni(II) centre must be due to a predominant contact contribution,
whereas those of the methine and methylene (or SiMe3 for 6)
protons must be a consequence of a larger dipolar input to the
hyperfine shift. The differences in chemical shift for the protons
of compound 6 when compared with their counterparts in the
series 7–10 can be attributed to small conformational differences
imposed by the steric bulk of the SiMe3 group, which modifies the
pseudocontact interaction of these protons with the metal centre,
which, in turn, induces different dipolar shift contributions.
Assuming that the relaxation takes place predominantly
through space by a dipolar mechanism, the longitudinal re-
1
laxation rates (T1−1) of a given H nucleus in a paramagnetic
metal complex should be inversely proportional to the metal–
proton distance to the sixth (dM · · · H6), according to the Solomon
equation.24 Therefore, relaxation rates and distance values can be
−1
−1
calculated from each other using the equation T1 calc = T1 ref
−6
(dM · · · H/dM · · · H ref
)
and a given resonance as reference. Table 3
This journal is
The Royal Society of Chemistry 2006
Dalton Trans., 2006, 5379–5389 | 5383
©

View Article Online
7–10 situates the benzyl ring above the pz-Me5 protons. A feasible
explanation for these observations is that the phenyl ring shields
the electron magnetic field at the pz-Me5 protons in 7–10, thus
explaining their upfield shifts. At high temperatures the ratio of
molecules in the lowest-energy conformations would decrease and
the phenyl shielding becomes less efficient.
The accessibility of the metal centres is an important question
in metallodendrimers, particularly in catalytic reactions, and
depends on the conformation adopted by the dendritic arms in
solution. Following the calculation of distances discussed above,
we wanted to know if the paramagnetic nickel centre might be
used as a probe to get insights into the disposition of the dendritic
arms. Unfortunately, the longer distances to the metal atom and
the conformational flexibility of the dendritic part of the molecule
complicate the data interpretation. Several conformations of
similar energy with very different Ni · · · H distances are possible,
and the diamagnetic contributions to the relaxation rates are no
longer negligible. For the third-generation dendrimer 10, we first
evaluated the paramagnetic relaxation rate (T1 para−1) subtracting
the diamagnetic part (T1 dia−1) from the measured overall rate
(T1−1). The diamagnetic rates were taken as those measured in
the free ligand 5, and Ni · · · H distances were calculated from
−1
T1 para according to the Solomon equation (Table 5). Secondly,
a search of optimized structures corresponding to local minima
was carried out by MM2 calculations, and the average distances
ꢀ
derived from them [calculated as (1/d6)average
=
(1/d6) because
of the dependence of rates on 1/d6] were compared with the
values estimated by NMR spectroscopy. Fig. 4 shows one of the
optimized structures whose average Ni · · · H distances (Table 5)
correlate acceptably (R2 = 0.89) with those calculated from T1
values, and therefore Fig. 4 gives a good picture of the accessibility
of the metal centre in dendrimer 10.
Fig. 3 a) Observed chemical shift dependence versus reciprocal tempera-
ture for the most affected protons in complex 7. The lines represent linear
square-fit data (Curie law). b) Temperature dependence of the chemical
shift of the pz-Me5 protons, where the Curie least-square fit is shown by
the dotted line.
Fig. 4 Space-filling model for G3 dendrimer 10.
preceding functions. A look at an expanded plot (Fig. 3b) reveals
that this resonance shifts upfield to the diamagnetic region at low
temperatures (Curie behavior), but the slope inverts at 266 K and
the resonance shifts downfield at higher temperatures (anti-Curie
behavior).25 On the other hand, we have pointed out above that
the hyperfine shift of pz-Me5 protons in complexes 7–10 is small in
comparison with that observed for the trimethylsilyl derivative 6.
Finally, we have postulated that the lowest-energy conformation in
Conclusions
Bis(3,5-dimethylpyrazol-1-yl)methane can be functionalized at the
methylene bridge with dendritic Fre´chet-type wedges of up to the
third generation. These didentate ligands readily form nickel(II)
paramagnetic complexes [NiBr2{RCH(3,5-Me2pz)2}] upon reac-
tion with [NiBr2(DME)]. The nickel centre is tetrahedral and the
5384 | Dalton Trans., 2006, 5379–5389
This journal is
The Royal Society of Chemistry 2006
©

View Article Online
Table 4 Temperature dependence of chemical shifts for complex 7 in CDCl3
Curie plotb
Polynomial plotc
Assignment
da (ppm)
dint (ppm)
a × 10−3/K−1
ddia (ppm)
aꢀ × 10−3/K−1
bꢀ × 10−3/K−2
pz-H4
69.3
21.4
4.1
−8.2
−9.1
5.0
−18.5
26.0
5.71
2.03
2.20
6.20
3.87
7.17
6.95
7.17
12.9
2.8
1.75
0.88
pz-Me3
pz-Me5
CH
−10.0
9.3
d
d
e
e
11.3
15.2
7.6
6.9
7.1
−5.8
−7.2
−0.77
0.08
−3.0
−1.1
−0.55
0.07
−0.37
−0.82
−0.03
0.00
CH2
Ph, ortho
Ph, meta
Ph, para
7.20
7.16
0.02
−0.02
0.00
a Observed chemical shifts at 293 K. b Fit of change in chemical shift with temperature, according to the Curie Law (dobs = dint + a/T), where dint is the
intercept at infinite temperature and a is the Curie slope. The correlation coefficient is greater than 0.99 in each case. c Fit of change in chemical shift with
temperature, according to a polynomial function (dobs = ddia + aꢀ/T + bꢀ/T2), where ddia is the observed diamagnetic shift in the free ligand. d Fitting was
unreliable. e Erroneous fitting with intercept at ddia = 2.20.
Table 5 Ni · · · H distances calculated from T1 data for 10
c
d
T1a/ms
T1 parab/ms
Ni · · · H (calc) /A
Ni · · · H (model) /A
˚
˚
Assignment
pz-H4
CH
CH2
37
23
5
23
345
84
408
570
211
575
945
380
854
37
24
5
5.1e
4.8
3.7
4.8
8.0
6.2
8.3
9.3
8.0
9.2
10.1
8.8
9.1
5.1
4.3
3.4
4.6
8.2
7.7
7.8
11.4
8.2
8.7
11.1
8.5
9.0
G0-o-Ar
G0-p-Ar
G1-CH2
G1-o-Ar
G1-p-Ar
G2–CH2
G2-o-Ar
G2-p-Ar
G3–CH2
o-Ph
24
542
122
689
1344
556
1281
2112
982
1194
a T1 values obtained in CDCl3 at 293 K and 300 MHz. b Estimated as T1para = T1 − T1 dia−1, where the T1 dia values were taken to be those of the
−1
−1
free ligand 5. c Average Ni · · · H distances (dNi · · · H) calculated from the measured proton paramagnetic relaxation rates (T1 para) with respect to that of a
1/6
reference resonance (T1 ref), according to the equation: dNi · · · H = dNi · · · H ref (T1 para/T1 ref
e Taken as reference.
)
(see ref. 24). The pyrazolyl proton pz-H4 was used as reference,
ꢀ
d Distances in the model for 10 represented in Fig. 4. In the case of equivalent protons, the average distance calculated as (1/d6)average
=
(1/d6) is given.
Ni–(NN)2–C metallacycle adopts a boat structure in the solid
state, which remains in solution, with the R groups in an axial
position. The paramagnetic nature of these nickel complexes does
H2C(3,5-Me2pz)2,3b,c [NiBr2(DME)],26 and dendritic benzyl bro-
mides Gn–Br,15 were prepared according to literature procedures.
Solvents were previously dried and distilled under argon as
described elsewhere.27 NMR spectra were recorded on Varian
Unity 500+, VR-300, or 200 NMR spectrometers. Chemical shifts
(d) are reported in ppm relative to SiMe4, and were referenced
1
not preclude their characterization by H NMR spectroscopy as
they yield spectra with fairly well-resolved and sharp signals with
chemical shifts spread over a relatively narrow spectral window.
In fact, the presence of the paramagnetic centre turns out to be a
useful probe for gathering extra structural information that is not
available in diamagnetic compounds. Thus, the conformational
aspects of the nickel surroundings, as well as the average distances
of the distal protons to the nickel atom, can be deduced in solution
from longitudinal relaxation time determinations. Further work
with this type of dendronized bis(pyrazolyl)methane ligand is
currently underway.
1
with respect to 13C and residual H resonances of the deuterated
solvents. Coupling constants (J) are given in Hz. The following
abbreviations/notations are used: Ph refers to aromatic ring of
terminal benzyl groups, Ar to internal rings of benzyl ethers,
and ipso refers to the first ring-position on going from the
didentate pyrazolyl ligand. Longitudinal relaxation times (T1)
were measured using the inversion–recovery pulse sequence (180◦–
s–90◦) method. IR spectra were recorded with a Perkin-Elmer FT-
IR Spectrum-2000 spectrophotometer. Elemental analyses were
performed by the Microanalytical Laboratories of the University
of Alcala´ on a Heraeus CHN-O-Rapid microanalyzer. ESI and
MALDI-TOF mass spectra were recorded by the Research
Services at the Universidad Auto´noma de Madrid (SIDI) in
Applied Biosystems spectrometers, API Qstar Pulsar I or 4700
Proteomics Analyzer, respectively, using MeOH + 0.1% HCOOH
for ligands and MeCN for nickel complexes as the ionizing phase
Experimental
General remarks
All operations were performed under argon using Schlenk or dry-
box techniques. Unless otherwise stated, reagents were obtained
from commercial sources and used as received. The compounds
This journal is
The Royal Society of Chemistry 2006
Dalton Trans., 2006, 5379–5389 | 5385
©

View Article Online
(ESI), or with dithranol as matrix (MALDI-TOF). Molecular
modeling was carried out using Chem 3D Pro version 5.0 for Mac
(CambridgeSoft, Cambridge, USA, 1999) and MM2 force field
computations.
CH), 6.18 (d, 4JH,H = 2.2 Hz, 2 H, o-Ar), 6.44 (t, 4JH,H = 2.2 Hz,
1
1 H, p-Ar), 7.3–7.4 ppm (m, 10H, Ph). 13C{ H} NMR (CDCl3):
d 11.0 (pz-Me5), 13.7 (pz-Me3), 40.2 (CH2), 69.8 (PhCH2O), 71.6
(CH), 101.2 (p-Ar), 106.3 (pz-C4), 108.1 (o-Ar), 127.4 and 128.5
(o- and m-Ph), 127.9 (p-Ph), 136.8 (ipso-Ph), 138.7 (ipso-Ar), 139.6
5
3
=
(pz-C ), 147.9 (pz-C ), 159.7 (m-Ar). IR (KBr): m 1558 (s, C N),
Syntheses
−1
=
1595 and 1453 (s, C C), 1261 and 1026 cm (vs, C–O–C). MS
(ESI+ in MeOH/0.1% HCOOH): m/z 507 [M + H]+, 411 [M −
Me2pz]+, 315 [M − 2Me2pz-H]+.
Preparation
of
(trimethylsilyl)bis(3,5-dimethylpyrazolyl)-
methane [Me3SiCH(3,5-Me2pz)2] (1). nBuLi (3.1 mL, 1.6 M in
hexanes, 4.96 mmol) was slowly added, from a funnel equipped
with a bubbler, to a solution of H2C(3,5-Me2pz)2 (1.00 g,
4.9 mmol) in THF (40 mL) at −78 ◦C and the mixture was stirred
for 2 h at that temperature. An excess of SiMe3Cl was then added
(0.76 mL, 6.0 mmol) and the reaction mixture was allowed to
warm to room temp. and stirred overnight. After removal of the
solvent under vacuum the residue was extracted into pentane
(2 × 15 mL), and compound 1 was isolated as a pale-yellow solid
by evaporation of the filtrates to dryness under vacuum. Yield:
1.098 g (81%). Anal. Calc. for C14H24N4Si (276.46): C, 60.82; H,
8.75; N, 20.27%. Found: C, 60.72; H, 8.39; N, 20.05%. 1H NMR
(CDCl3): d 0.20 (s, 9 H, SiMe3), 1.94 (s, 6 H, pz-Me3), 2.16 (s, 6
Preparation of G2-CH(3,5-Me2pz)2 (4). This compound was
prepared according to the procedure described above for 2, from
LiCH(3,5-Me2pz)2 (0.87 mmol), prepared in situ, and dendritic
wedge G2–Br (0.7 g, 0.87 mmol) in THF. Ligand 4 was purified
by addition of pentane to a stirred diethyl ether solution, which
caused it to precipitate as an off-white solid. Yield: 0.632 g (78%).
Anal. Calc. for C60H58N4O6 (931.14): C, 77.40; H, 6.28; N, 6.02%.
Found: C, 76,98; H, 6.37; N, 5.91%. 1H NMR (CDCl3): d 2.01 (s,
6 H, pz-Me3), 2.20 (s, 6 H, pz-Me5), 3.78 (d, JH,H = 7.3 Hz, 2 H,
CH2), 4.80 (s, 4 H, ArCH2O), 5.01 (s, 8 H, PhCH2O), 5.71 (s, 2 H,
pz-H4), 6.13 (t, JH,H = 7.3 Hz, 1 H, CH), 6.17 (d, 4JH,H = 2.0 Hz, 2
1
H, pz-Me5), 5.74 (s, 2 H, pz-H4), 5.87 ppm (s, 1 H, CH). 13C{ H}
H, G0-o-Ar), 6.44 (t, 4JH,H = 2.0 Hz, 1 H, G0-p-Ar), 6.54 (t, 4JH,H
=
NMR (CDCl3): d −1.2 (SiMe3), 10.8 (pz-Me5), 13.5 (pz-Me3),
4
2.1 Hz, 2 H, G1-p-Ar), 6.61 (d, JH,H = 2.1 Hz, 4 H, G1-o-Ar),
68.5 (CH), 106.1 (pz-C4), 139.7 (pz-C5), 146.6 ppm (pz-C3). IR
1
7.3–7.4 ppm (m, 20H, Ph). 13C{ H} NMR (CDCl3): d 11.0 (pz-
−1
=
=
Me5), 13.8 (pz-Me3), 40.2 (CH2), 69.7 (ArCH2O), 70.1 (PhCH2O),
71.7 (CH), 101.3 (G0-p-Ar), 101.4 (G1-p-Ar), 106.2 (G1-o-Ar),
106.3 (pz-C4), 108.1 (G0-o-Ar), 127.5 and 128.6 (o- and m-Ph),
128.0 (p-Ph), 136.7 (ipso-Ph), 138.8 (G1-ipso-Ar), 139.3 (pz-C5),
139.6 (G0-ipso-Ar), 147.9 (pz-C3), 159.6 (G0-m-Ar), 160.1 ppm
(KBr): m 1617 (m, C C), 1550 cm (s, C N). MS (ESI+ in
MeOH/0.1% HCOOH): m/z 277 [M + H]+, 181 [M − Me2pz]+,
85 [M − 2Me2pz-H]+.
Preparation of (benzyl)bis(3,5-dimethylpyrazolyl)methane [G0-
CH(3,5-Me2pz)2] (2). nBuLi (3.1 mL, 1.6 M in hexanes,
4.96 mmol) was added dropwise to a solution of H2C(3,5-Me2pz)2
(1.00 g, 4.9 mmol) in THF (15 mL) at −78 ◦C. The reaction mixture
was stirred for 2 h at that temperature, then PhCH2Br was added
(0.6 mL, 5.0 mmol) and the solution allowed to warm to room
temp. The solvent was removed under vacuum, the crude solid
dissolved in diethyl ether (30 mL) and washed with water (2 ×
20 mL), and the organic layer dried with MgSO4. Removal of the
volatiles followed by recrystallization from pentane led to ligand
2 as a white solid. Yield: 1.138 g (79%). Anal. Calc. for C18H22N4
(294.40): C, 73.44; H, 7.53; N, 19.03%. Found: C, 73.21; H, 7.47;
N, 18.95%. 1H NMR (CDCl3): d 2.03 (s, 6 H, pz-Me3), 2.20 (s, 6 H,
pz-Me5), 3.87 (d, JH,H = 7.3 Hz, 2 H, CH2), 5.71 (s, 2 H, pz–H4),
6.20 (t, JH,H = 7.3 Hz, 1 H, CH), 6.95 (m, 2 H, m-Ph), 7.17 ppm (m,
=
=
(G1-m-Ar). IR (KBr): m 1559 (s, C N), 1595 and 1451 (vs, C C),
1293 and 1262 (s, C–O–Cas), 1156 and 1040 cm−1 (vs, C–O–Cs).
MS (ESI+ in MeOH/0.1% HCOOH): m/z 931 [M + H]+, 835
[M − Me2pz]+, 739 [M − 2Me2pz-H]+.
Preparation of G3-CH(3,5-Me2pz)2 (5). Dendritic wedge 5 was
prepared according to the procedure described above for 2, starting
from LiCH(3,5-Me2pz)2 (0.6 mmol), prepared in situ, and G3–
Br (1.0 g, 0.6 mmol) in THF. Purification was carried out by
addition of pentane to a stirred toluene solution, which caused
precipitation as an off-white solid. Yield: 0.775 g (72%). Anal.
Calc. for C116H106N4O14 (1780.14): C, 78.27; H, 6.00; N, 3.15%.
Found: C, 78.14; H, 6.09; N, 2.90%. 1H NMR (CDCl3): d 2.00 (s, 6
H, pz-Me3), 2.20 (s, 6 H, pz-Me5), 3.79 (d, JH,H = 7.3 Hz, 2 H, CH2),
4.81 (s, 4 H, G1-ArCH2O), 4.94 (s, 8 H, G2-ArCH2O), 5.00 (s, 16
H, PhCH2O), 5.71 (s, 2 H, pz-H4), 6.13 (t, JH,H = 7.3 Hz, 1 H, CH),
6.17 (d, 4JH,H = 2.2 Hz, 2 H, G0-o-Ar), 6.44 (t, 4JH,H = 2.2 Hz, 1 H,
1
3 H, o- and p-Ph). 13C{ H} NMR (CDCl3): d 11.2 (pz-Me5), 14.0
(pz-Me3), 40.1 (CH2), 72.2 (CH), 106.2 (pz-C4), 126.6 (p-Ph), 128.1
and 129.1 (o- and m-Ph), 136.3 (ipso-Ph), 139.2 (pz-C5), 147.6 (pz-
3
−1
=
=
C ). IR (KBr): m 1557 (s, C N), 1604 (m, C C), 1453 cm (s,
G0-p-Ar), 6.51 (t, 4JH,H = 2.2 Hz, 2 H, G1-p-Ar), 6.54 (t, 4JH,H
=
+
=
C C). MS (ESI+ in MeOH/0.1% HCOOH): m/z 295 [M + H] ,
4
2.2 Hz, 4 H, G2-p-Ar), 6.59 (d, JH,H = 2.2 Hz, 4 H, G1-o-Ar),
199 [M − Me2pz]+.
4
6.66 (d, JH,H = 2.2 Hz, 8 H, G2-o-Ar), 7.3–7.4 ppm (m, 40H,
1
Preparation of G1-CH(3,5-Me2pz)2 (3). This compound was
prepared according to the procedure described above for 2, from
LiCH(3,5-Me2pz)2 (1.3 mmol), prepared in situ, and dendritic
wedge G1–Br (0.50 g, 1.3 mmol) in THF (20 mL). The resulting
yellow oil was stirred with pentane (15 mL) for several hours to
give compound 3 as a white solid. Yield: 0.50 g (76%). Anal. Calc.
for C32H34N4O2 (506.65): C, 75.86; H, 6.76; N, 11.06%. Found: C,
75.69; H, 6.80; N, 10.88%. 1H NMR (CDCl3): d 2.02 (s, 6 H, pz-
Me3), 2.21 (s, 6 H, pz-Me5), 3.79 (d, JH,H = 7.3 Hz, 2 H, CH2), 4.87
(s, 4 H, PhCH2O), 5.73 (s, 2 H, pz–H4), 6.12 (t, JH,H = 7.3 Hz, 1 H,
Ph). 13C{ H} NMR (CDCl3): d 11.0 (pz-Me5), 13.7 (pz-Me3), 40.2
(CH2), 69.8 (G1-ArCH2O), 70.0 (G2-ArCH2O), 70.1 (PhCH2O),
71.7 (CH), 101.2 (G0-p-Ar), 101.4 (G1-p-Ar), 101.6, (G2-p-Ar),
106.2 (overlapping G1-o-Ar, G2-o-Ar and pz-C4), 108.2 (G0-o-
Ar), 127.6 and 128.6 (o- and m-Ph), 128.0 (p-Ph), 136.8 (ipso-
Ph), 138.9 (G1-ipso-Ar), 139.1 (G2-ipso-Ar), 139.3 (pz-C5), 139.6
(G0-ipso-Ar), 147.9 (pz–C3), 159.7 (G0-m-Ar), 160.1 (G1-m-Ar),
=
160.2 ppm (G2-m-Ar). IR (KBr): m 1560 (s, C N), 1595 and 1452
(vs, C C), 1295 (s, C-O-Casym), 1155 and 1051 cm−1 (vs, C–O–Cs).
=
MS (ESI+ in MeOH/0.1% HCOOH): m/z 1781 [M + H]+.
5386 | Dalton Trans., 2006, 5379–5389
This journal is
The Royal Society of Chemistry 2006
©

View Article Online
(vbr. s, Dm1/2 = 99 Hz, 2 H, CH2), −8.5 (br. s, Dm1/2 = 24 Hz, 1 H,
Preparation of [NiBr2{Me3SiCH(3,5-Me2pz)2}] (6). A solution
of ligand 1 (80 mg, 0.29 mmol) in dichloromethane (15 mL) was
added at room temp. to a suspension of [NiBr2(DME)] (78 mg,
0.25 mmol) in the same solvent (15 mL), and the mixture stirred
for 2 h. During the course of the reaction the initial orange
suspension changed rapidly to red, evolving to a deep-blue final
color in 10 min. The solvent was removed in vacuo and the residue
washed with pentane (2 × 15 mL) to give 6 as a grayish purple
paramagnetic solid, which was recrystallized from toluene. Yield:
110 mg (89%). Anal. Calc. for C14H24Br2N4NiSi (494.96): C, 33.97;
H, 4.89; N, 11.32%. Found: C, 34.15; H, 4.86; N, 11.23%. 1H NMR
(CDCl3, 293 K): d −12.0 (br. s, Dm1/2 = 16 Hz, 1 H, CH), −4.4 (br. s,
Dm1/2 = 20 Hz, 9 H, SiMe3), 15.6 (s, Dm1/2 = 8 Hz, 6 H, pz-Me5),
CH), 3.9 (br. s, Dm1/2 = 24 Hz, 2 H, G0-o-Ar), 3.98 (s, 6 H, Dm1/2
=
6 Hz, pz-Me5), 5.18 (s, 8 H, Dm1/2 = 4 Hz, PhCH2O), 5.47 (s, 4
H, Dm1/2 = 6 Hz, ArCH2O), 6.42 (s, 1 H, Dm1/2 = 4 Hz, G0-p-Ar),
6.80 (s, 2 H, Dm1/2 = 5 Hz, G1-p-Ar), 7.04 (s, 4 H, Dm1/2 = 5 Hz,
G1-o-Ar), 7.38 (d, JH,H = 7.0 Hz, 4 H, p-Ph), 7.46 (t, JH,H = 7.2 Hz,
8 H, m-Ph), 7.56 (t, JH,H = 7.4 Hz, 8 H, o-Ph), 21.4 (vbr. s, 6 H,
Dm1/2 = 99 Hz, pz-Me3), 69.7 ppm (br. s, 2 H, Dm1/2 = 24 Hz, pz-
4
=
=
H ). IR (KBr): m 1559 (s, C N), 1595 (vs) and 1451 (s, C C), 1293
(m, C–O–Oasym), 1156 and 1046 cm−1 (vs, C–O–Cs). MS (ESI+ in
MeCN): m/z 1101 [M − 2Br + Me2pz + H2O]+, 1069 [M − Br]+,
960 [M + 4 − 2Br]2+, 931 [4 + H]+, 835 [4 − Me2pz]+, 739 [4 −
2Me2pz]+.
38.5 (vbr. s, Dm1/2 = 117 Hz, 6 H, pz-Me3), 64.7 ppm (br. s, Dm1/2
=
Preparation of [NiBr2{G3-CH(3,5-Me2pz)2}] (10). This com-
plex was synthesized as described above for 6, from ligand 5
(200 mg, 0.112 mmol) and [NiBr2(DME)] (35 mg, 0.113 mmol),
and was isolated as a purple solid from a mixture of toluene and
pentane. Yield: 160 mg (80%). Anal. Calc. for C116H106Br2N4NiO14
(1998.64): C, 69.71; H, 5.35; N, 2.80%. Found: C, 69.81; H, 5.10;
4
=
=
20 Hz, 2 H, pz-H ). IR (KBr): m 1625 (m, C C), 1552 (s, C N).
MS (ESI+ in MeCN): m/z 447 [M − 2Br + Me2pz + H2O]+, 415
[M − Br]+, 333 [M − 2Br − H]+, 277 [1 + H]+, 181 [1 − Me2pz]+.
Preparation of [NiBr2{G0-CH(3,5-Me2pz)2}] (7). Compound
7 was synthesized as described above for 6, starting from
didentate ligand 2 (106 mg, 0.360 mmol) and [NiBr2(DME)]
(110 mg, 0.356 mmol). Purple monocrystals of complex 7 were
obtained after recrystallization from CH2Cl2/pentane by the two-
layers diffusion technique. Yield: 166 mg (91%). Anal. Calc. for
C18H22Br2N4Ni (512.90): C, 42.15; H, 4.32; N, 10.92%. Found:
N, 2.54%. 1H NMR (CDCl3, 293 K): d −9.4 (vbr. s, 2 H, Dm1/2
=
108 Hz, CH2), −8.4 (br. s, 1 H, Dm1/2 = 25 Hz, CH), 3.9 (br. s, 2
H, Dm1/2 = 23 Hz, G0-o-Ar), 3.99 (s, 6 H, Dm1/2 = 7 Hz, pz-Me5),
5.01 (s, 16 H, Dm1/2 = 4 Hz, PhCH2O), 5.08 (s, 8 H, Dm1/2 = 5 Hz,
G2-ArCH2O), 5.46 (s, 4 H, Dm1/2 = 8 Hz, G1-ArCH2O), 6.41 (s, 1
H, Dm1/2 = 5 Hz, G0-p-Ar), 6.59 (s, 4 H, Dm1/2 = 6 Hz, G2-p-Ar),
6.73 (s, 2 H, Dm1/2 = 5 Hz, G1-p-Ar), 6.77 (s, 8 H, Dm1/2 = 5 Hz,
G2-o-Ar), 6.99 (s, 4 H, Dm1/2 = 6 Hz, G1-o-Ar), 7.30 (m, 24 H,
1
C, 42.67; H, 4.46; N, 10.64%. H NMR (CDCl3, 293 K): d −9.1
(vbr. s, Dm1/2 = 67 Hz, 2 H, CH2), −8.2 (br. s, Dm1/2 = 18 Hz, 1
H, CH), 4.13 (s, Dm1/2 = 4 Hz, 6 H, pz-Me5), 5.0 (br. s, Dm1/2
=
p-Ph and m-Ph), 7.38 (m, 16 H, o-Ph), 21.2 (vbr. s, 6 H, Dm1/2
=
19 Hz, 2 H, o-Ph), 7.16–7.19 (m, 3 H, m- and p-Ph), 21.4 (vbr. s,
Dm1/2 = 84 Hz, 6 H, pz-Me3), 69.3 ppm (br. s, Dm1/2 = 18 Hz, 2 H,
101 Hz, pz-Me3), 69.9 ppm (br. s, 2 H, Dm1/2 = 21 Hz, pz-H4).
=
=
4
−1
IR (KBr): m 1559 (s, C N), 1595 (vs) and 1448 (s, C C), 1293
(m, C–O–Oasym), 1155 and 1051 cm−1 (vs, C–O–Cs). MS (ESI+
in MeCN): m/z 1952 [M − 2Br + Me2pz + H2O]+, 1917 [M −
Br]+, 1809 [M + 5 − 2Br]2+, 1781 [5 + H]+, 919 [M − 2Br]2+. MS
(MALDI-TOF in dithranol): m/z 3917 [M2 − Br]+, 3837 [(M −
Br)2]+, 2096 [M + Br + H2O]+, 2078 [M + Br]+, 1918 [M − Br]+,
1837 [M − 2Br]+.
=
=
pz-H ). IR (KBr): m 1557 (s, C N), 1605 (m, C C) and 1463 cm
=
(s, C C). MS (ESI+ in MeCN): m/z 465 [M − 2Br + Me2pz +
H2O]+, 433 [M − Br]+, 371 [M − 2Br + H + H2O]+, 351 [M −
2Br − H]+, 199 [2-Me2pz]+.
Preparation of [NiBr2{G1-CH(3,5-Me2pz)2}] (8). Compound
8 was also synthesized as described above for 6, starting from
wedge 3 (210 mg, 0.414 mmol) and [NiBr2(DME)] (128 mg,
0.415 mmol). The purple solid was recrystallized from toluene.
Yield: 269 mg (90%). Anal. Calc. for C32H34Br2N4NiO2 (725.15):
C, 53.00; H, 4.73; N, 7.73%. Found: C, 53.27; H, 4.84; N, 7.52%.
1H NMR (CDCl3, 293 K): d −9.3 (vbr. s, Dm1/2 = 89 Hz, 2 H,
CH2), −8.2 (br. s, Dm1/2 = 23 Hz, 1 H, CH), 3.9 (br. s, 2 H, partially
overlapping, o-Ar), 4.02 (s, Dm1/2 = 6 Hz, 6 H, pz-Me5), 5.59 (s,
Dm1/2 = 5 Hz, 4 H, PhCH2O), 6.47 (s, Dm1/2 = 4 Hz, 1 H, p-Ar), 7.52
(d, 2 H, JH,H = 7.3 Hz, p-Ph), 7.64 (t, 4 H, JH,H = 7.4 Hz, m-Ph),
7.82 (t, 4 H, JH,H = 7.5 Hz, o-Ph), 21.4 (vbr. s, Dm1/2 = 102 Hz, 6 H,
X-Ray crystallographic studies
Single crystals of 7 suitable for an X-ray diffraction study were
obtained by slow diffusion of pentane into a toluene solution
of the nickel complex at room temperature. A summary of
crystal data, data collection, and refinement parameters for the
structural analysis is given in Table 6. A purple-blue crystal was
glued to a glass fiber and mounted on a Kappa-CCD Bruker-
Nonius diffractometer with an area detector, and equipped with
an Oxford Cryostream 700 unit; data were collected using graphite
pz-Me3), 69.6 ppm (br. s, Dm1/2 = 23 Hz, 2 H, pz-H4). IR (KBr): m
˚
monochromated Mo-Ka radiation (k = 0.71073 A) at 150 K, with
−1
=
=
1559 (s, C N), 1607, 1596 and 1464 (s, C C), 1293 and 1058 cm
(s, C–O–C). MS (ESI+ in MeCN): m/z 1151 [M + 3 − Br]+, 677
[M − 2Br + Me2pz + H2O]+, 645 [M − Br]+, 583 [M − 2Br + H +
H2O]+, 507 [3 + H]+, 411 [3 − Me2pz]+, 315 [3 − 2Me2pz]+.
an exposure time of 10 s per frame (five sets; 239 frames; phi
scans 2◦ scan-width). Raw data were corrected for Lorenz and
polarization effects.
The structure was solved by direct methods, completed by
subsequent difference Fourier techniques and refined by full-
matrix least-squares on F2 with SHELXL-97.28 Anisotropic
thermal parameters were used in the last cycles of refinement for
the non-hydrogen atoms. The hydrogen atoms were introduced
in the last cycle of refinement from geometrical calculations and
refined using a riding model. All the calculations were made using
the WINGX system.29
Preparation of [NiBr2{G2-CH(3,5-Me2pz)2}] (9). Dendritic
nickel complex 9 was synthesized as described above for 6, starting
from ligand 4 (162 mg, 0.174 mmol) and [NiBr2(DME)] (54 mg,
0.175 mmol), and was isolated as a purple solid from toluene used
as recrystallization solvent. Yield: 160 mg (80%). Anal. Calc. for
C60H58Br2N4NiO6 (1149.64): C, 62.69; H, 5.09; N, 4.87%. Found:
1
C, 62.35; H, 5.02; N, 4.79%. H NMR (CDCl3, 293 K): d −9.5
This journal is
The Royal Society of Chemistry 2006
Dalton Trans., 2006, 5379–5389 | 5387
©

View Article Online
Table 6 Crystal data and structure refinement for compound 7
Empirical formula
Formula weight
Color
C18H22N4Br2Ni
512.90
Purple-blue
150.0(2)
Temperature/K
˚
Wavelength/A
0.71073
Crystal system, space group
Monoclinic, P21/n
8.816(1)
16.318(1)
14.369(1)
90.00
97.29(1)
90.00
2050.2(3)
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
3
˚
Volume/A
Z, Calculated density/g cm−3
Absorption coefficient/mm−1
F(000)
4, 1.662
4.852
1024
Crystal size/mm
0.50 × 0.15 × 0.15
h ranges/◦
3.12 to 25.51
Limiting indices
−10 ≤ h ≤ 10, −19 ≤ k ≤ 19, −17 ≤ l ≤ 17
13369/3814 [Rint = 0.2191]
2098 [I > 2r(I)]
99.6
Reflections collected/unique
Reflections observed
Completeness to h (%)
Refinement method
Data/restraints/parameters
Goodness of fit on F2
Final R indices [I > 2r(I)]a
R indices (all data)
Full-matrix least-squares on F2
3813/0/246
0.968
R1 = 0.0641, wR2 = 0.1049
R1 = 0.1451, wR2 = 0.1252
0.753 and −0.931
−3
˚
Largest diff. peak and hole/e A
ꢀ
ꢀ
ꢀ
ꢀ
a R1 = ||Fo| − |Fc||/ |Fo|; wR2 = {[ w(Fo − Fc2)]/[ w(Fo2)2]}
.
2
1/2
CCDC reference number 606876.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b607090f
6 C. Pettinari and R. Pettinari, Coord. Chem. Rev., 2005, 249, 525.
7 (a) D. L. Reger, R. P. Watson, M. D. Smith and P. J. Pellechia,
Organometallics, 2005, 24, 1544; (b) B. S. Hammes, M. T. Kieber-
Emmons, J. A. Letizia, Z. Shirin, C. J. Carrano, L. N. Zakharov and
A. L. Rheingold, Inorg. Chim. Acta, 2003, 346, 227.
8 (a) A. Otero, J. Ferna´ndez-Baeza, A. Antin˜olo, J. Tejeda, A. Lara-
Sa´nchez, L. Sa´nchez-Barba, E. Mart´ınez-Caballero, A. M. Rodr´ıguez
and I. Lo´pez-Solera, Inorg. Chem., 2005, 44, 5336; (b) A. Otero, J.
Ferna´ndez-Baeza, A. Antin˜olo, J. Tejeda, A. Lara-Sa´nchez, L. Sa´nchez-
Barba, A. M. Rodr´ıguez and M. A. Maestro, J. Am. Chem. Soc.,
2004, 126, 1330; (c) A. Otero, J. Ferna´ndez-Baeza, A. Antin˜olo, F.
Carrillo-Hermosilla, J. Tejeda, A. Lara-Sa´nchez, L. Sa´nchez-Barba,
A. M. Rodr´ıguez and I. Lo´pez-Solera, Inorg. Chem., 2002, 41, 5193;
(d) A. Otero, J. Ferna´ndez-Baeza, J. Tejeda, A. Antin˜olo, F. Carrillo-
Hermosilla, E. D´ıaz-Barra, A. Lara-Sa´nchez and M. Ferna´ndez-Lo´pez,
J. Chem. Soc., Dalton Trans., 2000, 2367.
Acknowledgements
We gratefully acknowledge financial support from the DGI-
Ministerio de Educacio´n
CTQ2005-00795/BQU) and Comunidad de Madrid (project S-
0505/PPQ/0328-03). A. S.-M. is also indebted to the Universidad
de Alcala´ for financial support (FPI grant).
y
Ciencia of Spain (project
9 (a) C. Pettinari and R. Pettinari, Coord. Chem. Rev., 2005, 249, 663;
(b) A. Otero, J. Ferna´ndez-Baeza, A. Antin˜olo, J. Tejeda and A. Lara-
Sa´nchez, Dalton Trans., 2004, 1499.
References
1 (a) S. Trofimenko, J. Am. Chem. Soc., 1966, 88, 1842; (b) S. Trofimenko,
J. Am. Chem. Soc., 1967, 89, 3170; (c) S. Trofimenko, J. Am. Chem. Soc.,
1969, 91, 588; (d) S. Trofimenko, J. Am. Chem. Soc., 1969, 91, 3183.
2 S. Trofimenko, Scorpionates: The Coordination Chemistry of Polypyra-
zolylborate Ligands, Imperial College Press, London, 1999.
3 (a) D. L. Reger, T. C. Grattan, K. J. Brown, C. A. Little, J. J. S. Lamba,
A. L. Rheingold and R. D. Sommer, J. Organomet. Chem., 2000, 607,
120; (b) S. Julia´, P. Sala, J. del Mazo, M. Sancho, C. Ochoa, J. Elguero,
J.-P. Fayet and M.-C. Vertut, J. Heterocycl. Chem., 1982, 19, 1141; (c) S.
Julia, J. M. del Mazo, L. Avila and J. Elguero, Org. Prep. Proced. Int.,
1984, 16, 299.
4 (a) D. L. Reger and T. C. Grattan, Synthesis, 2003, 350; (b) D. L. Reger,
R. F. Semeniuc and M. D. Smith, J. Organomet. Chem., 2003, 666,
87; (c) D. L. Reger, R. F. Semeniuc and M. D. Smith, J. Chem. Soc.,
Dalton Trans., 2002, 476; (d) D. L. Reger, K. J. Brown and M. D. Smith,
J. Organomet. Chem., 2002, 658, 50.
10 Some reviews: (a) C. Gorman, Adv. Mater., 1998, 10, 295; (b) G. R.
Newkome, E. He and C. N. Moorefield, Chem. Rev., 1999, 99, 1689;
(c) I. Cuadrado, M. Mora´n, C. M. Casado, B. Alonso and J. Losada,
Coord. Chem. Rev., 1999, 193–195, 395; (d) F. J. Stoddart and T. Welton,
Polyhedron, 1999, 18, 3575; (e) M. A. Hearshaw and J. R. Moss, Chem.
Commun., 1999, 1; (f) H.-J. van Manen, F. C. J. M. van Veggel and
D. N. Reinhoudt, Top. Curr. Chem., 2001, 217, 121; (g) K. Onitsuka
and S. Takahashi, Top. Curr. Chem., 2003, 228, 39; (h) P. A. Chase,
R. J. M. K. Gebbink and G. van Koten, J. Organomet. Chem., 2004,
689, 4016; (i) O. Rossell, M. Seco and I. Angurell, C. R. Chim., 2003,
6, 803.
11 Some reviews on catalysis with dendrimers: (a) G. E. Oosterom, J. N. H.
Reek, P. C. J. Kamer and P. W. N. M. van Leeuwen, Angew. Chem.,
Int. Ed., 2001, 40, 1828; (b) D. Astruc and F. Chardac, Chem. Rev.,
2001, 101, 2991; (c) R. van Heerbeek, P. C. J. Kamer, P. W. N. M.
van Leeuwen and J. N. H. Reek, Chem. Rev., 2002, 102, 3717; (d) R.
Kreiter, A. W. Kleij, R. J. M. K. Gebbink and G. van Koten, Top. Curr.
Chem., 2001, 217, 163; (e) A. M. Caminade, V. Maraval, R. Laurent
and J. P. Majoral, Curr. Org. Chem., 2002, 6, 739; (f) A. Dahan and M.
5 (a) C. Santini, M. Pellei, G. Gioia Lobbia, A. Cingolani, R. Spagna
and M. Camalli, Inorg. Chem. Commun., 2002, 5, 430; (b) W. Kla¨ui, M.
Berghahn, G. Rheinwald and H. Lang, Angew. Chem., Int. Ed., 2000,
39, 2464.
5388 | Dalton Trans., 2006, 5379–5389
This journal is
The Royal Society of Chemistry 2006
©

View Article Online
Portnoy, J. Polym. Sci., Part A: Polym. Chem., 2005, 43, 235; (g) R.
van de Coevering, R. J. M. Klein Gebbink and G. van Koten, Prog.
Polym. Sci., 2005, 30, 474; (h) D. Me´ry and D. Astruc, Coord. Chem.
Rev., 2006, DOI: 10.1016/j.ccr.2005.11.012.
19 S. Tsuji, D. C. Swenson and R. F. Jordan, Organometallics, 1999, 18,
4758.
20 J. C. Jansen, H. van Koningsveld, J. A. C. van Ooijen and J. Reedijk,
Inorg. Chem., 1980, 19, 170.
21 N. Arroyo, F. Go´mez-de la Torre, F. A. Jalo´n, B. R. Manzano, B.
Moreno-Lara and A. M. Rodr´ıguez, J. Organomet. Chem., 2000, 603,
174.
12 (a) R. Andre´s, E. de Jesu´s, F. J. de la Mata, J. C. Flores and R. Go´mez,
J. Organomet. Chem., 2005, 690, 939; (b) V. Amo, R. Andre´s, E. de
Jesu´s, F. J. de la Mata, J. C. Flores, R. Go´mez, P. Go´mez-Sal and
J. F. C. Turner, Organometallics, 2005, 24, 2331; (c) J. M. Benito, S.
Are´valo, E. de Jesu´s, F. J. de la Mata, J. C. Flores and R. Go´mez,
J. Organomet. Chem., 2000, 610, 42; (d) J. M. Benito, E. de Jesu´s, F. J.
de la Mata, J. C. Flores, R. Go´mez and P. Go´mez-Sal, J. Organomet.
Chem., 2002, 664, 258.
13 (a) J. M. Benito, E. de Jesu´s, F. J. de la Mata, J. C. Flores and R.
Go´mez, Chem. Commun., 2005, 5217; (b) J. M. Benito, E. de Jesu´s, F. J.
de la Mata, J. C. Flores, R. Go´mez and P. Go´mez-Sal, Organometallics,
2006, DOI: 10.1021/om0509084; (c) J. M. Benito, E. de Jesu´s, F. J. de
la Mata, J. C. Flores and R. Go´mez, Organometallics, 2006, 25, 3045.
14 A. Sa´nchez-Me´ndez, G. F. Silvestri, E. de Jesu´s, F. J. de la Mata, J. C.
Flores, R. Go´mez and P. Go´mez-Sal, Eur. J. Inorg. Chem., 2004, 3287.
15 C. J. Hawker and J. M. J. Fre´chet, J. Am. Chem. Soc., 1990, 112, 7638.
16 (a) M. Helldo¨rfer, W. Milius and H. G. Alt, J. Mol. Catal. A: Chem.,
2003, 197, 1; (b) M. Helldo¨rfer, J. Backhaus and H. G. Alt, Inorg. Chim.
Acta, 2003, 351, 34; (c) R. J. Maldanis, J. S. Wood, A. Chandraselkaran,
M. D. Rausch and J. C. W. Chien, J. Organomet. Chem., 2002, 645, 158.
17 (a) W.-H. Sun, W. Zhang, T. Gao, X. Tang, L. Chen, Y. Li and X.
Jin, J. Organomet. Chem., 2004, 689, 917; (b) S. R. Wilson and Y. Wu,
Organometallics, 1993, 12, 1478.
22 (a) R. S. Drago, Physical Methods in Chemistry, W. Saunders, Philadel-
phia, 1977; (b) I. Bertini, P. Turano and A. J. Vila, Chem. Rev., 1993,
93, 2833.
1
23 Examples of H NMR spectra of nickel(II) complexes: (a) E. Szajna,
P. Dobrowolski, A. L. Fuller, A. M. Arif and L. M. Berreau, Inorg.
Chem., 2004, 43, 3988; (b) C. Belle, C. Bougault, M.-T. Averbuch, A.
Durif, J.-L. Pierre, J.-M. Latour and L. Le Pape, J. Am. Chem. Soc.,
2001, 123, 8053; (c) R. C. Holz, E. A. Evdokimov and F. T. Gobena,
Inorg. Chem., 1996, 35, 3808.
24 (a) I. Solomon, Phys. Rev., 1955, 99, 559; (b) L.-J. Ming, H. G. Jang
and L. Que, Jr., Inorg. Chem., 1992, 31, 359; (c) J. M. Brink, R. A. Rose
and R. C. Holz, Inorg. Chem., 1996, 35, 2878.
25 L. Banci, I. Bertini, C. Luchinat, R. Pierattelli, N. V. Shokhirev and
F. A. Walker, J. Am. Chem. Soc., 1998, 120, 8472.
26 L. G. L. Ward, in Inorganic Syntheses, ed. F. A. Cotton, McGraw-Hill
Book Company, New York, 1972, vol. 13, p. 154.
27 D. P. Perrin and W. L. F. Armarego, Purification of Laboratory
Chemicals, Pergamon Press, Oxford, 3rd edn, 1988.
28 G. M. Sheldrick, SHELXL_97. Program for Crystal Structure Refine-
ment, University of Go¨ttingen, Go¨ttingen,Germany, 1997.
29 L. J. Farrugia, WinGX System, J. Appl. Crystallogr., 1999, 32,
837.
18 S. M. Nelana, J. Darkwa, I. A. Guzei and S. F. Mapolie, J. Organomet.
Chem., 2004, 689, 1835.
This journal is
The Royal Society of Chemistry 2006
Dalton Trans., 2006, 5379–5389 | 5389
©