A novel one-dimensional nickel(II) alternating chain from discrete pyrazolate-based dinuclear complexes

Article abstract of DOI:10.1039/a708565f

Hexadentate dinucleating ligands that are based on a bridging pyrazolate bearing chelating side arms [3,5-(R₂NCH₂)₂C₃N₂H ₂; R₂N = Me₂N(CH₂)₂NMe (HL¹), Me₂N(CH₂)₂NMe (HL²)] reacted with NiCl₂·6H₂O to yield complexes ClNi(μ-Cl)(μ-L¹)NiCl 1 (Ni₂L¹Cl₃) and ClNi(μ-Cl)(μ-L²)NiCl 2 (Ni₂L²Cl₃), respectively. Depending on the side-arm chain length and the solvent used for crystallisation these complexes either crystallised as dicrete bimetallic units (1) or were assembled via di-μ-chloro linkages to form a tetranuclear compound [ClNi(μ-Cl)-(μ-L²)Ni(μ-Cl) ₂Ni(μ-Cl)(μ-L²)NiCl] 2a ([Ni₂L²Cl₃]₂) or a novel bridge-alternating one-dimensional chain [Ni(μ-Cl)(μ-L¹)Ni(μ-Cl)₂]_∞ 2b ([Ni₂L²Cl₃]_∞) in the solid state. Variable-temperature magnetic susceptibility measurements revealed antiferromagnetic coupling within the basic μ-chloro-μ-pyrazolato bridged bimetallic framework in all cases and also suggested antiferromagnetic superexchange propagated by the di-μ-chloro linkage in 2b. The latter result is rationalised on the basis of the specific geometric findings for this di-μ-chloro linkage, in particular the unusually large Ni-Cl-Ni angle [101.37(4)°].

Full text of DOI:10.1039/a708565f

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A novel one-dimensional nickel(II) alternating chain from discrete
pyrazolate-based dinuclear complexes
,
,a
b
a
a
a
Franc Meyer,* † Uwe Ruschewitz, Peter Schober, Björn Antelmann and Laszlo Zsolnai
a
Anorganisch-Chemisches Institut der Universität Heidelberg, Im Neuenheimer Feld 270,
D-69120 Heidelberg, Germany
b
Institut für Anorganische Chemie der RWTH Aachen, Prof.-Pirlet-Straße 1, D-52056 Aachen,
Germany
Hexadentate dinucleating ligands that are based on a bridging pyrazolate bearing chelating side arms [3,5-
1
2
(
R NCH ) C N H ; R N = Me N(CH ) NMe (HL ), Me N(CH ) NMe (HL )] reacted with NiCl ؒ6H O to yield
2 2 2 3 2 2 2 2 2 3 2 2 2 2 2
1 1 2 2
complexes ClNi(µ-Cl)(µ-L )NiCl 1 (Ni L Cl ) and ClNi(µ-Cl)(µ-L )NiCl 2 (Ni L Cl ), respectively. Depending
2
3
2
3
on the side-arm chain length and the solvent used for crystallisation these complexes either crystallised as dicrete
bimetallic units (1) or were assembled via di-µ-chloro linkages to form a tetranuclear compound [ClNi(µ-Cl)-
2
2
2
(
[
µ-L )Ni(µ-Cl) Ni(µ-Cl)(µ-L )NiCl] 2a ([Ni L Cl ] ) or a novel bridge-alternating one-dimensional chain
2 2 3 2
1 2
Ni(µ-Cl)(µ-L )Ni(µ-Cl) ] 2b ([Ni L Cl ] ) in the solid state. Variable-temperature magnetic susceptibility
2 ∞ 2 3 ∞
measurements revealed antiferromagnetic coupling within the basic µ-chloro-µ-pyrazolato bridged bimetallic
framework in all cases and also suggested antiferromagnetic superexchange propagated by the di-µ-chloro
linkage in 2b. The latter result is rationalised on the basis of the specific geometric findings for this di-µ-chloro
linkage, in particular the unusually large Ni᎐Cl᎐Ni angle [101.37(4)Њ].
In the last decade much research has been devoted to the study
of magnetic interactions between paramagnetic centres in
O
O
O
O
(1) SOCl2
1,2
exchange-coupled systems. Thereby the interest has extended
from dinuclear to oligonuclear and polymeric compounds, and
some of the more recent work in low-dimensional magnetism
has focused on one-dimensional exchange-alternating linear
HO
OH (2) 2 HNR –NEtPri
R2N
NR2
2
2
N
N
N
N
H
H
3–5
LiAlH4
chains with local spins S > ¹. The theoretical treatment of the6
¯
²
J-alternating S = 1 chain by Borrás-Almenar and co-workers
strongly stimulated experimental searches for systems of this
kind, however fully characterised examples have still remained
R2N
NR2
Me
3,4,7
rare
and in most cases are derived from the co-ordinative
N
N
4,8
H
versatility of the multidentate azido ligand. While the general
and most often pursued synthetic strategy, i.e. the mixing of an
appropriate metal salt with different types of potential ligands,
leaves the formation of the desired alternating one-dimensional
system to the whims of nature, a more systematic approach is
based on the linking of discrete bimetallic complexes with
Me
NR2 = N
NMe2 (HL1) or
N
NMe2 (HL2)
Scheme 1
7
accessible co-ordination sites. We recently described the syn-
3,5-dicarboxylic acid is converted into the corresponding
12
thesis and co-ordination potential of a series of dinucleating
bis(amide) by treatment with SOCl2 and subsequent reaction
pyrazolate ligands with chelating polyamino substituents in the
with the appropriate secondary amine in the presence of
9
i
3
- and 5-positions of the heterocycle, where characteristic
NEtPr . The resulting bis(amide) can then be reduced using
2
2
features of the resulting metal complexes like the metal–metal
separation or co-ordination numbers can be selectively tuned by
an excess of LiAlH to yield the polydentate ligand HL as a
4
slightly yellowish oil.
10
11
varying the side arm chain length and donor types. In the
present contribution we employ bimetallic systems of this kind
to construct a novel one-dimensional S = 1 alternating chain
For the synthesis of nickel() complexes the potential ligands
1
2
HL and HL are treated with 2 equivalents of NiCl ؒ6H O in
2
2
i
the presence of NEtPr 2 to afford green solutions of co-
ordination compounds Ni L Cl 1 and Ni L Cl 2, whose
1
1
2
from the assembly of basic dinickel() entities. Ligands L and
2 3 2 3
2
L (Scheme 1) were chosen in order to provide fewer donor sites
formations are corroborated by elemental analyses and by the
FAB-MS spectra. Although no definite conclusions can be
drawn from the solution UV/VIS spectra (Table 1), the ligand-
field transitions observed for 1 and 2 bear close resemblance
with those observed for complexes containing five-co-ordinate
than required to co-ordinatively saturate a nickel() ion in each
co-ordination compartment and thus allow for the requisite
linking of the pyrazolate-based dinuclear complexes.
II
13
high spin Ni ions. As expected, the band positions for 2 in
Results and Discussion
Syntheses and general characterisation
CH Cl and in CHCl are very similar, indicating the presence
2
2
3
II
of identical five-co-ordinate dinuclear Ni species in solution,
in contrast to the different solid-state structures obtained by
crystallisation from these two solvents (see below). A solid-state
UV/VIS spectrum of the green crystals of 2 obtained from
CH Cl –light petroleum (b.p. 40–60 ЊC) reveals three absorp-
2
The new ligand HL is synthesised analogously to the prepar-
1
9
ation of HL reported previously (Scheme 1). Thus pyrazole-
2
2
†
E-Mail: Franc@sun0.urz.uni-heidelberg.de
tions that differ significantly from the solution data, but are
J. Chem. Soc., Dalton Trans., 1998, Pages 1181–1186
1181

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Table 1 The UV/VIS data of the complexes
Table 2 Selected bond distances (Å) and angles (Њ) for complex 1
Ϫ1
Ϫ1
Ϫ1
ν
˜
/cm (ε/ cm
)
Ni(1)᎐N(1)
Ni(1)᎐N(3)
Ni(1)᎐N(4)
Ni(1)᎐Cl(1)
Ni(1)᎐Cl(2)
Ni(2)᎐N(2)
1.959(6)
2.163(6)
2.074(6)
2.292(2)
2.445(2)
1.961(6)
Ni(2)᎐N(5)
Ni(2)᎐N(6)
Ni(2)᎐Cl(2)
Ni(2)᎐Cl(3)
N(1)᎐N(2)
2.182(6)
2.068(6)
2.439(2)
2.294(2)
1.365(8)
3.918
a
a
b
c
1
2
2
b
13 660 (80), 23 640 (240)
12 990 (60), 22 270 (120)
13 070 (60), 22 320 (140)
8370 [ν ], 14 300 [ν ], 23 750 [ν ]
2
1
2
3
Ni(1) ؒ ؒ ؒ Ni(2)
a
b
c
In CHCl3. In CH Cl . Powder in Nujol.
2
2
N(1)᎐Ni(1)᎐N(3)
N(1)᎐Ni(1)᎐N(4)
N(1)᎐Ni(1)᎐Cl(1)
N(1)᎐Ni(1)᎐Cl(2)
N(3)᎐Ni(1)᎐N(4)
N(3)᎐Ni(1)᎐Cl(1)
N(3)᎐Ni(1)᎐Cl(2)
N(4)᎐Ni(1)᎐Cl(1)
N(4)᎐Ni(1)᎐Cl(2)
Cl(1)᎐Ni(1)᎐Cl(2)
N(2)᎐Ni(2)᎐N(5)
N(2)᎐Ni(2)᎐N(6)
78.8(2)
107.7(2)
153.4(2)
85.4(2)
94.5(2)
96.2(2)
159.8(2)
98.7(2)
102.2(2)
92.4(1)
78.6(2)
106.3(3)
N(2)᎐Ni(2)᎐Cl(2)
N(2)᎐Ni(2)᎐Cl(3)
N(5)᎐Ni(2)᎐N(6)
N(5)᎐Ni(2)᎐Cl(2)
N(5)᎐Ni(2)᎐Cl(3)
N(6)᎐Ni(2)᎐Cl(2)
N(6)᎐Ni(2)᎐Cl(3)
Cl(2)᎐Ni(2)᎐Cl(3)
Ni(1)᎐N(1)᎐N(2)
Ni(2)᎐N(2)᎐N(1)
86.1(2)
154.2(2)
94.9(2)
159.9(2)
95.5(2)
102.0(2)
99.1(2)
92.6(1)
131.5(4)
129.8(4)
Ni(1)᎐Cl(2)᎐Ni(2) 106.7(1)
Table 3 Selected bond distances (Å) and angles (Њ) for complex 2a
Ni(1)᎐N(1)
Ni(1)᎐N(3)
Ni(1)᎐N(4)
Ni(1)᎐Cl(1)
Ni(1)᎐Cl(1 )
Ni(1)᎐Cl(2)
Ni(2)᎐N(2)
1.987(3)
2.205(3)
2.182(3)
2.567(1)
2.378(1)
2.536(1)
1.950(3)
Ni(2)᎐N(5)
Ni(2)᎐N(6)
Ni(2)᎐Cl(2)
Ni(2)᎐Cl(3)
N(1)᎐N(2)
Ni(1) ؒ ؒ ؒ Ni(2)
Ni(1) ؒ ؒ ؒ Ni(1 )
2.246(3)
2.066(3)
2.435(1)
2.275(1)
1.349(4)
3.924
I
I
3.774
N(1)᎐Ni(1)᎐N(3)
N(1)᎐Ni(1)᎐N(4)
78.3(1)
97.0(1)
88.84(9)
N(2)᎐Ni(2)᎐N(5)
N(2)᎐Ni(2)᎐N(6)
N(2)᎐Ni(2)᎐Cl(2)
N(2)᎐Ni(2)᎐Cl(3)
N(5)᎐Ni(2)᎐N(6)
N(5)᎐Ni(2)᎐Cl(2)
N(5)᎐Ni(2)᎐Cl(3)
N(6)᎐Ni(2)᎐Cl(2)
N(6)᎐Ni(2)᎐Cl(3)
Cl(2)᎐Ni(2)᎐Cl(3)
Ni(1)᎐N(1)᎐N(2)
Ni(2)᎐N(2)᎐N(1)
77.5(1)
105.2(1)
88.64(9)
147.02(9)
84.7(1)
165.36(8)
99.08(8)
94.74(9)
107.11(9)
95.07(4)
131.8(2)
129.3(2)
N(1)᎐Ni(1)᎐Cl(1)
I
N(1)᎐Ni(1)᎐Cl(1 ) 169.09(8)
N(1)᎐Ni(1)᎐Cl(2)
N(3)᎐Ni(1)᎐N(4)
N(3)᎐Ni(1)᎐Cl(1)
85.20(8)
83.1(1)
Fig. 1 Cyclic voltammograms of complexes 1 (top) and 2 (bottom) in
n
Ϫ1
91.10(8)
99.22(8)
159.70(8)
170.77(8)
93.22(8)
87.35(8)
80.57(4)
CH Cl containing NBu PF (0.1 ) at scan speed 200 mV s
2
2
4
6
I
N(3)᎐Ni(1)᎐Cl(1 )
8
13c
N(3)᎐Ni(1)᎐Cl(2)
N(4)᎐Ni(1)᎐Cl(1)
typical for a d ion in a near-octahedral co-ordination sphere,
3
i.e. they are assigned to spin-allowed transitions from A to
T , T (F) and T (P), respectively (Table 1).
I
2g
N(4)᎐Ni(1)᎐Cl(1 )
3
3
3
2g
1g
1g
N(4)᎐Ni(1)᎐Cl(2)I
Both complexes 1 and 2 have been studied by cyclic voltam-
Cl(1)᎐Ni(1)᎐Cl(1 )
Ni(1)᎐Cl(2)᎐Ni(2) 104.23(4)
I
metry in the potential range 0.0 to 1.50 V vs. SCE in CH Cl
Cl(1)᎐Ni(1)᎐Cl(2) 100.31(4)
Ni(1)᎐Cl(1)᎐Ni(1 )
99.43(4)
2
2
I
Cl(1 )᎐Ni(1)᎐Cl(2)
99.18(4)
(
E
Fig. 1). Complex 1 shows a reversible oxidation wave at
₁
= ϩ1.26 V (∆E = Epc Ϫ Epa = 120 mV at scan speed 200 mV
₂
�
Ϫ1
s ), presumably corresponding to the formation of a mixed-
valence Ni Ni compound. In contrast, the cyclic voltam-
mogram of 2 displays an oxidation process at Ep = ϩ1.37 V
(
III
II
ox
Ϫ1
200 mV s ) that is irreversible at low scan rates but changes its
shape with increasing scan rates to become quasi-reversible and
Ϫ1
yields E ₂� = 1.33 V (∆E = 200 mV at scan rate 800 mV s ). It
₁
thus transpires that the longer and more flexible ligand side
arms in 1 are more favourable for the adaptation to an oxidised
III
II
Ni Ni species than the smaller sized chelate rings enforced by
the shorter side arms in 2.
Description of the solid-state structures
Fig. 2 View of the molecular structure of complex 1 (30% probability
ellipsoids). In the interests of clarity all hydrogen atoms have been
omitted
Complex 1 crystallises in the orthorhombic space group Pbca
with two CHCl solvent molecules per bimetallic entity in the
3
unit cell. Selected bond distances and angles are given in Table
2
, the molecular structure is depicted in Fig. 2. It shows discrete
solvent system. In the former case the complex crystallises in
¯
dinuclear molecules in which two nickel ions are bridged by
both the pyrazolate of the primary ligand L and a chlorine
atom. Each nickel centre is found to be five-co-ordinate in an
approximately square pyramidal co-ordination environment
with the outermost nitrogen donors N(4) and N(6) in the apical
positions. The structure of 1 is thus very similar to those of a
related cobalt() compound reported previously. Interestingly,
the species Ni L Cl yields orange-yellow crystals (2a) when a
solution of the complex in CHCl is layered with light petrol-
eum, but green crystals (2b) from the CH Cl –light petroleum
the triclinic space group P1 with two CHCl solvent molecules
3
1
per pyrazolate-based dinickel unit. The molecular structure of
2a is shown in Fig. 3, selected bond lengths and angles are listed
in Table 3.
The structural analysis shows that two pyrazolate-based
bimetallic entities similar to 1 are dimerised via two chlorine
atoms to form linear tetranuclear species containing two six-co-
ordinate nickel() ions in a distorted octahedral surrounding,
that are flanked by terminal five-co-ordinate nickel() ions
intermediate between SPY-5 (square pyramidal) and TBPY-5
9
2
2
3
3
2
2
1
182
J. Chem. Soc., Dalton Trans., 1998, Pages 1181–1186

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atom distances Ni(1)᎐Cl(2) [2.536(1) Å] and Ni(2)᎐Cl(2)
Table 4 Selected bond distances (Å) and angles (Њ) for complex 2b
[2.435(1) Å]. Similar variations have previously been observed
for a series of pyrazolate-bridged dinuclear complexes, in which
the co-ordination number asymmetry was imposed by different
chelating side arms in the 3- and 5-positions of the hetero-
cycle. Subtle modifications of the chelating side arms of a
dinucleating pyrazolate ligand in conjunction with the choice of
an appropriate solvent for crystallisation thus enable the con-
struction of complexes with differing extents of aggregation,
leading to either single bimetallic or oligomeric or even one-
dimensional polymeric solid-state structures.
I
Ni᎐N(1)
Ni᎐N(2)
Ni᎐N(3)
Ni᎐Cl(1)
Ni᎐Cl(2)
1.979(4)
2.197(4)
2.160(4)
2.525(1)
2.392(1)
Ni᎐Cl(2 )
2.592(1)
1.385(7)
3.857
II
N(1)᎐N(1 )
Ni ؒ ؒ ؒ Ni
Ni ؒ ؒ ؒ Ni
I
11
II
4.028
N(1)᎐Ni᎐N(2)
N(1)᎐Ni᎐N(3)
N(1)᎐Ni᎐Cl(1)
N(1)᎐Ni᎐Cl(2)
77.9(1)
97.7(2)
85.1(1)
168.9(1)
90.4(1)
82.9(1)
159.8(1)
99.8(1)
88.6(1)
N(3)᎐Ni᎐Cl(1)
N(3)᎐Ni᎐Cl(2)
N(3)᎐Ni᎐Cl(2 )
88.8(1)
92.8(1)
I
166.7(1)
98.94(5)
102.49(3)
78.63(4)
105.73(6)
101.37(4)
131.5(1)
Cl(1)᎐Ni᎐Cl(2)
I
I
N(1)᎐Ni᎐Cl(2 )
Cl(1)᎐Ni᎐Cl(2 )
Magnetic properties of the complexes
I
N(2)᎐Ni᎐N(3)
N(2)᎐Ni᎐Cl(1)
N(2)᎐Ni᎐Cl(2)
Cl(2)᎐Ni᎐Cl(2 )
II
Ni᎐Cl(1)᎐Ni
Ni᎐Cl(2)᎐Ni
The temperature-dependence of the magnetic susceptibilities
for powdered samples of 1, 2a and 2b is plotted in Fig. 5. The χm
I
I
II
N(2)᎐Ni᎐Cl(2 )
Ni᎐N(1)᎐N(1 )
value is found to increase with decreasing temperature, reaching
a maximum at around 35 K (1), 30 K (2a) or 25 K (2b) and then
tending towards zero at lower temperatures, while the χ T vs. T
m
curve continuously decreases upon cooling, this behaviour
being indicative of overall antiferromagnetic coupling in all
cases.
For the dinuclear complex 1 the experimental data have been
fitted to the theoretical expression for the isotropic spin-
Hamiltonian H = Ϫ2J ؒ S ؒ S2 (with S = S = 1) including a
1
1
2
molar fraction p of uncoupled paramagnetic impurity [equation
14
(
1)] where N refers to the temperature-independent para-
α
χm = [χm,dim(1 Ϫ p)] ϩ (2χm,mono p) ϩ 2Nα
(1)
Ϫ6
3
Ϫ1
1
magnetism [100 × 10 cm mol per nickel() ion ], χ
=
m,dim
2
2
(
Ng µ /kT)ؒ[2exp(2J/kT) ϩ 10exp(6J/kT)]/[1 ϩ 3exp(2J/kT) ϩ
exp(6J/kT)] and χm,mono = 2Ng µ /3kT, all other parameters
B
Fig. 3 View of the molecular structure of the tetranuclear unit of
complex 2a. Details as in Fig. 2
2
2
5
B
have their usual meaning. Although powder measurements are
not ideally suited for a thorough analysis of S = 1 dinuclear
systems, the intradimer exchange term J often proved to be the
14,15
dominant term in the spin-Hamiltonian.
Accordingly, the
experimental results for 1 are satisfactorily modelled despite
the neglect of both a zero-field splitting parameter D and inter-
dimer interactions zЈJЈ, if data only down to 10 K are included
in the fitting procedure. The best parameters obtained are
Ϫ1
J = Ϫ13.5 cm , g = 1.96 and p = 2.9% [agreement factor
calc
obs
2
obs
m
2
Ϫ4
R = Σ(χm Ϫ χm ) /Σ(χ ) = 3.1 × 10 ], the value of J thus
being in reasonable agreement with the exchange interactions
Ϫ1
observed in the range 8.1–13.1 cm for a series of related
dinuclear nickel() complexes with both a pyrazolato and a
Fig. 4 Part of the one-dimensional alternating chain structure of
complex 2b. Details as in Fig. 2
11
chloro bridge. The unusually low value of the g factor (being
even smaller if no paramagnetic impurity is included) is prob-
ably due to zero-field splitting effects which were not taken into
account.
(
trigonal bipyramidal). These findings seem surprising, as there
is no obvious geometric or steric distinction between the two
pyrazolate-bridged metal centres in a dinuclear framework like
To our knowledge no analytical expression for the magnetic
properties of a linear Ni complex with alternating bridges has
1
that would restrict a possible increase in co-ordination num-
4
hitherto been reported in the literature. In a preliminary
approach, we employed the dimer model [equation (1)] for an
analysis of the susceptibility data of 2a [Fig. 5(b)] to obtain a
ber to only one of them. Indeed, simply the use of CH Cl2
2
instead of CHCl as the solvent for crystallisation induces the
3
formation of polymeric one-dimensional alternating chains
built from bimetallic pyrazolate complexes that are linked via
dichloro bridges (2b). Compound 2b crystallises in space group
C2/c with one half CH Cl solvent molecule per nickel atom in
Ϫ1
good quality fit with J = Ϫ11.1 cm , g = 2.12 and p = 3.5%
Ϫ5
(
R = 5.5 × 10 ). However, these values have to be interpreted
with caution, as it cannot be assumed that the magnetic
superexchange propagated by the dichloro linkage between the
two bimetallic pyrazolate-based units is negligible, particularly
in view of the results obtained for 2b.
2
2
the crystal lattice. A part of the chain structure of 2b is shown
in Fig. 4, selected bond lengths and angles are listed in Table 4.
A comparison of 2a and 2b reveals that the change in co-
ordination number from five to six is accompanied by a signifi-
cant lengthening of the mean nickel–ligand atom distances
and an increase of the Ni ؒ ؒ ؒ Ni separation within the basic
pyrazolate-bridged skeleton [d(Ni ؒ ؒ ؒ Ni) = 3.924 (2a), 4.028 Å
As expected, attempts to use equation (1) in order to adjust
the experimental susceptibility data for compound 2b only gave
Ϫ1
Ϫ3
a poor fit (J = Ϫ10.9 cm , g = 2.01, p = 4.0%; R = 1.4 × 10 ).
Borrás-Almenar et al. recently reported analytical expressions
for J-alternating chain systems with local spin S = 1 derived
(
2b)]. The bond lengths Ni᎐Clbridge for the pyrazolate-based
N
units are found to be 2.445(2)/2.439(2) Å in 1 versus 2.525(1) Å
in 2b, and bond lengths close to these values are also observed
for the respective five-co-ordinate and six-co-ordinate nickel
ions in the tetranuclear species 2a, where the co-ordination
number asymmetry thus gives rise to considerably different
from the Hamiltonian H = Ϫ2J
(S ؒ S
ϩ αS ؒ S2iϪ1) for
͚
2i
2i ϩ 1 2i
i = 1
6a
both cases of either overall antiferromagnetic (AF/AF) or
alternating antiferromagnetic/ferromagnetic (AF/F) coup-
ling, where the alternance parameter α relates the two nearest
6b
J. Chem. Soc., Dalton Trans., 1998, Pages 1181–1186
1183

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to be considered. By applying the expression for the AF/AF
6a
case a good quality fit with optimised coupling parameters
Ϫ1
Ϫ1
Ϫ4
J = Ϫ9.1 cm , g = 2.11 and αJ = Ϫ7.7 cm (R = 1.9 × 10 ) is
obtained [solid line in Fig. 5(c)]. In view of the results men-
Ϫ1
tioned above we assign the larger value of J = Ϫ9.1 cm to the
coupling within the basic pyrazolate-bridged unit. The obvious
decrease of the magnitude of the magnetic exchange within the
pyrazolate-based dinuclear core when going from five-co-
Ϫ1
ordinate (1: J = Ϫ13.5 cm ) to mixed five/six-co-ordinate (2a:
Ϫ1
J = 11.1 cm ) and overall six-co-ordinate nickel() ions (2b:
Ϫ1
J = Ϫ9.1 cm ) is in accord with the observed trend for other
µ-chloro-µ-pyrazolato dinickel systems with differing co-
11
ordination numbers and might be related to a more efficient
orbital overlap for the five-co-ordinate species caused by the
slightly shorter Ni᎐Npyrazolate and Ni᎐Clbridge bond lengths (see
above). Previous investigations on di-µ-chloro-bridged nickel()
16,17
dimers
have revealed antiferromagnetic behaviour in the
case of five-co-ordinate SPY-5 metal ions, but ferromagnetic
superexchange for TBPY-5 and six-co-ordinate metal ions.
Attempts to model the experimental data for 2b by the AF/F
Ϫ1
expression with optimised parameters J = Ϫ12.6 cm , g = 1.96
Ϫ1
and αJ = ϩ9.1 cm however led to a clearly worse fit [R =
Ϫ4
6
.4 × 10 ; dashed line in Fig. 5(c)], thus suggesting a negative
value for both J and αJ in the present case.
This latter choice is further corroborated when considering
the above-mentioned trend in the magnitude of |J|, i.e. its
expected decrease with increasing Ni᎐Cl bridge bond lengths
within the pyrazolate-bridged dinickel() units. Additional
support for the AF/AF situation stems from a ligand-field
analysis of the solid-state UV/VIS data of 2b with regard to the
significantly different g values obtained from the two fitting
Ϫ1
procedures. A value ∆oct ≈ 8370 cm can be deduced from the
ν absorption band (Table 1), and a calculated Racah parameter
1
Ϫ1
B ≈ 863 cm then results from the consideration of an octa-
Ϫ1
hedral strong-field coupling scheme. Taking 15B = 15 615 cm
2ϩ
3
18
for the gaseous ion Ni ( P), this leads to a nephelauxetic
19
2
ratio β of 0.829. From the expression λ = Ϫ0.27 ؒB /Dq we
Ϫ1
calculated the spin-orbit coupling constant λ to be Ϫ240 cm ,
which implies a reasonable reduction of 28% compared to the
Ϫ1 20
free ion value of Ϫ335 cm . Using the relationship g = 2 Ϫ
8
λ/10Dq a g value of 2.23 can thus be derived from the UV/VIS
data, which is in reasonable agreement with the analysis of the
magnetic measurements only if one assumes AF/AF behaviour.
This unexpected result of overall AF/AF superexchange
may be rationalised on the basis of the specific geometric
conditions for the di-µ-chloro-linkage in 2b as revealed by
the crystallographic analysis. As J results from the sum of
an antiferromagnetic contribution JAF mainly dependent on
the overlap integral between two magnetic orbitals centred
on each metal ion, and a ferromagnetic contribution JF
depending on the two-electron exchange integral, Kahn and
17
co-workers pointed out that for angles Ni᎐Cl᎐Ni close to
0Њ the overlap integral becomes very small and JAF is
9
therefore expected to be weak whatever the Ni᎐Cl bond
length may be. Accordingly, the bridging angles for the ferro-
magnetically coupled OC-6 dinickel() complexes reported
Fig. 5 Temperature-dependence of the magnetic susceptibilities for
complexes (a) 1; (b) 2a (per dinuclear unit); (c) 2b (per nickel ion). The
solid line in each case represents the calculated curve. For complex 2b
16,17
in the literature were generally found in the range 92–97Њ.
I
In contrast
a
considerably larger Ni᎐Cl(2)᎐Ni angle
6
the curves are calculated using the alternating chain equations with
[
101.37(4)Њ] is observed for 2b, indicating that in the present
AF/AF coupling (——) and AF/F coupling (– – –) (see text)
case JAF might not vanish but play a dominant role and
consequently lead to observable antiferromagnetic exchange
coupling propagated via the di-µ-chloro-linkage. It should
however be emphasised that additional factors, e.g. the pro-
neighbour exchange interactions and 2N is the number of
interacting spins. Considering the results for 1 and 2a as well as
for the related dinickel() complexes described recently, in par-
ticular for a µ-chloro-µ-pyrazolato dinickel() compound with
nounced asymmetry in the NiCl Ni framework of 2b {d[Ni᎐
2
I
Cl(2)] = 2.392(1), d [Ni᎐Cl(2 )] = 2.592(1) Å} and the severe
11
both metal ions in a distorted octahedral surrounding, it can
be assumed that antiferromagnetic coupling occurs within the
pyrazolate-based dimeric cores. However, the type of magnetic
exchange propagated via the dichloro bridges in 2b is not
known a priori, and thus both AF/AF and AF/F behaviour has
distortion of the octahedral co-ordination sphere around the
nickel() ions may also contribute to the specific magnetic
properties of 2b, and further work is definitely needed for a
conclusive understanding of this type of alternating chain
compound.
1
184
J. Chem. Soc., Dalton Trans., 1998, Pages 1181–1186

View Article Online
Table 5 Crystal data and refinement details for complexes 1, 2a and 2b
1
2a
2b
Formula
Mr
C H Cl N Ni ؒ2CHCl
786.0
C H Cl N Ni ؒ2CHCl
757.9
C H Cl N Ni ؒCH Cl
2
604.1
1
7
35
3
6
2
3
15 31
3
6
2
3
15 31
3
6
2
2
Crystal size/mm
Crystal system
Space group
0.40 × 0.30 × 0.30
Orthorhombic
Pbca
0.40 × 0.30 × 0.20
Triclinic
P1
0.30 × 0.30 × 0.30
Monoclinic
C2/c
¯
a/Å
b/Å
c/Å
α/Њ
β/Њ
γ/Њ
U/Å
ρcalc/g cm
Z
19.308(2)
13.031(2)
25.771(2)
11.038(4)
11.857(3)
12.847(3)
70.30(1)
77.64(2)
86.96(3)
1546(1)
1.624
17.950(3)
13.374(2)
13.277(2)
130.70(1)
3
6484(1)
1.610
8
2416(1)
1.661
4
Ϫ3
1
F(000)
T/K
µ(Mo-Kα)/mm
Scan mode
3216
200
1.925
ω
768
200
2.015
ω
1248
200
2.127
ω
Ϫ1
hkl Ranges
0–22, 0–15, 0–30
3.8–50.0
5634
0–13, ±13, Ϫ14 to 15
3.6–50.0
5606
Ϫ18 to 23, Ϫ9 to 17, Ϫ17 to 13
4.2–50.5
2262
2
θ Range/Њ
Measured reflections
Observed reflections [I > 2σ(I)]
Refined parameters
3535
344
4429
351
1966
141
Maximum, minimum residual
electron density/e Å
Ϫ3
0.732, Ϫ0.809
0.059
0.140
0.715, 0.801
0.037
0.087
1.361, Ϫ1.267
0.044
0.131
R1
2
wR2 (refinement on F )
Goodness of fit
1.027
1.038
2.455
Conclusion
Syntheses
2
Bimetallic entities composed of dinucleating ligand systems
that leave potentially accessible co-ordination sites at both co-
ordinated metal centres are valuable building blocks for the
formation of higher nuclearity complexes with alternating
bridges. The extent of aggregation of the basic dinuclear units
is varied by a fine-tuning of the primary ligand core and by the
choice of appropriate conditions for crystallisation, yielding
either a dinuclear, tetranuclear or infinite one-dimensional
alternating chain compound of nickel() from the set of
pyrazolate-based ligands employed in the present work. Tem-
perature-dependent magnetic data reveal antiferromagnetic
coupling between the nickel() centres for all the µ-chloro-µ-
pyrazolato cores as well as for the di-µ-chloro-linkage of the
alternating chain compound 2b. The latter result, which con-
trasts the usual ferromagnetic superexchange in di-µ-chloro-
bridged six-co-ordinate nickel(), may stem from the specific
geometric conditions observed in the solid-state structure of 2b,
in particular the unusually large bridging angle Ni᎐Cl᎐Ni. We
plan to study further examples of these new types of J-
alternating polynuclear complexes in order to enable a more
conclusive understanding of their properties.
HL . Pyrazole-3,5-dicarboxylic acid monohydrate (2.7 g, 15.5
mmol) was converted into 3,5-bis(chloroformyl)pyrazole by
12
means of SOCl
.
2
This was taken up in thf (150 ml) and a
solution of 1-(dimethylamino)-2-(methylamino)ethane (5.8 ml,
45.6 mmol) and ethyl(diisopropyl)amine (8.0 ml, 45.9 mmol) in
thf (80 ml) was added dropwise with stirring. The reaction mix-
ture was left at room temperature overnight, then filtered and
evaporated to dryness. The resulting crude oil was redissolved
in thf (40 ml) and filtered again. After removal of all volatile
material under vacuum 4.4 g (88%) of the bis(amide) remained
as a yellowish oil, δ
CH ), 3.05, 3.27 (4 H, 2s, br, E-/Z-CONCH
CONCH
37.8, 45.7, 45.9, 46.3 (CH
(pyrazole C ), 142.5 (br, pyrazole C ) and 163.5 (br, CO). A
solution of the bis(amide) (4.3 g, 13.3 mmol) in thf (80 ml) was
(CDCl
) 2.27 (12 H, s, CH
), 3.67 (4 H, m,
) and 6.98 (1 H, s, pyrazole H ); δ (CDCl ) 34.7,
), 46.9, 49.7, 56.7, 57.7 (CH ), 110.4
), 2.56 (4 H, m,
H
3
3
2
3
4
2
C
3
3
2
4
3/5
added dropwise to a stirred suspension of LiAlH (2.0 g, 52.7
4
mmol) in thf (200 ml). After the addition was completed the
reaction mixture was heated at reflux for 5 h, then stirred at
room temperature overnight and finally, while cooling to 0 ЊC, it
was carefully quenched with water (10 ml). All inorganic pre-
cipitates were separated by filtration and washed twice with thf
(
50 ml). The combined filtrates were evaporated to dryness and
Experimental
the resulting crude oil was redissolved in light petroleum (b.p.
All manipulations were carried out under an atmosphere of dry
nitrogen by employing standard Schlenk techniques. Solvents
were dried according to established procedures. The compound
40–60 ЊC) (50 ml) and filtered again. Evaporation of all volatile
material under vacuum afforded 3.0 g (76%) of the product as a
Ϫ1
slightly yellow oil, ν
˜
max/cm 3189m (br), 2944–2769s, 1575w,
1
9
HL was synthesised according to the reported method.
1
463s, 1123m and 1030s (film); δ (CDCl ) 2.25 (12 H, s, CH ),
H 3 3
Microanalyses: Mikroanalytische Laboratorien des Organisch-
Chemischen Instituts der Universität Heidelberg; IR spectra:
Bruker IFS 66 FTIR; FAB- and EI-MS spectra: Finnigan
MAT 8230; UV/VIS/NIR spectra: Perkin-Elmer Lambda 19;
magnetic measurements: Quantum Design SQUID magneto-
meter MPMS-5S; the sample device has been described
2.30 (6 H, s, CH
3
), 2.48 (8 H, m, CH ), 3.62 (4 H, s, CH )
2 2
and 6.70 (s, 1 H, CH); δC(
CDCl ) 43.2 (CH ), 46.0 (CH ), 54.4
3
3
3
4
(
CH ), 55.0 (CH ), 57.8 (CH ), 104.3 (pyrazole C ) and 146.2
2
2
2
ϩ
3/5
ϩ
(
pyrazole C ); m/z 296 (M , 14%), 238 (M Ϫ CH NMe ,
2
2
2ϩ
2
ϩ
8
1
0), 195 [M Ϫ MeN(CH ) NMe , 98] and 58 (CH NMe
,
2
2
2
2
00).
21
recently. All susceptibility measurements were performed at 1,
Ϫ4
1
1
0 and 20 kG (1 G = 10 T). The field-dependence of the data
Complex 1. A solution of HL (0.26 g, 0.80 mmol) in EtOH
(20 ml) was treated with NiCl ؒ6H O (0.38 g, 1.60 mmol) and
was found to be small, and the 10 kG data were used for further
analyses.
2
2
ethyl(diisopropyl)amine (0.16 ml, 0.92 mmol) and the resulting
J. Chem. Soc., Dalton Trans., 1998, Pages 1181–1186
1185

View Article Online
light green solution was stirred at room temperature for 2 h
while a precipitate formed. The volume of the reaction mixture
was then reduced to ≈10 ml under vacuum and stirred for a
further 5 h at 0 ЊC. Finally the solid was filtered off, washed
with cold EtOH and dried under vacuum to yield 0.25 g (0.46
References
1
2
O. Kahn, Molecular Magnetism, VCH, Weinheim, 1993.
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3 E. Coronado, M. Drillon, A. Fuertes, D. Beltran, A. Mosset and
J. Galy, J. Am. Chem. Soc., 1986, 108, 900; A. Escuer, R. Vicente
and X. Solans, J. Chem. Soc., Dalton Trans., 1997, 531.
1
mmol, 57%) of light green Ni L Cl . Crystals of 1 were
2
3
obtained by layering a solution of the complex in CHCl with
3
4
R. Vicente, A. Escuer, J. Ribas and X. Solans, Inorg. Chem., 1992,
1, 1726; A. Escuer, R. Vicente, M. S. El Fallah, J. Ribas, X. Solans
light petroleum (Found: C, 37.45; H, 6.55; Cl, 19.56; N, 15.46.
C H Cl N Ni requires C, 37.31; H, 6.45; Cl, 19.43; N,
3
1
7
35
3
6
2
and M. Font-Bardia, J. Chem. Soc., Dalton Trans., 1993, 2975;
A. Escuer, R. Vicente, J. Ribas, M. S. El Fallah, X. Solans and
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Ϫ1
1
1
1
5.36%); ν˜ /cm 2974w, 2905, 2862m, 1514w, 1469s, 1312m,
199m, 1033s, 976m, 826s, 762s and 506w; m/z 511 (M Ϫ Cl,
ϩ
00%).
2
Complexes 2a and 2b. A solution of HL (0.17 g, 0.58 mmol)
in EtOH (20 ml) was treated with NiCl ؒ6H O (0.27 g, 1.14
2
2
mmol) and ethyl(diisopropyl)amine (0.11 ml, 0.63 mmol) and
the resulting orange-yellow solution was stirred at room tem-
perature for 3 h. The volume of the reaction mixture was then
reduced to ≈7 ml under vacuum and cooled to Ϫ20 ЊC, causing
the precipitation of a green-yellow solid. This was filtered off,
7
A. Escuer, R. Vicente, X. Solans and M. Font-Bardia, Inorg.
Chem., 1994, 33, 6007.
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washed with cold EtOH and dried under vacuum to yield 0.15 g
8
2
(
0.29 mmol, 50%) of Ni L Cl . Orange-yellow crystals of 2a
2
3
were obtained by layering a solution of the complex in CHCl3
with light petroleum (Found: C, 29.42; H, 4.95; Cl, 33.70; N,
9
F. Meyer, S. Beyreuther, K. Heinze and L. Zsolnai, Chem. Ber./
Recl., 1997, 130, 605.
1
3
1
4
2.61. C H Cl N Ni ؒCHCl requires C, 30.09; H, 5.05; Cl,
15 31 3 6 2 3
Ϫ1
10 F. Meyer, K. Heinze, B. Nuber and L. Zsolnai, J. Chem. Soc., Dalton
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11 M. Konrad, F. Meyer, K. Heinze and L. Zsolnai, J. Chem. Soc.,
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3.31; N, 13.16%); ν
˜ /cm 2980m, 2887m, 1516w, 1460s, 1330m,
029m, 1007m, 963m, 810m, 749s, 664m, 603w, 488m and
2
ϩ
2
63w; m/z 483 (Ni L Cl , 100%). Layering a solution of the
2
crude complex in CH Cl with light petroleum afforded green
2
2
1
2 C. Acerete, J. M. Bueno, L. Campayo, P. Navarro and M. I.
Rodriguez-Franco, Tetrahedron, 1994, 50, 4765.
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C H Cl N Ni ؒCH Cl requires C, 31.81; H, 5.51; Cl, 29.34;
1
5
31
3
6
2
2
2
Ϫ1
13 (a) M. Ciampolini, N. Nardi and G. P. Speroni, Coord. Chem. Rev.,
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029m, 1007m, 963m, 809m, 774m, 728s, 698m, 600w, 487w
and 461w.
˜ /cm 2967w, 2881m, 1507w, 1454s, 1328s, 1263m,
1
966, 1, 222; (b) C. Furlani, Coord. Chem. Rev., 1968, 3, 141;
1
(
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1
1
4 C. J. O’Connor, Prog. Inorg. Chem., 1982, 29, 203.
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Structure determinations. The measurements were carried
out on a Siemens (Nicolet Syntex) R3m/v four-circle diffract-
ometer with graphite-monochromated Mo-Kα (λ = 0.710 73 Å)
radiation. All calculations were performed with a micro-VAX
computer using the SHELXTL PLUS software package.
Structures were solved by direct methods with the SHELXS 86
and refined with the SHELX 93 programs. An absorption
correction (ψ scan, ∆ψ = 10Њ) was applied to all data. Atomic
coordinates and anisotropic thermal parameters of the non-
hydrogen atoms were refined by full-matrix least-squares calcu-
lations. The hydrogen atoms were placed at calculated positions
and constrained to ride on the atoms they were attached to.
Table 5 compiles the data for the structure determinations.
CCDC reference number 186/886.
1
22
1
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22
1
9, 170; C. P. Landee and R. D. Willett, Inorg. Chem., 1981, 20,
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1
7 I. Bkouche-Waksman, Y. Journaux and O. Kahn, Transition Met.
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1
2
2
See http://www.rsc.org/suppdata/dt/1998/1181/ for crystallo-
graphic files in .cif format.
Acknowledgements
2
46, 139.
2
2 G. M. Sheldrick, SHELXTL PLUS, Program Package for Structure
Solution and Refinement, Siemens Analytical Instruments,
Madison, WI, 1990; SHELX 93, Program for Crystal Structure
Refinement, Universität Göttingen, 1993; SHELXS 86, Program for
Crystal Structure Solution, Universität Göttingen, 1986.
We are grateful to Professor Dr. G. Huttner for his generous and
continuous support of our work as well as to the Deutsche
Forschungsgemeinschaft (Habilitandenstipendium for F. M.)
and the Fonds der Chemischen Industrie for financial support.
Professor Dr. H. Lueken and the DFG are sincerely thanked for
allowing the use of the SQUID magnetometer.
Received 27th November 1997; Paper 7/08565F
1
186
J. Chem. Soc., Dalton Trans., 1998, Pages 1181–1186