Nickel(II) N2X2 Schiff-base complexes incorporating pyrazole (X = NH, O or S): Syntheses and characterization

Article abstract of DOI:10.1039/a703258g

A series of four-co-ordinate nickel(II) N₂X₂ complexes (X = NH, O or S) with tetradentate Schiff-base ligands incorporating pyrazole has been prepared in order to compare the influence on the nickel(II) centre of the donor atoms in N₂(NH)₂ and N₂S₂ complexes versus N₂O₂ complexes. In each case, two identical parts of the ligand are linked by a two- or three-carbon aliphatic chain (n = 2 or 3). Crystal structures have been determined for a n = 3 N₂(NH)₂ complex, for two n = 3 N₂S₂ complexes and for a n = 2 N₂O₂ complex. All of the complexes have been investigated in solution by spectroscopic methods (UV/VIS/NIR, ¹H NMR). The n = 2 and 3 N₂(NH)₂ complexes and the n = 2 N₂S_i complexes are fully diamagnetic. An extension of the chain linking the two halves of the ligand from two to three carbon atoms induces a spin-equilibrium process (S = 0 ? S = 1) for the resulting N₂S₂ complex (ΔG = 19-26 kJ mol^-1 at 50°C). Ligand-field parameters have been derived from the electronic spectra and the electrochemical properties of the N₂S₂ complexes have been investigated by cyclic voltammetry. From these results it is clear that low-spin character is emphasized in N₂(NH)₂ and N₂S₂ complexes of nickel(II) when compared with the corresponding N₂O₂ complexes.

Full text of DOI:10.1039/a703258g

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Nickel(II) N2X2 Schiff-base complexes incorporating pyrazole
(X ؍ NH, O or S): syntheses and characterization†
Bibhutosh Adhikhari,a Oren P. Anderson,*,b Agnete la Cour,*,a,b Rita Hazell,c Susie M. Miller,b
Carl E. Olsend and Hans Toftlunda
a Department of Chemistry, Odense University, DK-5230 Odense M, Denmark
b Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523, USA
c Department of Chemistry, Aarhus University, DK-8000 Århus C, Denmark
d Department of Chemistry, The Royal Veterinary and Agricultural University,
DK-1871 Frederiksberg C, Denmark
A series of four-co-ordinate nickel() N2X2 complexes (X = NH, O or S) with tetradentate Schiff-base ligands
incorporating pyrazole has been prepared in order to compare the influence on the nickel() centre of the donor
atoms in N2(NH)2 and N2S2 complexes versus N2O2 complexes. In each case, two identical parts of the ligand are
linked by a two- or three-carbon aliphatic chain (n = 2 or 3). Crystal structures have been determined for a n = 3
N2(NH)2 complex, for two n = 3 N2S2 complexes and for a n = 2 N2O2 complex. All of the complexes have been
investigated in solution by spectroscopic methods (UV/VIS/NIR, 1H NMR). The n = 2 and 3 N2(NH)2 complexes
and the n = 2 N2S2 complexes are fully diamagnetic. An extension of the chain linking the two halves of the ligand
from two to three carbon atoms induces a spin-equilibrium process (S = 0
S = 1) for the resulting N2S2
complex (∆G = 19–26 kJ molϪ1 at 50 ЊC). Ligand-field parameters have been derived from the electronic spectra
and the electrochemical properties of the N2S2 complexes have been investigated by cyclic voltammetry. From
these results it is clear that low-spin character is emphasized in N2(NH)2 and N2S2 complexes of nickel(II) when
compared with the corresponding N2O2 complexes.
It is well documented that most bis(bidentate ligand)nickel()
N2X2 Schiff-base complexes incorporating pyrazole1 exhibit
pseudo-tetrahedral co-ordination (X = NH,1a,b O1c,e or S1b,f,g).
In contrast, bis(bidentate ligand)nickel() N2X2 Schiff-base
complexes based on salicylaldehydes, hydroxynaphthaldehydes
or β-diketones generally exhibit planar co-ordination spheres
and low-spin (S = 0) magnetic behavior2 unless forced into a
pseudo-tetrahedral configuration with high-spin (S = 1) or
NH, n = 2–106d–f) and incorporating aromatic carbon rings are
all diamagnetic. Some results are available for tetradentate lig-
and nickel() N2X2 complexes based on thiosalicylaldehydes,
thioacetylacetone and other thio-β-diketones, and diamines
(n = 2–4).3e,7,8a The influence of sulfur seems to be similar to
thatestablished in the bis(bidentate ligand) complexes. Thetetra-
dentate ligand N2O2 salicylaldimine complex in which the link-
ing unit is 2,5-dimethylhexane (n = 4) is fully paramagnetic,
whereas the corresponding N2S2 complex is diamagnetic.8a
Furthermore, tetradentate ligand N2O2 salicylaldimine com-
plexes in which the linking unit is biphenyl8b,c exhibit spin
equilibria, while an N2S2 analogue is diamagnetic.8d
In contrast to these systems that incorporate aromatic
carbon rings the biphenyl-bridged nickel() N2S2 complexes
incorporating pyrazole rings exhibit spin equilibria.8d Introduc-
tion of aromatic heterocycles as a component of Schiff-base
chemistry1g has improved the possibilities of studying the
influence of ligand modifications on the properties of metal
complexes. For example, some of us have reported the proper-
ties of a series of tetradentate ligand nickel() N2O2 ketoimine
and aldimine Schiff-base complexes incorporating pyrazole in
which related halves of the ligands were linked by aliphatic
carbon chains (n = 2 or 3).9a In another study9b one of us
investigated a related series of N2S2 ketoimine complexes. In the
present paper we report the preparation and properties of
tetradentate ligand nickel() N2(NH)2, N2S2, and N2O2 com-
plexes with aldimine Schiff-base ligands based on pyrazole
(n = 2 or 3; see structure A). We do so in order to allow com-
parisons with the N2O2 aldimine9a and N2S2 ketoimine9b
analogues, and with the corresponding N2X2 Schiff-base
complexes (n = 2 or 3) incorporating unsaturated carbon rings,
β-thioketoamines or β-iminoketoamines.
spin-equilibrium (S = 0
S = 1) magnetic behavior by bulky
substituents on the nitrogen donor atoms.2b,3a–c
Bis(bidentate ligand)nickel() N2S2 Schiff-base complexes,
including the complexes that contain pyrazole, tend to be stabil-
1e,3a
ized more in the low-spin state1b,3a,d,e,4a than the N2O2
N2(NH)2
and
1b,2b,3b
analogues. This may be ascribed to the weak
S ؒ ؒ ؒ S bonding normally observed4a,b in the thiolate complexes,
which results in the formation of a ligand more like a tetra-
dentate ligand in bis(bidentate ligand) complexes and a more
planar co-ordination geometry. The tendency of sulfur atoms
to bond weakly to nearby sulfur atoms is also seen in metal-free
systems,4c–e and in a complex the metal may bring about the
proximity of the sulfur atoms necessary for the development of
such a weak S ؒ ؒ ؒ S bond. The S ؒ ؒ ؒ S bonding forces the bis-
(bidentate ligand) N2S2 complexes to be cis1f,g,4a,b (the angle
between the N᎐M᎐S and S᎐M᎐N planes, θ <90Њ). Bis(bidentate
ligand) N2O2 and N2(NH)2 complexes seem to prefer the trans
configuration1b,2c,5 (θ > 90Њ) due to the steric requirements of
substituents on the imine nitrogen donor atoms.
Comparative studies of the influence of the donor atom X on
the spin state of NiII in tetradentate ligand N2X2 Schiff-base
complexes have been almost unavailable since a tetradentate
ligand tends to stabilize the planar low-spin form irrespective of
X. Tetradentate ligand nickel() N2X2 complexes bridged by
unsubstituted aliphatic carbon chains (X = O, n = 2–12;6a–c X =
Results and Discussion
† Supplementary data available (No. SUP 57293, 8 pp.): NMR data,
thermodynamic parameters, cyclovoltammetric data. See J. Chem. Soc.,
Dalton Trans., 1997, Issue 1.
Syntheses and identification
Yields and some analytical data are given in Table 1 (NMR data
J. Chem. Soc., Dalton Trans., 1997, Pages 4539–4548
4539

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R1
N
R1
N
Table 1 Yields and analytical data for the complexes
X
N
X
N
N
N
Analyses (%)b
Yielda
EI mass
spectrum
m/zc
Ni
R2
R2
R3
Complex
(%)
C
H
N
A
1d
87
62.25
4.45
16.45
620
648
482
496
524
496
654
682
392
406
434
516
544
530
558
516
530
558
(62.60)
67.10
(67.50)
59.65
(59.65)
59.95
(59.85)
60.80
(61.20)
57.60
(58.15)
63.20
(63.20)
65.00
(64.40)
42.50
(42.75)
44.35
(44.25)
45.65
(45.50)
55.75
(55.70)
56.75
(57.25)
49.85
(49.80)
57.85
(57.95)
54.55
(54.10)
56.45
(4.50)
5.35
(5.35)
5.05
(5.00)
5.30
(5.30)
5.75
(5.80)
5.45
(5.75)
4.30
(4.25)
4.90
(4.70)
4.60
(4.60)
5.10
(4.95)
5.55
(5.75)
4.30
(4.30)
4.85
(4.80)
4.05
(4.00)
5.05
(5.05)
4.35
(4.20)
4.65
(16.45)
16.95
(17.00)
23.10
(23.20)
22.15
(22.35)
21.00
(21.15)
21.45
(21.00)
12.50
(12.60)
11.90
(12.15)
21.20
(21.40)
20.35
(20.65)
18.60
(18.75)
16.35
(16.25)
14.95
(15.40)
13.40
(13.55)
14.85
(15.00)
15.35
(15.60)
15.70
n
X
R1
R2
R3
-(CH2)3-
2
3d
4
20
28
88
88
85
1a
2
3a
4a
5
3
3
2
3
3
3
3
3
2
3
3
2
3
3
3
2
3
3
2
NH
NH
NH
NH
NH
NH
S
Ph
Ph
Ph
Ph
Ph
Me
Ph
Ph
Me
Me
Me
Ph
Ph
Ph
Ph
Me
Me
Me
Ph
Ph
Ph
Me
Me
Me
Ph
Ph
Ph
Me
Me
Me
Me
Me
Me
Me
Ph
Ph
Ph
Me
-CHMeCH2CHMe-
-(CH2)2-
-(CH2)3-
-CHMeCH2CHMe-
-(CH2)3-
5
6
7
-(CH2)3-
6e
7f
8
S
-CHMeCH2CHMe-
-(CH2)2-
9
S
79
(59)
83
(28)
89
(49)
90
(37)
51
(46)
88
(55)
55
(22)
89
(53)
85
10
S
-(CH2)3-
8f
11
12
13
14
15
16
17
18
19b
S
-CHMeCH2CHMe-
-(CH2)2-
S
9d
10
11
12d
13
14d
15
16f
17
18
S
-CHMeCH2CHMe-
-(CH2)3-
S
S
-CHMeCH2CHMe-
-(CH2)2-
S
S
-(CH2)3-
S
-CHMeCH2CHMe-
-CHMeCH2-
O
a First presented in ref. 10. b First presented in ref. 9(a).
are available as SUP 57293).‡ Crystals of the complexes tend to
incorporate the solvent used for recrystallization, as has been
observed for other MIIN2X2 Schiff-base complexes containing
1,3-disubstituted pyrazolyl rings (n = 3).9–11,12a Even after drying
the complexes under vacuum, the results of elemental analyses
can reflect small amounts of occluded solvent (see Experi-
mental section and Table 1). Impurities were not observed in
the proton NMR spectra of the complexes. Moreover, the
(n = 3) N2(NH)2 complexes reported herein take up small
amounts of water (according to microanalysis) as also seen in
similar n = 4 N2(NH)2 complexes.1b
For the complexes bridged by dimethyl-substituted chains,
the racemic mixture of the least-soluble diastereomer (∆SS/
ΛRR, where ∆ and Λ refer to the absolute configuration of the
co-ordination sphere, R and S to the configurations about the
chiral centers in the dimethyl-substituted aliphatic chain) could
in general be separated from other diastereomers by fractional
crystallization. For example, a crystal from the first precipitate
of complex 5 contained the ∆SS and ΛRR forms as established
by X-ray diffraction (see below). These forms have C2 sym-
metry, and the 1H NMR spectra of the solutions obtained
by redissolving the first precipitates of complexes 2, 5, 11, 13, 15
and 18 are simple, with only one resonance observed for each
kind of proton. Subsequent precipitates contain mixtures of
diastereomers in every case. Complex 8 was isolated only as a
mixture of diastereomers.
(50)
77
(6)
90
(59)
54
(56.50)
58.10
(57.95)
(4.55)
5.10
(5.05)
(15.80)
14.85
(15.00)
(23)
Data for complex 19 were presented in ref. 9(a). a From ligand. Yields
of pro-ligands in parentheses. b Calculated values in parentheses.
c Crystal solvent, CHCl3, found in complexes 1 (0.5), 7 (0.15) and 14
(0.75); CH2Cl2, in 6 (0.1), 8 (0.1) and 16 (0.25); or water, in 2 (0.5), 4
(0.25), 5 (0.25) and 6 (1.0), is not included in the molecular ion, Mϩ.
d Recrystallized from EtOH–CHCl3. e Recrystallized from MeOH–
CH2Cl2. f Recrystallized from EtOH–CH2Cl2.
(also seen in the electronic spectra of similar n = 4 N2S2 com-
plexes1b,11). Crystals of 10 grown from a dmso solution contain
only four-co-ordinate complexes (see below) but this may
only indicate that the four-co-ordinate form crystallizes more
readily. The low Lewis acidity of four-co-ordinate N2(NH)2 and
N2S2 Schiff-base complexes of NiII compared with similar N2O2
complexes is also observed for other families of N2X2 com-
plexes with conjugated chelate rings.3e,6d–f
Structures of complexes 5, 10, 14 and 19
Selected bond distances and angles are listed in Table 2. The
molecular structure and the asymmetric unit of complex 5 are
shown in Fig. 1(a) and 1(b). The molecular structure of 10 is
shown in Fig. 2, the asymmetric unit of 14 in Fig. 3, and the
molecular structure and a packing diagram of 19 are seen in
Fig. 4(a) and 4(b).
In complexes 5 and 14 the asymmetric unit contains two
nickel() complexes, the ∆ and Λ forms respectively. In the
asymmetric unit of the centrosymmetric structure of 5 the ∆
form of the complex is found to have the S configuration, the Λ
form to have the R configuration at both chiral carbon atoms
[C(23) and C(25)]. Crystals of both complexes diffract poorly.
The molecules are bulky and difficult to pack economically, and
there are no strong intermolecular forces. The distance between
the nickel atoms of the asymmetric unit in complex 14 is
Proton NMR spectra in (CD3)2SO and electronic spectra
in Me2SO (dmso), dimethylformamide (dmf)10 and MeCN
suggest that the N2(NH)2 complexes do not increase their co-
ordination number in potentially co-ordinating solvents, in con-
trast with the corresponding N2O2 complexes.9a The electronic
spectrum of the n = 3 N2S2 complex 10 in dmso shows two weak
transitions at ≈1500 nm that increase with time, which suggests
that five- and four-co-ordinate complexes may be in equilibrium
‡ Complexes 3 and 4, along with the corresponding n = 4 complex, were
presented in ref. 10 as pyrazolyl 1-methyl-3-phenyl substituted; they
should have been 3-methyl-1-phenyl substituted. The entropy change
for the spin-equilibrium process for the n = 4 complex diphenyl pyra-
zolyl substituted in the same manner as in 1 was given in ref. 10 as 6 J
K
Ϫ1 mol; the correct value is 25 J KϪ1 molϪ1
.
4540
J. Chem. Soc., Dalton Trans., 1997, Pages 4539–4548

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Fig. 3 Asymmetric unit for complex 14 with 50% probability thermal
ellipsoids for non-hydrogen atoms. The numbering is as for complex 5
[Fig. 1(a)] except that the methyl substituents of the chain [C(26) and
C(27)] are omitted
Fig. 1 Molecular structure (a) and asymmetric unit (b) for complex 5
with 50% probability thermal ellipsoids for non-hydrogen atoms
Fig. 4 (a) Molecular structure of complex 19 with 50% probability
thermal ellipsoids for non-hydrogen atoms; one component of the dis-
ordered chain has been omitted for simplicity. (b) Packing diagram for
19; the intermolecular distances Ni ؒ ؒ ؒ O(8Ј) and Ni ؒ ؒ ؒ NiЈ are 3.372(3)
and 3.627(1) Å, respectively
Fig. 2 Molecular structure of complex 10 with 50% probability thermal
ellipsoids for non-hydrogen atoms
4.829(6) Å. Large atomic displacement parameters for C(23A)–
C(27A) in complex 5 are associated with the observed disorder
of the atoms connecting the two halves of the ligand; this dis-
order proved to be intractable to model.
The structures of complexes 5, 10 and 14 all show a tetra-
hedral twist from a planar co-ordination geometry. The angles θ
between the N᎐Ni᎐X planes are 17.8(5) and 15.7(6)Њ for the two
independent complexes in the asymmetric unit of 5, 6.9(1)Њ for
10 and 10.2(6) and 7.2(6)Њ for the two independent complexes
in the asymmetric unit of 14 (see Table 2). The co-ordination
geometry is close to that previously established9a for an n = 3
nickel() N2O2 phenyl ketoimine complex (θ = 12.7Њ)9a with
phenyl substituents on pyrazolyl groups in the same pattern as
in 1 and 7. In contrast, the n = 3 nickel() N2X2 Schiff-base
complexes incorporating aromatic carbon rings (benzene,
naphthalene) tend to be more planar,7b,13a,b,d and the dis-
tortions from planarity tend to be folds rather than tetrahedral
twists.13a,b,d The structure of 19 reveals a planar co-ordination
geometry (see Table 2). The packing diagram for 19 [Fig. 4(b)]
shows that this complex tends to form dimers in the solid
state, as seen for many other N2O2 Schiff-base complexes of NiII
and CuII.13a–c The Ni᎐L bond lengths in the four complexes
are similar to those of other nickel() N2X2 Schiff-base com-
plexes that are low spin in the solid state (X = NH,10,13d O9a,13a–c
J. Chem. Soc., Dalton Trans., 1997, Pages 4539–4548
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Table 2 Selected interatomic distances (Å) and angles (Њ) for complexes 5, 10, 14 and 19*
5
5A
10
14
14A
19
Ni᎐N(1)
Ni᎐N(11)
Ni᎐X(8)
1.903(6)
1.890(7)
1.864(5)
1.872(5)
1.32(1)
1.903(8)
1.906(6)
1.877(6)
1.859(6)
1.32(1)
1.37(1)
1.38(1)
1.324(9)
1.32(1)
1.40(1)
1.410(9)
1.329(8)
1.928(2)
1.942(2)
2.2057(7)
2.1926(7)
1.297(3)
1.422(4)
1.389(4)
1.725(3)
1.298(3)
1.418(4)
1.398(4)
1.725(3)
1.97(3)
1.91(3)
2.21(1)
2.21(1)
1.31(3)
1.39(4)
1.38(4)
1.77(3)
1.29(3)
1.44(4)
1.42(5)
1.71(3)
1.88(3)
1.89(3)
2.18(1)
2.24(1)
1.26(3)
1.41(4)
1.42(4)
1.65(3)
1.29(3)
1.37(4)
1.44(4)
1.65(3)
1.865(4)
1.868(4)
1.887(3)
1.874(3)
1.298(6)
1.386(6)
1.400(6)
1.274(5)
1.298(6)
1.377(7)
1.399(6)
1.278(5)
Ni᎐X(18)
N(1)᎐C(2)
C(2)᎐C(3)
C(3)᎐C(7)
C(7)᎐X(8)
N(11)᎐C(12)
C(12)᎐C(13)
C(13)᎐C(17)
C(17)᎐X(18)
1.37(1)
1.400(9)
1.324(8)
1.32(1)
1.39(1)
1.400(9)
1.329(8)
X ؒ ؒ ؒ X
N(1) ؒ ؒ ؒ N(11)
2.550(7)
2.674(9)
2.521(8)
2.735(9)
2.851(1)
2.702(3)
2.88(1)
2.70(3)
2.88(1)
2.70(4)
2.528(6)
2.487(6)
N(1)᎐Ni᎐N(11)
N(1)᎐Ni᎐X(8)
N(1)᎐Ni᎐X(18)
N(11)᎐Ni᎐X(8)
N(11)᎐Ni᎐X(18)
X(8)᎐Ni᎐X(18)
Ni᎐N(1)᎐C(2)
N(1)᎐C(2)᎐C(3)
C(2)᎐C(3)᎐C(7)
C(3)᎐C(7)᎐X(8)
Ni᎐X(8)᎐C(7)
Ni᎐N(11)᎐C(12)
N(11)᎐C(12)᎐C(13)
C(12)᎐C(13)᎐C(17)
C(13)᎐C(17)᎐X(18)
Ni᎐X(18)᎐C(17)
90.0(3)
93.8(9)
168.8(3)
164.7(4)
93.1(3)
92.0(3)
92.0(3)
167.0(4)
168.8(3)
93.5(3)
88.55(9)
96.16(6)
173.94(7)
173.26(7)
94.91(6)
80.83(3)
129.8(2)
124.8(2)
125.5(2)
128.9(2)
104.19(9)
129.7(2)
124.4(2)
124.4(2)
129.1(2)
103.19(9)
88(1)
95.0(8)
171.8(7)
171.5(8)
96.5(9)
81.4(4)
129(2)
124(3)
127(3)
126(3)
105(1)
132(3)
123(4)
129(3)
125(3)
107(1)
92(1)
94.7(8)
173.4(8)
171.9(8)
93.0(8)
81.3(4)
135(3)
123(3)
124(3)
129(3)
107(1)
133(2)
124(3)
122(3)
132(2)
101(1)
85.2(2)
95.8(2)
178.4(2)
179.0(2)
96.2(2)
86.1(2)
84.9(3)
82.8(1)
126.2(5)
125.7(7)
122.1(6)
126.3(6)
125.9(5)
127.0(6)
124.6(7)
121.6(6)
126.3(6)
124.9(4)
125.6(7)
126.1(8)
121.1(7)
126.4(6)
125.4(5)
126.1(6)
125.3(7)
122.3(7)
124.7(7)
127.0(5)
127.9(3)
122.5(4)
122.4(4)
130.4(4)
120.6(7)
126.8(4)
123.4(4)
122.5(5)
130.4(5)
121.0(3)
Angles θ (Њ) between planes
N(1)᎐Ni᎐X(8)
X(18)᎐Ni᎐N(11)
17.8(5)
15.7(6)
6.9(1)
10.2(6)
7.2(6)
0.7(3)
Selected torsion angles (Њ)
N(5)᎐N(6)᎐C(30)᎐C(31)
N(15)᎐N(16)᎐C(40)᎐C(41)
151.3(6)
Ϫ30.0(8)
Ϫ149.3(7)
140.7(7)
—
—
41(2)
32(2)
43(2)
47(2)
18.7(6)
Ϫ178.8(4)
* Estimated standard deviations (e.s.d.s) of the least significant digits are given in parentheses. For complexes 5 and 14 parameters for both molecules
of the asymmetric unit (5/5A, and 14/14A) are shown. The symbol X denotes N (5/5A), S (10, 14/14A) or O (19).
or S7b–d,13e–g). It is seen from torsion angles for 5 and 14 (Table 2)
Table 3 Thermodynamic parameters for the spin-equilibrium process*
that the pyrazole rings are not coplanar with the phenyl sub-
stituents. In 19 the phenyl rings are almost coplanar with the
∆H
∆S
∆G (50 ЊC)
pyrazole rings as also seen in a similar n = 3 N2O2 complex.9a
Complexes 10 and 19 were both crystallized from the potential
donor solvent dmso. However, in spite of spectroscopic indi-
cations that the four-co-ordinate complexes are in equilibrium
with five- [10, this work; see also refs. 1(b) and 11 for similar
cases] or six-co-ordinate complexes [19, ref. 9(a)] in solution,
the four-co-ordinate complex crystallizes more readily.
Complex
/kJ molϪ1
/J KϪ1 molϪ1
/kJ molϪ1
7
10
11
14
15
17
18
30.0 7.4
22.6 0.0
22.3
24.7 0.8
21.8
23.2 0.1
22.0
13.7 17.9
9.36 0.86
16.9
4.92 2.27
7.49
2.43 2.97
8.79
25.6 1.6
19.6 0.3
16.8
23.1 0.1
19.4
22.4 1.1
19.2
* Measured in CDCl3; e.s.d.s are given only for complexes bridged by
unsubstituted propylene chains (see the text). Correlation coefficients
of the van’t Hoff plots were >0.99.
Magnetic properties
All complexes are low spin in the solid state as indicated by the
magnetic moments (µ = 0.52–1.14 µB at 23 ЊC; µB ≈ 9.27 × 10Ϫ24
J TϪ1). The weak paramagnetism observed is mainly due to
temperature-independent paramagnetism.8d
increasing temperature (protons Hc in particular). The coupling
constant Ac is negative, corresponding to more positive δc values
with temperature, for the n = 3 N2S2 complexes and for other
pyrazolyl-containing N2X2 complexes1b,10,13f,g (n = 4, X = NH or
S). The temperature dependence in these cases is therefore
ascribed to conformational changes and not to a spin-
equilibrium process. Complex 19 has been shown previously9a
to be fully diamagnetic in non-donor solvents. The temperature-
Temperature-dependent (25–50 ЊC) 1H NMR spectra for
representative complexes in CDCl3 reveal that the n = 3 N2S2
complexes (see Table 3) are in spin equilibrium (S = 0
S = 1) in solution, as were the related N2O2 complexes.9a The
chemical shifts of protons such as the chain proton, Ha, closest
to the nitrogen donor atom and of the imine proton, Hc, are
temperature dependent; the chemical shift becomes more posi-
tive due to an increase in the paramagnetic isotropic Fermi
contact shift with temperature. For the n = 2 N2S2 complexes
and for all N2(NH)2 complexes the shifts are practically tem-
perature independent or are at less positive δ values with
1
dependent H NMR spectra show that if n = 3 the nickel()
N2(NH)2 complexes are more stabilized in the low-spin state
than are both the nickel() N2O2 and N2S2 complexes.
The chemical shifts of protons Ha and Hc were used to evalu-
4542
J. Chem. Soc., Dalton Trans., 1997, Pages 4539–4548

View Article Online
ate equilibrium constants for the spin-equilibrium process for
the complexes listed in Table 3 by a method described previ-
ously9a (see Experimental section for details); ∆H and ∆S were
derived from the van’t Hoff plots (Table 3). For the complexes
in which the ligands contained dimethyl-substituted chains, a
coupling constant Aa, for a paramagnetic reference was not
available for Ha, and only the chemical shifts of Hc were used to
assess the thermodynamic parameters for these complexes (11,
15 and 18). The use of chemical shifts for Ha for the remaining
complexes yields results consistent with those obtained by the
use of chemical shifts for Hc. The parameters listed in Table 3
are for each complex the average values of those obtained from
the two van’t Hoff plots based on chemical shifts for Ha and Hc,
respectively. The uncertainties of the parameters are large, how-
ever, for the least paramagnetic spin-equilibrium system, the
diphenylpyrazolyl-containing complex 7.
In general, the high-spin state is less favoured for the N2S2
complexes listed in Table 3 than in the n = 3 N2O2 ketoimine
complexes incorporating pyrazole,9a mainly due to more posi-
tive enthalpy changes (see also Fig. 5 below). As was the case
for the N2O2 complexes,9a the entropy changes are small and
probably result from the few degrees of rotational freedom
gained for the ligand substituents in the slightly less crowded
pseudo-tetrahedral configuration. The more positive ∆S values
for the complexes bridged by methyl-substituted chains (11, 15
and 18) than for the unsubstituted analogues (10, 14 and 17) are
expected for steric reasons.
Fig. 5 Plots of ∆G(50 ЊC) and ∆H (inset) against ∆1/B. Complexes
labelled with an asterisk are the nickel() N2O2 ketoimine complexes of
ref. 9(a) analogous to 7 (Ph2), 14 (PhMe) and 17 (MePh) where the
substituents are those of pyrazolyl
chain length are small and comparable with the substitution
effects. Similar weak effects of a chain extension have been
observed for other N2S2 Schiff-base complexes.7b,14b,c However,
as seen for analogous nickel() N2(NH)2 complexes,1b,10 the
high-spin population of the nickel() N2S2 complexes increases
dramatically on extension of the chain from three to four
carbon atoms.1b,8d,13f,g
The quotient ∆1/B establishes the following trend in ligand-
field strength: X = O < S ≈ N. This is the same order established
by the thermodynamic measurements of N2O2 and N2S2 com-
plexes (see Fig. 5 and discussion of NMR results above). From
these results, it should be clear that low-spin character is
emphasized in N2(NH)2 and N2S2 complexes of nickel() when
compared with the corresponding N2O2 complexes.
Electronic spectra
The ligand-field spectra (see Table 4) are similar to those of the
N2O2 analogues,9a and the assignments for those analogues9a
have been used in the present work. (D4h symmetry is assumed
with the donor atoms on the Cartesian x and y axes.) Ligand-
field parameters were derived using the method of ref. 9(a).
1
Since the B1g transition is seen only as a shoulder, the
Ligand-field parameters derived from reported spectra for
other n = 2 or 3 N2X2 Schiff-base complexes are listed in Table 4
for comparison. The N2(NH)2 and N2S2 complexes incorporat-
ing benzene, [Ni(asaltn)],6f [Ni(tsalen)],7b [Ni(tsaltn)],7b or thio-
acetylacetonimine, [Ni(tacacen)],7d are significantly more stabli-
ized in the low-spin state than the pyrazole-based complexes in
agreement with the results obtained for the N2O2 complexes.9a
The parameters for the N2S2 ketoimine (12keto and 14keto) com-
plexes9b analogous to 12 and 14 and for the n = 2 or 3 N2S2
aldimine complexes incorporating cyclopentene [Ni(pent-en),
Ni(pent-tn)]14b
uncertainties of the ligand-field parameters derived in this work
are larger than for the N2O2 complexes previously studied.9a
In our earlier work,9a the spectra of the n = 3 N2O2 pro-
ligands were only slightly influenced by complexation with NiII.
In contrast, the spectra of the N2(NH)2 and the N2S2 pro-
ligands in the present work were more substantially changed
(see Table 5) on complexation with NiII. Therefore, more
covalency in the M᎐L bonds and stronger nephelauxetic effects
are expected for the N2(NH)2 and N2S2 complexes. In fact, such
effects have been observed1b,d,8d in related systems and are in
accord with expected trends.15a Values of B in related complexes
have been found to be influenced by substitution on the ali-
phatic chain.1b Similar influences are assumed to apply to the
complexes investigated in this work.
are comparable with those of N2S2 aldimine
complexes investigated in this work. Parameters for the low-
spin form of some n = 4 or 5 spin-equilibrium systems, [Ni(bi-
Ph)],8d [Ni(pent-bn)],14b [Ni(pent-pn)],14c are also listed. With
B ≈ 850 cmϪ1 for the paramagnetic tetrahedral form the change
of spin-pairing energy16 in going from S = 0 to 1 is about
Ϫ12 000 cm
For N2(NH)2 systems (1–6) the effect of a chain extension
from two to three carbon atoms is a significant decrease of the
Ϫ1, and a tetrahedral field ∆Td of ≈5000 cmϪ1 is
6d–f,14a
9a
ligand field. This is normal for N2(NH)2
and N2O2
expected for the pseudo-tetrahedral form in the spin-
equilibrium mixture. This is close to the ∆Td of ≈4500 cmϪ1
found for n = 4 pyrazole-based N2S2 spin-equilibrium
systems.1b,8d
Charge-transfer (CT) and intraligand (IL) transitions and
spectral data for the nickel() complexes and for the protonated
pro-ligands are given in Table 5. The intensities and the resem-
blance with the protonated pro-ligands suggest that the transi-
Schiff-base complexes. In nickel() N2(NH)2 complexes
incorporating pyrazole an increase in chain length to four
carbon atoms induces the spin-equilibrium process at temper-
atures significantly below room temperature.1b,10
Based on the eσ and eπ|| values,§ the N2(NH)2 ligands are both
stronger σ and π donors than are the N2O2 analogues;9a based
on the ∆1 and ∆1/B values, the ligand fields and ligand-field
strengths are larger in the N2(NH)2 complexes. These tendencies
are expected6d and are in accord with the temperature-
dependent 1H NMR results above and in ref. 9(a).
tions below 320 nm are π → π* IL transitions. For the N2
S
2
complexes the transition between 335 and 350 nm is also
ascribed to a π → π* IL transition, as the first transition
(ε ≈ 10 000 dm3 molϪ1 cmϪ1) is found at that position for zinc()
complexes with similar ligands.12a,13g The remaining bands are
assigned to CT transitions.
The N2S2 ligands are both weaker σ and π donors than are
the N2O2 and N2(NH)2 analogues. The effects of an increase in
§ Only the in-plane π interaction can be evaluated from the spectra, as
1
the Eg transition was not observed. For the N2O2 complexes, the in-
Electrochemistry of the N2S2 complexes
and out-of-plane π interactions are similar.15b For the N2(NH)2 com-
plexes, the nature of the out-of-plane π interaction is uncertain.15b
Previous work has shown that electrochemical processes for the
J. Chem. Soc., Dalton Trans., 1997, Pages 4539–4548
4543

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Table 4 Ligand-field transitions and parametersa
Transition
3Γ
1A2g
1B1g
eσ
eπ ||
∆
1
eσ/eπ
∆1/B
b
Complex
X = NH
1c (n = 3)
16 000
(125)
15 950
(123)
16 780
(147)
15 675
(105)
21 275 (sh)
(295)
21 055 (sh)
(280)
7965
7895
8265
7880
7830
7830
8080
1345
1310
1375
1365
1360
1315
1345
18 515
18 445
19 295
18 180
18 050
18 230
18 860
5.9
6.0
6.0d
5.8
5.8
6.0
6.0d
29.6
29.5
30.9
29.1
28.9
29.2
30.2
2c (n = 3)
3c (n = 2)
4c (n = 3)
21 000 (sh)
(180)
20 835 (sh)
(225)
20 835 (sh)
(175)
5c (n = 3)
15 550
(104)
6c (n = 3)
15 725
(124)
[Ni(asaltn)]c,e (n = 3)
16 3506f
X = S
7f (n = 3)
15 455
(108)
15 625
(114)
15 290
(108)
15 085
(97)
15 315
(107)
15 455
(73)
15 505
(101)
15 360
(100)
15 500 (sh)
(100)
15 410
(111)
15 290
(143)
15 430
(142)
20 000 (sh)
(160)
19 610 (sh)
(170)
20 000 (sh)
(130)
19 085 (sh)
(120)
18 690 (sh)
(145)
20 000 (sh)
(105)
20 000 (sh)
(140)
19 400 (sh)
(170)
19 050 (sh)
(175)
20 000 (sh)
(155)
19 410 (sh)
(140)
19 050 (sh)
(165)
7515
7455
7515
7240
7180
7515
7515
7335
7290
7515
7335
7285
1170
1060
1215
1055
930
17 865
18 125
17 685
17 500
17 640
17 865
17 905
17 765
18 010
17 805
17 685
17 915
6.4
7.0
6.2
6.9
7.7
6.4
6.5
6.9
7.6
6.3
6.8
7.4
29.8
29.0
29.5
29.1
28.2
29.8
29.8
29.6
28.8
29.7
29.5
28.7
8c (n = 3)
9f (n = 2)
10f (n = 3)
11c (n = 3)
12f (n = 2)
13f (n = 3)
14f (n = 3)
15c (n = 3)
16f (n = 2)
17f (n = 3)
18c (n = 3)
12 820 (sh)
(25)
12 500 (sh)
(30)
1170
1160
1060
965
13 070 (sh)
(35)
12 500 (sh)
(25)
1185
1080
985
12 660 (sh)
(30)
12keto f,g (n = 2)
15 6009b
15 4109b
≈14 5008d
16 6707b
16 5307b
16 5307d
15 15014b
14 95014b
14 75014b
14 40014c
7580
7335
6890
8030
7800
7970
7390
7145
6995
6850
1185
1050
920
1255
1115
1245
1155
1020
935
18 000
17 805
17 000
19 070
18 780
18 930
17 550
17 355
17 245
16 890
6.4d
7.0d
7.5d
6.4d
7.0d
6.4d
6.4d
7.0d
7.5d
7.5d
30.0
29.7
27.2
31.8
31.3
31.3
29.3
28.9
27.6
27.0
14keto f,g (n = 3)
[Ni (biPh)]c,h (n = 4)
[Ni (tsalen)]f,i (n = 2)
[Ni (tsaltn)]f,j (n = 3)
[Ni (tacacen)]f,k (n = 2)
[Ni (pent-en)]f,l (n = 2)
[Ni (pent-tn)]f,l (n = 3)
[Ni (pent-bn)]c,l (n = 4)
[Ni (pent-pn)]c,l (n = 5)
915
a Measured in CHCl3 at 25 ЊC; energies in cmϪ1, absorption coefficients listed in parentheses in dm3 molϪ1 cmϪ1; sh = shoulder; the ground state is
1A1g. b ∆1 = 3eσ Ϫ 4eπ. c B = 625 cmϪ1 d Estimated. e o-Aminobenzylideneimine complex.6f f B = 600 cmϪ1 g Ketoimine complex9b pyrazolyl substi-
. .
tuted in the same manner as for 12 and 14. h Biphenyl-bridged complex incorporating pyrazole.8d i Ref. 7(b). H2tsalen = N,NЈ-
Bis(thiosalicylidene)ethane-1,2-diamine. j Ref. 7(b). H2tsaltn = N,NЈ-Bis(thiosalicylidene)propane-1,3-diamine. k Ref. 7(d ). H2tacacen = 4,4Ј-
Ethylenedinitrilobis(pentane-2-thione). l Refs. 14(b) and 14(c).
N2S2 Schiff-base pyrazolyl-containing ligands and zinc() com-
plexes are very slow.12a Voltammetric data for the nickel() N2S2
complexes in acetonitrile are listed in Table 6. Results for the
n = 3 N2O2 ketoimine complex analogous to 14 (NiN2O2)9a
measured under the same conditions are listed for comparison.
However, most N2O2 complexes described in ref. 9(a) are too
insoluble in acetonitrile for electrochemical measurements.
Results for the n = 2 or 3 N2S2 ketoimine complexes9b analo-
gous to 12 and 14 (12keto, 14keto) are also listed.
process, and in every case NiII is quasi-reversibly reduced to NiI
(see Fig. 6 for complex 14).
The pyrazole-based complexes listed in Table 6 are more
easily reduced than the nickel() N2S2 complexes incorporating
unsaturated carbon rings.14b,17a This arises from (1) weaker
ligand fields and (2) stronger nephelauxetic effects for the for-
mer complexes.
Regarding the ligand-field effects, the MIIN2S2 complexes
previously investigated9b,14b,17b,c showed reduction potentials
inversely correlated with ligand-field strengths. For example, an
extension of the chains in tetradentate ligand complexes
decreases the ligand field and increases the potential for the
reduction of the metal. This same trend is observed for the n = 2
The NiII᎐NiIII oxidation is in most cases partly obscured by a
wave arising from ligand oxidation (Eox = 0.80 to 0.92,
₁
Ered = Ϫ0.39 to Ϫ0.11, E = 0.17 to 0.48, ∆E = 0.91 to 1.33 V).
₂
For 12 and 14 the metal oxidation is a reversible one-electron
4544
J. Chem. Soc., Dalton Trans., 1997, Pages 4539–4548

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Table 5 Charge-transfer and intraligand transitionsa
Complex
X = NH
1
2
397 (6160), 380 (sh) (5650), 320 (sh) (21 900), 278 (56 100)
395 (sh) (5610), 383 (5790), 330 (sh) (16 400), 293 (48 000)
439 (2170), 391 (5490), 375 (sh) (4410), 310 (sh) (23 800), 290 (sh) (28 400), 265 (40 800)
394 (5220), 385 (sh) (5090), 310 (sh) (20 200), 268 (37 200)
390 (sh) (4410), 377 (4660), 325 (sh) (21 900), 287 (50 500)
395 (6330), 380 (sh) (5860), 320 (sh) (15 000), 279 (44 400)
327 (20 000), 265 (20 000)
3
4
5
6
HLb
X = S
7
8
400 (sh) (2510), 350 (sh) (10 400), 297 (69 000), 259 (87 100)
400 (sh) (2680), 350 (sh) (9010), 299 (57 200), 260 (76 900)
400 (sh) (2150), 381 (3170), 340 (sh) (7170), 320 (sh) (8130), 290 (sh) (35 700), 267 (68 900)
380 (sh) (2420), 340 (sh) (6710), 295 (34 900), 265 (64 600)
380 (sh) (2380), 340 (sh) (6680), 297 (27 600), 264 (58 900)
392 (2530), 350 (sh) (8360), 335 (9440), 293 (44 000), 260 (46 900)
395 (2580), 345 (sh) (9080), 337 (9250), 291 (52 100), 256 (48 400)
410 (sh) (2140), 350 (sh) (8060), 310 (sh) (29 300), 288 (47 700), 258 (49 700)
395 (sh) (2180), 350 (sh) (7110), 310 (sh) (22 800), 291 (31 000), 285 (sh) (30 300), 255 (sh) (38 900)
405 (sh) (2400), 380 (sh) (3710), 345 (sh) (12 260), 331 (13 400), 286 (52 600)
390 (sh) (2450), 345 (sh) (10 800), 294 (55 900), 284 (56 500)
9
10
11
12
13
14
15
16
17
18
H2Lc
390 (sh) (2450), 340 (sh) (8880), 294 (31 500), 281 (31 100)
≈400 (≈10 000), ≈320 (≈20 000), ≈290 (>20 000)
a Measured in CHCl3 at 25 ЊC; wavelengths in nm, absorption coefficients in parentheses in dm3 molϪ1 cmϪ1
.
b Pro-ligand (X = NH) incorporating
pyrazole [from ref. 1(a)]. c Pro-ligands for complexes 7–18 (X = S).
Table 6 Voltammetric results for the nickel() N2S2 complexesa
₁
Complex
n
∆E/V
E /V
₂
7b
3
3
2
3
3
2
3
3
2
3
3
2
3
3
0.071
0.072
0.083
0.075
0.125
0.085
0.065
0.082
0.091
0.080
0.081
0.060
0.058
0.14
Ϫ1.143(3)
Ϫ1.131(3)
Ϫ1.339(6)
Ϫ1.178(2)
Ϫ1.205(6)
Ϫ1.308(11)
Ϫ1.124(2)
Ϫ1.129(2)
Ϫ1.327(3)
Ϫ1.158(6)
Ϫ1.160(2)
Ϫ1.24
8b
9b
10c
11c
12b
14c
15c,d
16b
17c
18c
e
12keto
14keto
NiN2O2
e
Ϫ1.16
Ϫ1.27
b,d,f
a Data for the NiII᎐NiI reduction are listed; e.s.d.s of the least significant
digits are given in parentheses; measured in MeCN vs. Ag᎐AgCl; sweep
rate 100 mV sϪ1; voltammetric data for the N2(NH)2 complexes 1, 3 and
4 measured in dmf are given in ref. 10. b Complex concentration 0.5
Fig. 6 Cyclic voltammogram for complex 14 in MeCN vs. Ag–AgCl;
the sweep rate is 100 mV sϪ1. The anodic process shown by the peaks at
₁
0.823 and Ϫ0.39 V is assigned to ligand oxidation (E = 0.22, ∆E = 1.21
₂
V); the anodic process shown by the peaks at 1.158 and 1.098 V is
mmol dmϪ3 c Complex concentration 1 mmol dmϪ3 d The process is
. .
assigned to a reversible NiII᎐NiIII oxidation (E = 1.127, ∆E = 0.060 V),
₁
partly irreversible: reduction peak > reoxidation peak. e The N2S2
ketoimine complexes9b pyrazolyl substituted as in 12 and 14 (measured
in MeCN at a mercury-film electrode). f Nickel() N2O2 ketoimine
complex9a pyrazolyl substituted as in 14 measured under the same con-
ditions as for the N2S2 complexes described in this work; voltammetric
results for the N2O2 complexes in CH2Cl2 are given in ref. 9(a).
₂
see Table 6 for the metal-based reductions.
for nickel() complexes with incorporated pyridinium are
not available. This order is expected based on the electron-
withdrawing effects of the incorporated rings. Ligand-field con-
siderations alone (see Table 4) would predict similar potentials
for the complexes incorporating cyclopentene and pyrazole.
Additionally, comparisons of differently 1,3-disubstituted
pyrazolyl-containing complexes with the same chain length
(Table 6) show that the phenyl-substituted complexes [7, 8, 14,
15, 17 and 18 (n = 3); 12 and 16 (n = 2)] in spite of stronger
ligand fields (Table 4) are easier to reduce than the dimethyl-
substituted complexes [10 and 11 (n = 3), 9 (n = 2)]. This is also
seen in a series of n = 4 nickel() N2S2 complexes incorporating
pyrazole11 and is ascribed to the inductive effects of the
substituents.
or 3 nickel() N2S2 complexes of the present work and of refs.
9(b) and 14(b). The inverse correlation between the reduction
potential and the ligand-field strength has been ascribed to an
easier access for the incoming electron to a less antibonding
2
2
metal σ* (dx Ϫ y ) orbital in the less ligand-field-stabilized
complex.9b
Stronger nephelauxetic effects lower the interelectronic
repulsions on the metal ion, which then becomes more easily
reducible.15a The electron-withdrawing effects of the nitrogen
atoms of the pyrazole rings reduce the σ-electron density on the
metal and lower the magnitude of B.13g In previously investi-
gated n = 4 MIIN2S2 Schiff-base complexes (M = Ni or Cu)
incorporating cyclopentene,14b,17b pyrazole,13g isoxazole,13g or
pyridinium17c the half-wave potentials for the CuII᎐CuI reduc-
tion are Ϫ640, Ϫ407, 3 and 200 mV, respectively. The same
order is found for the NiII᎐NiI reduction,13g,14b except that data
Experimental
Materials
Most chemicals used were reagent grade and commercially
available, used as received. Solvents used for analytical
J. Chem. Soc., Dalton Trans., 1997, Pages 4539–4548
4545

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Table 7 Crystal data and details of data collection and structure refinement for complexes 5, 10, 14 and 19
5
10
14
19
Formula
M
Crystal system
Space group
C27H30N8Ni
525.30
Monoclinic
P21/c
C15H20N6NiS2
407.20
Monoclinic
P21/c
C25H24N6NiS2
531.33
Monoclinic
P21
C25H24N6NiO2
493.16
Monoclinic
P21/c
a/Å
b/Å
c/Å
23.1532(4)
10.8755(1)
22.0478(3)
105.607(1)
5347.0(1)
0.42 × 0.28 × 0.08
1.30
13.1393(3)
9.5415(2)
14.0097(4)
94.185(1)
1751.69(7)
0.40 × 0.28 × 0.08
1.54
10.165(9)
22.04(1)
11.146(8)
92.03(4)
2496(3)
0.46 × 0.34 × 0.26
1.41
8.6235(2)
13.2402(3)
19.5375(2)
93.263(1)
2227.11(8)
0.40 × 0.20 × 0.10
1.47
β/Њ
U/Å3
Crystal size/mm
Dc/g cmϪ3
Z
8
7.56
4
13.6
4
9.65
4
9.07
µ/cmϪ1
Transmission factors
θ Limits/Њ
0.697–0.862
1–23
0.702–0.862
2–23
—
0–20
h, ϩk, ϩl
2 every 50
4
2405
1548
0.641–0.942
2–28
h, k, l
Octants collecteda
Standard reflections measured
Fall-off corrected for (%)
No. of unique data
No. data with I/σ(I)>2
No. variables
R
h, k,
l
h, k,
l
—
—
—
—
—
—
7647b
6123b
650
2513b
2217b
218
5389b
3012b
335
210
0.081c
0.195c
1.022
0.2
0.027c
0.068c
1.052
0.001
0.087d
0.085d
1.71
0.068c
0.153c
1.041
1.0
wR
S
(∆/σ)max
0.3
0.62, Ϫ0.45
∆ρmax, ∆ρmin/e ÅϪ3
1.43,e Ϫ0.75
0.38, Ϫ0.30
0.56, Ϫ0.48
a Followed by merging in the case of complexes 5, 10 and 19. b Merged data. c Refinement on F2; R = Σ(|Fo| Ϫ |Fc|)/Σ|Fo|, wR = [Σw(Fo2 Ϫ Fc2)2/
¹
Σw(Fo )2]², wϪ1 = σ2Fo2 ϩ (0.0546P)2 ϩ 30.8007P for 5, σ2Fo2 ϩ (0.0380P)2 ϩ 2.0751P for 10 and σ2Fo2 ϩ (0.0722P)2 ϩ 2.7095P for 19; P =
2
¹
¹
(Fo2 ϩ 2Fc2)/3. d Refinement on F; R = Σ(|Fo| Ϫ |Fc|)/Σ|Fo|, wR = [Σw(|Fo| Ϫ |Fc|)2/Σw|Fo|2]², wϪ1 = {[σc(F2) ϩ 1.03F2]² Ϫ |F|}2. e 1.35 Å from C(24).
purposes were spectroscopic grade. Acetonitrile used for
electrochemical measurements was refluxed over phosphorus
pentaoxide and distilled immediately before use.
ately before use. The electrolyte solution (0.1 mol dmϪ3) was
scanned before adding complex to check its purity.
Electronic absorption spectra were obtained in a 1 cm quartz
cuvette on a thermostatted Shimadzu UV-3100 apparatus. The
electron impact mass spectra were obtained on a Finnigan Mat
SSQ710 or a Varian Mat 311A apparatus. Magnetic suscepti-
bilities were measured in the solid state at 23 ЊC by using a
Sherwood Scientific magnetic susceptibility balance. Dia-
magnetic corrections were made by using Pascal’s constants.
Elemental analyses were performed at the H. C. Ørsted Insti-
tute, University of Copenhagen. The complexes were dried 24 h
in vacuum before elemental analyses.
Preparations
2,3-Diaminobutane,18 2,4-diaminopentane,19 the N2S2 Schiff
bases,12a and the N2S2 complexes13g were prepared according to
literature methods.
Nickel(II) N2(NH)2 complexes. The following general pro-
cedure was used. A suspension of the appropriate o-aminopyr-
azolecarbaldehyde (3.8 mmol) prepared according to ref. 20
was stirred in absolute ethanol (7 cm3) at reflux temperature.
The appropriate diamine (3.8 mmol) and nickel() acetate tetra-
hydrate (3.8 mmol) were added. The mixture was stirred at
reflux temperature for 20 min. In most cases the dark green com-
plex precipitated during that period. The product was filtered
off and washed with 96% ethanol on the filter-paper. In the case
of 5 the product-containing solution was concentrated to about
half the original volume, and the complex crystallized after a
few days. It was collected and used without further purification.
Determination of thermodynamic parameters for the spin-
equilibrium process
Equilibrium constants for the spin-equilibrium process in
CDCl3 were evaluated from the temperature-dependent 1H
NMR spectra of protons Ha and Hc over the temperature range
25–50 ЊC by the method described in ref. 9(a). The chemical
shifts for the protons of a spin-equilibrium system vary almost
linearly with temperature when the system is close to the
diamagnetic ground state.1b This was the case in this work, and
the diamagnetic reference points used (δa,dia and δc,dia) are the
chemical shifts of Ha and Hc at Ϫ10 ЊC found by extrapolation
from the two lowest-temperature measurements. Coupling con-
stants found for similar pyrazolyl-containing nickel() com-
plexes1b were used: Aa, Ac, R1 = R2 = Ph Ϫ1.432, Ϫ3.500 G;
R1 = R2 = Me Ϫ1.432, Ϫ3.643 G; R1 = Ph, R2 = Me Ϫ1.480,
Ϫ3.354 G; R1 = Me, R2 = Ph Ϫ1.780, Ϫ4.303 G (1G = 2.8
MHz).
Physical measurements
Proton NMR spectra of pro-ligands and complexes in CDCl3
and (CD3)2SO were obtained on Bruker AC250 spectrometers
equipped with VT1000 temperature controllers. The un-
certainty of the temperature was less than 1 ЊC. Assignments
were accomplished by comparisons with similar systems; SiMe4
was used as internal standard.
Electrochemical data for the N2S2 complexes in MeCN (0.5–1
mmol dmϪ3) were collected under N2 by cyclic voltammetry
at room temperature using microelectrodes from BAS100
(platinum working and counter electrodes, a water-based Ag–
AgCl reference electrode). Ferrocene was used as an external
Crystallography
The diffraction experiments employed Mo-Kα radiation
(λ = 0.710 73 Å). Crystals of complexes 10 and 19 were grown
from dmso, those of 14 from ethanol. For complex 5, crystals
from the first precipitate collected after the preparation (see
above) were used for the diffraction experiment. All crystals
II
III
₁
standard [E (Fe ᎐Fe ) = 500 mV under the given conditions].
₂
The sweep rate was 50–300 mV sϪ1. The supporting electrolyte,
NBu4PF6, and the complexes were dried in vacuum immedi-
4546
J. Chem. Soc., Dalton Trans., 1997, Pages 4539–4548

View Article Online
O. Simonsen, unpublished work; ( f ) A. la Cour, B. Adhikhari,
were dark green. Data were collected for 14 at 21 ЊC, and for 5,
10 and 19 at Ϫ110 ЊC. The routine XP in SHELXTL21a was used
to prepare the diagrams. Details of the X-ray diffraction
experiments are given in Table 7.
H. Toftlund and A. Hazell, Inorg. Chim. Acta, 1992, 202, 145;
(g) A. D. Garnovskii, A. L. Nivorozhkin and V. I. Minkin, Coord.
Chem. Rev., 1993, 126, 1.
2 (a) S. Yamada, Coord. Chem. Rev., 1966, 1, 415; (b) R. Knorr and
A. Weiss, Chem. Ber., 1981, 114, 2104; (c) J. M. Fernandez-G., M. J.
Rosales-Hoz, M. F. Rubio-Arroyo, R. Salcedo, F. Toscano and
A. Vela, Inorg. Chem., 1987, 26, 349.
3 (a) D. H. Gerlach and R. H. Holm, J. Am. Chem. Soc., 1969, 91,
3457; (b) W. S. Sheldrick, R. Knorr and H. Polzer, Acta Crystallogr.,
Sect. B, 1979, 35, 739; (c) M. Schumann and H. Elias, Inorg. Chem.,
1985, 24, 3187; (d) E. Uhlemann, G. Hinsche, H. Braunschweig and
M. Weissenfels, Z. Anorg. Allg. Chem., 1970, 377, 321; (e) I. Bertini,
L. Sacconi and G. P. Speroni, Inorg. Chem., 1972, 11, 1323.
4 (a) V. A. Kogan, N. N. Kharabaev, O. A. Osipov and S. G. Kochin,
J. Struct. Chem., 1981, 22, 96; (b) O. P. Anderson, J. Becher,
H. Frydendahl, L. F. Taylor and H. Toftlund, J. Chem. Soc., Chem.
Commun., 1986, 699; (c) C. T. Pedersen, Sulfur Rep., 1980, 1, 1;
(d) C. T. Pedersen, Phosphorus Sulfur Silicon, Relat. Elem., 1991, 58,
17; (e) J. Roncali, L. Rasmussen, C. Thobie-Gautier, P. Frère,
H. Brisset, M. Sallé, J. Becher, O. Simonsen, T. K. Hansen,
A. Benahmed-Gasmi, J. Orduna, J. Garin, M. Jubault and A.
Gorgues, Adv. Mater., 1994, 6, 841.
Room-temperature structure of complex 14. For complex 14
data were collected on a Huber four-circle diffractometer. The
data were processed as previously described21b and were cor-
rected for Lorentz-polarization effects. The structure was
solved by direct methods (SHELXS 8621c) and refined by least-
squares full-matrix refinement using
a modification of
ORFLS.21d,e Given the limitations of the data set it was
appropriate to use constraints during the refinements in order
to limit the number of parameters: (1) all four phenyl groups
were constrained to mm2 symmetry; (2) displacement par-
ameters were constrained to the TLS rigid-body model as
described in ref. 21( f ) for the entire molecule, except for the
nickel() atom and the atoms of the propylene bridge; (3) all
hydrogen atoms were placed in calculated positions, with dis-
placement parameters 10% larger than those of the atom they
were attached to. Methyl groups were refined as rigid rotors
with idealized distances and angles; rotation of this rigid rotor
about the C᎐C bond was allowed. Atomic scattering factors
were from ref. 21(g).
5 E. Frason, C. Panattoni and L. Sacconi, J. Phys. Chem., 1959, 63,
1908; S. C. Chang, D. Y. Park and N. C. Li, Inorg. Chem., 1968, 7,
2144; E. E. Castellano, O. J. R. Hodder, C. K. Prout and P. J. Sadler,
J. Chem. Soc. A, 1971, 2620; L. L. Merritt, jun., C. Guaret and
A. E. Lessor, jun., Acta Crystallogr., 1956, 9, 253.
6 (a) W. C. Hoyt and G. W. Everett, jun., Inorg. Chem., 1969, 8, 2030;
(b) G. M. Mockler, G. W. Chaffey, E. Sinn and H. Wong, Inorg.
Chem., 1972, 11, 1308; (c) M. A. A. F. de C. T. Carrondo, B. de
Castro, A. M. Coelho, D. Doningues, C. Freire and J. Morais, Inorg.
Chim. Acta, 1993, 205, 157; (d) M. Green and P. A. Tasker, J. Chem.
Soc. A, 1970, 2531; (e) M. Green and P. A. Tasker, Inorg. Chim.
Acta, 1971, 5, 65; ( f ) B. M. Higson and E. D. McKenzie, J. Chem.
Soc. A, 1971, 269.
7 (a) L. Beyer, M. Pulst, H. R. Paul and E. Hoyer, J. Prakt. Chem.,
1975, 317, 265; (b) H. S. Jensen, V. McKee and C. J. McKenzie,
personal communication; (c) T. Yamamura, M. Tadokoro and
R. Kuroda, Chem. Lett., 1989, 1246; (d) P. R. Blum, R. M. C. Wei
and S. C. Cummings, Inorg. Chem., 1974, 13, 450; (e) D. Attanasio,
G. Dessy, V. Fares and G. Pennesi, Mol. Phys., 1980, 40, 269.
8 (a) H. R. Engeseth, D. R. McMillin and E. L. Ulrich, Inorg. Chim.
Acta, 1982, 67, 145; (b) M. J. O’Connor, R. E. Ernst and R. H.
Holm, J. Am. Chem. Soc., 1968, 90, 4561; (c) M. J. O’Connor and
R. H. Holm, Prog. Inorg. Chem., 1971, 14, 241; (d) H. Frydendahl,
H. Toftlund, J. Becher, J. C. Dutton, K. S. Murray, L. F. Taylor,
O. P. Anderson and E. R. T. Tiekink, Inorg. Chem., 1995, 34, 4467.
9 (a) A. la Cour, M. Findeisen, R. Hazell, L. Hennig, C. E. Olsen and
O. Simonsen, J. Chem. Soc., Dalton Trans., 1996, 3437 and refs.
therein; (b) L. Hennig, R. Kirmse, O. Hammerich, S. Larsen,
H. Frydendahl, H. Toftlund and J. Becher, Inorg. Chim. Acta, 1995,
234, 67.
Low-temperature structures of complexes 5, 9 and 19. For
complexes 5, 10 and 19 the entire data sets were obtained over 6
(5 and 10) or 10 h (19) by using a Siemens SMART-CCD X-ray
system. Data were processed by using Siemens SMART system
software and the Siemens SAINT program for integration of
data frames. Corrections for Lorentz-polarization effects were
applied. Data were corrected for absorption (SADABS21h). The
structures were solved by direct methods.21a The coordinates
and anisotropic displacement parameters for all non-hydrogen
atoms were refined by full-matrix least-squares methods.21a
Hydrogen atoms were included in the structural model at calcu-
lated positions. The carbon atoms bridging the two halves of
the ligand in 19 were disordered; the site occupation factors for
these atoms were fixed at 50% in the final cycles of refinement.
For these bridge atoms of 19 a constraint was applied to the
bond between C(20A) and C(22A) to fix its length at a reason-
able value.
CCDC reference number 186/717.
Acknowledgements
10 A. N. Nivorozhkin, H. Toftlund, P. L. Jørgensen and L. E.
Nivorozhkin, J. Chem. Soc., Dalton Trans., 1996, 1215.
We gratefully acknowledge Mette Maagaard and Niels J.
Poulsen, Aarhus University (Aa. U.), for their participation in
the structure determination of complex 5, Svend U. Hansen,
Karsten N. Koch and Lene Mogensen (Aa. U.) for their partici-
pation in the structure determination of 14, Carsten Buch and
Birthe Haack, Odense University (O. U.), for technical assist-
ance with collection of NMR spectra, Inge Pedersen and Ole T.
Sørensen (O. U.) for the mass spectra, the Danish Natural
Science Research Council and the Carlsberg Foundation for the
Huber diffractometer at Aa. U., and the National Institutes of
Health (U. S. Grant Number 1510RR10547-01) for the Siemens
SMART-CCD diffractometer at Colorado State University.
11 A. la Cour, Ph. D. Thesis, Odense University, Odense, 1996.
12 (a) O. P. Anderson, A. la Cour, M. Findeisen, L. Hennig, L. F.
Taylor, O. Simonsen and H. Toftlund, J. Chem. Soc., Dalton Trans.,
1997, 111; (b) C. P. Brock and J. D. Dunitz, Chem. Mater., 1994, 6,
1118; (c) K. Bernardo, S. Leppard, A. Robert, G. Commenges,
F. Dahan and B. Meunier, Inorg. Chem., 1996, 35, 387.
13 (a) F. Akhtar and M. G. B. Drew, Acta Crystallogr., Sect. B, 1982,
38, 1149; (b) M. G. B. Drew, R. N. Prasad and R. P. Sharma, Acta
Crystallogr., Sect. C, 1985, 41, 1755; (c) F. Akhtar, Acta Crystallogr.,
Sect. B, 1981, 37, 84; (d) N. A. Bailey, E. D. McKenzie and
J. M. Worthington, J. Chem. Soc., Dalton Trans., 1974, 1363;
(e) E. M. Martin, R. D. Bereman and P. Singh, Inorg. Chem., 1991,
30, 957; ( f ) A. L. Nivorozhkin, L. E. Konstantinovsky, L. E.
Nivorochkin, V. I. Minkin, T. G. Takhirov, O. A. Diachenko and
D. B. Tagiev, Izv. Akad. Nauk SSSR, Ser. Khim., 1990, 327 (in
Russian); (g) A. la Cour, M. Findeisen, A. Hazell, R. Hazell and
G. Zdobinsky, J. Chem. Soc., Dalton Trans., 1997, 121.
14 (a) J. H. Weber, Inorg. Chem., 1967, 6, 258; (b) E. M. Martin,
R. D. Bereman and J. Dorfman, Inorg. Chim. Acta, 1990, 176, 247;
(c) E. M. Martin and R. D. Bereman, Inorg. Chim. Acta, 1991, 188,
221.
15 (a) M. Gerloch, Coord. Chem. Rev., 1990, 99, 117; (b) A. Ceulemans,
M. Dendooven and L. G. Vanquickenborne, Inorg. Chem., 1985, 24,
1159.
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J. Chem. Soc., Dalton Trans., 1997, Pages 4539–4548