The effects of distal ligand substitution on the copper(II)/ bis-(2,6-dipyrazol-1-ylpyridine) centre

Article abstract of DOI:10.1039/b103142m

The complex [Cu(L⁴)₂](BF₄)₂ (3; L⁴ = 2-[pyrazol-1-yl]-6-[3-{2,4,6-trimethylphenyl}pyrazol- 1-yl]pyridine) has been synthesised. Complex 3 crystallises in two crystal forms from MeNO₂/Et₂O. The monoclinic α-form contains crystallographically ordered, pseudo-Jahn-Teller elongated {d_y2-_z2}¹ Cu(II) ions that are structurally very similar to [CH(L²)₂](BF₄)₂ (1; L² = 2,6-dipyrazol-1-ylpyridine). The Cu ion in the orthorhombic β-polymorph at 180 K exhibits Cu-N bond lengths that are rather different from the α-form, and which could correspond to a disordered {d_y2-_z2}¹ Cu(II) centre; or, to a static, rhombically compressed ion with a {d_z2}¹ ground state. Variable temperature EPR data favour the former interpretation. A crystal structure at 31 K of [Cu(L³)₂](BF₄)₂ (2; L³ = 2,6-bis-[3-{2,4,6-trimethylphenyl}pyrazol-1-yl]pyridine), a genuine {d_z2}¹ ion, is also described. Comparison of the crystallographic data of 1-3 shows that the {d_y2-_z2}¹-to-{d_z2} ¹ ground state change occurs concomitantly with only a small z-axis compression.

Full text of DOI:10.1039/b103142m

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The effects of distal ligand substitution on the copper(II)/
bis-(2,6-dipyrazol-1-ylpyridine) centre†
Nayan K. Solanki,a Michael A. Leech,b Eric J. L. McInnes,c Jing P. Zhao,c Frank E. Mabbs,c
Neil Feeder,a Judith A. K. Howard,b John E. Davies,a Jeremy M. Rawsona and
Malcolm A. Halcrow*d
a Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge,
UK CB2 1EW
b Department of Chemistry, University of Durham, South Road, Durham, UK DH1 3LE
c CW EPR Service Centre, Department of Chemistry, University of Manchester, Oxford Road,
Manchester, UK M13 9PL
d School of Chemistry, University of Leeds, Woodhouse Lane, Leeds, UK LS2 9JT.
E-mail: M.A.Halcrow@chem.leeds.ac.uk
Received 6th April 2001, Accepted 30th May 2001
First published as an Advance Article on the web 3rd July 2001
The complex [Cu(L4)2](BF4)2 (3; L4 = 2-[pyrazol-1-yl]-6-[3-{2,4,6-trimethylphenyl}pyrazol-1-yl]pyridine) has
been synthesised. Complex 3 crystallises in two crystal forms from MeNO2/Et2O. The monoclinic α-form contains
1
2
2
crystallographically ordered, pseudo-Jahn–Teller elongated {dy Ϫ z } Cu() ions that are structurally very similar to
[Cu(L2)2](BF4)2 (1; L2 = 2,6-dipyrazol-1-ylpyridine). The Cu ion in the orthorhombic β-polymorph at 180 K exhibits
1
2
2
Cu–N bond lengths that are rather different from the α-form, and which could correspond to a disordered {dy
}
Ϫ z
1
2
Cu() centre; or, to a static, rhombically compressed ion with a {dz } ground state. Variable temperature EPR data
favour the former interpretation. A crystal structure at 31 K of [Cu(L3)2](BF4)2 (2; L3 = 2,6-bis-[3-{2,4,6-trimethyl-
1
2
phenyl}pyrazol-1-yl]pyridine), a genuine {dz } ion, is also described. Comparison of the crystallographic data of 1–3
1
1
2
2
2
shows that the {dy Ϫ z } -to-{dz } ground state change occurs concomitantly with only a small z-axis compression.
change in electronic ground state in [Cu(L3)2](ClO4)2 (2), which
Introduction
1
2
adopts an unusual {dz } ground state in the solid and solution
phases.4,5 Although on symmetry grounds the Jahn–Teller
theorem does not apply to 2, this complex should still exhibit
an axial or rhombic structural compression along the molec-
ular z-axis (Scheme 1). This result is significant because, prior
to our work, there was only one authenticated, homoleptic
six-coordinate Cu() compound that exhibits a compressed
molecular structure,7,8 namely the 1-D coordination polymer
KAlCuF6;9 some other Cu-doped lattices also exhibit this
In common with the stereochemically similar complexes
2
[Cu(terpy)2]X2 (XϪ = BrϪ, PF6Ϫ),1 [Cu(dien)2](NO3)2 and
[Cu(L1R)2](BF4)2 (R = Me, Cy),3 [Cu(L2)2](BF4)2 (1) adopts a
1
2
2
{dy
}
electronic ground state in the solid state and in
Ϫ z
solution (Scheme 1).4,5 This corresponds structurally to a
pseudo-Jahn–Teller elongation of the molecular x-axis.4,6 In
solid 1, this structural elongation is dynamically disordered
over the molecular x- and y-axes, which results in unique
thermally induced Jahn–Teller switching behaviour involving a
crystal phase change at 41 K.6 We have shown that ligand sub-
stitution adjacent to the distal, pyrazole N-donors results in a
property.10 Hence, it is clearly of interest to define the spectro-
1
2
scopic and structural properties of the near-homoleptic {dz }
species 2.
Scheme 1 Molecular axes for [CuL2]2ϩ (L = L2–L4).
In order to examine further the steric factors that contribute
1
2
towards the {dz } ground state of 2, we have now prepared
[Cu(L4)2](ClO4)2 (3). We report here variable temperature EPR
† Non-SI unit employed: 1 G = 10Ϫ4 T.
DOI: 10.1039/b103142m
J. Chem. Soc., Dalton Trans., 2001, 2083–2088
This journal is © The Royal Society of Chemistry 2001
2083

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Table 1 Selected bond lengths (Å) and angles (Њ) for α-[Cu(L4)2]-
(ClO4)2ؒ2CH3NO2 (α-3ؒ2CH3NO2) and β-[Cu(L4)2](ClO4)2ؒxCH3NO2
(β-3ؒxCH3NO2)
and crystallographic studies of this complex, which define its
electronic ground state. A low temperature X-ray structure
determination of 24 is also described. These results now allow
us to elucidate changes in the molecular structure of this Cu()
center that occur concomitantly with the change in ground
state from {dy Ϫ z } to {dz } . The molecular axis convention
shown in Scheme 1 is used for the rhombic complexes described
in this paper, with the short, intermediate and long Cu–N
bonds being defined as lying near the molecular z-, y- and
x-axes respectively.
α-3ؒ2CH3NO2
β-3ؒxCH3NO2
1
1
2
2
2
150 K
30 K
1.9668(14)
2.0928(14)
2.1012(14)
2.0343(14)
2.2741(14)
2.3048(13)
180 K
Cu–N(1)
Cu–N(2)
Cu–N(3)
Cu–N(4)
Cu–N(5)
Cu–N(6)
1.962(2)
2.089(2)
2.101(2)
2.023(2)
2.258(2)
2.301(2)
1.985(4)
2.173(4)
2.150(4)
1.996(4)
2.186(4)
2.220(4)
Results and discussion
The new ligand L4 was prepared by the sequential coupling of
2,6-dibromopyridine with one molar equivalent of potassium
pyrazolide, followed by one molar equivalent of potassium
3{5}-(2,4,6-trimethylphenyl)pyrazolide, according to the
method of Jameson and Goldsby.11 Complexation of
Cu(ClO4)2ؒ6H2O by 2 molar equivalents of L4 in MeNO2 yields
a lime green solution, from which feathery green microcrystals
were obtained in good yield following concentration of the
solution and addition of Et2O. The dried solid analysed cleanly
as the desired product [Cu(L4)2](ClO4)2 (3), a formulation
confirmed by positive ion FAB mass spectrometry (m/z = 722,
corresponding to [63Cu(L4)2 ϩ H]ϩ) and IR spectroscopy, which
N(1)–Cu–N(2)
N(1)–Cu–N(3)
N(1)–Cu–N(4)
N(1)–Cu–N(5)
N(1)–Cu–N(6)
N(2)–Cu–N(3)
N(2)–Cu–N(4)
N(2)–Cu–N(5)
N(2)–Cu–N(6)
N(3)–Cu–N(4)
N(3)–Cu–N(5)
N(3)–Cu–N(6)
N(4)–Cu–N(5)
N(4)–Cu–N(6)
N(5)–Cu–N(6)
77.98(9)
78.04(9)
179.48(9)
108.62(9)
101.37(9)
155.81(9)
103.33(9)
93.89(9)
88.28(9)
100.82(9)
90.74(9)
99.53(9)
75.70(9)
74.43(9)
149.73(9)
78.10(5)
78.08(5)
176.22(5)
107.75(5)
102.36(5)
156.01(5)
103.48(5)
93.78(5)
88.54(5)
100.47(5)
90.80(5)
99.26(5)
75.67(5)
74.34(5)
149.63(5)
77.32(16)
76.99(16)
175.95(16)
107.65(15)
99.85(15)
153.65(16)
102.51(15)
91.98(15)
92.46(14)
103.53(16)
90.24(17)
97.59(16)
76.39(16)
76.11(15)
152.46(15)
Ϫ
showed the presence of L4 and ClO4 only. The d–d maxima
exhibited by 3 in MeCN [νmax = 6.0 × 103 cmϪ1 (εmax = 35 MϪ1
cmϪ1), 13.5 (48), 23.5 (108)] are very similar to those shown by 1
[6.3 (33), 13.9 (46), 24.4 (119)] and 2 [5.9 (44), 13.8 (52), 23.5
(sh)].4
Complex 3 is difficult to crystallise. However, diffusion of
Et2O vapor into a small volume (ca. 0.3 cm3) of a solution
of 3 in MeNO2 sometimes yielded clusters of green needles,
from which fragments suitable for X-ray analysis could be
cleaved. Examination of several crystals from different samples
obtained in this way revealed the presence of two solvated
modifications: monoclinic [Cu(L4)2](ClO4)2ؒ2CH3NO2 (α-3ؒ
2CH3NO2) and orthorhombic [Cu(L4)2](ClO4)2ؒxCH3NO2
(β-3ؒxCH3NO2; x ≈ 2.0). The uncertain composition of the
β-polymorph reflects severe solvent disorder in this form (see
below). Under the microscope, needles of the α-modification
have a flatter cross-section compared to the β-polymorph,
although they are indistinguishable to the naked eye.
Crystals of α-3ؒ2CH3NO2 contain one cation, two anions
and two fully occupied solvent sites per asymmetric unit, all
lying on general positions. The six-coordinate complex at 150 K
has a distribution of bond lengths that clearly arises from a
crystallographically ordered Jahn–Teller elongated dication, the
axis of elongation lying along the N(5)–Cu(1)–N(6) vector
(Table 1, Fig. 1). At 30 K, the molecular structure of the com-
plex is almost identical to that at 150 K, consistent with its
being non-fluxional in the crystal. Interestingly, the distance
Cu(1)–N(5) has lengthened slightly at 30 K, from 2.258(2) Å
at 150 K to 2.2741(14) Å; the reason for this is uncertain. All
other metric parameters at the Cu ion are crystallographically
indistinguishable at the two temperatures. The Cu–N distances
and N–Cu–N angles in α-3ؒ2CH3NO2 resemble those in the
low-temperature phase of 1 (Table 1).6 The only manifestation
of steric repulsion between the 2,4,6-trimethylphenyl substi-
tuents of one L4 ligand in 3 and the pyridine ring of the other,
is a slight elongation of the bonds Cu(1)–N(2) and Cu(1)–N(3)
in 3, which at 30 K are 0.029(2)–0.037(2) Å longer than the
equivalent bonds in 1 at 31 K.
Fig. 1 View of the [Cu(L4)2]2ϩ complex dication in the crystal of α-
3ؒ2CH3NO2 at 31 K, showing the atom numbering scheme adopted.
The structure of the complex dication in β-3ؒxCH3NO2 is visually
almost identical to that shown here, and uses the same atom numbering
scheme. Thermal ellipsoids are at the 50% probability level, while all
non-H atoms have been omitted for clarity.
this modification at 180 K is also six-coordinate, but with Cu–N
distances that differ significantly from those of α-3ؒ2CH3NO2
(Table 1). Rather, the coordination sphere in this structure
more closely resembles that of 2 (see below).7 In principle, this
could reflect the presence of either: a dynamically or statically
disordered pseudo-Jahn–Teller elongated Cu() ion with a
1
2
2
{dy
}
ground state; or a rhombically compressed Cu()
Ϫ z
1
8,12
2
centre analogous to 2, with a {dz } ground state.
In order to address this ambiguity, the vibrational amplitudes
〈d2〉 were calculated for both polymorphs of 3.8 This parameter
corresponds to the difference in the mean-square displacement
parameters (MSDAs) of a given N atom and the Cu atom along
their common vector (eqn. 1 and 2).
The structure of β-3ؒxCH3NO2 suffers badly from twinning
problems, and from static anion and solvent disorder that
reflects the presence of channels of solvent molecules within the
crystal lattice, running parallel to the unit cell c-vector. Follow-
ing unsuccessful attempts to collect data from several different
crystals at several temperatures, we were able to obtain one
useful refinement of this polymorph at 180 K. The Cu() ion in
(1)
〈d2〉 = MSDA(N) Ϫ MSDA(Cu)
(2)
2084
J. Chem. Soc., Dalton Trans., 2001, 2083–2088

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Table 2 Calculated 〈d2〉 (104 Å2) values for 2 and 3. A negative value of 〈d2〉 arises when MDSA(N) < MDSA(Cu). Estimated errors on 〈d2〉 are
20 × 104 Å2
T /K
Cu(1)–N(1)
Cu(1)–N(2)
Cu(1)–N(3)
Cu(1)–N(4)
Cu(1)–N(5)
Cu(1)–N(6)
2
150a
31
150
31
Ϫ95
24
24
33
Ϫ14
8
31
62
47
65
34
40
Ϫ1
42
Ϫ69
23
16
33
7
18
31
34
37
80
15
38
Ϫ16
29
Ϫ40
3, α-polymorph
3, β-polymorph
180
101
a Taken from ref. 4.
Table 3 Q-Band EPR data for solid 3. Hyperfine couplings are to
63,65Cu and are in G. Estimated errors in g are 0.002 for the major spin,
and 0.005 for the minor spin. Estimated errors in A are 3 G
where Uij is an element of the 3 × 3 matrix of thermal
parameters and ni, nj are elements of the vector describing the
bond. It has been suggested that statically or dynamically
disordered Cu–N bonds generally exhibit 〈d2〉 >0.010 Å2, while
ordered N-donor atoms afford 〈d2〉 values that are smaller than
this value.13 However, care must be taken using this criterion,
since 〈d2〉 for fluxional Cu–X (X = N, O) bonds decreases
markedly with decreasing temperature.13,14 In our experience,
this can lead to Cu–N bonds that (from variable temperature
crystallography) are clearly dynamically disordered, but which
show 〈d2〉 < 0.010 Å2 at lower temperatures.15 All the 〈d2〉 values
for the Cu–N bonds in β-3ؒxCH3NO2 are somewhat greater
than for the α-polymorph, although only for the bond Cu(1)–
N(3) in the β-form is 〈d2〉 greater than the putative 0.010 Å2
Major species
Minor species
T /K
g1
g2
g3
A1
g1
g2
g3
296
220
180
80
40
20
2.280
2.286
2.290
2.296
2.296
2.296
2.298
2.300
2.281
2.116
2.106
2.100
2.092
2.089
2.087
2.087
2.087
2.099
2.050
2.050
2.050
2.050
2.058
2.058
2.057
2.057
2.051
130
130
130
135
135
135
135
135
137
2.200
2.200
2.200
2.200
2.200
2.200
2.250
2.250
—
2.200
2.200
2.200
2.200
2.200
2.200
2.116
2.126
—
2.050
2.050
2.020
2.020
2.020
2.020
2.030
2.030
—
10
5
120a
threshold value (Table 2). Overall, these data are inconclusive;
1
a Solution spectrum run in 10 : 1 MeCN–toluene.
2
2
however, they do not rule out a dynamic {dy
}
ground
Ϫ z
state for β-3ؒxCH3NO2 (see below). A possible alternative
explanation for the different Cu–N distances in the polymorphs
of 3 might be the presence of a cocrystallised impurity in the
β-form. However, no residual electron density that might arise
from such a contaminant was detected in the Fourier map for
this structure, while the displacement ellipsoids of all the atoms
in the [Cu(L4)2]2ϩ dication are consistent with it being fully
crystallographically occupied.
Powder EPR spectra of 3 were obtained at Q-band over the
range 5–296 K, using a mixture of the two polymorphs of this
compound (see above). At all temperatures, the spectra could
only be simulated assuming the presence of two distinct spin
systems, populated in a ratio of approximately 3 : 1 which did
not vary detectably with temperature (Table 3, Fig. 2). The
major species has the form of a resolved molecular spectrum,
with a rhombic g1 > g2 ≈ g3 > ge pattern and A1{63,65Cu}
1
2
2
coupling that are clearly consistent with a {dy Ϫ z } ground
state.16 The rhombicity of this signal decreases slightly as the
temperature is lowered. However, the observed g-values are
clearly consistent with this species having a static, rather than
fluxional, axis of pseudo-Jahn–Teller elongation. The minor
spin is apparently axial at 290 K with an ‘inverse’ g⊥ > g|| > ge
pattern. The g-values of this minor signal do not vary signific-
antly with temperature until below 20 K. However, at 10 K this
signal has changed substantially, to a rhombic g1 > g2 > g3 > ge
pattern (Fig. 2). This is the behaviour expected for an exchange-
averaged spectrum, reflecting the fluxionality of a pseudo-
Jahn–Teller axis of elongation (as in 1).1–3,8,12 The lack of
observable hyperfine coupling in the minor spectrum is also
Fig. 2 Q-Band EPR spectra of powdered 3 at (a) 290 and (b) 5 K.
consistent with this interpretation. The alternative description
1
2
of this species, as a static {dz } species analogous to 2, is ruled
study to confirm this suggestion, which we have not yet
been able to achieve. In frozen 10 : 1 MeCN–toluene solution,
the same sample of 3 yields a single EPR peak, which
out because of the variation of this spectrum at low temper-
atures, and because g3 > ge.
It is clear from these measurements that solid 3 contains
resembles the major signal from the powder spectrum and
1
1
2
2
2
2
a mixture of static and fluxional {dy Ϫ z } spins. This is con-
sistent with the observation of a static, rhombically elon-
gated molecular structure in the α-3ؒ2CH3NO2 modification
is assignable to a pseudo-Jahn-Teller elongated {dy
}
Ϫ z
Cu() ion (Table 3).16
For comparison, a structure determination of 2ؒ2CH3NO2
was carried out at 31 K (Table 4, Fig. 3). All bond lengths and
angles to copper in this structure are crystallographically
identical to those we have previously reported for this
compound at 180 K.4 In addition, all the Cu–N bonds in 2 at
both temperatures give 〈d2〉 <0.010 Å2 (Table 2). These data
and a potentially fluxional Cu() centre in β-3ؒxCH3NO2.
1
2
2
The assignment of β-3ؒxCH3NO2 as a dynamic {dy
}
Ϫ z
Cu() compound is not inconsistent with the crystallographic
MSDA data for this polymorph (Table 2; see above). How-
ever, it would require a variable temperature crystallographic
J. Chem. Soc., Dalton Trans., 2001, 2083–2088
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Table 4 Selected bond lengths (Å) and angles (Њ) for [Cu(L3)2](ClO4)2ؒ
2CH3NO2 (2ؒ2CH3NO2) at 31 K
difficult, because these will be dictated by conformational strain
within the tridentate ligand as well as by the electronic con-
figuration at Cu. The bonds Cu(1)–N(1) and Cu(1)–N(4) are
both shorter in 3 than in 2. However, the contraction of Cu(1)–
N(1) [0.008(2) Å] is much smaller than for Cu(1)–N(4) [0.054(2)
Å], because for the former bond ligand conformational effects
will oppose the electronically imposed z-axis compression in
3 compared to 2, while for Cu(1)–N(4) they will reinforce it.
Taking an average of these two values to cancel the ligand
Cu–N(1)
Cu–N(2)
Cu–N(3)
Cu–N(4)
Cu–N(5)
Cu–N(6)
1.9586(14)
2.1371(14)
2.1463(14)
1.9799(14)
2.2117(15)
2.2503(15)
N(1)–Cu–N(2)
N(1)–Cu–N(3)
N(1)–Cu–N(4)
N(1)–Cu–N(5)
N(1)–Cu–N(6)
N(2)–Cu–N(3)
N(2)–Cu–N(4)
N(2)–Cu–N(5)
N(2)–Cu–N(6)
N(3)–Cu–N(4)
N(3)–Cu–N(5)
N(3)–Cu–N(6)
N(4)–Cu–N(5)
N(4)–Cu–N(6)
N(5)–Cu–N(6)
78.00(6)
78.14(6)
179.46(6)
103.44(5)
102.70(5)
156.08(5)
101.51(5)
95.38(5)
91.08(5)
102.34(5)
91.78(5)
92.47(5)
76.82(5)
77.05(5)
153.84(5)
1
1
2
2
2
conformational strain, it can be said that the {dy Ϫ z } –{dz }
transition occurs concomitantly with a 0.031(3) Å shortening
of the two z-axis Cu–N bonds, which is somewhat smaller than
the structural changes in the xy-plane.
A similar comparison between the structures of 16 and 2 at
31 K leads to an identical trend to that described above. The
more rhombic stereochemistry of 1 means that the changes in
Cu–N bond length close to the x- and y-axes between 1 and 2
are greater compared to between 2 and 3. Interestingly, however,
the two pairs of compounds give rise to similar estimated z-axis
contractions, of 0.025–0.030 Å. We have also reported a similar,
but again more pronounced, trend between the structures of
1
1
1
2
2
2
the {dy Ϫ z } complex [Cu(L Cy)2](BF4)2, and the {dz } [Cu-
(L1But)2](BF4)2.3 Hence, on the basis of this study and the work
in ref. 3, it can be concluded that the structural distortion
1
2ϩ
2
arising from a {dz } configuration in [CuL2] (L = meridional
tris-imine ligand) complexes is probably best considered as an
elongation of the Cu–N bonds within the molecular xy-plane,
rather than as a z-axis compression.
Experimental
Unless stated otherwise, all manipulations were performed
in air. 2-Pyrazol-1-yl-6-bromopyridine,11 3{5}-(2,4,6-trimethyl-
phenyl)pyrazole,17 1 and 2 were prepared by the literature
procedures.4 KH (35 wt% suspension in mineral oil, Aldrich)
was washed twice with freshly distilled n-hexane under N2, and
dried in vacuo, before use. Cu(ClO4)2ؒ6H2O (Avocado) and all
other solvents (analytical grade) were used as supplied, except
that diglyme was dried over sodium before use.
Syntheses
Fig. 3 View of the [Cu(L3)2]2ϩ complex dication in the crystal of
2ؒ2CH3NO2 at 31 K, showing the atom numbering scheme adopted.
Details as for Fig. 1.
2-(Pyrazol-1-yl)-6-(3-{2,4,6-trimethylphenyl}pyrazol-1-yl)-
pyridine (L4). To a suspension of KH (1.0 g, 2.6 × 10Ϫ2 mol) in
diglyme (80 cm3) under N2 was added 3{5}-(2,4,6-tri-
methylphenyl)pyrazole (4.8 g, 2.6 × 10Ϫ2 mol), resulting in the
vigorous evolution of H2. The mixture was stirred for 1 h at
55 ЊC. 2-Pyrazol-1-yl-6-bromopyridine (5.7 g, 2.0 × 10Ϫ2 mol)
was then added, and the resultant mixture stirred at 130 ЊC for 5
days. After cooling, an equal volume of water was added to the
mixture, yielding an off-white precipitate, which was filtered
and washed twice with water. Recrystallisation from CH2Cl2–
MeOH yielded a pale yellow microcrystalline solid. Yield 2.9 g,
45% (Found: C, 72.7; H, 5.8; N, 20.9; calc. for C20H19N5: C, 72.9;
H, 5.8; N, 21.3%). Mp 140–142 ЊC. EI–MS: m/z 329 [M]ϩ, 185
[3{5}-mesitylpyrazole]ϩ, 145 [M Ϫ (3{5}-mesitylpyrazole) ϩ
confirm our earlier conclusion that 2 is not fluxional in the solid
state.
Concluding remarks
In order to determine the structural effects of the unusual elec-
tronic ground state in 2, the most instructive comparison
is between 2ؒ2CH3NO2 and the α-modification of 3ؒ2CH3NO2
at 31 K, since the latter will have a steric environment that
most closely resembles that of 2. For ease of comparison, the
numbering scheme for the Cu and N atoms in these two struc-
tures is identical (Figs. 1 and 3). The two Cu–N bonds close to
the molecular y-axis (Scheme 1) in 2, Cu(1)–N(2) and Cu(1)–
N(3), are each longer compared to those in 3 by 0.045(2) Å. The
two x-axis Cu–N bonds in 2 are correspondingly shorter than in
1
H]ϩ. NMR (CDCl3, 293K): H, δ 8.64 (d, 2.2, 1H, PzMes H5),
8.61 (d, 2.3, 1H, PzH H5), 7.83–7.92 (m, 3H, Py H3–H5), 7.78 (br
s, 1H, PzH H3), 6.97 (s, 2H, Ph H3/5), 6.52 (br s, 1H, PzH H4), 6.43
(d, 2.2 Hz, 1H, PzMes H4), 2.34 (s, 3H, CH3), 2.21 (s, 6H, CH3);
13C, δ 153.8, 150.2, 150.1 (Py C2 ϩ Py C6 ϩ PzMes C3), 142.4,
141.4 (PzH C3 ϩ Py C4), 137.9 (Ph C1), 137.4 (Ph C2/6), 130.3 (Ph
C4), 128.3 (Ph C3/5), 127.3, 127.0 (PzHC5 ϩ PzMes C5), 109.5,
109.5 (Py C3 ϩ Py C5), 109.0, 108.0 (PzH C4 ϩ PzMes C4), 21.1
(Mes CH3), 20.6 (Mes CH3).
3, by 0.055(2) Å for Cu(1)–N(5) and 0.062(2) Å for Cu(1)–N(6).
1
2
2
Hence, it is apparent that the change from a {dy Ϫz } to
1
2ϩ
a {dz } ground state in [CuL2] (L = L3, L4) is reflected in
a convergence of the Cu–N bond lengths along the molecular
x- and y-axes, by roughly equivalent amounts.
2
Other things being equal, lengthening of the distal M–N
bonds to a meridional tridentate ligand generally results in a
concomitant elongation of the central M–N bond.3 Hence,
quantifying the behaviour of Cu–N bonds parallel to the
molecular z-axis upon changing the Cu ion ground state is more
Bis-[2-(pyrazol-1-yl)-6-(3-{2,4,6-trimethylphenyl}pyrazol-1-
yl)pyridine]copper(II) diperchlorate (3). A solution of L4 (0.80 g,
2.4 × 10Ϫ3 mol) and Cu(ClO4)2ؒ6H2O (0.44 g, 1.2 × 10Ϫ3 mol) in
MeNO2 (30 cm3) was stirred for 10 min at room temperature.
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J. Chem. Soc., Dalton Trans., 2001, 2083–2088

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Table 5 Experimental details for the single crystal structure determinations in this study
[Cu(L3)2](ClO4)2ؒ
2CH3NO2
(2ؒ2CH3NO2)
α-[Cu(L4)2](ClO4)2ؒ
2CH3NO2
(α-3ؒ2CH3NO2)
α-[Cu(L4)2](ClO4)2ؒ
2CH3NO2
(α-3ؒ2CH3NO2)
β-[Cu(L4)2](ClO4)2ؒ
2.5CH3NO2
(β-3ؒ2.0CH3NO2)
Formula
Mr
T /K
C60H64Cl2CuN12O12
1279.67
31(2)
C42H44Cl2CuN12O12
1043.33
150(2)
C42H44Cl2CuN12O12
1043.33
30(2)
C60H64Cl2CuN12O12
1279.67
180(2)
Crystal class
Space group
Monoclinic
P21/c
Monoclinic
P21/c
Monoclinic
P21/c
Orthorhombic
Pbcn
a/Å
b/Å
c/Å
12.0031(15)
19.684(2)
24.967(3)
99.955(2)
5810.1(12)
4
18.6594(6)
18.1992(6)
14.8395(4)
111.396(2)
4692.0(3)
4
18.5980(13)
18.2528(12)
14.6183(10)
111.2280(10)
4625.7(5)
4
40.780(6)
18.023(3)
13.673(3)
—
5810.1(12)
4
β/Њ
V/Å3
Z
µ(Mo-Kα, mmϪ1
)
0.543
0.654
0.663
0.543
Measured reflections
Independent reflections
47634
12155
24012
8214
33683
9384
16792
8872
Rint
0.038
0.032
0.085
0.061
0.047
0.123
0.029
0.030
0.084
0.044
0.089
0.274
R(F)a
wR(F 2)b
4
a R = Σ[|Fo| Ϫ |Fc|]/Σ|Fo|. b wR = [Σw(Fo2 Ϫ Fc2)/ΣwFo ]1/2
.
The lime green solution was filtered and concentrated to ca.
5 cm3. Vapor diffusion of Et2O into this solution afforded
feathery green microcrystals. Yield 0.97 g, 86% (Found: C, 52.1;
H, 4.3; N, 15.2; calc. for C40H38Cl2CuN10O8: C, 52.2; H, 4.2; N,
15.2%). FAB-MS: m/z 722 [63Cu(L4)2 ϩ H]ϩ, 392 [63Cu(L4)]ϩ,
330 [L4ϩH]ϩ. UV/vis/NIR (CH3CN): νmax 103 cmϪ1 (εmax, MϪ1
cmϪ1) 6.0 (35), 13.5 (48), 23.5 (108), 30.1 (sh), 30.5 (27,800),
36.1 (sh), 36.8 (30,200), 40.5 (sh), 43.4 (sh), 44.8 (sh).
CAUTION: although we have experienced no difficulties in
handling 3, metal-organic perchlorates are potentially explosive
and should only be prepared or manipulated in small
quantities.
except the minor anion disorder orientation were modeled
anisotropically.
ꢁ-[Cu(L4)2](ClO4)2ؒxCH3NO2 (ꢁ-3ؒxCH3NO2; x ≈ 2.0) at
180 K. The lattice of this structure contains channels of solvent
molecules running parallel to the z-axis of the unit cell, giving
rise to substantial anion and solvent disorder. One perchlorate
anion was disordered by rotation about one Cl–O bond, with
the remaining three O atoms each being distributed over three
equally occupied sites. The second anion was disordered over
two sites, with a 0.7 : 0.3 occupancy ratio. All Cl–O distances
within the disordered anion were restrained to 1.43(2) Å, and
non-bonded O ؒ ؒ ؒ O distances within a particular disorder
orientation to 2.34(2) Å. Four different solvent molecules were
refined: one wholly occupied ordered site lying across a crystal-
lographic C2 axis; one wholly occupied site, whose O atoms
were disordered over two equally occupied orientations; one
site with occupancy 0.75; and one with occupancy 0.25, with
one O atom lying on a crystallographic C2 axis. All N–O
distances were restrained to 1.22(2) Å. All non-H atoms with
occupancy ≥ 0.5 were modeled anisotropically.
Single crystal X-ray structure determinations
Single crystals of 2 and 3 were grown by vapour diffusion of
Et2O into solutions of the complexes in MeNO2. Experimental
details from the structure determinations are given in Table 5.
All X-ray data at 31 K were obtained using a Siemens SMART
diffractometer fitted with an Oxford Cryosystems HELIX
helium cooling device. All measurements at temperatures equal
to or above 100 K were collected using an Enraf Nonius
KappaCCD diffractometer fitted with an Oxford Cryosystems
nitrogen cooling device. Crystals were mounted on a glass
(T ≥ 100 K) or nylon (T ≤ 50 K) fibre using a drop of perfluoro-
polyether oil. All structures were solved by direct methods
(SHELXS 8618) and refined by full matrix least-squares on F 2
(SHELXL 9719), with H atoms placed in calculated positions.
Mean-square diplacement parameters were calculated using
the program THMA11,20 incorporated into the WinGX suite of
crystallographic software.21
Other measurements
Infrared spectra were obtained from Nujol mulls pressed
between KBr plates, in the range 400–4000 cmϪ1 using a Perkin-
Elmer Paragon 1000 spectrophotometer. UV/visible spectra
were obtained with Perkin-Elmer Lambda 12 or Lambda 900
spectrophotometers, in 1 cm quartz cells. All NMR spectra
were run on a Bruker ARX250 spectrometer, operating at 250.1
(1H) and 62.9 MHz (13C). Electron impact and positive ion fast
atom bombardment mass spectra were performed on a Kratos
MS890 spectrometer, the FAB spectra employing a 3-NOBA
matrix. CHN microanalyses were performed by the University
of Cambridge Department of Chemistry microanalytical
service. Melting points are uncorrected. Q-Band EPR spectra
were obtained using a Bruker ESP300E spectrometer, fitted
with an ER5106QT resonator and ER4118VT cryostat. Spec-
tral simulations were performed using in-house software which
has been described elsewhere.22
CCDC reference numbers 164481–164484.
See http://www.rsc.org/suppdata/dt/b1/b103142m/ for crys-
tallographic data in CIF or other electronic format.
[Cu(L3)2](ClO4)2ؒ2CH3NO2 (2ؒ2CH3NO2) at 31
K and
ꢀ-[Cu(L4)2](ClO4)2ؒ2CH3NO2 (ꢀ-3ؒ2CH3NO2) at 30 K. No
disorder was detected during refinement of either of these
structures, and no restraints were applied to the final models.
All non-H atoms were refined anisotropically.
ꢀ-[Cu(L4)2](ClO4)2ؒ2CH3NO2 (ꢀ-3ؒ2CH3NO2) at 150 K. One
perchlorate anion was disordered over two distinct orientations,
which were modeled with a 0.75 : 0.25 occupancy ratio. All
Cl–O distances within the disordered anion were restrained
to 1.41(2) Å, and non-bonded O ؒ ؒ ؒ O distances within a
particular disorder orientation to 2.27(2) Å. All non-H atoms
Acknowledgements
Financial support is acknowledged from the Royal Society
(London) for a research fellowship to M. A. H., the EPSRC
(N. K. S., M. A. L.), ICI R&T Division (N. K. S.), the
University of Cambridge and the University of Leeds.
J. Chem. Soc., Dalton Trans., 2001, 2083–2088
2087

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