Double-helical dinuclear copper(I) and mononuclear copper(II) complexes of a compartmental tetradentate bridging ligand: Crystal structures and spectroscopic properties

Article abstract of DOI:10.1039/a802280a

Reaction of 3-(2-pyridyl)pyrazole with α,α′-dibromoxylene affords a new tetradentate ligand L having two bidentate pyridyl-pyrazole compartments linked by an o-CH₂C₆H₄CH₂ spacer. With Cu^I, L forms a d

Full text of DOI:10.1039/a802280a

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Double-helical dinuclear copper(I) and mononuclear copper(II)
complexes of a compartmental tetradentate bridging ligand: crystal
structures and spectroscopic properties
James S. Fleming, Karen L. V. Mann, Samantha M. Couchman, John C. Jeffery,
Jon A. McCleverty* and Michael D. Ward*,†
School of Chemistry, University of Bristol, Cantock’s Close, Bristol, UK BS8 1TS
Reaction of 3-(2-pyridyl)pyrazole with α,αЈ-dibromoxylene affords a new tetradentate ligand L having two
bidentate pyridyl-pyrazole compartments linked by an o-CH2C6H4CH2 spacer. With CuI, L forms a dinuclear
double helicate [Cu2L2][PF6]2 in which both ligands are bridging both pseudo-tetrahedral metal ions, which are
5.05 Å apart. However L does not support a double helical architecture with CuII. Reaction of L with CuCl2
affords [CuLCl][BF4] which is trigonal bipyramidal both in the solid state (by X-ray crystallography) and
in solution (by EPR spectroscopy). Reaction of L with Cu(MeCO2)2ؒH2O affords [Cu2L3][BF4]4 which was
characterised by mass spectroscopy and elemental analysis. By analogy with the (known) nickel() analogue,
this complex has one ligand acting as a bis-bidentate bridge between two {CuL}2ϩ fragments in which L acts as
a terminal tetradentate chelating ligand. Recrystallisation of this afforded a few crystals of the decomposition
product [CuL(MeOH)][CuL(SiF6)][BF4]2, which contains trigonal-bipyramidal [CuL(MeOH)]2ϩ and six-co-
ordinate [CuL(SiF6)] fragments, the latter displaying the first known example of the hexafluorosilicate anion
acting as a bidentate chelating F,F-donor.
Of all the commonly studied transition metals, copper has a
particular appeal because of the different stereoelectronic pref-
erences of its two common oxidation states.1 The CuI/CuII
N
N
interconversion is often accompanied by a pronounced geo-
metric change, and if the conversion is reversible this struc-
tural change may be the basis of an allosteric effect in which
the geometric change at the metal alters the properties of a
more remote site in the complex.2 Conversely, control of the
geometric environment around copper centres allows control of
the redox potential of the CuI–CuII couple. This is in part the
basis of the redox properties of type 1 copper proteins3 and
many synthetic analogues,4 where the rigidity of a carefully
designed ligand environment can impose a geometry on the
metal which is anywhere between the extremes preferred by CuI
and CuII and thereby control the redox potential.
In this paper we describe the synthesis of the tetradentate
ligand L, containing two bidentate chelating pyridyl-pyrazolyl
fragments linked by a flexible o-CH2C6H4CH2 spacer, and its
co-ordination behaviour with CuI and CuII. This ligand is a new
member of a class of ligand which we have been studying
recently in which two bidentate pyridyl-pyrazolyl fragments are
linked to a variety of spacer groups such as {BH2}Ϫ,5 {CH2}6
and {PO2}Ϫ,7 to give flexible ligands which can either co-
ordinate to a single metal ion as a tetradentate chelate, or can
co-ordinate each bidentate arm to a separate metal centre to give
bridged polynuclear species. The results presented here show
that L can bridge two metal ions to give a double helicate with
CuI, but acts as a tetradentate chelate to a single metal centre in
its structurally characterised complexes with CuII. There are
other examples in the literature of pairs of copper-() and -()
complexes in which the co-ordination mode of a tetradentate
ligand is similarly dependent on the stereoelectronic preferences
of the metal centre.8
N
N
N
N
L
chemical studies has been described in previous papers. 3-(2-
Pyridyl)pyrazole was prepared as previously described.9
Preparations
Ligand L. A mixture of α,αЈ-dibromoxylene (5.50 g, 20.8
mmol), 3-(2-pyridyl)pyrazole (6.40 g, 44.1 mmol), NBun OH
4
(25 drops of 40% aqueous solution), aqueous NaOH (10 , 35
cm3) and toluene (200 cm3) was heated with stirring to 85 ЊC for
24 h. After cooling, the organic layer was washed with water
and then dried over MgSO4. Removal of the solvent in vacuo
afforded a pale yellow oil which afforded a white powder on
recrystallisation from CH2Cl2–Et2O (6.7 g, 82%). EI mass spec-
trum: m/z 392 (Mϩ, 5%), 246 (loss of one bidentate pyridyl-
1
pyrazolyl arm, 100%). H NMR (300 MHz, CDCl3): δ 8.61
(2 H, d, J 4.8, pyridyl H6), 7.94 (2 H, d, J 8.0, pyridyl H3), 7.68
(2 H, td, J 7.8, 1.7, pyridyl H4), 7.33 (2 H, d, J 2.3, pyrazolyl H4
or H5), 7.30 (2 H, m, phenyl), 7.17 (4 H, m, phenyl and pyridyl
H5), 6.91 (2 H, d, J 2.3; pyrazolyl H5 or H4) (Found: C,
73.0; H, 5.1; N, 21.3. C24H20N6 requires C, 73.5; H, 5.1; N,
21.4%). X-Ray quality crystals were grown by slow diffusion of
diethyl ether vapour into a concentrated CH2Cl2 solution of L.
[Cu2L2][PF6]2. Under an atmosphere of N2, a solution of L
(0.139 g, 0.35 mmol) in dry CH2Cl2 (7 cm3) was added dropwise
to a solution of [Cu(MeCN)4][PF6] (0.132 g, 0.35 mmol) in dry
CH2Cl2 (10 cm3) to afford an orange solution. After stirring at
room temperature for 2 h, the solution was reduced in volume
and hexane added to precipitate a yellow solid which was
filtered off and dried in vacuo (yield: 0.20 g, 95%). Electro-
spray MS: m/z 1657.6 [7%, {Cu3L3(PF6)2}ϩ], 1449.5 [4%,
{Cu2L3(PF6)}ϩ], 1056.7 [12%, {Cu2L2(PF6)}ϩ], 455.0 [100%,
{Cu2L2}2ϩ] (Found: C, 47.5; H, 3.5; N, 13.6. C48H40Cu2F12N12P2
Experimental
General details
Instrumentation used for routine spectroscopic and electro-
† E-Mail: mike.ward@bristol.ac.uk
J. Chem. Soc., Dalton Trans., 1998, Pages 2047–2052
2047

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Table 1 Crystallographic data for the four crystal structuresa
Compound
L
[Cu2L2][PF6]2ؒ0.5CH2Cl2
[CuLCl][BF4]
[CuL(MeOH)][CuL(SiF6)][BF4]2
Formula
M
C24H20N6
392.46
C24.5H21ClCuF6N6P
643.43
C24H20BClCuF4N6
578.26
C49H44B2Cu2F14N12OSi
1259.75
System, space group
a/Å
b/Å
c/Å
Monoclinic, P21/c
10.1831(10)
24.930(3)
8.1034(7)
102.111(6)
2011.4(3)
4
Tetragonal, P43212
16.3084(14)
16.308(2)
19.627(2)
90
5220.1(10)
8
Monoclinic, P21/n
16.333(2)
8.492(2)
17.002(2)
90.423(9)
2358.1(6)
4
Monoclinic, P21/n
10.135(2)
18.355(4)
27.362(5)
99.284(14)
5024(2)
β/Њ
U/Å3
Z
4
Dc/g cmϪ3
1.296
0.081
1.637
1.071
1.629
1.099
1.666
0.976
µ/mmϪ1
F(000)
824
2600
1172
2552
Crystal size/mm
Reflections collected:
total, independent, Rint
Data, restraints, parameters
Final R1, wR2b,c
Weighting factorsc
Largest peak, hole/e ÅϪ3
0.6 × 0.3 × 0.3
11 978, 4566, 0.0334
0.5 × 0.3 × 0.1
33 732, 6003, 0.0426
0.3 × 0.2 × 0.1
23 437, 5371, 0.0471
0.3 × 0.2 × 0.1
50 899, 11 519, 0.1098
4563, 0, 271
6003, 0, 357
5371, 0, 371
11 517, 111, 749
0.0628, 0.1247
0.0296, 8.9760
ϩ0.755, Ϫ0.603
0.0524, 0.1464
0.0596, 0.9333
ϩ0.325, Ϫ0.373
0.0535, 0.1576
0.0893, 4.5299
ϩ0.684, Ϫ0.682
0.0362, 0.1078
0.0573, 0.0708
ϩ0.429, Ϫ0.489
a Details in common: temperature for data collection, 173 K; upper 2θ limit for data collection, 55Њ. b Structure was refined on Fo2 using all data; the
¹
2
value of R1 is given for comparison with older refinements based on Fo with a typical threshold of F у 4σ(F). c wR2 = [Σ[w(Fo2 Ϫ Fc2)2]/Σw(Fo )2]²
2
2
where wϪ1 = [σ2(Fo ) ϩ (aP)2 ϩ bP] and P = [max(Fo ,0) ϩ 2Fc2]/3.
requires C, 48.0; H, 3.3; N, 14.0%). X-Ray quality crystals were
grown by slow diffusion of diethyl ether vapour into a concen-
trated CH2Cl2 solution of [Cu2L2][PF6]2.
eters, data collection and refinement. In all cases the structures
were solved by conventional direct methods and refined by the
full-matrix least-squares method on all F2 data using the
SHELXTL 5.03 package on a Silicon Graphics Indy com-
puter.11 Absorption corrections were applied using SADABS.12
Non-hydrogen atoms were refined with anisotropic thermal
parameters; hydrogen atoms were included in calculated posi-
tions and refined with isotropic thermal parameters riding on
those of the parent atom.
The structural determinations of L and [CuLCl][BF4] were
straightforward and presented no problems. In [Cu2L2][PF6]2ؒ
0.5CH2Cl2 the carbon atom of the half-molecule of CH2Cl2 in
the asymmetric unit lies astride a twofold axis. The space group
is chiral, indicating that spontaneous resolution of the helicates
occurs on crystallisation; the Flack parameter was zero within
experimental error, indicating a correct assignment of the abso-
lute configuration. In [CuL(MeOH)][CuL(SiF6)][BF4]2 one of
the [BF4]Ϫ anions was disordered over two orientations. These
could be resolved successfully, but similarity restraints were
applied to the bonded B᎐F distances and non-bonded F ؒ ؒ ؒ F
separations to keep the geometries of the disordered components
reasonable.
[CuLCl][BF4]. A solution of L (0.146 g, 0.37 mmol) in
MeOH (8 cm3) was added to a solution of CuCl2 (0.050 g, 0.37
mmol) in MeOH (6 cm3). The mixture was stirred at room tem-
perature for 1 h and then filtered to remove traces of unidenti-
fied insoluble material. Addition of an aqueous solution of
NaBF4 to the dark green solution afforded a precipitate of the
product which was filtered off and dried in vacuo (yield: 0.10 g,
50%). Fast-atom bombardment MS: m/z 490 [40%, {CuLCl}ϩ],
455 [100%, {CuL}ϩ] (Found: C, 49.5; H, 3.5; N, 14.6. C24H20B-
ClCuF4N6 requires C, 49.8; H, 3.5; N, 14.5%). X-Ray quality
crystals were grown by slow diffusion of diethyl ether vapour
into a concentrated solution of [CuLCl][BF4] in MeCN–
acetone (1:1).
[Cu2L3][BF4]4. A solution of L (0.090 g, 0.23 mmol) in
MeOH (5 cm3) was added dropwise to
a solution of
[Cu(MeCO2)2]ؒH2O (0.030 g, 0.15 mmol) in MeOH (5 cm3).
After stirring the mixture at room temperature for 1 h to give a
clear solution, the product was precipitated by addition of
aqueous NaBF4 to give a blue-green solid which was filtered off
and dried in vacuo (yield: 0.10 g, 74%). Fast-atom bombard-
ment MS: m/z 1542 (100%); see Results and Discussion section
for interpretation. Electrospray MS: m/z 1171.5 [1%,
{Cu2L2(BF4)3}ϩ], 825.6 [1%, {Cu2L3(BF4)4}2ϩ], 455.4 [100%,
{CuL}ϩ] (Found: C, 50.9; H, 3.7; N, 15.1. C24H20BClCuF4-
N6ؒ2H2O requires C, 51.2; H, 3.8; N, 14.9%). Recrystallisation
by slow diffusion of diethyl ether vapour into a concentrated
solution of [Cu2L3][BF4]4 in acetone–MeOH (1:1) afforded
X-ray quality crystals of what proved to be [CuL(MeOH)]-
[CuL(SiF6)][BF4]2 (see Results and Discussion for explanation).
CCDC reference number 186/992.
See http://www.rsc.org/suppdata/dt/1998/2047/ for crystallo-
graphic files in .cif format.
Results and Discussion
Synthesis and crystal structure of the ligand L
The new ligand L was simply prepared in high yield by reaction
of 2 equivalents of 3-(2-pyridyl)pyrazole with α,αЈ-dibromoxyl-
ene under phase-transfer conditions. Satisfactory 1H NMR and
mass spectral data confirmed the formulation of the product.
As mentioned in the introduction a particular advantage of
using chelating fragments with terminal pyrazol-3-yl groups is
the ease with which several of these groups can be linked to a
common spacer via deprotonation of the pyrazolyl N1 position.
We can easily envisage extension of this general route to the
preparation of multinucleating ligands in which several chelat-
ing arms are linked to central spacers such as 1,3,5-tris-
(bromomethyl)benzene, C(CH2Br)4, and so on.
X-Ray crystallography
Suitable crystals were quickly transferred from the mother-
liquor to a stream of cold N2 at Ϫ100 ЊC on a Siemens SMART
diffractometer fitted with a CCD-type area detector, and data
were in all cases collected at Ϫ100 ЊC to a 2θ limit of 55Њ using
graphite-monochromatised Mo-Kα radiation.
A detailed
experimental description of the methods used for data
collection and integration using the SMART system has been
published.10 Table 1 contains a summary of the crystal param-
The crystal structure of L is in Fig. 1. The two pyridyl-
pyrazolyl arms are approximately trans coplanar, with angles of
6Њ between the mean planes of rings 6 and 7 [where ring 6
2048
J. Chem. Soc., Dalton Trans., 1998, Pages 2047–2052

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Table 2 Selected bond lengths (Å) and angles (Њ) for [Cu2L2][PF6]2ؒ
0.5CH2Cl2
Cu(1)᎐N(11)
Cu(1)᎐N(61)
2.022(4)
2.028(4)
Cu(1)᎐N(51)
Cu(1)᎐N(21)
2.050(4)
2.049(4)
N(11)᎐Cu(1)᎐N(61) 123.4(2)
N(11)᎐Cu(1)᎐N(21) 81.3(2)
N(61)᎐Cu(1)᎐N(21) 134.3(2)
N(11)᎐Cu(1)᎐N(51) 134.7(2)
N(61)᎐Cu(1)᎐N(51) 80.5(2)
N(21)᎐Cu(1)᎐N(51) 110.1(2)
expected for a complex between a ligand with four donor atoms
and a metal ion with a preference for four-co-ordination. The
mass spectral data showed the complex to be dimeric, i.e.
[Cu2L2][PF6]2, and the crystal structure (Fig. 2, Table 2) showed
it to be a dinuclear double helicate13 with both ligands bridging
both metal ions.
The complex dication lies on a C2 axis, such that both metal
ionsareequivalent. TheCuI ionhastheusualpseudo-tetrahedral
co-ordination with an angle of 71.5Њ between the two Cu(NN)2
mean planes. Aromatic π-stacking interactions are evident
between the ligands: specifically, the near-planar pyridyl-
pyrazolyl fragment comprising rings 1 and 2 stacks with its
symmetry-related equivalent, with separations of 3.3–4.1 Å
between the atoms of one fragment and the mean plane of the
other; similarly, the pyrazolyl ring N(61)–C(65) overlaps with
its symmetry-related equivalent N(61A)–C(65A), with a separ-
ation of ca. 3.4 Å. Such interactions are common in double
helicates with aromatic ligands, and the structure of [Cu2L2]-
[PF6]2 is generally typical of double helicates of CuI with bis-
bidentate ligands.13 The metal–metal separation is 5.051(3) Å
which reflects the length and flexibility of the o-CH2C6H4CH2
spacer group. It is clearly not possible for the two bidentate sites
of one ligand to co-ordinate to a single metal ion in such a way
that the two CuN2 planes are mutually perpendicular, which
would confer the required pseudo-tetrahedral geometry on the
metal in a mononuclear complex. This is in contrast to some
related ligands described recently containing two bipyridyl
fragments with a large and flexible spacer between them, which
are capable of forming mononuclear pseudo-tetrahedral
complexes in this way.14
Fig. 1 Crystal structure of the new ligand L
1
In solution the H NMR spectrum, although rather broad,
was consistent with the presence of four-fold symmetry such
that eight aromatic and one aliphatic (᎐CH2᎐) proton environ-
ments were present. Electrochemical studies revealed no evi-
dence for a CuI–CuII couple at the potentials accessible with our
combination of solvent and base electrolyte.
Synthesis and crystal structure of [CuLCl][BF4]
Reaction of L with CuCl2, followed by treatment of the solu-
tion with NaBF4, afforded a material with empirical formula
(from elemental analysis) [CuLCl][BF4]. In this case however
the mass spectral data were also consistent with a monomeric
complex rather than a dimeric one, and this was confirmed crys-
tallographically (Fig. 3, Table 3). The ligand L is now acting as
a tetradentate chelate, co-ordinating all four donor atoms to
the single metal centre which has an approximately trigonal-
bipyramidal geometry. According to the method of Addison
et al. for classifying five-co-ordinate geometries intermediate
between square pyramidal (C4v) and trigonal bipyramidal
(D3h),15 the parameter τ is 0.63 (where a value of 1 denotes
perfect trigonal-bipyramidal symmetry and 0 denotes perfect
square pyramidal symmetry) indicating an intermediate struc-
ture but one that lies more towards the trigonal bipyramidal
limit. The three equatorial ligands are therefore Cl, N(41)
and N(71); the two axial ligands are N(51) and N(61), and the
four atoms Cu(1), Cl, N(41) and N(71) are almost exactly
coplanar (maximum deviation from mean plane, 0.006 Å). The
principal distortion from ideal trigonal-bipyramidal geometry
arises from a distortion in the equatorial plane, which has an
Fig. 2 Two views of the crystal structure of the dication of
[Cu2L2][PF6]2: (a) the entire structure showing the labelling scheme; (b)
a simplified view emphasising the helical geometry (four of the atoms
on the central phenyl ring of each ligand have been omitted for clarity)
denotes atoms N(61)–C(65), etc.], and 20Њ between rings 3
and 4. The bond lengths and angles within the molecule are
unremarkable. There are close intermolecular contacts in the
range 3.2–3.6 Å between overlapping phenyl rings of adjacent
molecules. From the conformation of the molecule in the
crystal structure it is apparent that the two CH2 hinges allow
a substantial degree of flexibility, such that the two bidentate
fragments can vary their separation, which will be of signifi-
cance for determining the mode of co-ordination of the ligand
(see later).
Synthesis and crystal structure of [Cu2L2][PF6]2
Reaction of L with 1 equivalent of [Cu(MeCN)4][PF6] afforded
a yellow compound whose elemental analysis indicates the stoi-
chiometry [CuL][PF6], i.e. a 1:1 metal:ligand ratio as would be
J. Chem. Soc., Dalton Trans., 1998, Pages 2047–2052
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Table 3 Selected bond lengths (Å) and angles (Њ) for [CuLCl][BF4]
Cu(1)᎐N(61)
Cu(1)᎐N(51)
Cu(1)᎐N(71)
1.996(2)
2.014(2)
2.090(2)
Cu(1)᎐N(41)
Cu(1)᎐Cl
2.141(2)
2.2878(8)
N(61)᎐Cu(1)᎐N(51) 171.74(9)
N(71)᎐Cu(1)᎐N(41) 109.72(8)
N(61)᎐Cu(1)᎐N(71)
N(51)᎐Cu(1)᎐N(71)
N(61)᎐Cu(1)᎐N(41)
N(51)᎐Cu(1)᎐N(41)
79.73(8)
95.45(8)
96.06(8)
79.17(8)
N(61)᎐Cu(1)᎐Cl
N(51)᎐Cu(1)᎐Cl
N(71)᎐Cu(1)᎐Cl
N(41)᎐Cu(1)᎐Cl
96.21(6)
91.96(7)
133.95(6)
116.31(6)
Fig. 3 Crystal structure of the cation in [CuLCl][BF4]
Fig. 4 X-Band EPR spectra of (a) [CuLCl][BF4] and (b) [Cu2L3][BF4]2
as frozen glasses at 77 K
expanded angle Cl᎐Cu(1)᎐N(71) [133.95(6)Њ] and a compressed
N(41)᎐Cu(1)᎐N(71) angle [109.72(8)Њ]. This could arise in part
from the steric bulk of the chloride ligand and from the
geometric constraints inherent in co-ordination on all four
donor atoms of L to a single metal centre. However Hathaway
et al. have recently shown for a wide variety of five-co-ordinate
CuN4X complexes that geometries along the pathway between
regular trigonal-bipyramidal and square-based pyramidal
geometries can occur by a vibronic coupling mechanism,16 and
the geometry observed for the [CuLCl]ϩ cation is exactly
consistent with this. The mean planes of the two bidentate
pyrazolyl-pyridyl arms are inclined at an angle of 71Њ to one
another. It is clear from this structure that the o-CH2C6H4CH2
linking group of L is sufficiently flexible to allow the two
bidentate arms to chelate to one metal ion, as well as bridging
two metal ions as we saw in the double helicate with CuI.
consistent with formation of a dinuclear complex [Cu2L3]-
[BF4]2. The dinuclear formation was confirmed by the electro-
spray mass spectrum, which showed (very weak) peaks corres-
ponding to the fragments {Cu2L2(BF4)3} and {Cu2L3(BF4)4}.
Because of the weakness of this spectrum we also investigated
the FAB mass spectrum of [Cu2L3][BF4]2 and obtained a very
strong signal at m/z 1542. There are two possible interpretations
for this. The fragment {Cu3L3(BF4)2} has the most intense
peak of its isotope envelope at m/z 1541, and the fragment
{Cu2L3(BF4)(noba)} has the most intense peak of its isotope
envelope at m/z 1543 (Hnoba is the matrix 3-nitrobenzyl-
alcohol used in the spectrum, and it is not uncommon to find
molecules of deprotonated noba co-ordinating to complex
fragments to give adducts of this sort). The isotopic patterns of
these two possible assignments are not sufficiently different for
it to be obvious which is the correct assignment, although the
latter (with a Cu2L3 ratio) is in agreement with the ES mass
spectrum and the elemental analysis. We can envisage two
possible structures for the {Cu2L3} stoichiometry. The first is a
triple helicate, in which each of three ligands bridges both metal
ions.19 The second is a structure in which each one ligand is
wholly co-ordinated to each metal ion {as in [CuLCl][BF4]
above}, and one ligand bridges the two copper() centres. We
recently structurally characterised [Ni2L3][PF6]4 and found it
to be of this second structural type, which suggests that the
copper() complex may follow suit,20 and the crystallographic
evidence described below indirectly supports this.
We used EPR and electronic spectroscopy to examine the
complex in solution. The frozen solution X-band EPR spec-
trum of [CuLCl][BF4] [Fig. 4(a)] is characteristic of a complex
2
with a dz ground state arising from a predominantly trigonal
bipyramidal geometry,17,18 with parameters of g|| = 2.00,
g⊥ = 2.19, A|| = 78 G and A⊥ = 109 G (G = 10Ϫ4 T). Even in a
good donor solvent therefore the five-co-ordinate structure is
retained, as solvation would inevitably lead to a six-co-ordinate
2 Ϫ y2
structure with a dx
ground state and a quite different
spectrum. The electronic spectrum (dmf solution) shows the
d–d transition at λmax = 825 nm (ε = 240 dm3 molϪ1 cmϪ1) and
a strong ligand-centred transition at 291 nm (ε = 26 300 dm3
molϪ1 cmϪ1); the relatively low energy and high intensity of the
d–d transition is indicative of a non-tetragonal geometry and
therefore also consistent with the solid-state structure.18
The electronic spectrum of [Cu2L3][BF4]2 in MeCN showed
the expected d–d transition at 759 nm (ε = 160 dm3 molϪ1
cmϪ1), with a second transition visible as a shoulder at about
900 nm. Intense ligand-centred transitions occur at 303 and 252
nm (ε = 31 000 and 72 000 dm3 molϪ1 cmϪ1 respectively). The
frozen solution EPR spectrum [thf–MeCN (1:1) at 77 K],
shown in Fig. 4(b), has the parameters g|| = 2.28, g⊥ = 2.07,
A|| = 156 G which are characteristic of a basically tetragonal
Synthesis of [Cu2L3][PF6]2 and the crystal structure of
[CuL(MeOH)][CuL(SiF6)][BF4]2
It seemed possible that formation of the relatively simple
mononuclear complex [CuLCl][BF4] arose in part from the
presence of the co-ordinated chloride ligand. We therefore
investigated the reaction of L with copper() acetate in
MeOH, and following precipitation of the complex as its
[BF4]Ϫ salt isolated a material whose elemental analysis was
geometry with a dx
ground state.1 There is no evidence in the
2 Ϫ y2
EPR spectrum for any interaction between the metal centres,
but this is not surprising given the likely large separation
between them; in [Ni2L3][BF4]2 the metal–metal separation
2050
J. Chem. Soc., Dalton Trans., 1998, Pages 2047–2052

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Table 4 Selected bond lengths (Å) and angles (Њ) for [CuL(MeOH)]-
[CuL(SiF6)][BF4]2
Cu(1)᎐N(121)
Cu(1)᎐N(151)
Cu(1)᎐N(141)
Cu(1)᎐N(111)
Cu(1)᎐F(34)
Cu(1)᎐F(31)
1.952(4)
1.982(4)
2.089(4)
2.100(4)
2.203(3)
2.355(3)
Cu(2)᎐N(221)
Cu(2)᎐N(251)
Cu(2)᎐N(241)
Cu(2)᎐N(211)
Cu(2)᎐O(1)
1.957(4)
1.996(4)
2.033(4)
2.066(4)
2.242(4)
N(121)᎐Cu(1)᎐N(151)
N(121)᎐Cu(1)᎐N(141)
N(151)᎐Cu(1)᎐N(141)
N(121)᎐Cu(1)᎐N(111)
N(151)᎐Cu(1)᎐N(111)
N(141)᎐Cu(1)᎐N(111)
N(121)᎐Cu(1)᎐F(34)
N(151)᎐Cu(1)᎐F(34)
N(141)᎐Cu(1)᎐F(34)
N(111)᎐Cu(1)᎐F(34)
N(121)᎐Cu(1)᎐F(31)
N(151)᎐Cu(1)᎐F(31)
N(141)᎐Cu(1)᎐F(31)
N(111)᎐Cu(1)᎐F(31)
F(34)᎐Cu(1)᎐F(31)
N(221)᎐Cu(2)᎐N(251)
N(221)᎐Cu(2)᎐N(241)
N(251)᎐Cu(2)᎐N(241)
N(221)᎐Cu(2)᎐N(211)
N(251)᎐Cu(2)᎐N(211)
N(241)᎐Cu(2)᎐N(211)
N(221)᎐Cu(2)᎐O(1)
N(251)᎐Cu(2)᎐O(1)
N(241)᎐Cu(2)᎐O(1)
N(211)᎐Cu(2)᎐O(1)
175.9(2)
98.9(2)
80.6(2)
80.4(2)
96.2(2)
117.4(2)
97.20(13)
86.88(13)
92.52(12)
150.08(13)
87.89(13)
94.32(13)
154.20(12)
88.24(13)
61.84(9)
174.6(2)
99.4(2)
Fig. 5 Crystal structure of the [CuL(MeOH)]2ϩ fragment of [CuL-
(MeOH)][CuL(SiF6)][BF4]2
81.0(2)
80.6(2)
102.9(2)
134.1(2)
88.66(14)
86.2(2)
110.9(2)
115.0(2)
[BF4]Ϫ because partial hydrolysis in the presence of atmospheric
moisture generates traces of HF which reacts with glassware (in
this case the vial used for the recrystallisation),21 and although
not common there are a few examples in which the hexafluoro-
silicate ion is known to act as a bridging ligand by co-ordinating
a fluorine atom to each of two different metal ions.21,22 There
are however no previously reported examples of the [SiF6]2Ϫ
anion acting as a bidentate chelating ligand to a single metal
ion. The two Cu᎐F distances [Cu(1)᎐F(31) 2.355(3), Cu(1)᎐
F(34) 2.203(3) Å; average 2.28 Å] are significantly longer than
those observed with simple fluoride ligands (ca. 1.9–1.95 Å),23
but significantly shorter than the ‘semico-ordinate’ distances of
ca. 2.6 Å arising from weak interactions with e.g. [BF4]Ϫ
anions.24 The geometry of the complex is further distorted by
the four-membered chelate ring, with the angle F(31)᎐Cu(1)᎐
F(34) being 61.84(9)Њ and the ‘opposite’ angle N(111)᎐Cu(1)᎐
N(141) being correspondingly large at 117.4(2)Њ. This geometry
is characteristic of a cis-distorted octahedron, typified by
numerous complexes of the form [Cu(chelate)2(OO)]ϩ, where
‘OO’ is a trigonal planar bidentate anion such as carboxylate or
nitrate which also forms a four-membered chelate ring.25 The
inequivalence of the Cu᎐F distances involving the [SiF6]2Ϫ
ligand is typical of these cis-distorted octahedra and suggests
Fig. 6 Crystal structure of the [CuL(SiF6)] fragment of [CuL-
(MeOH)][CuL(SiF6)][BF4]2
is 9.36 Å. Significantly however there is no evidence for
dissociation of the complex into mononuclear {CuL(solv)}2ϩ
(solv = solvent) fragments, which {cf. the crystal structure
below, and the structure above of [CuLCl]ϩ} would be trigonal
bipyramidal and have a quite different EPR spectrum.
Recrystallisation of pale green [Cu2L3][BF4]2 by slow diffu-
sion of diethyl ether vapour into a concentrated solution of the
complex in acetone–MeOH (1:1) afforded a large amount of
pale green powder, which was unchanged [Cu2L3][BF4]2, and a
small number of dark green crystals which on investigation
proved to be the decomposition product [CuL(MeOH)][CuL-
(SiF6)][BF4]2 (Figs. 5 and 6, Table 4). It is reasonable to suppose
that if [Cu2L3][BF4]2 has the structure proposed above, with
two terminal ligands and one bridging, dissociation of the bis-
bidentate bridging ligand would liberate mononuclear {CuL}2ϩ
fragments which would fill their co-ordination spheres with
whatever ligands are available in the reaction mixture.
The [CuL(MeOH)]2ϩ fragment (Fig. 5) is approximately tri-
gonal bipyramidal with the axial ligands being N(221) and
N(251), and the equatorial ligands being O(1), N(211) and
N(241). The τ value is 0.68, with the principal distortion from
trigonal-bipyramidal geometry arising from a distortion in the
trigonal plane where the three angles are 134.1(2), 100.9(2) and
115.0(2)Њ, resulting in a distortion towards a square-pyramidal
geometry in which O(1) would be the apical ligand. Apart from
differences arising from the different steric bulk of the MeOH
ligand compared to chloride, this complex has a similar
structure to that of [CuLCl]ϩ.
a
distortion towards a Jahn–Teller elongated geometry.
Unfortunately only a few single crystals of this decomposition
product appeared so further spectroscopic characterisation was
not possible.
Conclusion
The new ligand L was simply prepared by reaction of 3-(2-
pyridyl)pyrazole with α,αЈ-dibromoxylene; this is a potentially
general route to a wide variety of new multinucleating
ligands. Whereas L acts as a bridging ligand between two
metal ions in the double helicate [CuI2L2][PF6]2, it is also capable
of co-ordinating all four donor atoms to one metal centre in
various five- and six-co-ordinate mononuclear copper()
The neutral fragment [CuL(SiF6)], containing a bidentate
hexafluorosilicate anion, is more unusual (Fig. 6). Hexafluoro-
silicate anions can arise in compounds originally containing
J. Chem. Soc., Dalton Trans., 1998, Pages 2047–2052
2051

View Article Online
13 C. Piguet, G. Bernardinelli and G. Hopfgartner, Chem. Rev., 1997,
97, 2005; D. Philp and J. F. Stoddart, Angew. Chem., Int. Ed. Engl.,
1996, 35, 1155; E. C. Constable, Prog. Inorg. Chem., 1994, 42, 67.
14 A. M. W. Cargill Thompson, I. Blandford, H. Redfearn, J. C. Jeffery
and M. D. Ward, J. Chem. Soc., Dalton Trans., 1997, 2661; Y.-J. Fu,
H. Yang, D.-F. Wang, W.-X. Tang, B. M. Wu and T. C. W. Mak,
Polyhedron, 1997, 16, 1505.
complexes. Of these [CuL(SiF6)] is of particular interest as it
contains the first example of a chelating hexafluorosilicate
anion.
Acknowledgements
15 A. W. Addison, T. N. Rao, J. Reedijk, J. van Rijn and G. C.
Verschoor, J. Chem. Soc., Dalton Trans., 1984, 1349.
16 G. Murphy, C. O’Sullivan, B. Murphy and B. Hathaway, Inorg.
Chem., 1998, 37, 240; G. Murphy, P. Nagle, B. Murphy and
B. Hathaway, J. Chem. Soc., Dalton Trans., 1997, 2645; G. Murphy,
C. Murphy, B. Murphy and B. Hathaway, J. Chem. Soc., Dalton
Trans., 1997, 2653.
17 M. Duggon, N. Ray, B. Hathaway, G. Tomlinson, P. Brint and
K. Pelin, J. Chem. Soc., Dalton Trans., 1980, 1342; R. Barbucci,
A. Bencini and D. Gatteschi, Inorg. Chem., 1997, 16, 2117;
A. Bencini, I. Bertini, D. Gatteschi and A. Scozzafava, Inorg. Chem.,
1978, 17, 3194; R. Barbucci and M. J. M. Campbell, Inorg. Chim.
Acta, 1975, 15, L15.
We thank the EPSRC for financial support, and Dr. J. P. Maher
for assistance with the EPR spectroscopy.
References
1 B. J. Hathaway, Comprehensive Coordination Chemistry, eds.
G. Wilkinson, R. D. Gillard and J. A. McCleverty, Pergamon,
Oxford, 1987, vol. 5, p. 533.
2 P. L. Jones, J. C. Jeffery, J. P. Maher, J. A. McCleverty, P. H. Rieger
and M. D. Ward, Inorg. Chem., 1997, 36, 3088.
3 W. Kaim and B. Schwederski, Bioinorganic Chemistry: Inorganic
Elements in the Chemistry of Life, Wiley, Chichester, UK, 1995.
4 K. K. Nanda, A. W. Addison, R. J. Butcher, M. R. McDevitt, T. N.
Rao and E. Sinn, Inorg. Chem., 1997, 36, 134; E. Müller, G.
Bernardinelli and J. Reedijk, Inorg. Chem., 1996, 35, 1952; C. F.
Martens, A. P. H. J. Schenning, M. C. Feiters, H. W. Berens, J. G. M.
van der Linden, G. Admiraal, P. T. Buerskens, H. Koojiman, A. L.
Spek and R. J. M. Noelte, Inorg. Chem., 1995, 34, 4735; S. Knapp,
T. P. Keenan, X. Zhang, R. Fikar, J. A. Potenza and H. J. Schugar,
J. Am. Chem. Soc., 1990, 112, 3452; W. E. Davis, A. Zask,
K. Nakanishi and S. J. Lippard, Inorg. Chem., 1985, 24, 3737;
J. Gouteron, S. Jeannin, Y. Jeannin, J. Livage and C. Sanchez,
Inorg. Chem., 1984, 23, 3387.
5 D. A. Bardwell, J. C. Jeffery, P. L. Jones, J. A. McCleverty,
E. Psillakis, Z. Reeves and M. D. Ward, J. Chem. Soc., Dalton
Trans., 1997, 2079; P. L. Jones, K. J. Byrom, J. C. Jeffery, J. A.
McCleverty and M. D. Ward, Chem. Commun., 1997, 1361; D. A.
Bardwell, J. C. Jeffery, J. A. McCleverty and M. D. Ward, Inorg.
Chim. Acta, 1998, 267, 323.
18 N. Wei, N. N. Murthy and K. D. Karlin, Inorg. Chem., 1994, 33,
6093 and refs. therein.
19 C. Provent, S. Hewage, G. Brand, G. Bernardinelli, L. J.
Charbonnière and A. F. Williams, Angew. Chem., Int. Ed. Engl.,
1997, 36, 1287; M. J. Hannon, C. L. Painting, A. Jackson,
J. Hamblin and W. Errington, Chem. Commun., 1997, 1807;
N. Yoshida and K. Ichikawa, Chem. Commun., 1997, 1091; D. L.
Caulder and K. N. Raymond, Angew. Chem., Int. Ed. Engl., 1997,
36, 1440; V. A. Grillo, E. J. Seddon, C. M. Grant, G. Aromí, J. C.
Bollinger, K. Folting and G. Christou, Chem. Commun., 1997, 1561;
R. W. Saalfrank, A. Dresel, V. Seitz, S. Trummer, F. Hampel,
M. Teichert, D. Stalke, C. Stadler, J. Daub, V. Schünemann and
A. X. Trautwein, Chem. Eur. J., 1997, 3, 2058; M. Albrecht and
O. Blau, Chem. Commun., 1997, 345.
20 J. S. Fleming, K. L. V. Mann, C.-A. Carraz, E. Psillakis, J. C. Jeffery,
J. A. McCleverty and M. D. Ward, Angew. Chem., in the press.
21 P. J. van Koningsbruggen, J. G. Haasnoot, R. A. G. de Graaff and
J. Reedijk, J. Chem. Soc., Dalton Trans., 1993, 483.
22 S. Subramanian and M. Zaworotko, Angew. Chem., Int. Ed. Engl.,
1995, 34, 2127; R. A. J. Driessen, F. B. Hulsbergen, W. J. Vermin and
J. Reedijk, Inorg. Chem., 1982, 21, 3594; F. D. Keij, R. A. G. de
Graaff, J. G. Haasnoot, J. Reedijk and E. Pedersen, Inorg. Chim.
Acta, 1989, 156, 65; E. W. Ainscough, A. M. Brodie, J. D. Ranford
and J. M. Waters, Inorg. Chim. Acta, 1995, 236, 83.
6 K. L. V. Mann, J. C. Jeffery, J. A. McCleverty, P. Thornton and
M. D. Ward, J. Chem. Soc., Dalton Trans., 1998, 89.
7 E. Psillakis, J. C. Jeffery, J. A. McCleverty and M. D. Ward, J. Chem.
Soc., Dalton Trans., 1997, 1645.
8 J.-P. Gisselbrecht, M. Gross, J.-M. Lehn, J.-P. Sauvage, R. Ziessel,
C. Piccini-Leopardi, J. M. Arrieta, G. Germain and M. van
Meersche, Nouv. J. Chim., 1984, 8, 659; Y. Yao, M. W. Perkovic,
D. P. Rillema and C. Woods, Inorg. Chem., 1992, 31, 3956.
9 A. J. Amoroso, A. M. W. Cargill Thompson, J. C. Jeffery, P. L.
Jones, J. A. McCleverty and M. D. Ward, J. Chem. Soc., Chem.
Commun., 1994, 2751; H. Brunner and T. Scheck, Chem. Ber., 1992,
125, 701.
10 P. L. Jones, A. J. Amoroso, J. C. Jeffery, J. A. McCleverty,
E. Psillakis, L. H. Rees and M. D. Ward, Inorg. Chem., 1997, 36, 10.
11 SHELXTL 5.03 program system; Siemens Analytical X-Ray
Instruments, Madison, WI, 1995.
23 J. van Rijn and J. Reedijk, Recl. Trav. Chim. Pays-Bas, 1984, 103, 78;
W. C. Velthuizen, J. G. Haasnoot, A. J. Kinneging, F. J. Rietmeijer
and J. Reedijk, J. Chem. Soc., Chem. Commun., 1983, 1366; H. G.
van Schnering, Z. Anorg. Allg. Chem., 1967, 353, 13.
24 H. Nakai, Bull. Chem. Soc. Jpn., 1983, 56, 1637; J. Foley,
D. Kennefick, D. Phelan, S. Tyagi and B. J. Hathaway, J. Chem. Soc.,
Dalton Trans., 1983, 2333; A. S. Batsanov, P. Hubberstey and C. E.
Russell, J. Chem. Soc., Dalton Trans., 1994, 3189.
25 See pp. 611–612 in ref. 1.
12 SADABS: A program for absorption correction with the Siemens
SMART system; G. M. Sheldrick, University of Göttingen, 1996.
Received 24th March 1998; Paper 8/02280A
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