Localization and delocalization in a mixed-valence dicopper helicate

Article abstract of DOI:10.1021/ic061504m

The coordination chemistry of the tetradentate pyridyl-thiazole (py-tz) N-donor ligand 6,6′-bis(4-phenylthiazol-2-yl)-2,2′-bipyridine (L¹) has been investigated. Reaction of L¹ with equimolar copper(II) ions results in the formation of the single-stranded mononuclear complex [Cu(L¹)(ClO₄)₂] (1), whereas reaction with copper(I) ions results in the double-stranded dinuclear helicate [Cu ₂(L¹)₂][PF₆]₂ (2). Both complexes were characterized by X-ray crystallography, UV-vis spectroscopy, and electrospray ionization mass spectroscopy (as well as ¹H NMR spectroscopy for diamagnetic 2). Complex 2 is redox-active and, upon one-electron oxidation, forms the stable tricationic mixed-valence helicate [Cu₂(L¹)₂]³⁺ (3). This species can also be prepared in situ by combining [Cu(MeCN)₄][BF₄], [Cu-(H₂O)₆][BF₄]₂, and L¹ in a 1:1:2 ratio in nitromethane. X-ray crystallographic analysis of 3 provides structural evidence for the presence of an internuclear Cu-Cu bond, with an even distribution of spin density across the two Cu centers. Room-temperature UV-vis spectroscopy is consistent with this finding; however, frozen-glass EPR spectroscopic investigations suggest solvatochromic behavior at 110 K, with the [Cu₂]³⁺ core varying from localized to delocalized depending on the solvent polarity.

Full text of DOI:10.1021/ic061504m

Inorg. Chem. 2007, 46, 2417−2426
Localization and Delocalization in a Mixed-Valence Dicopper Helicate
John C. Jeffery,*,† Thomas Riis-Johannessen,*,† Callum J. Anderson,† Christopher J. Adams,†
Adam Robinson,† Stephen P. Argent,‡ Michael D. Ward,‡ and Craig R. Rice§
School of Chemistry, UniVersity of Bristol, Bristol BS8 1TS, U.K., Department of Chemistry,
UniVersity of Sheffield, Sheffield S3 7HF, U.K., and Department of Chemical and Biological
Sciences, UniVersity of Huddersfield, Huddersfield HD1 3DH, U.K.
Received August 9, 2006
The coordination chemistry of the tetradentate pyridyl-thiazole (py-tz) N-donor ligand 6,6
2,2
′-bis(4-phenylthiazol-2-yl)-
′
-bipyridine (L1) has been investigated. Reaction of L1 with equimolar copper(II) ions results in the formation of
the single-stranded mononuclear complex [Cu(L1)(ClO4)2] (1), whereas reaction with copper(I) ions results in the
double-stranded dinuclear helicate [Cu2(L1)2][PF6]2 (2). Both complexes were characterized by X-ray crystallography,
UV−
vis spectroscopy, and electrospray ionization mass spectroscopy (as well as 1H NMR spectroscopy for
diamagnetic 2). Complex 2 is redox-active and, upon one-electron oxidation, forms the stable tricationic mixed-
+
valence helicate [Cu2(L1)2]3 (3). This species can also be prepared in situ by combining [Cu(MeCN)4][BF4], [Cu-
(H2O)6][BF4]2, and L1 in a 1:1:2 ratio in nitromethane. X-ray crystallographic analysis of 3 provides structural evidence
for the presence of an internuclear Cu−Cu bond, with an even distribution of spin density across the two Cu
centers. Room-temperature UV
−
vis spectroscopy is consistent with this finding; however, frozen-glass EPR
spectroscopic investigations suggest solvatochromic behavior at 110 K, with the [Cu2]3 core varying from localized
to delocalized depending on the solvent polarity.
+
Introduction
metals into a polynuclear array1 and (ii) probing the
mechanistic and energetic aspects of multicomponent self-
assembly processes at a fundamental level.2
Of the many elaborate metallosupramolecular architectures
described in literature reports over the past 30 years, helicates
are among those most frequently addressed.1-4 Their ubi-
quitous structural motifs lend them significant aesthetic and
biomimetic appeal,1a-c,3 while from a synthetic perspective,
they offer vast potential for (i) incorporating two or more
Ever since early studies by the groups of Lehn,3 Con-
stable,4 and Potts5 established the versatile complexation
properties of the oligo-pyridine derivatives, a wide range of
polydentate ligands has been developed for the formation
of helicate complexes.1 Notable examples include the bis-
catecholate O-donor ligands of Raymond et al.,1d,6 the
benzenedithiol ligands of Hahn et al.,7 the pyrazole-phenol
N,O donors of Ward et al.,8 and the pyridyl-benzimidazole/
amido mixed N,N/N,O donors of Piguet et al.1c,2,9 By careful
consideration of the donor-atom disposition and structural
* To whom correspondence should be addressed. E-mail:
T.Riis-Johannessen@bristol.ac.uk.
† University of Bristol.
‡ University of Sheffield.
§ University of Huddersfield.
(1) (a) Hannon, M. J.; Childs, L. J. Supramol. Chem. 2004, 16, 7. (b)
Albrecht, M. Chem. ReV. 2001, 101, 3457. (c) Piguet, C.; Bernardinelli,
G.; Hopfgartner, G. Chem. ReV. 1997, 97, 2005. (d) Yeh, R. M.; Davis,
A. V.; Raymond, K. N. In ComprehensiVe Coordination Chemistry
II; McCleverty, J. A., Meyer, T. J., Eds.; Elsevier Ltd.: Oxford, U.K.,
2004; Vol. 7, p 327.
(2) (a) Piguet, C.; Borkovec, M.; Hamacek, J.; Zeckert, K. Coord. Chem.
ReV. 2005, 249, 705. (b) Hamacek, J.; Borkovec, M.; Piguet, C. Chem.
Eur. J. 2005, 11, 5217. (c) Hamacek, J.; Borkovec, M.; Piguet, C.
Chem. Eur. J. 2005, 11, 5227.
(5) (a) Potts, K. T.; Keshavarz-K, M.; Tham, F. S.; Abrun˜a, H. D.; Arana,
C. R. Inorg. Chem. 1993, 32, 4422. (b) Potts, K. T.; Keshavarz-K,
M.; Tham, F. S.; Abrun˜a, H. D.; Arana, C. R. Inorg. Chem. 1993, 32,
4436. (c) Potts, K. T.; Keshavarz-K, M.; Tham, F. S.; Gheysen Raiford,
K. A. (d) Abrun˜a, H. D.; Arana, C. R. Inorg. Chem. 1993, 32, 4422;
Potts, K. T.; Keshavarz-K, M.; Tham, F. S.; Abrun˜a, H. D.; Arana,
C. R. Inorg. Chem. 1993, 32, 4450.
(6) See, for example: (a) Yeh, R. M.; Raymond, K. N. Inorg. Chem.
2006, 45, 1130. (b) Caulder, D. L.; Raymond, K. N. J. Chem. Soc.,
Dalton Trans. 1999, 1185. (c) Caulder, D. L.; Raymond, K. N. Angew.
Chem., Int. Ed. Engl. 1997, 36, 1440.
(3) (a) Lehn J.-M. Supramolecular ChemistrysConcepts and PerspectiVes;
VCH: Weinheim, Germany, 1995. (b) Lehn, J.-M. Angew. Chem.,
Int. Ed. Engl. 1990, 29, 1304.
(4) Constable, E. C. In ComprehensiVe Supramolecular Chemistry;
Atwood, J. L., Davies, J. E. D., Lehn, J.-M., MacNicol, D. D., Vo¨gtle,
F., Eds.; Pergamon Press: Oxford, U.K., 1996; Vol. 9, p 213.
(7) See, for example: (a) Hahn, F. E.; Isfort, C. S.; Pape, T. Angew. Chem.,
Int. Ed. 2004, 43, 4807. (b) Hahn, F. E.; Kreickmann, T.; Pape, T.
Dalton Trans. 2006, 769.
10.1021/ic061504m CCC: $37.00
Published on Web 02/28/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 7, 2007 2417

Jeffery et al.
Chart 1. Schematic Diagram of Ligands L1-L3
Ba2+ or Sr2+ ions by the µ-[18]crown-6 macrocycles in the
dimercury helicate [Hg2(L3)2]4+, for example, causes rapid
disassembly of the helicate. Within the context of ligand
programming, the remote crown ether sites on L3 thus offer
the potential for the ligand to be reprogrammed (from
double- to single-stranded coordination modes) by the
sequential binding of specific s-block cations.10a
In this article, we report several complexes of the related
ligand L1 with copper(I) and (II) ions and discuss their
solution and solid-state structures. Our results demonstrate
how the replacement of terminal methyl groups (in L2) with
phenyl groups (in L1) can dramatically alter the coordination
properties of the tetradentate pyridyl-thiazole donor domains
common to both ligands. In particular, self-assembly of L1
with CuI ions leads to the formation of a redox-active [Cu2-
(L1)2]2+ helicate, the (reversible) electrochemical oxidation
of which is accompanied by the formation of a Cu-Cu bond
and subsequent flattening of the helical pitch. Both members
of the redox-related [Cu2(L1)2]2+/3+ pair have been structur-
ally characterized by X-ray crystallography, and the stable
tricationic [Cu2(L1)2]3+ helicate was further probed using
UV-vis and EPR spectroscopies. The latter complex is, to
the best of our knowledge, the first structurally characterized
cupric helicate to exhibit a significant intermetallic bonding
interaction.
rigidity of the ligand backbone, many of these ligands have
been designed such that adjacent donor atoms in the sequence
are prevented from coordinating to a common metal ion.
Accordingly, they can be considered as being programmed
to partition into distinct binding sites, a feature that militates
in favor of the formation of polynuclear helicate structures.3
As part of our ongoing investigations into the use of ortho-
linked pyridyl-thiazole-based ligands for the assembly and
control of helicate structures, we have recently been exploit-
ing simple tetradentate N donors (L1-L3) of the type shown
in Chart 1.10 The appeal of these ligands lies in both their
synthetic accessibility11 and their capacity to adopt one of
two distinct coordination modes depending on the preference
of the metal ion for a particular coordination number or
geometry.10c The monotopic ligand L2 behaves in a similar
manner to 2,2′:6′,2′′:6′′,2′′′-quaterpyridine (qtpy):12 it forms
single-stranded mononuclear complexes of the type [M(L2)]2+
with transition metal dications and double-stranded dinuclear
[M2(L2)2]2+ complexes with the group XI monocations CuI
and AgI.10c The behavior displayed by the ditopic crown ether
derivative L3, however, is more remarkable.10a,b Binding of
Results and Discussion
Synthesis and Crystal Structure of [Cu(L1)(ClO4)2] (1).
The copper(II) complex of L1 was prepared by reacting
equimolar amounts of ligand with [Cu(H2O)6][ClO4]2 in
nitromethane. Slow diffusion of diethyl ether vapor into the
resulting solution afforded dark green microcrystals, for
which electrospray ionization mass spectroscopy and el-
emental analysis suggested the formulation [Cu(L1)(ClO4)2].
The solid-state structure was established by a single-crystal
X-ray diffraction to be the single-stranded copper(II) complex
[Cu(L1)(ClO4)2]‚MeNO2 (1‚MeNO2; Figure 1).
(8) See, for example: (a) Ronson, T. K.; Adams, H.; Riis-Johannessen,
T.; Jeffery, J. C.; Ward, M. D. New J. Chem. 2006, 30, 26. (b) Ronson,
T. K.; Adams, H.; Ward, M. D. Inorg. Chim. Acta 2005, 358, 1943.
(9) See, for example: (a) Zeckert, K.; Hamacek, J.; Senegas, J.-M.; Dalla-
Favera, N.; Floquet, S.; Bernardinelli, G.; Piguet, C. Angew. Chem.,
Int. Ed. 2005, 44, 1. (b) Torelli, S.; Delahaye, S.; Hauser, A.;
Bernardinelli, G.; Piguet, C. Chem. Eur. J. 2004, 10, 3503. (c) Floquet,
S.; Borkovec, M.; Bernardinelli, G.; Pinto, A.; Leuthold, L.-A.;
Hopfgartner, G.; Imbert, D.; Bu¨nzli, J.-C. G.; Piguet, C. Chem. Eur.
J. 2004, 10, 1091. (d) Floquet, S.; Ouali, N.; Bocquet, B.; Bernardinelli,
G.; Imbert, D.; Bunzli, J.-C. G.; Hopfgartner, G.; Piguet, C. Chem.
Eur. J. 2003, 9, 1860.
(10) (a) Baylies, C. J.; Harding, L. P.; Jeffery, J. C.; Riis-Johannessen, T.;
Rice, C. R. Angew. Chem., Int. Ed. 2004, 43, 4515. (b) Baylies, C. J.;
Riis-Johannessen, T.; Harding, L. P.; Jeffery, J. C.; Moon, R.; Rice,
C. R.; Whitehead, M. Angew. Chem., Int. Ed. 2005, 44, 6909. (c) Riis-
Johannessen, T.; Jeffery, J. C.; Robson, A. P. H.; Rice, C. R.; Harding,
L. P. Inorg. Chim. Acta 2005, 358, 2781.
(11) (a) Rice, C. R.; Wo¨rl, S.; Jeffery, J. C.; Paul, R. L.; Ward, M. D. J.
Chem. Soc., Dalton Trans. 2001, 550. (b) Rice, C. R.; Wo¨rl, S.; Jeffery,
J. C.; Paul, R. L.; Ward, M. D. Chem. Commun. 2000, 1529.
(12) (a) Constable, E. C.; Elder, S. M.; Healy, J.; Tocher, D. A. J. Chem.
Soc., Dalton Trans. 1990, 1169. (b) Maslen, E. N.; Raston, C. L.;
White, A. H. J. Chem. Soc., Dalton Trans. 1975, 323. (c) Henke, Von,
W.; Kremer, S.; Reinen, D. Z. Anorg. Allg. Chem. 1982, 491, 124.
(d) Constable, E. C.; Elder, S. M.; Healy, J.; Ward, M. D.; Tocher, D.
A. J. Am. Chem. Soc. 1990, 112, 4590. (e) Constable, E. C.; Elder, S.
M.; Tocher, D. A. Polyhedron 1992, 11, 1337. (f) Constable, E. C.;
Elder, S. M.; Hannon, M. J.; Martin, A.; Raithby, P. R.; Tocher, D.
A. J. Chem. Soc., Dalton Trans. 1996, 2423.
The structure of 1 closely resembles that of the previously
characterized copper(II) complex of L2 [Cu(L2)(H2O)(ClO4)]-
[ClO4],10c except that in the former, both axial sites on the
metal are occupied by perchlorate anions, leading to a charge-
neutral complex in the solid state. The respective Cu-O(1A)
and Cu-O(2A) distances of 2.573(2) and 2.532(2) Å are
-
long and reminiscent of the pseudocoordinate CuII‚‚‚BF4
interactions previously reported for the tetrafluoroborate salt
of bis(2,2′-bipyridine)copper(II), in which weakly bound
anions occupy the remote axial sites of a Jahn-Teller
distorted copper(II) system.13
Ligand L1 coordinates to the equatorial plane of the CuII
center via four N donors (Figure 1), and the all-cisoid
N-donor conformation brings the two terminal Ph rings into
close proximity to one another at the open end of the
complex. Far from causing strain at this end of the molecule,
however, the interplanar distance of ca. 3.3 Å tends to suggest
the presence of offset π-π stacking interactions between
(13) Foley, J.; Kennefick, D.; Phelan, D.; Tyagi, S.; Hathaway, B. J. Chem.
Soc., Dalton Trans. 1983, 2333.
2418 Inorganic Chemistry, Vol. 46, No. 7, 2007

Localization in a Mixed-Valence Dicopper Helicate
Figure 1. Two views of the solid-state structure of [Cu(L1)(ClO4)2] (1) with emphasis on (a) intraligand stacking interactions and (b) planar tetradentate
coordination of the ligand to the CuII center.
Table 1. Selected Bond Lengths (Å) and Angles (deg) for 1
Cu(1)-N(21)
Cu(1)-N(41)
2.027(2)
1.947(2)
Cu(1)-N(31)
Cu(1)-N(51)
1.940(2)
2.029(2)
N(31)-Cu(1)-N(41) 79.18(7)
N(41)-Cu(1)-N(21) 157.18(7) N(31)-Cu(1)-N(51) 157.79(7)
N(41)-Cu(1)-N(51) 80.70(7) N(21)-Cu(1)-N(51) 120.79(7)
N(31)-Cu(1)-N(21) 80.60(7)
the ligand termini (Figure 1).14 A general pattern of long
Cu-N(tz) and short Cu-N(py) bonds is apparent (Table 1),
and with a Cu-N range 1.940(2)-2.029(2) Å, all metal-
L1 bonds are significantly shorter than the equivalent bonds
in [Cu(L2)(H2O)(ClO4)], for which the Cu-N separations
lie in the range of 1.977(2)-2.189(2) Å.10c
EPR Spectroscopy. The X-band EPR spectrum of 1 in
nitromethane (110 K) shows two broad features with g values
of g| ) 2.28 and g ) 2.08. The low-field feature is further
split into a poorly resolved quartet indicating hyperfine
coupling to one Cu nucleus (I ) 3/2) with a coupling constant
of A| ≈ 140 G. These parameters are entirely typical of a
copper(II) complex with approximate axial symmetry and a
Figure 2. Solid-state structure of 2 showing (a) the (P)-enantiomer of the
complex cation 22+ and (b) a chain of three adjacent cations 22+ (viewed
along the crystallographic b axis) to emphasize intermolecular stacking
interactions.
Table 2. Selected Bond Lengths (Å) and Angles (deg) for 2
Cu(1)-N(21)
Cu(1)-N(31)
Cu(1)-N(81)
Cu(1)-N(91)
2.053(3)
2.090(3)
2.066(4)
2.076(3)
Cu(2)-N(41)
Cu(2)-N(51)
Cu(2)-N(101)
Cu(2)-N(111)
2.058(4)
2.055(3)
2.055(4)
2.053(4)
1
2
15
2
(dx -y ) ground-state configuration.
Synthesis and Crystal Structure of 2. The copper(I)
complex of L1 was similarly prepared by reacting equimolar
amounts of [Cu(MeCN)4][PF6] and ligand in nitromethane
to afford a dark red solution. Diffusion of diethyl ether into
the solution afforded a dark red crystalline material for which
the electrospray mass spectrum (of crystals dissolved in
nitromethane) showed peaks for only the dicopper(I) complex
[Cu2(L1)2]2+. The structure was confirmed by X-ray crystal-
lographic analysis as the dicopper(I) double-stranded helicate
[Cu2(L1)2][PF6]2‚MeNO2‚MeCN (Figure 2). Again, the struc-
ture closely resembles that of the L2 analogue [Cu2(L2)2]2+,
with the two ligands being partitioned into two bis-bidentate
py-tz binding domains by rotation of ca. 37.5° about the
central py-py bond.10c
The metals are coordinated by one py-tz binding domain
from each ligand, and the lack of interaction with either
solvent or anion species gives the two CuI centers essentially
identical flattened tetrahedral geometries. (The dihedral
angles θ between coordinating CuNN planes are ca. 66° in
each case.) With interligand separations in the range of 3.3-
3.7 Å, the structure also appears to exhibit extensive π-π
N(21)-Cu(1)-N(81) 134.93(13) N(111)-Cu(2)-N(51) 134.65(14)
N(21)-Cu(1)-N(91) 117.05(13) N(111)-Cu(2)-N(101) 81.40(14)
N(81)-Cu(1)-N(91) 80.87(15) N(51)-Cu(2)-N(101) 111.57(14)
N(21)-Cu(1)-N(31) 80.72(12) N(111)-Cu(2)-N(41) 114.89(13)
N(81)-Cu(1)-N(31) 110.46(13) N(51)-Cu(2)-N(41)
81.98(15)
N(91)-Cu(1)-N(31) 142.79(13) N(101)-Cu(2)-N(41) 142.11(14)
stacking interactions between overlapping py-tz rings on
adjacent ligand strands.14
The feature that most distinguishes the solid-state structure
of the present complex 22+ from that of previously reported
[Cu2(L2)2]2+ is the reduction in helical pitch ∆ of ca. 0.87 Å
observed when the terminal methyl group is replaced by a
phenyl substituent. This corresponds to a Cu‚‚‚Cu separation
of 2.933(1) Å in 22+ {cf. 3.805(1) Å for [Cu2(L2)2]2+}, and
the resulting compression allows both metals to achieve a
much more uniform distribution of Cu-N separations (Table
2). The change is also apparent from the decrease in py-py
dihedral angle of ca. 20° on going from [Cu2(L2)2]2+ to 22+.10c
Despite having a notably shorter Cu‚‚‚Cu separation than
both [Cu2(L2)2]2+ and previously reported [Cu2(qtpy)2]2+,12f
the value of 2.933(1) Å for 22+ is still greater than the
Cu‚‚‚Cu distance observed in metallic copper (2.56 Å) and
lies outside the range generally accepted as constituting a
(14) Hunter, C. A.; Lawson, K. R.; Perkins, J.; Urch, C. J. Chem. Soc.,
Perkin Trans. 2 2001, 651.
(15) Hathaway, B.; Billing, D. E. Coord. Chem. ReV. 1970, 5, 143.
Inorganic Chemistry, Vol. 46, No. 7, 2007 2419

Jeffery et al.
Figure 3. Cyclic voltammogram of 22+ in MeNO2.
Figure 4. Evolution of the electronic (UV-vis) absorption spectrum during
electrochemical oxidation of 22+ with an applied potential of +0.8 V (T )
-20 °C). The inset shows the appearance of the broad transition in the
near-IR region. Red and blue lines show the initial and final states,
respectively.
significant bonding interaction (ca. 2.4-2.7 Å).16 It is
therefore probable that the contraction in Cu‚‚‚Cu separation
for 2 occurs to enable optimal overlap between the π-π
stacked ligand strands, although the potential influence of
crystal packing interactions on solid-state geometry is also
worth taking into consideration.34
The crystal packing arrangement of cations in the crystal
structure of 2 is also of interest. Overlap between the tz and
phenyl rings on adjacent molecules generates extended
columns of π-π stacked helicates that propagate in the
crystallographic 101 direction (Figure 2b). The columns
retain their screw sense throughout the entire lattice, but
because it exhibits inversion symmetry, the crystal as a whole
contains an equal number of both (P)- and (M)-configured
helices.
1H NMR Spectroscopy. The 300-MHz 1H NMR spectra
of 22+ in CD3NO2 shows a well-resolved set of seven
resonances, with three pyridyl and one thiazole resonance
in the range δ 7.0-7.9 and three phenyl resonances in the
range δ 6.9-7.5. This is consistent with the formation of a
D2-symmetric double-stranded species, for which facile
rotation of the terminal phenyl moieties renders the protons
in the ortho and meta positions (relative to the tz ring)
equivalent on the NMR time scale.
complex in other common organic solvents precluded further
electrochemical characterization.
The cyclic voltammogram is consistent with fully revers-
ible one-electron oxidation of 22+ to give a stable tricationic
species for which the same overall dicopper double-stranded
helicate topology is retained. This behavior contrasts with
that of the L2 analogue [Cu2(L2)2]2+, which under the same
conditions displayed a pseudoreversible redox process (peak-
peak separation ∆Ep was ca. 200 mV at a scan rate of 200
mV s-1), indicative of at least partial disassembly to give
the mononuclear [CuII(L2)]2+ oxidation product.10c
UV-vis spectroelectrochemical investigations were carried
out on a nitromethane solution containing 22+ (ca. 0.1 mM).
The electronic spectrum of the dicopper(I) starting material
shows two metal-to-ligand charge-transfer (MLCT) bands
at ca. 420 nm (ꢀ ) 3500 M-1 cm-1) and 550 nm (ꢀ ) 2150
M-1 cm-1) and an additional intense band at ca. 340 nm
corresponding to a ligand-based π-π* transition. The relative
intensities and energies of two MLCT featuressespecially
the relatively intense band at 550 nmsare consistent with
the solution structure of 22+ retaining distorted tetrahedral
copper(I) geometries (θ ≈ 66°) similar to those observed in
the solid state.17
When the dicopper(I) helicate was oxidized using an
applied potential of +0.8 V, successive scans showed a
gradual depletion of both MLCT bands and the appearance
of a shoulder at ca. 390 nm on the (red-shifted) ligand-based
transition (Figure 4). Aside from the latter, the final spectrum
contains only two broad bands at ca. 490 nm (ꢀ ) 1600 M-1
cm-1) and 1470 nm (ꢀ ) 1500 M-1 cm-1). The overlaid
spectra show two isosbestic points at 360 and 420 nm,
indicating clean conversion between only two species. In
particular, the weak d-d transition at 650 nm (ꢀ ) 300 M-1
cm-1), characteristic of the mononuclear complex 1, is absent
in the final scan, and the possibility that electrochemical
oxidation causes the helicate to disassemble into mononuclear
copper(II) species can thus be excluded.
The chemical shifts of the py and tz resonances of 22+
appear upfield with respect to those of the single-stranded
mononuclear complexes [M(L2)]2+ (where M ) ZnII, CdII
or HgII),10c and all fall within the characteristic range (ca. δ
7.0-8.0) for the double-stranded helicates [M2(L2)2]2+
1
(where M ) CuI, AgI).10c The H NMR spectrum of 22+ is
thus typical of a helicate structure in which the py-tz protons
on one ligand strand are held within the shielding regions
of the aromatic rings on the other strand.
Cyclic Voltammetry and UV-Vis Spectroelectrochem-
istry. Cyclic voltammetry of 22+ in MeNO2 showed a fully
reversible redox process at a potential of 0.72 V (V s SCE)
(Figure 3). This behavior was maintained for scan rates
ranging from 20 mV s-1 to 2 V s-1, and the peak-peak
separation ∆Ep was ca. 90 mV at a scan rate of 200 mV s-1.
No further redox activity was observed within the solvent-
accessible potential window, and the poor solubility of the
(16) van Koten, G.; Noltes, J. G. In ComprehensiVe Organometallic
Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E., Eds.; Pergamon
Press: New York, 1982; Vol. 2, Chapter 14, p 772.
(17) Ichinaga, A. K.; Kirchhoff, J. R.; McMillin, D. R.; Dietrich-Buchecker,
C. O.; Marnot, P. A.; Sauvage, J.-P. Inorg. Chem. 1987, 26, 4290.
2420 Inorganic Chemistry, Vol. 46, No. 7, 2007

Localization in a Mixed-Valence Dicopper Helicate
The appearance of the lowest-energy band at λ ) 1470
nm (Figure 4, inset) can be attributed to one of the
following: (i) a d-d transition within one CuIIN4 chro-
mophore, (ii) an interaction between the two Cu centers in
a localized (class II) mixed-valence dicopper system, or (iii)
a transition within the d-orbital manifold of a delocalized
(class III) [Cu2]3+ core. The first of these scenarios is very
unlikely given that both its intensity (ꢀ ) 1500 M-1 cm-1)
and its wavelength (1470 nm) are significantly higher than
those typically expected for d-d transitions even in tetra-
hedrally distorted CuIIN4 chromophores; these normally
appear in the range of 700-1200 nm with extinction
coefficients in the range 100 e ꢀ e 1000 M-1 cm-1.18 On
this basis, the band is tentatively assigned to a transition
within the [Cu2]3+ chromophore, although whether it corre-
sponds to an intervalence charge-transition (IVCT) or simply
an allowed transition between two orbitals encompassing
both Cu ions (e.g., the HOMO f SOMO transition) is not
immediately obvious. The former scenario would imply that
the structure has a valence-localized CuI-CuII configuration,
corresponding to a class II system in the Robin and Day
classification of mixed-valence compounds, whereas the latter
would imply a delocalized average-valence Cu1.5-Cu1.5 state,
characteristic of solvent-decoupled class III behavior.19
A simple analysis of the bandwidth at half-height, ∆νj1/2,
can provide a certain degree of insight into the nature of the
transition. According to Hush theory, the relationship
Figure 5. (M)-enantiomer of the complex cation (B)-33+. The symmetry
operator for generating equivalent atoms is -x, y, 1.5 - z.
Table 3. Selected Bond Lengths (Å) and Angles (deg) for 3a
A
B
Cu(1)-Cu(2)
Cu(1)-N(21)
Cu(1)-N(31)
Cu(2)-N(41)
Cu(2)-N(51)
2.450(1)
2.092(2)
1.969(2)
1.972(2)
2.058(2)
Cu(3)-Cu(4)
Cu(3)-N(81)
Cu(3)-N(91)
Cu(4)-N(101)
Cu(4)-N(111)
2.430(1)
2.057(2)
1.992(2)
1.979(2)
2.066(2)
N(31)-Cu(1)-N(31) 156.97(13) N(91)-Cu(3)-N(91)
N(31)-Cu(1)-N(21) 107.79(9) N(91)-Cu(3)-N(81)
153.84(12)
109.72(8)
82.86(8)
N(31)-Cu(1)-N(21) 83.09(9)
N(91)-Cu(3)-N(81)
N(21)-Cu(1)-N(21) 124.72(12) N(81)-Cu(3)-N(81)
123.81(12)
N(41)-Cu(2)-N(41) 155.60(12) N(101)-Cu(4)-N(101) 161.16(12)
N(41)-Cu(2)-N(51) 82.45(9)
N(101)-Cu(4)-N(111) 83.00(9)
∆νj1/2(calc) ≈ (2310 × ∆νjop)1/2 ≈ ∆νj1/2(obs)
(1)
N(41)-Cu(2)-N(51) 109.36(9) N(101)-Cu(4)-N(111) 106.51(9)
N(51)-Cu(2)-N(51) 123.48(12) N(111)-Cu(4)-N(111) 120.52(13)
(where ∆νj1/2 is the bandwidth at half-height and ∆νjop is the
a Symmetry operator for generating equivalent atoms: -x, y, 1.5 - z.
transition energy, both in cm-1) should hold for valence-
localized (class II) systems, whereas in delocalized (class
as MeCN, DMF, and DMSO compete with the ligand for
coordination of the metal ions. This is evidenced by
precipitation of the highly insoluble ligand.) Such a failure
to respond to changes in solvent polarity is further diagnostic
of class III or solvent-decoupled class II behavior.
20
III) systems, ∆νj1/2(obs) is typically much smaller than ∆νj1/2(calc)
.
When applied to the low-energy band at 1470 nm, the
observed value of ∆νj1/2(obs) ) 2402 cm-1 appears significantly
lower than that predicted by eq 1 (∆νj1/2(calc) ≈ 3960 cm-1),
supporting the notion that the dicopper trication adopts a fully
delocalized (class III) structure in solution.
X-ray Crystallography. The solid-state structure of 3 was
confirmed by X-ray crystallography as the double-stranded
helicate [Cu2(L1)2][BF4]3‚1/3H2O (3‚1/3H2O). The structure
was solved in the centrosymmetric space group C2/c (Z′ )
1), and the asymmetric unit consists of two unique Cu(L1)
fragments, each of which grows by 2-fold rotation (about
the Cu‚‚‚Cu axis) to generate two crystallographically
independent complex cations (A)- and (B)-[Cu2(L1)2]3+. One
of the complex units is shown in Figure 5, and selected metric
parameters for both complex units are listed in Table 3. The
overall 3+ complex charge and associated mixed-/average-
valence [Cu2(L1)2]3+ identity can be unambiguously con-
firmed by the presence of three noncoordinating tetrafluo-
roborate anions per complex cation, each of which was
refined satisfactorily with full crystallographic site occupa-
tion.
In attempts to synthesize this tricationic complex directly,
[Cu(MeCN)4][BF4], [Cu(H2O)6][BF4]2, and L1 were com-
bined in a 1:1:2 ratio in nitromethane. Diffusion of diethyl
ether into the resulting solution afforded dark brown crystals,
the electrospray mass spectrum of which was consistent with
the expected formulation [Cu2(L1)2][BF4]3 (3). The UV-
vis spectrum of [Cu2(L1)2][BF4]3 in nitromethane was found
to be identical to that of the oxidized species present at the
end point of the spectroelectrochemical investigations de-
scribed above. Although the complex is either insoluble or
unstable in most other (pure) organic solvents, no significant
changes in band position or intensity were observed for
spectra recorded in 1:1 nitromethane/toluene or nitromethane/
dichlorobenzene mixtures. (Note that donor solvents such
The packing arrangement of 33+ trications within the lattice
is similar to that observed in the crystal structure of the
dicopper(I) helicate 22+. End-on overlap between the terminal
tz-phenyl rings on adjacent molecules generates infinite
columns of π-π stacked helicates that propagate along the
(18) (a) Davis, W. D.; Zask, A.; Nakanishi, K.; Lippard, S. J. Inorg. Chem.
1985, 24, 3737. (b) Gouge, E. M.; Geldard, J. F. Inorg. Chem. 1978,
17, 270.. (c) Yokoi, H.; Addison, A. W. Inorg. Chem. 1977, 16, 1341.
(19) Robin, M. B.; Day, P. AdV. Inorg. Chem. Radiochem. 1967, 10, 247.
(20) (a) Hush, N. S. Prog. Inorg. Chem. 1967, 8, 391. (b) Creutz, C. Prog.
Inorg. Chem. 1983, 30, 1.
Inorganic Chemistry, Vol. 46, No. 7, 2007 2421

Jeffery et al.
Table 4. Calculated d2 (× 10-4 Å2) Values for All Cu-N Bonds in
crystallographic b axis. Likewise, each column contains
helicates of only one screw sense, (P)- or (M)-, such that its
chirality is maintained throughout the lattice. The crystal as
a whole, however, is once again racemic because of the
presence of inversion symmetry.
the Two Independent Complex Cations (A)- and (B)-33+ a
A
B
Cu(1)-N(21)
-6
-4
7
Cu(1)-N(81)
Cu(1)-N(91)
Cu(2)-N(101)
Cu(2)-N(111)
-34
29
12
Cu(1)-N(31)
Cu(2)-N(41)
Cu(2)-N(51)
The dicopper double-stranded helicate 33+ has a structure
almost identical to that of the dicopper(I) complex 22+. The
ligands are similarly disposed about the central Cu-Cu axes
in both complexes, conferring flattened tetrahedral geometries
at the metal centers with CuNN dihedral angles θ in the range
of 64-67° and Cu-N separations in the relatively narrow
range of 1.969(2)-2.092(2) Å. The Cu-Npy bonds in 33+
are marginally shorter than the remaining Cu-N(tz) separa-
tions (cf. 22+). The feature that most distinguishes the two
complexes, however, is (again) a reduction, ∆, of ca. 0.49
Å in Cu‚‚‚Cu separation on going from the dicopper dication
22+ to the trication 33+. This reduction is far greater than
can be explained by simple crystal packing effects. Moreover,
the corresponding Cu‚‚‚Cu separations of 2.450(1) and 2.430-
(1) Å for the two crystallographically independent A and B
cations both lie toward the short end of the range generally
accepted as constituting a bonding interaction (ca. 2.4-2.7
Å).16
7
-18
a Negative d2 values arise when the displacement of the Cu atom along
a Cu-N bond is greater than that of the coordinating N-donor atom.
tions for each set of four Cu-N bonds falling in the range
2.02-2.03 Å. This implies that each pair of Cu centers has
an approximately equal share of the electron spin density,
as a more asymmetric distribution would give rise to two
Cu(NN)2 chromophores (e.g., one CuIN4 and one CuIIN4 set)
with nonequivalent geometries reflecting the different charge
densities at the two metal centers. However, these observa-
tions do not account for the potential effects of dynamic or
static disorder within the crystal structure [i.e., a time- or
space-averaged representation of two extreme CuI(NN)2 and
CuII(NN)2 states could make all copper centers appear
identical], and concrete conclusions can be drawn only once
these possibilities have been ruled out. A brief analysis of
the atomic displacement parameters of each Cu(NN)2 chro-
mophore is therefore necessary in order to determine whether
the ligand atoms show more displacement along the M-L1
bonds than do the metal atoms to which they are attached,
a likely scenario if the N-donor atoms alternate between two
different positions in the crystal.22 Although visual inspection
of the anisotropic displacement parameters can provide some
useful information regarding this disorder, a more rigorous
approach considers the mean-square displacement amplitudes
(MSDAs). These are parameters that more accurately de-
scribe the displacement of a given atom in a specified
crystallographic direction, e.g., along a metal-ligand bond.22
Although unprecedented in metallohelicate chemistry,
comparable Cu‚‚‚Cu separations have been observed in the
crystal structures of a series of related [Cu2(L)]3+ cryptate
systems (where L are macrotricyclic aza-cryptand ligands).21
In the average-valence Cu1.5‚‚‚Cu1.5 form, complexes of this
type contain two Cu ions encapsulated within the macro-
tricyclic host cavity, with internuclear separations in the range
2.36-2.45 Å.21 Quantum chemical MO calculations have
also been carried out in several cases, the results of which
suggest that the unpaired electron occupies an orbital
2
2
(SOMO) with significant σ* antibonding Cu dz -Cu dz
2
2
character, whereas the in-phase Cu dz -Cu dz combination
forms the basis of a fully occupied σ bonding orbital.21a,h
With structural evidence further supplemented by Raman,
electronic, and ESR spectroscopic investigations, many of
the cryptate systems were concluded to benefit from rela-
tively strong Cu‚‚‚Cu σ-bonding interactions, a feature with
which the observed Cu‚‚‚Cu contraction in 33+ is also clearly
compatible.
3
3
U n n
i j
∑ ∑
ij
MSDA ) i)1 j)1
(2)
(3)
2
|n|
∆MSDA ) MSDA(ligand) - MSDA(metal) ) d2
The crystal structure of 33+ also provides further evidence
for the high level of spin delocalization expected from the
UV-vis measurements. Comparison of the Cu-N separa-
tions from each Cu(NN)2 chromophore reveals minimal
variation in bond length (Table 3), the mean Cu-N separa-
The MSDAs for the Cu and N atoms in each Cu(NN)2
chromophore are obtained from eq 2, where Uij is the ijth
element of the 3 × 3 tensor of anisotropic displacement
parameters (Å2) and ni and nj are elements of the vector |n|
describing the relevant Cu-N bond.23 The d2 values for
the various Cu-N bonds can then be computed from eq 3
to give an indication of the extent to which the atoms of
each Cu-N pair are disordered along the bond mutually
connecting them. For ordered systems, the M-N bonds
generally exhibit values of d2 , 0.01 Å,2 whereas for
dynamically or statically disordered systems, d2 g 0.01
Å2.24 As is apparent from Table 4, the d2 values obtained
(21) (a) Barr, M. E.; Smith, P. H.; Antholine, W. E.; Spencer, B. J. Chem.
Soc., Chem. Commun. 1993, 1649. (b) Harding, C.; McKee, V.;
Nelson, J. J. Am. Chem. Soc. 1991, 113, 9684. (c) Franzen, S.;
Miskowski, V. M.; Shreve, A. P.; Wallace-Williams, S. E.; Woodruff,
W. H.; Ondrias, M. R.; Barr, M. E.; Moore, L.; Boxer, S. G. Inorg.
Chem. 2001, 40, 6375. (d) Al-Obaidi, A.; Baranovi_c, G.; Coyle, J.;
Coates, C. G.; McGarvey, J. J.; McKee, V.; Nelson, J. Inorg. Chem.
1998, 37, 3567. (e) LeCloux, D. D.; Davydov, R.; Lippard, S. J. Inorg.
Chem. 1998, 37, 6814. (f) Harding, C.; Nelson, J.; Symons, M. C. R.;
Wyatt, J. J. Chem. Soc., Chem. Commun. 1994, 2499. (g) Farrar, J.
A.; McKee, V.; Al-Obaidi, A. H. R.; McGarvey, J. J.; Nelson, J.;
Thomson, A. J. Inorg. Chem. 1995, 37, 3567. (h) Farrar, J. A.; Grinter,
R.; Neese, F.; Nelson, J.; Thomson, A. J. J. Chem. Soc., Dalton Trans.
1997, 4083.
(22) Falvello, L. R. J. Chem. Soc., Dalton Trans. 1997, 4463.
(23) Dunitz, J. D.; Schomaker, V.; Trueblood, K. N. J. Chem. Phys. 1988,
92, 856.
(24) Hathaway, B. J. Struct. Bonding (Berlin) 1984, 57, 55.
2422 Inorganic Chemistry, Vol. 46, No. 7, 2007

Localization in a Mixed-Valence Dicopper Helicate
Figure 6. X-band EPR spectrum of 33+ in MeNO2 at 110 K: (a)
experiment and (b) simulation.
Figure 7. (a) Frozen-glass X-band EPR spectrum of 33+ recorded in a
1:1 MeNO2/toluene mixture (110 K). The asterisk * highlights the weak
inflection at ca. 3580 G. (b) Spectrum a corrected by subtraction of a small
amount of the spectrum obtained in MeNO2 (Figure 6a). (c) Simulation of
spectrum b based on hyperfine coupling to two identical Cu (I ) 3/2) nuclei
(g ) 2.17, g| ) 2.08, and A ≈ 62 G).
for both (A)- and (B)-33+ all fall well within the range
expected for ordered systems, and it can therefore be
concluded that the uniform distribution of Cu-N bond
lengths reflects a genuinely high level of spin delocalization
in the solid-state complex.
3
hyperfine coupling to one Cu nucleus (I ) /2) with a
EPR Spectroscopy. The EPR spectra of compounds
displaying mixed- or average-valence behavior can yield a
wealth of information regarding electronic structure and spin
distribution over a wide range of conditions. For dicopper
systems with one fully delocalized unpaired electron, hy-
perfine coupling of the electron to two equivalent Cu (I )
3/2) nuclei often gives a characteristic seven-line spectrum.
In contrast, in valence-localized states, the electron interacts
with only one nucleus on the EPR time scale, and hyperfine
coupling constant of Az ≈ 150 G.
The spectrum of 33+ in a 1:1 nitromethane/toluene mixture
is somewhat different from the above, and a total of seven
lines can just about be discerned on the main feature, in
addition to a weak inflection that appears at ca. 3580 G
(Figure 7a). The spectrum could be interpreted as representa-
tive of a fully spin-delocalized system, with g ) 2.17 and
g| ) 2.08 and hyperfine coupling of the unpaired electron
3
3
to two equivalent Cu (I ) /2) nuclei on the low-field
coupling to one Cu (I ) /2) nucleus results in a four-line
component (A ≈ 62 G) to give the expected septet. (A| was
not included in this simulation.) However, equally satisfac-
tory simulations were obtained using a valence-localized
model, which involves coupling to one Cu (I ) 3/2) nucleus,
with gx ) 2.25, gy ) 2.15, and gz ) 2.04 and hyperfine
coupling constants for the two low-field components of Ax
) 45 and Ay ) 60 G. In this interpretation, although the
electron is coupled to only one nucleus, the hyperfine
splitting observed on both the (sufficiently well resolved)
gx and gy components gives rise to two overlapping quartets
that, in turn, resemble a poorly resolved septet.
spectrum. Although this is the general case, it should be noted
that EPR observation of mixed- or average-valence behavior
can be extremely sensitive to the experimental conditions
and several systems have been reported in which the degree
of spin delocalization is strongly dependent on temperature.
This effect normally takes the intuitive form: on reaching
sufficiently low temperatures, a delocalized (ambient-tem-
perature) system gets trapped in a Valence-localized state.25
However, the converse was also observed by Ward et al.,
who reported the first mixed-valence tricopper complex in
which spin delocalization occurred only below 120 K, the
valence-localized form becoming more abundant at higher
temperatures.26
The X-band EPR spectra of 33+ in pure nitromethane
(MeNO2), 1:1 nitromethane/toluene (MeNO2/tol), and 1:1
nitromethane/dichlorobenzene (MeNO2/DCB) mixtures were
recorded at 110 K. The frozen-glass spectra of 33+ in pure
MeNO2 and a 1:1 MeNO2/DCB mixture are essentially
identical and fully consistent with a spin-localized structure
(Figure 6). In both cases, gx ) 2.25, gy ) 2.18, and gz )
2.01, giving a gx g gy > gz > 2.00 pattern characteristic of
a copper(II) complex with approximate axial symmetry and
a (dz2)1 ground-state configuration.15 The feature at low field
is further split into a well-resolved quartet, indicating
It is worth noting that neither of the two models described
above accounts for the weak inflection observed at ca. 3580
G. Given its proximity to the highest-field feature in the four-
line spectra obtained for pure MeNO2 and MeNO2/DCB
glasses (Figure 6), this feature could be due to trace amounts
of the (apparently) valence-localized species observed in the
latter. Indeed, the inflection is effectively remoVed by
subtracting a small amount (ca. 10%) of the MeNO2 spectrum
(Figure 6) from that recorded in the MeNO2/tol glass (Figure
7a) to give the corrected spectrum shown in Figure 7b. This
also improves somewhat the resolution of the low-field
feature and likewise its resemblance to the simulation based
3
on coupling to two equivalent Cu (I ) /2) nuclei (Figure
7c).
The degree of spin delocalization in [Cu2(L1)2]3+ thus
appears highly susceptible to variations in both phase and
conditions. UV-vis spectra and X-ray crystallographic data
suggest class III behavior for the (ambient-temperature)
(25) See, for example: (a) Gagne´, R. R.; Koval, C. A.; Smith, T. J.;
Cimolino, M. C. J. Am. Chem. Soc. 1979, 101, 4571. (b) Long, R. C.;
Hendrickson, D. N. J. Am. Chem. Soc. 1983, 105, 1513.
(26) Jones, P. L.; Jeffery, J. C.; Maher, J. P.; McCleverty, J. A.; Rieger, P.
H.; Ward, M. D. Inorg. Chem. 1997, 36, 3088.
Inorganic Chemistry, Vol. 46, No. 7, 2007 2423

Jeffery et al.
solution-phase, and (low-temperature, 100 K) solid-state
forms, but in the frozen glass (110 K), EPR measurements
begin to expose solvatochromic tendencies. Spin delocal-
ization is observed when the spectrum is recorded in a 1:1
MeNO2/tol glass, whereas spectra recorded in pure MeNO2
or 1:1 MeNO2/DCB glasses show only a spin-localized
species.
adjacent strands of (phenyl-substituted) L1 imparts the
necessary flexibility on 22+ to accommodate structural
contraction. Indeed, given the ease with which 33+ undergoes
self-assembly in situ, it is not unreasonable to assume that
the interligand stacking and Cu-Cu bonding interactions in
33+ are of a complementary nature.
Experimental Section
An attractive explanation for this behavior requires
consideration of the overall solvent polarity in each system.
Exchanging pure MeNO2 for a MeNO2/tol mixture ef-
fectively constitutes a significant reduction in polarity of the
medium surrounding each trication in both the solution and
frozen glass. Clearly, at room temperature, this change in
polarity has a negligible influence on the spin distribution.
At 110 K, however, the combination of low temperature and
a highly polar MeNO2 glass (the dielectric constants for
MeNO2, DCB, and toluene are 39.4, 2.8, and 1.0, respec-
tively) results in the trapping of a valence-localized state (on
the EPR time scale). On introducing toluene, a system
crossover from spin-localized to spin-delocalized begins to
occur as the glass is no longer sufficiently polar to fully
stabilize the large dipole moment associated with a valence-
localized state. That the resolution of the (delocalized) seven-
line spectrum can be improved by removing a ca. 10%
contribution from the (valence-localized) four-line spectrum
is, however, also consistent with the retention of a small
quantity of the valence-localized form.
Characterization and Physical Measurements. Electron impact
(EI) mass spectra were recorded on a VG Autospec instrument.
Electrospray ionization mass spectra were recorded on either a
continual-flow VG Quattro II triple quadrupole mass spectrometer
(with Z-spray source and cone voltages in the range of 25-40 V)
or a Bruker-Daltronics Apex 4e 7.0T (FT-MS) instrument; analytes
were dissolved in acetonitrile or nitromethane at concentrations of
ca. 0.1 mg mL-1. UV-vis spectra were recorded using a Perkin-
Elmer Lambda 19 UV-vis spectrometer. Solutions were analyzed
in the 0.1-1.0 mmol concentration range in quartz cells of path
length 10 or 1 mm. One-dimensional 1H NMR spectra were
recorded on Jeol GX400 and Jeol GX270 spectrometers. EPR
spectra were recorded on a Bruker ESP300E spectrometer. Mea-
surements were made at 110 K in frozen nitromethane, 1:1
nitromethane/toluene, and 1:1 nitromethane/dichlorobenzene glasses.
Data analysis and spectral simulations were performed using
WINEPR and SimFonia software, respectively.27 Electrochemical
measurements were performed using an EG&G model 273A
potentiostat linked to a computer with EG&G model 273 research
electrochemistry software, in conjunction with a three-electrode cell.
The auxiliary electrode was a platinum wire, and the working
electrode was a platinum disc (1.6-mm diameter). The reference
electrode was an aqueous saturated calomel electrode separated from
the solution by a fine-porosity frit and an agar bridge saturated
with KCl. Compounds were analyzed in 1 mM concentrations with
[NnBu4][PF6] (0.1 M) as the supporting electrolyte. Ferrocene (E°
) 0.47 V) or diacetylferrocene (E° ) 0.97 V) was added at the
end of each experiment as an internal standard. Unless otherwise
stated, (Ep)ox and (Ep)red values are quoted at a scan rate of 200
mV s-1. UV-vis/NIR spectroelectrochemical measurements were
carried out using a Cary Varian 5000 spectrophotometer in
conjunction with an optically transparent thin-layer electrode
(OTTLE) cell fitted with platinum gauze, platinum wire, and Ag/
AgCl working, counter, and reference electrodes, respectively. The
temperature was maintained at -20 °C throughout data collection.
Potentials were applied using an EG&G model 273A potentiostat
linked to a computer with EG&G model 273 research electrochem-
istry software. UV-vis/NIR spectra were recorded every 3-5 min
until electrolysis was complete. Magnetic measurements were
carried out using a 5T Quantum Design SQUID magnetometer.
The temperature was varied from 2 to 300 K at a rate of 2 K/min
under an applied field of 1 T.
Conclusion
The mononuclear copper(II) complex [Cu(L1)(ClO4)2] (1)
and the dinuclear helicate redox-related pair [Cu2(L1)2]2+/3+
(22+ and 33+, respectively) have been isolated and character-
ized using a combination of X-ray crystallography; cyclic
1
voltammetry; and ESI-MS, UV-vis, H NMR, and EPR
spectroscopies. The solid-state structures of 1 and 22+ closely
resemble those of their previously reported (methyl-
substituted) L2 analogues.10c The impact of replacing the
terminal Me groups in L2 for phenyl groups in L1 is instead
manifested in the electrochemical behavior of 22+. Whereas
oxidation of [Cu2(L2)2]2+ is pseudoreversible and causes
partial disassembly of the helicate structure, 22+ readily loses
one electron to form a stable tricationic helicate [Cu2(L1)2]3+
(33+). The latter displays an average-valence electronic
structure under ambient conditions and can be synthesized
in situ by combining a 1:1:2 ratio of [Cu(MeCN)4][BF4],
[Cu(H2O)6][BF4]2, and L1 in nitromethane. X-ray crystal-
lographic analysis of the resulting compound also provides
evidence of a Cu-Cu bonding interaction.
X-ray Crystallography. Crystals were visually inspected for
defects and singularity under a binocular microscope fitted with a
polarizing attachment. Suitable crystals were then coated with epoxy
resin, mounted on a glass fiber, and quickly transferred to a Bruker-
AXS PROTEUM (CCD area detector) diffractometer under a stream
of cold N2 gas. Preliminary scans were employed to assess crystal
quality, lattice symmetry, ideal exposure time, etc., prior to
collection of a sphere or hemisphere (for low- and high-symmetry
crystal systems, respectively) of diffraction intensity data using
We suggest that the formation and apparent stability of
average-valence 33+ can be attributed to the π-π stacking
ability of the terminal phenyl substituents on L1. With Me
groups in the same positions in L2, the [Cu2(L2)2]2+ helicate
is constrained to adopt conformations in which steric
congestion between the Me groups on one ligand and the
py rings on the other is reduced. This is liable to prevent
the helical pitch of [Cu2(L2)2]2+ from contracting sufficiently
to allow Cu-Cu bond formation. In contrast, the heightened
potential for extensive π-π overlap to occur between
(27) (a) Bruker WINEPR System, version 2.11; Bruker-Franzen Analytik
GmbH, 1990-1996. (b) WINEPR SimFonia, version 1.25; Bruker
Analytische Messtechnik GmbH, 1994-1996.
2424 Inorganic Chemistry, Vol. 46, No. 7, 2007

Localization in a Mixed-Valence Dicopper Helicate
Table 5. Crystal Data and Refinement Parameters for 1-3
1·MeNO2
2·MeNO2·MeCN
3·1/3H2O
empirical formula
formula weight
temperature
C29H21Cl2CuN5O10S2
798.07
100(2) K
C59H42Cu2F12N10O2P2S4
1468.29
100(2) K
C56H36.66B3Cu2F12N8O0.3S4
1341.31
100(2) K
wavelength
1.54178 Å
1.54178 Å
1.54178 Å
crystal system
space group
unit cell dimensions
monoclinic
P21/n
monoclinic
P2/c
monoclinic
C2/c
a ) 15.8162(2) Å
b ) 12.9136(2) Å
c ) 16.8236(2) Å
R ) 90°
a ) 18.7741(8) Å
b ) 19.6516(8) Å
c ) 17.3283(8) Å
R ) 90°
a ) 27.2491(6) Å
b ) 18.7615(5) Å
c ) 24.5663(6) Å
R ) 90°
â ) 114.0750(10)°
γ ) 90°
â ) 115.311(2)°
γ ) 90°
â ) 121.9390(10)°
γ ) 90°
volume
Z (Z)
density (calcd)
absorption coefficient
F(000)
3137.22(7) Å3
4 (1)
5779.4(4) Å3
10657.8(5) Å3
8 (1)
4 (1)
1.690 Mg/m3
1.687 Mg/m3
1.672 Mg/m3
4.388 mm-1
3.615 mm-1
3.261 mm-1
1620
2968
5405
crystal size
θ range
0.3 × 0.2 × 0.1 mm
0.2 × 0.05 × 0.02 mm
0.3 × 0.2 × 0.1 mm
3.23-70.04°
2.25-70.22°
3.03-70.16°
index ranges
-19 e h e 18, -15 e k e 15,
-17 e l e 19
14802
5147 (Rint ) 0.0316)
5147/0/526
-22 e h e 22, -23 e k e 23,
-21 e l e 21
42831
10738 (Rint ) 0.0723)
10738/0/763
-33 e h e 33, -22 e k e 22,
-22 e l e 29
29837
8828 (Rint ) 0.0296)
8828/0/777
reflections collected
independent reflections
data/restraints/parameters
GOF on F2
S ) 1.034
S ) 1.046
S ) 1.046
R indices [I > 2σ(I)]a-c
R indices (all data)a,b,d
weighting schemeb
R1 ) 0.0315, wR2 ) 0.0826
R1 ) 0.0355, wR2 ) 0.0853
a ) 0.0525, b ) 1.1740
0.349 and -0.552 eÅ-3
R1 ) 0.0567, wR2 ) 0.1304
R1 ) 0.0985, wR2 ) 0.1455
a ) 0.0676, b ) 0.7040
0.592 and -0.746 eÅ-3
R1 ) 0.0404, wR2 ) 0.1121
R1 ) 0.0444, wR2 ) 0.1155
a ) 0.0671, b ) 18.3342
1.033 and -0.658 eÅ-3
largest diff peak and hole
2
a Structure was refined on Fo using all data; the value of R1 is given for comparison with older refinements based on Fo with a typical threshold of F
2
2
2
2
2
g 4σ(F). b wR2 ) [∑[w(Fo2 - Fc )2]/ ∑w(Fo )2]1/2, where w-1 ) [σ2(Fo ) + (aP)2 + bP] and P ) [max(Fo ,0) + 2Fc ]/3. c 4577 reflections with I > 2σ(I).
d All 5147 data.
SMART operating software.28 Intensities were then integrated from
several series of exposures (each exposure covering 0.3° in ω),
merged, and corrected for Lorentz and polarization effects using
SAINT software.29 Solutions were generated by conventional heavy-
atom Patterson or direct methods and refined by full-matrix
nonlinear least-squares on all F2 data, using SHELXS-97 and
SHELXL software, respectively (as implemented in the SHELXTL
suite of programs).30 Empirical absorption corrections were applied
based on multiple and symmetry-equivalent measurements using
SADABS,31 and where stated, the scattering contributions from
diffuse solvent moieties were removed using the SQUEEZE
function in PLATON.32 All structures were refined until conver-
gence (max shift/esd <0.01), and in each case, the final Fourier
difference map showed no chemically sensible features. Crystal-
lographic refinement parameters are summarized in Table 5.
Synthesis of 6,6′-Bis(4-phenylthiazol-2-yl)-2,2′-bipyridine (L1).
An excess of chloroacetophenone was added to a two-necked round-
bottom flask, fitted with a condenser, containing a suspension of
6,6′-bis(thiocarboxyamide)-2,2′-bipyridine33 (0.873 g, 3.16 mmol)
in ethanol (50 mL). The mixture was brought to reflux and stirred
for 4 h, during which two more portions of chloroacetophenone
were added. The reaction mixture was left at reflux for an additional
4 h, resulting in total loss of the initial yellow color. Cooling
followed by concentration under reduced pressure gave an off-white
precipitate that was collected by filtration and suspended in aqueous
ammonia solution overnight. The solid was then filtered and washed
in cold ethanol to give 6,6′-bis(4-phenylthiazol-2-yl)-2,2′-bipyridine
(L1) as a tan powder (yield: 80%). Found C, 70.65; H, 3.59; N,
11.11%. C28N4S2H18 requires C, 70.86; H, 3.82; N, 11.81%. High-
resolution EI-MS m/z 474.096153 ([M]+) (calcd for C28H18N4S2,
474.097290; deviation, 2.4 ppm). The poor solubility of L1 in
common organic solvents precluded characterization by NMR
spectroscopy.
Metal Complexes. The complexes were prepared by combining
equimolar amounts (0.029 mmol) of L1 with an appropriate metal
salt in the minimum amount (5-10 mL) of solvent (nitromethane
or acetonitrile). For [Cu2(L1)2][BF4]3, a 1:1:2 ratio of [Cu(H2O)6]-
[BF4]2, [Cu(MeCN)4][BF4], and ligand was used. The suspensions
were then gently heated, sonicated for ca. 10 min, filtered, and
concentrated in vacuo (if necessary). Slow diffusion of diethyl ether
into the resulting solutions afforded crystals suitable for X-ray
structural analysis. Caution: Perchlorate salts are potentially
explosiVe and should be handled with due care. Those complexes
described below that were isolated as perchlorates were prepared
in only small quantities (10-20 mg), and we did not encounter
any problems with them.
Data for [Cu(L1)(ClO4)2]‚MeNO2 (1): Found C, 43.63; H, 3.10;
N, 8.68%. CuC28H18N4S2O8Cl2‚MeNO2 requires C, 43.64; H, 2.65;
N, 8.78%. ESI-MS m/z 268.5 {[Cu(L1)]2+}, 505.5 {[Cu(L1)2]2+},
635.9 {[Cu(L1)(ClO4)]+}. High-resolution ESI-MS m/z 635.9756400
{[Cu(L1)(ClO4)]+} (calcd for CuC28H18N4S2ClO4, 635.9748517;
deviation, 1.2 ppm).
(28) SMART Diffractometer Control Software; Bruker Analytical X-ray
Instruments Inc.: Madison, WI, 1998.
(29) SAINT Integration Software; Siemens Analytical X-ray Instruments
Inc.: Madison, WI, 1994.
(30) SHELXTL Program System, version 5.1; Bruker Analytical X-ray
Instruments Inc.: Madison, WI, 1998.
(31) Sheldrick, G. M. SADABS: A Program for Absorption Correction with
the Siemens SMART System; University of Go1ttingen: Go1ttingen,
Germany, 1996.
(32) Spek, A. L. Acta. Crystallogr. 1990, A46, C-34. PLATONsA
Multipurpose Crystallographic Tool; Utrecht University: Utrecht, The
Netherlands, 2003.
(33) For synthesis, see: Baxter, P. N. W.; Connor, J. A.; Schweizer, W.
B.; Wallis, J. D. J. Chem. Soc., Dalton Trans. 1992, 3015.
(34) We note that an almost identical Cu‚‚‚Cu separation of 2.92 Å was
observed by Thummel et al. in the dicopper(I) helicate of a tetradentate
bis-1,10-phenanthroline ligand. See: Riesgo, E. C.; Hu, Y.-Z.;
Thummel, R. P. Inorg. Chem. 2003, 42, 6648.
Inorganic Chemistry, Vol. 46, No. 7, 2007 2425

Jeffery et al.
Data for [Cu2(L1)2][PF6]2‚MeNO2‚MeCN (2): ESI-MS m/z 538.0
{[Cu2(L1)2]2+}, 1221.0 {[Cu2(L1)2(PF6)]+}. High-resolution ESI-
MS m/z 1363.9905520 {[Cu2(L1)2(PF6)2]+} (calcd for Cu2C56H36N8-
S4P2F12, 1363.9815910; deviation, 6.5 ppm). 1H NMR (300 MHz,
CD3CN) δ 7.91 (4H, t, J ) 7.8, pyridyl H4), 7.88 (4H, dd, J ) 7.8,
2.4 pyridyl H3), 7.80 (4H, s, thiazole H5), 7.45 (8H, d, J ) 7.8,
ortho-phenyl), 7.06 (4H, t, J ) 7.8, para-phenyl), 7.00 (4H, dd, J
) 6.3, 2.4, pyridyl H5), 6.94 (8H, d, J ) 7.8, meta-phenyl).
Data for [Cu2(L1)2][BF4]3‚1/3H2O (3): ESI-MS m/z 358.6 {[Cu2-
(L1)2]3+}, 505.5 {[Cu(L1)2]2+}, 538.0 {[Cu2(L1)2]2+}, 582.0 {[Cu2-
(L1)2(BF4)]2+}, 1163.1 {[Cu2(L1)2(BF4)]+}, 1250.1 {[Cu2(L1)2-
Acknowledgment. We thank the EPSRC and the Uni-
versity of Bristol for financial support and the School of
Chemistry (University of Bristol) Mass Spectroscopy service
for obtaining mass spectra of all compounds reported herein.
We also thank the EPSRC National EPR service for
conducting preliminary EPR measurements.
Supporting Information Available: Crystallographic data for
1-3 (atomic coordinates, anisotropic displacement parameters,
refinement details, etc.) in cif format and a thermal ellipsoid plot
of the mixed-valence compound 3. This material is available free
of charge via the Internet at: http://pubs.acs.org.
(BF4)2]+}. High-resolution ESI-MS m/z 358.0177470 {[Cu2(L1)2]3+
}
(calcd for Cu2C56H36N8S4, 358.0173775; deviation, 1.0 ppm).
Crystalline samples of 3 displayed typical Curie-type paramagnetic
behavior, with a linear dependence of 1/M on T (or of M on 1/T)
according to the Curie law M/H ) C/T.
IC061504M
2426 Inorganic Chemistry, Vol. 46, No. 7, 2007