Redox cycling of segmented copper helicates [17]

Full text of DOI:10.1021/ja981284e

J. Am. Chem. Soc. 1998, 120, 9973-9974
Redox Cycling of Segmented Copper Helicates
9973
Abdelkrim El-ghayouy,1 Anthony Harriman,1 Andre´ De Cian,2
Jean Fischer,2 and Raymond Ziessel1*
Ecole Europe´enne de Chimie, Polyme´res, Mate´riaux
1 rue Blaise Pascal, 67008 Strasbourg Cedex, France
ReceiVed April 16, 1998
In the past decade many wondrous examples of metallohelicates
have been described.3 Although these architectures are fascinating
to behold, few applications have been found for metallohelicates
or related structures and most attention has focused on their
relevance to supramolecular chemistry.4 In particular, because
dissociation of the helicate competes with reversible redox
cycling,5 useful catalytic functions have not been identified despite
the close proximity of redox-active metal centers.4 With copper-
(I) helicates, for example, oxidative dissociation is promoted by
the need to change the geometry around the metal center from
tetrahedral6 to a 5- or 6-coordinate ligand field.7 Even the
binuclear mixed-valence copper helicate formed from quinquepyr-
idine,8 which shows reversible one-electron oxidation and reduc-
tion steps in cyclic voltammetry, changes nuclearity on reduction
to the copper(I) species. We now introduce a simple strategy,
based on a multitopic ligand able to satisfy the coordination
demands of both copper(I) and copper(II) cations, that allows self-
assembly of binuclear copper helicates displaying exceptional
redox stability. It might be anticipated that such systems will
display useful catalytic functions.9
Figure 1. Schematic representation of the changes in coordination and/
or structure at each stage of oxidation of the binuclear copper helicate.
[Cu2L2](BF4)2, 12+, was prepared in 96% yield by treating the
free ligand L (Figure 1) in CH2Cl2 with copper(I) tetrafluoroborate
in acetonitrile at 20 °C. The X-ray crystal structure shows that
the two copper(I) cations are separated by 3.278 Å and that the
complex is a helicate (Figure 2A). Each metal center, having
pseudotetrahedral geometry, is coordinated to N atoms provided
by a single imino (IN) function and three pyridine (PN) residues,
with the two cations being equivalent. The free IN atom that
resides 2.629 Å from the copper(I) cation is unlikely to be
coordinated to the metal even if the cis conformation is indicative
of some electronic interaction.10 In solution, however, 15N-HMBC
NMR spectroscopy resolves only three distinct N atoms. This
suggests that the two bound ligands glide across the metal centers,
causing their equivalence on the NMR time scale, so that the
complex persists in a state of dynamic fluctuation in solution
(Figure 1). A similar conclusion is reached from FT-IR and both
1H and 13C NMR spectra recorded in solution where only a single
imino group can be resolved.
Figure 2. (A) ORTEP view of the X-ray crystal structure of the binuclear
copper(I) complex 12+ showing the helical arrangement of the ligands L
with the uncoordinated imino functions residing in the cis conformation.
(B) ORTEP view of the crystal structure of the mixed-valence species
13+ showing coordinative asymmetry in the solid state.
Cyclic voltammetry studies made in acetonitrile show, in
addition to several quasi-reversible reduction processes associated
with the ligands (E1/2 ) -1.18 ( 0.02 V, ∆Ep ) 69 mV; E1/2
)
-1.42 ( 0.02 V vs SCE, ∆Ep ) 103 mV), two successive, quasi-
reversible oxidation steps. Controlled potential coulommetry
showed each oxidation step corresponds to removal of a single
electron. Consequently, the first process (E1/2 ) 0.15 ( 0.01 V
vs SCE, ∆Ep ) 78 mV) forms the mixed-valence species 13+
while the second step (E1/2 ) 0.87 ( 0.02 V vs SCE, ∆Ep ) 84
mV) generates the binuclear copper(II) complex 14+
. The
(1) Current address: Laboratoire de Chimie, d’Electronique et Photonique
Mole´culaires, Ecole Europe´enne de Chimie, Polyme`res et Mate´riaux (ECPM).
(2) Current address: Laboratoire de Cristallochimie et de Chimie Struc-
turale, Institut Le Bel.
difference in potential between removal of the first and second
electrons (∆E1/2 ) 720 mV) is due, in part, to the Coulombic
effect associated with the increased charge since the metal centers
must remain in close proximity. That the two metal centers are
electronically coupled is apparent from the appearance of a
Gaussian-shaped, intervalence charge-transfer (IVCT) absorption
(3) Lehn, J.-M. Supramolecular Chemistry; VCH Publishers: Weinheim,
1995.
(4) Piguet, C.; Bernardinelli, G.; Hopfgartner, G. Chem. ReV. 1997, 97,
2005.
(5) Gisselbrecht, J.-P.; Gross, M.; Lehn, J.-M.; Sauvage, J.-P.; Ziessel, R.;
Piccinni-Leopardt, C.; Arrieta, J. M.; Germain, G.; Van Meersche, M. NouV.
J. Chim. 1984, 8, 661.
band centered at 1400 nm (ꢀmax ≈ 37 ( 5 M-1 cm-1; ∆ν1/2
≈
1950 ( 100 cm-1). Spectroelectrochemistry made at fixed
potentials of 0.50 and 1.10 V vs Ag°, respectively, showed no
detectable loss of either 13+ or 14+ over 5 min standing while the
IVCT absorption band of 13+ was stable over many days. This
stability is in marked contrast to most other metallohelicates which
undergo substantial structural rearrangement during redox cy-
cling.4,5,8,11
(6) (a) Burke, P. J.; McMillan, D. R.; Robinson, W. R. Inorg. Chem. 1980,
19, 1211. (b) Healy, P. C.; Engelhardt, L. M.; Patrick, V. A.; White A. H. J.
Chem. Soc., Dalton Trans. 1985, 2541.
(7) (a) Albrecht-Gary, A.-M.; Dietrich-Buchecker, C. O.; Saad, Z.; Sauvage,
J.-P. J. Am. Chem. Soc. 1988, 110, 1467. (b) Collin, J.-P.; Gasvind, P.;
Sauvage, J.-P. Chem. Commun. 1996, 2005.
(8) Potts, K. T.; Kashavarz, K. M.; Than, F. S.; Abruna, H. D.; Arana, C.
Inorg. Chem. 1993, 32, 4442, 4450.
(9) Kaim, W.; Rall, J. Angew. Chem., Int. Ed. Engl. 1996, 35, 43.
(10) All X-ray structural data reported for structurally relevant imines show
a trans conformation while MO calculations made at the PM3 level indicate
that a trans configuration is the more stable structure for L.
(11) A further example of a redox stable copper helicate has been
reported: Ho, P. K. K.; Peng, S. M.; Wong, K. Y.; Che, C. M. J. Chem. Soc.,
Dalton Trans. 1996, 1829.
S0002-7863(98)01284-0 CCC: $15.00 © 1998 American Chemical Society
Published on Web 09/12/1998

9974 J. Am. Chem. Soc., Vol. 120, No. 38, 1998
Communications to the Editor
Chemical oxidation of 12+ with ferrocenium cations formed
the mixed-valence species with a bimolecular rate constant of
(4.5 ( 0.6) × 105 M-1 s-1 in acetonitrile containing ammonium
tetrafluoroborate (0.1 M). Immediately after one-electron oxida-
tion, we propose that the ancillary imino N atom coordinates to
the newly formed copper(II) center, giving a 5-coordinate species
(Figure 1). Isolated crystals of 13+, however, show that the
copper(II) center is 6-coordinate in the solid state with the ligand
field being supplied by two IN and four PN atoms (Figure 2B).
There is a concomitant shift in the ligand field around the copper-
(I) center from three PN and one IN atoms to two PN and two IN
atoms. The solid-state structure clearly demands a substantial
internal rearrangement. This is not the case in solution, however,
where the IVCT absorption band indicates that the two metal
centers must possess similar coordination shells. Analyzing the
IVCT band in terms of an asymmetric structure12 gives an
electronic coupling matrix element (HMM) of ca. 140 cm-1 but
requires the energy difference between the terminals to be only
ca. 0.2 eV. This is considered too small to reflect the markedly
disparate terminals seen in the crystal structure. Conversely,
analyzing the IVCT band as a symmetric system13 requires the
metal centers to be strongly coupled (HMM ≈ 3570 cm-1) with
full electron delocalization, and this is consistent with their close
spacing. The high reversibility encoded in the cyclic voltammo-
grams also indicates that oxidation in solution is not accompanied
by large-scale structural reorganization. This hypothesis was
confirmed by EPR spectroscopy made in frozen solution where
hydrazine restored the original copper(I) helicate in essentially
quantitative (>95%) yield.
The overall stability constant for 12+ was measured by titration
of L with Cu+ in acetonitrile (log â ) 18.5) and by dissociative
titration17 with 2,9-dimethyl-1,10-phenanthroline (log â ) 20.3).
Stepwise formation of the copper(I) helicate is not cooperative
since the individual stability constants do not increase progres-
sively:
L + Cu+ f Cu(L)+
log K1 ) 7.05
log K2 ) 3.55
log K3 ) 7.90
L + Cu(L)+ f Cu(L)2
+
Cu(L)2+ + Cu+ f Cu2(L)2
2+
The difficult step involves attachment of a second ligand to
the mononuclear fragment Cu(L)+, possibly for stereochemical
reasons. A dissociative titration made with 2,9-dimethyl-1,10-
phenanthroline after chemical oxidation with phenoxathiin hexachlo-
roantimonate established that 14+ was considerably less stable
(log â ) 9.80) than 12+. The mixed-valence species 13+ does
not disproportionate in CH3CN while adding an equimolar amount
of 12+ to a solution of 14+ shows that the equilibrium lies firmly
on the side of the mixed-valence species with the compropor-
tionation constant being ca. 1 × 1012. Quantitative analysis of
the resultant absorption spectral profile shows 13+ (log â . 26.4)
to be the more stable complex, as expected from the cyclic
voltammetry results.
The present system has two copper cations ca. 3.3 Å apart and
can cycle reversibly over four redox levels spanning some 2.3
eV. In solution, a symmetrical 5-coordinate geometry around
the copper(II) center(s) is favored over the asymmetrical 6-co-
ordinate geometry preferred in the solid state. Such structures
are made possible by the segmented nature of L and because the
IN atoms coordinate strongly to the metal center. Computer
molecular modeling studies suggest that access to the metal centers
is partially blocked by the terminal anisole groups and this could
exaggerate the preference for a 5-coordinate arrangement. The
internal flexibility, especially at the copper(I) level, and the
ambivalent binding properties of L provide the foundation for
the kinetic inertness in solution, and this might provide a means
by which to design catalytically active copper helicates.
the spectral pattern14 corresponds to a dx -y ground state having
a distorted 5-coordinate environment around the copper(II)
cation.15 These various findings indicate that the 5-coordinate
species is favored in solution but not in the solid (Figure 1).
Oxidation of both 12+ and 13+ with phenoxathiin hexachloro-
antimonate in CH3CN/CH2Cl2 (4/1) resulted in quantitative
formation of the fully oxidized product 14+ under conditions where
it was stable for about 1 h. Formation of 14+ could restore the
molecular symmetry, giving two identical 5-coordinate copper-
(II) cations, or polarize the molecule into 4- and 6-coordinate
terminals. The various kinetic spectroscopic studies show no
indication of a large-scale transformation accompanying oxidation,
and it is concluded that the metal centers in 14+ adopt 5-coordinate
structures. Consequently, we attribute the remarkable redox
stability of the metallohelicate to the presence of a terminal imino
function that can readily provide the coordination demands of
the copper(II) cation. Indeed, without the accessory, imine
oxidation of the copper(I) helicate causes rapid dissociation into
mononuclear copper(II) species.16 Treatment of 13+ and 14+ with
2
2
Acknowledgment. We thank Dr. J.-P. Gisselbrecht for helpful
discussions and for providing spectroelectrochemical facilities, Dr. P.
Turek for performing all the EPR measurements, and Dr. N. Kyritsakas
for solving the X-ray structures. Financial support from CNRS and ECPM
is gratefully acknowledged.
(12) (a) Hush, N. S. Prog. Inorg. Chem. 1967, 8, 391. (b) Creutz, C. Prog.
Inorg. Chem. 1983, 30, 1.
Supporting Information Available: The synthesis and characteriza-
tion of 12+ and 13+, UV-vis absorption spectra for the helicates, cyclic
voltammograms, spectroelectrochemical changes, an EPR spectrum
recorded for the mixed-valence species, analysis of IVCT absorption band,
and methodology used to determine stability constants (22 pages, print/
PDF). See any current masthead page for ordering information and Web
access instructions.
(13) For the IVCT absorption band the oscillator strength (f) is 0.00033,
the dipole strength (M) is 0.066 Å, the degree of electron delocalization (R2)
is 0.0004, and the amount of charge transferred (1 - 2R2) is unity.
(14) The main EPR parameters are: g (parallel) ) 2.010, A (parallel) )
122 G, g (perpendicular) ) 2.25, A (perpendicular) ) 42 G. A distorted
5-coordinate geometry is supported by the observation that {g (parallel)/A
(parallel) ) 176 cm-1}.
(15) Lee, D.-H.; Murthy, N.; Karlin, K. D. Inorg. Chem. 1997, 36, 5785.
(16) For example, the binuclear copper(I) helicate described in ref 17
undergoes rapid dissociation on one-electron oxidation. The rate of dissociation
is too fast to be measured by stopped-flow techniques.
JA981284E
(17) Ziessel, R.; Harriman, A.; Suffert, J.; Youinou, M. T.; De Cian, A.;
Fischer, J. Angew. Chem., Int. Ed. Engl. 1997, 36, 2509.