Altering the Balance between Ligand-Based Radical Anion Formation and Dechelation in Electrochemically Reduced Binuclear Copper(I) Complexes: A Resonance Raman Spectroelectrochemical Study

Article abstract of DOI:10.1021/ic9715052

The electrochemistry and spectral properties of a series of mono- and binuclear complexes with bridging ligands based on 2,3-di(2-quinolyl)quinoxaline are reported. The ligands are 2,3-di(2-quinolyl)quinoxaline (dqq), 6,7-dimethyl-2,3-di(2-quinolyl)quinoxaline (dqqMe₂), and 6,7-dichloro-2,3-di(2-quinolyl)quinoxaline (dqqCl₂). The complexes are [Cu(dqq)(PPh₃)₂]BF₄, 1·[BF₄]; [Cu(dqqMe₂)(PPh₃)₂]BF₄, 2·[BF₄]; [Cu(dqqCl₂)(PPh₃)₂]BF₄, 3·[BF₄]; [(PPh₃)₂Cu(dqq)Cu(PPh₃)₂](BF ₄)₂,4·[BF₄]₂; [(PPh₃)₂Cu(dqqMe₂)Cu(PPh₃) ₂](BF₄)₂, 5·[BF₄]₂; [(PPh₃)₂Cu(dqqCl₂)-Cu(PPh₃) ₂](BF₄)₂, 6·[BF₄]₂. The mononuclear complexes reduce at the metal and dechelate, as evidenced by UV/vis spectroelectrochemistry. Reduction of the binuclear complexes results in ligand-based radical union formation for 4 and 6 but decomposition of 5 to 2. The reduction species are identified using resonance Raman spectroscopy. The structures of [Cu(PPh₃)₂(C₁₆H₁₄Cl ₂N₄)][BF₄] (3·[BF₄]) and [(Cu(PPh₃)₂)₂(C₂₆H ₁₄Cl₂N₄)][BF₄] ₂·2CH₂-Cl₂ (6·[BF₄]₂) were determined by single-crystal X-ray diffraction, 3·[BF₄] crystallized in the monoclinic space group P1? with cell dimensions a = 10.956(2) A?, b = 15.278(3) A?, c = 16.032(3) A?, α = 100.342(8)°, β = 95.291(13)°, γ = 93.968(12)°, Z = 2, ρ_calcd = 1.431 g/cm³, and R(F₀) = 0.0589. 6·[BF₄]₂ crystallized in the, monoclinic space group C2/c with cell dimensions a = 21.295(4) A?, b = 24.322(5) A?, c = 20.034(4) A?, β = 112.64(3)°, Z = 8, ρ_calcd = 1.486 g/cm³, and R(F₀) = 0.0422.

Full text of DOI:10.1021/ic9715052

4452
Inorg. Chem. 1998, 37, 4452-4459
Altering the Balance between Ligand-Based Radical Anion Formation and Dechelation in
Electrochemically Reduced Binuclear Copper(I) Complexes: A Resonance Raman
Spectroelectrochemical Study
Simon E. Page and Keith C. Gordon*,†
Department of Chemistry, University of Otago, P.O. Box 56, Dunedin, New Zealand
Anthony K. Burrell*
Department of Chemistry, Massey University, Private Bag 11122, Palmerston North, New Zealand
ReceiVed December 2, 1997
The electrochemistry and spectral properties of a series of mono- and binuclear complexes with bridging ligands
based on 2,3-di(2-quinolyl)quinoxaline are reported. The ligands are 2,3-di(2-quinolyl)quinoxaline (dqq), 6,7-
dimethyl-2,3-di(2-quinolyl)quinoxaline (dqqMe2), and 6,7-dichloro-2,3-di(2-quinolyl)quinoxaline (dqqCl2). The
complexes are [Cu(dqq)(PPh3)2]BF4, 1‚[BF4]; [Cu(dqqMe2)(PPh3)2]BF4, 2‚[BF4]; [Cu(dqqCl2)(PPh3)2]BF4, 3‚[BF4];
[(PPh3)2Cu(dqq)Cu(PPh3)2](BF4)2, 4‚[BF4]2; [(PPh3)2Cu(dqqMe2)Cu(PPh3)2](BF4)2, 5‚[BF4]2; [(PPh3)2Cu(dqqCl2)-
Cu(PPh3)2](BF4)2, 6‚[BF4]2. The mononuclear complexes reduce at the metal and dechelate, as evidenced by
UV/vis spectroelectrochemistry. Reduction of the binuclear complexes results in ligand-based radical anion
formation for 4 and 6 but decomposition of 5 to 2. The reduction species are identified using resonance Raman
spectroscopy. The structures of [Cu(PPh3)2(C26H14Cl2N4)][BF4] (3‚[BF4]) and [(Cu(PPh3)2)2(C26H14Cl2N4)][BF4]2‚2CH2-
Cl2 (6‚[BF4]2) were determined by single-crystal X-ray diffraction. 3‚[BF4] crystallized in the monoclinic space
group P1h with cell dimensions a ) 10.956(2) Å, b ) 15.278(3) Å, c ) 16.032(3) Å, R ) 100.342(8)°, â )
95.291(13)°, γ ) 93.968(12)°, Z ) 2, Fcalcd ) 1.431 g/cm3, and R(Fo) ) 0.0589. 6‚[BF4]2 crystallized in the
monoclinic space group C2/c with cell dimensions a ) 21.295(4) Å, b ) 24.322(5) Å, c ) 20.034(4) Å, â )
112.64(3)°, Z ) 8, Fcalcd ) 1.486 g/cm3, and R(Fo) ) 0.0422.
Introduction
We are interested in the behavior of copper(I) complexes with
polypyridyl ligands that may bridge metal centers. Our stimulus
for these studies comes from the use of copper(I) polypyridyl
complexes in the construction of supramolecular assemblies.1
These may be used in solar energy harvesting systems. The
utility of an assembly constructed from copper(I) polypyridyl
units, with regard to solar energy applications, lies in the
multichromophoric nature of the system coupled with the ability
to transduce energy along the assembly. This is well established
in ruthenium(II) and rhenium(I) polynuclear complexes.2 Re-
cently a number of copper(I) complexes have been studied that
also undergo energy transduction and charge-separation.3 If
copper(I) systems are to be used for such applications then it is
critically important to understand how the building blocks of
the assembly behave with respect to electron transfer. Very
Figure 1. Ligands and numbering system used.
little is known about the behavior of copper(I) complexes with
bridging ligands in regard to electrochemical reduction.4 In
many copper(I) complexes reduction leads to dechelation of the
complex.5
The bridging ligands used in this study are based on 2,3-di-
(2-quinolyl)quinoxaline (dqq), and the extensive π conjugation
of these ligands makes them good electron acceptors. The
ligands are shown in Figure 1.
* Corresponding authors.
† E-mail address: kgordon@alkali.otago.ac.nz.
(1) (a) Baxter, P.; Lehn, J.-M.; DeCain, A.; Fischer, J. Angew. Chem.,
Int. Ed. Engl., 1993, 32, 69. (b) Marquis-Rigault, A.; Dupont-Gervais,
A.; Van Dorsselaer, A.; Lehn, J.-M. Chem. Eur. J. 1996, 2, 1395. (c)
Lehn, J.-M.; Rigault, A.; Siegel, J.; Harrowfield, J.; Chevrier, B.;
Moras, D. Proc. Natl. Acad. Sci. U.S.A. 1987, 84, 2565. (d) Lehn,
J.-M.; Rigault, A. Angew. Chem., Int. Ed. Engl. 1988, 27, 1095. (e)
Lehn, J.-M.; Harding, M. M.; Koert, U.; Marquis-Rigault, A.; Piguet,
C.; Siegel, J. HelV. Chim. Acta 1991, 74, 594. (f) Lehn, J.-M.
Supramolecular Chemistry; VCH: Weinheim, 1995; Chapter 9.
(2) Balzani, V.; Juris, A.; Venturi, M.; Campagna, S.; Serroni, S. Chem.
ReV. 1996, 96, 759.
We have investigated a series of mono- and binuclear
complexes with the [Cu(PPh3)2]+ moiety. This is a particularly
(3) Ruthkosky, M.; Kelly, C. A.; Zaros, M. C.; Meyer, G. J. J. Am. Chem.
Soc. 1997, 119, 12004 and references therein.
(4) Kohlman, S.; Kaim, W. Inorg. Chem. 1987, 26, 1469.
(5) Sauvage, J.-P. J. Am. Chem. Soc. 1988, 111, 7791.
S0020-1669(97)01505-X CCC: $15.00 © 1998 American Chemical Society
Published on Web 08/07/1998

Electrochemically Reduced Binuclear Copper(I) Complexes
Inorganic Chemistry, Vol. 37, No. 17, 1998 4453
7′, 7′′); 7.557 (d, J ) 7.7 Hz, 2H, 5′, 5′′); 7.781 (dd, J ) 7.4, 1.7 Hz,
2H, 8′, 8′′); 8.029 (s, 2H, 5, 8); 8.071 (d, J ) 8.6 Hz, 2H, 3′, 3′′);
8.205 (d, J ) 8.6 Hz, 2H, 4′, 4′′). Anal. Calcd: C, 81.5; H, 4.9; N,
13.6. Found: C, 81.0; H, 4.9; N, 13.4. Yield 61%.
6,7-Dichloro-2,3-(2′,2′′)diquinolylquinoxaline (dqqCl2). 1H NMR
(CDCl3): δ 7.508 (m, 6H, 5′, 5′′, 6′, 6′′, 7′, 7′′); 7.810 (dd, J ) 7.6,
1.7 Hz, 2H, 8′, 8′′); 8.099 (d, J ) 8.6 Hz, 2H, 3′, 3′′); 8.265 (d, J )
8.6 Hz, 2H, 4′, 4′′); 8.407 (s, 2H, 5, 8). Anal. Calcd: C, 68.9; H, 3.1;
N, 12.4. Found: C, 68.7; H, 2.8; N, 12.3. Yield 75%.
[(PPh3)2Cu(dqq)]BF4 (1‚[BF4]). Anal. Calcd: C, 70.3; H, 4.4; N,
5.3. Found: C, 70.3; H, 4.5; N, 5.5. Yield 30%.
[(PPh3)2Cu(dqqMe2)]BF4 (2‚[BF4]). Anal. Calcd: C, 70.1; H, 4.6;
N, 5.2. Found: C, 70.3; H, 4.6; N, 4.9. Yield 25%.
useful species to use in spectroscopic studies because the PPh3
ligands do not possess chromophores in the visible region which
greatly simplifies spectral interpretation.6
The crystal structures for a mononuclear and its corresponding
binuclear complex are presented. These show the structural
differences resulting from addition of the second metal center.
These changes may be related to the electrochemical properties.
It is also possible to compare the structural differences between
the reported dqq ligands and complexes with the previously
studied dpq-based ligands (dpq is 2,3-di(2-pyridyl)quinoxaline).
We find that all of the mononuclear complexes dechelate when
reduced. For the binuclear complexes dechelation or reduction
of the ligand may occur depending on the substituents of the
ligand. These properties are very surprising in view of the low
reduction potentials for the binuclear complexes. In a study of
the related [(PPh3)2Cu(dpq)Cu(PPh3)2]2+ complex it was found
that bridging ligand (BL) reduction occurred.7 For that complex
the reduction potential was ca. -0.9 V vs SCE. All of the
binuclear complexes reported herein are much easier to reduce,
the lowest reduction potential lying at -0.6 V vs SCE. Our
results suggest that the dechelation versus BL reduction
processes are balanced by electronic and structural factors.
[(PPh3)2Cu(dqqCl2)]BF4 (3‚[BF4]). Anal. Calcd: C, 66.0; H, 3.9;
N, 5.0. Found: C, 66.0; H, 4.1; N, 5.1. Yield 56%.
[(PPh3)2Cu(dqq)Cu(PPh3)2](BF4)2 (4‚[BF4]2). Anal. Calcd: C,
67.9; H, 4.4; N, 3.2. Found: C, 67.4; H, 4.1; N, 3.2. Yield 85%.
[(PPh3)2Cu(dqqMe2)Cu(PPh3)2](BF4)2 (5‚[BF4]2). Anal. Calcd: C,
67.5; H, 4.5; N, 3.2. Found: C, 67.7; H, 4.3; N, 3.0. Yield 85%.
[(PPh3)2Cu(dqqCl2)Cu(PPh3)2](BF4)2 (6‚[BF4]2). Anal. Calcd: C,
65.3; H, 4.1; N, 3.1. Found: C, 65.3; H, 4.6; N, 3.0. Yield 90%.
Physical Measurements. A Perkin-Elmer Lambda-19 spectropho-
tometer was used for collection of electronic absorption spectra. This
was calibrated with a Ho2O3 filter and spectra were run with a 2 nm
resolution.
Experimental Section
For electrochemical and spectroscopic measurements solvents of
spectroscopic grade were used. These were further purified by
distillation and were stored over 5 Å molecular sieves. The supporting
electrolytes used in the electrochemical measurements were tetrabu-
tylammonium perchlorate (TBAP) and tetrabutylammonium hexafluo-
rophosphate (TBAH). These were purified by repeated recrystalliza-
tions from ethanol/water for TBAP or ethyl acetate/ether for TBAH.11
Cyclic voltammograms (CVs) were obtained from argon-purged
degassed solutions of compound (ca. 1 mM) with 0.1 M concentration
of TBAP or TBAH present. The electrochemical cell consisted of a
1.6 mm diameter platinum working electrode embedded in a Kel-F
cylinder with a platinum auxiliary electrode and a saturated potassium
chloride calomel reference electrode. The potential of the cell was
controlled by an EG&G PAR 273A potentiostat with model 270
software.
NMR spectra were recorded using a Varian 200 MHz NMR.
Raman scattering was generated using a Spectra-Physics model 166
argon ion laser. The sample was held in a spinning NMR tube or an
optically transparent thin-layer electrode (OTTLE) cell, and the
scattering was collected in a 135° backscattering geometry. The
irradiated volume was imaged into a Spex 750M spectrograph using a
two-lens arrangement.12 The spectrograph was equipped with an 1800
g/mm holographic grating which provided a dispersion of 0.73 nm/
mm. The Raman photons were detected using a Princeton Instruments
liquid nitrogen cooled 1152-EUV charge-coupled detector controlled
by a Princeton Instruments ST-130 controller. CSMA v2.4 software
(Princeton Instruments) was used to control the CCD, and spectra were
analyzed using GRAMS/32 (Galactic Industries Corp.) software.
Spectral windows were approximately 18 nm wide and were calibrated
using emission lines from a neon lamp or from an argon ion laser. The
calibrations were checked by measuring the Raman band frequencies
for known solvents.13 It was found that, for the data reported herein,
the calibrations were accurate to 1 cm-1. Rayleigh and Mie scattering
from the sample was attenuated using a Notch filter (Kaiser Optical
Systems Inc.) of appropriate wavelength. A polarization scrambler was
placed in front of the spectrograph entrance slit. A 150 µm slit width
was used on the spectrograph, and this gave a resolution of ap-
proximately 6 cm-1 with 457.9 nm excitation.
Synthesis. Ligands were prepared by the Schiff base condensation
of a diamino compound with 2,2′-quinadil.8 This was prepared in an
analogous fashion to 2,2′-pyridyl from 2-quinoline carboxaldehyde.9
In a typical preparation 0.3 g (1 × 10-3 mol) of 2,2′-quinadil and 1
× 10-3 mol of the appropriate diaminobenzene were suspended in 100
mL of ethanol (freshly distilled from Mg/I2)10 and refluxed for 1 h. An
orange-red color change was observed in the solution phase. After
cooling, the solvent was removed under vacuum and the ligand was
recrystallized from ethanol.
Mononuclear complexes were prepared by the rapid mixing of
equimolar solutions (CHCl3) of ligand and [Cu(MeCN)2(PPh3)2](BF4).
In a typical preparation 75.6 mg (1 × 10-4 mol) of [Cu(MeCN)2(PPh3)2]-
(BF4) was dissolved in CHCl3 and made up to 100 mL in a volumetric
flask. The same was done with 1 × 10-4 mol of the appropriate ligand.
Rapid mixing resulted in a yellow-orange solution. The solvent was
removed under vacuum and the complex was recrystallized from
methanol.
Binuclear complexes were prepared by addition of 2 mol equiv of
[Cu(MeCN)2(PPh3)2](BF4) to 1 mol equiv of ligand in CH2Cl2. In a
typical preparation 0.24 g (3.2 × 10-4 mol) of [Cu(MeCN)2(PPh3)2]-
(BF4) was added to a stirring solution of ligand (1.6 × 10-4 mol) in
dichloromethane (20 mL). A rapid color change to dark red was
observed. After 5 min of stirring the volume of the solution was
reduced under vacuum and the complex was crystallized from the
solution by slow ether diffusion.
2,3-(2′,2′′)-Diquinolylquinoxaline (dqq). 1H NMR (CDCl3): δ
7.452 (m, 2H, 6′,6′′); 7.500 (m, 2H, 7′, 7′′); 7.561 (dd, J ) 7.6, 1.6
Hz, 2H, 5′, 5′′); 7.784 (dd, J ) 7.4, 1.9 Hz, 2H, 8′, 8′′); 7.848 (dd, J
) 6.4, 3.4 Hz, 2H, 6, 7); 8.106 (d, J ) 8.5 Hz, 2H, 3′, 3′′); 8.226 (d,
J ) 8.5 Hz, 2H, 4′, 4′′); 8.287 (dd, J ) 6.5, 3.4 Hz, 2H, 5, 8). Anal.
Calcd: C, 81.2; H, 4.2; N, 14.6. Found: C, 80.9; H, 3.9; N, 14.7.
Yield 70%.
6,7-Dimethyl-2,3-(2′,2′′)diquinolylquinoxaline (dqqMe2). 1H NMR
(CDCl3): δ 2.550 (s, 6H, -CH3); 7.440 (m, 2H, 6′,6′′); 7.492 (m, 2H,
(6) Gordon, K. C.; McGarvey, J. J. Inorg. Chem. 1991, 30, 2986.
(7) Gordon, K. C.; Al-Obaidi, A. H. R. Jayaweera, P. M.; McGarvey, J.
J.; Malone, J. F.; Bell, S. E. J. J. Chem. Soc., Dalton Trans. 1996,
1591.
The electronic absorption spectra of reduced species were measured
using an OTTLE cell with a platinum grid as the working electrode.14
(8) Goodwin, H. A.; Lions, F. J. Am. Chem. Soc. 1959, 81, 6415.
(9) (a) Buehler, C. A.; Harris, J. O. J. Am. Chem. Soc. 1950, 72, 5015-
5016. (b) Kaplin, H. J. Am. Chem. Soc. 1941, 63, 2654.
(10) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of
Laboratory Chemicals, 2nd ed.; Pergamon Press: Oxford, 1980; p
552.
(11) House, H. O.; Feng, E.; Peet, N. P. J. Org. Chem. 1971, 36, 2371.
(12) Strommen, D. P.; Nakamoto, K. Laboratory Raman Spectroscopy; John
Wiley & Sons, Inc.: New York, 1984.
(13) Ferraro, J. R.; Nakamoto, K. Introductory Raman Spectroscopy;
Academic Press Inc.: San Diego, CA, 1994.

4454 Inorganic Chemistry, Vol. 37, No. 17, 1998
Page et al.
Table 2. Electrochemical Data for Reduction Waves of Ligands
Table 1. Crystal Data for 3‚[BF4] and 6‚[BF4]2
and Complexes in CH2Cl2 at Room Temperature (25 °C)a
identification
code
3‚[BF4]
6‚[BF4]2‚2CH2Cl2
compound
dqqb
dqqMe2
E°′ vs SCE/V
empirical formula C62H44BCl2CuF4N4P2
C51H41BCl5CuF4N2P2
1071.40
293(2)
0.71073
monoclinic
C2/c
21.295(4)
24.322(5)
20.034(4)
90
112.64(3)
90
-1.71
-1.75
-1.31
-0.80c
-0.53
-0.61
-0.32
b
fw
1128.20
203(2)
0.71073
triclinic
P1h-
10.956(2)
15.278(3)
16.032(3)
100.342(8)
95.291(13)
93.968(12)
2618.6(9)
2
temp, K
wavelength, Å
crystal system
space group
a, Å
dqqCl2
3
4
5
6
b, Å
c, Å
a Reduction potentials are given versus SCE. Supporting electrolyte
TBAP or TBAH. All waves are fully reversible, unless indicated,
showing a ratio of currents for the anodic and cathodic peaks of unity
and peak separation between anodic and cathodic waves of ca. 60 mV
(ref 35). b Measured using DMF as solvent. c Does not show reversible
electrochemistry.
R, (deg)
â, (deg)
γ, (deg)
V, Å3
9576.8(3)
8
Z
F(calcd), (Mg/m)
µ, mm-1
final R indices
[F > 4σ(F)]
1.431
0.641
1.486
0.857
Table 3. Electronic Absorption Data for Ligands and Complexes
in CH2Cl2
a
R(Fo) ) 0.0651,
R(Fo) ) 0.0422,
Rw(Fo ) ) 0.1590a
Rw(Fo ) ) 0.1023a
2
2
compound
λ/nm (ꢀ × 10-3/dm3 mol-1 cm-1
)
a wR2 ) [([w(Fo2 - Fc2)2]/([w(Fo )2]]1/2 where w-1 ) [σ2(Fo ) + (aP)2
+ bP] for 3‚[BF4] (a ) 0.0; b ) 0.0) and 6‚[BF4]2‚2CH2Cl2 (a )
2
2
dqq
dqqMe2
dqqCl2
1
2
3
4
5
6
255 (89)
261 (87)
260 (88)
267 (82)
273 (54)
272 (83)
265 (83)
261 (86)
267 (88)
334 (27)
324 (26)
349 (24)
0.0549; b ) 35.0216), P ) [max(Fo ,0) + 2Fc2]/3. The structure was
2
310s (25)
315s (19)
311s (26)
335 (27)
338 (29)
337 (25)
368 (16)
380 (14)
374 (19)
374s (20)
379s (22)
380s (19)
420s (3.3)
2
refined of Fo using all data; R1 ) ∑||Fo| - |Fc||/∑|Fo|.
417s (3.9)
437 (3)
For Raman spectra of reduced species a similar cell was employed.
Initial measurements found that the signal-to-noise of the Raman spectra
were reduced because of reflection off the platinum grid. This problem
was alleviated by removing a portion of the center of the grid (ca. 2
mm × 4 mm) and aligning the laser to irradiate the solution in that
region.
474 (4.8)
461 (5.3)
503 (3.7)
a Superscript “s” denotes shoulder.
0.710 73 Å) in the range 3 e 2θ e 40° by the ω scan motion with
index ranges 0 e h e 19, 0 e k e 23, -18 e l e 17. A total of 4609
reflections were collected, of which 4450 were unique (Rint ) 0.0541).
The data were corrected for Lorentz, and polarization effects. No
correction for extinction was applied. Scattering factors are included
in SHELX-93.15 Systematic monitoring of three check reflections
showed no systematic crystal decay and no correction was applied.
The position of the Cu atom was determined from a Patterson synthesis.
Calculations were carried out using an IBM-compatible computer and
SHELX-93.1 The remaining non-hydrogen atoms were located by
application of a series alternating least-squares cycles and difference
Fourier maps. All non-hydrogen atoms were refined with anisotropic
displacement parameters. Hydrogen atoms were included in the
structure factor calculations at idealized positions but were not
subsequently refined. Final least-squares refinement of 624 parameters
Crystallography. Single crystals of 3‚[BF4] were grown by the
slow diffusion of diethyl ether into a solution of 3‚[BF4] dissolved in
dichloromethane. A yellow platelike crystal with approximate dimen-
sions 0.15 × 0.25 × 0.20 mm3 was selected and attached to the end of
a glass fiber and cooled to -70 °C in a nitrogen stream. Crystal-
lographic data are summarized in Table 1. Intensity data were collected
using an Siemens SMART diffractometer (263 K, Mo KR X-radiation,
graphite monochromator, λ ) 0.710 73 Å). The data collection
nominally covered over a hemisphere of reciprocal space, by a
combination of three sets of exposures; each set had a different (angle
for the crystal and each exposure covered 0.3° in ω. The crystal-to-
detector distance was 4.94 cm. Coverage of the unique set is over
97% complete to at least 26° in θ. Crystal decay was monitored by
repeating the initial frames at the end of data collection and analyzing
the duplicate reflections, no decay was observed and no correction was
applied. The data with index ranges -13 e h e 13, -15 e k e 19,
-20 e l e 18 were employed for refinement. A total of 11 533
reflections were collected, of which 6749 were unique (Rint ) 0.1108).
The data were corrected for Lorentz, and polarization effects. Scattering
factors are included in SHELX-93.13 No correction for extinction was
applied. The position of the Cu atom was determined from a Patterson
synthesis. Calculations were carried out using a IBM-compatible
computer and SHELX-93.13 The remaining non-hydrogen atoms were
located by application of a series alternating least-squares cycles and
difference Fourier maps. All non-hydrogen atoms were refined with
anisotropic displacement parameters. Hydrogen atoms were included
in the structure factor calculations at idealized positions but were not
subsequently refined. Final least-squares refinement of 685 parameters
2
resulted in residuals, R(Fo) of 0.0422 and Rw(Fo ) of 0.1023 [Fo > 4σ-
2
(Fo)]. After convergence the quality-of-fit on Fo was 0.978 and the
highest peak in the final difference map was 0.420.
Results
Electrochemistry. Electrochemical data for the ligands and
complexes are shown in Table 2. The mononuclear complexes
show irreversible reduction waves. The binuclear complexes
have reversible reductions. The E°′ values follow 6 > 4 > 5.
The ligand reductions lie in the same order, with the most
electron deficient ligand, dqqCl2, being the easiest to reduce
and the electron rich dqqMe2 the most difficult.
Electronic Spectra. The electronic spectral data for the
ligands and complexes are presented in Table 3. The lowest
transition for the ligands lies in the 320-330 nm region. This
is assigned as a π f π* transition. The energy of this transition
is lowest for dqqCl2 and highest for dqqMe2. A further
transition is observed at ca. 260 nm. No n f π* transitions are
seen for these ligands. Such transitions are generally weak.16
2
resulted in residuals, R(Fo) of 0.0651 and Rw(Fo ) of 0.1590 [F > 4σ-
(F)] (4801 reflections). After convergence the quality-of-fit on F2 was
0.964, and the highest peak in the final difference map was 0.603.
Single crystals of 6‚[BF4]2‚2CH2Cl2, were grown by the slow
diffusion of diethyl ether into a solution of 6‚[BF4]2 dissolved into
dichloromethane. A red rod-shaped crystal with approximate dimen-
sions 0.45 × 0.65 × 0.69 mm3 was secured to the end of a glass fiber
with cyanoacrylate glue. Crystallographic data are summarized in Table
1, and all other relevant data are available as Supporting Information.
Intensity data were collected using an Enraf-Nonius CAD-4 diffrac-
tometer (293 K, Mo KR X-radiation, graphite monochromator, ()
(14) Babaei, A.; Connor, P. A.; McQuillan, A. J.; Umapathy, S. J. Chem.
Educ. 1997, 74, 1200.
(15) Sheldrick, G. M. SHELXL-93-97; Institut fu¨r Anorganische Chemie
der Universita¨t Go¨ttingen: Germany, 1993.
(16) Badger, G. M.; Pettit, R. J. Chem. Soc. 1952, 1874.

Electrochemically Reduced Binuclear Copper(I) Complexes
Inorganic Chemistry, Vol. 37, No. 17, 1998 4455
Figure 2. Resonance Raman spectra (λexc ) 488 nm, 25 mW) of (upper
trace) 1 (2 mM) in CH2Cl2; (middle trace) 4 (2 mM) in CH2Cl2; and
(lower trace) 4- (2 mM) in CH2Cl2. Solvent bands are labeled S.
Figure 3. Resonance Raman spectra (λexc ) 488 nm, 25 mW) of (upper
trace) 2 (2 mM) in CH2Cl2; (middle trace) 5 (2 mM) in CH2Cl2; and
(lower trace) 5- (2 mM) in CH2Cl2. Solvent bands are labeled S.
The mononuclear complexes show a long wavelength shoul-
der of moderate intensity (ꢀ ∼ 3000 M-1 cm-1). This is
assigned as a metal-to-ligand charge-transfer (MLCT) transition.
The energy of this transition follows the same pattern as that
for the free ligands; 3 has the lowest energy. It is interesting
to note that the ꢀ drops as the transition energy decreases.
The binuclear complexes also show MLCT transitions as the
lowest energy visible absorption. This is consistent with the
more positive E°′ values for reduction, which indicates that the
presence of the second metal stabilizes the reduction. The
reduction in (with lower transition energy for the MLCT band
is also observed.
Resonance Raman Spectra. Resonance Raman spectra for
the mononuclear complexes generated with 488 nm excitation
are shown in Figures 2-4. These spectra are similar, all three
complexes show bands at about 1594 and 1378 cm-1. Other
bands shift with substitution at the quinoxaline ring. The
strongest band in 1 and 2 lies at 1460 cm-1, this shifts to 1450
cm-1 in 3. Also of note is a very intense band at ca. 1530
cm-1 in the spectrum of 1 and 3.
All of the bands observed in the binuclear complexes are seen
in the corresponding mononuclear spectra, with some small
shifts in wavenumber values. However, the relative enhance-
ments of the bands are quite different from mono- to binuclear
complexes.
The resonance Raman spectra of the binuclear complexes
show strong excitation wavelength dependence.17 For 4 bands
at 1323 and 1559 cm-1 are enhanced into the red with a band
at 1510 cm-1 showing strongest enhancement with blue excita-
tion. For 5 the 1549 cm-1 band shows strongest enhancement
with 514.5 nm excitation. The 1511 cm-1 shows greater
Figure 4. Resonance Raman spectra (λexc ) 488 nm, 25 mW) of (upper
trace) 3 (2 mM) in CH2Cl2; (middle trace) 6 (2 mM) in CH2Cl2; and
(lower trace) 6- (2 mM) in CH2Cl2. Solvent bands are labeled S.
enhancement at 457 nm. For 6 the 1545, 1450, 1354, and 1324
cm-1 bands have increased enhancements out to 514.5 nm. The
bands at 1515 and 1400 cm-1 show maximum enhancement at
457.9 nm. This strongly suggests that the broad absorption
feature in the electronic spectrum of the complexes is made up
of at least two transitions, the population of which results in
different geometry changes on the bridging ligand. That is, the
nature of the acceptor orbital of the BL or the donor MO of the
metal is significantly different for the two transitions.
(17) The relative intensities of the complex bands may be parameterized
by peak area (Icomplex band) divided by the peak area of an internal
standard band in the solution (Isolvent). In the case of solutions of 4-6
the solvent bands may act as the internal standard. The intensities of
the complex and solvent bands were determined from the peak area
corrected for baseline variations. The solvent band used was the 702
cm-1 peak as this is a strong feature and does not overlap with other
solvent or complex bands. Some of the complex bands’ enhancement
factors could not be determined beause of overlapping features.
Excitation profiles for 4-6 are provided in supporting material.
Spectroelectrochemistry. Changes in the electronic spectra
of dqqCl2 and 3 upon electrochemical reduction are shown in
Figure 5.
For dqqCl2 the reduction (Figure 5a) leads to the growth in
of a band at 551 nm (ꢀ ∼ 2000 M-1 cm-1). The series of spectra
measured at differing potentials across the reduction wave show

4456 Inorganic Chemistry, Vol. 37, No. 17, 1998
Page et al.
P(2). The only apparent reason for this distortion is the
geometry of the ligand. The geometry of the ligand is
interesting with the sections that are coordinating to the Cu(I)
(the dichloroquinoxaline and one of the quinoline groups) being
held in a slightly bent (15.5(3)° plane-to-plane) arrangement.
This bending of the ligand is toward P(2) which appears to react
by moving away to a more equatorial position. The structure
in Figure 6 (right side) shows a view down the coordinating
part of the ligand and the deviation of the free quinoline by
44.6(1)° from the mean plane of the rest of the ligand.
The molecular structure of 6‚[BF4]2‚2CH2Cl2 is shown in
Figure 7. The two sides of the molecule are related by
symmetry operation (C2 rotation) and both coppers are therefore
identical. In comparison to the structure of 3 the ligand has
adopted a different geometry with significant deviations from
planarity for the two sections of the ligand, the dichloroqui-
noxaline and the quinoline groups. The angle between the
dichloroquinoxaline and the quinoline is 38.4(1)(and is probably
due to unfavorable interactions between the protons on C3′ and
C3′′.5 The copper is coordinated in a distorted tetrahedral
geometry with a Cu-Cu′ distance at 6.892(9) Å. The bond
lengths for the Cu-N and Cu-P bonds are all within the range
reported for similar compounds.15 The distortion of the
geometry around the Cu atom is far more pronounced in 6 than
was observed in 3. The P(1)-Cu-P(2) angle is 124.30(8)° with
P(1) being significantly more axial in nature that P(2). The
angle between P(1) and the mean Cu-N(1,2) plane is 122.8-
(1)°. In contrast, the angle between P(2) and the mean Cu-
N(1,2) plane is 112.9(1)°. In this case the distortion is caused
by an interaction between one of the phenyl rings on P(2) and
the dichloroquinoxaline section of the ligand. The phenyl ring
stacks above, and below, the pyrazine ring at an average distance
of 3.536 Å, within range expected for π stacking.
Figure 5. Changes in the electronic absorption spectrum of (a) dqqCl2
(1mM) in CH2Cl2 as a function of applied potential, 0.0 to -1.6 V vs
Ag wire, and (b) 3 (1 mM) in CH2Cl2 as a function of applied potential,
0.0 to -1.0 V vs Ag wire.
an isosbestic point at 381 nm. This suggests a single process
is the dominant reaction probed in the OTTLE cell.
The reduction of 3 (Figure 5b) results in a bleaching of the
MLCT band at 437 nm. The resultant spectrum is identical to
that of the free ligand, dqqCl2.
The binuclear complexes undergo electrochemical reduction.
The absorption spectra (see Supporting Information for spectra)
show depletion of the MLCT bands for the parent species with
the reduced species absorbing to the blue. Isosbestic points are
observed at 310 nm for 4 f 4-; 319 nm for 5 f 5-; 320 nm
for 6 f 6-. For 5- the observed spectrum is similar to that of
the corresponding mononuclear complex, 2. Furthermore, the
spectral changes for 4 and 6 are fully reversible; those observed
for 5 are not.
As a result of the varying factors controlling the solid-state
structures of 3 and 6 the ligand has quite different orientations.
In 3 one of the quinoline lobes are effectively planar to the
dichloroquinoxaline whereas in 6 neither of the two quinoline
lobes is planar to the dichloroquinoxaline.
The resonance Raman spectra, generated with 488 nm
excitation, of 4-6 and their reduced products are shown in
Figures 2-4. Also shown are the resonance Raman spectra of
the corresponding mononuclear species generated with 488 nm
excitation.
Electrochemistry and Electronic Spectra. The electro-
chemical data for the reduction potentials of the ligands show
that the most electron deficient ligand is easiest to reduce. For
the mononuclear systems there is no reversible reduction
observed.
The presence of two copper(I) sites, as in the binuclear
complexes, stabilizes the system to reduction. The E°′ values
show the same pattern as the ligands. This suggests the
reduction in the binuclear complexes is occurring at the ligand
or involves a redox orbital with significant ligand character.
The wavelength of the lowest energy electronic transition for
4-6 follow the reduction potential values. That is, the complex
which is easiest to reduce, 6, has the lowest energy transition,
at 503 nm. A qualitative relationship between the E°′ for
reduction and the transition energy is indicative of an MLCT
transition.19
Application of a reducing potential to the samples results in
changes in the observed Raman spectra. The spectra of the
reduced species contain no signal from the parent species. For
4 (Figure 2), reduction results in the depletion of the neutral
species bands at 1559, 1510, and 1461 cm-1. For 5 (Figure 3),
bands at 1511 and 1363 cm-1 disappear upon reduction. For 6
(Figure 4), the intense neutral species bands at 1545, 1515, 1450,
1382, and 1354 cm-1 are all bleached upon reduction.
Discussion
Crystallography. The molecular structure of the cationic
portion of 3 is shown in Figure 6. The ligand is quite large but
no significant steric interactions are evident. The copper is held
in a distorted tetrahedral environment with the Cu-P and Cu-N
bond lengths being within the range reported for related
complexes.5,18 The P(1)-Cu-P(2) is 120.87(6)°, but the
phosphine coordination is not symmetrical with respect to the
mean Cu-N(1,2) plane. P(1) is slightly more axial, by 6°, than
As the λmax for the MLCT band increases, the ꢀ is observed
to fall. McMillin et al.20 examined the electronic spectroscopy
of a series of copper(I) polypyridyl complexes. Using theories
developed by Mulliken et al.21 they demonstrated that the
(18) (a) Barron, P. F.; Dyason, J. C.; Engelhardt, L. M.; Healy, P. C.; White,
A. H. Aust. J. Chem. 1985, 38, 261. (b) Engelhardt, L. M.;
Pakawatchchai, C.; White, A. H.; Healy, L. M. J. Chem. Soc., Dalton,
Trans. 1985, 125. (c) Diez, J.; Gamasa, M. P.; Gimeno, J.; Tiripicchio,
A.; Tiripicchio, C. J. Chem. Soc., Dalton Trans. 1987, 1275. (d)
Ainschough, E. W.; Baker, E. N.; Brader, M. L.; Brodie, M. L.;
Ingham, S. L.; Waters, J. M.; Hanna, J. V.; Healy, P. C. J. Chem.
Soc., Dalton Trans. 1991, 1243. (e) Ainscough, E. W.; Brodie, A.
M.; Ingham, S. L.; Waters, J. M. J. Chem. Soc., Dalton Trans. 1994,
215.
(19) Juris, A.; Campagna, S.; Bidd, I.; Lehn, J.-M.; Ziessel, R. Inorg. Chem.
1988, 27, 4007.
(20) Phiffer, C. C.; McMillin, D. R. Inorg. Chem. 1986, 25, 1329.
(21) (a) Mulliken, R. S. J. Am. Chem. Soc. 1952, 74, 811. (b) Day, P.;
Sanders, N. J. Chem. Soc. A 1967, 1530. (c) Day, P.; Sanders, N. J.
Chem. Soc. A 1967, 1536.

Electrochemically Reduced Binuclear Copper(I) Complexes
Inorganic Chemistry, Vol. 37, No. 17, 1998 4457
Figure 6. Molecular geometry of the cationic part of 3‚[BF4]. For clarity part of the phenyl rings of the PPh3 groups have been omitted from the
left view. Ellipsoids are shown at the 50% probability level.
Figure 7. Molecular geometry of cationic part of 6‚[BF4]‚2CH2Cl2. For clarity, part of the phenyl rings of the PPh3 groups has been omitted from
the left view. The two right views have been reduced in size with respect to the left view, and the intramolecular π stacking is evident. Ellipsoids
are shown at the 50% probability level.
transition intensity, as measured by ꢀ, could be related to the
dipole length of the transition. The distance between the
acceptor orbital, BL π* MO, and the donor site, the metal, is
related to the intensity of the transition between the two orbitals.
The greater the distance, the larger the ꢀ value.
In terms of the binuclear complexes, in this study, there are
two related phenomena which may be explained within the
context of the Mulliken analysis. The transitions energies are
reduced along with the ꢀ, or dipole length. The dqq-based
ligands used in this study possess two differing ring systems,
each has two quinoline rings and a quinoxaline or substituted
quinoxaline ring. The former are less electron deficient than
the latter. Thus one might expect the acceptor orbital in an
MLCT transition to have lower energy the greater its quinoxaline
ring character. However, the greater the contribution of the
quinoxaline ring to the acceptor MO the shorter the dipole
length, by virtue of the fact that the quinoxaline is directly bound
to the donor metal center. For the quinoline rings only one is
bound to the donor metal site, the other being attached to the
remote metal center. An acceptor MO with quinoline character
will have a longer dipole length but a higher transition energy.
Clearly the actual nature of the of the donor and acceptor
MOs is more complex than presented, nevertheless, this
approach offers a qualitative insight into the nature of the MLCT
transition. Furthermore, it is interesting to note the studies on
the related dpq copper(I) systems reveal a similar pattern of
behavior. The ꢀ for 4-6 are greater than those reported for
the corresponding dpq-based complexes of Yam.22 This is
consistent with the idea of dipole length being related to the
distance between donor and acceptor orbitals. The larger
quinoline rings provide extended distance for the π* MO for
BL from the metal center.
(22) Yam, V. W.; Lo, K. K. J. Chem. Soc. Dalton Trans. 1995, 499.

4458 Inorganic Chemistry, Vol. 37, No. 17, 1998
Page et al.
Resonance Raman Spectra. The large number of atoms in
these complexes, coupled with the low symmetry mean that
there are many Raman active modes of vibration. All of the
complexes show a large number of bands in the resonance
Raman spectra in the 1000-1600 cm-1 region. A number of
empirical points may be made about the form of the resonance
Raman spectra:
(i) Each complex shows a distinct and unique spectral
signature, this includes mono and binuclear complexes with the
same BL and the mono- and binuclear series of compounds.
(ii) The frequencies of the observed bands are little shifted
on going from mono- to binuclear complex, although the
enhancement patterns are severely perturbed.
(iii) The observed spectra for the dqq based systems are
significantly different from those of the related dpq complexes.
By comparing the spectra of related complexes it is possible
to suggest assignments for the BL vibrations as being associated
with either the quinoline or quinoxaline ring systems. The fact
that the frequencies of the observed bands do not shift
significantly on going from mono- to binuclear complex implies
that the normal modes associated with the observed bands are
BL in nature.
For [Cu(dpq)(PPh3)2]+ and Re(CO)3Cl(dpq)23 the following
bands are observed 1329, 1402, and 1471 cm-1 and 1325, 1360,
and 1467 cm-1. If these spectra are compared to that of 1, two
of the bands lie close in wavenumber to the dpq bands; these
are at 1331 and 1397 cm-1. In 1 a strong band is also present
at 1461 cm-1, shifted significantly from the 1471 observed in
[Cu(dpq)(PPh3)2]+. The 1330 and 1400 cm-1 bands appear to
be associated with the quinoxaline, the 1460 cm-1 with the
quinoline. Consistent with this assignment the quinoxaline
bands shift with substitution. The spectrum of 2 shows bands
at 1332 and 1346 cm-1, there is no band at 1400 cm-1, and the
1462 cm-1 is unshifted from 1. On going to 3 the 1330 cm-1
region has bands at 1324, 1343, and 1354 cm-1, a band is
present at 1398 cm-1, and a band at 1463 cm-1 is weak with a
stronger band at 1450 cm-1. The reduction in enhancement of
the 1460 cm-1 band in 3 in comparison to 1 and 2 is consistent
with the electronic absorption spectra which show that 3 has
the lowest energy transition of the mononuclear complexes and
the lowest ꢀ. The reduced enhancement of a band associated
with the quinoline ring suggests that the acceptor MO in the
MLCT transition, the π* bridging ligand orbital has greater
quinoxaline character; the reduction in ꢀ is consistent with this
because the dipole length is shorter to the quinoxaline ring than
the center of the two quinoline rings.
The spectra for binuclear complexes of the related dpq
bridging ligand with ruthenium(II),24 copper(I),25 and rhenium-
(I)26 are similar to one another. For [(Cu(PPh3)2)2(dpq)]2+ bands
lie at 1286, 1319, 1364, 1471, 1493, 1566, and 1606 cm-1. For
the ruthenium(II) complex bands appear at 1273, 1308, 1357,
1461, 1488, 1562, and 1597 cm-1. For the rhenium(I) complex
bands are observed at 1293, 1321, 1366, 1467, 1497, 1558, and
1598 cm-1. The SERS spectrum of quinoxaline shows a series
of bands lying close in wavenumber to those reported above,
for this reason the above bands are attributed to quinoxaline.22
In 4 bands are observed at 1298, 1323, 1366, 1400, 1485, 1559,
and 1591 cm-1 which show stronger enhancement on going to
red excitation. Furthermore these bands are shifted on going
to 5 and 6, consistent with them being quinoxaline-based modes.
In contrast, a number of bands appear common to 4-6 which
do not appear in the spectra of [(Cu(PPh3)2)2(dpq)]2+ and
[Re(CO)3Cl]2(dpq) and show diminished enhancement with red
excitation. These bands are weak in comparison to the
quinoxaline-based modes. They lie at approximately 1335,
1380, 1510, and 1564 cm-1. Some of these bands lie close in
wavenumber to those of 2,2′-biquinoline (biq) as observed in
[Cu(biq)2]+.27 The biq modes lie at 1334, 1384, 1464, 1551,
and 1601 cm-1. Although, the similarities between the vibra-
tional spectra of quinoline and quinoxaline make such distinc-
tions difficult,28 it appears correct to state that those bands that
are strongly enhanced at 514.5 nm are similar to bands observed
in complexes with dpq and shift with substitution at the
quinoxaline ring. Most of the bands that show diminished
enhancement at 514.5 nm are less sensitive to substitution at
the quinoxaline ring and lie close to bands of [Cu(biq)2]+.
This suggests that, under the MLCT absorption band, there
are two transitions: one terminating on a ligand MO with
significant quinoxaline character and a second, at higher energy,
terminating on a ligand MO with more quinoline character.
The 1510 cm-1 band is notable by its absence in spectra of
dpq or biq. It is perhaps a delocalized mode coupling the two-
ring systems. However, its relatively small shifting with
quinoxaline substitution suggests significant quinoline character.
Spectroelectrochemistry. The reduction of dqqCl2 (Figure
5a) results in increased absorption across the visible region. This
is typical of radical anion species of polypyridyl ligands.29
Reduction of 3 (Figure 5b) does not result in ligand based
radical anion formation. A bleaching of the MLCT band is
observed as the parent species is reduced. The resulting
spectrum is identical to that for the free ligand. This is
consistent with the reduction occurring at the metal Cu(I) f
Cu(0), which results in the dissociation of the complex. This
behavior is common in copper(I) polypyridyl systems;30 how-
ever, it is surprising that it occurs in a complex that is easily
reduced and has an electron-deficient ligand. The clearest
example of ligand-based reduction in copper(I) polypyridyl
complexes occurs for [Cu(dmbinap)2]+ (dmbinap ) 3,3′-
dimethylene-2,2′-bi-1,8-naphthyridine).31 Those complexes are
considerably more difficult to reduce than 3.
The reduction of the binuclear complexes (4-6) results in
depletion of MLCT absorptions of the parent species. The
reduced complexes do not correspond to the free-ligand spectra.
For 5 f 5- the reduced species appears to possess an electronic
absorption spectrum similar to 2. For 4- and 6- the spectra do
not correspond to the mononuclear complexes 1 and 3. This
suggests that 5, which possesses an electron rich BL is separating
into the mononuclear complex upon reduction.
5 + e- f 2 + Cu + 2PPh3
The resonance Raman spectrum of 5- confirms the formation
of 2 as a reduction product. The resonance Raman spectra of
5- and 2 are identical (Figure 3).
(27) Leupin, P.; Scha¨lphfer, C. W. J. Chem. Soc., Dalton Trans. 1983,
1635.
(28) Carrano, J. T.; Wait, S. C., Jr. J. Mol. Spectrosc. 1973, 46, 401.
(29) Shida, T. Electronic Absorption Spectra of Radical Ions; Elsevier:
(23) Waterland, M. W.; Simpson, T. J.; Gordon, K. C.; Burrell, A. K. J.
Chem. Soc. Dalton Trans., in press.
Amsterdam, 1988.
(24) Cooper, J. B.; MacQueen, D. B.; Petersen, J. D.; Wertz, D. W. Inorg.
Chem. 1990, 29, 3701.
(25) Gordon, K. C.; McGarvey, J. J. Chem. Phys. Lett. 1990, 173, 443.
(26) Simpson, T. J.; Gordon, K. C. Inorg. Chem. 1995, 34, 6232.
(30) Masood, M. A.; Zacharias, P. S. J. Chem. Soc., Dalton Trans. 1991,
111.
(31) Scott, S. M.; Gordon, K. C.; Burrell, A. K. Inorg. Chem. 1996, 35,
2452.

Electrochemically Reduced Binuclear Copper(I) Complexes
Inorganic Chemistry, Vol. 37, No. 17, 1998 4459
For 4 and 6 reduction does not result in the formation of the
respective mononuclear complexes. The resonance Raman
spectrum of 4- (Figure 2) shows a very strong band at 1532
cm-1 with weaker bands at 1570 and 1592 cm-1. Although
the 1532 cm-1 band is coincident with a band in the corre-
sponding mononuclear complex, 1, the strongest band for 1 at
1461 cm-1 is not present in 4-.
appear stable do so by having very electron-deficient ligands,29
thus forming radical anion species, or by having ligands which
encapsulate the reduced metal such that dechelation does not
occur.4 [(Cu(PPh3)2)2(dpq)]2+ forms a dpq-based radical anion
species upon reduction. Yet it is significantly harder (0.3 V)
to reduce than 5. The ability to form a ligand-based radical
anion species is not simply related to ease of reduction. The
most probable reason for the instability of 5- is the steric bulk
of the dqqMe2 ligand. However, if the steric effect dominated
the nature of the electrochemical products then both 4 and 6
would dissociate upon reduction. By considering electronic and
steric factors together it is possible to provide an explanation
consistent with our results.
In the case of 5 the methyl groups appear to cause the
reducing electron to reside in an MO which is localized at the
quinoline rings because of the electron donating methyl groups
on the quinoxaline. Thus the reduced electron will form a redox
MO with significant charge density on the quinoline and
quinoxaline rings. Assuming the π* ligand orbital occupied
by the reducing electron is bonding with respect to the inter-
ring linkage then reduction will lead to a flattening out of the
ligand and dissociation of the complex. The observation of a
flattening of ligands with reduction has been observed in 2,2′-
bipyridine- and 4,4′-bipyridine-based systems.34
A similar pattern is observed for 6-. The most intense bands
in the resonance Raman spectrum lie at 1374 and 1536 cm-1
.
Both of these bands lie close to resonance Raman bands of 3
(1377 and 1532 cm-1). However, the strongest features in the
resonance Raman spectrum of 3 with 488 nm excitation lie at
1450 and 1398 cm-1. These are absent in the spectrum of 6-.
For 4 and 6 the reduction appears to be based on the bridging
ligand. It is interesting to note that the electronic absorption
-
spectrum of dqqCl2 is unlike that of 6-. This may be due to
the differing geometries of these two species. Single-crystal
X-ray studies of the related dpq ligand show it to have the
pyridyl rings with the N pointed in to each other.32 This is
clearly not the configuration adopted by the ligand in 3 or 6.
The resonance Raman spectra of 4- and 6- are similar but
not identical. These spectra are unlike those of dpq and biq
radical anion systems24,33 suggesting the redox MO involves
both quinoline and quinoxaline ring systems. Recently the
resonance Raman spectra for a series of reduced dirhenium(I)
complexes with dpq-based bridging ligand systems were
reported.24 In these complexes the spectra of the reduced species
were identical. This finding was explained in terms of a
localized redox molecular orbital in which the electron density
was concentrated at the pyridyl ring systems for the differing
ligands. The quinoline ring systems present in 4 and 6 also
offer the possibility of electron localization; indeed the greater
electron accepting ability of the quinoline over pyridyl ring may
favor such a process. However, the spectral evidence suggests
this has not occurred. There are a number of possible reasons
for this:
(i) The two [Cu(PPh3)2]+ units are insufficiently stabilizing
to BL-. For the binuclear Re(CO)3Cl systems it is known that
the rhenium centers very strongly stabilize electron density
suggesting the π* MO of the ligand has a lot of metal character.
The lesser stabilizing ability of the coppers may mean the
electron acceptor MO has mostly ligand character. In that case
stabilization of charge at the most electron deficient ring system
would be anticipated.
(ii) The quinolines reduce the planarity of the ligand with
respect to the three ring systems. This may reduce the ability
of the two quinoline rings to stabilize the reducing electrons
charge. This would lead to greater charge density for that
electron at the quinoxaline ring system.
The suggestion that the redox MO is not strongly localized
at the quinoline ring system is consistent with the electronic
spectral data. The data may be interpreted in terms of the optical
MO in 6 extending over the quinoxaline more than the quinoline
rings.
The dissociation of 5- to 2 provides an insight into the factors
the control stability of reduced species formed from copper(I)
complexes.
Until recently, very few copper(I) polypyridyl complexes
showed ligand-based reduction electrochemistry. Those that
For 4 and 6 the reducing electron will reside over the
quinoxaline ring, lowering the geometric distortions and result-
ing in less destabilization of the complex. This would occur
more with the dqqCl2 ligand than the dqq system. This is
consistent with the different resonance Raman spectra for 4-
and 6-.
Conclusions
This work provides the first example of dechelation versus
ligand based radical anion formation in the electrochemically
reduced forms of a closely related group of complexes.
Substitution effects on the quinoxaline ring system alter the
nature of the acceptor MO or optical MO for the MLCT
transition. They also appear to change the charge density
distribution for the MO occupied by the reducing electron. It is
clear that the use of such units in the construction of polynuclear
copper(I) assemblies requires careful thought as both steric and
electronic factors play a role in determining the outcome of
reduction at one these units.
Acknowledgment. Support from the New Zealand Lottery
Commission and the University of Otago Research Committee
for the purchase of the Raman spectrometer is gratefully
acknowledged. S.E.P. thanks the University of Otago for a
postgraduate scholarship. This work was supported, in part, by
the New Zealand Public Good Science Fund (Contract No.
UOO-508). We are grateful for support of this work from the
Massey University Research Fund. We thank Associate Profes-
sor C. E. F. Rickard (University of Auckland) for data collection
on 3‚[BF4].
Supporting Information Available: Crystallographic data, reso-
nance Raman spectra of 4, 5, and 6 at 457.9, 488, and 514.5 nm
excitation, changes in the electronic absorption spectra of 4, 5, and 6
as a function of reducing potential, and resonance Raman spectra of
1-3 at 457.9 nm excitation are available (20 pages). Ordering
information is given on any current masthead page.
IC9715052
(32) Richter, M. M.; Brewer, K. J. Inorg. Chem. 1990, 29, 3926.
(33) (a) Bungon, P.; Hester, R. E. Chem. Phys. Lett. 1983, 102, 537. (b)
Al-Obaidi, A. H. R.; Gordon, K. C.; McGarvey, J. J.; Bell, S. E. J.;
Grimshaw, J. J. Phys. Chem. 1993, 97, 10942. (c) Gordon, K. C.;
McGarvey, J. J. Chem. Phys. Lett. 1989, 162, 117.
(34) (a) Danzer, G. D.; Golus, J. A.; Kincaid, J. R. J. Am. Chem. Soc.
1993, 115, 8643. (b) Schoonover, J. R.; Pingyun, C.; Bates, W. D.;
Dyer, R. B.; Meyer, T. J. Inorg. Chem. 1994, 33, 793.
(35) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals
and Applications; Wiley: New York, 1980.