Synthesis and characterization of several dicopper(I) complexes and a spin-delocalized dicopper(I,II) mixed-valence complex using a 1,8-naphthyridine-based dinucleating ligand

Article abstract of DOI:10.1021/ic0005339

The synthesis of dicopper(I) complexes [Cu₂(BBAN)(MeCN)₂](OTf)₂ (1), [Cu₂(BBAN)(py)₂](OTf)₂ (2), [Cu₂(BBAN)(1-Me-Bzlm)₂](OTf)₂ (3), [Cu₂(BBAN)(1-Me-Im)₂](OTf)₂ (4), and [Cu₂(BBAN)(μ-O₂CCPh₃)](OTf) (5), where BBAN = 2,7-bis((dibenzylamino)methyl)-1,8-naphthyridine, py = pyridine, 1-Me-Im = 1-methylimidazole, and 1-Me-BzIm = 1-methylbenzimidazole, are described. Short copper-copper distances ranging from 2.6151(6) to 2.7325(5) A were observed in the solid-state structures of these complexes depending on the terminal ligands used. The cyclic voltammogram of compound 5 dissolved in THF exhibited a reversible redox wave at E₁/₂ = -25 mV vs Cp₂Fe⁺/Cp₂Fe. When complex 5 was treated with 1 equiv of silver(I) triflate, a mixed-valence dicopper(I,II) complex [Cu₂(BBAN)(μ-O₂CCPh₃)(OTf)](OTf) (6) was prepared. A short copper-copper distance of 2.4493(14) A observed from the solid-state structure indicates the presence of a copper-copper interaction. Variable-temperature EPR studies showed that complex 6 has a fully delocalized electronic structure in frozen 2-methyltetrahydrofuran solution down to liquid helium temperature. The presence of anionic ligands seems to be an important factor to stabilize the mixed-valence dicopper(I,II) state. Compounds 1-4 with neutral nitrogen-donor terminal ligands cannot be oxidized to the mixed-valence analogues either chemically or electrochemically.

Full text of DOI:10.1021/ic0005339

Inorg. Chem. 2000, 39, 5225-5231
5225
Synthesis and Characterization of Several Dicopper(I) Complexes and a Spin-Delocalized
Dicopper(I,II) Mixed-Valence Complex Using a 1,8-Naphthyridine-Based Dinucleating
Ligand
Chuan He and Stephen J. Lippard*
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
ReceiVed May 19, 2000
The synthesis of dicopper(I) complexes [Cu2(BBAN)(MeCN)2](OTf)2 (1), [Cu2(BBAN)(py)2](OTf)2 (2),
[Cu2(BBAN)(1-Me-BzIm)2](OTf)2 (3), [Cu2(BBAN)(1-Me-Im)2](OTf)2 (4), and [Cu2(BBAN)(µ-O2CCPh3)](OTf)
(5), where BBAN ) 2,7-bis((dibenzylamino)methyl)-1,8-naphthyridine, py ) pyridine, 1-Me-Im ) 1-methylimi-
dazole, and 1-Me-BzIm ) 1-methylbenzimidazole, are described. Short copper-copper distances ranging from
2.6151(6) to 2.7325(5) Å were observed in the solid-state structures of these complexes depending on the terminal
ligands used. The cyclic voltammogram of compound 5 dissolved in THF exhibited a reversible redox wave at
E1/2 ) -25 mV vs Cp2Fe+/Cp2Fe. When complex 5 was treated with 1 equiv of silver(I) triflate, a mixed-valence
dicopper(I,II) complex [Cu2(BBAN)(µ-O2CCPh3)(OTf)](OTf) (6) was prepared. A short copper-copper distance
of 2.4493(14) Å observed from the solid-state structure indicates the presence of a copper-copper interaction.
Variable-temperature EPR studies showed that complex 6 has a fully delocalized electronic structure in frozen
2-methyltetrahydrofuran solution down to liquid helium temperature. The presence of anionic ligands seems to
be an important factor to stabilize the mixed-valence dicopper(I,II) state. Compounds 1-4 with neutral nitrogen-
donor terminal ligands cannot be oxidized to the mixed-valence analogues either chemically or electrochemically.
Introduction
spin fully localized on one metal center.9 The small number of
class II, mixed-valence dicopper compounds usually exhibit
Copper is used widely as a metal cofactor to perform different
functions in biology. One example is the spin-delocalized
dinuclear mixed-valence copper(I,II) cluster identified in the CuA
site of cytochrome c oxidase (CcO) and nitrous oxide reductase
(N2OR), which mediates electron transfer. X-ray crystallographic
studies on the CuA sites in CcO and N2OR revealed short
copper-copper distances (∼2.5 Å) indicative of metal-metal
bonding.1-4 The presence of such metal-metal interactions was
previously unprecedented in biological systems. The unpaired
spin in the CuA site is fully delocalized over two copper ions,5,6
as revealed by an EPR signal with seven-line hyperfine coupling
patterns arising from spin interactions with both I ) 3/2 copper
ions.7,8
temperature-dependent spin exchange phenomena. The unpaired
spin is trapped at low temperature, as indicated by EPR
spectroscopy for these systems.10,11 Type III mixed-valence
dicopper(I,II) complexes have unpaired spin delocalized over
two copper centers even at liquid-helium temperature.
To date three types of class III dicopper complexes have been
prepared that reproduce features of the fully delocalized mixed-
valence CuA site (Table 1). A tridentate N2S ligand was used
to assemble a bis(µ-thiolato)dicopper(I,II) compound, where
the unpaired electron was fully delocalized over two copper
atoms.12,13 The Cu-Cu separation of 2.9306(9) Å in the struc-
ture of this compound is long compared to the short Cu-Cu
distance of 2.5 Å in the CuA center. An irreversible one-electron
reduction or oxidation wave was recorded at E1/2 ) +350 mV
vs SCE for this complex in methanol. Several class III
delocalized mixed-valence dicopper compounds with short
Cu-Cu distances of 2.415(1), 2.419(1), and 2.448(1) Å were
synthesized using octaazacryptand macrobicyclic ligands.14-16
A reversible redox potential of +310 mV vs Ag/AgCl in DMF
Fully delocalized, type III mixed-valence copper complexes
such as the CuA site are rare in inorganic chemistry. Most mixed-
valence dicopper complexes belong to class I with the unpaired
(1) Iwata, S.; Ostermeier, C.; Ludwig, B.; Michel, H. Nature 1995, 376,
660-669.
(2) Tsukihara, T.; Aoyama, H.; Yamashita, E.; Tomizaki, T.; Yamaguchi,
H.; Shinzawa-Itoh, K.; Nakashima, R.; Yaono, R.; Yoshikawa, S.
Science 1995, 269, 1069-1074.
(3) Tsukihara, T.; Aoyama, H.; Yamashita, E.; Tomizaki, T.; Yamaguchi,
H.; Shinzawa-Itoh, K.; Nakashima, R.; Yaono, R.; Yoshikawa, S.
Science 1996, 272, 1136-1144.
(9) Dunaj-Jurco, M.; Ondrejovic, G.; Melnik, M.; Garaj, J. Coord. Chem.
ReV. 1988, 83, 1-28.
(10) Long, R. C.; Hendrickson, D. N. J. Am. Chem. Soc. 1983, 105, 1513-
(4) Brown, K.; Tegoni, M.; Prudeˆncio, M.; Pereira, A. S.; Besson, S.;
Moura, J. J.; Moura, I.; Cambillau, C. Nat. Struct. Biol. 2000, 7, 191-
195.
1521.
(11) Gagne´, R. R.; Koval, C. A.; Smith, T. J.; Cimolino, M. C. J. Am.
Chem. Soc. 1979, 101, 4571-4580.
(5) Farrar, J. A.; Neese, F.; Lappalainen, P.; Kroneck, P. M. H.; Saraste,
M.; Zumft, W. G.; Thomson, A. J. J. Am. Chem. Soc. 1996, 118,
11501-11514.
(12) Houser, R. P.; Young, V. G., Jr.; Tolman, W. B. J Am. Chem. Soc.
1996, 118, 2101-2102.
(13) Abbreviations: LiPrdacoS ) 1-isopropyl-5-(ethylthiolato)-1,5-diazocy-
clooctane; L2 ) N(CH2CH2NdCHCHdNCH2CH2)3N.
(14) Harding, C.; Nelson, J.; Symons, M. C. R.; Wyatt, J. J. Chem. Soc.,
Chem. Commun. 1994, 2499-2500.
(6) Gamelin, D. R.; Randall, D. W.; Hay, M. T.; Houser, R. P.; Mulder,
T. C.; Canters, G. W.; de Vries, S.; Tolman, W. B.; Lu, Y.; Solomon,
E. I. J. Am. Chem. Soc. 1998, 120, 5246-5263.
(7) Neese, F.; Zumft, W. G.; Antholine, W. E.; Kroneck, P. M. H. J. Am.
Chem. Soc. 1996, 118, 8692-8699.
(15) Harding, C.; McKee, V.; Nelson, J. J. Am. Chem. Soc. 1991, 113,
9684-9685.
(16) Barr, M. E.; Smith, P. H.; Antholine, W. E.; Spencer, B. J. Chem.
Soc., Chem. Commun. 1993, 1649-1652.
(8) Lappalainen, P.; Aasa, R.; Malmstro¨m, B. G.; Saraste, M. J. Biol.
Chem. 1993, 268, 26416-26421.
10.1021/ic0005339 CCC: $19.00 © 2000 American Chemical Society
Published on Web 10/25/2000

5226 Inorganic Chemistry, Vol. 39, No. 23, 2000
He and Lippard
Table 1. Structural, Spectroscopic, and Electrochemical Data for Selected Examples of Class III Mixed-Valence Dicopper Complexes
a Only ranges were reported for the absorption maxima and molar absorptivities. b No esd reported. c All parameters were derived from simulated
spectra. d Potential vs NHE. e Potential vs Cp2Fe+/Cp2Fe in THF. f Potential vs Ag/AgCl in DMF. See ref 18 for scaling literature potentials to
potentials vs Cp2Fe+/Cp2Fe in THF.
(-206 mV vs Cp2Fe+/Cp2Fe in THF) was reported for one of
these compounds.15 The third example of class III delocalized
mixed-valence Cu(1.5)-Cu(1.5) compounds comes from our
laboratory. These species were assembled by using the conver-
gent bis(carboxylate) ligand, m-xylylenediamine bis(Kemp’s
triacid imide) (H2XDK).17,18 The metal-metal distances in these
dicopper(I,II) XDK complexes are ∼2.4 Å, and a reversible
redox couple with E1/2 of -218 mV vs Cp2Fe+/Cp2Fe in THF
was observed for one of them.18
The reversible redox potentials recorded for two families of
class III mixed-valence dicopper complexes are very close to
Figure 1. Schematic representation of BBAN.
each other when converted to same scale (Table 1), even though
very different ligand sets were used. The negatively charged
carboxylate groups in the all-oxygen bis(carboxylate) complex
do not lower the redox potential of the system compared to the
neutral all-nitrogen ligands available in the octaazacryptand
compound. More experimental details are required, however,
before any generalizations can be made about this and other
properties of the fully delocalized dicopper(I,II) mixed-valence
system. In the present study we have prepared a class III mixed-
valence dicopper complex containing both nitrogen and oxygen
donor ligands. The physical properties of this complex were
investigated, with special attention paid to the redox potentials,
and compared with previous examples to help elucidate further
the nature of their mixed-valence-character.
2,7-Bis((dibenzylamino)methyl)-1,8-naphthyridine (BBAN)
(Figure 1) was employed as the tetradentate dinucleating ligand
in this work. The 1,8-naphthyridine ligand scaffold affords
short metal-metal distances, as reported previously.19-21
A
series of dicopper(I) compounds [Cu2(BBAN)(MeCN)2](OTf)2
(1), [Cu2(BBAN)(py)2](OTf)2 (2), [Cu2(BBAN)(1-Me-BzIm)2]-
(OTf)2 (3), [Cu2(BBAN)(1-Me-Im)2](OTf)2 (4), and [Cu2(BBAN)-
(19) Tikkanen, W. R.; Kru¨ger, C.; Bomben, K. D.; Jolly, W. L.; Kaska,
W. C.; Ford, P. C. Inorg. Chem. 1984, 23, 3633-3638.
(20) Boelrijk, A. E. M.; Neenan, T. X.; Reedijk, J. J. Chem. Soc., Dalton
Trans. 1997, 4561-4570.
(17) LeCloux, D. D.; Davydov, R.; Lippard, S. J. J. Am. Chem. Soc. 1998,
120, 6810-6811.
(18) LeCloux, D. D.; Davydov, R.; Lippard, S. J. Inorg. Chem. 1998, 37,
6814-6826.
(21) He, C.; Lippard, S. J. J. Am. Chem. Soc. 2000, 122, 184-185.

A Family of Dicopper(I) Complexes
Inorganic Chemistry, Vol. 39, No. 23, 2000 5227
7.14-7.40 (m, 22 H), 7.46 (d, J ) 7 Hz, 2 H), 7.63 (d, J ) 7 Hz, 2
H), 7.66-7.80 (m, 2 H), 8.11 (d, J ) 7 Hz, 2 H), 8.80 (d, J ) 4 Hz,
4 H). FTIR (KBr, cm-1): 3072 (m), 3029 (w), 2929 (w), 2862 (w),
2839 (w), 1666 (s), 1519 (m), 1498 (m), 1488 (m), 1449 (s), 1436
(m), 1353 (w), 1282 (s), 1222 (m), 1153 (s), 1069 (w), 1029 (s), 971
(m), 879 (m), 856 (m), 788 (m), 755 (s), 700 (s), 630(s). Anal. Calcd
for 2, C50H46N6O6S2F6Cu2: C, 53.04; H, 4.10; N, 7.42. Found: C, 52.93;
H, 3.91; N, 7.24.
(µ-O2CCPh3)](OTf) (5), where py ) pyridine, 1-Me-Im )
1-methylimidazole, and 1-Me-BzIm ) 1-methylbenzimidazole,
were prepared and their chemical and electrochemical oxida-
tions investigated. A mixed-valence complex [Cu2(BBAN)-
(µ-O2CCPh3)(OTf)](OTf) (6) was obtained when complex 5 was
treated with 1 equiv of silver(I) triflate. The physical properties
of complex 6 were studied and compared with those of previous
compounds.
[Cu2(BBAN)(1-Me-BzIm)2](OTf)2 (3). A portion of BBAN (30 mg,
0.055 mmol) in THF (0.5 mL) was added to a solution of [Cu(CH3-
CN)4](OTf) (41.2 mg, 0.11 mmol) in THF (1 mL). The color of the
solution turned to pink. A portion of 1-methylbenzimidazole (14.4 mg,
0.11 mmol) was added, and the solution turned yellow. After 30 min
of stirring, the solvent was removed under vacuum. The residue was
dissolved in CH2Cl2. Purple blocks suitable for X-ray crystallography
were obtained by vapor diffusion of Et2O into the CH2Cl2 solution.
The product became a blue powder after collection and drying under
Experimental Section
General Procedures and Methods. All reagents were obtained from
commercial suppliers and used without further purification unless
otherwise noted. THF was distilled from sodium benzophenone ketyl
under nitrogen. Pentane and Et2O were purified by passing through
activated Al2O3 columns under nitrogen. Dichloromethane was distilled
from CaH2 under nitrogen. Fourier transform infrared spectra were
recorded on a Bio-Rad FTS135 instrument. UV-vis spectra were
recorded on a Varian 1-E spectrophotometer. The ligand BBAN was
synthesized according to procedures reported elsewhere.22 [Cu(CH3-
CN)4](OTf) was prepared by following a known procedure.23 All air-
sensitive manipulations were carried out either in a nitrogen-filled
Vacuum Atmospheres drybox or by standard Schlenk line techniques.
Electrochemistry. Cyclic voltammetric measurements were per-
formed in a Vacuum Atmospheres drybox under N2 with an EG&G
model 263 potentiostat. A three-electrode configuration consisting of
a 1.75 mm2 platinum working electrode, a Ag/AgNO3 (0.1 M in CH3-
CN) reference electrode, and a coiled platinum wire auxiliary electrode
was used. The supporting electrolyte was 0.5 M (Bu4N)(PF6). Sample
concentrations of 2 mM were used for typical experiments. Scan rate
profiles were conducted for each sample, at scan speeds of 25-150
mV/s, and compared to that obtained for Cp2Fe under identical
conditions. All cyclic voltammograms were externally referenced to
Cp2Fe.
1
vacuum (32 mg, 47%). H NMR (CDCl3, 300 MHz): δ 3.54-4.00
(m, broad, 14 H), 4.45 (s, broad, 4 H), 7.08 ∼ 7.46 (m, 24 H), 7.63 (d,
J ) 8 Hz, 2 H), 7.89 (d, J ) 7.8 Hz, 2 H), 8.11 (d, J ) 7 Hz, 2 H),
8.92 (s, 2 H). FTIR (KBr, cm-1): 3110 (m), 3087 (w), 3062 (m), 3030
(m), 2927 (w), 2891 (w), 2853 (m), 1607 (s), 1523 (s), 1477 (w), 1459
(s), 1369 (m), 1277 (s), 1224 (m), 1153 (s), 1070 (w), 1030 (s), 974
(w), 877 (w), 856 (w), 747 (s), 703 (s), 638 (s). Anal. Calcd for 3,
C56H52N8O6S2F6Cu2: C, 54.32; H, 4.23; N, 9.05. Found: C, 54.02; H,
4.33; N, 8.96.
[Cu2(BBAN)(1-Me-Im)2](OTf)2 (4). A portion of BBAN (20 mg,
0.036 mmol) in THF (0.5 mL) was added to a solution of [Cu(CH3-
CN)4](OTf) (27.5 mg, 0.073 mmol) in THF (1 mL). The color of the
solution turned to pink. A portion of 1-methylimidazole (5.8 µL, 0.073
mmol) was added, and the solution turned yellow. After 30 min of
stirring, the solvent was removed under vacuum. The residue was
dissolved in CH2Cl2. Layering Et2O on top of the CH2Cl2 solution for
1 week yielded green blocks suitable for X-ray crystallography (19
1
mg, 46%). H NMR (CDCl3, 300 MHz): δ 3.78 (s, 8 H), 3.84 (s, 6
EPR Spectroscopy. The EPR spectrum was collected as frozen
2-methyltetrahydrofuran glass on a Bruker model 300 ESP X-band
spectrometer operating at 9.37 GHz and running WinEPR software.
Low temperature was maintained at 77 K with a specially designed
coldfinger filled with liquid N2 and at liquid-helium temperature with
an Oxford Instruments EPR 900 crystat. Simulation was carried out
with the WINEPR SimFonia program package.24 Simultaneous coupling
of the unpaired spin to two I ) 3/2 copper centers was incorporated
into the fit.
H), 4.24 (s, 4 H), 6.90-6.98 (broad, 2 H), 7.08-7.48 (m, broad, 20
H), 7.56-7.72 (m, broad, 4 H), 8.23 (broad, 2 H), 8.37 (d, J ) 7 Hz,
2 H). FTIR (KBr, cm-1): 3136 (m), 3126 (w), 3132 (w), 3062 (w),
2962 (w), 2843 (w), 1608 (s), 1541 (m), 1522 (m), 1497 (w), 1458
(m), 1427 (m), 1368 (w), 1264 (m), 1224 (m), 1160 (s), 1114 (m),
1099 (w), 1029 (s), 970 (w), 857 (w), 819 (w), 791 (w), 749 (s), 703
(m), 657 (m), 638 (s). Anal. Calcd for 4, C52H40N8O6S2F6Cu2: C, 50.65;
H, 4.25; N, 9.85. Found: C, 50.27; H, 4.10; N, 9.77.
[Cu2(BBAN)(µ-O2CCPh3)](OTf) (5). A portion of BBAN (20 mg,
0.036 mmol) in THF (0.5 mL) was added to a solution of [Cu(CH3-
CN)4](OTf) (27.5 mg, 0.073 mmol) in THF (1 mL). The solution turned
pink, and a solution of sodium triphenylacetate (11.3 mg, 0.036 mmol)
in THF (0.5 mL) was added. The color of the solution turned to green.
After 30 min of stirring, the solvent was removed under vacuum. Green
plates suitable for X-ray crystallography were obtained from pentane/
[Cu2(BBAN)(CH3CN)2](OTf)2 (1). A portion of BBAN (50 mg,
0.091 mmol) in THF (0.5 mL) was added to a solution of [Cu(CH3-
CN)4](OTf) (68.7 mg, 0.182 mmol) in THF (1 mL). The solution turned
pink, and the product began to precipitate. After 1 h of stirring, the
pink product was isolated and dried under vacuum (80.5 mg, 84%).
Single crystals suitable for X-ray crystallographic study were obtained
1
by diffusion of Et2O into a saturated CH2Cl2 solution at -30 °C. H
1
Et2O/benzene (27 mg, 67%). H NMR (CDCl3, 300 MHz): δ 3.70-
NMR (CDCl3, 300 MHz): δ 2.17 (s, 6 H), 4.13 (s, 8 H), 4.22 (s, 4 H),
7.10-7.26 (m, 12 H), 7.32-7.50 (m, 8 H), 7.62 (broad, 2 H), 8.26 (d,
J ) 8 Hz, 2 H). FTIR (KBr, cm-1): 3085 (w), 3064 (w), 3032 (w),
2932 (w), 2859 (w), 2290 (w), 1610 (m), 1560 (w), 1264 (s), 1226
(m), 1152 (s), 1032 (s), 877 (w), 852 (w), 791 (w), 754 (m), 706 (m),
638 (s). Anal. Calcd for 1, C44H42N6O6S2F6Cu2: C, 50.04; H, 4.01; N,
7.96. Found: C, 49.86; H, 4.20; N, 7.81.
4.30 (m, broad, 12 H), 7.00-7.46 (m, 29 H), 7.58-7.70 (m, 6 H),
8.40 (broad, 2 H), 8.57 (broad, 2 H), 7.62 (broad, 2 H), 8.26 (d, J )
8 Hz, 2 H). FTIR (KBr, cm-1): 3080 (m), 3032 (m), 2920 (m), 2848
(w), 1617 (s), 1606 (s), 1551 (s), 1494 (m), 1479 (s), 1448 (m), 1380
(s), 1274 (s), 1268 (s), 1224 (w), 1161 (s), 1149 (s), 1031 (s), 998 (s),
873 (s), 755 (s), 743 (s), 701 (s), 638 (s). UV-vis (THF, λmax, nm (ꢀ,
M-1 cm-1)): 640 (96). Anal. Calcd for 5, C59H51N4O5SF3Cu2: C, 63.71;
H, 4.62; N, 5.04. Found: C, 63.59; H, 4.45; N, 4.96.
[Cu2(BBAN)(py)2](OTf)2 (2). A portion of BBAN (50 mg, 0.091
mmol) in THF (0.5 mL) was added to a solution of [Cu(CH3CN)4]-
(OTf) (68.7 mg, 0.182 mmol) in THF (1 mL). The color of the solution
turned to pink. A portion of pyridine (15 µL, 0.182 mmol) was added,
and the solution turned yellow. After 30 min of stirring, the solvent
was removed under vacuum. The residue was dissolved in CH2Cl2.
Crystallization by vapor diffusion of Et2O into this CH2Cl2 solution
yielded green-brown blocks suitable for X-ray crystallography (78 mg,
[Cu2(BBAN)(µ-O2CCPh3)(OTf)](OTf) (6). A portion of BBAN (20
mg, 0.036 mmol) in THF (0.5 mL) was added to a solution of [Cu-
(CH3CN)4](OTf) (27.5 mg, 0.073 mmol) in THF (1 mL). The solution
turned pink, and a solution of sodium triphenylacetate (11.3 mg, 0.036
mmol) in THF (0.5 mL) was added. The color of the solution turned
to yellow. After 30 min of stirring, a solution of AgOTf (9.4 mg, 0.036
mmol) in THF (0.5 mL) was added. The mixture was allowed to stir
for 5 h. Solvent was removed under vacuum. The residue was extracted
with CH2Cl2 and filtered. Deep purple plates suitable for X-ray
crystallography were obtained from pentane/Et2O/CH2Cl2 (11 mg, 24%).
FTIR (KBr, cm-1): 3085 (w), 3065 (m), 3031 (w), 2940 (w), 2920
(w), 2871 (w), 1616 (s), 1604 (m), 1541 (m), 1496 (m), 1458 (m),
1
76%). H NMR (CDCl3, 300 MHz): δ 3.76 (s, 8 H), 4.43 (s, 4 H),
(22) He, C.; Lippard, S. J. Tetrahedron 2000, 56, 8245-8252.
(23) Kubas, G. J. Inorg. Synth. 1979, 19, 90-92.
(24) WINEPR-SimFonia, 1.25; Bruker Analytik GmbH: Karlsruhe, FRG,
1994-1996.

5228 Inorganic Chemistry, Vol. 39, No. 23, 2000
Table 2. Summary of X-ray Crystallographic Data
He and Lippard
1‚2CH2Cl2
2
3‚CH2Cl2
4‚Et2O
5‚1.5C6H6
6‚CH2Cl2
formula
C46H46N6O6
Cl4F6S2Cu2
1225.89
C50H46N6O6
F6S2Cu2
1132.13
C2/c
15.490(5)
15.846(5)
21.070(5)
C57H54N8O6
Cl2F6S2Cu2
1323.18
P1h
C52H58N8O7
F6S2Cu2
1212.26
P1h
C68H60N4O5
F3Scu2
C60.50H52N4O8
ClF6S2Cu2
1303.71
fw
1229.34
space group
a, Å
C2/c
C2/c
P21/c
16.9286(3)
15.9582(1)
21.2111(3)
12.8261(5)
15.0676(6)
16.2679(6)
85.8140(10)
83.8290(10)
69.9740(10)
2934.5(2)
2
11.8651(2)
12.5037(2)
20.9961(4)
72.8750(10)
88.5070(10)
68.7790(10)
2763.62(8)
2
41.2217(10)
13.9946(4)
20.7004(6)
22.5642(2)
22.3852(4)
23.5102(4)
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
111.5570(10)
101.369(5)
102.3350(10)
95.5300(10)
5329.36(13)
4
5070(3)
4
11 666.0(6)
8
11819.8(3)
8
85
1.465
0.912
0.0743
0.1751
T, °C
-85
-85
-85
-85
-85
F
calcd, g cm-3
1.528
1.150
0.0447
0.1261
1.483
0.998
0.0326
0.0971
1.497
1.457
1.400
0.830
0.0571
0.1558
µ(Mo KR), mm-1
0.963
0.923
Rb
0.0533
0.0443
wR2c
0.1615
0.1259
a Observation criterion: I > 2σ(I). b R ) Σ||Fo| - |Fc||/Σ|Fo|. c wR2 ) {Σ[w(Fo - Fc2)2]/Σ[w(Fo )2]}1/2
.
2
2
1450 (m), 1397 (s), 1274 (s), 1235 (s), 1219 (s), 1160 (s), 1150 (s),
1296 (s), 1025 (s), 917 (w), 881 (w), 850 (w), 815 (w), 789 (w), 749
(s), 700 (s), 637 (s). UV-vis (THF, λmax, nm (ꢀ, M-1 cm-1)): 875
(3775), 660 (1450), 550 (1425), 500 (1583). Anal. Calcd for 6,
C60H51N4O8S2F6Cu2: C, 57.14; H, 4.08; N, 4.44. Found: C, 56.86; H,
4.24; N, 4.28.
different terminally bound ligands were prepared as illustrated
in Scheme 1. Treatment of the ligand BBAN with two equiv of
[Cu(CH3CN)4](OTf) in THF gave 1 as a pink solid in high yield
(84%). A clear yellow solution was obtained when 2 equiv of
pyridine was added to the slurry of 1 in THF. Subsequent
crystallization from CH2Cl2/EtO2 afforded complex 2. Com-
plexes 3-5 were prepared by following same procedure except
that 5 was crystallized from pentane/Et2O/benzene. As shown
in Figure 2, each copper in [Cu2(BBAN)(MeCN)2](OTf)2 (1)
is ligated to one nitrogen atom of 1,8-naphthyridine, one tertiary
amine nitrogen atom and one terminally bound acetonitrile
molecule. Each copper has square-planar coordination geometry.
A short copper-copper distance of 2.6879(7) Å was observed
from the crystal structure. Dicopper(I) complexes 2-5 have
structural features very similar to those of 1 (Figures 2 and 3),
and selected bond distances and angles for all five complexes
are listed in Table 3. The copper-copper distances among
complexes 2-4 vary with the different donating strengths of
the terminal ligands. The most donating ligand, 1-methylimi-
dazole in 4, affords the longest copper-copper distance of
2.7328(5) Å, whereas the least donating terminal ligand, pyridine
in 2, affords the shortest copper-copper distance of 2.6574(6)
Å (Table 3). The terminal ligand-copper bond distances vary
with the reverse trend. The SOMO of a dicopper complex with
same geometry has considerable Cu dx -y /Cu dx -y σ-antibond-
ing character, as revealed from our previous extended Hu¨ckel
molecular orbital calculations.18 The extra electron density
donated into this antibonding orbital by terminally bound ligands
apparently weakens the copper-copper interaction. With a
bridging carboxylate group, the copper-copper distance of
2.6151(6) Å in 5 is, not surprisingly, the shortest among all
compounds. Interactions involving π-π stacking between
terminal aromatic ligands (∼3.4 Å) may also contribute to the
shortening of copper-copper distances in 2-4 compared to 1.
In the latter, which has a longer copper-copper distance than
those in 2 and 3, such an interaction cannot occur.
Collection and Reduction of X-ray Data. X-ray crystallographic
studies (Table 2) were carried out on a Bruker CCD diffractometer
with graphite-monochromatized Mo KR radiation (λ ) 0.710 73 Å)
controlled by a Pentium-based PC running the SMART software
package.25 Single crystals were mounted at room temperature on the
ends of quartz fibers in Paratone N oil and data were collected at 193
K in a stream of cold N2 maintained by a Bruker LT-2A nitrogen
cryostat. Data collection and reduction protocols are described in detail
elsewhere.26 The structures were solved by direct methods and refined
on F2 by using the SHELXTL software package.27 Empirical absorption
corrections were applied by using the SADABS program.28 Structures
were checked for higher symmetry with the PLATON program.29 All
non-hydrogen atoms were refined anisotropically. Hydrogen atoms were
assigned idealized locations and given an isotropic thermal parameter
1.2 times the thermal parameter of the carbon atom to which they were
attached.
In the structure of 3, one chlorine atom of a solvent dichloromethane
was disordered over three positions, Cl(1), Cl(1A), and Cl(1B), refined
at occupancies of 0.4, 0.3, and 0.3, respectively. The other chlorine
atom was also disordered over three positions, Cl(2), Cl(2A), and
Cl(2B), refined at occupancies of 0.5, 0.4, and 0.1, respectively. In the
structure of 5, the carbon and fluorine atoms of the triflate molecule
were disordered over two sets of positions, C(59), F(1), F(2), and F(3)
and C(59B), F(1B), F(2B), and F(3B), refined at occupancies of 0.6
and 0.4, respectively. Two asymmetric units share a solvent benzene
molecule. This benzene molecule was also disordered over two sets of
positions, C(7sA), C(8sA), and C(9sA) and C(7sB), C(8sB), and C(9sB),
respectively, each refined at half-occupancy.
2
2
2
2
Results and Discussion
Preparation and Structural Characterization of
Dicopper(I) Complexes: [Cu2(BBAN)L2](OTf)2, L ) MeCN
(1), py (2), 1-Me-BzIm (3), and 1-Me-Im (4); [Cu2(BBAN)-
(µ-O2CCPh3)](OTf) (5). Dicopper(I) complexes of BBAN with
Preparation and Structural Characterization of the Mixed-
Valence Complex [Cu2(BBAN)(µ-O2CCPh3)(OTf)](OTf) (6).
Complex 6 was prepared by one electron oxidation of 5 with 1
equiv of silver(I) triflate in THF (Scheme 1). After the resulting
precipitate was filtered off, deep purple crystals of 6 were
obtained from pentane/Et2O/CH2Cl2. The crystal structure of 6
is shown in Figure 3, and selected bond distances and angles
are listed in Table 3. One copper atom in 6 maintains square-
planar coordination geometry. The other copper is five-coor-
dinate with pyramidal geometry and a terminally bound triflate.
(25) SMART V5.05; Bruker AXS: Madison, WI, 1998.
(26) Feig, A. L.; Bautista, M. T.; Lippard, S. J. Inorg. Chem. 1996, 25,
6892-6898.
(27) Sheldrick, G. M. SHELXTL97-2: Program for the Refinement of
Crystal Structures; University of Go¨ttingen: Go¨ttingen, Germany,
1997.
(28) Sheldrick, G. M. SADABS: Area-Detector Absorption Correction;
University of Go¨ttingen: Go¨ttingen, Germany, 1996.
(29) Spek, A. L. PLATON, A Multipurpose Crystallographic Tool; Utrecht
University: Utrecht, The Netherlands, 1998.

A Family of Dicopper(I) Complexes
Inorganic Chemistry, Vol. 39, No. 23, 2000 5229
Figure 2. ORTEP diagrams of [Cu2(BBAN)(MeCN)2](OTf)2 (1), [Cu2(BBAN)(py)2](OTf)2 (2), [Cu2(BBAN)(1-Me-BzIm)2](OTf)2 (3), and [Cu2-
(BBAN)(1-Me-Im)2](OTf)2 (4), showing the 40% probability thermal ellipsoids for all non-hydrogen atoms (left to right and top to bottom).
Scheme 1
The copper-copper distance of 2.4500(15) Å is significantly
shorter than that in the dicopper(I) compound 5. The observed
asymmetric dicopper coordination is probably a consequence
of the sterically bulky BBAN ligand, which prevents simulta-
neous binding of two terminal triflate ligands.
Physical Properties of the Mixed-Valence Complex
[Cu2(BBAN)(µ-O2CCPh3)(OTf)](OTf) (6). The UV-vis spec-
trum of 6 exhibits an intense peak at 875 nm (ꢀ ) 3775 M-1
cm-1), a peak at 500 nm (ꢀ ) 1583 M-1 cm-1), and two
shoulders at 660 nm (ꢀ ) 1450 M-1 cm-1) and 550 nm (ꢀ )
1425 M-1 cm-1), as shown in Figure 4. The absorption features
are close to those observed in class III mixed-valence macro-
cyclic octaazacryptand complexes. The sharp intense peak at
875 nm is red-shifted by about 140 nm, and the peak at 500
nm is blue-shifted by about 120 nm compared to those of the
all-nitrogen octaazacryptand-based compounds.14-16 The spec-

5230 Inorganic Chemistry, Vol. 39, No. 23, 2000
He and Lippard
Table 3. Selected Bond Lengths (Å) and Angles (deg)a
bond lengths
bond angles
1
2
3
Cu(1)‚‚‚Cu(1A) 2.6879(7) N(2)-Cu(1)-Cu(1A)
150.93(6)
157.25(10)
111.70(9)
84.04(6)
Cu(1)-N(1)
Cu(1)-N(2)
Cu(1)-N(3)
1.955(2)
2.187(2)
1.867(2)
N(1)-Cu(1)-N(3)
N(2)-Cu(1)-N(3)
N(1)-Cu(1)-Cu(1A)
Cu(1)‚‚‚Cu(1A) 2.6574(7) N(1)-Cu(1)-Cu(1A)
164.04(4)
175.85(7)
99.09(6)
84.06(5)
Cu(1)-N(1)
Cu(1)-N(2)
Cu(1)-N(3)
1.9542(17) N(1)-Cu(1)-N(3)
2.3583(16) N(2)-Cu(1)-N(3)
1.9308(17) N(1)-Cu(1)-Cu(1A)
Cu(1)‚‚‚Cu(2)
Cu(1)-N(1)
Cu(1)-N(3)
Cu(1)-N(5)
Cu(2)-N(2)
Cu(2)-N(4)
Cu(2)-N(7)
2.6716(7) N(3)-Cu(1)-Cu(2)
161.99(8)
163.45(8)
178.72(17)
176.08(17)
1.944(3)
2.349(3)
1.928(4)
1.937(3)
2.403(3)
1.923(4)
N(4)-Cu(2)-Cu(1)
N(1)-Cu(1)-N(5)
N(2)-Cu(2)-N(7)
4
5
6
Cu(1)‚‚‚Cu(2)
Cu(1)-N(1)
Cu(1)-N(3)
Cu(1)-N(5)
Cu(2)-N(2)
Cu(2)-N(4)
Cu(2)-N(7)
2.7328(5) N(3)-Cu(1)-Cu(2)
1.9483(19) N(4)-Cu(2)-Cu(1)
2.379(2)
1.909(2)
1.946(2)
2.352(2)
1.909(2)
163.61(5)
162.19(6)
174.24(9)
169.98(10)
N(1)-Cu(1)-N(5)
N(2)-Cu(2)-N(7)
Cu(1)‚‚‚Cu(2)
Cu(1)-N(1)
Cu(1)-N(3)
Cu(1)-O(1)
Cu(2)-N(2)
Cu(2)-N(4)
Cu(2)-O(2)
2.6151(6) N(3)-Cu(1)-Cu(2)
167.55(7)
158.12(10)
169.42(11)
168.48(11)
1.929(3)
2.303(3)
1.891(2)
1.976(3)
2.221(3)
1.948(2)
N(4)-Cu(2)-Cu(1)
N(1)-Cu(1)-O(1)
N(2)-Cu(2)-O(2)
Cu(1B)‚‚‚Cu(2B) 2.4493(14) N(3B)-Cu(1B)-Cu(2B) 162.8(2)
Cu(1B)-N(1B)
Cu(1B)-N(3B)
Cu(1B)-O(1B)
Cu(1B)-O(3B)
Cu(2B)-N(2B)
Cu(2B)-N(4B)
Cu(2B)-O(2B)
1.911(7)
2.090(6)
1.872(5)
2.257(6)
1.907(7)
2.113(6)
1.883(5)
N(4B)-Cu(2B)-Cu(1B) 168.98(18)
N(1B)-Cu(1B)-O(1B) 171.8(3)
N(2B)-Cu(2B)-O(2B) 170.9(3)
N(1B)-Cu(1B)-O(3B)
O(1B)-Cu(1B)-O(3B)
96.1(2)
91.3(2)
Cu(2B)-Cu(1B)-O(3B) 95.23(17)
a Numbers in parentheses are estimated standard deviations
of the last significant figure. Atoms are labeled as indicated in Figures
2 and 3.
Figure 3. ORTEP diagrams of [Cu2(BBAN)(µ-O2CCPh3)](OTf)2 (5),
[Cu2(BBAN)(µ-O2CCPh3)(OTf)](OTf) (6), and core fragment of 6,
showing the 40% probability thermal ellipsoids for all non-hydrogen
atoms (top to bottom).
trum of 6 compares well with those of the XDK-based class III
mixed-valence complexes, except that the peak around 875 nm
is much more intense.18 The appearance of several shoulders
in the spectrum of 6 is not fully understood.
The X-band EPR spectrum of 6 was measured in a frozen
solution (2-methyltetrahydrofuran glass) at 77 and 4 K (Figure
5). A seven-line hyperfine coupling pattern on the lowest field
component can be clearly distinguished. The two higher field
components have overlapping features. The seven-line pattern
arises from the coupling of the unpaired electron to a pair of I
) 3/2 copper nuclei. The pattern of the spectrum is very similar
to that measured at 4 K (Figure 5). The temperature-independent
behavior confirms the fully delocalized class III nature of
compound 6. The spectrum taken at 77 K was simulated to
afford g values and copper hyperfine coupling constants of g1
) 2.002, g2 ) 2.100, g3 ) 2.222, A1 ) 30 G, A2 ) 60 G,
and A3Cu ) 143 G. These values resemble those obtained from
bis(carboxylate) model complexes (Table 1). These two types
of complexes share similar structural features. Complex 6 can
be viewed as a member of the bis(carboxylate) mixed-valence
family except that one such bridging group is replaced by a
masked carboxylate, the neutral bridging 1,8-naphthyridine unit
of BBAN.
Figure 4. UV-vis spectrum of [Cu2(BBAN)(µ-O2CCPh3)(OTf)](OTf)
(6), 0.4 mM in THF.
Electrochemistry. The cyclic voltammograms of compounds
1-5 were recorded in tetrahydrofuran. No reversible redox
waves were observed for 1-4. Attempts to oxidize chemically
complexes 1-4 with 1 equiv of silver(I) triflate in THF and
CH2Cl2 did not yield purple solutions characteristic of a mixed-
valence dicopper(I,II) species. A mixed-valence dicopper(I,II)
complex apparently cannot be stabilized with the ligand donor
sets in these compounds. A chemically reversible (Ipa/Ipc ∼ 1)
and electrochemically quasireversible (∆Ep ) 150 mV, scan
rate ) 75 mV/s) redox wave was observed for 5, however, with
E1/2 ) -25 mV vs Cp2Fe+/Cp2Fe (Figure 6). This E1/2 value
falls in the positive end of the range characteristic of blue copper
Cu
Cu

A Family of Dicopper(I) Complexes
Inorganic Chemistry, Vol. 39, No. 23, 2000 5231
attributed to stabilization of the two-coordinate, linear
dicopper(I) geometry by the convergent bis(carboxylate) XDK
ligand. Although not reversible, a potential at E1/2 ) +350 mV
vs SCE in MeOH was reported for the bis(µ-thiolato)dicopper-
(I,II) compound. This potential can be scaled to approximately
-210 mV vs Cp2Fe+/Cp2Fe in THF. The redox potentials
(-206, -218, and -210 mV), observed in these earlier model
compounds with completely different ligand environments, are
thus very close to each other.
Complex 5 on the other hand exhibits a very different redox
potential compared to the model complexes prepared so far.
The E1/2 value of 5, which contains only a single anionic
carboxylate bridge, is almost 200 mV higher than that of the
XDK complexes. The change of one carboxylate in mixed-
valence XDK complexes to the 1,8-naphthyridine nitrogen
analogue shifts the redox potential by +200 mV. The presence
of one additional anionic carboxylate in the XDK complexes
thus renders the dicopper(I) core easier to oxidize. The CuA
site has a redox potential of approximately +240 mV vs NHE,
corresponding to -560 mV vs Cp2Fe+/Cp2Fe in THF.30-32 With
a pair of very donating anionic thiolate bridges and two His
ligands, CuA site has a low redox potential compared to all
model compounds, since addition of an anionic carboxylate
ligand can cause a -200 mV change of the redox potential in
these similar types of systems.
The neutral all-nitrogen ligands in 1-4 cannot stabilize
the mixed-valence CuICuII state. In fact, the mixed-valence
dicopper(I,II) octaazacryptand complexes are not stable in
organic solvents and decay upon standing.15 It appears that
anionic ligands are preferred to stabilize the mixed-valence
dicopper(I,II) core. Two thiolate groups are used in nature, and
carboxylate groups are utilized in 6 and in the XDK dicopper
complexes.
Figure 5. X-band EPR spectra for [Cu2(BBAN)(µ-O2CCPh3)(OTf)]-
(OTf) (6): (A) frozen solution (2 mM in 2-Me-THF) at 77 K,
experimental and simulated data; (B) frozen solution (2 mM in 2-Me-
THF) data at 4 K.
Conclusions
A family of dicopper(I) complexes with short copper-copper
distances have been assembled by using a 1,8-naphthyridine-
based dinucleating ligand BBAN and different terminal ligands.
Chemical and electrochemical oxidation studies of these com-
plexes revealed that, with neutral nitrogen-donor terminal
ligands, the mixed-valence dicopper(I,II) analogues could not
be stabilized. A fully delocalized mixed-valence dicopper(I,II)
complex was prepared by chemically oxidizing a carboxylate-
bridged dicopper(I) complex. Frozen solution EPR studies
demonstrate the fully delocalized nature of this complex down
to 4 K. A cyclic voltammetric study indicated a reversible redox
wave with an E1/2 value that is +200 mV compared to those of
previous model compounds. The E1/2 values of the mono-
(carboxylate) compound prepared in this work and the structur-
ally very similar bis(carboxylate) XDK compounds reveal the
contribution from an anionic carboxylate group to the redox
potential of the mixed-valence system to be ∼ -200 mV.
Anionic ligands are important factors in stabilizing the mixed-
valence state and controlling the redox potential.
Figure 6. Cyclic voltammogram of 2 mM Cu2(BBAN)(µ-O2CCPh3)]-
(OTf) (5) in THF with 0.5 M (Bu4N)(PF6) as supporting electrolyte
and a scan rate of 75 mV/s.
proteins, ∼+200 to +800 mV vs NHE or ∼ -600 to 0 mV vs
Cp2Fe+/Cp2Fe in THF.30,31
The E1/2 value of the previously reported macrocyclic
octaazacryptand complexes is +310 mV vs Ag/AgCl in DMF,
which corresponds to -206 mV vs Cp2Fe+/Cp2Fe in THF.18
With the anionic dicarboxylate ligand XDK, the E1/2 value of
the mixed-valence compound is -218 mV vs Cp2Fe+/Cp2Fe
in THF. The anionic bis(carboxylate) ligand does not seem to
decrease the redox potential of the CuIICuI/CuICuI couple com-
pared to the all-nitrogen octaazacryptand ligands. This result is
Acknowledgment. This work was supported by a grant from
the National Science Foundation. We thank Dr. M. Merkx for
assistance with the EPR measurement.
Supporting Information Available: X-ray crystallographic files,
in CIF format, for complexes 1-6. This material is available free of
charge via the Internet at http://pubs.acs.org.
IC0005339
(30) Reinhammar, B. R. M. Biochim. Biophys. Acta 1972, 275, 245-259.
(31) Tronson, D. A.; Ritchie, G. A. F.; Nicholas, D. J. D. Biochim. Biophys.
Acta 1973, 310, 331-343.
(32) Immoos, C.; Hill, M. G.; Sanders, D.; Fee, J. A.; Slutter, C. E.;
Richards, J. H.; Gray, H. B. JBIC 1996, 1, 529-531.