Synthesis, structure, and electrochemical properties of copper(I) complexes with S/N homoscorpionate and heteroscorpionate ligands

Article abstract of DOI:10.1021/ic048221j

Dinuclear Cu(I) complexes with bifunctionalized homoscorpionate ligands, hydrotris(thioxotriazolyl)borato [Li(Tr^Me,o-Py) (1) and Li(Tr ^Mes,Me) (2)], and the heteroscorpionate ligand hydro[bis(thioxotriazolyl)-3-(2-pyridyl)pyrazolyl]borato [K(Br ^Mespz^o-Py)] (3) were synthesized and crystallographically characterized. The complexes [Cu(Tr^Mes,Me)]₂ (4) and [Cu(Tr^Me,o-Py)]₂ (5) exhibit a similar coordination geometry where every metal is surrounded by three thioxo groups in a trigonal arrangement. The presence of a [B-H...Cu] three-center-two-electron interaction in both compounds causes the overall coordination to become tetrahedrally distorted (S₃H coordination for each metal). The complex [Cu(Br^Mespz^o-Py)]₂ (6) presents a trigonal geometry in which the metals interact with two thioxo groups and a bridging pyrazolyl nitrogen atom. A weak contact with a pyridine nitrogen atom completes the coordination of the metals (S₂N,N′ coordination for each metal). [Cu(Tr^Mes,Me)]₂, [Cu(Tr ^Me,o-Py)]₂, and [Cu(Br^Mespz ^o-Py)]₂ exhibit fluxional behavior in solution as evidenced by variable-temperature NMR spectroscopy, and for 5 and 6 two species in equilibrium [in the ratio 2/1 for 5 (CDCl₃) and 3/2 for 6 (CD ₂Cl₂)] are distinguishable in the ¹H NMR spectra at 270 K. 2D-NOESY spectra recorded at 270 K assisted in the attribution of solution molecular geometries for each isomer of 5 and 6. The free energy of activation (ΔG?_Tc) was determined for both equilibria from the evaluation of the coalescence temperature. DFT calculations were performed to describe plausible molecular geometry for the minor isomer of 5 and 6 and to propose a possible mechanism of interconversion between major and minor isomers. Cyclic voltammograms were recorded in CH ₂Cl₂ (3 and 6) or CH₂Cl₂/CH ₃CN (1/1, v/v) (2, 4, and 5) solutions using 0.1 M TBAHFP or TBAOTf as supporting electrolytes. [Cu(Tr^MES,Me)]₂, [Cu(Tr ^Me,o-Py)]₂, and [Cu(Br^Mespz ^o-Py)]₂ exhibit a quasi-reversible Cu(I)/Cu(II) redox behavior with E_pa = +719 mV and E_pc = +538 mV for 4, E_pa = +636 mV and E_pc = -316 mV for 5, and E_pa = +418 mV and E_pc = -319 mV for 6.

Full text of DOI:10.1021/ic048221j

Inorg. Chem. 2005, 44, 4333−4345
Synthesis, Structure, and Electrochemical Properties of Copper(I)
Complexes with S/N Homoscorpionate and Heteroscorpionate Ligands
Roberto Cammi, Marcello Gennari, Marco Giannetto, Maurizio Lanfranchi, Luciano Marchio`,*
Giovanni Mori, Cristiano Paiola, and Maria Angela Pellinghelli
Dipartimento di Chimica Generale ed Inorganica, Chimica Analitica, Chimica Fisica, Parco area
delle Scienze 17a, 43100 Parma, Italy
Received December 17, 2004
-
Py
Dinuclear Cu(I) complexes with bifunctionalized homoscorpionate ligands, hydrotris(thioxotriazolyl)borato [Li(TrMe,o
)
(1) and Li(TrMes,Me) (2)], and the heteroscorpionate ligand hydro[bis(thioxotriazolyl)-3-(2-pyridyl)pyrazolyl]borato
[K(BrMespzo Py)] (3) were synthesized and crystallographically characterized. The complexes [Cu(TrMes,Me)]2 (4) and
-
[Cu(TrMe,o Py)]2 (5) exhibit a similar coordination geometry where every metal is surrounded by three thioxo groups
-
in a trigonal arrangement. The presence of a [B−H‚‚‚Cu] three-center−two-electron interaction in both compounds
causes the overall coordination to become tetrahedrally distorted (S3H coordination for each metal). The complex
-
[Cu(BrMespzo Py)]2 (6) presents a trigonal geometry in which the metals interact with two thioxo groups and a
bridging pyrazolyl nitrogen atom. A weak contact with a pyridine nitrogen atom completes the coordination of the
-
-
metals (S2N,N′
coordination for each metal). [Cu(TrMes,Me)]2, [Cu(TrMe,o Py)]2, and [Cu(BrMespzo Py)]2 exhibit fluxional
behavior in solution as evidenced by variable-temperature NMR spectroscopy, and for 5 and 6 two species in
1
equilibrium [in the ratio 2/1 for 5 (CDCl3) and 3/2 for 6 (CD2Cl2)] are distinguishable in the H NMR spectra at 270
K. 2D-NOESY spectra recorded at 270 K assisted in the attribution of solution molecular geometries for each
isomer of 5 and 6. The free energy of activation (∆
GqTc) was determined for both equilibria from the evaluation of
the coalescence temperature. DFT calculations were performed to describe plausible molecular geometry for the
minor isomer of 5 and 6 and to propose a possible mechanism of interconversion between major and minor
isomers. Cyclic voltammograms were recorded in CH2Cl2 (3 and 6) or CH2Cl2/CH3CN (1/1, v/v) (2, 4, and 5)
-
solutions using 0.1 M TBAHFP or TBAOTf as supporting electrolytes. [Cu(TrMes,Me)]2, [Cu(TrMe,o Py)]2, and
-
[Cu(BrMespzo Py)]2 exhibit a quasi-reversible Cu(I)/Cu(II) redox behavior with Epa ) +719 mV and Epc ) +538 mV
for 4, Epa ) +636 mV and Epc ) −316 mV for 5, and Epa ) +418 mV and Epc ) −319 mV for 6.
Introduction
tion: type 1 (T1); type 2 (T2); type CuA. The unusual
coordination of T1 centers, intermediate between trigonal
and distorted tetrahedral, is characterized by strong bonds
between the metal and three donor atoms on a plane (N-
His, N-His, S-Cys) and by a weaker interaction with a
fourth axial ligand (usually S-Met). T1 sites are subdivided
in T1 trigonal (typical of blue copper proteins, such as
plastocyanin,6 azurin,7 and ceruloplasmin8) and T1 distorted
Cupredoxins are small copper proteins (10-20 kDa)
soluble in water, which control electron flow from donor to
acceptor biomolecules in respiratory and photosynthetic
processes of many bacteria and plants.1-4 In the natural state
they contain intensely colored Cu(II)-thiolate sites and are
classified in three types,5 according to the metal coordina-
* Author to whom correspondence should be addressed. E-mail:
marchio@unipr.it.
(1) Adman, E. T. AdV. Protein Chem. 1991, 42, 145-197.
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10.1021/ic048221j CCC: $30.25
Published on Web 05/12/2005
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 12, 2005 4333

Cammi et al.
Chart 1
tetrahedral (blue nitrite reductasi9), depending on the effec-
tive geometry of the metal. Copper geometry and donor atom
sets have important consequences on the function of these
proteins. According to the entatic state theory,10 the metal
coordination geometry is imposed by the protein matrix,
which forces the Cu(II) site to adopt a geometry similar to
that needed for requirements of Cu(I). This would favor a
reversible and fast electron transfer since it is associated with
low reorganization energy during the redox process.11 Other
authors have questioned the entatic-state theory and suggest
that the metal geometry is mainly dictated by the coordination
environment and donor set [S(thiolate), S(thioether), N(imi-
dazole)] instead of being determined by the protein
framework.12-15
Several Cu(II) and Cu(I) complexes, which model T1 sites
of cupredoxins, are reported in the literature. Some of these
models are represented by dinuclear Cu(I) complexes with
hydrotris(pyrazolyl)borato ligands (TpRR′) where the metal
exhibits a trigonal geometry16 whereas other systems achieve
a tetrahedral copper coordination by means of a facially
bound Tp ligand and additional monodentate donor
groups.17-19 N2S2 donor ligands based on the biphenyl
platform20 have been especially designed to provide a rigid
and tetrahedral ligand framework around Cu(II) to meet the
requirements of the entatic-state theory. Other systems are
based on the bis(benzimidazoles)-thioether ligands which
are able to give a wide variety of metal coordination spanning
from the planar trigonal to the trigonal bipyramidal.21,22
Recently, bulky â-diketiminate ligands associated with a
thiolate group have been able to reproduce the unusual
trigonal geometry at Cu(II) occurring in type I centers.23 In
these low molecular weight complexes the ligand topology,
the donor set, and the metal coordination geometry imposed
by the ligands modulate the redox activity of the Cu(II)/Cu-
(I) couple.
This work regards the synthesis of Cu(I) complexes
with the homoscorpionate hydrotris(thioxotriazolyl)borato
[(TrMe,o-Py)- and (TrMes,Me)-] ligands and the heteroscorpi-
onate24 hydro[bis(thioxotriazolyl)-3-(2-pyridyl)pyrazolyl]bo-
rato [(BrMespzo-Py)-] ligand (Chart 1) aimed at obtaining
models for T1 sites. Even though these ligands may adopt
rather different modes of coordination by virtue of the
different substituents on the triazoline rings, the dinuclear
Cu(I) complexes exhibit quite similar solid-state structures
with respect to the metal geometry. In fact, the metals adopt
a trigonal geometry, which is distorted toward the tetrahedral
one by a [B-H‚‚‚Cu] interaction for [Cu(TrMes,Me)]2 and
[Cu(TrMe,o-Py)]2 and by a Cu‚‚‚Npyridine interaction for
[Cu(BrMespzo-Py)]2. Many examples of dinuclear Cu(I) com-
plexes with Tp ligands have been reported where the metal
adopts a linear or trigonal16,25 coordination according to the
steric constraints determined by bulky substituents placed
on different parts of the ligands. The high flexibility of
(9) Inoue, T.; Gotowda, M.; Deligeer; Kataoka, K.; Yamaguchi, K.;
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1998, 124, 876-879.
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59, 498-505. (b) Williams, R. J. P. Eur. J. Biochem. 1995, 234, 363-
381.
(TrMe,o-Py)-, (TrMes,Me)-, and (BrMespzo-Py -
) may limit the
influence of the steric hindrance on the metal geometry. As
a consequence, it is possible to infer that the resulting
coordination environments are mainly dictated by the metal
requirements and only to a less extent by ligand constraints.
(11) Marcus, R. A.; Sutin, N. W. Biochim. Biophys. Acta 1985, 811, 265-
322.
(12) Messerschmidt, A.; Prade, L.; Kroes, S. J.; Sanders-Loehr, J.; Huber,
R.; Canters, G. W. Proc. Natl. Acad. Sci. U.S.A. 1998, 95, 3443-
3448.
(13) (a) Ryde, U.; Olsson, M. H. M.; Pierloot, K.; Roos, B. O. J. Mol.
Biol. 1996, 261, 586-596. (b) Ryde, U.; Olsson, M. H. M.; Roos, B.
O.; Borin, A. C. Theor. Chem. Acc. 2001, 105, 452-462.
(14) Ryde, U.; Olsson, M. H. M.; Pierloot, K. Theor. Comput. Chem. 2001,
9, 1-55.
(21) Balamurugan, R.; Palaniandavar, M. Inorg. Chem. 2001, 40, 2246-
2255.
(22) Vaidyanathan, M.; Balamurugan, R.; Sivagnanam, U.; Palaniandavar,
M. J. Chem. Soc., Dalton Trans. 2001, 3498-3506.
(23) (a) Holland, P. L.; Tolman, W. B. J. Am. Chem. Soc. 1999, 121, 7270-
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B.; Hodgson, K. O.; Tolman, W. B.; Solomon, E. I. J. Am. Chem.
Soc. 2000, 122, 11632-11648.
(24) The definition homoscorpionate and heteroscorpionate have been
adopted in analogy to the poly(pyrazolyl)borate nomenclature: Tro-
fimenko, S. Chem. ReV. 1993, 93, 943-980.
(25) (a) Yoon, K.; Parkin, G. Polyhedron 1995, 14, 811-821. (b) Kiani,
S.; Long, J. R.; Stavropoulos, P. Inorg. Chim. Acta 1997, 263, 357-
366. (c) Mealli, C.; Arcus, C. S.; Wilkinson, J. L.; Marks, T. J.; Ibers,
J. A. J. Am. Chem. Soc. 1976, 98, 711-718. (d) Carrier, S. M.;
Ruggiero, C. E.; Tolman, W. B. J. Am. Chem. Soc. 1992, 114, 4407-
4408.
(15) Guckert, J. A.; Lowery, M. D.; Solomon, E. I. J. Am. Chem. Soc.
1995, 117, 2817-2844.
(16) Carrier, S. M.; Ruggiero, C. E.; Houser, R. P.; Tolman, W. B. Inorg.
Chem. 1993, 32, 4889-4899.
(17) Randall, D. W.; George, S. D.; Hedman, B.; Hodgson, K. O.; Fujisawa,
K.; Solomon, E. I. J. Am. Chem. Soc. 2000, 122, 11620-11631.
(18) Kitajima, N.; Fujisawa, K.; Moro-oka, Y. J. Am. Chem. Soc. 1990,
112, 3210-3212.
(19) Kitajima, N.; Fujisawa, K.; Tanaka, M.; Moro-oka, Y. J. Am. Chem
Soc. 1992, 114, 9232-9233.
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4334 Inorganic Chemistry, Vol. 44, No. 12, 2005

Cu(I) Complexes with S/N Scorpionate Ligands
100; 715, 6 [M+]. Anal. Calcd for C36H43N9S3BLi (Mr ) 715.74):
C, 60.41; H, 6.05; N, 17.61. Found: C 59.89; H 5.96; N 17.31.
Preparation of KBrMes,Mepzo-Py (3). 4-Mesityl-3-methyl-5-
thioxotriazole (1.00 g, 4.28 mmol), 3-(2-pyridyl)pyrazole (0.351
g, 2.42 mmol), KBH4 (0.119 g, 2.21 mmol), and toluene (50 mL)
were added to a Schlenk flask connected to a water condenser and
to an oil-bubbler. While the solution was being stirred, the
temperature was raised to 150 °C. A white precipitate formed after
1 night. The mixture was allowed to react for 9 days. It was then
filtered, and the solid was washed with hot toluene (20 mL) and
with a 2/1 dichloromethane/ethanol mixture (30 mL), vacuum-dried,
and collected (0.797 g, 1.21 mmol, 56%). IR (KBr pellet, cm-1):
3018 w, 2920 m, 2854 w, 2441 w [ν(BH)], 1595 m, 1491 m, 1429
m, 1409 s, 1382 m, 1359 s, 1323 m, 1299 m, 1265 m, 1205 m. 1H
NMR (300 MHz, 298 K, DMSO-d6): δ 1.84 (s, 6H, CH3), 1.93 (s,
6H, CH3), 1.95 (s, 6H, CH3), 2.29 (s, 6H, CH3), 4.92 (s br, 1H,
BH), 6.67 (d, J ) 2 Hz, 1H, CH pz), 7.00 (s, 4H, Mes), 7.20 (m,
1H, CH py), 7.59 (d, J ) 2 Hz, 1H, CH pz), 7.76 (t, J ) 7.8 Hz,
1H, CH py), 7.98 (d, J ) 7.8 Hz, 1H, CH py), 8.52 (d, J ) 4.5 Hz,
1H, CH py). 13C NMR (75 MHz, 298 K, DMSO-d6): δ 11.07
(CH3), 17.34 (CH3), 17.44 (CH3), 20.59 (CH3), 102.15 (CH pz),
118.85 (CH py), 121.09 (CH py), 128.61 (CH Mes), 128.73 (CH
Mes), 130.89 (C quat), 134.65 (C quat), 135.77 (CH pz), 136.07
(CH py), 137.98 (C quat), 145.64 (C quat), 148.79 (CH py), 154.06
(C quat), 168.75 (CdS). ESI-MS (cone 26 V, negative ion
mode, 1/1 CH3CN/MeOH, m/z, I %): 620.1, 100 [L] [L )
(BrMes,Mepzo-Py)-]. Anal. Calcd for C32H35N9S2BK (Mr ) 659.72):
C, 60.41; H, 5.35; N, 17.61. Found: C, 59.95; H, 5.61; N 17.70.
Preparation of [Cu(TrMes,Me)]2 (4). A solution of Cu(MeCN)4BF4
(0.132 g, 0.419 mmol) in methanol (3 mL) was added to a stirred
mixture of LiTrMes,Me (0.300 g, 0.419 mmol) in methanol (20 mL).
The suspension became dark immediately, and it cleared after a
few minutes giving a white precipitate. After 2 h the solid was
filtered off, washed with methanol (5 mL), vacuum-dried, and
collected (0.117 g, 0.076 mmol, 36%). IR (KBr pellet, cm-1): 3009
w, 2966 w, 2949 w, 2920 m, 2857 w, 2491 w [ν(BH)], 1609 w,
1576 w, 1488 m, 1407 s, 1371 m, 1328 s, 1302 m. 1H NMR (300
MHz, 298 K, CDCl3): δ 1.93 (s, 36H, CH3), 1.98 (s, 18H, CH3),
2.29 (s, 18H, CH3), 6.90 (s, 12H, CH Mes). 13C NMR (75 MHz,
298 K, CDCl3): δ 11.20 (o-CH3 Mes), 17.88 (p-CH3 Mes), 21.06
(CH3 triazoline), 128.97 (CH Mes), 130.17 (C quat), 136.30 (C
quat), 138.97 (C quat), 147.62 (C quat), 167.80 (CdS). ESI-MS
(cone 50 V, positive ion mode, 1/1 CH3CN/MeOH, m/z, I %):
3127.8, 10 [CuL]4K+; 2380.1, 12 [Cu4L3]+; 2355.2, 32 [CuL]3K+;
2339.5, 13 [CuL]3Na+; 1607.7, 68 [Cu3L2]+; 1583.3, 85 [CuL]2K+;
1567.2, 43, [CuL]2Na+; 834.7, 7 [Cu2L]+; 810.6, 100 [CuL]K+;
794.6, 63 [CuL]Na+ [L ) (TrMes,Me)-]. Anal. Calcd for [C36H43N9S6-
BCu]2 (Mr ) 1544.68): C, 55.98; H, 5.61; N, 16.32. Found: C
55.43; H 5.94; N 15.93. Colorless crystals suitable for X-ray
structure determination were obtained from an acetonitrile solution,
corresponding to [Cu(TrMes,Me)]2‚4CH3CN (4a).
By means of cyclic voltammetry the main electrochemical
parameters of the redox couple Cu(I)/Cu(II) have been
evaluated for the complexes, and they are discussed in
relation to solid state and solution structural properties,
obtained by X-ray diffration and variable-temperature and
2D NMR spectroscopy, respectively.
Experimental Section
General Procedures. All reagents were purchased from Sigma-
Aldrich except mesityl isothiocyanate,26 3-(2-pyridyl)pyrazole,27
LiTrMe,o-Py (1),28 and [Cu(CH3CN)4]BF4,29 which were prepared
using methods reported in the literature. All solvents were com-
mercially available and were used without further purification except
THF and toluene, which were distilled on sodium-benzophenone
and sodium, respectively, and stored under nitrogen. Acetonitrile
and methanol were stored on molecular sieves 4 Å (1-2 mm) and
degassed prior to use. Syntheses of tripodal ligands and complexes
were performed in inert gas (N2) using Schlenk techniques. CI-
MS spectra were recorded on a Finnigan 1020 mass spectrometer
in positive-ion mode with a quadrupole MATTSSQ 710. ESI-MS
spectra were recorded on a Micromass ZMD spectrometer. The
solutions were analyzed in both positive and negative ionization
mode. Infrared spectra were recorded as KBr pellets from 4000 to
400 cm-1 on a Perkin-Elmer FT-IR Nexus. Elemental analyses were
performed on a Carlo Erba EA 1108 automatic analyzer.
Preparation of 4-Mesityl-3-methyl-5-thioxotriazole. Mesityl
isothiocyanate (4.8 g, 27 mmol) and acetic hydrazide (2.2 g, 27
mmol) were dissolved in absolute ethanol (35 mL). The solution
was heated to reflux and stirred. After about 2.5 h a white solid
precipitated. Heating was stopped after 4.5 h, and the solid was
filtered off and washed with absolute ethanol (20 mL). It was then
dissolved in aqueous NaOH (2 M, 90 mL) and refluxed for 7 h.
The white suspension was filtered, and the solution was treated
with HCl (at first 10 M and then 3 M) until neutral pH. The white
solid that formed was filtered off, washed with H2O, vacuum-dried,
and collected (4.8 g, 21 mmol, 78%). IR (KBr pellet, cm-1): 3116
1
s, 3062 m, 2941 s, 1584 w, 1496 m, 1328 s. H NMR (300 MHz,
298 K, CDCl3): δ 2.07 (s, 6H, CH3), 2.11 (s, 3H, CH3), 2.37 (s,
3H, CH3), 7.06 (s, 2H, CH Mes), 11.95 (bs, 1H, NH). MS (CI,
m/z, I %): 233, 50 [M+]; 234, 100 [MH+]. Anal. Calcd for
C12H15N3S (Mr ) 233.33): C, 61.77; H 6.48; N, 18.01. Found: C,
61.49; H, 6.48; N, 17.67.
Preparation of LiTrMes,Me (2). 4-Mesityl-3-methyl-5-thioxo-
triazole (4.000 g, 17.14 mmol), LiBH4 (0.125 g, 5.45 mmol), and
toluene (80 mL) were mixed together in a Schlenk flask connected
to a water condenser and to an oil-bubbler. During stirring, the
temperature was increased to 135 °C. After some hours a white
solid began to precipitate. After 3 days, the hot mixture was filtered
and the solid washed with hot toluene (3 × 20 mL), vacuum-dried,
and collected (3.7 g, 5.2 mmol, 95%). IR (KBr pellet, cm-1): 3010
w, 2920 m, 2854 w, 2550 w [ν(BH)], 1610 m, 1583 m, 1490 s,
Preparation of [Cu(TrMe,o-Py)]2 (5). [Cu(MeCN)4]BF4 (0.109
g, 0.346 mmol) was added to a stirred suspension of LiTrMe,o-Py
(0.204 g, 0.344 mmol) in methanol (30 mL). The mixture instantly
became yellow. After 1 h the yellow solid was filtered off, washed
with methanol (3 mL), vacuum-dried, and collected (0.181 g, 0.139
mmol, 81%). IR (KBr pellet, cm-1): 3306 m, 3274 s, 3207 s, 2962
w, 2443 w br [ν(BH)], 1652 m, 1639 m, 1622 m, 1477 s, 1411 s,
1350 s, 1142 s, 1078 s, 1014 s. 1H NMR (300 MHz, 270 K, CDCl3;
see discussion for signals attribution; the integration of the signals
is reported separately for each isomer): δ 4.06 (s, 18H, CH3
triazoline, e), 4.11 (s, 18H, CH3 triazoline, e′), 7.30 (m, 6H/6H,
CH py, b/b′), 7.64 (m, 6H, CH py, c), 7.74 (t, J ) 6.6 Hz, 6H, CH
1
1410 vs, 1323 s, 1136 m, 1015 m. H NMR (300 MHz, 298 K,
DMSO-d6): δ 1.85 (m, 9H, CH3), 1.96 (m, 18H, CH3), 2.30 (s,
9H, CH3), 4.93 (s br, 1H, BH), 7.00 (m, 6H, CH Mes). 13C NMR
(75 MHz, 298 K, DMSO-d6): δ 11.26 (CH3), 17.49 (CH3), 20.65
(CH3), 128.65 (CH Mes), 131.13 (C quat), 136.06 (C quat), 137.87
(C quat), 145.13 (C quat), 168.95 (CdS). MS (CI, m/z, I %): 233,
(26) Jochims, C. J. Chem. Ber. 1968, 101, 1746.
(27) Brunner, H.; Scheck, T. Chem. Ber. 1992, 125, 701-709.
(28) Gennari, M.; Giannetto, M.; Lanfranchi, M.; Marchio`, L.; Pellinghelli,
M. A.; Tegoni, M. Polyhedron 2004, 23, 1829-1835.
(29) Leftin, J. H. Chem. Abstr. 1967, 66, 46487e.
Inorganic Chemistry, Vol. 44, No. 12, 2005 4335

Cammi et al.
Table 1. Summary of X-ray Crystallographic Data for 4a, 5a, and 6a
4a
5a
6a
empirical formula
fw
C80H98B2Cu2N22S6
1708.86
C52H48B2Cl12Cu2N24S6
1775.60
C69H80B2Cl10Cu2N18S4
1792.95
color, habit
cryst size, mm
cryst system
space group
a, Å
b, Å
c, Å
colorless, block
0.25 × 0.08 × 0.08
triclinic
colorless, block
0.45 × 0.35 × 0.20
triclinic
colorless, block
0.30 × 0.20 × 0.15
monoclinic
C2/c
31.568(9)
13.690(3)
21.026(7)
90
P1h
P1h
11.130(1)
13.395(2)
14.998(2)
92.233(3)
90.078(3)
94.367(2)
2227.8(5)
1
12.542(1)
13.070(1)
14.215(1)
92.892(2)
115.559(1)
105.716(1)
1985.6(3)
1
R, deg
â, deg
94.118(7)
90
9063
γ, deg
V, Å3
Z
4
T, K
293(2)
1.274
0.672
1.53-27.02
13 389/3378
0.0507
293(2)
1.485
1.148
1.62-27.99
21 694/5077
0.0525
293(2)
1.314
0.904
1.62-27.13
12 929/2891
0.0565
F(calc), Mg/m3
µ, mm-1
θ range, deg
no. of rflcns/obsvns F > 4σ(F)
R1a
wR2a
0.0632
0.1212
0.0875
a R1 ) Σ||Fo| - |Fc||/Σ|Fo|; wR2 ) [Σ[w(Fo
Fc )2]/Σ[w(Fo )2]]1/2, w ) 1/[σ2(Fo ) + (aP)2 + bP], where P ) [max(Fo ,0) + 2Fc ]/3.
2
-
2
2
2
2
2
py, c′), 7.93 (d, J ) 7.5 Hz, 6H, CH py, d), 8.11 (d, J ) 7.8 Hz,
6H, CH py, d′), 8.61 (m, 6H/6H CH py, a/a′). 13C NMR (75 MHz,
298 K, CDCl3): δ 33.96 (CH3 triazoline), 124.06 (CH py), 124.18
(CH py), 124.45 (CH py), 136.63 (CH py), 148.54 (CH py), 167.74
(CdS), quaternary carbons not detected. Anal. Calcd for [C24H22-
N12S3BCu]2 (Mr ) 1298.13): C, 44.41; H, 3.42; N, 25.90. Found:
C, 44.66; H, 3.49; N, 25.69. Colorless crystals suitable for X-ray
structure determination were obtained from a chloroform solution,
corresponding to [Cu(TrMe,o-Py)]2‚4CHCl3 (5a).
1.000 (6a). The structures were solved by direct methods (SIR9731)
and refined on F2 with full-matrix least squares (SHELXL-9732),
using the Wingx software package.33 Non-hydrogen atoms were
refined anisotropically, and the hydrogen atoms were placed at their
calculated positions, except for the hydrogens of the B-H groups,
which were found and refined. One of the two independent solvent
molecules of crystallization (CHCl3) in 5a was found disordered
in two positions with site occupancy factors of 0.8 and 0.2,
respectively. For 6a, four molecules of CH2Cl2/dinuclear complex
were found, while the contribution of a fifth molecule was modeled
with the program SQUEEZE PLATON.34 The programs ORTEP3
for Windows35 and Parst36,37 were also used. X-ray data are reported
in Table 1.
NMR Experiments. NMR spectra were recorded on a Bruker
300 MHz Avance spectrometer. Chemical shifts are reported in
ppm, referred to residual solvent protons (CDCl3, CD2Cl2, 1,1,2,2-
tetrachloroethane-d2, CD3CN, DMSO-d6). 13C signals were assigned
by means of (13C-1H)-HETCOR and DEPT (distortionless en-
hancement of polarization transfer) experiments. Two-dimensional
experiments (HETCOR, COSY, and NOESY) were recorded using
standard Bruker pulse sequences. Various 2D-NOESY experiments
were recorded changing the mixing time (τm) in the range 0.2-0.8
s for 5 and 6, and the best enhancement of the negative NOE peaks
was obtained using a τm of 0.6 s. NOESY peaks intensities were
determined by measuring peak volumes, and the distances between
nuclei were determined according to the formula
Preparation of [Cu(BrMes,Mepzo-Py)]2 (6). [Cu(MeCN)4]BF4
(0.092 g, 0.292 mmol) was added to a stirred suspension of
KBrMes,Mepzo-Py (0.160 g, 0.242 mmol) in acetonitrile (50 mL). The
mixture became bright yellow. After 2.5 h it was filtered, and the
yellow solid was washed with acetonitrile (2 mL), vacuum-dried,
and collected (90 mg, 0.066 mmol, 54%). IR (KBr pellet, cm-1):
2966 w, 2920 w, 2859 w, 2497 w br [ν(BH)], 1597 w, 1578 w,
1489 m, 1433 m, 1414 s, 1369 m, 1327 m, 1204 m, 1114 m, 1092
1
m, 1033 m. H NMR (300 MHz, 270 K, CD2Cl2; see discussion
for signals attribution; the integration of the signals is reported
separately for each isomer): δ 1.95 (s, 12H, CH3 Mes, c, e), 2.00
(s, 12H, CH3 triazoline, f), 2.02 (s, 12H, CH3 Mes, c, e), 2.33 (s,
12H, CH3 Mes, d), 6.75 (m br, 2H, CH m′), 6.81 (d, J ) 2.1 Hz,
2H/2H, CH h/h′), 6.97 (s, 2H, CH Mes, a, b), 7.04 (s, 2H, CH
Mes, a, b), 7.19 (m br, 2H/2H CH py, m/l′), 7.69 (m, J ) 7.8 Hz,
2H/2H CH py, l/i′), 7.77 (d, J ) 7.8 Hz, 2H CH py, i), 8.01 (s, br,
2H, CH py, n′), 8.35 (d, J ) 4.2 Hz, 2H, CH py, n), 8.48 (s, br,
2H, CH pz, g′), 8.55 (d, J ) 3.0 Hz, 2H CH pz, g). Some signals
of the minor isomer (c′, d′, e′, f′, a′, b′) could not be detected as
they are covered by the corresponding signals of the major isomer.
Anal. Calcd for [C32H35N9S2BCu]2 (Mr ) 1366.00): C, 56.18; H,
5.16; N, 18.42. Found: C, 55.80; H, 4.92; N, 17.98. Yellow crystals
suitable for X-ray structure determination were obtained from a
dichloromethane solution, corresponding to [Cu(BrMes,Mepzo-Py)]2‚
5CH2Cl2 (6a).
1/6
Vref
rij ) rref
(V )
ij
where Vij and rij are the NOE cross-peak volume and the interproton
(30) Area-Detector Absorption Correction; Siemens Industrial Automation,
Inc.: Madison, WI, 1996.
(31) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giaco-
vazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna,
R. J. Appl. Crystallogr. 1999, 32, 115.
X-ray Crystallography. The X-ray structure analysis on single
crystals for the compounds [Cu(TrMes,Me)]2‚4CH3CN (4a),
[Cu(TrMe,o-Py)]2‚4CHCl3 (5a), and [Cu(BrMes,Mepzo-Py)]2‚5CH2Cl2
(6a) was performed on a Smart Bruker AXS 1000 area-detector
diffractometer (Mo KR ) 0.710 73 Å). An absorption correction
was applied using the program SADABS30 with trasmission factors
in the ranges 0.809-1.000 (4a), 0.696-1.000 (5a), and 0.907-
(32) Sheldrick, G. M. SHELX97, Programs for Crystal Structure Analysis,
release 97-2; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(33) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837.
(34) Van der Sluis, P.; Spek, A. L. Acta Crystallogr., Sect. A 1990, 46,
194-201.
(35) Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565.
4336 Inorganic Chemistry, Vol. 44, No. 12, 2005

Cu(I) Complexes with S/N Scorpionate Ligands
Scheme 1
distance of two protons i and j and Vref is NOE cross-peak volume
between two protons at a known distance rref. The distances between
protons in compound 5 were determined by comparison between
the Hd/Hc cross-peak intensity and the known Hd‚‚‚Hc distance of
2.35 Å, whereas for compound 6 the distances between protons
were determined by comparison between the Hg/Hh cross-peak
intensity and the known Hg‚‚‚Hh distance of 2.49 Å. Variable-
temperature NMR experiments were recorded at 10 K intervals
outside the coalescence regions and at 5 K intervals in proximity
to the coalescence regions. These experiments were performed in
the 210-350 K range (CDCl3 as solvent) for 5 and in the 182-
340 K range (CD2Cl2 as solvent) for 6 in controlled atmosphere
valve NMR tubes. Free energies of activation (kJ/mol) for unequally
populated sites38 were calculated from the coalescence temperatures
(Tc) and the frequency differences (δν) between the coalescing
signals (Hd/Hd′ for 5 and Hn/Hn′ for 6) extrapolated at the
coalescence temperatures, using the formula
6 all the mesityl groups were approximated to hydrogen atoms and
for both compounds the methyl groups on the triazoline rings were
also approximated to hydrogen atoms. The gradient-corrected hybrid
density functional B3LYP42,43 and double-ú basis set LANL2DZ
with Hay and Wadt effective core potential (ECP)44,45 were used
for the description of the model system. Hartree-Fock with a
minimum basis set LANL2MB with Hay and Wadt ECP were used
for the “real system”. For the geometry optimization of the model
system, a polarization p-function for the hydrogen atoms of the
B-H moiety and a polarization d-function for all sulfur atoms were
added in the basis sets to give a better description of the interactions
of these groups with the metals. Single point energy calculations
were performed for all complexes using the B3LYP density
functional and the LANL2DZ basis set.
Electrochemistry. Cyclic voltammetry (CV) and differential
pulse voltammetry (DPV) were performed in a three-electrode cell
with a Pt disk as working electrode, a Pt rod as counter electrode,
and a 0.1 M Ag/AgCl/KCl (E0 ) +281 mV) as reference electrode,
by using a computerized electrochemical workstation consisting of
an Autolab PGSTSAT 20 potentiostat (Ecochemie, Utrecht, The
Netherlands) controlled by the GPES 4.9 software. Cyclic volta-
mmograms were recorded on freshly prepared CH2Cl2 (3 and 6) or
1/1 CH2Cl2/CH3CN (v/v) (2, 4, and 5) solutions containing 0.1 M
supporting electrolyte [tetra-n-butylammonium hexafluorophosphate
(TBAHFP) or tetra-n-butylammonium trifluoromethanesulfonate
(TBAOTf)] and purged with N2 for 2 min. All experiments were
performed under identical conditions for different scan rates (50-
1000 mV/s).
∆Gcq ) aTc[9.972 + log(Tc/δν)]
a ) 1.914 × 10-2 kJ/mol
Two- and one-dimensional FIDs were processed using the MestReC
program suite.39
Quantum Chemical Calculations. All the calculations have
been performed with the Gaussian 01 program suite.40 Geometry
optimization was performed for compounds 5 and 6 using initial
coordinates derived from X-ray data (5b and 6b) and on model
geometries proposed on the basis of two- and one-dimensional NMR
experiments (5c and 6c). Since quantum mechanical studies on these
dinuclear complexes were computationally demanding, we em-
ployed the two layers ONIOM technique also known as IMOMO.41
The details about the ONIOM layers defined for the molecules in
exam are reported in the Supporting Information. For the model
system of 5 only the pyridine groups involved in the proposed
fluxional process (one for each ligand) were retained on the
triazoline rings whereas the remaining pyridine rings were ap-
proximated to hydrogen atoms. In the model system of compound
(36) Nardelli, M. Comput. Chem. 1983, 7, 95.
(37) Nardelli, M. J. Appl. Crystallogr. 1995, 28, 659.
(38) (a) Sandstrom, J. Dynamic NMR Spectroscopy; Academic Press:
London, 1982. (b) Kost, D.; Carlson, E. H.; Raban, M. Chem.
Commun. 1971, 656-657. (c) Shanan-Atidi, H.; Bar-Eli, K. H. J. Phys.
Chem. 1970, 74, 961-963.
(39) Cobas, J. C.; Sardina, F. J. Nuclear magnetic resonance data processing.
MestRe-C: A software package for desktop computers. Concepts
Magn. Reson. 2003, 19A, 80-96.
(40) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Kudin, K. N.; Burant, J. C.; Millam, J. M.; Stratmann, R. E.; Tomasi,
J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.;
Iyengar, S.; Petersson, G. A.; Ehara, M.; Toyota, K.; Nakatsuji, H.;
Adamo, C.; Jaramillo, J.; Cammi, R.; Pomelli, C.; Ochterski, J.; Ayala,
P. Y.; Morokuma, K.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.;
Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A.
D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul,
A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko,
A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D.
J.; Keith, T.; Al-Laham, M. A.; Peng, C. A.; Nanayakkara, A.;
Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M.
W.; Andres, J. L.; Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.;
Pople, J. A. Gaussian 01, DeVelopment Version, revision B.01;
Gaussian, Inc.: Pittsburgh, PA, 2001.
Results and Discussion
The synthesis of the tripodal ligand LiTrMe,o-Py (1)28 and
LiTrMes,Me (2) were performed at high temperature in toluene
by reacting triazoles and LiBH4 in 3/1 ratio, Scheme 1. The
method extensively employed to produce hydrotris(pyra-
zolyl)borate,46 hydrotris(mercaptoimidazolyl)borate,47 and
hydrotris(thioxotriazolyl)borate,48 which involves the reaction
between borohydrides and heterocycles in the melt, could
not be followed here due to the high and prohibitive melting
temperatures of triazoles (>200 °C). Different strategies were
attempted to synthesize the heteroleptic ligand 3:
(42) Becke, A. D. Phys. ReV. A 1988, 38, 3098.
(43) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(44) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299.
(45) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284.
(46) Trofimenko, S. Inorg. Synth. 1970, 12, 99. Trofimenko, S. Chem. ReV.
1993, 93, 943-980.
(41) (a) Humbel, S.; Sieber, S.; Morokuma, K. J. Chem. Phys. 1996, 105,
1959-1967. (b) Svensson, M.; Humbel, S.; Froese, R. D. J.;
Matsubara, T.; Sieber, S.; Morokuma, K. J. Phys. Chem. 1996, 100,
19357-19363. (c) Dapprich, S.; Komaromi, I.; Byun, K. S.; Moro-
kuma, K.; Frisch, M. J. J. Mol. Struct. (THEOCHEM) 1999, 461-
462, 1-21.
(47) Reglinski, J.; Garner, M.; Cassidy, I. D.; Slavin, P. A.; Spicer, M. D.;
Armstrong, D. R. J. Chem. Soc., Dalton Trans. 1999, 2119-2126.
(48) Bailey, P. J.; Lanfranchi, M.; Marchio`, L.; Parsone, S. Inorg. Chem.
2001, 40, 5030-5035.
Inorganic Chemistry, Vol. 44, No. 12, 2005 4337

Cammi et al.
Scheme 2
(1) A step procedure was used that gives the hydrobis-
and is directed toward the B-H group. This type of
molecular structure has been already reported for the
dinuclear complex [Cu(TrEt,Me)]2 in which the peripheral part
of the ligand (ethyl and methyl groups) provides a limited
steric hindrance.52 From a comparison of these molecular
structures, it may be inferred that when these ligands are
S3H bound with an inverted geometry (with the B-H
pointing toward the metal as in the reported molecular
structures) the substituents on the triazoline rings play a
minor role in determining the coordination geometry. It
should be noted, however, that, in 4a, the mesityl groups
are blocked in a rigid position (perpendicular to the triazoline
ring) to minimize the steric repulsion between the ortho
methyl groups on the mesityl ring and the CdS and methyl
group on the triazoline ring. This may limit the steric
constraints that the mesithyl group may impose over the
coordination geometry. As far as 5a is concerned, the
pyridine rings are pointing away from the coordination
environment and therefore they are not directly influencing
the copper(I) coordination. The distance between the two
copper centers is 5.401(1) Å for 4a and 5.308(1) Å for 5a
ruling out any metal-metal interaction.
(thioxotriazolyl)borato first and the final product in a second
phase from a reaction with 3-(2-pyridyl)pyrazole, which is
analogous with similar synthesis reported in the literature.49,50
This procedure gave no results as (a) reacting 4-mesityl-3-
methyl-5-thioxotriazole with LiBH4 or NaBH4 in 2/1 ratio
in THF at reflux condition gave a mixture of products
together with unreacted reagents and (b) reacting 4-mesityl-
3-methyl-5-thioxotriazole with LiBH4 in 2/1 ratio (toluene,
50 °C) gave hydrobis(thioxotriazolyl)borato in good yield
but the scarce solubility of this product in toluene even at
high temperatures did not allow it to react with 1 equiv of
3-(2-pyridyl)pyrazole.
(2) The successful synthesis of 3 was obtained by a one-
pot reaction between KBH4, 3-(2-pyridyl)pyrazole, and
4-mesityl-3-methyl-5-thioxotriazole in 1/1/2 ratio (toluene,
150 °C), according to a modification of a reported method
(Scheme 1).51
1-3 are air-stable white solids. The neutral Cu(I) com-
plexes were prepared at room temperature by reacting [Cu-
(MeCN)4]BF4 with 1-3 (Scheme 2). The stability of the
ligands and of the resulting complexes in methanol allowed
the synthesis in this solvent for 4 and 5.
A relevant difference with respect to the previously
described molecular structures is exhibited by 6a as the third
“arm” of the heteroscorpionate ligand is represented by a
N,N chelating pyrazolyl-2-pyridyl group. In the dinuclear
complex each metal is in a trigonal geometry distorted toward
the tetrahedral one and with the apical position occupied by
the pyridine nitrogen atom. The overall coordination around
copper may be described as S2N,N′ with each ligand S,S
chelating on a metal center and bridging with the pyrazolyl
nitrogen atom to a second metal. The copper atoms lie out
of the S,S,N trigonal plane of 0.0672(6) Å, and they are
directed toward the pyridine nitrogen atoms. Consequently,
for this compound there is no evidence of a [B-H‚‚‚Cu]
interaction as the H‚‚‚Cu separation is 2.83(3) Å and the
B-H distance (0.97(3) Å) is significantly shorter than those
found in compounds 4a and 5a. However, the Cu-Npyridine
distance of 2.547(4) Å is consistent with a weak coordinative
interaction. The distance between the two metals is 4.862-
(1) Å, and even though it is shorter than in 4a and 5a, it
does not indicate a metal-metal interaction.
Crystal Structures. The Ortep diagrams of the three
complexes are reported in Figures 1-3, and selected bond
lengths and angles, in Tables 2-4. The coordination
geometry of the metal for 4a and 5a are almost equivalent,
with each copper(I) bound by three sulfur atoms in a distorted
trigonal planar geometry. The complexes are dinuclear, and
each ligand is S,S chelated on a metal center and bridges
with the third sulfur atom a second metal. The Cu-S
distances are in the range 2.253(1)-2.355(1) Å for 4a and
2.252(1)-2.305(1) Å for 5a. The distortion of the coordina-
tion geometry is evident from the analysis of the coordination
bond angles: for 4a they vary from 97.57(4) to 143.04(5)°
whereas for 5a they vary in a more narrow range from
111.54(4) to 124.63(4)°. The presence of a [B-H‚‚‚Cu]
interaction for both compounds is responsible for a distortion
of the trigonal geometry toward the tetrahedral one. In fact,
the Cu-H separation is of 1.99(3) Å (4a) and 1.94(4) Å
(5a) and the metal lies out of the plane defined by the three
sulfur atoms of 0.1138(5) Å for 4a and 0.2237(5) Å for 5a
1
Solution Structures. The H NMR spectrum of 4 in the
temperature range 220-310 K in CDCl3 (Supporting Infor-
mation) is characterized by a single set of sharp signals
(49) Kimblin, C.; Hascall, T.; Parkin, G. Inorg. Chem. 1997, 36, 5680-
5681.
(50) Alvarez, H. M.; Tran, T. B.; Richter, M. A.; Alyounes, D. M.;
Rabinovich, D.; Tanski, J. M.; Krawiec, M. Inorg. Chem. 2003, 42,
2146-2156.
(51) Shu, M.; Walz, R.; Wu, B.; Seebacher, J.; Vahrenkamp, H. Eur. J.
Inorg. Chem. 2003, 2502-2511.
(52) Careri, M.; Elviri, L.; Lanfranchi, M.; Marchio, L.; Mora, C.;
Pellinghelli, M. A. Inorg. Chem. 2003, 42, 2109-2144.
4338 Inorganic Chemistry, Vol. 44, No. 12, 2005

Cu(I) Complexes with S/N Scorpionate Ligands
Figure 1. ORTEP drawing of 4a at the 30% thermal ellipsoids level.
Hydrogen atoms, except those of the B-H groups, have been removed for
clarity.
Figure 3. ORTEP drawing of 6a at the 30% thermal ellipsoids level.
Hydrogen atoms, except those of the B-H groups, have been removed for
clarity.
Table 2. Experimental Selected Bond Lengths (Å) and Angles (deg)
with Esd’s in Parentheses for 4aa
Cu-S(1)
Cu-S(2)
Cu-S(3)′
Cu-H
C(11)-S(1)
C(12)-S(2)
2.355(1)
2.265(1)
2.253(1)
1.99(3)
1.687(4)
1.694(4)
C(13)-S(3)
B-H
B-N(11)
B-N(12)
B-N(13)
1.697(4)
1.27(3)
1.546(5)
1.539(5)
1.541(5)
S(1)-Cu-S(2)
S(1)-Cu-S(3)′
S(2)-Cu-S(3)′
S(1)-Cu-H
S(2)-Cu-H
S(3)′-Cu-H
B-H-Cu
118.56(4)
97.57(4)
143.04(5)
84.1(7)
91.8(8)
100.0(7)
130(2)
N(11)-B-N(12)
N(11)-B-N(13)
N(12)-B-N(13)
N(11)-B-H
107.5(1)
111.3(3)
108.0(3)
110(1)
108(1)
113(1)
N(12)-B-H
Figure 2. ORTEP drawing of 5a at the 30% thermal ellipsoids level.
Hydrogen atoms, except those of the B-H groups, have been removed for
clarity.
N(13)-B-H
a Equivalent position: ′ ) 1 - x, 1 - y, 2 - z.
despite the presence in the crystal structure of three distinct
environments (corresponding to the three arms of each
ligand) for both ligands in the dinuclear complex. The
equivalence of the ligand arms observed in the NMR
spectrum may result from a rapid interconversion of the
triazolyl environments due either to a fast mononuclear-
dinuclear equilibrium or to a fluxional process involving only
the dinuclear complex. 1H NMR at different concentrations
(10-2-10-4 M based on the dinuclear complex) show that
the chemical shifts are concentration independent, and this
would support a fluxional dinuclear complex hypothesis.
A different situation is encountered upon dissolution of
crystals of 5a and 6a in various deuterated solvents (CDCl3,
CD2Cl2, 1,1,2,2-tetrachloroethane-d2), as their 1H NMR
spectra show the presence of two species, Figures 4 and 5.
The assignment of the peaks for every isomer for 5 and 6
was made by means of 2D-HETCOR (1H-13C) and 2D-
COSY NMR spectroscopy (Supporting Information). As far
as 5 is concerned, variable-temperature NMR experiments
were performed in the range 350-210 K in CDCl3. At 270
K two isomers are clearly distinguishable in the ratio major
isomer/minor isomer of ∼2. Increasing the temperature leads
to the coalescence of their signals, and above 335 K a single
set of frequencies is observed. Furthermore, upon a decrease
of the temperature below 220 K there is no substantial
variation of the signals of the major isomer whereas, for the
minor isomer, coalescence of its peaks occurs at 230 K and
below this temperature three different environments can be
observed. This confirms the presence of two equilibria, one
involving the major and minor isomers (Tc ) 335 K) and
the other concerning only the minor isomer (Tc ) 230 K).
Two hypotheses can be made to explain this behavior: (1)
The major isomer corresponds to a species with a symmetric
structure as in the case of a mononuclear complex with the
ligand S3 bound, a trigonal geometry around the metal, and
Inorganic Chemistry, Vol. 44, No. 12, 2005 4339

Cammi et al.
Table 3. Experimental and Calculated Selected Bond Lengths (Å) and Angles (deg) with Esd’s in Parentheses for 5a-ca
X-ray (5a)
calcd (5b)
calcd (5c)
X-ray (5a)
calcd (5b)
calcd (5c)
Cu-S(11)
Cu-S(12)
Cu-S(13)′
Cu-H
Cu-N(23)
Cu-N(43)′
C(11)-S(11)
2.305(1)
2.285(1)
2.252(1)
1.94(4)
2.411
2.384
2.363
2.004
2.462
2.397
C(12)-S(12)
C(13)-S(13)
B-H
B-N(11)
B-N(12)
B-N(13)
1.698(3)
1.708(3)
1.15(3)
1.536(4)
1.537(4)
1.555(4)
1.707
1.713
1.205
1.542
1.537
1.557
1.714
1.677
1.197
1.536
1.555
1.560
2.473
2.126
2.200
1.706
1.700(3)
1.702
S(11)-Cu-S(12)
S(11)-Cu-S(13)′
S(12)-Cu-S(13)′
S(11)-Cu-H
120.95(4)
111.54(4)
124.64(4)
86.0(8)
125.22
106.69
123.62
84.39
120.24
S(11)-Cu-N(43)′
S(12)-Cu-N(43)′
N(23)-Cu-N(43)′
S(11)-Cu-N(23)′
S(12)-Cu-N(23)′
N(11)-B-H
103.55
95.23
77.27
138.59
100.61
109.14
108.95
108.51
S(12)-Cu-H
90.1(9)
86.82
S(13)′-Cu-H
110.5(9)
110.2(3)
110.1(2)
111.5(2)
121.35
110.35
109.67
111.45
109(1)
109(1)
108(2)
136(2)
109.20
108.59
107.49
136.32
N(11)-B-N(12)
N(11)-B-N(13)
N(12)-B-N(13)
108.08
113.15
108.94
N(12)-B-H
N(13)-B-H
B-H-Cu
a Equivalent position: ′ ) -x, -y, -z.
Table 4. Experimental and Calculated Selected Bond Lengths (Å) and Angles (deg) with Esd’s in Parentheses for 6a-ca
X-ray (6a)
calcd (6b)
calcd (6c)
X-ray (6a)
calcd (6b)
calcd (6c)
Cu-S(1)
Cu-S(2)
Cu-N(23)′
Cu-N(33)′
Cu-H
2.268(2)
2.245(1)
1.985(3)
2.547(4)
2.83(3)
2.408
2.440
2.070
2.226
2.730
1.702
2.385
2.400
2.022
C(12)-S(2)
B-H
B-N(11)
B-N(12)
B-N(13)
1.679(5)
0.97(3)
1.537(6)
1.538(6)
1.520(6)
1.704
1.191
1.556
1.561
1.547
1.706
1.200
1.556
1.555
1.537
2.416
1.707
C(11)-S(1)
1.686(4)
S(1)-Cu-S(2)
S(1)-Cu-N(23)′
S(2)-Cu-N(23)′
S(1)-Cu-H
120.95(4)
111.54(4)
124.64(4)
77.2(9)
113.21
119.42
126.55
76.38
116.89
116.00
120.71
76.37
84.87
133.03
N(11)-B-N(12)
N(11)-B-N(13)
N(12)-B-N(13)
N(11)-B-H
109.7(4)
109.7(4)
111.0(4)
111(2)
104(2)
111(2)
109.25
110.24
108.35
108.94
110.11
109.94
122.24
109.19
110.61
110.07
108.44
109.26
109.23
126.65
S(2)-Cu-H
68.7(9)
84.86
N(12)-B-H
N(23)′-Cu-H
S(1)-Cu-N(33)′
S(2)-Cu-N(33)′
N(23)′-Cu-N(33)′
124.2(9)
102.6(1)
99.0(1)
133.04
107.87
93.09
N(13)-B-H
B-H-Cu
126(2)
71.5(1)
78.69
a Equivalent position: ′ ) -x + 1/2, -y + 1/2, -z + 1.
Figure 4. Temperature dependence of the 1H NMR spectrum of 5 in
CDCl3: left, pyridine protons; right, methyl protons.
a normal conformation of the ligand (with the B-H vector
pointing outward). This could be possible thanks to the high
flexibility of the ligand that chelates with eight-membered
chelating rings and may be able to envelope the metal. The
minor isomer presents a structure similar to the one reported
in the solid state with nonequivalence between the three arms
of the tripod, which becomes evident when decreasing the
temperature below 230 K. (2) The major isomer corresponds
Figure 5. Temperature dependence of the 1H NMR spectrum of 6 in CD2-
Cl2: left, aromatic protons; right, methyl protons.
to the structure reported in the solid state but with great
stereochemical mobility that makes the three arms of the
ligand equivalent in the temperature range under study,
analogous with the behavior of compound 4. The minor
isomer is another dinuclear complex, the nonsymmetric
4340 Inorganic Chemistry, Vol. 44, No. 12, 2005

Cu(I) Complexes with S/N Scorpionate Ligands
Scheme 3
structure of which is evident below 230 K as three sets of
signals appear. To distinguish between the two scenarios,
mononuclear-dinuclear (1) or dinuclear-dinuclear (2) equi-
To help in the assignment of the molecular structures of
the two isomers in solution, a 2D-NOESY NMR spectrum
was recorded at 270 K in CDCl3 (Supporting Information).
At this temperature significant chemical exchange among
the pyridine protons was evident from the positive cross-
peaks in the NOESY spectrum. Intense negative cross-peaks
are found for the protons of the same pyridine rings for the
two conformers (separation of ∼2.3 Å). More interestingly,
a weak cross-peak is found between the methyl protons and
the Ha proton for the major isomer that correspond to an
interproton distance of 3.74 Å, in good agreement with the
equivalent separation found in the solid-state structure (mean
value ) 3.80(3) Å), Figure 6. This would confirm that for
the major isomer the pyridine ring is oriented as in the solid-
state structure (with the pyridine nitrogen facing the methyl
group). For the minor isomer, the absence of a strong cross-
peak between the methyl protons (He′) and the Hd′ proton
could derive from a competition between the tendency of
the pyridine to coordinate the copper center and the repulsion
that would consequently arise between Hd′ and the methyl
group. This would result in a structure in which the pyridine
is nearly perpendicular to the triazoline plane exhibiting an
incipient coordination to the metal. The free energy of
1
libria, H NMR spectra were recorded at concentrations in
the 10-2-10-4 M range at 298 K (Supporting Information).
These experiments show that the ratio of the integrals of the
major and minor isomers is unvaried in this concentration
range suggesting that the equilibrium with Tc ) 335 K would
involve two different dinuclear species (hypothesis 2) with
an equilibrium constant of 0.5 for the formation of the minor
isomer.53 A possible structure of the minor isomer could
involve the coordination of the ligand in a S2N,N′ mode so
that it chelates S,S on a metal center and bridges on a second
metal with the triazoline and pyridine nitrogen atoms deriving
form the third arm. This would be in agreement with the
observation that the signals in the pyridine region are at low
fields compared to the corresponding signals of the major
isomer, as a consequence of the involvement of the pyridine
in the metal coordination. The resulting dinuclear structure
would be centrosymmetric, but each ligand would present
three different environments for each arm, giving rise to three
sets of signals below 230 K. Increasing the temperature above
this value may lead to the equivalence of the three arms of
each ligand in accordance with the single set of frequencies
observed in the NMR spectra. If this attribution is correct,
the equilibrium with Tc ) 335 K (between the two dinuclear
complexes) would imply a rotation of the B-N bond for
one arm of the ligand to replace the CdS group with the
triazoline nitrogen atom in the copper coordination, Scheme
3. Moreover, if the pyridine is taking part in the metal
activation derived from the coalescence temperature (∆Gq
)
335
for the process going from the major isomer to the minor
coordination, it has to rotate of ∼150° around the Ctriazoline
-
Cpyridine bond according to the value of the torsion angle
[N(2)-C(2)-C(3)-N(4)] of -150.3(4)° found for the solid-
state structure.54 Nevertheless, this would bring the Hd′ proton
and the methyl (He′) on the triazoline ring in very close
proximity, and this may represent a destabilizing factor for
the structure of the minor isomer, Scheme 3. The proof that
this N,N′ chelation is possible comes from the structure of
[Ni(TrMe,o-Py)2] in which the ligand was in fact S,N,N′ bound
to the metal even though it adopted a normal conformation.28
(53) This value is obtained by the ratio of the integrals of the Hd and Hd′
protons (signals with no overlap).
(54) According to the X-ray structure of 5a the pyridine rings are oriented
in a way that places the nitrogen atom close to the methyl group of
the triazoline ring and not on the same side of the triazoline nitrogen
atom.
Figure 6. Aromatic and methyl regions of the NOESY NMR spectrum
of 5 at 270 K in CDCl3. Solid lines denote negative cross-peaks. See Scheme
3 for signal attribution.
Inorganic Chemistry, Vol. 44, No. 12, 2005 4341

Cammi et al.
Scheme 4
more difficult to extract information relative to the solution
structure of the minor isomer as chemical exchange pro-
cesses, affecting both positive and negative cross-peaks,
hinder the interpretation. Moreover, decreasing the temper-
ature to block the exchange processes leads to the disap-
pearance of the minor species. According to the observation
that the signals of the minor isomer in the aromatic region
are at higher fields with respect to the corresponding proton
signals of the major isomer, a molecular structure for the
minor isomer can be proposed in which the pyridine is not
coordinated to the metal. This can be achieved by a rotation
along the Cpyridine-Cpyrazole bond and by the approaching of
the B-H moiety that pushes the pyridine nitrogen atom
backward, Scheme 4. In this structure, the stabilization would
come form the [B-H‚‚‚Cu] interaction and by the disap-
pearance of the Hi/Hh steric repulsion. In both molecular
structures the ligand would present three distinct environ-
ments for each arm and a high fluxional behavior has to be
considered to explain the single set of signals for each species
Figure 7. Aromatic region of the NOESY NMR spectrum of 6 at 270 K
in CD2Cl2. Solid lines denote negative cross-peaks; dashed lines denote
positive cross-peaks. See Scheme 4 for signal attribution.
isomer is 70.0 kJ/mol, whereas for the reverse process is
68.0 kJ/mol, Scheme 3.
For compound 6, variable-temperature NMR experiments
were performed in the range 340-182 K in CD2Cl2. At 280
K two isomers are clearly distinguishable with the ratio major
isomer/minor isomer of 1.67 and the coalescence of their
signals is observed at 320 K. Furthermore, decreasing the
temperature below 250 K leads to the progressive disap-
pearance of the minor isomer. Again, the question were we
facing a mononuclear-dinuclear or dinuclear-dinuclear
in the 182-340 K temperature range. The ∆Gq for the
320
process going from the major isomer to the minor isomer is
64.2 kJ/mol whereas for the reverse process it is 62.8 kJ/
mol (Scheme 4).
1
equilibrium arose, and H NMR spectra were recorded at
concentrations in the 10-2-10-4 M range at 280 K (Sup-
porting Information). The concentration-independent integral
ratio confirmed the presence of an equilibrium involving only
two dinuclear species with an equilibrium constant of 0.6
for the formation of the minor isomer at 280 K. A
2D-NOESY spectrum was recorded at 270 K to assign
molecular structures in solution, Figure 7. At this tempera-
ture, the exchange rate between the two isomers is not
negligible and positive cross-peaks are in fact evident in the
pyridine, mesityl, and methyl regions and are complementing
the 2D-COSY peaks attribution. Various negative cross-peaks
are helpful in the assignment of the major isomer molecular
structure. Strong negative cross-peaks are observed between
the doublet centered at 6.81 ppm (Hh) and the multiplet
centered at 7.75 ppm (Hi) and between the doublet at 8.32
ppm (Hn) and the singlet at 1.99 ppm (Hc/e) corresponding
to an interproton distance of 2.46 and 2.62 Å, respectively.
A cross-peak of intermediate intensity was also found for
the multiplet centered at 7.66 ppm (Hl) and Hh.55 These cross-
peaks would confirm that in solution the molecular structure
of the major isomer corresponds to the solid-state one. It is
DFT Calculations. To support the proposed fluxional
processes evidenced by NMR spectroscopy for 5 and 6, DFT
geometry optimizations were performed starting from the
geometry derived from the X-ray structural analysis (5b, 6b)
and on geometries proposed according to two-dimensional
and variable-temperature NMR spectroscopy (5c, 6c). The
b structures correspond to the major isomers whereas the c
structures correspond to the minor isomers as found in the
NMR experiments. The drawings corresponding to the 5c
and 6c optimized structures are reported in Figures 8 and 9,
and the geometric parameters (bond distances and angles)
for the optimized structures 5b,c and 6b,c are reported in
Tables 3 and 4. The 5b calculated structures agree very well
with the experimental data obtained by X-ray diffraction.
Nevertheless, an increase of the metal-S bond distances is
exhibited probably due to the lack of the crystal packing
forces present in the solid-state structures. In the 5c structure
the coordination at the metal may be described as S2N,N′ in
which a CdS bridging group has been replaced by a bridging
Ntriazoline-Npyridine chelate system. Consequently, the H‚‚‚Cu
separation has increased to 2.437 Å diminishing the strength
(55) The interproton distances found in the solid-state structure are as
follows: D(Hh/Hi) ) 2.50 Å; D(Hn/Hc/e) ) 2.67 Å.
of the [B-H‚‚‚Cu] interaction. The repulsive (CH3)triazoline-
4342 Inorganic Chemistry, Vol. 44, No. 12, 2005

Cu(I) Complexes with S/N Scorpionate Ligands
but still too large (2.730 Å) to account for a possible [B-H‚
‚‚Cu] interaction. For the 6c calculated structure, the torsion
angle τ[N(23)-C(33)-C(43)-N(33)] is 165.8° as a result
of the pyridine ring rotation along the Cpyrazole-Cpyridine bond
with respect to 6b. The H‚‚‚Cu (2.416 Å) separation is ∼0.4
Å shorter than in the X-ray structure and ∼0.3 Å shorter
than in 6b, and this may be in agreement with a [B-H‚‚‚
Cu] interaction. This is also confirmed by the fact that the
two metals in 6c move away from the plane defined by the
sulfur and nitrogen atoms and are directed toward the B-H
groups. On the basis of these calculations, different copper-
(I) coordinations have been reported, namely S3H, S2N,N′,
S2N,H, and it appears that the most stable coordinative
environments are the S3H (4 and 5) and S2N,H (6) as may
expected for a soft ion such as Cu(I).
The calculated energy difference (∆Ecalc) at the B3LYP/
LANL2DZ level between the major and the minor isomers
is 26.5 kJ/mol in 5 (∆G335(expt) ) 2 kJ/mol) and 2.2 kJ/
mol in 6 (∆G320(expt) ) 1.4 kJ/mol). Whereas there is a
good accordance between the experimental and calculated
energy difference for the two isomers of 6, compound 5
presents a more marked difference between the ∆G335 and
the ∆Ecalc values.56 This poor agreement may be the result
of (a) the level of calculation, which may not be accurate
enough to give a ∆Ecalc that reproduces the experimental
value, and (b) the solvent effect, since the calculation are
performed for an isolated system whereas the experimental
∆GTc are obtained in solution and the solvent can have a
strong influence in stabilizing an isomer with respect to the
other. In fact, for 5, the ratio 5b/5c is ∼8 in CD2Cl2, ∼2 in
CDCl3, and ∼0.7 in 1,1,2,2-tetrachloroethane-d2.
Figure 8. Optimized structure for the minor isomer 5c.
Voltammetric Studies. The cyclic voltammograms of
4-6 are reported in Figure 10. Two main features appear
from a comparison of the voltammograms of the three
compound: (1) high anodic peak potential values (Epa); (2)
quasi-reversible behavior of the Cu(I)/Cu(II) couple.57 The
high Epa values can be explained by considering the environ-
ment that surrounds the metal in the three compounds. In
fact, with regard to the X-ray structures 4a and 5a three sulfur
atoms are present in the copper coordination sphere along
with a [B-H‚‚‚Cu] interaction, whereas for 6a two sulfur
atoms and two nitrogen atoms represent the donor set. Even
if one takes into account fluxional processes, at least two
sulfur atoms are present in the coordination sphere of the
metal, and this may be a factor that stabilizes Cu(I) with
respect to oxidation.
Figure 9. Optimized structure for the minor isomer 6c.
Hd′ interaction that arises as a consequence of the N,N′
coordination is evidenced by the relatively high value of the
torsion angle τ[N(23)-C(23)-C(33)-N(43)] of -18.8° (cf.
0° for an ideal conjugated and chelate aromatic system such
as bipyridine). The corresponding torsion angles of sulfur
bound ligand arms present absolute values of ∼160° in
agreement with the values reported in the X-ray structure,
and this may be a compromise between the stabilization that
arises from the coplanarity/conjugation of the triazoline and
pyridine rings and the repulsion between the Npyridine lone
pair and the triazoline methyl group. With respect to the
X-ray structure, the 6b calculated structure shows an increase
in the metal-S bond distances and only a slight elongation
of the metal-Npyrazole one, whereas a more marked decrease
(∼0.3 Å) for metal-Npyridine separation is observed. This
would suggest a more pronounced interaction between the
pyridine and Cu(I) if compared to the experimental structure.
As a consequence of this interaction the torsion angle
τ[N(23)-C(33)-C(43)-N(33)] is very small (1.7°). The H‚
‚‚Cu separation is ∼0.1 Å shorter than in the X-ray structure
The voltammogram of 4 shows an anodic peak at +719
mV correlated to a +538 mV cathodic one,58 and they are
assigned to the Cu(I)/Cu(II) couple, since its parent com-
(56) The energetics does not include zero-point energy or entropy correc-
tions.
(57) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals
and Applications; John Wiley & Sons: New York, 1980.
(58) The correlation between the cathodic and anodic peaks has been
performed by holding the potential at +800 mV for about 5 min.
Restarting the scan in the cathodic direction resulted in an increase of
the current of the correlated peak at +538 mV. The correlation between
the anodic and cathodic peaks for compounds 5 and 6 has been
performed in the same way by holding the potential at +0.700 and
+0.500 mV, respectively.
Inorganic Chemistry, Vol. 44, No. 12, 2005 4343

Cammi et al.
stabilization. This is in accordance with the fluxional
behavior of the complex as evidenced by NMR spectroscopy
since the proposed structure of the minor isomer implies the
coordination of one arm of the ligand in the N,N′ fashion, a
preferable coordinative situation for copper(II). The new
species formed probably rearranges only a little in the
following reduction; in fact, the subsequent anodic peak
occurs at a lower potential (+123 mV) with respect to the
initial species. This is confirmed by the fact that the
occurrence of the anodic peak at +123 mV is particularly
evident at the third scan of the potential in the anodic
direction.
Complex 6 exhibits the anodic peak at +418 mV and the
cathodic one at -319 mV attributable to the Cu(I)/Cu(II)
couple, and the ∆Ep (+737 mV) is consistent with a quasi-
reversible redox process. In the parent compound 3 two
irreversible anodic events occur at higher potential (+770
and +1160 mV). Furthermore, a minor structural reorganiza-
tion could be proposed for 6 if compared to 5 [∆Ep(5) >
∆Ep(6)]. Also for this compound the electrochemical features
could be explained according to its fluxional properties in
agreement with the fact that, in 6, only a weak-interacting
pyridine nitrogen atom is displaced from the metal whereas,
for 5, the coordination environment is substantially different
for the two species in equilibrium (S3H and S2N,N′) requiring
a greater reorganization energy. It has been reported that in
some cases TBAHFP used as supporting electrolyte may
interact with the species that undergo the electrochemical
process and can alter their electrochemical behavior.16 To
confirm that this was not the case, the voltammograms were
recorded in the same experimental conditions but using
TBAOTf as supporting electrolyte without any appreciable
changes to their redox features. This would support the
hypothesis that the quasi-reversibility of the Cu(I)/Cu(II)
redox couple is mainly due to ligand rearrangement con-
temporary with the electrochemical events.
Figure 10. Cyclic voltammograms of (a) 10-4 M solutions of 4 dissolved
in 1/1 CH2Cl2/CH3CN (v/v) overimposed with the DPV scan (in the +0.25/
+0.85 V window), (b) 10-3 M solutions of 5 dissolved in 1/1 CH2Cl2/
CH3CN (v/v), and (c) 10-3 M solutions of 6 dissolved in CH2Cl2. Supporting
electrolyte: 0.1 M TBAHFP. Reference electrode: 0.1 M Ag/AgCl/KCl.
Scan rate: 100 mV/s.
Conclusions
Cu(I) complexes with homoscorpionate [(TrMes,Me -
) and
(TrMe,o-Py)-] and heteroscorpionate ligands [(BrMes,Mepzo-Py)-]
were synthesized. The X-ray characterization reveals that
these compounds have a dinuclear structure in the solid state.
The compounds [Cu(TrMes,Me)]2 and [Cu(TrMe,o-Py)]2 exhibit
equivalent coordination environment at copper (S3H type
pound (2) shows two irreversible anodic peaks only at higher
potentials (+960 and +1224 mV). The redox couple Cu(I)/
Cu(II) is quasi-reversible, as shown by ∆Ep values (161 mV),
implying a certain structural reorganization of the metal
environment associated with the electron-transfer process.
The scarce solubility of 4 in CH2Cl2/CH3CN (1/1) (concen-
trated ∼ 10-4 M) resulted in a difficult detection of the
electrochemical process, and to support the CV findings, a
differential pulse voltammetry (that enhances the faradaic
component of the signals) was performed on the same
sample, which confirms the occurrence of the anodic and
catodic events. Complex 5 shows rather different redox
behavior. An anodic peak at +636 mV is correlated to a
cathodic one at -316 mV, and they are assigned to the Cu(I)/
Cu(II) couple. The quasi-reversibility of the procress (∆Ep
) 952 V) can be rationalized in terms of considerable
coordination sphere modification resulting in copper(II)
supported by a [B-H‚‚‚Cu] interaction). The ligand (TrMes,Me -
presents minor coordination capabilities (CdS groups) if
compared to (TrMe,o-Py -
for which an involvement of the
)
)
N,N′ chelate system is also possible. Nevertheless, the similar
solid-state structures for these compounds is in accordance
with preference of Cu(I) for a sulfur-rich environment. This
is also confirmed by DFT calculations, which suggest that
the preferred Cu(I) environments in these systems are S3H
and S2N,H. Nevertheless, the competition between the CdS
and N,N′ systems for the Cu(I) coordination is evidenced
by the solution fluxional behavior of [Cu(TrMe,o-Py)]2, as two
species in equilibrium are present with a ratio that is also
dependent on the deuterated solvents used. The proposed
structure for the minor isomer of [Cu(TrMe,o-Py)]2 would
4344 Inorganic Chemistry, Vol. 44, No. 12, 2005

Cu(I) Complexes with S/N Scorpionate Ligands
correspond to a species in which a CdS group has been
replaced by a N,N′ system. This ligand flexibility (ability to
display different donor sets) translates into nonreversible
Cu(I)/Cu(II) redox processes due to stabilization of Cu(II)
by involvement of N atoms for the metal coordination.
Moreover, compounds [Cu(TrMes,Me)]2 and [Cu(TrMe,o-Py)]2
exhibit high Epa values due to Cu(I) stabilization by the soft
ligand environment. From this point of view, an improvement
of this situation has been achieved with the ligand
(BrMes,Mepzo-Py)-. In fact, in the coordination sphere of
[Cu(BrMes,Mepzo-Py)]2, the S2N donor set is always present
(even taking into account its fluxional behavior), and this
grants a minor Epa value for the couple Cu(I)/Cu(II) with
respect to [Cu(TrMes,Me)]2 and [Cu(TrMe,o-Py)]2. However, a
direct comparison with E° values of cupredoxins containing
T1 copper sites is not possible as a consequence of the
nonreversible properties of the Cu(I)/Cu(II) couple for the
three compounds. Moreover, there is an additional question
to be considered: in the blue copper protein the metal is
forced into an energized state (entactic) by the protein
framework that does not change conformation during the
redox process. According to this theory, this protein rigidity
at the active site is an essential requirement for the correct
functioning of these proteins as electron carriers. Unfortu-
nately, the model systems used in this work fail to satisfy
this requirement, as they possess great stereochemical
mobility. In fact, even though the geometry of the various
isomers is essentially distorted tetrahedral as in cupredoxins
T1 sites, the donor atom set changes from S3H to S2N,N′ in
5 and from S2N,N′ to S2N,H in 6 with the result that the redox
processes are not reversible. With this respect we are
currently investigating new Cu(I) model systems based on
more conformationally rigid ligands that preserve the cu-
predoxins T1 center donor set.
Acknowledgment. This work was supported by the
Ministero dell’Istruzione, dell’Universita` e Ricerca (Rome,
Italy).
Supporting Information Available: Experimental details in-
cluding preparation schemes, NMR spectroscpy, and X-ray crystal-
lography (PDF). This material is available free of charge via the
Internet at http://pubs.acs.org. Other supplementary crystallographic
data for this paper are found in CCDC 256810-256812
(www.ccdc.cam.ac.uk/conts/retrieving.html).
IC048221J
Inorganic Chemistry, Vol. 44, No. 12, 2005 4345