Mononuclear and dinuclear copper(I) complexes of bis(3,5-dimethylpyrazol-1- yl)methane: Synthesis, structure, and reactivity

Article abstract of DOI:10.1021/ic050568e

The synthesis and structures of a mononuclear copper(I) carbonyl complex [Cu(OClO₃)(CO)(H₂CPz′₂)] (3) and a dinuclear copper(I) carbonyl complex [{Cu(CO) (H₂CPz′₂)}₂(μ-pyrazine)](ClO ₄

Full text of DOI:10.1021/ic050568e

Inorg. Chem. 2005, 44, 6122−6128
Mononuclear and Dinuclear Copper(I) Complexes of
Bis(3,5-dimethylpyrazol-1-yl)methane: Synthesis, Structure, and
Reactivity
Chang-Chuan Chou,†,‡ Chan-Cheng Su,*,† and Andrew Yeh§
Department of Chemistry, National Taiwan Normal UniVersity, Taipei 116, Taiwan,
Center for General Education, Chang Gung Institute of Technology, Tao-Yuan 333, Taiwan, and
Department of Chemistry, Tunghai Christian UniVersity, Taichung 407, Taiwan
Received April 12, 2005
The synthesis and structures of a mononuclear copper(I) carbonyl complex [Cu(OClO3)(CO)(H2CPz′2)] (3) and a
dinuclear copper(I) carbonyl complex
bis(3,5-dimethylpyrazol-1-yl)methane, are described. These two compounds were generated by the carbonylation
of the corresponding copper(I)-acetonitrile complexes, [Cu(H2CPz2)(MeCN)](ClO4) (1) and Cu(H2CPz2)-
(MeCN) 2 µ-pyrazine)](ClO4)2 (2). Alternatively, treatment of mononuclear 1 and 3, respectively, with pyrazine in a
molar ratio of 2:1 produces the pyrazine-bridged dinuclear CuI complexes 2 and 4. Each of the complexes 1
4 can
react with PPh3 to generate a common three-coordinated copper(I) complex [Cu(PPh3)(H2CPz2)](ClO4) (5). The
structures of complexes 1 C(CO) bond
5 were all confirmed by X-ray crystallography. Comparison of the CuI
distances and the CO stretching frequencies of 3 and 4 indicates the back-donating properties of d (Cu)−π*(pyrazine)
[{Cu(CO)(H2CPz2′)}2(µ-pyrazine)](ClO4)2 (4), where H2CPz2′ )
′
[{
′
}
(
−
′
−
−
π
bonds in 4, and accordingly, stabilizes the mixed-valence species generated from 2. Complex 3, stabilized by the
strong interaction between copper(I) ion and perchlorate counteranion (CuI
precursor for polynuclear copper(I) carbonyl complexes.
−O(ClO4) ) 2.240(3) Å), is a potential
Introduction
(HC(3-(t-Bu)pz)3)]PF6.6 The structures of these complexes
have been determined by X-ray diffraction analyses. How-
ever, Cu(I) carbonyl complexes that have been structurally
confirmed to contain bis(3,5-disubstituted pyrazolyl) biden-
tate ligands have not been reported. The lowering of the
ligand dentisity from three to two may result in opening a
potential coordination site on the central copper and therefore
increase the reactivity of the Cu(I) complexes with bidentate
pyrazolyl derivatives.7 Structural information on such com-
plexes would be worthy of further pursuit in order to better
understand their chemical and electronic properties.
Copper(I) carbonyl complexes have been extensively
studied because of their importance in bioinorganic chem-
istry1 and their potential use in catalysis.2 Since the structural
characterization of the neutral copper(I) carbonyl complex
[Cu(HB(pz)3(CO))] in 1975,3 where HB(pz)3- ) hydrotris-
(1-pyrazolyl)borate, the ensuing copper(I) carbonyl com-
plexes containing tris(3,5-disubstituted pyrazolyl) tripodal
ligands, in which tunable steric and/or electronic effects are
evident at the 3 and/or 5 positions of the pyrazole ring, have
been reported. Typical examples include [Cu(HB(3,5-(CF3)2-
pz)3)(CO)],4 [Cu(HB(3,5-(i-Pr)2pz)3)(CO)],5 and [Cu(CO)-
In our previous studies8 of the reactions of bis(1-pyra-
zolyl)methane (H2CPz2) with [Cu(CH3CN)4]+, an intriguing
air-stable bisacetonitrile Cu(I) complex, [Cu(H2CPz2)(MeCN)2]-
* To whom correspondence should be addressed. E-mail: chefv003@
scc.ntnu.edu.tw.
† National Taiwan Normal University.
(5) Kitajima, N.; Fujisawa, K.; Fujimoto, C.; Moro-oka, Y.; Hashimoto,
S.; Kitagawa, T.; Toriumi, K.; Tatsumi, K.; Nakamura, A. J. Am.
Chem. Soc. 1992, 114, 1277.
‡ Chang Gung Institute of Technology.
§ Tunghai Christian University.
(1) Kretzer, R. M.; Ghiladi, R. A.; Lebeau, E. L.; Liang, H.-C.; Karlin,
K. D. Inorg. Chem. 2003, 42, 3016, and the references therein.
(2) Solomon, E. I.; Jones, P. M.; Maj, J. A. Chem. ReV. 1993, 93, 2623.
(3) Churchill, M. R.; DeBoer, B. G.; Rotella, F. J.; Abu Salah, O. M.;
Bruce, M. I. Inorg. Chem. 1975, 14, 2051.
(6) Reger, D. L.; Collins, J. E. Organometallics 1996, 15, 2029.
(7) (a) Pasquali, M.; Floriani, C.; Gaetani-Manfredotti, A. Inorg. Chem.
1980, 19, 1191. (b) Pasquali, M.; Marini, G.; Floriani, C.; Gaetani-
Manfredotti, A.; Guastini, C. Inorg. Chem. 1980, 19, 2525.
(8) Chou, C.-C.; Su, C.-C.; Tsai, H.-L.; Lii, K.-H. Inorg. Chem. 2005,
44, 628.
(4) Dias, H. V. R.; Lu, H.-L. Inorg. Chem. 1995, 34, 5380.
6122 Inorganic Chemistry, Vol. 44, No. 17, 2005
10.1021/ic050568e CCC: $30.25
© 2005 American Chemical Society
Published on Web 07/19/2005

Copper(I) Complexes of Bis(3,5-dimethylpyrazol-1-yl)methane
500.13 MHz): δ 8.59 (s, 4H, pyrazine), 6.01 (s, 4H, CH), 5.97 (s,
4H, CH2), 2.38 (s, 12H, CH3), 2.13 (s, 12H, CH3), 1.96 (s, 6H,
NCCH3) ppm. 13C NMR (CD3CN, 125.8 MHz): δ 150.9 (C of
pyrazolyl), 146.3 (C of pyrazine), 142.0 (C of pyrazolyl), 107.2
(CH of pyrazolyl), 58.3 (CH2), 13.7 (CH3), 11.2 (CH3) ppm. Anal.
Calcd for C30H42Cl2Cu2N12O8 (2): C, 40.18; H, 4.72; N, 18.74.
Found: C, 40.52; H, 4.74; N, 18.45.
(ClO4), and an air-sensitive Y-shaped monoacetonitrile Cu-
(I) complex, [Cu(H2CPz2)(MeCN)](ClO4), were prepared.
This finding prompted us to attempt the preparation of a
related monoacetonitrile Y-shaped Cu(I) complex, [Cu(H2-
CPz′)(MeCN)](ClO4) (1), and investigate its reactivity with
2
respect to small molecules such as pyrazine, carbon mon-
oxide, triphenylphosphine, and dioxygen.9 Herein we present
the synthesis, characterization, molecular structures, and
reactions of some of the aforementioned complexes.
[Cu(OClO3)(CO)(H2CPz′)] (3). A suspension of complex 1
2
(0.204 g, 0.50 mmol) in CH2Cl2 (5 mL) was stirred for ∼10 min
while CO is bubbled in at room temperature. The resulting
transparent solution was layered with Et2O (saturated with CO) to
Experimental Section
yield 0.18 g (91%) of white crystals. IR (Nujol): νCO 2108 cm-1
,
1
νClO 1115, 1052 cm-1. H NMR (CD2Cl2, 400.13 MHz): δ 6.03
-
Materials and Methods. All operations were carried out under
N2 by means of standard Schlenk and vacuum-line techniques.
Organic solvents were dried by standard procedures and distilled
under N2 before use. Triphenylphosphine, pyrazine, and CO gas
4
(s, 2H, CH), 6.00 (s, 2H, CH2), 2.39 (s, 6H, CH3), 2.35 (s, 6H,
CH3) ppm. 13C NMR (CD2Cl2, 100.6 MHz): δ 173.9 (CO), 152.4
(C of pyrazolyl), 142.1 (C of pyrazolyl), 107.3 (CH of pyrazolyl),
56.8 (CH2), 14.3 (CH3), 11.3 (CH3) ppm. Anal. Calcd for C12H16-
ClCuN4O5 (3): C, 36.24; H, 4.08; N, 16.17. Found: C, 36.22; H,
3.95; N, 16.24.
were purchased and used as received. The ligand, H2CPz′,10 and
2
the copper(I) starting material, [Cu(MeCN)4](ClO4),11 were prepared
according to literature procedures. Warning: Perchlorate com-
pounds are potentially explosiVe! Extreme care must be taken when
working with perchlorate complexes, and only small quantities
should be handled.
[{Cu(CO)(H2CPz′)}2(µ-pyrazine)](ClO4)2 (4). Method 1. A
2
suspension of complex 2 (0.204 g, 0.50 mmol) in CH2Cl2 (5 mL)
was stirred while CO is bubbled in at room temperature. The CO
bubbling was continued until the solvent was removed completely.
Method 2. To a CH2Cl2 (5 mL) solution of complex 3 (0.160 g,
0.40 mmol) was added, dropwise, a solution of pyrazine (0.017 g,
0.21 mmol) in 5 mL of CH2Cl2 at room temperature for ca. 20 min
under a CO atmosphere. The resulting solution was layered with
Physical Measurements. IR spectra were recorded on a Perkin-
Elmer Paragon 500 IR spectrometer. NMR spectra were obtained
on Bruker Avance 400 (1H, 400; 13C, 100 MHz) and Bruker Avance
500 (1H, 500; 13C, 126 MHz) FT NMR spectrometers using CD2-
1
Cl2 (δ 5.32 ppm for H NMR and δ 54.00 ppm for 13C NMR) or
1
CD3CN (δ 1.94 ppm for H NMR and δ 1.39 ppm for 13C NMR)
Et2O saturated with CO to yield 0.15 g (86%) of pale-yellow
1
crystals. IR (Nujol): νCO 2119 cm-1, νClO 1098, 625 cm-1. H
-
as the lock solvent and internal standard. Mass spectra were acquired
on a Finnigan TSQ 700 spectrometer. Cyclic voltammetric mea-
surements were carried out on a potentiostat (EG&G Potentiostat/
Galvan Model 273) at room temperature in acetonitrile solutions
(10-3 M). The supporting electrolyte was tetra-n-butylammonium
perchlorate (0.1 M). A three-electrode assembly comprised of a
glassy-carbon working electrode, a platinum auxiliary electrode,
and a Ag/AgCl reference electrode was used. All of the measure-
ments were referenced externally to Cp2Fe. Elemental analyses were
performed on a Heraeus CHN-OS Rapid Elemental Analyzer by
the microanalysis laboratories of National Chung Hsin University.
4
NMR (CD2Cl2, 500.13 MHz): δ 8.66 (s, 4H, pyrazine), δ 6.03 (s,
4H, CH), 5.89 (s, 4H, CH2), 2.40 (s, 12H, CH3), 2.32 (s, 12H, CH3)
ppm. 13C NMR (CD2Cl2, 125.8 MHz): δ 173.7 (CO), 152.0 (C of
pyrazolyl), 146.6 (C of pyrazine), 142.4 (C of pyrazolyl), 107.3
(CH of pyrazolyl), 57.2 (CH2), 14.3 (CH3), 11.3 (CH3) ppm. Anal.
Calcd for C28H36Cl2Cu2N10O10 (4): C, 38.63; H, 4.17. Found: C,
38.55; H, 4.11.
[Cu(PPh3)(H2CPz′)](ClO4) (5). Method 1. To a CH2Cl2 (10
2
mL) solution of complex 1 or 3 (0.30 mmol) was added PPh3 (0.08
g, 0.31 mmol) followed by stirring for 30 min at room temperature.
The resulting solution was concentrated and layered with Et2O to
yield 0.18 g (95%) of white crystals.
[Cu(H2CPz′)(MeCN)](ClO4) (1). A 5 mL CH2Cl2 solution of
2
[Cu(MeCN)4]ClO4 (0.164 g, 0.50 mmol) and H2CPz′ (0.102 g,
2
0.50 mmol) was stirred at room temperature for 30 min under N2.
The white precipitates formed were filtered and washed with Et2O
to yield 0.18 g (88%) of product. Single crystals suitable for X-ray
Method 2. To a CH2Cl2 (10 mL) solution of complex 2 or 4
(0.20 mmol) was added PPh3 (0.11 g, 0.42 mmol) followed by
stirring for 30 min at room temperature. The resulting solution was
structure determination were grown from CH2Cl2/hexane. IR
concentrated and layered with Et2O to yield 0.22 g (87%) of white
1
(Nujol): νCN 2275 cm-1; νClO 1097, 623 cm-1. H NMR (CD2-
-
-
1
crystals. IR (Nujol): νClO 1096, 623 cm-1. H NMR (CD2Cl2,
4
4
Cl2, 500.13 MHz): δ 6.07 (s, 2H, CH), 6.01 (s, 2H, CH2), 2.42 (s,
500.13 MHz): δ 7.54-7.45 (m, 15H, phenyl), 6.32 (s, 2H, CH),
5.99 (s, 2H, CH2), 2.49 (s, 6H, CH3), 1.89 (s, 6H, CH3) ppm. 13C
NMR (CD2Cl2, 125.8 MHz): δ 152.26 (C of pyrazolyl), 142.93
(C of pyrazolyl), 134.30 (C of phenyl), 134.18 (C of phenyl), 131.73
(C of phenyl), 131.63 (C of phenyl), 131.40 (C of phenyl), 129.92
(C of phenyl), 129.83 (C of phenyl), 107.54 (CH of pyrazolyl),
57.99 (CH2) 14.19 (CH3), 11.39 (CH3) ppm. Anal. Calcd for C29H31-
ClCuN4O4P (5): C, 55.33; H, 4.93; N, 8.90. Found: C, 55.12; H,
4.86; N, 8.75.
X-ray Crystallography. Crystal data and collection parameters
are listed in Table 1. The selected bond lengths and angles are given
in Table 2. Data collection was carried out on a Bruker SMART
CCD diffractometer, using graphite-monochromated Mo KR radia-
tion. The structures were solved by direct methods using SHELXL-
97,12 completed by subsequent difference Fourier syntheses, and
refined by full-matrix least-squares procedures. All of the non-
hydrogen atoms were refined with anisotropic temperature factors.
6H, CH3), 2.33 (s, 3H, NCCH3), 2.27 (s, 6H, CH3) ppm. Positive
ESI-MS: 307.9 (M+, 100%), 267.4(M+ - NCMe, 8%). Anal. Calcd
for C13H19ClCuN5O4 (1): C, 38.24; H, 4.83; N, 17.15. Found: C,
38.07; H, 4.69; N, 16.88.
[{Cu(H2CPz′)(MeCN)}2(µ-pyrazine)](ClO4)2 (2). To a stirred
2
solution of complex 1 (0.163 g, 0.40 mmol) in 10 mL CH2Cl2 was
added, dropwise, a solution of pyrazine (0.017 g, 0.21 mmol) in 5
mL CH2Cl2 at room temperature. After reaction for 1 h, the resulting
orange-yellow precipitates were filtered and washed with n-hexane
to give 0.16 g (89%) of product. Single crystals suitable for X-ray
structure determination were grown from CH3CN/Et2O. IR (Nu-
1
jol): νCN 2274 cm-1; νClO 1092, 623 cm-1. H NMR (CD3CN,
-
4
(9) Chou, C.-C.; Su, C.-C. To be published.
(10) Julia, S.; Mazo, J. del; Sancho, M.; Ochoa, C.; Elguero, J.; Fayet J.-
P.; Vertut, M.-C. J. Heterocycl. Chem. 1982, 19, 1141.
(11) Kubas, G. J. Inorg. Synth. 1990, 28, 68.
Inorganic Chemistry, Vol. 44, No. 17, 2005 6123

Chou et al.
Table 1. Crystal Structure and Refinement Data for 1-5
1
2
3
4
5
empirical formula
fw
C13H19ClCuN5O4
408.32
C30H42Cl2Cu2N12O8
896.74
C12H16ClCuN4O5
395.28
C28H36Cl2Cu2N10O10
870.65
C29H31ClCuN4O4P
629.54
temp (K)
wavelength (Å)
crystal system
space group
a (Å)
100(2)
200(2)
0.71073
triclinic
100(2)
200(2)
0.71073
triclinic
293(2)
0.71073
monoclinic
P21/c
8.6670(2)
13.0930(2)
14.6660(4)
90
95.1150(10)
90
1659.54(6)
4
0.71073
triclinic
P1h
0.71073
triclinic
P1h
P1h
P1h
7.60200(10)
11.3620(2)
12.1490(3)
83.9930(10)
75.5730(10)
73.3260(10)
972.91(3)
1
7.7670(3)
9.7260(4)
12.1940(5)
112.1980(10)
97.5550(10)
103.242(2)
805.55(6)
2
7.7460(7)
10.5840(9)
11.0030(12)
83.844(3)
83.124(4)
84.848(4)
887.71(15)
1
8.3966(5)
10.3157(7)
18.923(1)
102.178 (1)
96.048 (1)
106.382 (1)
1513.2(2)
2
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
D
calcd (Mg/m3)
1.634
1.506
1.531
1.293
1.630
1.551
1.629
1.417
1.382
0.903
abs coeff (mm-1
)
θ range (deg)
2.09-25.02
2.50-25.00
2.32-25.02
2.56-25.29
1.12-28.25
no. of reflns collected
no. of independent reflns
R(int)
17739
13990
9001
10158
16104
2928
0.0596
3402
0.0487
2797
0.0705
3122
0.1189
7184
0.0687
no. of data/restraints/ params
2928/0/218
1.094
0.0475
0.1390
0.886, -0.732
3402/0/244
1.079
0.0430
0.1251
0.693, -0.489
2797/0/208
1.139
0.0449
0.1230
0.885, -0.519
3122/0/235
1.022
0.0702
0.1527
0.547, -0.591
7184/0/361
0.826
0.0623
0.1471
0.564, -0.441
GOF on F2
R1a,b
wR2a,c
max, min peaks (e Å-3
)
a Observation criterion: I > 2σ(I). b R1 ) ∑(Fo - Fc)/∑Fo. c wR2 ) {∑[w(Fo - Fc)2]/∑[w(Fo)2]}1/2
Table 2. Selected Bond Lengths (Å) and Angles (deg)a
.
of H2CPz′ at room temperature for ca. 30 min under N2.
2
After treatment of 1 with 0.5 equiv of pyrazine in CH2Cl2,
complex 2 was formed as an orange-yellow precipitate in
30 min. The addition of CO to a suspension of 1 or 2 in
CH2Cl2 gave the colorless soluble mononuclear CuI-carbonyl
3 or light yellow dinuclear biscarbonyl 4, respectively. The
uptake of CO was rapid for 1 or 2, as evidenced by the
instantaneous solubilization of the suspension of 1 or 2 upon
the addition of CO. The reverse reactions took place readily
by bubbling N2 into a CH2Cl2 solution of 3 or 4, allowing
the starting 1 or 2 to be recovered. Complex 4 was also
isolated from the reaction of 3 with 0.5 equiv of pyrazine.
When PPh3 was added to 1, no νCN was detected for 5,
consistent with the replacement of MeCN by PPh3. While
PPh3 was added to 3, CO gas was released instantaneously
and no νCO was detected for 5, indicative of the replacement
of CO by PPh3. Similar reactions were observed for 2 and
4. The overall reactions are depicted in Scheme 1.
bond lengths
bond angles
1
2
Cu1-N1
Cu1-N4
Cu1-N5
1.988(3)
2.006(3)
1.879(4)
N1-Cu1-N5
N4-Cu1-N5
N1-Cu1-N4
136.0(1)
128.3(1)
95.2(1)
Cu1-N1
Cu1-N4
Cu1-N5
Cu1-N6
N5-C12
N5-C13
C12-C13A
2.037(3)
2.042(3)
2.040(3)
1.972(3)
1.338(4)
1.339(4)
1.372(5)
N1-Cu1-N4
N1-Cu1-N5
N1-Cu1-N6
N4-Cu1-N5
N4-Cu1-N6
N5-Cu1-N6
94.2(1)
111.9(1)
118.3(1)
113.5(1)
114.6(1)
104.6(1)
3
4
Cu1-N1
Cu1-N4
Cu1-C12
Cu1-O2
C12-O1
2.007(3)
2.001(3)
1.806(5)
2.240(3)
1.124(6)
N1-Cu1-N4
N4-Cu1-C12
N1-Cu1-O2
O2-Cu1-C12
N1-Cu1-C12
N4-Cu1-O2
Cu1-C12-O1
95.3(1)
129.7(2)
103.5(1)
102.4(2)
126.3(2)
92.4(1)
176.8(4)
Cu1-N1
Cu1-N4
Cu1-N5
Cu1-C14
C14-O1
N5-C12
N5-C13
C12-C13A
2.043(6)
2.046(6)
2.056(5)
1.835(8)
1.121(8)
1.334(8)
1.329(8)
1.38(1)
N1-Cu1-N4
N1-Cu1-N5
N1-Cu1-C14
N4-Cu1-N5
N4-Cu1-C14
N5-Cu1-C14
Cu1-C14-O1
93.7(2)
107.3(2)
113.2(3)
107.1(2)
118.4(3)
114.8(3)
171.9(7)
The molecular structure of [Cu(H2CPz′)(MeCN)](ClO4)
2
(1) is shown in Figure 1a. The complex cation adopts a
Y-shaped trigonal planar structure with the copper ion
slightly deviated (0.0779 Å) from the N1-N4-N5 best plane
-
toward the discrete ClO4 at a distance of 3.181 Å. The
5
Cu1-N1
Cu1-N4
Cu1-P1
2.020(6)
2.018(6)
2.171(2)
N1-Cu1-N4
P1-Cu1-N1
P1-Cu1-N4
95.0(2)
132.1(2)
130.3(2)
CuI-N(MeCN) bond distance of 1.879(4) Å is close to those
previously reported for Y-shaped Cu(I) complexes.13 The
mean CuI-N(Pz′) bond distance of 1.997 ( 0.009 Å is
significantly longer than those of Sorrell’s pyrazole-derived
three-coordinate Cu(I) complexes (1.874-1.895 Å),14 which
a Numbers in parentheses are estimated standard deviations of the last
significant figure. Atoms are labeled as indicated in Figures 1 and 2.
All hydrogen atoms were located geometrically and refined in the
riding mode. Additional crystallographic data as CIF files are
available as Supporting Information.
(12) Sheldrick, G. M. SHELXL97, version 97-2; University of Gottingen,
Gottingen, Germany, 1997.
(13) Spencer, D. J. E.; Reynolds, A. M.; Holland, P. L.; Jazdzewski, B.
A.; Duboc-Toia, C.; Pape, L. L.; Yokota, S.; Tachi, Y.; Itoh, S.;
Tolman, W. B. Inorg. Chem. 2002, 41, 6307.
Results and Discussion
(14) (a) Sorrell, T. N.; Malachowski, M. R.; Jameson, D. L. Inorg. Chem.
1982, 21, 3250. (b) Sorrell, T. N.; Malachowski, M. R. Inorg. Chem.
1983, 22, 1883.
The white complex 1 was generated in high yield by the
reaction of [Cu(MeCN)4](ClO4) with an equimolar amount
6124 Inorganic Chemistry, Vol. 44, No. 17, 2005

Copper(I) Complexes of Bis(3,5-dimethylpyrazol-1-yl)methane
Figure 1. ORTEP diagrams of copper(I) complexes (a) 1, (b) cation of 2, (c) 3, and (d) cation of 4 showing 50% probability thermal ellipsoids for all
non-hydrogen atoms.
Scheme 1. Reactions of Copper(I) Complexes 1-5 in CH2Cl2
can be attributed to the different coordination geometry
adopted by Sorrell’s complexes (T-shaped, N(Pz′)-Cu-
N(Pz′) ) 156°). Accordingly, the CuI-N(Pz′) bonds in 1
tend to become weakened. Compound 1 was also character-
ized by means of ESI-MS (+) spectrometry. The m/z (%) at
308 (M+, 100) and 267 (M+-MeCN, 8) indicates the
presence of one-coordinated MeCN. The νCN at 2275 cm-1
(IR, Nujol mull) observed for 1 also suggests the presence
of a coordinated MeCN, consistent with the X-ray structure
of 1. After treatment of 1 with pyrazine, a νCN band at 2274
cm-1 was observed for 2, consistent with the coordinated
MeCN, as evidenced by the X-ray structure of 2 (vide infra).
Inorganic Chemistry, Vol. 44, No. 17, 2005 6125

Chou et al.
Table 3. Selected CO Vibrational and Cu-C Structural Data for Copper(I) Carbonyl Complexes with Pyrazole-Derived Polydentate Ligands
complexes
[Cu{HB(Pz)3}(CO)]
[Cu{HB(3,5-(CF3)2Pz)3}(CO)]
[Cu{HB(3,5-(CH3)2Pz)3}(CO)]
[Cu(CO){HC(3,5-(CH3)2Pz)3}]PF6
[Cu(CO){HC(3,5-(t-Bu)2Pz)3}]PF6
[Cu(OClO3)(CO){H2C(3,5-(CH3)2Pz)2}] (3)
ν
CO, cm-1
Cu-C (Å)
ref
3, 19
4
20
6
6
2083
2137
2066
2113
2100
2108
1.765(14)
1.808(4)
1.778(10)
1.806(5)
this work
The X-ray structure of the centrosymmetric complex
cation of [{Cu(H2CPz′)(MeCN)}2(µ-pyrazine)](ClO4)2 (2)
2
is shown in Figure 1b. Each Cu(I) is coordinated to four N
donor atoms in a distorted tetrahedral geometry. The
CuI-N(Pz′) bonds are slightly elongated by 0.043 Å,
compared with the corresponding mean bond distance of 1.
The CuI-N(MeCN) bond distance of 1.978(3) Å is signifi-
cantly elongated by 0.1 Å, compared with that of 1, but is
nearly identical to those of the Cu-N in [Cu(MeCN)4]+ (1.98
( 0.01 Å).15 The CuI-N(µ-pyrazine) bond distance of 2.042-
(3) Å is close to the literature values for CuII-N(µ-pyrazine)
(2.02-2.06 Å).16 Note that the pyrazine is bonded equato-
rially to the imposed boat conformation of the Cu(N-N)2C
ring, while the acetonitrile is bound axially. The intramo-
lecular and nearest intermolecular Cu-Cu distances are
6.872(1) and 6.831(2) Å, respectively.
Figure 2. ORTEP diagram of the complex cation of 5 showing 30%
probability thermal ellipsoids for all non-hydrogen atoms.
methane copper(I) carbonyl complex, [Cu(CO){HC(3,5-
(CH3)2Pz)3}]PF6.
The X-ray structure of the centrosymmetric complex cation
The molecular structure of [Cu(OClO3)(CO)(H2CPz′)]
2
(3), as shown in Figure 1c, is best described as a trigonally
distorted tetrahedron with two nitrogen atoms of the boat
fragment Cu(N-N)2C, an equatorial carbonyl group, and an
axial oxygen atom from the perchlorate anion bound to the
central CuI. Owing to the strong interaction between the CuI
cation and the perchlorate anion, the copper ion deviates
0.3219 Å from the N1-N4-C12 best plane toward the
ClO4- anion at a distance of 2.240(3) Å, which is very close
to the upper limit of Cu(I)-O single bonds (2.23 Å).14b
Similar to the perchlorate-coordinated Cu(I) complex [Cu-
(OClO3)(CO)(NH(py)2)] [py ) 2-pyridyl, νCO ) 2110 cm-1,
Cu-C(CO) ) 1.808(2) Å],17 the splitting perchlorate IR
bands of 3 appeared at 1052 and 1115 cm-1, suggesting a
unidentate coordination mode, in agreement with the X-ray
data. The CO stretching frequency at 2108 is in agreement
with a terminal carbonyl in 3. It is noteworthy that the NH-
(py)2 (pKa ) 5.3 for Hpy)18 would be expected to be a better
of [{Cu(CO)(H2CPz′)}2(µ-pyrazine)](ClO4)2 (4), as shown
2
in Figure 1d, is quite different from that of 2. The bridging
pyrazine binds to the boat fragments in 4 in an axial
orientation. Similar to complex 3, the carbonyl groups are
situated at equatorial sites. However, the grossly longer
CuI-C(CO) bond distance of 1.835(8) Å and the consider-
ably high CO stretching frequency at 2119 cm-1 suggest a
poorer back π-bonding from Cu to CO in 4. Evidently, this
is caused by dπ(Cu) to pπ*(pyrazine) back π-bonding.
Moreover, compared with complex 2, the CuI-N(pyrazine)
bonds in 4 are slightly elongated by 0.016 Å and the average
bond distance among the pyrazine ring is virtually unaffected,
suggesting that the dπ(Cu) to pπ*(pyrazine) back π-bonding
in 4 is somewhat lessened. This can be attributed the
difference in bonding properties of MeCN and CO, i.e.,
MeCN is a moderately strong σ-donor and a very weak
π-acceptor, whereas CO is a weak σ-donor and strong
π-acceptor.6
σ-donor than H2CPz′ (pKa ) 2.5 for HPz),18 and therefore,
2
the CuI-O(ClO4) bond distance in 3 would be expected to
be shorter than that in [Cu(OClO3)(CO)(NH(py)2)] (2.429-
(1) Å).17 The perchlorate anion plays a key role in a subtle
electronic balance required for the central CuI upon binding
the carbonyl molecules. Accordingly, the bond distance of
Cu-C(CO) and stretching frequencies observed for these two
complexes are nearly equal. Evaluation of the N-donor ability
The molecular structure of the complex cation of [Cu-
(PPh3)(H2CPz′)](ClO4) (5) is shown in Figure 2. The
2
complex cation adopts a Y-shaped trigonal planar structure
with the copper ion slightly deviated (0.186 Å) from the N1-
-
N4-P1 best plane toward the discrete ClO4 at a distance
of 2.574 Å.21 The coordination sphere of the central copper-
(I) is similar to that of the related three-coordinate copper-
(I) complex [Cu((mpz)2B(mpz)2)(P(cy)3)].22 (mpz ) 3-me-
of the bidentate ligand H2CPz′ through the CO stretching
2
frequency is perturbed by the coordinated perchlorate anion.
As shown in Table 3, the value of νCO for 3 is unexpectedly
lower than that of the related tris(3,5-dimethylpyrazolyl)-
(19) Bruce, M. I.; Ostazewski, A. P. P. J. Chem. Soc., Dalton Trans. 1973,
2433.
(20) Mealli, C.; Arcus, C. S.; Wilkinson, J. L.; Marks, T. J.; Ibers, J. A. J.
Am. Chem. Soc. 1976, 98, 711.
(21) (a) Karlin, K. D.; Haka, M. S.; Cruse, R. W.; Meyer, G. J.; Farooq,
A.; Gultneh, Y.; Hayes, J. C.; Zubieta, J. J. Am. Chem. Soc. 1998,
110, 1196. (b) Patch, M. G.; Choi, H.; Chapman, D. R.; Bau, R.;
McKee, V.; Reed, C. A. Inorg. Chem. 1990, 29, 110.
(15) Csoregh, I.; Kierkegaard, P.; Norrestam, R. Acta Crystallogr. 1975,
B31, 314.
(16) Oshio, H. J. Chem. Soc., Dalton Trans. 1990, 2985.
(17) Thompson, J. S.; Whitney, J. F. Inorg. Chem. 1984, 23, 2813.
(18) Sorrell, T. N. Tetrahedron 1989, 45, 3.
6126 Inorganic Chemistry, Vol. 44, No. 17, 2005

Copper(I) Complexes of Bis(3,5-dimethylpyrazol-1-yl)methane
Table 4. CV Data and Comproportionation Constants for MeCN-Dicopper(I) Complexes
a
complex
E1/2
Kcom
solvent
MeCN
ref
[Cu2(L2)(MeCN)2](ClO4)2
+140
+568
+41
+516
+164
+548
2 × 107
1 × 108
3 × 106
28
29
b
c
[Cu2(Ph4bdptz)(MeCN)2](OTf)2
[Cu2(H2CPz′)2(MeCN)2(µ-pyrazine)](ClO4)2 (2)
MeCN
MeCN
this work
2
a mV (vs Cp2Fe+/Cp2Fe). b L2 ) 5,5,16,16-tetramethyl-23,24-dioxa-3,7,14,18-tetraazatricyclo[18.2.1.19,12]tetracosa-1(22),2,7,9,11,13,18,20-octaene.
c Ph4bdptz ) 1,4-bis[bis(6-phenyl-2-pyridyl)methyl]phthalazine. OTf ) triflate.
thylpyrazol-1-yl; cy ) cyclohexyl) The low basicity of the
neutral bidentate H2CPz′ ligand results in a large bite angle
2
of H2CPz′ [95.0(2)°] and a short Cu-P bond distance
2
(2.171 Å) for complex 5, to comply with the electronic and
geometrical requirements of the nearly planar three-
coordinate geometry. The formation of the three-coordinated
complex stems mainly from the good σ-donating and
π-accepting properties of the PPh3. The steric repulsion
between the methyl groups of the H2CPz′ ligand and the
2
bulky PPh3 leads to the slight lengthening of the average
Cu-N bond distances (2.019 Å) and, consequently, the
suppressing of the bite angle of 5 as compared with those
of the other two mononuclear copper(I) complexes, 1 [95.2-
(1)°] and 3 [95.3(1)°].
Figure 3. Cyclic voltammogram of 2 in MeCN (1 × 10-3 M). Scan rate
The redox properties of 1, 2, and 5 were examined by
) 20 mV/s, electrolyte ) (Bu4N)(ClO4) (0.1 M).
cyclic voltammetry in MeCN. Compounds 1 and 5 exhibited
a quasireversible wave (cf. Figures S1 and S2) with E1/2
potentials of +187 mV (∆Ep ) 202 mV) and +224 mV (∆Ep
) 276 mV), respectively. Compared to other Y-shaped
CuI-N(MeCN) complexes with the strong electron-donating
â-diketiminate ligands (-122 to -242 mV versus Fc/Fc+),13
the E1/2 potential of compound 1 is apparently more positive
and closer to that of the three-coordinate CuI-N(MeCN)
complex with a neutral N-donor ligand, [Cu2(L2)(MeCN)2]-
(ClO4)2, as shown in Table 4. Evidently, the redox potentials
of Cu(I) complexes are greatly influenced by the coordinated
bidentate ligand and the coligands as well. While the anionic
coligands of â-diketiminate-copper complexes depress the
E1/2 potentials,13 the neutral coligand PPh3 of compound 5
raises the E1/2 value.
Two well-separated quasireversible waves (Figure 3) with
E1/2 potentials of +164 mV (∆Ep ) 174 mV) and +548 mV
(∆Ep ) 226 mV) for complex 2 suggest that no significant
structural reorganization occurs during the redox process on
the electrochemical time scale. These waves are believed to
be associated with the CuICuI to CuICuII and CuICuII to
CuIICuII processes. The high redox potentials comparable
with the compounds listed in Table 4 are probably due to
the weak N-donor bidentate ligand, its “environment ef-
fect”,23 and the stabilization effect of the MeCN solvent
toward Cu(I),29 making the oxidation to Cu(II) more difficult.
106. This corresponds to an additional stability for the mixed
valence ion of 4.4 kcal mol-1, with respect to its isovalent
species. The contributions to the stability of the mixed
valence compounds have been discussed previously on the
basis of the interaction between metal centers, MLCT back-
bonding stabilization, and the energy difference between the
mixed valence and isovalent states due to electrostatics.25,26
In the present system, the electrostatic factor is rather small
because the binuclear complexes are made up of moieties
of like charges. The electrostatic contribution, as calculated
from the appropriate ion pair formation constants,27 is 0.18
kcal mol-1. Apparently, the stabilization of the mixed valence
ion arises predominantly from electronic delocalization and
dπ-π* back-bonding. Unfortunately, the mixed-valence
species could not be obtained as of this writing. In fact, such
a situation is similar to those reported for the bis-MeCN
dicopper(I) compounds listed in Table 4. Their Kcom values
are even larger than the present one.
In summary, we report herein on a novel three-coordinate
Cu(I)-NCMe complex, [Cu(H2CPz′)(MeCN)](ClO4) (1), and
2
a perchlorate coordinated Cu(I)-carbonyl complex, [Cu-
(OClO3)(CO)(H2CPz′)] (3), which were first explored in
2
(24) The value of Kcom was obtained from ∆E1/2 (0.384 V) via the
equation: ∆E1/2 ) 0.0591 log Kcom. Gagne, R. R.; Koval, C. A.; Smith,
T. J.; Cimolino, M. C. J. Am. Chem. Soc. 1979, 101, 4571.
(25) Sutton, J. E.; Taube, H. Inorg. Chem. 1981, 20, 3125.
(26) Yeh, A.; Haim, A. J. Am. Chem. Soc. 1985, 107, 369.
(27) Miralles, A. J.; Armstrong, R. E.; Haim, A. J. Am. Chem. Soc. 1977,
99, 1416.
(28) Yates, P. C.; Drew, M. G. B.; Trocha-Grimshaw, J.; McKillop, K. P.;
Nelson, S. M.; Ndifon, P. T.; McAuliffe, C. A.; Nelson, J. J. Chem.
Soc., Dalton Trans. 1991, 1973.
Kcom
CuICuIL2+ + CuIICuIIL4+ y z 2CuICuIIL3+
(1)
The comproportionation constant for eq 1,24 as calculated
from the appropriate reduction potentials, is Kcom ) 3.1 ×
(22) Pellei, M.; Pettinari, C.; Santini, C.; Skelton, B. W.; Somers, N.; White,
A. H. J. Chem. Soc., Dalton Trans. 2000, 3416.
(23) Sorrell, T. N.; Jameson, D. L. Inorg. Chem. 1982, 21, 1014.
(29) Kuzelka, J.; Mukhopadhyay, S.; Spingler, B.; Lippard, S. J. Inorg.
Chem. 2004, 43, 1751.
Inorganic Chemistry, Vol. 44, No. 17, 2005 6127

Chou et al.
the synthesis of the symmetric dicopper(I) complexes
different types of dinuclear and polynuclear copper(I) car-
bonyl complexes for potential applications in catalysis is
currently in progress.
[{Cu(H2CPz′)(MeCN)}2(µ-pyrazine)](ClO4)2 (2) and [{Cu-
2
(CO)(H2CPz′)}2(µ-pyrazine)](ClO4)2 (4), although binucle-
2
ating ligands30 or macrocyclic polydentate ligands31 are
commonly employed in the preparation of dicopper(I)
complexes. In fact, complex 3 is a promising precursor,
namely a source of [Cu(CO)]+, for the preparation of
biscarbonyl dicopper(I) species, for example, [{Cu(CO)(H2-
Acknowledgment. Financial support of the National
Science Council of the Republic of China is highly appreci-
ated. We thank Prof. Dr. W. -Z. Lee for valuable discussion
and assistance on standard Schlenk line manipulations and
Mr. T.-S. Kuo for collecting X-ray data. Our gratitude also
goes to the Academic Paper Edition Clinic, NTNU.
CPz′)}2(µ-4,4′-bpy)](ClO4)2(CH2Cl2)2.32 An exploration of
2
(30) (a) Karlin, K. D.; Tykela´r, Z.; Farooq, A.; Haka, M. S.; Ghosh, P.;
Cruse, R. W.; Gultneh, Y.; Hayes, J. C.; Toscano, P. J.; Zubieta, J.
Inorg. Chem. 1992, 31, 1436. (b) Lee, D.-H.; Wei, N.; Tykela´r, Z.;
Murthy, N. N.; Zubieta, J.; Karlin, K. D.; Kaderli, S.; Jung, B.;
Zuberbu¨hler, A. D. J. Am. Chem. Soc. 1995, 117, 12498. (c) Kodera,
M.; Katayama, K.; Tachi, Y.; Kano, K.; Hirota, S.; Fujinami, S.;
Suzuki, M. J. Am. Chem. Soc. 1999, 121, 11006.
Supporting Information Available: X-ray crystallographic data
in CIF format for compounds 1-5 and CV diagrams of 1 (Figure
S1) and 5 (Figure S2). This material is available free of charge via
the Internet at http://pubs.acs.org.
(31) (a) Rockcliffe, D. A.; Martell, A. E. Inorg. Chem. 1993, 32, 3143. (b)
Utz, D.; Heinemann, F. W.; Hampel, F.; Richens, D. T.; Schindler, S.
Inorg. Chem. 2003, 42, 1430. (c) Costa, M.; Xifra, R.; Llobet, A.;
Sola`, M.; Robles, J.; Parella, T.; Stoeckli-Evans, H.; Neuburger, M.
Inorg. Chem. 2003, 42, 4456.
IC050568E
(32) Chou, C.-C.; Su, C.-C. Unpublished results.
6128 Inorganic Chemistry, Vol. 44, No. 17, 2005