Dinuclear copper(I) complexes of tris(3,5-dimethylpyrazol-1-yl)methane: Synthesis, structure, and reactivity

Article abstract of DOI:10.1016/j.jorganchem.2007.05.006

The reaction of [Cu(NCMe)₄](BF₄) with equimolar amounts of the tris(substituted-pyrazolyl)methane ligand HCPz₃ or HC(3,5-Me₂Pz)₃ yields the respective salts [Cu(HCPz₃)(NCMe)](BF₄) (1a) or [Cu(HC(3,5-Me₂Pz)₃)(NCMe)](BF₄) (1). The acetonitrile ligand of 1 can be replaced by prazine, 4,4′-dipyridine or 1,4-diisocyanobenzene to yield related mononuclear complexes [Cu(HC(3,5-Me₂Pz)₃)(pyrazine)](BF₄) (2), [Cu(HC(3,5-Me₂Pz)₃)(4,4′-bipyridine)] (BF₄) (3) or [Cu(HC(3,5-Me₂Pz)₃)(1,4- CNC₆H₄NC)](BF₄) (7), respectively. A series of binuclear copper(I) complexes {[Cu(HC(3,5-Me₂Pz)₃)]₂(μ -BL)}(BF₄)₂ (4, BL = pyrazine; 5, BL = 4,4′-dipyridine; 8, BL = 1,4-diisocyanobenzene) were prepared by treating equal molar ratio of 1 with related mononuclear complexes 2, 3 and 7. In addition, binuclear copper(I) complexes were also prepared from treatment of 2 equiv of 1 with the related bridge ligand. Both of 4 and 5 reformed mononuclear starting complex 1 in acetonitrile solution. However, the more robust complex 8 was stable in acetonitrile solutions. The structure of complexes 1a, 4, 5, and 7 were confirmed by X-ray crystallography. The redox properties of 4 and 8 were examined by cyclic voltammetry and exhibited two quasi-reversible waves suggesting that no significant structural reorganization occurs during the redox process on the electrochemical time scale.

Full text of DOI:10.1016/j.jorganchem.2007.05.006

Journal of Organometallic Chemistry 692 (2007) 3676–3684
www.elsevier.com/locate/jorganchem
Dinuclear copper(I) complexes of tris(3,5-dimethylpyrazol-1-yl)
methane: Synthesis, structure, and reactivity
*
Sodio C.N. Hsu , Howard H.Z. Chen, I-Jung Lin, Jung-Jung Liu, Po-Yu Chen
Faculty of Medicinal and Applied Chemistry, Kaohsiung Medical University, Kaohsiung 807, Taiwan
Received 6 March 2007; received in revised form 18 April 2007; accepted 1 May 2007
Available online 8 May 2007
Abstract
The reaction of [Cu(NCMe)4](BF4) with equimolar amounts of the tris(substituted-pyrazolyl)methane ligand HCPz3 or HC(3,5-
Me2Pz)3 yields the respective salts [Cu(HCPz3)(NCMe)](BF4) (1a) or [Cu(HC(3,5-Me2Pz)3)(NCMe)](BF4) (1). The acetonitrile ligand
of 1 can be replaced by prazine, 4,40-dipyridine or 1,4-diisocyanobenzene to yield related mononuclear complexes [Cu(HC(3,5-
Me2Pz)3)(pyrazine)](BF4) (2), [Cu(HC(3,5-Me2Pz)3)(4,40-bipyridine)] (BF4) (3) or [Cu(HC(3,5-Me2Pz)3)(1,4- CNC6H4NC)](BF4) (7),
respectively. A series of binuclear copper(I) complexes {[Cu(HC(3,5-Me2Pz)3)]2(l -BL)}(BF4)2 (4, BL = pyrazine; 5, BL = 4,40-dipyri-
dine; 8, BL = 1,4-diisocyanobenzene) were prepared by treating equal molar ratio of 1 with related mononuclear complexes 2, 3 and
7. In addition, binuclear copper(I) complexes were also prepared from treatment of 2 equiv of 1 with the related bridge ligand. Both
of 4 and 5 reformed mononuclear starting complex 1 in acetonitrile solution. However, the more robust complex 8 was stable in aceto-
nitrile solutions. The structure of complexes 1a, 4, 5, and 7 were confirmed by X-ray crystallography. The redox properties of 4 and 8
were examined by cyclic voltammetry and exhibited two quasi-reversible waves suggesting that no significant structural reorganization
occurs during the redox process on the electrochemical time scale.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Copper; Diisocyanobenzene; Tris(3,5-dimethylpyrazol-1-yl)methane
1. Introduction
of some copper(I)-containing proteins were obtained by
X-ray crystallography or NMR spectroscopy [6–9]. In
those structures, tris(pyrazolyl)methane, hydrotris(pyraz-
olyl)borate and other N3 ligand donor sets coordinate to
the copper(I) centers are employed because of their facially
coordinating ability [6,8–11]. Our interest is in developing
copper(I) complexes containing nitrogen donor tripodal
ligand sets [12], which are very important in bioinorganic
chemistry area. Here, we report a series of novel mononu-
clear and binuclear copper(I) complexes containing
tris(3,5-dimethylpyrazol-1-yl)methane ligand related to
the fundamental copper coordination chemistry.
During the past few decades, the structures and func-
tions of some copper proteins have been elucidated, and
the results of these studies have been recognized as among
the most remarkable advances in biochemistry and bioin-
organic chemistry [1,2]. Additionally, synthesis of low
molecular weight models for copper containing proteins
and their reactions with small molecule substrates are an
important approach to understand catalytic mechanism
of copper enzymes [3–5]. Model complexes not only pro-
vide a better understanding of the biological molecules
but also assist in the development of new homogeneous
catalysts for selective oxidations under mild conditions
and also provide fundamental knowledge in copper(I)
coordination chemistry which is necessary for a deeper
understanding of copper enzymes. The model structures
2. Experimental
2.1. Materials
*
All manipulations were carried out under an atmosphere
of purified dinitrogen with standard Schlenk techniques.
Corresponding author. Tel.: +886 7 3121101x6984; fax: +886 7 3125339.
E-mail address: sodiohsu@kmu.edu.tw (S.C.N. Hsu).
0022-328X/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2007.05.006

S.C.N. Hsu et al. / Journal of Organometallic Chemistry 692 (2007) 3676–3684
3677
Chemical reagents were purchased from Aldrich Chemical
Company Ltd., Lancaster Chemicals Ltd., or Fluka Ltd.
All the reagents were used without further purification,
apart from all solvents that were dried over Na (Et2O, hex-
ane, THF) or CaH2 (CH2Cl2, CH3CN) and then thor-
oughly degassed before use. [Cu(NCMe)4](BF4), HCPz3,
and HC(3,5-Me2pz)3 were prepared according to the liter-
ature procedures [13,14].
[Cu(HC(3,5-Me2Pz)3)]+. The spectroscopy data of this
compound has been reported by Reger et al. [10].
2.5. [Cu(HC(3,5-Me2Pz)3)(pyrazine)](BF4) (2)
A solution of 1 (0.505 g, 1.03 mmol) in 40 mL methanol
and 10 mL acetone was added to a solution of pyrazine
(0.482 g, 5.15 mmol) in 20 mL methanol followed by stir-
ring for 3 h at room temperature. The reaction mixture
solution was reduced to <8 mL and 20 mL of ether was
added followed by to yield yellow precipitates. The yellow
precipitates were filtered and washed with ether to yield
2.2. Measurements
IR spectra were recorded on a Perkin–Elmer System
2000 FT-IR spectrometer. UV–Visible spectra were
recorded on a Jasco V-560 spectrophotometer using 1 cm
path length quartzcell. 1H NMR spectra were acquired
on a Varian Gemini-200 proton/Carbon FT NMR or a
Varian Gemini-500 proton/Carbon FT NMR spectrome-
ter. ESI mass spectra were collected on a Bruker BioTOF
Q Quadrupole-TOF mass spectrometer. Fast-atom bom-
bardment (FAB) mass spectra were recorded by using a
VG Blotch-5022 mass spectrometer. Elemental analyses
were performed on a Heraeus CHN-OS Rapid Elemental
Analyzer. Cyclic voltammetry were measured at a scan rate
of 100 mV/s on 10ꢀ3 M MeCN solutions using 0.1 M
(Bu4N)(OTf) as supporting electrolyte and referenced to
Fc+/0. A platinum wire counter electrode, a glassy carbon
working electrode, and an Ag/AgPF6 (MeCN) reference
electrode were used.
1
0.380 g (70%) of product. H NMR (d6-acetone): d 8.991
(s, 4H, pyrazine), 8.051 (s, 1H, HC(3,5-Me2Pz)3), 6.147
(s, 3H, 4-H Pz), 2.699 (s, 9H, 3,5Me-Pz), 2.248 (s, 9H,
3,5Me-Pz ). Positive FAB-Mass: m/z = 433 [Cu(HC(3,5-
Me2Pz)3)(pyrazine)]+, 361 [Cu(HC(3,5-Me2Pz)3)]+. Anal.
Calc. for C20H26BCuF4N8: C, 45.42; H, 4.96; N, 21.19.
Found: C, 45.05; H, 5.08; N, 21.15%.
2.6. [Cu(HC(3,5-Me2Pz)3)(4,40-dipyridine)](BF4) (3)
A solution of 1 (0.503 g, 1.02 mmol) in 10 mL acetone
was added to a solution of 4,40-dipyridine (0.819 g,
5.11 mmol) in 20 mL methanol followed by stirring for
3 h at room temperature. The reaction mixture solution
was reduced to <8 mL and 20 mL of ether was added fol-
lowed by to yield yellow precipitates. The yellow precipi-
tates were filtered and washed with ether to yield 0.463 g
1
2.3. [Cu(HCPz3)(NCMe)](BF4) (1a)
(75%) of product. H NMR (d6-acetone): d 8.990 (s, 4H,
4,40-dipyridine), 8.073 (s, 4H, 4,40-dipyridine), 8.072 (s,
1H, HC(3,5-Me2Pz)3), 6.154 (s, 3H, 4-H Pz), 2.706 (s,
9H, 3,5Me-Pz), 2.244 (s, 9H, 3,5Me-Pz). Positive FAB-
Mass: m/z = 517 [Cu(HC(3,5-Me2Pz)3)(4,40-dipyridine)]+,
361 [Cu(HC(3,5-Me2Pz)3)]+. Anal. Calc. for C26H30BCu-
F4N8: C, 51.62; H, 5.00; N, 18.52. Found: C, 51.14; H,
5.05; N, 18.53%.
A
solid mixture of [Cu(NCMe)4](BF4) (0.31 g,
0.80 mmol) and HCPz3 (0.17 g, 0.80 mmol) was charged
in a 100 mL flask and suspended in CH2Cl2 (5 mL). After
it was stirred for 1 h, the reaction mixture was treated with
hexanes (20 mL). The mixture was filtered, and the remain-
ing 0.24 g (81%) pale yellow solid and dried under vacuum.
1H NMR (d6-acetone): d 2.45 (s, 3H, CH3CN), 6.52 (dd,
3H, HCPz3 ), 7.91 (dd, 3H, HCPz3 ), 8.35 (dd, 3H,
HCPz3), 9.33 (s, 1H, HCPz3); ESI-Mass: m/z = 318.1
[Cu(HCPz3)(NCMe)]+, 277.2 [Cu(HCPz3)]+. Anal. Calc.
for C12H13BCuF4N7: C, 35.53; N, 24.17; H, 3.23. Found:
C, 35.53; N, 23.77; H, 3.28%.
2.7. [Cu2(HC(3,5-Me2Pz)3)2 (l-pyrazine)](BF4)2 (4)
Method A. A solution of 1 (0.258 g, 0.553 mmol) in
30 mL acetone was added to a solution of 2 (0.278 g,
0.55 mmol) in 60 mL methanol and followed by stirring
for 3 h at room temperature. The reaction mixture solution
was reduced to 3 mL to yield yellow precipitates. The yel-
low precipitates were filtered and washed with ether to
yield 0.386 g (71%) of product. Single crystals suitable for
X-ray structure determination were grown from acetone/
ether. 1H NMR (d6-acetone): d 9.038 (s, 4H, pyrazine),
8.039 (s, 2H, HC(3,5-Me2Pz)3), 6.139 (s, 6H, 4-H Pz),
2.692 (s, 18H, 3,5Me-Pz), 2.24 (s, 18H, 3,5Me-Pz). Anal.
Calc. for C36H48B2Cu2F8N14: C, 44.23; H, 4.95; N, 20.06.
Found: C, 44.54; H, 4.85; N, 19.83%.
2.4. [Cu(HC(3,5-Me2Pz)3)(NCMe)](BF4) (1)
A solution of HC(3,5-Me2Pz)3 (0.298 g, 1.0 mmol) in
10 mL acetone was added to a solution of [Cu(NC-
Me)4](BF4) (0.375 g, 1.0 mmol) in 20 mL acetone followed
by stirring for 3 h at room temperature. The reaction mix-
ture solution was reduced to <8 mL and 40 mL of ether
was added followed by to yield white precipitates. The white
precipitates were filtered and washed with ether to yield
0.376 g (78%) of product. H NMR (d6-acetone): d 2.50
(s, 3H, CH3CN), 2.30, 2.65 (s, s; 9H, 9H; 3,5Me-Pz), 6.15
(s, 3H, 4-H Pz ), 7.91 (s, 1H, HC(3,5-Me2Pz)3); ESI-Mass:
1
Method B. A solution of 1 (0.553 g, 1.0 mmol) in 30 mL
acetone was added to a solution of pyrazine (0.04 g,
0.5 mmol) in 20 mL methanol and followed by stirring
for 3 h at room temperature. The reaction mixture solution
m/z = 402.5
[Cu(HC(3,5-Me2Pz)3)(NCMe)]+,
361.2

3678
S.C.N. Hsu et al. / Journal of Organometallic Chemistry 692 (2007) 3676–3684
was reduced to 5 mL to yield yellow precipitates. The yel-
low precipitates were filtered and washed with ether to
yield 0.325 g (66%) of product.
trile): d 7.759 (s, 6H, 1,4-CNC6H4NC and HC(3,5-
Me2Pz)3), 6.076 (s, 6H, 4-H Pz), 2.513 (s, 18H, 3,5Me-
Pz), 2.353 (s, 18H, 3,5Me-Pz). 13C NMR (d3-acetonitrile):
d 152.349 (s, C of 3,5Me-Pz), d 148.691 (s, br, 1,4
ꢀ CNC6H4NC), d 142.697 (s, C of 3,5Me-Pz), d 129.313
(s, 1,4-CNC6H4NC), d 128.553 (s, 1,4-CNC6H4NC), d
107.553 (s, C of 3,5Me-Pz), d 68.566 (s, HC(3,5-Me2Pz)3),
d 13.924 (s, 3,5Me-Pz), d 11.216 (s, 3,5Me-Pz). Positive
FAB-Mass: m/z = 937 [(Cu(HC(3,5-Me2Pz)3))2(1,4-CNC6-
H4NC)]2+ ðBF4ꢀÞ, 426 [(Cu(HC(3,5-Me2Pz)3))2(1,4-
CNC6H4NC)]2+, 489 [Cu(HC(3,5-Me2Pz)3)(1,4-CNC6H4-
NC)]+, 361 [Cu(HC(3,5-Me2Pz)3)]+. IR(nujol) m(CN) =
2158 cmꢀ1. Anal. Calc. for C40H48B2Cu2F8N14: C, 46.84;
H, 4.72; N, 19.12. Found: C, 46.65; H, 4.70; N, 19.32%.
Method B. A solution of 1,4-diisocyanobenzene (0.012 g,
0.094 mmol) in 20 mL acetonitrile was added to a solution
of 1 (0.109 g, 0.22 mmol) in 20 mL acetonitrile followed by
stirring for 2 h at room temperature. The reaction mixture
solution was reduced to <8 mL to yield white precipitates.
The white precipitates were filtered and washed with ether
to yield 0.042 g (40%) of product.
2.8. [Cu2(HC(3,5-Me2Pz)3)2 (l-4,40-dipyridine)](BF4)2
(5)
Method A. A solution of 1 (0.134 g, 0.286 mmol) in
20 mL acetone was added to a solution of 3 (0.173 g,
0.286 mmol) in 60 mL methanol and followed by stirring
for 3 h at room temperature. The reaction mixture solution
was reduced to 5 mL to produce yellow precipitates. The
yellow precipitates were collected and washed with ether
to yield 0.201 g (67%) of product. Single crystals suitable
for X-ray structure determination were grown from ace-
1
tone/ether. H NMR (d6-acetone): d 9.180 (s, 4H, 4,40-
dipyridine), 8.258 (s, 4H, 4,40-dipyridine), 8.073 (s, 2H,
HC(3,5-Me2Pz)3), 6.156 (s, 6H, 4-H Pz), 2.704 (s, 18H,
3,5Me-Pz), 2.248 (s, 18H, 3,5Me-Pz). Anal. Calc. for
C42H52B2Cu2F8N14: C, 47.88; H, 4.97; N, 18.61. Found:
C, 47.99; H, 5.06; N, 18.52%.
Method B. A solution of 1 (0.280 g, 0.573 mmol) in
30 mL acetone was added to a solution of 4,40-dipyridine
(0.045 g, 0.288 mmol) in 20 mL methanol and followed
by stirring for 3 h at room temperature. The reaction mix-
ture solution was reduced to 5 mL to yield yellow precipi-
tates. The yellow precipitates were filtered and washed with
ether to yield 0.191 g (63%) of product.
2.11. Crystallography
All crystals were mounted on a thin glass fiber by using
oil (Paratone-N, Exxon) before being transferred to the dif-
fractometer. Diffraction data for [Cu(HCPz3)(NCMe)]
(BF4) (1a), [Cu2(HC(3,5-Me2Pz)3)2(l- pyrazine)](BF4)2
(4), [Cu2(HC(3,5-Me2Pz)3)2(l-4,40- dipyridine)](BF4)2 (5),
and [Cu(HC(3,5-Me2Pz)3)(1,4- CNC6H4NC)](BF4) (7)
were collected at 150(3) or 200(2) K on a Bruker Nonius
Kappa CCD diffractometer (Mo Ka radiation,
2.9. [Cu(HC(3,5-Me2Pz)3)(1,4-CNC6 H4NC)](BF4) (7)
A solution of 1 (0.250 g, 0.51 mmol) in 20 mL acetone
was added to a solution of 1,4-diisocyanobenzene (0.131
g, 1.02 mmol) in 20 mL acetone followed by stirring for
3 h at room temperature. The white precipitates formed
were filtered and washed with ether to yield 0.179 g (17%)
of product. Single crystals suitable for X-ray structure
˚
k = 0.71073 A). Data processing was performed with the
integrated program package SHELXTL [15]. All structures
were solved using direct methods and refined using full-
matrix least squares on F2 using the program SHELXL-97
[16]. All non-hydrogen atoms were refined anisotropically.
Hydrogen atoms were placed using a riding model and
included in the refinement at calculated positions. The data
were corrected for absorption on the basis of W scans. A
summary of relevant crystallographic data for 1a, 4, 5,
and 7 is provided in Table 1.
1
determination were grown from acetone/ether. H NMR
(d6-acetone): d 8.035 (s, 1H, HC(3,5-Me2Pz)3), 7.801 (dd,
2H, 1,4-CNC6H4NC), 7.717 (s, 2H, 1,4-CNC6H4NC),
6.117 (s, 3H, 4-H Pz), 2.699 (s, 9H, 3,5Me-Pz), 2.289 (s,
9H, 3,5Me-Pz). Positive FAB-Mass: m/z = 488 [Cu(HC-
(3,5-Me2Pz)3)(1,4-CNC6H4NC)]+, 361 [Cu(HC(3,5-Me2-
Pz)3)]+. IR(nujol) m(CN) = 2150 cmꢀ1. Anal. Calc. for
C24H27BCuF4N8: C, 49.88; H, 4.71; N, 19.39. Found: C,
49.65; H, 4.66; N, 19.28%.
3. Results and discussion
The copper(I) complex [Cu(HC(3,5-Me2Pz)3)(NC-
Me)](BF4) (1) and [Cu(HCPz3)(NCMe)](BF4) (1a) were
generated in high yield by the reaction of [Cu(NC-
Me)4](BF4) with an equimolar amount of HC(3,5-Me2Pz)3
and HCPz3 at room temperature for ca. 60 min under N2.
The molecular structure of the complex cation of
[Cu(HCPz3)(NCMe)](BF4) (1a) is shown in Fig. 1. The
coordination geometry around the copper atom is best
described as a distorted tetrahedron with a facial tridentate
ligand chelated. The intraligand N–Cu–N angles are
restrained by the chelate rings to 87.83ꢁ, and the average
angles from nitrogen atoms of acetonitrile donor to the
2.10. [Cu2(HC(3,5-Me2Pz)3)2 (l-1,4-
CNC6H4NC)](BF4)2 (8)
Method A. A solution of 1 (0.140 g, 0.286 mmol) in
20 mL acetone was added to a solution of 7 (0.165 g,
0.286 mmol) in 60 mL methanol and followed by stirring
for 2 h at room temperature. The reaction mixture solution
was reduced to 1 mL to produce white precipitates. The
white precipitates were collected and washed with ether
1
to yield 0.202 g (69%) of product. H NMR (d3-acetoni-

S.C.N. Hsu et al. / Journal of Organometallic Chemistry 692 (2007) 3676–3684
3679
Table 1
Crystallographic data for [Cu(HCPz3)(NCMe)](BF4) (1a), [Cu2(HC(3,5-Me2Pz)3)2(l-pyrazine)](BF4)2 (4), [Cu2(HC(3,5-Me2Pz)3)2(l-4,40- dipyri-
dine)](BF4)2 (5), and [Cu(HC(3,5-Me2Pz)3)(1,4- CNC6H4NC)](BF4) (7)
1a
4
5
7
Chemical formula
Crystal system
Space group
Fw
C24H26B2Cu2F8N14
Monoclinic
P21/c
C42H60B2Cu2F8N14O2
Triclinic
C44H56B4Cu2F14N14O2
Monoclinic
C2/c
1249.35
27.2373(3)
7.69130(10)
31.4124(4)
90
109.1720(10)
90
1337.3(7)
4
C27H32BCuF4N8O
Monoclinic
P21/n
ꢀ
P1
811.29
1093.74
634.96
˚
a (A)
21.1842(3)
10.8143(3)
15.4268(1)
90
109.8743(7)
90
3323.67(9)
4
1.621
10.7719(2)
11.55800(10)
11.7128(2)
65.5150(10)
80.5630(10)
72.3490(10)
1263.41(3)
1
1.438
0.0468
0.1310
1.046
13.8840(4)
15.1800(4)
15.0230(4)
90
99.9170(10)
90
3119.92(15)
4
1.352
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
3
˚
V (A )
Z
Dcalc (g cmꢀ3
Ra
)
1.335
0.0712
0.2065
1067
0.0387
0.0875
1.004
0.0768
0.2036
1.063
Rwa
GOF
a
P
P
P
P
2
1=2
I > 2r(I), R = iFoj-j Foi jFoj. Rw ¼ ½ð wðjF oj ꢀ jF cjÞ = wF 2oÞꢁ
.
pyrazolyl nitrogen atoms is 126.61ꢁ. The Cu–N distances
ligands (i.e. pyrazine and 4,40-dipyridyl) with 1. The reverse
reactions took place by adding acetonitrile into an acetone
solution of 2 and 3, allowing the starting 1 to be recovered.
The binuclear complexes, [Cu2(HC(3,5-Me2Pz)3)2(l-pyra-
zine)](BF4)2 (4) and [Cu2(HC(3,5-Me2Pz)3)2(l-4,40-dipyri-
dine)](BF4)2 (5), were also prepared by using equal molar
ratio of 1 and mononuclear complexes 2 or 3, respectively.
In addition, 4 and 5 can also be prepared by the 2:1 molar
ratio of 1 and its related ligand. In the present of acetoni-
trile donor, both of 4 and 5 reformed mononuclear starting
complex 1. The overall reactions are depicted in Scheme 1.
The X-ray structures of cations of [Cu2(HC(3,5-
for the tridentate ligand vary from 2.053(2) to
˚
˚
2.1377(19) A (average 2.087 A). The fourth Cu–N distance
to the nitrogen atom of acetonitrile is much shorter at
˚
1.871(2) A, which is similar to previously reported complex
[Cu(HC(3-ButPz)3)(NCMe)](PF6) [10]. This may imply
that the steric hindrance at the third position of the pyraz-
olyl rings does not significantly affect the bonding distance
and strength of the acetonitrile complexes because this
ligand is not very space demanding. The ESI-MS(+) spec-
trometry data of complexes 1 and 1a show the molecular
ion peaks and their corresponding fragment ion peaks,
which lose one-coordinated MeCN. The mCN at 2273 and
2275 cmꢀ1 observed for 1 and 1a also suggests the presence
of a coordinated MeCN, consistent with the X-ray struc-
ture of 1a Table 2.
Me2Pz)3)2(l-pyrazine)](BF4)2
(4)
and
[Cu2(HC(3,
5-Me2Pz)3)2(l-4,40-dipyridine)](BF4)2 (5) are shown in
Fig. 2. The cꢀomplexes 4 and 5 consist of dinuclear cations
and two BF4 anions. In the cations, the two Cu(I) centers
are bridged by pyrazine or 4,40-dipyridine, forming a
The mononuclear complexes, [Cu(HC(3,5-Me2Pz)3)
(pyrazine)](BF4) (2) and [Cu(HC(3,5-Me2Pz)3)(4,40-dipyri-
dine)](BF4) (3), were prepared by the reaction of excess
Table 2
˚
Selected bond length (A) and angles (ꢁ) of 1a, 4, 5, and 7
1a
2.1377(19) 2.119(2)
2.053(2) 2.056(3)
2.0707(19) 2.141(2)
4
5
7
Cu(1)-N(1)
Cu(1)–N(3)
Cu(1)–N(5)
Cu(1)–N(7)
2.077(2)
2.133(3)
2.078(14)
1.931(4)
2.094(4)
2.062(4)
2.056(4)
1.871(2)
1.941(3)
Cu(1)–C(17)
1.819(5)
C(17)–N(7)
C(24)–N(8)
1.163(7)
1.189(8)
121.46(9)
130.27(8)
128.11(9)
N(1)–Cu(1)–N(7)
N(3)–Cu(1)–N(7)
N(5)–Cu(1)–N(7)
N(1)–Cu(1)–C(17)
N(3)–Cu(1)–C(17)
N(5)–Cu(1)–C(17)
N(1)–Cu(1)–N(3)
N(1)–Cu(1)–N(5)
N(3)–Cu(1)–N(5)
117.09(10) 135.72(15)
141.7(11) 121.17(5)
119.57(10) 122.67(15)
122.3(2)
130.26(19)
129.0(2)
85.59(8)
88.79(7)
89.14(8)
89.24(10)
88.50(9)
86.20(10)
87.79(13)
87.39(13)
88.62(13)
87.55(16)
87.22(16)
86.75(16)
Numbers in parentheses are estimated standard deviations of the last
significant figure.
Fig. 1. Crystal structure of the cation of [Cu(HCPz3)(NCMe)](BF4) (1a).

3680
S.C.N. Hsu et al. / Journal of Organometallic Chemistry 692 (2007) 3676–3684
+
+
N N
N N
bridge ligand
MeCNa
N
Cu
C Me
HC
N
N
N
N
HC
Cu BL
N
N
N
N
[Cu(HC(3,5-Me2Pz)3)(pyrazine)]BF4 (2)
[Cu(HC(3,5-Me2Pz)3)(4,4'-bipyridine)]BF4 (3)
[Cu(HC(3,5-Me2Pz)3)(1,4-CNC6H4NC)]BF4 (7)
[Cu(HC(3,5-Me2Pz)3)(NCMe)]BF4 (1)
0.5 eq. bridge ligand
CO(g)b
(1)
MeCNc
2+
+
N N
N N
N N
CH
HC
Cu BL Cu
N
N
N
N
N
N
N
N
HC
Cu
C O
N
N
N
N
[Cu2(HC(3,5-Me2Pz)3)2(μ-pyrazine)](BF4)2 (4)
[Cu2(HC(3,5-Me2Pz)3)2(μ-4,4'-bipyridine)](BF4)2 (5)
[Cu2(HC(3,5-Me2Pz)3)2(μ-1,4-CNC6H4NC)](BF4)2 (8)
[Cu(HC(3,5-Me2Pz)3)(CO)]BF4 (6)
,
N
BL(bridge ligand) =
,
N
C N
N
N
N C
a only 2 and 3, deterted by 1H NMR monitoring. b detected by IR. c only 4 and 5, deterted by 1H NMR monitoring.
Scheme 1. Reactions of copper(I) complexes.
centrosymmetrical homobinuclear complexes. Copper cen-
ter of 4 and 5 is four-coordinate binding a tridentate
HC(3,5-Me2Pz)3 ligand plus the nitrogen atom of pyrazine
or 4,40-dipyridine in a distorted tetrahedral geometry,
respectively. The local geometry around the copper center
of 4 and 5 is relatively similar to that of 1a. The average
The molecular structure of the complex cation of
[Cu(HC(3,5-Me2Pz)3)(1,4-CNC6H4NC)](BF4) (7) is shown
in Fig. 3. The copper(I) ion has a distorted tetrahedral
geometry with ligation to the tridentate HC(3,5-Me2Pz)3
ligand plus a carbon atom of 1,4-diisocyanobenzene. The
Cu–N(Pz) bond distances of 7 are shorter than 1a, 4 and
5 which may imply isocyanide having a strong r-donor role
toward copper(I) center to increase Cu–N(Pz) back bond-
ing ability. The Cu–C bond of isocyanide is short
Cu–N(Pz) distances for the tridentate ligand of
4
˚
˚
(2.105 A) and 5 (2.096 A) are slightly longer to 1a
˚
(2.087 A). The Cu–N(Pz) bond distances of 4 and 5 are also
˚
slightly longer, compared with the corresponding mean
bond distances of Cu(HB(3,5-Me2Pz)3)(pyrazine) which
bears a negative tridentate ligand bond to copper(I) atom
[17]. The Cu–N(bridge) bond distances are slightly shorter
(1.819 A) and compares well with the shortest reported in
the literature [20–24]. In addition, the C(17)–N(7) distance
˚
(1.163 A) of copper bonded isocyanide is slightly shorter
˚
than the C(24)–N(8) distance (1.189 A) of unbonded isocy-
˚
by 0.1 A, compared with that of Cu(HB(3,5-Me2Pz)3)(pyr-
anide indicating that the dominant mode of bonding of iso-
cyanide is r-donation with only minor contributions from
p–acceptor interaction. The 1H NMR spectrum of 7 exhib-
its well separated AA0BB0 pattern for benzene ring protons,
which is characteristic of two difference coordinated envi-
ronments on the para-diisocyanobenzene ring protons,
one by copper coordinated and the other one free. The
CNR stretching frequencies of 7 appear at 2150 and
2125(shoulder) cmꢀ1, also suggesting two different bonding
mode, in agreement with the X-ray data.
The binuclear complex [Cu2(HC(3,5-Me2Pz)3)2(l-1,4-
CNC6H4NC)](BF4)2 (8) was prepared by using equal molar
ratio of 1 and mononuclear complexes 7. Treatment of
1 with 0.5 equiv. of 1,4-diisocyanobenzene also gives binu-
clear complex 8. Complexes 7 and 8 do not take place
reverse reactions by adding acetonitrile because the
azine) [17] and the literature values for Cu(I)–N(l-pyra-
˚
zine) (2.022–2.056 A) [18,19]. This may be a consequence
of the expected stronger r-donation from the pyrazine or 4,
40-dipyridine ligand to copper atom between positive
[Cu(HC(3,5-Me2Pz)3)] moiety than the neutral [Cu(HB
(3,5-Me2Pz)3)] moiety.
The addition of CO to a solution of 1–5 in CH2Cl2 gave
the mononuclear CuI-carbonyl [Cu(HC(3,5-Me2Pz)3)(CO)]
(BF4) (6), which was previously reported by Reger et al. as
ꢀ
its PF6 derivative [10]. The CO stretching frequency at
2112 cmꢀ1 is in agreement with a terminal carbonyl in 6.
On the other hand, treatment of 1 with 1,4-diisocyanoben-
zene (isoelectron to CO) affords a mononuclear complex
[Cu(HC(3,5-Me2Pz)3)(1,4-CNC6H4NC)](BF4) (7) as an
colorless precipitate.

S.C.N. Hsu et al. / Journal of Organometallic Chemistry 692 (2007) 3676–3684
3681
Fig. 2. ORTEP representation of the crystal structures of the cation of [Cu2(HC(3,5-Me2Pz)3)2](l-pyrazine)](BF4)2 (4) (up) and [Cu2(HC(3,5-
Me2Pz)3)2](l-4,40-dipyridine)](BF4)2 (5) (down).
Fig. 3. ORTEP representation of the crystal structures of the cation of [Cu(HC(3,5-Me2Pz)3)(1,4-CNC6H4NC)](BF4) (7).
copper-isocyanide bond is stronger than copper–pyrazine
(pyridine) bond. We are not able to grow single crystal for
8 to obtain its crystallographic data. However, the 1H
NMR spectrum of complex 8 shows a centrosymmetrical
environment of 1,4-isocyanobenzene ring protons, which is
significant difference with 7. The CNR stretching frequency
only at 2158 cmꢀ1 is also in agreement with a centrosymmet-
rical homobinuclear bridging isocyanide ligand in 8.
Isocyanides are potential ligand-directed probes of meta-
lloprotein coordination due to their strong IR absorption

3682
S.C.N. Hsu et al. / Journal of Organometallic Chemistry 692 (2007) 3676–3684
and preference for low-valence metal coordination chemis-
try [25,26]. Although the isocyanide ligand is isoelectronic
with CO, its mode of bonding is rather different. Metal
carbonyls exhibit strong p backbonding between the elec-
tron-rich low-valence metal and the empty antibonding
orbitals of p symmetry on CO, which results in a weaken-
ing of the CO bond and a decrease in carbonyl stretching
frequency, often well below the free-ligand value. In con-
trast, isocyanides are poorer p acceptors and stronger r-
donors. In Cu(I) isocyanide complexes, the frequency of
the CNR stretch generally lies above the free-ligand value,
implying that the dominant mode of bonding is r-donation
with only minor contributions from p–acceptor interac-
tions. This is probably due to the inability of the closed
d10 configuration to effectively engage in backbonding.
Table 3 lists several copper(I) complexes containing N3
ligands and isocyanide donor. We observe the DmCN of
complexes 7 (32) and 8 (40) are higher than the value
observed for similar complex [Cu(HB(Pz)3)(CNBut)] (17),
which is containing N3 anionic ligand [27]. This imply that
N3 neutral ligand complexes 7 and 8 have more r-donation
character in the isocyanide bonding mode than N3 anionic
ligand complex [Cu(HB(Pz)3)(CNBut)]. On the other hand,
The unusually high DmCN value observed for [Cu(HB(3,5-
(CF3)2Pz)3)(CNBut)] (58) is a direct result of the fluorina-
tion of the tris(pyrazolyl)borate ligand [22].
Several related copper(I) complexes containing bis(pyr-
azolyl)methane (N2 ligand) and bridge by pyrazine ligand
are reported very recently [19]. In complexes 1a, 4, 5, and
7, tris(pyrazolyl)methane coordinate by three nitrogen
atoms with tripod scorpionate formation (N3 ligand)
compare with N2 ligand as shown in Scheme 2. The coor-
dinating ability of tris(pyrazolyl)methane and bis(pyraz-
olyl)methane are consequence of favorable electronic and
geometric factors. For example, the bidentate chelating
N2 ligand bis(pyrazolyl)methane react with [Cu(NCMe)4]+
afforded the monomeric air stable bis chelates [Cu-
(H2CPz2)2]+ and mono chelate air-sensitive Y-shaped
[Cu(H2CPz2)(NCMe)]+ [19,28]. However, the N3 ligand
tris(pyrazolyl)methane react with copper(I) source
[Cu(NCMe)4]+ only afford a stable monomeric complex
1a. The raising of the ligand density from two to three will
Me
Me
Me
Me
N N
N N
Me
HC
Me
M
M
H2C
Me
N
N
N
N
N
N
Me
Me
Me
tripodal scorpionate
bidenate chelate
Scheme 2. Structure comparison for ligand tris(pyrazolyl)methane and
bis(pyrazolyl)methane.
result in increase the stability if the copper(I) complexes
with tripodal scorpionate ligand. Indeed, complexes 2–8
are air- and moisture-stable in solid state. In solution, com-
plexes 2–5 will gradually react with dioxygen and turn to
green in 5 h at room temperature. The isocyanide com-
plexes 7 and 8 are air stable in both solid and solution state.
These observations are similar related to complex 1, which
reacts with dioxygen to give a l-g2:g2-peroxo complex,
[Cu2(HC(3,5-Me2Pz)3)2 (l-O2)](PF6)2, at ꢀ80 ꢁC to +7 ꢁC
and then gradually decomposes to a bis(l-hydroxo) com-
plex, [Cu2(HC(3,5-Me2Pz)3)2 (l-OH)2](PF6)2 [11]. On the
other hand, the air-sensitive bis(pyrazolyl)methane cop-
per(I) complex [Cu(H2CPz2)(NCCH3)](ClO4) react with
dioxygen also gave the bis(l-hydroxo) complex [28].
The redox properties of 4 and 8 were examined by cyclic
voltammetry in methanol solution under N2. Complexes 4
and 8 (Fig. 4) exhibited two quasi-reversible waves suggest-
ing that no significant structural reorganization occurs dur-
ing the redox process on the electrochemical time scale.
These two quasi-reversible redox waves can be ascribed
to electrode processes in the following equations:
½CuICuIꢁ ꢀ ½CuICuIIꢁ þ eꢀ
½CuICuIIꢁ ꢀ ½CuIICuIIꢁ þ eꢀ
2þ
ð1aÞ
ð1bÞ
Kcom
4þ
3þ
ðCuIBLCuIÞ þ ðCuIIBLCuIIÞ ꢀ 2ðCuIBLCuIIÞ
ð2Þ
Table 3
Cyanide stretching frequencies for copper(I) isocyanide complexes con-
taining N3 ligands
mCN (cmꢀ1
)
DmCN
Ref.
1,4-Diisocyanobenzene
tBuNC
2118
2138
2155
2196
[25]
[22]
[27]
[22]
This
work
This
work
[Cu(HB(Pz)3)(CNtBu)]
[Cu(HB(3,5-(CF3)2Pz)3)(CNtBu)]
[Cu(HC(3,5-Me2Pz)3)
(1,4- CNC6H4NC)]BF4 (7)
[Cu(HC(3,5-Me2Pz)3)]2
(l-1,4- CNC6H4NC)(BF4)2 (8)
17
58
32
2150, 2125(sh)
2158
40
DmCN, difference of cyanide stretching frequencies between coordinating
isocyanide and free ligand.
Fig. 4. CV of 8 in MeOH (1 · 10ꢀ3M). Scan rate = 20 mV/s,electro-
lyte = (Bu4N)(OTf) (0.1 M).

S.C.N. Hsu et al. / Journal of Organometallic Chemistry 692 (2007) 3676–3684
3683
Table 4
plex 1. However, complex 8 does not take place ligand
exchange reactions by adding acetonitrile. The synthesis
and characterization of those compounds have provided
fundamental knowledge about copper(I) complexes con-
taining nitrogen donor ligand, necessary for a deeper
understanding of the copper enzymes in particular as well
as pyrazolyl complexes in general.
CV data and comproportionation constant for dicopper complexes
Complex
Ea1/2
DE Kcom
Ref.
[30]
[Cu2(TDTO)(MeCN)2](ClO4)2b
+140 124 2 · 107
+568 122
[Cu2(Ph4bdptz)(MeCN)2](OTf)2b
+41
+516 124
122 1 · 108
[29]
[19]
[Cu2(H2C(3,5-Me2Pz)2)2(MeCN)2
(l-pyrazine)](ClO4)
+164 174 3 · 106
+548 226
Acknowledgements
[Cu(HC(3,5-MePz)3)]2(l-pyrazine)
(BF4)2(4)
ꢀ54
+354 186
146 8 · 106
This
work
Financial support of the National Science Council of
the Republic of China is highly appreciated. We thank
Mr. T.-S. Kao for collecting X-ray data.
[Cu(HC(3,5-MePz)3)]2(l-1,
4-CNC6H4NC) (BF4)2(8)
ꢀ32
+347 222
164 2.6 · 106 This
work
a
mV (vs. Cp2Fe+/Cp2Fe).
b
Appendix A. Supplementary material
TDTO, 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; Ph4bdptz, 1,4-bis
[bis(6-phenyl-2-pyridyl)methyl]phthalazine; OTf, triflate.
CCDC 625281, CCDC 603434, CCDC 603436 and
CCDC 603435 contain the supplementary crystallographic
data for 1a, 4, 5 and 7. These data can be obtained free of
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.
html, or from the Cambridge Crystallographic Data Cen-
tre, 12 Union Road, Cambridge CB2 1EZ, UK; fax:
(+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.jorganchem.
2007.05.006.
Electrochemical studies on a series of dicopper(I) com-
plexes supported by nitrogen donor ligands (Table 4),
including the complex [Cu2(H2C(3,5- Me2Pz)2)2(MeCN)2
(l-pyrazine)](ClO4)2, also showed well-resolved sequential
one-electron redox waves at comparably high potentials
than complexes 4 and 8 [19,29,30]. Redox potentials of cop-
per complexes are influenced by many factors, including
the type of donor atoms and the geometry of coordinated
complexes. Here, complexes 4 and 8 have same type coord-
inational geometry around copper center and donor atoms
but different donor numbers to complex [Cu2(H2C(3,5-
Me2Pz)2)2(MeCN)2(l-pyrazine)](ClO4)2. The relatively
low redox potentials of 4 and 8 are probably due to the
facial tridentate N-donor vs. bidentate N-donor ligand,
which making the oxidation to Cu(II) more easy. It also
can be explained by the N3 ligand of tris(pyrazolyl)meth-
ane is more electron donating compared to the N2 ligand
of bis(pyrazolyl)methane.
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.
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