Copper(II) complexes of bidentate ligands containing nitrogen and sulfur donors: Synthesis, structures, electrochemistry and catalytic properties

Article abstract of DOI:10.1016/j.ica.2008.06.008

Two nitrogen and sulfur containing ligands, 1-methyl-4-((4-methylimidazol-5-yl)methylthio)benzene (NS-mim) (1) and 1-methyl-4-(2-pyridylmethylthio)benzene (NS-mpy) (2) were synthesized and a series of their Cu(II) complexes, 3-10, prepared. The imidazole-

Full text of DOI:10.1016/j.ica.2008.06.008

Inorganica Chimica Acta 362 (2009) 1247–1252
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Copper(II) complexes of bidentate ligands containing nitrogen and sulfur
donors: Synthesis, structures, electrochemistry and catalytic properties
a
a
a
a
Mitchell R. Malachowski a, , Mark E. Adams , Daniel Murray , Ryan White , Nadia Elia ,
*
Arnold L. Rheingold b, Lev N. Zakharov b, Richard S. Kelly c
a Department of Chemistry, University of San Diego, Alcala Park, San Diego, CA 92110, USA
b Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA 92093, USA
c Department of Chemistry, East Stroudsburg University, East Stroudsburg, PA 18301, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Two nitrogen and sulfur containing ligands, 1-methyl-4-((4-methylimidazol-5-yl)methylthio)benzene
(NS-mim) (1) and 1-methyl-4-(2-pyridylmethylthio)benzene (NS-mpy) (2) were synthesized and a series
of their Cu(II) complexes, 3–10, prepared. The imidazole-containing complexes (3–6) have the form
[Cu(NS-mim)2(solvent)2](X)2 where X = ClO4, BF4and [Cu(NS-mim)2(Y)2] where Y = Cl or Br and the pyr-
idine-containing complexes (7–10) have the form [Cu(NS-mpy)2]X2 (where X = ClO4, BF4) and [Cu2(NS-
mpy)2Y4] (where Y = Cl or Br). These complexes were characterized by a combination of elemental
analysis, FAB-MS and electrochemistry. The X-ray structure of the imidazole-containing [Cu(NS-
mim)2(DMF)2](ClO4)2 (3) was determined and it showed the copper(II) coordinated only by the nitrogen
donors while the sulfurs remain uncoordinated. In comparison, the X-ray structure of the pyridine-
containing [Cu2(NS-mpy)2(Cl)4] (9) shows a dinuclear copper(II) complex with the nitrogens and the
Received 5 January 2008
Received in revised form 10 June 2008
Accepted 10 June 2008
Available online 17 June 2008
Keywords:
Copper complexes
Biological models
Blue copper proteins
Imidazole
sulfurs coordinated along with a terminal chloride and two l-chloro atoms bridging the coppers. Cyclic
Pyridine
voltammetry studies indicated that the complexes undergo quasi-reversible one-electron reductions in
acetonitrile at potentials between 0.31 and 0.51 V versus SCE. The complexes were found to be active
for the oxidation of di-tert-butyl catechol (DTBC) with the rate dependent on the ligand and the counter-
ion present.
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
We are involved in a systematic study of multidentate ligands
and their copper(II) and copper(I) complexes in order to better
The existence of metal complexes at the active site of numerous
biological systems has fueled tremendous interest in preparing
synthetic metal complexes. Among the ligand sets of interest are
those that contain both sulfur and nitrogen atoms due to the pres-
ence of these donors in a number of metalloproteins [1]. Some of
the most important of these are the blue copper proteins that are
involved in electron transfer [2–4]. Blue copper proteins, such as
plastocyanin and azurin, function as biological electron carriers.
Models for the blue copper proteins require the preparation of
ligands that include sulfur and nitrogen donors [5–7]. In order to
prevent disulfide formation or to reproduce the unusual distorted
tetrahedral geometry, steric constraints often are built into the li-
gand sets or thioethers are used in place of the thiolate sulfur.
Among the best synthetic models are those that include hindered
pyrazolylborate compounds as they have been shown to recreate
many of the properties of the naturally occurring compounds [8].
understand copper-containing biological systems [9–13]. In our
earlier work, we showed that the presence of sulfur donors and a
biphenyl moiety led to spontaneous reduction of Cu(II) to a Cu(I)
complex but in the absence of the sulfur donors, no spontaneous
reduction was found [10]. The premise of that work was that due
to the dihedral twist angle in the biphenyl ring, it would impart
a non-planar geometry around the copper resulting in more tetra-
hedral-like geometries. To further probe whether the biphenyl ring
is essential for this spontaneous reduction process for these types
of ligand sets, in this work we prepared the copper complexes from
the corresponding bidentate NS ligands that are based on a ben-
zene ring rather than the more sterically encumbering biphenyl
moiety.
We report a variation on the synthesis of the imidazole-contain-
ing ligand, 1-methyl-4-((4-methylimidazol-5-yl)methylthio)ben-
zene (NS-mim) (1) [14], and a series of its copper(II) complexes
of the form [Cu(NS-mim)2(solvent)2](X)2 where X = ClO4, BF4and
[Cu(NS-mim)2(Y)2] where Y = Cl or Br. In order to assess the impor-
tance of the biologically relevant imidazole in these complexes, we
also have made the new pyridine-containing complexes
* Corresponding author.
E-mail address: malachow@sandiego.edu (M.R. Malachowski).
0020-1693/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2008.06.008

1248
M.R. Malachowski et al. / Inorganica Chimica Acta 362 (2009) 1247–1252
[Cu(NS-mpy)2]X2 (where X = ClO4, BF4) and [Cu2(NS-mpy)2Y4]
(where Y = Cl or Br) from the previously prepared 1-methyl-4-(2-
pyridylmethylthio)benzene (NS-mpy) ligand [10]. The molecular
structures of [Cu(NS-mim)2DMF2](ClO4)2 (3) and [Cu2(NS-
mpy)2(Cl)4] (9) have been determined. The electrochemistry of
the complexes has been determined by cyclic voltammetry and
the catalytic properties of the complexes towards the oxidation
of 3,5-di-tert-butylcatechol studied.
with cold EtOH to yield 3 (0.12 g, 38.8%). X-ray quality crystals
were grown by slow diffusion of ether into a solution of the com-
plex in DMF. Anal. Calc. for C24H28Cl2CuN4O8S2: C, 41.23; H, 4.05; N,
8.01, Cu, 9.09. Found: C, 40.88; H, 4.02; N, 7.97, Cu, 9.20%. Mass
spectrum (FAB MS): m/z (relative intensity) 598 (22), 499 (94),
375 (10), 281 (100), 220 (90).
2.4. Synthesis of [Cu(NS-mim)2](BF4)2 (4)
To 0.20 g (0.92 mmol) of NS-mim in 15 mL of hot EtOH was
added a solution of 0.16 g (0.46 mmol) of Cu (BF4)2 ꢂ 6H2O in
20 mL of EtOH. The solution was filtered and cooled to room tem-
perature. Dark green crystals of 4 (0.15 g, 47.3%) formed that were
collected and washed with cold EtOH. Anal. Calc. for
C24H28B2CuF8N4S2: C, 42.77; H, 4.20; N, 8.31, Cu, 9.43. Found: C,
42.42; H, 4.21; N, 8.55, Cu, 9.34%. Mass spectrum (FAB MS): m/z
(relative intensity) 499 (65), 375 (22), 281 (100), 220 (78), 154
(18).
2. Experimental
2.1. Materials and methods
All reagents and solvents were purchased from commercial
sources and used as received unless noted otherwise. 1-methyl-
4-(2-pyridylmethylthio)benzene was prepared by the literature
method [10]. C, H, N and Cu chemical analyses were performed
at Desert Analytical, Tucson, AZ. IR spectra were recorded on a Jas-
co 480 instrument. Mass spectra were run at the Nebraska Center
for Mass Spectrometry in Lincoln, NE. 1H and 13C NMR were re-
corded on a Varian 400 MHz instrument.
2.5. Synthesis of [Cu(NS-mim)2(Cl)2] (5)
To 0.20 g (0.92 mmol) of NS-mim in 15 mL of hot EtOH was
added a solution of 0.078 g (0.46 mmol) of CuCl2 ꢂ 2H2O in 10 mL
of EtOH. The solution was filtered and cooled to room temperature.
Dark green crystals of 5 (0.15 g, 58.3%) formed that were collected
and washed with cold EtOH. Anal. Calc. for C24H28Cl2CuN4S2: C,
50.47; H, 4.95; N, 9.81, Cu, 11.12. Found: C, 50.22; H, 5.03; N,
9.78, Cu, 11.01%. Mass spectrum (FAB MS): m/z (relative intensity)
536 (32), 534 (28), 499 (77), 375 (34), 281 (100), 220 (62), 151 (67).
Cyclic voltammetric data were collected using a Cypress Sys-
tems Model 1090 electrochemical analyzer (Cypress Systems, Law-
rence, KS). All scans were done at 0.050 V/s with 3 mm glassy
carbon electrodes (BAS, West Lafayette, IN) in acetonitrile (Aldrich,
99.5% spectrophotometric grade) that contained 0.10 M tetrabutyl-
ammonium hexafluorophosphate (Aldrich, 98%) as the supporting
electrolyte. The glassy carbon electrodes were polished with 0.3
and 0.05 lm alumina on microcloth pads (all Buehler, Lake Bluff,
IL), sonicated for 5 s in distilled water, and dried carefully before
introduction into the electrochemical cell.
2.6. Synthesis of [Cu(NS-mim)2(Br)2] ꢂ 2H2O (6)
A three-electrode system was used in all the measurements,
with potentials recorded versus a zero-leakage Ag+/AgCl reference
electrode (SDR2, World Precision Instruments, Sarasota, FL). The
potential of this reference was measured daily versus an aqueous
To 0.20 g (0.92 mmol) of NS-mim in 15 mL of hot EtOH was
added a solution of 0.10 g (0.46 mmol) of CuBr2 in 10 mL of EtOH.
The solution was filtered and cooled to room temperature. Brown
crystals of 6 (0.10 g, 34.2%) formed that were collected and washed
with cold EtOH. Anal. Calc. for C24H32Br2CuN4O2S2: C, 41.41; H,
4.64; N, 8.04, Cu, 9.13. Found: C, 41.50; H, 4.58; N, 8.00, Cu,
9.27%. Mass spectrum (FAB MS): m/z (relative intensity) 499 (82),
375 (77), 281 (100), 220 (39), 151 (89).
saturated calomel electrode (SCE) and an appropriate correction
0
made so that the Eꢀ values for each complex could be reported ver-
sus SCE. A platinum wire served as the auxiliary electrode. The
electrochemical cell consisted of a glass vial of ca. 10.0 mL volume
with a fitted Teflon cap. All solutions were sparged with solvent-
saturated nitrogen prior to data collection.
Caution: Although there were no incidents in our laboratory,
transition metal perchlorates may explode violently. They should
be prepared in small quantities and handled with care.
2.7. Synthesis of [Cu(NS-mpy)2](ClO4)2 (7)
A solution of 0.30 g (1.3 mmol) of NS-mpy in 10 mL of absolute
ethanol was treated with 0.24 g (0.70 mmol) of Cu(ClO4)2 ꢂ 6H2O
in 10 mL of absolute ethanol. The resulting solution was heated
to reflux, filtered and allowed to cool to room temperature. Large
brown crystals of 7 (0.25 g, 51.3%) formed overnight that were
filtered and washed with cold EtOH. Anal. Calc. for C26H26Cl2Cu-
N2O8S2: C, 45.05; H, 3.79; N, 4.04, Cu, 9.17. Found: C, 45.40; H,
3.91; N, 4.13, Cu, 9.33%. Mass spectrum (FAB MS): m/z (relative
intensity) 495 (30), 493 (23), 371 (8), 307 (31), 216 (32), 154
(100).
2.2. Synthesis of 1-methyl-4-((4-methylimidazol-5-
yl)methylthio)benzene (NS-mim) (1)
To 17.80 g (0.143 mol) of p-thiocresol in 250 mL of acetic acid
was added 21.30 g (0.143 mol) of 4-methyl-5-imidazole methanol
hydrochloride. The solution was refluxed for 3 h. One hundred mil-
lilitres of hot EtOH were added followed by 200 mL of NH4OH. The
solution was cooled to ꢁ10 °C overnight. Crystals formed which
were collected and recrystallized from EtOH to yield 24.4 g
(78.3%) of colorless crystals, mp 133–134 °C. 1H NMR dH (CDCl3,
400 MHz) 2.06 (3H, s), 2.32 (3H, s), 4.01 (2H, s), 6.24 (1H, broad
s). 7.07–7.42 (4H, m). 13C NMR d (CDCl3,): 135.5, 135.2, 134.8,
133.4, 131.7, 129.4, 129.3, 126.8, 126.6, 28.6, 25.3, 12.5.
2.8. Synthesis of [Cu(NS-mpy)2](BF4)2 (8)
A solution of 0.30 g (1.3 mmol) of NS-mpy in 10 mL of absolute
ethanol was treated with 0.24 g (0.70 mmol) of Cu(BF4)2 ꢂ 6H2O in
10 mL of absolute ethanol. The resulting solution was heated to re-
flux, filtered and allowed to cool to room temperature. Small
brown crystals of 8 (0.14 g, 29.3%) formed overnight that were
filtered and washed with cold EtOH. Anal. Calc. for
C26H26B2CuF8N2S2: C, 46.76; H, 3.93; N, 4.20, Cu, 9.51. Found: C,
46.33; H, 3.78; N, 3.93, Cu, 9.86%. Mass spectrum (FAB MS): m/z
(relative intensity) 493 (54), 371 (20), 307 (31), 216 (63), 154
(100).
2.3. Synthesis of [Cu(NS-mim)2(DMF)2](ClO4)2 (3)
To 0.20 g (0.92 mmol) of NS-mim in 15 mL of hot EtOH was
added a solution of 0.17 g (0.46 mmol) of Cu(ClO4)2 ꢂ 6H2O in
20 mL of EtOH. The solution was filtered and cooled to room tem-
perature. Green plates formed that were collected and washed

M.R. Malachowski et al. / Inorganica Chimica Acta 362 (2009) 1247–1252
1249
2.9. Synthesis of [Cu2(NS-mpy)2(Cl)4] (9)
ested in seeing if any of them would lead to spontaneous reduction
of the copper(II) upon complexation. Treatment of NS-mim, 1, with
copper(II) salts led to isolation of mononuclear copper(II) com-
plexes with the formula [Cu(NS-mim)2](X)2where X = ClO4, BF4,
or [Cu(NS-mim)2(X)2] where X = Cl or Br. For 3, two molecules of
DMF were also found complexed to the copper(II) ion. On the other
hand, treatment of NS-mpy, 2, with the same copper salts led to
isolation of either mononuclear or dinuclear complexes depending
on the ability of the counterion to complex. Reaction with
Cu(ClO4)2 ꢂ 6H2O or Cu(BF4)2 ꢂ 6H2O led to mononuclear complexes
with the formula [Cu(NS-mpy)2](X)2while reaction with CuCl2 or
CuBr2 led to dinuclear complexes with the formula [Cu2(NS-
mpy)2(X)4] where X = Cl or Br. The stoichiometry of the compounds
has been established from the elemental analysis. The infrared
spectra of all the compounds show the presence of the ligand
bands, and for complexes 3–4 and 7–8, the presence of the BF4
(1050 cmꢁ1) and ClO4 (1100 cmꢁ1) anions, respectively. Regardless
of the particular structure formed, what is most important is that
in all cases copper(II) complexes were isolated rather than the cop-
per(I) form so it is clear that the biphenyl ring was essential in
leading to the spontaneous reduction of our previously prepared
copper(I) complexes [10].
A solution of 0.30 g (1.3 mmol) of NS-mpy in 10 mL of absolute
ethanol was treated with 0.24 g (0.70 mmol) of CuCl2 ꢂ 2H2O in
10 mL of absolute ethanol. The resulting solution was heated to re-
flux, filtered and allowed to cool to room temperature. Lime green
crystals of 9 (0.19 g, 46.9%) formed overnight that were filtered and
washed with cold ethanol. X-ray quality crystals were grown by
slow diffusion of pentane into a solution of the compound dis-
solved in MeOH. Anal. Calc. for C26H26Cl4Cu2N2S2: C, 44.63; H,
3.75; N, 4.00, Cu, 18.17. Found: C, 44.61; H, 3.98; N, 4.10, Cu,
18.28%. Mass spectrum (FAB MS): m/z (relative intensity) 593
(54), 530 (38), 528 (42), 509 (22), 495 (30), 493 (30), 307 (30),
216 (60), 154 (100).
2.10. Synthesis of [Cu2(NS-mpy)2(Br)4] (10)
A solution of 0.30 g (1.3 mmol) of NS-mpy in 10 mL of absolute
ethanol was treated with 0.20 g (0.70 mmol) of CuBr2 in 10 mL of
absolute ethanol. The resulting solution was heated to reflux, fil-
tered and allowed to cool to room temperature. Brown crystals
of 10 (0.19 g, 41.2%) formed overnight that were filtered and
washed with cold EtOH. Anal. Calc. for C26H26Br4Cu2N2S2: C,
35.59; H, 2.99; N, 3.19, Cu, 14.49. Found: C, 35.44; H, 3.12; N,
3.26, Cu, 14.67%. Mass spectrum (FAB MS): m/z (relative intensity)
593 (32), 530 (77), 528 (54), 509 (20), 495 (12), 493 (45), 307 (55),
216 (23), 154 (100).
3.2. Mass spectrometry
Fast atom bombardment mass spectrometry (FAB-MS) has been
shown to be a useful technique for analysis of metal complexes
[16]. This technique has been used here to help characterize com-
plexes 3–10 by using both the molecular ion present and the frag-
mentation patterns obtained. The presence of two NS-mim ligands
per copper can be seen clearly in the mass spectrum of 3–6 with
the [Cu(NS-mim)2]+ found at 499 m/z. In the case of complex 3, a
small peak corresponding to [Cu(NS-mim)2](ClO4)+ was observed
at m/z 598. For the NS-mpy complexes, 7–10, two NS-mpy ligands
per copper can be seen in the mass spectrum with [Cu(NS-mpy)2]+
found at 493 m/z for the Cl and ClO4 complexes. No evidence of any
exogenous donors is found in any of the NS-mpy complexes. The
fragmentation patterns found for complexes 3–10 are also consis-
tent with the presence of their respective ligands (Fig. 1).
2.11. Kinetic measurements
Kinetic experiments were carried out at 25 °C by mixing meth-
anolic solutions of 1 mM of the metal complex (0.3 mL) and 2.0 mL
of DTBC (0.1 M). The appearance of the product, 3,5-di-tert-butyl-
quinone, was detected at 400 nm (e
= 1900 Mꢁ1 cmꢁ1) and initial
rates were calculated from the linear slope.
2.12. X-ray crystallographic analysis
Single crystals of compounds 3 and 9 were mounted on quartz
capillaries by using Paratone oil and were cooled in a nitrogen
stream on the diffractometer. Data were collected on a Bruker P4
diffractometer with an APEX area detector. Peak integrations were
performed with the Bruker SAINT software package. Absorption cor-
rections were applied using the program SADABS. Space group deter-
minations were performed by the program XPREP. The structures
were solved by direct methods and refined with the SHELXTL soft-
ware package [15]. Hydrogen atoms were both fixed at calculated
positions or located from difference maps and refined with isotro-
pic thermal parameters, and all non-hydrogen atoms were refined
anisotropically unless otherwise noted.
3.3. Crystallographic analysis
Single crystals of [Cu(NS-mim)2(DMF)2](ClO4)2 (3) suitable for
X-ray diffraction structural determination were grown by vapor
diffusion of ether into a solution of the complex in DMF while
[Cu2(NS-mpy)2(Cl)4] (9) was crystallized by vapor diffusion of pen-
tane into CH3OH. The molecular structure of complex 3 is shown in
Fig. 2. It crystallizes in a monoclinic crystal system with space
group P2(1)/n while 9 (shown in Fig. 3) is triclinic with space group
ꢀ
P1. The relevant details of the crystals, data collection and struc-
ture refinement are found in Table 1. Selected bond distances
and bond angles are presented in Tables 2 and 3. The complete list
of bond distances, bond angles, atomic coordinates and anisotropic
thermal parameters are presented in Supplementary material.
3. Results and discussion
3.1. Synthesis and spectroscopic studies
Pyridine and imidazole donors have found extensive use in a
variety of multidentate ligands complexed to metal ions. The NS-
mim ligand was prepared in one step by treating 4-methylthiocre-
sol with 4-hydroxymethylimidazole resulting in isolation of 1 [14].
Imidazolylmethylation of thiols has been shown to occur for a vari-
ety of thiols, and the procedure smoothly led to the product here.
Compound 2 was prepared as previously reported [10].
S
S
N
N
HN
The Cu(II) complexes 3–10 were prepared in straightforward
fashion from the appropriate ligand. We made a large number of
different copper complexes from various anions as we were inter-
NS-mim
NS-mpy
Fig. 1. NS-mpy and NS-mim ligands.

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M.R. Malachowski et al. / Inorganica Chimica Acta 362 (2009) 1247–1252
Table 2
Select bond lengths (Å) and angles (°) for [Cu(NS-mim)2(DMF)2](ClO4)2 (3)
Cu(1)–N(1A)
Cu(1)–N(1)
Cu(1)–O(1)
Cu(1)–O(1A)
1.958(2)
1.958(2)
1.9897(17)
1.9897(17)
N(1A)–Cu(1)–N(1)
N(1A)–Cu(1)–O(1)
N(1)–Cu(1)–O(1)
N(1A)–Cu(1)–O(1A)
N(1)–Cu(1)–O(1A)
O(1)–Cu(1)–O(1A)
180.00(13)
92.13(8)
87.87(8)
87.87(8)
92.13(8)
180.00(11)
Table 3
Select bond lengths (Å) and angles (°) for [Cu2(NS-mpy)2(Cl)4] (9)
Cu(1)–N(1)
Cu(1)–Cl(2)
Cu(1)–Cl(1)
Cu(1)–S(1)
Cu(1)–Cl(1A)
Cu(1A)–Cl(1)
2.0355(16)
2.2373(6)
2.2881(5)
2.3603(5)
2.6965(6)
2.6965(6)
Fig. 2. Structural diagram of [Cu(NS-mim)2(DMF)2](ClO4)2 (3) with partial atom.
numbering schemes (ORTEP, 50% probability ellipsoids).
N(1)–Cu(1)–Cl(2)
N(1)–Cu(1)–Cl(1)
Cl(2)–Cu(1)–Cl(1)
N(1)–Cu(1)–S(1)
Cl(2)–Cu(1)–S(1)
Cl(1)–Cu(1)–S(1)
N(1)–Cu(1)–Cl(1A)
Cl(2)–Cu(1)–Cl(1A)
Cl(1)–Cu(1)–Cl(1A)
S(1)–Cu(1)–Cl(1A)
Cu(1)–Cl(1)–Cu(1A)
162.57(5)
94.23(5)
93.85(2)
83.82(5)
88.27(2)
177.849(18)
87.86(5)
106.89(2)
94.402(17)
84.621(17)
85.598(17)
As can be seen in Figs. 2 and 3, the structures of complexes 3
and 9markedly differ. We did not anticipate the dramatic differ-
ences in their structures as we did not see the same variability in
the previously prepared biphenyl analogs. In the mononuclear
complex 3, the structure shows the copper to be four-coordinate
with the two imidazole nitrogens coordinated along with two mol-
ecules of DMF coordinating via the oxygens. Although the ligand is
potentially tetradentate, the Cu–S distance of 2.955 Å indicates
that the sulfurs are non-bonding [17,18]. The copper(II) is found
in a nearly ideal square planar environment as evidenced by the
bond angles. The Cu–Nim bond distances (1.958 Å) and Cu–O bond
distances (1.9897 Å) are in the expected range for these bonds [19].
In contrast, complex 9 is a five-coordinate dimer with each cop-
per coordinated by NSCl3 donors. This is a result of the copper coor-
dinating to only one NS-mpy ligand, a terminal Cl and two bridging
Fig. 3. Structural diagram of [Cu2(NS-mpy)2(Cl)4] (9) with partial atom numbering
schemes (ORTEP, 50% probability ellipsoids).
Table 1
Crystal data and structure refinement for [Cu(NS-mim)2(DMF)2] (ClO4)2 (3) and
[Cu2(NS-mpy)2(Cl)4] (9)
3
9
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
C30H42Cl2CuN6O10S2
845.26
100(2)
0.71073
monoclinic
P2(1)/n
10.3776(8)
7.6819(6)
23.3602(18)
110.350(2)
95.181(2)
102.478(2)
1858.6(2)
4
C26H26Cl4Cu2N2S2
699.49
100(2)
0.71073
triclinic
ꢀ
P1
7.8055(5)
8.7399(5)
11.4430(7)
107.6010(10)
98.8160(10)
109.7810(10)
671.15(7)
1
l-chloro donors. In this case, the Cu–S distance is 2.36 Å, a value
b (Å)
c (Å)
clearly within bonding distance [20]. The coordination geometry
around the CuNSCl3 is best described as trigonal bipyramidal dis-
torted square based pyramidal (TBDSBP) as revealed by the magni-
a
(°)
b (°)
c
(°)
Volume (Å3)
tude of the trigonality index
and trigonal bipyramidal geometries, the
s
of 0.25. For perfect square pyramidal
values are zero and
s
Z
Dcalc (Mg/m3)
Absorption coefficient (mmꢁ1
F(000)
1.510
0.905
878
1.731
2.159
354
unity, respectively [21]. The corners of the square plane are occu-
pied by the pyridine nitrogen, the sulfur, the terminal chlorine
and Cl(1) with Cl(1A) in the axial position. As expected, the bond
distance of the axially coordinated Cl(1A) is considerably longer
)
Crystal size (mm)
Reflections collected
0.25 ꢃ 0.20 ꢃ 0.16
0.30 ꢃ 0.22 ꢃ 0.08
15494
4886
Independent reflections (Rint
Data/restraints/parameters
Goodness-of-fit on F2
)
4227 (0.0268)
4227/0/316
1.115
R1 = 0.0520,
wR2 = 0.1323
R1 = 0.0571,
wR2 = 0.1357
3039 (0.0177)
3039/0/215
1.043
R1 = 0.0309,
wR2 = 0.0834
R1 = 0.0328,
wR2 = 0.0854
0.633 and ꢁ0.568
(2.6965 Å) than the other
2.2881 Å. The Cu–Npy distances fall within the ranges expected
(2.00–2.11 Å) [20].
What is most important about the structures is that they show
that spontaneous reduction of the copper(II) salt to a copper(I)
complex does not take place for these complexes. This is in com-
parison to the corresponding biphenyl-based N2S2-mim and
N2S2-mpy complexes where copper(I) complexes were isolated
l-chloro donor, Cl(1) found at
Final R indices [I > 2
r(I)]
R indices (all data)
Largest difference in peak and hole 0.740 and ꢁ0.352
(e Åꢁ3
)

M.R. Malachowski et al. / Inorganica Chimica Acta 362 (2009) 1247–1252
1251
Table 4
previous oxidation studies [11–13]. We and others have shown
that there is a complex interplay between the structural and elec-
tronic properties of the copper(II) complexes and their ability to
serve as effective catalysts for the conversion of 3,5-di-tert-butyl-
catechol to 3,5-di-tert-butylquinone [23–29].
Cyclic voltammetry data for the reduction of copper(II) complexes and catalytic
activities for complexes 3–10
0
Eꢀ (V vs. SCE)
Activity (mmol substrate per
mg catalyst per min)
[Cu(NS-mim)2(DMF)2](ClO4)2 (3)
[Cu(NS-mim)2](BF4)2 (4)
[Cu(NS-mim)2(Cl)2] (5)
[Cu(NS-mim)2(Br)2] (6)
[Cu(NS-mpy)2](ClO4)2 (7)
[Cu(NS-mpy)2](BF4)2 (8)
[Cu2(NS-mpy)2(Cl)4] (9)
[Cu(NS-mpy)2(Br)2] (10)
0.47
0.46
0.31
0.38
0.48
0.48
0.47
0.51
.0305
.0438
.0502
.0523
.0506
.0545
.0435
.0768
The reactivity of the Cu(II) complexes 3–10 towards 3,5-di-tert-
butylcatechol under catalytic conditions (0.3 mL of a 1.0 ꢃ 10ꢁ3 M
methanol solution of catalyst and 2.0 mL of a 0.1 M methanolic
solution of 3,5-di-tert-butylcatechol) in the presence of atmo-
spheric O2 was studied using electronic spectroscopy by following
the appearance of the absorption maximum of the 3,5-di-tert-
butylquinone at 390 nm over time. The results of the oxidations
are presented in Table 4.
As can be seen, all of the complexes catalyze the oxidation pro-
cess with the rate varying from a high of 0.0768 for 10 to a low of
0.0305 for 3. In some respects, we are surprised that these com-
plexes catalyze the oxidation at all as their reduction potentials
are much more positive than other complexes we have studied [11].
Although in the past we have made comparisons between pyr-
azole, imidazole and pyridine donors, we hesitate to make any here
as the structures of the complexes are so vastly different. However,
it is routinely assumed that a vacant coordination site needs to be
present on the metal prior to the initiation of the reaction as elec-
tron transfer from catechol to copper can happen only after a cop-
per catecholate intermediate is formed. The results for these
complexes suggest that a vacant site is available for binding as
the complexes have at least modest catalytic properties.
from copper(II) precursors [10]. It is clear that the presence of the
nitrogen and sulfur donors and a biphenyl moiety is necessary for
the spontaneous reduction process to occur for these ligand sets.
3.4. Electrochemistry
The reduction potentials for complexes 3–10 were measured by
cyclic voltammetry at a glassy carbon electrode in acetonitrile. The
cyclic voltammetry data are shown in Table 4. The average of the
peak potentials is taken as an approximation of the formal poten-
tial, E . For all the complexes, a one-electron reduction is observed
ꢀ
with the reduced form of the complexes displaying high chemical
stability on the time scale of the cyclic voltammetry experiment.
Ratios of reverse to forward currents (ipa/ipc) are close to 1.0 for
the complexes at a scan rate of 0.050 V/s.
4. Conclusions
Surprisingly, the E , values for the NS-mpy complexes, 7–10, are
ꢀ
We have synthesized and characterized two series of copper(II)
complexes formed from NS ligands that were inspired by the blue
copper proteins although we have drifted far from modeling bio-
logical systems. One ligand contains thioether and imidazole do-
nors and the second ligand has a thioether and a pyridine donor.
In neither case does the complexation of the ligand with copper(II)
lead to spontaneous reduction of the copper(II) as was found for
the related ligands containing the biphenyl moiety [10]. It is clear
from these results that the biphenyl ring is essential in enforcing
particular geometrical results on the metal that led to the sponta-
neous reduction.
almost identical in all cases and only show a range of 0.03 V. In
other Cu(II) complexes we have studied, notable differences occur
as the nature of any exogenous donors or even counterions is chan-
ged [9–11,13]. For the NS-mim complexes, 3–6, the spread of val-
ues is greater (0.16 V) but still quite modest. The presence of sulfur
in the ligands raises the potential compared to other complexes.
The electrochemical data show that the reduction potentials of
the bound copper(II) species depend more on the nature of the li-
gand than on the counterion or exogenous donor.
In general, the reduction potential of the NS-mpy complexes are
slightly more positive than the NS-mim complexes. This probably
results from the fact that pyridine has a pKb = 8.7 while imidazole
has a pKb = 7.0. Analogous to results found for comparisons of
other nitrogenous donors [11,22], replacing imidazole by pyridine
in the ligands should result in a more stable Cu(I) form for the pyr-
idine complexes because of its higher pKb. This feature will affect
the reduction potentials of the complexes by shifting the E1/2 val-
ues of the pyridine complex more positive compared to their imid-
azole-containing counterparts, a result that is borne out by the
data.
Acknowledgements
We are grateful to the Research Corporation and the University
of San Diego Faculty Research Grants for financial support of this
work.
Appendix A Supplementary material
CCDC 690102 and 690103 contain the supplementary crystallo-
graphic data for 3 and 9. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.ccdc.-
cam.ac.uk/data_request/cif. Supplementary data associated with
this article can be found, in the online version, at doi:10.1016/
j.ica.2008.06.008.
Direct comparisons of the electrochemical properties of com-
0
plex 7 (Eꢀ = 0.48 V) with the previously prepared biphenyl N2S2-
0
mpy complex [10] that had a Eꢀ of 0.77 V shows the large effect
of the biphenyl ring in shifting the potential to a more positive
value and that is most certainly the reason for the spontaneous
reduction of the biphenyl complex in preference to isolation of
the copper(II) form for the benzene-derived complex 7.
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