Interconversion between Bis(μ-thiolato)dicopper(II) and disulfide-bridged dicopper(I) complexes mediated by chloride ion

Article abstract of DOI:10.1021/ja027397m

An unprecedented clean interconversion between a novel bis(μ-thiolato)dicopper(II) complex (1) and a disulfide-bridged dicopper(I) complex (3) through a disulfide-bridged (μ-chloro)dicopper(I) complex (2) mediated by chloride ion has been established for

Full text of DOI:10.1021/ja027397m

Published on Web 10/01/2002
Interconversion between Bis(µ-thiolato)dicopper(II) and Disulfide-Bridged
Dicopper(I) Complexes Mediated by Chloride Ion
Yoshiki Ueno, Yoshimitsu Tachi, and Shinobu Itoh*
Department of Chemistry, Graduate School of Science, Osaka City UniVersity, 3-3-138 Sugimoto,
Sumiyoshi-ku, Osaka 558-8585, Japan
Received June 20, 2002
Transition metal-sulfur bonds are ubiquitous not only in
metalloenzymes but also in man-made functional materials and
catalysts.1 The majority of the sulfur atoms serve as terminal or
bridging ligands for transition metal ions to construct various types
of MmSn molecular architectures (M ) transition metal).1-3 Thus,
regulation of the structure and reactivity of MmSn has long been an
important and attractive subject in both biochemical and industrial
researches.1 With this view, electronic effects of the sulfur ligands
Figure 1. ORTEP drawing of 1 showing 50% probability thermal-ellipsoid.
The counteranions and hydrogen atoms are omitted for clarity.
on the structures and redox properties of the metal centers have
been studied extensively.1 However, little attention has thus far been
focused on the redox chemistry of the sulfur ligands themselves in
such systems.4
We report herein an unprecedented clean interconversion between
a novel bis(µ-thiolato)dicopper(II) complex and a disulfide-bridged
dicopper(I) complex mediated by chloride ion, providing a new
redox chemistry in the MmSn systems (Scheme 1).5 The bis(µ-
thiolato)dicopper(II) core structure in compound 1 is relatively rare
among the MmSn complexes and has recently attracted much
attention as a model compound of the CuA sites of cytochrome c
oxidase (CcO) and nitrous oxide reductase (N2OR).6-13
Treatment of ligand 2L with 2 equiv of [CuI(CH3CN)4](ClO4)
in acetone under anaerobic conditions (Ar) gave an air-sensitive
dark brown-colored dicopper complex 1 in 81% yield.14 The crystal
structure of 1 is shown in Figure 1, which definitely confirms the
bis(µ-thiolato)dicopper(II) structure.15 Thus, the disulfide bond of
ligand 2L is reductively cleaved by the reaction with two cuprous
Figure 2. Spectral change for the titration of complex 1 (1.0 × 10-4 M)
with Me4N+Cl- in CH2Cl2 at 25 °C: (a) spectrum of 1, (b) Cu:Cl ) 2:1,
(c) Cu:Cl ) 2:2. (Inset) Absorbance change at 813 nm.
ions to generate the bis(µ-thiolato)dicopper(II) core. Complex 1
has a C2 symmetry, and the cupric ion exhibits four-coordinate
square planar geometry.16 The Cu2S2 core is largely bent along the
S-S axis (angle of the two least-squares planes consisted of Cu(1)-
N(1)-N(2)-S(1)-S(1)* and Cu(1)*-N(1)*-N(2)*-S(1)-S(1)*
is 115.5 °), and the distances between the two sulfur atoms and
two cupric ions are 3.13 and 2.80 Å, respectively. These structural
features of the Cu2S2 core of 1 is similar to those of the reported
bis(µ-thiolato)dicopper(II) complexes with N3 capping ligands,
although the copper(II) center in those cases exhibits a distorted
five-coordinate square pyramidal geometry.9,13 Compound 1 is EPR-
silent17 and exhibits characteristic absorption bands at 351 nm (ꢀ
) 12 300 M-1 cm-1), 516 (1200), and 813 (2400) (Spectrum a in
Figure 2). The low-energy absorption band at 813 nm can be
attributed to a thiolate-to-copper(II) charge-transfer transition.18,19
Addition of an external ligand such as chloride ion into a CH2-
Cl2 solution of 1 resulted in color change from dark brown to pale
yellow. In Figure 2 is shown the spectral change for the titration
of complex 1 with Me4N+Cl-. Spectrum a due to 1 was converted
to spectrum b, when 1 equiv of Cl- relative to complex 1 (thus,
Scheme 1
Cu:Cl ) 2:1) was added. Addition of another 1 equiv of Cl- (totally
2 equiv, thus Cu:Cl ) 2:2) gave spectrum c, whereas further
addition of Cl- did not affect spectrum c as shown in the inset of
Figure 2. From the final reaction mixture of a preparative scale
experiment, the product exhibiting spectrum c was successfully
isolated.14
* To whom correspondence should be addressed. E-mail: shinobu@
sci.osaka-cu.ac.jp.
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10.1021/ja027397m CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
can induce the drastic changes in the structure and redox state of
the bridging sulfur and oxygen ligands.
Acknowledgment. This work was financially supported in part
by Grants-in-Aid for Scientific Research on Priority Area (No.
11228206) and Grants-in-Aid for Scientific Research (No. 13480189)
from the Ministry of Education, Culture, Sports, Science and
Technology, Japan.
Supporting Information Available: Experimental details about the
synthetic procedures of the ligand (2L) and the copper complexes, X-ray
structural determination, summary of X-ray crystallographic data
(Tables S1 and S3), selected bond lengths and angles (Tables S2 and
S4), ESI-MS (Figure S1), and cyclic voltammogram of complex 1
(Figure S2) (PDF). X-ray crystallographic file in CIF format. This
material is available free of charge via the Internet at http://pubs.acs.org.
Figure 3. ORTEP drawing of 3 showing 50% probability thermal-ellipsoid.
The hydrogen atoms and acetone molecules are omitted for clarity.
The product was a disulfide-bridged dicopper(I) complex [(CuI-
Cl)2(2L)] (3) as shown in Figure 3, demonstrating that the disulfide
bond originally existed in the starting ligand 2L was regenerated
in 3 by oxidative coupling of the thiolate groups of 1, while the
two cupric ions in 1 are reduced to the dicopper(I) state in 3. This
is reflected on the disappearance of the characteristic LMCT bands
of 1 as shown in Figure 2 (spectrum c). The complex 3 also has a
C2 symmetry, and the copper(I) ions, separated by 4.11 Å each
other, exhibit a distorted tetrahedral geometry with a N2SCl donor
set. The S-S and the Cu-S bond lengths are 2.07 and 2.36 Å,
respectively, which are nearly the same to those of the reported
disulfide copper(I) complexes.13,20,21
Although crystal structure of intermediate 2 has yet to be
obtained, the stoichiometry of Cu:Cl ) 2:1 for generation of 2 as
well as the complete disappearance of the absorption bands in the
visible region (spectrum b in Figure 2) suggest that it is a disulfide-
bridged (µ-chloro)dicopper(I) complex as illustrated in Scheme 1.22
In fact, the ESI-MS analysis of intermediate 2 gave a set of
prominent peaks with a mass and isotope distribution pattern that
is consistent with the molecular formulation of [CuI2(Cl)(2L)]+.14
Notably, the disulfide-dicopper(I) complex 3 was converted
stepwise into the bis(µ-thiolato)dicopper(II) complex 1 through
intermediate 2, when 3 was treated with AgBF4 in acetone.23 Thus,
spectrum c due to 3 was first converted to the spectrum of 2 (b in
Figure 2), which further changed to the spectrum of 1 (a in Figure
2) by the gradual addition of AgBF4 into an acetone solution of 3.
In this case, Ag+ withdraws Cl- first from 3 giving 2 and then
from the resulting intermediate 2 to give 1 as indicated in Scheme
1. Thus, the conversion between 1 and 3 through 2 is reversible
(Scheme 1).
Although the mechanism of each process has yet to be
investigated in detail, the present result is the first example of a
clean interconversion between 2RS- and RSSR on a distinct
dicopper unit. Coordination of the external ligand such as Cl- to
the cupric ion in 1 may induce a geometric change of the copper
from the square planar to tetrahedral by kicking out one of the
sulfur atoms of the thiolato ligands. This may induce electron
transfer from the thiolate group to Cu(II), resulting in formation of
the disulfide-bridged (µ-chloro)dicopper(I) complex 2.22 Further
addition of Cl- to 2 gives the disulfide-dicopper(I) complex 3.
The interconversion between 1 and 3 is reminiscent of the one
between the bis(µ-oxo)dicopper(III) and the (µ-η2:η2-peroxo)-
dicopper(II) complexes, a key process in copper/dioxygen chem-
istry.24,25 Thus, these results clearly demonstrate that a small
perturbation in the coordination environment of the dicopper units
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