Angewandte
Chemie
DOI: 10.1002/anie.200704690
Copper–Sulfur Complexes
Novel Reactivity of Side-On (Disulfido)dicopper Complexes
Supported by Bi- and Tridentate Nitrogen Donors:Impact of Axial
Coordination**
Itsik Bar-Nahum, John T. York, Victor G. Young,Jr., and William B. Tolman*
The distinctive structure of the [(histidine) Cu (m -S)] cluster
7
4
4
[1]
in the “Cu ” active site of nitrous oxide reductase and the
Z
[
2]
[3]
intriguing proposals for its redox and catalytic reactivity
have inspired synthetic model studies aimed at understanding
the properties of copper–sulfur complexes supported by N-
[4]
donor ligands. Of the complexes structurally characterized
[
5–11]
[9,10]
2
2
to date,
(
those with end-on (m-1,2)
or side-on (m-h :h )
2
+
disulfido)dicopper(II), {Cu (S )} , cores are the most
2
2
common, with examples of the latter having been
Figure 1. Molecular structures of 1 (left) and the dicationic portion of
(right; ellipsoids set at 50% probability, H atoms excluded, only
[11a]
isolated
with
tris(pyrazolyl)hydroborate,
bis-
2
[
11c]
[11b,d]
(
pyridyl)methylamine,
b-diketiminate,
ligands. Reactivity studies of {Cu (S )} cores
and anilido-
heteroatoms labeled). Selected interatomic distances [] for 1: S1–S1A
2.105(4), Cu1–S1 2.2279(18), Cu1–S1A 2.2406(19), Cu1–N1 2.018(6),
Cu1–N2 2.024(6), Cu1–O1 2.344(6), Cu1–Cu1A 3.92. For 2: S1–S1A
2.165(4), Cu1–S1 2.2037(19), Cu1–S1A 2.2044(18), Cu1–N1 2.012(5),
Cu1–N2 2.214(6), Cu1–N3 2.009(6), Cu1–Cu1A 3.84.
[
11b,d]
2+
imine
2
2
supported by tri- or tetradentate pyridylamines have been
[
9b,11c]
reported,
and differences between the end- and side-on
types were attributed to contrasting nucleophilic and electro-
philic character. Further insights into the reactivity of copper–
sulfur complexes are needed to understand structure–func-
tion relationships among clusters with variable nuclearities
and supporting ligands. Ultimately, such fundamental knowl-
edge will facilitate efforts to elucidate possible mechanisms of
N O reduction by Cu and synthetic models.
[
12]
Me pda and Cu(O SCF ) (1:1 molar ratio) and subsequent
4
3
3 2
recrystallization of the resulting orange precipitate from
CH Cl /pentane. Using an alternative method, green-brown
[(Me tacn) Cu (m-h :h -S )](SbF ) (2) was synthesized by
addition of S8 to a CH Cl2 solution of [(Me tacn)Cu-
(MeCN)]SbF . X-ray structures of the complexes revealed
6
{Cu (m-h :h -S )} cores with pentacoordinate copper ions in
2 2
distorted square-pyramidal geometries (Figure 1). Whereas
the axial N donors in 2 are disposed in anti fashion and the
Cu S core is flat, in 1 the axial CF SO ligands are syn and
2
2
2
2
3
2
2
2
6 2
2
Z
2
3
2
2
[12]
Herein we report the characterization of two new (m-h :h -
2
2
2+
disulfido)dicopper(II) complexes (1 and 2, Figure 1) sup-
ported by neutral peralkylated bi- or triamine donors as well
as a comparative investigation of their reactivity with a
variety of substrates. This study has revealed redox processes
previously unseen for copper–sulfur complexes, including
phenol, hydroquinone, and catechol oxidations (some involv-
ꢀ
2
2
3
3
the Cu S core is puckered (dihedral angle between CuS
2
2
2
planes 1688). Comparison of the SꢀS, CuꢀCu, and CuꢀS
2
2
ing S-atom insertions), as well as conversion of a {Cu (m-h :h -
distances in the two complexes indicates that 2 exhibits a
greater degree of SꢀS bond activation and shorter, stronger
2
2
+
3+
I
S2)} core to a {Cu (m-S) } moiety by Cu -induced SꢀS bond
3
2
[11d]
scission. The reactivity of the complexes supported by the bi-
and triamine donor ligands differs, pointing to interesting
CuꢀS bonds than 1. As described in a previous analysis,
these differences are consistent with the electron-donating
2
2
2+
influences of supporting ligands on the {Cu (m-h :h -S )}
capabilities of their supporting ligands (Me tacn > Me pda).
2
2
3
4
core properties.
Compound 1, [(Me pda) Cu (O SCF ) (m-h :h -S )], was
Thus, greater electron donation results in more effective
2
2
II
2ꢀ
[10]
Cu d !S s* back-bonding
and longer SꢀS, shorter
4
2
2
3
3
2
2
xy
2
prepared by addition of Na S (0.5 equiv) to a THF solution of
CuꢀS, and shorter CuꢀCu distances.
2
2
Spectroscopic and electrochemical data provide further
[
*] Dr. I. Bar-Nahum, J. T. York, Dr. V. G. Young,Jr., Prof. W. B. Tolman
Department of Chemistry and
Center for Metals in Biocatalysis
University of Minnesota
support for these notions. Both 1 and 2 are diamagnetic, as
revealed by sharp pe aks between d = 1–10 ppm in H NMR
1
spectra (RT, CD Cl ) and, as measured for 1, EPR silence (X-
2
2
band, 5 K). Conductivity measurements indicate that 1 is a 1:1
207 Pleasant St. SE, Minneapolis, MN 55455 (USA)
ꢀ
electrolyte in CH Cl ; thus, dissociation of one O SCF ion
2
2
3
3
Fax: (+1)612-624-7029
E-mail: tolman@chem.umn.edu
occurs in solution. UV/Vis spectra (CH Cl ) exhibit bands
2
2
ꢀ
1
ꢀ1
with lmax = 370 nm (1, e = 11000m cm ) or 397 nm (2, e =
[
**] We thank the National Institutes of Health (grant GM47365 to
W.B.T.) and the University of Minnesota (dissertation fellowship to
J.T.Y.) for financial support of this research.
ꢀ
1
ꢀ1
2ꢀ
II
1
4300m cm ) attributable to S !Cu ligand-to-metal
2
[10,11d]
charge transfer (LMCT);
the 27-nm shift to longer
wavelength (lower energy) for 2 is consistent with the greater
Supportinginformation for this article is available on the WWW
under http://www.angewandte.org or from the author.
electron density provided by its Me tacn ligands. The relative
3
Angew. Chem. Int. Ed. 2008, 47, 533 –536
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
533