A new class of (μ-η2:η2-disulfido) dicopper complexes: Synthesis, characterization, and disulfido exchange

Article abstract of DOI:10.1021/ic049811k

Rare examples of (μ-η²:η ²-disulfido)dicopper complexes have been prepared from Cu(I) and Cu(II) complexes of β-diketiminate and anilido-imine supporting ligands. A novel byproduct derived from sulfur functionalization of the methine position of a β-diketiminate ligand was identified. DFT calculations on [(LCu) ₂X₂] (L = β-diketiminate, X = O or S) complexes rationalize the absence of a bis(μ-sulfido)dicopper isomer, [Cu ₂(μ-S)₂]²⁺, in the synthetic reactions, yet predict that a [Cu₂(μ-S)₂]⁰ core is a stable product of 2-electron reduction of the [Cu₂(μ-η ²:η²-S₂)]²⁺ unit. Exchange of the disulfido ligand was discovered upon reaction of a (μ-η²: η²-disulfido)dicopper complex with a Cu(I) reagent.

Full text of DOI:10.1021/ic049811k

Inorg. Chem. 2004, 43, 3335−3337
A New Class of (µ-η2:η2-Disulfido)dicopper Complexes: Synthesis,
Characterization, and Disulfido Exchange
,‡
Eric C. Brown,† Nermeen W. Aboelella,† Anne M. Reynolds,† Gabriel Aullo´n,‡ Santiago Alvarez,* and
,†
William B. Tolman*
Department of Chemistry and Center for Metals in Biocatalysis, UniVersity of Minnesota,
207 Pleasant Street SE, Minneapolis, Minnesota 55455, and Departament de Qu´ımica Inorga`nica
and Centre de Recerca en Qu´ımica Teo`rica, UniVersitat de Barcelona, AVda. Diagonal 647,
08028 Barcelona, Spain
Received February 13, 2004
Rare examples of (µ-η2:η2-disulfido)dicopper complexes have been
prepared from Cu(I) and Cu(II) complexes of â-diketiminate and
anilido−imine supporting ligands. A novel byproduct derived from
sulfur functionalization of the methine position of a â-diketiminate
mental questions concerning the properties and redox
behavior of discrete copper-sulfide complexes. Indeed, Cux-
(µ-S) compounds that feature low coordinate, N-donor
supported copper ions with oxidation states > +1,6 as found
in the N2OR active site, are unknown. Likewise, synthetic
protocols for disulfido complexes [Cux(µ-S2)] are poorly
developed, with examples limited to preparations of (µ-η1:
η1-disulfido)- and (µ-η2:η2-disulfido)dicopper(II,II) com-
pounds supported by tetra- and tridentate N-donors, respec-
tively.7-9 Herein we present preliminary results of a syste-
matic exploratory study aimed at the preparation and charac-
terization of high valent copper-sulfide complexes supported
by low denticity N-donor ligands. We have developed prepar-
ative routes to a new class of µ-η2:η2-disulfido complexes
with bidentate N-donor ligation, examined aspects of their
bonding via DFT calculations, and discovered an intriguing
ligand was identified. DFT calculations on [(LCu)2X ] (L )
2
â-diketiminate, X ) O or S) complexes rationalize the absence
of a bis(µ-sulfido)dicopper isomer, [Cu2(µ-S)2]2+, in the synthetic
reactions, yet predict that a [Cu2(µ-S)2]0 core is a stable product
of 2-electron reduction of the [Cu2(µ-η2:η2-S )]2+ unit. Exchange
2
of the disulfido ligand was discovered upon reaction of a (µ-η2:
η2-disulfido)dicopper complex with a Cu(I) reagent.
Copper-sulfur coordination in metallobiomolecules is
generally limited to copper-thiolates, as exemplified by the
ubiquitous class of CuI,II-SR electron transfer sites (cupre-
doxins)1 and the CuIx(SR)y clusters found in metallothion-
eins.2 Synthetic models of these various sites have provided
extensive fundamental insights into the chemistry of Cu-
SR units in a variety of supporting ligand contexts.3 Recently,
a unique, redox active tetracopper-sulfide cluster was
identified in the bacterial denitrification enzyme nitrous oxide
reductase (N2OR)4 and shown to be the site of N2O
reduction.5 This discovery of the first example of a multiple
valence state Cux(µ-S) unit in biology raises many funda-
(4) (a) Brown, K.; Djinovic-Carugo, K.; Haltia, T.; Cabrito, I.; Saraste,
M.; Moura, J. J. G.; Moura, I.; Tegoni, M.; Cambillau, C. J. Biol.
Chem. 2000, 275, 41133. (b) Rasmussen, T.; Berks, B. C.; Sanders-
Loehr, J.; Dooley, D. M.; Zumft, W. G.; Thomson, A. J. Biochemistry
2000, 39, 12753. (c) Alvarez, M. L.; Ai, J.; Zumft, W.; Sanders-Loehr,
J.; Dooley, D. M. J. Am. Chem. Soc. 2001, 123, 576. (d) Chen, P.;
DeBeer George, S.; Cabrito, I.; Antholine, W. E.; Moura, J. J. G.;
Moura, I.; Hedman, B.; Hodgson, K. O.; Solomon, E. I. J. Am. Chem.
Soc. 2002, 124, 744. (e) Chen, P.; Cabrito, I.; Moura, J. J. G.; Moura,
I.; Solomon, E. I. J. Am. Chem. Soc. 2002, 124, 10497. (f) Oganesyan,
V. S.; Rasmussen, T.; Fairhurst, S.; Thomson, A. J. Dalton Trans.
2004, 996.
(5) (a) Ghosh, S.; Gorelsky, S. I.; Chen, P.; Cabrito, I.; Moura, J. J. G.;
Moura, I.; Solomon, E. I. J. Am. Chem. Soc. 2003, 125, 15708. (b)
Chan, J. M.; Bollinger, J. A.; Grewell, C. L.; Dooley, D. M. J. Am.
Chem. Soc. 2004, 126, 3030.
(6) Numerous copper(I)-sulfide/disulfide/persulfide complexes are known.
For examples, see ref 3d and the following: (a) Ramli, E.; Rauchfuss,
T. B.; Stern, C. L. J. Am. Chem. Soc. 1990, 112, 4043 and references
therein. (b) Dehnen, S.; Eichho¨fer, A.; Fenske, D. Eur. J. Inorg. Chem.
2002, 279. (c) Yam, V. W.-W.; Lo, K. K.-W.; Wang, C.-R.; Cheung,
K.-K. J. Phys. Chem. A 1997, 101, 4666.
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Zakharov, L. N.; Rheingold, A. L.; Solomon, E. I.; Karlin, K. D. J.
Am. Chem. Soc. 2003, 125, 1160.
(8) Fujisawa, K.; Moro-oka, Y.; Kitajima, N. J. Chem. Soc., Chem.
Commun. 1994, 623.
* Authors to whom correspondence should be addressed. E-mail:
tolman@chem.umn.edu (W.B.T.); santiago.alvarez@qi.ub.es (S.A.).
† University of Minnesota.
‡ Universitat de Barcelona.
(1) Lu, Y. In ComprehensiVe Coordination Chemistry II; McCleverty, J.
A., Meyer, T. J., Eds.; Elsevier: Amsterdam, 2004; Vol. 8, pp 91-122.
(2) Gonza´lez-Duarte, P. In ComprehensiVe Coordination Chemistry II;
McCleverty, J. A., Meyer, T. J., Eds.; Elsevier: Amsterdam, 2004;
Vol. 8, pp 213-228.
(3) Copper-thiolate electron transfer center models: (a) Kitajima, N.;
Fujisawa, K.; Moro-oka, Y. J. Am. Chem. Soc. 1990, 112, 3210. (b)
Holland, P. L.; Tolman, W. B. J. Am. Chem. Soc. 2000, 122, 6331.
(c) Houser, R. P.; Young, V. G., Jr.; Tolman, W. B. J. Am. Chem.
Soc. 1996, 118, 2101. Cu(I)-thiolate clusters: (d) Henkel, G.; Krebs,
B. Chem. ReV. 2004, 104, 801. (e) Dance, I. G. Polyhedron 1986, 5,
1037.
(9) Chen, P.; Fujisawa, K.; Helton, M. E.; Karlin, K. D.; Solomon, E. I.
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10.1021/ic049811k CCC: $27.50 © 2004 American Chemical Society
Published on Web 04/29/2004
Inorganic Chemistry, Vol. 43, No. 11, 2004 3335

COMMUNICATION
reaction between members of the class and an added CuI
2-
complex involving exchange of the µ-S2 unit.
Initial syntheses involved treatment of the complexes
[LCuIICl]n10 with (TMS)2S in CH3CN, using a â-diketiminate
A or a backbone-protected analogue B11 as supporting ligand
L (Scheme 1). Instead of isolating targeted product(s) with
Scheme 1. Syntheses of Complexes
Figure 2. Representation of the X-ray crystal structure of 2, with all non-
hydrogen atoms shown as 50% thermal ellipsoids. Selected bond distances
(Å) and angles (deg): Cu1-S1, 2.2011(17); Cu1-S1A, 2.2113(18); Cu1-
N1, 1.880(5); Cu1-N2, 1.922(5); Cu1‚‚‚Cu1A, 3.8446(16); S1-S1A, 2.165-
(3); S1-Cu1-N1, 103.5(17); S1-Cu1-N2, 158.8(17); S1A-Cu1-N1,
162.2(17); S1A-Cu1-N2, 100.1(17); N1-Cu1-N2, 97.7(2); S1-Cu1-
S1A, 58.8(8); Cu1-S1-Cu1A, 121.2(7); S1A-S1-Cu1, 60.9(7).
and S-S′ distances consistent with a single bond (2.2007(11)
Å for 1 and 2.165(3) Å for 2), yet longer than the S-S′
distance in the only other reported (µ-η2:η2-disulfido)-
dicopper(II,II) complex (2.073(4) Å).8 UV-vis spectra of 1
and 2 (Figures S5 and S6, Supporting Information) contain
multiple intense features tentatively ascribed to ligand based
2-
π f π* and/or S2 f CuII LMCT transitions,9 but exact
Cu(µ-S)Cu units, however, we isolated [(LCu)2(µ-S2)] (1 and
2) in low yields (17% and 29% based on copper, respec-
tively). Formation of the disulfido bond suggests a redox
process between CuII and the sulfur ligand(s). This notion
was corroborated by identification of LCuI(CH3CN)12 as a
coproduct for the case L ) B and by isolation of a small
amount of a novel byproduct for the case L ) A that was
identified by X-ray crystallography (Figure 1). This
byproduct (3) results from reduction of copper and addition
of sulfur to the central methine carbon of the â-diketiminate
ligand, yielding a CuI4(SR)4 cluster where R is a diimine-
thiolate. Related derivatizations at the methine position of
â-diketiminates have been reported,13 which may be avoided
by using ligands such as B.
assignments await more detailed analysis.14 Both 1 and 2
are EPR silent and exhibit sharp 1H NMR spectra, indicative
of strong antiferromagnetic coupling between the CuII ions.
In contrast to previous findings for oxygen analogues,15
[Cu2(µ-S)2]2+ isomers of the [Cu2(µ-S2)]2+ cores of 1 and 2
were notably absent from the aforementioned synthetic
reactions. In order to shed light on this structural preference,
DFT calculations on the series [(LCu)2X2] were performed,
for X ) O or S and L ) the simplified â-diketiminate
HN(CH)3NH-.16,17 Two energetic minima were found for X
) O, corresponding to different O-O distances (1.44 and
2.36 Å), in agreement with previous experimental and
theoretical results15 that indicate similar stabilities for some
(µ-η2:η2-peroxo)- and bis(µ-oxo)dicopper cores. For X ) S,
however, only one minimum energy structure was found with
a short S-S ) 2.18 Å, in close agreement with the S-S
distances in 1 and 2. This result suggests an intrinsic
Complexes 1 and 2 were more efficiently prepared by
1
addition of /8 equiv of S8 to LCu(CH3CN)10a,12 in CH3CN
(Scheme 1), providing the products in 54% and 59% isolated
yields, respectively. X-ray crystal structures of both com-
plexes (2, Figure 2; 1, Figure S3, Supporting Information)
show 4-coordinate, square planar metal centers with similar
Cu-Cu distances (3.799(5) Å for 1 and 3.8446(16) Å for 2)
(10) (a) [(A)CuCl]n and [(A)Cu(CH3CN)]: 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. (b) [(2)CuCl]n exists as a 3-coordinate monomer (n ) 1) by
X-ray crystallography, but formation of a dimer at low temperature is
indicated by EPR spectroscopy (Figure S4, Supporting Information).
(11) Hayes, P. G.; Welch, G. C.; Emslie, D. J. H.; Noack, C. L.; Piers, W.
E.; Parvez, M. Organometallics 2003, 22, 1577.
(12) (B)Cu(CH3CN) was independently synthesized and fully characterized,
including by an X-ray crystal structure. See Supporting Information
for details.
(13) Yokota, S.; Tachi, Y.; Itoh, S. Inorg. Chem. 2002, 41, 1342 and
references therein.
(14) 1: UV-vis (THF) [λmax, nm (ꢀ, M-1 cm-1)]: 284 (16600), 328
(41400), 432 (24200), 541 (900), 806 (300). 2: UV-vis (THF) [λmax
,
nm (ꢀ, M-1 cm-1)]: 287 (31900), 448 (31600), 633 (1300), 865 (400).
(15) (a) Que, L., Jr.; Tolman, W. B. Angew. Chem., Int. Ed. 2002, 41,
1114. (b) Mirica, L. M.; Ottenwaelder, X.; Stack, T. D. P. Chem. ReV.
2004, 104, 1013.
Figure 1. Drawing of 3 and a representation of its heteratom core from
an X-ray crystal structure, shown as 50% thermal ellipsoids. See Supporting
Information for interatomic distances and angles.
(16) See Supporting Information for details.
(17) A more complete discussion of the calculations will be presented
elsewhere: Aullo´n, G.; Alvarez, S. Submitted for publication.
3336 Inorganic Chemistry, Vol. 43, No. 11, 2004

COMMUNICATION
Scheme 2. Disulfido Exchange Reaction
of the observation of incomplete formation of 1 when <4
equiv of (A)Cu(CH3CN) or different solvents (that solubilize
all the involved species) were used. To our knowledge, the
reaction in Scheme 2 is unprecedented in copper chemistry,
although it resembles a recently reported20 metal-metal
exchange reaction of a (µ-disulfido)dimanganese com-
plex.21
In conclusion, synthetic protocols for a new class of
(µ-η2:η2-disulfido)dicopper(II,II) complexes have been de-
veloped, the lack of isomeric bis(µ-sulfido)dicopper(III,III)
products in these reactions has been rationalized through DFT
calculations, and a new disulfido exchange reaction has been
discovered. Further spectroscopic, mechanistic, and reactivity
studies are planned, with a view toward developing prepara-
tive routes toward µ-sulfido species relevant to the N2OR
active site.
instability for the [Cu2(µ-S)2]2+ core, an intriguing contrast
to what has been found for isoelectronic complexes with
group 10 metals.18 On the other hand, future synthetic efforts
are inspired by the additional finding that 2-electron reduction
of [(LCu)2(µ-S2)] results in scission of the S-S bond (S-S
) 3.24 Å) to yield a [Cu2(µ-S)2]0 core as the energetic
minimum, with structural parameters (Cu-Cu ) 3.34 Å,
Cu-S ) 2.32 Å) that are similar to those of known bis(µ-
thiolato)dicopper(II,II) complexes (Cu-Cu ) 3.34 Å, Cu-S
) 2.33 Å).19
Finally, in an initial exploration of the reactivity of the
new disulfido complexes, we discovered that treatment of 2
with (A)Cu(CH3CN) in CH3CN at room temperature cleanly
generated 1 and (B)Cu(CH3CN) (Scheme 2). The unantici-
pated exchange process is drawn as an equilibrium because
Acknowledgment. We thank the NIH (GM47365 to
W.B.T.; NRSA award GM68307 to E.C.B.), the NSF
(predoctoral fellowship to N.W.A.), the University of Min-
nesota (Louise T. Dosdall fellowship to A.M.R.), and MCT
(BQU2002-04033-C02-01 to S.A. and RyC research contract
to G.A.) for financial support.
Supporting Information Available: Experimental details,
spectroscopic data, representations of the X-ray structures of (B)-
Cu(CH3CN), (B)CuCl, and 1, perspective views of theoretically
optimized structures, cross sections of potential energy surfaces
(PDF), and X-ray crystallographic information (CIF). This material
is available free of charge via the Internet at http://pubs.acs.org.
IC049811K
(20) Adams, R. D.; Captain, B.; Kwon, O.; Miao, S. Inorg. Chem. 2003,
42, 3356.
(18) (a) Aullo´n, G.; Hamidi, M.; Lledo´s, A.; Alvarez, S. Inorg. Chem., in
press. (b) Mas-Balleste´, R.; Capdevila, M.; Gonza´lez-Duarte, P.;
Hamidi, M.; Lledo´s, A.; Me´gret, C.; de Montauzon, D. Dalton Trans.
2004, 706.
(19) (a) Houser, R. P.; Halfen, J. A.; Young, V. G., Jr.; Blackburn, N. J.;
Tolman, W. B. J. Am. Chem. Soc. 1995, 117, 10745. (b) Ueno, Y.;
Tachi, Y.; Itoh, S. J. Am. Chem. Soc. 2002, 124, 12428.
(21) Insertion of metal units into the S-S bond of disulfido ligands is more
common. For example, see: (a) Cowie, M.; DeKock, R. L.; Wagen-
maker, T. R.; Seyferth, D.; Henderson, R. S.; Gallagher, M. K.
Organometallics 1989, 8, 119. (b) Kuwata, S.; Mizobe, Y.; Hidai, M.
J. Am. Chem. Soc. 1993, 115, 8499. (c) Adams, R. D.; Kwon, O.;
Smith, M. D. Inorg. Chem. 2002, 41, 1658. (d) Adams, R. D.; Miao,
S. Organometallics 2003, 22, 2492.
Inorganic Chemistry, Vol. 43, No. 11, 2004 3337