Ligand structural effects on Cu2S2 bonding and reactivity in side-on disulfido-bridged dicopper complexes

Article abstract of DOI:10.1021/ic061589r

To assess supporting ligand effects on S-S bond activation, a series of [Cu₂(μ-η²:η²-S₂)] ²⁺ complexes supported by various β-diketiminate or anilido-imine ligands (L) were synthesized via the reaction of Cu(I) precursors LCu(CH₃CN) with S₈. For the cases where L = β-diketiminate, the syntheses were complicated by formation of clusters [Cu(SR)]₄, where SR represents the ligand functionalized by sulfur at the central methine position. The [CU₂(μ-η²: η²-S₂)]²⁺ products were characterized by X-ray crystallography and electronic absorption and resonance Raman spectroscopy. Correlations among the Cu-S, Cu-Cu, and S-S distances and the ν(S-S) values were observed and interpreted within the framework of a previously described bonding picture (Chen, P.; Fujisawa, K.; Helton, M. E.; Karlin, K. D.; Solomon, E. I. J. Am. Chem. Soc. 2003, 125, 6394). Comparison of these data to those for other relevant species revealed a remarkable degree of S-S bond activation in the compounds supported by the β-diketiminate and anilido-imine ligands, which through strong electron donation increase backbonding from the copper ions into the S-S σ* orbital and cause S-S bond weakening. Reactions of one of the complexes supported by an anilido-imine ligand with PPh₃ and xylyl isocyanide were explored, revealing facile transfer of sulfur to PPh₃ but only displacement of sulfur to yield a LCu(I)-CNAr (Ar = xylyl) complex with the isocyanide.

Full text of DOI:10.1021/ic061589r

Inorg. Chem. 2007, 46, 486−496
Ligand Structural Effects on Cu2S2 Bonding and Reactivity in Side-On
Disulfido-Bridged Dicopper Complexes
Eric C. Brown, Itsik Bar-Nahum, John T. York, Nermeen W. Aboelella, and William B. Tolman*
Department of Chemistry and Center for Metals in Biocatalysis, UniVersity of Minnesota,
207 Pleasant Street Southeast, Minneapolis, Minnesota 55455
Received August 22, 2006
+
To assess supporting ligand effects on S
by various -diketiminate or anilido-imine ligands (L) were synthesized via the reaction of Cu(I) precursors LCu-
(CH3CN) with S8. For the cases where L ) â-diketiminate, the syntheses were complicated by formation of clusters
−S bond activation, a series of [Cu2(µ- :η
η2 2-S2)]2 complexes supported
â
[Cu(SR)]4, where SR represents the ligand functionalized by sulfur at the central methine position. The [Cu2(
µ
-
+
η2
:
η
2-S2)]2 products were characterized by X-ray crystallography and electronic absorption and resonance Raman
spectroscopy. Correlations among the Cu−S, Cu−Cu, and S−S distances and the ν(S−S) values were observed
and interpreted within the framework of a previously described bonding picture (Chen, P.; Fujisawa, K.; Helton, M.
E.; Karlin, K. D.; Solomon, E. I. J. Am. Chem. Soc. 2003, 125, 6394). Comparison of these data to those for other
relevant species revealed a remarkable degree of S
-diketiminate and anilido-imine ligands, which through strong electron donation increase backbonding from the
copper ions into the S * orbital and cause S S bond weakening. Reactions of one of the complexes supported
by an anilido-imine ligand with PPh3 and xylyl isocyanide were explored, revealing facile transfer of sulfur to PPh3
but only displacement of sulfur to yield a LCu(I) CNAr (Ar xylyl) complex with the isocyanide.
−S bond activation in the compounds supported by the
â
−S
σ
−
−
)
Introduction
only been identified in one copper-containing enzyme, nitrous
oxide reductase (N2OR), which produces N2 during microbial
denitrification.8 Recent structural9 and spectroscopic8,10 stud-
ies have revealed the catalytic site of N2OR (“CuZ”) to
contain a novel (µ4-sulfido)tetracopper cluster ligated by
multiple histidine donors. This cluster can exist in multiple
oxidation states, the most important of which appear to be
the fully reduced Cu(I)4 and mixed-valent Cu(I)xCu(II)4-x
forms.10,11 Provocative mechanistic proposals for the enzyme
Copper ions coordinated by multiple histidine N-donor and
sulfur-containing thiolate or sulfide ligands comprise the
active sites of numerous functionally important metallopro-
teins. Much interest has focused on the mononuclear type 1
and binuclear CuA electron-transfer sites,1,2 for which the
significance of highly covalent copper(II)-thiolate interac-
tions has been demonstrated through detailed structural,
spectroscopic, and synthetic modeling studies.2,3 Copper(I)-
thiolate centers also are prevalent in proteins that store and
transport copper, as well as those that regulate gene
transcription to ensure proper copper ion homeostasis.4-6
Although ubiquitous in the biochemistry of iron,7 sulfide has
(4) (a) Henkel, G.; Krebs, B. Chem. ReV. 2004, 104, 801-824. (b)
Gonza´lez-Duarte, P. In ComprehensiVe Coordination Chemistry II;
McCleverty, J. A., Meyer, T. J., Eds.; Elsevier: Amsterdam, 2004;
Vol. 8, pp 213-228.
(5) Rosenzweig, A. C. Acc. Chem. Res. 2001, 34, 119-128.
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659. (b) Beinert, H. J. Biol. Inorg. Chem. 2000, 5, 2-15. (c) Holm,
R. H. In ComprehensiVe Coordination Chemistry II; McCleverty, J.
A., Meyer, T. J., Eds.; Elsevier: Amsterdam, 2004; Vol. 8, pp 61-
90.
(8) Chen, P.; Gorelsky, S. I.; Ghosh, S.; Solomon, E. I. Angew. Chem.,
Int. Ed. 2004, 43, 4132-4140.
* To whom correspondence should be addressed. E-mail:
tolman@chem.umn.edu.
(1) Lu, Y. In ComprehensiVe Coordination Chemistry II; McCleverty, J.
A., Meyer, T. J., Eds.; Elsevier: Amsterdam, 2004; Vol. 8, pp 91-
122.
(2) For selected lead references, see: (a) Randall, D. W.; Gamelin, D.
R.; LaCroix, L. B.; Solomon, E. I. J. Biol. Inorg. Chem. 2000, 5, 16-
19. (b) Gray, H. B.; Malmstrom, B. G.; Williams, R. J. P. J. Biol.
Inorg. Chem. 2000, 5, 551-559. (c) Solomon, E. I.; Szilagyi, R. K.;
DeBeer-George, S.; Basumallick, L. Chem. ReV. 2004, 104, 419-
458.
(9) (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, 41133-41136. (b) Haltia, T.; Brown, K.; Tegoni,
M.; Cambillau, C.; Saraste, M.; Mattila, K.; Djinovic-Carugo, K.
Biochem. J. 2003, 369, 77-88.
(3) Tolman, W. J. Biol. Inorg. Chem. 2006, 11, 261-271 and references
therein.
486 Inorganic Chemistry, Vol. 46, No. 2, 2007
10.1021/ic061589r CCC: $37.00
© 2007 American Chemical Society
Published on Web 12/14/2006

Side-On Disulfido-Bridged Dicopper Complexes
have been put forth using computational methods,8,12 key
features of which include unusual side-on coordination of
N2O to an edge of the Cu(I)4 form of the cluster and µ4-
sulfide-mediated electron delocalization in higher oxidation
states. Stimulated by the unusual properties of and the
mechanistic hypotheses put forth for the (µ4-sulfido)tetra-
copper site of N2OR, we have begun to explore the sulfur
chemistry of copper complexes supported by N-donor
ligands.13 We aim to provide fundamental chemical insights
into the properties of copper-sulfur species in general and,
in particular, into less common ones14 that feature copper in
oxidation states greater than +1. An ultimate goal is to
prepare useful models of the N2OR catalytic site with which
to evaluate its spectroscopic properties and reactivity.15
Previous synthetic efforts have yielded sulfur-containing
Cu(II or III) products with either the S-S bond intact (e.g.,
disulfido(2-), 1 and 2,16-18 or disulfido(‚1-), 313c) or broken
(e.g., sulfide, 4),13b depending on the nature of the N-donor
ligand and the reaction conditions (Figure 1). The µ-η1:η1-
and µ-η2:η2-disulfidodicopper complexes 116 and 2 (L )
TpiPr2 or Me2NPY2)17,18 are close counterparts of peroxodi-
copper analogs with identical tetra- or tridentate supporting
ligands,19-21 and the bonding interactions in the Cu2O2 and
Cu2S2 cores are similar as determined from comparative
spectroscopic/theoretical studies.22 The side-on µ-η2:η2 com-
Figure 1. Copper(II or III)-sulfur complexes.
(10) (a) Rasmussen, T.; Berks, B. C.; Sanders-Loeh, J.; Dooley, D. M.;
Zumft, W. G.; Thomson, A. J. Biochemistry 2000, 39, 12753-12756.
(b) Alvarez, M. L.; Ai, J.; Zumft, W.; Sanders-Loeh, J.; Dooley, D.
M. J. Am. Chem. Soc. 2001, 123, 576-587. (c) Chen, P.; DeBeer, G.
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,
744-745. (d) Chen, P.; Cabrito, I.; Moura, J. J. G.; Moura, I.;
Solomon, E. I. J. Am. Chem. Soc. 2002, 124, 10497-10507. (e)
Oganesyan, V. S.; Rasmussen, T.; Fairhurst, S.; Thomson, A. J. Dalton
Trans. 2004, 996-1002.
(11) (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, 15708-15709.
(b) Chan, J. M.; Bollinger, J. A.; Grewell, C. L.; Dooley, D. M. J.
Am. Chem. Soc. 2004, 126, 3030-3031.
(12) Gorelsky, S. I.; Ghosh, S.; Solomon, E. I. J. Am. Chem. Soc. 2006,
128, 278-290.
(13) (a) Brown, E. C.; Aboelella, N. W.; Reynolds, A. M.; Aullo´n, G.;
Alvarez, S.; Tolman, W. B. Inorg. Chem. 2004, 43, 3335-3337. (b)
Brown, E. C.; York, J. T.; Antholine, W. E.; Ruiz, E.; Alvarez, S.;
Tolman, W. B. J. Am. Chem. Soc. 2005, 127, 13752-13753. (c) York,
J. T.; Brown, E. C.; Tolman, W. B. Angew. Chem., Int. Ed. 2005, 44,
7745-7748.
(14) Cu(I)-sulfur complexes are numerous, see ref 4a and (a) Dance, I.
G. Polyhedron 1986, 5, 1037-1104. (b) Ramli, E.; Rauchfuss, T. B.;
Stern, C. L. J. Am. Chem. Soc. 1990, 112, 4043-4044. (c) Yam, V.
W.-W.; Lo, K. K.-W.; Wang, C.-R.; Cheung, K.-K., J. Phys. Chem.
A 1997, 101, 4666-4672. (d) Lee, Y.; Sarjeant, A. A. N.; Karlin, K.
D. Chem. Commun. 2006, 621-623.
Figure 2. Schematic molecular orbital energy-level diagram for the (µ-
η2:η2-peroxo/sulfido)dicopper core (adapted from ref 22).
(15) Karlin, K. D. Science 1993, 261, 701-708.
(16) Helton, M. E.; Chen, P.; Paul, P. P.; Tyeklar, Z.; Sommer, R. D.;
Zakharov, L. N.; Rheingold, A. L.; Solomon, E. I.; Karlin, K. D. J.
Am. Chem. Soc. 2003, 125, 1160-1161.
(17) Fujisawa, K.; Moro-oka, Y.; Kitajima, N. J. Chem. Soc., Chem.
Commun. 1994, 623-624.
(18) Helton, M. E.; Maiti, D.; Zakharov, L. N.; Rheingold, A. L.; Porco,
J. A., Jr.; Karlin, K. D. Angew. Chem., Int. Ed. 2006, 45, 1138-1141.
(19) Tyeklar, Z.; Jacobson, R. R.; Wei, N.; Murthy, N. N.; Zubieta, J.;
Karlin, K. D. J. Am. Chem. Soc. 1993, 115, 2677-2689.
(20) Kitajima, N.; Fujisawa, K.; Fujimoto, C.; Moro-oka, Y.; Hashimoto,
S.; Kitagawa, T.; Toriumi, K.; Tatsumi, K.; Nakamura, A. J. Am.
Chem. Soc. 1992, 114, 1277-1291.
(21) Henson, M. J.; Vance, M. A.; Zhang, C. X.; Liang, H.-C.; Karlin, K.
D.; Solomon, E. I. J. Am. Chem. Soc. 2003, 125, 5186-5192.
(22) Chen, P.; Fujisawa, K.; Helton, M. E.; Karlin, K. D.; Solomon, E. I.
J. Am. Chem. Soc. 2003, 125, 6394-6408.
plexes most pertinent here (2, L ) TpiPr2, and its peroxo
congener) exhibit a pair of intense charge-transfer (CT)
electronic absorption bands and low S-S and O-O stretch-
ing frequencies of 500 and ∼760 cm-1, respectively, that
indicate weak bonds (i.e., extensive activation of the S2 and
O2 moieties). These features have been rationalized by a
common core-bonding model illustrated in Figure 2.22
2-
According to this picture, the O22-/S2 π* orbital that lies
in the plane of the Cu2O2/Cu2S2 core (π*σ) interacts strongly
2-
with the Cu dxy orbitals, while the out-of-plane O22-/S2
Inorganic Chemistry, Vol. 46, No. 2, 2007 487

Brown et al.
Taken together, these results provide new insights into ligand
influences on S-S bond activation by copper ions.
Experimental Section
General Considerations. All solvents and reagents were ob-
tained from commercial sources and used as received unless noted
otherwise. The solvents tetrahydrofuran (THF), pentane, diethyl
ether (Et2O), acetonitrile (CH3CN), and CH2Cl2 were passed though
solvent purification columns (Glass Contour, Laguna, CA, or
MBraun) prior to use. All metal complexes were prepared and stored
in a Vacuum Atmospheres inert atmosphere glove box under a dry
nitrogen atmosphere or were manipulated using standard Schlenk
line techniques. Labeled elemental sulfur (34S, 99% enrichment)
was purchased from Cambridge Isotope Laboratories, Inc. Com-
plexes of the general formula R(R’LR′′2)Cu(CH3CN),23 as well as
(HL’iPr2)Cu(CH3CN),24 (MeL’iPr2)Cu(CH3CN),25 [H(Me2LEt2)Cu]2-
(S2) (2b),13a and [(HL’iPr2)Cu]2(S2) (2i),13a were prepared via
previously reported methods (for ligand nomenclature, see Figure
3).Thecomplex(HL’Me2)Cuwaspreparedsimilarlyto(HL’iPr2)Cu(CH3-
1
CN), characterized by H NMR spectroscopy, and used directly
for the synthesis of [(HL’Me2)Cu]2(S2) (2h) (Supporting Informa-
tion).
Physical Methods. NMR spectra were recorded on a Varian VI-
300 or VXR-300 spectrometer. Chemical shifts (δ) for H NMR
Figure 3. Ligands and abbreviations used in this work.
1
spectra were referenced to residual protium in the deuterated solvent.
UV-vis spectra were recorded on a HP8453 (190-1100 nm) diode
array spectrophotometer; extinction coefficients were determined
from Beers’ Law plots. Resonance Raman spectra were recorded
on an Acton 506 spectrometer using a Princeton Instruments LN/
CCD-11100-PB/UVAR detector and ST-1385 controller interfaced
with Winspec software. A Spectra Physics BeamLok 2065-7S Ar
Laser provided excitation at 457.9 nm. The spectra were obtained
at -196 °C using a backscattering geometry. Samples were frozen
in NMR tubes in thermal contact with a Dewar flask containing
liquid nitrogen. Raman shifts were externally referenced to liquid
indene. Elemental analyses were performed by Robertson Microlit
(Madison, NJ).
π* orbital (π*v) remains basically unperturbed (nonbonding
with respect to the Cu ions). The Cu-O/S bonding is thus
dominated by highly covalent, strong σ-donation from the
filled O22-/S22- π*σ orbital into the Cu dxy set. An additional
interaction occurs between the Cu dxy orbitals and the empty
O22-/S22- σ* orbital. Essentially a backbonding interaction,
it results in a lowering of the predominantly Cu-based
2-
HOMO energy and rationalizes the weakening of the O2
/
S22- bond. The electronic CT absorption bands derive from
excitations out of the π*σ + xy + xy and π*v orbitals, with
the former being more intense and at higher energy. A
detailed comparative study22 showed that the Cu2S2 core of
2 (L ) TpiPr2) exhibits greater metal-ligand covalency (π*σ/
Cu) and backbonding (Cu/σ*) than its peroxide analog,
corresponding to a more significant weakening (activation)
of the S-S bond.22
[H(Me2LMe2)Cu]2(S2) (2a). [H(Me2LMe2)]Cu(CH3CN) (43.6 mg,
0.12 mmol) in CH3CN (4 mL) was added to elemental sulfur (3.8
mg, 0.015 mmol) in CH3CN (2 mL). The reaction mixture was
stirred for 30 min, during which a brown precipitate formed. The
precipitate was collected, washed with CH3CN (4 mL), and dried
under reduced pressure (13.4 mg, 44%). Allowing the reaction
mixture to stir for >1 h resulting in the conversion of [H(Me2LMe2)-
Cu]2(S2) to [Cu(SR)]4 [R ) H(Me2LMe2)] (5a); a 1H NMR spectrum
of this residue matched that of 5a prepared independently (see
We recently communicated the synthesis and characteriza-
tion of two examples of µ-η2:η2-disulfide complexes sup-
ported by bidentate N-donor ligands (2, L ) b or i, Figure
3).13a Their S-S bond distances (2.2007(11) and 2.165(3)
Å, respectively) are distinctly longer than that of 2, L ) TpiPr2
or Me2NPY2, (2.073(4) Å and 2.117(2) Å, respectively),
suggesting an even greater degree of S-S bond weakening
because of the nature of the supporting ligands. Here, we
present a more complete study in which we assess the X-ray
structures, UV-vis and resonance Raman spectra, and
aspects of the reactivity of a series of µ-η2:η2-disulfide
complexes supported by bidentate N-donor ligands with
variable steric and electronic properties (Figure 3). Inter-
pretations of the spectroscopic properties within the context
of the bonding picture illustrated in Figure 2, correlations
of Cu2S2 core structural parameters and S-S stretching
frequencies, and reactivity different from that previously
reported for 2, L ) Me2NPY2 (Figure 1)18 are discussed.
1
below). H NMR (C6D6, 300 MHz): δ 6.90-7.01 (m, 12H), 4.79
(s, 2H), 2.03 (s, 24H), 1.44 (s, 12 H). UV-vis (THF) [λmax, nm (ꢀ,
M-1 cm-1)]: 211 (18500), 327 (19200), 352 (12000), 427 (8200),
540 (400), 804 (130). Repeated attempts to obtain a satisfactory
elemental analysis (CHN) for 2a failed; we attribute this to the
variable small amounts of sulfur impurities that we were unable to
remove by recrystallization because of the competing formation of
5a. Structural assignment thus rests on the X-ray crystal structure
and the spectroscopic data.
[H(tBu2LiPr2)Cu]2(S2) (2c). [H(tBu2LiPr2)]Cu(CH3CN) (45.3 mg,
0.075 mmol) in pentane (3 mL) was added to elemental sulfur (1.2
(23) Spencer, D. J. E.; Reynolds, A. M.; Holland, P. J.; Jazdzewski, B. A.;
Duboc-Toia, C.; Le Pape, L.; Yokota, S.; Tachi, Y.; Itoh, S.; Tolman,
W. B. Inorg. Chem. 2002, 41, 6307-6321.
(24) Hayes, P. G.; Welch, G. C.; Emslie, D. J. H.; Noack, C. L.; Piers, W.
E.; Parvez, M. Organometallics 2003, 22, 1577-1579.
(25) Reynolds, A. M.; Gherman, B. F.; Cramer, C. J.; Tolman, W. B. Inorg.
Chem. 2005, 44, 6989-6997.
488 Inorganic Chemistry, Vol. 46, No. 2, 2007

Side-On Disulfido-Bridged Dicopper Complexes
1
mg, 0.0047 mmol) in pentane (2 mL). The reaction mixture was
stirred for 1 h and filtered though Celite, and the filtrate was
concentrated under reduced pressure to ∼2 mL. Storage of the
orange solution at -20 °C resulted in the formation of yellow and
brown crystals in a ratio of about 30:1. The brown material was
identified as [H(tBu2LiPr2)Cu]2(S2) by X-ray crystallography on a
selected crystal, but a pure bulk sample has not been obtained to
date. Raman spectroscopy was performed on an initial reaction
solution prepared in benzene, which was frozen in liquid N2
immediately after preparation.
[Ph(H2LEt2)Cu]2(S2) (2d). [Ph(H2LEt2)]Cu(CH3CN) (112.0 mg,
0.22 mmol) in CH3CN (5 mL) was added to elemental sulfur (7.0
mg, 0.027 mmol) in CH3CN (2 mL). The reaction mixture was
stirred for 3 h, during which a green precipitate formed. The
precipitate was collected, washed with CH3CN (8 mL), and dried
under reduced pressure (88.7 mg, 40%). 1H NMR (C6D6, 300
MHz): δ 7.58 (s, 4H), 6.92-7.11 (m, 22 H), 2.51 (quartet, J )
7.5 Hz, 16H), 1.07 (t, J ) 7.5 Hz, 24H). UV-vis (THF) [λmax, nm
(ꢀ, M-1cm-1)]: 295 (49385), 387 (34376), 421 (23470), 588 (1260),
813 (310). Anal. Calcd for C58H66Cu2N4S2: C, 68.95; H, 6.58; N,
5.55. Found: C, 68.67; H, 6.43; N, 5.40.
pressure (67.0 mg, 67%). H NMR (C6D6, 300 MHz): δ 7.41 (s,
2H), 6.96-7.07 (m, 8H), 6.77-6.89 (m, 8H), 6.36 (d, J ) 8.4 Hz,
2H), 6.26 (m, 2H), 2.09 (s, 12H), 1.91 (s, 12 H). UV-vis (THF)
[λmax, nm (ꢀ, M-1 cm-1)]: 256 (49 700), 282 (31 300), 435 (29 100),
604 (1300), 815 (400). Anal. Calcd for C46H46Cu2N4S2: C, 65.30;
H, 5.48; N, 6.62. Found: C, 64.83; H, 5.42; N, 6.49.
[(MeL’iPr2)Cu]2(S2) (2j). (MeL’iPr2)Cu(CH3CN) (51.8 mg, 0.09
mmol) in toluene (5 mL) was added to elemental sulfur (3.0 mg,
0.012 mmol) in toluene (2 mL). The reaction mixture was stirred
for 3 h and filtered though Celite, and the volume was reduced to
2 mL. The addition of 20 mL of CH3CN and storage at -20 °C
resulted in the precipitation of a dark green solid. The green solid
was collected and dried under reduced pressure (17.0 mg, 33%).
1H NMR (C6D6, 300 MHz): δ 7.40 (d, J ) 9.6 Hz, 2H), 6.85-
7.30 (m, 12 H), 6.80 (m, 2H), 6.41 (d, J ) 8.7 Hz, 2H), 6.31 (m,
2H), 3.25 (septet, J ) 6.6 Hz, 4H), 2.87 (septet, J ) 6.6 Hz, 4H),
1.85 (s, 6H), 1.22 (m, 24H), 1.12 (d, J ) 6.9 Hz, 12H), 0.97 (d, J
) 6.9 Hz, 12H). UV-vis (THF) [λmax, nm (ꢀ, M-1 cm-1)]: 250
(32 200), 292 (15 800), 437 (17 700), 460 (21 600), 651 (1000),
877 (120). Anal. Calcd for C64H82Cu2N4S2: C, 69.97; H, 7.52; N,
5.10. Found: C, 69.35; H, 7.06; N, 4.62.
[Ph(H2LiPr2)Cu]2(S2) (2e). [Ph(H2LiPr2)]Cu(CH3CN) (46.5 mg,
0.082 mmol) in CH3CN (3 mL) was added to elemental sulfur (2.6
mg, 0.01 mmol) in CH3CN (2 mL). The reaction mixture was stirred
for 2 h, during which a green precipitate formed. The precipitate
was collected, washed with CH3CN (8 mL), and dried under reduced
[Cu(SR)]4 [R ) H(Me2LMe2)] (5a). H(Me2LMe2)Cu(CH3CN)
(90.0 mg, 0.22 mmol) in a 1:1 toluene/CH3CN (4 mL) solution
was added to elemental sulfur (7.14 mg, 0.028 mmol) in CH3CN
(2 mL). The reaction mixture was stirred for 2 h, resulting in the
development of a tan precipitate. The solid was collected, washed
with CH3CN (3 × 5 mL), and dried under reduced pressure (50
mg, 57%). 1H NMR (CD2Cl2, 300 MHz): δ 6.80-7.00 (m, 24H),
3.90 (s, 4H), 2.09 (broad s, 24H), 2.03 (broad s, 12H), 1.96 (broad
s, 12 H), 1.73 (broad s, 12 H), 1.65 (broad s, 12H). Anal. Calcd
for C84H100Cu4N8S4: C, 62.89; H, 6.28; N, 6.99. Found: C, 62.47;
H, 5.85; N, 6.88.
1
pressure (32.0 mg, 70%). H NMR (C6D6, 300 MHz): δ 7.66 (s,
4 H), 7.04-7.22 (m, 14 H), 6.98 (d, J ) 7.6 Hz, 8 H), 3.26 (septet,
J ) 6.9 Hz, 8 H), 1.06 (d, J ) 6.8 Hz, 24 H), 1.01 (d, J ) 6.8 Hz,
24 H). UV-vis (THF) [λmax, nm (ꢀ, M-1 cm-1)]: 299 (52 760),
386 (32 410), 432 (24 350), 612 (1290). Anal. Calcd for C66H82-
Cu2N4S2: C, 70.61; H, 7.36; N, 4.99. Found: C, 70.32; H, 7.62;
N, 4.72.
Reactions of 2h with PPh3. Two equivalents of PPh3 (7.1 mg,
0.027 mmol) were dissolved in C6D6 (1 mL), and the mixture was
added to [(HL’Me2)Cu]2(S2) (2h) (11.4 mg, 0.013 mmol). After the
[3,5-(CF3)2C6H3(H2LMe2)Cu]2(S2)
(2f).
[3,5-(CF3)2C6H3-
(H2LMe2)]Cu(CH3CN) (65.2 mg, 0.11 mmol) in CH3CN (4 mL) was
added to elemental sulfur (3.5 mg, 0.014 mmol) in CH3CN (3 mL).
The reaction mixture was stirred for 4 h, during which a brown
precipitate formed. The precipitate was collected, washed with CH3-
CN (5 mL), and dried under reduced pressure (38.0 mg, 59%). 1H
NMR (C6D6, 300 MHz): δ 7.56 (s, 2H), 7.41 (s, 4H), 7.26 (s, 4H),
6.96 (m, 4H), 6.88 (d, J ) 7.4 Hz, 8H), 1.99 (s, 24H). UV-vis
(THF) [λmax, nm (ꢀ, M-1 cm-1)]: 319 (60 500), 377 (32 900), 421
(25 500), 547 (1400), 827 (600). Anal. Calcd for C54H46-
Cu2F12N4S2: C, 55.43; H, 3.96; N, 4.79. Found: C, 55.16; H, 4.23;
N, 4.58.
1
mixture was stirred for 1 h, H and 31P{1H} NMR spectra were
obtained, which showed formation of 1 equiv of triphenylphosphine
sulfide, 1 equiv of (HL’Me2)Cu(PPh3), and 0.5 equiv of unreacted
2h. The identity of (HL’Me2)Cu(PPh3) was confirmed by indepen-
dently reacting (HL’Me2)Cu(CH3CN) (9.2 mg, 0.021 mmol) with
PPh3 (5.6 mg, 0.021 mmol) in C6D6 (1 mL). 1H NMR (C6D6, 300
MHz): δ 7.97 (d, J ) 2.7 Hz, 1H), 6.80-7.10 (m, 23H), 6.66 (d,
J ) 8.7 Hz, 1H), 6.41 (t, J ) 7.6 Hz, 1H), 2.23 (s, 6H), 2.03 (s,
6H). 31P{1H} NMR (C6D6, 121.372 MHz): δ 6.81. A similar
reaction of PPh3 with 2h, using 4 equiv of phosphine, yielded
[3,5-(CF3)2C6H3(H2LiPr2)Cu]2(S2) (2g). [3,5-(CF3)2C6H3-
(H2LiPr2)]Cu(CH3CN) (190.0 mg, 0.27 mmol) in CH3CN (6 mL)
was added to elemental sulfur (8.6 mg, 0.034 mmol) in CH3CN (3
mL). The reaction mixture was stirred for 2 h, during which a green
precipitate formed. The precipitate was collected, washed with CH3-
CN (2 × 6 mL), and dried under reduced pressure (172.0 mg, 91%).
1H NMR (C6D6, 300 MHz): δ 7.67 (s, 4H), 7.60 (s, 4H), 7.53 (s,
2H), 7.11 (t, J ) 7.9 Hz, 4H), 6.94 (d, J ) 7.86 Hz, 8H), 3.19
(septet, J ) 6.58 Hz, 8H), 1.02 (apparent t: two overlapping d,
J ) 6.58 Hz, 48H). UV-vis (THF) [λmax, nm (ꢀ, M-1 cm-1)]: 322
(54 300), 381 (26 200), 434 (22 800), 575 (1300), 830 (290). Anal.
Calcd for C70H78Cu2F12N4S2: C, 60.29; H, 5.64; N, 4.02. Found:
C, 59.99; H, 5.52; N, 3.95.
2
equiv of triphenylphosphine sulfide and
2 equiv of
(HL’Me2)Cu(PPh3).
Reaction of 2h with Xylyl Isocyanide. Two equivalents of xylyl
isocyanide (3.7 mg, 0.028 mmol) were dissolved in C6D6 (1 mL)
and added to [(HL’Me2)Cu]2(S2) (2h) (12.0 mg, 0.014 mmol). After
1
the mixture was stirred for 1 h, the only species observed by H
NMR spectroscopy was the Cu(I)-xylyl isocyanide adduct. The
identity of this adduct was confirmed by independently reacting
(HL’Me2)Cu(CH3CN) (11.4 mg, 0.026 mmol) with xylyl isocyanide
(3.5 mg, 0.026 mmol) in C6D6 (1 mL). 1H NMR (C6D6, 300
MHz): δ 7.96 (s, 1H), 6.90-7.30 (m, 8H), 6.68 (d, J ) 8.7 Hz,
1H), 6.59 (t, J ) 7.5 Hz, 1H), 6.43 (t, J ) 7.0 Hz, 1H), 6.39 (d,
J ) 7.8 Hz, 2H), 2.48 (s, 6H), 2.31 (s, 6H), 1.65 (s, 6H).
[(HL’Me2)Cu]2(S2) (2h). (HL’Me2)Cu(CH3CN) (102.0 mg, 0.24
mmol) in CH3CN (4 mL) was added to elemental sulfur (7.6 mg,
0.03 mmol) in CH3CN (2 mL). The reaction mixture was stirred
for 3 h, during which a green precipitate formed. The precipitate
was collected, washed with CH3CN (6 mL), and dried under reduced
X-ray Crystallography. Crystals of the appropriate size were
chosen and placed on the tip of a 0.1 mm diameter glass fiber and
mounted on a Siemens SMART Platform CCD diffractometer for
data collection at 173(2) K. Data collections were carried out using
Mo KR radiation (graphite monochomator) with a detector distance
Inorganic Chemistry, Vol. 46, No. 2, 2007 489

Brown et al.
Scheme 1. Synthesis of Complexes
Figure 4. X-ray structure of 2d, showing all non-hydrogen atoms as 50%
thermal ellipsoids.
unit cell. The final full-matrix least-squares refinement converged
to R1 ) 0.0450 and wR2 ) 0.1031 (F2, all data).
[Ph(H2LEt2)Cu]2(S2) (2d). Green crystals suitable for X-ray
crystallography were grown from pentane at -20 °C. The final
full-matrix least-squares refinement converged to R1 ) 0.0386 and
wR2 ) 0.0825 (F2, all data).
[Ph(H2LiPr2)Cu]2(S2) (2e). Green crystals suitable for X-ray
crystallography were grown by vapor diffusion of pentane into a
toluene solution at -20 °C. The carbon atoms of one isopropyl
group were found to be disordered over two positions with a 52:
48 occupancy ratio. Two molecules of toluene are also present
within the asymmetric unit. The final full-matrix least-squares
refinement converged to R1 ) 0.0348 and wR2 ) 0.0747 (F2, all
data).
[3,5-(CF3)2C6H3(H2LMe2)Cu]2(S2) (2f). Brown crystals suitable
for X-ray crystallography were grown from vapor diffusion of
pentane into a CH2Cl2 solution at -20 °C. The fluorine atoms of
one of the CF3 groups were found to be disordered over two
positions with an 84:16 occupancy ratio. The final full-matrix least-
squares refinement converged to R1 ) 0.0419 and wR2 ) 0.0962
(F2, all data).
[3,5-(CF3)2C6H3(H2LiPr2)Cu]2(S2) (2g). Green crystals suitable
for X-ray crystallography were grown from pentane at -20 °C.
The fluorine atoms of one of the CF3 groups were found to be
disordered over two positions with a 90:10 occupancy ratio. Highly
disordered solvent was found that could not be modeled ap-
propriately. The reflection contributions of the solvent were
removed using the program PLATON, function SQUEEZE,29 from
which it was determined that there were 143 electrons in a volume
of 578.3 Å3 per unit cell. The final full-matrix least-squares
refinement converged to R1 ) 0.0380 and wR2 ) 0.1039 (F2, all
data).
[(HL’Me2)Cu]2(S2) (2h). Green crystals suitable for X-ray
crystallography were grown by vapor diffusion of pentane into a
toluene solution at -20 °C. Two molecules of toluene are present
within the asymmetric unit. The final full-matrix least-squares
refinement converged to R1 ) 0.0410 and wR2 ) 0.0997 (F2, all
data).
of 4.9 cm. The intensity data were corrected for absorption and
decay (SADABS).26 Final cell constants were calculated from the
xyz centroids of strong reflections from the actual data collection
after integration (SAINT).27 The CIFs given in the Supporting
Information show additional crystal and refinement information.
The structures were solved by direct methods using SHELXL-9728
software. All non-hydrogen atoms were refined with anisotropic
displacement parameters. All hydrogen atoms were placed in ideal
positions and refined as riding atoms with relative isotropic
displacement parameters. Pertinent details for each structure are
noted below (see Table S1 for a summary of crystallographic data
and the CIFs for full crystallographic information).
[H(Me2LMe2)Cu]2(S2) (2a). Red crystals suitable for X-ray
crystallography were grown by vapor diffusion of pentane into a
toluene solution at -20 °C. The final full-matrix least-squares
refinement converged to R1 ) 0.0321 and wR2 ) 0.0870 (F2, all
data).
[H(tBu2LiPr2)Cu]2(S2) (2c). Red crystals suitable for X-ray
crystallography were grown from pentane at -20 °C. The carbon
atoms of one isopropyl group were found to be disordered over
two positions with a 79:21 occupancy ratio. A pentane solvent
molecule was also disordered over two positions, as well as over
a 2-fold symmetry axis. Attempts to model the disorder were
unsuccessful so the reflection contributions of the solvent were
removed using the program PLATON, function SQUEEZE,29 which
calculated that there are 171 electrons in a volume of 852 Å3 per
[(MeL’iPr2)Cu]2(S2) (2i). Green crystals suitable for X-ray
crystallography were grown from pentane at -20 °C. Highly
disordered solvent was found that could not be modeled ap-
propriately. The reflection contributions of the solvent were
removed using the program PLATON, function SQUEEZE,29 from
which it was determined that there were 283 electrons in a volume
of 1484 Å3 per unit cell. The final full-matrix least-squares
refinement converged to R1 ) 0.0446 and wR2 ) 0.1183 (F2, all
data).
(26) For an empirical correction for absorption anisotropy, see: Blessing,
R. Acta Crystallogr. 1995, A51, 33.
(27) SAINT, version 6.2; Bruker Analytical X-Ray Systems: Madison, WI,
2001.
(28) SHELXTL, version 6.10; Bruker Analytical X-Ray Systems: Madison,
WI, 2000.
(29) Spek, A. L. Platon, A Multipurpose Crystallographic Tool; Utrecht
University: Utrecht, The Netherlands, 2002.
490 Inorganic Chemistry, Vol. 46, No. 2, 2007

Side-On Disulfido-Bridged Dicopper Complexes
Table 1. Selected Interatomic Distances (Å) and Angles (deg)a
[H(Me2LMe2)Cu]2(S2) (2a)
Cu1-N1
Cu1-N2
Cu1-S1
1.8964(17)
1.8994(17)
2.1842(6)
Cu1-S1A
2.1868(6)
2.2140(10)
3.7687(5)
S1-S1A
Cu1‚‚‚Cu1A
S1-Cu1-S1A
60.87(2)
N1-Cu1-S1-S1A
3.84(17)
[H(Me2LEt2)Cu]2(S2)b (2b)
Cu1-N1
Cu1-N2
Cu1-S1
1.9065(18)
1.9101(18)
2.1930(6)
Cu1-S1A
2.1974(6)
2.2007(11)
3.7991(5)
S1-S1A
Cu1‚‚‚Cu1A
S1-Cu1-S1A
60.16(3)
N-Cu1-S1-S1A
-0.07(18)
[H(tBu2LiPr2)Cu]2(S2) (2c)
Cu1-N1
Cu1-N2
Cu1-S1
1.942(2)
1.936(2)
2.2572(6)
Cu1-S2
2.2674(6)
2.1242(13)
3.9950(7)
S1-S2
Cu1‚‚‚Cu1A
S1-Cu1-S2
56.00(3)
N-Cu1-S1-S2
-32.40(18)
[Ph(H2LEt2)Cu]2(S2) (2d)
Cu1-N1
Cu1-N2
Cu1-S1
1.910(2)
1.909(2)
2.1951(10)
Cu1-S1A
2.1940(10)
2.2138(13)
3.7899(12)
S1-S1A
Cu1‚‚‚Cu1A
Figure 5. Plot of Cu-Cu, S-S, and Cu-S distances (Å, from Table 2)
determined by X-ray crystallography for the µ-η2:η2-disulfidodicopper
complexes (blue ) complexes supported by tri- and tetradentate ligands;
green ) complexes supported by bidentate â-diketiminate and anilido-imine
ligands). For label nomenclature, see Figures 1 and 3.
S1-Cu1-S1A
60.58(3)
N2-Cu1-S1-S1A
-5.0(2)
[Ph(H2LiPr2)Cu]2(S2) (2e)
1.9054(16)
1.9127(16)
2.1984(6)
Cu1-N1
Cu1-N2
Cu1-S1
Cu1-S1A
2.2051(6)
2.2007(10)
3.8143(5)
S1-S1A
Cu1‚‚‚Cu1A
S1-Cu1-S1A
59.97(2)
N2-Cu1-S1-S1A
-6.27(17)
[3,5-(CF3)2C6H3(H2LMe2)Cu]2(S2) (2f)
Cu1-N1
Cu1-N2
Cu1-S1
1.912(2)
1.906(2)
2.1978(8)
Cu1-S1A
2.1976(8)
2.2013(15)
3.8045(7)
S1-S1A
Cu1‚‚‚Cu1A
S1-Cu1-S1A
60.11(4)
N1-Cu1-S1-S1A
-4.6(3)
[3,5-(CF3)2C6H3(H2LiPr2)Cu]2(S2) (2g)
Cu1-N1
Cu1-N2
Cu1-S1
1.9213(18)
1.9047(17)
2.2060(9)
Cu1-S1A
2.1941(9)
2.2060(12)
3.8072(10)
S1-S1A
Cu1‚‚‚Cu1A
S1-Cu1-S1A
60.18(2)
N1-Cu1-S1-S1A
-6.3(2)
[(HL’Me2)Cu]2(S2) (2h)
Cu1-N1
Cu1-N2
Cu1-S1
1.925(2)
1.893(2)
2.1936(8)
Cu1-S1A
2.1916(8)
2.2130(15)
3.7858(7)
S1-S1A
Cu1‚‚‚Cu1A
S1-Cu1-S1A
60.62(3)
N1-Cu1-S1-S1A
-6.6(2)
[(HL’iPr2)Cu]2(S2)b (2i)
Cu1-N1
Cu1-N2
Cu1-S1
1.880(5)
1.922(5)
2.2011(18)
Cu1-S1A
2.2113(18)
2.165(3)
3.8446(16)
S1-S1A
Cu1‚‚‚Cu1A
S1-Cu1-S1A
58.78(8)
N-Cu1-S1-S1A
-3.9(5)
[(MeL’iPr2)Cu]2(S2) (2j)
Figure 6. (a) X-ray structure of [Cu(SR)]4 (R ) H(Me2LMe2)), showing
all non-hydrogen atoms as 50% thermal ellipsoids. (b) Comparison of the
cores of the structures of [Cu(SR)]4 (left, R ) H(Me2LMe2); right,
H(Me2LEt2)) with selected interatomic distances shown (Å): green, Cu; blue,
N; yellow, S; white, C. Note that the complex on the left features a C4
axis, so only one set of unique distances are listed.
Cu1-N1
Cu1-N2
Cu1-S1
1.889(2)
1.929(2)
2.2278(6)
Cu1-S2
2.2224(6)
2.1691(13)
3.8857(8)
S1-S2
Cu1‚‚‚Cu1A
S1-Cu1-S2
58.34(3)
N2-Cu1-S1-S2
1.5(2)
a
Estimated standard deviations in parentheses. “A” refers to symmetry-
related atoms. b These structures have been reported previously.13a
[Cu(SR)]4 [R ) H(Me2LMe2)] (5a). Yellow crystals suitable for
X-ray crystallography were grown by vapor diffusion of pentane
into a CH2Cl2 solution at -20 °C. Highly disordered solvent was
found that could not be modeled appropriately. The reflection
contributions of the solvent were removed using the program
PLATON, function SQUEEZE,29 from which it was determined
that there were 110 electrons in a volume of 1819 Å3 per unit cell.
The final full-matrix least-squares refinement converged to R1 )
0.0570 and wR2 ) 0.1326 (F2, all data).
Results
Synthesis of Complexes. With one exception, the series
of µ-η2:η2-disulfidodicopper(II) complexes 2a-j were iso-
lated as either brown or green solids from the reaction of
the Cu(I) precursors (a-j)Cu(CH3CN) with S8 in CH3CN,
Inorganic Chemistry, Vol. 46, No. 2, 2007 491

Brown et al.
Table 2. Cu2S2 Core Distances (Å) and Vibrational Frequencies (cm-1) for (µ-η2:η2-disulfido)dicopper Complexes
S-S
Cu‚‚‚Cu
av Cu-S
ν(S-S)
∆ν(34S)
ref
2a
2b
2c
2d
2e
2f
2g
2h
2.214(10)
2.2007(11)
2.1242(13)
2.2138(13)
2.2007(10)
2.2013(15)
2.2060(12)
2.2130(15)
2.165(3)
3.7687(5)
3.7991(5)
3.9950(7)
3.7899(12)
3.8143(5)
3.8045(7)
3.8072(10)
3.7858(7)
3.8446(16)
3.8857(8)
4.028(3)
2.186
2.195
2.262
2.195
2.202
2.198
2.200
2.193
2.206
2.225
2.264
2.233
442
443
454
424
435
441
428
432
440
443
500
9
10
13
10
8
9
12
8
this work
13a
this work
this work
this work
this work
this work
this work
13a
this work
17, 22
18
2i
2j
13
11
12
2.1691(13)
2.073(4)
2.117(2)
2 (L ) TpiPr2
)
2 (L ) Me2NPY2)
3.9336(10)
toluene, or pentane (Scheme 1).30 The products were
characterized by H NMR, UV-vis, and resonance Raman
activation changes; stronger bonding of the copper centers
to the S22- moiety accompanies a decreased S-S bond order.
The relationship between the Cu2S2 bonding and the sup-
porting ligand attributes is analyzed below (Discussion).
1
spectroscopy, as well as by CHN analysis and X-ray
crystallography. The exceptional case was [H(tBu2LiPr2)Cu]2-
(S2) (2c, Figure 2), which we were unable to isolate as an
analytically pure bulk sample, although crystals suitable for
an X-ray structural determination were obtained. Isolation
of samples of the complexes lacking a substituent on the
central methine position of the â-diketiminate ligand (e.g.,
2a and b) required shorter reaction times to prevent conver-
sion to the Cu(I) clusters [Cu(SR)]4 (5a and b, Scheme 1),
which were fully characterized in two instances (R )
H(Me2LEt2), 5b,13a and H(Me2LMe2), 5a). Exclusive formation
of 5a was possible using a CH3CN/toluene mixture as solvent
and allowing the solution to stir for >1 h.
X-ray Structures. (a) Disulfido Complexes. X-ray crystal
structures of 2a-j were determined, with 2b and i having
been reported previously.13a Because of their general similar-
ity, only that of 2d is shown here (Figure 4); thermal ellipsoid
representations of 2a, c, e-h, and j are provided as
Supporting Information (Figures S1-S7). Selected interac-
tomic distances and angles for all of the complexes are
summarized in Table 1.
(b) Clusters [Cu(SR)]4 (R ) H(Me2LMe2), 5a, or
H(Me2LEt2), 5b). The new X-ray structure of 5a is presented
in Figure 6a, and its core is compared to that of the previously
reported complex13a 5b in Figure 6b. The atom connectivity
in 5a and 5b is the same; both complexes feature 3-coor-
dinate Cu(I) ions bridged by thiolate sulfur atoms derived
from functionalization of the original â-diketiminate ligand
at the central methine carbon. The complexes adopt signifi-
cantly different geometries, however, and thus can be
envisioned as conformational isomers (notwithstanding the
different Me vs Et substituents). As indicated in Figure 6b,
the [Cu4(SR)4] core of 5a (which features a C4 molecular
axis of symmetry) is expanded relative to the folded core of
5b, such that the former features Cu‚‚‚Cu distances ∼0.7 Å
longer than in the latter. The puckering of the core in 5b is
accompanied by significantly more acute Cu-S-Cu angles
(∼75°) relative to those in 5a (∼100°), a variation that
indicates significant flexibility in the bonding of the thiolates
to the Cu(I) ions.
In each complex, the copper centers are coordinated to
two nitrogen atoms of the supporting ligand and both sulfur
atoms of the µ-η2:η2-disulfido bridge in a square planar
geometry. Some deviation of the Cu(II) ion from a square
planar shape is observed for 2c, presumably because of the
large steric constraints of the supporting ligand. This
deviation is evident by the significant distortion of the
N-Cu-S-S torsion angle (N2-Cu-S1-S2 ) -32.4°)
from the idealized value of ∼0°. Overall, the complexes in
the series exhibit similar planar Cu2S2 core geometries.
Nonetheless, structural variation is evident upon comparison
of the Cu2S2 core parameters (Cu-Cu, S-S, and Cu-S
distances) for 2a-j and other examples from the literature
(Table 2, Figure 4). The plot in Figure 5 shows that the core
parameters for most of the â-diketiminate complexes are
clustered close to one another (lower left), but across the
entire set there is a correlated trend, such that the S-S
distance decreases as the Cu-Cu and Cu-S distances
increase. This trend implies that as the supporting N-donor
ligand is varied, the core bonding and extent of S-S bond
Spectroscopy on Disulfido Complexes. (a) Absorption.
The disulfido complexes 2a-j are deeply colored because
of multiple low-energy features in their absorption spectra.
Plots of the spectra are presented in Figure S8 for those
complexes that could be isolated analytically pure in bulk;
the data for 2f and 2g are presented for illustrative purposes
in Figure 7a. In general, similar spectral features are observed
for all the complexes, which we assign by reference to the
detailed analysis published for 2 (L ) TpiPr2).22 The spectrum
of 2 (L ) TpiPr2) contains two intense features at ∼28 000
(ꢀ ≈ 31 200 M-1 cm-1) and ∼21 000 cm-1 (ꢀ ≈ 3700 M-1
cm-1) attributed to π* f Cu(II) CT transitions originating
from the π*σ + xy + xy (hereafter designated as π*σ) and
π*v orbitals, respectively (Figure 2). In addition, weaker d
f d transitions at ∼15 000 (ꢀ ≈ 230 M-1 cm-1) and ∼10 300
cm-1 (ꢀ ∼ 130 M-1cm-1) were reported. The spectra for
complexes 2a-j also exhibit intense bands between 20 000-
28 000 cm-1. In the case of the â-diketiminate complexes
exemplified by 2f and 2g (Figure 7a), these are clearly
resolved; for example, the maxima for 2g are at ∼23 000
and ∼26 400 cm-1. By analogy to the assignments for 2 (L
) TpiPr2), the features are assigned as the out-of-plane
(30) On the basis of 1H NMR spectroscopy, the Cu(I) complexes of ligands
a and h lack a CH3CN coligand.
492 Inorganic Chemistry, Vol. 46, No. 2, 2007

Side-On Disulfido-Bridged Dicopper Complexes
Figure 7. UV-vis absorption spectra of (a) â-diketiminate disulfido complexes 2f (dashed line) and 2g (solid line), and (b) anilido-imine disulfido complexes
2h (long dashed line), 2i (solid line), and 2j (short dashed line). All spectra were measured in THF at ambient temperature. Proposed assignments are
indicated (see text).
disulfide π*v f Cu(II) and the in-plane disulfide π*σ f Cu-
(II) CT transitions, respectively. The spectra also contain less
intense features between 13 000-18 000 cm-1 that may be
assigned as Cu d f d transitions. In general, the absorption
features for the â-diketiminate complexes occur at higher
energy than those of 2 (L ) TpiPr2). Thus, the lower energy
π*v f Cu(II) CT has a maximum at an energy of >23 000
cm-1 for 2a-g, which is g2000 cm-1 greater than that of
the corresponding feature at ∼21 000 cm-1 for 2 (L ) TpiPr2).
Similarly, the d f d transitions for the â-diketiminate
Figure 8. Resonance Raman spectra (λex ) 457.9 nm = 21 900 cm-1) of
compounds appear ∼1000-2000 cm-1 higher in energy than
frozen benzene solutions (77 K) of [Ph(H2LEt2)Cu]2(S2) (2d, solid line for
the ∼15 000 cm-1 band for 2 (L ) TpiPr2). These discrep-
32S, dashed line for 34S).
ancies may be traced to differences in the strength of the
Scheme 2. Reactions of 2h (L ) HL′Me2, h)
Cu-S bonding, as described below (Discussion). Such Cu-S
bonding differences also are likely to be the cause of more
subtle variation of absorption spectral features seen when
data for complexes with similar supporting ligands are
compared. For example, the disulfide π* f Cu(II) CT and
d f d bands for 2f are shifted to higher energy relative to
2g (Figure 7a).
The absorption spectra for the anilido-imine disulfido
complexes 2h-j (Figure 7b) are unique, insofar as their π*
f Cu(II) CT bands appear to overlap and to lie at lower
energy than the â-diketiminate analogs. The d f d features
for 2 (L ) TpiPr2). This latter value is similar to those reported
for a wide range of disulfido complexes of transition metals
also are shifted to lower energy. Proper assignments of these
spectra and rationales for their differences await more
(Table S2). Thus, the data indicate an especially high degree
complete spectroscopic studies, which have yet to be
of S-S bond activation for the series of compounds 2a-j.
Reactivity. For the purposes of drawing some preliminary
performed.
(b) Resonance Raman. Using an excitation wavelength
comparisons to the reactivity reported for 2 (L ) Me2-
of 457.9 nm that falls within the π*v f Cu(II) CT transition,
NPY2),18 reactions of one of the disulfido complexes (2h)
we obtained resonance Raman spectra of complexes 2a-j
with PPh3 and xylyl isocyanide were explored (Scheme 2).
prepared with 32S8 or 34S8. The spectra encompassing the
1
Monitoring by H and 31P NMR spectroscopy showed that,
region where sulfur-isotope-sensitive features were observed
as seen for 2 (L ) Me2NPY2),18 treatment of 2h with 4 equiv
(300-550 cm-1) are provided as Figures S9-S18; for
of PPh3 yielded 2 equiv of SdPPh3 and 2 equiv of the Cu-
illustration, the spectrum for 2d is shown in Figure 8. All
(I)-phosphine adduct (HL′Me2)Cu(PPh3). This complex was
the spectra contain a sharp sulfur-isotope-sensitive peak (∆ν-
identified on the basis of comparison to other known
(34S) ) 8-13 cm-1) that on the basis of precedent22 is
examples with â-diketiminate supporting ligands,31 as well
assigned to a predominantly ν(S-S) vibrational mode. The
peak positions and isotope shifts are listed in Table 2. The
as by independent synthesis from (HL′Me2)Cu(CH3CN). A
ν(S-S) values for 2a-j fall within a narrow range (424-
(31) Reynolds, A. M.; Lewis, E. L.; Aboelella, N. W.; Tolman, W. B. Chem.
454 cm-1) notably lower than ν(S-S) ) 500 cm-1 reported
Commun. 2005, 2014-2016.
Inorganic Chemistry, Vol. 46, No. 2, 2007 493

Brown et al.
similar reaction of 2h with 2 equiv of PPh3 resulted in
incomplete conversion to 1 equiv of the phosphine adduct
and 1 equiv of SdPPh3, with half of 2h remaining. This
suggests similar efficiency for S-atom transfer to PPh3 and
trapping of Cu(I) by PPh3, which differs from the results
reported for 2 (L ) Me2NPY2), where 2 equiv of PPh3 were
reported to yield 2 equiv of SdPPh3.18,32
also cause an increase in the energy splitting of the frontier
orbitals, notably a lowering of the π*σ + dxy + dxy and the
HOMO concomitant with a raising of the LUMO (π*σ +
dxy - dxy). Experimentally, these shifts are indicated by
increases in the energies of the π*v f Cu(II) and π*σ f
Cu(II) CT transitions in absorption spectra.22 In addition,
X-ray crystallographic and vibrational spectroscopic data
enable the extent of Cu-S bond strengthening and S-S bond
weakening to be directly discerned. Observation of appropri-
ate correlations among the experimental parameters provides
support for the bonding picture and enables the specific role
of ligand structural variation on S-S bond activation to be
determined.
The reactions of 2h with xylyl isocyanide, O2, and CO
also proceeded differently than the same reactions with 2
(L ) Me2NPY2). While S-atom transfer was reported for
reaction of 2 (L ) Me2NPY2) with xylyl isocyanide to yield
ArNdCdS,18 we observed clean conversion of 2h to the
Cu(I) adduct of the unfunctionalized isocyanide, (HL′Me2)-
2-
The X-ray structural and resonance Raman data show that
the degree of S-S bond activation in the [Cu2(µ-η2:η2-S2)]2+
complexes of bidentate, anionic â-diketiminate and anilido-
imine ligands is generally greater than in other disulfido
complexes of copper and other transition metals. As il-
lustrated in Figure 5 (Table 2), with one exception (2c), the
complexes 2a-j feature shorter Cu-S, longer Cu-Cu, and
longer S-S distances than 2 (L ) ii or Me2NPY2). The
correlated trends are consistent with the bonding picture in
Figure 2; shorter Cu-S distances indicate stronger Cu-S
bonds, which result in shorter Cu-Cu distances and a
lowering of the S-S bond order (longer S-S distance)
Cu(CNAr) (no effort to determine the fate of the S2
fragment was made). This complex was identified by
comparison to other examples,45 as well as by independent
synthesis. Complex 2h was unreactive with O2 (1 atm, ∼1
h, room temperature, THF, UV-vis) or CO (1 atm, 20 min,
room temperature, C6D6, NMR), in contrast to 2 (L ) Me2-
NPY2), which yielded peroxodicopper and Cu(I)-CO com-
plexes, respectively.18
Discussion
The reaction of S8 with Cu(I) complexes of a series of
anionic, bidentate N-donor ligands (Figure 3) yielded a series
of (µ-η2:η2-disulfido)dicopper complexes 2a-j. In the case
of the â-diketiminate ligands, in particular those that contain
an unsubstituted methine position (a-c), the syntheses are
complicated by further reaction to yield the clusters 5, which
were isolated for ligands a and b. A related sensitivity of
â-diketiminate ligands toward reaction at the methine posi-
tion has been reported previously.33 The ligand functional-
ization is avoided entirely when anilido-imine ligands h-j
are used. X-ray structures of 5a and 5b reveal that, despite
their different alkyl substituents, they are conformational
isomers. Their cyclic Cu4(SR)4 topologies differ with respect
to their Cu-Cu distances and Cu-S-Cu angles, such that
5b is puckered relative to 5a.
2-
through greater backbonding into the S2 σ* orbital.
Complexes 2c, 2j, and 2 (L ) TpiPr2 or Me2NPY2), which
exhibit Cu-S > 2.21 Å, Cu-Cu > 3.85 Å, and S-S <
2.17 Å, are at one extreme in Figure 5. Notably, the S-S
distance in 2 (L ) TpiPr2) of 2.073(4) Å is close to that of
H2S2 (2.055 Å), both of which fall within the range of values
reported for a large sampling of transition metal disulfido
complexes (∼2.00-2.08 Å, Table S2).34 Complexes 2a, d,
and h are at the other extreme, with Cu-S < 2.195 Å, Cu-
Cu < 3.79 Å, and S-S > 2.21 Å values that are indicative
of an extraordinary degree of S-S bond activation. Indeed,
a search of the CSD35 revealed only a few examples of
2-
complexes with S2 ligands featuring S-S bond distances
of >2.2 Å. The S22- ligand is bound to three or more metal
ions in several of these,36 leaving three cases with µ-η2:η2-
disulfido moieties bridging two Ni (S-S ) 2.298 and 2.209
Å)37 or Re ions (S-S ) 2.228 Å).38
A primary goal of this work is to assess the structural and
spectroscopic properties of the disulfido complexes 2a-j to
understand supporting ligand effects on S-S bond activation.
The relevant experimental data are best understood by
recourse to the bonding picture for the [Cu2(µ-η2:η2-S2)]2+
core (and peroxo analogs) shown in Figure 2.22 According
to this picture, increased Cu-S bonding interactions results
in greater backbonding from the filled Cu dxy orbitals into
The generally high degree of S-S bond activation in 2a-j
is further supported by the resonance Raman spectra, which
feature peaks attributed to predominantly ν(S-S) modes on
the basis of their positions and 34S-isotope shifts. The peak
positions for 2a-j are >45 cm-1 lower than that of 1 or 2
2-
the empty S2 σ* orbital and a lowering of the S-S bond
order (i.e., greater S-S bond activation). Stronger bonding
interactions between the Cu ions and the disulfide fragment
(34) Mu¨ller, A.; Jaegermann, W.; Enemark, J. H. Coord. Chem. ReV. 1982,
46, 245-280.
(35) Cambridge Structure Database, version 5.27, November 2005.
(36) (a) Cp2Ni2Mn4(CO)14(µ6-S2)(µ3-S)2, S-S ) 2.2573(12) Å. Adams,
R. D.; Miao, S.; Smith, M. D.; Farach, H.; Webster, C. E.; Manson,
J.; Hall, M. B. Inorg. Chem. 2004, 43, 2515-2525. (b) [(Cp*Ru)3-
(µ3-S2)(µ3-S)(µ2-SEt)]2+, S-S ) 2.210(3) Å. Houser, E. J.; Krautsc-
heid, H.; Rauchfuss, T. B.; Wilson, S. R. Chem. Commun. 1994,
1283-1284.
(32) Trapping of released sulfur (e.g., as S8) by free PPh3 is a possible
pathway,32a which only further mechanistic studies can validate or
refute. Bartlett, P. D.; Meguerian, G. J. Am. Chem. Soc. 1956, 78,
3710-3715.
(33) (a) Yokota, S.; Tachi, Y.; Itoh, S. Inorg. Chem. 2002, 41, 1342-
1344. (b) Jazdzewski, B. A.; Holland, P. L.; Pink, M.; Young, V. G.,
Jr.; Spencer, D. J. E.; Tolman, W. B. Inorg. Chem. 2001, 40, 6097-
6107. (c) Radzewich, C. E.; Coles, M. P.; Jordan, R. F. J. Am. Chem.
Soc. 1998, 120, 9384-9385. (d) Fekl, U.; Kaminsky, W.; Goldberg,
K. I. J. Am. Chem. Soc. 2001, 123, 6423-6424.
(37) (a) Pleus, R. J.; Waden, H.; Saak, W.; Haase, D.; Pohl, S. J. Chem.
Soc., Dalton Trans. 1999, 2601-2610. (b) Mealli, C.; Midollini, S.
Inorg. Chem. 1983, 22, 2785-2786.
(38) Rakowski DuBois, M.; Jagirdar, B. R.; Dietz, S.; Noll, B. C.
Organometallics 1997, 16, 294-296.
494 Inorganic Chemistry, Vol. 46, No. 2, 2007

Side-On Disulfido-Bridged Dicopper Complexes
2 (L ) TpiPr2) fall within the regime of the sampling of
typical transition metal disulfido complexes (purple circles,
bond order formally equal to 1). These generally feature
weaker S-S bonds than that of S2- (green circle, bond order
≈1.5) and S2 (bond order ) 2).
The weak S-S bonds in 2a-j may be attributed to the
presence of the strongly electron-donating â-diketiminate and
anilido-imine supporting ligands, which are effective at
inducing back-donation from the copper ions into the S-S
σ* orbital. Accordingly, this effect is decreased in 2 (L )
TpiPr2 or Me2NPY2) because the supporting ligands are
poorer electron donors, resulting in shorter and stronger S-S
bonds. The differences in the electron-donating capabilities
of the supporting ligands are also manifested in differences
in the Cu-S bonding, as revealed experimentally by the
energies of the π f Cu(II) CT and d f d transitions in
electronic absorption spectra. These transition energies are
generally higher in 2a-j than in 2 (L ) TpiPr2 or Me2NPY2),
reflecting greater splitting of the frontier molecular orbitals
and stronger Cu-S bonding (Figure 2).
Figure 9. Plot of the S-S distance (Å) versus 1/ν2/3 (cm2/3), where ν )
S-S vibrational mode. The various S22- complexes shown as purple circles
are listed in Table S2. The dashed line is a linear least-squares fit of the
data to eq 1, with slope ) CS-S ) 63.95 and intercept ) dSdS ) 1.063
(R ) 0.93).
(L ) TpiPr2), consistent with lower S-S bond orders for the
former class. The data may be further analyzed by application
of Badger’s Rule (eq 1), an empirical relationship
The relative electron-donating power of â-diketiminate/
anilido-imine versus tris(pyrazolyl)hydroborate ligands was
identified previously as a key determinant of the electronic
structures of 1:1 Cu/O2 adducts (peroxo-Cu(III) vs superoxo-
Cu(II))42 and Cu-thiolate models of type 1 copper electron-
transfer sites.43 Differences in the relative stability of bis(µ-
oxo)- versus µ-η2:η2-peroxodicopper isomers have also been
linked to the electron-donating capabilities of these support-
ing ligands.44 A finding of particular significance here is the
sole observation of bis(µ-oxo)dicopper complexes with the
â-diketiminate ligands a, b, d-g (which stabilize the formal
Cu(III) state).45 In contrast, analogous bis(µ-sulfido)dicopper-
(III) cores appear not to be accessible, which has been
verified by theoretical calculations.13a Despite the S-S bond
weakening evident in 2a-j, the lower electronegativity of
sulfur relative to oxygen renders cleavage of the S-S bond
in the disulfidodicopper core less favorable than that of the
O-O bond in the peroxodicopper congener.
Cij
ν2e/3
re )
+ dij
(1)
between an equilibrium bond distance (re) and its associ-
ated stretching frequency (νe) in a series of related species.39
Usually applied to small polyatomic molecules, it has also
recently been found to be useful for assessing heme and non-
heme iron-oxygen bonds.40 A plot of S-S distance versus
1/ν2/3 is shown in Figure 9 for 2a-j (red circles), 2 (L )
TpiPr2) (black triangles), 1 (green triangles), and a large
sampling from the large class of known transition metal
disulfido complexes (purple circles, listed in Table S2). Also
shown are data for Na2S2 and S2,41 taken as representative
of S-S single and SdS double bonds, respectively (black
circles). Further perspective is provided by a data point (green
circle) corresponding to the µ-1,2-disulfido(‚1-) moiety in
a recently reported dicopper complex.13c The data fit reason-
ably well to eq 1 with parameters C ) 63.95 and d ) 1.063
(dashed line, R ) 0.93), although the scatter about the line
is greater than that reported for a similar plot of Fe-O bonds
in heme species over similar ranges of distances (∆ ≈ 0.3
Å) and stretching frequencies (∆ ≈ 300-350 cm-1).40 We
speculate that this may be caused by the ν(S-S) features
not always being pure S-S stretches, a notion supported by
calculations reported for 2 (L ) TpiPr2) that indicate that its
500 cm-1 mode has 63% S-S and 37% Cu-S character.22
With this caveat in mind, a number of broad conclusions
can nonetheless be drawn from the linear correlation in
Figure 9. Although tightly clustered, the data for 2a-j lie at
one extreme, indicative of S-S bond orders even smaller
than that in Na2S2 (bond order formally equal to one, but
with an S-S bond considered to be slightly elongated
because of lone pair-lone pair repulsions). Complexes 1 and
Differences in the degree of S-S bond activation within
the series 2a-j are most clearly evident from the bond
distances (Figure 5)46 and can generally be attributed to steric
effects. For those ligands with relatively small aryl substit-
uents (R′′ ) Me or Et; a, b, d, f, h) or with larger iPr groups
but which feature R′ ) H (thus allowing the aryl group to
(42) (a) Cramer, C. J.; Tolman, W. B.; Theopold, K. H.; Rheingold, A. L.
Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 3635-3640. (b) Aboelella,
N. W.; Kryatov, S. V.; Gherman, B. F.; Brennessel, W. W.; Young,
V. G., Jr.; Sarangi, R.; Rybak-Akimova, E. V.; Hodgson, K. O.;
Hedman, B.; Solomon, E. I.; Cramer, C. J.; Tolman, W. B. J. Am.
Chem. Soc. 2004, 126, 16896-16911. (c) Sarangi, R.; Aboelella, N.;
Fujisawa, K.; Tolman, W. B.; Hedman, B.; Hodgson, K. O.; Solomon,
E. I. J. Am. Chem. Soc. 2006, 128, 8286-8296.
(43) Randall, D. W.; DeBeer, S.; Holland, P. L.; Hedman, B.; Hodgson,
K. O.; Tolman, W. B.; Solomon, E. I. J. Am. Chem. Soc. 2000, 122,
11632-11648.
(44) Mirica, L. M.; Ottenwaelder, X.; Stack, T. D. P. Chem. ReV. 2004,
104, 1013-1045.
(45) 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, 6307-6321.
(39) Badger, R. M. J. Chem. Phys. 1935, 3, 710-714.
(40) Green, M. T. J. Am. Chem. Soc. 2006, 128, 1902-1906.
(41) Mu¨ller, A.; Jaegermann, W. Inorg. Chem. 1979, 18, 2631-2633.
(46) A clear trend in ν(S-S) values for 2a-j is less obvious, perhaps also
because these vibrational modes are not pure S-S stretches.
Inorganic Chemistry, Vol. 46, No. 2, 2007 495

Brown et al.
‘bend back’; e, g, i), the full electronic donor influence of
the ligands is manifested by longer S-S, shorter Cu-S, and
shorter Cu-Cu distances. The ligands c and j are signifi-
cantly more sterically congested because of the presence of
iPr aryl (R′′) substituents and Me or tBu backbone (R′)
groups.47 We postulate that this congestion is the underlying
cause of the relatively longer Cu-S and Cu-Cu distances
and the decreased level of S-S bond activation in their
disulfido complexes. These steric effects for c and j have
precedent in Cu/O2 chemistry, since reactions of O2 with
Cu(I) complexes of these ligands yield 1:1 adducts42b,48
instead of bis(µ-oxo) complexes seen with a, b, and d-g.45
Preliminary investigation of the reactivity of one of the
disulfido complexes (2h) showed some similarity to that
previously reported for 2 (L ) Me2NPY2),18 although
significant differences also were seen (Scheme 2). Thus,
S-atom transfer to PPh3 with trapping of the resulting Cu(I)
sites by excess phosphine was observed for both, but the
results with only 2 equiv of PPh3 differed. For 2 (L ) Me2-
NPY2), complete S-atom transfer to yield 2 equiv of Sd
PPh3 was reported,18 but we found that 2h yielded 1 equiv
of SdPPh3 and 1 equiv of the Cu(I)-phosphine adduct, with
half of 2h left unreacted. These results imply that S-atom
transfer from 2 (L ) Me2NPY2) outpaces trapping of the
Cu(I) complex by PPh3, wheareas for 2h the two processes
are more closely competitive. Poorer S-atom transfer capa-
bilities for 2h relative to 2 (L ) Me2NPY2) are also
suggested by the results of reactions with xylyl isocyanide,
which for the former gave the Cu(I) adduct of the isocyanide,
but for the latter yielded the product of S-atom incorporation,
ArNdCdS. In addition, the lack of reactivity of 2h with O2
and CO contrasts with reactions of these reagents with 2 (L
) Me2NPY2), which yielded peroxodicopper or Cu(I)-CO
species, respectively.
structural correlation of the Cu-S, S-S, and Cu-Cu
distances within the [Cu2(µ-η2:η2-S2)]2+ core (Figure 5), the
S-S distances also correlate inversely with the ν(S-S) value
measured by resonance Raman spectroscopy (Badger’s rule,
Figure 9). A comparison to a range of other species that
feature S2n- units places 2a-j at the extreme of weak S-S
bonding (long S-S and low ν(S-S)) that corresponds to
particularly powerful S-S bond activation. These phenom-
ena, as well as the observation of increased energies for π
f Cu(II) CT and d f d transitions in electronic absorption
spectra, may be understood in terms of a previously proposed
bonding picture (Figure 2). According to this picture, large
frontier orbital energy splittings result from strong Cu-S
bonding, with the powerful electron-donating capabilities of
the â-diketiminate and anilido-imine ligands underlying
2-
increased backbonding into the S2 σ* orbital, which
weakens the S-S bond. These ligand effects are mitigated
by steric hindrance provided by bulky substituents, which
yield complexes with longer Cu-Cu and shorter S-S
distances.
The [Cu2(µ-η2:η2-S2)]2+ complexes supported by â-diketim-
inate or anilido-imine ligands also exhibit unique reactivity.
Sulfur transfer to the central methine carbon of the â-diketim-
inate ligands yields [Cu(SR)]4 clusters 5, which may adopt
different conformations depending on the ligand substituents.
This reaction is prevented when the anilido-imine ligands
are used (2h-j). A preliminary reactivity study of 2h
revealed facile sulfur transfer to added PPh3, but only
displacement without sulfur insertion upon reaction with
xylyl isocyanide, and no reactions with O2 or CO. In
comparison to 2 (L ) Me2NPY2), the sulfur moiety in 2h
appears to be less reactive, with trapping of the copper ions
to yield Cu(I)-X (X ) PPh3 or ArNC) complexes being
more facile.
Conclusions
Acknowledgment. We thank the NIH for financial
support of this research (GM47365 to W.B.T. and GM68307
to E.C.B.) and Dr. Lyndal Hill for assistance with data
analysis and preparation of Figure 5.
Through the synthesis and detailed structural and spec-
troscopic characterization of the series of complexes 2a-j
that feature the [Cu2(µ-η2:η2-S2)]2+ core, the degree of S-S
bond activation as a function of supporting ligand was
assessed. In general, the strong electron donating power of
the â-diketiminate and related anilido-imine ligands results
in strong Cu-S interactions that are correlated with weak
S-S bonds and short Cu-Cu distances. In addition to the
Supporting Information Available: Crystallographic data in
CIF format, experimental information for (HL′Me2)Cu, tables
showing a summary of the crystallographic data and the S-S bond
lengths and vibrational frequencies, and figures showing the X-ray
crystal structure of 2a, c, e-h, and j, the electronic absorption
spectra the the disulfido dicopper complexes, and the resonance
Raman spectra of 2a-j. This material is available free of charge
via the Internet at http://pubs.acs.org.
(47) Smith, J. M.; Lachicotte, R. J.; Holland, P. L. Chem. Commun. 2001,
1542-1543.
(48) (a) Aboelella, N. W.; Lewis, E. A.; Reynolds, A. M.; Brennessel, W.
W.; Cramer, C. J.; Tolman, W. B. J. Am. Chem. Soc. 2002, 124,
10660-10661. (b) Reynolds, A. M.; Gherman, B. F.; Cramer, C. J.;
Tolman, W. B. Inorg. Chem. 2005, 44, 6989-6997.
IC061589R
496 Inorganic Chemistry, Vol. 46, No. 2, 2007