A μ-η2:η2-disulfide dicopper(II) complex from reaction of S8 with a copper(I) precursor: Reactivity of the bound disulfur moiety

Article abstract of DOI:10.1002/anie.200503216

(Chemical Equation Presented) Side on: Elemental sulfur (S₈) reacts with a tridentate chelate (MePY2)^Me2N copper(I) compound, generating a side on μ-η²:η²-disulfido bridged complex [{Cu^II-(MePY2)^Me2N}₂(S₂ ^2-)]²⁺ (see structure: yellow S, green Cu, blue N, gray C); its reactivity towards PPh₃, RNC, CO, O₂, PhCH ₂Br, and a tetradentate chelating ligand are described.

Full text of DOI:10.1002/anie.200503216

Communications
Bioinorganic Chemistry
DOI: 10.1002/anie.200503216
A m-h2:h2-Disulfide Dicopper(ii) Complex from
Reaction of S8 with a Copper(i) Precursor:
Reactivityof the Bound Disulfur Moiety**
Matthew E. Helton, Debabrata Maiti, Lev N. Zakharov,
Arnold L. Rheingold, John A. Porco, Jr., and
Kenneth D. Karlin*
Copper chalcogenide compounds including clusters are of
continuing interest in structural, synthetic, and materials
chemistry,[1,2] while examples of copper/inorganic-sulfur
[*] Dr. M. E. Helton, D. Maiti, Prof. K. D. Karlin
Department of Chemistry
Johns Hopkins University
3400 North Charles Street, Baltimore, MD 21218 (USA)
Fax: (+1)410-516-7044
E-mail: karlin@jhu.edu
L. N. Zakharov,[+] Prof. A. L. Rheingold[+]
Department of Chemistry and Biochemistry
University of Delaware
Newark, DE 19716 (USA)
Prof. J. A. Porco, Jr.
Department of Chemistry and
Center for Chemical Methodology and
Library Development
Boston University
Boston, MA 02215 (USA)
[+] Current address:
Department of Chemistry and Biochemistry
University of California San Diego
La Jolla, CA 92093 (USA)
[**] K. D. Karlin is grateful to the US National Institute of Health (NIH,
GM 28962) for financial support of the research. M. E. Helton
acknowledges support fromthe NIH for a postdoctoral fellowship.
We also acknowledge Dr. Andreas Heutling (Boston University) for
carrying out preliminary studies.
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active-site entities in biology were recently discovered in the
case of two metalloenzymes.[3,4,5] Nitrous oxide reductase
(N2OR) has an active-site {(histidine)7Cu4S} cluster (called
CuZ), where nitrous oxide binds and is reduced in the terminal
step in bacterial denitrification (N2O + 2eꢀ + 2H+!H2O +
N2).[3,4] The other case occurs in the active site of aerobic
organism carbon monoxide dehydrogenase (CO +
H2OQCO2 + 2H+ + 2eꢀ), with a molybdopterin and copper
containing Mo-S-CuI-Scysteine moiety.[5] With these findings,
there has been a surge of interest in relevant copper
coordination chemistry, with goals including 1) the generation
of discreet and tractable copper sulfide complexes, 2) the
elucidation of their structure and spectroscopy, and 3) deter-
mination of copper sulfide compound chemistry, that is, redox
and/or atom-transfer reactivity patterns.
Figure 1. X-ray structure of the cation of 1-[B(C6F5)4]2·2CH2Cl2 (hydro-
Our own interests are focused towards chemistry poten-
tially relevant to N2OR.We previously reported the use of
elemental sulfur as a reagent to generate new copper sulfide
complexes with nitrogen containing co-ligands.In the reac-
tion of a copper(i) complex of tris(2-pyridylmethyl)amine
(TMPA) with S8, the first example of an end-on bound m-1,2-
disulfido dicopper(ii) complex was discovered.[6,7] Herein, we
report rather that, with a tridentate ligand, N,N-bis{2-[2-
gen atoms are omitted for clarity; yellow S, green Cu, blue N, gray C).
Selected bond lengths [Š] and angles [8]: S(1)–S(1A) 2.117(2), Cu(1)–
S(1) 2.2249(12), Cu(1)–S(1A) 2.2420(12), Cu(1)–N(1) 1.978(4), Cu(1)–
N(3) 2.225(4), Cu(1)–N(4) 1.978(4). Cu(1A)···Cu(1) 3.9336(10); N(1)-
Cu(1)-S(1) 96.53(12), N(3)-Cu(1)-S(1) 105.18(10), N(4)-Cu(1)-S(1)
151.30(11), N(1)-Cu(1)-S(1A) 147.50(12), N(3)-Cu(1)-S(1A)
104.59(10), N(4)-Cu(1)-S(1A) 98.93(11), S(1A)-S(1)-Cu(1) 62.12(5),
S(1)-Cu(1)-S(1A) 56.57(5), S(1A)-S(1)-Cu(1A) 61.31(6), Cu(1)-S(1)-
Cu(1A) 123.43(5).
(N’,N’-4-dimethylamino)pyridyl]ethyl}
methylamine
((MePY2)Me N), and S8 a side-on bound m-h2:h2-disulfido
2
dicopper(ii) compound is formed, [{CuII(MePY2)Me N}2-
lmax [nm] (e, mꢀ1 cmꢀ1) at 315 (13900), 395 (3700), 460
(1850), and 630 (460).There is a noticeable difference in the
charge-transfer energy and intensity for these transitions of
complex 1 relative to Kitajimaꢀs[10] and Tolmanꢀs[11] com-
2
(S22ꢀ)]2+ (1).It is notable that the pattern of structures, end-
on versus side-on binding with a tetradentate versus triden-
tate ligand for copper, exactly follows that already known for
peroxo dicopper(ii) complexes formed in ligand–CuI/O2
reactivity investigations.[8,9] We also report the reactivity of
1 towards exogenous substrates; (reversible) displacement of
bound sulfur can be effected by a number of reagents, and
substrate-sulfur-atom transfer can also be demonstrated.
Kitajima, Fujisawa, and co-workers have also generated a
side-on bound analogue of 1 using a tridentate hydrotris(3,5-
diisopropyl-1-pyrazolyl)borate (HB(3,5-iPr2pz)3) ligand; the
disulfide moiety formed through thermal decomposition
plexes, most likely because the (MePY2)Me N ligand is neutral
2
whereas the other systems have ligands with a ꢀ1 charge.
As mentioned above, an understanding of the reactivity of
copper sulfur (disulfide in this case) complexes is of consid-
erable interest, but almost no information is yet available.
Preliminary studies, the first of their kind, are described
below.The reaction of 1 with PPh3 leads to nearly quantitative
=
yields of triphenylphosphine sulfide (S PPh3; Scheme 1),
assuming the reaction leads to formal “release” of zero-valent
ꢀ
(through C S bond cleavage and follow-up chemistry) of a
thiolate copper(ii) complex.[10] More recently, Tolman,
Alvarez, and co-workers employed bidentate b-diketiminate
and anilido imine supporting ligands and ligand–copper(i)
reactions with either trimethylsilylsulfide or S8 to form m-
h2:h2-disulfido dicopper(ii) compounds.[11] The study pre-
sented herein complements this investigation, but 1 shows a
considerably more diverse reactivity than has been previously
known.
The X-ray crystal structure of 1-[B(C6F5)4]2·2CH2Cl2
(Figure 1) reveals that the copper ions in the centrosymmetric
binuclear complex are pentacoordinate, in slightly distorted
square-pyramidal environments (t = 0.06);[12] both pyridyl
nitrogen atoms (N(1) and N(4)) and the disulfur unit are
equatorial ligands.The remaining alkylamino nitrogen N(3) is
in the axial position.The nonbonding Cu···Cu (39. 336(10) Š)
ꢀ
distance and the S S (2.117(2) Š) bond in 1 fall in the middle
of a relatively narrow range of values seen for the other
known disulfido dicopper(ii) complexes.[6,10,11]
The UV/Vis absorption spectrum of 1-[B(C6F5)4]2·CH2Cl2
in acetone at reduced temperature contains multiple features
Scheme 1. Reactivity of the {Cu2S2} core in 1 towards exogenous substrates.
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Communications
sulfur and reduction of copper ion to give [CuI(MePY2)Me N]+.
chemistry, including reduction of 1 or analogues, could lead to
new copper sulfide species capable of N2O reduction chemis-
try.[3,16]
2
With excess (4 equiv) PPh3, the copper(i) is trapped as the
phosphine complex [CuI(MePY2)Me N(PPh3)]+ (2; Scheme 1).
2
Sulfur atom transfer from 1 also occurs in a near quantitiative
manner (GC and GC/MS analysis) when treated with 2,6-
dimethylphenylisocyanide, leading to the corresponding iso- Experimental Section
Preparation of 1-[B(C6F5)4]2·CH2Cl2: Solid elemental sulfur (0.0045 g,
thiocyanate (Scheme 1).Such chemistry is known for other
1.75 ” 10ꢀ5 mol) was added to
a
stirred solution of [{CuI-
metal sulfur compounds.[13]
((MePY2)Me N}][B(C6F5)4] (0.150 g, 1.40 ” 10ꢀ4 mol) prepared in dry/
deoxygenated CH2Cl2 (5 mL).The color of the solution changed from
near-colorless to dark red-brown over the course of a few hours; this
solution was allowed to stir for at least 1 day.Addition of dry/
deoxygenated pentane (75 mL) precipitated dark colored solid 1
which was collected by filtration through a coarse porosity Schlenk
filter-frit.The solid was redissolved in CH 2Cl2 and filtered through a
KimWipe plugged pipette and then the solvent was removed under
vacuum, and this process was repeated 5 times to confirm the removal
2
Interestingly, there is no reaction of 1 with benzyl bromide
(Scheme 1).This observation, combined with the results of
the PPh3 and isocyanide reactivity, indicates that the disulfur
moiety in 1 is electrophilic.In fact, this reactivity directly
contrasts with what we otherwise observe for an end-on m-1,2-
disulfido complex where nucleophilic chemistry occurs; with
a modified TMPA ligand, TMPAR, we observe that [{CuII-
(TMPAR)}2(S2)]2+ + 2PhCH2Br!2[CuII(TMPAR)Br]+ +
of all residual S8.The product was finally precipitated out with
I
PhCH2SSCH2Ph.Thus, our findings with ligand–Cu /S8 reac-
II
pentane.After collection by filtration, the solid product [{Cu
-
tivity appear to parallel those for ligand–CuI/O2 chemistry, as
side-on bound peroxo dicopper(ii) complexes are generally
electrophilic compared to end-on bound peroxides (e.g., as
with TMPA) which are nucleophilic towards exogenous
substrates.[8,14]
2
(MePY2)Me N}2(S22ꢀ)][B(C6F5)4]2·CH2Cl2 (0.080 g; 50%) was dried
under vacuum.Elemental analysis (%) calcd for
C87H60B2Cl2Cu2F40N10S2: C 45.65, H 2.64, N 6.12; found: C 45.97, H
2.72, N 6.11.
X-ray crystal structure analysis of 1-[B(C6F5)4]2·2CH2Cl2: A red
crystal suitable for X-ray diffraction was obtained from dichloro-
methane layered with pentane.Crystal data: C 88H62B2Cl4Cu2F40N10S2,
Other reactions also provide insight concerning the nature
of the CuII disulfide moiety in 1.Dioxygen displaces the
¯
Mr = 2374.10, triclinic, space group P1 (no.2), a = 11.3778(7), b =
disulfur moiety in 1, leading to the already well established m-
14.9669(9), c = 15.8101(9) Š, a = 106.7870(10), b = 103.6420(10), g =
h2:h2-peroxo complex [{CuII(MePY2)Me N}2(O22ꢀ)]2+ (lmax
=
107.3320(10)8, V= 2302.9(2)C3, Z = 1, 1calcd = 1.712 gcmꢀ3
,
m =
2
0.758 mmꢀ1, MoKa radiation (l = 0.71070 Š), T= 173 K, R1 = 0.074,
Rw = 0.088. CCDC-283434 contains the supplementary crystallo-
graphic data for this paper.These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
360 nm; Scheme 1).[15] When CO is bubbled through an
acetone solution of 1, the reddish color is bleached as a result
of the formation of the previously reported[15] carbonyl
2
complex,
[CuI(MePY2)Me N(CO)]+
(nCO = 2077 cmꢀ1)
PPh3 reactions: 31P NMR spectroscopy of the reaction mixture of
1-[B(C6F5)4]2·2CH2Cl2 with 4 equiv PPh3 in CDCl3 with P(Mes)3 as
(Scheme 1).That elemental sulfur is reformed in the reaction
of 1 with CO is indicated by the reversible nature of the
reaction.Application of a vacuum/Ar purge to the solution
(which removes the carbon monoxide) leads to a change from
colorless back to reddish as concomitant growth in the
absorption features arising to 1 occurs.This carbonylation/
sulfur-release/CO-removal cycle can be repeated several
times.
=
reference: (400 MHz); d = 43.92 (S PPh3), ꢀ1.69 (2), ꢀ4.62 (PPh3),
=
ꢀ35.91 ppm (P(Mes)3).On the basis of integration, the ratio of S
PPh3/[2+PPh3] ꢁ 1:1.2 showed the reaction yield was 91%. 31P NMR,
2 in CDCl3 with P(Mes)3 as reference (400 MHz): d = ꢀ1.72(2),
ꢀ35.92 ppm (P(Mes)3). 1H NMR, 2 in CDCl3 (400 MHz): d = 7.8 (2H,
d, J = 6.0 Hz), 7.41–7.34 (15H, m, PPh3), 6.29 (2H, d, J = 2.8 Hz), 6.25
(2H, d, J = 6.4 Hz), 2.97 (8H, m), 2.57 (8H, m), 2.26 ppm (3H, s).
Elemental analysis (%) calcd for [CuI(MePY2)Me N(PPh3)]-
2
Addition of the tetradentate ligand TMPA to a THF
solution of 1 causes an immediate color change to purple, the
solution exhibiting a UV/Vis feature at 568 nm ascribed to the
known complex [{CuII(TMPA)}2(S22ꢀ)]2+ (Scheme 1).[6,7] This
result implies the greater thermodynamic stability of [{CuII-
(B(C6F5)4)·(C5H12)0.9: C 56.30, H 3.95, N 5.01; found: 56.24, H 3.82,
N 4.89.
Received: September 9, 2005
Published online: December 30, 2005
(TMPA)}2(S22ꢀ)]2+ compared to [{CuII(MePY2)Me N}2(S22ꢀ)]2+
2
(1) and may reside with the tetradentate (versus tridentate)
nature of the ligand.Tolman and co-workers have also
demonstrated related disulfide exchange reactions.[11]
In conclusion, a {Cu2S2} core with bound side-on m-h2:h2-
Keywords: atomtransfer · bioinorganic chemistry · copper ·
N ligands · sulfur
.
2ꢀ
S2 moiety has been synthesized and characterized and the
reactivity with exogenous substrates has been explored.This
represents the first example of a sulfur atom-transfer reaction
from a copper sulfur moiety to substrates.Striking analogies
between copper dioxygen and copper sulfur chemistry appear
to be emerging.The use of elemental sulfur in reactions with
ligand–copper(i) complexes seems to be a lucrative avenue for
future generation of copper sulfur entities.Future investiga-
tions will include further structural–spectroscopic interrog-
ations and elaboration of the scope of sulfur atom-transfer
reactivity of this and other dicopper disulfide moieties.Redox
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