Carbon-sulfur bond formation between a ruthenium-coordinated thiyl radical and methyl ketones

Article abstract of DOI:10.1002/anie.200462713

A metal-coordinated thiyl radical was revealed by the electrochemical oxidation of a ruthenium(III)-thiolate in acetone (and related ketones). The subsequent reaction between the radical intermediate and acetone resulted in C-S bond formation and ultimately a ruthenium(III)-thioether product (see picture). A mechanism that involves reaction of the enol tautomer of acetone with the radical is proposed. (Figure Presented)

Full text of DOI:10.1002/anie.200462713

Angewandte
Chemie
À
C S Bond Formation
Carbon–Sulfur Bond Formation between a
Ruthenium-Coordinated Thiyl Radical and
Methyl Ketones**
Selma Poturovic, Mark S. Mashuta, and
Craig A. Grapperhaus*
The oxidation of metal–thiolate complexes to yield disulfides
is well-known.[1–4] This process may be irreversible, as with the
oxidation of [Fe(SPh)(CO)(PPh3)(h5-C5H5)]PF6 to form the
disulfide-bridged binuclear product [(Fe(CO)(PPh3)(h5-
C5H5)2(m-SSPh2)]2+ [5]
or reversible, as with the oxidation of
,
([RuIIIL], L = 1,4,7-tris(4-tert-butyl-2-mercaptobenzyl)-1,4,7-
triazacyclononane), which yields an isolable, intermolecular
disulfide complex.[6] In both cases, the oxidation is consistent
with a metal–thiyl radical intermediate. Recent efforts have
focused on stabilizing metal-coordinated thiyl radicals.[7–10]
Wieghardt and co-workers synthesized and compared phenyl-
thiyl and phenoxyl radicals coordinated to CoIII centers (and
other metal centers) with tacn-based ligands (tacn = 1,4,7-
triazacyclononane).[7] The phenylthiyl radical displays
increased reactivity compared with the phenoxyl analogue.
In the former the unpaired electron is localized on the sulfur
[*] S. Poturovic, Dr. M. S. Mashuta, Prof. Dr. C. A. Grapperhaus
Department of Chemistry
University of Louisville
Louisville, KY 40292 (USA)
Fax: (+1)502-852-8149
E-mail: grapperhaus@louisville.edu
[**] This research was supported by the National Science Foundation
(CHE-0238137). CCD X-ray equipment was purchased through
funds provided by the Kentucky Research Challenge Trust Fund.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2005, 44, 1883 –1887
DOI: 10.1002/anie.200462713
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atom, whereas in the latter the electron delocalizes over the
thioether product 6 (in acetone). Further oxidization to RuIII
(formation of 7) occurs at the potential applied to the cell.
Thus, the oxidation of 2 to 7 is a two-electron ECE
(electrode–chemical–electrode) process with no change in
phenyl ring. Earlier Hꢀckel molecular orbital (MO) calcu-
lations by Darensbourg and co-workers for RS·[{Cr(CO)5}2],
in which R is a tert-butyl group, also conclude that the singly
occupied molecular orbital (SOMO) is concentrated largely
on the 3p orbital of the sulfur atom.[11] Moreover, the
unpaired electron in noncoordinated thiyls is also localized
on the sulfur atom as confirmed by recent density functional
theory (DFT) calculations of the cysteine-derived radical.[12]
As expected, localization leads to enhanced reactivity. For
example, phenylthiyl radicals decay at a rate of approximately
109 mÀ1 sÀ1 to disulfide.[13]
À
the formal oxidation state of the metal center and C S bond
formation.
The oxidation of 2 in acetone was performed by bulk
electrolysis in a controlled-temperature electrochemical cell
(T= À178C) equipped with a quartz window for in situ UV/
Vis monitoring. On the basis of coulometric measurements
and UV/Vis spectroscopy (Figure 1), the oxidation is unequiv-
Herein, we report the novel reactivity of an electrophilic,
oxidized metal–thiolate (metal-coordinated thiyl radical)
À
with ketones that results in C S bond formation. Electro-
philic behavior contrasts the well-studied reactivity of nucleo-
philic metal–thiolates and presents a new method for the
synthesis of metal–sulfur complexes.[14] Previously, we
reported the oxidation of [Ru(dppbt)3] (2; dppbt = 2-diphe-
Figure 1. Electronic spectra obtained during bulk oxidation of 2 to 7 at
620 mV in acetone at À178C. Data were collected every 50 mC
(0.10 electron equivalents).
ocally assigned as two electrons per metal center, which is in
contrast to the one-electron oxidation of 2 in acetonitrile to
give 4. During initial stages of the oxidation in acetone, the
charge-transfer bands of 2 at 540 and 1041 nm decrease in
intensity with a concomitant increase in intensity at 850 nm,
which is attributed to the formation of 3. This observation for
the oxidation in acetone is identical to that for the oxidation
in acetonitrile.[15] However, upon decay of the band at 850 nm,
a new band at 735 nm, which is assigned to complex 7,
emerges in stark contrast to the silent visible spectrum in
acetonitrile associated with 4.
Square-wave voltammograms recorded after bulk oxida-
tion of 2 in acetonitrile and acetone are consistent with the
assignments of 4 as RuII–disulfide and 7 as RuIII–thioether
(Figure 2). UV/Vis-monitored spectroelectrochemistry con-
firms a reversible one-electron reduction of 7 to give 6 at an
applied potential of + 200 mV (see Supporting Information).
The RuIII/RuII couple of 7/6 is shifted by + 813 mV relative to
the thiolato precursors 2/1, consistent with previously
reported shifts associated with S alkylation.[16] In contrast, a
shift of + 1151 mV in the RuIII/RuII potential for the disulfide
complexes 5/4 indicates the modification of two anionic
thiolates to form two neutral sulfur donors. All complexes
assigned as RuIII display rhombic electron paramagnetic
resonance (EPR) spectra with g values near 2 as shown in
Table 1. The notable exception is the metal-coordinated
radical 3, which is EPR-silent, consistent with the coupling
of the ruthenium spin (S = 1/2) with that of the thiyl radical
(S = 1/2).
nylphosphinobenzenethiolate) in acetonitrile to form the
observable metal-coordinated RuIII–thiyl radical complex
3.[15] In this case, complex 3 is short-lived and decays to
form the proposed RuII–disulfide complex 4. The overall
oxidation of 2 to 4 proceeds by a coupled two-electron ligand
oxidation and one-electron metal-centered reduction. At an
applied potential of 620 mV, an EC mechanism (EC is short
for electrode step–chemical step) is observed. Complex 4 can
be further oxidized to its RuIII derivative, 5, but only at more
positive potentials (+ 806 mV).
Oxidation of 2 at 620 mV in acetone or related ketones
also initially yields 3. At this point, the pathways in
acetonitrile and ketones diverge as shown in Scheme 1.
À
Decay of 3 in ketones results in C S bond formation between
the thiyl radical and the solvent molecule to yield the RuII–
The electrospray-ionization mass spectrum (ESI-MS) of
complex 7 displays a parent ion with m/z = 1038, which is
consistent with the proposed structure (see Supporting
Scheme 1. Oxidation pathways for 1 in acetonitrile (top) and acetone
(bottom).
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Figure 3. ORTEP representation of 6 (thermal ellipsoids are set at the
40% level of probability).[23] Chlorobenzene molecules have been omit-
ted. Selected bond lengths [ꢀ] and angles [8]: Ru-P1 2.3545(10), Ru-P2
2.2939(11), Ru-P3 2.3626(10), Ru-S1 2.3971(10), Ru-S2 2.3826(10), Ru-
S3 2.4151(10), S3-C55 1.813(4); P2-Ru-P1 95.00(4), S2-Ru-P1 90.16(4),
S2-Ru-S1 175.57(4), C55-S3-Ru 108.98(13).
Figure 2. Comparison of square-wave voltammograms for solutions of
a) 1/2 (1 mm) in acetonitrile, b) 4/5 (1 mm) in acetonitrile, and c) 6/7
(1 mm) in acetone. Voltammograms were obtained at a glassy carbon
electrode referenced versus Ag+/Ag0.
À
À
the Ru Sthioether bond length for Ru S3, 2.4151(10) ꢁ, as is the
case in 14.
Table 1: Formal oxidation-state assignments, charge-transfer (CT) band
energies, and EPR g values for 1–11.
Scheme 2 details a proposed pathway for the reaction of 3
Complex
Formal
CT absorbance
g values[a]
with acetone.[25] The enol tautomer of acetone acts as a
oxidation state
lmax (e)
1
2
3
II
–
–
III
III
II
1041 (3020)[b]
2.12, 2.06, 2.04
–
–
850 (4800)
–
4
5
6
7
8
9
10
11
III
II
III
II
III
II
III
688 (2338)
2.09, 2.05, 2.03
–
2.09, 2.04, 2.01
–
2.15, 2.04, 2.02
–
2.15, 2.06, 2.01
–
735 (1921)[c]
–
707 (557)[d]
–
697 (106)[e]
[a] Obtained from frozen solution at 77 K. [b] Acetonitrile solution at
À178C. [c] Acetone solution at À178C. [d] 2-Butanone solution at
À178C. [e] Acetophenone solution recorded at 198C.
Information). Complex 6, the RuII derivative of 7, was also
prepared chemically by alkylation of 1 with chloroacetone in
acetonitrile. The ESI-MS, UV/Vis, spectroelectrochemical,
and EPR measurements of 7/6 from the two routes are
indistinguishable, which is consistent with the oxidation-
Scheme 2. Proposed pathway for the oxidation of 2 in acetone.
À
induced C S bond formation and the proposed structures
given above.
nucleophile and attacks the metal-coordinated thiyl radical to
result in the reduction of the RuIII center to RuII. Deproto-
nation of the RuII species yields the RuII–thioether product 6.
At the potentials necessary for oxidation of 2 to form 3, 6 is
further oxidized to the observed product 7. To support this
pathway, 2 was oxidized in the presence of the trimethylsilyl
enol ether derivative of acetone. The oxidation is a two-
electron process and yields a product identified spectroscopi-
cally as 7. Notably, no thiyl radical intermediate is observed
that is consistent with the expected rate increase with
increased enol concentration.
The X-ray crystal structure of 6[17–22] (Figure 3)[23] confirms
the alkylation of a single sulfur site to yield a neutral
ruthenium complex in a triphosphine-dithiolate-thioether
donor set. The chelating ligands arrange in a meridional
fashion as seen in 1 and its methylene-bridged dialkylated
derivative [Ru(dppbt){CH2(dppbt)2}]+ (14).[24] The ketone
functional group of the thioether donor is directed away from
À
the metal center. The Ru P bond lengths range from
2.2939(11) to 2.3626(10) ꢁ, similar to the bonds in 1 and 14.
À
À
The Ru Sthiolate bond lengths for Ru S1, 2.3971(10) ꢁ, and
To support the pathway proposed in Scheme 2, 3 was
generated under a variety of conditions. Similar ketones (2-
À
Ru S2, 2.3826(10) ꢁ, in 6 are not significantly shorter than
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standard Schlenk techniques unless otherwise stated. Chloroacetone,
chlorotrimethylsilane, sodium iodide, and triethylamine were
obtained from Aldrich and used as received. (Isopropenyloxy)tri-
methylsilane was prepared according to published methods.[31]
6: Chloroacetone (0.015 g, 16 mmol) was added with a micro-
syringe to a yellow solution of 1 (0.188 g, 12.4 mmol) in acetonitrile
(100 mL). The reaction solution was stirred for 1 hour. Removal of
solvent under reduced pressure yielded a yellow residue. The crude
product was dissolved in THF and gravity filtered to remove residual
PPNCl. Removal of solvent from the filtrate yielded 6 in 43% yield.
X-ray-quality needle-like crystals were obtained from liquid diffusion
of chlorobenzene/ether. Elemental analysis (%) calcd for
C57H47OP3S3Ru: C 65.94, H 4.56, found: C 63.48, H 4.37; IR (KBr
pellet): n˜3047(m), 1707(m), 1429(m), 1082(s), 693(s), 518 cmÀ1 (s).
Compound 6 can also be prepared by electrochemical reduction of 7
in acetone (1 mm, À17 Æ 38C) at an applied potential of + 200 mV.
7: Bulk oxidation of 3 in acetone at 620 mV (À178C) yields 7 by a
two-electron oxidation. The product was characterized by UV/Vis
and EPR spectroscopy, and square-wave voltammetry. Electronic
absorption: lmax = 735 nm (e = 1921 cmÀ1mÀ1).
butanone and acetophenone) generate products that are
closely related to 6 and 7 (see complexes 8–11; Ph = phenyl).
À
Acetonitrile, malononitrile, or THF, which have C H bond
enthalpies similar to or lower than that of acetone, but are not
II
À
enolizable, yield only Ru –disulfide 4 and there is no C S
bond formation.[26] Thus, a pathway that involves an initial
hydrogen atom abstraction step is not consistent with the
observed data.
The reaction of 3 with ketones resembles the previously
noted reactivity of disulfide-bridged metal clusters with
acetone.[27,28] C S bond formation was observed at the
À
bridging disulfide upon addition of acetone. It was suggested
that the enol tautomer of acetone acts as a nucleophile and
attacks the disulfide, whereas the cluster is reduced by two
electrons. This reduction is spread over two metal centers.
À
Finally, the C S bond formation between the enol
tautomer of acetone and 3 is reminiscent of the reaction
between ethylene and nickel dithiolene reported by Stiefel
and co-workers (Scheme 3).[29] In both cases, an oxidized
Received: November 24, 2004
Published online: February 11, 2005
Keywords: ketones · radicals · ruthenium · sulfur
.
[1] M. T. Ashby, J. H. Enemark, D. L. Lichtenberger, Inorg. Chem.
1988, 27, 191.
[2] C. A. Grapperhaus, M. Y. Darensbourg, Acc. Chem. Res. 1998,
31, 451.
[3] L. E. Maelia, M. Millar, S. A. Koch, Inorg. Chem. 1992, 31, 4594.
[4] J. C. Noveron, R. Herradora, M. M. Olmstead, P. K. Mascharak,
Inorg. Chim. Acta 1999, 285, 269.
Scheme 3. Reversible ethylene binding to nickel dithiolene.[29]
[5] P. M. Treichel, L. D. Rosenhein, Inorg. Chem. 1984, 23, 4018.
[6] B. Albela, E. Bothe, O. Brosch, K. Mochizuki, T. Weyhermꢀller,
K. Wieghardt, Inorg. Chem. 1999, 38, 5131.
[7] S. Kimura, E. Bill, E. Bothe, T. Weyhermꢀller, K. Wieghardt, J.
Am. Chem. Soc. 2001, 123, 6025.
[8] D. Herebian, E. Bothe, E. Bill, T. Weyhermꢀller, K. Wieghardt,
J. Am. Chem. Soc. 2001, 123, 10012.
[9] P. Ghosh, E. Bill, T. Weyhermꢀller, F. Neese, K. Wieghardt, J.
Am. Chem. Soc. 2003, 125, 1293.
mononuclear metal–sulfur complex is attacked by an ene
nucleophile, which results in reduction at the metal center and
À
C S bond formation. Reduction of the nickel center pro-
À
motes C S bond cleavage and release of ethylene. Further
studies by Geiger on related systems revealed that reductive
dissociation of the olefin proceeds by a two-electron ECE
mechanism.[30] Current efforts are focusing on the reaction of
3 with additional substrates.
[10] C. H. Hsieh, I. J. Hsu, C. M. Lee, S. C. Ke, T. Y. Wang, G. H. Lee,
Y. Wang, J. M. Chen, J. F. Lee, W. F. Liaw, Inorg. Chem. 2003, 42,
3925.
[11] J. Springs, C. P. Janzen, M. Y. Darensbourg, J. C. Calabrese, P. J.
Krusic, J. N. Verpeaux, C. Amatore, J. Am. Chem. Soc. 1990, 112,
5789.
[12] M. van Gastel, W. Lubitz, G. Lassmann, F. Neese, J. Am. Chem.
Soc. 2004, 126, 2237.
[13] G. N. R. Tripathi, Q. Sun, D. A. Armstrong, D. M. Chipman,
R. H. Schuler, J. Phys. Chem. 1992, 96, 5344.
[14] E. C. Constable, Metals and Ligand Reactivity, 2nd ed., VCH,
Weinheim, 1996.
Experimental Section
Physical and Spectroscopic Methods: IR spectra were obtained by
using a Thermo Nicolet Avatar 360 spectrometer with a 4 cmÀ1
resolution. X-band EPR spectra were collected on a Bruker EMX
EPR spectrometer at 77 K in a Suprasil quartz dewar. Spectra were
simulated with SimFonia. An Agilent8453 diode array spectrometer
was used for electronic absorption spectroscopy. Elemental analyses
were performed by Midwest Microlab (Indianapolis). Mass spectra
were recorded at the University of Louisville Mass Spectrometry
Core Laboratory. All electrochemical experiments were performed
with an EG&E273 potentiostat/galvenostat. Electrochemical and
spectroelectrochemical measurements were carried out in the 10 mL
cell designed by E. Bꢂthe of the Max Planck Institute fꢀr Bio-
anorganische Chemie, Mꢀlheim (Germany), as described previ-
ously.[15]
[15] C. A. Grapperhaus, S. Poturovic, Inorg. Chem. 2004, 43, 3292.
[16] P. J. Farmer, J. H. Reibenspies, P. A. Lindahl, M. Y. Dare-
nsbourg, J. Am. Chem. Soc. 1993, 115, 4665.
[17] Crystal data for 6: yellow needle, orthorhombic, space group
Pbca, a = 24.832(2), b = 10.9955(10), c = 41.949(4) ꢁ, V=
11453.7(18) ꢁ3, 1calcd = 1.465 gcmÀ3, Z = 8. Data were collected
on a Bruker SMART APEX CCD using MoKa radiation. For all
13702 unique reflections (R(int) = 0.056), the final anisotropic
full-matrix least-squares refinement on F2 for 713 variables data
converged at R1 = 0.06 and wR2 = 0.11 with a GOF of 1.09.
CCDC-256671 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from
Synthetic Methods: Syntheses of complexes 1–5 have been
previously reported.[15,24] 2-Butanone, and acetophenone were pur-
chased from Aldrich and were freshly distilled immediately prior to
use. Syntheses were performed under a N2 atmosphere by using
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Chemie
The Cambridge Crystallographic Data Centre via www.ccdc.
cam.ac.uk/data_request/cif.
[18] SHELXL-97. Program for the Refinement of Crystal Structures,
University of Gꢂttingen, Germany, 1997.
[19] SHELXTL (v6.12), Program Library for Structure Solution and
Molecular Graphics, Bruker Advanced X-ray Solutions, Inc.,
Madison, WI, 2001.
[25] An alternate pathway in which complex 3 is regarded as a RuIV
–
thiolate would yield the same product. Attack by the enol
tautomer at sulfur would result in the two-electron reduction of
the metal. The radical pathway is favored based on the
spectroscopic assignment of 3 as a metal-coordinated thiyl
radical[15] and the high sulfur character in the HOMO of
nucleophilic metal thiolates.[1]
[20] SMART (v.5628), Bruker Advanced X-ray Solutions, Inc.,
Madison, WI, 2002.
[21] SAINT (v6.36), Bruker Advanced X-ray Solutions, Inc., Mad-
ison, WI, 2002.
[22] SADABS (v2.02), Area Detector Absorption Correction, Uni-
versity of Gꢂttingen, Germany, 2001.
[23] ORTEP-3 for Windows: L. J. Farrugia, J. Appl. Crystallogr. 1997,
30, 565.
[26] J. A. Kerr, Chem. Rev. 1966, 66, 465.
[27] A. Venturelli, T. B. Rauchfuss, A. K. Verma, Inorg. Chem. 1997,
36, 1360.
[28] K. Matsumoto, H. Sugiyama, Acc. Chem. Res. 2002, 35, 915.
[29] K. Wang, S. E. Stiefel, Science 2001, 291, 106.
[30] W. E. Geiger, Inorg. Chem. 2002, 41, 1360.
[31] P. Cazeau, F. Duboudin, F. Moulines, O. Babot, J. Dunogues,
Tetrahedron 1987, 43, 2075.
[24] C. A. Grapperhaus, S. Poturovic, M. S. Mashuta, Inorg. Chem.
2002, 41, 4309.
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