Alkyne addition to a metal-stabilized thiyl radical: Carbon-sulfur bond formation between 1-octyne and [Ru(SP)3]+

Article abstract of DOI:10.1002/ejic.201101016

The metal-stabilized thiyl radical complex [Ru(SP)₃] ⁺, [Ru-1]⁺, {SP = 2-(diphenylphosphanyl)benzenethiolate} adds 1-octyne across the cis-sulfur sites to yield the S-alkylated dithiolene product [Ru-1·octyne]⁺. The product complex exists as a pair of inseparable geometric isomers, which were characterized by X-ray crystallography, ³¹P NMR spectroscopy, and cyclic voltammetry. By using electrochemical methods, the rate constant for 1-octyne addition was determined as 8.7 × 10³ M^-1 s^-1. Copyright

Full text of DOI:10.1002/ejic.201101016

SHORT COMMUNICATION
DOI: 10.1002/ejic.201101016
Alkyne Addition to a Metal-Stabilized Thiyl Radical: Carbon–Sulfur Bond
Formation between 1-Octyne and [Ru(SP)3]+
Kagna Ouch,[a] Mark S. Mashuta,[a] and Craig A. Grapperhaus*[a]
Keywords: Alkynes / C–S bond formation / Oxidation / Sulfur / Electrochemistry
The metal-stabilized thiyl radical complex [Ru(SP)3]+, [Ru-
1]+, {SP = 2-(diphenylphosphanyl)benzenethiolate} adds 1-
octyne across the cis-sulfur sites to yield the S-alkylated di-
thiolene product [Ru-1·octyne]+. The product complex exists
as a pair of inseparable geometric isomers, which were char-
acterized by X-ray crystallography, 31P NMR spectroscopy,
and cyclic voltammetry. By using electrochemical methods,
the rate constant for 1-octyne addition was determined as
8.7ϫ103
M .
–1 s–1
ing regimes with equilibrium constants over 20 orders of
magnitude are accessible within a 240 mV potential win-
dow.[2]
Introduction
We have recently reported a series of studies on the ad-
dition of alkenes to the oxidized metal-thiolate complexes
[M(SP)3]+ {M = Ru, Re; SP = 2-(diphenylphosphanyl)-
benzenethiolate}.[1–4] Although formally in the +4 oxi-
dation state, the complexes are regarded as metal-stabilized
thiyl radicals because of the delocalization of spin density
over the metal and sulfur donors, Scheme 1.[4] Two of the
S ligands are optimally positioned for C–S bond forming
reactions with alkenes, which generate diamagnetic RuII-di-
thioether complexes. The reaction is initiated by electro-
chemical (Scheme 2 top) or chemical (Scheme 2 middle)
oxidation of [Ru-1]–. Similar alkene additions are reported
with other oxidized ruthenium thiolates[5] and oxidized
metal-dithiolenes.[6–9] For [Ru-1]+, the alkene addition rate
constant varies from 4.3ϫ107 m–1 s–1 to 2.9ϫ103 m–1 s–1,
and higher values are observed for “electron-rich” al-
kenes.[1] While alkene addition to [Ru-1]+ is irreversible,
equilibrium binding constants for ethylene to [Re-1]n+ (n =
0, 1, 2) depend on the complex charge. Three distinct bind-
Scheme 2. Reactivity of [Ru-1]+ with alkenes and alkynes.
Herein, we describe carbon–sulfur bond forming reac-
tions between [Ru-1]+ and 1-octyne (Scheme 2 bottom).
Alkyne addition across the thiolate donors of neighboring
SP chelates yields a new tetradentate, S-alkylated dithiolene
ligand. The intraligand addition of alkynes across sulfur do-
nors of Mo-tris(dithiolene)s was employed by the Fekl
group to selectively displace a single dithiolene donor.[10]
Similarly, Yan and co-workers detected a variety of metal-
free alkyne disulfuration and hydrosulfuration products fol-
lowing alkyne addition to a dinuclear Co-thiolate.[11] Dis-
Scheme 1. Metal-coordinated thiyl radical complex [Ru-1]+ with
calculated spin density values (italics).[4]
[a] Department of Chemistry, University of Louisville,
2320 South Brook Street, Louisville, KY 40292, USA
Fax: +1-502-852-8149
E-mail: grapperhaus@louisville.edu
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201101016.
Eur. J. Inorg. Chem. 2012, 475–478
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
475

K. Ouch, M. S. Mashuta, C. A. Grapperhaus
SHORT COMMUNICATION
crete, isolable metal-coordinated alkyne addition products those observed in alkene addition products such as
were also reported by Yan by using related dinuclear Ru- [Ru-1·C2H4]+. The S2–C55–C56–S3 torsion angle, 8.3(8)°,
thiolates.[12,13] In that case, the formal oxidation states in approaches zero as expected for an S-alkylated dithiolene
the Ru precursor are +2 and +4, and alkyne addition occurs chelate; in contrast to the torsion angle of –47.3(3)° for the
at the more oxidized center.
dithioether fragment in [Ru-1·C2H4]+. The C55–C56 bond
length, 1.322(10) Å, matches the values of 1.312(9) and
1.394(10) Å reported by Yan in their alkyne addition prod-
ucts and is consistent with a C–C double bond.[12,13]
Results and Discussion
Oxidation of [Ru-1]– with ferrocenium hexafluorophos-
phate (2 equiv.) in the presence of 1-octyne yields the dia-
magnetic Ru(II) complex [Ru-1·octyne]PF6 as a dark
orange product. Recrystallization from THF/hexane gives
pure [Ru-1·octyne]PF6 as a yellow solid. The ESI-MS spec-
trum displays a parent peak at m/z = 1091.18, consistent
with the theoretical value, m/z = 1091.37, for the desired
complex, Figure S1.
Single crystals of [Ru-1·octyne]PF6 for X-ray diffraction
studies were obtained by evaporation of chlorobenzene/hex-
ane mixtures. The asymmetric unit consists of co-crys-
tallized mixtures of two geometric isomers (Scheme 2 bot-
tom). Unlike the related alkene addition product [Ru-1·p-
methylstyrene]PF6, the isomers of [Ru-1·octyne]PF6 could
not be separated by crystallization or other methods.[1]
Analyses of multiple crystals consistently reveal co-crys-
tallized products. The isomers differ based on the relative
position of the hexyl side chain, which can be located either
at the R1 (nearest S trans to S, major) or R2 (nearest S trans
to P, minor) position. An ORTEP[14] representation of the
major [Ru-1·octyne]+ isomer is provided in Figure 1.
Table 1. Selected bond lengths [Å] and angles [°] of [Ru-1·
octyne]+, [Ru-1·C2H4]+,[3] and [Ru-1]–.[15]
[Ru-1·octyne]+
[Ru-1·C2H4]+
[Ru-1]–
Ru–S1
Ru–S2
Ru–S3
Ru–P1
Ru–P2
Ru–P3
S2–C55
2.3890(14)
2.3663(15)
2.3106(15)
2.3294(14)
2.3422(14)
2.3823(14)
1.798(8)
2.3856(9)
2.3749(9)
2.3365(9)
2.3290(9)
2.3965(10)
2.3648(9)
1.836(4)
2.402(1)
2.445(1)
2.394(1)
2.295(1)
2.353(1)
2.340(1)
S3–C56
1.796(7)
1.843(4)
C55-C56
S2–Ru–S3
Ru–S2–C55
Ru–S3–C56
S2–C55–C56–S3
1.322(10)
86.22(6)
103.1(3)
105.2(3)
8.3(8)
1.510(5)
87.71(3)
104.52(12)
103.59(12)
–47.3(3)
89.77(2)
The 31P NMR of crystalline [Ru-1·octyne]+ displays a
pair of second-order spectra, Figure S2. The relative inten-
sities, chemical shifts, and coupling constants for the major
(60%) (δ1 = 57.2, δ2 = 42.8, δ3 = 35.8 ppm; J12 = J13 = 31
and J23 = 313 Hz) and minor (40%) isomers (δ1 = 57.2, δ2
= 43.0, δ3 = 36.2 ppm; J12 = J13 = 31 and J23 = 313 Hz)
are similar to values previously reported for the two isomers
of [Ru-1·p-methylstyrene]+.[1] In that study, the isomer with
the substituted carbon positioned nearest the S trans to S
was identified as the major alkene addition product. This is
consistent with the higher calculated spin densities[4] on the
S trans to P (0.31) as compared to the S trans to S (0.21)
and Tedder’s Rules for radical alkene addition.[16] Given
similar product ratios, we assign the major isomer of [Ru-
1·octyne]+ likewise.
The cyclic voltammogram of [Ru-1·octyne]+ reveals a
single, reversible redox event at +300 mV (vs. ferrocenium/
ferrocene) [Figure 2 (top)] assigned to the RuIII/II couple.
No other redox couples are observed, which indicates that
both isomers have the same potential. Further, the RuII/I
couple lies outside the solvent window, which precludes the
possibility of reduction promoted release of 1-octyne. As
shown in Table 2, the redox couple is shifted by +1130 mV
with respect to the RuIII/II couple in the thiolate precursor
Figure 1. ORTEP[14] representation of [Ru-1·octyne]+ showing a
single isomer with the hexyl side chain (C57–C62) extending from
C56. For clarity, the co-crystallized isomer with a hexyl side chain
originating from C55 is not shown (see Supporting Information).
The [Ru-1·octyne]+ complex contains a pseudo-octahe- [Ru-1]–.[17] Similar large shifts are observed for the related
dral Ru ion in a meridional P3S3 donor environment. The dithioether derivatives.[1,3,4,15]
Ru–S and Ru–P bond lengths range from 2.3106(15) to
The addition of 1-octyne to electrochemically generated
2.3890(14) Å and 2.3294(14) to 2.3823(14) Å, respectively, [Ru-1]+ was monitored by cyclic voltammetry. As shown
in accord with related structures, Table 1.[1,3,15] The alkynyl in Figure 2 (bottom), the cyclic voltammogram of [Ru-1]–
carbon atoms of 1-octyene provide a bridge, C55 and C56, recorded in the presence of 1-octyne displays anodic peaks
between the cis-sulfur donors, S2 and S3. The S2–C55 and associated with the oxidation of [Ru-1]– to [Ru-1] at
S3–C56 bond lengths of 1.798(8) and 1.796(7) Å, respec- –830 mV and of [Ru-1] to [Ru-1]+ at –60 mV. Following the
tively, confirm covalent attachment of 1-octyne to the second oxidation, 1-octyne rapidly adds to [Ru-1]+ to yield
metal-stabilized thiyl radical core with values similar to [Ru-1·octyne]+, as identified by its RuIII/II redox couple at
476
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Eur. J. Inorg. Chem. 2012, 475–478

Alkyne Addition to a Metal-Stabilized Thiyl Radical
Conclusions
The previously reported addition reaction of alkenes to
the metal-stabilized thiyl radical [Ru-1]+ has been extended
to alkynes. By using similar chemical oxidation strategies, 1-
octyne is efficiently added to [Ru-1]+ to yield an S-alkylated
dithiolene ligand that remains coordinated to the metal cen-
ter. As with alkenes, the addition of alkyne to the Ru core
is irreversible, which is attributable to the stability of the
RuII oxidation state in a pseudo-octahedral environment.
The rate constant for 1-octyne addition was found to be
approximately 100 times lower than those of the corre-
sponding alkenes, in line with the electrophilic character of
our metal-stabilized thiyl radical. We are currently explor-
ing the addition of other alkynes and related unsaturated
compounds to establish the scope and limits of this reactiv-
ity.
Experimental Section
All reactions were performed under an inert nitrogen or argon at-
mosphere by using standard Schlenk techniques. Solvents were
purified, dried, and freshly distilled and degassed immediately prior
to use. The ruthenium thiolate precursors HNEt3[Ru-1][19] and
PPN[Ru-1][15] were prepared as described previously. All other
chemicals were purchased from Aldrich and used as received. Ele-
mental analyses were obtained from Midwest Microlab, LLC in
Indianapolis, Indiana. Electrochemical measurements were per-
formed with a PAR 273A potentiostat/galvanostat in dry, degassed
acetonitrile with 0.1 m tetrabutylammonium hexafluorophosphate
(TBAHFP) as supporting electrolyte. Additional details of the elec-
trochemical measurements are described in the Supporting Infor-
mation.
Figure 2. (top) Cyclic voltammogram of [Ru-1·octyne]+ (1.0 mm in
CH3CN with 0.1 m TBAHFP) at a scan rate of 200 mV/s. (bottom)
Cyclic voltammogram of [Ru-1]– (2.0 mm in CH3CN with 0.1 m
TBAHFP) in the presence of 1-octyne (200 mm) at a scan rate of
200 mV/s. Potentials are referenced vs. the ferrocenium/ferrocene
couple.
Table 2. Electrochemical comparison of [Ru-1]–, [Ru-1·octyne]+,
and [Ru-1·C2H4]+.
Redox couple
E1/2 (ΔE) /
Formal oxidation
states
Ref.
mV[a]
[17]
[Ru–1]0/–
–830 (95)
+300 (75)
+285 (80)
RuIII/II
RuIII/II
RuIII/II
[Ru-1·octyne]2+/+
[Ru-1·C2H4]2+/+
this work
Crystallographic Studies: Crystals of [Ru-1·octyne]PF6 adequate for
X-ray diffraction studies were grown from chlorobenzene/hexane.
[Ru(C62H55P3S3)]+[PF6]–: yellow prism, monoclinic, space group
P21/n, a = 12.0367(4), b = 24.2458(10) Å, c = 20.1572(9) Å, β =
98.291(3)°, V = 5821.2(7) Å3, rcalcd. = 1.409 g/cm3, Z = 4. Data
were collected on an Agilent Technologies/Oxford Diffraction
Gemini CCD diffractometer at 100 K by using Mo-Kα radiation.
For 8452 reflections, IϾ2σ(I) [R(int) 0.061], the final anisotropic
full-matrix least-squares refinement on F2 for 679 variables con-
verged at R1 = 0.078 and wR2 = 0.133 with a GOF of 1.06. Full
details of the data collection and structure refinement are provided
in the Supporting Information. CCDC-844993 contains the supple-
mentary crystallographic 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.
[3]
[a] Potentials vs. ferrocenium/ferrocene reference. ΔE = Epc – Epa
at a scan rate of 200 mV/s. Measurements were recorded in dry,
degassed acetonitrile with 0.1 m tetrabutylammonium hexafluoro-
phosphate as supporting electrolyte.
+300 mV. The rapid, irreversible binding of 1-octyne to
[Ru-1]+ is also evident by the decrease in cathodic current
for the [Ru-1]+/0 and [Ru-1]+/0 couples.
The rate constant for 1-octyne addition to [Ru-1]+ was
evaluated by using methods previously described for alkene
addition. A series of cyclic voltammograms were recorded
over a range of scan rates, 100–1000 mV/s, for solutions
containing [Ru-1]– and 1-octyne. The data were simulta-
[Ru-1·octyne]PF6: To a yellow solution of HNEt3[Ru-1] (100 mg,
neously fitted over all scan rates by using the Digisim soft- 0.0935 mmol) in dry acetonitrile (40 mL) was added 1-octyne
ware package[18] to determine a second-order rate constant
(1.38 mL, 9.35 mmol) by a syringe. The solution was cooled to 0 °C
of 8.7ϫ103 m–1 s–1 for the alkyne addition reaction. This
in an ice bath, and a blue solution of ferrocenium hexafluorophos-
phate (61.9 mg, 0.0187 mmol) in acetonitrile (30 mL) was slowly
value is two orders of magnitude lower than the corre-
added by a cannula. The resulting solution was stirred overnight,
sponding alkene addition rate constant for 1-hexene to [Ru-
during which time the solution gradually developed a dark orange
1]+, 7(2) ϫ105 m–1 s–1.[1] A 100–1000-fold decrease in rate
color. The solvent was removed by rotary evaporation to yield an
constant is typical for electrophilic substitutions of compar-
able alkynes and alkenes. This corroborates our previous
observation based on substituent effects of alkene addition
to [Ru-1]+ that our metal-stabilized thiyl radical complex is
electrophilic.
oily orange residue. The crude product was washed with an excess
of hot water (≈300 mL) and diethyl ether (100 mL). The crude
product was recrystallized from THF/hexane. Yield: 49 mg
(0.040 mmol, 43%). X-ray quality crystals were obtained by slow
evaporation of a 1:2 chlorobenzene/hexane mixture. E1/2 (RuIII
/
Eur. J. Inorg. Chem. 2012, 475–478
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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477

K. Ouch, M. S. Mashuta, C. A. Grapperhaus
SHORT COMMUNICATION
RuII) = +356 mV. +ESI-MS: calcd. for C62H56P3S3Ru m/z (Z = 1)
= 1091.37; found: 1091.18. C62H56F6P4RuS3 (1236.26): calcd. C
60.23, H 4.57; found C 58.03, H 3.72. Electronic absorption: λmax
(ε [cm–1], m–1) = 305 (9527), 361 nm (shoulder). FTIR (KBr pellet):
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Received: September 23, 2011
Published Online: Dezember 1, 2011
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Eur. J. Inorg. Chem. 2012, 475–478