Selective and reversible base-induced elimination of a ruthenium dithioether yields a vinyl metallosulfonium complex

Article abstract of DOI:10.1021/ic300983x

Chemical oxidation of tris(2-diphenylphosphinebenzenethiolato)ruthenate(II) [Ru-1]^- with ferrocenium hexafluorophosphate in the presence of ethylene yields [(2-diphenylphosphinebenzenethiolato)(ethane-1,2-diylbis(thio-2, 1-phenylene)diphenylphosphine)ruthenium(II)] hexafluorophosphate, [Ru-1·C₂H₄]PF₆, from addition of the alkene across cis sulfur sites. The [Ru-1·C₂H ₄]⁺ complex displays a single redox couple at +794 mV versus ferrocenium/ferrocene. ¹H NMR of [Ru-1·C ₂H₄]⁺ displays ethylene resonances at δ = 1.29 (td, 1H), 1.59 (td, 1H), 2.78 (dd, 1H), and 3.03 (dd, 1H). In the presence of base [Ru-1·C₂H₄]⁺ is selectively deprotonated at the pseudoequatorial proton on the carbon α to the sulfur trans to phosphorus, yielding the vinyl metallosulfonium derivative [Ru-1·C₂H₃]. ¹H and ³¹P NMR spectra of [Ru-1·C₂H₃] are temperature dependent, associated with inversion of the sulfur lone pair at the vinyl metallosulfonium. The activation energy for the fluxional process calculated using density functional theory (B3LYP/LANL2DZ+6-31g) of 14.36 kcal/mol is consistent with the experimentally determined value of 13.08 kcal/mol. The complex [Ru-1·C₂H₃] crystallizes as yellow blocks in the triclinic space group P-1 with unit cell dimensions of a = 11.2718(5) A?, b =12.0524(3) A?, c = 23.6075(10) A?, α = 101.715(3)°, β = 98.154(4)°, and γ = 105.209(3)°. Addition of hydrochloric acid to [Ru-1·C₂H₃] regenerates [Ru-1·C ₂H₄]⁺. Addition of DCl confirms the selectivity of this reverse reaction.

Full text of DOI:10.1021/ic300983x

Article
pubs.acs.org/IC
Selective and Reversible Base-Induced Elimination of a Ruthenium
Dithioether Yields a Vinyl Metallosulfonium Complex
Rajat Chauhan, Mark S. Mashuta, and Craig A. Grapperhaus*
Department of Chemistry, University of Louisville, 2320 South Brook Street, Louisville, Kentucky 40292, United States
S
* Supporting Information
A B S T R A C T : C h e m i c a l o x i d a t i o n o f t r i s ( 2 -
diphenylphosphinebenzenethiolato)ruthenate(II) [Ru-1]− with
ferrocenium hexafluorophosphate in the presence of ethylene
yields [(2-diphenylphosphinebenzenethiolato)(ethane-1,2-
diylbis(thio-2,1-phenylene)diphenylphosphine)ruthenium(II)]
hexafluorophosphate, [Ru-1·C2H4]PF6, from addition of the
alkene across cis sulfur sites. The [Ru-1·C2H4]+ complex
displays a single redox couple at +794 mV versus ferrocenium/
ferrocene. 1H NMR of [Ru-1·C2H4]+ displays ethylene
resonances at δ = 1.29 (td, 1H), 1.59 (td, 1H), 2.78 (dd,
1H), and 3.03 (dd, 1H). In the presence of base [Ru-1·C2H4]+
is selectively deprotonated at the pseudoequatorial proton on the carbon α to the sulfur trans to phosphorus, yielding the vinyl
metallosulfonium derivative [Ru-1·C2H3]. 1H and 31P NMR spectra of [Ru-1·C2H3] are temperature dependent, associated with
inversion of the sulfur lone pair at the vinyl metallosulfonium. The activation energy for the fluxional process calculated using
density functional theory (B3LYP/LANL2DZ+6-31g) of 14.36 kcal/mol is consistent with the experimentally determined value
of 13.08 kcal/mol. The complex [Ru-1·C2H3] crystallizes as yellow blocks in the triclinic space group P-1 with unit cell
dimensions of a = 11.2718(5) Å, b =12.0524(3) Å, c = 23.6075(10) Å, α = 101.715(3)°, β = 98.154(4)°, and γ = 105.209(3)°.
Addition of hydrochloric acid to [Ru-1·C2H3] regenerates [Ru-1·C2H4]+. Addition of DCl confirms the selectivity of this reverse
reaction.
reported by Goh and Webster.5 In the current manuscript, we
INTRODUCTION
■
Base-induced elimination of organic trialkyl sulfoniums (R3S+)
report the selective deprotonation/protonation and reactivity of
to yield alkenes and sulfides is well known, Scheme 1.1
a cationic metal−dithioether complex prepared by oxidation-
induced alkene addition to a metal−thiolate precursor.
The carbon−sulfur bond forming reaction between organic
thiyl radicals and alkenes is well established with applications
including cis/trans isomerization, sulfide synthesis, and
polymerization.7−10 Similar reactions have been reported
between oxidized metal−sulfur complexes including metal
dithiolenes11−16 and metal−thiolates.17−22 The former has
been known since the 1960s,12 although the report of reversible
ethylene addition to nickel−dithiolenes by Wang and Stiefel
renewed interest in these complexes.11 The promise of these
complexes in ethylene purification was never realized due in
large part to complications arising from deleterious interligand
addition, which is favored over the initially reported intraligand
product. Recent elegant studies by Fekl detailed the necessity
for an anionic, reduced metal complex in the reaction mixture
to obtain the latter.13 In related studies, Fekl also noted
oxidation-induced addition of ethylene to a Mo tris-
(dithiolene)14 and dienes to Pt bis(dithiolene).16
Scheme 1. Base-Induced Eliminations
Similarly, metal coordination of thioethers imparts partial
sulfonium character on the sulfur donor, leading to base
sensitivity of the resulting “metallosulfonium complex”. Several
cationic metal−dithioether complexes with five-membered
chelate structures readily undergo base-induced elimination,
yielding metal complexes with one thiolate and one vinyl
substituent, Scheme 1.2−6 The resulting vinyl metallosulfonium
complexes are susceptible to further reactivity. Examples
include the reversible deprotonation/protonation of [Co-
(TTCN)2]3+ with a pKa near 4.0 in aqueous solution by
Blake et al.3 and the intramolecular C−C coupling between a
C6Me6 ring and the vinyl sulfide in the presence of excess base
Our studies focused on the trischelate complexes [Ru-
(DPPBT)3]−, [Ru-1]−, and Re(DPPBT)3, Re-1 (DPPBT =
Received: May 11, 2012
Published: June 29, 2012
© 2012 American Chemical Society
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al.25 The complex [Ru-1·C2H4]PF6 was previously observed upon
electrochemical oxidation of PPN[Ru-1] in the presence of ethylene.20
[Ru-1·C2H4][PF6]. In an ice bath at 0 °C a yellow solution of
HNEt3[Ru-1] (100 mg, 0.0935 mmol) in acetonitrile (40 mL) was
saturated with ethylene gas by purging the solution via syringe for 3−5
min. A blue solution of ferrocenium hexafluorophosphate (0.0619 g,
0.187 mmol) in acetonitrile (30 mL) was added by cannula transfer.
The resulting green solution was stirred for 3 h at 0 °C followed by
removal of solvent by rotary evaporation. The crude yellow−green
residue was washed with an excess of hot water (300 mL) and diethyl
ether (25 mL). The product is spectroscopically identical to the
previously reported derivative.20 Yield: 0.065 g (60%). E1/2 vs Fc+/Fc
diphenylphosphinobenzenethiolate), which precludes intra-
ligand addition and orients two of the thiolate donors in a
position that favors interligand addition, Scheme 2. Oxidation
Scheme 2. Ligand-Centered Reactivity of [Ru-1]− and Its
Derivatives
1
(RuIII/RuII) = +794 mV. H NMR (500 MHz, CD3CN): δ 1.29 (td,
1H, J = 14, 14, 4 Hz, SCHHaxCH2S), 1.59 (td, 1H, J = 14, 14, 4 Hz,
SCH2CHHaxS), 2.78 (dd, 1H, J = 14, 4 Hz, SCH2CHHeqS), 3.03 (dd,
1H, J = 14, 4 Hz, SCHHeqCH2S), 6.32−8.29 (m, 42H, SC6H4P-
(C6H5)2). Selected 13C{1H} NMR (176 MHz, CD3CN): δ 36.2 (s,
SCH2CH2S), 44.2 (s, SCH2CH2S). 31P{1H} NMR (162 MHz,
CD3CN): δ 40.3 (J = 30, 304 Hz, Pax1) 37.5 (J = 30, 304 Hz, Pax2),
61.0 (J = 30, 30 Hz, Peq).
[Ru-1·C2H3]. To a yellow-green solution of [Ru-1·C2H4][PF6] (100
mg, 0.087 mmol) in acetonitrile (40 mL) was added an 0.18 M
solution of KOH in methanol (0.49 mL, 0.087 mmol). The resulting
solution was stirred for 30 min, during which time the color changed
to golden yellow. Solvent was removed by rotary evaporation to yield a
yellow residue. The crude product was washed with an excess of water
(300 mL). Yield: 0.084 g (96%). X-ray quality crystals were obtained
by addition of 30 mg of crude product to 3 mL of toluene. Slow
evaporation yielded golden yellow crystals. E1/2 vs Fc+/Fc (RuIII/RuII)
= −250 mV. +ESI-MS for RuP3S3C56H45, [M + H]+ 1009.0978 found,
1009.1018 calcd. Anal. Calcd for RuP3S3C56H45·C8H7: C, 68.77; H,
4.86. Found: C, 68.22; H, 4.77. 1H NMR (500 MHz, CD3CN): δ 4.01
(d, 1H, J = 16 Hz, SCHCHHcis), 4.40 (d, 1H, J = 9 Hz, SCH
CHHtrans), 4.94 (dd, 1H, J = 16, 9 Hz, SCHCH2), 6.32−8.29 (m,
42H, SC6H4P(C6H5)2). Selected 13C{1H} NMR (176 MHz, CD3CN):
δ 120.0 (s, SCHCH2). 31P{1H} NMR (162 MHz, CD3CN): δ 55.4
(J = 30, 310 Hz, Pax1) 48.7 (J = 30, 310 Hz, Pax2), 58.3 (J = 30, 30 Hz,
Peq).
of [Ru-1]− to the formally Ru(IV) derivative [Ru-1]+ results in
a significant increase in spin density on the sulfur donors,
leading to our assignment of the complex as a metal-stabilized
thiyl radical.21,23 The [Ru-1]+ complex readily reacts with
alkenes,18−20 methyl ketones,24 and alkynes17 across the cis-
sulfur sites to yield isolable metal−thioether complexes. The
rate of alkene addition is fastest for electron-donating alkenes,
consistent with an electrophilic metal-stabilized thiyl radical.18
While the reaction is irreversible for [Ru-1]+, reversible
ethylene addition was observed for the Re derivative, [Re-1]+.19
In our prior studies we established the addition of a variety of
alkenes to [Ru-1]+ by electrochemical methods.20 However,
few of these complexes have been prepared on a synthetic scale
and isolated for further reactivity studies. In the current article,
we report the synthesis of the ethylene addition product [Ru-
1·C2H4]+ by chemical methods and explore its acid/base
chemistry in solution, Scheme 2. The complex has previously
been prepared by nucleophilic addition of 1,2-dibromoethane
to the anionic complex [Ru-1]−.20 However, this route is only
accessible for products with readily available dihaloalkane
precursors, whereas the methods reported herein are applicable
to a variety of alkenes including styrenes as previously
reported.18 In the presence of base, [Ru-1·C2H4]+ is selectively
deprotonated to the vinyl metallosulfonium derivative [Ru-
1·C2H3]. Addition of acid reverses the reaction. This reaction
represents an initial survey of the reactivity of vinyl metal-
losulfonium complexes, which may have relevance for the
functionalization of olefins via ligand-centered reactivity.
[Ru-1·C2H3D]Cl. To a golden yellow solution of [Ru-1·C2H3] (100
mg, 0.099 mmol) in benzene (10 mL) was added a 35% solution of
DCl in D2O (9.57 μL, 0.099 mmol). The resulting solution was heated
at reflux for 1 h, during which time the color changed to chartreuse
yellow. Solvent was removed by rotary evaporation to yield a yellow-
green residue. The crude product was washed with an excess of water
(300 mL) and diethyl ether (25 mL). Yield: 0.093 g (90%). E1/2 vs
Fc+/Fc (RuIII/RuII) = +794 mV. +ESI-MS for C56H45DP3S3Ru: [M]+
1
1010.1104 found, 1010.1081 calcd. H NMR (500 MHz, CD3CN): δ
1.29 (t, 1H, J = 14, 14 Hz, SCHHaxCHDS), 1.59 (dd, 1H, J = 14, 4 Hz,
SCH2CDHaxS), 3.03 (dd, 1H, J = 14, 4 Hz, SCHHeqCHDS), 6.32−
8.29 (m, 42H, SC6H4P(C6H5)2). Selected 13C{1H} NMR (176 MHz,
CD3CN): δ 35.8 (t, JCD = 20.4 Hz, SCH2CHDS), 44.2 (s,
SCH2CHDS). 31P{1H} NMR (162 MHz, CD3CN): δ 40.3 (J = 30,
304 Hz, Pax1) 37.5 (J = 30, 304 Hz, Pax2), 61.0 (J = 30, 30 Hz, Peq).
Physical Methods. Mass spectra were collected by the Mass
Spectrometry Application and Collaboration Facility in the Chemistry
Department at Texas A&M University. Elemental analyses were
performed by Midwest Microlab (Indianapolis). All electrochemical
measurements were performed using a PAR 273 potentiostat/
galvanostat with a three-electrode cell (glassy carbon working
electrode, platinum wire counter electrode, and Ag/Ag ion reference
electrode). Reported potentials are scaled versus a ferrocenium/
ferrocene (Fc+/Fc) standard (0.00 V), which was determined using
ferrocene as an internal standard. 1H NMR and gCOSY spectra,
referenced to TMS, were recorded on a Varian 500 MHz
spectrometer. 31P NMR spectra were recorded on a Varian 400
MHz spectrometer and are referenced to 85% H3PO4. 13C and
gHSQCAD NMR spectra were recorded on a Varian 700 MHz
spectrometer. The activation energy (ΔG†) for the fluxional NMR
process was calculated using the following equation26
EXPERIMENTAL SECTION
■
Materials and Reagents. All reagents were obtained from
commercially available sources and used as received unless otherwise
noted. All solvents were dried and freshly distilled using standard
techniques under a nitrogen atmosphere and degassed using the
freeze−pump−thaw method. Reactions were conducted using stand-
ard Schlenk techniques under a nitrogen atmosphere or in an argon-
filled glovebox unless otherwise noted. The complex HNEt3[Ru-1]
was prepared according to methods previously reported by Dilworth et
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ΔG† = aT[9.972 + log(T /Δν)]
where a = 4.575 × 10−3 kcal/mol, Tc = coalescence temperature, and
Δν = frequency difference at the low-temperature limit.
Crystallographic Stuides. A yellow block crystal of [Ru-1·C2H3]
was cut to dimensions 0.25 × 0.13 × 0.10 mm3 and mounted on a
glass fiber for collection of X-ray data on an Agilent Technologies/
Oxford Diffraction Gemini CCD diffractometer. The CrysAlisPro27
CCD software package (v 171.35.11) was used to acquire a total of
Table 1. Crystal Data and Structure Refinement for [Ru-
1·C2H3]·2.5 C7H8
c
empirical formula
fw
C73.50H61P3RuS3
1234.38
temp.
100.1 K
wavelength
cryst syst
0.7107 Å
triclinic
space group
unit cell dimens
P-1
913 30-s frame ω-scan exposures of data at 100(1) K to a 2θmax
=
a = 11.2718(5) Å
b = 12.0524(3) Å
c = 23.6075(10) Å
α = 101.715(3)°
β = 98.154(4)°
γ = 105.209(3)°
2965.04(21) Å3
2
59.20° using monochromated Mo Kα radiation (0.71073 Å) from a
sealed tube. Frame data were processed using CrysAlisPro27 RED to
determine final unit cell parameters: a = 11.2718(5) Å, b = 12.0524(3)
Å, c = 23.6075(10) Å, α = 101.715(3)°, β = 98.154(4)°, γ =
105.209(3)°, V = 2965.04(21) Å3, Dcalcd = 1.383 Mg/m3, Z = 2 to
produce raw hkl data that were corrected for absorption (transmission
min/max = 0.926/0.952; μ = 0.496 mm−1) using SCALE3
ABSPACK.28 The structure was solved by Patterson methods in the
space group P-1 using SHELXS-9029 and refined by least-squares
methods on F2 using SHELXL-9729 incorporated into the
SHELXTL30 (v 6.14) suite of programs. The disordered toluene
solvent was modeled with a one-half occupancy set of carbon atoms
(C80−C86) using an appropriate set of constraints; the second set of
one-half occupancy carbon atoms (C80a−C86a) involved in the
disorder is generated by symmetry through an inversion center. All
other non-hydrogen atoms were refined with anisotropic atomic
displacement parameters. The three hydrogen atoms (H55, H56a, and
H56b) of the vinyl group were located by difference maps and refined
isotropically. Remaining hydrogen atoms were placed in their
geometrically generated positions and refined as a riding model.
Phenyl H’s were included as fixed contributions with U(H) = 1.2U11
(attached C atom), while methyl groups were allowed to ride (the
torsion angle which defines its orientation was allowed to refine) on
the attached C atom, and these atoms were assigned U(H) = 1.5U11.
For 12 354 reflections I > 2σ(I) [R(int) 0.040] the final anisotropic full
matrix least-squares refinement on F2 for 735 variables converged at
R1 = 0.038 and wR2 = 0.076 with a GOF of 1.04. Crystal data and
structure refinement parameters for [Ru-1·C2H3] are provided in
Table 1. Selected bond distances and bond angles are listed in Table 2.
Computational Methodology. Geometry optimization and
frequency calculations for all complexes were performed using the
Gaussian 09 suite of programs.31 Density functional theory (DFT)
calculations employed the B3LYP functional. For these calculations
the 6-31g basis set was used for carbon, hydrogen, sulfur, and
phosphorus atoms, while the LANL2DZ basis set was used for
ruthenium. Input coordinates were taken from crystallographic data for
the neutral complex [Ru-1·C2H3]. Optimized coordinates for [Ru-
1·C2H3], its structural isomer, and their truncated derivates are
provided in Tables S1−S6, Supporting Information. To investigate the
rotation and inversion of the vinyl group, the Ru−S2−C55−C56 and
Ru−S2−C19−C55 torsion angles were frozen, respectively, at various
positions and the structure reoptimized.
vol.
Z
density (calcd)
abs coeff
F(000)
1.383 Mg/m3
0.496 mm−1
1278
cryst color, habit
cryst size
yellow cut-block
0.25 × 0.13 × 0.10 mm3
theta range for data collection 3.37−29.62°
index ranges
reflns collected
indep reflns
−15 ≤ h ≤ 14, −16 ≤ k ≤ 15, −31 ≤ l ≤ 31
67 874
15 439 [R(int) = 0.041]
completeness to theta = 29.62° 92.4%
completeness to theta = 27.04° 99.8%
abs corr
semiempirical from equivalents
max/min transmission
refinement method
data/restraints/parameters
goodness-of-fit on F2
final R indices [I > 2σ(I)]
R indices (all data)
largest diff. peak and hole
0.952/0.926
full-matrix least-squares on F2
15439/7/735
1.043
a
R1 = 0.0383, wR2 = 0.0763
R1 = 0.0562, wR2 = 0.0849
0.763 and −0.736 e.Å−3
a
R1 = Σ∥Fo| − |Fc2∥/Σ|Fo|. wR2 = {Σ[w(Fo − Fc2)2]/Σ[w(Fo )2]}1/2
,
2
2
where w = q/σ2(Fo ) + (qp) 2 + bp. GOF = S = {Σ[w(Fo2 − Fc2) 2]/(n
− p)}1/2, where n is the number of reflections and p is the number of
parameters refined.
Table 2. Selected Bond Distances (Angstroms) and Angles
20
(degrees) for [Ru-1·C2H4]+ and [Ru-1·C2H3]
[Ru-1·C2H4]+
[Ru-1·C2H3]
experimental
experimental
computational
Ru−S1
Ru−S2
Ru−S3
Ru−P1
Ru−P2
Ru−P3
S2−C55
S3−C56
C55−C56
S1−Ru−S2
S1−Ru−S3
S2−Ru−P3
P1−Ru−P2
Ru−S2−C55
2.3856(9)
2.3749(9)
2.3365(9)
2.3648(9)
2.3965(10)
2.3290(9)
1.836(4)
2.3932(5)
2.4014(5)
2.3992(5)
2.3440(6)
2.3640(6)
2.2853(5)
1.787(2)
2.503
2.541
2.515
2.455
2.488
2.409
1.864
RESULTS AND DISCUSSIONS
■
Synthesis and Characterization. A series of complexes
has been prepared through successive ligand-centered reactions
based on the tris(diphenylphosphinobenzenethiolato)
(DPPBT) metal complex [Ru-1]−, Scheme 2. The dithioether
complex [Ru-1·C2H4]+ can be prepared in 60% yield upon
chemical oxidation of [Ru-1]‑ with two equivalents of
ferrocenium hexafluorophosphate (FcPF6) in the presence of
ethylene. Deprotonation of [Ru-1·C2H4]+ with KOH in
acetonitrile yields the vinyl metallosulfonium derivative [Ru-
1·C2H3] in an elimination reaction in 96% yield. A single
structural isomer with the vinyl substituent on the sulfur trans
to phosphorus is obtained. The reaction does not proceed
under aqueous conditions and in basic methanolic solutions,
consistent with the greater basicity of hydroxide in acetonitrile
1.843(4)
1.510(5)
1.313(4)
95.030(18)
178.19(2)
171.59(2)
168.455(18)
112.23(8)
1.332
93.048
87.06(3)
173.73(3)
172.87(3)
168.81(3)
104.52(12)
176.796
173.048
167.143
111.325
as compared to hydrogen-bonding solvents.32,33 Acidification of
[Ru-1·C2H3] with HCl or HBr regenerates [Ru-1·C2H4]+.
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The vinyl metallosulfonium product [Ru-1·C2H3] is easily
distinguished from its dithioether precursor [Ru-1·C2H4]+ by
changes in color, solubility, and redox potential. The golden-
yellow, neutral [Ru-1·C2H3] has enhanced solubility in
nonpolar solvents, such as benzene and toluene, as compared
to the chartreuse yellow, cationic precursor [Ru-1·C2H4]+. The
vinyl metallosulfonium complex displays a [Ru-1·C2H3]+/0
redox couple at −250 mV versus Fc+/Fc, Figure 1. This formal
No reaction occurs upon addition of a large excess of base to
solutions of [Ru-1·C2H3] in the absence of air even under
prolonged reflux conditions. However, under aerobic con-
ditions excess base induces C−S bond cleavage in both [Ru-
1·C2H4]+ and [Ru-1·C2H3] upon heating, Scheme 3. Depend-
Scheme 3. Aerobic Reaction of [Ru-1·C2H3] with Excess
Base
ing on the length of the reaction, [Ru-1]− and/or its S-
oxygenated derivatives35 are recovered as the only metal-
containing products. NMR data (1H, 13C, and gHSQC; Figures
S15−S17, Supporting Information) of the observed organic
product are consistent with C−C bond cleavage of the ethylene
bridge to yield formate. The reaction pathway remains elusive
but is proposed to proceed via nucleophilic attack at the vinyl
metallosulfonium under strongly basic conditions similar to
observations reported by Goh and Webster.5
Structural Determination. Yellow block-shaped crystals of
[Ru-1·C2H3] in the triclinic space group P-1 were obtained
upon slow evaporation of toluene solution of the vinyl
metallosulfonium complex. Crystal data and structure refine-
ment details are listed in Table 1. The vinyl metallosulfonium
complex [Ru-1·C2H3] contains a six-coordinated Ru(II) ion
ligated by three PS chelates in a pseudo-octahedral environ-
ment as shown in the ORTEP36 representation in Figure 2. The
three sulfur donors are arranged in a meridional fashion as in
related complexes.17−20,25,37 The vinyl substituent is located at
S2, which sits trans to P3. The two thiolate sulfurs, S1 and S3,
are trans to each other.
Figure 1. Square wave voltammograms of [Ru-1]− (top), [Ru-
1·C2H3] (center), and [Ru-1·C2H4]+ (bottom) in dichloromethane or
acetonitrile with 0.1 M tetrabutylammonium hexafluorophosphate as
supporting electrolyte. Potentials referenced to Fc+/Fc using an
internal standard.
RuIII/II potential is exactly centered between those previously
reported for [Ru-1·C2H4]2+/+ at +330 mV and [Ru-1]0/− at
−830 mV. The results are consistent with a large (+580 mV)
cathodic shift upon substitution of an anionic thiolate donor
with a neutral thioether.34 Cyclic voltammetry of [Ru-1·C2H3]
confirms the oxidation is quasi-reversible with a peak-to-peak
potential difference, ΔEp, of 80 mV at a scan rate of 100 mV/s,
Figure S3, Supporting Information. The two products cannot
be discriminated by +ESI-MS. The dithioether complex [Ru-
1·C2H4]+ displays an m/z 1009.1015 amu consistent with the
theoretical value of 1009.1018 amu for the M+ ion. The vinyl
complex [Ru-1·C2H3] displays a similar peak at 1009.0978 with
the same theoretical value for the [M + H]+ ion.
Reversible deprotonation of the ethylene linker in [Ru-
1·C2H4]+ proceeds selectively at the carbon alpha to the sulfur
trans to phosphorus. Deuteration of [Ru-1·C2H3] with DCl
yields the monodeuterated dithioether complex [Ru-
1·C2H3D]+, which displays an m/z peak in the +ESI-MS at
1010.1104, which is shifted by 1.0089 relative to [Ru-1·C2H4]+.
Further deprotonation of [Ru-1·C2H3D]+ regenerates the vinyl
metallosulfonium complex [Ru-1·C2H3] with no incorporation
of deuterium. The results, corroborated by NMR investigations,
indicate selective deprotonation of a single proton in the
ethylene bridge. As described in the NMR section below, the
reaction occurs only at the hydrogen in the pseudoequatorial
position on the carbon alpha to the sulfur trans to phosphorus.
Figure 2. ORTEP36 representation of [Ru-1·C2H3].
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Comparison of the structure of [Ru-1·C2H3] with previously
reported crystallographic data of [Ru-1·C2H4]+ reveals a similar
RuP3S3 core with clear distinctions associated with deprotona-
tion at C55. Selected bond distances and angles are listed in
Table 2. The Ru−P bond distances are shorter by 0.02−0.04 Å
in [Ru-1·C2H3] relative to [Ru-1·C2H4]+. Conversion of S3
from a thioether in [Ru-1·C2H4]+ to a thiolate in [Ru-1·C2H3]
results in a bond distance increase of 0.06 Å from 2.3365(9) to
2.3992(5) Å. There is also an increase in the Ru−S2 bond
distance of 0.03 Å from 2.3749(9) to 2.4014(5) Å. The C55−
C56 bond distance decreases from 1.510(5) to 1.313(4) Å,
consistent with formation of a CC double bond, confirming
deprotonation of dithioether. Cleavage of the 5-membered
Ru−S2−C55−C56−S3 ring upon deprotonation opens the
Ru−S2−C55 bond angle from 104.52(12)° in [Ru-1·C2H4]+ to
112.23(8)° in [Ru-1·C2H3]. Deprotonation also allows
expansion of the S1−Ru−S2 and S1−Ru−S3 bond angles
from 87.06(3)° to 95.030(18)° and 173.73(3)° to 178.19(2)°,
respectively.
between four ethylene protons. The coupling constants are J13
= 14 Hz, J12 ≈ J34 = 14 Hz, J14 ≈ J23 = 4 Hz, and J24 ≈ 0 Hz.
The aromatic region between δ = 6.2 and 8.5 exhibits a
complex multiplet of peaks associated with the nine phenyl
rings and a total integration proportional to the four ethylene
protons. The 31P NMR spectrum of [Ru-1·C2H4]+ is second
order with chemical shifts and coupling constants as previously
reported,20 see Table 3. The 13C NMR of [Ru-1·C2H4]+ shows
the ethylene carbons C55 (36.0, s) and C56 (44.2, s) along
with nine phenyl rings carbons in region of δ = 110−180 ppm.
Substitution of a single ethylene proton by deuterium, as
shown in Scheme 2, occurs selectively at the pseudoequatorial
position on C55, H4. The 1H NMR of [Ru-1·C2H3D]+ displays
three of the resonances associated with the ethylene bridge in
[Ru-1·C2H4]+ (H1 (1.29, t); H2 (3.03, dd); H3 (1.59, dd))
with the peak at δ = 2.78 ppm notably absent. The presence of
deuterium at the pseudoequatorial position on C55 is further
confirmed by gCOSY NMR, Figure S9, Supporting Informa-
tion, which clearly shows the expected coupling pattern. The
13C NMR of [Ru-1·C2H3D]+ also confirms the selective
deuterium labeling at C55 with a slight upfield shift of the peak
to δ = 35.8 ppm and a coupling constant of 20.4 Hz.
Deuteration at C55 has no effect on the 31P NMR spectrum of
[Ru-1·C2H3D]+.
NMR Investigations. The dithioether complex [Ru-
1·C2H4]+, its monodeuterated derivative [Ru-1·C2H3D]+, and
the vinyl metallosulfonium complex [Ru-1·C2H3] have been
thoroughly characterized by NMR techniques. A summary of
1
pertinent H, 13C, and 31P chemical shift values and coupling
The NMR spectra of [Ru-1·C2H3] are complicated by a
fluxional process associated with the vinyl substituent. At low
temperature, 31P NMR shows two partially overlapping second-
order spectra, Figure 3. The first isomer (53%) has chemical
constants are summarized in Table 3.
The 1H NMR spectrum of [Ru-1·C2H4]+ displays four
unique resonances for the ethylene bridge assigned as H1 (1.29,
td), H2 (3.03, dd), H3 (1.59, td), and H4 (2.78, dd). The
pseudoaxial protons H1 and H3 are located upfield with respect
to the pseudoequatorial protons H2 and H4. gCOSY NMR,
Figure S6, Supporting Information, clearly shows the coupling
Table 3. Selected NMR Parameters for [Ru-1·C2H4]+, [Ru-
1·C2H3D]+, and [Ru-1·C2H3] (δ, ppm; J, Hertz)
1H NMR
a
H1
H2
H3
H4
1.29 (1H, td)
3.03 (1H, dd)
1.59 (1H, td)
2.78 (1H, dd)
H1
H2
H3
1.29 (1H, t)
H1
H2
H3
4.01 (1H, d)
4.40 (1H, d)
4.94 (1H, dd)
a
a
3.03 (1H, dd)
1.59 (1H, dd)
Figure 3. Variable-temperature 31P NMR spectra of [Ru-1·C2H3].
a
J13 = J12 = 14 Hz
J23 = 4 Hz
J13 = 16 Hz
a
shifts values for P1, P2, and P3 of δ = 65.6, 50.8, and 57.8 ppm,
respectively, with coupling constants J12 = 303 Hz, J13 = J23 = 30
Hz. In the second component (47%) the chemical shifts of P1
and P2 are significantly upfield at δ = 48.9 and 44.5 ppm. The
J13 = J12 = J34 = 14 Hz
J14 = J23 = 4 Hz
13C NMR
J23 = 9 Hz
b
C1
C2
44.2 (s)
36.2 (s)
C1
C2
44.2 (s)
35.8 (t)
C1
120 (s)
P3 resonance, δ = 57.9 ppm, and the coupling constants, J12
=
299 Hz and J13 = J23 = 32 Hz, are similar to the first isomer. As
the temperature is increased, the two sets of resonances begin
to converge with coalescence occurring at 303 K. At higher
temperatures, a single set of resonances is observed, Figure 3
and Table 3.
Fluxional behavior of the vinyl substituent in [Ru-1·C2H3] is
also observed in the 1H NMR, Figure S11, Supporting
Information. As detailed in the Computational Studies section
below, this is attributed to lone-pair inversion at the vinyl
metallosulfonium sulfur. At low temperature (223 K), one set
of vinyl resonances is clearly visible with chemical shift values of
JC2D = 20.4 Hz
31P NMR
c
e
e
e
e
d
e
e
e
P1
P2
P3
40.3
P1
P2
P3
40.3
37.5
61.0
P1
P2
P3
55.4
48.7
58.3
c
c
e
e
d
d
37.5
61.0
c
d
J12 = 304 Hz
J12 = 304 Hz
J12 = 310 Hz
c
d
J13 = J23 = 30 Hz
J13 = J23 = 30 Hz
J13 = J23 = 30 Hz
a
Resonances for one of two isomers observed at 223 K. Peaks
associated with the other isomer are obscured by the phenyl
b
c
d
resonances. Recorded at 343 K. Reference 20. Recorded at 363
e
K. Second-order spectrum (see text).
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31P NMR. The barrier is also consistent with previously
reported sulfur lone pair inversions in metal thioether
complexes,40−42 which display inversion barriers less than
those typically observed in pyramidal sulfoniums, ∼27−30
kcal/mol,43 and significantly lower than comparable sulf-
oxides.44 An alternate fluxional process attributed to rotation
of the vinyl substituent about the S2−C55 bond angle has too
low of an activation barrier, 4.38 kcal/mol (Figure S4,
Supporting Information), to be considered.
A third potential fluxional process involves vinyl group
transfer between S2 and S3. However, this is readily negated by
energy calculations. The optimized structure of the crystallo-
graphically observed isomer with the vinyl group on S2 (trans
to P3) is stabilized by 6.78 kcal/mol relative to the isomer with
the vinyl substituent on S3 (trans to S1). This difference in
thermodynamic energies is too large to observe the disfavored
isomer in the 31P NMR spectra regardless of the associated
kinetic barrier (not calculated).
δ = 4.01, 4.40, and 4.94 ppm for H1, H2, and H3, respectively.
The coupling between the three resonances is confirmed by
gCOSY NMR, Figure S12, Supporting Information. The
second set of vinyl resonances is hidden in the aromatic
region. As in the 31P NMR, coalescence is observed upon an
increase in temperature followed by a single set of resonances at
higher temperatures, Table 3 and Figure S11, Supporting
Information. The first set of resonances is further upfield than
the chemical shift range from δ = 4.7 to 6.6 ppm observed in
other vinyl metallosulfonium complexes.2−6 This is consistent
with shielding effects of the aromatic diphenyl substituents of
the ligand. As suggested by a reviewer, the upfield shift of the
vinyl protons could also be due to a linkage isomer with an η2-
coordinated vinyl substituent. While we cannot completely
discount this possibility, similar Ru complexes display proton
resonances in the range from δ = 1.3 to 3.0 ppm,38 which is
significantly further upfield than that observed in [Ru-1·C2H3].
From the variable-temperature 31P NMR data, the activation
barrier associated with the fluxional process was calculated as
13.08 kcal/mol.39 Several possible origins of the fluxional
process were considered including vinyl group transfer between
S2 and S3, rotation of the vinyl substituent about the S2−C55
bond axis, and lone-pair inversion at S2. To discriminate
between these possibilities, a series of DFT investigations was
conducted.
A comparison of the two optimized structural isomers reveals
a more crowded steric environment for the vinyl group when it
is on S3 as compared to S2. To quantify the steric versus
electronic effects, truncated computational models of each
isomer with the diphenyl substituents of the DPPBT ligand
replaced with methyl (DMPBT) or hydrogen (PBT) were
calculated, Table 4. The results show a consistent preference for
Computational Studies. The B3LYP exchange-correlation
functional with the LANL2DZ basis set was used for Ru and
the 6-31g basis set for all other atoms to optimize the structure
of [Ru-1·C2H3] and its derivatives. The calculated metal−
ligand bond distances reproduce the experimental values for
[Ru-1·C2H3] within 0.11−0.14 Å, Table 2. Calculated bond
angles reproduce experimental values within 2−3°, Table 2.
Inversion of the S2 lone pair was modeled by systematic
variation of the Ru−S2−C19−C55 torsion angle, Figure 4. In
the X-ray crystal structure this angle is observed as −116.06°,
which corresponds with the calculated minimum of −115.6°. A
second minimum is observed near +120° as expected for a
lone-pair inversion on S2. The energy barrier between these
two minima of 14.36 kcal/mol is close to the measured
activation barrier, 13.08 kcal/mol, for the fluxional process by
Table 4. Calculated Relative Energies of Vinyl
Metallosulfonium Complexes with the Substituent on the
Sulfur Trans to Sulfur (tS) or Trans to Phosphorus (tP) for
[Ru-1·C2H3] (left) and Its Truncated Derivatives
[(DMPBT)2(DMPBT·C2H3)Ru] (center) and
a
[(PBT)2(PBT·C2H3)Ru] (right)
ΔHtS − ΔHtP
TΔStS - TΔStP
ΔGtS − ΔGtP
6.36
−0.43
6.78
3.21
−1.74
4.95
4.03
0.19
3.83
a
DMPBT = dimethylphosphinebenzenethiolate, PBT = phosphine-
benzenethiolate.
the vinyl group on S2 relative to S3, although the energy gap
decreases with decreased steric bulk on the ligand. While the
steric contribution may facilitate formation of one isomer over
the other, clearly electronic effects are significant in dictating
the position of the vinyl group on the sulfur trans to P as
opposed to trans to S. Efforts to prepare these less sterically
hindered derivatives are underway.
CONCLUSIONS
In this study we have shown that thioether coordination to a
metal center may impart “sulfonium-like” ligand-centered
■
Figure 4. Calculated relative free energies of [Ru-1·C2H3] as a
function of the Ru−S2−C19−C55 torsion angle representative of a
sulfur lone-pair inversion at S2.
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reactivity such as base-induced eliminations. These complexes
can be considered as metalloderivatives of trialkyl sulfoniums or
metallosulfoniums. The cationic metal−dithioether complex
[Ru-1·C2H4]+ has a five-membered MS2C2 ring that has been
shown to be particularly base sensitive in related complexes.
The ring contains four hydrogen atoms (H1, H2, H3, and H4)
as potential sites for deprotonation, Figure 5. The two
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: grapperhaus@louisville.edu.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Donors of the American Chemical Society
Petroleum Research Fund (51566-ND3) for support of this
work. We thank the Kentucky Research Challenge Trust Fund
for the purchase of CCD X-ray equipment and upgrade of our
X-ray facility.
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* Supporting Information
+ESI-MS and NMR spectra of [Ru-1·C2H4]+, [Ru-1·C2H3],
and [Ru-1·C2H3D], cyclic voltammogram of [Ru-1·C2H3], and
optimized Cartesian coordinates in pdf format, and crystallo-
graphic data for [Ru-1·C2H3] in CIF format (CCDC 881016).
This material is available free of charge via the Internet at
http://pubs.acs.org.
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