Metal-stabilized thiyl radicals as scaffolds for reversible alkene addition via C-S bond formation/cleavage

Article abstract of DOI:10.1021/ic200416y

The one-electron oxidation of metal thiolates results in an increased oxidation state of the metal ion or the formation of a sulfur-based, thiyl radical in limiting extremes. For complexes with highly covalent M-S bonds, the unpaired electron may be delocalized over the metal and the sulfur, yielding a metal-stabilized thiyl radical. Oxidation of the metal thiolate precursors [Ru(DPPBT)₃]^-, [Ru-1]^-, and Re(DPPBT) ₃, Re-1 (DPPBT = diphenylphosphinobenzenethiolate), generates metal-stabilized thiyl radicals that react with alkenes to yield dithioether-metal products. Alkene addition to [Ru-1]⁺ and [Re-1]⁺ is symmetry-allowed due to the meridional arrangement of the DPPBT chelates. Combined bulk electrolysis and cyclic voltammetry experiments reveal the addition of alkenes to [Ru-1]⁺ as an irreversible process with experimentally determined rate constants ranging from 4.6(5) × 10⁷ M^-1 s^-1 for electron-rich alkenes to 2.7(2) × 10⁴ M^-1 s^-1 for electron-poor alkenes. Rate constants for cyclic alkenes range from 4(2) × 10⁷ to 2.9(3) × 10³ M^-1 s^-1. Chemical oxidation of [Ru-1]^- by ferrocenium hexafluorophosphate (FcPF₆) in the presence of m-methylstyrene or p-methylstyrene yields the dithioether complexes [Ru-1?m-methylstyrene]⁺ and [Ru-1?p- methylstyrene]⁺, respectively. Each complex was crystallized and the structure determined by single-crystal X-ray diffraction. ³¹P NMR of the samples reveals a major and minor product, each displaying a second-order spectrum. The oxidized intermediate [Re-1]⁺ binds alkenes reversibly with equilibrium binding constants that vary with the complex charge from 1.9 × 10^-11 M^-1 for n = 0 to 4.0 M^-1 for n = +1 to 2.5 × 10⁹ M^-1 for n = +2. The three binding regimes are separated by 240 mV. Crystalline samples of [Re-1?C ₂H₄]²⁺ are obtained upon chemical oxidation of Re-1 with silver hexafluorophosphate (AgPF₆) in the presence of ethylene. Strategies for the addition of alkenes to other metal-stabilized thiyl radicals are suggested.

Full text of DOI:10.1021/ic200416y

FORUM ARTICLE
pubs.acs.org/IC
Metal-Stabilized Thiyl Radicals as Scaffolds for Reversible Alkene
Addition via CꢀS Bond Formation/Cleavage
Kagna Ouch, Mark S. Mashuta, and Craig A. Grapperhaus*
Department of Chemistry, University of Louisville, Louisville, Kentucky 40292, United States
S Supporting Information
b
ABSTRACT: The one-electron oxidation of metal thiolates
results in an increased oxidation state of the metal ion or the
formation of a sulfur-based, thiyl radical in limiting extremes.
For complexes with highly covalent MꢀS bonds, the unpaired
electron may be delocalized over the metal and the sulfur,
yielding a metal-stabilized thiyl radical. Oxidation of the metal
thiolate precursors [Ru(DPPBT)3]ꢀ, [Ru-1]ꢀ,andRe(DPPBT)3,
Re-1 (DPPBT = diphenylphosphinobenzenethiolate), generates metal-stabilized thiyl radicals that react with alkenes to yield
dithioetherꢀmetal products. Alkene addition to [Ru-1]þ and [Re-1]þ is symmetry-allowed due to the meridional arrangement
of the DPPBT chelates. Combined bulk electrolysis and cyclic voltammetry experiments reveal the addition of alkenes to [Ru-
1]þ as an irreversible process with experimentally determined rate constants ranging from 4.6(5) ꢁ 107 Mꢀ1 sꢀ1 for electron-rich
alkenes to 2.7(2) ꢁ 104 Mꢀ1 sꢀ1 for electron-poor alkenes. Rate constants for cyclic alkenes range from 4(2) ꢁ 107 to 2.9(3) ꢁ
103 Mꢀ1 sꢀ1. Chemical oxidation of [Ru-1]ꢀ by ferrocenium hexafluorophosphate (FcPF6) in the presence of m-methylstyrene
or p-methylstyrene yields the dithioether complexes [Ru-1 m-methylstyrene]þ and [Ru-1 p-methylstyrene]þ, respectively.
3
3
Each complex was crystallized and the structure determined by single-crystal X-ray diffraction. 31P NMR of the samples reveals a
major and minor product, each displaying a second-order spectrum. The oxidized intermediate [Re-1]þ binds alkenes reversibly
with equilibrium binding constants that vary with the complex charge from 1.9 ꢁ 10ꢀ11 Mꢀ1 for n = 0 to 4.0 Mꢀ1 for n = þ1 to
2.5 ꢁ 109 Mꢀ1 for n = þ2. The three binding regimes are separated by 240 mV. Crystalline samples of [Re-1 C2H4]2þ are
3
obtained upon chemical oxidation of Re-1 with silver hexafluorophosphate (AgPF6) in the presence of ethylene. Strategies for the
addition of alkenes to other metal-stabilized thiyl radicals are suggested.
’ INTRODUCTION
nearly equally distributed between the metal and sulfur, a specific
site of oxidation cannot be defined, as the unpaired electron is
delocalized over both sites. We refer to these complexes as metal-
stabilized thiyl radicals.
Potential precursors of metal-stabilized thiyl radicals include
metalꢀthiolate complexes with highly nucleophilic sulfur cen-
ters. The nucleophilicity of metalꢀthiolates was attributed to
repulsive metalꢀsulfur dπꢀpπ interactions by Enemark and
Ashby more than 20 years ago.21 More recently, we noted that
metalꢀthiolates with nearly equal metal and sulfur character in
the highest occupied molecular orbital (HOMO) display en-
hanced nucleophilicity as compared to those with higher sulfur
character.22ꢀ24 The oxidation of such complexes is likely to lead
to metal-stabilized thiyl radicals.
The addition of organic thiyl radicals to unsaturated hydro-
carbons is well established with applications including cis/trans
isomerization, sulfide synthesis, and polymerization.25ꢀ28 As a
result, we elected to pursue the reactivity of metal-stabilized thiyl
radicals with alkenes. This work was largely inspired by the
reversible binding of ethylene to nickelꢀdithiolenes as a poten-
tial method for alkene purification reported by Wang and
This Forum Article highlights recent efforts in our group
toward the reversible addition of alkenes to metal-stabilized thiyl
radicals via carbonꢀsulfur bond formation/cleavage reactions.
As defined below, metal-stabilized thiyl radicals are discrete
oxidation products of metal complexes with “noninnocent”
thiolate ligands. They belong to a general class of reaction
products that may be obtained upon air oxidation of metalꢀthio-
late complexes, Scheme 1. Air oxidation of metalꢀthiolates can
proceed through multiple steps including one- or two-electron
events that are ligand- or metal-centered. Two-electron oxida-
tions involve O-atom transfer, leading to high-valent metal oxo
complexes1,2 or metal-coordinated sulfurꢀoxygenates3ꢀ6 with
relevance to cysteine dioxygenase,7,8 nitrile hydratase,4,6,9ꢀ11 and
thiocyanate hydrolase.4,6,9,12 One-electron oxidations yield oxi-
dized metal ions or thiyl radicals,13ꢀ20 which may yield disulfide
if not stabilized.
In this microreview, we focus on one-electron oxidations by
electrochemical methods or through the use of oxygen-free
chemical oxidants. In the simplest terms, the site of oxidation
depends on the composition of the molecular orbital that loses an
electron, Scheme 2. Molecular orbitals that are heavily localized
toward the metal or the sulfur yield the corresponding oxidized
metal or thiyl radical, accordingly. In cases where the orbital is
Received: February 28, 2011
Published: May 25, 2011
r
2011 American Chemical Society
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Inorganic Chemistry
FORUM ARTICLE
Scheme 1. Oxygen Sensitivity of MetalꢀThiolates
Scheme 3. Selected Reactions of [Ru-1]ꢀ
Scheme 2. Effect of Relative Metal and Sulfur Orbital En-
ergies on the Site of Oxidation
Figure 1. Stick representations of the metalꢀthiolate precursor com-
plexes [Ru-1]ꢀ and Re-1.
Stiefel.29 Later studies by Fekl et al.30 and Geiger31 indepen-
dently showed complications with the oxidation induced CꢀS
bond forming step and the reduction promoted CꢀS bond
cleavage. Nevertheless, the concept of reversible, redox-con-
trolled binding of alkenes to a metalꢀthiolate precursor re-
mained intriguing. Through a series of recent studies, Fekl et al.
obtained ethylene addition products with molybdenum dithio-
lenes that partially release the alkene in refluxing chloroform32
and diene addition to platinum dithiolenes upon oxidation.33
Also of note, Webster and co-workers reported alkene addition
to a ruthenium thiolate upon chemical oxidation.34 In the fol-
lowing pages, we will describe efforts from our group to develop a
system for redox-controlled, reversible binding of unsaturated
hydrocarbons to metal-stabilized thiyl radicals.
Figure 2. SOMO of Ru-1 showing significant contributions from the
Ru dxz orbital and the in-phase p orbitals on S2 and S3.
’ METAL-STABILIZED THIYL RADICALS
Our investigations with metal-stabilized thiyl radicals have
largely focused on the Ru and Re complexes [Ru(DPPBT)3]ꢀ,
[Ru-1]ꢀ, and Re(DPPBT)3, Re-1 (DPPBT = diphenylphosphino-
benzenethiolate), initially reported by Dilworth et al., Figure 1.35,36
Regarding the multiple redox events of Re-1, Dilworth et al.
noted that the ligand must “delocalize electron density in an
analogous manner to dithiolenes”.35 We became further inter-
ested in these complexes due to the reported solvent-dependent
O2 sensitivity of [HNEt3][Ru-1], which was noted to undergo
one-electron metal-centered oxidation to Ru-1 in dichloro-
methane and a six-electron sulfur-centered oxygenation to the
sulfenato(RSOꢀ)/sulfinato(RSO2ꢀ) derivative in toluene, di-
chloromethane, and methanol mixtures.36 We then synthesized
[PPN][Ru-1] (PPN = bis(triphenylphosphoranylidene)-
ammonium), which has improved solubility in organic solvents,
to further investigate its oxidation chemistry.37
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FORUM ARTICLE
Figure 3. Absorbance spectrum of [Ru-1]þ calculated by subtracting
one-half of the absorbance spectrum of Ru-1 prior to oxidation by bulk
electrolysis from a spectrum collected in situ during the oxidation after
one-half of the calculated charge had been released; ΔA = Ahalf-ox.
ꢀ
0.5Ainitial. A UVꢀvisible trace of the oxidation recorded every 0.1
electron equivalents has been previously reported.37
Figure 4. Qualitative overview of the frontier molecular orbitals of [Ru-
1]þ determined by density functional theory calculations.39 The highest
singly occupied molecular orbital (SOMO) is nearly orthogonal to the
SOMOꢀ1 and highest fully occupied molecular orbital (HOMO),
which are coplanar. The orientation of the sulfur p-type lone pairs
perpendicular to the respective PS chelate plane are shown along with
the assigned coordinate system in the lower left corner.
As shown in Scheme 3, [PPN][Ru-1] displays a variety of
metal- and ligand-centered reactions including alkylation,38
oxidation,37,39 and oxygenation.40 The Ru(II) complex [Ru-1]ꢀ
can be oxidized in two successive one-electron steps to the
formally Ru(III) and Ru(IV) derivatives Ru-1 and [Ru-1]þ. The
half-potentials for these events versus ferrocenium/ferrocene are
ꢀ830 mV and ꢀ20 mV for the [Ru-1]ꢀ/0 and [Ru-1]0/þ
couples, respectively. Sulfur-centered alkylation of [Ru-1]ꢀ in
dichloromethane at room temperature yields the dithioether
Scheme 4. Addition of Alkenes andMethyl Ketones to[Ru-1]þ
complex [Ru-1 CH2]þ, consistent with a high degree of
3
metalꢀsulfur interaction in the frontier molecular orbitals.
This suggests that oxidation of [Ru-1]ꢀ may involve both the
metal and sulfur.
The Ru-1 derivative can be isolated by chemical oxidation via
I2 or by electrochemical methods.37 The absorbance spectrum
displays low-energy bands at 540 (ε = 1090), 797 (ε = 200), and
1041 (ε = 3020) nm, consistent with previously reported metal-
coordinated thiyl radical complexes.14,20 The formal oxidation
state assignment of Ru(III) is further challenged by calculated
Mulliken atomic spin densities of 0.62 on Ru and 0.20 and 0.17
on the thiolate donors S2 and S3, respectively.39 Orbital analysis
of the singly occupied molecular orbital (SOMO) of Ru-1 reveals
significant contributions from the dxz orbital on Ru and the
coplanar, in-phase S2-pz and S3-px orbitals, Figure 2. The nearly
equal metal and sulfur contributions to the SOMO combined
with the approximately 60:40 distribution of spin density be-
tween Ru and the S donors is consistent with the assignment of
Ru-1 as a metal-stabilized thiyl radical.
Further oxidation of Ru-1 has been investigated using chemi-
cal and electrochemical methods. The addition of O2 to Ru-1 or
[Ru-1]ꢀ results in irreversible sulfur-centered oxygenation.40
The only detectable product after 24 h is the bis(sulfinato)-
ꢀ20 °C reveals an intermediate with an absorbance band at
859 nm, Figure 3, assigned as [Ru-1]þ, as supported by time-
dependent density functional theory.39 The low-energy band
initially grows in intensity but then decays prior to completion of
the one-electron oxidation. The final oxidation product is assigned
as a Ru(II) complex with an intramolecular disulfide bond.
Density functional theory investigations of [Ru-1]þ reveal a
nearly degenerate electronic ground state that is best described as
a singlet diradical.39 Significant spin localization is observed on
Ru (0.84), S1 (0.31), S2 (0.47), and S3 (0.21). A qualitative
overview of the frontier molecular orbitals, Figure 4, highlights
the π interactions between Ru d orbitals and S p orbitals in the xz,
yz, and xy planes. The configuration of the ground state is best
described as (S2(pz) ꢀ S3(px))2(dxz ꢀ S2(pz) ꢀ S3(px))1(dyz ꢀ
S1(py))1.
Ru(II) product, [Ru-1 O4]ꢀ. The isolation of an anionic Ru(II)
3
product from the neutral Ru-1 requires reduction along with
sulfur oxygenation. Through control reactions, we have shown
hydroxide to be a competent reductant of the RuIII sulfurꢀ
oxygenate [Ru-1 O4] in acetonitrile. Further oxidations of Ru-1
3
were conducted under anaerobic conditions.
The one-electron oxidation of Ru-1 by electrochemical meth-
ods is reversible, as determined by cyclic voltammetry (CV) with
a [Ru-1]þ/0 redox couple of ꢀ20 mV.37 Bulk oxidation of Ru-1
reveals an EC process with an irreversible chemical step (C) that
follows the reversible oxidation (E). A UVꢀvisible trace re-
corded during the bulk oxidation of Ru-1 in acetonitrile at
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FORUM ARTICLE
Figure 5. The 31P NMR spectrum of crystalline [Ru-1 p-methylstyrene]þ
3
in CD3CN. The spectrum matches the major product of the as-
isolated mixture. Chemical shifts and coupling constants are provided
in the text.
The diradical ground state of [Ru-1]þ delocalizes the un-
paired electrons of the metal-stabilized thiyl radicals over the
metal and sulfur donors in nearly orthogonal molecular orbitals.
This inhibits disulfide formation. In the absence of additional
substrates, the rate constant for the decay of [Ru-1]þ is
0.0101(6) sꢀ1 at 272 K.37 The relatively slow degradation of
[Ru-1]þ coupled with its significant sulfur-centered spin density
make it a prime candidate for reactivity studies with unsaturated
organic compounds.
Figure 6. ORTEP representations of the complex cations of [Ru-1 m-
3
’ IRREVERSIBLE ALKENE ADDITION
The oxidized intermediate [Ru-1]þ reacts with alkenes and
methylstyrene][PF6] (top) and [Ru-1 p-methylstyrene][PF6]
3
(bottom). Selected bond distances and angles are provided in Table 2.
methyl ketones to yield dithioether alkene addition products,
[Ru-1 alkene]þ, and monothioether ketone addition products,
3
3
[Ru-1 CH2C(O)R]þ, respectively, asshown inScheme4.39,41,42
step is proposed to proceed through the enol tautomer with
Through a series of bulk electrolysis experiments on freshly
prepared solutions of Ru-1, we established the addition of
alkenes to [Ru-1]þ via a one-electron EC mechanism. In the
electrochemical step, Ru-1 is oxidized to [Ru-1]þ. The addition
of the alkene in the following chemical step proceeds too rapidly
to detect [Ru-1]þ with UVꢀvisible spectroscopy.
rearrangement of the addition product to [Ru-1 CH2C(O)R]þ.
3
Since the [Ru-1 CH2C(O)R]2þ/þ redox couple, ꢀ60 mV, is
3
slightly more accessible than the [Ru-1]þ/0 couple, ꢀ20 mV, the
second oxidation proceeds at the applied potential. As with the
alkenes, the addition of methyl ketones to [Ru-1]þ is irreversible.
The synthesis of [Ru-1 alkene]þ derivatives by chemical
3
The ethylene addition product displays a [Ru-1 C2H4]2þ/þ
oxidation has not been previously reported. It can be readily
achieved upon the addition of mild chemical oxidants to [Ru-1]ꢀ
in the presence of the appropriate alkenes. To demonstrate this
reaction, we have prepared and isolated [Ru-1•m-
3
redox couple at þ285 mV assigned to the formal oxidation of
Ru(II) to Ru(III).41 A comparison of square voltammograms
recorded before and after oxidation reveals quantitative conver-
sion of [Ru-1 C2H4]þ to [Ru-1 C2H4]2þ. The other alkene
methylstyrene]þ and [Ru-1 p-methylstyrene]þ as described
3
3
3
products display similar potentials for the RuIII/II redox couple. A
in the Experimental Section. The complexes are obtained in 80%
yield from [Ru-1]ꢀ, two equivalents of FcPF6, and the respective
alkene. Analysis of the crude products by þESI-MS reveals a
single peak with m/z = 1099.20, consistent with the expected
ratio of 1099.15. Alkene addition is confirmed through the
observation of a single redox event at þ224 and þ284 mV for
[Ru-1 m-methylstyrene]þ and [Ru-1 p-methylstyrene]þ, re-
potential of þ280 mV is also observed for the [Ru-1 CH2]2þ/þ
3
couple of the methylene bridged complex isolated from the
alkylation of [Ru-1]ꢀ with dichloromethane.38 The large shift,
þ1110 mV, in the formal RuIII/II couple as compared to [Ru-1]ꢀ/0
is consistent with the conversion of two thiolate donors to
thioethers.23,43,44
3
3
The alkene addition product [Ru-1 alkene]þ is air and water
spectively. The RuIII/II couples are shifted by approximately
3
stable. There is no observed release of ethylene from [Ru-
þ1100 mV with respect to [Ru-1]0/ꢀ
.
1 C2H4]þ after prolonged purging with inert gases or overnight
Although the mass spectrum and cyclic voltammograms of
3
exposure to vacuum conditions. The stability of these complex is
attributed to the favorable low-spin, d6 electron configuration of
Ru(II) in a phosphine and thioether donor environment.
The reaction with ketones proceeds more slowly via a two-
electron ECE pathway.42 The first electrochemical step generates
[Ru-1]þ with the expected low energy band at 859 nm in the
absorbance spectrum. The addition of the ketone in the chemical
[Ru-1 p-methylstyrene]þ are consistent with a single product,
3
the 31P NMR spectrum of the “as isolated” compound re-
veals two reaction products with distinct spectra, Figure S1
(Supporting Information). The chemical shifts are tempera-
ture-independent, ruling out interconverting configurational
isomers. Instead, the peaks are assigned to a pair of structural
isomers that differ by the relative position of the aromatic
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FORUM ARTICLE
Table 1. Crystal Data and Structure Refinement for [Ru-1 m-methylstyrene][PF6] and [Ru-1 p-methylstyrene][PF6]
3
3
identification code
[Ru-1 m-methylstyrene][PF6]
[Ru-1 p-methylstyrene][PF6]
3
3
empirical formula
C
64.5H52Cl3F6P4RuS3
C75H62Cl2F6P4RuS3
1469.29
formula weight
temperature (K)
wavelength (Å)
crystal system
space group
unit cell dimensions
a (Å)
1368.54
100(2)
0.71073
monoclinic
P21
100(2)
0.71073
triclinic
P1
10.5855(4)
10.5684(8)
b (Å)
16.8551(5)
12.4018(9)
c (Å)
17.7704(4)
12.9434(6)
R (deg)
90
88.536(5)
β (deg)
104.576(3)
85.552(5)
γ (deg)
90
74.111(6)
vol (Å3)
3068.53(18)
2
1626.71
Z
1
dcalcd (Mg/m3)
abs coeff (mmꢀ1
1.481
1.500 Mg/m3
0.580 mmꢀ1
)
0.651
F(000)
cryst size (mm3)
1392
752
0.31 ꢁ 0.14 ꢁ 0.10
3.38 to 29.65
ꢀ14 e h e 13
ꢀ23 e k e 23
ꢀ24 e l e 22
50963
0.14 ꢁ 0.11 ꢁ 0.02 mm3
3.30 to 28.14°
ꢀ13 e h e 14
ꢀ15 e k e 16
ꢀ15 e l e 17
17453
θ range for data collection (deg)
index ranges
reflns collected
independent refections
completeness to theta max
absorption correction
max. and min transmission
refinement method
15701 [R(int) = 0.0498]
94.5%
11533[R(int) = 0.0610]
87.7%
SCALE3 ABSPACK
0.952 and 0.926
full-matrix least-squares on F2
15701/27/778
1.014
SCALE3 ABSPACK
0.996 and 0.931
full-matrix least-squares on F2
11553/3/530
1.09
data/restraints/params
goodness of fit on F2
final R indices [I > 2σ(I)]a,b
R indices (all data)a,b
R1 = 0.0438, wR2 = 0.1042
R1 = 0.0563, wR2 = 0.1136
R1 = 0.0807, wR2 = 0.1399
R1 = 0.1109, wR2 = 0.1536
largest diff. peak and hole (e Åꢀ3
)
1.372 and ꢀ0.564
1.660 and ꢀ0.955
3
a R1 = ∑||Fo| ꢀ |Fc||/∑|Fo|. b wR2 = {∑[w(Fo ꢀ Fc2)2]/∑[w(Fo ) ]}1\2, where w = q/σ2(Fo ) þ (qp) 2 þ bp. GOF = S = {∑[w(Fo ꢀ Fc2)2]/(n ꢀ p)}1/2
,
2
2 2
2
2
where n is the number of reflections and p is the number of parameters refined.
substituent in the addition product. The major component (65%)
displays a second-order spectrum with chemical shifts (ppm) of
δ1 = 59.2, δ2 = 39.2, and δ3 = 35.1 and coupling constants (Hz) of
J12 = J13 = 30 and J23 = 300. The chemical shifts and coupling
constants are similar to those of previously reported related
complexes.36,38,41,45 A similar second-order spectrum is observed
for the minor component (35%); δ1 = 54.7, δ2 = 44.4, δ3 =
35.9 ppm, J12 = J13 = 28, and J23 = 316 Hz. Crystallization of [Ru-
dichloromethane/hexane solutions. ORTEP46 representations
of the cationic complexes are shown in Figure 6. Details of the
data collection and refinement are summarized in Table 1. In
each instance, the complex maintains a meridional M(PS)3 core
structure with the addition of the alkene across S2 and S3,
similar to [Ru-1 C2H4]þ.41 To date, only one isomer of each
3
complex has been crystallized. In both cases, the X-ray structure
reveals the unsubstituted carbon, C55, bonded with S2, which is
the sulfur trans to P1. As shown in Table 2, there are no
significant differences in the primary coordination sphere of the
three complexes. Alkene coordination generates a five-member
Ru(1)ꢀS(2)ꢀC(55)ꢀC(56)ꢀS(3) puckered ring. The aver-
age S(2)ꢀC(55) and S(3)ꢀC(56) bond distances of 1.83(2)
and 1.86(2) Å are consistent with single bond formation
between the coordinated sulfur and the alkene. Also, the
average and C(55)ꢀC(56) bond distance of 1.50(2) Å con-
firms single bond character and covalent coordination of the
alkene to the metal-stabilized thiyl radical precursor.
1 p-methylstyrene]þ yields yellow plates, which display a 31P
3
NMR spectrum consistent with the major isomer, Figure 5.
Repeated attempts to crystallize the minor component have been
unsuccessful. The [Ru-1 m-methylstyrene]þ products are simi-
3
lar, with δ1 = 59.3, δ2 = 39.1, and δ3 = 35.2 ppm and J12 = J13 = 30
and J23 = 300 Hz for the major isomer and δ1 = 54.8, δ2 = 44.3, and
δ3 = 35.9 ppm and J12 = J13 = 32 and J23 = 318 Hz for the minor.
X-ray-quality yellow plate crystals of [Ru-1 p-methylstyrene]-
3
[PF6] and [Ru-1 m-methylstyrene][PF6] were obtained
3
through the evaporation of concentrated chlorobenzene or 1:1
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Inorganic Chemistry
FORUM ARTICLE
Table 2. Selected Bond Distances (Å) and Bond Angles (deg) for [Ru-1 C2H4]þ,41 [Ru-1 m-methylstyrene]þ, and
3
3
[Ru-1 p-methylstyrene]þ
3
[Ru-1 C2H4]þ
[Ru-1 m-methylstyrene]þ
[Ru-1 p-methylstyrene]þ
3
3
3
RuꢀS1
2.3856(9)
2.3749(9)
2.3365(9)
2.3290(9)
2.3965(10)
2.3648(9)
1.836(4)
1.843(4)
1.510(5)
87.71(3)
168.81(3)
87.06(3)
173.73(3)
114.0(3)
112.8(3)
2.3735(9)
2.3707(9)
2.3404(9)
2.3423(9)
2.3623(9)
2.3717(9)
1.818(4)
1.883(4)
1.501(6)
87.86(3)
170.64(3)
85.81(3)
172.50(3)
114.1(3)
112.0(3)
2.389(3)
2.373(3)
2.320(2)
2.333(3)
2.350(3)
2.391(3)
1.838(9)
1.867(10)
1.483(13)
87.67(9)
169.64(10)
87.14(9)
174.37(11)
113.8(7)
110.1(7)
RuꢀS2
RuꢀS3
RuꢀP1
RuꢀP2
RuꢀP3
S2ꢀC55
S3ꢀC56
C55ꢀC56
S2ꢀRuꢀS3
P2ꢀRuꢀP3
S1ꢀRuꢀS2
S1ꢀRuꢀS3
S2ꢀC55ꢀC56
S3ꢀC56ꢀC55
Scheme 5. EECE Mechanism for the Addition of Alkenes to
[Ru-1]ꢀ
Table 3. Second-Order Rate Constants for the Addition of
Selected Alkenes to [Ru-1]þ in Acetonitrile at 298 Ka
alkene
k (Mꢀ1 sꢀ1
)
monosubstituted alkenes
Figure 7. Cyclic voltammograms of [Ru-1]ꢀ in the absence (top) and
presence (bottom) of 2 M styrene. Data were collected in acetonitrile
with 0.1 M tetrabutylammonium hexafluorophosphate as a supporting
electrolyte. Potentials are referenced versus Fcþ/Fc.
styrene
4.6(5) ꢁ 107
2.0(1) ꢁ 107
2.6(8) ꢁ 106
7(2) ꢁ 105
n-propyl vinyl ether
t-butyl vinyl ether
n-hexene
In the crystal structures of both [Ru-1 m-methylstyrene]þ
acrylonitrile
2.7(2) ꢁ 104
3
and [Ru-1 p-methylstyrene]þ, the unsubstituted carbon of the
3
cyclic alkenes
styrene is bonded to S2 and the sulfur trans to P1. The S2 and S3
sites can also be differentiated by their calculated spin density,
which is more than twice as high on S2 (0.47) than on S3
(0.21). Since Tedder’s Rules47 for radical alkene addition
suggest that substitution should occur initially at the nonsub-
stituted carbon, we tentatively assign the crystal structures of
[Ru-1 m-methylstyrene]þ and [Ru-1 p-methylstyrene]þ to
norborene
4(2) ꢁ 107
6(1) ꢁ 106
2.9(3) ꢁ 103
cyclopentene
cyclohexene
a Rate constants were determined by simulation of cyclic voltammetry
data collected at multiple scan rates, as described in the Supporting
Information.
3
3
the major products observed in the 31P NMR.
To obtain further insight into the alkene addition mechanism,
CV was used to determine rate constants for the addition of
various alkenes to the oxidized intermediate [Ru-1]þ. The rapid
addition of alkenes to [Ru-1]þ results in significant changes in
the cyclic voltammograms of [Ru-1]ꢀ, Figure 7, that can be
simulated according to an EECE mechanism, Scheme 5. The first
two electrochemical steps are assigned to the oxidation of [Ru-1]ꢀ
to [Ru-1]þ in successive one-electron steps. The cathodic return
peak currents associated with these events are greatly diminished
due to the irreversible formation of [Ru-1 alkene]n during
3þ
oxidation. The rapid alkene addition to [Ru-1] in the chemical
step disrupts the equilibrium concentrations of [Ru-1]þ/0 and
shifts the apparent anodic potential to lower potentials. The final
electrochemical step is the oxidation of the alkene addition
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Scheme 6. Thermodynamic Squares for [Re-1]n and [Re-
1 C2H4]n (n = 0, þ1, þ2)
3
Figure 8. Absorbance spectra showing the reversible binding of ethy-
lene to [Re-1]þ. The absorbance bands at 390 and 581 nm decrease in
intensity upon the introduction of ethylene gas (top) until the bands
reach ∼46% of their initial intensity. The bands are restored by purging
the solution with nitrogen. Spectra were recorded every 30 s.
product by one electron from [Ru-1 alkene]þ to [Ru-1 alkene]2þ
As described in the Supporting Information, rate constant data
were extracted for a variety of alkenes using the DigiSim software
package.48
.
3
3
Experimentally determined rate constants for the addition of
various alkenes to [Ru-1]þ are summarized in Table 3. The data
are consistent with an electrophilic metal-stabilized thiyl radical.
The nonfunctionalized alkene, 1-hexene, displays a second-order
rate constant of 7(2) ꢁ 105 Mꢀ1 sꢀ1. The “electron-rich” alkenes
styrene, n-propyl vinyl ether, and t-butyl vinyl ether add more
rapidly, while the “electron-poor” acrylonitrile displays a reduced
rate constant. Similar observations for the addition of haloalkyl49
and t-butoxy radicals50 to alkenes were reported by Hosomi et al.
and Solomon et al., respectively. For cyclic alkenes, the rate
constant is related to the amount of ring strain in the alkene
on the release of ethylene from [Re(TTCN)]2þ confirmed a
drop in the energy barrier for CꢀS bond cleavage from 22 to
4 kcal/mol upon reduction.52,53 This 18 kcal/mol decrease in
activation energy was attributed to a combination of charge and
π-back-bonding effects. Related studies on the [Ru(TTCN)]2þ
derivative predict an activation energy for ethylene release of
44 kcal/mol, consistent with the observed stability of our Ru(II)
[Ru-1 alkene]þ complexes. As such, to achieve reversible alkene
3
binding, we turned our attention to the rhenium derivative, Re-1.
Unlike Ru-1, solutions of Re-1 are stable in the air and show no
detectable reaction with dichloromethane after one week. The
neutral complex displays two reversible oxidation events at ꢀ340
and þ420 mV assigned to the [Re-1]þ/0 and [Re-1]2þ/þ redox
couples, respectively.35,54 A [Re-1]0/ꢀ reduction event is also
observed at ꢀ1620 mV. Although Re-1 has the same valence
electron count as [Ru-1]þ, the neutral precursor does not react
with ethylene. This is attributable to the difference in charge of the
two complexes, which was predicted by Rothlisberger to signifi-
cantly influence the energy barrier for CꢀS bond cleavage.52
Oxidation of Re-1 to [Re-1]þ by chemical or electrochemical
methods yields a deep blue solution that reversibly binds
ethylene.54 As shown in Figure 8, the absorbance band at
581 nm decreases to 45% of its initial intensity within 150 s.
No further changes in the absorbance band are noted at longer
reaction times, indicating the process is in dynamic equilibrium
precursor; knorborene > kcyclopentene > kcylcohexene
.
Overall, a variety of alkenes can be added to [Ru-1] upon
oxidation to yield the corresponding dithioetherꢀRu(II) deri-
vatives. The addition proceeds via an electrophilic ruthenium-
stabilized thiyl radical, [Ru-1]þ, with the fastest rate constants
observed for “electron-rich” alkenes and cyclic alkenes with
significant ring strain. In all cases, the addition is irreversible.
The insurmountable barrier to CꢀS bond cleavage can be
attributed to the thermodynamic stability of the dithioetherꢀRu(II)
products. As such, we sought other derivatives that would prove
more amenable to CꢀS bond cleavage and reversible alkene
binding.
’ REVERSIBLE ALKENE ADDITION
Although our results with [Ru-1]þ demonstrated facile CꢀS
with an equilibrium binding constant of approximately 2.6 Mꢀ1
.
bond formation with alkenes to generate the desired [Ru-
Purging the solution with nitrogen restores the initial intensity
of the 581 nm band within 300 s. The process can be re-
peated multiple times with no detectable loss of binding/release
efficiency.
To refine the ethylene binding constant and establish rate
constants for ethylene binding/release, cyclic voltammograms of
1 alkene]þ, the thermodynamic stability of the product pre-
3
cluded reversibility. Reports of CꢀS bond cleavage with re-
lease of ethylene upon reduction of the Re(II) complex
[Re(TTCN)]2þ (TTCN = 1,4,7-trithiacyclononane) by Went
et al. provided a lead.51 Computational studies by Rothlisberger et al.
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[Re-1]þ were recorded under nitrogen and ethylene atmospheres.54
Simulation of the data over a range of scan rates from 100 to
1000 mV/s revealed rate constants for ethylene addition and
release of 0.12(2) Mꢀ1 sꢀ1 and 0.030(4) sꢀ1, respectively. The
equilibrium binding constant of 4.0(8) Mꢀ1 matches the pre-
diction from the UVꢀvisible study.
Using the experimentally determined redox potentials for
[Re-1]n and [Re-1 C2H4]n with the equilibrium binding con-
3
stant for ethylene to [Re-1]þ, we constructed the set of
thermodynamic boxes, Scheme 6.54 From these, we determined
equilibrium binding constants for ethylene to the neutral Re-1
complex, 1.9(4) ꢁ 10ꢀ11
through Scheme 6 from Re-1 to [Re-1 C2H4]2þ, the three
M ,
ꢀ1, and the dicationic [Re-1]2þ
2.5(9) ꢁ 109 Mꢀ1. As highlighted by following an ECE path
3
distinct ethylene binding regimes are accessible and only sepa-
rated by 240 mV. Mild oxidants can be employed with Re-1 to
induce strong ethylene binding, while mild reductants can induce
its release from [Re-1 C2H4]2þ. Using this approach, we have
3
isolated [Re-1 C2H4]2þ through the chemical oxidation of
Figure 9. Orientation of thiolate lone pairs in various chelate geome-
tries. The p-type lone pair on sulfur is placed orthogonal to the LS
chelate plane (shaded). The predicted alkene reactivity is based on the
symmetry of the thiolate lone pairs relative to the HOMO of an alkene,
as described in the text.
3
[Re-1] with AgPF6 in an ethylene saturated solution.
Attempts to measure rate constants for the addition of various
alkenes to [Re-1]þ by CV following the protocol established for
[Ru-1]þ were unsuccessful. Chemical oxidation of [Re-1]þ by
AgPF6 in the presence of styrene confirms the formation of [Re-
1 styrene]þ, but with a rate constant it is too slow to detect on
of common organic solvents, making the addition of alkenes to
[Ru-1]þ irreversible.
3
the time scale of the CV experiments. Preliminary UVꢀvisible
studies suggest a rate constant on the order of 10ꢀ5 Mꢀ1 sꢀ1
,
If the only requirements for alkene addition to a metal-
stabilized thiyl radical were the symmetry of the frontier molec-
ular orbitals and the accessibility of a suitable oxidation state of
the product, Re-1 would be expected to display similar reactivity
to [Ru-1]þ. It does not. As described by Rothlisberger et al., the
charge of the complex is also important. The low binding affinity
of Re-1 for ethylene is ascribed to the relatively high energy of
which is below our CV detection limit of 1 ꢁ 10ꢀ3 Mꢀ1 sꢀ1
.
The rate constant for styrene addition to [Re-1]þ is 4 orders
of magnitude slower than that of ethylene, indicating that
[Re-1]þ can selectively bind ethylene even in the presence of
other alkenes. Further, the rate constant for styrene addition to
[Re-1]þ is 12 orders of magnitude slower than that to [Ru-1]þ.
This would be consistent with a linear free energy relationship
between the rate constant and equilibrium binding constant with
fast irreversible addition of alkenes to [Ru-1]þ and slow,
reversible addition to [Re-1]þ.
the Re-1 C2H4 and Re-1 frontier orbitals with respect to the
3
HOMO of ethylene. Oxidation of the Re complexes to their
cationic derivatives lowers the energies of their molecular
orbitals, providing a better energetic match with the HOMO of
ethylene. For [Re(TTCN)2]3þ/2þ, a þ1 change in charge was
calculated to increase the energy barrier for ethylene release
by 18 kcal/mol.52 Similarly, in our system, oxidation of Re-1
to [Re-1]þ stabilizes ethylene binding by 15.4 kcal/mol. Further
oxidation of [Re-1]þ to [Re-1]2þ results in an additional
12.0 kcal/mol of stabilization.
’ CONCLUDING REMARKS
The addition of alkenes to the oxidized metalꢀthiolate com-
plexes [Ru-1]þ and [Re-1]þ is consistent with their assignments
as metal-stabilized thiyl radicals. The addition occurs selectively
across the S2/S3 sulfur pair, which is differentiated from S1 by
the directionality of their p-type “lone pair” orbitals, as shown in
Figure 4. The in-phase combination of the S2/S3 p-type orbitals
interacts in an antibonding capacity with the metal dyz orbital
generating a half-occupied, frontier molecular orbital. As shown
in Figure 4, this orbital has the proper symmetry to interact with
the HOMO of the approaching alkene.
With the insight gleamed from the [Ru-1]n and [Re-1]n systems,
we can predict general features that would promote alkene
addition to metal-stabilized thiyl radicals. To identify potential
metal-stabilized thiyl radicals, the nucleophilic character of the
coordinated thiolate can serve as a predictor of high metalꢀsulfur
contributions in the frontier molecular orbitals. The ligand
framework should be designed to provide a SOMO with the
proper symmetry to matche the alkenes’ HOMO. As shown in
Figure 9, this can be accomplished with cis-dithiolate metal
precursors with coplanar sulfur lone pairs dictated by chelation.
In an octahedral environment, bis-chelates or tetradentate do-
nors should adopt an “R-isomer” confirmation, while tridentate
ligands should be facially coordinating to promote the interaction
of the metal d orbital with coplanar sulfur p orbitals, as shown in
Figure 4. A “trans-isomer” orientation has perpendicular sulfur
lone pairs with an in-phase combination that can also overlap
with the HOMO of an alkene. A potential drawback of this
arrangement is the relative position of the symmetry preferred
molecular orbital below the out-of-phase Sꢀp combination.
Alkene addition to our metal-stabilized thiyl radicals initiates a
net two-electron transfer from the alkene to the metal-stabilized
thiyl radical complex, resulting in a two-electron reduction in the
formal oxidation state of the metal ion. For [Ru-1]þ, the formal
RuIV is readily reduced to RuII in the thermodynamically stable
[Ru-1 alkene]þ product. Although our computational studies
3
also find the requisite half-occupied frontier molecular in the
neutral Ru-1 complex, alkene addition does not occur, as the
putative Ru(I) product, Ru-1 alkene, is thermodynamically
3
unfavorable. As such, reduction of [Ru-1 alkene]þ to Ru-
3
1 alkene would be expected to initiate the CꢀS bond, but the
3
[Ru-1 alkene]þ/0 redox couple lies outside the solvent window
3
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Regarding oxidation and charge state preferences, alkene
addition requires a net two-electron reduction of the metal-
stabilized thiyl radical. The formal oxidation state of the reactive
complex should be high enough to yield an accessible oxidation
state in the alkene addition product. If the products’ oxidation
state is inaccessible, alkene addition will not occur. If the product
will be generated in the lowest accessible oxidation state, addition
will likely be irreversible. Finally, addition products with acces-
sible redox couples can be reduced to lower alkene binding
affinity and oxidized to enhance it. Future efforts to develop new
complexes to demonstrate alkene binding to metal-stabilized
thiyl radicals and the development of potential applications is
underway.
Crystallographic Studies. A yellow needle 0.31 ꢁ 0.14 ꢁ
0.10 mm3 crystal of [Ru-1 m-methylstyrene][PF6] was mounted on
3
a glass fiber for collection of X-ray data on an Agilent Technologies/
Oxford Diffraction Gemini CCD diffractometer. The CrysAlisPro55
CCD software package (v 171.33.34d) was used to acquire a total of 668
30-s frame ω-scan exposures of data at 100 K to a 2θ max = 59.30° using
monochromated Mo KR radiation (0.71073 Å) from a sealed tube.
Frame data were processed using CrysAlisPro55 RED to determine final
unit cell parameters: a = 10.5855(4) Å, b = 16.8551(5) Å, c = 17.7704(4) Å,
β = 104.576(3)°, V = 3068.53(18) Å3, Dcalc = 1.481 Mg/m3, and Z = 2
to produce raw hkl data that were then corrected for absorption
(transmission min./max. = 0.952/0.926; μ = 0.651 mmꢀ1) using
SCALE3 ABSPACK in CrysAlisPro.55 The structure was solved by
Patterson methods in the space group P21 using SHELXS-9056 and
refined by least-squares methods on F2 using SHELXL-9757 incorpo-
rated into the SHELXTL58 (v 6.14) suite of programs. Non-hydrogen
atoms were refined with anisotropic atomic displacement parameters
(except for the 20% occupancy F atoms, which were refined iso-
tropically). The hexafluorophosphate anion has a tumbling disorder
that was modeled with one 60% occupancy (F1aꢀF6a) group and two
20% occupancy (F1bꢀF6b and F1cꢀF6c) groups of fluorine atoms.
The ratio for the anion disorder was fixed at 60:20:20 after being
determined from unstable refinement of the F-atom occupancies. The
disordered full occupancy methylene chloride solvate was modeled
with two 50% occupancy groups, Cl1aꢀCl2a and Cl1bꢀCl2b, in
addition to the full occupancy C atom. The 1/2 occupancy methylene
chloride was modeled with two 25% occupancy groups, Cl3aꢀCl4a
and Cl3bꢀCl4b, and a 50% C atom. Hydrogen atoms were placed in
their geometrically generated positions and refined as a riding model.
Methylene, methine, and phenyl H’s were included as fixed contribu-
tions with U(H) = 1.2 ꢁ Ueq (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) = 1.5 ꢁ Ueq. For all 15 701 unique reflections (R(int) 0.049), the
final anisotropic full matrix least-squares refinement on F2 for 778 variables
converged at R1 = 0.056 and wR2 = 0.113 with a GOF of 1.01.
’ EXPERIMENTAL SECTION
Materials and Methods. All chemicals were purchased from
commercial sources (Aldrich) and used without further purification
unless otherwise noted. All solvents were purified utilizing the standard
methods and were freshly distilled and degassed using the freezeꢀ
pumpꢀthaw method immediately before use. The metalꢀthiolate
complexes PPN[Ru-1],37 HNEt3[Ru-1],36 and Re-135 were synthe-
sized according to published procedures. Deuterated acetonitrile was
purchased from Cambridge Isotope Laboratories, Inc. and used as
received. All reactions were conducted under anaerobic conditions by
using standard Schlenk techniques under a nitrogen atmosphere.
[Ru-1 m-methylstyrene][PF6]. To a yellow solution of HNEt3-
3
[Ru-1] (100 mg, 0.0935 mmol) in acetonitrile (40 mL) was added m-
methylstyrene (0.0621 mL, 0.468 mmol) via syringe. The solution was
cooled in an ice bath, and a blue solution of ferrocenium hexafluoropho-
sphate (0.0619 g, 0.0187 mmol) in acetonitrile (30 mL) was added by
cannula transfer. The resulting solution was stirred for 3 h, during which
time the color changed to a yellow-green. The solvent was removed by
rotary evaporation to yield a yellow residue. The crude product was
washed with an excess of hot water (∼300 mL) and diethyl ether
(25 mL). Yield: 0.090 g (82%). X-ray quality crystals were obtained
by the addition of 30 mg of crude product to a mixture of dichloro-
methane (1 mL) and hexane (1 mL). Slow evaporation yielded yellow
X-ray quality crystals. E1/2(RuIII/RuII) = þ224 mV. þESI-MS for
C63H52P3S3Ru, theoretical: m/z (Z = 1) 1099.15. Observed: 1099.20.
Element analysis for C65H56P4S3F6Ru, calculated: C, 61.35; H, 4.44.
Found: C, 61.50; H, 4.73.
After numerous attempts, crystals of [Ru-1 p-methylstyrene][PF6]
3
suitable for X-ray analysis were grown by slow evaporation of chlor-
obenzene solution and mounted on a CryoLoop for collection of X-ray
data on an Agilent Technologies/Oxford Diffraction Gemini CCD
diffractometer. X-ray structural analysis for [Ru-1 p-methylstyrene]-
3
[PF6] was performed on a 0.14 ꢁ 0.11 ꢁ 0.02 mm3 yellow plate using a
463 frame, 55-s frame ω-scan data collection strategy at 100 K to a 2θmax
=
56.30°. [Ru-1 p-methylstyrene][PF6] crystallizes in the space group
[Ru-1 p-methylstyrene][PF6]. The complex is prepared by
3
3
P1 with unit cell parameters a = 10.5684(8) Å, b = 12.4018(9) Å, c =
12.9434(6) Å, R = 88.536(5)°, β = 85.552(5)°, γ = 74.111(6)°, V =
1626.71(18) Å3, Z = 1, and Dcalc = 1.500 Mg/m3. A total of 11 553 raw
independent data were corrected for absorption (transmission min./
max. = 0.996/0.931; μ = 0.580 mmꢀ1). The structure was solved by
Patterson methods using SHELXTL58 as well as by direct methods
using SIR9759 and charge-flipping using OLEX2.22,23 All non-hydro-
gen atoms were refined with anisotropic atomic displacement parameters.
Hydrogen atoms were included as fixed contributions, as described above
the method described above except that p-methylstyrene (0.0618 mL,
0.468 mmol) was added in place of m-methylstyrene. Yield: 0.088 g
(80%). X-ray quality yellow plate crystals were obtained from the evapora-
tion of chlorobenzene solutions. E1/2(RuIII/RuII) = þ284 mV. þESI-MS
for C63H52P3S3Ru, theoretical: m/z (Z = 1) 1099.15. Observed:
1099.20. Element analysis for C65H56P4S3F6Ru, calculated: C, 61.35;
H, 4.44. Found: C, 60.99; H, 4.73.
Physical Methods. NMR spectra were recorded on a Varian 400 MHz
spectrometer and referenced to TMS (1H NMR) and 85% H3PO4
(31P NMR). Elemental analysis was performed by the Midwest
Microlab (Indianapolis, IN). Mass spectra were collected by the Mass
Spectrometry Application and Collaboration Facility in the Chemistry
Department at Texas A&M University. All electrochemical measure-
ments were performed by 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 standard (0.00 V),
which was determined using an internal standard. Full details regarding
the determination of rate constants by cyclic voltammetry are provided
in the Supporting Information.
for [Ru-1 m-methylstyrene][PF6]. Forreflections I>2σ(I) (R(int) 0.061),
3
the final anisotropic full matrix least-squares refinement on F2 for 530
variables converged at R1 = 0.080 and wR2 = 0.139 with a GOF of 1.09.
’ ASSOCIATED CONTENT
S
Supporting Information. Data fitting procedure for de-
b
termination of alkene addition rate constants, best-fit CV para-
meters, 31P NMR of [Ru-1 p-methylstyrene]þ as-isolated
3
showing major and minor products, and þESI-MS for [Ru-
1 m-methylstyrene]þ and [Ru-1 p-methylstyrene]þ in PDF
3
3
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format and crystallographic data in CIF format (CCDC 814546
and 814547). This material is available free of charge via the
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’ AUTHOR INFORMATION
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Corresponding Author
*E-mail: grapperhaus@louisville.edu.
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C.A.G. expresses sincere gratitude to the students and
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acknowledge the Donors of the American Chemical Society
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Science Foundation (CHE-0238137) for financial support
during the early stages of this project. M.S.M. thanks the
Department of Energy, grant number DE-FG02-08CH11538,
and the Kentucky Research Challenge Trust Fund for the
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