Terthienyl and poly-terthienyl ligands as redox-switchable hemilabile ligands for oxidation-state-dependent molecular uptake and release

Article abstract of DOI:10.1021/ja0030008

Mononuclear, dinuclear, and polymeric Ru(II) complexes formed from terthienylalkylphosphino redox-switchable hemilabile ligands demonstrate that this class of ligand provides electrochemical control over the electronic properties, coordination environments, and reactivities of bound transition metals. Specifically, [CpRuCO(κ²-3′-(2-diphenylphosphinoethyl)-5, 5″-dimethyl-2,2′:5′,2″-terthiophene)] [B(C₆H₃-3,5-(CF₃)₂)₄] (4a) exhibits a 3 orders of magnitude increase in binding affinity for acetonitrile upon terthienyl-based oxidation. FT-IR spectroelectrochemical experiments on 4a indicate that terthienyl-based oxidation removes electron density from the metal center, equivalent to approximately 11-17% of the electronic change that occurs upon direct oxidation of Ru(II) to Ru(III) in analogous complexes. The spectroelectrochemical responses of 4a were compared to those of dimeric and polymeric analogues of 4a. The spectroelectrochemistry of the dimer is consistent with two sequential, one-electron ligand-based oxidations, compared to only one in 4a. In contrast, the polymer exhibits spectroelectrochemical behavior similar to that of 4a. The polymer spectroelectrochemistry shows changes in the metal center electronic properties between two different states, reflective of two discrete oxidation states of the polymeric ligand backbone. We propose that the polymer backbone does not allow one to vary the electronic properties of the metal center through a continuous range of oxidation states due to charge localization within the metalated films. In an effort to explore the molecular uptake and release properties of 4a and its polymer analogue as a function of ligand oxidation state, the oxidation-state-dependent coordination chemistries of 4a and 4a⁺ with a variety of substrates were examined.

Full text of DOI:10.1021/ja0030008

J. Am. Chem. Soc. 2001, 123, 2503-2516
2503
Terthienyl and Poly-terthienyl Ligands as Redox-Switchable
Hemilabile Ligands for Oxidation-State-Dependent Molecular Uptake
and Release
Dana A. Weinberger,† Thomas B. Higgins,† Chad A. Mirkin,*,† Charlotte L. Stern,†
Louise M. Liable-Sands,‡ and Arnold L. Rheingold‡
Contribution from the Department of Chemistry, Northwestern UniVersity, 2145 Sheridan Road,
EVanston, Illinois 60208, and Department of Chemistry and Biochemistry, UniVersity of Delaware,
Newark, Delaware 19716
ReceiVed August 11, 2000
Abstract: Mononuclear, dinuclear, and polymeric Ru(II) complexes formed from terthienylalkylphosphino
redox-switchable hemilabile ligands demonstrate that this class of ligand provides electrochemical control
over the electronic properties, coordination environments, and reactivities of bound transition metals. Specifically,
[CpRuCO(κ2-3′-(2-diphenylphosphinoethyl)-5,5′′-dimethyl-2,2′:5′,2′′-terthiophene)][B(C6H3-3,5-(CF3)2)4] (4a)
exhibits a 3 orders of magnitude increase in binding affinity for acetonitrile upon terthienyl-based oxidation.
FT-IR spectroelectrochemical experiments on 4a indicate that terthienyl-based oxidation removes electron
density from the metal center, equivalent to approximately 11-17% of the electronic change that occurs upon
direct oxidation of Ru(II) to Ru(III) in analogous complexes. The spectroelectrochemical responses of 4a
were compared to those of dimeric and polymeric analogues of 4a. The spectroelectrochemistry of the dimer
is consistent with two sequential, one-electron ligand-based oxidations, compared to only one in 4a. In contrast,
the polymer exhibits spectroelectrochemical behavior similar to that of 4a. The polymer spectroelectrochemistry
shows changes in the metal center electronic properties between two different states, reflective of two discrete
oxidation states of the polymeric ligand backbone. We propose that the polymer backbone does not allow one
to vary the electronic properties of the metal center through a continuous range of oxidation states due to
charge localization within the metalated films. In an effort to explore the molecular uptake and release properties
of 4a and its polymer analogue as a function of ligand oxidation state, the oxidation-state-dependent coordination
chemistries of 4a and 4a+ with a variety of substrates were examined.
Introduction
reactivity.2-4 Previously, Wolf and Wrighton prepared poly-
[5,5′-(2-thienyl)-2,2′-bithiazole] and subsequently functionalized
the backbone of this conducting polymer with cationic Re
carbonyl fragments.4 Although only modest metal-centered
changes were noted upon polymer ligand-based oxidation, this
novel material provided important initial insight into the use of
conducting polymer films as substitutionally inert modalities
for electrochemically modulating the electronic nature of the
bound Re centers. One limitation of the aforementioned system
is that the organic polymer was functionalized with the metal
fragments after polymerization, leading to a heterogeneous
structure which was difficult to characterize. Furthermore,
although several well-characterized conducting polymer/transi-
tion metal coordination complex precursors and polymers
formed from them have been synthesized, there is no informa-
tion regarding the utility of these polymers for electrochemically
controlling the electronic properties and reactivity of the bound
metal centers.5-9
Redox-active ligands are beginning to be used extensively
to control ligand binding properties and transition metal
reactivity.1 Thus far, studies pertaining to such ligands have
focused almost exclusively upon metallocene-based structures
susceptible to one-electron oxidation/reduction reactions (e.g.,
ferrocene and cobaltocene). However, redox-active ligands with
multiple accessible oxidation states could provide greater control
and tailorability over the metal centers to which they are bound.
Our research group and others have pursued a decade-long
goal of preparing conducting polymer-metal complexes in an
effort to evaluate their potential as high-surface-area, electrode-
immobilized materials with redox properties that allow one to
electrochemically “tune”, or vary through a continuum of redox
states, the electron density at bound transition metal centers.2-4
It has been hypothesized that such an approach could lead to
materials with electrochemically variable transition metal
* To whom correspondence should be addressed.
† Northwestern University.
(5) Reddinger, J. L.; Reynolds, J. R. Chem. Mater. 1998, 10, 3-5.
(6) Martin, K. F.; Hanks, T. W. Organometallics 1997, 16, 4857-4860.
(7) Martin, K.; Dotson, M.; Litterer, M.; Hanks, T. W.; Veas, C. Synth.
Met. 1996, 78, 161-168.
‡ University of Delaware.
(1) Allgeier, A. M.; Mirkin, C. A. Angew. Chem., Int. Ed. Engl. 1998,
37, 894-908.
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353.
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10.1021/ja0030008 CCC: $20.00 © 2001 American Chemical Society
Published on Web 02/21/2001

2504 J. Am. Chem. Soc., Vol. 123, No. 11, 2001
Weinberger et al.
Scheme 1
Scheme 2
By extending the concept of redox-switchable hemilabile
ligands (RHLs)2,3,10-15 to conducting polymers, we have
designed ligands in which a polymer backbone can be used to
electrochemically modulate the electronic properties and coor-
dination spheres of bound transition metals. With RHLs, one
functional group on a bidentate ligand binds strongly to a chosen
transition metal, and another functional group binds weakly,
Scheme 1. The weakly bonding group also is in direct electronic
communication with a pendant redox-active group. Upon
oxidation and subsequent reduction of the pendant redox group,
the strength of the interaction between the weakly bonding group
and the metal center can be reversibly controlled. Significantly,
the decrease in RHL binding constant for the metal center upon
ligand-centered oxidation effectively increases the complex’s
binding affinity for substrates (L) in solution, Scheme 1. RHLs
thereby provide an electrochemical means of controlling both
the electronic and steric environments of metal centers. For this
reason, it is thought that such complexes could be useful for
electrochemically controllable catalytic or small-molecule uptake
and release reactions (i.e., molecular separations).1,16
Herein, we report the synthesis, characterization, and reactiv-
ity of mononuclear, dinuclear, and polynuclear Ru(II) complexes
formed from terthienyl-based RHLs. The design of these ligands
takes into account several key considerations. First, such ligands
are hemilabile when bound to Ru(II) via a substitutionally inert
phosphine group and a relatively substitutionally labile thienyl
moiety.17,18 Second, the lower oxidation/reduction potentials for
the ligand- compared to the metal-based redox reactions enable
one to address ligand-based redox properties separate from
metal-centered ones. Third, methyl-capped ligands allow for the
controlled electrochemical study of mononuclear RHL-metal
complexes, whereas hydrogen atoms in the terminal R-positions
allow for the oxidative polymerization of the transition metal
complexes formed from these ligands.19-30 The direct polym-
erization of the metal-containing monomer avoids the post-
polymerization functionalization of resultant polymers with the
metal centers of interest, which inherently leads to questions
regarding the degree and nature of functionalization.
Results
Synthesis and Characterization of Terthienyl-Based
Ligands, 1a and 1b. A methyl-capped terthienylalkylphosphine
ligand, 3′-(2-diphenylphosphinoethyl)-5,5′′-dimethyl-2,2′:5′,2′′-
terthiophene (1a), was synthesized14 in order to prepare metal-
ligand complexes suitable for solution electrochemical and
spectroelectrochemical studies. Polymerizable ligand-metal
complexes can be prepared from a hydrogen-atom-terminated
analogue of 1a, 3′-(2-diphenylphosphinoethyl)-2,2′:5′,2′′-ter-
thiophene (1b), which was synthesized by methods analogous
to those used for the preparation of 1a.14 These ligands were
fully characterized, and all data are consistent with their
proposed structural formulations.
Synthesis and Characterization of a Ru(II)-Terthienyl
Complex, 2a. Ligand 1a was complexed to Ru(II) to form
CpRuCOCl(3′-(2-diphenylphosphinoethyl)-5,5′′-dimethyl-2,2′:
5′,2′′-terthiophene) (2a), Scheme 2. Complex 2a was fully
characterized in solution and in the solid state, and the solution
spectroscopic data for this complex are consistent with its solid-
state structure, which was determined by a single-crystal X-ray
diffraction study (see Supporting Information). Moreover, the
data are similar to those reported for isoelectronic, non-thienyl-
group-containing complexes.31 For example, the FT-IR CO
stretching frequency at 1955 cm-1 for 2a in CH2Cl2 compares
(10) Singewald, E. T.; Mirkin, C. A.; Stern, C. L. Angew. Chem., Int.
Ed. Engl. 1995, 34, 1624-1627.
(11) Sassano, C. A.; Mirkin, C. A. J. Am. Chem. Soc. 1995, 117, 11379-
11380.
(12) Slone, C. S.; Mirkin, C. A.; Yap, G. P. A.; Guzei, I. A.; Rheingold,
A. L. J. Am. Chem. Soc. 1997, 119, 10743-10753.
(13) Allgeier, A. M.; Slone, C. S.; Mirkin, C. A.; Liable-Sands, L. M.;
Yap, G. P. A.; Rheingold, A. L. J. Am. Chem. Soc. 1997, 119, 550-559.
(14) Weinberger, D. A.; Higgins, T. B.; Mirkin, C. A.; Liable-Sands, L.
M.; Rheingold, A. L. Angew. Chem., Int. Ed. Engl. 1999, 38, 2565-2568.
(15) Allgeier, A. M.; Singewald, E. T.; Mirkin, C. A.; Stern, C. L.
Organometallics 1994, 13, 2928-2930.
(21) Engelmann, G.; Sto¨sser, R.; Kossmehl, G.; Jugelt, W.; Welzel,
H.-P. J. Chem. Soc., Perkin Trans. 2 1996, 2015-2019.
(22) Tanaka, K.; Matsuura, Y.; Oshima, Y.; Yamabe, T.; Hotta, S. Synth.
Met. 1994, 66, 295-298.
(23) Hill, M. G.; Penneau, J.-F.; Zinger, B.; Mann, K. R.; Miller, L. L.
Chem. Mater. 1992, 4, 1106-1113.
(24) Kirste, B.; Tian, P.; Kossmehl, G.; Engelmann, G.; Jugelt, W. Magn.
Reson. Chem. 1995, 33, 70-76.
(25) Zotti, G.; Schiavon, G.; Berlin, A.; Pagani, G. Chem. Mater. 1993,
5, 430-436.
(16) Slone, C. S.; Weinberger, D. A.; Mirkin, C. A. In The Transition
Metal Coordination Chemistry of Hemilabile Ligands; Karlin, K. D., Ed.;
John Wiley & Sons: New York, 1999; Vol. 48, pp 233-350.
(17) Benson, J. W.; Angelici, R. J. Organometallics 1992, 11, 922-
927.
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687.
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Soc. 1992, 114, 2728-2730.
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Chem. Soc. 1996, 118, 5480-5481.
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J. S.; Tour, J. M. Chem. Mater. 1994, 6, 327-332.
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Mater. 1992, 4, 1097-1105.
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Terthienyls as Redox-Switchable Hemilabile Ligands
J. Am. Chem. Soc., Vol. 123, No. 11, 2001 2505
well with the νCO band at 1958 cm-1 reported for CpRu(PPh3)-
(CO)(Cl)31 in CS2. The UV-vis spectrum of 2a exhibits a π-π*
transition with λmax at 353 nm (ꢀ ) 3.6 × 104 L‚mol-1‚cm-1),
which is very similar to the π-π* transition for ligand 1a (358
nm, ꢀ ) 3.8 × 104 L‚mol-1‚cm-1). The E1/2 value associated
with ligand-centered radical cation formation in 2a (510 mV
vs FcH/FcH+) is near the potential associated with oxidation
and reduction of 5,5′-dimethylterthiophene (vide infra).
Synthesis and Characterization of Complexes with a
Ru(II)-Terthienyl Interaction, 3a and 4a. Complex 2a
can be converted to a cationic complex in which one thienyl
moiety is bound to the Ru(II) center, [CpRuCO(κ2-3′-(2-diphenyl-
phosphinoethyl)-5,5′′-dimethyl-2,2′:5′,2′′-terthiophene)][BF4] (3a),
by stirring 2a in methylene chloride with AgBF4 at room
temperature for 1 h, Scheme 2. Chloride abstraction from 2a to
form 3a is accompanied by a change in the CO stretching
frequency from 1955 to 1993 cm-1. This change is a conse-
quence of decreased back-bonding between the CO ligand and
the metal center in the cationic complex 3a as compared to that
in 2a. Complex 3a was isolated and characterized in solution,
and all solution spectroscopic data are consistent with its
proposed structural formulation.
Complex 3a, dissolved in methylene chloride, exhibits long-
term stability (over three months) at room temperature, as
evidenced by 1H NMR, 31P{1H} NMR, and FT-IR spec-
troscopies. However, ligand-centered oxidation of 3a (effected
in a spectroelectrochemical cell by setting the working electrode
potential to 800 mV vs an Ag wire in 0.1 M n-Bu4NPF6 in
CH2Cl2) results in immediate decomposition of 3a. The nature
of this decomposition is unclear, but the decomposition does
not occur in a solution of 3a and 0.1 M n-Bu4NPF6 in CH2Cl2
in the absence of applied potential. The decomposition product
exhibits an FT-IR νCO band at 1955 cm-1, and although the
decomposition product was not fully characterized, the fre-
quency of this νCO band indicates the formation of a neutral
CpRu(CO)(PR3)X adduct.
shifted downfield compared to the analogous resonances for 5a
(δ 6.72, 6.68, respectively). These changes strongly indicate
that the shifted resonances are associated with the Ru-bound
thienyl ring, and therefore, we assign the resonances at δ 7.09
and 6.72 to H-3 and δ 6.95 and 6.68 to H-4 in 4a and 5a,
respectively. These data are consistent with the single-crystal
X-ray diffraction study of 4a,14 in which the terminal ring closest
to the methylene substituent is the metal-bound moiety (see
Supporting Information). If the thienyl ring were bound through
one of the double bonds, the proton resonances associated with
the coordinated carbon atoms would be expected to shift much
farther upfield to δ 3.0-3.5, as in complexes with η2-thiophene35
or η2-ethylene (12, vide infra) moieties.
Variable-temperature 31P{1H} NMR spectroscopy studies
provide further evidence for the η1 binding of the S atom in
the solution structure of 4a. These studies show that at room
temperature, the phosphorus resonance for 4a (δ 33.1) is due
to the signal averaging of two different resonances and,
therefore, two different forms of the complex which undergo
fast interconversion at room temperature on the NMR time scale.
A 31P{1H} NMR spectrum of 4a at -77 °C shows two unequally
populated resonances (δ 47.0, 29.4; ratio 1:21) that have been
assigned to two different diastereomers which interconvert by
pyramidal inversion at the bound sulfur. This type of process
has been observed with analogous thiophene complexes of Fe
and Ru.17,18,36,37 The coalescence temperature for this process
was not determined due to the low signal intensity of the
downfield resonance but was determined for the analogous
process in 4b (vide infra).
Synthesis and Characterization of a Polymerizable Ru(II)-
Terthienyl Complex, 2b. A Ru(II) complex of 1b, CpRu-
COCl(3′-(2-diphenylphosphinoethyl)-2,2′:5′,2′′-terthiophene) (2b),
was prepared via a method analogous to the one used for
preparing 2a, Scheme 2. Complex 2b was characterized in
solution, and the solution spectroscopic data for this complex
are consistent with its proposed formulation and are similar to
those reported for 2a (see Supporting Information).
To obtain an electrochemically stable version of 3a, an
alternative high-oxidation-potential, weakly coordinating coun-
terion was sought. Therefore, 4a was prepared from 3a via
counterion metathesis with Na[B(3,5-(CF3)2C6H3)4]32 (NaBAr4′).
Complex 4a was fully characterized in solution and in the solid
state by a single-crystal X-ray diffraction study (see Supporting
Information), and all data are consistent with its proposed
structural formulation. For example, the FT-IR νCO stretching
frequency of 4a at 1996 cm-1 is identical to the frequency
reported for an isoelectronic and isostructural complex, [Cp-
(CO)(PPh3)Ru(2,5-Me2thiophene)]BF4.17 In the UV-vis spec-
trum of 4a, the π-π* transition has a λmax at 352 nm with a
shoulder at 420 nm (ꢀ ) 2.7 × 104 L‚mol-1‚cm-1). Although
this shoulder is not present in 2a, the λmax values for 2a and 4a
are very similar, suggesting that the extent of conjugation does
not change significantly upon oligothienyl binding to the metal
Synthesis and Characterization of Polymerizable Com-
plexes with Ru(II)-Terthienyl Interactions, 3b and 4b.
Chloride abstraction from 2b with AgBF4 yields 3b, which
has been isolated and characterized in solution, Scheme 2.
Counterion metathesis of 3b with LiB(C6F5)4‚Et2O yields 4b,
a cationic complex with one thienyl group bound to the metal
center. Complex 4b was fully characterized in solution and in
the solid state, and its solution spectroscopic data are consistent
with its proposed structural formulation. The UV-vis spectrum
of 4b exhibits a π-π* transition with a λmax at 340 nm and a
shoulder at 420 nm (ꢀ ) 1.9 × 104 L‚mol-1‚cm-1). The λmax is
at a slightly higher energy than the analogous transition for 4a.
This difference is due to the greater electron-donating properties
of the methyl groups in 4a as compared to the hydrogen atoms
in 4b. The shoulders observed in the spectra of 4a and 4b are
not observed in the spectra of 2a and 2b. Although the
assignment of these shoulders has not been established conclu-
sively, they are present only for those complexes in which the
terthienyl moiety is bound directly to the metal, and similar
1
center. Additionally, the H NMR spectrum of 4a exhibits the
expected five thienyl resonances, with one set of ring resonances
shifted downfield when compared with analogous resonances
for the complex formed when CH3CN displaces the thienyl
group from the Ru center of 4a (5a, vide infra). Specifically,
one doublet at δ 7.09, which can be assigned to either H-3 or
H-3′′ on the basis of a characteristic terthienyl coupling constant
(J3-4 ) 3.6 Hz), and one doublet of doublets at δ 6.95 in 4a,
which can be assigned to either H-4 or H-4′′ on the basis of
coupling constants (J3-4 ) 3.6 Hz, J4-Me ) 1.6 Hz),33,34 are
(33) Rossi, R.; Carpita, A.; Ciofalo, M.; Lippolis, V. Tetrahedron 1991,
47, 8443-8460.
(34) Mac Eachern, A.; Soucy, C.; Leitch, L. C.; Arnason, J. T.; Morand,
P. Tetrahedron 1988, 44, 2403-2412.
(35) Cordone, R.; Harman, W. D.; Taube, H. J. Am. Chem. Soc. 1989,
111, 5969-5970.
(36) Draganjac, M.; Ruffing, C. J.; Rauchfuss, T. B. Organometallics
1985, 4, 1909-1911.
(32) Brookhart, M.; Grant, B.; Volpe, A. F. Organometallics 1992, 11,
3920-3922.
(37) Goodrich, J. D.; Nickias, P. N.; Selegue, J. P. Inorg. Chem. 1987,
26, 3424-3426.

2506 J. Am. Chem. Soc., Vol. 123, No. 11, 2001
Weinberger et al.
Figure 2. ORTEP diagram of 4b. The [B(3,5-(CF3)2C6H3)4] counterion
and hydrogen atoms are omitted for clarity, and thermal ellipsoids are
drawn at 30% probability.
Figure 1. Variable-temperature 31P{1H} NMR spectra of 4b in CD2Cl2.
The asterisk indicates residual 2b in solution.
shoulders have been noted in the spectra of polyalkylthiophenes
to which [CpFe(CO)2]+ fragments are bound to the S atoms.38
Complex 4b was synthesized with a perfluorinated aryl borate
counterion rather than the [B(3,5-(CF3)2C6H3)4] counterion used
for 4a, because the perfluorinated aryl borate yields higher
quality polymers than does the counterion in 4a (vide infra).
In the variable-temperature 31P{1H} NMR spectroscopy
studies of 4b in CD2Cl2, a single resonance at δ 40.2 is observed
at room temperature, whereas at -77 °C resonances are
observed at δ 45.2 and 28.8 with a signal intensity ratio of 28:1,
respectively, Figure 1. Similar to the assignment for 4a, these
resonances were assigned to two different diastereomers which
interconvert by pyramidal inversion at the bound sulfur. The
two different free energies of activation (∆Gq), calculated39 for
the interconversion of the two unequally populated diastereomers
at coalescence (206 K) in CD2Cl2, are 34.9 and 40.6 kJ/mol.
By comparison, pyramidal sulfur inversions of substituted
thiophenes bound in an η1 fashion to Ru17,18,36 and Fe37 centers
have been reported with ∆Gq values ranging from 39 to 43 kJ/
mol. The intensities and chemical shifts of the peaks in the
variable-temperature 31P{1H} NMR spectra of 4a and 4b suggest
that the preferred diastereomeric configuration for 4b in solution
is opposite to the one preferred by 4a.
Solid-State Structure of 4b. Crystals of 4b suitable for
single-crystal X-ray diffraction studies were grown by slow
diffusion of pentane into a methylene chloride solution of 4b,
Figure 2 (see Supporting Information). Despite several attempts
to recrystallize 4b, the best crystals obtained were very weak
and diffusely diffracting (I/σ ) 3.32). Nevertheless, a single-
crystal X-ray diffraction study of 4b (BAr4′ counterion) provides
connectivity data. It shows that the metal center has a tetrahedral
coordination environment and that the κ2 ligand binds through
the phosphino moiety and the terminal thienyl ring closest to
the methylene substituent (ring 1) in η1 fashion. The thienyl
Figure 3. Cyclic voltammogram of 4a in CH2Cl2/0.1 M n-Bu4NPF6
using an Au working electrode, Pt mesh counter electrode, and Ag
wire quasi-reference electrode at a scan rate of 200 mV/s.
rings have an antiparallel arrangement, and the Ru-S bond
length is 2.397(3) Å, similar to the Ru-S bond length in 4a
(2.3903(9) Å). The Ru-bound S atom has a pyramidal geometry,
which is apparent from the Ru-S-midpoint (the midpoint of
the vector between the carbon atoms R to the bound sulfur)
angle, which is 127.8°, and a calculation of the sum of the angles
formed by the S atom and its substituents (323.0°).17,18 Just as
low-temperature 31P{1H} NMR spectroscopy studies suggest that
4a and 4b have different preferred diasteromeric configurations
in solution, the absolute diastereomeric configuration of 4b in
the solid state is RuSSS/RuRSR,40 in contrast to that of 4a (RuRSS/
RuSSR).14
The inter-ring dihedral torsion angles in 4b and other pertinent
crystallographic data are summarized in Table 1. In 4b, the S
atom displacement is greatest for ring 1 and progressively
diminishes for rings 2 and 3. In 2a, the rings are generally
comparable in planarity to those in 4a and 4b, as evidenced by
S atom displacements and mean deviations of the thienyl atoms
from the planes they comprise. In 4b, the S-C bonds in the
ring which is bound to Ru are longer than the S-C bonds in
2a, and the average C-S bond length for each ring in 4b
decreases as the rings move farther away from the Ru atom. In
contrast to this trend in C-S bond lengths, the C-S-C bond
angles are not affected by binding of the S atom to Ru, similar
to the trend observed for 4a (see Supporting Information).
(38) Shaver, A.; Butler, I. S.; Gao, J. P. Organometallics 1989, 8, 2079-
2080.
(40) Ligand priority order: Ru (C5H5 > thiophene > phosphine > CO);
S (Ru, C-thiophene, C-CH3, lone pair). See: Stanley, K.; Baird, M. C. J.
Am. Chem. Soc. 1975, 97, 6598.
(39) Sandstro¨m, J. Dynamic NMR Spectroscopy; Academic Press:
London, 1982.

Terthienyls as Redox-Switchable Hemilabile Ligands
J. Am. Chem. Soc., Vol. 123, No. 11, 2001 2507
Table 1. (a) Crystallographic Data and (b) Crystal Structure Torsion Angles (deg) and Ring Planarity for Oligothienyl-Ru(II) Complexes 2a,
4a, 4b, and 6‚2H2O‚1/2CH2Cl2
(a) Crystallographic Data
2a
4a
4b
6‚2H2O‚1/2CH2Cl2
formula
FW
crystal system
space group
color, habit
a, Å
C34H30ORuClS3P
718.29
triclinic
P-1 (No. 2)
yellow, plate
9.932(2)
13.266(3)
13.482(3)
68.788(4)
72.794(4)
72.088(4)
1541.3(6)
0.25 × 0.25 × 0.05
153(1)
C66H42BF24OPRuS3
1546.03
triclinic
C64H38BF24OPRuS3
1517.98
triclinic
C64H50O2Ru2Cl2S6P2‚2H2O‚1/2CH2Cl2
1462.95
triclinic
P-1
P-1
P-1 (No. 2)
yellow, columnar
11.2287(6)
yellow, block
13.6821(3)
15.4088(2)
17.4920(3)
103.8463(2)
94.7346(7)
110.1334(8)
3306.33(12)
0.30 × 0.30 × 0.20
198(2)
yellow, plate
12.2724(2)
13.6667(2)
21.3680(3)
84.7148(7)
81.0736(9)
86.8583(5)
3522.48(10)
0.30 × 0.30 × 0.25
198(2)
b, Å
c, Å
16.1477(9)
18.7005(10)
77.5596(10)
79.9085(10)
86.6928(10)
3259.2(3)
R, deg
â, deg
γ, deg
V, Å3
dimensions, mm
temp, K
Z
0.59 × 0.11 × 0.04
153(1)
2
2
2
2
R(F)a
R1 ) 0.048
wR2b ) 0.118
[I > 3σ(I)]
1.48
R1 ) 0.0514
wR2c ) 0.1166
[I > 2σ(I)]
1.009
R1 ) 0.1097
wR2c ) 0.2666
[I > 2σ(I)]
1.384
R1 ) 0.072
wR2b ) 0.172
[I > 3σ(I)]
GOF on F2
3.25
(b) Crystal Structure Torsion Angles and Ring
complex
ring numberd
ring planarity,e Å
deviation of S from plane, Åf
ring connectivity
torsion angle, deg
2a
1
2
3
1
2
3
1
2
3
1
2
3
4
5
6
0.0041
0.0055
0.0091
0.0158
0.0044
0.0058
0.0114
0.0085
0.0121
0.0125
0.0080
0.0065
0.0079
0.0059
0.0161
0.0168
0.0182
0.0114
0.0687
0.0059
0.0252
0.0496
0.0317
0.0193
g
0.0320
0.0256
0.0191
0.0143
g
1-2
2-3
1-3
1-2
2-3
1-3
1-2
2-3
1-3
1-2
2-3
3-4
4-5
5-6
1-6
19.80
2.56
17.25
21.09
32.47
50.62
10.09
25.28
19.03
14.34
14.10
13.75
30.47
23.47
20.45
4a
4b
6
2
2
2
a R1 ) ∑||Fo| - |Fc||/∑|Fo|; wR2 ) [∑(w(Fo2 - Fc2)2)/∑w(Fo )2)]1/2
.
b w ) 1/(σ2F2). c w ) 1/[σ2(Fo ) + (aP)2 + bP], P ) [2Fc2 + Max(Fo ,0)]/
3. d The ring number in 2a, 4a, and 4b is defined such that ring one is the terminal ring closest to the methylene substituent, and the numbering
of the remaining rings is consecutive. For 6, the ring number corresponds to the number of the sulfur atom. e Mean deviation of the atoms from the
plane defined by the five thienyl atoms of each ring. f Deviation of the S atom from the plane defined by the four thienyl carbons in each ring.
g Rings are disordered.
Electrochemistry of 4a and 5a. The cyclic voltammetry of
4a in CH2Cl2/0.1 M n-Bu4NPF6 exhibits a chemically reversible,
one-electron, terthienyl-based oxidation/reduction wave at E1/2
) 735 mV, Figure 3, and an irreversible oxidation at 1180 mV
(not shown). All electrochemical data are internally referenced
and reported versus FcH/FcH+ unless noted otherwise, FcH )
(η5-C5H4)Fe(η5-C5H5). The first wave is attributed to ligand-
centered oxidation/reduction of 4a and reversible formation of
the radial cation 4a+. The one-electron nature of this process
was confirmed by variable scan rate cyclic voltammetry and
variable spin rate rotating disk electrode experiments.14 The
second oxidation is assigned to terthienyl-centered dication
formation. These assignments were made on the basis of a
comparison of the cyclic voltammetry data for 4a to the cyclic
voltammograms of model compounds.19,20 For example, [Cp-
(CO)(P(CH2CH3)Ph2)Ru(CH3CN)]BF4,14 a model complex for
the metal center in 4a, is oxidized irreversibly at 1335 mV in
CH3CN/CH2Cl2 (1:1). Additionally, 5,5′′-dimethylterthiophene,34
a model compound for the terthienyl-centered electrochemistry
of 4a, exhibits two reversible oxidation/reduction waves with
E1/2 values of 520 and 1020 mV, respectively, in CH2Cl2/0.1
M n-Bu4NPF6. These values are consistent with those reported
by Mann and co-workers for this same compound in CH3CN/
n-Bu4NBF419 and for 3′,4′-dibutyl-5,5′′-diphenylterthiophene in
CH2Cl2/n-Bu4NPF6.20
Upon chloride abstraction from 2a to yield 4a, the E1/2 for
the terthienyl-based radical cation formation increases by 225
mV due to a combination of effects, including the binding of
the oligothienyl backbone to a cationic metal and any changes
in the electronic nature of the thienyl rings due to metal-sulfur
binding (vide infra). Concomitantly, the metal oxidation po-
tential increases as expected for a cationic complex and is
outside the potential window of the solvent/supporting electro-
lyte solution, Table 2.
When CH3CN is added to a CH2Cl2 solution of 4a, the
complex is converted to [Cp(CH3CN)RuCO(3′-(2-diphenylphos-
phinoethyl)-5,5′′-dimethyl-2,2′:5′,2′′-terthiophene)][B(C6H3-3,5-
(CF3)2)4] (5a), a CH3CN adduct formed via displacement of
the η1-terthienyl moiety from the Ru center by CH3CN. A cyclic
voltammogram of 5a exhibits two waves. The lower potential
wave is assigned to reversible radical cation formation (E1/2
)
535 mV), and the higher potential wave is assigned to
irreversible dication formation (Epa ) 960 mV), Table 2. The
first wave for 5a has an E1/2 value that is 200 mV lower than
the first E1/2 for 4a, and this difference allows one to estimate
the quantitatiVe increase in the Ru(II) binding constant (K) for

2508 J. Am. Chem. Soc., Vol. 123, No. 11, 2001
Weinberger et al.
Table 2. Cyclic Voltammetrya and UV-Vis Spectroscopyb of Oligothienyl-Ru(II) Complexes
E1/2 (radical cation)/mV vs
FcH/FcH+ (Epa - Epc)
E1/2 (Ru(II/III))/mV vs
FcH/FcH+ (Epa - Epc)
E1/2 (dication)/mV vs
FcH/FcH+ (Epa - Epc)
UV-vis π-π*
transition λmax/nm
complex
2a
4a
5a
2b
4b
5b
6
7
510 (70)
735 (100)
535 (70)
680 (70)
c
c
d
945 (70)
1180 (Epa)
960 (Epa)
d
353
352, 420 (sh)
351
344
∼735 (Epa)d,e
∼765 (Epa)d,e
∼685 (Epa)d,e
365 (70)
c
c
d
d
340, 420 (sh)
341
430
456
430
645f (120)
645f (120)
820 (60)
605 (60)
520 (60)
c
c
8
415 (60)
poly-4b
typical E1/2 ) 540 mVe
a In 0.1 M n-Bu4NPF6 in CH2Cl2 and referenced vs internal FcH/FcH+ unless noted otherwise. b In CH2Cl2. c Outside of solvent window. d Cyclic
voltammogram response convoluted by simultaneous polymerization. e In 0.1 M LiB(C6F5)4‚Et2O in CH2Cl2. f Waves overlap.
CH3CN upon terthienyl-based oxidation. From the Nernst
equation, ∆E1/2 ) (RT/nF) ln(Kox/Kred),41 or Kox/Kred ) 2.4 ×
103. Alternatively, this ratio may be viewed as a measure of
the decrease in binding constant of the terthienyl RHL for Ru(II)
that accompanies ligand-based oxidation.
FT-IR Spectroelectrochemistry of 4a. Spectroelectrochemi-
cal FT-IR experiments of 4a in CH2Cl2/0.1 M NaBAr4′ provide
insight into the influence of terthienyl-based oxidation on the
electronic properties of the bound Ru(II) center. The νCO of a
metal carbonyl complex can be an excellent spectroscopic
indicator of the amount of electron density at a metal center; as
the electron-richness of a metal center decreases, CO stretching
frequencies typically increase.42 By setting the potential of the
working electrode in a spectroelectrochemical cell to 600 mV
vs an Ag wire (approximately 800 mV vs FcH/FcH+, external
reference), ligand-based oxidation of 4a can be effected. This
oxidation results in formation of 4a+ with a concomitant shift
in the CO stretching frequency to a higher wavenumber than
Figure 4. FT-IR spectroelectrochemical experiments: (a) 0.006 M 4a,
for 4a. This change can be visualized through difference spectra
0.03 M NaBAr′, CH2Cl2, Pt mesh counter, Ag wire quasi-reference,
obtained by subtracting the initial spectrum of 4a from the
and Au minigrid working electrodes. Potential held at 600 mV vs Ag
spectra acquired at 600 mV vs an Ag wire as a function of time.
wire for 9 min. (b and c) 0.005 M 7, 0.04 M LiB(C6F5)4‚Et2O, CH2Cl2,
The difference spectrum obtained in this way for 4a has a band
Pt wire counter, Ag/AgCl reference, Au minigrid working electrode.
with a positive peak at 2011 cm-1 and a negative peak at 1992
(b) Potential held at 700 mV vs FcH/FcH+ for 2 min. (c) Potential
cm-1. The absolute intensity values of the positive peak at 2011
held at 1100 mV vs FcH/FcH+ for 2 min. (d) Poly-4b, 0.04 M LiB-
cm-1 and the negative peak at 1992 cm-1 increase with time
(C6F5)4‚Et2O, CH2Cl2, Pt mesh counter, Ag/AgCl reference electrode,
and correspond to a buildup of 4a+ and a loss of 4a, respectively,
derivatized Au working electrode. Potential held at 800 mV vs FcH/
Figure 4a. Reduction of 4a+ for 15 min at 0 mV reverses the
process and results in a flat line in the difference spectrum,
indicating complete chemical reversibility, Figure 4e.
FcH+ for 2 min. (e) Typical difference spectrum after reduction of the
ligand in experiments a-d.
To determine how spectroelectrochemical difference bands
relate to actual FT-IR bands of new products, model difference
spectra were simulated from an authentic spectrum of 4a. First,
the x-axis of the spectrum of 4a was systematically altered to
generate model spectra with νCO values ranging from 1995 to
2025 cm-1. Second, the difference spectra between these model
spectra and the original spectrum of 4a were obtained. These
simulations show that if the separation between the peak
frequencies of the spectra of 4a and a new product is less than
20 cm-1, the peak frequencies in the experimental difference
spectra will not accurately represent the values of the actual
νCO bands. (The full width at half-maximum for the spectrum
of 4a is 19.3 cm-1). Additional simulations show that the
peak frequencies in the experimental difference spectra should
be independent of the extent of product formation. From
comparison of the difference spectra obtained experimentally
and by these simulations, it is estimated that the actual band
of 4a+ is 2006-2011 cm-1, or 10-15 cm-1 higher than the
νCO of 4a.
The magnitude of the change in νCO observed upon ligand-
based oxidation in 4a is substantial, considering that direct
spectroelectrochemical oxidation of the Ru(II) center in iso-
electronic Cp(CO)(P(CH2CH3)Ph2)RuCl results in a 90 cm-1
shift in νCO (νCO: [Ru(II)] ) 1955 cm-1; [Ru(III)] ) 2045
cm-1).14 This change in νCO recorded upon metal-centered
oxidation suggests that terthienyl-centered oxidation in 4a
corresponds to the removal of approximately 11-17% of an
electron from the Ru center in terms of electronic change at
the metal center.
Synthesis and Characterization of 4a+ and 5a+. In addition
to the electrochemical and spectroelectrochemical formation of
4a+ and 5a+, these complexes can be prepared by bulk
electrochemical oxidation of 4a and 5a, respectively, in a two-
compartment cell. The deep-blue paramagnetic complexes have
been characterized by FT-IR, UV-vis, and EPR spectroscopies
(see Supporting Information). For 4a+, the νCO band in the FT-
(41) Miller, S. R.; Gustowski, D. A.; Chen, Z.-H.; Gokel, G. W.;
Echegoyen, L.; Kaifer, A. E. Anal. Chem. 1988, 60, 2021-2024.
(42) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles
and Applications of Organotransition Metal Chemistry; University Science
Books: Mill Valley, CA, 1987.

Terthienyls as Redox-Switchable Hemilabile Ligands
J. Am. Chem. Soc., Vol. 123, No. 11, 2001 2509
Scheme 3
IR spectrum is at 2006 cm-1, consistent with the range of values
determined by simulation of the difference spectra resulting from
spectroelectrochemical oxidation of 4a. In contrast, for 5a+, the
νCO band in the FT-IR spectrum remains unchanged from that
of 5a, as expected due to the lack of direct communication
between the terthienyl moiety and the metal center in these
complexes. After oxidation of 4a and 5a, the solution UV-vis
spectra of the resulting materials typically exhibit broad bands
with λmax values of 656 and 685 nm, respectively, Table 2.43
These spectra differ significantly from the reported UV-vis
spectrum of the radical cation of 5,5′′-dimethylterthiophene (λmax
) 572 and 880 nm in CH3CN/0.1 M n-Bu4NBF4),44 and its π
dimer (λmax ) 466, 708, and 868 nm).23 The widths of the
transitions for 4a+ and 5a+ could be due, in part, to the
π-dimerization of 4a+ and 5a+.
Synthesis and Characterization of a Neutral Dinuclear
Complex, 6. To better understand the properties of the Ru(II)-
terthienyl complexes reported herein, dimeric analogues of these
complexes were synthesized. Complex 6, a dimeric analogue
of 2b, can be prepared by first oligomerizing 4b electrochemi-
cally in CH2Cl2/LiB(C6F5)4‚Et2O by repeatedly scanning the
potential of the working electrode between 0 and 900 mV vs
an Ag wire in a cyclic voltammetry experiment, Scheme 3. This
oxidation yields soluble analogues of 4b with n (number of
repeat units) values greater than 1. Removal of the methylene
chloride from the solution of oligomers, addition of acetone and
KCl (10 equiv), and stirring the resulting mixture for 1 week
under ambient conditions yields a solution of chloride adducts
of the initial metal complexes. From these oligomers, 6 can be
isolated in 66% yield by preparative scale thin-layer chroma-
tography using an alumina support and methylene chloride as
the eluent.
Figure 5. ORTEP diagram of 6‚2H2O‚1/2CH2Cl2 (hydrogen atoms are
omitted for clarity). Thermal ellipsoids are drawn at 30% probability.
6 (365 mV) is much lower than the one-electron oxidation/
reduction of 2a, Table 2. This decrease in E1/2 is due to an
increase in conjugation which results from the increased number
of thienyl rings in 6 compared to 2a. Dication formation for 6
also occurs at a lower potential than for 2a and overlaps the
metal-centered redox process of 6 (E1/2,apparent ) 645 mV, two-
electron process).
Solid-State Structure of 6‚2H2O‚1/2CH2Cl2. A crystal of
6‚2H2O‚1/2CH2Cl2 suitable for a single-crystal X-ray diffraction
study was grown by slow diffusion of pentane into a solution
of 6 in CH2Cl2. The crystal structure of 6‚2H2O‚1/2CH2Cl2 shows
that there are two tetrahedral metal centers bound via methylene
arms to a sexithienyl backbone and that the thienyl rings have
an antiparallel arrangement with all R linkages, Figure 5 and
Table 1 (see Supporting Information). The terminal rings in the
crystal structure of 6‚2H2O‚1/2CH2Cl2 were modeled as rota-
tionally disordered with an occupancy distribution of 50/50 and
35/65 parallel and antiparallel ring configurations for rings 1
and 6, respectively. The disordered sulfur and carbon atoms of
these rings were refined with g\tied isotropic thermal parameters.
Similar disorder in terminal thienyl rings in the crystal structures
of substituted oligothiophenes has been reported previously.46
The structure contains disordered H2O and CH2Cl2 molecules,
which were refined isotropically, while the non-hydrogen atoms
in the main molecule were refined anisotropically. The crystal
structure of 6‚2H2O‚1/2CH2Cl2 shows that the terminal rings of
4b, which are not bound to Ru, are covalently linked in the
oxidative coupling reaction which forms 6. The isolation of this
connectivity isomer in 66% yield suggests that this connection
of the thienyl rings is the preferred terthienyl linkage in the
electrochemical oligomerization of 4b. This preference is most
likely due to a combination of steric and electronic factors which
places the most available unpaired electron density on the
terminal, unbound thienyl ring in the monomer upon ligand-
based oxidation. The C-S bond lengths in the rings of 6 are
not elongated as in the rings of 4a and 4b, evidence that this
elongation is due to oligothienyl-Ru binding. Additionally, the
planarities of the thienyl rings in 6‚2H2O‚1/2CH2Cl2 are com-
parable to those of 2a, 4a, and 4b.
Complex 6 has been fully characterized in solution and in
the solid state, and its solution spectroscopic data are consistent
with its proposed structural formulation. For example, the
νCO band in the FT-IR spectrum of 6 is at 1955 cm-1, which
compares well to the νCO band for 2b. Also, the λmax in the
UV-vis spectrum of 6 is at 430 nm, which compares well
to a reported λmax for sexithiophene (432 nm in CHCl3).45 The
π-π* transition for 6 (λmax ) 430 nm) is at lower energy
than the π-π* transition for 2b (λmax ) 344 nm), as expected
for an oligomer with longer conjugation length. The E1/2
value associated with radical cation formation and reduction in
(43) Although the UV-vis spectrum of a typical sample of 4a+ exhibits
a broad band at 656 nm, sometimes this band is comprised of two bands at
579 nm and a broad band with λmax at approximately 654 nm.
(44) In our hands, the UV-vis spectrum of 5,5′′-dimethylterthiophene
in methylene chloride/0.1 M n-Bu4NPF6 has two transitions with λmax values
of 581 and 889 nm.
The torsion angles between adjacent, individual rings in the
solid-state structures of 2a, 4a, 4b, and 6 range from 2.56° to
32.47° and are summarized in Table 1b. These torsion angles
are greater than the torsion angles observed in unsubstituted
(45) Van Pham, C.; Burkhardt, A.; Shabana, R.; Cunningham, D.; Mark,
H.; Zimmer, H. Phosphorus, Sulfur Silicon 1989, 46, 153-168.
(46) Herrema, J. K.; Wildeman, J.; van Bolhuis, F.; Hadziioannou, G.
Synth. Met. 1993, 60, 239-248.

2510 J. Am. Chem. Soc., Vol. 123, No. 11, 2001
Weinberger et al.
oligothiophenes (6-9° for R-terthiophene,47 less than 1° for
R-octithiophene,48 and less than the esd for sexithiophene49) but
are comparable to the torsion angles observed in other previously
reported substituted oligothiophenes (5-11° in a dialkyl-
substituted sexithiophene46 and as large as 37.4° in a terminally
substituted sexithiophene49). Since 2a, 4a, 4b, and 6 all have
comparable torsion angles, it appears that η1-thienyl binding of
the oligothienyl ligand to the metal center does not cause
increases in the torsion angles compared to “free” oligothienyl
moieties. Significantly, all of the inter-ring dihedral torsion
angles in 2a, 4a, 4b, and 6 are less than 40°, the angle below
which ab initio calculations on polythiophene show that the
torsion does not significantly affect the electronic properties of
the polymer.50
Synthesis and Characterization of a Cationic Dinuclear
Complex, 7. When 6 is treated with LiB(C6F5)4‚Et2O for 1 week
in methylene chloride, 7 forms in quantitative yield, eq 1.
Complex 7 was characterized in solution, and its solution
Figure 6. Cyclic voltammogram of 7 in CH2Cl2/0.1 M n-Bu4NPF6
using an Au working electrode, Pt mesh counter electrode, and Ag
wire quasi-reference electrode at a scan rate of 100 mV/s.
complex; the E1/2 for dication formation in 7 is also higher
(approximately 175 mV) than the potential observed for dication
formation for 6. Additionally, the metal-centered oxidation of
7 is no longer accessible in CH2Cl2; this increase in the potential
for the metal-centered oxidation in 7 relative to metal-centered
oxidation in 6 is expected due to the replacement of an
anionichalide ligand with a neutral thienyl moiety. Significantly,
the waves associated with oxidation of the ligand-based radical
cation to the dication are reversible in the dinuclear, sexithienyl
complexes (7, 8) but irreversible in the mononuclear analogues
(4a, 5a) due to the increased stability of the cations on the longer
oligothiophene backbones.
By dissolving 7 in CH3CN, the bis-CH3CN adduct of 7,
complex 8, can be prepared in quantitative yield. This new
complex has been characterized in solution, and its spectroscopic
data are consistent with its proposed formulation (see Experi-
mental Section). The cyclic voltammogram of 8 shows two
reversible waves with E1/2 values of 415 and 605 mV, associated
with the formation of radical cation and dication, respectively.
Significantly, the E1/2 value associated with radical cation
formation in 8 is 105 mV lower in potential than the E1/2 for 7.
Therefore, the binding constant (K) of 7 for CH3CN increases
by a factor of 60 upon one-electron, ligand-based oxidation as
calculated from the Nernst equation. This binding constant
increase is smaller than the change in CH3CN binding constant
observed for the terthienyl complex 4a upon its oxidation. This
trend is expected for a single hole on an oligothiophene
backbonesif the hole is delocalized over more thienyl rings,
or able to move farther from the metal center, the electronic
change at the metal center will be smaller.
FT-IR Spectroelectrochemistry of 7. FT-IR spectroelectro-
chemistry of 7 was performed in CH2Cl2/0.04 M LiB(C6F5)4‚
Et2O by techniques analogous to those used for the spectro-
electrochemical experiments for 4a. Setting the working electrode
potential to 700 mV vs FcH/FcH+ (externally referenced) and
subtracting the initial spectrum from the resulting spectrum
yields a difference spectrum with a positive peak at 2006 cm-1
(associated with an increase in 7+ upon oxidation of 7) and a
negative peak at 1992 cm-1 (corresponding to loss of 7 upon
oxidation), Figure 4c. These difference peaks correspond to an
actual FT-IR band of 2000-2006 cm-1 for 7+, as determined
by simulation of the difference spectra as previously described.
Upon oxidation of 7 at 1100 mV, which is past the second
oxidation potential of 7, the difference band exhibits a positive
peak at 2011 cm-1 (corresponding to 7++ and 7+) and a negative
peak at 1994 cm-1 (corresponding to loss of 7 upon oxidation),
Figure 4b. Simulation of the spectra is an insufficient method
spectroscopic data are consistent with its proposed structural
formulation. The FT-IR spectrum of 7 in CH2Cl2 exhibits a νCO
band at 1996 cm-1, consistent with the data acquired for 4b.
The UV-vis spectrum for 7 shows evidence for a π-π*
transition with λmax ) 456 nm in CH2Cl2, consistent with this
analogous transition for other sexithiophene derivatives51 but
shifted to lower energy compared to 6. Although binding of
the Ru centers to the thienyl S atoms in 7 is expected to remove
electron density from the sexithienyl ligand, which should raise
the energy of the π-π* transition, the observed shift is likely
due to the reduction in rotational ring disorder that the metal-
thienyl interaction provides, as observed for other terthienyl-
Ru(II) complexes.9
Electrochemistry of 7 and 8. The cyclic voltammogram of
7 shows two reversible oxidation/reduction waves with E1/2
values of 520 and 820 mV, respectively, Figure 6. The first
wave for this complex is associated with ligand-based, one-
electron oxidation to form a radical cation, and the second wave
is attributed to dication formation by comparison with the cyclic
voltammetry for 4a and the cyclic voltammogram reported for
didodecylsexithiophene (E° ) 0.34, 0.54 V vs FcH/FcH+).52
The E1/2 for radical cation formation/reduction increases by 155
mV from 6 to 7 due to Ru bonding to the S atom in the latter
(47) van Bolhuis, F.; Wynberg, H.; Havinga, E. E.; Meijer, E. W.; Staring,
E. G. J. Synth. Met. 1989, 30, 381-389.
(48) Fichou, D.; Bachet, B.; Demanze, F.; Billy, I.; Horowitz, G.; Garnier,
F. AdV. Mater. 1996, 8, 500-504.
(49) Yassar, A.; Garnier, F.; Deloffre, F.; Horowitz, G.; Ricard, L. AdV.
Mater. 1994, 6, 660-663.
(50) Bre´das, J. L.; Street, G. B.; The´mans, B.; Andre´, J. M. J. Chem.
Phys. 1985, 83, 1323-1329.
(51) Martinez, F.; Voelkel, R.; Naegele, D.; Naarmann, H. Mol. Cryst.
Liq. Cryst. 1989, 167, 227-232.
(52) Ba¨uerle, P.; Segelbacher, U.; Gaudl, K.-U.; Huttenlocher, D.;
Mehring, M. Angew. Chem., Int. Ed. Engl. 1993, 32, 76-78.

Terthienyls as Redox-Switchable Hemilabile Ligands
J. Am. Chem. Soc., Vol. 123, No. 11, 2001 2511
for 7 days, the resulting film exhibits two FT-IR bands, νCN
to assign the frequency of the actual band for 7++ from the
difference band because the difference spectrum arises from a
multicomponent system. However, this FT-IR change upon
formation of the dication from the radical cation is consistent
with a more highly charged oligothienyl backbone which
withdraws more electron density from the bound Ru(II) centers.
Reduction of the doubly oxidized complex at -150 mV for
15 min shows the decrease in the absolute intensities of the
FT-IR difference peaks associated with the oxidized products,
confirming the reversibility of these electrochemical reactions,
Figure 4e.
Synthesis and Characterization of 7+ and 8+. Bulk
electrochemical oxidation of 7 and 8 allows for the preparation
of 7+ and 8+, respectively, which have been characterized by
UV-vis and EPR spectroscopies (see Supporting Information).
The spectra of the materials resulting from the one-electron
oxidation of 7 and 8 have bands at 762 (680 nm, sh) and 760
nm (700 nm, sh), respectively. These spectra compare well with
the reported spectrum for oxidized didodecylsexithiophene,
which exhibits a band at 775 nm and a shoulder at 685 nm; the
high-energy shoulder was attributed to the formation of the
π-dimer.52 By analogy, the shoulders observed in spectra of 7+
and 8+ are attributed to the diamagnetic π-dimer. The energies
of the π-π* transitions for sexithienyl complexes 7+ and 8+
are very similar, whereas the energies of the π-π* transitions
for 7 and 8 differ by 26 nm.
The UV-vis spectra for 7, 7+, 8, and 8+ compare well
with the spectra of previously reported sexithienyl compounds.
For example, for unsubstituted oligothiophenes in CHCl3, the
π-π* transitions for quaterthiophene, pentathiophene, and
sexithiophene are at 390, 416, and 434 nm, respectively.51 The
π-π* transitions for the corresponding quaterthiophene and
sexithiophene radical cations are at 614 and 775 nm, respec-
tively.53 For another series of trimethylsilyl-terminated oligo-
thiophenes, the quaterthienyl transition is at 394 nm, the
pentathienyl transition is at 422 nm, and the sexithienyl transition
is at 422 nm; the radical cation analogues are at 666, 743, and
779 nm, respectively.27 Significantly, the absorption bands for
7+ and 8+ are inconsistent with the reported spectra for oxidized
oligothiophenes with four rings, indicating that the binding of
an oligothienyl ring to the Ru center does not remove that ring
from conjugation with the unbound rings.
Synthesis and Characterization of a Polymeric Complex,
Poly-4b. Poly-4b has been electrochemically deposited onto an
Au foil electrode by electrochemically oxidizing 4b in CH2Cl2/
0.1 M LiB(C6F5)4‚Et2O in a cyclic voltammetry experiment.
The characteristic oxidative peak for this terthienyl compound
is observed at 765 mV vs FcH/FcH+ (external reference), and
the reversible oxidation/reduction wave for a typical deposited
polymer has an E1/2 value of 540 mV vs FcH/FcH+. The
specular reflectance FT-IR spectrum of poly-4b exhibits a single
metal carbonyl band at 1998 cm-1 (fwhm ) 34 cm-1), which
is almost identical in frequency to that of the monomer 4b (1997
cm-1). Since IR spectroscopy in this region probes the CO-
containing Ru fragments, the similarity of the νCO of the polymer
to that of the monomer suggests that all of the CO-containing
Ru centers are electronically similar to the monomer. EDS
shows, within experimental error, that the Ru:S ratio remains
constant before and after polymerization (polymer, 1.47 ( 0.05;
cast film of 4b, 1.47 ( 0.16), suggesting that no appreciable
leaching of the metal from the polymer occurs.14 When poly-
4b is immersed in a 0.04 M solution of (CH3)3CNC in CH2Cl2
)
2175 cm-1 and νCO ) 2008 cm-1. These bands are consistent
with those of the isonitrile adduct (9) formed upon reaction of
4a and (CH3)3CNC (νCN ) 2176 cm-1, νCO ) 2003 cm-1) (vide
infra). This reaction demonstrates the coordinative lability of
the polymeric thienyl ligand and provides further evidence that
the chemical nature of the film is similar to that of the monomer.
FT-IR Spectroelectrochemistry of Poly-4b. Although 4a
has only two and 7 has only three electrochemically accessible,
stable oligothienyl oxidation states, poly-4b might be expected
to have many due to the terthienyl repeat unit along the polymer
backbone. Spectroelectrochemical experiments were performed
on poly-4b by oxidizing the polymer, immobilized on a gold
foil working electrode, at 800 mV (external reference vs FcH/
FcH+) in a three-electrode electrochemical cell, removing the
electrode from the cell under potential control, and acquiring
the specular reflectance FT-IR spectrum. Upon polyterthienyl-
centered oxidation, an FT-IR difference band is observed with
a positive peak at 2017 cm-1 and a negative peak at 1995 cm-1
,
with a concomitant color change of the polymer from orange
to blue, Figure 4d. This oxidation process can be electrochemi-
cally reversed, as evidenced by the flat difference spectrum
obtained by subtracting the spectrum of poly-4b after re-
reduction at 0 mV for 2 min from poly-4b before oxidation,
Figure 4e. It is important to note that no changes in the FT-IR
spectrum are observed until the application of this threshold
potential and that continued increases in the absolute values of
the positive and negative difference peaks at 2017 and 1995
cm-1, respectively, are observed as the polymer is oxidized at
potentials up to 300 mV past the threshold potential (i.e., higher
oxidation potentials do not lead to a positive difference peak
with a higher νCO).
Independently prepared films of poly-4b have difference
bands associated with spectroelectrochemical oxidation with
positive difference peaks which range from 2017 to 2011 cm-1
.
However, all films exhibit the same qualitative behavior (i.e., a
single difference band at a threshold oxidation potential). The
value of the positive difference peak largely depends on the
peak frequency and width of the initial spectrum of poly-4b.
Simulation of the spectroelectrochemical difference spectra for
several independently prepared films of poly-4b using their
respective initial spectra shows that the actual νCO for each film
of oxidized poly-4b is approximately 10 cm-1 greater than that
for poly-4b, consistent with the changes observed upon oxidation
of 4a and 7.
The intensity of the carbonyl band in poly-4b decreases
reversibly by 30-40% upon polymer oxidation. Decreases in
metal-carbonyl and -isocyanide stretching band intensities
accompanying metal-centered oxidation processes have been
noted in the literature for metal-carbonyl and -isocyanide
complexes, respectively.54-57 This effect has been explained in
terms of the reduction in the metal-dπ to ligand-π* back-
bonding with increases in metal oxidation. This reduction
decreases the amount of charge transferred to the CO or CN
during the vibrational transition, thereby reducing the net dipole
moment change during that transition. The decrease in carbonyl
stretching band intensity upon ligand-centered oxidation in poly-
4b shows that ligand-based oxidation, in addition to metal-based
oxidation, can cause this effect.
(54) Kettle, S. F. A.; Paul, I. In Infrared Intensities of Metal Carbonyl
Stretching Vibrations; Stone, F. G. A., West, R., Eds.; Academic Press:
New York, 1972; Vol. 10, pp 199-236.
(55) Noack, K. HelV. Chim. Acta 1962, 45, 1847-1859.
(53) Caspar, J. V.; Ramamurthy, V.; Corbin, D. R. J. Am. Chem. Soc.
1991, 113, 600-610.
(56) Bullock, J. P.; Mann, K. R. Inorg. Chem. 1989, 28, 4006-4011.
(57) Brown, T. L.; Darensbourg, D. J. Inorg. Chem. 1967, 6, 971-977.

2512 J. Am. Chem. Soc., Vol. 123, No. 11, 2001
Weinberger et al.
Probing the Reactivity of 4a with Small Molecules with
Various Ligating Properties. To determine the oxidation-state-
dependent reactivity of 4a, its reactivity patterns were studied
by a series of NMR tube reactions, eq 2. Addition of 44 equiv
of CH3CN to 4a in CD2Cl2 at room temperature results in the
overnight formation of the CH3CN adduct, 5a. The metal-bound
CH3CN in 5a can be removed by heating 5a to 100 °C in vacuo
(<50 mTorr) for 5 days. Similarly, addition of 5 equiv of (CH3)3-
CNC to 4a in CD2Cl2 at room temperature for 12 days results
in the formation of the isocyanide adduct, 9. Complex 9 is
substitutionally inert with respect to displacement of the bound
(CH3)3CNC, even when it is heated to 100 °C in vacuo (<50
mTorr) for 5 days.
Acetone (100 equiv) does not react with 4a in CD2Cl2 at room
temperature. However, if 4a is dissolved in neat acetone-d6,
approximately 9% of the complex is converted to the acetone-
d6 adduct (10), which was determined by integration of the two
Cp resonances observed in the 1H NMR spectrum of the mixture
of 4a and 10. Removing the excess acetone in vacuo and
redissolving the resulting solid in CD2Cl2 results in quantitative
re-formation of 4a.
Figure 7. Variable-temperature 2H{1H} NMR spectra of 4a+ in
CH2Cl2/CD2Cl2 for binding studies. Sequential addition of (a) 0.5 equiv
of CD3CN, (b) 1 equiv of CD3CN, (c) 2 equiv of decamethylferrocene
as reducing agent. The asterisk indicates CD2Cl2 internal reference.
NMR experiments. Deuterium NMR resonances are often
1
sharper than H NMR resonances when the compounds being
studied are in the presence of paramagnetic species.58 Since
replacing hydrogen with deuterium in small molecule substrates
is not expected to significantly impact the steric or electronic
nature of the substrates, monitoring the 2H{1H} NMR spectrum
of deuterated ligands in the presence of 4a+ provides a
convenient way to examine the binding behavior of these ligands
2
to the metal center. For these VT H{1H} NMR experiments,
4a+ was prepared by bulk oxidation of 15 mg of 4a in a two-
compartment electrochemical cell with LiB(C6F5)4‚Et2O as the
supporting electrolyte. The resulting 4a+/electrolyte solution was
loaded into an air-free NMR tube in 0.4 mL of CH2Cl2 with 1
µL of CD2Cl2 as an internal standard. The 2H{1H} NMR spectra
of these solutions show only the CD2Cl2 reference at δ 5.32.
After addition of 0.5 equiv of CD3CN to a sample of 4a+ in
CH2Cl2/CD2Cl2, a new resonance at δ 2.53 is observed, Figure
7. Significantly, this resonance is shifted downfield relative to
the frequency observed for “free” CD3CN in CH2Cl2 (δ 1.93).
Upon cooling of the sample to -73 °C, the CD3CN resonance
splits into two separate resonances (δ 2.79, 2.10; downfield
resonance more intense). After addition of a second half of an
equivalent of CD3CN to the sample at room temperature, the
acetonitrile resonance moves upfield from δ 2.53 to 2.36. This
change corresponds to more “free” CD3CN in solution, which
causes the weighted average to shift upfield. Upon cooling of
the sample to -73 °C, the acetonitrile resonance splits into two
resonances at δ 2.77 and 2.07. In this case, the upfield peak is
more intense. These results are consistent with a chemical
exchange process in which the CD3CN is rapidly coordinating
to and dissociating from the metal center coordination sphere;
at room temperature the reaction is fast but at -73 °C the
exchange is slow on the NMR time scale. Significantly, after
addition of a reducing agent (decamethylferrocene, 2 equiv) to
the solution at room temperature, all of the bound CD3CN is
Carbon monoxide (2-4 psi) does not react with 4a in CD2Cl2
at room temperature nor under refluxing conditions. In com-
parison, CO reacts slowly with 4a in acetone-d6 at room
temperature or at reflux temperature to form the CO adduct of
4a, complex 11. Heating 11 to 100 °C in vacuo (<50 mTorr)
for 5 days does not result in its reconversion to 4a. Ethylene
(2-4 psi) does not react with 4a in CD2Cl2 under ambient
conditions, but it does react with 4a to form an ethylene adduct
(12) upon refluxing the same sample. This reaction can be
reversed to re-form 4a by heating a sample of 12 to 100 °C in
vacuo (<50 mTorr) for 5 days.
Although 1 equiv of thiophene does not react with 4a in
CD2Cl2 or acetone-d6, when 4a is dissolved in 10 mL of neat
thiophene for 1 day, an equilibrium is established between 4a
and the thiophene adduct of 4a (13). The product distribution
1
was characterized by H and 31P{1H} NMR spectra obtained
after the excess thiophene was removed in vacuo at room
temperature and the resulting solid was dissolved in CD2Cl2.
These spectra show a 1.1:1 ratio of 4a to 13. However, after
the sample is allowed to sit at room temperature for 1 day, the
bound thiophene is displaced from the metal coordination sphere
to yield 4a and metal-free thiophene exclusively. The mode of
coordination for thiophene cannot be established conclusively
due to overlap of the thienyl resonances associated with 4a and
13 in the 1H NMR spectrum of 4a and 13. It is likely, however,
that the coordination is through the S atom, based on the
coordination mode in very similar complexes17 and the lack of
resonances in the 1H NMR spectrum at δ 3-3.5, which would
be present for a complex with thiophene bound in an η2 fashion.
Probing the Reactivity of 4a+. The reactivity of paramag-
netic 4a+ with CD3CN, acetone-d6, 2,5-dideuteriothiophene, and
ethylene-d4 was examined by variable-temperature 2H{1H}
2
released, as evidenced by VT H{1H} NMR spectroscopy. It
should be noted that the spectra of control samples prepared
with 4a in place of 4a+ do not show significant temperature-
dependent changes.
(58) Johnson, A.; Everett, G. W. J. Am. Chem. Soc. 1972, 94, 1419-
1425.

Terthienyls as Redox-Switchable Hemilabile Ligands
J. Am. Chem. Soc., Vol. 123, No. 11, 2001 2513
Variable-temperature 2H{1H} NMR experiments designed to
probe the reactivity of 4a+ with acetone-d6, ethylene-d4, and
2,5-dideuteriothiophene59 were conducted analogously to those
for CD3CN. These studies indicate that 4a+ does not react with
1 equiv of ethylene-d4, acetone-d6, or 2,5-dideuteriothiophene
under the conditions examined.
steric interactions of substituents (steric interactions contribute
to increased twisting of the rings and therefore decreased
conjugation), and inductive electronic effects of the substituents.
The spectra of complexes 5a and 8, in which the oligothienyl
backbones are not coordinated to the metal centers, provide a
comparison for the spectroscopy of the metal-bound oligothienyl
complexes, 4a and 7, respectively. The λmax values for 4a and
5a, and 7+ and 8+, are very similar to each other, respectively,
suggesting that metal center binding does not significantly alter
the extent of ring conjugation for these complexes. Thus, the
combination of electrochemistry and UV-vis spectroscopy for
these complexes suggests that metal center binding of the
oligothienyl ligands primarily increases the ligand-based oxida-
tion potentials due to the electronic influence of the positively
charged metal centers.
Discussion
We have developed a new redox-switchable hemilabile ligand
(RHL), 1b, which allows for the preparation of an isolable,
mononuclear Ru(II) complex (4b) which can be oligomerized
to yield dimers, oligomers, and surface-immobilized conducting
polymer/metal hybrid materials. Also, model complex 4a, a
nonpolymerizable analogue of 4b, has been studied electro-
chemically in solution under controlled conditions. These
monomeric, oligomeric, and polymeric complexes have been
used to evaluate the influence that redox-active thienyl-based
ligands can have on the electronic and coordination environ-
ments of bound transition metal centers.
An important question to address is that of why the ligand-
centered oxidation/reduction potentials increase for these oligo-
thienyl complexes upon oligothienyl coordination to the Ru
centers (Table 2): is the increase due to inductive effects
associated with the cationic Ru centers or due to distortion of
the metal-coordinated thienyl moieties, which results in de-
creased conjugation of the oligothienyl units that comprise the
backbones of these ligands? Graf and Mann addressed this
question for [CpRu]+ fragments bound in an η5 manner to
oligothiophene backbones and concluded that the bonding of
the [CpRu]+ fragment to the oligothiophene effectively removes
the metal-bound ring from π conjugation with the remaining
unbound thiophene rings.60 This article addresses this question
for oligothiophenes bound in an η1 fashion to Ru(II) metal
centers.
If bonding between the metals and the oligothienyl groups
in 4a and 7 removes the bound rings from conjugation with the
other thienyl rings, then the oxidation potentials of 4a and 7
should be effectively those of bithienyl and quaterthienyl
compounds, respectively. In several reported series of homolo-
gous oligothiophenes, the difference between bi- and terthienyl
oxidation ranges from 150 to 250 mV, and the difference
between quater- and sexithienyl oxidation ranges from 80 to
110 mV.27,29,51 Thus, the differences in E1/2 values observed
for 4a and 5a (200 mV) and for 7 and 8 (105 mV) are consistent
with removal of bound rings from conjugation upon terthienyl
coordination to the Ru center. However, others have concluded
that binding positive Ru centers in an η6 fashion to phenyl
groups of phenyl-terminated terthiophenes increases the ter-
thienyl oxidation potential by 150 mV per metal center, mainly
due to the effects of the positive Ru center.60 Thus, the
magnitudes of the changes in potential between 5a and 4a and
between 7 and 8 also are consistent with inductive and
electrostatic effects of the Ru cations. Therefore, cyclic voltam-
metry, alone, does not provide conclusive evidence regarding
the reason for the ligand-centered oxidation/reduction potential
increases upon oligothienyl-metal binding.
The spectroelectrochemistry of 7 compared to that of 4a
shows that by increasing the length of the oligothienyl unit,
additional ligand-centered oxidations become electrochemically
accessible. Complex 7 provides an example of how sequential
oxidations can result in successive increases in CO stretching
frequency and, therefore, electronic changes at the metal center.
Note that for the dimer (7), two, one-electron oxidation processes
are required to effect a change in electronic character at the Ru
center comparable to the change observed for a single, one-
electron oxidation of the monomer (4a). Additionally, the fact
that the CO stretching frequency of 7+ is lower than that of
4a+ implies that the charge does not remain unsymmetrically
localized in 7+, which could result in one metal center feeling
the doping level equivalent to that in 4a+ and one metal center
remaining unaffected (and thus silent by difference spectros-
copy). This observation indicates that the charge is delocalized
over multiple rings in 7+. However, it is not known from these
experiments whether the extent of delocalization of the hole is
over three rings in the monomer and six rings in the dimer, or
over two rings in the monomer and the four central rings in the
dimer. Either explanation is consistent with the observed results
for the spectroelectrochemical oxidation of 4a and 7.
Poly-4b provides additional insight into the degree of
localization in these materials. In the FT-IR spectroelectro-
chemistry of poly-4b, a single quantized change in the νCO band
is observed upon oxidation of the polymer backbone. Although
several groups, including ours, have suggested that metal-
conducting polymer complexes might give access to systems
with tunable electrochemical properties based on extent of
oxidation,2-4 for this system, we do not observe gradual shifts
in CO stretching frequency to higher energy as poly-4b is
oxidized to higher potentials. These results are consistent with
a model in which the charged Ru fragment causes the electron
holes produced during oxidation of the polymer backbone to
remain localized on the uncomplexed thienyl rings, at least on
the time scale of the FT-IR experiment. The polymer consists
of monomer units connected in ring 1-ring 1, ring 1-ring 3,
and ring 3-ring 3 (as in 7) manners (Figure 8); each connection
provides for slightly different physical properties. However, the
average effect in the polymer film is such that localization
essentially transforms what ordinarily might be considered a
delocalized conducting polymer into a string of monomers,
giving rise to a single change in the spectroelectrochemical
response at the potential capable of causing oxidation of these
units. Consistent with this model, 7+ has a lower energy FT-IR
band than 4a+ because in 7+ the electron hole is delocalized
over the four internal rings not bound to Ru.
The energies of the π-π* transitions in the UV-vis spectra
of 4a, 5a, 7, 8, and their oxidized forms, which are known to
depend on the extent of ring conjugation in oligothiophene,27,29,51
also can be used to address this question. Several factors
influence the energy of oligothienyl π-π* transitions, including
chain length (increased length leads to increased conjugation),
(59) Delabouglise, D.; Garreau, R.; Lemaire, M.; Roncali, J. New J.
Chem. 1988, 12, 155-161.
(60) Graf, D. D.; Mann, K. R. Inorg. Chem. 1997, 36, 150-157.
This article also addresses the design of polymeric RHL
complexes for controlled small-molecule uptake and release.

2514 J. Am. Chem. Soc., Vol. 123, No. 11, 2001
Weinberger et al.
based oxidation of 4a to 4a+, this change will not always
translate into an observable change in the lability of the small
molecule with respect to the metal center. Thus, in the design
of RHL-metal complexes for molecular uptake and release,
the target substrate should be selected and the complex
specifically designed to (1) maximize the RHL effect and (2)
ensure that the oxidation-induced changes in the metal-RHL
binding interaction yield changes in the binding properties of
the metal center for the small molecule of interest.
Conclusions
Polymerizable, terthienyl-based RHLs, their corresponding
Ru(II) complexes, and polymers formed from these complexes
have been designed, synthesized, and characterized. These
complexes provide a model for understanding the factors that
affect hole delocalization within metal-bound oligothienyl units.
Moreover, the systematic study reported herein provides an
important lesson regarding the design of tunable ligands that
can be used to electrochemically modulate bound metal center
properties. Specifically, efforts directed at preparing conducting
polymer-metal hybrid materials for tunable redox control over
bound transition metal properties must balance (1) the need for
metal centers with high oxidation potentials and (2) the
observation that cationic repulsion between the bound metal
centers and the oxidized conducting polymer can effectively
localize holes within the conducting polymer backbone and limit
the tunability of metalated conducting polymer materials.
Notably, the results suggest that if one wants a truly delocalized
system where the incremental removal of charge from the
conducting polymer results in an incremental change in the
electronic properties of the bound metal centers, one should
reduce the ratio of metal centers to oligothienyl repeat units
within the polymeric structure.
Figure 8. Three possible monomer linkages within poly-4b: (a) ring
3-ring 3, (b) ring 1-ring 3, and (c) ring 1-ring 1.
To successfully realize redox-controlled molecular uptake and
release utilizing RHL-controlled metal center coordination sites,
several requirements (ideally) must be achieved: (1) target
substrates should not react with the RHL-metal complex in its
reduced state, (2) target substrates should react (quickly) with
the oxidized form of the RHL-metal complex to yield a metal-
substrate adduct, and (3) target substrates should dissociate
(quickly) from the metal center coordination sphere upon
reduction of the RHL-metal complex, Scheme 1. Additionally,
all complexes generated in this cycle must be chemically stable
with respect to each other, the solvent, and the target substrate.
For example, complexes of terthienyl-based redox-active ligands
cannot be used with nucleophilic solvents or target substrates,
including water, alcohols, and amines due to the ability of these
nucleophiles to react with the oxidized form of the terthienyl
ligand. These requirements necessitate a careful balance between
the thermodynamic and kinetic factors which influence the
oxidation-state-dependent binding properties of RHL-metal
complexes when designing complexes for molecular uptake and
release of target substrates.
Thermodynamic and kinetic factors govern the molecular
uptake and release properties of these complexes. A comparison
of the cyclic voltammogram of 4a to that of 5a shows that the
binding constant of the metal center for CH3CN increases by a
factor of 2 × 103 upon ligand-centered oxidation, and the
analogous experiments for 7 and 8 show that the binding
constant increases by a factor of 60. This comparison demon-
strates that the thermodynamic perturbation on the oligothienyl
backbone, in both cases, is significant upon ligand-based
oxidation.
Experimental Section
General. All reactions were carried out under a dry nitrogen
atmosphere using standard Schlenk techniques or in an inert-atmosphere
glovebox unless otherwise noted. Acetonitrile, dichloromethane, and
pentane were dried over calcium hydride. Tetrahydrofuran (THF)
and diethyl ether were dried over sodium/benzophenone. Acetone
(HPLC grade) was purchased from Acros Organics and used as
received. All solvents were distilled under nitrogen and saturated with
N2 prior to use. Deuterated solvents and ethylene-d4 were purchased
from Cambridge Isotope Laboratories and used without further purifica-
tion. LiB(C6F5)4‚Et2O was purchased from Boulder Scientific and used
as received. Silver(I) tetrafluoroborate, tert-butylisonitrile, carbon
monoxide, and ethylene were purchased from Aldrich Chemical Co.
and used as received. Thiophene and n-Bu4NPF6 were purchased from
Aldrich Chemical Co. and purified as previously reported61 and by
recrystallization three times from hot ethanol followed by heating in
vacuo for 3 days, respectively. Precious metals were purchased from
D. F. Goldsmith, Evanston, IL. A gold minigrid was purchased from
Buckbee Mears, St. Paul, MN.
2H{1H} NMR experiments were designed to assess the kinetic
component of RHL perturbation of the metal center’s small-
molecule binding properties upon ligand-based oxidation. These
experiments show that acetonitrile is kinetically labile with
respect to Ru(II) in 4a+, but not 4a, on the time scale of the
NMR experiment. Significantly, acetonitrile is the only one of
Physical Measurements. 1H, 31P{1H}, 19F{1H}, and 13C{1H} NMR
spectra were recorded on a Varian Gemini 300-MHz FT-NMR
spectrometer, a Varian Unity 400-MHz FT-NMR spectrometer, or a
Varian Mercury 300-MHz FT-NMR. 31P{1H} NMR spectra were
externally referenced relative to 85% H3PO4, 19F{1H} NMR spectra
were externally referenced relative to CFCl3, and 1H NMR spectra were
referenced relative to residual solvent peaks. To achieve accurate
integrations for cyclopentadiene-containing complexes, a 40-s pulse
delay between scans was used to allow all protons to relax. Variable-
2
the substrates examined by H{1H} NMR studies with 4a+
which reacts with 4a in a 1:1 ratio of 4a to substrate in
methylene chloride under ambient conditions. In contrast,
ethylene, thiophene, and acetone need more extreme conditions
to force the reaction with 4a to occur, such as elevated
temperature, coordinating solvents, or high concentration of
target substrates. This demonstrates that, although the binding
constant changes of the metal center for small molecules is
expected to increase by 3 orders of magnitude upon ligand-
1
temperature H and 31P{1H} NMR spectra were recorded on a Varian
Unity 400-MHz FT-NMR at 400 and 162 MHz, respectively, or a
Varian Mercury 300-MHz FT-NMR at 300 and 121 MHz, respectively.
(61) Spies, G. H.; Angelici, R. J. Organometallics 1987, 6, 1897-1903.

Terthienyls as Redox-Switchable Hemilabile Ligands
J. Am. Chem. Soc., Vol. 123, No. 11, 2001 2515
Variable-temperature 2H{1H} NMR spectra were recorded on a Varian
Unity 400-MHz FT-NMR at 61 MHz. FT-IR spectra were recorded
using a Nicolet 520 FT-IR spectrometer. UV-vis spectra were recorded
using an HP 8452A or HP 845x spectrometer. Electrochemical
measurements were carried out on either a PINE AFRDE4 or an
AFRDE5 bipotentiostat/galvanostat using Au, Pt, or glassy carbon disk
electrodes with a Pt mesh counter electrode and a Ag/AgNO3 (0.01 M
AgNO3 in CH3CN/0.1 M n-Bu4NPF6; BAS, West Lafayette, IN)
reference electrode, a Ag/AgCl reference electrode (Cypress Systems,
Lawrence, KS), or a silver wire quasi-reference electrode. All electro-
chemical data were referenced versus an internal FcH/[FcH]+ (Fc )
(η5-C5H5)Fe(η5-C5H4)) redox couple unless noted otherwise. Spectro-
electrochemistry (FT-IR) was performed in a cell consisting of NaCl
plates sandwiched together with a Teflon spacer.14 The electrodes placed
in the internal volume of the cell were a Pt mesh or Pt wire counter
electrode, a Ag wire or Ag/AgCl reference electrode, and a Au minigrid
working electrode for transmittance in situ monitoring of the oxidation
products. Electron impact (EI) and fast atom bombardment (FAB) mass
spectra were recorded using a Fisions VG 70-250 SE mass spectrometer.
Electrospray (ES) mass spectra were recorded using a Micromass
Quatro II electrospray triple-quadrapole mass spectrometer. EPR spectra
were measured using a modified Varian E-4 X-band spectrometer, and
the field was calibrated using diphenylpicrylhydrazyl (dpph) as a
standard. Elemental analyses were performed by Desert Analytics
Laboratory, Tuscon, AZ. Energy dispersive spectroscopy (EDS) was
performed on a Hitachi scanning electron microscope equipped with
an EDS detector. Compounds 1a-5a and 1b-4b were prepared as
previously reported.14
well-shaped electrochemical cell.62 A rectangle of Au foil forms the
bottom of the electrochemical cell, and the working, reference (Ag
wire), and counter (Pt mesh) electrodes are held in solution above the
working electrode. Cycling the potential between 0 and 900 mV vs
Ag wire for 48 h at 50 mV/s yields the surface-immobilized polymer
and a mixture of oligomers. Reduction of any oxidized species at -200
mV for several hours yields the orange polymer and oligomers in the
neutral oxidation state.
3′,4′′′′-Bis{CpRuCOCl(2-diphenylphosphinoethyl)}-2,2′:5′,2′′:
5′′,2′′′:5′′′,2′′′′:5′′′′, 2′′′′′-sexithiophene (6). The solvent is removed
from the resulting solution after a typical preparation of poly-4b and
the solutions from several polymer preparations are combined. Typi-
cally, a mixture of oligomers corresponding to 125 mg of monomer
(0.09 mmol) is combined in 5 mL of acetone under ambient conditions,
100 mg (1.3 mmol) of KCl is added, and the mixture is refluxed for 3
d. Removal of solvent and column chromatography on alumina using
methylene chloride as eluent followed by preparative TLC (alumina/
methylene chloride) allows for the isolation of a bright yellow band.
Recrystallization from CH2Cl2 (3 mL)/ pentane (20 mL) yields the
desired product in 66% yield as orange-yellow blocks. 1H NMR
(CD2Cl2): δ 7.61-7.41 (m, 20 H); 7.28 (dd, 2 H, J ) 0.6, 3.0);
7.10-7.07 (m, 6 H); 7.01-6.97 (m, 4 H); 4.83 (s, 10 H, Cp); 3.21-
3.13 (m, 2 H, CH2); 2.99-2.91 (m, 2 H, CH2); 2.71-2.58 (m, 4 H,
CH2). 31P{1H} NMR (CD2Cl2): δ 45.2 (s). 13C{1H} NMR (CD2Cl2):
δ 205.5 (d, JC-P ) 15.1, CO); 139.8 (s); 139.6 (s); 138.6 (s); 138.0 (s);
136.3 (s); 136.2 (s); 135.5 (s); 135.3 (s); 134.3 (s); 134.2 (s); 133.9
(s); 133.3 (s); 131.7 (s); 131.5 (s); 131.4 (s); 130.4 (s); 129.2 (s); 129.0
(s); 128.9 (s); 128.8 (s); 128.1 (s); 126.8 (s); 126.8 (s); 126.1 (s); 124.9
(s); 86.3 (s, Cp); 30.1 (d, JC-P ) 30.2, CH2CH2P); 24.4 (s, CH2CH2P)
(phenyl and thienyl resonances not assigned). IR (CH2Cl2): νCO ) 1955
cm-1. MS (FAB+) [M+]: calcd for C64H50Cl2O2P2Ru2S6, m/z 1377.9;
found, m/z 1378.6. UV-vis λmax (CH2Cl2): 430 nm.
[CpRuCO(K2-3′-(2-diphenylphosphinoethyl)-5,5′′-dimethyl-2,2′:
5′,2′′-terthiophene)][B(C6F5)4]2 (4a+). In
a typical experiment,
10-20 mg of 4a was dissolved in 6 mL of CH2Cl2 with 50 mg of
LiB(C6F5)4‚Et2O (0.07 mmol) as electrolyte and added to one side of
a two-compartment cell which held a working electrode (Pt mesh) and
a reference electrode (Ag/AgCl). Separated from the complex solution
by a frit was 50 mg of LiB(C6F5)4‚Et2O dissolved in 6 mL of CH2Cl2
and the counter electrode (Pt mesh). Electrochemical oxidation to a
potential beyond the first oxidation potential of the complex with stirring
overnight yielded the blue 4a+ from green 4a. Conversion was complete
as monitored by UV-vis spectroscopy. 1H NMR (CD2Cl2): featureless.
[3′,4′′′′-Bis{CpRuCO(K2-(2-diphenylphosphinoethyl))}-2,2′:5′,2′′:
5′′,2′′′:5′′′,2′′′′:5′′′′,2′′′′′-sexithiophene][B(C6F5)4]2 (7). Complex 6 (43
mg, 0.03 mmol) was dissolved in 5 mL of CH2Cl2 with LiB(C6F5)4‚
Et2O (35 mg, 0.046 mmol) and stirred under N2 for 7 d to abstract the
chloride anion. Filtration to remove LiCl, removal of solvent in vacuo,
and recrystallization from CH2Cl2 (5 mL)/ pentane (20 mL) yields 7
as a pure, air-stable, orange-red powder which has limited solubility
in CH2Cl2. 1H NMR (CD2Cl2): δ 7.44-7.26 (m, 26 H); 7.12 (d,
2 H, J ) 4); 6.98 (d, 2 H, J ) 4); 6.63 (s, 2 H); 4.90 (s, 10 H, Cp);
3.20-3.05 (m, 8 H, CH2CH2). 31P{1H} NMR (CD2Cl2): δ 39.9 (s).
19F{1H} NMR (CD2Cl2): δ -133.6 (s); -164.0 (t, J ) 20.2); -167.9
(s). IR (CH2Cl2): νCO ) 1996 cm-1. MS (ES) [M+]: calcd for
[C64H50S6Ru2P2O2][BC24F20], m/z 1986.9; found, m/z 1986.3. UV-vis
λmax (CH2Cl2): 456 nm.
31P{1H} NMR (CD2Cl2): featureless. 19F{1H} NMR (CD2Cl2):
δ
-133.2 (s), -163.4 (s), -167.4 (s). IR (CH2Cl2): νCO ∼2006 cm-1
(this frequency is a lower limit to the accurate value due to overlap of
the band with the band of any residual 4a). UV-vis λmax (CH2Cl2):
656 (br) nm. X-band EPR (CH2Cl2, 298K): g ) 2.003, width ) 30 G.
[Cp(CH3CN)RuCO(3′-(2-diphenylphosphinoethyl)-5,5′′-dimethyl-
2,2′:5′,2′′-terthiophene)][B(C6F5)4]2 (5a+). Complex 5a (6 mg, 4 ×
10-3 mmol) was electrochemically oxidized in a manner similar to the
formation of 4a+ with 50 mg of LiB(C6F5)4‚Et2O (0.05 mmol) in 6 mL
of CH2Cl2 on each side of the electrochemical cell. Oxidation overnight
yielded a deep blue-colored solution comprised of a mixture of the
target complex and electrolyte. Removal of solvent yielded a solid
mixture of 5a+ and electrolyte. UV-vis λmax (CH2Cl2): 685 nm (br).
X-band EPR (CH2Cl2, 298K): g ) 2.0025 ( 0.0005, width ) 30 G.
Acetonitrile Adduct of 4b: [Cp(CH3CN)RuCO(3′-(2-diphenyl-
phosphinoethyl)-2,2′:5′,2′′-terthiophene)][B(C6F5)4] (5b). This com-
plex was prepared by dissolving 4b (15 mg, 9.7 × 10-3 mmol) in 0.5
mL of CH3CN for 24 h under ambient conditions. Over the course of
the reaction, the orange solution became pale yellow. The CH3CN was
removed in vacuo and the product characterized. The CH3CN can be
quantitatively removed to re-form 4b by heating the sample to 100 °C
in vacuo for 5 d. 1H NMR (CD2Cl2): δ 7.58-7.37 (m, 10H, Ph); 7.36
[3′, 4′′′′-Bis{CpRuCO(K2-(2-diphenylphosphinoethyl))}-2,2′:5′,2′′:
5′′,2′′′:5′′′,2′′′′:5′′′′,2′′′′′-sexithiophene][B(C6F5)4]3 (7+). Complex 7 was
oxidized using the same apparatus as for oxidation of 4a, using 7 (3
mg, 1 × 10-3 mmol) with 6 mL of CH2Cl2 and 50 mg of LiB(C6F5)4‚
Et2O (0.07 mmol) on each side of the electrochemical cell. Overnight
oxidation yielded a color change from red to green. The solvent was
removed to yield a green solid which is a mixture of 7+ and electrolyte.
UV-vis λmax (CH2Cl2): 762 nm. EPR (CH2Cl2, 298 K): g ) 2.002.
[3′, 4′′′′-Bis{Cp(CH3CN)RuCO(2-diphenylphosphinoethyl)}-2,2′:
5′,2′′:5′′,2′′′:5′′′,2′′′′:5′′′′,2′′′′′-sexithiophene][B(C6F5)4]2 (8). Complex
7 (6 mg, 2 × 10-3 mmol) is dissolved in 1 mL of CH3CN and shaken
at room temperature under ambient conditions for 4 d with a change
from a red-orange sparingly soluble complex to a bright yellow, freely
soluble complex. Removal of CH3CN and addition of CD2Cl2 allows
for monitoring the 1H NMR spectroscopy which shows a 100%
spectroscopic yield. 1H NMR (CD2Cl2): δ 7.58-7.32 (m, 22 H); 7.09-
6.94 (m, 10 H); 5.02 (s, 10 H, Cp); 2.78 (m, 8 H, CH2CH2); 2.07 (s,
6 H, coordinated CH3CN). 31P{1H} NMR (CD2Cl2): δ 41.7 (s). 19F-
{1H} NMR (CD2Cl2): δ -133.4 (s); -163.9 (t, J ) 19.9); -167.8 (s).
IR (CH2Cl2): νCO ) 1993 cm-1. MS (ES) [M+]: calcd for [C68H56N2O2P2-
Ru2S6][BC24F20], m/z 2069.0; found, m/z 2069.2. UV-vis λmax (CH2-
Cl2): 430 nm.
(dd, 1H, J3-5 ) 1.0, J5-4 ) 5.3, 5′′ or 5 thienyl); 7.29 (dd, 1H, J3-5
)
1.0, J5-4 ) 5.3, 5′′ or 5 thienyl); 7.20 (dd, 1H, J3-5 ) 1.2, J3-4 ) 3.6,
3 or 3′′ thienyl); 7.06 (2 overlapping dd, 2H, 4 and 4′′ thienyl); 6.98
(overlapping dd and s, 2H, 4′ thienyl and 3 or 3′′); 5.03 (s, 5H, Cp);
2.78 (s (br), 4H, CH2CH2); 2.09 (s, 3H, CH3CN). 31P{1H} NMR (CD2-
Cl2): δ (s) 42.1. IR (CH2Cl2): νCO ) 1994 cm-1. MS (ES) [M+]: calcd
for C34H29NOPRuS3, m/z 696; found, m/z 696. UV-vis λmax (CH2Cl2):
) 341 nm.
(62) Sailor, M. J., Heinrich, J. L., Lauerhaas, J. M., Kamat, P. V., and
Meisel, D., Eds.; Elsevier Science: New York, 1997; Vol. 103, pp 209-
235.
Poly-4b. In a typical preparation, 4b (30 mg, 0.02 mmol), 70 mg of
LiB(C6F5)4‚Et2O (0.10 mmol), and 3 mL of CH2Cl2 are added to a

2516 J. Am. Chem. Soc., Vol. 123, No. 11, 2001
Weinberger et al.
[3′, 4′′′′-Bis{Cp(CH3CN)RuCO(2-diphenylphosphinoethyl)}-2,2′:
5′,2′′:5′′,2′′′:5′′′,2′′′′:5′′′′,2′′′′′-sexithiophene][B(C6F5)4]3 (8+). Elec-
trochemical oxidation of 8 (6 mg, 2.2 × 10-3 mmol) in a manner similar
to that described for 4a with 50 mg of LiB(C6F5)4‚Et2O and 6 mL of
CH2Cl2 on each side of the electrochemical cell yielded a green-colored
solution of 8+ and electrolyte. Removal of solvent yielded a green-
brown-colored solid mixture of 8+ and electrolyte. UV-vis λmax
(CH2Cl2): 763 nm. EPR (CH2Cl2, 298 K): g ) 2.0025 ( 0.0005.
tert-Butylisonitrile Adduct of 4a: [Cp((CH3)3CNC)RuCO(3′-
(2-diphenylphosphinoethyl)-5,5′′-dimethyl-2,2′:5′,2′′-terthiophene)]-
[B(C6H3-3,5-(CF3)2)4] (9). This complex was prepared in quantitative
spectroscopic yield by mixing 4a (15 mg, 9.7 × 10-3 mmol) and 5
equiv of (CH3)3CNC (5.5 µL) in 0.7 mL of CD2Cl2 at room temperature
for 12 d. Heating the sample to 100 °C in vacuo (<50 mTorr) for 5 d
temperature for 3 d yielded about 9% product conversion. Heating the
sample to 50 °C for 13 d yielded the desired product in 94%
1
spectroscopic yield. H NMR (CD2Cl2): δ 7.73 (br, 8 H, Ar′); 7.56
(br, 4 H, Ar′); 7.49-7.40 (m, 10 H, Ph); 6.96 (d, J3-4 ) 3.6, 1 H, 3 or
3′′ thienyl); 6.81 (s, 1 H, 4′ thienyl); 6.73 (d, J3-4 ) 3.6, 1 H, 3 or 3′′
thienyl); 6.69 (2 overlapping dd, 2 H, 4 and 4′′ thienyl); 5.36 (s, 5 H,
Cp); 2.83-2.73 (m, 4 H, CH2CH2); 2.47 (d, JMe-4 ) 0.6, 3 H, CH3);
2.46 (d, JMe-4 ) 1.2, 3 H, CH3). 31P{1H} NMR (CD2Cl2): δ 38.9 (s).
IR (CH2Cl2): νCO ) 2021, 2066 cm-1. UV-vis λmax (CH2Cl2): 348
nm. MS (ES) [M+]: calcd for C35H30O2PRuS3, m/z 711.0; found m/z
711.0.
Ethylene Adduct of 4a: [Cp(C2H4)RuCO(3′-(2-diphenyl-
phosphinoethyl)-5,5′′-dimethyl-2,2′:5′,2′′-terthiophene)][B(C6H3-
3,5-(CF3)2)4] (12). The addition of ethylene to an NMR tube containing
15 mg (9.7 × 10-3 mmol) of 4a in 0.7 mL of CD2Cl2 at room
temperature for 3 d yielded a minor amount of product. Heating the
sample to 50 °C for 22 d results in a final spectroscopic yield of
1
does not result in the re-formation of 4a. H NMR (CD2Cl2): δ 7.71
(br, 8 H, Ar′); 7.56 (br, 4 H, Ar′); 7.54-7.38 (m, 10 H, Ph); 6.96 (d,
J3-4 ) 3.3, 1 H, 3 or 3′′ thienyl); 6.83 (s, 1 H, 4′ thienyl); 6.72 (d, J3-4
) 3.3, 1 H, 3 or 3′′ thienyl); 6.68 (2 overlapping dd, 2 H, 4 and 4′′
thienyl); 5.12 (s, 5H, Cp); 2.76-2.70 (m, 4 H, CH2CH2); 2.48 (d, JMe-4
) 1.2, 3 H, CH3); 2.45 (d, JMe-4 ) 1.2, 3 H, CH3); 1.29 (s, 9H, CNC-
(CH3)3). 31P{1H} NMR (CD2Cl2): δ 42.7 (s). IR (CH2Cl2): νCO ) 2003
cm-1, νCN ) 2176 cm-1. UV-vis λmax (CH2Cl2): 354 nm. MS (ES)
[M+]: calcd for C39H39NOPRuS3, m/z 766; found, m/z 766.0.
Acetone-d6 Adduct of 4a: [Cp(CD3C(O)CD3)RuCO(3′-(2-di-
phenylphosphinoethyl)-5,5′′-dimethyl-2,2′:5′,2′′-terthiophene)][B-
(C6H3-3,5-(CF3)2)4] (10). This adduct can be generated by the addition
of 0.7 mL of acetone-d6 to 60 mg of 4a. 1H and 31P{1H} NMR
spectroscopies indicate approximately 9% conversion to the acetone
adduct. Due to the low concentration of 10 relative to that of 4a, the
full 1H NMR spectrum cannot be reported. However, the 1H NMR Cp
(δ 5.33) and 31P{1H} NMR resonances (δ 46.3) are distinguishable
from those of 4a in acetone (δ 5.27 and 41.5, respectively). The
assignment of 4a and 10 in acetone was made by examining the
variable-temperature 31P{1H} NMR spectra (see Supporting Informa-
tion). The resonance assigned to 4a at room temperature splits into
two separate resonances (δ 53.3 and 35.1) at -80 °C due to pyramidal
inversion of the bound S atom, which yields diastereomers with different
chemical shifts. In contrast, the resonance assigned to 10 remains sharp
and of constant intensity from room temperature to -80 °C. Removal
of acetone-d6 in vacuo quantitatively removes the bound acetone to
re-form 4a. It should be noted that the addition of acetone (100 equiv)
to a CD2Cl2 solution of 4a does not result in the formation of 10.
Confirmation of adduct formation was obtained by mass spectrometry
of 4a in acetone. However, the mode of acetone coordination has not
been determined. MS (ES) [M+]: calcd for C38H36O2PRuS3, m/z 741;
found, m/z 741.
1
93%. H NMR (CD2Cl2): δ 7.73 (br, 8 H, Ar′); 7.56 (br, 4 H, Ar′);
7.50-7.20 (m, 10 H, Ph); 6.96 (d, J3-4 ) 3.3, 1 H, 3 or 3′′ thienyl);
6.81 (s, 1 H, 4′ thienyl); 6.74 (d, J3-4 ) 3.5, 1 H, 3 or 3′′ thienyl); 6.71
(dd, J4-Me ) 0.9, J3-4 ) 3.6, 1 H, 4 or 4′′ thienyl); 6.69 (dd, J4-Me
)
0.9, J3-4 ) 3.6, 1 H, 4 or 4′′ thienyl); 5.21 (s, 5H, Cp); 3.15-3.08 (m,
4H, bound ethylene); 2.65 (s (br), 4 H, CH2CH2); 2.47 (s, 6 H, CH3).
31P{1H} NMR (CD2Cl2): δ 43.1 (s). IR (CH2Cl2): νCO ) 2010 cm-1
;
UV-vis λmax (CH2Cl2): 348 nm. MS (ES) [M+]: calcd for C36H34-
OPRuS3, m/z 711.1; found, m/z 711.0.
Thiophene Adduct of 4a: [Cp(C4H4S)RuCO(3′-(2-diphenyl-
phosphinoethyl)-5,5′′-dimethyl-2,2′:5′,2′′-terthiophene)][B(C6H3-
3,5-(CF3)2)4] (13). The addition of 1 equiv of thiophene (0.78 µL) to
an NMR tube containing 15 mg (9.7 × 10-3 mmol) of 4a in 0.7 mL of
CD2Cl2 did not yield any thiophene adduct after 3 d at room temperature
or with mild heating. The CD2Cl2 was removed, and acetone-d6 was
added. Similarly, no reaction occurred after 3 d at room temperature
or with mild heating. Dissolving 4a in neat thiophene (10 mL) at room
temperature for 1 d leads to formation of a thiophene adduct. Removal
of excess thiophene in vacuo and dissolution of the resulting product
in CD2Cl2 allows for characterization of the resulting thiophene adduct,
formed in 1:1.1 ratio to 4a. 1H NMR (CD2Cl2): δ 4.75 (s, Cp). 31P{1H}
NMR (CD2Cl2): δ 39.4. MS (ES) [M+]: calcd for C38H34OPRuS4, m/z
767; found, m/z 767.
Acknowledgment. We acknowledge the NSF (CHE-
0071885) for generously funding this research.
Supporting Information Available: Tables of crystal data,
structure solution and refinement, atomic coordinates, bond
lengths and angles, and anisotropic thermal parameters for 2a,
4a, 4b, and 6; spectroscopic data for 1a-5a, 1b-4b; EPR
spectra for 4a+, 5a+, 7+, and 8+ (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
CO Adduct of 4a: [Cp(CO)RuCO(3′-(2-diphenylphosphino-
ethyl)-5,5′′-dimethyl-2,2′:5′,2′′-terthiophene)][B(C6H3-3,5-(CF3)2)4]
(11). The addition of CO to an NMR tube containing 15 mg (9.7 ×
10-3 mmol) of 4a in 0.7 mL of CD2Cl2 at room temperature or at 50
°C for 3 d yielded no product. Removal of the solvent, addition of
acetone-d6 (0.6 mL), recharging the tube with CO, and shaking at room
JA0030008