Redox-regulated ethylene binding to a rhenium-thiolate complex

Full text of DOI:10.1021/ja8086483

Published on Web 12/12/2008
Redox-Regulated Ethylene Binding to a Rhenium-Thiolate Complex
Craig A. Grapperhaus,* Kagna Ouch, and Mark S. Mashuta
Department of Chemistry, UniVersity of LouisVille, LouisVille, Kentucky 40292
Received November 4, 2008; E-mail: grapperhaus@louisville.edu
In 2001 Wang and Stiefel proposed metal-assisted separation of
ethylene from feedstock utilizing electrochemically regulated C-S
bond formation/cleavage as an alternative to current energy intensive
methods.1 While initial results were promising, two major obstacles
were revealed in further studies.2,3 First, the major products of
alkene addition are the deleterious intraligand product dihydrodithiin
and complex degradation (the desired interligand addition is only
a minor product).2 Second, the reduction potential required for
ethylene release is significantly more negative than originally
anticipated making electrochemical release energy-intensive.3 Herein
we report a system that overcomes these obstacles with rapid and
reversible C-S bond formation/cleavage in a redox-regulated
process.
and 420 mV (E1ox2) versus ferrocene. Repeating the measurements
in the presence of ethylene revealed two additional events for
complex 2 at -100 mV (E2ox2) and -1010 mV (E2ox1) with currents
that are scan rate dependent, Figure S1. The scan rate dependence
was simulated using the DigiSim software package as described in
the Supporting Information.7,8 Average rate constant values for C-S
+
bond formation between [1ox1
]
and ethylene, kf, and C-S bond
cleavage for [2ox1]+, kr, were determined as (1.2 ( 0.2) × 10-1
M-1 s-1 and (3.0 ( 0.4) × 10-2 s-1, respectively. From these, K2
was calculated as 4.0 ( 0.8 M-1. From the four redox potentials
and the equilibrium constant K2, values for K1 and K3 were
determined as (1.9 ( 0.4) × 10-11 M-1 and (2.5 ( 0.9) × 109
M-1, respectively.
Previously we reported irreversible C-S bond formation between
Ru(DPPBT)3 (DPPBT ) 2-diphenylphosphino-benzenethiolate) and
alkenes (and selected ketones) upon oxidation.4,5 Once formed, the
C-S bond was stable in all accessible metal oxidation states
precluding C-S bond cleavage. In the current study, we report
reversible C-S bond formation/cleavage between ethylene and the
rhenium analogue [Re(DPPBT)3] (1) and its oxidized derivatives,
Scheme 1. The equilibrium constants for ethylene binding are
The large differences in equilibrium constants can be exploited
in solution studies. In the most reduced form, 1 and 2, equilibrium
strongly favors 1. As such, no spectral changes are observed upon
ethylene addition to solutions of 1. Likewise, solutions of 2
generated by reduction of [2ox1]+ or [2ox2
]
2+ result in rapid ethylene
+
release. In contrast, the one electron oxidized complexes [1ox1
]
and [2ox1]+ exist in equilibrium as a function of ethylene concentra-
tion. A slow ethylene purge through fresh solutions of [1ox1]+ rapidly
initiates C-S bond formation to yield [2ox1]+. As shown in Figure
+
Scheme 1. Square Representation of 1, 2, and Oxidized
Derivatives
1, the absorbance at 581 associated with [1ox1
]
rapidly decreases
Figure 1. Electronic spectra recorded during slow ethylene purge through
+
a solution of [1ox1
] at room temperature. Data collected every 30 s for 3
min and after 10 min (red trace after dilution to original volume). The
relative absorbance at 581 nm following alternating 10 min purges of
ethylene (E) and N2 (N) is shown in the inset.
oxidation state dependent with 1 strongly favoring C-S bond
cleavage while oxidation promotes C-S bond formation such that
K3 > K2 > K1.
The equilibrium constants K1, K2, and K3 were evaluated by cyclic
voltammetric methods in ethylene saturated solutions. The synthesis
of 1 was previously reported by Dilworth et al., and two reversible
one electron oxidations were noted.6 Cyclic voltammograms
to approximately one-half of its initial value within 2.5 min with
no further significant changes thereafter. Assuming ethylene satura-
tion (0.4642 M in CH2Cl2),9 K2 is estimated as 2.4 ( 0.1 M-1
consistent with its value from voltammetry studies. The C-S bonds
+
of [2ox1
]
can be easily cleaved by nitrogen purge within 10 min
restoring the initial spectrum of [1ox1]+. The C-S bond formation/
cleavage cycle was repeated 7 times with no significant changes in
efficiency, Figure 1 (inset).
recorded under nitrogen reveal these two events at -340 mV (E1ox1
)
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64 J. AM. CHEM. SOC. 2009, 131, 64–65
10.1021/ja8086483 CCC: $40.75
2009 American Chemical Society

C O M M U N I C A T I O N S
2+
The large value of K3 indicates a stable C-S bond in [2ox2
]
.
“locked on” and “locked off” modes, multiple cycles could not be
realized. In fact, even C-S bond cleavage has been challenging.
Recently reported studies with mixed-ligand Mo-dithiolenes yield
partial ethylene release after reflux in chloroform for 21 h.22
Equilibrium binding of alkenes, including ethylene at low temper-
atures, to ReS4- has been noted by Goodman and Rauchfuss.23 In
our current system, the kinetics of C-S bond formation/cleavage
are much faster than those of metal-dithiolenes and allow facile
trap and release of ethylene over the period of several minutes at
258 K. Thermodynamic control coupled with rapid C-S bond
formation/cleavage are ideal for metal-assisted ethylene separation
as first proposed by Wang and Stiefel. Yet, our system is not subject
to the complications that hindered their original system.
2+
Solutions of [2ox2
]
were prepared by bulk oxidation of 1 under
an ethylene atmosphere via an ECE mechanism. In situ monitoring
+
of the electronic spectra, Figure S2, reveals oxidation of 1 to [1ox1
]
in the initial electrochemical step. In the chemical step [1ox1]+ adds
2+
ethylene yielding [2ox1]+, which is subsequently oxidized to [2ox2
]
to complete the ECE process. Once formed, the C-S bond cannot
be cleaved in this oxidation state. No significant changes in the
2+
UV-visible spectrum of [2ox2
]
are observed upon prolonged
standing at room temperature, N2 purging, or exposure to vacuum
via repeated cycles of freeze-pump-thaw. C-S bond cleavage is
only facilitated by reduction to [2ox1]+ or 2, which rapidly releases
ethylene to give 1.
In addition to electrochemical methods, [2ox2
2+
]
was prepared
Acknowledgment. The authors thank the Donors of the
American Chemical Society Petroleum Research Fund (43917-AC3)
for support. M.S.M. thanks the Kentucky Research Challenge Trust
Fund for upgrade of our X-ray facilities.
by chemical oxidation (2.0 equiv of AgPF6) of 1 in ethylene
saturated solution. The structure of [2ox2][PF6]2 has been determined
by single-crystal X-ray techniques with details provided in the
Supporting Information.10-17 An ORTEP view of [2ox2
]
is
2+
presented in Figure 2 with selected bond distances and angles
Supporting Information Available: Experimental procedures,
voltammetry fitting parameters, cyclic voltammograms, and UV-visible
spectra in PDF format. X-ray structural data in CIF format. This material
is available free of charge via the Internet at http://pubs.acs.org.
provided in the figure caption.18
References
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Figure 2. ORTEP representation of [2ox2]
2+. Selected bond distances (Å):
Re(1)-S(1) 2.209(3); Re(1)-S(2) 2.435(3); Re(1)-S(3) 2.433(3); Re(1)-P(1)
2.420(3); Re(1)-P(2) 2.456(3); Re(1)-P(3) 2.465(3); C(55)–C(56) 1.499(14);
Selected bond angles (deg): S(1)-Re(1)-S(3) 170.00(9); P(1)-Re(1)-S(2)
174.01(9); P(2)-Re(1)-P(3) 162.68(9); S(3)-Re(1)-S(2) 84.37(10).
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(17) Crystal data for [2ox2][PF6]2: orange plate, monoclinic, space group C2/c,
a ) 29.009(18), b ) 22.577(18) Å, c ) 43.99(3) Å, ꢀ ) 96.182(17)°, V
) 28 643(35) Å3, Fcalcd ) 1.413 g/cm3, Z ) 8. Data were collected on a
Bruker SMART APEX CCD using Mo ΚR radiation. For all 25 546 unique
reflections (R(int) ) 0.073), the final anisotropic full-matrix least-squares
refinement on F2 for 822 variables converged at R1 ) 0.125, wR2 ) 0.184
with a GOF of 1.09. CCDC-699464 contains the supplementary crystal-
lographic data for this paper. Data can be obtained free of charge from
The Cambridge Crystallographic Data Center via www.ccdc.cam.ac.uk/
data_request.cif.
The stability of the C-S bonds as a function of oxidation state
was previously noted for Re (and Tc) complexes of 1,4,7-
trithiacylcononane.19 Theoretical investigations by Rothlisberger
related this effect to π-donation from a metal “t2g” orbital to a C-S
σ*.20,21 In our system, we propose similar changes in bonding are
responsible for the remarkably large changes in K as a function of
oxidation state and complex charge.
In summary, through the facile control of oxidation state by
electrochemical or chemical methods, ethylene can be in a “locked
on”, “locked off”, or in dynamic equilibrium with 1 or its oxidized
derivatives. The system is robust with no apparent changes in
efficiency after several cycles of C-S bond formation/cleavage.
Although metal-dithiolenes were previously reported to display
(18) Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565.
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2007, 46, 7644–7647.
(23) Goodman, J. T.; Rauchfuss, T. B. J. Am. Chem. Soc. 1999, 121, 5017–5022.
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