Reversible, electrochemically controlled binding of phosphine to iron and cobalt bis(dithiolene) complexes

Article abstract of DOI:10.1021/ic0624569

The homoleptic bis(dithiolene) complexes [M(S₂C ₂R₂)₂]₂ (M = Fe, Co; R = p-anisyl) undergo two successive reductions to form anions that display [M(S ₂C₂R₂)₂]₂^2- ? 2[M(S₂C₂R₂)₂]^1- solution equilibria. The neutral dimers react with Ph₃P to form square pyramidal [M(Ph₃P)(S₂C₂R ₂)₂]⁰. Voltammetric measurements upon [M(Ph₃P)(S₂C₂R₂)₂] ⁰ in CH₂Cl₂ reveal only irreversible features at negative potentials, consistent with Ph₃P dissociation upon reduction. Dissociation and reassociation of Ph₃P from and to [Fe(Ph₃P)-(S₂C₂R₂)₂] ⁰ is demonstrated by spectroelectrochemical measurements. These collective observations form the basis for a cycle of reversible, electrochemically controlled binding of Ph₃P to [M(S ₂C₂R₂)₂]₂ (M = Fe, Co; R = p-anisyl). All members of the cycle ([M(S₂C₂R ₂)₂]₂⁰, [M(S₂C ₂R₂)₂]₂^1-, [M(S ₂C₂R₂)₂]₂^2-, [M(S₂C₂R₂)₂]^1-, [M(Ph₃P)(S₂C₂R₂)₂]) for M = Fe, Co have been characterized by crystallography. Square planar [Fe(S ₂C₂R₂)₂]^1- is the first such iron dithiolene species to be structurally identified and reveals Fe-S bond distances of 2.172(1) and 2.179(1) A, which are appreciably shorter than those in corresponding square planar dianions.

Full text of DOI:10.1021/ic0624569

Inorg. Chem. 2007, 46, 5131−5133
Reversible, Electrochemically Controlled Binding of Phosphine to Iron
and Cobalt Bis(dithiolene) Complexes
Rongmin Yu, Kuppuswamy Arumugam, Ashok Manepalli, Yvon Tran, Russell Schmehl,
Heiko Jacobsen, and James P. Donahue*
Department of Chemistry, Tulane UniVersity, 6400 Freret Street,
New Orleans, Louisiana 70118-5698
Received December 21, 2006
The homoleptic bis(dithiolene) complexes [M(S2C2R2)2]2 (M
Co; R p-anisyl) undergo two successive reductions to form
anions that display [M(S2C2R2)2]2
)
Fe,
electrochemically controlled cycle for olefin binding and
release. This intriguing observation suggests the idea that
other homoleptic metal dithiolene complexes could display
analogous possibilities for reversible, electrochemically
controlled ligand binding. We report here our findings that
both iron and cobalt bis(dithiolene) complexes display such
behavior with square pyramidal phosphine adducts.
)
2-
-
T
2[M(S2C2R2)2]1 solution
equilibria. The neutral dimers react with Ph3P to form square
pyramidal [M(Ph3P)(S2C2R2)2]0. Voltammetric measurements upon
[M(Ph3P)(S2C2R2)2]0 in CH2Cl2 reveal only irreversible features at
negative potentials, consistent with Ph3P dissociation upon reduc-
tion. Dissociation and reassociation of Ph3P from and to [Fe(Ph3P)-
(S2C2R2)2]0 is demonstrated by spectroelectrochemical measure-
ments. These collective observations form the basis for a cycle of
reversible, electrochemically controlled binding of Ph3P to
Using the P4S10/acyloin method first devised by Schrauzer,4
the neutral p-anisyl-substituted compounds [M(S2C2(C6H4-
p-OCH3)2)2]2 (M ) Fe (1a), Co (1b), Figure 1) were
synthesized as microcrystalline solids that were readily
prepared as diffraction-quality single crystals by vapor
diffusion techniques. These neutral dimers readily undergo
[M(S2C2R2)2]2 (M
cycle ([M(S2C2R2)2]20, [M(S2C2R2)2]21-, [M(S2C2R2)2]22-, [M(S2C2R2)2]1-
[M(Ph3P)(S2C2R2)2]) for M Fe, Co have been characterized by
crystallography. Square planar [Fe(S2C2R2)2]1 is the first such
iron dithiolene species to be structurally identified and reveals Fe
) Fe, Co; R ) p-anisyl). All members of the
-
,
one-electron reduction by BH4 or Cp2Co to afford the
)
corresponding dimeric monoanions (2a and 2b). A second
one-electron reduction reveals the onset of solution equilibria
between dimeric dianions (3a, 3b) and monomeric monan-
ions (4a, 4b). The principal difference between the two
metals at this stage of reduction is that the cobalt system
displays greater access to its monomeric monoanion 4b in
the 3b T 4b equilibrium. The neutral dimers 1a and 1b are
readily cleaved by strong-field σ-donor ligands, such as Ph3P,
to form square pyramidal adducts 5a and 5b.
-
−S
bond distances of 2.172(1) and 2.179(1) Å, which are appreciably
shorter than those in corresponding square planar dianions.
The rich photo- and electrochemical activity that have long
been associated with transition metal dithiolene complexes
continue to stimulate a high degree of interest in their
applications, which include, to name a select few, their use
as conducting or magnetic materials1 and as sensing devices.2
Recently, Wang and Stiefel proposed an innovative applica-
tion for nickel bis(dithiolene) complexes in the selective
separation of olefins from hydrocarbon mixtures.3 Charge-
neutral nickel bis(dithiolene) complexes react with olefins
to form S,S-dialkylated adducts wherein the olefin has added
across the adjacent sulfur atoms of two different dithiolene
ligands. Wang and Stiefel noted that a one-electron reduction
released the olefin and generated a nickel bis(dithiolene)
monoanion, thereby forming the basis for a reversible,
Each of the iron dithiolene complexes (1a-5a) and cobalt
dithiolene complexes (1b-5b) has been authenticated by
X-ray crystallography at 100 K. Figure 1 presents thermal
ellipsoid plots at the 50% probability level for the iron
compounds 1a-5a; the corresponding cobalt compounds
1b-5b have similar structures, although they crystallized
with different unit cells and cell parameters. The use of a
constant dithiolene ligand throughout this set of structural
studies removes uncertainty about the significance of sub-
stituent effects in modulating the dithiolene ligand structural
parameters.
Recent structural and spectroscopic work reported by
Wieghardt and co-workers with iron complexes bearing the
benzene-1,2-dithiolate and 3,5-di-tert-butylbenzene-1,2-dithi-
* To whom correspondence should be addressed. E-mail: donahue@
tulane.edu.
(1) Faulmann, C.; Cassoux, P. Prog. Inorg. Chem. 2004, 52, 399.
(2) Pilato, R. S.; Van Houten, K. A. Prog. Inorg. Chem. 2004, 52, 369.
(3) Wang, K.; Stiefel, E. I. Science 2001, 291, 106.
(4) Schrauzer, G. N.; Mayweg, V. P.; Finck, H. W.; Heinrich, W. J. Am.
Chem. Soc. 1966, 88, 4604.
10.1021/ic0624569 CCC: $37.00
Published on Web 05/19/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 13, 2007 5131

COMMUNICATION
Figure 1. Electrochemical cycle for reversible binding of PR′3 ligand to Fe and CO bis(dithiolene) complexes. R ) p-anisyl, R′ ) Ph, and M ) Fe
(1a-5a) or Co (1b-5b). The 50% probability thermal ellipsoid plots are for the Fe compounds. The Co complexes are homologous to the corresponding
Fe compounds.
olate ligands5 and with the p-tolyl- and p-tert-butylphenyl-
substituted ligands6 has brought considerable clarity to the
description of bonding and redox processes in these com-
plexes. One of the salient contributions of their work is the
recognition that the redox processes [Fe(S2C2R2)2]2 + e- f
[Fe(S2C2R2)2]21- and [Fe(S2C2R2)2]21- + e- f [Fe(S2C2R2)2]22-
are ligand-based events involving conversion of thienyl
radical monoanions [•S-CdC-S- T SdC-C(•)-S-] to
fully reduced ene-1,2-dithiolate dianions (-S-CdC-S-).
The charge-neutral [Fe(S2C2R2)2]2 compounds are therefore
comprised of ferric ions, which are both coordinated by one
reduced ene-dithiolate dianion and one thienyl radical
monoanion.
Our structural characterization of 1a-5a and 1b-5b
corroborate and add to Wieghardt’s interpretation of the
electronic structure and redox processes of iron bis(dithi-
olene) complexes. Compound 1a is only the second charge-
neutral iron bis(dithiolene) dimer to be structurally charac-
terized, and it was found to crystallize on an inversion center
from a variety of solvent systems. The C-S and C-C bond
lengths of the dithiolene ligands of 1a clearly reveal a
different state of reduction for the terminal chelating ligand
in contrast to the ligand involved in the bridging interaction.
The S(1)-C(1) (1.708(3) Å) and S(2)-C(2) (1.707(3) Å)
bond distances are rather short and imply the participation
of the thione resonance structure. These values contrast with
the S(3)-C(3) (1.744(3)) and S(4)-C(4) (1.767(3)) bond
distances, which are more typical of pure C-S single bonds.
Furthermore, the C(1)-C(2) bond length (1.397(4) Å) is
somewhat longer than the C(3)-C(4) interatomic distance
(1.371(4) Å), which indicates partial single-bond character
according to resonance structure (b) in Figure 2. Thus, the
terminal dithiolene ligand S(1)-C(1)-C(2)-S(2) in 1a is
partially oxidized as a thienyl radical monoanion, while the
bridging dithiolene ligand S(3)-C(3)-C(4)-S(4) is a fully
reduced ene-1,2-dithiolate. In experimental hindsight, it is
plausible that the fully reduced, more electron rich dithiolene
would be preferred for bridging interactions between the
Lewis acidic Fe(III) centers.
Compound 2a crystallizes on a general position in P2/c
with C-S and C-C bond lengths that point toward terminal
dithiolene ligand S(7)-C(7)-C(8)-S(8) as the sole dithi-
olene ligand with thienyl radical monoanion character. The
C-S and C-C interatomic distances in 3a are fully
consistent with single and double bonds, respectively, as
expected for all dithiolene ligands in a fully reduced state.
The C-S and C-C bond lengths within the cobalt dimers
1b-3b are largely similar to those in 1a-3a except that 2b,
possibly because of a static disorder, reveals no clear
difference between its two terminal dithiolene ligands.
In the course of efforts to prepare diffraction-quality
crystals of dimeric dianion 3a, the mixed salt [Cp2Co]2a‚
[Cp2Co]4a (i.e., [Cp2Co][Fe(S2C2(C6H4-p-OCH3)2)2]2‚
[Cp2Co][Fe(S2C2(C6H4-p-OCH3)2)2]) was synthesized in which
the square planar monoanion 4a is in the same state of
reduction as dimeric 3a. Despite its formulation as a 1:1
mixture with 2a, this structural identification is the first for
a mononuclear iron bis(dithiolene) monoanion. The existence
of 4a indicates that, with the relatively electron rich p-anisyl-
substituted dithiolene ligand, the bridging interactions that
form dimeric 3a are weak enough to be disrupted by crystal
packing effects. The Fe-S bond lengths in 4a (2.172(1),
2.179(1) Å) are appreciably shorter than those in its
isoelectronic dimer 3a (2.2110(5), 2.2492(5) Å) and thus
provide an insight into the intrinsic Fe(III)-S(thiolate) bond
lengths in a square planar field without the strongly perturb-
(5) Ray, K.; Bill, E.; Weyhermu¨ller, T.; Wieghardt, K. J. Am. Chem. Soc.
2005, 127, 5641.
(6) Patra, A. K.; Bill, E.; Weyhermu¨ller, T.; Stobie, K.; Bell, Z.; Ward,
M. D.; McCleverty, J. A.; Wieghardt, K. Inorg. Chem. 2006, 45, 6541.
Figure 2. Possible oxidation states of a dithiolene ligand.
5132 Inorganic Chemistry, Vol. 46, No. 13, 2007

COMMUNICATION
ing effect of additional axial ligation. The Fe-S bond lengths
of 4a are also markedly shorter than those of the square
planar dianions [Fe(S2C6H3(o-CO2CH3))2]2- (2.2055(8)-
2.2072(8) Å)7 and [Fe(S2C6H4)2]2- (2.2242(3)-2.2263(3)
Å).8 This fact confirms that the reductions that form these
latter two species from the corresponding dimeric dianions
are metal-based processes (Fe(III) f Fe(II)).
McCleverty has reported voltammetric measurements for
5a and 5b, apparently in DMF, and described the existence
of one (5a) or two (5b) reversible reductions, upon scanning
to negative potentials.9 Our own measurements, conducted
in CH2Cl2 solution display only irreversible waves upon
scanning to reducing potentials, which is consistent with the
dissociation of axial Ph3P ligand and hence “destruction” of
compounds 5a and 5b. A similar behavior was very recently
noted by Wieghardt and McCleverty with [Fe(P(OPh)3)(S2C2-
Ph2)2].10
To more clearly demonstrate the reversibility of dissocia-
tion of Ph3P from 5a, a spectroelectrochemical controlled
potential coulometry experiment was undertaken. The UV-
vis absorption spectrum of 5a displays a strong, distinctive
absorption at 720 nm, which is attributed to an interligand
intervalence charge-transfer transition and is completely
absent in the reduced homoleptic iron bis(dithiolene) com-
plexes. Using an electrochemical cell fitted with a Pt screen
working electrode and with quartz windows for concomitant
spectroscopic interrogation, a solution sample of 5a was
poised at a reducing potential of -1.0 V vs Ag/AgCl while
the 325-800 nm region of the spectrum was monitored using
a diode array light source. Over a period of minutes (Figure
3A), the one-electron reduction of 5a transformed it into 3a,
as identified by its absorption spectrum. The absence of tight
isosbestic points is due to the 3a-4a solution equilibrium.
When disappearance of 5a was complete, bulk electrolytic
oxidation of the same sample was then done by switching
the working electrode to a +1.0 V potential vs Ag/AgCl.
As electrons were removed from 3a to transiently produce
1a, reaction of free Ph3P immediately formed 5a again
(Figure 3B). Over a period of minutes, 5a was completely
restored.
Figure 3. (A) Spectroscopic monitoring of the controlled potential (-1.0
V vs Ag/AgCl) coulometric reduction of 5a in CH2Cl2, which reveals its
conversion to 3a. The absence of clean isobestic points is due to the 3a T
4a equilibrium. (B) Re-formation of 5a from 3a and 2Ph3P by controlled
potential coulometric oxidation at +1.0 V.
(S2C2H2)2]0 results in partial or full occupancy of this orbital,
thereby rendering the system unavailable for interactions with
electron-pair donor ligands, be they phosphines or other types
of σ-donors.
The foregoing observations prove the feasibility of a fully
reversible, electrochemically controlled cycle for binding and
release of Ph3P to homoleptic iron and cobalt bis(dithiolene)
complexes. We are currently exploring the generality of this
behavior with other types of Lewis bases, in particular
heterocyclic imines and cyclic sulfides, and are studying
whether variation of the dithiolene ligand substituent can
afford a basis for selectivity for the type of Lewis base
adduct.
Acknowledgment. J.P.D. thanks Tulane University for
start-up funding support and the Petroleum Research Fund
(Award No. 545884G1) for financial support of this research.
A.M. thanks Tulane University for the support of a Georges
Lurcy Grant for undergraduate research. The late Professor
F. A. Cotton of Texas A&M is gratefully acknowledged for
granting access to his research facilities during fall 2005.
Prof. L. Cavallo is recognized for providing access to
computing facilities at Universita` di Salerno.
To obtain further insight into the basis for reversible
binding of Ph3P, we undertook BP86 density functional
calculations of suitable iron model complexes. The square
planar charge-neutral model compound [Fe(S2C2H2)2]0 has
an active state in energetic proximity to the ground state,
2
which is characterized by a LUMO (2ag) of dz character.
This orbital is favorably oriented for accepting an electron
lone pair from an exogenous donor. Reduction of [Fe-
Supporting Information Available: X-ray data for 1a-5a and
1b-5b in standard CIF format; thermal ellipsoid plots at the 50%
probability level for 1a-5a with complete atom labeling in PDF
format; a description of computational details and synthetic
procedures in PDF format. This material is available free of charge
via the Internet at http://pubs.acs.org.
(7) Sellman, D.; Becker, T.; Knoch, F. Chem. Ber. 1996, 129, 509.
(8) Ray, K.; Begum, A.; Weyhermu¨ller, T.; Piligkos, S.; van Slageren,
J.; Neese, F.; Wieghardt, K. J. Am. Chem. Soc. 2005, 127, 4403.
(9) McCleverty, J. A.; Ratcliff, B. J. Chem. Soc. A 1970, 1631.
(10) Patra, A. K.; Bill, E.; Bothe, E.; Chlopek, K.; Neese, F.; Weyhermu¨ller,
T.; Stobie, K.; Ward, M. D.; McCleverty, J. A.; Wieghardt, K. Inorg.
Chem. 2006, 45, 7877.
IC0624569
Inorganic Chemistry, Vol. 46, No. 13, 2007 5133