Synthesis of carboxylic acid-modified [FeFe]-hydrogenase model complexes amenable to surface immobilization

Article abstract of DOI:10.1021/om7003354

Model complexes of [FeFe]-hydrogenase bearing carboxylic acid functionalities have been designed for applications toward immobilization of hydrogen production electrocatalysts on amino-functionalized carbon electrode surfaces. Using carboxylic acid-substituted thiols, complexes incorporating the -COOH moiety into the thiolate linkers have been synthesized: (μ-SCH ₂CH₂COOH)₂[Fe(CO)₃]₂ (1) and the previously known (μ-(SCH₂)₂CHCOOH) [Fe(CO) ₃]₂ (2). Carboxylic acid units have also been introduced via ligand substitution with a tricarboxyethyl phosphine to generate (μ-pdt)[Fe(CO)₃] [Fe(CO)₂-{P(C₂H ₄COOH)₃}] (3). To mimic the linkage of complexes of these types to amino-functionalized monolayers, it has been demonstrated that 2 can be coupled with aniline in solution to generate (μ-(SCH₂) ₂-CHCONHPh)[Fe(CO)₃]₂ (4), and the stability of this linkage has been addressed. The carboxylic acid-substituted complex 2 undergoes ligand substitution with PMe₃ to generate (μ-(SCH ₂)₂CHCOOH)[Fe(CO)₂(PMe₃)] ₂ (5). Complex 5 can then be protonated in either methanol or tetrahydrofuran solvent to generate (μ-(SCH₂)₂CHCOOMe) (μ-H)[Fe(CO)₂(PMe₃)]₂}PF₆ (6) and (μ-(SCH₂)₂CHCOOH)(μ-H)[Fe(CO)₂(PMe ₃)]₂}PF₆ (7), respectively, demonstrating that a range of carboxy-functionalized complexes can be synthesized. Structural characterization and cyclic voltammetry of these complexes indicate that the carboxylic acid functionality has little effect on the structure and reactivity of the diiron dithiolate core.

Full text of DOI:10.1021/om7003354

3976
Organometallics 2007, 26, 3976-3984
Synthesis of Carboxylic Acid-Modified [FeFe]-Hydrogenase Model
Complexes Amenable to Surface Immobilization
Christine M. Thomas, Olaf Ru¨diger, Tianbiao Liu, Cody E. Carson, Michael B. Hall, and
Marcetta Y. Darensbourg*
Department of Chemistry, Texas A&M UniVersity, College Station, Texas 77843
ReceiVed April 5, 2007
Model complexes of [FeFe]-hydrogenase bearing carboxylic acid functionalities have been designed
for applications toward immobilization of hydrogen production electrocatalysts on amino-functionalized
carbon electrode surfaces. Using carboxylic acid-substituted thiols, complexes incorporating the -COOH
moiety into the thiolate linkers have been synthesized: (µ-SCH2CH2COOH)2[Fe(CO)3]2 (1) and the
previously known (µ-(SCH2)2CHCOOH)[Fe(CO)3]2 (2). Carboxylic acid units have also been introduced
via ligand substitution with a tricarboxyethyl phosphine to generate (µ-pdt)[Fe(CO)3][Fe(CO)2{P(C2H4-
COOH)3}] (3). To mimic the linkage of complexes of these types to amino-functionalized monolayers,
it has been demonstrated that 2 can be coupled with aniline in solution to generate (µ-(SCH2)2-
CHCONHPh)[Fe(CO)3]2 (4), and the stability of this linkage has been addressed. The carboxylic acid-
substituted complex 2 undergoes ligand substitution with PMe3 to generate (µ-(SCH2)2CHCOOH)[Fe-
(CO)2(PMe3)]2 (5). Complex 5 can then be protonated in either methanol or tetrahydrofuran solvent to
generate (µ-(SCH2)2CHCOOMe)(µ-H)[Fe(CO)2(PMe3)]2}PF6 (6) and (µ-(SCH2)2CHCOOH)(µ-H)[Fe(CO)2-
(PMe3)]2}PF6 (7), respectively, demonstrating that a range of carboxy-functionalized complexes can be
synthesized. Structural characterization and cyclic voltammetry of these complexes indicate that the
carboxylic acid functionality has little effect on the structure and reactivity of the diiron dithiolate core.
based on the well-known organometallics, (µ-SR)2[Fe(CO)2L]2,6
feature two dithiolate-bridged FeI centers and as many as six
carbonyl ligands. Carbonyl substitution chemistry and variation
of the dithiolate linkers has led to a large number of reported
[FeFe]-H2ase model complexes in recent years.7 Various Fe2S2
complexes have been shown to be competent electrocatalysts
for the production of hydrogen at fairly negative potentials in
the presence of weak acids.8 Optimization of electrocatalysis
by the diiron models has included introduction of hydrophilic
ligands such as PTA (phosphatriazaadamantane)9 and electro-
active ligands such as the N-heterocyclic carbene IMes (1,3-
Introduction
In biology, the reduction of protons to form dihydrogen
(2H+ + 2e- / H2) is reversibly catalyzed by the hydrogenases.1
These enzymes employ bimetallic active sites featuring dithiolate
bridged [FeFe] or [NiFe] scaffolds.2 While electrocatalytic
hydrogen production can be accomplished at a platinum
electrode, platinum metal is far too expensive and limited in
supply to be of practical use in a global hydrogen-based fuel
cell economy. For this reason, researchers have examined the
attachment of the hydrogenase enzymes to electrodes by either
surface adsorption3 or covalent linkage4 to assess the electro-
catalytic proton reduction and hydrogen oxidation abilities of
the enzymes.
Since the structure of [FeFe]-hydrogenase, [FeFe]-H2ase, was
reported,5 small-molecule Fe2S2 model complexes that closely
resemble the enzyme’s active site have been scrutinized in
efforts to better understand the mechanism of stepwise proton
reduction in binuclear catalytic sites. The model complexes,
(6) (a) Reihlen, H.; Gruhl, A.; Hessling, G. Liebigs Ann. Chem. 1929,
472, 268. (b) Seyferth, D.; Womack, G. B.; Archer, C. M.; Dewan, J. C.
Organometallics 1989, 8, 430-442.
(7) (a) Lyon, E. J.; Georgakaki, I. P.; Reibenspies, J. H.; Darensbourg,
M. Y. J. Am. Chem. Soc. 2001, 123, 3268-3278. (b) Lawrence, J. D.; Li,
H.; Rauchfuss, T. B.; Benard, M.; Rohmer, M.-M. Angew. Chem., Int. Ed.
2001, 40, 1768-1771. (c) Razavet, M.; Davies, S. C.; Hughes, D. L.; Pickett,
C. J. Chem. Commun. 2001, 847-848. (d) Tard, C.; Liu, X.; Ibrahim, S.
K.; Bruschi, M.; De Gioia, L.; Davies, S. C.; Yang, X.; W. L.-S.; Sawers,
G.; Pickett, C. J. Nature 2005, 433, 610-613. (e) Justice, A. K.; Zampella,
G.; De Gioia, L.; Rauchfuss, T. B.; van der Vlugt, J. I.; Wilson, S. R. Inorg.
Chem. 2007, 46, 1655-1664. (f) van der Vlugt, J. I.; Rauchfuss, T. B.;
Whaley, C. M.; Wilson, S. R. J. Am. Chem. Soc. 2005, 127, 16012-16013.
(8) (a) Gloaguen, F.; Lawrence, J. D.; Rauchfuss, T. B. J. Am. Chem.
Soc. 2001, 123, 9476-9477. (b) Chong, D.; Georgakaki, I. P.; Mejia-
Rodriguez, R.; Sanabria-Chinchilla, J.; Soriaga, M. P.; Darensbourg, M.
Y. J. Chem. Soc., Dalton Trans. 2003, 4158-4163. (c) Capon, J.-F.;
Gloaguen, F.; Schollhammer, P.; Talarmin, J. J. Electroanal. Chem. 2004,
566, 241-247. (d) Jiang, S.; Liu, J.; Shi, Y.; Wang, Z.; A° kermark, B.;
Sun, L. Dalton Trans. 2007, 896-902. (e) Morvan, D.; Capon, J.-F.;
Gloaguen, F.; Le Goff, A.; Marchivie, M.; Michaud, F.; Schollhammer, P.;
Talarmin, J.; Yahouanc, J.-J.; Pichon, R.; Karvarec, N. Organometallics
2007, 26, 2042-2052. (f) Duan, L.; Wang, M.; Li, P.; Na, Y.; Wang, N.;
Sun, L. Dalton Trans. 2007, 1277-1283.
* Corresponding author. E-mail: marcetta@mail.chem.tamu.edu.
(1) (a) Adams, M. W. W.; Stiefel, E. I. Science 1998, 282, 1842-1843.
(b) Cammack, R.; Frey, M.; Robson, R. Hydrogen as a Fuel: Learning
from Nature; Taylor and Francis: London, 2001.
(2) (a) Adams, M. W. W.; Stiefel, E. I. Curr. Opin. Chem. Biol. 2000,
4, 214-220. (b) Evans, D. J.; Pickett, C. J. Chem. Soc. ReV. 2003, 32,
268-275. (c) Darensbourg, M. Y.; Lyon, E. J.; Smee, J. J. Coord. Chem.
ReV. 2000, 206-207, 533-561.
(3) (a) Vincent, K. W.; Cracknell, J. A.; Parkin, A.; Armstrong, F. A.
Dalton Trans. 2005, 3397-3403. (b) Leger, C.; Jones, A. K.; Roseboom,
W.; Albracht, S. P. J.; Armstrong, F. A. Biochemistry 2002, 41, 15736-
15746.
(4) Ru¨diger, O.; Abad, J. M.; Hatchikian, E. C.; Fernandez, V. M.; De
Lacey, A. L. J. Am. Chem. Soc. 2005, 127, 16008-16009.
(5) (a) Nicolet, Y.; Piras, C.; LeGrand, P.; Hatchikian, C. E.; Fontecilla-
Camps, J. C. Structure 1999, 7, 13-23. (b) Peters, J. W.; Lanzilotta, W.
N.; Lemon, B. J.; Seefeldt, L. C. Science 1998, 282, 1853-1858.
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Darensbourg, M. Y. J. Am. Chem. Soc. 2004, 126, 12004-12014.
10.1021/om7003354 CCC: $37.00 © 2007 American Chemical Society
Publication on Web 07/04/2007

Carboxylic Acid-Modified [FeFe]-Hydrogenase Complexes
Organometallics, Vol. 26, No. 16, 2007 3977
Chart 1
Scheme 1
bis(2,4,6-trimethylphenyl)imidazol-2-ylidene).10 While the mild
conditions of nature (H2 production at pH ) 7 and E ) -0.4
V (vs NHE)) are yet to be achieved, various imaginative
approaches to a workable molecular electrocatalyst continue to
address the goal. To be of practical use, chemical tethers that
achieve immobilization on cheap electrode materials, most likely
graphite, must also be developed and their stability assessed.
With this goal in mind, Pickett and co-workers recently reported
some initial steps toward incorporating model complexes of the
active site of [FeFe]-hydrogenase into electrode-bound elec-
tropolymeric materials.11 Our approach utilizes a particularly
versatile method for generating amine monolayers on carbon
electrodes via electrochemical reduction of aryl diazonium salts
developed by Saveant and co-workers.12 Covalent amide link-
ages can be formed between such monolayers and molecules
bearing a carboxylic acid functionality. With this in mind, the
complexes shown in Chart 1, derivatized with carboxylic acid
functionalities suitable for attachment to amino-functionalized
electrode surfaces via carbodiimide-promoted acylation, have
been prepared. In a proof of principle, one of these complexes
has been coupled with aniline and the stability of the amido
linkage explored.
Results and Discussion
Synthesis and Characterization. Access to the carboxy-
thiolate-bridged complexes was achieved using routes similar
to those previously employed for simple alkyl thiolates.16
Refluxing a toluene solution of 3-mercaptopropionic acid and
Fe3(CO)12 results in formation of red-orange (µ-SCH2CH2-
COOH)2[Fe(CO)3]2 (1), isolated in 35% yield (Scheme 1).
Complex 1 is insoluble in most organic solvents and unstable
to highly polar solvents such as acetonitrile, DMF, and methanol;
however, satisfactory ESI-MS, combustion analysis, and IR data
confirmed its identity. Like other (µ-RS)2[Fe(CO)3]2 complexes,8b
the IR spectrum of 1 has diagnostic ν(CO) stretching frequencies
at 2071(m), 2037(s), and 1994(s) cm-1, with an additional band
at 1735(w) cm-1 for the carboxylic acid moiety (Table 1).
The 2-carboxy-1,3-propanedithiol17 and red (µ-(SCH2)2-
CHCOOH)[Fe(CO)3]2 (2)13 were prepared following literature
protocols. Shorter heating periods contributed to greater yields
of 2 (Scheme 1). The ν(CO) bands in the IR spectrum of 2 are
consistent with those of the parent (µ-pdt)[Fe(CO)3]2 complex
(Table 1), with an additional stretch observed at 1730 cm-1 for
the -COOH moiety. The 1H NMR spectrum (298 K, acetone-
d6) of complex 2 contains three diagnostic signals for the protons
on the propanedithiolate linker, indicative of the inequivalence
of the axial and equatorial protons of the iron dithiacyclohexane
rings. Similar behavior has been observed in the low-temperature
1H NMR spectrum of (µ-pdt)[Fe(CO)3]2,7a and in the case of 2,
it is likely that substitution at the 2-position of the propanedithi-
olate hinders ring interconversion.
Two strategies have been employed to prepare model
complexes with carboxylic acid functionalities. The first of these
incorporates a carboxylic acid group into the thiolate linkage
of the Fe2S2 complexes (Chart 1), making use of both mono-
dentate thiolate and chelating dithiolate bridging linkers.
Complex 2 and an amido derivative of it have been reported
by Volkers, Rauchfuss, and Wilson in a study of the co-
ordination properties of dihydroasparagusic acid or (HSCH2)2-
CHCO2H in diiron carbonyl and nickel derivatives.13 The
second approach is via substitution of a CO ligand with a
carboxy-substituted phosphine. It is important to note that it
has previously been demonstrated that functionalized di-
thiolate linkers can be used to covalently attach diiron com-
plexes to electrode surfaces,14 and functionalized dithiolates
and phosphines have been used to link diiron complexes to
photosensitizers.15
(10) Tye, J. W.; Lee, J.; Wang, H.-W.; Mejia-Rodriguez, R.; Reibenspies,
J. H.; Hall, M. B.; Darensbourg, M. Y. Inorg. Chem. 2005, 44, 5550-
5552.
(11) Ibrahim, S. K.; Liu, X.; Tard, C.; Pickett, C. J. Chem. Commun.
2007, 1535-1537.
(12) (a) Delamar, M.; Hitmi, R.; Pinson, J.; Saveant, J. M. J. Am. Chem.
Soc. 1992, 114, 5883-5884. (b) Allongue, P.; Delamar, M.; Desbat, B.;
Fagebaume, O.; Hitmi, R.; Pinson, J.; Saveant, J. M. J. Am. Chem. Soc.
1997, 119, 201-207.
(13) Volkers, P. I.; Rauchfuss, T. B.; Wilson, S. R. Eur. J. Inorg. Chem.
2006, 4793-4799.
Incorporation of a carboxylic acid into the diiron dithiolate
model complexes through substitution of a CO ligand with a
carboxy-functionalized phosphine has been used by Sun et al.
to generate (µ-pdt)[Fe(CO)3][Fe(CO)2(Ph2PCH2COOH)] and (µ-
(SCH2)2NC3H7[Fe(CO)3][Fe(CO)2(Ph2PCH2CH2COOH)].18 For
(14) Vijaikanth, V.; Capon, J.-F.; Gloaguen, F.; Schollhammer, P.;
Talarmin, J. Electrochem. Commun. 2005, 7, 427-430.
(15) (a) Ekstro¨m, J.; Abrahamsson, M.; Olson, C.; Bergquist, J.; Kaynak,
F. B.; Eriksson, L.; Sun, L.; Becker, H.-C.; Åkermark, B.; Hammarstro¨m,
L.; Ott, S. Dalton Trans. 2006, 4599-4606. (b) Wolpher, H.; Borgstro¨m,
M.; Hammarstro¨m, L.; Bergquist, J.; Sundstro¨m, V.; Styring, S.; Sun, L.;
Åkermark, B. Inorg. Chem. Commun. 2003, 6, 989-991.
(16) Winter, A.; Zsolnai, L.; Huttner, G. Z. Naturforsch. 1982, 37b,
1430-1436.
(17) Singh, R.; Whitesides, G. M. J. Am. Chem. Soc. 1990, 112, 1190-
1197.
(18) Dong, W.; Wang, M.; Wang, F.; Jin, K.; Yu, Z.; Sun, L. personal
communication.

3978 Organometallics, Vol. 26, No. 16, 2007
Thomas et al.
Table 1. Infrared Spectroscopic Data for Complexes 1-6 (in acetonitrile) as Well as Some Unfunctionalized Fe2S2 Complexes
for Comparison
complex
ν(CO)Fe (cm-1
)
ν(CdO) (cm-1
)
ref
(µ-SCH2CH2COOH)2[Fe(CO)3]2 (1)a
(µ-SEt)2[Fe(CO)3]2
2071(m), 2037(s), 1994(s)
2073(m), 2038(vs), 1992(s)
2077(m), 2034(s), 1992(s), 1978(sh)
2074(m), 2036(vs), 1995(s)
2041(s), 1981(s), 1966(sh), 1922(w)
2037(s), 1980(s), 1919(m)
2078(m), 2038(s), 1997(s)
1984(m), 1948(vs), 1903 (s)
1979(m), 1942(s), 1898(s)
2034(s), 1994(s)
1735
b
7a
b
7a
b
19
b
b
20
b
(µ-(SCH2)2CHCOOH) [Fe(CO)3]2 (2)a
(µ-pdt) [Fe(CO)3]2
1730
1733
(µ-pdt)[Fe(CO)3][Fe(CO)2{P(CH2CH2COOH)3}] (3)
(µ-pdt) [Fe(CO)3][Fe(CO)2(PMe3)]
(µ-(SCH2)2CHCONHPh)[Fe(CO)3]2 (4)
(µ-(SCH2)2CHCOOH)[Fe(CO)2(PMe3)]2 (5)a
(µ-pdt) [Fe(CO)2(PMe3)]2)
{(µ-(SCH2)2CHCOOH) (µ-H) [Fe(CO)2(PMe3)]2}PF6 (6)
{(µ-pdt)(µ-H) [Fe(CO)2(PMe3)]2}PF6
1690, 1600
1729
1739
2029(s), 1989(s)
20
b
a Spectrum recorded in tetrahydrofuran. This work.
our purposes, the commercially available phosphine P(CH2CH2-
COOH)3HCl was chosen in order to maximize the potential for
eventual surface attachment through amido linkages. Addition
of this phosphine to (µ-pdt)[Fe(CO)3]2 in acetonitrile in the
presence of 1 equiv of the decarbonylation agent ONMe3 leads
to formation of (CO)3Fe(µ-pdt)Fe(CO)2{P(C2H4COOH)3} (3)
in 80% yield. Complex 3 is characterized by a 31P NMR signal
at 52.9 ppm, and, similar to other monophosphine Fe2S2
complexes, the IR spectrum of 3 features stretches at 2041(s),
1981(s), 1966 (sh), and 1922 (w) cm-1 for the iron carbonyls.
A band at 1733 cm-1 is assigned to the CdO of the carboxylic
acids.
Coupling of Carboxy-Functionalized Diiron Dithiolates to
Amines in Solution. To mimic the linkage of the carboxylic
acid-substituted diiron complexes with surface-bound mono-
layers of aniline, complex 2 was coupled with aniline in solution
to generate the corresponding amide complex. Addition of
aniline to complex 2 in the presence of diisopropylcarbodiimide
as a coupling agent results in rapid formation of (µ-(SCH2)2-
CHCONHPh)[Fe(CO)3]2 (4) in high yield (Scheme 2).21 The
structure of amide 4 was confirmed by X-ray crystallography
(Vide infra). The IR spectrum of complex 4 is similar to that of
Even in the absence of acid, at temperatures higher than 60 °C,
gradual decomposition of complex 4 occurs. After 2 h of heating
in toluene, the IR spectrum revealed a decrease in the intensity
of the ν(CdONHR) stretch at 1690 cm-1 and the appearance of
a new broad stretch around 1730 cm-1
.
LigandSubstitutionandProtonationof(µ-(SCH2)2CHCOOH)-
[Fe(CO)3]2 (2). While the FeIFeI carbonyl derivatives 1, 2, and
3 are suitable for catalytic proton reduction at an electrode
surface, a carboxy-substituted complex suitable for the reverse
reaction (dihydrogen oxidation) is also desirable. In the [FeFe]-
H2ase enzyme as well as in the diiron model complexes,
requirements for H2 uptake are (1) oxidized iron to achieve d6
FeII and (2) an open site for H2 binding and activation.
Previously, it was shown that protonation of the disubstituted
phosphine complex (µ-pdt)[FeI(CO)2(PMe3)]2 leads to the
cationic FeII(µ-H)FeII complex.20 Under photolysis to remove
a CO ligand, this oxidized complex and its analogues are active
toward H2 uptake, as has been assayed by their ability to
facilitate H/D exchange reactions in D2/H2O mixtures.20,22 For
potential applications toward electrocatalytic hydrogen oxidation
chemistry, such ligand substitution and protonation reactions
have been pursued using the carboxy-substituted complex 2.
Reaction of complex 2 with 2 equiv of PMe3 at 50 °C for
several hours resulted in the formation of the disubstituted
complex (µ-(SCH2)2CHCOOH)[Fe(CO)2(PMe3)]2, complex 5,
in ca. 80% yield.23 The IR spectrum of complex 5 is consistent
with that reported earlier and in good agreement with that of
(µ-pdt)[Fe(CO)2(PMe3)]2, whose solid-state structure found the
PMe3 ligands to be in basal positions on each square-pyramidal
iron and transoid to each other.20 Complex 5 has diagnostic
31P NMR signals at 27.6 and 21.5 ppm. This inequivalence of
the two phosphorus atoms, observed only at the low temperature,
stopped-exchange region, in (µ-pdt)[Fe(CO)2(PMe3)]2 (at 29.6
and 25.7 ppm),22 likely arises from the fixed position of the
chair/boat iron dithiacyclohexane conformers even at room
temperature upon incorporation of the carboxylic acid moiety
of 5. Isomeric forms due to phosphine positions (axial/basal)
on individual iron centers is typically indicated by a difference
in 31P NMR chemical shift positions of over 10 ppm.22
2 with the exception of the disappearance of the ν(CdOOH
)
stretch at 1730 cm-1 and the appearance of two new bands at
1690 and 1600 cm-1 attributed to the amide functionality. An
analogous carboxamido derivative, (µ-(SCH2)2CHCONHEt)[Fe-
(CO)3]2, is reported to have bands at 1660 and 1563 cm-1 13
.
For electrode-supported catalytic applications, the stability
of the carboxylic amide tether under various conditions is of
obvious importance. Specifically, it is crucial that this linkage
be stable at negative potentials (Vide infra) and under moderately
acidic conditions. To assess this stability, complex 4 was
exposed to 10 equiv of acetic acid in THF solution and
monitored by IR spectroscopy for 48 h. No change was observed
in either the ν(CtOFe) or ν(CdONHR) bands under these
conditions. Likewise, complex 4 was found to be stable to 10
equiv of HCl for at least several hours. Exposure to larger
excesses of strong acid (dissolution in a 1:1 mixture of
concentrated HCl(aq) and THF) resulted in gradual decomposi-
tion of complex 4, as indicated by bleaching of the solution’s
color and formation of brown solids over the course of 1 h.
Under the conditions reported for protonation of (µ-pdt)[Fe-
(CO)2(PMe3)]2 (conc HCl, MeOH),22 complex 5 can be proto-
Scheme 2

Carboxylic Acid-Modified [FeFe]-Hydrogenase Complexes
Organometallics, Vol. 26, No. 16, 2007 3979
Table 2. Electrochemical Reduction Potentials (vs ferrocene) of Complexes 1-5a
complex
Epcb FeIFeI f FeIFe0
Epcb FeIFe0 f Fe0Fe0
(µ-SCH2CH2COOH)2[Fe(CO)3]2 (1)
-1.73 V
-1.64 V
-2.07 V
-1.67 V
-2.42 V
-2.37 V
-2.51 V
-2.44 V
-2.60 V
(µ-(SCH2)2CHCOOH)[Fe(CO)3]2 (2)
(µ-pdt)[Fe(CO)3][Fe(CO)2{P(CH2CH2COOH)3}] (3)b
(µ-(SCH2)2CHCONHPh)[Fe(CO)3]2 (4)
(µ-(SCH2)2CHCOOH)[Fe(CO)2(PMe3)]2 (5)c
n
a CO-saturated CH3CN solution (0.1 M Bu4NBF4) with a glassy carbon working electrode (A ) 0.071 cm2) referenced to Cp2Fe/Cp2Fe+ as an internal
b
c
standard. Counter electrode: Pt. Scan rate: 200 mV/s. See discussion on assignments in text. Under Ar atmosphere.
nated to generate a hydride-bridged FeIIFeII complex; however,
concomitant esterification of the carboxylic acid moiety occurs,
producing {(µ-(SCH2)2CHCOOMe)(µ-H)[Fe(CO)2(PMe3)]2}PF6
(6) in 76% yield. Infrared spectroscopy of 6 reveals two ν(CO)
stretches at 2034 and 1994 cm-1 and a stretch at 1739 cm-1
for the C(dO)OMe moiety. Protonation of 5 with HCl in THF
rather than methanol results in an FeIIFeII hydride complex with
conservation of the -COOH group, and {(µ-(SCH2)2CHCOOH)-
(µ-H)[Fe(CO)2(PMe3)]2}PF6 (7) can be isolated in 54% yield.
1
The H NMR spectra of 6 and 7 both feature a diagnostic
hydride resonance at -15.2 ppm with a 2JH-P coupling constant
of 22 Hz (for (µ-H)(µ-pdt)[Fe(CO)2(PMe3)]2+, δ ) -15.2 ppm
and 2JH-P ) 22 Hz).22 The 31P NMR spectra of 6 and 7 feature
two distinct PMe3 signals (23.9 and 21.7 ppm for 6; 25.1 and
21.8 for 7). Again the small difference reflects the slightly
different chemical environment of the two PMe3 ligands derived
Figure 1. Cyclic voltammograms of complexes 1, 2, and 4 (2.5
mM in 0.1 M Bu4NBF4 in MeCN, under CO atmosphere).
from the locked position of the µ-(SCH2)2CHCOOMe bridge.
n
-
A septet at -142.7 ppm is assigned to the PF6 counteranion.
feature two irreversible reductive processes (Figure 1).26 The
first reduction events at -1.73, -1.64, and -1.67 V for
complexes 1, 2, and 4, respectively, can be assigned as the one-
electron FeIFeI f FeIFe0 reduction on the basis of previous
assignments for the unfunctionalized complexes.8b Unlike the
(µ-pdt)[Fe(CO)3]2 parent, this reduction event is irreversible even
when more negative potentials are not applied or when the scan
rate is increased. An additional one-electron reduction, which
on the basis of the (µ-pdt)[Fe(CO)3]2 parent may be assigned
as FeIFe0 f Fe0Fe0, is also observed at -2.37 V for 1, -2.51
V for 2, and -2.60 V for 4. Complexities in the reductive region
of the CVs of simpler compounds have elicited detailed
electrochemical, spectroelectrochemical, and computational
study.27 Heinekey and co-workers reported that chemical
reduction of complexes of this type leads to a bimolecular
decomposition pathway involving Fe-S bond cleavage and the
production of tetrairon species with propanedithiolate spanning
two diiron units.28 Clearly the carboxylate moiety introduces
even more possibilities for complications in the compounds of
our current study.
Solution Electrochemistry. In order to realize the ultimate
goal of converting solution electrocatalysts to electrode surface-
supported catalysts, it is important to establish that the incor-
poration of tethers does not dramatically affect the redox
behavior of the diiron complexes. For this purpose, cyclic
voltammetry data were collected for complexes 1-7 for
comparison with the analogous unfunctionalized complexes.24
The cyclic voltammogram (CV) of the well-studied (µ-pdt)-
[Fe(CO)3]2 displays two reduction events at -1.74 V (quasi-
reversible) and -2.35 V (irreversible) versus the ferrocene/
ferrocenium couple.8b [Note: PreVious studies reported these
redox eVents relatiVe to the NHE in attempts to relate potentials
of the model complexes to the hydrogenase enzyme. The
uncertainty of standards in nonaqueous solVents leads to a
reconsideration and to the simpler protocol used here. Addition
of the 0.40 V conVersion factor25 conVerts the Fc/Fc+ referenced
Values to the NHE Values reported earlier.] The CVs of
complexes 1, 2, and 4 were recorded in CO-saturated CH3CN
solution, as these conditions give the most well-defined redox
waves for related compounds.8b Reduction potentials under these
conditions are listed in Table 2. CVs of the carboxylic acid
complexes 1 and 2 and the carboxyamido complex 4 essentially
Figure 2 displays the cyclic voltammogram of acetic acid in
MeCN, showing a broad response centered at -2.6 V that shows
a linear dependence on HOAc concentration. As expected,
complexes 1, 2, and 4 also have reductive events in this region,
likely due to the carboxylate moiety. In the presence of
increasing concentrations of acetic acid (pKa ) 22.6),29 the
current height of the first reductive wave remains relatively
constant for complexes 1, 2, and 4, indicating that the FeIFe0
(19) Li, P.; Wang, M.; He, C.; Li, G.; Liu, X.; Chen, C.; Åkermark, B.;
Sun, L. Eur. J. Inorg. Chem. 2005, 2506-2513.
(20) Zhao, X.; Georgakaki, I. P.; Miller, M. L.; Yarbrough, J. C.;
Darensbourg, M. Y. J. Am. Chem. Soc. 2001, 123, 9710-9711.
(21) Monitoring this reaction by 1H NMR spectroscopy (acetone-d6)
reveals quantitative conversion of the -COOH to -CONHPh within 15
min. Isolated yields are lower as a result of difficulties separating the amide
product from carbodiimide-derived side products.
(22) Zhao, X.; Georgakaki, I. P.; Miller, M. L.; Mejia-Rodriguez, R.;
Chiang, C.-Y.; Darensbourg, M. Y. Inorg. Chem. 2002, 41, 3917-3928.
(23) A similar reaction was observed previously (ref 13) using the
deprotonated carboxylate derivative NR4{(µ-pdt-COO-)[Fe(CO)3]2} to
generate NR4{(µ-pdt-COO-)[Fe(CO)2(PMe3)]2}.
(26) Additional features in the CV of complexes 1 and 2 likely arise
from the redox activity of the carboxylic acid moiety itself. A CV of
2-carboxy-1,3-propanedithiol is included in the Supporting Information.
(27) Cheah, M. H.; Borg, S. J.; Best, S. P. Inorg. Chem. 2007, 46, 1741-
1750.
(28) Aguirre de Carcer, I.; DiPasquale, A.; Rheingold, A. L.; Heinekey,
D. M. Inorg. Chem. 2006, 45, 8000-8002.
(29) Izutsu, K. Acid-Base Dissociation Constants in Dipolar Aprotic
SolVents; Blackwell Scientific Publications: Oxford, 1990; Vol. 35.
(24) The reduction potentials of complexes 2 and 5 have been previously
reported (ref 13).
(25) Gagne´, R. R.; Koval, C. A.; Lisensky, G. C. Inorg. Chem. 1980,
19, 2854-2855.

3980 Organometallics, Vol. 26, No. 16, 2007
Thomas et al.
n
Figure 3. Cyclic voltammograms of 3 (2.5 mM in 0.1 M Bu4-
NBF4 in MeCN under Ar) with increments of HCl (0, 25, 50, and
75 mM).
the potential of the FeIFeI/FeIFe0 couple shifts to more positive
potential when acid is added. We postulate that one (or both)
of the carboxylic acid moieties is deprotonated in acetonitrile
solution and that addition of acid reprotonates the carboxylate
and, thus, shifts the reduction potential.30 The CVs of all three
complexes 1, 2, and 4 show an increase in current with
increasing concentrations of HOAc at or near the second
reductive wave (-2.26, -2.51, and -2.43 V, respectively).
These values are from 100 to 350 mV more positive than that
of acetic acid itself, and the enhanced current is tentatively
assigned to the electrochemically generated Fe0Fe0 species.
The cyclic voltammogram of the carboxyethyl phosphine
complex 3 under an Ar atmosphere has three distinct irreversible
reductive features (Figure 3, Table 2). The first reduction at
-2.06 V corresponds to a FeIFeI/FeIFe0 couple. It is postulated
that the next reduction at -2.44 V is an FeIFe0/Fe0Fe0 process
and that the additional reductive feature at -2.67 V results from
the lability of the CO ligand and displacement by MeCN in the
Fe0Fe0 state, as has been discussed previously in the context of
(µ-pdt)[Fe(CO)3]2.8b,31 When increments of HOAc are added,
a small increase in peak height is observed for the first reduction
of 3, while the current response of the second reduction increases
more dramatically (see Supporting Information). This indicates
that for this complex the FeIFe0 redox state is modestly active
toward proton reduction, but the Fe0Fe0 state is significantly
more catalytically competent. When increments of HCl are
added, however, complex 3 becomes a competent electrocatalyst
at milder potentials (-1.9 V vs Fc, Figure 3). This positive shift
in potential is a result of the stronger acid’s ability to protonate
3 more readily in the FeIFe0 state prior to further reduction to
the Fe0Fe0 state.
Cyclic voltammetry of the phosphine-substituted complex 5
displays a single one-electron reduction (within the solvent
window of MeCN) at -2.42 V (Figure 4). In contrast to the
all-CO complexes, complex 5 catalyzes H2 production from
HOAc at this first FeIFeI f FeIFe0 event, as has been observed
(30) In support of this hypothesis, monitoring the IR spectrum of 1 in
acetonitrile solution reveals that the stretch attributed to the -COOH unit
decreases over time, and a new band at ∼1650 cm-1 corresponding to
-COO- appears (see Supporting Information).
(31) Additional support for this hypothesis is provided by the changes
in the relative peak heights of the two reductions at -2.44 and -2.67 V at
different scan rates: At faster scan rates (> 500 mV/s), the -2.44 V peak
dominates, while the two peak heights are nearly identical at slow scan
rates (100 mV/s). See Supporting Information for the corresponding
voltammograms.
Figure 2. Cyclic voltammograms of complexes 1, 2, and 4 (2.5
mM) with HOAc (0, 25, 50, 75, 100 mM) in 0.1 M Bu4NBF4 in
CO-saturated CH3CN. The CV of HOAc in MeCN at these
concentrations is shown as a control.
n
species generated at this potential is not active toward electro-
catalytic proton reduction (Figure 2). In the case of complex 1,

Carboxylic Acid-Modified [FeFe]-Hydrogenase Complexes
Organometallics, Vol. 26, No. 16, 2007 3981
n
Figure 4. Cyclic voltammograms of 5 (MeCN, 0.1 M BuNBF4) in the presence of 0, 25, 50, 75, and 100 mM HOAc (left) and HCl
(right).
Figure 5. Thermal ellipsoid representation (50% probability) of
(µ-pdt)[Fe(CO)3][Fe(CO)2{P(C2H4COOH)3}] (3). A solvent mol-
ecule (H2O) and all hydrogen atoms have been omitted for
Figure 6. Thermal ellipsoid representation (50% probability) of
(µ-(SCH2)2CHCONHPh)[Fe(CO)3]2 (4) showing (A) the solid-state
structure and (B) hydrogen bonding between two molecules in the
clarity.
for the parent (µ-pdt)[Fe(CO)2(PMe3)]2 complex.8b The CV of
unit cell. Solvent molecules (THF) have been omitted for clarity.
hydride-bridged FeIIFeII complex 7 features two irreversible
Table 3. Selected Interatomic Distances (Å) and Angles
reductive events: a reduction assigned as the FeIIFeII f FeII-
(deg) for Complexes 2, 3, 4, and 6
FeI process at -1.48 V and a reduction at -2.34 V, coincident
213
3
4
6
with the FeIFeI f FeIFe0 couple of complex 5 (see Supporting
Information). Thus, reduction of 7 likely leads to loss of the
hydride,8b and catalytic H2 production occurs from the product
of that hydride loss, i.e., 5 in the presence of HOAc. The cyclic
voltammograms of 5 in the presence of varying concentrations
of HCl, an acid strong enough to protonate complex 5 to
generate hydride 7, reveal electrocatalytic behavior at a more
positive potential coincident with the one-electron reduction
of 7.
Fe-Fe
2.5130(8)
67.83(2)
67.79(3)
85.21(3)
85.06(3)
N/A
2.5553(8)
68.67(4)
69.33(4)
85.02(5)
84.85(5)
2.212(1)
2.514(2)
67.76(7)
68.06(8)
85.06(9)
85.31(8)
N/A
2.6017(9)
70.23(3)
70.00(3)
84.50(4)
84.50(4)
2.235(1)
2.246(1)
Fe-S-Fe
S-Fe-S
Fe-P
Fe2S2C3 ring (Figure 6A). The interatomic distances and angles
of 4 are very similar to those for 2 (Table 3), with an Fe-Fe
distance of 2.514(2) Å. An intermolecular hydrogen-bonding
interaction is observed in the extended structure of 4 (Figure
6B) between the amido-carbonyl oxygen atom (O1) of one
molecule and the amide hydrogen of another (CdOsHN
distance ) 1.987(8) Å).
The solid-state structure of 6 confirms the presence of a
methyl ester rather than a carboxylic acid (Figure 7). Since the
ester functionality should not affect the overall geometry of the
diiron complex, the general structural features of ester complex
6 can be inferred for carboxylic acid complex 7 as well. The
bridging hydride of 6 was located in the difference Fourier map
and refined.32 As a result of the protonation of the Fe-Fe bond
to form an FeIIFeII complex, the Fe-Fe distance in 6 is modestly
Solid-State Structures of 3, 4, and 6. The molecular
structure of complex 3 reveals that the carboxyethyl-
phosphine occupies a basal position (Figure 5). As with other
monosubstituted Fe2S2 model complexes,9,10 the boat con-
formation of the metallodithiacyclohexane ring is oriented
toward the phosphine-substituted Fe center. The geometry
about each metal center, however, remains unaffected by this
seemingly more sterically disfavored frozen orientation. As
expected, the packing diagram shows an extensive H-bonding
array.
As was found in the molecular structure of 2 as well as its
ethylamido derivative13 the solid-state structure of 4 shows that
the aniline-derived carboxyamide moiety is also oriented in the
sterically favored equatorial position of the six-membered

3982 Organometallics, Vol. 26, No. 16, 2007
Thomas et al.
Laboratories, Inc. or Aldrich and used without further purification.
2-Carboxy-1,3-propanedithiol17 and (µ-pdt)[Fe(CO)3]27a were pre-
pared using literature methods. All other chemicals were purchased
from commercial vendors and used without further purification.
NMR spectra were recorded on Varian Mercury and Inova 300
MHz instruments. 1H NMR chemical shifts were referenced to
residual solvent. 31P NMR chemical shifts were referenced to 85%
H3PO4. Mass spectrometry was performed at the Laboratory for
Biological Mass Spectrometry at Texas A&M University. Elemental
analyses were performed by Canadian Microanalytical Service in
Delta, British Columbia, Canada. Infrared spectra were recorded
on a Bruker Tensor 27 instrument using a 0.1 mm NaCl cell.
X-ray Structure Determination. The X-ray data were collected
on a Bruker SMART 1000 CCD diffractometer and covered a
hemisphere of reciprocal space by a combination of three sets of
exposures. The space groups were determined on the basis of
systematic absences and intensity statistics. Structures were deter-
mined using direct methods with standard Fourier techniques
using the Bruker AXS software package.32 Anisotropic displace-
ment parameters were determined for all non-hydrogen atoms.
Hydrogen positions were calculated and refined with fixed isotropic
displacement parameters. Crystallographic data are summarized in
Table 4.
Figure 7. Thermal ellipsoid representation (50% probability) of
{(µ-(SCH2)2CHCOMe)(µ-H)[Fe(CO)2(PMe3)]2}PF6 (6). The PF6
counteranion has been omitted.
-
elongated from the FeIFeI complexes (2.6017(9) Å, compared
to ∼2.51-2.55 Å). The phosphine ligands both occupy positions
cis to the bridging hydride. As with the related {(µ-pdt)(µ-H)-
[Fe(CO)2(PMe3)]2}PF6 complex,20 the phosphines are oriented
transoid to each other.
Electrochemistry. Cyclic voltammetry measurements were
performed using a BAS 100A electrochemical workstation. All
voltammograms were obtained in a three-electrode cell under Ar
(or CO as noted) atmosphere at room temperature. The working
electrode was a glassy carbon disk (0.071 cm2), and a platinum
wire was used as the auxiliary electrode. The experimental reference
electrode was Ag/AgNO3, and measured potentials are reported
relative to the internal reference of Cp2Fe/Cp2Fe+ ) 0.00. Com-
parisons to earlier work in CH3CN solution may be made by adding
0.4 V to the values recorded here. The supporting electrolyte was
0.1 M nBu4NBF4. Increments of glacial acetic acid were added by
microsyringe.
Conclusions
In summary, diiron dithiolate model complexes of the active
site of [FeFe]-hydrogenase bearing carboxylic acid functional-
ities have been synthesized. We have estabilished that these
complexes are spectroscopically and structurally quite similar
to their unsubstituted relatives and have comparable electro-
chemical behavior in solution. In addition, it has been demon-
strated that these complexes can undergo the same ligand
substitution and protonation processes as their unsubstituted
relatives.
The carboxylic acid moieties of these complexes render them
amenable to potential applications such as immobilization on
electrode surfaces to design heterogeneous electrocatalysts. To
mimic the attachment of these complexes to an amino-
functionalized electrode surface, (µ-(SCH2)2CHCOOH)[Fe-
(CO)3]2 has been coupled to aniline in solution to generate a
carboxyamide linkage. Preliminary attempts to immobilize this
complex on an amino-functionalized highly ordered pyrolytic
graphite (HOPG) electrode surface have been successful, leading
to an electrochemical response very similar to the solution CV
data (see Supporting Information). However, the reductive signal
does not persist over repeated scans and electrocatalytic proton
reduction behavior has not been observed. This limited success
may be largely due to the irreversible nature of the reduction
of (µ-(SCH2)2CHCOOH)[Fe(CO)3]2. It further indicates that
additional protective films will be required to maintain stability
of such modified electrodes.
(µ-SCH2CH2COOH)2[Fe(CO)3]2 (1). Solid Fe3(CO)12 (5.00 g,
10.0 mmol) and 3-mercaptopropionic acid (2.34 g, 22 mmol) were
combined in toluene (30 mL) and refluxed under N2 for 15 min,
whereupon the solution color changed from green to red. The
reaction mixture was loaded onto a silica column (10 × 1 in.) and
eluted first with toluene to remove excess starting materials and
then with 1:1 toluene/ethyl acetate to elute the red-orange product.
Solvent was removed from the eluant in Vacuo to yield spectro-
scopically pure product as a highly insoluble red-orange solid (1.75
g, 35%). Due to the highly insoluble nature of 1, we were unable
1
to obtain satisfactory H NMR data in any common deuterated
solvents. IR (THF, cm-1): ν(CO) 2071 (m), 2037 (s), 1994 (s, br),
1735 (m, COOH). ESI-MS (CH3CN, neg mode): m/z 488.79 (M
- H+), 460.80 (M - CO - H+). Dec point: 149 °C. Anal. Calcd
for C12H10Fe2O10S2: C, 29.41; H, 2.06. Found: C, 28.99; H, 2.90.
Repeated analyses led to similar analytical results; we attribute this
to the difficulty in purification arising from the insolubility of this
complex.
(µ-(SCH2)2CHCOOH)[Fe(CO)3]2 (2). The following is a modi-
fication of the literature procedure.13 Solid Fe3(CO)12 (5.16 g, 10.2
mmol) and 2-carboxy-1,3-propanedithiol (1.56 g, 10.2 mmol) were
combined in toluene (30 mL) and refluxed under N2 for 15 min,
whereupon the solution color changed from green to red. The
reaction mixture was loaded onto a silica column (10 × 1.5 in)
and eluted with toluene to remove excess starting material as a
dark green solution. The red-orange product was then eluted with
2:1 toluene/ethyl acetate. Solvent was removed in Vacuo to yield
product as a red-orange microcrystalline solid (2.65 g, 60.5%). IR
(THF, cm-1): ν(CO) 2077 (m), 2034 (s), 1992 (s, br), 1978
Experimental Section
General Considerations. Tetrahydrofuran, methanol, hexanes,
acetonitrile, and toluene were degassed and dried by sparging with
N2 gas followed by passage through an activated alumina column.
Deuterated solvents were purchased from either Cambridge Isotope
(32) (a) Sheldrick, G. SHELXS-97: Program for Crystal Structure
Solution; Institut fu¨r Anorganische Chemie der Universita¨t: Gottingen,
Germany, 1977. (b) SAINT-PLUS, version 6.02; Bruker: Madison, WI,
1999. (c) SHELXTL, version 6.1; Bruker: Madison, WI, 1998.
1
(sh), 1730 (m, COOH). H NMR (300 MHz, acetone-d6): δ 3.01

Carboxylic Acid-Modified [FeFe]-Hydrogenase Complexes
Organometallics, Vol. 26, No. 16, 2007 3983
Table 4. Crystallographic Dataa for Complexes 3, 4‚THF, and 6
3‚H2O
4‚THF
6
empirical formula
fw
C17H23Fe2O12PS2
626.16
C20H19Fe2NO8S2
505.08
C15H27F6Fe2O6P3S2
686.10
cryst syst
space group
unit cell
a (Å)
b (Å)
c (Å)
triclinic
P1h
monoclinic
P21/c
monoclinic
P21/n
8.306(2)
8.306(2)
19.337(4)
98.805(3)
92.008(3)
117.698(3)
1222.8(4)
2
9.701(9)
26.87(2)
9.429(8)
90
104.42(2)
90
2379(4)
5
1.762
10.560(3)
19.478(5)
14.017(4)
90
112.13
90
2670(1)
4
1.706
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
d(calcd) (g/cm3)
R1b [I > 2σ(I)]
wR2c
1.652
0.0508
0.1207
0.0913
0.2188
0.0368
0.0946
b
c
2
a Obtained using graphite-monochromatized Mo KR radiation (λ ) 0.71073 Å) at 110(2) K for all structures. R1 ) ∑||Fo| - |Fc||/∑Fo. wR2 ) [∑[w(Fo
2
2
- Fc )2]∑w(Fo )2]]1/2
.
2
3
(dd, J ) 16.4 Hz, J ) 3.6 Hz, 2H, CH2ax), 2.24 (m, 1H, CH),
to 50 °C for 4 h under N2, monitoring by IR spectroscopy to ensure
that the reaction was complete. Upon cooling to room temperature,
volatiles were removed in Vacuo. The resulting air- and moisture-
sensitive red solids were washed with hexanes (2 × 25 mL) and
dried in Vacuo to yield 5 as a red microcrystalline solid (0.458 g,
79.2%). IR (THF, cm-1): 1984 (m), 1948 (vs), 1903 (m), 1729
(w). 1H NMR (300 MHz, acetonitrile-d3): δ 2.70 (m, 2H, CH2ax),
1.69 (m, 1H, CH), 1.50 (m, 20H, CH2eq and PMe3). 31P NMR (121.5
MHz, acetone-d6): δ 27.6 (s, PMe3), 21.5 (s, PMe3). Dec point:
108 °C. Anal. Calcd for C14H24Fe2O6P2S2: C, 31.96; H, 4.60.
Found: C, 32.22; H, 4.96.
{(µ-(SCH2)2CHCOOMe)(µ-H)[Fe(CO)2(PMe3)]2}PF6 (6). Solid
5 (0.268 g, 0.510 mmol) was dissolved in MeOH (10 mL). To this
was added concentrated HCl (10 mL, 12M) dropwise. The solution
gradually changed color from dark red to red-orange. After 45 min,
an aqueous solution of NH4PF6 (0.416 g in 5 mL of H2O) was
added dropwise to precipitate a red-orange solid. Solids were
collected via filtration, washed with H2O (2 × 10 mL) and Et2O
(2 × 20 mL), and dried in Vacuo to yield analytically pure product
(0.260 g, 75.7%). X-ray quality crystals were obtained by cooling
a concentrated methanol solution to -35 °C overnight. IR (MeCN,
cm-1): 2034 (s), 1994 (s), 1739 (w). 1H NMR (300 MHz, acetone-
d6): δ 3.69 (s, 3H, -COOMe), 3.31 (m, 2H, CH2ax), 2.82 (m, 1H,
CH), 2.44 (m, 2H, CH2eq), 1.75 (m, 18H, PMe3), -15.2 (dd, 2JP-H
) 21 Hz, 1H, µ-H). 31P NMR (121.5 MHz, acetone-d6): δ 23.9 (s,
PMe3), 21.7 (s, PMe3), -142.7 (septet, PF6). Dec point: 160 °C.
Anal. Calcd for C15H27F6Fe2O6P3S2: C, 26.26; H, 3.97. Found: C,
26.44; H, 3.96.
2
3
1.88 (dd, J ) 12.9 Hz, J ) 12.6 Hz, 2H, CH2eq). Dec point:
140 °C.
(µ-pdt)[Fe(CO)3][Fe(CO)2{P(C2H4COOH)3}] (3). A solution
of Me3NO (0.578 g, 0.77 mmol) in acetonitrile (5 mL) was added
to a solution of (µ-pdt)[Fe(CO)3]2 (0.300 g, 0.77 mmol) in
acetonirile (30 mL). After 20 min, tris(2-carboxyethyl)phosphine
hydrochloride (0.2207 g, 0.77 mmol) in degassed Mili-Q water was
transferred into the solution via cannula. The reaction mixture was
stirred at room temperature until IR monitoring indicated that there
was no intermediate remaining. After solvent removal in Vacuo,
the solid residue was washed with dry, degassed hexanes (2 × 20
mL) and degassed Mili-Q water (2 × 20 mL). An analytically pure
red solid was obtained in a yield of 80%. Crystals suitable for X-ray
analysis were grown from a layered ether/hexanes solution. IR
(acetonitirile, cm-1): 2041(s), 1981(s), 1966 (sh), 1922 (w), 1733-
(w). 1H NMR (300 MHz, acetone-d6): δ 12.41 (br, 3H, -COOH),
2.57(q, 6H, P-CH2-CH2-), 2.06 (q, 6H, P-CH2-CH2-), 1.81
(br, 2H, -CH2-CH2-CH2-), 1.64 (br, 4H, -CH2-CH2-CH2-
). 31P NMR (121.5 MHz, acetone-d6): δ 52.89. ESI-MS (CH3CN,
neg mode): m/z 607 (M - H+). Dec point: 144 °C. Anal. Calcd
for C17H21Fe2O11PS2 plus 1 equiv of H2O: C, 32.61; H, 3.70.
Found: C, 32.77; H, 3.57.
(µ-(SCH2)2CHCONHPh)[Fe(CO)3]2 (4). To a stirring solution
of 2 (0.5044 g, 1.174 mmol) in THF (20 mL) were added aniline
(0.11 mL, 1.2 mmol) and diisopropylcarbodiimide (0.18 mL, 1.2
mmol). The resulting solution gradually changed color from red to
orange. After 1 h, the solvent was removed in Vacuo. The remaining
orange solids were washed with hexanes (2 × 20 mL). The solids
were then purified by column chromatography (silica, 10 cm),
eluting first with toluene to remove impurities, then with 2:1
toluene/EtOAc to elute product. Solvent was removed from the
eluant in Vacuo. The remaining solids were suspended in hexanes
(30 mL) and THF was added dropwise until most of the solid had
dissolved. This solution was then filtered and cooled to -35 °C
overnight to yield analytically pure product as orange needles
(0.0945 g, 34%). Crystals suitable for X-ray diffraction were grown
via vapor diffusion of hexanes into a concentrated THF solution.
IR (MeCN, cm-1): 2078 (m), 2038 (s), 1997 (s, br), 1690 (w),
{(µ-(SCH2)2CHCOOH)(µ-H)[Fe(CO)2(PMe3)]2}PF6 (7). Solid
2 (0.404 g, 0.941 mmol) was dissolved in THF (25 mL). To this
was added neat PMe3 (0.24 mL, 2.4 mmol). The resulting solution
was heated to reflux for 4 h under N2, monitoring by IR
spectroscopy to ensure that the reaction was complete. Upon cooling
to room temperature, concentrated HCl (20 mL, 12 M) was added
dropwise. The solution gradually changed color from dark red to
red-orange. After 15 min, a saturated aqueous solution of NH4PF6
(10 mL) was added dropwise to precipitate a red-orange solid.
Solids were collected via filtration, washed with H2O (2 × 10 mL)
and Et2O (2 × 20 mL), and dried in Vacuo to yield product as an
orange powder (0.342 g, 54.1%). IR (THF, cm-1): 2032 (s), 1985
1
1600 (w). H NMR (300 MHz, acetone-d6): δ 9.31 (s, 1H, NH),
1
(s), 1732 (w). H NMR (300 MHz, acetone-d6): δ 3.34 (m, 2H,
7.56 (d, J ) 8.4 Hz, 2H, Ar), 7.27 (m, 2H, Ar), 7.08 (m, 1H, Ar),
3.01 (dd, J ) 39 Hz, 3 Hz, 2H, CH2ax), 2.36 (m, 1H, CH), 1.99
(m, 2H, CH2eq). Dec point: 130 °C. Anal. Calcd for C16H11Fe2-
NO7S2: C, 38.05; H, 2.20; N, 2.77. Found: C, 38.02; H, 2.01; N,
3.07.
CH2ax), 2.79 (m, 1H, CH), 2.44 (m, 2H, CH2eq), 1.71 (m, 18H,
PMe3), -15.2 (t, 2JP-H ) 22 Hz, 1H, µ-H). 31P NMR (121.5 MHz,
acetone-d6): δ 21.8 (s, PMe3), 25.1 (s, PMe3), -142.7 (septet, PF6).
Mp: 162 °C.
(µ-(SCH2)2CHCOOH)[Fe(CO)2(PMe3)]2 (5). Solid 2 (0.472 g,
1.10 mmol) was dissolved in THF (30 mL). To this was added
neat PMe3 (0.28 mL, 2.8 mmol). The resulting solution was heated
Acknowledgment. We acknowledge financial support
from the National Science Foundation (CHE-0616695 to

3984 Organometallics, Vol. 26, No. 16, 2007
Thomas et al.
M.Y.D. and CHE-0518047 to M.B.H.) with contributions from
the R.A. Welch Foundation (A-0924 to M.Y.D. and A-0648 to
M.B.H.). A grant from the Spanish Ministerio de Educacio´n y
Ciencia (projects CTQ2006-12097 and CTQ-2005-09105-C04-
01) funded Olaf Ru¨diger as a visiting scientist to TAMU.
as electrocatalysts are as follows: 1, 0.80 V (HOAc); 2, 1.05
V (HOAc); 3, 0.98 V (HOAc) 1.24 V (HCl); 4, 0.97 V (HOAc);
5, 0.96 V (HOAc) 0.82 V (HCl).
Supporting Information Available: Additional cyclic volta-
mmetry and IR data, and crystallographic tables for 3, 4, and 6.
This material is available free of charge via the Internet at
http://pubs.acs.org.
Note added in proof: According to a recent approach to
the evaluation of potentials in hydrogen-producing electrocata-
lysts in non-aqueous solvents (Felton, G. A. N.; Glass, R. S.;
Lichtenberger, D. L.; Evans, D. H. Inorg. Chem. 2006, 46,
9181-9184), the calculated overpotentials for complexes 1-5
OM7003354