Steps along the path to dihydrogen activation at [FeFe] hydrogenase structural models: Dependence of the core geometry on electrocatalytic proton reduction

Article abstract of DOI:10.1021/ic0623361

Differences in the rate of electrocatalytic proton reduction by Fe ₂(μ-PPh₂)₂(CO)₆, DP, and the linked phosphidobridged analogue Fe₂(μ,μ-PPh(CH ₂)₃PPh)(CO)₆, 3P, sugge

Full text of DOI:10.1021/ic0623361

Inorg. Chem. 2007, 46, 1741−1750
Steps along the Path to Dihydrogen Activation at [FeFe] Hydrogenase
Structural Models: Dependence of the Core Geometry on
Electrocatalytic Proton Reduction
Mun Hon Cheah, Stacey J. Borg, and Stephen P. Best*
School of Chemistry, UniVersity of Melbourne, Victoria, Australia 3010
Received December 7, 2006
Differences in the rate of electrocatalytic proton reduction by Fe2(
bridged analogue Fe2( -PPh(CH2)3PPh)(CO)6, 3P, suggest that dihydrogen elimination proceeds through a bridging
hydride. The reaction path was examined using electrochemical, spectroscopic, and in silico studies where reduction
µ-PPh2)2(CO)6, DP, and the linked phosphido-
µ,µ
of 3P gives a moderately stable monoanion [Kdisp(3P-)
)
13] and a distorted dianion. The monomeric formulation
-
of 3P- is supported by the form of the IR and EPR spectra. EXAFS analysis of solutions of 3P, 3P-, and 3P2
indicates a large increase in the Fe
of the 3P, 3P-, 3P2- redox series satisfactorily reproduce the IR spectra in the
(3P) and EXAFS-derived Fe Fe distances. Digital simulation of the electrocatalytic response for proton reduction
−Fe separation following reduction (from 2.63 to ca. 3.1−3.55 Å). DFT calculations
ν(CO) region and the crystallographic
−
indicates a low rate of dihydrogen evolution from the two-electron, two-proton product of 3P (H23P), with more
rapid dihydrogen evolution following further reduction of H23P. Because dihydrogen evolution is not observed upon
formation of H2DP, dihydrogen evolution at the two-electron-reduced level does not involve protonation of a hydridic
Fe
related to the DFT-calculated H
suggests a common reaction path for the thiolato- and phosphido-bridged diiron carbonyl compounds.
−H ligand. The rates of dihydrogen elimination from H2DP, H23P, and H2Fe2(
µ,µ-S(CH2)3S)(CO)6 (H23S) are
−H distances [H23S (1.880 Å) < H23P (2.064 Å) < H2DP (3.100 Å)], and this
Introduction
Although the diiron model compounds present problems
associated with overpotential, dioxygen sensitivity, and
poorly defined dihydrogen oxidation, the high efficiency of
the biological system provides inspiration for the develop-
ment of biomimetic chemistry that will translate into
technologies that will be of practical use. Moreover, the
availability of good structural and functional models6 of the
catalytic center of the enzyme presents the opportunity to
develop a thorough understanding of the chemistry of the
Together with the task of providing a conceptual frame-
work for understanding biological hydrogen activation, the
imperative of a shift to more sustainable technologiess
possibly with extensive use of dihydrogen as an energy
vectorshas stimulated investigations into the chemistry of
bridged diiron compounds that are related to the 2Fe3S
fragment of the [FeFe] hydrogenase active site.1 This activity
has been sustained, at least in part, by the observation that,
in addition to their structural and compositional relationships,
the compounds are also functional models of the enzyme,
catalyzing H2/D2 scrambling2,3 and proton reduction.4,5
(4) Gloaguen, F.; Lawrence, J. D.; Rauchfuss, T. B. J. Am. Chem. Soc.
2001, 123, 9476-9477. Capon, J.-F.; Gloaguen, F.; Schollhammer,
P.; Talarmin, J. Coord. Chem. ReV. 2005, 249, 1664-1676. 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. Ott, S.; Kritikos, M.; Åkermark, B.; Sun, L.; Lomoth,
R. Angew. Chem., Int. Ed. 2004, 43, 1006-1009.
* To whom correspondence should be addressed. Phone: -61-3-
83446505. Fax: -61-3-93475180. E-mail: spbest@unimelb.edu.au.
(1) Nicolet, Y.; Piras, C.; Legrand, P.; Hatchikian, C. E.; Fontecilla-Camps,
J. C. Structure 1999, 7, 13-23. Peters, J. W.; Lanzilotta, W. N.;
Lemon, B. J.; Seefeldt, L. C. Science 1998, 282, 1853-1858.
(2) Zhao, X.; Georgakaki, I. P.; Miller, M. L.; Mejia-Rodriguez, R.;
Chiang, C.-Y.; Darensbourg, M. Y. Inorg. Chem. 2002, 41, 3917-
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C. J. J. Am. Chem. Soc. 2004, 126, 16988-16999.
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2005, 249, 1641-1652. Tard, C.; Liu, X.; Ibrahim Saad, K.; Bruschi,
M.; De Gioia, L.; Davies, S. C.; Yang, X.; Wang, L.-S.; Sawers, G.;
Pickett, C. J. Nature 2005, 433, 610-613. Rauchfuss, T. B. Inorg.
Chem. 2004, 43, 14-26. Georgakaki, I. P.; Thomson, L. M.; Lyon,
E. J.; Hall, M. B.; Darensbourg, M. Y. Coord. Chem. ReV. 2003, 238-
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(3) Tye, J. W.; Hall, M. B.; Georgakaki, I. P.; Darensbourg, M. Y. AdV.
Inorg. Chem. 2004, 56, 1-26.
10.1021/ic0623361 CCC: $37.00
Published on Web 01/27/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 5, 2007 1741

Cheah et al.
LiBEt3H (1 M in THF, Aldrich) and p-toluenesulfonic acid, HOTs
(Merck), were obtained from commercial sources and used without
further purification. High-purity argon and nitrogen were obtained
from BOC Gases. Solvents were dried using standard procedures16
and distilled under an atmosphere of dinitrogen immediately prior
to use. Solutions used for electrochemical analysis were prepared
under a dinitrogen atmosphere either using standard Schlenk
techniques or with the aid of a Vacuum Atmospheres glove box.
The tetra-n-butlyammonium hexafluorophosphate (TBAPF6) used
for electrochemical measurements was synthesized and purified
using standard procedures.17
system and to calibrate the computational approaches directed
toward the elucidation of the enzymatic reaction path.
The enzymatic mechanism of [FeFe] hydrogenase has been
investigated using density functional theory (DFT), and two
different descriptions have emerged.7 Either dihydrogen
activation occurs at the “vacant” apical site of the iron atom
most distant from the 4Fe4S cube of the H cluster,8,9 in which
case the rate of reaction might be accelerated by a bridging
dithiomethylamine cofactor,10 or the reaction occurs opposite
the bridging ligand between the two iron atoms of the 2Fe3S
subsite.7,11 Although the facile formation of hydride-bridged
compounds by protonation of electron-rich dithiolate-bridged
diiron compounds is well documented,2,12 recent studies by
Rauchfuss and co-workers of [Fe2(µ, µ-SCH2CH2S)H(CO)2-
(PMe3)4]+ show clearly that the form with a bridging hydride
is much more inert to protonation and hydrogen elimination
than is the isomer with a terminal hydride.13 Whereas the
mechanistic detail of this reaction has not been resolved, the
high kinetic activity of hydrogenase enzymes would, at first
sight, appear to be inconsistent with formation of a kinetically
inert hydride-bridged intermediate.
Electrochemistry and Spectroelectrochemistry (SEC). Cyclic
voltammetry experiments were controlled using an Autolab PG-
STAT30 potentiostat with GPES software and were carried out in
a one-compartment glass cell using a 1-mm vitreous carbon working
electrode, double-jacketed silver wire pseudo-reference electrode,
and platinum wire counter electrode. The potential of the reference
electrode was determined using the ferrocenium/ferrocene (Fc+/
Fc) couple, and all potentials are quoted relative to the SCE
reference electrode. Against this reference, the Fc+/Fc couple occurs
at +0.56 V in THF.18 Electrochemical simulations were carried
out using the program Digisim (version 3.03, Bioanalytical
Systems).19
The current work explores the influence of the 2Fe2X core
geometry on electrocatalytic proton reduction. The strategy
employed draws on the difference in reduction chemistry
for thiolato- and phosphido-bridged compounds where, for
the latter, there is a tendency to give a product with a planar
2Fe2P core geometry.14 For the reduced 2Fe2P species, the
core geometry can be constrained by incorporation of a
linking group between the phosphorus atoms. The first part
of the article is concerned with characterization of the
reduction products of the propylene-linked compound. The
influence of this structural constraint on the chemistry is then
examined with reference to electrocatalytic proton reduction.
Differences between the rates of dihydrogen evolution from
the doubly reduced and protonated phosphido- and thiolato-
bridged diiron compounds is then used to evaluate the
influence of the core geometry on the catalytic reaction and
to cast light on the reaction path. These results are then
considered in the context of the proposed reaction path for
the enzyme.
Spectroelectrochemical experiments were performed using a
previously described20 purpose-built cell. All experiments were
conducted using a 3-mm-diameter vitreous carbon working elec-
trode, a silver pseudo-reference electrode, and a platinum foil
counter electrode. A PAR model 362 potentiostat was used to
control the potential, and a Powerlab 4/20 interface with Chart
V4.12 software (ADInstruments) was used to monitor the potential
and current response during SEC experiments.
Continuous-flow electrosynthesis experiments were conducted
using a cell described in the literature21 that incorporates reticulated
vitreous carbon as the working and counter electrodes and a jacketed
silver wire as a pseudo-reference electrode. Standard liquid chro-
matography fittings in conjunction with narrow-bore (0.2-mm-i.d.)
Teflon tubing was used to transfer solutions from the syringe pump
to the electrosynthesis cell and then to the sample cells. The extent
of electrosynthesis was monitored by IR spectroscopy.
EXAFS Sample Preparation and Analysis. Fe K-edge (7111.2
eV) X-ray absorption measurements were conducted using the
bending magnet source of beamline 20B at the KEK Photon Factory
Tsukuba, Tsukuba, Japan. A channel-cut Si(111) monochromator
with energy resolution (∆E/E) of ca. 2.4 × 10-4 provided the source
of monochromatic radiation, where higher-order harmonics at the
selected wavelength were rejected by detuning the monochromator
Experimental Details
General Procedures. Samples of Fe2(µ, µ-PPh(CH2)3PPh)(CO)6,
3P were prepared using literature methods15 and confirmed to be
pure by spectroscopic and electrochemical analyses. Samples of
1
by a factor of /2. Solutions were measured in fluorescence mode
using a 36-element Ge detector (Canberra). Data analysis was
conducted using the XFIT suite of programs,22 which incorporates
FEFF version 6.01.23 Details of the procedures followed for the
(7) Zampella, G.; Greco, C.; Fantucci, P.; De Gioia, L. Inorg. Chem. 2006,
45, 4109-4118.
(8) Cao, Z.; Hall, M. B. J. Am. Chem. Soc. 2001, 123, 3734-3742. Liu,
Z.-P.; Hu, P. J. Chem. Phys. 2002, 117, 8177-8180.
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(10) Fan, H.-J.; Hall, M. B. J. Am. Chem. Soc. 2001, 123, 3828-3829.
(11) Zhou, T.; Mo, Y.; Liu, A.; Zhou, Z.; Tsai, K. R. Inorg. Chem. 2004,
43, 923-930. Bruschi, M.; Fantucci, P.; De Gioia, L. Inorg. Chem.
2002, 41, 1421-149.
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Chemistry; Blackie Academic & Professional: London, 1997.
(17) Sawyer, D. T.; Sobkowiak, A.; Roberts, J. J. L. Electrochemistry for
Chemists, 2nd ed.; Wiley-Interscience: New York, 1995.
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589A-600A.
(12) Arabi, M. S.; Mathieu, R.; Poilblanc, R. J. Organomet. Chem. 1979,
177, 199-209.
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(21) Bondin, M. I.; Foran, G.; Best, S. P. Aust. J. Chem. 2001, 54, 705-
709.
(13) van der Vlugt, J. I.; Rauchfuss, T. B.; Whaley, C. M.; Wilson, S. R.
J. Am. Chem. Soc. 2005, 127, 16012-16013.
(14) Ginsburg, R. E.; Rothrock, R. K.; Finke, R. G.; Collman, J. P.; Dahl,
L. F. J. Am. Chem. Soc. 1979, 101, 6550-6562.
(22) Ellis, P. J. Xfit for Windows 95; Australian Synchrotron Research
Program: Sydney, Australia, 1996. Ellis, P. J.; Freeman, H. C. J.
Synchrotron Radiat. 1995, 2, 190-195.
(23) Zabinsky, S. I.; Rehr, J. J.; Ankudinov, A.; Albers, R. C.; Eller, M. J.
Phys. ReV. B: Condens. Matter Phys. 1995, 52, 2995-3009.
(15) Collman, J. P.; Rothrock, R. K.; Finke, R. G.; Moore, E. J.; Rose-
Munch, F. Inorg. Chem. 1982, 21, 146-156. Flood, T. C.; DiSanti,
F. J.; Campbell, K. D. Inorg. Chem. 1978, 17, 1643-1646.
1742 Inorganic Chemistry, Vol. 46, No. 5, 2007

Electrocatalysis by Strained 2Fe2P Carbonyls
analysis of related systems have been described previously.5,24,25
Samples of the reduced compounds were generated by continuous-
flow electrosynthesis using 5 mM (i.e., 10 mM in Fe) solutions of
3P in THF/0.2 M TBA[PF6]. The potential of the cell was controlled
using a PAR model 273A potentiostat, and the composition of the
solution was monitored, in line, by IR spectroscopy (Shimadzu,
Prestige), with the sample freeze-quenched in liquid nitrogen.
Instrumentation. IR SEC spectra were collected using a Biorad
FT175C FTIR spectrometer with a Ge/KBr beamsplitter and a
narrow-band MCT detector. Spectral subtraction and curve fitting
was performed using Grams/32 AI software (Galactic). Multicom-
ponent analysis was conducted using routines available within the
program Igor Pro (version 5.04B, Wavemetrics). A Bruker ECS106
X-band spectrometer was used to collect EPR spectra.
DFT Calculations. All reported calculations were conducted
using the hybrid functional B3LYP as implemented in the Gaussian
03 (G03, revision B.04) package26 using the all electron basis set
6-311+G(d) with diffuse and polarization functions from the
internal library of G03. Calculations were conducted on an analogue
of 3P in which the phenyl rings were replaced by methyl groups.
This simplification significantly reduced the time needed for the
calculations to reach convergence. Although a similar outcome
might have been obtained by restricting the conformational freedom
of the phenyl rings, previous calculations27 suggested that methyl
substitution provides a satisfactory alternative. The calculated
analogue of 3P is designated [3PM], where the subscript indicates
the replacement of the phenyl by methyl groups and the square
brackets designate a DFT-calculated species. All calculations were
performed as restricted spin singlets except for [3PM-], which was
treated as an unrestricted spin doublet. The crystal structure of 3P
was used as input for geometry optimization of [3PM]; subsequent
optimization of [3PM-] proceeded from the optimized geometry
of [3PM] with addition of a single electron specified and no further
constraints. Repeating the same procedure gave [3PM2-]. Vibrational
analyses were carried out as second derivatives and confirmed that
all optimized geometries were true minima.
Figure 1. (a) Cyclic voltammogram (100 mV/s) of 3P in THF/0.2 M
TBAPF6 (all potentials are given with reference to the SCE). (b) Differential
absorption IR spectra recorded during reduction of a 4.8 mM solution of
3P in THF/0.2 M TBAPF6. Spectra were recorded with a 1.5-s interval
following a potential step from 0 to -1.8 V where, for clarity, every fourth
spectrum is shown. (c) Time dependence of the concentrations of 3P, 3P-,
and 3P2- obtained by fitting the SEC spectra. The solid line corresponds
to the concentration of 3P- calculated using the concentrations of 3P and
3P2- and an equilibrium constant of 13 for the disproportionation of 3P-.
Atom Topological Model and atomic radii optimized for the PBE0/
6-31G(d) level of theory. All final geometry optimizations and
subsequent vibrational analyses were conducted within the PCM
framework described above.
Solvation effects were included using the PCM model28 as
implemented in G03 with THF as the implicit solvent. Molecular
cavities were built as solvent-excluding surfaces using the United
Results and Discussion
Electrochemistry of 3P. The electrochemical response of
3P is similar to that of Fe2(µ- PPh2)2(CO)6, DP, described
previously.25 The assignment of the single reduction wave
(-1.52 V, Figure 1a) to a two-electron process is suggested
by the relative magnitudes of the peak anodic and cathodic
currents relative to that obtained for oxidation of an equimo-
lar solution of ferrocene when measured in the same
experiment and is corroborated by the magnitude of the
charge transferred during exhaustive electrosynthesis of the
thin layer during SEC experiments (2.2 ( 0.2 electrons/
molecule). The conclusion is supported by analysis of the
IR-active ν(CO) bands of the reduction product, 3P2-,
described in the next section. Whereas alkali metal reduction
also yields solutions of 3P2-, it has not proved possible to
obtain a sample of this species in a form suitable for X-ray
structural characterization. Although the assignment of the
two-electron character of the reduction process of 3P is
unambiguous, the value of ∆Ep (ca. 137 mV, 100 mV s-1)
in THF is substantially larger than that obtained for the Fc+/
Fc couple (78 mV) when measured under the same condi-
tions. This indicates that the process does not involve
(24) Bondin, M. I.; Borg, S. J.; Cheah, M. H.; Foran, G.; Best, S. P. Aust.
J. Chem. 2006, 59, 263-272.
(25) Cheah, M. H.; Borg, S. J.; Bondin, M. I.; Best, S. P. Inorg. Chem.
2004, 43, 5635-5644.
(26) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin,
K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone,
V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G.
A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.;
Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R.
E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J.
W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.;
Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.;
Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari,
K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.;
Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.;
Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A.
Gaussian 03, revision B.04; Gaussian, Inc.: Wallingford CT, 2004.
(27) Baik, M.-H.; Ziegler, T.; Schauer, C. K. J. Am. Chem. Soc. 2000,
122, 9143-9154.
(28) Cammi, R.; Mennucci, B.; Tomasi, J. J. Phys. Chem. A 1999, 103,
9100-9108. Cammi, R.; Mennucci, B.; Tomasi, J. J. Phys. Chem. A
2000, 104, 5631-5637. Mennucci, B.; Cances, E.; Tomasi, J. J. Phys.
Chem. B 1997, 101, 10506-10517. Mennucci, B.; Tomasi, J. J. Chem.
Phys. 1997, 106, 5151-5158.
Inorganic Chemistry, Vol. 46, No. 5, 2007 1743

Cheah et al.
) 13, where the value of Kdisp was obtained from continuous-
flow electrosynthesis experiments described in the following
section. It is noted that the spectrum of 3P2- obtained by
subtraction is contaminated by a small concentration of 3P-,
and this accounts for the poorer agreement between the
observed and calculated concentrations of 3P- at longer
times. Experiments conducted at lower temperatures did not
give a more pronounced initial increase in the concentration
of 3P-, and this supports the proposition that the neutral,
monoanion, and dianion are in thermodynamic equilibrium.
Corresponding experiments conducted in CH3CN showed a
similar conversion of 3P to 3P2-, although without the
formation of significant concentrations of 3P- (Figure S2).
2 3P- h 3P + 3P2-
Kdisp
(1)
Analysis of the SEC results suggests that 3P- might be
formed by comproportionation of 3P and 3P2-. Support for
this proposition is provided by continuous-flow electrosyn-
thesis experiments in which the products are monitored, in
line, by IR spectroscopy. Spectra recorded at the resting
potential, at potentials sufficient to give quantitative conver-
sion to 3P2- and near the half-wave potential, are shown in
Figure 2. The spectrum of the intermediate product, 3P-,
can be obtained by subtraction (Figure 2d). The IR spectra
generated from the continuous-flow electrosynthesis experi-
ments match closely those extracted from the in situ SEC
experiments (Figure 1). Analysis of the IR spectra indicates
that the composition of the solution has a 3P/3P-/3P2- ratio
of 43:12:45, which gives Kdisp ) 13. This result is consistent
with the analysis of the SEC spectra (Figure 1c). The value
of Kdisp requires that the reduction potential of 3P is 67 mV
more negative than that of 3P-. In cases where the applied
potential is such as to give approximately equal concentra-
tions of 3P and 3P2-, the relative concentration of 3P- does
not change significantly if the flow rate of the solution (i.e.,
the time between electrosynthesis and collection of the IR
spectrum) is changed. Moreover, if the flow from the
electrosynthesis cell is diverted so as to give a static solution
in the IR cell, then the relative concentrations of the three
species are found not to change to a significant extent over
a 10-min period.
The monomeric form of 3P- is suggested both by the IR
spectrum and by the observation of a strong EPR signal from
a THF solution obtained by mixing equimolar amounts of
3P and 3P2-, where the latter was obtained by sodium re-
duction. At room temperature, a single resonance was observ-
ed in which the hyperfine coupling to 31P was not resolved.
An identical room-temperature EPR spectrum was obtained
from solutions generated by continuous-flow electrosynthesis,
where this approach allows the composition of the solution
to be established by IR spectroscopy. EPR spectra recorded
from these solutions give a single resonance at room
temperature (giso ) 1.995, Figure S3) and a rhombic spectrum
for frozen solutions (g1 ) 1.985, g2 ) 1.994, g3 ) 2.018).
Hyperfine coupling to 31P, a(31P), is evident for each of the
components of g (Figure 3). The g value obtained for 3P-
is similar to that reported for [Fe2(µ-PMe2)2(CO)6]1- (giso
Figure 2. Representative IR spectra recorded from a solution of 3P in
THF/TBAPF6 following continuous-flow electrosynthesis at potentials of
(a) 0, (b) -2.0, and (c) -1.5 V and (d) the spectrum of 3P- obtained by
the subtraction of the spectra of 3P and 3P2- from that obtained at potentials
near -1.5 V.
diffusion-controlled electron transfer, for which ∆Ep can, for
example, be modeled by a process involving two one-electron
steps where the rate of electron transfer is low for one of
the steps.
Spectroelectrochemistry (SEC) of 3P. Reduction of 3P
in THF at -1.8 V in a thin-layer SEC experiment leads to
quantitative formation of a well-defined product, 3P2-, with
an IR spectrum similar to that of the crystallographically
characterized complex DP2- (Figure 1b). The process is
chemically reversible, with near quantitative recovery of the
starting material obtained by application of a potential of
ca. -1.0 V (Figure S1). The absence of an isosbestic point
in the spectra recorded during reduction indicates the
formation of significant concentrations of at least one other
species during the conversion, where the transient appearance
of a band near 1940 cm-1 (Figure 1b) provides a clear marker
of such an intermediate. The IR spectrum of this species can
be extracted from the differential absorption SEC spectra.
The spectral profile of the intermediate is closely related to
that of the starting material but shifted to lower wavenumber
(Figure 2), suggesting retention of the core geometry with
the shift of the ν(CO) bands consistent with formation of
the one-electron-reduced species, 3P-. The spectra recorded
during reduction can be satisfactorily fitted to a combination
of the spectra of 3P, 3P-, and 3P2- (Figure 1c) with no other
species formed in significant concentrations. Although mix-
ing of the solution in the thin layer might complicate the
analysis, the observed concentration of 3P- is well modeled
by the disproportionation equilibrium of 3P- (eq 1) with Kdisp
1744 Inorganic Chemistry, Vol. 46, No. 5, 2007

Electrocatalysis by Strained 2Fe2P Carbonyls
Table 1. EXAFS Parameters and Refinement Statisticsa
3P
3P2-
2
E0/eV (S0 )
ø2 (REXAFS/%)
Fe-Fe/Å
-7.97 (0.78)
-12.87 (0.73)
3.14 (14.99)
3.55 (0.0011)
2.23 (0.0015)
1.75 (0.0008)
1.16 (0.0031)
1.60 (0.0031)
1.67 (10.28)
2.63 (0.0015)b [2.630]
2.21 (0.0013) [2.212c]
1.78 (0.0013) [1.791c]
1.16 (0.0028) [1.140c]
1.87 (0.0009) [1.828c]
Fe-µ-P/Å
Fe-C(O)/Å
C-O/Å
P-CR/Å
a A total of 13 independent parameters were refined over the range 1-13
Å-1 in k and 1-4 Å in R (25 independent points35). Details of the refinement
parameters are given in Table S1, and plots of the observed and calculated
EXAFS spectra are shown in Figure S4. Definitions of ø2 and REXAFS are
given in ref 25. b Debye-Waller factors (Å2) are given in parentheses, and
X-ray-determined values34 are given in square brackets. The error in the
internuclear contacts is dominated by the typical systematic error, assigned
a conservative consensus value of 0.02 Å.36 c Average value.
Figure 3. X-band EPR spectrum recorded from a frozen solution (120
K) of partly reduced 3P THF/TBAPF6 prepared by continuous-flow
electrosynthesis.
) 1.999),29 and the apparently small value of a(31P) is
consistent with the more highly resolved spectrum reported
in a subsequent study [a(31P) ) 4.5 G and a(1H) ) 1.4 and
<0.2 G].30 The EPR spectrum reported for [Fe2(µ- P(But)2)2-
(CO)5]1-, a decarbonylated analogue of 3P- that features an
Fe-Fe double bond, has a giso value of 2.037 and an a(31P)
value of 14.7 G.31 Hyperfine coupling to the protons of the
carbon atom bound directly to the P atom was not resolved
in the spectra. More extensive studies of diiron compounds
bridged by a single phosphido group, Fe2(µ-PR2)(CO)7, give
values of giso close to 2.05 and a(31P) of ca. 23 G, whereas
the hyperfine coupling to the protons of the dialkylphosphido
bridged compounds could not be resolved.32,33 In addition
to the structure in the frozen EPR spectrum, retention of the
two phosphorus atoms at the diiron core of 3P- is supported
by the transient (ca. 30 min) observation of a three-line EPR
spectrum immediately following introduction of low con-
centrations of dioxygen; addition of dioxygen to 3P- or 3P2-
results in solutions containing 3P. The three-line pattern is
well simulated by a doublet of doublets with a giso value of
2.045 and a(31P) values of 19.1 and 11.7 G. The inequiva-
lence of the two phosphorus atoms of the oxidation product
of 3P- might reflect a change in the coordination geometry
as would occur if one of the P atoms adopted a terminal
mode of coordination. The EPR spectrum of [FeCo(µ-PPh2)-
(CO)6(PPh3)]+ gives a three-line spectrum, which, although
reported as a triplet,33 can also be simulated by a doublet of
doublets.
obtained crystallographically.34 As expected, there is a
substantial increase in the Fe-Fe distance following reduc-
tion, although the resulting value is shorter for 3P2- than
for DP2-. Although these measurements provide good
estimates of the Fe-Fe and Fe-P distances, there are no
multiple-scattering paths that are sufficiently close to linear
(Fe-P-X angle >150°) to make a significant contribution
to the scattering to allow definition of the 2Fe2P core
geometry. Although the P-P separation, from P K-edge
EXAFS spectra, would resolve this problem, measurements
in the required energy range (2.14 keV) are not possible using
beamline 20B.
The product distribution obtained from electrochemical and
chemical reduction of 3P indicates that the disproportionation
equilibrium is rapidly established, and this limits the
concentration of 3P- available in solution. Even in THF,
where the relative stability of 3P- is greatest, the maximum
relative concentration of the anion amounts to only ca. 12%
of the total concentration of the 3P species. In this case, the
range of structural information for 3P- that can be extracted
from EXAFS analysis is restricted to the determination of
the Fe-Fe distance.
Two solutions having approximately equal concentrations
of 3P and 3P2- were used for collection of Fe K-edge X-ray
absorption spectra. IR analysis indicates the presence of
approximately 12% of 3P- for both solutions (Figure 2). The
simplest and most highly constrained method for extracting
the Fe-Fe separation for 3P- involves modeling the data
using the EXAFS-derived structural parameters for the
neutral and dianion forms with the populations of these
species fixed according to the composition determined using
IR spectroscopy. A single Fe-Fe interaction with population
fixed at 0.12 requires two parameters, and these, together
with E0 and S02 were refined. In cases where the initial Fe-
Fe distance was in the range of 2.8-3.4 Å, the refinement
proceeded to local minima with refined Fe-Fe distances of
2.76, 3.03, and 3.26 Å. The analysis was repeated using
several different relative populations of 3P and 3P2- with
ratios ranging between 40:48 and 48:40, and this showed
EXAFS of the 3P Redox Series. In the absence of
crystalline samples suitable for X-ray crystallographic analy-
sis, the basic details of the core geometry of the reduced
forms of 3P can be obtained using EXAFS techniques. The
EXAFS data of 5 mM THF solutions of 3P and 3P2- have
been modeled using approaches described in a recent work,24
and the results are summarized in Table 1. In the case of
3P, it is possible to verify the accuracy of the derived
parameters, which are in excellent agreement with those
(29) Dessy, R. E.; Kornmann, R. L.; Smith, C.; Haytor, R. J. Am. Chem.
Soc. 1968, 90, 2001-2004.
(30) Dessy, R. E.; Rheingold, A. L.; Howard, G. D. J. Am. Chem. Soc.
1972, 94, 746-752.
(31) van der Linden, J. G. M.; Heck, J.; Walther, B.; Boettcher, H. C. Inorg.
Chim. Acta 1994, 217, 29-32.
(32) Baker, R. T.; Krusic, P. J.; Calabrese, J. C.; Roe, D. C. Organometallics
1986, 5, 1506-1508.
(34) Reingold, A. L. Acta Crystallogr. C: Cryst. Struct. Commun. 1985,
41, 1043.
(35) Stern, E. A. Phys. ReV. B 1993, 48, 9825-9827.
(36) Gurman, S. J. J. Synchrotron Radiat. 1995, 2, 56-63.
(33) Baker, R. T.; Calabrese, J. C.; Krusic, P. J.; Therien, M. J.; Trogler,
W. C. J. Am. Chem. Soc. 1988, 110, 8392-8412.
Inorganic Chemistry, Vol. 46, No. 5, 2007 1745

Cheah et al.
Table 2. Summary of the Final Refinement Parameters from DFT
Structure Optimization for the [3PM] Redox Series
-
2-
[3PM]
2.677
[3PM
]
[3PM ]
Fe-Fe/Å
Fe-Pa/Å
3.142
2.299
1.783
1.779
2.724
116.0
3.638
2.334
1.754c
2.257
Fe-COapa/Å
Fe-CObasa/Å
P-P/Å
1.775 [1.768]b
1.800 [1.787]
2.803 [2.742]
98.3 [98.5]
2.591
139.2
dihedrald
a Average value. b X-ray value.34 c Apical/basal positions are not defined
for [3PM2-]. d Defined as the Fe-P-P-Fe dihedral angle.
Figure 5. Comparison of the wavenumbers of the IR-active V(CO) bands
of 3P (9), 3P- (2), and 3P2- (b)
by Tye et al.39 most likely reflects differences in the basis
sets and inclusion of solvation effects in the present study.
Although X-ray data are not available for 3P2-, previous
investigations have shown that this level of theory gives very
satisfactory results for the related compound, DP2-, for which
the calculated Fe-Fe distance is within 0.1 Å of that obtained
crystallographically.14, 27 After the systematic differences in
the calculated bond lengths have been taken into consider-
ation, there is good agreement between the EXAFS-derived
Fe-Fe and Fe-P bond lengths (∆r ) 0.089 and 0.103 Å)
for 3P2- (Tables 1 and 2), and there is excellent agreement
between the calculated and observed ν(CO) IR spectra
(Figure 5).
For 3P-, the Fe-Fe distance is poorly defined by the
EXAFS analysis and gives an estimate of ca. 3.1 Å. In terms
of the uncertainty associated with the extraction of this
parameter from the EXAFS spectrum of a species that
represents a minor component of the sample, the agreement
between the calculated and experimental Fe-Fe distances
for 3P- is considered to be satisfactory. Further validation
of the calculations is obtained from the good agreement
between the calculated and observed IR spectra (Figure 5).
It is apparent from the calculated structures that the
propylene linking group severely impacts the geometry of
the dianion and the large increase in Fe-Fe distance is
accommodated by a highly distorted structure (Figure 4).
Clearly, the propylene linker prevents the 2Fe2P core from
adopting a planar arrangement, with the dihedral angle of
the butterfly increasing from 98° for [3PM] to 139° for
[3PM2-]; a dihedral angle of 180° is obtained for the planar
core geometry of DP2-.14 The coordination environment
about each Fe center was calculated to be approximately
square-pyramidal, with one of the CO groups in the apical
position and distortion of the basal positions resulting from
a P-Fe-P angle of 67.5°. The C atoms bound directly to
the bridging P have significantly different nonbonded
contacts to the Fe atoms (3.400 and 3.625 Å), and addition-
ally, there is one Fe-Fe-C(O) angle close to 90° for each
Fe atom, giving an additional nonbonded contact closer than
4 Å. The simple model used to fit the EXAFS data does not
allow for distortions of this sort and, if reflected in the
Figure 4. DFT-optimized structures of the 3PM redox series; for clarity,
the hydrogen stoms have been omitted.
that the final Fe-Fe distance is not correlated with the
solution composition (Table S2).
DFT Calculations of the 3P Redox Series. Geometry
optimization of [3PM], [3PM-], and [3PM2-] proceeded to
give well-defined minima with the structures shown in Figure
4 and selected structural parameters given in Table 2. For
related dithiolato-bridged compounds, the reliability of the
calculations for prediction of the structure and IR spectra is
well established,7,8,37-39 and the extension of this approach
to the linked phosphido-bridged compound was assessed with
reference to 3P. There is close agreement between the details
of the X-ray and calculated structure, including the small
difference in Fe-C bond lengths for the basal and apical
CO groups and the 2Fe2P dihedral angle (Table 2). The
discrepancy between the Fe-Fe (0.048 Å) and Fe-P (0.046
Å) distances are basis-set-dependent, and the systematic
overestimation of these distances by ca. 0.05 Å for the neutral
compound is in keeping with earlier observations.24 The good
agreement between the observed and computed spectra
(Figures 5 and S5) is in keeping with recent studies of the
correlation between the computed and observed IR spectra
in the ν(CO) region.37,39 The different scaling factor that
would apply for the present study relative to that reported
(37) Zilberman, S.; Stiefel, E. I.; Cohen, M. H.; Car, R. J. Phys. Chem. B
2006, 110, 7049-7057.
(38) Zampella, G.; Bruschi, M.; Fantucci, P.; Razavet, M.; Pickett, C. J.;
De Gioia, L. Chem.-Eur. J. 2005, 11, 509-520. Fiedler, A. T.;
Brunold, T. C. Inorg. Chem. 2005, 44, 1794-1809. Fiedler, A. T.;
Brunold, T. C. Inorg. Chem. 2005, 44, 9322-9334. Bruschi, M.;
Zampella, G.; Fantucci, P.; De Gioia, L. Coord. Chem. ReV. 2005,
249, 1620-1640.
(39) Tye, J. W.; Darensbourg, M. Y.; Hall, M. B. J. Comput. Chem. 2006,
27, 1454-1462. Borg, S. J.; Tye, J. W.; Hall, M. B.; Best, S. P. Inorg.
Chem. 2007, 46, 384-394.
1746 Inorganic Chemistry, Vol. 46, No. 5, 2007

Electrocatalysis by Strained 2Fe2P Carbonyls
electron/proton additions as proposed5 for Fe2(µ,µ-S(CH2)3S)-
(CO)6 (3S) is not ruled out by this analysis, the additional
complication is not required for satisfactory modeling of the
voltammetry. The electrocatalytic current is limited by the
low rate of dihydrogen elimination from H23P, and further
reduction of this species results in rapid evolution of
dihydrogen.
It is important to note that electrocatalytic proton reduction
by 3P is markedly different from that of DP, where addition
of HOTs shifts the potential of the primary reduction wave
but no electrocatalytic response is obtained.25 The current
response of 3P is similar to that of 3S, in which dihydrogen
evolution follows two-electron reduction of the complex in
acid with more rapid dihydrogen evolution following further
reduction.5 The dependence on acid concentration of the
current at the potential of the primary reduction process
provides a measure of the relative rates of dihydrogen
elimination from the two-electron, two-proton products.
Under dinitrogen and comparable conditions of scan rate and
concentration, the ratio of the current obtained for 3S in the
presence and absence of 6 equiv of HOTs is 4.4, whereas
for 3P, the ratio is 1.7 [although, in the latter case, the two-
electron character of the process suggests that doubling the
ratio (to 3.4) is more appropriate]. In either case, the
difference in catalytic current obtained for process 1 indicates
a lower rate for dihydrogen elimination from H23P than from
H23S. Electrochemical simulation allows quantification of
the rate constants and suggests a difference of nearly a factor
of 2 for the first-order rate constants.
The reaction path given in Figure 6c allows for loss of
the catalyst through rearrangement of H3P- such that there
is no loss or gain of CO associated with formation of 3PW.
Consistent with this proposal, addition of CO does not lead
to a significant change in the current response for the
electrocatalytic reaction. This observation is in contrast to
that found for 3S for which there is strong inhibition of
electrocatalysis by CO.5 In that case, the predominant side
product of the reaction is a rearranged 7-CO species [Fe2(µ-
S(CH2)3SH)(µ-CO)(CO)6]1-.
SEC of 3P during Proton Reduction. The current and
spectroscopic responses to reduction and reoxidation of a
solution containing 3P with 10 equiv of HOTs are shown in
Figure 7. During the initial period of reduction (<45 s), there
is a significant current flow, but there is virtually no
spectroscopic change (Figure 7a). Analogous experiments
conducted in the absence of 3P show a much slower
depletion of the bands due to HOTs (<10%) for final
reduction potentials in the range from -1.6 to -2.2 V. These
observations require that the catalytic process includes the
neutral complex, i.e., dihydrogen evolution occurs from the
doubly reduced form of 3P. These results are in sharp
contrast to those obtained with DP, for which proton
reduction requires access to the three-electron-reduced level
and elimination of dihydrogen results in reduced forms of
DP.25
Figure 6. (a) Cyclic voltammetry of 3P (1.15 mM) in THF/0.2 M TBAPF6
with 0-6 equiv of HOTs and (b) Digisim19 simulation of the electrochem-
istry based on the scheme given in the inset (c). Full details of the simulation
parameters are given in Table S3.
structure of 3P2-, would contribute to the poorer fit obtained
for modeling of the EXAFS data of 3P2- relative to DP2-.
A small improvement in the refinement statistics for the
EXAFS modeling of 3P2- was obtained by elaboration of
the model to allow for these distortions; however, the
improvement was not sufficient to allow assignment of these
details of the structure.
Electrocatalytic Proton Reduction by 3P. Reduction of
3P in the presence of HOTs is accompanied by well-defined
electrocatalytic proton reduction waves with Epc equal to
-1.55 V (process 1) and -1.68 V (process 2) that dominate
at low and high acid concentrations, respectively, together
with an additional weak feature with Epc equal to -2.5 V
(Figure 6a). The voltammetry of 3P in the presence of HOTs
can be modeled satisfactorily using the scheme shown in
Figure 6c. At low acid concentrations, an EECC mechanism40
operates in which two one-electron transfer (E) steps precede
the chemical steps (C) where these involve protonation (×2)
and dihydrogen elimination. The difference in potential for
reduction of 3P and 3P- is fixed by the value of Kdisp
.
Although a contribution to the reaction path from sequential
As the HOTs concentration becomes depleted, a weak
additional set of bands appear transiently in the spectra with
wavenumbers and band profile similar to those of H2DP
(40) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals
and Applications; Wiley: New York, 1980.
Inorganic Chemistry, Vol. 46, No. 5, 2007 1747

Cheah et al.
Table 3. Summary of the Final Parameters from DFT Structural
Refinement of the Isomers of [H23PM] and cis-[H2DPM]
[H23PM]
down:down up:down
up:up
cis-[H2DPM]
∆G/(kcal mol-1
Fe-Fe/Å
)
2.63
0
1.55
3.653
2.342
1.792
1.824
2.632
141.0
1.512
4.334
3.619
2.343
1.790
1.821
2.613
137.0
1.518
2.066
3.621
2.342
1.792
1.824
2.626
138.0
1.516
4.311
3.634
2.327
1.798
1.816
2.906
176.6
1.524
3.104
Fe-Xa/Å
Fe-COa/Å
Fe-COaxiala/Å
X-X/Å
dihedralb
Fe-Ha/Å
H-H/Å
a Average value. b Defined as the Fe-P-P-Fe dihedral angle.
coordination sites normal to the 2Fe2P plane.41 Both cis and
trans isomers are obtained, in a relative population of 5:9.
The two strong IR bands reported for the compound (2095
and 2054 cm-1) match well the spectra reported for H2DP
(2069 vw, 2053 vs, 2007 s, and 2001 s)25 and H23P (Figure
7e), where the wavenumber shift is explained in terms of
the strongly electron-withdrawing CF3 groups. In the case
of 3P2-, protonation would give a canted edge-shared
bioctahedral geometry with CO and H ligands occupying
the axial sites of each Fe atom. These coordination sites are
distinguished according to whether the hydrogen atoms are
opposite (down) or adjacent (up) to the propylene group
linking the phosphorus atoms. Three isomers based on this
geometry are distinguished: up:up, up:down, and down:
down. DFT-based geometry optimization proceeded from
[3PM2-] with an additional proton added in a position
bridging the two Fe atoms. The final geometry for [H3PM]-
had a terminally bound hydride in an axial position where
up-[H3PM-] was calculated to be more stable than down-
[H3PM-] by 2.75 kcal mol-1. Geometry optimization fol-
lowing addition of a second proton proceeded to well-defined
minima for each of the isomers of [H23PM], with selected
structural parameters reported in Table 3. The calculated IR
spectra of the isomers of [H23PM] have band profiles similar
to each other and to that of the additional species formed
during reduction in the presence of HOTs (Figures 7e and
S6). The offset between the observed and calculated ν(CO)
frequencies is similar to that obtained for the corresponding
modes of 3P.
Dihydrogen elimination from H23P might involve proto-
nation of a basic terminally bound hydride or a dihydride
such as down:down-H23PM. The structures of H2DP and
H2FP provide qualified support for the latter alternative. The
striking difference in the rate of dihydrogen elimination from
H23P and H2DP might be reflected by the ground-state
geometries, and the calculated structures of down:down-
[H23PM] and cis-[H2DPM] are shown in Figure 8. Clearly,
the difference in the 2Fe2P core geometry has a significant
effect on the H-H distance (Table 3). Computational studies
of the reaction path for electrocatalytic proton reduction by
3S have recently been reported.42 The pathway is suggested
Figure 7. Top: Applied potential and current response from the SEC
experiment conducted on a sample of 3P with 10 equiv of HOTs. The
current is displayed as “fill to zero”. Bottom: (a-d) Differential absorbance
IR spectra recorded during the SEC experiment with the last spectrum of
each group highlighted. Spectra of additional species obtained by subtraction
of 3P and 3P2- from the SEC results (e) during the latter stages of reduction
at -2.2 V and (f) immediately prior to oxidation at +0.6 V.
(Figure 7e).25 During this period, there is gradual conversion
of 3P into two predominant products that are identified as
3PW and 3P2- (Figure 7b). The initial product, 3PW, has an
IR band at 1705 cm-1, suggesting rearrangement to give a
product with a bridging CO group. The ν(CO) band profile
is closely related to that of a previously identified protonated
two-electron-reduced product of DP (Figure 7f).25 The
geometry proposed for 3PW has bridging CO and phosphido
groups and a terminally bound phosphine ligand. A more
detailed study of the characterization of the structure and
chemistry of 3PW is to be the subject of a separate
publication. Formation of 3P2- occurs at longer times when
the proton concentration in the thin layer is sufficiently low.
Application of a potential of -1.0 V leads to selective
oxidation of 3P2- with recovery of 3P (Figure 7c). More
positive potentials are required to oxidize 3PW, and this leads
to recovery of ca. 90% of the 3P present at the start of the
experiment (Figure 7d).
DFT Investigation of H23P. Insight into the structure of
1
(41) Dobbie, R. C.; Whittaker, D. J. Chem. Soc., Chem. Commun. 1970,
796-797.
(42) Greco, C.; Zampella, G.; Bertini, L.; Bruschi, M.; Fantucci, P.; Gioia,
L. D. Inorg. Chem. 2007, 46, 108-116.
H23P can be gained from an earlier H NMR study of H2-
Fe2(µ-P(CF3)2)2(CO)6, H2FP, which indicated that the hydride
ligands are bound to different Fe atoms and occupy
1748 Inorganic Chemistry, Vol. 46, No. 5, 2007

Electrocatalysis by Strained 2Fe2P Carbonyls
The absence of depletion/growth features during the initial
phase of electrocatalytic proton reduction in thin-layer SEC
experiments (Figure 7a) can only be interpreted in terms of
a pathway for dihydrogen evolution from a two-electron-
reduced form of 3P. Electrochemical simulation of the
voltammetry supports this conclusion (Figure 6). Provided
that a core geometry similar to that of the dithiolato-bridged
analogues can be obtained, dihydrogen evolution from the
two-electron, two-proton form of the phosphido-bridged
compound occurs at a comparable rate. A reaction path for
dihydrogen evolution that involves protonation of a basic
terminal hydrido ligand followed by oxidative elimination
of dihydrogen would not be expected to have a high
sensitivity to the 2Fe2X core geometry. The protonation
chemistry of DP2- is well established with HDP- formed
in reactions with weak acids for which the IR spectra show
that a terminally bound hydride is formed,25 an observation
in keeping with DFT-based geometry optimization of H3P-.
Evidently, an Fe-Fe distance greater than 3.5 Å does not
favor a bridged hydride. Reaction with stronger acids leads
to formation of H2DP without elimination of dihydrogen.
Because the Fe-Fe distances are similar for the reduced
forms of both compounds, the observation of proton reduc-
tion by 3P and not DP would appear to rule out a reaction
path mechanism centered on a single iron atom, such as
protonation of a basic terminal hydrido ligand followed by
oxidative elimination of dihydrogen.
Figure 8. DFT-optimized geometries of selected protonated reduced forms
of 3P and DP; for clarity, non-hydride hydrogen atoms have been omitted.
to proceed through a bridging hydride, [H3S]-, where
protonation gives down:down-[H23S]. The transition state
for dihydrogen elimination is calculated to have dihydrogen
end-on-bound to one iron center and the second iron atom
weakly interacting with the nonbound hydrogen atom.42 The
calculation of the structure of down:down-[H23S] was
repeated so as to remove effects due to differences in basis
set and indicated shorter Fe-Fe and H-H distances (∆r )
0.202 and 0.181 Å) than for down:down-[H23PM]. The
differences in calculated geometry for the respective doubly
protonated, doubly reduced compounds, when taken together
with the rates of dihydrogen elimination, provide strong
support for the proposition that the electrocatalytic pathway
for proton reduction at more moderate potentials (process
1) proceeds through a down:down dihydride such that the
increase in rate is due to destabilization of the ground state
and/or stabilization of the end-on-bound dihydrogen transi-
tion state by the closer proximity of the nonbound iron atom.
Conclusions
DFT investigations of 3S suggest a path for electrocatalytic
proton reduction that involves a dihydride intermediate with
hydride ligands bound to different iron atoms. Similar species
were calculated for H23P and H2DP, where, in each case,
the productive isomer has the hydride ligands closest. The
transition state has an end-on-bound dihydrogen ligand
coordinated to one iron atom, with the nonbound iron and
hydrogen atoms interacting weakly. The distortion of the core
geometry by linking the bridging phosphido groups or by
the presence of dithiolate bridging ligands reduces the H-H
distance of the intermediate and gives a shorter nonbonded
Fe-H distance for the transition state. Although the differ-
ence in electronic structure for S and P bridging ligands also
makes a contribution, the relative rates of dihydrogen
elimination from H2DP (∼0), H23P (2.4), and H23S (4.0),
together with the spectroscopic and SEC data and DFT
calculations, are consistent with a reaction path that proceeds
through an Fea:Feb dihydride. Clarification of the reaction
path for bridged diiron compounds is important in terms of
providing a blueprint for the refinement of systems of this
sort for catalysis of proton reduction (and dihydrogen
oxidation).
Linking the bridging phosphido groups of Fe2(µ-PR2)2-
(CO)6 constrains the geometry of the 2Fe2P core, and this
alters the relative energies of the reduced products. Although
3P- is unstable with respect to disproportionation (Kdisp
)
13), a sufficient concentration can be obtained to allow IR
and EPR characterization, together with an estimation of the
Fe-Fe bond distance from EXAFS analysis.
The two-electron-reduced product has an IR spectrum
closely related to that of the crystallographically characterized
species DP2-, and the long Fe-Fe contact of 3.55 Å confirms
that the two electrons are added to an orbital of Fe-Fe
antibonding character. DFT calculations of the redox series
reproduce the structural parameters and IR spectra and
suggest that 3P2- has a distorted structure with a nonplanar
2Fe2P core geometry.
Even though 3P displays an electrocatalytic response for
proton reduction similar to that of 3S, the paths leading to
the respective two-proton, two-electron products are mark-
edly different, and this is demonstrated most clearly by the
one-electron (3S) and two-electron (3P) characters of the
primary reduction. Formation of H23S proceeds by successive
reduction and protonation steps5 (ECEC reaction), whereas
an EECC reaction provides a path to H23P. In the case of
3S, the predominant mode for loss of the catalyst involves
dimerization of 3S-, and this underlies CO inhibition of
electrocatalysis. There appears to be no comparable dimer-
ization products of 3P-, and there is no evidence for CO
inhibition of electrocatalysis.
Acknowledgment. S.P.B. gratefully acknowledges the
Australian Research Council for funding this research.
M.H.C. gratefully acknowledges the University of Melbourne
for the award of a scholarship, and S.J.B. acknowledges the
Inorganic Chemistry, Vol. 46, No. 5, 2007 1749

Cheah et al.
receipt of an Australian Postgraduate Research Award. The
EXAFS experiments were performed at the Australian
National Beamline Facility with support from the Australian
Synchrotron Research Program, which is funded by the
Commonwealth of Australia under the Major National
Research Facilities Program. Mr. Mark I. Bondin and Dr.
Garry Foran are thanked for expert assistance with the
EXAFS experiments. Support from the Victorian Institute
for Chemical Sciences High Performance Computing Facility
is gratefully acknowledged. We thank Professor Luca De
Gioia for providing a copy of his manuscript prior to
publication.
Supporting Information Available: IR SEC results for 3P;
EPR spectra of 3P- (RT); fits of the EXAFS data of 3P, 3P2-, and
a mixture of 3P, 3P-, and 3P2-; summary of parameters used in
the electrochemical simulation of electrocatalytic proton reduction
by 3P; final coordinates and calculated IR spectra for the DFT
structure optimized species. This material is available free of charge
via the Internet at http://pubs.acs.org.
IC0623361
1750 Inorganic Chemistry, Vol. 46, No. 5, 2007