Electrocatalytic proton reduction by phosphido-bridged diiron carbonyl compounds: Distant relations to the H-cluster?

Article abstract of DOI:10.1021/ic049746e

Intermediates formed during reduction of Fe₂(μ-PPh ₂)₂(CO)₆ (1) in the presence of protons have been identified by spectroelectrochemical, continuous-flow, and interrupted-flow techniques. The mechanism for electrocatalytic proton reduction suggested by these observations yields digital simulation of the voltammetry in close agreement with measurements conducted in THF over a range of acid concentrations. The mechanism for electrocatalytic proton reduction involves initial formation of the dianion, 1^2-, which is doubly protonated prior to further reduction and dihydrogen elimination. The IR spectra of the singly and doubly protonated forms of 1^2- indicate structures corresponding to [FeH(CO)₃(μ-PPh₂)₂Fe(CO) ₃]^- (1H^-) and FeH(CO)₃(μ-PPh ₂)₂FeH(CO)₃ (1H₂). The thiolato and dithiolato analogues of 1 exhibit electrocatalytic proton reduction associated with the two-electron reduction step, and this implies that the corresponding two-electron reduced doubly protonated species is unstable with respect to dihydrogen elimination. The stability of 1H₂ is most likely to be due to the weak interactions between the iron centers of the flattened {2Fe2P} core. Whereas 1H₂ is stable in the absence of a reducing potential, 1H^- rearranges rapidly to a product previously described as [Fe ₂(μ-PPh₂)(μ-CO)(PHPh₂)(CO) ₅]^- (1H^-_W). Another protonation product of 1^2-, previously formulated as [Fe₂(μ- PPh₂)₂(μ-CO)H(CO)₅]^-, has been reformulated as [Fe₂(μ-PPh₂)(μ-CO)-(CO) ₆]^- (2) on the basis of a range of spectroscopic measurements. Solution EXAFS measurements of 1, 1^2-, 1H ^-_W, and 2 are reported, and these yield model-independent Fe-Fe distances of 2.61 (1), 3.58 (1^2-), 2.58 (1H^- _W), and 2.59 A (2). The presence of an Fe-Fe bond for both 1H^-_W and 2 is a key aspect of the proposed structures, and this strongly supports the deductions based on spectroscopic evidence. The fits of the solution EXAFS to different structural models give statistics in agreement with the proposed structures.

Full text of DOI:10.1021/ic049746e

Inorg. Chem. 2004, 43, 5635−5644
Electrocatalytic Proton Reduction by Phosphido-Bridged Diiron
Carbonyl Compounds: Distant Relations to the H-Cluster?
Mun Hon Cheah, Stacey J. Borg, Mark I. Bondin, and Stephen P. Best*
School of Chemistry, UniVersity of Melbourne, Victoria 3010, Australia
Received February 27, 2004
Intermediates formed during reduction of Fe2(µ-PPh2)2(CO)6 (1) in the presence of protons have been identified by
spectroelectrochemical, continuous-flow, and interrupted-flow techniques. The mechanism for electrocatalytic proton
reduction suggested by these observations yields digital simulation of the voltammetry in close agreement with
measurements conducted in THF over a range of acid concentrations. The mechanism for electrocatalytic proton
reduction involves initial formation of the dianion, 12-, which is doubly protonated prior to further reduction and
dihydrogen elimination. The IR spectra of the singly and doubly protonated forms of 12- indicate structures
corresponding to [FeH(CO)3(µ-PPh2)2Fe(CO)3]- (1H-) and FeH(CO)3(µ-PPh2)2FeH(CO)3 (1H ). The thiolato and
2
dithiolato analogues of 1 exhibit electrocatalytic proton reduction associated with the two-electron reduction step,
and this implies that the corresponding two-electron reduced doubly protonated species is unstable with respect to
dihydrogen elimination. The stability of 1H is most likely to be due to the weak interactions between the iron
2
centers of the flattened {2Fe2P} core. Whereas 1H is stable in the absence of a reducing potential, 1H- rearranges
2
rapidly to a product previously described as [Fe2(µ-PPh2)(µ-CO)(PHPh2)(CO)5]- (1H- ). Another protonation product
W
of 12-, previously formulated as [Fe2(µ-PPh2)2(µ-CO)H(CO)5]-, has been reformulated as [Fe2(µ-PPh2)(µ-CO)-
(CO)6]- (2) on the basis of a range of spectroscopic measurements. Solution EXAFS measurements of 1, 12-,
1H- , and 2 are reported, and these yield model-independent Fe−Fe distances of 2.61 (1), 3.58 (12-), 2.58 (1H- ),
W
W
and 2.59 Å (2). The presence of an Fe−Fe bond for both 1H-W and 2 is a key aspect of the proposed structures,
and this strongly supports the deductions based on spectroscopic evidence. The fits of the solution EXAFS to
different structural models give statistics in agreement with the proposed structures.
Introduction
12-, that exhibits hydridic chemistry and has a proposed
structure (C, Scheme 1) that includes both terminal hydride
and bridging carbonyl ligands.7 Moreover, the mechanistic
The structure of the {2Fe2S} subsite of the H-cluster of
the [Fe]-hydrogenase enzyme, {2Fe2S}H,1 has provided a
stimulus for a more intimate understanding of the chemistry
of diiron carbonyl/cyanide compounds2,3 and raised their
profile as functional models of the enzyme3-6 and potential
abiological hydrogen activation catalysts. In this spirit we
have been drawn into the examination of dithiolato-bridged
diiron carbonyl compounds and in this investigation into the
chemistry of the related phosphido-bridged diiron carbonyl
compounds. The reasons that lead us to move from the more
biologically relevant dithiolato-bridged compounds center on
the report of a protonation product of [Fe2(µ-PPh2)2(CO)6]2-,
(2) Lyon, E. J.; Georgakaki, I. P.; Reibenspies, J. H.; Darensbourg, M.
Y. Angew Chem., Int. Ed. 1999, 38, 3178-3180. Schmidt, M.;
Contakes, S. M.; Rauchfuss, T. B. J. Am. Chem. Soc. 1999, 121, 9736-
9737. Liaw, W.-F.; Lee, N.-H.; Chen, C.-H.; Lee, C.-M.; Lee, G.-H.;
Peng, S.-M. J. Am. Chem. Soc. 2000, 122, 488-494. Razavet, M.;
Davies, S. C.; Hughes, D. L.; Pickett, C. J. Chem. Commun. 2001,
847-848. Gloaguen, F.; Lawrence, J. D.; Schmidt, M.; Wilson, S.
R.; Rauchfuss, T. B. J. Am. Chem. Soc. 2001, 123, 12518-12527.
Lyon, E. J.; Georgakaki, I. P.; Reibenspies, J. H.; Darensbourg, M.
Y. J. Am. Chem. Soc. 2001, 123, 3268-3278. George, S. J.; Cui, Z.;
Razavet, M.; Pickett, C. J. Chem.sEur. J. 2002, 8, 4037-4046.
Lawrence, J. D.; Rauchfuss, T. B.; Wilson, S. R. Inorg. Chem. 2002,
41, 6193-6195. Razavet, M.; Borg, S. J.; George, S. J.; Best, S. P.;
Fairhurst, S. A.; Pickett, C. J. Chem. Commun. 2002, 700-701. Liaw,
W.-F.; Tsai, W.-T.; Gau, H.-B.; Lee, C.-M.; Chou, S.-Y.; Chen, W.-
Y.; Lee, G.-H. Inorg. Chem. 2003, 42, 2783-2788. Razavet, M.;
Davies, S. C.; Hughes, D. L.; Barclay, J. E.; Evans, D. J.; Fairhurst,
S. A.; Liu, X.; Pickett, C. J. Dalton Trans. 2003, 586-595. Lawrence,
J. D.; Li, H.; Rauchfuss, T. B.; Benard, M.; Rohmer, M.-M. Angew
Chem., Int. Ed. 2001, 40, 1768-1771.
* Author to whom correspondence should be addressed. E-mail: spbest@
unimelb.edu.au. Fax: 61-3-93805187.
(1) Peters, J. W.; Lanzilotta, W. N.; Lemon, B. J.; Seefeldt, L. C. Science
1998, 282, 1853-1858. Nicolet, Y.; Piras, C.; Legrand, P.; Hatchikian,
C. E.; Fontecilla-Camps, J. C. Structure 1999, 7, 13-23.
10.1021/ic049746e CCC: $27.50 © 2004 American Chemical Society
Published on Web 08/11/2004
Inorganic Chemistry, Vol. 43, No. 18, 2004 5635

Cheah et al.
Scheme 1. Summary of the Reduction Chemistry Reported for 1a,7,10,14,15
a X-ray structures are available for 1 and 12- 11
indicated.
;
in all the other cases, the type of spectroscopic evidence used to support the proposed structures is
insights gained from the study of the phosphido-bridged
compounds will contribute to our understanding of the related
dithiolato-bridged compounds. The close similarity of the
ν(CO) bands of C and a product formed during the reduction
of dithiolato-bridged diiron carbonyl compounds8 supports
this proposition.
Electrochemical7,9 or chemical7,10 reduction of 1 results
in the formation of 12-, where both the neutral and reduced
products have been characterized structurally.11 For related
compounds the stability of the one-electron reduced product
is strongly dependent on the substituent on the phospho-
rus;12,13 in the case of the diphenylphosphido bridge the
monoanion is unstable with respect to disproportionation,
and this is reflected by failure to observe an EPR spectrum
from the reaction product of 1 and 12-. Protonation of 12-
has been the subject of studies by Collman and co-workers7
in 1982 and Wojcicki and co-workers.10,14,15 While the
reactions conducted in the two studies are related, the reaction
products obtained are spectroscopically distinct, this being
reflected by the proposed structures. In neither study nor in
our own work to date has either product been obtained in a
form suitable for crystallographic analysis. As far as we are
aware, no subsequent study has either sought to rationalize
the difference in the chemistry or to confirm the structure
of the reaction products. In addition to the presence of key
structural features relevant to {2Fe2S}H, C is also reported
to undergo hydride transfer chemistry1 as is summarized in
Scheme 1. These observations suggest that C may serve both
as a structural and functional model of {2Fe2S}H.
In this paper we describe IR spectroelectrochemical
experiments conducted under argon and carbon monoxide
that lead preferentially either to C or 1H-W. These experi-
ments have led to the identification of conditions that permit
the isolation of either product, and this has permitted the
collection of a broader range of spectroscopic and EXAFS
results. On the basis of these studies, a revised structure for
C is proposed. The effect of elevated proton concentration
on the electrochemistry of 1 is examined, and it is shown
that in the presence of sufficiently strong acids the system
is electrocatalytic in terms of proton reduction. Intermediates
(3) Darensbourg, M. Y.; Lyon, E. J.; Zhao, X.; Georgakaki, I. P. Proc.
Natl. Acad. Sci. U.S.A. 2003, 100, 3683-3688.
(4) Zhao, X.; Georgakaki, I. P.; Miller, M. L.; Mejia-Rodriguez, R.;
Chiang, C.-Y.; Darensbourg, M. Y. Inorg. Chem. 2002, 41, 3917-
3928. Gloaguen, F.; Lawrence, J. D.; Rauchfuss, T. B. J. Am. Chem.
Soc. 2001, 123, 9476-9477. Georgakaki, I. P.; Miller, M. L.;
Darensbourg, M. Y. Inorg. Chem. 2003, 42, 2489-2494.
(5) Gloaguen, F.; Lawrence, J. D.; Rauchfuss, T. B.; Benard, M.; Rohmer,
M.-M. Inorg. Chem. 2002, 41, 6573-6582.
(6) Chong, D.; Georgakaki, I. P.; Mejia-Rodriguez, R.; Sanabria-
Chinchilla, J.; Soriaga, M. P.; Darensbourg, M. Y. Dalton Trans. 2003,
4158-4163.
(7) Collman, J. P.; Rothrock, R. K.; Finke, R. G.; Moore, E. J.; Rose-
Munch, F. Inorg. Chem. 1982, 21, 146-56.
(8) Razavet, M. Ph.D. Thesis, University of East Anglia, Norwich, U.K.,
2002. Borg, S. J.; Behrsing, T.; Best, S. P.; Razavet, M.; Liu, X.;
Pickett, C. J. Manuscript in preparation.
(9) Dessy, R. E.; Kornmann, R. L.; Smith, C.; Haytor, R. J. Am. Chem.
Soc. 1968, 90, 2001-4.
(12) Baik, M.-H.; Ziegler, T.; Schauer, C. K. J. Am. Chem. Soc. 2000,
122, 9143-9154.
(10) Yu, Y. F.; Gallucci, J.; Wojcicki, A. J. Am. Chem. Soc. 1983, 105,
4826-8.
(13) van der Linden, J. G. M.; Heck, J.; Walther, B.; Boettcher, H. C. Inorg.
Chim. Acta 1994, 217, 29-32.
(11) Ginsburg, R. E.; Rothrock, R. K.; Finke, R. G.; Collman, J. P.; Dahl,
L. F. J. Am. Chem. Soc. 1979, 101, 6550-62.
(14) Shyu, S. G.; Wojcicki, A. Organometallics 1985, 4, 1457-9.
(15) Wojcicki, A. Inorg. Chim. Acta 1985, 100, 125-33.
5636 Inorganic Chemistry, Vol. 43, No. 18, 2004

Electrocatalytic Proton Reduction
Figure 2. Cyclic voltammetry of a 1 mM solution of 1 with HOTs: (a)
0 and 2 equiv; (b) 0-5 equiv (0.2 M (TBA)PF6/THF, ν ) 100 mV s-1
,
0.03 cm2 vitreous carbon working electrode). Digital simulation18 of these
experiments using the model given in Scheme 2 is given in the insets.
Figure 1. IR spectra in the ν(CO) region recorded during reduction of 1
(3 mM, 0.2 M (TBA)ClO4/THF) (a) under 0.4 MPa Ar, (b, c) with HOTs
(27 mM; 0.4 MPa Ar), and (d, e) with HOTs (27 mM; 0.4 MPa CO).
The spectral changes indicate a clean conversion of the
starting material to a single product with no significant
buildup of additional species during the course of the
reaction. As expected, the spectrum of the reaction product
matches that of 12- and reoxidation of the film of solution
in contact with the working electrode at a potential of -0.9
V leads to near quantitative recovery of the starting material
(Supporting Information).
formed in the course of the reactions have been identified
using spectroelectrochemical and continuous-flow elec-
trosynthetic techniques, and a reaction mechanism consistent
with the spectroscopic and electrochemical results is pre-
sented.
Whereas the reduction wave of 1 (Epc ) -1.4 V) is only
slightly shifted to more positive potentials on addition of
acid, the reoxidation wave is almost completely quenched
(Figure 2). A second reduction wave is observed with Epc ≈
-1.7 V (ν ) 100 mV s-1) having Ipc related to the
concentration of acid. At lower acid concentrations this wave
has two components. The absence of similar features in
analogous experiments conducted on p-toluenesulfonic acid
(HOTs) in the absence of 1 suggests that the wave at ca.
-1.7 V is due to electrocatalytic proton reduction with the
electrocatalyst being a protonated reduction product of 1.
Since 12- is known to react rapidly with acids much weaker
than HOTs,14 the appearance of a reoxidation wave requires
that the acid concentration be depleted near the working
electrode. The appearance of a reoxidation wave when the
potential is scanned more negative than -1.7 V (Figure 2)
is consistent with the consumption of protons during the
second reduction process.
Results and Discussion
Electrochemistry and Spectroelectrochemistry (SEC)
of 1. The phosphido-bridged diiron compounds, Fe2(µ-PR2)2-
(CO)6, undergo well-defined two-electron reductions to give
the corresponding dianion.7,9 The comproportionation reac-
tions of the neutral and dianion forms of the compound have
been found to yield observable concentrations (by EPR) of
the one-electron reduced product in the case where R ) Me16
but not for R ) Ph.11 The sensitivity of the disproportionation
equilibrium to the identity of the R group is also reflected
by density functional calculations.12 In the course of the
reduction the Fe-Fe single bond is lost and the {2Fe2P}
butterfly is replaced by a planar core geometry. This
structural change may be expected to contribute to a high
barrier to electron transfer contributing to the value of ∆Ep
(220 mV, 100 mV/s) for the two-electron reduction of 1 when
using a platinum working electrode.13 It is interesting that
the reduction appears to occur in a single two-electron step
when using a hanging-mercury-drop electrode (∆Ep ) 45
mV).7 IR spectra recorded during the reduction of 1 at -1.5
V in a thin-layer SEC experiment are shown in Figure 1a.
SEC experiments of 1 in the presence of 9 molar equiv of
HOTs in THF under an atmosphere of argon show clearly
the involvement of an additional species, 1A, in the initial
phase of the experiment (Figure 1b). At longer times a more
complex set of bands develop that may be attributed to a
(16) Dessy, R. E.; Rheingold, A. L.; Howard, G. D. J. Am. Chem. Soc.
1972, 94, 746-52.
mixture of 1H- and 12- with a small amount of C that
W
Inorganic Chemistry, Vol. 43, No. 18, 2004 5637

Cheah et al.
grows into the spectrum at longer reaction times (Figure 1c).
The addition of HOTs has no observable effect on the
depletion bands observed inthe IR-SEC experiments, indicat-
ing that HOTs in THF is not a sufficiently strong acid to
protonate 1. The formation of 12- in the latter stages of the
reduction confirms the interpretation of the electrochemistry,
i.e. that there is depletion of the acid concentration following
polarization of the working electrode at potentials more
negative than ca. -1.6 V. Since neither 1H-W nor C reacts
with protons to generate dihydrogen at an appreciable rate
and reduction of either species requires the application of
more negative potentials, it is unlikely that either 1H- or
W
C is an intermediate in the catalytic proton reduction reaction.
Reoxidation of the solution at -0.9 V results in the limited
recovery of 1, and this may be attributed to the reoxidation
of 12-. More strongly oxidizing potentials are required to
remove 1H-W from the spectrum (ca. -0.5 V) although this
does not lead to significant recovery of 1.
SEC experiments of 1 in the presence of 9 molar equiv of
HOTs in THF carried out under 0.68 MPa of CO initially
yield results that are indistinguishable from those obtained
under an argon atmosphere (Figure 1b,d). However, the
distribution of reduction products obtained following deple-
tion of the acid is markedly different (Figure 1c,e), where C
is the predominant product with lesser amounts of 12- and
only trace quantities of 1H-W formed. The current response
observed during SEC experiments conducted under argon
or CO is similar, as is the time evolution of products
(including 12-). This suggests that the rate of electrocatalytic
proton reduction is not affected by the relative concentrations
of 1H-W and C, supporting the argument that neither plays
an important role in the catalytic cycle. The marked change
in relative stability of 1H-W and C in experiments conducted
under argon and CO suggests a difference in CO stoichi-
ometry for the two compounds. Reoxidation at a potential
of -0.9 V results in conversion of 12- into 1. The application
of a more oxidizing potential, ca. -0.2 V, results in depletion
C with limited recovery of 1 together with another product
with an IR spectrum similar to that reported for Fe2(µ-PPh2)-
(CO)7.17
Figure 3. IR spectra in the ν(CO) region of 1 and its electrogenerated
products. The spectra of 1, 1H-W, and C were obtained from THF solutions
of isolated samples. The spectra of 12-, 1A, and 1D were obtained from
continuous-flow electrosynthesis experiments (0.2 M (TBA)ClO4/CH3CN)
conducted in the presence of HOTs (1A) or CH3COOH (1D). Whereas
samples of 12- were obtained pure, samples of 1A and 1D were obtained
as mixtures with 1 and, in the latter case, 1H-W. The spectra of 1A and 1D
were obtained by subtraction.
Scheme 2. Mechanism for Electrocatalytic Proton Reduction Used for
Electrochemical Simulationsa
Well-defined solutions of 1A can be generated by continu-
ous-flow electrosynthesis of 1-HOTs in the mole ratio 1:3
in CH3CN at a reduction potential of ca. -1.5 V. The
electrogenerated solution of 1A is stable with respect to
further reaction with the acid, and this suggests that 1A is
the initial reduction product formed in the presence of HOTs
and this is further reduced at -1.7 V in the electrochemical
experiments. The IR spectra of 1 and its stable reduction
products are shown in Figure 3. The ν(CO) bands of the
different compounds are highly sensitive to their charge state,
with the ν(CO) bands of 12- ca. 120 cm-1 lower than those
of 1. The ν(CO) bands of 1H-W and C occur at intermediate
wavenumbers, consistent with their relation to 1 by 2e-/1H+
reaction. The similarity of the ν(CO) bands of 1 and 1A
suggest that they share a common charge state; i.e., 1A is
related to 1 by either 1e-/1H+ or 2e-/2H+ reaction.
a In all cases common values of the diffusion coefficients (1 × 10-5
cm2 s-1), the rate of heterogeneous electron transfer (0.1 cm s-1), and the
R parameter (0.5) were used.
Electrochemical Simulation of the Reduction of 1. The
simplest mechanism for the reduction of 1 consistent with
the electrochemical and SEC results in THF in the presence
of HOTs is shown in Scheme 2. This is used in digital
simulations of the electrochemical experiments shown in
Figure 2. While the available voltammetric results are not
sufficient to permit quantitative determination of the kinetic
and thermodynamic parameters, they provide a test of the
validity of the proposed mechanism. To reduce the parameter
space available to the simulation the diffusion coefficients
(17) Baker, R. T.; Krusic, P. J.; Calabrese, J. C.; Roe, D. C. Organometallics
1986, 5, 1506-1508.
5638 Inorganic Chemistry, Vol. 43, No. 18, 2004

Electrocatalytic Proton Reduction
Scheme 3. Protonation Products of 12-
of all species were set at 1 × 10-5 cm2 s-1, the rates of
heterogeneous electron transfer were set to 0.1 cm s-1, and
the forward rate constant for proton transfer was assumed
to be fast and was arbitrarily set to 1 × 106 M-1 s-1. No
corrections have been included for the iR drop that is
apparent for the experimental data. The remaining kinetic
and thermodynamic parameters were adjusted so as to
reproduce the general features of the electrochemistry for a
range of acid concentrations (0-5 equiv).
by the wavenumbers of the ν(CO) bands. This can be most
clearly established by the observation of its singly protonated
precursor.
The small shift of Epc (ca. 20 mV) of the first reduction
wave following addition of acid may, in principle, result from
protonation of either of the initial reduction products, 1- or
12-. The calculated acid concentration dependence of the
current response is in better agreement with experiment if
the reduction process involves two one-electron reactions,
with protonation of 1- uncompetitive with the second
reduction step. If, as suggested by the wavenumbers of the
ν(CO) bands, 1A is in a neutral charge state, then this must
be related to 1 by 2e-/2H+ reaction (i.e. 1H2). The alternative
formulation of 1A, as 1H, would be inconsistent with Scheme
1 and can also be excluded since such a formulation would
imply that protonation of 1- leads to a shift of the reduction
potential to more negative values. This is contrary to the
large shift to more positive potentials observed to accompany
protonation of related compounds.5,19 The shoulder on the
wave associated with electrocatalytic proton reduction (Fig-
ure 2) implies the presence of a proton-assisted path for
dihydrogen elimination. This may be modeled in an overall
two-electron reduction with evolution of dihydrogen from
The protonation reactions of the highly reactive 12- species
are most conveniently examined in mixing experiments
where 12- is generated immediately prior to reaction. This
has been achieved using a continuous-flow electrosynthesis
cell coupled to a mixing valve with the reaction products
monitored, in-line, by IR spectroscopy. Reactions conducted
in CH3CN using 2, or more, molar equiv of HOTs result in
formation of relatively stable solutions of 1A. The addition
of 12- to 1 mol equiv of HOTs results in a more complex
reaction, with transient formation of 1A and predominant
formation of 1H-W. Reaction between 12- and CH3COOH
(pKa ) 28 in CH3CN20) yields 1H- even at large molar
W
excesses of the weak acid. IR spectra recorded from solutions
immediately after mixing contain bands that are due to an
intermediate species (1D) that subsequently rearranges to give
1H-W. The spectra of 12-, 1D, and 1A obtained in the
continuous-flow experiments are shown in Figure 3.
Shyu and Wojcicki have previously reported 31P, 2H, and
1H NMR studies of the low-temperature reduction of 1 by
Super-Hydride and the protonation of 12- by CF3COOH.
These indicate that both reactions produce 1H-W and proceed
through an intermediate formulated with a terminal hydride
on one of the iron atoms of 12- (Scheme 1).14 The IR
spectrum of 1D is consistent with such a formulation, with
the higher wavenumber set of ν(CO) bands (2038, 1981, and
1962 cm-1) associated with the Fe(CO)3 subunit at the site
of protonation. Protonation of the remaining iron atom
reestablishes the equivalence of the two fac-Fe(CO)3 frag-
ments thereby accounting for the similar pattern of bands
observed for 12- and 1A. Thus, the intermediate 1D and the
product 1A correspond to 1H- and 1H2 in Scheme 2, where
the previously reported14 NMR spectra of 1H- and the IR
spectra obtained in this study are consistent with the
structures shown in Scheme 3. The mixing experiments
conducted using CH3COOH and HOTs suggest a much larger
difference between Kb(12-) and Kb(1H-) than obtained from
modeling the electrochemistry. In this regard it is noted that
electrochemical modeling using Scheme 2 provides a lower
limit on Kb(12-). The involvement of a reaction pathway
involving reduction of 1H- can be tested by the observation
of electrocatalysis in acids that are insufficiently strong to
protonate this species. Whereas the addition of CH3COOH
to a THF solution of 1 has a marked effect on the reversibility
of the two-electron wave at -1.3 V, the presence of a large
molar excess of CH3COOH (>20-fold) does not lead to
2-
-
1H2 and 1H3 . Further investigation is needed to clarify
the details of this step of the reaction. The rearrangement of
1H- to 1H-W has previously been proposed on the basis of
NMR studies of the reaction between 1 and Super-Hy-
dride.14,15 This interconversion is consistent with the evolution
of IR spectra in the IR-SEC experiments. The formation of
C, which occurs at longer times for experiments conducted
under argon, is not modeled in Scheme 2. It is noted that a
product having an oxidation potential of ca. -0.5, similar
to that of 1HW-, is formed in cyclic voltammetric experi-
ments conducted with less than ca. 1 equiv of acid. This
wave is likely to be due to the oxidation of 1HW-, where at
higher acid concentrations this species is protonated.10 It is
-
the protonated form of 1HW (1HX-, Scheme 2) that is
oxidized at more positive potentials.
Despite having a common charge state Kb(1-) and Kb(1H-)
are required to be markedly different for the mechanism
shown in Scheme 2. Since protonation of 1- is not competi-
tive with reduction, Kb(1-) must be less than ca. 1, whereas
a large value of Kb(1H-) is required to reproduce the
reoxidation wave of 12- obtained after electrocatalytic proton
reduction (Figure 2). In this context it is critical to confirm
that the stable protonated product of 12- (i.e. 1A) corresponds
to the doubly protonated neutral species, 1H2, as inferred
(18) Rudolph, M.; Reddy, D. P.; Feldberg, S. W. Anal. Chem. 1994, 66,
589A-600A.
(19) Guedes da Silva, M. F. C.; Frausto da Sila, J. J. R.; Pombeiro, A. J.
L.; Amatore, C.; Verpeaux, J. N. NATO ASI Ser., Ser. C 1993, 385,
483-7.
(20) Izutsu, K. Acid-Base Dissociation Constants in Dipolar Aprotic
SolVents; Blackwell Scientific Publications: Oxford, U.K., 1990.
Inorganic Chemistry, Vol. 43, No. 18, 2004 5639

Cheah et al.
Table 1. Comparison of Spectroscopic Data for C and That Reported
for [Fe2(µ-PPh2)(µ-CO)(CO)6]-, 2
significant proton reduction or to a reduction process
attributable to 1H- at potentials more positive than -2.3 V.
This observation confirms the assignment of the reduction
wave with Epc ≈ -1.7 V to a process involving 1H2 and
permits the exclusion of mechanisms involving reduction of
1H-.
[(15C5)Li]Ca
[NEt4]2b
PPN2c
IRd/cm-1
THF
THF
Nujol
Nujol
2015 (ms)
1965 (vs)
1916 (s)
1904 (w, sh)
2010 (m)
1965 (vs)
1935 (s)
1920 (s)
2010 (s)
1965 (s)
1910 (s, br) 1928 (s)
1904 (w)
2016 (vs)
1964 (vs, br)
The failure of 12- to reduce protons and yield dihydrogen
when HOTs is used as the acid may be understood in terms
of a structure in which there is a planar {2Fe2P} core and
the two FeH(CO)3 centers interact weakly. While a folding
of the core may be expected to facilitate elimination of
dihydrogen with recovery of 1, it would appear that this
reaction does not proceed at an appreciable rate prior to
further reduction. Paradoxically, reaction of 12- with the
weaker acid CF3COOD is reported to give D2 in high isotopic
purity.7 In this case the acid is insufficiently strong to
generate 1A and rearrangement of 1H- leads to the formation
of a product that is able to act as a hydride donor. It is clear
from the electrochemistry (Figure 2) that this reaction is slow
compared with electrocatalysis associated with reduction of
1A.
1865 (s)
1705 (w, br) 1735 (s, br) 1724 (vs)
7.21-7.8 (mpt)
1892 (w)
1733 (w, br)
NMR, 1H/δ
7.31 (mpt, 2H)
7.61 (mpt, 3H)
NMR, 31P{H}e/δ 127 (CD3CN)
125.3 (CH2Cl2)
a This work. b Reference 22. c Reference 23. d s ) strong, m ) medium,
w ) weak, vs ) very strong, sh ) shoulder, br ) broad, mpt ) multiplet.
e Relative to H3PO4.
free phosphine where solutions prepared by CO saturation
of 1H- are marked by a strong “phosphine” odor.
W
Following an earlier study,24 two crystallographic reports
of diiron carbonyl compounds having a single bridging
phosphido group have previously been published.22,23 These
include the anion [Fe2(µ-PPh2)(µ-CO)(CO)6]-, 2, and the
method of preparation involved reaction of [Fe2(CO)8]2- with
PR2Cl followed by decarbonylation. The reported spectro-
scopic details (IR, NMR) are consistent with those obtained
for the sample of C (Table 1), and the structural parameters
are clearly consistent with the presence of an Fe-Fe bond
(Table 2). Despite the strong support for the assignment of
the structure of C to 2 provided by the ESI-MS and
Formulation of C. The differences in the SEC experi-
ments of 1 in the presence of a proton source under an
atmosphere of argon or CO suggest that C may be formed
cleanly from reactions conducted using CO-saturated sol-
vents. Indeed C can be obtained by saturating a solution of
1H-W with CO, this being the basis of an alternative method
for its preparation. Despite repeated attempts C was not
obtained in a form suitable for X-ray structural analysis and
its structure remains to be established. ESI-MS experiments
conducted on dilute solutions of C yielded, at low cone
voltages (5 V), a parent peak with m/z centered on 493 with
an isotopic distribution consistent with a monoanion of
formulation [Fe2(PPh2)(CO)7]-. At higher cone voltages (30-
50 V) increasing numbers of CO groups are fragmented from
the molecular ion (Supporting Information). In no experiment
was there any indication of the presence of an ion with m/z
) 651 as would be expected for the structure proposed for
1
spectroscopic details the report that the H NMR of C
includes a triplet (JHP ) 42.5) at δ ) -20 together with
hydridic reactions with protons, benzoyl chloride, and carbon
tetrachloride7 are inconsistent with such a structure.
The formation of C by reaction of 1H- and CO may
W
proceed by substitution of the terminal PHPh2 group, in
which case the 34 electron diiron product would retain the
Fe-Fe bond whereas addition of CO to 1H-W, or CO-
mediated rearrangement of 1H- to the structure proposed
W
for C (Scheme 1), would give a product without an Fe-Fe
bond. Thus, the Fe-Fe separation provides an additional
means of distinguishing between the different structures
proposed for C. In the absence of a crystalline sample metric
details of the Fe-Fe separation can be obtained from the
application of EXAFS techniques. Iron K-edge X-ray
fluorescence spectra were obtained from solutions of 1, 12-,
1H-W, and C and solid-state samples of 1 diluted in boron
nitride. In these experiments the integrity of the electro-
C (Scheme 1). Analogous experiments conducted on 1H-
W
yielded parent peaks centered on m/z ) 493 and 651, where
the relative intensities of the two peaks varied between
experiments. The isotopic distribution of the peaks centered
on m/z ) 651 match that required for the proposed structure.
1
The H and 31P NMR of isolated samples of C yield
resonances consistent with there being a single phosphorus
chemically (12-) and chemically (1H- and C) generated
W
environment.7 Solutions of C prepared by CO saturation of
samples was confirmed by in-line IR measurements collected
immediately prior to freezing the solution at 80 K.
1H- yield IR spectra indistinguishable from those of
W
1
isolated C; however, additional H (multiplets centered on
The models used to refine the EXAFS data are summarized
in Table 2. In all cases the number of refined parameters
was minimized while still allowing the main features of the
proposed structures to be represented. A discussion of the
alternate strategies for modeling the EXAFS results for diiron
7.34 and 7.52 ppm) and 31P (singlet at -41 ppm in proton
decoupled spectra) resonances are observed and these are
consistent with their assignment to diphenylphosphine.21 The
relative intensities of these resonances are less than those of
-
the PPh2 group of C, whereas a 1:1 ratio is expected if
conversion of 1H- to C involves the loss of PHPh2. This
W
(22) Ellis, J. E.; Chen, Y.-S. Organometallics 1989, 8, 1350-1361.
(23) Walther, B.; Hartung, H.; Bottcher, H.-C.; Baumeister, U.; Bohland,
U.; Reinhold: J.; Sieler, J.; Ladriere, J.; Schiebel, H.-M. Polyhedron
1991, 10, 2423-2435.
(24) Osterloh, W. T. Ph.D. Thesis, University of Texas, Austin, TX, 1982;
University Microfilms International: Ann Arbor, MI, 1982
difference may be explained in terms of the volatility of the
(21) Emsley, J.; Hall, D. The Chemistry of Phosphorus; John Wiley &
Sons: New York, 1976.
5640 Inorganic Chemistry, Vol. 43, No. 18, 2004

Electrocatalytic Proton Reduction
Table 2. Summary of the EXAFS Refinements of 1, 12-, 1H-W, and Ca
sampleb
12-
1 (solid)
1
1H-
C
W
modelc
k/Å-1
M1
M1
M1
M2
M3
1-15
0.75-3.8
-9.02
1-15
1-14
1-14.5
0.75-3
-7.33
0.91
0.57 [8.1]
2.58
(0.0034)
2.17
(0.0010)
1.77
(0.0031)
1.15
(0.0036)
2.25
(0.0047)
1.12
(0.0010)
2.23
1-14
R/Å
0.75-3.8
-7.24
0.73
0.75-4
0.75-3
E0/eV
-8.39
-5.68
2
S0
0.73
0.95
0.91
ø2 [RXAFS (%)]d
Fe-Fe/Åe
3.84 [17.5]
2.61 [2.630]f
(0.0025)
2.21 [2.212]av
(0.0028)
1.78 [1.781]av
(0.0010)
1.16 [1.140]av
(0.0023)
5.59 [17.4]
2.61
(0.0007)
2.21
(0.0009)
1.78
(0.0006)
1.15
3.33 [15.0]
3.58 [3.63]g
(0.0060)
2.26 [2.29]
(0.0034)
1.75 [1.75]
(0.0026)
1.17 [1.15]
(0.0036)
3.50 [3.45]
(0.0035)
1.49 [13.2]
2.59 [2.601]h
(0.0043)
2.18 [2.209]av
(0.0019)
1.78 [1.777]av
(0.0053)
1.16 [1.144]av
(0.0047)
2.13 [1.962]av
(0.0010)
1.31 [1.176]av
(0.0010)
Fe-P/Å
Fe-C/Å
C-O/Å
(0.0021)
3.52
(0.0018)
Fe-CR/b/Å
C-Ob/Å
Fe-Pt/Å
3.52 [3.543]av
(0.0027)
(0.0012)
a Fits of the EXAFS data in k and R space are included in the Supporting Information. b Unless otherwise stated data were collected from 3 to 5 mM
c
solutions in acetonitrile (1 and 12-) or THF (1H- and C).
W
d ø2 ) ∫k)0...∞[w(øobs(k) - øcalc(k))]2, RXAFS ) ∫k)0...∞[ø2/ø2
]
1/2; øobs(k) and øcalc(k) are the observed and calculated EXAFS and w is a weighting factor.27
calct0
e Crystallographic distances are given in brackets, and the Debye Waller factors, in parentheses. The errors in the internuclear contacts is dominated by the
typical systematic error, assigned a conservative consensus value of 0.02.28 f Fe-Fe and Fe-P are available for 1;11 other bond distances were obtained
from Fe2(µ-PhP(CH2)3PPh2)(CO)6.29 g [NEt4]212- 11 h [NEt4]22.22
.
carbonyl compounds of this sort is included in a separate
publication.25 Good internal consistency of the data was
obtained for repeated measurements of the same compound
and, in the case of 1, for samples measured in the solid and
solution state. In all cases the determinancy of the fit was
good, with the ratio of the number of refined parameters to
the number of independent points26 ranging between 1.3
(1H-W) and 2.4 (1), and the refinement statistics were
situation applies for the backscattering iron atom, and a
similar Fe-Fe separation is obtained from analysis of the
EXAFS data from 1H-W and C using any of the three models
described in Table 2 or for a model based on the structure
of C given in Scheme 1. On this basis the Fe-Fe distance
is determined with a high level of certainty. The Fe-Fe
distances obtained for 1H- and C are similar and are
W
marginally shorter than that obtained for 1. This observation
is consistent with the proposed structure for 1H-W; however,
the presence of an Fe-Fe bond for C is inconsistent with
the earlier proposed structure and is in excellent agreement
with that expected for structure 2. Moreover, the structural
parameters obtained from the analysis of C are in excellent
agreement with the crystallographic values obtained from
[NEt4]2 (Table 2).
2
satisfactory in all cases. The S0 and the Debye-Waller
factors refined to give physically reasonable values, the
exception being the somewhat low values of the Debye-
Waller factors obtained for the Fe-Fe and Fe-C interactions
for solutions of 1. There is excellent agreement between the
bond lengths obtained from EXAFS analysis of both 1 and
1
2- and the reported crystallographic determinations (Table
2). It is noteworthy that even the long nonbonding Fe-Fe
The reported hydride transfer properties of C are hard to
reconcile in terms of the revised structure, i.e. 2. It is
significant that in those studies C was generated in situ by
reaction of 12- with 1 equiv of protons and, accordingly,
the solutions would have contained both 2 and PHPh2.7 In
the absence of PHPh2 isolated samples of C react with
protons yielding a complex mixture of products that do not
include significant concentrations of 1. Moverover, experi-
ments in which an isolated sample of C is reacted with
protons in the presence of thiophenol yield a product with
ν(CO) bands intermediate between those of 1 and the bis-
(thiophenolato)-bridged analogue. A possible explanation of
distance of 12- is obtained with good accuracy.
Modeling the EXAFS data of 1H-W and C is complicated
by the absence of an unequivocal structural assignment, and
therefore, conclusions drawn from this analysis need to be
model independent. In cases where the backscattering atom
is significantly different in atomic number from the other
scattering atoms, then the calculated internuclear separation
will be largely insensitive to the details of the model. This
(25) Bondin, M. I.; Borg, S.; Foran, G.; Best, S. P. Manuscript in
preparation.
(26) Stern, E. A. Phys. ReV. B: Condens. Matter 1993, 48, 9825-9827.
Inorganic Chemistry, Vol. 43, No. 18, 2004 5641

Cheah et al.
the hydride transfer chemistry attributed to C is that this
proceeds through a hydridic species in equilibrium with 2
and PHPh2. A species of structure similar to that originally
proposed for C (Scheme 1) is an attractive candidate.
Investigations into the details of this aspect of the chemistry
are in progress.
in mode of CO coordination is kinetically important. The
comparative inertness of 1H2 is interesting given that the
isomer [FeH(CO)3(µ-PPh2)Fe(PHPh2)(CO)3] may be formed
10
in reactions between CF3COOH and 1H-
.
W
While the
structures of the protonated products of 12- are unlikely to
be as closely related to the structure of {2Fe2S}H as
suggested by the previously proposed structure of C, the
system exhibits remarkably well-behaved electrocatalytic
proton reduction behavior that, in THF, is amenable to
detailed analysis. The framework provided by such an
analysis is relevant to the corresponding reactions of a
broader range of bridged diiron compounds.
Conclusions
A combination of chemical and electrosynthetic techniques
have permitted identification of the intermediates formed in
the course of electrocatalytic proton reduction by 1, and this
together with digital simulation of the voltammetry suggest
the reaction mechanism shown in Scheme 2. Protonation of
Experimental Section
1
2- proceeds via attack at one of the iron atoms of the planar
{2Fe2P} core, and this product is either protonated to give
Chlorodiphenylphosphine (PPh2Cl) and Super-Hydride (LiBEt3H,
1 M in THF) were obtained from Aldrich and used without further
purification; iron pentacarbonyl was dried over calcium hydride,
vacuum distilled, and stored at 4 °C under dinitrogen before use.
Solvents were purified using standard procedures30 and distilled
under a dinitrogen atmosphere. Tetrahydrofuran (THF) was distilled
from sodium/benzophenone, acetonitrile from CaH2, and n-hexane
from freshly pressed sodium wire. Unless stated otherwise, all
reactions were carried out under an inert dinitrogen atmosphere
using standard Schlenk techniques or conducted within a glovebox
(Vacuum Atmospheres). Tetra-n-butylammonium perchlorate ((TBA)-
ClO4) and tetra-n-butylammonium hexafluorophosphate ((TBA)-
PF6) used as supporting electrolytes were prepared and purified
using standard methods.31 Literature methods were employed to
a relatively stable product, 1A or rearranges to give 1H-
.
W
The structure of 1H- previously proposed by Wojcicki10
W
is consistent both with the spectroscopic results and the
structural parameters obtained from EXAFS analysis. Dif-
ferences in the product distribution obtained in SEC experi-
ments of 1-HOTs conducted under argon or CO suggest
that CO replaces the terminally bound PHPh2 group to give
2. This formulation of the product is consistent with results
obtained by ESI-MS and a range of spectroscopic techniques.
The alternative formulations of C differ in terms of the
number of diphenylphosphido bridging groups, which leads
to differences in the Fe-Fe bonding. This aspect of the
structure has been probed by a series of EXAFS measure-
ments conducted on 1, together with electrochemically and
chemically generated samples of 12-, 1H-W, and C. While
the extent of the data obtained from these measurements is
insufficient to permit unambiguous structure characterization,
the Fe-Fe separation is obtained with high reliability and
this clearly indicates the presence of an Fe-Fe bond for 1,
1H-W, and C. These results confirm that the previously
proposed structure for C (Scheme 1)7 is not tenable and all
the structural parameters are in close agreement with those
expected for the revised structure, 2.
prepare Fe2(µ-PPh2)2(CO)6, 1,7 [Li(15-crown-5)][Fe2(µ-PPh2)(µ-
10
CO)(PHPh2)(CO)5], [Li(15C5)]1H-
,
and Na2[Fe2(µ-PPh2)2-
W
(CO)6]2-, Na212- 7
.
Caution! Perchlorate salts are potentially explosiVe. Solutions
containing (TBA)ClO4 as supporting electrolyte should not be
allowed to eVaporate to dryness.
[Li(15-crown-5)][Fe2(µ-PPh2)(µ-CO)(CO)6], [Li(15C5)]C, was
prepared by the following method. A solution of 1 in THF (0.025
g, 38.5 µmol) was treated with 1 equiv of LiBEt3H (38.5 µL, 1 M)
to yield a red solution of 1H- in greater than 90% purity (IR).
W
The solution was then saturated with CO and allowed to stand
overnight. A stoichiometric quantity of 15-crown-5 ether was added,
and following the addition of n-hexane, the crude product was
isolated as a deep-red oil. The product was washed with n-hexane
(3 × 1 mL) to remove traces of 1 to yield a spectroscopically clean
sample of [Li(15C5)]C. IR (THF): 2015, 1965, 1934, 1916, 1733
cm-1. The elemental analysis, while poor, confirmed an Fe:P ratio
of at least 2:1. Anal. Calcd for Fe2PC29H30O12Li: C, 48.8; H, 4.2;
Fe, 15.5; P, 4.3. Found: C, 50.05; H, 6.07; Fe, 12.6; P, 2.7.
Electrochemical measurements were conducted using either an
Autolab PGSTAT30 or a MacLab potentiostat. In the former case
the potentiostat was controlled using GPES software (Autolab); in
the latter case EChem software with a PowerLab interface (AD-
Instruments) was used. Experiments employed a standard three-
electrode geometry with a 2 mm diameter vitreous carbon disk as
the working electrode, a platinum gauze counter electrode, and a
Contrary to observations for the thiolato-bridged analogue,
where turnover occurs following two-electron reduction,6 12-
reacts with HOTs to give a doubly protonated product, 1A,
that is stable with respect to dihydrogen evolution. Further
reduction is required to achieve dihydrogen elimination at a
significant rate. No significant electrocatalytic proton reduc-
tion is evident for solutions of 1-HOTs in THF following
two-electron reduction. Whereas isolated samples of C do
not act as a source of H-, the observation of hydridic
chemistry from 12--CF3COOH mixtures7 suggests that one,
or more, of the rearrangement products are hydridic in
character. Further work is needed to delineate the chemistry
of the system so as to better understand the impact of
structural and electronic factors on the hydridic chemistry
of phosphido- and thiolato-bridged diiron compounds.
A key aspect of the chemistry of {2Fe2S}H is the change
in the mode of CO coordination with redox state and the
coupling of this structural change to the catalytic cycle. The
(27) Ellis, P. J.; Freeman, H. C. J. Synchrotron Radiat. 1995, 2, 190-195.
(28) Gurman, S. J. J. Synchrotron Radiat. 1995, 2, 56-63.
(29) Reingold, A. L. Acta Crystallogr., Sect. C: Cryst. Struct. Commun.
1985, 41, 1043.
(30) Armarego, W. L. F.; Perrin, D. D. Purification of Laboratory
Chemicals, 4th ed.; Butterworth-Heinemann: Oxford, U.K., 1996.
(31) Sawyer, D. T.; Sobkowiak, A.; Roberts, J. J. L. Electrochemistry for
Chemists, 2nd ed.; Wiley-Interscience: New York, 1995.
conversion of 1H- into 1H- ultimately involves such a
W
transformation, although it is not clear whether the change
5642 Inorganic Chemistry, Vol. 43, No. 18, 2004

Electrocatalytic Proton Reduction
jacketed silver wire pseudo-reference electrode. The potential of
the silver electrode was referenced against the ferrocenium/ferrocene
couple, and all potentials are reported relative to the saturated
calomel electrode (SCE), where in THF E°′(Fc+/Fc) ) 0.53 V.32
The working electrode was polished with 0.3 µm alumina and
sonicated in THF between experiments. Acid titration experiments
were performed by measured addition of a THF solution of
p-toluenesulfonic acid (0.2 M) together with the complex and
supporting electrolyte at the same conditions as used in the initial
electrochemical experiment. Electrochemical simulations were
performed using DIGISIM version 3.0.18
and the front window consisted of a 25 µm thick film of Teflon-
coated Kapton. The 1/16 in. Teflon tubing used to transfer solutions
to/from the cell were press-sealed into holes in the Teflon block.
For electrochemically generated samples (TBA)PF6 (0.2 M) was
used as the supporting electrolyte. For solutions in acetonitrile the
data were in some instances affected by crystallinity leading to the
loss of data over a narrow k range for different of the detector
elements. This problem was ameliorated by rapidly freezing the
sample in isopentane (140 K) before storage in liquid nitrogen prior
to loading into the closed-cycle helium Displex cryostat (10 K).
The quality of the samples was established by IR spectroscopy
(Shimadzu Prestige FTIR) using an in-line system that included
both the fluorescence EXAFS cell and a solution IR cell. The
sample was frozen immediately after transfer to the EXAFS cell.
Infrared spectroelectrochemical (IR-SEC) measurements were
made using a purpose built high-pressure SEC cell capable of
operating at elevated gas pressure.33 In all experiments vitreous
carbon was used as the working electrode. Measurements were
generally conducted at gas pressures in the range 275-680 kPa.
Spectra of the thin film of solution (10-20 µm) in contact with
the working electrode were recorded during electrolysis at 1-2 s
intervals. Spectra are presented in absorbance mode where the
reference spectrum was recorded immediately before the application
of an oxidizing or reducing potential. A PAR model 362 potentiostat
was used to control the potential during SEC experiments.
The solution sample of 12- was generated by continuous-flow
electrosynthesis.34 The potential for these experiments was con-
trolled using a PAR 273A potentiostat. Samples of 1H- and C
W
were generated chemically. The sample of 1H-W was prepared by
reaction of a 3.5 mM solution of 1 in THF with 1 mol equiv of
Super-Hydride. IR spectroscopy was used to confirm that the
reaction proceeded quantitatively and with no significant formation
of side products. Exposure of this sample to CO resulted in its
conversion to C. Owing to the presence of Li+ the product contains
a mixture of ion-paired (35%) and non-ion-paired (65%) forms of
C. The sample used for EXAFS measurements contained a total
Continuous-flow electrosyntheses were performed using a pur-
pose built airtight cell based on a design reported previously.34 The
working and counter electrodes were fashioned from reticulated
vitreous carbon, and the reference electrode consisted of a jacketed
silver wire or a no-leak Ag/AgCl reference electrode (Cypress
Systems), which was inserted into a small cavity in the working
electrode. Standard liquid chromatography valves, gastight syringes,
and fittings (Hamilton) in conjunction with thick-walled 1/16 in.
Teflon tubing permitted anaerobic transfer of solution to and from
the cell. The flow of solution to the cell was controlled using a
syringe pump that was typically operated at a flow rate of 5 µL
s-1. The cell was jacketed, and the temperature could be maintained
at -50 °C for the duration of the experiment. IR spectra were
collected using a Bio-Rad FTS 175C spectrometer which utilized
a glowbar source, Ge/KBr beam splitter, and a liquid-nitrogen-
cooled MCT detector. NMR spectra were recorded using a Varian
Unity Plus 400 MHz instrument; chemical shifts of 31P NMR spectra
were referenced relative to H3PO4. Electrospray mass spectra were
recorded using a Micromass Quattro II mass spectrometer in
negative ion mode. Deoxygenated acetonitrile was used as the
mobile phase, and N2, as the drying and nebulizing gas. Sample
concentrations were approximately 0.4 µM.
of 5-10% 1 and 1H- (IR, Supporting information).
W
Data reduction of experimental X-ray absorption spectra was
performed using the program XFIT.27,28 The photon energy was
calibrated against the absorption edge of Fe foil, where the peak in
the first derivative was assigned an energy of 7111.2 eV. Back-
ground removal from the EXAFS data was performed both
automatically and manually using the AUTOBK algorithm35
(implemented within Athena36) and XFIT27,28 spline packages,
respectively. The use of both approaches provided an additional
check on the reliability of the EXAFS function.35,37 The model
structures were refined using the software package XFIT,27,28 which
incorporates ab initio FEFF 6.0138 MS curved-wave calculations.
The calculations included MS paths with Rmax ) 4 Å (maximum
effective path length of 8 Å) and up to four legs. The plane wave
and curved wave path filter thresholds were set at 2% and 3% of
the strongest MS path, respectively. A nonlinear least-squares fitting
is used to vary the model and concurrently optimize the fit of the
calculated to the observed EXAFS.27 The goodness of fit residual
parameter (RXAFS) has been defined28 where an RXAFS value around
20% is considered a good fit to the non-Fourier-transformed data,
while an RXAFS value greater than 40% is poor. The random
(statistical) errors (σr) in the Fe-L bond lengths due to noise in
the EXAFS data were estimated by Monte Carlo calculations.27,28
These were combined with the typical systematic errors (σs),
assigned a conservative consensus value of 0.02 Å,28 to give the
maximum root-mean-square (rms) error ([σr2 + σs2]1/2). A descrip-
tion of the models used for data analysis together with details of
X-ray fluorescence measurements were made at a temperature
of ca. 10 K using beamline 20B (bending magnet) at the KEK
Photon Factory, Tskuba, Japan. A channel-cut Si(111) monochro-
mator provided an energy resolution (∆E/E) of ca. 2.4 × 10-4
,
and higher order harmonics of the selected wavelength were rejected
by detuning the monochromator by a factor of a half. EXAFS
measurements were conducted at the Fe K-edge (7111.2 eV). Solid
samples were dispersed in boron nitride, and data were measured
in transmission mode using an ion chamber detector. Solutions were
measured in fluorescence mode using a 10-element Ge detector
and were prepared so as to give a concentration of 3-5 mM (i.e.
6-10 mM in iron) in either acetonitrile or THF. The fluorescence
cell consisted of a Teflon block with a 1.5 × 1.5 × 10 mm channel,
2
the symmetry constraints and restraints on E0, S0 , and σ2 is available
as Supporting Information.
(35) Newville, M.; Livins, P.; Yacoby, Y.; Rehr, J. J.; Stern, E. A. Phys.
ReV. B: Condens. Matter 1993, 47, 14126-14131.
(36) Ravel, B. Athena, 0.8.019; University of Washington: Seattle, WA,
2001.
(37) Bridges, F.; Booth, C. H.; Li, G. G. Physica B (Amsterdam) 1995,
208, 209, 121-124. Li, G. G.; Bridges, F.; Booth, C. H. Phys. ReV.
B: Condens. Matter 1995, 52, 6332-6348.
(38) Zabinsky, S. I.; Rehr, J. J.; Ankudinov, A.; Albers, R. C.; Eller, M. J.
Phys. ReV. B: Condens. Matter 1995, 52, 2995-3009.
(32) Chang, D.; Malinski, T.; Ulman, A.; Kadish, K. M. Inorg. Chem. 1984,
23, 817-24. Connelly, N. G.; Geiger, W. E. Chem. ReV. 1996, 96,
877-910.
(33) Borg, S. J.; Best, S. P. J. Electroanal. Chem. 2002, 535, 57-64.
(34) Bondin, M. I.; Foran, G.; Best, S. P. Aust. J. Chem. 2001, 54, 705-
709.
Inorganic Chemistry, Vol. 43, No. 18, 2004 5643

Cheah et al.
Acknowledgment. We thank Professor Chris Pickett for
many insightful discussions throughout this investigation.
S.P.B. gratefully acknowledges the Australian Research
Council for funding this research. S.J.B. and M.I.B. ac-
knowledge the 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. We thank Dr. Garry Foran for
expert assistance with the EXAFS experiments.
Supporting Information Available: Spectroelectrochemical
plots, tables of model parameters, a description of the models,
EXAFS and FT plots, and a description of EXAFS sample
preparation with IR spectra. This material is available free of charge
via the Internet at http://pubs.acs.org.
IC049746E
5644 Inorganic Chemistry, Vol. 43, No. 18, 2004