Pendant bases as proton relays in iron hydride and dihydrogen complexes

Article abstract of DOI:10.1021/ja057242p

The complex trans-[HFe(PNP)(dmpm)(CH₃CN)]BPh₄, 3, (where PNP is Et₂PCH₂N(CH₃)CH ₂PEt₂ and dmpm is Me₂PCH₂PMe ₂) can be successively protonated in two steps using increasingly strong acids. Protonation with 1 equiv of p-cyanoanilinium tetrafluoroborate in acetone-d₆ at -80 °C results in ligand protonation and the formation of endo (4a) and exo (4b) isomers of frans-[HFe(PNHP)(dmpm)(CH ₃CN)]-(BPFu)₂. The endo isomer undergoes rapid intramolecular proton/hydride exchange with an activation barrier of 12 kcal/mol. The exo isomer does not exchange. Studies of the reaction of 3 with a weaker acid (anisidinium tetrafluoroborate) in acetonitrile indicate that a rapid intermolecular proton exchange interconverts isomers 4a and 4b, and a pK_a value of 12 was determined for these two isomers. Protonation of 3 with 2 equiv of triflic acid results in the protonation of both the PNP ligand and the metal hydride to form the dihydrogen complex [(H₂)Fe(PNHP) (dmpm)(CH₃CN)]³⁺, 11. Studies of related complexes [HFe(PNP)(dmpm)(CO)]⁺ (12) and [HFe(depp)(dmpm)(CH ₃CN)]⁺ (10) (where depp is bis(diethylphosphino)propane) confirm the important roles of the pendant base and the ligand trans to the hydride ligand in the rapid intra- and intermolecular hydride/proton exchange reactions observed for 4. Features required for an effective proton relay and their potential relevance to the iron-only hydrogenase enzymes are discussed.

Full text of DOI:10.1021/ja057242p

Published on Web 02/11/2006
Pendant Bases as Proton Relays in Iron Hydride and
Dihydrogen Complexes
Renee M. Henry,† Richard K. Shoemaker,† Daniel L. DuBois,*,‡ and
M. Rakowski DuBois*,†
Contribution from the Department of Chemistry and Biochemistry, UniVersity of Colorado,
Boulder, Colorado 80309, and National Renewable Energy Laboratory, 1617 Cole BouleVard,
Golden, CO 80401
Received October 24, 2005; E-mail: Mary.Rakowski-dubois@Colorado.edu
Abstract: The complex trans-[HFe(PNP)(dmpm)(CH3CN)]BPh4, 3, (where PNP is Et2PCH2N(CH3)CH2PEt2
and dmpm is Me2PCH2PMe2) can be successively protonated in two steps using increasingly strong acids.
Protonation with 1 equiv of p-cyanoanilinium tetrafluoroborate in acetone-d6 at -80 °C results in ligand
protonation and the formation of endo (4a) and exo (4b) isomers of trans-[HFe(PNHP)(dmpm)(CH3CN)]-
(BPh4)2. The endo isomer undergoes rapid intramolecular proton/hydride exchange with an activation barrier
of 12 kcal/mol. The exo isomer does not exchange. Studies of the reaction of 3 with a weaker acid
(anisidinium tetrafluoroborate) in acetonitrile indicate that a rapid intermolecular proton exchange
interconverts isomers 4a and 4b, and a pKa value of 12 was determined for these two isomers. Protonation
of 3 with 2 equiv of triflic acid results in the protonation of both the PNP ligand and the metal hydride to
form the dihydrogen complex [(H2)Fe(PNHP)(dmpm)(CH3CN)]3+, 11. Studies of related complexes [HFe-
(PNP)(dmpm)(CO)]+ (12) and [HFe(depp)(dmpm)(CH3CN)]+ (10) (where depp is bis(diethylphosphino)-
propane) confirm the important roles of the pendant base and the ligand trans to the hydride ligand in the
rapid intra- and intermolecular hydride/proton exchange reactions observed for 4. Features required for an
effective proton relay and their potential relevance to the iron-only hydrogenase enzymes are discussed.
Introduction
to form a terminal hydride ligand and protonated nitrogen
base.9
The recent determination of the structures of iron-only
hydrogenases has revealed that the active sites consist of
bimetallic iron complexes bridged by a 1,3-dithiolate ligand.1-8
Based on the scattering of X-rays alone, it is difficult to precisely
identify the three atom backbone in the dithiolate ligand of this
enzyme. Certain structural and spectroscopic features of the
enzyme have been interpreted to suggest that the central atom
of the chelate is a secondary amine as shown in structure 1.1 In
addition, DFT calculations have shown that the nitrogen atom
of the azadithiolate ligand is a competent base for assisting the
heterolytic cleavage of hydrogen, and its presence provides a
low energy pathway for the heterolytic cleavage of hydrogen
A number of recent model studies have described dithiolate10-22
or phosphido23,24 bridged diiron complexes, and in some cases
(9) Niu, S.; Thomson, L. M.; Hall, M. B. J. Am. Chem. Soc. 1999, 121, 4000-
4007.
† University of Colorado.
(10) Tye, J. W.; Lee, J.; Wang. H.-W.; Mejia-Rodriguez, R.; Reibenspies, J.
H.; Hall, M. B.; Darensbourg, M. Y. Inorg. Chem. 2005, 44, 5550-5552.
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(13) Justice, A. K.; Linck, R. C.; Rauchfuss, T. B.; Wilson, S. R. J. Am. Chem.
Soc. 2004, 126, 13214-13215.
‡ National Renewable Energy Laboratory.
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J. AM. CHEM. SOC. 2006, 128, 3002-3010
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Pendant Bases as Proton Relays
A R T I C L E S
these have included the attachment of a 4Fe4S cluster or the
presence of azadithiolate20,25 or azadiphosphido bridges.24
Electrochemical studies of [(µ-ADT)Fe2(CO)6] (where ADT )
SCH2NRCH2S, R ) p-bromobenzyl) which contains a diiron
core indicate that the presence of a bridging azadithiolate ligand
shifts the potential for hydrogen production to more positive
potentials by 230 mV compared to the complex [(µ-PDT)Fe2-
(CO)6] (where PDT ) SCH2CH2CH2S) which does not contain
a pendant nitrogen atom in the dithiolate bridge.20 These results
suggest that the pendant nitrogen atom of the dithiolate ligand
in [(µ-ADT)Fe2(CO)6] plays a role in the delivery of protons
to the catalytically active site. However the effects are not large,
and this complex still has a large overpotential of more than a
volt for H2 production. Other experimental11,26,27 and theoreti-
cal28,29 studies of model compounds indicate that bridging
hydride ligands may be involved in the catalytic cycle of
hydrogenase enzymes and that there are other possible low
energy pathways for the cleavage of hydrogen that do not require
a nitrogen in the dithiolate chelate. Finally, the role of
interactions between the active site and the surrounding protein
is not understood. As a result, there is no clear consensus on
the mechanism of operation of the active site of the iron-only
hydrogenase enzyme.
Studies of organometallic complexes have shown that pendant
bases are important in proton/hydride exchange reactions,30-37
which suggests an understanding of proton relays will be
necessary for developing efficient hydrogen production/
hydrogen oxidation catalysts. Our laboratories have previously
studied the reactivities of 2 and its corresponding hydride
derivative.38 It was found that the secondary amine in the PNP
ligand of 2 acts as a fast proton relay for intermolecular
exchange between D2O and the metal hydride. The exchange
was proposed to proceed through a nickel(IV) dihydride or a
nickel(II) dihydrogen intermediate, but neither of these species
could be detected. These results prompted us to study the role
of a pendant base in proton/hydride exchange in iron diphos-
phine complexes that are known to readily form dihydrogen
and hydride complexes.39-43 In this work we demonstrate
experimentally that a pendant nitrogen base in a six-membered
chelate ring can promote very rapid intra- and intermolecular
proton/hydride exchange in octahedral Fe(II) PNP complexes
and function as a proton relay for oxidized Fe(III) hydrides as
well. Our experimental results are consistent with models
proposed previously for iron-only hydrogenase enzymes based
on theoretical studies9 and structural inference.1
Results and Discussion
Previous work has shown that octahedral iron(II) complexes
containing two chelating PNP ligands and two monodentate
ligands adopt a cis geometry, while those containing one PNP
and one dmpm chelate favor trans structures.44 The complex
trans-[HFe(PNP)(dmpm)(CH3CN)](BF4)2, 3, was selected for
studying the role of the PNP ligand in hydride/proton exchange
reactions. Compound 3 is easily prepared in pure form, and its
crystal structure has been previously determined.44 This structure
indicates that the PNP ligand adopts a chair conformation in
the solid state. This same conformation is likely the most stable
in solution, but the boat conformation should also be easily
accessible. Previous work demonstrated that protonation of 3
in acetonitrile with even relatively weak acids such as anisi-
dinium tetrafluoroborate, which has a pKa in acetonitrile of 11.3,
leads to the evolution of hydrogen and the formation of the
dication [Fe(PNP)(dmpm)(CH3CN)2]2+. However NMR experi-
ments conducted at low temperatures have made it possible to
slow this hydrogen evolution reaction and observe the protonated
intermediates. Quantitative data have been obtained on the rate
of intramolecular proton/hydride exchange in this system.
Selected reactions of trans-[HFe(PNP)(dmpm)(CO)]+, 12, and
trans-[HFe(depp)(dmpm)(CH3CN)]+, 10, are also described for
comparison.
Intramolecular Proton/Hydride Exchange. Reaction of 3
with 1 equiv of p-cyanoanilinium tetrafluoroborate (pKa ) 7.6
in acetonitrile)45 in acetone-d6 at -80 °C results in the formation
of endo and exo isomers, 4a and 4b, as shown in eq 1. The
(19) Evans, D. J.; Pickett, C. J. Chem. Soc. ReV. 2003, 32, 268-275.
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A R T I C L E S
Henry et al.
The exchange mechanism is proposed to involve a proton
transfer from the PNHP+ ligand to the hydride ligand to form
a dihydrogen complex, 5, followed by rotation of the dihydrogen
ligand and heterolytic cleavage as shown in eq 2. Hydride/proton
Figure 1. (a) 400 MHz 1H NMR spectrum of the hydride region of 4a
and 4b recorded in acetone-d6 at -80 °C; the pentet centered at -20.06
ppm is assigned to 4a, and the pentet centered at -19.42 ppm is assigned
exchange is not observed for the exo isomer 4b because the
proton of the PNHP+ ligand cannot approach the hydride ligand
in the boat conformation. As a result a reaction analogous to
eq 2 cannot occur. Loss of hydrogen from the protonated
complexes is observed at room temperature over the course of
0.5 h with formation of [Fe(PNP)(dmpm)(CH3CN)2]2+ which
has been previously characterized. This reaction also presumably
proceeds through intermediate 5.
To obtain kinetic data on the proton/hydride exchange of 4a,
GOESY NMR experiments were performed on a mixture of 4a
and 4b in acetone-d6 at -60 °C. Inversion of the pentet at
-20.06 ppm resulted in the appearance of an inverted peak at
8.28 ppm. The ratio of the intensity of the inverted NH
resonance to the intensity of the inverted hydride resonance can
be plotted as a function of the mixing time and analyzed to
yield a rate constant of 7.3 s-1 at -60 °C (see Figure S2) and
a free energy of activation of 12 kcal/mol for the exchange of
the hydride and NH proton of 4a.46 This corresponds to an
exchange rate at room temperature of approximately 1 × 104
s-1. This is consistent with rapid exchange broadening the
hydride and NH resonances of 4a in the NMR spectrum at room
temperature.
In contrast, a GOESY NMR experiment in acetone-d6 at -60
°C, in which the hydride resonance at -19.42 ppm assigned to
the exo isomer 4b was inverted, did not result in the appearance
of any inverted peaks. No exchange is observed between the
metal hydride and the proton on the nitrogen of 4b, consistent
with the hydride resonance for this isomer being observed as a
pentet at room temperature.
Several examples of intramolecular interactions between a
metal hydride and a ligand proton in synthetic complexes have
been reported previously,30-37 and example complexes are
shown by structures 6-9. For complexes 6-8, intramolecular
1
to 4b. (b) 400 MHz H NMR spectrum of the NH region in acetone-d6 at
-80 °C; the singlet at 8.56 ppm is assigned to 4a, and the resonance at
8.03 ppm is assigned to 4b.
hydride region of the 1H NMR spectrum at -80 °C consists of
two pentets at -20.06 ppm (2JPH ) 47.0 Hz) and -19.42 ppm
(2JPH ) 46.2 Hz) assigned to 4a and 4b, respectively (Figure 1,
trace a). Corresponding to these two hydride resonances are two
singlets assigned to the NH protons of 4a and 4b at 8.56 and
8.03 ppm, respectively (Figure 1, trace b). The 31P NMR
spectrum at -80 °C also indicates the presence of two isomers
(see Experimental Section and Figure S1). Upon warming the
solution, the hydride resonance assigned to the endo isomer
broadens and disappears, while the hydride resonance assigned
to the exo isomer remains unchanged even at room temperature.
When the reaction with acid is repeated at room temperature in
acetonitrile, a single hydride pentet is observed at -19.66 ppm
(2JPH ) 46.2 Hz). This resonance is assigned to the exo isomer,
4b. A hydride resonance for the endo isomer is not observed
due to rapid exchange of the hydride ligand and the proton on
the PNHP ligand. The 31P NMR spectrum exhibits resonances
assigned to both the endo and exo isomers (see Experimental
Section) indicating that these two isomers are not interconverting
rapidly on the NMR time scale at room temperature. These data
confirm that 4a undergoes rapid intramolecular hydride/proton
exchange while 4b does not.
(40) Hills, A.; Hughes, D. L.; Jimenez-Tenorio, M.; Leigh, G. J. J. Chem. Soc.,
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Pendant Bases as Proton Relays
A R T I C L E S
conversion of the endo and exo isomers is observed. For
example, upon dissolving a mixture of anisidinium tetrafluoro-
borate (pKa ) 11.3 in acetonitrile)47 and 3 in acetonitrile-d3,
two broad resonances are observed in the 31P NMR spectrum
for the PNP (55.18 ppm) and dmpm (1.33 ppm) ligands at room
temperature and at -40 °C. These results are consistent with
the rapid protonation of 3 to form isomers 4a and 4b as shown
in reaction 1 and the reverse reaction so that an equilibrium
distribution is reached. In this case the observed chemical shifts
for the PNP and dmpm ligand are a weighted average of the
chemical shifts for 3, 4a, and 4b. No hydride resonance is
1
observed in the H NMR spectrum at room temperature. This
is attributed to rapid intramolecular exchange between the
hydride ligand and the PNHP ligand of 4a (reaction 2) and the
rapid intermolecular exchange of the protons of the PNHP
ligands of 4a and 4b with anisidinium (reaction 1). At -40 °C,
proton/hydride exchange is observed, but it is relatively slow
as indicated by the observation of distinct resonances for both
the hydride ligand and NH protons.30,31,33,35 In all of these cases,
it has been proposed that the intramolecular proton/hydride
exchange proceeds through a dihydrogen complex similar to
that proposed for 4a. Only complex 9 exhibits a fast intra-
molecular proton/hydride exchange36 comparable to that ob-
served for 4a. In this case it has been proposed that the proton/
hydride exchange proceeds through a trans dihydride interme-
diate and not through a dihydrogen complex.35 Our results for
4a demonstrate that a dihydrogen intermediate can also par-
ticipate in rapid (104 s-1) intramolecular hydride/proton ex-
change. A similar high rate of hydride/proton exchange (on the
order of 104 s-1) has been observed previously for the nickel
1
a broad pentet is observed at -20.07 ppm in the H NMR
spectrum. This indicates that the hydride ligand is not exchang-
ing sufficiently fast with the protons of anisidinium or the PNHP
ligand at this temperature to collapse the PH coupling (46.0
Hz). Hence the intermolecular exchange process between the
PNHP ligand of 4a and 4b and anisidinium (reaction 1) is faster
than the intramolecular exchange of the hydride and the proton
on the PNHP ligand of 4a (reaction 2) at -40 °C in acetonitrile.
When D2O is added to [HFe(CH3CN)(PNP)(dmpm)]+, com-
plete disappearance of the hydride resonance at -19.86 ppm
occurs in less than 5 min (the time required to record the
spectrum) in CD3CN. The observed exchange of deuterium with
the hydride ligand of 3 presumably involves the protonation of
the pendant base in an intermolecular process followed by rapid
intramolecular H/D exchange. To probe the role of the PNP
ligand in the D2O/hydride exchange of 3, we prepared the
complex [HFe(CH3CN)(depp)(dmpm)](BPh4), 10, in which the
NMe group of the PNP ligand has been replaced with a
methylene group of the depp ligand (depp ) 1,3-bis(dieth-
ylphosphino)propane). This complex has been fully character-
ized and exhibits spectral characteristics very similar to those
of 3 as expected (see Experimental Section). Addition of D2O
to [HFe(CH3CN)(depp)(dmpm)]+ results in only 60% incorpo-
ration of deuterium after 2.2 h. The much slower rate of
exchange of the depp complex supports the importance of the
N atom in the backbone of the PNP ligand for the rapid
intermolecular exchange of protons from the bulk solution into
the hydride position.
Observation of a Dihydrogen Complex by Protonation of
3 with Strong Acids. Protonation of 3 with fluoroboric acid in
acetone-d6 at -80 °C results in the formation of a mixture of
4a and 4b as well as a new product [(H2)Fe(CH3CN)(PNHP)-
(dmpm)]3+, 11, which is protonated both on the PNP ligand
and at the hydride ligand to form a dihydrogen complex, reaction
3. The presence of the dihydrogen ligand is indicated by the
disappearance of the hydride resonance of 3 at -19.85 ppm (a
triplet of triplet of doublets) and the appearance of a broad
resonance at -17.24 ppm assigned to a dihydrogen ligand. The
observation of an AA′XX′ pattern in the 31P NMR spectrum
indicates that a trans geometry is preserved in the product, with
the dihydrogen ligand trans to acetonitrile. As shown in reaction
3, both endo and exo isomers are possible for complex 11, but
complex [HNi(PNHP)(PNP)]2+ 38
but the nature of the inter-
,
mediate has not been established. At least part of the reason
for the relatively slow intramolecular exchange rates observed
for several of these complexes is the mismatch in pKa values
of the protonated ligand and the pKa of the incipient dihydrogen
or dihydride complex that results from proton transfer. In this
regard, the failure of complex 7 to undergo rapid proton/hydride
exchange is surprising. The ability to form either the protonated
nitrogen complex or the corresponding dihydrogen complex by
varying the nature of the phosphine ligand suggests the pKa
values of the ammonium group of 7 and the dihydrogen complex
resulting from proton transfer are very similar. In this case, the
failure to observe rapid proton/hydride exchange may be due
to the restricted motion of the nitrogen atom of the pendant
amine caused by its attachment to an extended aromatic
chelating ring system and not to a mismatch in pKa values. This
suggests two requirements for an effective proton relay: (1)
matched pKa values of the incipient dihydrogen or dihydride
complex and the protonated pendant base, and (2) a flexible
attachment of the base that permits close approach to the metal
center without requiring significant rearrangement of the inner
coordination sphere of the complex.
Intermolecular Proton Exchange. Interconversion of 4a and
4b involves deprotonation of the N atom of the PNHP+ ligand
to form 3, inversion at nitrogen, and reprotonation, an inter-
molecular proton exchange process. When a relatively strong
acid (cyanoanilinium, pKa ) 7.6 in acetonitrile) is used, the
protonation of the nitrogen atom of 3 is quantitative, and the
interconversion of the endo and exo isomers is slow even at
room temperature in acetonitrile-d3. For weaker acids with pKa
values close to that of 4a and 4b (see below), rapid inter-
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2257-2263.
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A R T I C L E S
Henry et al.
Figure 2. (a) Hydride region of 400 MHz 1H NMR spectrum of 11(HD) in acetone-d6 at -80 °C. (b) 400 MHz 2H NMR spectrum of 11(HD) proton-
coupled (bottom) and proton-decoupled (top) in acetone-d6 at -80 °C.
only one isomer is observed in the NMR spectra. We tentatively
assign this species as the endo isomer on the basis of changes
in chemical shifts similar to those of 4a.
in these two isomers in acetonitrile can be determined experi-
mentally. Reaction 1 is reversible when anisidinium tetrafluo-
roborate is used as the acid in acetonitrile. Loss of H2 also occurs
after protonation, but this reaction is much slower than the rate
at which equilibrium 1 is established. The observed 31P NMR
chemical shifts of the PNP ligand as a function of added
anisidinium tetrafluoroborate were used to determine an equi-
librium constant of 6.2 ( 0.2 for reaction 1 with B ) anisidine.
From this equilibrium constant, a pKa value of 12.1 ( 0.2 was
calculated for the N-protonated ligand in the mixture of 4a and
4b (see Experimental Section for details). This compares to pKa
values of 9.4 for trans-[Fe(PNHP)(dmpm)(CH3CN)2]3+ and 10.5
for trans-[HFe(PNHP)(dmpm)(CO)]2+ 44
Both charge and the
.
electron-withdrawing ability of the trans ligands exert an
influence on the acidity of the PNHP ligand.
The second protonation occurs on the hydride ligand of 4 to
form the dihydrogen complex 11. The pKa of the dihydrogen
ligand in 11 can be bracketed between those of triflic acid (2.6
in acetonitrile)50 and p-cyanoanilinium (7.5 in acetonitrile). The
pKa value for 11 is apparently similar to that of HBF4 in acetone,
because, as noted above, addition of tetrafluoroboric acid to a
solution of 3 in acetone-d6 at -80 °C results in the formation
of a mixture of 4a, 4b, and 11 based on 1H and 31P NMR spectra
(see Experimental Section).
When 3 is protonated with triflic acid in acetone-d6 at -80°C,
only 11(HD) is formed. The isotope composition arises from
H/D exchange of triflic acid with acetone-d6. In this case, a
1
1:1:1 triplet is observed in the H NMR spectrum at -17.08
ppm with 1JHD ) 30.4 Hz (trace a of Figure 2). In the 2H NMR
spectrum, a doublet (1JHD ) 30.4 Hz) and a singlet are observed
at -17.40 and -17.43 ppm, respectively (trace b of Figure 2).
The doublet is assigned to an HD ligand and the singlet to a D2
ligand. When the 2H NMR spectrum is 1H-decoupled, the
doublet collapses to a singlet as expected. Using relationships
Reactions of [HFe(CO)(PNP)(dmpm)](BF4), 12. Studies of
the analogous carbonyl hydride complex, 12, have shown that
the nature of the ligand trans to the hydride in these Fe(II)
complexes has a significant effect on the chemistry of the
hydride ligand. Protonation of 12 in acetone-d6 with excess triflic
acid results only in the protonation of the nitrogen atom of the
PNP ligand to form primarily the endo isomer, 13a (see reaction
1). In contrast to the protonation of 3, formation of a doubly
protonated dihydrogen complex is not observed in the NMR
spectrum at -80 °C. However dihydrogen is evolved from the
protonated complex at rt upon addition of excess triflic acid.
In addition, fast intra- and intermolecular exchange processes
which are proposed to involve a dihydrogen intermediate are
not observed for the carbonyl complex. Protonation of
[HFe(PNP)(dmpm)(CO)](BF4), 12, with tetrafluoroboric acid
in acetonitrile results in the formation of the endo (13a) and
exo (13b) isomers of [HFe(PNHP)(dmpm)(CO)]2+ analogous
1
between JHD and H-H bond distances suggested by Morris48
and Heinekey,49 an H-H bond distance of 0.92 Å can be
calculated for 11. This distance is in good agreement with
previously reported iron dihydrogen complexes.39-43
Thermodynamic Considerations. As described above, 3 can
be successively protonated in two steps using increasingly strong
acids. The first protonation occurs on the nitrogen atom of the
PNP ligand to form a nearly 50:50 mixture of endo and exo
isomers, 4a and 4b. The average pKa of the protonated ligand
(48) Maltby, P. A.; Schlaf, M.; Steinbeck, M.; Lough, A. J.; Morris, R. H.;
Klooster, W. T.; Koetzle, T. F.; Srivastava, R. C. J. Am. Chem. Soc. 1996,
118, 5396-5407.
(49) Luther, T. A.; Heinekey, M. Inorg. Chem. 1998, 37, 127-132.
(50) Fujinaga, T.; Sakamoto, I. J. Electroanal. Chem. 1977, 85, 185.
9
3006 J. AM. CHEM. SOC. VOL. 128, NO. 9, 2006

Pendant Bases as Proton Relays
A R T I C L E S
to 4a and 4b. However, hydride resonances for both 13a and
13b are observed at room temperature, indicating that rapid
intramolecular proton/hydride exchange is not occurring for
either isomer. Replacement of acetonitrile with CO trans to the
hydride ligand turns off the rapid intramolecular exchange
-
observed for 4a. Although the BF4 anion is known to affect
proton-transfer rates in some systems through ion-pairing
effects,51 the exchange between the proton of the PNHP ligand
and the hydride ligand is slow for 13 when HBF4 is used and
fast for 4a when [p-cyanoanilinium]BF4 is used. This indicates
that the anion is not playing a significant role in this proton/
hydride exchange.
The nature of the trans ligand in these octahedral iron
complexes can also have a major influence on the rate of
intermolecular H/D exchange. While Fe-H/D2O exchange is
very fast for the PNP complex containing an acetonitrile ligand
trans to the hydride, the rate of deuterium exchange into the
hydride position is orders of magnitude slower when the trans
ligand is CO. When excess D2O is added to [HFe(CO)(PNP)-
(dmpm)]+ in CD3CN or acetone-d6, no change is observed in
the hydride resonance at -7.18 ppm after 24 h.
These differences are attributed to the fact that a dihydrogen
ligand trans to a CO ligand is known to be much more acidic
than one trans to acetonitrile.52-55 As a result the energy barrier
for the formation of a dihydrogen intermediate by proton transfer
from the PNHP+ ligand is expected to be large in this system,
and formation of the doubly protonated dihydrogen complex
analogous to 11 is not observed.
Figure 3. (a) Cyclic voltammogram of a 3.0 mM solution of [HFe(PNP)-
(dmpm)(CH3CN)](BPh4), 3, in a 0.3 M solution of Bu4NBF4/CH3CN
(Ep ) -0.31 V). The scan rate is 0.2 V/s. (b) Cyclic voltammogram of a
3.0 mM solution of [HFe(depp)(dmpm)(CH3CN)](BPh4), 10, in a 0.3 M
solution of Bu4NBF4/CH3CN (E1/2 ) -0.33 V). The scan rate is 0.2 V/s.
Electrochemical Studies. Cyclic voltammograms of [HFe-
(PNP)(dmpm)(CH3CN)](BPh4), 3, and [HFe(depp)(dmpm)(CH3-
CN)](BPh4), 10, were obtained at 200 mV/s in acetonitrile
solutions (Figure 3). For 10 the one-electron oxidation at -0.33
V (∆Ep ) 69 mV) is reversible, while the FeII/FeIII oxidation
wave obtained for 3 under identical conditions is irreversible
(Ep ) -0.31 with Ep - Ep/2 ) 61 mV). For complex 3 there is
also an additional irreversible oxidation wave at -0.03 V. The
cyclic voltammograms of both compounds have been studied
as a function of scan rate. The wave assigned to the FeII/FeIII
couple of 10 remains reversible over scan rates of 0.2 to 2 V/s,
and a linear plot of peak current versus the square root of the
scan rate for 10 indicates that the electron-transfer process is
diffusion controlled. Scan rates below 0.2 V/s are quasi-
reversible with ic/ia less than 0.90, corresponding to a slow
proton migration to solution from the FeIII hydride.
disappears (Figure S4). These results are consistent with an EC
mechanism in which the one-electron oxidation of 3 is followed
by a fast chemical reaction. For a reversible electron-transfer
reaction followed by a fast irreversible chemical reaction, a 30
mV shift in the peak potential is expected for each 10-fold
increase in scan rate.56 For scan rates between 0.05 V/s and 5
V/s, a 65 mV shift toward a more positive potential is observed
for the first oxidation wave of 3 corresponding to a 32 mV shift
in Ep for each 10-fold increase in scan rate. The EC mechanism
proposed for 3 involves a reversible oxidation to form an FeIII
hydride (14) followed by rapid proton transfer from the iron to
the PNP ligand to form an FeI species (15), as shown in reaction
4. A cathodic wave is not observed for 14 until the scan rate
For complex 3, no return cathodic wave is observed after
the FeII/FeIII oxidation couple is recorded for scan rates up to
75 V/s. At 100 V/s a small cathodic wave that corresponds to
the FeIII/FeII couple is observed (Figure S3). At these higher
scan rates the second oxidation wave at -0.03 V nearly
(51) (a) Appelhans, L. N.; Zuccaccia, D.; Kovacevic, A.; Chianese, A. R.;
Miecznikowski, J. R.; Macchioni, A.; Clot, E.; Eisenstein, O.; Crabtree,
R. H. J. Am. Chem. Soc. 2005, 127, 16299-16311. (b) Basallote, M. G.;
Besora, M.; Duran, J.; Fernandez-Trujillo, M. J.; Lledos, A.; Manez, M.
A.; Maseras, F. J. Am. Chem. Soc. 2004, 126, 2320. (c) Gruet, K.; Clot,
E.; Eisenstein, O.; Lee, D. H.; Patel, B.; Macchioni, A.; Crabtree, R. H.
New J. Chem. 2003, 27, 80-87.
(52) Landau, S. E.; Morris, R. H.; Lough, A. J. Inorg. Chem. 1999, 38, 6060-
6068.
(53) Chin, B.; Lough, A. J.; Morris, R. H.; Schweitzer, C. T.; D’Agostino, C.
Inorg. Chem. 1994, 33, 6278-6288.
(54) Cappellani, E. P.; Drouin, S. D.; Jia, G.; Maltby, P. A.; Morris, R. H.;
Schweitzer, C. T. J. Am. Chem. Soc. 1994, 116, 3375-3388.
(55) Xu, Z.; Bytheway, I.; Jia, G.; Lin, Z. Organometallics 1999, 18, 1761-
1766.
9
J. AM. CHEM. SOC. VOL. 128, NO. 9, 2006 3007

A R T I C L E S
Henry et al.
previously. 1H NMR, 31P NMR, GOESY (Gradient Selected Overhauser
Effect Spectroscopy), and variable-temperature NMR spectra were
recorded on a Varian Inova 400 MHz spectrometer. 1H chemical shifts
(ppm) are reported relative to residual solvent protons, and 31P chemical
shifts (ppm) are referenced to external phosphoric acid. VT NMR
experiments were allowed to equilibrate for 5 min at each temperature
before spectra were recorded. In the GOESY experiments, the exchange
rate constants were determined by performing selective one-dimensional
dynamic exchange experiments using the selective DPFGSE (Double
Pulsed Field Gradient Spin-Echo) pulse sequence.59 SigmaPlot 2002
for Windows version 8.02 was used to curve-fit the data for the dynamic
exchange experiments.
reaches 100V/s and the time between the oxidation and reduction
events is 8 ms. This result is consistent with a half-life of
approximately 10 ms for this hydride species or a proton-transfer
rate from intermediate 14 of approximately 100 s-1. The second
oxidation wave observed at lower scan rates is assigned to the
FeI species 15 that results from proton transfer from Fe to N.
This species forms rapidly following oxidation, and as a result
it is observed at slower scan rates. To obtain more quantitative
data, double potential step chronoamperometric experiments57
were performed on complex 3. In this experiment the potential
was stepped from an initial potential of -0.56 V to a potential
of +0.15 V for a variable amount of time and then back to the
initial potential. From working curves and the ratio of the anodic
and cathodic currents (see Figure S5), a rate constant of (1.1 (
0.2) × 102 s-1 was obtained for the proton-transfer reaction in
the conversion of 14 to 15 shown in reaction 4. It is clear from
the different results for 3 and 10 that proton migration following
oxidation of the iron center is assisted by the PNP ligand.
Summary and Relevance to Fe-Only Hydrogenases. The
results described in this work establish that a nitrogen base in
the backbone of a six-membered chelate ring is capable of
serving as a very efficient proton relay in iron hydride and
dihydrogen complexes. The intramolecular proton/hydride ex-
change process for 4a is proposed to involve the heterolytic
cleavage of a dihydrogen ligand on iron in the same manner as
has been proposed for the distal iron atom of iron-only
hydrogenase. The pendant base in 3 also facilitates (a) inter-
molecular exchange of the N-H proton of the PNHP+ ligand
with protons in solution at rates faster than the intramolecular
exchange process, and (b) the transfer of the proton from iron
to solution upon oxidation of FeII to FeIII. These reactions
provide a potential model for the movement of protons from
the active site of the enzyme to the proton conduction channel.
Important requirements for an effective proton relay in these
synthetic systems have been found to include the matching of
the pKa values of the incipient dihydrogen complex with the
protonated pendant base and a flexible ligand which allows close
approach of the protonated base to the hydride ligand without
significant reorganization of the inner coordination sphere of
iron. Similar features are likely to be important in the enzyme
active site. In the system studied here, the elimination of H2
from intermediate 5 was found to be too slow to permit the
development of a rapid electrocatalytic cycle for proton reduc-
tion. Nevertheless, determining the features responsible for the
efficient movement of protons from a metal center to the bulk
solution will be important in designing synthetic catalysts for a
range of reactions that involve the transfer of both protons and
electrons to or from the substrate during reduction/oxidation
processes.
Electrochemical measurements were carried out at room temperature
in a 0.30 M Bu4NBF4/acetonitrile solution with 3.0 mM concentration
of iron complex under a nitrogen flow. Cyclic voltammetry and
chronoamperometric data were collected using a Cypress Systems model
CS-1200 computer-aided potentiostat with a three electrode system.
The working electrode was a glassy carbon disk with a 2 mm diameter,
and the counterelectrode was a glassy carbon rod with a 3 mm diameter.
The reference electrode was a silver/silver chloride wire in a 0.30 M
Bu4NBF4/acetonitrile solution with a Vycor tip. All potentials are
referenced to the ferrocene/ferrocenium couple which was used as an
internal standard.
Synthesis of [HFe(CH3CN)(depp)(dmpm)](BPh4), 10. A mixture
of depp58 (0.244 g, 1.11 mmol) and dmpm (0.150 g, 1.10 mmol) in
THF (5 mL) was added to a suspension of FeCl2 (0.140 g, 1.11 mmol)
in 25 mL of THF, and the resulting green solution was stirred for 3 h.
This solution was filtered onto a suspension of bis(triphenylphosphine)-
iminium borohydride (0.917 g, 1.66 mmol) in THF (25 mL) and an
orange solution with a white precipitate formed after stirring for 1.5 h.
The reaction mixture was filtered, and the solvent was removed forming
a red-orange tar. NaBPh4 (3.78 g, 11.05 mmol) and acetonitrile (150
mL) were added to the flask, and the resulting yellow solution was
stirred for 2 h. A yellow solid (0.533 g, 60%) precipitated on addition
of water. This solid was collected by filtration and washed with ether.
Yellow crystals were grown from acetonitrile/ether solutions at -15
°C (0.154 g, 17%). Anal. Calcd for C42H64BFeNP4: C, 65.22; H, 8.34;
N, 1.81. Found: C, 65.35; H, 8.60; N, 1.76. 31P NMR (CD3CN):
AA′XX′ spin pattern; δA ) 2.35 (dmpm), δX ) 42.60 (depp). 1H NMR
2
2
4
(CD3CN): -19.48 (ttd, JPH ) 45 Hz, JPH ) 40 Hz, JHH ) 3.6 Hz,
1.0 H, HFe); 1.05 (br m, 12.0 H, PCH2CH3); 1.45 (t), 1.53 (t) (2JPH
)
4.4 Hz, 13.0 H total, PCH3); 1.41 (m), 1.62 (m), 1.69 (m) (11.8 H
total, PCH2CH3 and PCH2CH2); 2.09 (br m, 1.3 H, PCH2CH2CH2P);
2.95 (m), 3.12 (m) (1 H each, PCH2P); 2.24 (s, 2.6 H, NCCH3); 6.84
(m), 6.99 (m), 7.27 (m) (17.8 H total, B(C6H5)4). MS, ESI (CH3CN;
m/z): 731 {[Fe(depp)(dmpm)](BPh4)}+, 413 {[HFe(depp)(dmpm)]}+.
IR (KBr, cm-1); νFeH ) 1829; νCN ) 2237. A cyclic voltammogram
recorded in 0.30 M Bu4NBF4/CH3CN solution at 200 mV/s consisted
of one irreversible oxidation wave at 0.56 V (Ep, Ep - Ep/2 ) 76 mV),
one reversible oxidation wave at -0.33 V (E1/2, ∆Ep ) 69 mV, ia/ic )
0.91), and one irreversible reduction wave at -2.54 V (Ep, Ep - Ep/2
) 56 mV). For comparison, the cyclic voltammogram of [HFe(CH3-
CN)(PNP)(dmpm)]BPh4 recorded under the same conditions consisted
of three irreversible oxidation waves at 0.53 V (Ep, Ep - Ep/2 ) 66
mV), -0.03 V, and -0.31 V (Ep, Ep - Ep/2 ) 61 mV) and an
irreversible reduction at -2.26 V(Ep, Ep - Ep/2 ) 96 mV).
Experimental Section
Selective 31P-Decoupled 1H NMR Spectra of [HFe(CH3CN)-
General Methods and Materials. All reactions were performed
using standard Schlenk techniques under argon or handled in a glovebox
under nitrogen, and all solvents were degassed with argon. Solvents
were dried by conventional distillation methods.44 Complex 344 and
bis(diethylphosphino)propane (depp)58 were synthesized as reported
1
(depp)(dmpm)](BPh4), 10. Selective 31P decoupled (depp) H NMR
2
3
(CD3CN): -19.48 (t, JPH ) 45 Hz, HFe), 1.03 and 1.08 (two t, JHH
2
) 7.6 Hz, PCH2CH3), 1.45 and 1.53 (two t, JPH ) 3.8 Hz, PCH3),
1.41, 1.62 and 1.69 (three m PCH2CH3 and PCH2CH2), 2.09 (br m, 1.3
H, PCH2CH2CH2P), 2.95 and 3.12 (two m, PCH2P), 2.24 (s, NCCH3).
(56) Bard, A. J.; Faulkner, L. R. Electrochemical Methods Fundamentals and
Applications, 2nd ed.; John Wiley & Sons Inc.: New York, 2001; p 236.
(57) Schwarz, W. M.; Shain, I. J. Phys. Chem. 1965, 69, 30-40. Bard, A. J.;
Faulkner, L. R.; Electrochemical Methods Fundamentals and Applications,
2nd ed.; John Wiley & Sons Inc.: New York, 2001; p 499.
(58) Curtis, C. J.; Miedaner, A.; Ellis, W. W.; DuBois, D. L. J. Am. Chem. Soc.
2002, 124, 1918-1925.
(59) Stott, K.; Keeler, J.; Van, Q. N.; Shaka, A. J. J. Magn. Reson. 1997, 125,
302-324.
9
3008 J. AM. CHEM. SOC. VOL. 128, NO. 9, 2006

Pendant Bases as Proton Relays
A R T I C L E S
1
2
Selective 31P decoupled (dmpm) H NMR (CD3CN): -19.48 (t, JPH
) 40 Hz, HFe), 1.05 (broad m, PCH2CH3), 1.45 and 1.53 (two s, PCH3),
1.41, 1.62 and 1.69 (three broad m, PCH2CH3 and PCH2CH2), 2.09 (br
Protonation of [HFe(PNP)(dmpm)(CH3CN)](BPh4), 3, with Ani-
sidinium Tetrafluoroborate. In a typical experiment [HFe(CH3CN)-
(PNP)(dmpm)](BPh4) (15 mg, 0.019 mmol) and anisidinium tetra-
fluoroborate (0.010 g, 0.047 mmol) were placed in an NMR tube and
dissolved in CD3CN (0.7 mL) at room temperature. 31P NMR, rt: 1.33
and 55.18 ([HFe(PNHP)(dmpm)(CH3CN)]2+, 70%) and -4.84 and
29.25 ([Fe(PNP)(dmpm)(CH3CN)2]2+, 30%). These shifts remained
constant for 4.0 h. No hydride resonance is observed due to exchange
of the proton on the PNHP+ ligand with the hydride and with the
2
m, 1.3 H, PCH2CH2CH2P), 2.95 (d, JHH ) 14.5 Hz) and 3.12 (dq,
2JHH ) 14.5 Hz JHH ) 3.6 Hz, PCH2P), 2.24 (s, NCCH3).
4
Observation of [Fe(D2)(CH3CN)(PNDP)(dmpm)]3+, 11(D2), and
[Fe(DH)(CH3CN)(PNDP)(dmpm)]3+, 11(HD). [HFe(CH3CN)(PNP)-
(dmpm)](BPh4) (0.015 g, 0.019 mmol) was dissolved in acetone-d6,
purged with H2, and cooled to -80 °C. Triflic acid (5.1 µL, 0.057
mmol) was syringed into the NMR tube, and the sample was kept at
-80 °C until VT NMR experiments were performed (0.5 h). 31P NMR
1
conjugate base in solution. At -40 °C, the hydride is seen in the H
NMR as a broad pentet centered at -20.07 (2JPH ) 46.0 Hz). The 31
NMR at -40 °C is similar to the room-temperature spectrum. The 31
P
P
1
(acetone-d6, -80 °C): -6.00 (m, dmpm); 41.60 (m, PNP). H NMR
(acetone-d6, -80 °C): -17.08 (1:1:1 t, 1JHD ) 30.4 Hz, [Fe(HD)(CH3-
CN)(PNHP)(dmpm)]2+). 2H NMR (acetone, -80 °C): -17.43 (s,
chemical shift of 55.18 ppm observed in the protonation experiment is
the weighted average of a rapidly exchanging mixture of [HFe(PNHP)-
(dmpm)(CH3CN)]2+ (4a and 4b) and [HFe(PNP)(dmpm)(CH3CN)]+ (3).
From this chemical shift value and the weighted average of 56.00 ppm
for the fully protonated PNHP ligand of [HFe(PNHP)(dmpm)(CH3-
CN)]2+ (see Observation of [HFe(CH3CN)(PNHP)(dmpm)]2+, 4a and
4b) and 44.86 ppm for the unprotonated PNP ligand of [HFe(CH3-
CN)(PNP)(dmpm)]+, an equilibrium constant (Keq ) {[HFe(CH3CN)-
(PNHP)(dmpm)]2+}{anisidine}/{[HFe(CH3CN)(PNP)(dmpm)]+}-
{anisidinium}) can be calculated for the reaction of [HFe(CH3CN)-
(PNP)(dmpm)]+ with anisidinium to form [HFe(CH3CN)(PNHP)-
(dmpm)]2+ and anisidine. Five similar experiments were carried out in
which varying ratios of anisidine/anisidinium were added to 3, and the
data were used to calculate a value of Keq of 6.2 ( 0.2. Addition of
log Keq to the pKa of anisidinium (11.3) gives a pKa value of 12.1 (
0.2 for [HFe(CH3CN)(PNHP)(dmpm)]+ in acetonitrile.
[Fe(D2)(CH3CN)(PNHP)(dmpm)]2+), -17.40 (d, JHD ) 30.4 Hz,
1
[Fe(HD)(CH3CN)(PNHP)(dmpm)]2+). This doublet collapsed to a
singlet when the spectrum was proton decoupled. The same results were
observed when triflic acid-d was used. Using this JHD value and dHH
) 1.42-0.0167(JHD), a value of 0.91 Å was calculated for the HH
bond distance.48 Using dHH ) 1.44-0.0168(JHD), a value of 0.93 Å
was calculated for the HH bond distance.49
1
Observation of [HFe(CH3CN)(PNHP)(dmpm)]2+, 4a and 4b.
[HFe(CH3CN)(PNP)(dmpm)](BPh4) (15 mg, 0.019 mmol) and p-
cyanoanilinium tetrafluoroborate (3.90 mg, 0.019 mmol) were ac-
curately weighed into an NMR tube. A 31P NMR spectrum was taken
immediately after this sample was dissolved in acetonitrile-d3 (0.7 mL)
at 23 ( 2 °C. 31P NMR (acetonitrile-d3, 20 °C): 54.87 (PNHP, endo,
37%), 59.00 (PNHP, exo, 31%), 0.67 (dmpm, endo), and 1.66 (dmpm,
exo). See text for the structures of the endo (4a) and exo isomers (4b).
The weighted average for the two PNHP chemical shifts is 56.00 ppm.
Additional resonances were observed and assigned to [Fe(PNHP)-
Kinetics of Intramolecular Hydride/Proton Exchange for [HFe-
(CH3CN)(PNHP)(dmpm)]2+. [HFe(CH3CN)(PNP)(dmpm)](BPh4) (0.015
g, 0.019 mmol) and p-cyanoanilinium tetrafluoroborate (0.0039 g, 0.019
mmol) were accurately weighed into an NMR tube and dissolved in
acetone-d6 (0.7 mL) at 0 °C. GOESY NMR (acetone-d6; -60 °C): The
pentet at -20.10 was selectively inverted (endo-[HFe(CH3CN)(PNHP)-
(dmpm)]2+), and an inverted exchange peak was observed at 8.28 (endo-
[HFe(CH3CN)(PNHP)(dmpm)]2+) (mixing times (s), {normalized in-
tegration of exchange peak}), 0.050 {0.15}, 0.075 {0.24}, 0.100 {0.30},
0.150 {0.42}, 0.200 {0.48}, 0.250 {0.57}, 0.300 {0.64}, 0.400 {0.75}.
The plot of [integration (exchange peak)/integration (selected peak)]
versus mixing time (s) gives a rate of 7.3 s-1 for the exchange of the
hydride with the proton of the endo isomer of the protontated PNHP
ligand. Using the relationship ∆G‡ ) aT[10.319 + log(T/k)], where a
) 4.575 × 10-2 (∆G‡ in kcal mol-1), T is temperature in Kelvin, and
k is the rate of exchange in s-1, a free energy of activation for exchange
(dmpm)(CH3CN)2]2+: -5.03 (m, dmpm), 29.35 (m, PNHP, 32%). H
NMR (acetonitrile-d3, 20 °C, ppm): -19.79 (br pentet, JPH ) 46.8
1
2
Hz).
1
This experiment was repeated at -80 °C in acetone-d6. H NMR:
2
2
-20.06 (HFe, JPH ) 47.0 Hz, endo, 4a); -19.42 (HFe, JPH ) 46.4
Hz, exo, 4b); 8.56 (br s, PNHP, endo); 8.03 (br s, PNHP, exo). 31P
NMR (acetone-d6, -80 °C): 53.03 (m, PNHP, endo, 66%), 58.56 (m,
PNHP, exo, 25%), 2.50 (m, dmpm, endo), 3.86 (m, dmpm, exo);
resonances at -3.78 and 31.31 for [Fe(PNHP)(dmpm)(CH3CN)2]2+
(9%) are also observed. The second acetonitrile in the last product arises
from an acetonitrile present in the crystal lattice.
Reaction of [HFe(CH3CN)(PNP)(dmpm)](BPh4), 3, with HBF4.
[HFe(CH3CN)(PNP)(dmpm)](BPh4) (15 mg, 0.019 mmol) was ac-
curately weighed into an NMR tube and dissolved in acetone-d6 (0.7
mL). The sample was cooled to -80 °C, and HBF4 (48% aqueous
solution) (7.45 µL, 0.057 mmol) was syringed into the NMR tube.
Spectra were recorded immediately. The distribution of products based
on integration of the 31P NMR spectra is endo-[HFe(PNHP)(dmpm)-
(CH3CN)]2+, 4a (71%), exo-[HFe(PNHP)(dmpm)(CH3CN)]2+, 4b
(21%), and [Fe(H2)(PNHP)(dmpm)(CH3CN)]3+, 11 (8%). 1H NMR,
-80 °C (hydride region): -17.24 (br s, 11); -20.19 (pentet, 4a) and
-19.4 (pentet, 4b). 31P NMR, -80 °C: -6.13 (dmpm), 41.05 (PNHP),
11; 2.38 (dmpm), 52.46 (PNHP) 4a; 3.64 (dmpm), 57.99 (PNHP) 4b.
Protonation of [HFe(CO)(PNP)(dmpm)]+, 12, with Triflic Acid.
The synthesis of [HFe(CO)(PNP)(dmpm)]+ consistently results in a
mixture of [HFe(CO)(PNP)(dmpm)]+/[Fe(Cl)(CO)(PNP)(dmpm)]+.44 In
a typical experiment, this mixture (15 mg, 0.024 mmol) in acetone-d6
of protons was calculated to be 12.0 ( 0.2 kcal mol-1 45
. No exchange
occurred, for mixing times of 0.10 s to 0.50 s, between the hydride
and the protonated PNHP ligand of the exo isomer when the pentet at
-19.42 ppm was selectively inverted.
HD Exchange of Fe-H with D2O. [HFe(CH3CN)(PNP)(dmpm)]-
(BPh4) (15 mg, 0.019 mmol) was placed into an NMR tube and
dissolved in 0.7 mL of CD3CN at 23 ( 2 °C. D2O (20 µL, 1.1 mmol)
was added to this sample. The disappearance of the hydride resonance
1
at -19.86 ppm was followed by H NMR by comparing the hydride
integral with that of the resonance of the PCH2CH3 group on the PNP
ligand. Complete deuterium incorporation occurred in less than 5 min.
Similar procedures were used to establish the rate of deuterium
incorporation for [HFe(CH3CN)(depp)(dmpm)](BPh4) and [HFe(CO)-
(PNP)(dmpm)]PF6. In the latter case, no deuterium incorporation
occurred at the hydride site over a period of 24 h. For [HFe(CH3CN)-
(depp)(dmpm)](BPh4), integration of the hydride resonance at -19.48
ppm indicated 60% deuterium incorporation after 2.2 h under conditions
identical to those described for [HFe(CH3CN)(PNP)(dmpm)](BPh4).
Double Potential Step Chromoamperometry Studies of 3. An
acetonitrile solution of [HFe(CH3CN)(PNP)(dmpm)]+ (3.0 mM in 0.3
M NEt4BF4) was used for these studies. The potential of the glassy
carbon electrode was stepped from an initial resting potential of -0.56
1
is treated with triflic acid (6.5 µL, 0.072 mmol) at -80 °C. H NMR
(acetone-d6, -80 °C): -7.68 (pentet, 2JPH ) 48.1 Hz, endo-HFe, 94%);
-7.07 (pentet, JPH ) 46.4 Hz, exo-HFe, 6%). 31P NMR (acetone-d6,
2
-80 °C): three complexes: 1.24 and 51.08 (endo isomer [HFe(CO)-
(PNHP)(dmpm)]2+, 13a, 61%); 3.02 and 55.03 (exo isomer [HFe(CO)-
(PNHP)(dmpm)]2+, 13b, 3%); and -7.55 and 37.57 [Fe(Cl)(CO)-
(PNHP)(dmpm)]2+ (36%). The last complex has been independently
synthesized and characterized previously.44
9
J. AM. CHEM. SOC. VOL. 128, NO. 9, 2006 3009

A R T I C L E S
Henry et al.
V to a potential of +0.15 V and then back to the initial potential. The
time of potential reversal, τ, was varied from 10 to 1500 ms; however
the best kinetic data were obtained for τ at 10 ms and 20 ms. Using
the working curves for the double potential step chronoamperometry
for an EC mechanism,57 a rate constant of (1.1 ( 0.2) × 102 s-1 was
calculated (average of two 10 ms and two 20 ms reversal times).
Department of Energy, Office of Science, Chemical and
Biological Sciences Division under Contract No. DE-AC36-
99GO10337.
Supporting Information Available: 31P NMR spectrum of
endo and exo isomers of [HFe(CH3CN)(PNHP)(dmpm)]BPh4,
4a and 4b; plot of GOESY data for 4a; cyclic voltammograms
of [HFe(CH3CN)(PNP)(dmpm)]BPh4, 3; double potential step
chronoamperometry of 3. This material is available free of
charge via the Internet at http://pubs.acs.org.
Acknowledgment. This research was supported by the
National Science Foundation, Grant Number CHE-0240106.
NMR instrumentation used in this work was supported in part
by the National Science Foundation CRIF program (CHE-
0131003). D.L.D. acknowledges the support of the United States
JA057242P
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3010 J. AM. CHEM. SOC. VOL. 128, NO. 9, 2006