Hydrogen activation by biomimetic diiron dithiolates

Article abstract of DOI:10.1021/ic900850u

Using the thermally stable salts of [Fe₂(SR)₂(CO) ₃(PMe₃)(dppv)]-BAr^F 4, we found that the azadithiolates [Fe₂(adtR)(CO)₃(PMe₃)-(dppv)] ⁺ react with high pressures of H₂ to give the hydride [Fe₂(μ-H)(adtR)(CO)₃(dppv)(PMe₃)]BAr ^F₄. The related oxadithiolate and propanedithiolate complexes are unreactive toward H₂. Molecular hydrogen is proposed to undergo heterolysis assisted by the amine followed by isomerization of an initially formed terminal hydride. Use of H₂ and D₂O gave the deuteride as well as the hydride, implicating protic intermediates.

Full text of DOI:10.1021/ic900850u

Inorg. Chem. 2009, 48, 7507–7509 7507
DOI: 10.1021/ic900850u
Hydrogen Activation by Biomimetic Diiron Dithiolates
Matthew T. Olsen, Bryan E. Barton, and Thomas B. Rauchfuss*
Department of Chemistry, University of Illinois, Urbana, Illinois 61801
Received May 1, 2009
Using the thermally stable salts of [Fe2(SR)2(CO)3(PMe3)(dppv)]-
Recently, we reported [Fe2(pdt)(CO)3(PMe3)(dppv)]BF4
([1]BF4, pdt = S2C3H6), a paramagnetic (S = 1/2) spin-
localized species that represents a useful structural model
for the Hox state of the binuclear active site [dppv = cis-1,
2-bis(diphenylphosphino)ethylene].7,8 With a vacant coordi-
nation site, Hox and its models are poised to activate H2.
Although [1]þ binds CO, it exhibits no discernible affinity
toward H2. The anticipated product of hydrogen activation,
[Fe2(μ-H)(pdt)(CO)3(PMe3)(dppv)]BF4 ([1H]BF4), was pre-
pared independently by protonation of the corresponding
FeIFeI precursor; a terminal hydride is initially formed that
rapidly isomerizes to bridging hydrides.9
BArF , we found that the azadithiolates [Fe2(adtR)(CO)3(PMe3)-
4
(dppv)]þ react with high pressures of H2 to give the hydride
[Fe2(μ-H)(adtR)(CO)3(dppv)(PMe3)]BArF . The related oxadithio-
4
late and propanedithiolate complexes are unreactive toward H2.
Molecular hydrogen is proposed to undergo heterolysis assisted by
the amine followed by isomerization of an initially formed terminal
hydride. Use of H2 and D2O gave the deuteride as well as the
hydride, implicating protic intermediates.
Hydrogenases are enzymes that catalyze the interconversion
of H2 with protons and reducing equivalents.1 Understanding
the reactivity of these enzymes via active-site models remains
topical,2 especially because these catalysts rely on inexpensive
first-row transition metals.3 Significant progress has been made
in [FeFe]-hydrogenase models,4,5 but nearly all studies to date
have focused on proton reduction.6 The opposite reaction,
hydrogen oxidation, has proven elusive. This lack of reactivity
is surprising because [FeFe]-hydrogenases are exceptionally
active toward H2 oxidation. The eventual translation of models
to applications in fuel cells requires progress in hydrogen
oxidation. Herein we report the first example of the activation
of H2 by a model for [FeFe]-hydrogenase, as well as the
associated advances that have facilitated this progress.
The thermal sensitivity of [1]BF4 severely limits studies of its
reactivity toward H2: it is unstable above 0 °C for more than
a few seconds. We have found that the corresponding
salt [1]BArF4 is stable in solution for days at room temperature
(BArF -=B(C6H3-3,5-(CF3)2)4-). The sensitivity of electrophilic
4
iron carbonyls toward fluorinated counterions is precedented.10
We confirmed that solutions of [1]BArF rapidly decomposed
4
upon treatment with [Bu4N]BF4. The robust salt [1]BArF ,
4
however, proved unreactive toward 1800 psi H2, even upon
addition of the bulky base 2,6-(tBu)2pyridine. Many bases such as
2,6-dimethylpyridine are not compatible with [1]BArF , causingit
4
to decompose to unidentified products. Apparently, H2 activa-
tion by Hox models is subject to a significant kinetic barrier.
The proposed azadithiolato cofactor has been shown to
significantly affect the acid-base properties of diiron dithio-
late complexes.11 Furthermore, pendant amine bases drama-
tically affect the rate of H2 uptake in Ni-based catalysts.12 We
*To whom correspondence should be addressed. E-mail: rauchfuz@uiuc.edu.
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4
As described below, we present evidence that such mixed-
valence azadithiolatodiiron complexes indeed activate H2.
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proven to be particularly sensitive: attempts to generate
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r
2009 American Chemical Society
Published on Web 07/15/2009
pubs.acs.org/IC

7508 Inorganic Chemistry, Vol. 48, No. 16, 2009
Olsen et al.
Table 1. Reactivity of [Fe2[(SCH2)2X](CO)3(PMe3)(dppv)]þ Derivatives toward
H2 (10 mM Toluene Solution, 1800 psi H2, 26 h)
Scheme 1. Thermodynamic Cycle for the Reaction 2[Fe2(SR)2(CO)3-
(PR3)3]þ þ H2 f 2[HFe2(SR)2(CO)3(PR3)3]þ
complex (BArF - salt)
yield of hydride (%)
4
[Fe2(pdt)(CO)3(PMe3)(dppv)]þ ([1]þ)
[Fe2(adt)(CO)3(PMe3)(dppv)]þ ([2]þ)
[Fe2(adt-Bn)(CO)3(PMe3)(dppv)]þ ([20]þ)
[Fe2(odt)(CO)3(PMe3)(dppv)]þ ([3]þ)
<5
∼30
∼85
<5
[Fe2(adt)(CO)3(PMe3)(dppv)]BF4 ([2]BF4, where adt =
(SCH2)2(NH)) by oxidation of 2 with FcBF4 resulted in
complex mixtures even at -78 °C. We found, however,
that solutions of [2]BArF4, generated from the treat-
13
ment of 2 with FcBArF
,
are stable for days at room
4
temperature. We also prepared the related benzyl derivative
[Fe2[(SCH2)2NCH2Ph](CO)3(PMe3)(dppv)]BArF4 ([20]BArF ),
4
which was used for the majority of our studies þfor reasons
described below. The spectroscopies for [1]þ, [2] , and [20]þ
are very similar (see the Supporting Information) and indicate
that one carbonyl is semibridging and the apical site on the
Fe(dppv) center is vacant. In [1]þ, νμ-CO is ∼30 cm-1 lower in
energy, suggesting that the carbonyl has more bridging
character. The unsaturated character of [20]þ was confirmed
by its rapid and reversible carbonylation to the adduct
[20(CO)]þ.
The formation of the μ-hydride [20(μ-H)]þ requires expla-
nation because the mechanism for H2 activation should
produce a terminal hydride adjacent to the adt.14,15 In an
independent experiment, protonation of 20 with [H(OEt2)]-
BArF4 was found to afford the ammonium salt [20H]þ, which
was characterized by IR and 31P NMR spectroscopies.16 This
ammonium compound was found to convert to [20(μ-H)]þ via
a first-order process (k = 3.9 ꢀ 10-4 s-1, 23 °C, CH2Cl2; eq 1).
The treatment of [2]BArF with 1800 psi H2 for 26 h
4
resulted in 30% conversion to the hydride [2(μ-H)]BArF
.
4
The product [2(μ-H)]þ is spectroscopically identical with
the hydride produced by protonation of 2 (see below).
Analogous but more efficient reactivity was found for the
tertiary amine [20]BArF4, which gave [20(μ-H)]BArF in
4
high yield. The mixed-valence oxadithiolate [Fe2(odt)-
(CO)3(PMe3)(dppv)]þ ([3]þ, where odt=(SCH2)2O) was also
found to be thermally stable, being comparable to [20]BArF
.
4
This tautomerizatþion is assumed to occur via the terminal
hydride ([20(t-H)] ), which is not observed. The related
terminal hydride [1(t-H)]þ, which is observable, isomerizes
to [1(μ-H)]þ (see the Supporting Information).14 Thus, unþder
the conditions of hydrogenation (hours, 25 °C), [20(μ-H)] is
the expected product.
It was, however, unreactive toward high pressures of
H2. Collectively, these results point to participation of the
amine in the activation of H2 by the cationic diiron complex
(Table 1). Under otherwise identical conditions but using
argon in place of hydrogen, [20]þ remain unchanged.
A number of controls were conducted to verify the signifi-
cance of our findings. 1H and 19F NMR analyses of reaction
The hydrogenation points to the stoichiometry shown in eq 2.
mixtures showed that BArF - and ferrocene remained un-
4
2½Fe2ðadtRÞðCOÞ ðPMe3ÞðdppvÞꢁþ
affected. No deuteride was detected when the reaction was
3
conducted in toluene-d8. When toluene solutions of [20]BArF
þH2 f2½HFe2ðadtRÞðCOÞ ðPMe3ÞðdppvÞꢁþ ð2Þ
4
3
were stored in the absence of H2 for several days, only trace
amounts of [20(μ-H)]þ formed, probably from a reaction
involving adventitious water. Thiþs interesting process was
confirmed by the treatment of [20] with D2O to give small
amountsþof [20(μ-D)]þ. In the same way that the adt com-
plexes [2] and [20]þ exhibit greater reactivity toward H2 than
does [1]þ, we found that [Fe2(adt)(CO)2(dppv)2]þ is reactive
toward H2 whereas the analogous propanedithiolate
[Fe2(pdt)(CO)2(dppv)2]þ is not.
Although this reaction appears homolytic on the basis of the
stoichiometry, the finding that it requires azadithiolate indicates
a heterolytic mechanism. A thermodynamic cycle highlights the
key steps; although the heterolytic cleavage of H2 is expected to
ꢀ
(14) Ezzaher, S.; Capon, J.-F.; Gloaguen, F.; Petillon, F. Y.; Schollhammer,
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Communication
Inorganic Chemistry, Vol. 48, No. 16, 2009 7509
Scheme 2. Proposed Mechanism of H2 Activation by [Fe2[(SCH2)2-
reduce [20]þ. The ammonium hydride [H20(t-H)]þ is, however,
expected19 to be a powerful reductant (∼-1.6 V vs Fc0/þ).
Heterolysis of [20(H2)]þ is apparently irreversible because [20]þ
was not observed to catalyze the formation of HD from H2 and
D2O under the conditions of the experiment. We thus propose
that heterolysis induces rapid electron transfer to give the
ammonium hydride [H20(t-H)]2þ and 20, the latter of which
rapidly deprotonates the former.
NR](CO)3(PMe3)(dppv)]þ
When a solution of [20]þ was treated with H2 (2000 psi) in
the presence of D2O, we obtained [20(μ-D)]þ and [20(μ-H)]þ in
a ratio of ∼4:1. Comparable results were obtained for the
corresponding reaction involving D2 and H2O, which gives
both [20(μ-D)]þ andþ[20(μ-H)]þ. Control experiments con-
firmed that [20(μ-H)] does not exchange with D2O. Proton
transfer from an initial dihydrogen complex [20(H2)]þ would
account for 50% of the exchange. The high degree of ex-
change is conþsistent with tautomerization between [20(μ-H)]þ
and [20(t-H)] (see eq 1). We have shown that terminal
hydrides are also kinetically inert in the absence of the amine
cofactor as indicated by the behavior of [20(t-H)]þ.11
The results presented in this Communication establish that
biomimetic diiron dithiolato complexes can thermally acti-
vate hydrogen. The findings point to a mechanism that
involves a coupling of the electron and proton transfers.
Optimized models will require manipulation of the steps in
Scheme 1, as well as suppressing the formation of the
μ-hydride, which is a thermodynamic sink. The present
results highlight the importance of the amine in the activation
of H2 by the diiron center.12,20
be endergonic, this process can be driven if the diiron cation is a
strong hydride acceptor. In this case, subsequent protonation
and redox events are required to make the overall process
energetically downhill (Scheme 1). In the protein, the stoichi-
ometry, of course, differs from that in eq 2 because the initial
heterolysis is coupled to oxidation by an Fe-S cluster and one
proton is released to the protein. þIn our system, the diiron
complexes can interact; half of [20] serves sequentially as an
oxidant and then a proton acceptor.
The proposed mechanism invokes the initial formation of
the adduct [20(H2)]þ (Scheme 2). Because exogenous CO binds
only weakly to [20]þ,8,17 coordination of H2 is expected18 to be
unfavorable. Binding of H2 initiates the heterolytic activation
sequence as implied by the requirement for the amine-contain-
ing dithiolate. We have shown that the redox couple for the CO
adduct [Fe2[(SCH2)2X](CO)4(PMe3)(dppv)]þ/2þ is ∼250 mV
more positive than the [Fe2[(SCH2)2X](CO)3(PMe3)(dppv)]0/þ
couple.7 Thus, we anticipate that [20(H2)]þ would be unable to
Acknowledgment. This research was supported by the
NIH. We thank Dr. D. L. DuBois (PNNL) for advice and
Dr. M. Nilges for the electron paramagnetic resonance
(EPR) spectra. M.T.O. was supported by a NIH-CBI
fellowship.
Supporting Information Available: Preparative and spectro-
scopic details, including EPR, IR, NMR, and electrospray
ionization mass spectrometry spectra. This material is available
free of charge via the Internet at http://pubs.acs.org.
(17) Thomas, C. M.; Liu, T.; Hall, M. B.; Darensbourg, M. Y. Chem.
Commun. 2008, 1563–1565.
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Academic/Plenum: New York, 2001.
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(20) Henry, R. M.; Shoemaker, R. K.; DuBois, D. L.; Rakowski DuBois,
M. J. Am. Chem. Soc. 2006, 128, 3002–3010.