[FeFe]-hydrogenase models and hydrogen: Oxidative addition of dihydrogen and silanes

Article abstract of DOI:10.1002/anie.200804400

(Chemical Equation Presented) Super model: Diiron dithiolates exist in nature in [FeFe]-hydrogenase, for the purpose of making or oxidizing molecular hydrogen. Evidence for how H₂ interacts with diiron complexes has been realized using model di

Full text of DOI:10.1002/anie.200804400

Communications
DOI: 10.1002/anie.200804400
Hydrogenase Models
[FeFe]-Hydrogenase Models and Hydrogen: Oxidative Addition of
Dihydrogen and Silanes**
Zachariah M. Heiden, Giuseppe Zampella, Luca De Gioia,* and Thomas B. Rauchfuss*
Intense interest has recently been focused on the biophysics[1]
and synthetic models of [FeFe]-hydrogenases.[2] Such studies
promise to contribute to the development of nonprecious
metal catalysts[3] for the production and utilization of hydro-
gen.[4] A challenge in current research is the resistance of
[FeFe]-hydrogenase models, diiron dithiolato carbonyl com-
plexes, to form complexes directly from H2, as well as related
Scheme 1. Structure of the active site of the [FeFe]-hydrogenases.
hydrogenic substrates. It is possible that the direct reaction of
diiron dithiolates with hydrogen-rich substrates has been
overlooked because it is considered too difficult or too
obvious, or both. Although some diiron complexes promote
isotopic H/D exchange under photochemical conditions,[5]
such reactions yield no isolable hydrides. A mixture of the
electron-rich diiron ethanedithiolate [Fe2(S2C2H4)(CO)4-
(PMe3)2] reacted with H2 in the presence of B(C6F5)3. In this
case the borane activated H2,[6] and the diiron center serves as
a base [Eq. (1)].[7]
(PPh2)2) because of its enhanced reactivity toward ligand
substitution.[9]
Density functional theory (DFT) calculations indicated
that substitution of CO by H2 in 1a is strongly disfavored by
29.2 kcalmolꢁ1. Assuming, however, that CO can be ejected
(for example, photochemically), the subsequent binding of H2
to the 32eꢁ [(dppv)(CO)Fe(S2C2H4)Fe(CO)2] complex
(Figure 1) is exothermic by 19.5 kcalmolꢁ1. The initial stable
2
ꢁ
product is predicted to feature an h -H2 ligand (H H 0.859 ꢀ,
compared to 0.74 ꢀ for free H2)[10] that would evolve to a
½Fe2ðS2C2H4ÞðCOÞ3ðPMe3Þ2ꢀ þ H2 þ BðC6F5Þ3 !
ð1Þ
ꢁ
dihydride species in which the H H bond is fully cleaved.
½ðm-HÞFe2ðS2C2H4ÞðCOÞ3ðPMe3Þ2ꢀHBðC6F5Þ3
Oxidative addition to give a cis-dihydride species is, energeti-
cally, slightly uphill from the initial H2 adduct. The subsequent
step to give [dppv(CO)Fe(S2C2H4)(m-H)Fe(CO)2H] is exo-
thermic by 10.3 kcalmolꢁ1.
We have now more directly examined the reactivity of
diiron dithiolato carbonyl compounds with H2 and with
organosilanes as surrogate substrates.[8]
The active site of Fe-only hydrogenase enzymes can be
described as [Fe2(SR)2(m-CO)(CO)2L3]z+, wherein the three
diatomic ligands on the distal iron atom (which are distal with
respect to the {4Fe–4S} cluster) are “rotated” by approx-
imately 608, thereby opening a coordination site trans to the
The energetics for the reaction of Ph2SiH2 with 1a is
initially quite similar to the case for H2. The h2-Ph2SiH2
ꢁ
complex is slightly activated (Si H 1.665 ꢀ compared to
1.417 ꢀ for SiH2(Mes)2). Significant differences are detected
in the oxidative cleavage of Ph2SiH2 compared to that for H2,
since oxidative addition is 3.3 kcalmolꢁ1 less endothermic
with the silane. The oxidative addition coupled to the
isomerization of the trans-hydridosilyl derivative is strongly
exothermic, by 10.4 kcalmolꢁ1, as it is for the H2 case. Notably,
the presence of donor ligands on the diiron center is crucial.
Calculations indicate that neither Ph2SiH2 nor H2 forms stable
derivatives with analogous complexes when the diphosphine
is replaced by two CO groups (see the Supporting Informa-
tion).
ꢁ
Fe Fe bond (Scheme 1). Hydrogen binds to this active site,
presumably as a s complex, prior to the release and oxidation
of H2. To examine the reactions of hydrogen, silanes, and
diiron dithiolato carbonyl compounds, we selected the
recently described, unsymmetrically substituted diiron dithio-
late complex [Fe2(S2C2H4)(CO)4(dppv)] (1a, dppv = cis-C2H2-
[*] Dr. G. Zampella, Prof. Dr. L. De Gioia
Department of Biotechnology and Bioscience
University of Milan-Biococca
Encouraged by the theoretical results, we attempted the
reaction of 1a with H2. No reaction occurred when a toluene
solution of 1a was treated with 1 atm H2 under reflux or
photolysis conditions. In THF, however, under photolysis
conditions, a sealed NMR sample of 1a and 1 atm H2 partially
converted into two hydride derivatives, exhibiting strongly
upfield resonances in the 1H NMR spectrum (see the
Supporting Information). The main product, 2a, is charac-
terized by a coupled doublet (d = ꢁ12.8 ppm, J = 18.4 Hz,)
and doublet of triplets (d = ꢁ14.9 ppm, J = 18.4, 31.2 Hz).
This species exhibits a single 31P NMR signal. We assign 2a as
the trans-dihydride (Scheme 2). A second product, charac-
Piazza della Scienza, 2, 20126 Milan (Italy)
E-mail: luca.degioia@unimib.it
Z. M. Heiden, Prof. Dr. T. B. Rauchfuss
Department of Chemistry, University of Illinois
600 S. Mathews Avenue, Urbana, IL 61801 (USA)
Fax: (+1)217-333-7355
E-mail: rauchfuz@uiuc.edu
[**] This work was supported by the National Institutes of Health. We
thank Bryan Barton and Aaron Justice for assistance.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200804400.
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Figure 1. Energetic profile for the gas-phase conversion of [Fe2(S2C2H4)(CO)4(dppv)] into the dihydride (values in parentheses) and the
diphenylsilyl hydride. Reaction energy values (kcalmolꢁ1) have been computed using density functional calculations at the BP86/TZVP level (see
the Supporting Information for details).
Similar photoreactions occurred, starting with the related
complex [Fe2(S2C2H4)(CO)4(dppbz)] (1b). The dppbz ligand,
1,2-bis(diphenylphosphino)benzene, is virtually isosteric with
dppv but is more rigid and, as a result, its derivatives
crystallized more readily. Irradiation of a toluene solution
containing Ph2SiH2 for 16 h resulted in the precipitation of an
orange solid. NMR and IR spectroscopic analysis indicated
that this species (3b) had a bridging hydride with a chemical
shift and P–H coupling constant (JP–H = 28.5 Hz) similar to the
value for the main product obtained from 1a in the presence
of Ph2SiH2.
Crystallographic analysis confirmed the structure of a silyl
hydrido complex with idealized Cs symmetry (Figure 2). The
hydride ligand, which was located crystallographically,
bridges the two metal centers and is slightly off center
Scheme 2. Reactivity of [Fe2(S2C2H4)(CO)4(P2)], where P2 =dppv (a) or
dppbz (b), with H2 and diphenylsilane.
(closer to the {Fe(dppbz)(CO)} site by 0.05 ꢀ). The silyl
ꢁ
ligand is apical, and the remaining Si H group is not bonded
to the iron atom. We assume that 3b arises from the
terized by a broad singlet at d = ꢁ14.4 ppm, was also detected.
Relative to toluene, THF is advantageous for this photo-
reaction because it aids the initial decarbonylation step.[11] As
shown by the calculations, decarbonylation is required for H2
reactivity, as CO binds more strongly than H2. The same
dihydride products formed more efficiently when NH3BH3
was used instead of H2 (see the Supporting Information).
The inefficient reactivity of H2 toward 1a led us to
investigate corresponding reactions involving silanes. DFT
analysis indicated that this process would be more favorable
(see above). No reaction occurred under reflux conditions,
even in the presence of the decarbonylation agent Me3NO.
Solutions of 1a in toluene were indeed found to readily react
with silanes upon photolysis. In the case of diphenylsilane, the
1H NMR spectrum of the crude product showed three hydride
resonances, indicating the formation of multiple products.
The reaction rate and product distribution ratio were
unaffected by the presence of an atmosphere of CO.
decarbonylation of the tetracarbonyl precursor 1b, followed
by formation of a transient s complex, which undergoes
intramolecular oxidative addition. The structure demon-
strates that the reactions are localized on the {Fe(CO)3}
center, not on the more electron rich {Fe(dppbz)(CO)} site.
The localization of reactivity on the unsubstituted Fe center is
also seen for the protonation[12] and the ligand substitution[9]
of this family of compounds.
As for 2a, we assume that 3b arises from formation of a
transient s complex, which undergoes intramolecular oxida-
tive addition. The selective formation of 3b arises from its low
solubility in toluene. Photolysis of solutions of 1b (in the
presence of Ph2SiH2) or 3b (in the absence of Ph2SiH2) in
1
THF afforded a new product, 4b (Scheme 2). The H NMR
spectrum of the hydride region of 4b displayed two signals, a
doublet and a doublet of triplets, integrating to 1:1, and again
these signals match those seen for the crude product of
photolysis of 1a and Ph2SiH2.
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NMR spectroscopic analysis of 4a and 4b indicated that
these species are stereochemically rigid over the range ꢁ80 to
208C. Relaxation-time (T1) measurements of the hydride
signals of complex 4a were found to be similar to those
ꢁ
reported for Fe H complexes at 1.34 and 0.71 s for the
hydride signals at d = ꢁ11.81 and ꢁ14.52, respectively.[15]
Complexes spectroscopically similar to 3a and 4a were
detected when PhSiH3 was used in place of Ph2SiH2 and
when [Fe2(S2C3H6)(CO)4(dppv)] was substituted for 1a (see
the Supporting Information).
In summary, this work provides the first evidence on the
product of the reaction of H2 with diiron dithiolato complexes
(Scheme 2). The reactivity is readily modeled computation-
ally and the overall patterns are confirmed using organo-
silanes. The results demonstrate that the diiron dithiolato
carbonyl complexes, which are robust and readily prepared,[16]
represent promising platforms for transformations of hydro-
gen-rich substrates.
Figure 2. Structure of [Fe2(S2C2H4)(m-H)(SiPh2H)(CO)3(dppbz)] (3b),
with thermal ellipsoids set at 35% probability. Phenyl thermal ellip-
soids and phenyl hydrogen atoms have been omitted for clarity.
Selected bond lengths [ꢀ], and angles [8]: Fe1–Fe2 2.6058(11), Fe1–
H1 1.6416(381), Fe1–S1 2.2697(11), Fe1–S2 2.2612(11), Fe1–
P1 2.1991(11), Fe1–P2 2.2115(11), Fe1–C1 1.7459(41), Fe2–H1 1.7094-
(375), Fe2–Si1 2.3025(12), Fe2–C2 1.7630(44), Fe2–C3 1.7553(44); Fe2-
Fe1-P1 109.14(4), Fe2-Fe1-P2 118.21(4), Fe2-Fe1-C1 140.09(12), Fe1-
Fe2-Si1 141.12(4), P1-Fe1-P2 86.78(4), P1-Fe1-C1 95.87(12), P2-Fe1-
C1 93.10(12), Si1-Fe2-C2 87.11(12), Si1-Fe2-C3 86.31(13).
Received: September 5, 2008
Published online: November 10, 2008
Keywords: agostic interactions · hydrogen · hydrogenase · iron ·
.
silanes
Crystallographic analysis revealed that 4a is formed from
ꢁ
3a by decarbonylation, reformation of Si H bonds, and a pair
ꢁ ꢁ
of Fe H Si s interactions spanning the diiron center. The
[13]
[1] M. Hambourger, M. Gervaldo, D. Svedruzic, P. W. King, D. Gust,
M. Ghirardi, A. L. Moore, T. A. Moore, J. Am. Chem. Soc. 2008,
130, 2015; J. C. Fontecilla-Camps, A. Volbeda, C. Cavazza, Y.
Nicolet, Chem. Rev. 2007, 107, 4273; K. A. Vincent, A. Parkin,
F. A. Armstrong, Chem. Rev. 2007, 107, 4366.
[2] D. J. Evans, C. J. Pickett, Chem. Soc. Rev. 2003, 32, 268; G. A. N.
Felton, A. K. Vannucci, J. Chen, L. T. Lockett, N. Okumura, B. J.
Petro, U. I. Zakai, D. H. Evans, R. S. Glass, D. L. Lichtenberger,
J. Am. Chem. Soc. 2007, 129, 12521.
ꢁ ꢁ
Fe H Si s interactions occupy two basal sites
[Fe2(SR)2L6] framework (Figure 3). Bridging silanes are
known in organoruthenium and organoiron complexes, for
of the
example,
[Cp*2Ru2(tBu2SiH2)(CO)2]
and
[Cp*2Fe2-
(tBu2SiH2)(H)2] (Cp* = C5Me5).[14] Variable-temperature
[3] C. Bolm, J. Legros, J. Le Paih, L. Zani, Chem. Rev. 2004, 104,
6217; S. Enthaler, K. Junge, M. Beller, Angew. Chem. 2008, 120,
3363; Angew. Chem. Int. Ed. 2008, 47, 3317.
[4] S. Fukuzumi, Eur. J. Inorg. Chem. 2008, 1351; A. J. Esswein,
D. G. Nocera, Chem. Rev. 2007, 107, 4022; J. Turner, G. Sverdrup,
M. K. Mann, P.-C. Maness, B. Kroposki, M. Ghirardi, R. J.
Evans, D. Blake, Int. J. Energy Res. 2008, 32, 379.
[5] X. Zhao, I. P. Georgakaki, M. L. Miller, R. Mejia-Rodriguez, C.-
Y. Chiang, M. Y. Darensbourg, Inorg. Chem. 2002, 41, 3917; X.
Zhao, C.-Y. Chiang, M. L. Miller, M. V. Rampersad, M. Y.
Darensbourg, J. Am. Chem. Soc. 2003, 125, 518.
[6] G. C. Welch, R. R. San Juan, J. D. Masuda, D. W. Stephan,
Science 2006, 314, 1124.
[7] K. Fauvel, R. Mathieu, R. Poilblanc, Inorg. Chem. 1976, 15, 976.
[8] S. C. Bart, E. Lobkovsky, P. J. Chirik, J. Am. Chem. Soc. 2004,
126, 13794; D. V. Gutsulyak, L. G. Kuzmina, J. A. K. Howard,
S. F. Vyboishchikov, G. I. Nikonov, J. Am. Chem. Soc. 2008, 130,
3732; G. J. Kubas, Metal Dihydrogen and s-Bond Complexes,
Kluwer Academic/Plenum, New York, 2001.
[9] A. K. Justice, G. Zampella, L. De Gioia, T. B. Rauchfuss, J. I.
van der Vlugt, S. R. Wilson, Inorg. Chem. 2007, 46, 1655.
[10] G. J. Kubas, Proc. Natl. Acad. Sci. USA 2007, 104, 6901.
[11] A. R. Ridley, A. I. Stewart, K. Adamczyk, H. N. Ghosh, B.
Kerkeni, Z. X. Guo, E. T. J. Nibbering, C. J. Pickett, N. T. Hunt,
Inorg. Chem. 2008, 47, 7453.
Figure 3. Structure of [Fe2(S2C2H4)(h2:h2-SiPh2H2)(CO)2(dppv)] (4a)
with thermal ellipsoids set at 35% probability. Phenyl thermal ellip-
soids and phenyl hydrogen atoms have been omitted for clarity.
Selected bond lengths [ꢀ], and angles [8]: Fe1–Fe2 2.512(2), Fe1–
H1 1.512(10), Fe1–S1 2.228(3), Fe1–S2 2.266(4), Fe1–P1 2.188(3), Fe1–
P2 2.196(4), Fe1–Si1 2.308(4), Fe2–C1 1.786(12), Fe2–H2 1.511(10),
Fe2–Si1 2.355(4), Fe2–C2 1.745(12), Si1–H1 1.625(19), Si1–
H2 1.626(19); Fe2-Fe1-P1 149.36(12), Fe2-Fe1-P2 117.54(12), Fe2-Fe1-
C1 141.68(39), Fe1-Fe2-Si1 58.31(10), Fe2-Fe1-Si1 56.49(10), Fe1-Si1-
Fe2 65.20(11), P1-Fe1-P2 86.51(13), H1-Si1-H2 145.52(57).
[12] S. Ezzaher, J.-F. Capon, F. Gloaguen, F. Y. Pꢁtillon, P. Scholl-
hammer, J. Talarmin, R. Pichon, N. Kervarec, Inorg. Chem. 2007,
46, 3426; D. Morvan, J.-F. Capon, F. Gloaguen, A. Le Goff, M.
9758
www.angewandte.org
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Angewandte
Chemie
Marchivie, F. Michaud, P. Schollhammer, J. Talarmin, J.-J.
Yaouanc, Organometallics 2007, 26, 2042.
[13] F. I. Adam, G. Hogarth, I. Richards, J. Organomet. Chem. 2007,
692, 3957.
[14] T. Takao, S. Yoshida, H. Suzuki, M. Tanaka, Organometallics
1995, 14, 3855.
[15] C. M. Thomas, J. C. Peters, Angew. Chem. 2006, 118, 790; Angew.
Chem. Int. Ed. 2006, 45, 776.
[16] H. Reihlen, A. von Friedolsheim, W. Ostwald, Justus Liebigs
Ann. Chem. 1928, 465, 72; P. I. Volkers, C. A. Boyke, J. Chen,
T. B. Rauchfuss, C. M. Whaley, S. R. Wilson, H. Yao, Inorg.
Chem. 2008, 47, 7002.
Angew. Chem. Int. Ed. 2008, 47, 9756 –9759
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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9759