Electron delocalization from the fullerene attachment to the diiron core within the active-site mimics of [FeFe]hydrogenase

Article abstract of DOI:10.1021/ic3007298

Attachment of the redox-active C₆₀(H)PPh₂ group modulates the electronic structure of the Fe₂ core in [(μ-bdt)Fe₂(CO)₅(C₆₀(H)PPh₂)]. The neutral complex is characterized by X-ray

Full text of DOI:10.1021/ic3007298

Communication
pubs.acs.org/IC
Electron Delocalization from the Fullerene Attachment to the Diiron
Core within the Active-Site Mimics of [FeFe]Hydrogenase
Yu-Chiao Liu,† Tao-Hung Yen,†,‡ Yu-Jan Tseng,§ Ching-Han Hu,§ Gene-Hsiang Lee,⊥
and Ming-Hsi Chiang*,†,‡
†Institute of Chemistry, Academia Sinica, Nankang, Taipei 115, Taiwan
‡Molecular Science and Technology Program, TIGP, Institute of Chemistry, Academia Sinica, Nankang, Taipei 115, Taiwan
§Department of Chemistry, National Changhua University of Education, Changhua 500, Taiwan
⊥Instrumentation Center, National Taiwan University, Taipei 106, Taiwan
S
* Supporting Information
complexes and even more negative in the phosphine-
ABSTRACT: Attachment of the redox-active
C60(H)PPh2 group modulates the electronic structure of
the Fe2 core in [(μ-bdt)Fe2(CO)5(C60(H)PPh2)]. The
neutral complex is characterized by X-ray crystallography,
IR, NMR spectroscopy, and cyclic voltammetry. When it is
reduced by one electron, the spectroscopic and density
functional theory results indicate that the Fe2 core is
partially spin-populated. In the doubly reduced species,
extensive electron communication occurs between the
reduced fullerene unit and the Fe2 centers as displayed in
the spin-density plot. The results suggest that the [4Fe4S]
cluster within the H cluster provides an essential role in
terms of the electronic factor.
substituted derivatives. To accommodate the potential differ-
ence between the sole {(μ-adt)Fe2(CO)3(CN)2} unit (adt =
azadithiolate) and the H cluster if voltammetric results of the
synthetic models are taken for reference, ligation of the
[4Fe4S] unit to the Fe2 core serves the purpose more than
merely the structural one (Figure 1).8
roton reduction/hydrogen oxidation is an important
P
process for energy cycling in biological systems. For a
variety of metalloenzymes, a subsidiary unit named hydro-
genase facilitates enzymatic reactions via the mediation of
hydrogen and reducing power. Understanding how hydro-
genases utilize protons, electrons, and molecular hydrogen in
an effective manner will assist chemists in the task of easing the
emerging energy crisis. Among the classes of hydrogenases,
[FeFe]hydrogenase is the most intriguing target according to
its high turnover frequency of hydrogen production (9000
molecules/s) at mild working potentials (−0.1 to −0.5 V vs
SHE).1 Numerous Fe2S2 complexes have been prepared in
recent years to model the active site of [FeFe]hydrogenase.2 As
indicated by synthetic work,3 computational simulations,4 and
biophysical studies,5 the aza nitrogen cofactor and rotated
geometry of the distal Fe center are mainly responsible for the
high catalytic efficiency.
Figure 1. (a) Proton−electron-transfer pathway within the active site
of [FeFe]hydrogenase and (b) the model complex bearing a redox
unit.
C60 is a C-based cluster that is able to uptake up to six
electrons in solution.9 In a 1,1,2,2-tetrachloroethane solution, it
can undergo one-electron reversible oxidation.10 In the aspect
of redox chemistry, C60 acts as an electron reservoir, which
mimics the [4Fe4S] unit in the active site of [FeFe]-
hydrogenase. Coordination of the fullerene to the Fe2 core
allows one to study the electronic influence exerted by the
reduced C60 unit. Herein, we report the generation of [(μ-
bdt)Fe2(CO)5(C60(H)PPh2)] (1; bdt = 1,2-benzenedithiolate),
which is characterized by spectroscopy and X-ray crystallog-
raphy. Under reducing conditions, the singly reduced species
[(μ-bdt)Fe2(CO)5(C60(H)PPh2)]•− ([1]•−) is spectroscopi-
cally characterized at low temperature. The density functional
theory calculations indicate that extensive electronic delocaliza-
tion occurs between the Fe2 core and the reduced fullerene
fragment upon the addition of two electrons. It is suggested
that the electronic structure of the Fe2 center within 1 is
modulated by the redox-active fullerene.
Less studied is the role of FeS clusters attached to the Fe2S2
core within a H cluster.6 It is proposed that the [4Fe4S]
attachment serves as a hub for electrons shuttling between the
Fe2S2 catalytic center and the interior FeS clusters 12−14 Å
away.7 Similar to the mitochondrial electron-transport cascade,
electron flux flows between the H and F clusters from one site
to the other in lower energy. The question is, how is electron
transfer between the [4Fe4S] attachment and the Fe2 site
initiated? Recent results indicate that proton reduction requires
a relatively negative working potential in diiron carbonyl
Received: April 9, 2012
Published: May 16, 2012
© 2012 American Chemical Society
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Communication
The treatment of C60(H)PPh2 with 1 equiv of [(μ-
bdt)Fe2(CO)6] in the presence of Me3NO results in the
formation of complex 1. The νCO IR signals of complex 1 have
the same band pattern in similar energy with the other
phosphine analogues,11 suggesting a similar structural skeleton
for complex 1. Crystallographic analysis of complex 1 confirms
that one of the CO groups is substituted by C60(H)PPh2
(Figure 2). The Fe−Fe bond distance of 2.4894(6) Å is
Figure 2. Molecular structure of 1. Only the hydrogen atom on
fullerene is shown for clarity. Selected bond length (Å) and angles
(deg): Fe−Fe, 2.4894(6); Fe−S, 2.2749(8); Fe−CCO,ap, 1.800(3); Fe−
CCO,ba, 1.790(3); Fe−P, 2.2449(8); C24−C25, 1.588(4); S−Fe−S,
80.24(3); S−Fe−Fe, 56.83(2); Fe−S−Fe, 66.34(2).
Figure 3. (a) Cyclic voltammogram of 1 (1 mM) in the CH2Cl2/o-
DCB solution (v = 100 mV/s, 0.1 M n-Bu4NPF6) at 256 K. (b)
Spectral changes from 1 (blue line) to [3]− (red line) monitored by in
situ IR spectroscopy in tetrahydrofuran at −50 °C.
comparable to those of 2.480(2), 2.4767(4), and 2.4917(4) Å
in [(μ-bdt)Fe2(CO)6],12 [(μ-bdt)Fe2(CO)5(PTA)] (PTA =
1,3,5-triaza-7-phosphaadamantane),13 and [(μ-bdt)-
Fe2(CO)5(PPh3)] (Figure S1 in the Supporting Information,
SI), respectively. In [(μ-bdt)Fe2(CO)5(L)] (L = C60(H)PPh2,
1; PPh3, 2; PTA) in which the phosphine ligands are of
different steric hindrance, coordination of the phosphines at the
apical position is mainly governed by the electronic factor. The
bulky fullerene fragment does exert some steric influence on the
structure. Compared with the dihedral angles between the basal
carbonyls (∠C2−Fe1−Fe2−C4 = ∠C3−Fe1−Fe2−C5 = 2.9°),
a large distortion between two apical ligands (∠C1−Fe1−Fe2−
P1 = 19.8°) is observed.
Four reversible reduction events are recorded for 1 under N2
at 256 K in the accessible electrochemical window of the
CH2Cl2/o-dichlorobenzene (o-DCB) solution (Figure 3a). The
first, second, and fourth one-electron processes at −1.11,
−1.48, and −2.14 V (vs Fc/Fc+), respectively, are ascribed to
the C60-based reductions by comparison with redox pairs of C60
and H3BC60(H)PPh2.14 The third reduction at −1.78 V is
assigned to the Fe-centered redox process. Its larger ΔE value
(the difference of Epc and Epa) of 0.24 V, compared to 0.14 and
0.12 V for the other three redox events and the internal Fc/Fc+
reference pair, respectively, suggests the occurrence of potential
inversion.15 The FeIFeI unit is initially singly reduced, which
triggers structural rearrangement with the most possible
scenario: rupture of the Fe−S bond.16 A new thermodynami-
cally stable species is achieved, which is reduced at the potential
close to the precedent reduction. Similarly, potential inversion
appears to occur upon removal of one electron from the FeIFeI
subset at Epa = +0.69 V. Oxidation of the C60 unit occurs at
+1.35 V. Like its parent fullerene molecule in the solution other
than 1,1,2,2-tetrachloroethane, it is an irreversible redox
process.17
methods. The pattern of the FTIR signals remains unchanged,
indicating no structural modification at low temperatures in the
singly reduced state. A shift of the νCO bands by 5 cm−1 to
lower energy (2047 vs, 1990 vs, 1978 s, 1938 w cm−1) confirms
that the added electron is mainly localized at the fullerene unit
at low temperature (Figure S4 in the SI). Reduction of 1 by
sodium in the presence of dibenzo-18-crown-6 ether at −70 °C
slowly led to a thermally sensitive green complex [(μ-
bdt)Fe2(CO)5(C60PPh2)]− ([3]−). Preparation of [3]− is also
achieved by the reaction of 1 and 1 equiv of KOtBu at low
temperature (Figure 3b). The vibrational band pattern of the
carbonyl groups in [3]− shows a close resemblance to that of
[1]•−, and they are similar in energy. Two species are distinct
by electron paramagnetic resonance (EPR) and electronic
spectra (Figure S5 in the SI). The EPR signals at 77 K are
different by 9 G: g = 1.999 for [1]•− and g = 1.995 for [3]−.18
In the visible and near-IR region, two excitations at 931 (sh)
and 1033 nm for [1]•− are observed, which are related to
characteristics of [C60]•−.19 They are blue-shifted to 899 (sh)
and 996 nm in [3]−. Two additional bands of [1]•− at 517 and
781 nm are not observed in [3]−. Instead, transitions of 611
and 649 nm are the features of [3]−. The addition of acetic acid
to the solution of [3]− does not lead to conversion to 1.
Recovery of 1 is completed in the presence of medium acids
(Figure S6 in the SI). Successful protonation occurs upon
contact with trifluoroacetic acid or hydrochloric acid.
The geometrical parameters of 1 that were optimized on the
B3LYP/6-31G(d) level are in fine agreement with those
obtained by X-ray crystallography. The lowest unoccupied
molecular orbital of 1 resides predominantly in the fullerene
unit, suggesting that the addition of the first electron into 1 is
the C60-based event (Figure S7 in the SI). For the singly
reduced species, the computed spin density (Figure 4) indicates
that the proximal Fe center is partially spin-populated, which is
Because of thermal instability, the red-brown product
resulting from the treatment of 1 with 1 equiv of Cp2Co,
presumably [1]•−, was characterized solely by spectroscopic
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Dr. Mei-Chun Tseng (AS) and Chia-Chen Tsai (NCKU) for
help with mass spectrometry and elemental analyses,
respectively.
REFERENCES
■
(1) Holm, R. H.; Kennepohl, P.; Solomon, E. I. Chem. Rev. 1996, 96,
2239−2314.
(2) Tard, C.; Pickett, C. J. Chem. Rev. 2009, 109, 2245−2274.
(3) (a) Singleton, M. L.; Bhuvanesh, N.; Reibenspies, J. H.;
Darensbourg, M. Y. Angew. Chem., Int. Ed. 2008, 47, 9492−9495.
(b) Ott, S.; Kritikos, M.; Åkermark, B.; Sun, L.; Lomoth, R. Angew.
Chem., Int. Ed. 2004, 43, 1006−1009. (c) Olsen, M. T.; Bruschi, M.;
De Gioia, L.; Rauchfuss, T. B.; Wilson, S. R. J. Am. Chem. Soc. 2008,
130, 12021−12030. (d) Barton, B. E.; Olsen, M. T.; Rauchfuss, T. B. J.
Am. Chem. Soc. 2008, 130, 16834−16835. (e) Liu, Y.-C.; Tu, L.-K.;
Yen, T.-H.; Lee, G.-H.; Chiang, M.-H. Dalton Trans. 2011, 40, 2528−
2541. (f) Chiang, M.-H.; Liu, Y.-C.; Yang, S.-T.; Lee, G.-H. Inorg.
Chem. 2009, 48, 7604−7612.
(4) Zampella, G.; Fantucci, P.; De Gioia, L. J. Am. Chem. Soc. 2009,
131, 10909−10917.
(5) (a) Fontecilla-Camps, J. C.; Volbeda, A.; Cavazza, C.; Nicolet, Y.
Chem. Rev. 2007, 107, 4273−4303. (b) Vincent, K. A.; Parkin, A.;
Armstrong, F. A. Chem. Rev. 2007, 107, 4366−4413.
(6) Tard, C.; Liu, X.; Ibrahim, S. K.; Bruschi, M.; De Gioia, L.;
Davies, S. C.; Yang, X.; Wang, L.-S.; Sawers, G.; Pickett, C. J. Nature
2005, 433, 610−613.
Figure 4. Spin-density plots of (a) [1]•− and (b) [1]2−.
(7) Peters, J. W.; Lanzilotta, W. N.; Lemon, B. J.; Seefeldt, L. C.
Science 1998, 282, 1853−1858.
reflected on the lengthened distance of the Fe−P bond in
[1]•−. Compared to the neutral species, the average 11 cm−1
shift toward lower energy for the computed νCO of [1]•−
indicates that the electron density about the Fe center is only
perturbed to a small extent and the majority of the spin is
localized at the fullerene unit. On the other hand, a
bathochromic shift of average 79 cm−1 is obtained for the
triplet state of the doubly reduced species, [1]2−. Figure 4
displays the spin-density plot of [1]2− in which the significant
extent of spin delocalization onto the Fe center is observed. It is
suggested that injection of the electron density from the doubly
charged fullerene anion of [1]2− into the then insulated Fe
molecular orbitals occurs. From the results, it is insinuated that
electron transfer from the F cluster to the [4Fe4S] unit within
the H cluster, followed by electron delocalization into the Fe
center, is a key pathway for proton reduction by the catalytic Fe
site.
(8) (a) Liu, Y.-C.; Lee, C.-H.; Lee, G.-H.; Chiang, M.-H. Eur. J. Inorg.
Chem. 2011, 1155−1162. (b) Camara, J. M.; Rauchfuss, T. B. J. Am.
Chem. Soc. 2011, 133, 8098−8101. (c) Camara, J. M.; Rauchfuss, T. B.
Nat. Chem. 2012, 4, 26−30.
(9) (a) Xie, Q.; Perez-Cordero, E.; Echegoyen, L. J. Am. Chem. Soc.
1992, 114, 3978−3980. (b) Echegoyen, L.; Echegoyen, L. E. Acc.
Chem. Res. 1998, 31, 593−601.
(10) Xie, Q.; Arias, F.; Echegoyen, L. J. Am. Chem. Soc. 1993, 115,
9818−9819.
(11) Liu, Y.-C.; Tu, L.-K.; Yen, T.-H.; Lee, G.-H.; Yang, S.-T.;
Chiang, M.-H. Inorg. Chem. 2010, 49, 6409−6420.
(12) Cabeza, J. A.; Martinez-Garcia, M. A.; Riera, V.; Ardura, D.;
Garcia-Granda, S. Organometallics 1998, 17, 1471−1477.
(13) Vannucci, A. K.; Wang, S.; Nichol, G. S.; Lichtenberger, D. L.;
Evans, D. H.; Glass, R. S. Dalton Trans. 2010, 39, 3050−3056.
(14) Yamago, S.; Yanagawa, M.; Mukai, H.; Nakamura, E.
Tetrahedron 1996, 52, 5091−5102.
(15) (a) Capon, J.-F.; Gloaguen, F.; Schollhammer, P.; Talarmin, J. J.
Electroanal. Chem. 2004, 566, 241−247. (b) Felton, G. A. N.;
Vannucci, A. K.; Chen, J.; Lockett, L. T.; Okumura, N.; Petro, B. J.;
Zakai, U. I.; Evans, D. H.; Glass, R. S.; Lichtenberger, D. L. J. Am.
Chem. Soc. 2007, 129, 12521−12530. (c) Felton, G. A. N.; Petro, B. J.;
Glass, R. S.; Lichtenberger, D. L.; Evans, D. H. J. Am. Chem. Soc. 2009,
131, 11290−11291.
In the present study, it is shown that the redox-active
attachment to the Fe2 core of 1 involves regulation of the
electronic structure of the Fe centers, which suggests the
essential role of the [4Fe4S] cluster within the H cluster in
terms of the electronic factor.
ASSOCIATED CONTENT
* Supporting Information
(16) Darchen, A.; Mousser, H.; Patin, H. J. Chem. Soc., Chem.
Commun. 1988, 968−970.
■
S
(17) Dubois, D.; Moninot, G.; Kutner, W.; Jones, M. T.; Kadish, K.
M. J. Phys. Chem. 1992, 96, 7137−7145.
(18) (a) Boyd, P. D. W.; Bhyrappa, P.; Paul, P.; Stinchcombe, J.;
Bolskar, R. D.; Sun, Y. P.; Reed, C. A. J. Am. Chem. Soc. 1995, 117,
2907−2914. (b) Boulas, P.; D’Souza, F.; Henderson, C. C.; Cahill, P.
A.; Jones, M. T.; Kadish, K. M. J. Phys. Chem. 1993, 97, 13435−13437.
(19) Cliffel, D. E.; Bard, A. J. J. Phys. Chem. 1994, 98, 8140−8143.
X-ray crystallographic data in CIF format and additional
experimental details and spectroscopic data. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: mhchiang@chem.sinica.edu.tw.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We are grateful for financial support from National Science
Council of Taiwan and Academia Sinica (AS). We also thank
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