Oxidative addition of thioesters to iron(0): active-site models for Hmd, nature's third hydrogenase

Article abstract of DOI:10.1021/om9004059

The thioester Ph₂PC₆H₄-2-C (O ) SPh reacts with Fe₂(CO)₉ to give [Ph₂PC₆H ₄C(O)]Fe(SPh) (CO)₃, a model for the CO-inhibited active site of the enzyme Hmd. T

Full text of DOI:10.1021/om9004059

3618 Organometallics 2009, 28, 3618–3620
DOI: 10.1021/om9004059
Oxidative Addition of Thioesters to Iron(0): Active-Site Models for Hmd,
Nature’s Third Hydrogenase
Aaron M. Royer, Thomas B. Rauchfuss,* and Danielle L. Gray
Department of Chemistry, University of Illinois at Urbana;Champaign, Urbana, Illinois 61801
Received May 17, 2009
Summary: The thioester Ph2PC6H4-2-C(O)SPh reacts with
Fe2(CO)9 to give [Ph2PC6H4C(O)]Fe(SPh)(CO)3, a mod-
el for the CO-inhibited active site of the enzyme Hmd. This
species, which reversibly decarbonylates to give a diiron
derivative, reacts with cyanide to give [[Ph2PC6H4C(O)]Fe-
(SPh)(CN)(CO)2]-.
single exchangeable residue, this active site is particularly
ripe for modeling, and indeed models have already been
described.5 Recently, however, the structural assignment of
the active site has been significantly revised.6 The new
analysis indicates that Fe is bound also to an acyl ligand,
provided by the GP cofactor (Figure 1). Acyl ligands are
rarely encountered in bioinorganic chemistry,7 and their
coexistence with thiolato ligands defines a novel platform
from the perspective of homogeneous catalysis. The new
structural information;i.e. Fe(SR)(N-donor)(CO)2(acyl);
provides sufficient information to enable the design of a first-
generation model for this active site, which is described
below.
In terms of retrosynthetic analysis, the structure of the Fe
center at the active site suggests that thioesters would
oxidatively add to Fe(0) reagents. The interaction of thioe-
sters with metal complexes has been intermittently investi-
gated,8 including studies suggesting that this interaction is of
prebiotic significance.9 The oxidative addition of a thioester
has been established for rhodium(I) complexes.10 Our ap-
proach focused on the use of a donor-functionalized thioe-
ster, which upon oxidative addition would simultaneously
deliver the Lewis base, thiolate, and acyl groups. To simplify
the analysis of the synthetic studies, we chose to use a
phosphine in place of the nitrogen heterocycle, since 31P
NMR analysis provides a convenient means to monitor
reactive intermediates. The probe thioester, Ph2PC6H4C(O)
SPh, was efficiently generated by condensation of 2-diphe-
nylphosphinobenzoic acid11 and benzenethiol. Related
phosphine thioesters are known12 but have been only lightly
studied.
One of the great surprises in bioinorganic chemistry was the
discovery of the iron carbonyl sites in the [NiFe]- and the
[FeFe]-hydrogenases.1 A third hydrogenase, Hmd (methylene-
tetrahydromethanopterin dehydrogenase), formerly thought to
be “metal-free,” was recently shown to also contain an iron
carbonyl center.2 Because it features only a single Fe center at
its active site, this enzyme is also called the [Fe]-hydrogenase.
Hmd represents possibly the last hydrogenase to be discovered:
it is strongly expressed only under special conditions (Ni defi-
ciency) and only by methanogenic archaea. The fact that three
genetically independent hydrogenases evolved similar catalytic
motifs underscores the versatility of the Fe-S-CO compounds
for reactions involving hydrogen.
Synthetic modeling of the active sites of the hydrogenases,
in parallel with biophysical studies, provides mechanistic
insights. Good progress is being made in the biomimicry of
the active sites of the [NiFe]- and [FeFe]-hydrogenases.3,4
The active site of Hmd consists of a Fe(CO)2 center bound to
a thiolate and the pyridine-like nitrogen of the guanylylpyr-
idinol (GP) cofactor. This ensemble, the Fe-GP cofactor, is
extractable from the protein via transthiolation using mer-
captoethanol. Because it is bound to the protein via this
*To whom correspondence should be addressed. E-mail: rauchfuz@
illinois.edu.
(1) Nicolet, Y.; Lemon, B. J.; Fontecilla-Camps, J. C.; Peters, J. W.
Trends Biochem. Sci. 2000, 25, 138–143.
(2) Shima, S.; Pilak, O.; Vogt, S.; Schick, M.; Stagni, M. S.; Meyer-
Klaucke, W.; Warkentin, E.; Thauer, R. K.; Ermler, U. Science 2008,
321, 572–575.
(6) Hiromoto, T.; Ataka, K.; Pilak, O.; Vogt, S.; Stagni, M. S.;
Meyer-Klaucke, W.; Warkentin, E.; Thauer, R. K.; Shima, S.; Ermler,
U. FEBS Lett. 2009, 583, 585–590.
(7) Dougherty, W. G.; Rangan, K.; O’Hagan, M. J.; Yap, G. P. A.;
Riordan, C. G. J. Am. Chem. Soc. 2008, 130, 13510–13511. Ragsdale, S.
W. J. Inorg. Biochem. 2007, 101, 1657–1666. Lindahl, P. A.; Graham, D.
E. Met. Ions Life Sci. 2007, 2, 357–415.
(3) Ohki, Y.; Yasumura, K.; Kuge, K.; Tanino, S.; Ando, M.; Li, Z.;
Tatsumi, K. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 7652–7657. Chiou,
T.-W.; Liaw, W.-F. C. R. Chim. 2008, 11, 818–833. Barton, B. E.;
Whaley, C. M.; Rauchfuss, T. B.; Gray, D. L. J. Am. Chem. Soc.
2009, 131, 6942–6943.
(4) Gloaguen, F.; Rauchfuss, T. B. Chem. Soc. Rev. 2009, 38,
100–108. Singleton, M. L.; Bhuvanesh, N.; Reibenspies, J. H.; Darens-
bourg, M. Y. Angew. Chem., Int. Ed. 2008, 47, 9492–9495. 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. 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, 610–614. Ezzaher, S.; Capon, J.-F.;
Gloaguen, F.; Petillon, F. Y.; Schollhammer, P.; Talarmin, J.; Kervarec,
(8) Li, C.-F.; Xiao, W.-J.; Alper, H. J. Org. Chem. 2009, 74, 888–890.
Arisawa, M.; Kubota, T.; Yamaguchi, M. Tetrahedron Lett. 2008, 49,
1975–1978. Liebeskind, L. S.; Srogl, J. J. Am. Chem. Soc. 2000, 122,
11260–11261. Villalobos, J. M.; Srogl, J.; Liebeskind, L. S. J. Am. Chem.
Soc. 2007, 129, 15734–15735. Yang, H.; Li, H.; Wittenberg, R.; Egi, M.;
Huang, W.; Liebeskind, L. S. J. Am. Chem. Soc. 2007, 129, 1132–1140.
Fukuyama, T.; Lin, S.-C.; Li, L. J. Am. Chem. Soc. 1990, 112, 7050–
7051. Wenkert, E.; Chianelli, D. J. Chem. Soc., Chem. Commun. 1991,
627–628.
::
::
(9) Huber, C.; Wachtershauser, G. Science 1998, 281, 670–672.
(10) Shaver, A.; Uhm, H. L.; Singleton, E.; Liles, D. C. Inorg. Chem.
1989, 28, 847–851.
N. Inorg. Chem. 2009, 48, 2–4.
(5) Obrist, B. V.; Chen, D.; Ahrens, A.; Schunemann, V.; Scopelliti,
::
(11) Hoots, J. E.; Rauchfuss, T. B.; Wrobleski, D. A. Inorg. Synth.
1982, 21, 175.
R.; Hu, X. Inorg. Chem. 2009, 48, 3514–3516. Wang, X.; Li, Z.; Zeng, X.;
Luo, Q.; Evans, D. J.; Pickett, C. J.; Liu, X. Chem. Commun. 2008, 3555–
3557. Yang, X.; Hall, M. B. J. Am. Chem. Soc. 2008, 130, 14036–14037.
::
(12) Burger, S.; Therrien, B.; Suss-Fink, G. Inorg. Chim. Acta 2004,
357, 1213–1218.
r
pubs.acs.org/Organometallics
Published on Web 05/28/2009
2009 American Chemical Society

Communication
Organometallics, Vol. 28, No. 13, 2009 3619
Figure 1. Structure of the CO-inhibited active site of Hmd (left)
and a general motif of proposed models (right).
Table 1. Comparison of νCO and νCN Bands for Hmd Derivatives
and Models
samplea
ν )
CO (cm-1
HmdCO
[Ph2PC6H4C(O)]Fe(SPh)(CO)3 (1)
2074, 2025, 2001
2075, 2020, 1981
2090 (νCN), 2020, 1956
2094 (νCN), 2013, 1954
2047 (νCN), 2017, 1956
2050 (νCN), 2012, 1954
HmdCN
[Fe(SPh)(Ph2PC6H4C(O)]2(CN)(CO)2]- (3)
Figure 2. Structure of Fe2(SPh)2[Ph2PC6H4C(O)]2(CO)3 (2).
˚
Hmd13CN
Selected distances (A): Fe1-C1, 1.760(3); Fe1-C2, 1.828(3);
[Fe(SPh)(Ph2PC6H4C(O)]2(13CN)(CO)2]-
Fe1-C3, 1.963(3); Fe1-P1, 2.2115(8); Fe1-S2, 2.3258(8);
Fe1-S1, 2.3581(8); Fe2-C34, 1.748(3); Fe2-C35, 1.937(3);
Fe2-O3, 2.0104(18); Fe2-P2, 2.2094(8); Fe2-S1, 2.3102(8);
Fe2-S2, 2.3972(8).
(313CN)
a Enzyme data from ref 18.
Scheme 1. Oxidative Addition of Thioester-Phosphines to Fe(0)
and Reactions of the Ferrous Acylthiolate
and thiolato acyls (Fe2(SR)(C(O)R0)(CO)6).15 Two acyl-
phosphine ligands are chelating, one on each Fe. One acyl
group is bridging, indicative of the basicity inherent in acyl
oxygen.15,16 In solution, 2 was found to isomerize over the
course of several hours, as indicated by the appearance of new
pairs of singlets in the 31P NMR spectrum. The isomerization
is attributed to the reorientation of the μ-SPh groups, a
well-known phenomenon (eq 2).14,17
We found that Fe2(CO)9, a source of the reactive Fe(CO)4
entity, reacted readily with the phosphine thioester to give a
new derivative 1 (31P NMR: δ 72.5). Compound 1 is assumed
to be the targeted acyliron(II) thiolate [Ph2PC6H4C(O)]Fe-
(SPh)(CO)3 (eq 1).
Treatment of solutions of 2 with 1600 psi of CO (24 h,
25 °C) resulted in its conversion back to 1. The carbonylation
of 2 provided us with highly pure samples of 1, allowing its
more definitive characterization. The IR spectrum of
1 resembles that for HmdCO, the CO-inhibited state of the
active site (Table 1).18
The presence of three νCO bands in the IR spectrum
established the facial geometry for 1, analogous to the case
for CO-inhibited Hmd.18 Exposure of a solution of 1 to an
atmosphere of 13CO resulted in rapid exchange of all sites to
give Fe(SPh)[Ph2PC6H4C(O)](13CO)3. The 31P NMR spec-
trum of this species confirmed the arrangement of the CO
ligands, since three separate 13C-31P couplings are observed,
(J = 58, 21, and 16 Hz). Treatment of Hmd with 13CO results
in stereoselective substitution,19 in contrast to our model.
The differing CO exchange pathways for the protein vs the
Fe2ðCOÞ9 þ Ph2PC6H4CðOÞSPhf
½Ph2PC6H4CðOÞꢀFeðSPhÞðCOÞ3 þ FeðCOÞ5 þ CO ð1Þ
Compound 1 proved to be labile: it partially decarbonylates
upon standing. It is known that Hmd also binds a third CO
ligand only weakly.13 When solutions of 1 were allowed to
decarbonylate under an inert gas, we obtained black, air-stable
crystals of derivative 2. Fresh solutions obtained from these
crystals gave a simple 31P NMR spectrum consistent with a pair
of uncoupled, nonequivalent phosphine ligands. As indicated
crystallographically, 2 has the formula Fe2(SPh)2[Ph2PC6H4C-
(O)]2(CO)3. Although it features ferrous centers, 2 is related
to the well-studied diiron(I) dithiolates (Fe2(SR)2(CO)6)14
(16) Ciganda, R.; Garralda, M. A.; Ibarlucea, L.; Pinilla, E.; Torres,
M. R. Dalton Trans. 2009, 4227–4235.
(17) King, R. B. J. Am. Chem. Soc. 1962, 84, 2460.
(18) Lyon, E. J.; Shima, S.; Boecher, R.; Thauer, R. K.; Grevels, F.-
W.; Bill, E.; Roseboom, W.; Albracht, S. P. J. J. Am. Chem. Soc. 2004,
126, 14239–14248.
(13) Shima, S.; Thauer, R. K. Chem. Record 2007, 7, 37–46.
(14) Maresca, L.; Greggio, F.; Sbrignadello, G.; Bor, G. Inorg. Chim.
Acta 1971, 5, 667–674.
(15) Seyferth, D.; Womack, G. B.; Archer, C. M.; Dewan, J. C.
Organometallics 1989, 8, 430–432.
(19) Shima, S.; Lyon, E. J.; Thauer, R. K.; Mienert, B.; Bill, E. J. Am.
Chem. Soc. 2005, 127, 10430–10435.

3620 Organometallics, Vol. 28, No. 13, 2009
Royer et al.
model may be due to the phosphine vs pyridine or may arise
from the role of the oxygen functionality on the 2-position of
the pyridine ring in the GP cofactor (see Figure 1).
24 Hz, indicating that CN is cis to the phosphine.20 Unlike
1, 3 is stable with respect to loss of CO.
In summary, we have shown that donor-functionalized
thioesters oxidatively add to Fe(0) to afford acyl ferrous
thiolates, which provide models for the inhibited
active site of Hmd. In view of the assembly strategy
described here, functional modeling of this active site
appears highly feasible. More broadly, the work shows
that thioesters are precursors to versatile organometallic
platforms.8-10,21
Analogous to the behavior of the enzyme,18 cyanide
forms a stable adduct from 1, giving [Fe(SPh)(Ph2PC6H4C-
(O)]2(CN)(CO)2]- (3) (Scheme 1 and Figure 2). This yellow-
ish species, which is highly sensitive to air, was initially
characterized by ESI-MS. The 31P NMR spectrum indicated
a single isomer. The IR data for the cyanide again match well
with the data for the cofactor. Using 13CN-, we generated
the corresponding labeled derivative. The close match of
the IR data for 3 and HmdCN is consistent with a low-spin
iron(II) center in Hmd (Table 1). From the 31P NMR
spectrum of 13CN-labeled 3, we obtained J(31P,13C) of
Acknowledgment. This research was supported by the
DoE and PRF. We thank Drs. S. Shima and R. K. Thauer
for advice.
(20) Rocchini, E.; Rigo, P.; Mezzetti, A.; Stephan, T.; Morris, R. H.;
Lough, A. J.; Forde, C. E.; Fong, T. P.; Drouin, S. D. Dalton Trans.
2000, 3591–3602.
(21) Zhang, J.; Wang, H.; Xian, M. J. Am. Chem. Soc. 2009, 131,
3854–3855.
Supporting Information Available: Text, figures, and a CIF
file giving preparative, spectroscopic, and crystallographic
details. This material is available free of charge via the Internet
at http://pubs.acs.org.