Evidence for the formation of terminal hydrides by protonation of an asymmetric iron hydrogenase active site mimic

Article abstract of DOI:10.1021/ic0703124

Treatment of [Fe₂(μ-pdt)(CO)₆] [pdt = S(CH ₂)₃S] with dppe (Ph₂PCH₂CH ₂PPh₂) in refluxing toluene affords the asymmetric complex [Fe₂(μ-pdt)(CO)₄

Full text of DOI:10.1021/ic0703124

Inorg. Chem. 2007, 46, 3426−3428
Evidence for the Formation of Terminal Hydrides by Protonation of an
Asymmetric Iron Hydrogenase Active Site Mimic
Salah Ezzaher, Jean-Franc
Philippe Schollhammer,* and Jean Talarmin
¸ois Capon, Fre´de´ric Gloaguen, Franc¸ois Y. Pe´tillon,
Chimie, Electrochimie Mole´culaires et Chimie Analytique, Faculte´ des Sciences,
UMR CNRS 6521, UniVersite´ de Bretagne Occidentale, 6 AVenue Le Gorgeu, CS 93837,
29238 Brest Cedex 3, France
Roger Pichon and Nelly Kervarec
SerVice de RMN, UFR Sciences et Techniques, UniVersite´ de Bretagne Occidentale, CS 93837,
29238 Brest Cedex 3, France
Received February 16, 2007
Treatment of [Fe2(
(Ph2PCH2CH2PPh2) in refluxing toluene affords the asymmetric
complex [Fe2( -pdt)(CO)4(dppe)] (1). Protonation of 1 with HBF4
Et2O in CH2Cl2 gives at room temperature the -hydrido derivative
[Fe2( -pdt)(CO)4(dppe)( -H)](BF4) (2). Monitoring the reaction by
1H, 31P, and 13C NMR at low temperature reveals unambiguously
that the process of the protonation of 1 implies terminal hydride
intermediates.
µ
-pdt)(CO)6] [pdt
)
S(CH2)3S] with dppe
that must drive the formation of H2 at the natural site or at
those of new electrocatalysts based on the diiron core.
Terminal hydride is proposed to be a major key intermediate
in these processes, but until now the first and sole precedent
of a terminal hydride had been obtained indirectly by the
addition of LiAlH4 to a bimetallic iron(II) complex.3 It has
been reported that the protonation of pdt (S(CH2)3S) or adt
(SCH2NRCH2S) species gives µ-hydride or N-protonated
derivatives.4 Recently, theoretical studies have pointed out
the interest in using asymmetric diiron systems as one of
the key structural features of the natural site.5 The use of
chelating bidendate ligands such as diphosphine is a way to
obtain such an asymmetry and to influence the basicity of
the site.6 The work reported here concerns new aspects of
the protonation of asymmetric dithiolatodiiron systems. The
appearance during the course of our writing of several
“ASAP” papers devoted to asymmetric diiron compounds7
prompted us to report our preliminary results on the study
µ
−
µ
µ
µ
Synthetic organometallic diiron molecules incorporating
some of the structural key features of the active site of the
iron-only hydrogenases are intensively developed with the
aim that a better understanding of the stereoelectronic control
exerted by the active site on the catalytic processes should
allow one to elaborate more efficient electrocatalysts. Some
satisfactory, rudimentary or sophisticated, structural models
with a combination of dithiolate or azadithiolate bridges and
various sets of terminal ligands, involving CO, CN-, RNC,
or PR3, have been synthesized, but few efficient catalysts of
proton reduction have been obtained.1 In this context, the
question of the activation of protons is far from being trivial
because it is the elementary step for a future efficient
electrocatalyst.2 The knowledge of the fate of the protons at
such a diiron site is primordial to understanding the processes
(3) van der Vlugt, J. I.; Rauchfuss, T. B.; Whaley, C. M.; Wilson, S. R.
J. Am. Chem. Soc. 2005, 127, 16012-16013.
(4) (a) Arabi, M. S.; Mathieu, R.; Poilblanc, R. J. Organomet. Chem. 1979,
177, 199-209. (b) Li, H.; Rauchfuss, T. B. J. Am. Chem. Soc. 2001,
124, 726-727. (c) Gloaguen, F.; Lawrence, J. D.; Rauchfuss, T. B.;
Benard, M.; Rohmer, M.-M. Inorg. Chem. 2002, 41, 6573-6582. (d)
Nehring, J. L.; Heinekey, D. M. Inorg. Chem. 2003, 42, 4288-4292.
(e) Wang, F.; Wang, M.; Liu, X.; Jin, K.; Dong, W.; Li, G.; Akermark,
B.; Sun, L. Chem. Commun. 2005, 3221-3223. (f) Dong, W.; Wang,
M.; Liu, X.; Jin, K.; Li, G.; Wang, F.; Sun, L. Chem. Commun. 2006,
305-307.
* To whom correspondence should be addressed. E-mail: schollha@
univ-brest.fr.
(5) Tye, J. W.; Darensbourg, M. Y.; Hall, M. B. Inorg. Chem. 2006, 45,
119-126.
(6) Dowa, J. R.; Zanotti, V.; Facchin, G.; Angelici, R. J. J. Am. Chem.
Soc. 1992, 114, 160-165.
(7) (a) Hogarth, G.; Richards, I. Inorg. Chem. Commun. 2007, 10, 66-
70. (b) Justice, A. K.; Zampella, G.; De Gioia, L.; Rauchfuss, T. B.;
van der Vlugt, J. I.; Wilson, S. R. Inorg. Chem. 2007, 46, 1655-
1664. (c) Duan, L.; Wang, M.; Li, P.; Na, Y.; Wang, N.; Sun, L. Dalton
Trans. 2007, DOI: 10.1039/b616645h. (d) Gao, W.; Ekstro¨m, J.; Liu,
J.; Chen, C.; Eriksson, L.; Weng, L.; Akermark, B.; Sun, L. Inorg.
Chem. 2007, 46, 1981-1991.
(1) (a) Rauchfuss, T. B. Inorg. Chem. 2004, 43, 14-26. (b) Georgakaki,
I. P.; Thomson, L. M.; Lyon, E. J.; Hall, M. B.; Darensbourg, M. Y.
Coord. Chem. ReV. 2003, 238-239, 255-266. (c) Evans, D. J.; Pickett,
C. J. Chem. Soc. ReV. 2003, 32, 268-275. (d) King, R. B.; Bitterwolf,
T. E. Coord. Chem. ReV. 2000, 206-207, 563-579. (e) Capon, J.-F.;
Gloaguen, F.; Schollhammer, P.; Talarmin, J. Coord. Chem. ReV. 2005,
249, 1664-1676.
(2) (a) Tye, J. W.; Darensbourg, M. Y.; Hall, M. B. THEOCHEM 2006,
45, 123-128. (b) Greco, C.; Zampella, G.; Bertini, L.; Bruschi, M.;
Fantucci, P.; De Gioia, L. Inorg. Chem. 2007, 46, 108-116.
3426 Inorganic Chemistry, Vol. 46, No. 9, 2007
10.1021/ic0703124 CCC: $37.00
© 2007 American Chemical Society
Published on Web 03/31/2007

COMMUNICATION
Scheme 1
of protonation of [Fe2(µ-pdt)(CO)4(dppe)] (1; dppe ) Ph2-
PCH2CH2PPh2), which shows clearly for the first time the
involvement of terminal hydride intermediates in this reac-
tion.
Figure 1. Structure of 1 showing 50% thermal ellipsoids. Selected
distances (Å) and angle (deg): Fe1-Fe2, 2.547(7); Fe1-P1, 2.231(1); Fe1-
P2, 2.190(1); P1-Fe1-P2, 87.7(4).
Treatment of [Fe2(µ-pdt)(CO)6] with dppe in refluxing
toluene for 5 h in the presence of Me3NO afforded the
asymmetric complex 1, which was obtained in moderate
yields (Scheme 1) after a subsequent purification workup.8
X-ray analysis of a single crystal of 1 obtained from a
diethyl ether solution establishes without any ambiguity the
asymmetric geometry of 1 and the binding mode of the
bidendate diphosphine to one iron atom in a basal-apical
position [Fe1-P1 2.231(1) Å, Fe1-P2 2.190(1) Å, and P1-
Fe1-P2 87.7(4)°; Figure 1].9 Distances and angles are
unexceptional. 1 is structurally closely analogous to other
known diiron(I) hexacarbonyl-pdt complexes.4,7 Two {Fe-
(CO)3} and {Fe(CO)P2} units are eclipsed and linked by a
direct Fe-Fe bond and a pdt group. The resulting coordina-
tion geometry around each iron atom is described as a
distorted square pyramid supplemented by the FeI-FeI single
bond required by the normal electron counting rule.
Compound 1 was also characterized by elemental analyses
and IR and NMR spectroscopies. Strong bands at 2019 and
1949 cm-1 are observed in the carbonyl region of the IR
spectrum of a CH2Cl2 solution of 1. Expected shifts of the
ν(CO) absorptions to lower frequencies relative to those of
the precursor [Fe2(µ-pdt)(CO)6] have also been pointed out.10
At 298 K, solutions of 1 contain two isomeric species with
relative abundances depending on the solvent. The 31P{1H}
NMR spectrum of 1 in CD2Cl2 shows two singlets at 89.6
and 75.0 ppm with relative 5:1 intensities. In toluene-d8, this
ratio is 2.5:1 and the two isomers do not interconvert readily
Scheme 2
these experiments can be explained by the presence of four
isomers of 1 in solution: two isomers (1ba-ap and 1′ba-ap
)
having the dppe group in a basal-apical (major) position
and two isomers (1ba-ba and 1′ba-ba) having this ligand in a
basal-basal (minor) position, as depicted in Scheme 2.
The 31P{1H} NMR spectrum at 190 K in toluene-d8
displays two different patterns of resonances for each type
of isomer. One of these patterns consists of two unresolved
AB signals of equal intensity, which are related to those of
two inequivalent phosphorus atoms; these are consistent with
boat and chair forms of the iron-dithiocyclohexane rings
of basal-apical isomers. The other pattern contains two
singlets of different intensity (2:1), in agreement with the
corresponding boat and chair forms of the basal-basal
isomers, having a symmetrically bonded dppe ligand (see
Figure a in the Supporting Information).
Protonation of 1 with HBF4‚Et2O in CH2Cl2 at 298 K was
carried out; it gave a red solution of the µ-hydride derivative
[Fe2(µ-pdt)(CO)4(dppe)(µ-H)](BF4) (2ba-ba). Compound 2ba-ba
was precipitated from the solution by the addition of diethyl
ether and recovered as a red powder (Scheme 3).8
The carbonyl absorptions in the IR spectrum of 2ba-ba are
logically shifted to higher energy [2097 (s), 2050 (s), 2036
(s), and 1971 (s) cm-1] relative to those of 1. The 1H NMR
spectrum of 2 in CD2Cl2 confirms that protonation occurs
at the Fe-Fe site with a characteristic high-field signal for
a Fe2(µ-H) group that appears as a triplet at -14.1 ppm with
1
between 293 and 335 K. The H NMR spectrum of 1
confirms the 31P NMR one; it displays two sets of resonances
expected for two isomeric forms of the µ-pdt and dppe
groups. 31P{1H} NMR studies at low temperature reveal that
fluxional processes,11 related to the dppe group and to the
µ-pdt, are operative in solution (Scheme 2). The results of
(8) Full details of the synthesis and characterization of compounds 1 and
2 are provided as Supporting Information.
(9) Crystal data for 1: C33H30Fe2O4P2S2‚CH2Cl2, fw ) 813.26, T ) 170
K, monoclinic, space group P21/c, a ) 10.7135(5) Å, b ) 10.7968(5)
Å, c ) 30.9134(15) Å, â ) 100.229(4)°, V ) 3519.0(3) Å3, Z ) 4.
Refinement of 415 parameters gave R1 ) 0.0474, wR2 ) 0.0896,
and |∆F| < 0.621 e‚Å-3 for all 7171 unique reflections. Crystal data
for 2ba-ba: C33H31BF4Fe2O4P2S2‚0.5CH2Cl2, fw ) 858.61, T ) 170
K, monoclinic, space group P21, a ) 11.9437(4) Å, b ) 9.6134(3) Å,
c ) 31.1947(10) Å, â ) 95.608(3)°, V ) 3564.6(2) Å3, Z ) 4.
Refinement of 497 parameters gave R1 ) 0.0386, wR2 ) 0.0881,
and |∆F| < 0.482 e‚Å-3 for all 7334 unique reflections. Further
crystallographic data for 1 and 2ba-ba are provided as Supporting
Information.
(10) Seyferth, D.; Womack, G. B.; Galagher, M. K.; Cowie, M.; Hames,
B. W.; Fackler, J. P.; Mazany, A. M. Organometallics 1987, 6, 283-
294.
(11) Zhao, X.; Georgakaki, I. P.; Miller, M. L.; Mejia-Rodriguez, R.;
Chiang, C.-Y.; Darensbourg, M. Y. Inorg. Chem. 2002, 41, 3917-
3928.
Inorganic Chemistry, Vol. 46, No. 9, 2007 3427

COMMUNICATION
Scheme 3
Scheme 4
Figure 2. View of the cation [Fe2(µ-pdt)(CO)4(dppe)(µ-H)]+ (2ba-ba) (50%
ellipsoids). Selected distance (Å) and angle (deg): Fe1-Fe2, 2.581(5); Fe1-
H, 1.627(3); Fe2-H, 1.640(4); Fe1-P1, 2.234(1); Fe1-P2, 2.238(1); P1-
Fe1-P2, 87.2(3).
a coupling constant of 21.0 Hz, consistent with the sym-
metrical and cis position of the two phosphorus atoms relative
to the µ-hydride in a basal-basal isomer. The 31P{1H} NMR
spectrum displays a signal at 78.7 ppm. An X-ray analysis
of 2ba-ba (Figure 2) confirms that protonation of 1 induces
the slippage of the chelated dppe to a basal-basal coordina-
tion in the protonated form.9
Protonation experiments at low temperature shed a par-
ticularly important insight into the process of formation of
2. Treatment of 1 (5 mg) with an excess (2-3 equiv) of
[(HOEt2)BF4] in a CD2Cl2 solution at 203 K resulted in the
very slow apparition of a signal at -4.33 ppm corresponding
to the formation of a terminal hydride at the distal iron atom
(3; which does not bind the dppe group; Figure 3).
Figure 3. Protonation of 1 at 203 K and a variable-temperature study of
the reaction.
increased by 298 K (overnight). When the protonation was
performed at 223 K in addition to the formation of the three
previous species (3, 2ba-ap, and 2ba-ba), an additionnal
intermediate 4 was detected (Figure c in the Supporting
Information). The identification of 4 with a terminal hydride
species resulting from the protonation of 1 at the proximal
iron atom, the one which supports the dppe ligand in a
basal-basal position, is strongly suggested by NMR data.
Indeed, a triplet is observed at -2.47 ppm with a coupling
1
constant JPH of 69 Hz in the H NMR spectrum of the
reaction mixture. The 31P{1H] NMR spectrum reveals an
additional singlet at 83.3 ppm.
The clean transformation of 1 into the intermediate 3
(Scheme 4) was complete within 5 h; this can be monitored
by 31P NMR by the disappearance of signals of 1 and the
appearance of a singlet at 60.1 ppm. An increase of the
temperature to 243 K induced the formation of µ-hydride
In summary, we show in this work the first NMR evidence
for the formation of terminal hydride intermediates by
protonation of an asymmetric model of the natural site of
iron-only hydrogenase. An IR study of these processes should
allow the determination in species 3 and 4 of whether the
CO group opposite the dithiolate bridging group is bridging
or terminally bound. Further experiments on protonation of
other asymmetric diiron complexes are in progress.
species with basal-basal (2ba-ba) and basal-apical (2ba-ap
)
1
diphosphine forms, which were detected in the H NMR
spectrum, at this temperature, as a triplet at -14.23 ppm
(JPH ) 21 Hz) and a doublet at -14.50 ppm (JPH ) 26 Hz).
The 31P NMR spectrum displayed a singlet at 77.7 ppm and
an AB pattern at 87.1 ppm (d, JPP ) 24.3 Hz) and 81.5 ppm
(d, JPP ) 24.3 Hz) (Figure b in the Supporting Information).
The two-dimensional heteronuclear multiple-bond correlation
1H-31P NMR spectrum shows a correlation between the
hydride observed as a triplet and the 31P singlet and between
the doublet at -14.50 ppm and the 31P AB system. This
mixture was stable at 243 K, and the complete transformation
of 2ba-ap into 2ba-ba was observed when the temperature
Acknowledgment. The authors thank the CNRS, the
ANR “PhotoBioH2”, and Universite´ de Bretagne Occidentale
for financial support.
Supporting Information Available: Synthetic, spectroscopic,
crystallographic, and analytical data for 1 and 2, NMR spectra, and
CIF files. This material is available free of charge via the Internet
at http://pubs.acs.org.
IC0703124
3428 Inorganic Chemistry, Vol. 46, No. 9, 2007