Influence of a pendant amine in the second coordination sphere on proton transfer at a dissymmetrically disubstituted diiron system related to the [2Fe]H subsite of [FeFe]H2ase

Article abstract of DOI:10.1021/ic801369u

Studies of the protonation of [Fe₂(CO)₄(Ar ²-PNP)(w-pdt)] (1; PNP = (Ph₂PCH₂) ₂NCH₃) by HBF₄ Et₂0 showed that the nature of the reaction product depends on

Full text of DOI:10.1021/ic801369u

Inorg. Chem. 2009, 48, 2-4
Influence of a Pendant Amine in the Second Coordination Sphere on
Proton Transfer at a Dissymmetrically Disubstituted Diiron System
Related to the [2Fe]H Subsite of [FeFe]H2ase
Salah Ezzaher, Jean-Franc¸ois Capon, Fre´de´ric Gloaguen, Franc¸ois Y. Pe´tillon,
Philippe Schollhammer,* and Jean Talarmin*
UniVersite´ Europe´enne de Bretagne, 12 aVenue JanVier, 35000 Rennes, France, and UniVersite´ de
Brest, CNRS, UMR 6521, “Chimie, Electrochimie Mole´culaires et Chimie Analytique”, ISSTB, CS
93837, 29238 Brest-Cedex 3, France
Nelly Kervarec
SerVice de RMN, UFR Sciences et Techniques, UniVersite´ de Bretagne Occidentale, CS 93837,
29238 Brest-Cedex 3, France
Received July 23, 2008
Studies of the protonation of [Fe2(CO)4(κ2-PNP)(µ-pdt)] (1; PNP
) (Ph2PCH2)2NCH3) by HBF4 ·Et2O showed that the nature of
the reaction product depends on whether the reaction is conducted
in acetone or in dichloromethane. In acetone, an N-protonated
form, 2, is isolated. Tautomerization of 2 in CH2Cl2 gives rise to a
µ-hydride species 3. Variable-temperature NMR experiments have
been performed to clarify the processes involved.
bridge would act as a proton relay in such a model,4 which
has led to the study of more sophisticated azadithiolate (adt)
mimics [Fe2(CO)6-2x(κ2-L2)x(µ-adt)] in order to combine the
effect of a chelating diphosphine with that of a pendant amine
in symmetrical or unsymmetrical substituted diiron
compounds.1b,5 Another powerful strategy for introducing a
pendant base in the second coordination sphere of a metal
center has been successfully developed by DuBois et al.6
by using base-containing diphosphines (PNP
)
t
(Et2PCH2)NR; R ) Me, Bu). We have extended this
approach to Fe2pdt compounds in order to examine the
influence of such a ligand on the proton and electron transfer
at a diiron site. Recently, works on [Fe2(CO)4(κ2-PNP)(µ-
pdt)] species7 and other diiron molecules featuring nitrogen
bases have appeared in the literature.8 More, during the
review process of the present work, a study by Sun and co-
workers concerning the protonation of [Fe2(CO)4(κ2-PNP)(µ-
Recent progress in the chemistry of organometallic diiron
molecules related to the active site of the iron-only hydro-
genases has shown that the use of chelating ligands such as
diphosphine,1 N-heterocyclic carbene,2 and phenanthroline3
favors the formation of terminal hydride species at a single
iron atom of bimetallic propanedithiolate (pdt) complexes
[Fe2(CO)6-2x(κ2-L2)x(µ-pdt)] (x ) 1, 2). Among the mech-
anisms that have been proposed for the uptake/production
of hydrogen by [FeFe]H2ase, one of the most attractive
involves the formation of a key intermediate featuring
hydrido-proton interaction. The amine of an azadithiolate
(4) (a) Nicolet, Y.; de Lacey, A. L.; Verne`de, X.; Fernandez, V. M.;
Hatchikian, C. E.; Fontecilla-Camps, J. C. J. Am. Chem. Soc. 2001,
123, 1596–1601. (b) Nicolet, Y.; Lemon, B. J.; Fontecilla-Camps, J. C.;
Peters, J. W. Trends Biochem. Sci. 2000, 25, 138–143. (c) Nicolet,
Y.; Cavazza, C.; Fontecilla-Camps, J. C. J. Inorg. Biochem. 2002,
91, 1–8.
(5) Ezzaher, S.; Orain, P.-Y.; Capon, J.-F.; Gloaguen, F.; Pe´tillon, F. Y.;
Roisnel, T.; Schollhammer, P.; Talarmin, J. Chem. Commun. 2008,
2547–2549.
(6) (a) Rakowski DuBois, M.; DuBois, D. L. C. R. Chimie 2008, 11, 805–
817. (b) Wilson, A. D.; Shoemaker, R. K.; Miedaner, A.; Muckerman,
J. T.; DuBois, D. L.; Rakowski DuBois, M. Proc. Natl. Acad. Sci.
U.S.A. 2007, 104, 6951–6958. (c) Curtis, C. J.; Miedaner, A.;
Ciancanelli, R.; Ellis, W. W.; Noll, B. C.; Rakowski DuBois, M.;
DuBois, D. L. Inorg. Chem. 2003, 42, 216–227.
(7) (a) Wang, N.; Wang, M.; Liu, T.; Zhang, T.; Darensbourg, M. Y.;
Sun, L. Inorg. Chem. 2008, 47, 6948–6955. (b) Adam, F. I.; Hogarth,
G.; Richards, I.; Sanchez, B. E. Dalton Trans. 2007, 2495–2498.
(8) (a) Xu, F.; Tard, C.; Wang, X.; Ibrahim, S. K.; Hughes, D. L.; Zhong,
W.; Zeng, X.; Luo, Q.; Liu, X.; Pickett, C. J. Chem. Commun. 2008,
606–608. (b) Wang, Z.; Jiang, W.; Liu, J.; Jiang, W.; Wang, Y.;
Åkermark, B.; Sun, L. J. Organomet. Chem. 2008, 693, 2828–2834.
* To whom correspondence should be addressed. E-mail: schollha@
univ-brest.fr (P.S.), jean.talarmin@univ-brest.fr (J.T.)
(1) (a) Ezzaher, S.; Capon, J.-F.; Gloaguen, F.; Pe´tillon, F. Y.; Scholl-
hammer, P.; Talarmin, J.; Pichon, R.; Kervarec, N. Inorg. Chem. 2007,
46, 3426–3428. (b) Barton, B. E.; Rauchfuss, T. B. Inorg. Chem. 2008,
47, 2261–2263. (c) Adam, F. I.; Hogarth, G.; Kabir, S. E.; Richards,
I. C. R. Chimie 2008, 11, 890–905. (d) Ezzaher, S.; Capon, J.-F.;
Gloaguen, F.; Pe´tillon, F. Y.; Schollhammer, P.; Talarmin, J.; Pichon,
R.; Kervarec, N. C. R. Chimie 2008, 11, 906–914.
(2) Morvan, D.; Capon, J.-F.; Gloaguen, F.; Le Goff, A.; Marchivie, M.;
Michaud, F.; Schollhammer, P.; Talarmin, J.; Yaouanc, J. J.; Kervarec,
N.; Pichon, R. Organometallics 2007, 26, 2042–2052.
(3) Orain, P.-Y.; Capon, J.-F.; Kervarec, N.; Gloaguen, F.; Pe´tillon, F. Y.;
Pichon, R.; Schollhammer, P.; Talarmin, J. Dalton Trans. 2007, 3754–
3755.
2 Inorganic Chemistry, Vol. 48, No. 1, 2009
10.1021/ic801369u CCC: $40.75  2009 American Chemical Society
Published on Web 12/01/2008

COMMUNICATION
Scheme 1a
a
Phenyl groups are omitted for clarity.
pdt)] (PNP ) (Ph2CH2)2NnPr) has appeared as a communi-
cation.9 Herein, we report our preliminay results on the
protonation of [Fe2(CO)4(κ2-PNP)(µ-pdt)] (1; PNP )
(Ph2CH2)2NMe), which show clearly the influence of a
pendant base lying in the outer coordination sphere of a
dissymmetrically disubstituted diiron core on proton transfer.
Our data give new and complementary insights into the
functioning of such molecules.
Figure 1. ORTEP of the cation 2·2CH3COCH3 (the counterion BF4- was
omitted for clarity). Selected bond lengths (Å) and angles (deg): Fe1-P2,
2.1812(10); Fe1-P1, 2.2101(9); Fe1-Fe2, 2.5687(7); C8-N50, 1.510(4);
C8-P1, 1.867(3); C9-N50, 1.500(4); C9-P2, 1.852(3); P2-Fe1-P1,
94.05(4); C9-N50-C51, 109.4(2); C9-N50-C8, 113.1(2); C51-N50-C8,
109.2(2).
Treatment of [Fe2(CO)6(µ-pdt)] with (Ph2PCH2)2NMe,6c
following reported procedures,1a afforded moderate yields
(55%) of 1. Expected IR and 1H NMR data were observed.10
The comparable electron-releasing properties of the PNP and
dppe ligands are shown by the similar potentials for the
reversible one-electron oxidation of 1 and of [Fe2(CO)4(κ2-
dppe)(µ-pdt)] in a CH2Cl2 electrolyte [E1/2 ) -0.21 and
-0.25 V (vs Fc+/Fc), respectively]. The 31P{1H} NMR
spectrum in CD2Cl2 displayed a singlet at 53.2 ppm that is
assigned on the basis of variable-temperature (VT) 31P{1H}
NMR experiments to a species having a basal-apical
arrangement of the diphosphine.10,11 Indeed, the 31P{1H}
NMR of 1 at 183 K evidenced the presence of an AB signal
(JPP ) 29 Hz). A coalescence process was noted at 213 K,
for which ∆G* of 41 kJ mol-1 was calculated.12 An X-ray
crystal structure of 1 showed a basal-apical coordination
of the diphosphine.10 The FeP2C2N metallacycle adopts a
chair conformation, with the methyl substituent of N1 in an
equatorial position. The Fe-Fe distance [2.564(4)Å] is
comparable to that found in similar complexes1-3,7 and the
P1-Fe1-P2 angle [93.38(18)°] is very close to that mea-
sured for an analogue where the diphosphine is
Ph2P(CH2)3PPh2.7b
by the addition of Et2O to the acetone solution. The shift of
the carbonyl bands by ca. 10 cm-1 is consistent with
protonation occurring at a ligand rather than at the Fe-Fe
site (Scheme 1).13 The 31P{1H} NMR in (CD3)2CO showed
a singlet at 60.0 ppm. Neither 1H nor 2D (1H-15N or 1H-1H)
NMR experiments allowed the detection of the signal of the
added proton. 2 is involved in a fluxional process that
exchanges the basal and apical positions of the phosphorus
atoms of the ligand.10
The X-ray crystal structure of 2 · 2CH3COCH3 confirms
that protonation occurred at the nitrogen atom of the
diphosphine (Figure 1).10 The diphosphine ligand occupies
basal-apical coordination sites, and the FeP2C2N metalla-
cycle retains a chair conformation. The Fe-Fe distance,
2.5687(7) Å, is quite the same as that in 1, which is consistent
with protonation at a remote site. It should be noted that
there is an interaction between the proton on the nitrogen
atom of the PNP ligand and the oxygen atom (O70) of one
of the acetone molecules (N50-O70 ) 2.815 Å).
Our investigation on the protonation of 1 by HBF4 · Et2O
revealed that the nature of the reaction product depends on
whether the reaction is conducted in acetone or in dichlo-
romethane. IR monitoring of the addition of 1 equiv of
HBF4 · Et2O to a solution of 1 in acetone showed the
immediate replacement of the ν(CO) bands of the starting
material by three new bands at higher energy [ν(CO) )
2028(s), 1957(s), and 1907(w) cm-1]. The resulting product
[Fe2(CO)4(κ2-PN(H)P)(µ-pdt)]+ (2) was isolated as a powder
Upon recording IR spectra of the isolated powder of 2 in
CH2Cl2, 2 underwent a transformation in this solvent. After
about 1 h, the CO bands of 2 were replaced by a new set of
strong bands at higher energy [ν(CO) ) 2099, 2052, 2036,
and 1961 cm-1], which suggested the formation of a hydride
species.13 This was confirmed by H NMR of a sample
1
prepared by solubilization of an isolated sample of 2 in
CD2Cl2. The 1H NMR spectrum showing a new ill-resolved
triplet at -13.1 ppm evidenced that the N-protonated
complex 2 was totally converted into a new product
[Fe2(CO)4(κ2-PNP)(µ-pdt)(µ-H)]+ (3) possessing a bridging
hydride and where the PNP ligand likely adopts a dibasal
coordination position (Scheme 2). This observation led us
to investigate the protonation of 1 in CH2Cl2 at RT. The
(9) Wang, N.; Wang, M.; Zhang, T.; Li, P.; Liu, J.; Sun, C. Chem.
Commun. 2008, 5800–5802.
(10) Full details of the synthesis and characterization including crystal data
of compounds 1-3 and supplementary figures (NMR and electro-
chemistry) are provided as Supporting Information.
(11) (a) Ezzaher, S.; Capon, J.-F.; Gloaguen, F.; Pe´tillon, F. Y.; Scholl-
hammer, P.; Talarmin, J. Inorg. Chem. 2007, 46, 9863–9872. (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.
(12) Schollhammer, P.; Pe´tillon, F. Y.; Poder-Guillou, S.; Talarmin, J.;
Muir, K. W.; Yufit, D. S. J. Organomet. Chem. 1995, 513, 181–192.
(13) Das, P.; Capon, J.-F.; Gloaguen, F.; Pe´tillon, F. Y.; Schollhammer,
P.; Talarmin, J.; Muir, K. W. Inorg. Chem. 2004, 43, 8203–8205.
Inorganic Chemistry, Vol. 48, No. 1, 2009 3

COMMUNICATION
Scheme 2a
a
Phenyl groups are omitted for clarity.
Figure 2. CV of 1 in (a) CH2Cl2[NBu4][PF6] and (b) (CH3)2CO[NBu4][PF6]
before (black line) and after (red line) the addition of 1 equiv of HBF4 ·Et2O
(vitreous carbon electrode, V ) 0.2 V s-1, potentials in V vs Fc+/Fc).
Figure 3. Protonation of 1 with 3 equiv of HBF4 ·Et2O in CD2Cl2 at 188
reaction of 1 with 1 equiv of HBF4 · Et2O in CH2Cl2,
monitored by IR, allows to isolate 3.
K and a VT 31P{1H} NMR study of the reaction.
The cyclic voltammetry (CV) of 1 was also examined in
(CH3)2CO and CH2Cl2 at room temperature in the presence
of HBF4.Et2O. Upon the addition of 1 equiv of HBF4 · Et2O
to a CH2Cl2[NBu4][PF6] solution of 1, new reductions are
observed at -1.26, -1.75, and -2.11 V, while the oxidation
is removed (Figure 2a), showing that 1 is quantitatively
protonated. The quasi-reversible system with E1/2red ) -1.26
V is due to the one-electron reduction of 3, as shown by a
comparison with the CV of an authentic sample of this
species under similar conditions.10 It should be noted that
the reduction of 3 also occurs at a potential very similar to
that of the hydrogen-bridged dppe analogue (E1/2red ) -1.27
V in CH2Cl2),14 which confirms that the presence of the
amine in the backbone of the chelating ligand does not affect
the redox potentials in a significant way.
The addition of HBF4 · Et2O to a (CH3)2CO[NBu4][PF6]
solution of 1 also gives rise to changes in the CV (Figure
2b). However, no peak is observed at a potential suggesting
the formation of a significant amount of 3. Instead, both the
irreversible reduction detected around -1.9 V and the quasi-
reversible one-electron oxidation at E1/2 ) -0.05 V are also
present in the CV of a fully characterized sample of 2 under
similar conditions.10 It can therefore be concluded that
treatment of 1 with 1 equiv of HBF4 · Et2O in acetone
afforded the N-protonated species 2. The fact that the stable
protonated form of 1 depends on the solvent was confirmed
by the following CV experiment. When a sample of the
this temperature gives the N-protonated form 2 as the major
product of the reaction (∼84%). A minor product (∼16%)
displays a singlet at 48.0 ppm that correlates with a triplet
at -14.4 ppm in the 1H NMR spectrum (2D 1H-31P HMBC
NMR). This indicates that the minor product possesses a
bridging hydride. The spectra obtained by warming the NMR
sample show both the fluxional process of 2 and its
consumption. A new singlet at 46.5 ppm, correlating with a
1
triplet at -14.0 ppm in the H NMR spectrum, appears at
243 K. The observation of two µ-hydride species in the
presence of an excess of acid in our experiments suggests
the formation of doubly protonated species featuring endo
and exo orientation of the NH bond.9
Finally, our preliminary results suggest that the amine
group of the diphosphine in 1 may act as a proton relay and
direct the transfer of protons. CV experiments also showed
that the formation of the [Fe2(CO)4(κ2-LL)(µ-pdt)(µ-H)]+
complex is much faster when LL ) PNP than when LL )
dppe.10 This is not likely to arise from a different basicity
of the Fe-Fe site in the complexes because the electron-
releasing properties of both LL ligands are similar. Therefore,
the higher protonation rate of 1 as compared to that of the
dppe analogue suggests a fast protonation at the nitrogen
atom, followed by a rapid N f Fe proton migration. Further
experiments are in progress in order to clarify the proton
exchange processes in such systems.
isolated N-protonated complex
2 was dissolved in
Acknowledgment. The authors thank the CNRS, the ANR
“CatH2′”, and the University of Brest for financial support.
S.E. was funded by MRES.
CH2Cl2[NBu4][PF6], CV revealed a fast conversion (within
the time of dissolution) into the µ-hydride isomer 3.10
VT 31P{1H} NMR experiments were performed in CD2Cl2.
The spectrum obtained at 188 K from a CD2Cl2 solution of
1 and HBF4 · Et2O (3 equiv) prepared at this temperature
revealed without any ambiguity that the protonation of 1 at
Supporting Information Available: Synthetic, spectroscopic,
crystallographic, and analytical data for 1-3, CV and NMR figures,
and a CIF file. This material is available free of charge via the
Internet at http://pubs.acs.org.
(14) Ezzaher, S.; Capon, J.-F.; Dumontet, N.; Gloaguen, F.; Pe´tillon, F. Y.;
Schollhammer, P.; Talarmin, J. Submitted for publication.
IC801369U
4 Inorganic Chemistry, Vol. 48, No. 1, 2009