Diiron species containing a cyclic PPh2N Ph2 diphosphine related to the [FeFe]H2ases active site

Article abstract of DOI:10.1039/c0cc03295f

A new dissymmetrically disubstituted diiron dithiolate species, [Fe ₂(CO)₄(κ²-P^Ph ₂N^Ph₂)(μ-pdt)] (pdt = S(CH₂) ₃S), was prepared by using a flexible cyclic

Full text of DOI:10.1039/c0cc03295f

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Diiron species containing a cyclic PPh NPh diphosphine related to the
2
2
[FeFe]H2ases active sitewz
a
a
b
Sondes Lounissi,b Jean-Franc
´ ´
¸ ois Capon, Frederic Gloaguen, Fatma Matoussi,
a
a
a
´
Franc¸ ois Y. Petillon, Philippe Schollhammer* and Jean Talarmin*
Received 16th August 2010, Accepted 18th October 2010
DOI: 10.1039/c0cc03295f
A new dissymmetrically disubstituted diiron dithiolate species,
[Fe2(CO)4(j2-PPh2NPh2)(l-pdt)] (pdt = S(CH2)3S), was prepared
by using a flexible cyclic base-containing diphosphine, 1,3,5,7-
tetraphenyl 1,5-diaza-3,7-diphosphacyclooctane (PPh2NPh
=
2
{PhPCH2NPh}2). Preliminary investigations of proton and
electron transfers on the diiron system have been done.
Despite the efforts of synthetic chemists to design diiron
molecules with some of the key features of the H cluster, the
organometallic active site of the [FeFe]-hydrogenase enzymes,1,2
the activity (high turnover and low overpotential) of this
natural system for building or breaking H2 has still not been
equalized by homogenous bioinspired [FeFe]-electrocatalysts.3
The role of a proton relay in the second sphere of coordination
of the active site is proposed to be essential in the catalytic
functioning of the H cluster. Such a proton relay has been
modeled by using azadithiolate bridges or base-containing
phosphine and diphosphine ligands.3,4 The introduction of a
pendant base by using functionalized diphosphine has been
widely developed by DuBois and Rakowski DuBois.5 Their
studies demonstrated great interest in using flexible base-
containing diphosphine of the type PNP ((R2PCH2)2NR0) or
Scheme 1 Synthesis of 1 and 2.
chromatography on a silica gel column (Scheme 1).y The
complex [Fe2(CO)4(k2-PPh2NPh2)(m-pdt)] (1) and two side-
products, 2 and 3, were obtained in ca. 49%, 6% and 2%
yields, respectively. They were characterised by 31P{1H} NMR
and IR spectroscopies, elemental analyses and by single crystal
X-ray analyses. The structure of 1 was confirmed by an
X-ray analysisz9 of a single crystal obtained from hexane–
dichloromethane solution (Fig. 1). This indicated a basal–apical
bidentate coordination of the diphosphine to one iron atom,
thereby forming boat and chair FePCNCP rings. 31P{1H} NMR
studies at low temperature indicated that typical fluxional
processes (Fig. Sa, ESIz), related to the cyclic diphosphine and
to the dithiolate bridge, are operative in solution.10
PR2NR0 ({RPCH2NR0}2) that can be tuned by the choice
2
of their substituents. Recently, we and others extended
this approach to propanedithiolate (pdt) diiron compounds
[Fe2(CO)4(k2-PNP)(m-pdt)] in order to examine the influence
of such a ligand on the proton and electron transfers at a
diiron site.6,7 Pursuing this approach, we have now combined
diiron systems with a cyclic diphosphine PPh2NPh2. We
report preliminary results concerning the formation of new
dissymmetrically disubstituted diiron and triiron dithiolate
species featuring cyclic base-containing diphosphine. Proto-
nation and electrochemical behaviours of the bimetallic
species have been investigated.
X-Ray analysis9 of a single crystal of a side product 3
revealed a typical structure of a monosubstituted dithiolate
diiron complex [Fe2(CO)5L(m-pdt)] with a monodentate cyclic
The reaction of the hexacarbonyl precursor [Fe2(CO)6(m-pdt)]
with PPh2NPh ({PhPCH2NPh}2),8 in refluxing toluene,
2
afforded a mixture of products which were separated by
a
´
UMR CNRS 6521, Universite de Bretagne Occidentale, C.S. 93837,
29238 Brest cedex 3, France. E-mail: schollha@univ-brest.fr,
jean.talarmin@univ-brest.fr; Fax: +33 0298017001
b
´
Departement de Chimie, Faculte des Sciences de Tunis,
Campus Universitaire, 2092, Tunis, Tunisia
´
w This article is a part of a ChemComm ‘‘Hydrogen’’ web-based
themed issue.
z Electronic supplementary information (ESI) available: Experimental
details, NMR and ORTEP figures, acid-dependence of the reduction
current for 1. CDCC 789266 (1), 789267 (3). For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/
c0cc03295f
Fig.
1 Ortep view (ellipsoids at 30% of probability level) of
[Fe2(CO)4(k2-PPh2NPh2)(m-pdt)] (1). Selected bond lengths (A) and
angles (1): Fe(1)–Fe(2), 2.5655(9); Fe(1)–P(1), 2.1862(13); Fe(1)–P(2),
2.1833(14); P(1)–Fe(1)–P(2), 82.41(5).
c
878 Chem. Commun., 2011, 47, 878–880
This journal is The Royal Society of Chemistry 2011

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diphosphine sulfide (L) (Fig. Sb, ESIz). The formation of this
product could arise from partial decomposition of dithiolate
diiron compounds.11
low temperature, of an intermediate species for which
1H NMR spectra do not display any hydride signal, which is
transformed upon warming into a doubly protonated species at
both diiron and nitrogen atoms (Fig. Se, ESIz). By comparison
with the protonation studies of the [Fe2(CO)4(k2-PNPMe)(m-pdt)]
analogue (PNPMe = {Ph2PCH2}2NMe),6 the above result
suggests that the initial protonation at an amine function of
the P2N2 ligand is followed by a N–H - Fe(m-H)Fe migration.
Upon addition of 1 equiv. of trifluoromethanesulfonic
(CF3SO3H, pKa = 2.6 in MeCN)17 or methanesulfonic acid
(CH3SO3H, pKa = 8.36;17 10.0 in MeCN18), the redox systems
Unfortunately, until now, only very small single crystals of 2
were obtained. For this reason, 2 could not be identified
without any ambiguity. A very poor X-ray analysis suggested
that the molecular structure of 2, proposed in Scheme 1, is a
trinuclear iron cluster [Fe3(CO)4(k2-PPh2NPh2)(m-CO)(m-pdt)2]
that is structurally analogous to [Fe3(CO)6(m-CO)(m-edt)2]
(edt = ethane dithiolate).12 Electron-counting rules suggest
a diamagnetic structure with two Fe–Fe bonds and a formal
Fe(I)Fe(II)Fe(I) unit bridged by two dithiolate groups. A
diphosphine ligand is chelated at one iron center and a semi-
bridging carbonyl group links two iron atoms. It is worth
noting that few examples of oligomeric Fe—SR–CO species
have been reported.13–15 Such a structure is also reminiscent of
mixed valent Fe(I)–Fe(II) systems, with a pyramidal inversion
of one iron atom, that have been recently reported in the
literature.16 The structure of 2 can also be structurally viewed
as a mixed valent Fe(I)–Fe(II) assembly linked to a {Fe(CO)3}
group through a metal–metal bond and the sulfur atoms of a
dithiolate bridge. The overall structure of 2 accords with its
elemental analysis and spectroscopic data.z In particular, the
31P{1H} NMR spectrum of 2 presents a doublet of doublets
with a coupling constant of 80 Hz (Fig. Sc, ESIz) which
accords with a bidentate diphosphine in basal–apical position.
The reactivity of [Fe2(CO)4(k2-PPh2NPh2)(m-pdt)] (1)
with protons was investigated in order to compare its activity
with that of the [Fe2(CO)4(k2-PNPMe)(m-pdt)] analogue
(PNPMe = {Ph2PCH2}2NMe).6
red
ox
of the initial complex (Ep = ꢀ2.35 V, E1/2 = ꢀ0.10 V;
potentials referred to the Fc/Fc+ couple) were replaced
by a reduction at ꢀ1.34 V, assigned to the bridging hydride
derivative by comparison with the reduction potential of an
authentic sample of this compound (see in ESIz). Further
additions of CH3SO3H (Fig. 2a) resulted in an increase of the
reduction current around ꢀ1.4 V, which suggests that catalytic
proton reduction occurred. In contrast, the addition of the
stronger CF3SO3H acid that first produced the bridging
hydride derivative led to the formation of the doubly protonated
[Fe2(CO)4(k2-HPPh2N2Ph)(m-pdt)(m-H)]2+, characterised by a
reduction at ꢀ1.2 V (Fig. 2b). However, the increase of the
The reactions of [Fe2(CO)4(k2-PPh2N2Ph)(m-pdt)] (1) with
acid were monitored by cyclic voltammetry (CV) in
1
CH2Cl2–[NBu4][PF6] and by H and 31P{1H} NMR spectro-
1
scopy at room temperature. The H NMR spectrum of 1, in
CD2Cl2 at 298 K, in the presence of 1 equiv. of CH3SO3H or
CF3SO3H shows a doublet at ꢀ13.5 ppm (JPH = 17.5 Hz) and
a triplet at ꢀ13.6 ppm (JPH = 20.0 Hz) with relative 3 : 1
intensities which are characteristic of the formation of
m-hydride species with basal–apical and dibasal diphosphine.10
The 31P{1H} NMR spectrum of this solution exhibits expected
signals: a singlet at 51.8 ppm and two doublets of equal
intensities at 64.5 ppm (d, JPP = 97 Hz) and 47.8 ppm
(d, JPP = 97 Hz).10 Two-dimensional heteronuclear multiple-
bond 1H–31P NMR experiments confirmed assignments of
these signals.
The transformation of 1 into a doubly protonated species, in
the presence of an excess of sufficiently strong acid, was
suggested by NMR experiments (Fig. Sd, ESIz). Indeed, the
treatment of 1 with two equivalents of CF3SO3H in a CD2Cl2
solution resulted in the formation of two other bridging
hydride species with dibasal and basal–apical diphosphine.
The 31P{1H} NMR spectrum in CD2Cl2 revealed a singlet at
29.4 ppm and two doublets at 37.0 ppm (d, JPP = 91 Hz) and
26.6 ppm (d, JPP = 91 Hz). Two typical signals, a triplet
at ꢀ15.0 ppm (JPH = 21.0 Hz) and a doublet at ꢀ13.9 ppm
(JPH = 16.5 Hz), with relative 1 : 2 intensities were detected in
Fig.
2
Cyclic voltammetry of [Fe2(CO)4(k2-PPh2NPh2)(m-pdt)]
1
the H NMR spectrum.
VT 1H and 31P{1H} NMR experiments in the presence of an
1.86 mM (a) in the absence and in the presence of CH3SO3H and
(b) in the absence and in the presence of CF3SO3H (CH2Cl2–[NBu4][PF6];
vitreous carbon electrode; v = 0.2 V sꢀ1; potentials are in V vs. Fc+/Fc).
excess of CF3SO3H or HBF4ꢁOEt2 indicated the formation, at
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 878–880 879

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reduction current around ꢀ1.2 V upon addition of acid is very
limited, which shows that essentially no proton reduction cata-
lysis occurred on the CV time scale at this potential. It should be
noted that as far as reduction of CH3SO3H is concerned, the
efficiency of 1 is not much different from that of the analogue
[Fe2(CO)4(k2-PNPPh)(m-pdt)] (see in ESIz, Fig. Sg).
2 Y. Nicolet, C. Piras, P. Legrand, C. E. Hatchikian and
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´
Comparison of the activities of PNP and P2N2 complexes
require now further full electrochemical and theoretical studies.
Further studies are now in progress in order to compare and to
understand the effect of various cyclic diphosphines PR2NR
2
on the proton and electron transfers at a dithiolate-bridged
diiron site as well as to obtain novel linear triiron clusters.
The authors thank the CNRS, the ANR ‘CatH2’, and
UBO for financial support. The ministere de l’Enseignement
Superieur, de la Recherche Scientifique et de la Technologie of
´
Tunisia is acknowledged for funding S. L. We are grateful to
Dr F. Michaud for the crystallographic measurements of 1 and
3 and to N. Kervarec for recording VT 1H and 31P{1H} NMR
experiments and two-dimensional NMR spectra on a Bruker
DRX 500 spectrometer.
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Notes and references
y Reaction of [Fe2(CO)6(m-pdt)] with PPh2NPh2: in a 100 mL round-
bottom flask, 0.6 g (1.55 ꢂ 10ꢀ3 mol) of [Fe2(CO)6(m-pdt)] were
reacted with PPh2NPh (0.633 g, 1.4 ꢂ 10ꢀ3 mol) in the presence of
2
2 equiv. (0.344 g, 3.1 ꢂ 10ꢀ3 mol) of Me3NO, 2H2O. The mixture was
refluxed in toluene for 18 h, after which time it was evaporated to
dryness under vacuum. The solid residue was chromatographed
on a silica gel column. Complexes 1–3 were eluted with hexane–
dichloromethane and dichloromethane–diethyl ether mixtures. They
were obtained as powders after evaporation of the solvent. Data for 1:
yield 0.537 g, 49%; anal. calcd for C35H34Fe2N2O4P2S2: C, 53.59; H,
4.37; N, 3.57%; found: C, 53.65; H, 4.68; N, 3.31%; IR (CH2Cl2,
cmꢀ1): nCO 2021(s), 1945(s), 1898(w); 31P{1H} NMR (CDCl3, 25 1C):
9
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d
55.2(br). Data for 2: yield 0.082 g, 6%; anal. calcd for
C39H40Fe3N2O5P2S4: C, 48.07; H, 4.14; N, 2.87%; found: C, 47.77;
H, 4.23; N, 2.77%; IR (CH2Cl2, cmꢀ1): nCO 2020(s), 1949(s), 1820(w);
31P{1H} NMR (CDCl3, 25 1C): d 56.4 (d, 80.0 Hz), 40.4(d, 80.0 Hz).
Data for 3: yield 0.024 g, 2%; anal. calcd for C36H34Fe2N2O5P2S3ꢁ
O(CH2CH3)2 C, 52.30; H, 4.83; N, 3.05%; found: C, 52.45; H,
4.56; N, 3.45%; IR (CH2Cl2, cmꢀ1): nCO 2042(s), 1983(vs), 1961(sh),
1923(w); 31P{1H} NMR (CD2Cl2, 25 1C): d 55.7 (d, 13.0 Hz), 32.8
(d, 13.0 Hz).
z Crystal data for 1: C71H70Cl2Fe4N4O8P4S4. M 1653.73, triclinic,
Pꢀ1, a = 10.6550(7), b = 12.7551(9), c = 14.3432(10) A, a =
103.409(6)1, b = 99.559(6)1, g = 99.746(6)1, Z = 1, V = 1825.5(2)
A3; rcal = 1.504 g cmꢀ3; m(Mo-Ka) = 1.149 mmꢀ1; l = 0.71073 A,
T = 170(2) K. 14161 reflections measured, 7431 unique (Rint = 0.0634)
used in refinement. R1 [7431 with I > 2s(I)] = 0.0659, wR2 (all data) =
0.1392. CCDC 789266.
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c
880 Chem. Commun., 2011, 47, 878–880
This journal is The Royal Society of Chemistry 2011