Lewis vs. Bronsted-basicities of diiron dithiolates: Spectroscopic detection of the "rotated structure" and remarkable effects of ethane- vs. propanedithiolate

Article abstract of DOI:10.1039/b700754j

The new complexes Fe₂(S₂C_nH _2n)(CO)₂(dppv)₂ (n = 2, 3; dppv = cis-1,2-C₂H₂(PPh₂)₂) form adducts with AlBr₃ and B(C₆F_5<

Full text of DOI:10.1039/b700754j

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Lewis vs. Brønsted-basicities of diiron dithiolates: spectroscopic
detection of the ‘‘rotated structure’’ and remarkable effects of ethane-
vs. propanedithiolate{
Aaron K. Justice,*a Giuseppe Zampella,b Luca De Gioia*b and Thomas B. Rauchfuss*a
Received (in Berkeley, CA, USA) 17th January 2007, Accepted 2nd March 2007
First published as an Advance Article on the web 11th April 2007
DOI: 10.1039/b700754j
this uncertainty, we turned to the use of Lewis acids to generate the
rotated structure.
The new complexes Fe2(S2CnH2n)(CO)2(dppv)2 (n = 2, 3; dppv =
cis-1,2-C2H2(PPh2)2) form adducts with AlBr3 and B(C6F5)3,
which adopt the ‘‘rotated structure’’ proposed for the active site
of the Fe-only hydrogenases—the propanedithiolate is signifi-
cantly more Lewis basic due to nonbonded interactions
between the dithiolate strap and the ligands on Fe.
Initial studies showed that bis- and tris(phosphine) complexes
Fe2(S2C2H4)(CO)4(PMe3)2 and Fe2(S2C2H4)(CO)3(dppv)(PMe3)7
do not form adducts with the strong8 Lewis acid AlBr3.
Apparently in such species, the CO sites are insufficiently basic
to cleave Al2Br6. The electron-rich dianion [Fe2(S2C2H4)
(CN)2(CO)4]22 does of course bind Lewis acids, but the nitrogen
centres on cyanide are the dominant basic sites,9 which precludes
interactions with CO. In order to conduct our experiments, we
required a complex substituted with several donor ligands, the
exteriors of which are not basic. We thus undertook the
preparation of the complexes Fe2(S2CnH2n)(CO)2(dppv)2 (eqn (1)).
Hard Lewis acids are known to bind bridging CO ligands in di-
and polynuclear metal carbonyl complexes.1 For example, the
affinity of AlEt3 for m-CO ligands is sufficiently strong that this
reagent converts [CpRu(CO)2]2 into Cp2Ru2(CO)2(m-COAlEt3)2.2
In this report we describe an unusual application of Lewis acids to
a difficult problem posed in the context of bioorganometallic
chemistry.3
Fe2(S2CnH2n)(CO)4(dppv) + dppv A
Fe2(S2CnH2n)(CO)2(dppv)2 + 2 CO
The active site of Fe-only hydrogenase enzymes can be described
as [Fe2(SR)2(m-CO)(CO)2L3]z, wherein the three diatomic ligands
on the distal iron are ‘‘rotated’’ by ca. 60u, thereby opening a
coordination site trans to the Fe–Fe bond (Scheme 1).4 This vacant
site is implicated in binding H2. Theoretical calculations indicate
that such rotated structures are only ca. 40 kJ mol21 higher in
energy than the conventional C2v isomer.5 Synthetic modeling
efforts have, however, failed to reproduce such rotated structures,
despite the preparation of hundreds of compounds of the type
Fe2(SR)2(CO)62nLn (L = CN2, PR3, SR2, CNR).6 In view of the
intensity of the experimental work, it would be reasonable to
question the plausibility of the rotated structures. To help resolve
(1)
1{ (n = 2); 2 (n = 3)
These deep green species, which are unique examples of
Fe2(SR)2(CO)2(PR3)4 derivatives, arise via the photochemical
reaction of dppv and Fe2(S2CnH2n)(CO)4(dppv).7 The IR spectra
for 1 and 2 are indistinguishable in the nCO region. The positions of
the bands (nCO = 1888, 1868 cm21) indicate that these complexes
are more electron-rich than Fe2(S2C2H4)(CO)3(dppv)(PMe3)
(nCO = 1943, 1892 cm21).7
The structure of 1 was established crystallographically (Fig. 1).
The species is noteworthy because of the presence of four donor
ligands on the diiron(I) center. The Fe2(S2CnH2n)(CO)62xLx
framework is similar to less substituted derivatives with respect
to Fe–Fe and Fe-ligand distances. Variable temperature 31P NMR
spectra confirm that 1 is fluxional in solution: one signal is
observed at room temperature (d 93.2) and an AB quartet at low
temperatures (d 95.8, JP–P = 21 Hz and 92.2, JP–P = 22 Hz). The
data are consistent with the degenerate interconversion of the
enantiomeric C2-symmetric isomers. Compound 2 is similarly
fluxional in solution, but the low temperature spectrum also
revealed, in addition to the C2-symmetric isomer, the presence of
20% of a C1-isomer wherein one dppv is axial/basal and the other
is dibasal (Scheme 2). The appearance of this second isomer is
ascribed to a destabilizing interaction between the central CH2
group of the propanedithiolate and one phenyl group of one dppv
ligand (see below).
Scheme 1
aDepartment of Chemistry, University of Illinois, Urbana, IL, USA.
E-mail: rauchfuz@uiuc.edu
bDepartment of Biotechnology and Biosciences, University of Milano-
Bicocca, Piazza della Scienza 1, 20126, Milan, Italy.
E-mail: luca.degioia@unimib.it
Addition of Al2Br6 to a CH2Cl2 solution of 1 at 220 uC induced
strong changes in both the IR and 31P NMR spectra. In particular,
the 31P NMR spectrum for 1?AlBr3 sharpened to four signals.
Following recently described empirical trends,7 signals at d 91.1
{ Electronic supplementary information (ESI) available: Details about
DFT calculations, atomic coordinates of starting and optimized structures.
See DOI: 10.1039/b700754j
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Chem. Commun., 2007, 2019–2021 | 2019

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solution containing two equiv. of 2 and half equiv. of Al2Br6
showed separate signals in the 31P NMR spectrum for unreacted 2
and 2?AlBr3. Thus, only one equivalent Lewis acid binds to 1 and
2 and exchange between the bound and free Lewis acid is slow on
the NMR time-scale.
Crucial evidence bearing on the structure of the Lewis acid
adduct was provided by IR spectroscopy. The nCO bands for 1, at
1888, 1868 cm21, shifted to 1960 and 1640 cm21 upon
complexation to AlBr3. The low energy band is characteristic of
the M2(m-COAlBr3) group.1 The 1960 cm21 band for the
Fe(dppv)(CO) center is ca. 80 cm21 higher energy than the
average of the two bands for 1. This ca. 80 cm21 shift is
comparable to that produced by protonation or 2e2 oxidation of a
diiron complex.11
Collectively the IR and 31P NMR data are consistent with the
stabilization of the rotated structure by the Lewis acid, which
‘‘pulls’’ a terminal CO ligand into the bridging position. Also DFT
calculations{ indicate that m-CO isomers are stabilized upon AlBr3
binding (Fig. 2), even though the rotated isomers are not predicted
to be the most stable (Table 1). Analogous results are obtained
with hybrid functionals (not shown), suggesting that the relative
basicity of CO groups in Fe2(S2CnH2n)(dppv)2(CO)2 complexes is
not fully satisfactorily predicted by DFT. The computed CO
stretching frequencies of the m-CO 1?AlBr3 adduct are, however,
much closer to experimental values than the corresponding
frequencies computed for the un-rotated 1?AlBr3 isomer (1930,
1632 and 1942, 1694 cm21, respectively). The possibility that AlBr3
could bind to the axial CO of (axial/basal)(dibasal) isomers was
also analysed by DFT calculations. However, the computed 31P
NMR spectra for these adducts (not shown) do not fit
experimental data.
Fig. 1 Structure of Fe2(S2C2H4)(CO)2(dppv)2, (1), with thermal ellip-
soids set at 35%. Phenyl ellipsoids and phenyl hydrogen atoms have been
˚
omitted for clarity. Selected bond lengths (A), and angles (deg): Fe(1)–
Fe(2), 2.5678(4); Fe(1)–S(1), 2.2611 (5); Fe(1)–S(2), 2.2485 (5); Fe(1)–P(1),
2.1732 (6); Fe(1)–P(2), 2.2026 (6); Fe(1)–C(2), 1.7357 (19); Fe(2)–P(3),
2.1769 (6); Fe(2)–P(4), 2.1958 (5); Fe(2)–C(1), 1.737 (2); Fe(2)–Fe(1)–P(1),
152.906 (19); Fe(2)–Fe(1)–P(2), 111.874 (18); Fe(2)–Fe(1)–C(2), 104.53 (6);
P(1)–Fe(1)–P(2), 87.93 (2); P(1)–Fe(1)–C(2), 92.70 (6); P(2)–Fe(1)–C(2),
91.61 (6).§
and 91.2 ppm are assigned to axial/basal dppv, and those at d 90.6
and 83.0 ppm, are assigned to dibasal dppv. Completely analogous
shifts were observed by 31P NMR spectroscopy for the
propanedithiolate 2?AlBr3. Addition of NEt3, to these solutions
regenerated 1 and 2, demonstrating that binding of the AlBr3 does
not destroy the Fe2(SR)2(CO)2(dppv)2 framework. We analyzed
the stoichiometry of the Lewis acid–base reaction by 19F NMR
spectroscopy using B(C6F5)3 as the Lewis acid.10 A solution
containing two equiv. B(C6F5)3 and one equiv. of 1, showed
separate comparably intense signals for the free (d 2129, 2145,
2162) and complexed (d 2137, 2160, 2166) Lewis acid. The
stoichiometry was also analyzed via 31P NMR spectroscopy. A
Fig. 2 DFT-optimized structure of Fe2(S2C2H4)(m-COAlBr3)(dppv)2(CO).
˚
Selected distances (A): Atoms are color-coded according to the
following scheme: light blue = iron, green = carbon, yellow = sulfur,
purple = phosphorus, red = oxygen, grey = aluminium, dark blue =
d
p
˚
bromine. Selected distances (A): Fe–Fe, 2.625; Fe -m-C, 1.697; Fe -
m-C, 2.604; Fed–S, 2.307, 2.316; Fep–S, 2.301, 2.340; Fep–CO, 1.751;
Fep–P, 2.241, 2.291; Fed–P, 2.213, 2.231.
Scheme 2
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Table 1 Relative stabilities of Fe2(S2CnH2n)(dppv)2(CO)2 and
Fe2(S2CnH2n)(dppv)2(COAlBr3)(CO) isomers
Relative stabilities
(kJ mol21
)
Compound
Fe2(S2C2H4)(dppv)2(m-CO)(CO)
Fe2(S2C2H4)(dppv)2(CO)2
41.4
0.0
Fe2(S2C2H4)(dppv)2(m-COAlBr3)(CO)
Fe2(S2C2H4)(dppv)2(COAlBr3)(CO)
Fe2(S2C3H6)(dppv)2(m-CO)(CO)
Fe2(S2C3H6)(dppv)2(CO)2
32.0
0.0
32.9
0.0
Fe2(S2C3H6)(dppv)2(m-COAlBr3)(CO)
Fe2(S2C3H6)(dppv)2(COAlBr3)(CO)
24.1
0.0
Scheme 3
We found that the propanedithiolate 2 is significantly more
Lewis basic than ethanedithiolate 1. When a solution containing
one equiv. each of 1 and 2 was treated with a 0.5 equiv. Al2Br6, we
observed the exclusive formation of 2?AlBr3. In order to probe the
relative thermodynamic preference of Al2Br6 for 2 vs. 1, we
examined the reaction of ten equiv. of 1, one equiv. 2, and two
equiv. of AlBr3. Even under these biased conditions, 2?AlBr3
formed quantitatively. Assuming that our detection limit is 5%,
this result indicates that the Lewis basicity of 2 is at least 3706
that of 1 at 254 K, corresponding to DDG y 10.9 kJ mol21
(eqn (2); Scheme 3, LA = Lewis acid).
Notes and references
1
{ For 1: H NMR (CD2Cl2): d 8.1–7.2 (m, 40H, 4.3 (s, 2H), 1.2 ppm (s,
4H). 31P NMR (CD2Cl2, 20 uC): d 93.2 ppm. 31P NMR (CD2Cl2, 260 uC):
d 95.8 (JP–P = 21 Hz), 92.2 ppm (JP–P = 22 Hz). IR (CH2Cl2): nCO = 1888,
1868 cm21. FD-MS: m/z = 1052.2 ([Fe2(S2C2H4)(CO)2(dppv)2]). Anal.
calcd for C56H48Fe2O2P4S2 (found): C, 63.87 (63.48); H, 4.60 (4.54).
§ Fe2(S2C2H4)(CO)2(dppv)2, (1), M = 1052.64, monoclinic, a = 10.9417(7),
3
˚
˚
b = 17.3546(10), c = 26.1713(15) A, b = 97.901(2), U = 4922.5(5) A , T =
193(2) K, space group P2(1)/n, Z = 4, m(Mo-Ka) = 0.847. 21096 reflections
were collected, R1 (I . 2s) = 0.034 and R1 = 0.0792 for all data. Rint is not
reported due to non-merohedral twinning. CCDC 634107. For crystal-
lographic data in CIF or other electronic format see DOI: 10.1039/
b700754j
<
:
1 AlBr3z21z2 AlBr3
:
(2)
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IR spectra for 1 and 2 in the nCO region are indistinguishable.
This similarity extends to their apparent Brønsted basicities:
treatment of a 1 : 1 solution of 1 and 2 with one equiv. of
H(OEt2)2BArF resulted in equal amounts of the hydrides 1H+
4
and 2H+.
The lack of a difference in Brønsted basicity in 1 and 2, directly
contrasts with their differing Lewis basicities. DFT analysis of the
reaction Fe2(S2CnH2n)(dppv)2(CO)2 + AlBr3 A Fe2(S2CnH2n)
(dppv)2(m-COAlBr3)(CO) shows that the formation of 2?AlBr3 is
favoured (by about 8 kJ mol21) relative to formation of 1?AlBr3.
This energetic difference arises from the steric clash between the
central methylene of the propanedithiolate and the dppv ligand in
2, an interaction which is partially relieved upon formation of the
rotated isomer. In contrast, nonbonding interactions in 1 and
1?AlBr3 are comparable, thus there is less driving force stabilizing
the rotated structure. This finding highlights an unsuspected
structural role played by alkanedithiolates in bimetallic complexes.
In summary, using a electron-rich diiron(I) dithiolate, we present
a unique case where Lewis acids stabilize a structure for a metal
carbonyl that has not been observed experimentally—except in a
protein. Furthermore, we show how nonbonding ligand–ligand
interactions can influence the Lewis basicity of other ligands.
This work was supported by the National Institutes of Health
and the Petroleum Research Fund.
This journal is ß The Royal Society of Chemistry 2007
Chem. Commun., 2007, 2019–2021 | 2021