Coordination and conformational isomers in mononuclear iron complexes with pertinence to the [FeFe] hydrogenase active site

Article abstract of DOI:10.1039/c3dt53268b

A series of six mononuclear iron complexes of the type [Fe(X-bdt)(P ^R₂N^Ph₂)(CO)] (P^R ₂N^Ph₂ = 1,5-diaza-3,7-diphosphaoctane, bdt = benzenedithiolate with X = H, Cl₂ or Me and R = Ph, Bn, Cyc or tert-Bu) was prepared. This new class of penta-coordinate iron complexes contains a free coordination site and a pendant base as essential structural features of the [FeFe]-hydrogenase active site. The bidentate nature of the P^R₂N^Ph₂ ligands was found to be crucial for the preferential formation of coordinatively unsaturated penta-coordinate complexes, which is supported by first principle calculations. IR-spectroscopic data suggest the presence of coordination isomers around the metal center, as well as multiple possible conformers of the P^R ₂N^Ph₂ ligand. This finding is further corroborated by X-ray crystallographic and computational studies. ³¹P{¹H}-NMR- and IR-spectroscopic as well as electrochemical measurements show that the electronic properties of the complexes are strongly, and independently, influenced by the P-substituents at the P^R₂N^Ph₂ ligand as well as by modifications of the bdt bridge. These results illustrate the advantages of this modular platform, which allows independent and selective tuning through site specific modifications. Potential catalytic intermediates, namely singly reduced and protonated complexes, have been further investigated by spectroscopic methods and exhibit remarkable stability. Finally, their general capacity for electro-catalytic reduction of protons to molecular hydrogen was verified.

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Coordination and conformational isomers in
mononuclear iron complexes with pertinence to
the [FeFe] hydrogenase active site†
Cite this: Dalton Trans., 2014, 43,
4537
Andreas Orthaber,‡a Michael Karnahl,‡§a Stefanie Tschierlei,¶a Daniel Streich,a
Matthias Stein*b and Sascha Ott*a
A series of six mononuclear iron complexes of the type [Fe(X-bdt)(PR2NPh2)(CO)] (PR2NPh = 1,5-diaza-
2
3,7-diphosphaoctane, bdt = benzenedithiolate with X = H, Cl2 or Me and R = Ph, Bn, Cyc or tert-Bu) was
prepared. This new class of penta-coordinate iron complexes contains a free coordination site and a
pendant base as essential structural features of the [FeFe]-hydrogenase active site. The bidentate nature
of the PR2NPh2 ligands was found to be crucial for the preferential formation of coordinatively unsaturated
penta-coordinate complexes, which is supported by first principle calculations. IR-spectroscopic data
suggest the presence of coordination isomers around the metal center, as well as multiple possible con-
formers of the PR2NPh ligand. This finding is further corroborated by X-ray crystallographic and compu-
2
tational studies. 31P{1H}-NMR- and IR-spectroscopic as well as electrochemical measurements show that
the electronic properties of the complexes are strongly, and independently, influenced by the P-substitu-
ents at the PR2NPh2 ligand as well as by modifications of the bdt bridge. These results illustrate the advan-
tages of this modular platform, which allows independent and selective tuning through site specific
modifications. Potential catalytic intermediates, namely singly reduced and protonated complexes, have
been further investigated by spectroscopic methods and exhibit remarkable stability. Finally, their general
capacity for electro-catalytic reduction of protons to molecular hydrogen was verified.
Received 20th November 2013,
Accepted 7th January 2014
DOI: 10.1039/c3dt53268b
www.rsc.org/dalton
Introduction
With a globally increasing energy demand and limited fossil
fuel resources, the development of efficient and sustainable
proton reduction catalysts is at the heart of a future “Hydrogen
Economy”.1 Particular enzymes, so-called hydrogenases
(H2ases),2 catalyze the reversible (inter)conversion of protons
to molecular hydrogen and thus provide inspiration for the
development of new synthetic catalysts.3,4 As H2ases are based
aDepartment of Chemistry, Ångström Laboratories, Uppsala University, Box 523,
75120 Uppsala, Sweden. E-mail: sascha.ott@kemi.uu.se; Fax: +46-18-471 6844;
Tel: +46-18-471 7340
Fig. 1 Schematic structure of the Hox state of [FeFe] H2ase active site
highlighting the free coordination site in the distal iron (Fed) (top left).
Hexa-coordiante (1 and I14) and penta-coordinate (II and III) mono-
nuclear model complexes of Fed.
bMax-Planck-Institute for Dynamics of Complex Technical System, Sandtorstr. 1,
39106 Magdeburg, Germany. E-mail: matthias.stein@mpi-magdeburg.mpg.de;
Fax: +49-391-6110 403; Tel: +49-391-611 0403
†Electronic supplementary information (ESI) available: Crystallographic, NMR
and IR data of the complexes 3a–d, 4a and 5a. UV/vis-SEC and electrochemical
on abundant metals in their active sites, functional synthetic
models may someday become alternatives to expensive noble
spectra of 3a as well as its catalytic activity. Further details concerning the DFT
calculations. CCDC 822967, 972155, and 972156. For ESI and crystallographic metal catalysts.5 Consequently, many attempts have been
data in CIF or other electronic format see DOI: 10.1039/c3dt53268b
made to prepare complexes that mimic the active sites of
‡Contributed equally and should be both considered as first authors.
hydrogenases, in particular those of [FeFe] H2ases (Fig. 1), in
§Present address: Leibniz-Institute for Catalysis at the University of Rostock,
structure and function.6
Albert-Einstein-Straße 29a, 18059 Rostock, Germany.
In the active site of [FeFe] H2ases, two low-valent Fe
centers are interconnected by a bidentate dithiolate bridge,
¶Present address: Institute of Physics, University of Rostock, Universitätsplatz 3,
18055 Rostock, Germany.
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with biologically unusual carbon monoxide and cyanide
ligands occupying most of the coordination sphere around
the metals. The presence of an open coordination site at the
Fed, i.e. the Fe center distal to the [4Fe4S] cluster, allows for
substrate binding. An amine in the second coordination
sphere is thought to shuttle protons to and from the active
site.7
To this end, a broad range of dinuclear complexes as struc-
tural and functional models of the [FeFe] H2ases have been
developed. Despite sometimes very close similarity to the
natural active site, catalysis with high turnover rates and low
overpotential remains elusive. One of the major reasons for
the lack of activity is the stability of hydrides that bridge the
two Fe centers.8,9 The formation of such bridging hydrides is
not observed in the enzyme as ligand rearrangements are pre-
cluded due to hydrogen bonding from the protein matrix to
the cyanide ligands, which is supported by experimental and
theoretical evidence.10–13
As the catalytically important elemental steps in hydrogen
formation are assumed to proceed exclusively at Fed
(Fig. 1),15–17 our interests veered towards developing mono-
nuclear complexes as functional models of Fed and by that
avoiding the potential occurrence of bridging hydrides.10,11,13,18
The desired structural features of these models include a
primary ligand environment with maximum similarity to Fed,
a free coordination site for substrate binding (i.e. protons,
hydrogen or hydride) and an amine in the second coordi-
nation sphere as the potential proton shuttle.19–22
The work presented in this paper summarizes our latest
results in this endeavor, and discusses experimental and
theoretical insights into factors that determine the conditions
under which Fe(II) complexes prefer to be penta- or hexa-
coordinate. A novel series of penta-coordinate Fe complexes
was prepared and their preferred coordination and confor-
mation geometry was investigated by spectroscopic techniques
and first principles DFT calculations. Further spectroscopic
and electrochemical investigations show that the complexes’
reduction potentials can be tuned by two orthogonal strategies
and that the complexes are catalysts for the electrochemical
reduction of protons at a relatively mild overpotential.
Scheme 1 One-step synthesis of hexa-coordinate iron complexes 1
(R = CH3) and 2 (R = C6H5). Reaction conditions: 3 h, 40 °C, 1 atm
CO, Et3N, CH3CN.
The trimethylphosphine (PMe3) analogue [(bdt)Fe(PMe3)2-
(CO)2] 1 was presented a few years later,24 and exhibits an equi-
valent coordination geometry as I. Complex 1 however features
stronger Fe–CO bonds and the penta-coordinate version
cannot be obtained by purging with argon. Penta-coordinate
Fe complexes with phosphine ligands could however be
afforded by tethering the phosphines into a bidentate ligand.
The resulting penta-coordinate (bdt)Fe(PNP)(CO) (III, PNP =
Ph2PCH2NRCH2PPh2,
R = CH3, CH2CF3) features both
P-ligands in the base of the square pyramidal geometry,
similar to the situation in II. Penta-coordinate II and III can
only be obtained when the sulfides are part of an aromatic
dithiolate (bdt) ligand, pointing towards a non-innocence of
this ligand.25,26
Considering the appeal of penta-coordinate iron complexes
as catalysts for proton/hydrogen conversion, our first objective
was to identify factors that determine their coordination
number and conformational freedom. Particular focus was on
whether or how the different donor strength and steric require-
ments of the PMe3 ligands in 1 compared to the PPh2 units
in the PNP ligand of III affect these properties. For this
purpose, [(bdt)Fe(PPh3)2(CO)2] 2, i.e. the PPh3 analogue of 1,
was prepared (Scheme 1).
Complex 2 is a hexa-coordinate species with an IR spectrum
that is almost identical to that of 1, thus suggesting that the
different electronic and spatial properties of PMe3 compared
to PPhn (n = 2, 3) do not determine whether a complex is
penta- or hexa-coordinate. Calculated symmetrical and anti-
symmetrical CO-stretching frequencies are given in Table 1.
The calculated IR parameters agree well with the measured
ones and indicate that the PMe3 vs. PPh3 ligand does not
significantly affect the CvO bond strengths and vibrations.
Yu et al. have shown that BP86/TZVP generally gives Fe–CO
stretching frequencies in better agreement with experiment
than B3LYP/TZVP calculations.27 This is also corroborated by
our results. The consideration of solvent effects by COSMO
(Conductor-Like-Screening Model)28 calculations does not lead
to an improvement of IR frequencies (see ESI†). When apply-
ing the scaling factor by Yu et al., IR frequencies of 1954 and
2001 cm−1 for 1 and 1941 and 1994 cm−1 for 2 were obtained
which agree well with the experimental data.
Results and discussion
More than a decade ago, Rauchfuss and co-workers reported
on a mononuclear Fe complex [(bdt)Fe(CN)2(CO)2]2− I (Fig. 1)
that contains two sulfide ligands, two cyanides and two CO
ligands.14 Interestingly, it was found that one CO ligand binds
very weakly and can be removed simply by purging with argon
to afford the penta-coordinate [(bdt)Fe(CN)2(CO)]2− (II). In
terms of coordination isomers, octahedrally coordinated I fea-
tures the cyanides trans to each other, while square pyramidal
II has the CO in the apical position (Fig. 1).14 Similarly,
Darensbourg et al. reported weak and reversible binding of a
third CO ligand to (2-amidothio-phenylate)Fe(CO)2P(Cy3)
giving a hexa-coordinate complex.23
In order to investigate the prevalence of hexa-coordinate
complexes 1 and 2, we determined the CO-binding free ener-
gies of the second CO ligand of complexes 1 and 2 by removing
one of the equatorial CO ligands to give putative penta-
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Table 1 Calculated and experimental CO stretching frequencies in solution, CO binding affinities for complexes 1 and 2 in acetonitrile and relative
free energy differences for pentacoordinate complexes
CO binding free
energyc relative to
Rel. free energyc
Cpd.
ν/cm−1 COasym
ν/cm−1 COsym
sp
tbp
sp vs. tbp
1
BP86
B3LYP
Exp.a
BP86
B3LYP
Exp.a
1920
1982
−39
−27
—
−27
−12
—
−14
−6
—
−5
+1
+25
+21
—
+22
+13
—
2019 (1954)b
1958
2071 (2001)b
2014
2
1911
1973
2020 (1941)b
1969
2074 (1994)b
2014
—
sp = square pyramidal; tbp = trigonal bipyramidal. a In DCM.18 b Scaled gas phase B3LYP/TZVP frequencies.27 c kcal mol−1
.
coordinate low-spin Fe(II) complexes 1–CO and 2–CO. As for all
five-coordinate 3d metal complexes, trigonal-bipyramidal (tbp)
and square-pyramidal (sp) coordination modes or hybrids
thereof can be expected. In general, the principal factors that
determine the configuration are electrostatic and steric repul-
sion between ligands, the nature of the metal–ligand bond
and the crystal field stabilization energy (omitting crystal
packing effects). With regard to ligand–ligand repulsion, it can
generally be expected that the trigonal bipyramid is the more
stable regular structure.
Formal removal of one equatorial CO ligand from hexa-
coordinate complexes 1 and 2 gives square-pyramidal 1–spCO and
–co
2
with almost unchanged P–Fe–P angles, for example, of
sp
177° in 1–spCO compared to 171° in 1. Calculated 1,2s–pCO feature a
bdt-S in the apical position which is different from the geome-
try of PNP complexes III which have a CO ligand in the axial
site instead. The square-pyramidal coordination modes in Fig. 2 Conceivable coordination isomers that arise from the formal
–CO
removal of one CO ligand from experimentally observed hexa-coordi-
1,2
are characterized as local minima on the potential
sp
nate complexes
bipyramidal.
1
and 2. sp
=
square pyramidal; tbp
=
trigonal
energy surface. There are, however, global energy minima
which stem from a distortion of the PMe3 and PPh3 ligands
that leads to changes in S–Fe–P angles from ∼90° to larger
angles. The result is a geometry that is best described as a
distorted trigonal-bipyramidal coordination in which the
trigonal plane is defined by the two P and one S (see Fig. 2,
bottom). The structural re-orientation is also characterized
–CO
sp
While coordination of the second CO ligand to 1,2
is
characterized by strong negative CO binding free energies
(see Table 1), exogenous CO binding to the ligand-distorted
–CO
tb
by a large change in the P–Fe–P angle from 163° in hexa-coor-
1,2
is very weak if not almost energetically unfavorable
–CO
tbp
dinate 2 to 102° in penta-coordinate 2
.
(data in the ESI†).
–CO
tbp
1,2
are both energetically and entropically favored
From the above considerations on monodentate phosphine
ligands, it is clear that tethering of the P-ligands into bidentate
motifs as in PNP ligands in III is essential to gain access to
penta-coordinate structures, especially if they are to display a
square-pyramidal coordination mode with the associated
vacant coordination site that is necessary for substrate
binding. A further potentially beneficial design feature for cata-
lysis is the inclusion of an amine in the second coordination
sphere. It has previously been shown that the capacity of the N
in the second coordination sphere to shuttle protons to a
metal site can be increased by introduction of a second six-
membered ring. In analogy to what has been observed for
related Ni(PR2NR′2)2 complexes, steric congestion in the result-
ing 1,5-diaza-3,7-diphosphaoctane ligands (PR2NR′2) promotes
a conformation of the complex in which one nitrogen base
–CO
(see ESI†) compared to their 1,2
analogues. For example,
sp
–CO
sp
1,2
is higher in Gibbs energy by 28 kcal mol−1 (BP86+D3/
TZVP) and 20 kcal mol−1 (B3LYP+D3/TZVP) compared to their
respective trigonal-bipyramidal analogue. The consideration of
solvent effects (acetonitrile) does not qualitatively change the
picture (21 kcal mol−1 BP86+D3/TZVP and 13 kcal mol−1
B3LYP+D3/TZVP). Whereas the consideration of the organic
solvent increases the difference in free energies by 5–6 kcal
mol−1 for 1–CO, it diminishes the free energy differences for
2–CO by 7 kcal mol−1. This is consistent with the increase in
solvent accessible surface for PPh3 vs. PMe3 ligands and the
decreased solubility of PPh3 in acetonitrile.
The different coordination modes of 1,2–CO also have a
large effect on the energetics of binding a sixth CO ligand.
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Fig. 3 Modular design of mononuclear pentacoordinate iron com-
plexes containing a PR2NPh ligand and its three different possible sites
(i)–(iii) of independent modifications.
2
Fig. 4 ORTEP representation of ligand PBn2NPh (thermal ellipsoids at a
2
probability level of 50%). Hydrogen atoms are omitted for clarity. The
solid state structure shows the conformational flexibility of this ligand
allowing to adjust the P⋯P distance according to the geometric require-
ments (see also structures of 3a and 3c). Symmetry related atoms are
created by a two-fold crystallographic symmetry (2 − x, 2 − y, z).
Selected distances and angles: P1–C1 1.8567(17), P1–C9 1.8655(16), P1–
C8 1.8686(16), P1⋯P1’ 4.0903(5), C1–P1–C9 97.43(8), C1–P1–C8
95.58(8), C9–P1–C8 99.11(8).
resides above the metal center.22,29 We thus decided to intro-
duce P2N2 ligands also to our penta-coordinate Fe complexes
and targeted structures of the general type 3, depicted in
Fig. 3.
A great advantage of this structural platform is that it offers
multiple possibilities to fine-tune the complexes’ electronic
properties (Fig. 3). Traditional and structurally more closely
related di-nuclear models of the [FeFe] H2ase active site are
more limited in this sense. In detail, these variations include
(i) the benzenedithiolate (bdt) bridge, (ii) the phosphine
ligand(s) and (iii) the substituent at the amine(s). The latter
predominantly changes the basicity of the amine site (i.e. as a
potential proton shuttle), while alterations of (i) and (ii) affect
the metal’s reduction potential. With this general architecture,
we aimed to investigate the potential of orthogonal tuning by
variation of sites (i) to (ii).
The motif could in principle also feature a free coordination
site for substrate binding, as well as a CO ligand that is a
useful IR probe to detect changes of the metal-oxidation state,
the influence of the modifications on the metal’s electronic
content, and potentially even conformational changes of the
6-membered FePCNCP heterocycles.
Fig. 5 Overview of the mononuclear penta-coordinate Fe(II)-com-
plexes, with its different substitutions at the bisphosphine ligand (3a R =
Ph; 3b: R = benzyl; 3c: R = cyclo-hexyl; 3d: R = tert-Bu; left) and at the
dithiolate bridge (4a: X = Cl, Y = H; 5a: X = H, Y = CH3; right side).
In this study, four different PR2NPh ligands were prepared
2
cyclic bisphosphine ligand (Scheme 2). The PR2NPh ligands
2
that carry identical N-phenyl substituent, but vary in the P-sub-
stituent (for a: R = Ph (phenyl), b: R = Bn (benzyl), c: R = Cyc
could be isolated by crystallization in moderate to good overall
yields. We were able to grow single crystals suitable for X-ray
t
(cyclo-hexyl) and d: R = Bu (tert-butyl)) (Scheme 2). The P2N2
diffraction of ligand PBn2NPh from a chloroform solution
2
ligands were synthesized according to published procedures
by the reaction of a primary phosphine with formaldehyde
(in situ generated from para-formaldehyde), resulting in the
corresponding bis(hydroxymethyl)phosphine, which was used
without further purification.30,31 This intermediate reacts in a
subsequent condensation with aniline, forming the desired
(Fig. 4).
To gain insight into the impact of the P-substituents on
relevant molecular properties, Fe complexes 3a–d were syn-
thesized by a one-pot reaction of Fe(II)Cl2, benzene-1,2-dithio-
late (bdt) and the respective PR2NPh ligand in an acetonitrile
2
solution containing triethylamine under an atmosphere of CO
(see Scheme SI-1 in the ESI†). Chromatographic workup with
deaerated solvents yields all complexes in excellent yields
(≥80%) as dark green solids (Fig. 5, left).
In order to investigate the impact of electronic variations
introduced by different substituents on the dithiolate bridge,
two additional iron complexes 4a and 5a were prepared using
3,6-dichloro-1,2-benzenedithiol (Cl2-bdt) and 4-methyl-1,2-
benzenedithiol (Me-bdt), respectively, together with the parent
Scheme 2 Synthesis of cyclic bisphosphine ligands a–d (PR2NPh2) start-
ing from primary phosphines and aniline: a: R = Ph (phenyl), b: R = Bn
(benzyl), c: R = Cyc (cyclo-hexyl) and d: R = tBu (tert-butyl).
PPh2NPh ligand a (Fig. 5, right, Scheme ESI-1†). While 4a was
2
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only obtained in moderate yields (49%), complex 5a could be
isolated in 85% yield, both as dark green solids. The lower
yield of the Cl2-bdt substituted complex is most likely related
to the electron withdrawing character of Cl2-bdt and is
in agreement with previous observations for dinuclear
[Fe2(dithiolate)(CO)6] analogues.15,24 While 3a–5a with P-
phenyl groups are fairly stable compounds, stability drops sig-
nificantly when changing to alkyl substituents at the phos-
phorus (Fig. 5). This behavior is in line with the expected
stability of aromatic vs. aliphatic phosphines and also explains
the lower yields for the aliphatic substituted complexes 3c and
3d (80 and 82%) compared to the aromatic derivatives 3a and
3b (94 and 96%).32
Table 2 Selected bond lengths (Å) and angles (°) for complexes 3a
(calculated and X-ray) and 3c
Compounds
3acryst (Exp.)
3acryst (Calc.)
3c (Exp.)
Fe1–C1
Fe1–S1/Fe1–S2
1.77
2.21/2.17
1.76
2.25/2.21
1.74/1.77
2.21/2.18
2.20/2.18
2.15/2.15
88.8
90.7
123.9
82.5
81.7
96.9
95.8
Fe1–P1/Fe1–P2
S1–Fe1–S2
S1–Fe1–P1
S2–Fe1–P1
S2–Fe1–C1
P1–Fe1–P2
S1–Fe1–P2
C1–Fe1–P2
2.14/2.17
88.7
93.1
123.9
83.4
80.5
2.16 ap/2.19 bas
88.7
103.3
119.3
83.4
83.0
88.5
96.8
93.1
92.0
31P{1H}-NMR spectroscopic investigations of all mono-
nuclear complexes show a single resonance in the expected
region, which indicates symmetric nature of the complex and/
or fast dynamic behavior in solution, which was also observed
for structurally related compounds.33–35 Electronic differences
of the phosphine ligands are well reflected in their 31P{1H}-
NMR shifts which range from +65 to almost +90 ppm. On the
other hand, variations of the bdt bridge only have a marginal
influence on 31P{1H}-NMR chemical shifts (3a: 68.0, 4a: 68.2,
and 5a: 68.1 ppm, Table 3), supporting the notion that ortho-
gonal tuning of the electronic properties of the central metal
using either strategy (i) or (ii) may be feasible.
ap = apical, bas = basal, Exp. = experimental, Calc. = calculated.
mononuclear iron complexes exhibit a coordination number of
five. To our great surprise, however, the apical position is not
occupied by a CO ligand as in the PNP complexes of type III,
but by one of the phosphorus centers of the P2N2 ligand. The
CO ligand lies in a position trans to one sulfur of the bdt
bridge instead.
The coordination environment also exhibits substantial
deviation from the ideal geometry and can be viewed as either
a distorted trigonal-bipyramidal or square-pyramidal coordi-
nation. A summary of relevant structural parameters from
crystallographic and theoretical investigations is given in
Table 2. The distorted geometry of 3a in the crystal (hereafter
denoted as 3acryst) facilitates a dense molecular packing of four
symmetry-related molecules per unit cell with CH⋯π-inter-
actions between the P-phenyl ring of PPh2NPh2 and an N-phenyl
of a neighboring molecule (see ESI†). In order to describe the
geometry of the complexes we employed Addison’s τ value.36
For complex 3a this value (0.22) supports a highly distorted
square-pyramidal complex.
Single crystals of complexes 3a and 3c suitable for X-ray
crystallographic analysis could be obtained, and their solid
state structure is displayed in Fig. 6. As expected, the
Considering that the ligand orientations in 3acryst are such
that proton shuttling from the N to the metal would not be
feasible, it was investigated whether 3acryst is also the preferred
coordination isomer in solution. In a conformational search, a
second minimum 3a′ was found in addition to the confor-
mation in 3acryst. In 3a′, the two bdt-S coordinate to the Fe-
center in the Fe–P–P plane and the CO ligand occupies an
apical position, resulting in a square-pyramidal coordination
sphere (see ESI†). The coordination in 3a′ thus resembles that
found in the PNP-type complexes III. Both 3acryst and 3a′ are
local minima in solution and in the gas phase and only differ
by rotation of the bdt-ligand. Both GGA (generalized gradient
approximation) and hybrid functionals give a very consistent
picture here. The square-pyramidal configuration 3a′ was
found to be lower in Gibbs energy by 6.6 kcal mol−1 and
6.8 kcal mol−1 for BP86+D3/TZVP, COSMO(acetonitrile) and
B3LYP+D3/TZVP, COSMO(acetonitrile), respectively. However,
calculations with a def2-TZVP basis set at the BP86+D3/
COSMO(acetonitrile) level show smaller energy differences of
max. 3 kcal mol−1 (vide infra).
Fig. 6 Molecular structures of 3a and 3c (top) and detailed view of the
Fe(II) coordination mode of 3a (bottom). Note that compound 3c crystal-
lizes with two independent complex molecules per unit cell and only
one is depicted here. Thermal ellipsoids are drawn at 50% probability
level. Hydrogen atoms and co-crystallized solvent molecules are
omitted for clarity.
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In the square-pyramidal 3a′ orientation, the Fe–CO distance
is 1.75 Å, which leads to a CvO stretching frequency of
2051 cm−1 (B3LYP+D3/TZP) and 2004 cm−1 (B3LYP+D3/TZVP
(acetonitrile)). In the distorted trigonal-bipyramidal coordi-
nation in 3acryst, the Fe–CO bond is longer (1.80 Å), which
leads to a lower CvO stretching frequency (2043 cm−1 in the
gas phase and 1995 cm−1 in the organic solvent). The calcu-
lated difference between the two coordination isomers is thus
about 8 cm−1 which would make it difficult, but not imposs-
ible, to distinguish them by FTIR spectroscopy (see ESI†).
The stabilization of 3a′ over the crystallized form 3acryst was
analyzed in detail. The major contribution to 3a′ being lower
in energy is from a reduction of steric repulsion of the bulky
bidentate ligands. Thermal corrections and entropic effects
favor the distorted trigonal-bipyramidal arrangement as in
3acryst over 3a′ but cannot compensate for the energetic differ-
ences. Both BP86 and B3LYP give a very consistent picture and
the effect of solvent consideration amounts to an extra stabili-
zation by 1.5–2 kcal mol−1 of 3a′.
Of the two isomers only 3a′, as opposed to 3acryst, contains
an open coordination site to afford a hexa-coordinate octa-
hedral complex. In 3acryst this coordination site is blocked by
the distorted trigonal-bipyramidally arranged bulky bidentate
–CO
ligand. Whereas the square-pyramidal complexes 1
and
sq
–CO
2
easily accommodate CO (see above), their trigonal bipyra-
sq
midal isomers are only weak CO-binders. This is in agreement
with the results obtained for 3a′.
Fig. 7 Top: normalized IR spectra of 3a (line), 3b (dashed), 3c (dotted)
and 3d (dash-dotted) in acetonitrile in the region of the carbonyl
stretching vibrations. Bottom: IR spectra of 3a in the absence of acid
(line), with 1 eq. (dashed), 2 eq. (dotted), and 5 eq. (dash-dotted) of triflic
acid and deprotonated with 5 eq. triethylamine (dash long dotted).
Acetonitrile at ambient temperatures.
3a′ is able to accommodate an additional CO ligand and
yield a hexa-coordinate octahedral complex. The calculated CO
ligand binding affinity for 3a′, however, shows that coordi-
nation of this additional sixth axial ligand, i.e. the second CO
ligand, to 3a′ is very weak, if at all, and may be thermodynami-
cally unfavorable by 0.5 (BP86) to 4.8 kcal mol−1 (B3LYP).
3acryst is not able to afford an additional CO ligand. It converts
to the structure of 3a′ with an extra CO.
Table 3 Summary of selected spectroscopic data for the mononuclear
complexes 3a–d, 4a and 5a
In the crystal structure of 3c we found two crystallographi-
cally independent entities in the unit cell. Interestingly, their
geometries differ significantly which further indicates the
possible co-occurrence of several isomers. While the structure
containing Fe1 features a τ-value of 0.25 and is similar to the
crystal structure of 3a (τ = 0.22), the second entity (containing
Fe2) is better described as a hybrid of a trigonal bipyramidal
and a square-pyramidal geometry (τ = 0.53).
In general the stretching vibrations of CO ligands are excel-
lent probes for changes in electron density of their coordinat-
ing metal centers. To test the hypothesis of orthogonal tuning,
the extent to which the different substituents at the P-centers
affect the electron density at the Fe center was investigated by
IR
31P-NMR
Echem
νCO, ACN [cm−1
]
δP [ppm]
νCO, ACN [cm−1
]
protonateda
Ered,1/2, ACN [V]
3a
3b
3c
3d
4a
5a
68.0
64.9
78.2
89.7
68.2
68.1
1939, 1919(sh)
1936
1931
1932
1946, 1925(sh)
1938, 1917(sh)
1956
1956
1943
1943b
n.d.c
1989
−1.65
−1.70
−1.78
−1.82
−1.57
−1.72
sh = shoulder, redox data vs. the Fc/Fc+ couple. a Protonated with
trifluoromethanesulfonic acid (triflic acid). b 3d with p-toluenesulfonic
acid (TsOH). c Not determined due to decomposition.
IR spectroscopy (Fig. 7, Table 3). The vibrational frequencies chloride substituents shifts νCO to slightly higher wave-
decrease in the order 3a (1939) > 3b (1936) > 3d (1932) and 3c numbers, from 1939 (3a) to 1946 cm−1 (4a), while a methyl
(1931 cm−1), which is in line with the increasing donor group has almost no effect (1938 cm−1, 5a).
strength of the respective phosphine ligands. Furthermore, the
Interestingly, the IR spectra of the complexes that contain
vibrational frequencies of 3a were compared with those of the phenyl substituents at the phosphorus center (3a–5a) do not
Cl2-bdt (4a) and Me-bdt (5a) analogues to elucidate the influ- exhibit only one absorption band, but contain a clearly visible
ence of variations in the dithiolate bridge on the metal center shoulder at lower energy (Fig. 7). This shoulder is not visible
(Table 3 and ESI†). The introduction of electron withdrawing in the IR spectra of single crystals of 3a–5a and thus has to
4542 | Dalton Trans., 2014, 43, 4537–4549
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rather similar, and only differ by max. 3 kcal mol−1 in Gibbs
energy. The energetically most favorable square-pyramidal
isomer is 3a′endo/exo-lp which gives rise to a CvO stretching fre-
cryst
quency of 1900 cm−1 while the corresponding 3a
does
endo-ph
not exhibit substantial differences in CO vibrations
(1899 cm−1). A second conformer, differing in the orientation
cryst
endo-lp
of the Ph at N1 (3a
) which has the lone pair of N1 pointing
towards the CO probe, causes a 12 cm−1 shift to lower wave-
numbers (1887 cm−1). However, similar carbonyl vibrations are
found also for the square-pyramidal isomer 3a′endo/exo-ph
(1886 cm−1) which differs from 3a′endo/exo-lp only in the orien-
tation of the remote N2-Ph substituent. Two additional confor-
mations could be identified that are shifted to significantly
higher (1922 cm−1) and lower (1877 cm−1) wavenumbers for
3a′endo/endo and 3a′exo/exo, respectively. Since we are interested in
relative free energy differences between conformers, small
differences of maximum 3.0 kcal mol−1 let us assume that the
experimentally observed CO stretching vibrations including
the observed shoulder are a superposition of all isomers. In
nitrogen substituted six-membered rings, both ring inversion
and nitrogen inversion are hard to discriminate experimentally
since they are both of similar barrier heights. Only upon
introduction of a planar sp2-hybridized carbon atom into
the six-membered ring, nitrogen inversion barriers of 8
and 9 kcal mol−1 could be obtained.38 Due to the high dimen-
sionality of the potential energy surface, barrier heights
for the individual inversion steps were not obtained but
we can estimate them to be of the order of ∼9–10 kcal mol−1
each.
Fig. 8 Potential coordination isomers 3a’ and 3acryst with different exo/
endo conformations of the 6-membered FePCNCP heterocycles as well
as different orientations of the N-Ph substituent.
arise from a process that is only enabled in solution. As described
above, potential coordination isomers in solution may offer
an explanation for the experimentally observed shoulder;
however, they hardly account for the Δν of 20 cm−1 between
the peak in the IR absorption and the shoulder.
Support for the assignment that the shoulder in the IR
spectrum of 3a–5a arises from different PPh2NPh confor-
2
The following computational analysis offers an alternative
explanation. It reveals a more complex picture that takes into
account the conformational flexibility of the P2N2 ligand, and
in particular the geometry of the N-substituent in the vicinity
of the CO ligand, as well as coordination isomers.
mations is provided by the observation that addition of acid
leads to an IR spectrum that is characterized by one single
band (Fig. 7, bottom). Complex 3a has multiple protonation
sites. Protonation of one of the thiolate S atoms is one possi-
bility. The second and more probable possibility is protonation
of one of the amine nitrogen atoms. No matter in which ring
conformation, protonation of either of the two nitrogen atoms
is possible. Their proton affinities are almost identical. Kineti-
cally preferred may be a shared proton between the amine
sites. This would enforce an exo/exo conformation of both
rings and explain the simplified IR spectrum of the protonated
form. In such a case, protonation leads to a shift of the main
band by 17 cm−1, which is well in agreement with compu-
tational results which predict a shift in CO stretching frequen-
The boat conformations of the heterocycle in which N(1)
points towards the CO ligand are termed endo-isomers 3a′endo
and 3aendo
cryst, while the chair conformations are termed exo
(Fig. 8) in analogy to the nomenclature used by DuBois and co-
workers.22,37 Further discrimination by the remote heterocycle,
including N(2), can be subdivided once more into endo and exo
conformers giving rise to e.g. endo/exo structures (Fig. 8).
During the calculations (BP86+D3/def2-TZVP/COSMO),
it was noticed that a simple endo/exo differentiation between
the two isomers is in some cases insufficient to fully
describe the conformations of the 6-membered ring systems,
cies to higher wavenumbers by
9
cm−1 (BP86+D3/TZVP
COSMO). In analogy to well-studied Ni(P2N2)2+ complexes, pro-
tonation can be expected to lead to a species where the proton
resides between the two six-membered rings. In this confor-
mation, the N(1) has to be in an exo orientation, and can thus
not inflict varying impacts on the CO ligand. The calculated
proton binding affinity for such a situation is −272 kcal mol−1
(BP86+D3/TZVP).
as the potential energy surface shows multiple minima.
cryst
endo
It was found, for example, that 3a
can exist in two
different geometries that differ in the orientation of the N-lone
pair (lp) or N-phenyl (ph) substituent relative to the CO ligand,
cryst
endo-lp
cryst
giving rise to isomers 3a
and 3a
(Fig. 8). The
endo-ph
former isomer was found to be the lowest in energy of all con-
sidered isomers, while the latter is 1.4 kcal mol−1 higher in
Gibbs energy. In general the different endo and exo isomers for
3a′ and 3acryst that are found on the PES are energetically
From the above considerations, it is clear that the CO
ligand is not only an indicator of changes to the electronic
density at the Fe center, but will also sense the presence of
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Fig. 10 Fully reversible reduction–reoxidation cycles of complex 3a in
acetonitrile solution, monitored by FTIR spectro-electrochemistry.
Time-depended FTIR spectra of 3ared induced by reduction of 3a at −1.8
V and reoxidation at −1.4 V vs. Ag/Ag+, respectively, measured against
the IR spectrum of 3a as the background. The reduction–reoxidation
cycles (first five of 20 cycles are shown, cycle length 100 sec) were
performed without any loss or decomposition of the compound.
Fig. 9 Top: cyclic voltammograms of 3a (line), 3b (dashed), 3c (dotted)
and 3d (dash-dotted). Bottom: CVs of 3a (line), 4a (dashed) and 5a
(dotted) with a concentration of 1.0 M in acetonitrile. Conditions: scan
rate of 100 mV s−1, 0.1 M (Bu)4NPF6 as a supporting electrolyte.
compounds’ 31P chemical shifts, despite the fact that the elec-
tronic properties of the iron centers are significantly influ-
enced. These findings clearly demonstrate that orthogonal
tuning of the complexes’ electronics is possible, either by
changes of the P-substituent or the introduction of bdt-substi-
tuents (strategies (i) and (ii) in Fig. 3).
coordination isomers in solution or conformational changes at
the nitrogen. Both nitrogen inversion that brings the phenyl
ring in close proximity to the CO ligand as well as rotation of
the bdt-ligand that converts a trigonal-bipyramidal to a square-
pyramidal coordination mode lead to distinct changes in νCO
vibration frequency.
The reversibility of the electrochemical reduction and the
stability of the produced anion were demonstrated by FTIR
spectro-electrochemistry on complex 3a (Fig. 10). When a con-
stant potential of −1.8 V vs. Ag/Ag+ is applied, the reduced
species 3ared is produced, giving rise to a new band located at
1850 cm−1 in the IR difference spectra with respect to the 3a
background spectrum. At the same time a negative signal
corresponding to depletion of 3a appears at 1939 cm−1. The
shift in IR absorption maximum to lower vibrational frequen-
cies for 3ared can be explained by the higher electron density at
the iron center resulting in stronger back-bonding to CO-based
π*-orbitals as compared to 3a. Quantitative re-oxidation of 3a
was achieved at a potential of −1.4 V vs. Ag/Ag+. No significant
decomposition of the catalyst was observed after 20 reduction–
reoxidation cycles, indicating high reversibility and a remark-
ably stable singly reduced species.
The electrochemical properties of 3a–d, 4a and 5a were
studied by cyclic voltammetry in acetonitrile solution (Fig. 9,
Table 3). All complexes show
a reversible one-electron
reduction assigned to the formal FeII/I-couple. The half-wave
potentials of compounds 3a–d decrease in the order of −1.65
(3a) > −1.70 (3b) > −1.78 (3d) > −1.82 V (3c) vs. Fc/Fc+, consist-
ent with the observed shifts in the IR absorption maxima of
the carbonyl stretching vibrations (Fig. 7; Table 3). The
reduction potentials are in agreement with the electron donat-
ing properties of the P-substituents and in the same range as
those of other penta-coordinate iron complexes.26,39 Further-
more, the influence of the substitution pattern at the bridge
was investigated. As expected, the electron withdrawing chlor-
ide substituents in 4a shift the reduction potential to a more
positive value (−1.57 V) whereas the electron donating methyl
group in 5a leads to a more negative reduction potential
(−1.72 V) compared to that of the unsubstituted complex 3a
(−1.65 V). As evident from 31P-NMR spectroscopy, substitution
at the dithiolate bridge has only marginal effects on the
The addition of increasing amounts of acetic acid (AcOH)
to an acetonitrile solution of compound 3a gives rise to
increasing cathodic peak currents in the cyclic voltammo-
grams that are attributable to catalytic turnover (Fig. 11). Back-
ground hydrogen production at glassy carbon is negligible at
this potential as confirmed by cyclic voltammetry and bulk
4544 | Dalton Trans., 2014, 43, 4537–4549
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observed shoulder in the IR spectrum of 3a. Protonation of the
PR2NPh2 ligand greatly restricts this conformational freedom.
The design of complexes of type 3 (Fig. 3) enables facile
and independent variations of the complexes’ properties
through specific modifications on the bdt bridge and the phos-
phine ligands. Spectroscopic studies clearly demonstrate the ortho-
gonal and independent tuning of the complexes’ properties by
variation of (i) the dithiolate bridge and (ii) the P-substituent.
Electrochemical experiments show electrocatalytic proton
reduction, even from weak acids like acetic acid at modest
overpotentials. The mononuclear complexes can be reversibly
protonated and reduced and are extraordinarily stable in their
ground, singly protonated and singly reduced states, making
them good candidates for further mechanistic investigations.
Fig. 11 Cyclic voltammograms of 3a (1.0 mM) in acetonitrile solution in
the absence (black) and presence (grey curves) of increasing AcOH con-
centrations (in steps of 50 eq. from 0 to 300 eq.). The increase in
current in the presence of acid indicates that the proton reduction is
catalyzed by the iron complex.
Experimental section
General information
If not stated otherwise all reactions were performed under an
inert argon atmosphere using standard Schlenk techniques.
All reagents were obtained from commercial suppliers and
used without further purification.
electrolysis control experiments at a constant potential of
−1.68 V vs. Fc/Fc+ (Fig. ESI-21†). Bulk electrolysis with 3a in
the presence of 100 equivalents of AcOH at the same constant
potential also leads to a substantially enhanced cathodic
current compared to experiments in the absence of 3a. Com- NMR spectroscopy
parison of the passed charge with the amount of produced
1H-NMR spectra were recorded on a JEOL Eclipse+ spectro-
hydrogen as monitored by means of in situ GC analysis
suggests a Faradaic yield close to unity and shows that 3a can
catalyze proton reduction even with very weak acids like acetic
acid (pKa = 22.3 in acetonitrile) at a mild overpotential of
merely 290 mV.
meter operating at a proton frequency of 400 MHz at 25 °C.
The spectra were referenced to a deuterated solvent as an
internal standard (CHCl3: δH = 7.26 ppm) and the measured
values for δ are given in ppm. 13C{1H}-NMR and 31P{1H}-NMR
measurements were recorded on the same instrument and
referenced internally to solvent peaks or externally to H3PO4(aq.)
(85%), respectively.
Conclusion
FTIR spectroscopy
IR absorption spectra of 3a–d, 4a and 5a were recorded in the
spectral range of 2100–1800 cm−1 with a resolution of 2 cm−1
on a Perkin Elmer SpectrumOne FTIR spectrometer and the
protonation studies were recorded on a Bruker (IFS 66 v/S)
FTIR spectrometer. The IR measurements were performed
with a liquid-sample-cell (Specac Omni-Cell) using CaF2
windows with 0.5 mm PTFE spacers in acetonitrile or
dichloromethane.
Based on the idea that functional mimics of the [FeFe] H2ase
only require the Fed site, a series of six mononuclear iron
model complexes was prepared. The different behavior of
monodentate vs. bidentate ligands in their tendency to form
hexa- and penta-coordinated complexes, respectively, could be
rationalized by first principle calculations. Simple phosphine
ligands, i.e. PMe3 and PPh3, form hexa-coordinate complexes,
and the loss of one CO ligand was calculated to be energeti-
cally unfavorable. The use of chelating PR2NPh bisphosphine
2
Electrochemistry
ligands leads to selective formation of coordinatively unsatu-
rated penta-coordinate Fe(II)-complexes with the general Cyclic voltammograms were obtained using an Autolab poten-
formula [Fe(X-bdt)(PR2NPh2)(CO)] (with X = H, Cl2 or Me and tiostat with a GPES electrochemical interface (Eco Chemie)
R = Ph, Bn, Cyc or tert-Bu). Two main coordination isomers of and a standard three electrode setup. The working electrode
this complex are found by ab initio calculations, namely a tri- was a glassy carbon disc (diameter 3 mm, freshly polished),
gonal-bipyramidal and a distorted square-pyramidal structure. while a glassy carbon stick was used as a counter electrode. As
Furthermore, the PR2NPh ligand leads to a variety of confor- a reference electrode a non-aqueous Ag/Ag+ electrode (CH
2
mational isomers depending on the orientation of the six- Instruments, 10 mM AgNO3 in acetonitrile) with a potential of
membered FePCNCP rings (endo/exo) as well as the orientation 80 mV (vs. the ferrocene/ferrocenium (Fc/Fc+) couple) was
of N-phenyl substituent (endo-lp/endo-Ph). The almost iso- used. Ferrocene was used as an internal standard and all
energetic isomers reveal significant shifts of the CO stretching reported potentials are quoted vs. the Fc/Fc+ couple. All
vibrations and may all contribute to the experimentally measurements were conducted with oven dried glassware,
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freshly distilled dry CH3CN and 0.1 M tetrabutylammonium- Bulk electrolysis with in situ GC detection
hexafluorophosphate (Bu)4NPF6 (Fluka, electrochemical grade)
as a supporting electrolyte.
All procedures were done in a glove box under an argon atmos-
phere. An Autolab potentiostat with a GPES electrochemical
interface (EcoChemie) was used with a three electrode setup
Spectro-electrochemistry
consisting of
a
non-aqueous Ag/Ag+ reference electrode
FTIR spectro-electrochemical experiments were performed in
an OTTLE-type (optically transparent thin-layer electrode) cell.
A platinum mesh was sandwiched as the working electrode
between CaF2 windows and the path length of the cell was
120 μm. The spectra were recorded on a Bruker FTIR spectro-
meter (IFS 66v/S) between 2200 and 1800 cm−1 and with a
resolution of 2 cm−1. UV/vis absorption spectro-electrochemi-
cal spectra were recorded between 250 and 800 nm on a diode
array UV/vis spectrometer (8543 spectrometer, Agilent) in
an acetonitrile solution. A quartz cell cuvette with a 10 mm
pathlength was used.
(+80 mV vs. Fc/Fc+), a platinum mesh counter electrode and a
0.44 cm2 glassy carbon rod working electrode. The counter
and reference electrodes were separated from the sample com-
partment by means of glass frits. The 130 ml gaseous head-
space was sampled in situ with a CP4900 Micro-GC (Varian)
equipped with a 5 Å molsieve column and argon as a carrier
gas. The GC was calibrated electrochemically in two separate
runs by reduction of acetic acid with a 2 mm diameter Pt disk
working electrode at −1.58 V vs. Fc/Fc+. Assuming 100%
charge to hydrogen conversion efficiency, the amount of pro-
duced hydrogen was calculated from the charge passed
through the electrode and the hydrogen peak area correlated
to this amount. (Bu)4NPF6 dried at 85 °C was used for 0.1 M
supporting electrolyte solutions in anhydrous acetonitrile. All
bulk electrolysis experiments were performed on 6 ml samples
at −1.68 V vs. Fc/Fc+ under rapid stirring. Acid backgrounds
were measured as averages of two independent runs. The over-
potential was determined from the applied potential of the
bulk electrolysis experiments as described by Artero et al.52
X-ray crystallography
Crystallographic data sets were collected from single crystal
samples mounted on a loop fibre and coated with N-paratone
oil (Hampton Research). The collection was performed using a
Bruker SMART APEX diffractometer equipped with an APEXII
CCD detector, a graphite monochromator and a 3-circle gonio-
meter. The crystal-to-detector distance was 5.0 cm, and the
data collection was carried out in 512 × 512 pixel mode. The
initial unit cell parameters were determined by a least-squares
fit of the angular setting of strong reflections, collected by a
30 degrees scan in 12 frames over three different parts of the
Synthesis
Preparation of the bisphosphine ligands PR2NPh2. The cyclic
reciprocal space (36 frames total). Cell refinement and data 1,5-diaza-3,7-diphosphacyclooctane ligands (PR2NPh2) were pre-
reduction were performed with SAINT V7.68A (Bruker AXS) on pared according to literature methods31, and the resulting data
the final data. Absorption correction was done by multi-scan were identical to those reported earlier.
methods using SADABS96 (Sheldrick). The structure was
The synthesis of the hexanuclear complex 2 [Fe(bdt)-
solved by direct methods and refined using SHELXL97.40 All (PPh3)2CO2] was performed under an Ar atmosphere starting
non-H atoms were refined by full-matrix least-squares with from the single precursors. Therefore, a Schlenk vessel was
anisotropic displacement parameters while hydrogen atoms loaded with FeCl2 (51 mg, 0.4 mmol), an excess of PPh3
were placed in idealized positions. Refinement of F2 was per- (4.0 mmol) and 13 ml freshly degassed acetonitrile. After this,
formed against all reflections. The weighted R-factor wR and a second Schlenk was filled with acetonitrile (5 ml), 1,2-benzene-
goodness of fit S are based on F2. Crystal structure data were dithiole (57 mg, 0.4 mmol) and Et3N (TEA, 0.15 ml). These
deposited at the Cambridge Crystallographic Data Centre solutions were combined and vigorously stirred for 3 h at
(CCDC 822967, 972155–972156).
room temperature, resulting in a deep red suspension. The
solid precipitate was filtered, washed and dried in vacuo. Yield:
55% (0.17 g), C44H34O2S2P2Fe1 (776.64 g mol−1), EA calcd:
Computational details
All calculations were performed with Orca version 2.9.41 The C 68.04, H 4.41, S 8.26; found: C 68.84, H 3.44, S 8.49. 1H-NMR
BP8642,43 GGA exchange-correlation functional and the B3LYP (400 MHz, CDCl3): δH = 7.23–7.71 (m, 32H, PPH3 and bdt), 8.17
hybrid44–47 exchange-correlation functionals were used (m, 2H, bdt) ppm. 31P{1H}-NMR (161.8 MHz, CDCl3): δP
=
together with dispersive corrections due to Grimme.48,49 An 60.1 ppm. IR (DCM): νCO = 2014 and 1940 cm−1
Ahlrichs TZVP basis set50 was used for initial structure optimi- General procedure for the preparation of [Fe(X-bdt)(PR2NPh2)-
.
zations. The def2-TZVP basis set51 was used for vibrational (CO)]complexes. In a Schlenk vessel FeCl2 (51 mg, 0.4 mmol)
assignment of conformers from numerically calculated second and one equivalent of the respective bisphosphane ligand
derivatives. Thermochemical corrections were applied for T = (PR2NPh2) (0.4 mmol) were mixed with 15 ml degassed aceto-
298.15 K and p = 1 atm. Solvent effects for acetonitrile were nitrile under a CO atmosphere. Subsequently, this suspension
included in a Conductor-Like-Screening Model (COSMO).28 was treated with a combined acetonitrile solution (5 ml) of the
The proton binding affinity is calculated as the Gibbs energy respective dithiol ligand (X-bdt, 0.4 mmol) and Et3N (TEA,
difference between protonated and unprotonated species at 0.15 ml). The reaction solution turned first quickly to violet
ambient temperature, normal pressure and in an implicit and then to dark green. After three hours of stirring and gently
solvation model.
heating at 40 °C the solvent was removed under reduced
4546 | Dalton Trans., 2014, 43, 4537–4549
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pressure. The solid residue was purified by chromatography on
[Fe(µ-Cl2bdt)(PPh2NPh2)(CO)] 4a (µ-Cl2bdt = 3,6-dichloro-1,2-
a silica gel column with a degassed CH2Cl2–n-hexane (8 : 2) benzenedithiolate). Yield:
mixture as an eluent. If necessary, a further purification could (747.43 g mol−1), EA calcd: C 56.24, H 4.05, N 3.75; found:
be obtained by recrystallization from CH2Cl2–n-heptane in the C 55.82, H 4.86, N 4.18. 1H-NMR (400 MHz, CDCl3): δH
absence of oxygen. Finally, a dark green powder was obtained 3.84–4.10 (m, 6H, CH2), 4.46 (t, J = 13.6 Hz, 1H, CH2), 4.92 (dt,
49%,
C35H30N2O1S2P2Cl2Fe1
=
after removal of all volatiles under vacuum.
J = 13.5, 4.0 Hz, 1H, CH2), 6.70 (m, 2H), 6.98 (t, J = 7.3 Hz, 1H),
[Fe(µ-bdt)(PPh2NPh2)(CO)] 3a (Ph = phenyl). Yield: 94%, 7.02–7.10 (m, 2H), 7.14–7.24 (m, 5H), 7.26–7.40 (m, 8H), 7.47
C35H32N2O1S2P2Fe1 (678.59 g mol−1), EA calcd: C 61.95, H (m, 1H), 7.61 (m, 1H) ppm. 13C{1H}-NMR (75.5 MHz, CDCl3):
4.75, N 4.13; found: C 61.69, H 4.83, N 4.10. 1H-NMR δC
=
53.17(d,
(400 MHz, CDCl3): δH = 3.91 (m, 4H, CH2), 4.04 (m, 2H, CH2), (161.8 MHz, CDCl3): δP = 68.1 ppm. IR (CH3CN): νCO
4.80 (m, 2H, CH2), 6.9–7.4 (m, 22H, phenyl), 8.15 (dd, J = 6.0, 1947 cm−1
3.2 Hz, 2H) ppm. 13C{1H}-NMR (75.5 MHz, CD2Cl2): δC = 53.09 [Fe(µ-Mebdt)(PPh2NPh2)(CO)] 5a (µ-Mebdt = 4-methyl-1,2-
J =
110.8 Hz, CH2), ppm. 31P{1H}-NMR
=
.
(dt, J = 38.2, 16.7 Hz, CH2) 128.88 (d, J = 10.1 Hz), 128.96 (d, J = benzenedithiolate). Yield: 85%, C36H32N2O1S2P2Fe1 (690.56 g
15.9 Hz) 129.39 (d, J = 23.0 Hz), 131.29 (t, J = 18.5 Hz), 131.95 mol−1), EA calcd: C 62.43, H 4.95, N 4.04; found: C 62.34, H
1
(t, J = 5.4 Hz), 152.25 (dt, J = 18.5, 8.8 Hz), 156.69 (t, J = 3.6 Hz, 5.37, N 3.73. H-NMR (400 MHz, CDCl3): δH = 2.40 (s, 3H, Me),
0H), 152.87 (t, J = 8.3 Hz), 214.96 (t, J = 23.1 Hz, CO) ppm. 3.88 (dt, J = 13.2, 2.9 Hz, 2H, CH2), 3.94 (t, J = 13.3 Hz, 2H,
31P{1H}-NMR (161.8 MHz, CDCl3): δP = 68.0 ppm. IR (CH3CN): CH2), 4.04 (dt, J = 13.3, 6.6 Hz, 2H, CH2), 4.80 (dt, J = 13.3, 3.9
νCO = 1939 cm−1
.
Hz, 2H, CH2), 6.95 (t, J = 7.3 Hz, 1H), 7.00–7.05 (m, 2H), 7.08
[Fe(µ-bdt)(PBn2NPh2)(CO)] 3b (Bn = benzyl). Yield: 96%, (dd, J = 8.7, 1.0 Hz, 2H), 7.16 (dd, J = 8.7, 1.0 Hz, 2H),
C37H36N2O1S2P2Fe1 (706.62 g mol−1), EA calcd: C 62.89, H 7.20–7.40 (m, 14H), 7.96 (br.s., 1H), 8.03 (d, J = 8.1 Hz,
5.14, N 3.96; found: C 62.33, H 5.30, N 3.86. 1H-NMR 1H) ppm. 13C{1H}-NMR (75.5 MHz, CD2Cl2): δC = 20.91
(400 MHz, CDCl3): δH = 3.43 (dd, J = 121.1, 14.1 Hz, 4H, CH2), (s, Me), 53.43 (dt, J = n.d., 19.0 Hz, CH2), 118.96, 119.57,
3.80 (s, 4H, benzyl), 3.82 (ddt, J = 146.5, 14.2, 3.4 Hz, 4H, CH2), 122.03, 122.61, 123.53, 128.95, 129.21 (t, J = 4.9 Hz), 129.51,
6.57 (dt, J = 59.9, 13.7 Hz, 4H), 6.86 (t, J = 7.3 Hz, 2H), 7.16 (m, 129.74 (d, J = 24.6 Hz), 131.61, 131.78 (d, J = 7.7 Hz), 132.32 (t,
4H), 7.20–7.30 (m, 18 H) ppm. 13C{1H}-NMR (75.5 MHz, J = 5.4 Hz), 152.65 (dt, J = 18.3, 8.7 Hz), 155.82 (d, J = 295.6
CDCl3): δC = 36.00 (ps. t, J = 7.4 Hz, benzyl), 51.28 (d, J = 355.7, Hz), 215.25 (d, J = 22.4 Hz; CO) ppm. 31P{1H}-NMR (161.8 MHz,
16.3 Hz, CH2), 117.94, 118.25, 121.61 (d, J = 27.0 Hz), 121.77, CDCl3): δP = 68.2 ppm. IR (CH3CN): νCO = 1938 cm−1
127.48, 129.19 (d, J = 13.0 Hz), 129.43 (d, J = 12.5 Hz), 129.97
.
(t, J = 2.0 Hz), 132.65 (t, J = 4.0 Hz), 151.98 (dt, J = 16.0, 7.4 Hz),
156.85 (t, J = 5.7 Hz), 214.96 (t, J = 22.8 Hz, CO) ppm. 31P{1H}- Funding sources
NMR (161.8 MHz, CDCl3): δP = 64.9 ppm. IR (CH3CN): νCO
=
A.O. is grateful to the Austrian Science Fund (FWF) for an
Erwin-Schrödinger fellowship (J3193-N17). M.K. thanks the
Wenner-Gren Foundation and S.T. thanks the German
Academic Exchange Service (DAAD) for a PostDoc fellowship.
M.S. is grateful to the Max-Planck-Society for the Advancement
of Science and the Excellence Initiative “Research Center
for Dynamic Systems: Biosystems Engineering” for financial
support. Further financial support from the Swedish Research
Council, the Knut and Alice Wallenberg Foundation and the
Swedish Energy Agency is gratefully acknowledged.
1936 cm−1
.
[Fe(µ-bdt)(PCyc2NPh2)(CO)] 3c (Cyc = cyclo-hexyl). Yield: 80%,
C35H44N2O1S2P2Fe1 (690.66 g mol−1), EA calcd: C 60.87, H
8.09, N 4.06; found: C 60.53, H 7.82, N 4.03. 1H-NMR
(400 MHz, CDCl3): δH = 0.75 (m, 2H), 1.08 (m, 4H), 1.36 (m,
4H), 1.55 (br.s. 6H), 1.72 (m, 4H), 3.31 (d, J = 13.3 Hz, 2H), 3.53
(d, J = 13.4 Hz, 2H), 3.65 (m, 2H), 4.47 (dt, J = 12.9, 3.9 Hz, 2H),
7.00 (q, J = 7.2 Hz, 2H), 7.08 (ps.t., J = 7.7 Hz, 4H), 7.14 (m, 1H)
7.33 (ps.t., J = 7.6 Hz, 4H), 8.12 (m, 1H) ppm. 13C{1H}-NMR
(75.5 MHz, CDCl3): δC = 26.45 (dt, J = 16.9, 6.1 Hz), 26.87,
27.76, 38.94 (t, J = 9.7 Hz), 49.88 (dt 16.8, 102.6 Hz), 119.06,
119.30, 121.23, 122.01, 122.35, 128.93, 129.45, 129.61, 152.90
(dt, J = 18.0, 8.5 Hz), 155.76 (t, J = 3.1 Hz), 214.82 (CO) ppm.
31P{1H}-NMR (161.8 MHz, CDCl3): δP = 78.2 ppm. IR (CH3CN):
Author contributions
The manuscript was written and approved through contri-
butions of all authors.
νCO = 1931 cm−1
.
[Fe(µ-bdt)(PtB2NPh2)(CO)] 3d (tB = tert-butyl). Yield: 82%,
C31H40N2O1S2P2Fe1 (638.59 g mol−1), EA calcd: C 58.31, H
6.31, N 4.39; found: C 58.30, H 6.40, N 4.50. 1H-NMR
(400 MHz, CDCl3): δH = 0.92 (m, 18H, t-Bu), 3.23 (d, J = 13.3
Hz, 2H, CH2), 3.53 (d, J = 13.5 Hz, 2H, CH2), 3.72 (m, 2H, CH2),
The authors declare no competing financial interest.
Acknowledgements
4.71 (m, 2H, CH2), 7.02 (m, 2H), 7.10–7.16 (m, 6H), 7.35 (dd, We acknowledge Dr Marie-Pierre Santoni (Uppsala University)
J = 16.2, 8.8 Hz, 4H), 8.12 (m, 2H) ppm. Due to the low solubi- for help in measuring the crystal structure of complex 3a and
lity of this complex no 13C NMR could be obtained. 31P{1H}- Mohammad Mirmohades (Uppsala University) for assistance
NMR (161.8 MHz, CDCl3): δP = 89.7 ppm. IR (CH3CN): νCO
1932 cm−1
=
with the IR protonation studies. Dr Reiner Lomoth is acknowl-
edged for valuable discussions.
.
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23 T. Liu, B. Li, C. V. Popescu, A. Bilko, L. M. Pérez, M. B. Hall
and M. Y. Darensbourg, Chem.–Eur. J., 2010, 16, 3083–
3089.
24 L. Schwartz, P. S. Singh, L. Eriksson, R. Lomoth and S. Ott,
C. R. Chim., 2008, 11, 875–889.
25 D. Streich, M. Karnahl, Y. Astuti, C. W. Cady,
L. Hammarström, R. Lomoth and S. Ott, Eur. J. Inorg.
Chem., 2011, 2011, 1106–1111.
26 M. Beyler, S. Ezzaher, M. Karnahl, M.-P. Santoni,
R. Lomoth and S. Ott, Chem. Commun., 2011, 47, 11662–
11664.
Notes and references
1 N. Armaroli and V. Balzani, Angew. Chem., Int. Ed., 2007,
46, 52–66.
2 V. Balzani, A. Credi and M. Venturi, ChemSusChem, 2008, 1,
26–58.
3 C. Tard, X. Liu, S. K. Ibrahim, M. Bruschi, L. D. Gioia,
S. C. Davies, X. Yang, L.-S. Wang, G. Sawers and
C. J. Pickett, Nature, 2005, 433, 610–613.
4 M. Y. Darensbourg, Nature, 2005, 433, 589–591.
5 S. Styring, Faraday Discuss., 2012, 155, 357–376.
6 T. Faunce, S. Styring, M. R. Wasielewski, G. W. Brudvig, 27 L. Yu, C. Greco, M. Bruschi, U. Ryde, L. De Gioia and
A. W. Rutherford, J. Messinger, A. F. Lee, C. L. Hill, M. Reiher, Inorg. Chem., 2011, 50, 3888–3900.
H. deGroot, M. Fontecave, D. R. MacFarlane, 28 A. Klamt and G. Schüürmann, J. Chem. Soc., Perkin Trans.
B. Hankamer, D. G. Nocera, D. M. Tiede, H. Dau, 2, 1993, 799–805.
W. Hillier, L. Wang and R. Amal, Energy Environ. Sci., 29 J. Y. Yang, S. E. Smith, T. Liu, W. G. Dougherty,
2013, 6, 1074–1076.
7 Y. Nicolet, C. Piras, P. Legrand, C. E. Hatchikian and
J. C. Fontecilla-Camps, Structure, 1999, 7, 13–23.
W. A. Hoffert, W. S. Kassel, M. R. DuBois, D. L. DuBois
and R. M. Bullock, J. Am. Chem. Soc., 2013, 135, 9700–
9712.
8 A. Jablonskyte, J. A. Wright and C. J. Pickett, Dalton Trans., 30 G. M. Jacobsen, R. K. Shoemaker, M. J. McNevin,
2010, 39, 3026–3034.
9 X. Zhao, I. P. Georgakaki, M. L. Miller, J. C. Yarbrough and
M. Rakowski DuBois and D. L. DuBois, Organometallics,
2007, 26, 5003–5009.
M. Y. Darensbourg, J. Am. Chem. Soc., 2001, 123, 9710– 31 G. Märkl, G. Y. Jin and C. Schoerner, Tetrahedron Lett.,
9711. 1980, 21, 1409–1412.
10 B. E. Barton and T. B. Rauchfuss, Inorg. Chem., 2008, 47, 32 T. E. Barder and S. L. Buchwald, J. Am. Chem. Soc., 2007,
2261–2263. 129, 5096–5101.
11 J. I. van der Vlugt, T. B. Rauchfuss, C. M. Whaley and 33 S. Ezzaher, A. Gogoll, C. Bruhn and S. Ott, Chem. Commun.,
S. R. Wilson, J. Am. Chem. Soc., 2005, 127, 16012–
16013.
12 A. R. Finkelmann, M. T. Stiebritz and M. Reiher, Chem.
Sci., 2014, 5, 215–221.
2010, 46, 5775–5777.
34 M. Karnahl, S. Tschierlei, O. F. Erdem, S. Pullen,
M.-P. Santoni, E. J. Reijerse, W. Lubitz and S. Ott, Dalton
Trans., 2012, 41, 12468–12477.
13 R. Zaffaroni, T. B. Rauchfuss, D. L. Gray, L. De Gioia 35 S. Lounissi, J.-F. Capon, F. Gloaguen, F. Matoussi,
and G. Zampella, J. Am. Chem. Soc., 2012, 134, 19260–
19269.
F. Y. Petillon, P. Schollhammer and J. Talarmin, Chem.
Commun., 2011, 47, 878–880.
14 T. B. Rauchfuss, S. M. Contakes, S. C. N. Hsu, 36 A. W. Addison, T. N. Rao, J. Reedijk, J. van Rijn and
M. A. Reynolds and S. R. Wilson, J. Am. Chem. Soc., 2001,
123, 6933–6934.
G. C. Verschoor, J. Chem. Soc., Dalton Trans., 1984, 1349–
1356.
15 P. E. M. Siegbahn, J. W. Tye and M. B. Hall, Chem. Rev., 37 U. J. Kilgore, M. P. Stewart, M. L. Helm, W. G. Dougherty,
2007, 107, 4414–4435.
W. S. Kassel, M. R. DuBois, D. L. DuBois and R. M. Bullock,
16 J.-F. Capon, F. Gloaguen, F. Y. Pétillon, P. Schollhammer
Inorg. Chem., 2011, 50, 10908–10918.
and J. Talarmin, Coord. Chem. Rev., 2009, 253, 1476– 38 J. M. Lehn, in Dynamic Stereochemistry, Springer, Berlin,
1494. Heidelberg, 1970, vol. 15/3, pp. 311–377.
17 D. L. DuBois and R. M. Bullock, Eur. J. Inorg. Chem., 2011, 39 J. M. Gardner, M. Beyler, M. Karnahl, S. Tschierlei, S. Ott
2011, 1017–1027.
and L. Hammarström, J. Am. Chem. Soc., 2012, 134, 19322–
18 S. Kaur-Ghumaan, L. Schwartz, R. Lomoth, M. Stein and
S. Ott, Angew. Chem., Int. Ed., 2010, 49, 8033–8036.
19 T. Liu, D. L. DuBois and R. M. Bullock, Nat. Chem., 2013, 5,
228–233.
19325.
40 G. Sheldrick, Acta Crystallogr., Sect. A: Found. Crystallogr.,
2008, A64, 112–122.
41 F. Neese, Wiley Interdiscip. Rev.: Comput. Mol. Sci., 2012, 2,
73–78.
20 A. D. Wilson, R. K. Shoemaker, A. Miedaner,
J. T. Muckerman, D. L. DuBois and M. R. DuBois, Proc. 42 A. D. Becke, Phys. Rev. A, 1988, 38, 3098–3100.
Natl. Acad. Sci. U. S. A., 2007, 104, 6951–6956. 43 J. P. Perdew, Phys. Rev. B: Condens. Matter, 1986, 33, 8822.
21 M. L. Helm, M. P. Stewart, R. M. Bullock, M. R. DuBois and 44 A. D. Becke, J. Chem. Phys., 1993, 98, 5648–5652.
D. L. DuBois, Science, 2011, 333, 863–866.
22 M. O’Hagan, M.-H. Ho, J. Y. Yang, A. M. Appel,
45 C. Lee, W. Yang and R. G. Parr, Phys. Rev. B: Condens.
Matter, 1988, 37, 1200–1211.
M. R. DuBois, S. Raugei, W. J. Shaw, D. L. DuBois and 46 P. J. Stephens, F. J. Devlin, C. F. Chabalowski and
R. M. Bullock, J. Am. Chem. Soc., 2012, 134, 19409–19424.
M. J. Frisch, J. Phys. Chem., 1994, 98, 11623–11627.
4548 | Dalton Trans., 2014, 43, 4537–4549
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
47 C. Lee, W. Yang and R. G. Parr, Phys. Rev. B: Condens. 50 A. Schäfer, H. Horn and R. Ahlrichs, J. Chem. Phys., 1992,
Matter, 1988, 37, 785–789. 97, 2571–2577.
48 S. Grimme, Wiley Interdiscip. Rev.: Comput. Mol. Sci., 2011, 51 F. Weigend and R. Ahlrichs, Phys. Chem. Chem. Phys., 2005,
1, 211–228. 7, 3297–3305.
49 S. Grimme, J. Comput. Chem., 2006, 27, 1787– 52 V. Fourmond, P.-A. Jacques, M. Fontecave and V. Artero,
1799.
Inorg. Chem., 2010, 49, 10338–10347.
This journal is © The Royal Society of Chemistry 2014
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