Synthesis of Mono-and Diiron Dithiolene Complexes as Hydrogenase Models by Dithiolene Transfer Reactions, Including the Crystal Structure of [{Ni(S2C2Ph2)}6]

Article abstract of DOI:10.1021/acs.organomet.8b00852

The dithiolene transfer reaction between the nickel bis(dithiolene) complex [Ni(S₂C₂Ph₂)₂] and iron carbonyls has been re-investigated, and the conditions for the production of the dinuclear product [Fe₂(μ-S₂C₂Ph₂)(CO)₆] have been optimized. Interception of a purple intermediate, thought to be [Fe(CO)₃(S₂C₂Ph₂)], in the reaction of [Fe(CO)₅] with [Ni(S₂C₂Ph₂)₂] by the addition of PPh₃ affords the new dark blue mononuclear complex [Fe(CO)₂(PPh₃)(S₂C₂Ph₂)] in good yield. The fate of the nickel dithiolene fragments in these reactions has also been established by crystallographic characterization of the hexamer [{Ni(S₂C₂Ph₂)}₆] and the trinuclear cluster [Ni₃(μ-S₂C₂Ph₂)₃(PPh₃)₂]. The substitution reactions of [Fe₂(μ-S₂C₂Ph₂)(CO)₆] with PPh₃ in the presence of Me₃NO to give monosubstituted [Fe₂(μ-S₂C₂Ph₂)(CO)₅(PPh₃)] and disubstituted [Fe₂(μ-S₂C₂Ph₂)(CO)₄(PPh₃)₂] are also reported.

Full text of DOI:10.1021/acs.organomet.8b00852

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Synthesis of Mono- and Diiron Dithiolene Complexes as
Hydrogenase Models by Dithiolene Transfer Reactions, Including
the Crystal Structure of [{Ni(S2C2Ph2)}6]
Harry Adams, Michael J. Morris,* Craig C. Robertson, and Helen C. I. Tunnicliffe
Department of Chemistry, University of Sheffield, Sheffield S3 7HF, United Kingdom
S
* Supporting Information
ABSTRACT: The dithiolene transfer reaction between the nickel
bis(dithiolene) complex [Ni(S2C2Ph2)2] and iron carbonyls has
been re-investigated, and the conditions for the production of the
dinuclear product [Fe2(μ-S2C2Ph2)(CO)6] have been optimized.
Interception of a purple intermediate, thought to be [Fe-
(CO)3(S2C2Ph2)], in the reaction of [Fe(CO)5] with [Ni-
(S2C2Ph2)2] by the addition of PPh3 affords the new dark blue
mononuclear complex [Fe(CO)2(PPh3)(S2C2Ph2)] in good yield.
The fate of the nickel dithiolene fragments in these reactions has
also been established by crystallographic characterization of the
hexamer [{Ni(S2C2Ph2)}6] and the trinuclear cluster [Ni3(μ-
S2C2Ph2)3(PPh3)2]. The substitution reactions of [Fe2(μ-
S2C2Ph2)(CO)6] with PPh3 in the presence of Me3NO to give
monosubstituted [Fe2(μ-S2C2Ph2)(CO)5(PPh3)] and disubsti-
tuted [Fe2(μ-S2C2Ph2)(CO)4(PPh3)2] are also reported.
INTRODUCTION
■
It is now generally recognized that the dwindling reserves of
fossil fuels and the impact of rising atmospheric CO2 on the
global climate pose a dual threat to the continued advancement
of the human race. One of the most promising solutions to
these problems is to use the almost inexhaustible energy
source, the sun, to power the photochemical splitting of water
into hydrogen and oxygen: the former could then be used as a
Figure 1. Structures of the dinuclear active sites of [FeFe]- and
[NiFe]-hydrogenase enzymes.
sustainable, green fuel as the only product of its combustion is
water. The development of the “hydrogen economy” faces
many technological challenges, not least in its storage,
transport, and efficiency of utilization; however, the effective
production of dihydrogen from water remains a key problem in
Diiron complexes that contain two bridging sulfur-based
this endeavor. Attention has mainly focused on the reduction
of protons to dihydrogen (4H+ + 4e− → 2H2) rather than the
ligands are very common, if not inevitable, products of the
reaction of iron carbonyls with sulfur-containing reagents.8 Of
associated oxidation (2H2O → O2 + 4H+ + 4e−). Although
the hundreds of hydrogenase active site models prepared, the
platinum is a good catalyst for this reaction, its expense and
vast majority involve compounds with this structural motif,
long-term unsustainability prompt the search for cheaper
particularly [Fe2(μ-SCH2XCH2S)(CO)6] (X = CH2, O, NH),
alternatives involving more earth-abundant metals.
Nature has solved this problem in typically efficient fashion
and their various phosphine-substituted derivatives (Figure
2).9−14 The ready accessibility of this unit also enables the
with the evolution of hydrogenase enzymes that can catalyze
incorporation of additional moieties of interest, for example,
the production of H2 from protons and electrons with
extremely high turnovers (>9000 s−1) and low overpotentials.1
redox-active components, light-harvesting antennae, or water-
solubilizing side chains.15−22 Although a number of these
Two types of hydrogenase, the [FeFe] and [NiFe], that occur
systems do catalyze the production of H2 from H+, they
in certain bacteria and primitive archaea are of most interest:
generally do so only at a large overpotential in strongly acidic
the elucidation of the structures of their active sites (shown in
Figure 1) has led to efforts by many synthetic chemists to
replicate the unusual structural features and functionality
Received: November 22, 2018
involved, with some success.2−7
© XXXX American Chemical Society
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dithiolene transfer reactions in the synthesis of both known
and new dithiolene complexes for some years55−57 and were
interested in re-investigating this route to compounds of type 1
as the availability of a large and diverse range of nickel
dithiolene complexes might allow the preparation of
compounds of this type that are unavailable by any of the
previous routes. In this paper, we report (i) the optimization of
the dithiolene transfer reaction, (ii) the modification of the
synthesis to afford mononuclear phosphine-substituted com-
plexes in one step, and (iii) the crystallographic character-
ization of the nickel cluster co-products of both reactions.
Figure 2. Typical structures of di- and mononuclear models for the
active site of the [FeFe]-hydrogenase.
solution (e.g., HClO4 or CF3CO2H) and at moderate turnover
rates.23−25
RESULTS AND DISCUSSION
■
In a more recent development, several groups have explored
the use of the more rigid 1,2-benzenedithiolate (bdt) or 3,4-
toluenedithiolate (tdt) ligands as the bridging group.26−31
Moreover, mononuclear iron complexes containing these
unsaturated ligands have shown good activity for catalytic
proton reduction. This is particularly the case for five-
coordinate complexes of the type [Fe(bdt)(CO)(L2)], where
L2 is a diphosphine ligand (Figure 2).32−39 It should also be
mentioned that bdt complexes of other metals, notably cobalt,
nickel, and molybdenum, have also shown activity for proton
reduction.40−45
Dithiolene Transfer Reactions: Iron Complexes. We
have concentrated on the complex [Fe2(μ-S2C2Ph2)(CO)6]
(1), first, because the appropriate precursor nickel bis-
(dithiolene) complex [Ni(S2C2Ph2)2] is readily available and,
second, because its spectroscopic data were recently reported
by Mousser et al., who unexpectedly isolated it in moderate
yield from the reaction of [Fe2(CO)9] with phenyl
dithiobenzoate.58 Their paper also includes the X-ray structure
determination of the complex, supplementing an earlier one by
Weber and Bryan.59
In Schrauzer’s original method, the bis(dithiolene) com-
plexes [M(S2C2Ph2)2] (M = Fe, Co, Ni) were heated with a
large (∼20-fold) excess of [Fe(CO)5] in refluxing benzene.54
We found that heating [Ni(S2C2Ph2)2] with a 4-fold excess of
[Fe(CO)5] in toluene at 80 °C overnight produced good yields
of 1 (>80% based on the Ni reagent transferring one dithiolene
ligand to the iron center), which can be isolated by
chromatography as an air-stable bright orange-red solid and
characterized by its IR, mass, 1H, and 13C NMR spectra, which
match those in the literature (Scheme 1).58 The reaction
between [Ni(S2C2Ph2)2] and [Fe(CO)5] can also be carried
out at 75 °C in THF in the presence of Me3NO (2 equiv),
giving an 88% yield of 1. It is well-known that both
[Fe2(CO)9] and [Fe3(CO)12] can also serve as sources of
iron carbonyl fragments in reactions with sulfur-containing
substrates, and as expected, the reactions of [Ni(S2C2Ph2)2]
with either of these reagents under similar conditions also gave
good yields of 1 (72 and 98%, respectively). In all of these
reactions, the only other significant product is the hexameric
nickel complex 2, described later.
Over 50 years after the initial breakthroughs of the 1960s,
the efficient synthesis of transition metal dithiolene complexes
continues to be an important objective in inorganic
chemistry.46 We were therefore interested in exploring iron
complexes similar to those in Figure 2 but containing related
dithiolene ligands to take advantage of the unique properties
conferred by the noninnocence of these ligands, in particular,
the accessibility of a range of redox states and the possibility of
incorporating additional redox-active substituents.
The bridging dithiolene complexes [Fe2(μ-SCR1CR2S)-
(CO)6] 1 (Figure 3) form a subset of the class of sulfur-
Figure 3. Structure of diiron hexacarbonyl dithiolene complexes.
In most of these reactions (except, curiously, the one
involving [Fe3(CO)12]), the reaction mixture initially changed
color from green to deep purple, then gradually to orange-
brown. The purple complex could not be characterized;
although small amounts remained at the end of the reaction
and could be collected from the chromatography column, it
decomposed to a green residue when the solvent was removed.
In the reaction carried out with Me3NO, the solution rapidly
turned dark purple on addition of the amine oxide, evolving
gas as it did so. We consider it most likely that the purple
intermediate is [Fe(CO)3(S2C2Ph2)]. Previously Balch and
McCleverty have prepared the analogous [Fe-
(CO)3{S2C2(CF3)2}], described as magenta;48,60 although it
exhibited a dimeric structure in the solid state,61 it was shown
to react with additional [Fe(CO)5] to give [Fe2(CO)6{μ-
S2C2(CF3)2}]. The only other isolable species of this type
appear to be the dithiooxamide complexes [Fe(CO)3{S2C2-
(NEt2)2}] and [Fe(CO)3{S2C2(NMeCH2CH2NMe)}], which
undergo a similar reaction.50 It therefore appears to be a
bridged diiron compounds. Several synthetic routes to these
exist, such as the reaction of iron carbonyls with bis-
(trifluoromethyl)dithiete (R1 = R2 = CF3),47,48 with dithiins
(R1 = Ph, R2 = H or Ph)49 or with dithiooxamides (R1 = R2 =
NEt2).50 Alternatively the complex [Fe2(μ-S2)(CO)6] can be
treated with activated alkynes under UV irradiation (R1 = R2 =
CO2Me, CF3)51,52 or with the acetylide anions R1CC−
followed by protonation of the intermediates (R1 = H, Ph,
Me, Bu, C5H11, SiMe3, R2 = H).53 Schollhammer and co-
workers demonstrated that one of these complexes, with R1 =
R2 = CO2Me, undergoes two reversible electrochemical
reductions and is able to reduce protons catalytically.52
The preparation of certain complexes of type 1 (R1 = R2 =
H, Ph, Me, p-C6H4Me, p-C6H4OMe) was also reported by
Schrauzer, who employed the reaction between [Fe(CO)5]
and the metal bis(dithiolenes) [M(SCR1CR2S)2] (M = Fe,
Co, Ni).54 However, experimental details were rather minimal,
and no yields were given. We have been studying the use of
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Scheme 1. Synthesis of the Iron Dithiolene Complexes by Dithiolene Transfer
Scheme 2. Substitution Reaction of Complex 1 with PPh3
reasonable proposal that [Fe(CO)3(S2C2Ph2)] is an inter-
mediate in the formation of 1 in our reaction.
(CO)2(PPh3)(S2C2Ph2)] 3 (Scheme 2). Compound 5 is air-
sensitive and decomposes to 3 during purification, but this
does not account for the total amount of this product: it is
already present in the reaction mixture before workup.
Compounds 3−5 were readily characterized by their
spectroscopic data and by comparison to other related species.
For example, Dixneuf and co-workers have previously shown
that the analogous disubstituted tetrathiooxalate complexes
[Fe2{μ-S2C2(SR)2}(CO)4(PPh3)2] (R = Me, Et), prepared by
reduction of [Fe(CO)2(PPh3)2(CS2R)]+, also decompose to
[Fe{S2C2(SR)2}(CO)2(PPh3)] on exposure to air; they also
crystallographically characterized both the dinuclear and
mononuclear products for R = Me.62 Numerous mono- and
disubstituted derivatives of the toluene−dithiolate complex
[Fe2(μ-S2C6H3Me)(CO)6] and the 1,3-propanedithiolate
complex [Fe2(μ-S2C3H6)(CO)6] with PPh3 have been
prepared and structurally characterized.10,26,27 By analogy
with these compounds, we expect the phosphine ligands to
occupy the apical positions in 4 and 5, as shown in Scheme
2.62,63
Both Balch and McCleverty also noted that [Fe-
(CO)3{S2C2(CF3)2}] reacted readily with phosphine ligands
to give [Fe(CO)2(L){S2C2(CF3)2}]. We therefore wondered if
it would be possible to intercept the intermediate by addition
of a phosphine ligand before it reacted to give 1. Addition of
triphenylphosphine to the dark purple solution produced from
[Ni(S2C2Ph2)2] and 1 equiv of [Fe(CO)5] in the presence of
Me3NO caused an immediate color change to green, and the
dark blue substituted complex [Fe(CO)2(PPh3)(S2C2Ph2)] 3
(vide infra) was isolated in 59% yield (Scheme 1). Hence, the
purple intermediate certainly reacts as if it were [Fe-
(CO)3(S2C2Ph2)], and we are currently exploring this
procedure as a useful route to other mononuclear iron
carbonyl dithiolene derivatives.
Substitution Reactions of 1 (R1 = R2 = Ph) with PPh3.
No reaction occurred on stirring a dichloromethane solution of
1 (R1 = R2 = Ph) with 1 or 2 equiv of PPh3 at room
temperature. However, on addition of 1 or 2 equiv of Me3NO,
the solution darkened rapidly and three products could be
isolated by column chromatography or TLC: the red
monosubstituted complex [Fe2(μ-S2C2Ph2)(CO)5(PPh3)] 4,
the brown disubstituted complex [Fe2(μ-S2C2Ph2)-
(CO)4(PPh3)2] 5, and the blue mononuclear species [Fe-
The crystal structure of mononuclear complex 3 was
determined and is shown in Figure 4, with bond lengths and
angles summarized in the caption. The complex is best
described as adopting a square-based pyramidal geometry with
the phosphine ligand in the apical position; the trigonality
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Figure 4. Molecular structure of [Fe(CO)2(PPh3)(S2C2Ph2)] 3 in the crystal. Selected bond lengths (Å) and angles (deg): Fe(1)−S(1) 2.1761(5),
Fe(1)−S(2) 2.1942(5), Fe(1)−P(1) 2.2158(6), Fe(1)−C(33) 1.791(2), Fe(1)−C(34) 1.791(2), C(33)−O(1) 1.142(2), C(34)−O(2) 1.142(2),
C(1)−C(2) 1.371(3); S(1)−Fe(1)−S(2) 88.18(2), S(1)−Fe(1)−P(1) 113.52(2), S(2)−Fe(1)−P(1) 94.05(2), C(33)−Fe(1)−S(1) 84.57(6),
C(33)−Fe(1)−S(2) 167.01(6), C(33)−Fe(1)−P(1) 98.72(6), C(33)−Fe(1)−C(34) 91.27(9), C(34)−Fe(1)−S(1) 152.59(6), C(34)−Fe(1)−
S(2) 90.23(6), C(34)−Fe(1)−P(1) 93.89(6).
index, τ, was 0.24.64 Interestingly, this is subtly different from
the structure of [Fe{S2C2(SMe)2}(CO)2(PPh3)], which,
although still square-based pyramidal with τ = 0.29, had a
CO ligand as the apical group.62 The origin of this difference
may be steric, as it places the bulky phosphine ligand further
away from the diphenyldithiolene. It has been known for many
years that a similar ruthenium complex, [Ru(CO)-
(PPh3)2{S2C2(CF3)2}], exists as two interconverting square
pyramidal isomers: one violet with an apical phosphine ligand
and the other orange with the CO ligand in the apical position;
very recently, the two forms have been shown to interconvert
via an observable trigonal bipyramidal intermediate.48,65−68
In a very recent paper, a contrasting result to ours was
obtained by Mousser and co-workers.69 Treatment of 1 with
P(OMe)3 in toluene at 45 °C gave mono- and disubstituted
dinuclear species analogous to 4 and 5, but instead of a
monosubstituted analogue of 3, the disubstituted mononuclear
complex [Fe(CO){P(OMe)3}2(S2C2Ph2)] was obtained. This
compound adopts a structure somewhat more toward trigonal
bipyramidal (τ = 0.44) in which both phosphite ligands occupy
equatorial positions, the dithiolene ligand spans the axial and
equatorial sites, and the CO is in the remaining axial position.
Clearly, the ligand arrangement in these mononuclear species
is very dependent on the steric and electronic properties of the
ligands concerned. Our attempts to induce a second
substitution in [Fe(CO)2(PPh3)(S2C2Ph2)] were hampered
by the fact that heating the complex in refluxing toluene, either
alone or with PPh3, unexpectedly caused its almost quantitative
transformation into the known bis(dithiolene) complex
[Fe(PPh3 )(S2 C2 Ph2 )2 ].5 4 , 7 0 In contrast, [Fe-
(CO)3{S2C2(CF3)2}] is known to undergo a second
phosphine substitution reaction under thermal conditions to
give [Fe(CO)(L)2{S2C2(CF3)2}].48,60
Because it appeared that 1 may be formed by the reaction of
[Fe(CO)3(S2C2Ph2)] with additional [Fe(CO)5], we also
attempted to prepare the monosubstituted complex 4 by the
reaction of complex 3 with an excess of [Fe(CO)5] and
Me3NO. Although 4 was formed in 39% yield, the major
product was the unsubstituted complex 1 (51%). No trace of
the purple band that we propose to be [Fe(CO)3(S2C2Ph2)]
was observed in this reaction; it therefore seems likely that the
initial product is indeed 4, but the PPh3 ligand is then replaced
by CO to give 1.
The electrochemical behavior of 1 was reported recently.69
The cyclic voltammogram of complex 3 is shown in Figure 5.
The compound displays a reversible reduction wave at −1.359
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eluted as a co-product in a yield similar to that of 1 (when
calculated on the basis of the starting nickel complex). It
therefore appears that only one dithiolene ligand from the
nickel starting material is transferred to the iron center, and the
remaining Ni(S2C2Ph2) fragments assemble into the hexamer.
This compound, formulated at the time as [{Ni(S2C2Ph2)}n],
was first isolated in the 1960s by Schrauzer in the reactions of
[Ni(S2C2Ph2)2] with alkynes. It was suggested that only one
dithiolene ligand per complex reacted (to give thiophenes),
leaving the remaining Ni(S2C2Ph2) fragment to associate into a
brown complex that they initially described as a tetramer (n =
4).71−73 In the following year, they isolated the same
compound as a byproduct in dithiolene transfer reactions
between [Ni(S2C2Ph2)2] and [M(CO)6] (M = Cr, Mo, W);74
much later, Holm and co-workers also observed it in the
related reaction of [Ni(S2C2Ph2)2] with [M(CO)3(NCMe)3]
(M = Mo, W) and described it as probably polymeric.75,76 The
reaction of [Ni(S2C2Ph2)2] with nickelocene to give [CpNi-
(S2C2Ph2)] also produced complex 2, and in this case, an ion
was observed in the mass spectrum at m/z 1806, indicating a
hexameric structure.77 We have previously observed its
formation in reactions of [Ni(S2C2Ph2)2] with carbonyl-
containing substrates,55 though interestingly, in other cases
involving dithiolene transfer to complexes containing halides
(such as [CpMo(CO)3Cl] or [RuCl2(PPh3)3]), it was not
produced, and the fate of the nickel in these reactions remains
unclear.56,57
Figure 5. Cyclic voltammogram of complex 3 in CH2Cl2 solution
(room temperature, scan rate 100 mV s−1).
V (vs Fc/Fc+) as well as some more complex oxidation
behavior. Addition of successive 100 equiv aliquots of acetic
acid did cause a small but consistent increase in the current of
the reduction wave (see Supporting Information), but we
conclude that the ability of 3 to catalyze the proton reduction
reaction under these conditions is weak at best.
Bis- or trisdithiolene complexes of the general formula
[M(S2C2R2)2] or [M(S2C2R2)3] are relatively common and
can be prepared for most of the transition metals.46 However,
monodithiolene complexes of the general type [M(S2C2R2)]n
with a 1:1 metal/dithiolene ratio are much rarer and are
Dithiolene Transfer Reactions: Nickel Complexes. In
addition to the iron complexes produced in the dithiolene
transfer reactions, we have also characterized the nickel clusters
that occur as co-products. In all of the dithiolene transfer
reactions giving 1, the brown complex [{Ni(S2C2Ph2)}6] 2 was
Figure 6. Molecular structure of complex 2 in the crystal.
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Figure 7. Schematic view of the framework of cluster 2 from the side (left) and top (right) of the molecule.
confined to four structurally characterized compounds: the
dimeric [{Tl(bdt)}2]2− (bdt = 1,2-benzenedithiolate,
S2C6H4),78 the tetramer [{Ag(mnt)}4]4−, mnt = 1,2-maleoni-
triledithiolate, S2C2(CN)2,79 and two hexameric palladium
complexes discussed in more detail below. It was therefore of
interest to structurally characterize 2, which is the most readily
available member of this series.
The molecule is chiral: as shown in Figure 7, when viewed
from above Ni(1), the dithiolene ligands of each sulfur in the
equatorial plane bridge to the nickel atom in a leftward
direction, whereas its enantiomer would have the opposite
direction (the molecule of Rawson’s Pd6 complex depicted in
that paper is the right-handed enantiomer).81
The cyclic voltammogram of hexamer 2 is shown in Figure
8. The complex displays two reversible reduction waves at
Crystallization of 2 was effected by diffusion of light
petroleum into a dichloromethane solution at room temper-
ature, giving brown needles. The structure is depicted in Figure
6 and is based on an octahedral arrangement of six nickel
atoms, each of which is in a square planar coordination
environment; if an imaginary cube is constructed such that the
nickel atoms lie in the centers of its faces, the sulfur atoms of
the dithiolene ligands occupy the centers of each edge of this
cube. Unique nickel atom Ni(1) at the top of the diagram
bears two chelating dithiolene ligands, the sulfur atoms of
which each bridge to one of the four nickel atoms in the
equatorial positions [Ni(3)−Ni(6)]. The other unique nickel
atom, Ni(2), at the bottom of the octahedron is bonded to
four different dithiolene ligands, each of which bridges to one
of the equatorial nickels, which are themselves therefore ligated
by three different dithiolenes. All of the Ni−Ni distances are
greater than 3.0 Å, precluding any significant metal−metal
bonding. The Ni−S distances within a chelating dithiolene ring
lie in the range of 2.16−2.19 Å, whereas the bridging Ni−S
distances are longer, between 2.23 and 2.27 Å.
Two closely related hexanuclear palladium dithiolene
complexes have been described. The first of these, [Pd-
{S2C2(CO2Me)2}]6, was isolated by Stiefel from the reaction
of [PdCl2(NCMe)2] with [Zn(tmeda){S2C2(CO2Me)2].80 Its
structure consists of an octahedral Pd6 core bridged by
dithiolene ligands but in a subtly different way such that the
structure has S6 symmetry and each Pd is bonded to three
different dithiolene ligands. More recently, Rawson and co-
workers explored the oxidative addition of tetrathiocins to
Pd(0) complexes, producing the benzenedithiolate cluster
[Pd{S2C6H2(OMe)2}]6, which has the same arrangement of
the dithiolate ligands as in 2.81
Figure 8. Cyclic voltammogram of complex 2 in CH2Cl2 solution
(room temperature, scan rate 100 mV s−1).
−1.028 and −1.593 V (vs Fc/Fc+) as well as two smaller
oxidation waves at 0.277 and 0.470 V, followed by a larger
wave (which may consist of two closely spaced oxidations) at
0.962 V. It is interesting to note that Stiefel’s [Pd-
{S2C2(CO2Me)2}]6 cluster showed four reversible reduction
waves, whereas Rawson’s Pd6 cluster showed only two
reductions and two oxidations (the oxidation behavior of
Stiefel’s cluster was not reported).80,81
The reactions carried out in the presence of PPh3 to give 3
did not, however, produce the hexamer 2. Instead, a new
cluster, [Ni3(μ-S2C2Ph2)3(PPh3)2] 6 (Figure 9), was isolated
in low yield as a green solid; its mass spectrum showed a
molecular ion centered on m/z 1426. The single-crystal X-ray
structure of this complex is shown in Figure 10, with important
bond lengths and angles detailed in the caption. The structure
consists of an equilateral triangle of nickel atoms; Ni(1) and
Ni(2), each of which bears a phosphine ligand are symmetri-
In the solid-state structure of 2, there is a two-fold axis of
symmetry which renders pairs of dithiolene ligands equivalent;
within each dithiolene ligand, the two carbons are not
equivalent. In the 13C NMR spectrum, one would therefore
expect to see six signals of equal intensity for the dithiolene
carbons and the same for the ipso carbons of the phenyl
substituents. A total of 12 signals are observed in this region,
implying that the solid-state structure is maintained in solution.
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cally bridged by sulfur atoms S(5) and S(6) of the unique
dithiolene ligand. In contrast, Ni(3) is ligated by two chelating
dithiolenes, which are bridging in the unusual μ3 coordination
mode: one sulfur of each ligand bridges to Ni(1) and the other
to Ni(2). The bridging of these two dithiolenes is much more
asymmetric: the average Ni(3)−S distance is 2.162 Å, whereas
the average Ni(1)/Ni(2) bond distance to the same four
sulfurs is 2.2905 Å. The geometry about Ni(3) is thus distorted
square planar, whereas the other two metals have a square
pyramidal coordination environment. Although the cluster has
52 electrons as opposed to the more usual 48 (which
corresponds to an 18 electron configuration for each metal
with three M−M single bonds), the three Ni−Ni distances are
all similar [2.4914(7)−2.5305(9) Å] and well within the range
observed for a bonding interaction, and the Ni−Ni−Ni angles
are close to 60°. The μ3 coordination of the dithiolene ligand,
in which the ligand largely retains its planarity, has been
Figure 9. Structure of [Ni3(μ-S2C2Ph2)3(PPh3)2] 6.
Figure 10. Molecular structure of complex 6 in the crystal. Selected bond lengths (Å): Ni(1)−Ni(2) 2.4914(7), Ni(1)−Ni(3) 2.5125(8), Ni(2)−
Ni(3) 2.5305(9), Ni(2)−S(1) 2.4767(12), Ni(3)−S(1) 2.1566(13), Ni(2)−S(2) 2.4500(14), Ni(3)−S(2) 2.1629(12), Ni(1)−S(3) 2.4959(12),
Ni(3)−S(3) 2.1621(11), Ni(1)−S(4) 2.4147(13), Ni(3)−S(4) 2.1657(13), Ni(1)−S(5) 2.2840(13), Ni(2)−S(5) 2.2883(12), Ni(1)−S(6)
2.2975(13) Ni(2)−S(6) 2.2924(12), Ni(1)−P(2) 2.2720(11), Ni(2)−P(1) 2.2759(12), C(19)−C(20) 1.356(6), C(65)−C(66) 1.344(6),
C(51)−C(52) 1.374(8).
G
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Article
observed previously in mixed-metal clusters by Nishihara.82,83
Moreover, two structurally very similar benzenedithiolate
clusters, [Ni3(μ-bdt)3(PPh3)2] and [Ni3(μ-bdt)3(PPh2Me)2],
both of which also feature relatively short Ni−Ni distances,
have been reported in the literature.84,85
Cluster 6 is evidently formed by the interception of the
Ni(S2C2Ph2) units produced in the dithiolene transfer reaction
by PPh3, thus preventing them from associating further into
hexamer 2. This suggests that a general route to such
compounds might involve generating such units in the
presence of other phosphines, a possibility we are currently
exploring.
[NBu4][PF6] (0.4 M) as supporting electrolyte. The compound
concentration was 2 mM unless stated otherwise. The solution was
saturated with N2 by bubbling the gas into the solution for a
minimum of 10 min. All redox potentials quoted in the figures and
results and discussion sections are versus the ferrocene/ferrocenium
couple (Fc/Fc+). Scan rates of 20−500 mVs−1 were used, with the
figure caption stating which was used to obtain the traces shown.
All chemicals were purchased from Sigma-Aldrich UK with the
exception of [Fe2(CO)9] and [Fe3(CO)12], which were previously
synthesized laboratory stock. Commercial Me3NO·2H2O was
rendered anhydrous by azeotropic distillation in toluene and stored
under argon. The complex [Ni(S2C2Ph2)2] was prepared by the
literature method.71
Caution. Although we have no evidence for the formation of highly
toxic [Ni(CO)4] during these reactions, this possibility should be borne in
mind during workup.86
CONCLUSIONS
■
Synthesis of [Fe2(μ-S2C2Ph2)(CO)6] (1; R1 = R2 = Ph). A
solution of [Ni(S2C2Ph2)2] (0.7713 g, 1.41 mmol) in toluene (50
cm3) was treated with [Fe(CO)5] (0.68 cm3, 5.03 mmol) and heated
to 80 °C for 23 h in a thermostatically controlled oil bath. The
initially green solution rapidly became dark purple and then more
slowly changed to an orange-red color. After cooling to rt and
addition of silica (about 5 g), the solvent was removed in vacuo. The
solid was then loaded onto a chromatography column. Elution with
light petroleum/CH2Cl2 (9:1) produced an orange-red band of
product 1 (0.6061 g, 1.16 mmol, 82% based on Ni). Elution with a
4:1 mixture of the same solvents gave an unidentified purple band,
which turns green on removal of the solvent, and with a 3:2 mixture, a
small band of starting nickel complex was eluted. Continued elution
with a 2:3 mixture of these solvents gave a brown band due to
[{Ni(S2C2Ph2)}6] 2 (0.3958 g, 0.219 mmol, 93% based on Ni). Two
further minor bands could be eluted with a 7:3 mixture of the same
solvents but remain unidentified.
We have confirmed that the dithiolene transfer reaction of
[Ni(S2C2Ph2)2] with iron carbonyls forms a viable route to the
dinuclear complex [Fe2(μ-S2C2Ph2)(CO)6], as described by
Schrauzer. Furthermore, we have demonstrated that the
reaction passes through an intermediate species, thought to
be [Fe(CO)3(S2C2Ph2)], that can be intercepted by reaction
with PPh3 to give useful yields of [Fe(CO)2(PPh3)(S2C2Ph2)]
in a convenient one-pot synthesis. We are currently exploring
the scope of this reaction with other dithiolene ligands and
phosphines and studying the electrochemical and catalytic
properties of the resulting five-coordinate complexes.
In addition, we have also structurally characterized the
hexameric nickel co-product [{Ni(S2C2Ph2)}6] 2, a compound
commonly observed in dithiolene transfer reactions. More
unexpectedly, we have shown that, in the presence of PPh3, the
aggregation of the Ni(dithiolene) fragments can be arrested
before they form 2, giving instead a new trinuclear cluster 6; in
this way, it may be possible to synthesize a range of different
clusters of intermediate nuclearity by the addition of different
ligands.
Alternatively, a solution of [Ni(S2C2Ph2)2] (0.7096 g, 1.30 mmol)
and [Fe(CO)5] (0.68 cm3, 5.03 mmol) in THF (100 cm3) was treated
with Me3NO (0.7896 g, 10 mmol). The green solution immediately
turned purple, accompanied by gas evolution. It was then heated at 75
°C for 23 h. Chromatographic workup as above gave 1 (0.5968 g, 1.14
mmol, 88% based on Ni).
1
Data for 1: IR (hexane) 2077m, 2042s, 2005vs, 1990w cm−1; H
EXPERIMENTAL SECTION
NMR δ 7.20, 7.02 (m, Ph); 13C NMR δ 207.8 (CO), 150.2 (CPh),
135.1 (Cipso), 128.9, 128.5, 127.3 (Ph); mass spectrum m/z 523 (M +
H)+. These data are in agreement with those reported previously.12
Data for 2: IR (ATR) 3054w, 2953w, 1594w, 1483w, 1441w,
■
All reactions were performed under an inert atmosphere of argon or
nitrogen using Schlenk techniques. Solvents for reactions were
purified with a Grubbs-type purification system manufactured by
Innovative Technology, Newburyport, MA. Chromatographic sepa-
rations were carried out on Geduran 60 silica under a positive
pressure of nitrogen; columns were initially made up in light
petroleum (40−60 °C fraction); polarity was increased by addition of
1
1175w, 1074w, 1029w, 761m, 738s, 692vs cm−1; H NMR δ 7.54−
6.98 (m, Ph); 13C NMR δ 145.7, 144.6, 141.6, 141.5, 141.4, 140.0,
138.6, 138.5, 138.4, 138.4, 138.0, 137.6 (CPh + Cipso), 130.2−125.3
(m, Ph); mass spectrum (ES+) m/z 1804. Anal. Calcd for
C84H60Ni6S12·2CH2Cl2: C, 52.27; H, 3.24; S, 19.45. Found: C,
52.43; H, 3.59; S, 18.84%.
increasing proportions of dichloromethane. The H (400 MHz), 13C
1
NMR (100 MHz), and 31P NMR (162 MHz) spectra were obtained
in CDCl3 solution on a Bruker Avance AVIIIHD machine having an
automated sample changer. Chemical shifts are given on the δ scale
Synthesis of [Fe2(μ-S2C2Ph2)(CO)6] (1; R1 = R2 = Ph) from
[Fe2(CO)9] or [Fe3(CO)12]. A solution of [Ni(S2C2Ph2)2] (0.8917 g,
1.64 mmol) and [Fe2(CO)9] (1.1897 g, 3.27 mmol) in toluene (50
cm3) was heated to 80 °C for 24 h. The color of the solution changed
rapidly to purple and ultimately orange-red as before. Separation of
the products by column chromatography as above produced 1
(0.6179 g, 1.18 mmol, 72% based on Ni). In a similar way, a solution
of [Ni(S2C2Ph2)2] (0.7462 g, 1.37 mmol) and [Fe3(CO)12] (0.9010
g, 1.78 mmol) in toluene (50 cm3) was heated at 80 °C for 24 h. In
this case, the solution did not pass through a purple stage but slowly
turned to dark orange-red. The product 1 was isolated as above
(0.7017 g, 1.34 mmol, 98% based on Ni).
1
relative to SiMe4 for H and 13C and relative to 85% H3PO4 for 31P.
The 13C{1H} NMR spectra were routinely recorded using an attached
proton test technique (JMOD or DEPT-Q pulse sequence). Mass
spectra were recorded on Waters LCT instruments operating in
electrospray (ES+) or atmospheric pressure chemical ionization (AP+)
mode. Solid-state IR spectra were recorded neat with a diamond ATR
device over the range of 4000−400 cm−1 and solution spectra in
CH2Cl2 solution over the range of 2200−1550 cm−1 on a
PerkinElmer Spectrum Two instrument. Elemental analyses were
carried out by the Microanalytical Service of the Department of
Chemistry.
Standard cyclic voltammetry was carried out using an Autolab
Potentiostat 100 attached to a computer using NOVA 1.10.5 software
to record the data. The data were processed using OriginPro 2018.
The experiments were performed at room temperature in a glass
sample tube using a glassy carbon working electrode, a platinum wire
counter electrode, and a Ag/AgCl reference electrode under a
nitrogen atmosphere. The solvent was dichloromethane containing
Synthesis of [Fe(CO)2(PPh3)(S2C2Ph2)] 3 from [Fe(CO)5]. A
solution of [Ni(S2C2Ph2)2] (0.5086 g, 0.937 mmol) and [Fe(CO)5]
(0.15 cm3, 1.09 mmol) in THF (100 cm3) was treated with Me3NO
(0.135 g, 1.76 mmol). A rapid color change to dark purple occurred.
The solution was stirred for 15 min, then triphenylphosphine (0.2800
g, 1.07 mmol) was added, causing a further change to green. The
solution was stirred overnight to ensure complete reaction. After
addition of silica (5 g), the solvent was removed in vacuo and the
H
DOI: 10.1021/acs.organomet.8b00852
Organometallics XXXX, XXX, XXX−XXX

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Article
residue loaded onto a column. Elution with light petroleum/CH2Cl2
(9:1) produced an orange-red band of 1 (30.1 mg, 0.06 mmol) and a
purple band, which was followed by a green band of residual
[Ni(S2C2Ph2)2] (41.7 mg, 8% recovery), eluted with a 17:3 mixture of
the same solvents. The major dark blue band of 3 was then eluted
with a 3:1 mixture of the same solvents; the compound was
recolumned to remove the last traces of [Ni(S2C2Ph2)2], giving a final
yield of 341.9 mg (0.51 mmol, 59% based on Ni). When the reaction
was repeated with 1 equiv of Me3NO, a slightly lower yield of 54%
was obtained.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00852.
■
S
1H, 13C and 31P NMR spectra and selected mass spectra
of compounds reported in this paper, with summary
tables of crystal data for complexes 2, 3, and 6 (PDF)
After the elution of the major product, the green complex [Ni3(μ-
S2C2Ph2)3(PPh3)2] 6 could be eluted with a mixture of CH2Cl2/
acetone (19:1). It proved difficult to obtain this complex in a pure
state due to persistent contamination by Ph3PO and Ph3PS
impurities, but washing with ethyl acetate followed by crystallization
gave a solid in which they were absent in the mass spectrum. The
yield so obtained was 70 mg (16%). Solutions gradually turn brown
and decompose on exposure to air.
Accession Codes
CCDC 1878620−1878622 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Data for 3: IR (CH2Cl2) 2006s, 1952s cm−1; 1H NMR δ 7.73−7.07
(m, Ph); 13C NMR δ 212.4 (d, J = 10 Hz, CO), 168.9 (CPh), 142.4
(Cipso of Ph), 133.3−127.1 (m, Ph); 31P NMR δ 67.6; mass spectrum
m/z 617 (M + H)+. Anal. Calcd for C34H25FeO2S2P: C, 66.23; H,
4.06; S, 10.39. Found: C, 65.70; H, 4.17; S, 10.29%.
AUTHOR INFORMATION
Corresponding Author
*E-mail: m.morris@sheffield.ac.uk.
■
ORCID
Data for 6: 1H NMR δ 7.35−6.82 (m, 58 H, Ph), 6.51 (d, 2 H, Ph);
13C NMR δ 171.5 (CPh), 150.2, 142.2, 141.2, 140.1 (CPh + Cipso),
Michael J. Morris: 0000-0001-8802-9147
Notes
The authors declare no competing financial interest.
134.6 (Ph), 131.2 (Cipso), 130.5−125.9 (m, Ph); 31P NMR δ 43.3 (s,
br); mass spectrum (ES+) m/z 1426 (M+); HRMS found m/z
1426.0485; calcd for C78H60Ni3P2S6 m/z 1426.0528.
Reaction of [Fe2(μ-S2C2Ph2)(CO)6] (1; R1 = R2 = Ph) with
PPh3. A solution of [Fe2(μ-S2C2Ph2)(CO)6] (0.5229 g, 1.00 mmol)
and triphenylphosphine (0.573 g, 2.18 mmol) in dichloromethane
(110 cm3) was treated with Me3NO (0.158 g, 2.10 mmol) and
allowed to stir at room temperature for 19 h. The solvent was
removed in vacuo and the residue absorbed onto silica and loaded
onto a column. Elution with light petroleum/CH2Cl2 (9:1) gave a red
band of the monosubstituted complex [Fe2(μ-S2C2Ph2)-
(CO)5(PPh3)] 4 (514.0 mg, 0.67 mmol, 67%). Elution with a 4:1
mixture of the same solvents gave a blue band due to the
mononuclear complex [Fe(S2C2Ph2)(CO)2(PPh3)] 3 (70.0 mg,
0.11 mmol, 11%). This was followed by a brown band of the
disubstituted complex [Fe2(μ-S2C2Ph2)(CO)4(PPh3)2] 5 (173.0 mg,
0.175 mmol, 17.5%), eluted with a 3:1 mixture of the same solvents.
ACKNOWLEDGMENTS
■
We thank the University of Sheffield for support, Dr. Simon
Parker for performing the electrochemical experiments, and
Mr. Thomas Wieczorek for assistance with some experiments.
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Data for 4: IR (hexane) 2052vs, 1996vs, 1985m, 1947w cm−1; H
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