Rapid, covalent addition of phosphine to dithiolene in a molybdenum tris(dithiolene). A new structural model for dimethyl sulfoxide reductase

Article abstract of DOI:10.1021/ic301031b

Triphenylphosphine (PPh₃) rapidly and reversibly adds to the bdt ligand in the molybdenum tris(dithiolene) complex Mo(tfd)₂(bdt) [tfd = S₂C₂(CF₃)₂; bdt = S ₂C₆H₄], turning chelating bdt into the monodentate zwitterionic ligand SC₆H₄SPPh₃. A second PPh₃ molecule fills the newly created open site in the crystallographically characterized product Mo(tfd)₂(SC ₆H₄SPPh₃)(PPh₃), which is a structural model for dimethyl sulfoxide (DMSO) reductase. While the complex is only a precatalyst for reduction of DMSO by PPh₃ (the initially low catalytic rate increases with time), Mo(tfd)₂(SMe₂) ₂ was found to be catalytically active without an induction period.

Full text of DOI:10.1021/ic301031b

Communication
pubs.acs.org/IC
Rapid, Covalent Addition of Phosphine to Dithiolene in a
Molybdenum Tris(dithiolene). A New Structural Model for Dimethyl
Sulfoxide Reductase
Neilson Nguyen,†,‡ Alan J. Lough,† and Ulrich Fekl*,†,‡
†Department of Chemistry, University of Toronto, Toronto, Ontario, Canada M5S 3H6
‡Department of Chemical and Physical Sciences, University of Toronto at Mississauga, Mississauga, Ontario, Canada L5L 1C6
S
* Supporting Information
sulfoxide (DMSO) reductase model. It is also a precatalyst for
DMSO reduction (discussed below).
ABSTRACT: Triphenylphosphine (PPh3) rapidly and
reversibly adds to the bdt ligand in the molybdenum
tris(dithiolene) complex Mo(tfd)2(bdt) [tfd =
S2C2(CF3)2; bdt = S2C6H4], turning chelating bdt into
the monodentate zwitterionic ligand SC6H4SPPh3. A
second PPh3 molecule fills the newly created open site
in the crystallographically characterized product Mo-
(tfd)2(SC6H4SPPh3)(PPh3), which is a structural model
for dimethyl sulfoxide (DMSO) reductase. While the
complex is only a precatalyst for reduction of DMSO by
PPh3 (the initially low catalytic rate increases with time),
Mo(tfd)2(SMe2)2 was found to be catalytically active
without an induction period.
When the known compound9 1 was treated with excess (4
equiv or more) PPh3, we observed the formation of a new
stable complex. As will be detailed below, an intermediate
species (2) is involved in the formation of the stable product
(3) from 1. A sharp singlet in the 19F NMR spectrum is
observed for 3, at −54.94 ppm, indicating that all CF3 groups
on the tfd ligands are equivalent either by symmetry or, as
discussed below, by rapid pseudorotation. Two sharp 31P
singlet peaks are seen at very different shifts, at 54.14 and 43.54
ppm. Magenta crystals of 3 were obtained from two different
solvents, benzene and chloroform, by directly reacting
concentrated solutions of 1 with an excess (ca. 3.7 equiv) of
PPh3. Figure 1 shows the structure of 3, where two phosphines
have added in a very unusual way.
hen transition-metal complexes undergo reactions, the
W
supporting ligands are normally “spectator ligands”.
They control the geometry and electronic structure of the
metal, whereas oxidation/reduction as well as bond-breaking
and bond-making occur at the metal. In comparably rare cases,
where frontier orbitals at the ligands are more accessible than
metal orbitals, a “non-innocent” behavior of the ligand is
observed.
Redox chemistry that involves electron transfer by reducing
the ligand has been known for decades for dithiolene (S2C2R2)
complexes of transition metals.1 Bond-making and bond-
breaking reactivity at dithiolene ligands appears much less
understood than electron transfer involving the ligands. While
the sulfur centers of dithiolenes can be expected to be attacked
by electrophiles (e.g., alkylating agents)2 and nucleophiles have
been reported to attack other ligands (for example, nitrides),3 it
is still very rare that a dithiolene ligand is attacked by a
nucleophile other than an alkene.4,5 Alkene additions to the
ligand6 have been reported for square-planar metal bis-
(dithiolene) complexes since the late 1960s for strained
alkenes7 and since 2001 for simple unstrained alkenes.8 Then,
in 2007 metal tris(dithiolene) complexes were also found to be
reactive to simple unstrained alkenes.9 However, alkenes are
often considered amphiphilic and are not clear-cut examples of
nucleophiles. We now report the addition of the nucleophile
triphenylphosphine (PPh3) to the bdt ligand in the
molybdenum tris(dithiolene) Mo(tfd)2(bdt) [1; tfd =
S2C2(CF3)2; bdt = S2C6H4], creating the zwitterionic ligand
SC6H4SPPh3. The product complex is a structural dimethyl
Figure 1. Molecular structure of 3, from X-ray crystallography on the
benzene solvate (30% probability ellipsoids). H and F atoms are
omitted for clarity. Selected distances and angles: Mo1−S1, 2.358(3);
Mo1−S2, 2.364(2); Mo1−S3, 2.335(2); Mo1−S4, 2.369(3); Mo1−
S5, 2.416(2); Mo1−P1, 2.564(2); S5−Mo1−P1, 76.24(7); S1−Mo1−
S2, 80.72(8); S3−Mo1−S4, 80.74(9). The structure of 3 in the
chloroform solvate is similar (Supporting Information).
One has attacked the bdt ligand, whereas the other one
coordinates to molybdenum, such that compound 3 is
Mo(tfd)2(SC6H4SPPh3)(PPh3). Apart from the molybdenum-
Received: May 18, 2012
Published: May 30, 2012
© 2012 American Chemical Society
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Inorganic Chemistry
Communication
bound phosphine, which is labile (see below), the first
coordination sphere of molybdenum in 3 (a thiolate ligand in
conjunction with two dithiolenes) is very similar to what is seen
for the DMSO reductase family of molybdenum oxotrans-
ferases10,11 or for nitrate reductase from desulfovibrio desulfur-
icans12 and also for a model complex for nitrate reductase from
Sarkar’s group, [Et4N][MoIV(PPh3)(p-MeSPh)(mnt)2].13 The
Mo−thiolate and Mo−P bond lengths are virtually identical
across the two complexes, at 2.416(2) versus 2.390(2) Å (this
work versus Sarkar’s complex) for Mo−thiolate and 2.564(2)/
2.586(2) Å for Mo−P. The Mo−dithiolene bond distances are
also very similar, with the average being 2.357(15) versus
2.364(13) Å. The same is true for thiolate−Mo−P angles, at
76.24(7)° versus 77.16(6)°.
The crystal structure shows no symmetry (C1), but flexibility
in the dangling SC6H4SPPh3 will lead to at least Cs symmetry in
solution. Furthermore, trigonal-prismatic molybdenum(IV)
complexes have access to a low-barrier “twisting” process in
which two distinct positions can rapidly exchange.14 This leads
to pseudo-C2v symmetry in solution, as is observed in the NMR
spectra of six-coordinate bis(dithiolene) complexes having two
different non-ditholene ligands.15,16 The positions occupied by
SC6H4SPPh3 and PPh3 are thus expected to exchange, and one,
averaged, CF3 environment is indeed seen in the 19F NMR
spectrum of 3, at −54.94 ppm. The connectivity of atoms is
unchanged in solution: one phosphine is coordinated to the bdt
ligand and the other one to molybdenum, as supported by 31P
NMR. The singlet at 43.54 ppm corresponds to the sulfur-
bonded phosphorus, and its shift is very similar to the 44.1 ppm
shift of sulfur-coordinated, monodentate SC6H4SPPh3 in a gold
dithiolene complex, reported as a synthetic byproduct in 11%
yield.5 While the formation of SC6H4SPPh3 has precedence, its
high-yielding and reversible formation is unprecedented. The
second singlet at 54.14 ppm corresponds to the molybdenum-
coordinated phosphorus, consistent with literature shifts (49.68
ppm).13
2.45 ppm (1H), while 3′ shows a sharp singlet at 2.42 ppm and
a broad singlet at 2.23 ppm. This is consistent with 2′ having
only one attached triarylphosphine, while 3′ has two. The broad
singlet at 2.23 ppm is assigned as the molybdenum-coordinated
phosphine. K for 3 ⇄ 2 + PPh3 was determined to be 2(1) ×
10−5 M at 29 °C (UV−vis; Supporting Information). When a
sufficiently diluted solution of 3 is equilibrated for ca. 30 min at
room temperature, some 2 can be seen by NMR. Further
dilution shifts the equilibrium more toward 2. Similarly, we
observe 3′ ⇄ 2′ + P(p-tolyl)3, where K = 6(5) × 10−5 M (29
°C). 3 is stable for at least a few days in the presence of excess
phosphine, while 2 slowly and irreversibly decays. Decay may
be due to dimerization.17 Inspired by the close structural
similarity of 3 to oxotransferase enzymes, we tested for
corresponding activity. In a stoichiometric oxotransfer reaction
(without extra phosphine), the addition of 16 equiv of DMSO
produced ∼1 equiv of dimethyl sulfide (DMS; relative to 3),
along with Ph3PO (1H and 31P NMR). The complex
decomposed slowly, as expected given the instability of 2.
However, the reaction was successfully made catalytic with the
addition of extra phosphine before the addition of DMSO.
We typically observed at least 80 turnovers in the presence of
112 equiv of PPh3 and 408 equiv of DMSO over the course of
∼2 days.
Preliminary kinetic studies using the p-tolyl system revealed
the rate of oxygen transfer to increase with time rather than
decrease. This induction period (see the Supporting
Information) indicates that the catalytically active species
might actually be a decomposition product of one of the
phosphine adducts. A possible pathway is the loss of the bdt-
derived ligand to give a Mo(tfd)2 complex with two labile
solvent molecules. Indeed, in a control experiment, the reaction
of Mo(tfd)2(DMS)2 (where DMS is very labile) with 6.3 equiv
of P(p-tolyl)3 and 59 equiv of DMSO yielded nearly
quantitative conversion of P(p-tolyl)3 to OP(p-tolyl)3 with
equivalent generation of DMS (see the Supporting Informa-
tion), where the catalytic rate was high from the outset, without
an induction period. It is unknown how exactly the bdt-derived
ligand is lost and what mechanistic cycle Mo(tfd)2 undergoes in
catalyzing O-atom transfer. A full mechanistic study will be
performed in the future.
The formation of 3 might be expected to proceed via
intermediate 2 (Scheme 1). When 1 was treated with between
Scheme 1
We conclude that nucleophilic addition to a noninnocent
ligand can open a coordinatively saturated complex and thus
enhance reactivity. In 1, phosphine can directly add to the bdt
ligand, creating a zwitterionic SC6H4SPPh3 monodentate
ligand, giving a complex that is a structural DMSO reductase
model and a precatalyst to a functional DMSO-reducing
system. Future work will expand on the mechanism of the
observed oxotransferase activity and the generality of the new,
logical but initially counterintuitive, approach to make an open
site: via the addition of a nucleophile but to the ligand.
1 and 2 equiv of PPh3, an additional species, 2, is indeed
observed, with a singlet in the 19F NMR spectrum at −55.09
ppm and a singlet in the 31P NMR spectrum at 47.58 ppm. The
addition of excess PPh3 (>2 equiv) leads to the disappearance
of these signals and the appearance of the signals for 3.
Computational modeling suggests that 2 is a five-coordinate
complex with square-pyramidal geometry (Supporting In-
formation). More support for the equilibrium in Scheme 1 is
obtained with a triarylphosphine having a methyl as a
spectroscopic “handle”: When tri-p-tolylphosphine is added to
1, the products 2′ and 3′ (Scheme 1) can be clearly
distinguished not only in their 31P and 19F NMR spectra but
also in their 1H NMR spectra. 2′ shows a single sharp singlet at
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental details and crystallographic information for
compound 3 in CIF format. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: ulrich.fekl@utoronto.ca.
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Inorganic Chemistry
Communication
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Funding by NSERC of Canada and by the University of
Toronto (U of T) is gratefully acknowledged. U.F. is a recipient
of an Early Researcher Award (Province of Ontario). We thank
David Armstrong (U of T) for performing density functional
theory studies on 2. Computations were performed on the
GPC supercomputer at the SciNet HPC Consortium. SciNet is
funded by the Canada Foundation for Innovation under the
auspices of Compute Canada, the Government of Ontario, the
Ontario Research FundResearch Excellence, and U of T.
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