Push-pull molybdenum trisdithiolenes allow rapid nonconventional binding of ethylene at ligand sulfur atoms

Article abstract of DOI:10.1002/anie.200702036

(Chemical Equation Presented) Adding to the mix: Mixed dithiolene complexes of molybdenum, such as [Mo-{S₂C₂(CF₃) ₂}₂{S₂(C₆H₄)}], bind ethylene rapidly, in a nonconventional fashion, at the sulfur atoms (see scheme). In the presence of additional donor ligands, substitution of the metal-bound 2,3-dihydro-1,4-dithiin occurs, providing a new approach to mixed-ligand molybdenum bis- and trisdithiolenes.

Full text of DOI:10.1002/anie.200702036

Communications
DOI: 10.1002/anie.200702036
Dithiolene Ligands
Push–Pull Molybdenum Trisdithiolenes Allow Rapid Nonconventional
Binding of Ethylene at Ligand Sulfur Atoms**
Daniel J. Harrison, Alan J. Lough, Neilson Nguyen, and Ulrich Fekl*
Molybdenum sulfides are used on a large industrial scale in
hydrodesulfurization (HDS) processes.[1] In addition,many
[2,3]
enzymes,such as oxotransferases
and nitrogenases,[4] rely
on molybdenum bound to sulfur-based ligands.[5] Owing to
their industrial and biological relevance,molybdenum com-
plexes with sulfur donors have received much attention.
Sulfur ligands can display unusual non-innocent behavior.[6]
For sulfur-ligated complexes of molybdenum and other
transition metals,increasing numbers of examples are surfac-
ing where reactions with organic substrates occur at the sulfur
atoms,rather than at the metal. Sulfur-centered reactivity has
been observed,for example,in complexes with bridging
sulfide groups ([{CpMo(m-S)}2S2CH2]; Cpꢀ = C5H5 ),[7] termi-
ꢀ
nal sulfide groups ([ReS4]ꢀ),[8] P,S chelates,[9] and sulfur
centers in metal-bound dithiolene ligands ([M(S2C2R2)2];
M = Ni,Pd,Pt).[10–14]
Scheme 1. Reactions of [Ni(tfd)2] with alkenes.
Ligand-centered reactions are less well understood than
reactions at metal centers,as illustrated by the reactivity of
the nickel bisdithiolene [Ni(tfd)2] (tfd = S2C2(CF3)2) towards
alkenes. While the binding of strained cyclic alkenes (e.g.,
norbornene) to the sulfur atoms of [Ni(tfd)2][11] and related
nickel bisdithiolenes[10] has been known for decades,similar
reactions of simple noncyclic mono-olefins (e.g.,ethylene)
were reported only in 2001,[12] and the mechanistic details
behind this reactivity are just beginning to become clear.[13,15]
In 2006,it was shown that alkene addition to [Ni(tfd) 2] occurs,
preferentially,in a symmetry-allowed,intraligand fashion to
yield decomposition products (substituted 2,3-dihydro-1,4-
dithiin and [{Ni(tfd)}x]; Scheme 1),unless monoanionic
[Ni(tfd)2]ꢀ is present.[13] The monoanion reverses the product
selectivity in favor of stable interligand adducts,by a
mechanism that is still under investigation.[13]
Alkene-addition products of metal bisdithiolenes were
initially proposed to be of the symmetry-allowed intraligand
type.[10] However,all characterized alkene adducts have
shown interligand binding.[11,13] In fact,intraligand addition
products,with a metal-bound dihydrodithiin,are generally
unstable and have not been directly observed for any metal.
The present work is the first to investigate the reactivity of the
well-known class of molybdenum trisdithiolenes towards
alkenes. We achieved rapid alkene binding by employing
unsymmetrical trisdithiolenes with electronically disparate
ligands attached to the central metal ion.
We first investigated the reactions of the previously
reported complexes [Mo(tfd)3][16] and [Mo(bdt)3][17] (bdt =
S2(C6H4)) with ethylene and found slow[18] reactions to give
corresponding dihydrodithiins and decomposed metal spe-
cies,presumably through intraligand alkene addition. We
reasoned that push–pull trisdithiolenes,with a combination of
electron-withdrawing and electron-donating dithiolene
ligands ([Mo(S2C2R2)2(S2C2R’2)]),would react more readily
with alkenes. For example,in a trisdithiolene complex with
one comparatively electropositive dithiolene ligand,the more
electron-withdrawing dithiolene groups are expected to pull
electron density from the more electron-releasing ligand.[19]
Thus,we propose that the electron-releasing ligand is partially
oxidized,relative to the electronegative ligands,and is poised
for [2+4] cycloaddition reactions at the dithiolene sulfur
centers (see below).
[*] D. J. Harrison, N. Nguyen, Prof. U. Fekl
University of Toronto Mississauga
3359 Mississauga Road North
Mississauga, ON L5L1C6 (Canada)
Fax: (+1)905-828-5425
E-mail: ulrich.fekl@utoronto.ca
Dr. A. J. Lough
X-ray Crystallography
University of Toronto
80 St. George Street
Toronto, ON M5S3H6 (Canada)
[**] This workwas supported by the Natural Sciences and Engineering
Research Council (NSERC) of Canada, the Canadian Foundation for
Innovation, the Ontario Research Fund, and the University of
Toronto.
Metal trisdithiolenes with different dithiolene ligands are
very rare and had not been crystallographically characterized
previously.[19,20] We synthesized the new compounds [Mo-
(bdt)2(tfd)] and [Mo(tfd)2(bdt)],using the procedures shown
in Scheme 2 (see the Supporting Information).
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
7644
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Chemie
Figure 2. a) MO interactions that lead to ligand bending along the
Scheme 2. Syntheses of [Mo(tfd)2(bdt)] and [Mo(bdt)2(tfd)].
S···S intraligand axes (distortion to lower symmetry).[22] b) Proposed
LUMO for [Mo(tfd)2(bdt)] and its symmetry-allowed interaction with
ethylene.
An X-ray crystal structure analysis of [Mo(bdt)2(tfd)]
reveals trigonal-prismatic geometry at the metal center
(Figure 1),[21] as has been observed in the solid-state structures
Hückel[17a] and Fenske–Hall[22] computations predict the
HOMO (a2’(Lvp)) and LUMO (a1’(dz2); left-hand part of
Figure 2a) to be close in energy. Further,by ligand bending
along the S···S axes (D3h!C3h distortion; Figure 2),these
orbitals can mix to form frontier orbitals with a mixture of
metal dz2 and ligand p (in-phase across intraligand sulfur
atoms) character.[22] For the mixed-ligand species,the sym-
metry will be further lowered compared to the C3h (homo-
leptic) case. We expect that the tfd sulfur atoms contribute
more to bonding MOs,whereas the bdt sulfur atoms should
have larger coefficients for antibonding MOs.
It is likely that the bdt sulfur atoms significantly contrib-
ute to the LUMO,as shown schematically for [Mo(tfd) 2(bdt)]
in Figure 2b. Also,since structural data show larger ligand-
bend angles for bdt compared to tfd,as well as evidence for a
partially oxidized character of bdt (Figure 1),the bdt ligands
appear to be more efficiently coupled to the dz2 orbital; we
propose that electron density from an alkene can be very
efficiently transferred to the metal and tfd ligands through the
p system of the bdt ligand.
Figure 1. Molecular structure of [Mo(bdt)2(tfd)]. One of two crystallo-
graphically independent molecules in the unit cell is shown, and
thermal ellipsoids are set at 30% probability. A ligand-bend angle is
defined as the angle between the ligand plane (least-squares plane
through S-C-C-S) and the metal coordination plane (through S-Mo-S).
ꢀ
ꢀ
Selected distances [Š] and angles [8]: Mo1 S1 2.367(1), Mo1 S2
ꢀ
ꢀ
ꢀ
2.366(1), Mo1 S3 2.371(1), Mo1 S4 2.359(1), Mo1 S5 2.363(1),
ꢀ
Mo1 S6 2.363(1); S1-Mo1-S2 81.44(4), S3-Mo1-S4 82.39(4), S5-Mo1-
S6 82.58(5); the average C C distances of the bdt ligands are given in
ꢀ
the inset, along with a limiting Lewis structure for oxidized[6] bdt.
Ligand-bend angles [8]: 17.9(1) for the tfd ligand, 23.8(1) for both bdt
ligands (values for the second molecule are 18.0(1) and 20.6(1),
respectively).
Both [Mo(bdt)2(tfd)] and [Mo(tfd)2(bdt)] react with
ethylene in the fashion predicted (Scheme 3). The reactions
of a number of homoleptic trisdithiolenes.[5] Also,the
dithiolene S2C2 planes are canted away from the MoS2
planes,along the intraligand S···S axes (“ligand bend”). This
ligand bending has been attributed to second-order Jahn–
Teller effects; for homoleptic trisdithiolenes (e.g.,[Mo-
(S2C2H2)3]),the distortion to lower symmetry ( D3h!C3h) is
likely driven by mixing of ligand p orbitals (Lvp) and the metal
dz2 orbital (Figure 2a).[22]
Scheme 3. Reactions of [Mo(tfd)2(bdt)] and [Mo(bdt)2(tfd)] with ethyl-
ene to form intraligand alkene adducts at the bdt ligands.
The reactions of the new mixed-ligand trisdithiolenes with
ethylene are predicted to proceed in an intraligand manner
(Figure 2b). As a first-order approximation,the known
molecular orbitals (MOs) of symmetrical (homoleptic) trisdi-
thiolenes can be used.[17a,22,23] The lowest unoccupied MO
(LUMO) of the metal complex and the highest occupied MO
(HOMO) of the alkene have symmetry properties allowing
synfacial intraligand addition. For homoleptic trisdithiolenes,
such as [Mo(S2C2H2)3],computed in D3h symmetry,extended
are rapid,[18] take place exclusively at the sulfur atoms of the
bdt ligand,and selectively form the intraligand adducts
[Mo(tfd)2{bdt(CH2CH2)}] and [Mo(bdt)(tfd){bdt(CH2CH2)}],
where
2,3-dihydro-1,4-benzodithiin
is
denoted
as
bdt(CH2CH2). Both adducts can be formed in high yield
(> 80%),if the reactions are conducted in weakly coordinat-
ing solvents (chlorinated or aliphatic hydrocarbons). In the
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7645

Communications
presence of more-coordinating solvents (e.g.,MeCN,THF),
decomposition through the loss of 2,3-dihydro-1,4-benzodi-
thiin is observed. [Mo(tfd)2{bdt(CH2CH2)}] is particularly
stable and can be kept in CDCl3 or CD2Cl2 at room
temperature for days. We investigated [Mo(tfd)2-
{bdt(CH2CH2)}] with respect to reversibility of ethylene
binding. Ethylene can be released from the metal-bonded
dihydrodithiin to regenerate the dithiolene ligand,[24] such
that [Mo(tfd)2(bdt)] is restored at mildly elevated temper-
ature:
refluxing
(21 h)
a
CDCl3
solution
of
[Mo(tfd)2{bdt(CH2CH2)}] under argon yielded a mixture of
unchanged [Mo(tfd)2{bdt(CH2CH2)}] (37%) and the alkene-
release product [Mo(tfd)2(bdt)] (41%),along with a small
amount of dihydrodithiin (12%; accompanied by correspond-
ing metal decomposition products; see the Supporting
Information). We expect that alkene dissociation from the
alkene adduct of [Mo(tfd)2(bdt)] is facilitated by back
ꢀ
donation of an occupied d orbital (likely dz2) into the C S
s* orbitals.[25]
X-ray quality crystals of [Mo(tfd)2{bdt(CH2CH2)}] were
grown by allowing ethylene to slowly diffuse into a toluene
solution of [Mo(tfd)2(bdt)].[21] Its structure provides the first
example of a 2,3-dihydro-1,4-dithiin chelating to a transition
metal using the sulfur atoms in the 1,4-positions (Figure 3).
Scheme 4. Reactions of [Mo(bdt)(tfd){bdt(CH2CH2)}] to form
[NEt4]2[Mo(bdt)(mnt)(tfd)] and [NEt4]2[MoO(bdt)(tfd)]. The insets
show the molecular structures of these products (thermal ellipsoids
+
are set at 30% probability; NEt4 cations are omitted). See the
Supporting Information for bond lengths, bond angles, and other
crystallographic details.
Dianionic [Mo(bdt)(mnt)(tfd)]2ꢀ is the first example of a
metal trisdithiolene with three different dithiolene ligands.
The mono-oxo bisdithiolene complex [MoO(bdt)(tfd)]2ꢀ is
rare in that it contains two different dithiolene groups (Holm
et al. reported [MoO(S2C2H2)(mnt)]2ꢀ [26]). Molybdenum–oxo
compounds with one or two dithiolene groups are of interest
as models for the active sites in a variety of oxygen-atom-
transfer enzymes.[2,3,5,26] Both [Mo(bdt)(mnt)(tfd)]2ꢀ and
[MoO(bdt)(tfd)]2ꢀ were isolated in crystalline form as their
+
NEt4 salts. X-ray structure determinations confirmed the
[21]
identity of the products (Scheme 4,insets).
Figure 3. Molecular structure of [Mo(tfd)2{bdt(CH2CH2)}]. Thermal
ellipsoids are set at 30% probability, and only one orientation is
shown for the rotationally disordered trifluoromethyl groups involving
In conclusion,we have prepared new mixed dithiolene
complexes of molybdenum. These compounds rapidly and
cleanly bind ethylene in a nonconventional intraligand
fashion to form metal-chelating 2,3-dihydro-1,4-dithiins.
This alkene-binding reaction was shown to be reversible for
[Mo(tfd)2(bdt)], consistent with the view that 2,3-dihydro-1,4-
benzodithiin can be regarded a protected form of ethylene.
2,3-Dihydro-1,4-dithiins have been previously used as pro-
tected forms of cis alkenes,and an excellent protocol for
alkene release involves a heterogeneous reaction at the
surface of Raney nickel.[27] Our demonstration of a 2,3-
dihydro-1,4-dithiin chelating to a transition metal might
provide a homogeneous model for the alkene-release step
in such reactions,and could also be of relevance to the
behavior of 1,4-disulfur heterocycles in hydrodesulfurization
reactions. Furthermore,we demonstrated that molybdenum-
coordinated 2,3-dihydro-1,4-benzodithiin can be labilized if
ꢀ
C7 and C8. Selected distances [Š] and angles [8]: Mo1 S1 2.329(3),
ꢀ
ꢀ
ꢀ
ꢀ
Mo1 S2 2.326(4), Mo1 S3 2.328(3), Mo1 S4 2.313(4), Mo1 S5
ꢀ
ꢀ
ꢀ
ꢀ
2.523(3), Mo1 S6 2.524(3), C9 C10 1.508(19), S5 C9 1.836(14), S6
C10 1.848(14); S1-Mo1-S2 82.3(1), S3-Mo1-S4 82.3(1), S5-Mo1-S6
72.4(1), C11-S5-C9 96.3(6), C12-S6-C10 97.9(1). Ligand-bend
angles [8], as defined in Figure 1: 0.4(4) for the tfd ligand including S1
and S2, 4.5(4)( for the tfd ligand including S3 and S4, 52.1(4)( for the
bdt–Mo substructure (using Mo1, S5, S6, C11, and C12).
We exploited the lability of the metal-coordinated dihy-
drodithiin
[NEt4]2[Mo(bdt)(mnt)(tfd)]
to
synthesize
the
new
compounds
and
(mnt = S2C2(CN)2)
[NEt4]2[MoO(bdt)(tfd)] in reasonable yields (67%). We
devised the synthetic procedures shown in Scheme 4 (see
the Supporting Information).
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Angew. Chem. Int. Ed. 2007, 46, 7644 –7647

Angewandte
Chemie
at room temperature: for [Mo(bdt)3], t1/2 > 2 h; for [Mo(tfd)3],
desired,which opens new synthetic routes to dithiolene
complexes of molybdenum possessing highly unsymmetrical
structures. We think that further development of this field can
provide both new insights into principles of reactivity,as well
as useful applications.
t
t
1/2 > 11 h (reaction not complete after 20 h); for [Mo(bdt)2(tfd)],
1/2 < 11 min; for [Mo(tfd)2(bdt)], t1/2 < 1 min (reaction complete
in 2 min).
[19] G. N. Schrauzer,V. P. Mayweg,W. Heinrich, J. Am. Chem. Soc.
1966, 88,5174.
[20] D. Argyropoulos,E. Lyris,C. A. Mitsopoulou,D. Katakis,
J.
Received: May 8,2007
Revised: July 5,2007
Published online: August 21,2007
Chem. Soc. Dalton Trans. 1997,615.
[21] Crystallographic data for the structures reported herein (for full
details,see the Supporting Information): CCDC-645333,CCDC-
645334,CCDC-645335,and CCDC-645336 (for the compounds
in the order they appear below) contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif. Data collection
on a Nonius Kappa CCD diffractometer using MoKa radiation
(l = 0.71073 Š); structure solution with direct methods; struc-
ture refinement on F2 against all reflections; R1 values reported
herein are for data with I > 2s(I); wR2 values are for all data.
[Mo(bdt)2(tfd)] (crystallizes with toluene): C23H16F6Mo1S6, Mr =
Keywords: chelates · ligand design · molybdenum ·
.
pericyclic reactions · S ligands
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¯
694.66,green needle,triclinic,space group P1, T= 150(1) K, a =
12.9104(5), b = 13.8875(7), c = 16.2347(9) Š, a = 106.797(2), b =
109.448(3), g = 96.042(3)8, Z = 4, R1 = 0.0468, wR2 = 0.1310,
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details on disorder,as well as twinning): C 26.50H20F12Mo1S6, Mr =
854.73,red plate,monoclinic,space group P21/n, T= 120(2) K,
a = 12.9582(9), b = 11.1308(6), c = 21.4456(15) Š, b = 91.851(3)8,
Z = 4,
R1 = 0.0954,
wR2 = 0.2638,GOF
= 1.100;
[Mo(bdt)(tfd)(mnt)]2ꢀ (two NEt4 cations,see the Supporting
Information for details on disorder in one cation):
C30H44F6Mo1N4S6, Mr = 862.99,dark red needle,monoclinic,
space group P21/c, T= 150(2) K, a = 13.2228(7), b = 19.4932(6),
c = 14.6543(8) Š, b = 90.1690(19)8, Z = 4, R1 = 0.0612, wR2 =
+
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+
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C26H44F6Mo1N2O1S4, Mr = 738.81,orange needle,monoclinic,
space group P21/n, T= 150(2) K, a = 11.6275(5), b = 18.5851(10),
c = 15.1232(5) Š, b = 94.295(3)8, Z = 4, R1 = 0.0536, wR2 =
0.1342,GOF = 1.025.
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(0.16 mm) with ethylene in CH2Cl2 (ethylene-saturated solvent)
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