Unprecedented metal template effect on the coupling of dithiafulvene moieties

Article abstract of DOI:10.1039/b311197k

Coordination of two dithiafulvenyldiphenylphosphines on a Mo(CO) ₄ fragment allows a carbon-carbon bond formation upon oxidation, leading to a novel type of metallacycle substituted by a redox active vinylogous tetrathiafulvalene.

Full text of DOI:10.1039/b311197k

Unprecedented metal template effect on the coupling of dithiafulvene
moieties
Dominique Lorcy,* Michel Guerro, Pascal Pellon and Roger Carlier
Synthèse et Electrosynthèse Organiques UMR-CNRS 6510-Université de Rennes 1, Campus de Beaulieu, Bât
10A, 35042 Rennes cedex, France. E-mail: dominique.lorcy@univ-rennes1.fr; Fax: (33) 2 23 23 67 38
Received (in Cambridge, UK) 15th September 2003, Accepted 6th November 2003
First published as an Advance Article on the web 25th November 2003
Coordination of two dithiafulvenyldiphenylphosphines on a
Mo(CO)4 fragment allows a carbon–carbon bond formation
upon oxidation, leading to a novel type of metallacycle
substituted by a redox active vinylogous tetrathiafulvalene.
an equivalent of cis-Mo(CO)4(piperidine)2 in CH2Cl2 gives the
complex 5 as a yellow crystalline solid. The same complex 5 can
also be prepared in a one-pot procedure directly from dithiafulvene
3 by adding directly in the medium where 4 was formed the
molybdenum carbonyl complex. Using this strategy, the complex
5† was obtained in excellent yield (Scheme 2).
Two concomitant polymorphs of 5 were isolated from crystal-
lization in MeOH (5a and 5b, Fig. 1).† X-ray crystal structure
analysis demonstrates that in both cases the two dithiafulvenes 4 are
coordinated to the metal center via the phosphorus atom and that the
molybdenum center is surrounded by two cis-coordinated ligands
4. The two crystalline forms differ only by the relative orientation
of the dithiafulvene cores.
New strategies have emerged recently for creating interactions
between redox active cores such as tetrathiafulvalenes (TTF) and
inorganic moieties. In particular, the chemistry of hybrid building
blocks where the donor molecule is linked to the metal via a
coordination function has developed rapidly.1–5 Along these lines,
the phosphine ligand offers numerous possibilities for coordinating
metals in their oxidized or reduced forms.2–4 In continuation of our
studies on dithiafulvene derivatives, and their ability to form
vinylogous tetrathiafulvalenes by oxidative coupling,6,7 we have
investigated the synthesis of a new dithiafulvene functionalized by
a diphenylphosphino moiety. Herein, we report the synthesis and
the redox properties of this novel redox active vinylphosphine as
well as its ability to coordinate the molybdenum Mo(CO)4
fragment. The structural and electrochemical properties of the
complex are presented together with the directing effect of the
metal on the coupling reaction, upon oxidation, of the two
coordinated vinylphosphines leading to a bidentate diphosphine
complex with a redox active vinylogous tetrathiafulvalene back-
bone.
The target molecule was prepared according to the chemical
pathway described in Scheme 1. Compared to methyldiphenylphos-
phine, it is indeed easy to generate a carbanion of methyldiphenyl-
phosphine borane 1 which is known to react with various
electrophiles.8 Therefore, in order to prepare the vinylphosphine 4,
we treated 1 with LDA and added the 4,5-dimethyl-2-piperidin-
1-yl-1,3-dithiol-2-ylium hexafluorophosphate 2 in the medium.
The piperidino leaving group can simply be removed by chroma-
tography of the reaction mixture on silica gel. Treatment of this
novel phosphine borane complex 3 with DABCO9 allows the
decomplexation of the phosphine borane and affords the dithia-
fulvene 4 as an air stable compound easily recrystallized in
MeOH.
The redox behavior of 3, 4 and 5 was investigated by cyclic
voltammetry in dichloromethane. For 3 and 4, only one irreversible
oxidation wave is observed, assigned to the oxidation of the
dithiafulvene core into the cation radical (Epa = 1.07 V for 3 and
Epa = 0.73 V vs. SCE for 4). While for 5 the oxidation of the
1
dithiafulvene core occurs at Epa = 0.60 V vs. SCE followed by a
2
second irreversible oxidation wave at higher potential (Epa = 1.35
V vs. SCE) corresponding to the oxidation of the metallic center.
The 340 mV potential anodic shift for 3 when compared to 4
indicates the electron withdrawing effect of the borane complexa-
tion. Contrariwise, the oxidation peak potential for 5 (e.g. 0.60 V)
is at a lower potential than the ligand itself (e.g. 0.73 V), indicating
an electron donating effect of the molybdenum redox center on the
dithiafulvene. This is a surprising feature as usually the complexa-
tion of the Mo(CO)4 fragment to a redox active phosphine, such as
In order to investigate the coordination chemistry of this new
phosphine ligand 4 we used cis-Mo(CO)4(piperidine)2.10 Indeed, in
mild conditions both piperidino ligands can be replaced by other
ligands such as phosphines in the cis position. Warming 4 with half
Scheme 2 Reagents and conditions: i) cis-Mo(CO)4(piperidine)2 (0.5
equiv.), CH2Cl2; ii) DABCO, toluene, 50 °C, 4 h, cis-Mo(CO)4(piperidine)2
(0.5 equiv.).
Fig. 1 Molecular structures of 5a and 5b (thermal ellipsoids set at 50%
probability, H atoms and phenyl rings omitted for clarity). Selected bond
lengths [Å] and angles [°] for 5a: Mo1–P1 2.5543 (12), Mo1–P2 2.5456
(11), P1–C1 1.795 (4), C1–C2 1.343 (5), P2–C11 1.798 (4), C11–C12 1.338
(6), P1–Mo–P2 91.65 (4), for 5b: Mo–P1 2.5756 (7), P1–C1 1.809 (3), C1–
C2 1.328 (4), P1–Mo–P1 99.08 (3).
Scheme 1 Reagents and conditions: i) LDA, THF, 280 °C, 2; ii)
chromatography on silica gel column; iii) DABCO, 4 h, toluene, 50 °C.
212
C h e m . C o m m u n . , 2 0 0 4 , 2 1 2 – 2 1 3
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

TTF2b or metallocene,11 induces a positive shift of about 200
mV.
molybdenum center: in both isomers 5a and 5b, the distances
between the carbon atoms involved in the formation of the
metallacycle do not exceed 3.3 Å. Therefore, upon oxidation, an
intramolecular coupling occurs leading to an unprecedented
bidentate diphosphine complex with a vinylogous tetrathiafulva-
lene backbone.
In conclusion, we have presented here a unique example of
carbon–carbon bond formation between two original vinylphos-
phine ligands in the coordination sphere of the Mo(CO)4 fragment.
Current work is devoted to studying the chelating ability of this
redox active phosphine ligand towards low and, even more
interesting, high oxidation state transition metal derivatives and the
formation of new metallacycles upon oxidation.
On the basis of our preceding studies concerning the oxidative
coupling of dithiafulvene derivatives which proceeds through a
radical cation–radical cation mechanism,6,7 we examined the
electrochemical oxidation of 3, 4 and 5 through macroscale
electrolyses. No product resulting from a carbon–carbon bond
formation upon electrochemical oxidation was observed. Next, we
tried the chemical oxidation of 3–5 with an oxidizing agent, tris(4-
bromophenyl)aminium hexachloroantimonate, which has proved
recently to be an efficient reactant for the synthesis of vinylogous
TTF starting from dithiafulvenes.12 The derivatives 3 and 4 do not
undergo intermolecular coupling upon oxidation, but instead we
mainly isolated the corresponding phosphine oxide. Treatment of 5
with two equivalents of (BrC6H4)3NSbCl6 followed by a reduction
with Na2S2O4 leads, after chromatography on a silica gel column,
to the metallacycle 6 resulting from an intramolecular carbon–
carbon bond formation upon oxidation (Scheme 3).
The X-ray molecular structure of 6 is shown in Fig. 2.† The five-
membered ring is not planar and adopts a half chair conformation
due to the steric interactions generated by the two dithiole rings.
The shortest S…S contact between the two dithiole rings is 3.12 Å
and the acute dihedral angle between the two dithiole rings (C2–
C1–C11–C12) amounts to 47.8(5)°. The redox behavior of 6 was
investigated by cyclic voltammetry. One reversible bielectronic
wave at Epa = 0.48 V vs. SCE corresponding to the reversible
oxidation of the vinylogous TTF core into the dication is observed
at a lower potential than the dithiafulvene core in 5. An irreversible
oxidation wave attributable to the oxidation of the metallic center
occurs at Epa = 1.25 V vs. SCE.
We thank Thierry Roisnel from the Centre de Diffractométrie X,
Université de Rennes 1, for X-ray structural determinations.
Notes and references
1
†
Selected data for 5: yellow crystals, mp 172 °C; yield 92%; H NMR
(300 MHz, CDCl3): d = 1.70 (s, 6H), 1.86 (s, 6H), 5.59 (d, 2H, JP–H = 20
Hz), 7.30–7.60 (m, 20H); 31P NMR (121 MHz, CDCl3): d = 24.1; Anal.
Calc. for C40H34MoO4P2S4: C 55.55, H 3.96, S 14.83. Found: C 55.07, H
4.04, S: 14.65%. Crystal data for compound 5a C40H34MoO4P2S4, M =
864.79, monoclinic, space group C2/c, a = 23.3608(5), b = 19.7947(4), c
= 19.4065(5) Å, b = 116.326(1)°, U = 8043.2(3) Å3, Z = 8, T = 293(2)
K, m(Mo-Ka) = 0.652 mm21, Dc = 1.428 g cm23, 17403 reflections
measured, of which 8861 independent (Rint = 0.0542), Rf = 0.048 [5247
data, I
> = 0.1365. Crystal data for compound 5b
2s(I)], wR(F2)
C
40H34MoO4P2S4, M = 864.79, orthorhombic, space group Pccn, a =
10.5290(1), b = 18.2873(2), c = 20.6979(3) Å, U = 3985.32(8) Å3, Z =
4, T = 293(2) K, m(Mo-Ka) = 0.658 mm21, Dc = 1.441 g cm23, 26818
reflections measured, of which 4470 independent (Rint = 0.0388), Rf
0.0363 [3077 data, I > 2s(I)], wR(F2) = 0.0857.
=
It is worth noting that in the absence of a metal template no
reaction was observed upon oxidation of the dithiafulvenyl ligand
4 or 3, either chemically or electrochemically. The coordination of
the dithiafulvenyl phosphine 4 brings closer the two radical cation
moieties, thanks to the cis position of the two ligands on the
Selected data for 6: mp 272 °C; yield 25%; 1H NMR (300 MHz, CDCl3)
d 1.67 (s, 6H), 2.00 (s, 6H), 7.10–7.90 (m, 20 H); 31P NMR (121 MHz,
CDCl3): d = 56.9; Anal. Calc. for C40H32MoO4P2S4: C 55.68, H 3.74, S
14.86. Found: C 55.56, H 3.81, S 14.84%. Crystal data for compound 6
¯
C40H32MoO4P2S4, M = 862.78, triclinic, space group P1, a = 11.5840(1),
b = 11.9101(1), c = 15.1839(2) Å, a = 82.692(1), b = 87.987(1), g =
70.523(1)°, U = 1958.89(3) Å3, Z = 2, T = 293(2) K, m(Mo-Ka) = 0.67
mm21, Dc = 1.463 g cm23, 34395 reflections measured, of which 8374
independent (Rint = 0.0393), Rf = 0.0398 [5466 data, I > 2s(I)], wR(F2)
= 0.0975; Nonius KappaCCD, MoKa radiation (l = 0.71073 Å). The
structures were solved by direct methods (SIR97) and refined by full-matrix
least squares on F2 (SHELXL-97). CCDC 215143 (5b), CCDC 215144 (5a)
and CCDC 215145 (6). See http://www.rsc.org/suppdata/cc/b3/b311197k/
for crystallographic data in .cif or other electronic format.
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Scheme 3 Reagents and conditions: i) 2 equiv. of (BrC6H4)3NSbCl6,
CH2Cl2, reflux, 1 h; ii) Na2S2O4, CH2Cl2, reflux, 1 h.
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Fig. 2 Molecular structure of 6 (thermal ellipsoids set at 50% probability, H
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C1–C2 1.371 (5), C1–C11 1.478 (4), C11–C12 1.375 (5), P1–Mo1–P2
79.12 (3), Mo1–P1–C1 107.71 (11), P1–C1–C11 113.0 (2), Mo1–P2–C11
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C h e m . C o m m u n . , 2 0 0 4 , 2 1 2 – 2 1 3
213