A convergent approach to the synthesis of multimetallic dithiolene complexes

Article abstract of DOI:10.1021/ic800880s

Controlled base hydrolysis of one or both of the protected 1,2-dithiolene chelates of 1,3,5,7-tetrathia-s-indacene-2,6-dione (O=CS₂C ₆H₂S₂C=O) enables the stepwise synthesis of di- and trimetallic complexes with 1,2,4,5-benzenetetrathiolate as the connector. Treatment of O=CS₂C₆H₂S₂C=O with MeO^-, followed by [NiBr₂(dcpe)] [dcpe = 1,2-bis(dicyclohexylphosphino)ethane], yields [(dcpe)Ni(S₂C ₆H₂S₂C=O)] (4). The reaction of 4 with EtO ^-, followed by [MX₂(dcpe)] (X = halide), yields [(dcpe)Ni-(S₂C₆H₂S₂)M(dcpe)] [M = Ni (5a), Pd (5b)]. Deprotection of the 1,3-dithiol-2-one group of 4, followed by introduction of 1/2 equiv of MX₂ and then I₂, yields the neutral trimetallic compounds [(dcpe)Ni(S₂C₆H ₂S₂)]₂M [M = Ni (6a), Pt (6b)]. Tetrahedralization at nickel is observed in 5a, which density functional theory calculations attribute to second-order Jahn-Teller effects, while 6a and 6b display an end-to-end folding of ～46°. A color darkening is observed in moving from 4 to compounds 6 due to the increasing size of the conjugated metal-organic π system. Intense, broad absorptions in the near-IR are observed for 6a and 6b.

Full text of DOI:10.1021/ic800880s

Inorg. Chem. 2008, 47, 5570-5572
A Convergent Approach to the Synthesis of Multimetallic Dithiolene
Complexes
Kuppuswamy Arumugam,† Rongmin Yu,†,‡ Dino Villagra´n,§ Thomas G. Gray,*,⊥ Joel T. Mague,†
and James P. Donahue*,†
Department of Chemistry, Tulane UniVersity, 6400 Freret Street, New Orleans, Louisiana 70118,
Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts AVenue,
Cambridge, Massachusetts 02139, and Department of Chemistry, Case Western ReserVe
UniVersity, 10900 Euclid AVenue, CleVeland, Ohio 44106
Received May 14, 2008
Controlled base hydrolysis of one or both of the protected 1,2-
dithiolene chelates of 1,3,5,7-tetrathia-s-indacene-2,6-dione
(OdCS2C6H2S2CdO) enables the stepwise synthesis of di- and
trimetallic complexes with 1,2,4,5-benzenetetrathiolate as the
connector. Treatment of OdCS2C6H2S2CdO with MeO-, followed
by [NiBr2(dcpe)] [dcpe ) 1,2-bis(dicyclohexylphosphino)ethane],
yields [(dcpe)Ni(S2C6H2S2CdO)] (4). The reaction of 4 with EtO-,
followed by [MX2(dcpe)] (X ) halide), yields [(dcpe)Ni-
(S2C6H2S2)M(dcpe)] [M ) Ni (5a), Pd (5b)]. Deprotection of the
tallic systems may be enhanced, or perhaps to some degree
tailored, in a metallodithiolene complex containing two or
more metal atoms. Noteworthy examples of dimetal com-
plexes with bis(dithiolene)-type bridging ligands include
[Cp2M]2(S2C6H2S2) (M ) Ti, Zr, Hf),4 [L2M]2(ttft) (L ) Cp,
Cp*, M ) Ti; L ) Ph3P, M ) Pt)5,6 [[Ni(S2C2S2C2-
(CO2Me)2)]2(tto)]n (n ) 0, 2),6,7 (Bu4N)2[[Ni(dmit)]2(tto)],8
6,9
[[(edo)Ni]2(tto)]2
and [Cp*Co(S2C6H2S2)CoCp*].10 In
-
two instances, trinickel species have been isolated and
characterized,9 albeit in low yield and as one component in
a mixture. A hindrance to the further pursuit of these
interesting types of compounds has been the absence of more
deliberate methods of synthesis that circumvent the formation
of insoluble metallodithiolene polymer and that are amenable
to a generalized preparative approach for a broad variety of
multimetal dithiolene molecules. One suggestion of such an
approach appeared in work by Purrington, Bereman, and co-
workers,11 who described the stepwise base hydrolysis of
4,4′-bis(1,3-dithiol-2-one) and the synthesis of dimetallic
complexes, including heterodimetallic complexes, joined by
the butadienetetrathiolate ligand. Described herein is a
controlled deprotection of a doubly protected bis(dithiolene)
ligand for the facile synthesis of di- and trimetallic dithiolene
bis(phosphine) complexes using 1,2,4,5-benzenetetrathiolate
as a bridging bis(dithiolene) ligand.
1
1,3-dithiol-2-one group of 4, followed by introduction of /2 equiv
of MX2 and then I2, yields the neutral trimetallic compounds
[(dcpe)Ni(S2C6H2S2)]2M [M ) Ni (6a), Pt (6b)]. Tetrahedralization
at nickel is observed in 5a, which density functional theory
calculations attribute to second-order Jahn-Teller effects, while
6a and 6b display an end-to-end folding of ∼46°. A color darkening
is observed in moving from 4 to compounds 6 due to the increasing
size of the conjugated metal-organic π system. Intense, broad
absorptions in the near-IR are observed for 6a and 6b.
The rich photophysical and redox behavior associated with
metal dithiolene complexes has stimulated a considerable
body of research into their possible application as conducting
materials,1 molecular magnets,1 sensing devices,2 and chro-
mophores in photocatalytic systems.3 The vast majority of
this work has focused upon monometallic systems. More
recently, some effort has been directed at the synthesis of
dimetallic dithiolene complexes with the idea that the
properties that have elicited so much attention in monome-
Scheme 1 summarizes the syntheses reported here of new
di- and trimetallic complexes using 1,2,4,5-benzenetetrathi-
(4) Ko¨pf, H.; Balz, H. J. Organomet. Chem. 1990, 387, 77.
(5) McCullough, R. D.; Belot, J. A. Chem. Mater. 1994, 6, 1396.
(6) ttft ) tetrathiafulvalenetetrathiolate(4-), tto ) tetrathiooxalate(2-);
dmit ) 2-thioxo-1,3-dithiole-4,5-dithiolate(2-); edo ) 5,6-dihydro-
1,4-dioxine-2,3-dithiolate(2-); dcpe ) 1,2-bis(dicyclohexylphosphi-
no)ethane.
* To whom correspondence should be addressed. E-mail: tgray@case.edu
(T.G.G.), donahue@tulane.edu (J.P.D.).
(7) Yang, X.; Doxsee, D. D.; Rauchfuss, T. B.; Wilson, S. R. J. Chem.
Soc., Chem. Commun. 1994, 821.
†
Tulane University.
‡ Current address: Fujian Institute of Research on the Science of Matter,
Chinese Academy of Sciences, Fuzhou, China 350002.
(8) Pullen, A. E.; Olk, R.-M.; Zeltner, S.; Hoyer, E.; Abboud, K. A.;
Reynolds, J. R. Inorg. Chem. 1997, 36, 958.
§
Massachusetts Institute of Technology.
Case Western Reserve University.
(9) Kubo, K.; Nakao, A.; Yamamoto, H. M.; Kato, R. J. Am. Chem. Soc.
2006, 128, 12358.
⊥
(1) Faulmann, C.; Cassoux, P. Prog. Inorg. Chem. 2004, 52, 399.
(2) Pilato, R. S.; Van Houten, K. A. Prog. Inorg. Chem. 2004, 52, 369.
(3) Cummings, S. D.; Eisenberg, R. Prog. Inorg. Chem. 2004, 52, 315.
(10) Nomura, M.; Fourmigue´, M. Inorg. Chem. 2008, 47, 1301.
(11) Keefer, C. E.; Purrington, S. T.; Bereman, R. D.; Boyle, P. D. Inorg.
Chem. 1999, 38, 5437.
5570 Inorganic Chemistry, Vol. 47, No. 13, 2008
10.1021/ic800880s CCC: $40.75  2008 American Chemical Society
Published on Web 06/03/2008

COMMUNICATION
Scheme 1. Synthesis of Di- and Trimetallic Complexes with
1,2,4,5-Benzenetetrathiolate as a Bridging Bis(dithiolene) Ligand (Cy )
cyclohexyl; X ) Halide)
olate as a connecting ligand. Of the various preparations for
protected forms of 1,2,4,5-benzenetetrathiol that have been
described, the method of Bechgaard for the preparation of
1,3,5,7-tetrathia-s-indacene-2,6-dione (3) is the most con-
venient and cost-effective.12 By a route beginning with
tetrachlorobenzene and proceeding through 1,2,4,5-tetrakis(2-
propylthio)benzene (2), multigram quantities of 1,3,5,7-
tetrathia-s-indacene-2,6-dione (3) are conveniently obtained.
The 1,3-dithiol-2-one moieties in this molecule are readily
unmasked by base hydrolysis to liberate one or both of the
arene-1,2-dithiolate chelates. The virtue of this controlled,
stepwise base hydrolysis of the protected benzenetetrathiolate
is the ability to avoid the formation of an insoluble metal
dithiolene polymer and instead to prepare well-defined metal
dithiolene complexes of increasing complexity through a
convergent-type synthesis.
The addition of 1 equiv of MeO- to a suspension of 3 in
1:1 MeOH/THF, followed by the addition of [NiBr2(dcpe)],6
enables the formation of the orange mononickel compound
[Ni(dcpe)(S2C6H2S2CdO)] (4) in 62% yield. Here, the use
of a substoichiometric 1 equiv of MeO- optimizes the
monodeprotection of 3 relative to double deprotection. The
use of 4 equiv of MeO- in THF/MeOH leads to full
deprotection of 3 and, following the addition of 2 equiv of
[NiBr2(dcpe)], to red-brown dinickel compound 5a in 65%
yield. Alternatively, 5a can be prepared in 47% yield from
isolated 4, which is treated with 2 equiv of EtO- in THF/
MeOH and then reacted with [NiBr2(dcpe)]. This ability to
hydrolyze the peripheral 1,3-dithiol-2-one group in 4 without
an adverse effect upon the remainder of the molecule allows
the opportunity to synthesize heterodimetallic compounds,
e.g., the mixed nickel-palladium compound 5b, by introduc-
tion of [PdCl2(dcpe)] to deprotected 4. The mixed-metal
composition of 5b has been confirmed, inter alia, by MALDI
mass spectrometry. Viewing 4 as a protected metallodithi-
olene “ligand” poses the idea of using 2 or 3 deprotected
equiv thereof to synthesize new, elaborated types of multi-
metallic bis- and tris(dithiolene) complexes. Thus, the mixing
of 2 equiv of deprotected 4 with 1 equiv of NiCl2 · 6H2O or
Figure 1. Thermal ellipsoid plots of [(dcpe)Ni(S2C6H2S2CdO)] (4; 50%),
[(dcpe)Ni]2(S2C6H2S2) (5a; 50%), and [[(dcpe)Ni(S2C6H2S2)]2Ni] (6a; 40%).
H atoms are omitted for clarity.
of PtCl2, followed by the addition of I2 to oxidize the
resulting dianion, produces charge-neutral trinickel com-
pound 6a or the mixed nickel-platinum species 6b. These
products are formed in moderate yields, are readily prepared
in highly crystalline form, and are amenable to characteriza-
tion by a variety of physical methods.
The new compounds shown in Scheme 1 have all been
authenticated by X-ray crystallography (Figure 1). The
structure of 4 reveals nearly square-planar coordination
geometry about nickel and a slight degree of bending of the
arene unit relative to the P2NiS2 mean plane. Compound 5a
shows more a pronounced deviation from planarity, with both
nickel centers tetrahedrally distorted and both P2NiS2 mean
planes folded toward the same side of the arene ring (Figure
1). The angle between the two P2NiS2 mean planes is 26°,
whilethemeandeviationsfromtheP(1)-P(2)-Ni(1)-S(1)-S(2)
and P(3)-P(4)-Ni(2)-S(3)-S(4) best-fit planes are 0.114
and 0.025 Å, respectively. In contrast, the mean deviation
from the P(1)-P(2)-Ni(1)-S(1)-S(2) plane in 4 is only
0.012 Å.
Two molecules of 5a occur in the asymmetric unit of the
unit cell in noncentric space group Pc and reside on general
positions. Both independent molecules reveal a similar type
and degree of nonplanarity, which suggests that this structural
feature is not incidental but rather indicative of specific
symmetry interactions within the electronic structure. The
heterodimetallic nickel-palladium compound 5b crystallizes
j
in the centric space group P1 with Ni and Pd atoms statically
disordered. The optimal site occupancy description for the
Ni atom over the two metal atom positions is a 40:60
distribution. Distortions that are qualitatively similar to those
(12) Larsen, J.; Bechgaard, K. J. Org. Chem. 1987, 52, 3285.
Inorganic Chemistry, Vol. 47, No. 13, 2008 5571

COMMUNICATION
Figure 2. Optimized D2, D2h, and C2h structures of 5a′ and their energies
relative to the D2h structure. Negative numbers indicate stabilization. Only
the D2 and C2h geometries are potential energy minima.
Figure 4. UV-vis and near-IR spectra in an N,N-dimethylformamide
solution of [(dcpe)Ni(S2C6H2S2)M(S2C6H2S2)Ni(dcpe)] (M ) Ni, red; M
) Pt, blue).
D2h symmetry is b1g; in C2h, it and the HOMO both become
bg, and they engage in CI. A structure (C2h) of lower
symmetry than D2h results. Both the D2 and C2h distortions
cause “tetrahedralization” at nickel and are instances of the
second-order Jahn-Teller instability.14 The bending distor-
tion observed in 5a, which is distinct from the tetrahedral-
ization at the metal atoms, has not been reproduced in the
calculations and may require a model compound that more
accurately simulates the basic nature and structural con-
straints of the dcpe ligand.
One obvious change in proceeding from compound 4 to
molecules 5 and then 6 is a progressive darkening in color
from orange to red-brown to brown-black. This color change
is expected as the conjugated π system is made more
extensive. The most prominent feature of the UV-vis-near-
IR spectra of molecules 6 (Figure 4) is the relatively intense,
low-energy absorption in the near-IR at 1159 and ∼1408
nm for 6a and at 1177 nm for 6b. It is possible that these
electronic absorptions are due to interligand intervalence
charge-transfer transitions involving fully reduced ene-1,2-
dithiolates and partially oxidized thienyl radical monoanions.
In summary, the present work describes an approach to
the synthesis of multimetal dithiolene complexes based upon
the selective monodeprotection of 1,3,5,7-s-indacene-2,6-
dione, which is a doubly protected form of 1,2,4,5-benzene-
tetrathiolate. It is probable that the synthetic approach by
which molecules 6a and 6b have been prepared can be
extended to the synthesis of “linear” pentametallic molecules.
The flexibility inherent to this method of synthesis may
permit the preparation of metallodithiolene compounds,
including heterometallic compounds, whose physical proper-
ties can be tailored with a very high degree of control.
Figure 3. View of 5a′ along the Ni-Ni axis in both the D2 and C2h point
groups. For both geometries, the central NiS4Ni unit is essentially planar.
In D2, the two P2Ni planes are staggered at 54.04° relative to each other
and at 27.02° in opposite directions relative to the central NiS4Ni plane. In
C2h, the two P2Ni planes are coincident and form a dihedral angle of 27.27°
with the NiS4Ni plane.
in 5a are observed in 5b, which lends further credence to
the possibility that these structural properties may be
attributable to more than crystal-packing effects.
Compounds 6a and 6b both crystallize in rhombohedral
j
space group R3 and reveal still greater bending from end to
end, with the angle between the terminal P2NiS2 mean planes
being 46°. The twisting in 6a, distinct from the bending, is
21° clockwise as determined by the torsion angle defined
by the P(1)-Ni(1) bond, the Ni(1)-Ni(2) interatomic vector,
and the Ni(2)-P(3) bond. Structurally, 6b is highly similar
to 6a, with bending and twisting angles of 47° and 24°,
respectively. It is likely that, if stereoelectronic effects are
indeed governing the structure of 5a and 5b, they are
pertinent to molecules 6a and 6b as well.
Density functional theory calculations13 were undertaken
to rationalize the distorted tetrahedral geometry about nickel
in 5a. A model complex 5a′ was chosen where H atoms
replace the alkyl substituents on phosphorus. A geometry
optimization within full D2h symmetry converged to an
unstable structure having two imaginary vibrational frequen-
cies, of Au and B3g symmetry (Figure 2). Deformation along
the Au mode leads to a D2 structure and that along the B3g
mode to a C2h geometry; both are energy minima by
harmonic frequency calculations. Figure 3 offers an alterna-
tive presentation of the D2 and C2h structures for further
emphasis of their differences. The C2h isomer is the lowest-
energy structure, being 0.038 eV (0.88 kcal mol-1) more
stable than the D2 geometry and 0.12 eV (2.8 kcal mol-1)
more stable than the idealized D2h complex. The distortions
to lower symmetry result from configuration interaction (CI).
The highest occupied Kohn-Sham orbital (HOMO) in D2h
symmetry is b2g, and the lowest unoccupied orbital (LUMO)
is b2u. In D2 symmetry, these both become b2 and CI between
them stabilizes a D2 structure over D2h. The LUMO + 1 in
Acknowledgment. Support from the Petroleum Research
Fund [Award Nos. 545884G1 (J.P.D.) and 42312-G3
(T.G.G.)], Tulane University, and CWRU is gratefully
acknowledged. Prof. Daniel G. Nocera of MIT is thanked
for access to instrumentation.
Supporting Information Available: Full crystallographic data
in CIF format and preparative procedures, computational details,
and Cartesian coordinates of optimized geometries. This material
is available free of charge via the Internet at http://pubs.acs.org.
IC800880S
(14) (a) Pearson, R. G. Proc. Natl. Acad. Sci. U.S.A. 1975, 72, 2104. (b)
Bersuker, I. B. Chem. ReV. 2001, 101, 1067.
(13) Full details appear in the Supporting Information.
5572 Inorganic Chemistry, Vol. 47, No. 13, 2008