D+-π-A- charge-transfer molecules based on tricyanoquinodimethane and diphosphine metal complexes

Article abstract of DOI:10.1021/ic8008333

A new family of D⁺-π-A^- chromophores in which the donor group is an organometallic complex and the acceptor group a tricyanoquinodimethane moiety has been synthesized by the reaction of diphosphinomethanide transition-metal complexes and 7,7′,8,8′- tetracyanoquinodimethane.

Full text of DOI:10.1021/ic8008333

Inorg. Chem. 2008, 47, 5540-5542
D+-π-A- Charge-Transfer Molecules Based on
Tricyanoquinodimethane and Diphosphine Metal Complexes
Javier Ruiz,*,† Maria J. Antón,† Maril´ın Vivanco,† Marta E. G. Mosquera,‡ and Roberto Quesada*,§
Departamento de Qu´ımica Orga´nica e Ino´rganica, Facultad de Qu´ımica, UniVersidad de OViedo,
33071 OViedo, Spain, Departamento de Qu´ımica Inorga´nica, UniVersidad de Alcala´, 28871 Alcala´
de Henares, Spain, and Departamento de Qu´ımica, UniVersidad de Burgos, 09001 Burgos, Spain
Received February 25, 2008
A new family of D+-π-A- chromophores in which the donor
group is an organometallic complex and the acceptor group a
tricyanoquinodimethane moiety has been synthesized by the
reaction of diphosphinomethanide transition-metal complexes and
7,7′,8,8′-tetracyanoquinodimethane.
ancillary ligands. Furthermore, redox-active metallic ions
could favor electron-transfer processes within these mol-
ecules. In this regard, we report here for the first time the
synthesis of organometallic analogues of such types of
D+-π-A- charge-transfer molecules, where the D group
is a metal complex, by means of the reaction of diphosphi-
nomethanide complexes with TCNQ. In the literature, there
are only a handful of examples involving reactions of
coordinated ligands and TCNQ, mainly leading to C-H
insertions, instead of the formation of the tricyanoquin-
odimethane fragment shown herein.5
The diphosphinomethanide complexes 1a-e react with 1
equiv of TCNQ at room temperature, yielding a mixture of
the neutral derivatives 5a-e and the cationic bis(diphe-
nylphosphino)methane complexes 4a-e (Scheme 1), which
are separated after chromatographic workup. Compounds of
types 4 and 5 display intense-green and dark blue-color,
respectively, in solution (Figure 1, left).
Compounds 5a-e result from the nucleophilic addition
of the methanide carbon to TCNQ to give the zwitterionic
complexes 3a-e followed by HCN elimination. This mech-
anism is supported by the insights provided by the reaction
between 1a and TCNQ at low temperature. Under these
conditions, 3a can be isolated in excellent yield as a yellow
solid and spectroscopically characterized.6 Upon standing in
solution at room temperature, this compound evolves to give
5a quantitatively, providing a convenient high-yield synthetic
route for this derivative. Compounds 3a-e can be compared
The bottom-up approach relies on the identification of
molecular building blocks with interesting optical, magnetic,
and electronic properties.1 Within this context, organic
chromophores derived from 7,7′,8,8′-tetracyanoquinodimethane
(TCNQ) have found diverse potential applications. Thus,
intramolecular charge-transfer molecules of the type
D+-π-A-, where D is an organic donor fragment and A a
tricyanoquinodimethane acceptor, have been extensively
studied as functional materials for dyes,2 nonlinear optics,3
and molecular rectifiers, notably hexadecylquinolinium tri-
cyanoquinodimethanide, C16H33Q-3CNQ.4 An interesting
variation in these systems would be the incorporation of a
metal complex as a D group because this could allow fine
control of the charge-transfer degree by adequately choosing
the metal atom, its oxidation state, and the nature of the
* To whom correspondence should be addressed. E-mail: jruiz@uniovi.es
(J.R.), rquesada@ubu.es (R.Q.).
†
Universidad de Oviedo.
Universidad de Alcala´.
Universidad de Burgos.
‡
§
(1) For example, see: (a) Wassel, R. A.; Gorman, C. B. Angew. Chem.,
Int. Ed. 2004, 43, 5120–5123. (b) Maruccio, G.; Cingolani, R.; Rinaldi,
R. J. Mater. Chem. 2004, 14, 542–554. (c) Creutz, C.; Brunschwig,
B. S.; Sutin, N. ComprehensiVe Coordination Chemistry II; McClev-
erty, J. A., Meyer, T. J., Eds.; Elsevier: Oxford, U.K., 2004; Vol. 7,
pp 731-777. (d) Hetch, S. Angew. Chem., Int. Ed. 2003, 42, 24–26.
(2) (a) Zaidi, N. A.; Bryce, M. R.; Cross, G. H. Tetrahedron Lett. 2000,
41, 4645–4649. (b) Bell, N. A.; Crouch, D. J.; Simmonds, D. J.; Goeta,
A. E.; Gelbrich, T.; Hursthouse, M. B. J. Mater. Chem. 2002, 12,
1274–1279.
(4) (a) Honciuc, A.; Otsuka, A.; Wang, Y.-H.; McElwee, S. K.; Woski,
S. A.; Saito, G.; Metzger, R. M. J. Phys. Chem. B 2006, 110, 15085–
15093. (b) Metzger, R. M. Chem. ReV. 2003, 103, 3803–3834. (c)
Metzger, R. M.; Chen, B.; Hoepfner, U.; Lakshmikantham, M. V.;
Vuillaume, D.; Kawai, T.; Wu, X.; Tachibana, H.; Hughes, T. V.;
Sakurai, H.; Baldwin, J. W.; Hosch, C.; Cava, M. P.; Brehmer, L.;
Ashwell, G. J. J. Am. Chem. Soc. 1997, 119, 10455–10466. (d)
Metzger, R. M. Acc. Chem. Res. 1999, 32, 950–957.
(3) (a) Katz, H. E.; Singer, K. D.; Sohn, J. E.; Dirk, C. W.; King, L. A.;
Gordon, H. M. J. Am. Chem. Soc. 1987, 109, 6561–6563. (b)
Szablewski, M.; Thomas, P. R.; Thornton, A.; Bloor, D.; Cross, G. H.;
Cole, J. M.; Howard, J. A. K.; Malagoli, M.; Meyers, F.; Bredas, J.-
L.; Wenseleers, W.; Goovaerts, E. J. Am. Chem. Soc. 1997, 119, 3144–
3154. (c) Beaudin, A. M. R.; Song, N.; Bai, Y.; Men, L.; Gao, J. P.;
Wang, Z. Y.; Szablewski, M.; Cross, G.; Wenseleers, W.; Campo, J.;
Goovaerts, E. Chem. Mater. 2006, 18, 1079–1084.
(5) For other reactions of TCNQ with coordinated ligands, see: (a) Ting,
P.-C.; Lin, Y.-C.; Lee, G.-H.; Cheng, M.-C.; Wang, Y. J. Am. Chem.
Soc. 1996, 118, 6433–6444. (b) Yamamoto, Y.; Settu, H.; Nishiyama,
K.; Kuge, K.; Tatsumi, K. Eur. J. Inorg. Chem. 2002, 9, 2447–2452.
(c) Yamamoto, Y.; Han, X.-H.; Sugawara, K.-i.; Nishimue, A. Angew.
Chem., Int. Ed. 1999, 38, 1242–1244.
(6) See the Supporting Information for details.
5540 Inorganic Chemistry, Vol. 47, No. 13, 2008
10.1021/ic8008333 CCC: $40.75  2008 American Chemical Society
Published on Web 05/30/2008

COMMUNICATION
Scheme 1a
Table 1. Summary of Selected IR and UV/Vis Data for Compounds
5a-e in a CH2Cl2 Solution
compound
5a
5b
5c
5d
5e
ν(Ct N) (cm-1
λmax (nm)
)
2189
680
2193
695
2187
682
2181
648
2186
674
delocalized on the tricyanoquinodimethane fragment and
these compounds are better described as charge-separated.
The IR spectra showed a strong absorption band correspond-
ing to the cyano groups of the quinone at much lower
frequencies (2193-2181 cm-1) than those corresponding to
the neutral TCNQ derivatives (∼2225 cm-1). The frequency
of this band drifted to lower wavenumbers as the donor
character of the ligands in the metal fragment augmented
(see Table 1), being in 5d (2181 cm-1) almost equal to that
exhibited by the radical anion TCNQ•- (2180 cm-1); this
indicates that the degree of charge transfer within this group
of complexes can be modulated by the ancillary ligands, as
determined by IR spectroscopy.10 The presence of electron-
donor and electron-acceptor moieties within the molecule
in the neutral complexes 5a-e resulted in the appearance
of a strong intervalence transfer band, responsible for
the intense-deep-blue color of the complexes in solution. The
UV/vis spectra of these compounds display a broad band
with two peaks or a shoulder at around 640 and 680 nm in
dichloromethane.6 A shift to low wavelength is observed in
the UV/vis major peak as the donor character of the ligands
increases (see Table 1). It is worth remarking that the
dramatic color change occurred on passing from compound
3a (yellow) to compound 5a (dark blue), which is probably
due to the formation of the π system in 5a linking the
organometallic complex and the tricyanoquinodimethane
residue, allowing the electronic communication between both
fragments. This result makes complexes 5a-e organome-
tallically analogous to C16H33Q-3CNQ and other related
charge-transfer organic molecules, furthering the possibility
of them being the donor moiety of a metal complex.11
An X-ray diffraction study was accomplished for 5b
(Figure 1, right),12 where it is worth noting the planarity of
the tricyanoquinodimethanide fragment, which is also close
to coplanar with the PCP skeleton, as well as the multiple-
bond character of the C1-C2 bond [1.385(4) Å], confirming
a
[M] ) (a) fac-[Mn(CNtBu)(CO)3]; (b) [Mn(CO)4]; (c) mer-[Mn-
(CO)3(PPh3)]; (d) trans-[Mn(CO)2dppm]; (e) mer-[FeI(CNtBu)3].
Figure 1. Left: Dichloromethane solutions (10-5 M) of compounds 4b
(left), 5b (center), and 6b (right). Right: X-ray crystal structure of compound
5b. Thermal ellipsoids drawn at the 30% probability level. Selected bond
lengths (Å) and angles (deg): P2-C1, 1.775(3); P3-C1, 1.786(3); C1-C2,
1.385(4); C2-C3, 1.406(4); C3-C4, 1.418(4); C3-C8, 1.413(4); C7-C8,
1.354(4); C4-C5, 1.341(4); C6-C7, 1.405(4); C5-C6, 1.405(5); C6-C9,
1.386(4); C2-C93, 1.455(4); C9-C92, 1.396(5); C9-C91, 1.413(5);
P2-C1-P3, 97.49(14); C1-C2-C3, 132.2(3); C1-C2-C93.
with the zwitterionic intermediate species proposed by
Braunstein et al. in the reaction of quinonoid compounds
with TCNQ;7 however, in marked contrast with that result,
in our case elimination of HCN is observed instead of the
corresponding insertion product into the C-H bond. Com-
pounds 4a-e are complex salts bearing the radical anion
TCNQ•- as the counterion, resulting from a redox process
between TCNQ and 1a-e.8 This process would lead to an
undetected radical ion pair (2a-e in Scheme 1), which yields
4a-e through a hydrogen atom abstraction from the solvent
(or traces of water). Also, formation of 3a-e through a C-C
coupling process in the proposed 2a-e cannot be ruled out.9
All of the spectroscopic features of compounds 5a-e
indicate that the negative charge of the ligand is mainly
(9) (a) Tolbert, L. M.; Sun, X.-J.; Ashby, E. C. J. Am. Chem. Soc. 1995,
117, 2681–2685. (b) Tanko, J. M.; Drumright, R. E.; Suleman, N. K.;
Brammer, L. E. J. Am. Chem. Soc. 1994, 116, 1785–1791. (b) Tanko,
J. M.; Drumright, R. E.; Suleman, N. K.; Brammer, L. E. J. Am. Chem.
Soc. 1994, 116, 1785–1791.
(10) Chapell, J. S.; Bloch, A. N.; Bryden, W. A.; Maxfield, M.; Poehler,
T. O.; Cowan, D. O. J. Am. Chem. Soc. 1981, 103, 2442–2443.
(11) (a) Coe, B. J. ComprehensiVe Coordination Chemistry II; McCleverty,
J. A., Meyer, T. J., Eds.; Elsevier-Pergamon: Oxford, U.K., 2004; Vol.
9, pp 621-687. (b) Low, P. J. Dalton Trans. 2005, 2821–2824.
(12) Crystal data for 5b: C40H24MnN3O4P2, Mr ) 727.5, crystal size 0.42
× 0.38 × 0.2 mm3, monoclinic, space group C2/c, a ) 28.024(7) Å,
b ) 18.174(5) Å, c ) 17.058(4) Å, ꢀ ) 104.822(11)°, V ) 8399(4)
Å3, Z ) 8, Fcalcd ) 1.151 mg m-3, µ ) 0.429 mm-1, Mo KR radiation
(λ ) 0.710 73 Å). Data collection was performed at 298(2) K.
Reflections collected/unique 33 352/9618 [R(int) ) 0.0897]. The final
cycle of full-matrix least-squares refinement based on 9618 reflections
and 452 parameters converged to final values of R1[F2 > 2σ(F2)] )
0.0569, wR2[F2 > 2σ(F2)] ) 0.1525, R1(F2) ) 0.1102, and wR2(F2)
(7) Braunstein, P.; Siri, O.; Taquet, J.-p.; Yang, Q.-Z. Angew. Chem., Int.
Ed. 2006, 45, 1393–1397.
(8) There are numerous examples of tetracyanoquinodimethane salts of
transition-metal complexes in the literature. For selected recent
references, see: (a) Ghazala, S. I.; Paul, F.; Toupet, L.; Roisnel, T.;
Hapiot, P.; Lapinte, C. J. Am. Chem. Soc. 2006, 128, 2463–2476. (b)
Chen, W.-H.; Reinheimer, E. W.; Dunbar, K. R.; Omary, M. A. Inorg.
Chem. 2006, 45, 2770–2772. (c) Feliz, M.; Llusar, R.; Uriel, S.; Vicent,
C.; Coronado, E.; Go´mez-Garc´ıa, C. J. Chem.sEur. J. 2004, 10, 4308–
4314.
) 0.1745. Largest diff. peak/hole +0.570/-0.335 e Å-3
.
Inorganic Chemistry, Vol. 47, No. 13, 2008 5541

COMMUNICATION
Scheme 2a
a
[M] ) (a) fac-[Mn(CNtBu)(CO)3]; (b) [Mn(CO)4]; (c) mer-[Mn-
(CO)3(PPh3)]; (d) trans-[Mn(CO)2dppm]; (e) mer-[FeI(CNtBu)3].
the presence of a π-electron bridge linking the donor and
acceptor moieties of the molecule.
The dicyanomethanide moiety present in the neutral
complexes 5a-d can be protonated by treatment with HBF4,
yielding the cationic derivatives 6a-d (Scheme 2). The
reaction is readily and fully reversible by treatment with
KOH and other bases, and this protonation-deprotonation
process can be repeated several times without degradation
of the complex (Figure 2). The IR spectrum of these
derivatives shows the disappearance of the intense band
assignable to the cyano groups of the quinone along with
the ν(CO) bands appearing at higher frequencies than those
corresponding to the starting materials. The appearance of a
new singlet signal around 5.2 ppm in the 1H NMR spectrum
supports the protonation occurring on the dicyanomethane
residue. Protonation of the ligand disrupts the intramolecular
charge-transfer process, and as a result, a dramatic change
of color from deep blue to light yellow occurred (Figure 1,
left). Similar changes are observed in the presence of Br2 as
a result of bromination of the tricyanoquinomethanide moiety
(7c in Scheme 2). This behavior suggests an easily switchable
system, which could allow pH-controlled electron transfer
within the molecule and its application as a halogen sensor.
In summary, we have reported in this paper a new entry
to charge-transfer molecules of the type D+-π-A-, which
are organometallically analogous to the well-known hexa-
decylquinolinium tricyanoquinodimethanide and other related
organic charge-transfer derivatives, whose potential applica-
Figure 2. Series of UV/vis spectra of compound 5b (in a dichloromethane
solution) recorded after successive additions of HBF4 and Et3N to generate
the protonated 6b and revert the reaction (Inset: variation of the absorbance
at 640 nm).
tion as functional materials (dyes, nonlinear optics, conduc-
tors, rectifiers, magnets, etc.) is widely recognized. Being
that the D+ fragment is derived from a metal complex
containing deprotonated dppm and taking into account the
plethora of known stable dppm complexes of transition
metals, it is expected that this methodology could be
extended to a variety of organometallic systems, allowing
the precise tuning of the electronic properties of D+ by
adequate choice of the ancillary ligands and the central
metallic ion.
Acknowledgment. This work was supported by the Spanish
Ministerio de Educacio´n y Ciencia (PGE and FEDER funding,
Project CTQ2006-10035) and the “Ramo´n y Cajal” contract
(R.Q.).
Supporting Information Available: Experimental procedures
and characterization data, UV/vis spectroscopy, and crystallographic
data (CIF) for 5b and 4a. This material is available free of charge
via the Internet at http://pubs.acs.org.
IC8008333
5542 Inorganic Chemistry, Vol. 47, No. 13, 2008