Rigid bis(tetrathiafulvalenes) doubly bridged by phosphino groups and derivatives: Synthesis and intramolecular mixed valence state

Article abstract of DOI:10.1021/om900107y

The synthesis and structural characterization of the λ⁵- bis(phosphine sulfide) and the bimetallic complexes bis[phosphino-M(CO) ₅] (M = Mo, W) of the 3,4-dimethyltetrathiafulvalene (orthoDMTTF)-based rigid dimer (PPh)₂(o-

Full text of DOI:10.1021/om900107y

Organometallics 2009, 28, 3691–3699 3691
DOI: 10.1021/om900107y
Rigid Bis(tetrathiafulvalenes) Doubly Bridged by Phosphino Groups and
Derivatives: Synthesis and Intramolecular Mixed Valence State
†
‡
‡
,‡
ꢀ ꢀ
ꢀ §
Ion Danila, Frederic Biaso, Helena Sidorenkova, Michel Geoffroy,* Marc Fourmigue,
E. Levillain,† and Narcis Avarvari*,†
†
ꢀ
ꢀ
ꢀ
Laboratoire Chimie et Ingenierie Moleculaire (CIMA), UMR 6200 CNRS-Universite d’Angers,
‡
UFR Sciences, Batiment K, 2 Boulevard Lavoisier, 49045 Angers, France, Department of Physical
^
Chemistry, University of Geneva, 30 Quai Ernest Ansermet, 1211 Geneva, Switzerland, and §Sciences
ꢀ
Chimiques de Rennes, UMR 6226 CNRS-Universite Rennes I, Campus de Beaulieu, 35042 Rennes, France
Received February 11, 2009
The synthesis and structural characterization of the λ5-bis(phosphine sulfide) and the bimetallic
complexes bis[phosphino-M(CO)5] (M = Mo, W) of the 3,4-dimethyltetrathiafulvalene (ortho-
DMTTF)-based rigid dimer (PPh)2(o-DMTTF)2, containing a central 1,4-dihydro-1,4-diphosphi-
nine ring, are described. Single-crystal X-ray analyses have been performed for the trans isomers
(PhPX)2(o-DMTTF)2 (X = S, Mo(CO)5, and W(CO)5) and for the cis isomer [PhPW(CO)5]2-
(o-DMTTF)2. Planar or slightly folded boat-type conformations are observed for the central six-
membered ring, together with different packings characterized by short intermolecular S
S
3 3 3
contacts. The optical signature of the oxidized species in the case of the free ligand (PPh)2-
(o-DMTTF)2 has been evidenced by UV-vis spectroelectrochemistry measurements. Solution
EPR measurements on the radical cation species of (PPh)2(o-DMTTF)2 definitely assess the full
delocalization of the unpaired electron over both electroactive TTF units, with an associated
coupling of 0.48 G with 12 equivalent protons. The EPR signal of the dication proves the radical
nature of this species, in favor of a triplet ground state. The radical cation of the cis-[PhPW(CO)5]2-
(o-DMTTF)2 isomer was also investigated by EPR, for which the observed hyperfine structure
demonstrates the extended delocalization of the electron, together with a larger coupling constant
with the phosphorus nuclei. DFT calculations for the radical cation of (PPh)2(o-DMTTF)2 afford a
boat-type conformation for the central ring and a SOMO consistent with a full delocalization of the
electron over both TTF units. Moreover, the calculations indicate that in the case of the dication of
(PPh)2(o-DMTTF)2 the triplet state is more stable by 11.7 kcal mol-1 than the singlet state.
Introduction
units, depending on the nature of the linkage between the
TTFs, and thus eventually allowing for the observation and
isolation, in either solution or solid state, of intramolecular
mixed valence species. In the corresponding crystalline radi-
cal cation salts or charge transfer complexes of bis(TTFs),
enhanced dimensionality, due to weaker intersite electro-
One of the most valuable strategies to achieve mixed
valence states in a controlled manner in the field of tetra-
thiafulvalenes (TTF),1 which represent a well-known class of
sulfur-rich organic electron donors extensively investigated
as precursors for molecular metals and superconductors,2
consists in the use of dimeric TTFs.3 Indeed, in such com-
pounds, through-bond or through-space interactions can
promote electronic communication between the redox-active
static repulsions and more numerous S S contacts, might
3 3 3
be observed, providing original architectures and unusual
band structures.4 Besides the direct linking of TTF units
through a single C-C bond,3a,5 a valuable strategy to
connect two TTFs in close vicinity consists in the utilization
of monatomic based linkers, thus allowing access to the
*Corresponding authors. (M.G.) E-mail: Michel.Geoffroy@
chiphy.unige.ch. (N.A.) Fax: (þ33)02 41 73 54 05. E-mail: narcis.
avarvari@univ-angers.fr.
ꢀ ꢁ
(4) (a) Bryce, M. R. J. Mater. Chem. 1995, 5, 1481. (b) Meziere, C.;
(1) (a) Segura, J. L.; Martin, N. Angew. Chem., Int. Ed. 2001, 40, 1372.
(b) Yamada, J.-L. TTF Chemistry: Fundamentals and Applications of
Tetrathiafulvalene; Springer-Verlag: Berlin, 2004.
(2) (a) Williams, J. M.; Ferraro, J. R.; Thorn, R. J.; Carlson, K. D.;
Geiser, U.; Wang, H. H.; Kini, A. M.; Whangbo, M.-H. Organic
Superconductors (Including Fullerenes), Synthesis, Structure, Properties
and Theory; Grimes, R. N., Ed.; Prentice-Hall: Englewoods Cliffs, NJ,
1992. (b) Ishiguro, T.; Yamaji, K.; Saito, G. Organic Superconductors;
Springer-Verlag, Heidelberg, 1998.
(3) (a) Becker, J. Y.; Bernstein, J.; Ellern, A.; Gershtenman, H.;
Khodorkovsky, V. J. Mater. Chem. 1995, 5, 1557. (b) Otsubo, T.;
Aso, Y.; Takimiya, K. Adv. Mater. 1996, 8, 203. (c) Iyoda, M.;
Hasegawa, M.; Miyake, Y. Chem. Rev. 2004, 104, 5085.
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Fourmigue, M.; Canadell, E.; Clerac, R.; Bechgaard, K.; Auban-Senzier,
P. Chem. Mater. 2000, 12, 2250. (c) Garcia-Yoldi, I.; Miller, J. S.; Novoa,
J. J. J. Phys. Chem. A 2009, 113, 484.
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Sato, T.; Kikuchi, K.; Ikemoto, I.; Mori, T. J. Mater. Chem. 1999, 9, 335.
(b) John, D. E.; Moore, A. J.; Bryce, M. R.; Batsanov, A. S.; Leech, M.
A.; Howard, J. A. K. J. Mater. Chem. 2000, 10, 1273. (c) Iyoda, M.;
Hara, K.; Kuwatani, Y.; Nagase, S. Org. Lett. 2000, 2, 2217. (d)
Kageyama, T.; Ueno, S.; Takimiya, K.; Aso, Y.; Otsubo, T. Eur. J.
Org. Chem. 2001, 2983. (e) Iyoda, M.; Hara, K.; Ogura, E.; Takano, T.;
Hasegawa, M.; Yoshida, M.; Kuwatani, Y.; Nishikawa, H.; Kikuchi,
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r
2009 American Chemical Society
Published on Web 06/15/2009
pubs.acs.org/Organometallics

3692 Organometallics, Vol. 28, No. 13, 2009
Danila et al.
general series formulated as X(TTF)2, with X = S or Se,6
Te,7 PPh, SiMe2 or Hg,8 and SbMe,9 where TTF is the parent
compound or substituted derivatives. However, these com-
pounds are inherently flexible, since it exists a priori the
possibility of free rotation to occur around the C-X simple
bonds and, hence, a decrease of the conjugation between the
redox-active sites through the spacer. Therefore, in order to
overcome this structural flexibility, more rigid derivatives
have been synthesized, upon attachment of the TTF units by
a second bridge. Accordingly, in the compounds X2(TTF)2
(X = S10 or Te11), the TTF units are connected through two
linkers, giving rise to a central six-membered ring, which can
be more or less distorted.
Scheme 1. Reactivity of 1
those of the free ligand 1. We report herein the synthesis and
structural characterization of the bis(P-sulfide) derivative of
1, along with bimetallic complexes based on the pentacarbo-
nyl fragments W(CO)5 and Mo(CO)5. Furthermore, the
intramolecular electronic communication between the TTF
units in 1 and its derivatives is investigated through solution
EPR measurements and theoretical calculations.
Results and Discussion
Note that analogous selenium-based donors have been
obtained by the use of two methyl-antimony (SbMe) linkers
between two tetraselenafulvalene (TSF) units.9
Synthesis and Crystal Structures. The synthesis of
(PhP)2(o-DMTTF)2, 1, as a mixture of cis (major, ∼90%)
and trans (minor, ∼10%) isomers, upon lithiation and sub-
sequent trapping with PhPCl2 has been previously de-
scribed.13 In the course of our experiments we noticed that
solutions in THF or toluene of pure cis-1 were slowly
evolving upon moderate heating at 70 °C toward enrichment
in trans-1. This behavior is very likely related to the pyrami-
dal inversion of the phosphines,14 for which the energy
barrier is generally lower when the phosphorus atom bears
aromatic rings and/or unsaturated substituents such as vinyl
or ethynyl groups and can be observed also in the case of
cyclic phosphines.15 The rationale for the weaker inversion
barrier can be explained on the basis of the stabilization of
the planar transition state provided by the overlap between
the lone pairs of the phosphorus atoms and the π vinylic
orbitals. Since the reactions we envisaged were expected to
occur upon thermal activation, we decided to investigate the
reactivity of the cis/trans mixture of 1 and then to eventually
proceed to the separation of the two isomers. The reaction
of 1 with a large excess of sulfur under reflux of THF during
24 h provides a mixture of phosphine sulfides cis-2 and trans-
2 in a ratio of 1.5:1, which is very difficult to separate by
column chromatography (Scheme 1). The progress of the
reaction was followed by 31P NMR spectroscopy, which
indicates a deshielding of the phosphorus atom resonances
when compared to those of cis/trans 1. Indeed, two close
singlets are now observed at þ11.6 ppm (cis) and þ12.0 ppm
(trans), while in the starting ligand 1 the singlets corresponding
to the cis and trans isomers appear at -21.5 and -25.6 ppm,
respectively. The composition of the reaction product mixture,
in which the proportion of trans-2 with respect to trans-1 in the
starting free phosphine is much larger, demonstrates that the
use of pure cis-1 or trans-1 in the sulfuration reaction is not
justified, since an inversion process occurs under these condi-
tions. Only fractions in which one or the other isomer were
dominant could be obtained after chromatographic workup,
yet slow evaporation of a CH2Cl2 solution enriched in trans-2
afforded single crystals of this isomer.
In this respect, we have recently described rigid bis(o-
DMTTFs) (o-DMTTF=ortho-dimethyltetrathiafulvalene),
in which the linkers were either SiMe2 and GeMe2 frag-
ments12 or phenylphosphino (PPh) groups.13 In the sili-
con- and germanium-based dimers we have proved the
extended electron delocalization in the radical cation state,
through theoretical calculations and experimental investiga-
tions, such as EPR spectroscopy, and have isolated the
corresponding mixed valence species in the solid state. On
the other hand, even though the cis isomer of the redox-
active 1,4-diphosphinine cis-(PPh)2(o-DMTTF)2 (cis-1)
shows in solution sequential one-electron oxidation into
radical cation and then dication species, its electrocrystalli-
zation provided a charge-localized crystalline salt with the
[Mo6O19]2- anion, in which one TTF unit was oxidized into
radical cation while the other TTF of the dimer was essen-
tially neutral. Moreover, the phosphorus atoms were found
to be oxidized into phosphine oxide.13 It is obvious that one
of the main interests related to the compound 1, which can
exist as a cis or trans isomer, is related to the reactivity and
the coordination properties of the phosphorus atoms. In-
deed, one might envisage the preparation of λ5-P derivatives
or bimetallic complexes in which the electronic properties of
the redox-active units are expected to vary with respect to
(6) Bryce, M. R.; Cooke, G.; Dhindsa, A. S.; Ando, D. J.;
Hursthouse, M. B. Tetrahedron Lett. 1992, 33, 1783.
(7) (a) Becker, J. Y.; Bernstein, J.; Dayan, M.; Shahal, L. J. Chem.
Soc., Chem. Commun. 1992, 1048. (b) Martin, J. D.; Canadell, E.;
Becker, J. Y.; Bernstein, J. Chem. Mater. 1993, 5, 1199.
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(8) Fourmigue, M.; Huang, Y.-S. Organometallics 1993, 12, 797.
(9) Ashizawa, M.; Yamamoto, H. M.; Nakao, A.; Kato, R. Tetra-
hedron Lett. 2006, 47, 8937.
(10) (a) Khodorkovsky, V. Y.; Becker, J. Y.; Bernstein, J. Synth. Met.
1993, 56, 1931. (b) Aqad, E.; Becker, J. Y.; Bernstein, J.; Ellern, A.;
Khodorkovsky, V.; Shapiro, L. J. Chem. Soc., Chem. Commun. 1994,
2775.
(11) (a) Wang, C.; Ellern, A.; Becker, J. Y.; Bernstein, J. Tetrahedron
Lett. 1994, 35, 8489. (b) Wang, C.; Ellern, A.; Khodorkovsky, V.;
Becker, J. Y.; Bernstein, J. J. Chem. Soc., Chem. Commun. 1994, 2115.
(c) Ojima, E.; Fujiwara, H.; Kobayashi, H.; Kobayashi, A. Adv. Mater.
1999, 11, 1527.
(12) Biaso, F.; Geoffroy, M.; Canadell, E.; Auban-Senzier, P.;
ꢀ
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(13) Avarvari, N.; Fourmigue, M. Chem. Commun. 2004, 2794.
(14) Baechler, R. D.; Mislow, K. J. Am. Chem. Soc. 1970, 92, 3090.
€
Levillain, E.; Fourmigue, M.; Avarvari, N. Chem.;Eur. J. 2007, 13, 5394.
(15) Markl, G.; Zollitsch, T.; Kreitmeier, P.; Prinzhorn, M.; Reithinger,
S.; Eibler, E. Chem.;Eur. J. 2000, 6, 3806.

Article
Organometallics, Vol. 28, No. 13, 2009 3693
Figure 1. View of trans-2 (H atoms omitted) (left) and side view of the molecule (right).
Table 1. Bond Distances and Folding Angles in the Compounds 2-4
trans-2 H2O
trans-3 C7H8
cis-4 CH2Cl2
trans-4 C7H8
3
3
3
3
Average Bond Distances (A)
P-M or P-S
1.940(3)
2.500(13)
2.482(2)
1.999(10)
2.043(10)
1.160(10)
1.146(10)
1.344(9)
1.341(9)
2.494(1)
2.017(5)
2.044(6)
1.137(6)
1.134(6)
1.341(6)
1.339(7)
M-C(CO) axial
M-C(CO) equatorial
CdO axial
2.014(7)
2.046(7)
1.132(8)
1.132(7)
1.344(7)
1.338(6)
CdO equatorial
central CdC of the TTF
diphosphinine CdC
1.342(8)
1.331(8)
Folding Angles (deg)
along S S axis,
internal cycle
27.5(3)
20.4(3)
19.8(3)
17.6(3)
3 3 3
30.9(2)
18.6(1)
0.122(2)
0.335(1)
(same molecule)
11.8(3)
0.0
0.0
0.076(1)
(two molecules)
17.3(3)
14.5(4)
0.208(2)
0.154(2)
(same molecule)
along P P axis
deviation of the P atoms from the mean
plane of the four C atoms of the central ring (A)
0.0
0.289(8)
3 3 3
The bis(phosphine sulfide) trans-2 crystallizes in the
monoclinic system, space group C2/c, with one independent
molecule in the asymmetric unit. Note the presence of a
crystallization water molecule, very likely the consequence of
the aerial evaporation process during which no special
conditions have been taken. The relative trans configuration
of the phenyl and sulfur substituents on the P atoms with
respect to the central six-membered ring was undoubtedly
established, thus allowing also the assignment of the 31P
NMR resonances of the two isomers (Figure 1).
Note the stronger downfield shift in 31P resonances in the
case of the Mo complexes, appearing at þ18.9 (cis) and
þ20.0 ppm (trans), when compared to the W ones, at þ0.9
(cis) and þ0.4 ppm (trans), a feature already observed in the
mononuclear diphosphine [(CO)4M(PPh2)2](o-DMTTF)
(M=Mo, W) complexes.16 The cis/trans mixture of [(CO)5-
Mo(PPh)]2(o-DMTTF)2 (3) could not be separated by column
chromatography, unlike the complexes [(CO)5W(PPh)]2-
(o-DMTTF)2, 4, for which an efficient separation of the cis
and trans isomers could be achieved. However, recrystalliza-
tion of cis/trans 3 from toluene afforded two types of crystals,
which were eventually manually separated. Only the trans-3
crystals proved to be suitable for X-ray diffraction analysis on
single crystals. The complex crystallizes in the triclinic system,
space group P1 (Table 2), with two independent half-mole-
cules, both located on inversion centers, in the asymmetric
unit, which further contains one-half of a crystallization
toluene molecule disordered about the inversion center. The
two molecules of bimetallic complexes A and B differ essen-
Selected bond lengths and angles are given in Table 1. The
C(2)dC(3) and C(10)dC(11) bonds (average 1.342 A), as
well as C(1)dC(8) and C(9)dC(16) (average 1.331 A) of
the central six-membered ring, are comparable with those in
cis-1 and lie in the typical range for neutral TTFs. It is
worthwhile noting the boat-like conformation of the 1,4-
diphosphinine cycle, with a folding angle of 18.6(1)°, which
is much smaller than the one observed in cis-1 (30.4(1)°).
Both TTFs units are strongly folded around the S(1) S(4)
3 3 3
(30.9(2)°) and S(5) S(8) (27.5(3)°) axes. The donors stack
along the c direction (see Supporting Information), with the
tially only by the folding angle along the internal S S hinge,
3 3 3
3 3 3
which amounts to11.8° for A and 20.4° for B, while the central
six-membered ring is perfectly planar for both of them
(Figure 2).
establishment of several intermolecular S S contacts
below 4.0 A, among which the shortest one, of 3.69 A,
corresponds to a PdS SdP interaction.
3 3 3
The complexes A and B stack alternatively along c, with
the setting of intermolecular S S contacts of 4.0 A between
3 3 3
The bimetallic complexes 3 (Mo) and 4 (W) have been
conventionally synthesized upon thermal displacement of
the THF ligand from the starting complexes (THF)M(CO)5
by the phosphine. Longer heating periods were necessary in
the case of the Mo-based complex, less reactive than the W
counterpart; therefore in the final reaction mixtures the ratio
cis/trans was different, as assessed by 31P NMR spectroscopy.
3 3 3
molecules belonging to neighboring columns, while within
the same column the S S distances are much longer
(Figure 3). It is thus clear that the system, despite the
3 3 3
ꢀ
(16) Avarvari, N.; Martin, D.; Fourmigue, M. J. Organomet. Chem.
2002, 643-644, 292.

3694 Organometallics, Vol. 28, No. 13, 2009
Table 2. Crystallographic Data, Details of Data Collection, and Structure Refinement Parameters
Danila et al.
trans-2 H2O
trans-3 C7H8
cis-4 CH2Cl2
trans-4 C7H8
3
3
3
3
formula
MW
cryst syst
space group
a (A)
C28H22O2P2S10
773.00
monoclinic
C2/c
46.8281(9)
10.7262(11)
15.665(3)
90
107.501(8)
90
7503.9(16)
8
1.368
0.697
293(2)
C48.5H34Mo2O10P2S8
1317.07
triclinic
P1
12.408 (2)
13.205(1)
9.364(1)
96.91(1)
104.46(1)
111.17(1)
2786.00(44)
2
C38.5H22Cl2P2O10S8W2
1373.14
triclinic
P1
12.237(2)
13.939(3)
14.864(3)
106.82(3)
105.47(3)
92.66(3)
2317.90(34)
2
C52H38P2O10S8W2
1508.94
monoclinic
P21/c
14.098(3)
12.607(3)
15.831(3)
90
100.44(3)
90
2767.12(15)
2
1.811
b (A)
c (A)
R (deg)
β (deg)
γ (deg)
V (A3)
Z
dcalcd (g cm-3
)
1.570
0.864
293(2)
1.967
5.499
293(2)
μ (mm-1
T (K)
)
4.570
293(2)
λ (A)
0.71073
0.068
0.214
0.71073
0.052
0.152
0.71073
0.037
0.056
0.71073
0.030
0.067
R(Fo)a
Rw(F2o)a
P
P
P
P
a R(Fo) = ||Fo| - |Fc||/ |Fo|; Rw(F2o) = [ [w(F2o - F2c)2]/ [w(F2o)2]]1/2
.
Figure 2. View of trans-3 molecule A (left) and side view of the molecule B (right) showing the planar conformation of the central ring
(H atoms omitted).
As expected, both cis-4 and trans-4 structures show octahe-
dral coordination geometry around the tungsten center,
as in the case of the molybdenum counterpart, with bond
lengths and angles in the normal range. The cis isomer
crystallizes in the triclinic system, space group P1, with one
independent molecule in the asymmetric unit in general
position, while the trans one crystallizes in the monoclinic
system, space group P21/c, with one independent half-mole-
cule in the asymmetric unit (Table 2). Both crystals contain
solvent molecules, i.e., methylene chloride in cis-4 and
toluene in trans-4. The 1,4-dihydrodiphosphinine ring in
the cis complex is less distorted than in the free ligand, with
the corresponding dihedral angle along the P(1) P(2)
3 3 3
hinge amounting now to 14.5(4)°, within a boat conforma-
tion (Figure 4). On the other hand, the trans isomer crystal-
lizes in a chairlike conformation, more pronounced than in
trans-3, with an essentially planar 1,4-dihydro-1,4-dipho-
sphinine ring, in which the P atoms lie 0.289(8) A above
and below the plane defined by the four carbon atoms of the
six-membered ring (Figure 4).
Figure 3. Packing diagram of trans-3 in the plane ab. The
shortest S S intermolecular interactions are established be-
3 3 3
tween donors of parallel columns along c.
important steric hindrance provided by the aromatic
rings and organometallic fragments, still accommodates
intermolecular short interactions between the redox-active
units.
Interestingly, the tungsten bimetallic complexes cis-4 and
trans-4 have different elution behavior, and thus they were
readily separated by column chromatography. Moreover,
suitable single crystals for X-ray analysis could be grown for
both isomers. Although the crystallization conditions were
basically the same, trans-3 and trans-4 are not isostructural.
Intermolecular S S distances amounting to 3.89-3.92
3 3 3
A are established in the structure of cis-4 through stacking of
molecules belonging to two parallel columns (Figure 5),
while these contacts are much longer (4.48 A) in the case of
trans-4 (Supporting Information).
It is interesting to note once again that the rigid bis(TTFs) can
accommodate rather short S S contacts thanks to the length
3 3 3
of the molecule overcoming the central steric hindrance due to
the metal-carbonyl fragments and the phenyl rings. Note that
these latter engage in π-π intermolecular interactions.

Article
Organometallics, Vol. 28, No. 13, 2009 3695
Figure 4. Molecular structure of cis-4 (left) and side view of trans-4 (right) showing the distortion of the central ring (H atoms omitted,
Ph groups omitted in trans-4).
Figure 6. UV-vis spectroelectrochemistry of cis-1.
intense one, occurring at lower wavelength.18 Note that these
Figure 5. Packing diagram of cis-4. Voids between TTF frag-
ments along the stacking direction are filled with solvent mole-
bands in our case are red-shifted with respect to those of the
parent TTF, and they compare very well with the experi-
cules (H atoms and solvent omitted).
mental values measured for the radical cation of the tetra-
Electrochemical Studies. As previously described,13 cyclic
voltammetry measurements performed on the cis-1 ligand
show splitting of both oxidation waves, consistent with the
stepwise formation of the radical cation, dication, trication,
and tetracation species upon reversible one-electron oxida-
tion processes. The potential difference ΔE amounts to 120
mV between the first two oxidation peaks and also between
the oxidation processes three and four. In order to get more
insight into the formation of the oxidized species of cis-1, we
have first carried out UV-vis spectroelectrochemistry mea-
surements.
Thin-layer cyclic voltammetry measurements (TLCV) re-
vealed that both the first and second pair of oxidation waves
correspond to two one-electron processes (see Experimental
Section). At potentials below 0.4 V one can observe only the
typical optical signature of a neutral TTF, with a λmax at 338
nm (Figure 6 and Supporting Information).17 Then, once the
radical cation is generated, two new absorption bands
centered at λmax 463 and 652 nm appear, with concomitant
disappearance of the band at 338 nm. These two main
absorptions are typical for TTF radical cation species and
have been assigned in the case of the simple TTFþ• radical
cation to a HOMO-1 f SOMO transition for the less
energetic one and a SOMO f π* transition for the most
methyltetrathiafulvalene (TMTTF).17a,19 Upon increasing
the potential up to 0.8 V, where very likely only the dicationic
[cis-1]2þ species is present, there is no noticeable difference in
the position of the bands between the mono- and the dica-
tion. This suggests that the dication also possesses radical
character, since the absorption spectra in the potential range
0.4-0.8 V are characteristic for TTF radical cations. By
further increasing the potential, the second couple of oxida-
tion processes starts to occur, the most significant change in
the optical spectra being the disappearance of the low-energy
band together with the blue shift of the high-energy band
toward λmax = 428 nm. The apparent lack of differences
between the absorption spectra of the tri- and tetracation is
quite surprising, when considering the likely radical nature
of the trication and the singlet ground state of the tetraca-
tion, by analogy with a TTF2þ. No absorption band was
observed beyond 850 nm at any potential, thus ruling out the
occurrence of intermolecular π-dimers. Moreover, neither
concentration nor scan rate dependence of the spectra was
detected.
Cyclic voltammetry measurements for the compounds 2-
4 reveal first that the transformation in the λ5-P derivative or
complexation does not perturb the electron donor properties
of the TTF moieties. Indeed, the oxidation potentials
are only slightly shifted toward more anodic values (Sup-
porting Information). Moreover, only splitting of the two
first oxidation processes is now observed, with ΔE values of
(17) (a) Torrance, J. B.; Scott, B. A.; Welber, B.; Kaufman, F. B.;
Seiden, P. E. Phys. Rev. B 1979, 19, 730. (b) Huchet, L.; Akoudad, S.;
€
Levillain, E.; Roncali, J.; Emge, A.; Bauerle, P. J. Phys. Chem. B 1998,
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M. J. Phys. Chem. A 2002, 106, 631.
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, E.; Merchan, M.; Rubio, M.; Viruela, P.
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G.; Gorgues, A.; Giffard, M. Chem. Commun. 2001, 2736.

3696 Organometallics, Vol. 28, No. 13, 2009
Danila et al.
90 mV for 2 and 105 mV in the case of 3 and 4. Subsequent
oxidation into 3þ and 4þ species is accompanied by adsorp-
tion phenomena, which probably hinder the observation of
wave splitting.
EPR Measurements. In order to investigate the electron
delocalization in cis-1 radical cation and dication species,
which are electrochemically available at rather low oxida-
tion potentials, we have performed solution EPR measure-
ments coupled with electrochemistry. Note that the EPR
technique allowed us to undoubtedly prove the full electron
delocalization in the case of the radical cations of silicon- and
germanium-bridged bis(TTFs) (XMe2)2(o-DMTTF)2 (X=
Si, Ge).12
The hyperfine structure observed after oxidation of cis-1
(Figure 7) is clearly consistent with the central part of a
spectrum exhibiting a coupling of 0.48 G with 12 magneti-
cally equivalent protons. Indeed, even though only 11 of the
13 expected lines are observed, the intensity distribution
follows accurately that of the simulated spectrum (Support-
ing Information). As confirmation of this analysis, a single
hyperfine coupling constant (Aiso=1.32 MHz) is detected on
the 1H ENDOR spectrum (inset Figure 7). We can therefore
confidently assign the hyperfine structure to the coupling
with four methyl groups. Moreover, the corresponding
isotropic constant amounts to half the value of the one
measured on the radical cation of (o-DMTTF)(PPh2)2 di-
phosphine, containing only two Me groups,20 and it com-
pares very well with that of 0.42 G observed in the case of
[(XMe)2(o-DMTTF)2]þ• (X=Si, Ge).12 This clearly demon-
strates that in [cis-1]þ• the unpaired electron is delocalized
over both TTF units, and hence a genuine intramolecu-
lar mixed valence state is generated, with each TTF bearing
a δ=þ0.5 mean charge. Note that, as also observed in the
case of the diphosphine (o-DMTTF)(PPh2)2, no feature
of the hyperfine structure is due to a coupling with the
phosphorus atoms, a likely proof that no spin density is
present on these P atoms. The same EPR spectrum is
observed when the oxidation is carried out chemically, with
1 equiv of NOBF4.
Further oxidation at higher potentials, or with 2 equiv of
chemical oxidant, such as to generate the dication of cis-1,
provokes a drastic modification of the spectrum: strong
additional lines are observed leading to a rather complex
spectrum (Supporting Information), which could not be
reasonably simulated. In frozen solution, no half-field signal,
characteristic for triplet species, could be detected, yet this is
not surprising when considering the delocalized nature of our
radicals. However, it is likely that no decomposition occurs
since this new spectrum shows reversibility to the previous
one, of [cis-1]þ•, upon lowering the potential, thus demon-
strating once again the radical nature of the [cis-1]2þ species.
Since among the derivatives 2-4, only the tungsten-based
bimetallic complex 4 could be separated as cis and trans
isomers by column chromatography, we have performed
EPR investigations on cis-4. The spectrum of [cis-4]þ• (Fig-
ure 8), electrochemically generated in a CH2Cl2 solution,
appears to be more complex than that of the free ligand. The
spectrum of such a large and fluxional radical ion is difficult
to simulate since the shape of the lines is likely sensitive
to internal motions. Moreover, as a consequence of the
Figure 7. EPR spectrum of [cis-1]þ• (CH2Cl2 sol., (TBA)PF6
0.2 M, E = þ0.4 V vs Ag, T = 300 K, ν=9426 MHz,); giso
=
2.008, Aiso = 0.48 G. Inset: ENDOR H of cis-1 oxidized by
1
1 equiv of NOBF4 in CH2Cl2; T=270 K, ν=9593 MHz, Aiso
1.32 MHz.
=
presence of metallic centers, the g-value (2.010) is appreci-
ably larger than for the free ligand; the probable small
anisotropy of the g-tensor is expected to also contribute to
the complexity of the spectrum of this slowly tumbling
radical. An acceptable simulation could nevertheless be
obtained by considering an isotropic coupling of 0.62 G with
12 equivalent protons and a coupling of 0.37 G with two
equivalent 31P nuclei.
It is clear that the functionalization at the P atoms induces
some changes in the spin distribution in the radical species of
2-4 with respect to that in the oxidized ligand [1]þ•; yet it is
very likely that full delocalization occurs in all the cases.
Theoretical Calculations. In order to have a deeper under-
standing of the electronic structures and to rationalize the
EPR results, we have performed unrestricted DFT calcula-
tions with the Gaussian03 package21 by using the B3LYP
functional22,23 and the 6-31þG* basis set for both the
geometry optimization and the prediction of the electronic
properties of (cis-1)þ•. For the sake of comparison, calcula-
tions have also been carried out using other functionals and
basis sets (see Experimental Section); the resulting values are
reported in the Supporting Information.
In contrast with the crystal structure of neutral cis-1,
which indicates a boat-type conformation of the central
(21) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.;
Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.;
Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson,
G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo,
C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin,
A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.;
Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.;
Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas,
O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.;
Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov,
B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.;
Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;
Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.;
Gonzalez, C.; Pople, J. A. Gaussian 03, Revision B.03; Gaussian, Inc.:
Pittsburgh, PA, 2003.
(20) Gouverd, C.; Biaso, F.; Cataldo, L.; Berclaz, T.; Geoffroy, M.;
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Levillain, E.; Avarvari, N.; Fourmigue, M.; Sauvage, F. X.; Wartelle, C.
Phys. Chem. Chem. Phys. 2005, 7, 85.
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(23) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785.

Article
Organometallics, Vol. 28, No. 13, 2009 3697
Figure 9. Optimized geometry of [cis-1]þ• (left) and SOMO
(right).
ably lies in the vibrational effects. There are 174 vibration
modes for 1þ•, and we have calculated the effect of some of
these modes on the 31P coupling constants by following the
method of Barone.26 It is found that most of these modes do
not affect the 31P-Aiso values; for some of them, however, a
slight decrease of the coupling is indeed calculated. This is the
case, for example, for the vibration at 1435 cm-1, corre-
sponding to an antisymmetrical combination of stretching of
the TTF double bonds, which leads to a decrease of 0.10 G of
the absolute value of 31P-Aiso. An exhaustive investigation
of all vibrations for such a large system is beyond the scope of
this work. It seems nevertheless reasonable to admit that the
real 31P hyperfine splitting is very close to zero. Moreover, it
should be remarked that in the radical cation of (o-DMTTF)-
Figure 8. Experimental (black) and simulated (red) EPR spec-
tra of [cis-4]þ• (CH2Cl2 sol., (TBA)PF6 0.2 M, E = þ0.45 V vs
31
Ag, T=300 K, ν=9426 MHz); giso=2.010, Aiso(1HMe)=0.62 G,
A
( P)=0.37 G.
iso
dihydrodiphosphine ring (dihedral angle CTTF1PPCTTF2
=
-154.2°), a previous optimization of this molecule13 had led
to a local minimum (Min 1, point group C1) characterized by
an almost planar conformation of the central ring (dihedral
angle CPPC = -177.1°). New optimization of this molecule
shows, however, that a second energy minimum (Min 2,
point group C2) is present. This energy minimum, Min 2, is
20
(PPh2)2 the calculated isotropic hyperfine coupling with the
phosphorus atoms is only 0.14 G, and hence not experimen-
tally observed, which is indicative of the variation of this
constant with the mutual orientation of the TTF and phosphino
groups.
hardly more stable than Min 1 (-0.75 kcal mol-1); in the
3
corresponding structure the central ring adopts a boat con-
formation and is strongly folded along the P P hinge
3 3 3
(dihedral angle CPPC=-138.3°) (Supporting Information).
The presence of these three structures for cis-1 suggests that
exchange processes likely occur in solution.
Furthermore, we have undertaken a theoretical study on
the dication [cis-1]2þ species, with the aim to estimate the
relative stability of the triplet and singlet states. Accordingly,
the calculations afford a triplet state that is more stable than
the singlet one by 11.7 kcal mol-1, with an overall equilib-
rium geometry quite similar to that of the radical cation, i.e.,
boat conformation of the central ring (folding angle
-129.1°) and planarity of the TTF units (Supporting In-
formation). These theoretical results are in agreement with a
full delocalization of the electron in the radical monocation
and with the radical character of the dication, as suggested by
the UV-vis and EPR spectroelectrochemistry measure-
ments (vide supra).
Geometry optimization of the radical monocation [cis-
1]þ• affords an equilibrium geometry characterized by a
distorted central six-membered ring within a boat-type con-
formation, with a folding angle along the P P hinge of
3 3 3
-135.9° and planar TTF fragments directed toward the
phenyl rings (Figure 9 and Supporting Information). The
singly occupied orbital (SOMO), delocalized on the two
redox-active units, with negligible contribution of P atoms,
consists of the antisymmetrical combination of TTF π
orbitals (Figure 9), while the SOMO-1 corresponds to the
symmetrical combination of these orbitals.
The calculated isotropic hyperfine coupling constant with
the lateral methyl protons amounts to 0.47 G, in agreement
with the experimental value of 0.48 G (vide supra). The
predicted 31P-Aiso values are equal to -1.27 G, while no
coupling with phosphorus is observed on the spectrum. This
discrepancy is likely caused by the well-known difficulty to
accurately predict the Fermi contact interaction with 31P
nuclei.24,25 The calculation of 31P-Aiso is especially critical
for 1þ• since for this radical there is no contribution of the
phosphorus orbitals to the SOMO and the 31P isotropic
coupling results only from spin polarization. Related to this
difficulty, the predicted couplings show some dependence
upon the functional and basis sets used for the calculations
(Supporting Information): for example at the BLYP/6-31G*
level the 1H and 31P couplings are equal to 0.6 and -0.8 G,
respectively. Another cause for the slight difference between
experimental and calculated 31P coupling constants prob-
Conclusions
In this study we have demonstrated first the versatility
of the redox-active bis(TTF)-1,4-dihydro-1,4-diphosphi-
nine 1, affording λ5-P derivatives or bimetallic complexes.
Single-crystal X-ray structures of some of these compounds
undoubtedly prove their cis or trans stereochemistry and
demonstrate a certain flexibility of the central six-mem-
bered ring, together with various supramolecular architec-
tures characterized by short intermolecular contacts, an
important prerequisite in view of preparing radical cation
salts or charge transfer complexes possessing conducting
and/or magnetic properties. UV-vis spectroelectrochem-
istry measurements on the cis-1 isomer allowed us to detect
the optical signature of the oxidized species up to the
tetracation. EPR investigations on the same compound,
coupled with theoretical calculations at the DFT level,
definitely prove the electron delocalization over both TTF
(24) Nguyen, M. T.; Creve, S.; Eriksson, L. A.; Vanquickenborne, L.
G. Mol. Phys. 1997, 91, 537.
(25) Barone, V.; Cimino, P.; Stendardo, E. J. Chem. Theory Comput.
2008, 4, 751.
(26) Barone, V.; Grand, A.; Minichino, C.; Subra, R. J. Chem. Phys.
1993, 99, 6787.

3698 Organometallics, Vol. 28, No. 13, 2009
Danila et al.
units in the radical cation [cis-1]þ• and the radical nature
of the dication [cis-1]2þ, for which the most stable ground
state is the triplet. EPR measurements demonstrate also the
electron delocalization in the bimetallic W-based complex
[cis-4]þ•. Current investigations in our groups deal with the
preparation of radical cation salts derived from these
new functional donors and also with the coordination
chemistry of the redox-active ligand 1 with various meta-
llic fragments, possibly endowed with magnetic or optical
properties.
X-ray Structure Determinations. Details about data collection
and solution refinement are given in Table 2. X-ray diffraction
measurements were performed on a Bruker Kappa CCD dif-
fractometer for trans-2 and trans-3 and on a Stoe Imaging Plate
System for cis-4 and trans-4, both operating with a Mo KR (λ=
0.71073 A) X-ray tube with a graphite monochromator. The
structures were solved (SHELXS-97) by direct methods and
refined (SHELXL-97) by full-matrix least-squares procedures
on F2.27 All non-H atoms of the donor molecules were refined
anisotropically, and hydrogen atoms were introduced at calcu-
lated positions (riding model), included in structure factor
calculations but not refined. For the structure trans-3 the carbon
atoms of the disordered toluene molecule were refined isotropi-
cally, while the corresponding hydrogen atoms were not intro-
duced. In the structure of cis-4 the methylene chloride molecule
is also highly disordered; therefore the carbon atom could not be
assigned. Finally, in the structure of trans-4 the crystallization
toluene molecule was not disordered; therefore the carbon
atoms were refined anisotropically, and hydrogen atoms were
introduced at calculated positions (riding model), included in
structure factor calculations but not refined. Crystallographic
data for the structures have been deposited in the Cambridge
Crystallographic Data Centre, deposition numbers CCDC
720073 (trans-2), CCDC 720074 (trans-3), CCDC 239192 (cis-
4), and CCDC 239191 (trans-4). The quality of the crystals of
cis-3 (thin needles) was not sufficient for an accurate determina-
tion of its structure; however the cell parameters could be
determined as follows: cis-3, red thin needles, triclinic system,
a=8.694(10) A, b=24.416(198) A, c=26.493(86) A, R = 63.24
(11)°, β = 89.88(16)°, γ=89.42(24)°, V=5021.16 A3.
Experimental Section
General Procedures. Reactions were carried out under nitro-
gen, THF was distilled from Na/benzophenone, and (THF)M-
(CO)5 (M=Mo, W) was freshly prepared by irradiation of M-
(CO)6 solutions in THF. A Stahler TNN 15/32 lamp was used,
with λ=254 nm and power consumption of 15 W. NMR spectra
were recorded on a Bruker Avance DRX 500 spectrometer
operating at 500.04 MHz for 1H, 125.75 MHz for 13C, and
202.39 MHz for 31P. Chemical shifts are expressed in parts per
million (ppm) downfield from external TMS. The following
abbreviation is used: s, singlet (NMR) or strong (IR). Elemental
analyses were performed by the “Service d’Analyse du CNRS”
at Gif/Yvette, France. Compound 1 was synthesized according
to the published procedure.13
Synthesis of (PhPS)2(o-DMTTF)2 (2). A large excess of sulfur
(360 mg, 11.2 mmol) was added to a solution of 1 (97 mg,
0.14 mmol) in THF (50 mL). The mixture was heated under
reflux until the disappearance of the 31P NMR signal of the
starting material (about 24 h). The solvents were then evapo-
rated, and the resulting solid was washed several times with
pentane. After chromatographic purification on a silica gel
column with methylene chloride/cyclohexane (1:1) as eluent, a
mixture of cis-2/trans-2 (1.5:1) was obtained as an orange solid
(80 mg, 72%). Crystals of trans-2, suitable for X-ray crystal-
lographic studies, were obtained upon evaporation of a methy-
lene chloride solution. 31P NMR (THF): δ 11.6 (s, cis), 12
(s, trans). Anal. Calcd for C28H22P2S10: C, 45.37; H, 2.99.
Found: C, 45.59; H, 2.85.
Electrochemical Studies. Cyclic voltammetry measurements
were performed using a three-electrode cell equipped with a
platinum millielectrode of 0.126 cm2 area, a silver wire pseudor-
eference electrode, and a platinum wire counter electrode. The
potential values were then readjusted with respect to the satu-
rated calomel electrode (SCE), using ferrocene as internal
reference. The electrolytic media involved a 0.1 mol L-1 solu-
3
tion of (n-Bu4N)PF6 in CH2Cl2. All experiments have been
performed at room temperature at 0.1 V s-1. Experiments have
3
been carried out with an EGG PAR 273A potentiostat with
positive feedback compensation.
Spectroelectrochemistry. Spectroelectrochemical experiments
on cis-1 were performed in a cell made of Teflon, in thin-layer
cyclic voltammetry (TLCV) conditions.28 The number of the
electrons involved in the redox processes has been precisely
determined by the use of dichloronaphtoquinone as internal
standard. A 2 mm diameter stationary Pt disk was used as the
working electrode and a Pt wire as counter electrode. A Lambda
19 Perkin-Elmer spectrophotometer was employed. The current
was provided by an EGG PAR 273A potentiostat; a scan rate
of 1.25 mV s-1 was used. HPLC-grade methylene chloride
was used as solvent and (n-Bu4N)PF6 (0.4 M) as supporting
electrolyte.
EPR Measurements. EPR and ENDOR spectra were re-
corded on a Bruker ESP 300 spectrometer (X-band) equipped
with a variable-temperature attachment B-VT-2000. Electro-
chemical oxidations at a controlled potential were performed by
using a homemade electrolytic cell. This quartz cell, similar to
the cell described by Fernando et al.,29 contains a silver wire as a
pseudo reference electrode; the working and counter electrodes
are in platinum. This cell can be positioned inside the variable-
temperature insert and allows measurement of EPR spectra at a
controlled voltage over a wide range of temperatures. All
measurements are carried out under a nitrogen atmosphere with
Synthesis of [(PhP)Mo(CO)5]2(o-DMTTF)2 (3). A solution of
(THF)Mo(CO)5 (38 mL), obtained by UV irradiation for 55 min
of Mo(CO)6 (2.64 g, 10 mmol) in THF (300 mL), was added to 1
(84 mg, 0.12 mmol) in THF (10 mL), and the mixture thus
obtained was heated at 50 °C for 8 h. After evaporation of
solvents, the crude product was purified by column chromatog-
raphy with silica gel (cyclohexane/toluene, 1:1) to afford 3 as a
brown solid (cis/trans 3:1) (70 mg, 49%). Different types of
crystals for the two isomers were obtained in toluene at low
temperature, and the mechanical separation of the isomers was
possible. Indeed, cis-3 crystallized as thin needles, while trans-3
crystallized as prisms. Only the crystals of trans-3 were suitable
for X-ray crystallographic studies. 31P NMR (THF): δ 18.9 (s;
cis), 20 (s, trans). IR (cm-1, KBr): 2077 s (CO), 2032 s (CO), 1954
s (CO). Anal. Calcd for C38H22Mo2O10P2S8: C, 39.72; H, 1.93.
Found: C, 39.58; H, 1.81.
Synthesis of [(PhP)W(CO)5]2(o-DMTTF)2 (4). 1 (0.1 g,
0.15 mmol) was stirred with an excess of (THF)W(CO)5,
obtained by UV irradiation for 65 min of W(CO)6 (3.52 g, 10
mmol) in THF (300 mL), at 50 °C during 4 h in THF. The
resulting isomeric bimetallic complexes (cis/trans 6:1) were
separated by column chromatography with a hexane/toluene
(60:40) mixture, eluting first the trans, then the cis isomer.
Overall yield: 0.18 g (92%). The trans isomer was recrystallized
from toluene, whereas the cis one was crystallized by slow
diffusion of pentane into a CH2Cl2 solution. 31P NMR (THF):
δ 0.9 (s, cis), 0.4 (s, trans). IR (cm-1, KBr): 2078 s (CO), 2034 s
(CO), 1928 s (CO). Anal. Calcd for C38H22O10P2S8W2: C, 34.45;
H, 1.68. Found: C, 34.24; H, 1.54.
(27) Sheldrick, G. M. Programs for the Refinement of Crystal
€
€
Structures; University of Gottingen: Gottingen, Germany, 1996.
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(29) Fernando, K. R.; McQuillan, A. J.; Peake, B. M.; Wells, J. J.
Magn. Reson. 1986, 68, 551.

Article
Organometallics, Vol. 28, No. 13, 2009 3699
rigorously degassed solutions. Chemical oxidations were per-
formed under a nitrogen atmosphere in a glovebox.
and the TZVP basis set (Supporting Information). Mole-
cular orbitals were represented by using the GaussView
program.31
Computational Details. Geometry optimization as well
as hyperfine coupling calculations were performed with the
Gaussian03 package21 using the B3LYP functional22,23 and
the 6-31þG* basis sets. Minima were characterized with har-
monic frequency calculations (no imaginary frequencies).
As reported in the Supporting Information, additional single-
point calculations were also run at the optimized geometry using
different functionals (LDA, BP86, BLYP, PBEPBE,
PW91PW91, B1LYP) and basis set functions (6-31G*, aug-
cc-pVDZ, N07D). In another set of calculations, the geometry
of the radical monocation was optimized with the Turbomole
package30 (B-P86 functional and SV(P) standard basis set), and
the properties were calculated with the B3LYP functional
Acknowledgment. Financial support from the Ministry
of Education and Research (grant to I.D.) and the French
Ministry of Foreign Affairs through a Germaine de Stael
::
2006-2007 (PAI 10613RJ) project is gratefully acknowl-
edged. This work was also supported by the CNRS
(France) and the Swiss National Science Foundation
(Switzerland).
Supporting Information Available: X-ray crystallographic
data for trans-2, trans-3, cis-4, and trans-4 in CIF format and
additional structural figures with bond distances and angles;
cyclic voltammetry curves; EPR details and figures; and theore-
tical calculation details. This material is available free of charge
via the Internet at http://pubs.acs.org.
€
€
(30) Ahlrichs, R.; Bar, M.; Haser, M.; Horn, H.; Kolmel, C. Chem.
€
Phys. Lett. 1989, 162, 165.
(31) GaussView 3.0; Gaussian Inc.: Pittsburgh, PA.