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Coordinated o-Dithio- and o-Iminothiobenzosemiquinonate(1) Radicals in [MII(bpy)(L.)](PF6) Complexes

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Macromolecules 1999, 32, 6989; c) R. Schmidt, T. Zhao, J.-B.
Green, D. J. Dyer, Langmuir 2002, 18, 1281.
O. Prucker, J. Habicht, I.-J. Park, J. R竓e, Mater. Sci. Eng. C
1999, 8� 291.
A. G?lzh?user, W. Eck, W. Geyer, V. Stadler, Th. Weimann, P.
Hinze, M. Grunze, Adv. Mater. 2001, 13, 806.
W. Geyer, V. Stadler, W. Eck, A. G?lzh?user, M. Grunze, M.
Sauer, T. Weimann, P. Hinze, J. Vac. Sci. Technol. B 2001, 19,
W. Eck, V. Stadler, W. Geyer, M. Zharnikov, A. G?lzh?user, M.
Grunze, Adv. Mater. 2000, 12, 805.
A. G?lzh?user, W. Geyer, V. Stadler, W. Eck, M. Grunze, K.
Edinger, T. Weimann, P. Hinze, J. Vac. Sci. Technol. B 2000, 18,
M. J. Lercel, H. G. Craighead, A. N. Parikh, K. Seshadri, D. L.
Allara, Appl. Phys. Lett. 1996, 68, 1504.
O. Nuyken, R. Weidner, Adv. Polym. Sci. 1986, 73�, 147, and
references therein.
N. Fery, R. Hoene, K. Hamann, Angew. Chem. 1972, 84, 359;
Angew. Chem. Int. Ed. 1972, 11, 337.
Another strategy to obtain this is to immobilize both termini of
the azo compound: a) G. Boven, M. L. C. M. Oosterling, G.
Challa, A. J. Schouten, Polymer 1990, 31, 2377; b) E. Carlier, A.
Guyot, A. Revillon, M.-F. Llauro-Darricades, R. Petiaud, React.
Polym. 1991, 16, 41.
It is suspected that the resonance-stabilized methylmalonodinitrile radical acts as a reversible termination agent in the
polymerization (P. C. Wieland, O. Nuyken, M. Schmidt, K.
Fischer, Macromol. Rapid Commun. 2001, 22, 1255). Related
studies are currently underway in our laboratories.
The external reflection FTIR spectra of the purified surfaces
(Soxhlet extraction, toluene, 12 h) confirmed that in all cases a
layer of covalently attached polystyrene was formed (typical
absorption bands: (n? in cm1): CH aromatic stretching: 3102,
3018, 3060, 3026, 2999; CH aliphatic stretching: 2922, 2848;
overtones of CH out-of-plane fundamentals for monosubstituted
aromatic rings: 1945, 1867, 1795; ring breathing: 1602, 1493,
1452, CH deformation modes (out-of-plane): 758, 698.
Quantitative analysis of the film thickness as a function of the
polymerization time and irradiation conditions are currently
underway. However, the film thickness steadily increases with
the time of polymerization, although not as rapidly as reported
in reference [17] for the photoinitiated polymerization in the
presence of a similar initiator.
The stencil mask was obtained from Quantifoil Micro Tools,
Jena, Germany.
Georg Albert PVD-Coatings, Heidelberg, Germany (
Methylmalonodinitrile was obtained by ammonolysis (R. Meyer,
P. Bock, Justus Liebigs Ann. Chem. 1906, 347, 98) and
subsequent dehydration (S. Starck, Ber. Dtsch. Chem. Ges. A
1934, 67, 42) of methylmalonic acid diethyl ester.
Radical Anion Complexes
Coordinated o-Dithio- and o-Iminothiobenzosemiquinonate(1) p Radicals in
[MII(bpy)(LC)](PF6) Complexes**
Prasanta Ghosh, Ameerunisha Begum, Diran Herebian,
Eberhard Bothe, Knut Hildenbrand,
Thomas Weyherm竘ler, and Karl Wieghardt*
Dedicated to Professor Gottfried Huttner
on the occasion of his 65th birthday
o-Benzosemiquinonate(1) p-radical anions are archetypical
open-shell, bidentate ligands in the coordination chemistry of
transition-metal ions[1] but for their o-dithio derivatives the
situation is not clear. The existence of S,S-coordinated
o-dithiobenzosemiquinonate(1) radical anions (rather than
their closed-shell, aromatic o-dithiolate dianions) has occasionally been suggested[2] to occur in some complexes but?at
times?has also been explicitly denied.[3] Clear experimental
evidence is lacking to date. Similarly, recently it has been
shown indirectly that o-iminobenzosemiquinonate(1) p
radical ions[4] and their sulfur analogues[5] as well as
o-diiminobenzosemiquinonate(1)[6] p radical anions
(Scheme 1) are coordinated in diamagnetic, square-planar
complexes [NiII(XC)2]. Density functional theoretical (DFT)
calculations have shown that the electronic structure of these
species is in accord with the description as singlet diradicals.[7]
Here we report the synthesis and characterization of
diamagnetic square-planar neutral complexes of PdII and PtII
containing a single aromatic (L)2 ligand and a 2,2?-bipyridine
ligand (Scheme 1): [MII(bpy)(X)]0 (1�. We show that
complexes 1�undergo a reversible, ligand-centered oneelectron oxidation yielding paramagnetic complexes
[MII(bpy)(XC)](PF6) (1 a�a) with a S � 1=2 ground state and
a coordinated p radical anionic ligand (LC).
The following complexes have been isolated as solid
materials: [PdII(bpy)(A)] (1), [PdII(bpy)(AC)](PF6) (1 a),
[PdIIbpy)(BC)](PF6) (2 a), [Pd(bpy)(C)] (3), [PdII(bpy)(D)]
(4), [PdII(bpy)(DC)](PF6) (4 a), [PtII(bpy)(E)] (5), and
[PtII(bpy)(EC)](PF6) (5 a).
The reaction of [M(bpy)Cl2] (M � Pd, Pt) and the
respective ligand H2[A], 篐2[E] (1:1) in CH3CN or THF in
the presence of two equivalents of a base (NaOCH3, BH4, or
NEt3) under argon led to the formation of the neutral species
1, 3, and 5, which were isolated as dark blue needles (1) or
dark green crystals (3 and 5). The oxidized species 1 a (dark
[*] Prof. Dr. K. Wieghardt, Dr. P. Ghosh, Dr. A. Begum, Dr. D. Herebian,
Dr. E. Bothe, Dr. K. Hildenbrand, Dr. T. Weyherm竘ler
Max-Planck-Institut f竢 Strahlenchemie
Stiftstrasse 34-36, 45470 M竘heim an der Ruhr (Germany)
Fax: (� 49) 208-306-3952
[**] This work was supported by the Fonds der Chemischen Industrie.
P. Ghosh thanks the Alexander von Humboldt Foundation for a
stipend. MII � Pd, Pt; bpy � 2,2?-bipyridine.
Angew. Chem. Int. Ed. 2003, 42, No. 5
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Scheme 1. Structures and redox scheme of the ligands L2 employed.
green) and 5 a (orange) were prepared by oxidation of the
neutral complexes 1 and 5 in CH2Cl2 with one equivalent of
ferrocenium hexafluorophosphate; 2 a was obtained by oxidation with air of the reaction mixture of 2 (after addition of
[N(nBu)4](PF6)). Complexes 4 and 4 a were described previously.[8] Owing to its extreme O2-sensitivity it was not
possible to prepare a pure sample of 2; 3 a was generated
electrochemically at 20 8C in CH2Cl2 solution and frozen
immediately with liquid nitrogen.
The crystal structures of 1, 5 (Figure 1), and 2 a (Figure 2)
were determined by X-ray structure analysis; those of 4, 4 a,[8]
and [PdII(bpy)(C6H4S2)][9] as an analogue of 3 are known.
Table 1 summarizes the bond lengths in the coordinated
dianions L2 and in their oxidized p-radical anions(LC). It is
clearly established, that the same geometric changes occur
upon oxidation of L2 to (LC): 1) In the dianions L2 the six
CC bonds of the ring are equidistant within 3s but in the
corresponding radicals these rings adopt a quinoid-type
structure with two alternating short CC distances (ca.
Figure 1. Structures of the neutral molecules in crystals of 1H3CN
(top) and 5 (bottom). Selected bond lengths [鋆 in 1: Pd-O(1)
1.973(4), Pd-O(2) 1.959(4), Pd-N(1) 2.001(5), Pd-N(2) 1.999(5) and in
5: Pt-N(1) 1.950(3), Pt-N(21) 2.013(3), Pt-N(32) 2.048(3), Pt-S(1)
1.37 �) and four longer CC distances (ca. 1.41 �). This is
also true for the o-aminothiophenolates (EC) and, as shown
here for the first time, for the o-dithiobenzosemiquinonates
(CC). 2) The CX and CY bonds are always ~ 0.04 � longer
in X,Y-coordinated dianions (single bonds) than in the radical
anions (double-bond character).
Table 1: Bond lengths [鋆 of X,Y-coordinated dianions L2 and of their radical anions (LC).
[Pd(AC)2] [16]
[W(CO)3(HNC6H4NH)]2 [17]
4 a[8]
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Angew. Chem. Int. Ed. 2003, 42, No. 5
The structure of 2 a is interesting because two ion pairs of
[Pd(bpy)(BC)]PF6 form a ?dimer? through p-stacking of two
(BC) radical anions (Figure 2). The centroid of the sixmembered ring of one (BC) ligand is only 2.923 � above the
carbon atom C(1) of the second ring. This accounts for the
observed diamagnetism of 2 a in the solid state.[10] In contrast,
in CH2Cl2 solution 2 a is paramagnetic (see below).
Figure 3 displays the cyclic voltammograms of 1, 2 a, 3,
and 5 in CH2Cl2 (0.1m [(nBu)4N](PF6)); that of 4 was reported
Table 2: Redox potentials of complexes 1, 2 a, 3� and [PtII(bpy)Cl2].[a]
E11=2 [V]
1=2 [V]
E31=2 [V]
1.80 (r)
1.91 (r)
1.87 (r)
1.90 (r)
1.86 (r)
1.61 (in DMF)
0.18 (r)
0.79 (r)
0.04 (r)
0.58 (r)
0.46 (r)
� 0.70 (irr)
0.01 (r)
� 0.90 (irr)
� 0.42 (r)
� 0.49 (r)
[a] Measured in CH2Cl2 (0.1 m [(nBu)4N](PF6)) versus Fc�/Fc at 20 8C and
and a scan rate of 200 mVs1. r � reversible, irr � irreversible.
previously.[8] Table 2 gives the redox potentials versus the
ferrocenium/ferrocene couple (Fc�/Fc). All complexes exhibit at least two, ligand-centered reversible one-electron transfer waves, E11=2 and E21=2, and a similar third wave, E31=2, which is
irreversible for 1 and 3. These waves are assigned to processes
shown in Scheme 2, in which (bpyC) is the radical anion of
2,2?-bipyridine (bpy) and (LBQ)0 is the neutral benzoquinone
form of the ligand L2.
Scheme 2. Electron transfer processes of the complexes investigated
by cyclic voltammometry.
Figure 2. Structure of 2 a in the crystal showing an ion pair dimer
{[Pd(bpy)(BC)]�(PF6)}2. Selected bond lengths [鋆: Pd-Nbpy 2.054(1),
2.025(1), Pd-NBC 1.976(1), 2.012(1). The dotted line represents the
shortest distance between the atom C(1) of the first (BC) ligand and
the centroid of the six-membered ring of the second (BC) ligand.
Figure 3. Cyclic voltammograms of 1, 2 a, 3, and 5 in CH2Cl2 (0.10 m
[(nBu)4N](PF6)) at a glassy carbon working electrode at a scan rate of
200 mVs1.
Angew. Chem. Int. Ed. 2003, 42, No. 5
Interestingly, E11=2 is nearly constant throughout the series
at ~ 1.87 V which is also observed for the reduction of
[PtII(bpy)Cl2] to EPR-active [PtII(bpyC)Cl2] .[11] E21=2 and E31=2
involve the successive oxidations of the aromatic dianions L2
(Scheme 1) to the semiquinonate radical anions (LC), and
further to the quinones (LBQ)0. Interestingly, the o-dithiolate
species 3 is the most difficult neutral species to oxidize,[12]
followed by the catecholate complex 1, whereas the o-diimino
species 2 is the easiest to oxidize.
The electronic spectra of the neutral complexes 1�(Figure 4) are dominated by a charge-transfer band in the
visible which was assigned to a ?mixed metal眑igand (M(L))
to ligand (bpy)? transition,[13] LLCT. Accordingly, the HOMO involves contributions from the metal ion and the
C6H4XY2 system, while the LUMO is localized on the bpy
ligand (p* orbital). Therefore, the energy of the LUMO
should be approximately constant throughout the series. If
this is the case, the redox potential E21=2 for the couple [neutral
complex]/[monocation]� should correlate with the energy of
this LLCT transition as shown in Figure 5.
Temperature-dependent (3�8 K) magnetic susceptibility measurements (SQUID magnetometer, 1 T) established
that solid samples of 1 a, 4 a,[8] and 5 a are paramagnetic with
an effective magnetic moment (meff) of 1.6�8 mB at 298 K
corresponding to one unpaired electron per formula unit. In
contrast, solid 2 a is diamagnetic (meff � 0.2 mB (10 K) and
0.8 mB per Pd center at 298 K corresponding to temperatureindependent paramagnetism, cTIP) because the N,N-coordinated ligands (BC) form dimers through p-stacking in the
solid state.
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with very small g anisotropy and without detectable hyperfine
splitting is observed. This p-radical anion is predominantly Scentered,[15] whereas in (AC), (BC), (DC), and (EC) the
unpaired electron is delocalized over the six-membered ring
(Scheme 3).
Scheme 3. Resonance structures of the p-radical ligands (AC), (BC),
(CC), (DC), and (EC).
Figure 4. Electronic spectra of the neutral species 1, 2, 3, and 5 and their
electrochemically generated one-electron oxidized forms 1 a, 2 a, 3 a, and 5 a
(0.10 m [(nBu)4N)(PF6)).
In summary, we have presented spectroscopic evidence
for the existence of the S,S- and N,S-coordinated p-radicals
o-dithiobenzosemiquinonates(1) and o-iminothiobenzosemiquinonates(1), respectively, in [M(bpy)(LC)]� complexes. These results necessitate a reformulation of oxidation
states and ligand oxidation levels of a number of o-dithiolato
transition-metal ion complexes.
Experimental Section
Figure 5. Correlation between the redox potential E21=2 for the couple
[neutral complex]/[monocation]� and the energy of the LLCT transitions of the neutral complexes.
CH2Cl2 solutions of 1 a, 2 a, 4 a, and 5 a as well as an
electrochemically generated solution of 3 a display typical pradical signals at g ~ 2.0 in the X-band EPR spectra. The
observed g values and hyperfine coupling constants are
summarized in Table 3. The spectrum of 3 a could only be
recorded in frozen solution below 60 K;[14] a rhombic signal
Table 3: X-band EPR spectra of [M(bpy)(XC)]� complexes in CH2Cl2
solution at 298 K.
Hyperfine coupling constant [G]
4 a[8]
aH � 3.5(2 H); a105Pd � 2.4
aN � 6.2, 5.8, aH � 5.1, 3.2
aN � 7.7; aH � 4.6; a105Pd � 3.56
a195Pt � 50
[a] Measured in frozen solution (CH2Cl2, 0.10 m [(nBu)4N](PF6)) at 60 K;
rhombic signal g1 � 2.018, g2 � 2.006, g3 � 1.993. [b] n.o. no hyperfine
coupling observed.
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Detailed procedures for the synthesis of complexes and their
characterizations using mass spectrometry and 1H NMR spectroscopy
will be reported later in a full paper. All new compounds 1, 1 a, 2 a, 3,
5, and 5 a gave satisfactory elemental analyses (C, H, N, S).
Crystal structure analysis data for 1: C24H28N2O2PdH3CN, Mr �
523.94, monoclinic, P21/c, a � 6.7891(6), b � 14.053(1), c �
25.259(3) �, b � 91.57(1)8, V � 2409.0(4) � Z � 4, 1calcd �
1.445 Mg m3, m(MoKa) � 0.797 mm1, F(000) � 1080, 12 870 reflections collected at 100(2) K, 4212 independent reflections, 290
parameters, GOF � 1.028, R1 � 0.066, wR2 � 0.1074. Crystal structure
analysis data for 2 a: C22H18F6N4PPd, Mr � 589.77, triclinic, P
1, a �
8.9146(3), b � 10.8276(4), c � 12.2280(4) �, a � 76.34(1), b � 83.24(1),
g � 66.57(1)8,
V � 1051.97(6) �
Z � 2,
1calcd � 1.862 Mg m3,
m(MoKa) � 1.031 mm , F(000) � 586, 39 386 reflections collected,
8052 independent reflections, 310 parameters, GOF � 1.032, R1 �
0.030, wR2 � 0.0606. Crystal structure analysis data of 5: C24H29N3SPt,
Mr � 586.65, monoclinic, P21/c, a � 10.4225(6), b � 12.6824(9), c �
16.4985(9) �, b � 91.00(1)8, V � 2180.5(2) � Z � 4, 1calcd �
1.787 Mg m3, m(MoKa) � 6.546 mm1, F(000) � 1152, 24 351 reflections collected, 8271 independent reflections, 271 parameters, GOF �
1.033, R1 � 0.036, wR2 � 0.079. CCDC-187891 (1), CCDC-187892
(2 a), and CCDC-187893 (5) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge
via (or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge
CB2 1EZ, UK; fax: (� 44) 1223-336-033; or
Received: June 21, 2002
Revised: October 10, 2002 [Z19586]
[1] C. G. Pierpont, C. W. Lange, Prog. Inorg. Chem. 1994, 41, 331.
[2] D. T. Sawyer, G. S. Srivatsa, M. E. Bodini, W. P. Schaefer, R. M.
Wing, J. Am. Chem. Soc. 1986, 108, 936.
[3] a) D. Sellmann, M. Geck, F. Knoch, G. Ritter, J. Dengler, J. Am.
Chem. Soc. 1991, 113, 3819; b) D. Sellmann, H. Binder, D.
H?ussinger, F. W. Heinemann, J. Sutter, Inorg. Chim. Acta 2000,
300�2, 829.
1433-7851/03/4205-0566 $ 20.00+.50/0
Angew. Chem. Int. Ed. 2003, 42, No. 5
[4] P. Chaudhuri, C. N. Verani, E. Bill, E. Bothe, T. Weyherm竘ler,
K. Wieghardt, J. Am. Chem. Soc. 2001, 123, 2213.
[5] D. Herebian, E. Bothe, E. Bill, T. Weyherm竘ler, K. Wieghardt,
J. Am. Chem. Soc. 2001, 123, 10 012.
[6] H.-Y. Cheng, C.-C. Lin, B.-C. Tzeng, S.-M. Peng, J. Chin. Chem.
Soc. 1994, 41, 775.
[7] V. Bachler, G. Olbrich, F. Neese, K. Wieghardt, Inorg. Chem.
2002, 41, 4179.
[8] X. Sun, H. Chun, K. Hildenbrand, E. Bothe, T. Weyherm竘ler, F.
Neese, K. Wieghardt, Inorg. Chem. 2002, 41, 4295.
[9] T. M. Cocker, R. E. Bachman, Inorg. Chem. 2001, 40, 1550.
[10] For a similar p-stacking of a metalloporphyrin p-radical cation
[Zn(tppC)(OClO3)]2 see: H. Song, N. P. Rath, C. A. Reed, W. R.
Scheidt, Inorg. Chem. 1989, 28, 1839.
[11] E. J. L. McInnes, R. D. Farley, S. A. Macgregor, K. J. Taylor, L. J.
Yellowlees, C. C. Rowlands, J. Chem. Soc. Faraday Trans. 1998,
94, 2985.
[12] In line with this observation it was recently reported that a
(thiophenolato)cobalt(iii) and its (phenolato)cobalt(iii) analogue are oxidized at 0.55 and 0.36 V versus Fc�/Fc, respectively
to generate the coordinated thinyl and phenoxy radicals. S.
Kimura, E. Bill, E. Bothe, T. Weyherm竘ler, K. Wieghardt, J.
Am. Chem. Soc. 2001, 123, 6025.
[13] a) J. A. Zuleta, J. M. Bevilacqua, D. M. Proserpio, P. D. Harvey,
R. Eisenberg, Inorg. Chem. 1992, 31, 2396; b) A. Vogler, H.
Kunkely, J. Hlavatsch, Inorg. Chem. 1984, 23, 506; c) A. Vogler,
H. Kunkely, J. Am. Chem. Soc. 1981, 103, 1559; d) T. R. Miller,
G. Dance, J. Am. Chem. Soc. 1973, 95, 6970.
[14] We rationalize this behavior by a fast dimerization at ambient
temperature of the radical 3 a with SS bond formation;
however, 3 a is stable in frozen solution at < 77 K.
[15] The EPR spectrum of the 2,4,6-tris(tert-butyl)phenylthiyl radical
is similar; it also lacks observable hyperfine splitting. Z. B.
Alfassi in S-Centered Radicals, Wiley, New York, 1999, p. 7.
[16] G. A. Fox, C. G. Pierpont, Inorg. Chem. 1992, 31, 3718.
[17] D. Darensbourg, K. K. Klausmeyer, J. H. Reibenspiess, Inorg.
Chem. 1996, 35, 1535.
Polymerization Mechanism Determined
[2.2]Paracyclophanes with Defined Substitution
Pattern?Key Compounds for the Mechanistic
Understanding of the Gilch Reaction to Poly(p-phenylene vinylene)s**
Jens Wiesecke and Matthias Rehahn*
Since the discovery of (semi)conductivity in organic polymers
in 1976,[1, 2] and electroluminescence in semiconducting polymers in the early 1990s,[3] much effort has been made to
determine the structure眕roperty relationships in conjugated
macromolecular systems. The knowledge of these correlations is essential for further systematic development for
application in light-emitting diodes (LEDs),[4] photovoltaic
cells,[5] and field-effect transistors (FETs).[6] Poly(p-phenylene
vinylene) (PPV) 1 is one of the most promising polymers for
LED applications. Furthermore, it can be prepared by many
synthetic methods in good to excellent yields.[7] The Gilch
route[8] has a few advantages compared to other routes such as
the Heck,[9] Wessling,[10] and Knoevenagel reactions.[11] These
advantages include easily accessible starting materials, mild
reaction conditions, and formation of film-forming products.
To be suitable for LED applications, the PPVs must be free of
structural defects.[12] Unfortunately, it is well-known that the
Gilch process generates a variety of characteristic defects
within the polymer chains, such as saturated ethylene bridges,
rodlike ethynylene subunits, and halogenated chain ends.[13]
Lowering the occurrence of such defects to a minimum level is
therefore of crucial importance and requires a profound
understanding of the Gilch reaction﹕ mechanism. If one
ignores some rather speculative ideas about mechanisms
occurring via carbene intermediates,[14] just one general
reaction scheme remains (Scheme 1): using KOtBu as the
base, the starting material 2 eliminates one HCl molecule. The
resulting ?real? monomer 3 polymerizes, leading to the
nonconjugated but chlorinated poly(p-xylylene) (PPX; 4). It
is particularly the mechanism of this chain growth process
which is presently under controversial discussion, although
the mechanisms of chain initiation, chain termination, and
cross-linking in this reaction are also unclear. For the final
step, there is agreement that 4 converts into PPV 1 by
elimination of a second equivalent of HCl.
For the polymerization reaction anionic,[15] radical,[16] and
simultaneous anionic and radical chain-growth processes
[*] Prof. Dr. M. Rehahn, Dipl.-Chem. J. Wiesecke
Technische Universit?t Darmstadt
Institut f竢 Makromolekulare Chemie
Petersenstrasse 22, 64287 Darmstadt (Germany)
Fax: (� 49) 6151-16-4670
[**] We thank the Fonds der Chemischen Industrie e.V. (FCI) and the
Vereinigung der Freunde der TU Darmstadt e.V. for financial
Supporting Information (experimental and analytical details) for
this article available on the WWW under or from the author.
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iminothiobenzosemiquinonate, dithiol, mii, pf6, coordinated, radical, complexes, bpy
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