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Controlling Valence Tautomerism of Cobalt Complexes Containing the Benzosemiquinone Anion as Ligand.

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[9] B. J. Hathaway, D. G. Holah, J. D. Poslethwaite, J. Chem. SOC.1961,
321 5-3218.
[lo] M.-T. Youinou, J. A. Osborn, J.-P. Collin. P. Lagrange. Inorg. Chem. 1986.
25. 453 -461.
[ l l ] A. Pfeil. J.-M. Lehn, J Cliem. Soc. Cliem. Commun. 1992. 838-840.
[I21 X-ray structure analysis of 4, perchlorate salt (C,,,H,,N,,O,,CI,Cu,),
A4 = 2265.9; deep-red crystals (0.150 x0.150 x0.120 mm'). monoclinic
space group C2!c with u = 28.627(8). h = 13.222(4), c = 28.097(8) A. 6 =
91.96(2) , V = 10629.7 A'. 2 = 4, Q ~ = 1.416
,g c m
~ - 3 .~ f(O00) = 4640,
{I = 24.19 cm-'. A crystal of 4 suitable for X-ray structure analysis was
selected on a cooled plate and mounted from the mother liquor.
61 72 intensity data were collected on a Philips PW 1100/16 diffractorneter
at - 100 'C with Cu,, radiation .; = 1.5418 A (Ni monochromator) (H,20
scan, 6 < 20 < 102 ~ )The
. systematic absences together with the E statistics and the NZ test suggested the space group to be C2:i. SuccesslLl
structure solution and refinement confirmed this choice. The structure was
solved by a combination of Patterson, difference Fourier. and full matrix
least-squares methods). The final conventional residuals were R = 0.0696
and R, = 0.0930 for 3266 reflections having I > 3a(l) and 677 variables.
Further details of the crystal structure investigation are available on request from the Director of the Cambridge Crystallographic Data Centre.
12 Union Road, Cambridge CB2 1EZ (UK). o n quoting the full journal
1131 M.-T Youinou, N. Rahmouni, J. Fischer. J. A. Osborn. AnRrw. Cl?fm.
1992, 104, 771 -773; Angeir. Ciimi. Inl. Ed. Erigl. 1992. 31. 733-735.
[I41 a) R. G. Vranka. E. L. Amma. J. Am. Clwi?.SOC.1966, 88. 4270-4271:
b) A. M. Manotti Lanfredi. A. Tiripicchio. A. Camus, N. Marsich. J
Clwin. Suc. Clwm. Cornmun. 1983. 1126- 1128, and references therein.
Controlling Valence Tautomerism of Cobalt
Complexes Containing the Benzosemiquinone
Anion as Ligand**
Several different types of transition metal complexes are
electronically labile, where electronic degeneracy o r neardegeneracy leads to vibronic interactions and an appreciable
sensitivity to the environment. Electronic lability is characteristic of mixed-valence,[" spin-crossover,"] and valencetautomerid3] metal complexes. Electronically labile complexes have been indicated as potential building blocks for molecular electronic devices.141Small collections of these molecules
could be expected to exhibit bistability; thus, as a result of an
external perturbation they could be made to flip-flop between two states. In fact, polycrystalline samples of Fe" spincrossover complexes have been shown to exhibit bistability
in the LIESST (light-induced excited spin state trapping)
effect discovered by Giitlich et al.151When a polycrystalline
spin-crossover complex is maintained at low temperatures
( < 50 K), the whole crystalline sample can be interconverted
between the stable low-spin state and the metastable highspin state with photons of different wavelengths. The
metastable high-spin state will remain indefinitely as long as
the sample is kept below about 50 K. In this paper we will
show that thermally driven bistability is possible for cobalt
complexes that exhibit valence tautomerism.
Complexes of transition metal ions with the benzosemiquinone and catecholate anions as ligands have been described
Prof. Dr. D. N . Hendrickson, D. M. Adams. Prof. A. Dei"'
Department of Chemistry-0506
University of California at San Diego
La Jolla. CA 92093-0506 (USA)
Telefax: Int. code + (619)534-4864
Prof. Dr. A. L. Rheingold
Department of Chemistry, University of Delaware
Newark, DE 19716 (USA)
On sabbatical leave from the Universiti di Firenze, Firenze (Italy).
This work is supported by the U.S. National Science Foundation
(CHE9115286 to D.N.H.) and the U.S. National Institutes of Health
(HL 13652 to D.N.H.).
VCH Verlu~.sgesrllsrhalrmbH, W-6940 Weirihrrm, 1993
Co" with two benzosemiquinonate anions (3,5-dtbsq- =
anion) as ligands and a
2.2'-bipyridine (bpy) ligand, and the other of Co"' with one
dtbsq and one catecholato (3,5-dtcat' -) ligand. The roomtemperature X-ray structure 16] of [Co"'(bp y)( 3,5-d tbsq)(3,5-dtbcat)] shows that the ligands are in two different
oxidation states and that the metal is trivalent. In order to
see if these valence tautomeric complexes exhibit bistability,
we systematically changed the bipyridine ligand to other
diiminium ligands. The goal was to find a compound that
exhibits the Co"-Co"' tautomeric transformation in the
solid state.
A series of complexes with the composition [Co(N N)(3,5dtbsq)("3,5-dtbq")], where "3,Sdtbq" is either the benzosLmiquinone anion or catecholate form of the ligand and the
N N diiminium ligand is 4,4-diphenyl-2,2'-bipyridine (dpbpy),
4,4-dimethyl-2,2'-bipyridine (dmbpy), 1,I 0-phenanthroline
(phen, complex I), 2,2'-bipyrimidine (bpym), o r 2,2'-bipyrazine (bpz, complex 2). was prepared. Magnetic susceptibility
[ C o ( p h e n ) ( 3 , 5 - d t b ~ q ) ~ ]1
By David M . Adarns, Andrea Dei, Arnold L. Rheingold,
and David N . Hendrickson*
in terms of localized b ~ n d i n g . ' ~
] of the most intriguing
aspects of these complexes is the thermally driven valence
tautomerism that has been observed for some complexes in
solution.[61One cobalt complex'61 exists in solution in two
valence tautomeric forms [Eq. (a)]. One complex consists of
data for polycrystalline samples of the dpbpy, dmbpy, and
bpy complexes show that they are Co"' complexes with one
unpaired electron on the dtbsq ligand (peffis 1.83-2.12 pB in
the 100-300 K range). On the other hand, the phen, bpym,
and bpz complexes have pefrvalues of 5.17,5.03, and 4.38 p B ,
respect%ely, at 300 K. These three complexes are clearly
[Co"(N N)(3,5-dtbsq),] complexes at 300 K in the solid state.
X-ray crystal structures were determined at 238 K for
1 . (C,H,CH,)['] and at 296 K for 2.['l Complex 2 has C,
symmetry, in which the two equivalent dtbsq ligands have
Co-0 bond lengths of 2.046(4) and 2.057(4) A, and Co-N
distance is 2.130(4) A. This is clearly a high-spin Co" complex with two dtbsq ligands. For comparison, the Co-0
bond lengths for the Co"' complex [Co(bpy)(3,5-dtbsq)(3,5dtbcat)] range from 1.851(6) to 1.906(6) (A) with Co-N
lengths of 1.940(7) and 1.957(7)
The fact that this
bipyridine complex is a Co"' complex was establishedL6]by
the bond lengths in the ligands, the presence of an intervalence charge-transfer electronic absorption band in the
near-IR region associated with the mixed-valence ligand
structure, and the observation of a relatively isotropic EPR
signal for an organic radical. The X-ray structure of the phen
complex 1 is shown in Figure 1. The two dtbsq ligands are
not related by symmetry; the Co-0 bond lengths of one are
2.004(4) and 2.011(4) A and of the other are 2.002(4) and
1.987(4) A. These values together with the average Co-N
bond length (2.082 A) suggest that 1 is intermediate between
the Co" and Co"' forms at 238 K. The values of C-0 bond
lengths for o-quinone-derived ligands are knownc3]to reflect
the oxidation state of the ligand: 1.29(1) A for a coordinated
benzosemiquinone anion and 1.35(1) for a catecholate
ligand. In the case of the phen complex, one ligand has C-0
lengths of 1.291(8) and 1.294(8)& and the other 1.302(7)
and 1.305(7) A. These two ligands appear to be dtbsq ligands. Perhaps at 238 K the phen complex has converted
from high-spin Co" into low-spin Co", and at lower temperatures it will finally experience the tautomeric shift to a Co"'
8 IO.OO+ ,2510
Angew. Clim?. In[. Ed. Engl. 1993, 32. No. 6
? ci351
of the electronic spectrum of [Co(dpbpy)(3,5-dtbsq)(3,5-dtbcat)] in toluene is shown in Figure 2. At room temperature
there is a band a t about 600 nm, which is characteristic of the
Co"' form of the complex. As the temperature is increased to
333 K a band at about 770 nm increases in intensity; this
band belongs to the Co" form of the complex. The temperature a t which there is approximately equal amounts of Co"
and Co"' complexes correlates well with the variation in redox potential of the diiminium ligand.
It is more difficult to find a complex which exhibits the
valence tautomeric transformation in the solid state. We
have now found the first example. In Figure 3 are shown
magnetic susceptibility data for a polycrystalline sample of
the cobalt-phen complex 1 . At low temperatures ( < 200 K)
Fig. 1. Structure of 1 . (C,H,CH,) with atom labeling scheme. The toluene
solvate molecule is not shown.
In the [Co(N N)(3,5-dtbsq)("3,5-dtbq")] series, three of
the complexes are Co" and three are Co"' tautomers in the
solid state at room temperature. The variation in properties
is understandable in view of the changes in reduction potential~~''for the series of diiminium ligands: -2.40 V (dmbpy);
-2.18V (bpy); -2.04V (phen); -1.80 (bpym); and
- 1.70 V (bpz). If the ligand is more easily reduced (i.e., has
a more positive reduction potential), it can stabilize the Co"
form of the complex by accepting electron density into its n*
orbital. Variable-temperature electronic absorption spectra
(320-820 nm) were run for all of the complexes dissolved in
toluene. In solution they all show temperature-dependent
spectra that indicate the presence of the valence tautomerism
equilibrium as in Equation (a). The temperature dependence
[cm- ' M - '1
1500 I
I [nm]
Fig. 2. Temperature dependence of the electronic absorptlon spectrum of a
toluene solution of [Co(dpbpy)(3.S-dtbsq)(3,5-dtbcat)l; at T = 298, 303. 308.
318. 328. 338, and 348 K . The molar extinction coefficient F is plotted versus
wavelength i .
Angew. Ch~wi.hi.Ed. Enzt. 1993, 32. N o . 6
Fig. 3. Plot of effective magnetic moment per molecule petrversus temperature T for a polycrystalline sample of 1 . (C,H,CH,). The sample was first
cooled to 30 K, and measurements were carried out as T was increased ( 0 ) .
After the measurement was completed at 320 K, data were collected (A) as the
sample T was decreased to 30 K.
pu,,,/molecule is 1.7 pB,whereas above about 270 K the value
is 5.1 ,uB.This complex undergoes a relatively abrupt transformation centered around approximately 240 K, which explains why the X-ray structure was done at 238 K. Thus, this
complex is [Co1"(phen)(3,5-dtbsq)(3,5-dtbcat)] below 200 K
and converts into [Co1'(phen)(3,5-dtbsq),] above about
270 K. It is important to emphasize that it is very necessary
to have the toluene solvate molecule present, for without it
the complex does not undergo the transformation. The sensitivity to solvate molecules observed for this valence-tautomeric transformation is reminiscent of solvate sensitivities
observed for spin-crossover['] and mixed-valence"] complexes.
The transformation seen for the Co-phen complex 1
(Fig. 2) is so abrupt that it probably involves a phase transition, which is somewhat substantiated by the small (ca. 5 K)
hysteresis measured (see Fig. 3). The valence-tautomeric
transformation Co" --t Co"' is entropy driven. At low temperatures in the Co"' complex an S = 1/2, ground state is
thermally populated, whereas at high temperatures in the
Co" complex one S = 512, one S = 112, and two S = 312
states are thermally populated, because of weak Co"-dtbsq
exchange interactions.[loJ Not only is there electronic entropy gain associated with the Co"'-to-Co" conversion, but
as the energy of the vibrations for the CoO,N, modes in the
VCH Ver/agsgaell.~chufi
mhH. W-6940 Wemheim, 1993
OS70-0833/93/0606-08818 10.00f .2Si0
Co" complex is higher, there is also a vibrational entropy
gain. Efforts are ongoing to prepare several more complexes
that exhibit the valence-tautomeric transformation in the
solid state.
Hydrogen Bonds as a Crystal Design Element
for Organic Molecular Solids with Intermolecular
Ferromagnetic Interactions**
By Esteve Hernandez, Montse Mas, Elies Molins,
Concepcid Rovira, and Jaume Veciana*
E.xperimenta1 Procedure
The synthesis of [Co(phen)(3,5-dtbsq),1 is given as a general scheme for preparation ofcobalt complexes containing benzosemiquinone anions as Iigands. All
reactions were carried out under an inert atmosphere of Ar in Schlenkware with
degassed reagent-grade solvents (Aldrich). The starting tetramer [Co4(3,S-dtbsq)J was prepared by a published procedure 1111. and a portion (0.646 g) suspended in 100niL of methylcyclohexane. This reactant and the ligand phen
(0.220 g i n 100 mL of methylcyclohexane) were dissolved in their separate solutions by heating to 100 32.The phen solution was then added dropwise over
30min to the tetramer solution. After half the solution had been added, a
purple microcrystalline precipitate was visible. After the addition was complete,
the solution was stirred for an additional 30 min at 100 "C. Upon cooling.
0.700 g of a purple microcrystalline solid was filtered off. This solid was recrysrallized by dissolving 0.200 gin 100 mL of toluene, filtering hot, and then slowly
evaporating the solution under a steady flow of N,gas. All cobalt complexes
gave good chemical analyses for C, H, N, and Co.
Received: January 25,1993 [Z 5826 IE]
German version: Angew. Cheni. 1993. 105.954
[ l ] D. N. Hendrickson in Mixed Vulenrj Sjxrernst Applicarrons in Chemisrrj,
Ph.ysics and Biology (Ed.: K. Prassides), Kluwer, Dordrecht. 1991,
Chap. 5, pp. 67-90; H. G . Jang, R. J. Wittebort, Y. Kaneko. M. Nakano,
M. Sorai. D. N. Hendrickson, Inorg. Chem. 1992. 31, 2265; M.S.
Mashuta, R. J. Webb, J. K. McCusker, E. A. Schmitt, K. J. Oberhdusen,
J. F. Richardson, R. M. Buchanan, D. N. Hendrickson, J Am. Chem. Soc.
1992, 114, 3815; R. J. Webb, T.-Y. Dong, R. K. Chadha, C. G . Pierpont,
D. N. Hendrickson, ibid. 1991, 113, 4806.
[2] P. Giitlich. Strucrure Bonding IBerlJn) 1981,44,83:P. Giitlich in Chemicul
Mossbuuer Spectroscopy (Ed.: R. H. Herber), Plenum. New York, 1984;
C. N. R &do, Int. Rev. P/ij.rr.s.Chem. 1985, 4, 19; P. Giitlich, A. Hauser.
Coord. Chem. Rev. 1990. 97. 1 ; E. Konig, Prog. Inorg. Chem. 1987. 35,
[3] C. G . Pierpont. S . K. Larson, S . R. Boone, Pure Appl. Chem. 1988, 60.
1331; C. G . Pierpont, R. M. Buchanan. Coord. Chem. Rev. 1981, 38,
[4] 0. Kahn. J. Krobert, C. Jay, Adv. Mafer. 1992, 4, 718; 0.Kahn. J.-P.
Launay, Chemrronics 1988, 3, 140; J.-P. Launay in Molecular Elecrronk
Devices I1 (Ed.: F. L. Carter), Dekker, New York. 1987.
[5] S . Decurtins, P. Giitlich, C. P. Kohler, H. Spiering. A. Hauser Chern. Phjs.
L e t f . 1984, 105. 1 ; P. Giitlich. A. Hauser. Pure Appl Chem. 1989. 61,
[6] R. M. Buchanan, C. G . Pierpont, J Am. Chem. Sue. 1980, 102. 4951.
[7] Crystaldata for 1 . C,H,CH,. C,,H,,CoN,O,
. C,H,, monoclinic, PZ,/c,
u = 10.425(3), h = 32.226(8). I = 13.454(4) A,
= 11l.39(2)",
4208.6(18)A, Z = 4 . ~ , , , , ~ = 1 . 2 1 8 g c m - ~p(MoK,)=4.48cm-',
238 K. Of 4787 data collected (Siemens P4. 4 " 5 28 s 42'). 4552 were
independent, and 2879 were observed (5uFo) The Co atom was located by
direct methods. All atoms were refined as in ref. [8]. R(F) = 0.0489,
R(wF) = 0.0629 1121.
[8] Crystal data for 2: C,,H,,CoN,O,.
orthorhombic, Ccc2. u = 16.233(6),
h = 26.894(10), c = 8.472(2)A, V = 3698.6(13) A'.
Z = 4. eCalcd
1.181 g ~ r n - p(MoKJ
= 5.00cm-'. T = 296 K. Of 1852 data collected
(Siemens P4, 4' 5 28 < 45 ), 1759 were independent. and 1307 were observed (3uFcJ. The noncentrosymmetric space group was chosen (instead
of Cccm) as required by the placement of a chiral trischelate complex on
a special position. The structure was solved intuitively by placing the Co
atom in an arbitrary position on the twofold axis. The correctness of the
reported hand was determined by the refinement of a multiplicative term
(1.11(8)) for AF'. All non-hydrogen atoms were refined with anisotropic
thermal parameters, and hydrogen atoms were treated as idealized contributions. R(F) = 0.0461, R(ivF) = 0.0504 [12]. SHELXTL-PC software
(G. Sheldrick. Siemens. Madison, WI. USA).
191 Y. Kawaiiishi. N. Kitamura. S. Tazuke, Inor<?.Chem. 1989.28.296s- 2975.
[lo] M. W. Lynch, R. M. Buchanan. C. G . Pierpont, D. N. Hendrickson, Inorg.
Chem. 1981, 20. 1038; A. Dei, D Gatteschi, Inorg. Chim. A c f u 1992,
198-200, 813
[ t i ] R. M. Buchanan, B. J. Fitzgerald, C. G. Pierpont, Inorg. Chem. 1979, 18,
[12] Further details of the crystal structures investigations are available from
the Director of the Cambridge Crystallographic Data Centre, 12 Union
Road, GB-Cambridge CB2 1EZ (UK), on quoting the complete journal
f> VCH Verlagsgesellrchafl mhH, W-6940 Weinheim. 1993
Ferromagnetism in purely organic molecular solids has
attracted considerable interest in the last few years."] U p to
now, only three open-shell organic molecules-free radicals
or radical ions-exhibiting such a magnetic property in the
solid state are known.['] In order to obtain this cooperative
property it is necessary not only to choose suitable openshell molecules but also to orient them properly relative to
each other in a three-dimensional network, because ferromagnetism is a three-dimensional p h e n ~ m e n o n . ' In
~ ] these
supramolecular organizations, the correct electronic mechanisms that give rise to the parallel, or ferromagnetic (FM),
spin alignment between neighboring molecules along three
(or two) directions of space must be e ~ t a b l i s h e d .Appropri~~]
ate structural modifications and chemical functionalizations
of stable open-shell molecules represent a promising approach to supramolecular organizations"] in which the natural predilection for antiparallel, o r antiferromagnetic (AF),
spin alignment is avoided.['.41 In fact, the use of crystal design elements provides one of the most direct ways to control
the relative orientations of molecules[61 and, therefore, of
their electronic interactions in crystalline and noncrystalline
solids.[7i Hence, basic studies devoted to the control of crystal packings and, through them, the magnetic properties of
organic molecular solids are in great demand.['I
We report here an example of the nitronyl nitroxide free
radical 1,['I in which hydrogen bonds have been used for the
first time as a crystal design element to obtain a crystalline
supramolecular organization showing intermolecular ferromagnetic interactions.
The study of the radical 1 seemed promising, because this
compound combined a priori most of the structural and
electronic prerequisites to show at least a short-range FM
ordering of hydrogen-bonded molecules: 1) semiempirical
molecular orbital (MO) calculations revealed strong electronic polarizations of NO and OH bonds (N+0.09-0-0.34
and 0-0.27-H +0.24),[91
indicating that the NO groups could
act as acceptors and the O H group as a donor in hydrogen
bonds; 2) the noncollinear relative orientation of NO and
OH groups, together with the strong directionality usually
observed for hydrogen bonds,I71 could favor the formation
in the solid state of a hydrogen-bonded network with a high
dimensionality, and 3) MO calculations based on the UHF
approximation['] showed that 1 fulfilled most of the electronic requirements that were considered relevant for the
[*] Dr. J. Veciana, E. Hernandez, M. Mas, Dr E. Molins, Dr. C. Rovira
Institut de Ciencia de Materials de Barcelona
Campus de la U.A.B., E-08193 Bellaterra (Spain)
[**I This work was supported by the Programa Nacional de Nuevos Materiales
(C.I.C. y T., Grant No. MAT 91/0553). We thank Dr. B. Martinez (Laboratori de Propietats Electriques i Magnetiques. ICMAB) for magnetic
0570-0833:93/0606-1882 S 10.00i .25/0
Angen. Chem. In/. Ed. Engl. 1993, 32, No. 6
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containing, valence, tautomeric, anion, complexes, cobalt, controlling, benzosemiquinone, ligand
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