Valence tautomerism in a o-benzoquinone adduct of a tetraazamacrocycle complex of manganese

Article abstract of DOI:10.1002/(SICI)1521-3773(19981116)37:21<3005::AID-ANIE3005>3.0.CO;2-2

Two different charge distributions of the complex cation [Mn(III)(cth)(diox)]⁺ (cth = a tetraazamacrocycle, diox = 3,5-di-tert- butyl-o-benzoquinone; structure shown in the picture) can be isolated by varying the counteranion: [Mn(III)(cth)(cat

Full text of DOI:10.1002/(SICI)1521-3773(19981116)37:21<3005::AID-ANIE3005>3.0.CO;2-2

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[5] a) R. M. Laine, Catal. Rev. Sci. Eng. 1983, 25, 459; b) J. R. Katzer, R. C.
Sivasubramanian, Catal. Rev. Sci. Eng. 1979, 20, 155; c) T. C. Ho, Catal.
Rev. Sci. Eng. 1988, 30, 117; d) Y. T. Shah, D. C. Cronauer, Catal. Rev.
Sci. Eng. 1979, 20, 209.
[6] See for example: a) R. D. Adams, S. B. Falloon, J. L. Perrin, J. A.
Queisser, J. H. Yamamoto, Chem. Ber. 1996, 129, 313; b) H. Imgar-
tinger, A. Weber, Tetrahedron Lett. 1997, 38, 2075; c) M. H. Chisholm,
S. T. Haubrich, J. C. Huffman, W. E. Streib, J. Am. Chem. Soc. 1997,
119, 1634; d) A. R. Katritzky, E. S. Ignatchenko, S. M. Allin, R. A.
Barcock, M. Siskin, C. W. Hudson, Energy Fuels 1997, 11, 160; e) A. J.
Arce, A. Karam, Y. Desanctis, R. Machado, M. V. Capparelli, J. Manzur
Inorg. Chim. Acta. 1997, 254, 119.
[7] a) J. L. Gary, G. E. Handwerk, Petroleum Refining: Technology and
Economics, 3rd ed., Marcel Dekker, New York, 1993; b) J. G. Speight,
The Chemistry and Technology of Petroleum, Marcel Dekker, New
York, 1983; c) B. C. Gates, Catalytic Chemistry, Wiley, New York, 1992;
d) C. N. Satterfield, C. M. Smith, M. Ingalls, Ind. Eng. Chem. Process
Des. Dev. 1985, 24, 1000; e) C. N. Satterfield, J. F. Cocchetto, Ind. Eng.
Chem. Process Des. Dev. 1981, 20, 53; f) F. Gioia, V. Lee, Ind. Eng.
Chem. Process Des. Dev. 1986, 25, 918; g) R. H. Fish in Aspects of
Homogeneous Catalysis (Ed.: R. Ugo), Kluver, Dordrecht, 1990, p. 342;
h) R. J. Kinney, W. D. Jones, R. G. Bergman, J. Am. Chem. Soc. 1978,
100, 7902.
[8] a) M. Tayebani, A. Kasani, K. Feghali, S. Gambarotta, G. Yap, Chem.
Commun. 1997, 20, 2001; b) M. Tayebani, K. Feghali, S. Gambarotta, C.
Bensimon, G. Yap, Organometallics 1997, 16, 5084; c) M. Tayebani, K.
Feghali, S. Gambarotta, G. Yap, Organometallics, in press; d) M.
Tayebani, S. Gambarotta, G. Yap, Organometallics, in press; e) K. A.
Hall, J. M. Mayer, J. Am. Chem. Soc. 1992, 114, 10402; f) R. R. Schrock,
M. L. Listeman, L. G. Sturgeoff, J. Am. Chem. Soc. 1982, 104, 4291.
formula [ML(diox)]Y (M ˆ Mn, Co; L ˆ tetraazamacrocyclic
ancillary ligand) in which the valence tautomers are charged
and thus allow the transition temperature to be tuned by using
suitable counterions Y; the Mn system is reported here.
The reaction between [Mn(cth)Cl2] (cth ˆ (Æ)l-5,7,7,
12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane) and
the 3,5-di-tert-butylcatecholate dianion (dtbcat) under inert
atmosphere yields the complex [Mn(cth)(dtbcat)]. Upon
‡
exposure to air the [Mn(cth)(diox)] cation is formed as the
initial oxidation product. If this cation is precipitated as the
tetraphenylborate salt, a yellow-brown product (1) is ob-
tained, whereas green microcrystalline 2 precipitates with
perchlorate. Analysis gave the general formulas [Mn(cth)-
(diox)]Y (Yˆ BPh4, ClO4). The two compounds have differ-
ent physical properties in the solid state, but the spectroscopic
properties of their solutions are identical.
Single crystals suitable for X-ray diffraction analysis were
obtained only for 1. The structure of the cation is shown in
Figure 1.[6] The Mn center is six-coordinate, the macrocyclic
ligand is bound in a folded configuration, and the benzoqui-
none acts as bidentate ligand. The features of the coordination
Valence Tautomerism in a o-Benzoquinone
Adduct of a Tetraazamacrocycle Complex of
Manganese**
Andrea Caneschi and Andrea Dei*
Among the molecular systems that exhibit electronic
bistability, the o-benzoquinone ± metal complexes, which are
characterized by valence tautomerism, are a relatively new
class of compounds.[1±3] Their main attractiveness resides in
the formal equivalence of their physical behavior with that of
the well-known spin-crossover metal complexes, whose re-
versible transformations can be utilized for creating address-
able memories.[4, 5] To date valence tautomerism has been
found only for a family of molecular o-benzoquinone
complexes of general formula [M(N N)(diox)2] (M ˆ Co,
Mn; N N ˆ diazine ligand; diox ˆ catecholato, semiquinone
anion); the transition temperatures Tc range from 100 to
350 K. We have now found a class of compounds of general
‡
Figure 1. ORTEP plot of [Mn(cth)(dtbcat)] ; the hydrogen atoms are
omitted for clarity. Selected bond lengths [Š] and angles [8]: Mn O1
1.881(4), Mn O2 1.876(4), Mn N1 2.311(5), Mn N2 2.117(5), Mn N3
2.313(5), Mn N4 2.129(5), O1 C17 1.369(7), O2 C22 1.356(7), C17 C22
1.398(8); O1-Mn-O2 85.3(2), O1-Mn-N1 90.8(2), O1-Mn-N2 173.4(2), O1-
Mn-N3 101.0(2), O1-Mn-N4 90.6(2), O2-Mn-N1 99.8(2), O2-Mn-N2
88.3(2), O2-Mn-N3 91.8(2), O2-Mn-N4 175.7(7).
polyhedron are fully consistent with the formulation of the
cation as [MnIII(cth)(dtbcat)] . The coordination geometry is
‡
strongly axially distorted, as expected for a high-spin d4
electronic configuration. The M N bond lengths are signifi-
cantly longer for the apical nitrogen atoms than for the basal
nitrogen atoms (2.31 and 2.12 Š, respectively) . The average
Mn O distance of 1.87 Š agrees well with those in other
metal(iii) catecholato derivatives.[7, 8] Finally, the structural
parameters of the coordinated benzoquinone are typical of a
catecholato dianion; the C O and C C bond lengths are
similar to those observed for other dtbcat derivatives.[1]
The IR spectrum of 1 is virtually superimposable on those
of the chromium, iron, and cobalt analogues, which can be
[*] Prof. A. Dei, Dr. A. Caneschi
Dipartimento di Chimica
Á
Universita di Firenze
Via Maragliano 75, I-50144 Firenze (Italy)
Fax: (‡39)55-354845
E-mail: adei@blu.chim1.unifi.it
[**] We are indebted to Professor D. Gatteschi for helpful criticism and
discussion. The financial support of MURST, CNR, and 3MD of the
EU network (contract no. ERB 4061 PL 97-0197) is gratefully
acknowledged.
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unambiguously described as containing catecholato adducts
of the trivalent metal ions.[9] However, the physical properties
of the green perchlorate salt 2 are consistent with semi-
quinonato (sq) character. Its IR spectrum is virtually super-
imposable on that of the nickel(ii) and zinc(ii) analogues,
which can be unambiguously formulated as containing
DMSO, the spectra of the yellow solutions are consistent with
‡
the presence of an [MnIII(cth)(dtbcat)] chromophore, where-
as in weakly polar solvents such as toluene or acetone the
spectra of the blue-green solutions are consistent with the
‡
presence of a [MnII(cth)(dtbsq)] chromophore. In the latter
solvents, both the energies and the intensities of the bands
‡
1
[MII(cth)(dtbsq)] cations (dtbsq ˆ 3,5-di-tert-butyl-o-semi-
at 13000 and 25700 cm
are similar to those of
quinone anion),[10] and its reflectance spectrum indicates the
presence of a semiquinonato chromophore.
[ZnII(cth)(dtbsq)] .[10] The other bands can be reasonably
‡
attributed to metal-to-ligand charge transfer (19000 and
1
The spectroscopic properties of both 1 and 2 are temper-
ature-dependent. Heating solid 1 causes a reversible color
change from yellow-brown to green, and cooling 2 in liquid
nitrogen changes its color reversibly from green to yellow.
Crystals of 1 are completely converted into the green form by
heating for 1 h at 360 ± 365 K under vacuum, and revert to the
yellow-brown form after a few minutes at room temperature.
The transition is noncooperative, as shown by the differential
scanning calorimetry curve.
23300 cm ) and to d--d transitions involving some ªtrip
multipletsº of the manganese(ii) ion.[11, 12] The electronic
spectra therefore suggest that this manganese ± benzoqui-
none adduct may be described by the two charge distributions
MnIII cat and MnII sq, depending on the donor power of the
solvent and the different solvation enthalpies of the two
tautomers.[13, 14]
1
Complex 1 is has a cT value of 3.1 emuKmol at 70 ±
300 K, as expected for a d4 metal ion in high-spin ground
state (Figure 3). Below 70 K cT decreases and attains a value
of 2.4 emuKmol 1 at 4.2 K. For 2, cT decreases monotonically
The electronic spectra of both compounds are strongly
solvent-dependent (Figure 2). In highly polar solvents such as
&
*
Figure 3. Change in cTas a function of temperature T for 1 ( ) and 2 ( ) in
a magnetic field of 5000 G.
1
1
from 3.5 emuKmol at 300 K to 3.1 emuKmol at 90 K.
Below 90 K the temperature dependence of 2 is similar to that
of 1, and a value of 2.4 emuKmol 1 is attained at 4.2 K. Since
cTof 2 continues to increase at room temperature, at the high-
temperature limit it greatly exceeds the value for a MnIII ion.
Therefore it can be suggested that at high temperature 2 is
stable as MnII sq. If the two paramagnetic species are not
1
coupled, cT should be 4.75 emuKmol . The decrease be-
tween 280 and 120 K might be due both to interconversion
between the MnII sq and MnIII cat forms and to antiferro-
magnetic coupling between the paramagnetic metal ion and
the radical ligand.[7, 8, 15] If the former were dominant, the
interconversion process should be of the noncooperative type.
The X-band EPR spectra of 1 and 2 at low temperatures show
several features which may be due to impurities. High-field
EPR investigations are in progress.
The system reported here follows the general pattern of
valence tautomerism,[1±3] in which an entropy-driven electron
transfer process involves two species with nearly degenerate
electronic states. The novelty of this system lies in the
possibility of tuning the thermodynamics of the electron
transfer process by varying the counterion.
Figure 2. Electronic spectra of [Mn(cth)(dtbcat)]BPh4 in acetone (- - - -)
and DMSO (ÐÐ) in the range of 8000 ± 20000 cm (a) and 20000 ±
30000 cm 1 (b).
1
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Experimental Section
A Novel Beryllium Thiolate Resulting from
N Si Bond Cleavage: Liberation of Ammonia
in the Reaction of Be[N(SiMe3)2]2 with HSPh**
[Mn(cth)Cl2] was prepared from [(NEt4)2MnCl4] (5 mmol) and cth
(6 mmol) in acetonitrile (30 mL). The reaction between dtbcat (1 mmol)
and [Mn(cth)Cl2] (1 equiv) in basic methanol (30 mL) under inert
atmosphere gave [Mn(cth)(dtbcat)]. Crystals of 1 were obtained by adding
an aereated solution of NaBPh4 in methanol, and 2 by adding an aqueous
solution of NaClO4. The inert atmosphere was removed and the resulting
suspensions were stirred for 1 h at room temperature. Complexes 1 and 2
were obtained in 80 and 70% yield, respectively.
Scott Chadwick, Ulrich Englich, and
Karin Ruhlandt-Senge*
In the course of our investigations into the synthesis and
solid-state structures of alkaline earth metal thiolates, we have
used the bis(trimethylsilyl)amides of beryllium, magnesium,
and calcium as hydrocarbon-soluble metalating agents.[1] We
anticipated clean metathesis reactions between the metal
amide and arenethiols with formation of the amine
HN(SiMe3)2. Here we present results that indicate compet-
itive acid/base reactions between hexamethyldisilazane and
unchanged thiol in the reaction of [Be{N(SiMe3)2}2] with
stoichiometric amounts of HSPh in the presence of Lewis
bases.
Received: October 7, 1997
Revised version: June 17, 1998 [Z11008IE]
German version: Angew. Chem. 1998, 110, 3220 ± 3222
Keywords: magnetic properties ´ manganese ´ O ligands ´
tautomerism
[1] C. G. Pierpont, C. W. Lange, Progr. Coord. Chem. 1993, 41, 381.
[2] P. Gütlich, A. Dei, Angew. Chem. 1997, 109, 2852; Angew. Chem. Int.
Ed. Engl. 1997, 36, 2734.
[3] D. M. Adams, A. Dei, A. L. Rheingold, D. N. Hendrickson, J. Am.
Chem. Soc. 1993, 115, 8221.
[4] P. Gütlich, A. Hauser, H. Spiering, Angew. Chem. 1994, 106, 2109;
Angew. Chem. Int. Ed. Eng. 1994, 33, 2024.
[5] O. Kahn, C. Jay-Martinez, Science 1998, 279, 44.
The chemistry of the beryllium ± oxygen bond is by far the
most extensively studied interaction between beryllium and
another element.[2] In contrast, information on beryllium
thiolates is scarce. In the only crystallographically character-
ized homoleptic beryllium thiolate, the sterically demanding
SMes* ligand (SMes* ˆ S-2,4,6-tBu3C6H2) afforded the three-
coordinate monomer [Be(thf)(SMes*)2].[3] A three-coordi-
nate monomeric structure was also identified in the mixed
alkylberyllium thiolate [Be(2,6-Mes2C6H3)(SMes*)(Et2O)]
(Mes ˆ 2,4,6-Me3C6H2), in which the metal center is also
stabilized by bulky ligands.[4] A four-coordinate beryllium
center was found in [Be(S2CNiPr2)2], in which both dithio-
carbamato ligands chelate the cation through the two thiolato
groups.[5] Finally, a small family of alkylberyllium thiolate
[6] Crystal data for 1: MnBC55.5H72N4O3.5, Mr ˆ 916.92, MoKa radiation
l ˆ 0.71069 Š, crystal dimensions 0.3  0.2  0.5 mm, triclinic, space
Å
group P1 (no. 2), a ˆ 11.225(2), b ˆ 11.545(1), c ˆ 21.800(2) Š, a ˆ
83.64(1), b ˆ 77.78(1), g ˆ 76.99(1)8, Z ˆ 2, Vˆ 2684(2) Š3, 1calcd
ˆ
1.134 gcm 3. Of 5346 reflections collected at 293 K, 5021 were unique
and observed. Final R values (I > 2sI): R1 ˆ 0.0663, wR2 ˆ 0.1715.
Hydrogen atoms were treated as fixed contributions in calculated
positions with isotropic thermal parameters B(H) ˆ 1.2 Beq(C). An
absorption correction based on a y scan was performed, and no decay
was observed. Crystallographic data (excluding structure factors) for
the structure reported in this paper have been deposited with the
Cambridge Crystallographic Data Centre as supplementary publica-
tion no. CCDC-100691. Copies of the data can be obtained free of
charge on application to CCDC, 12 Union Road, Cambridge
CB21EZ, UK (fax: (‡44)1223-336-033; e-mail: deposit@ccdc.cam.
ac.uk).
1
compounds have been shown by H NMR spectroscopy and
cryoscopic measurements to exist as monomeric or oligomeric
complexes, depending on the combination of ligands and
donor, although detailed structural data are not available.[6]
Here we report the synthesis and characterization of the
novel, ammonia-coordinated, crown ether templated beryl-
lium thiolate [{Be(SPh)2(py)(NH3)}2{[18]crown-6}] ´ 2C7H8 (1;
py ˆ pyridine), which is formed by a sequence of acid-
promoted N Si bond cleavage reactions and in situ trapping
of the liberated ammonia.
[7] A. S. Attia, C. G. Pierpont, Inorg. Chem. 1995, 34, 1172.
[8] A. S. Attia, C. G. Pierpont, Inorg. Chem. 1997, 36, 6184.
[9] C. Benelli, A. Dei, D. Gatteschi, L. Pardi, Inorg. Chim. Acta 1989, 163,
99.
[10] C. Benelli, A. Dei, D. Gatteschi, L. Pardi, Inorg. Chem. 1989, 28, 1476.
[11] A. B. P. Lever, Inorganic Electronic Spectroscopy, Elsevier, Amster-
dam, 1984.
The reaction of [Be{N(SiMe3)2}2][7] with two equivalents of
HSPh in toluene in the presence of [18]crown-6 gave a thick,
white suspension; addition of one equivalent of pyridine
generated a homogenous solution,[8] from which 1 was
obtained as colorless crystals, which were analyzed by
single-crystal X-ray diffraction. The identity of 1 was also
[12] C. Benelli, A. Dei, D. Gatteschi, H. Gudel, L. Pardi, Inorg. Chem.
1989, 28, 3089.
[13] A. Dei, Inorg. Chem. 1993, 32, 5730.
[14] The reactivity of this complex towards dioxygen is markedly depend-
ent on the solvent. Solutions in highly polar solvents, in which the
catecholato species is predominant, are stable for days; however, in
weakly polar solvents, in which the semiquinonato form is predom-
inant, the complex is quickly oxidized (t1/2 ˆ 30 min in acetonitrile).
The reaction products are oxo dimers of the manganese tetraazama-
crocycle complex and dtb-quinone (60%) as well as the extradiolic
cleavage oxidation products 3,5-bis(1,2-dimethylethyl)-2H-pyranone
(36%) and its isomer 4,6-bis(1,1 dimethylethyl)-2H-pyranone (4%).
[15] A. Dei, D. Gatteschi, Inorg. Chim. Acta 1992, 198 ± 200, 813.
[*] Prof. Dr. K. Ruhlandt-Senge, Dr. S. Chadwick, Dr. U. Englich
Department of Chemistry
1-014 Center for Science and Technology
Syracuse University
Syracuse NY 13244-4100 (USA)
Fax: (‡1)315-443-4070
E-mail: kruhland@syr.edu.
[**] This work was supported by the National Science Foundation (CHE-
9409446 and CHE-9702246) and the Deutsche Forschungsgemein-
schaft (postdoctoral stipendium for U.E.). Purchase of the X-Ray
diffractometer was made possible by grants from the NSF (CHE-95 ±
27898), the W. M. Keck Foundation, and Syracuse University.
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