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Polyanionic high-spin hydrocarbons with high symmetry
Synthetic Metals 102 (1999) 1.544-1545
Polyanionic high-spin hydrocarbons with high symmetry M.C.B.L. Shohoji’,
M.L.T.M.B.
aDepartment of Chemistry. Departments of %hemistry and
France”, M.C.R.L.R. Lazana ‘, S. Nakazawa b, K. Satob, D. Shiomi’, T Takui b, K. ltoh’
Instiruto ‘Material
Superior Science,
Tknico, Graduate
Universidade Tknica de Lkboa, School of Science, Osaka City
1096 Lisboa Codex, Portugal University, Osaka, 558-8585. Japan
Abstract The quartet ground state from trianionic decacyclene with 3-fold symmetry has been for the first time observed and the finestructure ESR spectra from random orientation have been completely analyzed. The observation is involved in a controversy with the well-documented results showing the spin-doublet ground state for the trianionic decacyclene. Keywords:
Electron spin resonance
1. Introduction As early as in 1925, on empirical grounds Hund formulated a famous rule for a criterion of judging what spin multiplicity of a given electronic configuration belongs to the lowest energy state in one-nuclear center systems. Application of Hund’s rule to molecular systems, which is called molecular Hund’s rule in this paper, appears in many text books, but the proof or validity of the application is only apparent. In molecular spin science and the research field of molecule-based magnetics, interpretations or molecular designs in terms of molecular Hund’s rule have frequently been invoked until so far. Recently, even statements of “violation of Hund’s rule” have appeared in the literature dealing with molecule-based magnetics. There is no violation of Hund’s rule, but there is the invalidity of a simple application of Hund’s rule to organic molecular open-shell systems. In this context, the wording “violation” is improper. The hydrocarbons with C, symmetry, e.g., coronene, decacyclene, 1,3,5triphenyl benzene, have two degenerate LUMOs near the zero energy level. When molecular Hund’s rule is simply applied to these systems, dianionic molecules are expected to be triplet ground state. But the experimental results showed a singlet ground state for dianionic coronene and triplet ground states for dianionic decacyclene and 1,3,5-triphenyl benzene[l]. Decacyclene has a pseudo degenerate molecular orbital ‘A state near two degenerate LUMOs according to Hiickel MO calculation. The experimental results showed that trianionic decacyclene was a doublet ground state[2]. In this paper we report that trianionic decacyclene is in a quartet ground state. 2. Experimental Polyanionic reduction of
decacyclene decacyclene
was generated by the chemical with alkali metals at ambient
temperature in 2-methyltetrahydrofuran(MTHF) including dicyclohexano-18-crown-6-ether (crown ether). The chemical reduction was carried out in a sealed-off glass apparatus with a cell for ESR measurements. ESR measurements were made with a JESFEZXG Xband ESR spectrometer at 1lOK and a Bruker ESP3CO spectrometer equipped with an Oxford ESR910 temperature controller below 1lOK.
/
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\/ Decacyclene
3. Results and Discussion The first reduction generated the metal-bridged high-spin cluster consisted of two monoanionic decacyclenes. Finestructure spectra from the intramolecular triplet dianionic decacyclene were also observed. The spectral simulations have been made by an exact numerical diagonalization of the eigenfield equation, where all the resonance fields and transition intensities for allowed and forbidden transitions have been calculated with Boltzmann distribution taken into account for the intensity evaluation. The experimentally determined finestructure constants are S=l, p]= 0.021Ocm’ and H-0, which agree with the documented values[l]. The further reduction yielded trianionic decacyclene with a spin quartet state. Figures 1 and 2 show the observed spectra at 90K and simulated finestructure spectra of the forbidden and allowed transitions, respectively, from trianionic decacyclene. These quartet-state fine-structure spectra have been for the first time observed from polyanionic hydrocarbons with 3-fold symmetry. It should be noted that as seen in Figure 1 the AMs = ~2 forbidden transitions appear in quite an anisotropic fashion for a
0379-6779/99/$ - see front matter 0 1999 Elsevier Science S.A. All rights reserved. PII: SO379-6779(98)00535-9
S. Nakazawa
et al. I Synthetic
----I 0.10
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0.14
0.16
0.18
Metals
102 (1999)
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1.544-1545
1545
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MAGNETlCFlELD/T Fig. 1. Fine-structure ESR spectra of trianionic decacyclene (forbidden transitions) (a) Observed spectrum (b) Simulated spectrum considerably large D-value and AMs = *3 forbidden transitions appear near O.llT. The strong peak appearing near 0.168 T in Figure 1 arises from the forbidden transitions of triplet-state species. The experimentally determined spin Hamiltonian parameters are as follows; S = 3/2, u= 0.0116 cm’, E]= O.Ocm’, and g = 2.001. The temperature dependence of ESR signal intensity indicated that the magnitude of the intensity increased as temperature decreased, so the observed quartet state was the ground state. Electronic spin and molecular structures of pluri-anionic decacyclenes and high spin clusters were studied. Assuming similar molecular structures for both dianionic and trianionic decacyclene since both have vanishing E-value, a value of (2.!& l)H represents a measure of spin-spin interactions for the intramolecular triplet state and quartet state: 0.0210 cm’ for S = 1 and 0.0232 cm’ for S = 3/2. This agreement shows that the observed quartet state originates in an intramolecular exchange interaction inherent in decacyclene trianion. The vanishing E value also indicates that the quartet state is of 3-fold symmetry. On the other hand, the triplet-state species with the smaller m values and nonvanishing E-values arise from intermolecular highspin clusters. Referred to the observation of the spin-quartet state from trianionic decacyclene, the present results are involved in a controversy with the well-established documented results[2]. Now a question arises why we have observed the quartet ground state. Without the help of crown ether we have never obtained the
0.30
0.32
0.34
, , , 0.36
MAGNETI C FIELD / T Fig. 2. Fine-structure ESRspectra of trianionic decacyclene (allowed transitions) (a) Observed spectrum (b) Simulated spectrum quartet-state spectra, suggesting that the ‘A state is stabilized remarkably by the influence of a change in polarity in solution, which is driven by the presence of crown ether with an alkaline ion. The stabilization drives the high-spin preference of the trianion in the ground state because of the additive n-spin polarization. 4. Conclusions The quartet ground state from trianionic decacyclene with 3fold symmetry has been for the first time observed and the finestructure ESR spectra from random orientation have been completely analyzed. The observation in the present study is involved in a controversy with the well-documented results showing the spin-doublet ground state for the trianionic decacyclene. A rational for the possible explanation has been given. The high spin clusters have been identified and their probable molecular structure is a sandwich tyPe. References [l] R.E. Jesse, P. Biloen, R. Prins, J.D.W. van Voorst, G.J. Hoijtink, Mol. Phys., 6 (1963) 633. [2] P. Brassem, R.E. Jesse, G.J. Hoijtink, Mol. Phys, 7 (1964) 587.
Polyanionic high-spin hydrocarbons with high symmetry M.C.B.L. Shohoji’,
M.L.T.M.B.
aDepartment of Chemistry. Departments of %hemistry and
France”, M.C.R.L.R. Lazana ‘, S. Nakazawa b, K. Satob, D. Shiomi’, T Takui b, K. ltoh’
Instiruto ‘Material
Superior Science,
Tknico, Graduate
Universidade Tknica de Lkboa, School of Science, Osaka City
1096 Lisboa Codex, Portugal University, Osaka, 558-8585. Japan
Abstract The quartet ground state from trianionic decacyclene with 3-fold symmetry has been for the first time observed and the finestructure ESR spectra from random orientation have been completely analyzed. The observation is involved in a controversy with the well-documented results showing the spin-doublet ground state for the trianionic decacyclene. Keywords:
Electron spin resonance
1. Introduction As early as in 1925, on empirical grounds Hund formulated a famous rule for a criterion of judging what spin multiplicity of a given electronic configuration belongs to the lowest energy state in one-nuclear center systems. Application of Hund’s rule to molecular systems, which is called molecular Hund’s rule in this paper, appears in many text books, but the proof or validity of the application is only apparent. In molecular spin science and the research field of molecule-based magnetics, interpretations or molecular designs in terms of molecular Hund’s rule have frequently been invoked until so far. Recently, even statements of “violation of Hund’s rule” have appeared in the literature dealing with molecule-based magnetics. There is no violation of Hund’s rule, but there is the invalidity of a simple application of Hund’s rule to organic molecular open-shell systems. In this context, the wording “violation” is improper. The hydrocarbons with C, symmetry, e.g., coronene, decacyclene, 1,3,5triphenyl benzene, have two degenerate LUMOs near the zero energy level. When molecular Hund’s rule is simply applied to these systems, dianionic molecules are expected to be triplet ground state. But the experimental results showed a singlet ground state for dianionic coronene and triplet ground states for dianionic decacyclene and 1,3,5-triphenyl benzene[l]. Decacyclene has a pseudo degenerate molecular orbital ‘A state near two degenerate LUMOs according to Hiickel MO calculation. The experimental results showed that trianionic decacyclene was a doublet ground state[2]. In this paper we report that trianionic decacyclene is in a quartet ground state. 2. Experimental Polyanionic reduction of
decacyclene decacyclene
was generated by the chemical with alkali metals at ambient
temperature in 2-methyltetrahydrofuran(MTHF) including dicyclohexano-18-crown-6-ether (crown ether). The chemical reduction was carried out in a sealed-off glass apparatus with a cell for ESR measurements. ESR measurements were made with a JESFEZXG Xband ESR spectrometer at 1lOK and a Bruker ESP3CO spectrometer equipped with an Oxford ESR910 temperature controller below 1lOK.
/
‘y -
\ @
\
/’
’
I\
\/ Decacyclene
3. Results and Discussion The first reduction generated the metal-bridged high-spin cluster consisted of two monoanionic decacyclenes. Finestructure spectra from the intramolecular triplet dianionic decacyclene were also observed. The spectral simulations have been made by an exact numerical diagonalization of the eigenfield equation, where all the resonance fields and transition intensities for allowed and forbidden transitions have been calculated with Boltzmann distribution taken into account for the intensity evaluation. The experimentally determined finestructure constants are S=l, p]= 0.021Ocm’ and H-0, which agree with the documented values[l]. The further reduction yielded trianionic decacyclene with a spin quartet state. Figures 1 and 2 show the observed spectra at 90K and simulated finestructure spectra of the forbidden and allowed transitions, respectively, from trianionic decacyclene. These quartet-state fine-structure spectra have been for the first time observed from polyanionic hydrocarbons with 3-fold symmetry. It should be noted that as seen in Figure 1 the AMs = ~2 forbidden transitions appear in quite an anisotropic fashion for a
0379-6779/99/$ - see front matter 0 1999 Elsevier Science S.A. All rights reserved. PII: SO379-6779(98)00535-9
S. Nakazawa
et al. I Synthetic
----I 0.10
0.12
0.14
0.16
0.18
Metals
102 (1999)
(4
0.20
1.544-1545
1545
4 o--0.30
0.32
(b)
0.34
0.36
0
5
II
---A--! I-+---
@-
I
0.10
0.12
.
,
0.14
’
,
0.16
*
,
0.18
I @ I @I,
7
0.20
MAGNETlCFlELD/T Fig. 1. Fine-structure ESR spectra of trianionic decacyclene (forbidden transitions) (a) Observed spectrum (b) Simulated spectrum considerably large D-value and AMs = *3 forbidden transitions appear near O.llT. The strong peak appearing near 0.168 T in Figure 1 arises from the forbidden transitions of triplet-state species. The experimentally determined spin Hamiltonian parameters are as follows; S = 3/2, u= 0.0116 cm’, E]= O.Ocm’, and g = 2.001. The temperature dependence of ESR signal intensity indicated that the magnitude of the intensity increased as temperature decreased, so the observed quartet state was the ground state. Electronic spin and molecular structures of pluri-anionic decacyclenes and high spin clusters were studied. Assuming similar molecular structures for both dianionic and trianionic decacyclene since both have vanishing E-value, a value of (2.!& l)H represents a measure of spin-spin interactions for the intramolecular triplet state and quartet state: 0.0210 cm’ for S = 1 and 0.0232 cm’ for S = 3/2. This agreement shows that the observed quartet state originates in an intramolecular exchange interaction inherent in decacyclene trianion. The vanishing E value also indicates that the quartet state is of 3-fold symmetry. On the other hand, the triplet-state species with the smaller m values and nonvanishing E-values arise from intermolecular highspin clusters. Referred to the observation of the spin-quartet state from trianionic decacyclene, the present results are involved in a controversy with the well-established documented results[2]. Now a question arises why we have observed the quartet ground state. Without the help of crown ether we have never obtained the
0.30
0.32
0.34
, , , 0.36
MAGNETI C FIELD / T Fig. 2. Fine-structure ESRspectra of trianionic decacyclene (allowed transitions) (a) Observed spectrum (b) Simulated spectrum quartet-state spectra, suggesting that the ‘A state is stabilized remarkably by the influence of a change in polarity in solution, which is driven by the presence of crown ether with an alkaline ion. The stabilization drives the high-spin preference of the trianion in the ground state because of the additive n-spin polarization. 4. Conclusions The quartet ground state from trianionic decacyclene with 3fold symmetry has been for the first time observed and the finestructure ESR spectra from random orientation have been completely analyzed. The observation in the present study is involved in a controversy with the well-documented results showing the spin-doublet ground state for the trianionic decacyclene. A rational for the possible explanation has been given. The high spin clusters have been identified and their probable molecular structure is a sandwich tyPe. References [l] R.E. Jesse, P. Biloen, R. Prins, J.D.W. van Voorst, G.J. Hoijtink, Mol. Phys., 6 (1963) 633. [2] P. Brassem, R.E. Jesse, G.J. Hoijtink, Mol. Phys, 7 (1964) 587.