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HF-EPR to monitor electron transfer in mixed valence dioxolene metal complexes
Chemical Physics Letters 368 (2003) 162–167 www.elsevier.com/locate/cplett
HF-EPR to monitor electron transfer in mixed valence dioxolene metal complexes Andrea Dei a, Dante Gatteschi a,*, Claudio Sangregorio a, Lorenzo Sorace b, Maria G.F. Vaz a a
Department of Chemistry, Universit a degli Studi di Firenze Via della Lastruccia 3, Sesto Fiorentino 50019, Italy b LCMI-CNRS 25, Avenue des Martyrs, Grenoble 38042, France Received 19 September 2002; in final form 8 November 2002
Abstract The reaction of Mn(II) salts with N ; N 0 -bis(3,5-di-tert-butyl-2-hydroxyphenyl)-1,3-phenylenediamine yields a dinuclear Mn2 L3 complex, which on the basis of its structural and magnetic characterisation can be formulated as a Mn(IV) derivative of both dinegative bis-iminosemiquinonato and mixed valence trinegative iminosemiquinonato–iminocatecholato forms of the reacting ligand. HF-EPR spectra at various frequencies suggest that the electron transfer process in this mixed valence species is frozen on the time scale of EPR at high frequency, whereas is still dynamic at lower frequencies. This spectral behaviour therefore indicates a thermally activated character of the electron transfer process, as expected in a class II mixed valence system. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction An important feature of 3d metal o-dioxolene complexes is their charge localised electronic character [1–3]. o-Dioxolenes can bind to metal ions in three different redox states, corresponding to catecholate, Cat, semiquinone, SQ, and quinone, Q, respectively. The combination of redoxactive character of the metal acceptor and the coordinated ligands may allow the formation of very peculiar molecular systems, thus providing *
Corresponding author. Fax: +39-055-4573372. E-mail addresses: dante.gatteschi@unifi.it (D. Gatteschi), mariavaz@unifi.it (M.G.F. Vaz).
unique examples for the investigation of intramolecular electron transfer (ET) processes [4–6]. These processes may in turn concern either the metal ion and the coordinated ligand and the ligands themselves if coordinated in different oxidation states, thus affording bistable systems of potential technological interest. Properly designed bis- and tris-dioxolene ligands have been recently synthesised, with the goal of obtaining extended magnetic structures [7–10]. These ligands show more complex redox chains than the simple o-dioxolenes. As an example, a bisdioxolene may exist in five different oxidation states, i.e., Cat–Cat, SQ–Cat, Q–Cat, Q–SQ and Q–Q, the SQ–Cat and Q–Cat species being mixed-
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 1 8 3 1 - 6
A. Dei et al. / Chemical Physics Letters 368 (2003) 162–167
valence radical ligands whose nature can be classified according to Robin and Day [11] as belonging to class I, II or III. It should be stressed that although the ET processes involving bridged metal ions in different oxidation states have been well investigated, there are few examples of similar studies involving mixed valence ligands. This gives exciting perspectives for observing temperature dependent phenomena and designing systems reminding the Aviram–Ratner [12,13] molecular diode prototypes. Some of us have recently shown how some complexes formed by a bis-dioxolene radical ligand SQ–Cat can be properly described as class III mixed valence systems [14]. We want to show here how a dinuclear compound of formula Mn2 L3 , where H4 L ¼ N ; N 0 -bis(3,5-di-tert-butyl2-hydroxyphenyl)-1,3-phenylenediamine, which can be formulated as a MnIV complex of both SQ–Cat(I) and SQ–SQ(II) ligands (see Schemes 1, 2) is a mixed valence compound belonging to class II where the ET process is frozen on the time scale of high frequency EPR spectroscopy, while it is still dynamic at the lower frequencies of conventional EPR.
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2. Experimental 2.1. Synthesis The ligand N ; N 0 -bis(3,5-di-tert-butyl-2-hydroxyphenyl)-1,3-phenylenediamine was synthesised as reported elsewhere [15]. MnCl2 and the above ligand were allowed to react in stoichiometric ratio 1:1.5 in refluxing acetonitrile for 5 h in the presence of triethylamine. The resulting product was filtered and recrystallised from acetonitrile/acetone, yielding deep-brown crystals after 24 h. The crystals satisfactorily analysed for Mn2 C102 H132 N6 O6 , calcd: C, 74.34; H, 8.07; N, 5.10; Mn, 6.67; found: C, 74.21; H, 8.14; N, 4.96; Mn 6.43. 2.2. Physical measurements Magnetic measurements were performed on a polycrystalline powder using a Cryogenic S600 SQUID magnetometer. The high field magnetisation curve was measured on a VSM from Oxford Instruments. HF-EPR spectra were recorded on a home-build spectrometer at the L.C.M.I.-C.N.R.S. in Grenoble, France. Sample was pressed in a pellet to prevent preferential orientation of the crystallites. Polycrystalline powder spectra were recorded at X-band on a Varian ESR9 spectrometer. 2.3. Crystal data and structure refinement
Scheme 1.
Scheme 2.
Data collection was made on a CCD-1K three circles Bruker diffractometer, Cu Ka radiation Þ, g€ ðk ¼ 1:54178A obel mirrors monochromator, x ) 2h scan, 293 K. Intensities were corrected for absorption (SADABS). Structure was successfully solved by direct methods (SIR97) [16] which gave the position of all non-hydrogen atoms but few carbon atoms of the tert-butyl groups. Remaining atoms were identified by successive Fourier difference syntheses using SH E L X L 97 [17]. The structure was refined against F 2 with full matrix least-squares refinement using 10 872 independent reflections (of which 7476 observed, I > 2rðIÞ), and 1120 parameters. Hydrogen atoms were added in calculated positions assuming idealised bond geometries, except for some H atoms of the
164
A. Dei et al. / Chemical Physics Letters 368 (2003) 162–167
m-phenylene that were located on a difference map. Anisotropic thermal factors were used for 113 of 118 non-hydrogen atoms. Details: Crystal dimensions: 0:15 0:20 0:60 mm3 . Crystal data: Mn2 C102 H132 N6 O6 ; formula weight 1648.02, monoclinic, space group C2/c, , b ¼ 15:442ð1Þ A , c ¼ 32:157ð2Þ a ¼ 39:581ð3Þ A , b ¼ 91:876ð4Þ°, V 19644(2) A 3 , Z ¼ 8, q A calcd ¼ 1:114 g=cm3 , l ¼ 2:497 cm1 , 2:23° < h < 55:60°. F(000) ¼ 7072, GOF on F 2 ¼ 0:967. Final R index ½I > 2rðIÞ R1 ¼ 0:0461, R index (all data) R1 ¼ 0:0787. Largest diff. peak and hole 0.527 and 3 . )0.365 e A
3. Results and discussion The X-ray structure of Mn2 L3 , 1, is shown in Fig. 1. The L ligands act as bis-bidentate bridging two six-coordinated manganese ions. The structural parameters, i.e., bond lengths Mn–O and Mn–N
and structural features of the ligands, suggest that this compound should be formulated as a MnIV derivative. It should be stressed that the three iminodioxolene rings surrounding each manganese ion are different: one shows the typical C–O, C1 –C2 and C–N bond lengths expected for an iminosemiquinonato, whereas those of the other two are intermediates between what expected for a semiquinonato and a catecholato ligand [18]. This charge distribution is the same observed in the mononuclear complex MnIV ðDBSQÞ2 (DBCat) (DBSQ and DBCat ¼ semiquinonato and catecholato forms of 3,6-di-tert-butyl-benzoquinone) [19]. Therefore two of the three bridging ligands have a mixed valence character but their nature in terms of Robin and Day classification is rather unclear, since the observed intermediate bond lengths for four of the six rings may originate either from electron delocalisation or structural disorder. The existence of an electron delocalisation process involving the radical ligands is clearly
) (standard Fig. 1. Molecular structure of 1. Hydrogen atoms and tert-butyl groups are omitted for clarity. Selected bond lengths (A deviations are given in parentheses): Mn1–O23 1.904(2), Mn1–O22 1.909(2), Mn1–N23 1.953(3), Mn1–N22 1.959(3), Mn1–N21 1.987(3), Mn1–O21 1.988(2), Mn2–O11 1.894(2), Mn2–O13 1.910(2), Mn2–N11 1.928(3), Mn2–N13 1.949(3), Mn2–O12 1.954(2), Mn2–N12 1.966(3), C111–N21 1.348(4), C131–N11 1.368(4), C216–N22 1.369(4), C231–N12 1.342(4), C311–N23 1.368(4), C336–N13 1.374(4), C111–C116 1.439(5), C131–C136 1.428(5), C211–C216 1.422(5), C231–C236 1.444(5), C311–C316 1.424(5), C336–C331 1.415(5), C111–O21 1.293(4), C136–O11 1.330(4), C211–O22 1.322(4), C236–O12 1.286(4), C316–O23 1.322(4), C331–O13 1.326(4).
A. Dei et al. / Chemical Physics Letters 368 (2003) 162–167
evidenced by the variable temperature-variable frequency EPR spectra shown in Fig. 2. The X-band spectra show a marked temperature dependence, with a line moving to low field on decreasing temperature and whose intensity passes through a maximum around 60 K. The assignment of the spectra is certainly not straightforward but there is no doubt that dynamic phenomena are monitored by the spectroscopic technique. On the other hand spectra measured at 285 GHz are easily interpreted on the basis of axial triplet spectra, and could be satisfactorily fitted using the spin hamiltonian H ¼ bSgH þ DS 2z with gk ¼ 2:004, g? ¼ 2:003, D ¼ 0:096 cm1 [20]. Except for some broadening of the features on increasing temperature their observed temperature dependency is the one expected on the basis of polarisation effects associated with the use of high field and high frequency [21] and no shift of the line position was observed up to 50 K. An intermediate situation is encountered for spectra recorded at 190 GHz whose high field line slowly shifts to higher fields on lowering temperature. These spectra could not be fitted by using a single set of parameters for different temperatures and large discrepancies were observed by employing the same parameter set of 285 GHz, thus suggesting that a dynamic process is still affecting the spectra at this frequency. The experimental data indicate that a low lying triplet state shows dynamic phenomena, presumably associated with the ET of the electron between the Cat and SQ sites which is comparable to the characteristic time of the EPR experiment [22].
165
The presence of a S ¼ 0 ground state with an excited triplet at ca. 7 cm1 is shown by the temperature dependence of vT shown in Fig. 3. At room temperature vT is ca. 0.9 cm3 mol1 K, much smaller than expected for two MnIV (S ¼ 3=2) and four SQ groups (S ¼ 1=2) vT ¼ 5:25 cm3 mol1 K. This is a clear indication of strong antiferromagnetic interactions operative in the compound. Actually the room temperature vT value is close to that expected for two weakly coupled S ¼ 1=2 states. On decreasing temperature vT steadily decreases indicating residual antiferromagnetic interactions. In fact v goes through a maximum at 6 K and its temperature dependence can be satisfactorily fit with a Bleaney–Bowers equation (J ¼ 6:1 cm1 ). This rough interpretation is confirmed by the field dependence of the magnetisation at 1.5 K (Fig. 4) which shows a field induced crossover between singlet and triplet ground state at ca. 7.5 T, corresponding to an energy gap of 7 cm1 . A detailed fit of the vT vs. T curve is in progress taking into account dynamic phenomena. ET in a magnetic system Cat–SQ determines the doubling of the spin levels, separated by the interaction energy B. If B ¼ 0 the two doublets are degenerate, while if B is large the two doublets have a large splitting. The former case corresponds to localisation, the latter to delocalisation of the unpaired electron [23]. In the simplifying assumption that the interaction energy parameter B is small, i.e., in a localised view, four resonating structure are possible in which one of the bridging ligands is in
Fig. 2. Temperature dependence of EPR spectra of 1 at different frequencies. (a) X-Band (9.23 GHz) spectra. The dashed line evidences the low field shifting of the line described in the text. (b) 285 GHz spectra with simulated spectra for 5 and 10 K (parameters reported in the text). (c) 190 GHz spectra.
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Fig. 3. vT vs. T data (full triangles) for 1 and best fit (continuous line). The inset shows the low temperature part of v vs. T curve and the best fit obtained by a Bleaney–Bowers equation. See text for details.
Fig. 4. Magnetisation curve of 1 at 1.5 K. The inset evidences that the field transition between singlet and triplet occurs at 7.5 T.
the state SQ–SQ, and the other two in the SQ–Cat one, each MnIV interacting with two semiquinones. On this basis a satisfactory fit could be obtained employing the spin Hamiltonian: H ¼ JMn–SQ ðSMn1 SSQ31 þ SMn1 SSQ21 þ SMn2 SSQ12 þ SMn2 SSQ32 Þ þ JSQ–SQ ðSSQ31 SSQ32 Þ 0 þ JSQ–SQ ðSSQ31 SSQ21 þ SSQ12 SSQ32 Þ;
ð1Þ
where the subscripts of SQ indicate the i ligand and the position close to the j Mn ion. Best fit parameters were JMn–SQ > 400 cm1 , JSQ–SQ ¼ 65
0 < 400 cm1 . The obtained 5 cm1 and JSQ–SQ values fully agree with the above given qualitative description, the strong antiferromagnetic interaction between each MnIV centres and two semiquinones leading to two weakly antiferromagnetically coupled S ¼ 1=2, with a singlet triplet gap of 7.2 cm1 . The observation of an antiferromagnetic coupling within the biradical is in agreement with the results reported for the similar Co(III) and Fe(III) derivatives [15]. On the other hand no acceptable fit could be obtained by assuming one of the spins to be completely delocalised over a bridging ligand.
A. Dei et al. / Chemical Physics Letters 368 (2003) 162–167
The analysis of the EPR spectra suggests that the spin multiplets are at least doubled by the ET process. The different spin triplets should have identical spin Hamiltonian parameters, but different orientations of the spin Hamiltonian tensors. This gives rise to a temperature and frequency dependence of the EPR spectra. At 5 K the spectra are still dynamic, meaning that the characteristic 1 time of the ET process, s, is shorter than ð2pmÞ at 9 GHz ð1:77 1011 sÞ, while the 285 GHz data show that it is longer than 5:6 1013 s. A more detailed description of the properties of this exciting molecule require measurements on single crystal which are actually in progress. Supporting information available Crystallographic data (excluding structure factors) for the structure reported in this Letter have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC-192860. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB21EZ, UK (fax: int.code +44-1223-336033; e-mail: deposit@ccdc. cam.ac.uk.
Acknowledgements The authors wish to thank A.-L. Barra for technical assistance in recording HF-EPR spectra. EU-network 3MD and MOLNANOMAG, CNR and MIUR are gratefully acknowledged for financial support. M.G.F.V. gratefully acknowledges for a 1-year grant the Brazilian Conselho Nacional de Desenvolvimento Cientifico e Tecnologico CNPq.
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References [1] C.G. Pierpont, R.M. Buchanan, Coord. Chem. Rev. 38 (1981) 87. [2] C.G. Pierpont, C.W. Lange, Prog. Inorg. Chem. 41 (1994) 331. [3] C.G. Pierpont, Coord. Chem. Rev. 219 (2001) 415. [4] P. Gutlich, A. Dei, Angew. Chem. Int. Ed. Eng. 37 (1997) 2734. [5] D.A. Shultz, in: J.S. Miller, M. Drillon (Eds.), Magnetism: from Molecules to Materials, Wiley-VCH, Weinheim, 2001. [6] C.G. Pierpont, Coord. Chem. Rev. 221 (2001) 99. [7] D.A. Shultz, A.K. Boal, D.J. Driscoll, J.R. Kitchin, G.N. Tew, J. Org. Chem. 60 (1995) 3578. [8] D.A. Shultz, A.K. Boal, G.T. Farmer, J. Am. Chem. Soc. 119 (1997) 3846. [9] A. Caneschi, A. Dei, H. Lee, D.A. Shultz, L. Sorace, Inorg. Chem. 40 (2001) 408. [10] A. Caneschi, A. Dei, P. Mussari, D.A. Shultz, L. Sorace, K. Vostrikova, Inorg. Chem. 41 (2002) 1086. [11] M.B. Robin, P. Day, Adv. Inorg. Chem. Radiochem. 10 (1967) 248. [12] M. Ratner, Nature 404 (2000) 137. [13] C. Joachim, J.K. Gimzewski, A. Aviram, Nature 408 (2000) 541. [14] A. Bencini, C.A. Daul, A. Dei, F. Mariotti, H. Lee, D.A. Shultz, L. Sorace, Inorg. Chem. 40 (2001) 1582. [15] A. Dei, D. Gatteschi, C. Sangregorio, L. Sorace, M.G.F. Vaz, Inorg. Chem. (submitted). [16] SIR97 - A. Altomare et al., J. Appl. Cryst. 32 (1999) 115. [17] SHELX97 – Programs for crystal structure analysis (release 97-2). G.M. Sheldrick, Instit€ ut f€ ur Anorganische Chemie der Universit€at, Tammanstrasse 4, D-3400 G€ ottingen, Germany, 1998. [18] P. Chaudhuri, C. Nazari Verani, E. Bill, E. Bothe, T. Weyherm€ uller, K. Wieghardt, J. Am. Chem. Soc. 123 (2001) 2213. [19] A.S. Attia, C.G. Pierpont, Inorg. Chem. 37 (1998) 3051. [20] C.J.H. Jacobsen, E. Pedersen, J. Villadsen, H. Weihe, Inorg. Chem. 32 (1993) 1216. [21] A.L. Barra, L.C. Brunel, D. Gatteschi, L. Pardi, R. Sessoli, Acc. Chem. Res. 31 (1998) 460. [22] K. M€ obius, Chem. Soc. Rev. 29 (2000) 129. [23] G. Blondin, J.J. Girerd, Chem. Rev. 90 (1990) 1359.
HF-EPR to monitor electron transfer in mixed valence dioxolene metal complexes Andrea Dei a, Dante Gatteschi a,*, Claudio Sangregorio a, Lorenzo Sorace b, Maria G.F. Vaz a a
Department of Chemistry, Universit a degli Studi di Firenze Via della Lastruccia 3, Sesto Fiorentino 50019, Italy b LCMI-CNRS 25, Avenue des Martyrs, Grenoble 38042, France Received 19 September 2002; in final form 8 November 2002
Abstract The reaction of Mn(II) salts with N ; N 0 -bis(3,5-di-tert-butyl-2-hydroxyphenyl)-1,3-phenylenediamine yields a dinuclear Mn2 L3 complex, which on the basis of its structural and magnetic characterisation can be formulated as a Mn(IV) derivative of both dinegative bis-iminosemiquinonato and mixed valence trinegative iminosemiquinonato–iminocatecholato forms of the reacting ligand. HF-EPR spectra at various frequencies suggest that the electron transfer process in this mixed valence species is frozen on the time scale of EPR at high frequency, whereas is still dynamic at lower frequencies. This spectral behaviour therefore indicates a thermally activated character of the electron transfer process, as expected in a class II mixed valence system. Ó 2002 Elsevier Science B.V. All rights reserved.
1. Introduction An important feature of 3d metal o-dioxolene complexes is their charge localised electronic character [1–3]. o-Dioxolenes can bind to metal ions in three different redox states, corresponding to catecholate, Cat, semiquinone, SQ, and quinone, Q, respectively. The combination of redoxactive character of the metal acceptor and the coordinated ligands may allow the formation of very peculiar molecular systems, thus providing *
Corresponding author. Fax: +39-055-4573372. E-mail addresses: dante.gatteschi@unifi.it (D. Gatteschi), mariavaz@unifi.it (M.G.F. Vaz).
unique examples for the investigation of intramolecular electron transfer (ET) processes [4–6]. These processes may in turn concern either the metal ion and the coordinated ligand and the ligands themselves if coordinated in different oxidation states, thus affording bistable systems of potential technological interest. Properly designed bis- and tris-dioxolene ligands have been recently synthesised, with the goal of obtaining extended magnetic structures [7–10]. These ligands show more complex redox chains than the simple o-dioxolenes. As an example, a bisdioxolene may exist in five different oxidation states, i.e., Cat–Cat, SQ–Cat, Q–Cat, Q–SQ and Q–Q, the SQ–Cat and Q–Cat species being mixed-
0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 1 8 3 1 - 6
A. Dei et al. / Chemical Physics Letters 368 (2003) 162–167
valence radical ligands whose nature can be classified according to Robin and Day [11] as belonging to class I, II or III. It should be stressed that although the ET processes involving bridged metal ions in different oxidation states have been well investigated, there are few examples of similar studies involving mixed valence ligands. This gives exciting perspectives for observing temperature dependent phenomena and designing systems reminding the Aviram–Ratner [12,13] molecular diode prototypes. Some of us have recently shown how some complexes formed by a bis-dioxolene radical ligand SQ–Cat can be properly described as class III mixed valence systems [14]. We want to show here how a dinuclear compound of formula Mn2 L3 , where H4 L ¼ N ; N 0 -bis(3,5-di-tert-butyl2-hydroxyphenyl)-1,3-phenylenediamine, which can be formulated as a MnIV complex of both SQ–Cat(I) and SQ–SQ(II) ligands (see Schemes 1, 2) is a mixed valence compound belonging to class II where the ET process is frozen on the time scale of high frequency EPR spectroscopy, while it is still dynamic at the lower frequencies of conventional EPR.
163
2. Experimental 2.1. Synthesis The ligand N ; N 0 -bis(3,5-di-tert-butyl-2-hydroxyphenyl)-1,3-phenylenediamine was synthesised as reported elsewhere [15]. MnCl2 and the above ligand were allowed to react in stoichiometric ratio 1:1.5 in refluxing acetonitrile for 5 h in the presence of triethylamine. The resulting product was filtered and recrystallised from acetonitrile/acetone, yielding deep-brown crystals after 24 h. The crystals satisfactorily analysed for Mn2 C102 H132 N6 O6 , calcd: C, 74.34; H, 8.07; N, 5.10; Mn, 6.67; found: C, 74.21; H, 8.14; N, 4.96; Mn 6.43. 2.2. Physical measurements Magnetic measurements were performed on a polycrystalline powder using a Cryogenic S600 SQUID magnetometer. The high field magnetisation curve was measured on a VSM from Oxford Instruments. HF-EPR spectra were recorded on a home-build spectrometer at the L.C.M.I.-C.N.R.S. in Grenoble, France. Sample was pressed in a pellet to prevent preferential orientation of the crystallites. Polycrystalline powder spectra were recorded at X-band on a Varian ESR9 spectrometer. 2.3. Crystal data and structure refinement
Scheme 1.
Scheme 2.
Data collection was made on a CCD-1K three circles Bruker diffractometer, Cu Ka radiation Þ, g€ ðk ¼ 1:54178A obel mirrors monochromator, x ) 2h scan, 293 K. Intensities were corrected for absorption (SADABS). Structure was successfully solved by direct methods (SIR97) [16] which gave the position of all non-hydrogen atoms but few carbon atoms of the tert-butyl groups. Remaining atoms were identified by successive Fourier difference syntheses using SH E L X L 97 [17]. The structure was refined against F 2 with full matrix least-squares refinement using 10 872 independent reflections (of which 7476 observed, I > 2rðIÞ), and 1120 parameters. Hydrogen atoms were added in calculated positions assuming idealised bond geometries, except for some H atoms of the
164
A. Dei et al. / Chemical Physics Letters 368 (2003) 162–167
m-phenylene that were located on a difference map. Anisotropic thermal factors were used for 113 of 118 non-hydrogen atoms. Details: Crystal dimensions: 0:15 0:20 0:60 mm3 . Crystal data: Mn2 C102 H132 N6 O6 ; formula weight 1648.02, monoclinic, space group C2/c, , b ¼ 15:442ð1Þ A , c ¼ 32:157ð2Þ a ¼ 39:581ð3Þ A , b ¼ 91:876ð4Þ°, V 19644(2) A 3 , Z ¼ 8, q A calcd ¼ 1:114 g=cm3 , l ¼ 2:497 cm1 , 2:23° < h < 55:60°. F(000) ¼ 7072, GOF on F 2 ¼ 0:967. Final R index ½I > 2rðIÞ R1 ¼ 0:0461, R index (all data) R1 ¼ 0:0787. Largest diff. peak and hole 0.527 and 3 . )0.365 e A
3. Results and discussion The X-ray structure of Mn2 L3 , 1, is shown in Fig. 1. The L ligands act as bis-bidentate bridging two six-coordinated manganese ions. The structural parameters, i.e., bond lengths Mn–O and Mn–N
and structural features of the ligands, suggest that this compound should be formulated as a MnIV derivative. It should be stressed that the three iminodioxolene rings surrounding each manganese ion are different: one shows the typical C–O, C1 –C2 and C–N bond lengths expected for an iminosemiquinonato, whereas those of the other two are intermediates between what expected for a semiquinonato and a catecholato ligand [18]. This charge distribution is the same observed in the mononuclear complex MnIV ðDBSQÞ2 (DBCat) (DBSQ and DBCat ¼ semiquinonato and catecholato forms of 3,6-di-tert-butyl-benzoquinone) [19]. Therefore two of the three bridging ligands have a mixed valence character but their nature in terms of Robin and Day classification is rather unclear, since the observed intermediate bond lengths for four of the six rings may originate either from electron delocalisation or structural disorder. The existence of an electron delocalisation process involving the radical ligands is clearly
) (standard Fig. 1. Molecular structure of 1. Hydrogen atoms and tert-butyl groups are omitted for clarity. Selected bond lengths (A deviations are given in parentheses): Mn1–O23 1.904(2), Mn1–O22 1.909(2), Mn1–N23 1.953(3), Mn1–N22 1.959(3), Mn1–N21 1.987(3), Mn1–O21 1.988(2), Mn2–O11 1.894(2), Mn2–O13 1.910(2), Mn2–N11 1.928(3), Mn2–N13 1.949(3), Mn2–O12 1.954(2), Mn2–N12 1.966(3), C111–N21 1.348(4), C131–N11 1.368(4), C216–N22 1.369(4), C231–N12 1.342(4), C311–N23 1.368(4), C336–N13 1.374(4), C111–C116 1.439(5), C131–C136 1.428(5), C211–C216 1.422(5), C231–C236 1.444(5), C311–C316 1.424(5), C336–C331 1.415(5), C111–O21 1.293(4), C136–O11 1.330(4), C211–O22 1.322(4), C236–O12 1.286(4), C316–O23 1.322(4), C331–O13 1.326(4).
A. Dei et al. / Chemical Physics Letters 368 (2003) 162–167
evidenced by the variable temperature-variable frequency EPR spectra shown in Fig. 2. The X-band spectra show a marked temperature dependence, with a line moving to low field on decreasing temperature and whose intensity passes through a maximum around 60 K. The assignment of the spectra is certainly not straightforward but there is no doubt that dynamic phenomena are monitored by the spectroscopic technique. On the other hand spectra measured at 285 GHz are easily interpreted on the basis of axial triplet spectra, and could be satisfactorily fitted using the spin hamiltonian H ¼ bSgH þ DS 2z with gk ¼ 2:004, g? ¼ 2:003, D ¼ 0:096 cm1 [20]. Except for some broadening of the features on increasing temperature their observed temperature dependency is the one expected on the basis of polarisation effects associated with the use of high field and high frequency [21] and no shift of the line position was observed up to 50 K. An intermediate situation is encountered for spectra recorded at 190 GHz whose high field line slowly shifts to higher fields on lowering temperature. These spectra could not be fitted by using a single set of parameters for different temperatures and large discrepancies were observed by employing the same parameter set of 285 GHz, thus suggesting that a dynamic process is still affecting the spectra at this frequency. The experimental data indicate that a low lying triplet state shows dynamic phenomena, presumably associated with the ET of the electron between the Cat and SQ sites which is comparable to the characteristic time of the EPR experiment [22].
165
The presence of a S ¼ 0 ground state with an excited triplet at ca. 7 cm1 is shown by the temperature dependence of vT shown in Fig. 3. At room temperature vT is ca. 0.9 cm3 mol1 K, much smaller than expected for two MnIV (S ¼ 3=2) and four SQ groups (S ¼ 1=2) vT ¼ 5:25 cm3 mol1 K. This is a clear indication of strong antiferromagnetic interactions operative in the compound. Actually the room temperature vT value is close to that expected for two weakly coupled S ¼ 1=2 states. On decreasing temperature vT steadily decreases indicating residual antiferromagnetic interactions. In fact v goes through a maximum at 6 K and its temperature dependence can be satisfactorily fit with a Bleaney–Bowers equation (J ¼ 6:1 cm1 ). This rough interpretation is confirmed by the field dependence of the magnetisation at 1.5 K (Fig. 4) which shows a field induced crossover between singlet and triplet ground state at ca. 7.5 T, corresponding to an energy gap of 7 cm1 . A detailed fit of the vT vs. T curve is in progress taking into account dynamic phenomena. ET in a magnetic system Cat–SQ determines the doubling of the spin levels, separated by the interaction energy B. If B ¼ 0 the two doublets are degenerate, while if B is large the two doublets have a large splitting. The former case corresponds to localisation, the latter to delocalisation of the unpaired electron [23]. In the simplifying assumption that the interaction energy parameter B is small, i.e., in a localised view, four resonating structure are possible in which one of the bridging ligands is in
Fig. 2. Temperature dependence of EPR spectra of 1 at different frequencies. (a) X-Band (9.23 GHz) spectra. The dashed line evidences the low field shifting of the line described in the text. (b) 285 GHz spectra with simulated spectra for 5 and 10 K (parameters reported in the text). (c) 190 GHz spectra.
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A. Dei et al. / Chemical Physics Letters 368 (2003) 162–167
Fig. 3. vT vs. T data (full triangles) for 1 and best fit (continuous line). The inset shows the low temperature part of v vs. T curve and the best fit obtained by a Bleaney–Bowers equation. See text for details.
Fig. 4. Magnetisation curve of 1 at 1.5 K. The inset evidences that the field transition between singlet and triplet occurs at 7.5 T.
the state SQ–SQ, and the other two in the SQ–Cat one, each MnIV interacting with two semiquinones. On this basis a satisfactory fit could be obtained employing the spin Hamiltonian: H ¼ JMn–SQ ðSMn1 SSQ31 þ SMn1 SSQ21 þ SMn2 SSQ12 þ SMn2 SSQ32 Þ þ JSQ–SQ ðSSQ31 SSQ32 Þ 0 þ JSQ–SQ ðSSQ31 SSQ21 þ SSQ12 SSQ32 Þ;
ð1Þ
where the subscripts of SQ indicate the i ligand and the position close to the j Mn ion. Best fit parameters were JMn–SQ > 400 cm1 , JSQ–SQ ¼ 65
0 < 400 cm1 . The obtained 5 cm1 and JSQ–SQ values fully agree with the above given qualitative description, the strong antiferromagnetic interaction between each MnIV centres and two semiquinones leading to two weakly antiferromagnetically coupled S ¼ 1=2, with a singlet triplet gap of 7.2 cm1 . The observation of an antiferromagnetic coupling within the biradical is in agreement with the results reported for the similar Co(III) and Fe(III) derivatives [15]. On the other hand no acceptable fit could be obtained by assuming one of the spins to be completely delocalised over a bridging ligand.
A. Dei et al. / Chemical Physics Letters 368 (2003) 162–167
The analysis of the EPR spectra suggests that the spin multiplets are at least doubled by the ET process. The different spin triplets should have identical spin Hamiltonian parameters, but different orientations of the spin Hamiltonian tensors. This gives rise to a temperature and frequency dependence of the EPR spectra. At 5 K the spectra are still dynamic, meaning that the characteristic 1 time of the ET process, s, is shorter than ð2pmÞ at 9 GHz ð1:77 1011 sÞ, while the 285 GHz data show that it is longer than 5:6 1013 s. A more detailed description of the properties of this exciting molecule require measurements on single crystal which are actually in progress. Supporting information available Crystallographic data (excluding structure factors) for the structure reported in this Letter have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC-192860. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB21EZ, UK (fax: int.code +44-1223-336033; e-mail: deposit@ccdc. cam.ac.uk.
Acknowledgements The authors wish to thank A.-L. Barra for technical assistance in recording HF-EPR spectra. EU-network 3MD and MOLNANOMAG, CNR and MIUR are gratefully acknowledged for financial support. M.G.F.V. gratefully acknowledges for a 1-year grant the Brazilian Conselho Nacional de Desenvolvimento Cientifico e Tecnologico CNPq.
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