Thermodynamics of valence tautomeric interconversion in a tetrachlorodioxolene:cobalt 1:1 adduct

Inorganica Chimica Acta 361 (2008) 3842–3846

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Thermodynamics of valence tautomeric interconversion in a tetrachlorodioxolene:cobalt 1:1 adduct Andrea Dei a, Alessandro Feis b, Giordano Poneti a, Lorenzo Sorace a,* a b

L.A.M.M, Department of Chemistry and INSTM Research Unit, Università di Firenze, Via Della Lastruccia 3, 50019 Sesto Fiorentino (FI), Italy Department of Chemistry, Università di Firenze, Via Della Lastruccia 3, 50019 Sesto Fiorentino (FI), Italy

a r t i c l e

i n f o

Article history: Received 18 February 2008 Accepted 27 February 2008 Available online 4 March 2008 This paper is dedicated to Prof. Dante Gatteschi. Keywords: Valence tautomerism Magnetism Electronic spectra Cobalt–dioxolene complexes

a b s t r a c t We report here the X-ray structure, solid state magnetic properties and temperature dependent solution electronic spectra of a 1:1 cobalt:tetrachlorodioxolene adduct. The study showed that in the solid state this system undergoes an intramolecular electron transfer above 300 K, passing from a Co(III)-Cat charge distribution to a Co(II)-SQ one (valence tautomeric interconversion). The occurrence of this process has also been evidenced in solution by the temperature dependent electronic spectra, showing the transition to take place around 250 K. This study also provided for the first time the thermodynamic parameters of the valence tautomeric process in a 1:1 adduct, confirming its entropy driven character. Ó 2008 Elsevier B.V. All rights reserved.

1. Introduction Intramolecular electron transfer involving 3d metal ions and dioxolene ligands is investigated since many years [1–19]. Among the several studied systems the cobalt derivatives are without any doubt the most attractive from a magnetic point of view. In all of the examples of this class to date reported the interconversion involves an intramolecular electron transfer between a six-coordinate diamagnetic cobalt(III) metal ion and a coordinated catecholato ligand yielding a cobalt(II)-semiquinonato species, the metal ion being in the high spin electronic configuration. In the simplest case of a 1:1 cobalt–dioxolene complex, the valence tautomeric transition is therefore associated with a change from a diamagnetic compound to a paramagnetic one, according to the equilibrium CoIII ðLÞCatþ ! CoII ðLÞðSQ Þþ

ð1Þ

where L is an ancillary ligand [20–23]. The change of the electronic population of the metal r orbitals occurring in (1) is associated with a significant change in Co–O bond lengths (about 0.18 Å). This makes the valence tautomeric process essentially entropy driven, as observed for spin crossover systems [24–26], the cobalt(III)catecholato species and the cobalt(II)-semiquinonato one being favored at low and high temperatures, respectively.

* Corresponding author. Tel.: +39 0554573336; fax: +39 0554573372. E-mail address: lorenzo.sorace@unifi.it (L. Sorace). 0020-1693/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2008.02.044

It is now well established that the transition between the two forms can be also induced by pressure changes [20,27] or by optical irradiation [1–3,14,17,22,23,28–30]. The latter aspect makes these compounds of potential technological interest as candidates for molecular switches. In analogy to what previously suggested for spin crossover complexes, the lifetime of the photoinduced metastable species was tentatively related with the critical temperature of the thermally induced interconversion process. In particular, it was suggested that the higher is the critical temperature, the faster is the decay from the photoinduced metastable state [22,23,26]. To obtain long living photoinduced state, a prerequisite for potential technological application, it is then crucial to be able to control the transition temperature (Tc) of the thermally induced interconversion, which is strictly related to the thermodynamic parameters of the system, Tc = DH/DS. Some of the investigations on valence tautomeric interconversions, most part of which concerns molecular dioxolene complexes of general formula Co(N–N)(diox)2 (N–N = diazine ligand, diox = 3,5- and 3,6-di-tert-butylcatecholato or the parent semiquinonato) [6–8,17,18] or related polynuclear complexes formed by tetraoxolene ligands acting as bis-bidentate [13,14], were therefore aimed at obtaining the thermodynamic quantities associated to equilibrium (1). In this laboratory we found that the charge distribution of a series of simple 1:1 cobalt–dioxolene adducts of general formula CoL(diox)Y (L = tetrazatetradentate ligand, diox = catecholato (Cat2) or semiquinonato (SQ) forms of 3,5-di-tert-butyl-1,2-dioxybenzene or 9,10-dioxy-phenanthrene, Y ¼ I ; PF6  ; BPh4  Þ was controlled by

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the nature of the dioxolene ligand and from that of the ancillary ligand [31,32]. As an example it was found that tetraazamacrocycles containing secondary amine donors stabilise the cobalt(III)-catecholato form, whereas those containing tertiary amine donors stabilise the cobalt(II)-semiquinonato species. The analysis of the electrochemical properties of these complexes led us to suppose that the best conditions for obtaining 1:1 cobalt–dioxolene cations undergoing valence tautomerism were reached by using CoL acceptors and dioxolene ligands showing similar CoIIIL/CoIIL and MIIL(SQ)/MIIL(Cat) redox potentials (MII = Ni, Zn). This approach was also used some years ago for analysing the energies of the LMCT transitions in some iron(III)-catecholato complexes [33,34]. Following this approach, some of us have recently shown that the observed and predictable trend in the redox potentials of a homologous family of 1:1 cobalt–dioxolene complexes can reasonably explain their photoswitchable and temperature dependent properties [21,23]. The steric hindrance induced by the successive introduction of methyl groups into 6-position of pyridine moieties of the ancillary ligand tpa (tris(2-pyridylmethyl)amine) was indeed found to modulate the redox properties of the metal acceptor in a series of cobalt complexes of general formula Co(Mentpa) (diox)PF6 (diox = 3,5-di-tert-butyl-1,2-dioxolene, n = 0, 1, 2, 3) [23, 35]. On its turn, this determines the charge distribution of the metal–dioxolene moiety at room temperature, which fits well with the expected one based on the calculated free energy changes for equilibrium (1). On these basis it was predicted that the tetrachorodioxolene complexes of Co(Mentpa) acceptors would adopt a Co(III)-catecholato charge distribution for n = 0, 1, 2, whereas Co(Me3tpa)(TCCat)PF6 could undergo valence tautomeric interconversion in solution and/or in the solid state. This manuscript concerns the structural and magnetic properties of this compound in the solid state and the temperature dependence of its electronic absorption spectra, which unambiguously show that the [Co(Me3tpa)(TCCat)]+ undergoes to an intramolecular electron transfer equilibrium in solution. This analysis provided for the first time the determination of the thermodynamic quantities associated to equilibrium (1) for these cationic 1:1 cobalt–dioxolene adducts. 2. Experimental 2.1. Synthesis Me3tpa ligand was prepared by using a general one step procedure, according to that recently reported for tpa [36]. The complex Co(Me3tpa)(TCCat) was obtained by mixing equimolar amounts of cobalt(II) chloride, Me3tpa ligand and tetrachlorocatechol in methanol in the presence of triethylamine. The yellow compound was filtered, dried in oven at 50 °C and then suspended in acetone. Upon addition of a stoichiometric amount of silver nitrate in the minimum quantity of water, a brown solution was obtained. The addition of an aqueous solution of potassium hexafluorophosphate yielded [Co(Me3tpa)(TCCat)]PF6, 1, as grey-brown powder, which was filtered and then recrystallised from acetone/diethyl ether or dichloromethane/hexane. Crystals of 1 suitable for X-ray analysis were obtained from a ethanol solution using dichloromethane as precipitating agent. 2.2. Electronic absorption spectra Electronic absorption spectra were measured with a Varian Cary 5 spectrophotometer. The temperature at the sample was lowered through the use of an Air Products and Chemicals closed-cycle helium cryostat and controlled by a Lake Shore 805 temperature controller.

2.3. Magnetic measurements The temperature dependence of the molar magnetic susceptibility, vM (obtained as the ratio of the magnetization with the field), of 1 was investigated in the range 2–380 K in an applied magnetic field of 1 T by a Quantum Design MPMS SQUID Magnetometer. Data were corrected for the intrinsic diamagnetism of the sample and sample holder contribution measured in the same temperature range. 2.4. X-ray crystallographic analysis X-ray data were collected at 295 K with an Oxford Diffraction Xcalibur3 diffractometer using Mo Ka radiation (k = 0.71073 Å). Data reduction was accomplished using CRYSALIS.RED p171.29.2 [37a]. Absorption correction was performed using both ABSGRAB and ABSPACK softwares included in the Crysalis package. The symmetry and systematic absences were found to be consistent with the monoclinic space groups Cc (no. 9) and C2/c (no. 15). The structure was solved in both space groups by direct methods (SIR92) [37b], which gave the positions of all non-hydrogen atoms. Due to the larger parameter/reflections ratio in space group no. 15, this was assumed in subsequent refinement cycles by full-matrix leastsquares on all F2 data using SHELXL 97 [37c]. Hydrogen atoms were included in calculated positions and allowed to ride on their parent atoms. A summary of the data collection and refinement process is collected in Table 1. 3. Description of the crystal structure The general features of the molecular structure of 1, obtained by refinement of X-ray crystallographic data, are in agreement with

Table 1 Crystal data and structure refinement for 1 Empirical formula Formula weight Temperature (K) Wavelength (Å) Crystal system Space group Unit cell dimensions a (Å) b (Å) c (Å) a (°) b (°) c (°) Volume (Å3) Z D(calc) (Mg/m3) Absorption coefficient (mm1) F(0 0 0) Crystal size (mm) h Range for data collection (°) Index ranges

Reflections collected Independent reflections (Rint) Completeness to h = 25° (%) Absorption correction Maximum and minimum transmission Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices (I > 2r(I)) R indices (all data) Largest differences in peak and hole (e Å3)

Co(C6O2Cl4)(C21N4H24)(PF6)  CH2Cl2 867.13 295(2) 0.71069 monoclinic C2/c 25.621(4) 17.765(4) 17.773(3) 90 114.694(11) 90 7350(2) 8 1.567 1.01 3488 0.44  0.41  0.05 3.77–28.9 32 6 h 6 33, 22 6 k 6 23, 23 6 l 6 23 25 561 8347 (0.0401) 99.2 semi-empirical from equivalents 0.951 and 0.615 full-matrix least-squares on F2 8347/0/436 1.039 R1 = 0.0767, wR2 = 0.2343 R1 = 0.128, wR2 = 0.2848 1.098 and 0.77

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analysis. On increasing temperature above 300 K, however, a relevant increase in vT is observed. This behaviour is typical for thermally induced valence tautomeric interconversion, which leads to the progressive formation of Co(II)-SQ species. This suggests that the transition temperature, defined as the temperature at which the two species are present in equimolar amount should be around 390 K. However, on increasing temperature above 360 K a very steep increase in vT is observed which we attribute to thermally induced polymerization of the sample. Indeed, on cooling back to 10 K and heating up again the transition appears to be irreversible. 5. Solution studies

Fig. 1. Diamond view of the cationic moiety of 1. Hydrogen atoms are omitted for clarity sake. Ellipsoids are shown at 50% probability.

those previously reported for 1:1 Co:dioxolene adducts with tpa derivatives as ancillary ligands [23]. The moiety Co(Me3tpa)(TCCat), which is shown in Fig. 1, is monopositively charged, the electric neutrality of the crystal being assured by one PF6  unit. The cobalt ion is six-coordinated in a cis-distorted pseudo-octahedral coordination and the tripodal ligand Me3tpa adopts a folded conformation around the metal ion, with the dioxolene ligand acting as bidentate. The values of the metal–donor bond lengths strongly indicates that at room temperature 1 is in the Co(III)-Cat charge distribution. Indeed, Co–N bond distances range between 1.95 and 2.01 Å, whereas Co–O ones amount to 1.88 and 1.89 Å. This result, which is in agreement with the outcome of magnetic data analysis, is confirmed by the analysis of the C1–C2 (1.41 Å) and C–O (1.31 Å) bond lengths of the dioxolene ligand, which gives strong indication of 1 being in the Co(III)-Cat form. 4. Magnetic measurements The temperature dependence of vT product (Fig. 2) indicates that, at low temperature and up to 300 K, 1 shows only a residual paramagnetic character, which we attribute to both the temperature independent contribution of Co(III) ion and to the presence of a weak and unavoidable impurity of Co(III) species in the sample. This results confirm the Co(III)-Cat charge distribution of 1 up to room temperature in solid state evidenced by X-ray structure

Fig. 2. Temperature dependence of vT product up to 380 K for 1 (full squares) and subsequent cooling (empty circles)–heating (full triangles) cycle, showing the irreversibility of the transition occurring above room temperature.

Previous studies on 3d metal complexes formed by tetrachlorodioxolene in both catecholato or semiquinonato oxidation states have shown that these compounds are unstable in solution [38,39]. Indeed, with the exception of complexes kinetically inert towards dissociation, the formation of 2,3-hexachloro-oxanthrenequinone, a product of the nucleophilic reaction between two dioxolene ligands, is often observed. These reactions are slow at room temperature, but cannot be neglected at higher temperatures. This precludes the possibility of carrying out temperature dependent spectroscopic studies in a temperature range above 300 K. As mentioned above, the electrochemical properties of the Co(Me3tpa)(TCCat)PF6 complex in acetonitrile solution, some of us recently reported [23], showed the occurrence of a quasi reversible one electron redox process at ca. 0.4 V vs. ferrocenium/ferrocene couple. We assigned this process to the metal centered CoIII(TCCat)/CoII(TCCat) couple on the basis of the features of the cyclic voltammetry trace and its room temperature electronic absorption spectrum (Fig. 3), which strongly suggests the existence of the CoIII(Me3tpa)(TCCat)+ chromophore as predominant species in such experimental conditions. Indeed the two weak bands occurring in the near infrared and violet regions of the spectrum were reasonably assigned to the symmetry forbidden LMCT transitions involving both the p(HOMO) and the highest energy p molecular orbitals of the ligand, respectively, and the empty eg (r in character) orbital of the metal ion [31]. The band at 19 200 cm1is assigned to the internal d–d transition (1A1 ? 1T1 in Oh), typical of the cobalt(III) six-coordinate chromophores [40]. The second expected d–d transition 1A1 ? 1 T2 in Oh cannot be clearly detected being obscured by the LMCT transition in the violet region. Similar electronic spectra were obtained in acetone and DMSO (Fig. 3) even if in this case the last charge transfer band is partially overlapped with solvent bands.

Fig. 3. Electronic spectra of 1 in different solvents at room temperature.

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Fig. 4. Temperature dependence of the UV–Vis absorption spectra of 1 in dichloromethane solution in the 200–300 K range. In the inset the variation of the fraction of Co(II)SQ species as a function of temperature, obtained by extrapolation from the 23 900 cm1 peak and corresponding best-fit curve, obtained with the model described in the text.

The observed value of the redox potential for the CoIII/CoII couple was thought to be of the same order of magnitude to that expected for the ligand centered CoII(TCSQ)/CoII(TCCat) couple and therefore we extended our investigation to other solvents. We found that the electronic spectra of this compound in other solvents like dichloromethane (Fig. 3) and 1,2-dichloroethane show features consistent with the presence of the cobalt(II)-semiquinonato redox isomer. Indeed the observed spectra are similar to those observed for Co(Me4cyclam)(DBSQ)+ and Co(Me3tpa)(DBSQ)+ chromophores, whose charge distribution has been well ascertained by magnetic and X-ray structural data [23,32]. Again we can tentatively assign the observed pattern of bands by taking into account the internal transitions of the semiquinonato ligands. Thus the transitions occurring at 13 750 and 23 900 cm1 can be attributed, as previously discussed, to internal ligand transitions [41]. The other bands we observe therefore are presumably charge transfer in origin, since d–d transitions in pseudo-octahedral cobalt(II) chromophores are very weak. Once compared with those observed for the Co(Me3tpa)(DBSQ)+ [23], the transitions occurring at 15 600 and 17 300 cm1 are assigned to MLCT transitions in agreement with the higher oxidizing power of the TCSQ anion with respect to the DBSQ one. The bands associated to these transitions are broad and relatively weak (e ca. 950 M1cm1). We suggest that these transitions might involve an electron transfer between the d metal orbital and the p SOMO of the TCSQ ligand. The more intense transition occurring in the violet region of the spectrum are more difficult to assign because of the overlap with the internal transition of the ligand. The different charge distribution observed in low donor solvents with respect to that observed in high donor ones is obviously due to different solvation free energy changes associated to the equilibrium: CoIII ðMe3 tpaÞðTCCatÞþ ! CoII ðMe3 tpaÞðTCSQ Þþ

ð2Þ

in the different solvents. Temperature dependent cyclic voltammetry experiments suggested that similar equilibria are controlled by the enthalpy changes, the high donor solvents stabilising the CoIIIcat species with respect to the CoII-SQ one. For the Fe(CTH)(diox)+ chromophore the contribution due to the different solvation enthalpies between 1,2-dicloroethane and acetonitrile was calculated of

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the order of 5 kJ mol1 [34]. The solvation entropy changes did not seem to substantially affect the equilibrium (<1 kJ mol1 at room temperature). Bearing this in mind, we found that the color of the acetonitrile solution of the Co(Me3tpa)(TCCat)PF6 complex shows a reversible changes upon warming. We did not perform any temperature dependent investigation on these solutions because of the side reactions we mentioned above and that we presumably observed in the solid state by warming the sample. However temperature dependent electronic spectra in dichloromethane showed the existence of spectral changes consistent with the existence of equilibrium (2) as shown in Fig. 4. The reversible character of an equilibrium involving two species is further supported by the presence of two isosbestic points. The observed spectral data were fitted using the regular solution model [42] taking into account that in the temperature range investigated just a partial conversion occurs (at 200 K we have 1% of high spin Co(II) species, while at 300 K 7% of Co(III) form is present). The thermodynamic quantities obtained by the best fit parameters (R2 = 0.99) were DH = 31.2 kJ mol1 and DS = 125 J K1 mol1, in agreement with the entropy driven character of equilibrium (2). The observed results are in good agreement with those observed for other valence tautomeric interconversion equilibria involving cobalt–dioxolene complexes. In particular these values well compare with those observed for Co(N–N)(diox)2 systems (N–N = diazine ligand, diox = catecholato or semiquinonato forms of 3,5-di-tert-butyl-1,2-dioxybenzene), which show enthalpy changes in the range 14.2–38.4 kJ mol1 and entropy changes in the range 60.8–134.0 J mol1 K1 [8,17,18,43], and for Co(Cat-NBQ)(Cat-NSQ) complex (Cat-N-BQ, Cat-N-SQ are the monoanion and dianion of 2-(2-hydroxy-3,5-di-tert-butylphenylimino)-4,6di-tert-butyl-cyclohexa-3,5-dienone, respectively) [44,45], which shows surprisingly similar thermodynamic quantities. It should be stressed, however, that the here reported data are the first ones concerned with this dioxolene ligand and, what is more important, with the simplest system undergoing valence tautomeric interconversion. Again, as previously suggested [17–19,32], the high value of the entropy change should be attributed to the change in the vibrational entropy contributions characterising the two redox isomers. As observed for all the other cobalt systems undergoing valence tautomeric equilibria, no evidence of the formation of a low-spin CoII-semiquinonato species was found, although subpicosecond transient absorption spectroscopy studies evidence the existence of a further species in the relaxation process of optically induced metastable state [30]. This species is expected to be characterized by a triplet electronic ground state, on the basis of the orthogonality of the dr(Co) and p(SQ) magnetic orbitals. This result is in full agreement with the observation of isosbestic points in the electronic spectrum of the present complex, i.e. no more than two species are involved in the interconversion equilibrium. This result is not surprising and its possible explanation has been discussed [46]. Acknowledgments The financial support of EU through NE-MAG-MANET (NMP3CT-2005-515767) and of Italian MIUR through PRIN and FIRB 2003 RBNE033KMA ‘‘Molecular compounds and hybrid nanostructured materials with resonant and non-resonant optical properties for photonic devices” is gratefully acknowledged. Appendix A. Supplementary material CCDC 675547 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from the

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