Microsoft Word - Augustin NBC 2015.doc 96 Nova Biotechnologica et Chimica 14-1 (2015) DOI 10.1515/nbec-2015-0019 © University of SS. Cyril and Methodius in Trnava MAGNETOSTRUCTURAL RELATIONSHIPS FOR Fe(III) SPIN CROSSOVER COMPLEXES PETER AUGUSTÍN1, ROMAN BOČA2 1Institute of Inorganic Chemistry, Faculty of Chemical and Food Technology, Slovak University of Technology, SK-812 37 Bratislava, Slovakia 2Department of Chemistry, University of SS. Cyril and Methodius in Trnava, SK-917 01 Trnava, Slovakia (roman.boca@stuba.sk) Abstract: Structural data for fifteen complexes of Fe(III) of a general formula [FeL5X], with pentadentate Schiff-base ligands L5 and unidentate coligands X−, were subjected to a statistical analysis. The multivariate methods such as Pearson correlation, cluster analysis and principal component analysis split the data into two clusters depending upon the low-spin and/or high-spin state of the complex at the temperature of the X-ray experiment. Some of these complexes exhibit a thermally induced spin crossover. The numerical analysis of the magnetic susceptibility and magnetization data for an enlarged set of Fe(III) spin crossover systems yields the enthalpy ΔH and entropy ΔS of the transition along with the transition temperature T1/2 and the solid state cooperativeness. The thermodynamic data show a mutual relationship manifesting itself by linear ΔS vs ΔH and T1/2 vs ΔH correlations. Key words: spin crossover, Fe(III) complexes, structural and magnetic data, magnetostructural relationships 1. Introduction A number of studies confirm a presence of thermally induced spin crossover between the low-spin (LS, S = 1/2) and high-spin (HS, S = 5/2) states in Fe(III) complexes (e.g. HALCROW et al., 2013). This process can be viewed as a unimolecular, entropy driven reaction for which ΔH > 0 and ΔS > 0 is characteristic. The minimum value of ΔS is represented by a pure electronic spin contribution ln(6 / 2)S RΔ = = 9.1 J K-1 mol-1 that is enriched by the vibrational contribution. The transition temperature, when the mole fraction obeys xLS = xHS = 0.5, is given by the ratio T1/2 = ΔH/ΔS and in order to get a sufficiently high value, say T1/2 > 100 K, the lowest estimate for the enthalpy of the spin transition is ΔH > 0.9 kJ mol-1. These thermodynamic data can be measured by calorimetry (SORAI, 2002) but they are available also from an analysis of magnetic data such as temperature dependence of the magnetic susceptibility (BOČA et al., 2003; PAVLIK et al., 2013). It was reported recently that the spin transition can be tuned by varying coligands in the basic skeleton of the complexes of [FeIIIL5X] type; here L5 stands for a pentadentate Schiff base whereas X− is a monodentate pseudohalido coligand (KRÜGER et al., 2013; KRÜGER et al., 2015; MASÁROVÁ et al., 2015; NEMEC et al., 2011; ŠALITROŠ et al., 2009). Although chemists try to rationalize ΔH by changing the ligand field strength, the aspects influencing the ΔS cofactor are unclear so far. To our hypothesis some magnetostructural relationships could bring an answer of how to get the spin crossover with a desired transition temperature. Bereitgestellt von Slovenská poľnohospodárska knižnica | Heruntergeladen 16.01.20 17:18 UTC Nova Biotechnologica et Chimica 14-1 (2015) 97 2. Material and methods The pentadentate ligands H2L 5 were prepared by a Schiff condensation between the substituted salicylaldehyde (Y-sal) and the asymmetric triamine pet = 1,6-diamino-4- azahexane as described elsewhere (KRÜGER et al., 2013; NEMEC et al., 2011). Their Fe(III) complexes [Fe(Y-salpet)X] were prepared by published recipes (see Fig. 1) where the Fe(III) salt was combined with H2L 5 and pseudohalide X− ligands. The elemental analysis confirmed the composition of the complexes. N OH N OH NH Y Y O OH Y N H NH2 NH22 + + Fe(III) + X− Fig. 1. Synthetic route for preparing Fe(III) complexes of the [FeL5X] type; Y = Cl and Br, X− = Cl−, N3−, NCO−, NCS− and NCSe−. Single crystals were subjected to the X-ray structure analysis and the results can be retrieved from the Cambridge Structural Database. The measured interatomic distances within the {FeN2N’O2X} chromophore are listed in Table 1. Table 1. Structural parameters for FeIII complexes under study.a Compound Class Fe-Nam Fe-Nim Fe-O Fe-X Σ α 1 [Fe(LBr)(Cl)]·0.5H2O H 2.188 2.088 1.959 2.352 62.3 63.1 2 [Fe(LBr)(NCS)] H 2.207 2.099 1.927 2.111 56.3 67.7 3 [Fe(LBr)(N3)]·MeOH H, SC 2.183 2.113 1.942 2.048 72.3 35.0 4 [Fe(LBr)(NCSe)] L, SC 2.013 1.952 1.884 1.944 27.8 85.9 5 [Fe(LCl)(Cl)]·0.25H2O H 2.199 2.095 1.958 2.361 66.9 62.2 6 [Fe(LCl)(NCO)] H 2.205 2.093 1.954 1.996 55.4 69.0 7 [Fe(LCl)(NCS)] L, SC 2.000 1.932 1.879 1.944 27.1 84.5 8 [Fe(LCl)(NCSe)] L, SC 2.001 1.932 1.881 1.952 26.9 85.5 9 [Fe(LCl)(CN)]·H2O L 2.012 1.934 1.903 1.959 19.3 65.9 10 [Fe(L)(CN)]·CH3OH L 2.000 1.922 1.880 1.962 24.6 76.0 11 [Fe(3-But,5-Me-L)(NCS)] H 2.191 2.096 1.916 2.100 56.0 75.6 12 [Fe(Lm)(NCO)]·MeOH H 2.218 2.078 1.946 2.081 54.0 73.3 13 [Fe(Lm)(N3)] H 2.211 2.085 1.936 2.085 57.4 58.1 14 [Fe(Lm)(CN)]·MeOH L 2.028 1.935 1.904 1.971 26.0 69.1 15 [Fe(Le)(CN)]·H2O L 2.010 1.944 1.904 1.975 20.8 66.4 a Distances in Å, angles Σ and α in deg. Class – classification according to the temperature of the X-ray experiment in comparison with spin state: H – high-spin, L – low spin, SC – spin crossover. Octahedral distortion angle Σ is calculated from twelve cis angles within the chromophore. Angle α – between planes of aromatic rings. Abbreviations: L = salpet, LCl = 5-Cl-salpet, LBr = 5-Br-salpet, Lm = 3-MeO-salpet, Le = 3-EtO-salpet. References: KRÜGER et al., 2013; KRÜGER et al., 2015; MASÁROVÁ et al., 2015; NEMEC et al., 2011; ŠALITROŠ et al., 2009. The SQUID magnetometer (Quantum Design, MPMS-XL7) was used for measurements of the magnetic data (temperature dependence of the magnetic Bereitgestellt von Slovenská poľnohospodárska knižnica | Heruntergeladen 16.01.20 17:18 UTC 98 Augustín, P. and Boča, R. susceptibility, and field dependence of the magnetization). The magnetic data for FeIII complexes referring either to the low-spin (LS) or the high-spin (HS) state was analyzed in terms of the Curie-Weiss law; to this end the g-factor, Weiss constant Θ, and the van Vleck term α were obtained. Some systems exhibit a thermally induced spin crossover and those data was analyzed by using Ising-like model with vibrations (equivalent to the thermodynamic regular solution model) giving rise the enthalpy ΔH and entropy ΔS of the spin transition along with the transition temperature T1/2 and the solid-state cooperativeness Γ (BOČA et al., 2003; PAVLIK et al., 2013). These data is listed in Table 2 along with some literature sources. Table 2. Thermodynamic parameters for spin crossover complexes under study.a Compound Type T1/2/K ΔH/kJ mol-1 ΔS/J K-1 mol-1 (Γ/kB)/K 1 [Fe(LCl)(NCS)] SC 280 5.77 20.6 180 2 [Fe(LCl)(NCSe)] SC 293 6.54 22.3 197 3 [Fe(LBr)(N3)]·CH3OH SC 142 1.64 11.5 76 4 [Fe(LBr)(NCSe)] SC 326 6.03 18.5 215 5 [Fe(Le)(NCS)] SC 84, 82 2.07 (5.0) 90 6 [Fe(5Cl-saldptn)py]BPh4 SC* 78 0.44 5.61 63 7 [Fe(3m-saldptn)py]BPh4 SC 273 4.54 16.6 90 8 [Fe(saldptn)py]BPh4 SC 310 5.42 17.5 150 9 [Fe(napet)(N3)]⋅MeOH SC 122, 117 1.53 11.2 99 10 [Fe(napet)(NCS)]⋅MeCN SC 151 1.92 12.5 87 11 [Fe(napet)(NCS)] SC 180 0.90 (5.0) 135 12 [Fe(napet)(NCO)] SC 155 2.54 16.3 102 13 [Fe(napet)(NCSe)]⋅MeCN SC 170 2.29 13.3 99 a SC – spin crossover between spins 1/2 5/2, SC – with hysteresis. SC* - spin crossover between spins 3/2 5/2. Abbr. for ligands: saldptn = N,N'-bis(2-hydroxybenzyliden)-1,7-diamino-4-azaheptane, napet = N,N'-bis(2-hydroxynapthylidene)-1,6-diamino-4-azahexane. References: BOČA et al., 2000; KRÜGER et al., 2013; KRÜGER et al., 2015; MASÁROVÁ et al., 2015; NEMEC et al., 2011; ŠALITROŠ et al., 2009. 3. Results and discussion The structural data was analyzed by using the statistical tools of the multivariate methods (STATGRAPHICS Centurion XV, 2006): the Cluster Analysis (CA), the Principal Component Analysis (PCA), and the Pearson Correlation (PC). The cluster analysis (using Ward’s method, distance according to the square Euclidean norm) shows that the data is split into two principal clusters according to their similarity (Fig. 2). Centroids of the two clusters are listed in Table 3. It can be seen that all distances in the cluster 1 are higher than in cluster 2 which allows concluding that the cluster 1 involves high-spin complexes (complexes which are high-spin at the temperature of the X-ray experiment). Then the cluster 2 contains low- spin complexes. Table 3. Centroids of the parameters according to the cluster analysis. Cluster Fe-Nam Fe-Nim Fe-O Fe-X Σ α 1 2.200 2.093 1.942 2.142 60.07 63.0 2 2.009 1.936 1.891 1.958 24.64 76.2 Bereitgestellt von Slovenská poľnohospodárska knižnica | Heruntergeladen 16.01.20 17:18 UTC Nova Biotechnologica et Chimica 14-1 (2015) 99 0 20 40 60 80 100 D is ta nc e 1 2 3 45 6 7 8 91011 12 13 1415 0 10 20 30 40 50 D is ta nc e Fe N am Fe X Fe N im Fe O S um al ph a Fig. 2. Dendograms of the cluster analysis. Left – for complexes; right – for distances and angles. Data according to Table 1. The PCA method performs a linear transformation of the original parameters into principal components that bear the maximum information about the variability of data. Here the first component involves 79 % and the second one 11 % of the variability, in total 90 %. The diagram (Fig. 3) shows which variables correlate, anticorrelate, or do not correlate. It follows that the interatomic distances Fe-Nam, Fe-Nim, Fe-O mutually correlate and also they correlate with the parameter Σ. On the contrary, α-parameter does not correlate with the others. FeX FeO alpha -3 -2 -1 0 1 2 3 Component 1 -3 -2 -1 0 1 2 3 C om po ne nt 2 FeNam FeNim Sum Component 1 C om po ne nt 2 Cluster H L 4 10 7 8 9 15 14 11 12 2 6 13 1 5 -3 -2 -1 0 1 2 3 -3 -2 -1 0 1 2 3 3 Fig. 3. Results of PCA. Left – a biplot with ray diagram, right – individual complexes classified into two clusters. PCA also brings information about the distribution of the original objects and the corresponding graph is also display in Fig. 3. It unambiguously identifies that the cluster 1 (squares) contains high-spin complexes whereas the cluster 2 (circles) the low-spin ones (at the temperature of the X-ray experiment). Finally, the PC analysis offers the pair correlation coefficients as listed in Table 4. Now it is seen that the greater Fe-Nam, the greater Fe-Nim and Fe-O. Also the octahedral distortion parameter Σ increases with distance of the amino nitrogen Fe-Nam and also imino nitrogen Fe-Nim. The angle of the aromatic rings α does not display any significant correlation with the other structural parameters. Bereitgestellt von Slovenská poľnohospodárska knižnica | Heruntergeladen 16.01.20 17:18 UTC 100 Augustín, P. and Boča, R. Table 4. Pair correlation (Pearson) coefficients for structural parameters. Fe-Nam Fe-X Fe-Nim Fe-O Σ α Fe-Nam 0.677 0.984 0.900 0.942 -0.515 Fe-X 0.677 0.683 0.757 0.732 -0.372 Fe-Nim 0.984 0.683 0.885 0.972 -0.569 Fe-O 0.900 0.757 0.885 0.869 -0.639 Σ 0.942 0.732 0.972 0.869 -0.613 α -0.515 -0.372 -0.569 -0.639 -0.613 The recorded temperature dependence of the effective magnetic moment for a number of complexes is displayed in Fig. 4 for a fixed pentadentate ligand L5 and varied coligand X−. It can be seen that with increasing crystal field strength of the coligand X− (according to the well-known spectrochemical series) the spin states of complexes move from the high-spin behavior through the spin crossover behavior to the low-spin state. T/K 0 50 100 150 200 250 300 350 400 μ e ff/ μ B 0 1 2 3 4 5 6 [Fe(LCl)(Cl)] [Fe(LCl)(NCO)] [Fe(LCl)(NCS)] [Fe(LCl)(NCSe)] [Fe(LCl)(CN)] T/K 0 50 100 150 200 250 300 350 400 μ e ff/ μ B 0 1 2 3 4 5 6 [Fe(LBr)(NCS)] [Fe(LBr)(N3)] [Fe(LBr)(NCSe)] Fig. 4. Comparison of the spin crossover in [FeIIIL5X] type complexes for a fixed Schiff-base ligand and varied coligand. For references see Table 2. T/K 0 50 100 150 200 250 300 350 400 μ e ff/ μ B 0 1 2 3 4 5 6 [Fe(5Br-salpet)(NCS)] [Fe(3MeO-salpet)(NCS)] [Fe(5Cl-salpet)(NCS)] T/K 0 50 100 150 200 250 300 350 400 μ e ff/ μ B 0 1 2 3 4 5 6 γ-[Fe(1anapet)(NCS)] α-[Fe(1anapet)(NCS)] [Fe(2anapet)(NCS)] Fig. 5. Comparison of the spin crossover in [FeIIIL5X] type complexes for a varied Schiff-base ligand and fixed coligand. For references see Table 2. The spin state of the complex can be influenced also by the Schiff-base ligand with a fixed coligand as shown in Fig. 5 for X− = NCS−. In order to find relationships among the thermodynamic parameters governing the spin crossover the multivariate statistical methods were applied: CA, PCA and PC. For Bereitgestellt von Slovenská poľnohospodárska knižnica | Heruntergeladen 16.01.20 17:18 UTC Nova Biotechnologica et Chimica 14-1 (2015) 101 this purpose the data listed in Table 2 were employed. The results of the cluster analysis are presented in Fig. 6. It can be seen that they span two clusters with centroids characterized in Table 5. The cluster 1 involves complexes (No 1, 2, 4, 7, and 8) for which near- and above-room temperature spin crossover has been observed. Dendrogram Ward's Method,Squared Euclidean 0 3 6 9 12 15 D is ta nc e 1 2 34 5 67 8 910 1112 13 Dendrogram Ward's Method,Squared Euclidean 0 0.5 1 1.5 2 2.5 3 D is ta nc e T H S J Fig. 6. Dendograms of the cluster analysis. Left – for complexes, right – for thermodynamic parameters. Data according to Table 2. Table 5. Centroids of the parameters according to the cluster analysis. Cluster T = T1/2 H = ΔH S = ΔS J = Γ 1 296.4 5.66 19.1 166.4 2 134.9 1.67 10.1 93.9 The PCA analysis yields the diagrams presented in Fig. 7. It can be seen that the transition temperature T1/2 (abbr. T) correlates with the enthalpy of the transition ΔH (abbr. H) whereas the correlation with the remaining parameters ΔS (abbr. S) and Γ (abbr. J) is much weaker. The diagram on the right shows the objects classified into individual clusters. H J -3 -2 -1 0 1 2 3 Component 1 -2 -1 0 1 2 C om po ne nt 2 T S Component 1 C om po ne nt 2 Cluster 1 2 7 8 1 4 2 6 5 11 9 3 10 13 -3 -2 -1 0 1 2 3 -2 -1 0 1 2 12 Fig. 7. Results of PCA. Left – a biplot with ray diagram, right – individual objects classified to the clusters (numbering according to Table 2). The Pearson correlation yields the pair correlation coefficients listed in Table 6. It is confirmed numerically that the highest correlation coefficient refers to the Tc – ΔH pair. Bereitgestellt von Slovenská poľnohospodárska knižnica | Heruntergeladen 16.01.20 17:18 UTC 102 Augustín, P. and Boča, R. Table 6. Pair correlation (Pearson) coefficients for thermodynamic parameters. T = T1/2 H = ΔH S = ΔS J = Γ T 0.922 0.826 0.824 H 0.922 0.890 0.826 S 0.826 0.890 0.675 J 0.824 0.826 0.675 The values of ΔS were plotted versus the ΔH and the corresponding graph is presented in Fig. 8 along with the linear regression curve and confidence intervals (95 %). Analogously, the values of T1/2 correlate with ΔH. These correlations were replotted on the right side of Fig. 8 when points with a problematic value of ΔS < 9.1 J K-1 mol-1 (complexes 5 and 11) were excluded form the correlation. The correlation is much improved. ΔH /kJ mol-1 0 1 2 3 4 5 6 7 Δ S /J K -1 m ol -1 0 5 10 15 20 25 ΔH /kJ mol-1 0 1 2 3 4 5 6 7 T 1/ 2/ K 0 50 100 150 200 250 300 350 400 ΔH /kJ mol-1 0 1 2 3 4 5 6 7 Δ S /J K -1 m ol -1 0 5 10 15 20 25 ΔH /kJ mol-1 0 1 2 3 4 5 6 7 T 1/ 2/ K 0 50 100 150 200 250 300 350 400 Fig. 8. Linear correlations T1/2 vs ΔH and ΔS vs ΔH. Right – reduced data omitting two problematic points. 4. Conclusions It has been demonstrated that the structural parameters within the {FeN2N’O2X} chromophore of the [FeIIIL5X] type complexes mutually correlate at a significant level. The complexes are classified into two clusters depending upon the low-spin and/or high-spin state of the complex at the temperature of the X-ray experiment. The thermodynamic data that characterize the spin crossover in the given class of Fe(III) complexes again show a mutual correlation confirmed by the multivariate methods. Again two principal clusters are visible, one for the systems exhibiting the spin crossover near- and above-room temperature, and the second for those showing the spin crossover below the room temperature. The enthalpy of the spin transition can be tuned by the coligand X− and consequently also the transition temperature is a function of ΔH. To this end, the coligands spanning the right side of the spectroscopic series Bereitgestellt von Slovenská poľnohospodárska knižnica | Heruntergeladen 16.01.20 17:18 UTC Nova Biotechnologica et Chimica 14-1 (2015) 103 cause an increase in the enthalpy of the spin transition, and consequently a rising of the transition temperature towards and above the room temperature. Acknowledgements: Slovak grant agencies (VEGA 1/0233/12, VEGA 1/0522/14 and APVV- 14-0078) are acknowledged for the financial support. References BOČA, R., LINERT, W.: Is there a need for new models of the spin crossover? Monats. Chemie, 134, 2003, 199-216. BOČA, R., FUKUDA, Y., GEMBICKÝ, M., HERCHEL, R., JAROŠČIAK, R., LINERT, W., RENZ, F., Yuzurihara J.: Spin crossover in mononuclear and binuclear iron(III) complexes with pentadentate Schiff−base ligands. Chem. Phys. Lett., 325, 2000, 411-419. CAMBRIDGE STRUCTURAL DATABASE, Cambridge Crystallographic Data Centre, Cambridge, CB2 1EZ, United Kingdom. HALCROW, M. A. (Ed.): Spin-Crossover Materials: Properties and Applications. Wiley, New York, 2013. KRÜGER, C., AUGUSTÍN, P., NEMEC, I., TRÁVNÍČEK, Z., OSHIO, H., BOČA, R., RENZ, F.: Spin crossover in iron(III) complexes with pentadentate Schiff-base ligands and pseudhalido coligands. Eur. J. Inorg. Chem., 2013, 902-915. KRÜGER, C., AUGUSTÍN, P., DLHÁŇ, Ľ., PAVLIK, J., MONCOĽ, J., NEMEC, I., BOČA, R., RENZ, F.: Iron(III) complexes with pentadentate Schiff-base ligands: Influence of crystal packing change and pseudohalido coligand variations on spin crossover. Polyhedron, 87, 2015, 194-201. MASÁROVÁ, P., ZOUFALÝ, J., MONCOĽ, J., NEMEC, I., PAVLIK, J., GEMBICKÝ, M., TRÁVNÍČEK, Z., BOČA, R., ŠALITROŠ, I.: Spin crossover and high spin electroneutral mononuclear iron(III) Schiff base complexes involving terminal pseudohalido ligands. New. J. Chem., 39, 2015, 508-519. NEMEC, I., HERCHEL, R., BOČA, R., TRÁVNÍČEK, Z., SVOBODA, I., FUESS, H., LINERT, W.: Tuning of spin crossover behaviour in iron(III) complexes involving pentadentate Schiff bases and pseudohalides. Dalton Trans., 40, 2011, 10090-10099. PAVLIK, J., BOČA, R.: Established static models of the spin crossover. Eur. J. Inorg. Chem., 2013, 697-709. SORAI, M.: Calorimetric investigations of phase transitions in which electrons are directly involved. J. Chem. Thermodyn., 34, 2002, 1207-1253. STATGRAPHICS CENTURION XV. Statpoint Inc., 2006. ŠALITROŠ, I., BOČA, R., DLHÁŇ, Ľ., GEMBICKÝ, M., KOŽÍŠEK, J., LINARES, J., MONCOĽ, J., NEMEC, I., PERAŠÍNOVÁ, L., RENZ, F., SVOBODA, I., FUESS, H.: Unconventional spin crossover in dinuclear and trinuclear iron(III) complexes with cyanido and metallacyanido bridges. Eur. J. Inorg. Chem., 2009, 3141-3154. Bereitgestellt von Slovenská poľnohospodárska knižnica | Heruntergeladen 16.01.20 17:18 UTC