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High-temperature spin-crossover in coordination compounds of iron(II) with tris(pyrazol-1-yl)methane
Inorganica Chimica Acta 363 (2010) 4059–4064
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High-temperature spin-crossover in coordination compounds of iron(II) with tris(pyrazol-1-yl)methane O.G. Shakirova a, L.G. Lavrenova a,c, V.A. Daletsky a, E.A. Shusharina a,c, T.P. Griaznova b, S.A. Katsyuba b,⇑, V.V. Syakaev b, V.V. Skripacheva b, A.R. Mustafina b, S.E. Solovieva b a b c
A.V. Nikolaev Institute of Inorganic Chemistry, Siberian Division, Russian Academy of Sciences, pr. Akademika Lavrent’eva 3, Novosibirsk 630090, Russia A.E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Centre of the Russian Academy of Sciences, Arbuzov Str. 8, 420088 Kazan, Russia Novosibirsk State University, Pirogov Str. 2, Novosibirsk 630090, Russia
a r t i c l e
i n f o
Article history: Received 21 May 2010 Received in revised form 29 July 2010 Accepted 7 August 2010 Available online 12 August 2010 Keywords: Inclusion p-Sulfonatothiacalix[4]arene Crystal structure IR NMR
a b s t r a c t Novel mononuclear Fe(II) complexes of tris(pyrazol-1-yl)methane [Fe{HC(pz)3}2]2+ with SiF6 2 and p-sulfonatothiacalix[4]arene (TCAS4) as counterions were obtained. The compounds were characterized by magnetic susceptibility method, IR and UV–Vis spectroscopy. The structure of [Fe{HC(pz)3}2]SiF6 has been analyzed by single-crystal X-ray diffraction. The 1H NMR spectroscopy measurements of [Fe{HC(pz)3}2]2(TCAS) in aqueous solution reveal the outer sphere inclusion of [Fe{HC(pz)3}2]2+ into the cyclophanic cavity of TCAS4. The temperature induced spin-crossover 1A1 , 5T2, accompanied by thermochromism, has been revealed from the temperature dependence of leff and IR spectra for both complexes. The comparative analysis of magnetochemical and spectroscopy data elucidates the effect of the cyclophanic counterion on the physico-chemical properties of Fe(II) complex. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Coordination compounds with d4–d7 electronic configuration in octahedral ligand environment have gained much attention due to their spin-crossover (SCO) behavior [1–8]. The reversible transfer from the low spin (LS) to high spin (HS) states can be induced by external input, such as temperature, pressure or light irradiation. Complexes with rather long lifetimes of LS and HS states are of great interest, both from a fundamental perspective and for potential application in molecular devices [8,9]. The complexes, which are changing their colour under SCO, gain the enhanced attention due to their particular importance for practical application. Complexes of Fe(II) with some polynitrogen-containing ligands, such as 1,2,4-triazole, tetrazole and their derivatives exemplify the thermochromic SCO complexes [5]. The SCO behavior, including abruptness, temperature and completeness of SCO transition, is greatly dependent on ligands’ structure (a nature of substituents is of particular importance), counterion and solvent effects [2,10– 14]. The large series of thermochromic complexes of Fe(II) with ⇑ Corresponding author. Tel.: +7 843 27 318 92; fax: +7 843 27 322 53. E-mail addresses: [email protected] (O.G. Shakirova), [email protected] (L.G. Lavrenova), [email protected] (V.A. Daletsky), [email protected] (E.A. Shusharina), [email protected] (T.P. Griaznova), [email protected], [email protected] (S.A. Katsyuba), [email protected] (V.V. Syakaev), [email protected] (V.V. Skripacheva), [email protected] (A.R. Mustafina), [email protected] (S.E. Solovieva). 0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2010.08.013
1,2.4-triazoles of formula Fe(4-Rtrz)3AnmH2O (R = H, NH2; n = 1, 2; m = 0, 1) was obtained by the Novosibirsk group [5,10–12]. These complexes are of polynuclear chain structure with ferrous ion being in the octahedral environment, where the coordination core is FeN6. Most of the complexes demonstrate an abrupt SCO 1 A1 , 5T2 with hysteresis on the temperature curves of the effective magnetic moment (leff) and thermochromism [5,10–12]. The temperature of SCO depends on the composition of the compounds and varies from 200 to 400 K. The series of Fe(II) complexes with tris(pyrazol-1-yl)methane (HC(pz)3, Scheme 1) [Fe{HC(pz)3}2]A2nH2O, A = Cl, Br, I, 1=2SO4 2 , n = 0–7 also demonstrate SCO and thermochromism, SCO temperatures (Tc) being within the range of 400–445 K [15]. The references [16–21] represent the synthesis of Fe(II) complexes of HC(pz)3 with various counterions, but some of these reports lack the magnetical measurements. The significant counter-ion and solvent effects on the SCO behavior (Tc in particular) were revealed for the series of [Fe{HC(pz)3}2]A2nH2O, A = Cl, Br, I, 1=2SO4 2 , n = 0–7 [15]. Though the effect of the counter ion is well known, it is not entirely predictable. Macrocyclic anions represent rather interesting alternative as counterions for SCO complexes, while at the moment these data are very rare, if any. The sulfonated derivatives of calix[n]arenes represent the series of highly charged polyanions, which can serve as counterions for [Fe{HC(pz)3}2]2+. p-Sulfonatothiacalix[4]arene (TCAS4) is one of the promising outer-sphere counterions for positively charged
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Scheme 1. Schematic representation of [Fe{HC(pz)3}2]2+ (a) and ligand HC(pz)3 (b).
1516, 1446 cm1 (ring stretchings), 715 cm1 (mSiF), 478 cm1 (dSiF). Anal. Calc. for C20H20F6FeN12Si (626.38): C, 38.4; H, 3.2; N, 26.8; Fe, 8.9. Found: C, 38.2; H, 3.4; N, 26.6; Fe, 8.5%. The single crystals suitable for X-ray diffraction were selected from the bulk of the precipitate and according to powder X-ray diffraction data they corresponded to a composition of polycrystalline sample. Scheme 2. Tetraanion of p-sulfonatothiacalix[4]arene (TCAS4).
complexes (Scheme 2) [22,27]. Therefore the present work extends the range of the outer sphere anions of [Fe{HC(pz)3}2]2+ by adding p-sulfonatothiacalix[4]arene (TCAS4) and SiF6 2 . The latter anion is chosen because of its previously revealed dramatic effect on SCO behavior of Fe(II) complexes, which was rather different for various ligands [11]. The most important peculiar feature of TCAS4 as the outer-sphere counterion is the presence of the hydrophobic cavity decorated with four ASO3Na groups, which easily dissociate in aqueous solutions, providing tetra-anion with high affinity towards positively charged metal complexes (Scheme 2). Thus the present work introduces the synthesis of [Fe{HC(pz)3}2]2+ complexes with TCAS4 and SiF6 2 as counterions and the comparative analysis of their magnetic and spectral properties.
2.2. Synthesis of [Fe{HC(pz)3}2]2(TCAS) (II) A weighed portion of [Fe{HC(pz)3}2]SiF6 (0.025 g, 0.04 mmol) was dissolved in distilled water (10 ml) under heating. A solution of Na4[TCAS] (0.0362 g, 0.04 mmol) in water (10 ml) was added to the solution of [Fe{HC(pz)3}2]SiF6. Both solutions were deoxygenated through bubbling of argon. Immediately after mixing of the initial solutions, a purple precipitate was formed, which was magnetically stirred for 1 h. The precipitate was transferred onto a filter, washed with water, dried in air and in a dessicator over anhydrone. Yield 70%. IR (nujol/perfluorinated nujol, cm1): 3374 cm1 (mOH); 3125, 2984 cm1 (mCH); 1562, 1516, 1446 cm1 (ring stretchings), 1280–1248 cm1 (mSO). Anal. Calc. for C64H52O16Fe2N24S8 (1781.44): C, 43.2; H, 2.9; N, 18.9; Fe, 6.3. Found: C, 43.3; H, 2.6; N, 18.3; Fe, 6.2%. 2.3. Methods
2. Experimental FeSO47H2O from Sigma was purified by recrystallization from acidified aqueous solution. Ascorbic acid from ICN Biomedical, Ba(NO3)2, (NH4)2SiF6, acetone from Aldrich were used as purchased. Tris(pyrizol-1-yl)methane was synthesized in accordance with the improved procedure [30]. Na4[TCAS] was synthesized according to procedure [31]. All aqueous solutions were prepared with bidistilled water. 2.1. Synthesis of [Fe{HC(pz)3}2]SiF6 (I) A 1 mmol (0.28 g) of FeSO47H2O was dissolved in 3 ml of distilled water, acidified by 0.1 g of ascorbic acid. A solution of Ba(NO3)2 (1 mmol, 0.26 g) in 7 ml of water was slowly added to the solution obtained. The precipitate of the barium sulfate was removed from solution. A solution of tris(pyrazol-1-yl)methane (0.43 g, 2 mmol) in acetone (5 ml) was added to the resulting aqueous solution of iron(II) nitrate and then an aqueous solution (7 ml) of (NH4)2SiF6 (2 mmol, 0.36 g) was rapidly added. Immediately after mixing of the initial solutions, a purple precipitate was formed, which was magnetically stirred for 1 h. The precipitate was transferred onto a filter, washed with water and ethanol, dried in air and in a dessicator over anhydrone. Yield 88%. IR (nujol/ perfluorinated nujol, cm1): 3129, 3053 cm1 (mCH); 1573–1533,
The elemental (C, H, N) analysis was performed on EURO EA 3000 (EuroVector). The analysis of metal content was performed by EDTA titration after thermal decomposition of complexes in a mixture of concentrated H2SO4 and HClO4 (1:2). X-ray diffraction study of complex I was carried out by a standard procedure on an automated four circle Bruker Nonius X8Apex diffractometer equipped with a two-dimensional CCD-detector at 150(2) K (Mo Ka-radiation, k = 0.71073 Å, graphite monochromator). The single crystal dimensions were 0.37 0.08 0.07 mm. The reflection intensities were measured by u scanning of narrow (0.5°) frames. The absorption corrections were applied empirically by the SADABS program. The structure was solved by the direct method and refined by full matrix least-squares in the anisotropic approximation for nonhydrogen atoms using SHELXTL software [32]. All hydrogen atoms were located geometrically. The X-ray experiment details and crystallographic characteristics for complex I are presented in Table 1. Table 2 contains interatomic distances for FeC20N12H20SiF6. The static magnetic susceptibility of polycrystalline samples was measured by a Faraday method in the temperature range of 275–500 K at an external magnetic field strength of up to 5 kOe. The effective magnetic moments were calculated as leff = (8v0 MT)1/2, where v0 M is the molar magnetic susceptibility, corrected for diamagnetism. The heating (cooling) rate in the field of SCO was 0.5 K/min.
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solution). The elemental analysis reveals the content of the precipitated complex as being [Fe{HC(pz)3}2]2[TCAS]. So, the interaction of [Fe{HC(pz)3}2]2+ with TCAS4 is in accordance with the Eq. (2):
Table 1 The X-ray experiment details and crystallographic characteristics for complex I. Empirical formula Formula weight Crystal system Space group Unit cell dimensions a c (Å) Volume (Å3/Z) Density (calculated) (g/cm3) F(0 0 0) Absorption coefficient (mm1) Theta range for data collection (°) Completeness to h = 25.00° Reflections collected Independent reflections/Rint Data/restraints/parameters Goodness-of-fit (GOF) on F2 R1/wR2 (I > 2r(I)) R1/wR2 (all data) Dqmax and Dqmin (e/Å3)
C20H20F6FeN12Si 626.42 Hexagonal R3
2½FefHCðpzÞ3 g2 2þ þ TCAS4 ! ½FefHCðpzÞ3 g2 2 ½TCAS
12.9311(3) 13.3679(4) 1935.83(9)/3 1.612 954 0.711 2.37–26.37 0.997 4680 880/0.0159 880/0/62 1.099 0.0230/0.0598 0.0246/0.0611 0.280 and 0.292
Table 2 Interatomic distances (d) for complex I. Bond
d (Å)
FeAN2 N1AN2 N1AC1 N1AC2 N2AC4 C2AC3 C4AC3 SiAF
1.9732(11) 1.3617(15) 1.4434(13) 1.3533(17) 1.3309(18) 1.371(2) 1.406(2) 1.6926(7)
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FTIR spectra of the samples prepared as nujol or perfluorinated nujol mulls were registered on Scimitar FTS 2000 (Varian, Inc.) spectrometer in the spectral range of 700–4000 cm1. IR spectra of the solid samples prepared as KBr pellets were registered on a FTIR spectrometer VECTOR-22 (BRUKER) in the 400–4000 cm1 range at an optical resolution of 4 cm1. The pellets were placed in a Bruker Euroterm variable temperature control unit. Diffuse reflection spectra (DRS) were registered on the scanning spectrophotometer UV-3101 PC (Shimadzu) at room temperature. The NMR experiments were performed on a Bruker Avance-600 spectrometer operating at 600.13 MHz (1H). The spectrometer was equipped with a Bruker multinuclear z-gradient probe head. The experiments were carried out at 298 K. The NMR spectra were recorded in D2O. The chemical shifts were referred to the signals for the residual protons of the deuterated solvent D2O (dH = 4.70 ppm). The 1H NMR spectra were recorded for the aqueous solution of [Fe{HC(pz)3}2]SiF6 alone (1.5 103 M) and in the presence of TCAS with TCAS:[Fe{HC(pz)3}2]2+ concentration ratio being 5:1 in the presence of ascorbic acid to prevent the oxidation of Fe(II).
ð2Þ
Both complexes are restrictedly soluble in water, ethanol, isopropanole, acetone and CH2Cl2, insoluble in benzene and toluene. X-ray diffraction measurements indicate that both complexes are in crystalline state. The single-crystal X-ray diffraction indicates that symmetry of (FeN6) moiety of complex [Fe{HC(pz)3}2]SiF6 is octahedral. Each Fe(II) ion is coordinated via three nitrogen atoms N(2) of two HC(pz)3 ligands (Fig. 1), the Fe. . .N(2) distance is 1.9732(11) Å. The structure is very similar with those of complexes [Fe{HC(pz)3}2]A2, where A = Cl, Br, I, 1=2SO4 2 [15]. The arrangement of complex cations [Fe{HC(pz)3}2]2+ is in accordance with a cubic compact packing mode, the SiF6 2 anions occupy all octahedral vacancies (Fig. 2). The IR spectra of I and II (Fig. 3) are very similar to each other in the ranges containing bands assigned to HC(pz)3 ligand. In the spectra of both complexes m(CAH) bands are shifted to the highfrequency range compared to the isolated ligand spectrum, which may be explained by changes in geometry of alkyl fragment of the ligand during the complexation. The bands 1428–1566 cm1 associated with the pyrazolil ring stretching–bending vibrations are also sensitive to the complexation: in the spectra of I and II they are observed in the higher frequency range 1446–1573 cm1 than in the spectra of free ligand, which indicates coordination of Fe2+ to N atoms in both compounds. The positions of SiAF stretching bands are indicative of the hexafluorosilicate anion as outer-sphere ligand. UV–Vis diffuse reflection spectra (DRS) of complexes I, II at room temperature exhibit one broad band in the spectral interval 450–550 nm with kmax = 480 nm, which can be assigned to 1 A1?1T2 d–d transition in a strong ligand field of distorted Oh symmetry. Unfortunately we have not succeeded in the growth of a crystal suitable for single-crystal X-ray analysis of [Fe{HC(pz)3}2]2[TCAS]. Therefore the 1H NMR measurements have been performed to reveal the outer sphere binding mode between [Fe{HC(pz)3}2]2+ and TCAS4 in solution. It is worth noting that four phenolic rings of TCAS4 form cone shaped cavity (Scheme 2), which is the reason of the inclusive binding between TCAS and positively charged metal complexes. The inclusion mode of the binding is well documented in the literature [22–29] for the outer sphere associates of TCAS4 with various positively charged complexes of Ni(II), Cu(II), Co(III). The main typical feature of such associates is the inclusion of metal complex into the cavity of calix[4]arene. Since the cavity is not large
3. Results and discussion The synthesis of [Fe{HC(pz)3}2]SiF6 was performed through the mixing of aqueous solution of FeSiF6 (Eq. (1)) with acetone solution of HC(pz)3 in the presence of ascorbic acid to prevent the oxidation of Fe2+.
FeSiF6 þ 2HCðpzÞ3 ! ½FefHCðpzÞ3 g2 SiF6
ð1Þ
Though the synthesis was performed at various Fe:HC(pz)3 concentration ratio, the elemental analysis revealed 1:2 stoichiometry ([Fe{HC(pz)3}2]SiF6). The synthesis of [Fe{HC(pz)3}2]2[TCAS] was performed by mixing of aqueous solutions of [Fe{HC(pz)3}2]SiF6 and Na4TCAS (oxygen was removed by bubbling argon through
Fig. 1. The atom numbering and the ORTEP view of the main structural units of FeC20N12H20SiF6 with 50% ellipsoid probability (independent part only). All hydrogen atoms are omitted for clarity.
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Scheme 3. Schematic representation of inclusion binding modes between TCAS and [Fe{HC(pz)3}2]2+ (a) and dissociated form of Fe(II) complex (b).
Fig. 2. Crystal structure of I. All organic ligands are omitted for clarity.
Fig. 3. IR spectra of Na4[TCAS] registered at room temperature (black line) and complexes I, II, registered at room temperature (blue lines), 373 K (green lines) and 433 K (red lines). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
enough to include the whole complex, only a fragment of the ligand environment can be included. This inclusion gives rise to rather peculiar changes in the 1H NMR spectra of metal complex due to the shielding effect of phenolic rings of the calix[4]arene cavity (Scheme 3). Thus the up-field shift of protons signals reveals the inclusion of metal complex into the cavity of calix[4]arene. It is also worth noting that such binding is rather tight due to efficient electrostatic attractive interactions provided by four ASO3 groups on the upper rim of TCAS. According to our previous data the binding constants of TCAS with mono, doubly and triply charged Co(III) complexes varies from 4 to 5 logarithmic units [27]. The NMR measurements were performed in D2O because of the good solubility in water of the Fe(II) complex and TCAS. Both 1H and 13C NMR spectra of [Fe{HC(pz)3}2]2+ possess two sets of signals (Fig. 4), which have been assigned to ligand molecules coordinated with Fe(II) (10 , 3, 4, 5) and to unbound ligand molecules (10 *, 3*, 4*, 5*). The presence of free ligands indicates the dissociation of Fe(II) complex according to equilibrium (3), which is in good agreement with its kinetic lability [19,33].
½FefHCðpzÞ3 g2 2þ ¡ ½FefHCðpzÞ3 g2þ þ HCðpzÞ3
ð3Þ
The values of chemical shifts are collected in Table 3 (see the numbering of atoms in Scheme 1). It is worth noting that the signals of iron complex protons undergo significant upfield shifts at the addition of 5-fold excess of TCAS, while very little if any effect of TCAS is observed for free ligand proton signals. The presence of the unbound ligand molecules in solution indicate that Fe(II) complex should exist in both complexed ([Fe{HC(pz)3}2]2+) and dechelated ([Fe{HC(pz)3}]2+) forms. Then three set of signals should be observed in the NMR spectra, if all three forms [Fe{HC(pz)3}2]2+, [Fe{HC(pz)3}]2+ and HC(pz)3 exist in solution. The presence of two set of signals cam be explained by the minor contribution of [Fe{HC(pz)3}2]2+ due to the equilibrium (3) being shifted to the right. The ratio of integral intensities of signals assigned to bound and free ligand molecules is close to 1:1, which confirms the minor contribution of [Fe{HC(pz)3}2]2+. Thus the experimentally observed up-field shifts are mainly resulted from the inclusion of [Fe{HC(pz)3}]2+ into the cavity of TCAS according to Scheme 3a. The inclusion mode represented in the Scheme 3a is in agreement with the values of the up-field shifts, being the less pronounced for H(1) and the most pronounced for H(4) and H(5) protons signals. It is worth noting that though significant dissociation of [Fe{HC(pz)3}2]2+ occurs in diluted solutions, no significant contribution of [Fe{HC(pz)3}]2+ has been revealed in solid state. This discrepancy can be explained by two reasons. The first one is that the precipitation of [Fe{HC(pz)3}2]2[TCAS] is faster than the dissociation of [Fe{HC(pz)3}2]2+. Thus complex [Fe{HC(pz)3}2]2[TCAS] is obtained from aqueous solutions at 4 mM of both the complex and the counterion, while in more diluted solutions (1.5–1 mM) used for the NMR measurements the dissociation is significant. The second reason is that complex [Fe{HC(pz)3}2]2[TCAS] can be less soluble in water than its dechelated counterpart ([Fe{HC(pz)3}]2[TCAS]), which should result in the predominance of [Fe{HC(pz)3}2]2[TCAS] in solid state. Nevertheless the NMR data obtained in solution enable to assume the most probable inclusion mode of [Fe{HC(pz)3}2]2+ into TCAS cavity (Scheme 3b). It is anticipated that the inclusion of [Fe{HC(pz)3}2]2+ into TCAS cavity, being possible in solution, should take place in solid state also. Some evidence in favor of the inclusion of [Fe{HC(pz)3}2]2+ into the cavity of TCAS4 in solid II is delivered by IR. For example, IR band 3374 cm1 of OH stretching vibrations of [TCAS]4 in the spectrum of solid II is blue-shifted relative to 3344 cm1 in the spectrum of solid Na4[TCAS], while the OH bending band is redshifted from 1395 to 1387 cm1, respectively. This indicates a weakening of the intramolecular H-bonds of the calixarene, which is most probably caused by the inclusion of the [Fe{HC(pz)3}2]2+
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Fig. 4. 1H NMR spectral changes observed for [Fe{HC(pz)3}2]SiF6 induced on addition of 5-fold excess of TCAS (D2O, 297 K).
Table 3 The values of chemical shifts of proton signals in 1H NMR spectra and carbon signals in 13C NMR spectra of solution of the Fe(II) complex itself and in the mixture with TCAS in D2O at 303 K and the values of upfield shifts of proton signals in the presence of TCAS. Atom number
Without TCAS
In the presence of TCAS
Complex
0
0
1 (1 *) 3(3*) 4(4*) 5(5*)
Free ligand
d 1H
d
13
9.053 8.497 7.148 7.855
71 143 115
C
Complex
d 1H
d
8.684 7.736 6.495 7.651
81 141 106 130
into the cavity of the TCAS anion. Some other changes found in the IR spectrum of II also suggest the inclusion (Fig. 3): (i) strong and very broad band with maxima at ca. 1204 and 1153 cm1 in the IR spectrum of [TCAS]Na4 transforms into several narrower bands at 1242, 1201, 1161 and 1144 cm1 in the spectrum of II; (ii) band at 1046 cm1, corresponding to SO3 bendings, shifts to 1034 cm1. The magnetochemical study of I, II showed that complexes exhibit high temperature SCO without hysteresis in the leff(T) dependence (Figs. 5 and 6). The SCO temperatures for I and II are equal to 335 and 450 K, respectively. Thus, Tc for II exceeds Tc for I considerably – by 115 K. At the same time SCO in complex II is less abrupt comparative to complex I. It is necessary to note that in the high temperature region (at 500 K) the decomposition of II begins, therefore leff(T) curve does not reach the plateau. The SCO in complex I is complete (the residual amounts of HS complexes are insignificant at room temperature). Residual part of HS form for II is more essential than for I. In both cases SCO is accompanied by thermochromism. The colour is changing from crimson to pink. It is worth noting that HS form of the previously known SCO Fe(II) complexes with 1,2,4-triazoles is white [11]. The pink colour of Fe(II) complexes with HC(Pz)3 can be explained by the residual amounts of the LS form at high temperatures. The above LS , HS transitions are reflected in temperature dependent IR spectra of I and II (Fig. 3). The most significant reversible changes occur in the spectral range 850–870 cm1, where heating of the samples causes dramatic grows of the band 850 cm1 and a red shift of the band 865 cm1, both effects being more pronounced in the IR spectrum of I. The ratio of intensities of the bands 851 and 861 in the IR spectrum of II at 433 K is
13
C
Free ligand
d 1H
Dd
d 1H
Dd
8.985 8.077 6.372 7.045
0.068 0.42 0.776 0.81
8.687 7.736 6.494 7.65
0.003 0 0.001 0.001
Fig. 5. The leff(T) dependence for compound I.
the same as in the spectrum of I at 393 K, which confirms that LS– HS transition is slower in the complex II compared to I (Fig. 3). More than that, the band 851 cm1, obviously belonging to HS form, presents in the spectrum of II at room temperature, whereas in the spectrum of I it appears at ca. 310–340 K only. This may be explained by the presence of both LS and HS forms in II at room temperature, while LS dominates in I at ambient conditions. With increasing of temperature several other changes in the spectra of I and II, caused by transition of Fe(II) from LS to HS, are observed.
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between the electronic system of the iron(II) ion and the phonon system of the lattice. Acknowledgements The work was partially supported by GC No. 02.740.11.0628 of FFP ‘‘SSESIR 2009-2013”. The authors thank I.V. Yushina (Nikolaev Institute of Inorganic Chemistry, Russia) for recording of DRS-spectra. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ica.2010.08.013. CCDC 773070 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif. Fig. 6. The leff(T) dependence for compound II.
References 1
The bands 1515 and 995 cm grow, while intensity of the bands 1450 and 1410 cm1 decreases. Practically all the bands shown in Fig. 3 undergo a red shift of ca. 5–10 cm1. Again, these changes are much more pronounced for the spectra of I in comparison with II. For example, ratio of intensities of the bands 1515 and 1410 cm1 changes from 1:4 to 1:9 in the spectrum of I, while in the spectrum of II the ratio changes from 1:3.5 to 1:4 only. This agrees with the above assumption about domination of the LS form at room temperature in I and presence of both LS and HS forms in II at the same conditions, and less abrupt LS , HS transition in the latter case. Less abrupt SCO in II comparative to I is most probably a consequence of the inclusion of [Fe{HC(pz)3}2]2+ into the cavity of TCAS4, which isolates the complex from the surrounding species and makes the character of the spin transition less cooperative relative to I. Though the crystal structure of [Fe{HC(pz)3}2]2[TCAS] has not been elucidated, it is obvious that only one of two Fe(II) complexes is included into the cavity of TCAS, while another [Fe{HC(pz)3}2]2+ cation is outside of the cavity. The unequal outer sphere environment of [Fe{HC(pz)3}2]2+ species, which are in and outside the cavity of TCAS4, should be mentioned as another possible reason of less abrupt SCO for complex II [6]. 4. Conclusion This paper describes synthesis and study of two novel mononuclear iron(II) complexes with tris(pyrazol-1-yl)methane, where hexafluorosilicate and p-sulfonatothiacalix[4]arene are used as the outer sphere counterions: [Fe{HC(pz)3}2]SiF6 and [Fe{HC(pz)3}2]2[TCAS]. Both complexes exhibit high-temperature spin-crossover without hysteresis in the leff(T) dependence. The SCO in both compounds is accompanied by the thermochromism. Tc-values for [Fe{HC(pz)3}2]SiF6 and [Fe{HC(pz)3}2]2(TCAS) are 335 and 450 K, respectively. Thus, the macrocyclic anion increases SCO Tc by 115 K in comparison with [Fe{HC(pz)3}2]SiF6. It should be noted that macrocyclic anion is used for the synthesis of SCO complexes for the first time. The 1H NMR data in solution reveal the inclusion of [Fe{HC(pz)3}2]2+ into the cavity of p-sulfonatothiacalix[4]arene. This leads to a less abrupt transition 1A1 , 5T2 and a significant increase of SCO temperature. Such a change in the nature of SCO can be attributed to the weakening of cooperative interactions
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Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica
High-temperature spin-crossover in coordination compounds of iron(II) with tris(pyrazol-1-yl)methane O.G. Shakirova a, L.G. Lavrenova a,c, V.A. Daletsky a, E.A. Shusharina a,c, T.P. Griaznova b, S.A. Katsyuba b,⇑, V.V. Syakaev b, V.V. Skripacheva b, A.R. Mustafina b, S.E. Solovieva b a b c
A.V. Nikolaev Institute of Inorganic Chemistry, Siberian Division, Russian Academy of Sciences, pr. Akademika Lavrent’eva 3, Novosibirsk 630090, Russia A.E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Centre of the Russian Academy of Sciences, Arbuzov Str. 8, 420088 Kazan, Russia Novosibirsk State University, Pirogov Str. 2, Novosibirsk 630090, Russia
a r t i c l e
i n f o
Article history: Received 21 May 2010 Received in revised form 29 July 2010 Accepted 7 August 2010 Available online 12 August 2010 Keywords: Inclusion p-Sulfonatothiacalix[4]arene Crystal structure IR NMR
a b s t r a c t Novel mononuclear Fe(II) complexes of tris(pyrazol-1-yl)methane [Fe{HC(pz)3}2]2+ with SiF6 2 and p-sulfonatothiacalix[4]arene (TCAS4) as counterions were obtained. The compounds were characterized by magnetic susceptibility method, IR and UV–Vis spectroscopy. The structure of [Fe{HC(pz)3}2]SiF6 has been analyzed by single-crystal X-ray diffraction. The 1H NMR spectroscopy measurements of [Fe{HC(pz)3}2]2(TCAS) in aqueous solution reveal the outer sphere inclusion of [Fe{HC(pz)3}2]2+ into the cyclophanic cavity of TCAS4. The temperature induced spin-crossover 1A1 , 5T2, accompanied by thermochromism, has been revealed from the temperature dependence of leff and IR spectra for both complexes. The comparative analysis of magnetochemical and spectroscopy data elucidates the effect of the cyclophanic counterion on the physico-chemical properties of Fe(II) complex. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Coordination compounds with d4–d7 electronic configuration in octahedral ligand environment have gained much attention due to their spin-crossover (SCO) behavior [1–8]. The reversible transfer from the low spin (LS) to high spin (HS) states can be induced by external input, such as temperature, pressure or light irradiation. Complexes with rather long lifetimes of LS and HS states are of great interest, both from a fundamental perspective and for potential application in molecular devices [8,9]. The complexes, which are changing their colour under SCO, gain the enhanced attention due to their particular importance for practical application. Complexes of Fe(II) with some polynitrogen-containing ligands, such as 1,2,4-triazole, tetrazole and their derivatives exemplify the thermochromic SCO complexes [5]. The SCO behavior, including abruptness, temperature and completeness of SCO transition, is greatly dependent on ligands’ structure (a nature of substituents is of particular importance), counterion and solvent effects [2,10– 14]. The large series of thermochromic complexes of Fe(II) with ⇑ Corresponding author. Tel.: +7 843 27 318 92; fax: +7 843 27 322 53. E-mail addresses: [email protected] (O.G. Shakirova), [email protected] (L.G. Lavrenova), [email protected] (V.A. Daletsky), [email protected] (E.A. Shusharina), [email protected] (T.P. Griaznova), [email protected], [email protected] (S.A. Katsyuba), [email protected] (V.V. Syakaev), [email protected] (V.V. Skripacheva), [email protected] (A.R. Mustafina), [email protected] (S.E. Solovieva). 0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2010.08.013
1,2.4-triazoles of formula Fe(4-Rtrz)3AnmH2O (R = H, NH2; n = 1, 2; m = 0, 1) was obtained by the Novosibirsk group [5,10–12]. These complexes are of polynuclear chain structure with ferrous ion being in the octahedral environment, where the coordination core is FeN6. Most of the complexes demonstrate an abrupt SCO 1 A1 , 5T2 with hysteresis on the temperature curves of the effective magnetic moment (leff) and thermochromism [5,10–12]. The temperature of SCO depends on the composition of the compounds and varies from 200 to 400 K. The series of Fe(II) complexes with tris(pyrazol-1-yl)methane (HC(pz)3, Scheme 1) [Fe{HC(pz)3}2]A2nH2O, A = Cl, Br, I, 1=2SO4 2 , n = 0–7 also demonstrate SCO and thermochromism, SCO temperatures (Tc) being within the range of 400–445 K [15]. The references [16–21] represent the synthesis of Fe(II) complexes of HC(pz)3 with various counterions, but some of these reports lack the magnetical measurements. The significant counter-ion and solvent effects on the SCO behavior (Tc in particular) were revealed for the series of [Fe{HC(pz)3}2]A2nH2O, A = Cl, Br, I, 1=2SO4 2 , n = 0–7 [15]. Though the effect of the counter ion is well known, it is not entirely predictable. Macrocyclic anions represent rather interesting alternative as counterions for SCO complexes, while at the moment these data are very rare, if any. The sulfonated derivatives of calix[n]arenes represent the series of highly charged polyanions, which can serve as counterions for [Fe{HC(pz)3}2]2+. p-Sulfonatothiacalix[4]arene (TCAS4) is one of the promising outer-sphere counterions for positively charged
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Scheme 1. Schematic representation of [Fe{HC(pz)3}2]2+ (a) and ligand HC(pz)3 (b).
1516, 1446 cm1 (ring stretchings), 715 cm1 (mSiF), 478 cm1 (dSiF). Anal. Calc. for C20H20F6FeN12Si (626.38): C, 38.4; H, 3.2; N, 26.8; Fe, 8.9. Found: C, 38.2; H, 3.4; N, 26.6; Fe, 8.5%. The single crystals suitable for X-ray diffraction were selected from the bulk of the precipitate and according to powder X-ray diffraction data they corresponded to a composition of polycrystalline sample. Scheme 2. Tetraanion of p-sulfonatothiacalix[4]arene (TCAS4).
complexes (Scheme 2) [22,27]. Therefore the present work extends the range of the outer sphere anions of [Fe{HC(pz)3}2]2+ by adding p-sulfonatothiacalix[4]arene (TCAS4) and SiF6 2 . The latter anion is chosen because of its previously revealed dramatic effect on SCO behavior of Fe(II) complexes, which was rather different for various ligands [11]. The most important peculiar feature of TCAS4 as the outer-sphere counterion is the presence of the hydrophobic cavity decorated with four ASO3Na groups, which easily dissociate in aqueous solutions, providing tetra-anion with high affinity towards positively charged metal complexes (Scheme 2). Thus the present work introduces the synthesis of [Fe{HC(pz)3}2]2+ complexes with TCAS4 and SiF6 2 as counterions and the comparative analysis of their magnetic and spectral properties.
2.2. Synthesis of [Fe{HC(pz)3}2]2(TCAS) (II) A weighed portion of [Fe{HC(pz)3}2]SiF6 (0.025 g, 0.04 mmol) was dissolved in distilled water (10 ml) under heating. A solution of Na4[TCAS] (0.0362 g, 0.04 mmol) in water (10 ml) was added to the solution of [Fe{HC(pz)3}2]SiF6. Both solutions were deoxygenated through bubbling of argon. Immediately after mixing of the initial solutions, a purple precipitate was formed, which was magnetically stirred for 1 h. The precipitate was transferred onto a filter, washed with water, dried in air and in a dessicator over anhydrone. Yield 70%. IR (nujol/perfluorinated nujol, cm1): 3374 cm1 (mOH); 3125, 2984 cm1 (mCH); 1562, 1516, 1446 cm1 (ring stretchings), 1280–1248 cm1 (mSO). Anal. Calc. for C64H52O16Fe2N24S8 (1781.44): C, 43.2; H, 2.9; N, 18.9; Fe, 6.3. Found: C, 43.3; H, 2.6; N, 18.3; Fe, 6.2%. 2.3. Methods
2. Experimental FeSO47H2O from Sigma was purified by recrystallization from acidified aqueous solution. Ascorbic acid from ICN Biomedical, Ba(NO3)2, (NH4)2SiF6, acetone from Aldrich were used as purchased. Tris(pyrizol-1-yl)methane was synthesized in accordance with the improved procedure [30]. Na4[TCAS] was synthesized according to procedure [31]. All aqueous solutions were prepared with bidistilled water. 2.1. Synthesis of [Fe{HC(pz)3}2]SiF6 (I) A 1 mmol (0.28 g) of FeSO47H2O was dissolved in 3 ml of distilled water, acidified by 0.1 g of ascorbic acid. A solution of Ba(NO3)2 (1 mmol, 0.26 g) in 7 ml of water was slowly added to the solution obtained. The precipitate of the barium sulfate was removed from solution. A solution of tris(pyrazol-1-yl)methane (0.43 g, 2 mmol) in acetone (5 ml) was added to the resulting aqueous solution of iron(II) nitrate and then an aqueous solution (7 ml) of (NH4)2SiF6 (2 mmol, 0.36 g) was rapidly added. Immediately after mixing of the initial solutions, a purple precipitate was formed, which was magnetically stirred for 1 h. The precipitate was transferred onto a filter, washed with water and ethanol, dried in air and in a dessicator over anhydrone. Yield 88%. IR (nujol/ perfluorinated nujol, cm1): 3129, 3053 cm1 (mCH); 1573–1533,
The elemental (C, H, N) analysis was performed on EURO EA 3000 (EuroVector). The analysis of metal content was performed by EDTA titration after thermal decomposition of complexes in a mixture of concentrated H2SO4 and HClO4 (1:2). X-ray diffraction study of complex I was carried out by a standard procedure on an automated four circle Bruker Nonius X8Apex diffractometer equipped with a two-dimensional CCD-detector at 150(2) K (Mo Ka-radiation, k = 0.71073 Å, graphite monochromator). The single crystal dimensions were 0.37 0.08 0.07 mm. The reflection intensities were measured by u scanning of narrow (0.5°) frames. The absorption corrections were applied empirically by the SADABS program. The structure was solved by the direct method and refined by full matrix least-squares in the anisotropic approximation for nonhydrogen atoms using SHELXTL software [32]. All hydrogen atoms were located geometrically. The X-ray experiment details and crystallographic characteristics for complex I are presented in Table 1. Table 2 contains interatomic distances for FeC20N12H20SiF6. The static magnetic susceptibility of polycrystalline samples was measured by a Faraday method in the temperature range of 275–500 K at an external magnetic field strength of up to 5 kOe. The effective magnetic moments were calculated as leff = (8v0 MT)1/2, where v0 M is the molar magnetic susceptibility, corrected for diamagnetism. The heating (cooling) rate in the field of SCO was 0.5 K/min.
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solution). The elemental analysis reveals the content of the precipitated complex as being [Fe{HC(pz)3}2]2[TCAS]. So, the interaction of [Fe{HC(pz)3}2]2+ with TCAS4 is in accordance with the Eq. (2):
Table 1 The X-ray experiment details and crystallographic characteristics for complex I. Empirical formula Formula weight Crystal system Space group Unit cell dimensions a c (Å) Volume (Å3/Z) Density (calculated) (g/cm3) F(0 0 0) Absorption coefficient (mm1) Theta range for data collection (°) Completeness to h = 25.00° Reflections collected Independent reflections/Rint Data/restraints/parameters Goodness-of-fit (GOF) on F2 R1/wR2 (I > 2r(I)) R1/wR2 (all data) Dqmax and Dqmin (e/Å3)
C20H20F6FeN12Si 626.42 Hexagonal R3
2½FefHCðpzÞ3 g2 2þ þ TCAS4 ! ½FefHCðpzÞ3 g2 2 ½TCAS
12.9311(3) 13.3679(4) 1935.83(9)/3 1.612 954 0.711 2.37–26.37 0.997 4680 880/0.0159 880/0/62 1.099 0.0230/0.0598 0.0246/0.0611 0.280 and 0.292
Table 2 Interatomic distances (d) for complex I. Bond
d (Å)
FeAN2 N1AN2 N1AC1 N1AC2 N2AC4 C2AC3 C4AC3 SiAF
1.9732(11) 1.3617(15) 1.4434(13) 1.3533(17) 1.3309(18) 1.371(2) 1.406(2) 1.6926(7)
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FTIR spectra of the samples prepared as nujol or perfluorinated nujol mulls were registered on Scimitar FTS 2000 (Varian, Inc.) spectrometer in the spectral range of 700–4000 cm1. IR spectra of the solid samples prepared as KBr pellets were registered on a FTIR spectrometer VECTOR-22 (BRUKER) in the 400–4000 cm1 range at an optical resolution of 4 cm1. The pellets were placed in a Bruker Euroterm variable temperature control unit. Diffuse reflection spectra (DRS) were registered on the scanning spectrophotometer UV-3101 PC (Shimadzu) at room temperature. The NMR experiments were performed on a Bruker Avance-600 spectrometer operating at 600.13 MHz (1H). The spectrometer was equipped with a Bruker multinuclear z-gradient probe head. The experiments were carried out at 298 K. The NMR spectra were recorded in D2O. The chemical shifts were referred to the signals for the residual protons of the deuterated solvent D2O (dH = 4.70 ppm). The 1H NMR spectra were recorded for the aqueous solution of [Fe{HC(pz)3}2]SiF6 alone (1.5 103 M) and in the presence of TCAS with TCAS:[Fe{HC(pz)3}2]2+ concentration ratio being 5:1 in the presence of ascorbic acid to prevent the oxidation of Fe(II).
ð2Þ
Both complexes are restrictedly soluble in water, ethanol, isopropanole, acetone and CH2Cl2, insoluble in benzene and toluene. X-ray diffraction measurements indicate that both complexes are in crystalline state. The single-crystal X-ray diffraction indicates that symmetry of (FeN6) moiety of complex [Fe{HC(pz)3}2]SiF6 is octahedral. Each Fe(II) ion is coordinated via three nitrogen atoms N(2) of two HC(pz)3 ligands (Fig. 1), the Fe. . .N(2) distance is 1.9732(11) Å. The structure is very similar with those of complexes [Fe{HC(pz)3}2]A2, where A = Cl, Br, I, 1=2SO4 2 [15]. The arrangement of complex cations [Fe{HC(pz)3}2]2+ is in accordance with a cubic compact packing mode, the SiF6 2 anions occupy all octahedral vacancies (Fig. 2). The IR spectra of I and II (Fig. 3) are very similar to each other in the ranges containing bands assigned to HC(pz)3 ligand. In the spectra of both complexes m(CAH) bands are shifted to the highfrequency range compared to the isolated ligand spectrum, which may be explained by changes in geometry of alkyl fragment of the ligand during the complexation. The bands 1428–1566 cm1 associated with the pyrazolil ring stretching–bending vibrations are also sensitive to the complexation: in the spectra of I and II they are observed in the higher frequency range 1446–1573 cm1 than in the spectra of free ligand, which indicates coordination of Fe2+ to N atoms in both compounds. The positions of SiAF stretching bands are indicative of the hexafluorosilicate anion as outer-sphere ligand. UV–Vis diffuse reflection spectra (DRS) of complexes I, II at room temperature exhibit one broad band in the spectral interval 450–550 nm with kmax = 480 nm, which can be assigned to 1 A1?1T2 d–d transition in a strong ligand field of distorted Oh symmetry. Unfortunately we have not succeeded in the growth of a crystal suitable for single-crystal X-ray analysis of [Fe{HC(pz)3}2]2[TCAS]. Therefore the 1H NMR measurements have been performed to reveal the outer sphere binding mode between [Fe{HC(pz)3}2]2+ and TCAS4 in solution. It is worth noting that four phenolic rings of TCAS4 form cone shaped cavity (Scheme 2), which is the reason of the inclusive binding between TCAS and positively charged metal complexes. The inclusion mode of the binding is well documented in the literature [22–29] for the outer sphere associates of TCAS4 with various positively charged complexes of Ni(II), Cu(II), Co(III). The main typical feature of such associates is the inclusion of metal complex into the cavity of calix[4]arene. Since the cavity is not large
3. Results and discussion The synthesis of [Fe{HC(pz)3}2]SiF6 was performed through the mixing of aqueous solution of FeSiF6 (Eq. (1)) with acetone solution of HC(pz)3 in the presence of ascorbic acid to prevent the oxidation of Fe2+.
FeSiF6 þ 2HCðpzÞ3 ! ½FefHCðpzÞ3 g2 SiF6
ð1Þ
Though the synthesis was performed at various Fe:HC(pz)3 concentration ratio, the elemental analysis revealed 1:2 stoichiometry ([Fe{HC(pz)3}2]SiF6). The synthesis of [Fe{HC(pz)3}2]2[TCAS] was performed by mixing of aqueous solutions of [Fe{HC(pz)3}2]SiF6 and Na4TCAS (oxygen was removed by bubbling argon through
Fig. 1. The atom numbering and the ORTEP view of the main structural units of FeC20N12H20SiF6 with 50% ellipsoid probability (independent part only). All hydrogen atoms are omitted for clarity.
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Scheme 3. Schematic representation of inclusion binding modes between TCAS and [Fe{HC(pz)3}2]2+ (a) and dissociated form of Fe(II) complex (b).
Fig. 2. Crystal structure of I. All organic ligands are omitted for clarity.
Fig. 3. IR spectra of Na4[TCAS] registered at room temperature (black line) and complexes I, II, registered at room temperature (blue lines), 373 K (green lines) and 433 K (red lines). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
enough to include the whole complex, only a fragment of the ligand environment can be included. This inclusion gives rise to rather peculiar changes in the 1H NMR spectra of metal complex due to the shielding effect of phenolic rings of the calix[4]arene cavity (Scheme 3). Thus the up-field shift of protons signals reveals the inclusion of metal complex into the cavity of calix[4]arene. It is also worth noting that such binding is rather tight due to efficient electrostatic attractive interactions provided by four ASO3 groups on the upper rim of TCAS. According to our previous data the binding constants of TCAS with mono, doubly and triply charged Co(III) complexes varies from 4 to 5 logarithmic units [27]. The NMR measurements were performed in D2O because of the good solubility in water of the Fe(II) complex and TCAS. Both 1H and 13C NMR spectra of [Fe{HC(pz)3}2]2+ possess two sets of signals (Fig. 4), which have been assigned to ligand molecules coordinated with Fe(II) (10 , 3, 4, 5) and to unbound ligand molecules (10 *, 3*, 4*, 5*). The presence of free ligands indicates the dissociation of Fe(II) complex according to equilibrium (3), which is in good agreement with its kinetic lability [19,33].
½FefHCðpzÞ3 g2 2þ ¡ ½FefHCðpzÞ3 g2þ þ HCðpzÞ3
ð3Þ
The values of chemical shifts are collected in Table 3 (see the numbering of atoms in Scheme 1). It is worth noting that the signals of iron complex protons undergo significant upfield shifts at the addition of 5-fold excess of TCAS, while very little if any effect of TCAS is observed for free ligand proton signals. The presence of the unbound ligand molecules in solution indicate that Fe(II) complex should exist in both complexed ([Fe{HC(pz)3}2]2+) and dechelated ([Fe{HC(pz)3}]2+) forms. Then three set of signals should be observed in the NMR spectra, if all three forms [Fe{HC(pz)3}2]2+, [Fe{HC(pz)3}]2+ and HC(pz)3 exist in solution. The presence of two set of signals cam be explained by the minor contribution of [Fe{HC(pz)3}2]2+ due to the equilibrium (3) being shifted to the right. The ratio of integral intensities of signals assigned to bound and free ligand molecules is close to 1:1, which confirms the minor contribution of [Fe{HC(pz)3}2]2+. Thus the experimentally observed up-field shifts are mainly resulted from the inclusion of [Fe{HC(pz)3}]2+ into the cavity of TCAS according to Scheme 3a. The inclusion mode represented in the Scheme 3a is in agreement with the values of the up-field shifts, being the less pronounced for H(1) and the most pronounced for H(4) and H(5) protons signals. It is worth noting that though significant dissociation of [Fe{HC(pz)3}2]2+ occurs in diluted solutions, no significant contribution of [Fe{HC(pz)3}]2+ has been revealed in solid state. This discrepancy can be explained by two reasons. The first one is that the precipitation of [Fe{HC(pz)3}2]2[TCAS] is faster than the dissociation of [Fe{HC(pz)3}2]2+. Thus complex [Fe{HC(pz)3}2]2[TCAS] is obtained from aqueous solutions at 4 mM of both the complex and the counterion, while in more diluted solutions (1.5–1 mM) used for the NMR measurements the dissociation is significant. The second reason is that complex [Fe{HC(pz)3}2]2[TCAS] can be less soluble in water than its dechelated counterpart ([Fe{HC(pz)3}]2[TCAS]), which should result in the predominance of [Fe{HC(pz)3}2]2[TCAS] in solid state. Nevertheless the NMR data obtained in solution enable to assume the most probable inclusion mode of [Fe{HC(pz)3}2]2+ into TCAS cavity (Scheme 3b). It is anticipated that the inclusion of [Fe{HC(pz)3}2]2+ into TCAS cavity, being possible in solution, should take place in solid state also. Some evidence in favor of the inclusion of [Fe{HC(pz)3}2]2+ into the cavity of TCAS4 in solid II is delivered by IR. For example, IR band 3374 cm1 of OH stretching vibrations of [TCAS]4 in the spectrum of solid II is blue-shifted relative to 3344 cm1 in the spectrum of solid Na4[TCAS], while the OH bending band is redshifted from 1395 to 1387 cm1, respectively. This indicates a weakening of the intramolecular H-bonds of the calixarene, which is most probably caused by the inclusion of the [Fe{HC(pz)3}2]2+
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Fig. 4. 1H NMR spectral changes observed for [Fe{HC(pz)3}2]SiF6 induced on addition of 5-fold excess of TCAS (D2O, 297 K).
Table 3 The values of chemical shifts of proton signals in 1H NMR spectra and carbon signals in 13C NMR spectra of solution of the Fe(II) complex itself and in the mixture with TCAS in D2O at 303 K and the values of upfield shifts of proton signals in the presence of TCAS. Atom number
Without TCAS
In the presence of TCAS
Complex
0
0
1 (1 *) 3(3*) 4(4*) 5(5*)
Free ligand
d 1H
d
13
9.053 8.497 7.148 7.855
71 143 115
C
Complex
d 1H
d
8.684 7.736 6.495 7.651
81 141 106 130
into the cavity of the TCAS anion. Some other changes found in the IR spectrum of II also suggest the inclusion (Fig. 3): (i) strong and very broad band with maxima at ca. 1204 and 1153 cm1 in the IR spectrum of [TCAS]Na4 transforms into several narrower bands at 1242, 1201, 1161 and 1144 cm1 in the spectrum of II; (ii) band at 1046 cm1, corresponding to SO3 bendings, shifts to 1034 cm1. The magnetochemical study of I, II showed that complexes exhibit high temperature SCO without hysteresis in the leff(T) dependence (Figs. 5 and 6). The SCO temperatures for I and II are equal to 335 and 450 K, respectively. Thus, Tc for II exceeds Tc for I considerably – by 115 K. At the same time SCO in complex II is less abrupt comparative to complex I. It is necessary to note that in the high temperature region (at 500 K) the decomposition of II begins, therefore leff(T) curve does not reach the plateau. The SCO in complex I is complete (the residual amounts of HS complexes are insignificant at room temperature). Residual part of HS form for II is more essential than for I. In both cases SCO is accompanied by thermochromism. The colour is changing from crimson to pink. It is worth noting that HS form of the previously known SCO Fe(II) complexes with 1,2,4-triazoles is white [11]. The pink colour of Fe(II) complexes with HC(Pz)3 can be explained by the residual amounts of the LS form at high temperatures. The above LS , HS transitions are reflected in temperature dependent IR spectra of I and II (Fig. 3). The most significant reversible changes occur in the spectral range 850–870 cm1, where heating of the samples causes dramatic grows of the band 850 cm1 and a red shift of the band 865 cm1, both effects being more pronounced in the IR spectrum of I. The ratio of intensities of the bands 851 and 861 in the IR spectrum of II at 433 K is
13
C
Free ligand
d 1H
Dd
d 1H
Dd
8.985 8.077 6.372 7.045
0.068 0.42 0.776 0.81
8.687 7.736 6.494 7.65
0.003 0 0.001 0.001
Fig. 5. The leff(T) dependence for compound I.
the same as in the spectrum of I at 393 K, which confirms that LS– HS transition is slower in the complex II compared to I (Fig. 3). More than that, the band 851 cm1, obviously belonging to HS form, presents in the spectrum of II at room temperature, whereas in the spectrum of I it appears at ca. 310–340 K only. This may be explained by the presence of both LS and HS forms in II at room temperature, while LS dominates in I at ambient conditions. With increasing of temperature several other changes in the spectra of I and II, caused by transition of Fe(II) from LS to HS, are observed.
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between the electronic system of the iron(II) ion and the phonon system of the lattice. Acknowledgements The work was partially supported by GC No. 02.740.11.0628 of FFP ‘‘SSESIR 2009-2013”. The authors thank I.V. Yushina (Nikolaev Institute of Inorganic Chemistry, Russia) for recording of DRS-spectra. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ica.2010.08.013. CCDC 773070 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif. Fig. 6. The leff(T) dependence for compound II.
References 1
The bands 1515 and 995 cm grow, while intensity of the bands 1450 and 1410 cm1 decreases. Practically all the bands shown in Fig. 3 undergo a red shift of ca. 5–10 cm1. Again, these changes are much more pronounced for the spectra of I in comparison with II. For example, ratio of intensities of the bands 1515 and 1410 cm1 changes from 1:4 to 1:9 in the spectrum of I, while in the spectrum of II the ratio changes from 1:3.5 to 1:4 only. This agrees with the above assumption about domination of the LS form at room temperature in I and presence of both LS and HS forms in II at the same conditions, and less abrupt LS , HS transition in the latter case. Less abrupt SCO in II comparative to I is most probably a consequence of the inclusion of [Fe{HC(pz)3}2]2+ into the cavity of TCAS4, which isolates the complex from the surrounding species and makes the character of the spin transition less cooperative relative to I. Though the crystal structure of [Fe{HC(pz)3}2]2[TCAS] has not been elucidated, it is obvious that only one of two Fe(II) complexes is included into the cavity of TCAS, while another [Fe{HC(pz)3}2]2+ cation is outside of the cavity. The unequal outer sphere environment of [Fe{HC(pz)3}2]2+ species, which are in and outside the cavity of TCAS4, should be mentioned as another possible reason of less abrupt SCO for complex II [6]. 4. Conclusion This paper describes synthesis and study of two novel mononuclear iron(II) complexes with tris(pyrazol-1-yl)methane, where hexafluorosilicate and p-sulfonatothiacalix[4]arene are used as the outer sphere counterions: [Fe{HC(pz)3}2]SiF6 and [Fe{HC(pz)3}2]2[TCAS]. Both complexes exhibit high-temperature spin-crossover without hysteresis in the leff(T) dependence. The SCO in both compounds is accompanied by the thermochromism. Tc-values for [Fe{HC(pz)3}2]SiF6 and [Fe{HC(pz)3}2]2(TCAS) are 335 and 450 K, respectively. Thus, the macrocyclic anion increases SCO Tc by 115 K in comparison with [Fe{HC(pz)3}2]SiF6. It should be noted that macrocyclic anion is used for the synthesis of SCO complexes for the first time. The 1H NMR data in solution reveal the inclusion of [Fe{HC(pz)3}2]2+ into the cavity of p-sulfonatothiacalix[4]arene. This leads to a less abrupt transition 1A1 , 5T2 and a significant increase of SCO temperature. Such a change in the nature of SCO can be attributed to the weakening of cooperative interactions
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