DEVELOPMENT OF SPIN CROSSOVER COMPLEXES BIS 2,6-BIS(PYRAZOL-3-YL)PYRIDINE (3-BPP) IRON(II) WITH TRIHALOACETATE, TRICHLOROSULFONATE AND TETRACYANONICKELATE(II) ANIONS

• Main Author: Fitriani
• Format: Dissertations
• Language: Indonesia
• Subjects: Kimia
• Online Access: https://digilib.itb.ac.id/gdl/view/83338
• Institution: Institut Teknologi Bandung

The [Fe(3-bpp)2]2+ complex with 3-bpp is a 2,6-bis(pyrazol-3-yl)pyridine ligand, which is one of the complexes that can undergo spin transition or Spin Crossover (SCO) phenomenon. The SCO phenomenon occurs when a complex compound changes its magnetic properties as a result of electron spin changes in the iron(II) ion, which is the central atom of the complex. At low temperatures, the iron(II) complex can be in a Low Spin (LS) state and it exhibits diamagnetic properties when all electrons are paired (t2g6 eg0). Meanwhile, the iron(II) complex can switch to a High Spin (HS) state at high temperatures and it exhibits paramagnetic properties with four unpaired electrons (t2g4 eg2). In the complex compound, the change in electron spin occurs because 3-bpp is a medium-strength ligand. Therefore, the LS ? HS transition is caused by an external factor, such as temperature change. Besides the strength of the ligand, the counter anion in the [Fe(3-bpp)2]2+ complex plays an important role in the SCO phenomenon, although its anion does not directly coordinate with the central Fe atom. The size of counter anion (X) in the [Fe(3-bpp)2]X complex has been investigated, and only the large-sized anions such as bromide and iodide produced the [Fe(3-bpp)2]2+complexes with SCO characteristics. The [Fe(3-bpp)2]2+complex with chloride anion did not display SCO characteristics, while the [Fe(3-bpp)2]2+complex with fluoride anion has not been found. However, the [Fe(3-bpp)2]2+complexes with polyatomic anions containing fluoride ions such as tetrafluoroborate and hexafluorophosphate have been observed and exhibited SCO characteristics. Furthermore, the use of anions containing trifluoride groups, such as trifluorosulfonate and trifluoroacetate, also contributed to the [Fe(3-bpp)2]2+ complexes with SCO characteristics. The research of counter anions in the [Fe(3-bpp)2]2+ complex has also been developed by using cyano complex anions such as [Fe(CN)5(NO)]2?, [Ag(CN)2]? dan [Au(CN)2]?, which exhibited SCO characteristics. Based on the literature study, the existence of counter anion in the [Fe(3-bpp)2]2+complexes, which does not directly coordinate with the central Fe atom, becomes an interesting research topic to be studied, especially regarding the factors of anion size, anion charge, and the presence of high electronegativity atoms in the counter anion. The use of different types of counter anions in the [Fe(3-bpp)2]2+ complexes still holds potential for further study in the development of SCO materials, particularly with large-sized anions such as trihaloacetate, trihalosulfonate, and cyano complex anions with a charge of -2 and a square planar structure. Therefore, this study aims to obtain new [Fe(3-bpp)2]2+ complexes with four types of counter anions, which are trifluoroacetate (CF3CO2 ?), trichloroacetate (CCl3CO2 ?), trichlorosulfonate (CCl3SO3 ?), and the complex anion of tetracyanonickelate(II) ([Ni(CN)4]??) that exhibit SCO characteristics. The synthesis of [Fe(3-bpp)2]2+complexes with various kinds of counter anions was carried out using two different synthesis methods. The [Fe(3-bpp)2]2+complexes with trifluoroacetate or trichloroacetate anions were synthesized by direct reaction of iron(II) salt with these anions. Meanwhile, the [Fe(3-bpp)2]2+complexes with trichlorosulfonate anion or tetracyanonickelate(II) complex anion were synthesized by the anion exchange reaction of chloride from the solution of [Fe(3-bpp)2]2+ complex with salts of these anions. From the synthesis results of [Fe(3-bpp)2]2+ complexes with various kinds of counter anions, the obtained new four complexes which have been successfully synthesized and confirmed their molecular formulas, consist of [Fe(3-bpp)2](CF3CO2)2·H2O; [Fe(3-bpp)2](CCl3CO2)2·H2O; [Fe(3-bpp)2](CCl3SO3)2·2H2O·0,5CH4 and [Fe(3-bpp)2][Ni(CN)4]·4H2O. Among all these complexes, the [Fe(3-bpp)2](CF3CO2)2·H2O complex exhibited an abrupt spin transition with a full low spin state. Contrary to the previous report of other research groups, a complex formula of [Fe(3-bpp)2](CF3CO2)2 has been synthesized. The magnetic study observed that the [Fe(3-bpp)2](CF3CO2)2 complex exhibited an abrupt spin transition, although it was not complete in achieving a full low spin state. In this study, the development of [Fe(3-bpp)2]2+ complex with trifluoroacetate anion produced a complex of [Fe(3-bpp)2](CF3CO2)2·H2O. The complex was obtained as solid powder with a red-brown color. Meanwhile, a single crystal of the complex with a brown-orange color was grown from the product recrystallization using methanol and chloroform solvents. The presence of one crystal water molecule in the complex was detected from thermogravimetric analysis, which showed a mass reduction of 3,02% at temperatures below 100 °C. Furthermore, the complex was stable up to 200 °C, but decomposed when it was heated up to that temperature. At room temperature (300 K), the complex exhibited paramagnetic properties with the complex's molar magnetic susceptibility (?MT) value of 3,03 emu mol?? K. This data indicated that around 83% of the iron(II) in the complex is in the high spin state. Single crystal X-ray Diffraction (XRD) analysis data also showed an average Fe–N bond length of 2,171 Å, which is consistent with the high spin state of the iron(II) complex. In the cooling process (230–180 K), the iron(II) complex changed into a low spin state with a sharp-discontinuous transition and the ?MT value of 0,34 emu mol?? K at 180 K. In the heating process, this complex produced the same ?MT value as in the cooling process. Based on these results, the [Fe(3-bpp)2](CF3CO2)2·H2O complex exhibited the SCO characteristics with an abrupt spin transition, and the transition temperature (T1/2) value occurred at about 216 K. The transition temperature indicates the temperature at which the high spin fraction is equal to the low spin fraction. The SCO phenomenon of this complex is accompanied by reversible color change (thermochromic effect) from red-brown at room temperature (in the high spin state) to dark brown upon cooling in liquid nitrogen (in the low spin state). The use of trichloroacetate anion, which is larger than trifluoroacetate anion in the [Fe(3-bpp)2]2+ complex, resulted in a complex of [Fe(3-bpp)2](CCl3CO2)2·H2O. This complex was obtained as reddish-black crystals and exhibited a low spin state with the ?MT value of 1,52 emu mol-1 K at 300 K. The ?MT value indicated a high spin fraction of around 40%. This finding suggested that the larger size of anion can reduce the high spin fraction of the [Fe(3-bpp)2]2+ complex with trihaloacetate group anion. Upon cooling to 13 K, the complex exhibited fully low spin with the ?MT value of 0,66 emu mol-1 K. The complex has an average Fe–N bond length of 1.955 Å, which is consistent with the low spin state of the iron(II) complex. The [Fe(3-bpp)2]2+ complex with trichlorosulfonate or triflate as counter anion produced a new complex, [Fe(3-bpp)2](CCl3SO3)2·2H2O·0,5CH4O. This complex was obtained as reddish-black crystals. The presence of crystal water molecules and methanol solvents in the complex was detected from the single crystal XRD analysis. The complex exhibited a low spin state with a high spin fraction of about 33%, or the ?MT value of 1,23 emu mol-1 K at 300 K. This ?MT value is lower than that of the [Fe(3-bpp)2]2+ complex with trichloroacetate anion. Upon cooling to 2 K, the complex showed fully low spin with the ?MT value of 0.10 emu mol-1 K. The low spin state of this complex is supported by an average Fe–N bond length of 1,956 Å. Upon heating to 340 K, the high spin fraction of the complex increased to about 46% with the ?MT value of 1.71 emu mol-1 K. However, upon heating above 340 K, the complex caused decomposition and it lost SCO characteristics. Based on this finding, the effect of anion size does not guarantee to display of the SCO phenomenon, and the stability factor of the [Fe(3-bpp)2]2+ complexes also plays a significant role in the SCO phenomenon. The use of cyano complex anion as a counter anion in the [Fe(3-bpp)2]2+ complex produced a complex with many water molecules, [Fe(3-bpp)2][Ni(CN)4]·4H2O. At room temperature, the complex was obtained as solid powder with a brick red color. The complex exhibited paramagnetic properties with a high spin fraction of about 75% and the ?MT value of 2,65 emu mol-1 K at 300 K. In the cooling process, the complex changed gradually into a low spin state with a high spin fraction of around 29% and the ?MT value of 1,37 emu mol-1 K at 12 K. The measurement of the ?MT value in the heating process obtained the same results as in the cooling process. Therefore, this complex exhibited the SCO characteristics with a gradual spin transition. The nature of the spin transition of this complex is also visually observed from the reversible color change of brick red at room temperature (in the high spin state) to a dark brown on cooling in liquid nitrogen (in the low spin state). Based on the magnetic properties measurement at various temperatures, the [Fe(3-bpp)2](CCl3CO2)2·H2O, [Fe(3-bpp)2](CCl3SO3)2·2H2O·0,5CH4O and [Fe(3-bpp)2][Ni(CN)4]·4H2O complexes exhibited the same gradual spin transition, respectively. Meanwhile, the [Fe(3-bpp)2](CCl3CO2)2·H2O complex displayed an abrupt spin transition, where the HS ? LS transition occurred abruptly in a short temperature range. The abrupt spin transition supports the functional material as sensors. Therefore, the [Fe(3-bpp)2](CCl3CO2)2·H2O complex can be further developed as a smart magnetic material in the future.