1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 z Inorganic Chemistry Studies on Sulfate Iron(II) Complex With Tris(pyrazol-1-yl) methane Exhibiting Spin Crossover in Aqueous Solutions Vasily V. Kokovkin,* [a, b] Igor V. Mironov, [a, b] Evgenii V. Korotaev, [a] Vladimir R. Shayapov, [a] Olga G. Shakirova, [c] and Ludmila G. Lavrenova [a, b] Using the methods of UV-, VIS- and near-IR spectrometry, potentiometry, static magnetic susceptibility and cyclic voltam- metry, an iron(II) complex with tris(pyrazol-1-yl)methane (L) having the composition of [FeL 2 ]SO 4 is studied in aqueous solution. In the solid state this compound exhibits spin cross- over 1 A 1 $ 5 T 2 . The protonation constants of L (log K 1H = 2.53, log K 2H = 1.80) and stability constants of the complex (log β(FeL 2 + ) � 4, log β(FeL 2 2 + ) = 8.62) at t = 25° C, I = 0.05M have been determined. The studies on μ eff (T) of [FeL 2 ]SO 4 in solution in the temperature range from 280 to 355 K have shown that the value of μ eff exhibits an increase with increasing temperature. This indicates that with increasing temperature a transition occurs from the low spin (LS) state of the complex to the high spin (HS) state thereof. Introduction The compounds that exhibit “bistability” properties, which means their ability of existing in two states with a sufficiently long lifetime, attract special researchers’ attention. This type of compounds involves the complexes of metals with d 3 - d 7 electron configuration that exhibit spin crossover (SCO), which means changing spin multiplicity either under the influence of external factors such as temperature, pressure, or under the impact of light with a certain wavelength. Most often, SCO is exhibited by iron(II) and iron(III) complexes with nitrogen-containing ligands. These complexes are rather prom- ising for practical applications as display elements, pressure sensors, optical switches etc. In the overwhelming majority of the complexes, changing spin multiplicity is accompanied by an abrupt change in the magnetic and optical properties. Monographs, [1,2] numerous reviews [3–9] and scientific papers are devoted to the investigation of this phenomenon. A wide range of methods is applied to the experimental investigation of SCO. Among them, the basic technique is represented by the measurement of static magnetic suscepti- bility, which allows one to obtain the effective magnetic moment (μ eff ) depending on temperature. The magnetochem- ical data are supplemented with the studies based on UV-Vis, IR-, Mössbauer and EXAFS-spectroscopy. Most often, the compounds that exhibit SCO are studied in the solid state. However, there are a number of papers devoted to the investigation of such complexes in solutions. [10–12] Iron(II) complexes with polynitrogen-containing ligands are of particular interest, since the spin transitions in the most part of them is accompanied by thermochromism, which signifi- cantly extends the potentialities of their practical application. These complexes can be used in the development of systems for recording and storing information, thermochromic indica- tors and labels, electroluminescent devices, temperature- sensitive materials in magnetic resonance imaging (MRI) etc. [13–18] Tris(pyrazol-1-yl)methanes represent a promising class of compounds for the synthesis of iron(II) magnetically active coordination compounds with polydentate nitrogen-containing heterocyclic ligands including the complexes that exhibit SCO. The tris(pyrazol-1-yl)methanes were shown to coordinate with metal ions through three N(2) atoms of pyrazole cycles in a tridentate-cyclic manner. [6,7] Owing to this type of coordination, two ligands in [FeL 2 ] 2 + form a FeN 6 coordination core, which is a prerequisite for SCO. Earlier, we synthesized an iron(II) sulfate complex with ligand L having the composition of [FeL 2 ]SO 4 ⋅2H 2 O and determined the crystal structure of complex [FeL 2 ]SO 4 ⋅7H 2 O. [6,19] The structure of coordination core [FeL 2 ] 2 + is presented in Scheme 1. The investigation of μ eff (T) showed that the compound exhibits a high temperature 1 A 1 $ 5 T 2 SCO with no hysteresis. The transition temperature under heating and cooling for a dehydrated analogue thereof is T c " = T c # = 400 K. It seemed worthwhile to investigate this complex in aqueous solutions using the methods of spectrometry in UV, visible and near-IR wavelength range, static magnetic susceptibility, potentiome- try, and cyclic voltammetry for determining the protonation constants of L and the stability constants of iron(II) complexes with this ligand, as well as for clarifying whether the complex retains the SCO in solutions. [a] Dr. V. V. Kokovkin, Prof. I. V. Mironov, Dr. E. V. Korotaev, Dr. V. R. Shayapov, Prof. L. G. Lavrenova Nikolaev Institute of Inorganic Chemistry SB RAS, 3 Lavrentiev Avenue, Novosibirsk, 630090, Russia E-mail: basil@niic.nsc.ru [b] Dr. V. V. Kokovkin, Prof. I. V. Mironov, Prof. L. G. Lavrenova Analytical Chemistry Department, Faculty of Natural Sciences, Novosibirsk National Research State University, 2 Pirogova Str., Novosibirsk, 630090, Russia [c] Dr. O. G. Shakirova Komsomolsky-na-Amure State Technical University, 27 Lenin Avenue, Komsomolsk-na-Amure, 681013, Russia Supporting information for this article is available on the WWW under https://doi.org/10.1002/slct.201901424 Full Papers DOI: 10.1002/slct.201901424 9360 ChemistrySelect 2019, 4,9360–9366 © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim