The article deals with coordination compounds of iron(II) that may exhibit thermally induced spin transition, known as spin crossover, depending on the nature of the coordinating ligand sphere. Spin transition in such compounds also occurs under pressure and irradiation with light. The spin states involved have different magnetic and optical properties suitable for their detection and characterization. Spin crossover compounds, though known for more than eight decades, have become most attractive in recent years and are extensively studied by chemists and physicists. The switching properties make such materials potential candidates for practical applications in thermal and pressure sensors as well as optical devices. The article begins with a brief description of the principle of molecular spin state switching using simple concepts of ligand field theory. Conditions to be fulfilled in order to observe spin crossover will be explained and general remarks regarding the chemical nature that is important for the occurrence of spin crossover will be made. A subsequent section describes the molecular consequences of spin crossover and the variety of physical techniques usually applied for their characterization. The effects of light irradiation (LIESST) and application of pressure are subjects of two separate sections. The major part of this account concentrates on selected spin crossover compounds of iron(II), with particular emphasis on the chemical and physical influences on the spin crossover behavior. The vast variety of compounds exhibiting this fascinating switching phenomenon encompasses mono-, oligoand polynuclear iron(II) complexes and cages, polymeric 1D, 2D and 3D systems, nanomaterials, and polyfunctional materials that combine spin crossover with another physical or chemical property.

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Spin state switching in iron coordination compounds

The three-dimensional polymeric compounds [Fe(pz)M(CN)4]·nH2O (pz = pyrazine; M = Ni(II), Pd(II), and Pt(II)) have been synthesized and characterized. They undergo strong cooperative spin transitions, large hysteresis loops, and dramatic color changes upon spin conversion. The two-dimensional homologues [Fe(py)2M(CN)4] (py = pyridine; M = Ni(II), Pd(II), and Pt(II)) also have been synthesized and characterized. In the latter case cooperativity is smaller than in the tri-dimensional derivatives, and consequently narrower hysteresis loops were observed.

Cooperative spin crossover behavior in cyanide-bridged Fe(II)-M(II) bimetallic 3D Hofmann-like networks (M = Ni, Pd, and Pt).

In der Ubergangsmetallchemie gibt es eine Klasse von Komplexverbindungen, bei denen eine Temperaturerniedrigung einen Wechsel im Spinzustand des Zentralatoms vom High-Spin- in den Low-Spin-Zustand bewirkt. Dabei andern sich die magnetischen und optischen Eigenschaften, uber die der thermische Spinubergang (auch Spincrossover genannt) sehr gut verfolgt werden kann. Dieses Phanomen tritt sowohl in flussiger Phase als auch im Festkorper auf. Eine herausragende Stellung nehmen Eisen(II) - Spincrossover - Verbindungen ein, in denen der Spinubergang im Festkorper auf sehr unterschiedliche Weise - graduell, abrupt, mit Hysterese oder stufenweise - verlaufen kann und mit Mosbauer- und optischer Spektroskopie, mit magnetischen Suszeptibilitats- und Warmekapazitatsmessungen sowie durch Kristallstrukturanalyse intensiv untersucht worden ist. Die kooperative Wechselwirkung zwischen den einzelnen Komplexmolekulen kann befriedigend durch elastische Eigenschaften und durch die Anderung von Gestalt und Volumen der Komplexmolekule beim Spinubergang erklart werden. Bei Untersuchungen an Eisen(II)-Spincrossover-Verbindungen konnte man beobachten, das sich der Low-Spin-Zustand mit grunem Licht in den High-Spin-Zustand umschalten last, der bei tiefen Temperaturen eine nahezu unendlich lange Lebensdauer haben kann (LIESST = Light-Induced Excited Spin State Trapping). Mit rotem Licht last sich der metastabile High-Spin- wieder in den Low-Spin-Zustand zuruckschalten. Der Mechanismus des LIESST-Effekts ist aufgeklart, die Zerfallskinetik im Detail untersucht und im Rahmen der Theorie strahlungsloser Ubergange verstanden. Anwendungen des LIESST-Effekts in der optischen Informationstechnik sind denkbar.

Thermisch und optisch schaltbare Eisen(II)‐Komplexe

Spin-state transitions in metal complexes which are driven by a change of temperature T or pressure p are always associated with a considerable reorganization of molecular geometry. The change involves metal-ligand bond lengths R, bond angles, and a variation of ligand orientation. In particular, the elongation 4R by up to ∼ 10% occurring in the course of the LS → HS conversion produces an expansion of molecular volume ΔV ≌ 25 A3 per metal atom. The average crystal structure for given values of T and p is reproduced by the fractional occupancy of the individual structures of the high-spin (HS) and low-spin (LS) isomer. The transitions are reasonably well described by a number of theoretical models which are equivalent to the Bragg and Williams approximation of the Ising model. The dynamics of the spin-state transitions in solution, based on measurements by ultrasonic and photo-perturbation techniques, is in general rapid with rate constants between 4 × 105 and 3 × 108 s−1. Similar results are obtained for the spin conversion in solid complexes where the line shape analysis of Mossbauer spectra based on the theory of Blume and Tjon is applied. The dynamics may be rationalized employing one-dimensional cross sections through Gibbs free-energy surfaces G = G(R), an alternative being the comparison of the results with quantum-mechanical calculations for a radiationless non-adiabatic multiphonon process.

Nature and dynamics of the spin-state interconversion in metal complexes

Thermal and light-induced spin crossover in iron(II) complexes

One of the authors (M.S.) wishes to express his sincere thanks to the Ministry of Education, Science and Culture for a Grant-in-Aid for Scientific Research. D.N.H. is grateful for funding from National Institutes of Health Grant HL16352.

Heat capacity and phase transitions of the mixed-valence compound [Fe3O(O2CCH3)6(py)3](py)

An automated adiabatic calorimeter for the temperature range 13 K to 530 K The heat capacities of benzoic acid from 15 K to 305 K and of synthetic sapphire from 60 K to 505 K

Heat capacity and dual spin-transitions in the crossover system [Fe(2-pic)3]Cl2·EtOH

The detailed nature of the spin-crossover phenomenon and phase transitions observed for the [Fe(3-OEt-SalAPA) 2 ]-(ClO 4 )-S (S is C 6 H 6 , C 6 H 6 , C 6 H 6 , C 6 H 5 Cl, C 6 H 5 Br or o-C 6 H 4 Cl 2 ) series is examined. The data base from the preceding paper together with an analysis of the crystal packing, variable-temperature solid-state 2 H NMR data for C 6 D 6 , C 6 D 5 Cl and C 6 D 5 Br solvates, and 13-300 K adiabatic calorimetry data are employed

Dynamics and phase transitions in spin-crossover complexes: detailed nature of phase transitions in the solvate series bis(3-ethoxysalicylideneaziridinopropylaminato)iron perchlorate.solvate

The effect of adding permanently distorted [Fe III 2 Co II O(O 2 CCH 3 ) 6 (py) 3 ] (py) (2) complexes to the lattice of the isostructural mixed-valence Fe III 2 Fe II O complex 1 to form solid solutions of [Fe IIi 2 Fe II 1−x Co II x O(O 2 CCH 3 ) 6 (py) 3 ] (py) is examined. Single-crystal 298 K X-ray structures are presented for [Fe 3 O(O 2 CCH 3 ) 6 (py) 3 ] (py) (1) and Fe III 2 Co II O complex 2. Both complexes crystallize in the rhombohedral space group R32. Complex 1 has a unit cell with a=17.558 (7) A and c=11.074 (4) A, where Z=3. Complex 2 crystallizes with a unit cell of a=17.583 (6) A and c=11.074 (3) A, with Z=3. Complex 2 is statically disordered, whereas complex 1 is dynamically disordered at 298 K. In both cases Fe 2 MO (M=Fe, Co) complexes and pyridine solvate molecules are alternatively stacked along the crystallographic C 3 axis

Kazutoshi Kaji

Papers

Heat capacity and phase transitions of the mixed-valence compound [Fe3O(O2CCH3)6(py)3](py)

An automated adiabatic calorimeter for the temperature range 13 K to 530 K The heat capacities of benzoic acid from 15 K to 305 K and of synthetic sapphire from 60 K to 505 K

Heat capacity and dual spin-transitions in the crossover system [Fe(2-pic)3]Cl2·EtOH

Dynamics and phase transitions in spin-crossover complexes: detailed nature of phase transitions in the solvate series bis(3-ethoxysalicylideneaziridinopropylaminato)iron perchlorate.solvate

Effect of quenched disorder on phase transitions in the mixed-valence solid solution [FeIII2FeII1-xCoIIxO(O2CCH3)6(py)3](py)