Spin-Crossover in a New Iron(II)/Di(pyrazolyl)pyridine Complex with a Terpyridine Embrace Lattice. Thermally Induced Excited Spin State Trapping and Clarification of a Structure–Function Correlation

Abstract

The complex salts [FeL 2]­X2 (1X 2 ; L = 2,6-di­{4-fluoropyrazol-1-yl}­pyridine; X– = BF4 – or ClO4 –) exhibit abrupt spin-transitions with narrow thermal hysteresis, at T 1/2 = 164 K (X– = BF4 –) and 148 K (X– = ClO4 –). The transition in 1­[ClO 4 ] 2 is complicated by efficient thermally induced excited spin-state trapping (TIESST) of its high-spin state below ca. 120 K, and the fully low-spin state was achieved only inside the magnetometer at a scan rate of 0.5 K min–1. Crystals of 1­[BF 4 ] 2 are tetragonal (P 21 c, Z = 2; phase 1) at 300 K but transform to a highly twinned monoclinic phase 2 (P21, Z = 2) at 285 ± 5 K. These are forms of the “terpyridine embrace” crystal lattice, which often affords cooperative spin-transitions in iron/di­(pyrazolyl)­pyridine complexes. Phase 2 of high-spin 1­[BF 4 ] 2 shows a significant temperature dependence by powder diffraction, which reflects increased canting of the monoclinic unit cell as the temperature is lowered. In contrast, 1­[ClO 4 ] 2 retains phase 2 between 100 and 300 K, and was crystallographically characterized in its thermally trapped metastable high-spin state at 100 K, as well as its thermodynamic high- and low-spin forms at higher temperatures. The spin-crossover transition temperature in 1­[ClO 4 ] 2 and related compounds correlates well with a parameter describing angular changes to the metal coordination sphere during the transition but not with other commonly used structural indices. The TIESST metastable high-spin state of 1­[ClO 4 ] 2 shows no single molecule magnet properties at 2 K.

Remote H/F Tuning of Cooperativity in (NN′O)2-Coordinate Iron(II) Complexes: Dynamic, Hysteretic, and Continuous Spin Crossover

“…This phenomenology, known as TIESST effect (temperature-induced excited spin state trapping), indicates freezing of a metastable HS state below its thermodynamic limits by rapid cooling. Trapped spin states can exist because of kinetic hindrance which prevents [HS→LS] relaxation, a case that is not uncommon in iron(II) systems . In the case of 1 we achieved retention of the metastable HS state down to 2 K by rapid cooling (gradual decrease of χ M T below 30 K is due to zero-field splitting; change in population of S = 2 energy sublevels).…”

“…Trapped spin states can exist because of kinetic hindrance which prevents [HS→LS] relaxation, a case that is not uncommon in iron(II) systems. 48 In the case of 1 we achieved retention of the metastable HS state down to 2 K by rapid cooling (gradual decrease of χ M T below 30 K is due to zero-field splitting; 49 change in population of S = 2 energy sublevels). Slow thermal annealing (0.3 K min −1 ; protocol by Letard et al 50 ) of the sample from 2 K induces full conversion from the metastable HS state to the LS state, which occurs sharply at T TIESST = 97 K (Figure 1b).…”

“…17,18 On the other hand, recent studies in the solution phase for different compound series have shown that the spin transition curves can be selectively affected by either solvent tuning, 19 alkyl chain length, 20 or the Hammett substitutent parameter. 21 In a rare success story, structure:function relationships could be derived which cover two different complex families, 22 that is, inner-salt [FeL n (NCS) 2 ] (L n : bidentate N-donating ligand) complexes as summarized by Guionneau et al 23 and salt-like [Fe(bpp R,Y ) 2 ] 2+ [bpp = 2,6-bis(pyrazol-1-yl)pyridine] put forward by Halcrow et al 24 Despite extensive studies for almost two decades in our group, mainly in terms of [4+2] coordination, a similarly comprehensive analysis of Jager-type ligand systems is hampered by the intrinsic thermal lability of the monodentate axial donors; the molecular factors of SCO in these systems have been recently discussed. 25 In order to suppress spurious thermal fragmentation, we decided to further extend our ligand family by keeping the fundamental entity (Schiff Base-like structure) but changing the donor topology to a bis-meridional (NN′O) 2 arrangement.…”

“…In a rare success story, structure:function relationships could be derived which cover two different complex families, that is, inner-salt [FeL n (NCS) 2 ] (L n : bidentate N -donating ligand) complexes as summarized by Guionneau et al and salt-like [Fe­(bpp R,Y ) 2 ] 2+ [bpp = 2,6-bis­(pyrazol-1-yl)­pyridine] put forward by Halcrow et al…”

Reversible Acid‐Base Responsive Fluorescence Changes of Solutions and Crystals Based on Anthracenyl Pyridyl Derivatives

“…For instance, heterocyclic molecules that contain nitrogen, oxygen, sulfur, and phosphorus atoms can cause electron movements upon external acid stimuli, such as the intramolecular charge transfer (ICT) effect, and lead to significant changes in the photophysical properties of luminophores [19,36–38] . Among the various acidochromic molecules, compounds bearing pyridines containing nitrogen atoms exhibit outstanding responsive properties and can form versatile intermolecular interactions, like metal‐coordination, electron‐acceptor, and hydrogen bonding [39–46] . Because the lone pair electrons on the nitrogen atom endow the pyridyl group with excellent capability of proton binding.…”