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

• Published in: Crystal growth & design 2022-11, Vol.22 (11), p.6809-6817
• Main Authors: Michaels, Evridiki, Capel Berdiell, Izar, Vasili, Hari Babu, Pask, Christopher M., Howard, Mark J., Cespedes, Oscar, Halcrow, Malcolm A.
• Format: Article
• Language: English
• ISSN: 1528-7483; 1528-7505
• DOI: 10.1021/acs.cgd.2c00980

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 421 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.