Lattice-solvent controlled spin transitions in iron(ii) complexes

Chandrasekar Rajadurai, Zhirong Qu, Olaf Fuhr, Balaji Gopalan, Robert Kruk, Mohammed Ghafari, Mario Ruben
2007 Dalton Transactions  
A series of spin transition (ST) iron(II) compounds of the type [Fe II L 2 ](X) 2 ·{S} 2 (where L is 4 -(4 -cyanophenyl)-1,2 :6 1 -bispyrazolylpyridine, X = ClO 4 − or BF 4 − , and S is acetonitrile) was synthesized and magnetically investigated. The effects of the removal of the lattice-solvent molecules and of their different positions relative to the iron(II) cations on the ST process were investigated. Crystallization yields orange block (A·{S} 2 ) crystals of the composition [Fe II (L) 2
more » ... tion [Fe II (L) 2 ](ClO 4 ) 2 ·{S} 2 , and two polymorphic compounds of the stoichiometry [Fe II (L) 2 ](BF 4 ) 2 ·{S} 2 as red coffin (B·{S} 2 ) and orange block (C·{S} 2 ) crystals. The Fe-N bond distances of A·{S} 2 (from 1.921(9) to 1.992(3) Å ; at 150 K), B·{S} 2 (from 1.943(2) to 2.017(2) Å ; at 180 K) and C·{S} 2 (from 1.883(3) to 1.962(3) Å ; at 180 K) indicate low spin (LS) states of the respective iron(II) ions. Notably, the observed small difference in the Fe-N distances at 180 K for the two polymorphs B·{S} 2 and C·{S} 2 are due to different positions of the acetonitrile molecules in the crystal lattices and illustrate the sensitivity of the spin transition properties on lattice-solvent effects. Variable-temperature single crystal X-ray studies display single-crystal thermochroism (red (LS) ↔ orange (HS)) for A·{S} 2 and B·{S} 2 and ca. 3.6% decrease in the unit cell volume of A·{S} 2 from 4403 Å 3 at 300 K to 4278 Å 3 at 150 K. The temperature dependent magnetic susceptibilities of A·{S} 2 and B·{S} 2 demonstrate systematic increase of the spin transition temperatures (T 1/2 ) and continuous decreases of the hysteresis loop width (DT 1/2 ) upon slow lattice-solvent exclusion.
doi:10.1039/b703700g pmid:17680043 fatcat:wd6c2acxpzau5ftdj66w75evpu