Spin State Modulation in Cobalt(II) Terpyridine Complexes by Co-Crystallization with 1,3,5-Triiodo-2,4,6-trifluorobenzene

Abstract Reversible water molecule-induced spin state inter-conversion for the mononuclear cobalt(II) complex [Co(terpy)2]I2·2H2O (1, terpy = 2,2′:6′,2′′-terpyridine) is reported along with its co-crystallization with 1,3,5-triiodo-2,4,6-trifluorobenzene (TITFB) to yield three types of halogen bonded frameworks, [Co(terpy)2][(TITFB)I2] (2), [Co(terpy)2][(TITFB)2I2] (3) and [Co(terpy)2][(TITFB)4(MeOH)I2] (4) (TITFB = 1,3,5-triiodo-2,4,6-trifluorobenzene). The magnetic properties of 1–4 have been investigated. While 1 exhibits gradual spin crossover (SCO) behavior, de-solvated [Co(terpy)2]I2 (1′) exhibits abrupt SCO behavior (T1/2 = 120 K) attributed to a change in its intermolecular interactions on dehydration. The crystal structures as well as the magnetic properties of 1 and 1′ can be switched reversibly via single-crystal to single-crystal (SCSC) transformations via hydration/dehydration processes. Co-crystallization of [Co(terpy)2]I2 with TITFB resulted in three types of halogen-bonded frameworks (2–4). While 2 exhibits incomplete abrupt spin transition (T1/2 = 56 K), 3 and 4 show incomplete gradual SCO behavior (attributed to stabilization of the LS state). The observed SCO behaviors are in accord with the structural distortions occurring in the respective [Co(terpy)2]2+ cations and resulting from their intermolecular interactions with the surrounding frameworks. These results illustrate the manner by which co-crystallization leading to halogen-bonded co-crystals in the present study can result in spin state modulation in SCO complexes.