Anion effects and spontaneous resolution of fluorescent tripyridinyl Co(II) spin crossover complexes

Three cobalt(II) complexes, [Co(Py-tpy)₂]X₂ (1, X = NO₃⁻; 2, X = BF₄⁻; 3, X = ClO₄⁻), were constructed from 4'-(pyren-1-yl)-2,2':6',2″-terpyridine to investigate the spin crossover (SCO) behavior and its potential coupling with luminescence. Complex 1 adopts a centrosymmetric structure, whereas complexes 2 and 3, with a large dihedral angle (~ 73°) between pyrene planes of two chelating Py-tpy ligands, crystallize in the chiral space group P2₁ from the N,N-dimethylformamide (DMF) solution. However, the crystallization of complex 3 in a MeOH-DCM mixture gave rise to the achiral form that is isostructural to complex 1. Magnetic measurements suggest that the desolvated complex 1-d exhibits an incomplete SCO with a ~ 20 K thermal hysteresis near room temperature. Complexes 2 and 3 show gradual SCO transitions modulated by desolvation. Temperature-dependent fluorescence emission spectra show the coexistence but no coupling effect of spin crossover and fluorescence for complex 1-d. This study clearly underscores the significant role of counter anions and the guest molecules in tuning SCO properties and inducing chirality in cobalt(II) terpyridine systems.