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

Three cobalt(II) complexes, [Co(Py-tpy)₂]X₂ (<strong>1</strong>, X = NO₃⁻; <strong>2</strong>, X = BF₄⁻; <strong>3</strong>, 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 <strong>1</strong> adopts a centrosymmetric structure, whereas complexes <strong>2</strong> and <strong>3</strong>, with a large dihedral angle (~73°) between pyrene planes of two chelating Py-tpy ligands, crystallize in the chiral space group<em> P</em>2₁ from the <em>N,N</em>-dimethylformamide (DMF) solution. However, the crystallization of complex <strong>3</strong> in a MeOH-DCM mixture gave rise to the achiral form that is isostructural to complex <strong>1</strong>. Magnetic measurements suggests that the desolvated complex <strong>1-d</strong> exhibits an incomplete SCO with a ~20 K thermal hysteresis near room temperature. Complexes <strong>2</strong> and <strong>3</strong> 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 <strong>1-d</strong>. 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.