Desolvation-Solvation-Induced Reversible On-Off Switching of Two Memory Channels in a Cobalt(II) Coordination Polymer: Overlay of Spin Crossover and Structural Phase Transition

The engineering of switchable materials with controllable stimuli-responsive multistability remains challenging in materials science. Herein, we present syntheses and structural and magnetic studies of a one-dimensional cobalt(II) coordination polymer[(enbzp)Co(bpy)](ClO4)(2)center dot MeOH center dot H2O(1; enbzp= N,N'-(ethane-1,2-diyl)bis(1-phenyl-1(pyridin-2-yl)methanimine, bpy = 4,4'-bipyridine) and its desolvated analogue [(enbzp)Co(bpy)](ClO4) 2 (2), obtained by reversible single-crystal-to-single-crystal (SCSC) transformation. Both complexes feature a rigid cationic chain with alternate enbzp-chelated Co(II) units and bpy linkers and exhibit incomplete and gradual spin crossover (SCO) behavior while, interestingly, additional thermally induced nonspin bistability was only observed in 2. Remarkably, the nonspin bistability shows an unprecedented scan-rate selectivity with a 75 K shift of center temperatures. At a rate above 5 K/min (fast-cooling), the transition takes place solely in the temperature range of 225-240K centered at about 230Kwithahysteresis loopof 14 K, while at rates below 0.5 K/min (slow-cooling), the dynamic bistabilitymoves to the room-temperature region (similar to 305 K) with wider hysteresis loops (similar to 26 K). Further studies revealed that the slow-cooling coupled transition occurs synchronouslywith the conformational swing of the cationic [(enbzp)Co(bpy)](2+) units and displacement of the [ClO4](-) anions. The fast-cooling situation, however, could not be followed due to the rapid and irreversible structural rearrangement toward the more thermodynamically stable phase, as confirmed by time-dependent structural characterization and magnetic relaxation studies. These physical properties strongly corroborate that the two separated thermalbistabilitystatesaswellas themultimagneticstates can be selected at will, based on the kinetics of subtle structural changes or rearrangements with different temperature-scan rates, which may be promoted to the memory devices armed with multichannels and functionalized in a desired manner. [GRAPHICS] .

Automated summary

The engineering of switchable materials with controllable stimuli-responsive multistability remains challenging. In this study, a one-dimensional cobalt(II) coordination polymer with reversible single-crystal-to-single-crystal transformation was synthesized and characterized. The material exhibits unique thermally induced nonspin bistability and scan-rate selectivity.