Variation in the zero-point energy difference via electrostatic interactions in Co(<sc>ii</sc>)-Cltpy-based spin-crossover molecular materials

Herein, newly synthesized Co(ii)-Cltpy-based spin-crossover molecular materials composed of ClO4-, BF4-, PF6-, and CF3SO3- are systematically investigated, emphasizing their structure-property interplay. The multi-faceted character of the ligand 4 '-chloro-2,2 ':6 ',2 ''-terpyridine (Cltpy) causes the Co(ii)-based molecular materials to show electronical behavior crucial to spin crossover involving crystallographic packing and disparity in the zero-point energy difference, Delta E0HL, through electrostatic interactions mediated by their cationic and anionic moieties as well as other dipolar and quadrupolar moieties. All crystalline coordination networks were found to be phase-pure, well-crystallized, and geometrically constrained, subsequently demonstrating fascinating crystal packing along with a gradual, incomplete temperature-dependent magnetic response and negligible elastic interactions among the Co(ii)-spin-crossover centers. The ClO4- and PF6- analogs showed comparatively strong intra-chain and inter-chain interactions, resulting in substantially strong effective crystal fields experienced by the Co(ii)-spin-crossover centers. This indicated large positive values of Delta E0HL and their distribution with considerable stabilization of the low-spin state over the high-spin state at higher temperatures, indicating an identical magnetic response close to the pure low-spin state. Alternatively, the BF4- analog demonstrated comparatively weak intra-chain interactions without having any inter-chain interactions, resulting in moderately weak effective crystal fields around the Co(ii)-spin-crossover centers. This signified negative values of Delta E0HL and their distribution towards stabilization of the high-spin state over the low-spin state, as exhibited by the enhanced magnetic response up to room temperature. In the case of the CF3SO3- analog, discrete molecular moieties were observed without any intra- and inter-molecular interactions, which was attributed to its weakest effective crystal-field strength among all the compounds reported herein, essentially signifying moderately large negative values of Delta E0HL and their distribution towards stabilization of the high-spin state over the low-spin state, as indicated by the rapid increase in the magnetic response to the greatest extent. Further, the EPR data of the reported compounds at 8 K showed an excellent fingerprint for the formation of the LS Co(ii). The significant variation in the polarity of the individual counter anions played a vital role in the alteration of the crystal packing, along with varying intra-, inter-molecular, and electrostatic interactions. The experimental results were further integrated with the newly developed physically interpreted theoretical model (electrostatic-mechanoelastic model) to highlight the role of electrostatic interactions in combination with the alteration in the crystallographic packing for successfully reproducing the experimental magnetic behavior.