Directed coordination of C/N-termini of cyano group in metal hexacyanoferrates to efficient palladium recovery: Enhanced adsorption affinity and selectivity.

N-termini Cyano group (CN) in metal hexacyanoferrates (MHCF) have been identified as specific-affinity sites for palladium (Pd), but C-termini CN do not effectively serve as Pd adsorption sites due to their stronger bonds with the metal ligands (M), which reduces the activity and density of CN. Herein, the optimization of directional coordination of cyano group C/N-termini by modulating the electronic structure of the M (Fe^II, Co^II, and Ni^II) in MHCF was investigated to reinforce the Pd recovery. Spectroscopic analyses and DFT calculations revealed that NiHCF exhibited N-site mono-coordination, whereas CoHCF displayed C-site mono-coordination due to spin-exchange interactions, leading to the strengthened N-Co^III bonds and weakened Fe^II-C bonds. Interestingly, FeHCF maintained N-coordination properties and showed C-coordination as the shift of the d-band center weakened the Fe^III-C bonds. Double-coordination of CN resulted in a higher adsorption-capacity and -rate than mono-coordination, which were attributed to its greater CN content and adsorption affinity, respectively. Additionally, all three MHCFs, particularly double-coordination FeHCF, demonstrated excellent selectivity in noble/base metal systems, good resistance to anionic interference, and reusability. The study underscores the pivotal role of M's electronic structure in the CN coordination environment, offering a novel strategy for the directional design of adsorbent sites.