II20 Mononuclear and icosanuclear spin-crossover complexes, [Fe] and [Fe20], were synthesized. [Fe] has a bis-chelate structure, while [Fe20] has a ring-shape structure comprising four [2 × 2] grid moieties and four mononuclear units.

Two carboxyl-substituted iron(II) grids, one protonated, [Fe4(HL)4](BF4)4•4MeCN•AcOEt (1), and the other deprotonated,benzoic acid), were synthesized. Single crystal X-ray structure analyses reveal both complexes have a tetranuclear [2 × 2] grid structure. 1 formed one-dimensional chains through intermolecular hydrogen bonds between the carboxylic acid units of neighboring grids, while 2 formed two-dimensional layers stabilized by p-p stacking interactions. 1 showed spin transition between 3HS-1LS and 1.5HS-2.5LS states around 200 K, while 2 showed spin crossover between 4LS and 2LS-2HS states above 300 K. A modified ITO electrode was fabricated by soaking the ITO in a solution of 1. The resultant electrode showed reversible redox waves attributed to original redox processes of Fe(II)/Fe(III).

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A mononuclear ruthenium complex, [RuII(L)(bpy)2](PF6), with a naphthoquinone-annelated imidazole ligand HL (2-(pyridin-2-yl)-1H-naphtho[2,3-d]imidazole-4,9-dione) was synthesized and structurally characterized. Electrochemical study reveals that the Ru complex shows four reversible redox waves at +0.98 V, −1.13 V, −1.53 V, and −1.71 V versus SCE in acetonitrile, which are assigned to Ru(II)/Ru(III), L−/L•2−, and two bpy/bpy•− redox couples, respectively. The redox potential of Ru(II)/Ru(III) was positively shifted upon the addition of trifluoromethanesulfonic acid due to protonation of the L− moiety, leading to stabilization of the Ru 4d orbital. In UV-vis absorption measurements for the Ru complex in acetonitrile, a metal-to-ligand charge transfer (MLCT) band was observed at 476 nm, which was shifted to 450 nm by protonation, which might be due to a decrease in the electron delocalization and stabilization of the π orbitals in L−. The blue shift of the MLCT band by protonation was associated with a shift of an emission band from 774 nm to 620 nm, which could be caused by the decreased electronic delocalization in the MLCT excited state. These electrochemical and spectroscopic changes were reversible for the protonation/deprotonation stimuli.