Ligand-directed assembly and transformation of hybrid oxo-vanadates into 3d-3d oxo-vanado-cuprates: structural diversification and superior electrochemical performance in a prototype liquid-configured-device-grade supercapacitor.

Oxo-vanadates, with their features of varied coordination and oxidation states, can form tunable structures, ranging from zero-dimensional ions or molecules to three-dimensional frameworks. Diphosphonate-functionalized oxovanadates, a recent inclusion into the family of oxovanadates, have formed diverse assemblies, particularly of mixed-valent oxovanadates. With pyridinyl-diphosphonate-functionalized oxovanadates, we have detected that the location of the nitrogen atom on the pyridine ring has strong structure- and symmetric-directing effects. To further explore the reactivity of the pyridinyl-dipshosphonate-functionalized oxo-vanadates, we reacted the mixed-valent oxo-vanadate polyanions [(VIVO2)(VV2O5)2{O3P-C(O)(CH2-n-C5NH4)-PO3}2]10- (n = 2 or 3) with copper(II) salt. This resulted in the formation of two novel 3d-3d mixed-metal hybrid oxo-vanado-cuprate polyanions: the cube-shaped [(CuII)2(VIVO2)2(VVO2)4{O3P-C(O)(CH2-2-C5NH4)-PO3}4]12- (CuV-1) and the sinusoidal one-dimensional chain [{CuII(H2O)2(CH3COO)}(VIVO2)(VV2O5)2{HO3P-C(O)(CH2-3-C5NH4)-PO3H}2]7- (CuV-2). These polyanions crystallized as (H3O)12[(CuII)2(VVO2)4(VIVO2)2{O3P-C(O)(CH2-2-C5NH4)-PO3}4]·5.5H2O (CuV-1a) and (H3O)7[{CuII(H2O)2(CH3COO)}(VIVO2)(VV2O5)2{HO3P-C(O)(CH2-3-C5NH4)-PO3H}2]·1.56H2O (CuV-2a), in the monoclinic space group of C2/c and orthorhombic space group of Pnna, respectively. The unique assembly of these mixed-metal polyanions is once again attributed to the strong structure-directing effects of the location of the nitrogen atom on the pyridinyl diphosphonate ligands, which resulted in isostructural oxo-vanadates capable of producing completely different assemblies. The oxidation states of the metal centres were established from bond valence sum (BVS) calculations and confirmed through multiple spectroscopic studies. Electrochemical energy-storage studies of the oxo-vanado-cuprate compounds revealed their enhanced redox activity and conductivity with respect to the parent oxo-vanadate compounds. Improvement was observed across all metrics, including the specific capacitance, energy density, power density, and cyclic stability. Hands-on implementation was performed using a fabricated liquid-type symmetric supercapacitor device to power LEDs and a small fan, making these compounds promising candidates for practical applications in high-performance supercapacitors with improved energy-storage efficiency.