pH-Controlled Assembly of Five New Organophosphorus Strandberg-Type Cluster-Based Coordination Polymers for Enhanced Electrochemical Capacitor Performance

Cluster-based coordination polymers (CCPs) have shown promise as capacitors. To investigate the relativity between capacitor performance and crystal structure, herein, five new CCPs based on organophosphorus Strandberg-type clusters were synthesized via in situ hydrothermal reactions at different pH's, namely, (H-2 bipy)(2) [(C6H5PO3)(2) Mo5O15]center dot 2H(2)O (1), (H-2 bipy)(1.5) [Cu (I) (bipy)(C6H5PO3)(2) Mo5O15]center dot H2O (2), H-2 [Cu-2(I) (bipy)(2.5) (C6H5PO3)(2) Mo5O15]center dot 2H(2)O (3), Na-2 [Cu-4(I) Cu-II(bipy)(4)(C6H5PO3)(2) (Mo5O15)(2)]center dot 15H(2)O (4), [Cu-2(II) (bipy)-(H2O)(4)(C6H5PO3)(2) Mo5O15] (5) (bipy = 4,4'-bipyridine). Compound 1 is a zero-dimensional monomer, in which the protonated bipy ligands as countercations combine Strandberg-type clusters by hydrogen bonding and zr-TC interaction forming a supramolecular layer. Compound 2 represents a unique one-dimensional (1D) channel chain structure linked by intermolecular hydrogen bonding and pi-pi interaction. Compounds 3 and 4 exhibit the first example of an interdigitated architecture based on organophosphorus Strandberg-type clusters [1D + 1D -> two-dimensional (2D) for 3 and 2D + 2D -> three-dimensional (3D) for 4]. Compound 5 displays a novel (3,4)-connected 3D microporous framework with (8(1).6(2)) (8(3).6(3)) topology. Notably, the more complicated structures of compounds 1-5 were obtained with an increase in pH. The isolation of five compounds is beneficial for our systematic understanding of the effect of pH on the assembly of CCPs. Organophosphorus Strandberg-type polyoxometalate clusters were explored as supercapacitor electrode materials for the first time. Compared with other CCPs in this work, compound 5 shows the highest specific capacitance, 160.9 F g(-1), at a current density of 2 A g(-1), and for favorable cycling stability, after 1000 cycles, the retention rate of the capacitance is 95.6% at 10 A g(-1).