Influence of methyl substitution on linear diboronic acids: toward spiroborate covalent organic framework formation in N,N-diethylformamide†

Recently, we reported the reconstruction of two-dimensional (2D) to three-dimensional (3D) covalent organic frameworks (COFs) via base-catalyzed boronate ester to spiroborate linkage conversion. In that work, we tentatively attributed the interlayer close-packing in the 2D BPDA-COF as the main cause for the long reaction time—40 days—required to complete the structure reconstruction in N,N-diethylformamide (DEF). Here, we address this hypothesis by designing methyl-substituted 4,4′-biphenyldiboronic acid (BPDA) with large molecular twist to weaken the packing between boronate esters. Experiments show that the spiroborate COF formation is accelerated by increased molecular twist in three linear diboronic acids linkers, with the pure 3D spiroborate phase obtained in 3 days via reaction of Co(II) 2,3,9,10,16,17,23,24-octahydroxyphthalocyaninato ((OH)₈PcCo) in N,N-diethylformamide (DEF). Mechanistic studies reveal that methyl-substituted linear diboronic acids are more liable to protodeboronation, which also contributes to the accelerated spiroborate structure formation.