Anion-aided Dynamic One-pot Self-assembly of Rectangular Metallacycles

Abstract

The tetracopper(I) complex [{Cu2(μ‐dppm)2}2(μ‐1,4‐O2CC6H4(CO2)2)](BF4)2 (1(BF4)2) and 1,2‐bis(4‐pyridyl)ethane (bpa) can establish a dynamic equilibrium in CH2Cl2. From the equilibrium mixture containing 1(BF4)2 and bpa with the molar ratio 1(BF4)2/bpa of 1:1, a supramolecular compound [{Cu2(μ‐dppm)2}2(μ‐1,4‐C6H4(CO2)2)(μ‐bpa)]2(BF4)4 (2(BF4)4) was obtained as single crystals. The crystal structure was determined by X‐ray crystallography to reveal presence of one normalB normalF 4 − anion inside a cationic rectangular metallacycle {2 ⊂ BF4 }3+. Both structural evidence and DFT‐calculated results indicate that the F atoms of the anion exert weak electrostatic attraction with hydrogen atoms of the bound bpa as the framework of the cationic metallacycle. The attractive interactions apparently play an important role in stabilizing some dynamically self‐assembled precursors so as to form the final anion‐included metallacycle. Without the electrostatic help from the anion, the self‐assembly of the empty metallacycle may be hindered by a rather large endothermic free energy. The favorable electrostatic stabilization is present not only for a normalB normalF 4 − anion but also for other anions such as normalP normalF 6 − , NO 3 − , and normalB normalF 4 − even when the flexible bpa is replaced by rigid 4,4′‐bipyridine (bpy). Based on the DFT results, the metallacycle 2(BF4)4 can be easily prepared in a one‐pot reaction of [Cu(MeCN)4](BF4) with three ligands.