Self-Assembly of a Pd₈ Macrocycle and Pd₁₂ Homochiral Tetrahedral Cages Using Poly(tetrazolate) Linkers

A two-dimensional molecular square (MC) was obtained by the self-assembly of a bis(tetrazole) linker, 4,4'-bis(1H-tetrazol-5-yl)-1,1'-biphenyl (H₂L), with a square-planar metal acceptor M [M = (tmeda)Pd(NO₃)₂, where tmeda = N,N,N',N'-tetramethylethane-1,2-diamine] in dimethyl sulfoxide (DMSO) followed by crystallization. The uncommon 2,3-binding mode through N atoms of the tetrazole rings in this assembly leads to the formation of an octanuclear molecular square. The molecular square MC [Pd₈(L)₄(NO₃)₈] is unstable in DMSO and slowly converts to a dynamic mixture of a 3D tetrahedral cage T1 [Pd₁₂(L)₆(NO₃)₁₂] and the macrocycle MC. A tetrahedral cage (T1) is formed by the usual 1,3-binding mode of the tetrazole rings. However, self-assembly of the T1 [Pd₁₂(L)₆(PF₆)₁₂] was possible to access in the pure form in a less polar solvent like acetonitrile. The pure T1 [Pd₁₂(L)₆(PF₆)₁₂] also converts to a mixture of T1 and MC in DMSO. Interestingly, when a tris(tetrazole) linker, tris(4-(1H-tetrazol-5-yl)phenyl)amine (H₃L), was treated with the acceptor M, it produced a tetrahedral nanocage T2 [Pd₁₂(L)₄(NO₃)₁₂] through 1,3-binding mode of the tetrazole rings without any trace of an octahedral cage through 2,3-binding mode of the tetrazole moieties.