Coordination Chemistry of Calix[3]Pyrrole-Related Macrocycles

Abstract

Calix[3]pyrrole is a ring-contracted porphyrinogen-like macrocycle in which three pyrrole units are linked by three sp3-carbon atoms at the a-positions. While synthetic routes to calix[3]pyrrole and its furan and thiophene derivatives have been established using oligoketone precursors,[1–3] their coordination chemistry remains fully unexplored. Here, the author reports the direct macrocyclization synthesis and metal complexation properties of novel thiazole-containing calix[3]pyrrole analogues that exhibit better metal chelation than that of calix[3]pyrrole.Treatment of calix[3]pyrrole (1) with n-butyl lithium (4 eq.) resulted in the deprotonation of all the NH protons to generate a tridentate trianionic ligand. Successive reaction with BBr3 furnished a boron(III)-complex 2 in 25% isolated yield. However, attempts to synthesize stable complexes with other metal ions were unsuccessful. The obtained boron complex 2 were unusually stable towards acids; even in pure trifluoroacetic acid (TFA) or methanesulfonic acid, no deboronation was observed, instead, protonation at a pyrrole carbon atom was detected by NMR spectroscopy.In order to generate isolable metal complexes with calix[3]pyrrole-related macrocycles, we have designed calix[1]pyrrole[2]thiazole (4). Calix[1]furan[2]thiazole (3), a precursor of 4, was synthesized by direct macrocyclization between bis(a-bromoketone) and 2,2-dimethylpropanebis(thioamide) in 60% yield. While calix[3]pyrrole (1)undergoes a strain-induced ring expansion reaction to give calix[6]pyrrole under acidic conditions, less strained 3 was stable in TFA, thus its furan moiety was hydrolyzed to a diketone moiety. Further Paal–Knorr pyrrole formation reaction led to the formation of 4 in 78% yield.When macrocycle 4 was mixed with Et2Zn, monoethyl zinc complex 5 was formed in 72% yield. Single crystal X-ray analysis revealed that 4 adopted a C s-symmetric cone conformation to behave as a monoanionic tridentate ligand. Due to the steric protection of the ethyl zinc center by meso-methyl groups, organozinc complex 5 was stable in the presence of excess water or alcohol at room temperature.Complexation of 4 with Pd(MeCN)4(BF4)2 afforded cationic palladium complex 6 in which macrocycle 4 adopted a partial cone conformation to chelate a Pd2+ ion as a neutral bidentate ligand. Similar complexation behavior was also observed with AgBF4, demonstrating conformational flexibility of macrocycle 4 upon metal complexation. Furthermore, coordination-driven self-assembly was confirmed for Ag+ complex of 4.In this presentation, the detailed synthesis and electronic properties of the metal complexes of calix[3]pyrrole-related macrocycles will be discussed.