Synthesis of PDK, a Novel Porphyrin-Linked Dicarboxylate Ligand

Article abstract of DOI:10.1021/jo000502v

A novel trinucleating ligand system H₄(PDK), for porphyrin-based diamine bis(Kemp's triacid)-imide, was constructed by using two molecules of Kemp's triacid and one molecule of porphyrin as building blocks. The dimeso-substituted octaalkylporphyrin unit, carrying bromomethyl groups at the ortho positions of two trans-positioned meso-phenyl groups, was synthesized from (3,3′-diethyl-4,4′-dimethyl-2,2′-dipyrrolyl)methane and α-bromo-o-tolualdehyde. The structures of both of the two atropisomers (α,α- and α,β-) of bromoporphyrin, H₂(α,α-BP) and H₂(α,β-BP), were determined by X-ray crystallography, with the α,β-isomer as the free base form and the α,α-isomer as the zinc complex. Both the α,α- and α,β-isomers of the bromomethyl porphyrins were converted to their aminomethyl derivatives, H₂(α,α-AP) and H₂(α,β-AP), through the corresponding imidoporphyrin intermediates, H₂(α,α-IP) and H₂(α,β-IP), by the Gabriel synthesis. The α,α- and α,β- aminoporphyrins were coupled with Kemp's triacid anhydride-chloride to form H₄(α,α-PDK) and H₄(α,β-PDK), respectively. Unlike the α,β-isomer, the structure of which was determined by X-ray crystallography for the zinc complex, H₄(α,α-PDK) has a remarkable and complicated solvent-dependent ¹H NMR spectrum that suggests the existence of hydrogen bonding interactions between two convergent, tethered Kemp's triacid units, as predicted in a modeling study. The convergent feature is essential for the target ligand H₄(α,α-PDK) to form trinuclear metal complexes with a bis(carboxylato) dimetallic unit sitting above a metalloporphyrin ring.