Synthesis and properties of 5,10-calixphyrin and its palladium-inserted complex

Abstract

The reaction of dipyrrolomethane and acetone in the presence of acid and oxidant affords a mixture of 5,15-calixphyrin and 5,10-calixphyrin. The 5,10-calixphyrin readily forms an organometallic Pd(II) complex in 84% yield when treated with Pd(OAc)2 in acetonitrile at reflux. The crystal structure revealed that the 5,10-calixphyrin macrocycle adopted a twisted conformation because of the existence of two sp3-hybridized meso-carbon atoms, which was an impediment in [Formula: see text]-delocalization throughout the macrocycle core. The optimized palladium complex displayed a tetrahedron geometry with reduced distortion throughout the macrocycle. The introduction of palladium metal to the parent calixphyrin scaffold was found to have profound effects on the photophysical and electrochemical properties. Significantly, palladium-inserted 5,10-calixphyrin complex exhibits favorable absorption bands in the visible-near-infrared region with a maximum peak at 672 nm, which is a 148 nm red-shift as compared with that of free base 5,10-calixphyrin. The Pd(II) complex exhibits two reversible reductions and two irreversible oxidations and is more electron-deficient than the corresponding free-base 5,10-calixphyrin. Time-dependent density functional theory studies supported the experimental observations.