Boron calixphyrin complexes: exploring the coordination chemistry of a BODIPY/porphyrin hybrid

Abstract

Boron complexes of calix[4]phyrins (1.1.1.1) were prepared by reacting the free-base ligands with BF₃·Et₂O. The reaction conditions can be efficiently tailored to produce mono- or di-boron calixphyrins. Mono-BF₂ calixphyrins with boron coordinating to either the dipyrrin, BF₂[H(Calix)], or dipyrromethane, BF₂[H(Calix)] and BF₂[H₂(Calix)]⁺, bonding sites were isolated. The dipyrromethane isomer, BF₂[H(Calix)], isomerises into BF₂[H(Calix)] which kinetic studies and DFT calculations indicate is an intramolecular process. Two isomers of B₂OF₂(Calix) were isolated, one isomer bonding via the dipyrrin sites with the FBOBF moiety in cisoid geometry, and the second isomer bonding via the dipyrromethane sites with the FBOBF moiety in transoid geometry. Although the cisoid/dipyrrin isomer was calculated to be most energetically favourable for B₂OF₂(Calix), the isolation of the transoid/dipyrromethane isomer is postulated to occur via the presumed intermediate (BF₂)₂(Calix), for which DFT indicated a preference for transoid/dipyrromethane geometry.