Mixed boron(III) and phosphorous(V) complexes of meso-triaryl 25-oxasmaragdyrins

Abstract

Two unprecedented mixed B^III/P^V complexes of meso-triaryl 25-oxasmaragdyrins were synthesized in appreciable yields under mild reaction conditions. These unusual 25-oxasmaragdyrin complexes containing one or two seven-membered heterocyclic rings comprised of five different atoms (B, C, N, O and P) were prepared by reacting B(OH)(Ph)-smaragdyrin and B(OH)₂-smaragdyrin complexes, respectively, with POCl₃ in toluene at reflux temperature. The products were characterized by HRMS and 1D- and 2D-NMR spectroscopy. X-ray crystallography of one of the mixed B^III/P^V smaragdyrin complexes indicated that the macrocycle is significantly distorted and contains a stable seven-membered heterocyclic ring within the macrocycle. The bands in the absorption and emission spectra were bathochromically shifted with reduced quantum yields and singlet-state lifetimes relative to the free base, meso-triaryl 25-oxasmaragdyrin. The mixed B^III/P^V complexes were difficult to oxidize but easier to reduce than the free base. The DFT-optimized structure of the 25-oxasmaragdyrin complex with two seven-membered heterocycles indicated that it was a bicyclic spiro compound with two half-chair-like conformers. This was in contrast to the chair-like conformation of the complex with a single seven-membered heterocyclic ring. Moreover, incorporation of a second phosphate group in the former case stabilized the bonding geometry and resulted in higher stability, which was reflected in the bathochromic shift of the absorption spectra, more-positive oxidation potential and less-negative reduction potential.