Synthesis, structure, spectral and electrochemical properties of B(OR) ₂-smaragdyrin complexes

A series of B(OR)₂-smaragdyrin complexes were synthesized by treating BF₂-smaragdyrin complex with various aliphatic and aromatic alcohols in the presence of AlCl₃ in CH₂Cl₂ at room temperature. The compounds are stable and characterized by various spectroscopic and electrochemical techniques. The X-ray structure solved for one of the B(OR)₂-smaragdyrin complex showed that the macrocycle is planar and the boron atom is in tetrahedral geometry coordinated to two pyrrole nitrogens of the macrocycle and two oxygen atoms of the axial -OR groups. ¹H NMR studies indicated that the alkoxy or aryloxy groups present on the boron atom experience the macrocyclic ring current effect and appear in the up-field region. The absorption properties of B(OR)₂-smaragdyrin complexes showed a strong absorption band at ∼700 nm along with other absorption bands in the 450-650 nm region like BF₂-smaragdyrin complex. Since B(OR)₂-smaragdyrin complexes are electron rich due to the presence of electron rich alkoxy and aryloxy groups, the B(OR) ₂-smaragdyrin complexes are easier to oxidize but difficult to reduce compared to BF₂-smaragdyrin. The B(OR)₂-smaragdyrin complexes are found to be weakly fluorescent than BF₂-smaragdyrin.