Borylated porphyrin and its metal complexes: Synthesis, electrochemistry and deprotection-protection strategy for anion sensing

Abstract

Free base borylated porphyrin (1) and its metal (Zn(II) (1a) and Cu(II) (1b)) complexes have been synthesized and characterized. The electronic spectral and electrochemical redox properties of 1-1b are found to be similar to that of MTPP (M=2H, Zn(II) and Cu(II)). The deprotection of borylated porphyrins was carried out by two different methods. The first one involves the use of trifluroacetic acid (TFA) and H2SO4 mixture to afford tetraphenylboronic acid porphyrin (3) whereas second one describes the formation of tetrabutylammonium decorated porphyrins (2-2b) via anion induced deprotection using TBAX (X=F−, CN−, H2PO4− and OAc−) followed by TFA/H2SO4 treatment. In presence of strong acids, the deprotection and demetallation proceed simultaneously for metal complexes (1a-1b and 2a-2b) to afford free base tetraphenylboronic acid porphyrin (3). The addition of neopentyl glycol to 2-2b and 3 regenerate the precursor porphyrins (1-1b) in quantitative yield with colorimetric changes. Further, Zn(II) borylated porphyrin (1a) was utilized as a qualitative tool for anion sensing as it shows colorimetric response and distinct spectral (UV–vis and fluorescence) changes toward various anions such as F−, CN−, H2PO4− and OAc− ions and 1a can be regenerated by addition of neopentyl glycol. The sensing ability of 1a is more specific for H2PO4− as ZnTPP doesn’t show any spectral and colorimetric response with H2PO4−.