β-Functionalized palladium(II) porphyrins: Facile synthesis, structural, spectral, and electrochemical redox properties

Abstract

Two [Formula: see text]-substituted Pd(II) porphyrins, (PdTPP(NO[Formula: see text]Ph2 and PdTPP(Ph)[Formula: see text], were synthesized and characterized by various spectroscopic techniques, and their spectral and electrochemical redox properties were investigated with respect to PdTPP. The single-crystal X-ray structure of PdTPP(NO[Formula: see text]-Ph2 revealed saddle-shape conformation of the macrocyclic core of porphyrin. DFT studies revealed nonplanar saddle shape conformation of PdTPP(Ph)4 with an average deviation of [Formula: see text]-pyrrole carbon atoms from the porphyrin mean plane ([Formula: see text]C[Formula: see text] = ± 0.751 Å) and also 24 core atoms ([Formula: see text]24 = ± 0.368 Å) as compared to PdTPP(NO[Formula: see text]Ph2 ([Formula: see text]C[Formula: see text] = 0.649 Å and [Formula: see text]24 = ± 0.317 Å) due to more [Formula: see text]-pyrrole substitution. Both PdTPP(NO[Formula: see text]Ph2 and PdTPP(Ph)4 exhibited 13-19 nm and 49-59 nm red-shift in the Soret and Q[Formula: see text] bands as compared to PdTPP, respectively. PdTPP(NO[Formula: see text]Ph2 exhibited a very high dipole moment (7.32 D) as compared to PdTPP(Ph)4 (0.239 D) due to the “push-pull” effect of electron donor (phenyl) and acceptor (-NO[Formula: see text] groups at the [Formula: see text]-pyrrole positions of the macrocycle. The redox potentials of PdTPP(NO[Formula: see text]Ph2 anodically shifted ([Formula: see text]E = 0.2-0.4V) as compared to PdTPP and PdTPP(Ph)4 due to the presence of [Formula: see text]-phenyl groups and the strong electron-withdrawing effect of the nitro group. Hence the HOMO-LUMO energy gap of PdTPP(NO[Formula: see text]Ph2 decreased by 0.20 V and 0.28 V as compared to PdTPP(Ph)4 and PdTPP, respectively.