Synthesis, electronic properties, structural studies, and catalytic activity of peripherally metalated β-η1-Pd(II)-thioalkyl porphyrazine

Abstract

The β-η¹-Pd(II)-thioethyl porphyrazine complex 1 was isolated as an intermediate in the Suzuki-Miyaura cross-coupling of β-bromo thioethyl porphyrazine with palladium complexes. The same complex was also synthesized via a direct reaction of the bromo porphyrazine with Pd(PPh₃)₄ and its structure was assessed by NMR and single-crystal X-ray diffraction analysis, representing one of the very few cases of crystallographic resolution of a thioalkyl porphyrazine obtained to date. This organometallic thioethyl porphyrazine derivative constitutes the first example of an η¹ organometallic complex of tetrapyrroles in which the metal is coordinated at the β position of the macrocycle rather than at the meso position, as observed in some porphyrin complexes. Spectroscopic and electrochemical studies, including UV-vis absorption and cyclic voltammetry, supported by time-dependent density functional theory calculations revealed that compared to symmetrically substituted thioalkyl porphyrazines, this complex exhibits a reduced HOMO-LUMO bandgap and a charge transfer transition involving the metal and the macrocycle at the typical tetrapyrrole Soret band. This behavior markedly contrasts with that observed in peripherally aryl-substituted thioalkyl porphyrazines, where a charge transfer HOMO-LUMO transition occurs between the aryl substituent and the macrocycle core in correspondence to the Q bands region. Furthermore, the complex demonstrated good catalytic activity in both Suzuki-Miyaura and Sonogashira Pd(II)-catalyzed coupling reactions of aryl halides with boronic acids or alkynes, respectively. In both reactions, the porphyrazine complex performed with comparable efficiency to the commonly used Pd(PPh₃)₂Cl₂ catalyst.