Synthesis and Electrochemical Studies of Porphyrin Dimers Linked by Metallocarbenes

Abstract

The functionalization of porphyrins at the meso positions by azoles led, after subsequent alkylation, to several precursors of N-heterocyclic carbenes. Porphyrin dimers linked by palladium (or rhodium) bis-carbene spacers were prepared and characterized. Spectroscopic data and X-ray structures showed that the coordination geometry for the two carbenes around the palladium linker was trans-anti. Electrochemical studies revealed significant electronic communication between the two porphyrins, despite the absence of conjugation pathways.

Synthesis, Characterization, and Electronic Properties of Porphyrins Conjugated with N-Heterocyclic Carbene (NHC)–Gold(I) Complexes

“…The formation of the peripheral [AuCl] complex was confirmed by 1 H NMR spectroscopy because the signal arising from the imidazolium proton at  = 11.07 ppm disappeared due to the formation of the C(2)-Au bond. Moreover, the signal of the carbene C(2) could be clearly observed at  = 182.3 ppm in its 13 C{ 1 H} NMR spectrum and is in good agreement with data reported in the literature for [(NHC)AuCl] complexes. 3 However, the same synthetic approach proved to be unsuccessful to obtain the desired complex [FbAuCl].…”

“…20 the two inner NH of the free base porphyrin was observed in its 1 H NMR spectrum. The signal of the carbene was also observed at  = 182.0 ppm in its 13 C{ 1 H} NMR spectrum. By using the same synthetic strategy, the complex [NiAuCl] was obtained in 34% yield.…”

“…12 Recently, Ruppert and coworkers avoided the use of spacers by introducing azoles directly onto meso positions. 13 The alkylation of these compounds afforded porphyrins with meso azolium units, which can further be deprotonated to obtain the corresponding NHC ligands and their metal complexes (Chart 1, C). In our group, we have synthesized porphyrins fused to imidazole and imidazolium units across two neighboring ,-pyrrolic positions.…”

Molecular Systems Combining Porphyrinoids and N‐Heterocyclic Carbenes

“…As it is not the case here ( meso NHC and porphyrin core mean plane are orthogonal), this effect may be attributed to intramolecular coulombic effects. Using the same kind of ligands, the bis(NHC) rhodium(I) complex was obtained by treating a meso ‐imidazolium porphyrin with [Rh(acac)(CO) 2 ] [32] . The obtained square planar bis(NHC) rhodium(I) complex with one iodide and one CO ligand ( ν CO =1943 cm −1 ) is chiral and, as a consequence, protons of the CH 2 groups are diastereotopic.…”

“…In addition to (benz)imidazol‐2‐ylidenes, Ruppert, Weiss and co‐workers also reported the synthesis of metal complexes with other NHC ligands on meso positions of porphyrins (Scheme 7). [32] For example, following a similar synthetic procedure using the Ullmann coupling reaction, the authors prepared a nickel(II) meso ‐1,2,4‐triazolyl‐substituted porphyrin, which was subsequently alkylated to obtain the 1,2,4‐triazolium‐substituted porphyrin salt. The corresponding bis(NHC) palladium(II) complex 17 was obtained by reacting the triazolium salt with Pd(OAc) 2 and KO t Bu.…”