Mono(NCN-pincer palladium)-metalloporphyrincatalysts: evidence for supramolecular bimetallic catalysis

Abstract

The synthesis and catalytic properties of ditopic mono-pincer-mono-porphyrin complexes were investigated. The statistical Adler condensation reaction of 3,5-bis(methoxymethyl)-4-bromo-benzaldehyde, p-tolylaldehyde, and pyrrole, furnished an AB₃-type tetraphenylporphyrin, containing three meso-p-tolyl groups and one meso-3,5-bis(methoxymethyl)-4-bromophenyl group. This material was converted into the ditopic ligand [2H(Br)], which comprises one porphyrin site and an NCN-pincer type ligand moiety. In order to metalate this compound in a stepwise, site-selective manner, two distinct synthetic routes were followed. Route A relies on the introduction of a metal in the porphyrin cavity followed by pincer metalation and a reversal of this order is employed for route B. For the hetero-bimetallic pincer-porphyrin target compounds, route A invariably proved to be the highest yielding alternative, giving pincer-porphyrin hybrids of general formula [M¹(M²X)] (M¹ = 2H, Mg, Co, Ni, Zn; M² = Pd, Br; X = Cl, Br). ¹⁹⁵Pt NMR spectroscopy revealed that the porphyrin metal has a modest influence on the electron density on the NCN-pincer Pt site. When the analogous cationic Pd complexes were used as Lewis acid catalysts for the double Michael addition between methyl vinyl ketone and ethyl α-cyanoacetate, it was noted that the catalytic activity did not depend on the central metal for M¹ = 2H, Ni, and Zn. However, when Mg occupied the porphyrin cavity, the rate of the reaction increased by a factor of six. Although a rate enhancement was observed when catalysis was conducted with a mixture of the two constituents of [Mg(PdOH₂)]BF₄ (i.e. MgTTP and [PdOH₂(NCN)]BF₄) this could not fully account for the rate enhancement. We believe that the rationale for this behaviour is dual, consisting of “cooperative dual catalysis” and supramolecular aggregation of two or more catalyst-substrate complexes.