Mono(NCN-pincer palladium)-metalloporphyrin catalysts: 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 AB(3)-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 [M(1)(M(2)X)] (M(1) = 2H, Mg, Co, Ni, Zn; M(2) = Pd, Br; X = Cl, Br). (195)Pt 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 alpha-cyanoacetate, it was noted that the catalytic activity did not depend on the central metal for M(1) = 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(PdOH(2))]BF(4) (i.e. MgTTP and [PdOH(2)(NCN)]BF(4)) 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.

Peripherally Metalated Porphyrins with Applications in Catalysis, Molecular Electronics and Biomedicine

“…Cationic Pd II complexes 12-M (M = Mg II , Ni II ,Z n II and 2H;S cheme 10) constructed from porphyrins bearing an NCN pincer-type ligand were tested as catalysts in the double Michael addition of ethyl-a-cyanoacetate with methyl vinyl ketone. [27] The observed catalytic activity did not depend of the porphyrin when M = Zn II ,N i II or 2H and was similar to the activity of the corresponding non-porphyrin complex. Interestingly,t he catalytic activity increasedb yafactor of six when M = Mg II .C ontrol experiments suggest that simultaneous activation of methyl vinyl ketone by coordinationo ft he carbonyl group to the Lewis acidic Mg II site and activation of ethyl-a-cyanoacetate bound to the cationic Pd II complex within supramolecular assemblies may explain the enhanced activity.I nterestingly,c ombining Mg II TTP and cationic [Pd(NCN)OH 2 ]BF 4 into one molecule 12-Mg II led to at hree-fold increase of its catalytic activity compared to that found for a1 :1 mixture of itsc onstituents.…”

“…However, the central metal ion can also be used as a secondary catalytic site for dual catalysis. Cationic Pd II complexes 12‐M (M=Mg II , Ni II , Zn II and 2H; Scheme ) constructed from porphyrins bearing an NCN pincer‐type ligand were tested as catalysts in the double Michael addition of ethyl‐α‐cyanoacetate with methyl vinyl ketone . The observed catalytic activity did not depend of the porphyrin when M=Zn II , Ni II or 2H and was similar to the activity of the corresponding non‐porphyrin complex.…”

Syntheses, structures, and characterizations of two new lead (II) supramolecular complexes containing 4-aminobenzenesulfonate ligand

“…The construction of supermolecule through coordination compound has attracted great interest in recent years owing to their applications in catalysis, chiral separations, and flask for chemical reactions [1][2][3]. Many novel supramolecular complexes can be constructed by two kinds of metal complexes: One is metal-organic frameworks [4][5][6][7][8]; another is monomeric complex [9].…”

“…Therefore, the combination of Pb II and 4-aminobenzenesulfonate was used to construct supramolecular complexes. Herein, we report two supramolecular complexes with 4-aminobenzenesulfonate ligand, {[Pb (4-abs) (4,4 -bipy) 2 (H 2 O) 3 ]·(4-abs)} n (1) and [Pb(4-abs) 2 (phen) 2 (H 2 O)] (2), where 4-abs is 4-aminobenzenesulfonate anion, 4,4 -bipy is 4,4 -bipyridine, phen is 1,10-phenanthroline. In these two complexes, the crystal structure shows π-π stacking and hydrogen bonding that give rise to supramolecular architectures.…”

Modulation of the catalytic activity of porphyrins by lipid-and surfactant-containing nanostructures

“…Different derivatives of synthetic porphyrins have been obtained to attain long-living species and improved catalysts. [4][5][6][7][8] Various factors influence the activity of metaloporphyrins. The structural factor modulating porphyrin activity encompasses pyrrole and equatorial ligands, as well as the central metal and the number and structure of their axial ligands.…”