Design and Performance of Rigid Nanosize Multimetallic Cartwheel Pincer Compounds as Lewis-Acid Catalysts

Abstract

Novel strategies for the preparation of rigid cartwheel pincer metal complexes have been developed. The aromatic backbone of these materials ensures a high rigidity, which is expected to be important for a high retention when these multimetallic nanosize complexes are applied as homogeneous catalysts in a nanomembrane reactor. The ligand precursors C 6 [C 6 H 3 (CH 2 Y) 2 -3,5] 6 (10, Y ) NMe 2 ; 11, Y ) SPh; 12, Y ) PPh 2 ; 13, Y ) pz ) pyrazol-1-yl) have been prepared in high yields from the key intermediate C 6 [C 6 H 3 (CH 2 Br) 2 -3,5] 6 (9). The hexakis(pincer) palladium(II) complexes 3,5-dimethylpyrazol-1-yl; 23) were superior to the P,C,P′-and S,C,S′-pincer complexes (1d, Y ) PPh 2 ; 1e, Y ) SPh). The nanosize cationic tri-N,C,N′ Pd II complex 23 was found to have a catalytic activity per catalytic site in the double Michael reaction of the same order of magnitude as the monopincer analogue 1a (k ) 279 × 10 -6 s -1 for 1a vs k ) 232 × 10 -6 s -1 for 23). The combination of the nanosize dimensions, the catalytic activity, and the high thermal and air stability makes these complexes excellent candidates for application in a continuous process in a nanomembrane reactor.

General Approach for Template‐Directed Synthesis of Macroheterocycles by Ring‐Closing Metathesis (RCM)

“…In earlier work, we have already reported the preparation of various shape-persistent templates with different dimensions and containing up to twelve metal-sites for ligand coordination. [7] Thus combining our shape-persistent macromolecular templates with a variety of bisolefin-substituted ligands (e.g., also with P-and S-coordination to the metal) can lead to a large number of macroheterocycles containing many functionalities, which can subsequently be used in host-guest chemistry. As described above, the stilbene-functionalized macroheterocycle 6e underwent photo-isomerization of the initially formed all-trans macrocycle to give a mixture of the corresponding trans-trans-cis, transcis-cis, and cis-cis-cis macrocycles.…”

“…[6] Recently, we reported the synthesis of macromolecular tri-and hexametallic ECE-pincer complexes (A -C, Figure 1) and investigated their use as recyclable homogeneous catalysts in a nanofiltration membrane reactor. [7] Two interesting features of these molecules are their shape-persistent backbones and their highly symmetric geometries, two very important features for a polynuclear macromolecular template in the controlled synthesis of macrocycles. We anticipated that coordinating bifunctional ligands to the metal centers of these complexes should result in the pre-organization of the functional groups in such a way that they can easily be interconnected to form macrocycles.…”

Platinum(II) and palladium(II) complexes with (N,N′) and (C,N,N′)− ligands derived from pyrazole as anticancer and antimalarial agents: Synthesis, characterization and in vitro activities

“…Few cyclopalladated complexes derived from pyrazole are known [37][38][39][40][41] and, to the best of our knowledge, their cytotoxicity has only been reported once [41]. On the other hand despite the interest for Pt(II) complexes, platinacycles with pyrazolyl ligands are scarce and studies on their biological activity have not been performed so far.…”

NCN–pincer palladium complexes with multiple anchoring points for functional groups

“…The catalytic activity of cationic complexes 1-4 was tested in this aldol reaction, applying initially dichloromethane as the commonly used solvent [39][40][41][42][43][44][45][46][47]. However, the low solubility of 4-hydroxymethyl-and 4-hydroxybenzaldehyde in this solvent led to extremely slow conversions.…”

“…Because of the reversible binding properties of the NCN-pincer platinum complexes with SO 2 and I 2 [1,16], and the concomitant reversible color change from colorless to orange and purple, respectively, these conjugates are interesting candidates as biomarkers and biosensors. We have extensively investigated the immobilization of NCN-pincer complexes onto soluble and molecular supports for catalyst recycling, for which supports as structurally diverse as benzene rings [17][18][19], dendrons [20], dendrimers [21][22][23][24][25][26][27], hyperbranched polymers [28][29][30][31], and dendronized polymers [32] were used to covalently, as well as non-covalently immobilize these complexes. One of the major applications for which the resulting hybrid materials are tested is their use in nano-filtration set-ups for the continuous as well as batch-wise operation and separation of homogeneous catalysts [33].…”