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

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 6H 3(CH 2Y) 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 6H 3 (CH 2Br) 2-3,5] 6 (9). The hexakis(pincer) palladium(II) complexes C 6[(PdX)-4-C 6H 2(CH 2Y) 2-3, 5] 6 (14, Y = SPh, L = Cl; 15, Y = PPh 2, L = Cl; 16, Y = pyrazol-1-yl, L = OAc; 17, Y = pyrazol-1-yl, L = Cl) have been prepared via direct electrophilic palladation of the corresponding ligands. The (tris)pincer ligand C 6H 3[Br-4-C 6H 3(CH 2 NMe 2) 2-3,5] 3-1,3,5 (20) was prepared via a triple-condensation reaction of 4-bromo-3,5-bis[(dimethylamino)methyl]acetophenone (19). Reaction of 20 with Pd(dba) 2 yielded the tripalladium complex C 6H 3[(PdBr)-4-C 6H 3(CH 2 NMe 2) 2-3,5] 3-1,3,5 (21). The crystal structure of 21 shows a propeller-like structure with D 3 symmetry and a fixed bromine-bromine distance of 17.4573(4) Å, approximately forming a triangle with a height of 15.2 Å. These nanosize cartwheel pincer metal complexes based on tridentate Y,C,Y′ pincer ligands have been used as homogeneous Lewis-acid catalysts. Moreover, the influence of the donor substituent Y on the catalytic activity of cationic mono-Y,C,Y′ Pd II complexes as Lewis-acid catalysts in the double Michael reaction between methyl vinyl ketone and ethyl α-cyanoacetate, as a model reaction, has been investigated. It was found that cationic N,C,N′-type pincer complexes (1a, Y = NMe 2; 1b, Y = pz; 1c, Y = pz* = 3,5-dimethylpyrazol-1-yl; 23) were superior to the P,C,P′- and S,C,S′-pincer complexes (1d, Y = PPh 2-Le, 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.