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[C6H3(CH2Y)(2)-3,5](6) (10, Y= NMe2; 11, Y = SPh; 12, Y = PPh2; 13, Y = pz = pyrazol-1-yl) have been prepared in high yields from the key intermediate C-6[C6H3(CH2Br)(2)-3,5](6) (9). The hexakis(pincer) palladium(II) complexes C-6[(PdX)-4-C2H2(CH2Y)(2)-3,5](6) (14, Y = SPh, L = Cl; 15, Y = PPh2, 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 C6H3 [Br-4-C6H3(CH2NMe2)(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 C6H3[(PdBr)-4-C6H3(CH2NMe2)(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) Angstrom, approximately forming a triangle with a height of 15.2 Angstrom. 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 a-cyanoacetate, as a model reaction, has been investigated. It was found that cationic N,C,N ' -type pincer complexes (1a, Y = NMe2; 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 (Id, Y = PPh2; 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 x 10(-6) s(-1) for 1a vs k = 232 x 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.