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

Harm P. Dijkstra,† Michel D. Meijer,† Jim Patel,‡ Rob Kreiter,† Gerard P. M. van Klink,† Martin Lutz,§ Anthony L. Spek,§‖ Allan J. Canty,‡ and Gerard van Koten*†
Debye Institute, Department of Metal-Mediated Synthesis, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands, School of Chemistry, University of Tasmania, Hobart, Tasmania 7001, Australia, and Bijvoet Center for Biomolecular Research, Department of Crystal and Structural Chemistry, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands
Organometallics, 2001, 20 (14), pp 3159–3168
DOI: 10.1021/om0101689
Publication Date (Web): June 8, 2001
Copyright © 2001 American Chemical Society
Organometallic and Organometalloidal Compounds

Abstract

Abstract Image

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 C6[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 C6[C6H3(CH2Br)2-3,5]6 (9). The hexakis(pincer) palladium(II) complexes C6[(PdX)-4-C6H2(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 D3 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‘ PdII 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 = 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 (1d, Y = PPh2; 1e, Y = SPh). The nanosize cationic tri-N,C,N‘ PdII 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.

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  • Published In Issue July 09, 2001
  • Received March 1, 2001

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