Ionic core-shell dendrimers with an octacationic core have been applied as noncovalent supports for homogeneous catalysts. Catalytically active arylpalladium complexes, which bear a tethered sulfato group, were noncovalently attached to the ionic core-shell dendritic supports via a straightforward ionexchange reaction under mild conditions. Diagnostic shifts in 1 H NMR and Overhauser contacts show that the sulfato groups of the catalysts are located close to the octacationic core of the dendritic support in the resulting assemblies. The location of the catalytic Pd(II) sites has been varied via two strategies: by increasing the dendrimer generation and/or by shortening of the sulfato tether. In addition, a metallodendritic assembly was prepared, which bears an alternative shell of apolar dodecyl groups. Both the dendrimer size and the nature of the dendritic shell have no influence on the binding properties of the dendritic supports, i.e., the octacationic dendrimers of generations 1-3 form discrete 1:8 assemblies with the arylpalladium complexes. The structural aspects and the nature of the metallodendritic assemblies have been studied by means of pulse gradient spin-echo NMR diffusion methods, Overhauser spectroscopy, and electron microscopy (TEM). These techniques showed that the dendritic supports and arylpalladium complexes are strongly associated in solution to give unimolecular assemblies of nanoscopic dimensions. Membrane dialysis can recover these metallodendritic assemblies due to their nanoscopic size. The catalytic performances of the metallodendritic assemblies are comparable, but slightly lower than the performance of the unsupported catalyst.
Arylpalladium (NCN-pincer) complexes [NBu 4 ][O 3 SO(CH 2 ) n SiMe 2 NCNPdCl] (n ) 3 (1a) and n ) 6 (1b)) with a tethered sulfato group as para-substituent were synthesized. These novel complexes combine an anionic sulfato group and a neutral NCN-Pd-Cl group in one molecule. Furthermore, 1a,b were converted into their corresponding cationic Pd-aqua complexes 9a,b, which bear both separated anionic sites and Pd cationic sites stabilized by counterions. These mixed, zwitterionic salts can be isolated devoid of counterions to give the pure unimolecular zwitterions [O 3 SO(CH 2 ) n SiMe 2 NCNPd(OH 2 )] (n ) 3 (2a) and n ) 6 (2b)) that aggregate in order to stabilize their ionic sites. The crystal structure of HO(CH 2 ) 6 -SiMe 2 NCNPdCl (8b) was revealed and used to study the effect of the tether group on the type of aggregate that is formed. The crystal structure of 8b comprises a dimeric structure, which is formed via noncovalent interactions, i.e., O-H‚‚‚Cl-Pd hydrogen bonds.
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