Pyrenoxy‐Based NCN‐Pincer Palladium( II ) Molecular Tweezers: Synthesis, Properties in Solution and Catalysis

A highly flexible molecular tweezer, (aqua)(2,6‐bis[(dimethylamino)methyl]‐4‐{3‐[(1‐pyrenyloxy)methyl]‐5‐(trimethylsilyl)benzyloxy}phenyl)palladium(II) tetrafluoroborate (1a), constructed from a para‐hydroxy‐functionalized NCN‐pincer palladium(II) complex {NCN−OH = [2,6‐(Me2NCH2)2C6H2‐4‐OH]}, a pyrenoxy unit and a xylyl spacer, was synthesized. X‐ray analysis of the structure of 1‐[3‐({3,5‐bis[(dimethylamino)methyl]‐4‐(trimethylsilyl)phenyloxy}methyl)‐5bromobenzyloxy]pyrene (10), a precursor ligand of 1a, showed a completely flattened conformation in the solid state, imposed by favorable intramolecular π‐stacking interactions. The molecular structure of 1a in solution proved to be fluxional on the NMR timescale. UV/Vis and (time‐resolved) fluorescence spectroscopy revealed that, in spite of the high flexibility, the catalyst and pyrenoxy units are in close proximity because of cation−π interactions. This was further supported by conformer searches using molecular mechanics (MM2). The binding affinity of picric acid towards the pyrenoxy moiety was studied by 1H NMR titrations, revealing association constants Ka in the order of 101 to 102 M−1. Tweezer 1a shows small rate enhancements in aldol condensations of aromatic aldehydes with methyl isocyanoacetate compared to (aqua)(2,6‐bis[(dimethylamino)methyl]4‐{[3‐(trimethylsilyl)benzyloxy}phenyl)palladium(II) tetrafluoroborate (2), which lacks the pyrenoxy unit. The tweezer was immobilized on a G2‐carbosilane dendrimer, generating the dendritic (aqua)(G2‐carbosilane)palladium complex 1b, showing a decreased catalytic activity compared to 1a. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)