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Pyrenoxy‐Based NCN‐Pincer Palladium( II ) Molecular Tweezers: Synthesis, Properties in Solution and Catalysis
Author(s) -
Slagt Martijn Q.,
Jastrzebski Johann T. B. H.,
Gebbink Robertus J. M. Klein,
Ramesdonk Hendrik J. van,
Verhoeven Jan W.,
Ellis Dianne D.,
Spek Anthony L.,
Koten Gerard van
Publication year - 2003
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/ejoc.200210601
Subject(s) - chemistry , palladium , moiety , intramolecular force , trimethylsilyl , aldol reaction , heteronuclear molecule , stereochemistry , tetrafluoroborate , medicinal chemistry , catalysis , nuclear magnetic resonance spectroscopy , organic chemistry , ionic liquid
Abstract 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‐(Me 2 NCH 2 ) 2 C 6 H 2 ‐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 1 H NMR titrations, revealing association constants K a in the order of 10 1 to 10 2 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)