Synthesis and Self-Assembly of NCN-Pincer Pd-Complex-Bound Norvalines

• Published in: Chemistry : a European journal Vol. 19; no. 37; pp. 12356 - 12375
• Main Authors: Ogata, Kazuki, Sasano, Daisuke, Yokoi, Tomoya, Isozaki, Katsuhiro, Yoshida, Ryota, Takenaka, Toshio, Seike, Hirofumi, Ogawa, Tetsuya, Kurata, Hiroki, Yasuda, Nobuhiro, Takaya, Hikaru, Nakamura, Masaharu
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 09.09.2013; WILEY‐VCH Verlag; Wiley; Wiley Subscription Services, Inc
• Subjects: Amino Acids - chemistry; Arrays; Carboxylic Acids - chemistry; Chemistry; Chemistry, Multidisciplinary; Coordination Complexes - chemical synthesis; Coordination Complexes - chemistry; Crystallography; Crystallography, X-Ray; Gels - chemistry; Hydrogen Bonding; metalated amino acid; metalated peptide; Models, Molecular; Molecular Structure; Palladium; Peptides - chemistry; Physical Sciences; Scattering, Small Angle; Science & Technology; Self assembly; Single crystals; supramolecular gel; Thermodynamics; Transformations; Valine - analogs & derivatives; Valine - chemistry; X-Ray Diffraction; X-rays; AMIDE-ONE VIBRATION; BIOLOGICALLY IMPORTANT LIGANDS; SOLID-PHASE SYNTHESIS; CYCLIC PEPTIDE MODEL; ALPHA-AMINO-ACIDS; ORGANIC LIQUIDS; metalated amino acid; metalated peptide; self-assembly; INFRARED-SPECTRA; palladium; SOLVENT-INDUCED AGGREGATION; ARENE-RUTHENIUM CHEMISTRY; METAL-COMPLEXES; supramolecular gel
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201301513

The NCN‐pincer Pd‐complex‐bound norvalines Boc‐D/L‐[PdCl(dpb)]Nva‐OMe (1) were synthesized in multigram quantities. The molecular structure and absolute configuration of 1 were unequivocally determined by single‐crystal X‐ray structure analysis. The robustness of 1 under acidic/basic conditions provides a wide range of N‐/C‐terminus convertibility based on the related synthetic transformations. Installation of a variety of functional groups into the N‐/C‐terminus of 1 was readily carried out through N‐Boc‐ or C‐methyl ester deprotection and subsequent condensations with carboxylic acids, R1COOH, or amines, R2NH2, to give the corresponding N‐/C‐functionalized norvalines R1‐D/L‐[PdCl(dpb)]Nva‐R2 2–9. The dipeptide bearing two Pd units 10 was successfully synthesized through the condensation of C‐free 1 with N‐free 1. The robustness of these Pd‐bound norvalines was adequately demonstrated by the preservation of the optical purity and Pd unit during the synthetic transformations. The lipophilic Pd‐bound norvalines L‐2, Boc‐L‐[PdCl(dpb)]Nva‐NH‐n‐C11H23, and L‐4, n‐C4H9CO‐L‐[PdCl(dpb)]Nva‐NH‐n‐C11H23, self‐assembled in aromatic solvents to afford supramolecular gels. The assembled structures in a thermodynamically stable single crystal of L‐2 and kinetically stable supramolecular aggregates of L‐2 were precisely elucidated by cryo‐TEM, WAX, SAXS, UV/Vis, IR analyses, and single‐crystal X‐ray crystallography. An antiparallel β‐sheet‐type aggregate consisting of an infinite one‐dimensional hydrogen‐bonding network of amide groups and π‐stacking of PdCl(dpb) moieties was observed in the supramolecular gel fiber of L‐2, even though discrete dimers are assembled through hydrogen bonding in the thermodynamically stable single crystal of L‐2. The disparate DSC profiles of the single crystal and xerogel of L‐2 indicate different thermodynamics of the molecular assembly process. Metalated amino acids: A series of NCN‐pincer Pd‐complex‐bound norvaline derivatives was successfully synthesized without loss of the optical purity and Pd unit. Efficient self‐assembly properties of these Pd–norvalines were found to afford well‐regulated Pd arrays both in the single crystal and in the supramolecular gel. A solvent‐dependent configuration control of the Pd array was corroborated by means of single‐crystal X‐ray crystallography and cryo‐TEM analysis.