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

Abstract

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, R(1)COOH, or amines, R(2)NH2 , to give the corresponding N-/C-functionalized norvalines R(1)-D/L-[PdCl(dpb)]Nva-R(2) 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.

Synthesis and Applications of (ONO Pincer)Ruthenium‐Complex‐Bound Norvalines

“…Similar ultrasound‐induced gelation was demonstrated in our earlier reports on Pd‐ and Pt‐bound glutamic acids and their peptides . Importantly, the observed sol/gel transition was completely reversible upon further heating (melting)/sonication cycles, thereby indicating that noncovalent‐bonding interactions, such as hydrogen bonding and π–π stacking, played a dominant role in the formation of the supramolecular gels ,. The formation of typical belt‐like supramolecular aggregates was observed in the SEM image of the xerogel of complex l ‐ 4 (Figure c).…”

Discrete and Polymeric Self‐Assembled Palladium(II) Complexes as Supramolecular Gelators

“…Ta kaya, Nakamura,a nd co-workers reportedt he gelation ability of [PdLCl]-type norvaline-appended NCN-pincer com- [18] in various aromatic solvents. The SEM images obtainedf or 7a and 7b in toluenec onfirmed the presence of fibrous aggregates (Figure 7b, c);i np articular, twisted fibers werer eportedf or the 7b/toluene system.Areversible gel-solc onversion was demonstrated by repeated heatinga nd cooling cycles.S elf-assembly of these gelators was due to hydrogen-bond formation by amide groups and p-p stacking of the NCN pincerunits.…”

“…[10,11] Self-assembled palladium(II) complexes were thus applieda st ectons in crystal engineering to make fascinating 1D, 2D, and 3D networks in the solid state. Mono-, bi-, and trinucleard iscrete complexes with variousc ompositions (PdL 2 ,P dL, Pd 2 L, Pd 2 L 2 ,P d 2 L 4 , Pd 3 L 6 ;L= ligand or metalloligand) are reported in the literature as gelators [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] and described below.T he molecular architectures of the complexes (gelators) are well characterized;h owever,f urthers tructuralc hanges during gelation cannot be ruled out. [10,11] In addition to the hierarchyi nt he solid state, aggregation of palladium(II)-based self-assembled coordination complexes to form metallogels is also known in the literature.…”

Determination of the electronic and structural configuration of coordination compounds by synchrotron-radiation techniques

“…for the determination of the pores size distribution [507][508][509][510][511]. SAXS was also employed to characterize amino-acid-based metal assemblies [512] and other supramolecular coordination compounds [339]. Finally, let us introduce an advanced XRS characterization strategy, i.e.…”