Interfacial and solvent effects govern the formation of tris(dibenzylidenacetone)dipalladium(0) microstructures

Light-responsive three-dimensional microstructures composed of azobenzene-based palladium complexes

We describe not only fleeting assembly of photoisomerizable azobenzene-based palladium complexes into microstructured crystalline architectures but also their light-responsive functions. A transformation in the crystalline morphology from two-dimensional (2D) parallelogram-like sheets to three-dimensional (3D) cuboid- or rhombus-like structures was achieved by changing the solvent from tetrahydrofuran (THF) to acetone and N,N-dimethylformamide (DMF). The sizes of the structures, ranging from a few hundred nanometers to several hundred micrometers, were also modified by varying the complex concentration. In stark contrast to the very stable 2D sheets in the THF-H2O suspensions, exposure of 3D structures in polar DMF-H2O suspensions to ultraviolet (UV) light led to fast disassembly of the structures into isolated metal complexes and further dissociation of free azobenzene ligands from the complexes. In acetone-H2O suspensions, interestingly, disassembly of 3D cuboid-like structures into isolated complex components occurred upon exposure to UV light without further dissociation of azobenzene ligands from the palladium complexes. Considering the photoisomerization ability of the azobenzene-based palladium complex in common organic solvents, the π-stacking interactions that support 3D structures are likely to be sufficiently weak that they might be broken by the UV-induced trans-to-cis isomerization in more polar solvent mixtures. As a consequence, disassembly proceeded under UV light irradiation. Moreover, the effect of solvent polarity on the UV-assisted dissociation (in DMF-H2O) may be associated with the coordination ability of solvent molecules with the metal center.

The elusive structure of Pd2(dba)3. Examination by isotopic labeling, NMR spectroscopy, and X-ray diffraction analysis: synthesis and characterization of Pd2(dba-Z)3 complexes

Pd(0)2(dba)3 (dba = E,E-dibenzylidene acetone) is the most widely used Pd(0) source in Pd-mediated transformations. Pd(0)2(dba-Z)3 (Z = dba aryl substituents) complexes exhibit remarkable and differential catalytic performance in an eclectic array of cross-coupling reactions. The precise structure of these types of complexes has been confounding, since early studies in 1970s to the present day. In this study the solution and solid-state structures of Pd(0)2(dba)3 and Pd(0)2(dba-Z)3 have been determined. Isotopic labeling ((2)H and (13)C) has allowed the solution structures of the freely exchanging major and minor isomers of Pd(0)2(dba)3 to be determined at high field (700 MHz). DFT calculations support the experimentally determined major and minor isomeric structures, which show that the major isomer of Pd(0)2(dba)3 possesses bridging dba ligands found exclusively in a s-cis,s-trans conformation. For the minor isomer one of the dba ligands is found exclusively in a s-trans,s-trans conformation. Single crystal X-ray diffraction analysis of Pd(0)2(dba)3·CHCl3 (high-quality data) shows that all three dba ligands are found over two positions. NMR spectroscopic analysis of Pd(0)2(dba-Z)3 reveals that the aryl substituent has a profound effect on the rate of Pd-olefin exchange and the global stability of the complexes in solution. Complexes containing the aryl substituents, 4-CF3, 4-F, 4-t-Bu, 4-hexoxy, 4-OMe, exhibit well-resolved (1)H NMR spectra at 298 K, whereas those containing 3,5-OMe and 3,4,5-OMe exhibit broad spectra. The solid-state structures of three Pd(0)2(dba-Z)3 complexes (4-F, 4-OMe, 3,5-OMe) have been determined by single crystal X-ray diffraction methods, which have been compared with Goodson's X-ray structure of Pd(0)2(dba-4-OH)3.