Sander J. Wezenberg

Material Applications for Salen Frameworks

Salens are among the most widely studied ligands in chemistry. They are primarily applied in homogeneous catalysis, a use that helps to increase our knowledge of environmentally benign and cost-attractive chemical and/or pharmaceutical processes. Lately, the interest in salen chemistry has shifted to the use of these scaffolds in various other applications in which the immense versatility of the salen framework as a molecular building block can be exploited. Herein, we highlight the most recent research involving the incorporation of salen motifs in new materials and sophisticated catalysts.

Effective chirogenesis in a bis(metallosalphen) complex through host-guest binding with carboxylic acids.

Transfer of chiral information through supramolecular interactions (chirogenesis) has been observed in many natural systems including DNA and proteins, and is nowadays widely used in the development of smart artificial and biomimetic materials. The induction of chirality in bis(metalloporphyrins) for example, has been successfully applied in assigning the absolute configuration of amines, diamines and aminoamides, aminoalcohols and epoxyalcohols, and diols by using a circular dichroism (CD) protocol. Effective chirality transfer with carboxylic acids, however, has proven to be highly difficult, and has only been achieved by using potassium carboxylate salts followed by tedious extractions, or by the addition of a huge excess of substrate to a metal-free host. The low efficiency of chiral induction with these previous methods is mainly due to the relatively weak host–guest interactions with carboxylic acid groups. To overcome this problem, we have designed a bis[Zn(salphen)] complex 1 (salphen = N,N’-phenylenebis(salicylimine)), which, similar to 2,2’-biphenol units, exists in dynamic equilibrium between two chiral conformations (S and R enantiomers; see Scheme 1). We reasoned that the energy barrier of rotation increases upon binding of a ditopic ligand to the Lewis acidic Zn centers. 12] Herein, we demonstrate that 1 binds very strongly with acetic acid and that axial chirality can be effectively induced by exchange for chiral a-substituted carboxylic acids, with the practical advantage that substrate derivatization or use of excessive substrate is not required. Compound 1 was prepared in a single step by reaction of a bis(salicylaldehyde) molecule with two equivalents of a monoimine precursor and Zn(OAc)2 in the presence of pyridine. Subsequent precipitation from MeOH afforded the product in excellent yield (73 %) and purity (see the Supporting Information). Characterization by NMR spectroscopy indicated the presence of one equivalent of acetic acid, which had formed as a by-product in the synthesis. Slow evaporation of a solution of the product in toluene/ MeCN 1:1 resulted in single crystals suitable for X-ray analysis (Figure 1). The solid-state structure revealed that the two Zn centers of 1 are bridged by AcOH (through the oxygen atoms of the carboxylic acid unit) to give a complex with 1:1 stoichiometry (1 AcOH). As anticipated, both the S and the R conformer were present in a 1:1 ratio in the unit cell; each conformer has the same dihedral angle (44.98) and Zn–O(acetate) distance (2.01 ). This distance is identical to that previously found in a related acetate-bridged complex. Since the exact position of the acidic proton of AcOH could not be resolved by X-ray diffraction, the proton was placed in Scheme 1. Conformational isomerism of 1. The tBu groups are omitted for clarity in the line drawings of the conformers. P denotes righthandedness and M left-handedness.

Extremely Strong Self-Assembly of a Bimetallic Salen Complex Visualized at the Single-Molecule Level

A bis-Zn(salphen) structure shows extremely strong self-assembly both in solution as well as at the solid-liquid interface as evidenced by scanning tunneling microscopy, competitive UV-vis and fluorescence titrations, dynamic light scattering, and transmission electron microscopy. Density functional theory analysis on the Zn(2) complex rationalizes the very high stability of the self-assembled structures provoked by unusual oligomeric (Zn-O)(n) coordination motifs within the assembly. This coordination mode is strikingly different when compared with mononuclear Zn(salphen) analogues that form dimeric structures having a typical Zn(2)O(2) central unit. The high stability of the multinuclear structure therefore holds great promise for the development of stable self-assembled monolayers with potential for new opto-electronic materials.

Colorimetric Discrimination between Important Alkaloid Nuclei Mediated by a Bis-Salphen Chromophore

A new colorimetric method is presented that allows for simple and effective discrimination between (iso)quinoline nuclei pertinent to many alkaloid species. The method is based on the reversible incorporation of Zn2+ ions into a bis-salphen chromophore under aqueous conditions. The metalation process is a function of the stability of the Zn-N (acceptor-donor) interaction and is accompanied by a clear colorimetric response.

Versatile approach toward the self-assembly of heteromultimetallic salen structures.

A general route is presented toward the template-directed preparation of self-assembled heteromultimetallic salen structures using noncovalent coordinative metal-ligand interactions. Various higher order assemblies have been studied in detail using a combination of NMR spectroscopy and X-ray crystallography.

Axial ligand control over monolayer and bilayer formation of metal-salophens at the liquid–solid interface

Nickel salophens exclusively form monolayers at a liquid-solid interface, while in contrast zinc salophens mainly self-assemble into bilayers via axial ligand self-coordination which can be disrupted by the addition of pyridine axial ligands.

Multi-State Regulation of the Dihydrogen Phosphate Binding Affinity to a Light- and Heat-Responsive Bis-Urea Receptor

A responsive bis-urea receptor can be switched between three isomers using light and heat as evidenced by (1)H NMR and UV-vis spectroscopy. Anion binding experiments ((1)H NMR titrations, ESI-MS) reveal a high selectivity for dihydrogen phosphate. Importantly, a large difference in binding affinity to the interchangeable isomers is observed, which is further rationalized by DFT calculations. As a consequence, the amount of bound substrate can be controlled via photo- and thermal isomerization in a three-step process.

Photocontrol of Anion Binding Affinity to a Bis-urea Receptor Derived from Stiff-Stilbene

Toward the development of photoresponsive anion receptors, a stiff-stilbene photoswitch has been equipped with two urea anion-binding motifs. Photoinduced E/Z isomerization has been studied in detail by UV–vis and NMR spectroscopy. Titration experiments (1H NMR) reveal strong binding of acetate and phosphate to the (Z)-isomer, in which the urea groups are closely together. Isomerization to the (E)-form separates the urea motifs, resulting in much weaker binding. Additionally, geometry optimizations by density functional theory (DFT) illustrate that oxo-anion binding to the (Z)-form involves four hydrogen bonds.

Visible-Light Excitation of a Molecular Motor with an Extended Aromatic Core

Exploring routes to visible-light-driven rotary motors, the possibility of red-shifting the excitation wavelength of molecular motors by extension of the aromatic core is studied. Introducing a dibenzofluorenyl moiety in a standard molecular motor resulted in red-shifting of the absorption spectrum. UV/vis and 1H NMR spectroscopy showed that these motors could be isomerized with light of wavelengths up to 490 nm and that the structural modification did not impair the anticipated rotary behavior. Extension of the aromatic core is therefore a suitable strategy to apply in pursuit of visible-light-driven molecular motors.

Asymmetric Synthesis of Second-Generation Light-Driven Molecular Motors

The enantiomeric homogeneity of light-driven molecular motors based on overcrowded alkenes is crucial in their application as either unidirectional rotors or as chiral multistate switches. It was challenging to obtain these compounds as single enantiomers via the established synthetic procedures due to loss of optical purity in the key step, i.e., the Barton–Kellogg olefination reaction. Searching for strategies to avoid racemization, a new class of light-driven molecular motors was designed, synthesized, and studied. The stereochemical integrity was fully preserved throughout the synthesis, and on the basis of photochemical and kinetic studies using UV/vis, CD, and 1H NMR spectroscopy, it was established that they still function properly as unidirectional molecular motors.