H. Tarik Baytekin

Mass spectrometric studies of non-covalent compounds: why supramolecular chemistry in the gas phase?

Supramolecular chemistry has progressed quite a long way in recent decades. The examination of non-covalent bonds became the focus of research once the paradigm that the observed properties of a molecule are due to the molecule itself was revised, and researchers became aware of the often quite significant influence of the environment. Mass spectrometry and gas-phase chemistry are ideally suited to study the intrinsic properties of a molecule or a complex without interfering effects from the environment, such as solvation and the effects of counterions present in solution. A comparison of data from the gas phase, i.e. the intrinsic properties, with results from condensed phase, i.e. the properties influenced by the surroundings of the molecule, can consequently contribute significantly to the understanding of non-covalent bonds. This review provides insight into the often-underestimated power of mass spectrometry for the investigation of supramolecules. Through example studies, several aspects are discussed, including determination of structure in solution and the gas phase, ion mobility studies to reveal the formation of zwitterionic structures, stereochemical issues, analysis of reactivity of supramolecular compounds in the condensed and in the gas phase, and the determination of thermochemical data.

What really drives chemical reactions on contact charged surfaces

Although it is known that contact-electrified polymers can drive chemical reactions, the origin of this phenomenon remains poorly understood. To date, it has been accepted that this effect is due to excess electrons developed on negatively charged surfaces and to the subsequent transfer of these electrons to the reactants in solution. The present study demonstrates that this view is incorrect and, in reality, the reactions are driven by mechanoradicals created during polymer-polymer contact.

Dendrimer Disassembly in the Gas Phase: A Cascade Fragmentation Reaction of Fréchet Dendrons

The mass spectrometric characterization of Fréchet-type dendrons is reported. In order to provide the charges necessary for electrospray ionization, dendrons bearing an OH group at the focal point can be deprotonated and observed in the negative ion mode. Alternatively, the corresponding bromides can be converted to quaternary ammonium ions that can easily be detected in the positive mode. If the latter ions are subjected to collision-induced dissociation experiments, a fragmentation cascade begins with the dissociation of the focal amine. The focal benzyl cation quickly decomposes in a fragmentation cascade from the focal point to the periphery until the peripheral benzyl (or naphthylmethyl) cations are formed. Five different mechanisms are discussed in detail, three of which can be excluded based on experimental evidence. The cascade fragmentation is reminiscent of self-immolative dendrimers.

Hierarchical Self-Assembly of Metallo-Supramolecular Nanospheres†

Self-assembly is an attractive bottom-up approach to complex architectures and nanomaterials because it reduces the synthetic effort and its reversibility ensures error correction. A variety of nano-objects, for example nanotubes, vesicles, and micelles, have been prepared. Vesicles and micelles form interalia from synthetic amphiphilic block copolymers, amphiphilic perylene bisimides, porphyrins, terpyridine, and b-cyclodextrin complexes, zwitterionic guanidiniocarbonyl pyrrole carboxylate, or inorganic transition metal oxide clusters. Very recently, the first examples of metallo-supramolecular compounds were reported, which assemble into vesicles. The self-assembly process depends on i) the nature of the binding sites and their positions in the building blocks, ii) subunit rigidity, and iii) environmental conditions. Here, we present evidence for the formation of vesicles from novel coordination oligomers synthesized by metal-directed self-assembly. Standard amide-bond formation between Hunter’s diamine 1 and isonicotinoyl chloride 2 yields the bent bidentate dipyridyl-substituted ligand 3 (Scheme 1A), which bears two diverging coordination sites. The assemblies (7a and 7b) were prepared with yields of 95% and 90% by mixing equimolar amounts of ligand 3 and the (dppp)M(OTf)2 precursors(corners)(dppp1⁄4 bis-(diphenylphosphino)-propane, OTf1⁄4 triflate; M1⁄4Pd(II) (6a) or M1⁄4Pt(II) (6b)) in CH2Cl2, stirring for 2 h, and precipitation of the products by slow addition of diethyl ether (Scheme 1B). Molecular modeling predicts the ligand curvature to be not ideal for formation of small 2:2 macrocycles from corners and ligands and one might expect a mixture of coordination oligomers to form to avoid the resulting strain. This is confirmed by electrospray-ionization mass spectrometry (ESIMS). For 7b, a quite complex mass spectrum is obtained