Pt-tetraethynylethene molecular scaffolding: synthesis and characterization of a novel class of organometallic molecular rods.

The series of monodisperse Pt-bridged TEE oligomers 3a-f was prepared by oxidative Glaser-Hay oligomerization of monomer 7 under end-capping conditions. These novel molecular rods extend in length from 3.3 nm (monomeric 3a) to 12.1 nm (hexameric 3 f). Their isolation was achieved by high performance gel permeation chromatography (GPC), and their purification was best monitored by analytical GPC in combination with matrix-assisted laser-desorption-ionization mass spectrometry (MALDI-TOF MS). The mass spectra of each oligomer revealed the molecular ion or its sodium complex as parent ion together with a clean, highly characteristic fragmentation pattern. Delayed addition of the end-capping reagent PhC(triple bond)CH to the oligomerization mixture afforded polymer 10 with an average of approximately 32 repeat units and a remarkably narrow molecular weight distribution (Mw/Mn=1.06), which is indicative of a living polymerization process. UV/Vis spectral data as well as measurements of the second hyperpolarizability gamma by third harmonic generation (THG) revealed a nearly complete lack of pi-electron delocalization along the oligomeric backbone. The Pt atoms act as true insulating centers, and the Pt-C(sp) bonds hardly possess any pi character. The synthesis of the molecular rods 3a-f provides another demonstration of the power of oxidative acetylenic homocouplings for the preparation of unusual nanoarchitecture.

Acetylenic Coupling: A Powerful Tool in Molecular Construction.

Acetylenic coupling is currently experiencing some of the most intensive study of its long history. Rigid and sterically undemanding di- and oligoacetylene moieties, which are frequently encountered in natural products, are finding increasing application as key structural elements in synthetic receptors for molecular recognition. Interesting electronic and optical properties of extensively pi-conjugated systems have spurred research into new linear oligoalkynes and acetylenic carbon allotropes. The synthetic challenges associated with these efforts have in turn spawned new methods. While classical Glaser conditions are still frequently used for homocoupling, the demand for increasingly selective heterocoupling conditions has provided the focus of research over the past decades. These efforts have undoubtedly been hampered by a relatively poor mechanistic understanding of these processes. More recently, palladium-catalyzed coupling methods have led to improvements in both the selectivity and reliability of acetylenic homo- and heterocouplings and paved the way for their application to ever more complicated systems. The variety of acetylenic coupling protocols, the current mechanistic understanding, and their application in natural product and targeted synthesis are discussed comprehensively for the first time in this review, with an emphasis on the most recently developed methods, and their application to the synthesis of complex macromolecular structures.