Diruthenium σ-Alkynyl Compounds: A New Class of Conjugated Organometallics

Reviewed herein are key aspects of the recent progress in the chemistry of σ-alkynyl compounds of Ru 2 (LL) 4, where LL is an N,N‘-bidentate bridging ligand. Synthetic methodologies for both the mono-alkynyl Ru 2 (II,III) and bis-alkynyl Ru 2 (III,III) compounds have been thoroughly investigated. The basic electronic properties of these Ru 2 (II,III) and Ru 2 (III,III) compounds were elucidated on the basis of solution voltammetry and absorption spectroscopy, which revealed rich redox characteristics and small energy gaps ranging from 1.2 to 1.6 eV. Facile electron transfer across the polyyn-diyl chains was demonstrated on the basis of Ru 2 (ap) 4 type compounds (ap is 2-anilinopyridinate), and a very small electronic decay constant (β) of 0.063 Å - 1 was determined. Also demonstrated was the long-range electronic coupling between Fc−Fc + termini bridged by a −(C⋮C) m −Ru 2 (DMBA) 4 −(C⋮C) n − fragment (DMBA is N,N ‘-dimethylbenzamidinate), and high hole mobility across the Ru 2 (III,III) unit was inferred. Illustrating the potential of metal−alkynyl compounds in molecular electronics, scanning tunneling microscopy (STM) studies of Ru 2 (ap) 4 (σ-OPE) embedded in an alkanethiol matrix revealed significantly improved molecular conductance compared to the OPE of comparable length (OPE is oligo(phenyleneethynylene)). Using the weak-base protocol, several donor−Ru 2 (DMBA) 4 −acceptor type compounds have been obtained, and their voltammetric characteristics are consistent with what are expected for a molecular diode. Ru 2 (ap) 4 (C 2 n R) type compounds undergo [2 + 2] cycloaddition/insertion reactions with TCNE, which drastically alter the electronic structures of metal−alkynyl compounds. Also described are the covalent modification chemistry both at the σ-alkynyl ligands and the periphery of N,N‘-bridging ligands and supramolecular chemistry based on the latter modification.