Coordinating Tectons. Experimental and Computational Infrared Data as Tools To Identify Conformational Isomers and Explore Electronic Structures of 4‑Ethynyl-2,2′-bipyridine Complexes

4-Ethynyl-2,2′-bipyridyl-substituted
ruthenium alkynyl complexes
have been prepared and used to access a range of binuclear homometallic
ruthenium and heterometallic ruthenium–rhenium complexes. These
have been characterized by a variety of spectroscopic and single-crystal
X-ray diffraction experiments. The IR spectra of a number of these
ruthenium alkynyls display multiple ν­(CC) bands in the
IR spectra, which are rationalized in terms of putative conformational
isomers, whose calculated infrared stretching frequencies are comparable
to those obtained experimentally. The mononuclear alkynyl ruthenium
complexes undergo reversible one-electron oxidations centered largely
on the alkynyl ligands, as inferred from the significant shift in
ν­(CC) frequency on oxidation, while the binuclear complex
[Ru­{CC-4-bpy-κ2-N,N′-RuClCp}­(dppe)­Cp*]+ undergoes initial
oxidation at the very electron rich {RuCl­(bpy)­Cp} fragment, causing
only a small change in ν­(CC). A combination of IR and
UV–vis spectroelectrochemical experiments, supported by quantum
chemical calculations on a selected range of conformers, led to the
classification of [Ru­{CC-4-bpy-κ2-N,N′-RuClCp}­(dppe)­Cp*]+ as a weakly coupled class II mixed-valence compound (Hab = 306 cm–1). These results indicate
that there is improved electronic communication through the 4-ethynyl-2,2′-bipyridyl
ligand in comparison to the analogous 5-ethynyl-2,2′-bipyridyl
complexes (Hab = 17 cm–1).