Structural characterization of metal–metal bonded polymer[Ru(L)(CO)2]n (L = 2,2′-bipyridine) in the solid state using high-resolution NMR and DFT chemical shift calculations

Abstract

The metal bonded ruthenium polymer [Ru⁰(bpy)(CO)₂]_n (bpy = 2,2′-bipyridine) is known to be a very promising and efficient solid material for catalysis applications, such as carbon dioxide electroreduction in pure aqueous media and the water–gas shift reaction. It also exhibits potential application for molecular electronics as a conductive molecular wire. The insolubility and relative air-sensitivity of [Ru⁰(bpy)(CO)₂]_n as well as the lack of monocrystals make its structural characterization very challenging. A first approach to determine the structure of this polymer has been obtained by ab initio X-ray powder diffraction, based on the known X-ray structure of [Ru(CO)₄]_n. In order to refine this structure, a non-conventional solid-state NMR study was performed. The results of this study are presented here. The comparison of high-resolution solid-state ¹³C NMR spectra of the polymer with those of the corresponding monomeric [Ru(bpy)(CO)₂Cl₂] or dimeric [Ru(bpy)(CO)₂Cl]₂ precursor complexes has shown a clear shift and splitting of carbonyl ligand resonances, which turns out to be linearly correlated with the redox state of the Ru (II, I or 0, respectively). Bipyridine resonances are also affected but in a non-trivial way. Finally, in the case of the dimer, it was found that the CO peak splitting (2.7 ppm) contains structural information, e.g. the ligand staggering angle. Based on DFT chemical shift calculations on corresponding model molecules (n = 1–2), all the described experimental observations could be reproduced. Moreover, upon extending these calculations to models of increasing length (n = 3–5), it turns out that information about the staggering angle between successive ligands is actually retained in the CO NMR computed peak splitting. Turning back to experiments, the CO broad signal measured for the wire could be decomposed into a major component (at 214.9 ppm) assigned to the internal CO ligands, and a minor doublet component (216.9 and 218.1 ppm) whose splitting (2.8 ppm) contains the staggering angle information. Finally, from the relative integrals of these three components, expected to be in the ratio 1 : 1 : n-2, it was possible to tentatively estimate the length n of the polymetallic wire (n = 7).