Direct observation of reversible bond homolysis by 2D EXSY NMR

Abstract

Bond homolysis is one of the most fundamental bond cleavage mechanisms. Thus, understanding of bond homolysis influences the development of a wide range of chemistry. Photolytic bond homolysis and its reverse process have been observed directly using time-resolved spectroscopy. However, direct observation of reversible bond homolysis remains elusive. Here, we report the direct observation of reversible Co-Co bond homolysis using two-dimensional nuclear magnetic resonance exchange spectroscopy (2D EXSY NMR). The characterization of species involved in this homolysis is firmly supported by diffusion ordered NMR spectroscopy (DOSY NMR). The unambiguous characterization of the Co-Co bond homolysis process enabled us to study ligand steric and electronic factors that influence the strength of the Co-Co bond. Understanding of these factors will contribute to rational design of multimetallic complexes with desired physical properties or catalytic activity.

Fig. 1. Methods to observe and prove reversible bond homolysis. (a) A conventional method: temperature-dependent reversible change of spectra. (b) This work: direct observation of exchange signals by 2D EXSY NMR.

Fig. 2. Isolable Co-metalloradical 1.

Fig. 3. Preparation and characterization of 2-Mes. (a) Preparation of 2-Mes. (b) SC-XRD structure of 2-Mes with 80% thermal ellipsoids. Hydrogen atoms, solvent molecules, and the second crystallographically independent 2-Mes molecule are omitted for clarity. (c) Variable temperature ¹H NMR spectra (500 MHz, C₆D₆) of 2-Mes showing the increasing concentration of radical species 2-Mes^* on increasing the temperature.

Fig. 4. Characterization of 2-Mes^*. (a) Trapping of 2-Mes^* by TEMPO. (b) Variable temperature EPR spectra (X-band, in C₆D₆) of 2-Mes^*. (c) Change of UV-Vis spectra (in C₆H₆) of a mixture of 2-Mes and 2-Mes^* on increasing the temperature from 10 to 50 °C. (d) 2D DOSY NMR spectrum (500 MHz, C₆D₆, 24.0 °C) of an equilibrium mixture of 2-Mes and 2-Mes^* in the presence of 4 and hexamethyldisiloxane (a signal at around 0 ppm) as internal standards.

Fig. 5. Direct 2D EXSY NMR observation of Co–Co bond homolysis. 2D EXSY NMR spectra (500 MHz, C₆D₆, 46.7 °C) of an equilibrium mixture of 2-Mes and 2-Mes^*, and assignment of exchanging signals. Red off-diagonal signals are due to NOE.

Fig. 6. van't Hoff plot for the Co–Co bond homolysis of 2-Mes plotted using NMR data. T: temperature (K), K_eq: [2-Mes^*]/[2-Mes] (mol L⁻¹), and error bar: standard deviation of three independent measurements.

Fig. 7. Structures of [Co(NHC)(CO)₃]₂ complexes.