Probing the speciation and electronic structure of organozinc reagents using X-ray spectroscopy Lewis Parker 1, J. M. Seymour 1 , N. O. Alblewi 1 , F. K. Towers Tompkins 1 , E. Gousseva 1 , M. Daw 1 , C. J. Clarke 2 , S. Hayama 3 , N. A. Besley 2 , C. D. Smith 1 , K. R. J. Lovelock 1 1 University of Reading, UK, 2 University of Nottingham, UK, 3 Diamond Light Source, UK Organozinc reagents play a crucial role in carbon-carbon bond formation (e.g. Negishi reaction) and Lewis acid catalysis. 1 However, despite its importance, there is an incomplete understanding of organozinc chemistry, for example how the choice of solvent or substituent affects reactivity. Therefore, understanding the speciation and electronic structure of these organozinc compounds in the liquid-phase reaction medium is key to elucidating their reactivity. Problematically, zinc is spectroscopically quiet, mainly due to the filled 3d shell; investigating organozinc reagents is very challenging using standard spectroscopies, e.g. UV-Vis, EPR, NMR. X-ray spectroscopy techniques can probe speciation and electronic structure. X-ray absorption spectroscopy has been used for many zinc-based compounds, 2 including a limited number of organozinc compounds; however, studies of zinc-based compounds using X-ray emission spectroscopy (XES) are very limited, 3 and seemingly non-existent for resonant inelastic X-ray scattering (RIXS). High energy resolution fluorescence detection XAS (HERFD-XAS), valence-to-core XES (VtC-XES) and RIXS were recorded on beamline I20 at Diamond Light Source and were successfully used to probe the liquid-phase speciation and valence electronic structure for a range of zinc samples. By combining these techniques, we have successfully linked both speciation and electronic structure to reactivity to improve our fundamental understanding of zinc reagents and catalysts in the solution phase. We observed that the stoichiometric ratio of coordinating solvents has a quantifiable effect on the speciation and electronic structure of diethylzinc (Et 2 Zn) (Figure 1). Results from time-dependent density functional theory (TDDFT) gave superb matches to both HERFD-XAS and XES, demonstrating that TDDFT can greatly enhance X-ray spectroscopy measurements of organozinc compounds. Using resonant techniques, we have quantified the effects of different substituents on the electronic structure of dialkylzinc species, R 2 Zn (R = C 6 F 5 , Ph, i- Pr, Et). Overall, we have emphatically demonstrated that this suite of X-ray spectroscopic techniques are effective tools for characterising the liquid-phase speciation and electronic structure of closed-shell diamagnetic complexes including zinc.
Figure 1. HERFD-XAS spectra for ZnEt 2 in hexane and in THF (two different concentrations). TDDFT calculated spectra and transitions are shown below for ZnEt 2 , ZnEt 2 (THF) and ZnEt 2 (THF) 2 References 1. Fazekas et al. , in Reference Module in Chemistry, Molecular Sciences and Chemical Engineering , 2021, DOI: 10.1016/ b978-0-12-820206-7.00090-1. 2. Penner-Hahn, Coord. Chem. Rev. , 2005, 249 , 161.Clarke et al. , J. Phys. Chem. A , 2019, 123 , 9552.
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