One of the most popular tools for the characterization of carbene empty orbitals is the 77Se NMR chemical shift of carbene-selenium adducts. A number of studies have shown good correlation between the electronic properties of coordinated carbenes and the 77Se NMR chemical shift of the adducts, validating the use of the approach. However, a few anomalies have been noted where seemingly innocent changes to carbene peripheral structure induce large downfield 77Se NMR chemical shifts, causing a loss of confidence in this popular method. In our study, we show that such unexpectedly downfield 77Se NMR chemical shifts are due to non-classical hydrogen bonds between carbene substituents and the coordinated Se atom. A number of carbene-Se adducts with varying steric bulk were characterized through NMR and single crystal X-ray analysis to demonstrate the steric conditions required to cause and avoid these unexpected interactions. These interactions can occur even with alkyl protons on robust adamantyl groups. Our study shows that a bulky substituent capable of forming a 6-membered ring through a H-Se interaction will exhibit a large downfield NMR chemical shift, independent of the electronic properties of the carbene itself. Such interactions must be accounted for or avoided when using 77Se NMR to measure the acidity of the carbene empty orbital.

Influence of C(sp3)H-selenium interactions on the 77Se NMR quantification of the π-accepting properties of carbenes

ACS Fall 2021

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