Probing Electronic Properties of Triazolylidenes through Mesoionic Selones, Triazolium Salts, and Ir-Carbonyl-Triazolylidene Complexes

Understanding and quantifying the electronic properties of ligands is important both from a fundamental point of view and also for their applications in various branches of chemistry. In this contribution, we present a comparativ-e study on the donor/ acceptor properties of 10 different triazolylidene-type mesoionic carbene ligands (MICs), vith the aim of investigating and deciphering the effects of substituents on the donor/ acceptor properties of such MIC ligands. Data from Se-77 NMR chemical shifts of the iazoline selones, the (1)J(C-H) coupling constants of the corresponding triazolium salts, and the Tolman electronic parameters (TEP) of the Ir-CO complexes are compared and contrasted. The data collected in this work show that the spread of both the TEP and the (1)J(C-H) coupling constants is relatively narrow to be able to significantly differentiate -between the donor properties of the various MICs. The Se-77 NMR shifts of the triazoline selones on the other hand show a broad spread, and it is in principle well-suited to understand the pi-accepting properties of the MICs as a function of sulistituents. However, this method can have other complications which are discussed in this work. DFT calculations are presented to spread light on the Se-77 NMF, shifts. Additionally, we present the first example of a Au-I complex with mesoionic triazoline selones, a class of ligands whose utility has remained underexplored in transition metal chemistry.