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 comparative study on the donor/acceptor properties of
10 different triazolylidene-type mesoionic carbene ligands (MICs),
with the aim of investigating and deciphering the effects of substituents
on the donor/acceptor properties of such MIC ligands. Data from 77Se NMR chemical shifts of the triazoline selones, the 1JC‑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 1JC‑H coupling constants
is relatively narrow to be able to significantly differentiate between
the donor properties of the various MICs. The 77Se NMR
shifts of the triazoline selones on the other hand show a broad spread,
and it is in principle well-suited to understand the π-accepting
properties of the MICs as a function of substituents. However, this
method can have other complications which are discussed in this work.
DFT calculations are presented to spread light on the 77Se NMR shifts. Additionally, we present the first example of a AuI complex with mesoionic triazoline selones, a class of ligands
whose utility has remained underexplored in transition metal chemistry.