Unveiling the Electronic Structure of the Bi(+1)/Bi(+3) Redox Couple on NCN and NNN Pincer Complexes

Low-valent group
15 compounds stabilized by pincer ligands have
gained particular interest, given their direct access to fine-tune
their reactivity by the coordination pattern. Recently, bismuth has
been employed in a variety of catalytic transformations by taking
advantage of the (+1/+3) redox couple. In this work, we present a
detailed quantum–chemical study on the electronic structure
of bismuth pincer complexes from two different families, namely, bis­(ketimine)­phenyl
(NCN) and triamide bismuthinidene (NNN). The use of the so-called
effective oxidation state analysis allows the unambiguous assignation
of the bismuth oxidation state. In contrast to previous studies, our
calculations suggest a Bi­(+1) assignation for NCN pincer ligands,
while Bi­(+3) character is found for NNN pincer complexes. Notably,
regardless of its oxidation state, the central bismuth atom disposes
of up to two lone pairs for coordinating Lewis acids, as indicated
by very high first and second proton affinity values. Besides, the
Bi–NNN systems can also accommodate two Lewis base ligands,
indicating also ambiphilic behavior. The effective fragment orbital
analysis of Bi and the ligand allows monitoring of the intricate electron
flow of these processes, revealing the noninnocent nature of the NNN
ligand, in contrast with the NCN one. By the dissection of the electron
density into effective fragment orbitals, we are able to quantify
and rationalize the Lewis base/acid character.