Quantifying the Donor Strength of Ligand-Stabilized Main Group Fragments

Unusual
binding properties, enabling the stabilization of elusive
species, and beneficial properties for homogeneous catalysts have
been predicted and demonstrated for ligand-stabilized main group fragments,
such as carbodiphosphoranes and -carbenes. However, the quantification
and comparison of their binding properties by experimental means still
represent major challenges. In this article, we describe a series
of iridium­(III) pincer complexes of the type [(PEP)­IrCl­(CO)­(H)]q enabling the quantification of the donor
strength of the central donor group E (q = 0, +1,
+2). Our investigations show that phosphine-stabilized boron­(I) and
carbon(0) compounds are exceptionally strong neutral donor groups
in comparison to common spectator ligands in homogeneous catalysis
such as carbenes and phosphines. Our experimental and computational
results for the first time allow and justify the comparison of the
donor strength of cationic, neutral, and anionic ligands. On the basis
of quantum chemical investigations, we further demonstrate that the
heavier homologues of phosphine-stabilized borylenes and carbon(0)
compounds exhibit slightly diminished donor properties.