Abstract
The coordination chemistry of an N-heterocyclic phosphenium
(NHP)-containing
bis(phosphine) pincer ligand has been explored with Pt0 and Pd0 precursors. Unlike previous compounds featuring
monodentate NHP ligands, the resulting NHP Pt and Pd complexes feature
pyramidal geometries about the central phosphorus atom, indicative
of a stereochemically active lone pair. Structural, spectroscopic,
and computational data suggest that the unusual pyramidal NHP geometry
results from two-electron reduction of the phosphenium ligand to generate
transition metal complexes in which the Pt or Pd centers have been
formally oxidized by two electrons. Interconversion between planar
and pyramidal NHP geometries can be affected by either coordination/dissociation
of a two-electron donor ligand or two-electron redox processes, strongly
supporting an isolobal analogy with the linear (NO+) and
bent (NO–) variations of nitrosyl ligands. In contrast
to nitrosyls, however, these new main group noninnocent ligands are
sterically and electronically tunable and are amenable to incorporation
into chelating ligands, perhaps representing a new strategy for promoting
redox transformations at transition metal complexes.