Periodicity in Structure, Bonding, and Reactivity for P-Block Complexes of a Geometry-Constraining Triamide Ligand

The use of pincer ligands to access non-VSEPR geometries at main-group centers is an emerging strategy for eliciting new stoichiometric and catalytic reactivity. As part of this effort, several different tridentate trianionic substituents have to date been employed at a range of different central elements, providing a patchwork dataset that precludes rigorous structure-function correlation. Here we report an analysis of periodic trends in structure (solid, solution, and gas phase), bonding, and reactivity based on systematic variation of the central element (P, As, Sb, or Bi) with retention of a single tridentate triamide substituent. In this homologous series, the central element can adopt either a bent or planar geometry. The tendency to adopt planar geometries increases descending the group with the phosphorus triamide (1) and its arsenic congener (2) exhibiting bent conformations, and the antimony (3) and bismuth (4) analogues exhibiting a predominantly planar structure in solution. This trend has been rationalized using the energy decomposition analysis. A rare phase-dependent dynamic covalent dimerization was observed for 3 and the associated thermodynamic parameters were established quantitatively. Planar geometries were found to engender lower LUMO energies and smaller band gaps as compared to bent ones, resulting in different reactivity patterns. These results provide a benchmark dataset to guide further research in this rapidly emerging area.