We reasoned that terminal PN coordination to kinetically labile, late first row transition metals would be enforced via the incorporation of such ligands into a rigid, multidentate framework. To date, however, very few multidentate PN scaffolds have been prepared, and these suffer from pronounced flexibility and concomitant ambiguity in their metal coordination chemistry. Hence, a tris-PN pro-ligand (H3 R L) based on a rigid triarylbenzene backbone was designed (Scheme 1A). Using a similar backbone, tris-alkoxide, -siloxide, and -carboxylate variants have been described, and tris-phosphine precursors (Int1) bearing phenyl-or adamantyl substituents are readily prepared (Supporting Information). However, attempts to convert Int1 to H3 R L via traditional routes, including the usage of Me3SiN3 6 or aminolysis of R3PCl2 derivatives proved unsuccessful; even after prolonged heating, incomplete conversion of Ph Int1 was observed upon exposure to excess Me3SiN3 as monitored by P NMR spectroscopy (Fig ). We hypothesize that steric effects preclude incorporation of three bulky Me3SiN groups onto this constrained framework. Scheme 1. (A) Synthesis of the tris-phosphinimine proligands H3 R L and (B) complexes 1 and 2 reported in this study. This observation underscores that the general application of PNs as ligands is limited by existing synthetic procedures for their preparation. We canvassed alternative reagents intended to effect the electrophilic amination of phosphines and identified O-(2,4-dinitrophenyl)-hydroxylamine as a non-hygroscopic, stable, and easily scalable reagent. The combination of this reagent with diverse mono-and diphosphines at room temperature results in quantitative conversion to the corresponding phospiniminium ions (R3PNH2 + ) as observed by P-NMR within 15 minutes, with isolated yields of 80-90% on gram scales (Supporting Information). Importantly, this reagent also quantitatively aminates all three phosphorus atoms of Int1 as monitored by P NMR spectroscopy. To facilitate isolation, anion exchange by treatment with excess Me3SiCl afforded the iminium chloride Int2 as an insoluble solid. Further treatment of Int2 with three equivalents of NaO t Bu produced the desired tris-phosphinimines in ~70% overall yield from Int1, providing efficient syntheses of the H3 R L pro-ligands.