Reversible Intermolecular E–H Oxidative Addition to a Geometrically Deformed and Structurally Dynamic Phosphorous Triamide

Abstract

The synthesis and reactivity of geometrically constrained tricoordinate phosphorus (σ(3)-P) compounds supported by tridentate triamide chelates (N[o-NR-C6H4]2(3-); R = Me or (i)Pr) are reported. Studies indicate that 2 (P{N[o-NMe-C6H4]2}) adopts a Cs-symmetric structure in the solid state. Variable-temperature NMR studies demonstrate a low-energy inversion at phosphorus in solution (ΔG(‡)(exptl)(298) = 10.7(5) kcal/mol), for which DFT calculations implicate an edge-inversion mechanism via a metastable C2-symmetric intermediate. In terms of reactivity, compound 2 exhibits poor nucleophilicity, but undergoes oxidative addition at ambient temperature of diverse O-H- and N-H-containing compounds (including alcohols, phenols, carboxylic acids, amines, and anilines). The resulting pentacoordinate adducts 2·[H][OR] and 2·[H][NHR] are characterized by multinuclear NMR spectroscopy and X-ray crystallography, and their structures (which span the pseudorotation coordinate between trigonal bipyramidal and square planar) are evaluated in terms of negative hyperconjugation. At elevated temperatures, the oxidative addition is shown to be reversible for volatile alcohols and amines.

A Stable but Highly Reactive Phosphine‐Coordinated Borenium: Metal‐free Dihydrogen Activation and Alkyne 1,2‐Carboboration

“…[16] Bertrand et al pioneered the field with ambiphilic acyclic and cyclic amino carbenes, [17] Aldridge and co-workers generalized the approach to aminoboryl and aminosilyl silylenes, [18] and Piers et al extended the field to highly electron-deficient antiaromatic boroles [2a] (see Refs. [19,20] for additional examples).…”

E–H Bond Activation of Ammonia and Water by a Geometrically Constrained Phosphorus(III) Compound

“…TheP 1 À O3 bond of 3 (1.774 (3) ) is significantly longer than both ligand P À O bonds and appreciably longer than the PÀO(Ph) bond in the phenol activation product of B,[B(H)(OPh)] (1.657 (2) ). [26] It is also slightly larger than the sum of the respective covalent radii but well within the sum of the Va nder Waals radii. [43,44] In contrast, the P À O (m-O) bonds of 4 are much shorter (1.601(2)-1.631(2) ), consistent with ag reater electrostatic interaction between the bridging oxo group and the two phosphorus centers.T he ligand PÀOand PÀNbonds of both species are comparable to those of 2.…”

“…[23,24] Furthermore, A can be used in conjunction with ammonia-borane to facilitate the catalytic reduction of azobenzene. [25] Related studies have since extended this approach to nonplanar phosphorus triamide (B)a nd diazadiphosphapentalene (C), [26,27] both of which activate E À Hb onds through al igand-assisted mechanism.…”

“…Furthermore,c omplementary doublets are detected in the 1 HNMR spectrum at d = 8.46 ppm and d = 2.16 ppm integrating to one (PH)and two protons (NH 2 ), respectively.T he proposed structure of 2 was confirmed through single-crystal X-ray diffraction studies (Figure 3). Although ammonia has been shown to react with both A and B [24,26] in ap rocess that constitutes af ormal oxidative addition, 2 is the first structurally authenticated example of such ac ompound.…”

“…[24] By contrast, the reaction products of B have been shown to retain af olded ligand geometry. [26] Thea xial disposition of the P À Ob onds of 2 results in their lengthening relative to the analogous bonds in 1,w hilst the increased valencye ffects ac ontraction of the P1ÀN1 bond length (average value of 1.701 ). Substitutional …”