On the Redox Reactivity of a Geometrically Constrained Phosphorus(III) Compound

The reactivity of a geometrically constrained phosphorus(III) complex bearing the N,N-bis(3,5-di-tert-butyl-2-phenolate)amide pincer ligand (P(ONO); 1) towards oxidants and reductants is explored. This compound can be readily oxidized to the phosphorus(V) dihalo-derivatives P(ONO)X₂ where X=Cl (2), Br (3) and I (4). Attempts at isolating the analogous difluoride through oxidation of 1 were unsuccessful yielding only the hydrofluoride P(ONO)(H)F (5), however P(ONO)F₂ (6) can be accessed via a halide exchange reaction of 2 with KF. Compound 2 can be employed as a precursor to novel cationic species through chloride ion displacement using strong Lewis bases. Thus, reaction of 2 with two or three molar equivalents of dimethylaminopyridine (DMAP) affords [P(ONO)(Cl)(DMAP)₂ ]⁺ (7) and [P(ONO)(DMAP)₃ ]²⁺ (8). Reaction of 2 with the weaker bidentate base 2,2'-bipyridine (bipy) affords [P(ONO)(Cl)(bipy)]⁺ (9), although this species was only accessible upon addition of a halide abstracting agent. The dicationic tris(pyridine) adduct [P(ONO)(py)₃ ]²⁺ (10) is also accessible by reaction of 4 with pyridine. Oxidation of 1 using oxygen gas proceeds slowly and allows for the observation of two compounds, a mixed valence dimeric phosphorus(III)/phosphorus(V) compound [P(ONO)(μ² -O)(μ² :κ¹ ,κ² -ONO)P] (11) and the fully oxidized species [P(ONO)(μ² -O)(μ² :κ¹ ,κ² -ONO)P(O)] (12). Finally, reaction of 1 using KC₈ results in the dimerization of the putative radical anion [P(ONO)]^.- through formation of a P-P bond to afford [P(ONO)]₂^2- (13). Reactions with TEMPO result in the formation of the trigonal bipyramidal species P(ONO)(TEMPO)₂ (14).