Base-induced dismutation of POCl3 and POBr3:: Synthesis and structure of ligand-stabilized dioxophosphonium cations

The interaction between POCl3 or POBr3 and pyridine or DMAP has been reinvestigated to clarify the discrepancies between previously published results concerning the Lewis acidity of phosphoryl halides and their behavior toward pyridine bases. The obtained results show that POCl3 virtually does not react with pyridine, while it does with 4-(dimethylamino)pyridine (DMAP), even in SO2 solution, to yield an ionic compound [(DMAP)(2)PO2]Cl.3SO(2) (1.3SO(2)). Its recrystallization from acetonitrile gives [(DMAP)(2)PO2]Cl.CH3CN (1.CH3CN). The POBr3 reacts readily with both DMAP and pyridine forming the analogous tribromides, [(DMAP)(2)PO2]Br-3 (2) and [(py)(2)PO2]Br-3 (3), respectively. Treatment of 3 with Me3SiOSO2CF3 in acetonitrile solution led to [(py)(2)PO2][CF3SO3].CH3CN (4), while the reaction between 1.CH3CN and Me3SiOPOF2 gave [(DMAP)(2)PO2][PO2F2] (5). The crystal structures of 1.CH3-CN, 1.3SO(2), 2, and 4 revealed that all four compounds are ionic containing the distorted tetrahedral cations [(DMAP)(2)PO2](+) and [(py)(2)PO2](+). Both ions represent a donor-stabilized form of the so far unknown cation [PO2](+). The geometry of [(DMAP)(2)PO2](+), optimized by density functional calculations at the B3LYP/6-31G(d,p) level, is in good agreement with X-ray structural data. The NBO analysis of natural atomic charges shows an extensive delocalization of the [PO2](+) intrinsic positive charge and indicates a contribution of the electrostatic attraction to the formation of N-P donor-acceptor bonds. According to a P-31 NMR study, the reactions of both phosphoryl halides with DMAP proceed via successive formation of the intermediates [(DMAP)POX2](+) and (DMAP)POX to give an equimlar mixture of [(DMAP)(2)PO2](+) and PX5 (X = Cl, Br) as the end products. The NMR spectroscopic identification of the cations [(DMAP)POX2](+) and [(DMAP)(2)PO2](+) was supported by ab initio calculations of their chemical shifts.