Intrinsic basicities of phosphorus imines and ylides: A theoretical study

A density functional theory (B3LYP/6-311+G**), ab initio (HF/3-21G*), and s emiempirical (PM3) study of intrinsic basicities, protonation energies, or protonation enthalpies of organic phosphorus imine (iminophosphorane) inclu ding phosphazene, phosphorus ylide (phosphorane), and phosphine superbases has been performed. The study shows that representatives of the first two c lasses of the above-mentioned organic superbases can reach the basicity lev el of the strongest inorganic superbases such as alkali-metal hydroxides, h ydrides, and oxides. The strongest organic phosphazene imine superbases are predicted to reach the gas- phase basicity level of ca. 300 kcal/mol (numb er of phosphorus atoms in, the system n greater than or equal to 7), wherea s the strongest organic phosphazene ylide superbases are estimated to have (at n greater than or equal to 5) gas-phase basicities around, or beyond 31 0-320 kcal/mol. The phosphine superbases, including the Verkade's bicyclic phosphines (proazaphosphatranes) are predicted to have a basicity comparabl e to P-2 phosphazenes or P-1 phosphorus ylides, whereas the respective proa zaphosphatrane imines and ylides are expected to be the strongest organic s uperbases which contain only a single phosphorus atom. Extremely high expec ted basicity values and handling preferences over inorganic superbases. mak e representatives of novel organic superbases possible partners for observi ng the spontaneous gas-phase proton transfer between neutral Bronsted super acids and -bases. For the comparison, the basicities of some alkali-metal s ubstituted ammonia, phosphine, phosphorus, and nitrogen ylides and imines h ave been also calculated.