Pitfalls and Limitations in Group 6 Triamidophosphine Chemistry: Cage-Closure Restrictions in Square Pyramidal Nitrides and Degradation via Spiro[4.4]-λ5-Amidophosphorane Formation

The benzylene-linked triaminophosphines P(C6H4-o-CH2NHXyl)3 ([A]H3) and P(CH2C6H4-o-NHPh)3 ([B]H3) were shown to react with (Me2N)3Mo≡N to afford the square pyramidal nitrido complexes 1 and 2, each comprising one residual dimethylamido moiety and one uncoordinated ligand sidearm. Only the [A]-coordinated complex 1 was found to eliminate the remaining dimethylamide upon heating. Instead of the closed-cage molybdenum(VI) nitride [A]Mo≡N, a cyclometalated molybdaziridine (3) was isolated, as the syn-alignment of the uncoordinated sidearm and the nitride in 1 inhibits a productive cage-closure reaction. Using [NPN]-coordinated model complexes, analogous syn-arrangements were found not only for molybdenum, but also for the related tungsten nitrides and the group 5 imides. Attempts to access the closed-cage Mo(VI) nitride [A]Mo≡N via transmetalation between [A]Sn(nBu) (9) and (tBuO)3Mo≡N, led to the formation of the spiro-[4.4]-λ5-amidophosphorane (10). These results contrast with the findings made with azatranes and methylene-linked exo-P-configured triamidophosphine complexes.