3<i>H</i>-Benzophosphepine Complexes:  Versatile Phosphinidene Precursors

The synthesis of a variety of benzophosphepine complexes [R = Ph, <i>t-</i>Bu, Me; ML<i><sub>n </sub></i>= W(CO)<sub>5</sub>, Mo(CO)<sub>5</sub>, Cr(CO)<sub>5</sub>, Mn(CO)<sub>2</sub>Cp] by two successive hydrophosphinations of 1,2-diethynylbenzene is discussed in detail. The first hydrophosphination step proceeds at ambient temperature without additional promoters, and subsequent addition of base allows full conversion to benzophosphepines. Novel benzeno-1,4-diphosphinanes were isolated as side products. The benzophosphepine complexes themselves serve as convenient phosphinidene precursors at elevated, substituent-dependent temperatures (>55 °C). Kinetic and computational analyses support the proposal that the phosphepine−phosphanorcaradiene isomerization is the rate-determining step. In the absence of substrate, addition of the transient phosphinidene to another benzophosphepine molecule is observed, and addition to 1,2-diethynylbenzene furnishes a delicate bidentate diphosphirene complex.