Stepwise Formation of a 1,3-Butadiene Analogue of Mixed Heavier Group 15 Elements.

The reaction of the phosphinidene complex [Cp*P{W(CO)5}2] (1 a) with di-tert-butylcarboimidophosphene leads to the P-C cage compound 6 and the Lewis acid-base adduct [Cp*P{W(CO)5}2(CNtBu)] (2 a). In contrast, the arsinidene complex shows a different reactivity. At low temperatures, the arsaphosphene complex [{W(CO)5}{η(2)-(Cp*)As=P(tBu)}{W(CO)5}] (3) is formed. At these temperatures, 3 reacts further with a second equivalent of carboimidophosphene to form [{W(CO)5}{η(2)-{(Cp*)(tBu)P}As=P(tBu)}{W(CO)5}] (5), probably by the insertion of a phosphinidene unit (tBuP) into an As-C bond. In contrast, at room temperature 3 reacts further by a radical-type reaction to form [{(tBu)P=As-As=P(tBu)}{W(CO)5}4] (4). Compound 4 is the first example of a neutral, 1,3-butadiene analogue containing only mixed heavier Group 15 elements. It consists of two P=As double bonds connected by arsenic atoms.