Cycloaddition Reactions of the Phosphinidene-Bridged Complex [Mo₂Cp(μ‑κ¹:κ¹,η⁵‑PC₅H₄)(CO)₂(η⁶‑HMes*)] with Diazoalkanes and Other Heterocumulenes

The
title phosphinidene complex reacted at room temperature with CS2 and SCNPh to give the phosphanyl derivatives [Mo2Cp­{μ-κ2P,S:κ1S′,η5-P­(CS2)­C5H4}­(CO)2(η6-HMes*)] and [Mo2Cp­{μ-κ2P,S:κ1P,η5-P­(C­(NPh)­S)­C5H4}­(CO)2(η6-HMes*)], respectively (Mes*
= 2,4,6-C6H2tBu3), which result from a [2 + 2] cycloaddition of a CS
bond in the organic reagent to the MoP bond of the phosphinidene
complex, with further insertion of S into the remaining Mo–P
bond, in the CS2 reaction. The title complex also reacted
with diazoalkanes N2CRR′ at room temperature to
give the corresponding phosphaalkene derivatives [Mo2Cp­{μ-η2:κ1P,η5-P­(CRR′)­C5H4}­(CO)2(η6-HMes*)]
(CRR′ = CH2, CPh2, CH­(SiMe3)). These products follow from a formal [2 + 1] cycloaddition of
the carbene CRR′ fragment to the MoP bond of the parent
compound but were shown to proceed through a [3 + 2] cycloaddition
of the diazoalkane molecule, followed by N2 elimination.
The diazomethane reaction allowed the identification at low temperature
of a stabilized form of the intermediate product, the phosphanyl complex
[Mo2Cp­{μ-κ2P,N:κ1P,η5-P­(CHN2H)­C5H4}­(CO)2(η6-HMes*)], which follows from a reversible 1,3-shift of a methylenic
H atom from C to N. It was concluded that all of the above cycloaddition
reactions are initiated by heteroatom coordination of the unsaturated
organic molecule to the MoCp­(CO)2 fragment in the parent
phosphinidene complex, this triggering the P–C bond formation
step which leads to the products eventually isolated. The structures
of the new complexes were determined by spectroscopic, diffractometric,
and (in some cases) density functional theory methods.