Phosphaalkenes as long-lived phosphorus cluster surface functional groups: intramolecular P=C addition to a niobium-supported P7 cage

The diniobium octaphosphorus complex (P8)Nb(OC(2)Ad]Mes)3]2 (1) (Ad = adamantylidene, Mes = 2,4,6-Me3C6H2) contains a reactive niobium phosphinidene moiety that can be exploited for metathetical scission of the NbP bond. When 1 is treated with aryl ketones, loss of ONb(OC(2)Ad]Mes)3(OEt2) (2) is observed along with concomitant formation of the corresponding phosphaalkene (RC6H4)2CPP7Nb(OC(2)Ad]Mes)3 (3-R). Complexes 3-R rearrange to incorporate the (RC6H4)2CP unit into the phosphorus cage, thereby generating a saturated organo-phosphorus cluster complexed to the niobium tris-enolate platform, (RC6H4)2CP8Nb(OC(2)Ad]Mes)3 (4-R). The structure of one such rearranged cluster 4-H, as determined by X-ray crystallography, is briefly discussed. An Eyring analysis of the first-order rearrangement of 3-H to 4-H gives activation parameters of DeltaH(double dagger) = 16.7 kcal/mol and DeltaS(double dagger) = -20.4 eu. A Hammett analysis of the phosphaalkene rearrangement, 3-R to 4-R, with substitution varying at the para positions of the aryl rings, reveals a linear relationship between the sigma values and the rearrangement rate constants. A concerted, asynchronous mechanism for the least-motion rearrangement of 3-H to 4-H is presented. When 1 is treated with alkyl ketones, similar loss of 2 and formation of the corresponding phosphaalkene is observed; however, the phosphaalkene complexes have considerably greater stability and are readily isolated.