Activation and functionalization of white phosphorus at rhodium:: Experimental and computational analysis of the [(triphos)Rh (η1:η2-P4RR′)]Y complexes (triphos = MeC(CH2PPh2)3; R = H, alkyl, aryl; R′=2 electrons, H, Me)

Thermal reaction of white phosphorus with [(triphos)RhH3] (1) in THF affords [(triphos)Rh(eta(1):eta(2)-P4H)] (2), triphos = MeC(CH2PPh2)(3). Similar complexes [(triphos)Rh(eta(1):eta(2)-P4R)] (R = Me (7), Et (8), Ph (9)) also form at lower temperature by the reaction of P-4 and [(triphos)Rh(R)(eta(2)-C2H4)] with elimination of ethene. In contrast, a double-insertion process follows the reaction of [(triphos)Rh(H) (eta(2)-C2H4)] and P-4 to generate tetraphosphido ethyl complex 8. Compounds 2, 7, 8 and 9 are thermally unstable and eventually decompose into the cyclotriphosphorus complex [(triphos)Rh(eta(3)-P-3)] (3) plus other unidentified phosphorus-containing species. Otherwise, PH3 or PH2R is generated in the presence of H-2. The formation of PH3 and 3 is quantitative starting from the precursor 2. The electrophilic attack of MeOTf or HBF4 on the P4R ligand in the complexes 2, 7-9 is regioselective, and yields a cationic product of formula [(triphos)Rh(eta(1):eta(2)- P4RR')](+). The direct attack on the substituted p-R phosphorus atom is demonstrated by crossing experiments. Complexes of the latter type have been isolated in the solid state for the combinations R = H and R' = Me (11) or R = Ph and R' = Me (12). The latter species, [(triphos)Rh(eta(1):eta(2)-P4PhMe')]OTf.2CH(2)Cl(2) (OTf = triflate), has been characterised by X-ray methods. The geometry at the metal is better described as a trigonal bipyramidal than pseudo-octahedral. In fact, the P4RR' unit acts as a bidentate ligand with its exocyclic PR2 donor group and the endocyclic, dihapto-coordinated P=P linkage. The latter group lies in the equatorial plane, in a similar way to a classic olefin ligand that is coordinated to a butterfly-shaped L4M fragment (M = d(8)). DFT calculations on a model of 2 and all possible protonated isomers confirm that double substitution at the exocyclic P-donor positions of the open P-4 unit is energetically favoured. A multinuclear and multidimensional NMR analysis confirms that this structure is maintained in solution for both the parent and the protonated compounds.