Comparison of Structure and Reactivity of Phosphine-Amido and Hemilabile Phosphine-Amine Chelates of Rhodium

A series of mono- and binuclear rhodium(I) complexes bearing ortho-phosphinoanilido and ortho-phosphinoaniline ligands has been synthesized. Reactions of the protic monophosphinoanilines, Ph2PAr or PhPAr2 (Ar = o-C6H4NHMe), with 0.5 equiv of [Rh(mu-OMe)(COD)](2) result in the formation of the neutral amido complexes, [Rh(COD)(P,N-Ph2PAr-)] or [Rh(COD)-(P,N-PhP(Ar-)Ar)] (Ar- = o-C6H4NMe-), respectively, through stoichiometrically controlled deprotonation of an amine by the internal methcoade ion. Similarly, the binudear complex, [Rh-2(COD)(2)(mu-P,N,P',N'-mapm(2-))] (mapm(2-) = Ar(Ar-)PCH2P(Ar-)Ar), can be prepared by reaction of the protic diphosphinoanifine, mapm (Ar2PCH2PAr2), with 1 equiv of [Rh(mu-OMe)(COD)](2). An analogous series of hernilabile phosphine-amine compounds can be generated by reactions of monophosphinoanilines, Ph2PAr' or PhPAr'(2) (AT' = o-C6H4NMe2), with 1 equiv of [Rh(NBD)(2)] [BF4] to generate [Rh(NBD)(P,N-Ph2PAr')][BF4] or [Rh(NBD)(P,N-PhPAr'(2))][BF4], respectively, or by reactions of diphosphinoanilines, mapm or dmapm (Ar'2PCH2PAr'(2)), with 2 equiv of the rhodium precursor to generate [Rh-2(NBD)(2)(mu-P,N,P',N'-mapm)][BF4](2) or [Rh-2(NBD)(2)(mu-P,N,P',N'-dmapm)][BF4](2), respectively. Displacement of the diolefin from [Rh(COD)(P,N-Ph2PAr)] by 1,2-bis(diphenylphosphino)ethane (dppe) yields [Rh(P,P'dppe)(P,N-Ph2PAr-)] which, while unreactive to H-2, reacts readily and irreversibly with oxygen to form the peroxo complex, [RhO2(P,P'-dppe)(P,N-Ph2PAr-)], and with iodomethane to yield [Rh'(CH3)(P,P'-dppe)(P,N-Ph2PAr)]. Hemilabile phosphine-amine compounds can also be prepared by reactions of [Rh(P,P'-dppe)(P,N-Ph2PAr-)] with Me3OBF4 or HBF4-Et2O, resulting in (thermodynamic) additions at nitrogen to form [Rh(P,P'-dppe)(P,N-Ph2PAr')][BF4] or [Rh(P,P'-dppe)(P,NPh2PAr)][BP4], respectively. The nonlabile phosphine-amido and hernilabile phosphine-amine complexes were tested as catalysts for the silylation of styrene. The amido species do not require the use of solvents in reaction media, can be easily removed from product mixtures by protonation, and appear to be more active than their hemilabile, cationic congeners. Reactions catalyzed by either amido or amine complexes favor dehydrogenative silylation in the presence of excess olefin, showing modest selectivities for a single vinylsilane product. The binuclear complexes, which were prepared in an effort to explore possible catalytic enhancemerAs of reactivity due to metal-metal cooperativity, are in fact somewhat less active than mononuclear species, discounting this possibility.