Forays into rhodium macrocyclic chemistry stabilized by a P2N2 donor set. Activation of dihydrogen and benzene.

Abstract

The reaction of the dilithium diamido-diphosphine macrocycle, Li2[N(SiMe2CH2P(Ph)CH2SiMe2)2N] (Li2[P2N2]) with [Rh(COD)Cl]2 generates the dirhodium macrocyclic compound, [P2N2][Rh(COD)]2 (where COD = η4-1,5-cyclooctadiene), wherein both rhodium-COD units are syn to each other and have square planar geometries. While this dirhodium derivative does react with H2, no clean products could be isolated. Upon reaction of Li2[P2N2] with [Rh(COE)2Cl]2 (where COE is η2-cyclooctene), the dilithium-dihodium derivative ([Rh(COE)][P2N2]Li)2(dioxane) forms, which was characterized by single-crystal X-ray analysis and NMR spectroscopy. The cyclooctene derivative reacts with dihydrogen in benzene to generate the dilithium-dirhodium-dihydride complex ([Rh(H)2][P2N2]Li)2(dioxane); also formed is the dilithium-dirhodium-phenylhydride complex ([Rh(C6H5)H][P2N2]Li)2(dioxane) via oxidative addition of a C-H bond of the solvent. The phenyl-hydride is eventually converted to the dihydride derivative via further reaction with H2. This process is complicated by adventitious H2O, which leads to the isolation of the amine-dihydride, Rh[P2N2H](H)2; drying of the H2 eliminates this side product. Nevertheless, careful addition of H2O to ([Rh(COE)][P2N2]Li)2(dioxane) results in protonation of one of the amido units and the formation of the rhodium-amine cyclooctene derivative, Rh[P2N2H](COE), which upon reaction with H2 generates the aforementioned amine-dihydride, Rh[P2N2H](H)2. The mechanism by which dihydrogen and C-H bonds of benzene are activated likely involves initial dissociation of cyclooctene from the 18-electron centers in ([Rh(COE)][P2N2]Li)2(dioxane), followed by H-H and C-H bond activation. The ability of one of the amido units of the P2N2 macrocycle to be protonated is a potentially useful proton storage mechanism and is of interest in other bond activation processes.