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.