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

The reaction of the dilithium diamido-diphosphine macrocycle, Li-2[N(SiMe2CH2P(Ph)CH2SiMe2)(2)N] (Li-2[P2N2]) with [Rh(COD)Cl](2) generates the dirhodium macrocyclic compound, [P2N2][Rh(COD)](2) (where COD = eta(4)-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 H-2, no clean products could be isolated. Upon reaction of Li-2[P2N2] with [Rh(COE)(2)Cl](2) (where COE is eta(2)-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 H-2. This process is complicated by adventitious H2O, which leads to the isolation of the amine-dihydride, Rh[P2N2H](H)(2); drying of the H-2 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 H-2 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.

Automated summary

The reaction of dilithium diamido-diphosphine macrocycle, Li-2[P2N2], with [Rh(COE)(2)Cl](2) generates the dilithium-dihodium derivative, which further reacts with dihydrogen to form different products. The presence of adventitious H2O leads to the isolation of an amine-dihydride side product, which can be eliminated by drying the H-2. Protonation of one of the amido units in the reaction mixture results in the formation of a rhodium-amine cyclooctene derivative, with further reaction with H-2 generating the amine-dihydride product.