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

The reaction of the dilithium diamido-diphosphine macrocycle, Li 2 [N(SiMe 2 CH 2 P(Ph)CH 2 SiMe 2 ) 2 N] (Li 2 [P 2 N 2 ]) with [Rh(COD)Cl] 2 generates the dirhodium macrocyclic compound, [P 2 N 2 ][Rh(COD)] 2 (where COD = η 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 [P 2 N 2 ] with [Rh(COE) 2 Cl] 2 (where COE is η 2 -cyclooctene), the dilithium-dihodium derivative ([Rh(COE)][P 2 N 2 ]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 ][P 2 N 2 ]Li) 2 (dioxane); also formed is the dilithium-dirhodium-phenylhydride complex ([Rh(C 6 H 5 )H][P 2 N 2 ]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 H 2 O, which leads to the isolation of the amine-dihydride, Rh[P 2 N 2 H](H) 2 ; drying of the H 2 eliminates this side product. Nevertheless, careful addition of H 2 O to ([Rh(COE)][P 2 N 2 ]Li) 2 (dioxane) results in protonation of one of the amido units and the formation of the rhodium-amine cyclooctene derivative, Rh[P 2 N 2 H](COE), which upon reaction with H 2 generates the aforementioned amine-dihydride, Rh[P 2 N 2 H](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)][P 2 N 2 ]Li) 2 (dioxane), followed by H-H and C-H bond activation. The ability of one of the amido units of the P 2 N 2 macrocycle to be protonated is a potentially useful proton storage mechanism and is of interest in other bond activation processes. A lithium-rhodium( i ) cyclooctene complex oxidatively adds dihydrogen and benzene to generate lithium-rhodium( iii ) dihydride and phenyl-hydride derivatives, respectively.