The reaction of mixtures of [Rh4(CO)12] and triphenylphosphite with carbon monoxide or syngas as studied by high-resolution, high-pressure NMR spectroscopy

The fragmentation and redistribution reactions of Rh4(CO)12-x{P(OPh)3}x] (x = 1-4) with carbon monoxide have been studied using high-resolution, high-pressure NMR spectroscopy. Under the conditions of efficient gas mixing in a high-pressure NMR bubble column, Rh4(CO)9{P(OPh)3}3] fragments to give mainly Rh2(CO)6{P(OPh)3}2]; Rh4(CO)11{P(OPh)3}] is also observed,implying redistribution of the phosphite ligand and/or recombination of the dimers to tetrameric clusters. Fragmentation ofRh4(CO)10{P(OPh)3}2] is found to be pressure-dependent giving predominantly Rh2(CO)6{P(OPh)3}2] at low CO pressure (1-40 bar), and increasing amounts of Rh2(CO)7{P(OPh)3}] at higher (40-80 bar) pressure. Using Syngas (CO : H2 (1 : 1)) instead of CO in the above fragmentations, homolytic addition of H2 to the dimer Rh2(CO)6{P(OPh)3}2] to give RhH(CO)3{P(OPh3}] and RhH(CO)2{P(OPh)3}2] is observed. The distribution of tetrameric species obtained is similar to that obtained under the same partial pressure of CO. On depressurisation/out-gassing of the sample, the original mixture of tetrameric clusters is obtained.