Synthesis and catalytic potential of coordinatively unsaturated rhodium hydride clusters incorporating chelating bidentate ligands

Abstract

The synthesis of three coordinatively unsaturated clusters of the formula [{R2PCH2CH2PR2}RhH]x (R = OMe and OEt: x = 4; R = O-i-pr: x = 2) is described starting from the appropriate 2-methylallylrhodium(1) derivatives and H2. The reaction of PMe3 with these clusters results in the formation of mononuclear complexes of the formula HRh(PMe3)2(R2PCH2CH2PR2). Olefins such as 1-hexene do not appear to react with the clusters although a slow isomerization to internal hexenes takes place. The tetranuclear clusters do not form a detectable adduct with H2 up to 4 atm, although exchange with D2 is observed. However, the dinuclear cluster adds one equivalent of H2 per dimer to generate, in situ. [{(i-prO)2PCH2CH2P(O-i-pr)2}2Rh2H4]. These clusters are catalytically active in the hydrogenation of 1-hexene with isomerization to internal hexenes being a competitive process. Although the evidence suggests that the dinuclear derivative does not fragment during the catalytic cycle, the catalytic activity of the tetranuclear clusters may be due to a smaller cluster present in low concentration, generated by fragmentation.