Cationic dinuclear rhodium complexes as catalyst precursors for the hydroformylation of alkenes

The dimeric cationic complexes [M2(μ-HL)2L′2(L″) 2]2+ [M = Rh or Ir; HL = 4-mercapto-1-methylpiperidine; L′ = L″ = 1/2 cyclo-octa-1,5-diene (cod) or CO or L′ = CO and L″ = PPh3] have been prepared as chloride and tetrafluoroborate salts. The activity of these complexes as hydroformylation and hydrogenation catalysts under mild conditions has been tested, in an attempt to elucidate the importance of functional groups in the ligand and the effect of counter ions. In hydroformylations, best results have been obtained with [Rh2(cod)2(μ-HL)2][BF4] 2 plus added PPh3 or P(OPh)3, with turnover rates of ca. 2.7 mmol of olefin per mmol of catalyst per minute, and conversions up to 80%; when P(OMe)3 was used, the activity decreased but the normal: iso-aldehyde ratio increased. An important anion effect was observed, namely that all the chlorides were found to be inactive. In hydrogenation, activities are only moderate but unaffected by the nature of the counter ion. However, the [BF4]- salts caused isomerization of the olefins, while the chlorides did not. The iridium complexes were found to be inactive.