Hydroformylation Catalysis: Two Rhodium Centers vs. One Cobalt

In 1993 we reported a dirhodium tetraphosphine hydroformylation catalyst system that represented one of the best examples of bimetallic cooperativity in homogeneous catalysis (Science, 1993, 260, 1784).  Understanding this catalyst system turned out to be a long process of discovery and updated proposals on the nature of the catalyst (Angew. Chem. Int. Ed., 1996, 35, 2253; JACS, 2003, 125, 11180).  Extensive in situ IR and NMR studies, in conjunction with DFT calculations, have finally shed light on what turns out to be a far more complicated catalyst system spanning three different dirhodium catalysts with different charges, oxidation states, and reactivities.  The most active form of this catalyst, [Rh₂(m-H)₂(CO)₂(rac-Et,Ph-P4)]²⁺, has a significant fragmentation problem due to the tetraphosphine ligand not chelating strongly enough.  A redesigned, super-chelating tetraphosphine ligand initially appeared to solve the fragmentation problem and generated a more active and selective dirhodium hydroformylation catalyst.  This, however, had a new Rh-induced tetraphosphine cleavage side reaction that led to catalyst deactivation.  Studies with a dicationic dicobalt tetraphosphine variant showed surprising hydroformylation activity under mild conditions for cobalt hydroformylation.  Studies clearly indicated an absence of bimetallic cooperativity and a shift to much simpler monometallic analogs.  This new class of highly active cationic cobalt(II) bisphosphine catalysts have similar hydroformylation and alkene isomerization activity to HCo(CO)₄, but a far greater temperature/pressure stability range.  They are about 100 times more active relative to the neutral HCo(CO)₃(PR₃) catalysts used by Shell Chemical.  These high-spin Co(II) catalysts have the general form: [HCo(CO)_x(P₂)](BF₄), where x = 1-3, and P₂ = chelating bisphosphine.  Unlike all other hydroformylation catalysts known this new class has higher activity with electron-donating alkylated phosphine ligands.  The high activity of these catalysts allow them to operate under far lower temperatures and pressures relative to HCo(CO)₄ and HCo(CO)₃(PR₃).  They are especially effective at hydroformylating internal branched alkenes to produce linear aldehydes (Science, 2020, 367, 542; JACS, 2023, 145, 19715).