The reaction of the tetraphosphine ligand rac-et,ph-P4 (et,ph-P4 = PEt2CH2CH2(Ph)PCH2P(Ph)CH2CH2PEt2) with 2 equiv. of [Rh(nbd)2]+ (nbd = norbornadiene) generates [rac-Rh2(nbd)2(et,ph-P4)]2+, which is the precursor to a highly active and regioselective hydroformylation catalyst system. The “key” catalyst species generated under H2/CO pressure has been proposed to be the dihydrido complex [rac-Rh2H2(ì-CO)2(et,ph-P4)]2+, and represents one of the most dramatic example of homobimetallic cooperativity in homogeneous catalysis. The addition of even small amounts of PPh3 to this dirhodium tetraphosphine catalyst causes a dramatic drop in the aldehyde linear-to-branched regioselectivity (25:1 to 3:1) in acetone solvent (90 oC, 6.1 bar, 1-hexene). Catalytic results are presented for differing amounts of added PPh3, along with comparisons to the monometallic Rh catalyst family, HRh(CO)x(PPh3)y (x = 1-3; y = 3 – x), generated from PPh3 and Rh(acac)(CO)2 (acac = acetylactonate). The results point to the extremely effective inhibition of the regioselective bimetallic hydroformylation mechanism and the formation of an inefficient monometallic catalyst cycle, but not fragmentation to generate free HRh(CO)(PPh3)2 catalyst.
The deactivation of the dirhodium catalyst by CO has been previously demonstrated, and in situ NMR spectroscopic studies have indicated that facile fragmentation is occurring at 90 oC under H2/CO pressure, but also at 25 oC, albeit at a slower rate, which can lead to complete catalyst deactivation. We are trying to address this problem through the design of a novel, far more strongly chelating tetraphosphine ligand rac-et,ph-P4-Ph (et,ph-P4-Ph = PEt2(o-C6H4)P(Ph)CH2(Ph)P(o-C6H4)PEt2). Extensive experimentation has been conducted toward the preparation of this ligand via various synthetic routes centered around sequential deprotonation/alkylation reactions, Grignard-mediated substitutions, aromatic nucleophilic substitutions, and palladium-catalyzed phosphorus-carbon coupling reactions. While these synthetic strategies have given some encouraging results, the new tetraphosphine ligand has been successfully prepared via the rather simple and efficient Grignard-mediated phosphorus-carbon coupling reaction of ClP(Ph)CH2(Ph)PCl with PEt2(o-C6H4)MgBr. The nature of the ligand has been confirmed and characterized by single crystal X-ray crystallography as the nickel complex meso-Ni2Cl4(et,ph-P4-Ph).