Rational Design of Highly Active Hybrid Phosphine-Phosphinite Pincer Iridium Catalysts for Alkane Metathesis

Both the bisphosphine and bisphosphinite pincer complexes ((PCP)-P-tBu4)IrH2 and ((POCOP)-P-tBu4)IrH2 can cocatalyze alkane metathesis in tandem with olefin metathesis catalysts, but the two complexes have different resting states during catalysis, suggesting that different steps are turnover-limiting in each case. This led to the hypothesis that a complex with intermediate properties would be catalytically more active than either of these two species. Accordingly, hybrid phosphine-phosphinite pincer ligands (PCOP) and the corresponding iridium complexes were synthesized (3c-e). In tandem with olefin-metathesis catalyst MoF12, ((PCOP)-P-tBu4)IrH2 displays significantly higher activity for the metathesis of n-hexane than does ((PCP)-P-tBu4)IrH2 or ((PCOP)-P-tBu4)IrH2. ((PCOPiPr2)-P-tBu2)IrH4 (3d) is even more active (>30-fold more active than ((PCOP)-P-tBu4)IrH2) and affords nearly 4.6 M alkane products after 8 h at 125 degrees C.