the intriguing behavior of an apparently simple coupling promoter ligand, PPh2(p-C6H4-C6F5), in their Pd complexes

Abstract

ABSTRACT: Ligand PPh2(p-C6H4-C6F5), LHF, is an example of monodentate biphenyl phosphine that allows for cis coor-dination of two phosphines to Pd, as in complex cis-[Pd(C6F5)2Pd(LHF)2] (A). At 25 °C complex A undergoes easy reductive elimination to decafluorobiphenyl and, competitively, isomerizes to trans-[Pd(C6F5)2Pd(LHF)2] via a 3-coordinated interme-diate [Pd(C6F5)2Pd(LHF)]. Unexpectedly the direct reductive C6F5–C6F5 elimination is faster from the 4-coordinated complex A than from the 3-coordinated intermediate. The reason of this is that two cis LHF ligands play the role of a chelate with a large bite angle with tetrahedral distortion. As a matter of fact, using LHF in excess gives ΔG‡Pf-Pf = 23.1 kcal.mol-1, which ranks its efficiency for coupling and formation of the corresponding Pd0 catalyst as better than XantPhos or PhPEWO-F, and about the same as tBuBrettPhos. On the other hand, complex (µ-Cl)2[Pd2(C6F5)2(LHF)2] (B), obtained by reaction of (µ-Cl)2[Pd2(C6F5)2(tht)2] with LHF, presents in the 19F NMR COSY spectrum a very intriguing through space coupling pattern of the Fortho atoms of the C6F5 group in LHF and the C6F3Cl2-3,5 group on Pd. The inter-mitent coupling mechanism proposed is based on the switching of π-π stacking of C6F5 with one or another Ph group of LHF, which gives rise to enantiomers at the chiral P atom. Rotation around the P–biphenyl bond under hindrance of rotation around the C–C6F5 bond produces the intriguing selective coupling observed.