Chelate effect on the structure and reactivity of electron-rich palladium complexes and its relevance to catalysis

Article abstract of DOI:10.1021/om00029a025

In order to clarify the origin of the "chelate effect" in catalysis by palladium, complexes of Pr₂P(CH₂)_nⁱPr₂P (n = 2, dippe; n = 3, dippp; n = 4, dippb), Ph₂P(CH₂)₃PPh₂ (dppp), and PⁱPr₂ⁿBu were prepared and their structures, dynamic properties, and reactivities were compared. Pd(dippe)₂ 1d is a coordinatively saturated complex, both in solution and in the solid state. X-ray characterization exhibits a distorted tetrahedral geometry. The dippe bite angle is 87.05°. The compound crystallizes in the orthorhombic space group Pnna with a = 16.713(3) A?, b = 17.561(3) A?, c = 11.116(2) A?, V = 3277(1) A?³, Z = 4. Pd(dippp)₂ (1a) and Pd(dippb)₂ (1e) are coordinatively unsaturated, trigonal complexes and are in equilibrium with the binuclear complexes LPd(η²-L)PdL, 1b and 1f, respectively. Whereas 1d does not exhibit dynamic behavior, 1a and 1e undergo fast, intramolecular phosphine exchange, a process which is not observed with 1b and 1f. The trigonal complexes (dippp)PdPⁱPr₂Bu (1c) and (PⁱPr₂ⁿBu)₃Pd were also prepared for comparison. The dippp complexes 1a-1c react with aryl chlorides to produce cis-(dippp)Pd(C₆H₄X)Cl as the major product and trans-(η¹-dippp)₂Pd(C₆H₄X)Cl as the minor one (X = 4-OMe, 4-Me, H, 3-OMe, 4-COMe, 4-CHO, 4-NO₂). In contrast, the dippb complex 1e oxidatively adds chlorobenzene to yield only the trans complex (η¹-dippb)₂Pd(Ph)Cl. Reaction monitoring reveals that the cis and trans complexes are formed in parallel pathways. Cis/trans equilibrium is on the cis side for dippp and on the trans side for dippb. Reactivity toward chlorobenzene follows the trend Pd(dippp)₂ > Pd(PⁱPr₂ⁿBu)₃ ? Pd(dippe)₂ ? Pd(dppp)₂. These results are interpreted in terms of chelate stability, ligand basicity, concentration of the active 14e species and effect of the P-Pd-P angle on its reactivity. The dippp ligand is unique in that it is the only one of those studied which results in Pd(0) complexes which (a) exhibit high reactivity in oxidative addition and (b) form cis complexes preferentially.