RESTRICTED ROTATION ABOUT THE METAL-ARYL BOND IN PLATINUM-ARYL COMPLEXES OF CHIRAL DIPHOSPHINES

Square-planar bromo aryl, iodo aryl, and diaryl platinum(II) diphosphine complexes were prepared from the corresponding Pt(0) ethene complex by direct reaction with the haloarene, or from the Pt(II) dichloride complex by reaction with 2 equiv of aryllithium. These complexes exhibit restricted rotation about the metal-aryl bond in different ways. The diastereotopic pair of ortho hydrogens of the arylplatinum moiety in (DIOP)Pt(3,5-dibromophenyl)(I) (3) were sharp and distinct in the H-1 NMR spectrum below ambient temperature. (DIOP)Pt(2-methoxyphenyl)(I) (6) exists as two distinct diastereomers discernable in the ambient-temperature 1H NMR spectrum. The bis(a-methoxyphenyl) complexes behaved similarly, since three distinct diastereomers were observed in solution for both the DIOP and BINAP complexes 11 and 13 (but not for the CHIRAPHOS complex 12). Only the BINAP complex 13 is dynamic on the NMR time scale at ambient temperature. The X-ray structure of complex 13 in one stereoisomeric anti form is reported: Monoclinic, space group C2, unit cell dimensions a = 36.08(5) Angstrom, b = 12.12(1) Angstrom, c = 12.45(2) Angstrom, beta = 104.5(1)degrees, Z = 4. The structure was refined to an R value of 0.063 (R(w) = 0.087) for 3011 observed reflections. Examination of the corresponding DPPF complex 15 indicates separate syn and anti diastereomers. In the course of the synthetic work, significant differences between the reactivity of the ligands was encountered, and trends in rotation barriers with changing complex structure are discussed.