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

Square-planar bromo aryl, iodo aryl, and diaryl platinum(II) diphosphi ne 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 exhib it restricted rotation about the metal-aryl bond in different ways. Th e 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-te mperature 1H NMR spectrum. The bis(a-methoxyphenyl) complexes behaved similarly, since three distinct diastereomers were observed in solutio n for both the DIOP and BINAP complexes 11 and 13 (but not for the CHI RAPHOS complex 12). Only the BINAP complex 13 is dynamic on the NMR ti me 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 r efined to an R value of 0.063 (R(w) = 0.087) for 3011 observed reflect ions. Examination of the corresponding DPPF complex 15 indicates separ ate syn and anti diastereomers. In the course of the synthetic work, s ignificant differences between the reactivity of the ligands was encou ntered, and trends in rotation barriers with changing complex structur e are discussed.