ATOMIC PROXIMITY DUE TO MOLECULAR CONGESTION - RATIONAL DESIGN OF BIS(PHOSPHITE) LIGANDS BY RESTRICTION OF MOLECULAR-MOTION

The synthesis and conformational analysis of the sterically congested bis(phosphite) ligand 2)O)(2)PO]C6H2}Et}-4,6-(t)Bu(2)C(6)H(2)O}(PhO)(2 )P (5) are reported. X-ray crystallographic, dynamic P-31{H-1} NMR, NO E, DNOE, CP-MAS P-31 NMR, and calculational studies of 5 as well as th e structurally related bis(phosphites) 1 and 6 suggest that the confor mational freedom of the molecule is severely restricted because of geo metric restraints due to steric congestion. A through-space mechanism of coupling is suggested to explain the observed eight-bond P-P J coup ling of 27.5 Hz in the P-31{H-1} NMR spectrum of 5, which is a result of the proximity of the two phosphorus atoms in 5. The results of this study support the contention that the restriction of molecular motion by steric congestion can be used to rationally design a ligand favori ng a particular disposition of phosphorus atoms.