ONIOM study of the coordination chemistry of Ag+ with the nitrogen-bridged Ligands Ph2P-NH-PPh2 and Ph2P-NCH3-PPh2:: Ligand chelation versus bridging

The coordination chemistry of Ag+ with the nitrogen-bridged ligands H2P-NR-PH2 and Ph2P-NR-PPh2 (R = H, CH3) has been studied at the B3LYP/6-31G(d)/ECP level where an effective core potential (ECP) replaces the core electrons of silver, The ONIOM method (B3LYP/ECP:STO-3G*) is used to model the effect of replacing hydrogens on phosphorus with phenyl groups (Ph2P-NR-PPh2, R = H (dppa) and R = CH3 (dppma). Free energy calculations predict that [Ag-2(dppa)(2)](2+) is favored over [Ag(dppa)(2)](2+) (R = H. less replusion), while formation of [Ag(dppma)](2+) is favored over [Ag-2(dppma)(2)](2+) (R = CH3. more repulsion), which is in agreement with experimental observations. The complexes of Ag+ with H2P-NR-PH2, R = H, CH3, do not show differential behavior, indicating that the presence of phenyl groups is important in determining the balance between chelation and bridging. The relationship of the present results to the Thorpe-Ingold effect is discussed. Calculations on [Ag-2(dppa)(3)](2+) indicated that there is a significant barrier to racemization (D-3 --> D-3h), which is due to steric repulsion between phenyl groups in the transition state.