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ONIOM Study of the Coordination Chemistry of Ag⁺ with the Nitrogen-Bridged Ligands Ph₂P−NH−PPh₂ and Ph₂P−NCH₃−PPh₂: Ligand Chelation versus Bridging

Michael L. McKee* and William E. Hill

Department of Chemistry, Auburn University, Auburn, Alabama 36849

J. Phys. Chem. A, 2002, 106 (25), pp 6201–6205

DOI: 10.1021/jp020820t

Publication Date (Web): May 25, 2002

In papers with more than one author, the asterisk indicates the name of the author to whom inquiries about the paper should be addressed.

Abstract

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 [Ag₂(dppa)₂]²⁺ is favored over [Ag(dppa)]⁺ (R = H, less replusion), while formation of [Ag(dppma)]⁺ is favored over [Ag₂(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 [Ag₂(dppa)₃]²⁺ indicated that there is a significant barrier to racemization (D₃ → D₃_h), which is due to steric repulsion between phenyl groups in the transition state.