Lubricants for Rigid Magnetic Media Based upon Cyclotriphosphazenes: Interactions with Lewis Acid Sites

Abstract

Lubricants such as polyperfluorinated ethers are used topically on rigid magnetic media, or computer disks, to provide a low friction interface. These materials are subject to degradation particularly in the presence of Lewis acids; however, the degradation may in part be mitigated by the addition of additives such as bis(4-fluorophenoxy)tetrakis(3-(trifluoromethyl)phenoxy)cyclotriphosphazene, [(NP)3(OC6H4CF3)4(OC 6H4F)2], or X-1P. This report explores how X-1P may preferentially interact with Lewis acid sites to impart protection to the host lubricant. Therefore, ab initio theoretical studies on the Lewis acid-base interactions between cyclotriphosphazene derivatives and AlF3 are performed. The theoretical results indicate that the strongest binding between X-1P and AlF3 is achieved when the endocyclic nitrogen atom bonds to the aluminum, providing a binding energy of the order of -55 kcal/mol. The magnitude of the binding energy indicates significant bonding as opposed to a dipole-dipole attraction. The lone pair of electrons on the phenoxy substituents also provide strong binding to AlF3, although not to the extent the nitrogen atoms do, with binding energies near -37 kcal/mol. Binding to fluorine is considerably smaller, near -8 kcal/mol. The population analyses indicate that the preferred nitrogen interaction involves the p-orbital that contains the in-plane lone pair of electrons. A computed reaction coordinate with [NPH2]3 and AlF3 gives every indication that the in-plane interaction is strongest and most stable on the potential energy surface. There is little desire on the part of the N3P3 ring itself to interact with Lewis acid sites parallel to the plane of the ring, i.e., with the p-orbitals housing the lone pair of electrons perpendicular to the plane of the ring. These orbitals instead provide weak π bonding with the phosphorus d-orbitals and are energetically well below the HOMO. The reason for the strong binding between AlF3 and the ring nitrogen atom originates from the polar, almost zwitterionic character of the endocyclic P-N bond, which polarizes the nitrogen atom negatively. All data lead to the conclusion that if the ring nitrogen in X-1P is sterically accessible, this will be the preferred binding site.