EVALUATIONS OF A GENERAL NMR METHOD, BASED ON PROPERTIES OF HETEROPOLY BLUES, FOR DETERMINING RATES OF ELECTRON-TRANSFER THROUGH VARIOUS BRIDGES - NEW MIXED-MIXED VALENCE COMPLEXES

Several examples of a new and potentially convenient NMR method for quantitative evaluations of electron exchange rates through various conducting molecular bridges have been investigated, with a view to elucidating some limitations and some conditions for success. This necessitated making several members of a new major general category of heteropoly complexes: the mixed-mixed valence species, wherein delocalized ''blue'' electrons are rapidly exchanging (similar to 10(-10)-10(11) s(-1)) among addenda atoms (e.g., W's) in a heteropoly entity while the same blue electrons are exchanging at a much slower rate (e.g., 10(2)-10(4) s(-1)) through the conducting bridge to another heteropoly entity. NMR spectra of heteroatoms in the bridged complex are employed. In the fast exchange region line-width analysis of the coalesced exchange peaks reveals exchange constants. In favorable cases, these may also be calculated from coalescence temperatures. In the slow exchange region, reasonable assumptions can lead to approximate values. Variations with temperature yield activation parameters. Nine bridged complexes were studied. Effects of diamagnetic, paramagnetic, rigid, and flexible bridges were investigated. Bridged complexes derived from the Keggin structure proved unsatisfactory, but derivatives of the Wells-Dawson structure gave good results. When the bridge is flexible, various countercations can hold the complex in a syn configuration, leading to anomalous exchange results; but use of hydrated Li+ as the only metallic cation present allows the electrostatic repulsions of the highly charged heteropoly entities to maintain an anti configuration, which yields consistent exchange rates.