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 qu antitative evaluations of electron exchange rates through various cond ucting molecular bridges have been investigated, with a view to elucid ating some limitations and some conditions for success. This necessita ted making several members of a new major general category of heteropo ly complexes: the mixed-mixed valence species, wherein delocalized ''b lue'' 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 sa me 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 th e fast exchange region line-width analysis of the coalesced exchange p eaks 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 wi th temperature yield activation parameters. Nine bridged complexes wer e studied. Effects of diamagnetic, paramagnetic, rigid, and flexible b ridges were investigated. Bridged complexes derived from the Keggin st ructure proved unsatisfactory, but derivatives of the Wells-Dawson str ucture gave good results. When the bridge is flexible, various counter cations can hold the complex in a syn configuration, leading to anomal ous exchange results; but use of hydrated Li+ as the only metallic cat ion present allows the electrostatic repulsions of the highly charged heteropoly entities to maintain an anti configuration, which yields co nsistent exchange rates.