ELECTRON-TRANSFER .41. RATE ENHANCEMENT BY LIGANDS IN WHICH CONJUGATION IS INTERRUPTED

Seven new carboxylatocobalt(III) complexes, each with a pyridine ring separated from the -COOCoIII group by a saturated unit, have been prepared and their reductions with Cr2+ and Eu2+ studied. Specific rates are well above those for reduction of the ordinary aliphatic and aromatic carboxylato complexes of (NH3)5CoIII when, and only when, a ~C(⎓0)NH- group lies in the γ position on the ring. Rate enhancement for the Cr2+ reduction of complexes of the type Inic+-C-COOCom (II and III), in which a single carbon separates the ring and the carboxyl, are particularly striking, exceeding those reported for a number of complexes in which the two functions are in direct conjugation. The rapid europium(II) reductions exhibit strong autocatalytic components, reflecting catalysis of the primary reactions by the ligand released. In these cases, very nearly linear kinetic decay curves demonstrate the specific rate of the uncatalyzed component to be nearly equal to that for Eu2+ reduction of the free ligand, which is the initiation step in the catalytic sequence. The properties of the Cr(III) products from the rapid Cr2+ reductions correspond to those of a -COOCrIII rather than a -C(NH2)=OCrIII complex, indicating coordination of chromium to the carbonyl of the -COOCoIII in the activated complexes and ruling out a two-step internal catalytic mechanism. The structures of the very rapid oxidants preclude rate enhancement by chelation or by conjugation of the usual type but appear to allow intervention of a homoallylic type intermediate such as X (or one featuring through-space interaction), in which chromium or europium is bound both to the lead-in carbonyl and, by π interaction, to the activated pyridine ring. It is further suggested that the latter interaction occurs with preliminary, but reversible, electron transfer to the ring. Similar intermediates may intervene in the Eu(II) reductions of the parent ligands, which have been found to be about 102 times as rapid as that of the protonated form of isonicotinamide, in which an +NH function replaces +N-C-COOH. © 1979, American Chemical Society. All rights reserved.