Bis(1,4,7-triazacyclononane)palladium(III): characterization and reactions of an unusually stable monomeric palladium(III) ion

The synthesis of the monomeric palladium(III) ion (Pd((9)aneN/sub 3/)/sub 2/)/sup 3+/ ((9)aneN/sub 3/: 1,4,7-triazacyclononane) as the hexafluorophosphate salt is described. Solutions of this ion exhibit exceptional kinetic stability in aqueous media as shown by the ESR spectrum of the d/sup 7/ ion. With Pd/sup II/((9)aneN/sub 3/)/sub 2//sup 2+/, electrochemical oxidation yields two one-electron cyclic voltammograms associated with oxidation to the Pd(III) and Pd(IV) states. A detailed analysis of the pH dependences of the electrode potentials is provided. An isotropic ESR signal is observed at low ionic strength. However, in saturated Li/sub 2/SO/sub 4/, a pseudoaxial spectrum is derived, owing to ion-pair formation. Stopped-flow kinetic experiments have been carried out with both Pd((9)aneN/sub 3/)/sub 2//sup 3+/ and Pd((9)aneN/sub 3/)/sub 2//sup 2+/ as reagents. Rates of reduction of the Pd(III) ion with Co((9)aneN/sub 3/)/sub 2//sup 2+/ and Co(sepulchrate)/sup 2+/ and rates of oxidation of the Pd(II) ion with Ni((9)aneN/sub 3/)/sub 2//sup 3+/ and Ni(Me(9)aneN/sub 3/)/sub 2//sup 3+/, when analyzed by using a Marcus cross-correlation, led to a value of (2 /plus minus/ 1) /times/ 10/sup /minus/6/ M/sup /minus/1/ s/sup /minus/1/ for the Pd((9)aneN/sub 3/)/sub 2//sup 3+/2+/ couple. The origin of this exceptionally low rate lies in the inner-sphere reorganization required to provide axial interaction of the noncoordinated nitrogens with the normally square-planar d/sup 8/ Pd(II) ion. This rate constant is contrasted with that of (7 /plus minus/ 3) /times/ 10/sup 2/ M/sup /minus/1/ s/sup /minus/1/ for the Pd((9)aneN/sub 3/)/sub 2//sup 4+/3+/ couple where both reagents are octahedral in configuration. 32 references, 4 figures, 6 tables.