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• 4730-54-5 1,4,7-triazacyclononane 

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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]aneN3)2]3+ ([9]aneN3: 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 d7 ion. With PdII([9]aneN3)22+, electrochemical oxidation yields two one-electron cyclic voltammograms associated with oxidation to the Pd(III) and Pd(IV) states. Electrode potentials are pH dependent owing to protonation (producing Pd(HL)(L)n+ and Pd(HL)2m+ (n = 3, 4; m = 4, 5)) of one or both of the ligands at an uncoordinated nitrogen site. For the nonprotonated couples, E°(Pd([9]aneN3)23+/2+) = 0.370 V (vs NHE) in ClO4- or NO3- media and is about 30 mV higher in acetate media. For the corresponding Pd([9]aneN3)24+/3+ couple, E° = 0.640 V (vs NHE). A detailed analysis of the pH dependences is provided. An isotropic ESR signal, giso = 2.077 (77 K) (giso = 2.08 (298 K)) is observed at low ionic strength. However, in saturated Li2SO4, a pseudoaxial spectrum is derived, g⊥ = 2.115 and g∥ = 2.007 (A = 26.2 G), owing to ion-pair formation. Stopped-flow kinetic experiments have been carried out with both Pd([9]aneN3)23+ and Pd([9]aneN3)22+ as reagents. Rates of reduction of the Pd(III) ion with Co([9]aneN3)22+ and Co(sepulchrate)2+ and rates of oxidation of the Pd(II) ion with Ni([9]aneN3)23+ and Ni(Me[9]aneN3)23+, when analyzed by using a Marcus cross-correlation, led to a value of (2 ± 1) × 10-6 M-1 s-1 for the Pd([9]aneN3)23+/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 d8 Pd(II) ion. This rate constant is contrasted with that of (7 ± 3) × 102 M-1 s-1 for the Pd([9]aneN3)24+/3+ couple where both reagents are octahedral in configuration. The data are compared to other d7/d6 exchanges, and the steric role of the ligand in the electron-transfer process is described.

Crystal and solution structure of the palladium(II) bis(1,4,7-triazacyclononane) ion: Evidence for rapid fluxional behavior in a macrocyclic complex

An unusual example of fluxionality at the normally inert Pd(II) center is described. The preparation and X-ray crystal structure of the Pd(non)22+ ion (non = 1,4,7-triazacyclononane) are presented. Pd(non)2(PF6)2 crystallizes in the space group P21/c with cell dimensions of a = 7.150 (1) ?, b = 16.675 (6) ?, c = 9.542 (3) ?, and β = 97.37 (2)° (Z = 2). In the solid state the ion displays square-planar geometry, Pd-N = 2.057 ?, with the third (noncoordinated) nitrogen atom in a configuration remote (～3.5 ?) from the metal center and the rings in an anti conformation. However, in solution at ambient temperatures, 13C NMR shows only a single resonance, explicable only by a rapid ring hopping of the Pd(II) around the triaza macrocycle. This is considered to be the first example of such behavior for a Pd(II) amine complex. Temperature-dependence studies have permitted identification of syn and anti conformations in equilibrium (Keq ≈ 2). Rate data for the interconversion have been obtained, and a possible mechanism is proposed. The fluxional behavior of this ion explains the ready oxidation by air in aqueous medium to a very stable hexacoordinate Pd(III) ion.