Head-to-head bis(α-pyridonato)-bridged bis(ethylenediamine) dipalladium(ii), HH-[Pd2(en)2(α-pyridonato) 2](ClO4)2, was synthesized and structurally characterized by X-ray crystallography. The 1H NMR spectra show that the head-to-head (HH) dimer produces the head-to-tail (HT) dimer and monomers ([Pd(en)(α-pyridone)2]2+, [Pd(en)(H 2O)(α-pyridone)]2+, [Pd(en)(H2O) 2]2+, etc.) in aqueous solution, and the relative amount of dimers to monomers is dependent on the total concentration of the HH dimer dissolved as well as the acidity of the solution. It was found that the formation of the HH and HT dimers from the monomers is fast, and the HT dimer is produced from the HH dimer only via coexisting monomers, i.e., there is no direct isomerization path between the HH and HT dimers. The kinetic analyses for the HH HT isomerization reaction with time-resolved 1H NMR measurements revealed that the reaction proceeds via first-order kinetics, which was explained based on a relaxation process. The rate determining step for HH HT isomerization is the reaction step between the mono-α-pyridone complex and the bis-α-pyridone complex, [Pd(en)(H2O)(α-pyridone)] 2+ + α-pyridone [Pd(en)(α-pyridone)2] 2+.
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