Heteropolynuclear Palladium Complexes with Pyrazolate and Its 3‐ tert ‐Butyl Derivatives: The Effect of Heterometal Ions on the Rate of Isomerization

The heteropolynuclear complexes [Pd 2 M′ 2 (μ‐pz) 6 ] (M′=Ag ( 1 ), Au ( 2 ); pzH=pyrazole), HT ‐[Pd 2 M′ 2 (μ‐3‐ t Bupz) 6 ] (M′=Ag ( 3 a ), Au ( 4 a ); 3‐ t BupzH=3‐ tert ‐butylpyrazole), and HH ‐[Pd 2 Au 2 (μ‐3‐ t Bupz) 6 ] ( 4 b ) have been prepared and some of them were structurally characterized. When 3‐ tert ‐butylpyrazolate was employed as a bridging ligand, two linkage isomers (head‐to‐tail ( HT ) and head‐to‐head ( HH )) arise from the difference in orientation of the substituent groups on the pyrazolate bridges between the two Pd atoms. 1 H NMR spectroscopy has been used to identify and to follow the reversible stereochemical rearrangement of the HH isomer of [Pd 2 Ag 2 (μ‐3‐ t Bupz) 6 ] ( 3 b ) to form the HT isomer 3 a in CDCl 3 and the HT isomer of [Pd 2 Au 2 (μ‐3‐ t Bupz) 6 ] ( 4 a ) to form the HH isomer 4 b in C 6 D 6 . Kinetic studies of the reaction have established the rate law to be −d( HH )/d t =d( HT )/d t = k 2 [ HH ]− k 1 [ HT ] for 3 b and −d( HT )/d t =d( HH )/d t = k 1 [ HT ]− k 2 [ HH ] for 4 a , where k 1 and k 2 denote the rate of isomerization from the HT to the HH isomer and that from the HH to the HT isomer, respectively. For typical runs at 50 °C in C 6 D 6 , k 1 =13.8×10 −5 s −1 , k 2 =18.6×10 −5 s −1 , and K eq = k 2 / k 1 =1.24 for 3 b , and k 1 =1.26×10 −5 s −1 , k 2 =3.52×10 −5 s −1 , and K eq = k 1 / k 2 =0.36 for 4 a . Temperature‐dependent rate measurements reveal Δ H ≠ and Δ S ≠ to be 100(1) kJ mol −1 and 0(3) J mol −1  K −1 for 3 b and 112(5) kJ mol −1 and 20(17) J mol −1  K −1 for 4 a , respectively. The rate of isomerization is essentially unaffected by the concentration of the complex or by the presence of neutral bridging ligands. These data and observations imply that the isomerization involves an intramolecular exchange process.