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

Abstract

The heteropolynuclear complexes [Pd(2)M'(2)(mu-pz)(6)] (M'=Ag (1), Au (2); pzH=pyrazole), HT-[Pd(2)M'(2)(mu-3-tBupz)(6)] (M'=Ag (3 a), Au (4 a); 3-tBupzH=3-tert-butylpyrazole), and HH-[Pd(2)Au(2)(mu-3-tBupz)(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)(mu-3-tBupz)(6)] (3 b) to form the HT isomer 3 a in CDCl(3) and the HT isomer of [Pd(2)Au(2)(mu-3-tBupz)(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)/dt=d(HT)/dt=k(2)[HH]-k(1)[HT] for 3 b and -d(HT)/dt=d(HH)/dt=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 degrees C in C(6)D(6), k(1)=13.8x10(-5) s(-1), k(2)=18.6x10(-5) s(-1), and K(eq)=k(2)/k(1)=1.24 for 3 b, and k(1)=1.26x10(-5) s(-1), k(2)=3.52x10(-5) s(-1), and K(eq)=k(1)/k(2)=0.36 for 4 a. Temperature-dependent rate measurements reveal DeltaH(not equal) and DeltaS(not equal) 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.

Coordination Versatility of Pyrazole‐Based Ligands towards High‐Nuclearity Transition‐Metal and Rare‐Earth Clusters

“…Molecular structure of [Eu 4 (L1m) 8 ]. [86] Tet-ranuclear heterobimetallic compounds with palladium(II) ions and silver(I) or gold(I) ions have also been reported, [52,87] as well as those with zinc(II) and sodium(I) ions. In these compounds, the pyrazolate anion bridges two dinuclear units.…”

“…A tetranuclear zirconium(IV) compound is obtained by conversion of a dinuclear compound in wet toluene 86. Tetranuclear heterobimetallic compounds with palladium(II) ions and silver(I) or gold(I) ions have also been reported,52,87 as well as those with zinc(II) and sodium(I) ions 88. Some of the tetranuclear compounds that are reported in the literature were minor side products that were only characterized by X‐ray crystallography 89,90…”

Pyrazoles and pyrazolides—flexible synthons in self-assembly

“…These isomerise in solution, by a process that does not involve the other pyrazolido ligands in the molecules. 143 Treatment of [Pd 2 M 2 (m-pz Me2 ) 6 ] (M + = Cu + , Ag + or Au + ) with another equivalent of AgClO 4 yields [{Pd 2 M 2 (k 1 :k 1 -mpz Me2 ) 4 (k 1 :k 1 :h 1 -m 3 -pz Me2 ) 2 } 2 Ag 2 (m-ClO 4 ) 2 ], in which the additional silver ions chelate to the intact clusters through h 1 p-interactions to the C4 atoms of one of the [PdM(m-pz Me2 ) 2 ] + units (Scheme 4). 142 While [Pt 2 Ag 4 (m-pz) 8 ] does not luminesce, [Pt 2 M 4 (m-pz Me2 ) 8 ] emits orange light when M + = Cu + and blue light when it is Ag + , with quantum yields approaching 1 in the solid-state.…”