Ligand exchange on pyrazolate-kappa N-mu-pyrazolate and pyrazole-kappa N-mu-pyrazolate binuclear nickel complexes

Compounds containing bridging pyrazolate and terminal pyrazolate or pyrazol e ligands of type Ni-2(PMe3)(2)(L-kappa N)(mu(2)-eta(1):eta(3)-CH2-o-C6H4)( mu(2)-L-kappa N,N') (L = 3,5-dimethylpyrazolate (Pz "), 3b, or 3,5-di-tert- butylpyrazolate (Pz**), 3c); Ni-2(PMe3)(2)(Pz**-kappa N)(mu(2-)eta(1);eta(3 )-CH2-o-C6H4)(mu(2)-Pz "-kappa N,N') , 4, and [Ni-2(PMe3)(2)(HPz "-kappa N) (mu(2)-eta(1):eta(3)-CH2-o-C6H4)(mu(2)-Pz-kappa NN')](X) (X = Br, 5, or BPh 4, 6) have been prepared. Spin saturation transfer experiments reveal a slo w motion of the terminal pyrazolate ligand of 3b and 4, consisting of a com bination of N,N-metallotropic shift and rotation around the Ni-N bond, but did not show any exchange of the bridging and terminal pyrazolate ligands. However, the mixed ligand derivative 4 can be obtained by conproportionatio n of 3b and Sc. This reaction reveals a slow intermolecular bridging-termin al as well as intermolecular pyrazolate exchange. In contrast with complexe s of type 3 and 4, the formally cationic 5 and 6 display dynamic NMR spectr a at room temperature due to their occurrence in solution as a mixture of r apidly exchanging isomers. Variable temperature NMR experiments have shown that 5 also undergoes a highly stereoselective HPz/Br- exchange.