Structural and dynamic studies in solution of anionic dinuclear azolato-bridged palladium(II) complexes

Detailed analysis of the 19F-NMR spectra of the boat-shaped complexes (NBu4)2[Pd2(μ-LL)2R4] (R=3,5-C6Cl2F3) (LL=pyrazolate (pz), 1; dimethylpyrazolate (dmpz), 2), HH- (head-to-head) and HT- (head-to-tail) (NBu4)2[Pd2(μ-LL)2R4] (LL=3-methylpyrazolate (mpz), 3; indazolate (indz), 4), (NBu4)2[Pd2(μ-dmpz)(μ-LL)R4] (LL=mpz, 6; indz, 7), and (NBu4)2[R2Pd(μ-LL)2Pd(C6F5)2] (LL=pz, 8; dmpz, 9) affords valuable structural information in solution. The substituents of the azolate ligands, which reduce the dihedral angle between the coordination planes of the two metals, place the endo Fortho of different PdR2 fragments at short distances. A single-crystal X-ray structure has been obtained for compound HT-4 and the distances between Fortho compared with that calculated from the NMR spectra. The study of the dynamic processes for these complexes reveals the involvement of different mechanisms such as: (a) rotation of the R groups, (b) boat–boat inversion of the six-membered Pd(μ-LL)2Pd ring, (c) σ-1,2-metallotropic shift, and (d) associative exchange of PdR2 fragments between 1 and mononuclear complexes.