Pd(II) Complexes with Polydentate Nitrogen Ligands. Molecular Recognition and Dynamic Behavior Involving Pd−N Bond Rupture. X-ray Molecular Structures of [{Pd(C6HF4)2}(bpzpm)] and [{Pd(η3-C4H7)}2(bpzpm)] (CF3SO3)2 [bpzpm = 4,6-Bis(pyrazol-1-yl)pyrimidine]

The ligands 4,6-bis(pyrazol-1-yl)pyrimidine (bpzpm) and 4,6-bis(4-methylpyrazol-1-yl)pyrimidine (Me−bpzpm) were synthesized and their reactions with some palladium derivatives explored. Mononuclear or dinuclear neutral or cationic complexes were obtained by reaction of the ligands with 1 or 2 equiv of Pd(C6XF4)2(cod) (cod = 1,5-cyclooctadiene; X = F, H) or the palladium fragment [Pd(η3-2-Me-C3H4)(S)2]+ (S = acetone). The reaction of the dinuclear derivatives with 1 equiv of the respective free ligand immediately led to the regeneration of the mononuclear complexes. Except in the case of the synthesis of [{Pd(C6HF4)2}{Pd(C6F5)2}(bpzpm)], where two similar metallic groups are present, all attempts to obtain dinuclear asymmetric complexes with two different palladium fragments failed. Instead, the dinuclear symmetric complexes were formed. This result could be considered as an example of molecular recognition with the ligand acting as a ditopic receptor. This behavior is comparable to chemical symbiosis but in this case applied to the ligand rather than to the metal center as occurs normally. The polyfluorophenyl rings are situated on average in a perpendicular orientation with respect to the coordination plane. Their restricted rotation results in several atropoisomers for the complexes with m-C6HF4. Different cross-reaction experiments were carried out, and these showed the mobility of the metallic fragments, with the more difficult process being that involving the more strongly bonded polyfluorophenyl palladium groups. By means of 1H NMR variable temperature studies, the interconversion of the two isomers of [{Pd(η3-C4H7)}2(bpzpm)]Tf2 (Tf = CF3SO3) was analyzed. In the case of [{Pd(η3-C4H7)}(bpzpm)]Tf the existence of two processes, an intramolecular apparent allyl rotation and an intermolecular exchange of the allylpalladium fragments, has been demonstrated. Different ΔG c ⧧ values at the coalescence temperatures have also been determined. An X-ray single-crystal analysis was carried out on [{Pd(η3-C4H7)}2(bpzpm)]Tf2, which crystallizes in the monoclinic system, space group I2/m, with a = 9.368(2), b = 16.191(3), c = 20.228(6) Å, β = 101.26(3), and Z = 4. Compound [{Pd(C6HF4)2}(bpzpm)] crystallized in the triclinic system, space group P1̄, with a = 8.845(6), b = 12.6609(9), c = 12.826(3) Å, α = 88.45(2), β = 74.36(3), γ = 89.32(2), and Z = 2.