The synthesis, structures, and solution equilibria of bis(pyrrole-2-aldimino)metal(II) complexes

Abstract

Synthesis of an extensive series of bis(pyrrole-2 akiimino)metal(II) complexes with M(II)-Co, Ni, Pd, Cu, and Zn and various alkyl groups (R) appended to the azomethine nitrogen has been effected by a nonaqueous chelation reaction in tetrahydrofuran. Preliminary single crystal X-ray results for complexes with R=-butyl reveal that cobalt, nickel, and zinc complexes are isomorphous, but appreciable differences in the cell constants of the nickel complex indicate that it is not truly isostructural with the tetrahedral cobalt and zinc complexes. The copper complex exists in two crystalline modifications, neither of which is isomorphous with the Co—Ni-Zn series. Spectral and magnetic studies in solution show that the t-butyl cobalt complex is tetrahedral whereas the corresponding copper complex is distorted from planarity to an unknown extent. Copper complexes with less bulky R groups are planar. The -butyl nickel complex is pseudo-tetrahedral: complexes with see-alkyl groups such as isopropyl are involved in a configurational equilibrium between planar and pseudo-tetrahedral forms. The paramagnetic nickel complexes show large isotropic proton hyperfine contact shifts. Spin density calculations for the coordinated ligand system are used as the basis of proton resonance assignments. It is concluded that in the pseudo-tetrahedral form spin imbalance exists in the highest filled ligand r.M0 and that in addition there isan underlying vie. imbalance in the highest filled α-MO₃ the result of which is observable in the proton resonance spectra. Then modynamic parameters characterizing the structural change have been obtained for the nickel complexes from the temperature dependence of the proton contact shifts. A quantitative comparison of the stabilization of tetrahedral Ni(II) by pyrrole-2-aldimine, salicylaldimine, and β-ketoamine ligand systems is presented.