Lanthanide(III)/actinide(III)differentiation in coordination of azine molecules to tris(cyclopentadienyl)complexes of cerium and uranium

摘要

Reaction of azine molecules L with the trivalent metallocenes [M(C_5H_4R)_3](M=Ce,U;R=Bu~t,SiMe_3) in toluene gave the Lewis base adducts[M(C_5H_4R)_3(L)](L=pyridine,3-picoline,3,5-lutidine,3-lutidine,3-chloropyridine,pyridazine,pyrimidine,pyrazine,3,5-dimethylpyrazine and s-triazine),except in the cases of M=U and L=3-chloropyridine,compounds of Ce(III) and U(III),i.e.[M(C_5H_4Bu~t)_3(L)](L=pyridine,picoline)and [M(C_5H_4SiMe_3)_3(L)](L=pyridine,lutidine,pyrimidine and dimethylpyrazine),the M-N and average M-C distances are longer for M=Ce than for M=U;however,within a series of azine adducts of the same methallocene,no significnat variationis noted in the M-N and average M-C distances.Theequilibria between [M(C_5H_4R)_3],L and [M(C_5H_4R)_3(L)] were studied by ~1H NMR spectroscopy.Th estability constants of the uranium complexes,K_UL,are greater than those of the cerium counterparts,K_CeL.The values of K_ML are much greater for R=SiMe_3 than for R=Bu~t and a liner correlation is foudn between the longarithms of K_ML and the hydrogen-bond basicity pK_(HB)seacle of the azines.thermodynamic parameters indicate that the enthalpy-entropy compensation effect holds for these complexation reacitions.Competition reactons of [Ce(C_5H_4R)_3] and [U(C_H_4R}_3] with L show that the selectivity of L in favour of U(III) increases with the pi donor character of the metallocene and is proporational to the pi accepting abilit of the azine molecule,measured by its reduction potential.