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

Reaction of azine molecules L with the trivalent metallocenes [M(C5H4R)(3)] (M=Ce, U; R=Bu-t, SiMe3) in toluene gave the Lewis base adducts [M(C5H4R)(3)(L)] (L=pyridine, 3-picoline, 3,5-lutidine, 3-chloropyridine, pyridazine, pyrimidine, pyrazine, 3,5-dimethylpyrazine and s-triazine), except in the cases of M=U and L=3-chloropyridine, pyridazine, pyrazine and s-triazine where oxidation of U(III) was found to occur. In the pairs of analogous compounds of Ce(III) and U(III), i.e. [M(C5H4But)(3)(L)] (L=pyridine, picoline) and [M(C5H4SiMe3)(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 metallocene, no significant variation is noted in the M-N and average M-C distances. The equilibria between [M(C5H4R)(3)], L and [M(C5H4R)(3)(L)] were studied by H-1 NMR spectroscopy. The stability 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=SiMe3 than for R=Bu-t and a linear correlation is found between the logarithms of K-ML and the hydrogen-bond basicity pK(HB) scale of the azines. Thermodynamic parameters indicate that the enthalpy-entropy compensation effect holds for these complexation reactions. Competition reactions of [Ce(C5H4R)(3)] and [U(C5H4R)(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 proportional to the pi accepting ability of the azine molecule, measured by its reduction potential.