An yttrium-based system to evaluate Lewis base coordination to an electropositive metal in a metallocene environment

The unsolvated bimetallic yttrium complex (C5Me5)(2)Y(mu-Cl)Y(C5Me5)(2)Cl (1) provides a convenient platform upon which to compare the coordination chemistry of oxygen-donor ligands and monomers with Lewis acidic metal ions. Reaction of I with 2 equiv of oxygen-containing substrates formed the monomeric complexes (C5Me5)(2)YCl(L) (L = THF (2), benzophenone (3), methyl methacrylate (4), epsilon-caprolactone (5), hexamethylphosphoramide (6), c-caprolactam (7), 1-methyl-2-pyrrolidinone (8), N,N'-dimethylpropyleneurea (9)). Each of these readily crystallize, which allows comparison of the Y-O interaction in the solid state. The X-ray data show that the numerical values of the measured Y-O lengths decrease in the order THF > benzophenone > methyl methacrylate > epsilon-caprolactone > hexamethyl-phosphoramide > E-caprolactam > 1-methyl-2-pyrrolidinone > NW-dimethylpropyleneurea, although these bond lengths span only a short range and some are the same within experimental error. In the case of epsilon-caprolactam, a bis adduct, (C5Me5)(2)YCl(epsilon-caprolactam)(2) (10), can be isolated from the reaction of I and excess E-caprolactam. H-1 NMR spectroscopy indicates that (C5Me5)(2)YCl(L)/L' ligand displacement reactions in solution with 2-9 follow a trend consistent with the bond lengths except for (C5Me5)(2)YCl(epsilon-caprolactam) (7), in which L participates in hydrogen bonding to the chloride.