An Yttrium-Based System to Evaluate Lewis Base Coordination to an Electropositive Metal in a Metallocene Environment

The unsolvated bimetallic yttrium complex (C5Me5)2Y(μ-Cl)Y(C5Me5)2Cl (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 1 with 2 equiv of oxygen-containing substrates formed the monomeric complexes (C5Me5)2YCl(L) (L = THF (2),
benzophenone (3), methyl methacrylate (4), ε-caprolactone (5), hexamethylphosphoramide
(6), ε-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 > ε-caprolactone > hexamethylphosphoramide > ε-caprolactam > 1-methyl-2-pyrrolidinone > N,N‘-dimethylpropyleneurea,
although these bond lengths span only a short range and some are the same within
experimental error. In the case of ε-caprolactam, a bis adduct, (C5Me5)2YCl(ε-caprolactam)2
(10), can be isolated from the reaction of 1 and excess ε-caprolactam. 1H NMR spectroscopy
indicates that (C5Me5)2YCl(L)/L‘ ligand displacement reactions in solution with 2−9 follow
a trend consistent with the bond lengths except for (C5Me5)2YCl(ε-caprolactam) (7), in which
L participates in hydrogen bonding to the chloride.