Comparisons between yttrium and titanium N-heterocyclic carbene complexes in the search for early transition metal NHC backbonding interactions

The d(0) yttrium N-heterocyclic carbene compound YL3 (L = OCMe2CH2[C{N(CHCH)NPr}]) has been made and structurally characterized. It adopts a mer configuration of the three bidentate ligands. A comparison of this with the isostructural d(1) titanium complex TiL3 is Made in order to seek experimental evidence of a pi-bonding contribution to the M-C bond. This has been augmented by DFT calculations. Experimentally, the metal radius-corrected Ti-C distance is shorter than the Y-C distance, suggesting a T-bonding contribution in the d(1) complex, but the computational data suggest that a shorter or bond might simply be formed by the more strongly polarizing titanium cation. From the potassium reduction of TiL(OPr')(3), only a byproduct arising from silicone grease activation was isolable, identified as a mixed-valent, multinuclear, d(0)/d(1) cluster [(TiL2)-L-III{Pr'OSiMe2O}K2OTiIV(OPr')(4)](2) in which the carbene ligands are bound to the Ti-III centers in preference to Ti-IV, with longer Ti-C distances than those found in TiL3.