Comparisons between Yttrium and Titanium N-Heterocyclic Carbene Complexes in the Search for Early Transition Metal NHC Backbonding Interactions

The d0 yttrium N-heterocyclic carbene compound YL3 (L = OCMe2CH2[C{N(CHCH)NPri}]) has been made and structurally characterized. It adopts a mer configuration of the three bidentate ligands. A comparison of this with the isostructural d1 titanium complex TiL3 is made in order to seek experimental evidence of a π-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 π-bonding contribution in the d1 complex, but the computational data suggest that a shorter σ bond might simply be formed by the more strongly polarizing titanium cation. From the potassium reduction of TiL(OPri)3, only a byproduct arising from silicone grease activation was isolable, identified as a mixed-valent, multinuclear, d0/d1 cluster [TiIIIL2{PriOSiMe2O}K2OTiIV(OPri)4]2 in which the carbene ligands are bound to the TiIII centers in preference to TiIV, with longer Ti−C distances than those found in TiL3.