Heteroleptic [M(CH₂C₆H₅)₂(I)(THF)₃] Complexes (M = Y or Er): Remarkably Stable Precursors to Yttrium and Erbium T-Shaped Carbenes

Heteroleptic dibenzyl yttrium and erbium iodides [Ln(Bn)2(I)(THF)3] [Ln = Y (1), Er (2); Bn = CH2C6H5] were prepared in high yields and are remarkable for their thermal stability and inertness toward ligand scrambling in Schlenk-type equilibria. A variable-temperature study of 1 revealed a dynamic process in solution attributed to the presence of three isomers, namely, cis-fac, cis-mer, and trans-mer, which were observed in a 0.11:1:0.05 ratio at 298 K, respectively. Only the isomer attributed as cis-mer was observed at 313 K. The synthetic utility of 1 and 2, which combines the potential benefits of protonolysis and salt elimination chemistry, was demonstrated by the facile synthesis of phosphorus-stabilized yttrium and erbium carbenes [Ln(BIPM)(I)(THF)2] [Ln = Y (3); Er (4); BIPM = {C(PPh2NSiMe3)}2−], which each contain unusual T-shaped carbene centers. DFT calculations on 3, BIPM, and Ph3PCPPh3 showed very similar frontier orbital compositions in all three examples. Although 3 and 4 are classified as carbene complexes, and NBO analysis is consistent with the BIPM ligand adopting the dipolar N−-P+-C2−-P+-N− resonance form, the possibility of categorizing 3 and 4 as captodative carbon(0) complexes of yttrium and erbium cannot be ruled out. Complexes 1−4 have been variously characterized by X-ray crystallography, multinuclear NMR spectroscopy, FTIR spectroscopy, room-temperature Evans method solution magnetic moments, and CHN microanalyses.