Synthesis and Characterization of Yttrium Methanediide Silanide Complexes

The salt metathesis reactions of the yttrium methanediide iodide complex [Y(BIPM)(I)(THF)2] (BIPM = {C(PPh2NSiMe3)2}) with the group 1 silanide ligand transfer agents MSiR3 (M = Na, R3 = tBu2Me or tBu3; M = K, R3 = (SiMe3)3) gave the yttrium methanediide silanide complexes [Y(BIPM)(SitBu2Me)(THF)] (1), [Y(BIPM)(SitBu3)(THF)] (2) and [Y(BIPM){Si(SiMe3)3}(THF)] (3). Complexes 1-3 provide rare examples of structurally authenticated rare earth metal-silicon bonds, and were characterized by single crystal X-ray diffraction, multinuclear NMR and ATR-IR spectroscopy, and elemental analysis. Density functional theory calculations were performed on 1-3 to probe their electronic structures further to reveal predominantly ionic Y–Si bonding. The computed Y–Si bonds show lower covalency than Y=C bonds, which are in turn best represented by Y+–C– dipolar forms due to the strong σ-donor properties of the silanide ligands investigated; these observations are in accord with experimentally-obtained 13C{1H} and 29Si{1H} NMR data for 1-3 and related Y(III) BIPM alkyl complexes in the literature. Preliminary reactivity studies were performed, with complex 1 treated separately with benzophenone, azobenzene and N,N′-dicyclohexyl-carbodiimide. 29Si{1H} and 31P{1H} NMR spectra of these reaction mixtures indicated that 1,2-migratory insertion of the unsaturated substrate into the Y–Si bond is favored, whilst for the latter substrate a [2 + 2] cycloaddition reaction also occurs at the Y=C bond to afford [Y{C(PPh2NSiMe3)2[C(NCy)2]-κ4C,N,N′,N′}{C(NCy)2(SitBu2Me)-κ2N,N′}] (4); these reactivity profiles complement and contrast with those of Y(III) BIPM alkyl complexes.