Synthesis, structure and hydrosilylation activity of neutral and cationic rare-earth metal silanolate complexes

Abstract

Rare-earth metal alkyl tri(tert-butoxy)silanolate complexes [Ln{μ,η²-OSi(O^tBu)₃}(CH₂SiMe₃)₂]₂ (Ln = Y (1), Tb (2), Lu (3)) were prepared via protonolysis of the appropriate tris(alkyl) complex [Ln(CH₂SiMe₃)₃(thf)₂] with tri(tert-butoxy)silanol in pentane. Crystal structure analysis revealed a dinuclear structure for 1 with square pyramidal geometry at the yttrium centre. The silanolate ligand coordinates in an η²-bridging coordination mode giving a 4-rung truncated ladder and non-crystallographic inversion centre. Addition of two equiv. of 12-crown-4 to a pentane solution of 1 or 3 respectively gave [Ln{OSi(O^tBu)₃}(CH₂SiMe₃)₂(12-crown-4)]·12-crown-4 (Ln = Y (4), Lu (5)). Crystal structure analysis of 5 showed a slightly distorted octahedral geometry at the lutetium centre. The silanolate ligand adopts an η¹-terminal coordination mode, whilst the crown ether unit coordinates in an unusual κ³-fashion. Reaction of 1–3 with [NEt₃H]⁺[BPh₄]⁻ in thf yielded the cationic derivatives [Ln{OSi(O^tBu)₃}(CH₂SiMe₃)(thf)₄]⁺[BPh₄]⁻ (Ln = Y (6), Tb (7) and Lu (8)); coordination of crown ether led to compounds of the form [Ln{OSi(O^tBu)₃}(CH₂SiMe₃)(L)(thf)_n]⁺[BPh₄]⁻ (Ln = Y, Lu, L = 12-crown-4, n = 1 (9, 10); Ln = Y, Lu, L = 15-crown-5, n = 0 (11, 12)). Reaction of 1 with [NMe₂PhH]⁺[B(C₆F₅)₄]⁻, [Al(CH₂SiMe₃)₃] or BPh₃ in thf gave the ion pairs [Y{OSi(O^tBu)₃}(CH₂SiMe₃)(thf)₄]⁺[A]⁻ ([A]⁻ = [B(C₆F₅)₄]⁻ (13), [Al(CH₂SiMe₃)₄]⁻ (14), [BPh₃(CH₂SiMe₃)]⁻ (15)), whilst two equiv. [NMe₂PhH]⁺[BPh₄]⁻ with 1 in thf produced the dicationic ion triple [Y{OSi(O^tBu)₃}(thf)₆]²⁺[BPh₄]⁻₂ (16). Crystal structure analysis revealed that 16 is mononuclear with pentagonal bipyramidal geometry at the yttrium centre. The silanolate ligand coordinates in an η¹-terminal fashion. All diamagnetic compounds have been characterized by NMR spectroscopy. 1, 3, 4, 6 and 13 were tested as olefin hydrosilylation pre-catalysts with a variety of substrates; 1 was found to be highly active in 1-decene hydrosilylation.