Homoleptic Trivalent Tris(alkyl) Rare Earth Compounds.

Homoleptic tris(alkyl) rare earth complexes Ln{C(SiHMe2)3}3 (Ln = La, 1a; Ce, 1b; Pr, 1c; Nd, 1d) are synthesized in high yield from LnI3THFn and 3 equiv of KC(SiHMe2)3. X-ray diffraction studies reveal 1a-d are isostructural, pseudo-C3-symmetric molecules that contain two secondary Ln↼HSi interactions per alkyl ligand (six total). Spectroscopic assignments are supported by comparison with Ln{C(SiDMe2)3}3 and DFT calculations. The Ln↼HSi and terminal SiH exchange rapidly on the NMR time scale at room temperature, but the two motifs are resolved at low temperature. Variable-temperature NMR studies provide activation parameters for the exchange process in 1a (ΔH⧧ = 8.2(4) kcal·mol-1; ΔS⧧ = -1(2) cal·mol-1K-1) and 1a-d9 (ΔH⧧ = 7.7(3) kcal·mol-1; ΔS⧧ = -4(2) cal·mol-1K-1). Comparisons of lineshapes, rate constants (kH/kD), and slopes of ln(k/T) vs 1/T plots for 1a and 1a-d9 reveal that an inverse isotope effect dominates at low temperature. DFT calculations identify four low-energy intermediates containing five ß-Si-H⇀Ln and one γ-C-H⇀Ln. The calculations also suggest the pathway for Ln↼HSi/SiH exchange involves rotation of a single C(SiHMe2)3 ligand that is coordinated to the Ln center through the Ln-C bond and one secondary interaction. These robust organometallic compounds persist in solution and in the solid state up to 80 °C, providing potential for their use in a range of synthetic applications. For example, reactions of Ln{C(SiHMe2)3}3 and ancillary proligands, such as bis-1,1-(4,4-dimethyl-2-oxazolinyl)ethane (HMeC(OxMe2)2) give {MeC(OxMe2)2}Ln{C(SiHMe2)3}2, and reactions with disilazanes provide solvent-free lanthanoid tris(disilazides).

Cerium-Catalyzed Hydrosilylation of Acrylates to Give α-Silyl Esters.

The homoleptic organocerium complex Ce{C(SiHMe2 )3 }3 (1) reacts with B(C6 F5 )3 to produce the zwitterionic bis(alkyl) hydridoborato Ce{C(SiHMe2 )3 }2 HB(C6 F5 )3 (2). NMR and IR spectroscopy and X-ray crystallography indicate that each alkyl ligand contains two bridging Ce↼H-Si interactions in both 1 and 2. Compound 2 serves as a precatalyst for the hydrosilylation of acrylates to give α-silyl esters at room temperature with a turnover number of 2200.

Magnesium-catalyzed hydrosilylation of α,ß-unsaturated esters.

ToMMgHB(C6F5)3 (1, ToM = tris(4,4-dimethyl-2-oxazolinyl)phenylborate) catalyzes the 1,4-hydrosilylation of α,ß-unsaturated esters. This magnesium hydridoborate compound is synthesized by the reaction of ToMMgMe, PhSiH3, and B(C6F5)3. Unlike the transient ToMMgH formed from the reaction of ToMMgMe and PhSiH3, the borate adduct 1 persists in solution and in the solid state. Crystallographic characterization reveals tripodal coordination of the HB(C6F5)3 moiety to the six-coordinate magnesium center with a ∠Mg-H-B of 141(3)°. The pathway for formation of 1 is proposed to involve the reaction of ToMMgMe and a PhSiH3/B(C6F5)3 adduct because the other possible intermediates, ToMMgH and ToMMgMeB(C6F5)3, react to give an intractable black solid and ToMMgC6F5, respectively. Under catalytic conditions, silyl ketene acetals are isolated in high yield from the addition of hydrosilanes to α,ß-unsaturated esters with 1 as the catalyst.

Direct hydrosilylation by a zirconacycle with ß-hydrogen.

Azasilazirconacycle Cp2Zr{κ(2)-N(SiHMe2)SiHMeCH2} (1) and formaldehyde react through an uncatalyzed addition reaction (hydrosilylation) to form an exocyclic methoxysilyl-substituted zirconacycle. Although 1 contains 2-center-2-electron SiH groups, this transformation parallels the reactions of non-classical [Cp2ZrN(SiHMe2)2](+) ([2](+)) with carbonyls. Reactions of 1 with a series of nucleophilic and electrophilic agents were explored, as well as reactions of related ß-SiH-containing silazidozirconium compounds, to develop a rationale for the unexpected hydrosilylation. For example, carbon monoxide and 1 react at the Zr-C bond to form Cp2Zr{κ(2)-OC(=CH2)SiHMeN(SiHMe2)} (7). The Lewis acid B(C6F5)3 also reacts at the Zr-C bond to give Cp2Zr{N(SiHMe2)SiHMeCH2B(C6F5)3} (8). OPEt3 and N,N-dimethylaminopyridine (DMAP) do not appear to interact with 1. In contrast, OPEt3 and DMAP react with non-classical compounds [2](+) and zwitterionic 8.