Homoleptic Trivalent Tris(alkyl) Rare Earth Compounds

Abstract

Homoleptic tris(alkyl) rare earth complexes Ln{C(SiHMe)} (Ln = La, 1a; Ce, 1b; Pr, 1c; Nd, 1d) are synthesized in high yield from LnITHF and 3 equiv of KC(SiHMe). X-ray diffraction studies reveal 1a-d are isostructural, pseudo-C-symmetric molecules that contain two secondary Ln↼HSi interactions per alkyl ligand (six total). Spectroscopic assignments are supported by comparison with Ln{C(SiDMe)} 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; ΔS = -1(2) cal·molK) and 1a-d (ΔH = 7.7(3) kcal·mol; ΔS = -4(2) cal·molK). Comparisons of lineshapes, rate constants (k/k), and slopes of ln(k/T) vs 1/T plots for 1a and 1a-d reveal that an inverse isotope effect dominates at low temperature. DFT calculations identify four low-energy intermediates containing five β-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(SiHMe) 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(SiHMe)} and ancillary proligands, such as bis-1,1-(4,4-dimethyl-2-oxazolinyl)ethane (HMeC(Ox)) give {MeC(Ox)}Ln{C(SiHMe)}, and reactions with disilazanes provide solvent-free lanthanoid tris(disilazides).

Silica-Supported Organolanthanum Catalysts for C–O Bond Cleavage in Epoxides

“…La{C(SiHMe2)3}3 was prepared by reaction of LaI3 and KC(SiHMe2)3}3. 34 SBA-type mesoporous silica nanoparticles (MSN) were prepared by P104-templated condensation of tetramethyl orthosilicate (TMOS) under acidic conditions. 42 The silica surface was partially dehydroxylated by heating under dynamic vacuum (1 mTorr) at 550 °C (MSN550) or 700 °C (MSN700) for 12 h. The surface areas and pore sizes, determined by N2 sorption, are 441 m 2 /g and 9.1 nm for MSN550 and 409 m 2 /g and 9.1 nm for MSN700 (see Table S1 in Supporting Information, which includes additional textural properties of both samples).…”

“…These assignments are aided by similar bands at 2110 cm -1 and 1829 cm -1 in the corresponding spectrum of 1, and supported by Hessian calculations of an energy-minimized gas-phase structure and the comparable solid-state structure determined by single-crystal X-ray diffraction. 34 The DRIFTS spectra indicate that at least some of the secondary interactions in 1 are also present after grafting reactions replacing one or two -C(SiHMe2)3 ligands with ≡Si-O-La linkages to the silica surface. The frequency of the La↼H-Si band in 1@MSN is higher than the corresponding signal in 1, which may reflect a higher ratio of H-Si to La↼H-Si in the surface species (see 1 JSiH discussion below).…”

“…The resonances labeled H1, H2, C1, C2 and Si1 can be assigned to 1@MSN (Figure 2A), based on the chemical shift values reported for 1 by solution-state NMR. 34 Note that C2 is represented by a single line in 1@MSN550 and 1@MSN700,…”

Interconverting Lanthanum Hydride and Borohydride Catalysts for C=O Reduction and C−O Bond Cleavage

“…[6] While hydroboration of aldehydes and ketones is catalyzed by alarge number of metal complexes, [7] fewer catalysts,m ainly magnesium [2f, 4, 8] and molybdenum compounds, [9] mediate hydroboration of esters. [11] In the present contribution, we demonstrate that the easily synthesized tris(alkyl)lanthanum compound La{C-(SiHMe 2 ) 3 } 3 (1) [12] catalyzes the hydroboration of esters and epoxides.W ith 5mol % 1 at room temperature,a lkyl esters, alkyl benzoates,a nd aryl benzoates are reduced to boryl esters.H ydrolysis gives the corresponding alcohol in high isolated yield (Table 1). [11] In the present contribution, we demonstrate that the easily synthesized tris(alkyl)lanthanum compound La{C-(SiHMe 2 ) 3 } 3 (1) [12] catalyzes the hydroboration of esters and epoxides.W ith 5mol % 1 at room temperature,a lkyl esters, alkyl benzoates,a nd aryl benzoates are reduced to boryl esters.H ydrolysis gives the corresponding alcohol in high isolated yield (Table 1).…”

Interconverting Lanthanum Hydride and Borohydride Catalysts for C=O Reduction and C−O Bond Cleavage

“…Moreover,h ydroboration of epoxides is uncommon, with reports of only an ickel catalyst (involving C À Oo xidative addition) [10] and an iron catalyst limited to aryl-substituted substrates. [11] In the present contribution, we demonstrate that the easily synthesized tris(alkyl)lanthanum compound La{C-(SiHMe 2 ) 3 } 3 (1) [12] catalyzes the hydroboration of esters and epoxides.W ith 5mol % 1 at room temperature,a lkyl esters, alkyl benzoates,a nd aryl benzoates are reduced to boryl esters.H ydrolysis gives the corresponding alcohol in high isolated yield (Table 1). Halogen-, nitro-, alkoxy-, and trifluoromethyl-substituted benzoate esters are reduced without affecting the functional group.O lefins are untouched, whereas La{N(SiMe 3 ) 2 } 3 is reported to catalyze addition of alkenes and HBpin.…”