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

Abstract

Single-site organolanthanum complexes supported on mesoporous silica nanoparticles, La{C(SiHMe2)3}n@MSNs, catalyze the ring-opening hydroboration reaction of aliphatic and styrenic epoxides with pinacolborane (HBpin). The surface-bound complexes, synthesized by reaction of the homoleptic tris(alkyl)lanthanum La{C(SiHMe2)3}3 and SBA-type MSN treated at 700 °C (MSN700), are mostly monopodal ≡SiO-La{C(SiHMe2)3}2 and contain an average of one bridging La↼H-Si per alkyl ligand. This structure was established through a combination of solid-state NMR (SSNMR) experiments, including J-resolved SiH coupling and quantitative 29Si measurements, diffuse reflectance IR, and elemental analysis. These rigorous analyses also established that grafting reactions in pentane provide a preponderance of ≡SiO-La{C(SiHMe2)3}2 sites and are superior to those in benzene and THF, and that grafting onto MSN treated at 550 °C (MSN550) results in a mixture of surface species. The single-site supported catalysts are more selective and in most cases more active than the homogeneous analogue, allow easy purification of products from the catalyst, are strongly resistant to leaching into solution phase, and may be recycled for reuse at least five times. After reaction of La{C(SiHMe2)3}n@MSN and HBpin, species including ≡SiO-La{C(SiHMe2)3}(H2Bpin) and ≡SiO-La{C(SiHMe2)3}{κ2-pinB-O(CMe2)2OBH3} are identified by detailed 1D and 2D 11B SSNMR experiments.ABSTRACT: Single-site organolanthanum complexes supported on mesoporous silica nanoparticles, La{C(SiHMe2)3}n@MSNs, catalyze the ring-opening hydroboration reaction of aliphatic and styrenic epoxides with pinacolborane (HBpin). The surface-bound complexes, synthesized by reaction of the homoleptic tris(alkyl)lanthanum La{C(SiHMe2)3}3 and SBA-type MSNs treated at 700 °C (MSN700), are mostly monopodal ≡SiO-La{C(SiHMe2)3}2 and contain an average of one bridging La↼H-Si per alkyl ligand. This structure was established through a combination of solid-state NMR (SSNMR) experiments, including J-resolved SiH coupling and quantitative 29 Si measurements, diffuse reflectance IR, and elemental analysis. These rigorous analyses also established that grafting reactions in pentane provide a preponderance of ≡SiO-La{C(SiHMe2)3}2 sites and are superior to those in benzene and THF, and that grafting onto MSN treated at 550 °C (MSN550) results in a mixture of surface species. The single-site supported catalysts are more selective and in most cases more active as the homogeneous analogue, allow easy purification of products from the catalyst, are strongly resistant to leaching into solution phase, and may be recycled for reuse at least five times. After reaction of La{C(SiHMe2)3}n@MSN and HBpin, species including ≡SiO-La{C(SiHMe2)3}(H2Bpin) and ≡SiO-La{C(SiHMe2)3}{κ 2 -pinB-O(CMe2)2OBH3} are identified by detailed 1D and 2D 11 B SSNMR experiments. AUTHOR INFORMATION Corresponding Authors

Reduction of Amides to Amines with Pinacolborane Catalyzed by Heterogeneous Lanthanum Catalyst La(CH2C6H4NMe2-o)3@SBA-15

“…Furthermore, hydrolysis of the resulting precipitate with CD 3 OD afforded BpinOD, a reaction outcome from HBpin with CD 3 OD (Figure S5 in the Supporting Information (SI)) . These observations showed that the composition of the resulting precipitate was a lanthanum hydride material like [La-H@SBA-15]·HBpin. ,, Remarkably, the resulting white precipitate, when isolated, exhibited activity toward amide reduction comparable to the active species generated in situ. In addition, the stoichiometric reaction between La­(CH 2 C 6 H 4 NMe 2 - o ) 3 @SBA-15 and C 6 H 4 CONH 2 in 1:5 molar ratio afforded CH 3 C 6 H 4 NMe 2 - o and a precipitate (Scheme b).…”

“…On the other hand, to overcome the drawbacks of the homogeneous catalysts in catalyst contamination in reaction outcomes and poor recyclability, increasing interest has been directed toward developing supported catalysts in surface organometallic chemistry (SOMC). − Such catalysts not only show the high activity of homogeneous catalysts but also display the ease of separation and recyclability of heterogeneous catalysts, allowing for better control of the whole synthetic process. Consequently, the immobilized heterogeneous rare-earth metal complexes have received much interest in SOMC and have demonstrated interesting performance in organic synthesis and polymerization. − Very recently, we developed the well-defined silica-grafted rare-earth metal benzyl materials Ln­(CH 2 C 6 H 4 NMe 2 - o ) 3 @SBA-15 (Scheme ) and found that they exhibited extremely high selectivity and good recyclable property in the transformation of primary arylsilanes into secondary arylsilanes through redistribution. , This interesting finding prompted us to explore their catalytic potential in other organic transformations. Inspired by the elegant work reported by the Marks group and the Chen group using rare-earth metal complexes Ln­[N­(SiMe 3 ) 2 ] 3 as catalysts in amide reduction, , it was envisaged that Ln­(CH 2 C 6 H 4 NMe 2 - o ) 3 @SBA-15 (Ln–C σ-bonded species) possesses promising catalytic performance in amide reduction owing to its bond disruption enthalpy comparable to the amide analogues (Ln–N σ-bonded complexes) .…”

Modeling Hydrogen Release from Water with Borane and Alane Catalysts: A Unified Reaction Valley Approach

“…A critical consideration in the catalytic H 2 production is the regeneration of the catalyst from the borate and aluminate byproducts, which is required to close the hydrogen cycle. This is very challenging due to the inefficiency in the process of breaking an O–B­(Al) bond in the byproduct to generate a B–H bond. − Utilizing the local mode analysis to inspect the intrinsic bond strength of O–B­(Al) bond(s) in the byproduct, we were able to shed some new light on the regenerability of the catalyst, which is a complex process and a comprehensive analysis is required, however, to further understand its proper mechanism.…”

For Whom a Puddle Is the Sea? Adsorption of Organic Guests on Hydrated MCM-41 Silica

“…A surface is a nanoscale film placed on a solid or liquid support. In the former case, whether it is about proteins on nanoparticles, − innovative materials, − heterogeneous catalysis, , or today’s practical application, − it is about specific effects induced by the surface of a rigid host on a mobile and/or flexible guest for predetermined purposes. Silica is probably the most common host for inorganic, organic, and biomolecules .…”