Lanthanide alkyl and silyl compounds: synthesis, reactivity and catalysts for green chemistry

The last few decades have witnessed enormous research in the field of organometallic lanthanide chemistry. Our research group has developed a few rare earth alkyl compounds containing tris(dimethylsilyl)methyl ligand and explored their reactivity. This thesis focusses on extending the study of lanthanide alkyl and silyl compounds to develop strategies for their synthesis and explore their reactivity and role as catalysts in processes such as hydrosilylation and cross-dehydrocoupling. Two novel ligands, alkyl, –C(SiHMe2)3 and silyl, –Si(SiHMe2)3 have been used to synthesize homoleptic organometallic lanthanide complexes. The silyl anion, KSi(SiHMe2)3, is prepared from the reaction of KOtBu and Si(SiHMe2)4. A single crystal X-ray diffraction study shows that KSi(SiHMe2)3 crystallizes as a chain of alternating K cations and Si(SiHMe2)3 anions with K coordinated to the central Si atoms and the three Si-H moieties oriented toward the next K atom. A series of lanthanum, cerium, praseodymium, neodymium, and samarium alkyl compounds and ytterbium and yttrium silyl compounds are synthesized and their characterization, reactivity and role as catalysts are described. These compounds are synthesized by salt metathesis reaction between the metal halide and an equiv. amount of KC(SiHMe2)3 and KSi(SiHMe2)3 ligands. The lanthanide tris(alkyl) compounds are solvent free compounds while lanthanide bis(silyl) compounds are THF coordinated and unstable at room temperature. All these compounds are highly air- and moisture-sensitive. Interestingly, spectroscopic characterization and X-ray analysis reveal that all the lanthanide alkyl compounds contain classical and non-classical β-SiH interactions with the metal center and undergo β-SiH abstraction by Lewis acids, such as B(C6F5)3 while the lanthanide silyl compounds lack such non-classical interactions with the metal center. The reactions of Ln{C(SiHMe2)3}3 (Ln = La, Ce, Pr, Nd) with one and two equiv. of B(C6F5)3 gives Ln{C(SiHMe2)3}2HB(C6F5)3, and LnC(SiHMe2)3{HB(C6F5)3}2, respectively and an equiv. amount of 1,3-disilacyclobutane dimer, {Me2Si-C(SiHMe2)2}2 as the by-product. The monocations, Ln{C(SiHMe2)3}2HB(C6F5)3 are used as catalysts for hydrosilylation of α,β- unsaturated esters to selectively yield α-silyl esters. α-Silyl esters are isolated in high yields from a range of α,β-unsaturated esters and hydrosilanes. The divalent bis(alkyl) lanthanide compounds, Ln{C(SiHMe2)3}2THF2 (Ln = Yb, Sm) are synthesized by salt metathesis of lanthanide halide and two equiv. of KC(SiHMe2)3 in THF. Reactions with one or two equiv. of B(C6F5)3 generate LnC(SiHMe2)3HB(C6F5)3 and an equiv. amount of 1,3-disilacyclobutane dimer, {Me2Si-C(SiHMe2)2}2. Ln{C(SiHMe2)3}2THF2 undergoes reaction with 1,3-di-tert-butylimidazol-2-ylidene (ImtBu) to yield Ln{C(SiHMe2)3}2ImtBu in non-polar solvent. A single crystal X-ray diffraction and spectroscopic study of Ln{C(SiHMe2)3}2THF2 and Ln{C(SiHMe2)3}2ImtBu reveal the presence of classical and non-classical interactions with the metal center. Ln{C(SiHMe2)3}2ImtBu (Ln = Yb, Sm) is an efficient catalyst for cross-dehydrocoupling of organosilanes and amines to yield silazanes at room temperature in high yields. Kinetic studies of the catalytic system indicate a first-order dependence on silane and amine concentrations. Lanthanide silyl compounds, Yb{Si(SiHMe2)3}2THF3 and Cl3Y{Si(SiHMe2)3}2(Et2O)].2K(Et2O)2 are synthesized by salt metathesis of lanthanide halide and two or three equiv. of KSi(SiHMe2)3 ligand. Yb{Si(SiHMe2)3}2THF3 is the first example of homoleptic Ln(II) silyl compound characterized by X-ray diffraction having trigonal bipyramidal geometry around the ytterbium center. Yb{Si(SiHMe2)3}2THF3 reacts with ancillary ligand, TlToM to yield ToM2Yb revealing lability of the silyl ligand

Homoleptic Divalent Dialkyl Lanthanide-Catalyzed Cross-Dehydrocoupling of Silanes and Amines

The rare-earth bis(alkyl) compound Sm{C(SiHMe2)3}2THF2 (1b) is prepared by the reaction of samarium(II) iodide and 2 equiv of KC(SiHMe2)3. This synthesis is similar to that of previously reported Yb{C(SiHMe2)3}2THF2 (1a), and compounds 1a,b are isostructural. Reactions of 1b and 1 or 2 equiv of B(C6F5)3 afford SmC(SiHMe2)3HB(C6F5)3THF2 (2b) or Sm{HB(C6F5)3}2THF2 (3b), respectively, and 1,3-disilacyclobutane {Me2Si-C(SiHMe2)2}2 as a byproduct. Bands from 2300 to 2400 cm–1 assigned to νBH in the IR spectra and highly paramagnetically shifted signals in the 11B NMR spectra of 2b and 3b provided evidence for Sm-coordinated HB(C6F5)3. Compounds 1a,b react with the bulky N-heterocyclic carbene (NHC) 1,3-di-tert-butylimidazol-2-ylidene (ImtBu) to displace both THF ligands and give three-coordinate monoadducts Ln{C(SiHMe2)3}2ImtBu (Ln = Yb (4a), Sm (4b)). Complexes 4a,b catalyze cross-dehydrocoupling of organosilanes with primary and secondary amines at room temperature to give silazanes and H2, whereas 1a,b are not effective catalysts under these conditions. Second-order plots of ln{[Et2NH]/[Ph2SiH2]} vs time for 4a-catalyzed dehydrocoupling are linear and indicate first-order dependences on silane and amine concentrations. However, changes in the experimental rate law with increased silane concentration or decreased amine concentration reveal inhibition by silane. In addition, excess ImtBu or THF inhibit the reaction rate. These data, along with the structures of 4a,b, suggest that the bulky carbene favors low coordination numbers, which is important for accessing the catalytically active species

Magnesium-catalyzed hydrosilylation of a,b-unsaturated esters

ToMMgHB(C6F5)3 (1, ToM = tris(4,4-dimethyl-2-oxazolinyl)phenylborate) catalyzes the 1,4-hydrosilylation of α,β-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,β-unsaturated esters with 1 as the catalyst

Zwitterionic Trivalent (Alkyl)Lanthanide Complexes in Ziegler-type Butadiene Polymerization

The organolanthanide complexes Ln{C(SiHMe2)3}3 (Ln = La, 1a; Ce, 1b; Pr, 1c; Nd, 1d) react with one or two equiv. of B(C6F5)3 to yield the well-defined zwitterionic species Ln{C(SiHMe2)3}2HB(C6F5)3 (Ln = La, 2a; Ce, 2b; Pr, 2c; Nd, 2d) or LnC(SiHMe2)3{HB(C6F5)3}2 (Ln = La, 3a; Ce, 3b; Pr, 3c; Nd, 3d), respectively. These complexes are shown to contain labile, bridging Si–H⇀Ln and o-F-\u3eLn interactions based upon the observation of low one-bond silicon-hydrogen coupling constants (1JSiH) in 1H NMR spectra of 2a and 3a, the presence of one set of C6F5 signals in the 19F NMR spectra of 2a and 3a, the detection of only m-F and p-F resonances in 19F NMR spectra of 2b-d and 3b-d, two νSiH bands in IR spectra of 2a-d, and X-ray crystallography analyses of 2b and 3d. In addition, a hexametertoluene molecule is coordinated to neodymium in 3d. Reactions of 1a and (AlMe3)2 yield labile adducts with an approximate stoichiometry of 1a·3AlMe3. Exchange between free and bound AlMe3 groups was observed in EXSY NMR experiments with greater than 3 equiv. of AlMe3. Compounds 2a-d and 3a-d, in the presence of AliBu3, are precatalysts for polymerization of butadiene. The neodymium alkyl 3d has the highest activity of the series, and its performance is consistent with chain transfer reactions with AliBu3

Lanthanide alkyl and silyl compounds: synthesis, reactivity and catalysts for green chemistry

The last few decades have witnessed enormous research in the field of organometallic lanthanide chemistry. Our research group has developed a few rare earth alkyl compounds containing tris(dimethylsilyl)methyl ligand and explored their reactivity. This thesis focusses on extending the study of lanthanide alkyl and silyl compounds to develop strategies for their synthesis and explore their reactivity and role as catalysts in processes such as hydrosilylation and cross-dehydrocoupling. Two novel ligands, alkyl, –C(SiHMe2)3 and silyl, –Si(SiHMe2)3 have been used to synthesize homoleptic organometallic lanthanide complexes. The silyl anion, KSi(SiHMe2)3, is prepared from the reaction of KOtBu and Si(SiHMe2)4. A single crystal X-ray diffraction study shows that KSi(SiHMe2)3 crystallizes as a chain of alternating K cations and Si(SiHMe2)3 anions with K coordinated to the central Si atoms and the three Si-H moieties oriented toward the next K atom. A series of lanthanum, cerium, praseodymium, neodymium, and samarium alkyl compounds and ytterbium and yttrium silyl compounds are synthesized and their characterization, reactivity and role as catalysts are described. These compounds are synthesized by salt metathesis reaction between the metal halide and an equiv. amount of KC(SiHMe2)3 and KSi(SiHMe2)3 ligands. The lanthanide tris(alkyl) compounds are solvent free compounds while lanthanide bis(silyl) compounds are THF coordinated and unstable at room temperature. All these compounds are highly air- and moisture-sensitive. Interestingly, spectroscopic characterization and X-ray analysis reveal that all the lanthanide alkyl compounds contain classical and non-classical β-SiH interactions with the metal center and undergo β-SiH abstraction by Lewis acids, such as B(C6F5)3 while the lanthanide silyl compounds lack such non-classical interactions with the metal center. The reactions of Ln{C(SiHMe2)3}3 (Ln = La, Ce, Pr, Nd) with one and two equiv. of B(C6F5)3 gives Ln{C(SiHMe2)3}2HB(C6F5)3, and LnC(SiHMe2)3{HB(C6F5)3}2, respectively and an equiv. amount of 1,3-disilacyclobutane dimer, {Me2Si-C(SiHMe2)2}2 as the by-product. The monocations, Ln{C(SiHMe2)3}2HB(C6F5)3 are used as catalysts for hydrosilylation of α,β- unsaturated esters to selectively yield α-silyl esters. α-Silyl esters are isolated in high yields from a range of α,β-unsaturated esters and hydrosilanes. The divalent bis(alkyl) lanthanide compounds, Ln{C(SiHMe2)3}2THF2 (Ln = Yb, Sm) are synthesized by salt metathesis of lanthanide halide and two equiv. of KC(SiHMe2)3 in THF. Reactions with one or two equiv. of B(C6F5)3 generate LnC(SiHMe2)3HB(C6F5)3 and an equiv. amount of 1,3-disilacyclobutane dimer, {Me2Si-C(SiHMe2)2}2. Ln{C(SiHMe2)3}2THF2 undergoes reaction with 1,3-di-tert-butylimidazol-2-ylidene (ImtBu) to yield Ln{C(SiHMe2)3}2ImtBu in non-polar solvent. A single crystal X-ray diffraction and spectroscopic study of Ln{C(SiHMe2)3}2THF2 and Ln{C(SiHMe2)3}2ImtBu reveal the presence of classical and non-classical interactions with the metal center. Ln{C(SiHMe2)3}2ImtBu (Ln = Yb, Sm) is an efficient catalyst for cross-dehydrocoupling of organosilanes and amines to yield silazanes at room temperature in high yields. Kinetic studies of the catalytic system indicate a first-order dependence on silane and amine concentrations. Lanthanide silyl compounds, Yb{Si(SiHMe2)3}2THF3 and Cl3Y{Si(SiHMe2)3}2(Et2O)].2K(Et2O)2 are synthesized by salt metathesis of lanthanide halide and two or three equiv. of KSi(SiHMe2)3 ligand. Yb{Si(SiHMe2)3}2THF3 is the first example of homoleptic Ln(II) silyl compound characterized by X-ray diffraction having trigonal bipyramidal geometry around the ytterbium center. Yb{Si(SiHMe2)3}2THF3 reacts with ancillary ligand, TlToM to yield ToM2Yb revealing lability of the silyl ligand.</p

Homoleptic Divalent Dialkyl Lanthanide-Catalyzed Cross-Dehydrocoupling of Silanes and Amines

The rare-earth bis(alkyl) compound Sm{C(SiHMe2)3}2THF2 (1b) is prepared by the reaction of samarium(II) iodide and 2 equiv of KC(SiHMe2)3. This synthesis is similar to that of previously reported Yb{C(SiHMe2)3}2THF2 (1a), and compounds 1a,b are isostructural. Reactions of 1b and 1 or 2 equiv of B(C6F5)3 afford SmC(SiHMe2)3HB(C6F5)3THF2 (2b) or Sm{HB(C6F5)3}2THF2 (3b), respectively, and 1,3-disilacyclobutane {Me2Si-C(SiHMe2)2}2 as a byproduct. Bands from 2300 to 2400 cm–1 assigned to νBH in the IR spectra and highly paramagnetically shifted signals in the 11B NMR spectra of 2b and 3b provided evidence for Sm-coordinated HB(C6F5)3. Compounds 1a,b react with the bulky N-heterocyclic carbene (NHC) 1,3-di-tert-butylimidazol-2-ylidene (ImtBu) to displace both THF ligands and give three-coordinate monoadducts Ln{C(SiHMe2)3}2ImtBu (Ln = Yb (4a), Sm (4b)). Complexes 4a,b catalyze cross-dehydrocoupling of organosilanes with primary and secondary amines at room temperature to give silazanes and H2, whereas 1a,b are not effective catalysts under these conditions. Second-order plots of ln{[Et2NH]/[Ph2SiH2]} vs time for 4a-catalyzed dehydrocoupling are linear and indicate first-order dependences on silane and amine concentrations. However, changes in the experimental rate law with increased silane concentration or decreased amine concentration reveal inhibition by silane. In addition, excess ImtBu or THF inhibit the reaction rate. These data, along with the structures of 4a,b, suggest that the bulky carbene favors low coordination numbers, which is important for accessing the catalytically active species.This is an article from Organometallics 35 (2016): 1674, doi: 10.1021/acs.organomet.6b00138. Posted with permission.</p

Direct hydrosilylation by a zirconacycle with β-hydrogen

Azasilazirconacycle Cp2Zr{κ2-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 β-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{κ2-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.This is a manuscript of an article published as Yan, KaKing, Aradhana Pindwal, Arkady Ellern, and Aaron D. Sadow. "Direct hydrosilylation by a zirconacycle with β-hydrogen." Dalton Transactions 43, no. 23 (2014): 8644-8653. DOI: 10.1039/C4DT00658E. Posted with permission.</p

Magnesium-catalyzed hydrosilylation of a,b-unsaturated esters

ToMMgHB(C6F5)3 (1, ToM = tris(4,4-dimethyl-2-oxazolinyl)phenylborate) catalyzes the 1,4-hydrosilylation of α,β-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,β-unsaturated esters with 1 as the catalyst.This article is from Chemical Science 6 (2015): 6901, doi: 10.1039/c5sc02435h. Posted with permission.</p