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(SiHMe 2)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(SiHMe 2)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 beta-SiH interactions with the metal center and undergo beta-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(C6F 5)3, and LnC(SiHMe2)3{HB(C6F 5)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 alpha,beta-unsaturated esters to selectively yield alpha-silyl esters. alpha-Silyl esters are isolated in high yields from a range of alpha,beta-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(C6F 5)3 generate LnC(SiHMe2)3HB(C6F 5)3 and an equiv. amount of 1,3-disilacyclobutane dimer, {Me2Si-C(SiHMe 2)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(SiHMe 2)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(Et 2O)].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}2THF 3 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}2THFu3 reacts with ancillary ligand, TlToM to yield ToM2Yb revealing lability of the silyl ligand.