Synthesis and reactivity of rare earth and alkaline earth metal complexes

Boteju, W Kasuni
Major Professor
Aaron D Sadow
Committee Member
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One of the aspects of our research group is developing new ligands, for instance -SiH containing alkyl, amide ligands and oxazoline based scorpionate type ligands for the synthesis of their rare earth metal, alkali earth metal and transition metal complexes which are used as catalyst precursors for organic transformation reactions.

Rare earth complexes of tris(dimethylsilyl)methyl ligand have been developed by our research group. In this thesis a new alkyl ligand is accessed by replacing one dimethylsilyl moiety with a phenyl group. The -SiH functionalized benzyl ligand HC(SiHMe2)2Ph is synthesized by reductive coupling of HCBr2Ph and SiHMe2Cl and the anion of the ligand [C(SiHMe2)2Ph]– is obtained by deprotonation of HC(SiHMe2)2Ph with KCH2Ph. An alternate route is introduced to obtain the same -SiH functionalized benzyl anion [C(SiHMe2)2Ph]– by reaction of KOtBu and (Me2HSi)3CPh which reacts via C–Si cleavage. LnI3·THFn and three equivalents of this carbanion combine to provide homoleptic tris(alkyl)lanthanide compounds Ln{C(SiHMe2)2Ph}3 (Ln = La, Ce, Pr, Nd) containing secondary metal-ligand interactions.

Rare earth compounds of –N(SiHMe2)tBu ligand have been synthesized by our research group. A new amide ligand is accessed by introducing an aryl group to the tertbutyl position, and this ligand is the amide version of the new alkyl ligand reported in this thesis. Further, addition of substituents on the aryl group allows to vary the steric properties of the new ligand series. Three new hydridosilazido ligands, –N(SiHMe2)Aryl (Aryl = Ph, 2,6-C6Me2H3 (dmp), 2,6-C6iPr2H3 (dipp)) and their homoleptic rare earth complexes Ln{N(SiHMe2)Aryl}3(THF)n (Ln = Sc, Y, Lu; Aryl = Ph, n = 2; Aryl = dmp, n = 1; Aryl = dipp, n = 0) are synthesized. NMR, IR and X-ray diffraction studies of the complexes show that Ln{N(SiHMe2)Ph}3(THF)2 contain only classical Si–H interactions with the metal center. Y{N(SiHMe2)dmp}3THF and Lu{N(SiHMe2)dmp}3THF contain three and two bridging Si–H interactions respectively. Three secondary Ln↼HSi interactions and one agostic CH bond per molecule are observed for planar Ln{N(SiHMe2)dipp}3 complexes. Further, the reaction of Ln{N(SiHMe2)dipp}3 with ketones provides the hydrosilylated product by inserting the C=O in bridging Ln↼H-Si rather than inserting into the Y–N bond or enolate formation. The insertion reaction is postulated to occur via an associative mechanism.

Ln{N(SiHMe2)dipp}3 undergo β-SiH abstraction by Lewis acids, such as B(C6F5)3. The reaction of Ln{N(SiHMe2)dipp}3 and one equivalent of B(C6F5)3 provides the cationic species Ln{κ2-N(dipp)SiMe2N(SiHMe2)dipp}{N(SiHMe2)dipp}HB(C6F5)3. The decomposition of that compound provides (Me2Si–Ndipp)2 and a rare earth adduct. A second order rate law is obtained for the decomposition of the cationic complex.

Tris(oxazolinyl)boratomagnesium alkyls (ToMMgR) have been developed by our research group and tested in various catalytic reactions for instance hydroboration and dehydrocoupling. In this thesis kinetic studies of the stoichiometric reaction between ToMMgR and HBpin are performed for comparison with other sigma bond metathesis type reactions. The reaction of ToMMgMe or ToMMgnPr with HBpin was too fast even at –78 °C to measure using NMR and IR kinetics. In contrast, ToMMgBn reacts with HBpin or DBpin more slowly at room temperature, and kinetic studies revealed a bimolecular rate law. The activation parameters obtained for the reaction of ToMMgBn with HBpin and DBpin are ΔH‡ = 13.08(0.02) kcal mol-1, ΔS‡ = –29.09(0.05) cal mol-1 K-1 and ΔH‡ 11.86(0.05) kcal mol-1, ΔS‡ = –33.44(0.15) cal mol-1 K-1 respectively. This study shows a kinetic isotope effect of 1.35 at 55 °C and it appears to be increased with increasing the temperature. The mechanistic investigations show that B-H/B-D bond cleavage is involved in the rate-determining step.