Synthesis, characterization, and reactivity of early-transition and rare-earth complexes
Date
2022-05
Authors
Chu, Yang Yun
Major Professor
Advisor
Sadow, Aaron D.
Huang, Wenyu
Rossini, Aaron J.
Slowing, Igor
VanVeller, Brett
Committee Member
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Abstract
A series of novel proligands C5H5CMe2CHPhOxR (R = Me2, CHMe2, CMe3) and their Group 4 metal complexes, (OxRCHPhCMe2C5H4)M(NMe2)3 and (C5H4CMe2CPhOxR)M(NMe2)2M(NMe2)4 (M = Zr, Ti, and Hf) obtained from mono- and doubly-deprotonation respectively are described. The NMR spectroscopic characteristics and X-ray diffraction analysis suggested the formation of observed Group 4 compounds, mono- or doubly deprotonated, are highly dependent on the steric effects from the substituents on the oxazoline ligand, metal center, and amide leaving group. The bulkier ligands such as 4-t-butyl-oxazoline or smaller metal center (i.e. Ti) gives only the mono-deprotonated (OxRCHPhCMe2C5H4)M(NMe2)3. Amine elimination from (OxRCHPhCMe2C5H4)M(NMe2)3 to give (C5H4CMe2CPhOxR)M(NMe2)2 is reversible for 4,4-dimethyl- and 4-isopropyl-oxazoline-based ligands in Zr or Hf complexes. Thermodynamic kinetic studies were carried out to investigate the plausible reaction mechanisms. DFT computational models indicate a single-step, non-linear transfer of the H between the benzylic position of the non-innocent, oxazoline-coordinated ligand and NMe2. Overall, these effects suggest that coordination of oxazoline to the metal center is a key part of the benzylic deprotonation and non-innocent behavior of the cyclopentadienyl-oxazoline ligand.
A series of solvent-free, homoleptic organometallic compounds mid-transition Ln{C(SiHMe2)3}3 and Ln{C(SiHMe2)2Ph}3 (Ln = Dy, Er) were synthesized. The characterizations by IR and X-ray diffraction analysis revealed the distinct structural features. The molecular structures of Ln{C(SiHMe2)3}3 (Ln = Dy, Er) adopt trigonal planar geometries with three alkyl ligands located in the equatorial plane. The structural features of Dy{C(SiHMe2)3}3 and Er{C(SiHMe2)3}3 are similar to the previously reported Ln{C(SiHMe2)3}3 (Ln = La, Ce, Pr, Nd; Y). Instead of three –C(SiHMe2)3 ligands, Dy{C(SiHMe2)2Ph}3 contains two –C(SiHMe2)3 and one η2-benzyl ligand in each alkyl group. The molecular structure of Dy{C(SiHMe2)2Ph}3 also adopts trigonal planar geometry coordination and reveals only two secondary interactions Ln↼H-Si in comparison to three in the previously reported Ln{C(SiHMe2)2Ph}3 (Ln = La, Ce, Pr, Nd; Y). In Dy{C(SiHMe2)2Ph}3, the five crystallographically independent molecules (Z = 20) each contain three π-coordinated phenyl groups in addition to either one or two secondary Dy↼H-Si interactions per molecule. These structures contrast the single crystallographically unique molecule in the previously reported La{C(SiHMe2)2Ph}3 (Z = 2). La{C(SiHMe2)2Ph}3 doped with Dy{C(SiHMe2)2Ph}3 adopts the crystallographic structure of either Ln{C(SiHMe2)2Ph}3 (Ln = Ce, Pr, Nd; P21/c) or La{C(SiHMe2)2Ph}3 (P-3).
The cationic Nd{C(SiHMe2)3{HB(C6F5)3}2 complexes, obtained by the activation of Nd{C(SiHMe2)3}3 with 2 equiv. of B(C6F5)3, were found active in the polymerization reaction of 1,3-butadiene in the presence of AliBu3 co-catalysts. This neodymium catalyst system was investigated in the aliphatic hydrocarbon solvent (i.e., heptane) in comparison to the aromatic solvent (i.e., toluene). This investigation has revealed the importance of the solvent effect on the butadiene polymerization as well as the effect of the catalyst concentration, butadiene concentration, ratio of neodymium to triisobutylaluminum, and reaction time. By altering each variable, high cis-selectivity with moderate dispersity was achieved. Even though the optimal conditions for the formation of high 1,4-cis-polybutadiene with low dispersity are not straightforward by direct comparison due to the co-dependency of each factor, the work has provided valuable insights.
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dissertation