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A Journey Across the Periodic Table: The Synthesis and Characterization of Main Group Metals Supported by Nitrogen- or Sulfur-Rich Ligands

Chakrabarti, Neena

In Chapter 1, I discuss the synthesis and characterization of lithium tris(pyrazolyl)hydroborato complexes, [TpR1,R2]Li. Group 1 [TpR1,R2]M complexes serve as key starting points to access many other main group and transition metal complexes; however, the synthesis and crystal structures of [Tp R1,R2]Li has not been reported. Molecular structures of [TpBut]Li and [TpBut,Me]Li show these complexes are trigonal pyramidal, an unusual geometry for lithium. These complexes are also able to bind small molecules to form four-coordinate pseudo-tetrahedral complexes, [Tp]Li-L (L = MeCN, pzButH, and H2O). The binding constants for the association of acetonitrile to [TpBut]Li and [TpBut,Me]Li are 0.84M-1 and 0.96M-1, respectively, indicating that the dissociation of MeCN is facile in solution. In addition, [TpBut,Me]Li serves as transmetallating agent to yield the cadmium halide complexes, [TpBut,Me]CdX (X = Cl, Br, I).
In Chapter 2, I discuss the synthesis and characterization of organometallic cadmium complexes supported by the nitrogen-rich multidentate ligands, tris(pyridylthio)methane, [Tptm]H; tris(1-methyl-imidazolylthio)methane, [TitmMe]H; and tris(1-methyl-benzimidazolylthio)methane, [TitmiPrBenz]H. These ligands are in the nascent stages of development and there are only a few metal [Tptm] and [TitmMe]
complexes in the literature. An investigation of the reactivity of [L]CdN(SiMe3)2, [L]CdOSiMe3, and [L]CdOSiPh3 ([L] = [Tptm], [TitmMe], [TitmiPrBenz]) shows these complexes provide access to a variety of organometallic cadmium complexes, [L]CdX, (X = OAc, Cl, Br, O2CH, NCO). The characterization of cadmium acetate and formate complexes is significant due to their structural similarity with the metal bicarbonate intermediate formed by zinc and cadmium-substituted carbonic anhydrase. In addition, the synthesis and characterization of cadmium methyl complexes, [L]CdMe, is discussed. The application of heat to a mixture of [TitmiPrBenz]H and CdMe2 results in isomerization of the ligand to [S3-TitmiPrBenz]CdMe. This sulfur-rich [S3-TitmiPrBenz] ligand is not reported in the literature and is ripe for further investigation. The solid state structures of these compounds provide a comparison with biologically relevant [Tp] or [Tm] cadmium methyl complexes in the literature.
In Chapter 3, I describe the synthesis and structural characterization of [BmButBenz]M (M = Na, K) and [BmRBenz]Ca(THF)2 (R = Me, But) are discussed. The sulfur-rich tripodal ligand tris(imidazolylthio)hydroborato, [Tm], was previously designed to serve as a softer version of the [Tp] ligand. Metal [Tm] complexes are prevalent in the literature and have often been used as molecular mimics of sulfur-rich enzyme active sites. Recently, the benzannulated [TmRBenz]M complexes were reported and were found to promote k3 coordination toward the metal center. To allow for an in-depth
investigation of the newly synthesized [BmRBenz] class of ligand, the [BmButBenz]M (M = Na, K, Ca) complexes were synthesized and compared to previously reported metal [BmMeBenz]M complexes. Additionally, the [BmMeBenz]2Ca(THF)2 was synthesized and characterized via X-ray diffraction. The molecular structure of [BmMeBenz]2Ca(THF)2 shows the complex is monometallic with an uncommon eight-coordinate dodecahedral calcium center. [BmMeBenz]2Ca(THF)2 is the first molecular structure of calcium coordinated to the [Tm] or [Bm] ligand class.
In Chapter 4, I discuss the synthesis and characterization of mercury alkyl complexes supported by the [TmMe], [BmR], [TmRBenz] and [BmRBenz] ligands (R = Me or But). As previously mentioned, [Tm]M complexes are considered biologically relevant molecular models of enzyme active sites. With this in mind, [TmBut]HgR (R = Me,Et) complexes have served as mimics for the mercury detoxification enzyme MerB. A previous study by our group showed that the adoption of multiple coordination modes of the ligand in [TmBut]HgR plays a significant role in the activation of the Hg-C bond toward protonolysis. The molecular structures of the [TmR], [BmR], [TmRBenz], and [BmRBenz] mercury alkyl complexes show that they adopt various coordination modes, ranging from k1 to k3. Preliminary competition experiments in which benzenethiol was added to [TmR]HgEt and [TmRBenz]HgEt indicate that the Hg-C bond in [TmMeBenz]HgEt was cleaved faster than that in [TmMe]HgEt. Conversely, the Hg-C bond in [TmBut]HgEt was cleaved faster than that in [TmButBenz]HgEt, indicating that benzannulation and the size of the R-group on the [Tm] ligand play important roles in Hg-C bond cleavage.


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More About This Work

Academic Units
Thesis Advisors
Parkin, Gerard
Ph.D., Columbia University
Published Here
July 7, 2014
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