2014 Theses Doctoral

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.