2018 Theses Doctoral
Main Group and Transition Metal Complexes Supported by Carbon, Sulfur, and Selenium Donor Ligands
This thesis explores the synthesis, characterization, and reactivity of main group and transition metal complexes that feature ligands with carbon, sulfur, and selenium donor atoms. Specifically, the carbon donor ligands explored include the carbodiphosphorane, (Ph₃P)₂C, and the analytical reagent, nitron, which behaves like an N-heterocyclic carbene in solution. The sulfur ligands include the amino acids cysteine and glutathione, and the tripodal tris(2-mercapto-1-t-butylimidazolyl)hydroborato ligand, of which the latter provides an [S₃] coordination environment. Finally, the selenium donor ligands explored comprise the phenylselenolate, [PhSe]–, and the selenobenzimidazole, H(sebenzimᴹᵉ).
Chapter 1 investigates the chemistry of two-coordinate mercury alkyl complexes supported by sulfur and selenium ligands. The first part of Chapter 1 examines the structure of the amino acid complexes, (Cys)HgMe and (GS)HgMe, which indicate that both complexes possess linear geometries. Additionally, 1H NMR studies confirm the labile nature of the cysteinato ligand in (Cys)HgMe. More specifically, in the presence of excess cysteine, exchange is observed, a result that is of relevance to mercury toxicity and detoxification. The second part of Chapter 1 examines the exchange reactions of the phenylselenolate mercury alkyl complexes, PhSeHgR (R = Me, Et), as well as their propensity to undergo protolytic Hg–C bond cleavage. The results from these experiments indicate that coordination by selenium promotes protolytic cleavage of Hg–C bonds more rapidly than compared to the sulfur analogues.
Expanding the metal centers to include the lighter group 12 metals, Chapter 2 investigates ligand exchange between zinc, cadmium, and mercury in a sulfur-rich coordination environment as provided by the [S₃] tris(2-mercapto-1-t-butylimidazolyl)hydroborato ligand. Similar to the Schlenk equilibrium, alkyl group exchange between the same metal center is observed as demonstrated by the formation of [Tmᴮᵘᵗ]MMe via treatment of [Tmᴮᵘᵗ]₂M with Me₂M (M = Zn, Cd). Additionally, alkyl group exchange between different metals centers is also possible. For example, a mixture of [Tmᴮᵘᵗ]ZnMe and Me₂Cd form an equilibrium mixture with [Tmᴮᵘᵗ]CdMe and Me₂Zn. Furthermore, transfer of the [TmBut] ligand between the metal centers is possible too. This is demonstrated by the transfer of [Tmᴮᵘᵗ] from mercury to zinc in the methyl system, [Tmᴮᵘᵗ]HgMe/Me₂Zn. Additionally, transfer of [Tmᴮᵘᵗ] from zinc to mercury is also observed upon treatment of [Tmᴮᵘᵗ]₂Zn with HgI₂ to afford [TmᴮᵘᵗHgI and [Tmᴮᵘᵗ]ZnI, thereby indicating that the nature of the co-ligand has a profound effect on the thermodynamics of ligand exchange.
Chapter 3 explores the coordination chemistry of the selenium donor ligand, H(sebenzimᴹᵉ). H(sebenzimᴹᵉ) is able to coordinate metal centers through the selenium atom in a dative fashion, and, depending upon the metal center, up to four H(sebenzimMᴹᵉ) ligands can coordinate the same metal. Additionally, H(sebenzimᴹᵉ) can be deprotonated to form [sebenzimᴹᵉ]–, allowing for the potential of an LX coordination mode, which results in bridging complexes for the metal compounds investigated. In regards to the metal centers investigated in Chapter 3, H(sebenzimᴹᵉ) has been demonstrated to be an effective ligand for Pd, Ni, Zn and Cd.
Chapter 4 investigates the various structural polymorphs of the carbodiphosphorane, (Ph₃P)₂C. More specifically, previous crystal structures of (Ph₃P)₂C have demonstrated that the P–C–P bond angle is highly bent. This is consistent with simple VSEPR theory, which predicts a bent geometry for compounds possessing a coordination number of two and two lone pairs of electrons. However, Chapter 4 details the characterization of a new linear form of (Ph₃P)₂C. DFT calculations indicate that the energy required to bend the P–C–P bonds of (Ph₃P)₂C over the range of 130˚-180˚ is less than 1.0 kcal mol–1. Analysis of the Natural Localized Molecular Orbitals (NLMOs) indicates that upon bending of the P–C–P bond angle, the σ-type lone pair NLMO on the central carbon atom is stabilized, while the two P–C bonding orbitals NLMOs are destabilized. The differential behavior of the lone-pair and bonding orbitals upon bending is one component that provides a simple rationalization for the flexibility of (Ph₃P)₂C.
In view of the fact that carbodiphosphoranes possess two lone pairs of electrons on the central carbon atom, (Ph₃P)₂C is an effective ligand for a variety of metals and nonmetals. Chapter 5 investigates the reactivity of (Ph₃P)₂C towards the main group alkyl metal complexes, Me₃E (E = Al, Ga), Me₂M (M = Mg, Zn, Cd), and MeHgI, as well as Mg[N(TMS)2]₂. Additionally, the reactivity of (Ph₃P)₂C towards transition metal complexes was also investigated. (Ph₃P)₂C is capable of coordinating in several different ways, a couple of which include forming a Lewis acid/base adduct, and ortho metalation of one of the phenyl groups.
Lastly, Chapter 6 expands the coordination chemistry of nitron. Nitron, which is used as a quantitative analytical reagent, has recently been shown to behave like an NHC in solution. This is attributed to the presence of the carbenic tautomer of nitron when placed in solution. Thus, nitron effectively coordinates metal centers through the central carbon atom. Chapter 6 outlines (i) the synthesis and structural characterization of nickel, palladium, and iridium complexes that feature nitron as a ligand, and (ii) the ability of the corresponding iridium complexes to serve as catalysts for the dehydrogenation of formic acid and the hydrosilylation of aldehydes.
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More About This Work
- Academic Units
- Thesis Advisors
- Parkin, Gerard
- Ph.D., Columbia University
- Published Here
- September 7, 2018