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The Utility Of Low-Valent Niobium In Small Molecule Activation And The Study Of First Row Transition Metal Complexes Supported By Tetradentate Redox Active Ligands

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Abstract

The low-valent complex (silox)3NbPMe3 (silox = tBu3SiO) was synthesized and tested for reactivity towards small molecules. It was discovered that exposure to CO generated the complexes (silox)3Nb=C=C=O (3-Nb), (silox)3Nb=O (2-Nb), and [(silox)3Nb]2([mu]-C2) (4-Nb) in various ratios depending on reaction stoichiometry. The formation of [(silox)3Nb]2([mu]-CO) (5-Nb) was discovered as a byproduct in most reactions. Treatment of (silox)3NbPMe3 with potassium under a dinitrogen atmosphere afforded the dinitrogen complex (silox)3NbNNNb(silox)3 (8-Nb). Reaction of (silox)3NbCl (10-Nb) with NH3 afforded (silox)3NbIV(NH3)Cl, with ammonia binding parameters of [INCREMENT]H = 20.4 ± 2.3 kcal/mol and [INCREMENT]S = 39 ± 7 e.u. The nickel complex {dmp(PI)2}Ni (dmp(PI)2 = Me2C(CH2=Npy)2) was synthesized and subjected to chemical oxidation and reduction to synthesize a 5membered redox series. Through electronic structure study, it was determined that in these complexes the metal center typically remained NiII, with the possible exception of the cationic complex {dmp(PI)2}Ni, and redox changes were primarily ligand-centered. The chemistry of dmp(PI)2 was extended to iron, and {dmp(PI)2}FePMe3 was synthesized. As with the nickel analogue, the complex was determined to be a MII center bound to a dianionic ligand framework, with similar redox behavior as the nickel species. A related tetradentate [beta]-diketiminate-based ligand incorporating two pyridinemethylene units, 2,4-bis[(E)-(2-pyridyl)methylideneamino]pentane (H{nn(PM)2}), was synthesized and metallated to form {nn(PM)2}FeN(TMS)2 (1-N(TMS)2). 1-N(TMS)2 could be further derivatized to form {nn(PM)2}FeX (X = Cl, N3). One of the methylene fragments of the ligand backbone was prone to facile deprotonation and allowed synthesis of {nn(PM)(PI)}FeLL' (LL' = (PMe3)2, (PMe2Ph)2, (PMe3)CO; 2LL') and {nn(PM)(PI)}FeL (L = PMe3, PMePh2, PPh3, CO; 3-L). Electronic structure studies suggested 2-LL' and 3-L existed as FeII metal centers bound to dianionic {nn(PM)(PI)}2-, with strong metal-ligand covalency in 3-L. Oxidation studies on 2(PMe3)2 showed two successive ligand-based 1e- oxidations. The same tetradentate [beta]diketiminate-based ligand was applied to other transition metals of the first row and allowed synthesis of {nn(PM)2}VCl2 (1-VCl2) and {nn(PM)2}Co (1-Co), both of which formed through unusual ligand exchange or disproportionation events, and {nn(PM)(PI)}M (M = Cr, 4-Cr; M = Ni, 4-Ni), which were generated through double deprotonation of the H{nn(PM)2} ligand.

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2014-08-18

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Redox-Active; Low-valent Niobium; First-row Transition metal

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Committee Chair

Wolczanski, Peter Thomas

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Collum, David B
Coates, Geoffrey

Degree Discipline

Chemistry and Chemical Biology

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Ph. D., Chemistry and Chemical Biology

Degree Level

Doctor of Philosophy

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Government Document

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dissertation or thesis

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