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Amidinates and Guanidinates as Supporting Ligands for Expanding the Reactivity of Thorium and Uranium Complexes

  • Author(s): Settineri, Nicholas Salvatore
  • Advisor(s): Arnold, John
  • et al.

Chapter 1:

A series of thorium and uranium tris-amidinate complexes featuring a rare O-bound terminal phosphaethynolate (OCP1-) ligand were synthesized and fully characterized. These are the first actinide complexes featuring the OCP1- moiety, and the coordination through the O-atom donor is unique compared to all previously reported metal complexes, which feature P-bound OCP1- ligands. The cyanate (OCN1-) and thiocyanate (SCN1-) analogs were prepared for structural comparison and feature preferential N-coordination to the metal center. Calculations suggest the binding in all these complexes is charge-driven. The thorium OCP complex reacts with Ni(COD)2 to yield a heterobimetallic adduct as addition across the C≡P triple bond is observed; this new species features an unprecedented reduced OCP1- bent fragment which bridges the two metal centers.

Chapter 2:

A new thorium monoalkyl complex, Th(CH2SiMe3)(BIMA)3 (2.2; BIMA = MeC(NiPr)2), undergoes insertion of chalcogen atoms resulting in a series of thorium chalcogenolate complexes, Th(ECH2SiMe3)(BIMA)3 (where E = S, SS, Se, Te; 2.5-2.8). Complex 2.6 represents the first alkyl disulfide thorium species and illustrates the ability of 2.2 to undergo controllable, stoichiometric atom insertion. All complexes have been characterized by 1H and 13C{1H} NMR spectroscopy, FTIR, EA, and melting point, and in the case of 2.1, 2.2, and 2.4-2.8, X-ray crystallography. X-ray diffraction studies reveal the η2-coordination mode of the sulfur atoms in 2.6, the acute nature of the Th-Se-C bond in 2.7, and the first molecular Th-Te single bond in 2.8. Insertion was achieved by balancing the thermodynamic driving force of chalcogenolate formation versus the BDE of the pnictogen-chalcogen bond in the transfer reagent. Utilizing Me3NO as an oxygen atom transfer reagent led to C-H activation and SiMe4 elimination rather than oxygen atom insertion, resulting in the alkoxide complex Th(OCH2NMe2)(BIMA)3 (2.4).

Chapter 3:

The reactivity of the thorium monoalkyl complex Th(CH2SiMe3)(BIMA)3 (2.2; BIMA = MeC(NiPr)2) with various small molecules is presented. While steric congestion prohibits the insertion of N,N’-diisopropylcarbodiimide into the Th-C bond in 2.2, the first thorium tetrakis(amidinate) complex, Th(BIMA)4 (3.1), is synthesized via an alternative salt metathesis route. Insertion of p-tolyl azide leads to the triazenido complex Th[(p-tolyl)NNN(CH2SiMe3)-κ2N1,2](BIMA)3 (3.2), which undergoes thermal decomposition to the amido species Th[(p-tolyl)N(SiMe)3](BIMA)3 (3.3). The reaction of 2.2 with 2,6-dimethylphenylisocyanide results in the thorium iminoacyl complex Th[η2-(C=N)-2,6-Me2-C6H3(CH2SiMe3)](BIMA)3 (3.4), while the reaction with isoelectronic CO leads to the products Th[OC(=CH2)SiMe3](BIMA)3 (3.5) and Th[OC(NiPr)C(CH2SiMe3)(C(Me)N(iPr))O-κ2O,O′](BIMA)2 (3.6), the latter being the result of CO coupling and insertion into an amidinate ligand. Protonolysis is achieved with several substrates, producing amido (3.8), aryloxide (3.9), phosphide (3.10, 3.11), acetylide (3.12), and cationic (3.13) complexes. Ligand exchange with 9-borabicyclo[3.3.1]nonane (9-BBN) results in the formation of the thorium borohydride complex (BIMA)3Th(μ-H)2[B(C8H14)] (3.14). Complex 2.2 also reacts under photolytic conditions to eliminate SiMe4 and produce Th(BIMA)2(BIMA*) [3.15, BIMA* = (iPr)NC(CH2)N(iPr)], featuring a rare example of a dianionic amidinate ligand. Complexes 3.1, 3.2, 3.4, 3.5, and 3.11-3.15 were characterized by 1H and 13C{1H} NMR spectroscopy, FTIR, EA, melting point and X-ray crystallography. All other complexes were identified by one or more of these spectroscopic techniques.

Chapter 4:

A series of uranium tris-guanidinate complexes is presented, including the first homoleptic U(III) guanidinate species (4.2). Reaction of 4.2 with diphenyldiazomethane results in the two-electron oxidation of U(III) to U(V), producing the first isolable U(V) diphenyldiazomethane complex (4.3), featuring a short U-Nimido bond. Corresponding U(V) imido (4.4), U(V) oxo (4.5), and U(IV) azido (4.6) complexes were also synthesized for spectroscopic comparison. All complexes were characterized by 1H NMR spectroscopy, FTIR, EA, melting point, and X-ray crystallography.

Chapter 5:

Thorium dialkyl complexes featuring bis-amidinate and bis-guanidinate frameworks are reported, and their ability to insert molecular oxygen is evaluated. The bis-amidinate system Th(BTBA)2(CH2SiMe3)2 (5.2; BTBA = PhC(NSiMe3)2) is able to undergo instantaneous oxygen insertion to form the corresponding dialkoxide complex (5.3), as well as selenium atom insertion to form the diselenolate (5.4). The bis-guanidinate system Th(TIG)2(CH2SiMe3)2 (5.6; TIG = iPr2NC(NiPr)2) undergoes oxygen insertion at a significantly slower rate, and preliminary mechanistic studies suggest a radical-type mechanism is at play. All complexes were characterized by 1H and 13C{1H} NMR spectroscopy, FTIR, EA, melting point, and X-ray crystallography.

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