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f-Block Complexes of Chelating Tetraphenolates and their Applications in Catalytic Dinitrogen Functionalization


Chapter 1 reports dinuclear actinide (An) complexes supported by the meta-phenyl-bridged tetraphenolate ligand mTP, and their ability to catalyze the N2 reduction reaction (N2RR). The bis-UIV metallacycle [U2(mTP)2] (1.1) possesses a rectangular, ‘letterbox’-shaped cavity between the two UIV cations. When treated with potassium reductant, 1.1 can mediate the reduction of N2 by four electrons; concomitant intramolecular deprotonation of ligand benzylic C–H leads to the formation of a hydrazido [N2H2]2− ligand in K4[U2(μ-η2:η2-N2H2)(mTP−)] (1.3). We also demonstrate that 1.1 can catalyze the selective formation of HN(SiMe3)2, a secondary silylamine, from N2 under ambient conditions, the first catalyst to achieve this transformation. This also represents the first example of a homogeneous f-block N2RR catalyst. A possible reaction mechanism, which was computed via DFT by our collaborator, is discussed.

Chapter 2 extends our investigation in f-block-mediated N2RR to the lanthanides (Ln). The bis-SmIII metallacycle K2[Sm2(mTP)2(THF)2] (2.1) is also capable of catalyzing the N2RR, the first 4f complex to do so. Compared to 1.1, 2.1 catalyzes the formation of the tertiary silylamine N(SiMe3)3 from N2. This suggests that the N2RR catalyzed by 2.1 is mechanistically different to that by 1.1, in that the mTP benzylic C–H is not deprotonated. The bis-SmIII platform complex [Sm2(mTP)I2(THF)6] (2.2) converts to 2.1 under the reaction conditions, further reinforcing the significance of the metallacyclic geometry in this system. Surprisingly, a wider series of dinuclear platform complexes [Ln2(mTP)I2(THF)n] (2.3; Ln = La, Nd, Dy, Lu), [Th2(mTP)Cl4(THF)5(dme)] (2.4) and [U2(mTP)I4(THF)4] (2.5) can also mediate the N2RR, despite showing no evidence of interconversion to metallacyclic complexes. Preliminary characterization data of the reduced N2 species are discussed, including EPR spectroscopy and magnetic susceptibility measurements that suggest the existence of a ligand radical.

Chapter 3 describes a series of An(IV) complexes supported by the pTP ligand, the para-phenyl-bridged counterpart of mTP. In the absence of donor solvents, the mononuclear complexes [An(pTP)] (An = Th 3.1, An = U 3.2) are formed, in contrast to the dinuclear mTP complexes described in Chapter 1. Both 3.1 and 3.2 possess rare η6-AnIV–arene interactions, which were investigated both experimentally, and computationally via DFT by our collaborators. Electronic structure calculations of the reduced analogues [3.1]− and [3.2]−, as well as the isoelectronic Yb counterpart [Yb(pTP)]− (3.15), were able to locate the unpaired electron density in each case and identify potential molecular qubit candidates. Addition of Lewis bases and monoanionic ligands also allowed us to study the effect of axial donor strength.

Chapter 4 investigates the application of the mTP ligand in stabilizing the uranyl(VI) dication, (UO2)2+. The platform complex [(UO2)2(mTP)(THF)3] (4.1) and metallacyclic [Na(15-c-5)(py)]4[(UO2)2(mTP)2] (4.5) were synthesized and fully characterized, locating two uranyl dications in close proximity. The U(VI) to U(IV) reduction of 4.1 can be mediated by the simple f-block halide SmI2(THF)2, but is concomitant with ligand dissociation. Complex 4.1 is also capable of mediating hydrogen atom abstraction reactions from hydrocarbons under visible light irradiation.

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