Synthesis, structure, and reactivity of low-valent lanthanide metallocenes

<p dir="ltr">The use of cyclopentadienyl ligands in organolanthanide chemistry has led to many advances in the applications of low-valent lanthanide complexes and in small-molecule activation. Metallocene complexes of the classical divalent lanthanides samarium, europium and ytterbium metallocenes were the first to be isolated owing to the relatively high stability of their respective 4f<sup>6</sup>, 4f<sup>7</sup> and 4f<sup>14</sup> ions. Isolating divalent metallocenes and other divalent compounds of the other members of lanthanide series has proven to be challenging due to their highly reactive nature. However, by 2013, ‘non- classical’ divalent lanthanides compounds have been isolated for the full 4f series excluding promethium, with metallocenes the focus of many notable developments.</p><p dir="ltr">Most studies on the reactivity of divalent lanthanide metallocenes have focused on the reduction of organic substrates, such as <i>N</i>-heterocycles. The first aim of this research is to directly reduce the b- diketiminate-ligated dinitrosyliron complexes (DNICs) [(NacNac<sup>Ar</sup>)Fe(m-NO)<sub>2</sub>] (Ar = mesityl, 2,6- diisopropylphenyl) with the divalent metallocenes [LnCp*<sub>2</sub>] (Ln = Sm, Yb) to yield [(NacNac<sup>Ar</sup>)Fe(m- NO)<sub>2</sub>LnCp*<sub>2</sub>]. Previous work in the Layfield group on trivalent metallocenes has shown that, in the case of the Dipp-substituted heterobimetallic complexes, elimination of N<sub>2</sub>O occurs to give the oxo-bridged complexes [(NacNac<sup>Dipp</sup>)Fe(m-O)LnCp*<sub>2</sub>]. This work explores the idea that the same reactivity could be induced by divalent lanthanides whilst also investigated the impact of lanthanide ionic radius on the chemistry.</p><p dir="ltr">The second aim of the work comprising this thesis was to develop the idea that divalent-like reactivity could be possible for trivalent lanthanides using a ‘masked’ approach. Here, we show that isocarbonyl- bridged lanthanide-iron complexes can indeed act as masked divalent species, transferring two electrons to the test substrate phenazine, yielding [{(Cp*<sub>2</sub>Ln}<sub>2</sub>(m-C<sub>12</sub>H<sub>8</sub>N<sub>2</sub>)].</p><p dir="ltr">The final aim of the work presented here was to use classical divalent lanthanide metallocenes as reducing agents towards the extended <i>N</i>-heterocycle hexaazatrinaphthylene, which could generate trivalent lanthanide complexes of a multiradical ligand. Thus, the resulting complexes [{Cp*<sub>2</sub>Ln}<sub>3</sub>(R<sub>6</sub>HAN)] (R = H, Me<sub>6</sub>) were synthesized and fully characterized, including a multireference ab initio computational study.</p>