Towards the Stabilisation of Subvalent Heavy Alkaline Earth Metal Complexes

In 2007, the fundamental understanding of the chemistry of the Alkaline Earth elements (AE = Mg-Ba) was challenged with the inaugural isolation of a low-oxidation state magnesium(I) complex. The number of isolable magnesium(I) complexes has since flourished, and the chemistry of such molecular compounds has been developed to include industrially relevant transformations. Despite significant efforts, the low-oxidation state chemistry of the heavy AE elements (Ca-Ba) is far less developed, though has shown hugely promising indications of the synthetic power of these molecular species. The work here discussed focuses on further developing the low-oxidation state chemistry of the AE elements, with a particular focus on the heavier congeners (Ca-Ba). To achieve this, a molecular design comprising a highly equatorial AE(II) precursor is proposed. In this work, both monoanionic and neutral AE(II) complexes that satisfy this molecular design are reported.

Chapter 2 describes the synthesis, characterisation, and reduction chemistry of a series of monoanionic AE(II) tris(amide) ‘ate’ complexes. This work is supported by a computational study that verifies the proposed molecular design.

Chapter 3 expands the molecular design to include neutral AE(II) complexes featuring bulky cyclic (alkyl)(amino)carbene (CAAC) ligands. The synthesis, characterisation and preliminary reduction chemistry of these complexes is reported. The organometallic complexes reported herein include isolated examples structurally authenticated Sr–CAAC and Ba–CAAC complexes. A further computational study is provided.

Chapter 4 describes the use of mechanochemistry – an emerging, solid-state technique – for the reductions of the monoanionic AE(II) complexes previously discussed in Chapter 2. Mechanochemistry has been proven to allow access to highly reactive molecular species that are otherwise inaccessible when using conventional solution-based techniques. Attempts to prepare a low-oxidation state Ca(I) complex led to the synthesis of a heterobimetallic Ca/K electride. The synthesis, characterisation and reactivity of this material is reported.