Developments in Alkaline Earth Hydride Chemistry: Towards Novel Heterobimetallic Complexes

Abstract

Since the seminal report of the first heteroleptic calcium hydride in 2007, a suite of coordinatively unsaturated alkaline earth (Ae) hydrides have been reported. These complexes have demonstrated their synthetic utility in small-molecule activation processes and have also been exploited to gain entry to a diverse range of heterobimetallic complexes. These heterobimetallic alkaline earth–main group metal partnerships introduce highly polarised Ae–M bonds that induce nucleophilic character at the installed main-group metal. This work has been reviewed in Chapter One.
Chapter Two extends the existing reactivity of β-diketiminato magnesium and calcium hydrides towards organonitrogen substrates containing nitrogen-nitrogen multiple bonds. The alkaline earth hydrides were found to undergo hydride addition, deprotonation and reductive elimination reactions, depending on the nature of the substrate. The reactivity was shown to both parallel and diverge from related low oxidation state β-diketiminato magnesium(I) dimers, dependent on the alkaline earth metal employed, with the calcium hydride able to act as a surrogate source of Ca(I).
Chapter Three describes the reactivity of β-diketiminato Ae hydrides towards heavy group 14 dimetallanes or hydrides, respectively. This resulted in metathesis/deprotonation to form in particular the first heteroleptic Mg–Ge and Ca–Ge species, which act as soluble sources of the germyl anion. Attempts to extend the chemistry towards lead were thwarted by facile Pb–C/Ae–H exchange to afford the corresponding Ae(II) phenyl species. Subsequent reactivity of the isolated germanides and related stannanides with unsaturated electrophiles demonstrated nucleophilic reactivity directed by the tetrylide anions.
Chapter Four details the reactivity of β-diketiminato magnesium and calcium hydrides with low valent group 14 hydrides in the +2 oxidation state to afford the first examples of heterobimetallic bridging hydrides between a Sn(II) centre and calcium or magnesium. The reactivity of these heterobimetallic complexes was assessed with a wide range of unsaturated substrates, generally providing separate homometallic products. Cooperative reactivity was observed towards hex-1-ene, yielding mixed hexyl hydride Ae–Sn heterobimetallics. For calcium, this reaction is selective for a monoalkyl species, whereas for magnesium, a second insertion of hex-1-ene readily affords a Mg–Sn(IV) dihexyl species. These reactions were investigated further by Density Functional Theory (DFT) calculations and show evidence for synergistic bond activation between each metal.
Chapter Five explores the role of kinetically stabilising ligands in the isolation of heteroleptic alkaline earth compounds to prevent deleterious Schlenk equilibria, which becomes increasingly important as group 2 is descended. Super-bulky β-diketiminato ligands employing 2,4,6-tricyclohexylanilido (TCHP) motifs at one or both of the chelating N atoms were utilised to synthesise a wide range of both solvent-free and THF-coordinated heteroleptic alkaline earth (Mg, Ca, Sr, Ba) hexamethyldisilazide compounds, and isolation of the complete series of alkaline earth hydrides via σ-bond metathesis with phenylsilane. Initial reactivity studies of the bis-TCHP substituted calcium hydride complex demonstrated that increased steric bulk largely inhibits reactivity with substrates, though reaction with carbon monoxide affords a cis-ethenediolate complex.
Chapter Six discusses the precedence established from the research conducted in this thesis, and potential directions that future work in these areas could take to further develop these aspects of organometallic alkaline earth chemistry.
Chapter Seven presents a concise summary of the experimental details pertinent to this thesis, both for synthetic manipulations of materials, and X-ray crystallographic processing and analysis of samples, across both the University of Bath and Monash University.

Date of Award	26 Jun 2024
Original language	English
Awarding Institution	University of Bath Monash University
Supervisor	Michael Hill (Supervisor), Cameron Jones (Supervisor) & David Liptrot (Supervisor)