Synthesis and Reactivity of Low-valent Aluminyl Anion and Group 14-Derived Heterobimetallic Complexes

The work detailed in this thesis describes the synthesis and reactivity of heterobimetallic complexes based on the low-valent potassium aluminyl compound [(NON)AlK] ([NON]2- = [O(SiMe2NDipp)2]2-, Dipp = 2,6-iPr2-C6H3) and group 14 fragments [(Am)MCl] (M = Ge, Sn, Pb; [Am]- = [tBu(NDipp)2]-). Compounds with heterobimetallic bonds are often sought after for their applications in catalysis and functionalising organic molecules, and the field of aluminium-group 14 species has recently emerged as a rapidly developing area of interest. Chapter One comprises a comprehensive literature review on heterobimetallic complexes derived from either an aluminyl anion or a tetrylene unit. The aluminyl anion is a low-valent, anionic aluminium(I) complex typically charge-balanced with a group 1 cation such as potassium. Tetrylenes are ‘heavy’ analogues of carbenes, which are divalent carbon(II)-based compounds, using elements such as germanium, tin, and lead. Both of these classes of compound are isoelectronic to one-another and are sought out for synthesising heterobimetallic compounds as they make strong L-type donor ligands and provide Lewis acidic properties to the complex. Heterobimetallic species are of recent interest as they possess enhanced, ‘cooperative’, reactivity in comparison to monometallic and homobimetallic systems due to factors such as their uniquely polarised metal-metal bonds and proximate reactive sites. Several aluminyl and tetrylene-derived heterobimetallic compounds have been described in this Chapter, with the importance of the partner metals emphasised by examples where changing only one metal allows activation of substrates previously inaccessible to the system. Chapter Two describes the use of potassium aluminyl [(NON)AlK] in forming aluminium-group 14 heterobimetallic complexes with group 14 amidinate chlorides and an acyclic germylene. Reaction of [(NON)AlK] with group 14 species [(Am)MCl] (M = Ge, Sn, Pb) formed a series of heterobimetallic compounds [(NON)Al-M(Am)] with unsupported Al-M bonds, including the first structurally authenticated example of an Al-Pb bond. Analysis using spectroscopic (NMR, UV-Vis, and Mössbauer), single crystal X-ray diffraction, and computational (DFT, QTAIM, TD-DFT) methods confirmed the Al-M bond as a σ-bond between a Lewis basic aluminium and Lewis acidic group 14 centre, with a minor back-donation from the germanium s-orbital lone pair donor NBO to acceptor NBOs on aluminium. This was followed by reacting [(NON)AlK] with acyclic germylene [{(SiMe3)2N2}Ge] to form an aluminacyclodigermene complex K[(Ge2)(Al(NON))(N(SiMe3)2)] with an AlGe2 ring. Analysis with X-ray diffraction and computational methods suggested this complex to be an η2-coordinated digermyne with Ge(I) centres. Further reduction over time of this compound with [(NON)AlK] afforded complex K3[(Ge4)(Al(NON))2(N(SiMe3)2)] with an internal distorted tetrahedral nido Ge(-I) cluster. Performing this reaction in methylcyclohexane instead afforded compound K2[((NON)Al)2(Ge)2] with a formal digermanium(0) unit and an unusually long Ge=Ge double bond. Chapter Three examines the reactivity of [(NON)Al-M(Am)] (M = Ge, Sn) with chalcogen elements E (E = O, S, Se, Te). The addition of N2O to [(NON)Al-M(Am)] afforded mono-μ-oxo species [(NON)Al(μ-O)Sn(Am)] and bis-μ-oxo compound [(NON)Al(μ-O)2Ge(Am)]. The controlled syntheses of the remaining mono- and bis-μ-chalcogenide complexes [(NON)Al(μ-E)nM(Am)] was made possible using catalytic amounts of a nBu3P chalcogen transfer agent. Analysis by NMR spectroscopy and single crystal X-ray diffraction concluded the chalcogen inserts into the Al-M bond via oxidation at the aluminium centre from Al(I) to Al(III), with a sequential oxidation of M(II) to M(IV) by a second equivalent of chalcogen. Notably, in the mono-μ-chalcogenide species, comparison of bond lengths suggest the Al-E bond decreases in length while descending the group, with a Al=Te double bond present in compound [(NON)Al(μ-Te)Sn(Am)] marking the third shortest reported Al-Te interaction. Bond comparison of the bis-μ-chalcogenide species confirmed intermediate M-E bond lengths between M-E single and M=E double bonds, suggesting this series consists of paired [(NON)Al]+ cations and [(Am)M(E)E]- heavy carboxylate anion analogues. Chapter Four describes the reactivity of [(NON)Al-M(Am)] (M = Ge, Sn) with organic substrates including heteroallenes (CO2, diisopropylcarbodiimide, and cyclohexyl isocyanate), MesN3, and unsaturated carbon chains such as alkenes and alkynes. Addition of CO2 to [(NON)Al-Ge(Am)] forms the bimetallated carboxylate [(NON)Al(μ-CO2)Ge(Am)], while [(NON)Al-Sn(Am)] reacts with additional CO2 to form a carbonate [(NON)Al(μ-CO3)Sn(Am)]. Addition of the other heteroallenes to the [(NON)Al-M(Am)] compounds yielded inserted amidinate, and imidinate complexes, while insertion of MesN3 yields a triazenide intermediate [(NON)Al(μ-N3Mes)Ge(Am)] or azido adduct [(NON)Al(μ-NMes)Sn(Am)] dependent on metal M. Addition of ethylene or alkynes (1-hexyne and phenyl acetylene) would bridge the aluminium and M centres to form novel frustrated Lewis pairs. The further reactivity of these frustrated Lewis pairs were examined against the same small organic molecules, as well as chalcogen elements E (E = O, S, Se, Te). Addition of CO2 to [(NON)Al(μ-PhCCH)Ge(Am)] and [(NON)Al(μ-C2H4)Ge(Am)] bridges the Al/Ge space, confirming frustrated Lewis pair activity. Addition of DIC to [(NON)Al(μ-C2H4)Ge(Am)] instead inserts into the Al-C bond in a C-C bond forming reaction. The addition of cyclohexyl isocyanate to [(NON)Al(μ-PhCCH)M(Am)] formed divergent products, bridging the Al/Ge space while inserting into the Al-C bond in the tin analogue, which suggests tuneability towards different insertions based on the group 14 metal. Introducing CO gas to [(NON)Al(μ-PhCCH)Ge(Am)] demonstrated insertion into the Am ligand, linking the aluminium and germanium centres with a mono-μ-oxo moiety. Reaction of MesN3 with [(NON)Al(μ-PhCCH)Ge(Am)] also added into the Al/Ge space; however, rearrangement of the NON ligand to NNO was observed. The addition of chalcogens E in the presence of a catalytic amount of nBu3P to [(NON)Al(μ-PhCCH)Ge(Am)] inserted the chalcogen to form a series of AlC2EGe five-membered heterocycles. The fluorescent properties of bridged Al/Ge compounds [(NON)Al(μ-PhCCH)Ge(Am)], [(NON)Al(μ-PhCCH)(μ-CO2)Ge(Am)], [(NON)Al(μ-PhCCH)(μ-O)Ge(CAm)], [(NON)Al(μ-PhCCH)(μ-E)Ge(Am)] (E = O, S) were examined with spectroscopic (UV-Vis, emission, excitation) methods, finding a large increase in photoluminescent quantum yield when analysing the [(NON)Al(μ-PhCCH)(μ-E)Ge(Am)] compounds compared to the rest of the series. Chapter Five explores the synthesis of potassium aluminyl [(NON)AlK] to form aluminium-iron heterobimetallic complexes with iron chlorides. Reaction of [(NON)AlK] with iron complexes [(BDIR)FeCl2Li(THF)2] (R = Mes, Dipp) and [(Am)FeCl2Li(THF)3] afforded a series of heterobimetallic species [(NON)Al-Fe(BDIR)] and [(NON)Al-Fe(Am)] with unsupported Al-Fe bonds. Analysis using Evans Method 1H NMR and Mössbauer spectroscopy concluded the Al-Fe series to possess Fe(II) high-spin centres in the solution phase, with potential change to a lower spin-state in the solid phase and at lower temperatures. Addition of two equivalents of cyclohexyl isocyanate to the Al-Fe series coupled the isocyanate units together to form bimetallated oxamide complexes [(NON)Al(μ-CyNCO)2Fe(BDIR)] and [(NON)Al(μ-CyNCO)2Fe(Am)]. Single-crystal X-ray diffraction confirmed the iron centre in compounds [(NON)Al(μ-CyNCO)2Fe(BDIR)] is distorted tetrahedral, while the centre is square planar in [(NON)Al(μ-CyNCO)2Fe(Am)].