Bonding and Reactivity of Gallium and Aluminium Coordination Complexes
Authors:   , Cummins Struan 
Published: December 2021

<p>This thesis describes the synthesis, structures and reactivities of gallium and aluminium complexes supported by β-diketiminato ligands ([CR{C(R)N(R’)}₂]-, abbrev. [(BDIR’)]-). Chapter 1 gives a general introduction into the trends and properties that distinguish the heavier p-block elements from their lighter counterparts. An introduction into the theory of multiple bond formation, both homonuclear and heteronuclear, in the heavy p-block elements is provided and a summary of the sterically demanding ligands required to stabilise these complexes is introduced. The β-diketiminato ligand framework utilised in this study is introduced and the methods of generation of low valent gallium and aluminium complexes supported by the BDIDIPP ligand are discussed. Chapter 2 discusses the reactivity of the complex BDIDIPPGa with diazo- compounds in the quest to isolate a complex with a formal gallium-carbon double bond. BDIDIPPGa reacts with two equivalents of both trimethylsilyldiazomethane and diazofluorene, presumably through the target gallium-carbon double bond intermediate. No reaction is observed with di-tert-butyldiazomethane, while BDIDIPPGa catalyses the decomposition of diphenyldiazomethane into tetraphenylethene. Three new β-diketiminato gallium(I) complexes were synthesised: ArBDIDIPPGa, BDIAr*Ga and BDIAr’Ga. ArBDIDIPPGa also reacted with two equivalents of trimethylsilyldiazomethane, presumably through the target gallium-carbon double bond intermediate. BDIAr*Ga and BDIAr’Ga both inserted into the C-H bond of trimethylsilyldiazomethane to give BDIAr*Ga(H)C(N2)SiMe₃ and BDIAr’Ga(H)C(N2)SiMe₃ respectively. Upon addition of diazofluorene to BDIAr*Ga, one of the aromatic protons of the BDIAr* ligand was abstracted by the diazofluorene, resulting in coordination of one of the flanking phenyl groups to the gallium centre. Chapter 3 discusses an investigation into the formation of formal double bonds between aluminium and phosphorus, and gallium and phosphorus. The proposed ‘deprotonation/elimination’ method, reacting BDIDIPPM(PHAr)Cl (M = Al, Ga Ar = Ph, Mes) with nBuLi, resulted in the formation of intractable mixtures of products. Direct synthesis by the addition of MesPLi₂ to BDIDIPPMCl₂ (M = Al, Ga) resulted in the formation of BDIDIPPM(PHMes)Cl (M = Al, Ga). Changing the elimination product to TMS-Cl, through the synthesis of BDIDIPPM(P(TMS)Ph)Cl (M = Al, Ga), resulted in the synthesis of BDIDIPPAl(P(TMS)Ph)Cl, which showed no signs of elimination occurring upon heating to 110 °C. BDIDIPPGa(P(TMS)Ph)Cl could not be isolated, potentially as the complex was undergoing the desired elimination of TMS-Cl, but the resulting complex was decomposing. Changing the elimination product to ethane, through the synthesis of BDIDIPPAl(PHMes)Et, resulted in no sign of elimination occurring upon heating to 110 °C. Reduction of BDIDIPPMCl₂ (M = Al, Ga) in the presence of bistrimethylsilylacetylene, as part of the synthesis of BDIDIPPMLi₂ (M = Al, Ga) salts, was unsuccessful, as was the reaction of BDIDIPPGa with bistrimethylsilylacetylene. Reduction of MesPCl₂ with potassium metal in the presence of BDIDIPPGa resulted in an intractable mixture of products, reduction with magnesium resulted in the formation of (MesP)₃ and (MesP)₄. Addition of MesPH₂ to BDIDIPPGa resulted in the formation of BDIDIPPGa(H)P(H)Mes, which did not undergo H₂ elimination at 110 °C. The synthesis of BDIDIPPAl was unsuccessful as the product could not be isolated cleanly. The synthesis of ArBDIDIPPAl resulted in the intramolecular rearrangement of the ligand to give a five-membered aluminium containing ring. The synthesis of BDIAr*Al stalled at the formation of BDIAr*Al(Me)I due to the steric bulk of the ligand blocking the second substitution of iodine from occurring. Chapter 4 discusses the reactivity of the primary phosphanide complexes BDIDIPPAl(PHMes)Cl, BDIDIPPAl(PHMes)Et and BDIDIPPGa(H)P(H)Mes with phenyl acetylene, 4-nitro-phenyl isocyanate, phenyl isothiocyanate, dicyclohexyl carbodiimide, cyclohexene, benzophenone, benzaldehyde, selenium, sulfur, and methyl iodide. Reactivity was not observed for phenyl acetylene, dicyclohexyl carbodiimide or benzophenone with any of the phosphanides. Reactivity with the phosphanides was observed with cyclohexene, however rapid decomposition of the products occurred and they were unable to be identified. BDIDIPPAl(PHMes)Cl and BDIDIPPGa(H)P(H)Mes showed no reactivity with benzaldehyde, however, the ethyl ligand of BDIDIPPAl(PHMes)Et reacted with the aldehyde proton, eliminating ethane and substituting the PhC(O)- ligand onto the aluminium centre. Reactivity with the phosphanides was observed with both sulfur and selenium, however multiple different products were formed, none of which were successfully isolated. Reactivity between the phosphanides and methyl iodide was observed, with the P-M bond appearing to be cleaved and formation of a M-I bond occurring. 4-nitro-phenyl isocyanate and phenyl isothiocyanate underwent insertion reactions into the M-P bond, however only BDIDIPPAl(Cl)N(4-NO₂-Ph)C(O)P(H)Mes was able to be isolated and fully characterised. Finally, chapter 5 summarises the results of this research and provides an outlook at the future direction of this field of research.</p>