Tuning Decomposition Temperature: A Structural Study of Ligand Directed Bonding and Fluxionality

In the first study of its kind towards the design and synthesis of easy-to-handle aluminium precursors that decompose at temperatures <200 °C: informed ligand choice and structural design of the compounds has caused inbuilt fluxionality leading to a markable decrease in the onset of decomposition temperatures. Eight thiourea ligands [L¹H-L⁸H] were chosen with the steric bulk on the N atoms of these ligands varied systematically [L^1-4H: RN(H)CS(NMe₂) and L^5-8H: RN(H)CS(NEt₂); R=Me (L¹H and L⁵H), Et (L²H and L⁶H), ⁱPr (L³H and L⁷H) and Ph (L⁴H and L⁸H). Three families of aluminium compounds were synthesised by the reaction of these thiourea ligands with trimethylamine alane [Al(L^x)₃ (1-7), trimethylaluminium [MeAl(L^x)₂] (8-11) and triethylaluminium [EtAl(L^x)₂] (12-14) respectively. The three most spatially encumbered compounds (Al(L³)₃ (2), Al(L⁶)₃ (5) and Al(L⁷)₃ (6) are highly fluxional in solution and displayed lengthening of the Al-N bond as compared to the other compounds. Both factors directly affect the activation temperature of these compounds. The remaining compounds were not shown to display any of these behaviours and consequently have higher thermal decomposition temperatures. SCXRD, ¹H and ¹³C{¹H} NMR, variable temperature ¹H NMR, MS and EA have been used to study the structure and solution dynamics of 1-14. This has directly been linked to the decomposition profiles of the compounds to assess their viability as precursors, evidencing that what we see in the solution state is present in the solid state too. Density functional theory calculations have been carried out to elucidate the various bonding modes observed for compounds 1-7. Tandem MS has been employed to better understand the breakdown of the molecules.