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

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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 [L1-4H: RN(H)CS(NMe2) and L5-8H: 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(Lx)3 (1-7), trimethylaluminium [MeAl(Lx)2] (8-11) and triethylaluminium [EtAl(Lx)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.