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

TL;DR

This study explores how ligand design affects the decomposition temperature of aluminum compounds, aiming to create easy-to-handle precursors that decompose below 200°C.

Contribution

This is the first study to demonstrate ligand-directed design of aluminum compounds with low decomposition temperatures and inherent fluxionality.

Findings

Highly fluxional compounds with longer Al−N bonds showed significantly lower decomposition temperatures.

Systematic ligand variation revealed a direct link between structural dynamics and thermal decomposition profiles.

Solution-state dynamics observed via NMR and SCXRD correlate with solid-state decomposition behavior.

Abstract

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 [L
1
H–L
8
H] were chosen with the steric bulk on the N atoms of these ligands varied systematically [L
1−4
H: RN(H)CS(NMe2) and L
5−8
H: RN(H)CS(NEt2); R=Me (L
1
H and L
5
H), Et (L
2
H and L
6
H), iPr (L
3
H and L
7
H) and Ph (L
4
H and L
8
H). 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. 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.

In this work, we have investigated the design and synthesis of aluminium compounds via systematic variation of ligand type. This is the first example of a ligand directed design and study of aluminium compounds which decompose at low temperatures (<200 °C). Their dynamic properties have also been investigated, with a focus on the decomposition profiles.

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