Tuning reactivity through implementation of the HSAB concept in oxygen- and sulphur-bridged Al/P and Ga/P FLPs

Sulphur-bridged frustrated Lewis pairs (FLPs) of the type Bis₂E-S-P^tBu₂ (ESP; E = Al, Ga) were synthesised in analogy to their oxygen-bridged E-O-P analogues (EOP). An exchange reaction between AlSP and GaOP affords selectively and in accordance with the concept of hard and soft acids and bases (HSAB) the inverted systems AlOP and GaSP. Reactivity studies towards small molecules, such as CO₂, CS₂, SO₂, N₂O, and propylene sulphide, revealed differences in adduct formations. The adducts EXP·CX₂ and EXP·SO₂ (X = O, S) consist of five-membered heterocylces. The oxidation products EXP·X are four-membered rings; they result from the reaction of the EXP with N₂O and/or propylene sulfide (under loss of propene), except the reaction of AlSP with propylene sulfide that forms a six-membered ring with the whole substrate molecule. The FLP GaSP is exceptional because the formation of its CO₂ adduct is temperature-dependent, confirmed by variable-temperature NMR studies, and its adduct GaSP·CS₂ has two structural isomers. All CS₂ adducts impress with different colours in solution.