N-Heterocyclic carbene and cyclic (alkyl)(amino)carbene adducts of gallium hydrides, gallium chlorides and gallium hydrochlorides

Abstract

The synthesis and characterization of N-heterocyclic carbene (NHC) and cyclic (alkyl)(amino)carbene (cAAC) gallane and chlorogallane adducts of the type (NHC)·GaH₃ (NHC = Me₂Im^Me1, iPr₂Im 2, iPr₂Im^Me3 and Dipp₂Im^H4; Me₂Im^Me = 1,3,4,5-tetra-methyl-imidazolin-2-ylidene; R₂Im = 1,3-di-organyl-imidazolin-2-ylidene; Dipp = 2,6-diisopropylphenyl; Dipp₂Im^H = 1,3-bis(2,6-diisopropylphenyl)-imidazolidin-2-ylidene), (NHC)·GaH₂Cl (NHC = iPr₂Im^Me9, Dipp₂Im 10 and Dipp₂Im^H11; iPr₂Im^Me = 1,3 diisopropyl-4,5-dimethyl-imidazolin-2-ylidene), (NHC)·GaHCl₂ (NHC = iPr₂Im^Me12, Dipp₂Im 13 and Dipp₂Im^H14) and (cAAC^Me)·GaHCl₂15 is reported. Compounds 1–3 and 9–11 are unstable in solution as heating to the boiling temperature of toluene (110 °C) leads to decomposition into elemental gallium and the corresponding dihydroaminal NHC–H₂. The reaction of the mono-NHC adducts with a second equivalent of NHC also afforded decomposition and formation of NHC–H₂, whereas the reaction of the NHC-stabilized gallanes with one equivalent cAAC^Me leads to an insertion of the cAAC^Me carbene carbon atom into the Ga–H bond. The synthesis and characterization of (NHC)·GaH₂(cAAC^MeH) (NHC = Me₂Im^Me5, iPr₂Im^Me6 and Dipp₂Im 7), the products of an oxidative addition of (NHC)·GaH₃ to cAAC^Me, are presented. The adduct (Dipp₂Im^H)·GaH₃4 reacts with two equivalents of cAAC^Me under the release of Dipp₂Im^H and insertion of two cAAC^Me molecules into the Ga–H bond to give the bisalkylgallane (cAAC^MeH)₂GaH 8. If one or two hydrogen atoms of the NHC gallane adducts are replaced with an electron-withdrawing chloride substituent, a selective insertion of cAAC^Me into the Ga–H bond occurs only for the adducts of sterically less demanding NHCs such as iPr₂Im^Me. The reaction of cAAC^Me with (iPr₂Im^Me)·GaH₂Cl 9 and (iPr₂Im^Me)·GaHCl₂12 afforded (iPr₂Im^Me)·GaHCl(cAAC^MeH) 16 and (iPr₂Im^Me)·GaCl₂(cAAC^MeH) 17. The reaction of cAAC^Me with (NHC)·GaHCl₂ and (NHC)·GaH₂Cl (NHC = Dipp₂Im, Dipp₂Im^H), adducts of the sterically more demanding NHCs, leads to an extrusion of the NHC to (cAAC^Me)·GaHCl₂15 and (cAAC^MeH)₂GaCl 18.