Unsaturated main group element compounds are capable of extraordinary reactivity. In particular, aluminium (I) species and silicon (II) species are well known to enact challenging small molecule activations. But little is known about their combined chemistry, with only a small number of unsaturated (or low oxidation-state) aluminium-silicon compounds reported, the first after the start of this work in 2022. Meanwhile, metal-silylene and unsaturated element-silicon compounds are well established and have exhibited unique modes of reactivity, including metal-ligand (or rather element-silicon) cooperativity. Bis-silylenes can act as a source of low oxidation-state silicon, since they are molecules containing two silicon (II) donor sites. They have been applied successfully to the synthesis of many interesting transition metal and main group complexes, but so far not to the more electropositive main group metals. To expand on both the chemistry of bis-silylenes and aluminium-silicon compounds, this project set out to explore the synthesis of unsaturated aluminium-silylene species, derived from bis-silylenes, and study their reactivity. Three bis-silylene ligands are investigated in this work, denoted as SiII (spacer) SiII where SiII = PhC (NtBu) 2Si and the spacers are 9, 9-dimethylxanthene (Xant), o-carborane (CCcage) or 2, 7, 9, 9-tetramethylacridane (NAcrid). The first bis-silylene investigated was SiII (Xant) SiII, used to prepare the aluminium (III) cation complex SiII (Xant) SiIIAlI2+I. Reaction of the iodide complex with Collman’s reagent, K2Fe (CO) 4, gave the iron (0) -stabilized iodoaluminium (I) complex SiII (Xant) SiIIAl (I) Fe (CO) 4. Reaction with other reducing agents led to no isolable products. Therefore, another bis-silylene SiII (CCcage) SiII with subtly different properties was investigated next and used to synthesize the aluminium (III) complex SiII (CCcage) SiIIAlI2+I with a closo-carborane backbone. Due to the redox-noninnocence of carboranes, one- or two-electron reduction with KC8 reduced the backbone to access the radical anion and nido dianionic forms, respectively, thus establishing a redox series with three ligand oxidation states characterized. Reaction instead with K/C10H8 unexpectedly gave the 1, 4-napthalene derivative K[SiII (CCcage) SiIIAl (C10H8) ]2. This indicated the formation of transient low-valent aluminium intermediates that are trapped in a 1+4 cycloaddition with C10H8. The carborane backbone was found to be important for the stability of the fleeting unsaturated intermediates. To increase the chance of stabilizing a highly reactive low-valent aluminium center, the anionic bis-silylene SiII (NAcrid) SiII was then employed for the synthesis of a pincer silylene aluminium complex bearing a more stabilizing Al-N covalent bond. The resulting pentacoordinate aluminium (III) complex SiII (NAcrid) SiIIAlI2 was reacted with KC8 giving the unexpected silicon (IV) -aluminium (III) four-membered ring compound [SiII (NAcrid) SiIVAl]2. This product indicated the initial formation of an unsaturated aluminium-silicon intermediate that undergoes a 2+2 cycloaddition reaction to give the ring compound. Since all attempts to trap the unsaturated intermediate failed, Collman’s reagent was applied again, enabling isolation of the silylene-aluminium (I) -iron (0) complex SiII (NAcrid) SiIIAlFe (CO) 4, which was studied with DFT calculations. The calculations predicted that the unsaturated intermediate was most likely a silylene-aluminylene species, with a lone pair of electrons residing predominantly at the aluminium center. Further study of SiII (NAcrid) SiIIAlFe (CO) 4 revealed that it transforms in solution into a second aluminium (I) -iron (0) complex SiII (NAcrid) SiIIAlFe (CO) 3 due to silylene transfer to the Fe center. Reactivity studies revealed that the second Al-Fe complex was more reactive. It reacted with Lewis bases (DMAP and OPEt3), alkyl halides, aniline and alcohols. The sigma-alane SiII (NAcrid) SiIIAl (NDipp) (mu-2-H) Fe (CO) 3, resulting from N-H activation by the Al-Fe bond, was isolated and characterized.
Artemis Saddington (Thu,) studied this question.