Frustrated Lewis Pairs from Al-E Bonds (E=Ge, Sn, Pb)

The potassium aluminyl K[Al(NON)] ([NON]=[O(SiMeNDipp)], Dipp=2,6-iPrCH) reacts with group 14 chloroamidinates E(Am)Cl (E=Ge, Sn, Pb. [Am]=[tBuC(NDipp)]) to form (NON)Al-E(Am) Lewis pairs with unsupported Al-E bonds, including the first structurally authenticated Al-Pb bond. Analysis using spectroscopic (NMR, UV-vis and Mössbauer for E=Sn), X-ray diffraction and computational (DFT, QTAIM, TD-DFT) methods conclude an Al-E σ-bond derived from a Lewis basic Al and a Lewis acidic tetrylene, with back-donation from the E s-orbital lone pair donor NBO to acceptor NBOs on Al that are derived from s/p-orbitals. The reaction of the Al-Ge compound with CO forms the dioxocarbene (NON)Al(μ-OC)Ge(Am), whilst under the same conditions the Al-Sn compound reacts with additional CO to form the carbonate, (NON)Al(μ-CO)Sn(Am). Addition of ethene to the Al-E (E=Ge, Sn) compounds introduces an ethylene bridge between the aluminium and group 14 atoms forming a frustrated Lewis pair (FLP). Reaction of the (Al/Ge) FLP with CO forms an activation product with new Al-O and Ge-C bonds, whilst reaction with iPrN=C=NiPr proceeds with insertion into the Al-C bond to form an aluminium amidinate.