Elusive Aluminyl Anion NHC Analogue Enables Isolation of Quasi-Linear and 2π-Aromatic Biradical B4 Fragments

Abstract

The [DAE]2– ligand (N,N′-bis(2,6-diisopropylphenyl)-1,2-diamidoethylene) has been successfully employed to generate the first stable boryl and gallyl anions, [(DAE)E]− (E = B, Ga), however an analogous aluminyl anion is yet to be reported. While the reduction of DAB (N,N′-bis(2,6-diisopropylphenyl)-1,4-diazabutadiene) and AlI3 with KC8 in a 2:2:3 ratio yields the dialane(4) [(DAE)Al]2 (1), the 2:2:5 reaction in toluene generates the corresponding aluminyl anion dimer [(η4-DAE)AlK(η6-tol)]2 (2tol), which displays a small but non-negligible degree of Al–Al bonding. Recrystallization from hexane, Et2O or in the presence of 18-crown-6 provides the solvent-free coordination polymer [{(η4-DAE)AlK}2]n (2), the Al–Al-bonded dimer [(η4-DAE)AlK(OEt2)]2 (2Et2O), and the trigonal-planar crown-ether complex [(DAE)AlK(18-crown-6)]2 (2crown), respectively. Hydrogenation (1 bar) of 2tol yields the dihydroaluminate coordination polymer 4. The reduction of B4(NMe2)4Br2 by 2tol results in the formation of complex 5, comprising a planar, pseudolinear B4 chain capped at each end by two NMe2 groups and η2,η′2,μ2-bridging between two [(DAE)Al] moieties. X-ray crystallographic and DFT analyses suggest some triple-bond character for the central B2 unit and weak, highly polarized Al–(η2-B2) bonding. Energy decomposition analysis in combination with natural orbitals for chemical valence (EDA-NOCV) reveals that 5 is best viewed as a triplet biradical bis(diaminoboryl)diborene stabilized by two [(DAE)Al]• radicals, with one-electron donor–acceptor rather than electron-sharing bonding. A further two-electron reduction results in rearrangement to a planar diamond-shaped 1,3-dialuminyl-2,4-diamino-substituted 2π-aromatic B4 ring, complex 6. Once more, EDA-NOCV shows that 6 is best described as a triplet biradical [B4(NMe2)2] ring stabilized by two [(DAE)Al(NMe2)]•– radical-anion donors.