Stepwise Reduction at Magnesium and Beryllium: Cooperative Effects of Carbenes with Redox Non-Innocent α-Diimines

Abstract

In the past two decades, the organometallic chemistry of the alkaline earth elements has experienced a renaissance due in part to developments in ligand stabilization strategies. In order to expand the scope of redox chemistry known for magnesium and beryllium, we have synthesized a set of reduced magnesium and beryllium complexes and compared their resulting structural and electronic properties. The carbene-coordinated alkaline earth−halides, ( Et2 CAAC)-MgBr 2 (1), (SIPr)MgBr 2 (2), ( Et2 CAAC)BeCl 2 (3), and (SIPr)BeCl 2 (4) [ Et2 CAAC = diethyl cyclic(alkyl)(amino) carbene; SIPr = 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazole-2-ylidene] were combined with an α-diimine [2,2bipyridine (bpy) or bis(2,6-diisopropylphenyl)-1,4-diazabutadiene ( Dipp DAB)] and the appropriate stoichiometric amount of potassium graphite to form singly-and doubly-reduced compounds ( Et2 CAAC)MgBr( Dipp DAB) (5), ( Et2 CAAC)MgBr(bpy) ( 6), ( Et2 CAAC)Mg( Dipp DAB) ( 7), ( Et2 CAAC)Be(bpy) (8), and (SIPr)Be(bpy) (9). The doubly-reduced compounds 7−9 exhibit substantial π-bonding interactions across the diimine core, metal center, and π-acidic carbene. Each complex was fully characterized by UV−vis, FT-IR, X-ray crystallography, 1 H, 13 C, and 9 Be NMR, or EPR where applicable. We use these compounds to highlight the differences in the organometallic chemistry of the lightest alkaline earth metals, magnesium and beryllium, in an otherwise identical chemical environment.

Chemistry of alkaline earth metals: It is not all ionic and definitely not boring!

“…Some recent examples on how to obtain Grignard compounds use one or two N-heterocyclic carbene ligands on magnesium halides [21]. In contrast to beryllium, magnesium typically tends to have higher coordination numbers, as exemplified in the carbene chemistry of both metal ions [22]. Indeed, with the same N-heterocarbene ligand, the magnesium compound rather forms a dimer with bridging halide anions and coordination number 4, while beryllium forms a monomer with coordination number 3 (Fig.…”

“…While compounds with hetero metal-metal bonds, like in Cu-Mg or the recent Mg-Pd compounds described by Crimmin et al [30,31] or the above-mentioned Mg + -compounds [22][23][24] as well as the hydrides with sterically demanding ligands [27], feature coordination number 3 or 4 (tetrahedral) for the alkaline earth metal, the Mg 2+ ion, like for beryllium, can also be coordinated in a square planar fashion in e.g. porphyrins [32].…”

“…Top: Comparison of coordination number for Mg 2+ (left) and Be 2+ (right) with the same N-heterocyclic carbene ligand (adapted from[22]); Bottom: Reduction of such complexes in presence of chelating N-donor ligands to form radical species (adapted with permission from[22]). …”

N-Heterocyclic Carbene-Supported Aryl- and Alk- oxides of Beryllium and Magnesium

“…In addition, we have prepared related magnesium alkoxide dimers, [(sIPr)Mg(OEt)Brl] 2 (4) and [(sIPr)Mg(OEt)Me] 2 (5). In nearly all of these cases, the synthetic route began with the preparation of a ligand stabilized metal halide complexes of Be or Mg, of which a number of examples are known [47,48].…”

Reduction of a 1,4-diazabutadiene and 2,2’-bipyridine using magnesium(I) compounds

“…While the one and two-electron reductions of such compounds have previously been achieved by a number of methods (e.g. reaction with activated elemental magnesium, and alkali metal reductions of magnesium halides in the presence of the unsaturate), product selectivity can be a problem (Freeman et al, 2019;Gao et al, 2011;Gardiner et al, 1994;Liu et al, 2009;Ren et al, 2017). We believed that the unique combination of properties that magnesium(I) reagents hold could lend them to the selective oneelectron reduction of 1,4-diazabutadienes (DAB) and 2,2'-bipyridine (bipy).…”