Carbon‐Halogen Bond Activation with Powerful Heavy Alkaline Earth Metal Hydrides

Abstract

Reaction of [( DIPeP BDI)SrH] 2 with C 6 H 5 X (X = Cl, Br, I) led to hydride-halogenide exchange ( DIPeP BDI = HC[(Me) phenyl] 2 ). Conversion rates increase with increasing halogen size (F < Cl < Br < I). Reaction of [( DIPeP BDI)SrH] 2 with C 6 H 5 F was slow and ill-defined but addition of C 6 H 4 F 2 gave smooth hydride-fluoride exchange. After addition of THF the full range of Sr halogenides was structurally characterized: [( DIPeP BDI)SrX • THF] 2 (X = F, Cl, Br, I). Mixtures of AeN" 2 and PhSiH 3 in situ formed less defined but more robust Ae metal hydride clusters (Ae x N" y H z , Ae = Ca, Sr, Ba and N" = N(SiMe 3 ) 2 ) which are able to hydrodefluorinate C 6 H 5 F. Conversion rates increase with increasing metal size (Ca < Sr < Ba). Also alkylfluorides (1-Fhexane, F-cyclohexane, 1-F-adamantane) could be converted but, due to solubility problems of the Ba species, the fastest conversion was found for Sr. These AeN" 2 /PhSiH 3 mixtures also converted SF 6 at room temperature to give undefined decomposition products. Addition of Me 6 Tren to a SrN" 2 /PhSiH 3 led to crystallization of [Sr 6 N" 2 H 9 • (Me 6 Tren) 3+ ][SrN" 3 À ]; Me 6 Tren = tris [2-(dimethylamino)ethyl]amine). After hydrodefluorination, Sr 6 N" 4 F 8 • (Me 6 Tren) 2 was formed and structurally characterized. Dissolution in THF led to cluster growth and the larger cluster Sr 16 N" 8 F 24 • (THF) 12 is structurally characterized. DFT calculations support that hydrodehalogenation of halobenzenes follows a concerted nucleophilic aromatic substitution mechanism (cS N Ar).

Diverse Reactivity of a Ca(I) Synthon

“…The dihedral angles between the Ca 2 X 2 and N 2 Ca planes gradually decrease descending the halogen group: F 74.1(1)°>Cl 64.2(1)°>Br 24.0(1)°>I 8.5(1)° (Figure 2b). Although the same trend was also observed in a series of comparable Sr halogenide complexes, [( DIPeP BDI)SrX ⋅ (THF)] 2 (X=F, Cl, Br, I), [30] we have currently no clear‐cut explanation for these geometric changes. The metal coordination spheres are governed by electrostatic and steric repulsion between ligands, as is generally the case for highly ionic s ‐block metal complexes.…”

“…The latter were obtained by hydrodehalogenation of PhX with [( DIPeP BDI)SrH] 2 . This direct X − /H − exchange likely follows a c S N Ar mechanism [30] . Based on formation of biphenyl, we propose a similar nucleophilic mechanism for the reaction of VIII with PhX (Scheme 2).…”

“…[( DIPeP BDI)SrX • (THF)] 2 (X = F, Cl, Br, I), [30] we have currently no clear-cut explanation for these geometric changes. The metal coordination spheres are governed by electrostatic and steric repulsion between ligands, as is generally the case for highly ionic s-block metal complexes.…”

“…This direct X À / H À exchange likely follows a cS N Ar mechanism. [30] Based on formation of biphenyl, we propose a similar nucleophilic mechanism for the reaction of VIII with PhX (Scheme 2). Indeed, the same reaction with D-labeled VIII, ( DIPeP BDI) 2 Ca 2 (C 6 D 6 ), gave C 6 D 5 -C 6 H 5 as one of the products.…”

Beryllium compounds for the carbon–halogen bond activation of phenyl halides: the role of non-innocent ligands

“…They found that the conversion rate increases with the metal size (Ca < Sr < Ba) and halogen size (F < Cl < Br < I). 34 Herein, using (CAAC) 2 Be, NtBuNtBuBe…”