Silyl–Hydrosilane Exchange at a Magnesium Triphenylsilyl Complex Supported by a Cyclen-Derived <i>NNNN</i>-Type Macrocyclic Ligand

Abstract

The magnesium triphenylsilyl complex [(Me₃TACD)Mg(SiPh₃)] (2) was synthesized from magnesium bis(triphenylsilyl) [Mg(SiPh₃)₂(THF)₂]·THF (1; THF = tetrahydrofuran) and the NNNN-type macrocyclic amidotriamine proligand (Me₃TACD)H ((Me₃TACD)H = Me₃[12]aneN₄ = 1,4,7-trimethyl-1,4,7,10-tetraazacyclododecane). Treating 2 with AlR₃ (R = Me, Et) gave the magnesium triphenylsilyl complexes with "blocked" amido function [(Me₃TACD·AlR₃)Mg(SiPh₃)] (3a: R = Me; 3b: R = Et). Instead of forming a Mg-H bond upon reaction with dihydrogen or hydrosilanes, 2 and 3a,b underwent rapid silyl-silane exchange with hydrosilanes. Treating the ethyl complex [(Me₃TACD·AlEt₃)MgEt] with H₃SiPh gave [(Me₃TACD·AlEt₃)MgH] (4), albeit not in a reproducible manner. The silyl-hydrosilane exchange allows access to other magnesium silyls of the type [(Me₃TACD)Mg(SiR'₃)] (5a: SiR'₃ = SiH₂Ph; 5b: SiR'₃ = SiHPh₂) and [(Me₃TACD·AlR₃)Mg(SiR'₃)] (6a: SiR'₃ = SiH₂Ph, R = Me; 6b: SiR'₃ = SiH₂Ph, R = Et; 7a: SiR'₃ = SiHPh₂, R = Me; 7b: SiR'₃ = SiHPh₂, R = Et). The reaction of 2 with H₂SiMePh in THF at room temperature resulted in an equilibrium (K_eq ≈ 1). Protonolysis of 2 with Brønsted acids (HX) 2,5-di-tert-butylphenol, phenylacetylene, acetophenone, aniline, and triethylammonium chloride each gave a compound [(Me₃TACD)Mg(X)] with the conjugated base coordinated at the magnesium along with HSiPh₃. The magnesium silyls 1, 2, and 7b as well as the magnesium hydride 4 contain a distorted square-pyramidal magnesium center according to single-crystal X-ray diffraction.