Intramolecular donor-stabilized tetra-coordinated germanium(iv) di-cations and their Lewis acidic properties

Abstract

We report the first examples of intramolecular phosphine-stabilized tetra-coordinated germanium(IV) di-cationic compounds: [L^iPr₂Ge][CF₃SO₃]₂3^iPr and [L^Ph₂Ge][CF₃SO₃]₂3^Ph (L^iPr = 6-(diisopropylphosphanyl)-1,2-dihydroacenaphthylene-5-ide; L^Ph = 6-(diphenylphosphanyl)-1,2-dihydroacenaphthylene-5-ide). The step wise synthetic strategy involves the isolation of neutral and mono-cationic Ge(IV) precursors: [L^iPr₂GeCl][X] (X = GeCl₃1^iPr, OTf 2^iPr), [L^Ph₂GeCl₂] 1^Ph and [L^Ph₂GeCl][OTf] 2^Ph. Both 3^iPr and 3^Ph exhibit constrained spiro-geometry. DFT studies reveal the dispersion of di-cationic charges over P–Ge–P sites. Anion or Lewis base binding occurs at the Ge site resulting in relaxed distorted trigonal bipyramidal/tetrahedral geometry. 3^iPr and 3^Ph activate the Si–H bond initially at the P-site. The hydride ultimately migrates to the Ge-site rapidly giving [L^Ph₂GeH][CF₃SO₃] 3^PhH, while sluggishly forming [L^iPr₂GeH][CF₃SO₃] 3^iPrH. Compounds 3^iPr and 3^Ph were tested as catalysts for the hydrosilylation of aromatic aldehydes. While catalytic hydrosilylation proceeded via the initial Et₃Si–H bond activation in the case of 3^iPr, compound 3^Ph as a catalyst showed a masked Frustrated Lewis Pair (FLP) type reactivity in the catalytic cycle.