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

We report the first examples of intramolecular phosphine-stabilized tetra-coordinated germanium(iv) di-cationic compounds: [L iPr 2 Ge][CF 3 SO 3 ] 2 3 iPr and [L Ph 2 Ge][CF 3 SO 3 ] 2 3 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 iPr 2 GeCl][X] (X = GeCl 3 1 iPr , OTf 2 iPr ), [L Ph 2 GeCl 2 ] 1 Ph and [L Ph 2 GeCl][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 Ph 2 GeH][CF 3 SO 3 ] 3 Ph H, while sluggishly forming [L iPr 2 GeH][CF 3 SO 3 ] 3 iPr H. Compounds 3 iPr and 3 Ph were tested as catalysts for the hydrosilylation of aromatic aldehydes. While catalytic hydrosilylation proceeded via the initial Et 3 Si-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.