Pyramidal Dicationic Ge(II) Complexes with Homoleptic Neutral Pnictine Coordination: A Combined Experimental and Density Functional Theory Study

An unusual series of Ge­(II) dicationic species with homoleptic phosphine and arsine coordination, [Ge­(L)]­[OTf]₂, L = 3 × PMe₃, triphos (MeC­(CH₂PPh₂)₃), triars (MeC­(CH₂AsMe₂)₃), or κ³-tetraphos (P­(CH₂CH₂PPh₂)₃) (OTf^– = O₃SCF₃^–) have been prepared by reaction of [GeCl₂(dioxane)] with L and 2 mol equiv of Me₃SiOTf in anhydrous CH₂Cl₂ (or MeCN for L = triars, triphos). X-ray crystal structures are reported for [Ge­(PMe₃)₃]­[OTf]₂, [Ge­(triars)]­[OTf]₂, and [Ge­(κ³-tetraphos)]­[OTf]₂, confirming homoleptic P₃- or As₃-coordination at Ge­(II) in each case and with the discrete OTf^– anions providing a charge balance. The Ge–P/As bond lengths are significantly shorter than those in neutral germanium­(II) dihalide complexes with diphosphine or diarsine coordination. Solution NMR spectroscopic data indicate that the complexes are labile in solution. Using excess AsMe₃ and [GeCl₂(dioxane)] gives only the neutral product, [Ge­(AsMe₂)₂(OTf)₂], the crystal structure of which shows four coordination at Ge­(II), via two As donor atoms and an O atom from two κ¹-OTf^– ligands; further weak, long-range intermolecular interactions give a chain polymer. The electronic structure of the [Ge­(PMe₃)₃]²⁺ dication has been investigated using density functional theory (DFT) calculations. The computed geometrical parameters for this dication are in good agreement with the experimental X-ray crystallographic values in [Ge­(PMe₃)₃]­[OTf]₂. The results also indicate that the pyramidal arrangement of the [Ge­(PMe₃)₃]²⁺ (computed P–Ge–P angle 96.8° at the B3LYP-D3 level) arises from a balance between electronic energy (<i>E</i>_elec) contributions, which favor a lower P–Ge–P angle, and nuclear–nuclear contributions (<i>E</i>_nn), which favor a higher P–Ge–P angle, to the total energy (<i>E</i>_TOT). An Atoms in Molecules (AIM) analysis reveals that one reason why <i>E</i>_elec decreases as the P–Ge–P angle decreases is because of C···H and H···H interactions between atoms on different CH₃ groups. The stability of the [Ge­(PMe₃)₃]²⁺ dication is enhanced by the distribution of a significant part of the positive charge on Ge²⁺ to the atomic centers of the PMe₃ ligands. Similar results were obtained for [Ge­(AsMe₃)₃]­[OTf]₂, showing the tris-AsMe₃ complex to be less stable compared to the PMe₃ analogue. Related calculations were also performed for the neutral [Ge­(PMe₃)₂(OTf)₂] and [Ge­(AsMe₃)₂(OTf)₂] complexes.