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

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journal contribution
posted on 28.07.2021, 13:03 by Rhys P. King, Victoria K. Greenacre, William Levason, John M. Dyke, Gillian Reid
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 (Eelec) contributions, which favor a lower P–Ge–P angle, and nuclear–nuclear contributions (Enn), which favor a higher P–Ge–P angle, to the total energy (ETOT). An Atoms in Molecules (AIM) analysis reveals that one reason why Eelec 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.