Edge-versus vertex-inversion at trigonal pyramidal ge(II) centers-a new aromatic anchimerically assisted edge-inversion mechanism

Theoretical calculations reveal that the model phosphagermylenes {(Me)P(C6H4-2-CH2NMe2)}GeX [X = F (1F), Cl (1Cl), Br (1Br), H (1H), Me (1Me)], which are chiral at both the phosphorus and pyramidal germanium(II) centers, may be subject to multiple inversion pathways which result in interconversion between enantiomers/diastereomers. Inversion via a classical vertex-inversion process (through a trigonal planar transition state) is observed for the phosphorus center in all compounds and for the germanium center in 1H, although this latter process has a very high barrier to inversion (221.6 kJ mol?1); the barriers to vertex-inversion at phosphorus increase with decreasing electronegativity of the substituent X. Transition states corresponding to edge-inversion at germanium (via a T-shaped transition state) were located for all five compounds; for each compound two different arrangements of the substituent atoms [N and X axial (1XN?X) or P and X axial (1XP?X)] are possible, and two distinct transition states were located for each of these arrangements. In the first of these (1XN?XPlanar and 1XP?XPlanar), inversion at germanium is accompanied by simultaneous planarization at phosphorus; these transition states are stabilized by p??p? interactions between the phosphorus lone pair and the vacant pz-orbital at germanium. In the alternative transition states (1XN?XFolded and 1XP?XFolded), the phosphorus atoms remain pyramidal and inversion at germanium is accompanied by folding of the phosphide ligand such that there are short contacts between germanium and one of the ipso-carbon atoms of the aromatic ring. These transition states appear to be stabilized by donation of electron density from the ?-system of the aromatic rings into the vacant pz-orbital at germanium. The barriers to inversion via 1XP?XPlanar and 1XP?XFolded are rather high, whereas the barriers to inversion via 1XN?XPlanar and 1XN?XFolded are similar to those for inversion at phosphorus, clearly suggesting that the most important factor in stabilizing these transition states is the ?-withdrawing ability of the substituents, rather than ?-donation of lone pairs or donation of ?-electron density from the aromatic rings into the vacant pz-orbital at germanium.