Edge- versus vertex-inversion at trigonal pyramidal Ge(II) centers--a new aromatic anchimerically assisted...

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 (1X(N-X)) or P and X axial (1X(P-X))] are possible, and two distinct transition states were located for each of these arrangements. In the first of these (1X(N-X)(Planar) and 1X(P-X)(Planar)), inversion at germanium is accompanied by simultaneous planarization at phosphorus; these transition states are stabilized by pπ-pπ interactions between the phosphorus lone pair and the vacant p(z)-orbital at germanium. In the alternative transition states (1X(N-X)(Folded) and 1X(P-X)(Folded)), 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 π-system of the aromatic rings into the vacant p(z)-orbital at germanium. The barriers to inversion via 1X(P-X)(Planar) and 1X(P-X)(Folded) are rather high, whereas the barriers to inversion via 1X(N-X)(Planar) and 1X(N-X)(Folded) are similar to those for inversion at phosphorus, clearly suggesting that the most important factor in stabilizing these transition states is the σ-withdrawing ability of the substituents, rather than π-donation of lone pairs or donation of π-electron density from the aromatic rings into the vacant p(z)-orbital at germanium.