Synthesis, Structures, and Dynamic Behavior of Intramolecularly Base-Stabilized Diphosphatetrylenes Containing a Five-Membered Chelate Ring

The reactions between GeCl2(1,4-dioxane) and 2 equiv of [{(Me3Si)2CH}(C6H4-2-NMe2)P]K and between SnCl2 and 2 equiv of [{(Me3Si)2CH}(C6H4-2-NMe2)P]Li give the corresponding diphosphatetrylenes [{(Me3Si)2CH}(C6H4-2-NMe2)P]2E [E = Ge (5), Sn (6)] as orange crystalline solids in good yield. X-ray crystallography reveals that 5 and 6 adopt similar, but not identical structures in the solid state; each crystallizes as a discrete monomeric diphosphatetrylene, with one chelating phosphide ligand and one ligand bound solely through its P atom. The two compounds differ in the orientation of the noncoordinated nitrogen atom: in 5 the Ge···N distance (>5 Å) is clearly too great for bonding, whereas in 6 the Sn···N distance is just 3.306(3) Å, implying a weak Sn−N bonding interaction. In solution both 5 and 6 exhibit dynamic behavior. However, whereas 5 undergoes both chelating−terminal ligand exchange and epimerization via inversion at phosphorus, compound 6 adopts a pseudo-trigonal-bipyramidal structure in solution at low temperatures. DFT calculations suggest that such a species, the putative intermediate in the associative mechanism for chelating−terminal ligand exchange, is a low-energy minimum on the potential energy surface.