Dynamic Behavior of Intramolecularly Base-Stabilized Phosphatetrylenes. Insights into the Inversion Processes of Trigonal Pyramidal Geramanium(II) and Tin(II) Centers

The reaction between SnCl2 and either 1 or 2 equiv of the lithium salt [{(Me3Si)2CH}(C6H4-2-CH2NMe2)P]Li gives the heteroleptic compound [{(Me3Si)2CH}(C6H4-2-CH2NMe2)P]SnCl (7) and the homoleptic, intramolecularly base-stabilized diphosphastannylene [{(Me3Si)2CH}(C6H4-2-CH2NMe2)P]2Sn (8), respectively, in good yields. The solid state structure of 8 shows that the tin(II) center is three-coordinate, bound by the N and P atoms of a chelating phosphide ligand and the P atom of a second phosphide ligand. Both 7 and 8 are highly dynamic in solution. Variable-temperature NMR spectra suggest that compound 7 and its germanium analogue 5 are subject to two distinct dynamic processes in polar solvents, which are attributed to the formation of adducts between either 5 or 7 and the free phosphine {(Me3Si)2CH}(C6H4-2-CH2NMe2)PH (9) and interconversion between diastereomers of these adducts. Adduct formation is observed only in polar solvents and may be associated with the formation of weakly bound [[{(Me3Si)2CH}(C6H4-2-CH2NMe2)P]E(L)]+···Cl− ion pairs in solution. The dynamic behavior of 8 has been studied by multielement and variable-temperature NMR experiments; at high temperatures there is rapid equilibrium between diastereomers, but at low temperatures a single diastereomer predominates and exchange between the chelating and terminal phosphide ligands is frozen out. DFT calculations on the model compound {(Me)(C6H4-2-CH2NMe2)P}SnCl (7a) suggest that epimerization occurs either through a vertex-inversion process at phosphorus [Einv = 65.3 kJ mol−1] or an edge-inversion process at tin [Einv = 141.0 kJ mol−1], of which the former is clearly favored. DFT calculations on the model complex {(Me)(C6H4-2-CH2NMe2)P}2Sn (8a) indicate that the lowest energy dynamic process involves exchange between the chelating and terminal phosphide ligands via a pseudotrigonal bipyramidal intermediate [E = −12.6 kJ mol−1]. Inversion at tin in 8a (via an unusual hybrid edge/vertex-inversion process) is calculated to have a barrier of 206.3 kJ mol−1, whereas the barriers to vertex-inversion at phosphorus are 59.4 and 51.0 kJ mol−1 for the chelating and terminal phosphorus atoms, respectively.