Derivatization of Tris(trimethylsilyl)heptaphosphane

Through SiP bond cleavage, the reaction of P7(SiMe3)3 with one equivalent of KOtBu or LiOtBu afforded different isomers of the heptaphosphanide anion [P7(SiMe3)2]−. With LiOtBu, concomitant inversion at an equatorial (silylated) phosphorus atom occurred and the Cs symmetric isomer characterized by a mirror plane formed. With KOtBu, inversion did not occur and the resulting asymmetric anion with C1 symmetry formed. With NaOtBu, a mixture of both isomers was obtained. The symmetries and structures of the anions were elucidated with 31P{1H} and 29Si{1H} NMR spectroscopy, and relative stabilities were calculated employing the B3LYP/6-31+G* method. The reaction of KP7(SiMe3)2 or LiP7(SiMe3)2 with 1,2-dichlorotetramethyldisilane led to (SiMe3)2P7SiMe2SiMe2P7(SiMe3)2, a molecule composed of two P7-cages connected by a disilane bridge. It can also be obtained through silyl exchange using P7(SiMe3)3 and ClMe2SiSiMe2Cl. The compound was characterized with 31P and 29Si-NMR spectroscopy and elemental analysis. Treatment of P7(SiMe3)3 with HypCl (Hyp = hypersilyl = Si(SiMe3)3) in DME led to the quantitative formation of Hyp2P7SiMe3. Single crystal X-ray diffraction as well as 31P and 29Si-NMR spectroscopy proves the presence of a heteroleptically substituted heptaphosphane cage. Quantum chemical HF and B3LYP/6-31G* calculations of equilibrium structures for the two possible isomers of P7(SiMe3)3 (sym and asym) reveal that asym is destabilized by about 30–40 kJ mol−1, which explains why its formation could not be observed. The phosphorus inversion barrier for the sym → asym transition is calculated as 60–70 kJ mol−1.