Supersilylated Tetraphosphene Derivatives M2[t-Bu3SiPPPPSi-t-Bu3] (M = Li, Na, Rb, Cs) and Ba[t-Bu3SiPPPPSi-t-Bu3]: Reactivity and Cis-Trans Isomerization

The tetraphosphenediides M-2[t-Bu3SiPPPPSi-t-Bu-3] (M = Li, Na, K) were accessible by the reaction of P-4 with the silanides M[Si-t-Bu-3] (M = Li, Na, K), whereas M-2[t-Bu3SiPPPPSi-t-Bu-3] (M = Rb, Cs) were obtained from the reaction of RbCI and CsF with Na-2[t-Bu3SiPPPPSi-tBu(3)]. P-31 NMR experiments revealed that, in tetrahydrofuran, Na-2[t-Bu3SiPPPPSi-t-Bu-3] adopts a cis configuration. However, treatment of Na-2[t-Bu3SiPPPPSi-t-Bu-3] with 18-crown-6 led to the formation of [Na(18-crown-6)(thf)(2)](2)[t-Bu3SiPPPPSi-t Bu-3] that possesses a trans configuration in the solid state. The ion-separated tetraphosphenediide [Na(18-crown-6)(thf)(2)](2)[t-Bu3SiPPPPSi-t-Bu-3] was analyzed using X-ray crystallography (monoclinic, space group P2(1)/n). The reaction of Na-2[t-Bu3SiPPPPSi-t-Bu-3] with Bal(2) gave, conveniently, the corresponding barium derivative Ba[t-Bu3SiPPPPSi-t-Bu-3]. However, addition of Aul to the tetraphosphenediide Na-2[t-Bu3SiPPPPSi-t-Bu-3] yielded 1,3-diiodo-2,4-disupersilyl-cyclotetraphosphane (monoclinic, space group C2/c), which is an isomer of disupersilylated diiodotetraphosphene. A further isomeric derivative of disupersilylated tetraphosphene, the 3,5-disupersilyl-2,2-di-tert-butyl-2-stanna-bicyclo[2.1.0(1,4)]pentaphosphane, which possesses a phosphanylcyclotriphosphane structure, was obtained by the reaction of Na-2[t-Bu3SiPPPPSi-t-Bu-3] with t-Bu2SnCl2. Calculations revealed that the acyclic cis and trans isomers of the dianions [HPPPPH](2-) and [H3SiPPPPSiH3](2-) are thermodynamically more stable than the cyclic isomers with a phosphanylcyclotriphosphane or a cyclotetraphosphane structure. However, the neutral cyclic isomers of H4P4 and H-2(H3Si)(2)P-4 represent more stable structures than the cis- and trans-tetraphosphenes H2P-P=P-PH2 and (H3Si)HP-P=P-PH(SiH3), respectively. In addition, the molecular orbitals (MOs) of the silylated cis- and trans-tetraphosphene dianions of [H3SiPPPPSiH3](2-), which are comparable with those of the ion-separated supersilylated tetraphosphenediide [t-Bu3SiPPPPSi-t-Bu-3](2-), show the highest occupied antibonding pi*MO (HOMO). The HOMO is represented by the (P-z-P-z+P-z-P-z) pi* MO.