The Triphosphide (tBu3Si)2P3Na: Formation, X‐ray and Ab initio Structure Analyses, Protonation and Oxidation to Triphosphane (tBu3Si)2P3H and Hexaphosphanes (tBu3Si)4P6☆

The violet THF adduct (tBu3Si)2P3Na(THF)4 (1a) of the triphosphide (tBu3Si)2P3Na (1) is prepared, (i) by protolysis of the tetraphosphide (tBu3Si)2P4Na2 (2) with an equimolar amount of CF3CO2H in THF (transformation of 2 into 1), (ii) by the reaction of tBu3SiNa and the oligophosphane (tBu3SiP3)n in THF (building-down of Pn), and (iii) by the action of tBu3SiNa on PCl3 in THF (building-up of Pn). According to X-ray structural analysis, the SiPPPSi skeleton of the anionic part [tBu3SiPPPSitBu3]– of 1a is W-shaped with two P–P 11/2 bonds; in addition, 1a contains a planar deltoid P3Na backbone with the 4 THF molecules coordinated to Na. The protolysis of 1a leads to cyclotriphosphane (tBu3Si)2P3H (11), and the oxidation of 1a leads to 1,1′-bicyclotriphosphane (tBu3Si)2P3–P3(SitBu3)2 (12) as the main product and bicyclo[3.1.0]hexaphosphane (tBu3Si)4P6 (13) as an isomer of 12. The structures of 11, 12, and 13 as well as the structure of 1a have been unambiguously determined by 31P-NMR studies. Ab initio structure and energy calculations show that the acyclic P3H2– with allylic conjugation and P–P 11/2 bonds is thermodynamically more stable than the cyclic isomer whereas, in contrast, the neutral compound P3H3 prefers the cyclic form. The exceptional downfield shift of the NMR signal of the central phosphorus in 1a is in agreement with DFT NMR calculations for a model compound [H3SiPPPSiH3]Na with P–P 11/2 bonds.