HP 4 - and (CH 2 )P 3 - Anions Form Four-membered Rings with an Allyl Moiety ? An ab initio /NMR study

Abstract

On the energy hypersurfaces of the anions HP4 - and CH2P3 - at the RMP2(fc) /6-31+G(d) level, the isomers with triphosphaallyl moiety are the lowest energy structures. For these free 1-X-2,4-(PB)2-3-PA - anions characteristic 31P NMR chemical shifts, are predicted to be (for X = PH, 1, δ31P(PA) = 517, δ31P(PB) = 424, and δ31P(PX) = 50; for X = CH2, 4, δ31P(PA) = 611, δ31P(PB) = 450). The observed δexp 31P values for HP4 - (Na/K, DME) completely disagree with the δ31P calculated at GIAO/MP2/6-311+G(d) //RMP2(fc) /6-31+G(d) for structure 1. The rotational average of the phosphinidyltriphosphirene structures (P3-PH-, 3) agree better with the δexp 31P than those with a bicyclo[1.1.0]hydrogentetraphosphanide backbone, 2. MO analysis can rationalize the extreme endo/exo effect (Δδ31P = 455 ppm) on the chemical shift in the exocyclic PH group of 3. The lowest energy geometry of the anion 3 has Erel of 31 kJ mol-1 relative to 1. The most favored 3 + Na+ structure is only 15 kJ mol-1 above the lowest energy HP4Na minimum, 2 + Na+ with Na+ in endo and H in exo orientation of the bicyclo-P4 framework (Erel of 1 + Na+ is 13 kJ mol-1). In most HP4Na structures the Na+ changes the 31P NMR chemical shifts towards higher field with respect to the bare anions.