Alleviating Strain in Organic Molecules by Incorporation of Phosphorus: Synthesis of Triphosphatetrahedrane

Abstract

Phosphatetrahedranes (^<i>t</i>BuCP)₂ and (^<i>t</i>BuC)₃P were recently reported and represent the first tetrahedranes containing a mixed carbon/phosphorus core. Herein, we report that tetrahydrofuran (THF) solutions of the parent triphosphatetrahedrane HCP₃ may be generated in 31% yield (NMR internal standard yield) by combining [Na(THF)₃][P₃Nb(ODipp)₃] (Dipp = 2,6-diisopropylphenyl), INb(ODipp)₃(THF), and bromodichloromethane in thawing THF. While HCP₃ was found to be stable in dilute THF solutions for extended periods of time, the concentration of the solution at -40 °C led to the formation of a black precipitate, which has been tentatively assigned as a polymerized form of HCP₃. HCP₃ reacts readily with (dppe)Fe(Cp*)Cl (dppe = 1,2-bis(diphenylphosphino)ethane, Cp*= η⁵-C₅Me₅) in the presence of Na[BPh₄] to form a purple cationic iron complex of triphosphatetrahedrane (50% yield), which was structurally characterized in a single-crystal X-ray diffraction experiment. Additionally, we present a series of homodesmotic equations analyzed via quantum chemical calculations that suggest triphosphatetrahedrane is the least strained of the mixed C/P phosphatetrahedranes.