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

Phosphatetrahedranes (tBuCP)2 and (tBuC)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.