Pnictogen-substituted tetrahedranes are pnictogen-containing analogues of tetrahedranes with the formula RxCxPn4−x (Pn = N, P, As, Sb, Bi). Computational work has indicated that the incorporation of pnictogens to the tetrahedral core alleviates the ring strain of tetrahedrane. Although theoretical work on pnictogen-substituted tetrahedranes has existed for decades, only the phosphorus-containing species have been synthesized. These species exhibit novel reactivities, most often through ring-opening and polymerization pathways. Phosphatetrahedranes are of interest as new retrons for organophosphorus chemistry. Their strain also make them of interest in the development of energy-dense compounds.
History Tetra-tert-butyltetrahedrane (tBu4C4) was reported in 1978 by Maier and coworkers Other Platonic solid species, like cubane and dodecahedrane had been reported by that time. As of 2023, the unencumbered tetrahedrane (H4C4) has yet to be synthesized. The substitution of carbons in the tetrahedrane core is implicit by the stability of white phosphorus and yellow arsenic. Mixed tetrahedral pnictogen molecules include AsP3 and (PbBi3)−. Elements on the extreme ends of the pnictogen family have not yet been observed in a tetrahedral Pn4 configuration, however. Nitrogen's orbitals lack diffusivity, and bismuth’s orbitals undergo minimal hybridization due to relativistic contraction. Computational studies into mixed pnictogen-tetrel tetrahedranes have suggested that pnictogen-substituted tetrahedranes are more stable than their all tetrel counterparts decades before their first synthesis.
In 2019, Wolf and coworkers synthesized di-tert-butyldiphosphatetrahedrane (tBu2C2P2) by the reaction of nickel catalyst with phosphaalkynes. In 2020, Cummins and coworkers announced tri-tert-butylmonophosphatetrahedrane (tBu3C3P). In 2021, Cummins and coworkers published the synthesis of triphosphatetrahedrane (HCP3), completing the set of tetrahedral molecules with carbon- and phosphorus-containing cores.
Phosphatetrahedrane Synthesis Despite the similarities of their structures, the syntheses of phosphatetrahedranes differ sharply.
Tri-tert-butylmonophosphatetrahedrane tBu3C3P was prepared by the reaction of tri-tert-butyl cyclopropenium with the triphenylborane adduct of an "P-" equivalent.
An improved synthesis was improved with alternative sources of "P-" equivalent.
Di-tert-butyldiphosphatetrahedrane In 2019, Wolf and coworkers reported the synthesis of tBu2C2P2 through the use of a metal catalyst. Ni(IPr)(CO)3, upon addition of 1 equivalent of tert-butylphosphaacetylene (tBuCP), loses two carbon monoxide ligands. The addition of a second equivalent of tBuCP generates the 1,3-diphosphacyclobutadiene ligand, now binding with η4 hapticity. Density functional theory calculations into the catalytic cycle suggest that the 1,3-diphosphacyclobutadiene isomerizes into the desired tetrahedrane. Upon addition of a final tBuCP, (tBu2C2P2) is released and the catalytic cycle can begin again.
Triphosphatetrahedrane Cummins and coworkers reported the synthesis of HCP3 in 2021. Due to the similarity of HCP3 to AsP3, the [NbII(ODipp)3(P3)]− previously shown to be a retron for AsP3 was used for the synthesis of HCP3. To add a -CH group to [P3]3-, bromodichloromethane undergoes halogen abstraction, leaving a carbon-centered radical. The niobium complex then undergoes P3 transfer to yield HCP3. The use of bromodichloromethyl trimethylsilane instead of bromodichloromethane in this process yields trimethylsilyl triphosphatetrahedrane ((Me3S)CP3).
Reactivity
Tri-tert-butylmonophosphatetrahedrane
Lewis Acid-Induced Reactions Addition of W(CO)5(THF) to tBu3C3P generates a phosphorus-containing housene analogue. The addition of 0.2 equivalents of triphenylborane in benzene can produce several cycloadducts. In the absence of exogenous reagents, tBu3C3P dimerizes into a ladderane-like compound with a P-P bond. In the presence of excess styrene or an atmosphere of ethylene, [4 + 2] cycloadditions occur to give 1-phosphabicyclo[2.2.0]hexenes.
Silylene Reaction The cage opening of tBu3C3P can be induced by PhC(NtBu)2SiN(SiMe3)2 over the course of 24 hours to generate the dark red phosphasilene PhC(NtBu)2Si=P(tBu3C3).
Ylide Reaction Reaction of tBu3C3P with the ylide Ph3P=CH2 over 48 hours and with heat induces cage opening in the same manner as the silylene reaction to generate H2C=P(tBuC)3. Reaction of this product with tBu3C3P generates the symmetric product (tBuC)3P(C)P(tBuC)3.
Formation of Phosphirane tBu3C3P is a retron for phosphirane synthesis. Upon reaction with Ni(COD)2 (COD = cycloocta-1,5-diene) catalyst in triisopropylphosphine, cage opening occurs. Like the silylene and ylide reactions, the phosphorus bridges the (tBuC)3 and the alkene components. The phosphate undergoes cycloaddition with the double bond to form the phosphirane moiety. This reaction pathway has been demonstrated for styrene, ethylene, and neohexene. Furthermore, this reaction pathway is also capable of synthesizing vinyl-substituted phosphirane as evidenced by tBu3C3P and cyclohexa-1,3-diene.
Ligand Substitution tBu3C3P can be used to replace the ethylene ligand of (Ph3P)Pt(C2H4) in melting THF.
Di-tert-butyldiphosphatetrahedrane
Dimerization Reactions Above the melting point of tBu2C2P2 (–32 °C), tBu2C2P2 dimerizes into another ladderane-like structure but it is prone to decomposition. This reaction can be hampered by keeping tBu2C2P2 under its melting point and/or by keeping the tBu2C2P2 concentration low. tBu2C2P2 can also be dimerized using nickel complexes to form a variety of exotic structures. tBu2C2P2 reacted with 1 equivalent of Ni(CpR)(IPr) (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene, R = H, CH3, 4-(CH3CH2)-C6H4) generates 0.5 equivalent of a tetracyclo-compound. Upon addition of another equivalent of the same nickel complex, a butterfly-like geometry is adopted, with two nickel atoms coordinated to opposite phosphorus atoms and two coordinated to adjacent phosphorus atoms on different four membered rings. This butterfly-structured compound is a dark red color. The reaction to the butterfly structure is believed to depend on kinetic access to the middle P-P bond. Bulky substituents on CpR kinetically hinder the P-P bond cleavage and transformation into the butterfly-structured product.
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