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.

tBu2C2P2 can also be reacted with Ni(IMes)2 (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene) in toluene to produce 0.5 equivalents of an asymmetric compound with two hexacoordinate Ni atoms, a Ni-Ni bond, and two weak P-P interactions. This product is an intermediate for further chemistry. Heating the product at 60 °C for 3 hours causes the expulsion of a di-tert-butylacetylene and the reformation of P-P bonds. Another reaction pathway involves the addition of 3 equivalents of CO to the product, leading to the production of Ni(IMes)(CO)3 and the ladderane-like compound described at the beginning of this section. A third reaction pathway involves the addition of hexachloroethane. This produces a 1,2-diphosphocyclobutadiene ring (vide supra) that is coordinated to both nickel atoms. This third reaction pathway also produces the ladderane analogue.

Ligand Substitution In solution with coordination complexes, tBu2C2P2 can cause ligand substitution. Most of these reactions cause cage-opening. The reaction of tBu2C2P2 with [K(crown-6)][Fe(anthracene)2] in toluene and THF causes the expulsion of one anthracene and the cage opening of tBu2C2P2 to form the replacement 1,2-diphosphacyclobutadiene ligand. This cage opening is due to P-C bond cleavage. Mono-ligand substitution is also observed in the reaction of tBu2C2P2 with [(DippBIAN)M(COD)] (Dipp = 2,6-diisopropylphenyl, BIAN = bis(arylimino)acenaphthene, M = Fe, Co). The cobalt product, upon reaction with Cy2PCl (Cy = cyclohexyl), forms a 1,2,3-triphospholium ligand. tBu2C2P2 can also displace the toluene in Ni(toluene)(IPr). Toluene is replaced by the aforementioned ladderane analogue in a η2-fashion.

Double-ligand substitution is seen in the reaction of tBu2C2P2 with [Co(COD)2][K(THF)0.2]. The major product has its ligands doubly substituted by 1,2-diphosphacyclobutadiene. A minor product with double substitution by 1,3-diphosphacyclobutadiene ligand is also observed. However, the cobalt complex with one 1,2-diphosphacyclobutadiene ligand and one 1,3-diphosphacyclobutadiene ligand is not observed; this is likely due to steric clash between the tert-butyl substituents. The preference for 1,2-diphosphacyclobutadiene makes tBu2C2P2 a potentially valuable retron as phosphalkynes are known to produce 1,3-diphosphacyclobutadiene.

Only two of tBu2C2P2's ligand substitution reactions are known to preserve the tetrahedral cage. Reacting (pftb)[Ag(CH2Cl2)2] (pftb = Al[PFTB]− = Al[OC(CF3)3]4−) with tBu2C2P2 in lightless conditions leads to the generation of a disilver complex wherein each of the two tBu2C2P2 ligates to one silver atom and each of the two ladderane analogues (vida infra) ligates to both silver atoms. By reacting Ni(CO)4 in THF at –80 °C and in the absence of light, three tBu2C2P2 molecules coordinate to Ni in an η2-fashion. Calculations by Intrinsic Bond Orbital (IBO) theory suggest that the coordination occurs through a 3-center-2-electron bond.

Reactions with N-Heterocyclic Carbenes tBu2C2P2 can be used a retron to form phosphirenes or phosphaalkenes with the addition of 1 equivalent or 2 equivalents of N-heterocyclic carbenes (NHC), respectively. Upon the addition of 1 equivalent of IPr, IMes, or MesDAC (1,3-bis(2,4,6-trimethylphenyl)diamidocarbene), tBu2C2P2 undergoes ring opening at one phosphorus atom's P-C bonds, creating structures with a bridging P-P bond between the NHC and the phosphirene. IBO calculations and crystallographic evidence support the assignment of double bonding to the P=C bond to resultant molecule. This reaction is very slow, taking several weeks to reach completion.

Upon the addition of 2 equivalents of TMC (2,3,4,5-tetramethylimidazolin-2-ylidene) in benzene, both phosphorus atoms bond to THC. The P-P bond is broken. A double bond also forms between the two carbons of tBu2C2P2, generating a phosphaalkene. This reaction happens significantly faster, with a reported speed of 1 hour.

Selectivity between the two reactions is suggested to be achieved by changing the steric bulk of the NHC used. A bulky NHC should prefer generating a phosphirene, whereas a smaller NHC should prefer generating a phosphaalkene.

Triphosphatetrahedrane

Reaction with (dppe)Fe(Cp*)Cl HCP3, upon addition of [(dppe)Fe(Cp*)]Cl (dppe = 1,2-bis-(diphenylphosphino)ethane) in sodium tetraphenylborate and THF, undergoes salt metathesis and produces [(dppe)Fe(Cp*)(HCP3)][BPh4]. This product is crystallizable, producing purple crystals. This product is prone to decomposition back to HCP3.

Theoretical Work

Azatetrahedranes

Bonding Parameters Ring and cage strain results in poor angular overlap of orbitals, leading to non-linear bonding. Due to the interest in tetrahedrane and their azatetrahedral analogues as highly strained molecules, Politzer and Seminario introduced the "bond deviation index" to determine the deviation of the bond path — defined as the path following maxima between nuclei — and the linear bond between the nuclei.

The strain of experienced by H4C4 is calculated to be partially alleviated upon the substitution of carbon atoms by nitrogen atoms. The bond deviation index decreases with the number of nitrogens in azatetrahedranes from 0.114 to 0.087 from 0 to 2 nitrogen atoms. The engendered stability is countered by the propensity of the N-N bond in the strained system to escape as dinitrogen. This is evidenced by the calculated bond length: 1.59 Å, which is much higher than that of aromatic N-N bonds: 1.21-1.36 Å.