Pnictogen-substituted tetrahedranes

[1] Computational work has indicated that the incorporation of pnictogens to the tetrahedral core alleviates the ring strain of tetrahedrane.

The first synthetic tetrahedral molecule, tetra-tert-butyltetrahedrane (tBu4C4) was reported in 1978 by Maier and coworkers[6] following the synthesis of other Platonic solid species, like cubane and dodecahedrane.

The tert-butyl substituents were used to encumber the tetrahedral core and quell the radical-mediated ring-opening of an otherwise kinetically stable but thermodynamically strained molecule via the corset effect.

In 1990, calculations on azatetrahedranes suggested positive correlation between the number of nitrogens in the tetrahedral core and thermodynamic stability.

[2] In the same vein, calculations done in 2010 on pnictacubanes suggested positive correlation between the number of phosphoruses in the cuboidal core and the thermodynamic stability.

[10] In 2019, Wolf and coworkers synthesized the first pnictogen-substituted tetrahedrane: di-tert-butyldiphosphatetrahedrane (tBu2C2P2), produced from the reaction of nickel catalyst with phosphaalkynes.

[3] In 2021, Cummins and coworkers published the synthesis of triphosphatetrahedrane (HCP3), completing the set of tetrahedral molecules with carbon- and phosphorus-containing cores.

[5] Despite the presentation of phosphatetrahedranes as a series of incrementally changing tetrahedrane derivatives, their syntheses are vastly different.

The addition of tri-tert-butyl cyclopropenium ion produces the thermally stable cyclopropenyl phosphine intermediate.

[3] An improved version of tri-tert-butylmonophosphatetrahedrane synthesis, where the anthracene is replaced by two trimethylsilyl groups, was reported by Cummins and coworkers a year later.

Ni(IPr)(CO)3, upon addition of 1 equivalent of tert-butylphosphaacetylene (tBuCP), loses two carbon monoxide ligands.

Density functional theory calculations into the catalytic cycle suggest that the 1,3-diphosphacyclobutadiene isomerizes into the desired tetrahedrane.

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.

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).

[5] Ring and cage strain results in poor angular overlap of orbitals, leading to non-linear bonding.

[18][19] On the basis of the higher electronegativity of nitrogen than carbon, azatetrahedranes have less negative electrostatic potentials at their C-C bonds than H4C4, leading to greater stability against electrophilic attacks.

However, essentially thermodynamically neutral nitro group rotation leads to small amounts of C-N lengthening,[21] weaking the interaction and localizing the electrons.

[23] Due to the instability of many azatetrahedranes, isodesmic comparisons to azacyclobutadiene analogues have been used to determine which core structures are the most synthetically feasible.

Alkorta, Elguero, and Rozas reported that every member of the azatetrahedrane core series is always slightly more unstable than their azacyclobutadiene analogue(s).

[2] Jursic's calculations suggest that the energetic differential between the azatetrahedrane and the azacyclobutadiene starts off large and decreases as the number of nitrogen atoms increase in the cage.

Furthermore, Jursic's calculations suggest that tetraazatetrahedrane may be slightly more stable (difference of 4.4 kcal/mol at 0 K and the CBSQ level of theory) than its azacyclobutadiene analogue.

The more diffuse orbitals of phosphorus versus carbon also favor the tetrahedral structure's σ-interactions over the planar phosphacyclobutadiene's π-interactions.

Calculations with one of the tert-butyl substituents with a methyl, ethyl, or isopropyl group result in net repulsion due to the loss of HHBs.

[5] Non-Lewis donation of electron density from the tetrahedral core to the tert-butyl substituents also stabilizes tBu3C3P according to natural bond orbital theory.

[27] Schaefer and coworkers, in light of the synthesis of tBu3C3P, ran calculations on the mono-pnictogen-substituted tetrahedrane series, represented by R3C3Pn (R = H, tBu, Pn = N, P, As, Sb, Bi).

The H-[C-C-C plane] angle increases from 9.1° to 31.1°, which is also attributed to the diffusivity of the heavier congener's orbitals.

The C-C-C ring becomes increasingly more negatively charged with the heavier pnictogens.Isodesmic reactions show greater stabilization of the cage structure due to the diffusivity of the pnictogen's orbitals, although even with bismuth, the mono-pnictogen-substituted tetrahedrane is unstable.

For example, electron density is increasingly transferred from the Pn-C bonds into the Pn lone pair in the heavier congeners.

The former set of interactions stabilize the tetrahedral core most when the substituent is an electron-withdrawing group (e.g., fluoride), although decreased electron density in C-C and C-Pn can facilitate cage-opening as well.

All currently synthesized pnictogen-substituted tetrahedranes in the scientific literature as of 2023.
Synthesis of tri- tert -butylmonophosphatetrahedrane with anthracene leaving group.
Synthesis of di- tert -diphosphatetrahedrane.
Synthesis of triphosphatetrahedrane.
Lewis acid-induced reactions of triphosphatetrahedranes.
Monophosphatetrahedrane reaction with silylene.
Monophosphatetrahedrane reaction with Ph 3 P=CH 2 ylide.
Formation of phosphiranes from monophosphatetrahedrane.
Monophosphatetrahedrane substitutes an ethylene ligand.
Stepwise dimerization of diphosphatetrahedrane with Ni(Cp)(IPr).
Di- tert -butyl-diphosphatetrahedrane reaction with Ni(IMes) 2 and further reactions.
Diphosphatetrahedrane ligand substitution of anthracene forms 1,2-diphosphacyclobutadiene analogue.
Diphosphatetrahedrane ligand substitution of cycloocta-1,5-diene forms triphosphacyclopentane analogue.
Diphosphatetrahedrane ligand substitution of toluene forms ladderane analogue.
Diphosphatetrahedrane ligand substitution of cycloocta-1,5-diene forms diphosphacyclobutadiene analogues.
Diphosphatetrahedrane ligand substitution with [Ag(C 2 H 2 Cl 2 ) 2 ](pftb).
Reaction of diphosphatetrahedrane with Ni(CO) 4 yields intact diphosphatetrahedrane ligands.
Diphosphatetrahedrane reactivity with NHCs is controlled by sterics.
Triphosphatetrahedrane reaction with (dppe)Fe(Cp*)Cl.
Intrinsic bond orbital analysis of monophosphatetrahedrane cage closing. Note that the intrinsic bond orbital corresponding P-Cl σ bond is localized on the chloride anion upon cage closing.
Molecular graph of tri- tert -butylmonophosphatetrahedrane optimized at the B3LYP-D3/6-31G** level of theory shows significant bond deviations.
Depiction of the corset effect in tri-tert-butylmonophosphatetrahedrane where the attractive interaction between hydrogen atoms of neighboring tert -butyl groups are highlighted in red.
Tabulated bond angles and lengths of the mono-pnictogen-substituted tetrahedrane series calculated at the CCSD(T)/aug-cc-pwCVTZ(-PP) level of theory.