Hexaphosphabenzene

Hexaphosphabenzene is a hypothetical molecular allotrope of phosphorus and analogue of benzene, with chemical formula P6. It is expected to share the planar structure of benzene, to which it is valence-isoelectronic, due to resonance stabilization and its sp2 nature. The pure substance has not been synthesized, but it has been studied computationally and some complexes have been synthesized.

Predicted stability Although several other allotropes of phosphorus are stable, no evidence for the existence of P6 has been reported. Preliminary ab initio calculations on the trimerisation of P2 leading to the formation of the cyclic P6 were performed, and it was predicted that hexaphosphabenzene would decompose to free P2 with an energy barrier of 13−15.4 kcal mol−1, and would therefore not be observed in the uncomplexed state under normal experimental conditions. The presence of an added solvent, such as ethanol, might lead to the formation of intermolecular hydrogen bonds which may block the destabilizing interaction between phosphorus lone pairs and consequently stabilize P6. The moderate barrier suggests that hexaphosphabenzene could be synthesized from a [2+2+2] cycloaddition of three P2 molecules. Currently, this is a synthetic endeavour which remains to be conquered.

Synthesis

Isolation of hexaphosphabenzene was first achieved within a triple-decker sandwich complex in 1985 by Scherer et al. Amber coloured, air-stable crystals of [{(η5-Me5C5)Mo}2(μ,η6-P6)] are formed by reaction of [CpMo(CO)2/3]2 with excess P4 in dimethylbenzene, albeit with a yield of approximately 1%. The crystal structure of this complex is a centrosymmetric molecule, and both five-membered rings as well as the central bridge-ligand P6 ring are planar and parallel. The average P–P distance for the hexaphosphabenzene within this complex is 2.170 Å. Thirty years later, Fleischmann et al. improved the synthetic yield of [{(η5-Me5C5)Mo}2(μ,η6-P6)] up to 64%. This was achieved by increasing the reaction temperature of the thermolysis of [CpMo(CO)2/3]2 with P4 to approximately 205 °C in boiling diisopropylbenzene, thus favouring the formation of [{(η5-Me5C5)Mo}2(μ,η6-P6)] as the thermodynamic product. Several analogues of this P6 triple‐decker complex where the coordinating metal and η5-ligand has been varied have also been reported. These include P6 triple‐decker complexes for Ti, V, Nb, and W, whereby the synthetic method is still based on the originally reported thermolysis of [CpM(CO)2/3]2 with P4.

Electron count

If one regards the planar P6 ring as a 6π electron donor ligand, then [{(η5-Me5C5)Mo}2(μ,η6-P6)] is a triple-decker sandwich complex with 28 valence electrons. If P6, similar to C6H6, is taken as a 10π electron donor, a 32 valence electron count may be obtained. In most triple-decker complexes with an electron count ranging from 26 to 34, the structure of the middle ring is planar ([{(η5-Cp)M}2(μ,η6-P6)] with M = Mo, Sc, Y, Zr, Hf, V, Nb, Ta, Cr, and W). In the 24 valence electron [{(η5-Cp)Ti}2(μ,η6-P6)] complex, however, a distortion is observed, and the P6 ring is puckered. Calculations have concluded that completely filled 2a*and 2b* orbitals in 28 valence electron complexes lead to a planar symmetrical P6 middle ring. In 26 valence electron complexes, the occupancy of either 2a*or 2b* results in in-plane or bisallylic distortions and an asymmetric planar middle ring. The puckering of P6 in 24 valence electron complexes is due to the stabilization of 5a, as well as that conferred by the tetravalent oxidation state of Ti in [{(η5-Cp)Ti}2(μ,η6-P6)].

Reactivity

One-electron oxidation The reactivity of [{(η5- Me5C5)Mo}2(μ,η6-P6)] toward silver and copper monocationic salts of the weakly coordinating anion [Al{OC(CF3)3}4]− ([TEF]) was studied by Fleischmann et al. in 2015. Addition of a solution of Ag[TEF] or Cu[TEF] to a solution of [{(η5- Me5C5)Mo}2(μ,η6-P6)] in chloroform results in oxidation of the complex, which can be observed by an immediate colour change from amber to dark teal. The magnetic moment of the dark teal crystals determined by the Evans NMR method is equal to 1.67 μB, which is consistent with one unpaired electron. Accordingly, [{(η5- Me5C5)Mo}2(μ,η6-P6)]+ is detected by ESI mass spectrometry. The crystal structure of the teal product shows that the triple‐decker geometry is retained during the one‐electron oxidation of [{(η5- Me5C5)Mo}2(μ,η6-P6)]. The Mo—Mo bond length of the [{(η5- Me5C5)Mo}2(μ,η6-P6)]+ cation is 2.6617(4) Å; almost identical to the bond length determined for the unoxidized species at 2.6463(3) Å. However, the P—P bond lengths are strongly affected by the oxidation. While the P1—P1′ and P3—P3′ bonds are elongated, the remaining P—P bonds are shortened compared to the average P—P bond length of about 2.183 Å in the unoxidized species. Therefore, the middle deck of the 27 valence electron [{(η5- Me5C5)Mo}2(μ,η6-P6)]+ complex can best be described as a bisallylic distorted P6 ligand, intermediate between the 28 valence electron complexes with a perfectly planar symmetrical ring, and those with 26 valence electrons displaying a more amplified in-plane distortion. Density functional theorem (DFT) calculations confirm that this distortion is due to depopulation of the P bonding orbitals upon oxidation of the triple-decker sandwich complex.

Cu[TEF] & Ag[TEF]

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