Hexaphosphabenzene is a valence isoelectronic analogue of

benzene Benzene is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar ring with one hydrogen atom attached to each. Because it contains only carbon and hydrogen atoms, ...

and is expected to have a similar planar structure due to

resonance stabilization In chemistry, resonance, also called mesomerism, is a way of describing bonding in certain molecules or polyatomic ions by the combination of several contributing structures (or ''forms'', also variously known as ''resonance structures'' or '' ...

. Although several other allotropes of phosphorus are stable, no evidence for the existence of has been reported. Preliminary ab initio calculations on the

trimerisation In chemistry, a trimer (; ) is a molecule or polyatomic anion formed by combination or association of three molecules or ions of the same substance. In technical jargon, a trimer is a kind of oligomer derived from three identical precursors often ...

of leading to the formation of the cyclic were performed, and it was predicted that hexaphosphabenzene would decompose to free with an

energy barrier In chemistry and physics, activation energy is the minimum amount of energy that must be provided for compounds to result in a chemical reaction. The activation energy (''E''a) of a reaction is measured in joules per mole (J/mol), kilojoules p ...

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 A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...

, such as

ethanol Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an Alcohol (chemistry), alcohol with the chemical formula . Its formula can be also written as or (an ethyl ...

, might lead to the formation of intermolecular

hydrogen bond In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a ...

s which may block the destabilizing interaction between

phosphorus Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...

lone pair In chemistry, a lone pair refers to a pair of valence electrons that are not shared with another atom in a covalent bondIUPAC ''Gold Book'' definition''lone (electron) pair''/ref> and is sometimes called an unshared pair or non-bonding pair. Lone ...

s and consequently stabilize . The moderate

barrier A barrier or barricade is a physical structure which blocks or impedes something. Barrier may also refer to: Places * Barrier, Kentucky, a community in the United States * Barrier, Voerendaal, a place in the municipality of Voerendaal, Netherl ...

suggests that hexaphosphabenzene could be synthesized from a +2+2

cycloaddition In organic chemistry, a cycloaddition is a chemical reaction in which "two or more Unsaturated hydrocarbon, unsaturated molecules (or parts of the same molecule) combine with the formation of a cyclic adduct in which there is a net reduction of th ...

of three 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 organometallic chemistry, a sandwich compound is a chemical compound featuring a metal bound by haptic, covalent bonds to two arene (ring) ligands. The arenes have the formula , substituted derivatives (for example ) and heterocyclic derivat ...

in 1985 by Scherer et al. Amber coloured, air-stable crystals of sub>2(μ,η⁶-P₆)are formed by reaction of with excess in dimethylbenzene, albeit with a yield of approximately 1%. The

crystal structure In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystal, crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric pat ...

of this complex is a

centrosymmetric In crystallography, a centrosymmetric point group contains an inversion center as one of its symmetry elements. In such a point group, for every point (x, y, z) in the unit cell there is an indistinguishable point (-x, -y, -z). Such point groups ...

molecule, and both five-membered rings as well as the central bridge-ligand 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 sub>2(μ,η⁶-P₆)up to 64%. This was achieved by increasing the reaction temperature of the

thermolysis Thermal decomposition, or thermolysis, is a chemical decomposition caused by heat. The decomposition temperature of a substance is the temperature at which the substance chemically decomposes. The reaction is usually endothermic as heat is req ...

of with to approximately 205 °C in boiling

diisopropylbenzene In organic chemistry, the diisopropylbenzenes constitute a group of aromatic hydrocarbons, whose chemical structure consists of a benzene ring () with two isopropyl groups () as substituents. Through their different arrangement, they form three st ...

, thus favouring the formation of sub>2(μ,η⁶-P₆)as the thermodynamic product. Several analogues of this triple‐decker

complex Complex commonly refers to: * Complexity, the behaviour of a system whose components interact in multiple ways so possible interactions are difficult to describe ** Complex system, a system composed of many components which may interact with each ...

where the coordinating metal and η⁵-ligand has been varied have also been reported. These include triple‐decker complexes for Ti, V, Nb, and W, whereby the synthetic method is still based on the originally reported

of with .

Electron count

Qualitative molecular orbital diagram for triple-decker sandwich complexes

If one regards the planar ring as a 6π

electron donor In chemistry, an electron donor is a chemical entity that donates electrons to another compound. It is a reducing agent that, by virtue of its donating electrons, is itself oxidized in the process. Typical reducing agents undergo permanent chem ...

ligand, then sub>2(μ,η⁶-P₆)is a triple-decker

with 28

valence electron In chemistry and physics, a valence electron is an electron in the outer shell associated with an atom, and that can participate in the formation of a chemical bond if the outer shell is not closed. In a single covalent bond, a shared pair forms ...

s. If , similar to C₆H₆, is taken as a 10π

, a 32

count may be obtained. In most triple-decker complexes with an

electron count Electron counting is a formalism used for classifying compounds and for explaining or predicting electronic structure and chemical bond, bonding. Many rules in chemistry rely on electron-counting: *Octet rule is used with Lewis structures for mai ...

ranging from 26 to 34, the structure of the middle ring is planar ( sub>2(μ,η⁶-P₆)with M = Mo, Sc, Y, Zr, Hf, V, Nb, Ta, Cr, and W). In the 24 valence electron sub>2(μ,η⁶-P₆)complex, however, a distortion is observed, and the ring is puckered. Calculations have concluded that completely filled 2a*and 2b* orbitals in 28

complexes lead to a planar symmetrical middle ring. In 26

complexes, the occupancy of either 2a*or 2b* results in in-plane or bisallylic distortions and an asymmetric planar middle ring. The puckering of in 24

complexes is due to the stabilization of 5a, as well as that conferred by the

tetravalent In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with other atoms when it forms chemical compounds or molecules. Description The combining capacity, or affinity of an ...

oxidation state In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. C ...

of Ti in sub>2(μ,η⁶-P₆)

Reactivity

One-electron oxidation

The reactivity of sub>2(μ,η⁶-P₆)toward

silver Silver is a chemical element with the Symbol (chemistry), symbol Ag (from the Latin ', derived from the Proto-Indo-European wikt:Reconstruction:Proto-Indo-European/h₂erǵ-, ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, whi ...

and

copper Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...

monocationic

salts In chemistry, a salt is a chemical compound consisting of an ionic assembly of positively charged cations and negatively charged anions, which results in a compound with no net electric charge. A common example is table salt, with positively cha ...

of the weakly coordinating anion l₄sup>− ( EF was studied by Fleischmann et al. in 2015. Addition of a solution of Ag EFor Cu EFto a solution of sub>2(μ,η⁶-P₆)in

chloroform Chloroform, or trichloromethane, is an organic compound with chemical formula, formula Carbon, CHydrogen, HChlorine, Cl3 and a common organic solvent. It is a colorless, strong-smelling, dense liquid produced on a large scale as a precursor to ...

results in

oxidation Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...

of the complex, which can be observed by an immediate colour change from amber to dark teal. The

magnetic moment In electromagnetism, the magnetic moment is the magnetic strength and orientation of a magnet or other object that produces a magnetic field. Examples of objects that have magnetic moments include loops of electric current (such as electromagnets ...

of the dark teal

crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...

s determined by the Evans NMR method is equal to 1.67 μB, which is consistent with one unpaired electron. Accordingly, sub>2(μ,η⁶-P₆)sup>+ 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

of sub>2(μ,η⁶-P₆) The Mo—Mo bond length of the sub>2(μ,η⁶-P₆)sup>+ 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

. 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 sub>2(μ,η⁶-P₆)sup>+ 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 orbital In theoretical chemistry, the bonding orbital is used in molecular orbital (MO) theory to describe the attractive interactions between the atomic orbitals of two or more atoms in a molecule. In MO theory, electrons are portrayed to move in waves. ...

s upon oxidation of the triple-decker

Cu EF& Ag EF

To avoid

of sub>2(μ,η⁶-P₆) further reactions were performed in

toluene Toluene (), also known as toluol (), is a substituted aromatic hydrocarbon. It is a colorless, water-insoluble liquid with the smell associated with paint thinners. It is a mono-substituted benzene derivative, consisting of a methyl group (CH3) at ...

to decrease the redox potential of the cations. This resulted in a bright orange coordination product upon reaction with

, although a mixture also containing the dark teal

product was obtained upon reaction with

. Single‐crystal X‐ray analysis reveals that this product displays a distorted square‐planar coordination environment around the central cation through two side‐on coordinating P—P bonds. The Ag—P distances are approximately 2.6 Å, whereas the Cu—P distances are determined to be approximately 2.4 Å. The P—P bonds are therefore elongated to 2.2694(16) Å and 2.2915(14) Å upon coordination to

and

, respectively, whilst the remaining P—P bonds are unaffected. In another experiment Cu EFis treated with sub>2(μ,η⁶-P₆)in pure

and the solution shows the bright orange color of the complex cation u([₂(μ,η⁶-P₆)₂.html" ;"title="sub>2(μ,η⁶-P₆)">u(

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Synthesis

Electron count

Reactivity

One-electron oxidation

Cu EF& Ag EF

__Tl_EF

Tl EF

Jahn-Teller distortion

Isomers

Suppression

References