Phosphaalkyne

thumb|right|Molecular structure of , a phosphaalkyne.

In chemistry, a phosphaalkyne (IUPAC name: alkylidynephosphane) is an organophosphorus compound containing a triple bond between phosphorus and carbon with the general chemical formula . Phosphaalkynes are the heavier congeners of nitriles, though, due to the similar electronegativities of phosphorus and carbon, possess reactivity patterns reminiscent of alkynes. Due to their high reactivity, phosphaalkynes are not found naturally on earth, but the simplest phosphaalkyne, phosphaethyne () has been observed in the interstellar medium.

Synthesis

From phosphine gas

The first of preparation of a phosphaalkyne was achieved in 1961 when Thurman Gier produced phosphaethyne by passing phosphine gas at low pressure over an electric arc produced between two carbon electrodes. Condensation of the gaseous products in a −196 °C (−321 °F) trap revealed that the reaction had produced acetylene, ethylene, and phosphaethyne, which were identified by infrared spectroscopy.

alt=A scheme showing the conversion of phosphine gas to HCP, acetylene, and ethylene, following passage through an electric arc produced by carbon electrodes.|center|thumb|497x497px|Gier's 1961 synthesis of phosphaethyne from low-pressure phosphine via electric discharge by carbon electrodes.

By elimination reactions

Elimination of hydrogen halides

alt=Scheme showing the flash pyrolysis of a generically substituted dichloromethylphospine to yield a substituted phosphaalkyne.|thumb|319x319px|Synthesis of substituted phosphaalkynes by flash pyrolysis of substituted dichloromethylphosphines. Here, R = , , Cl, or F.

Following the initial synthesis of phosphaethyne, it was realized that the same compound can be prepared more expeditiously via the flash pyrolysis of methyldichlorophosphine (), resulting in the loss of two equivalents of hydrogen chloride. This methodology has been utilized to synthesize numerous substituted phosphaalkynes, including the methyl, vinyl, chloride, derivatives. Fluoromethylidynephosphane () can also be prepared via the potassium hydroxide promoted dehydrofluorination of trifluoromethylphosphine (). It is speculated that these reactions generally proceed via an intermediate phosphaethylene with general structure RClC=PH. This hypothesis has found experimental support in the observation of by ³¹P NMR spectroscopy during the synthesis of .

Elimination of chlorotrimethylsilane

The high strength of silicon–halogen bonds can be leveraged toward the synthesis of phosphaalkynes. Heating bis-trimethylsilylated methyldichlorophosphines () under vacuum results in the expulsion of two equivalents of chlorotrimethylsilane and the ultimate formation of a new phosphaalkyne. This synthetic strategy has been applied in the synthesis of 2-phenylphosphaacetylene and 2-trimethylsilylphosphaacetylene. As in the case of synthetic routes reliant upon the elimination of a hydrogen halide, this route is suspected to involve an intermediate phosphaethylene species containing a C=P double bond, though such a species has not yet been observed. tertiary alkyl, secondary alkyl, phosphaalkynes in good yields.

By rearrangement of a putative phospha-isocyanide

Dihalophospaalkenes of the general form , where X is Cl, Br, or I, undergo lithium-halogen exchange with organolithium reagents to yield intermediates of the form . These species then eject the corresponding lithium halide salt, LiX, to putatively give a phospha-isocyanide, which can rearrange, much in the same way as an isocyanide, to yield the corresponding phosphaalkyne.

Simulation suggests that simple isophosphiles are not triple-bonded between P and C; instead both atoms bear a lone pair.

Other methods

It has been demonstrated by Cummins and coworkers that thermolysis of compounds of the general form leads to the extrusion of (anthracene), triphenylphosphine, and the corresponding substituted phosphaacetylene: . Unlike the previous method, which derives the phosphaalkyne substituent from an acyl chloride, this method derives the substituent from a Wittig reagent.

center|thumb|825x825px|Synthesis of phosphaalkynes from an anthracene based phosphine chloride and a Wittig reagent, as demonstrated by Cummins and coworkers. Here, R = [[hydrogen|H, Me, Et, ⁱPr, or ^sBu. By bond length metrics, most structurally characterized alkyl and aryl substituted phosphaalkynes contain triple bonds between carbon and phosphorus, as their bond lengths are either equal to or less than the theoretical bond distance.

{| class="wikitable" style="text-align: center; margin-left: auto; margin-right: auto; border: none; width: 35%"

|+

Table of some representative C-P bond lengths in several substituted phosphaalkynes with general form .

!R

!Bond Length (Å)

|-

|H

|1.5442

|-

|Me

|1.542(2)

|-

|triphenylmethyl

|1.533(3)

|}

The carbon-phosphorus bond order in phosphaalkynes has also been the subject of computational inquiry, where quantum chemical calculations have been utilized to determine the nature of bonding in these molecules from first principles. In this context, natural bond orbital (NBO) theory has provided valuable insight into the bonding within these molecules. Lucas and coworkers have investigated the electronic structure of various substituted phosphaalkynes, including the cyaphide anion (), using NBO, natural resonance theory (NRT), and quantum theory of atoms in molecules (QTAIM) in an attempt to better describe the bonding in these molecules. For the simplest systems, and , NBO analysis suggests that the only relevant resonance structure is that in which there is a triple bond between carbon and phosphorus. For more complex molecules, such as and , the triple bonded resonance structure is still the most relevant, but accounts for only some of the overall electron density within the molecule (81.5% and 72.1%, respectively). This is due to interactions between the two carbon-phosphorus pi-bonds and the C-H or C-C sigma-bonds of the substituents, which can be visualized by inspecting the C-P pi-bonding molecular orbitals in these molecules.

center|thumb|883x883px|One of two degenerate pi-bonds in various phosphaalkyne species showing the interactions between C-P pi-bonds and substituent sigma bonds in and , but not in the cyaphide anion or in H-C≡P. Surfaces were calculated at the [[B3LYP level of theory using the def2-tzvpp basis set in ORCA. Molecules shown are (from left to right) the cyaphide anion, , , and . Geometries utilized in creating this figure are those reported by Lucas and coworkers. [3+2] cycloadditions, and [4+2] cycloadditions. This reactivity is summarized in graphical format below, which includes some examples of 1,2-addition reactivity (which is not a form of cycloaddition).

[[File:Phosphaalkyne Reactivity pinwheel.tif|center|thumb|790x790px|A graphic showing some prototypical reactivity espoused by the phosphaalkyne functional group, including 1,2-additions, [2+1] cycloadditions, [2+3] cycloadditions, and [2+4] cycloadditions. The phosphaalkyne core is shown in orange throughout the graphic.]]

Oligomerization

The pi-bonds of phosphaalkynes are weaker than most carbon-phosphorus sigma bonds, rendering phosphaalkynes reactive with respect to the formation of oligomeric species containing more sigma bonds. These oligomerization reactions are triggered thermally, or can be catalyzed by transition or main-group metals.

thumb|311x311px|Synthesis of a cuboidal phosphaalkyne tetramer by heating a kinetically stable phosphaalkyne.

Uncatalyzed

Phosphaalkynes with small substituents (H, F, Me, Ph, etc.) undergo decomposition at or below room temperature by way of polymerization/oligimerization to yield mixtures of products which are challenging to characterize. The same is largely true of kinetically stable phosphaalkynes, which undergo oligomerization reactions at elevated temperature. In spite of the challenges associated with isolating and identifying the products of these oligimerizations, however, cuboidal tetramers of tert-butylphosphaalkyne and tert-pentylphosphaalkyne have been isolated (albeit in low yield) and identified following heating of the respective phosphaalkyne.

Computational chemistry has proved a valuable tool for studying these synthetically complex reactions, and it has been shown that while the formation of phosphaalkyne dimers is thermodynamically favorable, the formation of trimers, tetramers, and higher order oligomeric species tends to be more favorable, accounting for the generation of intractable mixtures upon inducing oligomerization of phosphaalkynes experimentally.

Metal-mediated

Unlike thermally initiated phosphaalkyne oligomerization reactions, transition metals and main group metals are capable of oligomerizing phosphaalkynes in a controlled manner, and have led to the isolation of phosphaalkyne dimers, trimers, tetramers, pentamers, and even hexamers.

[[File:Phosphaalkyne oligomers2.tif|center|thumb|569x569px|Some of the reported phosphaalkyne oligomers generated upon treatment of a phosphaalkyne (usually ) with a transition metal or main group metal complex. Note that several of these species are unstable in their free forms, and instead exist stably only when bound to a transition metal. In this figure, the • symbols individually represent one C-R unit, and are utilized for clarity.