Redox-Triggered Reversible Interconversion of a Monocyclic and a Bicyclic Phosphorus Heterocycle

Recent advances in the chemistry of the phosphaethynolate and arsaethynolate anions

Over the past decade, the reactivity of 2-phosphaethynolate (OCP^-), a heavier analogue of the cyanate anion, has been the subject of momentous interest in the field of modern organometallic chemistry. It is used as a precursor to novel phosphorus-containing heterocycles and as a ligand in decarbonylative processes, serving as a synthetic equivalent of a phosphinidene derivative. This perspective aims to describe advances in the reactivities of phosphaethynolate and arsaethynolate anions (OCE^-; E = P, As) with main-group element, transition metal, and f-block metal scaffolds. Further, the unique structures and bonding properties are discussed based on spectroscopic and theoretical studies.

Analysis of Non-innocence of Phosphaquinodimethane Ligands when Charge and Aromaticity Come into Play

Several phosphaquinodimethanes and their M(CO)5 complexes (M=Cr, Mo, W) and model derivatives have been theoretically investigated regarding the quest of non-innocence. Computed structural and electronic properties of the P-Me/NH2 substituted phosphaquinodimethanes and tungsten complexes revealed an interesting non-innocent ligand behaviour for the radical anion complexes with distonic ion character and a strong rearomatization of the middle phenyl ring. The latter was further probed taking also geometric aromaticity (HOMA) and quinoid distortion parameters (HOMQc) into account, as well as NICS(1). Furthermore, the effect of the P-substitution was investigated for real (or plausible) complexes and their free ligands focusing on the resulting aromaticity at the middle phenyl ring and vertical one-electron redox processes. The best picture of ligand engagement in redox changes was provided by representing NICS(1) values versus HOMA and the new geometric distortion parameter HOMQc8.

η3 -Coordination and Functionalization of the 2-Phosphaethynthiolate Anion at Lanthanum(III)*

We present the η3 -coordination of the 2-phosphaethynthiolate anion in the complex (PN)2 La(SCP) (2) [PN=N-(2-(diisopropylphosphanyl)-4-methylphenyl)-2,4,6-trimethylanilide)]. Structural comparison with dinuclear thiocyanate-bridged (PN)2 La(μ-1,3-SCN)2 La(PN)2 (3) and azide-bridged (PN)2 La(μ-1,3-N3 )2 La(PN)2 (4) complexes indicates that the [SCP]- coordination mode is mainly governed by electronic, rather than steric factors. Quantum mechanical investigations reveal large contributions of the antibonding π*-orbital of the [SCP]- ligand to the LUMO of complex 2, rendering it the ideal precursor for the first functionalization of the [SCP]- anion. Complex 2 was therefore reacted with CAACs which induced a selective rearrangement of the [SCP]- ligand to form the first CAAC stabilized group 15-group 16 fulminate-type complexes (PN)2 La{SPC(R CAAC)} (5 a,b, R=Ad, Me). A detailed reaction mechanism for the SCP-to-SPC isomerization is proposed based on DFT calculations.

The Chemistry of the 2-Phosphaethynolate Anion

In all likelihood the first synthesis of the phosphaethynolate anion, PCO^- , was performed in 1894 when NaPH₂ was reacted with CO in an attempt to make Na(CP) accompanied by elimination of water. This reaction was repeated 117 years later when it was discovered that Na(OCP) and H₂ are the products of this remarkable transformation. Li(OCP) was synthesized and fully characterized in 1992 but this salt proved to be too unstable to allow for a detailed investigation of its chemistry. It was not until the heavier analogues of this lithium salt were isolated, Na(OCP) and K(OCP) (both of which are remarkably stable and can be even dissolved in water), that the chemistry of this new functional group could be explored. Here we review the chemistry of the 2-phosphaethynolate anion, a heavier phosphorus-containing analogue of the cyanate anion, and describe the wide breadth of chemical transformations for which it has been thus far employed. Its use as a ligand, in decarbonylative and deoxygenative processes, and as a building block for novel heterocycles is described. In the mere twenty-six years since Becker first reported the isolation of this remarkable anion, it has become a fascinating reagent for the synthesis of a vast library of, often unprecedented, molecules and compounds.

Synthesis and reactivity of rare-earth metal phosphaethynolates

Over the course of the last six years, research on the synthesis and reactivity of molecular metal phosphaketenes (M-PCO) has gained increasing attention. However, lanthanide complexes of the heavier group 15 cyanate analogue PCO^- have not been investigated so far. Herein we present exemplar studies on the nature and reactivity of rare-earth phosphaethynolato-complexes using three characteristic representatives of the rare-earth metals: Y, Nd and Sm. Our investigations comprise both +2 and +3 redox states, and one defined amidinate-based ligand set, as well as novel reaction pathways in the presence of the sequestering agents 18-crown-6 and 2,2,2-crypt.

Simple Synthesis of Functionalized 2-Phosphanaphthalenes

A simple synthesis of sodium 2-phosphanaphthalene-3-olate (1) based on the extrusion of N₂ from phthalazine using Na[OCP] is reported. This heterocycle can be readily functionalized at the negatively charged oxygen center using a variety of electrophilic substrates. The coordination chemistry of both 1 and its neutral derivatives was explored, revealing their facile use as P-donor ligands for late-transition-metal complexes.

An isolable magnesium diphosphaethynolate complex

The reaction of magnesium chloride with two equivalents of sodium phosphaethynolate, Na[OCP]·(dioxane)_2.5 (1), yields a magnesium diphosphaethynolate complex, [(THF)₄Mg(OCP)₂] (3). The formation of compound 3 goes through a monosubstituted chloromagnesium phosphaethynolate Mg(OCP)Cl (2). The structure of 3 was determined via a single crystal X-ray diffraction study. For comparison, we also report the structure of a monomeric sodium phosphaethynolate complex, [Na(OCP)(dibenzo-18-crown-6)] (4).

Borane-Stabilized Isomeric Dimers of the Phosphaethynolate Anion

The reactions of the phosphaethynolate anion ([PCO]^- ) with a range of boranes were explored. BPh₃ and [PCO]^- form a dimeric anion featuring P-B bonds and is prone to dissociation at room temperature. The more Lewis acidic borane B(C₆ F₅ )₃ yields a less symmetric dimer of [PCO]^- with P-B and P-O bonds. Less sterically demanding HB(C₆ F₅ )₂ and H₂ B(C₆ F₅ ) boranes form a third isomer with [PCO]^- featuring both boranes bound to the same phosphorus atom. Despite the unexpected thermodynamic preference for P-coordination, computational data illustrate that electronic and steric features impact the binding modes of the resulting dianionic dimers.

A Monoanionic Arsenide Source: Decarbonylation of the 2-Arsaethynolate Anion upon Reaction with Bulky Stannylenes

We report fundamental studies on the reactivity of the 2-arsaethynolate anion (AsCO- ), a species that has only recently become synthetically accessible. The reaction of AsCO- with the bulky stannylene Ter2 Sn (Ter=2,6-bis[2,4,6-trimethylphenyl]phenyl) is described, which leads to the unexpected formation of a [Ter3 Sn2 As2 ]- cluster compound. On the reaction pathway to this cluster, several intermediates were identified and characterized. After the initial association of AsCO- to Ter2 Sn, decarbonylation occurs to give an anion featuring monocoordinate arsenic, [Ter2 SnAs]- . Both species are not stable under ambient conditions, and [Ter2 SnAs]- rearranges to form [TerSnAsTer]- , an unprecedented anionic mixed Group 14/15 alkene analogue.

The reactivity of acyl chlorides towards sodium phosphaethynolate, Na(OCP): a mechanistic case study

The reaction of Na(OCP) with mesitoyl chloride delivers an ester functionalized 1,2,4-oxadiphosphole in a clean and P-atom economic way. The reaction mechanism has been elucidated by means of detailed NMR-spectroscopic, kinetic and computational studies. The initially formed acyl phosphaketene undergoes a pseudo-coarctate cyclization with an (OCP)- anion under the loss of carbon monoxide to yield a five-membered ring anion. Subsequently, the nucleophilic attack of the formed heterocyclic anion on a second acyl chloride molecule results in the 1,2,4-oxadiphosphole. The transient acyl phosphaketene is conserved during the reaction in the form of four-membered ring adducts, which act as a reservoir. Consequently, the phosphaethynolate anion has three different functions in these reactions: it acts as a nucleophile, as an en-component in [2 + 2] cycloadditions and as a formal P- transfer reagent.

Aminotroponiminates as tunable, redox-active ligands: reversible single electron transfer and reductive dimerisation

Aminotroponiminates (atis) are shown to be redox-active ligands. Under strongly reducing conditions, the result of electron transfer can be controlled by the choice of the metal bound to the ati ligand. Either reversible electron transfer or a reductively induced dimerisation is observed. The latter reaction is (regio- and diastereo-) selective and chemically reversible.

The 2-Arsaethynolate Anion: Synthesis and Reactivity Towards Heteroallenes

The synthesis and isolation of the 2-arsaethynolate anion, AsCO(-) , and its subsequent reactivity towards heteroallenes is reported. Reactions with ketenes and carbodiimides afford four-membered anionic heterocycles in formal [2+2] cycloaddition reactions. By contrast, reaction with an isocyanate yielded a 1,4,2-diazaarsolidine-3,5-dionide anion and the unprecedented cluster anions As10 (2-) and As12 (4-) . These preliminary reactivity studies hint at the enormous potential synthetic utility of this novel anion, which may be employed as an arsenide (As(-) ) source.

A very peculiar family of N-heterocyclic phosphines: unusual structures and the unique reactivity of 1,3,2-diazaphospholenes

This Perspective gives an account of the peculiar electronic and molecular structures of N-heterocyclic phosphines featuring either a single 1,3,2-diazaphospholene (DAP) ring with an exocyclic P-substituent X or two DAP rings linked by a P-P bond (bis-diazaphospholenyls), respectively, and their impact on the chemical properties of these molecules. The bonding situation in simple DAPs is epitomized by strong hyperconjugation between endocyclic π-type electrons and the exocyclic P-X bond. This interaction may induce a perceptible ionic polarization of the P-X bond which can persist even in the limit of a vanishing electronegativity gradient between P and X, and becomes visible in unusual geometric distortions of molecular structures and a unique chemical behaviour. Structural distortions are particularly evident in bond lengthening effects in P-halogen and P-phosphino derivatives R2P-DAP (with R2P ≠ DAP) which span the whole range from covalent molecules to contact ion pairs with a close relation to frustrated Lewis-pairs. The most significant impact on the chemical properties is found for P-phosphino- and P-hydrogen derivatives where reactions at substantially accelerated rates or totally new reaction modes can be observed, and new stoichiometric and first catalytic processes exploiting these features are currently emerging. The recently discovered bis-diazaphospholenyls differ from the simple derivatives as their central bond remains unpolarised as a consequence of the symmetric molecular structure. The occurrence of low-energy P-P bond homolysis that was nonetheless observed in one case is according to the results of thermochemical studies of P-P bond fission reactions attributable to the effects of steric congestion and induces chemical reactivity that can be considered complementary to that of the simple R2P-DAPs. Some concluding remarks will pay attention to a facet of DAP reactivity that has so far been widely neglected but is currently receiving increasing attention, namely well-defined ring-opening processes.

Ambient-Temperature Synthesis of 2-Phosphathioethynolate, PCS-, and the Ligand Properties of ECX- (E = N, P; X = O, S)

A synthesis of the 2-phosphathioethynolate anion, PCS-, under ambient conditions is reported. The coordination chemistry of PCO-, PCS- and their nitrogen-containing congeners is also explored. Photolysis of a solution of W(CO)6 in the presence of PCO- [or a simple ligand displacement reaction using W(CO)5(MeCN)] affords [W(CO)5(PCO)]- (1). The cyanate and thiocyanate analogues, [W(CO)5(NCO)]- (2) and [W(CO)5(NCS)]- (3), are also synthesised using a similar methodology, allowing for an in-depth study of the bonding properties of this family of related ligands. Our studies reveal that, in the coordination sphere of tungsten(0), the PCO- anion preferentially binds through the phosphorus atom in a strongly bent fashion, while NCO- and NCS- coordinate linearly through the nitrogen atom. Reactions between PCS- and W(CO)5(MeCN) similarly afford [W(CO)5(PCS)]-; however, due to the ambidentate nature of the anion, a mixture of both the phosphorus- and sulfur-bonded complexes (4a and 4b, respectively) is obtained. It was possible to establish that, as with PCO-, the PCS- ion also coordinates to the metal centre in a bent fashion.

Isolation of Au-, Co-η1PCO and Cu-η2PCO complexes, conversion of an Ir-η1PCO complex into a dimetalladiphosphene, and an interaction-free PCO anion

Sodium phosphaethynolate reacts with [MCl(PDI)] (M = Co, Ir; PDI = pyridinediimine) to give metallaphosphaketenes, which in the case of iridium rearranges into a dimetalladiphosphene.

Sodium phosphaethynolate reacts with [MCl(PDI)] (M = Co, Ir; PDI = pyridinediimine) to give metallaphosphaketenes, which in the case of iridium rearranges into a dimetalladiphosphene, via CO migration from phosphorus to the metal. Two different bonding modes of the PCO anion to CAAC-coinage metal complexes [CAAC: cyclic (alkyl)(amino)(carbene)] are reported, one featuring a strong Au–P bond and the other an η² coordination to copper. The gold complex appears to be mostly unreactive whereas the copper complex readily reacts with various organic substrates. A completely free PCO anion was structurally characterized as the [Cu(L_a)₂]⁺ (OCP)^– salt. It results from the simple displacement of the PCO unit of the cationic (CAAC)Cu(PCO) complex by a second equivalent of CAAC.

Simple access to ionic liquids and organic salts containing the phosphaethynolate (PCO−) and Zintl (Sb113−) anions

A simple and almost quantitative synthesis of the first salts of the PCO⁻ anion with organic cations is presented. Moreover, we discovered a simple synthesis for [P(nBu)₃Me]⁺₃[Sb₁₁]³⁻, the first organic cation salt comprising the Zintl-anion [Sb₁₁]³⁻.

N-Heterocyclic carbene phosphaketene adducts as precursors to carbene–phosphinidene adducts and a rearranged π-system

Sodium phosphaethynolate, Na(OCP), is demonstrated to be successful for the synthesis of NHC–phosphinidene adducts and a linear π-conjugated molecule.

Uranium and thorium complexes of the phosphaethynolate ion

New tris-amidinate actinide (Th, U) complexes containing a rare O-bound terminal phosphaethynolate (OCP ^- ) ligand were synthesized and fully characterized. The cyanate (OCN ^- ) and thiocyanate (SCN ^- ) analogs were prepared for comparison and feature a preferential N-coordination to the actinide metals. The Th(amid)₃(OCP) complex reacts with Ni(COD)₂ to yield the heterobimetallic adduct (amid) ₃ Th(μ-η ¹ (O):η ² (C,P)-OCP)Ni(COD) featuring an unprecedented reduced (OCP ^- ) bent fragment bridging the two metals.

The phosphaethynolate anion reacts with unsaturated bonds: DFT investigations into [2+2], [3+2] and [4+2] cycloadditions

Density functional theory (DFT) calculations were carried out to investigate the [2+2], [3+2] and [4+2] cycloadditions of the phosphaethynolate anion (PCO(-)). The results reveal that the electronic properties of different unsaturated compounds play a crucial role in reactivity and regioselectivity.

Exploiting the Brønsted acidity of phosphinecarboxamides for the synthesis of new phosphides and phosphines

We demonstrate that the synthesis of new N-functionalized phosphinecarboxamides is possible by reaction of primary and secondary amines with PCO(-) in the presence of a proton source. These reactions proceed with varying degrees of success, and although primary amines generally afford the corresponding phosphinecarboxamides in good yields, secondary amines react more sluggishly and often give rise to significant decomposition of the 2-phosphaethynolate precursor. Of the new N-derivatized phosphinecarboxamides available, PH2C(O)NHCy (Cy = cyclohexyl) can be obtained in sufficiently high yields to allow for the exploration of its Brønsted acidity. Thus, deprotonating PH2C(O)NHCy with one equivalent of potassium bis(trimethylsilyl)amide (KHMDS) gave the new phosphide [PHC(O)NHCy](-). In contrast, deprotonation with half of an equivalent gives rise to [P{C(O)NHCy}2](-) and PH3. These phosphides can be employed to give new phosphines by reactions with electrophiles, thus demonstrating their enormous potential as chemical building blocks.

Cyclo-oligomerization of isocyanates with Na(PH2) or Na(OCP) as "P-" anion sources

We show that the 2-phosphaethynolate anion, OCP^-, is a simple and efficient catalyst for the cyclotrimerization of isocyanates. This process proceeds step-wise and involves five-membered heterocycles, namely 1,4,2-diazaphospholidine-3,5-dionide anions and spiro-phosphoranides as detectable intermediates, both of which were also found to be involved in the catalytic conversion. These species can be considered as adducts of a phosphide anion with two and four isocyanate molecules, respectively, demonstrating that the OCP^- anion acts as a formal "P^-" source. The interconversion between these anionic species was found to be reversible, allowing them to serve as reservoirs for unique phosphorus-based living-catalysts for isocyanate trimerization.

Synthesis of anionic phosphorus-containing heterocycles by intramolecular cyclizations involving N-functionalized phosphinecarboxamides

We report that the 2-phosphaethynolate anion (PCO(-)) reacts with propargylamines in the presence of a proton source to afford novel N-derivatized phosphinecarboxamides bearing alkyne functionalities. Deprotonation of these species gives rise to novel five- and six-membered anionic heterocycles resulting from intramolecular nucleophilic attack of the resulting phosphide at the alkyne functionality (via 5-exo-dig or 6-endo-dig cyclizations, respectively). The nature of the substituents on the phosphinecarboxamide can be used to influence the outcome of these reactions. This strategy represents a unique approach to phosphorus-containing heterocylic systems that are closely related to known organic molecules with interesting bio-active properties.

Phosphide delivery to a cyclotrisilene

The reactivity of the 2-phosphaethynolate anion (PCO⁻) towards a cyclic trisilene (cSi₃(Tip)₄) is reported. The result is the net activation of the P=C and Si=Si multiple bonds of the precursors affording a heteroatomic bicyclo[1.1.1]pentan-2-one analogue ([P(CO)Si₃(Tip)₄]⁻; 1). This reaction can be interpreted as the formal addition of a phosphide and a carbonyl across the Si=Si double bond. Photolytic decarbonylation of 1 results in the incorporation of the phosphide vertex into the cyclotrisilene scaffold, yielding a congener of the cyclobutene anion with considerable allylic character.