Formation of a Uranium-Bound η1-Cyaphide (CP–) Ligand via Activation and C–O Bond Cleavage of Phosphaethynolate (OCP–)

A Monometallic Bis(cyaphido) Complex

The first example of a bis(cyaphido) complex, trans-[Ru(dppe)2(C≡P)2], is described, unequivocally demonstrating the synthetic accessibility and stability of complexes that feature more than one cyaphido ligand. Synthesis is achieved from the precedent cation [Ru(dppe)2(C≡P)]+ via sequential coordination and desilylation of the phosphaalkyne Me3SiC≡P. The heteroleptic analogue trans-[Ru(dppe)2(C≡N)(C≡P)] is also prepared from the same cation and NaCN; both cyaphido complexes are structurally characterized, enabling the first direct comparison of cyaphide with cyanide, its isoelectronic and isolobal counterpart. This demonstrates an enhanced π-acidity for -C≡P over -C≡N, while computational studies reveal also a higher π-donor character for the cyaphido ligand.

Tetravalent Uranium and Thorium Complexes: Elucidating Disparate Reactivities of AnIVCl2 (An = U, Th) Supported by a Pyridine-Decorated Dianionic Ligand

Although synthesis, reactivity, and bonding of U(IV) and Th(IV) complexes have been extensively studied, direct comparison of fully analogous compounds is rare. Herein, we report corresponding complexes 1-U and 1-Th, in which U(IV) and Th(IV) are supported by the tetradentate pyridine-decorated dianionic ligand N₂NN' (1,1,1-trimethyl-N-(2-(((pyridin-2-ylmethyl)(2-((trimethylsilyl)amino)benzyl)amino)methyl)phenyl)silanamine). Although 1-U and 1-Th are structurally very similar, they display disparate reactivities with TMS₃SiK (tris(trimethylsilyl)silylpotassium). The reaction of (N₂NN')UCl₂ (1-U) and 1 equiv of TMS₃SiK in THF unexpectedly formed [Cl(N₂NN')U]₂O (2-U) featuring an unusual bent U-O-U moiety. In contrast, a salt elimination reaction between (N₂NN')ThCl₂ (1-Th) and 1 equiv of TMS₃SiK led to thorium complex 2-Th, in which the pyridyl group has undergone a 1,4-addition nucleophilic attack. Complex 2-Th serves as a synthon for preparing dimetallic bis-azide complex 3-Th by reaction with NaN₃. The complexes were characterized by X-ray crystal diffraction, solution NMR, FT-IR, and elemental analysis. Computations of the formation mechanism of 2-U from 1-U suggest reduced U(III) as a key intermediate for promoting the cleavage of the C-O bonds of THF. The inaccessible nature of Th(III) as an intermediate oxidation state explains the very different reactivity of 1-Th versus 1-U. Given that reactants 1-U and 1-Th and products 2-U and 2-Th all comprise tetravalent actinides, this is an unusual case of very disparate reactivity despite no net change in the oxidation state. Complexes 2-U and 3-Th provide a basis for the synthesis of other dinuclear actinide complexes with novel reactivity and properties.

Metal-Mediated Oligomerization Reactions of the Cyaphide Anion

The cyaphide anion, CP- , is shown to undergo three distinct oligomerization reactions in the coordination sphere of metals. Reductive coupling of Au(IDipp)(CP) (IDipp=1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) by Sm(Cp*)2 (OEt2 ) (Cp*=1,2,3,4,5-pentamethylcyclopentadienyl), was found to afford a tetra-metallic complex containing a 2,3-diphosphabutadiene-1,1,4,4-tetraide fragment. By contrast, non-reductive dimerization of Ni(SIDipp)(Cp)(CP) (SIDipp=1,3-bis(2,6-diisopropylphenyl)-imidazolidin-2-ylidene; Cp=cyclopentadienyl), gives rise to an asymmetric bimetallic complex containing a 1,3-diphosphacyclobutadiene-2,4-diide moiety. Spontaneous trimerization of Sc(Cp*)2 (CP) results in the formation of a trimetallic complex containing a 1,3,5-triphosphabenzene-2,4,6-triide fragment. These transformations show that while cyaphido transition metal complexes can be readily accessed using metathesis reactions, many such species are unstable to further oligomerization processes.

Formation, Reactivity and Decomposition of Aryl Phospha-Enolates

Two lithium phospha-enolates [RP=C(Siⁱ Pr₃ )OLi]₂ were prepared by reaction of triisopropyl silyl phosphaethynolate, ⁱ Pr₃ SiPCO, with aryl lithium reagents LiR (R=Mes: 1,3,5-trimethyl phenyl; or Mes*: 1,3,5,-tri-tertbutyl phenyl). Monomer/dimer aggregation of the enolates can be modulated by addition of 12-crown-4. Substitution of lithium for a heavier alkali metal was achieved through initial formation of a silyl enol ether, followed by reaction with KO^t Bu to form the corresponding potassium phospha-enolate [MesP=C(Siⁱ Pr₃ )OK]₂ . On addition of water, the enolates are protonated to afford RP=C(Siⁱ Pr₃ )(OH). For the sterically less demanding system (R=Mes), this phospha-enol rapidly tautomerises to the corresponding acyl phosphine MesP(H)C(Siⁱ Pr₃ )(O), which on heating extrudes CO. In contrast, bulkier phospha-enol (R=Mes*) is stable to rearrangement at room temperature and thermally decomposes to RH and ⁱ Pr₃ SiPCO.

Facile Synthesis of Uranium Complexes with a Pendant Borane Lewis Acid and 1,2-Insertion of CO into a U-N Bond

In this contribution, we illustrate uranium complexes bearing a pendant borate (i.e. 1 and 2) or a pendant borane (i.e. 3 and 4) moiety via reaction of the highly strained uranacycle I with various 3-coordinate boranes. Complexes 3 and 4 represent the first examples of uranium complexes with a pendant borane Lewis acid. Moreover, complex 3 was capable of activation of CO, delivering a new CO activation mode, and an abnormal CO 1,2-insertion pathway into a U-N bond. The importance of the pendant borane moiety was confirmed by the controlled experiments.

Revealing the Role of the Cyaphide Ion as a Bridging Ligand in Heterometallic Complexes

The synthesis of heterometallic transition metal complexes featuring bridging cyaphide ions (C≡P⁻) is reported. These are synthesized from reactions of Au(IDipp)(CP) (IDipp=1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene) with electron‐rich, nucleophilic transition metal reagents, affording Au(IDipp)(μ₂−C≡P)Ni(^MeIⁱPr)₂ (^MeIⁱPr=1,3‐diisopropyl‐4,5‐dimethylimidazol‐2‐ylidene) and Au(IDipp)(μ₂−C≡P)Rh(Cp*)(PMe₃). These studies reveal that, in contrast to the cyanide ion, bimetallic cyaphido complexes strongly favor a η¹ : η² coordination mode that maximizes the interaction of the second metal (Ni, Rh) with the π‐manifold of the ion (and not the phosphorus atom lone pair). End‐on bridging can be effectively unlocked by blocking the π‐manifold, as demonstrated by reaction of Au(IDipp)(μ₂−C≡P)Rh(Cp*)(PMe₃) with an electrophilic transition metal reagent, W(CO)₅(THF), which affords the heterotrimetallic compound Au(IDipp)(μ₃−C≡P)[Rh(Cp*)(PMe₃)][W(CO)₅].

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.

Arsinocarbyne reactivity

The reactivity of the tungsten diphenylarsinocarbyne [W(CAsPh₂)(CO)₂(Tp*)] (1; Tp* = hydrotris(dimethylpyrazolyl)borato) is described. The pyramidal arsenic coordinates to a selection of 5d metal centres, forming heterobi- or trimetallic complexes with osmium(II), iridium(III), platinum(II) and gold(I). In the latter case, the WC bond provides a competitive site for gold(I) coordination. Treatment with MeOSO₂CF₃ results in methylation at arsenic to give the first example of an arsoniocarbyne, [W(CAsPh₂CH₃)(CO)₂(Tp*)]O₃SCF₃, for which only the WC bond remains available for gold(I) coordination.

Synthesis, Structure and Reactivity of a Cyapho-Cyanamide Salt

We describe a facile synthesis of the cyapho-cyanamide salt [Na(18-crown-6)][N(CN)(CP)] from reaction of [Na(18-crown-6)][PH₂ ] (18-crown-6=1,4,7,10,13,16-hexaoxacyclooctadecane) with dimethyl N-cyanocarbonimidate, (MeO)₂ C=N(CN). The reaction proceeds with elimination of two equivalents of methanol. Careful tuning of the reaction conditions allowed for the isolation and characterization of the N-cyano(carboximidate)phosphide intermediate [HP{C(OMe)N(CN)}]^- . Due to the adverse effects of methanol in these reaction mixtures, a bulk scale synthesis of [Na(18-crown-6)][N(CN)(CP)] could be achieved by addition of a base (LiHMDS) to neutralize the resulting alcohol. Further reactivity studies of this anion reveal that functionalization at the phosphorus atom is viable to yield a new family of cyanide-functionalised phosphorus heterocycles.

Phosphorus-Atom Transfer from Phosphaethynolate to an Alkylidyne

A low-spin and mononuclear vanadium complex, (Me nacnac)V(CO)(η2 -P≡Ct Bu) (2) (Me nacnac- =[ArNC(CH3 )]2 CH, Ar=2,6-i Pr2 C6 H3 ), was prepared upon treatment of the vanadium neopentylidyne complex (Me nacnac)V≡Ct Bu(OTf) (1) with Na(OCP)(diox)2.5 (diox=1,4-dioxane), while the isoelectronic ate-complex [Na(15-crown-5)]{([ArNC(CH2 )]CH[C(CH3 )NAr])V(CO)(η2 -P≡Ct Bu)} (4), was obtained via the reaction of Na(OCP)(diox)2.5 and ([ArNC(CH2 )]CH[C(CH3 )NAr])V≡Ct Bu(OEt2 ) (3) in the presence of crown-ether. Computational studies suggest that the P-atom transfer proceeds by [2+2]-cycloaddition of the P≡C bond across the V≡Ct Bu moiety, followed by a reductive decarbonylation to form the V-C≡O linkage. The nature of the electronic ground state in diamagnetic complexes, 2 and 4, was further investigated both theoretically and experimentally, using a combination of density functional theory (DFT) calculations, UV/Vis and NMR spectroscopies, cyclic voltammetry, X-ray absorption spectroscopy (XAS) measurements, and comparison of salient bond metrics derived from X-ray single-crystal structural characterization. In combination, these data are consistent with a low-valent vanadium ion in complexes 2 and 4. This study represents the first example of a metathesis reaction between the P-atom of [PCO]- and an alkylidyne ligand.

A Cyaphide Transfer Reagent

The cyanide ion plays a key role in a number of industrially relevant chemical processes, such as the extraction of gold and silver from low grade ores. Metal cyanide compounds were arguably some of the earliest coordination complexes studied and can be traced back to the serendipitous discovery of Prussian blue by Diesbach in 1706. By contrast, heavier cyanide analogues, such as the cyaphide ion, C≡P^–, are virtually unexplored despite the enormous potential of such ions as ligands in coordination compounds and extended solids. This is ultimately due to the lack of a suitable synthesis of cyaphide salts. Herein we report the synthesis and isolation of several magnesium–cyaphido complexes by reduction of ⁱPr₃SiOCP with a magnesium(I) reagent. By analogy with Grignard reagents, these compounds can be used for the incorporation of the cyaphide ion into the coordination sphere of metals using a simple salt-metathesis protocol.

Charge frustration in ligand design and functional group transfer

Molecules with different resonance structures of similar importance, such as heterocumulenes and mesoionics, are prominent in many applications of chemistry, including 'click chemistry', photochemistry, switching and sensing. In coordination chemistry, similar chameleonic/schizophrenic entities are referred to as ambidentate/ambiphilic or cooperative ligands. Examples of these had remained, for a long time, limited to a handful of archetypal compounds that were mere curiosities. In this Review, we describe ambiphilicity - or, rather, 'charge frustration' - as a general guiding principle for ligand design and functional group transfer. We first give a historical account of organic zwitterions and discuss their electronic structures and applications. Our discussion then focuses on zwitterionic ligands and their metal complexes, such as those of ylidic and redox-active ligands. Finally, we present new approaches to single-atom transfer using cumulated small molecules and outline emerging areas, such as bond activation and stable donor-acceptor ligand systems for reversible 1e^- chemistry or switching.

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.

Indirect Access to Carbene Adducts of Bismuth- and Antimony-Substituted Phosphaketene and Their Unusual Thermal Transformation to Dipnictines and [(NHC)2OCP][OCP]

The synthesis and thermal redox chemistry of the first antimony (Sb)– and bismuth (Bi)–phosphaketene adducts are described. When diphenylpnictogen chloride [Ph₂PnCl (Pn = Sb or Bi)] is reacted with sodium 2-phosphaethynolate [Na[OCP]·(dioxane)_x], tetraphenyldipnictogen (Ph₂Pn–PnPh₂) compounds are produced, and an insoluble precipitate forms from solution. In contrast, when the N-heterocyclic carbene adduct (NHC)–PnPh₂Cl is combined with [Na[OCP]·(dioxane)_x], Sb– and Bi–phosphaketene complexes are isolated. Thus, NHC serves as an essential mediator for the reaction. Immediately after the formation of an intermediary pnictogen–phosphaketene NHC adduct [NHC–PnPh₂(PCO)], the NHC ligand transfers from the Pn center to the phosphaketene carbon atom, forming NHC–C(O)P-PnPh₂ [Pn = Sb (3) or Bi (4)]. In the solid state, 3 and 4 are dimeric with short intermolecular Pn–Pn interactions. When compounds 3 and 4 are heated in THF at 90 and 70 °C, respectively, the pnictogen center Pn^III is thermally reduced to Pn^II to form tetraphenyldipnictines (Ph₂Pn–PnPh₂) and an unusual bis-carbene-supported OCP salt, [(NHC)₂OCP][OCP] (5). The formation of compound 5 and Ph₂Pn–PnPh₂ from 3 or 4 is unique in comparison to the known thermal reactivity for group 14 carbene–phosphaketene complexes, further highlighting the diverse reactivity of [OCP]⁻ with main-group elements. All new compounds have been fully characterized by single-crystal X-ray diffraction, multinuclear NMR spectroscopy (¹H, ¹³C, and ³¹P), infrared spectroscopy, and elemental analysis (1, 2, and 5). The electronic structure of 5 and the mechanism of formation were investigated using density functional theory (DFT).

An N-heterocyclic carbene (NHC) was used to support the otherwise unstable Ph₂Sb—P=C=O and Ph₂Bi—P=C=O moieties. Exploration of the thermal chemistry of these NHC−phosphaketene adducts reveals the formation of the salt [NHC₂OCP][OCP]. This present work demonstrates the thermal chemistry of the 2-phospaethynolate anion with heavier pnictogens (Sb and Bi).

Phosphine Carboxylate-Probing the Edge of Stability of a Carbon Dioxide Adduct with Dihydrogenphosphide

We present a new adduct of carbon dioxide with dihydrogenphosphide, that may be prepared either by direct reaction of NaPH₂ with carbon dioxide or by hydrolysis of the phosphaethynolate ion (PCO^- ). In this hydrolysis transformation, a new mechanism is proposed for the electrophilic reactivity of the phosphaethynolate ion. Protonation to form phosphine carboxylic acid (PH₂ COOH) and functionalization to form esters is shown to increase the strength of the P-C interaction, allowing for comparisons to be drawn between this species and the analogous carbamic (NH₂ COOH) and carbonic acids (H₂ CO₃ ). Functionalization of the oxygen atom is found to stabilize the phosphine carboxylate while also allowing solubility in organic solvents whereas phosphorus functionalization is shown to facilitate decarboxylation. Substituent migration occurs in some cases.

The "Hidden" Reductive [2+2+1]-Cycloaddition Chemistry of 2-Phosphaethynolate Revealed by Reduction of a Th-OCP Linkage

The reduction chemistry of the newly emerging 2‐phosphaethynolate (OCP)⁻ is not well explored, and many unanswered questions remain about this ligand in this context. We report that reduction of [Th(Tren^TIPS)(OCP)] (2, Tren^TIPS=[N(CH₂CH₂NSiPrⁱ₃)]³⁻), with RbC₈ via [2+2+1] cycloaddition, produces an unprecedented hexathorium complex [{Th(Tren^TIPS)}₆(μ‐OC₂P₃)₂(μ‐OC₂P₃H)₂Rb₄] (5) featuring four five‐membered [C₂P₃] phosphorus heterocycles, which can be converted to a rare oxo complex [{Th(Tren^TIPS)(μ‐ORb)}₂] (6) and the known cyclometallated complex [Th{N(CH₂CH₂NSiPrⁱ₃)₂(CH₂CH₂SiPrⁱ₂CHMeCH₂)}] (4) by thermolysis; thereby, providing an unprecedented example of reductive cycloaddition reactivity in the chemistry of 2‐phosphaethynolate. This has permitted us to isolate intermediates that might normally remain unseen. We have debunked an erroneous assumption of a concerted fragmentation process for (OCP)⁻, rather than cycloaddition products that then decompose with [Th(Tren^TIPS)O]⁻ essentially acting as a protecting then leaving group. In contrast, when KC₈ or CsC₈ were used the phosphinidiide C−H bond activation product [{Th(Tren^TIPS)}Th{N(CH₂CH₂NSiPrⁱ₃)₂[CH₂CH₂SiPrⁱ₂CH(Me)CH₂C(O)μ‐P]}] (3) and the oxo complex [{Th(Tren^TIPS)(μ‐OCs)}₂] (7) were isolated.

Base induced isomerisation of a phosphaethynolato-borane: mechanistic insights into boryl migration and decarbonylation to afford a triplet phosphinidene

We report on the (tert-butyl)isocyanide-catalysed isomersation of a phosphaethynolato-borane, [B]OCP ([B] = N,N′-bis(2,6-diisopropylphenyl)-2,3-dihydro-1H-1,3,2-diazaboryl), to its linkage isomer, a phosphaketenyl-borane, [B]PCO. Mechanistic insight into this unusual isomerisation was gained through a series of stoichiometric reactions of [B]OCP with isocyanides and theoretical calculations at the Density Functional Theory (DFT) level. [B]PCO decarbonylates under photolytic conditions to afford a novel boryl-substituted diphosphene, [B]P Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> P[B]. This reaction proceeds via a transient triplet phosphinidene which we have been able to observe spectroscopically by Electron Paramagnetic Resonance (EPR) spectroscopy.

We report on the (tert-butyl)isocyanide-catalysed isomersation of a phosphaethynolato-borane, [B]OCP, to its linkage isomer, a phosphaketenyl-borane, [B]PCO.

Controlled Reactivity of Terminal Cyaphide Complexes: Isolation of the 5-Coordinate [Ru(dppe)2(C≡P)]. Inorg Chem 2019;58:14800-14807. [PMID: 31633922 DOI: 10.1021/acs.inorgchem.9b02471]

The novel cyaphide complex trans-[Ru(dppe)₂Me(C≡P)] is obtained in excellent yields and exhibits the first instance of controlled reactivity of any terminal cyaphide complex. Its treatment with ZnX₂/PPh₃ effects selective metathesis of the methyl moiety to afford the unprecedented halocyaphide complexes trans-[Ru(dppe)₂(X)(C≡P)] (X = Cl, Br, I), which are structurally characterized (X = Cl, Br). Exemplified with the trans-bromide, these compounds are susceptible to substitution of the halides by nucleophilic reagents-illustrated with Me₂Mg-and also readily undergo halide abstraction by TlOTf to afford the first hypocoordinate cyaphide complex, viz., [Ru(dppe)₂(C≡P)]·OTf, which is isolable in bulk and exhibits good stability. NMR spectroscopic and crystallographic data reveal the latter to adopt a square-pyramidal geometry with an accessible coordinate vacancy, which is susceptible to the addition of nucleophiles. This is illustrated analytically by reactions with Me₂Mg and LiC≡CPh and with its facile bulk carbonylation to afford trans-[Ru(dppe)₂(CO)(C≡P)]⁺.

A μ-Phosphido Diiron Dumbbell in Multiple Oxidation States

The reaction of the ferrous complex [LFe(NCMe)2 ](OTf)2 (1), which contains a macrocyclic tetracarbene as ligand (L), with Na(OCP) generates the OCP- -ligated complex [LFe(PCO)(CO)]OTf (2) together with the dinuclear μ-phosphido complex [(LFe)2 P](OTf)3 (3), which features an unprecedented linear Fe-(μ-P)-Fe motif and a "naked" P-atom bridge that appears at δ=+1480 ppm in the 31 P NMR spectrum. 3 exhibits rich redox chemistry, and both the singly and doubly oxidized species 4 and 5 could be isolated and fully characterized. X-ray crystallography, spectroscopic studies, in combination with DFT computations provide a comprehensive electronic structure description and show that the Fe-(μ-P)-Fe core is highly covalent and structurally invariant over the series of oxidation states that are formally described as ranging from FeIII FeIII to FeIV FeIV . 3-5 now add a higher homologue set of complexes to the many systems with Fe-(μ-O)-Fe and Fe-(μ-N)-Fe core structures that are prominent in bioinorganic chemistry and catalysis.

Trapping of a Highly Bent and Reduced Form of 2-Phosphaethynolate in a Mixed-Valence Diuranium-Triamidoamine Complex

The chemistry of 2-phosphaethynolate is burgeoning, but there remains much to learn about this ligand, for example its reduction chemistry is scarce as this promotes P-C-O fragmentations or couplings. Here, we report that reduction of [U(Tren^TIPS )(OCP)] (Tren^TIPS =N(CH₂ CH₂ NSiPrⁱ ₃ )₃ ) with KC₈ /2,2,2-cryptand gives [{U(Tren^TIPS )}₂ {μ-η² (OP):η² (CP)-OCP}][K(2,2,2-cryptand)]. The coordination mode of this trapped 2-phosphaethynolate is unique, and derives from an unprecedented highly reduced and highly bent form of this ligand with the most acute P-C-O angle in any complex to date (P-C-O ∡ ≈127°). The characterisation data support a mixed-valence diuranium(III/IV) formulation, where backbonding from uranium gives a highly reduced form of the P-C-O unit that is perhaps best described as a uranium-stabilised OCP^2-. radical dianion. Quantum chemical calculations reveal that this gives unprecedented carbene character to the P-C-O unit, which engages in a weak donor-acceptor interaction with one of the uranium ions.

Finding a soft spot for vanadium: a P-bound OCP ligand

Transmetallation studies with the phosphaethynolate ion, [OCP]^-, have largely resulted in coordination according to classical Lewis acid-base theory. That is, for harder early transition metal ions, O-bound coordination has been observed, whereas in the case of softer late transition metal ions, P-bound coordination predominates. Herein, we report the use of a V(iii) complex, namely [(nacnac)VCl(OAr)] (1) (nacnac^- = [ArNC(CH₃)]₂CH; Ar = 2,6-ⁱPr₂C₆H₃), to transmetallate [OCP]^- and bind via the P-atom as [(nacnac)V(OAr)(PCO)] (2), the first example of a 3d early transition metal that binds [OCP]^-via the P-atom. Full characterization studies of this molecule including HFEPR spectroscopy, SQuID measurements, and theoretical studies are presented.

Heavier pnictogens - treasures for optical electronic and reactivity tuning

We highlight recent advances in organopnictogen chemistry contrasting the properties of lighter and heavier pnictogens. Exploring new bonding situations, discovering unprecedented reactivities and producing fascinating opto-electronic materials are some of the most prominent directions of current organopnicogen research. Expanding the chemical toolbox towards the heavier group 15 elements will continue to create new opportunities to tailor molecular properties for small molecule activation/reactivity and materials applications alike. This frontier article illustrates the elemental substitution approach in selected literature examples.

From As-Zincoarsasilene (LZn-As=SiL') to Arsaethynolato (As≡C-O) and Arsaketenylido (O=C=As) Zinc Complexes

The reactivity of the As-zincosilaarsene LZn-As=SiL' A (L=[CH(CMeNDipp)₂ ]^- , Dipp=2,6-ⁱ Pr₂ C₆ H₃ , L'=[{C(H)N(2,6-ⁱ Pr₂ -C₆ H₃ )}₂ ]^2- ) towards small molecules was investigated. Due to the pronounced zwitterionic character of the Si=As bond of A, it undergoes addition reactions with H₂ O and NH₃ , forming LZnAs(H)SiOH(L') 1 and LZnAs(H)SiNH₂ (L') 2. Oxygenation of A with N₂ O at -60 °C furnishes the deep blue 1,2-disiloxydiarsene, [LZnOSi(L')As]₂ 4, presumably via dimerization of the arsinidene intermediate LZnOSi(L')As 3. Oxygenation of A with CO₂ leads to the monomeric arsaethynolato siloxido zinc complex LZnOSi(L')(OC≡As) 5, essentially trapping the intermediary arsinidene 3 with liberated CO following initial oxidation of the Si=As bond. DFT calculations confirm the ambident coordination mode of the anionic [AsCO] ligand in solution, with the O-arsaethynolato [As≡C-O]^.- in 5, and the As-arsaketenylido ligand mode [O=C=As]^- present in LZnO-Si(L')(-As=C=O) 5' akin to the analogous phosphorus system, [PCO]^- .

Cyaarside (CAs- ) and 1,3-Diarsaallendiide (AsCAs2- ) Ligands Coordinated to Uranium and Generated via Activation of the Arsaethynolate Ligand (OCAs- )

Reaction of the trivalent uranium complex [((^Ad,Me ArO)₃ N)U(DME)] with one molar equiv [Na(OCAs)(dioxane)₃ ], in the presence of 2.2.2-crypt, yields [Na(2.2.2-crypt)][{((^Ad,Me ArO)₃ N)U^IV (THF)}(μ-O){((^Ad,Me ArO)₃ N)U^IV (CAs)}] (1), the first example of a coordinated η¹ -cyaarside ligand (CAs^- ). Formation of the terminal CAs^- is promoted by the highly reducing, oxophilic U^III precursor [((^Ad,Me ArO)₃ N)U(DME)] and proceeds through reductive C-O bond cleavage of the bound arsaethynolate anion, OCAs^- . If two equiv of OCAs^- react with the U^III precursor, the binuclear, μ-oxo-bridged U₂ ^IV/IV complex [Na(2.2.2-crypt)]₂ [{((^Ad,Me ArO)₃ N)U^IV }₂ (μ-O)(μ-AsCAs)] (2), comprising the hitherto unknown μ:η¹ ,η¹ -coordinated (AsCAs)^2- ligand, is isolated. The mechanistic pathway to 2 involves the decarbonylation of a dimeric intermediate formed in the reaction of 1 with OCAs^- . An alternative pathway to complex 2 is by conversion of 1 via addition of one further equiv of OCAs^- .

Cyaphide–alkynyl complexes: metal–ligand conjugation and the influence of remote substituents

A series of cyaphide–alkynyl complexes exhibits significant cooperativity between the electron accepting “–CP” ligand and remote alkynyl substituents, indicative of long-range communication.

Salen supported Al–O–CP and Ga–PCO complexes

The synthesis and reactivity of salen supported OCP adducts of aluminium and gallium is reported.

From an Fe2P3 complex to FeP nanoparticles as efficient electrocatalysts for water-splitting

In large-scale, hydrogen production from water-splitting represents the most promising solution for a clean, recyclable, and low-cost energy source. The realization of viable technological solutions requires suitable efficient electrochemical catalysts with low overpotentials and long-term stability for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) based on cheap and nontoxic materials. Herein, we present a unique molecular approach to monodispersed, ultra-small, and superiorly active iron phosphide (FeP) electrocatalysts for bifunctional OER, HER, and overall water-splitting. They result from transformation of a molecular iron phosphide precursor, containing a [Fe2P3] core with mixed-valence FeIIFeIII sites bridged by an asymmetric cyclo-P(2+1) 3- ligand. The as-synthesized FeP nanoparticles act as long-lasting electrocatalysts for OER and HER with low overpotential and high current densities that render them one of the best-performing electrocatalysts hitherto known. The fabricated alkaline electrolyzer delivered low cell voltage with durability over weeks, representing an attractive catalyst for large-scale water-splitting technologies.

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.