On the Ambiphilic Reactivity of Geometrically Constrained Phosphorus(III) and Arsenic(III) Compounds: Insights into Their Interaction with Ionic Substrates

Metallomimetic C-F Activation Catalysis by Simple Phosphines

Delivering metallomimetic reactivity from simple p-block compounds is highly desirable in the search to replace expensive, scarce precious metals by cheap and abundant elements in catalysis. This contribution demonstrates that metallomimetic catalysis, involving facile redox cycling between the P(III) and P(V) oxidation states, is possible using only simple, cheap, and readily available trialkylphosphines without the need to enforce unusual geometries at phosphorus or use external oxidizing/reducing agents. Hydrodefluorination and aminodefluorination of a range of fluoroarenes was realized with good to very good yields under mild conditions. Experimental and computational mechanistic studies show that the phosphines undergo oxidative addition of the fluoroaromatic substrate via a Meisenheimer-like transition state to form a fluorophosphorane. This undergoes a pseudotransmetalation step with a silane, via initial fluoride transfer from P to Si, to give experimentally observed phosphonium ions. Hydride transfer from a hydridosilicate counterion then leads to a hydridophosphorane, which undergoes reductive elimination of the product to reform the phosphine catalyst. This behavior is analogous to many classical transition-metal-catalyzed reactions and so is a rare example of both functional and mechanistically metallomimetic behavior in catalysis by a main-group element system. Crucially, the reagents used are cheap, readily available commercially, and easy to handle, making these reactions a realistic prospect in a wide range of academic and industrial settings.

Ligand-enforced geometric constraints and associated reactivity in p-block compounds

The geometry at an element centre can generally be predicted based on the number of electron pairs around it using valence shell electron pair repulsion (VSEPR) theory. Strategies to distort p-block compounds away from these predicted geometries have gained considerable interest due to the unique structural outcomes, spectroscopic properties or reactivity patterns engendered by such distortion. This review presents an up-to-date group-wise summary of this exciting and rapidly growing field with a focus on understanding how the ligand employed unlocks structural features, which in turn influences the associated reactivity. Relevant geometrically constrained compounds from groups 13-16 are discussed, along with selected stoichiometric and catalytic reactions. Several areas for advancement in this field are also discussed. Collectively, this review advances the notion of geometric tuning as an important lever, alongside electronic and steric tuning, in controlling bonding and reactivity at p-block centres.

Geometrically Deformed and Conformationally Rigid Phosphorus Trisamides Featuring an Unsymmetrical Backbone

Nonclassical P(III) centers have attracted much attention in recent years. Incorporating a P(III) center in a rigid bicyclic platform offers a particularly attractive way to invoke significant geometric distortion of the phosphorus atom that may in turn induce unusual reactivity. Although still relatively scarcely explored, phosphorus centers enforced in a non-C3 symmetry have gained significant traction lately. However, the current scaffolds are based on a relatively limited set of design principles and ligand platforms associated therewith. This work is focussed on the synthesis as well as versatile oxidation, addition and coordination chemistry of a geometrically distorted P(III) species featuring a synthetically modular, nonsymmetric trisamine platform derived from 2-(methylamino)-N-(2-(methylamino)phenyl)benzenesulfonamide.

Reactivity of a Strictly T-Shaped Phosphine Ligated by an Acridane Derived NNN Pincer Ligand

The steric tuning of a tridentate acridane-derived NNN pincer ligand allows for the isolation of a strictly T-shaped phosphine that exhibits ambiphilic reactivity. Well-defined phosphorus-centered reactivity towards nucleophiles and electrophiles is reported, contrasting with prior reports on this class of compounds. Reactions towards oxidants are also described. The latter result in the two-electron oxidation of the phosphorus atom from +III to +V and are accompanied by a strong geometric distortion of the NNN pincer ligand. By contrast, cooperative activation of E-H (HCl, HBcat, HOMe) bonds proceeds with retention of the phosphorus redox state. When using H2 O as a substrate, the reaction results in the full disassembly of H2 O to its constituent atoms, highlighting the potential of this platform for small molecule activation reactions.

Dual Reactivity of a Geometrically Constrained Phosphenium Cation

A geometrically constrained phosphenium cation in bis(pyrrolyl)pyridine based NNN pincer type ligand (1⁺) was synthesized, isolated and its preliminary reactivity was studied with small molecules. 1⁺ reacts with MeOH and Et₂NH, activating the O−H and N−H bonds via a P‐center/ligand assisted path. The reaction of 1⁺ with one equiv. of H₃NBH₃ leads to its dehydrogenation producing 5. Interestingly, reaction of 1⁺ with an excess H₃NBH₃ leads to phosphinidene (P^I) species coordinating to two BH₃ molecules (6). In contrast, [1⁺][OTf] reacts with Et₃SiH by hydride abstraction yielding 1‐H and Et₃SiOTf, while [1⁺][B(C₆F₅)₄] reacts with Et₃SiH via an oxidative addition type reaction of Si−H bond to P‐center, affording a new P^V compound (8). However, 8 is not stable over time and degrades to a complex mixture of compounds in matter of minutes. Despite this, the ability of [1⁺][B(C₆F₅)₄] to activate Si−H bond could still be tested in catalytic hydrosilylation of benzaldehyde, where 1⁺ closely mimics transition metal behaviour.

Metal-Free N-H Bond Activation by Phospha-Wittig Reagents

N‐containing molecules are mostly derived from ammonia (NH₃). Ammonia activation has been demonstrated for single transition metal centers as well as for low‐valent main group species. Phosphinidenes, mono‐valent phosphorus species, can be stabilized by phosphines, giving so‐called phosphanylidenephosphoranes of the type RP(PR′₃). We demonstrate the facile, metal‐free NH₃ activation using ArP(PMe₃), affording for the first time isolable secondary aminophosphines ArP(H)NH₂. DFT studies reveal that two molecules of NH₃ act in concert to facilitate an NH₃ for PMe₃ exchange. Furthermore, H₂NR and HNR₂ activation is demonstrated.

Intramolecular C–N bond activation by a geometrically constrained PIII-centre

First examples of the insertion of a geometrically constrained PIII ambiphilic center into C–N bonds.

Calix[4]pyrrolato Aluminates: The Effect of Ligand Modification on the Reactivity of Square-Planar Aluminum Anions

Structural constraint represents an attractive tool to modify p-block element properties without the need for unusual oxidation or valence states. The recently reported methyl-calix[4]pyrrolato aluminate established the effect of forcing a tetrahedral aluminum anion into a square-planar coordination mode. However, the generality of this structural motif and any consequence of ligand modification remained open. Herein, a systematic ligand screening was launched, and the class of square-planar aluminum anions was extended by two derivatives that differ in the meso-substitution at the calix[4]pyrrolato ligand. Strikingly, this modification provoked opposing trends in the preference for a Lewis acidic binding mode with σ-donors versus the aluminum-ligand cooperative binding mode with carbonyls. Insights into the origin of these counterintuitive experimental observations were provided by computation and bond analysis. Importantly, this rationale might allow to exploit mode-selective binding for catalytic rate control.

Enthalpy-Controlled Insertion of a "Nonspectator" Tricoordinate Phosphorus Ligand into Group 10 Transition Metal-Carbon Bonds

Insertion of a tricoordinate phosphorus ligand into late metal-carbon bonds is reported. Metalation of a P^P-chelating ligand (L1), composed of a nontrigonal phosphorous (i.e., P(III)) triamide moiety, P(N(o-N(Ar)C6H4)2, tethered by a phenylene linker to a -PiPr2 anchor, with group 10 complexes L2M(Me)Cl (M = Ni, Pd) results in insertion of the nontrigonal phosphorus site into the metal-methyl bond. The stable methylmetallophosphorane compounds thus formed are characterized spectroscopically and crystallographically. Metalation of L1 with (cod)PtII(Me)(Cl) does not lead to a metallophosphorane but rather to the standard bisphosphine chelate (κ2-L1)Pt(Me)(Cl). These divergent reactivities within group 10 are rationalized by reference to periodic variation in M-C bond enthalpies.

Chemoselectivity for B-O and B-H Bond Cleavage by Pincer-Type Phosphorus Compounds: Theoretical and Experimental Studies

Selective cleavage of the B-O bond or B-H bond in HBpin can be achieved by adjusting the pincer ligand of a phosphorus(III) compound guided by a combination of theoretical prediction and experimental verification. Theoretical calculations reveal that a pincer-type phosphorus compound with an [ONO]^3- ligand reacts with HBpin, leading to cleavage of the stronger B-O bonds (ΔG°^⧧ = 23.2 kcal mol^-1) rather than the weaker B-H bond (ΔG^°⧧ = 26.4 kcal mol^-1). A pincer-type phosphorus compound with a [NNN]^3- ligand reacts with HBpin, leading to the weaker B-H bond cleavage (ΔG°^⧧ = 16.2 kcal mol^-1) rather than cleavage of the stronger B-O bond (ΔG°^⧧ = 33.0 kcal mol^-1). The theoretical prediction for B-O bond cleavage was verified experimentally, and the final products were characterized by NMR, HRMS, and single-crystal X-ray diffraction. The chemoselectivity of B-O bond cleavage was also observed in the presence of B-C or B-B bonds in borane substrates.

Pincer-Type Ligand-Assisted Catalysis and Small-Molecule Activation by non-VSEPR Main-Group Compounds

In 2005, a facile dihydrogen activation was reported by the Power group using an alkyne analog of germanium [ArGe≡GeAr; Ar=2,6-Trip₂ -C₆ H₃ (Trip=2,4,6-ⁱ Pr₃ -C₆ H₂ )]. After that, a significant progress has been made in the activation of various small molecules by main-group compounds, and a variety of stoichiometric and catalytic processes have been formulated using the p-block elements. In this regard, compounds containing low-valent main-group elements with a frontier orbitals of relatively small energy gaps or compounds forming frustrated Lewis pair (FLP) became quite successful. In spite of these promising stoichiometric and catalytic transformations, redox-cycling catalysts based on main-group elements remain extremely rare. Recently, it has been observed that pincer type ligands supported geometry constrained main-group compounds are capable of acting as redox catalysts similar to those of the transition metals. In this review, we focus on the synthesis and the structural aspects of the geometry constrained main-group compounds using pincer ligands. Emphasis has been placed on their applications on catalytic activity and small molecules activation.

Recent developments in the chemistry of non-trigonal pnictogen pincer compounds: from bonding to catalysis

The combination of well-established meridionally coordinating, tridentate pincer ligands with group 15 elements affords geometrically constrained non-trigonal pnictogen pincer compounds. These species show remarkable activity in challenging element-hydrogen bond scission reactions, such as the activation of ammonia. The electronic structures of these compounds and the implications they have on their electrochemical properties and transition metal coordination are described. Furthermore, stoichiometric and catalytic bond forming reactions involving B-H, N-H and O-H bonds as well as carbon nucleophiles are presented.

Validating the Biphilic Hypothesis of Nontrigonal Phosphorus(III) Compounds

Constraining σ³ -P compounds in nontrigonal, entatic geometries has proven to be an effective strategy for promoting biphilic oxidative addition reactions more typical of transition metals. Although qualitative descriptions of the impact of structure and symmetry on σ³ -P complexes have been proposed, electronic structure variations responsible for biphilic reactivity have yet to be elucidated experimentally. Reported here are P K-edge XANES data and complementary TDDFT calculations for a series of structurally modified P(N)₃ complexes that both validate and quantify electronic structure variations proposed to give rise to biphilic reactions at phosphorus. These data are presented alongside experimentally referenced electronic structure calculations that reveal nontrigonal structures predicted to further enhance biphilic reactivity in σ³ -P ligands and catalysts.

A Tautomeric λ3/λ5-Phosphane Pair and Its Ambiphilic Reactivity

The central phosphorus atom of a novel hydroxyl-functionalized triarylphosphane was shown to reversibly insert into one of the molecule's O-H bonds, which forms the basis for a tautomeric λ³/λ⁵-phosphane equilibrium. For the first time, this equilibrium was detected for a λ³-triarylphosphane and the underlying dynamic process was elucidated by NMR spectroscopy. On the basis of reactivity studies, a nucleophilic character was assigned to the minor species present in solution, the λ³-phosphane. Upon methylation, for example, the λ³-form was selectively removed from the equilibrium and converted to the corresponding phosphonium salt. However, upon generation of an alkoxide via proton abstraction, the electrophilic character of the λ⁵-phosphane in the equilibrium became evident since the alkoxide was found to attack the molecule's phosphorus atom. This intramolecular reaction led to the selective formation of a new anionic λ⁶-hydridospirophosphane.

Ambiphilic geometrically constrained phosphenium cation

In this work the synthesis of a geometrically constrained phosphenium cation is shown. In contrast to previously reported phosphenium cations, the geometrical constriction of the P-center in this cation makes it ambiphilic and reactive towards small molecules such as H2O, ROH and NH3.

On the Redox Reactivity of a Geometrically Constrained Phosphorus(III) Compound

The reactivity of a geometrically constrained phosphorus(III) complex bearing the N,N-bis(3,5-di-tert-butyl-2-phenolate)amide pincer ligand (P(ONO); 1) towards oxidants and reductants is explored. This compound can be readily oxidized to the phosphorus(V) dihalo-derivatives P(ONO)X₂ where X=Cl (2), Br (3) and I (4). Attempts at isolating the analogous difluoride through oxidation of 1 were unsuccessful yielding only the hydrofluoride P(ONO)(H)F (5), however P(ONO)F₂ (6) can be accessed via a halide exchange reaction of 2 with KF. Compound 2 can be employed as a precursor to novel cationic species through chloride ion displacement using strong Lewis bases. Thus, reaction of 2 with two or three molar equivalents of dimethylaminopyridine (DMAP) affords [P(ONO)(Cl)(DMAP)₂ ]⁺ (7) and [P(ONO)(DMAP)₃ ]²⁺ (8). Reaction of 2 with the weaker bidentate base 2,2'-bipyridine (bipy) affords [P(ONO)(Cl)(bipy)]⁺ (9), although this species was only accessible upon addition of a halide abstracting agent. The dicationic tris(pyridine) adduct [P(ONO)(py)₃ ]²⁺ (10) is also accessible by reaction of 4 with pyridine. Oxidation of 1 using oxygen gas proceeds slowly and allows for the observation of two compounds, a mixed valence dimeric phosphorus(III)/phosphorus(V) compound [P(ONO)(μ² -O)(μ² :κ¹ ,κ² -ONO)P] (11) and the fully oxidized species [P(ONO)(μ² -O)(μ² :κ¹ ,κ² -ONO)P(O)] (12). Finally, reaction of 1 using KC₈ results in the dimerization of the putative radical anion [P(ONO)]^.- through formation of a P-P bond to afford [P(ONO)]₂^2- (13). Reactions with TEMPO result in the formation of the trigonal bipyramidal species P(ONO)(TEMPO)₂ (14).

Stable Open-Shell Phosphorane Based on a Redox-Active Amidodiphenoxide Scaffold

The synthesis and redox reactivity of pentacoordinate phosphorus compounds incorporating a redox-active ONO amidodiphenoxide scaffold [ONO = N,N-bis(3,5-di-tert-butyl-2-phenoxide)amide] are described. Dichloro- and diphenylphosphoranes, 2·Cl₂ and 2·Ph₂, respectively, are synthesized and crystallographically characterized. Cyclic voltammograms of 2·Cl₂ show only a single irreversible oxidation (E_pa = +0.83 V vs Cp₂Fe^0/+), while the diphenyl analogue 2·Ph₂ is reversibly oxidized at lower applied potential (E_1/2 = +0.47 V vs Cp₂Fe^0/+). Chemical oxidation of 2·Ph₂ with AgBF₄ produces the corresponding radical cation [2·Ph₂]^•+, where electron paramagnetic resonance spectroscopy and density functional theory calculations reveal that the unpaired spin density is largely ligand-based and is highly delocalized throughout the ONO framework of the paramagnetic species. The solid-state structures indicate only minor geometrical changes between the neutral 2·Ph₂ and oxidized [2·Ph₂]^•+ species, consistent with fast self-exchange electron transfer, as observed by NMR line-broadening experiments.

On the Ambiphilic Reactivity of Geometrically Constrained Phosphorus(III) and Arsenic(III) Compounds: Insights into Their Interaction with Ionic Substrates

The ambiphilic nature of geometrically constrained Group 15 complexes bearing the N,N‐bis(3,5‐di‐tert‐butyl‐2‐phenolate)amide pincer ligand (ONO³⁻) is explored. Despite their differing reactivity towards nucleophilic substrates with polarised element–hydrogen bonds (e.g., NH₃), both the phosphorus(III), P(ONO) (1 a), and arsenic(III), As(ONO) (1 b), compounds exhibit similar reactivity towards charged nucleophiles and electrophiles. Reactions of 1 a and 1 b with KOtBu or KNPh₂ afford anionic complexes in which the nucleophilic anion associates with the pnictogen centre ([(tBuO)Pn(ONO)]⁻ (Pn=P (2 a), As (2 b)) and [(Ph₂N)Pn(ONO)]⁻ (Pn=P (3 a), As (3 b)). Compound 2 a can subsequently be reacted with a proton source or benzylbromide to afford the phosphorus(V) compounds (tBuO)HP(ONO) (4 a) and (tBuO)BzP(ONO) (5 a), respectively, whereas analogous arsenic(V) compounds are inaccessible. Electrophilic substrates, such as HOTf and MeOTf, preferentially associate with the nitrogen atom of the ligand backbone of both 1 a and 1 b, giving rise to cationic species that can be rationalised as either ammonium salts or as amine‐stabilised phosphenium or arsenium complexes ([Pn{ON(H)O}]⁺ (Pn=P (6 a), As (6 b)) and [Pn{ON(Me)O}]⁺ (Pn=P (7 a), As (7 b)). Reaction of 1 a with an acid bearing a nucleophilic counteranion (such as HCl) gives rise to a phosphorus(V) compound HPCl(ONO) (8 a), whereas the analogous reaction with 1 b results in the addition of HCl across one of the As−O bonds to afford ClAs{(H)ONO} (8 b). Functionalisation at both the pnictogen centre and the ligand backbone is also possible by reaction of 7 a/7 b with KOtBu, which affords the neutral species (tBuO)Pn{ON(Me)O} (Pn=P (9 a), As (9 b)). The ambiphilic reactivity of these geometrically constrained complexes allows some insight into the mechanism of reactivity of 1 a towards small molecules, such as ammonia and water.