A Phosphanyl-Phosphagallene that Functions as a Frustrated Lewis Pair

Synthesis and Reactivity of Germyl-Substituted Gallapnictenes

Decarbonylation of the phospha- and arsaketenyl germylenes (L)GeECO (E = P, As; L = CH[C(Me)NAr]2, Ar = 2,6-iPr2C6H3) in the presence of the Ga(I) precursor (L)Ga afforded the corresponding germyl-substituted gallaphosphene 1 and gallaarsene 3, respectively. Both 1 and 3 are examples of unsaturated chains with heavier group 13/15/14 elements. The germylene center and the polarized Pn = Ga (Pn = P or As) double bond provide multiple sites for small-molecule activation. For example, gallaarsene 3 reacted with adamantyl azide in a formal [3 + 2]-cyclization to give 4 containing a GaAsN3 heterocycle, clearly underlining the analogy between the As = Ga and C-C multiple bonds. By contrast, the reaction of 3 with Me-I afforded the 1,3-addition product 5, which indicates frustrated Lewis pair character in 3. DFT calculations indicate that the Ga-P/As bonds are highly polarized toward the pnictogen center. EDA-NOCV calculations further support this description and additionally shed light on ambiguous bonding scenarios in 4 and 5. These calculations prove that orbital interactions are outweighed by electrostatic interactions, resulting in polar bonds with significant ionic character.

The Diverse Reactivity of an Al(I) Carbenoid Towards Phosphorus-Based Ylides. The Crucial Role of Co-Crystallised Lithium Halide

Attempts to generate NacNacAl-based (NacNac=[ArNC(Me)CHC(Me)NAr]-; Ar=2,6-iPr2C6H3) alkylidenes via reactions with trimethyl(methylidene), triphenyl(methylidene) and triethyl(ethylidene) phosphiranes led to new and unexpected reactivity patterns. Reaction with Me3P=CH2 resulted in the intermolecular deprotonation of the NacNac -CCH3 backbone. Reaction with Ph3P=CH2*LiI afforded an equimolar mixture of NacNacAlH2 and the ortho-metalated adduct, NacNac'Al(η(C,C)2-CH2=PPh2(o-C6H4)) (NacNac'=[ArNC(Me)CHC(=CH2)NAr]2-). Finally, the use of Et3P=CHMe*LiBr resulted in the fragmentation of the NacNac moiety.

Synthesis of a Transient Cationic Phosphaborene and its Trapping by Facile Aliphatic C-H Bond Activation

The synthesis of a transient cationic phosphaborene [(Mes*)P=B(CAAC)]+ (Mes*=2,4,6,-tri-tert-butylphenyl, CAAC=cyclic alkylamino carbene) by halide abstraction from the B-brominated analogue is reported. This species was found to undergo rapid and selective intramolecular aliphatic C-H bond activation to yield a phosphinoborenium cation, which undergoes facile deprotonation to give a cyclic base-stabilized phosphaborene. Computational investigation of the mechanism of C-H activation indicates a boron-centred activation route with an exceptionally low barrier of 8 kJ mol-1, followed by a nearly barrierless hydride migration from boron to phosphorus. This underscores the enhanced reactivity of phosphaborenes compared to their lighter iminoborane analogues and indicates that steric stabilization alone is likely insufficient to isolate persistent cationic phosphaborenes, necessitating the use of alternative design strategies.

A Crystalline Unsupported Phosphagallene and Phosphaindene

The synthesis and isolation of TerP═GaTer and TerP═InTer (Ter = 2,6-Dipp2-C6H3; Dipp = 2,6-diisopropylphenyl) is reported. These compounds feature unsupported P═Ga and P═In double bonds and two-coordinate triel element centers. Key to the stabilization of such compounds is the steric bulk of the terphenyl substituents, which serve to shield the highly reactive P═E bonds (E = Ga, In) and prevent further aggregation. When smaller aromatic substituents are employed on the phosphorus-containing precursor, the cyclic compounds Mes*P(ETer)2 (Mes* = 2,4,6-tBu3-C6H2) are isolated instead. These species contain weakly aromatic three-membered rings. The presence of an external base (PMe3) is required in order to stabilize a phosphagallene when the smaller Mes* substituent is used, allowing for the isolation of Mes*P═GaTer(PMe3).

Diphosphaenones: beyond the phosphorus analogue of enones

Phosphaenones, like their carbon analogue enones (C[double bond, length as m-dash]C-C[double bond, length as m-dash]O), are promising building blocks for synthetic chemistry and materials science. However, in contrast to the α- and β-phosphaenones, structurally and spectroscopically well-defined diphosphaenones (DPEs) are rare. In this study, we disclose the isolation and spectroscopic characterization of N-heterocyclic vinyl (NHV) substituted acyclic DPEs 3a,b [NHV-P[double bond, length as m-dash]P-C(O)-NHV]. X-ray diffraction methods allowed determination of the structures, which show a central planar trans P[double bond, length as m-dash]P-C[double bond, length as m-dash]O configuration. Compound 3a behaves like classical enones and shows 1,4-addition across the P[double bond, length as m-dash]P-C[double bond, length as m-dash]O unit, which proceeds in a stepwise manner. In contrast, 3a exhibits also 1,2-addition across the P[double bond, length as m-dash]P but not the C[double bond, length as m-dash]O double bond, which differentiates it from enones.

A Phosphanyl Phosphagermene and its Reactivity

Reaction of a nucleophilic germylene Ge[CH(SiMe3)2]2 with the phosphanyl phosphaketene [{(H2C)(NDipp)}2P]PCO induces decarbonylation to form a phosphanyl phosphagermene [{(H2C)(NDipp)}2P]P=Ge[CH(SiMe3)2]2 (1; Dipp=2,6-diisopropyl-phenyl). Addition of CO2 or MeCN to 1 results in [3+2]-cycloaddition reactions to afford five-membered heterocycles. This mode of reactivity is reminiscent of that observed for frustrated Lewis pairs, with the pendant phosphanyl group acting as a base and the germanium center as a Lewis acid. Contrastingly, 1,2-addition across the P=Ge bond was observed when using ammonia, small primary amines (NH2 nP), or metal complexes (e. g. Au(PPh3)Cl and ZnEt2). These latter reactions allow for the one-step synthesis of metal phosphide complexes.

Homogeneous and Heterogeneous Frustrated Lewis Pairs for the Activation and Transformation of CO2

Due to the serious ecological problems caused by the high CO2 content in the atmosphere, reducing atmospheric CO2 has attracted widespread attention from academia and governments. Among the many ways to mitigate CO2 concentration, the capture and comprehensive utilization of CO2 through chemical methods have obvious advantages, whose key is to develop suitable adsorbents and catalysts. Frustrated Lewis pairs (FLPs) are known to bind CO2 through the interaction between unquenched Lewis acid sites/Lewis base sites with the O/C of CO2, simultaneously achieving CO2 capture and activation, which render FLP better potential for CO2 utilization. However, how to construct efficient FLP targeted for CO2 utilization and the mechanism of CO2 activation have not been systematically reported. This review firstly provides a comprehensive summary of the recent advances in the field of CO2 capture, activation, and transformation with the help of FLP, including the construction of homogeneous and heterogeneous FLPs, their interaction with CO2, reaction activity, and mechanism study. We also illustrated the challenges and opportunities faced in this field to shed light on the prospective research.

Trapping an Elusive Phosphanyl-Phosphaalumene

We describe our efforts to access a compound with an Al=P double bond by reaction of Al(Nacnac) towards [H2CN(Dipp)]2P(PCO) (Nacnac=HC[C(Me)N(Dipp)]2; Dipp=2,6-iPr2C6H3). Our observations are consistent with the formation of a transient phosphanyl-phosphaalumene at low temperatures (-70 °C), however this species was found to readily undergo intramolecular C-H activation of the β-diketiminato ligand upon warming to room temperature. The reactivity of the transient complex toward small molecules including dihydrogen, carbon dioxide, phosphaketenes, amines and silanes could be explored at low temperatures, showcasing that the target compound can react as both a frustrated Lewis pair (via the pendant phosphanyl moiety) or in hydroelementation reactions of the Al=P bond. The elusive target molecule could be trapped by addition of a Lewis base (tetrahydrofuran) affording an isolable molecular species that reacts in an analogous fashion to the base-free compound.

Quo Vadis CO2 Activation: Catalytic Reduction of CO2 to Methanol Using Aluminum and Gallium/Carbon-based Ambiphiles

We report on so-called "hidden FLPs" (FLP: frustrated Lewis pair) consisting of a phosphorus ylide featuring a group 13 fragment in the ortho position of a phenyl ring scaffold to form five-membered ring structures. Although the formation of the Lewis acid/base adducts was observed in the solid state, most of the title compounds readily react with carbon dioxide to provide stable insertion products. Strikingly, 0.3-3.0 mol% of the reported aluminum and gallium/carbon-based ambiphiles catalyze the reduction of CO2 to methanol with satisfactory high selectivity and yields using pinacol borane as stoichiometric reduction equivalent. Comprehensive computational studies provided valuable mechanistic insights and shed more light on activity differences.

Gallaphosphene L(Cl)GaPGaL: A novel phosphinidene transfer reagent

Gallaphosphene L(Cl)GaPGaL 1 (L=HC[C(Me)N(Ar)]2 ; Ar=2,6-iPr2 C6 H3 ) reacts with N-heterocyclic carbenes R NHC (R NHC=[CMeN(R)]2 C; R=Me, iPr) to R NHC-coordinated phosphinidenes R NHC→PGa(Cl)L (R=Me 2 a, iPr 2 b) and with isonitriles RNC (R=iPr, Cy) to 1,3-phosphaazaallenes L(Cl)GaP=C=N-R (R=iPr 3 a, Cy 3 b), respectively. Quantum chemical calculations reveal that 2 a/2 b possess two localized lone pair of electrons, whereas 3 a/3 b only show one localized lone pair as was reported for gallaphosphene 1. 2 b reacts with 2.5 equivalents of a borane (THF ⋅ BH3 ) to the NHC-stabilized phosphinidene-borane complex [iPr NHC→P(BH2 )]2 (BH3 )3 4 with concomitant formation of LGa(H)Cl 5. 2-5 are characterized by heteronuclear (1 H, 13 C{1 H}, 31 P{1 H}) NMR and IR spectroscopy, elemental analysis, and single crystal X-ray diffraction (sc-XRD).

Hydroelementation and Phosphinidene Transfer: Reactivity of Phosphagermenes and Phosphastannenes Towards Small Molecule Substrates

We describe the facile synthesis of [(Me3 Si)2 CH]2 E=PMes* (E=Ge, Sn) from the reaction of the tetrylenes with the phospha-Wittig reagent, Me3 P-PMes*. Their reactivity towards a range of substrates with protic and hydridic E-H bonds (E=N, O, Si) is described. In addition to hydroelementation reactions of the E=P bonds, we show that these compounds, particularly [(Me3 Si)2 CH]2 Sn=PMes*, also act as base-stabilized phosphinidenes, allowing phosphinidene transfer to other nucleophiles.

Advances in CO2 activation by frustrated Lewis pairs: from stoichiometric to catalytic reactions

The rise of CO2 concentrations in the environment due to anthropogenic activities results in global warming and threatens the future of humanity and biodiversity. To address excessive CO2 emissions and its effects on climate change, efforts towards CO2 capture and conversion into value adduct products such as methane, methanol, acetic acid, and carbonates have grown. Frustrated Lewis pairs (FLPs) can activate small molecules, including CO2 and convert it into value added products. This review covers recent progress and mechanistic insights into intra- and inter-molecular FLPs comprised of varying Lewis acids and bases (from groups 13, 14, 15 of the periodic table as well as transition metals) that activate CO2 in stoichiometric and catalytic fashion towards reduced products.

Structures, Bonding Analyses and Reactivity of a Dicationic Digallene and Diindene Mimicking trans-bent Ditetrylenes

The reaction of bisdicyclohexylphosphinoethane (dcpe) and the subvalent MI sources [MI (PhF)2 ][pf] (M=Ga+ , In+ ; [pf]- =[Al(ORF )4 ]- ; RF =C(CF3 )3 ) yielded the salts [{M(dcpe)}2 ][pf]2 , containing the first dicationic, trans-bent digallene and diindene structures reported so far. The non-classical MI ⇆MI double bonds are surprisingly short and display a ditetrylene-like structure. The bonding situation was extensively analyzed by quantum chemical calculations, QTAIM (Quantum Theory of Atoms in Molecules) and EDA-NOCV (Energy Decomposition Analysis with the combination of Natural Orbitals for Chemical Valence) analyses and is compared to that in the isoelectronic and isostructural, but neutral digermenes and distannenes. The dissolved [{Ga(dcpe)}2 ]2+ ([pf]- )2 readily reacts with 1-hexene, cyclooctyne, diphenyldisulfide, diphenylphosphine and under mild conditions at room temperature. This reactivity is analyzed and rationalized.

Donor-Acceptor Activation of Carbon Dioxide

The activation and functionalization of carbon dioxide entails great interest related to its abundance, low toxicity and associated environmental problems. However, the inertness of CO2 has posed a challenge towards its efficient conversion to added-value products. In this review we discuss one of the strategies that have been widely used to capture and activate carbon dioxide, namely the use of donor-acceptor interactions by partnering a Lewis acidic and a Lewis basic fragment. This type of CO2 activation resembles that found in metalloenzymes, whose outstanding performance in catalytically transforming carbon dioxide encourages further bioinspired research. We have divided this review into three general sections based on the nature of the active sites: metal-free examples (mainly formed by frustrated Lewis pairs), main group-transition metal combinations, and transition metal heterobimetallic complexes. Overall, we discuss one hundred compounds that cooperatively activate carbon dioxide by donor-acceptor interactions, revealing a wide range of structural motifs.

Photolysis of Phosphaketenyltetrylenes with a Carbazolyl Substituent

Phosphaketenes of divalent group 14 compounds can potentially serve as precursors for the synthesis of heavy multiple-bond systems. We have employed the dtbp Cbz substituent (dtbp Cbz=1,8-bis(3,5-ditertbutylphenyl)-3,6-ditertbutylcarbazolyl) to prepare such phosphaketenyltetrylenes [(dtbp Cbz)EPCO] (E=Ge, Sn, Pb). While the phosphaketenyltetrylenes are stable at ambient conditions, they can be readily decarbonylated photolytically. For the germylene and stannylene derivatives, dimeric diphosphene-type products [(dtbp Cbz)EP]2 (E=Ge, Sn) were obtained. In contrast, photolysis of the phosphaketenylplumbylene, via isomerisation of the [(dtbp Cbz)PbP] intermediate to [(dtbp Cbz)PPb], afforded an unsymmetric and incompletely decarbonylated product [(dtbp Cbz)2 Pb2 P2 CO] formally comprising a [(dtbp Cbz)PPb] and a [(dtbp Cbz)PbPCO] moiety.

Base-Stabilized Phosphinidene Oxide, Imide and Sulfide

Oxidation of a base-stabilized phosphinidene (κ2 -NNP)P (12, NNP=phosphinoamidinate) with N2 O afforded a labile phosphinidene oxide (κ2 -NNP)P=O (16) which was characterized by NMR spectroscopy. Further oxidation of 16 by N2 O or reaction of 12 with two equivalents of pyridine oxide afforded the isolable dioxide (κ2 -NNP)PO2 which was characterized by NMR and SC XRD. Trapping of 16 with tolyl isocyanate resulted in P=O/N=C metathesis, eventually affording a urea-ligated phosphine (κ1 -NNP)P(NTol)2 C=O (17) The mechanism of this reaction was elucidated by DFT calculations. Reactions of phosphinidene 12 with azides generated transient imines (NNP)P=NR, which in the case of R=Tol underwent cycloaddition with tolyl Isocyanate to afford the urea product 17, and in the case of R=SiMe3 reacts with N3 SiMe3 via the addition of N-Si across the P=N bond affording, after the extrusion of dinitrogen, a P,N-heterocyclic compound. Both products of the reactions with azides have been fully characterized, both in solution and the solid-state. Finally, reaction of phosphinidene 12 with one equivalent of sulfur resulted in the isolation of the base-stabilized phosphinidene sulfide (κ2 -NNP)P=S that has also been fully characterized.

Stannyl phosphaketene as a synthon for phosphorus analogues of β-lactams

The reaction of the stannyl phosphaketene (Nacnac)SnPCO 1 (Nacnac = CH{(CMe)(2,6-iPr2C6H3N)}2) with B(C6F5)3 produced the 1,4-addition product of (Nacnac)SnPCO(B(C6F5)3). However, the corresponding reactions in the presence of dimethyl maleate, diisopropyl fumarate or diethyl-but-2-ynedioate gave [2+2] addition yielding four-membered phosphacycles, ((Nacnac)Sn(MeO2C))CHPC(OB(C6F5)3)CH(CO2Me), [(C6F5)3B)PC(OSn)C(CO2Me)CH(CO2Me)]2, (Nacnac)Sn(iPrO2C)CC(OAl(C6F5)3)P[CH(CO2iPr)CH2(CO2iPr)]CH(CO2iPr), and (Nacnac)SnP (EtO2CCC(CO2Et))CO(B(C6F5)3), respectively. In contrast, the corresponding reaction of phenylacetylene gave the FLP-addition product (Nacnac)SnOC(P)C(Ph)CH(B(C6F5)3). Collectively, this reactivity demonstrates that the stannyl phosphaketene 1 can act as a synthon for P-analogues of β-lactam derivatives.

On the Reactivity of Mes*P(PMe3 ) towards Aluminum(I) Compounds - Evidence for the Intermediate Formation of Phosphaalumenes

Phosphaalumenes are the heavier isoelectronic analogs of alkynes and have eluded facile synthesis until recently. We have reported that the combination of a phosphinidene transfer agent, Ar TerP(PMe3 ) (Ar Ter=2,6-Ar2 -C6 H3 ), with (Cp*Al)4 (Cp*=C5 (CH3 )5 ) afforded the phosphaalumenes Ar TerPAlCp* as isolable, violet, thermally stable compounds. In here we describe attempts to utilize Mes*P(PMe3 ) (Mes*=2,4,6-tBu3 -C6 H2 ) as a phosphinidene source in combination with different Al(I) precursors, namely Dip NacnacAl (Dip Nacnac=HC[C(Me)NDip]2 , Dip=2,6-iPr2 -C6 H3 ), (Cp*Al)4 and Cp3t Al (Cp3t =1,2,4-tBu3 -C5 H2 ). In all cases the formation of phosphaalumenes was not observed, however, their intermediate formation is indicated by formation of the dimer [Cp*Al(μ-PMes*)]2 (2) and C-H-bond activation products along the putative P=Al bond, giving unusual 1,2-P,Al-tetrahydronaphtalene derivatives 1 and 4, clearly underlining the role the sterically demanding group on phosphorus plays in these transformations. The reactivity studies are supported by theoretical studies, demonstrating a thermodynamic preference for the C-H activation products. Additionally, we show that there are potential pitfalls in the synthesis of Cp*2 AlH, the precursor to make (Cp*Al)4 and give recommendations how to circumvent these.

Photochemical formation and reversible base-induced cleavage of a phosphagallene

The reactivity of Cp*Ga (Cp* = C₅Me₅) towards phosphanylidenephosphoranes of the type ^ArTerP(PMe₃) (^ArTer = ^DipTer 2,6-(2,6-iPr₂C₆H₃)₂C₆H₃), ^TipTer 2,6-(2,4,6-iPr₃C₆H₂)₂C₆H₃ was investigated. While no thermal reaction was observed (in line with DFT results), irradiation at 405 nm at low temperatures resulted in the formation of phosphagallenes ^DipTerP = GaCp* (1a) and ^TipTerP = GaCp* (1b) accompanied by release of PMe₃. When warming the reaction mixture to ambient temperatures without irradiation, the clean re-formation of ^ArTerP(PMe₃) and Cp*Ga in a second-order reaction was observed. Upon removal of PMe₃, 1a and 1b were isolated and fully characterized. Both derivatives were found to be labile and decomposed to the phosphafluorenes 2a and 2b, indicating generation of the transient phosphinidene ^ArTerP along with Cp*Ga. First reactivity studies show that CO₂ and H₂O cleanly reacted with 1a, affording ^DipTerPCO (3) and ^DipTerPH₂ (4), respectively.

Stabilisation and reactivity studies of donor-base ligand-supported gallium-phosphides with stronger binding energy: a theoretical approach

Gallium phosphide is a three-dimensional polymeric material of the hetero-diatomic GaP unit, which has a wurtzite type structure, and captivating application as a light emitting diode (LED). As a result, there is a constant search for suitable precursors to synthesise GaP-based materials. However, the corresponding monomeric species is exotic in nature due to the expected Ga[triple bond, length as m-dash]P multiple bond. Herein, we report on the theoretical studies of stability, chemical bonding, and reactivity of the monomeric gallium phosphides with two donor base ligands having tuneable binding energies. We have performed detailed investigations using density functional theory at three different levels (BP86/def2-TZVPP, B3LYP/def2-TZVPP, M06-2X/def2-TZVPP), QTAIM and EDA-NOCV (BP86-D3(BJ)/TZ2P, M06-2X/TZ2P) to analyse various ligand-stabilised GaP monomers, which revealed the synthetic viability of such species in the presence of stable singlet carbenes, e.g., cAAC, and NHC as ligands [cAAC = cyclic alkyl(amino) carbene, NHC = N-heterocyclic carbene] due to the larger bond dissociation energy compared to a phosphine ligand (PMe₃). The calculated bond dissociation energies between a pair of ligands and the monomeric GaP unit are found to be in the range of 87 to 137 kcal mol^-1, predicting their possible syntheses in the laboratory. Further, the reactivity of such species with metal carbonyls [Fe(CO)₄, and Ni(CO)₃] have been theoretically investigated.

On the Reactivity of Phosphaalumenes towards C-C Multiple Bonds

Heterocycles containing group 13 and 15 elements such as borazines are an integral part of organic, biomedical and materials chemistry. Surprisingly, heterocycles containing P and Al are rare. We have now utilized phosphaalumenes in reactions with alkynes, alkenes and conjugated double bond systems. With sterically demanding alkynes 1,2-phosphaalumetes were afforded, whereas the reaction with HCCH or HCCSiMe₃ gave 1,4-phosphaaluminabarrelenes. Using styrene saturated 1,2-phosphaalumates were formed, which reacted further with additional styrene to give different regio-isomers of 1,4-aluminaphosphorinanes. Using ethylene, a 1,4-aluminaphosphorinane is obtained, while with 1,3-butadiene a bicyclic system containing an aluminacyclopentane and a phosphirane unit was synthesized. The experimental work is supported by theoretical studies to shed light on the mechanism governing the formation of these heterocycles.

How long are Ga⇆Ga double bonds and Ga-Ga single bonds in dicationic gallium dimers?

Syntheses and characterization of two salts [(L)GaGa(L)][pf]₂ ([pf]^- = [Al(OR^F)₄]^-; R^F = C(CF₃)₃) are reported. They include the first dicationic digallene [(L)Ga⇆Ga(L)]²⁺ (L = CDP^Ph = C(PPh₃)₂) and a digallane [(L)Ga-Ga(L)]²⁺ (L = [NacNac^Mes]^-). The CDP^Ph-supported digallene dication includes a trans-bent [L-GaGa-L]²⁺ bond that is analogous to neutral R-GaGa-R molecules and related to Robinson's famous "Digallyne" [R-GaGa-R]^2-. The dicationic digallane [(L)Ga-Ga(L)]²⁺ is analogous to the widely used "Jones magnesium dimer", but includes a very short Ga^II-Ga^II single bond.

Influence of the Element and Substituent Effects on the Reactivity of Catching Reactions of Difluorocarbene by Benzene-Bridged and Group-13/Group-15-Based Frustrated Lewis Pairs

The trapping reactions of CF₂ by benzene-bridged Group-13/P-based and B/Group-15-based frustrated Lewis pairs (FLPs) have been computationally investigated based on density functional theory. Interestingly, our theoretical calculations predict that the capture of CF₂ by all five Group-13/P-based FLPs is energetically feasible. However, in the B/Group-15-based FLPs, only the phosphorus-based B/P-FLP can trap CF₂ from kinetic and thermodynamical viewpoints. According to the analyses of the activation strain model, it can be known that the atomic radius of the G15 element (Lewis base) of benzene-bridged B/Group-15-FLP plays an important role in controlling the reactivity of the CF₂ catching reactions, whereas the atomic radius of the Group-13 center (Lewis acid) does not play a role in influencing the activation barrier of these CF₂ catching reactions. Our theoretical findings based on sophisticated methods suggest that the forward bonding is the FLP-to-CF₂ interaction, the LP (Group-15-donor) → vacant p-π-orbital (CF₂), which was quantitatively proved to be strong in such present CF₂ catching reactions. However, the back bonding is the CF₂-to-FLP interaction, the empty σ-orbital (Group-13-acceptor) ← sp²-σ-orbital (CF₂), which was verified to be relatively weak. Our theoretical pieces of evidence reveal that the stronger electron-donating ability of the substituents is attached to the Lewis basic center and can make the reaction barrier of the benzene-bridged Group-13/Group-15-based FLP-related compound catching CF₂ smaller and more exothermic.

A Free Phosphaborene Stable at Room Temperature

Free phosphaborenes (R-P═B-R) are PB analogues of alkynes, and their isolation is a long-sought-after goal. Herein, we demonstrate that the combination of a π-donating and a π-accepting substituent with bulky flanking arene rings enables the isolation of a crystalline free phosphaborene 5 at room temperature. This electron push-pull cooperation, combined with the kinetic protection, hinders its inherent tendency to oligomerize. This species features a PB double bond consisting of a conventional σ bond and a delocalized π bond. The lone pair of electrons at P slightly contributes to the PB bonding. Preliminary reactivity studies show that 5 undergoes facile (cyclo)addition reactions with p-methyl benzaldehyde, p-fluoroacetophenone, and carbon disulfide, the last of which results in facile PB double bond cleavage. Our strategy has a significant impact on the future synthesis of ambiphilic heterodiatomic multiply bonded main group species.

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, H2 NR and HNR2 activation is demonstrated.

Heavier group 13/15 multiple bond systems: synthesis, structure and chemical bond activation

Heavier group 13/15 multiple bonds have been under investigation since the late 80s and to date, several examples have been published, which shows the obsoleteness of the so-called double bond rule. Especially in the last few years, more and more group 13/15 multiple bonds became synthetically feasible and their application in terms of small molecule activation has been demonstrated. Our group has recently shown that the combination of the pnictinidene precursor ^DipTer-Pn(PMe₃) (Pn = P, As) in combination with Al(I) synthons afforded the first examples of phospha- and arsaalumenes as isolable and thermally robust compounds. This feature article is intended to show the recent developments in the field, to outline early synthetic approaches and to discuss strategies to unlock the synthetic potential of these elusive chemical bonds.

Selective 1,2 addition of polar X-H bonds to the Ga-P double bond of gallaphosphene L(Cl)GaPGaL

Gallaphosphene L(Cl)GaPGaL 1 (L = HC[C(Me)N(2,6-i-Pr₂-C₆H₃)]₂) reacts at ambient temperature with a series of polar X-H bonds, i.e. ammonia, primary amines, water, phenol, thiophenol, and selenophenol, selectively with 1,2 addition at the polar Ga-P double bond. The gallium atom serves as electrophile and the phosphorous atom is protonated in all reactions. The resulting complexes L(Cl)GaP(H)Ga(X)L (X = NH₂2, NHi-Pr 3, NHPh 4, OH 5, OXyl 6, SPh 7, SePh 8) were characterized by IR and heteronuclear (¹H, ¹³C{¹H}, ³¹P{¹H}) NMR spectroscopy, elemental analysis, and single-crystal X-ray diffraction.

Cyclo-Dipnictadialanes

Using the AlI precursor Cp3t Al in conjunction with triphosphiranes (PAr)3 (Ar=Mes, Dip, Tip) we have succeeded in preparing Lewis base-free cyclic diphosphadialanes with both the Al and P atoms bearing three substituents. Using the sterically more demanding Dip and Tip substituents the first 1,2-diphospha-3,4-dialuminacyclobutanes were obtained, whereas with Mes substituents [Cp3t Al(μ-PMes)]2 is formed. This divergent reactivity was corroborated by DFT studies, which indicated the thermodynamic preference for the 1,2-diphospha-3,4-dialuminacyclobutane form for sterically more demanding groups on phosphorus. Using Cp*Al we could extend this concept to the corresponding cyclic diarsadialanes [Cp*Al(μ-AsAr)]2 (Ar=Dip, Tip) and additionally add the phosphorus variants [Cp*Al(μ-PAr)]2 (P=Mes, Dip, Tip). The reactivity of one variant [Cp3t Al(μ-PPh)]2 towards NHCs was tested and resulted in double NHC-stabilised [Cp3t (IiPr2 )Al(μ-PPh)]2 .

Significant Insight into the Origin of Reaction Barriers Determining Dihydrogen Activation by G13-P-P (G13 = Group 13 Element) and G15-P-Ga (G15 = Group 15 Element) Frustrated Lewis Pairs

The heterolytic cleavage of H₂ by multiply bonded phosphorus-bridged G13-P-P-Rea (G13 = B, Al, Ga, In, and Tl) and G15-P-Ga-Rea (G15 = N, P, As, Sb, and Bi) frustrated Lewis pairs (FLPs) has been theoretically investigated using density functional theory calculations. For the above nine FLP-type molecules, our theoretical findings suggest that only Al-P-P-Rea, Ga-P-P-Rea, and In-P-P-Rea can undergo the energetically feasible H₂ activation reaction from kinetic and thermodynamic viewpoints. Our study based on the activation strain model (ASM) reveals that gaining a better orbital overlap between G13-P-P-Rea and G15-P-Ga-Rea molecules and H₂ affected the reaction barriers through the atomic radius of G13 and G15. According to our energy decomposition analysis-natural orbitals for chemical valence (EDA-NOCV) results, the bonding of these H₂ activation reactions involving G13-P-P-Rea and G15-P-Ga-Rea is dominated by the donor-acceptor interaction (singlet-singlet interaction) rather than the electron-sharing interaction (triplet-triplet interaction). Moreover, our EDA-NOCV evidence reveals that the best description for the above bonding situations is the lone pair(G15) → σ*(H₂) interaction rather than the empty p-π-orbital(G13) ← σ(H₂) interaction. In particular, the findings in this work based on theoretically calculated geometries and the corresponding relative free energies of the stationary points combined with the results from the above sophisticated methods nicely agree with the famous Hammond postulate.

Reversible and Irreversible [2+2] Cycloaddition Reactions of Heteroallenes to a Gallaphosphene

[2+2] Cycloaddition reactions of gallaphosphene L(Cl)GaPGaL 1 (L=HC[C(Me)N(2,6-i-Pr2 C6 H3 )]2 ) with carbodiimides [C(NR)2 ; R=i-Pr, Cy] and isocyanates [RNCO; R=Et, i-Pr, Cy] yielded four-membered metallaheterocycles LGa(Cl)P[μ-C(X)NR]GaL (X=NR, R=i-Pr 2, Cy 3; X=O, R=Et 4, i-Pr 5, Cy 6). Compounds 4-6 reversibly react with CO2 via [2+2] cycloaddition at ambient temperature to the six-membered metallaheterocycles LGa(Cl)P[μ-C(O)O]-μ-C(O)N(R)GaL (R=Et 7, i-Pr 8, Cy 9). Compounds 2-9 were characterized by IR and heteronuclear (1 H, 13 C{1 H}, 31 P{1 H}) NMR spectroscopy and elemental analysis, while quantum chemical calculations provided a deeper understanding on the energetics of the reactions.

Contrasting E-H Bond Activation Pathways of a Phosphanyl-Phosphagallene

The reactivity of the phosphanyl-phosphagallene, [H2 C{N(Dipp)}]2 PP=Ga(Nacnac) (Nacnac=HC[C(Me)N(Dipp)]2 ; Dipp=2,6-i Pr2 C6 H3 ) towards a series of reagents possessing E-H bonds (primary amines, ammonia, water, phenylacetylene, phenylphosphine, and phenylsilane) is reported. Two contrasting reaction pathways are observed, determined by the polarity of the E-H bonds of the substrates. In the case of protic reagents (δ- E-Hδ+ ), a frustrated Lewis pair type of mechanism is operational at room temperature, in which the gallium metal centre acts as a Lewis acid and the pendant phosphanyl moiety deprotonates the substrates. Interestingly, at elevated temperatures both NH2 i Pr and ammonia can react via a second, higher energy, pathway resulting in the hydroamination of the Ga=P bond. By contrast, with hydridic reagents (δ+ E-Hδ- ), such as phenylsilane, hydroelementation of the Ga=P bond is exclusively observed, in line with the polarisation of the Si-H and Ga=P bonds.

Vibrational spectrum and photochemistry of phosphaketene HPCO

The vibrational spectra of the simplest phosphaketene HPCO and its isotopologue DPCO in solid Ar-matrices at 12.0 K have been analyzed with the aid of the computations at the CCSD(T)-F12a/cc-pVTZ-F12 level using configuration-selective vibrational configuration interaction (VCI). In addition to the four IR fundamentals, four overtone and ten combination bands have been unambiguously identified. Furthermore, the photochemistry of HPCO in the matrix has been investigated for the first time. Upon UV-light irradiation (365 or 266 nm), CO-elimination occurs by forming the parent phosphinidene HP that can be trapped by ˙NO to yield the elusive phosphinimine-N-oxyl radical HPNO˙. In contrast, an excimer laser (193 nm) irradiation of HPCO causes additional decomposition to H˙ and ˙PCO with concomitant formation of the long-sought phosphaethyne HOCP.

A Thermally Stable Magnesium Phosphaethynolate Grignard Complex

The 2-phosphaethynolate (OCP) anion has found versatile applications across the periodic table but remains underexplored in group 2 chemistry due to challenges in isolating thermally stable complexes. By rationally modifying their coordination environments using 1,3-dialkyl-substituted N-heterocyclic carbenes (NHCs), we have now isolated and characterized thermally stable, structurally diverse, and hydrocarbon soluble magnesium phosphaethynolate complexes (2, 4^Me, and 8-10), including the novel phosphaethynolate Grignard reagent (2^iPr). The methylmagnesium phosphaethynolate and magnesium diphosphaethynolate complexes readily activate dioxane with subsequent H-atom abstraction to form [(NHC)MgX(μ-OEt)]₂ [X = Me (3) or OCP (8 and 9)] complexes. Their reactivities increased with the Lewis acidity of the Mg²⁺ cation and may be attenuated by Lewis base saturation or a slight increase in carbene sterics. Solvent effects were also investigated and led to the surreptitious isolation of an ether-free sodium phosphaethynolate (NHC)₃Na(OCP) (6), which is soluble in aromatic hydrocarbons and can be independently prepared by the reaction of NHC and [Na(dioxane)₂][OCP] in toluene. Under forcing conditions (105 °C, 3 days), the magnesium diphosphaethynolate complex (NHC)₃Mg(OCP)₂ (10) decomposes to a mixture of organophosphorus complexes, among which a thermal decarbonylation product [(NHC)₂P^I][OCP] (11) was isolated.

Adducts of the Parent Boraphosphaketene H2 BPCO and their Decarbonylative Insertion Chemistry

The first examples of Lewis base adducts of the parent boraphosphaketene (H2 B-PCO) and their cyclodimers are prepared. One of these adducts is shown to undergo mild decarbonylation and phosphinidene insertion into a B-C bond of a borole, forming very rare examples of 1,2-phosphaborinines, B/P isosteres of benzene. The strong donor properties of these 1,2-phosphaborinines are confirmed by the synthesis of their π complexes with the Group 6 metals.

Controlling the Lewis Acidity and Polymerizing Effectively Prevent Frustrated Lewis Pairs from Deactivation in the Hydrogenation of Terminal Alkynes

Two strategies were reported to prevent the deactivation of Frustrated Lewis pairs (FLPs) in the hydrogenation of terminal alkynes: reducing the Lewis acidity and polymerizing the Lewis acid. A polymeric Lewis acid (P-BPh₃) with high stability was designed and synthesized. Excellent conversion (up to 99%) and selectivity can be achieved in the hydrogenation of terminal alkynes catalyzed by P-BPh₃. This catalytic system works quite well for different substrates. In addition, the P-BPh₃ can be easily recycled.

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.

Multi-Talented Gallaphosphene for Ga-P-Ga Heteroallyl Cation Generation, CO2 Storage, and C(sp3 )-H Bond Activation

Gallaphosphene L(Cl)GaPGaL (2; L=HC[C(Me)N(2,6-i-Pr2 C6 H3 )]2 ), which is synthesized by reaction of LGa(Cl)PCO (1) with LGa, reacts with [Na(OCP)(dioxane)2.5 ] to LGa(OCP)PGaL (3), whereas chloride abstraction with LiBArF 4 yields [LGaPGaL][BArF 4 ] (4; BArF 4 =B(C6 F5 )4 ). 4 represents a heteronuclear analog of the allyl cation according to quantum chemical calculations. Remarkably, 2 reversibly reacts with CO2 to yield L(Cl)Ga-P[μ-C(O)O]2 GaL (5), while reactions with acetophenone and acetone selectively give compounds 6 and 7 by C(sp3 )-H bond activation.