Phosphaalkenes with Inverse Electron Density

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).

Triphosphiranes as phosphinidene-transfer agents - synthesis of regular and chelating NHC phosphinidene adducts

In this contribution we describe the general use of aryl-substituted triphosphiranes (Ar3P3; Ar = Mes, Dip, Tip) as phosphinidene transfer reagents towards N-heterocyclic carbenes (NHCs) to give a library of twelve N-heterocyclic carbene phosphinidene adducts of the type ArPNHC (NHCPs), in which the NHCs have varying steric profiles, allowing a systematic evaluation of their structural and NMR-spectroscopic properties. In the next series of experiments we utilized 1,3- and 1,4-phenylene bridged bis-NHCs to access a new class of chelating bis(NHCP)s, of which three derivatives could be structurally characterized. The 1,4-phenylene derivatives were shown to be susceptible to P-CNHC bond cleavage when irradiated with an LED (396 nm), providing a rare example of phosphinidene release from NHCPs. The coordination chemistry of 1,3-phenylene bridged bis(NHCP)s towards GeCl2(dioxane) and GaI3 was investigated and revealed the formation of ion-separated cationic complexes, with significant charge transfer from the ligand to the metal center according to NBO analyses.

Synthesis and Ligand Properties of Chelating Bis(N-heterocyclic carbene)-Stabilized Bis(phosphinidenes)

NHC-phosphinidene (NHCP) adducts are an emerging class of ligands with proven binding ability for main group and transition metal elements. They possess electron-rich P atoms with two lone pairs (LPs) of electrons, making them interesting platforms for the formation of multimetallic complexes. We describe herein a modular, high-yielding synthesis of bis(NHCP)s, starting from alkylidene-bridged bis(NHC)s ((IMe)2CnH2n; n = 1,3) and triphosphirane (PDip)3 (Dip = 2,6-iPr2C6H3) as phosphinidene transfer reagent. The coordination chemistry of [{DipP(IMe)}2CH2], 1, was studied in detail, and complexes [1·FeBr2] and [1·Rh(cod)]Cl were prepared, showing that the ligand has a flexible bite angle. The dicarbonyl complex [1·Rh(CO)2]Cl, with an average value for the CO stretching frequency of 2029 cm-1, indicates a strongly donating ligand when compared to related complexes. The binding ability of the remaining two phosphorus LPs was demonstrated with AuCl(SMe2), giving the heterotrimetallic complex [1·(AuCl)2·Rh(cod)]Cl. Moreover, [1·Rh(cod)]X (X- = Cl, B(3,5-(CF3)2-C6H3)4) was tested in the catalytic hydrogenation of methyl-Z-α-acetamidocinnamate (MAC) and dimethyl itaconate (ItMe2), revealing that the chloride complex was inactive, while the BArF complex demonstrated moderate activity. Additionally, [1·Rh(cod)]Cl was shown to be moderately air- and moisture-stable, slowly decomposing to the corresponding NHC-stabilized bis-dioxophosphorane, which was independently synthesized by treating the free ligand with dry O2.

Formation of a Hexaphosphido Cobalt Complex through P-P Condensation

The reaction between diphosphorus derivatives [(Cl ImDipp )P2 (Dipp)]OTf (1[OTf]) and [(Cl ImDipp )P2 (Dipp)Cl] (1[Cl]) with the cyclotetraphosphido cobalt complex [K(18c-6)][(PHDI)Co(η4 -cyclo-P4 )] (2) leads to the formation of complex [(PHDI)Co{η4 -cyclo-P6 (Dipp)(Cl ImDipp )}] (3), which features an unusual hexaphosphido ligand [Cl ImDipp =4,5-dichloro-1,3-bis(2,6-diisopropylphenyl)imidazol-2-yl, Dipp=2,6-diisopropylphenyl, 18c-6=18-crown-6, PHDI=bis(2,6-diisopropylphenyl)phenanthrene-9,10-diimine]. Complex 3 was obtained as a crystalline material with a moderate yield at low temperature. Upon exposure to ambient temperature, compound 3 slowly transforms into two other compounds, [K(18c-6)][(PHDI)Co(η4 -P7 Dipp)] (4) and [(PHDI)Co{cyclo-P5 (Cl ImDipp )}] (5). The novel complexes 3-5 were characterized using multinuclear NMR spectroscopy and single-crystal X-ray diffraction. To shed light on the formation of these compounds, a proposed mechanism based on 31 P NMR monitoring studies is presented.

Cationic Phosphinidene as a Versatile P1 Building Block: [LC-P]+ Transfer from Phosphonio-Phosphanides [LC-P-PR3]+ and Subsequent LC Replacement Reactions (LC = N-Heterocyclic Carbene)

Cationic imidazoliumyl(phosphonio)-phosphanides [LC-P-PR3]+ (1a-e+, LC = 4,5-dimethyl-1,3-diisopropylimidazolium-2-yl; R = alkyl, aryl) are obtained via the nucleophilic fragmentation of tetracationic tetraphosphetane [(LC-P)4][OTf]4 (2[OTf]4) with tertiary phosphanes. They act as [LC-P]+ transfer reagents in phospha-Wittig-type reactions, when converted with various thiocarbonyls, giving unprecedented cationic phosphaalkenes [LC-P═CR2]+ (5a-f[OTf]) or phosphanides [LC-P-CR(NR2')]+ (6a-d[OTf]). Theoretical calculations suggest that three-membered cyclic thiophosphiranes are crucial intermediates of this reaction. To test this hypothesis, treatment of [LC-P-PPh3]+ with phosphaalkenes, that are isolobal to thioketones, permits the isolation of diphosphirane salts 11a,b[OTf]. Furthermore, preliminary studies suggest that the cationic phosphaalkene [LC-P═CPh2]+ may be employed to access rare examples of η2-P═C π-complexes with Pd0 and Pt0 when treated with [Pd(PPh3)4] and [Pt(PPh3)3] for which analogous complexes of neutral phosphaalkenes are scarce. The versatility of [LC-P]+ as a valuable P1 building block was showcased in substitution reactions of the transferred LC-substituent using nucleophiles. This is demonstrated through the reactions of 5a[OTf] and 6c[OTf] with Grignard reagents and KNPh2, providing a convenient, high-yielding access to MesP═CPh2 (16) and otherwise difficult-to-synthesize 1,3-diphosphetane 17 and P-aminophosphaalkenes.

Counterintuitive Chemistry: Carbene Stabilization of Zero-Oxidation State Main Group Species

Carbenes have evolved from transient laboratory curiosities to a robust, diverse, and surprisingly impactful ligand class. A variety of different carbenes have significantly contributed to the development of low-oxidation state main group chemistry. This Perspective focuses upon advances in the chemistry of carbene complexes containing main group element cores in the formal oxidation state of zero, including their diverse synthetic strategies, unusual bonding and structural motifs, and utility in transition metal coordination chemistry and activation of small molecules.

Gradual Donor Stabilization of a Transient Ferrocene Bridged Bisphosphanyl Phosphenium Cation

A transient phosphenium cation embedded into a [3]ferrocenophane scaffold was formed via chloride abstraction. The cation has been trapped with phosphane, carbene, and silylene donors resulting in stable adducts or bond activation of the ferrocenophane bridge. In the absence of donors, dimerization of the phosphenium cation to the corresponding dication is observed or P-C bond activation with migration of a substituent leading to a putative phosphoniodiphosphene. Using 1,3-di-tert-butylimidazol-2-silylene as the donor, further reaction of the initially formed chlorosilane leads to activation of a P-P bond of the ferrocenophane scaffold with ring expansion of the ansa-bridge. The donor formation and bonding situation are investigated by density functional theory calculations as well as experimental methods (e.g., NMR spectroscopy and X-ray crystallography).

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.

Isolation of an Annulated 1,4-Distibabenzene Diradicaloid

The first 1,4-distibabenzene-1,4-diide compound [(ADC)Sb]₂ (5) based on an anionic dicarbene (ADC) (ADC=PhC{N(Dipp)C}₂ , Dipp=2,6-iPr₂ C₆ H₃ ) is reported as a bordeaux-red solid. Compound 5, featuring a central six-membered C₄ Sb₂ ring with formally Sb^I atoms may be regarded as a base-stabilized cyclic bis-stibinidene in which each of the Sb atoms bears two lone-pairs of electrons. 5 undergoes 2 e-oxidation with Ph₃ C[B(C₆ F₅ )₄ ] to afford [(ADC)Sb]₂ [B(C₆ F₅ )₄ ]₂ (6) as a brick-red solid. Each of the Sb atoms of 6 has an unpaired electron and a lone-pair. The broken-symmetry open-shell singlet diradical solution for (6)²⁺ is calculated to be 2.13 kcal mol^-1 more stable than the closed-shell singlet. The diradical character of (6)²⁺ according to SS-CASSCF (state-specific complete active space self-consistent field) and UHF (unrestricted Hartree-Fock) methods amounts to 36 % and 39 %, respectively. Treatments of 6 with (PhE)₂ yield [(ADC)Sb(EPh)]₂ [B(C₆ F₅ )₄ ]₂ (7-E) (E=S or Se). Reaction of 5 with (cod)Mo(CO)₄ affords [(ADC)Sb]₂ Mo(CO)₄ (8).

Carbene-Anchored Boryl- and Stibanyl-Phosphaalkenes as Precursors for Bis-Phosphaalkenyl Dichlorogermane and Mixed-Valence AgI/AgII Phosphinidenide

Cyclic alkyl(amino) carbene (cAAC)-anchored boryl- and stibanyl-phosphaalkenes with general formula cAAC = P-ER₂ [E = B, R = (NⁱPr₂)₂ (3a-c); E = Sb, R = 2,4,6-triisopropylphenyl (5a-b)] have been synthesized and utilized as precursors for the bis-phosphaalkenyl dichlorogermane [(cAAC = P)₂GeCl₂] (6) and the first molecular example of a neutral polymeric mixed-valence Ag^I/Ag^II phosphinidenide complex [(cAACP)₂Ag₄^IAg^IICl₄]_n (7). All compounds have been characterized by single-crystal X-ray diffraction and further investigated by nuclear magnetic resonance (NMR), mass spectrometric analysis, and UV-vis/fluorescence measurements. The paramagnetic complex 7 has been characterized by ESR spectroscopy. Cyclic voltammetry studies of compounds 3/5 have suggested possible one-electron quasi-reversible reductions, indicating their redox noninnocent behavior in solution. Quantum chemical studies revealed the electron-sharing nature of the P-B and P-Sb σ bonds in compounds 3 and 5, and the polar C_cAAC = P bonds in compounds 3, 5, and 6 prevailing their phosphaalkene structures over phosphinidenes.

Metathesis Reactions of a NHC-Stabilized Phosphaborene

The BP unsaturated unit is a very attractive functional group as it provides novel reactivity and unique physical properties. Nonetheless, applications remain limited so far due to the bulky nature of B/P-protecting groups, required to prevent oligomerization. Herein, we report the synthesis and isolation of a N-heterocyclic carbene (NHC)-stabilized phosphaborene, bearing a trimethylsilyl (TMS) functionality at the P-terminal, as a room-temperature-stable crystalline solid accessible via facile NHC-induced trimethylsilyl chloride (TMSCl) elimination from its phosphinoborane precursor. This phosphaborene compound, bearing a genuine B=P bond, exhibits a remarkable ability for undergoing P-centre metathesis reactions, which allows the isolation of a series of unprecedented phosphaborenes. X-ray crystallographic analysis, UV/Vis spectroscopy, and DFT calculations provide insights into the B=P bonding situation.

Synthesis, Coordination Chemistry, and Mechanistic Studies of P,N-Type Phosphaalkene-Based Rh(I) Complexes

The synthesis of P,N-phosphaalkene ligands, py-CH═PMes* (1, py = 2-pyridyl, Mes* = 2,4,6-tBu-C₆H₂) and the novel quin-CH═PMes* (2, quin = 2-quinolinyl) is described. The reaction with [Rh(μ-Cl)cod]₂ produces Rh(I) bis(phosphaalkene) chlorido complexes 3 and 4 with distorted trigonal bipyramidal coordination environments. Complexes 3 and 4 show a pronounced metal-to-ligand charge transfer (MLCT) from Rh into the ligand P═C π* orbitals. Upon heating, quinoline-based complex 4 undergoes twofold C-H bond activation at the o-tBu groups of the Mes* substituents to yield the cationic bis(phosphaindane) Rh(I) complex 5, which could not be observed for the pyridine-based analogue 3. Using sub- or superstoichiometric amounts of AgOTf the C-H bond activation at an o-tBu group of one or at both Mes* was detected, respectively. Density functional theory (DFT) studies suggest an oxidative proton shift pathway as an alternative to a previously reported high-barrier oxidative addition at Rh(I). The Rh(I) mono- and bis(phosphaindane) triflate complexes 6 and 7, respectively, undergo deprotonation at the benzylic CH₂ group of the phosphaindane unit in the presence of KOtBu to furnish neutral, distorted square-planar Rh(I) complexes 8 and 9, respectively, with one of the P,N ligands being dearomatized. All complexes were fully characterized, including multinuclear NMR, vibrational, and ultraviolet-visible (UV-vis) spectroscopy, as well as single-crystal X-ray and elemental analysis.

A Carbene-Stabilized Boryl-Phosphinidene

Herein, the synthesis and characterization of the carbene-stabilized boryl phosphinidenes 1-3 are reported. Compounds 1-3 are obtained by reacting Me-cAAC=PK (Me2 -cAAC=dimethyl cyclic(alkyl)(amino)carbene) and dihaloaryl borane in toluene. All three compounds were characterized by X-ray crystallography. Quantum mechanical studies indicated that these compounds have two lone pairs on the P center viz., an σ-type lone pair and a "hidden" π-type lone pair. Hence, these compounds can act as double Lewis bases, and the basicity of the π-type lone pair is higher than the σ-type lone pair.

N-Heterocyclic Carbene-Phosphinidenide Complexes as Hydroboration Catalysts

The reactions of the N-heterocyclic carbene-phosphinidene adducts (NHC)PSiMe₃ and (NHC)PH with the dinuclear ruthenium and osmium complexes [(η⁶-p-cymene)MCl₂]₂ (M = Ru, Os) afforded the half-sandwich complexes [(η⁶-p-cymene){(NHC)P}MCl] and [(η⁶-p-cymene){(NHC)PH}MCl₂] with two- and three-legged piano-stool geometries, respectively (NHC = IDipp, IMes; IDipp = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene; IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene). The complexes were initially tested as precatalysts for the hydroboration of benzonitrile, and the most active species, the ruthenium complex [(η⁶-p-cymene){(IMes)P}RuCl], was further used for the efficient hydroboration of a wide range (ca. 50 substrates) of nitriles, carboxylic esters, and carboxamides in neat pinacolborane (HBpin) under comparatively mild reaction conditions (60-80 °C, 3-5 mol % catalyst loading). Preliminary mechanistic and kinetic studies are reported, and stoichiometric reactions with HBpin indicate the initial formation of the monohydride complex [(η⁶-p-cymene){(IMes)P}RuH] as the putative catalytically active species.

A PH-functionalized dicationic bis(imidazolio)diphosphine

Reaction of the iodide salt of a secondary imidazolio-iodophosphine [(L)PHI]I (L⁺ = 1,3-diarylimidazolium-yl) with an imidazolio-phosphide (L)PH in the presence of GaI₃ afforded the isolable salt of a dicationic, bis(imidazolio)-substituted dihydro-diphosphine [(L)₂P₂H₂][GaI₄]₂. Non-preparative formation of the cationic diphosphines was also observed upon spontaneous "dehalo-coupling" of [(L)PHI]⁺, or in reactions of [(L)PHI]I and (L)PH in the absence of GaI₃. Further reaction of [(L)₂P₂H₂]²⁺ with (L)PH produced an iodide salt of a known (bis)imidazolio-diphosphide monocation [(L)₂P₂H]⁺. The identity of cationic diphosphines and diphosphides was established by single-crystal X-ray diffraction studies. NMR spectroscopy revealed that dications [(L)₂P₂H₂]²⁺ exist as a mixture of meso- and rac-diastereomers in solution. Computational studies confirmed the stereochemical assignment of the isomers observed, and gave insight into the bonding situation of the diphosphine dications.

Isoselective Polymerization of rac-Lactide by Aluminum Complexes of N-Heterocyclic Carbene-Phosphinidene Adducts

The N-heterocyclic carbene-phosphinidene adducts (NHC)PH were reacted with AlMe3 in toluene to afford the monoaluminum complexes [{(IDipp)PH}AlMe3 ] and [{(IMes)PH}AlMe3 ] (IDipp=1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene, IMes=1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene). In contrast, the dialuminum complex [{(Me IMes)PH}(AlMe3 )2 ] was obtained for Me IMes=1,3-bis(2,4,6-trimethylphenyl)-4,5-dimethylimidazolin-2-ylidene. These complexes served as initiators for the efficient ring-opening polymerization of rac-lactide in toluene at 60 °C. High degrees of isoselectivity were found for the poly(rac-lactide) obtained in the presence of the monoaluminum complexes (Pm up to 0.92, Tm up to 191 °C), whereas almost atactic polymers were produced by the dialuminum complex. Detailed mechanistic studies reveal that the polymerization proceeds via a coordination-insertion mechanism with the carbene-phosphinidene ligands acting as stereodirecting groups.

Metalloradical Cations and Dications Based on Divinyldiphosphene and Divinyldiarsene Ligands

Metalloradicals are key species in synthesis, catalysis, and bioinorganic chemistry. Herein, two iron radical cation complexes (3‐E)GaCl₄ [(3‐E)^.+ = [{(IPr)C(Ph)E}₂Fe(CO)₃]^.+, E = P or As; IPr = C{(NDipp)CH}₂, Dipp = 2,6‐iPr₂C₆H₃] are reported as crystalline solids. Treatment of the divinyldipnictenes {(IPr)C(Ph)E}₂ (1‐E) with Fe₂(CO)₉ affords [{(IPr)C(Ph)E}₂Fe(CO)₃] (2‐E), in which 1‐E binds to the Fe atom in an allylic (η³‐EEC_vinyl) fashion and functions as a 4e donor ligand. Complexes 2‐E undergo 1e oxidation with GaCl₃ to yield (3‐E)GaCl₄. Spin density analysis revealed that the unpaired electron in (3‐E)^.+ is mainly located on the Fe (52–64 %) and vinylic C (30–36 %) atoms. Further 1e oxidation of (3‐E)GaCl₄ leads to unprecedented η³‐EEC_vinyl to η³‐EC_vinylC_Ph coordination shuttling to form the dications (4‐E)(GaCl₄)₂.

Di-tert-butyldiphosphatetrahedrane as a building block for phosphaalkenes and phosphirenes

The remarkable 'mixed' diphosphatetrahedrane (tBuCP)2 (1) - which is both the elusive dimeric form of the phosphaalkyne tBuCP and an isolobal analogue of the important industrial feedstock P4 - was recently isolated for the first time; however, its chemistry remains unexplored. Herein we report that treatment of 1 with various N-heterocyclic carbenes readily yields unusual, unsaturated organophosphorus motifs. These results demonstrate the significant potential of 1 as a building block for the synthesis of previously unknown organophosphorus compounds.

2-(Dimethylamino)phosphinine: A Phosphorus-Containing Aniline Derivative

The yet unknown 2-amino-substituted λ³ ,σ² -phosphinines are phosphorus-containing aniline derivatives. Calculations show that the strong interaction of the π-donating NR₂  group with the aromatic system results in a high π-density at the phosphorus atom. We could now synthesize 2-N(CH₃ )₂ -functionalized phosphinines, starting from a 3-N(CH₃ )₂ -substituted 2-pyrone and (CH₃ )₃ Si-C≡P. Their reaction with CuBr⋅S(CH₃ )₂ affords Cu^I  complexes with the first example of a neutral phosphinine acting as a rare bridging μ₂ -P-4e donor-ligand between two Cu^I  centers. Our experimental and theoretical investigations show that 2-aminophosphinines are missing links in the series of known 2-donor-functionalized phosphinines.

Synthesis and Coordination Ability of a Donor-Stabilised Bis-Phosphinidene

Chelating phosphines have long been a mainstay as efficient directing ligands in transition-metal catalysis. Low-valent derivatives, namely chelating phosphinidenes, are to date unknown, and could lead to chelating complexes containing more than one metal centre due to the intrisic capacity of phosphinidenes to bind two metal fragments at one P-centre. Here we describe the synthesis of the first such chelating bis-phosphinidene ligand, XantP2 (2), generated by the reduction of a diphosphino xanthene derivative, Xant(PH2 )2 (1) with iPr NHC (iPr NHC=[:C{N(iPr)C(H)}2 ]). Initial studies have shown that this novel chelating ligand can act as a bidentate ligand towards element dihalides (i.e. FeCl2 , ZnI2 , GeCl2 , SnBr2 ), forming cationic complexes with the tetryl elements. In contrast, XantP2 demonstrates an ability to bind multiple metal centres in the reaction with CuCl, leading to a cationic Cu3 P3 ring complex, with Cu centres bridged by phosphinidene arms. Density Functional Theory calculations show that 2 indeed holds 4 lone pairs of electrons, shedding further light on the coordination capacity for this novel ligand class through observation of directionality and hybridisation of these electron pairs.

Two complementary approaches for the synthesis and isolation of stable phosphanylphosphaalkenes

A new series of compounds formed in a one-step  reaction (Phospha–Wittig  or phospha–Peterson), containing the CP–P moiety was synthesized and characterized. These species are stable under atmospheric conditions and are resistant to hydrolysis.

Isolation of Elusive Electrophilic Phosphinidene Complexes with π-Donor N-Heterocyclic Vinyl Substituents

Phosphinidene complexes of the general formula RPM(CO)_n (R = an alkyl or aryl group; M = a transition metal) are electrophilic and thermally unstable. Thus, the isolation of these elusive species for structural elucidations remains a challenge. Herein, we report the first terminal phosphinidene complexes [{(NHC)C(Ph)}P]Fe(CO)₄ [NHC = IPr = C{(NDipp)CH}₂ for 3; Me-IPr = C{(NDipp)CMe}₂ for 4; Dipp = 2,6-iPr₂C₆H₃; NHC = N-heterocyclic carbene] as red crystalline solids containing a π-donor N-heterocyclic vinyl (NHV) substituent at the phosphorus atom. Calculations reveal donor-acceptor type bonding between phosphorus and iron atoms in 3 and 4. The P → Fe donation represents ∼70% of the orbital interaction, whereas the Fe → P π-back-donation corresponds to ∼15% of the orbital interaction. The phosphorus atoms in 3 and 4 carry charges of +0.65e and +0.64e, respectively, indicating the electrophilic character of the phosphinidene {(NHC)C(Ph)}P moiety. Accordingly, 3 reacts with an NHC nucleophile (IMe₄) to yield the Lewis adduct [{(NHC)C(Ph)}P(IMe₄)]Fe(CO)₄ (5) [IMe₄ = C(NMeCMe)₂]. The coordination of an electron-rich NHC (IMe₄) to the phosphorus atom in 5 precludes the π-electron density transfer from the NHV to the phosphorus atom. Thus, the C_IPr-C_vinyl and C_vinyl-P bonds of 5 become shorter and longer, respectively, compared to those of 3.

Stretching the P-C Bond. Variations on Carbenes and Phosphanes

The stability and the structure of adducts formed between four substituted phosphanes (PX3, X:H, F, Cl, and NMe2) and 11 different carbenes have been investigated by DFT calculations. In most cases, the structure of the adducts depends strongly on the stability of the carbene itself, exhibiting a linear correlation with the increasing dissociation energy of the adduct. Carbenes of low stability form phosphorus ylides (F), which can be described as phosphane → carbene adducts supported with some back-bonding. The most stable carbenes, which have high energy lone pair, do not form stable F-type structures but carbene → phosphane adducts (E-type structure), utilizing the low-lying lowest unoccupied molecular orbital (LUMO) of the phosphane (with electronegative substituents), benefiting also from the carbene-pnictogen interaction. Especially noteworthy is the case of PCl3, which has an extremely low energy LUMO in its T-shaped form. Although this PCl3 structure is a transition state of rather high energy, the large stabilization energy of the complex makes this carbene-phosphane adduct stable. Most interestingly, in case of carbenes with medium stability both F- and E-type structures could be optimized, giving rise to bond-stretch isomerism. Likewise, for phosphorus ylides (F), the stability of the adducts G formed from carbenes with hypovalent phosphorus (PX-phosphinidene) is in a linear relationship with the stabilization of the carbene. Adducts of carbenes with hypervalent phosphorus (PX5) are the most stable when X is electronegative, and the carbene is highly nucleophilic.

Dinuclear gold(i) complexes: from bonding to applications

The last two decades have seen a veritable explosion in the use of gold(i) complexes bearing N-heterocyclic carbene (NHC) and phosphine (PR₃) ligands.

N-Heterocyclic Carbene Adducts of Main Group Elements and Their Use as Ligands in Transition Metal Chemistry

N-Heterocyclic carbenes (NHC) are nowadays ubiquitous and indispensable in many research fields, and it is not possible to imagine modern transition metal and main group element chemistry without the plethora of available NHCs with tailor-made electronic and steric properties. While their suitability to act as strong ligands toward transition metals has led to numerous applications of NHC complexes in homogeneous catalysis, their strong σ-donating and adaptable π-accepting abilities have also contributed to an impressive vitalization of main group chemistry with the isolation and characterization of NHC adducts of almost any element. Formally, NHC coordination to Lewis acids affords a transfer of nucleophilicity from the carbene carbon atom to the attached exocyclic moiety, and low-valent and low-coordinate adducts of the p-block elements with available lone pairs and/or polarized carbon-element π-bonds are able to act themselves as Lewis basic donor ligands toward transition metals. Accordingly, the availability of a large number of novel NHC adducts has not only produced new varieties of already existing ligand classes but has also allowed establishment of numerous complexes with unusual and often unprecedented element-metal bonds. This review aims at summarizing this development comprehensively and covers the usage of N-heterocyclic carbene adducts of the p-block elements as ligands in transition metal chemistry.

Isolation of Cyclic(Alkyl)(Amino) Carbene-Bismuthinidene Mediated by a Beryllium(0) Complex

The long-sought carbene-bismuthinidene, (CAAC)Bi(Ph), has been synthesized. Notably, this represents both the first example of a carbene-stabilized subvalent bismuth complex and the extension of the carbene-pnictinidene concept to a non-toxic metallic element (Bi). The bonding has been investigated by single-crystal X-ray diffraction studies and DFT calculations. This report also highlights the hitherto unknown reducing and ligand transfer capability of a beryllium(0) complex.

Synthesis of Heteroleptic Phosphorus(I) Cations by P+ Transfer

Reported are general synthetic approaches for the syntheses of asymmetrically substituted phosphorus(I) cations by P⁺ transfer from [dppeP]⁺ (dppe = 1,2-bis(diphenylphosphino)ethane). The first method grants access to acyclic derivatives and is accomplished by the sequential substitution of dppe using first a sterically encumbered ligand which cannot form a stable homoleptic complex, followed by a second equivalent of a less sterically demanding ligand. The second method grants access to cyclic derivatives and utilizes asymmetric hybrid phosphine/N-heterocyclic carbene ligands. Interplay between the different ligand types and their stoichiometries relative to those of [dppeP]⁺ also allows for the isolation of symmetrical derivatives with pendant phosphines.

NHCs in Main Group Chemistry

Since the discovery of the first stable N-heterocyclic carbene (NHC) in the beginning of the 1990s, these divalent carbon species have become a common and available class of compounds, which have found numerous applications in academic and industrial research. Their important role as two-electron donor ligands, especially in transition metal chemistry and catalysis, is difficult to overestimate. In the past decade, there has been tremendous research attention given to the chemistry of low-coordinate main group element compounds. Significant progress has been achieved in stabilization and isolation of such species as Lewis acid/base adducts with highly tunable NHC ligands. This has allowed investigation of numerous novel types of compounds with unique electronic structures and opened new opportunities in the rational design of novel organic catalysts and materials. This Review gives a general overview of this research, basic synthetic approaches, key features of NHC-main group element adducts, and might be useful for the broad research community.

Recent Advances in Imidazoliumyl-Substituted Phosphorus Compounds

This review aims to highlight the recent developments in the chemistry of selected imidazoliumyl-substituted phosphorus compounds. The synthetic approaches for their preparation with phosphorus in various oxidation states and coordination environments are discussed. Their intriguing properties and versatile chemistry strongly depends on the bonding motif at the P atoms, which is given special focus.

Versatile Coordination Modes of Triphospha-1,4-pentadiene-2,4-diamine

1,3,5-Triphospha-1,4-pentadiene-2,4-diamine reacts with [M(CO)₄L] (M = Mo, L = nbd (norbornadiene); M = W, L = 2 CH₃CN) to give the chelate complexes [M(CO)₄(PMes{C(NHCy)PMes}₂-κ P¹ ,P³)]. In contrast, an unusual intramolecular rearrangement occurred with [Cu(CH₃CN)₄]PF₆ leading to the dimeric copper(I) complex [Cu(CNCy){PHMesPMesC(NHCy)PMes-κ P¹ ,P³}]₂(PF₆)₂. The mechanism of the rearrangement was supported by quantum-mechanical calculations. The transition-metal complexes were characterized by multinuclear NMR spectroscopy, mass spectrometry, infrared spectroscopy, and X-ray crystallography.

Reactivity studies of an imine-functionalised phosphaalkene; unusual electrostatic and supramolecular stabilisation of a σ2λ3-phosphorus motif via hydrogen bonding

Protonation with strong acids at an imine over addition to a phosphaalkene; resulting adducts display hydrogen bonding.

Accessing multimetallic complexes with a phosphorus(i) zwitterion

We present the synthesis of a zwitterionic triphosphenium molecule, ^tBu(C₅H₂)(PPh₂)₂P^I (L), which can act as a single- or multidentate ligand with group 6, 7, 8 and 9 metal carbonyl complexes. Group 6, [M(CO)₅L] complexes are formed under photolytic conditions, where the metal is bound at the P(i) center. In the case of Mo(CO)₆, the bimetallic complex [M(CO)₅LMo(CO)₃] is generated, which features bonds to both the phosphorus(i) center and the cyclopentadienyl moiety of the molecule. Interestingly, group 7 and 9 metal carbonyl dimers generate bimetallic complexes in the form [M₂(CO)_nL], where both metal centers are bound at the phosphorus(i) center. A detailed analysis of these molecules is provided, including X-ray diffraction, multinuclear NMR, infrared spectroscopy and computational investigations.