Carbamoylphosphinidene: A Phosphorus Analogue of Carbonyl Nitrene.

Carbonyl nitrenes are versatile intermediates that have been extensively characterized; however, their phosphorus analogues remain largely unknown. Herein, we report the observation of a rare example of carbonyl phosphinidene NH2C(O)P, which was generated through the photolytic (193 nm) dehydrogenation of phosphinecarboxamide (NH2C(O)PH2) in a solid N2-matrix at 12 K. The characterization of NH2C(O)P in the triplet ground state with matrix-isolation IR and ultraviolet-visible (UV-vis) spectroscopy is supported by comprehensive isotope labeling experiments (D and 15N) and quantum chemical calculations. Upon visible-light irradiation at 680 nm, NH2C(O)P inserts into dihydrogen by the reformation of NH2C(O)PH2 with concomitant isomerization to the more stable aminophosphaketene (NH2PCO). Additionally, the photoisomerization of NH2C(O)PH2 to NH2C(OH) = PH along with decomposition by yielding hydrogen-bonded complexes HNCO···PH3 and HPCO···NH3 has been observed in the matrix.

Ligand Centered Reactivity of a Transition Metal Bound Geometrically Constrained Phosphine.

The electronic properties, coordination chemistry and reactivity of metal complexes of a planar (C2v symmetric) acridane-derived geometrically constrained phosphine, P(NNN), are described. On complexation to metal centers, the phosphine was found to adopt a distorted trigonal pyramidal structure with a high barrier to pyramidal inversion (22.3 kcal/mol at 298 K for Au[P(NNN)]Cl). Spectroscopic data and theoretical calculations carried out at the density functional level of theory indicate that P(NNN) is a moderate σ-donor, with significant π-acceptor properties. Despite the distortion undergone by the phosphorus atom on coordination to metal centers, the P(NNN) ligand retains its ability to react with small molecule substrates with polar E-H bonds (MeOH, NH2Ph, NH3). It does so in a concerted fashion across one of the P-N bonds, and reversibly in the case of amine substrates. This cooperative bond activation chemistry may ultimately prove beneficial in catalysis.

Metal Influence on Cyaphide-Azide 1,3-Dipolar Cycloaddition Reactions: Aromaticity and Activation Strain.

The factors governing 1,3-dipolar cycloaddition reactions involving C≡P-containing compounds are computationally explored in detail using quantum chemical tools. To this end, the parent process involving tBuN3 and tBuCP is analyzed and compared to the analogous reaction involving organometallic cyaphide complexes (metal=Au, Pt, Ge, Mg), in order to understand the role of the metal fragment in such transformations. It is found that while the metal fragment does not significantly influence the aromaticity of the corresponding concerted transition states or the regioselectivity of the transformation, it may modify the reactivity of the cyaphide complexes (i. e. Ge and Mg cyaphide complexes are comparatively more reactive). The computed reactivity trends and the factors behind the regioselectivity of the cycloaddition reaction are quantitatively analyzed with the help of the activation strain model in combination with the energy decomposition analysis method.

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 ß-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.

Publisher Correction: Synthesis, structure and reaction chemistry of a nucleophilic aluminyl anion.

In Fig. 1 of this Letter, the hydrogen (H) atoms attached to each of the two nitrogen (N) atoms in the chemical structure of (NON)H2 were inadvertently missing. The original figure has been corrected online.

Synthesis, structure and reaction chemistry of a nucleophilic aluminyl anion.

The reactivity of aluminium compounds is dominated by their electron deficiency and consequent electrophilicity; these compounds are archetypal Lewis acids (electron-pair acceptors). The main industrial roles of aluminium, and classical methods of synthesizing aluminium-element bonds (for example, hydroalumination and metathesis), draw on the electron deficiency of species of the type AlR3 and AlCl31,2. Whereas aluminates, [AlR4]-, are well known, the idea of reversing polarity and using an aluminium reagent as the nucleophilic partner in bond-forming substitution reactions is unprecedented, owing to the fact that low-valent aluminium anions analogous to nitrogen-, carbon- and boron-centred reagents of the types [NX2]-, [CX3]- and [BX2]- are unknown3-5. Aluminium compounds in the +1 oxidation state are known, but are thermodynamically unstable with respect to disproportionation. Compounds of this type are typically oligomeric6-8, although monomeric systems that possess a metal-centred lone pair, such as Al(Nacnac)Dipp (where (Nacnac)Dipp = (NDippCR)2CH and R = t Bu, Me; Dipp = 2,6- i Pr2C6H3), have also been reported9,10. Coordination of these species, and also of (η5-C5Me5)Al, to a range of Lewis acids has been observed11-13, but their primary mode of reactivity involves facile oxidative addition to generate Al(III) species6-8,14-16. Here we report the synthesis, structure and reaction chemistry of an anionic aluminium(I) nucleophile, the dimethylxanthene-stabilized potassium aluminyl [K{Al(NON)}]2 (NON = 4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene). This species displays unprecedented reactivity in the formation of aluminium-element covalent bonds and in the C-H oxidative addition of benzene, suggesting that it could find further use in both metal-carbon and metal-metal bond-forming reactions.

Controlled cluster expansion at a Zintl cluster surface.

Reaction of the tris-hypersilyl nonagermanide Zintl cluster salt, K[Ge9 (Hyp)3 ] (Hyp=Si(SiMe3 )3 ) with [Rh(η2 ,η2 -L)Cl]2 (L=1,5-cyclooctadiene, COD; norbornadiene, NBD) afforded eleven- and twelve-vertex homo-multimetallic clusters by cluster core expansion. Using a stepwise procedure, starting from the Zintl cluster [Rh(COD){Ge9 (Hyp)3 }] and [Ir(COD)Cl]2 , this methodology was expanded for the synthesis of eleven-vertex hetero-multimetallic clusters. A mechanism for the formation of these first examples of closo eleven-vertex Zintl clusters is proposed, informed by density functional theory calculations.

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 KOt 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.

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.

Aluminium and Gallium Silylimides as Nitride Sources.

Terminal aluminium and gallium imides of the type K[(NON)M(NR)], bearing heteroatom substituents at R, have been synthesised via reactions of anionic aluminium(I) and gallium(I) reagents with silyl and boryl azides (NON=4,5-bis(2,6-diisopropyl-anilido)-2,7-di-tert-butyl-9,9-dimethyl-xanthene). These systems vary significantly in their lability in solution: the N(Sii Pr3 ) and N(Boryl) complexes are very labile, on account of the high basicity at nitrogen. Phenylsilylimido derivatives provide greater stabilization through the π-acceptor capabilities of the SiR3 group. K[(NON)AlN(Sit BuPh2 )] offers a workable compromise between stability and solubility, and has been completely characterized by spectroscopic, analytical and crystallographic methods. The silylimide species examined feature minimal π-bonding between the imide ligand and aluminium/gallium, with the HOMO and HOMO-1 orbitals effectively comprising orthogonal lone pairs centred at N. Reactivity-wise, both aluminium and gallium silylimides can act as viable sources of nitride, [N]3- , with systems derived from either metal reacting with CO to afford cyanide complexes. By contrast, only the gallium system K[(NON)Ga{N(SiPh3 )}] is capable of effecting a similar transformation with N2 O to yield azide, N3 - , via formal oxide/nitride metathesis. The aluminium systems instead generate RN3 via transfer of the imide fragment [RN]2- .

An Aluminium Imide as a Transfer Agent for the [NR]2- Function via Metathesis Chemistry.

The reactions of a terminal aluminium imide with a range of oxygen-containing substrates have been probed with a view to developing its use as a novel main group transfer agent for the [NR]2- fragment. We demonstrate transfer of the imide moiety to [N2 ], [CO] and [Ph(H)C] units driven thermodynamically by Al-O bond formation. N2 O reacts rapidly to generate the organoazide DippN3 (Dipp=2,6-i Pr2 C6 H3 ), while CO2 (under dilute reaction conditions) yields the corresponding isocyanate, DippNCO. Mechanistic studies, using both experimental and quantum chemical techniques, identify a carbamate complex K2 [(NON)Al-{κ2 -(N,O)-N(Dipp)CO2 }]2 (formed via [2+2] cycloaddition) as an intermediate in the formation of DippNCO, and also in an alternative reaction leading to the generation of the amino-dicarboxylate complex K2 [(NON)Al{κ2 -(O,O')-(O2 C)2 N-(Dipp)}] (via the take-up of a second equivalent of CO2 ). In the case of benzaldehyde, a similar [2+2] cycloaddition process generates the metallacyclic hemi-aminal complex, Kn [(NON)Al{κ2 -(N,O)-(N(Dipp)C(Ph)(H)O}]n . Extrusion of the imine, PhC(H)NDipp, via cyclo-reversion is disfavoured thermally, due to the high energy of the putative aluminium oxide co-product, K2 [(NON)Al(O)]2 . However, addition of CO2 allows the imine to be released, driven by the formation of the thermodynamically more stable aluminium carbonate co-product, K2 [(NON)Al(κ2 -(O,O')-CO3 )]2 .

Cyaphide-Azide 1,3-Dipolar Cycloaddition Reactions: Scope and Applicability.

Several examples of the cyaphide-azide 1,3-dipolar cycloaddition reaction to afford metallo-triazaphospholes are reported. The gold(I) triazaphospholes Au(IDipp)(CPN3 R) (IDipp=1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene; R=t Bu, Ad, Dipp), magnesium(II) triazaphospholes, {Mg(Dipp NacNac)(CPN3 R)}2 (Dipp NacNac=CH{C(CH3 )N(Dipp)}2 , Dipp=2,6-diisopropylphenyl; R=t Bu, Bn), and germanium(II) triazaphosphole Ge(Dipp NacNac)-(CPN3 t Bu) can be prepared straightforwardly, under mild conditions and in good yields, in a manner reminiscent of the classic alkyne-azide click reaction (albeit without a catalyst). This reactivity can be extended to compounds with two azide functional groups such as 1,3-diazidobenzene. It is shown that the resulting metallo-triazaphospholes can be used as precursors to carbon-functionalized species, including protio- and iodo-triazaphospholes.

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.

Dioxygen Splitting by a Tantalum(V) Complex Ligated by a Rigid, Redox Non-Innocent Pincer Ligand.

The reaction of TaMe3 Cl2 with the rigid acridane-derived trisamine H3 NNN yields the tantalum(V) complex [TaCl2 (NNNcat )]. Subsequent reaction with dioxygen results in the full four-electron reduction of O2 yielding the oxido-bridged bimetallic complex [{TaCl2 (NNNsq )}2 O]. This dinuclear complex features an open-shell ground state due to partial ligand oxidation and was comprehensively characterized by single crystal X-ray diffraction, LIFDI mass spectrometry, NMR, EPR, IR and UV/VIS/NIR spectroscopy. The mechanism of O2 activation was investigated by DFT calculations revealing initial binding of O2 to the tantalum(V) center followed by complete O2 scission to produce a terminal oxido-complex.

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.

An Inorganic Click Reaction for the Synthesis of Interlocked Molecules.

We describe the use of the cyaphide-azide 1,3-dipolar cycloaddition reaction for the synthesis of a new class of inorganic rotaxanes containing gold(I) triazaphosphole stoppers. Electron-deficient bis-azides, which thread perethylated pillar[5]arene in aromatic solvents, readily react with two equivalents of Au(IDipp)(CP) (IDipp=1,3-bis-(2,6-diisopropylphenyl)-imidazol-2-ylidene) to afford interlocked molecules via an inorganic click reaction. These transformations proceed in good yields (ca. 65 %) and in the absence of a catalyst. The resulting organometallic rotaxanes are air- and moisture-stable and can be purified by column chromatography under aerobic conditions. The targeted rotaxanes were characterized by multi-element nuclear magnetic resonance (NMR) spectroscopy, mass-spectrometry, and single-crystal X-ray diffraction.

The Chemistry of the Cyaphide Ion.

We review the known chemistry of the cyaphide ion, (C≡P)- . This remarkable diatomic anion has been the subject of study since the late nineteenth century, however its isolation and characterization eluded chemists for almost a hundred years. In this mini-review, we explore the pioneering synthetic experiments that first allowed for its isolation, as well as more recent developments demonstrating that cyaphide transfer is viable in well-established salt-metathesis protocols. The physical properties of the cyaphide ion are also explored in depth, allowing us to compare and contrast the chemistry of this ion with that of its lighter congener cyanide (an archetypal strong field ligand and important organic functional group). Recent studies show that the cyaphide ion has the potential to be used as a versatile chemical regent for the synthesis of novel molecules and materials, hinting at many interesting future avenues of investigation.

Coordination and Homologation of CO at Al(I): Mechanism and Chain Growth, Branching, Isomerization, and Reduction.

Homologation of carbon monoxide is central to the heterogeneous Fischer-Tropsch process for the production of hydrocarbon fuels. C-C bond formation has been modeled by homogeneous systems, with [CnOn]2- fragments (n = 2-6) formed by two-electron reduction being commonly encountered. Here, we show that four- or six-electron reduction of CO can be accomplished by the use of anionic aluminum(I) ("aluminyl") compounds to give both topologically linear and branched C4/C6 chains. We show that the mechanism for homologation relies on the highly electron-rich nature of the aluminyl reagent and on an unusual mode of interaction of the CO molecule, which behaves primarily as a Z-type ligand in initial adduct formation. The formation of [C6O6]4- from [C4O4]4- shows for the first time a solution-phase CO homologation process that brings about chain branching via complete C-O bond cleavage, while a comparison of the linear [C4O4]4- system with the [C4O4]6- congener formed under more reducing conditions models the net conversion of C-O bonds to C-C bonds in the presence of additional reductants.

Synthesis, Structure and Reactivity of a Cyapho(dicyano)methanide Salt.

We describe the synthesis of a cyapho(dicyano)methanide salt, [K(18-crown-6)][C(CN)2 (CP)], from reaction of [Na(18-crown-6)][PH2 ] (18-crown-6=1,4,7,10,13,16-hexaoxacyclooctadecane) with 1,1-diethoxy-2,2-dicyanoethylene (EtO)2 C=C(CN)2 . The reaction proceeds through a Michael addition-elimination pathway to afford [Na(18-crown-6)][HP{C(OEt)=C(CN)2 }]. Addition of a strong, non-nucleophilic base (KHMDS) to this intermediate results in the formation of [K(18-crown-6)][C(CN)2 (CP)]. Subsequent reactivity studies reveal that the cyapho(dicyano)methanide ion is susceptible to protonation with strong acids to afford the parent acid HC(CN)2 (CP). The reactivity of the cyaphide moiety in [C(CN)2 (CP)]- was explored through coordination to metal centers and in cycloaddition reactions with azides.

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(Me Ii Pr)2 (Me Ii Pr=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 ].