Putting cyaphide in its place: determining the donor/acceptor properties of the κC-cyaphido ligand.

The synthesis of group 9 pyridine-diimine complexes M(DippPDI)X and [M(DippPDI)L]+ (M = Co, Rh; DippPDI = 1,1'-(pyridine-2,6-diyl)bis(N-(2,6-diisopropylphenyl)ethan-1-imine); X = CP-, CCH-; L = CO, t BuNC) bearing a series of strong-field ligands, including the cyaphide ion (C[triple bond, length as m-dash]P-), is reported. A combined experimental and computational comparative study of the group 9 PDI cyaphide complexes Co(DippPDI)(CP) and Rh(DippPDI)(CP), as well as the N-heterocyclic carbene (NHC) gold(i) cyaphide complex Au(IDipp)(CP) (IDipp = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene), reveals the σ donor and π acceptor properties of the κC-cyaphido ligand, and allow us to suggest a position for this ion in the spectrochemical series.

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.

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.

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 .

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.

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.

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.

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.

Zintl cluster supported low coordinate Rh(i) centers for catalytic H/D exchange between H2 and D2.

Ligand exchange reactions of [Rh(COD){η4-Ge9(Hyp)3}] with L-type nucleophiles such as PMe3, PPh3, IMe4 (IMe4 = 1,3,4,5-tetramethylimidazol-2-ylidene) or [W(Cp)2H2] result in the displacement of the COD ligand to afford clusters with coordinatively unsaturated trigonal pyramidal rhodium(i) centers [Rh(L){η3-Ge9(Hyp)3}]. These species can be readily protonated allowing access to cationic rhodium-hydride complexes, e.g. [RhH(PPh3){η3-Ge9(Hyp)3}]+. These clusters act as catalysts in H/D exchange between H2 and D2 and alkene isomerisation, thereby illustrating that metal-functionalized Zintl clusters are active in both H-H and C-H bond activation processes. The mechanism of H/D exchange was probed using parahydrogen induced polarization experiments.

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

The synthesis of heterometallic transition metal complexes featuring bridging cyaphide ions (C≡P- ) is reported. These are synthesized from reactions of Au(IDipp)(CP) (IDipp=1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) with electron-rich, nucleophilic transition metal reagents, affording Au(IDipp)(µ2 -C≡P)Ni(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 ].

Reduction of tert-butylphosphaalkyne and trimethylsilylnitrile with magnesium(I) dimers.

We report on the reactivity of magnesium(I) dimers, [Mg(nacnac)]2 (nacnac = HC[C(Me)N(2,6-iPr2C6H3)]2 ([DippLMg]2) and HC[C(Me)N(2,4,6-Me3C6H2)]2 ([MesLMg]2)), towards the phosphaalkyne tBuCP. The steric profile of the magnesium(I) dimer results in selectivity for different products. The larger diisopropylphenyl derivative yields exclusively the monomeric dimagnesiated phosphaalkene [DippLMg]PC(tBu)([DippLMg]) (1), while the mesityl derivative facilitates reductive coupling of two phosphaalkyne equivalents to give access to the 1,3-diphosphacyclobutadienediide [MesLMg]2[(tBu)2C2P2](2). The reactivity differs in coordinating solvents such as THF, which allowed for the observation of C-P coupled products. For sake of comparison, reactions of magnesium(I) compounds with Me3SiCN were carried out. In contrast to the reactions involving tBuCP, these afforded 1,3-diazabutadienediyl complexes via reductive coupling and silyl migration processes.

Coinage metal aluminyl complexes: probing regiochemistry and mechanism in the insertion and reduction of carbon dioxide.

The synthesis of coinage metal aluminyl complexes, featuring M-Al covalent bonds, is reported via a salt metathesis approach employing an anionic Al(i) ('aluminyl') nucleophile and group 11 electrophiles. This approach allows access to both bimetallic (1 : 1) systems of the type ( t Bu3P)MAl(NON) (M = Cu, Ag, Au; NON = 4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene) and a 2 : 1 di(aluminyl)cuprate system, K[Cu{Al(NON)}2]. The bimetallic complexes readily insert heteroallenes (CO2, carbodiimides) into the unsupported M-Al bonds to give systems containing a M(CE2)Al bridging unit (E = O, NR), with the µ-κ1(C):κ2(E,E') mode of heteroallene binding being demonstrated crystallographically for carbodiimide insertion in the cases of all three metals, Cu, Ag and Au. The regiochemistry of these processes, leading to the formation of M-C bonds, is rationalized computationally, and is consistent with addition of CO2 across the M-Al covalent bond with the group 11 metal acting as the nucleophilic partner and Al as the electrophile. While the products of carbodiimide insertion are stable to further reaction, their CO2 analogues have the potential to react further, depending on the identity of the group 11 metal. ( t Bu3P)Au(CO)2Al(NON) is inert to further reaction, but its silver counterpart reacts slowly with CO2 to give the corresponding carbonate complex (and CO), and the copper system proceeds rapidly to the carbonate even at low temperatures. Experimental and quantum chemical investigations of the mechanism of the CO2 to CO/carbonate transformation are consistent with rate-determining extrusion of CO from the initially-formed M(CO)2Al fragment to give a bimetallic oxide that rapidly assimilates a second molecule of CO2. The calculated energetic barriers for the most feasible CO extrusion step (ΔG ‡ = 26.6, 33.1, 44.5 kcal mol-1 for M = Cu, Ag and Au, respectively) are consistent not only with the observed experimental labilities of the respective M(CO)2Al motifs, but also with the opposing trends in M-C (increasing) and M-O bond strengths (decreasing) on transitioning from Cu to Au.

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.

Probing the Extremes of Covalency in M-Al bonds: Lithium and Zinc Aluminyl Compounds.

Synthetic routes to lithium, magnesium, and zinc aluminyl complexes are reported, allowing for the first structural characterization of an unsupported lithium-aluminium bond. Crystallographic and quantum-chemical studies are consistent with the presence of a highly polar Li-Al interaction, characterized by a low bond order and relatively little charge transfer from Al to Li. Comparison with magnesium and zinc aluminyl systems reveals changes to both the M-Al bond and the (NON)Al fragment (where NON=4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene), consistent with a more covalent character, with the latter complex being shown to react with CO2 via a pathway that implies that the zinc centre acts as the nucleophilic partner.

A Cyaphide Transfer Reagent.

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

Cluster expansion and vertex substitution pathways in nickel germanide Zintl clusters.

We describe the reactivity of the hypersilyl-functionalized Zintl cluster salt K[Ge9(Hyp)3] towards the nickel reagents Ni(COD)2 and Ni(Cp)2, which gives rise to markedly different complexes. In the case of Ni(COD)2 (COD = 1,5-cyclooctadiene), a dianionic sandwich-like cluster [Ni{Ge9(Hyp)3}2]2- (1) was obtained, in line with a simple ligand substitution reaction of COD by [Ge9(Hyp)3]-. By contrast, when an analogous reaction with Ni(Cp)2 (Cp = cyclopentadienyl) was performed, vertex substitution of the [Ge9(Hyp)3]- precursor was observed, giving rise to the nine-vertex nido-cluster (Cp)Ni[Ge8(Hyp)3] (2). This is the first instance of vertex substitution at a hypersilyl-functionalized Zintl cluster cage. The electrochemical behavior of these compounds was explored and showed reversible redox behaviour for both clusters.

Novel primary phosphinecarboxamides derived from diamines.

We describe the synthesis of N-functionalised phosphinecarboxamides obtained by reaction of the 2-phosphaethynolate anion (PCO-) with diamines, specifically hydrazine, methylenediamine and ethylenediamine, in the presence of acid. The resulting neutral compounds can be deprotonated to generate phosphide anions that, when further reacted with electrophiles, form secondary phosphines.

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.

Thermoneutral N-H Bond Activation of Ammonia by a Geometrically Constrained Phosphine.

A geometrically constrained phosphine bearing a tridentate NNS pincer ligand is reported. The effect of the geometric constraint on the electronic structure was probed by theoretical calculations and derivatization reactions. Reactions with N-H bonds result in formation of cooperative addition products. The thermochemistry of these transformations is strongly dependent on the substrate, with ammonia activation being thermoneutral. This represents the first example of a molecular compound that reversibly activates ammonia via N-H bond scission in solution upon mild heating.

Pincer-Supported Gallium Complexes for the Catalytic Hydroboration of Aldehydes, Ketones and Carbon Dioxide.

Gallium hydrides stabilised by primary and secondary amines are scarce due to their propensity to eliminate dihydrogen. Consequently, their reactivity has received limited attention. The synthesis of two novel gallium hydride complexes HGa(THF)[ON(H)O] and H2 Ga[µ2 -ON(H)O]Ga[ON(H)O] ([ON(H)O]2- =N,N-bis(3,5-di-tert-butyl-2-phenoxy)amine) is described and their reactivity towards aldehydes and ketones is explored. These reactions afford alkoxide-bridged dimers through 1,2-hydrogallation reactions. The gallium hydrides can be regenerated through Ga-O/B-H metathesis from the reaction of such dimers with pinacol borane (HBpin) or 9-borabicyclo[3.3.1]nonane (9-BBN). These observations allowed us to target the catalytic reduction of carbonyl substrates (aldehydes, ketones and carbon dioxide) with low catalyst loadings at room temperature.