Mixed-Valence Iron Complexes Containing End-On Bridging Cyaphide Ions.

Iron cyanide compounds are among the oldest known synthetic coordination compounds, dating back to the early 18th century. By contrast, iron complexes of the cyaphide ion (C≡P-)─a heavy valence isoelectronic analog of the cyanide ion─are unknown. Herein we report the synthesis of highly stable mono- and bis-cyaphide complexes of iron(II), namely Fe(depe)2(Cl)(CP) and Fe(depe)2(CP)2 (depe = 1,2-bis(diethylphosphino)ethane). These iron(II) cyaphide complexes are capable of further coordination to iron(I) in a hitherto unknown linear coordination geometry, affording the conjugated multimetallic mixed-valence complexes [{Fe(depe)2}2(µ-CP)(N2)][BArF4]2 and [{Fe(depe)2}3(µ-CP)2][OTf]2.

A Transient Iron Carbide Generated by Cyaphide Cleavage.

Despite their potential relevance as molecular models for industrial and biological catalysis, well-defined mononuclear iron carbide complexes are unknown, in part due to the limited number of appropriate C1 synthons. Here, we show the ability of the cyaphide anion (C≡P-) to serve as a C1 source. The high spin (S = 2) cyaphide complex PhB(tBuIm)3Fe-C≡P (PhB(tBuIm)3- = phenyl(tris(3-tert-butylimidazol-2-ylidene)borate) is readily accessed using the new cyaphide transfer reagent [Mg(DippNacNac)(CP)]2 (DippNacNac = CH{C(CH3)N(Dipp)}2 and Dipp = 2,6-di(iso-propyl)phenyl). Phosphorus atom abstraction is effected by the three-coordinate Mo(III) complex Mo(NtBuAr)3 (Ar = 3,5-Me2C6H3), which produces the known phosphide (tBuArN)3Mo≡P along with a transient iron carbide complex PhB(tBuIm)3Fe≡C. Electronic structure calculations reveal that PhB(tBuIm)3Fe≡C adopts a doublet ground state with nonzero spin density on the carbide ligand. While isolation of this complex is thwarted by rapid dimerization to afford the corresponding diiron ethynediyl complex, the carbide can be intercepted by styrene to provide an iron alkylidene.

Softer Is Better for Titanium: Molecular Titanium Arsenido Anions Featuring Ti≡As Bonding and a Terminal Parent Arsinidene.

We introduce the arsenido ligand onto the TiIV ion, yielding a remarkably covalent Ti≡As bond and the parent arsinidene Ti═AsH moiety. An anionic arsenido ligand is assembled via reductive decarbonylation involving the discrete TiII salt [K(cryptand)][(PN)2TiCl] (1) (cryptand = 222-Kryptofix) and Na(OCAs)(dioxane)1.5 in thf/toluene to produce the mixed alkali ate-complex [(PN)2Ti(As)]2(µ2-KNa(thf)2) (2) and the discrete salt [K(cryptand)][(PN)2Ti≡As] (3) featuring a terminal Ti≡As ligand. Protonation of 2 or 3 with various weak acids cleanly forms the parent arsinidene [(PN)2Ti═AsH] (4), which upon deprotonation with KCH2Ph in thf generates the more symmetric anionic arsenido [(PN)2Ti(As){µ2-K(thf)2}]2 (5). Experimental and computational studies suggest the pKa of 4 to be ∼23, and the bond orders in 2, 3, and 5 are all in the range of a Ti≡As triple bond, with decreasing bond order in 4.

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.

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.

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.

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.

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.

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 ].

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.

A Tris(3-pyridyl)stannane as a Building Block for Heterobimetallic Coordination Polymers and Supramolecular Cages.

The systematic assembly of supramolecular arrangements is a persistent challenge in modern coordination chemistry, especially where further aspects of complexity are concerned, as in the case of large molecular mixed-metal arrangements. One targeted approach to such heterometallic complexes is to engineer metal-based donor ligands of the correct geometry to build 3D arrangements upon coordination to other metals. This simple idea has, however, only rarely been applied to main group metal-based ligand systems. Here, we show that the new, bench-stable tris(3-pyridyl)stannane ligand PhSn(3-Py)3 (3-Py=3-pyridyl) provides simple access to a range of heterometallic SnIV /transition metal complexes, and that the presence of weakly coordinating counter anions can be used to build discrete molecular arrangements involving anion encapsulation. This work therefore provides a building strategy in this area, which parallels that of supramolecular transition metal chemistry.

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.

Isolation of the elusive [Ru(bipy)3]+: a key intermediate in photoredox catalysis.

Photoredox catalysis has flourished in recent years, but due to its widespread utility applications have grown faster than mechanistic understanding. In this report we help to address this deficit by isolating and characterising one of the intermediates of the iconic photocatalyst [Ru(bipy)3]2+, and testing its initial photoreactivity towards common substrates.