Cyclophanes as Platforms for Reactive Multimetallic Complexes

Alkyl backbone variations in common β-diketiminate ligands and applications to N-heterocyclic silylene chemistry

We report the extension of the common β-diketimine proligand class, RArnacnacH (HC(RCNAr)2H), where R is an alkyl group such as Et or iPr, plus Ph, and Ar is a sterically demanding aryl substituent such as Dip = 2,6-diispropylphenyl, Dep = 2,6-diethylphenyl, Mes = 2,4,6-trimethylphenyl or mesityl, Xyl = 2,6-dimethylphenyl, via one-pot condensation procedures. When a condensation reaction is carried out using the chemical dehydrating agent PPSE (polyphosphoric acid trimethylsilylester), β-diketiminate phosphorus(V) products such as (iPrMesnacnac)PO2 can also be obtained, which can be converted to the respective proligand iPrMesnacnacH via alkaline hydrolysis. The RArnacnacH proligands can be converted to their alkali metal complexes with common methods and we have found that deprotonation of iPrDipnacnacH is significantly more sluggish than that of related β-diketimines with smaller backbone alkyl groups. The basicity of the RArnacnac- anions can play a role in the success of their salt metathesis chemistry and we have prepared and structurally characterised the EtDipnacnac-derived silicon(II) compounds (EtDipnacnac)SiBr and (EtDipnacnac')Si, where EtDipnacnac' is the deprotonated variant MeCHC(NDip)CHC(NDip)Et.

Cluster dynamics of heterometallic trinuclear clusters during ligand substitution, redox chemistry, and group transfer processes

Stepwise metalation of the hexadentate ligand tbsLH6 (tbsLH6 = 1,3,5-C6H9(NHC6H4-o-NHSiMe2tBu)3) affords bimetallic trinuclear clusters (tbsL)Fe2Zn(thf) and (tbsL)Fe2Zn(py). Reactivity studies were pursued to understand metal atom lability as the clusters undergo ligand substitution, redox chemistry, and group transfer processes. Chloride addition to (tbsL)Fe2Zn(thf) resulted in a mixture of species including both all-zinc and all-iron products. Addition of ArN3 (Ar = Ph, 3,5-(CF3)2C6H3) to (tbsL)Fe2Zn(py) yielded a mixture of two trinuclear products: (tbsL)Fe3(μ3-NAr) and (tbsL)Fe2Zn(μ3-NAr)(py). The two imido species were separated via crystallization, and outer sphere reduction of (tbsL)Fe2Zn(μ3-NAr)(py) resulted in the formation of a single product, [2,2,2-crypt(K)][(tbsL)Fe2Zn(μ3-NAr)]. These results provide insight into the relationship between heterometallic cluster structure and substitutional lability and could help inform both future catalyst design and our understanding of metal atom lability in bioinorganic systems.

Mixed valence triiron complexes from the conjugation of [FeIFeI] and [FeII] complexes via intermolecular carbyne/alkyne coupling

We present a new synthetic strategy for obtaining mixed-valence triiron complexes where the metal centers are bridged by a novel, highly functionalized hydrocarbyl ligand. The alkynyl-vinyliminium complexes [Fe2Cp2(CO)(μ-CO){μ-η1:η3-C(X-CCH)CHCNMe2}]CF3SO3 (X = 4-C6H4, [2a1]CF3SO3; X = (CH2)3, [2a2]CF3SO3) were synthesized in almost quantitative yields from the aminocarbyne precursor [Fe2Cp2(CO)2(μ-CO){μ-CNMe2}]CF3SO3, [1a]CF3SO3, and the di-alkynes HCC-X-CCH. Then, the ferracycle [Fe(Cp)(CO){C(NMe2)CHC(4-C6H4CCH)C(O)}], 4a1, was produced in 47% yield from the cleavage of [2a1]CF3SO3 promoted by pyrrolidine. Subsequent reactions of the acetonitrile adducts [Fe2Cp2(CO)(μ-CO)(NCMe){μ-CNMe(R)}]CF3SO3 (R = Me, [1aACN]CF3SO3; R = Xyl, [1bACN]CF3SO3) ([FeIFeI]) with 4a1 ([FeII]) at room temperature resulted in the formation of [FeIFeIFeII] complexes [Fe2Cp2(CO)(μ-CO){μ-η1:η3-C(X-CCHC(NMe2)FeCp(CO)CO)CHCNMe(R)}]CF3SO3 (R = Me, [5a1]CF3SO3; R = Xyl, [5b1]CF3SO3) in yields ranging from 56% to 64%. The new products were characterized by IR and multinuclear NMR spectroscopy, and the structures of [2a2]CF3SO3 and 4a1 were confirmed by single crystal X-ray diffraction. Cyclic voltammetry and spectroelectrochemical studies on [5a1]+ have revealed that reduction and oxidation events occur almost independently at the [FeIFeI] and [FeII] units, respectively. This observation underscores a minimal electronic interaction between the two fragments within the triiron complex. Accordingly, DFT studies pointed out that the HOMO and LUMO orbitals are predominantly localized in the two distinct compartments of [5a1]+.

Metal-Tuned Ligand Reactivity Enables CX2 (X = O, S) Homocoupling with Spectator Cu Centers

Ligand non-innocence is ubiquitous in catalysis with ligands in synthetic complexes contributing as electron reservoirs or co-sites for substrate activation. The latter chemical non-innocence is manifested in H+ storage or relay at sites beyond the metal primary coordination sphere. Reaction of a competent CO2-to-oxalate reduction catalyst, namely, [K(THF)3](Cu3SL), where L3- is a tris(β-diketiminate) cyclophane, with CS2 affords tetrathiooxalate at long reaction times or at high CS2 concentrations, where otherwise an equilibrium is established between the starting species and a complex-CS2 adduct in which the CS2 is bound to the C atom on the ligand backbone. X-ray diffraction analysis of this adduct reveals no apparent metal participation, suggesting an entirely ligand-based reaction controlled by the charge state of the cluster. Thermodynamic parameters for the formation of the aforementioned Cligand-CS2 bond were experimentally determined, and trends with cation Lewis acidity were studied, where more acidic cations shift the equilibrium toward the adduct. Relevance of such an adduct in the reduction of CO2 to oxalate by this complex is supported by DFT studies, similar effects of countercation Lewis acidity on product formation, and the homocoupled heterocumulene product speciation as determined by isotopic labeling studies. Taken together, this system extends chemical non-innocence beyond H+ to effect catalytic transformations involving C-C bond formation and represents the rarest example of metal-ligand cooperativity, that is, spectator metal ion(s) and the ligand as the reaction center.

Mössbauer and Nuclear Resonance Vibrational Spectroscopy Studies of Iron Species Involved in N-N Bond Cleavage

Diketiminate-supported iron complexes are capable of cleaving the strong triple bond of N2 to give a tetra-iron complex with two nitrides (Rodriguez et al., Science, 2011, 334, 780-783). The mechanism of this reaction has been difficult to determine, but a transient green species was observed during the reaction that corresponds to a potential intermediate. Here, we describe studies aiming to identify the characteristics of this intermediate, using a range of spectroscopic techniques, including Mössbauer spectroscopy, electronic absorption spectroscopy, Raman spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, and nuclear resonance vibrational spectroscopy (NRVS) complemented by density functional theory (DFT) calculations. We successfully elucidated the nature of the starting iron(II) species and the bis(nitride) species in THF solution, and in each case, THF breaks up the multiiron species. Various observations on the green intermediate species indicate that it has one N2 per two Fe atoms, has THF associated with it, and has NRVS features indicative of bridging N2. Computational models with a formally diiron(0)-N2 core are most consistent with the accumulated data, and on this basis, a mechanism for N2 splitting is suggested. This work shows the power of combining NRVS, Mössbauer, NMR, and vibrational spectroscopies with computations for revealing the nature of transient iron species during N2 cleavage.

A Review of Crystalline Multibridged Cyclophane Cages: Synthesis, Their Conformational Behavior, and Properties

This paper reviews the most stable conformation of crystalline three-dimensional cyclophane (CP) achieved by self-assembling based on changing the type of aromatic compound or regulating the type and number of bridging groups. [3_n]cyclophanes (CPs) were reported to form supramolecular compounds with bind organic, inorganic anions, or neutral molecules selectively. [3_n]cyclophanes ([3_n]CPs) have stronger donor capability relative to compound [2_n]cyclophanes ([2_n]CPs), and it is expected to be a new type of electron donor for the progress of fresh electron conductive materials. The synthesis, conformational behavior, and properties of crystalline multi-bridge rings are summarized and discussed.

Molecular Capacitors: Accessible 6- and 8-Electron Redox Chemistry from Dimeric "Ti(I)" and "Ti(0)" Synthons Supported by Imidazolin-2-Iminato Ligands

Reduction of the diamagnetic Ti(III)/Ti(III) dimer [Cl₂Ti(μ-NIm^Dipp)]₂ (1) (NIm^Dipp = [1,3-bis(Dipp)imidazolin-2-iminato]^-, Dipp = C₆H₃-2,6-ⁱPr₂) with 4 and 6 equiv of KC₈ generates the intramolecularly arene-masked, dinuclear titanium compounds [(μ-N-η⁶-Im^Dipp)Ti]₂ (2) and {[(Et₂O)₂K](μ-N-μ-η⁶:η⁶-Im^Dipp)Ti}₂ (3), respectively, in modest yields. The compounds have been structurally characterized by X-ray crystallographic analysis, and inspection of the bond metrics within the η⁶-coordinated aryl substituent of the bridging imidazolin-2-iminato ligand shows perturbation of the aromatic system most consistent with two-electron reduction of the ring. As such, 2 and 3 can be assigned respectively as possessing metal centers in formal Ti(III)/Ti(III) and Ti(II)/Ti(II) oxidation states. Exploration of their redox chemistry reveal the ability to reduce several substrate equivalents. For instance, treatment of 2 with excess C₈H₈ (COT) forms the novel COT-bridged complex [(Im^DippN)(η⁸-COT)Ti](μ-η²:η³-COT)[Ti(η⁴-COT)(NIm^Dipp)] (4) that dissociates in THF solutions to give mononuclear (Im^DippN)Ti(η⁸-COT)(THF) (5). Addition of COT to 3 yields heterometallic [(Im^DippN)(η⁴-COT)Ti(μ-η⁴:η⁵-COT)K(THF)(μ-η⁶:η⁴-COT)Ti(NIm^Dipp)(μ-η⁴:η⁴-COT)K(THF)₂]_n (6). Compounds 4 and 5 are the products of the 4-electron oxidation of 2, while 6 stands as the 8-electron oxidation product of 3. Reduction of organozides was also explored. Low temperature reaction of 2 with 4 equiv of AdN₃ gives the terminal and bridged imido complex [(Im^DippN)Ti(═NAd)](μ-NAd)₂[Ti(NIm^Dipp)(N₃Ad)] (7) that undergoes intermolecular C-H activation of toluene at room temperature to afford the amido compound [(Im^DippN)Ti(NHAd)](μ-NAd)₂[Ti(C₆H₄Me)(NIm^Dipp)] (8-tol). These complexes are the 6-electron oxidation products of the reaction of 2 with AdN₃. Furthermore, treatment of 3 with 4 equiv of AdN₃ produces the thermally stable Ti(III)/Ti(III) terminal and bridged imido [K(18-crown-6)(THF)₂]{[(Im^DippN)Ti(NAd)](μ-NAd)₂K[Ti(NIm^Dipp)]} (10). Altogether, these reactions firmly establish 2 and 3 as unprecedented Ti(I)/Ti(I) and Ti(0)/Ti(0) synthons with the clear capacity to effect multielectron reductions ranging from 4 to 8 electrons.

Sidechain Metallopolymers with Precisely Controlled Structures: Synthesis and Application in Catalysis

Inspired by the cooperative multi-metallic activation in metalloenzyme catalysis, artificial enzymes as multi-metallic catalysts have been developed for improved kinetics and higher selectivity. Previous models about multi-metallic catalysts, such as cross-linked polymer-supported catalysts, failed to precisely control the number and location of their active sites, leading to low activity and selectivity. In recent years, metallopolymers with metals in the sidechain, also named as sidechain metallopolymers (SMPs), have attracted much attention because of their combination of the catalytic, magnetic, and electronic properties of metals with desirable mechanical and processing properties of polymeric backbones. Living and controlled polymerization techniques provide access to SMPs with precisely controlled structures, for example, controlled degree of polymerization (DP) and molecular weight dispersity (Đ), which may have excellent performance as multi-metallic catalysts in a variety of catalytic reactions. This review will cover the recent advances about SMPs, especially on their synthesis and application in catalysis. These tailor-made SMPs with metallic catalytic centers can precisely control the number and location of their active sites, exhibiting high catalytic efficiency.

Mapping the Reactivity of Dicobalt Bridging Nitrides in Constrained Geometries

Low-nuclearity nitrides of the late transition metals are rare and reactive molecular species, with little experimental precedent. The first putative examples of dicobalt bridging nitrides, [(nPDI2)Co2(μ-N)(PMe3)2][OTf]3 (n[Co2N]3+; PDI = pyridyldiimine; n = 2 or 3, representing the length of the aliphatic chain linking PDI imino groups), were reported recently and shown to undergo a range of intramolecular reaction pathways, including N-H bond formation, C-H bond insertion, and P═N bond formation at the bridging nitride. The specific mode of reactivity changed with the phase of the reaction and the size of the macrocycle used to support the transient species. The present contribution offers a computational investigation into both the geometric and electronic structures of these nitrides as well as the factors governing their reaction selectivity. The compounds n[Co2N]3+ exhibit μ-N-based lowest unoccupied molecular orbitals (LUMOs) that are consistent with subvalent, electrophilic nitrides. The specific orientations of the LUMOs induce ring-size-dependent stereoelectronic effects, thereby causing the product selectivity observed experimentally. Notably, the nitrides also exhibit a degree of nucleophilicity at μ-N by way of a high-energy, μ-N-based lone pair. This ambiphilic character appears to be a direct result of the constrained environment imposed by the folded-ligand geometries of n[Co2N]3+. When combined with the experimental findings, these data led to the conclusion that the folded-ligand isomers are the reactive species and that the constrained geometry imposed by the macrocyclic ligand plays an important role in controlling the reaction outcome.

Tuning metal-metal interactions for cooperative small molecule activation

Cluster complexes have attracted interest for decades due to their promise of drawing analogies to metallic surfaces and metalloenzyme active sites, but only recently have chemists started to develop ligand scaffolds that are specifically designed to support multinuclear transition metal cores. Such ligands not only hold multiple metal centers in close proximity but also allow for fine-tuning of their electronic structures and surrounding steric environments. This Feature Article highlights ligand designs that allow for cooperative small molecule activation at cluster complexes, with a particular focus on complexes that contain metal-metal bonds. Two useful ligand-design elements have emerged from this work: a degree of geometric flexibility, which allows for novel small molecule activation modes, and the use of redox-active ligands to provide electronic flexibility to the cluster core. The authors have incorporated these factors into a unique class of dinucleating macrocycles (nPDI2). Redox-active fragments in nPDI2 mimic the weak-overlap covalent bonding that is characteristic of M-M interactions, and aliphatic linkers in the ligand backbone provide geometric flexibility, allowing for interconversion between a range of geometries as the dinuclear core responds to the requirements of various small molecule substrates. The union of these design elements appears to be a powerful combination for analogizing critical aspects of heterogeneous and metalloenzyme catalysts.

Boron: the first p-block element to fix inert N2 all the way to NH3

Boron, the fifth lightest element, in its sub-valent state in the form of borylene is able to activate inert dinitrogen all the way to the ammonium ion. The entire conversion has been established through a successive reduction-cum-protonation sequence, through the isolation of all intermediate species involving addition of two electrons and two protons. The activation of dinitrogen by the ambiphilic borylene is a parallel tactic to that of the well-known Haber-Bosch process. This chemistry can be potentially extrapolated to the activation of similar small molecules by low valent compounds of boron and other p-block elements.

Synthetic Factors Governing Access to Tris(β-diketimine) Cyclophanes versus Tripodal Tri-β-aminoenones

Tris(β-diketimine) cyclophanes are an important ligand class for investigating cooperative multimetallic interactions of bioinorganic clusters. Discussed herein are the synthetic factors governing access to tris(β-diketimine) cyclophanes versus tripodal tri-β-aminoenones. Cyclophanes bearing Me, Et, and MeO cap substituents and β-Me, Et, or Ph arm substituents are obtained, and a modified condensation method produced α-Me β-Me cyclophane. These operationally simple procedures produce the ligands in gram quantities and in 22-94% yields.

Pentanuclear Scaffold: A Molecular Platform for Small-Molecule Conversions

Small-molecule conversions involving multielectron transfer processes enable the conversion of earth-abundant materials into valuable chemicals and are regarded as a solution for environmental and energy shortage problems. In this context, the development of artificial catalysts that promote these reactions is an important research target. In nature, metalloenzymes that contain multinuclear metal complexes as active sites are known to efficiently catalyze reactions under mild conditions. Therefore, using multinuclear metal complexes as artificial catalysts can be an attractive strategy for small-molecule conversions involving multielectron transfer processes. However, multinuclear-metal-complex-based catalysts for these reactions have not been well established. In this Account, we describe our recent advances in the development of multinuclear metal complexes as catalysts for small-molecule conversion, mainly focusing on water oxidation. As small-molecule conversions involving multielectron transfer processes consists of two essential processes, (1) the transfer of multiple electrons and (2) the formation/cleavage of covalent bond(s), catalysts for these reactions should facilitate both steps. Therefore, we assumed that the assembly of redox-active metal ions and the cooperative effect of neighboring coordinatively unsaturated metal ions can promote these processes. On the basis of this assumption, we employed a pentanuclear metal complex as a molecular scaffold for the catalyst. The scaffold has a pentanuclear structure with quasi-D₃ symmetry and consists of a [M₃(μ₃-X)] core (X = O^2- or OH^-) wrapped by two [M(μ-bpp)₃] units (Hbpp = 3,5-bis(2-pyridyl)pyrazole). The metal ions in the triangular core are coordinatively unsaturated, whereas the metal ions at the apical positions are coordinatively saturated. In other words, the pentanuclear scaffold possesses multiple redox-active centers and coordinatively unsaturated sites. It should also be noted that the electron transfer ability of the complex changes dramatically depending on the identity of the constituent metal ions. The iron derivative of the pentanuclear scaffold was found to serve as an electrocatalyst for water oxidation (2H₂O → O₂ + 4e^- + 4H⁺) with a high reaction rate and excellent robustness. The substitution of metal ions in the pentanuclear scaffold to cobalt ions resulted in the development of a catalyst for CO₂ reduction. Furthermore, we investigated the effect of substituents on the ligands of the pentanuclear iron complex and succeeded in precisely manipulating the electron transfer possess. These results clearly demonstrate that the pentanuclear scaffold is an attractive platform for catalysts for small-molecule conversions. Additionally, the intrinsic features of the multinuclear catalytic system, which are totally different from those of conventional mononuclear-metal-complex-based catalysts, are disclosed. In reactions mediated by multinuclear complexes, the multinuclear core can initially accumulate the charge required for catalysis to reach the catalytically active state. Subsequently, the catalyst in the active state reacts with the substrate, initiating electron transfer to the substrate and rearrangement of covalent bonds in the substrate to afford the product. In such a mechanism, the desired number of electrons can be transferred to the substrates in an on-demand fashion, and the formation of undesired chemical species in the targeted catalysis may be prevented. This feature of multinuclear-metal-complex-based catalysts will achieve demanding small-molecule conversions with a high reaction rate, selectivity, and durability.

Ligand architecture for triangular metal complexes: a high oxidation state Ni3 cluster with proximal metal arrangement

A new multidentate tetraanionic ligand platform for supporting trinuclear transition metal clusters has been developed. Two trisphenoxide phosphinimide ligands bind three Ni centers in a triangular arrangement. The phosphinimide donors bridge in μ3 fashion and the phenoxides complete a pseudo-square planar coordination sphere around each metal center. Electrochemical studies reveal two pseudo-reversible oxidation events at notably low potentials (-0.80 V and +0.05 V). The one electron oxidized species was characterized structurally, and it is assigned as a NiIII-containing cluster.

Unusual cyanide and methyl binding modes at a dicobalt macrocycle following acetonitrile C-C bond activation

This communication describes the C-C bond activation of acetonitrile and the trapping of the methyl and cyanide fragments by macrocyclic, dicobalt complexes. Both products display unique structural features as a result of the constraints imposed by the macrocycle. The bridged species [(3PDI2)Co2(μ-CN)(PMe3)2][OTf] ([Co2CN]+) exhibits atypical Co-CN-Co binding, and upon either phosphine dissociation or oxidation, the flexible ligand framework is able to switch between different binding modes of μ-cyanide. Further, the bridging methyl species [(3PDI2)Co2(μ-CH3)(PMe3)][OTf] ([Co2CH3]+) is the first structurally characterized dicobalt complex with a bridging methyl group.

Using nature's blueprint to expand catalysis with Earth-abundant metals

Numerous redox transformations that are essential to life are catalyzed by metalloenzymes that feature Earth-abundant metals. In contrast, platinum-group metals have been the cornerstone of many industrial catalytic reactions for decades, providing high activity, thermal stability, and tolerance to chemical poisons. We assert that nature's blueprint provides the fundamental principles for vastly expanding the use of abundant metals in catalysis. We highlight the key physical properties of abundant metals that distinguish them from precious metals, and we look to nature to understand how the inherent attributes of abundant metals can be embraced to produce highly efficient catalysts for reactions crucial to the sustainable production and transformation of fuels and chemicals.

Unexpected reactivity of a PONNOP 'expanded pincer' ligand

We report the synthesis, characterization and coordination chemistry of a new naphthyridine-derived phosphinite PONNOP expanded pincer ligand. As envisioned, the dinucleating ligand readily binds two copper(i) centers in close proximity, but undergoes an unexpected rearrangement in the presence of nickel(ii) salts to form an interesting PONNP pincer platform.

Evaluating Metal Ion Identity on Catalytic Silylation of Dinitrogen Using a Series of Trimetallic Complexes

We report catalytic silylation of dinitrogen to tris(trimethylsilyl)amine by a series of trinuclear first row transition metal complexes (M = Cr, Mn, Fe, Co, Ni) housed in our tris(β-diketiminate) cyclophane (L ^3- ). Yields are expectedly dependent on metal ion type ranging from 14 to 199 equiv NH₄ ⁺/complex after protonolysis for the Mn to Co congeners, respectively. For the series of complexes, the number of turnovers trend observed is Co > Fe > Cr > Ni > Mn, consistent with prior reports of greater efficacy of Co over Fe in other ligand systems for this reaction.

Assembly of High-Spin [Fe3] Clusters by Ligand-Based Multielectron Reduction

The hexanuclear [Na₁₂Fe₆(tris-cyclo-salophen)₂(THF)₁₄], 1-THF, and the trinuclear [Na₆Fe₃(tris-cyclo-salophen)(py)₉], 1-py, Fe(II) clusters can be easily assembled in one step from the ligand-based reduction of the [Fe^II(salophen)(THF)] complex. These complexes consist of triangular cores where three Fe(II) ions are held together, within range of bonding interaction, by the hexa-amide, hexaphenolate macrocyclic ligand tris-cyclo-salophen^12-. The tris-cyclo-salophen^12- ligand is perfectly suited for binding three Fe(II) centers at short distances, allowing for strong magnetic coupling between the Fe(II) centers. The macrocyclic ligand is generated by the reductive coupling of the imino groups of three salophen ligands, resulting in three new C-C bonds. The six electrons stored in the ligand become available for the reduction of carbon dioxide with selective formation of carbonate.

Cooperative H2 Activation on Dicopper(I) Facilitated by Reversible Dearomatization of an "Expanded PNNP Pincer" Ligand

A naphthyridine-derived expanded pincer ligand is described that can host two copper(I) centers. The proton-responsive ligand can undergo reversible partial and full dearomatization of the naphthyridine core, which enables cooperative activation of H2 giving an unusual butterfly-shaped Cu4 H2 complex.

Redox active mixed-valence hexamanganese double-cubane complexes supported by tetravanadates

A double-cubane-type hexamanganese complex that is reminiscent of the structure of an oxygen evolution center was synthesized using a polyoxovanadate ligand.