Cluster self-assembly and anion binding by metal complexes of non-innocent thiazolidinyl-thiolate ligands

Zn^II and Fe^II chloride complexes of a di(methylthiazolidinyl)pyridine ligand were deprotonated to form the corresponding thiolate complexes supported by redox-active iminopyridine moieties. The thiolate donor groups are nucleophilic and reactive toward oxidants, electrophiles, and protons, while the pendant thiazolidine rings are available for hydrogen bonding. Anion exchange with the weakly-coordinating triflate anion resulted in self-assembly of the iminopyridine complexes to form a trimeric [M₃S₃] cluster. Hydrogen bonding closely associates anions with this trimetallic core.

Access to Metal Centers and Fluxional Hydride Coordination Integral for CO2 Insertion into [Fe3(μ-H)3]3+ Clusters

CO₂ insertion into tri(μ-hydrido)triiron(II) clusters ligated by a tris(β-diketiminate) cyclophane is demonstrated to be balanced by sterics for CO₂ approach and hydride accessibility. Time-resolved NMR and UV-vis spectra for this reaction for a complex in which methoxy groups border the pocket of the hydride donor (Fe₃H₃L², 4) result in a decreased activation barrier and increased kinetic isotope effect consistent with the reduced sterics. For the ethyl congener Fe₃H₃L¹ (2), no correlation is found between rate and reaction solvent or added Lewis acids, implying CO₂ coordination to an Fe center in the mechanism. The estimated hydricity (50 kcal/mol) based on observed H/D exchange with BD₃ requires Fe-O bond formation in the product to offset an endergonic CO₂ insertion. μ₃-hydride coordination is noted to lower the activation barrier for the first CO₂ insertion event in DFT calculations.

Dinitrogen Fixation: Rationalizing Strategies Utilizing Molecular Complexes

Dinitrogen (N2 ) is the most abundant gas in Earth's atmosphere, but its inertness hinders its use as a nitrogen source in the biosphere and in industry. Efficient catalysts are hence required to ov. ercome the high kinetic barriers associated to N2 transformation. In that respect, molecular complexes have demonstrated strong potential to mediate N2 functionalization reactions under mild conditions while providing a straightforward understanding of the reaction mechanisms. This Review emphasizes the strategies for N2 reduction and functionalization using molecular transition metal and actinide complexes according to their proposed reaction mechanisms, distinguishing complexes inducing cleavage of the N≡N bond before (dissociative mechanism) or concomitantly with functionalization (associative mechanism). We present here the main examples of stoichiometric and catalytic N2 functionalization reactions following these strategies.

Cleavage of cluster iron-sulfide bonds in cyclophane-coordinated FenSm complexes

Reaction of the tri(μ-sulfido)triiron(iii) tris(β-diketiminate) cyclophane complex, Fe3S3LEt/Me (1), or of the di(μ-sulfido)diiron(iii) complex Fe2S2HLEt/Me (5), with the related tri(bromide)triiron(ii) complex Fe3Br3LEt/Me (2) results in electron and ligand redistribution to yield the mixed-ligand multiiron complexes, including Fe3Br2SLEt/Me (3) and Fe2Br2SHLEt/Me (4). The cleavage and redistribution observed in these complexes is reminiscent of necessary Fe-S bond cleavage for substrate activation in nitrogenase enzymes, and provides a new perspective on the lability of Fe-S bonds in FeS clusters.

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.

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.

Remote Ligand Modifications Tune Electronic Distribution and Reactivity in Site-Differentiated, High-Spin Iron Clusters: Flipping Scaling Relationships

We report the synthesis, characterization, and reactivity of [LFe₃O(^RArIm)₃Fe][OTf]₂, the first Hammett series of a site-differentiated cluster. The cluster reduction potentials and CO stretching frequencies shift as expected on the basis of the electronic properties of the ligand: electron-donating substituents result in more reducing clusters and weaker C-O bonds. However, unusual trends in the energetics of their two sequential CO binding events with the substituent σ_p parameters are observed. Specifically, introduction of electron-donating substituents suppresses the first CO binding event (ΔΔH by as much as 7.9 kcal mol^-1) but enhances the second (ΔΔH by as much as 1.9 kcal mol^-1). X-ray crystallography, including multiple-wavelength anomalous diffraction, Mössbauer spectroscopy, and SQUID magnetometry, reveal that these substituent effects result from changes in the energetic penalty associated with electronic redistribution within the cluster, which occurs during the CO binding event.

Isolation of chloride- and hydride-bridged tri-iron and -zinc clusters in a tris(β-oxo-δ-diimine) cyclophane ligand

A cyclophane ligand (H6L) bearing three β-oxo-δ-diimine arms and the corresponding tri-iron and -zinc complexes in which the metal ions are bridged by either chlorides, viz. Fe3Cl3(H3L) (1) and Zn3Cl3(H3L) (2), or hydrides, viz. Fe3H3(H3L) (3), Zn3H3(H3L) (4), were synthesized and characterized. 1 adopts a chair-shaped C3v-symmetric [Fe3(μ-Cl)3]3+ cluster wherein only one hemisphere of the ligand is metallated and the other three ketoimine sites remain protonated as evidenced by single crystal X-ray diffraction and vibrational and NMR spectroscopic analyses. 3 and 4 were synthesized by substitution of the bridging chlorides in 1 and 2 using KBEt3H and are accessed with retention of the three protonated ketoimine sites.

Dinitrogen Activation and Functionalization using β-Diketiminate Iron Complexes

Iron catalysts are adept at breaking the N-N bond of N₂, as exemplified by the iron-catalyzed Haber-Bosch process and the iron-containing clusters at the active sites of nitrogenase enzymes. This Minireview summarizes recent work that has identified a well-characterized set of multi-iron complexes that are capable of breaking and functionalizing N₂, and are amenable to detailed mechanistic studies. We discuss the redox balancing, the potential intermediates during N₂ activation, the variation of alkali metal reductant, the reversibility of N₂ cleavage, and the formation of N-H and N-C bonds from N₂.

Cyclophanes as Platforms for Reactive Multimetallic Complexes

Multimetallic cofactors supported by weak-field donors frequently function as reaction centers in metalloproteins, and many of these cofactors catalyze small molecule activation (e.g., N₂, O₂, CO₂) with prominent roles in geochemical element cycles or detoxification. Notable examples include the iron-molybdenum cofactor of the molybdenum-dependent nitrogenases, which catalyze N₂ fixation, and the NiFe₄S₄ cluster and the Mo(O)SCu site in various carbon monoxide dehydrogenases. The prevailing proposed reaction mechanisms for these multimetallic cofactors relies on a cooperative pathway, in which the oxidation state changes are distributed over the aggregate coupled with orbital overlap between the substrate and more than one metal ion within the cluster. Such cooperativity has also been proposed for chemical transformations at the surfaces of heterogeneous catalysts. However, the design details that afford cooperative effects and allow such reactivity to be harnessed effectively in homogeneous synthetic systems remain unclear. Relatedly, hydride donors ligated to these metal cluster cofactors are suggested as precursors to the state that reacts with substrates; here too, however, the reactivity of hydride-decorated clusters supported by weak-field ligands is underexplored. Inspired by the reactivity potential of multimetallic assemblies evidenced in biological systems, approaches to design, synthesize, and evaluate reactivity of polynuclear metal compounds have been actively explored. In a similar vein to the templating function afforded by enzyme active sites, a carefully engineered organic ligand can be employed to control metal nuclearity of the complex and the local coordination environment of each metal center. This Account presents our efforts within this field, beginning with ligand design considerations followed by a survey of observed small molecule activation by trimetallic cyclophanates. We highlight the distinct reactivity outcomes accessed by multimetallic compounds as compared to aggregates that assemble in reaction mixtures from monometallic precursors. Contributing to the opportunity for programmed cooperativity in these designed multimetallic compounds, the cyclophane also dictates the orientation of substrate binding and metal-substrate interactions, which has a prominent influence on reactivity. For example, the dinitrogen-tricopper(I) cyclophanate reacts with dioxygen with markedly different results as compared to monocopper compounds. As an unexpected outcome, one series of tricopper compounds were discovered to be competent catalysts for carbon dioxide reduction to oxalate-a formally one-electron process-hinting at an inherently broader reaction scope for weak-field clusters at lowering the barrier for one-electron pathways as well as multielectron redox transformations. Further reflecting the role of the ligand in tuning reactivity, the trimetallic trihydride cluster compounds, [M₃(μ-H)₃]³⁺ (M = Fe^II, Co^II, Zn^II), demonstrate substrate specificity for CO₂ over various other unsaturated molecules and surprising stability toward water. This series reflects the role of the local environment of a shallow ligand pocket to control substrate access. Summed together, the systems described here evidence the anticipated cooperative reactivity accessed in designed multimetallic species vs self-assembled monometallic systems (e.g., O₂ activation and O atom transfer) as well as control of substrate access by seemingly subtle structural effects. Indeed, future efforts aim to interrogate the limits of cooperativity in these systems as well as the role of ligand dynamics and sterics on reactivity.

A bio-inspired approach to ligand design: folding single-chain peptoids to chelate a multimetallic cluster

Synthesis of biomimetic multimetallic clusters is sought after for applications such as efficient storage of solar energy and utilization of greenhouse gases. However, synthetic efforts are hampered by a dearth of ligands that are developed for multimetallic clusters due to current limitations in rational design and organic synthesis. Peptoids, a synthetic sequence-defined oligomer, enable a biomimetic strategy to rapidly synthesize and optimize large, multifunctional ligands by structural design and combinatorial screening. Here we discover peptoid oligomers (≤7 residues) that fold into a single conformation to provide unprecedented tetra- and hexadentate chelation by carboxylates to a [Co4O4] cubane cluster. The structures of peptoid-bound cubanes were determined by 2D NMR spectroscopy, and their structures reveal key steric and side-chain-to-main chain interactions that work in concert to rigidify the peptoid ligand. This efficient ligand design strategy holds promise for creating new scaffolds for the abiotic synthesis and manipulation of multimetallic clusters.

Thermally Persistent High-Spin Ground States in Octahedral Iron Clusters

Chemical oxidation and reduction of the all-ferrous (^HL)₂Fe₆ in THF affords isostructural, coordinatively unsaturated clusters of the type [(^HL)₂Fe₆] ⁿ: [(^HL)₂Fe₆][BArF₂₄] (1, n = +1; where [BArF₂₄]^- = tetrakis[(3,5-trifluoromethyl)phenyl]borate), [Bu₄N][(^HL)₂Fe₆] (2a, n = -1), [P][(^HL)₂Fe₆] (2b, n = -1; where [P]⁺ = tributyl(1,3-dioxolan-2-ylmethyl)phosphonium), and [Bu₄N]₂[(^HL)₂Fe₆] (3, n = -2). Each member of the redox-transfer series was characterized by zero-field ⁵⁷Fe Mössbauer spectroscopy, near-infrared spectroscopy, single-crystal X-ray crystallography, and magnetometry. Redox-directed trends are observed when comparing the structural metrics within the [Fe₆] core. The metal octahedron [Fe₆] decreases marginally in volume as the molecular reduction state increases as gauged by the Fe-Fe_avg distance varying from 2.608(11) Å ( n = +1) to 2.573(3) ( n = -2). In contrast, the mean Fe-N distances and ∠Fe-N-Fe angles correlate linearly with the [Fe₆] oxidation level, or alternatively, the changes observed within the local Fe-N₄ coordination planes vary linearly with the aggregate spin ground state. In general, as the spin ground state ( S) increases, the Fe-N(H)_avg distances also increase. The structural metric perturbations within the [Fe₆] core and measured spin ground states were rationalized extending the previously proposed molecular orbital diagram derived for (^HL)₂Fe₆. Chemical reduction of the (^HL)₂Fe₆ cluster results in an abrupt increase in spin ground state from S = 6 for the all-ferrous cluster, to S = ¹⁹/₂ in the monoanionic 2b and S = 11 for the dianionic 3. The observation of asymmetric intervalence charge transfer bands in 3 provides further evidence of the fully delocalized ground state observed by ⁵⁷Fe Mössbauer spectroscopy for all species examined (1-3). For each of the clusters examined within the electron-transfer series, the observed spin ground states thermally persist to 300 K. In particular, the S = 11 in dianionic 3 and S = ¹⁹/₂ in the monoanionic 2b represent the highest spin ground states isolated up to room temperature known to date. The increase in spin ground state results from population of the antibonding orbital band comprised of the Fe-N σ* interactions. As such, the thermally persistent ground states arise from population of the resultant single spin manifolds in accordance with Hund's rules. The large spin ground states, indicative of strong ferromagnetic electronic alignment of the valence electrons, result from strong direct exchange electronic coupling mediated by Fe-Fe orbital overlap within the [Fe₆] cores, equivalent to a strong double exchange magnetic coupling B for 3 that was calculated to be 309 cm^-1.

Selective Synthesis of Site-Differentiated Fe4S4 and Fe6S6 Clusters

Obtaining rational control over the structure and nuclearity of metalloclusters is an ongoing challenge in synthetic Fe-S cluster chemistry. We report a new family of tridentate imidazolin-2-imine ligands L(NIm^R)₃ that can bind [Fe₄S₄]²⁺ or [Fe₆S₆]³⁺ clusters, depending on the steric profile of the ligand and the reaction stoichiometry. A high-yielding synthetic route to L(NIm^R)₃ ligands (where R is the imidazolyl N substituents) from trianiline and 2-chloroimidazolium precursors is described. For L(NIm^Me)₃ (tris(1,3,5-(3-( N, N-dimethyl-4,5-diphenylimidazolin-2-imino)phenylmethyl))benzene), metalation with 1 equiv of [Ph₄P]₂[Fe₄S₄Cl₄] and 3 equiv of NaBPh₄ furnishes a mixture of products, but adjusting the stoichiometry to 1.5 equiv of [Ph₄P]₂[Fe₄S₄Cl₄] provides (L(NIm^Me)₃)Fe₆S₆Cl₆ in high yield. Formation of an [Fe₆S₆]³⁺ cluster using L(NIm^Tol)₃ (tris(1,3,5-(3-( N, N-bis(4-methylphenyl)-4,5-diphenylimidazolin-2-imino)phenylmethyl))benzene) is not observed; instead, the [Fe₄S₄]²⁺ cluster [(L(NIm^Tol)₃)(Fe₄S₄Cl)][BPh₄] is cleanly generated when 1 equiv of [Ph₄P]₂[Fe₄S₄Cl₄] is employed. The selectivity for cluster nuclearity is rationalized by the orientation of the imidazolyl rings whereby long N-imidazolyl substituents preclude formation of [Fe₆S₆]³⁺ clusters but not [Fe₄S₄]²⁺ clusters. Thus, the structure and nuclearity of L(NIm^R)₃-bound Fe-S clusters may be selectively controlled through rational modification the ligand's substituents.

Chalcogen Impact on Covalency within Molecular [Cu3(μ3-E)]3+ Clusters (E = O, S, Se): A Synthetic, Spectroscopic, and Computational Study

Reaction of the tricopper(I)-dinitrogen tris(β-diketiminate) cyclophane, Cu₃(N₂)L, with O-atom-transfer reagents or elemental Se affords the oxido-bridged tricopper complex Cu₃(μ₃-O)L (2) or the corresponding Cu₃(μ₃-Se)L (4), respectively. For 2 and 4, incorporation of the bridging chalcogen donor was supported by electrospray ionization mass spectrometry and K-edge X-ray absorption spectroscopy (XAS) data. Cu L_2,3-edge X-ray absorption data quantify 49.5% Cu 3d character in the lowest unoccupied molecular orbital of 2, with Cu 3d participation decreasing to 33.0% in 4 and 40.8% in the related sulfide cluster Cu₃(μ₃-S)L (3). Multiedge XAS and UV/visible/near-IR spectra are employed to benchmark density functional theory calculations, which describe the copper-chalcogen interactions as highly covalent across the series of [Cu₃(μ-E)]³⁺ clusters. This result highlights that the metal-ligand covalency is not reserved for more formally oxidized metal centers (i.e., Cu^III + O^2- vs Cu^II + O^-) but rather is a significant contributor even at more typical ligand-field cases (i.e., Cu₃^II/II/I + E^2-). This bonding is reminiscent of that observed in p-block elements rather than in early-transition-metal complexes.

Catalytic Silylation of Dinitrogen by a Family of Triiron Complexes

A series of triiron complexes supported by a tris(β-diketiminate)cyclophane (L ^3- ) catalyze the reduction of dinitrogen to tris(trimethylsilyl)amine using KC₈ and Me₃SiCl. Employing Fe₃Br₃ L affords 83 ± 7 equiv. NH₄ ⁺/complex after protonolysis, which is a 50% yield based on reducing equivalents. The series of triiron compounds tested evidences the subtle effects of ancillary donors, including halides, hydrides, sulfides, and carbonyl ligands, and metal oxidation state on N(SiMe₃)₃ yield, and highlight Fe₃(μ₃-N)L as a common species in product mixtures. These results suggest that ancillary ligands can be abstracted with Lewis acids under reducing conditions.

Synthesis, Characterization, and Thermal Properties of N-alkyl β-Diketiminate Manganese Complexes

A series of N, N'-dialkyl-β-diketiminato manganese(II) complexes was synthesized and characterized by single crystal X-ray diffraction, UV-vis and FTIR spectroscopy, and then assayed for volatility, thermal stability, and surface reactivity relevant to vapor-phase film growth processes. Bis( N, N'-dimethyl-4-amino-3-penten-2-imine) manganese(II), 1, and bis( N- N'-diisopropyl-4-amino-3-penten-2-imine) manganese(II), 2, specifically, emerge as the most promising candidates, balancing volatility (sublimation temperatures < 100 °C at 100 mTorr) with coordinative unsaturation and reactivity, as revealed by rapid release of ligand in the presence of a silica surface. Good correlation is observed between buried volume calculations and relative surface reactivity data, indicating that metal availability resulting from sterically open ligand alkyl substituents increases surface reactivity. The thermal stability, volatility, and reactivity exhibited by these compounds render them promising precursors for the growth of manganese oxide films via vapor-phase growth processes.

Shape-Persistent [4+4] Imine Cages with a Truncated Tetrahedral Geometry

The synthesis of shape-persistent organic cage compounds is often based on the usage of multiple dynamic covalent bond formation (such as imines) of readily available precursors. By careful choice of the precursors geometry, the geometry and size of the resulting cage can be accurately designed and indeed a number of different geometries and sizes have been realized to date. Despite of this fact, little is known about the precursors conformational rigidity and steric preorganization of reacting functional groups on the outcome of the reaction. Herein, the influence of conformational rigidity in the precursors on the formation of a [4+4] imine cage with truncated tetrahedral geometry is discussed.

Correlating Bridging Ligand with Properties of Ligand-Templated [MnII3X3]3+ Clusters (X = Br-, Cl-, H-, MeO-)

Polynuclear manganese compounds have garnered interest as mimics and models of the water oxidizing complex (WOC) in photosystem II and as single molecule magnets. Molecular systems in which composition can be correlated to physical phenomena, such as magnetic exchange interactions, remain few primarily because of synthetic limitations. Here, we report the synthesis of a family of trimanganese(II) complexes of the type Mn₃X₃L (X = Cl^-, H^-, and MeO^-) where L^3- is a tris(β-diketiminate) cyclophane. The tri(chloride) complex (2) is structurally similar to the reported tri(bromide) complex (1) with the Mn₃X₃ core having a ladder-like arrangement of alternating M-X rungs, whereas the tri(μ-hydride) (3) and tri(μ-methoxide) (4) complexes contain planar hexagonal cores. The hydride and methoxide complexes are synthesized in good yield (48% and 56%) starting with the bromide complex employing a metathesis-like strategy. Compounds 2-4 were characterized by combustion analysis, X-ray crystallography, X-band EPR spectroscopy, SQUID magnetometry, and infrared and UV-visible spectroscopy. Magnetic susceptibility measurements indicate that the Mn₃ clusters in 2-4 are antiferromagnetically coupled, and the spin ground state of the compounds (S = 3/2 (1, 2) or S = 1/2 (3, 4)) is correlated to the identity of the bridging ligand and structural arrangement of the Mn₃X₃ core (X = Br, Cl, H, OCH₃). Electrochemical experiments on isobutyronitrile solutions of 3 and 4 display broad irreversible oxidations centered at 0.30 V.

Intramolecular C-H and C-F Bond Oxygenation by Site-Differentiated Tetranuclear Manganese Models of the OEC

The dangler manganese center in the oxygen-evolving complex (OEC) of photosystem II plays an important role in the oxidation of water to dioxygen. Inspired by the structure of the OEC, we synthesized a series of site-differentiated tetra-manganese clusters [LMn3(PhPz)3OMn][OTf]x (2: x = 2; 3: x = 1) that features an apical manganese ion-distinct from the others-that is appended to a trinuclear manganese core through an μ4-oxygen atom bridge. This cluster design was targeted to facilitate studies of high-valent Mn-oxo formation, which is a proposed step in the mechanism for water oxidation by the OEC. Terminal Mn-oxo species-supported by a multinuclear motif-were targeted by treating 2 and 3 with iodosobenzene. Akin to our previously reported iron complexes, intramolecular arene hydroxylation was observed to yield the C-H bond oxygenated complexes [LMn3(PhPz)2(OArPz)OMn][OTf]x (5: x = 2; 6: x = 1). The fluorinated series [LMn3(F2ArPz)3OMn][OTf]x (8: x = 2; 9: x = 1) was also synthesized to mitigate the observed intramolecular hydroxylation. Treatment of 8 and 9 with iodosobenzene results in intramolecular arene C-F bond oxygenation as judged by electrospray ionization mass spectrometry. The observed aromatic C-H and C-F hydroxylation is suggestive of a putative high-valent terminal metal-oxo species, and it is one of the very few examples capable of oxygenating C-F bonds.

Reactivity of hydride bridges in a high-spin [Fe3(μ-H)3]3+ cluster: reversible H2/CO exchange and Fe-H/B-F bond metathesis

The triiron trihydride complex Fe₃H₃L (1) [where L^3– is a tris(β-diketiminate)cyclophanate] reacts with CO and with BF₃·OEt₂ to afford (Fe^ICO)₂Fe^II(μ₃-H)L (2) and Fe₃F₃L (3), respectively.

The triiron trihydride complex Fe₃H₃L (1) [where L^3– is a tris(β-diketiminate)cyclophanate] reacts with CO and with BF₃·OEt₂ to afford (Fe^ICO)₂Fe^II(μ₃-H)L (2) and Fe₃F₃L (3), respectively. Variable-temperature and applied-field Mössbauer spectroscopy support the assignment of two high-spin (HS) iron(i) centers and one HS iron(ii) ion in 2. Preliminary studies support a CO-induced reductive elimination of H₂ from 1, rather than CO trapping a species from an equilibrium mixture. This complex reacts with H₂ to regenerate 1 under a dihydrogen atmosphere, which represents a rare example of reversible CO/H₂ exchange and the first to occur at high-spin metal centers, as well as the first example of a reversible multielectron redox reaction at a designed high-spin metal cluster. The formation of 3 proceeds through a previously unreported net fluoride-for-hydride substitution, and 3 is surprisingly chemically inert to Si–H bonds and points to an unexpectedly large difference between the Fe–F and Fe–H bonds in this high-spin system.

A three-coordinate Fe(ii) center within a [3Fe-(μ3-S)] cluster that provides an accessible coordination site

A μ3-sulfide bridged triiron cluster(ii,ii,iii) supported by a cyclophane ligand undergoes metal-based reduction to yield an all-ferrous species. The latter complex incorporates a three-coordinate iron center that provides an accessible coordination site to a solvent molecule.

Guest–Host Interaction of Coinage Metals in π-Rich Cavities

The complexation of coinage metal cations with [2.2.2]paracyclophane and deltaphane has been investigated by means of density functional theory (DFT) calculations employing the PBE0-D3 hybrid functional, which incorporates explicit dispersion corrections to account for the weak intermolecular forces that are important in the systems studied. Natural bond orbital (NBO) analyses, Bader's Atoms in Molecules theory analyses as well as localised molecular orbital – energy decomposition analyses (LMO-EDAs) have been carried out to further investigate the electronic structure and bonding of the complexes. It was found that both cyclophanes bind strongest with gold ions, followed closely by copper ions and lastly silver ions. The two fragments interact in a non-covalent fashion in these complexes and the metal preferentially resides at the periphery of the molecular cavity of the cyclophane.

A [3Fe–3S]3+ cluster with exclusively μ-sulfide donors

A [3Fe–3(μ-S)]³⁺ cluster is reported in which each ferric center has a distorted trigonal pyramidal geometry, with an S = 1/2 ground state for the cluster and unusually anisotropic hyperfine coupling constants as determined by variable temperature magnetometry and Mössbauer spectroscopy.

Insights into small molecule activation by multinuclear first-row transition metal cyclophanates

The rational design of trimetallic transition metal clusters supported by a trinucleating cyclophane ligand, L³⁻, and the reactivities of these complexes with dinitrogen and carbon dioxide are discussed.

On the non-innocence of “Nacnacs”: ligand-based reactivity in β-diketiminate supported coordination compounds

While β-diketiminate (BDI or ‘nacnac’) ligands have been widely adopted to stabilize a wide range of metal ions in multiple oxidation states and coordination numbers, in several occurrences these ligands do not behave as spectators and participate in reactivity.

Reactivity of Hydride Bridges in High-Spin [3M-3(μ-H)] Clusters (M = FeII, CoII)

The designed [3M-3(μ-H)] clusters (M = Fe(II), Co(II)) Fe3H3L (1-H) and Co3H3L (2-H) [where L(3-) is a tris(β-diketiminate) cyclophane] were synthesized by treating the corresponding M3Br3L complexes with KBEt3H. From single-crystal X-ray analysis, the hydride ligands are sterically protected by the cyclophane ligand, and these complexes selectively react with CO2 over other unsaturated substrates (e.g., CS2, Me3SiCCH, C2H2, and CH3CN). The reaction of 1-H or 2-H with CO2 at room temperature yielded Fe3(OCHO)(H)2L (1-CO2) or Co3(OCHO)(H)2L (2-CO2), respectively, which evidence the differential reactivity of the hydride ligands within these complexes. The analogous reactions at elevated temperatures revealed a distinct difference in the reactivity pattern for 2-H as compared to 1-H; Fe3(OCHO)3L (1-3CO2) was generated from 1-H, while 2-H afforded only 2-CO2.

Alkali metal control over N-N cleavage in iron complexes

Though N2 cleavage on K-promoted Fe surfaces is important in the large-scale Haber-Bosch process, there is still ambiguity about the number of Fe atoms involved during the N-N cleaving step and the interactions responsible for the promoting ability of K. This work explores a molecular Fe system for N2 reduction, particularly focusing on the differences in the results obtained using different alkali metals as reductants (Na, K, Rb, Cs). The products of these reactions feature new types of Fe-N2 and Fe-nitride cores. Surprisingly, adding more equivalents of reductant to the system gives a product in which the N-N bond is not cleaved, indicating that the reducing power is not the most important factor that determines the extent of N2 activation. On the other hand, the results suggest that the size of the alkali metal cation can control the number of Fe atoms that can approach N2, which in turn controls the ability to achieve N2 cleavage. The accumulated results indicate that cleaving the triple N-N bond to nitrides is facilitated by simultaneous approach of least three low-valent Fe atoms to a single molecule of N2.