A Triad of Highly Reduced,Linear Iron Nitrosyl Complexes: {FeNO}8–10

Given the importance of Fe–NO complexes in both human biology and the global nitrogen cycle, there has been interest in understanding their diverse electronic structures. Herein a redox series of isolable iron nitrosyl complexes stabilized by a tris(phosphine)borane (TPB) ligand is described. These structurally characterized iron nitrosyl complexes reside in the following highly reduced Enemark–Feltham numbers: {FeNO}⁸, {FeNO}⁹, and {FeNO}¹⁰. These {FeNO}^8–10 compounds are each low‐spin, and feature linear yet strongly activated nitric oxide ligands. Use of Mössbauer, EPR, NMR, UV/Vis, and IR spectroscopy, in conjunction with DFT calculations, provides insight into the electronic structures of this uncommon redox series of iron nitrosyl complexes. In particular, the data collectively suggest that {TPBFeNO}^8–10 are all remarkably covalent. This covalency is likely responsible for the stability of this system across three highly reduced redox states that correlate with unusually high Enemark–Feltham numbers.     

Mechanistic Insight into the Nitric Oxide Dioxygenation Reaction of Nonheme Iron(III)–Superoxo and Manganese(IV)–Peroxo Complexes

Reactions of nonheme Fe^III–superoxo and Mn^IV–peroxo complexes bearing a common tetraamido macrocyclic ligand (TAML), namely [(TAML)Fe^III(O₂)]^2? and [(TAML)Mn^IV(O₂)]^2?, with nitric oxide (NO) afford the Fe^III–NO₃ complex [(TAML)Fe^III(NO₃)]^2? and the Mn^V–oxo complex [(TAML)Mn^V(O)]^? plus NO₂^?, respectively. Mechanistic studies, including density functional theory (DFT) calculations, reveal that M^III–peroxynitrite (M=Fe and Mn) species, generated in the reactions of [(TAML)Fe^III(O₂)]^2? and [(TAML)Mn^IV(O₂)]^2? with NO, are converted into M^IV(O) and ^.NO₂ species through O?O bond homolysis of the peroxynitrite ligand. Then, a rebound of Fe^IV(O) with ^.NO₂ affords [(TAML)Fe^III(NO₃)]^2?, whereas electron transfer from Mn^IV(O) to ^.NO₂ yields [(TAML)Mn^V(O)]^? plus NO₂^?.     

Structural and Spectroscopic Characterization of a High‐Spin {FeNO}6 Complex with an Iron(IV)−NO− Electronic Structure

Although the interaction of low‐spin ferric complexes with nitric oxide has been well studied, examples of stable high‐spin ferric nitrosyls (such as those that could be expected to form at typical non‐heme iron sites in biology) are extremely rare. Using the TMG₃tren co‐ligand, we have prepared a high‐spin ferric NO adduct ({FeNO}⁶ complex) via electrochemical or chemical oxidation of the corresponding high‐spin ferrous NO {FeNO}⁷ complex. The {FeNO}⁶ compound is characterized by UV/Visible and IR spectroelectrochemistry, Mössbauer and NMR spectroscopy, X‐ray crystallography, and DFT calculations. The data show that its electronic structure is best described as a high‐spin iron(IV) center bound to a triplet NO^? ligand with a very covalent iron?NO bond. This finding demonstrates that this high‐spin iron nitrosyl compound undergoes iron‐centered redox chemistry, leading to fundamentally different properties than corresponding low‐spin compounds, which undergo NO‐centered redox transformations.     

System with Potential Dual Modes of Metal–Ligand Cooperation: Highly Catalytically Active Pyridine‐Based PNNH–Ru Pincer Complexes

Metal–ligand cooperation (MLC) plays an important role in catalysis. Systems reported so far are generally based on a single mode of MLC. We report here a system with potential for MLC by both amine–amide and aromatization–dearomatization ligand transformations, based on a new class of phosphino–pyridyl ruthenium pincer complexes, bearing sec‐amine coordination. These pincer complexes are effective catalysts under unprecedented mild conditions for acceptorless dehydrogenative coupling of alcohols to esters at 35 °C and hydrogenation of esters at room temperature and 5 atm H₂. The likely actual catalyst, a novel, crystallographically characterized monoanionic de‐aromatized enamido–Ru^II complex, was obtained by deprotonation of both the N?H and the methylene proton of the N‐arm of the pincer ligand.     

First Stabilization of 14‐Electron Rhodium(I) Complexes by Hemichelation

Hemichelation is emerging as a new mode of coordination where non‐covalent interactions crucially contribute to the cohesion of electron‐unsaturated organometallic complexes. This study discloses an unprecedented demonstration of this concept to a Group 9 metal, that is, Rh^I. The syntheses of new 14‐electron Rh^I complexes were achieved by choosing the anti‐[(η⁶:η⁶‐fluorenyl){Cr(CO)₃}₂] anion as the ambiphilic hemichelating ligand, which was treated with [{Rh(nbd)Cl}₂] (nbd=norbornadiene) and [{Rh(CO)₂Cl}₂]. The new T‐shaped Rh^I hemichelates were characterized by analytical and structural methods. Investigations using the methods of the DFT and electron‐density topology analysis (NCI region analysis, QTAIM theory) confirmed the closed‐shell, non‐covalent and attractive characters of the interaction between the Rh^I center and the proximal Cr(CO)₃ moiety. This study shows that, by appropriate tuning of the electronic properties of the ambiphilic ligand, truly coordination‐unsaturated Rh^I complexes can be synthesized in a manageable form.     

A Highly Stable Nonhysteretic {Cu2(tebpz) MOF+water} Molecular Spring

A molecular spring formed by a hydrophobic metal–organic framework Cu₂(tebpz) (tebpz=3,3′,5,5′‐tetraethyl‐4,4′‐bipyrazolate) and water is presented. This nanoporous heterogeneous lyophobic system (HLS) has exceptional properties compared to numerous reported systems of such type in terms of stability, efficiency, and operating pressure. Mechanical and thermal energetic characteristics as well as stability of the system are discussed and compared in detail with those of other previously reported HLS.     

Coordination Polymer Nanobamboos of {FexIn1−x}‐MIL‐88B: Induced Formation of a Virtual In‐MIL‐88B

A precise fabrication of nanobamboo structures made from hybrid coordination polymers of the type {Fe_xIn_1?x}‐MIL‐88B is demonstrated. The compositions of the hybrid coordination polymer nanobamboos of {Fe_xIn_1?x}‐MIL‐88B (x=0.06, 0.19, or 0.75) are regulated by altering the amount of metal ions used in the reactions. Interestingly, the formation of a virtual In‐MIL‐88B (precise structure, {Fe_0.06In_0.94}‐MIL‐88B), which cannot be created in a typical reaction, is induced by the assistance of a Fe‐MIL‐88B structure. The a and c cell parameters of {Fe_0.06In_0.94}‐MIL‐88B are calculated at 10.95 and 19.86 Å, respectively. These values of {Fe_0.06In_0.94}‐MIL‐88B are larger than those of pure Fe‐MIL‐88B owing to the large ionic size of In³⁺ within the framework.     

Structural and Photophysical Study on Heterobimetallic Complexes with d8–d10 Interactions Supported by Carborane Ligands: Theoretical Analysis of the Emissive Behaviour

Heterobimetallic complexes of formula [M{(PPh₂)₂C₂B₉H₁₀}(S₂C₂B₁₀H₁₀)M′(PPh₃)] (M=Pd, Pt; M′=Au, Ag, Cu) and [Ni{(PPh₂)₂C₂B₉H₁₀}(S₂C₂B₁₀H₁₀)Au(PPh₃)] were obtained from the reaction of [M{(PPh₂)₂C₂B₁₀H₁₀}(S₂C₂B₁₀H₁₀)] (M=Pd, Pt) with [M′(PPh₃)]⁺ (M′=Au, Ag, Cu) or by one‐pot synthesis from [(SH)₂C₂B₁₀H₁₀], (PPh₂)₂C₂B₁₀H₁₀, NiCl₂ ? 6 H₂O, and [Au(PPh₃)]⁺. They display d⁸–d¹⁰ intermetallic interactions and emit red light in the solid state at 77 K. Theoretical studies on [M{(PPh₂)₂C₂B₉H₁₀}(S₂C₂B₁₀H₁₀)Au(PPh₃)] (M=Pd, Pt, Ni) attribute the luminescence to ligand (thiolate, L)‐to‐“P₂‐M‐S₂” (ML′) charge‐transfer (LML′CT) transitions for M=Pt and to metal (M)‐to‐“P₂‐M‐S₂” (ML′) charge‐transfer (MML′CT) transitions for M=Ni, Pd.     

A New Iron‐Based Carbon Monoxide Oxidation Catalyst: Structure–Activity Correlation

A new iron‐based catalyst for carbon monoxide oxidation, as a potential substitute for precious‐metal systems, has been prepared by using a facile impregnation method with iron tris‐acetylacetonate as a precursor on γ‐Al₂O₃. Light‐off and full conversion temperatures as low as 235 and 278 °C can be reached. However, the catalytic activity strongly depends on the loading; lower loadings perform better than higher ones. The different activities can be explained by variations of the structures formed. The structures are thoroughly characterized by a multimethodic approach by using X‐ray diffraction, Brunauer–Emmett–Teller surface areas, and Mössbauer spectroscopy combined with diffuse reflectance UV/Vis and X‐ray absorption spectroscopy. Consequently, isolated tetrahedrally coordinated Fe³⁺ centers and phases of AlFeO₃ are identified as structural requirements for high activity in the oxidation of carbon monoxide.     

Unusual Transformation from a Solvent‐Stabilized 1D Coordination Polymer to a Metal–Organic Framework (MOF)‐Like Cross‐Linked 3D Coordination Polymer

An unusual 1D‐to‐3D transformation of a coordination polymer based on organic linkers containing highly polar push–pull π‐conjugated side chains is reported. The coordination polymers are synthesized from zinc nitrate and an organic linker, namely, 2,5‐bis{4‐[1‐(4‐nitrophenyl)pyrrolidin‐2‐yl]butoxy}terephthalic acid, which possesses highly polar (4‐nitrophenyl)pyrrolidine groups, with high dipole moments of about 7 D. The coordination polymers exhibit an unusual transformation from a soluble, solvent‐stabilized 1D coordination polymer into an insoluble, metal–organic framework (MOF)‐like 3D coordination polymer. The coordination polymer exhibits good film‐forming ability, and the MOF‐like films are insoluble in conventional organic solvents.     

Conformational Adaptation of β-Peptide Foldamers for the Formation of Metal–Peptide Frameworks

Metal-coordinated frameworks derived from small peptidic ligands have received much attention thanks to peptides’ vast structural and functional diversity. Various peptides with partial conformational preferences have been used to build metal–peptide frameworks, however, the use of conformationally constrained β-peptide foldamers has not been explored yet. Herein we report the first metal-coordination-mediated assembly of β-peptide foldamers with 12-helical folding propensity. The coordination of Ag⁺ to the terminal pyridyl moieties afforded a set of metal–peptide frameworks with unique entangled topologies. Interestingly, formation of the network structures was accompanied by notable conformational distortions of the foldamer ligands. As the first demonstration of new metal–peptide frameworks built from modular β-peptide foldamers, we anticipate that this work will be an important benchmark for further structural evolution and mechanistic investigation.     

Unusual Stabilization of an Intermediate Spin State of Iron upon the Axial Phenoxide Coordination of a Diiron(III)–Bisporphyrin: Effect of Heme–Heme Interactions

The binding of a series of substituted phenols as axial ligands onto a diiron(III)? bisporphyrin framework have been investigated. Spectroscopic characterization revealed high‐spin states of the iron centers in all of the phenolate complexes, with one exception in the 2,4,6‐trinitrophenolate complex of diiron(III)? bisporphyrin, which only stabilized the pure intermediate‐spin (S=3/2) state of the iron centers. The average Fe? N (porphyrin) and Fe? O (phenol) distances that were observed with the 2,4,6‐trinitrophenolate complex were 1.972(3) Å and 2.000(2) Å, respectively, which are the shortest and longest distances reported so far for any Fe^III? porphyrin with phenoxide coordination. The alternating shift pattern, which shows opposite signs of the chemical shifts for the meta versus ortho/para protons, is attributed to negative and positive spin densities on the phenolate carbon atoms, respectively, and is indicative of π‐spin delocalization onto the bound phenolate. Electrochemical data reveals that the E_1/2 value for the Fe^III/Fe^II couple is positively shifted with increasing acidity of the phenol. However, a plot of the E_1/2 values for the Fe^III/Fe^II couple versus the pK_a values of the phenols shows a linear relationship for all of the complexes, except for the 2,4,6‐trinitrophenolate complex. The large deviation from linearity is probably due to the change of spin for the complex. Although 2,4,6‐trinitrophenol is the weakest axial ligand in the series, its similar binding with the corresponding Fe^III? monoporphyrin only results in stabilization of the high‐spin state. The porphyrin macrocycle in the 2,4,6‐trinitrophenolate complex of diiron(III)? bisporphyrin is the most distorted, whilst the “ruffling” deformation affects the energy levels of the iron d orbitals. The larger size and weaker binding of 2,4,6‐trinitrophenol, along with heme? heme interactions in the diiron(III)? bisporphyrin, are responsible for the larger ring deformations and eventual stabilization of the pure intermediate‐spin states of the iron centers in the complex.     

Core–Shell Hollow Microspheres of Magnetic Iron Oxide@Amorphous Calcium Phosphate: Synthesis Using Adenosine 5′‐Triphosphate and Application in pH‐Responsive Drug Delivery

Drug nanocarriers with magnetic targeting and pH‐responsive drug‐release behavior are promising for applications in controlled drug delivery. Magnetic iron oxides show excellent magnetism, but their application in drug delivery is limited by low drug‐loading capacity and poor control over drug release. Herein, core–shell hollow microspheres of magnetic iron oxide@amorphous calcium phosphate (MIO@ACP) were prepared and investigated as magnetic, pH‐responsive drug nanocarriers. Hollow microspheres of magnetic iron oxide (HMIOs) were prepared by etching solid MIO microspheres in hydrochloric acid/ethanol solution. After loading a drug into the HMIOs, the drug‐loaded HMIOs were coated with a protective layer of ACP by using adenosine 5′‐triphosphate (ATP) disodium salt (Na₂ATP) as stabilizer, and drug‐loaded core–shell hollow microspheres of MIO@ACP (HMIOs/drug/ACP) were obtained. The as‐prepared HMIOs/drug/ACP drug‐delivery system exhibits superparamagnetism and pH‐responsive drug‐release behavior. In a medium with pH 7.4, drug release was slow, but it was significantly accelerated at pH 4.5 due to dissolution of the ACP shell. Docetaxel‐loaded core–shell hollow microspheres of MIO@ACP exhibited high anticancer activity.