Mononuclear Nonheme Iron(III)‐Iodosylarene and High‐Valent Iron‐Oxo Complexes in Olefin Epoxidation Reactions

High‐spin iron(III)‐iodosylarene complexes are highly reactive in the epoxidation of olefins, in which epoxides are formed as the major products with high stereospecificity and enantioselectivity. The reactivity of the iron(III)‐iodosylarene intermediates is much greater than that of the corresponding iron(IV)‐oxo complex in these reactions. The iron(III)‐iodosylarene species—not high‐valent iron(IV)‐oxo and iron(V)‐oxo species—are also shown to be the active oxidants in catalytic olefin epoxidation reactions. The present results are discussed in light of the long‐standing controversy on the one oxidant versus multiple oxidants hypothesis in oxidation reactions.     

Stabilization of the Low‐Spin State in a Mononuclear Iron(II) Complex and High‐Temperature Cooperative Spin Crossover Mediated by Hydrogen Bonding

The tetrapyridyl ligand bbpya (bbpya=N,N‐bis(2,2′‐bipyrid‐6‐yl)amine) and its mononuclear coordination compound [Fe(bbpya)(NCS)₂] ( 1 ) were prepared. According to magnetic susceptibility, differential scanning calorimetry fitted to Sorai’s domain model, and powder X‐ray diffraction measurements, 1 is low‐spin at room temperature, and it exhibits spin crossover (SCO) at an exceptionally high transition temperature of T_1/2=418 K. Although the SCO of compound 1 spans a temperature range of more than 150 K, it is characterized by a wide (21 K) and dissymmetric hysteresis cycle, which suggests cooperativity. The crystal structure of the LS phase of compound 1 shows strong N?H???S intermolecular H‐bonding interactions that explain, at least in part, the cooperative SCO behavior observed for complex 1 . DFT and CASPT2 calculations under vacuum demonstrate that the bbpya ligand generates a stronger ligand field around the iron(II) core than its analogue bapbpy (N,N′‐di(pyrid‐2‐yl)‐2,2′‐bipyridine‐6,6′‐diamine); this stabilizes the LS state and destabilizes the HS state in 1 compared with [Fe(bapbpy)(NCS)₂] ( 2 ). Periodic DFT calculations suggest that crystal‐packing effects are significant for compound 2 , in which they destabilize the HS state by about 1500 cm^?1. The much lower transition temperature found for the SCO of 2 compared to 1 appears to be due to the combined effects of the different ligand field strengths and crystal packing.     

Low‐Spin Pseudotetrahedral Iron(I) Sites in Fe2(μ‐S) Complexes

Fe^I centers in iron–sulfide complexes have little precedent in synthetic chemistry despite a growing interest in the possible role of unusually low valent iron in metalloenzymes that feature iron–sulfur clusters. A series of three diiron [(L₃Fe)₂(μ‐S)] complexes that were isolated and characterized in the low‐valent oxidation states Fe^II? S? Fe^II, Fe^II? S? Fe^I, and Fe^I? S? Fe^I is described. This family of iron sulfides constitutes a unique redox series comprising three nearly isostructural but electronically distinct Fe₂(μ‐S) species. Combined structural, magnetic, and spectroscopic studies provided strong evidence that the pseudotetrahedral iron centers undergo a transition to low‐spin S=1/2 states upon reduction from Fe^II to Fe^I. The possibility of accessing low‐spin, pseudotetrahedral Fe^I sites compatible with S^2? as a ligand was previously unknown.     

Catalytic Water Oxidation by Ruthenium(II) Quaterpyridine (qpy) Complexes: Evidence for Ruthenium(III) qpy‐N,N′′′‐dioxide as the Real Catalysts

Polypyridyl and related ligands have been widely used for the development of water oxidation catalysts. Supposedly these ligands are oxidation‐resistant and can stabilize high‐oxidation‐state intermediates. In this work a series of ruthenium(II) complexes [Ru(qpy)(L)₂]²⁺ (qpy=2,2′:6′,2′′:6′′,2′′′‐quaterpyridine; L=substituted pyridine) have been synthesized and found to catalyze Ce^IV‐driven water oxidation, with turnover numbers of up to 2100. However, these ruthenium complexes are found to function only as precatalysts; first, they have to be oxidized to the qpy‐N,N′′′‐dioxide (ONNO) complexes [Ru(ONNO)(L)₂]³⁺ which are the real catalysts for water oxidation.     

A Unified Treatment of the Relationship Between Ligand Substituents and Spin State in a Family of Iron(II) Complexes

The influence of ligands on the spin state of a metal ion is of central importance for bioinorganic chemistry, and the production of base‐metal catalysts for synthesis applications. Complexes derived from [Fe(bpp)₂]²⁺ (bpp=2,6‐di{pyrazol‐1‐yl}pyridine) can be high‐spin, low‐spin, or spin‐crossover (SCO) active depending on the ligand substituents. Plots of the SCO midpoint temperature (T ) in solution vs. the relevant Hammett parameter show that the low‐spin state of the complex is stabilized by electron‐withdrawing pyridyl (“X”) substituents, but also by electron‐donating pyrazolyl (“Y”) substituents. Moreover, when a subset of complexes with halogeno X or Y substituents is considered, the two sets of compounds instead show identical trends of a small reduction in T for increasing substituent electronegativity. DFT calculations reproduce these disparate trends, which arise from competing influences of pyridyl and pyrazolyl ligand substituents on Fe‐L σ and π bonding.     

On‐Surface Reactive Planarization of Pt(II) Complexes

A series of Pt(II) complexes with tetradentate luminophores has been designed, synthesized, and deposited on coinage metal surfaces with the aim to produce highly planar self‐assembled monolayers. Low‐temperature scanning tunneling microscopy (STM) and density functional theory (DFT) calculations reveal a significant initial nonplanarity for all complexes. A subsequent metal‐catalyzed separation of the nonplanar moiety at the bridging unit via the scission of a C?N bond is observed, leaving behind a largely planar core complex. The activation barrier of this bond scission process is found to depend strongly on the chemical nature of both bridging group and coordination plane, and to increase from Cu(111) through Ag(111) to Au(111).     

Gold(III) Complexes of Asymmetrically Aryl‐Substituted 1,2‐Dithiolene Ligands Featuring Potential‐Controlled Spectroscopic Properties: An Insight into the Electronic Properties of bis(Pyren‐1‐yl‐ethylene‐1,2‐dithiolato)Gold(III)

The electrochemical, UV/Vis–NIR absorption, and emission‐spectroscopic features of (TBA⁺)( 1 ⁻) and the corresponding neutral complex 1 were investigated (TBA⁺=tetrabutylammonium; 1 ⁻=[Au^III(Pyr,H‐edt)₂]⁻; Pyr,H‐edt²⁻=pyren‐1‐yl‐ethylene‐1,2‐dithiolato). The intense electrochromic NIR absorption (λ_max=1432 nm; ε=13000 M ⁻¹ cm⁻¹ in CH₂Cl₂) and the potential‐controlled visible emission in the range 400–500 nm, the energy of which depends on the charge of the complex, were interpreted on the grounds of time‐dependent DFT calculations carried out on the cis and trans isomers of 1 , 1 ⁻, and 1 ²⁻. In addition, to evaluate the nonlinear optical properties of 1 ^x− (x=0, 1), first static hyperpolarizability values β_tot were calculated (β_tot=78×10⁻³⁰ and 212×10⁻³⁰ esu for the cis isomer of 1 ⁻ and 1 , respectively) and compared to those of differently substituted [Au(Ar,H‐edt)₂]^x− gold dithiolenes [Ar=naphth‐2‐yl ( 2 ), phenyl ( 3 ); x=0, 1].     

A cis‐Divacant Octahedral and Mononuclear Iron(IV) Imide

A rare, low‐spin Fe^IV imide complex [(pyrr₂py)Fe?NAd] (pyrr₂py^2?=bis(pyrrolyl)pyridine; Ad=1‐adamantyl) confined to a cis‐divacant octahedral geometry, was prepared by reduction of N₃Ad by the Fe^II precursor [(pyrr₂py)Fe(OEt₂)]. The imide complex is low‐spin with temperature‐independent paramagnetism. In comparison to an authentic Fe^III complex, such as [(pyrr₂py)FeCl], the pyrr₂py^2? ligand is virtually redox innocent.     

Polarized Neutron Diffraction to Probe Local Magnetic Anisotropy of a Low‐Spin Fe(III) Complex

We have determined by polarized neutron diffraction (PND) the low‐temperature molecular magnetic susceptibility tensor of the anisotropic low‐spin complex PPh₄[Fe^III(Tp)(CN)₃]?H₂O. We found the existence of a pronounced molecular easy magnetization axis, almost parallel to the C₃ pseudo‐axis of the molecule, which also corresponds to a trigonal elongation direction of the octahedral coordination sphere of the Fe^III ion. The PND results are coherent with electron paramagnetic resonance (EPR) spectroscopy, magnetometry, and ab initio investigations. Through this particular example, we demonstrate the capabilities of PND to provide a unique, direct, and straightforward picture of the magnetic anisotropy and susceptibility tensors, offering a clear‐cut way to establish magneto‐structural correlations in paramagnetic molecular complexes.     

(Aminocarbene)(Divinyltetramethyldisiloxane)Iron(0) Compounds: A Class of Low‐Coordinate Iron(0) Reagents

The combined use of aminocarbene and divinyltetramethyldisiloxane (dvtms) as supporting ligands enables the access of unprecedented low‐coordinate iron(0) alkene compounds [L_nFe(η²:η²‐dvtms)] (L=N‐heterocyclic carbene (NHC) or cyclic (alkyl)(amino)carbene (CAAC), n=1 or 2) from the reactions of FeCl₂ with alkali‐metal reducing agents, free aminocarbene ligands, and dvtms. The iron(0) species deliver their {L_nFe⁰} fragments to perform redox reactions with Ph₂SiH₂, S₈, Se, and DippN₃, furnishing novel aminocarbene‐supported iron(IV) silylene, all‐ferrous iron–sulfur/selenium cubanes, and bis(imido)iron(IV) compounds. These conversions demonstrate the potential synthetic utility of the carbene‐supported iron(0) complexes as a valuable class of low‐coordinate iron(0) reagents.     

Spectrophotometric Determination of Iron(III) as Azide Complexes in Aqueous Tetrahydrofuran

Abstract

A simple, sensitive and alternative method for the spectrophotometric determination of iron(III) has been established. The procedure is based on the formation of iron-azide complexes in 60% (v/v) tetrahydrofuran/ water medium. The high sensitivity obtained in this method is due to the use of an interesting absorption band not previously reported in the literature. In the recommended conditions, absorbances for the ferric complexes are measured at 400 nm where the molar absorptivity is 1.52 × 10⁴ 1 mol^?1 cm^?1. The organic solvent used increases the sensitivity and the stability of the measurements. The precision is shown by the average deviation of about 0.3%. This system obeys Beer's law and is suitable for iron(III) determination in the concentration range from 0.6 to 3.2 mg 1^?1 (ppm). The best experimental conditions were determined studying the different factors involved. The influence of various diverse ions was also studied.     

Low‐Valent Iron(I) Amido Olefin Complexes as Promotors for Dehydrogenation Reactions

Fe^I compounds including hydrogenases show remarkable properties and reactivities. Several iron(I) complexes have been established in stoichiometric reactions as model compounds for N₂ or CO₂ activation. The development of well‐defined iron(I) complexes for catalytic transformations remains a challenge. The few examples include cross‐coupling reactions, hydrogenations of terminal olefins, and azide functionalizations. Here the syntheses and properties of bimetallic complexes [MFe^I(trop₂dae)(solv)] (M=Na, solv=3 thf; M=Li, solv=2 Et₂O; trop=5H‐dibenzo[a,d]cyclo‐hepten‐5‐yl, dae=(N‐CH₂‐CH₂‐N) with a d⁷ Fe low‐spin valence‐electron configuration are reported. Both compounds promote the dehydrogenation of N,N‐dimethylaminoborane, and the former is a precatalyst for the dehydrogenative alcoholysis of silanes. No indications for heterogeneous catalyses were found. High activities and complete conversions were observed particularly with [NaFe^I(trop₂dae)(thf)₃].     

Iron(III)‐Complexes Engaged in the Biochemical Processes in Seawater. II. Voltammetry of Fe(III)‐Malate Complexes in Model Aqueous Solution

Characteristics of iron(III) complexes with malic acid in 0.55 mol L^?1 NaCl were investigated by voltammetric techniques. Three iron(III)‐malate redox processes were detected in the pH range from 4.5 to 11: first one at ?0.11 V, second at ?0.35 V and third at ?0.60 V. First process was reversible, so stability constants of iron(III) and iron(II) complexes were calculated: log K₁(Fe^III(mal))=12.66±0.33, log β₂(Fe^III(mal)₂)=15.21±0.25, log K₁(Fe^II(mal))=2.25±0.36, and log β₂(Fe^II(mal)₂)=3.18±0.32. In the case of second and third reduction process, conditional cumulative stability constants of the involved complexes were determined using the competition method: log β(Fe(mal)₂(OH)_x)=15.28±0.10 and log β(Fe(mal)₂(OH)_y)=27.20±0.09.     

Comment on “Stabilization of Low‐Valent Iron(I) in a High‐Valent Vanadium(V) Oxide Cluster”

A recent Communication in this journal reported the stabilization of low‐valent iron(I) in a fully oxidized polyoxovanadate. With no ligand‐field argument to support such an assignment, a re‐evaluation of the data accompanied by detailed computational analysis reveals the redox chemistry is localized to the polyoxovanadate, and when reduced, instigates a spin transition at iron.     

Mononuclear Nonheme High‐Spin (S=2) versus Intermediate‐Spin (S=1) Iron(IV)–Oxo Complexes in Oxidation Reactions

Mononuclear nonheme high‐spin (S=2) iron(IV)–oxo species have been identified as the key intermediates responsible for the C?H bond activation of organic substrates in nonheme iron enzymatic reactions. Herein we report that the C?H bond activation of hydrocarbons by a synthetic mononuclear nonheme high‐spin (S=2) iron(IV)–oxo complex occurs through an oxygen non‐rebound mechanism, as previously demonstrated in the C?H bond activation by nonheme intermediate (S=1) iron(IV)–oxo complexes. We also report that C?H bond activation is preferred over C=C epoxidation in the oxidation of cyclohexene by the nonheme high‐spin (HS) and intermediate‐spin (IS) iron(IV)–oxo complexes, whereas the C=C double bond epoxidation becomes a preferred pathway in the oxidation of deuterated cyclohexene by the nonheme HS and IS iron(IV)–oxo complexes. In the epoxidation of styrene derivatives, the HS and IS iron(IV) oxo complexes are found to have similar electrophilic characters.