Di-Iron(II) [2+2] Helicates of Bis-(Dipyrazolylpyridine) Ligands: The Influence of the Ligand Linker Group on Spin State Properties

Four bis[2-{pyrazol-1-yl}-6-{pyrazol-3-yl}pyridine] ligands have been synthesized, with butane-1,4-diyl (L¹ ), pyrid-2,6-diyl (L² ), benzene-1,2-dimethylenyl (L³ ) and propane-1,3-diyl (L⁴ ) linkers between the tridentate metal-binding domains. L¹ and L² form [Fe₂ (μ-L)₂ ]X₄ (X^- =BF₄ ^- or ClO₄ ^- ) helicate complexes when treated with the appropriate iron(II) precursor. Solvate crystals of [Fe₂ (μ-L¹ )₂ ][BF₄ ]₄ exhibit three different helicate conformations, which differ in the torsions of their butanediyl linker groups. The solvates exhibit gradual thermal spin-crossover, with examples of stepwise switching and partial spin-crossover to a low-temperature mixed-spin form. Salts of [Fe₂ (μ-L² )₂ ]⁴⁺ are high-spin, which reflects their highly twisted iron coordination geometry. The composition and dynamics of assembly structures formed by iron(II) with L¹ -L³ vary with the ligand linker group, by mass spectrometry and ¹ H NMR spectroscopy. Gas-phase DFT calculations imply the butanediyl linker conformation in [Fe₂ (μ-L¹ )₂ ]⁴⁺ influences its spin state properties, but show anomalies attributed to intramolecular electrostatic repulsion between the iron atoms.

Coordination Chemistry of Bulky Aminopryridinates with Main Group and Transition Metals

The coordination chemistry of bidentate aminopyridinato ligands (ApH), in particular 2-aminopyridines, is a highly popular area of research. Due to easy accessibility and versatility, 2-aminopyridines have played a prominent role as alternatives to cyclopentadienyl ligands in coordination chemistry. Easily modifiable steric bulks and the ability for fine-tuning of electronic effects have allowed researchers to control not only the metal-to-ligand stoichiometry but also the properties of their metal complexes. Previously, ligand redistribution was frequently observed for ligands of small steric demands. Bulky aminopyridinato ligands refer to ligands that incorporate alkyl-substituted phenyl groups at the amine/amido nitrogen and at the sixth position of the pyridine ring. The steric crowding allowed the stabilization of transition metals in unusually low oxidation conditions. One of the remarkable developments, for example, is the stabilization of metal-metal quintuple bonds by these ligands, thus providing a diamagnetic platform to study such systems chemically. Application of metal aminopyridinates in homogeneous catalysis has also broadened considerably in recent years. This review provides a comprehensive account of advances made with such ligands since their development for main group and transition elements.

Diarylphosphate as a New Route for Design of Highly Luminescent Ln Complexes

Organophosphate-chloride complexes [{(2,6-ⁱPr₂C₆H₃-O)₂POO}₂LnCl(CH₃OH)₄]·2CH₃OH, Ln = Nd (1), Eu (2), Gd (3), and Tb (4) have been obtained and structurally characterized. Their reaction with 2,2':6',2″-terpyridine leads to the formation of 1:1 adducts ([{(2,6-ⁱPr₂C₆H₃-O)₂POO}₂LnCl(terpy)(H₂O)₂(CH₃OH)], Ln = Eu (5), Gd (6), Tb (7) with exception of Nd, where tris-diisopropylphenylphosphate complex [{(2,6-ⁱPr₂C₆H₃-O)₂POO}₃Nd) (terpy)(H₂O)(CH₃OH)] (8) was obtained due to the ligand metathesis. A bright luminescence observed for the Eu and Tb organophosphate complexes is the first example of an application of organophosphate ligands for 4f-ions luminescence sensitization. Photophysical properties of all complexes were analyzed by optical spectroscopy and an energy transfer scheme was discussed. A combination of two types of ligands into the coordination sphere (phosphate and phenanthroline) allows designing the Eu surrounding with very high intrinsic quantum yield QEuEu (0.92) and highly luminescent Ln complexes for both visible and near-infrared (NIR) regions.

The 1,6,7,12-Tetraazaperylene Bridging Ligand as an Electron Reservoir and Its Disulfonato Derivative as Redox Mediator in an Enzyme-Electrode Process

The homodinuclear ruthenium(II) complex [{Ru(l-N₄ Me₂ )}₂ (μ-tape)](PF₆ )₄ {[1](PF₆ )₄ } (l-N₄ Me₂ =N,N'-dimethyl-2,11-diaza[3.3](2,6)-pyridinophane, tape=1,6,7,12-tetraazaperylene) can store one or two electrons in the energetically low-lying π* orbital of the bridging ligand tape. The corresponding singly and doubly reduced complexes [{Ru(l-N₄ Me₂ )}₂ (μ-tape^.- )](PF₆ )₃ {[2](PF₆ )₃ } and [{Ru(l-N₄ Me₂ )}₂ (μ-tape^2- )](PF₆ )₂ {[3](PF₆ )₂ }, respectively, were electrochemically generated, successfully isolated and fully characterized by single-crystal X-ray crystallography, spectroscopic methods and magnetic susceptibility measurements. The singly reduced complex [2](PF₆ )₃ contains the π-radical tape^.- and the doubly reduced [3](PF₆ )₂ the diamagnetic dianion tape^2- as bridging ligand, respectively. Nucleophilic aromatic substitution at the bridging tape in [1]⁴⁺ by two sulfite units gave the complex [{Ru(l-N₄ Me₂ )}₂ {μ-tape-(SO₃ )₂ }]²⁺ ([4]²⁺ ). Complex dication [4]²⁺ was exploited as a redox mediator between an anaerobic homogenous reaction solution of an enzyme system (sulfite/sulfite oxidase) and the electrode via participation of the low-energy π*-orbital of the disulfonato-substituted bridging ligand tape-(SO₃ )₂^2- (E_red1 =-0.1 V versus Ag/AgCl/1 m KCl in water).

Spin States of Homochiral and Heterochiral Isomers of [Fe(PyBox)2 ]2+ Derivatives

The following iron(II) complexes of 2,6-bis(oxazolinyl)pyridine (PyBox; L^H ) derivatives are reported: [Fe(L^H )₂ ][ClO₄ ]₂ (1); [Fe((R)-L^Me )₂ ][ClO₄ ]₂ ((R)-2; L^Me =2,6-bis{4-methyloxazolinyl}pyridine); [Fe((R)-L^Ph )₂ ][ClO₄ ]₂ ((R)-3) and [Fe((R)-L^Ph )((S)-L^Ph )][ClO₄ ]₂ ((RS)-3; L^Ph =2,6-bis{4-phenyloxazolinyl}pyridine); and [Fe((R)-L^iPr )₂ ][ClO₄ ]₂ ((R)-4) and [Fe((R)-L^iPr )((S)-L^iPr )][ClO₄ ]₂ ((RS)-4; L^iPr =2,6-bis{4-isopropyloxazolinyl}pyridine). Solid (R)-3⋅MeNO₂ exhibits an unusual very gradual, but discontinuous thermal spin-crossover with an approximate T1/2 of 350 K. The discontinuity around 240 K lies well below T1/2 , and is unconnected to a crystallographic phase change occurring at 170 K. Rather, it can be correlated with a gradual ordering of the ligand conformation as the temperature is raised. The other solid compounds either exhibit spin-crossover above room temperature (1 and (RS)-3), or remain high-spin between 5-300 K [(R)-2, (R)-4 and (RS)-4]. Homochiral (R)-3 and (R)-4 exhibit more twisted ligand conformations and coordination geometries than their heterochiral isomers, which can be attributed to steric clashes between ligand substituents [(R)-3]; or, between the isopropyl substituents of one ligand and the backbone of the other ((R)-4). In solution, (RS)-3 retains its structural integrity but (RS)-4 undergoes significant racemization through ligand redistribution by ¹ H NMR. (R)-4 and (RS)-4 remain high-spin in solution, whereas the other compounds all undergo spin-crossover equilibria. Importantly, T1/2 for (R)-3 (244 K) is 34 K lower than for (RS)-3 (278 K) in CD₃ CN, which is the first demonstration of chiral discrimination between metal ion spin states in a molecular complex.

Highly Enantioselective Iron-Catalyzed cis-Dihydroxylation of Alkenes with Hydrogen Peroxide Oxidant via an Fe(III) -OOH Reactive Intermediate

The development of environmentally benign catalysts for highly enantioselective asymmetric cis-dihydroxylation (AD) of alkenes with broad substrate scope remains a challenge. By employing [Fe(II) (L)(OTf)2 ] (L=N,N'-dimethyl-N,N'-bis(2-methyl-8-quinolyl)-cyclohexane-1,2-diamine) as a catalyst, cis-diols in up to 99.8 % ee with 85 % isolated yield have been achieved in AD of alkenes with H2 O2 as an oxidant and alkenes in a limiting amount. This "[Fe(II) (L)(OTf)2 ]+H2 O2 " method is applicable to both (E)-alkenes and terminal alkenes (24 examples >80 % ee, up to 1 g scale). Mechanistic studies, including (18) O-labeling, UV/Vis, EPR, ESI-MS analyses, and DFT calculations lend evidence for the involvement of chiral Fe(III) -OOH active species in enantioselective formation of the two C-O bonds.

Synthesis of a dimeric magnesium(I) compound by an Mg(I)/Mg(II) redox reaction

The synthesis of dimeric magnesium(I) compounds of the general type RMgMgR (R=monoanionic substituent) is still a challenging synthetic task and limited to few examples with sterically demanding ligands with delocalized CN-frameworks that all have been accessed by Na or K metal reduction of magnesium(II) halide precursors. Here we report on the synthesis of a novel diiminophosphinato magnesium(I) compound that has been synthesized by a facile redox reaction using a known magnesium(I) complex. The synthetic strategy may be applicable to other ligand systems and can help expand the class of low oxidation state magnesium complexes even if reductions with Na or K are unsuccessful. The new dimeric magnesium(I) complex has been structurally characterized and undergoes a C-C coupling reaction with tert-butylisocyanate.