Synthesis, Structure and Reactivity of a Mononuclear N,N,O-Bound Fe(II) α-Keto-Acid Complex.

A bulky, tridentate phenolate ligand (ImPh2 NNOtBu ) was used to synthesise the first example of a mononuclear, facial, N,N,O-bound iron(II) benzoylformate complex, [Fe(ImPh2 NNOtBu )(BF)] (2). The X-ray crystal structure of 2 reveals that the iron centre is pentacoordinate (τ=0.5), with a vacant site located cis to the bidentate BF ligand. The Mössbauer parameters of 2 are consistent with high-spin iron(II), and are very close to those reported for α-ketoglutarate-bound non-heme iron enzyme active sites. According to NMR and UV-vis spectroscopies, the structural integrity of 2 is retained in both coordinating and non-coordinating solvents. Cyclic voltammetry studies show that the iron centre has a very low oxidation potential and is more prone to electrochemical oxidation than the redox-active phenolate ligand. Complex 2 reacts with NO to form a S=3 /2 {FeNO}7 adduct in which NO binds directly to the iron centre, according to EPR, UV-vis, IR spectroscopies and DFT analysis. Upon O2 exposure, 2 undergoes oxidative decarboxylation to form a diiron(III) benzoate complex, [Fe2 (ImPh2 NNOtBu )2 (µ2 -OBz)(µ2 -OH)2 ]+ (3). A small amount of hydroxylated ligand was also observed by ESI-MS, hinting at the formation of a high-valent iron(IV)-oxo intermediate. Initial reactivity studies show that 2 is capable of oxygen atom transfer reactivity with O2 , converting methyl(p-tolyl)sulfide to sulfoxide.

Facile electrochemical affinity measurements of small and large molecules.

A novel miniaturized sensor for electrochemical detection that contains graphene- and gold nanoparticles was functionalized with proteins. Using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) it was possible to observe and quantify interactions of molecules with these proteins. The protein binders included carbohydrate ligands as small as carbohydrates up to COVID-19 spike protein variants engaged in protein-protein interactions. The system uses off-the-shelf sensors combined with an affordable potentiostat and yet is sensitive enough for small ligand binding.

Hydrogen Evolution Electrocatalysis with a Molecular Cobalt Bis(alkylimidazole)methane Complex in DMF: a Critical Activity Analysis.

[Co(HBMIMPh2 )2 ](BF4 )2 (1) [HBMIMPh2 =bis(1-methyl-4,5-diphenyl-1H-imidazol-2-yl)methane] was investigated for its electrocatalytic hydrogen evolution performance in DMF using voltammetry and during controlled potential/current electrolysis (CPE/CCE) in a novel in-line product detection setup. Performances were benchmarked against three reported molecular cobalt hydrogen evolution reaction (HER) electrocatalysts, [Co(dmgBF2 )2 (solv)2 ] (2) (dmgBF2 =difluoroboryldimethylglyoximato), [Co(TPP)] (3) (TPP=5,10,15,20-tetraphenylporphyrinato), and [Co(bapbpy)Cl](Cl) (4) [bapbpy=6,6'-bis-(2-aminopyridyl)-2,2'-bipyridine], showing distinct performances differences with 1 being the runner up in H2 evolution during CPE and the best catalyst in terms of overpotential and Faradaic efficiency during CCE. After bulk electrolysis, for all of the complexes, a deposit on the glassy carbon electrode was observed, and post-electrolysis X-ray photoelectron spectroscopy (XPS) analysis of the deposit formed from 1 demonstrated only a minor cobalt contribution (0.23 %), mainly consisting of Co2+ . Rinse tests on the deposits derived from 1 and 2 showed that the initially observed distinct activity was (partly) preserved for the deposits. These observations indicate that the molecular design of the complexes dictates the features of the formed deposit and therewith the observed activity.

E-selective Semi-hydrogenation of Alkynes under Mild Conditions by a Diruthenium Hydride Complex.

The synthesis, characterization and catalytic activity of a new class of diruthenium hydrido carbonyl complexes bound to the tBu PNNP expanded pincer ligand is described. Reacting tBu PNNP with two equiv of RuHCl(PPh3 )3 (CO) at 140 °C produces an insoluble air-stable complex, which was structurally characterized as [Ru2 (tBu PNNP)H(µ-H)Cl(µ-Cl)(CO)2 ] (1) using solid-state NMR, IR and X-ray absorption spectroscopies and follow-up reactivity. A reaction with KOtBu results in deprotonation of a methylene linker to produce [Ru2 (tBu PNNP* )H(µ-H)(µ-OtBu)(CO)2 ] (3) featuring a partially dearomatized naphthyridine core. This enables metal-ligand cooperative activation of H2 analogous to the mononuclear analogue, [Ru(tBu PNP*)H(CO)]. In contrast to the mononuclear system, the bimetallic analogue 3 catalyzes the E-selective semi-hydrogenation of alkynes at ambient temperature and atmospheric H2 pressure with good functional group tolerance. Monitoring the semi-hydrogenation of diphenylacetylene by 1 H NMR spectroscopy shows the intermediacy of Z-stilbene, which is subsequently isomerized to the E-isomer. Initial findings into the mode of action of this system are provided, including the spectroscopic characterization of a polyhydride intermediate and the isolation of a deactivated species with a partially hydrogenated naphthyridine backbone.

Combining metal-metal cooperativity, metal-ligand cooperativity and chemical non-innocence in diiron carbonyl complexes.

Several metalloenzymes, including [FeFe]-hydrogenase, employ cofactors wherein multiple metal atoms work together with surrounding ligands that mediate heterolytic and concerted proton-electron transfer (CPET) bond activation steps. Herein, we report a new dinucleating PNNP expanded pincer ligand, which can bind two low-valent iron atoms in close proximity to enable metal-metal cooperativity (MMC). In addition, reversible partial dearomatization of the ligand's naphthyridine core enables both heterolytic metal-ligand cooperativity (MLC) and chemical non-innocence through CPET steps. Thermochemical and computational studies show how a change in ligand binding mode can lower the bond dissociation free energy of ligand C(sp3)-H bonds by ∼25 kcal mol-1. H-atom abstraction enabled trapping of an unstable intermediate, which undergoes facile loss of two carbonyl ligands to form an unusual paramagnetic (S = ) complex containing a mixed-valent iron(0)-iron(i) core bound within a partially dearomatized PNNP ligand. Finally, cyclic voltammetry experiments showed that these diiron complexes show catalytic activity for the electrochemical hydrogen evolution reaction. This work presents the first example of a ligand system that enables MMC, heterolytic MLC and chemical non-innocence, thereby providing important insights and opportunities for the development of bimetallic systems that exploit these features to enable new (catalytic) reactivity.

Structurally Modelling the 2-His-1-Carboxylate Facial Triad with a Bulky N,N,O Phenolate Ligand.

We present the synthesis and coordination chemistry of a bulky, tripodal N,N,O ligand, ImPh2 NNOtBu (L), designed to model the 2-His-1-carboxylate facial triad (2H1C) by means of two imidazole groups and an anionic 2,4-di-tert-butyl-subtituted phenolate. Reacting K-L with MCl2 (M = Fe, Zn) affords the isostructural, tetrahedral non-heme complexes [Fe(L)(Cl)] (1) and [Zn(L)(Cl)] (2) in high yield. The tridentate N,N,O ligand coordination observed in their X-ray crystal structures remains intact and well-defined in MeCN and CH2 Cl2 solution. Reacting 2 with NaSPh affords a tetrahedral zinc thiolate complex, [Zn(L)(SPh)] (4), that is relevant to isopenicillin N synthase (IPNS) biomimicry. Cyclic voltammetry studies demonstrate the ligand's redox non-innocence, where phenolate oxidation is the first electrochemical response observed in K-L, 2 and 4. However, the first electrochemical oxidation in 1 is iron-centred, the assignment of which is supported by DFT calculations. Overall, ImPh2 NNOtBu provides access to well-defined mononuclear, monoligated, N,N,O-bound metal complexes, enabling more accurate structural modelling of the 2H1C to be achieved.

Nickel is a Different Pickle: Trends in Water Oxidation Catalysis for Molecular Nickel Complexes.

The development of novel water oxidation catalysts is important in the context of renewable fuels production. Ligand design is one of the key tools to improve the activity and stability of molecular catalysts. The establishment of ligand design rules can facilitate the development of improved molecular catalysts. In this paper it is shown that chemical oxidants can be used to probe oxygen evolution activity for nickel-based systems, and trends are reported that can improve future ligand design. Interestingly, different ligand effects were observed in comparison to other first-row transition metal complexes. For example, nickel complexes with secondary amine donors were more active than with tertiary amine donors, which is the opposite for iron complexes. The incorporation of imine donor groups in a cyclam ligand resulted in the fastest and most durable nickel catalyst of our series, achieving oxygen evolution turnover numbers up to 380 and turnover frequencies up to 68 min-1 in a pH 5.0 acetate buffer using Oxone as oxidant. Initial kinetic experiments with this catalyst revealed a first order in chemical oxidant and a half order in catalyst. This implies a rate-determining oxidation step from a dimeric species that needs to break up to generate the active catalyst. These findings lay the foundation for the rational design of molecular nickel catalysts for water oxidation and highlight that catalyst design rules are not generally applicable for different metals.

Bioinspired Non-Heme Iron Complexes: The Evolution of Facial N, N, O Ligand Design.

Iron-containing metalloenzymes that contain the 2-His-1-Carboxylate facial triad at their active site are well known for their ability to activate molecular oxygen and catalyse a broad range of oxidative transformations. Many of these reactions are synthetically challenging, and developing small molecular iron-based catalysts that can achieve similar reactivity and selectivity remains a long-standing goal in homogeneous catalysis. This review focuses on the development of bioinspired facial N,N,O ligands that model the 2-His-1-Carboxylate facial triad to a greater degree of structural accuracy than many of the polydentate N-donor ligands commonly used in this field. By developing robust, well-defined N,N,O facial ligands, an increased understanding could be gained of the factors governing enzymatic reactivity and selectivity.

On the Ability of Nickel Complexes Derived from Tripodal Aminopyridine Ligands to Catalyze Arene Hydroxylations.

The development of catalysts for the selective hydroxylation of aromatic C-H bonds is an essential challenge in current chemical research. The accomplishment of this goal requires the discovery of powerful metal-based oxidizing species capable of hydroxylating inert aromatic bonds in a selective manner, avoiding the generation of non-selective oxygen-centered radicals. Herein we show an investigation on the ability of nickel(ii) complexes supported by tripodal tetradentate aminopyridine ligands to catalyze the direct hydroxylation of benzene to phenol with H2O2 as oxidant. We have found that modifications on the ligand structure of the nickel complex do not translate into different reactivity, which differs from previous findings for nickel-based arene hydroxylations. Besides, several nickel(ii) salts have been found to be effective in the oxidation of aromatic C-H bonds. The use of fluorinated alcohols as solvent has been found to result in an increase in phenol yield; however, showing no more than two turn-overs per nickel. These findings raise questions on the nature of the oxidizing species responsible for the arene hydroxylation reaction.

Electrocatalytic Proton Reduction by a Cobalt Complex Containing a Proton-Responsive Bis(alkylimdazole)methane Ligand: Involvement of a C-H Bond in H2 Formation.

Homogeneous electrocatalytic proton reduction is reported using cobalt complex [1](BF4 )2 . This complex comprises two bis(1-methyl-4,5-diphenyl-1H-imidazol-2-yl)methane (HBMIM Ph 2 ) ligands that contain an acidic methylene moiety in their backbone. Upon reduction of [1](BF4 )2 by either electrochemical or chemical means, one of its HBMIM Ph 2 ligands undergoes deprotonation under the formation of dihydrogen. Addition of a mild proton source (acetic acid) to deprotonated complex [2](BF4 ) regenerates protonated complex [1](BF4 )2 . In presence of acetic acid in acetonitrile solvent [1](BF4 )2 shows electrocatalytic proton reduction with a kobs of ≈200 s-1 at an overpotential of 590 mV. Mechanistic investigations supported by DFT (BP86) suggest that dihydrogen formation takes place in an intramolecular fashion through the participation of a methylene C-H bond of the HBMIM Ph 2 ligand and a CoII -H bond through formal heterolytic splitting of the latter. These findings are of interest to the development of responsive ligands for molecular (base)metal (electro)catalysis.

Enantioselective C-H Lactonization of Unactivated Methylenes Directed by Carboxylic Acids.

The formidable challenges of controlling site-selectivity, enantioselectivity, and product chemoselectivity make asymmetric C-H oxidation a generally unsolved problem for nonenzymatic systems. Discrimination between the two enantiotopic C-H bonds of an unactivated methylenic group is particularly demanding and so far unprecedented, given the similarity between their environments and the facile overoxidation of the initially formed hydroxylation product. Here we show that a Mn-catalyzed C-H oxidation directed by carboxylic acids can overcome these challenges to yield γ-lactones in high enantiomeric excess (up to 99%) using hydrogen peroxide as oxidant and a Brønsted acid additive under mild conditions and short reaction times. Coordination of the carboxylic acid group to the bulky Mn complex ensures the rigidity needed for high enantioselectivity and dictates the outstanding γ site-selectivity. When the substrate contains nonequivalent γ-methylenes, the site-selectivity for lactonization can be rationally predicted on the basis of simple C-H activation/deactivation effects exerted by proximal substituents. In addition, discrimination of diastereotopic C-H bonds can be modulated by catalyst design, with no erosion of enantiomeric excess. The potential of this reaction is illustrated in the concise synthesis of a tetrahydroxylated bicyclo[3.3.1]nonane enabled by two key, sequential γ-C-H lactonizations, with the latter that fixes the chirality of five stereogenic centers in one step with 96% ee.

Crystal structure of tetra-kis-(tetra-hydro-furan-κO)bis-(tri-fluoro-methane-sulfonato-κO)iron(II).

The title compound, [Fe(CF3SO3)2(C4H8O)4], is octa-hedral with two tri-fluoro-methane-sulfonate ligands in trans positions and four tetra-hydro-furane mol-ecules in the equatorial plane. By the conformation of the ligands the complex is chiral in the crystal packing. The compound crystallizes in the Sohncke space group P212121 and is enanti-omerically pure. The packing of the mol-ecules is determined by weak C-H⋯O hydrogen bonds. The crystal studied was refined as a two-component inversion twin.

Mononuclear iron(ii) complexes containing a tripodal and macrocyclic nitrogen ligand: synthesis, reactivity and application in cyclohexane oxidation catalysis.

Two novel tripodal ligands L1 and L2 based on a tris(methylpyridyl)amine (TPA) motif have been prepared and reacted with two different iron(ii) salts. The ligand L1 contains a bis(amino-phenyl)-TPA group whereas the macrocyclic ligand L2 displays two different coordinating cores, namely TPA and pyridine-dicarboxamide. The resulting mononuclear complexes 1-4 have been characterized in the solid state and in solution by spectroscopic and electrochemical methods. All complexes are high spin and mainly pentacoordinated. X-ray diffraction analyses of the crystals of complexes 2 and 3 demonstrate that the coordination sphere of the iron(ii) centre adopts either a distorted bipyramidal-trigonal or square pyramidal geometry. In the absence of an exogenous substrate, oxidation of complex 2 by H2O2 induces an intramolecular aromatic hydroxylation, as shown by the X-ray structure of the resulting dinuclear complex 2'. Catalytic studies in the presence of a substrate (cyclohexane) show that the reaction process is strongly impacted by the macrocyclic topology of the ligand and the nature of the counter-ion.

1,2-Addition of Diethylzinc to a Bis(Imidazolyl)ketone Ligand.

In this study, the selective 1,2-addition of diethylzinc to the ketone functionality of BMdiPhIK [bis(1-methyl-4,5-diphenylimidazolyl)ketone] is shown. The reaction product is isolated in a dimeric form with a planar Zn2(µ-O)2-motif keeping the two monomers together. This compound can serve as a model for reactive intermediates in the catalytic alkylation of ketones with diorganozinc reagents. Hydrolysis of this binuclear zinc compound leads to isolation of the C-alkylated product in 89 % yield. A reaction pathway is proposed in which BMdiPhIK initially coordinates to diethylzinc as a bidentate bis(nitrogen) ligand. This is followed by the homolytic cleavage of the Zn-Et bond and in-cage recombination of the Et-radical and the Zn-coordinated ligand-centered radical, which is mainly localized on the carbonyl moiety of the ligand.

A Cptt-Based Trioxo-Rhenium Catalyst for the Deoxydehydration of Diols and Polyols.

Trioxo-rhenium complexes are well known catalysts for the deoxydehydration (DODH) of vicinal diols (glycols). In this work, we report on the DODH of diols and biomass-derived polyols using CpttReO3 as a new catalyst (Cptt=1,3-di-tert-butylcyclopentadienyl). The DODH reaction was optimized using 2 mol % of CpttReO3 and 3-octanol as both reductant and solvent. The CpttReO3 catalyst exhibits an excellent activity for biomass-derived polyols. Specifically, glycerol is almost quantitatively converted to allyl alcohol and mucic acid gives 75 % of muconates at 91 % conversion. In addition, the loading of CpttReO3 can be reduced to 0.1 mol % to achieve a turn-over number as high as 900 per Re when using glycerol as substrate. Examination of DODH reaction profiles by NMR spectroscopy indicates that catalysis is related to Cp-ligand release, which raises questions on the nature of the actual catalyst.

Periodic Trends in the Binding of a Phosphine-Tethered Ketone Ligand to Fe, Co, Ni, and Cu.

π-Coordinating ligands are commonly found in intermediate structures in homogeneous catalysis, and are gaining interest as supporting ligands for the development of cooperative catalysts. Herein, we systematically investigate the binding of the ketone group, a strongly accepting π ligand, to mid-to-late metals of the first transition series. To this end, the coordination of 2,2'-bis(diphenylphosphino)benzophenone (Ph dpbp), which features a ketone moiety flanked by two strongly binding P-donor groups, to Fe, Co, Ni, and Cu was explored. The ketone moiety does not bind to the metal in MII complexes, whereas MI complexes (Fe, Co, Ni) adopt an η2 (C,O) coordination. A structural and computational investigation of periodic trends in this series was performed. These data suggest that the coordination of the ketone to MI can mostly be described by the resonance extremes of the Dewar-Chatt-Duncanson model, that is, the π complex and the metallaoxacycle extreme, with a possible minor contribution from a ketyl radical resonance structure in the case of the iron complex.

Noninnocent ß-Diiminate Ligands: Redox Activity of a Bis(alkylimidazole)methane Ligand in Cobalt and Zinc Complexes.

A new ß-diiminate ligand (the bis(1-methyl-4,5-diphenyl-1H-imidazol-2-yl)methane anion, BMIMPh2- ) is introduced, in which the ligand framework bears an extended imidazole-based π-system in conjugation with a formal ß-diketiminates (NacNac) backbone. Bis-ligated transition metal complexes (Co, Zn) featuring this anionic ligand undergo a series of four consecutive single-electron oxidations that are all ligand-based. The singly and doubly oxidized complexes can be synthesized on a preparative scale and have been fully characterized by various spectroscopic techniques. This is in sharp contrast to the corresponding NacNac-based complexes in which only singly oxidized complexes were isolated and characterized. Single crystal X-ray structure determination revealed a correlation between the intra-ligand metrical parameters and the oxidation state of BMIMPh2- . These structural changes in the ligand framework make BMIMPh2- as a perceptible non-innocent ligand in contrast to NacNac type ligands.

Combination of Scanning Probe Microscopy and Coordination Chemistry: Structural and Electronic Study of Bis(methylbenzimidazolyl)ketone and Its Iron Complex.

Here, we report the bulk synthesis of [FeII(BMBIK)Cl2] bearing the redox noninnocent bis(methylbenzimidazolyl)ketone (BMBIK) ligand and the synthesis of the similar complex [FeI(BMBIK)]+ on a Au(111) surface using lateral manipulation at the atomic level. Cyclic voltammetry and scanning tunneling spectroscopy are shown to be useful techniques to compare the coordination compound in solution with the one on the surface. The total charge, as well as the oxidation and spin state of [FeI(BMBIK)]+, are investigated by comparison of the shape of the lowest unoccupied molecular orbital (LUMO), visualized by tunneling through the LUMO, with theoretical models. The similar reduction potentials found for the solution and surface compounds indicate that the major effect of lowering the LUMO upon coordination of BMBIK to the iron center is conserved on the surface. The synthesis and analysis of [FeI(BMBIK)]+ using scanning tunneling microscopy, scanning tunneling spectroscopy, and atomic force microscopy are the first steps toward mechanistic studies of homogeneous catalysts with redox noninnocent ligands at the single molecule level.

Aryl Radical Geometry Determines Nanographene Formation on Au(111).

The Ullmann coupling has been used extensively as a synthetic tool for the formation of C-C bonds on surfaces. Thus far, most syntheses made use of aryl bromides or aryl iodides. We investigated the applicability of an aryl chloride in the bottom-up assembly of graphene nanoribbons. Specifically, the reactions of 10,10'-dichloro-9,9'-bianthryl (DCBA) on Au(111) were studied. Using atomic resolution non-contact AFM, the structure of various coupling products and intermediates were resolved, allowing us to reveal the important role of the geometry of the intermediate aryl radicals in the formation mechanism. For the aryl chloride, cyclodehydrogenation occurs before dehalogenation and polymerization. Due to their geometry, the planar bisanthene radicals display a different coupling behavior compared to the staggered bianthryl radicals formed when aryl bromides are used. This results in oligo- and polybisanthenes with predominantly fluoranthene-type connections.

The B(C6F5)3-Catalyzed Tandem Meinwald Rearrangement-Reductive Amination.

A system of three coupled catalytic cycles enabling the one-pot transformation of epoxides to amines via Meinwald rearrangement, imine condensation, and imine reduction is described. This assisted tandem catalysis is catalyzed by B(C6F5)3 resulting in the first tandem Meinwald rearrangement-reductive amination protocol. The reaction proceeds in nondried solvents and yields ß-functionalized amines. In particular, ß-diarylamines are obtained in high yields.