Electrocatalysis of the Oxygen Reduction Reaction by Copper Complexes with Tetradentate Tripodal Ligands

The tetradentate tripodal ligand scaffold is capable of supporting the expected geometries of the copper ion during the oxygen reduction reaction (ORR) catalysis. As such, we probed the reactivity of copper complexes with these types of ligands by electronically and structurally tweaking the tris(pyridin 2-ylmethyl)amine (tmpa) scaffold by progressively replacing the terminal pyridines with carboxylate donors. This work shows that systems with one carboxylato donor (bpg = bis(pyridin-2-ylmethyl)glycine), (bpp = (3-(bis(pyridin-2-ylmethyl)amino)propanoic acid)) are active in electrocatalyzing the homogeneous ORR under circumneutral aqueous conditions. Turnover frequencies in the range from 105 to 106 s-1, on par with that for Cu-tmpa under identical conditions, were obtained. It is noteworthy that the CuII/CuI redox potentials for the Cu-bpg, Cu-bpp, and Cu-tmpa systems in phosphate-buffered water (pH 7, under Ar) are similar at -0.409, -0.375, and -0.401 V vs Ag/AgCl, respectively. This is rationalized by the influence of the Lewis acidity of the copper ions on the water coligand. Corroborating this are pKa values for [Cu(tmpa)(H2O)]2+, Cu(bpg)(H2O)]+, and [Cu(bpp)(H2O)]+ of 6.6, 8.8, and 10.2, respectively. Thus, the overall charge of the solution species for all three complexes will be +1 at pH 7 and this will be an important determinant for the redox potentials and, in turn, the catalytic overpotentials, which are also similar. A cis carboxylato donor offers H-bonding possibilities for exogenous resting state water and intermediate hydroperoxo coligands. This is reflected by the higher pKa values for Cu-bpp and Cu-bpg compared with that for Cu-tmpa, with the Cu-bpp system furnishing the least strained H-bonding.

A Click Chemistry-Based Artificial Metallo-Nuclease

Artificial metallo-nucleases (AMNs) are promising DNA damaging drug candidates. Here, we demonstrate how the 1,2,3-triazole linker produced by the Cu-catalysed azide-alkyne cycloaddition (CuAAC) reaction can be directed to build Cu-binding AMN scaffolds. We selected biologically inert reaction partners tris(azidomethyl)mesitylene and ethynyl-thiophene to develop TC-Thio, a bioactive C3 -symmetric ligand in which three thiophene-triazole moieties are positioned around a central mesitylene core. The ligand was characterised by X-ray crystallography and forms multinuclear CuII and CuI complexes identified by mass spectrometry and rationalised by density functional theory (DFT). Upon Cu coordination, CuII -TC-Thio becomes a potent DNA binding and cleaving agent. Mechanistic studies reveal DNA recognition occurs exclusively at the minor groove with subsequent oxidative damage promoted through a superoxide- and peroxide-dependent pathway. Single molecule imaging of DNA isolated from peripheral blood mononuclear cells shows that the complex has comparable activity to the clinical drug temozolomide, causing DNA damage that is recognised by a combination of base excision repair (BER) enzymes.

Sulfonamido-Pincer Complexes of Cu(II) and the Electrocatalysis of O2 Reduction

Heteroleptic copper complexes of an asymmetrical pincer ligand containing a central anionic sulfonamide donor (pyridine-2-yl-sulfonyl)(quinolin-8-yl)-amide (psq), which contains a central anionic sulfonamido donor have been prepared. Meridional κ3-N,N″,N‴ binding with the co-ligands acetate, chloride, or acetonitrile (MeCN), trans to the central sulfonamido N-donor, is revealed by the X-ray crystal structures of [Cu(OAc)(psq)(H2O)], [CuCl(psq)]2, and [Cu(psq)(MeCN)](PF6). Either overall distorted square pyramidal or octahedral geometries of the copper atom are satisfied by coordinated water in the case of the acetate complex or interactions with periphery sulfonamido oxygen atoms on adjacent molecules in the dimeric chloride and 1D polymeric acetonitrile complexes. The cyclic voltammogram (CV) of [Cu(OAc)(psq)(H2O)] shows a quasi-reversible CuII/CuI reduction at -0.930 V (vs Fc+/Fc0, MeCN), and an irreversible CuII/CuI reduction for [Cu(psq)(MeCN)](PF6) is seen at -0.838 V. This signal is split into two quasi-reversible redox processes on the addition of 2,2,2-trifluoroethanol (TFE). This suggests that TFE pushes a solution equilibrium toward a dimeric acetate complex analogous to [CuCl(psq)]2, which shows two quasi-reversible waves at -0.666 V and -0.904 V vs Fc+/Fc0 consistent with its dimeric solid-state structure. A comparison of the CVs of [Cu(OAc)(psq)(H2O)] under either a N2 or an O2 atmosphere revealed that this complex catalyzes turnover electro-reduction of O2 to H2O2 and H2O. The rate of reaction increases on addition of a weak organic acid, and a coulombic efficiency of 48% for H2O2 was determined by iodometric titration. We propose that a CuI complex formed on electroreduction binds O2 to yield an intermediate superoxide complex. On electron and proton transfer to this species, a bifurcated route back to the O2-activating CuI complex is feasible with either release of H2O2 or O-O cleavage resulting in the liberation of H2O. The CuI complex is regenerated by subsequent reduction and protonation to close the cycle.

Structure Activity Relationships for Reversible O2 Chemisorption by the Solid Phases of Co(salen) and Co(3F-salen)

The potential of solid-state materials comprising Co(salen) units for concentrating dioxygen from air was recognized over 80 years ago. While the chemisorptive mechanism at the molecular level is largely understood, the bulk crystalline phase plays important, yet unidentified roles. We have reverse crystal-engineered these materials and can for the first time describe the nanostructuring requisite for achieving reversible O₂ chemisorption by Co(3R-salen) R = H or F, the simplest and most effective of the many known derivatives of Co(salen). Of the six phases of Co(salen) identified, α-ζ: α = ESACIO, β = VEXLIU, γ, δ, ε, and ζ (this work), only γ, δ, ε, and ζ are capable of reversible O₂ binding. Class I materials (phases γ, δ, and ε) are obtained by desorption (40-80 °C, atmospheric pressure) of the co-crystallized solvent from Co(salen)·(solv), solv = CHCl₃, CH₂Cl₂, or 1.5 C₆H₆. The oxy forms comprise between 1:5 and 1:3 O₂:[Co] stoichiometries. Class II materials achieve an apparent maximum of 1:2 O₂:Co(salen) stoichiometries. The precursors for the Class II materials comprise [Co(3R-salen)(L)·(H₂O)_x], R = H, L = pyridine, and x = 0; R = F, L = H₂O, and x = 0; R = F, L = pyridine, and x = 0; R = F, L = piperidine, and x = 1. Activation of these depends on the desorption of the apical ligand (L) that templates channels through the crystalline compounds with the Co(3R-salen) molecules interlocked in a Flemish bond brick pattern. The 3F-salen system produces F-lined channels proposed to facilitate O₂ transport through the materials through repulsive interactions with the guest O₂. We postulate that a moisture dependence of the activity of the Co(3F-salen) series is due to a highly specific binding pocket for locking in water via bifurcated hydrogen bonding to the two coordinated phenolato O atoms and the two ortho F atoms.

Probing Noncovalent Interactions in [3,3]Metaparacyclophanes

Arene-arene interactions are fundamentally important in molecular recognition. To precisely probe arene-arene interactions in cyclophanes, we designed and synthesized (2,6-phenol)paracyclophanes and (2,6-aniline)paracyclophanes that possess two aromatic rings in close proximity. Fine-tuning the aromatic character of one aromatic ring by fluorine substituents enables investigations on the intramolecular interactions between the electron-rich phenol and aniline with tetra-H- and tetra-F-substituted benzene. pK_a measurements revealed that the tetra-F-template increases the acidity of the phenol (ΔpK_a = 0.55). X-ray crystallography and computational analyses demonstrated that all [3,3]metaparacyclophanes adopt cofacial parallel conformations, implying the presence of π-π stacking interactions. Advanced quantum chemical analyses furthermore revealed that both electrostatic interactions and orbital interactions provide the key contribution to the structure and stability of [3,3]metaparacyclophanes.

Facile transmetallation of [SbIII(DOTA)]− renders it unsuitable for medical applications

The antimony(iii) complex of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate (DOTA) has been prepared and its exceptionally low stability observed. The Sb(iii) ion in Na[Sb(DOTA)]·4H₂O shows an approximately square antiprismatic coordination geometry that is close to superimposable to the Bi(iii) geometry in [Bi(DOTA)]⁻ in two phases containing this anion, Na[Bi(DOTA)]·4H₂O, [H₃O][Bi(DOTA)]·H₂O for which structures are also described. Interestingly, DOTA itself in [(H₆DOTA)]Cl₂·4H₂O·DMSO shows the same orientation of the N₄O₄ metal binding cavity reflecting the limited flexibility of DOTA in an octadentate coordination mode. In 8-coordinate complexes it can however accommodate M(iii) ions with r_ion spanning a relatively wide range from 87 pm (Sc(iii)) to 117 pm (Bi(iii)). The larger Bi³⁺ ion appears to be the best metal–ligand size match since [Bi(DOTA)]⁻ is associated with greater complex stability. In the solution state, [Sb(DOTA)]⁻ is extremely susceptible to transmetallation by trivalent ions (Sc(iii), Y(iii), Bi(iii)) and, significantly, even by biologically important divalent metal ions (Mg(ii), Ca(ii), Zn(ii)). In all cases just one equivalent is enough to displace most of the Sb(iii). [Sb(DOTA)]⁻ is resistant to hydrolysis; however, since biologically more abundant metal ions easily substitute the antimony, DOTA complexes will not be suitable for deployment for the delivery of the, so far unexploited, theranostic isotope pair ¹¹⁹Sb and ¹¹⁷Sb.

The antimony(iii) complex of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate (DOTA) has been prepared and its exceptionally low stability observed.

Probing the Lewis Acidity of Boronic Acids through Interactions with Arene Substituents

Boronic acids are Lewis acids that exist in equilibrium with boronate forms in aqueous solution. Here we experimentally and computationally investigated the Lewis acidity of 2,6‐diarylphenylboronic acids; specially designed phenylboronic acids that possess two flanking aromatic rings with tunable aromatic character. Hammett analysis of 2,6‐diarylphenylboronic acids reveals that their Lewis acidity remains unchanged upon the introduction of EWG/EDG at the distant para position of the flanking aromatic rings. Structural and computational studies demonstrate that polar‐π interactions and solvation effects contribute to the stabilization of boronic acids and boronate forms by aromatic rings. Our physical‐organic chemistry work highlights that boronic acids and boronates can be stabilized by aromatic systems, leading to an important molecular knowledge for rational design and development of boronic acid‐based catalysts and inhibitors of biomedically important proteins.

Cooperative Co-Activation of Water and Hypochlorite by a Non-Heme Diiron(III) Complex

Aqueous solutions of the iron(III) complex of N,N,N'-tris(2-pyridylmethyl)ethylenediamine-N'-acetate (tpena) react with hypochlorite (ClO^-) to produce the reactive high-valent [Fe^IV(O)(tpena)]⁺. Under catalytic conditions, in bicarbonate-buffered media (pH 8) with a set ionic strength (10 mM NaCl), kinetic analysis shows that two equivalents of [Fe^IV(O)(tpena)]⁺ per one ClO^- are produced, with benign chloride ions the only byproduct. An unprecedented supramolecular activation of ClO^- by {(HCO₃)⊂[(tpena)Fe^III(μ-O)Fe^III(Htpena)]}²⁺ is proposed. This mode of activation has great advantage for use in the catalytic oxidation of C-H bonds in water since: (i) the catalyst scaffold is protected from oxidative degradation and (ii) undesirable radical side reactions which produce toxic chlorinated compounds are circumvented by this novel coactivation of water and ClO^-. The unique activation mechanism by the Fe-tpena system makes possible the destruction of organic contaminants as an add-on technology to water disinfection by chlorination, demonstrated here through (i) the catalytic oxidation of micropollutant metaldehyde, and (ii) mineralization of the model substrate formate. The resting-state speciation at pH 3, 5, 7, and 9, as well as the catalytically active iron speciation are characterized with Mössbauer and EPR spectroscopy and supported by DFT calculations. Our study provides fundamentally new insights into the design and activation mode of iron-based catalysts relevant to applications in water remediation.

Preparation of organocobalt(iii) complexes via O2 activation

The coupling of selective C-H activation with O₂ activation is an important goal for organic synthesis. New experimental and computational results, along with the results from experimental work accumulated over many decades, now unequivocally link O₂ activation with C-H activation by the classic Co(salen) complexes. A common holistic mechanistic framework can rationalise the formation of ostensibly diverse peroxo, superoxo, organo and alkoxide complexes of Co^III(salen). DFT calculations show that cobalt(iii)superoxo, dicobalt(iii)peroxo and cobalt(iii)hydroperoxo complexes are all viable intermediates as participants in hydrogen atom transfer reactions, whereas a Co(iv)oxo intermediate is unlikely. The reaction conditions will determine the pathway followed and all pathways are initiated through the initial formation of a superoxo complex, Co^III(salen)(O₂˙)(MeOH) (EPR: g = 2.025, A = 19 G). Organo and alkoxide ligands are derived from solvent media and the trends in reactivity reveal that combination of the pK_a and BDE of the C-H of the respective solvent substrates are important. These data explain why landmark, structurally characterized, μ₂-η¹,η²-peroxide and η¹-superoxide Co(salen)-O₂ adducts were predominantly isolated from solvents with high C-H pK_a values (DMSO, DMF, DMA).

Do Sulfonamides Interact with Aromatic Rings?

Aromatic rings form energetically favorable interactions with many polar groups in chemical and biological systems. Recent molecular studies have shown that sulfonamides can chelate metal ions and form hydrogen bonds, however, it is presently not established whether the polar sulfonamide functionality also interacts with aromatic rings. Here, synthetic, spectroscopic, structural, and quantum chemical analyses on 2,6-diarylbenzenesulfonamides are reported, in which two flanking aromatic rings are positioned close to the central sulfonamide moiety. Fine-tuning the aromatic character by substituents on the flanking rings leads to linear trends in acidity and proton affinity of sulfonamides. This physical-organic chemistry study demonstrates that aromatic rings have a capacity to stabilize sulfonamides via through-space NH-π interactions. These results have implications in rational drug design targeting electron-rich aromatic rings in proteins.

Engineering the Oxidative Potency of Non-Heme Iron(IV) Oxo Complexes in Water for C-H Oxidation by a cis Donor and Variation of the Second Coordination Sphere

A series of iron(IV) oxo complexes, which differ in the donor (CH₂py or CH₂COO^-) cis to the oxo group, three with hemilabile pendant donor/second coordination sphere base/acid arms (pyH/py or ROH), have been prepared in water at pH 2 and 7. The ν_Fe═O values of 832 ± 2 cm^-1 indicate similar Fe^IV═O bond strengths; however, different reactivities toward C-H substrates in water are observed. HAT occurs at rates that differ by 1 order of magnitude with nonclassical KIEs (k_H/k_D = 30-66) consistent with hydrogen atom tunneling. Higher KIEs correlate with faster reaction rates as well as a greater thermodynamic stability of the iron(III) resting states. A doubling in rate from pH 7 to pH 2 for substrate C-H oxidation by the most potent complex, that with a cis-carboxylate donor, [Fe^IVO(Htpena)]²⁺, is observed. Supramolecular assistance by the first and second coordination spheres in activating the substrate is proposed. The lifetime of this complex in the absence of a C-H substrate is the shortest (at pH 2, 3 h vs up to 1.3 days for the most stable complex), implying that slow water oxidation is a competing background reaction. The iron(IV)═O complex bearing an alcohol moiety in the second coordination sphere displays significantly shorter lifetimes due to a competing selective intramolecular oxidation of the ligand.

Photosynthesis of a Dihydroimidazopyridine Chelate Shines Light on the Reactions of a Photoactivated Iron(III) Complex with O2

The high-spin (S = ⁵/₂) meridional diastereoisomer of [Fe^III(tpena)]²⁺ (tpena = N,N,N'-tris(2-pyridylmethyl)ethylendiamine-N'-acetate), mer-[Fe(tpena)]²⁺, undergoes photolytic CO₂ release to produce an iron(II) intermediate of a radical dihydroimidazopyridine ligand (L^•). The structure of this unprecedented transient iron(II)(L^•) complex is supported by UV-vis and Mössbauer spectroscopies, DFT calculations, as well as the X-ray structural characterization of an μ-oxo iron(III) complex of the oxidized derivative of L^•, namely, [Fe^III₂O(Cl)₂(L⁺)₂](ClO₄)₄(MeCN)₂ (L⁺ = 2-(2-(bis(pyridin-2-ylmethyl)amino)ethyl)-2,3-dihydro-1H-imidazo[1,5-a]pyridin-4-ium). [Fe^III₂O(Cl)₂(L⁺)₂]⁴⁺ is obtained only in the absence of O₂. Under aerobic conditions, O₂ will intercept the iron(II)(L^•) complex to form a putative Fe(III)-alkylperoxide complex which cascades to an iron(II) complex of SBPy3 (SBPy3 = N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-2-aldimine). Thus, through different oxidative pathways, the unknown ligand L⁺ or SBPy3 forms by loss of a one-carbon-atom or a two-carbon-atom unit, respectively, from the glycyl arm of tpena. Acceleration of the photodecarboxylation step is achieved by addition of thiocyanate because of transient formation of a more photoreactive NCS^- adduct of [Fe(tpena)]²⁺. This has allowed for kinetic observation of the reaction of [Fe^II(L^•)]²⁺ with O₂ which is, unexpectedly, promoted also by light. We propose that this corresponds to the energy needed for the conversion of the ring-closed radical ligand L^• to a ring-opened tautomer to allow for O₂ insertion between the C and Fe atoms of the iron(II) complex.

Probing Halogen-π versus CH-π Interactions in Molecular Balance

Molecular balances based on the dibenzobicyclo[3.2.2]nonane template enable probing of the competition between halogen-π and CH-π interactions. Structural, NMR spectroscopic, and computational analyses revealed that the π system can favorably interact both with C-X or C-H functionalities, depending on the size of the functional group.

Through-Space Polar-π Interactions in 2,6-Diarylthiophenols

The front cover artwork is provided by Marijn Maas from the group of Prof. Jasmin Mecinović (University of Southern Denmark). The image shows the stabilization of thiols by aromatic rings, as a result of energetically favorable SH-π interactions in a designed small molecule and in proteins. Read the full text of the Article at 10.1002/cphc.202000132.

Metal-Organic Frameworks with Hexakis(4-carboxyphenyl)benzene: Extensions to Reticular Chemistry and Introducing Foldable Nets

Nine metal-organic frameworks have been prepared with the hexagon-shaped linker 1,2,3,4,5,6-hexakis(4-carboxyphenyl)benzene (H6cpb) by solvothermal reactions in dimethylformamide (dmf) or dimethylacetamide (dmac) with acetic acid or formic acid as modulators: [Bi2(cpb)(acetato)2(dmf)2]·2dmf CTH-6 forms a rtl-net; 2(H2NMe2)[Cu2(cpb)] CTH-7 forms a kgd-net; [Fe4(cpb)(acetato)2(dmf)4] CTH-8 and [Co4(cpb)(acetato)2(dmf)4] CTH-9 are isostructural and form yav-nets; 2(HNEt3)[Fe2(cpb)] CTH-10 and the two polymorphs of 2(H2NMe2)[Zn2(cpb)]·1.5dmac, Zn-MOF-888 and CTH-11, show kgd-nets; [Cu2(cpb)(acetato)2(dmf)2]·2dmf, CTH-12, forms a mixed coordination and hydrogen-bonded sql-net; and 2(H2NMe2)[Zn2(cpb)] CTH-13, a similarly mixed yav-net. Surface area values (Brunauer-Emmett-Teller, BET) range from 34 m2 g-1 for CTH-12 to 303 m2 g-1 for CTH-9 for samples activated at 120 °C in dynamic vacuum. All compounds show normal (10-fold higher) molar CO2 versus N2 uptake at 298 K, except the 19-fold CO2 uptake for CTH-12 containing Cu(II) dinuclear paddle-wheels. We also show how perfect hexagons and triangles can combine to a new 3D topology laf, a model of which gave us the idea of foldable network topologies, as the laf-net can fold into a 2D form while retaining the local geometry around each vertex. Other foldable nets identified are cds, cds-a, ths, sqc163, clh, jem, and tfc covering the basic polygons and their combinations. The impact of this concept on "breathing" MOFs is discussed. I2 sorption, both from gas phase and from MeOH solution, into CTH-7 were studied by time of flight secondary ion mass spectrometry (ToF-SIMS) on dried crystals. I2 was shown to have penetrated the crystals, as layers were consecutively peeled off by the ion beam. We suggest ToF-SIMS to be a method for studying sorption depth profiles of MOFs.

Through-Space Polar-π Interactions in 2,6-Diarylthiophenols

Molecular recognition between polar groups and aromatic molecules is fundamentally important to rational drug design. Although it has been well established that many polar functionalities interact with electron-rich aromatic residues through energetically favorable polar-π interactions, there is a limited understanding of the association between thiols and aromatic systems. Herein we report physical-organic chemistry studies on 2,6-diarylthiophenols that possess the central thiophenol ring and two flanking aromatic rings with tunable electronic properties caused by substituents at distant para position. Hammett analysis revealed that pK_a values and proton affinities correlate well with Hammett sigma values of substituents. Additional energy decomposition analysis supported the conclusion that both through-space SH-π interactions and S^- -π interactions contribute to intramolecular stabilization of 2,6-diarylthiophenols.

Structure-Activity Relationship Studies of Tetrahydroquinolone Free Fatty Acid Receptor 3 Modulators

Free fatty acid receptor 3 (FFA3, previously GPR41) is activated by short-chain fatty acids, mediates health effects of the gut microbiota, and is a therapeutic target for metabolic and inflammatory diseases. The shortage of well-characterized tool compounds has however impeded progress. Herein, we report structure-activity relationship of an allosteric modulator series and characterization of physicochemical and pharmacokinetic properties of selected compounds, including previous and new tools. Two representatives, 57 (TUG-1907) and 63 (TUG-2015), showed improved solubility and preserved potency. Of these, 57, with EC50 = 145 nM and a solubility of 33 μM, showed high clearance in vivo but is a preferred tool in vitro. In contrast, 63, with EC50 = 162 nM and a solubility of 9 μM, showed lower clearance and seems better suited for in vivo studies. Using 57, we demonstrate for the first time that FFA3 activation leads to calcium mobilization in murine dorsal root ganglia.

Redox- and EPR-Active Graphene Diiron Complex Nanocomposite

A mixed valence diiron(II/III) complex with the ligand 2,6-bis{bis[(2-pyridinylmethyl)amino]methyl}phenol (bppH) has been covalently anchored onto graphene using a mild in situ microwave-assisted diazonium coupling through an aryl amino precursor and isoamyl nitrite. A dinuclear iron complex is then formed by complexation of the grafted bppH-graphene material with iron(II) in the presence of dioxygen. X-ray photoelectron spectroscopy (XPS), atomic force microscopy, cyclic voltammetry, scanning transmission electron microscopy, energy-dispersive X-ray spectroscopy, and electron paramagnetic resonance (EPR) spectroscopy confirm the formation of the anchored ligand and derivative diiron complexes. Semiquantitative XPS analysis shows an average bppH ligand bulk loading of 0.33 mmol/g, corresponding to a significant 20.7 wt % of the functionalized material consisting of grafted moieties. EPR measurements reveal the existence of a strong isotropic S = 1/2 spin center associated with the graphene lattice, together with a much weaker S = 5/2 signal, associated with the iron(III) center of the grafted complex. The grafted complex is redox-active with surface-confined Fe^IIFe^II → Fe^IIFe^III (+0.56 V vs NHE), Fe^IIFe^III → Fe^IIIFe^III (+0.73 V), and Fe^IIIFe^III → Fe^IIIFe^IV (+0.95 V) redox processes accessible, with an estimated surface coverage of 58 pmol cm^-2 established from the electrochemical measurements.

NO sorption, in-crystal nitrite and nitrate production with arylamine oxidation in gas-solid single crystal to single crystal reactions

A bridging nitrite and a nitrate counter anion per Co2 site are generated in-crystal and an arylamine group on the ligand scaffold is oxidised to a nitro group when nitric oxide (NO) is chemisorbed by molecular crystals of cobalt complexes.

Reversible and Vapochromic Chemisorption of Ammonia by a Copper(II) Coordination Polymer

The single crystal X-ray structure determination of {[Cu(tpt)(o-phthalate)]·31/3(C2H2Cl4)}n (tpt=2,4,6-tri-4-pyridyl-1,3,5-triazine, C2H2Cl4=1,1,2,2-tetrachloroethane=TCE) shows a 3D network in which CuII centres are linked by 3-connecting tpt ligands with the topology of a 12,3 net. CuII centres are further linked by o-phthalate dianions. The copper coordination geometry is square pyramidal, with o-phthalate oxygen donors trans to each other in the basal plane and the remaining positions taken by the pyridines of three linking tpt units. The solvent accessible void space is ~65%. The pale blue-green crystalline desolvate, obtained by heating to 200°C or washing the TCE solvate with acetone is formulated as [Cu(tpt)(o-phthalate)]n. Powder X-ray diffraction and electron paramagnetic resonance spectroscopy show that the crystal structure and the CuII geometry changes upon desolvation. The crystalline desolvated phase sorbs two equivalents of ammonia per copper ion. The adduct, mauve [Cu(tpt)(o-phthalate)(NH3)2]n, shows reasonable crystallinity and is stable up to ~150°C under ambient conditions before the reversible desorption (minimum 10 cycles) of the guest ammonia. The colour change and high desorption temperature, along with changes in g values, is suggestive of chemisorption in two steps with Cu–ammine bonding in the loaded phase.

Photoinduced O2-Dependent Stepwise Oxidative Deglycination of a Nonheme Iron(III) Complex

The iron(III) complex [Fe(tpena)]²⁺ (tpena = N, N, N'-tris(2-pyridylmethyl)ethylendiamine- N'-acetate) undergoes irreversible O₂-dependent N-demethylcarboxylation to afford [Fe^II(SBPy3)(MeCN)]²⁺ (SBPy3 = N, N-bis(2-pyridylmethyl)amine- N-ethyl-2-pyridine-2-aldimine), when irradiated with near-UV light. The loss of a mass equivalent to the glycyl group in a process involving consecutive C-C and C-N cleavages is documented by the measurement of the sequential production of CO₂ and formaldehyde, respectively. Time-resolved UV-vis absorption, Mössbauer, EPR, and Raman spectroscopy have allowed the spectroscopic characterization of two iron-based intermediates along the pathway. The first of these, proposed to be a low-spin iron(II)-radical ligand complex, reacts with O₂ in the rate-determining step to produce a putative alkylperoxide complex. DFT calculations suggest that this evolves into an Fe(IV)-oxo species, which can abstract a hydrogen atom from a cis methylene group of the ligand to give the second spectroscopically identified intermediate, a high-spin iron(III)-hydroxide of the product oxidized ligand, [Fe^III(OH)(SBPy₃)]²⁺. Reduction and exchange of the cohydroxo/water ligand produces the crystallographically characterized products [Fe^II(SBPy3)(X)]^2+/3+, X = MeCN, [Zn(tpena)]⁺.

Tetrameric and polymeric silver complexes of the omeprazole scaffold; synthesis, structure, in vitro and in vivo antimicrobial activities and DNA interaction

Two complexes [AgI(pmtbH)]₄ (1) and {[Ag₄(pmtbH)₄(NO₃)₄·2X}_n (2) (where pmtbH is 2-[(2-pyridinylmethyl)thio]-1H-benzimidazole and X is H₂O or MeOH) were synthesised and structurally characterised. Complex 2 showed therapeutic potential against Candida albicans, Escherichia coli, Staphylococcus aureus and Pseudomonas aeruginosa but complex 1 did not show significant activity in vitro. Further in vivo studies using larvae of the insect Galleria mellonella indicated that complex 2 significantly stimulates the immune system and that pre-treatment with the complex offers appreciable protection against all three bacteria. Real-time flow cytometry data support the observed antimicrobial profile of complex 2 and suggest the antimicrobial response may be linked to a form of bacterial programmed cell death (PCD). Complex 2 was found to interact with DNA in the bacterial and fungal cells but it did not cleave plasmid DNA isolated from the three bacteria.

Electrochemically Generated cis-Carboxylato-Coordinated Iron(IV) Oxo Acid-Base Congeners as Promiscuous Oxidants of Water Pollutants

The nonheme iron(IV) oxo complex [Fe^IV(O)(tpenaH)]²⁺ and its conjugate base [Fe^IV(O)(tpena)]⁺ [tpena^- = N,N,N'-tris(2-pyridylmethyl)ethylenediamine-N'-acetate] have been prepared electrochemically in water by bulk electrolysis of solutions prepared from [Fe^III₂(μ-O)(tpenaH)₂](ClO₄)₄ at potentials over 1.3 V (vs NHE) using inexpensive and commercially available carbon-based electrodes. Once generated, these iron(IV) oxo complexes persist at room temperature for minutes to half an hour over a wide range of pH values. They are capable of rapidly decomposing aliphatic and aromatic alcohols, alkanes, formic acid, phenols, and the xanthene dye rhodamine B. The oxidation of formic acid to carbon dioxide demonstrates the capacity for total mineralization of organic compounds. A radical hydrogen-atom-abstraction mechanism is proposed with a reactivity profile for the series that is reminiscent of oxidations by the hydroxyl radical. Facile regeneration of [Fe^IV(O)(tpenaH)]²⁺/ [Fe^IV(O)(tpena)]⁺ and catalytic turnover in the oxidation of cyclohexanol under continuous electrolysis demonstrates the potential of the application of [Fe^III(tpena)]²⁺ as an electrocatalyst. The promiscuity of the electrochemically generated iron(IV) oxo complexes, in terms of the broad range of substrates examined, represents an important step toward the goal of cost-effective electrocatalytic water purification.

Assignment of solid-state 13C and 1H NMR spectra of paramagnetic Ni(II) acetylacetonate complexes aided by first-principles computations

Recent advances in computational methodology allowed for first-principles calculations of the nuclear shielding tensor for a series of paramagnetic nickel(II) acetylacetonate complexes, [Ni(acac)₂L₂] with L = H₂O, D₂O, NH₃, ND₃, and PMe₂Ph have provided detailed insight into the origin of the paramagnetic contributions to the total shift tensor. This was employed for the assignment of the solid-state ^1,2H and ¹³C MAS NMR spectra of these compounds. The two major contributions to the isotropic shifts are by orbital (diamagnetic-like) and contact mechanism. The orbital shielding, contact, as well as dipolar terms all contribute to the anisotropic component. The calculations suggest reassignment of the ¹³C methyl and carbonyl resonances in the acac ligand [Inorg. Chem.53, 2014, 399] leading to isotropic paramagnetic shifts of δ(¹³C) ≈ 800-1100 ppm and ≈180-300 ppm for ¹³C for the methyl and carbonyl carbons located three and two bonds away from the paramagnetic Ni(II) ion, respectively. Assignment using three different empirical correlations, i.e., paramagnetic shifts, shift anisotropy, and relaxation (T₁) were ambiguous, however the latter two support the computational results. Thus, solid-state NMR spectroscopy in combination with modern quantum-chemical calculations of paramagnetic shifts constitutes a promising tool for structural investigations of metal complexes and materials.

Noncovalent Halogen Bonding as a Mechanism for Gas-Phase Clustering

Gas-phase clustering of nonionizable iodylbenzene (PhIO₂) is attributed to supramolecular halogen bonding. Electrospray ionization results in the formation of ions of proton-charged and preferably sodium-charged clusters assignable to [H(PhIO₂) _n ]⁺, n = 1-7; [Na(PhIO₂) _n ]⁺, n = 1-6; [Na₂(PhIO₂) _n ]²⁺, n = 7-20; [HNa(PhIO₂) _n ]²⁺, n = 6-19; [HNa₂(PhIO₂) _n ]³⁺, n = 15-30; and [Na₃(PhIO₂) _n ]³⁺, n = 14-30. The largest cluster detected has a supramolecular mass of 7147 Da. Electronic structure calculations using the M06-2X functional with the 6-311++G(d,p) basis set for C, H, and O, and LANL2DZ basis set for I and Na predict 298 K binding enthalpies for the protonated and sodiated iodylbenzene dimers and trimers are greater than 180 kJ/mol. This is exceptionally high in comparison with other protonated and sodiated clusters with well-established binding enthalpies. Strongly halogen-bonded motifs found in the crystalline phases of PhIO₂ and its derivatives serve as models for the structures of larger gas-phase clusters, and calculations on simple model gas-phase dimer and trimer clusters result in similar motifs. This is the first account of halogen bonding playing an extensive role in gas-phase associations. Graphical Abstract ᅟ.

Evidence of molybdenum association with particulate organic matter under sulfidic conditions

The geochemical behavior of molybdenum (Mo) in the oceans is closely linked to the presence of sulfide species in anoxic environments, where Fe availability may play a key role in the Mo scavenging. Here, we show that Mo(VI) is reduced in the presence of particulate organic matter (represented by sulfate-reducing bacteria). Molybdenum was immobilized at the surface of both living cells and dead/lysed cells, but not in cell-free control experiments. Experiments were carried out at four different Mo concentrations (0.1 to 2 mm) to yield cell-associated Mo precipitates with little or no Fe, consisting of mainly Mo(IV)-sulfide compounds with molecular structures similar to Mo enzymes and to those found in natural euxinic sediments. Therefore, we propose that Mo removal in natural sulfidic waters can proceed via a non-Fe-assisted pathway that requires particulate organic matter (dead or living sulfate-reducing bacteria). This pathway has implications for global marine Mo cycling and the current use of Mo-based proxies for paleo-environmental investigations.

Copper-promoted methylene C–H oxidation to a ketone derivative by O2

The oxime-dipyridyl ligand on a copper complex is slowly oxygenated at the benzylic C–H bond in air.

A coordinatively flexible hexadentate ligand gives structurally isomeric complexesM2(L)X3(M= Cu, Zn;X= Br, Cl)

Polypyridyl multidentate ligands based on ethylenediamine backbones are important metal-binding agents with applications in biomimetics and homogeneous catalysis. The seemingly hexadentate tpena ligand [systematic name:N,N,N′-tris(pyridin-2-ylmethyl)ethylenediamine-N′-acetate] reacts with zinc chloride and zinc bromide to form trichlorido[μ-N,N,N′-tris(pyridin-2-ylmethyl)ethylenediamine-N′-acetato]dizinc(II), [Zn2(C22H24N5O2)Cl3], and tribromido[μ-N,N,N′-tris(pyridin-2-ylmethyl)ethylenediamine-N′-acetato]dizinc(II), [Zn2Br3(C22H24N5O2)]. One ZnIIion shows the anticipated N5O coordination in an irregular six-coordinate site and is linked by ananticarboxylate bridge to a tetrahedral ZnX3(X= Cl or Br) unit. In contrast, the CuIIions in aquatribromido[μ-N,N,N′-tris(pyridin-2-ylmethyl)ethylenediamine-N′-acetato]dicopper(II)–tribromido[μ-N,N,N′-tris(pyridin-2-ylmethyl)ethylenediamine-N′-acetato]dicopper(II)–water (1/1/6.5) [Cu2Br3(C22H24N5O2)][Cu2Br3(C22H24N5O2)(H2O)]·6.5H2O, occupy two tpena-chelated sites, one a trigonal bipyramidal N3Cl2site and the other a square-planar N2OCl site. In all three cases, electrospray ionization mass spectra were dominated by a misleading ion assignable to [M(tpena)]+(M= Zn2+and Cu2+).

Reduction of hypervalent iodine by coordination to iron(iii) and the crystal structures of PhIO and PhIO2

Iodine L₃-edge XANES spectra using reference compounds with formal iodine oxidation states spanning −1 to +7 show that iodine in an Fe(iii) coordinated iodosylbenzene (PhIO) is reduced compared to parent hypervalent PhIO which has been structurally characterized.

Reversible Guest Binding in a Non-Porous Fe(II) Coordination Polymer Host Toggles Spin Crossover

Formation of either a dimetallic compound or a 1 D coordination polymer of adiponitrile adducts of [Fe(bpte)](2+) (bpte=[1,2-bis(pyridin-2-ylmethyl)thio]ethane) can be controlled by the choice of counteranion. The iron(II) atoms of the bis(adiponitrile)-bridged dimeric complex [Fe2 (bpte)2 (μ2 -(NC(CH2 )4 CN)2 ](SbF6 )4 (2) are low spin at room temperature, as are those in the polymeric adiponitrile-linked acetone solvate polymer {[Fe(bpte)(μ2 -NC(CH2 )4 CN)](BPh4 )2 ⋅Me2 CO} (3⋅Me2 CO). On heating 3⋅Me2 CO to 80 °C, the acetone is abruptly removed with an accompanying purple to dull lavender colour change corresponding to a conversion to a high-spin compound. Cooling reveals that the desolvate 3 shows hysteretic and abrupt spin crossover (SCO) S=0↔S=2 behaviour centred at 205 K. Non-porous 3 can reversibly absorb one equivalent of acetone per iron centre to regenerate the same crystalline phase of 3⋅Me2 CO concurrently reinstating a low-spin state.