Labile Coordination Sites Research Articles

Aliphatic C[sbnd]H azidation by Mn based mimics of α-ketoglutarate dependent enzymes

α-Ketoglutarate dependent nonheme halogenases catalyze regioselective functionalization of aliphatic CH bonds via H abstraction by the ferryl site of the [X−FeIV=O] intermediate, followed by radical rebound to an adjacent halogen ligand X at the same Fe center. Herein we report biomimetic nonheme Mn complexes, possessing two labile cis-α coordination sites and capable of catalyzing the enantioselective azidation of C(sp3)−H groups. Proper ligand environment of the active sites imposes steric constraints on HO−Mn rebound, thus accounting for preferential C-radical rebound to the adjacent N3−Mn ligand. Such mechanism of selectivity control is analogous to that operating in the prototypical α-ketoglutarate-dependent nonheme iron halogenases.

Secondary Bracing Ligands Drive Heteroleptic Cuboctahedral PdII12 Cage Formation

The structural complexity of self-assembled metal-organic capsules can be increased by incorporating two or more different ligands into a single discrete product. Such complexity can be useful, by enabling larger, less-symmetrical, or more guests to be bound. Here we describe a rational design strategy for the use of subcomponent self-assembly to selectively prepare a heteroleptic cage with a large cavity volume (2631 Å3) from simple, commercially available starting materials. Our strategy involves the initial isolation of a tris(iminopyridyl) PdII3 complex 1, which reacts with tris(pyridyl)triazine ligand 2 to form a heteroleptic sandwich-like architecture 3. The tris(iminopyridyl) ligand within 3 serves as a "brace" to control the orientations of the labile coordination sites on the PdII centers. Self-assembly of 3 with additional 2 was thus directed to generate a large PdII12 heteroleptic cuboctahedron host. This new cuboctahedron was observed to bind multiple polycyclic aromatic hydrocarbon guests simultaneously.

Well‐defined Nickel P3C Complexes as Hydrogenation Catalysts of N‐Heteroarenes Under Mild Conditions

AbstractThe reactivity of metal complexes can be primarily controlled by their coordinating ligands. Electronic effects and geometric restrictions imposed by ligands, prototypically modify their reactivity, but are not always straightforward accessible. In the course of the study, we explored new C3‐symmetric organometallic nickel (II) complexes ((P3C)Ni)) with the particularity of having a metalated apical carbon trans to a labile coordination site readily available as a catalytic site. The synthetic methodology allowed to tune the electronic properties by introducing substituents on the phosphines. We studied the nature of the C−Ni bond by X‐ray absorption spectroscopy (XAS) and DFT and applied them to the catalytic hydrogenation of quinolines with a good chemical scope and group tolerance. 1H‐NMR monitoring, isotopic labelling, and computational studies are consistent with a heterolytic activation of the dihydrogen molecule after forming a σ‐H2 adduct trans to the metalated carbon.

Tuning Receptor Properties of Metal-Amyloid Beta Complexes. Studies on the Interaction between Ni(II)-Aβ5-9 and Phosphates/Nucleotides.

Amyloid-beta (Aβ) peptides, potentially relevant in the pathology of Alzheimer’s disease, possess distinctive coordination properties, enabling an effective binding of transition-metal ions, with a preference for Cu(II). In this work, we found that a N-truncated Aβ analogue bearing a His-2 motif, Aβ5–9, forms a stable Ni(II) high-spin octahedral complex at a physiological pH of 7.4 with labile coordination sites and facilitates ternary interactions with phosphates and nucleotides. As the pH increased above 9, a spin transition from a high-spin to a low-spin square-planar Ni(II) complex was observed. Employing electrochemical techniques, we showed that interactions between the binary Ni(II)–Aβ5–9 complex and phosphate species result in significant changes in the Ni(II) oxidation signal. Thus, the Ni(II)–Aβ5–9 complex could potentially serve as a receptor in electrochemical biosensors for phosphate species. The obtained results could also be important for nickel toxicology.

Molecular bixbyite-like In12-oxo clusters with tunable functionalization sites for lithography patterning applications.

Indium oxides have been widely applied in many technological areas, but their utilization in lithography has not been developed. Herein, we illustrated a family of unprecedented In12-oxo clusters with a general formula [In12(μ4-O)4(μ2-OH)2(OCH2CH2NHCH2CH2O)8(OR)4X4]X2 (where X = Cl or Br; R = CH3, C6H4NO2 or C6H4F), which not only present the largest size record in the family of indium-oxo clusters (InOCs), but also feature the first molecular model of bixbyite-type In2O3. Moreover, through the labile coordination sites of the robust diethanolamine-stabilized In12-oxo core, these InOCs can be accurately functionalized with different halides and alcohol or phenol derivatives, producing tunable solubility. Based on the high solution stability as confirmed by ESI-MS analysis, homogeneous films can be fabricated using these In12-oxo clusters by the spin-coating method, which can be further used for electron beam lithography (EBL) patterning studies. Accordingly, the above structural regulations have significantly influenced their corresponding film quality and patterning performance, with bromide or p-nitrophenol functionalized In12-oxo clusters displaying better performance of sub-50 nm lines. Thus, the here developed bixbyite-type In12-oxo cluster starts the research on indium-based patterning materials and provides a new platform for future lithography radiation mechanism studies.

Tuning the electronic properties of tetradentate iron-NHC complexes: Towards stable and selective epoxidation catalysts

Two sets of bio-inspired non-heme iron complexes, each comprising of an FeII and FeIII species, bearing 16-membered macrocyclic tetradentate N-heterocyclic carbene ligands are reported. The complexes exhibit trans labile coordination sites, are characterized by means of NMR, ESI-MS, elemental analysis, SC-XRD and cyclic voltammetry and assessed as olefin epoxidation (pre-)catalysts applying H2O2 as oxidant. Sc(OTf)3 and AcOH are evaluated as Lewis and Brønsted acidic additives, respectively, resulting in partially noticeable improvement in catalytic performance. Hereby, complex 2b shows high stability (TON = 1000 at 20 °C), high temperature tolerance and advances in the more challenging epoxidation of terminal and functionalized olefins. Furthermore, in-depth DFT calculations are conducted to put the catalysts’ structural and electronic features into relation with the catalytic results.

Mixed tetradentate NHC/1,2,3-triazole iron complexes bearing cis labile coordination sites as highly active catalysts in Lewis and Brønsted acid mediated olefin epoxidation

Two bio-inspired non-heme iron complexes bearing mixed tetradentate N-heterocyclic carbene/1,2,3-triazole ligands with cis labile coordination sites are reported. The compounds are studied in olefin epoxidation catalysis using H2O2 as oxidant. Sc(OTf)3, HClO4 and HOAc are applied as additives resulting in significant improvement of catalytic performance. Under optimized conditions the most active catalyst exhibits activities of 76,000 turnovers per hour, which is the highest reported value for an iron(II) catalyst. The complexes reveal comparably high stability and enable the challenging epoxidation of functionalized olefins. These results prove 1,2,3-triazoles to be promising and tunable ligands for iron catalyzed oxidation reactions.

Synthesis and Binding Properties of a Tren-Capped Hexahomotrioxacalix[3]arene.

The straightforward synthesis of a new hexahomotrioxacalix[3]arene-based ligand capped by a tren subunit was developed and the binding properties of the corresponding zinc complex were explored by NMR spectroscopy. Similarly to the closely related calix[6]tren-based systems, the homooxacalixarene core ensures the mononuclearity of the zinc complex and the metal center displays a labile coordination site for exogenous guests. However, very different host-guest properties were observed: i) in CDCl3 , the zinc complex strongly binds a water molecule and is reluctant to recognize other neutral guests, ii) in CD3 CN, the exo-coordination of anions prevails. Thus, in strong contrast to the calix[6]tren-based systems, the coordination of neutral guests that thread through the small rim and fill the polyaromatic cavity was not observed. This unique behaviour is likely due to the fact that the 18-membered ethereal macrocycle is too small to let a molecule threading through it. This work illustrates the key role played by the second coordination sphere in the binding properties of metal complexes.

Sn13-Oxo Clusters with an Open Hollow Structural Motif and Decorated by Different Functional Ligands.

The first open hollow Sn13-oxo cluster family has been successfully prepared and characterized. These Sn13 clusters contain a {Sn7} moiety that is similar to the basic structure unit of rutile SnO2. Interestingly, the Sn13 clusters show labile coordination sites on the edge, which could be functionalized by different ligands. With the different decorated types of functionalized ligands, the open hollow Sn13 clusters present different structural details and framework diameters. The presented results provide a new open hollow structural motif of tin-oxo clusters and also a good platform for their ligand functionalization.

Crystal structures and supramolecular architectures of ONO donor hydrazone and solvent exchangeable dioxidomolybdenum(VI) complexes derived from 3,5-diiodosalicyaldehyde-4-methoxybenzoylhydrazone: Hirshfeld surface analysis and interaction energy calculations

A tridentate ONO donor aroylhydrazone, 3,5-diiodosalicyaldehyde-4-methoxybenzoylhydrazone (H2DSMB), and two of its cis-MoO2 complexes, [MoO2(DSMB)(H2O)](DMF)2 (1) and [MoO2(DSMB)(py)] (2), have been synthesized and characterized by different physico-chemical methods. The molecular structures were determined by single crystal X-ray diffraction studies. The aroylhydrazone H2DSMB crystallizes in a monoclinic space group with two identical molecules in the asymmetric unit. The complex [MoO2(DSMB)(H2O)](DMF)2 (1) crystallizes in a triclinic space group whereas the other complex, [MoO2(DSMB)(py)] (2), crystallizes in an orthorhombic crystal system. The distorted octahedral geometry around the Mo(VI) central atom is satisfied by the di-deprotonated dianionic hydrazone moiety and two oxido oxygen atoms, the sixth labile coordination site being occupied by oxygen/nitrogen atoms from solvent molecules. The importance of van der Waals interactions in the formation of the elementary structure obtained from the single crystal X-ray diffraction was authenticated by Hirshfeld surface analysis. The individual types of intermolecular contacts and their impact on the crystal packing was investigated by the analysis of the Hirshfeld surfaces and the signatures on the two-dimensional fingerprint plots. Interaction energy calculations reveal that the dispersion energy framework is dominant over other energy frameworks. The thermal behavior and the nature of the water molecule in the complex [MoO2(DSMB)(H2O)](DMF)2 (1) is discussed. Furthermore, in vitro cytotoxicity studies of the synthesized aroylhydrazone and its Mo(VI) complexes against the lymphoma ascites cell line are also included.

Bpytrisalen/Bpytrisaloph: A Triangular Platform That Spatially Arranges Different Multiple Labile Coordination Sites.

Complexes that possess multiple metals in their proximity exert useful functions such as multivalent capture and catalytic activities. However, it is often challenging to arrange multiple metals with regular distances and to maintain the reactive coordination sites. We have now designed and synthesized organic macrocyclic ligands, bpytrisalen and bpytrisaloph, as platforms to spatially arrange different kinds of metals and their coordination sites. Metals coordinating to the bpy N2 units of the triangular ligands are assembled in the cavity with their labile coordination sites directed inward. Meanwhile, the metals at the salen/saloph N2O2 units have axial coordination sites that are vertically pointing out of the triangle. As a result, planar homo- and heterohexanuclear complexes with different kinds of coordination sites are obtained. Furthermore, a double-decker structure was selectively constructed upon coordinating a ditopic linker ligand to the axial coordination sites of the salen units, which shows the orthogonality of the coordination sites at the bpy and salen units. The consistency in the array of metal centers and their coordination sites irrespective of the elements is a desirable feature in the pursuit of applications for the multinuclear complexes.

Biomimetic catalytic activity and structural diversity of cobalt complexes with N3O-donor Schiff base ligand

This report describes the syntheses and structural characterizations of four new cobalt(III) compounds (1–4) derived from a tetradentate Schiff base ligand and their catalytic oxidation of different o-aminophenols in aerobic condition. X-ray crystal structural studies reveal that both pseudohalide ions and solvents used for the syntheses have significant influence on the structural diversity in these systems. All these compounds show the diverse catalytic activity towards the aerial oxidation of different o-aminophenols. As expected, the substitutionally labile coordination sites available for substrate binding is responsible for significantly higher catalytic activity of 1 and 2 then others. Literature reports reveal that although significant attention has been made to the catalytic oxidation of some substituted o-aminophenols in terms of the isolations and characterizations of their oxidised products with the native enzyme and its models, but the detailed kinetic studies were not available in the literature. Herein, we have examined the detailed kinetic studies of the aerobic oxidation of two substituted o-aminophenols, namely 2-amino-5-methylphenol and 2-amino-4-methylphenol, using 1 and 2 as catalysts. ESI mass spectrometry provides significant information to get insight into the mechanistic details and further disclose that the methyl substation in aromatic ring does not inhibit formation of the complex-substrate aggregate but it resulted the phenoxazine chromophore instead of phenoxazinone chormophore.

Metal Coordination to Unsymmetric 1,n′‐Disubstituted Ferrocene Histidine Peptides

In this study, the coordination chemistry of unsymmetrical 1,n′‐substituted ferrocene (Fc) peptide conjugates is explored, where the peptide substituents are two different histidine‐containing peptides. The resulting Fc‐conjugates serve as ligands for the formation of metal complexes, with the intention to generate complexes that possess a labile coordination site. The following Fc[CO‐His(Trt)‐OMe][COOMe] (1), Fc[CO‐His(Trt)‐Gly‐OMe][CO‐Gly‐OMe] (2), Fc[CO‐His(Trt)‐His(Trt)‐OMe][CO‐His(Trt)‐OMe] (3), and Fc[CO‐His(Trt)‐Gly‐His(Trt)‐OMe][CO‐Gly‐His(Trt)‐OMe] (4) were synthesized and characterized. Each compound forms discrete complexes with metal divalent metal ions Zn2+, Co2+ and Cd2+, where a single metal ion is complexed by the metallo‐ligand, as was confirmed spectroscopically. The redox properties of the Fc group are influenced by metal coordination to the peptide substituents and anodic shifts occur upon the addition of metal ions. These follow the order Zn2+ > Cd2+ > Co2+. NMR experiments indicated imidazole rings are the site of metal coordination. Fc‐peptide conjugates display a switchable axial chirality that is dependent on metal complexation.

Tetrahedral Copper(II) Complexes with a Labile Coordination Site Supported by a Tris-tetramethylguanidinato Ligand.

A new tridentate N3 ligand (TMG3tach) consisting of cis,cis-1,3,5-triaminocyclohexane (tach) and three N,N,N',N'-tetramethylguanidino (TMG) groups has been developed to prepare copper complexes with a tetrahedral geometry and a labile coordination site. Treatment of the ligand with CuIIX2 (X = Cl and Br) gave copper(II)-halide complexes, [CuII(TMG3tach)Cl]+ (2Cl) and [CuII(TMG3tach)Br]+ (2Br), the structures of which have been determined by X-ray crystallographic analysis. The complexes exhibit a four-coordinate structure with C3v symmetry, where the labile halide ligand (X) occupies a position on the trigonal axis. 2Br was converted to a methoxido-copper(II) complex [CuII(TMG3tach)(OMe)](OTf) (2OMe), also having a similar four-coordinate geometry, by treating it with an equimolar amount of tetrabutylammonium hydroxide in methanol. The methoxido-complex 2OMe was further converted to the corresponding phenolato-copper(II) (2OAr) and thiophenolato-copper(II) (2SAr) complexes by ligand exchange reactions with the neutral phenol and thiophenol derivatives, respectively. The electronic structures of the copper(II) complexes with different axial ligands are discussed on the basis of EPR spectroscopy and DFT calculations.

Molecular recognition by multiple metal coordination inside wavy-stacked macrocycles

Most biological and synthetic receptors for small organic molecules employ a combination of relatively weak intermolecular interactions such as hydrogen bonds. A host compound that utilizes stronger yet reversible bonding in a synergistic manner could realize precise recognition, but the regulation and spatial arrangement of such reactive interaction moieties have been a challenge. Here, we show a multinuclear zinc complex synthesized from a macrocyclic ligand hexapap, which inwardly arranges labile metal coordination sites for external molecules. The metallomacrocycle forms a unique wavy-stacked structure upon binding a suitable length of dicarboxylic acids via multipoint coordination bonding. The saddle-shaped deformation and dimerization realize the differentiation of the interaction moieties, and change of guest-binding modes at specific metal coordination sites among the many present have been achieved utilizing acid/base as external stimuli.

Bandgap Engineering of Titanium–Oxo Clusters: Labile Surface Sites Used for Ligand Substitution and Metal Incorporation

AbstractThrough the labile coordination sites of a robust phosphonate‐stabilized titanium–oxo cluster, 14 O‐donor ligands have been successfully introduced without changing the cluster core. The increasing electron‐withdrawing effect of the organic species allows the gradual reduction of the bandgaps of the {Ti6} complexes. Transition‐metal ions are then incorporated by the use of bifunctional O/N‐donor ligands, organizing these {Ti6} clusters into polymeric structures. The coordination environments of the applied metal ions show significant influence on their visible‐light adsorption. Both the above structural functionalizations also tune the photocatalytic H2 production activities of these clusters. This work provides a systematic bandgap engineering study of titanium–oxo clusters, which is important not only for their future photocatalytic applications, also for the better understanding of the structure–property relationships.

Bandgap Engineering of Titanium-Oxo Clusters: Labile Surface Sites Used for Ligand Substitution and Metal Incorporation.

Through the labile coordination sites of a robust phosphonate-stabilized titanium-oxo cluster, 14 O-donor ligands have been successfully introduced without changing the cluster core. The increasing electron-withdrawing effect of the organic species allows the gradual reduction of the bandgaps of the {Ti6 } complexes. Transition-metal ions are then incorporated by the use of bifunctional O/N-donor ligands, organizing these {Ti6 } clusters into polymeric structures. The coordination environments of the applied metal ions show significant influence on their visible-light adsorption. Both the above structural functionalizations also tune the photocatalytic H2 production activities of these clusters. This work provides a systematic bandgap engineering study of titanium-oxo clusters, which is important not only for their future photocatalytic applications, also for the better understanding of the structure-property relationships.

Characterization of Water Coordination to Ferrous Nitrosyl Complexes with fac-N2O, cis-N2O2, and N2O3 Donor Ligands.

Electron paramagnetic resonance (EPR) experiments were done on a series of S = (3)/2 ferrous nitrosyl model complexes prepared with chelating ligands that mimic the 2-His-1-carboxylate facial triad iron binding motif of the mononuclear nonheme iron oxidases. These complexes formed a comparative family, {FeNO}(7)(N2Ox)(H2O)3-x with x = 1-3, where the labile coordination sites for the binding of NO and solvent water were fac for x = 1 and cis for x = 2. The continuous-wave EPR spectra of these three complexes were typical of high-spin S = (3)/2 transition-metal ions with resonances near g = 4 and 2. Orientation-selective hyperfine sublevel correlation (HYSCORE) spectra revealed cross peaks arising from the protons of coordinated water in a clean spectral window from g = 3.0 to 2.3. These cross peaks were absent for the {FeNO}(7)(N2O3) complex. HYSCORE spectra were analyzed using a straightforward model for defining the spin Hamiltonian parameters of bound water and showed that, for the {FeNO}(7)(N2O2)(H2O) complex, a single water conformer with an isotropic hyperfine coupling, Aiso = 0.0 ± 0.3 MHz, and a dipolar coupling of T = 4.8 ± 0.2 MHz could account for the data. For the {FeNO}(7)(N2O)(H2O)2 complex, the HYSCORE cross peaks assigned to coordinated water showed more frequency dispersion and were analyzed with discrete orientations and hyperfine couplings for the two water molecules that accounted for the observed orientation-selective contour shapes. The use of three-pulse electron spin echo envelope modulation (ESEEM) data to quantify the number of water ligands coordinated to the {FeNO}(7) centers was explored. For this aspect of the study, HYSCORE spectra were important for defining a spectral window where empirical integration of ESEEM spectra would be the most accurate.

HYSCORE Analysis of the Effects of Substrates on Coordination of Water to the Active Site Iron in Tyrosine Hydroxylase.

Tyrosine hydroxylase is a mononuclear non-heme iron monooxygenase found in the central nervous system that catalyzes the hydroxylation of tyrosine to yield L-3,4-dihydroxyphenylalanine, the rate-limiting step in the biosynthesis of catecholamine neurotransmitters. Catalysis requires the binding of tyrosine, a tetrahydropterin, and O₂ at an active site that consists of a ferrous ion coordinated facially by the side chains of two histidines and a glutamate. We used nitric oxide as a surrogate for O₂ to poise the active site iron in an S = ³/₂ {FeNO}⁷ form that is amenable to electron paramagnetic resonance (EPR) spectroscopy. The pulsed EPR method of hyperfine sublevel correlation (HYSCORE) spectroscopy was then used to probe the ligands at the remaining labile coordination sites on iron. For the complex formed by the addition of tyrosine and nitric oxide, TyrH/NO/Tyr, orientation-selective HYSCORE studies provided evidence of the coordination of one H₂O molecule characterized by proton isotropic hyperfine couplings (A(iso) = 0.0 ± 0.3 MHz) and dipolar couplings (T = 4.4 and 4.5 ± 0.2 MHz). These data show complex HYSCORE cross peak contours that required the addition of a third coupled proton, characterized by an A(iso) of 2.0 MHz and a T of 3.8 MHz, to the analysis. This proton hyperfine coupling differed from those measured previously for H₂O bound to {FeNO}⁷ model complexes and was assigned to a hydroxide ligand. For the complex formed by the addition of tyrosine, 6-methyltetrahydropterin, and NO, TyrH/NO/Tyr/6-MPH₄, the HYSCORE cross peaks attributed to H₂O and OH⁻ for the TyrH/NO/Tyr complex were replaced by a cross peak due to a single proton characterized by an A(iso) of 0.0 MHz and a dipolar coupling (T = 3.8 MHz). This interaction was assigned to the N₅ proton of the reduced pterin.

Nitrosyl–heme and anion–arene complexes: structure, reactivity and spectroscopy

Abstract Two topics are selected and illustrated to exemplify (i) a biological and (ii) an organic ionic intermediate. The reactivity behavior of NO adducts with ferric and ferrous hemes has shown remarkable similarities when examined in the gas phase, demonstrating that the largely different NO affinity displayed in solution and in biological media is due to the different coordination environment. In fact, ferrous hemes present a vacant or highly labile axial coordination site, prone to readily bind NO. The vibrational signatures of the NO ligand have also been probed in vacuo for the first time in the nitrosyl complexes deriving from ferrous and ferric hemes under strictly comparable five-coordination at the metal center. Negatively charged σ-adducts, from the association of anions with 1,3,5-trinitrobenzene, an exemplary π-electron-deficient arene, have been probed by IRMPD spectroscopy and found to display variable binding motifs from a strongly covalent σ-adduct (Meisenheimer complex) to a weakly covalent σ-complex, depending on the anion basicity.