Polynuclear transition metal complexes of metal⋯metal-bridging compartmental pyrazolate ligands

An octa-nuclear nickel(II) pyrazolate cluster with a cubic Ni8 core and its methyl- and n-octyl-functionalized derivatives

The mol-ecular and crystal structure of a discrete [Ni8(μ4-OH)6(μ-4-Rpz)12]2- (R = H; pz = pyrazolate anion, C3H3N2 -) cluster with an unprecedented, perfectly cubic arrangement of its eight Ni centers is reported, along with its lower-symmetry alkyl-functionalized (R = methyl and n-oct-yl) derivatives. Crystals of the latter two were obtained with two identical counter-ions (Bu4N+), whereas the crystal of the complex with the parent pyrazole ligand has one Me4N+ and one Bu4N+ counter-ion. The methyl derivative incorporates 1,2-di-chloro-ethane solvent mol-ecules in its crystal structure, whereas the other two are solvent-free. The compounds are tetra-butyl-aza-nium tetra-methyl-aza-nium hexa-μ4-hydroxido-dodeca-μ2-pyrazolato-hexa-hedro-octa-nickel, (C16H36N)(C4H12N)[Ni8(C3H3N2)12(OH)6] or (Bu4N)(Me4N)[Ni8(μ4-OH)6(μ-pz)12] (1), bis-(tetra-butyl-aza-nium) hexa-μ4-hydroxido-dodeca-μ2-(4-methyl-pyrazolato)-hexa-hedro-octa-nickel 1,2-di-chloro-ethane 7.196-solvate, (C16H36N)2[Ni8(C4H5N2)12(OH)6]·7.196C2H4Cl2 or (Bu4N)2[Ni8(μ4-OH)6(μ-4-Mepz)12]·7.196(ClCH2CH2Cl) (2), and bis-(tetra-butyl-aza-nium) hexa-μ4-hydroxido-dodeca-μ2-(4-octylpyrazolato)-hexa-hedro-octa-nickel, (C16H36N)2[Ni8(C11H19N2)12(OH)6] or (Bu4N)2[Ni8(μ4-OH)6(μ-4-nOctpz)12] (3). All counter-ions are disordered (with the exception of one Bu4N+ in 3). Some of the octyl chains of 3 (the crystal is twinned by non-merohedry) are also disordered. Various structural features are discussed and contrasted with those of other known [Ni8(μ4-OH)6(μ-4-Rpz)12]2- complexes, including extended three-dimensional metal-organic frameworks. In all three structures, the Ni8 units are lined up in columns.

Jellyfish-type Dinuclear Hafnium Azido Complexes: Synthesis and Reactivity

Di- and multinuclear hafnium complexes bridged by ligands have been rarely reported. In this article, a novel 3,5-disubstituted pyrazolate-bridged ligand LH5 with two [N2 N]2- -type chelating side arms was designed and synthesized, which supported a series of dinuclear hafnium complexes. Dinuclear hafnium azides [LHf2 (μ-1,1-N3 )2 (N3 )2 ][Na(THF)4 ] 3 and [LHf2 (μ-1,1-N3 )2 (N3 )2 ][Na(2,2,2-Kryptofix)] 4 were further synthesized and structurally characterized, featuring two sets of terminal and bridging azido ligands like jellyfishes. The reactivity of 3 under reduction conditions was conducted, leading to a formation of a tetranuclear hafnium imido complex [L1 Hf2 (μ1 -NH)(N3 ){μ2 -K}]2 5. DFT calculations revealed that the mixed imido azide 5 was generated via an intramolecular C-H insertion from a putative dinuclear HfIV -nitridyl intermediate.

A tetracobalt(II) cluster with a two vertex truncated dicubane topology endogenously supported by carboxylate-based (2-pyridyl)methylamine ligands: magneto-structural and DFT studies

A reaction between CoCl2 and L3-(CO2-)2 (2 : 1 stoichiometry) in CH3OH affords a discrete complex [CoII4-{L3-(CO2-)2}2(μ3-OCH3)2(CH3OH)2(H2O)2Cl2] (1) [L3-(CO2-)2 = 3-[N-{2-(pyridin-2-yl)methyl}amino]-bis(propionate)]. The structure of 1 reveals two terminal mononuclear CoII{L3-(CO2-)2}Cl units connected by a dimeric CoII2(μ3-OCH3)2(CH3OH)2(H2O) unit present in the centre through two methoxo (μ3-OCH3)- and two carboxylate (μ-1,1-OCO-) bridges affording a tetranuclear coordination cluster of Co(II) with a defective dicubane topology. In 1, Co1 (terminal) has distorted octahedral CoIIN2O3Cl and the central Co2 has CoIIO6 coordination. Such coordination arrangements afford the observed topology. Variable-temperature magnetic studies reveal anti-ferromagnetic coupling in 1. Three magnetic exchange interactions (one anti-ferromagnetic and two ferromagnetic: J1 = +3.3 cm-1 (Co⋯Co 3.176 Å; μ-1,1-OCO- and μ3-OCH3 bridges), J2 = -2.5 cm-1 (Co⋯Co 3.228 Å; μ-1-OCO- and μ3-OCH3 bridges) and J3 = +10.6 cm-1 (Co⋯Co 3.084 Å; two μ3-OCH3 bridges)) have been identified, with the inclusion of the orbital reduction parameter (α = Aκ = 1.38), spin-orbit coupling (λ = -158 cm-1) and axial distortion (energy gap Δ = -975 cm-1 between singlet and doublet levels), rationalized by density functional theory (DFT) calculations.

Electrocatalytic CO2 Reduction with a Binuclear Bis-Terpyridine Pyrazole-Bridged Cobalt Complex

A pyrazole-based ligand substituted with terpyridine groups at the 3 and 5 positions has been synthesized to form the dinuclear cobalt complex 1, that electrocatalytically reduces carbon dioxide (CO₂ ) to carbon monoxide (CO) in the presence of Brønsted acids in DMF. Chemical, electrochemical and UV-vis spectro-electrochemical studies under inert atmosphere indicate pairwise reduction processes of complex 1. Infrared spectro-electrochemical studies under CO₂ and CO atmosphere are consistent with a reduced CO-containing dicobalt complex which results from the electroreduction of CO₂ . In the presence of trifluoroethanol (TFE), electrocatalytic studies revealed single-site mechanism with up to 94 % selectivity towards CO formation when 1.47 M TFE were present, at -1.35 V vs. Saturated Calomel Electrode in DMF (0.39 V overpotential). The low faradaic efficiencies obtained (<50 %) are attributed to the generation of CO-containing species formed during the electrocatalytic process, which inhibit the reduction of CO₂ .

Tuning of Cr-Cr Magnetic Exchange through Chalcogenide Linkers in Cr2 Molecular Dimers

A set of three Cr-dimer compounds, Cr₂Q₂(en)₄X₂ (Q: S, Se; X: Br, Cl; en: ethylenediamine), with monoatomic chalcogenide bridges have been synthesized via a single-step solvothermal route. Chalcogenide linkers mediate magnetic exchange between Cr³⁺ centers, while bidentate ethylenediamine ligands complete the distorted octahedral coordination of Cr centers. Unlike the compounds previously reported, none of the chalcogenide atoms are connected to extra ligands. Magnetic susceptibility studies indicate antiferromagnetic coupling between Cr³⁺ centers, which are moderate in Cr₂Se₂(en)₄X₂ and stronger in Cr₂S₂(en)₄Cl₂. Fitting the magnetic data requires a biquadratic exchange term. High-frequency EPR spectra showing characteristic signals due to coupled S = 1 spin states could be interpreted in terms of the "giant spin" Hamiltonian. A fourth compound, Cr₂Se₈(en)₄, has a single diatomic Se bridge connecting the two Cr³⁺ centers and shows weak ferromagnetic exchange interactions. This work demonstrates the tunability in strength and type of exchange interactions between metal centers by manipulating the interatomic distances and number of bridging chalcogenide linkers.

Tris(Imidazolyl) Dicopper(I) Complex and its Reactivity to Exert Catalytic Oxidation of Sterically Hindered Phenol Substrates via a [Cu2O]2+ Core

The Cu ion ligated with histidine residues is a common active site motif of various Cu-containing metalloenzymes exerting versatile catalytic oxidation reactions. Due to the scarce of structurally characterized biomimetic...

Enhancing Chemo- and Stereoselectivity in C-H Bond Oxygenation with H2O2 by Nonheme High-Spin Iron Catalysts: The Role of Lewis Acid and Multimetal Centers

Spin states of iron often direct the selectivity in oxidation catalysis by iron complexes using hydrogen peroxide (H₂O₂) on an oxidant. While low-spin iron(III) hydroperoxides display stereoselective C-H bond hydroxylation, the reactions are nonstereoselective with high-spin iron(II) catalysts. The catalytic studies with a series of high-spin iron(II) complexes of N4 ligands with H₂O₂ and Sc³⁺ reported here reveal that the Lewis acid promotes catalytic C-H bond hydroxylation with high chemo- and stereoselectivity. This reactivity pattern is observed with iron(II) complexes containing two cis-labile sites. The enhanced selectivity for C-H bond hydroxylation catalyzed by the high-spin iron(II) complexes in the presence of Sc³⁺ parallels that of the low-spin iron catalysts. Furthermore, the introduction of multimetal centers enhances the activity and selectivity of the iron catalyst. The study provides insights into the development of peroxide-dependent bioinspired catalysts for the selective oxygenation of C-H bonds without the restriction of using iron complexes of strong-field ligands.

Tantalum(iv) pyrazolate: new wine in the old wineskin

Homoleptic binuclear Ta(iv) pyrazolate [Ta₂(pz)₈] with Ta–Ta distance of 3.1895(7) Å was obtained in a reaction of Ta(NMe₂)₅ with unsubstituted pyrazole Hpz.

Influence of the chain length and metal : ligand ratio on the self-organization processes of Cu2+ complexes of [1 + 1] 1H-pyrazole azamacrocycles

Three new [1 + 1] macrocycles formed by the reaction of 1H-3,5-bis(chloromethyl)pyrazole with the tosylated amines 1,4,7,10-tetraazadecane (L1), 1,4,8,11-tetraazaundecane (L2) and 1,5,10,14-tetraazatetradecane (L3) are described. Potentiometric studies and HR-ESI-Mass spectrometry show the formation of dimeric binuclear Cu²⁺ complexes whose organization depends on the type of hydrocarbon chains connecting the amine groups. Furthermore, trinuclear or/and tetranuclear complexes are formed depending also on the length of the polyaminic bridge and on the sequence of the hydrocarbon chains. The crystal structures of the [2 + 2] [Cu₂(H(H_-1L2))₂](ClO₄)₄·4H₂O (1) and [Cu₂(H_-1L2)₂](ClO₄)₂ (2) complexes show in both of them two macrocycles self-assembled by the metal ions which interconnect their pyrazolate fragments that behave as bis(monodentate) ligands. While in 1 one central amine of each macrocycle binds to the axial position of a distorted square-pyramid and the other ones remain protonated, in 2 all the amine groups are involved in the coordination giving rise to a strongly distorted octahedral geometry. Paramagnetic ¹H NMR measurements support that these structures also form in solution. Interestingly, tetranuclear complexes [Cu₄(H_-1L4)₂(OH)_2.08](ClO₄)_2.92Br_0.54Cl_0.46 (3) and [Pd_2.39Cu_1.61(H_-1L4)₂(OH)₂](ClO₄)₂Cl_1.33Br_0.67·2.87H₂O (4) have been isolated for the macrocycle containing the 1,5,9,13-tetraamine chain (L4). 3 has two binucleating units, one of them formed by the pyrazolate moieties and their neighbouring secondary amines and the other by the two central amines of both macrocycles. This latter Cu²⁺ coordination site is completed by two hydroxide anions as bridging ligands. 4 was obtained from a solution prepared to achieve full formation of the dimeric cage [Cu₂(H_-1(HL4))₂]⁴⁺ by addition of K₂PdCl₄. The Pd²⁺ ion due to its softer acidic characteristics displaces the Cu²⁺ ions from the pyrazolate site. UV-vis spectroscopy suggests that the exchange is completed at room temperature after one hour.

Metal-metal cooperative bond activation by heterobimetallic alkyl, aryl, and acetylide PtII/CuI complexes

We report the selective formation of heterobimetallic Pt^II/Cu^I complexes that demonstrate how facile bond activation processes can be achieved by altering the reactivity of common organoplatinum compounds through their interaction with another metal center. The interaction of the Cu center with the Pt center and with a Pt-bound alkyl group increases the stability of PtMe₂ towards undesired rollover cyclometalation. The presence of the Cu^I center also enables facile transmetalation from an electron-deficient tetraarylborate [B(Ar^F)₄]⁻ anion and mild C–H bond cleavage of a terminal alkyne, which was not observed in the absence of an electrophilic Cu center. The DFT study indicates that the Cu center acts as a binding site for the alkyne substrate, while activating its terminal C–H bond.

The selective formation of heterobimetallic Pt^II/Cu^I complexes demonstrates how facile bond activation processes can be achieved by altering the reactivity of common organoplatinum compounds through their interaction with another metal center.

Simultaneous Spin-Crossover Transition and Conductivity Switching in a Dinuclear Iron(II) Coordination Compound Based on 7,7',8,8'-Tetracyano-p-quinodimethane

The reaction of Fe(OAc)₂ and Hbpypz with neutral TCNQ results in the formation of [Fe₂ (bpypz)₂ (TCNQ)₂ ](TCNQ)₂ (1), in which Hbpypz=3,5-bis(2-pyridyl)pyrazole and TCNQ=7,7',8,8'-tetracyano-p-quinodimethane. Crystal packing of 1 with uncoordinated TCNQ and π-π stacking of bpypz^- ligands produces an extended two-dimensional supramolecular coordination assembly. Temperature dependence of the dc magnetic susceptibility and heat capacity measurements indicate that 1 undergoes an abrupt spin crossover (SCO) with thermal spin transition temperatures of 339 and 337 K for the heating and cooling modes, respectively, resulting in a thermal hysteresis of 2 K. Remarkably, the temperature dependence of dc electrical transport exhibits a transition that coincides with thermal SCO, demonstrating the thermally induced magnetic and electrical bistability of 1, strongly correlating magnetism with electrical conductivity. This outstanding feature leads to thermally induced simultaneous switching of magnetism and electrical conductivity and a magnetoresistance effect.

Mixed ligand complexes of silver(i) supported by highly fluorinated pyrazolates, and chelating and bridging N-heterocycles

A range of silver complexes featuring all nitrogen coordination environments involving pyrazolates and chelating and non-chelating bidentate N-heterocycles are reported.

Structural diversity of ethylzinc derivatives of 3,5-substituted pyrazoles

Equimolar reactions of Et2Zn with 3,5-dimethylpyrazole (H-pzMe2), 3,5-di-iso-propylpyrazole (H-pziPr2), 3,5-di-tert-butylpyrazole (H-pztBu2) and indazole (H-ind) were investigated in toluene or tetrahydrofuran (as a coordinating solvent). A series of diverse ethylzinc pyrazolates and indazolates were identified using advanced NMR spectroscopy and the single crystal X-ray diffraction techniques. The NMR experiments indicate that dimeric moieties of the general formula [EtZn(pz)]2 or [Et2Zn2(pz)2(THF)] are favoured in solution. Nevertheless, these types of complexes are kinetically labile and tend to undergo ligand scrambling reactions according to the Schlenk equilibrium. For example, the alkyl substituents in the pzMe2 and pziPr2 ligands do not appear to be a strong determinant of the dimeric moieties and the composition of the isolated complexes by crystallisation from the parent reaction mixture varies between spiro-type tri- and tetranuclear aggregates, [Et2Zn3(pz)4(THF)x] (x = 0 or 2) and [Et2Zn4(pz)6(THF)2], respectively. The nonstoichiometric formula of these organozinc derivatives is likely related to both the Schlenk-type equilibria and solubility of the respective moieties. In turn, the high steric demands of the 3,5-di-tert-butylpyrazolate ligand promote the dimeric form in solution and the solid state. Interestingly, the ethylzinc indazolate complex also does not undergo a redistribution reaction and yields a dimer.

A fluorescent Cu(ii) complex as a dual functional sensor for selective and sensitive detection of acetone and Cd(ii) pollutants

In this study, Cu²⁺ perchlorate complexes of 3,5-dimetylpyrazole(dmpy), namely [Cu(dmpy)₃(H₂O)₂](H₂O)(ClO₄)₂ (1), were prepared and characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA), infrared (IR) spectroscopy, elemental analysis, mass spectrometry, and ultraviolet visible (UV-Vis) spectroscopy. It was determined that Cu²⁺ coordinated to the nitrogen atoms of three dmpy molecules as well as to the oxygen atoms of two H₂O molecules in a square pyramidal geometry. To balance the charge, two uncoordinated perchlorate anions and one H₂O molecule were also present in the lattice. The resulting Cu²⁺ complex was involved in extensive hydrogen bonding, which stabilized the structure. The fluorescence properties of complex 1 were studied in acetonitrile at room temperature upon the addition of different organic solvents. It was confirmed that the complex exhibited remarkable acetone selectivity via a fluorescence quenching mechanism. At low concentrations, the fluorescence intensities were nearly completely quenched via a turn-off mode. Moreover, complex 1 displayed exceptional selectivity, fast detection time, and high sensitivity for Cd²⁺ in aqueous solutions via a fluorescence enhancement mechanism (turn-on mode). Hence, the prepared complex showed potential for application in Cd²⁺ detection. Notably, in an aqueous solution, complex 1 exhibited highly selective fluorescence enhancement effects (turn-on mode) with respect to Cd²⁺ and was not affected by the interference of other metal ions.

The prepared complex 1 exhibited highly sensitive and selective fluorescence quenching (turn-off) effect in the presence of acetone as well as fluorescence enhancement (turn-on) effect toward Cd²⁺ ions in aqueous solutions.

Effect of metal ion substitution on the catalytic activity of a pentanuclear metal complex

A pentanuclear cobalt complex that consists of five cobalt ions and six bpp⁻ ligands (Co5, Hbpp = 3,5-bis(2-pyridyl)pyrazole) was newly developed. The obtained complex can catalyze CO₂ reduction under electrochemical and photochemical conditions.

Phosphine-functionalised tris(pyrazolyl)methane ligands and their mono- and heterobimetallic complexes

The synthesis, characterisation and reactivity of new phosphine-functionalised tris(pyrazolyl)methane ligands (TpmPR₂, 2a-c, with R = Ph, nBu, iPr) are presented. The reaction of 2a-c with [Rh(CO)₂Cl]₂ furnished N,P-heterochelate carbonyl complexes (3a-c), which were used to quantify the donor abilities of the ligands via IR spectroscopy. The coordination flexibility was demonstrated by treating representative members of this new ligand class with [CpRu(acn)₃][PF₆] (acn = acetonitrile), [(tht)AuCl] (tht = tetrahydrothiophene) and [Pd(allyl)Cl]₂ providing either a N,N,P-heteroscorpionate complex (4) or the P-coordinated complexes (5,6) without any involvement of the pyrazolyl entities. With 2a and [Pd(cod)Cl₂], another P,N-heterochelate complex (7) was obtained, which served as a precursor for a heterobimetallic complex containing palladium and copper (8). Detailed NMR spectroscopic and X-ray crystallographic investigations have been performed on all new complexes.

Crystal structure of dihydrazinium 1H-pyrazole-3,5-dicarboxylate, C5H12N6O4

C5H12N6O4, monoclinic, P21/n (no. 14), a = 4.3368(6) Å, b = 15.483(2) Å, c = 13.8852(19) Å, β = 97.714(3)°, V = 923.9(2) Å3, Z = 4, R gt(F) = 0.0411, wR ref(F 2) = 0.1109, T = 200(2) K.

Highly Selective Removal of Pb(II) by a Pyridylpyrazole-β-ketoenol Receptor Covalently Bonded onto the Silica Surface

Efficient materials capable of capturing toxic metals from water are widely needed. Herein, a new pyridylpyrazole-β-ketoenol receptor, X-ray diffraction analyzed, was covalently incorporated into the silica surface to produce solid and recyclable adsorbent particles. The new material, fully characterized, revealed extremely efficient removal of toxic metals from water, with a selectivity order of Pb(II) > Zn(II) > Cu(II) > Cd(II). The adsorption was exceptionally rapid at optimum pH and concentrations, showing Pb(II) removal of 93 mg g-1 within 5 min and maximum Pb(II) adsorption of 110 mg g-1 after only 20 min. Sorption isotherms agreed well with the Langmuir model suggesting a monolayer adsorption, whereas kinetics agreed with the pseudo-second-order model suggesting a chemisorption binding mechanism. Thermodynamics of adsorption were fitted with an endothermic and spontaneous process. The material, recyclable for at least five cycles, is therefore promising for the cleanup of water polluted by toxic metals, especially lead.

Versatile coordination architectures of products generated by the in situ reaction of a doubly bis(2-pyridyl)pyrazolate bridged dinuclear copper(ii) complex with tetracyanoethylene

Reaction of doubly bpypz⁻ bridged Cu(ii) dinuclear complex and TCNE gave three versatile coordination assemblies with TCNE solvolysis derivatives, depending on reaction solvents.

Iron-based trinuclear metal-organic nanostructures on a surface with local charge accumulation

Coordination chemistry relies on harnessing active metal sites within organic matrices. Polynuclear complexes-where organic ligands bind to several metal atoms-are relevant due to their electronic/magnetic properties and potential for functional reactivity pathways. However, their synthesis remains challenging; few geometries and configurations have been achieved. Here, we synthesise-via supramolecular chemistry on a noble metal surface-one-dimensional metal-organic nanostructures composed of terpyridine (tpy)-based molecules coordinated with well-defined polynuclear iron clusters. Combining low-temperature scanning probe microscopy and density functional theory, we demonstrate that the coordination motif consists of coplanar tpy's linked via a quasi-linear tri-iron node in a mixed (positive-)valence metal-metal bond configuration. This unusual linkage is stabilised by local accumulation of electrons between cations, ligand and surface. The latter, enabled by bottom-up on-surface synthesis, yields an electronic structure that hints at a chemically active polynuclear metal centre, paving the way for nanomaterials with novel catalytic/magnetic functionalities.

Selective Synthesis of a Series of Isostructural MII CuI Heterobimetallic Complexes Spontaneously Assembled by an Unsymmetrical Naphthyridine-Based Ligand

Metal-metal cooperation is integral to the function of many enzymes and materials, and model complexes hold enormous potential for providing insights into the capabilities of analogous multimetallic cores. However, the selective synthesis of heterobimetallic complexes still presents a significant challenge, especially for systems that hold the metals in close proximity and feature open or reactive coordination sites for both metals. To address this issue, a rigid, naphthyridine-based dinucleating ligand featuring distinct binding environments was synthesized. This ligand enables the selective synthesis of a series of M^II Cu^I bimetallic complexes (M=Mn, Fe, Co, Ni, Cu, Zn), in which each metal center exclusively occupies its preferred binding pocket, from simple chloride salts. The precision of this selectivity is evident from cyclic voltammetry, ESI-MS and anomalous X-ray diffraction measurements.

Cyclometalated Ir(III) Complex as a Metalloligand and a Selective Cu(II) Sensor: Synthesis and Structural Characterization of a Heterometallic Tetranuclear Ir(III)/Cu(II) Complex

A cyclometalated Ir(III) complex, [Ir(ppy)2(LH)] (2) [LH2 = 1,2-bis(pyridine-2-carboxamido)benzene; ppyH = 2-phenylpyridine] was synthesized and structurally characterized. [Ir(ppy)2(LH)] contains free donor sites and functions as a metalloligand. Accordingly, it interacts with a Cu(II) salt to afford the heterometallic tetranuclear complex, [{Ir(ppy)2(L)}2{Cu2Cl2}] (3). In the latter, a Cu2Cl2 dimer bridges the two cyclometalated Ir(III) units, resulting in a IrIII-CuII-CuII-IrIII motif. 2 also functions as a selective and reversible sensor for Cu2+, as revealed by quenching of its emission and subsequent revival of the luminescence signal upon addition of EDTA.