Polytopic Ligands Research Articles

Synthesis and EPR Studies of Zinc and Copper TPMA Metal Complexes Containing TEMPO Functionalities

A new family of stable radical‐containing polytopic ligands based on tris(2‐pyridylmethyl)amines TPMA architecture and their corresponding metal complexes have been synthesized. These molecular systems offered the possibility to investigate how various spin carriers, such as the metal ion and the organic radical, influence their EPR properties mainly through the empirical ratio of peak heights d1/d. Moreover, it has been possible to analyze how different conformations of TEMPO radical units in the molecular skeleton affect the metallic system.

Photoluminescence and magnetic properties of isostructural europium(III), gadolinium(III) and terbium(III) oxamate-based coordination polymers.

Developing and investigating advanced multifunctional materials with magnetic properties as candidates for assembling spin qubits for quantum computing is imperative. A new polytopic ligand based on oxamate and aniline was used to promote the synthesis of three neutral homometallic lanthanide-coordinated polymers. New complexes with the formula {Ln(phox)3(DMSO)2(H2O)}n, where Ln = Eu3+ (1), Gd3+ (2), and Tb3+ (3) [phox = N-(phenyl)oxamate and DMSO = dimethylsulfoxide], were synthesized and well characterized by spectroscopic methods as well as X-ray crystallographic analysis. All crystalline structures comprise neutral zigzag chains. The lanthanide ions are linked by three phox ligands, in which two oxygen atoms from two different ligands are responsible for connecting the trivalent lanthanide ions, and one phox ligand completes the coordination sphere in a bis-bidentate mode, together with two DMSO molecules and one water coordination molecule. The coordination sphere of lanthanide ions consisted of spherical capped square antiprism (CSAPR-9) symmetry. The magnetic properties of 1-3 were investigated in the 2-300 K temperature range. The dynamic (ac) magnetic properties of 2 reveal a frequency dependence involving the phonon bottleneck mechanism below 33 K under nonzero applied dc magnetic fields, resulting in an example of a field-induced single-molecule magnet. Solid-state photophysical measurements for Eu3+ (1) and Tb3+ (3) complexes indicate that the N-(phenyl)oxamate ligands are very efficient in sensitizing the lanthanide(III) ions in the visible region of the electromagnetic spectrum. Compounds 1 and 3 exhibited an emission in the red and green regions, respectively. Experimental results and theoretical calculations using the Sparkle/RM1 method support a quantum efficiency of ∼72% for 1, suggesting its potential as a candidate for light conversion molecular devices (LCMDs).

(R,R)-Tartrate as a polytopic ligand for UO22+: Mono- and diperiodic coordination polymers including di- and tetranuclear subunits

(R,R)-Tartaric acid (H4tart) has been reacted with uranyl nitrate hexahydrate under solvo-hydrothermal conditions, in the presence of either [Ni(R,S-Me6cyclam)]2+ (R,S-Me6cyclam = 7(R),14(S)-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane), quinH+ (quinuclidinium), or [Ni(bipy)3]2+ (bipy = 2,2ʹ-bipyridine), to give three complexes characterized by their crystal structure. [H2NMe2]2[(UO2)2(Htart)2(HCOO)2Ni(R,S-Me6cyclam)]⋅2H2O (1) crystallizes as a monoperiodic coordination polymer in which dimeric [UO2(Htart)]22– subunits are linked by nickel(II) cations. The structure of the dimers matches that expected from solution experiments but not previously found in the solid state. Both [Hquin][(UO2)2(tart)(CH3COO)] (2) and [Ni(bipy)3][(UO2)2(tart)(CH3COO)]2 (3) crystallize as diperiodic networks based on tetranuclear [(UO2)2(tart)(CH3COO)]22– subunits which contain a central U2O2 ring involving μ2-bridging alkoxide groups. Further chelation of the two lateral uranyl cations by the carboxylate groups of adjacent subunits yields a sql-type network. Due to the different bulk of the counterions, the layers are quasi-planar in 2 and corrugated in 3. Comparison of these polynuclear subunits with those formed by the related ligands citrate, R,S-malate and R-citramalate shows that while the dimeric form is common to all, the tetranuclear subunit is specific to the tartrate ligand.

Pyrogallate‐Based Metal‐Organic Framework with a Two‐Dimensional Secondary Building Unit

AbstractWe report a metal–organic framework (MOF) with a rare two‐dimensional (2D) secondary building unit (SBU). The SBU comprises mixed‐valent Fe2+ and Fe3+ metal ions bridged by oxygen atoms pertaining to the polytopic ligand 3,3′,4,4′,5,5′‐hexahydroxybiphenyl, which also define the iron‐oxide 2D layers. Overall, the anionic framework exhibits rare topology and evidences strong electronic communication between the mixed‐valence iron sites. These results highlight the importance of dimensionality control of MOF SBUs for discovering new topologies in reticular chemistry, and especially for improving electronic communication within the MOF skeleton.

Pyrogallate-Based Metal-Organic Framework with a Two-Dimensional Secondary Building Unit.

We report a metal-organic framework (MOF) with a rare two-dimensional (2D) secondary building unit (SBU). The SBU comprises mixed-valent Fe2+ and Fe3+ metal ions bridged by oxygen atoms pertaining to the polytopic ligand 3,3',4,4',5,5'-hexahydroxybiphenyl, which also define the iron-oxide 2D layers. Overall, the anionic framework exhibits rare topology and evidences strong electronic communication between the mixed-valence iron sites. These results highlight the importance of dimensionality control of MOF SBUs for discovering new topologies in reticular chemistry, and especially for improving electronic communication within the MOF skeleton.

Tripodal 1,2,3-triazole click ligand based on the triphenylphosphine oxide platform: atrane-type lanthanide complexes in solutions

Tris[2-(1-phenyl-1,2,3-triazol-4-ylmethoxy)phenyl]-phosphine oxide, a novel tripodal polytopic ligand produces, stable complexes with La(NO3)3 and Lu(NO3)3. According, to IR, multinuclear NMR and DFT data, the complexes, have atrane-type structure with tetradentate O,N,N,Ncoordination, of ligand in MeCN solutions.

Architecture of a dinuclear Co(II) complex based on 3-amino-1,2,4-triazole-5-carboxylic acid: molecular structure, thermal behavior, optical properties, and DFT calculations

The dinuclear compound [Co2(Hatzc)2(H2O)6](NO3)2·4H2O containing the polytopic ligand Hatzc‒ = 3-amino-4H-1,2,4-triazole-5-carboxylate was synthesized and characterized. Single-crystal X-ray diffraction showed that two monoanionic N,O-chelating Hatzc‒ ligands bridge two Co(II) ions in a µ-N1,N2 fashion. Both Co(II) centers are further coordinated by three H2O ligands making up an octahedral coordination. A complex network of intra- and intermolecular O‒H···O and N‒H···O hydrogen bonding interactions characterize the structure. Hirshfeld surface analysis, ATR–FTIR, and FT-Raman spectroscopy in combination with density functional theory (DFT) calculation of vibrational modes confirm this multitude of H bonds. Thermogravimetric and differential thermal analysis experiments under an argon atmosphere in the temperature range of 25–900 °C showed thermal stability till about 100 °C and multi-step decomposition yielding Co2O3 at about 280 °C. The octahedral environment of Co(II) was confirmed by UV–vis absorption spectroscopy, which shows weak absorption bands at around 520 nm assignable to electronic d‒d transitions alongside with π‒π* transitions dominating the 250–350 nm range. Time-dependent DFT calculated excited states agree very well in terms of energy match with the observed transitions. The direct and indirect optical band gap values were determined using the Tauc plot method to 3.33 eV and 2.75 eV, respectively, indicating semiconducting properties.

Coordination polymers fabricated from Cd(NO3)2andN,N′,O-pincer-type isonicotinoylhydrazone-based polytopic ligands – an insight from experimental and theoretical investigations

Three new Cd(ii) coordination polymers based on isonicotinohydrazide ligands (HLI,HLII) differing in the presence of a methyl unit have been obtained and extensively characterized by experimental and computational approaches.

Coordination Properties of Hydroxyisophthalic Acids: Topological Correlations, Synthesis, Structural Analysis, and Properties of New Complexes.

Hydroxyisophthalic acids are valuable polytopic ligands for the design of functional materials based on coordination polymers due to the variety of charges and coordination modes they possess. Herein, we describe the synthesis, thermal stability, nonlinear optical (NLO) and spectroscopic properties of five novel coordination compounds, [K2 L(H2 O)2 ], [MgL(H2 O)2 ] ⋅ 3H2 O, [CaL(H2 O)3 ], [SrL(H2 O)3 ] ⋅ H2 O, [BaL(H2 O)(H2 O)5 ], and one salt, (NH4 )2 L ⋅ 2H2 O, with 4,5,6-trihydroxyisophthalic acid (H2 L), which has not been tested in assembling crystalline coordination networks before. The peculiarities of the structural organization of the compounds were analyzed and compared with those for other hydroxyisophthalates. The coordination properties of hydroxyisophthalic acids were studied from the topological point of view, and a comparative topological analysis of coordination and H-bonded networks was performed. Structural correlations revealed in this study could be useful for the design of hydroxyisophthalate-based coordination networks, including porous metal-organic frameworks, proton conductors, and NLO materials.

Biomass-derived rctt-3,4-di-2-furanyl-1,2-cyclobutanedicarboxylic acid: a polytopic ligand for synthesizing green metal-organic materials

Biomass is an abundant and environmentally friendly source for materials that can be used in a multitude of applications in the effort to replace petrochemicals. Furfural and malonic acid are biomass-sourced platforms that can be utilized in the synthesis of biobased compounds; rctt-3,4-di-2-furanyl-1,2-cyclobutanedicarboxylic acid (CBDA-2) is one such compound. In this study, CBDA-2 has been introduced into metal-organic materials chemistry as a semi-rigid polytopic ligand. This compound has been utilized as a polytopic ligand in the formation of two different 2D coordination polymers with Cu2+ and Co2+ as the metal centers via a conventional solution method. Both complexes have been characterized by X-ray crystal structure determination and showed visual thermochromic behaviors. This study demonstrates that CBDA-2 is a promising green building block in coordination chemistry.

Coordination capacity of Keggin anions as polytopic ligands: case study of [VNb12O40]15.

Reaction of Na9H4[VNb12O40{NbO(CO3)}2] with [(C6H6)RuCl2]2 (molar ratio {VNb12} : {(C6H6)Ru} = 1 : 4) in aqueous solution gives a mixture of [α-{(C6H6)Ru}4VNb12O40]7- and [α-{(C6H6)Ru}3VNb12O40]9-. Direct acetone diffusion into mother liquor leads to crystallization of Na6H[α-{(C6H6)Ru}4VNb12O40]·41.25H2O (1), characterized by single crystal X-ray diffraction (SCXRD). This anion has four organometallic fragments coordinated to the α-Keggin type [VNb12O40]15- backbone in different manner. Three {(C6H6)Ru}2+ groups cap triangular faces and one group a rectangular face of [VNb12O40]15-. Equilibrated mixture of [α-{(C6H6)Ru}4VNb12O40]7- and [α-{(C6H6)Ru}3VNb12O40]9- was studied by 1H DOSY NMR, HPLC-ICP-AES and HPLC-ESI-MS combined techniques. Direct chromatographic separation of these complexes results in unexpected transformation of both species into [α-{(C6H6)Ru}5VNb12O40]5-, isolated and characterized as Na5[α-{(C6H6)Ru}5VNb12O40]·16H2O (2). This anion contains five coordinated organometallic groups occupying both triangular and rectangular faces.

Hierarchically Porous Carbons Derived from Nonporous Coordination Polymers.

Hierarchically porous carbons (HPCs) with multimodal pore systems exhibit great technological potentials, especially in the fields of heterogeneous catalysis, energy storage, and conversion. Here, we establish a simple and general approach to HPCs by carbonization of nonporous coordination polymers that are produced by mixing metal salts with polytopic ligands in alkaline aqueous solutions at room temperature. The proposed approach is applicable to a wide scope of ligand molecules (18 examples), thus affording the synthesized HPCs with high diversity in porosity, morphology, and composition. In particular, the prepared HPCs exhibit high specific surface areas (up to 2647 m2 g-1) and large pore volumes (up to 2.39 cm3 g-1). The HPCs-supported atomically dispersed Fe-Nx catalysts show much-improved fuel cell cathode performance over the micropore-dominated carbon black-supported catalysts, demonstrating the structural superiority of the HPCs for enhancing the mass transport properties.

Solid-state 1D → 3D transformation of polynitrile-based coordination polymers by dehydration reaction.

In crystal structures of two chain coordination polymers [M(tcnopr3OH)2(H2O)2] (M = NiII and CoII; tcnopr3OH- = [(NC)2CC(O(CH2)3OH)C(CN)2]-) based on a N,O or N,N'-bridge polynitrile ligand, the parallel chains are connected via, respectively, C[triple bond, length as m-dash]NH-O and O-HO hydrogen bonds between uncoordinated functional groups of the ligand and coordinated water molecules. Upon heating, both solids undergo dehydration accompanied by degradation of their single crystals. Powder X-ray diffraction showed that non-isostructural triclinic single crystals transformed to isostructural monoclinic compounds. The solid-state reaction yielded 3D coordination polymers [M(tcnopr3OH)2] (M = NiII and CoII) based on a N,N',O-connected tcnopr3OH-. Although previously tens of complexes based on tcnopr3OH and similar anions were synthesized and X-ray characterized, none of these contain a tridentate polynitrile ligand. Thus, this study provides evidence that solid-state reactions allow obtaining novel coordination modes of polynitrile ligands. The possible pathways for the transformation of H-bonded networks to 3D coordination polymers are discussed on the basis of the topological approach. Applicability of the topological approach to predict possible networks of solid-state reaction products based on the crystal structures of initial compounds is demonstrated.

Ligands with Two Monoanionic N,N-Binding Sites: Synthesis and Coordination Chemistry.

Polytopic ligands have become ubiquitous in coordination chemistry because they grant access to a variety of mono‐ and polynuclear complexes of transition metals as well as rare‐earth and main‐group elements. Nitrogen‐based ditopic ligands, in which two monoanionic N,N‐binding sites are framed within one molecule, are of particular importance and are therefore the primary focus of this review. In detail, bis(amidine)s, bis(guanidine)s, bis(β‐diimine)s, bis(aminotroponimine)s, bis(pyrrolimine)s, and miscellaneous bis(N,N‐chelating) ligands are reviewed. In addition to the general synthetic protocols, the application of these ligands is discussed along with their coordination chemistry, the multifarious binding modes, and the ability of these ligands to bridge two (or more) metal(loids).

Fragmentation of a One-Dimensional Zinc Coordination Polymer and Partial Reassembly Evidenced by Mass Spectrometry

We report here the rare observation of a one-dimensional coordination polymer, {[Zn5(bit)4Cl6]·2(CH3CN)}n (1, Hbit = 1-methyl-2-(1H-1,2,3-triazol-4-yl)-1H-benzo[d]imidazole), having one mode of binding for the polytopic ligand and three different polyhedra, tetrahedron, square-base pyramid, and octahedron, for the metal node. The formation process of 1 involves the in situ decarboxylation of Hbitc (5-(1-methyl-1H-benzo[d]imidazol-2-yl)-1H-1,2,3-triazole-4-carboxylic acid) to Hbit. Apart from one bridging μ2-Cl, all the other chlorine atoms act as a terminal ligand to complete the polyhedra. While 1 is almost insoluble in most organic solvent, electrospray ionization mass spectrometry of its ultrasound-treated DMSO suspension found the two main species [Zn(bit)]+ at 0 eV and [Zn2(bit)3]+ at elevated in-source energies. Density functional theory calculation suggests a potential formation process involving the sequence: [Zn(bit)]+→ [Zn2(bit)3]+ + [Zn2(bit)2Cl]+ → [Zn3(bit)4Cl2]+. The luminescence intensity e...

Polynuclear Cu(i) and Ag(i) phosphine complexes containing multi-dentate polytopic ligands: syntheses, crystal structures and photoluminescence properties.

A series of polynuclear metal complexes, [Cu2(L1)(PPh3)4](ClO4)2 (1), [Cu3(L2)(PPh3)6](ClO4) (2), [Cu3(L3)(PPh3)6] (3), [Ag2(L1)(PPh3)4](BF4)2 (4), [Ag4(L2)2(PPh3)6] (5) and [Ag3(L3)(PPh3)5] (6), have been obtained from the reactions of the highly conjugated bridging ligands 2,3-bis(2-pyridyl)pyrazine (L1), 2,3-bis(2-tetrazoyl)pyrazine (H2L2) and 2,3-bis(2-tetrazoyl)imidazole (H3L3) with [Cu(MeCN)4]ClO4 and AgBF4, respectively. Their crystal structures have been determined by X-ray crystallography and their photophysical properties have been investigated in detail. Complexes 1 and 3 show photoluminescence in CH2Cl2 solution, while all the complexes exhibit obvious luminescence in the solid state; detailed photophysical studies and density functional theory calculations of these complexes have revealed that their lowest energy absorptions and emissions are predominantly derived from either metal-to-ligand charge-transfer (MLCT) or intraligand (IL) excited states.

A stimuli responsive multifunctional ZMOF based on an unorthodox polytopic ligand: reversible thermochromism and anion triggered metallogelation.

A novel Cd(ii)-ZMOF with a unique sodalite topology has been successfully designed and synthesized using a flexible polytopic compartmental ligand. The microporous complex contains 1D hexagonal channels with large void space for the accommodation of guest molecules. This work demonstrates a new paradigm for designing and functionalizing zeolite-type frameworks. The triconnected linker forms coordination polymer gels in the presence of Cd2+ and the gelation was controlled by the presence of specific anions. They possess good thermal stability and exhibit thixotropic behavior. Optical properties revealed that the complex is exclusively thermochromic and undergoes a reversible transition at 80 °C, changing its color from yellow to orange red. Owing to the large voids in the framework, the complex can serve as a host for use in dye adsorption. Thus this paper offers a new MOF material with exceptional chromic behavior, gelation properties and adsorption capability for the development of high performance multifunctional materials.

Metal directed self-assembly of Tetranuclear $$\hbox {Cu}^{\mathrm{II}}$$ Cu II and $$\hbox {Ni}^{\mathrm{II}}$$ Ni II clusters

The self-assembly of two pyridine hydrazone based tritopic ligands appended with terminal carboxylate groups with $$\hbox {Cu}^{\mathrm{II}}$$ and $$\hbox {Ni}^{\mathrm{II}}$$ have been investigated. These polytopic ligands with tridentate coordination pockets were designed to produce homoleptic [ $$3\times 3$$ ] grid complexes on reaction with transition metals. Despite the formation of anticipated metallogrids as the final self-assembly outcome, metal ion geometric preferences and ligand flexibility lead to the formation of tetranuclear clusters in the self-assembly process with $$\hbox {Cu}^{\mathrm{II}}$$ and $$\hbox {Ni}^{\mathrm{II}}$$ metal ions. These results illustrate the dynamic nature of the metal–ligand interactions and flexible nature of the ligand backbone in coordination self-assembly. The synthesis, structure and magnetic properties of three tetraanuclear species {[ $$\hbox {Cu}_{4}(\hbox {H}_{2}\hbox {L}^{\mathrm{1b}})_{2}(\hbox {OTf})_{4}(\hbox {OH})_{2}(\hbox {H}_{2}\hbox {O})_{2}$$ ] $$\cdot 6\hbox {H}_{2}\hbox {O}\}_{\mathrm{n}}$$ (1), [ $$\hbox {Ni}_{4}(\hbox {L}^{\mathrm{2a}})_{2}(\hbox {OCH}_{3})_{4}]{\cdot }4\hbox {H}_{2}\hbox {O}$$ (2), [ $$\hbox {Cu}_{4}(\hbox {L}^{\mathrm{2b}})_{2}(\hbox {OTf})_{4}(\hbox {H}_{2}\hbox {O})_{4}]{\cdot }6\hbox {H}_{2}\hbox {O}$$ (3) involving two tritopic ligands with central pyridine framework are described. Synopsis: Metal Directed Self-Assembly of Tetra Nuclear $$\hbox {Cu}^{\mathrm{II}}$$ and $$\hbox {Ni}^{\mathrm{II}}$$ Clusters Avinash Lakma, Sayed Muktar Hossain, Rabindranath Pradhan and Akhilesh Kumar Singh xxxx-xxxx Three tetranuclear non-grid complexes have been synthesized by reacting two pyridine hydrazone-based tritopic ligands with $$\hbox {Cu}^{\mathrm{II}}$$ and $$\hbox {Ni}^{\mathrm{II}}$$ salts. All complexes are spectroscopically and structurally characterized and their magnetic properties have been studied.

Metal-Stabilized Quinoidal Dibenzo[ g, p]chrysene-Fused Bis-dicarbacorrole System.

We report here a metal complexation-based strategy that permits access to a highly stable expanded porphyrin-type quinoidal polycyclic aromatic hydrocarbons (PAH). Specifically, double insertion of Pd(II) ions into a dibenzo[ g, p]chrysene-fused bis-dicarbacorrole (bis-H3) gives rise to a bis-metalated species (bis-Pd) that undergoes a facile benzenoid-quinonoid transformation. In contrast to what is true for the corresponding mono-Pd(II) complex, which has organic radical character, well resolved 1H NMR and 19F NMR spectra are seen for bis-Pd. This complex is also electron paramagnetic resonance (EPR) silent over a range of temperatures. On the basis of crystallographic analyses, Raman spectroscopic studies, harmonic oscillator model of aromaticity (HOMA), and nucleus-independent chemical shift (NICS) calculations, we suggest that the dibenzo[ g, p]chrysene bridge in bis-Pd has quinoidal character and that the system as a whole is a closed shell species. As expected for a quinoidal system, bis-Pd is characterized by a lowest energy absorption band that is shifted into the NIR (λmax = ca. 1420 nm (ε > 1.5 × 105 M-1 cm-1) for bis-Pd vs 780 nm (ε < 5.0 × 103 M-1 cm-1) for bis-H3). On the other hand, bis-Pd displays solvent dependent ground state and transient absorption spectral features. Such findings provide support for a zwitterionic resonance contribution to what is a predominantly a quinonoid-type ground state. The use of specific metalation to fine-tune the electronic features of polytopic ligands, as reported here, opens the door to what might be a potentially generalizable approach to the design of quinoidal PAH structures with long wavelength solvatochromic absorption features.

Exploring the reversible host-guest chemistry of a crystalline octanuclear Ag(i) metallosupramolecular macrocycle formed from a simple pyrazinylpyridine ligand.

Here we report the synthesis of two new 2-(2'-pyrazinyl)pyridine ligands, and explore their coordination chemistry with Cu(ii) and Ag(i) ions, leading to the discovery of metallosupramolecular architectures of surprising complexity. Differing only in the substitution pattern of a nitrophenyl substituent attached to the 4-pyridyl position, ligands L1 and L2 both form sulfate-bridged dinuclear complexes on reaction with copper sulfate, without coordination of the nitrogen atom at the pyrazine 4-position. However, on reaction with Ag(i), both ligands adopt bridging coordination modes which lead to dramatically different outcomes. Ligand L2 reacts with AgCF3SO3 to give a densely packed coordination polymer 3 of unusually low symmetry (Z' = 7), while the structural isomer L1 instead gives an octanuclear macrocycle 4 in the crystalline state. Macrocycle 4, containing crystallographically-defined solvent molecules within its central cavity as well as in the interstitial spaces, readily undergoes solvent exchange in a single-crystal-to-single-crystal transformation. This allows a direct comparison of guest solvent affinity from single component and mixed-component exchange solutions using a combination of X-ray diffraction, NMR and TGA techniques, revealing the fully reversible uptake and exchange preferences of this material.