Titanium-Oxo Clusters with Bi- and Tridentate Organic Ligands: Gradual Evolution of the Structures from Small to Big

Homometallic titanium oxo clusters are one of the most important groups of metal oxo clusters, with more than 300 examples characterized by X-ray structure analyses. Most of them are uncharged and are obtained by partial hydrolysis and condensation of titanium alkoxo derivatives. The cluster cores, ranging from 3 to >50 titanium atoms, are stabilized by organic ligands. Apart from residual OR groups, carboxylato and phosphonato ligands are most frequent. The article critically reviews and categorizes the known structures and works out basic construction principles by comparing the different cluster types.

A water-stable molecular cadmium phosphonate bearing 2-(2-pyridyl)benzimidazole as a highly sensitive luminescence sensor for the selective detection of bisphenol AF and bisphenol B

A highly water-stable molecular cadmium phosphonate bearing 2-(2-pyridyl)benzimidazole has been used as a sensor platform for the luminescence detection of bisphenol AF (BPAF) and bisphenol B (BPB) in water with good sensitivity and selectivity.

Optimizing the Proton Conductivity of Fe-Diphosphonates by Increasing the Relative Number of Protons and Carrier Densities

Proton conductive materials have attracted extensive interest in recent years due to their fascinating applications in sensors, batteries, and proton exchange membrane fuel cells. Herein, two Fe-diphosphonate chains (H₄-BAPEN)_0.5·[Fe^III(H-HEDP)(HEDP)_0.5(H₂O)] (1) and (H₄-TETA)₂·[Fe^III₂Fe^II(H-HEDP)₂(HEDP)₂(OH)₂]·2H₂O (2) (HEDP = 1-hydroxyethylidenediphosphonate, BAPEN = 1,2-bis(3-aminopropylamino)ethane, and TETA = triethylenetetramine) with different templating agents were prepared by hydrothermal reactions. The valence states of the Fe centers were demonstrated by ⁵⁷Fe Mössbauer spectra at 100 K, with a high-spin Fe^III state for 1 and mixed high-spin Fe^III/Fe^II states for 2. Their magnetic properties were determined, which featured strong antiferromagnetic couplings in the chain. Importantly, the proton conductivity of both compounds at 100% relative humidity was explored at different temperatures, with 2.79 × 10^-4 S cm^-1 at 80 °C for 1 and 7.55 × 10^-4 S cm^-1 at 45 °C for 2, respectively. This work provides an opportunity for improving proton conductive properties by increasing the relative number of protons and the carrier density using protonated flexible aliphatic amines.

New Directions in Metal Phosphonate and Phosphinate Chemistry

In September 2018, the First European Workshop on Metal Phosphonates Chemistry brought together some prominent researchers in the field of metal phosphonates and phosphinates with the aim of discussing past and current research efforts and identifying future directions. The scope of this perspective article is to provide a critical overview of the topics discussed during the workshop, which are divided into two main areas: synthesis and characterisation, and applications. In terms of synthetic methods, there has been a push towards cleaner and more efficient approaches. This has led to the introduction of high-throughput synthesis and mechanochemical synthesis. The recent success of metal–organic frameworks has also promoted renewed interest in the synthesis of porous metal phosphonates and phosphinates. Regarding characterisation, the main advances are the development of electron diffraction as a tool for crystal structure determination and the deployment of in situ characterisation techniques, which have allowed for a better understanding of reaction pathways. In terms of applications, metal phosphonates have been found to be suitable materials for several purposes: they have been employed as heterogeneous catalysts for the synthesis of fine chemicals, as solid sorbents for gas separation, notably CO2 capture, as materials for electrochemical devices, such as fuel cells and rechargeable batteries, and as matrices for drug delivery.

Catechol thioethers with physiologically active fragments: Electrochemistry, antioxidant and cryoprotective activities

A number of asymmetrical thioethers based on 3,5-di-tert-butylcatechol containing sulfur atom bonding with physiologically active groups in the sixth position of aromatic ring have been synthesized and the electrochemical properties, antioxidant, cryoprotective activities of new thioethers have been evaluated. Cyclic voltammetry was used to estimate the oxidation potentials of thioethers in acetonitrile. The electrooxidation of compounds at the first stage leads to the formation of o-benzoquinones. The antioxidant activities of the compounds were determined using 2,2'-diphenyl-1-picrylhydrazyl radical (DPPH) assay, experiments on the oxidative damage of the DNA, the reaction of 2,2'-azobis(2-amidinopropane hydrochloride) (AAPH) induced glutathione depletion (GSH), the process of lipid peroxidation of rat liver (Wistar) homogenates in vitro, and iron(II) chelation test. Compounds 1-9 have greater antioxidant effectiveness than 3,5-di-tert-butylcatechol (CatH₂) in all assays. The variation of physiologically active groups at sulfur atom allows to regulate lipophilic properties and antioxidant activity of compounds. Thioethers 3, 4 and 7 demonstrate the combination of radical scavenging, antioxidant activity and iron(II) binding properties. The researched compounds 1-9 were studied as possible cryoprotectants of the media for cryopreservation of the Russian sturgeon sperm. Novel cryoprotective additives in cryomedium reduce significantly the content of membrane-permeating agent (DMSO). A cryoprotective effect of an addition of the catechol thioethers depends on the structure of groups at sulfur atom. The cryoprotective properties of compounds 3, 4 and 7 are caused by combination of catechol fragment, bonded by a thioether linker with a long hydrocarbon chain and a terminal ionizable group or with a biologically relevant acetylcysteine residue.

Zinc-diphosphonates with extended dipyridine units: synthesis, structures, in situ reactions, and photochromism

We report two zinc-diphosphonates bearing extended dipyridine units. In situ reactions and photochromism are observable in the title compounds.

Self-Assembled Bilayers on Nanocrystalline Metal Oxides: Exploring the Non-Innocent Nature of the Linking Ions

Self-assembled bilayers on nanocrystalline metal oxide films are an increasingly popular strategy for modulating electron and energy transfer at dye-semiconductor interfaces. A majority of the work to date has relied on Zr^II and Zn^IV linking ions to assemble the films. In this report, we demonstrate that several different cations (Cd^II, Cu^II, Fe^II, La^III, Mn^II, and Sn^IV) are not only effective in generating the bilayer assemblies but also have a profound influence on the stability and photophysical properties of the films. Bilayer films with Zr^IV ions exhibited the highest photostability on both TiO₂ and ZrO₂. Despite the metal ions having a minimal influence on the absorption/emission energies and oxidation potentials of the dye, bilayers composed of Cu^II, Fe^II, and Mn^II exhibit significant excited-state quenching. The excited-state quenching decreases the electron injection yield but also, for Cu^II and Mn^II bilayers, significantly slows the back electron transfer kinetics.

Inorganic Phosphate and Arsenate within New Tetranuclear Copper and Zinc Complexes: Syntheses, Crystal Structures, Magnetic, Electrochemical, and Thermal Studies

Three, PO₄ ^3-/HPO₄ ^2- and AsO₄ ^3--incorporated, new tetranuclear complexes of copper(II) and zinc(II) ions have been synthesized and fully characterized. In methanol-water, reactions of H₃cpdp (H₃cpdp = N,N'-Bis[2-carboxybenzomethyl]-N,N'-Bis[2-pyridylmethyl]-1,3-diaminopropan-2-ol) with copper(II) chloride in the presence of either NaOH/Na₂HPO₄·2H₂O or KOH/Na₂HAsO₄·7H₂O lead to the isolation of the tetranuclear complexes Na₃[Cu₄(cpdp)₂(μ₄-PO₄)](OH)₂·14H₂O (1) and K₂[Cu₄(cpdp)₂(μ₄-AsO₄)](OH)·16²/₃H₂O (2), respectively. Similarly, the reaction of H₃cpdp with zinc(II) chloride in the presence of NaOH/Na₂HPO₄·2H₂O yields a tetranuclear complex, Na(H₃O)₂[Zn₄(cpdp)₂(μ₄-HPO₄)]Cl₃·12¹/₂H₂O (3). All complexes are characterized by single-crystal X-ray diffraction and other analytical techniques, such as Fourier transform infrared and UV-vis spectroscopy, thermogravimetric and electrochemical studies. The solid-state molecular framework of each complex contains two monocationic [M₂(cpdp)]⁺ (M = Cu, Zn) units, which are exclusively coordinated to either phosphate/hydrogen phosphate or arsenate groups in a unique mode. All three complexes exhibit a μ₄:η¹:η¹:η¹:η¹ bridging mode of the PO₄ ^3-/HPO₄ ^2-/AsO₄ ^3- groups, with each bridging among four metal ions. The thermal properties of all three complexes have been investigated by thermogravimetric analysis. Low-temperature magnetic studies of complexes 1 and 2 disclose moderate antiferromagnetic interactions mediated among the copper centers through alkoxide and phosphate/arsenate bridges. Electrochemical studies of complexes 1 and 2 in dimethylformamide using cyclic voltammetry reveal the presence of a fairly assessable one-electron metal-based irreversible reduction and one quasireversible oxidation couple.

Phosphonate Based High Nuclearity Magnetic Cages

Transition metal based high nuclearity molecular magnetic cages are a very important class of compounds owing to their potential applications in fabricating new generation molecular magnets such as single molecular magnets, magnetic refrigerants, etc. Most of the reported polynuclear cages contain carboxylates or alkoxides as ligands. However, the binding ability of phosphonates with transition metal ions is stronger than the carboxylates or alkoxides. The presence of three oxygen donor sites enables phosphonates to bridge up to nine metal centers simultaneously. But very few phosphonate based transition metal cages were reported in the literature until recently, mainly because of synthetic difficulties, propensity to result in layered compounds, and also their poor crystalline properties. Accordingly, various synthetic strategies have been followed by several groups in order to overcome such synthetic difficulties. These strategies mainly include use of small preformed metal precursors, proper choice of coligands along with the phosphonate ligands, and use of sterically hindered bulky phosphonate ligands. Currently, the phosphonate system offers a library of high nuclearity transition metal and mixed metal (3d-4f) cages with aesthetically pleasing structures and interesting magnetic properties. This Account is in the form of a research landscape on our efforts to synthesize and characterize new types of phosphonate based high nuclearity paramagnetic transition metal cages. We quite often experienced synthetic difficulties with such versatile systems in assembling high nuclearity metal cages. Few methods have been emphasized for the self-assembly of phosphonate systems with suitable transition metal ions in achieving high nuclearity. We highlighted our journey from 2005 until today for phosphonate based high nuclearity transition metal cages with V(IV/V), Mn(II/III), Fe(III), Co(II), Ni(II), and Cu(II) metal ions and their magnetic properties. We observed that slight changes in stoichiometry, reaction conditions, and presence or absence of coligand played crucial roles in determining the final structure of these complexes. Most of the complexes included are regular in geometry with a dense arrangement of the above-mentioned metal centers in a confined space, and a few of them also resemble regular polygonal solids (Archimedean and Platonic). Since there needs to be a historical approach for a comparative study, significant research output reported by other groups is also compared in brief to ensure the potential of phosphonate ligands in synthesizing high nuclearity magnetic cages.

Alkylzinc diorganophosphates: synthesis, structural diversity and unique ability to incorporate zincoxane units

New tetrameric alkylzinc organophosphates of a drum-like structure and novel aggregates based on alkylzinc organophosphates with incorporated zincoxane units were synthesized and structurally characterized.

Monitoring organic reactions by UF-NMR spectroscopy

Standard 2D NMR experiments suffer from the many t1 increments needed for spectra with sufficient digital resolution in the indirect dimension. Despite the different methodological approaches to overcome this problem, these increments have prevented studies of fast reactions. The development of ultrafast NMR (UF-NMR) has decisively speeded up the time scale of standard NMR to allow the study of organic reactions as they happen in real time to reveal mechanistic details. This mini-review summarizes the results achieved in monitoring organic reactions through this exciting technique.

Phosphonate-substituted zirconium oxo clusters

ABSTRACT

The phosphonate-substituted zirconium oxo clusters Zr6O2(OBu)12(O3PPh)4 and Zr7O2(OiPr)12(O3PCH2CH2CH2Br)6, with octahedrally coordinated Zr atoms, were synthesized by reaction of zirconium alkoxides with phosphonic acid bis(trimethylsilyl) esters. The basic structural motif are Zr3O(µ2-OR)3(OR)3 units which are connected in different ways.

GRAPHICAL ABSTRACT

Titanium oxo/alkoxo clusters with both phosphonate and methacrylate ligands

ABSTRACT

The clusters Ti5O(OiPr)11(OMc)(O3PR)3 (OMc = methacrylate; R = Et, CH2CH2CH2Br) and Ti10(OiPr)16(OMc)4(O3PCH2CH=CH2)10 were obtained when Ti(OiPr)4 was reacted with the corresponding bis(trimethylsilyl) phosphonate and methacrylic acid. Oxo clusters of the composition Ti6O4(OiPr)10(OMc)2(O3PR)2, with a variety of groups R (Et, Ph, CH=CH2, CH2Ph, CH2CH=CH2, CH2CH2CH2Br, CH2CH2CN, CH2C(O)Me, CH2CH2OC(O)C(Me)=CH2), were formed instead, when a stoichiometric amount of water was added to the reaction mixture.

GRAPHICAL ABSTRACT

Discrete and polymeric cobalt organophosphates: isolation of a 3-D cobalt phosphate framework exhibiting selective CO2 capture

Auxiliary ligand assisted control over the structural diversity has been achieved in the case of cobalt(ii) organophosphates.

Nanoscopic molecular magnets

This review highlights fundamental concepts and synthetic strategies of SMMs and selected examples of 3d, 4f, 5f and mixed 3d–4f, 4d–5d and 3d–5f based SMMs are discussed.

Synthesis, crystal structures and magnetic properties of a family of manganese phosphonate clusters with diverse structures

Four manganese clusters containing tert-butylphosphonate ligands with diverse structures have been synthesized and characterized.

Molecular and polymeric zinc(II) phosphonates: isolation of an octanuclear ellipsoidal ensemble

The reaction of zinc(II) perchlorate with trichloromethyl phosphonic acid at room temperature afforded, upon crystallization, a two-dimensional layered coordination polymer possessing a dinuclear repeat unit, [{Zn2(Cl3CPO3)2(H2O)3}·1.5H2O]n (1). Modification of the above reaction by involving a co-ligand afforded the tetranuclear complex, [{Zn4(η(1)-DMPzH)6(Cl3C-PO3)2}(μ-OH)2(ClO4)2] (2). The molecular structure of 2 reveals that the tetranuclear core is non-planar and consists of three contiguous inorganic rings which include one 8-membered Zn2P2O4 ring and two six-membered Zn2PO3 rings. Replacement of Zn(ClO4)2·6H2O with ZnCl2 under the same reaction conditions that afforded 2 allowed the formation of the dinuclear complex [{(ZnCl)2(η(2)-Pz)2(Cl3CPO3)}(Et3NH)2] (3). 3 possesses a bicyclic core containing a seven-membered Zn2N2O2P ring. In 3, the phosphoryl oxygen atom (P=O) is involved in a bifurcated hydrogen bonding interaction with the triethylammonium cation. The reaction of ZnCl2 and 2,3,5,6-(Me)4C6HCH2PO3H2 afforded the octanuclear complex [Zn8(Cl)6{2,3,5,6-(Me)4C6HCH2PO3}6(Et3N)2](Et3NH)2]·2n-hexane·3H2O (4). The core of 4 is ellipsoid-shaped with the end-end polar distance (C-C) being ~20 Å.

Acetic Acid Mediated Synthesis of Phosphonate-Substituted Titanium Oxo Clusters

New phosphonate/acetate-substituted titanium oxo/alkoxo clusters were prepared from Ti(OiPr)4 and bis(trimethylsilyl) phosphonates in the presence of acetic acid, which served to generate water in situ through ester formation. The process led to clusters with a higher degree of condensation than in previously known phosphonate-substituted titanium oxo clusters. The clusters [Ti6O4(OiPr)10(OAc)2(O3PR)2] (OAc = acetate) were obtained for a large variety of functional and non-functional groups R under a range of reaction conditions. This cluster type, which is also retained in solution, therefore appears to be very robust. Two other clusters, [Ti5O(OiPr)11(OAc)(O3PCH2CH2CH2Br)3] and [Ti5O3(OiPr)6(OAc)4(O3P-xylyl)2], were only isolated in special cases.

Four new trinuclear {Cu3(μ3-OH)(oximate)3}2+ clusters: crystal structure and magnetic behaviour

Four new triangular copper(ii) complexes with the fragment {Cu₃(μ₃-OH)(oximate)₃}²⁺ are reported. They exhibit strong antiferromagnetic coupling and antisymmetric exchange interaction.

Di-, tri- and tetranuclear molecular vanadium phosphonates: a chloride encapsulated tetranuclear bowl

Synthesis and characterization of three dinuclear, two dinuclear and one tetranuclear vanadium phosphonates is being reported. These include a chloride encapsulated tetranuclear bowl-shaped complex.

Formation of a non-porous cobalt-phosphonate framework by small pH change in the preparation of the microporous STA-16(Co)

The initial pH of the suspension was found to determine whether the porous STA-16(Co) phase or a non-porous Co(ii)-phosphonate pillared layer structure was formed.

Can anisotropic exchange be reliably calculated using density functional methods? A case study on trinuclear Mn(III)-M(III)-Mn(III) (M=Fe, Ru, and Os) cyanometalate single-molecule magnets

Density functional studies have been performed on a set of trinuclear single-molecule magnets (SMMs) of general formula [{Mn2(5-Br salen)2(MeOH)2}M(CN)6](NEt4) (M=Fe(III) (1), Ru(III) (2) and Os(III) (3); 5-Brsalen=N,N'-ethylenebis(5-bromosalicylidene)iminato anion). We have computed the orbital-dependent exchange interaction for all three complexes for the first time using DFT and complete active space self-consistent field (CASSCF) methods. DFT calculations yield the anisotropic exchange as J(ξξ)=3.5 cm(-1) for 1; J(ξξ)=12.1 cm(-1), J(ζζ)=-6.9 cm(-1) and J(ηη)=-14 cm(-1) for 2; and J(ξξ)=23.7 cm(-1) and J(ζζ) =-11.1 cm(-1) for 3. The computed values are in agreement with the experimental report, and this suggests that the established methodology can be used to compute the anisotropic exchange in larger clusters. Our calculations reiterate the fact that the exchange is described by a three-axis anisotropic exchange for complexes 2 and 3 as evidenced by the experiments. A stronger exchange coupling as we move down the periodic table from 3d to 5d is reproduced by our calculations, and the origin of this enhancement in the exchange interaction has been probed by using molecular orbital analysis. The electronic origin of different types of exchange observed in this series is found to be related to the energy difference between possible degenerate pairs and the nature of orbital interactions. By computing the exchange interaction, the single-ion anisotropy of Mn(III) and zero-field splitting of the S=9/2 ground state of complexes 1-3 using CASSCF and/or DFT methods, we have attempted to shed light on the issue of anisotropic exchange and the barrier height for the magnetisation reversal in SMMs. Comprehensive magneto-structural correlations have been developed to offer clues on how to further enhance the barrier height in this class of SMMs.