Recent reports on vanadium based coordination polymers and MOFs

Coordination polymers (CP) and metal-organic frameworks (MOF) have become a topic of immense interest in this century primarily because of the structural diversity that they offer. This structural diversity results in their multifaceted utility in various fields of science and technology such as catalysis, medicine, gas storage or separation, conductivity and magnetism. Their utility inspires a large variety of scientists to engage with them in their scientific pursuit thus creating a buzz around them in the scientific community. Metals capable of forming CPs and MOFs are primarily transition metals. Among them vanadium-based CPs and MOFs demand detailed discussion because of the unique nature of vanadium which makes it stable in many oxidation states and coordination number. Vanadium’s versatility imparts additional structural marvel and usefulness to these CPs and MOFs.

Toward Advanced Functional Systems: Honeycomb-Like Polymeric Surfaces Incorporating Polyoxovanadates with Surface-Appended Copper-Cyclam Complexes

In this work the immobilization of hybrid polyoxometalates (POMs) onto functional polymeric surfaces is exposed and discussed. Thus, various hybrid polymer‒inorganic films were prepared by anchoring selected hybrid POMs onto tailored polymeric surfaces that consisted of breath figures (BFs) made of polystyrene-b-poly(acrylic acid)/polystyrene (PS-b-PAA/PS) blends. Functionalization of the BF films was performed by selective arrangement of acrylic acid groups of the amphiphilic block copolymer on the surface pores because of their affinition for the water condensed during breath figure formation. These carboxylic acid functional groups contained within the PAA blocks were then employed to anchor [Cu(cyclam)][{Cu(cyclam)}₂(V₁₀O₂₈)]·10H2O (1-CuV10) and [{Cu(cyclam)}(VO₃)₂]·5H₂O (1-CuV1), hybrid POMs by immersing the films into aqueous solutions of the in situ formed hybrid clusters, resulting in the hybrid films BF1 and BF2, respectively. Superficial analysis of these hybrid polymeric films was carried out by the sophisticated ion beam-based technique time-of-flight secondary ion mass spectrometry (ToF-SIMS) that was revealed to be an excellent method for the superficial compositional mapping of patterned surfaces.

Thermostructural Behavior in a Series of Lanthanide-Containing Polyoxotungstate Hybrids with Copper(II) Complexes of the Tetraazamacrocycle Cyclam: A Single-Crystal-to-Single-Crystal Transformation Study

A series of 14 isostructural [Cu(cyclam)]₂[{Cu(cyclam)}₄{(α-GeW₁₁O₃₉)Ln(H₂O)(OAc)}₂]·18H₂O (1-Ln, where Ln = La-Lu; cyclam = 1,4,8,11-tetraazacyclotetradecane) polyoxometalate-based hybrids reported herein represent (i) the first example of a two-dimensional covalent hybrid lattice involving the [{(α-XW₁₁O₃₉)Ln(H₂O)(OAc)}₂] ^n- archetype and (ii) the first structural characterization of such a dimeric polyoxotungste for Ln = La and Pr as well as for the combination of X = Ge and Ln = Ce, Nd, Sm, or Lu. Compounds 1-Ln have been characterized by elemental analyses, infrared spectroscopy, and thermogravimetric analysis, and their thermostructural behavior has been monitored by powder and single-crystal X-ray diffraction. The title compounds undergo two single-crystal-to-single-crystal transformations triggered by thermal dehydration leading to the [{Cu(cyclam)}₆{(α-GeW₁₁O₃₉)Ln(H₂O)(OAc)}₂]·4H₂O intermediate (2-Ln, where Ln = Eu or Er) and [Cu(cyclam)]_0.5[{Cu(cyclam)}_5.5{(α-GeW₁₁O₃₉)Ln(OAc)}₂] (3-Ln, where Ln = Ce or Eu) final anhydrous phases, the latter evidencing a coordinatively unsaturated derivative of the dimeric archetype for the first time. These transitions involve formation and disruption of Cu-O_POM bonds that result in different {Cu(cyclam)}²⁺ moieties grafting onto and being released from Keggin surfaces, which reduces the dimensionality of 1-Ln to one-dimensional covalent assemblies for 2-Ln and 3-Ln. While all 3-Ln phases rehydrate fully upon exposure to air for 24 h, the kinetics governing the crystal transitions back toward 1-Ln through 2-Ln depend on the nature of Ln. Under ambient moisture, the anhydrous structures fully revert back to the parent framework for Ln = La-Sm, while the samples containing Eu to Lu afford mixtures of 1-Ln and 2-Ln and require immersion in water for the structural reversion to reach completion. Single-crystal X-ray diffraction analyses of the rehydrated 1R-Ln samples (Ln = Ce, Eu, and Er) support these observations.

Enantiopure versus Racemic Mixture in Reversible, Two-Step, Single-Crystal-to-Single-Crystal Transformations of Copper(II) Complexes

The reaction of chiral sodium complexes, ¹_∞ [Na(S-valmetH)]⋅H₂ O (1-S) and ¹_∞ [Na(R-valmetH)]⋅H₂ O (1-R), with copper(II) acetate affords chiral one-dimensional coordination polymers with the formulas ¹_∞ [Cu(S-valmet)(H₂ O)]⋅H₂ O (2-S) and ¹_∞ [Cu(R-valmet)(H₂ O)]⋅H₂ O (2-R) (R/S-valmetH₂ are Schiff base proligands resulting from the condensation reactions between o-vanillin and R/S-methionine). The copper ions are connected by the carboxylato groups belonging to the amino-acid moieties, resulting in infinite chains showing syn-anti out-of-plane bridging mode. The circular dichroism spectra of 1-S, 1-R, 2-S, and 2-R confirm their enantiomeric nature. Compounds 2-S and 2-R undergo a two-step single-crystal-to-single-crystal transformation, with the elimination of the lattice and coordinated water molecules: ¹_∞ [Cu(S-valmet)(H₂ O)]⋅H₂ O (2-S)→¹_∞ [Cu(S-valmet)]⋅H₂ O (3-S⋅H₂ O)→¹_∞ [Cu(S-valmet)] (3-S) and ¹_∞ [Cu(R-valmet)(H₂ O)]⋅H₂ O (2-R)→¹_∞ [Cu(R-valmet)]⋅H₂ O (3-R⋅H₂ O)→¹_∞ [Cu(R-valmet)] (3-R), respectively. During these transformations, every pair of face-to-face chains present in 2-S (or 2-R) has been "zipped up" into a chiral double chain through the removal of the aqua ligands and their replacement by the carboxylato oxygen atoms from the neighboring chain. Consequently, each carboxylato group now bridges three copper ions. The conversion of the single chains, 2-S and 2-R, into the double chains, 3-S and 3-R, is accompanied by a change of the strength of the exchange interactions between the copper ions: weak antiferromagnetic couplings are observed in compound 2-S (J/k_B =-1.23(5) K, H=-2J ΣS_i S_i+1 ) and relatively strong in compound 3-S (J/k_B =-76.0(8) K). When the racemic mixture of the ligands, R,S-valmetH₂ , is employed, in the same experimental conditions, a racemic mixture of mononuclear compounds, [Cu(R,S-valmet)(H₂ O)₂ ]⋅H₂ O (4-RS), is obtained. Compound 4-RS also undergoes a SCSC transformation with the elimination of the lattice and one of the coordinated water molecules, resulting in a racemic mixture of chiral chains, ¹_∞ [Cu(R-valmet)(H₂ O)]⋅¹_∞ [Cu(S-valmet)(H₂ O)] (5-RS). In this compound, the coupling of the copper(II) ions within the chains is weak and ferromagnetic (J/k_B =+0.10(2) K). These results prove that the chirality of the valmetH₂ ligands (optically pure or racemic mixture) plays a key role in the self-assembly process of the copper(II) complexes.

A Robust Open Framework Formed by Decavanadate Clusters and Copper(II) Complexes of Macrocyclic Polyamines: Permanent Microporosity and Catalytic Oxidation of Cycloalkanes

The first decavanadate-based microporous hybrid, namely, [Cu(cyclam)][{Cu(cyclam)}2(V10O28)]·10H2O (1, cyclam = 1,4,8,11-tetraazacyclotetradecane) was prepared by reaction of (VO3)(-) anions and {Cu(cyclam)}(2+) complexes in NaCl (aq) at pH 4.6-4.7 and characterized by elemental analyses, thermogravimetry, and X-ray diffraction (powder, single-crystal) techniques. Compound 1 exhibits a POMOF-like supramolecular open-framework built of covalent decavanadate/metalorganic layers with square-like voids, the stacking of which is aided by interlamellar cementing complexes and generates water-filled channels with approximate cross sections of 10.4 × 8.8 Å(2). The framework is robust enough to remain virtually unaltered upon thermal evacuation of all water molecules of hydration, as demonstrated through single-crystal X-ray diffraction studies on the anhydrous phase 1a. This permanent microporosity renders interesting functionality to 1, such as selective adsorption of CO2 over N2 and remarkable activity as heterogeneous catalyst toward the H2O2-based oxidation of the highly-stable, tricyclic alkane adamantane.

Temperature-induced single-crystal-to-single-crystal transformation of a binuclear Mn(ii) complex into a 1D chain polymer

The new binuclear Mn(ii) complex can transform into a new 1D chain structure in a SCSC transformation via dehydration.