Switching of magnetic properties by topotactic reaction in a 1D CN-bridged Ni(II)-Nb(IV) system.

Two 1D CN-bridged assemblies: the nearly straight Li2[Ni(cyclam)][Nb(CN)8]·7.5H2O (1) chains and the zigzag-shaped Li2[Ni(cyclam)][Nb(CN)8]·2H2O (2) chains, are obtained in the reaction between [Ni(cyclam)]2+ and [Nb(CN)8]4- in warm concentrated LiCl water solution. Both compounds are composed of alternating bimetallic Ni(II)-Nb(IV) chains and contain incorporated lithium cations, which compensate the negative charge of the coordination skeleton. The straight chain 1 (Ni-Nb-Ni angle = 153.2°) can be reversibly dehydrated under dry nitrogen flow at room temperature to an intermediate dihydrate phase 1d and further transformed to the zigzag-shaped chain 2 (Ni-Nb-Ni angle = 86.6°) by annealing at 150 °C. The process can be reversed by exposure to high humidity at room temperature, upon which 2 is converted back to 1. This water sorption-induced breathing effect is accompanied by changes in magnetic properties, most notably reflected in different values of saturation magnetization and critical field of metamagnetic transition, which indicate that both intra- and inter-chain interactions are affected by the structure reorganization.

Crystal Engineering and Photomagnetic Studies of CN-Bridged Coordination Polymers Based on Octacyanidometallates(IV) and [Ni(cyclam)]2.

A series of new CN-bridged coordination networks of different dimensionality and topology was obtained through the modification of reaction conditions between [Ni(cyclam)]2+ (cyclam = 1,4,8,11-tetraazacyclotetradecane) and [W(CN)8]4-. The factors determining the reaction pathway are temperature and addition of the LiCl electrolyte. The products include three negatively charged frameworks incorporating Li+ guests: the 1D Li2[Ni(cyclam)][W(CN)8]·6H2O (1) straight chain, the 1D Li2[Ni(cyclam)][W(CN)8]·2H2O (2) zigzag chain, and the 2D Li2[Ni(cyclam)]3[W(CN)8]2·24H2O (3) honeycomb-like network, as well as the 3D two-fold interpenetrating [Ni(cyclam)]5[Ni(CN)4][W(CN)8]2·11H2O (4) network and the 1D [Ni(cyclam)][Ni(CN)4]·2H2O (5) chain, which result from partial decomposition of the starting complexes. Together with the previously characterized 3D [Ni(cyclam)]2[W(CN)8]·16H2O (6) network, they constitute the largest family of CN-bridged coordination polymers obtained from the same pair of building blocks. All compounds exhibit paramagnetic behavior because of the separation of paramagnetic nickel(II) centers through the diamagnetic polycyanidometallates. However, the presence of the photomagnetically active octacyanidotungstate(IV) ions allowed observation of the magnetic superexchange after the violet light excitation (405 nm) for compound 3, which constitutes the first example of the photomagnetic effect in a NiII-[WIV(CN)8] system. The photomagnetic investigations for fully hydrated and dehydrated sample of 3, as well as for the isostructural octacyanidomolybdate(IV)-based network are discussed.

Tuning of magnetic properties of the 2D CN-bridged NiII-NbIV framework by incorporation of guest cations of alkali and alkaline earth metals.

The reaction between [Ni(cyclam)]2+ (cyclam = 1,4,8,11-tetraazacyclotetradecane) and [Nb(CN)8]4- in concentrated water solutions of different s-block metal salts leads to the formation of 2-dimensional honeycomb-like coordination networks of the formula Mx[Ni(cyclam)]3[Nb(CN)8]2·nH2O (x = 2: M = Li+, Na+; x = 1: M = Mg2+, Ca2+, Sr2+, Ba2+). The CN-bridged Ni-Nb coordination layers are intersected by channels filled with crystallisation water molecules and guest mono- or di-valent metal cations, which compensate the negative charge of the framework. The structural details and crystal symmetry vary between the networks, depending on the arrangement of the water molecules and the intermolecular interactions enforced by the guest cations. All compounds show long range magnetic order arising from superexchange interactions between paramagnetic NiII (s = 1) and NbIV (s = 1/2) centres through CN-bridges within the layers and weaker inter-layer interactions mediated by H-bonds. The ordering temperature as well as the coercive field of the magnetic hysteresis can be tuned by the type of guest cation, with the highest values achieved for Mg2+ and the lowest for Na+.

Reversible Humidity-Driven Transformation of a Bimetallic {EuCo} Molecular Material: Structural, Sorption, and Photoluminescence Studies.

Functional molecule-based solids built of metal complexes can reveal a great impact of external stimuli upon their optical, magnetic, electric, and mechanical properties. We report a novel molecular material, {[EuIII(H2O)3(pyrone)4][CoIII(CN)6]}·nH2O (1, n = 2; 2, n = 1), which was obtained by the self-assembly of Eu3+ and [Co(CN)6]3- ions in the presence of a small 2-pyrrolidinone (pyrone) ligand in an aqueous medium. The as-synthesized material, 1, consists of dinuclear cyanido-bridged {EuCo} molecules accompanied by two H-bonded water molecules. By lowering the relative humidity (RH) below 30% at room temperature, 1 undergoes a single-crystal-to-single-crystal transformation related to the partial removal of crystallization water molecules which results in the new crystalline phase, 2. Both 1 and 2 solvates exhibit pronounced EuIII-centered visible photoluminescence. However, they differ in the energy splitting of the main emission band of a 5D0 → 7F2 origin, and the emission lifetime, which is longer in the partially dehydrated 2. As the 1 ↔ 2 structural transformation can be repeatedly reversed by changing the RH value, the reported material shows a room-temperature switching of detailed luminescent features including the ratio between emission components and the emission lifetime values.

Solvent- and Temperature-Driven Photoluminescence Modulation in Porous Hofmann-Type SrII-ReV Metal-Organic Frameworks.

A unique family of three-dimensional (3D) luminescent SrII-ReV metal-organic frameworks (MOFs), {[SrII(MeOH)5][ReV(CN)4(N)(bpen)0.5]·MeOH}n [1·MeOH; N3- = nitrido ligand, bpen = 1,2-bis(4-pyridyl)ethane, and MeOH = methanol], {[SrII(MeOH)4][ReV(CN)4(N)(bpee)0.5]·2MeOH}n [2·MeOH; bpee = 1,2-bis(4-pyridyl)ethylene], and {[SrII(bpy)0.5(MeOH)2][ReV(CN)4(N)(bpy)0.5]}n (3·MeOH; bpy = 4,4'-bipyridine), is reported. They are obtained by the molecular self-assembly of Sr2+ ions with tetracyanidonitridorhenate(V) metalloligands, [ReV(CN)4(N)]2-, and pyridine-based organic spacers (L = bpen, bpee, bpy). Such a combination of molecular precursors results in bimetallic SrII-ReV cyanido-bridged layers further bonded by organic ligands into pillared Hofmann-type coordination skeletons. Because of the formation of {ReV-(L)-ReV} moieties providing emissive metal-to-ligand charge-transfer states, 1·MeOH-3·MeOH exhibit solid-state room-temperature photoluminescence tunable from green to orange by the applied organic ligand. The most stable MOF of 3·MeOH, based on the alternating {ReV-(bpy)-ReV} and {SrII-(bpy)-SrII} linkages, exhibits three interconvertible, variously solvated phases, methanol-solvated 3·MeOH, hydrated {[SrII(bpy)0.5(H2O)2][ReV(CN)4(N)(bpy)0.5]·0.6H2O}n (3·H2O), and desolvated {[SrII(bpy)0.5][ReV(CN)4(N)(bpy)0.5]}n (3). Their formation was correlated with water and methanol vapor sorption properties investigated for 3·H2O. The solvent content affects the luminescence mainly by tuning the emission energy within the series of 3·MeOH, 3·H2O, and 3. All of the obtained compounds exhibit temperature-driven modulation of luminescence, including the shift of the emission maximum and lifetime. The thermochromic luminescent response was found to be sensitive to the presence and type of solvent in the crystal lattice. This work shows that the construction of [ReV(CN)4(N)]2--based MOFs is an efficient route toward advanced solid luminophores tunable by external stimuli such as solvent or temperature.

Proton Conductive Luminescent Thermometer Based on Near-Infrared Emissive {YbCo2} Molecular Nanomagnets.

Lanthanide(III)-based coordination complexes have been explored as a source of bifunctional molecular materials combining Single-Molecule Magnet (SMM) behavior with visible-to-near-infrared photoluminescence. In pursuit of more advanced multifunctionality, the next target is to functionalize crystalline solids based on emissive molecular nanomagnets toward high proton conductivity and an efficient luminescent thermometric effect. Here, a unique multifunctional molecule-based material, (H5O2)2(H)[YbIII(hmpa)4][CoIII(CN)6]2·0.2H2O (1, hmpa = hexamethylphosphoramide), composed of molecular {YbCo2}3- anions noncovalently bonded to acidic H5O2+ and H+ ions, is reported. The resulting YbIII complexes present a slow magnetic relaxation below 6 K and room temperature NIR 4f-centered photoluminescence sensitized by [Co(CN)6]3- ions. The microporous framework, built on these emissive magnetic molecules, exhibits a high proton conductivity of the H-hopping mechanism reaching σ of 1.7 × 10-4 S·cm-1 at 97% relative humidity, which classifies 1 as a superionic conductor. Moreover, the emission pattern is strongly temperature-dependent which was utilized in achieving a highly sensitive single-center luminescent thermometer with a relative thermal sensitivity, Sr > 1% K-1 in the 50-175 K range. This work shows an unprecedented combination of magnetic, optical, and electrical functionalities in a single phase working as a proton conductive NIR-emissive thermometer based on Single-Molecule Magnets.

Proton-Conducting Humidity-Sensitive NiII-NbIV Magnetic Coordination Network.

The 2D coordination network (NH4)2[NiII(cyclam)]3[NbIV(CN)8]2·21H2O (1·21H2O) was obtained on a cation-assisted synthetic pathway. The reaction between [Ni(cyclam)]2+ and [Nb(CN)8]4- in the presence of excess of NH4Cl resulted in the formation of negatively charged coordination layers with the simultaneous incorporation of the NH4+ cations into the microporous channels of the structure. 1·21H2O network can be partly dehydrated in a single-crystal-to-single-crystal structural transformation to give (NH4)2[NiII(cyclam)]3[NbIV(CN)8]2·14H2O (1·14H2O). The dehydration-induced structural changes, in particular the deformation of CN--bridges and the disruption of interlayer interactions, give rise to the solvatomagnetic effect. Fully hydrated 1·21H2O phase is a ferrimagnet with a critical temperature of magnetic ordering of 7.6 K and a narrow magnetization hysteresis loop, while 1·14H2O hydrate is an antiferromagnet with Tc = 7.2 K and metamagnetic transition at 6.3 kOe. Thanks to the presence of the NH4+ ions in the structure, the proton conductivity of ∼4 × 10-5 S cm-1 (295 K, 100% relative humidity, RH) is observed with the activation energy of 0.80 eV.