Lanthanide Hexacyanidoruthenate Frameworks for Multicolor to White-Light Emission Realized by the Combination of d-d, d-f, and f-f Electronic Transitions.

We report an effective strategy toward tunable room-temperature multicolor to white-light emission realized by mixing three different lanthanide ions (Sm3+, Tb3+, and Ce3+) in three-dimensional (3D) coordination frameworks based on hexacyanidoruthenate(II) metalloligands. Mono-lanthanide compounds, K{LnIII(H2O)n[RuII(CN)6]}·mH2O (1, Ln = La, n = 3, m = 1.2; 2, Ln = Ce, n = 3, m = 1.3; 3, Ln = Sm, n = 2, m = 2.4; 4, Ln = Tb, n = 2, m = 2.4) are 3D cyanido-bridged networks based on the Ln-NC-Ru linkages, with cavities occupied by K+ ions and water molecules. They crystallize differently for larger (1, 2) and smaller (3, 4) lanthanides, in the hexagonal P63/m or the orthorhombic Cmcm space groups, respectively. All exhibit luminescence under the UV excitation, including weak blue emission in 1 due to the d-d 3T1g → 1A1g electronic transition of RuII, as well as much stronger blue emission in 2 related to the d-f 2D3/2 → 2F5/2,7/2 transitions of CeIII, red emission in 3 due to the f-f 4G5/2 → 6H5/2,7/2,9/2,11/2 transitions of SmIII, and green emission in 4 related to the f-f 5D4 → 7F6,5,4,3 transitions of TbIII. The lanthanide emissions, especially those of SmIII, take advantage of the RuII-to-LnIII energy transfer. The CeIII and TbIII emissions are also supported by the excitation of the d-f electronic states. Exploring emission features of the LnIII-RuII networks, two series of heterobi-lanthanide systems, K{SmxCe1-x(H2O)n[Ru(CN)6]}·mH2O (x = 0.47, 0.88, 0.88, 0.99, 0.998; 5-9) and K{TbxCe1-x(H2O)n[Ru(CN)6]}·mH2O (x = 0.56, 0.65, 0.93, 0.99, 0.997; 10-14) were prepared. They exhibit the composition- and excitation-dependent tuning of emission from blue to red and blue to green, respectively. Finally, the heterotri-lanthanide system of the K{Sm0.4Tb0.599Ce0.001(H2O)2[Ru(CN)6]}·2.5H2O (15) composition shows the rich emission spectrum consisting of the peaks related to CeIII, TbIII, and SmIII centers, which gives the emission color tuning from blue to orange and white-light emission of the CIE 1931 xy parameters of 0.325, 0.333.

Site Selectivity for the Spin States and Spin Crossover in Undecanuclear Heterometallic Cyanido-Bridged Clusters.

Polynuclear molecular clusters offer an opportunity to design new hierarchical switchable materials with collective properties, based on variation of the chemical composition, size, shapes, and overall building blocks organization. In this study, we rationally designed and constructed an unprecedented series of cyanido-bridged nanoclusters realizing new undecanuclear topology: FeII[FeII(bzbpen)]6[WV(CN)8]2[WIV(CN)8]2·18MeOH (1), NaI[CoII(bzbpen)]6[WV(CN)8]3[WIV(CN)8]·28MeOH (2), NaI[NiII(bzbpen)]6[WV(CN)8]3[WIV(CN)8]·27MeOH (3), and CoII[CoII(R/S-pabh)2]6[WV(CN)8]2[WIV(CN)8]2·26MeOH [4R and 4S; bzbpen = N1,N2-dibenzyl-N1,N2-bis(pyridin-2-ylmethyl)ethane-1,2-diamine; R/S-pabh = (R/S)-N-(1-naphthyl)-1-(pyridin-2-yl)methanimine], of size up to 11 nm3, ca. 2.0 × 2.2 × 2.5 nm (1-3) and ca. 1.4 × 2.5 × 2.5 nm (4). 1, 2, and 4 exhibit site selectivity for the spin states and spin transition related to the structural speciation based on subtle exogenous and endogenous effects imposed on similar but distinguishable 3d metal-ion-coordination moieties. 1 exhibits a mid-temperature-range spin-crossover (SCO) behavior that is more advanced than the previously reported SCO clusters based on octacyanidometallates and an onset of SCO behavior close to room temperature. The latter feature is also present in 2 and 4, which suggests the emergence of CoII-centered SCO not observed in previous bimetallic cyanido-bridged CoII-WV/IV systems. In addition, reversible switching of the SCO behavior in 1 via a single-crystal-to-single-crystal transformation during desolvation was also documented.

Octacyanidometallates for multifunctional molecule-based materials.

Octacyanidometallates have been successfully employed in the design of heterometallic coordination systems offering a spectacular range of desired physical properties with great potential for technological applications. The [M(CN)8]n- ions comprise a series of complexes of heavy transition metals in high oxidation states, including NbIV, MoIV/V, WIV/V, and ReV. Since the discovery of the pioneering bimetallic {MnII4[MIV(CN)8]2} and {MnII9[MV(CN)8]6} (M = Mo, W) molecules in 2000, octacyanidometallates were fruitfully explored as precursors for the construction of diverse d-d or d-f coordination clusters and frameworks which could be obtained in the crystalline form under mild synthetic conditions. The primary interest in [M(CN)8]n--based networks was focused on their application as molecule-based magnets exhibiting long-range magnetic ordering resulting from the efficient intermetallic exchange coupling mediated by cyanido bridges. However, in the last few years, octacyanidometallate-based materials proved to offer varied and remarkable functionalities, becoming efficient building blocks for the construction of molecular nanomagnets, magnetic coolers, spin transition materials, photomagnets, solvato-magnetic materials, including molecular magnetic sponges, luminescent magnets, chiral magnets and photomagnets, SHG-active magnetic materials, pyro- and ferroelectrics, ionic conductors as well as electrochemical containers. Some of these materials can be processed into the nanoscale opening the route towards the development of magnetic, optical and electronic devices. In this review, we summarise all important achievements in the field of octacyanidometallate-based functional materials, with the particular attention to the most recent advances, and present a thorough discussion on non-trivial structural and electronic features of [M(CN)8]n- ions, which are purposefully explored to introduce desired physical properties and their combinations towards advanced multifunctional materials.

Room-Temperature Bistability in a Ni-Fe Chain: Electron Transfer Controlled by Temperature, Pressure, Light, and Humidity.

Bistable and stimuli-responsive molecule-based materials are promising candidates for the development of molecular switches and sensors for future technologies. The CN-bridged {NH4 [Ni(cyclam)][Fe(CN)6 ]⋅5 H2 O}n chain exists in two valence states: NiII -FeIII (1HT ) and NiIII -FeII (1LT ) and shows unique multiresponsivity under ambient conditions to various stimuli, including temperature, pressure, light, and humidity, which generate measurable response in the form of significant changes in magnetic susceptibility and color. The electron-transfer phase transition 1LT ↔1HT shows room-temperature thermal hysteresis, can be induced by irradiation, and shows high sensitivity to small applied pressure, which shifts it to higher temperatures. Additionally, it can be reversibly turned off by dehydration to the {NH4 [NiII (cyclam)][FeIII (CN)6 ]}n (1 d) phase, which features the NiII -FeIII valence state over the whole temperature range, but responds to pressure by yielding NiIII -FeII above 1.06 GPa.

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.

Guest-Dependent Pressure-Induced Spin Crossover in FeII 4 [MIV (CN)8 ]2 (M=Mo, W) Cluster-Based Material Showing Persistent Solvent-Driven Structural Transformations.

Discrete molecular species that can perform certain functions in response to multiple external stimuli constitute a special class of multifunctional molecular materials called smart molecules. Herein, cyanido-bridged coordination clusters {[FeII (2-pyrpy)2 ]4 [MIV (CN)8 ]2 }⋅4 MeOH⋅6 H2 O (M=Mo (1 solv), M=W (2 solv) and 2-pyrpy=2-(1-pyrazolyl)pyridine are presented, which show persistent solvent driven single-crystal-to-single-crystal transformations upon sorption/desorption of water and methanol molecules. Three full desolvation-resolvation cycles with the concomitant change of the host molecules do not damage the single crystals. More importantly, the Fe4 M2 molecules constitute a unique example where the presence of the guests directly affects the pressure-induced thermal spin crossover (SCO) phenomenon occurring at the FeII centres. The hydrated phases show a partial SCO with approximately two out-of-four FeII centres undergoing a gradual thermal SCO at 1 GPa, while in the anhydrous form the pressure-induced SCO effect is almost quenched with only 15 % of the FeII centres undergoing high-spin to low-spin transition at 1 GPa.

Europium(III) Photoluminescence Governed by d8-d10 Heterometallophilic Interactions in Trimetallic Cyanido-Bridged Coordination Frameworks.

We report an efficient pathway toward sensitization of red room temperature EuIII emission by the charge-transfer (CT) states related to d8-d10 heterometallophilic interactions achieved by the simultaneous application of tetracyanidometallates of PtII/PdII and dicyanidometallates of AuI/AgI in the construction of a trimetallic d-d-f assembly. The combination of Eu3+, [MII(CN)4]2- (M = Pt, Pd), and [MI(CN)2]- (M = Au, Ag) ions along with 4,4'-bipyridine N,N'-dioxide (4,4'-bpdo) results in four novel isostructural 2D {[EuIII(4,4'-bpdo)(H2O)2][MII(CN)4]}·[MI(CN)2]·H2O (MII/MI = Pt/Au, 1; Pt/Ag, 2; Pd/Au, 3; Pd/Ag, 4) coordination networks. They are built of hybrid coordination layers, based on cyanido-bridged {EuIII[MII(CN)4]}n square grids coexisting with metal-organic {EuIII(4,4'-bpdo)}n chains, with the further attachment of [MI(CN)2]- ions through metallophilic {MII-MI} interactions. This results in dinuclear {MIIMI} units generating an orange emissive metal-to-metal-to-ligand charge-transfer (MMLCT) state, whose energy is tuned by the applied d8-d10 metal centers. Thanks to these CT states, 1-4 exhibit room temperature red EuIII photoluminescence enhanced by energy transfer from {MIIMI} units, with the additional role of 4,4'-bpdo also transferring the energy to lanthanides. These donor CT states lying in the visible range successfully broaden the available efficient excitation range up to 500 nm. The overall emission quantum yield ranges from 8(1)% for 4 to 15(2)% for 1, with the intermediate values for 2 and 3 relatively high among the reported EuIII-based compounds with tetracyanido- and dicyanidometallates. We found that the sensitization efficiency is equally high for all compounds because of the similar energies of the CT states, while the main differences are related to the observed emission lifetimes ranging from ca. 80 µs for 4 to 120-130 µs for 2 and 3 to ca. 180 µs for 1. This phenomenon was correlated with the energies of the vibrational states, e.g., cyanide stretching vibrations, responsible for nonradiative deactivation of EuIII excited states, which are the highest for the Pd/Ag pair of 4 and the lowest for the Pt/Au pair in 1. Thus, the heaviest pair of PtII/AuI cyanide metal complexes is proven to be the best candidate for the sensitization of room temperature EuIII luminescence.

Chiral Photomagnets Based on Copper(II) complexes of 1,2-Diaminocyclohexane and Octacyanidomolybdate(IV) Ions.

Chiral photomagnets compose a class of multifunctional molecule-based materials with light-induced alteration of magnetization and chiral properties. The rational design and synthesis of such assemblies is a challenge, and only few such systems are known. Herein, the remarkable octacyanide-bridged enantiomeric pair of 1-D chains [Cu((R,R)-chxn)2]2[Mo(CN)8]·H2O (1R) and [Cu((S,S)-chxn)2]2[Mo(CN)8]·H2O (1S) exhibiting enantiopure structural helicity, which results in optical activity in the 350-800 nm range as confirmed by natural circular dichroism (NCD) spectra, is reported. The photomagnetic effects of 1R, 1S, and 1rac result from the blue light excitation (436 nm) of the photomagnetically active octacyanidomolybdate(IV) ions. In the excited state MoIVHS centers with S = 1 couple antiferromagnetically with the neighboring CuII centers with JCuMo values of -1.3, -1.0, and -1.1 cm-1 for 1R, 1S, and 1rac, respectively. The values of thermal relaxation energy barriers have been estimated as 142 and 356 K for 1R and 1S, being comparable with the energy range of the thermal bath. The value for 1rac reveals a significantly lower value of 75 K. On the basis of these results the value of gMoHS has been estimated to be in the range 4.8-5.8.

Octacyanidorhenate(V) Ion as an Efficient Linker for Hysteretic Two-Step Iron(II) Spin Crossover Switchable by Temperature, Light, and Pressure.

A two-step hysteretic FeII spin crossover (SCO) effect was achieved in programmed layered Cs{[Fe(3-CNpy)2 ][Re(CN)8 ]}⋅H2 O (1) (3-CNpy=3-cyanopyridine) assembly consisting of cyanido-bridged FeII -ReV square grid sheets bonded by Cs+ ions. The presence of two non-equivalent FeII sites and the conjunction of 2D bimetallic coordination network with non-covalent interlayer interactions involving Cs+ , [ReV (CN)8 ]3- ions, and 3-CNpy ligands, leads to the occurrence of two steps of thermal SCO with strong cooperativity giving a double thermal hysteresis loop. The resulting spin-transition phenomenon could be tuned by an external pressure giving the room-temperature range of SCO, as well as by visible-light irradiation, inducing an efficient recovery of the high-spin FeII state at low temperatures. We prove that octacyanidorhenate(V) ion is an outstanding metalloligand for induction of a cooperative multistep, multiswitchable FeII SCO effect.

Site-Selective Photoswitching of Two Distinct Magnetic Chromophores in a Propeller-Like Molecule To Achieve Four Different Magnetic States.

Magnetic photoswitching is a highly important but relatively rare phenomenon for enabling optical writing/reading of the magnetic state of a molecule. In this work, an unprecedented site-selective double photoswitching is reported from the assembly of two different "photomagnetic chromophores" into a single hexanuclear molecule: namely, a spin-crossover Fe(II) center exhibiting light-induced excited spin state trapping (LIESST) and a photochemically active octacyanometalate(IV) unit. Four different magnetization levels are accessible through the appropriate combination of violet/red light and temperature, results that highlight the potential of photomagnetic molecules as future molecular memory cells.

Dehydration-Hydration Switching of Single-Molecule Magnet Behavior and Visible Photoluminescence in a Cyanido-Bridged DyIIICoIII Framework.

Microporous magnets compose a class of multifunctional molecule-based materials where desolvation-driven structural transformation leads to the switching of magnetic properties. Herein, we present a special type of microporous magnet where a dehydration-hydration process within a bimetal coordination framework results in the switching of emissive DyIII single-molecule magnets (SMMs). We report a three-dimensional (3-D) cyanido-bridged coordination polymer, {[DyIII(H2O)2][CoIII(CN)6]}·2.2H2O (1), and its dehydrated form of {DyIII[CoIII(CN)6]} (2), which was obtained through a reversible single-crystal-to-single-crystal transformation. Both phases are composed of paramagnetic DyIII centers alternately arranged with diamagnetic hexacyanidocobaltates(III). The hydrated phase contains eight-coordinated [DyIII(µ-NC)6(H2O)2]3- complexes of a square antiprism geometry, while the dehydrated form contains six-coordinated [DyIII(µ-NC)6]3- moieties of a trigonal prism geometry. This change in coordination geometry results in the generation of DyIII single-molecule magnets in 2, whereas slow magnetic relaxation effect is not observed for DyIII sites in 1. The D4d-to-D3h symmetry change of DyIII complexes produces also the shift of photoluminescent color from nearly white to deep yellow thanks to the modulation of emission bands of f-f electronic transitions. A combined approach utilizing dc magnetic data and low-temperature emission spectra confirmed an axial crystal field of trigonal prismatic DyIII complexes in 2, which produces an Orbach type of slow magnetic relaxation. Therefore, we present a unique route to the efficient switching of SMM behavior and photoluminescence of DyIII complexes embedded in a 3-D cyanido-bridged framework.

Photoluminescent Lanthanide(III) Single-Molecule Magnets in Three-Dimensional Polycyanidocuprate(I)-Based Frameworks.

Three-dimensional bimetallic cyanido-bridged frameworks, [LnIII (2,2'-bipyridine N,N'-dioxide)2 (H2 O)][CuI 2 (CN)5 ]⋅5 H2 O (Ln=Dy, 1; Yb, 2), are reported. They exhibit the effect of slow relaxation of magnetization, leading to a magnetic hysteresis loop, and sensitized visible-to-near-infrared photoluminescence. Both physical properties are related to the eight-coordinated lanthanide(III) complexes embedded in the unprecedented coordination skeleton composed of symmetry-breaking polycyanidocuprate linkers. The three-dimensional d-f cyanido-bridged network was shown to serve as an efficient coordination scaffold to achieve emissive lanthanide single-molecule magnets.

Wide-Range UV-to-Visible Excitation of Near-Infrared Emission and Slow Magnetic Relaxation in LnIII(4,4'-Azopyridine-1,1'-dioxide)[CoIII(CN)6]3- Layered Frameworks.

Trivalent lanthanide ions combined with two molecular linkers, organic 4,4'-azopyridine-1,1'-dioxide (apdo), and inorganic hexacyanidocobaltate(III), gave a series of magnetoluminescent coordination polymers, [{LnIII(apdo)(H2O)4}{CoIII(CN)6}]·2H2O (Ln = Nd, 1; Tb, 2; Dy, 3; Er, 4; Tm, 5; Yb, 6). They are hybrid organic-inorganic layered frameworks composed of cyanido-bridged {Ln2(µ-NC)4Co2} squares linked by Ln-apdo-Ln bridges into a coordination network of a mixed 4- and 8-metal ring topology. Lanthanide(III) complexes, [LnIII(µ-apdo)2(H2O)4(µ-NC)2]+, of a distorted dodecahedral geometry are isolated by diamagnetic [CoIII(CN)6]3- and apdo linkers. As a result, 1-6 reveal field-induced slow relaxation of magnetization, with typical temperature-dependent relaxation of a single-ion origin for NdIII-containing 1, DyIII-containing 3, and YbIII-containing 6. The related alternate-current magnetic data were precisely analyzed, indicating the multiple magnetic relaxation pathways, including a direct process, strong quantum tunneling of magnetization, non-negligible Raman processes, and crucial two-phonon Orbach thermal relaxation. The thermal energy barriers of the Orbach process, Δ E/ kB, are 15.1(9) K with τ0 = 9.8(9) × 10-6 s at Hdc = 4500 Oe, 16.1(8) K with τ0 = 9.0(9) × 10-5 s at Hdc = 1500 Oe, and 17.3(6) K with τ0 = 3.2(7) × 10-6 s at Hdc = 700 Oe, for 1, 3, and 6, respectively, proving the single-molecule magnet (SMM) behavior. Because of the presence of [Co(CN)6]3-, 1-6 show strong UV absorption, while the chromophoric apdo leads to the strong absorption in the visible range. As a result, the visible 4f/3d metal-centered emission is quenched, but the near-infrared luminescence from NdIII and YbIII is observed in 1 and 6, respectively. It is realized by Co-to-Ln metal-to-metal, and apdo-to-Ln ligand-to-metal energy transfers; thus, broad UV-to-visible excitation can be explored. Compounds 1-6 form a novel family of functional bimetallic assemblies, incorporating NIR-emissive SMMs as presented for NdCo (1) and YbCo (6) derivatives.

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.

Effect of Noble Metals on Luminescence and Single-Molecule Magnet Behavior in the Cyanido-Bridged Ln-Ag and Ln-Au (Ln = Dy, Yb, Er) Complexes.

Self-assembly of lanthanide(III) complexes of 2,2':6',2″-terpyridine (terpy) with dicyanidoargentate(I) and dicyanidoaurate(I) anions in water results in the formation of six isostructural dinuclear systems [LnIII(terpy)(H2O)(NO3)2][MI(CN)2] (LnIII/MI = Dy/Ag, 1; Dy/Au, 2; Yb/Ag, 3; Yb/Au, 4; Er/Ag, 5; Er/Au, 6). They form three-dimensional supramolecular networks based on dinuclear molecules linked by hydrogen bonds, π-π interactions, and argentophilic (Ag···Ag) or aurophilic (Au···Au) interactions. All of the assemblies show complex solid-state strong UV and weak vis-NIR absorption due to overlapping contributions from 2,2':6',2″-terpyridine, dicyanidoargentate(I), dicyanidoaurate(I), and lanthanide(III) ions. Moreover, they exhibit excitation-wavelength-dependent multicolor photoluminescence ranging from bright white to blue via yellow, green, and cyan colors due to variable contributions from the dicyanidometalate and ligand. Assemblies 3-6 show NIR emission originating from YbIII and ErIII metal centers. Furthermore, compounds 1-6 and their magnetically diluted samples are magnetic-field-induced single-molecule magnets with energy barriers of up to 35 K. The effect of noble metal substitution on the magnetic properties of particular lanthanide ions is described. The influence on the thermal anisotropic energy barrier, which relates to the strength of the magnetic anisotropy, depends on the type of lanthanide used. The Ag-to-Au substitution enhances the anisotropy of the prolate YbIII ion and decreases it for the oblate DyIII ion. It also modifies the strength of dipolar interactions affecting the slow magnetic relaxation processes.

In Situ Ligand Transformation for Two-Step Spin Crossover in FeII[MIV(CN)8]4- (M = Mo, Nb) Cyanido-Bridged Frameworks.

We report a unique synthetic route toward the multistep spin crossover (SCO) effect induced by utilizing the partial ligand transformation during the crystallization process, which leads to the incorporation of three different FeII complexes into a single coordination framework. The 3-acetoxypyridine (3-OAcpy) molecules were introduced to the self-assembled FeII-[MIV(CN)8]4- (M = Mo, Nb) system in the aqueous solution which results in the partial hydrolysis of the ligand into 3-hydroxypyridine (3-OHpy). It gives two novel isostructural three-dimensional {FeII2(3-OAcpy)5(3-OHpy)3[MIV(CN)8]}· nH2O (M = Mo, n = 0, FeMo; M = Nb, n = 1, FeNb) coordination frameworks. They exhibit an unprecedented cyanido-bridged skeleton composed of {Fe3M2} n coordination nanotubes bonded by additional Fe complexes. These frameworks contain three types of Fe sites differing in the attached organic ligands, [Fe1(3-OAcpy)4(µ-NC)2], [Fe2(3-OHpy)4(µ-NC)2], and [Fe3(3-OAcpy)3(3-OHpy)(µ-NC)2], which lead to the thermal two-step FeII SCO, as proven by X-ray diffraction, magnetic susceptibility, UV-vis-NIR optical absorption, and 57Fe Mössbauer spectroscopy studies. The first step of SCO, going from room temperature to the 150-170 K range with transition temperatures of 245(5) and 283(5) K for FeMo and FeNb, respectively, is related to Fe1 sites, while the second step, occurring at the 50-140 K range with transition temperatures of 70(5) and 80(5) K for FeMo and FeNb, respectively, is related to Fe2 sites. The Fe3 site with both 3-OAcpy and 3-OHpy ligands does not undergo the SCO at all. The observed two-step SCO phenomenon is explained by the differences in the ligand field strength of the Fe complexes and the role of their alignment in the coordination framework. The simultaneous application of two related pyridine derivatives is the efficient synthetic route for the multistep FeII SCO in the cyanido-bridged framework which is a promising step toward rational design of advanced spin transition molecular switches.

A Photomagnetic Sponge: High-Temperature Light-Induced Ferrimagnet Controlled by Water Sorption.

"Converting" light energy to magnetization is the attribute of molecule-based compounds called photomagnets and is inaccessible for conventional magnetic solids. The design and synthesis of such compounds, however, is a formidable challenge, and only a few examples are known, all with rather low magnetic ordering temperatures well below the boiling point of liquid nitrogen. Herein, a cyanide-bridged coordination polymer, {[MnII(imidazole)]2[WIV(CN)8]} n, exhibiting the highest light-induced magnetic ordering temperature ever observed and a magnetic hysteresis loop up to 90 K is reported. The photomagnetic effect results from the blue light excitation (450 nm) of the constituent octacyanotungstate(IV) moiety, which then couples magnetically with manganese(II), resulting in light-induced ferrimagnetic ordering. The reported coordination framework shows also outstanding water sorption properties that are strongly correlated with the photomagnetic functionality. The photoswitching observed in the anhydrous state is completely quenched by the reversible capture of water, with the fully hydrated phase becoming practically non-photomagnetic.

Connecting Visible Photoluminescence and Slow Magnetic Relaxation in Dysprosium(III) Octacyanidorhenate(V) Helices.

Functional crystalline materials based on bimetallic cyanido-bridged {[DyIII(4-Mephen)(dmf)4][MV(CN)8]}·0.5H2O (M = Re, 1; Mo, 2; W, 3; 4-Mephen = 4-methyl-1,10-phenanthroline) helices have been prepared. 1 is the first heterometallic coordination polymer incorporating an unexplored [ReV(CN)8]3- ion. Implementation of the ReV-based diamagnetic analogue of broadly investigated paramagnetic [MoV(CN)8]3- and [WV(CN)8]3- ions into the d-f coordination framework results in yellow photoluminescence originating from 4F9/2 → 6H J f-f electronic transitions of DyIII sensitized by 4-Mephen, and field-induced slow magnetic relaxation related to the single-ion anisotropy of the dysprosium(III) complexes. We prove that [ReV(CN)8]3- can work as a noninnocent metalloligand in the preparation of emissive 4f-metal-based single-molecule magnets.

Irradiation Temperature Dependence of the Photomagnetic Mechanisms in a Cyanido-Bridged CuII2MoIV Trinuclear Molecule.

We report a new bimetallic cyanido-bridged trinuclear complex [CuII(enpnen)]2[MoIV(CN)8]·6.75H2O (1) (enpnen = N,N'-bis(2-aminoethyl)-1,3-propanediamine) that shows reversible photomagnetic effect. The photo-induced increase of magnetization is characterized by the irradiation temperature-dependent shapes of the χM T( T) plots and different magnetization values at low temperature in high magnetic field, suggesting multiple photoexcited states. The photomagnetic effect in 1 is explained through two possible processes simultaneously: the light-induced metal-to-metal charge transfer (MMCT) in the CuII-NC-MoIV pair and the light-induced excited spin-state trapping (LIESST) effect in MoIV center. A numerical model assuming the simultaneous existence of three possible states after irradiation: the MMCT CuI-NC-MoV-CN-CuII state, the LIESST CuII-NC-MoIVHS-CN-CuII state, and the ground-state CuII-NC-MoIVLS-CN-CuII was applied to the data and resulted in Cu-Mo exchange coupling constants J1MMCT = 11 cm-1 and J2LIESST = 109 cm-1 for the MMCT and LIESST mechanisms induced states, respectively. Fractions of respective states after irradiations at different temperatures were also calculated, shedding light on the influence of irradiation temperature on the photomagnetic mechanism. The proposed model can provide the interpretative framework for testing and refinement of the mechanism of photomagnetic effect in other coordination networks with cyanido-bridged Cu-[Mo(CN)8]4- linkages.

Dehydration-Triggered Charge Transfer and High Proton Conductivity in (H3O)[NiIII(cyclam)][MII(CN)6] (M = Ru, Os) Cyanide-Bridged Chains.

The coexistence of dehydration-driven charge transfer, magnetic interactions, and high proton conductivity was found in two bimetallic alternating CN-bridged chains {(H3O)[NiIII(cyclam)][MII(CN)6]·5H2O} n (M = Ru (1), Os (2); cyclam = 1,4,8,11-tetraazacyclotetradecane). Dehydration of these materials causes structural transformation and triggers charge transfer between the metal centers: NiIII-NC-MII → NiII-NC-MIII. The CT process, whose extent is tuned by the change of the anionic building block, causes significant increase of magnetic moment, appearance of antiferromagnetic interactions, and noticeable changes in color. The high conductivity values of σ = 1.09 × 10-3 (1) and 1.12 × 10-3 S cm-1 (2) at 295 K and 100% relative humidity allow the classification of the materials as superionic conductors. The proton conduction occurs according to the Grotthuss mechanism as a hopping of protons between H-bonded water molecules due to the presence of the H3O+ ions, which compensate negative charge of the coordination chains.