Barbara Sieklucka

Photomagnetism in Clusters and Extended Molecule-Based Magnets

Photomagnetism in molecular systems is a new development in molecular magnetism. It traces back to 1982 and 1984 when a transient effect and then the light-induced excited-spin-state-trapping effect was discovered in spin-crossover complexes. The present contribution gives a definition of the phenomenon, a process that changes the magnetism of a (molecular) system after absorption of a photon. It is limited to the discussion of photomagnetism based on metal-metal electron transfer in clusters and extended molecule-based magnets. The paper is organized around the main pairs of spin bearers, which allowed us to evidence and to study the phenomenon: Cu-Mo, Co-Fe, and Co-W. For each metallic pair, we report and discuss the conditions of appearance of the effect and its characteristics, both in extended structures and in molecular units: structure, spectroscopy, magnetism, thermodynamics and kinetics, and applications. We conclude with some brief prospects. The field is in rapid expansion. We are convinced that the interaction of photons with magnetized matter, to provide original magnetic properties, will meet more and more interest in the future.

Double Switching of a Magnetic Coordination Framework through Intraskeletal Molecular Rearrangement

Molecular systems which undergo reversible structural transformations on application of external stimuli and show additional strongly intertwined physical phenomena are fundamental to the generation of nanoscale multifunctional molecular devices. Multifunctional molecular systems are recognized as potentially revolutionary magnetic, electric, and magneto-optical materials with possible applications in data storage/processing at the molecular level, as molecular switching devices in gas processing systems, and molecular sensors. In the field of porous molecular magnetic materials, the structural versatility of coordination chemistry has allowed the engineering of materials having novel topologies and remarkable properties. Magnetic coordination compounds with high magnetic ordering temperatures Tc exceeding the boiling point of liquid nitrogen are achievable in cyanidoor tetracyanoethylenebridged molecular solids. Incorporation of relatively large organic molecules into the structures of the former led to hybrid systems showing multifunctionality but usually with significantly lower Tc due to the lower ratio of bridging/ terminal CN ligands. Controlled dehydration of selected cyanido-bridged hybrids may result in substantial increase of Tc due to structural transformations involving terminal CN groups. However, there have been practically no reports on rational utilization of the terminal CN ligands and large organic molecules incorporated into the molecular framework to impart multifunctionality on these materials. Recently, we recognized the great potential of terminal CN groups and successfully exploited it in an Mn-imidazole– [Nb(CN)8] magnetic spongelike material to reversibly increase its critical temperature from 25 to 62 K. Here we present a cyanido-bridged molecular magnet in which the coordinated organic molecules and terminal CN ligands are appropriately arranged to provide a unique coexistence of molecule-specific porosity and doubly switchable high ordering temperature in one material. Orange platelike crystals of {[Mn(pydz)(H2O)2][Mn (H2O)2][Nb (CN)8]·2H2O}n (1; pydz = pyridazine, C4H4N2) were crystallized from an aqueous solution of MnCl2·4 H2O, pyridazine, and K4[Nb(CN)8]·2H2O (for details, see Supporting Information). Samples of 1 can be stored in a closed vessel for several months without decomposition. The structure of 1 was determined by single-crystal X-ray diffraction analysis (space group P21/c ; for details, see Supporting Information and CCDC 810685). The cyanido-bridged Mn2Nb skeleton of 1 (Figure 1a) consists of Mn2-NC-Nb square-grid motifs (in the bc plane) cross-linked at Nb centers by Mn1-NC-Nb ladders (along the a axis). Both Mn1 and Mn2 centers in 1 are octahedral (coordination number cn = 6), but their coordination spheres are different (Figure S2, top in the Supporting Information). Mn1 is coordinated by three nitrogen atoms of CN ligands (mer), one nitrogen atom of pyridazine (N11), and two aqua ligands in trans geometry. Mn1 belongs exclusively to the ladder motifs. The coordination sphere of Mn2, on the contrary, is purely inorganic and comprises four cyanido ligands in the equatorial plane and two aqua ligands in trans geometry. Mn2 belongs to the square-grid motifs. The only terminal CN ligand of the [Nb(CN)8] moiety (C4N4) is involved in a strong hydrogen bond with oxygen atom O11 of the aqua ligand coordinated to Mn1 (N4 Mn1 4.37 ). The noncoordinating nitrogen atom of the pyridazine ligand (N16) is involved in a hydrogen-bond to the aqua ligand (O22) of Mn2 (N16 Mn2 4.19 ; Figure 1a, bottom and Figure S3a in the Supporting Information). The local hydrogen-bonding system is completed by the H2O molecule of crystallization (O2) bound to O22 and N4. Such an arrangement is very promising from the point of view of topotactic reactivity of the terminal CN and pyridazine ligands. [*] Dr. D. Pinkowicz, Dr. R. Podgajny, Dr. B. Gaweł, Dr. W. Nitek, Prof. Dr. W. Łasocha, M. Oszajca, Prof. Dr. B. Sieklucka Faculty of Chemistry Jagiellonian University Ingardena 3, 30-060 Krak w (Poland) Fax: (+ 48)12-634-0515 E-mail: pinkowic@chemia.uj.edu.pl barbara.sieklucka@uj.edu.pl Homepage: http://www.chemia.uj.edu.pl/znmm/

Enforcing Multifunctionality: A Pressure-Induced Spin-Crossover Photomagnet

Photomagnetic compounds are usually achieved by assembling preorganized individual molecules into rationally designed molecular architectures via the bottom-up approach. Here we show that a magnetic response to light can also be enforced in a nonphotomagnetic compound by applying mechanical stress. The nonphotomagnetic cyano-bridged Fe(II)-Nb(IV) coordination polymer {[Fe(II)(pyrazole)4]2[Nb(IV)(CN)8]·4H2O}n (FeNb) has been subjected to high-pressure structural, magnetic and photomagnetic studies at low temperature, which revealed a wide spectrum of pressure-related functionalities including the light-induced magnetization. The multifunctionality of FeNb is compared with a simple structural and magnetic pressure response of its analog {[Mn(II)(pyrazole)4]2[Nb(IV)(CN)8]·4H2O}n (MnNb). The FeNb coordination polymer is the first pressure-induced spin-crossover photomagnet.

Magnetic Spongelike Behavior of 3D Ferrimagnetic {[MnII(imH)]2[NbIV(CN)8]}n with Tc = 62 K

Fully reversible room temperature dehydration of 3D {Mn(II)2(imH)2(H2O)4[Nb(IV)(CN)8] x 4 H2O}n (1; imH = imidazole) of Tc = 25 K results in the formation of 3D ferrimagnet {[Mn(II)(imH)]2[Nb(IV)(CN)8]}n (2), with Tc = 62 K, the highest ever known for octacyanometalate-based compounds. The dramatic magnetostructural modifications in 2 provide the first example of magnetic spongelike behavior in an octacyanometallate-based assembly.

Magnetic clusters based on octacyanidometallates

Octacyanidometallates make an important branch of cyanide-based molecular magnets that not only follow the trends in modern magnetochemistry and materials science but also have stimulated these fields from the very beginning and still blaze a trail by introducing new concepts such as heterotrimetallic systems, new functionalities and cross-effects like photo-switching of magneto-optical properties and setting new records in magnetic ordering temperatures or magnetic exchange interactions. The following paper focuses on a special class of molecular magnets which, to the best of our knowledge, have not been reviewed so far: octacyanide-based magnetic clusters. A complete list of all known molecules incorporating octacyanides with an extensive discussion of their structures, topologies and magnetic properties, with special attention paid to multifunctional systems, is provided. Several milestone-clusters are discussed thoroughly to emphasize their particular importance in the development of crystal engineering and molecular magnetism.

Towards high Tc octacyanometalate-based networks

We present an overview of very recent advances in the engineering of magnetic networks based on octacyanometalates. The selected magnetic networks of CuIIWV, NiIIWV and MnIILNbIV (L – organic bridging linker) illustrate the possible strategies for tuning of the magnetic characteristics. The combination of magnetic ordering for 2D (two-dimensional) and 3D (three-dimensional) networks together with the solvent sensitivity of a cyano-bridged framework resulted in the development of a novel 3D {[MnII(imH)]2[NbIV(CN)8]} assembly with magnetic sponge character, characterized by Tc of 62 K, the highest ever observed for octacyanometalate-based networks.

Co–NC–W and Fe–NC–W Electron‐Transfer Channels for Thermal Bistability in Trimetallic {Fe6Co3[W(CN)8]6} Cyanido‐Bridged Cluster

The design and construction of switchable materials attracts tremendous interest owing to the potential in information storing and processing or molecular sensing. The archetypal examples involve a diversity of Fe-, 6] Feor Cobased spin-crossover (SCO) compounds, Co-catecholate/semiquinone systems, 11] as well as d-d bimetallic and sd-d trimetallic cyanide-bridged systems revealing chargetransfer-induced spin transitions (CTIST). Some of these compounds, for example Prussian blue analogues, are particularly promising from the point of view of photoswitching between nonmagnetic and magnetized (that is, TB, TC) states, owing to magnetic coupling through molecular bridges in discrete species 15] and extended networks. 18] Such bistability also emerged in the magnetochemistry of octacyanidometalates, exploiting metal-to-metal electron transfer in CoL[W(CN)8] 3 (L = pyrimidine, 4-methylpyridine) or canonical SCO in FeL[Nb(CN)8] 4 extended networks (L = 4-pyridinealdoxime). A magnetic hysteresis loop with a coercivity of 1–3 T were observed in an optically excited low-temperature metastable phase. As a continuing effort to obtain innovative bistable systems, we explored the simultaneous embedding of Co and Fe cations into one octacyanido-bridged coordination skeleton. We have engineered and isolated the novel trimetallic {Co3Fe II 6[W (CN)8]6(MeOH)24}·x MeOH (1) material built of nanosized (ca. 20 ) pentadecanuclear six-capped body-centered cubic Co3Fe6W6 clusters with MeOH molecules of crystallization (see the Supporting Information). The {M9M’ V 6(CN)48(L)24}·n solv compound family (M = Mn, Co, Ni; M’= Mo, W; L = blocking ligands; Figure 1) reveal high-

Nature of Magnetic Interactions in 3D {[MII(pyrazole)4]2[NbIV(CN)8]·4H2O}n (M = Mn, Fe, Co, Ni) Molecular Magnets

The self-assembly of [Nb(IV)(CN)(8)](4-) with different 3d metal centers in an aqueous solution and an excess of pyrazole resulted in the formation of four 3D isostructural compounds {[M(II)(pyrazole)(4)](2)[Nb(IV)(CN)(8)].4H(2)O}(n), where M(II) = Mn, Fe, Co, and Ni for 1-4, respectively. All four assemblies crystallize in the same I4(1)/a space group and show identical cyanido-bridged structures decorated with pyrazole molecules coordinated to M(II) centers. All four compounds show also long-range magnetic ordering below 24, 8, 6, and 13 K, respectively. A thorough analysis of the structural and magnetic data utilizing the molecular field model has allowed for an estimation of the values of coupling constants J(M-Nb) attributed to the one type of M(II)-NC-Nb(IV) linkage existing in 1-4. The J(M-Nb) values increase monotonically from -6.8 for 1 through -3.1 for 2 and +3.5 for 3, to +8.1 cm(-1) for 4 and are strongly correlated with the number of unpaired electrons on the M(II) metal center. Average orbital contributions to the total exchange coupling constants J(M-Nb) have also been identified and calculated: antiferromagnetic J(AF) = -21.6 cm(-1) originating from the d(xy), d(xz), and d(yz) orbitals of M(II) and ferromagnetic J(F) = +15.4 cm(-1) originating from d(z(2)) and d(x(2)-y(2)) orbitals of M(II). Antiferromagnetic interaction is successively weakened in the 1-4 row with each additional electron on the t(2g) level, which results in a change of the sign of J(M-Nb) and the nature of long-range magnetic ordering from ferrimagnetic in 1 and 2 to ferromagnetic in 3 and 4.

Coordination polymers based on octacyanometalates(IV,V)(M = Mo, W) and aliphatic polyamine copper(II) tectons with [N3] donor atom sets

The cyano-bridged [CuII(tetrenH2)]2[WIV(CN)8]2·5H2O (tetren = tetraethylenepentaamine) (1), [CuII(tetrenH2)][CuII(tetrenH)][WV(CN)8][WIV(CN)8]·2.5H2O (2), [CuII(dien)]2[WIV(CN)8]·4H2O (dien = diethylenetriamine) (3) and its isomorphous molybdenum(IV) analogue (4) have been prepared and structurally characterised. 1 and 2 are built from the W2Cu2(μ-CN)4 squares extended into 1-D structure by cyano-bridges. 2-D 3 and 4 form a square grid pattern with tungsten atoms in the corners and –CN–Cu(dien)–NC– linkages on the edges of the squares. The magnetic behaviour of 1 and 3 indicates the presence of two isolated CuII spins S = 1/2 with a very weak antiferromagnetic coupling through the diamagnetic NC–WIV–CN bridges in the low temperatures. Assembly 2 exhibits a weak ferromagnetic interaction between CuII and WV isolated by diamagnetic [WIV(CN)8]4− spacer from another CuII centre within WV–CN–CuII–NC–WIV–CN–CuII unit and the antiferromagnetic interaction between the CuII2WVWIV units.

Multifunctionality in Bimetallic LnIII[WV(CN)8]3− (Ln=Gd, Nd) Coordination Helices: Optical Activity, Luminescence, and Magnetic Coupling

Two chiral luminescent derivatives of pyridine bis(oxazoline) (Pybox), (SS/RR)-iPr-Pybox (2,6-bis[4-isopropyl-2-oxazolin-2-yl]pyridine) and (SRSR/RSRS)-Ind-Pybox (2,6-bis[8H-indeno[1,2-d]oxazolin-2-yl]pyridine), have been combined with lanthanide ions (Gd(3+), Nd(3+)) and octacyanotungstate(V) metalloligand to afford a remarkable series of eight bimetallic CN(-)-bridged coordination chains: {[Ln(III)(SS/RR-iPr-Pybox)(dmf)4]3[W(V)(CN)8]3}n ⋅dmf⋅4 H2O (Ln = Gd, 1-SS and 1-RR; Ln = Nd, 2-SS and 2-RR) and {[Ln(III)(SRSR/RSRS-Ind-Pybox)(dmf)4][W(V)(CN)8]}n⋅5 MeCN⋅4 MeOH (Ln = Gd, 3-SRSR and 3-RSRS; Ln = Nd, 4-SRSR and 4-RSRS). These materials display enantiopure structural helicity, which results in strong optical activity in the range 200-450 nm, as confirmed by natural circular dichroism (NCD) spectra and the corresponding UV/Vis absorption spectra. Under irradiation with UV light, the Gd(III)-W(V) chains show dominant ligand-based red phosphorescence, with λmax ≈660 nm for 1-(SS/RR) and 680 nm for 3-(SRSR/RSRS). The Nd(III)-W(V) chains, 2-(SS/RR) and 4-(SRSR/RSRS), exhibit near-infrared luminescence with sharp lines at 986, 1066, and 1340 nm derived from intra-f (4)F3/2 → (4)I9/2,11/2,13/2 transitions of the Nd(III) centers. This emission is realized through efficient ligand-to-metal energy transfer from the Pybox derivative to the lanthanide ion. Due to the presence of paramagnetic lanthanide(III) and [W(V)(CN)8](3-) moieties connected by cyanide bridges, 1-(SS/RR) and 3-(SRSR/RSRS) are ferrimagnetic spin chains originating from antiferromagnetic coupling between Gd(III) (SGd = 7/2) and W(V) (SW = 1/2) centers with J1-(SS) = -0.96(1) cm(-1), J1-(RR) =-0.95(1) cm(-1), J3-(SRSR) = -0.91(1) cm(-1), and J3-(RSRS) =-0.94(1) cm(-1). 2-(SS/RR) and 4-(SRSR/RSRS) display ferromagnetic coupling within their Nd(III)-NC-W(V) linkages.