Cyanide-bridge Fe-Fe and Fe-Co molecular squares: structures and electrochemistry of

Stimuli-responsive magnetic materials: impact of spin and electronic modulation

Addressing molecular bistability as a function of external stimuli, especially in spin-crossover (SCO) and metal-to-metal electron transfer (MMET) systems, has seen a surge of interest in the field of molecule-based magnetic materials due to their enormous potential in various technological applications such as molecular spintronics, memory and electronic devices, switches, sensors, and many more. The fine-tuning of molecular components allow the design and synthesis of materials with tailored properties for these vast applications. In this Feature Article, we discuss a part of our research work into this broad topic, pertaining to the recent discoveries in the field of switchable molecular magnetic materials based on SCO and MMET systems, along with some historical background of the area and related accomplishments made in recent years.

Effects of cis/trans-Configuration and Ligand Substitution of the Cyanidometal Bridge on Metal to Metal Charge Transfer Properties in Mixed Valence Complexes

To investigate the effects of cis/trans-configuration of the cyanidometal bridge and the electron donating ability of the auxiliary ligand on the cyanidometal bridge on metal to metal charge transfer (MMCT) in cyanidometal-bridged mixed valence compounds, two groups of trinuclear cyanidometal-bridged compounds cis/trans-[Cp(dppe)Fe(μ-NC)Ru(4,4'-dmbpy)₂ (μ-CN)Fe(dppe)Cp][PF₆ ]_n (n=2 (cis/trans-1[PF₆ ]₂ ), 3 (cis/trans-1[PF₆ ]₃ ), 4 (cis/trans-1[PF₆ ]₄ )) and cis/trans-[Cp(dppe)Fe(μ-NC)Ru(bpy)₂ (μ-CN)Fe(dppe)Cp][PF₆ ]₃ (cis/trans-2[PF₆ ]₃ ) were synthesized and fully characterized. The experimental results indicate that for these one- and two-electron oxidation mixed valence compounds, the trans-configuration compounds are more beneficial for MMCT than the cis-configuration compounds, and increasing the electron donating ability of the auxiliary ligand on the cyanidometal bridge is also conductive to MMCT. Moreover, compounds cis/trans-1[PF₆ ]_n (n=3, 4) and cis/trans-2[PF₆ ]₃ belong to localized compounds by analyzing the experimental characterization results, supported by the TDDFT calculations.

Temperature dependence of spherical electron transfer in a nanosized [Fe14] complex

The study of transition metal clusters exhibiting fast electron hopping or delocalization remains challenging, because intermetallic communications mediated through bridging ligands are normally weak. Herein, we report the synthesis of a nanosized complex, [Fe(Tp)(CN)3]8[Fe(H2O)(DMSO)]6 (abbreviated as [Fe14], Tp-, hydrotris(pyrazolyl)borate; DMSO, dimethyl sulfoxide), which has a fluctuating valence due to two mobile d-electrons in its atomic layer shell. The rate of electron transfer of [Fe14] complex demonstrates the Arrhenius-type temperature dependence in the nanosized spheric surface, wherein high-spin centers are ferromagnetically coupled, producing an S = 14 ground state. The electron-hopping rate at room temperature is faster than the time scale of Mössbauer measurements (<~10-8 s). Partial reduction of N-terminal high spin FeIII sites and electron mediation ability of CN ligands lead to the observation of both an extensive electron transfer and magnetic coupling properties in a precisely atomic layered shell structure of a nanosized [Fe14] complex.

Two-electron redox-active tricyano iron(ii) complex with 2,4,6-tris(2-pyrimidyl)-1,3,5-triazine as a building block for coordination polymers

A new tricyano iron(ii) building unit, [{K(H2O)4}{FeII(CN)3(L)}]·3H2O (1), was synthesized by the reaction of Moor's salts with 2,4,6-tris(2-pyrimidyl)-1,3,5-triazine (L) as a capping ligand. X-ray structural analysis reveals that the mononuclear iron(ii) tricyano complex consists of one ligand and three cyanide groups, with K+ ions coordinated between neighboring units to form a one-dimensional chain network structure. 1 shows two reversible redox waves at +0.713 and -0.849 V vs. SCE, which are assigned as Fe(ii)/Fe(iii) and L/L˙- processes, respectively. One-dimensional 4,2-ribbon chain type coordination polymers, [MII(H2O)2{FeII(CN)3(L)}2]·6H2O (M = Fe (2) and Mn (3)), were synthesized by using 1 as a building unit. Cryomagnetic studies reveals that 2 and 3 show paramagnetic behaviour of S = 2 and 5/2, respectively. On the other hand, reaction of 1 and Gd(NO3)3·6H2O forms another one-dimensional chain, [GdIII(NO3)2(H2O)3{FeII(CN)3(L)}]·0.5H2O·2CH3OH (4), showing S = 7/2 paramagnetic behaviour.

Kineticomechanistic Study of the Redox pH Cycling Processes Occurring on a Robust Water-Soluble Cyanido-Bridged Mixed-Valence {CoIII/FeII}2 Square

A kineticomechanistic study of reversible electron-transfer processes undergone by the water-soluble, cyanido-bridged mixed-valence [{Co^III{(Me)₂(μ-ET)cyclen}}₂{(μ-NC)₂Fe^II(CN)₄}₂]^2- square has been carried out. The oxidation reaction consists of a two-step process with the participation of a solvent-assisted outer-sphere complex, as a result of the establishment of hydrogen bonds that involve the oxo groups of the oxidant (peroxodisulfate) and the terminal cyanido ligands of the tetrametallic square. The formally endergonic reduction reaction of the fully oxidized ([{Co^III{(Me)₂(μ-ET)cyclen}}₂{(μ-NC)₂Fe^III(CN)₄}₂]) core by water, producing hydrogen peroxide from water even at low pH values, is also a two-step process. Each one of these processes requires a set of two preequilibria involving the association of OH^- and its subsequent deprotonation by a further OH^- anion. The structure of the square compound in its fully protonated form has also been determined by X-ray diffraction and shows the existence of strong hydrogen-bonding interactions, in agreement with the rather high basicity of the terminal cyanido ligands. Likewise, density functional theory calculations on the tetrametallic complex showed zones with negative electrostatic potential around the Fe^II centers of the square that would account for the establishment of the hydrogen bonds found in the solid state. Spectroelectrochemistry experiments demonstrated the singular stability of the {Co^III/Fe^II}₂^2- complex, as well as that of their partially, {Co₂^III/Fe^IIIFe^II}^-, and fully, {Co^III/Fe^III}₂, oxidized counterparts because no hysteresis was observed in these measurements.

Ligand dependent topology and spontaneous resolution in high-spin cyano-bridged Ni3W2 clusters

Two high-spin pentanuclear NiW clusters with diimine blocking ligands have been obtained: {[Ni(4,4'-MeObpy)2]3[W(CN)8]2}·12H2O (1) and {[Ni(phen)2(H2O)][Ni(phen)2]2[W(CN)8]2}·7H2O (2) (4,4'-MeObpy = 4,4'-dimethoxy-2,2'-bipyridine, phen = 1,10-phenanthroline). Despite the similarity of the building blocks and synthetic conditions the compounds show different topologies of the cluster core: 1 is a trigonal bipyramid while 2 is a decorated square. Both cluster structures are chiral with either ΔΔΔ or ΛΛΛ configuration around all three Ni centres. In 1 spontaneous resolution occurs and it crystallises in the P212121 space group forming a conglomerate containing both types of enantiomorphic crystals. 1Δ and 1Λ are the first pair of enantiomorphic structures of cyano-bridged clusters of trigonal bipyramidal topology obtained with achiral bidentate blocking ligands. 2 crystallises as a racemic compound in a centrosymmetric space group P1[combining macron] with both enantiomers present in the structure. 2 is an exceptional square-motif containing structure with an identical stereoconfiguration of all complex cations within one cluster. Ferromagnetic interactions are present in both clusters resulting in the ground spin state S = 4.

Switchable Fe/Co Prussian blue networks and molecular analogues

Switchable Fe/Co Prussian blue compounds and their low dimensional analogues displaying thermally and photo-induced electron transfer phenomena are reviewed.

Cyanide-bridged NiCr and alternate NiFe-NiCr magnetic ultrathin films on functionalized Si(100) surface

Sequential growth in solution (SGS) was performed for the magnetic cyanide-bridged network obtained from the reaction of Ni(H(2)O)(2+) and Cr(CN)(6)(3-) (referred to as NiCr) on a Si(100) wafer already functionalized by a Ni(II) complex. The growth process led to isolated dots and a low coverage of the surface. We used the NiFe network as a template to improve the growth of the magnetic network. We elaborated alternate NiFe (paramagnetic)-NiCr (ferromagnetic) ultrathin films around 6 nm thick. The magnetic behaviour confirmed the alternate structure with the ferromagnetic zones isolated between the paramagnetic ones since the evolution of the blocking temperature is consistent with the evolution of the layers' thickness expected from the SGS process.

Self-assembled Co(II) molecular squares incorporating the bridging ligand 4,7-phenanthrolino-5,6:5',6'-pyrazine

Three high-spin tetranuclear cobalt(II) complexes have been prepared with the bridging ligand 4,7-phenanthrolino-5,6:5',6'-pyrazine (ppz) through metal-ion directed self-assembly. The complexes differ by the incorporation of three different coordinating anions: chloride, thiocyanide and selenocyanide. The physical properties of these complexes have been investigated in detail.

A Family of Mixed-Metal Cyanide Cubes with Alternating Octahedral and Tetrahedral Corners Exhibiting a Variety of Magnetic Behaviors Including Single Molecule Magnetism

A series of structurally related pseudocubic metal cyanide clusters of Re(II) and 3d metal ions [{MX}4{Re(triphos)(CN)3}4] (M = Mn, Fe, Co, Ni, Zn; X = Cl, I, -OCH3) have been prepared, and their magnetic and electrochemical properties have been probed to evaluate the effect of changing the identity of the 3d metal ion. Electrochemistry of the clusters reveals several rhenium-based oxidation and reduction processes, some of which result in cluster fragmentation. The richest electrochemistry was observed for the iron congener, which exists as the Re(I)/Fe(III) cluster at the resting potential and exhibits six clear one-electron reversible redox couples and two, closely spaced one-electron quasi-reversible processes. The [{MnIICl}4{ReII(triphos)(CN)3}4] complex exhibits single molecule magnetism with a fast tunneling relaxation process observed at H = 0 determined by micro-SQUID magnetization measurements. A comparative evaluation of the magnetic properties across the series reveals that the compounds exhibit antiferromagnetic coupling between the metal ions, except for [{NiIICl}4{ReII(triphos)(CN)3}4] that shows ferromagnetic behavior. Despite the large ground-state spin value of [{NiIICl}4{ReII(triphos)(CN)3}4] (S = 6), only manganese congeners exhibit SMM behavior to 1.8 K.

Structural and magnetic studies on cyano-bridged rectangular Fe2M2 (M = Cu, Ni) clusters

Using the tricyano precursor, (Bu4N)[(Tp)Fe(CN)3] (Tp = Tris(pyrazolyl) hydroborate) (1), four new tetranuclear clusters, [(Tp)Fe(CN)3Cu(Tp)]2.2H2O (2), [(Tp)Fe(CN)3Cu(bpca)]2.4H2O (3) (bpca = bis(2-pyridylcarbonyl)amidate anion), [(Tp)Fe(CN)3Ni(tren)]2(ClO4)2.2H2O (4) (tren = tris(2-amino)ethylamine), and [(Tp)Fe(CN)3Ni(bipy)2]2[(Tp)Fe(CN)3]2.6H2O (5) (bipy = 2,2'-bipyridine), have been synthesized and structurally characterized. The four clusters possess similar square structures, where FeIII and MII (M = CuII or NiII) ions alternate at the rectangle corners. There exist intermolecular - stacking interactions through pyrazolyl groups of Tp- ligands in complexes 2 and 4, which lead to 1D chain structures. Complex 5 shows a 3D network structure through the coexistence of - stacking effects and hydrogen-bonding interactions. Magnetic studies show intramolecular ferromagnetic interactions in all four clusters. The exchange parameters are +11.91 and +1.38 cm(-1) for clusters 2 and 3, respectively, while uniaxial molecular anisotropy can be detected in complex 3 due to the distorted core in its molecular structure. Complex 4 has a ground state of S = 3 and shows SMM behavior with an effective energy barrier of U = 18.9 cm(-1). Unusual spin-glass-like dynamic relaxations are observed for complex 5.

Structural and Magnetic Diversity in Cyano-Bridged Bi- and Trimetallic Complexes Assembled from Cyanometalates and [M(rac-CTH)]n+ Building Blocks (CTH = d,l-5,5,7,12,12,14-Hexamethyl-1,4,8,11-tetraazacyclotetradecane)

Seven new cyano-bridged heterometallic systems have been prepared by assembling [M'(rac-CTH)]n+ complexes (M' = CrIII, NiII, CuII), which have two cis available coordination positions, and [M(CN)6]3- (M = FeIII, CrIII) and [Fe(CN)2(bpy)2]+ cyanometalate building blocks. The assembled systems, which have been characterized by X-ray crystallography and magnetic investigations, are the molecular squares (meso-CTH-H2)[{Ni(rac-CTH)}2{Fe(CN)6)}2].5H2O (2) and [{Ni(rac-CTH)}2{Fe(CN)2(bpy)2}2](ClO4)4.H2O (5), the bimetallic chain [{Ni(rac-CTH)}2{Cr(CN)6)}2Ni(meso-CTH)].4H2O (3), the trimetallic chain [{Ni(rac-CTH)}2{Fe(CN)6)}2Cu(cyclam)]6H2O (4), the pentanuclear complexes [{Cu(rac-CTH}3{Fe(CN)6}2].2H2O (6) and [{Cu(rac-CTH)}3{Cr(CN)6)}2].2H2O (7), and the dinuclear complex [Cr(rac-CTH)(H2O)Fe(CN)6].2H2O (8). With the exception of 5, all compounds exhibit ferromagnetic interaction between the metal ions (JFeNi = 12.8(2) cm-1 for 2; J1FeCu= 13.8(2) cm-1 and J2FeCu= 3.9(4) cm-1 for 6; J1CrCu= 6.95(3) cm-1 and J2CrCu= 1.9(2)cm-1 for 7; JCrFe = 28.87(3) cm-1 for 8). Compound 5 exhibits the end of a transition from the high-spin to the low-spin state of the octahedral FeII ions. The bimetallic chain 3 behaves as a metamagnet with a critical field Hc = 300 G, which is associated with the occurrence of week antiferromagnetic interactions between the chains. Although the trimetallic chain 4 shows some degree of spin correlation along the chain, magnetic ordering does not occur. The sign and magnitude of the magnetic exchange interaction between CrIII and FeIII in compound 8 have been justified by DFT type calculations.