Syntheses, Structures, and Magnetic Properties of Heterobimetallic Clusters with Tricyanometalate and π-Conjugated Ligands Containing 1,3-Dithiol-2-ylidene

Observation of protonation-induced intra-molecular metal-to-metal charge transfer in cyano-bridged [Fe-CN-Mn/Ni] dinuclear complexes

A key challenge in the design of magnetic molecules with intramolecular charge transfer behavior is to obtain reversible magnetic bistability triggered by external stimuli. Here, we show that two dinuclear metal complexes, [(bbp)Fe(CN)3Mn(Py5Me2)]·2.5CH3OH (4) and [(bbp)Fe(CN)3Ni(Py5Me2)]·2.5CH3OH (5) (Py5Me2 = 2,6-bis(1,1-di(pyridine-2-yl)ethyl)pyridine, H2bbp = 2,6-bis(benzimidazole-2-yl)pyridine), were self-assembly synthesized by (Bu4N)2[(bbp)FeIII(CN)3] and [Mn(Py5Me2)(OH2)](ClO4)2 or [Ni(Py5Me2)(OH2)](ClO4)2, respectively. Complexes 4 and 5 exhibited intramolecular metal-to-metal charge transfer with the addition of acids or bases in solution by UV-visible spectrophotometric measurements and electrochemistry studies, and concomitant switching of the {FeIII(μ-CN)MnII/NiII} state to the {FeII(μ-CN)MnIII/NiIII} state.

Tunable Assembly and Magnetic Interactions in Two Cyano-bridged FeIII–CoII Bimetallic Complexes

Abstract

Using a new semi-rigid tetradentate ligand and two cyanometalate building blocks with different steric hindrance, a Fe2Co triangular unit [(Tp)FeIII(CN)3]2CoII(bpmb)·2H2O [1; Tp = hydrotris(pyrazolyl)borate; bpmb = 1,2-bis(3-(2-pyridyl)pyrazol-1-ylmethyl)benzenes] and a Fe2Co2 square unit {[(pzTp)FeIII(CN)3]2[CoII(bpmb)]2}·[(pzTp)FeIII(CN)3]2·4H2O [2; pzTp = tetrakis(pyrazolyl)borate] were synthesized via tunable assembly. Both compounds adopt cis arrangement because of the structural distorted semi-rigid bpmb. Tetranuclear square of 2 was formed due to larger steric hindrance building block of [(pzTp)FeIII(CN)3]− compared with [(Tp)FeIII(CN)3]− in trinculear 1. Magnetic measurements show that antiferromagnetic interactions dominate in 1 and 2. The fitting results of J values suggest a positive correlation between magnetic orbitals overlap of FeIII and CoII ions and Co–N≡C bond angles.

Graphical Abstract

Two cis arrangement cyanometalate [Fe-Co] clusters were synthesized via tunable assembly with a new semi-rigid tetradentate ligand. The tetranuclear cluster was formed because of larger steric hindrance of pzTp. Both compounds exhibit dominant antiferromagnetic couplings. The fitting results of J values suggest a positive correlation between magnetic orbitals overlap of FeIII and CoII ions and Co–N≡C bond angles.

Evidence of protonation induced intra-molecular metal-to-metal charge transfer in a highly symmetric cyanido bridged {Fe2Ni2} molecular square

The possibility to induce intra-molecular metal-to-metal charge transfer in a cyanido bridged tetranuclear square shaped {Fe₂Ni₂} complex by employing protonation as an external stimulant is explored. Two cyanido bridged square shaped tetranuclear complexes, [{Ni(TPA)(μ₂-NC)₂Ni(CN)₂}₂]·4H₂O (2) and [{Ni(TPA)(μ₂-NC)₂Fe(bbp)(CN)}₂]·10H₂O (3) [TPA = tris(3,5-dimethylpyrazol-1-ylmethyl)amine and H₂bbp = bis(2-benzimidazolyl)pyridine], are synthesized and characterized. Low temperature magnetic measurements reveal that complex 3 has dominant ferromagnetic interactions between low-spin Fe^III (S = 1/2) and high-spin Ni^II (S = 1) ions. UV-visible spectrophotometric measurements and electrochemical studies establish that reversible intra-molecular metal-to-metal electron transfer can be triggered in 3 upon the addition of either an acid or a base.

Chiral heterobimetallic chains from a dicyanideferrite building block including a π-conjugated TTF annulated ligand

The π-conjugated tetrathiafulvalene (TTF) annulated ligand was introduced into a dicyanometallate for the first time, leading to the synthesis of the versatile redox-active dicyanideferrite building block [(n-Bu)₄N][Fe(TTFbp)(CN)₂] (H₂TTFbp = N-(2-(4,5-bis(methylthio)-1,3-dithiol-2-ylidene)-5-(picolinamido) benzo[d][1,3]dithiol-6-yl) picol inamide). The incorporation of the new precursor with chiral Mn^III Schiff-base complexes resulted in two enantiopure one-dimensional complexes, [Mn((R,R)-salphen)Fe(TTFbp)(CN)₂]_n (2-(RR)) and [Mn((S,S)-salphen)Fe(TTFbp)(CN)₂]_n (2-(SS)) (Salphen = N,N'-1,2-diphenylethylene-bis(salicylideneiminato) dianion), which were synthesized and structurally characterized. Circular dichroism (CD) and vibrational circular dichroism (VCD) spectra confirmed the enantiomeric nature of the optically active complexes, and structural analyses revealed the formation of neutral cyanide-bridged double chains in 2-(RR) and 2-(SS). Solution and solid state CV studies revealed the redox-active characteristics of the complexes. Antiferromagnetic couplings were detected between Fe^III and Mn^III centers within a chain, and a field-induced magnetic phase transition was observed (T_N = 4.8 K). The introduction of electroactivity and chirality into cyanide-bridged complexes with interesting magnetic properties leads the way towards new multifunctional materials.

Magnetic clusters based on octacyanidometallates

A complete review of all known octacyanide-based clusters with an extensive discussion of their structures, magnetic properties and multifunctionality.

Synthesis, crystal structures and magnetic properties of mer-cyanideiron(iii)-based 1D heterobimetallic cyanide-bridged chiral coordination polymers

Two pairs of 1D cyanide-bridged heterometallic chiral enantiomer complexes have been synthesized, structurally characterized and magnetically investigated.

Single-molecule magnet engineering: building-block approaches

Tailoring the specific magnetic properties of any material relies on the topological control of the constituent metal ion building blocks. Although this general approach does not seem to be easily applied to traditional inorganic bulk magnets, coordination chemistry offers a unique tool to delicately tune, for instance, the properties of molecules that behave as "magnets", the so-called single-molecule magnets (SMMs). Although many interesting SMMs have been prepared by a more or less serendipitous approach, the assembly of predesigned, isolatable molecular entities into higher nuclearity complexes constitutes an elegant and fascinating strategy. This Feature article focuses on the use of building blocks or modules (both terms being used indiscriminately) to direct the structure, and therefore also the magnetic properties, of metal ion complexes exhibiting SMM behaviour.

Evolvement of molecular nanomagnets in China

Molecular nanomagnets have been undergoing development for 20 years since the first single-molecule magnet (SMM), Mn₁₂Ac, was characterized as the molecule-behaved magnet. The multi-disciplinary scientists promoted the magnetic characteristics to be more suitable for use in information science and spintronics. The concept of molecular nanomagnets has also evolved to include single-chain magnets (SCMs), single-ion magnets (SIMs) and even magnetic molecules that showed only slow magnetic relaxation, in addition to the initial cluster-type SMMs. In this review, several aspects, including SMMs, SCMs and SIMs, are introduced briefly through some representative examples. In particular, the contribution of Chinese chemists is highlighted in the design, synthesis and understanding of various types of molecular nanomagnets.

Cyanide-bridged W(V)Mn(III) bimetallic chains composed of a blocked W hexacyanide precursor: geometry-related magnetic couplings and magnetostructural correlation

Five one-dimensional bimetallic W(V)Mn(III) complexes 1-5, consisting of [W(CN)6(bpy)](-) anions and [Mn(Schiff base)](+) cations, were prepared. The central coordination geometry around each W atom is determined as a distorted dodecahedron (DD) for 1 and 2, and a distorted square antiprism (SAPR) for 3-5. Magnetic analyses demonstrate that compounds 1, 4, and 5 exhibit antiferromagnetic interactions between magnetic centers, which are different from the ferromagnetic couplings in 2 and 3. For the distorted DD geometry, the Mn-N(ax) (ax = axial) bond length increases when moving from 1 to 2, with the Mn-N(ax)-C(ax) angle remaining constant. The elongation of the bond length is responsible for the reduction in orbital overlap and consequent ferromagnetic coupling in 2. In comparison, for 3-5 with the distorted SAPR geometry, given that the Mn-N(ax) bond lengths are similar across all the samples, the increase in the Mn-N(ax)-C(ax) angles accounts for the enhanced magnetic strength. Notably, a correlation between structure and magnetic exchange coupling is established for the first time in W(V)Mn(III) bimetallic systems based on the [W(CN)6(bpy)](-) precursor.

Synthesis, structure, and magnetic properties of three 1D chain complexes based on high-spin metal-cyanide clusters: [Mn(III)6M(III)] (M = Cr, Fe, Co)

On the basis of high-spin metal-cyanide clusters of Mn(III)(6)M(III) (M = Cr, Fe, Co), three one-dimensional (1D) chain complexes, [Mn(salen)](6)[Cr(CN)(6)](2)·6CH(3)OH·H(2)O (1), [Mn(5-CH(3))salen)](6)[Fe(CN)(6)](2)·2CH(3)CN·10H(2)O (2), and [Mn(5-CH(3))salen)](6)[Co(CN)(6)](2)·2CH(3)CN·10H(2)O (3) [salen = N,N'-ethylenebis(salicylideneiminato) dianion], have been synthesized and characterized structurally as well as magnetically. Complexes 2 and 3 are isomorphic but slightly different from complex 1. All three complexes contain a 1D chain structure which is comprised of alternating high-spin metal-cyanide clusters of [Mn(6)M](3+) and a bridging group [M(CN)(6)](3-) in the trans mode. Furthermore, the three complexes all exhibit extended 3D supramolecular networks originating from short intermolecular contacts. Magnetic investigation indicates that the coupling mechanisms are intrachain antiferromagnetic interactions for 1 and ferromagnetic interactions for 2, respectively. Complex 3 is a magnetic dilute system due to the diamagnetic nature of Co(III). Further magnetic investigations show that complexes 1 and 2 are dominated by the 3D antiferromagnetic ordering with T(N) = 7.2 K for 1 and 9.5 K for 2. It is worth noting that the weak frequency-dependent phenomenon of AC susceptibilities was observed in the low-temperature region in both 1 and 2, suggesting the presence of slow magnetic relaxations.