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The construction of large and complex supramolecular architectures through self‐assembly is at the forefront of contemporary coordination chemistry. Notwithstanding great success in various systems using anionic bridges (e.g., O2− or S2−) or organic ligands (e.g., pyridine or carboxylate ligands), the assembly of large cyanide‐bridged clusters with increasing nuclearity remains a formidable synthetic challenge. In this study, it is achieved in preparing two heterometallic cyanometallate clusters with unprecedented complexity, [Fe20Co20] (1) and [Fe12Co15] (2), by creating the "flexibility" through a versatile ligand of bis((1H‐imidazol‐4‐yl)methylene)hydrazine (H2L) and low‐coordinate cobalt. Complex 1 features a super‐square array of four cyanide‐bridged [Fe4Co4] cube subunits as the corners that are interconnected by four additional [FeCo] units, resulting in a torus‐shaped architecture. Complex 2 contains a lantern‐like core‐shell cluster with a triple‐helix kernel of [Co3L3] enveloped by a [Fe12Co12] shell. The combined structure analysis and mass spectrometry study reveal a hierarchical assembly mechanism, which sheds new light on constructing cyanometallate nanoclusters with atomic precision. Moreover, complex 1 undergoes a thermally induced electron‐transfer‐coupled spin transition (ETCST) between the diamagnetic {FeIILS(µ‐CN)CoIIILS} and paramagnetic {FeIIILS(µ‐CN)CoIIHS} configurations (LS = low spin, HS = high spin) above room temperature, representing the largest molecule displaying electron transfer and spin transition characteristic. [ABSTRACT FROM AUTHOR]

Switchable molecules with magnetic bistability that can function at standard conditions are highly desirable from the perspective of practical applications. Over the last decade, significant progress has been made in modulating thermally induced electron-transfer-coupled spin transition (ETCST) events between diamagnetic {FeIILS(μ-CN)CoIIILS} and paramagnetic {FeLS(μ-CN)-CoIIHS} electron configurations (named LS and HS, respectively, for short) within the Fe/Co mixed-valence cyanometallate complexes. Herein, we prepared a novel [Fe2Co2] square complex [(pzTp)Fe(CN)3Co(bnbpen)]2[NO3]2·4CH3OH·2.5H2O (1·Solv) and its desolvated phase [(pzTp)Fe(CN)3Co(bnbpen)]2[NO3]2 (1), obtained via single-crystal-to-single-crystal (SCSC) transformation. Complex 1·Solv remaining in the diamagnetic state below room temperature exhibits an irreversible ETCST behavior accompanied by desolvation upon heating. Notably, complex 1 displays an unprecedented rate-dependent ETCST behaviour involving with two diamagnetic LS states (a dynamically favoured LS1 state and a thermodynamically favoured LS2 state), between which a net one-way transition from LS1 to LS2 is accessible likely via a structural phase transition (SPT) above 255 K. Consequently, two well separated transition channels (HS ↔ LS1 at around 255 K and HS ↔ LS2 in the temperature range of 317–350 K) are achieved according to either fast (> 20 K/min) or slow rate (< 1 K/min), respectively. At a moderate rate of 3 K/min, complex 1 shows a wide hysteresis of 66 K centered at 328 K, the largest yet reported for discrete Fe/Co cyanometallate complexes. Complex 1 represents the first ETCST example coupled with dynamic SPT, providing a new strategy for manipulating bistability within this fascinating cyanometallate family. [ABSTRACT FROM AUTHOR]

The creation of magnetically switchable materials that concurrently incorporate spin crossover (SCO) and a structural phase transition (SPT) presents a significant challenge in materials science. In this study, we prepared four structurally related cobalt(II)-based SCO compounds: two one-dimensional (1D) chains of {[(enbzp)Co(μ-L)](ClO4)2·sol}n (L = bpee, sol = 2MeOH·H2O, 1; L = bpea, sol = none, 2; enbzp = N,N′-(ethane-1,2-diyl)bis(1-phenyl-1-(pyridin-2-yl)methanimine); bpee = 1,2-bis(4-pyridyl)ethylene; and bpea = 1,2-bis(4-pyridyl)ethane) and their discrete segments, [{(enbzp)Co}2(μ-L)](ClO4)4·2MeOH (L = bpee, 3; L = bpea, 4). In all of these complexes, each Co(II) center is equatorially chelated by the planar tetradentate ligand enbzp and connected to a chain or dinuclear structure through bpee or bpea ligands along its axial direction. All of the complexes, including their desolvated phases, displayed overall incomplete and gradual SCO properties. Interestingly, the desolvated phase of 1 exhibited an additional non-spin magnetic transition characterized by wide room-temperature hysteresis (>40 K), which was reversible and rate-dependent, showcasing the synergy between SCO and SPT manifested through slow kinetics. We discuss the possible reasons for the distinct features and our findings demonstrate that the combination of a rigid polymeric framework with flexible substituents holds promise for achieving synergy between SCO and SPT. [ABSTRACT FROM AUTHOR]

The engineering of intermolecular interaction is challenging but critical for magnetically switchable molecules. Here, we prepared two cyanide‐bridged [Fe4Co4] cube complexes via the alkynyl‐ and alcohol‐functionalized trispyrazoyl capping ligands. The alkynyl‐functionalized complex 1 exhibited a thermally‐induced incomplete metal‐to‐metal electron transfer (MMET) behaviour at around 220 K, while the mixed alkynyl/alcohol‐functionalized cube of 2 showed a complete and abrupt MMET behaviour at 232 K. Remarkably, both compounds showed a long‐lived photo‐induced metastable state up to 200 K. The crystallographic study demonstrated that the incomplete transition of 1 was likely due to the possible elastic frustration originating from the competition between the anion‐propagated elastic interactions and inter‐cluster alkynyl‐alkynyl & CH‐alkynyl interactions, whereas the latter are eliminated in 2 as a result of the partial substitution by the alcohol‐functionalized ligand. Additionally, the introduction of chemically distinguishable cobalt centers within the cube unit of 2 did not lead to a two‐step but a one‐step transition, possibly because of the strong ferroelastic intramolecular interaction through the cyanide bridges. [ABSTRACT FROM AUTHOR]

Recently, a family of [Fe2Co2] molecular capsules that display tunable electron transfer-coupled spin transition (ETCST) behavior were reported via a smart approach through Schiff-base condensation of aldehyde-functionalized 2,2-bipyridines (bpyCHO) and 1,7-heptanediamine (H2N(CH2)7NH2). Here, three more capsule complexes {[(TpR)Fe(CN)3]2[Co(bpyC＝N(CH2)nN＝Cbpy)]2[ClO4]2}·n(solvent) (1, TpR = Tp*, n = 5, sol = 8DMF; 2, TpR = TpMe, n = 9, sol = 5MeCN; and 3, TpR = Tp*, n = 11, sol = 5MeCN), where Tp* = hydridotris(3,5-dimethylpyrazol-1-yl)borate and TpMe = hydridotris(3-methylpyrazol-1-yl)borate are reported, demonstrating a successful extension of such an approach with other alkyldiamines of different lengths. Combined X-ray crystallographic, infrared spectroscopic and magnetic studies reveal incomplete electron transfer with either changing temperature or upon light exposure. [ABSTRACT FROM AUTHOR]

It has been recognized that both the ligand fields and intermolecular interactions may greatly impact the electron-transfer-coupled spin transition (ETCST) events in switchable magnetic materials; however, the engineering of these factors within a given system is still challenging. In this article, we chose the 4,4′-substituent 2,2′-bipyridine derivatives as chelating ligands according to their increasing electron-donating strength and incremental potential for forming hydrogen bonds (bpyCHO,CH3(L1) < bpyCH2OH,CH3 (L2) < bpyCH2OH,CH2OH (L3)), and prepared three new [Fe2Co2] complexes, {[(Tp*)Fe(CN)3Co(L)2]2[ClO4]2}·Sol (1, L = L1, Sol = 4MeCN·2H2O; 2, L = L2, Sol = 3MeCN; 3, L = L3, Sol = 4MeOH; Tp* = hydrotris(3,5-dimethylpyrazol-1-yl)borate). X-ray crystallography studies revealed that all the complexes share similar cyanide-bridged [Fe2Co2] square compositions except for the different substituted groups of L ligands, which led to the clearly evidenced intercluster hydrogen bonds between the neighbouring hydroxyl groups in 2 and 3. As a result, 1 remained in the paramagnetic [Fe III,LS2 Co II,HS2 ] state over the whole temperature range, while 2 and 3 showed complete ETCST behaviour with the transition temperatures (T1/2) being 221 and 294 K, respectively. [ABSTRACT FROM AUTHOR]

Two lanthanide single-molecule magnets (SMMs) [Dy3(μ3-OH)(HL-1)3(H2O)3](NO3)2·3H3O (1, H3L-1 = (E)-3-(((8-hydroxyquinolin-2-yl)methylene)amino)propane-1,2-diol) and [Dy6(μ3-OH)4(H2L-2)4(HL-2)2(L-2)2] (2, H3L-2 = (E)-2-hydroxy-N′-(2-hydroxy-3-methoxybenzylidene)benzohydrazide) were synthesized and characterized structurally and magnetically. Complex 1 contains a triangular Dy3 core in which the three Dy3+ ions share a μ3-OH− anion and the deprotonated ligands of (HL-1)2− serve both capping and bridging functions, while 2 displays a centrosymmetric hexanuclear DyIII structure with two similar Dy3 triangular cores ligated by two fully deprotonated (L-2)3− ligands, each of which shares two μ3-OH− anions. All the DyIII ions are eight-coordinated with quasi D2d or C2v symmetry. Magnetic studies reveal that 1 exhibited two-step magnetic relaxation under an applied dc field of 800 Oe, with effective energy barriers of 40.1 and 31.0 K for the slow relaxation (SR) and fast relaxation regimes (FR), respectively. Meanwhile, 2 only showed a tail of slow magnetic relaxation at above 2 K. Ab initio calculations have been carried out to show the nature of their different magnetic properties. [ABSTRACT FROM AUTHOR]

Manipulating the transition temperature (T1/2) of spin-crossover (SCO) complexes capable of fulfilling practical criteria through different synthetic strategies is one of the main focuses in the field of molecular magnetism. The reaction of the tricyanometallate precursor [(Tp*)FeIII(CN)3]− and Fe(II) salt with the "facially" tridentate ligand tris(2-pyridyl)phosphine oxide (TPPO) and NCE− anions afforded three isostructural {Fe III2 Fe II2 } square complexes {[(Tp*)FeIII(CN)3]2[FeII(TPPO)]2[NCE]2}·Sol (E = S, Sol = 2CH3OH·6H2O, 1; E = Se, Sol = 2MeCN·2CH2Cl2·2H2O, 2; E = BH3, Sol = 4CH3OH·2MeCN, 3). Detailed structural analysis, variable-temperature IR analysis, magnetic susceptibility measurements and DFT calculations revealed that all compounds exhibit complete and one-step SCO behaviour between the {Fe III,LS2 Fe II,HS2 } and {Fe III,LS2 Fe II,LS2 } electronic states. As the ligand field increases from NCS− to NCSe− to NCBH3−, T1/2 shifts dramatically from 214 to 250 to 288 K for 1, 2 and 3, respectively, demonstrating another effective way to tune the SCO properties of the [FeIII–CN–FeII] systems through the introduction of NCE− co-ligands. [ABSTRACT FROM AUTHOR]