Pre-programmed self-assembly of polynuclear clusters

Four Redox Isomers of a [3 × 3] Copper-Iron Heterometal Grid

A mixed-valence heterometallic nonanuclear [3 × 3] grid complex, [CuI2CuII6FeIII(L)6](BF4)5·MeOH·9H2O (1; MeOH = methanol), was synthesized by a one-pot reaction of copper and iron ions with multidentate ligand 2,6-bis[5-(2-pyridinyl)-1H-pyrazol-3-yl]pyridine (H2L). 1 showed five quasi-reversible one-electron redox processes centered at +0.74, +0.60, +0.39, +0.27, and -0.13 V versus SCE, assignable to four CuI/CuII processes and one FeII/FeIII couple, respectively. The two-electron-oxidized species [CuII8FeIII(L)6](PF6)7·4MeOH·7H2O (12eOx), the two-electron-reduced species [CuI4CuII4FeIII(L)6](PF6)3·2H2O (12eRed), and the three-electron-reduced species [CuI4CuII4FeII(L)6](PF6)2·5MeOH·H2O (13eRed) were isolated electrochemically. The four redox isomers were characterized by single-crystal X-ray analysis, SQUID magnetometry, and Mössbauer spectroscopy.

Tuning the spin-crossover properties of FeII4L6 cages via the interplay of coordination motif and linker modifications

Despite the increasing number of spin-crossover FeII-based cages, the interplay between ligand modifications (e.g. coordination motif substituents and linker) is not well-understood in these multinuclear systems, limiting rational design. Here, we report a family of FeII4L6 spin-crossover cages based on 2,2'-pyridylbenzimidazoles where subtle ligand modifications lowered the spin crossover temperature in CD3CN by up to 186 K. Comparing pairs of cages, CH3 substituents on either the coordination motif or phenylene linker lowered the spin-crossover temperature by 48 K, 91 K or 186 K, attributed to electronic effects, steric effects and a combination of both, respectively. The understanding of the interplay between ligand modifications gained from this study could be harnessed on the path towards the improved rational design of spin-crossover cages.

Structure, Optical and Magnetic Properties of Two Isomeric 2-Bromomethylpyridine Cu(II) Complexes [Cu(C6H9NBr)2(NO3)2] with Very Different Binding Motives

Two isomeric 2-bromomethylpyridine Cu(II) complexes [Cu(C₆H₉NBr)₂(NO₃)₂] with 2-bromo-5-methylpyridine (L¹) and 2-bromo-4-methylpyridine (L²) were synthesized as air-stable blue materials in good yields. The crystal structures were different with [Cu(L¹)₂(NO₃)₂] (CuL¹) crystallizing in the monoclinic space group P2₁/c, while the 4-methyl derivative CuL² was solved and refined in triclinic P1¯. The orientation of the Br substituents in the molecular structure (anti (CuL¹) vs. syn (CuL²) conformations) and the geometry around Cu(II) in an overall 4 + 2 distorted coordination was very different with two secondary (axially elongated) Cu-O bonds on each side of the CuN₂O₂ basal plane in CuL¹ or both on one side in CuL². The two Br substituents in CuL² come quite close to the Cu(II) centers and to each other (Br⋯Br ~3.7 Å). Regardless of these differences, the thermal behavior (TG/DTA) of both materials is very similar with decomposition starting at around 160 °C and CuO as the final product. In contrast to this, FT-IR and Raman frequencies are markedly different for the two isomers and the UV-vis absorption spectra in solution show marked differences in the π-π* absorptions at 263 (CuL²) or 270 (CuL¹) nm and in the ligand-to-metal charge transfer bands at around 320 nm which are pronounced for CuL¹ with the higher symmetry at the Cu(II) center, but very weak for CuL². The T-dependent susceptibility measurements also show very similar results (µ_eff = 1.98 µ_B for CuL¹ and 2.00 µ_B for CuL² and very small Curie-Weiss constants of about -1. The EPR spectra of both complexes show axial symmetry, very similar averaged g values of 2.123 and 2.125, respectively, and no hyper-fine splitting.

Syntheses, structures, and magnetic properties of a series of Mn-M-Mn trinuclear complexes with different spin configurations

A series of trinuclear complexes, [MnII2Y^III(L)₂(HL)₂(NO₃)₃][Y^III(NO₃)₅]·7H₂O (1'), [MnII2Gd^III(HL)₄(NO₃)₄]₂[MnII2Gd^III(L)(HL)₃(NO₃)₄][Gd^III(NO₃)₅]₄·2(o-Xy)·12H₂O (2') and [MnII3(L)(HL)₂(NO₃)₄](NO₃)·1.25(p-Xy) (3'), were synthesized using a β-diketone ligand HL (HL = 1,3-bis(pyridin-2-yl)propane-1,3-dione). X-ray structural analyses revealed that each complex has a trinuclear core with an Mn(II)-M-Mn(II) arrangement (M = Y^III (1), Gd^III (2), and Mn^II (3)). In 1' with a diamagnetic Y(III) ion, negligible antiferromagnetic interactions between terminal Mn(II) ions are operative. On the other hand, 2' shows ferromagnetic interactions between Mn(II) and Gd(III) ions, affording a spin ground state of S_T = 17/2. The homometallic Mn(II)₃ complex of 3' has an S_T = 5/2 spin ground state resulting from the antiferromagnetic interactions between neighboring Mn(II) ions. The maximum magnetic entropy change (-ΔS_m) of 1'-3' was estimated to be 12.3, 24.8, and 8.0 J kg^-1 K^-1, respectively.

A ring of grids: a giant spin-crossover cluster

Mononuclear and icosanuclear spin-crossover complexes, [Fe^II(HL)₂](BF₄)₂ (1) and [FeII20(L)₂₄](BF₄)₁₆ (2), were synthesized using an asymmetric multidentate ligand (HL). 1 has a bis-chelate structure with two protonated ligands, while 2 has a ring-shape structure comprising four [2 × 2] grid moieties and four mononuclear units.

A Ferromagnetically Coupled Octanuclear Manganese(III) Cluster: A Single-Molecule Magnet with a Spin Ground State of S = 16

An octanuclear manganese complex, [Mn^III₈(μ₄-O)₄(L)₄(OMe)₄(OAc)₄(OCH₂CH₂NH₃)₄] [1; H₂L = 3-(dimethoxymethyl)-2-hydroxybenzoic acid], was synthesized with an extended cubane core structure consisting of eight Mn ions bridged by O atoms. Cryomagnetic studies revealed that 1 showed a single-molecule-magnet behavior with an S = 16 spin ground state.

Effect of metal ion substitution on the catalytic activity of a pentanuclear metal complex

A pentanuclear cobalt complex that consists of five cobalt ions and six bpp⁻ ligands (Co5, Hbpp = 3,5-bis(2-pyridyl)pyrazole) was newly developed. The obtained complex can catalyze CO₂ reduction under electrochemical and photochemical conditions.

A triple-triangle cluster derived from a simple tridentate ligand

The heptanuclear ferric cluster, [FeIII7(O)6(HL)5(H2L)(H2O)8](BF4)4 (H2L = 2,6-bis(1,5-diphenyl-1H-pyrazol-3-yl)pyridine), was synthesized by the reaction of the planar tridentate ligand H2L with iron(ii) tetrafluoroborate hexahydrate. The cluster has a corner-shared triple-triangle core structure connected by three μ-O atoms, and it has a spin-frustrated ferrimagnetic spin ground state of S = 19/2.

Heterometallic Ru2Co2 [2 × 2] Grid with Localized Single Molecule Magnet Behavior

Metal complexes with a [n × n] gridlike structure are discussed as attractive building blocks for various materials chemistry applications in molecular nanotechnology and electronics, which often rely on the grids' magnetic and redox properties. Most of the known metallogrids are homometallic, though heterometallic systems that comprise two or more different metals promise higher level functionalities. However, heterometallic [n × n] grids are relatively rare, mostly because of the more challenging synthetic strategies. To that end a new heterometallic [2 × 2] grid complex [L₄Ru₂Co₂](BF₄)₄ (2) based on a known pyrazolate-bridged bis(tridentate) compartmental N-donor ligand [L]^- is presented in this work, along with its doubly oxidized congener [L₄Ru₂Co₂](BF₄)₆ (3). In order to prevent scrambling of the different metal ions, a stepwise synthetic approach was implemented in which an inert Ru^II "corner complex" [(HL)₂Ru](BF₄)₂ (1) was isolated first, followed by addition of the more labile Co^II. This exclusively yields the desired [L₄Ru₂Co₂]⁴⁺ with anti-topology, viz., with the Ru^II and Co^II ions situated at opposite corners of the [2 × 2] grid, as confirmed by single crystal X-ray diffraction. 2 can be sequentially oxidized four times, first at the Co vertices and then at the Ru vertices. ¹H NMR spectroscopy as well as ESI mass spectrometry evidenced integrity of the [L₄Ru₂Co₂]^4+/6+ grids in solution. Structural and magnetic analyses revealed that paramagnetic 2 features LS-Ru^II and HS-Co^II ions (LS = low-spin, HS = high-spin) whereas LS-Ru^II and LS-Co^III ions are present in diamagnetic 3. The LS-Ru^II ions in 2 serve to magnetically isolate the HS-Co^II whose coordination geometry is strongly distorted from octahedral. A large and negative axial zero-field splitting value (D = -64 cm^-1) for the local S = 3/2 ions is shown to lead to single molecule magnetic (SMM) properties characterized by a barrier to spin inversion of U_eff = 8.8 cm^-1 and a single relaxation process with τ_o = 3.1 × 10^-5 s. Transition metal [2 × 2] grid complexes showing SMM behavior are extremely rare, and this is the first heterometallic 3d/4d grid system featuring such a magnetic signature.

Heterometallic grids: synthetic strategies and recent advances

Supramolecular metallogrid complexes are metalloclusters involving metal ions in planar array arrangements and organic ligands in perpendicular arrangements at each corner of the metal sites. Such essentially metal ion arrays have attracted great attention in the last three decades owing to their variety of interesting optical, electronic and magnetic properties. Among them, metallogrids containing more than one type of metal, that is heterometallic grids, are rare but have more potential to engineer a higher level functionality into one molecule. However, until now, only dozens of heterometallic grids have been reported without any specific review. Herein, we aim to give an overview of the assembly strategies, the physicochemical properties, and finally some perspectives on the future development of heterometallic grids.

Carboxylic Acid Functionalized Spin-Crossover Iron(II) Grids for Tunable Switching and Hybrid Electrode Fabrication

Two carboxyl-substituted iron(II) grids, one protonated, [Fe₄(HL)₄](BF₄)₄·4MeCN·AcOEt (1), and the other deprotonated, [Fe₄(L)₄]·DMSO·EtOH (2), where H₂L = 4-{4,5-bis[6-(3,5-dimethylpyrazol-1-yl)pyrid-2-yl]-1 H-imidazol-2-yl}benzoic acid, were synthesized. Single-crystal X-ray structure analyses reveal that both complexes have a tetranuclear [2 × 2] grid structure. 1 formed one-dimensional chains through intermolecular hydrogen bonds between the carboxylic acid units of neighboring grids, while 2 formed two-dimensional layers stabilized by π-π-stacking interactions. 1 showed spin transition between the 3HS-1LS and 1.5HS-2.5LS states around 200 K, while 2 showed spin-crossover between the 4LS and 2LS-2HS states above 300 K. A modified indium-tin oxide (ITO) electrode was fabricated by soaking the ITO in a solution of 1. The resultant electrode showed reversible redox waves attributed to the original redox processes of iron(II)/iron(III).