An octanuclear [Co(ii)2–Co(iii)2]2 interlocked grid—example of an inorganic [2]catenane

Self-assembly of fish-bone and grid-like CoII-based single-molecule magnets using dihydrazone ligands with NNN and NNO pockets

Three Co^II complexes, [Co₂(H₂L¹)₂](ClO₄)₄·4CH₃OH (1), [Co₂(H₄L²)₂](ClO₄)₄ (2) and [Co₄(H₄L²)₄](ClO₄)₈ (3), were constructed by the self-assembly of the symmetrical dihydrazone ligands H2L1 and H4L2 with Co^II ions under different synthetic conditions. The fish-bone-like complex 1 was obtained using the ligand H2L1 in methanol via the solvothermal method, while the self-assembly of H4L2 with Co^II ions is solvent-dependent, producing the fish-bone-like complex 2 and [2 × 2] grid-like complex 3. Magnetic susceptibility measurements and theoretical calculations reveal that the large negative D values for the three complexes stem from their easy-axis magnetic anisotropy. Ac magnetic susceptibility measurements disclosed field-induced slow magnetic relaxation behaviors and the presence of Raman and/or direct processes of the three complexes at various applied dc fields.

Synthesis of Novel Tritopic Hydrazone Ligands: Spectroscopy, Biological Activity, DFT, and Molecular Docking Studies

Polytopic organic ligands with hydrazone moiety are at the forefront of new drug research among many others due to their unique and versatile functionality and ease of strategic ligand design. Quantum chemical calculations of these polyfunctional ligands can be carried out in silico to determine the thermodynamic parameters. In this study two new tritopic dihydrazide ligands, N’2, N’6-bis[(1E)-1-(thiophen-2-yl) ethylidene] pyridine-2,6-dicarbohydrazide (L1) and N’2, N’6-bis[(1E)-1-(1H-pyrrol-2-yl) ethylidene] pyridine-2,6-dicarbohydrazide (L2) were successfully prepared by the condensation reaction of pyridine-2,6-dicarboxylic hydrazide with 2-acetylthiophene and 2-acetylpyrrole. The FT-IR, 1H, and 13C NMR, as well as mass spectra of both L1 and L2, were recorded and analyzed. Quantum chemical calculations were performed at the DFT/B3LYP/cc-pvdz/6-311G+(d,p) level of theory to study the molecular geometry, vibrational frequencies, and thermodynamic properties including changes of ∆H, ∆S, and ∆G for both the ligands. The optimized vibrational frequency and (1H and 13C) NMR obtained by B3LYP/cc-pvdz/6-311G+(d,p) showed good agreement with experimental FT-IR and NMR data. Frontier molecular orbital (FMO) calculations were also conducted to find the HOMO, LUMO, and HOMO−LUMO gaps of the two synthesized compounds. To investigate the biological activities of the ligands, L1 and L2 were tested using in vitro bioassays against some Gram-negative and Gram-positive bacteria and fungus strains. In addition, molecular docking was used to study the molecular behavior of L1 and L2 against tyrosinase from Bacillus megaterium. The outcomes revealed that both L1 and L2 can suppress microbial growth of bacteria and fungi with variable potency. The antibacterial activity results demonstrated the compound L2 to be potentially effective against Bacillus megaterium with inhibition zones of 12 mm while the molecular docking study showed the binding energies for L1 and L2 to be −7.7 and −8.8 kcal mol−1, respectively, with tyrosinase from Bacillus megaterium.

Self-Assembly Synthesis of a [2]Catenane CoII Single-Molecule Magnet

We describe herein the self-assembly synthesis of an octanuclear Co^II [2]catenane {[Co₄ (H₂ L)₆ ]₂ ¹⁶⁺ } formed by the mechanical interlocking of two {[Co₄ (H₂ L)₆ ]⁸⁺ } rectangles of unprecedented topology. Subtle manipulation of the synthetic conditions allows the isolation of a mixed-valence [Co₂ ^III /Co₂ ^II ]¹⁰⁺ non-catenated rectangle. The Co^II centers in the [2]catenane exhibit slow relaxation of their magnetic moment, i. e. single-molecule magnet properties, dominated by quantum tunneling and Raman relaxation processes. This work shows that metallo-supramolecular chemistry can precisely control the organization of single-molecule magnets in topologically complex arrangements.

One-Pot Self-Assembly of Dinuclear, Tetranuclear, and H-Bonding-Directed Polynuclear Cobalt(II), Cobalt(III), and Mixed-Valence Co(II)/Co(III) Complexes of Schiff Base Ligands with Incomplete Double Cubane Core

The reaction of 2,6-diformyl-4-methylphenol (DFMF) with 1-amino-2-propanol (AP) and tris(hydroxymethyl)aminomethane (THMAM) was investigated in the presence of Cobalt(II) salts, (X = ClO₄⁻, CH₃CO₂⁻, Cl⁻, NO₃⁻), sodium azide (NaN₃), and triethylamine (TEA). In one pot, the variation in Cobalt(II) salt results in the self-assembly of dinuclear, tetranuclear, and H-bonding-directed polynuclear coordination complexes of Cobalt(III), Cobalt(II), and mixed-valence Co^IICo^III: [Co₂^III(H₂L⁻¹)₂(AP⁻¹)(N₃)](ClO₄)₂ (1), [Co₄(H₂L⁻¹)₂(µ₃-1,1,1-N₃)₂(µ-1,1-N₃)₂Cl₂(CH₃OH)₂]·4CH₃OH (2), [Co₂^IICo₂^III(HL⁻²)₂(µ-CH₃CO₂)₂(µ₃-OH)₂](NO₃)₂·2CH₃CH₂OH (3), and [Co₂^IICo₂^III (H₂L1²⁻)₂(THMAM⁻¹)₂](NO₃)₄ (4). In 1, two cobalt(III) ions are connected via three single atom bridges; two from deprotonated ethanolic oxygen atoms in the side arms of the ligands and one from the1-amino-2-propanol moiety forming a dinuclear unit with a very short (2.5430(11) Å) Co-Co intermetallic separation with a coordination number of 7, a rare feature for cobalt(III). In 2, two cobalt(II) ions in a dinuclear unit are bridged through phenoxide O and μ₃-1,1,1-N₃ azido bridges, and the two dinuclear units are interconnected by two μ-1,1-N₃ and two μ₃-1,1,1-N₃ azido bridges generating tetranuclear cationic [Co₄(H₂L⁻¹)₂(µ₃-1,1,1-N₃)₂(µ-1,1-N₃)₂Cl₂(CH₃OH)₂]²⁺ units with an incomplete double cubane core, which grow into polynuclear 1D-single chains along the a-axis through H-bonding. In 3, HL²⁻ holds mixed-valent Co(II)/Co(III) ions in a dinuclear unit bridged via phenoxide O, μ-1,3-CH₃CO₂⁻, and μ₃-OH⁻ bridges, and the dinuclear units are interconnected through two deprotonated ethanolic O in the side arms of the ligands and two μ₃-OH⁻ bridges generating cationic tetranuclear [Co₂^IICo₂^III(HL⁻²)₂(µ-CH₃CO₂)₂(µ_3-OH)₂]²⁺ units with an incomplete double cubane core. In 4, H₂L1⁻² holds mixed-valent Co(II)/Co(III) ions in dinuclear units which dimerize through two ethanolic O (μ-RO⁻) in the side arms of the ligands and two ethanolic O (μ₃-RO⁻) of THMAM bridges producing centrosymmetric cationic tetranuclear [Co₂^IICo₂^III (H₂L1⁻²)₂(THMAM⁻¹)₂]⁴⁺ units which grow into 2D-sheets along the bc-axis through a network of H-bonding. Bulk magnetization measurements on 2 demonstrate that the magnetic interactions are completely dominated by an overall ferromagnetic coupling occurring between Co(II) ions.

Synthesis, structure and phenoxazinone synthase-like activity of three unprecedented alternating CoII–CoIII 1D chains

Three new one-dimensional Co^II/Co^III chains have been synthesized using N-salicylidene-l-alanine/N-salicylidene-l-phenylalanine and 2,2′-bipyridine/1,10-phenanthroline. All three complexes exhibit phenoxazinone synthase-like activity.

A Six-Crossing Doubly Interlocked [2]Catenane with Twisted Rings, and a Molecular Granny Knot

A molecular 6 2 3 link (a six crossing, doubly interlocked, [2]catenane with twisted rings) and a 31 #31 granny knot (a composite knot made up of two trefoil tangles of the same handedness) were constructed by ring-closing olefin metathesis of an iron(II)-coordinated 2×2 interwoven grid. The connections were directed by pendant phenyl groups to be between proximal ligand ends on the same faces of the grid. The 6 2 3 link was separated from the topoisomeric granny knot by recycling size-exclusion chromatography. The identity of each topoisomer was determined by tandem mass spectrometry and the structure of the 6 2 3 link confirmed by X-ray crystallography, which revealed two 82-membered macrocycles, each in figure-of-eight conformations, linked through both pairs of loops.

Syntheses, crystal structures and magnetic properties of two mixed-valence Co(iii)Co(ii) compounds derived from Schiff base ligands: field-supported single-ion-magnet behavior with easy-plane anisotropy

Two μ-phenoxo-μ_1,1-azide dinuclear Co^IIICo^II complexes [Co^III(N₃)₂L¹(μ_1,1-N₃)Co^II(N₃)]·MeOH (1) and [Co^III(N₃)₂L²(μ_1,1-N₃)Co^II(N₃)]·MeOH (2) (HL¹ and HL² are two Schiff base ligands having N₂O-N₂O compartments) both possess one hexacoordinate Co(iii) and one pentacoordinate Co(ii) center. DC magnetic susceptibility and magnetization measurements show an appreciable amount of positive magnetic anisotropy (D/hc∼ 40 cm^-1) that is also confirmed by ab initio CASSCF calculations. AC susceptibility measurements of 1 reveal that it exhibits a slow magnetic relaxation with two relaxation channels. The external magnetic field supports the low-frequency (LF) channel that escapes on heating more progressively than the high-frequency (HF) branch. The relaxation time is as slow as τ = 255 ms at T = 1.9 K and B_DC = 0.6 T, where the LF mole fraction is 69%. The complex 2 also displays similar field-supported slow magnetic relaxation with two relaxation channels.

Coordination-Driven Self-Assembly Using Ditopic Pyridyl-Pyrazolyl Donor and p-Cymene Ru(II) Acceptors: [2]Catenane Synthesis and Anticancer Activities

Coordination-driven self-assembly of m-bis[3-(4-pyridyl)pyrazolyl]xylene (L) and [(p-cymene)₂Ru₂(OO∩OO)₂(OTf)₂] (A₁) (OO∩OO = 6,11-dioxido-5,12-naphthacenedione) in methanol resulted in a mixture of [2]catenane 1 and macrocycle 2, and self-assembly in nitromethane resulted in pure macrocycle 2, whereas the coordination-driven self-assembly of L and similar acceptors [(p-cymene)₂Ru₂(OO∩OO)₂(OTf)₂] [OO∩OO = 5,8-dioxido-1,4-naphthoquinonnato (A₂); 2,5-dioxido-1,4-benzoquinonato (A₃); oxalato (A₄)] resulted in the formations of monomeric macrocycles 3-5, respectively. All self-assembled macrocycles were obtained in excellent yields (>90%) as triflate salts and were fully characterized by multinuclear NMR, elemental analysis, and electrospray ionization mass spectrometry (ESI-MS). The structures of [2]catenane 1 and macrocycles 5 were confirmed by single-crystal X-ray diffraction analysis. The X-ray structure of 1 confirmed an edge-to-face interaction between the tetracene moiety in parallel-displaced π-π stacks (3.5 Å), and CH···π (2.5 Å) stabilizes the [2]catenane topology. Macrocycles 2-5 were assessed for anticancer activities using human cancer cell lines of different origins, and the macrocycle 3 was found to exhibit the best inhibitory effect and to do so in a dose-dependent manner. Further examination with the Tali apoptosis assay suggested the growth inhibitory effect of 3 involved the induction of the programmed cell death, and this suggestion was supported by observations of PARP and caspase 3 cleavage after treating cells with 3. In addition, exposure to 3 increased the expression of Bax and repressed the expression of Bcl-2, thus indicating the involvement of macrocycle 3 upstream of Bax and Bcl-2 in the apoptotic signaling pathway. Macrocycle 3 also tended to repress metastasis as evidenced by changes in the transcriptional expressions E- and N-cadherin (markers of metastasis). Furthermore, a stability assay demonstrated macrocycle 3 remained stable at high concentration.

Tetranuclear Lanthanide Complexes Containing a Hydrazone-type Ligand. Dysprosium [2 × 2] Gridlike Single-Molecule Magnet and Toroic

A multidentate hydrazone-type ligand (Z,Z)-bis(1-(pyridin-2-yl)-1-amino-methylidene)oxalohydrazide (H₂L) was utilized in the synthesis of three new isomorphous tetranuclear complexes of the general formula [Ln₄(HL)₄(H₂L)₂(NO₃)₄](NO₃)₄·4CH₃OH (Ln = Gd^III, 1, Tb^III, 2, Dy^III, 3) with the gridlike [2 × 2] topology. The analysis of the static magnetic data revealed weak anti-ferromagnetic interaction among lanthanide(III) atoms, whereas dynamic magnetic data led to the observation of the single-molecule magnet behavior in zero static magnetic field for the Dy₄ compound 3 with U_eff = 42.6 K and τ₀ = 1.50 × 10^-5 s. The theoretical CASSCF calculations supported also the presence of the net toroidal magnetic moment, which classifies compound 3 also as a single-molecule toroic.

Conformation driven in situ interlock: from discrete metallocycles to infinite polycatenanes

A novel conformation driven self-assembly system, where four metallocycles with different conformations have been in situ self-assembled, has been reported. Interestingly, only square metallocycles can further interlock into polycatenanes. However, rectangular and rhombus metallocycles fail to overcome such an entropically unfavourable process, which constitutes an obstacle to the formation of polycatenanes.

Mixed valence mono- and hetero-metallic grid catenanes

Here, we report on the multicomponent self-assembly and single crystal X-ray diffraction study of a series of three interlocked mixed valence mono- and hetero-metallic [2]-catenanes made of [2 × 2] metallo-grids. They show unique structural features and highlight the essential roles of both the Cu(ii)/Cu(i) pair and of the conformationally adaptable organic ligands for achieving catenation of grids.

Constructing supramolecular grids: from 4f square to 3d–4f grid

Novel supramolecular 4f and 3d–4f grids were constructed using dihydrazone based ligands, where the 3d–4f grid represents the first example of this type and shows SMM behavior.

Programmed multiple complexation for the creation of helical structures from acyclic phenol-bipyridine oligomer ligands

Two new multidentate ligands, H3L(1) and H4L(2), possessing bipyridine-phenol repetitive units were designed so that the multi-metal complexation could produce a single-helical structure in a pre-programmed fashion. The ligands were synthesized by successive palladium-catalyzed coupling reactions. The complexation of H3L(1) with zinc(II) and nickel(II) acetate afforded [L(1)Zn2(OAc)] and [(L(1))2Ni4](OAc)2, respectively. Each of the ligand moieties in these complexes formed a one-turn single helix. The zinc(II) complex [L(1)Zn2(OAc)] underwent a helix compression-extension motion in solution. The complexation of the H3L(1) ligand with iron(III) chloride gave a dinuclear complex [(HL(1))2Fe2Cl2] with a non-helical dimeric structure. The longer ligand H4L(2) afforded a trinuclear complex [L(2)Zn3(OAc)2] with a 1.5-turn single-helical structure upon complexation with zinc(II) acetate. The reaction of the H4L(2) ligand with cobalt(II) acetate under aerobic conditions gave a mixed valence complex [L(2)Co3(OAc)3(OMe)], which had two trivalent and one divalent cobalt ions. The structural features of the trinuclear complexes significantly depended on the metals; [L(2)Co3(OAc)3(OMe)] had a helical pitch of 7.6 Å, which was almost twice that of [L(2)Zn3(OAc)2] (4.0 Å).

Tuning transverse anisotropy in Co(III)-Co(II)-Co(III) mixed-valence complex toward slow magnetic relaxation

Two cobalt mixed-valence complexes with different substituents have been prepared and structurally characterized by single-crystal X-ray diffraction to alter slow magnetic relaxation by tailoring the transverse anisotropy. The trinuclear complexes [(L(1))4Co3(H2O)2](NO3)4·CH3OH·5H2O (1-NO3) and [(L(2))4Co3(H2O)2](NO3)4·6H2O (2-NO3) feature a distorted octahedral Co(II) strongly hindered in a trinuclear Co(III)-Co(II)-Co(III) mixed-valence array. Detailed magnetic studies of 1-NO3 and 2-NO3 have been conducted using direct- and alternating-current magnetic susceptibility data. In accordance with variable-field magnetic susceptibility data at low temperatures, high-field electron paramagnetic resonance (HF-EPR) spectroscopy reveals the presence of an easy-plane anisotropy (D > 0) with a significant transverse component, E, in complexes 1-NO3 and 2-NO3. These findings indicate that the onset of the variation of distortion within complex 2-NO3 leads to a suppression of quantum tunneling of the magnetization within the easy plane, resulting in magnetic bistability and slow relaxation behavior. Consequently, the anisotropy energy scale associated with the relaxation barrier, 5.46 cm(-1) (τo = 1.03 × 10(-5) s), is determined by the transverse E term. The results demonstrate that slow magnetic relaxation can be switched through optimization of the transverse anisotropy associated with magnetic ions that possess easy-plane anisotropy.

Slow magnetic relaxation in Co(III)-Co(II) mixed-valence dinuclear complexes with a Co(II)O5X (X = Cl, Br, NO3) distorted-octahedral coordination sphere

The reaction of the multisite coordination ligand (LH4) with CoX2·nH2O in the presence of tetrabutylammonium hydroxide affords a series of homometallic dinuclear mixed-valence complexes, [Co(III)Co(II)(LH2)2(X)(H2O)](H2O)m (1, X = Cl and m = 4; 2, X = Br and m = 4; 3, X = NO3 and m = 3). All of the complexes have been structurally characterized by X-ray crystallography. Both cobalt ions in these dinuclear complexes are present in a distorted-octahedral geometry. Detailed magnetic studies on 1-3 have been carried out. M vs H data at different temperatures can be fitted with S = 3/2, the best fit leading to D(3/2) = -7.4 cm(-1), |E/D| < 1 × 10(-3), and g = 2.32 for 1 and D(3/2) = -9.7 cm(-1), |E/D| <1 × 10(-4), and g = 2.52 for 2. In contrast to 1 and 2, M vs H data at different temperatures suggest that compound 3 has comparatively little magnetic anisotropy. In accordance with the large negative D values observed for compounds 1 and 2, they are single-molecule magnets (SMMs) and exhibit slow relaxation of magnetization at low temperatures under an applied magnetic field of 1000 Oe with the following energy barriers: 7.9 cm(-1) (τo = 6.1 × 10(-6) s) for 1 and 14.5 cm(-1) (τo = 1.0 × 10(-6) s) for 2. Complex 3 does not show any SMM behavior, as expected from its small magnetic anisotropy. The τo values observed for 1 and 2 are much larger than expected for a SMM, strongly suggesting that the quantum pathway of relaxation at very low temperatures is not fully suppressed by the effects of the applied field.

Strategies and tactics for the metal-directed synthesis of rotaxanes, knots, catenanes, and higher order links

More than a quarter of a century after the first metal template synthesis of a [2]catenane in Strasbourg, there now exists a plethora of strategies available for the construction of mechanically bonded and entwined molecular level structures. Catenanes, rotaxanes, knots and Borromean rings have all been successfully accessed by methods in which metal ions play a pivotal role. Originally metal ions were used solely for their coordination chemistry; acting either to gather and position the building blocks such that subsequent reactions generated the interlocked products or by being an integral part of the rings or "stoppers" of the interlocked assembly. Recently the role of the metal has evolved to encompass catalysis: the metal ions not only organize the building blocks in an entwined or threaded arrangement but also actively promote the reaction that covalently captures the interlocked structure. This Review outlines the diverse strategies that currently exist for forming mechanically bonded molecular structures with metal ions and details the tactics that the chemist can utilize for creating cross-over points, maximizing the yield of interlocked over non-interlocked products, and the reactions-of-choice for the covalent capture of threaded and entwined intermediates.

Getting harder: cobalt(III)-template synthesis of catenanes and rotaxanes

The synthesis of catenanes and rotaxanes using the hard trivalent transition metal ion cobalt(III) as a template is reported. Tridentate dianionic pyridine-2,6-dicarboxamido ligands, each with two terminal alkene groups, coordinate Co(III) in a mutually orthogonal arrangement such that entwined or interlocked molecular architectures are produced by ring-closing olefin metathesis. Double macrocyclization of two such ligands bound to Co(III) afford a non-interlocked "figure-of-eight" complex in 42% yield, the structure determined by X-ray crystallography. Preforming one macrocycle and carrying out a single macrocyclization of the second bis-olefin with both ligands attached to the Co(III) template led to the isomeric [2]catenate in 69% yield. The mechanically interlocked structure was confirmed by X-ray crystallography of both the Co(III) catenate and the metal-free catenand. A Co(III)-template [2]rotaxane was assembled in 61% yield by macrocyclization of the bis-olefin ligand about an appropriate dianionic thread. For both catenanes and rotaxanes, removal of the metal ion via reduction under acidic conditions to the more labile Co(II) gave neutral interlocked molecules with well-defined co-conformations stabilized by intercomponent hydrogen bonding.

New Hybrid [2]Catenanes Based on a 4,4‘-Bipyridinium Ligand

The self-assembly-mediated synthesis of metallomacrocycles 4a and 4b from (en)M(NO3)2 (M = Pd, Pt) and bipyridinium ligand 3 is described. The reaction is templated by disodium p-phenyldiacetic dicarboxylate, which is inserted into the macrocyclic cavity. Similarly, the self-assembly process between ligand 3, (en)M(NO3)2 (M = Pd, Pt), and the macrocyclic polyether 6 resulted in the formation of hybrid catenanes 7a and 7b. In the [2]catenanes, the circumrotation of the macrocyclic polyether through the cavity of the metallocycle is slow on the 1H NMR time scale.

A new infinite inorganic [n]catenane from silver and bis(2-methylimidazolyl)methane ligand

A new type of 1D infinite inorganic [n]catenane framework was self-assembled by silver nitrate and bis(2-methylimidazolyl)methane, while a comparatively one-dimensional zigzag cationic chain was generated by AgNO(3) and bis(imidazol-1-yl)methane.