Coordination complexes of 15-membered pentadentate aza, oxoaza and thiaaza Schiff base macrocycles “Old Complexes Offer New Attractions”

The optical properties of 3 + 3 macrocyclic Schiff base thin material obtained by the Molecular Beam Epitaxy method

3 + 3 optically active macrocyclic Schiff bases were synthesized in the reaction between 4-tert-butyl-2,6-diformylphenol with (1R,2R)-(+)-1,2-diphenylethylenediamine (S1) or (1S,2S)-(-)-1,2-diphenylethylenediamine (S1a). The new compounds were spectroscopically characterised by NMR, IR, X-ray (S1a), UV-Vis and fluorescence spectroscopy. The S1a molecule creates channels with distances between oxygen atoms ranging from 5.8-6.3 Å and sufficiently large to host acetonitrile molecule. Both compounds exhibit green-yellow emission in solution and solid state. Thin layers of the S1 compound obtained via Molecular Beam Epitaxy (MBE) were characterised by scanning electron microscopy with energy-dispersive X-ray spectroscopy SEM/EDS and atomic force microscopy (AFM). The optical properties of the S1/Si thin material were analysed using spectroscopic ellipsometry (SE), fluorescence spectroscopy and synchrotron radiation (SR). The time constant for the decay investigated under SR, denoted by τ1, was determined to be approximately 1.02 ns, suggesting a fast deactivation process of the excited electronic state in the S1/Si material. The ellipsometric analysis of the S1/Si layer showed semiconducting behaviour with pronounced absorption features in the UV range, attributed to π → π* and n → π* transitions, characteristic of Schiff bases. The band-gap energy, determined using the Tauc method, is 3.46 ± 0.01 eV. These analyses highlight the material's potential in applications requiring precise control of optical properties. In the emission spectrum of S1a, a significant emission peak of approximately 561 nm indicates the presence of a prominent emissive process within this wavelength. The S1a compound is emissive in the yellow-green region of the spectrum and has a longer decay time, which suggests that it can be used in sensing optical technologies.

Spin crossover iron complexes with spin transition near room temperature based on nitrogen ligands containing aromatic rings: from molecular design to functional devices

During last decades, significant advances have been made in iron-based spin crossover (SCO) complexes, with a particular emphasis on achieving reversible and reproducible thermal hysteresis at room temperature (RT). This pursuit represents a pivotal goal within the field of molecular magnetism, aiming to create molecular devices capable of operating in ambient conditions. Here, we summarize the recent progress of iron complexes with spin transition near RT based on nitrogen ligands containing aromatic rings from molecular design to functional devices. Specifically, we discuss the various factors, including supramolecular interactions, crystal packing, guest molecules and pressure effects, that could influence its cooperativity and the spin transition temperature. Furthermore, the most recent advances in their implementation as mechanical actuators, switching/memories, sensors, and other devices, have been introduced as well. Finally, we give a perspective on current challenges and future directions in SCO community.

Trigonal prismatic coordination geometry imparted by a macrocyclic ligand: an approach to large axial magnetic anisotropy for Co(II)

Large uniaxial magnetic anisotropy, expressed by a negative value of the axial zero-field splitting parameter D, has been achieved in a series of trigonal prismatic Co(II) complexes with the general formula [Co(L)X]Y, where L = 1,5,13,17,22-pentaazatricyclo[15.2.2.17,11]docosa-7,9,11(22)-triene, X = Cl-(1a,b), Br-(2), N3-(3), NCO-(4), NCS-(5), NCSe-(6), and Y = Cl-(1), Br-(2), NCS-(4), NCSe-(5), ClO4-(3,6). Complexes 1-6 are six-coordinate with the distorted trigonal prismatic geometry imparted by the pentadentate pyridine-/piperazine-based macrocyclic ligand L and by one monovalent coligand X-. Based on magnetic studies, all complexes 1-6 exhibit strong magnetic anisotropy with negative D-values ranging from about -20 to -41 cm-1. This variation in D (i.e. the increase of magnetic anisotropy) parallels the trend obtained by theoretical calculations and the lesser distortion of the coordination sphere with respect to the trigonal prismatic reference geometry. AC magnetic susceptibility investigations revealed field-induced single-molecule magnet behaviour for all complexes except Cl- derivative 1. The series investigated represents a rare example of Co(II) complexes with a robust trigonal prismatic geometry.

Unprecedented synthesis of a 14-membered hexaazamacrocycle

The transformation of 3-[(ethoxymethylene)amino]-1-methyl-1H-pyrazole-4-carbonitrile into the 14-membered macrocycle, 2,10-dimethyl-2,8,10,16-tetrahydrodipyrazolo[3,4-e:3',4'-l][1,2,4,8,9,11]hexaazacyclotetradecine-4,12-diamine, by the reaction with excess hydrazine under various conditions was studied in detail. The reaction proceeded through the initial formation of 4-imino-2-methyl-2,4-dihydro-5H-pyrazolo[3,4-d]pyrimidin-5-amine followed by dimerization to give the final macrocycle. A convenient synthesis of the latter starting from 4-imino-2-methyl-2,4-dihydro-5H-pyrazolo[3,4-d]pyrimidin-5-amine was developed. A plausible pathway for the macrocycle self-assembly is discussed. Some features of the structure and reactivity of the obtained macrocycle are outlined.

Oligonuclear Metal Complexes with Schiff Base Ligands

As stated by two of the seven papers [...].

Pentagonal-bipyramidal 4d and 5d complexes with unquenched orbital angular momentum as a unique platform for advanced single-molecule magnets: current state and perspectives

This article overviews the current state and prospects of the concept of advanced single-molecule magnets (SMMs) based on low-spin (S = 1/2) pentagonal-bipyramidal (PBP) 4d³ and 5d³ complexes with unquenched orbital angular momentum. This approach is based on the unique property of PBP 4d³ and 5d³ complexes to cause highly anisotropic spin coupling of perfect uniaxial symmetry, -J_zSziSzj - J_xy(SxiSxj + SyiSyj), regardless of the local geometric symmetry. The M(4d/5d)-M(3d) exchange-coupled pairs in the apical positions of the PBP complexes produce Ising-type exchange interactions (|J_z| > |J_xy|), which serve as a powerful source of uniaxial magnetic anisotropy of a SMM cluster. In polynuclear heterometallic 4d/5d-3d complexes embodying PBP 4d/5d units and high-spin 3d ions, anisotropic Ising-type exchange interactions produce a double-well potential with high energy barriers U_eff, which is controlled by the anisotropic exchange parameters J_z, J_xy. Theoretical analysis shows that the barrier is proportional to the difference |J_z - J_xy| and to the number n of the apical 4d/5d-3d pairs in a SMM cluster, U_eff ∝ |J_z - J_xy|n, which provides an opportunity to scale up the barrier U_eff and blocking temperature T_B up to the record values. A novel family of 4d/5d complexes with forced PBP coordination provided by structurally rigid planar pentadentate Schiff-base ligands in the equatorial plane is discussed as a better alternative to the cyanometallates. The possibility of a significant increase in the anisotropic exchange parameters J_z, J_xy in PBP complexes with monoatomic apical μ-bridging ligands is examined. The basic principles of molecular engineering the highest barrier through anisotropic exchange interactions of PBP 4d/5d complexes are formulated. The theoretical and experimental results taken together indicate that the concept of high-performance SMMs based on 4d/5d PBP complexes with unquenched orbital angular momentum is an attractive alternative to the currently dominant lanthanide-based SMM strategy.

Tunable Construction of Sandwich-Type Double-[1 + 1] and Half-Folded [2 + 2] Schiff-Base Complexes Controlled by the Combination of Primary and Secondary Template Effects

The first-row transition-metal ions Mn²⁺-Cu²⁺ could serve as effective templates to construct three types of double-[1 + 1], [2 + 2], and [1 + 1] Schiff-base dinuclear macrocyclic complexes in the presence of dialdehydes with different pendant arms and a common 1,8-diamine. The extremely flexible nature of macrocyclic ligands allows for the multiple template-directed syntheses, but the final products could be finely tuned by the subtle variations of Mn²⁺-Cu²⁺ ions in a 3d-electronic configuration, radius, and coordination number/geometry as well as the auxiliary (pendant-armed and anionic) template effect at the same time. Two borderlines are observed at the Co²⁺ ion for forming double-[1 + 1] and [2 + 2] metallacycles involving the H₂pdd precursor and the [1 + 1] Cu²⁺ complex for double-[1 + 1] and [2 + 2] macrocycles containing the H₂hpdd unit, respectively. The structural diversity is originated from the non-perfect match between [1 + 1]/[2 + 2] Schiff-base macrocycles and dinuclear metal centers; hence, a compromise between the metal coordination modes and alterations of the ligand conformation takes place.

Different Modes of Acid-Promoted Cyclooligomerization of 4-(4-Thiosemicarbazido)butan-2-one Hydrazone: 14-Membered versus 28-Membered Polyazamacrocycle Formation

Unprecedented self-assembly of a novel 14-membered cyclic bis-thiosemicarbazone or/and a 28-membered cyclic tetrakis-thiosemicarbazone upon acid-promoted cyclooligomerization of 4-(4-thiosemicarbazido)butan-2-one hydrazone has been discovered. A thorough study of the influence of various factors on the direction of macrocyclization provided the optimal conditions for the highly selective formation of each of the macrocycles in excellent yields. Plausible pathways for macrocyclizations have been discussed. The macrocycle precursor was prepared by the reaction of readily available 4-isothiocyanatobutan-2-one with an excess of hydrazine.

Lanthanide SMMs Based on Belt Macrocycles: Recent Advances and General Trends

Enhancement of axial magnetic anisotropy is the central objective to push forward the performance of Single-Molecule Magnet (SMM) complexes. In the case of mononuclear lanthanide complexes, the chemical environment around the paramagnetic ion must be tuned to place strongly interacting ligands along either the axial positions or the equatorial plane, depending on the oblate or prolate preference of the selected lanthanide. One classical strategy to achieve a precise chemical environment for a metal centre is using highly structured, chelating ligands. A natural approach for axial-equatorial control is the employment of macrocycles acting in a belt conformation, providing the equatorial coordination environment, and leaving room for axial ligands. In this review, we present a survey of SMMs based on the macrocycle belt motif. Literature systems are divided in three families (crown ether, Schiff-base and metallacrown) and their general properties in terms of structural stability and SMM performance are briefly discussed.

Multinuclear Ni(II) and Cu(II) complexes of a meso 6 + 6 macrocyclic amine derived from trans-1,2-diaminocyclopentane and 2,6-diformylpyridine

Four hexanuclear chloride and sulphate Ni(II) and Cu(II) complexes 1, 2, 4 and 5 and one tetranuclear nitrate Cu(II) complex 3 have been synthesised from appropriate metal salts and 6 + 6 octadecaaza macrocyclic ligands. All obtained coordination compounds have been characterised by elemental analysis, spectroscopic methods (ESI MS, NMR and EPR), magnetic susceptibility measurements and X-ray crystallography. Their X-ray crystal structures reveal different coordination modes of metal cations involved in the obtained centro-symmetrical coordination compounds. The conformational folding of the macrocyclic ligand adopted in the respective complexes depends on the number of metal cations bound within the macrocycle but not on their type. The cavities of these multinuclear complexes might be occupied by solvent molecules and counter anions bound by hydrogen bonds or might be empty in the case where the macrocyclic ring of the ligand is squeezed in the middle. All obtained Ni(II) and Cu(II) coordination compounds are paramagnetic. This has been proved by their ¹H NMR and EPR spectra and magnetic measurements. Direct current (DC) variable-temperature magnetic susceptibility measurements on the polycrystalline samples of 1-5 were carried out in the temperature range of 1.8-300 K. The magnetic behaviour of 1 and 2 is dominated by the magnetic anisotropy of the nickel(II) ion masking the magnetic interactions between magnetic centres. The magnetic data of 3-5 reveal small antiferromagnetic interactions within the Cu₄ and Cu₆ units. EPR experiments for 3-5 show, at 9.6 and 34 GHz frequencies, that the predominant contribution to the orbitals occupied by the unpaired electrons in the ground state originates from d_x²-y².

A general and stereoselective approach to 14-membered cyclic bis-semicarbazones involving BF3-catalyzed amidoalkylation of 2-(trimethylsilyloxy)propene

A general and stereoselective five-step approach to 14-membered cyclic bis-semicarbazones, 5,12-diaryl-7,14-dimethyl-1,2,4,8,9,11-hexaazacyclotetradeca-7,14-diene-3,10-diones, starting from aldehyde semicarbazones has been developed. The key intermediates, 4-(3-oxobut-1-yl)semicarbazones, were prepared by BF₃-catalyzed amidoalkylation of 2-(trimethylsilyloxy)propene with 4-[(aryl)(methoxy)methyl]- or 4-[(aryl)(tosyl)methyl]semicarbazones. Treatment of these intermediates with excess of hydrazine gave hydrazones of 4-(3-oxobut-1-yl)semicarbazones or 4-(3-oxobut-1-yl)semicarbazides, which in the presence of TsOH were converted into the target macrocycles. All steps of this approach could be scaled up easily to the multi-gram level.

Magnetic anisotropy of transition metal and lanthanide ions in pentagonal bipyramidal geometry

The magnetic anisotropy associated with a pentagonal bipyramidal (PBP) coordination sphere is examined on the basis of experimental and theoretical investigations. The origin and the characteristics of this anisotropy are discussed in relation to the electronic configuration of the metal ions. The effects of crystal field, structural distortion, and a second-coordination sphere on the observed anisotropies for transition meal and lanthanide ions are outlined. For the Ln derivatives, we focus on compounds showing SMM-like behavior (i.e. slow relaxation of their magnetization) in order to highlight the essential chemical and structural parameters for achieving strong axial anisotropy. The use of PBP complexes to impart controlled magnetic anisotropy in polynuclear species such as SMMs or SCMs is also addressed. This review of the magnetic anisotropies associated with a pentagonal bipyramidal coordination sphere for transition metal and lanthanide ions is intended to highlight some general trends that can guide chemists towards designing a compound with specific properties.

Syntheses, crystal structures and Hirshfeld surface analysis of (Z)-3-[(3-acetyl-2-hydroxyphenyl)amino]-2-bromoprop-2-enal and a novel ZnII complex

A novel zero-dimensional dinuclear zinc complex, di-μ-acetato-1:2κ4 O:O′-(μ-2-acetyl-6-{[(Z)-2-bromo-3-oxoprop-1-en-1-yl]azanidyl}phenolato-1κ2 O 1,O 2:2κ3 O 1,N,O 6)(N,N-dimethylacetamide-1κO)dizinc(II), [Zn2(C11H8BrNO3)(CH3COO)2(C4H9NO)] or [Zn2(L)(CH3COO)2(DMA)], 1, was synthesized using (Z)-3-[(3-acetyl-2-hydroxyphenyl)amino]-2-bromoprop-2-enal (H2 L), which was synthesized from 1-(3-amino-2-hydroxyphenyl)ethanone and 2-bromomalonaldehyde. H2 L and 1 were characterized by single-crystal X-ray diffraction, FT–IR spectroscopy and elemental analysis. Theoretical calculations of the bond orders and excited state of H2 L confirmed that there is extensive electron delocalization in the H2 L molecules. Single-crystal X-ray diffraction shows that the two Zn atoms are pentacoordinated in distorted trigonal bipyramidal configurations in the crystals of 1. The thermogravimetric analysis of 1 shows that the main frame of the complex remains stable to about 190 °C. Powder X-ray diffraction (PXRD) analysis shows that 1 possesses high purity and acid and alkali resistance. The intermolecular interactions of H2 L and 1 were analyzed using Hirshfeld surface analysis and the results indicate that the H...H and O...H interactions of H2 L and 1 play a considerable role in stabilizing the self-assembly process.

When the Metal Makes the Difference: Template Syntheses of Tridentate and Tetradentate Salen-Type Schiff Base Ligands and Related Complexes

The reaction of organic molecules mediated by a metal center (template synthesis) can result in a final connectivity that may differ from the one obtained in the absence of the metal. The condensation of carbonyl fragments with primary amines form C=N iminic bonds, the so-called Schiff bases, which can act as ligands for the templating metal center by means of the lone pair on the nitrogen atom. This review focuses on the template methods for the reaction between a carbonyl compound (mainly salicylaldehyde) and a primary aliphatic diamine able to prevent the double condensation on both amine groups and obtain tridentate N2O ligands. These adducts, still having one free amino group, can further react, yielding tetradentate salen-type Schiff base ligands. A screening over the transition metals able to show such a template effect will be presented, with particular attention to copper(II), together with their peculiar reactivity and the available crystal structure of the metal complexes and related coordination geometries.

Accessing water processable cyanido bridged chiral heterobimetallic Co(ii)-Fe(iii) one dimensional network

A water processable cyanido bridged extended chiral heterobimetallic Co(ii)-Fe(iii) network is assembled. The unusual water processability of the coordination polymer originates from dangling hydrophilic substituents. The present approach offers a simple route to impart solution processability to cyanido bridged molecular magnetic materials.

Specific detection and determination of cysteine by a luminescent samarium macrocycle-based fluorescent probe platform

A samarium macrocycle-based chemosensor bearing two imine bonds and a Sm(iii) emitter provides an indirect approach to detect and determine cysteine.

Alignment of Axial Anisotropy in a 1D Coordination Polymer shows Improved Field Induced Single Molecule Magnet Behavior over a Mononuclear Seven Coordinated FeII Complex

Herein, we report a CN-bridged alternating Fe^II -Ni^II 1D chain to ensure the alignment of axial anisotropy and improve the single molecule magnet (SMM) behavior in seven coordinated Fe^II compound. The chain was constructed from hepta coordinated Fe(II) complex as an anisotropic building unit and diamagnetic nickel tetra cyanate as a bridging ligand. The magnetic measurements show the easy-axis anisotropy of the seven coordinated Fe(II) complex and field induced SMM behavior with spin reversal energy barrier U_eff =61(2) K (42 cm^-1 ) and pre-exponential relaxation time τ₀ =1.9×10^-8  s. The detailed analysis of the relaxation dynamics discloses that the Orbach process plays an important role in slow relaxation of magnetization for this compound. Notably, this example represents a remarkable energy barrier observed in hepta coordinated Fe(II) SMMs. The ab initio calculations estimate the magnitude of axial anisotropy and show the parallel orientation of the anisotropic axis throughout the 1D polymeric chain. In addition, it is also reported that the presence of weak π accepter ligands in the distorted axial position enhance the easy-axis anisotropy.

Late first-row transition metal complexes of a 17-membered piperazine-based macrocyclic ligand: structures and magnetism

A 17-membered piperazine-based macrocyclic ligand LdiProp (1,5,13,17,22-pentaazatricyclo[15.2.2.17,11]docosa-7,9,11(22)-triene) was resynthesized in high yield by using a linear pump. Its Mn(ii), Fe(ii), Co(ii) and Ni(ii) complexes of the general formula [MnLdiProp(ClO4)2] (1), [FeLdiProp(CH3CN)](ClO4)2 (2), [CoLdiProp(CH3CN)](ClO4)2 (3), [NiLdiProp](ClO4)2 (4) were prepared and thoroughly characterized. X-ray diffraction analysis confirmed that Mn(ii) complex 1 has capped trigonal prismatic geometry with a coordination number of seven, Fe(ii) and Co(ii) complexes 2 and 3 are trigonal prismatic with a coordination number of six and Ni(ii) complex 4 has square pyramidal geometry with a coordination number of five. The decrease of the coordination number is accompanied by a shortening of M-N distances and an increase of torsion of the piperazine ring from the equatorial plane. Magnetic measurement reveals moderate anisotropy for 4 and rather large magnetic anisotropy for 2 and 3 (axial zero-field splitting parameter D(Ni) = 9.0 cm-1, D(Fe) = -14.4 cm-1, D(Co) = -25.8 cm-1, together with rather high rhombicity). Co(ii) complex 3 behaves as a field-induced SMM with a combination of Raman and direct or Orbach and direct relaxation mechanisms. Obtained magnetic data were extensively supported by theoretical CASSCF calculations. The flexibility and rather large 17-membered macrocyclic cavity of ligand LdiProp could be responsible for the variation of coordination numbers and geometries for the investigated late-first row transition metals.

Homo- and Hetero-Oligonuclear Complexes of Platinum Group Metals (PGM) Coordinated by Imine Schiff Base Ligands

Chemistry of Schiff base (SB) ligands began in 1864 due to the discovery made by Hugo Schiff (Schiff, H., Justus Liebigs Ann. der Chemie 1864, 131 (1), 118-119). However, there is still a vivid interest in coordination compounds based on imine ligands. The aim of this paper is to review the most recent concepts on construction of homo- and hetero-oligonuclear Schiff base coordination compounds narrowed down to the less frequently considered complexes of platinum group metals (PGM). The combination of SB and PGM in oligonuclear entities has several advantages over mononuclear or polynuclear species. Such complexes usually exhibit better electroluminescent, magnetic and/or catalytic properties than mononuclear ones due to intermetallic interactions and frequently have better solubility than polymers. Various construction strategies of oligodentate imine ligands for coordination of PGM are surveyed including simple imine ligands, non-innocent 1,2-diimines, chelating imine systems with additional N/O/S atoms, classic N2O2-compartmental Schiff bases and their modifications resulting in acyclic fused ligands, macrocycles such as calixsalens, metallohelical structures, nano-sized molecular wheels and hybrid materials incorporating mesoionic species. Co-crystallization and formation of metallophilic interactions to extend the mononuclear entities up to oligonuclear coordination species are also discussed.

Structural, magnetic, redox and theoretical characterization of seven-coordinate first-row transition metal complexes with a macrocyclic ligand containing two benzimidazolyl N-pendant arms

A structurally new heptadentate derivative of a 15-membered pyridine-based macrocycle containing two benzimidazol-2-yl-methyl N-pendant arms (L = 3,12-bis((1H-benzimidazol-2-yl)methyl)-6,9-dioxa-3,12,18-triazabicyclo[12.3.1]octadeca-1(18),14,16-triene) was synthesized and its complexes with the general formula [M(L)](ClO4)2·1.5CH3NO2 (M = MnII (1), FeII (2), CoII (3) and NiII (4)) were thoroughly investigated. X-ray crystal structures confirmed that all complexes are seven-coordinate with axially compressed pentagonal bipyramidal geometry having the largest distortion for NiII complex 4. FeII, CoII and NiII complexes 2, 3 and 4 show rather large magnetic anisotropy manifested by moderate to high obtained values of the axial zero-field splitting parameter D (7.9, 40.3, and -17.2 cm-1, respectively). Magneto-structural correlation of the FeII, CoII and NiII complexes with L and with previously studied structurally similar ligands revealed a significant impact of the functional group in pendant arms on the magnetic anisotropy especially that of the CoII and NiII complexes and some recommendations concerning the ligand-field design important for anisotropy tuning in future. Furthermore, complex 3 showed field-induced single-molecule magnet behavior described with the Raman (C = 507 K-n s-1 for n = 2.58) relaxation process. The magnetic properties of the studied complexes were supported by theoretical calculations, which very well correspond with the experimental data of magnetic anisotropy. Electrochemical measurements revealed high positive redox potentials for M3+/2+ couples and high negative redox potentials for M2+/+ couples, which indicate the stabilization of the oxidation state +ii expected for the σ-donor/π-acceptor ability of benzimidazole functional groups.

A Constrained and "Inverted" [3+3] Salphen Macrocycle with an ortho-Phenylethynyl Substitution Pattern

A [3+3] Schiff-base salphen macrocycle (7 a) was synthesized by imine condensation between ortho-phenylenediamine and ortho-phenylethynyl-bridged bis(5-salicylaldehyde) precursors. The triangular-shaped macrocycle 7 a has a nonclassical (or "inverted") design in which the N₂ O₂ coordination pockets are located at the sides instead of the corners. Compound 7 a could be synthesized in a reasonably good yield (64 %) considering the steric constraints imposed by the ortho substitution pattern. Subsequent zinc metalation afforded the corresponding Zn metallomacrocycle 7 b. Spectroscopic experiments evidenced weak (7 a) to strong (7 b) self-aggregation behavior in solution. Their ability to self-organize at the supramolecular level was further studied in the solid state by AFM and TEM, which revealed the formation of large bundles of fibers with lengths of several micrometers and widths of nanometers.

A ferromagnetic Ni(ii)–Cr(iii) single-chain magnet based on pentagonal bipyramidal building units

The first example of a ferromagnetic Ni(ii)–Cr(iii) single-chain magnet fashioned using pentagonal bipyramidal Ni(ii) complexes with Ising-type anisotropy.

Representation of a photosensitive Schottky barrier diode made with hetero-dinuclear cobalt(iii)/sodium building blocks

A hetero-dinuclear cobalt(iii)/sodium complex has been synthesized and characterized. The complex which has been used to construct photosensitive Schottky devices, shows interesting C–H⋯π(N₃) interactions in the solid state.

Palladium(ii) complexes of tetradentate donor–acceptor Schiff base ligands: synthesis and spectral, structural, thermal and NLO properties

Structural and NLO behavior of push–pull palladium(ii) complexes of metallocenyl-containing asymmetric Schiff base ligands.

Twisted Schiff-base macrocycle showing excited-state intramolecular proton-transfer (ESIPT): assembly and sensing properties

A Schiff-base macrocyclic host showing ESIPT-based AIEE and nanoparticle assembly.

A Series of Field-Induced Single-Ion Magnets Based on the Seven-Coordinate Co(II) Complexes with the Pentadentate (N3O2) H2dapsc Ligand

A series of five new mononuclear pentagonal bipyramidal Co(II) complexes with the equatorial 2,6-diacetylpyridine bis(semicarbazone) ligand (H2dapsc) and various axial pseudohalide ligands (SCN, SeCN, N(CN)2, C(CN)3, and N3) was prepared and structurally characterizated: [Co(H2dapsc)(SCN)2]∙0.5C2H5OH (1), [Co(H2dapsc)(SeCN)2]∙0.5C2H5OH (2), [Co(H2dapsc)(N(CN)2)2]∙2H2O (3), [Co(H2dapsc)(C(CN)3)(H2O)](NO3)∙1.16H2O (4), and {[Co(H2dapsc)(H2O)(N3)][Co(H2dapsc)(N3)2]}N3∙4H2O (5). The combined analyses of the experimental DС and AC magnetic data of the complexes (1–5) and two other earlier described those of this family [Co(H2dapsc)(H2O)2)](NO3)2∙2H2O (6) and [Co(H2dapsc)(Cl)(H2O)]Cl∙2H2O (7), their theoretical description and the ab initio CASSCF/NEVPT2 calculations reveal large easy-plane magnetic anisotropies for all complexes (D = + 35 − 40 cm‒1). All complexes under consideration demonstrate slow magnetic relaxation with dominant Raman and direct spin–phonon processes at static magnetic field and so they belong to the class of field-induced single-ion magnets (SIMs).

Successive modification of polydentate complexes gives access to planar carbon- and nitrogen-based ligands

Polydentate complexes containing combinations of nitrogen and carbon (N and C) ligating atoms are among the most fundamental and ubiquitous molecules in coordination chemistry, yet the formation of such complexes with planar high-coordinate N/C sites remains challenging. Herein, we demonstrate an efficient route to access related complexes with tetradentate CCCN and pentadentate CCCCN and NCCCN cores by successive modification of the coordinating atoms in complexes with a CCCC core. Combined experimental and computational studies reveal that the rich reactivity of metal-carbon bonds and the inherent aromaticity of the metallacyclic skeletons play key roles in these transformations. This strategy addresses the paucity of synthetic approaches to mixed N/C planar pentadentate chelating species and provides valuable insights into the synthesis of carbon-based high-coordinate complexes. Furthermore, the resulting complexes are the examples of organometallic species with combined photoacoustic, photothermal, and sonodynamic properties, which makes them promising for application in related areas.

Two three-dimensional [MoIII(CN)7]4−-based magnets showing new topologies and ferrimagnetic ordering below 80 K

Two 3D [Mo^III(CN)₇]⁴⁻-based magnets with new topologies and ferrimagnetic ordering below 80 K have been prepared using different amides as auxiliary ligands.

Spin-crossover in iron(ii)-Schiff base complexes

A collective overview of iron(ii)-Schiff base complexes, showing abrupt and hysteretic SCO suitable for device applications, and the structure–property relationships governing the SCO of the complexes in the solid-state is presented.

Synthetic Evolution of the Multifarene Cavity from Planar Predecessors

The stepwise evolution of curved multifarene structures from planar precursors is demonstrated, highlighting three architectural design elements: 1) employment of various aromatic units, 2) changing the hybridization of the linking atoms from sp² to sp³ , and 3) rigidification of the system by the introduction of five-membered rings. Similar design elements have been employed to transform graphene sheets into curved carbon structures. Specifically, the stepwise synthetic evolution of multifarene[2+2], which has a curved, quite rigid structure, begins with a planar, tetraimine precursor, conversion to pairs of vicinal diamines, and the transformation of these pairs to cyclic thiourea groups. This process was probed by NMR spectroscopy and X-ray crystallography. Since varying the carbonylation conditions resulted in carbamates or thiocarbamates rather than the urea or thiourea isomers, the isomeric interconversion was studied both experimentally and by DFT computations. The carbamate versus urea preference was found to reflect either kinetic or thermodynamic control, respectively.

Synthesis and properties of a trinuclear copper(II) complex of a ligand with phthalocyanine and Schiff-base coordination sites

A trinuclear copper(II) phthalocyanine complex was synthesized by chelate coordination of the peripherally introduced Schiff-base nitrogen and phenoxide oxygen on the Cu(pc) core to a copper(II) ion. The magnetic, spectral and electrochemical properties were compared with those of the precursor mononuclear Cu(pc) complex with the NO chelate coordination site and a mononuclear Schiff-base copper(II) complex corresponding to the central bis-chelated unit of the title trinuclear complex. A stronger aggregating nature of the trinuclear complex compared with the precursor mononuclear Cu(pc) complex was confirmed by the spectral change of the Q band feature coming from the coordination to the copper(II) ion in dichloromethane. Two successive pc-ring reduction waves were not observed for the trinuclear complex in dichloromethane containing TBP(PF[Formula: see text], alternatively showing an irreversible wave in the reduction side. The central bis-chelated copper(II) ion was considered to play an important role for the redox behavior of the trinuclear complex.

A Novel Manganese Complex with Mixed Ligands: Preparation, Structure, Photoluminescent and Semiconductive Properties

A manganese complex with mixed ligands, [Mn(2,2′-biim)2(4,4′-bipy)] n[Mn(2,2′-biim)3]2 n6 n(ClO4)•8 nH2O (2,2′-biim = 2,2′-biimidazole; 4,4′-bipy = 4,4′-bipyridine), has been obtained by a hydrothermal procedure and its structure determined by single-crystal X-ray crystallography. A two-dimensional (2-D) supramolecular layer is established through hydrogen-bonding interactions. Solid-state photoluminescence measurement reveals that this complex shows a blue light emission. Solid-state diffuse reflectance determination uncovers the presence of a narrow optical band gap of 2.18 eV in this complex.

A new nitrogen rich open chain diazine ligand system: synthesis coordination chemistry and magneto-structural studies

The synthesis and coordination chemistry of a new series of open chain diazine based ligands (L3aH₂, L3bH₂, and L3cH₂) is reported. The ligands comprise a central disubstituted bipyridine moiety with two bridging alkoxide oxygen donors, together with diazine and pyridine terminal groups strategically located to coordinate three metal centres. Reactions of L3aH₂ and L3bH₂ with CuX₂ (X^- = ClO₄, Cl, NO₃) yield a trinuclear complex (1) and 1-D copper chains (2, 3). In these complexes the ligands bind copper ions via N_bipyridne, trans N_diazine, O_hydrazone, and N_pyridne donors while vacant sites are occupied by counter ions or solvent molecules (methanol, water, acetonitrile). Reaction of L3cH₂ with MnCl₂ affords a linear trinuclear Mn complex (4), where the Mn(ii) ions are connected via μ₂-O_hydrazone linkers with no N-N bridging. Reaction of L3aH₂ with Fe(SO₃CF₃)₂ yields a tetranuclear mixed valence Fe complex (5), in which both trans N-N and O_hydrazone bridging is observed. Magnetic studies reveal the presence of moderate to strong antiferromagnetic interactions in complexes 1-4 while a mix of ferromagnetic and antiferromagnetic interactions is observed in complex 5.