Solvation/desolvation induced reversible distortion change and switching between spin crossover and single molecular magnet behaviour in a cobalt(II) complex

Coexistence and switching between spin-crossover (SCO) and single molecular magnet (SMM) behaviours in one single complex may lead to materials that exhibit bi-stable and stimuli sensitive properties in a wide temperature range and under multiple conditions; unfortunately, the conflict and dilemma in the principle of approaching SCO and SMM molecules make it particularly difficult; at low temperature, low spin (LS) SCO molecules possess highly symmetrical geometry and isotropic spins, which are not suitable for SMM behaviour. Herein, we overcome this issue by using a rationally designed Co(II) mononuclear complex [Co(MeOphterpy)2] (ClO4)2 (1; MeOphterpy = 4'-(4-methoxyphenyl)-2,2':6',2''-terpyridine), the magnetic properties of which reversibly respond to desolvation and solvation. The solvated structure reinforced a low distortion of the coordination sphere via hydrogen bonding between ligands and methanol molecules, while in the desolvated structure a methoxy group flipping occurred, increasing the distortion of the coordination sphere and stabilising the HS state at low temperature, which exhibited a field-induced slow magnetic relaxation, resulting in a reversible switching between SCO and SMM properties within one molecule.

Field-Induced Slow Magnetic Relaxation in CoII Cyclopropane-1,1-dicarboxylates

New Co^II substituted malonate field-induced molecular magnets {[Rb₆Co₃(cpdc)₆(H₂O)₁₂]∙6H₂O}_n (1) and [Cs₂Co(cpdc)₂(H₂O)₆]_n (2) (where cpdc²⁻ stands for cyclopropane-1,1-dicarboxylic acid dianions) were synthesized. Both compounds contain mononuclear bischelate fragments {Co^II(cpdc)₂(H₂O)₂}²⁻ where the quasi-octahedral cobalt environment (CoO₆) is complemented by water molecules in apical positions. The alkali metal atoms play the role of connectors between the bischelate fragments to form 3D and 2D polymeric structures for 1 and 2, respectively. Analysis of dc magnetic data using the parametric Griffith Hamiltonian for high-spin Co^II supported by ab initio calculations revealed that both compounds have an easy axis of magnetic anisotropy. Compounds 1 and 2 exhibit slow magnetic relaxation under an external magnetic field (H_DC = 1000 and 1500 Oe, respectively).

Slow-Relaxation Behavior of a Mononuclear Co(II) Complex Featuring Long Axial Co-O Bond

Co(II) mononuclear complex with different coordination geometry would display various of field-induced single-ion magnet (SIM) behaviors. Here, we identify a field-induced single-ion magnet in a mononuclear complex Co(H₂DPA)₂·H₂O (H₂DPA = 2,6-pyridine-dicarboxylic acid) by the hydrothermal method. The long axial Co-O coordination bond (Co1‧‧‧O3) can be formed by Co1 and O3. Therefore, Co(II) ion is six-coordinated in a distorted elongated octahedron. AC magnetization susceptibilities show that the effective energy barrier is up to 43.28 K. This is much larger than most mononuclear Co(II). The distorted elongated octahedron caused by the axial Co-O coordination bond is responsible for the high effective energy barrier. The distribution of electron density in Co1 and O3 atoms in the long axial bond would influence the magnetic relaxation process in turn. Our work deepens the relationship between the effective energy barrier and the weak change of ligand field by long axial bonds, which would facilitate constructing SIM with high energy temperature.

Novel 'main-part' isostructuralism in metal complexes with 1-methylimidazole: crystal structures, energy calculations and magnetic properties

Two new transition metal complexes with 1-methylimidazole (1-MeIm) and azide as ligands, namely, [Co(1-MeIm)₄(N₃)₂] (1) and [Ni(1-MeIm)₄(N₃)₂] (2), have been synthesized and characterized by IR, Raman, UV-Vis and XPS spectroscopy. Their crystal structures were solved by single-crystal X-ray diffraction. The supramolecular self-assembly of the two complexes is governed by non-classical C-H⋯N hydrogen bonds and C-H⋯π interactions. Lattice energies and intermolecular interaction energies for various molecular pairs are quantified using the PIXEL method. DFT computational studies to assess the binding energy through modern tools like non-covalent interaction (NCI plots) analysis and reduced density gradient (RDG) analysis have also been carried out. A detailed analysis of geometric descriptors revealed the existence of quasi-isostructural pairs or 'main-part' isostructuralism in a series formed by 1, 2, and a related cadmium complex, being more evident in the 1/2 pair. DFT studies using theoretical models have been used to disclose the relative importance of the H-bond and C-H⋯π noncovalent interactions. Magnetic measurements for compound 1 show weak ferrimagnetic coupling between adjacent M(II) centers, mediated by H-bonding and C-H⋯π non-covalent interactions.

Influence of F-position and solvent on coordination geometry and single ion magnet behavior of Co(II) complexes

Three mononuclear Co(II) complexes with the compositions of [Co(L₁)₂] (1), [Co(L₂)₂(CH₃CN)] (2) and [Co(L₃)₂] (3) (HL₁ = 2-((E)-(2-fluorobenzylimino)methyl)-4,6-dibromophenol, HL₂ = 2-((E)-(3-fluorobenzylimino)methyl)-4,6-dibromophenol and HL₃ = 2-((E)-(4-fluorobenzylimino)methyl)-4,6-dibromophenol) were prepared and structurally determined. The changes in the F-positions in the ligands and solvents led to the formation of these products with various coordination geometries. Both complexes 1 and 3 are four-coordinated and their coordination geometries can be described as tetrahedron and seesaw, whereas complex 2 is five coordinated with a coordination configuration in between trigonal bipyramid and square pyramid. Static magnetic studies reveal that all these complexes exhibit considerable easy-axis magnetic anisotropy. The easy-axis magnetic anisotropy of 1 and 3 mainly derives from the first quartet excited state, whereas that of 2 primarily originates from the first, third and fourth quartet excited states established by theoretical calculations. All the resulting complexes display field-induced slow magnetic relaxation. Complex 3 represents the first Co(II) single ion magnet with a seesaw coordination geometry. Ab initio calculations predict that the magnetic anisotropy will enhance when the seesaw coordination geometry varies from distortion to regulation.

Magnetic anisotropy in square pyramidal cobalt(II) complexes supported by a tetraazo macrocyclic ligand

Two five-coordinate mononuclear Co(ii) complexes [Co(12-TMC)X][B(C6H5)4] (L = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane (12-TMC), X = Cl- (1), Br- (2)) have been studied by X-ray single crystallography, magnetic measurements, high-frequency and -field EPR (HF-EPR) spectroscopy and theoretical calculations. Both complexes have a distorted square pyramidal geometry with the Co(ii) ion lying above the basal plane constrained by the rigid tetradentate macrocyclic ligand. In contrast to the reported five-coordinate Co(ii) complex [Co(12-TMC)(NCO)][B(C6H5)4] (3) exhibiting easy-axis anisotropy, an easy-plane magnetic anisotropy was found for 1 and 2via the analyses of the direct-current magnetic data and HF-EPR spectroscopy. Frequency- and temperature-dependent alternating-current magnetic susceptibility measurements demonstrated that complexes 1 and 2 show slow magnetic relaxation at an applied dc field. Ab initio calculations were performed to reveal the impact of the terminal ligands on the nature of the magnetic anisotropies of this series of five-coordinate Co(ii) complexes.

Co(II)-Based single-ion magnets with 1,1'-ferrocenediyl-bis(diphenylphosphine) metalloligands

Herein, we report on investigations of magnetic and spectroscopic properties of three heterobimetallic Fe(ii)-Co(ii) coordination compounds based on the tetracoordinate {CoP2X2} core encapsulated by dppf metalloligand, where X = Cl (1), Br (2), I (3), dppf = 1,1'-ferrocenediyl -bis(diphenylphosphine). The analysis of static magnetic data has revealed the presence of axial magnetic anisotropy in compounds (1) and (2) and this was further confirmed by high-frequency electron spin resonance (HF-ESR) spectroscopy. Dynamic magnetic data confirmed that (1) and (2) behave as field-induced Single-Ion Magnets (SIMs). Together with bulk studies, we have also tested the possibility of depositing (2) as thick films on Au(111), glass, and polymeric acetate by drop-casting as well as thermal sublimation, a key aspect for the development of future devices embedding these magnetic objects.

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.

Halogen Bonding in New Dichloride-Cobalt(II) Complex with Iodo Substituted Chalcone Ligands

The synthesis and properties of new chalcone ligand 4I-L ((2E)-1-[4-(1H-imidazol-1-yl)phenyl]-3-(4-iodophenyl)prop-2-en-1-one) and tetracoordinate Co(II) complex [Co(4I-L)2Cl2], (1a), are reported in this article. Upon recrystallization of 1a, the single crystals of [Co(4I-L)4Cl2]·2DMF·3Et2O (1b) were obtained and crystal structure was determined using X-ray diffraction. The non-covalent interactions in 1b were thoroughly analyzed and special attention was dedicated to interactions formed by the peripheral iodine substituents. The density functional theory (DFT), atoms in molecule (AIM) and noncovalent interaction (NCI) methods and electronic localization function (ELF) calculations were used to investigate halogen bond formed between the iodine functional groups and co-crystallized molecules of diethyl ether.

Stereochemistry of coordination polyhedra vs. single ion magnetism in penta- and hexacoordinated Co(ii) complexes with tridentate rigid ligands

A tridentate ligand L (2,6-bis(1-(3,5-di-tert-butylbenzyl)-1H-benzimidazol-2-yl)pyridine) was synthesized and used for the preparation of three pentacoordinated Co(ii) complexes of formula [Co(L)X₂] (where X = NCS^- for 1, X = Cl^- for 2 and X = Br^- for 3) and one ionic compound 4 ([Co(L)₂]Br₂·2CH₃OH·H₂O) containing a hexacoordinated Co(ii) centre. Static magnetic data were analysed with respect to the spin (1-3) or the Griffith-Figgis (4) Hamiltonian. Ab initio calculations enable us to identify the positive axial magnetic anisotropy parameter D accompanied by a significant degree of rhombicity in the reported complexes. Also, magneto-structural correlation was outlined for this class of compounds. Moreover, all four compounds exhibit slow relaxation of magnetisation at an applied static magnetic field with either both low- and high-frequency relaxation channels (3) or a single high-frequency relaxation process (1, 2 and 4). The interplay between the stereochemistry of coordination polyhedra, magnetic anisotropy and the relaxation processes was investigated and discussed in detail.

Effect of linear and non-linear pseudohalides on the structural and magnetic properties of Co(ii) hexacoordinate single-molecule magnets

Field-induced single-molecule magnets based on Co(ii) hexacoordinate complexes comprising pseudohalides are reported.