Field-Induced Slow Magnetization Relaxation of a Tetrahedral S=2 FeIIS4-Containing Complex

In the work described herein, the spin relaxation properties of the mononuclear tetrahedral S=2 [Fe{(SPiPr2)2N}2] complex (1) were studied by employing static and dynamic magnetic measurements at liquid helium temperatures. In the absence of an external direct current (DC) magnetic field, 1 exhibits fast magnetization relaxation. However, in the presence of external magnetic fields of a few kOe, slow relaxation is induced as monitored by alternating current (AC) magnetic susceptibility measurements up to 10 kHz, in the temperature range 2-5 K. Analysis of the temperature dependence of the corresponding relaxation time reveals contributions by Quantum Tunnelling of Magnetization, and the Direct and Orbach processes in the magnetization relaxation mechanism of 1. The energy barrier, Ueff, of the Orbach process, as determined by this analysis, is compared with that related to the zero-field splitting parameters of 1 which were previously determined by high- frequency and -field electron paramagnetic resonance and Mössbauer spectroscopies.

Trapping an unprecedented octacoordinated iron(II) complex with neutral bis-tetrazolylpyridyl ligands and solvent molecules

Iron(II) can show a very rich coordination chemistry with concomitant modulation of its properties as promising functional materials. Metalation of the neutral tridentate nitrogen-donor mer-coordinating ligand 2,6-bis(2-(methyl)-2H-tetrazol-5-yl)pyridine (Me2btp) with Fe(ClO4)2·6H2O through accurate solvent polarity control enables the selective crystallization of [FeHS/LS(Me2btp)2](ClO4)2·MeCN·2.75H2O (2HS/LS·MeCN·2.75H2O) as red rods, where half of the iron(II) centres resides in the low spin (LS, S = 0) state and the other half is in the high spin (HS, S = 2) state. The red rods spontaneously convert into yellow crystals once removed from the mother liquor and exposed to air due to solvent rearrangement within the crystal packing; these new crystals can be assigned to [FeHS(Me2btp)2](ClO4)2·solvent (2HS·solvent) where all the iron(II) centres are now blocked in the HS state, as confirmed by magnetic measurements. The polarity of the crystallization solvent, together with the maintenance of the crystals within the mother liquor, are pivotal for the reactivity and interconversion of different species. Indeed, upon long standing in solution, 2HS/LS·MeCN·2.75H2O converts to another form of red crystals belonging to [FeLS(Me2btp)2][FeHS(Me2btp)(MeCN)2(H2O)](ClO4)4·MeCN (2LS·3HS·MeCN), as confirmed by single crystal X-ray diffraction data. In this co-crystal, the iron(II) in 2 resides in the LS state at all temperatures while the iron(II) in 3 is blocked in the HS state. Well-formed yellow crystals could be also isolated among the red crystals of 2HS/LS·MeCN·2.75H2O, and they could be identified as the unprecedented octacoordinated species [Fe(Me2btp)2(MeCN)(H2O)](ClO4)2·H2O (1·H2O) by single-crystal X-ray diffraction. These yellow crystals are stable in the air, but slowly convert into 2LS·3HS·MeCN if kept in the mother liquor for about one week. 1·H2O can be considered the trapped intermediate in the solid state during the conversion of [FeHS(Me2btp)2]2+ into [FeHS(Me2btp)(MeCN)2(H2O)]2+ in solution, where the two tridentate ligands in the starting species can unfold to accommodate coordinated MeCN and H2O molecules, as confirmed by theoretical calculations, and eventually one of the two Me2btp is completely replaced by the solvent.

Slow magnetic relaxation in 8-coordinate Mn(II) compounds

The design and synthesis of high-spin Mn(II)-based single-molecule magnets (SMMs) have not been well developed to a great extent, as compared with a large number of SMMs based on the other first row transition metal complexes. In light of our success in designing Fe(II), Co(II) and Fe(III)-based SMMs with a high coordination number of 8, it is of great interest to design Mn(II) analogues with such a strategy. In this contribution, four Mn(II) compounds, [MnII(Ln)2](ClO4)2 (1-4) were obtained from reactions of neutral tetradentate ligands, L1-L4, with hydrated MnII(ClO4)2 (L1 = 2,9-bis(carbomethoxy)-1,10-phenanthroline, L2 = 2,9-bis(carbomethoxy)-2,2'-dipyridine, L3 = N2,N9-dibutyl-1,10-phenanthroline-2,9-dicarboxamide, L4 = 6,6'-bis(2-(tert-butyl)-2H-tetrazol-5-yl)-2,2'-bipyridine). Their crystal structures have been determined by X-ray crystallography and it clearly shows that the Mn(II) centers in these compounds have an oversaturated coordination number of 8. Their magnetic properties have been investigated in detail; to our surprise, all of these Mn(II) compounds show interesting slow magnetic relaxation behaviors under an applied direct current field, although they have very small negative D values.

Syntheses, Structures, and Magnetic Properties of Three Cyano-Bridged FeII-MoIII Single-Molecule Magnets

Three new cyano-bridged FeII-MoIII complexes assembled from the [MoIII(CN)7]4- unit, FeII ions, and three pentadentate N3O2 ligands, namely {[Fe2H3(dapab)2][Mo(CN)6]}n·2H2O·3.5MeCN (1), [Fe(H2dapb)(H2O)][Fe(Hdapb)(H2O)][Mo(CN)6]·4H2O·3MeCN (2), and [Fe(H2dapba)(H2O)]2[Mo(CN)7]·6H2O (3) (H2dapab = 2,6-diacetylpyridine bis(2-aminobenzoylhydrazone), H2dapb = 2,6-diacetylpyridine bis(benzoylhydrazone), H2dapba = 2,6-diacetylpyridine bis(4-aminobenzoylhydrazone)), have been synthesized and characterized. Single-crystal structure analyses suggest that complex 1 contains a one-dimensional (1D) chain structure where two FeII ions are bridged by the in situ generated [MoIII(CN)6]3- unit through two trans-cyanide groups into trinuclear Fe2IIMoIII clusters that are further linked by the amino of the ligand into an infinite chain. Complexes 2 and 3 are cyano-bridged Fe2IIMoIII trinuclear clusters with two FeII ions connected by the [MoIII(CN)6]3- and [MoIII(CN)7]4- units, respectively. Direct current magnetic studies confirmed the ferromagnetic interactions between the cyano-bridged FeII and MoIII centers and significant easy-axis magnetic anisotropy for all three complexes. Furthermore, complexes 1-3 exhibit slow magnetic relaxation under a zero dc field, with relaxation barriers of 42.3, 21.6, and 14.4 K, respectively, making them the first examples of cyano-bridged FeII-MoIII single-molecule magnets.

Slow magnetic relaxation in mononuclear octa-coordinate Fe(II) and Co(II) complexes from a Bpybox ligand

Two 3d transition metal mononuclear complexes, [(FeL₂)(ClO₄)₂]₂·CH₃CN (1) and (CoL₂)(ClO₄)₂·2CH₃CN (2), have been prepared from a rigid tetradentate bpybox (L = 6,6'-bis(2,5-dihydrooxazol-4-yl)-2,2'-bipyridine) ligand. Single crystal X-ray diffraction analyses together with the help of calculations show that both compounds are octa-coordinate. Direct current magnetic studies reveal their significant magnetic anisotropy. Impressively, field-induced relaxation of magnetism is observed in the two complexes and the apparent anisotropy barriers are 14.1 K for 1 and 21.6 K for 2, respectively. Theoretical calculations reveal that two Fe(II) centers in 1 have small negative D values of -4.897 and -4.825 cm^-1 and relatively small E values of 0.646 and 0.830 cm^-1, indicating a uniaxial magnetic anisotropy. In contrast, the D and E values in the Co(II) center of 2 are 46.42 cm^-1 and 11.51 cm^-1, featuring a rhombic anisotropy. This work demonstrates that field-induced slow magnetic relaxation in 3d transition metal complexes with high coordination numbers can be manipulated through rigid ligand design.

Slow magnetic relaxation in a trigonal-planar mononuclear Fe(II) complex

Based on a β-diketiminate ligand, an iron(III) tetrahedral high-spin complex, [LFeIII(Cl)2] (1), and an iron(II) high-spin triangular planar complex, [LFeIICl] (2), have been synthesized and structurally characterized. Also, complex 1...

Modulation of the architectures and magnetic dynamics in pseudotetrahedral cobalt(II) complexes

Two β-Diketiminate cobalt(II) compounds of formula [LCo(μ-Cl)]2∙2C6H14 (1) and [LCoClPy]∙0.5C7H8∙0.5C6H14 (2) (L = [PhC-(PhCN-Dip)2]−, Dip = 2,6-iPr2C6H3) have been synthesized and structurally characterized by single crystal X-ray diffraction. Compound 1...

Slow magnetic relaxation in high-coordinate Co(II) and Fe(II) compounds bearing neutral tetradentate ligands

The first-row transition metal compounds, [M^II(L1)₂](ClO₄)₂ (M = Ni (1); Co (2)), have been prepared by treatment of a neutral tetradentate ligand (L1 = N²,N⁹-dibutyl-1,10-phenanthroline-2,9-dicarboxamide) with metal perchlorate salts in MeOH. Both compounds have been structurally characterized by X-ray crystallography and it was found that the coordination numbers are 6 and 7, respectively. The reaction of 6,6'-bis(2-^tbutyl-tetrazol-5-yl)-2,2'-bipyridine (L2) with hydrated Fe^II(ClO₄)₂ afforded a 8-coordinate Fe(II) compound, [Fe^II(L2)₂](ClO₄)₂ (3); however its reaction with hydrated Co^II(ClO₄)₂ resulted in 6-coordinate [Co^II(L2)₂](ClO₄)₂. It is interesting to observe field-induced slow magnetic relaxation in the 7-coordinate Co(II) compound 2 and 8-coordinate Fe(II) compound 3, which further supports the validity of designing high coordination number compounds as single-molecule magnets. Direct current magnetic studies demonstrate that 2 has a very large positive D value (56.2 cm^-1) and a small E value (0.66 cm^-1), indicating easy plane magnetic anisotropy. Consistent with the larger D value, an effective spin-reversal barrier of U_eff = 100 K (71.4 cm^-1) is obtained, which is the highest value reported for 7-coordinate Co(II) complexes with a pentagonal bipyramidal geometry. In contrast, 8-coordinate Fe(II) compound 3 exhibits uniaxial magnetic anisotropy.

Tuning Magnetic Anisotropy in a Class of Co(II) Bis(hexafluoroacetylacetonate) Complexes

Tuning the magnetic anisotropy of metal ions remains highly interesting in the design of improved single-molecule magnets (SMMs). We herein report synthetic, structural, magnetic, and computational studies of four mononuclear Co^II complexes, namely [Co(hfac)₂ (MeCN)₂ ] (1), [Co(hfac)₂ (Spy)₂ ] (2), [Co(hfac)₂ (MBIm)₂ ] (3), and [Co(hfac)₂ (DMF)₂ ] (4) (MeCN=acetonitrile, hfac=hexafluoroacetylacetone, Spy=4-styrylpyridine, MbIm=5,6-dimethylbenzimidazole, DMF=N,N-dimethylformamide), with distorted octahedral geometry constructed from hexafluoroacetylacetone (hfac) and various axial ligands. By a building block approach, complexes 2-4 were synthesized by recrystallization of the starting material of 1 from various ligands containing solution. Magnetic and theoretical studies reveal that 1-4 possess large positive D values and relative small E parameters, indicating easy-plane magnetic anisotropy with significant rhombic anisotropy in 1-4. Dynamic alternative current (ac) magnetic susceptibility measurements indicate that these complexes exhibit slow magnetic relaxation under external fields, suggesting field-induced single-ion magnets (SIMs) of 1-4. These results provide a promising platform to achieve fine tuning of magnetic anisotropy through varying the axial ligands based on Co(II) bis(hexafluoroacetylacetonate) complexes.

Design and programmable self-assembly of an eight-coordinate Cu(ii) heterometallic structure via a dipalladium corner as a building block

An eight-coordinate (8C) [Cu(ii)O₈] heteromultimetallic complex displaying a distorted dodecahedral structure has been synthesized via the coordination-driven programmable self-assembly of dipalladium metalloligands containing heteroditopic ligands.

Structure, magnetic anisotropy and relaxation behavior of seven-coordinate Co(ii) single-ion magnets perturbed by counter-anions

A series of mononuclear seven-coordinate complexes with the same coordination unit [Co(BPA-TPA)]²⁺ (BPA-TPA = pentapyidyldiamine) display the different slow magnetic relaxation processes perturbed by the variation of the counter anions.

Two Four-Coordinate and Seven-Coordinate CoII Complexes Based on the Bidentate Ligand 1, 8-Naphthyridine Showing Slow Magnetic Relaxation Behavior

For a long time, the cobalt(II) complex ([Co(napy)₄ ](ClO₄ )₂ ) (napy=1, 8-naphthyridine) has been considered as an eight-coordinate complex without any structural proof. After careful considerations, two complexes [Co(napy)₂ Cl₂ ] (1) and [Co(napy)₄ ](ClO₄ )₂ (2) based on the bidentate ligand napy were synthesized and structurally characterized. X-ray single-crystal structural determination showed that the cobalt(II) center in [Co(napy)₂ Cl₂ ] (1) is four-coordinate with a tetrahedral geometry (T_d ), while [Co(napy)₄ ](ClO₄ )₂ (2) is seven-coordinate rather than eight-coordinate with a capped trigonal prism geometry (C_2v ). Direct-current (dc) magnetic data revealed that complexes 1 and 2 possess positive zero-field splitting (ZFS) parameters of 11.08 and 25.30 cm^-1 , respectively, with easy-plane magnetic anisotropy. Alternating current(ac) susceptibility measurements revealed that both complexes showed slow magnetic relaxation behaviour. Theoretical calculations demonstrated that the presence of easy-plane magnetic anisotropy (D>0) for complexes 1 and 2 is in agreement with the experimental data. Furthermore, these results pave the way to obtain four-coordinate and seven-coordinate cobalt(II) single-ion magnets (SIMs) by using a bidentate ligand.