Field-Induced Slow Magnetic Relaxation in a Mononuclear Manganese(III)-Porphyrin Complex

Thermal Jahn-Teller Distortion Changes and Slow Relaxation of Magnetization in Mn(III) Schiff Base Complexes

The impact that the anion and alkyl group has on the electronic structures and magnetic properties of four mononuclear Mn(III) complexes is explored in [Mn(salEen-Br)2]Y (salEen-Br = 2-{[2-(ethylamino)ethylimino]methyl}-4-Br-phenol; Y = ClO4- 1 and BF4-·1/3CH2Cl2 2) and [Mn(salBzen-Br)2]Y (salBzen-Br = 2-{[2-(benzylamino)ethylimino]methyl}-4-Br-phenol; Y = ClO4- 3 and BF4- 4). X-ray structures of [Mn(salEen-Br)2]ClO4·0.45C6H14 1-hexane, [Mn(salEen-Br)2]BF4·0.33CH2Cl2·0.15C6H14 2-dcm-hexane, and 3-4 reveal that they crystallize in ambient conditions in the monoclinic P21/c space group. Lowering the temperature, 2-dcm-hexane uniquely exhibits a structural phase transition toward a monoclinic P21/n crystal structure determined at 100 K with the unit cell trebling in size. Remarkably, at room temperature, the axially elongated Jahn-Teller axis in 2-dcm-hexane is poorly defined but becomes clearer at low temperature after the phase transition. Magnetic susceptibility measurements of 1-4 reveal that only 3 and 4 show slow relaxation of magnetization with Δeff/kB = 27.9 and 20.7 K, implying that the benzyl group is important for observing single-molecule magnet (SMM) properties. Theoretical calculations demonstrate that the alkyl group subtly influences the orbital levels and therefore very likely the observed SMM properties.

Axial Coordinated Manganese(III) Porphyrin/Tetraazaporphyrin - 4-(10-phenylanthracen-9-yl)Pyridine Dyads: Self-Assembly, Structure and Spectral Properties in Ground and Excited States

Self-assembly of new donor-acceptor systems based on (5,10,15,20-tetraphenylporphinato)manganese(III)/(5,10,15,20-tetra-4-tert-butylphenylporphinato)manganese(III)/(octakis(4-tert-butylphenyl)tetraazaporphinato)manganese(III) acetate ((AcO)MnTPP/(AcO)MnTBPP/(AcO)MnTAP) and 4-(10-phenylanthracen-9-yl)pyridine (PyAn) was studied using fluorescence spectroscopy and mass spectrometry. It was found that the coordination complexes of 1 : 1 composition (dyads) are formed in toluene. The spectral properties, the chemical structures and redox behavior of the dyads were described using 1H NMR, IR, ESR spectroscopy and cyclic voltammetry, respectively. The dynamic processes and the characteristics in the excited state of the dyads were obtained using the femtosecond transient absorption spectroscopy method. Density functional theory (DFT), time-dependent DFT methods were used to elucidate the dyad electronic structures and to establish the differences in their frontier molecular orbitals. The analysis of the lambda parameter and the distance of hole-pair interaction was indicated more favorable charge transfer between the macrocycle and the axial PyAn fragment in (AcO)(PyAn)MnTAP. The calculated values of the zero-field splitting parameters D and E/D, together with the g tensors of the lowest spin-orbit state for (AcO)MnTPP and (AcO)(PyAn)MnTPP were obtained using the combination of DFT and Multireference Perturbation Theory (CASSCF/NEVPT2) simulations. The data obtained develop the fundamental basis in the field of photovoltaics and show the prospects for the study of molecular systems of this class.

Slow magnetic relaxation in two mononuclear Mn(II) complexes not governed by the over-barrier Orbach process

Two hexacoordinate Mn(II) complexes containing a chelating residue of hexafluoroacetylacetone and (Cl-substituted) 4-benzylpyridine show DC magnetic functions typical for S = 5/2 spin systems: g ∼ 2, D - small. The AC susceptibility confirms a field supported slow magnetic relaxation in which the over-barrier Orbach relaxation process does not play a role. Both systems possess two or three slow relaxation channels.

Ni12 tetracubane cores with slow relaxation of magnetization and efficient charge utilization for photocatalytic hydrogen evolution

We report two Ni₁₂ multicubane topologies enclosed in the polyanions [Ni₁₂(OH)₉(WO₄)₃(PO₄)(B-α-PW₉O₃₄)₃]^21-{Ni₁₂W₃₀} and [Ni₁₂(OH)₉(HPO₄)₃(PO₄)(B-α-PW₉O₃₄)(A-α-PW₉O₃₄)₂]^21-{Ni₁₂W₂₇} that magnetically behave as Ni₁₂ units clearly distinguishing them from typical Ni₄ cubanes as shown by magnetic studies together with high field and frequency electron paramagnetic resonance (HFEPR). Beyond the unprecedented static properties, {Ni₁₂W₃₀} shows the unusual coexistence of slow relaxation of the magnetization and a diamagnetic ground state (S = 0), providing the unique opportunity of studying the essentially elusive magnetic relaxation behavior in excited states. The cubane-topology dependent activity of {Ni₁₂W₃₀} and {Ni₁₂W₂₇} as homogeneous HER photocatalysts unveils the structural key features significant for the design of photocatalysts with efficient charge utilization exemplified by high quantum yields (QY) of 10.42% and 8.36% for {Ni₁₂W₃₀} and {Ni₁₂W₂₇}, respectively.

Record High Magnetic Anisotropy in Three-Coordinate MnIII and CrII Complexes: A Theoretical Perspective

Ab initio calculations performed in two three-coordinate complexes [Mn{N(SiMe₃)₂}₃] (1) and [K(18-crown-6) (Et₂O)₂][Cr{N(SiMe₃)₂}₃] (2) reveal record-high magnetic anisotropy with the D values -64 and -15 cm^-1, respectively, enlisting d⁴ ions back in the race for single-ion magnets. A detailed spin-vibrational analysis performed of 1 and 2 suggests dominance under barrier relaxation due to the flexible coordination spheres around the metal ion. Furthermore, several in silico models were constructed by varying the nature of donor atoms based on the X-ray structure of 1 and 2, unveiling much larger anisotropy and robust single-ion magnet (SIM) characteristics for some of the models offering design clues for low-coordinate transition-metal SIMs.

Generation of a Hetero Spin Complex from Iron(II) Iodide with Redox Active Acenaphthene-1,2-Diimine

The reaction of the redox active 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene (dpp-BIAN) and iron(II) iodide in acetonitrile led to a new complex [(dpp-BIAN)Fe^III₂] (1). Molecular structure of 1 was determined by the single crystal X-ray diffraction analysis. The spin state of the iron cation in complex 1 at room temperature and the magnetic behavior of 1 in the temperature range of 2–300 K were studied using Mossbauer spectroscopy and magnetic susceptibility measurements, respectively. The neutral character of dpp-BIAN in 1 was confirmed by IR and UV spectroscopy. The electrochemistry of 1 was studied in solution and solid state using cyclic voltammetry. The generation of the radical anion form of the dpp-BIAN ligand upon reduction of 1 in a CH₂Cl₂ solution was monitored by EPR spectroscopy.

Investigation of vanadium(III) and vanadium(IV) compounds supported by the linear diaminebis(phenolate) ligands: correlation between structures and magnetic properties

A family of oxidovanadium(iv) compounds containing linear diaminebis(phenolate (salans) L1-5 ligands (L1 = [MeNCH2CH2NMe(CH2-4-CMe2CH2CMe3-C6H3O)2]2-; L2 = [MeNCH2CH2NMe(CH2-4-CH3-C6H3O)2]2-; L3 = [MeNCH2CH2NMe(CH2-4-Cl-C6H3O)2]2-; L4 = {MeNCH2CH2NMe[CH2-4,6-(CH3)2-C6H2O]2}2-; and L5 = {MeNCH2CH2NMe[CH2-4,6-(Br)2-C6H2O]2}2-) and non-oxidovanadium(iii) with L2,4 and acac ligands has been prepared and characterized by chemical and physical techniques. Reactions of [VO(acac)2] with ligand precursors H2L2,4 in toluene or hexane afforded vanadium(iii) compounds [V(L-κ4ONNO)(acac)] (1, L2; 2, L4), while the use of acetonitrile or ethanol led to the formation of dimeric oxidovanadium(iv) [(VO)2(μ-L-κ4ONNO)2] (3, L1; 4, L2; 5, L3) and monomeric [VO(L-κ4ONNO)] (6, L4, 7, L5) compounds. As shown by X-ray crystallography, compounds 1 and 2 are monomeric, in which the chelating ligands afford octahedral cis-α geometry at the vanadium center. In the dimeric structures of 3-5, the six-coordinate vanadium centers are bridged via two oxygen atoms of the L1-3 ligands while the L4,5 ligands generate square pyramidal structures of the monomeric 6 and 7 compounds. HFEPR studies allowed the determination of the spin Hamiltonian parameters of the S = 1 spin state of the monomeric V(iii) and dimeric V(iv), and S = ½ in monomeric V(iv) compounds. Magnetic measurements of 3-5 indicated weak ferromagnetic metal-metal exchange interactions. A reaction course for the deoxygenation and reduction of vanadyl-salan compounds is proposed.

X-ray Structure and Magnetic Properties of Heterobimetallic Chains Based on the Use of an Octacyanidodicobalt(III) Complex as Metalloligand

The assembly of [Co2III(μ-2,5-dpp)(CN)8]2− anions and [MII(CH3OH)2(DMSO)2]2+ cations resulted into the formation of two heterobimetallic 1D coordination polymers of formula [MII(CH3OH)2(DMSO)2(μ-NC)2Co2III(μ-2,5-dpp)(CN)6]n·4nCH3OH [M = CoII (1)/FeII (2) and 2,5-dpp = 2,5-bis(2-pyridyl)pyrazine. The [Co2III(μ-2,5-dpp)(CN)8]2− metalloligand coordinates the paramagnetic [MII(CH3OH)2(DMSO)2]2+ complex cations, in a bis-monodentate fashion, to give rise to neutral heterobimetallic chains. Cryomagnetic dc (1.9–300 K) and ac (2.0–13 K) magnetic measurements for 1 and 2 show the presence of Co(II)HS (1) and Fe(II)HS (2) ions (HS – high-spin), respectively, with D values of +53.7(5) (1) and −5.1(3) cm−1 (2) and slow magnetic relaxation for 1, this compound being a new example of SIM with transversal magnetic anisotropy. Low-temperature Q-band EPR study of 1 confirms that D value is positive, which reveals the occurrence of a strong asymmetry in the g-tensors and allows a rough estimation of the E/D ratio, whereas 2 is EPR silent. Theoretical calculations by CASSCF/NEVPT2 on 1 and 2 support the results from magnetometry and EPR. The analysis of the ac magnetic measurements of 1 shows that the relaxation of M takes place in the ground state under external magnetic dc fields through dominant Raman and direct spin-phonon processes.

Magnetic Properties and Electronic Structure of the S = 2 Complex [MnIII{(OPPh2)2N}3] Showing Field-Induced Slow Magnetization Relaxation

The high-spin S = 2 Mn(III) complex [Mn{(OPPh₂)₂N}₃] (1_Mn) exhibits field-induced slow relaxation of magnetization (Inorg. Chem. 2013, 52, 12869). Magnetic susceptibility and dual-mode X-band electron paramagnetic resonance (EPR) studies revealed a negative value of the zero-field-splitting (zfs) parameter D. In order to explore the magnetic and electronic properties of 1_Mn in detail, a combination of experimental and computational studies is presented herein. Alternating-current magnetometry on magnetically diluted samples (1_Mn/1_Ga) of 1_Mn in the diamagnetic gallium analogue, [Ga{(OPPh₂)₂N}₃], indicates that the slow relaxation behavior of 1_Mn is due to the intrinsic properties of the individual molecules of 1_Mn. Investigation of the single-crystal magnetization of both 1_Mn and 1_Mn/1_Ga by a micro-SQUID device reveals hysteresis loops below 1 K. Closed hysteresis loops at a zero direct-current magnetic field are observed and attributed to fast quantum tunneling of magnetization. High-frequency and -field EPR (HFEPR) spectroscopic studies reveal that, apart from the second-order zfs terms (D and E), fourth-order terms (B₄^m) are required in order to appropriately describe the magnetic properties of 1_Mn. These studies provide accurate spin-Hamiltonian (sH) parameters of 1_Mn, i.e., zfs parameters |D| = 3.917(5) cm^-1, |E| = 0.018(4) cm^-1, B⁰₄ = B₄² = 0, and B₄⁴ = (3.6 ± 1.7) × 10^-3 cm^-1 and g = [1.994(5), 1.996(4), 1.985(4)], and confirm the negative sign of D. Parallel-mode X-band EPR studies on 1_Mn/1_Ga and CH₂Cl₂ solutions of 1_Mn probe the electronic-nuclear hyperfine interactions in the solid state and solution. The electronic structure of 1_Mn is investigated by quantum-chemical calculations by employing recently developed computational protocols that are grounded on ab initio wave function theory. From computational analysis, the contributions of spin-spin and spin-orbit coupling to the magnitude of D are obtained. The calculations provide also computed values of the fourth-order zfs terms B₄^m, as well as those of the g and hyperfine interaction tensor components. In all cases, a very good agreement between the computed and experimentally determined sH parameters is observed. The magnetization relaxation properties of 1_Mn are rationalized on the basis of the composition of the ground-state wave functions in the absence or presence of an external magnetic field.

Advanced Paramagnetic Resonance Studies on Manganese and Iron Corroles with a Formal d4 Electron Count

Metallocorroles wherein the metal ion is Mn^III and formally Fe^IV are studied here using field- and frequency-domain electron paramagnetic resonance techniques. The Mn^III corrole, Mn(tpfc) (tpfc = 5,10,15-tris(pentafluorophenyl)corrole trianion), exhibits the following S = 2 zero-field splitting (zfs) parameters: D = -2.67(1) cm^-1, |E| = 0.023(5) cm^-1. This result and those for other Mn^III tetrapyrroles indicate that when D ≈ - 2.5 ± 0.5 cm^-1 for 4- or 5-coordinate and D ≈ - 3.5 ± 0.5 cm^-1 for 6-coordinate complexes, the ground state description is [Mn^III(Cor^3-)]⁰ or [Mn^III(P^2-)]⁺ (Cor = corrole, P = porphyrin). The situation for formally Fe^IV corroles is more complicated, and it has been shown that for Fe(Cor)X, when X = Ph (phenyl), the ground state is a spin triplet best described by [Fe^IV(Cor^3-)]⁺, but when X = halide, the ground state corresponds to [Fe^III(Cor^•2-)]⁺, wherein an intermediate spin (S = ³/₂) Fe^III is antiferromagnetically coupled to a corrole radical dianion (S = ¹/₂) to also give an S = 1 ground state. These two valence isomers can be distinguished by their zfs parameters, as determined here for Fe(tpc)X, X = Ph, Cl (tpc = 5,10,15-triphenylcorrole trianion). The complex with axial phenyl gives D = 21.1(2) cm^-1, while that with axial chloride gives D = 14.6(1) cm^-1. The D value for Fe(tpc)Ph is in rough agreement with the range of values reported for other Fe^IV complexes. In contrast, the D value for Fe(tpc)Cl is inconsistent with an Fe^IV description and represents a different type of iron center. Computational studies corroborate the zfs for the two types of iron corrole complexes. Thus, the zfs of metallocorroles can be diagnostic as to the electronic structure of a formally high oxidation state metallocorrole, and by extension to metalloporphyrins, although such studies have yet to be performed.

Slow magnetic relaxation in hexacoordinated cobalt(ii) field-induced single-ion magnets

To gain an insight into the factors affecting the enhancement of the energy barrier in SMM/SIM, hexacoordinate pseudohalide Co(ii) complexes based on the tridentate ligand were investigated.

Coligand effects on the architectures and magnetic properties of octahedral cobalt(ii) complexes with easy-axis magnetic anisotropy

Coligand effects lead to two mononuclear octahedral Co(ii) complexes exhibiting easy-axis magnetic anisotropies and distinct magnetic properties.

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.

Mononuclear manganese(iii) complexes with reduced imino nitroxide radicals by single-electron transfer and intermolecular hydrogen bonds as an intramolecular structural driving force

One-electron transfer from Mn(ii) ions to an imino nitroxide radical gives mononuclear Mn(iii) complexes of the reduced amino imine-oxide form for which crystal structures evidence hydrogen bonds networks acting as a stabilizing driving-force.

Modulation of the magnetic anisotropy of octahedral cobalt(ii) single-ion magnets by fine-tuning the axial coordination microenvironment

The alteration of the axial N-donor ligands leads to two octahedral Co(ii) SIMs with varying easy-plane magnetic anisotropies and dynamic magnetic behaviors.

Slow magnetic relaxation in a high-spin pentacoordinate Fe(iii) complex

A mononuclear pentacoordinate iron(iii) complex shows slow magnetic relaxation with three relaxation channels.

Isostructural M(ii) complexes (M = Mn, Fe, Co) with field-induced slow magnetic relaxation for Mn and Co complexes

We herein report the synthetic, structural, theoretical, and magnetic studies on three isostructural complexes, [M(L)2(CH3OH)2] (M = Mn (Mn), Fe (Fe), and Co (Co); HL = 2,6-bis(pyrazole-1-yl)pyridine-4-carboxylic acid). From single crystal X-ray crystallography, it is found that the complexes crystallized in the same space group (C2/c) and had seven-coordinate pentagonal bipyramidal structures. From direct current (dc) and alternating current (ac) magnetic susceptibility measurements, Mn and Co were found to undergo field-induced slow magnetic relaxation with two relaxation pathways. To elucidate the origin of the slow magnetic relaxation phenomena of Mn, electron paramagnetic resonance (EPR) measurements and theoretical calculations were performed. The EPR measurements were performed on polycrystalline powder samples, and the following parameters were obtained by simulating the EPR data: giso = 2.00 and small zero field splitting parameter D = -0.13 cm-1. To the best of our knowledge, this is the first example of a seven-coordinate mononuclear Mn(ii) complex undergoing slow magnetic relaxation.

A mononuclear five-coordinate Co(ii) single molecule magnet with a spin crossover between the S = 1/2 and 3/2 states

Although a great number of single-ion magnets (SIMs) and spin-crossover (SCO) compounds have been discovered, multifunctional materials with the combination of SCO and SIM properties are extremely scarce. Here magnetic studies have been carried out for a mononuclear, five-coordinate cobalt(ii) complex [Co(3,4-lut)4Br]Br (1) with square pyramidal geometry. Direct-current magnetic measurement confirms the spin transition between the S = 1/2 and 3/2 states in the range of 150-290 K with a small hysteresis loop. Frequency- and temperature-dependent alternating-current magnetic susceptibility reveals slow magnetization relaxation under an applied dc field of 3000 Oe. The work here presents the first instance of the five-coordinate mononuclear cobalt(ii)-based SIM exhibiting the thermally induced complete SCO.

The influence of pseudohalide ligands on the SIM behaviour of four-coordinate benzylimidazole-containing cobalt(ii) complexes

Three, mononuclear complexes of the formula [Co(bmim)2(SCN)2] (1), [Co(bmim)2(NCO)2] (2) and [Co(bmim)2(N3)2] (3) [bmim = 1-benzyl-2-methylimidazole] were prepared and structurally analyzed by single-crystal X-ray crystallography. The cobalt(ii) ions in 1-3 are tetrahedrally coordinated with two bmim molecules and two pseudohalide anions. The angular distortion parameter δ was calculated and the SHAPE program (based on the CShM concept) was used for 1-3 to estimate the angular distortion from an ideal tetrahedron. The molecules of 1-3 are effectively separated, and the values of the shortest distance of cobalt-cobalt are 8.442(6) and 6.774(8) Å for 1, 10.349(8) and 10.716(8) Å for 2 and 6.778(1) and 9.232(1) Å for 3. Direct current (dc) magnetic susceptibility measurements on the crushed crystals of 1-3 were carried out in the temperature range 1.9-295 K. The variable-temperature magnetic data of 1-3 mainly obey the zero-field splitting effect (D) of the 4A2 ground term of the tetrahedral cobalt(ii) complexes (2D being the energy gap between the |±1/2 and |±3/2 levels of the spin). The analysis of their magnetic data through the Hamiltonian H = D[S2z - S(S + 1)/3] + E(Sx2 - Sy2) + gβHS led to the following best-fit parameters: g = 2.29, D = -7.5 cm-1 and E/D = 0.106 (1), g = 2.28, D = + 6.3 cm-1 and E/D = 0.007 (2) and g = 2.36, D = + 6.7 cm-1 and E/D = 0.090 (3). The signs of D for 1-3 were confirmed by Q-band EPR spectra on powdered samples in the temperature range 4.0-20 K. Field-induced SIM behaviour was observed for 1-3 below 4.0 K, and the frequency-dependent maxima of χ''M were observed for 1 and only incipient signals of χ''M occurred for 2 and 3. The values of the exponential factor (τ0) and activation energy (Ea) for 1-3 which were obtained from the Arrhenius plot suggest a single relaxation process characteristic of an Orbach mechanism.

Single Ion Magnets from 3d to 5f: Developments and Strategies

Single-ion magnets (SIMs), exhibiting slow magnetization relaxation in the absence of the magnetic field, originate from their single spin-carrier centre. In pursuit of high-performance magnetic properties, such as high spin-reversal barrier and high blocking temperature, various metal centres were investigated to establish SIMs, including 3d and 5d transition metal ions, 4f lanthanide ions, and 5f actinide ions, which possess unique zero-field splitting and magnetic properties. Therefore, proper ligand field is of great importance to different types of metals. In the given great breakthroughs since the first SIM, [Pc₂ Tb]^- (Pc=dianion of phthalocyanine), was reported, strategies of ligand field design have emerged. In this review, the developments of SIMs with different metal centres are summarized, as well as the possible strategies.

Cyanide-bridged coordination polymers constructed from lanthanide ions and octacyanometallate building-blocks

Self-assembling lanthanide ions and octacyanometallate building-blocks in the presence of pyrazine-2,3-dicarboxylate acid leads to 3D dense networks.

Field-induced slow magnetic relaxation of two 1-D compounds containing six-coordinated cobalt(ii) ions: influence of the coordination geometry

Two 1-D Co(ii) compounds with different coordination geometries of the same coordination sphere exhibited field-induced slow relaxation magnetization with different D values.

One-dimensional cobalt(ii) coordination polymer featuring single-ion-magnet-type field-induced slow magnetic relaxation

Single-ion-magnet-type field-induced double magnetic relaxation was observed in a one-dimensional cobalt(ii) coordination polymer which shows easy-axis anisotropy with D = −33.9 cm⁻¹ and an energy barrier of U_eff = 38.8 K.

Field-Induced Co(II) Single-Ion Magnets with mer-Directing Ligands but Ambiguous Coordination Geometry

Three air-stable Co(II) mononuclear complexes with different aromatic substituents have been prepared and structurally characterized by single-crystal X-ray diffraction. The mononuclear complexes [Co(H₂L1)₂]·2THF (1), [Co(HL2)₂] (2), and [Co(H₂L3)₂]·CH₂Cl₂ (3) (where H₃L1, H₂L2, and H₃L3 represent 3-hydroxy-naphthalene-2-carboxylic acid (6-hydroxymethyl-pyridin-2-ylmethylene) hydrazide, nicotinic acid (6-hydroxymethyl-pyridin-2-ylmethylene) hydrazide, and 2-hydroxy-benzoic acid (6-hydroxymethyl-pyridin-2-ylmethylene) hydrazide, respectively) feature a distorted mer octahedral coordination geometry. Detailed magnetic studies of 1-3 have been conducted using direct and alternating current magnetic susceptibility data. Field-induced slow magnetic relaxation was observed for these three complexes. There are few examples of such behavior in (distorted) octahedral coordination geometry (OC) Co(II) mononuclear complexes with uniaxial anisotropy. Analysis of the six-coordinate Co(II) mononuclear single-ion magnets (SIMs) in the literature using the SHAPE program revealed that they all show what is best described as distorted trigonal prismatic (TRP) coordination geometry, and in general, these show negative D zero-field splitting (ZFS) values. On the other hand, all the Co(II) mononuclear complexes displaying what is best approximated as distorted octahedral (OC) coordination geometry show positive D values. In the new Co(II) mononuclear complexes we describe here, there is an ambiguity, since the rigid tridentate ligands confer what is best described for an octahedral complex as a mer coordination geometry, but the actual shape of the first coordination sphere is between octahedral and trigonal prismatic. The negative D values observed experimentally and supported by high-level electronic structure calculations are thus in line with a trigonal prismatic geometry. However, a consideration of the rhombicity as indicated by the E value of the ZFS in conjunction with the SHAPE analysis shows that in this case it is difficult to distinguish between the OC and TRP descriptions.

Reversible solvatomagnetic switching in a single-ion magnet from an entatic state

A vast impact on molecular nanoscience can be achieved using simple transition metal complexes as dynamic chemical systems to perform specific and selective tasks under the control of an external stimulus that switches "ON" and "OFF" their electronic properties. While the interest in single-ion magnets (SIMs) lies in their potential applications in information storage and quantum computing, the switching of their slow magnetic relaxation associated with host-guest processes is insufficiently explored. Herein, we report a unique example of a mononuclear cobalt(ii) complex in which geometrical constraints are the cause of easy and reversible water coordination and its release. As a result, a reversible and selective colour and SIM behaviour switch occurs between a "slow-relaxing" deep red anhydrous material (compound 1) and its "fast-relaxing" orange hydrated form (compound 2). The combination of this optical and magnetic switching in this new class of vapochromic and thermochromic SIMs offers fascinating possibilities for designing multifunctional molecular materials.

Three novel mononuclear Mn(iii)-based magnetic materials with square pyramidal versus octahedral geometries

Single crystal X-ray diffraction studies reveal that complexes 1 and 2 have square pyramidal geometry and 3 has octahedral geometry, which showed a dependence of negative anisotropy (D) values on the electronic, geometry and packing effects.

Slow Magnetic Relaxations in Cobalt(II) Tetranitrate Complexes. Studies of Magnetic Anisotropy by Inelastic Neutron Scattering and High-Frequency and High-Field EPR Spectroscopy

Three mononuclear cobalt(II) tetranitrate complexes (A)₂[Co(NO₃)₄] with different countercations, Ph₄P⁺ (1), MePh₃P⁺ (2), and Ph₄As⁺ (3), have been synthesized and studied by X-ray single-crystal diffraction, magnetic measurements, inelastic neutron scattering (INS), high-frequency and high-field EPR (HF-EPR) spectroscopy, and theoretical calculations. The X-ray diffraction studies reveal that the structure of the tetranitrate cobalt anion varies with the countercation. 1 and 2 exhibit highly irregular seven-coordinate geometries, while the central Co(II) ion of 3 is in a distorted-dodecahedral configuration. The sole magnetic transition observed in the INS spectroscopy of 1-3 corresponds to the zero-field splitting (2(D² + 3E²)^1/2) from 22.5(2) cm^-1 in 1 to 26.6(3) cm^-1 in 2 and 11.1(5) cm^-1 in 3. The positive sign of the D value, and hence the easy-plane magnetic anisotropy, was demonstrated for 1 by INS studies under magnetic fields and HF-EPR spectroscopy. The combined analyses of INS and HF-EPR data yield the D values as +10.90(3), +12.74(3), and +4.50(3) cm^-1 for 1-3, respectively. Frequency- and temperature-dependent alternating-current magnetic susceptibility measurements reveal the slow magnetization relaxation in 1 and 2 at an applied dc field of 600 Oe, which is a characteristic of field-induced single-molecule magnets (SMMs). The electronic structures and the origin of magnetic anisotropy of 1-3 were revealed by calculations at the CASPT2/NEVPT2 level.

Crystal structure of 3,5-di-methyl-pyridine N-oxide dihydrate

In the title compound, also known as 3,5-lutidine N-oxide dihydrate, C7H9NO·2H2O, the N-O bond is weakened due to the involvement of the O atom as an acceptor of hydrogen bonds from the two water mol-ecules of crystallization present in the asymmetric unit. Fused R35(10) ring motifs based on O-H⋯O hydrogen bonds form chains in the [010] direction, which are further connected by weak C-H⋯O inter-molecular contacts. As a result, the lutidine mol-ecules are stacked in an efficient manner, with π-π contacts characterized by a short separation of 3.569 (1) Å between the benzene rings.

Magnetic Anisotropy of Tetrahedral CoII Single-Ion Magnets: Solid-State Effects

This study reports the static and dynamic magnetic characterization of two mononuclear tetrahedral Co^II complexes, [Co{ⁱPr₂P(E)NP(E)ⁱPr₂}₂], where E = S (CoS₄) and Se (CoSe₄), which behave as single-ion magnets (SIMs). Low-temperature (15 K) single-crystal X-ray diffraction studies point out that the two complexes exhibit similar structural features in their first coordination sphere, but a disordered peripheral ⁱPr group is observed only in CoS₄. Although the latter complex crystallizes in an axial space group, the observed structural disorder leads to larger transverse magnetic anisotropy for the majority of the molecules compared to CoSe₄, as confirmed by electron paramagnetic resonance spectroscopy. Static magnetic characterization indicates that both CoS₄ and CoSe₄ show easy-axis anisotropy, with comparable D values (∼-30 cm^-1). Moreover, alternating-current susceptibility measurements on these Co^II complexes, magnetically diluted in their isostructural Zn^II analogues, highlight the role of dipolar magnetic coupling in the mechanism of magnetization reversal. In addition, our findings suggest that, despite their similar anisotropic features, CoS₄ and CoSe₄ relax magnetically via different processes. This work provides experimental evidence that solid-state effects may affect the magnetic behavior of SIMs.

Single-ion magnet behaviour in mononuclear and two-dimensional dicyanamide-containing cobalt(ii) complexes

The magneto-structural features of one mononuclear and two layered Co(ii) complexes being new examples of Single-Ion Magnets (SIMs) are reported.

A Mn(iii) single ion magnet with tridentate Schiff-base ligands

Single ion magnet behaviour is reported for a mononuclear Mn(iii) ion with tridentate Schiff-base ligands revealing that the size of the axial anisotropy is the highest reported to date.

{CoIIIMnIII}ncorrugated chains based on heteroleptic cyanido metalloligands

The use of cyanide-bearing complexes affords one-dimensional coordination polymers with the formulas {[Mn^III(salen)(μ-NC)₂Co^III(4,4-dmbipy)(CN)₂]·H₂O}_n(1) and {[Mn^III(salen)(μ-NC)₂Co^III(dmphen)(CN)₂]}_n(2) .