Theoretical Magnetic Relaxation and Spin-Phonon Coupling Study in a Series of Molecular Engineering Designed Bridged Dysprosocenium Analogues.

A detailed computational study of hypothetical sandwich dysprosium double-decker complexes, bridged by various numbers of aliphatic linkers, was performed to evaluate the effect of the structural modifications on their ground-state magnetic sublevels and assess their potential as candidates for single-molecule magnets (SMMs). The molecular structures of seven complexes were optimized using the TPSSh functional, and the electronic structure and magnetic properties were investigated using the complete active space self-consistent field method (CASSCF). Estimates of the magnetic moment blocking barrier (Ueff) and blocking temperatures (TB) are reported. In addition, a new method based on computed derivatives of effective demagnetization barriers Ueff with respect to vibrational normal modes was introduced and applied to evaluate the impact of spin-phonon coupling on the SMM properties. On the basis of the computed parameters, we have identified promising candidates with properties superior to those of the existing single-molecule magnets.

Tetracoordinate Co(II) complexes with semi-coordination as stable single-ion magnets for deposition on graphene.

We present a theoretical and experimental study of two tetracoordinate Co(II)-based complexes with semi-coordination interactions, i.e., non-covalent interactions involving the central atom. We argue that such interactions enhance the thermal and structural stability of the compounds, making them appropriate for deposition on substrates, as demonstrated by their successful deposition on graphene. DC magnetometry and high-frequency electron spin resonance (HF-ESR) experiments revealed an axial magnetic anisotropy and weak intermolecular antiferromagnetic coupling in both compounds, supported by theoretical predictions from complete active space self-consistent field calculations complemented by N-electron valence state second-order perturbation theory (CASSCF-NEVPT2), and broken-symmetry density functional theory (BS-DFT). AC magnetometry demonstrated that the compounds are field-induced single-ion magnets (SIMs) at applied static magnetic fields, with slow relaxation of magnetization governed by a combination of quantum tunneling, Orbach, and direct relaxation mechanisms. The structural stability under ambient conditions and after deposition was confirmed by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Theoretical modeling by DFT of different configurations of these systems on graphene revealed n-type doping of graphene originating from electron transfer from the deposited molecules, confirmed by electrical transport measurements and Raman spectroscopy.

Reversible Spin-State Switching and Tuning of Nuclearity and Dimensionality via Nonlinear Pseudohalides in Cobalt(II) Complexes.

The self-assembly of a macrocyclic tetradentate ligand, cobalt(II) tetrafluoroborate, and nonlinear pseudohalides (dicyanamide and tricyanomethanide) has led to two cobalt(II) complexes, {[Co(L)(µ1,5-dca)](BF4)·MeOH}n (1) and [Co2(L)2(µ1,5-tcm)2](BF4)2 (2) (L = N,N'-di-tert-butyl-2,11-diaza[3,3](2,6)pyridinophane; dca- = dicyanamido; tcm- = tricyanomethanido). Both complexes were characterized by single-crystal X-ray diffraction, spectroscopic, magnetic, and electrochemical studies. Structural analyses revealed that 1 displays a one-dimensional (1D) coordination polymer containing [Co(L)]2+ repeating units bridged by µ1,5-dicyanamido groups in cis positions, while 2 represents a discreate dinuclear cobalt(II) molecule bridged by two µ1,5-tricyanomethanido groups in a cis conformation. Both complexes have a CoN6 coordination environment around each cobalt center offered by the tetradentate ligand and cis coordinating bridging ligands. Complex 1 exhibits a high-spin (S = 3/2) state of cobalt(II) in the temperature range of 2-300 K with a weak ferromagnetic coupling between two dicyanamido-bridged cobalt(II) centers. Interestingly, complex 2 exhibits reversible spin-state switching associated with spin-spin coupling. Complexes 1 and 2 also exhibit interesting redox-stimuli-based reversible paramagnetic high-spin cobalt(II) to diamagnetic low-spin cobalt(III) conversion, offering an additional way to switch magnetic properties. A detailed theoretical calculation was consistent with the stated results.

ON/OFF Photoswitching and Thermoinduced Spin Crossover with Cooperative Luminescence in a 2D Iron(II) Coordination Polymer.

A 2D coordination polymer, {[Fe(L)2(NCSe)2]·6MeOH·14H2O}n (1; L = 2,5-dipyridylethynylene-3,4-ethylenedioxythiophene), has been synthesized based on a redox active luminescence ligand. 1 possesses a 2D [4 × 4] square-grid network where the iron(II) center is in a FeN6 octahedral coordination environment. 1 displays reversible thermoinduced high-spin (HS; S = 2) to diamagnetic low-spin (LS; S = 0) ON/OFF spin-state switching with a T1/2 value of 150 K. Interestingly, optical reflectivity and photomagnetic studies at 10 K under light irradiation revealed an efficient conversion to a photoinduced metastable HS excited state from a LS ground state. Remarkably, the photoexcited HS state can be reversibly switched ON and OFF by using 625 and 850 nm light-emitting-diode lights. Intriguingly, the thermal dependence of the luminescence intensity of the maximum emission at 524 nm for 1 shows a minimum at around the spin-crossover (SCO) temperature, indicating a cooperative nature between the SCO and luminescence properties. Theoretical calculations confirmed the above findings.

Selective Coordination of Self-Assembled Hexanuclear [Ni4Ln2] and [Ni2Mn2Ln2] (Ln = DyIII, TbIII, and HoIII) Complexes: Stepwise Synthesis, Structures, and Magnetic Properties.

The versatility for a unique aggregation of heterometallic 3d-4f and 3d-3d'-4f ions by the new Schiff base ligand 2-{[(2-hydroxy-3-methoxybenzyl)imino]methyl}phenol (H2L) providing O3N donors has been examined. A series of complexes having the general formula [Ln2Ni4(L)4(µ1,3-CH3CO2)2(µ3-OH)4(MeOH)2]·xCH3OH·yH2O [where Ln = DyIII, x = 4, y = 0 (1), Ln = TbIII, x = 0, y = 4 (2), and Ln = HoIII, x = 4, y = 0 (3)] were obtained from the sequential use of lanthanide(III) nitrate salts and Ni(CH3CO2)2·4H2O. The incorporation of two different 3d ions and one 4f ion in the same coordination aggregate was achieved through the isomorphic replacement of two NiII centers by MnIII ions as second group of cationic complexes, [Ln2Ni2Mn2(L)4(µ1,3-CH3CO2)2(µ3-OH)4(MeOH)2](NO3)2·2CH3OH [where Ln = DyIII (4), TbIII (5), and HoIII (6)]. Direct-current (dc) magnetic susceptibility studies hint to the possibility of ferromagnetic interactions occurring in the aggregates, whereas alternating-current susceptibility measurements find both the DyIII analogues, 1 and 4, to show out-of-phase components at zero applied dc field, characteristic of single-molecule-magnet (SMM) behavior. Micro-SQUID studies reveal open hysteresis loops for 1, corroborating its SMM character. Further detailed complete-active-space self-consistent-field and density functional theory calculations were also performed, supporting the experimental findings in complexes 1 and 4.

Effect of Coordination Geometry on Magnetic Properties in a Series of Cobalt(II) Complexes and Structural Transformation in Mother Liquor.

The three Co(II) complexes [Co(bbp)2][Co(NCS)4]·4DMF (1), [Co(bbp)(NCS)2(DMF)]·2DMF (2), and [Co(bbp)(NCS)2] (3) have been synthesized and characterized by single-crystal X-ray diffraction, magnetic, and various spectroscopic techniques. Complexes 1 and 3 are obtained by the reaction of Co(NCS)2 with 2,6-bis(1H-benzo[d]imidazol-2-yl)pyridine (bbp), and complex 1 undergoes a structural transformation to form complex 2. A single-crystal X-ray study revealed that complex 1 is comprised of two Co(II) centers, a cationic octahedral Co(II) unit and an anionic tetrahedral Co(II) unit, while the Co(II) ion is in a distorted-octahedral environment in 2. Moreover, in complex 3, the Co(II) ion is in a distorted-square-pyramidal geometry. The effect of coordination geometry on the magnetic properties was studied by both static and dynamic magnetic measurements. Direct current (dc) magnetic susceptibility measurements showed that all of the Co(II) ions are in high-spin state in these complexes. Alternating current (ac) magnetic susceptibility measurements indicated that complexes 2 and 3 display slow relaxation of magnetization in an external dc magnetic field, while complex 1 displayed no such property. EPR experiments and theoretical calculations were consistent with the above findings.

The Structural and Magnetic Properties of FeII and CoII Complexes with 2-(furan-2-yl)-5-pyridin-2-yl-1,3,4-oxadiazole.

Two novel coordination compounds containing heterocyclic bidentate N,N-donor ligand 2-(furan-2-yl)-5-(pyridin-2-yl)-1,3,4-oxadiazole (fpo) were synthesized. A general formula for compounds originating from perchlorates of iron, cobalt, and fpo can be written as: [M(fpo)2(H2O)2](ClO4)2 (M = Fe(II) for (1) Co(II) for (2)). The characterization of compounds was performed by general physico-chemical methods-elemental analysis (EA), Fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance (NMR) in case of organics, and single crystal X-ray diffraction (sXRD). Moreover, magneto-chemical properties were studied employing measurements in static field (DC) for 1 and X-band EPR (Electron paramagnetic resonance), direct current (DC), and alternating current (AC) magnetic measurements in case of 2. The analysis of DC magnetic properties revealed a high spin arrangement in 1, significant rhombicity for both complexes, and large magnetic anisotropy in 2 (D = -21.2 cm-1). Moreover, 2 showed field-induced slow relaxation of the magnetization (Ueff = 65.3 K). EPR spectroscopy and ab initio calculations (CASSCF/NEVPT2) confirmed the presence of easy axis anisotropy and the importance of the second coordination sphere.

Deposition of Tetracoordinate Co(II) Complex with Chalcone Ligands on Graphene.

Studying the properties of complex molecules on surfaces is still mostly an unexplored research area because the deposition of the metal complexes has many pitfalls. Herein, we probed the possibility to produce surface hybrids by depositing a Co(II)-based complex with chalcone ligands on chemical vapor deposition (CVD)-grown graphene by a wet-chemistry approach and by thermal sublimation under high vacuum. Samples were characterized by high-frequency electron spin resonance (HF-ESR), XPS, Raman spectroscopy, atomic force microscopy (AFM), and optical microscopy, supported with density functional theory (DFT) and complete active space self-consistent field (CASSCF)/N-electron valence second-order perturbation theory (NEVPT2) calculations. This compound's rationale is its structure, with several aromatic rings for weak binding and possible favorable π-π stacking onto graphene. In contrast to expectations, we observed the formation of nanodroplets on graphene for a drop-cast sample and microcrystallites localized at grain boundaries and defects after thermal sublimation.

Are Inorganic Single-Molecule Magnets a Possibility? A Theoretical Insight into Dysprosium Double-Deckers with Inorganic Ring Systems.

Computational studies of sandwich dysprosium double-decker complexes [Dy(L)2]+/3+ as candidates for single-ion magnets with several inorganic aromatic ring systems (P5-, N5-, B3N3H6, B3P3H6, B3S3H3) have been performed. The molecular structures were optimized with the TPSSh functional, and the ground state properties were investigated with the complete active space SCF method (CASSCF) complemented by the dynamic correlation dressed correction (DCD-CAS(2)) or NEVPT2. Besides the evaluation of the magnetic moment blocking barrier, the impact of the molecular vibration on the relaxation of magnetization was also inspected. We were able to make predictions about the performance of those molecules as single-molecule magnets, where estimated effective energy barrier, Ueff, values are as high as 1475 K in the case of [Dy(N5)2]-, which is the most anisotropic complex from our choice of studied compounds, making them a potentially very effective carbon-free alternative to organometallic double-decker dysprosocenium high-temperature single-molecule magnets.

Two Types of Hexanuclear Partial Tetracubane [Ni4Ln2] (Ln = Dy, Tb, Ho) Complexes of Thioether-Based Schiff Base Ligands: Synthesis, Structure, and Comparison of Magnetic Properties.

New Schiff base ligand H2L containing N2O4S donor atoms has been explored for its ability to provide complexes of selected 3d and 4f metal ions. Room temperature reaction of H2L with NiCl2·6H2O and Ln(NO3)3·5H2O in the presence of Et3N in MeCN-MeOH (2:1) medium resulted in [Ni4Ln2(L)2(µ-Cl)2(µ3-OH)4(H2O)6]Cl4·2H2O (where Ln = Dy3+ (1), Tb3+ (2), and Ho3+ (3) and H2L = 2-((2-(2-(2-hydroxy-3-methoxybenzylideneamino)ethylthio)ethylimino)methyl)-6-methoxyphenol). Use of Ni(SCN)2·4H2O during synthesis provided SCN- ions for bridging and terminal coordination in [Ni4Ln2(L)2(µ-NCS)2(µ3-OH)4(NCS)4(H2O)2]·xMeOH·yH2O (where Ln = Dy3+ (4), x = 2, y = 4; Tb3+ (5) and Ho3+ (6), x = 0, y = 14.1). All six complexes possess a hexanuclear defective tetracubane topology having exchangeable bridging groups. The study of direct current magnetic susceptibility measurements revealed that the Ni(II) ions are engaged in ferromagnetic interaction with the DyIII, TbIII, and HoIII ions and have significant magnetic anisotropy in all six complexes. Alternating current susceptibility measurements confirmed that both of the two types of compounds qualify as zero-field single-molecule magnets (SMMs), and the effective barrier for the reversal of the magnetic moment was found to be in the range Ueff = 23-31 K for 1-2 and 4-5, respectively. Detailed insight into the electronic structure and magnetic properties was calculated using DFT- and CASSCF-based analyses. The found isotropic exchange parameter (J) values are JNi-Ni = -4.7 cm-1 for 1 and JNi-Ni = +29.2 cm-1 for 4 and clearly indicate that the µ-NCS-bridge is a better candidate than µ-Cl for ferromagnetic exchange interactions. Out of the six complexes, only complex 5 displays TbIII centered emission peaks at 451 and 480 nm.

Polynuclear Iron(II) Complexes with 2,6-Bis(pyrazol-1-yl)pyridine-anthracene Ligands Exhibiting Highly Distorted High-Spin Centers.

Two bis-tridentate ligands L1 and L2 that contain 2,6-bis(pyrazol-1-yl) pyridine N-donor embraces introduced on a anthracene-acetylene backbone were used for the synthesis of a tetranuclear compound [Fe4(L1)4](CF3SO3)8·7CH3CN (1) and a hexanuclear compound [Fe6(L2)6](CF3SO3)12·18CH3NO2·9H2O (2). The polynuclear structures of both complexes were confirmed by X-ray diffraction studies, which revealed a [2 + 2] grid-like complex cation for 1 and a closed-ring hexagonal molecular architecture for the complex cation in 2. Although both compounds contain anthracene moieties arranged in a face-to-face manner, attempts at [4 + 4] photocyclization remain unsuccessful, which can be explained either by steric restraints or by inhibition of the photo-cycloaddition. Magnetic studies identified gradual and half-complete thermal spin crossover in the tetranuclear grid 1, where 50% of ferrous atoms exhibit thermal as well as photoinduced spin state switching and the remaining half of iron(II) centers are permanently blocked in their high-spin state. On the contrary, the hexanuclear compound 2 exhibits complete blocking in a high-spin state. Analysis of the magnetic data reveals the zero-field splitting parameter | D| ≈ 6-8 cm-1 with a large rhombicity for all high-spin iron(II) atoms in 1 or 2. The electronic structures and the magnetic anisotropies were also investigated by the multireference CASSCF/NEVPT2 method, and intramolecular exchange interactions were calculated by density functional theory methods.

Single-Chain Magnet Based on 1D Polymeric Azido-Bridged Seven-Coordinate Fe(II) Complex with a Pyridine-Based Macrocyclic Ligand.

Peculiar magnetic behavior was found for 1D-polymeric seven-coordinate pentagonal bipyramidal Fe(II) complex {[Fe(L)(µ1,3-N3)](ClO4)} n (1) with a pentadentate macrocyclic ligand L (3,12,18-triaza-6,9-dioxabicyclo[12.3.1]octadeca-1(18),14,16-triene) coordinated in the pentagonal equatorial plane and with end-to-end bridging azido ligands in axial positions. The static and dynamic magnetic data revealed that spin-canting in the 1D-chain of 1 results in the single-chain magnet (SCM) behavior with high spin-reversal energy barrier Ueff (Δτ) = 87.5 K, exhibiting magnetic hysteresis below 4 K and coexistence with the metamagnetism altogether resulting in weak 3D-ferromagnetic behavior. This is the first reported example of the exclusively azido-bridged homospin Fe(II)-based SCM.

Impact of Halogenido Coligands on Magnetic Anisotropy in Seven-Coordinate Co(II) Complexes.

The structural and magnetic features of a series of mononuclear seven-coordinate CoII complexes with the general formula [Co(L)X2], where L is a 15-membered pyridine-based macrocyclic ligand (3,12,18-triaza-6,9-dioxabicyclo[12.3.1]octadeca-1(18),14,16-triene) and X = Cl- (1), Br- (2), or I- (3), were investigated experimentally and theoretically in order to reveal how the corresponding halogenido coligands in the apical positions of a distorted pentagonal-bipyramidal coordination polyhedron may affect the magnetic properties of the prepared compounds. The thorough analyses of the magnetic data revealed a large easy-plane type of the magnetic anisotropy (D > 0) for all three compounds, with the D-values increasing in the order 35 cm-1 for 3 (I-), 38 cm-1 for 1 (Cl-), and 41 cm-1 for 2 (Br-). Various theoretical methods like the Angular Overlap Model, density functional theory, CASSCF/CASPT2, CASSCF/NEVPT2 were utilized in order to understand the observed trend in magnetic anisotropy. The D-values correlated well with the Mayer bond order (decreasing in order Co-I > Co-Cl > Co-Br), which could be a consequence of two competing factors: (a) the ligand field splitting and (b) the covalence of the Co-X bond. All the complexes also behave as field-induced single-molecule magnets with the spin reversal barrier Ueff increasing in order 1 (Cl-) < 2 (Br-) < 3 (I-); however, taking into account the easy-plane type of the magnetic anisotropy, the Raman relaxation process is most likely responsible for slow relaxation of the magnetization. The results of the work revealed that the previously suggested and fully accepted strategy employing heavier halogenido ligands in order to increase the magnetic anisotropy has some limitations in the case of pentagonal-bipyramidal CoII complexes.

Late First-Row Transition-Metal Complexes Containing a 2-Pyridylmethyl Pendant-Armed 15-Membered Macrocyclic Ligand. Field-Induced Slow Magnetic Relaxation in a Seven-Coordinate Cobalt(II) Compound.

The 2-pyridylmethyl N-pendant-armed heptadentate macrocyclic ligand {3,12-bis(2-methylpyridine)-3,12,18-triaza-6,9-dioxabicyclo[12.3.1]octadeca-1,14,16-triene = L} and [M(L)](ClO4)2 complexes, where M = Mn(II) (1), Fe(II) (2), Co(II) (3), Ni(II) (4), and Cu(II) (5), were prepared and thoroughly characterized, including elucidation of X-ray structures of all the compounds studied. The complexes 1-5 crystallize in non-centrosymmetric Sohncke space groups as racemic compounds. The coordination numbers of 7, 6 + 1, and 5 were found in complexes 1-3, 4, and 5, respectively, with a distorted pentagonal bipyramidal (1-4) or square pyramidal (5) geometry. On the basis of the magnetic susceptibility experiments, a large axial zero-field splitting (ZFS) was found for 2, 3, and 4 (D(Fe) = -7.4(2) cm(-1), D(Co) = 34(1) cm(-1), and D(Ni) = -12.8(1) cm(-1), respectively) together with a rhombic ZFS (E/D = 0.136(3)) for 4. Despite the easy plane anisotropy (D > 0, E/D = 0) in 3, the slow relaxation of the magnetization below 8 K was observed and analyzed either with Orbach relaxation mechanism (the relaxation time τ0 = 9.90 × 10(-10) s and spin reversal barrier Ueff = 24.3 K (16.9 cm(-1))) or with Raman relaxation mechanism (C = 2.12 × 10(-5) and n = 2.84). Therefore, compound 3 enlarges the small family of field-induced single-molecule magnets with pentagonal-bipyramidal chromophore. The cyclic voltammetry in acetonitrile revealed reversible redox processes in 1-3 and 5, except for the Ni(II) complex 4, where a quasi-reversible process was dominantly observed. Presence of the two 2-pyridylmethyl pendant arms in L with a stronger σ-donor/π-acceptor ability had a great impact on the properties of all the complexes (1-5), concretely: (i) strong pyridine-metal bonds provided slight axial compression of the coordination sphere, (ii) substantial changes in magnetic anisotropy, and (iii) stabilization of lower oxidation states.

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

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

Pentacoordinate and Hexacoordinate Mn(III) Complexes of Tetradentate Schiff-Base Ligands Containing Tetracyanidoplatinate(II) Bridges and Revealing Uniaxial Magnetic Anisotropy.

Crystal structures and magnetic properties of polymeric and trinuclear heterobimetallic MnIII···PtII···MnIII coordination compounds, prepared from the Ba[Pt(CN)4] and [Mn(L4A/B)(Cl)] (1a/b) precursor complexes, are reported. The polymeric complex [{Mn(L4A)}2{µ4-Pt(CN)4}]n (2a), where H2L4A = N,N'-ethylene-bis(salicylideneiminate), comprises the {Mn(L4A)} moieties covalently connected through the [Pt(CN)4]2- bridges, thus forming a square-grid polymeric structure with the hexacoordinate MnIII atoms. The trinuclear complex [{Mn(L4B)}2{µ-Pt(CN)4}] (2b), where H2L4B = N,N'-benzene-bis(4-aminodiethylene-salicylideneiminate), consists of two [{Mn(L4B)} moieties, involving pentacoordinate MnIII atoms, bridged through the tetracyanidoplatinate (II) bridges to which they are coordinated in a trans fashion. Both complexes possess uniaxial type of magnetic anisotropy, with D (the axial parameter of zero-field splitting) = -3.7(1) in 2a and -2.2(1) cm-1 in 2b. Furthermore, the parameters of magnetic anisotropy 2a and 2b were also thoroughly studied by theoretical complete active space self-consistent field (CASSCF) methods, which revealed that the former is much more sensitive to the ligand field strength of the axial ligands.

Structural, magnetic, and redox diversity of first-row transition metal complexes of a pyridine-based macrocycle: well-marked trends supported by theoretical DFT calculations.

A series of first-row transition metal complexes with 15-membered pyridine-based macrocycle (3,12,18-triaza-6,9-dioxabicyclo[12.3.1]octadeca-1(18),14,16-triene = L) was prepared ([M(II)(L)Cl2], where M = Mn, Co, Ni, Zn (1, 3, 4, 6); [Fe(III)(L)Cl2]Cl (2), [Cu(II)(L)Cl]Cl (5)) and thoroughly characterized. Depending on the complexated metal atom, the coordination number varies from 7 (Mn, Fe, Co), through 5 + 2 for Ni and 4 + 1 for Cu, to 5 for Zn accompanied by changes in the coordination geometry from the pentagonal bipyramid (1-4) to the square pyramid (5 and 6). Along the series, the metal-oxygen distances were prolonged in such manner that their bonding character was investigated, apart from X-ray structural analysis, also by ab initio calculations (Mayer's bond order, electron localization function), which confirmed that, in 4 and 5, two and one oxygen donor atoms are semicoordinated, respectively, and one and two oxygen atoms are uncoordinated in 5, and 6, respectively. On the basis of the temperature variable magnetic susceptibility measurements, 1 and 2 behave as expected for 3d(5) high-spin configuration with negligible zero-field splitting (ZFS). On the other hand, a large axial ZFS (D(Co) ≈ 40 cm(-1), D(Ni) ≈ -6.0 cm(-1)) was found for 3 and 4, and rhombic ZFS (E/D ≈ 0.15) for 4. Antiferromagnetic exchange coupling was observed for 4 and 5 (J(Ni) = -0.48 cm(-1), and J(Cu) = -2.43 cm(-1), respectively). The obtained results correlate well with ab initio calculations of ZFS parameters as well as J-values, which indicate that the antiferromagnetic exchange is mediated by hydrogen bonds. The complexes were also investigated by cyclic voltammetry in water or acetonitrile. A quasi-reversible couple Mn(II)/Mn(III) at 1.13/0.97 V, an almost reversible couple Fe(II)/Fe(III) at 0.51/0.25 V, and a one-step/multistep reduction/oxidation of Cu(II) complex 5 at -0.33 V/0.06-0.61 V were detected.

Experimental and Theoretical Investigations of Magnetic Exchange Pathways in Structurally Diverse Iron(III) Schiff-Base Complexes.

The synthesis, and the structural and magnetic properties, of the following new iron(III) Schiff base complexes with the {O',N,O″}-chelating ligand H2L (2-hydroxyphenylsalicylaldimine) are reported: K[FeL2]·H2O (1), (Pr3NH)[FeL2]·2CH3OH (2), [FeL(bpyO2) (CH3OH)][FeL2]·CH3OH (3), [Fe2L3(CH3OH)]·2CH3OH·H2O (4), and [{Fe2L2}(µ-OH)2{FeL(bpyO2)}2][BPh4]2·2H2O (5), where Pr3NH(+) represents the tripropylammonium cation and bpyO2 stands for 2,2'-bipyridine-N-dioxide. A thorough density functional theory (DFT) study of magnetic interactions (the isotropic exchange) at the B3LYP/def-TZVP level of theory was employed, and calculations have revealed superexchange pathways through intramolecular/intermolecular noncovalent contacts (π-π stacking, C-H···O and O-H···O hydrogen bonds, diamagnetic metal cations) and/or covalent bonds ((µ-O(Ph), µ-OH) or bis(µ-O(Ph)) bridging modes), which helped us to postulate trustworthy spin Hamiltonians for magnetic analysis of experimental data. Within the reported family of compounds 1-5, the mediators of the antiferromagnetic exchange can be sorted by their increasing strength as follows: π-π stacking (J(DFT) = -0.022 cm(-1)/J(mag) = -0.025(4) cm(-1) in 2) < C-H···O contacts and π-π stacking (J(DFT) = -0.19 cm(-1)/J(mag) = -0.347(9)cm(-1) in 1) < O-H···O hydrogen bonds (J(DFT) = -0.53 cm(-1)/J(mag) = -0.41(1) cm(-1) in 3) < bis(µ-O(Ph)) bridge (J(DFT) = -13.8 cm(-1)/J(mag) = -12.3(9) cm(-1) in 4) < (µ-O(Ph), µ-OH) bridge (J(DFT) = -18.0 cm(-1)/J(mag) = -17.1(2) cm(-1) in 5), where J(DFT) and J(mag) are the isotropic exchange parameters derived from DFT calculations, and analysis of the experimental magnetic data, respectively. The good agreement between theoretically calculated and experimentally derived isotropic exchange parameters suggests that this procedure is applicable also for other chemical and structural systems to interpret magnetic data properly.

Slow magnetic relaxation in octahedral cobalt(II) field-induced single-ion magnet with positive axial and large rhombic anisotropy.

Pseudooctahedral mononuclear cobat(II) complex [Co(abpt)2(tcm)2] (1), where abpt = 4-amino-3,5-bis(2-pyridyl)-1,2,4-triazole and tcm = tricyanomethanide anion, shows field-induced slow relaxation of magnetization with U = 86.2 K and large axial and rhombic single-ion zero-field-splitting parameters, D = +48(2) cm(-1) and E/D = 0.27(2) (D = +53.7 cm(-1) and E/D = 0.29 from ab initio CASSCF/NEVPT2 calculations), thus presenting a new example of a field-induced single-ion magnet with transversal magnetic anisotropy.

Disklike hepta- and tridecanuclear cobalt clusters. Synthesis, structures, magnetic properties, and DFT calculations.

The synthesis, structure and magnetic properties are reported of two disklike mixed-valence cobalt clusters [Co(III)Co(II)6(thmp)2(acac)6(ada)3] (1) and [Co(III)2Co(II)11(thmp)4(Me3CCOO)4(acac)6(OH)4(H2O)4](Me3CCOO)2·H2O (2). Heptanuclear complex 1 was prepared by solvothermal reaction of cobalt(II) acetylacetonate (Co(acac)2), 1,1,1-tris(hydroxymethyl)-propane (H3thmp), and adamantane-1-carboxylic acid (Hada), whereas by substituting Hada with Me3CCO2H, tridecanuclear complex 2 was obtained with an unexpected [Co(III)2Co(II)11] core. The core structures of 1 and 2 are related to each other: that of 1 arranges as a centered hexagon of a central Co(III) ion surrounded by a [Co(II)6] hexagon, while that of 2 can be described as a larger oligomer based on two vertex-sharing [Co(III)Co(II)6] clusters. Variable-temperature direct-current magnetic susceptibility measurements demonstrated overall ferromagnetic coupling between the Co(II) ions within both clusters. The magnetic exchange (J) and magnetic anisotropy (D) values were quantified with appropriate spin-Hamiltonian models and were also supported by density functional theory calculations. The presence of frequency-dependent out-of-phase (χM″) alternating current susceptibility signals at temperatures below 3 K suggested that 2 might be a single-molecule magnet.