Field-induced Single-ion Magnet Research Articles

Slow relaxation of the magnetization in a {CoIIIMnIII} heterometallic brick-wall network

The use of the cyanide-bearing dicobalt(III) complex (PPh4)2[Co2III(μ−2,5-dpp)(CN)8] as a metalloligand towards [Mn(salen)(H2O)]ClO4 afforded the heterobimetallic two-dimensional compound of formula [{MnIII(salen)}2{(μ-NC)4Co2III(μ−2,5-dpp)(CN)4}]n (1) [PPh4+ = teraphenylphosphonium cation, 2,5-dpp = 2,5-bis(2-pyridyl)pyrazine and H2salen = N,N’-ethylenebis(salicylideneimine)] whose structure has been determined by single crystal X-ray diffraction. Compound 1 exhibits a neutral brick-wall structure, where each [Co2III(μ−2,5-dpp)(CN)8]2− unit adopts a tetrakis-monodentate bridging mode towards four {MnIII(salen)}+ fragments through four of its eight cyanide ligands. Each cobalt(III) ion is six-coordinate with one pyrazine- and one pyridine-nitrogen atoms of the 2,5-dpp molecule and four cyanide-carbon atoms describing a somewhat distorted octahedral surrounding. The two crystallographically independent manganese(III) ions are also six-coordinate in an elongated octahedral environment with two imino-nitrogen and two phenoxo-oxygen atoms occupying the equatorial positions and two cyanide-nitrogen atoms filling the axial sites. Variable-temperature direct- (1.9–300 K) and alternating-current (2.0–5.0 K) magnetic measurements of 1 show the occurrence of magnetically isolated manganese(III) ions with a local negative zero-field splitting (D = −5.87 cm−1) and slow magnetic relaxation, this compound being a new example of field-induced Single-Ion Magnet (SIM). The analysis of the ac magnetic data of 1 under an applied dc field of 5000 G reveals that the value of the energy barrier (Ea) to reverse the magnetization direction and that of the pre-exponential factor (τ0) are 10.8 cm−1 and 2.7 × 10−8 s, respectively. These values compare well with those reported for the few examples of manganese(III)-based SIMs nowadays.

Field-induced single-ion magnet based on a quasi-octahedral Co(II) complex with mixed sulfur-oxygen coordination environment.

Synthesis and characterization of structure and magnetic properties of the quasi-octahedral complex (pipH2)[Co(TDA)2] 2H2O (I), (pipH22+ = piperazine dication, TDA2- = thiodiacetic anion) are described. X-ray diffraction studies reveal the first coordination sphere of the Co(II) ion, consisting of two chelating tridentate TDA ligands with a mixed sulfur-oxygen strongly elongated octahedral coordination environment. SQUID magnetometry, frequency-domain Fourier-transform (FD-FT) THz-EPR spectroscopy, and high-level ab initio SA-CASSCF/NEVPT2 quantum chemical calculations reveal a strong "easy-plane" type magnetic anisotropy (D ≈ +54 cm-1) of complex I. The complex shows field-induced slow relaxation of magnetization at an applied DC field of 1000 Oe.

Magnetic investigations of monocrystalline [Co(NCS)2(L)2]n: new insights into single-chain relaxations.

A large single crystal of a compound from the family of coordination polymer [Co(NCS)2(L)2]n chains was synthesized and its magnetic properties are reported. [Co(NCS)2(4-(3-phenylpropyl)pyridine)2]n is ferromagnetic with Tc = 3.39 K. Single-ion ab initio calculations predict an almost Ising-type magnetic anisotropy and the direction of the magnetic easy-axis nearly along the Co-Npy bond of the apical pyridine-based co-ligand. Both predictions are confirmed by single-crystal magnetic measurements. The magnetic relaxation of the single crystal sample significantly differs from the powder sample data, and clearly shows the presence of two separate relaxation processes. The process dominant below 3.2 K demonstrates a single chain magnet (SCM) behaviour, with a crossover between single-wall and two-wall processes, in spite of the fact that the system is ferromagnetically ordered. The faster process that dominates just below Tc is attributed to spin waves. Micromagnetic Monte Carlo simulations of the investigated compound show that the dipolar field cancels for some chains located at the border between 3-dimensional domains. Such chains are responsible for the measured ac signal, and demonstrate the SCM behaviour. The quantitative analysis of the SCM relaxation time is supported by preparing and examining a corresponding diamagnetically diluted compound, [CoxCd1-x(NCS)2(4-(3-phenylpropyl)pyridine)2]n (x = 0.013), which behaves as a field-induced single-ion magnet. The relaxation pathways for single Co(ii) spins are determined to be Raman, direct, and quantum tunneling processes, which were included in an improved approach to describe the magnetic relaxation in the Co(ii)-based SCM compound.

Field-induced single ion magnet behaviour of discrete and one-dimensional complexes containing [bis(1-methylimidazol-2-yl)ketone]-cobalt(II) building units.

We describe herein the first examples of six-coordinate CoII single-ion magnets (SIMs) based on the β-diimine Mebik ligand [Mebik = bis(1-methylimidazol-2-yl)ketone]: two mononuclear [CoII(Rbik)2L2] complexes and one mixed-valence {CoIII2CoII}n chain of formulas [CoII(Mebik)(H2O)(dmso)(μ-NC)2CoIII2(μ-2,5-dpp)(CN)6]n·1.4nH2O (3) [L = NCS (1), NCSe (2) and 2,5-dpp = 2,5-bis(2-pyridyl)pyrazine (3)]. Two bidentate Mebik molecules plus two monodentate N-coordinated pseudohalide groups in cis positions build somewhat distorted octahedral surroundings around the high-spin cobalt(II) ions in 1 and 2. The diamagnetic [CoIII2(μ-2,5-dpp)(CN)8]2- metalloligand coordinates the paramagnetic [CoII(Mebik)(H2O)(dmso)]2+ complex cations in a bis-monodentate fashion to afford neutral zigzag heterobimetallic chains in 3. Ab initio calculations, and cryomagnetic dc (2.0-300 K) and ac (2.0-12 K) measurements as well as EPR spectroscopy for 1-3 show the existence of magnetically isolated high-spin cobalt(II) ions with D values of 59.84-89.90 (1), 66.32-93.90 (2) and 70.40-127.20 cm-1 (3) and field-induced slow relaxation of the magnetization, being thus new examples of SIMs with transversal magnetic anisotropy. The analysis of their relaxation dynamics reveals that the relaxation of the magnetization occurs by the Raman (with values of the n parameter covering the range 6.0-6.8) and direct spin-phonon processes.

Field-induced single-ion magnets exhibiting tri-axial anisotropy in a 1D Co(II) coordination polymer with a rigid ligand 4,4'-(buta-1,3-diyne-1,4-diyl)dibenzoate.

Herein a 1D Co(II) coordination polymer of formula [Co(η1-L1)(η2-L1)(py)2(H2O)]n (CoCP) has been synthesised using the rigid H2L1 proligand, containing a long spacer bearing two triple bonds. Single-crystal X-ray diffraction showed that Co(II) adopts a distorted octahedral geometry. The state-averaged complete active self-consistent field (SA-CASSCF) calculation showed that the ground state of CoCP is a high spin quartet with a highly multiconfigurational character of its electronic structure. Due to the large intra- and intermolecular distances between the spin carriers, the magnetic interactions are negligible and the zero-field splitting (ZFS) effects of cobalt(II) ions are predominant. This behavior was confirmed by direct current (DC) magnetic measurements and theoretical calculations using the broken-symmetry approach. Quantum chemical calculations indicate that CoCP has a negative axial component possessing mixed tri-axial anisotropy. The DC magnetic susceptibility data were fitted with a Griffith-Figgis Hamiltonian and the obtained parameters are in good agreement with those simulated by the ab initio calculation. Alternating current (AC) magnetic measurements showed a field induced slow magnetic relaxation in CoCP, which is attributed to the hyperfine interaction effects.

Slow Relaxation of the Magnetization in a {Co <sup>III</sup>Mn <sup>III</sup>} Heterometallic Brick-Wall Network

The use of the cyanide-bearing dicobalt(III) complex (PPh 4 ) 2 [Co 2 III (m-2,5-dpp)(CN) 8 ] as a metalloligand towards [Mn(salen)(H 2 O)]ClO 4 afforded the heterobimetallic two-dimensional compound of formula [{Mn III (salen)} 2 {(m-NC) 4 Co 2 III (m-2,5-dpp)(CN) 4 }] n (1) [PPh 4 + = teraphenylphosphonium cation, 2,5-dpp = 2,5-bis(2-pyridyl)pyrazine and H 2 salen = N,N’ -ethylenebis(salicylideneimine)] whose structure has been determined by single crystal X-ray diffraction. Compound 1 exhibits a neutral brick-wall structure, where each [Co 2 III (m-2,5-dpp)(CN) 8 ] 2- unit adopts a tetrakis-monodentate bridging mode towards four {Mn III (salen)} + fragments through four of its eight cyanide ligands. Each cobalt(III) ion is six-coordinate with one pyrazine- and one pyridine-nitrogen atoms of the 2,5-dpp molecule and four cyanide-carbon atoms describing a somewhat distorted octahedral surrounding. The two crystallographically independent manganese(III) ions are also six-coordinate in an elongated octahedral environment with two imino-nitrogen and two phenoxo-oxygen atoms occupying the equatorial positions and two cyanide-nitrogen atoms filling the axial sites. Variable-temperature direct- (1.9-300 K) and alternating-current (2.0-5.0 K) magnetic measurements of 1 show the occurrence of magnetically isolated manganese(III) ions with a local negative zero-field splitting ( D = -5.87 cm -1 ) and slow magnetic relaxation, this compound being a new example of field-induced Single-Ion Magnet (SIM). The analysis of the ac magnetic data of 1 under an applied dc field 5000 G reveals that the value of the energy barrier ( E a ) to reverse the magnetization direction and that of the pre-exponential factor (t 0 ) are 10.8 cm -1 and 2.7 x 10 -8 s, respectively. These values compare well with those reported for the few examples of manganese(III)-based SIMs nowadays.

Observation of field-induced single-ion magnet behavior in an octahedral dysprosium complex with a strong ligand field

A mononuclear six-coordinate DyIII complex exhibiting slow magnetic relaxation provides a rare case for octahedral lanthanide complexes with SMM behavior.

Cobalt(II) Complexes Based on Benzylmalonate Anions Exhibiting Field-Induced Single-Ion Magnet Slow Relaxation Behavior

The reaction of (NBu4)2Bzmal (where Bzmal2− is benzylmalonate dianion) with Co(OAc)2∙4H2O gives the [Co(Bzmal)(EtOH)(H2O)]n 2D-polymer (1). The addition of 2,2′-bipyridine (bpy) to the starting system results in the [Co(Bzmal)(bpy)2]·H2O·EtOH molecular complex (2). Their molecular and crystal structures were analyzed by single-crystal X-ray crystallography. An analysis of the static magnetic data supported by the SA-CASSCF/NEVPT2 calculations revealed the presence of easy-plane magnetic anisotropy in both complexes. The AC susceptibility data confirm that both complexes show a slow field-induced (HDC = 1000 Oe) magnetic relaxation behavior.

Nd3+ and Yb3+ complexes with N-(diphenylphosphoryl)pyrazine-2-carboxamide as UV-NIR radiation converters and single-ion magnets

The increased interest in designing bifunctional magnetic-luminescent compounds stems from their promising application potential. Appropriately designed lanthanide coordination compounds were found suitable for this purpose. This work describes the structural, optical and magnetic properties of photostable lanthanide coordination compounds (Nd, Yb) with N-(diphenylphosphoryl)pyrazine-2-carboxamide (HL) acting as an antenna. Optical properties of the compounds were characterized by IR, high-resolution absorption and emission spectroscopies at 300, 77 and 5 K. The trinuclear coordination compounds (labeled as Na2LnL4), with a crystal structure stiffened by eight oxygen bridges with eight-coordinated lanthanide ion, as well as six- and seven-coordinated Na ions, were found to be excellent converters of UV radiation into the NIR. The ligand-to-metal energy transfer mechanisms in Na2NdL4 were discussed. Na2YbL4 reached the highest (QYbL = 5.8%) overall emission quantum yield among the coordination compounds of Yb3+ with organic ligands in solid state. The static and dynamic magnetic properties of Na2NdL4 and Na2YbL4 were studied, revealing that the compounds exhibit behavior of field-induced Single Ion Magnets (SIMs) with the effective energy barrier value of 27.4 K (Na2NdL4) and 26.2 K (Na2YbL4). The anisotropic barrier was extracted from static magnetism and optical properties analysis, giving an insight into the magneto-optical correlation. We demonstrated that proper design of ligand structure and coordination compound architecture allows to obtain molecular magnets with optimized infrared emission.

Mononuclear Yb and Dy single molecule magnets based on the terpyridine derivatives

The reaction of terpyridine derivatives L1/L2 with Ln(NO3)3 led to the formation of three new mononuclear complexes with formulae [LnL1(CH3OH)(H2O)(NO3)2]NO3•2CH3OH (Ln=Yb, 1 ; Ln=Dy, 2 ) and [DyL2(NO3)3(H2O)]•2CH3OH ( 3 ). Complexes 1 , 2 and 3 were characterized by crystal structure determination, magnetic properties study. Ac magnetic susceptibility measurements reveal that all complexes exhibited field-induced single-ion magnet (SIM) behavior.

Observation of field-induced single-ion magnet behavior in a mononuclear DyIII complex by co-crystallization of a square-planar CuII complex

Employing Cu(acac)2 to react with Dy(hfac)3(H2O)2 (where acac− = acetylacetonate and hfac− = 1,1,1,5,5,5-hexafluoroacetylacetonate), the first true 3d-4f metal-complex cocrystal with the composition of [Cu(acac)2·Dy(hfac)3(H2O)2] that only consists of reactants, was acquired and structurally characterized. Magnetic investigations revealed that the 4f coformer Dy(hfac)3(H2O)2 does not show slow magnetic relaxation behavior even under an applied dc field of 2000 Oe down to 1.9 K, while the obtained 3d-4f cocrystal displays evident slow magnetic relaxation behavior associated with single-ion magnet (SIM) characteristic under the same dc field. This phenomenon is further supported and elucidated by the results of theoretical calculations together with the analyses of their crystal structures. Thus our results first indicate that 3d-4f co-crystallization technique is a useful tool for improving the magnetic performance of lanthanide complexes.

Complexes of Cobalt(II) Iodide with Pyridine and Redox Active 1,2-Bis(arylimino)acenaphthene: Synthesis, Structure, Electrochemical, and Single Ion Magnet Properties.

Complexes [(dpp-BIAN)0CoIII2]·MeCN (I) and [(Py)2CoI2] (II) were synthesized by the reaction between cobalt(II) iodide and 1,2-bis(2,6-diisopropylphenylimino)acenaphthene (dpp-BIAN) or pyridine (Py), respectively. The molecular structures of the complexes were determined by X-ray diffraction. The Co(II) ions in both compounds are in a distorted tetrahedral environment (CoN2I2). The electrochemical behavior of complex I was studied by cyclic voltammetry. Magnetochemical measurements revealed that when an external magnetic field is applied, both compounds exhibit the properties of field-induced single ion magnets.

Field-Induced Single-Ion Magnet Phenomenon in Hexabromo- and Hexaiodorhenate(IV) Complexes

Two mononuclear ReIV complexes of general formula (PPh4)2[ReX6] [PPh4+ = tetraphenylphosphonium cation, X = Br (1) and I (2)] have been prepared and structurally and magnetically characterised. Both compounds crystallise in the triclinic system with space group Pī. Their structures are made up of hexahalorhenate(IV), [ReX6]2−, anions, and bulky PPh4+ cations. Each ReIV ion in 1 and 2 is six-coordinate and bonded to six halide ions in a quasi regular octahedral geometry. In their crystal packing, the [ReX6]2− anions are well separated from each other through the organic cations, generating alternated anionic and cationic layers, and no intermolecular Re−X···X−Re interactions are present. Variable-temperature dc magnetic susceptibility measurements performed on microcrystalline samples of 1 and 2 show a very similar magnetic behaviour, which is typical of noninteracting mononuclear ReIV complexes with S = 3/2. Ac magnetic susceptibility measurements reveal the slow relaxation of the magnetisation in the presence of external dc fields for 1 and 2, hence indicating the occurrence of the field-induced single-ion magnet (SIM) phenomenon in these hexabromo- and hexaiodorhenate(IV) complexes.

Pentagonal Bipyramidal Ln(III) Complexes Containing an Axial Phosphine Oxide Ligand: Field-induced Single-ion Magnetism Behavior of the Dy(III) Analogues

A series of neutral homologous complexes [(L)Ln(Cy3PO)Cl] {where Ln = Gd (1), Tb (2), Dy (3), and Er (5)} and [(L)Dy(Ph3PO)Cl] (4) [H2L = 2,6-diacetylpyridine bis-benzoylhydrazone] were isolated. In these complexes, the central lanthanide ion possesses a pentagonal bipyramidal geometry with an overall pseudo D5h symmetry. The coordination environment around the lanthanide ion comprises of three nitrogen and two oxygen donors in an equatorial plane. The axial positions are taken up by a phosphine oxide (O donor) and a chloride ion. Among these compounds, the Dy(III) (3 and 4) analogues were found to be field-induced single-ion magnets.

Adducts of Tris(alkyl) Holmium(III) Showing Magnetic Relaxation.

In the series of the adducts of tris(alkyl) HoIII complexes, Ho(CH2SiMe3)3(THF)2 (1Ho-THF, Me = methyl) can exhibit slow magnetic relaxation under a zero applied direct current (DC) field with the energy barrier Δ/kB of 76 K, which is one of the highest in the non-Kramers ion HoIII-based single-ion magnets (SIMs). The DC field-dependence of relaxation time for 1Ho-THF indicates the occurrence of direct relaxation process at low temperature under certain DC fields. 1Ho-THF stands out in the series of 1Ln-THF (Ln = Tb, Dy, Ho, Er, Tm), wherein Dy congener is another SIM in the absence of a DC field with the relaxation barrier of 40 K, while Tb and Er congeners are field-induced SIMs. Further substitutions of the two trans-THF in 1Ho-THF by other neutral ligands such as quinuclidine, tricyclohexylphosphine oxide, and 3,5-lutidine afforded Ho(CH2SiMe3)3(quinuclidine)2 (2Ho-QN), Ho(CH2SiMe3)3(OPCy3)2 (3Ho-OPCy3), and Ho(CH2SiMe3)3(lutidine)3 (4Ho-Lut), respectively. The former two possess analogous structures to 1Ho-THF with two trans-arranged neutral ligands, and the latter one has three cis-lutidine coordinated. The fast quantum tunneling effect swamps the magnetic relaxation for the above three derivatives, so slow relaxation could not be observed under experimental conditions, even when a certain DC field was applied.

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 CoII 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.

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.

Slow magnetic relaxation in penta-coordinate cobalt(II) field-induced single-ion magnets (SIMs) with easy-axis magnetic anisotropy.

Two penta-coordinate complexes of the general formula [Co(Ln)(NCS)]ClO4, where L1 = {bis[(3,5-dimethyl-1H-pyrazol-1-yl)ethyl]-[(3,4-dimethoxypyridin-2-yl)methyl]}amine and L2 = {bis[(3,5-dimethyl-1H-pyrazol-1-yl)ethyl]-[(4-methoxy-3,5-dimethylpyridin-2-yl)methyl]}amine, have been synthesized and thoroughly characterized. Each of the cobalt(ii) atoms is penta-coordinated in the {CoN5} donor set with a distorted square-pyramidal geometry in [Co(L1)(NCS)]ClO4·MeOH (1), while the vicinity of the central atom can be described as a distorted trigonal-bipyramidal geometry in [Co(L2)(NCS)]ClO4 (2) as revealed using the SHAPE analysis. Differences in interatomic parameters among the cobalt(ii) and donor atoms in 1 and 2 have definite impact on the magnetic features of both compounds. The complexes show an easy-axis magnetic anisotropy (D = -38.5 cm-1 for 1 and D = -8.5 for 2), and both complexes reveal a large rhombicity with E/D = 0.21 for 1 and E/D = 0.29 for 2. The ZFS parameters (g, D and E) were also calculated using CASSCF/NEVPT2 methods and they are in good agreement with those determined from experimental data. A frequency dependent out-of-phase susceptibility has been observed in external magnetic field (Bdc = 0.1 T) revealing the following parameters of slow relaxation of magnetization for 1: energy of the spin reversal barrier, Ueff = 16.0 cm-1 (Ueff/kB = 23.0 K) and the relaxation time, τ0 = 1.28 × 10-6 s. In the case of complex 2, no maxima of frequency dependent out-of-phase susceptibility have been observed and thus, the value of Ueff = 17 cm-1 has been estimated using the expression Ueff = |D| × (S2 - 1/4). It has been demonstrated that the degree of substitution and the type of substituents on the pyridyl moieties of the tripodal ligands (L1 and L2) used in these penta-coordinate cobalt(ii) complexes have significant impact on structural and magnetic features.

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.

Tuning magnetic anisotropy by the π-bonding features of the axial ligands and the electronic effects of gold(i) atoms in 2D {Co(L)2[Au(CN)2]2}nmetal–organic frameworks with field-induced single-ion magnet behaviour

AuIatoms play an important role in determining the anisotropy of CoIInodes in 2D AuI–CoIIfield-induced SIMs.