Received 4th October 2015,Accepted 26th April 2016
DOI: 10.1039/c5dt03871e
www.rsc.org/dalton
Heterometallic trinuclear {CoIII2 Ln
III} (Ln = Gd, Tb,Ho and Er) complexes in a bent geometry.Field-induced single-ion magnetic behaviorof the ErIII and TbIII analogues†
Joydeb Goura,a Jamie Brambleby,b Craig V. Topping,c Paul A. Goddard,*b
Ramakirushnan Suriya Narayanan,d Arun Kumar Bard andVadapalli Chandrasekhar*a,d
Through the use of a multi-site compartmental ligand, 2-methoxy-6-[{2-(2-hydroxyethylamino)ethyl-
imino}methyl]phenol (LH3), the family of heterometallic, trinuclear complexes of the formula
[CoIII2 Ln(L)2(μ-O2CCH3)2(H2O)3]·NO3·xMeOH·yH2O has been expanded beyond Ln = DyIII to include GdIII (1),
TbIII (2), HoIII (3) and ErIII (4) for 1, 3 and 4 (x = 1; y = 1) and for 2 (x = 0; y = 2). The metallic core of these
complexes consists of a (CoIII–LnIII–CoIII) motif bridged in a bent geometry resulting in six-coordinated
distorted CoIII octahedra and nine-coordinated LnIII monocapped square-antiprisms. The magnetic
characterization of these compounds reveals the erbium and terbium analogues to display a field induced
single-ion magnetic behavior similar to the dysprosium analogue but at lower temperatures. The energy
barrier for the reversal of the magnetization of the CoIII2 Tb
III analogue is Ueff ≥ 15.6(4) K, while for the
CoIII2 Er
III analogue Ueff ≥ 9.9(8) K. The magnetic properties are discussed in terms of distortions of the 4f
electron cloud.
Introduction
Heterometallic polynuclear complexes, particularly those con-taining 3d/4f metal ions, have been receiving increasing atten-tion in recent years, especially because of their interestingmagnetic properties arising out of the interaction between theparamagnetic 3d and 4f metal ions suggesting possible appli-cations in information storage, magnetic refrigeration andquantum computing.1 Members of this family have beenshown to be single-molecule magnets (SMMs) and are particu-larly intriguing as an avenue towards high density informationstorage and spintronics.1 Furthermore, they are also known to
display the property of quantum tunnelling of magnetization(QTM).1 A recent development in this area has been the realiz-ation that replacement of the paramagnetic 3d metal ion by adiamagnetic species can lead, in some instances, to complexeswith better magnetic properties.2 In such cases, where a SMMbehaviour is seen, the complexes are called single-ion-magnets(SIMs) since their magnetic properties are essentially deemedto be derived from the lanthanide metal ion.2 Theoreticalstudies, by Rajaraman3 and co-workers have revealed that therole of the diamagnetic metal ion is to make the oxide ligandsmore hard and hence allow a tighter binding with the lantha-nide metal ion causing a substantial separation betweenthe ground and excited electronic states of the latter. Pre-viously, we have utilized the phosphorus-supported ligandSP[N(Me)NvCH–C6H3–2-OH–3-OMe]3 for the preparation ofheterometallic trinuclear complexes (Chart 1) which showedSMM properties.4 Interestingly, in these complexes, thearrangement of the trimetallic core is perfectly linear. Also the3d metal ions are paramagnetic. More recently, in a prelimi-nary communication, we reported a heterometallic {CoIII2 Dy}complex, containing diamagnetic CoIII ions that showed aSMM behaviour.5 We now report the full details of thesestudies. Accordingly, the synthesis, structure and magnetismof [CoIII2 Ln(L)2(μ-O2CCH3)2(H2O)3]·NO3·xMeOH·yH2O [Ln =
†Electronic supplementary information (ESI) available: Additional figures,BVS calculation table and bond length and bond angle tables. CCDC1429323–1429326 for 1–4. For ESI and crystallographic data in CIF or other elec-tronic format see DOI: 10.1039/c5dt03871e
aDepartment of Chemistry, Indian Institute of Technology Kanpur, Kanpur-208016,
IndiabDepartment of Physics, University of Warwick, Gibbet Hill Road, Coventry, CV4 7AL,
UK. E-mail: [email protected] of Physics, Clarendon Laboratory, University of Oxford, Oxford,
Parks Road, OX1 3PU, UKdNational Institute of Science Education and Research Bhubaneswar, P.O. Jatni,
GdIII (1), TbIII (2), HoIII (3) and ErIII (4) for 1, 3 and 4 (x = 1;y = 1) and for 2 (x = 0; y = 2)] are reported herein. All thecomplexes (1–4) possess a bent trimetallic {CoIII2 Ln} motif.Among these complexes, the ErIII and TbIII analogues show afield-induced SIM behavior. In general, SMMs containing ErIII
are quite rare.6
Experimental sectionReagents and general procedures
Solvents and other general reagents used in this work werepurified according to standard procedures.7 Co(OAc)2·4H2O,N-(2-hydroxyethyl)ethylenediamine and o-vanillin were obtainedfrom S.D. Fine Chemicals, India. Gd(NO3)3·6H2O, Tb(NO3)3·5H2O,Ho(NO3)3·5H2O and Er(NO3)3·5H2O were obtained from SigmaAldrich (USA) and used as received. The ligand 2-methoxy-6-[{2-(2-hydroxyethylamino)ethylimino}methyl]phenol (LH3) wasprepared by a reported procedure.5,8
Instrumentation
Melting points were measured using a JSGW melting pointapparatus and are uncorrected. IR spectra were recorded asKBr pellets on a Bruker Vector 22 FT IR spectrophotometeroperating at 400–4000 cm−1. Elemental analyses of the com-pounds were obtained from the Thermoquest CE instrumentsCHNS-O, EA/110 model. Electrospray ionization mass spec-trometry (ESI-MS) spectra were recorded on a MicromassQuattro II triple quadrupole mass spectrometer. 1H NMRspectra were recorded in CDCl3 solutions on a JEOL JNMLAMBDA 400 model spectrometer operating at 400.0 MHz.Chemical shifts are reported in parts per million (ppm) andreferenced with respect to internal tetramethylsilane (1H).
DC magnetometry
Powdered samples of Co2Gd, Co2Tb, Co2Ho and Co2Er weredispersed in Vaseline to prevent the samples from moving andplaced inside a gelatin capsule. The capsule was fixed inside aplastic drinking straw and attached to the end of a brass rod.The sample’s magnetic moment (M) was then measured in anapplied field of μ0H = 0.1 T for temperatures in the range 1.8 ≤T ≤ 300 K, with a Quantum Design SQUID magnetometer. Inthe linear limit, the molar susceptibility (χmol) was deducedfrom this measurement using χmol = M/nH, where n is thenumber of moles of the sample. An isothermal measurementof the magnetic moment at 2 K was also recorded for appliedfields up to μ0H = 7 T.
AC susceptibility
AC susceptibility measurements were performed on powderedsamples. For Co2Gd measurements were recorded on coolingfrom 10 K with an AC measurement system inserted into aQuantum Design Physical Property Measurement System atthe University of Oxford. An AC field of amplitude μ0HAC =0.4 mT and frequencies in the range 15 ≤ f ≤ 1500 Hz wasapplied, and data were recorded in DC fields of μ0HDC = 0 and0.1 T. In the case of Co2Er, the frequency range was extendedto 10 kHz to investigate the onset of SIM behaviour at lowtemperatures. This behaviour was also repeatable with areduced AC-field amplitude, indicating that no sample heatingoccurred on the application of the AC-field for the measure-ments presented in this study.
For Co2Tb, AC measurements were performed in aQuantum Design PPMS at the Materials Characterisation Lab-oratory at the ISIS Neutron and Muon Source. Data wererecorded in the frequency range 15 ≤ f ≤ 10 000 Hz for μ0HAC =0.4 mT. Through a field-dependent study of the AC suscepti-bility at 2 K, we found it necessary to increase the DC-fieldabove 0.1 T to induce maxima in the out-of-phase suscepti-bility component within the experimentally accessible temp-erature range. The temperature dependence of this sample wastherefore performed in an applied field of μ0HDC = 0 T and0.15 T. The low temperature SIM behaviour observed in thelatter case was reproducible when recorded with a reduced AC-field amplitude of 0.1 mT.
For the measurements performed in the PPMS, wait timesextending up to 45 s were included following a change in theAC frequency during the measurement, to ensure reliability ofthe data across the measured frequency range.
For Co2Ho, measurements were made with a QuantumDesign SQUID magnetometer at the University of Oxford. Thesample exhibited no field-induced slow relaxation for frequen-cies in the range 10 ≤ f ≤ 1000 Hz and amplitude μ0HAC =0.4 mT, and with DC fields of μ0HDC = 0 and 0.1 T.
X-ray crystallography
Single-crystal X-ray structural studies of 1–4 were performed ona Bruker SMART APEX CCD diffractometer equipped with anOxford low-temperature attachment. Data were collected using
a graphite-monochromated MoKα radiation (λα = 0.71073 Å).The crystals did not degrade/decompose during the data col-lection. Data collection and reduction were performed usingthe SMART and SAINT programs respectively.9a–c The structuresolving and refinement were performed using SHELXT andSHELXL-2014/7 programs9d,e of the WinGX software packa-ge.9f–h Refinements were carried out with full-matrix least-
squares methods against F2. All of the hydrogen atoms werefixed at idealized positions and a riding model was used. Thebest fit models for the current data sets are satisfactorily good.In spite of our best efforts to obtain the best quality data, theCH2CH2 moieties of the ligand and the interstitial solventmolecules are highly disordered. Hence, we had to employrestrains/constrains for disorder modeling. One of the C atoms
Table 1 Crystal data and structure refinement parameters of 1–4
b (wR2c) 0.0365 (0.1036) 0.0601 (0.1873) 0.0350 (0.097) 0.0432 (0.1194)
a The formula and the formula weights include the un-modelled H atoms and the masked solvent molecules also. b R1 = ∑|Fo − Fc|/∑Fo.c wR2 =
∑[w(Fo2 − Fo
2)]2/w(Fo2)2]1/2.
Fig. 1 ORTEP diagrams (with 50% ellipsoid probability) of 1–4. The atoms of the asymmetric units are labelled. Disordered parts/H atoms/anion/solvent molecules are omitted for clarity. Colour codes: Gd = cyan; Tb = orange; Ho = yellow; Er = pink, Co = green, O = red; N = blue and C =light-grey.
in each of the CH2CH2 moieties is highly disordered as athermal ellipsoid and hence its electron density was parti-tioned into two positions. In 2, one such C atom with partialoccupancy is modelled with isotropic displacement refine-ment. For each complex, the asymmetric unit contains onehalf-occupied NO3
− counter anion which is intertwined withone disordered water molecule. The intertwined disorderedwater molecule possesses four sites. The remaining solventmolecules are too disordered to locate their positions andhence are squeezed out using the PLATON/SQUEEZE pro-gram.9i The solvent accessible void volumes and possiblesqueezed out electron counts are described in the corres-ponding CIFs. The crystallographic data in CIF formats areprovided in the ESI.† Crystal data and refinement parametersare summarized in Table 1. The selected bond parameters aresorted in Table S1 (ESI†). The ORTEP diagrams of all the com-plexes are portrayed in Fig. 1 and the representative asym-metric unit is as shown in Fig. 2. All of the mean planeanalyses and molecular drawings were obtained usingDiamond (version 3.2k).9j
SynthesisGeneral procedure for the synthesis of metal complexes 1–4
A general protocol was employed for the synthesis of the metalcomplexes (1–4) as follows. LH3 (1 eq.) was dissolved in metha-nol (20 mL). Ln(NO3)3·nH2O (1 eq.) was added to this solution.The reaction mixture was stirred for 30 minutes. At this stage,Co(OAc)2·4H2O(2 eq.) and tetramethyl ammonium hydroxide(Me4NOH) (2 eq.) were added to the reaction mixture andstirred for a further 12 hours at room temperature to afford aclear brown solution. The solvent was removed from the reac-tion mixture and the residue obtained was washed with diethylether, dried, re-dissolved in methanol, filtered and kept in avial for crystallization. After 15 days, brown block-shaped crys-tals suitable for X-ray diffraction were isolated. Specific detailsof each reaction and the characterization data of the productsobtained are given below.
Compartmental ligands containing o-vanillin have been shownto be effective for the preparation of both heterometallic (3d/4f) or homometallic (3d and 4f) complexes.10 The multi-sitecoordination ligand, derived from o-vanillin, 2-methoxy-6-[{2-(2-hydroxyethylamino)ethylimino}methyl]phenol (LH3) con-tains five coordination sites: one phenolic oxygen, one –OMe,one imino nitrogen and one secondary amine nitrogen alongwith a terminal, flexible, –NCH2CH2OH arm. An examination
Fig. 2 Asymmetric unit of 1. The coordinated atoms and metal centresare highlighted with a ball-and-stick model. Hydrogen atoms and otherdisordered part/anion/solvent molecules are omitted for clarity. Colourcode: C = grey.
of this ligand reveals the presence of two distinct coordinationbinding sites. While one of these sites is expected to prefer atransition metal ion, the other site, owing to the presence ofthe –OMe group is likely to bind to a lanthanide metal ion.This hypothesis was successfully tested by us earlier andinvolved the preparation of heterometallic {FeIII4 LnIII
2 } 11 and{CoIII2 DyIII} 5 complexes. Accordingly, the sequential reaction of2-methoxy-6-[{2-(2-hydroxyethylamino)ethylimino}methyl]-phenol (LH3) with various lanthanide metal salts followedby the reaction with Co(OAc)2·4H2O in a 1 : 1 : 2 stoichio-metric ratio in the presence of tetramethylammoniumhydroxide (Me4NOH) afforded trinuclear heterobimetallicCo2Ln complexes with the general formula, [CoIII2 Ln(L)2-(μ-O2CCH3)2(H2O)3]·NO3·xMeOH·yH2O [Ln = GdIII (1), TbIII (2),HoIII (3) and ErIII (5)] for 1, 3 and 4 (x = 1; y = 1) and for 2 (x = 0;y = 2) (Scheme 1). The presence of CoIII in these complexes isattributed to the ready aerial oxidation of CoII.
ESI-MS spectra of 1–4 reveal that they retain their molecularintegrity in solution as indicated by the presence of parention peaks at m/z (mass/charge ratio): 1014.0833, [CoIII2 Gd-(L)2(μ-O2CCH3)2(H2O)3 + 3CH3OH]+; 960.0533, [CoIII2 Tb-(L)2(μ-O2CCH3)2(H2O)3 + CH3CN]
+ respectively. The ESI-MS spec-trum of 1, as a representative example, is given in Fig. 3a and band those of the other complexes 2–4 are given in the ESI.†
Molecular structures of 1–4
The molecular structures of 1–4 were determined by single-crystal X-ray crystallography which reveals that these are iso-structural. All the complexes crystallized in the monoclinicsystem, C2/c (Z = 4). The crystallographic parameters of thesecomplexes are given in Table 1. All the complexes are mono-cationic and contain nitrate counter anions. In view of the struc-tural similarities of these complexes, the molecular structureof 1 is described as a representative example. The structuraldetails of all other compounds are given in the ESI.†
The asymmetric unit of 1 contains half of the total mole-cule, namely, [CoGd0.5(L)(μ-OAc)(OH2)1.5] (Chart 2 and Fig. 2).The molecular structure of 1 is given in Scheme 1 and Fig. 1.The various coordination modes of all the participatingligands in 1–4 are summarized in Chart 3. The trimetallic coreof the complex contains a bent {CoIII2 GdIII} motif with the GdIII
ion placed in the centre and connected on either side to a CoIII
through the intervention of various bridging ligands viz.,[NCH2CH2O]
−, a phenoxide and an acetate. The coordinationaround CoIII is completed by two ethylenediamino nitrogenatoms and a water molecule. The coordination around thecentral lanthanide is completed by the methoxide groups andone water molecule. It is interesting to compare the structuralfeatures of the current family of {CoIII2 LnIII} complexes with thepreviously known {CoII2Ln
III} 4 complexes. In the latter the CoII
ion has a 3N,3O coordination environment while the lantha-Scheme 1 Synthesis of the trinuclear heterometallic complexes 1–4.
Fig. 3 (a) ESI-MS spectrum of complex 1 (experimental) and (b) simulated isotopic pattern for the parent ion peak of 1.
nide ion has a 12O environment. Also, in the latter, the centralLnIII is connected to the peripheral CoIII through three phen-oxide bridges (Chart 1 and Table 2). The metric parametersinvolved in the current instance and in the literature examplesare different and these values are summarized in Table 2.From these data we can see that the Co–N and Co–O bond
distances are shorter in the current instance, because CoIII isinvolved, while in the reported case, because CoII is present,these distances are longer. On the other hand, Ln–O bond dis-tances in the current instance and in the literature precedentsare very similar. The most important variation, however, is inthe geometry of the Co–Ln–Co motif. While in the presentinstance the geometry is bent and the Co–Ln–Co bond anglesare in the range, 99.75(5)–100.81(4), in [L′CoIILnCoII′L] (L′H3 =SP[N(Me)NvCH–C6H3–2-OH–3-OMe]) the Co–Ln–Co geometryis perfectly linear. This difference is mainly because of thenature of the ligands involved. In the case of the phosphorus-supported ligand system the three hydrazone arms anchoredto the phosphorus center force a linear geometry. The moreflexible ligand, in the current instance, allows a more relaxedbent geometry.
A further description of the molecular structure of 1 is inorder. Thus, it may be noticed that on either end a [L]3− ligandholds a dinuclear Gd–Co motif together. Thus, the methoxygroup binds to the GdIII centre (Gd1–O1, 2.687(3) Å) while thephenolate bridges a CoIII and the GdIII ions [Co1–O2, 1.878(2)Å; Gd1–O2, 2.385(2) Å]. An imino nitrogen is bound to a CoIII
center [Co1–N1, 1.875(3) Å]. Interestingly, the secondary aminemotif of the ligand is deprotonated in the presence of tetra-methylammonium hydroxide and the resulting coordinationcenter binds exclusively with CoIII [Co1–N2, 1.903(3) Å]. Inaddition, the asymmetric unit contains one η1,η1 coordinatedacetate bridged ligand that bridges the Co/Gd pairs [Co1–O5,1.923(3) Å; Gd1–O4, 2.360(3) Å]. Another interesting obser-vation is that the terminal, flexible, –NCH2CH2OH arm (ethylene-diamine motif ) of the ligand is deprotonated under thereaction conditions and helps in bridging CoIII and GdIII
[Co1–O3, 1.898(3) Å; Gd1–O3, 2.389(3) Å]. The oxidation state ofCoIII ions was confirmed by BVS analysis (ESI†).12 Thus the twoCoIII centers in 1 are equivalent and hexacoordinate (2N,4O) ina distorted octahedral geometry (Fig. 4a) and the GdIII center
Chart 2 Asymmetric unit of complex 1.
Chart 3 Coordination binding modes of L3− and MeCO2− ligands in the
present study.
Table 2 Comparison of bond lengths (Å) and bond angles (°) between the trinuclear [CoIII–LnIII–CoIII] complexes reported here and the earlierreported [CoII–LnIII–CoII] (ref. 4) complex
is nine-coordinated in a distorted monocapped square anti-prism geometry (Fig. 4b). A mean plane analysis of core 1reveals that two CoIII and GdIII ions are present in the sameplane (Fig. 5) and are arranged in the vertices of an isoscelestriangle. The CoIII–CoIII bond distance is 5.086(2) Å while theCoIII–GdIII distance is 3.325(1) Å.
Magnetic studies
DC magnetometry. The DC susceptibility of each sample,plotted as χmolT vs. T, is shown in Fig. 6a. The data have beencorrected for core diamagnetism, estimated from the appropri-ate Pascal’s constants.13 In the high temperature limit, thesamples have approximately constant values of χmolT indicat-ing that the moments are in the paramagnetic phase. Thevalues of χmolT at 300 K are within approximately 10% of thatexpected for independent lanthanide ions, μ0NAμeff
2/3kB, whereμeff = gJ[J ( J + 1)]1/2 μB is the effective moment (gJ is the Landég-factor and J is the total-spin quantum number). This indicatesthat the magnetism in each sample is due to the lanthanideions alone and that the octahedrally coordinated CoIII ions arein the low-spin (S = 0) state. The difference in the measuredand expected moments is representative of the error in deter-mining the mass of the powdered samples after dispersingthem in Vaseline, and may also include a small diamagneticcontribution to the measurement from the sample holder andVaseline.
For Co2Gd, the flat response of χmolT down to low tempera-tures is indicative of paramagnetic moments. This is character-istic of GdIII ions, in which the ground-state 4f 8S7/2 multiplet
has a zero angular-momentum component and, as a result, aspherically symmetric electron distribution. The anisotropy ofmoments in lanthanide complexes arises from the lack ofspherical asymmetry of the lanthanide ion’s 4f electron wavefunctions in the presence of the ligand field,14 which thereforeimplies that the GdIII moments will exhibit a Heisenbergbehaviour.
If exchange interactions were present between GdIII ions,χmolT would deviate away from a horizontal line (see e.g. ref.15). The flat form of χmolT therefore also suggests thatexchange interactions between the GdIII ions are weak. Giventhat GdIII contains the maximum number of unpaired elec-trons in the 4f orbitals of any lanthanide ion, the implicationis that spin-density may become delocalised in more directionsin the crystal compared to the TbIII, HoIII and ErIII congeners.This suggests that each of these remaining members of thisisostructural series is also likely to have weak exchange inter-actions between lanthanide ions, such that the magnetic pro-perties of Co2Tb, Co2Ho and Co2Er are dominated by single-ion behaviour.
In the case of TbIII, HoIII and ErIII, the product χmolT beginsto decrease from its high temperature value below about100 K. This is attributed to the thermal depopulation of thecrystalline-electric field (CEF) splitting of the lanthanideenergy levels permitted by the non-zero orbital angularmomentum in these ions, and the reduced symmetry of the
Fig. 4 Coordination geometry around the metal centers (a) CoIII centerdistorted octahedron and (b) GdIII center distorted monocapped squareanti-prism geometry in 1.
Fig. 5 Mean planes of 1. Isosceles triangular arrangement of the metalions.
Fig. 6 (a) The product χmolT vs. temperature, T, for Co2Gd, Co2Tb,Co2Er and Co2Ho. The GdIII sample remains paramagnetic down to 2 K,whilst for the remaining samples, χmolT is approximately constant atroom temperature and begins to decrease below about 100 K, which weattribute to the thermal depopulation of crystalline electric field splitlevels in the lanthanide ions. A diamagnetic correction of −6.0 × 10−9
m3 mol−1 for 1, and −6.1 × 10−9 m3 mol−1 for 2–4 was subtracted fromthe susceptibility data, estimated from ref. 13. (b) Magnetization (in Bohrmagnetons per lanthanide ion) vs. applied field for Co2Gd, Co2Tb,Co2Ho and Co2Er at 2 K. The solid line is a fit to a Brillouin function. Thedotted lines are a guide to the eye.
electron distribution that results. We found this depopulationto be most pronounced for Co2Ho. For the ErIII sample, theshape of χmolT is similar to that published5 for the isostruc-tural DyIII compound. This result arises from the fact that bothmaterials are Kramers ions (with total-spin angular momen-tum J = 15/2), a property which necessitates a bistable ground-state in each ErIII and DyIII ion, implying that at low tempera-tures the magnetic properties in both samples will tendtowards the behaviour of a two-level system.
The non-Kramers members of the series, TbIII ( J = 6) andHoIII ( J = 8), do not necessarily exhibit a ground-state doublet.However, the shape of χmolT for Co2Tb resembles that of theKramers ion samples in the series, whereas the shape forCo2Ho is distinct in that χmolT decreases rapidly for tempera-tures below about 25 K. This result suggests that the ligandenvironment around TbIII promotes a degenerate ground-stateand hence a similar temperature dependence of the magneticmoment as compared to DyIII and ErIII. The pronounced down-turn in χmolT below 25 K for the HoIII sample could indicatethat the energy level distribution is different as compared toTbIII, allowing for the possibility of a singlet ground state inHoIII. We test this description of the lowest lying energy levelswith low temperature AC susceptibility measurements (seebelow).
The DC magnetization of the four samples at 2 K is dis-played in Fig. 6b. By comparing the measured moment parallelto the field at μ0H = 7 T (of these powdered samples), to theexpected saturation moment (Table 3), we find significantlyreduced moments per ion in the case of ErIII, TbIII and HoIII.This supports the presence of single-ion anisotropy on anenergy scale much larger than the Zeeman energy from theapplied field, in agreement with the χmolT data.
For Co2Gd, the magnetization (M) has been fitted to a Bril-louin function (see for instance ref. 15), and since the orbitalangular momentum is expected to be zero, the g-factor wasfixed to g = 2. The resultant value of the total angular momen-tum was found to be J = 3.3, which is close to the value of 7/2expected for GdIII assuming the orbital moment to be zero forthis complex. This result reinforces the conclusion that thesystem is paramagnetic down to 2 K, whereas the low tempera-ture magnetization of the remaining samples cannot be accu-rately represented with a paramagnetic model.
AC susceptibility. The zero field AC susceptibility for Co2Gdis shown in Fig. 7a and b. The in-phase component of suscep-tibility (χ′) remained paramagnetic in form for temperaturesdown to 2 K and AC-frequencies up to 1500 Hz, which isemphasised by the fact that the out-of-phase component (χ″) iszero under these conditions. This implies that there is noenergy barrier for changing the direction of the GdIII
moments, which further supports the suggestion that thismaterial lacks single-ion anisotropy. Applying a μ0HDC = 0.1 Tfield induced a non-zero χ″ as the temperature is lowered(Fig. 7c and d). This effect becomes more pronounced, andpersists to higher temperatures, when the frequency of theoscillating field is increased.
The effect of the DC-field on the measured AC susceptibilitycan be explained within the paramagnetic model. For a para-magnetic spin system in thermal equilibrium, a DC-field intro-duces anisotropy to the Hamiltonian and lifts the degeneracyof the mJ states of a free ion to produce a series of Zeemanlevels (see e.g. ref. 15) populated with a Boltzmann distributionacross the sample (here mJ is the z-component of the totalangular momentum where the z-direction is defined to be par-allel to the field). When an AC-field is applied to the system,the total-field at each ion continuously changes. This pertur-bation induces a net portion of the moments to reorient, so asto occupy different mJ-levels and return the system to equili-brium.16 On a local level, such a process can occur via inter-actions of the GdIII ions with phonons.16 As the temperature islowered, the reduced probability of phonon interactionsincreases the time taken for the system to reach a new equili-brium state as the field changes, leading to an out-of-phasesusceptibility. The temperature at which this effect becomesmeasurable is expected to be higher for a larger AC-frequency,
Table 3 The saturation moment, gJJ, expected for GdIII, TbIII, ErIII andHoIII compared to the measured moment at 2 K and 7 T, M (μ0H = 7 T).In the absence of interactions between the lanthanide ions, the obser-vation that the moment at 7 T is significantly less than the saturation forCo2Tb, Co2Ho and Co2Er indicates the presence of a large magneticanisotropy in these samples compared to the energy scale of theapplied field
Sample gJJ (μB) M (μ0H = 7 T) (μB)
Co2Gd 7 6.5Co2Tb 9 4.4Co2Ho 10 4.7Co2Er 9 4.4
Fig. 7 AC susceptibility vs. temperature for Co2Gd. In-phase (a) andout-of-phase (b) susceptibility vs. temperature with μ0HDC = 0 T andμ0HAC = 0.4 mT. (Note the change in scale.) In-phase (c) and out-of-phase (d) components vs. temperature under the application of a DC-field μ0HDC = 0.1 T. The colours indicate data collected with differentfrequencies for the oscillating field.
since the short time-scale of the oscillating field requires a fastrelaxation mechanism for the spins to remain in phase withthe AC-field.
The AC susceptibility of Co2Er, recorded in a zero-appliedDC-field, is shown in Fig. 8a and b. On cooling, the in-phasecomponent continuously rises, and below 5 K the out-of-phasecomponent shows that the AC-response is frequency-depen-dent, such that χ″ tends to be enhanced with increasing AC-fre-quency. At 2 K, however, χ″ begins to decrease for f ≥ 8000 Hz.Overall, the small out-of-phase component and rising in-phasecomponent are indicative of a fast relaxation mechanism forthe ErIII moments. In addition, this relaxation rate is fielddependent since the application of a DC-magnetic fieldinduces χ′ to exhibit a maximum, which tends to move tolower temperatures as the frequency of the AC-frequency isreduced (Fig. 8c). Furthermore, χ″ is significantly increasedfrom the corresponding zero-field value (Fig. 8d), and for fre-quencies f ≥ 3000 Hz, a maximum in χ″ is observed.
Since ErIII is a Kramers ion, we compare the AC suscepti-bility to that of the isostructural DyIII compound,5 a knownSIM for which maxima in χ′ are also observed as the sample iscooled. These maxima occur at a temperature for which χ″ isincreasing, and the out-of-phase component only showed amaximum itself as the temperature was cooled further. Thesimilarity of the AC susceptibility of Co2Er to that of Co2Dy isgood evidence for SIM behaviour in the new ErIII material.
For ErIII ions in zero-field, there are two main competingrelaxation pathways for magnetization: (i) the quantum tunnel-ling of magnetization (QTM) between the two degenerateground-states;17 and (ii) an Orbach process in which an ErIII
ion in one of the ground-states absorbs a phonon to overcomea thermal barrier, then emits a second phonon to decay to thedegenerate ground state on the other side of the barrier.18,19 Inzero field, QTM often dominates the relaxation of moments,17
and this acts to suppress the out-of-phase component in ameasurement of the AC susceptibility. When a DC-field isapplied, the degeneracy of the ground state doublet is lifted,reducing the relaxation rate due to QTM and consequentlyincreasing the influence of the relaxation process mediated byphonons, giving rise to the slow relaxation of moments at lowtemperatures.
To determine the number of active relaxation pathwayspresent over the temperature range measured for the data col-lected with μ0HDC = 0.1 T, a Cole–Cole plot of χ′ vs. χ″ for Co2Eris constructed (Fig. 9). This shows that isotherms in the χ′–χ″
plane appear as single arcs, which become larger as thesample is cooled. This shape is indicative of phonon-relaxationvia one relaxation pathway (see for instance ref. 18), and is ana-logous to the DyIII sample.5
The data at 2.0, 2.6 and 3.0 K have been collated fromrepeat experiments, combining measurements recorded attemperatures which vary by ±100 mK along each of the respec-tive arcs. Grouping the data in this way allows each arc to befitted to the modified Debye model,20 for which
where χT is the isothermal susceptibility (measured in the DC-limit), χS is the adiabatic susceptibility (measured at high fre-quencies) and alpha measures the distribution of the relax-ation time.21 This form is expected to be obeyed for SIMs witha single relaxation pathway. The fits (solid lines in Fig. 9) are agood representation of the data and further support the sug-gestion of SIM behaviour at low temperatures.
Having established a single phonon-assisted relaxationpathway, Arrhenius analysis can be used to estimate the size of
Fig. 8 (a) In-phase susceptibility vs. temperature for Co2Er in a zero DCfield. (b) Corresponding out-of-phase susceptibility vs. temperature forCo2Er. (c) In-phase susceptibility vs. temperature for Co2Er in an appliedDC field, μ0HDC = 0.1 T. (d) Corresponding out-of-phase susceptibilityvs. temperature for Co2Er. The colours indicate data collected withdifferent AC-frequencies.
Fig. 9 The Cole–Cole plot of the AC susceptibility data for Co2Er col-lected in μ0HDC = 0.1 T. For each isotherm, frequency increases onmoving from the isothermal limit, where χ’ is greatest, towards the adia-batic limit as χ’ decreases. The solid lines are free fits to eqn (1). Thefitted value of χS was found to be approximately temperature indepen-dent, with an average value of χS = 0.42(2) × 10−5 m3 mol−1, whilst α
varied from 0.2 to 0.1 for temperatures from 2.0 to 3.0 K.
the barrier to relaxation, Ueff. To do so, the temperature depen-dence of the relaxation time τ can be extracted from the databy applying the modified Debye model. Within this model, themaximum in χ″ vs. T (at constant f ) occurs once the relaxationtime satisfies the condition τ = 1/(2πf ).16 Strictly, this is truewithin the Debye model if χT is assumed to be temperatureindependent,16 which is a reasonable first approximationgiven that the peaks are observed over a narrow temperaturewindow.
The maxima in χ″ vs. T from Fig. 8d are defined to be thetemperature at which dχ″/dT = 0, whilst the main source oferror arises from the discreteness of the data. This error is esti-mated as half of the interval between the two measured datapoints which fall on either side of the maximum as definedabove.
A plot of the logarithm of the relaxation time vs. the inversetemperature of the peak in χ″ is shown in Fig. 10. The data canbe modelled with the Arrhenius law, given by18,19
ln τ ¼ ln τ0 þ Ueff
kBTð2Þ
where τ0 is the relaxation time measured in the high tempera-ture limit. This law will be obeyed at high temperatures, wherethe high probability of phonon interactions ensures that theOrbach relaxation mechanism dominates the sample’sresponse to an AC-field. We expect that as the temperature islowered, the relaxation rate will tend towards a tempera-ture independent value as QTM becomes the prominentmechanism.
The measured relaxation time tends to increase on cooling,ruling out the scenario where QTM dominates the relaxationprocess. However, since the peaks in χ″ are only measured overa narrow temperature range, this allows the data to be in tworegimes: (i) a temperature window in which the Orbachprocess alone dominates, and (ii) an intermediate regionbetween the two extremes where either phonon relaxation orQTM processes dominate.
This situation limits the quantitative conclusions that canbe made. To emphasise this, we present two linear fits to thedata, and the results are presented in Table 4. In linear fit 1,the high temperature portion of the data is fitted. On includ-ing the low temperature peaks in linear fit 2, the fitted value ofthe energy barrier is reduced, whilst the relaxation timeincreases. Higher AC-frequencies would be needed to differen-tiate between cases (i) and (ii) outlined above. This would pushthe peaks in χ″ to higher temperatures, and help determinewhether the gradient of ln τ vs. 1/T would become steeper, witha smaller y-axis intercept, if the Arrhenius plot was extended toa lower 1/T. Given the experimentally available temperaturesand frequencies, we therefore conclude that Co2Er has τ0 ≤ 8 ×10−7 s, and Ueff ≥ 9.9(8) K.
The AC susceptibility components of Co2Tb, χ′ and χ″, wereinvestigated as a function of the applied field at 2 K [Fig. 11aand b]. In contrast to Co2Er, the data show that the out-of-
Fig. 10 Logarithmic plot of the relaxation time τ versus inverse temp-erature for Co2Er in a DC field of μ0HDC = 0.1 T, as measured from theposition of the peak in χ’’ vs. T in Fig. 8d. The data were fitted to eqn (2)over two temperature ranges and the results are given in Table 4.
Table 4 Results of fitting the data in Fig. 10 to eqn (2) over two temp-erature ranges
Fit ln τ0 τ0 (s) Ueff/kB (K)
Linear fit 1 −14.1(3) ≈8 × 10−7 9.9(8)Linear fit 2 −13.4(2) ≈2 × 10−6 8.0(5)
Fig. 11 (a) In-phase susceptibility vs. applied DC-field for Co2Tb at 2 K for selected frequencies in the range 20 ≤ f ≤ 10 000 Hz. (b) Out-of-phasesusceptibility vs. applied DC-field for Co2Tb. χ’’ exhibits a maximum as a function of frequency for fixed applied DC fields μ0HDC ≥ 0.1 T. (c) TheCole–Cole plot of Co2Tb collected in μ0HDC = 0.15 T. Isotherms for T ≤ 3.5 K have been fitted to eqn (1), with the resultant parameters displayed inTable 6.
phase susceptibility of the TbIII sample does not show a strongmaximum as a function of frequency up to 10 kHz in a fixedapplied DC-field of μ0HDC = 0.1 T (dotted line 1). On increasingthe DC-field beyond this point, the measurement of χ″ at 10kHz reduces most rapidly such that χ″ exhibits a maximum asa function of frequency in a constant field. For μ0HDC = 0.15 T(dotted line 2), a strong signal can be measured, which showsa maximum in χ″ vs. f at the lowest temperatures accessible. Asa result, this field is used to investigate the temperature depen-dence of the sample.
In a zero applied DC-field, the in-phase susceptibility risescontinuously on cooling [Fig. 12a]. This measurement is fre-quency independent, evident from a negligible out-of-phasecomponent across the temperature range investigated andhighlights that the system exhibits a fast relaxation of the TbIII
moments [Fig. 12b]. In an applied-field of μ0HDC = 0.15 T[Fig. 12c and d] a large out-of-phase component is induced.This is consistent with a model in which the sample exhibits adoubly degenerate ground state, such that QTM dominates therelaxation process in zero-field. This fast relaxation is alle-viated by the DC-field, allowing SIM behaviour to be observed.The applied field is large enough to induce peaks in χ″ atexperimentally accessible temperatures and are observedbelow 3.5 K.
The maxima in χ″ from these constant frequency measure-ments were extracted through differentiating the data (asdescribed above) and an Arrhenius plot of the sample relax-ation time is shown in Fig. 13. The data were fitted to eqn (2)over two separate temperature ranges, and the fitted para-meters are displayed in Table 5. To aid in the interpretation ofthese results, a Cole–Cole plot is constructed [Fig. 11c] from
constant temperature cuts to the AC susceptibility data inFig. 12c and d.
On cooling to 3 K, the arcs mapped out by isotherms in theχ′–χ″ plane become increasing large in a comparable mannerto Co2Er and Co2Dy,
5 whilst below 2.5 K they begin to contract.To investigate this behaviour, the data below 3.5 K were fittedto eqn (1) and the resultant fitted parameters are given inTable 6.
The four isotherms in the range 2 to 3.5 K are well rep-resented with the modified Debye model, indicating that thesample behaves as a SIM with a unique thermally activatedrelaxation pathway. The value of α, which models the distri-bution of the relaxation time, increases on cooling. Given this,and combining it with the observation that the out-of-phasecomponent is reduced at low temperatures, indicates that thesample behaviour below 2.5 K is increasingly influenced by anadditional non-thermally activated relaxation mechanism,such as QTM. This is known to be present in the sample fol-lowing the discussion of Fig. 12.
Fig. 12 AC susceptibility vs. temperature for Co2Tb. (a) In-phase sus-ceptibility vs. temperature for Co2Tb in a zero DC field. (b) Corres-ponding out-of-phase susceptibility vs. temperature for Co2Tb. (c) In-phase susceptibility vs. temperature for Co2Tb in an applied DC field,μ0HDC = 0.15 T. (d) Out-of-phase susceptibility vs. temperature forCo2Tb, with μ0HDC = 0.15 T. The colours indicate data collected withdifferent AC-frequencies.
Fig. 13 Logarithm of the sample relaxation time vs. inverse temperaturefor Co2Tb. The data have been fitted to eqn (2) over two temperatureregions and the resultant parameters extracted from the fits are given inTable 5.
Table 5 Results of fitting the data in Fig. 13 to eqn (2) over two temp-erature ranges
Fit ln τ0 τ0 (s) Ueff/kB (K)
Linear fit 1 −15.9(1) ≈1 × 10−7 15.6(4)Linear fit 2 −15.1(1) ≈3 × 10−7 13.3(3)
Table 6 Results of fitting the isotherms for 2 ≤ T ≤ 3.5 K in Fig. 11c toeqn (1)
The Arrhenius plot, however, was relatively insensitive to achange in the sample behaviour at 2.5 K. Nevertheless, theparameters extracted from the high-temperature portion of thedata (linear fit 1 in Fig. 13) are less influenced by the possibleonset of the additional relaxation mechanisms that emerge atlow temperatures, therefore we estimate τ ≤ 1 × 10−7 s and Ueff
≥ 15.6(4) K for Co2Tb.Our conclusion that the non-Kramers ion TbIII exhibits a
ground state doublet can be discussed in terms of the ligandenvironment developed for the compounds in this study. Gen-erally, a doublet ground state is permitted provided that theligand field around the TbIII ion is strictly axial.14,22 In the caseof Co2Tb, the TbIII ions occupy specific positions in the mono-clinic (C2/c) unit-cell where the point group is determined tobe 2. This 2-fold rotational, or axial, symmetry of the ligand-field permits a doubly degenerate ground-state for the TbIII
ions.The AC susceptibility of Co2Ho, measured in a zero DC-
field, is shown in Fig. 14a and b. We find that χ′ increases asthe sample is cooled, and the measurement above 3 K is insen-sitive to the AC-frequencies ≤ 1000 Hz. Below 3 K, a weak fre-quency-dependence of the magnetic properties is evident inχ″, which tends to increase with the AC-frequency. We notethat the out-of-phase response is two orders of magnitudeweaker than that of Co2Er, which we attribute to the relaxationrate in Co2Ho being comparatively fast in the measured temp-erature range. Furthermore, under the application of a smallDC-field [Fig. 14c and d] the data strongly resemble the corres-ponding measurements in zero-field, showing that the relax-ation rate is field independent. This absence of a measurablefield-induced behaviour is likely to result from the energy-levelstructure in the non-Kramers HoIII ion and is a direct contrastto the TbIII sample.
The conclusions from DC susceptibility and magnetizationmeasurements [Fig. 6(a) and (b)] point to the presence ofsingle-ion anisotropy in Co2Ho which is of a comparableenergy scale as that seen in the TbIII and ErIII complexes. Toreconcile this with the field-independent relaxation rate in thissample, the implication is that QTM is not the dominant relax-ation mechanism in zero-field. This can occur for the non-Kramers HoIII ions, if each ion exhibits a singlet ground-state.Such a state would necessarily have a mJ = 0 quantum number,which has easy-plane anisotropy and (in the classical picture)results in no energy barrier to reorient the moment within thisplane (here mJ is the z-component of the total angular momen-tum, where the z-axis is defined by the local ligand field). Thisleads to a small out-of-phase component of the susceptibilitythat would be unaffected by an applied field,23 provided thatthe energy scale of the CEF producing the singlet ground stateis much larger than the applied field, a condition which theDC magnetometry indicates is satisfied. Furthermore, theabsence of a field induced SIM behaviour has been an indi-cator of a singlet ground-state for non-Kramers ions in similarsystems.23,24
Magnetostructural properties
The shared structural form of the distorted monocappedsquare-antiprism LnIIIO9 coordination sphere for each com-pound is shown in Fig. 15. The LnIII ion occupies a site withpoint group 2, where the rotation axis is aligned along [010]such that there are five unique LnIII–O distances within thecoordination sphere. These distances are labelled (a)–(e) in thefigure and are given for compounds 1–4 in Table 7. For com-parison, the values for the published5 isostructural single-ionmagnet, CoIII2 DyIII [Ueff = 88(8) K], are also given.
The LnIII–O distances are similar in magnitude across eachrow of Table 7 implying that there are only weak distortions ofthe geometry of the coordination sphere across this series ofcompounds. Moreover, the size of the coordination sphereparameterised by the average LnIII–O distance is comparablefor all five compounds. We therefore conclude that the magni-
Fig. 14 (a) In-phase susceptibility vs. temperature for Co2Ho in a zeroDC field. We see a very weak frequency dependence of the in-phasecomponent only below T ≤ 3 K. (b) Out-of-phase susceptibility vs.temperature for Co2Ho. Note the change in scale. In-phase (c) and out-of-phase (d) susceptibility vs. temperature for Co2Ho in the applied DC-field μ0HDC = 0.1 T. The colours indicate data collected with differentAC-frequencies.
Fig. 15 The monocapped distorted square antiprism arrangement ofthe O9 coordination sphere around each LnIII ion. The LnIII occupiessites with a point group of 2 where the rotation axis along [010] is suchthat there are five unique LnIII–O distances, labelled with (a)–(e).
tude of the axial CEF at the LnIII sites is likely to be similar ineach case such that the principal factor that determines themagnetic properties of the samples will be how this CEF fieldinteracts with the unique 4f electron cloud distribution of eachlanthanide ion.14
It has been reported that uniaxial single-ion properties oflanthanide ions with oblate free electron distributions can beencouraged for sandwich-like geometries of the surroundingligands.14 The square-antiprism arrangement of ligands in thecompounds here provides such an environment, as is advo-cated by the Kramers system CoIII2 DyIII in which this coordi-nation geometry led to a uniaxial SIM behaviour with Ueff =88(8) K.5 The TbIII ion also has an oblate electron cloud, sogiven that a uniaxial anisotropy was promoted for DyIII in asimilar coordination sphere, it follows that the CEF inter-action in the TbIII sample will result in a similar nature of an-isotropy for the sample. This is consistent with the suggestionfor the uniaxial behaviour of this new system in the sectionabove.
Despite this, the thermal energy barrier in CoIII2 TbIII issmaller than in the DyIII material, with the lower bound esti-mated to be 15.6(4) K. This result may be associated with thesymmetry of the CEF at the LnIII sites. Whilst the 2-foldrotation symmetry of the coordination sphere was sufficient topromote a relatively large thermal barrier within the Kramersion DyIII, for which a degenerate ground-state is guaranteed,the non-Kramers TbIII ions additionally require a strictly axialcrystal-field in order to obtain a bistable ground state.14 Thepoint group at the TbIII lattice sites of 2 is a low-symmetryenvironment, and this weakly axial lattice site is likely to leadto a smaller barrier for the TbIII congener.
As discussed in the previous section, the spherical 4f elec-tron cloud for GdIII resulted in no measureable single-ion an-isotropy for T > 2 K in this material. By comparison to TbIII
and DyIII, both ErIII and HoIII have only weakly prolate andoblate distortions of the 4f electron clouds respectively.14 Byanalogy with the results of the GdIII sample, we thereforeexpect weaker single-ion properties for these two materialsgiven the similarity of the local lanthanide ion environmentsin each compound. In the case of ErIII, this argument is con-sistent with the energy barrier being the smallest of the uni-
axial SIMs in this series. Furthermore, the weak SIM behaviouris in keeping with the result that the coordination geometry ofthe LnIII–O9 environments promotes a strong single-ion behav-iour in oblate ions, such that the prolate distortion within theErIII 4f electron distribution is suboptimal for uniaxial behav-iour. An equatorial ligand geometry14 with a high degree ofrotational symmetry is preferable to observe SIM behaviour forthis ion. For HoIII, the combination of the non-Kramers ion inthe weakly axial ligand environment coupled with the moreisotropic electron distribution relative to TbIII may result in thelack of uniaxial behaviour being detected for this ion in thisparticular ligand environment.
Conclusion
In summary, we have utilized a compartmental ligand,2-methoxy-6-[{2-(2-hydroxyethylamino)ethylimino}methyl]-phenol (LH3), to synthesize the heterometallic SIM containinga CoIII diamagnetic metal ion. We have measured the DC andAC susceptibility of four lanthanide containing complexesCo2Ln
III (Ln = GdIII, TbIII, HoIII and ErIII). Co2Gd is well rep-resented by a paramagnet model in the temperature range 2 ≤T ≤ 300 K, which we attribute to the spherical symmetry of thespin-only 8S7/2 ground-state. This in turn leads to no single-ionanisotropy in this sample. The DC magnetic properties of thismaterial therefore allow us to conclude that there are onlyweak interactions between lanthanide centres in these com-plexes so that the magnetic properties are determined by thesingle-ion behaviour of the lanthanide ions alone. The DCmagnetic properties of Co2Tb, Co2Ho and Co2Er all suggestthe presence of crystalline-electric field splitting of the energylevels which is ≈100 K.
The non-Kramers ion Co2Tb shows evidence for fieldinduced SIM properties emerging at low temperatures, whichresults from the axial ligand field at the TbIII sites permitting abistable ground state necessary for SIM behaviour. We esti-mate an energy barrier of Ueff ≥ 15.6(4) K for this sample. Thesimilarity of the zero-field and in-field AC susceptibility ofCo2Ho suggests that the lowest lying energy level is the mJ = 0state, such that quantum tunnelling of magnetization is notpresent as a relaxation mechanism in this compound. Co2Ershows strong evidence for field induced SIM behaviour and wesuggest that the energy barrier is Ueff ≥ 9.9(8) K.
Acknowledgements
We thank the Department of Science and Technology, India,and the Council of Scientific and Industrial Research, India,for financial support. V. C. is thankful to the Department ofScience and Technology, for a JC Bose fellowship. J. G. thanksthe Council of Scientific and Industrial Research, India, for aSenior Research Fellowship. Work done in the UK is supportedby the EPSRC. PAG thanks University of Oxford for the pro-vision of a visiting lectureship.
Table 7 LnIII–O distances for compounds 1–4 using the labellingscheme given in Fig. 15. The results are compared to the published5
material CoIII2 Dy
III in the last column. The last row gives the average ofthe nine LnIII–O distances as a measure of the size of the coordinationsphere
We would like to thank Gavin Stenning for help on theQuantum Design PPMS instrument in the Materials Character-isation Laboratory at the ISIS Neutron and Muon Source. CVTthanks Dharmalingham Prabhakaran and Stephen Blundell,the University of Oxford (UK), for experimental assistance anduseful discussions.
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