Single-molecule magnetism arising from cobalt(II) nodes of a crystalline sponge

The magnetic susceptibility measurements were obtained using a Quantum Design Superconducting Quantum Interference Device (SQUID) magnetometer MPMS-XL7 that functions between 1.8 and 300 K for direct current (dc) applied fields ranging from -7 to 7 T. Measurements were performed on polycrystalline samples of 14.7, 5.4 and 8.6 mg of complexes 1, 2 and 3, respectively. Compound 1 was measured in paraffin oil to prevent transformation of 1 to 2, while compounds 2 and 3 were wrapped in a polyethylene membrane. Alternating current (ac) susceptibility measurements were performed under an oscillating ac field of 3.78 Oe and ac frequencies that ranged from 0.1 to 1500 Hz. Ferromagnetic impurities that were absent in all samples were investigated by collecting magnetization data at 100 K. All magnetic data were corrected for sample holders as well as other diamagnetic contributions.


Introduction
In recent years, the drive towards molecular materials that behave as small nanomagnets has relied on the use of metal ions to generate non-zero spin ground states.The combination of large spin ground states with magnetic anisotropy can give rise to the magnet-like behaviour of slow relaxation of the magnetization.Molecular materials exhibiting such behaviour are commonly referred to as single-molecule magnets (SMMs) or single-ion magnets (SIMs) for polynuclear and mononuclear complexes, respectively. 1When considering 3d transition metal ions, magnetic anisotropy is commonly achieved by unquenched orbital angular momentum due to the unequal filling of the d orbitals. 2In this regard, Co II ions in an octahedral ligand field are particularly interesting due to degenerate t 2g levels that are partially occupied, and thus orbital angular momentum is not quenched.An additional key parameter in the rational design of SMMs is control over the intermolecular interactions.Such interactions often hinder a precise understanding of the origin of the relaxation modes in SMMs, and moreover, can impede the observation of SMM-like behaviour. 3Consequently, several different approaches have been established in order to minimize intermolecular interactions.Initially, the synthetic strategy consisted of incorporating a shell of peripheral protecting diamagnetic ligands and/or separating the spin carriers by large organic counterions. 4Another approach involves the magnetic dilution method which incorporates a paramagnetic ion into a diamagnetic system, effectively isolating a single paramagnetic metal center. 5A more recent strategy, however, involves fixing the metal centres in place through the use of rigid linkers that play the role of organic spacers. 6Subsequently, we can modulate the linkers to increase or decrease the space between spin carriers, leading to high dimensionality networks.Thus, metal-organic frameworks (MOFs) provide a fascinating approach at potentially enhancing SMM properties.While MOFs are generally associated with applications based on gas storage and separation, 7 the incorporation of magnetic moment carriers within the framework of a MOF, through either paramagnetic metal centres or radical linkers, can be an effective strategy towards fine-tuning the magnetic interactions between neighbouring moment carriers. 8ecently, a new subclass of MOFs, the so-called ''crystalline sponges'', were described in which guest encapsulation occurs in a single-crystal-to-single-crystal fashion, permitting the subsequent use of X-ray diffraction techniques to elucidate the crystal structure of the guest compound. 9Our investigations on the cobalt-containing MOF { [(Co(NCS) , where TPT is 2,4,6-tris(4-pyridyl)-1,3,5-triazine, revealed two solid-state-to-solid-state transformations that significantly alter the structure and composition of the crystalline sponge. 10Nevertheless, we were intrigued by the potential of 1 to exhibit slow relaxation of the magnetization due to the octahedral ligand field of the Co II ions which promotes significant magnetic anisotropy as a result of unquenched first-order orbital angular momentum.
Herein, we report the SMM behaviour of a crystalline sponge, which reveals the first example of a three-dimensional network built from Co II SIMs as nodes.The exciting advancement of incorporating metal nodes, which behave as individual nanomagnets, into porous solids, effectively yielding light-weight materials, is particularly appealing for use as spin qubits in quantum computation.Indeed, SMMs prove to be attractive candidates for high-density information storage, due in part to their molecular nature and long coherence times. 11The discovery of a crystalline sponge exhibiting SMM behaviour paves the way for novel guest encapsulation studies, where both diaand paramagnetic guests can influence the overall slow magnetic relaxation dynamics.In the present work, we have evaluated the magnetic properties of MOF 1 and the compounds obtained therefrom through solid-state transformations.All compounds exhibit frequency-dependent out-of-phase peaks or tails of signals, suggestive of SMM behaviour.

Syntheses and structure
In accordance with previously reported methods, 10 the parent MOF 1 is obtained by carefully layering a methanol solution of Co(NCS) 2 (40 mM, 1.0 mL) on top of a solution of TPT (6.3 mg) dissolved in 4.0 mL of o-dichlorobenzene and 1.0 mL of MeOH.After a period of 1 week, orange block-like single-crystals can be obtained alongside a microcrystalline pink powder, identified as compound 3.In order to obtain compound 2, the singlecrystals of 1 can simply be left out of solution under ambient conditions for a period of 24 h, yielding the green semiamorphous material.The molecular structures of the three compounds studied herein will be presented succinctly, as they are described in detail elsewhere, 10 however the main features which are of importance to the magnetic properties are presented.Compound 1 exhibits a 3D porous network, assembled by monomeric units of Co II in a slightly distorted octahedral coordination environment (Fig. 1).The TPT ligands take up the equatorial positions, while the axial positions are occupied by nitrogen-bound thiocyanate anions.The Co II ions are wellisolated, with the closest CoÁ Á ÁCo separation being 13.39 Å, which occurs through the TPT ligand (Fig. S1 in the ESI †).Subsequently, we expect zero or minimal magnetic interactions between the metal centres.We have previously demonstrated that the removal of the single-crystals of 1 from solution, results in an irreversible transformation to a semiamorphous material in which the surface Co II ions undergo a change in coordination environment from octahedral to tetrahedral. 10This singlecrystal-to-amorphous phase transition leads to the formation of { [(Co(NCS) 2 ) 3 (k 0-3 -TPT) 4 ]Ác(H 2 O)} n (2).The third compound studied in the present work, is obtained by evaporation of the MeOH layer during synthesis of 1, and yields the densely packed layered structure { [(Co(NCS)  1).In this case, the Co II ions remain in a distorted octahedral symmetry, however, two TPT ligands have been replaced by coordinated water and methanol molecules.While the nearest intralayer CoÁ Á ÁCo separation in 3 is 13.35 Å, the closest metal-metal distance is 8.37 Å and occurs between adjacent sheets.

Static magnetic properties
An analysis of the magnetic properties of 1 allows us to elucidate the effects of structural collapse due to solvent evaporation, as observed in 2, and of structural reorganization in 3, on the overall magnetic behaviour.It is important to note that magnetic measurements of 1 were performed in paraffin oil in order to prevent solvent evaporation and to maintain its structural integrity.Variable temperature direct current (dc) susceptibility measurements were performed at 1000 Oe in the temperature range of 1.8-300 K using a SQUID magnetometer (Fig. 2).The room temperature wT products are 3.06, 2.64 and 2.94 cm 3 K mol À1 for compounds 1, 2 and 3, respectively.These values, while higher than the anticipated spin-only value for S = 3/2 of 1.88 cm 3 K mol À1 , still fall in an acceptable range when compared to other experimentally observed high-spin Co II ions with significant magnetic anisotropy. 12The wT values remains fairly constant down to 200 K for all compounds investigated, before gradually decreasing upon further cooling.In all cases, the decrease of the wT product is most likely a consequence of magnetic anisotropy and/or thermal depopulation of the excited states rather than antiferromagnetic interactions due to the large distance separating the Co II ions.This is especially valid for 1 and 3, with the closest CoÁ Á ÁCo distances being 13.39 Å and 8.37 Å, respectively.
For 2, due to the structural rearrangement it is not possible to definitively rule out intermolecular interactions, however, based on the fact that 2 also contains tetrahedral Co II ions, nonnegligible anisotropy can be expected.To confirm the presence of magnetic anisotropy, field dependent magnetization measurements (M vs. H) and reduced magnetization studies were performed on all compounds presented herein (Fig. S2-S4 in the ESI †).In all cases, the magnetization curves reveal a rapid and steady increase of the magnetization at 1.8 K without clear saturation at 7 T.The nonsaturation, as well as the non-superimposition of the isotemperature lines in the M vs. H/T data, clearly confirms the presence of significant magnetic anisotropy.

Dynamic magnetic properties
In recent years, mononuclear cobalt complexes with significant anisotropy were found to exhibit SMM-like behaviour. 13,14This behaviour is primarily arising from the inherent magnetic anisotropy of the metal centre which is strongly influenced by the ligand field and coordination geometry/environment.To investigate potential slow relaxation of the magnetization dynamics, temperature dependent alternating current (ac) susceptibilities were measured under applied fields of 0 and 1000 Oe for compounds 1-3 (Fig. S5-S7 in the ESI †).For all compounds, an ac signal was only present under applied dc fields of 1000 Oe.This is generally indicative of the presence of significant quantum tunnelling of the magnetization due to non-negligible transverse anisotropy (E).In the case of 1, the emergence of a clear peak, rather than merely tails of peaks, as in the case of 2 and 3, encouraged us to further examine the magnetic properties arising from this compound.The optimum applied dc field for 1, where the minimum of the characteristic frequency was observed, was determined to be H dc = 600 Oe (Fig. S8 in the ESI †).In the ac susceptibility data, the shifting of the peaks towards lower frequencies with decreasing temperatures is indicative of superparamagnet-like slow relaxation of a fieldinduced SMM (Fig. 3).In order to reproduce the temperature dependence of the relaxation time, we initially considered the thermally-activated Orbach process.The fit of the linear portion of the Arrhenius plot afforded an effective energy barrier for the reversal of the magnetization of 7.0 K and t 0 = 8.68 Â 10 À6 s (Fig. S9 in the ESI †).This observable barrier is rather small, yet comparable to other mononuclear Co II SMMs.2b,13 Recent energy barriers reported for Co II -based SIMs with structural dimensionalities greater than zero are summarized in Table 1.Such behaviour is in agreement with the predicted positive D value for octahedral d 7 Co II cations, as demonstrated in a number of prior studies.2b,15 It is important to note however, that the activation energy derived from the Arrhenius plot is  This journal is © The Royal Society of Chemistry 2017 several orders of magnitude smaller than the energy gap between the ground and excited level doublets (vide infra).Thus, the absence of excited states with similar values to the activation energy allows us to preclude an Orbach mechanism, and consider uniquely the contributions from direct/Raman processes.The resulting fit of the t À1 versus T plot is in good agreement with the experimental data, and yields A dir = 1.43 Â 10 8 s À1 K À1 T À4 , B Raman = 0.94 s À1 K À8.97 and n = 8.97 (Fig. S10 in the ESI †).Subsequently, we confirm that the contribution of the onephonon direct process dominates below 3 K, while the Raman exponent is in good agreement with the expected value of n = 9 for a Kramers system.2b,15,16 To the best of our knowledge, 1 represents the first case of a 3D Co II -based network exhibiting SIM behaviour.The Cole-Cole plot (w 00 vs. w 0 ) of 1 was employed to confirm the presence of a single relaxation process (Fig. S11 in the ESI †).At fixed temperatures between 1.8 and 4 K, semicircular plots were obtained and fitted using a generalized Debye model, yielding a parameters in the range of 0.01-0.13,indicating a narrow distribution of relaxation times.
The disparity in the generation of a frequency dependent signal based on the application of an external applied field is often attributed to dipolar/hyperfine interactions and zero-field tunneling.In the case of 1, the large metal-metal separations would strongly suggest that the latter plays a significant part in suppressing SMM behavior at zero applied field.Nevertheless, slow magnetic relaxation can be revealed through the application of an external field.Thus, we were interested in the magnetic field dependence of the relaxation times.The Cole-Cole plot of the variable-field ac magnetic susceptibility data was fitted using a generalized Debye model (Fig. S12 in the ESI †).The data could be fitted to give a r 0.085 for the iso-field scans and a narrow distribution of relaxation times.This strongly suggests that the observed slow relaxation dynamics are dominated by a single process, and that t remains relatively constant up to 1.6 T, as observed in other cobalt SIMs (Table S1 in the ESI †).15b The observable difference in the generation of slow magnetic relaxation between 1 and 2 may be attributed to the change in coordination geometry from octahedral to tetrahedral.In theory, first-order orbital angular momentum, the principal contributor to magnetic anisotropy, is absent in a perfect tetrahedral geometry.However, it has been demonstrated that some distorted tetrahedral complexes exhibit non-negligible barriers even at zero applied dc fields due to the mixing of the electronic ground state and the anisotropic excited states. 17The sign of the anisotropy is often dictated by the ligand field around the metal centre.In our case, due to the amorphous nature of 2, it is not possible to identify any distortion in the coordination environment, and consequently, magneto-structural correlations cannot be performed.Nevertheless, through ac susceptibility measurements we can unequivocally conclude that 2 displays different structural features than the parent MOF 1.When comparing the magnetic behaviours of 1 and 3, the weak ac signal observed for 3 can again be attributed to a change in the coordination environment of the Co II ions.In comparison to 1, two TPT nitrogen atoms are replaced by two oxygen atoms from coordinated H 2 O and MeOH molecules.This change induces a weak ligand field around the metal centre and a smaller separation of the t 2g and e g sets.Such a variation in the electronic configuration is known to lead to a change in the local anisotropy of the metal centre (i.e.sign and strength), which subsequently leads to weaker spin-orbital coupling.This results in a change of the superparamagnetic properties through a decrease of the energy barrier for magnetization reversal.

Ab initio studies
Ab initio calculations were performed on 1 and 3 in order to gain additional insight into the electronic and magnetic structures of the compounds presented herein.The magnetic properties of the low-lying states of complexes 1 and 3 were analyzed by means of an ab initio multireference methodology; the computed second-order anisotropy parameters and excitation energies are collected in Table 2.These values have been obtained from two different electronic structure calculations that have been carried out with the ORCA 18 and MOLCAS 19 software packages.ORCA produces two sets of results: CASSCF and NEVPT2 (which introduces the dynamic correlation effects), both including spin-orbit contributions.On the other hand MOLCAS has been only able to provide CASSCF results, including spin-orbit effects  In all cases, a 3/2 ground state is found for both complexes before including the spin-orbit effects.In these conditions, the calculations show the existence of low-lying spin-orbit free excited states (d in Table 2) with close energies to the ground state, showing a direct correlation of such energy difference with the calculated D value.This is also confirmed by the anisotropic g-values for the ground state of 1 and 3 (Table 2, the full g tensors can be found in Tables S2-S7, ESI †).Once the spinorbit effects are included, a set of Kramers' doublets (KDs, D) is obtained for each complex.In the case of compound 1, there are two low-lying KDs at around 280 and 450 cm À1 , which may participate in the spin relaxation processes (vide infra).This situation changes slightly for complex 3; while the first KD lies low at around 210 cm À1 , the second excited state is quite higher in energy (650 cm À1 ).A full list of the excited state energies, both with (D) and without (d) including spin-orbit contributions is collected in Tables S2-S7 (ESI †).These data again show relatively high similarities between the employed theoretical methods.
Extracting the excitation energies from CASSCF calculations is relatively easy and fast; nevertheless, identifying the metal d-orbitals involved in such transitions is not straightforward.For that reason, using a single-determinant wavefunction calculation (DFT) is often the method of choice for obtaining a qualitative explanation of the excitation processes, in which the orbital composition is much easier to rationalize.By doing this, the excitation energies correspond to electronic transitions from the highest energy doubly occupied orbital to the higher energy semioccupied b-orbitals.The DFT calculations of complexes 1 and 3 have been carried out with the Gaussian09 package. 20The final d-orbital splitting of the studied complexes, which allows the location of the lowest energy transitions, is shown in Fig. 4. As may be observed, the degeneracy of the t 2g orbitals is broken and one of those moves up in energy, far from the last doubly occupied orbitals.In the case of complex 1, the last doubly occupied orbital is d xy and the first semioccupied orbital is d yz (or d xz , because those cannot be distinguished).Since these orbitals have a different |m l | value i.e.AE2 and AE1, respectively, the D value should be positive.2b,21 The reverse situation is found in complex 3, in this case, the highest energy doubly occupied orbital is d yz (or d xz ), while the lowest energy semioccupied orbital is d xy .As before, a transition between these orbitals entails a change in |m l |, thus producing a positive D value.The computed d-orbital splitting schemes confirm that the ligand field i.e. the separation between t 2g and e g orbitals is smaller in compound 3, in agreement with the experimental observations.
The computed relative energies of the lowest-lying KDs and the spin relaxation pathways of 1 and 3 are shown in Fig. 5.In both cases, the spin relaxation mechanisms show a plausible pathway via a direct quantum tunneling (QTM) in the ground state under zero applied dc fields, as proposed from similar experiments.The matrix elements of the transition magnetic moments between states 1À and 1+ are 1.19 and 0.93 for 1 and 3, respectively, much higher than the 0.1 value required to be associated with an efficient relaxation mechanism.In the case of 1, the first two KDs may be accessible (approx.280 and 450 cm À1 ) and able to participate in alternative relaxation pathways, either thermally assisted-QTM or Orbach processes.In complex 3 there is only one low-lying KD at around 210 cm À1 (the second lowest KD is located at almost 650 cm À1 ); the alternative thermally assisted-QTM and Orbach spin relaxation processes seem plausible but are probably not able to compete with the ground state QTM.These relaxation processes provide

Conclusions
We have reported the magnetic properties of three Co II compounds, which were fully characterized through dc, ac susceptibility measurements and electronic structure calculations.Interestingly, a 3D crystalline sponge displays single-molecule magnet-like behaviour under applied static fields, where each node individually acts as a nanomagnet.We have also demonstrated that the magnetlike behaviour of these nodes can be fine-tuned via manipulation of the coordination environment of the Co II ions.Thus, changes in the coordination sphere of metal centres in extended networks could be easily monitored through their magnetic properties.Furthermore, we can envision how the magnetization dynamics of the porous host could be tuned by guest exchange and similarly, how the intercalation of guest molecules could be detected via magnetism for novel sensor-based application.

Fig. 1
Fig. 1 (a) Packing arrangement of 1, illustrating the large pore dimensions of the 3D network.(b) View of the 2D planar sheet arrangement of 3, with individual sheets displayed in orange and blue.Colour code: pink (Co), blue (N), red (O), yellow (S).Carbon atoms are represented as stick model for clarity.Hydrogen atoms and solvent molecules are omitted for clarity.

Fig. 2
Fig. 2 Temperature dependence of the magnetic susceptibility for compounds 1-3 in a wT vs. T plot at 1000 Oe.

Fig. 3
Fig.3Frequency dependence of the in-phase w 0 (top) and out-of-phase w 00 (bottom) magnetic susceptibilities for 1, under an applied optimum dc field of H dc = 600 Oe.Lines serve as guides for the eyes.
introduced with the SO-RASSI method.As expected for octahedral Co II complexes, large and positive D values are found.2b The calculated zero-splitting D parameters remain very similar regardless of the method employed and are larger for compound 1.These computed values are not unusual since the spin relaxation mechanisms that depend on the lattice effects, and should contribute to reduce the D values, cannot be captured in a single-molecule calculation.The calculated D tensors for all compounds are very similar (Tables S2-S7, ESI †) and show almost the same orientation (Fig. S13, ESI †).

Fig. 5
Fig. 5 Lowest Kramers' doublets and ab initio computed relaxation mechanism in 1 (left) and 3 (right).The thick black lines imply KDs as a function of their magnetic moment along the main anisotropy axis.Red lines indicate the magnetization reversal mechanism.The blue lines correspond to ground state QTM and thermally assisted-QTM via the first and second excited KD, and green and purple lines show possible Orbach relaxation processes.The values close to the arrows indicate the matrix elements of the transition magnetic moments (above 0.1 an efficient spin relaxation mechanism is expected). 21

Table 1
Compilation of the energy barriers of recent octahedral Co II SIMs with extended structures (in one, two or three dimensions)

Table 2
CASSCF and NEVPT2 (Orca code) and CASSCF (Molcas code) computed D, |E| (in cm À1 ), and g-values for the ground state of complexes 1 and 3. d and D (in cm À1 ) are the computed first excitation energies before and after including the spin-orbit effects, respectively.The D value corresponds to the energy difference between the ground and the first excited Kramers' doublets a ORCA/CASSCF.b ORCA/NEVPT2.c MOLCAS/CASSCF.