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Received 00th January 20xx,Accepted 00th January 20xx

DOI: 10.1039/x0xx00000x

www.rsc.org/

A Large Barrier Single-Molecule Magnet Without Magnetic MemoryMarcus J. Giansiracusa, Susan Al-Badran, Andreas K. Kostopoulos, George F. S. Whitehead, David Collison, Floriana Tuna, Richard E. P. Winpenny* and Nicholas F. Chilton*

In memory of Professor Paul O’Brien FRS, a great friend and colleague

We report a six coordinate DyIII single-molecule magnet (SMM) with an energy barrier of 1110 K for thermal relaxation of magnetization. The pure compound shows no retention of magnetization even at 2 K.

Single-molecule magnets (SMMs) are molecules that show slow relaxation of magnetisation, and hence retain magnetisation in the absence of an external field. The dominant relaxation process in SMMs was believed to be via a thermally activated process over an energy barrier (Ueff), called the Orbach process, and several lanthanide-based SMMs have been reported with remarkable properties such as Ueff > 1000 K.1–6 Most fall into two classes: they are either seven-coordinate with pentagonal bipyramidal coordination geometries with strong axial crystal fields,2,4 or they are [DyCpR

2]+ cations (where CpR is a substituted cyclopentadienyl ligand) where the strong axial crystal field is provided by the cyclopentadienyl ligands.1,5,6 There are no examples of other coordination geometries supporting such a high Ueff. Another feature common to most of these high Ueff SMMs is that they are air- and moisture-sensitive and require high synthetic skill to prepare and handle.

Here we report a compound [Dy(DiMeQ)2Cl3(H2O)] (DiMeQ = 5,7-dimethyl-8-oxoquinolinium, Figure 1) 1, prepared from reaction of DyCl3·6H2O with 5,7-dimethyl-8-hydroxyquinoline in a molar ratio of 1:2 in methanol in air (see ESI for details). Compound 1 crystallizes in space group P-1 and features a six-coordinate DyIII ion bound to two trans DiMeQ ligands through the deprotonated phenoxide group, three mer chloride anions and a water molecule (Figure 1a). The structure is solved in P-1 with an inversion centre located at the DyIII ion (Table S1). Therefore, the H2O and chloride trans to the water are crystallographically disordered. In a single molecule there is always a single water present as this leads to a Cl---OH2 hydrogen-bonding interaction which results in a H-bonded network as the disposition of H2O and Cl are reversed in a neighbouring column (Figure 1b).

Fig. 1 (a) Crystal structure of 1 with the intra-molecular H-bonds. (b) Packing of 1 showing the inter-molecular H-bonds and -stacking interactions. H-atoms not involved in H-bonding omitted for clarity.

This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 1

M. J. Giansiracusa, S. Al-Badran, Dr. A. K. Kostopoulos, Dr. G. F. S. Whitehead, Prof. D. Collison, Dr. F. Tuna, Prof. Richard E. P. Winpenny and Dr. Nicholas F. ChiltonThe School of Chemistry, The University of ManchesterOxford Road, Manchester, M13 9PL, United KingdomEmail: richard.winpenny@manchester.ac.uk, nicholas.chilton@manchester.ac.uk

S. Al-BadranChemistry Department, College of Science, Basrah University, Basrah, Iraq

Electronic Supplementary Information (ESI) available: [details of any supplementary information available should be included here]. See DOI: 10.1039/x0xx00000x

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The coordinating DiMeQ ligand binds in a zwitterionic form, due to the deprotonation of the phenoxide and the protonation of nitrogen in the quinoline ring. The trans-disposition of the DiMeQ ligands gives a crystallographically-imposed O-Dy-O bond angle of 180° with a Dy-O bond length of 2.150(4) Å. The trans equatorial Dy-Cl bonds are 2.681(2) Å, and the third Cl ligand trans to the neutral water ligand (2.32(1) Å) has a bond length of 2.897(8) Å. There are strong intra-molecular N-H…Cl hydrogen bonds of 2.322(2) Å, which support the near perfect octahedral coordination geometry (validated using Shape software, see ESI Table S2).7,8

The inter-molecular hydrogen-bonding interactions between the water and a bound chloride on the adjacent molecule (O-H…Cl 2.244(9) Å) lead to chains running through the structure; the Dy…Dy distance is 7.1829(6) Å along these chains. -stacking of the DiMeQ ligands interlocks these chains, where the closest C…C contacts between carbon atoms of adjacent chains is 3.382(9) Å (Figure 1b). The strong intermolecular interactions lead to a very rigid, closely packed 2D structure.

Magnetic studies of 1 give a χMT product of 13.2 cm3 mol-1 K at 300 K, consistent with a Dy III ion with large crystal field splitting (Figure S3), and below 7 K it drops rapidly to 7.8 cm3

mol-1 K.9 The magnetization data saturate at a value of 4.9 NAμB

by 3 T, suggesting an mJ = ±15/2 ground doublet. We note that both χMT at 300 K and the saturation magnetisation value at 7 T and 1.8 K are slightly lower than theory predicts for a well-isolated mJ = ±15/2 doublet (Figures 2 and S8); this is either due to an uncertainty of the sample mass on the order of 3% (ca. 0.7 mg) or to overestimation of the axiality of the electronic structure by CASSCF-SO calculations (i.e. the ground state is more mixed); previous studies10-13 have noted that long-range electrostatic interactions can affect the crystal field (CF) potential, which have not been included here. AC susceptibility measurements in zero DC field reveal strong frequency-dependent peaks in the out-of-phase susceptibility up to 68 K (Figure S4). Fitting the AC susceptibility data from 2 – 68 K to a generalized Debye model gives low alpha values (α < 0.2) consistent with a single relaxation process (Figure S4-5, Table S4).14

Fig. 2 Magnetic hysteresis measurements of 1 performed at 1.8 and 5 K showing no remanent magnetisation at either temperature, insets with expansion around 0 T for clarity. Field sweep rate of ~15 Oe s1.

The relaxation rates were fitted using Equation 1, which contains three terms for Orbach relaxation, Raman relaxation and quantum tunnelling of magnetisation (QTM), respectively.

1τ= 1τ0e

−U eff

T +CTn+ 1τQTM

(1)

The best fit parameters (goodness-of-fit based on log[τ−1]) for 1: Ueff = 1110(50) K, τ0 = 10-11.3(2) s, C = 5(1) x 10-3 s-1 K-n, n = 3.32(7) and τQTM = 0.0244(9) s (Figure 3, Table S3). This is the highest Ueff value reported for a six-coordinate compound, larger than that previously reported for [Dy(bipm)2]- (813 K; bipm = {C(PPh2NSiMe3)2}2-).15

Fig. 3 Fitting of relaxation rate data for 1 (pure, black points) and 1@1Y (dilute, red points) using the parameters given in the text. Fits: for 1, overall (light green), Orbach (black), Raman (orange), QTM (light blue); for 1@1Y, overall (dark green), Raman (red), QTM (dark blue).

The environment of the Dy site in 1 is highly anisotropic and stabilizes the large |mJ| projections of the ground Dy(III) multiplet, as shown by an electrostatic calculation (Figure S7).16

To obtain the electronic structure of the Dy centre in 1, we performed complete active space self-consistent field spin-orbit (CASSCF-SO) calculations (see ESI for details, Table S5-6).

The low-lying CF states are almost pure mJ functions, quantised along the gz direction of the ground doublet which deviates by 2.10° from the O-Dy-O vector (Table S5); consequently, the first two excited doublets have highly axial principal g-values with minimal deviation in the directions of the largest g-value for the two Kramers doublets. This suggests that Orbach relaxation is most likely to occur via the third excited state, which has a highly rhombic g-matrix. This state has an energy of 1095 K above the ground state, in excellent agreement with the experimental Ueff obtained from fitting the AC susceptibility data.

ZFC/FC susceptibility measurements with an applied field of 100 Oe give curves that almost perfectly overlay (Figure S9), and hysteresis measurements show rapid closing of hysteresis loops at zero-field with no remanent magnetisation (Figure 2). These data indicate that the blocking temperature (TB) of 1 is < 1.8 K.

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We probed the effect of dipolar interactions between Dy III

sites through dilution experiments. A doped sample (1@1Y) was synthesized with a 1:19 molar ratio between DyCl3.6H2O and YCl3.6H2O, and purity of the bulk sample was confirmed by powder X-ray diffraction (Figure S2); elemental analysis shows around 7% concentration of Dy. This concentration results in partial isolation of the DyIII sites, with around 56% expected to have no paramagnetic neighbours. The carbon-content found in the doped sample is slightly low (45.9% against 46.8% calculated); C-analysis can be low for complexes of heavier elements as some can be trapped rather than escaping as CO2

on burning.The AC susceptibility data for 1@1Y were obtained; the

Orbach region could not be studied due to the weak signal at high temperatures. Therefore fitting the AC susceptibility data for 1@1Y does not require the first term in equation [1]; we obtain Raman parameters of C = 4(1) x 10-4 s-1 K-n and n = 4.0(1), and the quantum tunnelling of magnetization (QTM) is slower with τQTM = 1.9(3) s (Figures 3, S10-11, Table S7-8). These numbers are significantly different from the best fit parameters for the pure compound 1, and this led us to examine the fits more closely. The Raman parameters for 1 and 1@1Y are statistically different (95% confidence intervals given in Tables S3 and S7), and the measured data in Figure 3 are clearly following a different slope. We do not believe such a difference has been reported previously. For the QTM relaxation process there is an almost two orders of magnitude change in the QTM timescale between 1 and 1@1Y. For 1@1Y we see a small magnetic hysteresis up to 5 K (Figure S12) along with a maximum in the ZFC susceptibility curve at 6 K (Figure S13). The doped sample therefore shows retention of magnetisation, albeit at low temperatures, and the pure sample does not. It is tempting to explain this memory effect by the change in the Raman and/or QTM relaxation rates.

We also examined if the nearby 1H nuclei of the bound water molecule had any specific influence on the SMM properties by preparing a partially deuterated sample using CD3OD as a solvent with DyCl3·6D2O in the synthetic process to obtain 1d. The presence of the D2O ligand was confirmed by IR spectroscopy (Figure S1), revealing the absence of the O-H stretch. However, magnetic measurements of 1d are nearly identical to 1 (Figure S14-19, Table S9-10), thus hyperfine coupling to these nearby 1H nuclei is not the main source of the rapid relaxation. We are also confident that 161/163Dy hyperfine is not responsible based on recent work removing the Dy-based nuclear spins in high Ueff DyIII SMMs.17-19

While 1 joins the ranks of monometallic SMMs with energy barriers over 1000 K (Table S11), it does not show any appreciable retention of magnetisation in the pure form. In another paper,20 we show that there is a correlation between the temperature at which magnetisation is retained, and switch, which is the relaxation time at the point where the Raman and Orbach processes have the same rate. In 1, τ Switch is found to be 1.36 x 10-4 s, which is the lowest known for any SMM with Ueff > 1000 K. In the [DyCpR

2]+ SMMs that have the highest blocking temperatures,τ Switch is typically 50 s (Table S11).20 To state this in another way: the Raman process varies in rate by

over five orders of magnitude between SMMs with high and low blocking temperatures.

Previously, it has been postulated that a flexible lattice is responsible for the rapid relaxation observed in some poorly performing SMM systems.21 In 1 there is a rigid lattice due to H-bonds and -stacking interactions, therefore this explanation does not seem to fit here. Also, partial deuteration of the sample has no effect; this would be expected to change intra- and inter-molecular H-bonds and hence rigidity, and this does not change the magnetic behaviour. Therefore, we do not presently understand how we could control Raman relaxation through chemistry; this could be the key to producing SMMs that retain magnetisation to higher temperatures.

Compound 1 is unusual in having a Ueff > 1000 K, but essentially no memory. It is intriguing in another regard, as it is the one of only two SMMs to our knowledge2 with a very large Ueff that can be prepared and handled in air. Perhaps this holds out the possibility that other lanthanide SMMs that are not hugely air-sensitive could be made that have both large Ueff

and that retain magnetisation to high temperatures.We thank The University of Manchester and EPSRC

National EPR Facility for access to the SQUID magnetometer, and the EPSRC for funding an X-ray diffractometer (grant number EP/K039547/1). M.J.G. thanks The University of Manchester for a President’s Doctoral Scholarship. S.A.-B. thanks the Higher Committee for Education Development in Iraq (HCED) for the award of a research scholarship. N.F.C thanks The Ramsay Memorial Trust for a Research Fellowship. R.E.P.W. thanks the EPSRC for an Established Career Fellowship (EP/R011079/1) and the European Research Council for an Advanced Grant (ERC-2017-ADG-786734).

Conflicts of interestThe authors declare no conflict of interest.

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