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Correlating Blocking Temperatures in Single Molecule Magnets with Raman Relaxation

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Correlating Blocking Temperatures in Single Molecule Magnets with Raman Relaxation doi.org/10.26434/chemrxiv.7067669.v1 Correlating Blocking Temperatures in Single Molecule Magnets with Raman Relaxation Marcus J. Giansiracusa, Andreas Kostopoulos, George F. S. Whitehead, David Collison, Floriana Tuna, Richard Winpenny, Nicholas Chilton Submitted date: 10/09/2018 • Posted date: 11/09/2018 Licence: CC BY-NC-ND 4.0 Citation information: Giansiracusa, Marcus J.; Kostopoulos, Andreas; F. S. Whitehead, George; Collison, David; Tuna, Floriana; Winpenny, Richard; et al. (2018): Correlating Blocking Temperatures in Single Molecule Magnets with Raman Relaxation. ChemRxiv. Preprint. We report a six coordinate DyIII single-molecule magnet (SMM) with an energy barrier of 1110 K for thermal relaxation of magnetization. The sample shows no retention of magnetization even at 2 K and this led us to find a good correlation between the blocking temperature and the Raman relaxation regime for SMMs. The key parameter is the relaxation time (ᵰ ) at the point where switch the Raman relaxation mechanism becomes more important than Orbach. File list (1) Dy-mon_final_for ChemRxiv.pdf (1.77 MiB) view on ChemRxiv download file Correlating Blocking Temperatures in Single Molecule Magnets with Raman Relaxation Marcus J. Giansiracusa, Susan Al-Badran, Andreas K. Kostopoulos, George F. S. Whitehead, David Collison, Floriana Tuna, Richard E. P. Winpenny*, and Nicholas F. Chilton* Dedications Abstract: We report a six coordinate DyIII single-molecule magnet length to the anionic DiMeQ oxygen donors at 2.150(4) Å. The (SMM) with an energy barrier of 1110 K for thermal relaxation of trans equatorial Dy-Cl bonds are 2.681(2) Å, and the third Cl magnetization. The sample shows no retention of magnetization even ligand trans to the neutral water ligand (2.32(1) Å) has a bond at 2 K and this led us to find a good correlation between the blocking length of 2.897(8) Å. There are strong intramolecular N-H…Cl temperature and the Raman relaxation regime for SMMs. The key hydrogen bonds of 2.322(2) Å which support the near perfect parameter is the relaxation time ( � ) at the point where the "#$%&' octahedral coordination geometry (validated using Shape Raman relaxation mechanism becomes more important than Orbach. software, see ESI).[8,9] There are intermolecular hydrogen-bonding interactions Several lanthanide-based single molecule magnets (SMMs) between the water and a bound chloride on the adjacent have now been reported with energy barriers for reversal of [1–5] molecule (O-H…Cl 2.244(9) Å) leading to chains running through magnetization (Ueff) greater than 1000 K. However, the the structure; the Dy…Dy distance is 7.1829(6) Å along these temperature at which these high-barrier SMMs retain chains. p-stacking of the DiMeQ ligands interlocks these chains, magnetization differ markedly. The dysprosocenium cation ttt ttt t where the closest C…C contacts between carbon atoms of [Dy(Cp )2][B(C6F5)4] (Cp = C5H2 Bu3-1,2,4), shows magnetic adjacent chains is 3.382(9) Å (Figure 1b). The strong hysteresis up to 60 K and has a blocking temperature (TB) of 40 [1] t intermolecular interactions lead to a very rigid, closely packed K, while [Dy(O Bu)2(py)5][BPh4] has a higher Ueff barrier yet [4] 2D structure. exhibits hysteresis only up to 4 K, and TB = 14 K. Here we use the conventional definition of TB as the peak in the zero-field cooled (ZFC) magnetic susceptibility of a superparamagnet.[6] One feature of these new high-barrier SMMs is that even this simple definition of TB has become questionable as the observed [7] behavior near TB varies significantly. Here we report a compound with a very high Ueff that shows negligible hysteresis even at 2 K. The compound has the formula [Dy(DiMeQ)2Cl3(H2O)] (DiMeQ = 5,7-dimethyl-8- hydroxyquinoline, Figure 1) 1, and was prepared through reaction of DyCl3.6H2O and 5,7-dimethyl-8-hydroxyquinoline in a molar ratio of 1:2 in methanol (see ESI for details); the isostructural [Y(DiMeQ)2Cl3(H2O)] 1Y, can be prepared by a similar route. Compound 1 crystallizes in 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 coordinating DiMeQ ligand has a neutral charge due to the deprotonation of the phenoxide and the protonation of nitrogen in the quinolate ring. The centrosymmetric structure with trans-disposition of the DiMeQ ligands gives an O-Dy-O bond angle of 180 ° with the Dy-O bond Figure 1. (a) Crystal structure of 1 with the intra-molecular H-bonds. (b) Packing of 1 showing the inter-molecular H-bonds and p-stacking interactions. H-atoms not involved in H-bonding omitted for clarity [a] M. J. Giansiracusa, S. Al-Badran, Dr. A. K. Kostopoulos, Dr. G. F. S. Whitehead, Prof. D. Collison, Dr. F. Tuna, Prof. E.J. L. McInnes, 3 -1 Prof. Richard E. P. Winpenny*, and Dr. Nicholas F. Chilton Magnetic studies of 1 give a χMT product of 13.2 cm mol K at The School of Chemistry, The University of Manchester 300 K, as expected for a DyIII ion (Figure S3).[10] The Oxford Road, Manchester, M13 9PL, United Kingdom magnetization data saturate to a value of 4.9 N μ by 3 T, Email: [email protected], A B [email protected] consistent with an mJ = ±15/2 ground state. AC susceptibility measurements in zero field reveal strong frequency-dependent S. Al-Badran peaks in the out-of-phase susceptibility up to 68 K (Figure S4). Chemistry Department, College of Science, Basrah University, Basrah, Iraq Fitting the Cole-Cole data from 2 – 68 K to a generalized Debye model yields low alpha values (α < 0.2) indicating a single Supporting information for this article is given via a link at the end of the document. relaxation process (Figure S4-5, Table S3). Fitting the relaxation -12 rates to Equation 1 yields Ueff = 1110(50) K with τ0 = 2(1) x 10 unobservable due to the weak signal at high temperatures -3 -1 -n s, C = 5(1) x 10 s K , n = 3.32(7) and τQTM = 0.0244(9) s (Figure 2). Furthermore, we observe a shift in the quantum (Figure 2, S6). tunneling of magnetization (QTM) to a slower rate of τQTM = 2.0(3) s, indicating that dipolar fields have an influence on QTM (Figure 2, S9-10, Table S6). Hysteresis measurements reveal a slight ./011 opening of the loops (Figure S11) along with a maximum in the ) ) 6 ) = � 2 + �� + (1) ZFC curve at 6 K (Figure S12). * *, *728 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).[11,12] To obtain quantitative insight into the electronic structure, we performed complete active space self-consistent field spin-orbit (CASSCF-SO) calculations on the structure of 1. The low lying crystal field (CF) states are almost pure mJ functions (Table S4); consequently, the first two doublets have highly axial principal g- values with minimal deviation in the directions of the largest g- value for the two Kramers doublets. Magnetic relaxation is most likely to occur via the third excited state which has an almost easy-plane g-tensor; this state has an energy of 1037 K (726 cm- 1) above the ground state, in excellent agreement with the experimental energy barrier obtained from fitting the AC data. ZFC/FC measurements with an applied field of 100 Oe yield curves that almost perfectly overlay (Figure S8), and hysteresis Figure 3. Magnetic hysteresis measurements of 1 performed at 1.8 and 5 K showing no remanent magnetization at either temperature, insets with zoom measurements show rapid closing of hysteresis loops at zero- around 0 T for clarity. Field sweep rate of ~15 Oe/s. field with no remanent magnetization (Figure 3). These data indicate that there is no magnetic blocking observable at 1.8 K Despite dilution, the performance of this SMM is unexpectedly 1 or higher temperatures; thus TB < 1.8 K. poor given Ueff > 1000 K. We examined if the nearby H 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 S13-18, Table S7), thus hyperfine coupling to these nearby 1H nuclei is not the source of the poor performance. We are also confident that 161/163Dy hyperfine is not responsible based on our recent work removing the Dy-based nuclear spins in SMMs.[13] While 1 joins the few reported SMMs with energy barrier over 1000 K (Table 1, S7 and Figure S19), it does not retain magnetization: we felt understanding why 1 is so poor might help develop better SMMs. There is clearly no good correlation Figure 2. Combined fitting of relaxation rate data for 1 (black points) and 1@1Y (red points) using the same Orbach and Raman parameters with unique QTM. between Ueff and TB (Figure S20); the short-coming of Ueff as the Orbach regime (black), Raman (red), QTM pure (light blue) and dilute (dark blue) defining figure of merit has been highlighted previously in the and overall fits for pure (light green) and dilute (dark green).
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