Fine Tuning the Energy Barrier of Molecular Nanomagnets via Lattice Solvent Molecules

Solvents play important roles in our lives, they are also of interest in molecular materials, especially for molecular magnets. The solvatomagnetic effect is generally used for trigger and/or regulation of magnetic properties in molecule-based systems, however, molecular nanomagnets showing solvatomagnetic effects are very difficult to obtain. Here we report four 3d-4f heterometallic cluster complexes containing ROH lattice solvent molecules, [Cu3Tb2(H3L)2(OAc)2(hfac)4]∙2ROH {H6L = 1,3-Bis[tris(hydroxymethyl)methylamino]propane, hfac− = hexafluoroacetylacetonate; R = CH3, 1; R = C2H5, 2; R = C3H7, 3; R = H, 4}. Single-molecule magnet (SMM) properties of these four complexes were observed to be dependent on the ROH lattice solvent molecule. There is an interesting magneto-structural correlation: the larger the R group, the higher the energy barrier. For the first time, the solvatomagnetic effect is used for the continuous fine adjustment of the energy barrier of 0D molecular nanomagnets. Additionally, [Cu3Dy2(H3L)2(OAc)2(hfac)4]∙2MeOH (5), an analogue of [Cu3Tb2(H3L)2(OAc)2(hfac)4]∙2MeOH (1), is also reported for comparison.

The external Cu atom, in a distorted square-pyramidal configuration, is coordinated with two N atoms and two μ-O atoms from one H 3 L 3− ligand, forming the base of the pyramid; whereas the third μ-O atom from the same H 3 L 3− ligand occupying the apical site. The central Cu 2+ ion is coordinated by two μ 3 -O atoms and four μ-O atoms from two H 3 L 3− ligands, generating a distorted octahedral geometry, in which two μ-O atoms bridging the central Cu atom and the external Cu atom are in the Jahn-Teller axis' direction, with the long Cu-O bond distance of 2.665 Å for complex 1.
There These weak intermolecular interactions play important roles in not only stabilization of crystal structures but also adjustment of magnetic properties for complexes 1-4.
Complex 5 has the same structure as 1, but Dy instead of Tb is used (Fig. S1, SI). The Dy-O bond distance (average 2.357 Å) in 5 is slightly smaller than the Tb-O bond length (average 2.368 Å) in 1 owing to the lanthanide contraction effect. The Dy(III) coordination polyhedron can also be described as a triangular dodecahedron with the deviation value of 0.975 from the ideal D 2d symmetry (Table S5, SI). This value is a little smaller than that of 1 (1.015), indicating that the Dy(III) coordination polyhedron in 5 is closer to a triangular dodecahedron than the Tb(III) coordination polyhedron in 1. The deviation value from the ideal D 2d symmetry for a biaugmented trigonal prism is 1.735 for 5, also a little smaller than that of 1 (1.756 Magnetic properties. The direct current (dc) variable-temperature magnetic susceptibility of complexes 1-4 was measured at 1000 Oe applied field (Fig. 3). The room temperature χT values of the complexes 1 (24.91 cm 3 K mol −1 ), 2 (24.85 cm 3 K mol −1 ), 3 (24.84 cm 3 K mol −1 ) and 4 (24.90 cm 3 K mol −1 ) are slightly larger than the theoretical value of 24.77 cm 3 K mol −1 for three noninteracting Cu 2+ ions (g = 2.0) and two uncoupled Tb 3+ ions ( 7 F 6 , J = 6, L = 3, S = 3, g = 3/2). As shown in Fig. 3, upon cooling, the χT product almost keeps a constant value or just slightly lowers; however, below about 50 K, a rapid rise appears until reaches the maximum values of 53.92 cm 3 K mol −1 at 6.0 K for 1, 45.14 cm 3 K mol −1 at 4.0 K for 2 and 49.80 cm 3 K mol −1 at 4.0 K for 3; the χT values then decline to 49.34 cm 3 K mol −1 at 2.0 K for 1, 43.48 cm 3 K mol −1 at 2.0 K for 2 and 47.48 cm 3 K mol −1 at 2.0 K for 3. Exceptionally, complex 4 does not reach the maximum value until 2.0 K (44.60 cm 3 K mol −1 ). These magnetic behaviours are very similar to those of {Tb 2 Cu 3 (H 3 L) 2 X n } (X = OAc − and NO 3 − ) 35 , suggesting that all four complexes are also ferromagnetic. The small difference in dc magnetic susceptibilities of 1-4 means that there is a solvatomagnetic effect in this [Cu 3 Tb 2 (H 3 L) 2 (OAc) 2 (hfac) 4 ] SMM system. The solvatomagnetic effect could also be detected by alternating current (ac) magnetic susceptibility investigations. Both the in-phase (χ′, Fig. S2, SI) and the out-of-phase (χ′′, Fig. 4) of variable-temperature ac magnetic susceptibility for 1-4 are frequency-dependent in zero dc field, indicating slow magnetic relaxation typical for SMMs. Such thermally induced relaxation was fitted with the Arrhenius law, τ = τ 0 exp(U eff /kT), extracting U eff /k values of 30 (Fig. 5a). All four τ 0 values are within the normal range for SMMs/SIMs (10 −5 -10 −11 s) 13 . A comparison of the effective barrier value for complexes 1-4 with the R group of the ROH lattice solvent molecules (R = H, 4; R = CH 3 , 1; R = C 2 H 5 , 2 and R = C 3 H 7 , 3) reveals an important magneto-structural correlation for this [Cu 3 Tb 2 (H 3 L) 2 (OAc) 2 (hfac) 4 ] SMM system: The larger the R group in ROH, the higher the energy barrier of the [Cu 3 Tb 2 (H 3 L) 2 (OAc) 2 (hfac) 4 ]•2ROH SMM (Fig. 5b). It is noteworthy that either the U eff /k value of 2 or the U eff /k value of 3 is one of the largest values so far for the Cu-Tb heterometallic SMMs in zero dc field, just smaller than  that of (NMe 4 ) 2 [Tb 2 Cu 3 (H 3 L) 2 (NO 3 ) 7 (CH 3 OH) 2 ](NO 3 ) (36 K) 35 ; the U eff /k value of 1 is also remarkable, which is comparable with that of [Cu 2 (valpn) 2 Tb 2 (N 3 ) 6 ]·2CH 3 OH [H 2 valpn = 1,3-propanediylbis(2-iminomethylene -6-methoxyphenol)] (30.1 K, H dc = 0 Oe) 38 . In many cases [39][40][41][42][43][44] , a dc field is necessary for 3d-4f heterometallic complexes to display magnetic relaxation because of the obvious quantum-tunnelling effects.
Simplified theoretical investigations by Murrie group suggested that the magnetic bistability in the [Cu 3 Tb 2 (H 3 L) 2 X n ] system is not because of single-ion behaviours, and both the Cu···Cu and Cu···Tb ferromagnetic interactions maybe quench the tunnel splitting, which are similar to acting as an internal applied field, inducing to zero-field SMM behaviours 35 . Nevertheless, the difference of the Tb 3+ coordination configurations has influence on the SMM characteristics 35 . Owing to great difficulty for theoretical calculation and comparison of the Cu···Cu and Cu···Tb ferromagnetic couplings 35 , we tried to make a magneto-structural correlation for complexes 1-4 using the deviation value from the ideal D 2d symmetry of the biaugmented trigonal prism for the Tb 3+ ion and the intermolecular distance as two main structural parameters. As shown in Table 1, the coordination configuration of the Tb 3+ ions is closer to the biaugmented trigonal prism from 1 to 3, the corresponding energy barrier value becomes larger from 1 to 3, indicating the biaugmented trigonal prism configuration in the [Cu 3 Tb 2 (H 3 L) 2 (OAc) 2 (hfac) 4 ] SMM system is the dominant configuration; but 4 is a bit unusual, its deviation value (1.735) is comparable with that of 1 (1.756), which suggests that other structural factors such as intermolecular distances need to be considered; as shown in Table 1, the longer the intermolecular distance (defined by the shortest Cu central …Cu central separation), the higher the energy barrier; which is in line with the magneto-structural correlation using the R group itself, because larger ROH lattice solvent molecules may enhance intermolecular distances correspondingly.
The SMM properties of 1-4 were also evaluated by the parameter Φ = (ΔT f /T f )/Δ(logf) 45 , where f represents the frequency and T f the peak temperature of χ″ curve; the Φ values of 1, 2, 3 and 4 are 0.18, 0.17, 0.17 and 0.21, respectively, which support the superparamagnet behaviour of these SMMs (Φ > 0.1), but exclude any spin glass properties (Φ ≈ 0.01) 45 . Further determinations of ac magnetic susceptibility revealed that the variable-frequency χ″ signals of 1-4 are evidently temperature-dependent (Fig. 6), confirming slow magnetic relaxation of SMMs. The χ″ vs χ′ plots show classical half-circular curves for all four complexes, indicating a single magnetic relaxation process (Fig. S3, SI). These Cole-Cole plots could be fitted with a generalized Debye model 46,47 . The α values are smaller than 0.07 for 2-4, suggesting a single relaxation mechanism; while the α values for 1 are from 0.10 to 0.22, indicating a relatively narrow distribution of the relaxation time. In addition, no any hysteresis was observed in the M vs H plot at 1.9 K for 1-4 (Fig. S4, SI).
The variable-frequency ac magnetic susceptibility study of 5 revealed that the χ″ signals of 5 are temperature-dependent (Fig. 7d), confirming the SMM behavior of 5. The Cole-Cole plots were fitted to a generalized Debye model (Fig. S6, SI) 46,47 , giving the α values of 0.01-0.08 for 5, suggesting the magnetic relaxation happens via a single relaxation process. Additionally, the M vs H plot of 5 shows no any hysteresis at 1.9 K (Fig. S7, SI).

Conclusions
In summary, a mixed OAc − /hfac − co-ligands' synthesis strategy was adopted to prepare 3d-4f heterometallic SMMs based on the 1,3-Bis[tris(hydroxymethyl)methylamino]propane ligand (H 6 L). The ROH lattice solvent molecules (R = H, CH 3 , C 2 H 5 and C 3 H 7 ) in the [Cu 3 Tb 2 (H 3 L) 2 (OAc) 2 (hfac) 4 ] SMM system have great influences on the energy barrier; the larger the R group, the higher the energy barrier. We predict that the larger ROH molecule may enlarge the intermolecular distance and can help to change the coordination configuration of the Ln(III) ions through the hydrogen bonding interaction between the ROH lattice solvent molecule and the [Cu 3 Tb 2 (H 3 L) 2 (OAc) 2 (hfac) 4 ] main-structural molecule. Our work demonstrates that solvatomagnetic effects can be used to continuously fine-tune energy barriers in SMMs. The discovery is bound to have significances in enhancing and turning energy barriers of molecular nanomagnets via chemical methods such as using lattice-solvent effects. Cu(ClO 4 ) 2 ·6H 2 O (0.375 mmol) in 20 mL of MeOH, was added Tb(OAc)(hfac) 2 (H 2 O) 2 (0.15 mmol), a blue solution was formed after being stirred for 10 min; Et 3 N (0.75 mmol) was then added dropwise, the resultant solution was stirred for 3 h at room temperature and turned violet. Violet plate-like X-ray quality crystals were obtained through slow evaporation of the filtrate at room temperature over 1 week. Yield (25%). Anal. Calcd (%) for C 48

Synthesis of [Cu 3 Tb 2 (H 3 L) 2 (OAc) 2 (hfac) 4 ]•2EtOH (2).
The same synthetic procedure for complex 1 was followed, but using ethanol instead of methanol. Violet plate-like X-ray quality crystals were obtained through slow evaporation of the filtrate at room temperature over 10 days. Yield (27%). Anal. Calcd (%) for C 50   SCIENtIfIC REPORTS | 7: 15483 | DOI:10.1038/s41598-017-15852-1 X-ray crystallography. A single crystal with dimensions 0.261 × 0.093 × 0.025 mm 3 of 1, 0.178 × 0.063 × 0.024 mm 3 of 2, 0.183 × 0.125 × 0.031 mm 3 of 3, 0.108 × 0.067 × 0.025 mm 3 of 4, and 0.134 × 0.125 × 0.027 mm 3 of 5 was picked out to mount on a Bruker SMART APEX-CCD diffractometer with Mo-K α radiation (λ = 0.71073 Å) for data collection at 173(2) K. Empirical absorption corrections from ϕ and ω scan were applied. Cell parameters were calculated by the global refinement of the positions of all collected reflections for five complexes. The structures were solved by direct methods and refined by a full matrix least-squares technique based on F 2 using with the SHELX-2014 program package. All hydrogen atoms were set in calculated positions and refined as riding atoms, and all non-hydrogen atoms were refined anisotropically. CCDC 1574978-1574982 contain the supplementary crystallographic data, which can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Crystal data for 1: P−1, a = 10.086 (2)