Field-Induced Single Molecule Magnets of Phosphine- and Arsine-Oxides

ORIGINAL RESEARCH published: 12 September 2018 doi: 10.3389/fchem.2018.00420 Frontiers in Chemistry | www.frontiersin.org 1September 2018 | Volume 6 | Article 420 Edited by: Carlos Lodeiro, Universidade Nova de Lisboa, Portugal Reviewed by: Guo-Hong Tao, Sichuan University, China Sukhendu Mandal, Indian Institute of Science Education and Research, Thiruvananthapuram, India *Correspondence: Matilde Fondo [email protected] Specialty section: This article was submitted to Inorganic Chemistry, a section of the journal Frontiers in Chemistry Received: 29 June 2018 Accepted: 24 August 2018 Published: 12 September 2018 Citation: Fondo M, Corredoira-Vázquez J, García-Deibe AM, Sanmartín-Matalobos J, Herrera JM and Colacio E (2018) Field-Induced Single Molecule Magnets of Phosphineand Arsine-Oxides. Front. Chem. 6:420. doi: 10.3389/fchem.2018.00420 Field-Induced Single Molecule Magnets of Phosphineand Arsine-Oxides Matilde Fondo1*, Julio Corredoira-Vázquez1, Ana M. García-Deibe1, Jesús Sanmartín-Matalobos1, Juan Manuel Herrera2and Enrique Colacio2 1Departamento de Química Inorgánica, Facultade de Química, Universidade de Santiago de Compostela, Santiago de Compostela, Spain, 2Departamento de Química Inorgánica, Facultad de Ciencias, Universidad de Granada, Granada, Spain The coordination chemistry of dysprosium and terbium toward phosphine and arsine oxides was further explored. Thus, the new nitrate [M(NO3)3(Ph3PO)3] (M=Tb, 1; Dy, 2), [Dy(NO3)3(EtOH)(Ph3XO)2] (X=P, 3; As, 4), chloride [DyCl2(Ph3AsO)4]Cl (5), triflate [Dy(OTf)2(MePh2PO)4]OTf (6; OTf =triflate) and hexafluoroacetylacetonate [M(hfa)3(Ph3PO)2] (hfa =hexafluoroacetylacetonate; M=Tb, 7; Dy, 8) complexes were isolated and fully characterized. The crystal structures of 1·CH3CN, 2·CH3CN, 4, 5·2.75EtOH·1.25H2O, 6,7, and 8show MO9cores in 1, 2, and 4, with highly distorted geometry, between spherical capped square antiprism and muffin-like, hexacoordinated environments for the dysprosium ions in 5and 6, with octahedral geometry, and octacoordination for the lanthanoid metals in 7and 8, with geometry closer to square antiprism. Comparison of the magnetic behavior of all the complexes allows analyzing which metal ion (Tb or Dy), phosphine or arsine oxide, or anionic ligand favor more the slow relaxation of the magnetization. Alternating current magnetic measurements show that only 2,4, and 8present slow relaxation of the magnetization in the presence of an external magnetic field, 8being the complex with the highest Ueff (44.85 K) of those described herein. Keywords: terbium, dysprosium, triphenylarsine oxide, phosphine oxide, single ion magnet INTRODUCTION The observation for the first time of slow magnetic relaxation in mononuclear lanthanoid complexes (TBA)[Pc2Ln] (TBA =But 4N+; Pc =phthalocyanide; LnIII =Tb or Dy) (Ishikawa et al., 2003) provided a real breakthrough in molecular magnetism, opening the field of single ion magnets (SIMs) in 2003. This field has received growing attention since its origins, given that the non-trivial memory effect and quantum character in SIMs renders them as potential ultra-high density data storage medium and spintronic devices (Woodruff et al., 2013; Shiddiq et al., 2016; Lu et al., 2017). Accordingly, the huge amount of work devoted to this research during the last 15 years has led to many remarkable advances. Thus, Rinehart and Long published in 2011 (Rinehart and Long, 2011) a benchmark study where they provided a clear explanation of how the electronic structure of f-elements can in theory be manipulated to create new single molecule magnets (SMMs). In this study, they give the relationship between the coordination environment of lanthanoid ions and the magnetic anisotropy of the complex and, therefore, they suggest that one can match an appropriate ligand field to maximize magnetic anisotropy on the basis of the shapes (oblate or prolate) of the 4f-shell electron density distributions. For the oblate Tb(III) and Dy(III) ions, theoretical calculations Fondo et al. SMMs of Phosphineand Arsine-Oxides established that the optimum environment to maximize the anisotropy is the axial one (Ungur and Chibotaru, 2011). Thus, the optimum geometry will be lineal but the coordination number two seems too low to be stabilized by the lanthanoid ions. Accordingly, in the absence of this possibility, the coordination number 7, with pentagonal bipyramidal geometry, was the most explored one. On the basis of these starting theoretical studies, much experimental work was done and continuous attainments relating to anisotropic energy barriers (Ueff) and blocking temperatures (TB) were achieved in the field of SIMs. Hence, the highest Ueff described up to now is 1815 K (Ding et al., 2016), a value significantly higher than 331 K reported for the first SIM (But 4N)[Tb(Pc)2] (Ishikawa et al., 2003). Besides, the blocking temperatures have been continuously increasing, from the initial 1.7 K (Ishikawa et al., 2005) thorough the 20 K achieved in 2016 for an-air stable [Dy(Cy3PO)2(H2O)5]Br3complex (Chen et al., 2016) to the astonishing 60 K recently reported for the metallocene complex [(Cpttt)2Dy][B(C6F5)4] (Goodwin et al., 2017; Guo et al., 2017). In spite of these advances, it should be noted that most of the SIMs with high Ueff and/or TBare air-unstable, and this is a handicap that must be surpassed. It must be noted that a series of simple, easy to obtain, and air stable phosphine oxide complexes [Dy(R3PO)2(H2O)5]X3(R =Cy3or CyPh2, X =Cl, Br or I) show blocking temperatures between 19 and 20K (Chen et al., 2016, 2017), the largest ones among SMMs if the 60K reported for the unstable [(Cpttt)2Dy][B(C6F5)4] is excluded. Accordingly, it seems that the coordination chemistry of lanthanoids with phosphine oxides is really interesting from the magnetic point of view. This coordination chemistry has been extensively studied, mainly by Platt (2017), but, in spite of this, the magnetic behavior of these compounds is still poorly investigated. Therefore, taking into account all the above considerations, we have decided to revise and extent the study of the coordination chemistry of Dy and Tb with phosphine and arsine oxides, and to investigate the magnetic behavior of the obtained compounds. The comparison of the magnetic properties of the isolated complexes between them allows establishing some patterns. MATERIALS AND METHODS General All chemical reagents and solvents were purchased from commercial sources and used as received without further purification. Elemental analyses of C, H and N were recorded on a Carlo Erba EA 1108 analyzer. Infrared spectra were performed in the range 4000–500 cm−1on a Varian 670 FT/IR spectrophotometer in the ATR mode. Syntheses of the Complexes [Tb(NO3)3(Ph3PO)3] (1): To a solution of Tb(NO3)3·5H2O (0.130g, 0.30 mmol) in acetonitrile (10 mL), triphenylphosphine oxide, (0.167g, 0.60 mmol) and acetonitrile (10mL) were added. The mixture was stirred for 4 h at room temperature and the resultant colorless solution was left to slowly evaporate, until single crystals of 1·CH3CN precipitated. The single crystals were filtered, and they lose the acetonitrile solvate on drying to yield 1. Yield (based on Ph3PO): 0.22g (93%). M.W.: 1179.80. Anal. calcd. for C54H45TbN3O12P3: C 54.97, H 3.84, N 3.56%. Found: C 54.89, H 3.59, N 3.95%. IR (ATR, ˜ν/cm−1): 1120, 1153 (P=O), 1305 (NO− 3). The same product is isolated when Tb(NO3)3·5H2O and Ph3PO are mixed in 1:3 molar ratio in acetonitrile. [Dy(NO3)3(Ph3PO)3] (2) was obtained in a similar way to 1: amounts of Dy(NO3)3·6H2O (0.23 g, 0.66 mmol) and Ph3PO (0.360g, 1.32 mmol). Single crystals of 2·CH3CN were isolated in the same way as those of 1·CH3CN, which lose the acetonitrile solvate on drying to yield 2. Yield (based on Ph3PO): 0.403g (77%). M.W.: 1183.37. Anal. calcd. for C54H45DyN3O12P3: C 54.81, H 3.83, N 3.55%. Found: C 54.84, H 3.40, N 3.53%. IR (ATR, ˜ν/cm−1): 1120, 1153 (P=O), 1305 (NO− 3). The same product is isolated when Dy(NO3)3·6H2O and Ph3PO are mixed in 1:3 molar ratio in acetonitrile. [Dy(NO3)3(EtOH)(Ph3PO)2] (3): A hot solution of Dy(NO3)3·6H2O (0.137 g, 0.30 mmol) in ethanol (5 mL) was added to a hot solution of triphenylphosphine oxide (0.170 g, 0.60 mmol) in ethanol (5 mL). The mixture was stirred at room temperature for 24 h, when a colorless powder precipitated. The solid was filtered and dried in air. Yield: 0.250 g (88%). M.W.: 951.15. Anal. calcd. for C38H36DyN3O12P2: C 47.98, H 3.81, N 4.42%. Found: C 47.90, H 3.81, N 4.29%. IR (ATR, ˜ν/cm−1): 1119, 1150 (P=O), 1308 (NO− 3), 3346 (OH). [Dy(NO3)3(EtOH)(Ph3AsO)2] (4) was obtained in the same way as 3: amounts of Dy(NO3)3·6H2O (0.23 g, 0.66 mmol) and Ph3AsO (0.425 g, 1.32 mmol). The mixture yields a solution that by slow evaporation gives rise to single crystals of 4. Yield: 0.313g (46%). M.W.: 1039.05. Anal. calcd. for C38H36DyN3O12As2: C 43.92, H 3.49, N 4.04%. Found: C 44.44, H 3.35, N 3.92%. IR (ATR, ˜ν/cm−1): 902, 924 (As=O), 1313 (NO− 3), 3334 (OH). [DyCl2(Ph3AsO)4]Cl·1.25H2O (5·1.25H2O): To an ethanol (10 mL) solution of DyCl3·6H2O (0.460 g, 1.22 mmol), Ph3AsO (0.77g, 2.395 mmol) was added. The mixture was stirred under reflux for 1 h and the colorless solution was concentrated in a rotaevaporator to ca. 2 mL, yielding an oil. After standing for 24 h, single crystals of 5·2.75EtOH·1.25H2O, suitable for X-ray diffraction studies, were isolated. The crystals were filtered and dried in air, losing the ethanol solvate to give rise to 5·1.25H2O. Yield (based on Ph3AsO): 0.74 g (78%). M.W.: 1579.93. Anal. calcd. for C72H62.5AsCl3DyO5.25: C 54.68, H 3.95%. Found: C 54.27, H 3.33%. IR (ATR, ˜ν/cm−1): 886 (As=O), 3264 (OH). The same product is isolated when DyCl3·6H2O and Ph3AsO are mixed in 1:4 molar ratio in ethanol. [Dy(OTf)2(MePh2PO)4](OTf)·THF (6·THF): Dy(OTf)3 (0.423g, 0.694 mmol) was dissolved in THF (7 mL), and methyldiphenylphosphine oxide (0.600 g, 1.387 mmol) was added. The mixture was stirred at room temperature for 24 h, and the resultant colorless solution was concentrated in a rotaevaporator up to ca. 2 mL, yielding an oil. The oil was left to stand for 10 days at room temperature until single crystals of 6·THFwere obtained. Yield: 0.250 (47%). M.W.: 1546.63. Anal. calcd. for C59H60DyF9O14P4S3: C 45.78, H 3.88, S 6.21%. Found: C 45.79, H 3.65, S 5.91%. IR (ATR, ˜ν/cm−1): 633 (δSO3), 1020, 1262 (SO3), 1156, 1220 (CF3), 1132 (P=O). Frontiers in Chemistry | www.frontiersin.org 2September 2018 | Volume 6 | Article 420 Fondo et al. SMMs of Phosphineand Arsine-Oxides The same product is isolated when Dy(OTf)3and MePh2PO are mixed in 1:4 molar ratio in THF. [Tb(hfa)3(Ph3PO)2] (7): Terbium chloride hexahydrate (0.14 g, 0.36 mmol) was dissolved in distilled water (5 mL). An ethanol solution (20 mL) of hexafluoroacetylacetone (0.22g, 1.08 mmol) was added to the aqueous solution. An ethanolic tetramethylammonium hydroxide solution (0.1 M) was added dropwise until pH 7 was reached. After stirring the mixture at room temperature for 6 h, the solvent was eliminated in a rotaevaporator. The white solid that precipitated was dissolved in methanol (30 mL), and triphenylphosphine oxide (0.10 g, 0.36 mmol) was added. The mixture was heated under reflux while stirring for 6 h. Slow evaporation of the resultant solution yields single crystals of 7, suitable for X-ray diffraction studies. The crystals were filtered and dried in air. Yield (based on Ph3PO): 0.070 (29%). M.W.: 1336.65. Anal. calcd. for C51H33TbF18O8P2: C 45.83, H 2.49%. Found: C 45.79, H 2.42%. IR (ATR, ˜ν/cm−1): 1138 (P=O), 1650 (C=O), 1250, 1160 (CF3). The same product is isolated when TbCl3·6H2O, hfah and Ph3PO are mixed in 1:3:2 molar ratio in water/ethanol. [Dy(hfa)3(Ph3PO)2] (8) was obtained in a similar way to 7: amounts of DyCl3·6H2O (0.400 g, 1.07 mmol), Ph3PO (0.298, 1.07 mmol) and hfah (0.664 g, 3.21 mmol). Single crystals of 8are isolated in the same way as those of 7. Yield 0.210 (29%). M.W.: 1340.22. Anal. calcd. for C51H33DyF18O8P2: C 45.70, H 2.48%. Found: C 45.38, H 2.86%. IR (ATR, ˜ν/cm−1): 1136 (P=O), 1652 (C=O), 1250, 1161(CF3). The same product is isolated when DyCl3·6H2O, hfah and Ph3PO are mixed in 1:3:2 molar ratio in water/ethanol. Magnetic Measurements Magnetic susceptibility direct current (dc) and alternating current (ac) measurements for 1–8were performed with a Quantum Design SQUID MPMS-XL-5 susceptometer. The magnetic susceptibility dc data were recorded at temperatures ranging from 2 to 300 K, under a magnetic field of 1000 Oe. Magnetization measurements under magnetic fields of 0– 50000 Oe at 2.0 K were also recorded. Diamagnetic corrections were estimated from Pascal’s Tables. ac susceptibility data were registered with an oscillating ac field of 3.5 Oe and ac frequency of 1400 Hz, under diverse applied static fields (Hdc =0 or 1,000) for all the reported compounds. In the case of 2and 8,ac susceptibility measurements were moreover recorded under a dc field of 1000 Oe at ac frequencies in the range 50–1400 Hz. Crystallographic Refinement and Structure Solution Crystal data and details of refinement are given in Table S1. Single crystals of 1·CH3CN, 2·CH3CN, 4,5·2.75EtOH·1.25H2O, 6·THF, 7and 8were obtained as detailed above. Data were collected at 100 K on a Bruker Kappa APEXII CCD diffractometer, employing graphite monochromated Mo-kα(λ =0.71073 Å) radiation. Multi-scan absorption corrections were applied using SADABS (Blessing, 1995; Krause et al., 2015). The structures were solved by standard direct methods, using SHELXT (Sheldrick, 2015a), and then refined by full-matrix leastsquares techniques on F2, using the program package SHELXL (Sheldrick, 2015b) from the program package SHELX (Sheldrick, 2008). All non-hydrogen atoms were refined anisotropically, with the exception of some atoms with low occupation sites. Most of the hydrogen atoms were included in the structure factor calculations in geometrically idealized positions. However, nondisordered hydrogen atoms, which could be potentially involved in H-bonding schemes, were mostly located in the corresponding Fourier maps. These H atoms were freely refined, or with thermal parameters derived from their parent atoms. CCDC 1850933–1850938 and 1851725 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Center via www.ccdc.cam.ac.uk/ structures. RESULTS AND DISCUSSION Synthesis and Spectroscopic Characterization As previously stated, the chemistry of lanthanoids with phosphine oxides is quite well studied and many complexes with different metal ion:phosphine oxide ratios were isolated (Platt, 2017). These previous studies show that the stoichiometry of the complexes seems to be dependent on the nature of the metal salt employed, and on the molar quantities of salt and phosphine oxide in the reaction mixture, but also on the cone angle of the phosphine (Tolman, 1977). Recently, it has been reported that complexes of formulas [Ln(Cy3PO)2(H2O)5]X3 (Ln =La, Dy, Er, Yb, Lu; X =Cl or Br) (Lees and Platt, 2014, 2015) can be easily obtained. These complexes have pentagonal bipyramidal geometry, and some of them are air stable single ion magnets, with high blocking temperatures (Chen et al., 2016). Nevertheless, similar pentagonal bipyramidal complexes were not reported for Ph3PO, what would lead to evaluate the influence of the aromatic vs. the aliphatic ring in the magnetic behavior of the complexes. With these considerations in mind, different dysprosium and/or terbium salts were initially mixed with Ph3XO (X =P or As) or MePh2PO in 1:2 molar ratio, with the intention of isolating complexes of stoichiometry Ln(R3XO)2Y5(R3=Ph3or MePh2; X =P or As; Y =OH2 and/or monodentate anion). The obtained results indicate that the salt:R3XO molar ratio plays a secondary role in the stoichiometry of the isolated complexes (Figure 1), and that in this case the obtaining of compounds with coordination number 7 and pentagonal bipyramidal geometry was not possible. Thus, mixing of Ln(NO3)2·6H2O (Ln =Dy or Tb) with Ph3XO (X =P or As) in 1:2 molar ratio leads to isolate [Ln(NO3)3(Ph3PO)3] or [Dy(NO3)3(EtOH)(Ph3XO)2] compounds, as a function of the solvent employed in the reaction, as previously described for Ph3PO (Cousins and Hart, 1967). Accordingly, it also seems that the nature of the X atom (P or As) in the Ph3XO donor does not play any role in the stoichiometry of the isolated complexes. Besides, it should be noted that the compounds [Ln(NO3)3(Ph3PO)3]1and 2are also obtained when the salts and oxides are mixed in 1:3 molar ratio, and that these complexes Frontiers in Chemistry | www.frontiersin.org 3September 2018 | Volume 6 | Article 420 Fondo et al. SMMs of Phosphineand Arsine-Oxides FIGURE 1 | Reaction scheme for isolation of the metal complexes. present the same stoichiometry as those reported for Ln =La, Nd, Eu, Er, Tm, and Yb with Cy3PO (Hunter et al., 2007). When the nitrate salt was changed by dysprosium chloride, and Ph3AsO was added in 1:2 molar ratio, the hydrated complex [DyCl2(Ph3AsO)4]Cl·1.25H2O was obtained (Figure 1). Its stoichiometry is the same described for [DyCl2(Ph3PO)4]Cl·2H2O (Glazier et al. 2004), obtained by mixing of DyCl3and Ph3PO in 1:4 molar ratio, but clearly differs from that of [Dy(Cy3PO)2(H2O)5]Cl3(Lees and Platt, 2014), which also has been obtained by mixing of DyCl3and Cy3PO in 1:4 molar ratio in ethanol. Accordingly, comparison of this experiment with the reactions with dysprosium nitrate and with previous results (Glazier et al., 2004; Hunter et al., 2007;Lees and Platt, 2014) clearly show that the cone angle of the phosphine plays a fundamental role in the stoichiometry of phosphine or arsine oxide complexes, but that the similarity between the chemistry of Cy3PO and Ph3XO also depends on the nature of the anion of the salt employed. Since three nitrate donors or two of the chloride anions are coordinated to the lanthanoid metal center in 1-4and related complexes (Glazier et al., 2004; Hunter et al., 2007; Bowden et al., 2014), a salt of a non-coordinating anion, such as triflate, was chosen, in order to see the influence of the coordinating ability of the anion of the salt in the final stoichiometry, and in the hope Frontiers in Chemistry | www.frontiersin.org 4September 2018 | Volume 6 | Article 420 Fondo et al. SMMs of Phosphineand Arsine-Oxides of isolating complexes of stoichiometry [Dy(R3XO)2(H2O)5]Y3. In this way, mixing of dysprosium triflate with MePh2PO in 1:2 molar ratio, and in a non-dried non-coordinating solvent such as THF, yields [Dy(OTf)2(MePh2PO)4]OTf, which also has a molar ratio metal:oxide of 1:4. Thus, this experiment shows that, in spite of the quantity of water present in the non-dried THF solvent, the dysprosium atom links to two poorly coordinating triflate ions, preventing the coordination of water to the metal center. Accordingly, this result resembles in part those obtained for Ce, Nd and Lu triflate complexes of Ph3PO (Fawcett et al., 2002; Berthet et al., 2003;), where the stoichiometry is the same, but for the Ce and Nd derivatives the coordination number is 7, with one triflate acting as a bidentate chelate ligand and the other one as a monodentate donor. Finally, the reactivity of Dy and Tb toward Ph3PO, in 1:1 molar ratio and in the presence of hexafluoroacetylacetone, was studied, given that a [Eu(tmh)3(RPh3PO)] (tmh = 2,2,6,6-tetramethylheptane-3,5-dione, R =H, m-Me or p-Me) complex, with coordination number seven, was previously reported (Yanagisawa et al., 2017). As Eu3+is significantly bigger than Tb3+and Dy3+, the isolation of Tb and Dy complexes with coordination number 7 was expected even in the presence of the less sterically hindered hexafluoroacetylacetone donor. Nevertheless, this synthesis leads to complexes [M(hfa)3(Ph3PO)2] (M =Tb, Dy) where the metal:oxide ratio is 1:2, as it occurs in the previously described [Eu(hfa)3(Ph3PO)2] and related compounds (Hasegawa et al., 2013), suggesting that for this kind of complex the size of the lanthanoid ion has a poor influence in the stoichiometry of the isolated compound. As a summary, comparison of the results described herein with those previously described for Cy3PO (Chen et al., 2016, 2017) seems to indicate that the lower cone angle for triphenylphosphine compared with tricyclohexylphosphine prevents the isolation of pentagonal bipyramidal complexes of triphenylphosphine oxide with dysprosium or terbium. In the same way, hexafluoroacetylacetone allows coordination of two phosphine oxide ligands, leading to coordination number of 8 for the lanthanoid atom while the more sterically hindered 2,2,6,6-tetramethylheptane-3,5-dione only allows linking one phosphine ligand, leading to coordination number 7 (Yanagisawa et al., 2017). Therefore, this study clearly shows that not only the cone angle of the phosphine oxides but also the volume of the auxiliary ligands are fundamental factors in controlling the coordination number in this type of complex, the salt:oxide metal ratio, and even the size of the lanthanoid ion, playing secondary roles. All the compounds described herein were fully characterized by analytical techniques, IR spectroscopy and by single crystal X-ray diffraction studies, except 3. In addition, direct and alternating current magnetic measurements were recorded for all the compounds. The IR spectra of all the phosphine oxide complexes show strong bands in the range 1119–1153 cm−1, which can be assigned to the P=O stretching vibration (Bowden et al., 2012). For complexes 1,2, and 3there are two bands in this range, indicating the existence of two types of non-equivalent phosphine oxide. Nevertheless, complexes 6-8present just one band at ca. 1135 cm−1, which suggest that all the phosphine oxide ligands present in the compounds are equivalent. Complexes 4and 5·1.25H2O show strong bands about 900 cm−1, which can be assigned to As=O vibrations (Levason et al., 2001). In 4two bands are observable, also suggesting two nonequivalent arsine oxide ligands, while 5·1.25H2O only shows an As=O band, in agreement with the equivalence of the four arsine oxide donors present in the complex. In addition, the synthetized complexes also present bands related to the auxiliary nitrate, triflate or hexafluoroacetylacetonate ligands. Thus, complexes 1-4 show a strong band at ca. 1305 cm−1, assigned to the ν1(N-O) vibration of the nitrate ligand, acting as bidentate (Bowden et al., 2012). Complex 6show many bands (633, 1020, 1262, 1156, and 1220 cm−1) related to S-O or C-F vibrations, which suggest the presence of triflate (Johnston and Shiver, 1993). Finally, the IR spectra of 7and 8present strong bands at 1652 cm−1, assigned to the stretching frequency of the C=O group, and at 1250 and 1161, in agreement with vibrations of the CF3and C-H moieties, all of which clearly suggest the presence of the hexafluoroacetylacetonate ligand (Richardson et al., 1968). X-Ray Diffraction Studies [Tb(NO3)3(Ph3PO)3]·CH3CN (1·CH3CN) and [Dy(NO3)3(Ph3PO)3]·CH3CN (2·CH3CN) The crystal structures of 1·CH3CN and 2·CH3CN are very similar and they will be discussed together. The unit cell of each complex contains neutral [M(NO3)3(Ph3PO)3] (M = Tb or Dy) molecules, in addition to acetonitrile as solvate. Ellipsoid diagrams for 1and 2are shown in Figure 2,Figure S1, respectively, and main distances and angles in Table S2. Both complexes crystallize in the P21/c group, with no symmetry elements relating the different ligands. In the [M(NO3)3(Ph3PO)3] molecules, three phosphine oxide monodentate ligands and three bidentate chelate nitrate donors complete the coordination sphere of the metal ions. Thus, in both complexes, the lanthanoid ion is in an O9environment. Calculations of the degree of distortion of the LnO9core with respect to a perfect nine-vertex polyhedron using the SHAPE software (Llunell et al., 2005, 2010; Ruiz-Martínez et al., 2008) lead to shape measurements between spherical capped square antiprism and muffin-like, but closer to spherical capped square antiprism (Figure 3,Table S3). In these polyhedra, all the distances and angles are within their usual range for lanthanoid complexes with phosphine oxide and nitrate donors (Bowden et al., 2011), showing M-OPh3PO distances notably shorter than the M-Onitrate ones (Table S2). [Dy(NO3)3(EtOH)(Ph3AsO)2] (4) An ellipsoid diagram for 4is shown in Figure 4 and main bond distances and angles in Table S4. The crystal structure of 4resembles that of 2. Besides, it should be noted that 4is isomorphous with the previously described and crystallographically characterized [La(NO3)3(EtOH)(Ph3AsO)2] complex (Levason et al., 2001). Thus, three bidentate chelate nitrate ligands, two monodentate Ph3AsO donors and an ethanol molecule fill the O9coordination sphere of the dysprosium Frontiers in Chemistry | www.frontiersin.org 5September 2018 | Volume 6 | Article 420 Fondo et al. SMMs of Phosphineand Arsine-Oxides FIGURE 2 | Ellipsoids diagram (50% probability) for 1. Only the Tb, P, and donor O-atoms have been labeled, for clarity. Color code: Tb, light blue; C, gray; H, light gray; N, dark blue; P, orange; O, red. FIGURE 3 | Coordination polyhedron for Dy in 2, illustrating the distorted spherical capped square antiprism geometry shown by the metal ion in 1and 2. ion. SHAPE calculations indicate that the geometry about the metal ion is also between spherical capped square antiprism and muffin-like, as in 1and 2, but in this case the muffin-like disposition seems to be a bit more stable (Table S2). All the distances and angles in this complex are within the usual range, but it should be noted that the Dy-Oarsine oxide distances are a bit shorter than the corresponding Dy-Ophospine oxide ones in 2, FIGURE 4 | Ellipsoids diagram (50% probability) for 4. Only the Dy, As, and donor O-atoms have been labeled, for clarity. Color code: Dy, light blue; As, violet; C, gray; H, light gray; N, dark blue; O, red. and this can be a consequence of the smaller electronegativity of arsenic with respect to phosphorous, and, therefore, to the greater donor strength of the arsine oxide, given that the geometry of both complexes is very similar. Finally, it is worth of mention that this complex shows a hydrogen bond between one oxygen atom (O6) of one nitrate Frontiers in Chemistry | www.frontiersin.org 6September 2018 | Volume 6 | Article 420 Fondo et al. SMMs of Phosphineand Arsine-Oxides donor and the ethanol ligand of a neighboring unit. This hydrogen bond is reciprocal and gives rise to a pseudodimer, with a Dy···Dy distance of 7.1870(9) Å (Figure 5). [DyCl2(Ph3AsO)4]Cl·2.75CH3CH2OH·1.25H2O (5·2.75 CH3CH2OH·1.25H2O) An ellipsoid diagram for [Dy(Ph3AsO)4(Cl)2]Cl is shown in Figure 6 and main distances and angles in Table S5. The asymmetric unit of 5·2.75CH3CH2OH·1.25H2O contains a [DyCl2(Ph3AsO)4]Cl molecule, and ethanol and water as solvates. The [DyCl2(Ph3AsO)4]+cation shows the Dy(III) ion FIGURE 5 | Hydrogen bond for 4, showing the pseudodimer. FIGURE 6 | Ellipsoids diagram (50% probability) for [DyCl2(Ph3AsO)4]+in 5. Only the Dy, As, Cl and donor O-atoms have been labeled, for clarity. Color code: Dy, light blue; As, violet; C, gray; Cl, green; H, light gray; O, red. joined to four Ph3AsO ligands through their O-atom, and to two chloride ions. Thus, the dysprosium atom is in an O4Cl2 environment, with octahedral geometry. In this polyhedron, the chloride ions occupy the axial sites, what produces an elongation of the octahedron. Although the octahedral geometry is not very common for lanthanide ions, given that they tend to achieve higher coordination numbers, the structure of [DyCl2(Ph3AsO)4]Cl is very similar to that previously described for other [LnX2(R3PO)4]X (X =Cl or Br) compounds (Glazier et al., 2004; Bowden et al., 2010; Platt, 2017), with distances and angles in the usual range, and that do not merit further consideration. [Dy(OTf)2(MePh2PO)4]OTf·THF (6·THF) The unit cell of 6·THF contains two crystallographically different but chemically equivalent [Dy(OTf)2(MePh2PO)4]+cations, which will be called 6A and 6B, respectively, and CF3SO− 3 counterions and THF as solvate. Each [Dy(OTf)2(MePh2PO)4]+ cation has an inversion center, located at the dysprosium atom. Besides, one of these [Dy(OTf)2(MePh2PO)4]+cations, the one containing Dy2 (6B), has the coordinated triflate anions disordered over two sites (80 and 20% occupancy). An ellipsoid diagram for one of the [Dy(MePh2PO)4(OTf)2]+ ions (6A) is shown in Figure 7 and main bonds distances and angles in Table S6. In both [Dy(MePh2PO)4(OTf)2]+cations, the DyIII ions are coordinated to four phosphine oxides and to two triflate ions, which act as monodentate ligands. This gives rise to an O6octahedral environment about the dysprosium center. It should be noted that three lanthanoid complexes of empirical formula [Ln(OTf)2(Ph3PO)4](OTf)(Ln =Ce, Nd, Lu) have been FIGURE 7 | Ellipsoids diagram (50% probability) for [Dy(MePh2PO)4(OTf)2]+ in 6A. Only the Dy, P, and donor O-atoms have been labeled, for clarity. Color code: Dy, light blue; C, gray; F, fluorescent yellow; H, light gray; O, red; P, orange. Frontiers in Chemistry | www.frontiersin.org 7September 2018 | Volume 6 | Article 420 Fondo et al. SMMs of Phosphineand Arsine-Oxides previously reported, and in Ce and Nd complexes one of the triflate ligands acts as bidentate chelate and the other one as monodentate (Fawcett et al., 2002; Berthet et al., 2003), leading to coordination number 7, while in 6both triflate donors act as monodentate. In fact, the structure of 6is more similar to that of [Lu(OTf)2(Ph3PO)4](OTf) (Fawcett et al., 2002), where the metal ion is also in an octahedral environment. This seems to be related to the smaller size of DyIII compared with CeIII and NdIII that, in spite of the less sterically hindered phosphine oxide in 6(MePh2PO) respect to the Ce and Nd complexes (Ph3PO), cannot allocate seven atoms in its surroundings. The shortest Dy-O distances in both 6A and 6B cations correspond to Dy-Ophosphine oxide, the weakest bonds being the Dy-Otriflate ones, as expected in view of the poor ability of the triflate anions to act as ligands. Thus, the polyhedra are axially elongated. Nevertheless, it should be noted that while in 6A all the Dy-Ophosphine oxide distances are very close, in 6B the strength of these bonds differs significantly (Dy2-O21 distance of ca. 2.19 Å and Dy2-O20 distance of ca. 2.24 Å) and the Dy-Otriflate bond is a bit stronger in 6B than in 6A, showing a less elongated octahedron. [Tb(hfa)3(Ph3PO)2] (7) and [Dy(hfa)3(Ph3PO)2] (8) The crystal structures of both complexes are very similar, and they will be discussed together. The main difference between them is that the fluoride atoms of one CF3moiety of one triflate ligand are disordered over two sites (56/44%) in 7, while this does not occur in 8. Ellipsoids diagrams for 7and 8are shown in Figure S2,Figure 8, respectively, and main bond distances and angles in Table S7. FIGURE 8 | Ellipsoids diagram (50% probability) for 8. Only the Dy, P, and donor O-atoms have been labeled, for clarity. Color code: Dy, light blue; C, gray; F, fluorescent yellow; H, light gray; O, red; P, orange. The structure of both compounds resembles that of [Eu(hfa)3(Ph3PO)2] (Hasegawa et al., 2013). Thus, in both complexes the lanthanoid ion is coordinated to three bidentate chelate hexafluoroacetylacetonate anions and to two phosphine oxides, giving rise to an O8environment. Calculations of the degree of distortion of the MO8environment with respect to an ideal eight-vertex polyhedron with the SHAPE software (Llunell et al., 2005, 2010; Ruiz-Martínez et al., 2008), indicates that the geometry is closer to square antiprism in both cases. In these polyhedra, all the distances and angles are in the expected range and do not merit further consideration. Magnetic Properties The direct current (dc) magnetic susceptibility studies of all the complexes (1-8) were recorded under a magnetic field of 1000 Oe in the temperature range 2–300 K. The plots of χMT vs. T for the complexes are shown in Figure S3. At room temperature (300 K), the χMTproducts for the terbium complexes 1and 7are in the range 11.81–12.37 cm3kmol−1while for the dysprosium complexes 2-6 and 8vary from 14.05 to 14.93 cm3kmol−1. All these values are similar to the expected ones for one isolated TbIII (4f8,7F6) or DyIII (4f9,6H15/2) ion of 11.82 and 14.17 cm3kmol−1, respectively. The experimental χMTvalues remain basically constant (1, 2), decreases very slowly (6-8) or decrease a bit quickly (35·1.25H2O) upon cooling up to 50 K, and more rapidly below this temperature in all cases. This behavior appears to be mostly owing to the depopulation of the MJsublevels of the lanthanoid ion, and possibly to very weak intermolecular interactions, which could be the cause of the more marked decrease in χMTat low temperature. The variation of the magnetization with the field at 2 K for 1-8 (Figure S3) are akin in all cases, and shows a quite fast increase of the M/NµBvalue at low field, without attaining saturation at 5 T. The M/NµBvalue at the largest applied field of 5 T (in the 5.07– 5.77 Ms/NµBrange) is remarkably lower than the predictable one for isolated TbIII or DyIII ions (Ms/NµB=gjJ), which can be ascribed to the crystal-field effects that promote a significant magnetic anisotropy. Alternating current (ac) magnetic susceptibility measurements were also done for microcrystalline samples of compounds 1-8, with the aim of knowing their low-temperature magnetic relaxation behavior. No one of the compounds show out-of-phase ac susceptibilities (χ′′) peaks at a frequency of 1400Hz at zero field, indicating the absence of SMM behavior. Nevertheless, it should be noted that, sometimes, application of an external static field (Ruiz et al., 2012), in order to eliminate the mixing of the ground ±Ms levels, can partially or completely suppress the quantum tunneling of the magnetization (QTM), allowing to observe the slow relaxation process through the real thermally activated energy barrier (U). Therefore, this approximation was tested with the aim of checking the quantum tunneling effects. Accordingly, variable temperature alternating current (ac) susceptibilities were measured for all the compounds at 1400Hz and under a small dc field of 1000 Oe. In these conditions, the χ”vs. T curve only shows a clear net peak for 2,4, and 8 Frontiers in Chemistry | www.frontiersin.org 8September 2018 | Volume 6 | Article 420 Fondo et al. SMMs of Phosphineand Arsine-Oxides (Figure S4), but in the case of 4the peak appears at 2.5 K at a frequency of 1400 Hz. Thus, in view of these results, given that the magnetic response of 4seems too weak, variable frequency and temperature ac susceptibilities were measured only for 2 and 8with the application of an experimental optimal dc field of 1000 Oe. As shown in Figure 9, in these conditions 2and 8show frequency and temperature-dependence of the out-ofphase susceptibility, with maxima for χ” below 5 and 5.8 K for 2and 8, respectively. This indicates that both complexes (2and 8) are fieldinduced single ion magnets. In addition, it should be noticed that Figure 9 reveals that χ” goes to zero below the maxima FIGURE 9 | Frequency dependence of out-of-phase component of the ac susceptibility (χ”) under a dc applied field of 1000 Oe at different temperatures for 2 (A) and 8 (C); and Arrhenius plot for 2 (B) and 8 (D). The solid lines represent the best fit to simple Arrhenius or Orbach-Raman equation. FIGURE 10 | Cole–Cole plot in 1000 Oe dc field for 2 (A) and 8 (B). Frontiers in Chemistry | www.frontiersin.org 9September 2018 | Volume 6 | Article 420