Lanthanide Triangles Supported by Radical Bridging Ligands

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Lanthanide Triangles Supported by Radical Bridging Ligands Brian S. Dolinar, Dimitris I. Alexandropoulos, Kuduva R. Vignesh, Tia'Asia James, and Kim R. Dunbar J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b12495 • Publication Date (Web): 19 Dec 2017 Downloaded from http://pubs.acs.org on December 19, 2017

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Lanthanide Triangles Supported by Radical Bridging Ligands Brian S. Dolinar, Dimitris I. Alexandropoulos, Kuduva R. Vignesh, Tia’Asia James, Kim R. Dunbar* Department of Chemistry, Texas A&M University, College Station, TX 77842-3012, United States Supporting Information Placeholder ABSTRACT: The first examples of metallacycles containing lanthanide ions bridged by radicals are reported. The molecular • triangles [Ln3(hfac)6(bptz -)3] (Ln = DyIII, YIII; hfac = 1,1,1,5,5,5hexafluoro-2-4-pentanedionate; bptz = 3,6-bis(2-pyridyl)-1,2,4,5tetrazine) consist of lanthanide ions bridged by bptz radical anion • (bptz - ) ligands. Magnetic susceptibility measurements and • CASSCF calculations performed on [Dy3(hfac)6(bptz -)3] reveal the presence of antiferromagnetic coupling between the DyIII cen• ters and the bptz - ligands with J = -6.62 cm-1. Supramolecular chemistry, the chemistry of non-covalent forces, is one of the most fascinating interdisciplinary areas of chemistry, physics and biological research.1 The weak intermolecular interactions that control the formation of supramolecular entities are often complemented by spatially and directionally oriented coordination bonds.2 The geometry adopted by the metal ion, typically octahedral, square planar or tetrahedral, combined with the denticity and spatial orientations of the bridging ligands greatly influences the ensuing self-assembled systems. This strategy3 has led to metallacyclic architectures, ranging from molecular triangles to cages,4,5 capsules,6 and various polyhedral structures.1,4,6-7 The most common metallacyclic motif is the molecular square,3,7a,7f,7g with many examples appearing in the literature since the pioneering studies of Stang7e,8 and Fujita.7c The formation of far less common molecular triangles and pentagons can be favored by utilizing rigid bridging ligand geometries or the presence of non-coordinating species in the reaction mixture.2,5c,9 Research in our laboratory determined that the 3,6-bis(2-pyridyl)1,2,4,5-tetrazine (bptz) ligand is capable of supporting both molecular squares and pentagons of the type [M4(MeCN)8(bptz)4]8+ and [M5(MeCN)10(bptz)5]10+ (M = FeII, NiII).5b-g In these compounds, the nuclearity of the metallacycle is dictated by the size and shape of the anions that engage in anion-π interactions. Predominantly, bridging ligands of metallacyles are closedshell ligands. The ability of bptz to be reduced to form a radical • anion, (bptz -) and its history of use in metallacycles make it ideal for generating radical-bridged metallacycles.10 Currently, only three examples of transition metal metallacycles with radical bridging ligands have been reported, viz., [Cp2Co]6[Mn6(N,OL)6] (N,OL = radical form of 4,5-bis(pyridine-2-carboxamido)-1,2catechol) reported by the Harris group and the molecular triangle, • • [Co3(dbm)3(bptz -)3], and square, [Co4(dbm)4(bptz -)4], (dbm = 1,3-diphenyl-1,3-propanedionate) recently prepared by us.11 In these cases, strong direct magnetic coupling is observed between the metal ions and the radical bridging ligand. Lanthanide metallacycles are scarce. In stark contrast to the plethora of oxide, hydroxide, and carboxylate bridged polynuclear rare earth complexes in the literature,12 only one example of a

metallacyclic Ln3 triangle has been reported.12e Polynuclear lanthanide complexes exhibit very weak superexchange interactions between lanthanide ions through diamagnetic linkers. Hence, their magnetic properties often reflect single-ion effects of isolated 4f spin centers.13,14 Magnetic interactions in 4f metal complexes are greatly enhanced by employing radical bridges as evidenced by reports of dinuclear and trinuclear complexes bridged by a single radical-bearing ligand that exhibit superior single molecule magnetic properties.15 Currently, no examples of radical-bridged lanthanide metallacyclic triangles exist. Our recent success in synthesizing neutral- and radical-bridged lanthanide complexes, [[Dy(tmhd)3]2(bptz)] and • [Cp2Co][[Dy(tmhd)3]2(bptz -)] (tmhd = 1,1,6,6-tetramethylhexanedionate)15f inspired us to explore this chemistry with different supporting ligands, These efforts led to the isolation of a radical-bridged lanthanide metallacyclic triangle • [Dy3(hfac)6(bptz -)3] (2-Dy, hfac = 1,1,1,5,5,5-hexafluoro-2-4pentanedionate). This molecule is an unexpected product of the one-electron reduction of the neutral-bridged dinuclear species [[Dy(hfac)3]2(bptz)] (1-Dy). The compounds were characterized by X-ray crystallography, SQUID magnetometry, and CASSCF calculations.

Scheme 1. The synthetic route employed for the synthesis of 1-Ln and 2-Ln. Compound 1-Dy was prepared by reacting Dy(hfac)3·2H2O with bptz in toluene. The two reactants were dissolved separately in refluxing toluene and combined, quickly forming an intensely colored red solution of 1-Dy. Upon cooling to 0° C, large X-ray quality crystals of 1-Dy formed, which were collected by filtration and washed with toluene. Single crystal X-ray diffraction revealed the structure of 1-Dy (Figure 1a). Both Dy atoms of 1-Dy are

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bonded to three hfac ligands and connected via a neutral bptz ligand, giving a square antiprismatic coordination geometry similar to that of [[Dy(tmhd)3]2(bptz)].15f Reduction of 1-Dy with Cp2Co in Et2O afforded a dark brown solution and a pale-yellow precipitate. Addition of pentane to the reaction mixture led to the co-precipitation of two different crystals – one dark orange-brown, and the other pale yellow. Single crystal diffraction of the dark orange crystals gives the structure of 2-Dy (Figure 1b). The two products were separated by extraction with toluene. After removing the toluene under vacuum, crystals of 2-Dy were grown by diffusion of pentane into a Et2O solution of the residue.

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angles of the octahedral metal centers in the triangle • [Co3(dbm)3(bptz -)3] (88.0° – 92.5°).11b As a result, the bptz ligands in 2-Dy are less distorted (average dihedral angle 3.8°) as • compared to [Co3(dbm)3(bptz -)3] (average dihedral angles ~10°).11b The pale yellow crystals that co-precipitate with 2-Dy were shown to be [Cp2Co][Dy(hfac)4] by X-ray crystallography, consistent with the stoichiometry in Scheme 1. Formation of this salt requires one of the Dy centers from 1-Dy to sequester an hfac ligand from the other Dy center, resulting in a coordinatively un• saturated intermediate “Dy(hfac)2(bptz -)”, which can oligomerize to form higher nuclearity species, such as 2-Dy. The static DC magnetic susceptibility properties of 1-Dy and 2-Dy were measured from 300 K to 2 K under an applied field of 0.1 T (Figure 2). Compound 1-Dy exhibits a χT value of 28.91 emu K mol-1, which is in excellent agreement with the theoretical value for two non-interacting DyIII ions (MJ = 15/2, χT = 14.2 emu K mol-1). As the temperature is lowered from 300 K to 6 K, χT decreases to 24.4 emu K mol-1 and then sharply decreases to 23.1 emu K mol-1 at 2 K, consistent with the magnetic susceptibility data found for the related complex [[Dy(tmhd)3]2(bptz)].

Figure 2. The χT vs. T plots for 1-Dy, 2-Dy, and 2-DyY measured under an applied field of 0.1 T.

Figure 1. Structures of a) 1-Dy and b) 2-Dy. The purple atoms are Dy, blue (N), red (O), green (F), black (C). Hydrogen atoms are omitted for clarity. The DyIII ions of 2-Dy are bonded to two hfac ligands and two • bptz - ligands, leading to a square antiprismatic geometry. The nitrogen atoms of the tetrazine rings (Ntz) of the bptz ligands exhibit Ntz-Ntz bond distances ranging from 1.376(9) Å to 1.399(9) Å, significantly longer than the corresponding distances of neutral bptz (~1.33 Å).10,15f,16 The elongated Ntz-Ntz bond distances and the charge balance of the structure are consistent with the assignment of the bptz ligand as a one-electron reduced radical. The average intramolecular Dy···Dy distances range from 7.54 Å to 7.58 Å with Dy···Dy···Dy angles ranging from 59.8°-60.3°, giving a nearly perfect equilateral triangle. The square antiprismatic coordination geometry of the DyIII ions results in Ntz-Dy-Ntz bond angles of 70.8(2)°, 72.2(3)°, 73.9(3)° which are closer to the 60° angles required for an equilateral triangle than the Ntz-Co-Ntz

2-Dy exhibits different magnetic properties than 1-Dy. At 300 K, 2-Dy has a χT value of 41.9 emu K mol-1, which is less than the expected value for three uncoupled DyIII ions (MJ = 15/2, χT = 14.2 emu K mol-1) and three uncoupled organic radicals (g = 2, S = ½, χT = 0.375 emu K mol-1). Upon cooling, χT decreases to a value of 33.65 emu K mol-1 at 16 K, increases to 33.76 emu K mol-1 at 5 K, and finally decreases to 32.79 emu K mol-1 at 2 K. The increase in χT between 16 K and 5 K is consistent with anti• ferromagnetic coupling between the bptz - ligands and the DyIII ions. In order to assess the importance of intermolecular interactions in 2-Dy, its YIII analog, [Y3(hfac)6(bptz•-)3] (2-Y) was prepared. 2-Y was synthesized analogously to 2-Dy from [[Y(hfac)3]2(bptz)] (1-Y), and isolated in similar yields. Crystals of a 1:7 [Dy3(hfac)6(bptz•-)3]:[Y3(hfac)6(bptz•-)3] (2-DyY) were obtained by slow diffusion of pentane into a 1:7 2-Dy:2-Y mixture dissolved in Et2O. The DC magnetic susceptibility of 2-DyY was measured between 300 K and 2 K under an applied field of 0.1 T (Figure 2). At 300 K, the value of χT is 6.52 emu K mol-1, which is in agreement with the theoretical value expected for a 1:7 ratio of 2-Dy:2-Y (6.24 emu K mol-1). The value of χT decreases to 5.73 emu K mol-1 at 40 K, and then increases to a value of 6.60 emu K mol-1 at 3.0 K, maintaining that value until 2 K. This behavior further supports the assignment of antiferromagnetic coupling between the bptz•- ligands and the DyIII ions.


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Figure 3. a) Out-of-phase signals for 1-Dy between 2-15 K. b) Arrhenius plot of relaxation times of 1-Dy. c) relaxation pathways of 1-Dy. d-f) magnetic relaxation pathways of Dy1, Dy2, and Dy3 centers in 2-Dy. Blue lines are Kramers doublets (KDs) as a function of computed magnetic moment, red arrows are QTM/TA-QTM pathways, and green/purple arrows are Orbach/Raman relaxation pathways. The numbers along the arrows are mean absolute values for the corresponding matrix element of transition magnetic moment. AC magnetic susceptibility measurements were performed on 1-Dy, 2-Dy, and 2-DyY. Compound 1-Dy (Figure 3a) exhibits out-of-phase signals from 2 K to 15 K under a zero applied DC field. At 2 K, the data exhibit a peak in χ” at 10 Hz which is temperature independent until 4 K at which temperature the peak becomes temperature dependent. A Cole-Cole plot of these data (Figure S5) were fit using CC-fit to obtain relaxation times τ for each temperature.17 Fitting the linear region of the Arrhenius plot of ln τ vs. 1/T (Figure 3b) gave a thermal relaxation barrier of 60 K. In contrast, 2-Dy exhibits only the beginning of an out-ofphase signal. While the increased magnetic coupling resulting from a radical bridging ligand typically improves SMM behavior, other factors can be an influence.18 Moreover, dilution of 2-Dy with 2-Y did not improve the SMM properties (Figure S7). In order to better understand the magnetic data, we employed CASSCF/RASSI/SINGLE_ANISO calculations as well as POLY_ANISO using MOLCAS 8.0 (computational details, SI). These calculations explain the different AC data for 1-Dy and 2Dy. For 1-Dy, the calculated gz value of 19.56 for the symmetry equivalent DyIII sites is close to the Ising-limit value of 20, suggesting strong axial magnetic anisotropy (Table S2). The ground state Kramers doublets (KDs) of both the DyIII ions in 1-Dy are primarily mJ = ±15/2 in character with small mJ = ±11/2 contributions (Figure 3c), suggesting a plausible mechanism for quantum tunneling of magnetization (QTM). The first excited state KDs are highly mixed with contributions from the mJ = ±13/2, ±11/2, and ±9/2 states, resulting in a large Thermally Assisted QTM (TAQTM) process that facilitates magnetic relaxation with an energy barrier of 108.3 cm-1. Thus, these calculations accurately predict the SMM behavior of 1-Dy but overestimate the thermal energy barrier, likely due to the exclusion of intermolecular and hyperfine interactions from the calculation.19 For 2-Dy, two of the Dy sites (Table S2,) are axial, with gz values of ~19.0 and relatively small transverse gx and gy values. The Dy3 site (Table S2) exhibits larger gx and gy values, providing a pathway for QTM. The ground state KDs of Dy1 and Dy2 are similar to those of 1-Dy and are primarily mJ = ±15/2 in character with small mJ = ±11/2 contributions (Figure 3d-e). The ground state KDs of Dy3 have large contributions from all mJ states consistent with the observed QTM (Figure 3f). Using the lowest energy states of single DyIII ions, the DC magnetic susceptibilities of 1-Dy and 2-Dy were fit using

POLY_ANISO by systematically varying the JDy-rad and JDy-Dy (computational details, SI) coupling constants (Figure 2, Table S5).20 The JDy-rad of 2-Dy is large with a value of -6.62 cm-1 that is comparable to those found in [(Cp*2Gd)2(tppz•-)][BPh4] and [K(crypt-2,2,2)][(Cp*2Gd)2(tppz•-)] (tppz = 2,3,5,6-tetra(2pyridyl)-pyrazine),15c and it is among the highest coupling constants reported for a Ln-radical system.15d This antiferromagnetic coupling gives rise to a ferrimagnetic ground state in which the three DyIII ions are magnetically aligned and the three radicals are anti-aligned. An intermolecular coupling constant zJ of -0.06 cm-1 was necessary to obtain a good fit with the data and is consistent with the observed differences in the susceptibility of 2-Dy and the magnetically diluted 2-DyY sample (vide supra). In summary, the first example of a radical-bridged lanthanide containing metallacycle is reported. The metallacycle consists of a • triangle of DyIII ions connected by bptz - ligands. SQUID magnetometry reveals that this compound exhibits significant antiferromagnetic coupling between the radicals and the DyIII ions. Calculations confirm the origin of the antiferromagnetic coupling with a coupling constant of -6.62 cm-1. The successful synthesis of this compound and demonstration of coupling bode well for the synthesis of higher nuclearity lanthanide supramolecular compounds with appreciable magnetic interactions between the lanthanide ions and superior SMM properties.

ASSOCIATED CONTENT Supporting Information Experimental details, calculations, magnetic and crystallographic data. This data is available free of charge on the internet at pubs.acs.org.

AUTHOR INFORMATION Corresponding Author Email: [email protected]

ACKNOWLEDGMENTS We gratefully acknowledge funding from the National Science Foundation (CHE-1310574). K.R.V. would like to thank Dr. Liviu Ungur, University of Leuven for his assistance with POLY_ANISO. The X-ray diffractometer and SQUID magne-



tometer used in this research were purchased with funds provided by the Texas A&M University Vice President of Research. We would like to thank the HPRC at Texas A&M for providing computing resources.

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Lanthanide Triangles Supported by Radical Bridging Ligands

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