Binuclear Lanthanide-Radical Complexes Featuring Two Centers

Synopsis. Binuclear complexes with formula [Ln2(hfac)6(H2O)2(dppnTEMPO)] (LnIII = Gd, Tb, and Dy) have been obtained using the paramagnetic ligand 1-p...
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Binuclear Lanthanide-Radical Complexes Featuring Two Centers with Different Magnetic and Luminescence Properties Samira G. Reis,† Matteo Briganti,†,‡ Stéphane Soriano,∥,§ Guilherme P. Guedes,⊥ Sergiu Calancea,† Carmen Tiseanu,# Miguel A. Novak,⊗ Miguel A. del Á guila-Sánchez,◇ Federico Totti,‡ Fernando Lopez-Ortiz,◇ Marius Andruh,*,△ and Maria G. F. Vaz*,† †

Universidade Federal Fluminense, Instituto de Quı ́mica, 24020-150, Niterói, Rio de Janeiro, Brazil Universidade Federal Fluminense, Instituto de Fı ́sica, 24210-346, Niterói, Rio de Janeiro, Brazil ‡ Università degli Studi di Firenze, via della Lastruccia 3, 50019, Sesto Fiorentino, Firenze, Italy § Consiglio Nazionale delle Ricerche − Istituto di Chimica dei Composti Organo-Metallici, via Madonna del Piano 10, 50019, Sesto Fiorentino, Firenze, Italy ⊥ Universidade Federal Rural do Rio de Janeiro, Departamento de Química, 23870-000, Seropédica, Rio de Janeiro, Brazil # National Institute for Laser, Plasma and Radiation, Str. Atomistilor 409, 077125-Magurele, Romania ⊗ Universidade Federal do Rio de Janeiro, Instituto de Física, Rio de Janeiro, 21941-972, Rio de Janeiro, Brazil ◇ ́ Area de Química Orgánica, Universidad de Almería, Crta. Sacramento s/n, 04120, Almería, Spain △ Inorganic Chemistry Laboratory, Faculty of Chemistry, University of Bucharest, 020464, Bucharest, Romania ∥

S Supporting Information *

ABSTRACT: Binuclear complexes with general formula [Ln2(hfac)6(H2O)2(dppnTEMPO)] (LnIII = Gd, Tb, and Dy) have been obtained using the paramagnetic ligand 1piperidinyl-4-[(diphenylphosphinyl)amino]-2,2,6,6-tetramethyl (dppnTEMPO) as a bridge. One of the lanthanide ions is ferromagnetically coupled with the TEMPO moiety. Two of the complexes (Dy and Tb) show slow relaxation of the magnetization, and the non-magneto-equivalence of the two LnIII ions was clearly observed. The ab initio CASSCF calculations were employed to confirm this behavior, as well as to rationalize the Ln−Rad interaction. The simulations of the magnetic properties were allowed by the insights given by the calculations. The inequivalence of the TbIII ions was also proved by emission spectroscopy.

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molecular entities in the crystal;5 or (ii) the oligonuclear species containing two crystallographically/chemically nonequivalent metal centers.6 The known examples of compounds with two relaxation processes are obtained rather serendipitously. In this paper, we report on a binuclear lanthanide complex family, with nonequivalent metal centers, which was designed using as a ligand the TEMPO molecule functionalized with an additional phosphinic amide coordinating group: 1-piperidinyl-4[(diphenylphosphinyl)amino]-2,2,6,6-tetramethyl (dppnTEMPO) (Chart 1). This molecule can act as a bridging ligand through the two oxygen atoms.7 In order to explore this potentiality, we have now investigated the coordination of dppnTEMPO to lanthanide ions, because of two of their important properties: magnetism and luminescence. The

INTRODUCTION

After the characterization of the first single molecule magnet (SMM),1 the research area of molecular nanomagnets experienced a fast growth in the past decade. Most of these systems, characterized by magnetic hysteresis of molecular origin, are homo- and heterometallic oligonuclear or metalradical complexes.2 In 2003, Ishikawa published the first mononuclear complex showing slow relaxation of the magnetization,3 opening the route toward the systematic investigation of the so-called single ion magnets (SIMs). More recently, it has been shown that the combination of three different spin carriers within the same molecular entity represents a valuable synthetic strategy leading to SMMs.4 From the huge number of SMMs characterized to date, there are several examples showing two relaxation processes. Such a behavior has been addressed to one of the following causes: (i) coexistence of two crystallographically different, either mono- or oligonuclear, © XXXX American Chemical Society

Received: July 12, 2016

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DOI: 10.1021/acs.inorgchem.6b01616 Inorg. Chem. XXXX, XXX, XXX−XXX

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with the SHELXS and SHELXL program packages.13 All atoms except hydrogens were refined anisotropically. Hydrogen atoms were set in calculated positions and refined as riding atoms. Additionally, for compound 2 the absence of other phases was ruled out by powder Xray diffraction (PXRD). The experimental pattern is in good agreement with the simulated one obtained from single-crystal data (Figure S1). Magnetic Measurements. Dc magnetic measurements were performed on a Cryogenic Sx600 SQUID magnetometer and on a VSM option of a Quantum Design PPMS in the temperature range 2− 290 K. The powder sample was pressed in Teflon tape and placed in a gelatin capsule, and the magnetic data were corrected for the contribution of the sample addenda. The sample diamagnetism correction was estimated from Pascal’s constants. Ac measurements were performed on a Quantum Design PPMS susceptometer. Luminescence Measurements. The luminescence measurements were done in the solid state using powder samples. The luminescence and excitation spectra were measured with a Fluoromax 4 spectrofluorometer (Horiba). The emission and excitation slits varied between 0.05 and 1 nm. The luminescence decays were measured by using the “decay by delay” feature of the phosphorescence mode. The time-resolved emission spectra were also recorded using a wavelength tunable (from 210 to 2300 nm) NT340 Series EKSPLA OPO (Optical Parametric Oscillator) operated at 10 Hz as excitation light source. The tunable wavelength laser had a narrow line width 2σ(I)) Final wR(F2) values (I > 2σ(I)) Final R1 values (all data) Final wR(F2) values (all data) Goodness of fit on F2 CCDC Deposition

smaller ionic radii when compared with GdIII and TbIII ions. The Ln1−O1−N1 and Ln2−O9−P1 bond angles are closer to each other for 1 and 2 (141.7(3)° and 152.5(8)° for 1, and 143.6(4) and 154.0(3)° for 2, respectively), while the analogous bond angles in dysprosium derivative 3 are 156.7(4) and 171.0(2)°. In the dppnTEMPO ligand, the P1− O9 bond length (distances of 1.502(3) Å for 1, 1.496(4) Å for 2 and 1.499(4) Å for 3) is somewhat elongated with respect to the uncoordinated (N-alkyl)-P,P-diphenylphosphinic amides, whose average is 1.485 Å,19 due to the coordination to the metal ions. In the isomorphic compounds 1 and 2, the intramolecular Ln1···Ln2 and the shortest intermolecular distances Ln1···Ln1i and Ln2···Ln2i are respectively 10.157(1), 5.624(1), and 5.816(1) Å for 1 and 10.2638(6), 5.7329(5), and 5.8713(5) for 2 (i = −x, y, 1/2 − z and ii = −x, −y, −z). Due to the differences observed in the crystal packing of 3 with respect to 1 and 2, the intermolecular distances involving the pairs Ln1··· Ln2iii and Ln1iii···Ln2 (iii = x, 1/2 − y, 1/2 + z) are equal to 5.8428(5) Å, while the intramolecular Ln1···Ln2 is the shortest in the family of compounds (9.9683(6) Å). The crystal packing in 1−3 is stabilized by weak Csp2−H···F contacts and a network of hydrogen bonds involving hfac (fluorine and oxygen atoms) and coordinated water molecules of neighboring molecules, which leads to a supramolecular zigzag chain formed by [Ln2(hfac)6(H2O)2dppnTEMPO] units, as shown in Figure 2 for the terbium derivative. Magnetic Properties. The magnetic properties of the compounds have been investigated in the temperature ranges

initio single ion anisotropy. The exchange coupling between the magnetic centers was computed within the Lines model15−18 (see SI).

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RESULTS AND DISCUSSION Crystal Structures. The reaction between [Ln(hfac)3] and dppnTEMPO radical [Ln = Gd (1), Tb (2), and Dy (3)] affords the expected binuclear complexes: [Ln2(hfac)6(H2O)2(dppnTEMPO)]. A summary of the data collection and selected bond distances and angles are listed in Tables 1 and 2. Compounds 1, 2, and 3 are isostructural, and the crystal structures consist of binuclear neutral species (Figure 1) in which the metal ions are bridged by the dppnTEMPO ligand. Both lanthanide ions show a distorted square antiprismatic coordination geometry; one lanthanide ion (Ln1) is coordinated by eight oxygen atoms: six arising from three hfac chelating ligands (O2−O7), one from the dppnTEMPO radical nitroxide (Gd1−O1 = 2.333(4); Tb1−O1 = 2.322(1) and Dy1−O1 = 2.284(2) Å), and one from a water molecule (Gd1−O8 = 2.382(3); Tb1−O8 = 2.371(4) Å, and Dy1−O8 = 2.349(4) Å). The Ln1−O1 bond lengths are typical for Gd, Tb, and Dy coordinated to nitroxide radical, as reported elsewhere.4d The other metal ion (Ln2) is also octacoordinated by eight oxygen atoms: six from hfac (O10−O16), one from a water molecule (Gd2−O16 = 2.405(4); Tb2−O16 = 2.401(5); Dy2−O16 = 2.396(3) Å), and the last one from the dppnTEMPO radical phosphinic amide group (Gd2−O9 = 2.274(3), Tb2−O9 = 2.258(4), and Dy2−O9 = 2.233(4) Å). As could be observed, the Ln−O bond lengths are slightly shorter for the dysprosium containing complex due to the C

DOI: 10.1021/acs.inorgchem.6b01616 Inorg. Chem. XXXX, XXX, XXX−XXX

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GdIII has a 8S7/2 ground term, with no orbital contribution, the magnetic behavior of compound 1 could be investigated on the basis of an isotropic Heisenberg spin Hamiltonian, Ĥ = −JHeis(S⃗Gd1·S⃗Rad1), considering only the magnetic coupling between one GdIII and the radical. The solid line in Figure 3 shows the best fit found with g = 2.000 ± 0.001; J = 2.7 ± 0.1 cm−1; and zJ′ = −2 × 10−2 ± 2 × 10−4 cm−1, where zJ′ corresponds to the mean-field intermolecular magnetic interaction parameter. The value of the coupling parameter between GdIII and the radical lies within the range already observed for other compounds.20 The ferromagnetic coupling is also confirmed by the ab initio calculations, even if a stronger magnitude of the interaction was found with a computed J value of 8.4 cm−1. The global behavior of χMT vs T is quite similar for both compounds 2 and 3. On lowering temperature down to around 15 K, χMT decreases, then increases slightly between 15 and 7 K, and finally drops at lower temperatures. For compounds containing lanthanide ions other than GdIII, the depopulation of the crystal-field split MJ states (Stark sublevels) occurs simultaneously with possible magnetic exchange interaction. In the high temperature range, the decrease of χMT values is dominated by the crystal field states, which persist below 15 K. Therefore, the slight increase below 15 K can be attributed to a ferromagnetic interaction between the radical and one lanthanide ion, as observed in compound 1 and in several other compounds.20b,c,21 This is confirmed by CASSCF calculations with no spin−orbit contributions on the model unit Ln1-dppnTEMPO, mapping the interaction on an Heisenberg Hamiltonian. In these cases, a JHeis ferromagnetic interaction has been computed for 2 and 3, leading respectively to JTb‑rad = 11.4 cm−1 and JDy‑rad = 1.7 cm−1. Below 6 K, the χMT drop can be due, on the other hand, to intermolecular antiferromagnetic dipolar interaction between lanthanide ions. Finally, the lack of a superposition of the M vs H/T data on a single master curve and the low magnetization value of 9.5 and 12.2 μBmol−1 at 90 kOe (Figure S4), respectively, for 2 and 3, supports the presence of a significant magnetic anisotropy. In this regard, CASSCF/RASSI-SO calculations on model complexes with quenched spin moment on the radical by adding an extra electron22 have also been performed (see SI). Strong local easy axes for Tb1 and Tb2 have been computed and are reported in Figure 4. In the case of Tb1, the easy axis

Table 2. Selected Bond Lengths (in Å) and Angles (in deg) for [Ln2(hfac)6(H2O)2(dppnTEMPO)] Compounds Label

Ln = Gd (1)

Ln = Tb (2)

Ln = Dy (3)

Ln1O1 Ln1O2 Ln1O3 Ln1O4 Ln1O5 Ln1O6 Ln1O7 Ln1O8 Ln2O9 Ln2O10 Ln2O11 Ln2O12 Ln2O13 Ln2O14 Ln2O15 Ln2O16 O1N1 O9P1 P1N2 Ln1O1N1 Ln2O9P1 Ln1O1N1C16 Ln1O1N1C22 Ln2O9P1C40 Ln2O9P1N2 Ln1···Ln2

2.333(4) 2.383(4) 2.365(4) 2.379(4) 2.323(3) 2.459(4) 2.383(3) 2.382(3) 2.274(3) 2.449(3) 2.420(3) 2.351(3) 2.400(3) 2.373(4) 2.413(3) 2.405(4) 1.287(6) 1.502(3) 1.637(4) 141.7(3) 152.5(2) −105.8(5) 93.7(5) 48.4(5) −73.4(4) 10.157(1)

2.322 (4) 2.356 (4) 2.373 (5) 2.356 (4) 2.309 (4) 2.446 (4) 2.374 (4) 2.371 (4) 2.258 (4) 2.433 (4) 2.407 (4) 2.340 (4) 2.374 (4) 2.359 (4) 2.418 (4) 2.401 (4) 1.293(7) 1.496(4) 1.632(6) 143.6(4) 154.0(3) 107.7(6) −92.5(7) 69.2(6) −52.6(7) 10.2638(6)

2.284(4) 2.383(4) 2.315(5) 2.314(4) 2.359(4) 2.457(4) 2.318(4) 2.349(4) 2.233(4) 2.370(4) 2.412(4) 2.326(4) 2.302(4) 2.364(4) 2.385(4) 2.396(3) 1.304(8) 1.499(4) 1.631(6) 156.7(4) 171.0(2) 116(1) −86(1) −72(2) 47(2) 9.9683(6)

2−290 K for 1−2 and 2−220 K for 3. The plots of χMT vs T are displayed in Figure 3 (χM is the molar magnetic susceptibility). At the highest temperature, the values of χMT (16.1, 23.9, and 27.5 cm3 mol−1K for compounds 1, 2, and 3, respectively) are close to the expected ones for three uncoupled spin carriers: two LnIII ions and one radical (16.1, 24.0, and 28.7 cm3 mol−1 K respectively). For compound 1, on lowering temperature, χMT increases up to 16.7 cm3 mol−1 K around 6 K, resulting from a ferromagnetic interaction between one GdIII ion and the radical, and then decreases slightly at lower temperatures, coming from antiferromagnetic intermolecular dipolar interactions. Since

Figure 1. Molecular structure of [Ln2(hfac)6(H2O)2(dppnTEMPO)] (Ln = Gd (1), Tb (2), and Dy (3). Hydrogen atoms were omitted for the sake of clarity. D

DOI: 10.1021/acs.inorgchem.6b01616 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. Details of the crystal packing of 2, showing the supramolecular zigzag chain formed by 1 units. Selected distances: O8···O4i = 3.010(6) Å; O8···O6i = 2.845(6) Å; O16···F23ii = 2.952(6) Å, and O16···F28ii = 3.020(7) Å (i = −x + 1, y, −z − 1/2; ii = −x + 1, −y, −z). Hydrogen atoms, dppnTEMPO methyl groups, and part of the hfac ligands were omitted for clarity.

Detailed orientations are reported in Table S3 (SI). The relative orientation of the two easy axes is almost orthogonal, indicating the different local coordinative environment and their orientations with respect to the crystal frame of the two TbIII ions. The same procedure was employed for the calculation of single ion anisotropy of the dysprosium ions in 3. The calculation evidenced for both ions ground Kramers’ doublets well isolated in energy from the first excited state with a very strong axial character of the g-tensor, with gz = 19, near the theoretical limit of 20, as shown in Tables S4 and S5 (SI). The orientation of the main magnetic axes with relation to the coordinative environment is similar to compound 2 (Figure S5): the axis calculated for Dy1 is almost orthogonal to the N− O bond while the other one relative to Dy2 is almost parallel to the P−O group (see Figure S3). The results from the CASSCF/RASSI-SO calculations for 2 were employed to simulate the magnetic χMT vs T data.15−18 For compound 2, only the exchange interaction between Tb1 and the coordinated isotropic S = 1/2 of the radical was taken into account, as ab initio calculation showed no magnetic interactions with Tb2. Our model also considered the dipolar interactions between lanthanide ions at a distance less than 6 Å in the crystal packing. The best simulation was obtained with a ferromagnetic J = 5.3 cm−1 (green solid line in Figure 3). The experimental curve is well reproduced: the simulation shows the peak at around 7 K and the same slope at higher temperatures. Within this theoretical framework of a pure Ising-type local magnetization on each LnIII ion in compounds 2−3, the χMT vs T data were fitted below 15 K considering Seff = 1/2 with anisotropic g values for each lanthanide ion21b,23 (see Tables S3 and S5). Furthermore, due to the short intermolecular distances between lanthanide ions, the intermolecular dipolar interaction had to be considered between the lanthanide ions Ln1 and Ln2 of different molecules (JDipolar). The solid red lines in Figure 3 show the best fits obtained for compounds 2−3 using the MagProp routine in the DAVE software suite.24 The parameters found for 2 are grad = 2 (fixed), gTb,x = gTb,y = 0 (fixed), gTb,z = 18.6 ± 0.1, JIsing = (15.6 ± 1.8) cm−1, and JDipolar = −(1.0 ± 0.1) cm−1. The parameters found for 3 are grad = 2 (fixed), gDy,x = gDy,y = 0 (fixed), gDy,z = 19.62 ± 0.02, JIsing = (7.8 ± 0.6) cm−1, and JDipolar = −(0.61 ± 0.05) cm−1. In both cases, the magnetic exchange interaction between the radical and the Ln1 ion is ferromagnetic and is higher in 2 than in 3, as

Figure 3. Plots of χMT vs T for compounds 1−3, measured at H = 1 kOe for 1 and at 200 Oe for 2−3. The red lines correspond to the best fits for 1−3 (vide text). The green line represents the best simulation (vide text).

Figure 4. Computed easy axis for the Tb1 and Tb2 for compound 2. Terbium, carbon, nitrogen, oxygen, phosphorus, and hydrogen are purple, brown, blue, red, orange, and white, respectively. For the sake of clarity fluorine atoms are omitted and only the hydrogens atoms of the water molecules are shown. The crystallographic orthogonal reference is also shown.

points nearly orthogonal to the NO direction while for the Tb2 the easy axis is almost parallel to the PO group. This can be visualized by considering both axes directions lying parallel to the square faces of the antiprismatic coordination environment. E

DOI: 10.1021/acs.inorgchem.6b01616 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry expected from the first-principles calculations. The ab initio computed JHeis values for 2 and 3 are in good agreement with experimental fit values. Moreover, the exchange coupling constants are close to the ones found in other compounds.21 In order to compare the simulation with the fit, the relationship between the two exchange coupling constants was extrapolated by matching the energy difference between the parallel and antiparallel configurations as a function of J. Considering the description of the lanthanide ion as a Seff = 1/2 spin within the Ising model, the obtained JIsing (15.6 cm−1) is lower than J (Jising = 6J, for 2 , where J = 5.3 cm‑1) and greater than JHeis (11.4 cm−1) but still comparable. Finally, the JDipolar dipolar magnetic interaction parameters are of the right order of magnitude, considering two effective half spins. In order to find a qualitative rationalization of the ferromagnetic isotropic exchange coupling constants obtained for isostructural 1, 2, and 3, we have also analyzed the magnetic orbitals involving the radical π* orbital and f metal orbitals. Unexpectedly, only one f orbital of the metal ions interacts considerably with the π* orbital: the f 0 orbital in the spherical basis. This is more evident for 2 and 3, where an increased overlap between the two orbitals is observed in the same order (see Figure 5). Indeed, a bonding and antibonding interaction

Ln1−O1−N1 angle, 141.7°). Indeed, in such a geometrical arrangement, the π* points to the f 0 nodal plane leading to a quasi-orthogonal situation. Finally, we verified the possible presence of other exchange mechanisms in addition to the direct exchange mechanism by analyzing the composition of computed natural orbitals for the three compounds. From such a calculation, it came out that the Ln(III) f orbitals are completely localized while the π* are very slightly delocalized on the Ln(III) 5d and 6s orbitals in all the systems. Such findings support that the direct mechanism is the driving exchange interaction25 with respect to the charge transfer one as observed for Cu(II)−Ln(III).26 Such a qualitative analysis points out that in the case of the interaction of NO radical it could be possible to selectively modulate the magnitude of the exchange interaction by choosing the Ln1 ion and by acting on geometrical parameters such as the Ln1−O1−N1 angle. This result suggests a closer resemblance of the lanthanides ions with transition metals than expected with regard to their interactions with nitroxides radicals. Dynamic Magnetic Properties. The dynamic magnetic properties of compounds 2 and 3 were investigated. The ac magnetic susceptibility measurements under zero dc field show no frequency dependence for 2 and a very slight one for 3 (Figure S6), suggesting quantum tunneling relaxation of the magnetization (QTM) in zero field. This effect is avoided by applying a dc magnetic field of 2.5 kOe, as observed by the frequency dependence of the out-of-phase ac susceptibilities for both compounds (Figure 6). It is noteworthy that two maxima are observed for the highest frequencies for 3, showing that two relaxation processes are active in this range of temperatures and frequencies. Furthermore, the presence of a second relaxation process is also observed for compound 2 when looking specifically at the 3 and 10 kHz χ″ signals with a hump at low temperatures. The occurrence of two different relaxation processes for 2 and 3 is also perceptible from isothermal ac susceptibility measurements varying the ac frequency (Figure S7). In these measurements, as the temperature is decreased, the shape of the curves changed, especially at low frequencies, reflecting more than one pathway for relaxation. The Cole−Cole plots obtained from the same sets of data show also the occurrence of two relaxation processes. Unfortunately within the accessible range of frequencies and temperature, it was not possible to resolve two different maxima (Figures S7−S8). In lanthanide-based complexes, relaxation mechanisms can be affected by local dipole−dipole interactions, hindering QTM or speeding up relaxation, enhancing or decreasing SMM properties.2d,20b,27 Nevertheless, the external applied field for the ac experiments is above the related dipolar fields. Moreover, magnetic relaxation is also very sensitive to the local molecular symmetry and to tiny distortions in the coordination geometry. With two LnIII ions in different environments for both compounds, confirmed by the ab initio calculations and by luminescent properties (see below), one relaxation process may be attributed to a SIM (Ln2) and the other to a SMM-like Ln1-radical moiety. Luminescent Properties of Compound 2. The inequivalence of the two terbium(III) ions seen in the magnetic measurements was further confirmed by analysis of the emission properties measured at 80 K (Figure 7). Under UV excitation, the typical TbIII emission related to 5D4 − 7FJ (J = 6, 5, 4, and 3) transitions was detected. The strongly overlapped excitation spectra monitoring TbIII emissions at 540 and 542

Figure 5. Magnetic orbitals involving the radical π* orbital and f 0 metal orbital. The isodensity value is set to 0.02 ebohr−3. For compounds 2 and 3, bonding (left) and antibonding (right) interactions between the π* and f 0 orbitals are visible.

between the two orbitals is observed as a result of an effective energy matching. Moreover, it can also be evidenced that a smaller overlap between the two magnetic orbitals is observed for 2 compared to 3. Indeed, due to a larger difference in Ln1− O1−N1, 143.6° vs 156.7°, and a slightly shorter Ln1−O1 distance, 2.32 Å vs 2.28 Å, the π* orbital in 3 can interact via an effective π-like interaction with f 0. On the other hand, a more ferromagnetic exchange is expected for 2 by virtue of a smaller σ-like overlap given by the smaller Ln1−O1−N1 angle and larger Ln1−O1 distance. A different scenario is present for 1. In such a case, the π* and f 0 are well localized, suggesting a less effective interaction (longest Ln1−O1, 2.33 Å, and smaller F

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ligands to Tb centers.28 Differences in the emission spectra of Tb1 and Tb2 centers could be found only by use of time-gated emission spectroscopy. Taking into account the high affinity of the phosphinoyl group (PO) for lanthanide ions,29 we expect that the interaction of the dppnTEMPO ligand with Tb2, through the PO group, is stronger than the one with Tb1, through the nitronyl (O−N) group. Consequently, the Tb2 ion is characterized by a more rigid structure that reduces the nonradiative relaxation of the excited state.30 Moreover, the two phenyl groups, which are closer to Tb2, reinforce the antenna effect toward this metal center. Despite efforts, the emission decays measured at 540 and 542 nm were only poorly separated, likely due to the small selectivity in both excitation and emission. The estimated lifetimes of Tb1 and Tb2 are 0.68 and 0.78 ms, respectively.

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CONCLUSIONS Compounds 1−3 described herein represent the first examples in lanthanide chemistry of a phosphinic amide−TEMPO heterotopic ligand. Derivatives of GdIII, DyIII, and TbIII were synthesized and characterized by X-ray and static and dynamic magnetic measurements. On the basis of the computed natural orbitals and geometrical considerations, a qualitative understanding of the experimental and, to a lesser extent, of the computed exchange constant at the CASSCF level was found. The terbium and dysprosium derivatives illustrated that such compounds show interesting magnetic properties arising from the presence of metal ions with different chemical environments. The results of the photophysical measurements for the terbium derivative also confirm the presence of two distinct luminescent centers. These results stress once again the necessity of the interplay between different experimental techniques and ab initio calculations in order to get a clearer understanding of the complex magnetic mechanisms involved in the lanthanide ion containing systems.

Figure 6. Temperature dependence of χ′′ at H = 2.5 kOe for different frequencies for (a) 2 and (b) 3.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01616. Experimental powder X-ray pattern for 2. Additional details of ab initio calculations. Isothermal magnetization M versus H/T curves for 2 and 3. In-phase and out-ofphase ac signals for compounds 2 and 3. Referred Cole− Cole plots for compounds 2 and 3 (PDF) CIF data for C51H38F36Gd2N2O16P (CIF) CIF data for C51H38F36N2O16PTb2 (CIF) CIF data for C51H38Dy2F36N2O16 P (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*Prof. Marius Andruh: e-mail: [email protected]. *Dr. Maria G. F. Vaz: e-mail: [email protected]ff.br.

Figure 7. Characteristic excitation and emission spectra as well as the emission decays of Tb1 and Tb2 centers in complex 2. Excitation/ emission wavelengths and delays after the laser pulse are indicated in the figure.

Notes

The authors declare no competing financial interest. CCDC-No. 1489554, 1038261, and 1489555 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_ request/cif.

nm differ by the relative contribution of ca. 300 and 338 nm based absorptions assigned to 1ππ* absorption of hfac and dppnTEMPO ligands, respectively. The spectra indicate an efficient intramolecular energy transfer from the triplet states of G

DOI: 10.1021/acs.inorgchem.6b01616 Inorg. Chem. XXXX, XXX, XXX−XXX

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ACKNOWLEDGMENTS The authors acknowledge the financial support from FAPERJ, CAPES, CNPq, MINECO, FEDER (Project CTQ2014-57157P), and CAPES/CSF-PVE (Project 88881.030358/2013-01). We are also grateful to LDRX-UFF and LabCri-UFMG for the use of crystallographic facilities. M.A. and S.S. thank CNPq. F.T. acknowledges Roberta Sessoli for fruitful discussions and the European Research Council Grant MolNanoMas (grant no. 267746) and CENAPAD-SP (proj 627). M.B. acknowledges the FAPERJ (proj E-26/200.104/2016).

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