Synthesis and Characterization of Anionic Lanthanide(III) Complexes

Publication Date (Web): December 13, 2018. Copyright © 2018 American Chemical Society. *E-mail: [email protected] (N.G.). Phone: (417) ...
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Synthesis and Characterization of Anionic Lanthanide(III) Complexes with a Bidentate Sulfonylamidophosphate (SAPh) Ligand Iryna Olyshevets,† Nataliia Kariaka,† Kateryna Znovjyak,† Nikolay Gerasimchuk,*,‡ Sergey Lindeman,§ Sergii Smola,∥ Maksym Seredyuk,† Tetiana Yu. Sliva,† and Vladimir M. Amirkhanov*,† †

Department of Chemistry, Taras Shevchenko National University of Kyiv, 64/13 Volodymyrska Street, Kyiv 01601, Ukraine Department of Chemistry, Missouri State University, Temple Hall 456, Springfield, Missouri 65879, United States § Marquette University, Wehr Chemistry Building, 1414 West Clybourn Street, Milwaukee, Wisconsin 53233,United States ∥ A. V. Bogatsky Physicochemical Institute, National Academy of Sciences of Ukraine, Odessa 65080, Ukraine Inorg. Chem. Downloaded from pubs.acs.org by COLUMBIA UNIV on 12/13/18. For personal use only.

‡

S Supporting Information *

ABSTRACT: A series of new anionic lanthanide(III) complexes with the general formula NEt4[LnL4] (1-Ln; HL = dimethyl[(4-methylphenyl)sulfonyl]amidophosphate; Ln = La, Nd, Eu), were synthesized and characterized by IR, UV− vis, and NMR spectroscopies, the differential scanning calorimetry method, thermogravimetric and X-ray analysis, and photoluminescence measurements. Single-crystal structures of NEt4[EuL4] (1-Eu) were determined at 293 and 100 K and evidenced the single-crystal-to-single-crystal phase transition. Both phases are in the monoclinic crystal system in centrosymmetric groups of the same Laue class. The room temperature structure is in C2/c (No. 15), while lowtemperature structure is in the P21/c (No. 14) space groups. The coordination environment geometry around the central europium(III) ion is a distorted square antiprism in both polymorphs, while the peripheral methoxy and tolyl groups show different orientations. This phenomenon indicates the occurrence of a thermally driven second-order phase transition during the cooling−heating process. The europium(III) complex exhibits an unusual emission spectrum, clearly dominated by the 5D0−7F4 bands, and an emission decay time equaling 3.5 ms, being among the highest values known for europium coordination compounds.

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“chelate claw” size and is able to tightly bind large metal cations. The luminescence efficiency of such complexes can be increased by an appropriate choice of substituents of the ligand because, in this way, the position of the triplet ligand level can be tuned to give good energy transfer between the β-diketone ligand (or CAPh or SAPh) and the lanthanide ion.3 Two types of complexes may be obtained, depending on the ratio of metal ions to β-diketonate (or CAPh or SAPh) ions in the product; tris complexes with a Ln:ligand = 1:3 ratio or tetrakis complexes with a Ln:ligand = 1:4 ratio.4,5 The neutral moiety [Ln(ligand)3] demonstrates the possibility of expanding the coordination sphere of the lanthanide ion by oligomerization or by the formation of heteroligand complexes with additionally coordinated solvent molecules or neutral organic ligands, such as 1,10-phenanthroline, 2,2-bipyridine, etc. The latter can serve as an additional antenna, providing an efficient pathway for excitation of the lanthanide ion. In the case of [Ln(ligand)4]− complexes, additional tuning of the lumines-

INTRODUCTION Anionic chelating ligands were found to be excellent groups for strong binding of a variety tripositive metal cations including lanthanides. The main feature of the complexes formed is their solid-state and especially aqueous solution stability toward hydrolysis reactions. Monoanionic ligands generated a large group of neutral LnL3 complexes, which gained practical application for separation and purification purposes. The generalized chemical formulas of those widely used chelators are depicted in Scheme 1. During past decades, there has been considerable interest in the luminescence properties of lanthanide complexes containing β-diketone ligands or their structural analogues, carbacylamidophosphates (CAPh) and sulfonylamidophosphates (SAPh) (Scheme 1).1,2 The latter anion possesses larger Scheme 1. Generalized Chemical Formulas of Widely Used Chelators for Binding Oxophilic Metal Cations

Special Issue: Innovative f-Element Chelating Strategies Received: October 8, 2018

© XXXX American Chemical Society

A

DOI: 10.1021/acs.inorgchem.8b02846 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Scheme 2), which are close analogues of the previously reported Na[LnL4] compounds.12,13 The role of the outersphere cation on the lanthanide tetrakis complex properties is discussed.

cence properties can be achieved by changing the outer-sphere cation, which adjusts the crystal structure of the complex and affects the local coordination geometry around the metal ion. Earlier, it was shown that outer-sphere interactions may significantly change the bond lengths between atoms in the inner coordination sphere and thus noticeably alter the luminescent properties.6 Additionally, it was found that the energy of extended noncovalent interactions occurring in the second coordination sphere is comparable to that of the Ln− ligand bonds.6 Many recent studies have been focused on the combination of ionic liquids (ILs) with lanthanide ions, with the final goal being to utilize obtained multifunctional materials for spectroscopic applications.7,8 ILs were found to be excellent solvents for many ionic coordination compounds, including metal salts and complexes. Several studies indicate that the most straightforward way for obtaining new ILs with specific properties due to the incorporation of a metal ion can be realized, and their properties are tuned by a judicious combination of the chelating organic cations and/or anions and metal ions.9,10 Therefore, the actual problem is the selection of appropriate cations and the design of ligands for improving the spectroscopic properties of lanthanide complexes. A large variety of SAPh (Scheme 1) have been synthesized and investigated, which justified them as promising ligands for the preparation of complexes with useful spectral and biological properties.2 Along this line, during the past few years, much effort has been devoted to the synthesis of new SAPh ligands and to investigations of their coordination behavior and photophysical properties in complexes with fmetal ions.4a,11,12 Upon complexation with lanthanide ions, SAPh ligands form a six-membered chelate ring Ln−O−S−N− P−O, with PO and SO2 groups coordinated to the metal center (Scheme 2) and additionally able to form supra-

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EXPERIMENTAL PART

General Considerations. All precursor reagents were purchased from commercial sources and used without further purification. Solvents were dried and purified by standard methods.15 Elemental analysis (C, H, and N) was performed on an EL III Universal CHNOS elemental analyzer. The melting points were measured using the Thiele oil-filled tube apparatus. Instrumentation. Spectroscopic Measurements. The IR spectra were recorded at room temperature using a PerkinElmer Spectrum BX spectrometer, on samples homogenized under pressure in KBr pellets. The NMR spectra were recorded on a Varian Mercury 400 NMR spectrometer at room temperature. The electronic spectra of solid samples of the obtained compounds were obtained at ambient conditions, as diffuse-reflectance spectroscopy (DRS) spectra, with the help of a UV−vis Specord M 40 Carl Zeiss spectrophotometer equipped with an integrating sphere. The emission and excitation spectra of the studied complexes were measured using a Horiba Fluorolog FL 3-22 spectrofluorimeter at 298 and 77 K. Both emission and excitation slits were set to 10 nm width. The f → f luminescence decay time measurements at room temperature were carried out on a FL-1040 phosphorimeter accessory to the above Fluorolog 3-22 instrument, from Horiba Jobin Yvon. The xenon−mercury arc lamp, in pulse mode with a 3 μs bandwidth, was used for measurements. Thermal Analyses. Thermogravimetric analysis (TGA) was performed using a synchronous Shimadzu DTG-60Hs TGA/differential thermal analysis (DTA) analyzer. A typical sample of 5−10 mg was heated to 800 °C in the alumina crucible under a steady argon flow (300 mL/min), with a heating rate of 10 °C/min. The crystalline powdery Al2O3 was used as a standard compound. Differential scanning calorimetry (DSC) measurements were performed on a Mettler Toledo TGA/SDTA 821e under a protective nitrogen atmosphere with a rate of 10 K/min. The DSC data were analyzed with Netzsch Proteus software, with an overall accuracy of 0.2 K in temperature and 2% in heat flow. X-ray Analysis. We were able to grow suitable crystals for X-ray analysis only for the 1-Eu complex, using a slow evaporation of solutions method that produces large, well-shaped crystals. It turned out that single crystals of 1-Eu were very sensitive to the cooling process in the diffractometer; they cracked and shattered easily if the cooling rate was higher than 2°/min. That fact immediately raised our suspicion that there might be a low-temperature phase transition that is a cause of such sensitivity. Therefore, an X-ray diffraction (XRD) experiment was first done at 293 K, using an APEX2 diffractometer equipped with a SMART CCD area detector, a molybdenum tube (λ = 0.71073 Å), and a highly oriented graphite monochromator. A suitable crystal of the compound was mounted on a thin glass fiber, or placed into the CryoLoop, and then attached to the copper pin positioned on the goniometer. The intensity data were collected in ωscan mode and then integrated from four series of 364 exposures, each covering 0.5° at 30 s of exposure time, with the total data set being a sphere. The space group determination was done with the aid of XPREP software.16 The absorption correction was performed using values from face-indexed crystals and numerical values obtained from the set of images recorded with the video microscope (Figure S1), followed by the SADABS program that was included in the Bruker AXS software package.17,18 The cryosystem cooling rate of the APEX2 diffractometer was too high to bring even a very small single crystal to low temperatures. Therefore, a second diffractometer, the Oxford SuperNova Dual Atlas, which is capable of very slow cooling of the sample, was used for the data collection experiment. Cu Kα radiation (λ = 1.54184 Å) was used for this experiment. The crystal was kept at 100.05(10) K during data collection. Using OLEX-2,19 the structure was solved with the ShelXS20 structure solution program using direct methods and refined with the XL20 refinement package using least

Scheme 2. Schematic Representation of Synthesized Tetraethylammonium Salts of [LnL4] Complexes

molecular bonds with outer-sphere cations. Moreover, the electronic structure of a ligand can be tuned by varying the substituents near the functional chelate core, which gives a valuable tool for the elaboration of “effective antennae” systems. It was shown that Ln-SAPh complexes exhibit relatively efficient ligand-to-metal energy transfer and strong metal-centered emission, which makes them potentially useful as effective energy converters.13,14 Here, we report synthesis and thermal, structural, and spectroscopic characterizations of a series of novel SAPh-based complexes of the NEt4[LnL4] composition (HL = dimethyl[(4-methylphenyl)sulfonyl]amidophosphate; Ln = La, Nd, Eu; B

DOI: 10.1021/acs.inorgchem.8b02846 Inorg. Chem. XXXX, XXX, XXX−XXX

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those of Na[LnL4] (193−196 °C). On the basis of these results, it may be assumed that replacing of the Na+ cation by [NEt4]+ leads to some weakening of intermolecular interactions in the obtained anionic complexes. All of the synthesized complexes are soluble in water, methanol, 2propanol, acetone, dimethyl sulfoxide (DMSO), and chloroform (CHCl3) and poorly soluble in nonpolar solvents (cyclohexane and toluene). Safety Note! Although we have not encountered any problems during many years of laboratory work and handling, special care should be taken during work with organophosphorus compounds because of their known and expected high toxicity. Thus, all synthetic procedures must be carried out under a ventilation hood. All compounds reported here are watersoluble, and that emphasizes the absolute necessity for wearing protective gloves at all times when working with these compounds.

squares on weighted F2 values for all reflections. Hydrogen atoms in the structure of the room temperature polymorph of 1-Eu-293 were placed in their idealized positions according to the sp2 or sp3 hybridization states of the hosting carbon atoms and refined isotropically. However, the positions of the hydrogen atoms in the low-temperature polymorph 1-Eu-100 were located from the electron density difference maps and refined by the “riding” model with Uiso = nUeq of the carrier atom (n = 1.5 for methyl groups and 1.2 for other hydrogen atoms). The structures reported herein did not have apparent errors and are well-refined. It should be noted, however, that, in the structure of the room temperature polymorph 1-Eu-293, four atoms demonstrate significant size of their thermal ellipsoids: two methoxy groups at P1−C8 and O3 and at P2−C18 and O8 (Figure S6). Their librational components of the TLS model commonly used for the refinement were too high and generated B-type alerts in the checkCIF report for this structure, which were addressed in the Results and Discussion section of the paper. In summary, the structures reported herein did not have apparent errors and are wellrefined. Thermal ellipsoid drawing of the crystal structures 1-Eu-293 and 1-Eu-100 was accomplished with the help of OLEX-2 software at a 50% thermal ellipsoid probability level,21,22 while packing diagrams were done using Mercury software.23 Both crystal structures have been deposited at CCDC under the following numbers: 1033142 for 1-Eu293 and 1033143 for 1-Eu-100. Synthetic Procedures. Synthesis of HL and NaL. Compound HL was synthesized and characterized according to the reported procedures.2 The sodium salt NaL was obtained by the reaction between 1 equiv of sodium carbonate and 2 equiv of HL in a water/2propanol (1:3) medium, with heating and stirring until complete dissolution of the sodium carbonate. The resulting solution was evaporated under vacuum and recrystallized from absolute 2propanol. The crystals of NaL were filtered, washed with hexane, and dried in open air. Yield: 90%. Anal. Calcd for C9H13NNaO5PS: C, 35.88; H, 4.35; N, 4.65. Found: C, 35.91; H, 4.36; N, 4.68. 1H NMR (DMSO-d6, 25 °C): δ 2.46 (s, 3H, CH3), 3.26 (d, 6H, OCH3, 3JP−H = 11.3 Hz), 7.12 (m, 2Hβ, C6H4), 7.77 (m, 2Hα, C6H4), with the actual NMR spectra (including 13C{1H}) presented in Figures S14−S17. Synthesis of Complexes 1-Ln (Ln = La, Nd, Eu). Complexes were prepared in a simple two-step procedure according to the equations

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RESULT AND DISCUSSION Crystal Structures of 1-Eu. The crystal structure of 1-Eu was determined at 293 K (further as 1-Eu-293) and 100 K (further as 1-Eu-100). Crystal and refinement data are presented in Table 1, while selected bond lengths and angles are summarized in Table 2, with a comparative view of both

Table 1. Crystal and Refinement Data for the 1-Eu Complex C44H72EuN5O20P4S4 at Two Different Temperatures T = 293(2) K fw wavelength used (Å) cryst syst space group unit cell dimens a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) cell volume (Å3) Z Dcalcd (g cm−3) abs coeff, μ (mm−1) F(000) cryst size (mm3) θ range for data collection (deg) indexes range

Ln(NO3)3 · nH 2O + 4NaL → Na[LnL4] + 3NaNO3↓ + nH 2O (1) Na[LnL4] + NEt4Cl → NEt4[LnL4] + NaCl↓

(2)

In the first step, the sodium salt of the sulfonylamidophosphate ligand (NaL) was used as a precursor to obtain Na[LnL4] complexes with lanthanides.24,25 To a solution of Ln(NO3)3·nH2O (0.1 mmol) in 2propanol (10 mL) under stirring was added a solution of NaL (0.4 mmol) in 2-propanol (15 mL). The mixture was refluxed for 10 min and cooled, and the sodium nitrate was filtered off. The resulting stock solution of Na[LnL4] of known concentration was used further on, for the synthesis of other NEt4[LnL4] complexes. A solution of NEt4Cl (1 mmol) in 2-propanol (5 mL) was added to the above Na[LnL4] filtrate and stirred for an additional 10 min at room temperature. The resulting mixture was filtered to remove slowly precipitating sodium chloride, and the filtrate was left in air, at room temperature, in a beaker in a desiccator, for slow solvent evaporation and crystal growth. Crystals of 1-Ln formed after 2−3 days and were filtered, washed with cold 2-propanol, and finally dried. Yields: 70− 80%. Anal. Calcd for 1-La (C44H72LaN5O20P4S4): C, 38,24; H, 5,25; N, 5,07. Found: C, 38,67 (38,51); H, 5,39 (5,32); N, 5,02 (5,10). 1H NMR (DMSO-d6, 25 °C): δ 2.33 (s, 3H, CH3), 3.42 (d, 6H, OCH3, 3 JP−H = 11.3 Hz), 7.24 (m, 2Hβ, C6H4), 7.70 (m, 2Hα, C6H4). The 13 C NMR (DMSO-d6, 25 °C): δ 20.88 (C5, CH3), 52.32 (2C6, OCH3), 125.71 (2C4, C6H4), 128.54 (2C3, C6H4), 139.97 (C2, C6H4), 144.10 (C1, C6H4). For the interested reader, both the 1H and 13 C{1H} NMR spectra for this complex are shown in Figures S16 and S17. The melting points for the synthesized lanthanide complexes 1-La, 1-Nd, and 1-Eu are in the range of 185−187 °C and are lower than

reflns collected indep reflns reflns with I > 2σ(I) R(int) restraints/data/ param GOF on F2 final R indices [I > 2σ(I)] R indices (all data) largest diff peak/ hole (e Å−3) structure volume, A3 (%) C

T = 100(1) K

1395.15 0.71073 (Mo)

1395.15 1.54184 (Cu)

monoclinic C2/c (No. 15)

monoclinic P21/c (No. 14)

28.276(2) 10.6780(9) 24.4149(2) 90.00 124.699(1) 90.00 6060.6(8) 4 1.529 13.51

24.3901(2) 10.59625(8) 24.2138(2) 90.00 111.1837(9) 90.00 5835.04(8) 4 1.588 10.718

2872 0.5 × 0.1 × 0.07 2.19−27.18

2872 0.21 × 0.1 × 0.06 3.67−73.32

−36 ≤ h ≤ +36, 13 ≤ k ≤ +13, −31 ≤ l ≤ +31 33415 6694 5915

−30 ≤ h ≤ +29, −12 ≤ k ≤ +12, −30 ≤ l ≤ +26 37689 11494 10907

0.0326 0/3610/719

0.0278 0/11494/719

1.033 R1 = 0.0317, wR2 = 0.0741 R1 = 0.0392, wR2 = 0.0795 0.353/−0.57

1.043 R1 = 0.0284, wR2 = 0.0729

1.23/−0.54

3696.2 (60.82)

3762.4 (64.48)

R1 = 0.0304, wR2 = 0.0745

DOI: 10.1021/acs.inorgchem.8b02846 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 2. Selected Bond Lengths (Å) and Angles (deg) for 1-Eu-293, 1-Eu-100, and Na[EuL4]a.13 1-Eu-100

Na[EuL4]a

1-Eu-293

Eu1−O2(S) Eu1−O3(P) Eu1−O6(S) Eu1−O8(P) Eu1−O11(S) Eu1−O13(P) Eu1−O16(S) Eu1−O18(P) Eu···Eu

2.463(2) 2.351(2) 2.475(2) 2.336(2) 2.484(2) 2.340(2) 2.436(2) 2.366(2) 10.596(2)

O2−Eu1−O3 O6−Eu1−O8 O11−Eu1−O13 O16−Eu1−O18 S1−N1−P1 S2−N2−P2 S3−N3−P3 S4−N4−P4

73.10(5) 72.76(5) 74.98(5) 75.40(5) 125.14(5) 127.01(5) 125.86(5) 124.36(5)

Eu1−O6 Eu1−O1 Eu1−O4 Eu1−O9

2.352(2) 2.355(2) 2.462(2) 2.493(2)

Eu1−O11(S) Eu1−O13(P) Eu1−O21(S) Eu1−O23(P)

2.494(2) 2.339(2) 2.479(2) 2.345(2)

Eu···Eu

10.678(2)

Eu···Eu

11.232(2)

O11−Eu1−O13 O21−Eu1−O23

73.10(7) 73.68(7)

S11−N11−P11 S21−N21−P21

122.94(7) 124.32(7)

a

Data from ref 13.

polymorphs presented in Figure 1. As mentioned above in the Experimental Part, the default procedure for collecting

experimental data sets at the conventional low-temperature cryostat setting of the APEX 2 diffractometer immediately led to crystal shattering, despite its great stability at room temperature. We interpreted this behavior as an indication of a possible phase transition. Indeed, the structures are different and represent two different polymorphs, shown in Figure 1, with the crystal packings for both 1-Eu-293 and 1-Eu-100 shown in Figure S3. Lowering the temperature induced an anomalously large contraction along the a direction, compared to those along the b and c directions (Figure S4). It should be noted that the β angle also significantly decreased in the lowtemperature polymorph (Figure S4). Also, we should note that in the room temperature structure of polymorph 1-Eu-293 four atoms demonstrate significant size of their thermal ellipsoids: two methoxy groups at P1−C8 and O3 and at P2−C18 and O8. Their librational components of the TLS model commonly used for the refinement were too large and generated B-type alerts in the checkCIF report for this structure. However, careful analysis of these fragments did not provide sufficient grounds for these atoms modeling as twopositional disorder because pivoting phosphorus atoms P1 and P2 had normal Us values. Therefore, in order to preserve the original data and minimize data altering, we did not apply for those dangling group SIMU/DELU restraints and did not “beautify” them using suitable EADP and ISOR commands from similar atoms. Thus, rather large room temperature oscillations of the methoxy groups at the P1 and P2 atoms and the absence of disorder are evident from the structure at 100 K, where all thermal ellipsoids are normal. Therefore, observed considerable librations of C8 and O3 and of C18 and O8 atoms evidence the presence of some void near these groups in the unit cell along the a direction, which contracts the most upon cooling of the crystal to 100 K (Figure S4). The structure of 1-Eu differs at 293 and 100 K, as is evident from the calculated powder XRD patterns (Figure S5). The complex 1-Eu-293 as a room temperature polymorph has crystallized in the monoclinic more symmetric space group C2/c (No. 15), while the low-temperature polymorph adopted lower monoclinic P21/c symmetry (No. 14). Thus, in the former group, an additional symmetry element is present: a 2-

Figure 1. Fragment of the crystal structure of 1-Eu showing the anionic part only, with CH3 groups of methoxy fragments at the phosphorus atoms as well as hydrogen atoms being omitted for clarity: (A) structure at 293 K in the C2/c group; (B) at 100 K in the P21/n group. D

DOI: 10.1021/acs.inorgchem.8b02846 Inorg. Chem. XXXX, XXX, XXX−XXX

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Examination of the low-temperature structure of 1-Eu-100 immediately shows that now all four six-membered metallochelate rings becomes unique (Figures 1 and S10−S12). That is the direct result of the lattice metric change, which forces the europium and N1c atoms out of special into general positions. Again, there are two types of conformations of those rings: boatlike and skewed (twisted; Figure S11). At 100 K, the structure is much more compact compared to the room temperature polymorph and it occupies 64.5% of the unit cell volume, leading to a ∼4% difference (Table 1). According to the geometrical criteria proposed by PoraiKoshits and Aslanov,29 the europium(III) ion coordination polyhedron can be interpreted as a square antiprism (Table S2 and Figures S12 and S13), slightly twisted toward the square prism. Additionally, some tetrahedral distortion is observed. The Eu−O distances (Table 2) were found to be in the range of 2.352(2)−2.493(2) Å for 1-Eu-293 and 2.336(2)−2.484(2) Å for 1-Eu-100, being comparable with those of related complexes.11,13,14 The Eu−O(P) bonds (O1 and O6 atoms for 1-Eu-293 and O3, O8, O13, and O18 atoms for 1-Eu-100) are shorter compared to Eu−O(S) (Table 2) because of the higher affinity of the phosphoryl group to the lanthanide ion. Tetraethylamonium cations are located in the outer sphere, and the balance is the charge of the anionic complex molecules. Compared to the recently reported complex Na[EuL4],13 the primary and expected effects of the substitution of Na+ by a bulkier [NEt4]+ consist of increasing distances between the closest metal centers (Table 2). Furthermore, in 1-Eu-100, the tetraethylamonium cations show two contacts with oxygen atoms of the phosphoryl groups, involved in coordination with the europium ion, which leads to elongation of the respective Eu−O(P) bond lengths compared to the Na[EuL4] complex (Table S3). Additionally, compared to Na[EuL4], in 1-Eu-100, the nitrogen atoms of the chelating rings are not bound with the cation, which apparently produces increasing S−N−P angles in 1-Eu-100 (Table 2). Besides that, the coordination polyhedron changes upon substitution from a distorted dodecahedron in Na[EuL4] to a distorted antiprism in 1-Eu. Calorimetric Studies. Characterization of the phase behavior was performed by DSC) (Figure 3). During the cooling/heating runs, the heat capacity anomalies show

fold proper rotation axis, which makes group No. 15 more symmetrical than group No. 14.26 This temperature-induced symmetry change is in line with the previously observed phase transitions favoring higher-symmetry structure at higher temperatures.27 Despite the phase transition, the inner part of the complex molecules remains largely intact; the changes consider the special orientation of peripheral tolyl and methoxy groups (Figure 2). Each europium(III) ion of the [EuL4]−

Figure 2. Structure overlay of the complex anion of 1-Eu-293 (red line) and 1-Eu-100 (blue).

complex anion is bound to two oxygen atoms belonging to the phosphoryl and sulfonyl groups of four bidentate chelate SAPh ligands, completing the inner coordination sphere of the lanthanide ion to eight and formation of the [EuO8] coordination polyhedron. Lanthanide ions, in general, are strongly oxophilic, similar to the main groups II and III metal cations, which represent hard acids.28 Hence, there is no surprise in the formation of the acac-type six-membered metallochelate rings (Figure 1). At first, let us describe the structure of the 1-Eu-293 complex. To begin with, it should be noted that the ASU (asymmetric unit) of this complex consists of the half-molecule (Figure S6). This is because both Eu1 and N1c occupy special positions in the lattice on the 2-fold axis (Figure S7). All bonds and angles in the ligand, as well as in the tetraethylammonium countercation, are normal and will not be commented on. Four metallochelate rings, because of the symmetry considerations, form two pairs (Figure S7). Thus, there are two boatlike shapes with a dihedral angle of ∼19° and two skewed (twisted) six-membered cycles (Figures S8 and S9). Except for the van der Waals and electrostatic forces between the cations and anionic europium complexes, there are no other noticeable intermolecular interactions. The anions and cations are bound through weak C−H···O and C−H···π intermolecular bonds (Tables S1 and S3), forming chains along the the b axis. Surprisingly enough, no π−π-stacking interactions occur during crystal packing, although tolyl fragments are present in abundance in the structure. The overall packing of this polymorph in the crystal is rather loose, with ∼60.2% of unit cell volume occupied (Table 1). The selection of a block crystal that is ∼2.8 times smaller by volume (Table 1) and very slow cooling allowed determination of the crystal structure of the low-temperature polymorph.

Figure 3. Low-temperature DSC traces for 1-Eu upon heating and cooling at 10 K min−1. E

DOI: 10.1021/acs.inorgchem.8b02846 Inorg. Chem. XXXX, XXX, XXX−XXX

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the anionic form. The intense bands with maxima at 1340 and 1235 cm−1 for HL were assigned to νas(SO2) and ν(PO) vibrations, respectively. In the case of the sodium salt and 1Ln, these bands are shifted to lower frequencies (Table 3), which can be explained by the deprotonation and coordination of the ligand to the metal ions.

considerable hysteresis, with peaks located at 165.0(2)/ 186.0(2) K, respectively. The average enthalpy change ΔH is 0.20(4) kJ mol−1, and the entropy change ΔS is 1.20(4) J K−1 mol−1. Considering the order of the thermodynamic values, apparently the phase transition does not involve a significant structural change of the compound, which is supported by the single-crystal X-ray data. A pronounced exothermic effect during the sample’s cooling is registered at 165 K, with the same magnitude endothermic effect registered upon the sample’s warming at 186 K. Therefore, the actual phase transition temperature is determined to be ∼175 ± 2 K. Thermal Analysis Results. The thermal stability is an important characteristic that determines the possibility of compound practical applications. The thermal stability of 1-Eu was investigated using TGA in the temperature range of 16− 800 °C (Figure 4). According to the TGA curve, there is no

Table 3. Selected Assigned Vibration Frequencies in the IR Spectra of Solid Samples of Synthesized Compounds frequency, cm−1 vibration

HL

NaL

1-La

1-Nd

1-Eu

νas(SO2) ν(PO) νs(SO2)

1340 1235 1170

1271 1175 1130

1241 1172 1123

1243 1171 1221

1243 1172 1123

For the neodymium tetrakis complex 1-Nd, the DRS spectra in the regions of 4I9/2 → 2P1/2 (A) and hypersensitive 4I9/2 → 4 G5/2 and 2G7/2 (B) transitions were studied at room temperature (Figure 5). The absorption bands in the region

Figure 5. DRS spectrum of 1-Nd in the regions of transitions 4I9/2 → 2 P1/2 (A) and 4I9/2 → 4G5/2 and 2G7/2 (B).

Figure 4. DTA and TGA weight loss traces for 1-Eu..

obvious weight loss of up to 185 °C. If heated above 185 °C, the sample melts (endothermic effect at 187 °C) and starts to decompose abruptly at 211 °C. The decomposition is accompanied by two exothermic and one endothermic effects. The total weight loss after heating of the sample to 800 °C is 68%, and at the boundary value of the experimental temperature, the mass of the sample has not reached a constant value. Analysis of the IR spectrum of the solid residual in the crucible after TGA investigation reveals the formation of a mixture of europium(III) orthophosphate and metaphosphate. Spectroscopic Properties. Recently reported IR spectroscopic investigations of CAPh and their complexes are suitable for preliminary analysis of the coordination modes of these ligands.30 Interpretation of the IR spectra of the compounds based on HL is more complicated because of the proximity of absorption bands of ν(SO2)as, ν(SO2)s, and ν(PO) vibrations. On the basis of the principles listed in ref 30 and the correlation between the synthesized complexes and related compounds with CAPh5,31 and SAPh,11,32−34 assignment of the absorption bands in the IR spectra of the obtained complexes was done as discussed below. The broad absorption band located at 2950−3000 cm−1 in the IR spectrum of HL was assigned to ν(N−H). This band is absent in the spectra of the sodium salt of the ligand and complexes, suggesting ligand coordination to the metal ions in

of 430−435 nm were assigned to electron transitions from the main 4I9/2 level (band with a maximum at 428.4 nm) and populated at room temperature crystal-field sublevels.12,35,36 The singlet band with a maximum at 428.4 nm in the region of transition 4I9/2 → 2P1/2 indicates the presence of one optical center in the complex. According to the number and intensity ratio of the absorption bands in the neodymium complex spectra, in the region of the hypersensitive transition of 4I9/2 → 4G5/2 and 2 G7/2 (560−610 nm), one can draw conclusions about the symmetry of the closest environment of the central atom, which in our case is characteristic for neodymium complexes with a coordination number of eight.5b−e,12,37 The solid-state excitation and emission spectra of the 1-Eu complex are shown in Figures 6 and 7, respectively. The excitation spectra (Figure 6) consist of a narrow f → f transition of the europium(III) ion and a broad band at 250− 300 nm, arising from absorption transitions to the ligand singlet states. Because of the small separation between the europium ground state 7F0 and the first excited state 7F1, transitions from the 7F1 level are observed. Decreasing the temperature causes a reduction of the first excited-state population, and as a result, transitions from this level almost disappeared at 77 K. The narrow f−f transitions dominate in the spectrum, indicating weak sensitization of europium(III) F

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symmetry of the europium(III) ion environment is not typical for known CAPh- and SAPh-based tetrakis complexes, for which the red/orange ratio lies in the range 3−9.39−42 The band of the 5D0 → 7F4 transition dominates in the spectra under study, which is not typical for europium coordination compounds, and has never been observed before for SAPhbased europium(III) complexes. However, for CAPh-based europium(III) complexes, some luminescent spectra with a high intensity of the 5D0 → 7F4 transition were reported recently.39,41,42 Such an abnormally high intensity observed for the 5D0 → 7F4 transition is a consequence of the behavior of the Ωλ intensity parameters in a highly polarizable chemical environment with local symmetry, corresponding to a coordination polyhedron that is slightly distorted from D4d.43,44 The 1-Eu complex also exhibited 5D1 emission, as displayed in Figure S2. The emission decay time (τobs) of the 5D0 state of the europium complex was measured and found to be temperature- and excitation-wavelength-independent. The decay curves can best be fitted by single-exponential functions. The values of the calculated radiative lifetime τRAD and intrinsic quantum yield QLnLn for the 1-Eu complex, and some other SAPh-, CAPh-, or β-diketone-based europium tetrakis complexes, are given in Table 4. The intrinsic quantum yield was calculated from the lifetimes

Figure 6. Excitation spectra of a solid sample of 1-Eu at 298 and 77 K.

Q Ln Ln = τobs/τRAD

whereby τRAD is the radiative lifetime of the 5D0 state, estimated from the emission data using the following equation 1/τRAD = AMD,0n3(Itot/IMD),45 with AMD,0 being a constant equal to 14.65 s−1, n considered to be 1.5, and Itot/IMD the ratio of the total integrated emission from the Eu(5D0) level to the integrated intensity of the MD transition 5D0 → 7F1.46 It was found that, in comparison to the earlier known europium(III) tetrakis(SAPh) complexes, the emission decay time for 1-Eu is notably higher (Table 4). This can be explained by the higher symmetry of the coordination environment in 1-Eu. The calculated intrinsic quantum yield for 1-Eu equals 74%, is comparable with other similar SAPh- and CAPh-based tetrakis complexes, and is notably higher compared to β-diketonates.

Figure 7. Emission spectra of a solid sample of 1-Eu at 298 and 77 K (λexc = 267 nm).

luminescence by organic ligands. Inefficient sensitization of europium(III) luminescence by dimethyl[(4-methylphenyl)sulfonyl]amidophosphate was explained earlier, by poor resonance conditions between the respective excited states of the donor and acceptor.13 However, participation of the ligandto-metal charge-transfer state in europium(III) luminescence quenching cannot be completely excluded. Being excited into the SAPh ligand singlet state (λexc = 267 nm) at 298 and 77 K, the europium complex (Figure 7) showed emission with narrow bands corresponding to the 5D0 → 7FJ (J = 0−4) transitions of europium(III). In spite of the existence of two different polymorphs (see above X-ray data) of the complex, the spectra obtained at different temperatures were very similar, which can be explained by the similarity of the europium ion environment in both polymorphs. The assumption about the high-symmetry coordination environment for europium can be concluded from the absence of the 5 D0 → 7F0 transition and relatively low value of the 5D0 → 7 F2/5D0 → 7F1 transitions intensity ratio (red/orange ratio), which equals to 1.20 at 298 K and 0.82 at 77 K, respectively. In the emission spectrum of 1-Eu, three peaks observed for the 5 D0 → 7F1 transition and one peak observed for the 5D0 → 7F2 transition may suggest a distorted D4d symmetry.38 Such a high

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CONCLUSIONS (1) Three new anionic lanthanide tetrakis complexes with dimethyl[(4-methylphenylsulfonyl)]amidophosphate and [NEt4]+ as a counterion were obtained and characterized. On the example of the europium complex, it was shown that substitution of sodium counterion of the [LnL4]− complex anion by [NEt4]+ leads to a change of the lanthanide ion environment and noticeable alteration of the luminescent properties. (2) The synthesized complexes possess high thermal stability and rather good solubility in polar organic solvents and water. (3) X-ray analysis of the 1-Eu complex revealed a singlecrystal-to-single-crystal phase transition, resulting in two different structures at 293 and 100 K, which were also confirmed by calorimetric measurements. At both temperatures, the geometry around the central europium(III) ion is a distorted square antiprism, although the peripheral methoxy and tolyl groups show different spatial orientations. Consequently, in 1-Eu-100, the neighboring complexes are G

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Inorganic Chemistry Table 4. Photophysical Data for Some Europium(III) Tetrakis Complexes Measured at 298 K

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connected by short contacts CH···O between moieties of SAPh ligands, while in 1-Eu-293, these short contacts disappear because of the phase transition and thermal expansion. (4) The europium complex exhibits red luminescence with a unusual spectrum clearly dominated by a band of the 5D0 → 7 F4 transition. The emission spectra of 1-Eu confirm the high

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02846. Single crystal of 1-Eu, emission spectrum of 1-Eu-293, fragment of the crystal packing of 1-Eu, unit cell constants, powder XRD patterns, molecular structure and numbering scheme of 1-Eu-293, geometry analysis, intermolecular bonds, coordination polyhedron determination, intermolecular contacts, analysis of planes/ dihedral angles, orthogonal views, and 13C{1H} and 1H NMR spectra (PDF)

symmetry of the europium(III) ion environment, which is in agreement with the crystal data. (5) The emission decay time of NEt4[EuL4] (3.5 ms) is notably longer in comparison with that of the known to date europium tetrakis(SAPh), while the intrinsic quantum yield of the compound is comparable (74%; Table 4). These findings warrant further studies of this complex for possible lightemitting-diode applications.

Accession Codes

CCDC 1033142−1033143 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The H

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A comparative study of their spectral properties. Dalton Trans. 2015, 44, 15508. (e) Kariaka, N. S.; Trush, V. A.; Gawryszewska, P.; Dyakonenko, V. V.; Shishkina, S. V.; Sliva, T. Yu.; Amirkhanov, V. M. Spectroscopy and Structure of [LnL3bipy] and [LnL 3phen] Complexes with CAPh Type Ligand Dimethylbenzoylamidophosphate. J. Lumin. 2016, 178, 392. (6) Puntus, L. N.; Lyssenko, K. A.; Antipin, M. Yu.; Bünzli, J.-C. G. Role of Inner- and Outer-Sphere Bonding in the Sensitization of EuIIILuminescence Deciphered by Combined Analysis of Experimental Electron Density Distribution Function and Photophysical Data. Inorg. Chem. 2008, 47, 11095. (7) Driesen, K.; Nockemann, P.; Binnemans, K. Ionic liquids as solvents for near-infrared emitting lanthanide complexes. Chem. Phys. Lett. 2004, 395, 306. (8) Lunstroot, K.; Driesen, K.; Nockemann, P.; Gorller-Walrand, C.; Binnemans, K.; Bellayer, S.; Le Bideau, J.; Vioux, A. Luminescent Ionogels Based on Europium-Doped Ionic Liquids Confined within Silica-Derived Networks. Chem. Mater. 2006, 18, 5711. (9) Binnemans, K. Ionic Liquid crystals. Chem. Rev. 2005, 105, 4148. (10) (a) Mudring, A.-V.; Tang, S.-F. Ionic Liquids for Lanthanide and Actinide Chemistry. Eur. J. Inorg. Chem. 2010, 2010, 2569. (b) Pereira, C. C. L.; Dias, S.; Coutinho, I.; Leal, J. P.; Branco, L. C.; Laia, C. A. T. Europium(III) Tetrakis(β-diketonate) Complex as an Ionic Liquid: A Calorimetric and Spectroscopic Study. Inorg. Chem. 2013, 52, 3755. (11) Moroz, O.; Trush, V.; Znovjyak, K.; Konovalova, I.; Omelchenko, I.; Sliva, T.; Shishkin, O.; Amirkhanov, V. Synthesis and crystal structures of new potential chelating sulfonylamidophosphate ligands. J. Mol. Struct. 2012, 1017, 109. (12) Kasprzycka, E.; Trush, V. A.; Jerzykiewicz, L.; Amirkhanov, V. M.; Gawryszewska, P. Structural and spectroscopic properties of Nd complexes with sulfonylamidophosphate type ligands. J. Lumin. 2016, 170, 348. (13) Kasprzycka, E.; Trush, V. A.; Amirkhanov, V. M.; Jerzykiewicz, L.; Malta, O. L.; Legendziewicz, J.; Gawryszewska, P. Contribution of Energy Transfer from the Singlet State to the Sensitization of Eu3+ and Tb3+ Luminescence by Sulfonylamidophosphates. Chem. - Eur. J. 2017, 23, 1318. (14) Kasprzycka, E.; Trush, V. A.; Amirkhanov, V. M.; Jerzykiewicz, L.; Gawryszewska, P. Structural and photophysical properties of lanthanide complexes with N-(diphenylphosphoryl)-4-methylbenzenesulfonamide. Opt. Mater. 2014, 37, 476. (15) Gordon, A. J.; Ford, R. A. The Chemists Companion. A Handbook of Practical Data, Techniques, and References; John Wiley & Sons: New York, 1972; p 560. (16) Software Suite for APEX2; Bruker AXS: Madison, WI, 2013. (17) Blessing, R. H. An Empirical Correction for Absorption Anisotropy. Acta Crystallogr., Sect. A: Found. Crystallogr. 1995, 51, 33. (18) Sheldrick, G. M. SADABS, 2.03 ed.; University of Göttingen: Göttingen, Germany, 1999. (19) Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.; Howard, J. A. K.; Puschmann, H. Complete Structure Solution, Refinement and Analysis Program. J. Appl. Crystallogr. 2009, 42, 339. (20) Sheldrick, G. M. A short history of SHELX. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112. (21) Farrugia, L. J. ORTEP-3 for Windows - a version of ORTEP-III with a Graphical User Interface (GUI). J. Appl. Crystallogr. 1997, 30, 565. (22) Burnett, M. N.; Johnson, C. K. ORTEP-III: Oak Ridge thermal ellipsoid plot program for crystal structure illustrations; Oak Ridge National Laboratory: Oak Ridge, TN, 1996. (23) Mercury Software, 3.2 ed.; CCDC: Cambridge, England, 2010. (24) Moroz, O. V.; Shishkina, S. V.; Trush, V. A.; Sliva, T. Yu.; Amirkhanov, V. M. Catena-Poly[neodymate(III)bis[-dimethyl(phenylsulfonyl)amidophosphato]sodium(I)bis[-dimethyl(phenylsulfonyl)amidophosphato]] Acta Cryst. Acta Crystallogr., Sect. E: Struct. Rep. Online 2007, 63, m3175. (25) Gawryszewska, P.; Moroz, O. V.; Trush, V. A.; Kulesza, D.; Amirkhanov, V. M. Structure and sensitized near-infrared lumines-

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (N.G.). Phone: (417) 836-5165. *E-mail: [email protected] (V.M.A.). Phone: (044) 239 3392. ORCID

Nikolay Gerasimchuk: 0000-0003-4867-6475 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to Dr. Victor A. Trush (Taras Shevchenko National University of Kyiv) for assistance in the synthesis of ligands and to Dr. Ievgen V. Odynets (Taras Shevchenko National University of Kyiv) for help with TGA measurements. N.G. is grateful to the College of Natural and Applied Sciences of Missouri State University for ongoing support of the X-ray Diffraction Laboratory and to Victoria Barry for technical support during the manuscript preparation.

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