Control of Lanthanide Coordination Environment: Synthesis, Structure

Jun 21, 2016 - Luminescence decay curves of 1tBu under (a) N2 (red, 1180 μs) and air (blue, 40 μs) and (b) air (enlarged view) in THF at room temper...
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Control of Lanthanide Coordination Environment: Synthesis, Structure, and Oxygen-Sensitive Luminescence Properties of an Eight-Coordinate Tb(III) Complex Hidetaka Nakai,*,†,‡,§ Juncheol Seo,† Kazuhiro Kitagawa,†,§ Takahiro Goto,† Kyoshiro Nonaka,†,§ Takahiro Matsumoto,†,‡,§ and Seiji Ogo*,†,‡,§ †

Department of Chemistry and Biochemistry, Graduate School of Engineering, ‡Center for Small Molecule Energy, and §International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan S Supporting Information *

ABSTRACT: Coordination environment of the Tb3+ ion in oxygensensitive luminescent complexes can be successfully controlled through the size of alkyl substituents on ligands {(RMeArOH)4cyclen} (R = tBu or Me; cyclen = 1,4,7,10-tetraazacyclododecane); a newly prepared eightcoordinate complex 1tBu shows higher oxygen sensitivity (KSV = 17 600) and lower luminescence quantum yield (Φ = 0.67 under N2) than those of the previously reported seven-coordinate analogues 1 Me and [{(MeMeArO)3tacn}TbIII(THF)] (KSV = 12 600 and 8300, Φ = 0.91 and 0.91 under N2, respectively; tacn = 1,4,7-triazacyclononane; THF = tetrahydrofuran). The oxygen-sensitive mechanism is discussed on the basis of the photophysical properties of the corresponding Gd(III) complexes.

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position of the phenol ring, {(tBuMeArOH)4cyclen}, provides an eight-coordinate Tb(III) complex, [H{(tBuMeArO)4cyclen} TbIII] (1tBu, Figure 1c). Thus, the coordination environment of the Tb3+ ion in the oxygen-sensitive luminescent complexes can be controlled through the size of the alkyl substituents on the phenol moieties: the CNs in 1Me and 1tBu are 7 and 8, respectively. Taking this golden opportunity, we investigated the unexplored relationship between the coordination environment and the oxygen-sensitive luminescence properties. Herein, we report the synthesis, structure, and oxygen-sensitive luminescence properties of the N4O4 eight-coordinate Tb(III) complex 1tBu; the obtained results are compared with those of the previously reported N4O3 and N3O4 seven-coordinate complexes, 1Me, and its tacn-based analogue [{(MeMeArO)3tacn}TbIII(THF)] (2; THF = tetrahydrofuran; Figure 1d),4b respectively.

INTRODUCTION In metal complexes, the functions such as luminescence, magnetic, and catalytic properties closely relate to coordination environment of the central metals: control of the coordination environment is important fundamentally and practically. However, especially in f-element complexes, it is still challenging to control the coordination environment because of the flexible coordination number (CN) of the f-element ions (CN = 2−12; the general coordination ranges of designed macrocyclic ligands are 6−12).1−3 Some derivatives of the macrocyclic polyamines such as 1,4,8,11-tetraazacyclotetradecane (cyclam), 1,4,7,10-tetraazacyclododecane (cyclen), and 1,4,7-triazacyclononane (tacn) have been successfully employed as multidentate ligands to control the coordination environment as well as the functions in felement chemistry.1c,d,3 In this context, we recently developed a cyclen-based tetrakis-phenol bearing methyl (Me) groups on ortho and para positions of the phenol ring, {(MeMeArOH)4cyclen} (Figure 1a).4a This cyclen-derivative provides a highly luminescent and highly oxygen-sensitive terbium(III) complex (luminescence quantum yield (Φ) = 0.91 under N2, Φ = 0.031 under air) with an extendable phenol pendant arm, [{(MeMeArOH)(MeMeArO)3cyclen}TbIII] (1Me, Figure 1b). In 1Me, the potentially N4O4-octadentate ligand unprecedentedly coordinates to the Tb3+ ion with N4O3-heptadentate fashion (CN = 7). In our efforts to understand this unique coordination chemistry, we have now found that a new cyclen-based tetrakis-phenol bearing tert-butyl (tBu) group on the ortho © XXXX American Chemical Society

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RESULTS AND DISCUSSION Synthesis and Characterization. The N4O4-octadentate ligand {(tBuMeArO)4cyclen}4− was newly synthesized through a Mannich reaction of cyclen with 2-(tert-butyl)-4-methylphenol a n d w a s i s o l a t e d i n t h e t e t ra -p r o t o n a t e d fo rm ({(tBuMeArOH)4cyclen} = 1,4,7,10-tetrakis{3-(tert-butyl)-5methyl-2-hydroxybenzyl}-1,4,7,10-tetraazacyclododecane) in 8 5 % y i e l d (Scheme 1) . T h e p r o t o n a t e d lig a n d {(tBuMeArOH)4cyclen} was characterized by X-ray diffraction Received: March 30, 2016

A

DOI: 10.1021/acs.inorgchem.6b00800 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. An ORTEP drawing of {(tBuMeArOH)4cyclen} with 50% probability ellipsoids. Hydrogen atoms, except O−H, are omitted for clarity. Selected bond lengths [Å]: O1−C7 = 1.3688(14), O2−C19 = 1.3729(14), N1···H1 = 1.974, N2···H2 = 2.033. Figure 1. (a) Cyclen-based tetrakis phenols {(RMeArOH)4cyclen} (R = Me 4a and tBu), (b) a seven-coordinate Tb(III) complex [{(MeMeArOH)(MeMeArO)3cyclen}TbIII] (1Me),4a (c) an eight-coordinate Tb(III) complex [H{(tBuMeArO)4cyclen}TbIII] (1tBu), and (d) a seven-coordinate Tb(III) complex [{(MeMeArO)3tacn}TbIII(THF)] (2).4b

{(MeMeArOH)4cyclen} instead of {(tBuMeArOH)4cyclen}, the use of 4.7 or 3.2 equiv of base led to the seven-coordinate complex (1Me). Thus, the coordination environment can be controlled not by the synthetic conditions but by the size of the alkyl substituents on the ligand (tert-butyl or methyl). The molar conductances (Λm < 2 S cm2 mol−1) of 1tBu in THF, CH2Cl2, CH2Cl2/MeOH (1/1), and CH2Cl2/pyridine (20/1) indicate that 1tBu acts as nonelectrolytes,5 implying that the blue H atom (Figure 1c) does not dissociate in solution. In addition, (i) 1tBu is not soluble in H2O; (ii) the cation exchange reaction of 1tBu with Na+, K+, or Et4N+ does not proceed. Those results suggest that the obtained species, 1tBu, is not H+[{(tBuMeArO)4cyclen}TbIII]− but [H{(tBuMeArO)4cyclen}TbIII], in which one H atom is bound to the four oxygen atoms of the phenolato (tBuMeArO−) moieties. The Tb(III) complex 1tBu was further characterized by 1H NMR (Figure S2 in the Supporting Information), electrospray ionization mass (Figure 3) spectroscopy (ESI-MS), and X-ray

Scheme 1. Preparation of the Cyclen-Based Tetrakis Phenol {(tBuMeArOH)4cyclen} and Its Terbium(III) Complex [H{(tBuMeArO)4cyclen}TbIII] (1tBu)

Figure 3. (a) Negative-ion ESI-MS of 1tBu dissolved in CH2Cl2/ MeOH (1/1). (b) The signal at m/z 1031.5 corresponds to [1tBu − H]−. (c) Calculated isotopic distribution for [1tBu − H]−.

analysis (Figure 2) and 1H NMR, 13C NMR, UV−vis, and luminescence spectroscopy (Figure S1 in the Supporting Information). The obtained data are comparable to those of the previously reported {(MeMeArOH)4cyclen} bearing the “Me group” on the ortho position of the phenol ring.4a Reaction of {(tBuMeArOH)4cyclen} with Tb(OTf)3 in dimethylformamide (DMF) in the presence of KOH (4.7 equiv) at room temperature led to the formation of 1tBu as a white powder (53%, Scheme 1). In this reaction, the use of 3.2 equiv of base also led to the eight-coordinate complex (1tBu, 20% yield). Meanwhile, in the reaction using

crystallography. The 1H NMR spectrum of 1tBu exhibits the expected nine paramagnetically shifted and broadened signals between 182 and −123 ppm. Colorless crystals (1tBu·2C5H5N) suitable for X-ray diffraction analysis were obtained from a saturated CH2Cl2/C5H5N (20/1) solution of 1tBu at room temperature. The solid-state molecular structure of 1tBu is depicted in Figure 4, along with selected interatomic data. The elusive H atom (Figure 1c, blue) could not be located in the Xray diffraction analysis. The trivalent terbium ion in 1tBu is coordinated by four nitrogen and four oxygen atoms. A B

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The lifetimes of 1tBu in CH3OH and CD3OD were determined to be 1270 and 1490 μs, respectively, which contrast with those (1350 and 1360 μs, respectively)4a of 1Me having no coordinated OH bond. The number of OH vibrators in inner coordination sphere (q) can be estimated by using the wellestablished Horrocks’ equation: q = A(1/τH − 1/τD), where A = 8.4 for the Tb3+ ion in MeOH with an uncertainty of ±0.5 MeOH molecules).7 The q for 1tBu is calculated to be 0.98, which is in agreement with the proposed structure (Figure 1c). This could be also compatible with a bound methanol solvent molecule. It is known that luminescence spectra of Sm(III) complexes are discriminative concerning coordination sphere.8 A comparison of emission spectra of Sm analogue, [H{(tBuMeArO)4cyclen}SmIII] (Sm_1tBu), in THF and CH3OH (Figure S4 in the Supporting Information) supports that the difference in the lifetimes does not come from coordinated solvent but from the H/D bound to the four tBuMeArO−.9 These observations indicate that, even in solution, the {(tBuMeArO)4cyclen}4− ligand coordinates to the Tb3+ ion with the N4O4-octadentate fashion found in the crystalline state (Figure 4). Optical Properties. The optical properties of the N4O4 eight-coordinate complex 1tBu are summarized in Table 1,

Figure 4. An ORTEP drawing of 1tBu with 50% probability ellipsoids. Hydrogen atoms and cocrystallized solvents (pyridine) are omitted for clarity. Selected bond lengths [Å] and angles [deg]: Tb1−N1 = 2.669(5), Tb1−O1 = 2.277(4), O1−Tb1−O1′ = 77.50(10), O1− Tb1−N1 = 71.11(15), N1−Tb1−N1′ = 67.52(10). (inset) The coordination polyhedron of the Tb3+ ion in 1tBu.

Table 1. Photophysical Data for 1tBu, 1Me, and 2 in THF at Room Temperature

systematic analysis of the coordination geometry of the eightcoordinate Tb3+ ion in 1tBu was performed using the SHAPE program for the stereochemical analysis of molecular fragments by means of continuous shape measures (CSM).6 The square antiprism geometry is suggested by the low CSM value: 1.880 and 2.380 for the square antiprism and triangular dodecahedron, respectively (Table S1 in the Supporting Information). Notably, the average Tb−O(ArO) bond distance in 1tBu (2.277(4) Å) is much longer than those found in the sevencoordinate analogues 1Me (2.180(3) Å) and 2 (2.189(3) Å).4 The X-ray crystallographic results show that the sterically demanding tert-butyl groups in 1tBu contribute to the eightcoordinate structure of the Tb3+ ion (Figure S3 in the Supporting Information). This is a successful example for control of the lanthanide coordination environment. It is noteworthy that the solution properties of 1tBu are different from those of 1Me. (i) Unlike 1Me having the reactive phenol pendant arm (Figure 1b),4a under the same reaction conditions, the reaction of 1tBu with dimethyl sulfate (Me2SO4) dose not afford an o-methylation product (Figure 5). (ii) If a H atom is bound to the four tBuMeArO−, the OH bond should have an effect on luminescence lifetimes (τ) of the Tb3+ ion in 1tBu.

λmax (nm) ε (M−1 cm−1)a Φ (%) under N2b,c Φ (%) under airb,c τ (μs) under N2b,c τ (μs) under airb,c KSV (M−1)b,c T1 (cm−1)f τ (μs) of T1f a

1tBu

1Me

2

301 12 000 67 1.9 1180 40 17 600 22 270 118

299d 15 000d 91d 3.1d 1070d 40d 12 600d 22 720 105

302e 15 000e 91e 5.4e 840e 40e 8300e 22 940 68

Recorded at ∼2.0 × 10−5 M (cell length = 10.0 mm). bRecorded at ∼3.0 × 10−5 M. cMaximum errors in Φ, τ, and KSV are within ±10%. d Data from ref 4a. eData from ref 4b. fThe lowest triplet-state energies were estimated from the phosphorescence spectra of the corresponding Gd(III) complexes in the crystalline state (the spectrum was acquired with a delay time of 50 μs). a

together with those of the previously reported N4O3 and N3O4 seven-coordinate analogues 1Me and 2, respectively. The UV− vis absorption spectrum of 1tBu in THF at room temperature shows an absorption band corresponding to the π → π* transition of the phenolato (tBuMeArO−) moieties (Figure 6, black).10 The band is the same as those observed in 1Me and 2 (Table 1) and is slightly red-shifted relative to that of {(tBuMeArOH)4cyclen} (λmax = 285 nm, ε = 10 × 103 M−1 cm−1, Figure S1 in the Supporting Information). The luminescence spectrum (λex = 300 nm) of 1tBu under N2 in THF at room temperature shows the seven bands at 494, 543, 586, 622, 657, 672, and 681 nm corresponding to the 5D4 → 7FJ (J = 6, 5, 4, 3, 2, 1, and 0, respectively) transitions of the Tb3+ ion (Figure 6, red).3c,11 The excitation spectrum of 1tBu, which was monitored at 543 nm (5D4 → 7F5 transition of the Tb3+ ion), is identical with the absorption spectrum of 1tBu in THF (Figure 6, red dot). As expected, the luminescence of 1tBu is oxygen-sensitive (Figure 6, blue), and its intensities reversibly respond to alternating changes in the oxygen concentration

Figure 5. Negative-ion ESI-MS (MeOH) of the reaction mixture of (a) 1tBu or (b) 1Me with Me2SO4. The signals at m/z 1031.5, 989.3, and 975.3 correspond to [1tBu − H]−, [o-methylated-1Me ([{(MeMeArOMe)(MeMeArO)3cyclen}TbIII]) + MeSO4]−, and [1Me + MeSO4]−, respectively. C

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Figure 6. UV−vis absorption (black), excitation (red dot), and corrected luminescence spectra (under N2 (red) and air (blue), λex = 300 nm) of 1tBu in THF at room temperature (3.1 × 10−6 M). The excitation spectrum was detected at 543 nm. (inset) Reversible responses of the luminescence intensity of 1tBu (3.1 × 10−6 M) to alternating N2 and air exposures: the luminescence was monitored at 543 nm.

Figure 8. Stern−Volmer plots of the luminescence intensity (I0/I) against the oxygen concentration [O2] for 1tBu (●; 3.7 × 10−6 M, KSV = 17 600 M−1, R2 = 0.9983), 1Me (■; 5.5 × 10−6 M, KSV = 12 600 M−1, R2 = 0.9974) and 2 (▲; 4.1 × 10−6 M, KSV = 8300 M−1, R2 = 0.9989). The I0 and I are the luminescence intensities at an O2 concentration of 0.00 M and at the indicated O2 concentrations, respectively.

(under N2 and air; Figure 6, inset). The luminescence quantum yields of 1tBu under N2 and air were determined to be Φ = 0.67 and 0.019, respectively (maximum errors in Φ and τ are within ±10%). The luminescence lifetimes (τ) of 1tBu under N2 and air were determined to be 1180 and 40 μs, respectively (Figure 7).

oxygen sensitivity. To estimate the energy level of T1 in 1tBu, the gadolinium(III) complex having the same ligand as 1tBu, [H{(tBuMeArO)4cyclen}GdIII] (Gd_1tBu), was synthesized, and its phosphorescence spectrum was measured in 2-MeTHF at 77 K (Figure 9a). From the obtained spectrum, T1 in 1tBu was estimated to be 25 840 cm−1. Intriguingly, even at room temperature, Gd_1tBu shows the phosphorescence in both THF and the crystalline state (Figure 9b,c). Furthermore, it was surprisingly found that a new phosphorescence is overlapped with the originally observed phosphorescence at 77 K. Thus, the T1 in 1tBu is not 25 840 but 22 270 cm−1, which is estimated

Figure 7. Luminescence decay curves of 1tBu under (a) N2 (red, 1180 μs) and air (blue, 40 μs) and (b) air (enlarged view) in THF at room temperature. The decay was monitored at 543 nm (λex = 300 nm). Fitted by single exponential curves (black).

The oxygen sensitivity, which is characterized by Stern− Volmer quenching constant (KSV), is obtained from the following equation: I0/I = 1 + KSV[O2] (I0 and I are the luminescence intensities at an O2 concentration of 0.00 M (under N 2 ) and at the indicated O 2 concentrations, respectively; [O2] is oxygen concentration). The Stern−Volmer plot (I0/I vs [O2]) of 1tBu exhibits good linearity (R2 = 0.9983) in the O2 concentration range of 0.00 M (under N2) to 1.01 × 10−2 M (under O2) in THF (Figure 8). The KSV of 1tBu is higher than those of 1Me and 2 (12 600 and 8300 M−1, respectively): the oxygen sensitivity of 1tBu is 1.4 and 2.1 times higher than those of 1Me and 2, respectively.12 Those differences are significant for oxygen sensor applications (maximum error in KSV is within ±10%):13a the oxygensensitive luminescence properties can be finely modulated by the coordination environment. Although there are many factors that control the oxygen sensitivity, the unique coordination environment including the long Tb−O(ArO) bond distance in 1tBu (vide supra, Figure 4) would contribute to the higher oxygen sensitivity among 1tBu, 1Me, and 2. In the oxygen-sensitive luminescent lanthanide complexes, the energy level of the lowest triplet ligand-centered excited state (T1) is one of the crucial factors determining the Φ and

Figure 9. Corrected luminescence spectra (λex = 250 nm) of Gd_1tBu (a) in 2-MeTHF at 77 K, (b) in THF at room temperature, and (c) in the crystalline state at room temperature. The spectra were acquired with delay times of 0 (black) and 50 μs (red). D

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H and 13C NMR spectra were recorded on a JEOL JNM-ESC400 FT-NMR spectrometer in CDCl3 and CD2Cl2. Chemical shifts were referenced to protio solvent impurities (1H: δ 7.26, 13C: δ 77.2 ppm in CDCl3, 1H: δ 5.32 ppm in CD2Cl2) and are reported in parts per million. Conductivity measurements were performed with a FiveEasy Conductivity Meter with InLab720 electrode (Mettler Toledo) using 1.0 × 10−3 M solutions of 1tBu in THF, CH2Cl2, CH2Cl2/MeOH (1/ 1), and CH2Cl2/pyridine (20/1). ESI-MS data were obtained by a JEOL JMS-T100ESI AccuTOF LC-plus. UV−vis absorption spectra were measured in a JASCO V-670 UV−visible-NIR Spectrophotometer (cell length = 10.0 mm). Elemental analyses were performed using a Yanaco CHN-coder MT-5. Luminescence properties (corrected luminescence and excitation spectra, luminescence lifetimes, and luminescence quantum yields) were measured using a Horiba Jobin Yvon FluoroMax-4P Spectrofluorometer (the measurements were independently repeated at least three times). Luminescence quantum yields were measured by the relative comparison method with quinine bisulfate in 0.5 M H2SO4 (Φ = 0.60) chosen for the standard (maximum error in the reported values is within ±10%, which is confirmed by the cross-calibration of the standard).15 Terbium(III) emission was measured between λ = 450 and 700 nm, corresponding to the 5D4 → 7FJ (J = 6−0) transitions. The general equation used for determination of relative quantum yields is given as Qx/Qst = [Ast(λ)/Ax(λ)][Ix/Ist][nx2/nst2], where A is absorbance at an excitation wavelength, I is the integrated luminescence intensity, and n is the refractive index. The subscripts x and st represent the sample and standard, respectively. For measuring oxygen-sensing properties, the standard gas mixtures (0, 30, 50, 75, and 100% of O2) and air (20.9% of O2) were passed through the cuvette to equilibrate the oxygen content to the respective concentrations (flow rate >5 mL min−1; time >5 min). The oxygen concentrations (1.01, 0.75, 0.50, 0.30, and 0.00 × 10−2 M at 100, 75, 50, 30, and 0% of O2, respectively) reported in Figure 8 were calculated from the mole fraction solubility of oxygen in THF at 101.325 kPa partial pressure of gas (8.16 × 10−4 at 298.15 K).16 X-ray Crystallography. All measurements were made on a Rigaku/ MSC Saturn CCD diffractometer with confocal monochromated Mo Kα radiation (λ = 0.710 75 Å; Table 2). Data were collected and processed using CrystalClear17 software (Rigaku). The data were corrected for Lorentz and polarization effects. Empirical absorption corrections were applied. The structures were solved by direct methods: SIR-9218 and expanded using a Fourier technique. All calculations were performed using the CrystalStructure19 crystallographic software package except for refinement, which was performed using SHELXL Version2014/6.20 All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were refined using the riding model. 1tBu: the pyridine (C5H5N) molecules with half occupancies are included. The elusive H atom (Figure 1c, blue) could not be located in the X-ray diffraction analysis. Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre (CCDC). CCDC reference numbers 1417829 ({(tBuMeArOH)4cyclen}) and 1439554 (1tBu). Synthesis of {(tBuMeArOH)4cyclen}. A mixture of 1,4,7,10tetraazacyclododecane (0.50 g, 2.9 mmol) and paraformaldehyde (0.44 g, 14.7 mmol) in toluene (50 mL) was stirred for 6 h at 50 °C and 12 h at room temperature. To the resulting pale yellow solution was added 2-(tert-butyl)-4-methylphenol (2.38 g, 14.5 mmol). The mixture was stirred for 11 h at 50 °C and 37 h at room temperature. The white precipitate was isolated by filtration and washed with MeOH. Yield: 2.16 g (85%). Single crystals suitable for X-ray diffraction analysis were obtained from a saturated solution of {(tBuMeArOH)4cyclen} in MeOH/CH2Cl2 (50/1) at room temperature. 1H NMR (400 MHz, CDCl3): δ 9.71 (4H, s, CH2(tBu)(Me)C6H2OH), 6.98 (4H, s, CH2(tBu)(Me)C6H2OH), 6.48 (4H, s, CH2(tBu)(Me)C6H2OH), 3.56 (8H, s, CH2(tBu)(Me)C6H2OH), 2.79 (16H, s, C8H16N4), 2.20 (12H, s, CH2(tBu)(Me)C6H2OH), 1.37 (36H, s, CH2(tBu)(Me)C6H2OH). 13C NMR (100 MHz, CDCl3): δ 153.70, 136.57, 127.69, 127.58, 127.09, 121.80, 60.18, 50.91, 34.63, 29.64, 20.86. UV−vis (THF): λmax/nm (ε/M−1 cm−1) =

from the phosphorescence spectrum of Gd_1tBu in the crystalline state (the spectrum was acquired with a delay time of 50 μs). Thus, energy gap (ΔE) between the lowest ligandcentered and metal-centered (Tb3+, 5D4: 20 490 cm−1) levels in 1tBu is found to be 1780 cm−1 (Table 1). This value is consistent with those found in the previously reported oxygensensitive terbium(III) complexes (ΔE < 3500 cm−1): the oxygen-sensitive mechanism of 1tBu would be explained by the well-known mechanism that involves the thermally activated back-energy transfer.13c,14 At this present, we assume that the new phosphorescence is attributed to triplet excimers of the three phenolato moieties in 1tBu. The luminescence decay curves of Gd_1tBu (λex = 250 nm), which were recorded at 387 and 449 nm in THF at room temperature, can be fitted by using single- and biexponential curves, respectively (at 387 nm: τ = 14 μs, R2 = 0.9997; at 449 nm: τ1 = 14 μs, amplitude A1 = 0.909 and τ2 = 118 μs, amplitude A2 = 0.091, R2 = 0.9999; Table 1). Thus, the lifetimes for the phosphorescence (λem at 387 and 449 nm) of Gd_1tBu were determined to be τ = 14 and 118 μs, respectively. The same treatments for [{(MeMeArOH)(MeMeArO)3cyclen} Gd III ] (Gd_1 Me ) 4a and [{( MeMe ArO) 3 tacn}Gd III (THF)] (Gd_2)4b show that the lowest triplet levels of the ligands in 1Me and 2 are estimated to be not T1 = 25 9204a but 22 720 cm−1 and T1 = 26 4604b but 22 940 cm−1, respectively (Table 1, Figures S5 and S6 in the Supporting Information). The energy gaps in 1Me and 2 are found to be ΔE = 2230 and 2450 cm−1, respectively. The lifetimes for the phosphorescence (λem at 386 and 440 nm for Gd_1Me and λem at 378 and 436 nm for Gd_2) were determined to be τ = 12 and 105 μs for Gd_1Me and τ = 67 and 68 μs for Gd_2, respectively. The obtained T1 and their τ for Gd_1tBu, Gd_1Me, and Gd_2 (T1 = 22 270, 22 720, and 22 940 cm−1; τ = 118, 105, and 68 μs) are parallel to the luminescence quantum yields (Φ = 0.67, 0.91, and 0.91 under N2) and oxygen sensitivities (KSV = 17 600, 12 600, and 8300, respectively).

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CONCLUSIONS In conclusion, we have demonstrated that, by using the cyclenbased tetrakis-phenol ligand system, the coordination environment as well as the oxygen-sensitive properties of the luminescent terbium(III) complexes can be controlled through the size of alkyl substituents on the ligand: this provides a clue to understand the unexplored relationship between the coordination environment and the oxygen-sensitive luminescence properties. Recently, the oxygen-sensitive lanthanide complexes are appealing for the oxygen sensor applications.13 Thus, our findings offer attractive new insight into not only the synthetic strategies for control of the coordination environment, namely, functions in f-element chemistry but also the construction of high-performance oxygen sensor based on the luminescent lanthanide complexes.

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EXPERIMENTAL SECTION

Materials and General Methods. All experiments were performed under a dry nitrogen or argon atmosphere using standard Schlenk techniques or a glovebox. Tb(OTf)3, Gd(OTf)3, and Sm(OTf)3 were purchased from Aldrich. Standard gas mixtures containing 0, 30, 50, 75, and 100% of O2 balanced with N2 (100, 70, 50, 25, and 0%, respectively) were purchased from Sumitomo Seika Chemicals Co, Ltd. All other chemicals were obtained from commercial sources and used as received unless otherwise noted. E

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64.98; H, 7.92; N, 5.36%. Gd_1Me (Yield: 59%): UV−vis (THF): λmax/nm (ε/M−1 cm−1) = 299 (15 000). Anal. Calcd for Gd_1Me + 3MeOH (C47H69N4O7Gd): C, 58.84; H, 7.25; N, 5.84%. Found: C, 58.75; H, 7.12; N, 5.85%. Gd_2 (Yield: 70%): UV−vis (THF): λmax/ nm (ε/M−1 cm−1) = 303 (15 000). Anal. Calcd for Gd_2 (C37H50N3O4Gd): C, 58.62; H, 6.65; N, 5.54%. Found: C, 58.62; H, 6.61; N, 5.52%. Reaction of 1tBu or 1Me with Me2SO4. A mixture of the Tb complex {1tBu (95 mg, 0.09 mmol) or 1Me (80 mg, 0.09 mmol)} and sodium methoxide (6 mg, 0.11 mmol) in DMF (5 mL) was stirred for 0.5 h at room temperature. To the mixture was added dropwise dimethyl sulfate (Me2SO4; 10 μL, 0.11 mmol). The resulting solution was stirred for 2 h at room temperature. The obtained reaction mixture was analyzed by ESI-MS (MeOH; Figure 5).

Table 2. Crystal Data and Structure Refinement Details for {(tBuMeArOH)4cyclen} and 1tBu formula Fw crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z μ (cm−1) F(000) Dcalcd (g/cm3) temperature (K) reflections collected independent reflection data/parameters R1 [I > 2σ(I)] wR2 (all data) goodness-of-fit Flack parameter

{(tBuMeArOH)4cyclen}

1tBu

C56H84N4O4 877.30 triclinic P1 (No. 2) 10.3798(16) 10.4741(18) 13.053(2) 84.364(8) 68.693(6) 76.419(8) 1285.0(4) 1 0.70 480.00 1.134 93 15 605 5846 (Rint = 0.0254) 5846/299 0.0432 0.1233 1.082

C56H80N4O4Tb·2C5H5N 1190.40 tetragonal I4 (No. 79) 18.4142(12) 18.4142(12) 10.3199(9) 90 90 90 3499.3(4) 2 10.53 1250.00 1.130 108 14 233 3833 (Rint = 0.0347) 3833/206 0.0353 0.0922 1.092 0.015(7)

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00800. Table of continuous shape measure (CSM) values for the Tb3+ in 1tBu, UV−vis absorption and luminescence spectra of {(tBuMeArOH)4cyclen}, 1H NMR spectrum of 1tBu, coordination polyhedrons and space-filling models of 1tBu, 1Me, and 2, luminescence spectra of Sm_1tBu, and additional luminescence spectra of Gd_1Me and Gd_2 (PDF) X-ray crystallographic data for {(tBuMeArOH)4cyclen} and 1tBu (CIF)

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285 (10 000). Anal. Calcd for {(tBuMeArOH)4cyclen} (C56H84N4O4): C, 76.67; H, 9.65; N, 6.39%. Found: C, 76.40; H, 9.64; N, 6.39%. Synthesis of [H{(tBuMeArO)4cyclen}TbIII] (1tBu). A mixture of {(tBuMeArOH)4cyclen} (100 mg, 0.11 mmol) and Tb(OTf)3 (69 mg, 0.11 mmol) in DMF (30 mL) was stirred for 5 min at room temperature. To the mixture was added dropwise a 1.0 M KOH aqueous solution (512 μL, 0.5 mmol). The resulting solution was stirred for 20 h at room temperature. The solvent was removed under reduced pressure, and the product was extracted with CH2Cl2. The CH2Cl2 was removed under reduced pressure. The obtained white microcrystalline solid was washed with a small amount of MeOH. Yield: 62 mg (53%). Single crystals (1tBu·2C5H5N) suitable for X-ray diffraction analysis were obtained from a saturated solution of 1tBu in CH2Cl2/C5H5N (20/1) at room temperature. 1H NMR (400 MHz, CD2Cl2, 25 °C, Figure S2 in the Supporting Information): δ = 181.18 (1H, s, Δν1/2 = 2103 Hz), 121.54 (4H, s, Δν1/2 = 882 Hz), 93.62 (8H, s, Δν1/2 = 312 Hz), 37.26 (8H, s, Δν1/2 = 122 Hz), 35.81 (12H, s, Δν1/2 = 109 Hz), 1.13 (4H, s, Δν1/2 = 109 Hz), −66.01 (36H, s, Δν1/2 = 394 Hz), −103.50 (4H, s, Δν1/2 = 502 Hz), −122.19 (4H, s, Δν1/2 = 461 Hz). UV−vis (THF): λmax/nm (ε/M−1 cm−1) = 301 (12 000). ESI-MS (CH2Cl2/MeOH (1/1)): m/z 1031.5 [1tBu − H]− {relative intensity (I) = 100% in the range of m/z 200−2000}. Anal. Calcd for 1tBu (C56H81N4O4Tb): C, 65.10; H, 7.90; N, 5.42%. Found: C, 64.82; H, 7.94; N, 5.48%. Synthesis of [H{(tBuMeArO)4cyclen}SmIII] (Sm_1tBu). The samarium complex having the same ligands as 1tBu was synthesized by the methods similar to that for 1tBu using the Sm(OTf)3 instead of Tb(OTf)3. Sm_1tBu (Yield: 49%): UV−vis (THF): λmax/nm (ε/M−1 cm−1) = 301 (13 000). Anal. Calcd for Sm_1tBu (C56H81N4O4Sm): C, 65.64; H, 7.97; N, 5.47%. Found: C, 65.46; H, 7.96; N, 5.46%. Syntheses of [H{( t B u M e ArO) 4 cyclen}Gd I I I ] (Gd_1 t B u ), [{( M e M e ArOH)( Me Me ArO) 3 cyclen} Gd I II ] (Gd_1 Me ), 4 a and [{(MeMeArO)3tacn}GdIII(THF)] (Gd_2).4b The gadolinium complexes having the same ligands as 1tBu, 1Me, and 2 were synthesized by the methods similar to those for 1tBu, 1Me, and 2, respectively, using the Gd(OTf)3 instead of Tb(OTf)3. Gd_1tBu (Yield: 44%): UV−vis (THF): λmax/nm (ε/M−1 cm−1) = 301 (12 000). Anal. Calcd for Gd_1tBu (C56H81N4O4Gd): C, 65.21; H, 7.92; N, 5.43%. Found: C,

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (H.N.) *E-mail: [email protected]. (S.O.) Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI Grant Nos. JP26000008 (Specially Promoted Research), JP26410074, JP26810038, JP15J03538, and JP15H00953 (Scientific Research on Innovative Areas “Stimuli-responsive Chemical Species for the Creation of Functional Molecules”, No. 2408), and by the World Premier International Research Center Initiative from the Ministry of Education, Culture, Sports, Science and Technology (Japan).

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DOI: 10.1021/acs.inorgchem.6b00800 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.6b00800 Inorg. Chem. XXXX, XXX, XXX−XXX