Deboronation-Induced Turn-on Phosphorescent ... - ACS Publications

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Deboronation-Induced Turn-on Phosphorescent Sensing of Fluorides by Iridium(III) Cyclometalates with o‑Carborane Nguyen Van Nghia, Jihun Oh, Jaehoon Jung,* and Min Hyung Lee* Department of Chemistry and EHSRC, University of Ulsan, Ulsan 44610, Republic of Korea S Supporting Information *

ABSTRACT: Heteroleptic tris-cyclometalated Ir(III) complexes bearing an o-carborane at the 4- or 5-position in the phenyl ring of the ppy ligand (closo-1 and -2) were prepared and characterized. The X-ray crystal structure of closo-1 reveals the fac arrangement of the three C∧N chelates around the Ir atom. Treatment of closo complexes with fluoride anions led to selective deboronation of the closo-carborane cage, producing the corresponding nido-carborane-substituted complexes (nido-1 and -2). Whereas closo-1 and -2 were almost nonemissive in THF, nido-1 and -2 were highly phosphorescent (ΦPL = 0.94−0.95). Theoretical studies suggested that, while the emission quenching in closo-1 can be ascribed to the substantial involvement of o-carborane in the 3MLCT excited state, the intraligand charge transfer (3ILCT) state from the nido-carborane to pyridyl moieties is responsible for the efficient phosphorescence in nido-1. The addition of fluoride to the buffered THF/H2O solution (1/1, v/v, pH 7) of closo-1 and -2 under mild heating led to strong emission intensity, allowing the turn-on phosphorescence detection of fluoride in aqueous medium at the ppb level.

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INTRODUCTION Fluoride sensing has received great attention during the past decade because of the detrimental effect of fluoride in physiological systems.1−10 In particular, detection of fluoride at low concentrations, such as the parts per million level, in aqueous media has been an important issue from a practical point of view. While recent studies on highly Lewis acidic organoboron and organoantimony(V) compounds, such as cationic boranes and stiboniums,11−21 and on nanomaterialassisted fluorescent sensors22−24 have elegantly demonstrated fluoride sensing in aqueous media, phosphorescent sensors based on heavy-metal complexes conjugated with a receptor unit have also been of great interest due to their advantageous photophysical properties such as large Stokes shifts, long emission lifetimes, and high quantum efficiency.25−45 These properties of phosphorescent sensors are also beneficial for attaining high signal to noise ratios and for easy separation of the undesired fluorescence noise in the medium.40,46 Among the reported examples, the “off−on”-type phosphorescent sensors, such as Ir(III)−triaryboryl conjugates, could be most promising owing to their low detection limits.47,48 Despite their turn-on response toward fluoride, however, these complexes are incompatible with aqueous media and their phosphorescence quantum efficiency drastically drops under aerated conditions, thereby limiting their practical application. In principle, the design of “off−on”-type phosphorescent sensors is to utilize complexes that are nonemissive in their dormant states but become highly emissive in the presence of fluoride. Although the foregoing examples were also based on © XXXX American Chemical Society

this concept, the complexes were not strictly nonemissive. Bearing this in mind, we searched for nonemissive Ir(III) cyclometalates having a fluoride receptor unit and found that Ir(III) cyclometalates with o-carborane (1,2-closoC2B10H12)49−56 could be suitable for this purpose because these complexes are almost nonemissive in polar media due to the variable nature of carboranyl C−C bonds and the rotational mobility of carborane in solution.51,52,54,57−63 Furthermore, it is well-known that closo-carborane cages are susceptible to nucleophilic anions such as hydroxide and fluoride.64−71 Thus, one may expect that the changes in the electronic structures of closo-carboranes upon conversion to their anionic nido forms may lead to significant changes in the photophysical properties of the complexes. Indeed, we and others have reported emission changes of closo-carboranyl luminophores after degradation of the carborane cage.54,72−78 In this study, we prepared heteroleptic tris-cyclometalated Ir(III) complexes with a closo-carborane (closo-1 and -2) and investigated the changes in their photophysical properties upon degradation of the carborane cage by fluoride. It turned out that closo-1 and -2 may constitute a new type of highly sensitive “off−on” phosphorescent sensors for fluoride in aqueous medium. Special Issue: Tailoring the Optoelectronic Properties of Organometallic Compounds with Main Group Elements Received: February 23, 2017

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DOI: 10.1021/acs.organomet.7b00139 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

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RESULTS AND DISCUSSION Synthesis and Characterization. Heteroleptic tris-cyclometalated Ir(III) complexes with a 4- or 5-o-carboranesubstituted phenylpyridyl ligand (closo-1 and -2) were prepared by means of cyclometalation reactions of (ppy)2Ir(acac) with the ligands n-(2-HCB)ppyH (n = 4, 1a; n = 5, 2a; CB = ocarboran-1-yl; ppy = 2-phenylpyridinato-C2,N)56 in glycerol under heating (Scheme 1). The reactions at ca. 160 °C

ppy ligand. The structural parameters around the Ir atom, such as bond lengths and angles, are in a range similar to that reported for fac-Ir(ppy)3,79 presumably due to peripheral substitution of one carborane cage. While the carboranyl C− C bond distance (1.684(6) Å) is elongated in comparison to that of the parent o-carborane (ca. 1.63 Å),80 it is comparable to those observed for bis- and tris(2-H-o-carborane)-substituted complexes (Ccage−Ccage = 1.65−1.69 Å).52,55,81 The torsion angle (ψ = CPh−CPh−C1(CB)−C2(CB)) of 92.5(5)° indicates that the carboranyl C−C bond axis is almost perpendicular to the phenyl group. Next, to investigate the photophysical properties of nido complexes in detail, we isolated nido complexes by treating closo-1 and -2 with excess (4 equiv) ntetrabutylammonium fluoride (TBAF) in refluxing THF (Scheme 1).70,74,82−84 The reactions led to selective deboronation of the closo-carborane cage, yielding nido-carboranesubstituted complexes (nido-1 and -2) in high yields (ca. 80%). The broad singlet proton resonances of these complexes, centered at δ ca. −2.6 and −2.3 ppm, respectively, indicated the B−H−B bridging hydrogen of nido-carborane. The 11B NMR signals were also shifted upfield into the range of δ ca. −10 to −36 ppm typical of nido-carboranyl boron atoms. Photophysical and Electrochemical Properties. To compare the photophysical properties of closo complexes with those of nido complexes, UV/vis absorption and PL experiments were carried out in degassed toluene and THF (Figure 2, Figures S5 and S6 in the Supporting Information, and Table 1); spectra of nido complexes were acquired in THF only due to poor solubility in toluene. All complexes showed broad lowenergy absorption at 350−450 nm, extending to 500 nm. This

Scheme 1. Synthesis of closo-1 and -2 and nido-1 and -2a

Conditions: (i) glycerol, 160 °C, 30 h (35% for closo-1, 36% for closo2); (ii) TBAF, THF, reflux, 24 h (81% for nido-1, 80% for nido-2).

a

produced closo-1 or -2 as the major product; higher temperatures produced a mixture of ligand redistribution complexes. The 1H NMR spectra exhibited one singlet resonance at δ ca. 3.6 and 3.9 ppm, respectively, typical of the o-carboranyl C−H proton. Broad B−H proton signals in the region of δ ca. 1−3 ppm and 11B NMR signals at δ ca. −2 to −13 ppm also confirmed the presence of closo-carborane. An Xray diffraction study revealed the molecular structure of closo-1 (Figure 1). The structure clearly showed the fac arrangement of the three C∧N chelates around the Ir atom, and the carborane cage was appended at the 4-position of the phenyl ring in the

Figure 1. X-ray crystal structure of closo-1 (40% thermal ellipsoids). H atoms and solvent molecules are omitted for clarity. Selected bond lengths (Å) and angles (deg). Ir(1)−C(2) 2.028(4), Ir(1)−C(13) 2.018(4), Ir(1)−C(24) 2.004(4), Ir(1)−N(1) 2.130(3), Ir(1)−N(2) 2.125(3), Ir(1)−N(3) 2.132(3), C(34)−C(35) 1.684(6); C(2)− Ir(1)−N(1) 79.19(15), C(13)−Ir(1)−N(2) 79.23(14), C(24)− Ir(1)−N(3) 79.26(15), CPh−CPh−Ccage−Ccage 92.5(5).

Figure 2. (top) UV/vis absorption (5 × 10−5 M in THF) and (bottom) PL spectra of closo-1 and -2 and nido-1 and -2 in degassed THF (1 μM, solid line) and in degassed buffered THF/H2O (1/1, v/v, 10 mM HEPES, pH 7; 1 μM, dashed line). λex is 384 nm for closo- and nido-1, 380 nm for closo-2, and 386 nm for nido-2. B


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Organometallics Table 1. Photophysical and Electrochemical Data for closo-1,2 and nido-1,2 λem/nma,b b

compd closo-1 closo-2 nido-1 nido-2

λabs/nm (ε/10−3 M−1 cm−1) THFa 380 (12.3), 402 375 (11.5), 405 339 (17.3), 383 460 (3.0) 385 (12.8), 414 −5

−6

(10.1), 461 (2.5) (7.7), 454 (2.7) (13.8), 416 (7.7), (8.9), 462 (3.3)

toluene 298 K/ 77 K 524/512 509/493

ΦPLd

THF 298 K/ 77 K

THF/H2O 298 K

f

f

525/513 513/500f 522/506b

526 519f 522b

528/510b

528b

c

toluene 0.13 0.42

THF

THF/ H2Oc

τ/μs

Eox/Ve

Ered/Ve

0.003 0.085 0.94

0.001 0.040 0.42

1.04 0.59g 1.63h

0.36 0.36i 0.25i

−1.93k −1.99k −2.78i

0.95

0.43

2.00h

0.11/0.28j

−2.72i

g

i

5 × 10 M. 1 × 10 M. In degassed buffered THF/H2O (1/1, v/v, 10 mM HEPES, pH 7). At 298 K; fac-Ir(ppy)3 as a standard (ΦPL = 0.97).86 In DMF (1 × 10−3 M, scan rate 100−200 mV/s) with reference to a Fc/Fc+ redox couple. fIn 2 × 10−5 M. gIn toluene. hIn THF. iReversible (E1/2). j Irreversible (Eonset). kQuasi-reversible (Eonset). a

b

c

d

e

absorption is similar to that of fac-Ir(ppy)3 and thus can be mainly assigned to the metal to ligand charge transfer (1,3MLCT) transition (see TD-DFT results below).85 closo-1 and -2 yielded featureless PL spectra in fluidic solutions, indicating that the lowest triplet (T1) excited state has 3MLCT character, whereas the structured bands observed at 77 K pointed to the involvement of the 3LC state in the emission. The absorption and PL spectra of closo-2 displayed higherenergy bands relative to those of closo-1, probably owing to the increased HOMO stabilization by 5-o-carborane substitution in closo-2.52,56 Although the quantum efficiency of closo-2 is greater than that of closo-1 in both solvents, both complexes are poorly emissive in THF, particularly in the case of closo-1 (ΦPL = 0.003 in THF). Indeed, these emission features are similar to those observed for bis- and tris(2-R-o-carborane)-substituted Ir(III) complexes and could be attributed to the substantial contribution of the o-carborane moiety to the excited states, leading to facile carboranyl C−C bond variations for efficient emission quenching.49,51,52,54−56,81 In sharp contrast to the weak emission of the closo complexes in THF, the nido complexes exhibited strong emission, with very high quantum efficiency in THF (ΦPL = 0.94−0.95; Figure 2). The emission lifetimes (τ) of 1.63 and 2.00 μs observed for nido-1 and -2, respectively, are consistent with a phosphorescence origin. The broad and featureless band shape is indicative of the 3CT state as the emitting T1 state of nido-1 and -2. Interestingly, whereas the 0−0 phosphorescence band of nido-1 at 77 K was hypsochromically shifted relative to that of closo-1 (λem 506 vs 513 nm), nido-2 exhibited a bathochromic shift relative to closo-2 (λem 510 vs 500 nm). As a result, the emission of nido-2 was red-shifted relative to that of nido-1, opposite to the trend observed between the corresponding closo complexes. It is likely that, because of a negative charge, the electron-donating effect of nido-carborane in nido-2 would elevate the energy of the HOMO electron density through the 5-position of the ppy ligand, resulting in the reduction of a band gap (red shift). In contrast, the nido-carborane cage may have a greater influence on the LUMO through the 4-position in nido-1, which could increase the band gap of the complex (blue shift). The photophysical properties of nido complexes were further investigated in aqueous solution to see whether the highly emissive nature of the nido complexes is retained in aqueous systems, which if so would allow their practical application in sensing. As shown in Figure 2 and Table 1, the phosphorescent properties of nido-1 and -2 in buffered THF/H2O mixed solvents (1/1, v/v, 10 mM HEPES, pH 7) are very similar to those observed in THF, except for an overall decrease in quantum efficiency, which nonetheless remained quite high (ΦPL = 0.42−0.43). Note that the quantum efficiency of closo

complexes also decreased with the increased solvent polarity, implying that the relative quantum efficiency of the nido complexes in comparison to the closo complexes remains similar in THF/H2O. These results may thus indicate that the conversion of closo complexes to nido complexes could provide high turn-on phosphorescence response toward fluoride anions in aqueous solution. The electrochemical properties of all closo and nido complexes were examined by cyclic voltammetry (Figure 3

Figure 3. Cyclic voltammograms of closo-1 and -2 and nido-1 and -2 (1.0 × 10−3 M in DMF, scan rate = 100 mV/s for oxidation and 200 mV/s for reduction).

and Table 1). closo-1 and -2 underwent reversible oxidation at 0.36 V, typical for oxidation centered on the dπ(Ir) orbitals. In the case of the nido complexes, however, whereas nido-1 displayed reversible oxidation at 0.25 V, nido-2 showed two irreversible peaks at Epa = 0.20 and 0.48 V. Because the nido derivatives of the 1a and 2a ligands (nido-1a and -2a) underwent two irreversible oxidations (Epa = ca. 0.48−0.50 and 0.75−0.85 V) attributable to the anionic nido-carborane (Figure S8 in the Supporting Information),82,83 the reversible oxidation of nido-1 can be mainly ascribed to the Ir-centered oxidation, and the irreversible oxidations of nido-2 could be attributed to both the Ir center and nido-carborane moieties. Note that the first oxidation potentials of nido-1 and -2 were cathodically shifted relative to those of closo complexes owing to the anionic nido-carborane. Moreover, the 5-substituted nido2 has a first oxidation potential smaller than that of nido-1 by ca. 0.14 V, indicating a higher HOMO level (Table 1). On the other hand, whereas closo-1 and -2 displayed carboranecentered, quasi-reversible reduction at −1.93 and −1.99 V, respectively, both nido-1 and -2 showed only pyridyl-centered, reversible reduction at −2.78 and −2.72 V, very similar to the reduction potential of fac-Ir(ppy)3 (E1/2 = −2.70 V).85 Owing C


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Organometallics to the presence of an anionic nido-carborane in the nido-1a and -2a ligand fragments, the pyridyl reduction in nido-1 and -2 would mainly occur at the pure ppy ligands, with slight cathodic shifts relative to the pyridyl reduction in fac-Ir(ppy)3. The more negative potential of nido-1 relative to that of nido-2 could be ascribed to the effect of nido-carborane on LUMO destabilization through the 4-position of the ppy ligand. However, the difference is very small (ca. 0.06 V) because the LUMO is mainly located at the ppy ligands, which are less affected by the remote nido-carborane moiety. Consequently, the difference between the band gaps of nido-1 and -2 is more attributable to their HOMO levels, and these electrochemical results are consistent with the emission wavelengths of nido-1 and -2. Theoretical Calculations. TD-DFT calculations were performed on both the ground state (S0) and lowest triplet excited state (T1) optimized structures of closo- and nido-1 to gain insight into their photophysical and electrochemical properties (Figure 4 and Figures S12−S15 and Tables S2−S7

similar to those observed in the S0 state (Figure S13), both the HOMO and LUMO in the T1 state of nido-1 are localized in 1a (HOMO → LUMO (80%)). This finding indicates intraligand CT (3ILCT) character of phosphorescence from the nidocarborane to pyridyl moieties (Figure 4, right). The computed phosphorescence wavelengths (T1 → S0) of closo- and nido-1 (525.2 and 520.1 nm), which shows the blue-shifted emission of nido-1, are also comparable to the observed 0−0 phosphorescence wavelengths (513 and 506 nm in THF at 77 K). Fluoride Sensing Properties. Finally, we tested the feasibility of using closo-1 and -2 in the turn-on phosphorescence sensing of fluoride. To this end, the emission change of a solution of closo-1 in aerated, buffered THF/H2O (1/1, v/v, 10 mM HEPES, pH 7; 1 μM) was monitored first in the presence of fluoride (2 equiv) at elevated temperature. closo-1 was nonemissive under these conditions, but upon heating at 60 °C, green phosphorescence bands began to grow as a result of the conversion of closo-1 to nido-1 (Figure 5a). Although this

Figure 4. Frontier molecular orbital diagrams and energies (eV) from DFT calculations (PCM in THF) of (left) closo-1 and nido-1 in their ground state (S0) geometries and (right) of nido-1 in its lowest triplet state (T1) geometry. The transition energy (in nm) was calculated using the TD-B3LYP method.

in the Supporting Information). The lowest energy absorption in closo-1 is characterized by the HOMO → LUMO (96%) transition. Whereas the HOMO is mainly localized in the dπ(Ir) orbital (50.2%), the LUMO has an exclusive contribution from ligand 1a (91.4%), with an appreciable contribution from the carborane moiety (8.0%). This indicates LUMO stabilization by the o-carborane substitution as well as the MLCT nature of the transition. Indeed, the computed MLCT contribution of the transition was found to be 48.2%. In contrast, the HOMO in nido-1 is delocalized over dπ(Ir) and 1a, with a large contribution from the nido-carborane moiety (25.4%), whereas the LUMO is located at the one of the ppy ligands. This feature is in accordance with the observed Ir(III)-centered oxidation and pyridyl-centered reduction. The lowest energy absorption, which is dominated by the HOMO → LUMO (67%) and HOMO-1 → LUMO (27%) transition, is CT (1MLCT/1LLCT) in nature. The two transitions also possessed MLCT contributions of 27.7% and 31.3%, respectively, which are smaller than that in closo-1 (48.2%). This in turn reflects substantial contribution of LLCT to the lowest energy absorption in nido-1. In the case of phosphorescence, however, although closo-1 shows a 3MLCT transition between MOs

Figure 5. Spectral changes in the phosphorescence of aerated buffered THF/H2O solutions of closo-1 (1/1, v/v, 10 mM HEPES, pH 7; 1 μM; λex 384 nm) (a) in the presence of 2 equiv of fluoride, upon heating at 60 °C over time, (b) in the presence of different amounts of fluoride, after heating at 60 °C for 6 h, and (c) in the presence of 4 equiv of TBAF or TBACN, after heating at 60 °C for 6 h. Insets show photographs of the solution of closo-1 under UV illumination (right) before and (left) after degradation.

change was not instantaneous, the emission intensity gradually increased for 6−7 h, reaching ca. 90% of the intensity of pure nido-1. Owing to the high quantum efficiency of nido-1, the emission was still very strong under aerated conditions, showing a bright green emission (Figure 5a, inset). Deboronation followed by turn-on response was also efficient in the presence of submicromolar quantities of fluoride (Figure 5b). Considering the concentration of fluoride in the solution, the observed turn-on response corresponds to the detection of fluoride at the ppb (parts per billion) concentration level (3.8− D


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suitable for X-ray structural determination were obtained by slow evaporation of the solution in CH2Cl2/MeOH. 1H NMR (CDCl3): δ 7.91 (d, J = 8.2 Hz, 2H), 7.84 (d, J = 1.1 Hz, 1H), 7.70−7.56 (m, 7H), 7.53−7.48 (m, 2H), 7.02−6.81 (m, 9H), 6.66−6.62 (m, 2H), 3.57 (s, 1H, CCB-H), 3.4−1.4 (br, 10H, B-H). 13C NMR (CDCl3): δ 166.6, 165.1, 161.7, 160.2, 160.0, 147.3, 147.1, 147.0, 145.4, 143.6, 143.5, 137.0, 136.8, 136.3, 136.2, 136.1, 134.1, 133.6, 130.1, 129.9, 124.1, 124.0, 123.3, 122.9, 122.2, 122.1, 122.0 120.2, 120.0, 119.4, 119.3, 119.0, 118.9 (Ar-C), 77.2, 59.4 (CB-C). 11B NMR (CDCl3): δ −3.0 (1B), −5.2 (br, 1B), −9.5 (2B), −11.4 (4B), −13.6 (br, 2B). Dec pt: 220 °C. Anal. Calcd for C35H34B10IrN3: C, 52.75; H, 4.30; N, 5.27. Found: C, 52.56; H, 4.29; N, 5.01. HR MS: m/z calcd for C35H34B10IrN3, 799.3313; found, 799.3315. Synthesis of fac-[5-(2-H-1,2-C2B10H10)ppy]Ir(ppy)2 (closo-2). This compound was prepared in a manner analogous to the synthesis of closo-1 using (ppy)2Ir(acac) (0.42 g, 0.70 mmol) and 2a (0.21 g, 0.70 mmol). Column chromatography on silica gel using CH2Cl2/ hexane (1/1, v/v) as eluent gave a yellow powder of closo-2 (0.20 g, 36%). 1H NMR (CDCl3): δ 7.89−7.82 (m, 3H), 7.75 (d, J = 2.1 Hz, 1H), 7.65−7.52 (m, 6H), 7.49 (d, J = 5.5 Hz, 2H), 6.94−6.78 (m, 10H), 6.73 (dd, J = 7.4, 1.1 Hz, 1H), 3.88 (s, 1H, CCB-H), 3.4−1.4 (br, 10H, B-H). 13C NMR (CDCl3): δ 166.5, 166.4, 165.4, 165.3, 160.1, 159.9, 147.3, 147.1, 146.9, 144.5, 143.6, 137.1, 137.0, 136.7, 136.5, 136.4, 136.3, 136.2, 130.2, 130.1, 130.0, 128.5, 124.1, 124.0, 123.3, 122.8, 122.1, 120.2, 120.1, 119.1, 119.0, 118.9, 118.8 (Ar−C), 79.0, 61.2 (CB-C). 11B NMR (CDCl3): δ −2.2 (1B), −5.1 (br, 1B), −9.5 (2B), −10.7 (4B), −12.7 (br, 2B). Dec pt: 250 °C. Anal. Calcd for C35H34B10IrN3: C, 52.75; H, 4.30; N, 5.27. Found: C, 52.70; H, 4.01; N, 5.02. HR MS: m/z calcd for C35H34B10IrN3, 799.3313; found, 799.3317. Synthesis of [NBu4][{4-(2-H-1,2-C2B9H10)ppy}Ir(ppy)2] (nido1). closo-1 (35 mg, 0.04 mmol) and n-tetrabutylammonium fluoride (TBAF) (50.5 mg, 0.16 mmol) were dissolved in THF, and the mixture was refluxed for 24 h. After the mixture was cooled to room temperature, the solvent was evaporated, and the remaining residue was purified by column chromatography on alumina using CH2Cl2/ ethyl acetate (1/1, v/v) as eluent to give a yellow powder of nido-1 (33.3 mg, 81%). 1H NMR (CD2Cl2): δ 7.99−7.96 (m, 2H), 7.87 (d, J = 8.4, 1H), 7.78−7.54 (m, 8H), 7.46 (dd, J = 8.1, 2.8 Hz, 1H), 6.99− 6.84 (m, 10H), 6.70 (dd, J = 8.5, 1.8 Hz, 1H), 3.4−1.4 (br, 9H, B-H), 3.16 (t, J = 8.2, 8H), 2.01 (s, 1H, CCB-H), 1.64−1.58 (m, 8H), 1.51 (sext, J = 7.1 Hz, 8H), 1.07 (t, J = 7.2 Hz, 12H). −2.58 (br s, 1H, B-HB). 13C NMR (CD2Cl2): δ 166.7, 166.6, 159.5, 156.0, 153.7, 147.2, 147.1, 147.0, 143.8, 143.7, 140.3, 138.6, 138.3, 137.1, 136.9, 136.5, 136.1, 135.9, 135.5, 129.7 129.6, 129.4, 128.5, 124.3, 124.2, 124.1, 123.2, 123.1, 122.2, 121.9, 119.2, 118.9, 118.3 (Ar-C), 59.0, 23.9, 19.7, 13.4 (NBu4) (CB-C signals were not observed). 11B NMR (CD2Cl2): δ −10.0 (2B), −13.4 (1B), −15.9 (1B), −19.3 (2B), −23.3 (1B), −33.0 (1B), −35.9 (1B). Dec pt: 170 °C. Anal. Calcd for C51H70B9IrN4: C, 59.55; H, 6.86; N, 5.45. Found: C, 59.28; H, 6.90; N, 5.22. HR MS: m/z calcd for [M − NBu4]− (C35H34B9IrN3), 788.3219; found, 788.3223. Synthesis of [NBu4][{5-(2-H-1,2-C2B9H10)ppy}Ir(ppy)2] (nido2). This compound was prepared in a manner analogous to the synthesis of nido-1 using closo-2 (40 mg, 0.05 mmol) and TBAF (63 mg, 0.2 mmol). Column chromatography on alumina using CH2Cl2/ ethyl acetate (1/1, v/v) as eluent gave a yellow powder of nido-2 (41.2 mg, 80%). 1H NMR (CDCl3): δ 7.85−7.78 (m, 3H), 7.63−7.43 (m, 9H), 6.89−6.74 (m, 10H), 6.53 (d, J = 7.9 Hz, 1H), 3.3−1.5 (br, 9H, B−H), 3.13 (t, J = 7.9, 8H), 2.35 (s, 1H, CCB-H), 1.60−1.49 (m, 8H), 1.41 (sext, J = 7.2 Hz, 8H), 0.95 (t, J = 7.2 Hz, 12H), −2.28 (br s, 1H, B-H-B). 13C NMR (CDCl3): δ 167.1, 166.7, 166.6, 163.0, 162.9, 161.8, 161.6, 156.2, 147.4, 147.2, 147.1, 145.5, 143.8, 143.7, 140.8, 138.0, 137.4, 137.2, 137.1, 136.5, 135.9, 135.8, 130.1, 129.6, 129.2, 123.8, 122.0, 121.9, 121.5, 119.6, 119.5, 118.9, 118.7 (Ar−C), 59.0, 24.0, 19.7, 13.7 (NBu4) (CB-C signals were not observed). 11B NMR (CDCl3): δ −10.4 (2B), −13.8 (br, 1B), −16.6 (br, 1B), −18.8 (2B), −22.5 (br, 1B), −32.9 (1B), −36.0 (1B). Dec pt: 180 °C. Anal. Calcd for C51H70B9IrN4: 59.55; H, 6.86; N, 5.45. Found: 59.37; H, 6.81; N, 5.21.

76 ppb). closo-2 exhibited a similar but lesser turn-on phosphorescence response toward fluoride, owing to the residual weak emission of closo-2 (Figure S9 in the Supporting Information). Furthermore, closo-1 did not show any measurable response toward cyanide (e.g., 4 equiv),87 which is the anion most competing with fluoride in a variety of Lewis acid chemosensing systems, in both THF and buffered solutions (Figure 5c and Figure S10 in the Supporting Information). This experiment also attests to the fact that the hydroxide ion at pH 7 does not degrade the o-carborane cage in closo-1. Although a relatively long response time (ca. 6 h) is observed, in comparison to those of other reaction-based fluoride sensors (0.5−4 h),22,24 these results indicate that iridium(III) cyclometalates with o-carborane, such as closo-1 and -2 in this study, can be utilized as a fluoride-selective, highly sensitive “off−on”-type phosphorescent sensor.

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CONCLUSION We have synthesized and characterized heteroleptic triscyclometalated Ir(III) complexes bearing an o-carborane at the 4- or 5-position in the phenyl ring of the ppy ligand (closo-1 and -2). These closo complexes underwent facile deboronation of the closo-carborane cage, producing the corresponding nidocarborane-substituted complexes (nido-1 and -2). While closo-1 and -2 were almost nonemissive in THF, nido-1 and -2 exhibited phosphorescence with very high quantum efficiency (ΦPL = 0.94−0.95). Theoretical studies suggested that the nidocarborane to pyridyl 3ILCT state is responsible for the efficient phosphorescence in nido-1. These photophysical changes accompanied by conversion from closo to nido complexes allowed the turn-on phosphorescent sensing of fluoride in aqueous medium at the ppb level.

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EXPERIMENTAL SECTION

General Considerations. All operations were performed under an inert nitrogen atmosphere using standard Schlenk and glovebox techniques. Anhydrous grade solvents (Aldrich) were dried by passing them through an activated alumina column and stored over activated molecular sieves (5 Å). Spectrophotometric-grade toluene and tetrahydrofuran (THF) were used as received. Commercial reagents were used without further purification after purchase. 4-(2-H-1,2C2B10H10)ppyH (1a),56 5-(2-H-1,2-C2B10H10)ppyH (2a),56 and (ppy)2Ir(acac)88 were synthesized according to the reported procedures. Deuterated solvents from Cambridge Isotope Laboratories were used. NMR spectra were recorded on a Bruker 300 AM spectrometer (300.13 MHz for 1H, 75.48 MHz for 13C, 96.29 MHz for 11 B) at ambient temperature. Chemical shifts are given in parts per million (ppm) and are referenced against external Me4Si (1H, 13C) and BF3·OEt2 (11B). Elemental analyses were performed on a Flash 2000 elemental analyzer (Thermo Scientific) by the Research Facilities Center at the University of Ulsan. Mass spectra were obtained using a JEOL JMS700 high-resolution FAB-mass spectrometer (HR FAB-MS) at the Korea Basic Science Institute, Daegu, Korea. Melting (mp) or decomposition points (dec pt) were measured by an SMP30 Melting Point Apparatus (Stuart Equipment). Cyclic voltammetry experiments were performed using an AUTOLAB/PGSTAT 101 system. Synthesis of fac-[4-(2-H-1,2-C2B10H10)ppy]Ir(ppy)2 (closo-1). (ppy)2Ir(acac) (0.40 g, 0.67 mmol) and 1a (0.19 g, 0.67 mmol) were dissolved in glycerol (20 mL), and the mixture was heated at 160 °C for 30 h under a nitrogen atmosphere. After the mixture was cooled to room temperature, water (30 mL) was added. The resulting yellow powder was filtered and washed with water, and the solid was extracted with CH2Cl2. The solvent was evaporated off under reduced pressure, and the residue was purified by column chromatography on silica gel using ethyl acetate/hexane (1/1, v/v) as eluent. Drying in vacuo afforded a yellow powder of closo-1 (0.18 g, 35%). Single crystals E


Article

Organometallics HR MS: m/z calcd for [M − NBu4]− (C35H34B9IrN3), 788.3219; found, 788.3223. X-ray Crystallography. A specimen of suitable size and quality was coated with Paratone oil and mounted onto a glass capillary. The crystallographic measurement was performed using a Bruker Apex IICCD area detector diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at 100 K. A hemisphere of reflection data was collected as ϕ and ω scan frames with 0.3°/frame and an exposure time of 10 s/frame. Cell parameters were determined and refined by the SMART program.89 Data reduction was performed using SAINT software.90 The data were corrected for Lorentz and polarization effects. An empirical absorption correction was applied using the SADABS program.91 The structure was solved by direct methods, and all non-hydrogen atoms were subjected to anisotropic refinement by full-matrix least squares on F2 by using the SHELXTL/ PC package.92 Hydrogen atoms were placed at their geometrically calculated positions and refined riding on the corresponding carbon atoms with isotropic thermal parameters. Detailed crystallographic data of closo-1 are given in Table S1 in the Supporting Information. Cyclic Voltammetry. Cyclic voltammetry measurements were carried out in DMF (1 × 10−3 M) with a three-electrode cell configuration consisting of platinum working and counter electrodes and an Ag/AgNO3 (0.01 M in CH3CN) reference electrode at room temperature. Tetra-n-butylammonium hexafluorophosphate (0.1 M in DMF) was used as the supporting electrolyte. The redox potentials were recorded at a scan rate of 100−200 mV/s and are reported with reference to the ferrocene/ferrocenium (Fc/Fc+) redox couple. Photophysical Measurements. UV/vis absorption and PL spectra were recorded on Varian Cary 100 and HORIBA FluoroMax-4P spectrophotometers, respectively. Solution PL spectra were performed in degassed toluene, THF, and buffered THF/H2O (1/1, v/v, 10 mM HEPES, pH 7) solutions with a 1 cm quartz cuvette (typically, 1 μM). Low-temperature PL measurements were carried out with 5 mm diameter quartz tubes that were placed in a Dewar flask filled with liquid nitrogen (77 K). Solution quantum efficiencies were measured with reference to that of fac-Ir(ppy)3 (ΦPL = 0.97 in toluene).86 The reported value of a refractive index was used in the calculation of quantum efficiencies in a mixed THF/H2O (1/1, v/v) solvent.93 Emission lifetimes were measured using the time-correlated single-photon counting (TCSPC) method on a FS5 spectrophotometer (Edinburgh Instruments) equipped with an EPL-375 ps pulsed diode laser at 298 K. The PL spectra of closo-1 and -2 in the presence of anions over time were obtained under aerated conditions. A set of tightly sealed vials containing the solution of closo-1 or -2 and anions was heated at 60 or 80 °C. Each vial was then taken out of the heating bath every 1 h. PL measurements were carried out after cooling the solution to room temperature. Theoretical Calculations. Computational studies were performed to investigate the electronic structures and transition of closo-and nido1 using the density functional theory (DFT) method implemented in the Gaussian 09 program suite.94 The ground (S0) state geometries were optimized using the B3LYP functional.95 The 6-31G(d,p) basis set was used for all atoms except for the iridium atom, which was treated with the LANL2DZ effective core potential (ECP) and corresponding basis set.96 To include the solvent effect, the polarizable continuum model using the integral equation formalism (IEFPCM) was considered in the calculations.97 The time-dependent density functional theory (TD-DFT) method, i.e., IEFPCM-TD-B3LYP/631G(d,p), was employed not only to optimize the lowest-lying triplet excited state (T1) but also to calculate the electronic transition energies, including electron correlation effects.98 Compositions of molecular orbitals and %MLCT were analyzed using the AOMix program.99

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Experimental and computational data (PDF) Crystallographic data of closo-1 (CIF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail for J.J.: [email protected]. *E-mail for M.H.L.: [email protected]. ORCID

Min Hyung Lee: 0000-0003-2977-183X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Basic Science Research Program (NRF-2014R1A1A2056364 for M.H.L.) and from the Priority Research Center Program (2009-0093818 for M.H.L.), through the National Research Foundation of Korea (NRF), is gratefully acknowledged. J.J. acknowledges an NRF grant funded by the Ministry of Science, ICT and Future Planning (NRF-2015R1C1A1A01052947).

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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.organomet.7b00139. F


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