Design of Luminescent Isocyano Rhenium(I) Complexes

Oct 24, 2018 - ... Chi-On Ng , Shek-Man Yiu , and Chi-Chiu Ko*. Department of Chemistry, City University of Hong Kong , Tat Chee Avenue, Kowloon , Hon...
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Design of Luminescent Isocyano Rhenium(I) Complexes: Photophysics and Effects of the Ancillary Ligands Kin-Cheung Chan,† Ka-Ming Tong,† Shun-Cheung Cheng, Chi-On Ng, Shek-Man Yiu, and Chi-Chiu Ko* Department of Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China

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S Supporting Information *

ABSTRACT: Despite the well-reported MLCT [dπ(M) → π*(CNR)] transitions in the isocyano transition metal complexes, emissive complexes with phosphorescence derived from MLCT [dπ(M) → π*(CNR)] were not extensively studied. To provide insights into the design strategy of phosphorescent rhenium(I) complexes with an emissive 3 MLCT [dπ(Re) → π*(CNR)] excited state, a series of pentaisocyano rhenium(I) complexes have been synthesized. In contrast to most of the reported penta- or hexaisocyano rhenium(I) complexes with unsubstituted or alkyl- or monohalo-substituted phenylisocyanide ligands, which only exhibit photoluminescence in 77 K glassy medium, the solutions of all of these complexes were found to show phosphorescence at room temperature. Detailed study on their emission properties revealed that they are derived from the 3MLCT [dπ(Re) → π*(CNR)] excited state mixed with LL′CT character. It has been shown that the strong electron-withdrawing substituents on the isocyanide ligands can lower the energy of the MLCT [dπ(Re) → π*(CNR)] state and raise the deactivating ligand-field state. These effects are the crucial criteria to render the pentaisocyano rhenium(I) complexes emissive. Moreover, the emission properties in terms of energy, lifetime, and quantum yields can also be enhanced by the ancillary ligand.

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INTRODUCTION

only display phosphorescence in 77 K glassy medium, but they are generally nonemissive at room temperature.3a,5 To understand the deactivation of the 3MLCT [dπ(Re) → π*(CNR)] excited state and design phosphorescent rhenium(I) complexes derived from the 3MLCT [dπ(Re) → π*(CNR)] excited state, a series of rhenium(I) complexes with monodentate isocyanide ligands of diverse electronic natures {[Re(CNR)5X]; CNR = 2,4,6-Cl3C6H2NC, 4-(SF5)C6H4NC, 3,5-(CF3)2C6H3NC, and 4-(EtOOC)C6H4NC; X = I, CN, CNB(C6F5)3} have been synthesized. The detailed photophysical and electrochemical properties of these complexes have also been reported.

Transition metal complexes with metal-to-ligand charge transfer (MLCT) excited state have been extensively reported because they found applications in many different areas.1 Most of the MLCT excited states showing rich photophysical and photochemical behavior are designed based on the π-conjugated multidentate ligands; study of the emissive MLCT excited state of monodentate ligands is much less reported.2 Previously, we have designed readily tunable luminescent rhenium(I) diimine luminophores with isocyanide ligands.3 Our spectroscopic study has revealed that these isocyano rhenium(I) diimine complexes exhibit both MLCT [dπ(Re) → π*(CNR)] and [dπ(Re) → π*(diimine)] transitions.3 As the MLCT [dπ(Re) → π*(diimine)] exited state is lower-lying, the phosphorescence of these complexes is derived from the MLCT [dπ(Re) → π*(diimine)] exited state. Although the emissive MLCT [dπ(M) → π*(CNR)] excited state was reported in the 1970s,2a−c luminescent complexes and their outstanding photoredox properties derived from this type of excited state only gained popularity in recent years owing to the development of metal complexes with bidentate isocyanide ligands by Wenger and co-workers.4 Surprisingly, rhenium(I) complexes with these bidentate isocyanide ligands are not strongly luminescence.4g Similarly, most of the reported penta- and hexaisocyano rhenium(I) complexes with monodentate isocyanide ligands © XXXX American Chemical Society

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RESULTS AND DISCUSSION Synthesis and Characterization. The substituted phenylisocyanide ligands with electron-withdrawing substituents were synthesized from their corresponding formamide by adopting the method developed by Luo et al.,13 in which triphenylphosphine and iodine were used for dehydration. Compared to the most commonly used dehydration reactions, as presented by Ugi et al.,12 this method is more effective in terms of the reaction yields as well as the purity of the crude products. These advantages are particularly noticeable in the preparation of Received: September 6, 2018

A

DOI: 10.1021/acs.inorgchem.8b02536 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Synthetic Route for Pentaisocyano Rhenium(I) Complexes

from linearity. This is likely attributed to the π-back-bonding interaction between the rhenium metal center and isocyanide ligands, as well as the steric repulsion between the adjacent isocyanide ligands. A careful analysis of the bonding parameters of the axial isocyanide ligand and the equatorial isocyanide ligands revealed that the average Re−C distance of equatorial isocyanide ligands (2.03 Å) is slightly longer than that of axial isocyanide ligands (1.98 Å). Moreover, the average CN−C bond angle of equatorial isocyanides (169.4°) is slightly larger than that of axial isocyanide ligands (167.6°). This is suggestive of a stronger π-back-bonding interaction from the metal center toward the axial ligand because the corresponding trans ligand (anionic ligand) is competing to a relatively lesser extent for πback-bonding interaction, due to the weaker π-accepting ability of the anionic ligands, compared to the neutral isocyanide ligands.3a−c UV−vis Spectroscopy. The electronic absorption properties of the pentaisocyano rhenium(I) complexes (1−4) in dichloromethane have been investigated. Their UV−vis absorption spectra are shown in Figure 2, and the absorption data are summarized in Table 1. All of the complexes showed intense absorptions in the high-energy UV region (λ ≤ 350 nm), with an extinction coefficient in the order of 103 dm3 mol−1 cm−1, which are assigned to be the LC (π → π*) transitions of substituted phenylisocyanide ligands.5b,8 Apart from the very intense LC transitions, the complexes also display poorly resolved absorption shoulders in the near UV−visible region (379−440 nm), which are assigned to MLCT [dπ(Re) → π*(CNR)] transitions as similar to those of penta- and hexaisocyano rhenium(I) complexes,5b isocyano rhenium(I) diimine complexes,3 and homoleptic isocyano complexes.2a−c The sensitivity of this absorption shoulder to the polarity of the solvent medium is in agreement with the charge transfer nature of the MLCT transition (Figure S1). Although the absorptivity of MLCT transitions is usually in the order of 104 dm3 mol−1 cm−1, the much larger extinction coefficient of these MLCT [dπ(Re) → π*(CNR)] transitions can be rationalized by the additive effect of the MLCT transitions of the five isocyanide ligands as well as the overlapping with the slightly higher energy LC transitions. This assignment is in agreement with the computation and electrochemical results. The assignment of the lowest energy absorption shoulders to the MLCT [dπ(Re) → π*(CNR)] transitions is also supported by the sensitivity of the absorption energy of these shoulders toward changing the ancillary ligand. In general, these MLCT absorptions are gradually blue-shifted upon increasing the πaccepting ability of the ancillary anionic ligand from iodide to

phenylisocyanides with electron-withdrawing substituents, such as trifluoromethyl and pentafluorosulfanyl groups. All isocyano rhenium(I) complexes were prepared from the ligand substitution reactions of the hexaiodorhenate(IV) precursor [ReI6]2−, with corresponding isocyanide ligands and the subsequent reduction reaction by hydrazine (Scheme 1). With the iodo pentaisocyano rhenium(I) complex precursors (1a−4a), the iodide ligand can be readily replaced through the halide abstraction with AgOTf and the coordination of another ancillary ligand, such as cyanide, to afford the cyano complexes (1b−4b). The isocyanotris(pentafluorophenyl)borato complexes (1c−4c) can be prepared by the reactions between the cyano complexes with tris(pentafluorophenyl)borane in dichloromethane under an inert atmosphere using a similar procedure to that for other isocyanoborato complexes.3g,6 Further purification of these complexes was achieved by the slow diffusion of n-hexane or n-pentane into concentrated dichloromethane or diethyl ether solutions of the complexes, which give analytically pure complexes as yellow to colorless crystals. All of the complexes have been characterized by 1H and 19 F NMR spectroscopy, IR spectroscopy, mass spectrometry, and elemental analysis. Complexes 1c, 2a, 2c, 3c, and 4a were also structurally characterized by X-ray crystallography. IR and NMR Spectroscopy. The cyanide, isocyanotris(pentafluorophenyl)borate, and isocyanide ligands were characterized by the IR active CN stretches in the range of 2034− 2202 cm−1, which are in the typical range of these ligands reported in other related complexes.6,7 These complexes were also characterized by 1H and 19F NMR spectroscopy. With a C4 symmetry of the complexes, the four equatorial isocyanide ligands are chemically equivalent but in a different chemical environment compared to the axial isocyanide ligand. Therefore, two sets of 1H and 19F NMR signals of the isocyanide ligands with the integral ratio of 4:1 were observed in the spectra of these complexes. The octahedral coordination geometry and the arrangement of the isocyanide ligands in 1c, 2a, 2c, 3c, and 4a were unambiguously confirmed by the X-ray crystal structures. X-ray Crystal Structure Determination. The perspective drawings of 1c, 2a, 2c, 3c, and 4a are shown in Figure 1. The crystal structure determination data, and selected bond distances and bond angles are summarized in Tables S1, S2 and Tables S3−S7 (Supporting Information), respectively. In these crystal structures, the complexes adopted an octahedral geometry with the bond angles at the rhenium metal center (L− Re−L′) close to 90° or 180°. As for the bond angles CN−C of isocyanide and isocyanoborate ligands, they are in the range of 157.3−177.8° and 164.9−174.8°, respectively, and deviated B

DOI: 10.1021/acs.inorgchem.8b02536 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Perspective drawings of (a) 1c, (b) 2a, (c) 2c, (d) 3c, and (e) 4a with atomic labeling. Hydrogen atoms have been omitted for clarity. Thermal ellipsoids are shown at the 30% probability level.

cyanide and to isocyanoborate. This is reflected in the trends of the energy of these absorption shoulders in the order of 1a (440 nm) < 1b (400 nm) < 1c (383 nm); 2a (400 nm) < 2b (396 nm) < 2c (381 nm); 3a (400 nm) < 3b (391 nm) < 3c (379 nm); 4a (417 nm) < 4b (408 nm) < 4c (397 nm) (Figure 2). These trends can be rationalized by the better stabilization of dπ(Re) by the ancillary ligand with better π-accepting ability (iodide < cyanide < isocyanoborate). On the other hand, these MLCT absorption shoulders for complexes with the same ancillary ligand also show the expected energy dependence on the πaccepting ability of isocyanide ligands (Figure 3). Emission Spectroscopy. The emission properties of the pentaisocyano rhenium(I) complexes in dichloromethane solution at 298 and 77 K EtOH/MeOH (4:1 v/v) glass medium

have been investigated. In contrast to the reported pentaisocyano rhenium(I) complexes,3a,5b such as complexes 5a−7a, which are nonemissive in the solution state at room temperature, the solutions of complexes 1−4 with different anionic ancillary ligands [I (1a−4a), CN (1b−4b), and CNB(C6F5)3 (1c−4c)] display blue to yellow phosphorescence with a structureless emission band (Figure 4) upon excitation with λ < 400 nm. The emission data are summarized in Table 2. These emission properties are assigned to phosphorescence mainly derived from the 3MLCT [dπ(Re) → π*(CNR)] state mixed with LL′CT character. The assignment is supported by the energy dependency on the π-accepting ability on the anionic ancillary ligand as well as the solvent dependency (Figure S2), similar to the situation regarding MLCT absorptions. C

DOI: 10.1021/acs.inorgchem.8b02536 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 2. Overlaid UV−vis absorption spectra of (a) 1a−1c, (b) 2a− 2c, (c) 3a−3c, and (d) 4a−4c in dichloromethane at 298 K.

Figure 4. Normalized emission spectra of (a) 1a−1c, (b) 2a−2c, (c) 3a−3c, and (d) 4a−4c at 298 K in dichloromethane.

Table 1. UV−vis Absorption Data for 1a−1c, 2a−2c, 3a−3c, and 4a−4c in CH2Cl2 Solution

Table 2. Emission Data for 1a−1c, 2a−2c, 3a−3c, 4a−4c, and 5a−7a

absorption λabs/nm (ε/M−1 cm−1) 1a 1b 1c 2a 2b 2c 3a 3b 3c 4a 4b 4c 5a 6a 7a

253 (69 780), 302 (52 495), 343 (65 670), 368 sh (49 005), 399 sh (28 315), 440 sh (13 035) 248 sh (68 345), 255 (74 795), 264 (63 135), 338 (79 155), 360 sh (65 565), 400 sh (36 555) 253 (81 065), 328 (81 250), 350 sh (63 560), 373 sh (46 430), 383 sh (37 060) 269 sh (41 035), 339 (47 885), 359 sh (40 900), 400 sh (15 095) 261 sh (43 475), 335 (58 850), 353 sh (54 555), 396 sh (26 760) 254 sh (54 020), 324 (66 070), 349 (58 635), 365 sh (51 205), 381 sh (28 340) 275 (48 590), 336 (56 470), 362 sh (43 090), 400 sh (15 745) 263 (42 950), 330 (59 935), 349 sh (53 810), 391 sh (26 315) 258 (55 190), 321 (67 575), 346 (58 220), 379 sh (27 090) 273 sh (59 700), 360 (70 575), 417 sh (26 985) 246 (87 895), 267 sh (57 140), 369 (74 065), 408 sh (44 395) 244 (92 320), 262 sh (70 025), 345 (74 040), 363 (75 040), 397 sh (37 425) 244 (80 510), 273 (68 400), 329 (84 505), 361 sh (48 200), 398 sh (18 510) 235 (56 880), 275 (61 740), 319 (66 720), 356 sh (27 240), 377 sh (16 420) 241 (66 860), 269 (53 145), 325 (74 340), 361 sh (32 375), 381 sh (22 195)

medium (T/K) 1a 1b 1c 2a 2b 2c 3a 3b 3c 4a 4b 4c 5ac 6ac 7ac

CH2Cl2a b

(298) glass (77) CH2Cl2a (298) glassb (77) CH2Cl2a (298) glassb (77) CH2Cl2a (298) glassb (77) CH2Cl2a (298) glassb (77) CH2Cl2a (298) glassb (77) CH2Cl2a (298) glassb (77) CH2Cl2a (298) glassb (77) CH2Cl2a (298) glassb (77) CH2Cl2a (298) glassb (77) CH2Cl2a (298) glassb (77) CH2Cl2a (298) glassb (77) glassb (77) glassb (77) glassb (77)

emission λem/nm (τo)

quantum yield Φem × 103

510 ( 4c. This corroborates that the replacement of the iodo ancillary ligand with the stronger π-accepting cyano or isocyanoborato ancillary ligand could destabilize the deactivating LF excited state and explains the enhanced emission properties for complexes with better π-accepting anionic ancillary ligand.

Figure 7. A plot of Eem (298 K, CH2Cl2) versus potential difference of the metal-centered oxidation and the isocyanide ligand-based reduction with linear least-squares fit.

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Computational Studies. To have a better understanding of the nature of the emissive excited state of these pentaisocyano rhenium(I) complexes and the influence of the electronic effects of isocyanide ligands, DFT calculations on selected iodo complexes (1a−5a) have been carried out. To gain insights into the effects of the anionic ancillary ligand on various electronic states, DFT calculations on 4b and 4c have also been performed. Although the computed energy gaps between the optimized T1 and S0 states of the complexes are underestimated compared to the experimental values, the trend along these complexes is consistent with that of the experimental results (Table S8, Supporting Information). The contour maps of the electron density of singly occupied molecular orbitals (SOMOs) of complexes 1a−5a, 4b, and 4c are shown in Figure S4 (Supporting Information). According to the contour maps, the lower-energy SOMOs of all of these complexes mainly consist of the dπ orbital of rhenium mixed with some contribution from the π orbital of the ancillary ligand, whereas the higher-energy SOMOs are mostly contributed by the π* orbitals of the isocyanide ligands. These results are consistent with the assignment of the predominantly 3MLCT [dπ(Re) → π*(CNR)] emission origin with LL′CT character. The influence of the electronic properties of the isocyanide ligands on the lowest triplet state can be elucidated from the calculated MO energy-level diagrams of 5a and 1a (Figure 8). The increase in the π-accepting ability of isocyanide ligands from (4-ClC6H4NC) in 5a to (2,4,6-Cl3C6H2NC) in 1a should lead to the stabilization of both SOMOs. Due to the direct attachment of electron-withdrawing substituents to the isocyanide ligands, the higher-energy SOMO (mainly the π*(CNR) orbital) is significantly more stabilized than the

CONCLUSION To conclude, a series of charge-neutral luminescent pentaisocyano rhenium(I) complexes {[Re(CNR)5X]; CNR = 2,4,6Cl3C6H2NC, 4-(SF5)C6H4NC, 3,5-(CF3)2C6H3NC, and 4(EtOOC)C6H4NC; X = I, CN, CNB(C6F5)3} have been synthesized and characterized by IR spectroscopy, 1H NMR, 19F NMR spectroscopy, ESI-mass spectrometry, and elemental analysis. In contrast to most of the reported penta- or hexaisocyano rhenium(I) complexes with unsubstituted or alkyl- or monohalo-substituted phenylisocyanide ligands, which only exhibit emission in 77 K glassy medium, the solutions of all of these complexes were found to show photoluminescence at room temperature. Detailed photophysical and electrochemical study together with DFT calculation revealed that the emission of these complexes is derived from the 3MLCT [dπ(Re) → π*(CNR)] excited state mixed with some LL′CT character. The presence of strong electron-withdrawing substituents on the isocyanide ligands, which can lower the MLCT [dπ(Re) → π*(CNR)] and raise the LF deactivating state, is found to be crucial in the design of emissive isocyano rhenium(I) complexes. On the other hand, the emission properties in terms of energy, lifetime, and quantum yields can also be enhanced by the anionic ancillary ligand. The emission colors of these pentaisocyano rhenium(I) complexes ranged from deep blue to yellow, and importantly, their emission bands remain structureless in the blue region. This work highlights the importance of the ligand design on the energy of various electronic states of the transition metal F

DOI: 10.1021/acs.inorgchem.8b02536 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Hz, 2H, m-phenyl H’s of the axial isocyanide ligand), 7.47 (d, J = 7.3 Hz, 8H, m-phenyl H’s of the equatorial isocyanide ligands), 8.01 (d, J = 8.1 Hz, 2H, o-phenyl H’s of the axial isocyanide ligand), 8.07 (d, J = 7.3 Hz, 8H, o-phenyl H’s of the equatorial isocyanide ligands). IR (KBr disc), v/ cm−1: 1717 ν(CO); 2060 (br), 2159 ν(CN). ESI−MS: m/z 1227 [M + K]+. Elemental analyses, Calcd for 4a (found) %: C 50.51 (50.81) H 3.81 (3.91) N 5.89 (5.85). General Synthetic Procedure for [Re(CNR)5CN]. A mixture of [Re(CNR)5I] (500 mg, 1.0 mol equiv), KCN (1.2 mol equiv), and AgOTf (1.2 mol equiv) in degassed ethanol (20 mL) were heated at 40 °C overnight under an inert atmosphere of argon. The resulting mixture was filtered, and the solvent was removed under reduced pressure. The residue was purified by column chromatography on silica gel using a mixture of dichloromethane and acetone as eluent. [Re(CNC6H2Cl3-2,4,6)5(CN)] (1b). Yield: 307 mg, 0.25 mmol, 66%. 1 H NMR (400 MHz, CDCl3, 298 K): δ 7.33 (s, 2H, m-phenyl H’s of the axial isocyanide ligand), 7.38 (s, 8H, m-phenyl H’s of the equatorial isocyanide ligands). IR (KBr disc), v/cm−1: 2034 (br), 2163 ν(CN). ESI−MS: m/z 1283 [M + K]+. Elemental analyses, Calcd for 1b· 0.5Et2O (found) %: C 35.61 (35.82) H 1.18 (1.14) N 6.56 (6.94). {Re[CNC6H4(SF5)-4]5(CN)} (2b). Yield: 252 mg, 0.19 mmol, 54%. 1H NMR (300 MHz, CDCl3, 298 K): δ 7.33 (d, J = 8.9 Hz, 2H, m-phenyl H’s of the axial isocyanide ligand), 7.47 (d, J = 8.5 Hz, 8H, m-phenyl H’s of the equatorial isocyanide ligands), 7.79 (m, 10H, o-phenyl H’s of the axial and equatorial isocyanide ligands). 19F NMR (282 MHz, CDCl3) δ 82.89 (m, 5F, axial F’s of SF5), 63.05 (m, 20F, equatorial F’s of SF5). ESI−MS: m/z 1397 [M + K]+. IR (KBr disc), v/cm−1: 2051 (br), 2165 ν(CN). Elemental analyses, Calcd for 2b·0.5C6H14 (found) %: C 33.43 (33.33) H 1.94 (1.95) N 6.00 (6.12). {Re[CNC6H3(CF3)-3,5]5(CN)} (3b). Yield: 275 mg, 0.20 mmol, 59%. 1 H NMR (300 MHz, CDCl3, 298 K): δ 7.74 (s, 2H, o-phenyl H’s of the axial isocyanide ligand), 7.80 (s, 1H, p-phenyl H’s of the axial isocyanide ligand), 7.88 (s, 12H, o- and p-phenyl H’s of the equatorial isocyanide ligands). 19F NMR (282 MHz, CDCl3) δ −63.14 (s, 24F, CF3 F’s of the equatorial isocyanide ligands), −63.18 (s, 6F, CF3 F’s of the axial isocyanide ligand). IR (KBr disc), v/cm−1: 2060 (br), 2130 ν(CN). ESI−MS: m/z 1446 [M + K]+. Elemental analyses, Calcd for 3b (found) %: C 39.24 (39.18) H 1.07 (1.38) N 5.97 (5.89). {Re[CNC6H4(COOEt)-4)]5(CN)} (4b). Yield: 210 mg, 0.19 mmol, 46%. 1 H NMR (300 MHz, CDCl3, 298 K): δ 1.40 (m, 15H, CH3 H’s of the ethyl groups), 4.38 (m, 10H, CH2 H’s of the ethyl groups), 7.31 (d, J = 8.3 Hz, 2H, m-phenyl H’s of the axial isocyanide ligand), 7.44 (d, J = 8.4 Hz, 8H, m-phenyl H’s of the equatorial isocyanide ligands), 8.06 (m, 10H, o-phenyl H’s of the axial and equatorial isocyanide ligands). ESI− MS: m/z 1127 [M + K]+. IR (KBr disc), v/cm−1: 1717 ν(CO); 2056 (br), 2164 ν(CN). Elemental analyses, Calcd for 4b (found) %: C 56.29 (56.29) H 4.17 (4.39) N 7.72 (7.46). General Procedure for [Re(CNR)5CNB(C6F5)3]. A mixture of [Re(CNR)5(CN)] (50 mg, 1.0 mol equiv) and B(C6F5)3 (1.2 mol equiv) was dissolved in anhydrous dichloromethane (20 mL) under an inert atmosphere of argon. The resulting solution was stirred overnight at room temperature. After removing the solvent under reduced pressure, the residue was purified by column chromatography on silica gel using a solvent mixture of hexane and dichloromethane as eluent. Further purification was achieved by the slow diffusion of pentane or hexane into the concentrated dichloromethane or diethyl ether solutions of the complexes to yield yellow to white crystalline solids depending on the nature of the isocyanide ligands. {Re(CNC6H2Cl3-2,4,6)5[CNB(C6F5)3]} (1c). Yield: 55 mg, 0.03 mmol, 78%. 1H NMR (400 MHz, CDCl3, 298 K): δ 7.38 (s, 2H, m-phenyl H’s of the axial isocyanide ligand), 7.40 (s, 8H, m-phenyl H’s of the equatorial isocyanide ligands). 19F NMR (376 MHz, CDCl3) δ −133.25 (dd, J = 23.3, 7.9 Hz, 6F, o-phenyl F’s of the isocyanoborate), −160.02 (t, J = 20.4 Hz, 3F, p-phenyl F’s of the isocyanoborate), −165.55 (td, J = 22.6, 7.8 Hz, 6F, m-phenyl F’s of the isocyanoborate). IR (KBr disc), v/cm−1: 2052 (br), 2153, 2202 ν(CN). ESI−MS: m/z 1796 [M + K]+. Elemental analyses, Calcd for 1c·0.5Et2O (found) %: C 37.50 (37.71) H 0.84 (0.75) N 4.69 (4.72). {Re[CNC6H4(SF5)-4]5[CNB(C6F5)3]} (2c). Yield: 36 mg, 0.02 mmol, 52%. 1H NMR (300 MHz, CDCl3, 298 K): δ 7.39 (d, J = 8.8 Hz, 8H, m-

complexes. Judicious functionalization of the isocyanide ligands could lead to the design of luminescent isocyano rhenium(I) complexes from the 3MLCT [dπ(Re) → π*(CNR)] excited state. Moreover, it also provides strategies to tune the energy of the emissive excited state and improve the phosphorescent properties in terms of quantum yield and lifetime. This should lead to a new class of luminescent materials developed from the transition metal isocyanides.

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EXPERIMENTAL SECTION

Materials and Reagents. Rhenium metal powder, silver trifluoromethanesulfonate (AgOTf), and potassium cyanide (KCN) were purchased from International Laboratory Company Ltd. Tris(pentafluorophenyl)borane [B(C6F5)3] and hydrazine hydrate were purchased from Acros Organics. These reagents were used without further purification. All solvents were of analytical reagent grade and were used without further purification. Potassium hexaiodorhenate(IV) and substituted phenylisocyanides 2,4,6-Cl3C6H2NC, 4-SF5C6H4NC, 3,5-(CF3)2C6H3NC, and 4-(EtOOC)C6H4NC were prepared according to our previously reported procedures3a,d using the methods developed by Ugi12 and Luo.13 The previously reported pentaisocyano rhenium(I) complexes, Re(CNC6H4Cl-4)5I] (5a), [Re(CNC6H4F4)5I] (6a), and [Re(CNC6H4(CH3)2-2,6)5I] (7a) were prepared according to our reported procedure.3a General Synthetic Procedure for [Re(CNR)5I] (1a−4a). These complexes were synthesized based on our previously reported procedures.3a,d To an acetone (10 mL) solution of K2[ReI6] (500 mg, 1.0 mol equiv) at 0 °C, isocyanide ligand (CNR) (5.5 mol equiv) was added. The resulting solution was stirred overnight at room temperature. After removing the solvent under reduced pressure, the residue was redissolved in methanol (10 mL). Hydrazine hydrate (2 mL) was slowly added to the resulting mixture and stirred for 30 min. Thereafter, dichloromethane (30 mL) was added and washed with water (10 mL × 3). After removal of solvent, the residue was purified by column chromatography on silica gel using a mixture of petroleum ether and dichloromethane as eluent. Further purification was achieved by the slow diffusion of pentane or hexane into concentrated dichloromethane or diethyl ether solutions of the complexes to give yellow crystalline solids. [Re(CNC6H2Cl3-2,4,6)5I] (1a). Yield: 440 mg, 0.33 mmol, 67%. 1H NMR (300 MHz, CDCl3, 298 K): δ 7.28 (s, 2H, m-phenyl H’s of the axial isocyanide ligand), 7.38 (s, 8H, m-phenyl H’s of the equatorial isocyanide ligands). IR (KBr disc), v/cm−1: 2041 (br), 2160 ν(CN). ESI−MS: m/z 1384 [M + K]+. Elemental analyses, Calcd for 1a (found) %: C 31.25 (31.51) H 0.75 (1.10) N 5.21 (5.29). {Re[CNC6H4(SF5)-4]5I} (2a). Yield: 270 mg, 0.19 mmol, 38%. 1H NMR (300 MHz, CDCl3, 298 K): δ 7.23 (d, J = 9.0 Hz, 2H, m-phenyl H’s of the axial isocyanide ligand), 7.50 (d, J = 9.0 Hz, 8H, m-phenyl H’s of the equatorial isocyanide ligands), 7.73 (d, 2H, J = 9.0 Hz, o-phenyl H’s of the axial isocyanide ligand), 7.80 (d, J = 9.0 Hz, 8H, o-phenyl H’s of the equatorial isocyanide ligands). 19F NMR (282 MHz, CDCl3) δ 83.67 (m, 5F, axial F’s of SF5), 63.30 (m, 20F, equatorial F’s of SF5). IR (KBr disc), v/cm−1: 2054 (br), 2157 ν(CN). ESI−MS: m/z 1458 [M + H]+. Elemental analyses, Calcd for 2a·0.5Et2O (found) %: C 29.71 (29.89) H 1.68 (1.38) N 4.68 (4.92). {Re[CNC6H3(CF3)-3,5]5I} (3a). Yield: 448 mg, 0.30 mmol, 61%. 1H NMR (300 MHz, CDCl3, 298 K): δ 7.63 (s, 2H, o-phenyl H’s of the axial isocyanide ligand), 7.70 (s, 1H, p-phenyl H’s of the axial isocyanide ligand), δ 7.86 (s, 4H, p-phenyl H’s of the equatorial isocyanide ligands), 7.88 (s, 8H, o-phenyl H’s of the equatorial isocyanide ligands). 19 F NMR (282 MHz, CDCl3) δ −63.14 (s, 24F, CF3 F’s of the equatorial isocyanide ligands), −63.18 (s, 6F, CF3 F’s of the axial isocyanide ligand). IR (KBr disc), v/cm−1: 2065 (br), 2129 ν(CN). ESI−MS: m/z 1508 [M + H]+. Elemental analyses, Calcd for 3a· 0.5Et2O (found) %: C 36.52 (36.56) H 1.30 (1.21) N 4.53 (4.56). {Re[CNC6H4(COOEt)-4)]5I} (4a). Yield: 210 mg, 0.18 mmol, 36%. 1H NMR (300 MHz, CDCl3, 298 K): δ 1.39 (m, 15H, CH3 H’s of the ethyl groups), δ 4.37 (m, 10H, CH2 H’s of the ethyl groups), δ 7.23 (d, J = 8.1 G

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Inorganic Chemistry phenyl H’s of the equatorial isocyanide ligands), 7.44 (d, J = 8.9 Hz, 2H, m-phenyl H’s of the axial isocyanide ligand), 7.82 (m, 10H, o-phenyl H’s of the axial and equatorial isocyanide ligands). 19F NMR (282 MHz, CDCl3) δ 84.09 (m, 5F, axial F’s of SF5), 62.86 (m, 20F, equatorial F’s of SF5), −133.86 (dd, J = 23.8, 8.6 Hz, 6F, o-phenyl F’s of the isocyanoborate), −158.83 (t, J = 20.4 Hz, 3F, p-phenyl F’s of the isocyanoborate), −164.84 (td, J = 23.1, 7.8 Hz, 6F, m-phenyl F’s of the isocyanoborate). IR (KBr disc), v/cm−1: 2057 (br), 2148, 2198 ν(C N). ESI−MS: m/z 1909 [M + K]+. Elemental analyses, Calcd for 2c· 0.5Et2O (found) %: C 35.27 (34.94) H 1.32 (1.54) N 4.41 (4.13). {Re[CNC6H3(CF3)-3,5]5[CNB(C6F5)3]} (3c). Yield: 30 mg, 0.02 mmol, 44%. 1H NMR (300 MHz, CDCl3, 298 K): δ 7.80 (s, 8H, o-phenyl H’s of the equatorial isocyanide ligands), 7.82 (s, 2H, o-phenyl H’s of the axial isocyanide ligand), 7.88 (s, 1H, p-phenyl H’s of the axial isocyanide ligand), 7.91 (s, 4H, p-phenyl H’s of the equatorial isocyanide ligands). 19 F NMR (282 MHz, CDCl3) δ −63.11 (s, 6F, CF3 F’s of the axial isocyanide ligand), −63.34 (s, 24F, CF3 F’s of the equatorial isocyanide ligands), −134.54 (dd, J = 23.1, 8.0 Hz, 6F, o-phenyl F’s of the isocyanoborate), −158.66 (t, J = 20.3 Hz, 3F, p-phenyl F’s of the isocyanoborate), −164.96 (td, J = 22.1, 7.6 Hz, 6F, m-phenyl F’s of the isocyanoborate). IR (KBr disc), v/cm−1: 2064 (br), 2127, 2206 ν(C N). ESI−MS: m/z 1959 [M + K]+. Elemental analyses, Calcd for 3c· 0.5Et2O (found) %: C 40.51 (40.53) H 1.03 (0.88) N 4.29 (4.39). {Re[CNC6H4(COOEt)-4)]5[CNB(C6F5)3]} (4c). Yield: 30 mg, 0.02 mmol, 41%. 1H NMR (300 MHz, CDCl3, 298 K): δ 1.41 (t, J = 7.1 Hz, 15H, CH3 H’s of the ethyl groups), 4.41 (d, J = 7.1 Hz, 10H, CH2 H’s of the ethyl groups), 7.37 (d, J = 8.1 Hz, 2H, m-phenyl H’s of the axial isocyanide ligand), 8.09 (m, 10H, o-phenyl H’s of the axial and equatorial isocyanide ligands). 19F NMR (282 MHz, CDCl3) δ −133.76 (dd, J = 23.9, 8.3 Hz, 6F, o-phenyl F’s of the isocyanoborate), −159.30 (t, J = 20.5 Hz, 3F, p-phenyl F’s of the isocyanoborate), −165.03 (td, J = 23.1, 8.0 Hz, 6F, m-phenyl F’s of the isocyanoborate). IR (KBr disc), v/cm−1: 1722 ν(CO), 2066 (br); 2144, 2193 ν(C N). ESI−MS: m/z 1639 [M + K]+. Elemental analyses, Calcd for 4b· 0.5CH2Cl2 (found) %: C 50.82 (50.73) H 2.82 (3.09) N 5.12 (4.86). Physical Measurements and Instrumentation. 1H and 19F NMR spectra were recorded on a Bruker AV300 (300 MHz) or AV400 (400 MHz) FT-NMR spectrometer. Chemical shifts (δ, ppm) were reported relative to tetramethylsilane (Me4Si). IR spectra were obtained from KBr discs by using a PerkinElmer Spectrum 100 FTIR spectrophotometer. All ESI mass spectra were recorded on a PE-SCIEX API-3200 single quadrupole mass spectrometer. Elemental analyses of all compounds were performed on an Elementar Vario MICRO Cube elemental analyzer. Electronic absorption spectra were recorded on a Hewlett-Packard 8452A diode array spectrophotometer. Steady-state emission at room temperature and at 77 K were recorded on an Edinburgh Instruments FLS980 fluorescence spectrometer and LP920 laser flash photolysis system with a flashlamp pumped Q-switched Nd:YAG laser using 355 nm excitation; lifetimes were measured with the photomultiplier tube of the instruments. Measurements of the EtOH/MeOH (4:1 v/v) glass samples at 77 K were carried out with dilute EtOH/MeOH sample solutions contained in a quartz tube inside a liquid nitrogen-filled quartz optical Dewar flask. Solutions were rigorously degassed on a high-vacuum line in a two-compartment cell with no less than four successive freeze pump-thaw cycles. The luminescence quantum yields were determined by using the opticaldilution method as described by Demas and Crosby using quinine sulfate in aqueous sulfuric acid (0.05 M) as reference standard.14 Cyclic voltammetry measurements were performed by using a CH instrument, Inc., Model CHI 620 Electrochemical Analyzer. Electrochemical measurements were performed in acetonitrile solutions with 0.1 M n Bu4NPF6 as the supporting electrolyte at room temperature with a glassy carbon electrode (CH Instruments, Inc.) with a diameter of 0.3 mm as a working electrode, a platinum wire coil as the counter electrode, and a Ag/AgNO3 (0.1 M in acetonitrile) electrode (CH Instruments, Inc.) as the reference electrode. The working electrode surface was polished with an 1 μm α-alumina slurry and then a 0.3 μm α-alumina slurry (Linde) on a microcloth (Buehler Co.). The ferrocenium/ferrocene couple (FeCp2+/0) was used as the internal

reference. All solutions for electrochemical studies were deaerated with prepurified nitrogen gas prior to measurements. X-ray Crystal Structure Determination. The crystal structures were determined on an Xcalibur, Sapphire3, Gemini Ultra diffractometer using graphite monochromatized Cu Kα (λ = 1.54178 Å) radiation. The structures were solved by direct methods employing the SHELXL-97 program15 on a PC. Re and many non-H atoms were located according to direct methods. The positions of other non-H atoms were found after successful refinement by full-matrix leastsquares using the SHELXL-97 program on a PC.15 All non-H atoms were refined anisotropically in the final stage of least-squares refinement. The positions of hydrogen atoms were calculated based on riding mode with thermal parameters equal to 1.2 times that of the associated carbon atoms, and participated in the calculation of final R indices. Computational Details. All calculations were done using the Gaussian 09 package, version B.01.16 The structures of the ground state (S0) and the lowest triplet state (T1) of the complexes 1a−4a, 4b, 4c, and [Re(CNC6H4Cl-4)5I] (5a)3a were optimized using Density Functional Theory (DFT) with the B3LYP hybrid functional.17 A combination of 6-31+G(d) (for elements H, B, C, N, O, F, S, and Cl) and LANL2DZ18 (for Re and I) basis sets was employed. Vibrational analysis was performed to ensure that all optimized structures are located at potential energy minima. The effect of the solvent was taken account by the polarized continuum model with integral equation formalism (IEF-PCM).19

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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.8b02536. Crystal and structure determination data, selected bond lengths and bond angles, computed energy gaps between the optimized T1 and S0 states, and contour maps of the electron density of SOMOs of complexes 1a−5a, 4b, and 4c (PDF) Accession Codes

CCDC 1822767−1822771 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 Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (+852)-3442-6958. Fax: (+852)-3442-0522. ORCID

Chi-Chiu Ko: 0000-0002-2426-3206 Author Contributions †

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by the Hong Kong University Grants Committee Area of Excellence Scheme (AoE/P-03-08) and General Research Fund (Project Nos. CityU 11303515 and CityU 11306217) from the Research Grants Council of the Hong Kong SAR, China. K.-C.C. acknowledges receipt of a University Postgraduate Studentship and a Research Tuition Scholarship administrated by City University of Hong Kong. H

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strong reductant for photoredox catalysis. Angew. Chem., Int. Ed. 2016, 55, 11247−11250. (b) Büldt, L. A.; Wenger, O. S. Chromium(0), molybdenum(0), and tungsten(0) isocyanide complexes as luminophores and photosensitizers with long-lived excited states. Angew. Chem., Int. Ed. 2017, 56, 5676−5682. (c) Büldt, L. A.; Larsen, C. B.; Wenger, O. S. Luminescent Ni0 diisocyanide chelates as analogues of CuI diimine complexes. Chem. - Eur. J. 2017, 23, 8577−8580. (d) Büldt, L. A.; Wenger, O. S. Chromium complexes for luminescence, solar cells, photoredox catalysis, upconversion, and phototriggered NO release. Chem. Sci. 2017, 8, 7359−7367. (e) Büldt, L. A.; Wenger, O. S. Luminescent complexes made from chelating isocyanide ligands and earth-abundant metals. Dalton Trans 2017, 46, 15175−15177. (f) Büldt, L. A.; Guo, X.; Vogel, R.; Prescimone, A.; Wenger, O. S. A Tris(diisocyanide)chromium(0) Complex Is a Luminescent Analog of Fe(2,2′-Bipyridine)32+. J. Am. Chem. Soc. 2017, 139, 985−992. (g) Larsen, C. B.; Wenger, O. S. Photophysics and photoredox catalysis of a homoleptic rhenium(I) tris(diisocyanide) complex. Inorg. Chem. 2018, 57, 2965−2968. (5) (a) Stacy, N. E.; Conner, K. A.; McMillin, D. R.; Walton, R. A. Studies of the photoinduced ligand substitution reactions of [Re(CNxyl)6]+ (xyl = 2,6-dimethylphenyl) in the presence of halide ions. Inorg. Chem. 1986, 25, 3649−3652. (b) Villegas, J. M.; Stoyanov, S. R.; Reibenspies, J. H.; Rillema, D. P. Spectroscopic and computational investigations of the temperature-dependent emission behavior of [Re(CNx)5Cl] and [Re(CNx)6]+ complexes. Organometallics 2005, 24, 395−404. (6) (a) Chu, W.-K.; Yiu, S.-M.; Ko, C.-C. Neutral luminescent bis(bipyridyl) osmium(II) complexes with improved phosphorescent properties. Organometallics 2014, 33, 6771−6777. (b) Chan, K.-C.; Chu, W.-K.; Yiu, S.-M.; Ko, C.-C. Synthesis, characterization, photophysics and electrochemical study of luminescent iridium(III) complexes with isocyanoborate ligands. Dalton Trans. 2015, 44, 15135−15144. (c) Chan, K.-C.; Cheng, S.-C.; Lo, L. T.-L.; Yiu, S.M.; Ko, C.-C. Luminescent charge-neutral copper(I) phenanthroline complexes with isocyanoborate ligand. Eur. J. Inorg. Chem. 2018, 2018, 897−903. (7) (a) Vei, I. C.; Pascu, S. I.; Green, M. L. H.; Green, J. C.; Schilling, R. E.; Anderson, G. D. W.; Rees, L. H. Synthesis and study of new binuclear compounds containing bridging (μ-CN)B(C6F5)3 and (μNC)B(C6F5)3 systems. Dalton Trans 2003, 2550−2557. (b) Zhou, Y.; Xiao, H.-P.; Kang, L.-C.; Zuo, J.-L.; Li, C.-H.; You, X.-Z. Synthesis and characterization of neutral iron(II) and ruthenium(II) complexes with the isocyanotriphenylborate ligand. Dalton Trans 2009, 10256−10262. (8) (a) Nalepa, R. A.; Laposa, J. D. Luminescence of phenylisocyanide and perdeuterophenyllisocyanide at 77K in polycrystalline matrices. J. Lumin. 1974, 8, 429−442. (b) Larowe, J. E.; McMillin, D. R.; Stacy, N. E.; Tetrick, S. M.; Walton, R. A. Synthetic and spectroscopic studies on the hexakis(ary1 isocyanide) complexes of ruthenium(II) [Ru(CNAr)6](PF6)2. Inorg. Chem. 1987, 26, 966−968. (9) Hummel, P.; Oxgaard, J.; Goddard, W. A., III; Gray, H. B. Ligand field strengths of carbon monoxide and cyanide in octahedral coordination. J. Coord. Chem. 2005, 58 (1), 41−45. (10) (a) Stufkens, D. J. The remarkable properties of α-diimine rhenium tricarbonyl complexes in their metal-to-ligand charge-transfer (MLCT) excited states. Comments Inorg. Chem. 1992, 13, 359. (b) Kotch, T. G.; Lees, A. J.; Fuerniss, S. J.; Papathomas, K. I.; Snyder, R. W. Luminescence rigidochromism of fac-ClRe(CO)3(4,7Ph2phen) (4,7-Ph2phen = 4,7-diphenyl-1,10-phenanthroline) as a spectroscopic probe in monitoring polymerization of photosensitive thin films. Inorg. Chem. 1993, 32, 2570−2575. (11) (a) Worl, L. A.; Duesing, R.; Chen, P.; Ciana, L. D.; Meyer, T. J. Photophysical properties of polypyridyl carbonyl complexes of rhenium(I). J. Chem. Soc., Dalton Trans. 1991, 849−858. (b) Baiano, J. A.; Murphy, W. R., Jr. Nonradiative decay in rhenium(I) monometallic complexes of 2,3-Di-2-pyridylpyrazine. Inorg. Chem. 1991, 30, 4594−4598. (c) Ng, C.-O.; Yiu, S.-M.; Ko, C.-C. Synthesis, characterization, and photophysical study of luminescent rhenium(I) diimine complexes with various types of N-heterocyclic carbene ligands. Inorg. Chem. 2014, 53, 3022−3031.

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