Luminescent Rhenium(I) Phenanthroline Complexes with a

Article pubs.acs.org/Organometallics

Luminescent Rhenium(I) Phenanthroline Complexes with a Benzoxazol-2-ylidene Ligand: Synthesis, Characterization, and Photophysical Study Chi-Chiu Ko,* Chi-On Ng, and Shek-Man Yiu Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, People’s Republic of China S Supporting Information *

ABSTRACT: A series of luminescent rhenium(I) phenanthroline complexes containing benzoxazol-2-ylidene ligands with the general formula {Re(CO)3(phen)[CN(X)C6H4-2-O]}+ and cis,trans-{Re(CO)2(phen)(L)[CN(H)C6H4-2-O]}+ (X = H, methyl; phen = 1,10-phenanthroline; L = PPh3, PPh2Me, P(OEt)3) have been synthesized and characterized. The X-ray crystal structures of most of the carbene complexes and some of their synthetic precursors have also been determined. A new synthetic methodology for the preparation of dicarbonyl rhenium diimine synthetic precursors with a labile acetonitrile ligand, [Re(CO)2(phen)(PR3)(MeCN)]+, was developed. Photophysical study shows that these carbene complexes display a green to red 3MLLCT [dπ(Re) → π*(N−N)] phosphorescence at room temperature. The N-deprotonations of the benzoxazol-2-ylidene ligand in these complexes were investigated.

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INTRODUCTION Transition metal complexes with N-heterocyclic carbene (NHC) were first documented in 1968.1 However, they have not received significant attention until the first report of a stable NHC ligand2 and the subsequent application studies of these complexes for various catalytic properties.3 Compared with free NHC ligands, the carbene ligand in transition metal complexes is much more stable, as its lone-pair electrons on the carbene carbon become unavailable after coordination. Moreover, upon coordination to metal centers with d-electrons, the pπ orbital of the carbene C is stabilized by delocalization of the filled pπ orbital of the adjacent heteroatom and filled dπ orbital of its coordinated metal center. In addition to their unique catalytic properties,3 many NHC transition metal complexes also display interesting photophysical and luminescent properties.4 The ease of functionalization, unique electronic properties, and robustness of NHC ligands have made them ideal ligands in the design of tunable luminescent NHC transition metal complexes with enhanced stability and novel photophysical properties.4 As metal isocyanide complexes are excellent synthetic precursors for NHC metal complexes,5 we have extended our recent work on luminescent isocyano rhenium(I) diimine complexes6 to synthesize a new class of tunable luminescent rhenium(I) © 2012 American Chemical Society

diimine complexes with NHC ligands. It is anticipated that these NHC rhenium(I) diimine complexes will possess better photoand thermal stability, as NHC ligands are much less susceptible to nucleophilic attack compared to isocyanide.3m,7 Herein, we report the synthesis, structures, photophysics, and electrochemistry of a series of carbonyl rhenium(I) phenanthroline complexes with a benzoxazol-2-ylidene ligand. A new synthetic methodology for dicarbonyl rhenium(I) diimine synthetic precursors, cis,trans-[Re(CO)2(diimine)(PR3)(MeCN)]+, was also described. Physical Measurements and Instrumentation. 1H and 13 C NMR spectra were recorded on a Bruker 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 Perkin-Elmer Spectrum 100 FTIR spectrophotometer. All positive-ion ESI mass spectra were recorded on a PE-SCIEX API 150 EX single quadrupole mass spectrometer. Elemental analyses of all compounds were performed on an Elementar Vario MICRO Cube elemental analyzer. Received: June 12, 2012 Published: October 2, 2012 7074

dx.doi.org/10.1021/om300526e | Organometallics 2012, 31, 7074−7084

7075

a

C22H13N3O4Re PF6 714.52 133(2) 12.1681(3) 11.0649(3) 17.4182(4) 90.00 91.646(2) 90.00 2344.20(10) yellow monoclinic P21/n 4 1368 2.025 0.4 × 0.3 × 0.05 1.5418 11.57 4.4 ≤ θ ≤ 67.0° (h: −14 to 13; k: −13 to 11; l: −20 to 18) 99.8% 8686 4172 4091 334 0.030 0.076 1.14 0.002 +0.88, −2.25

C22H12N3O4Re 568.55 133(2) 23.6676(13) 11.3775(2) 17.8694(17) 90.00 127.018(9) 90.00 3842.0(4) yellow monoclinic C2/c 8 2176 1.966 0.5 × 0.4 × 0.1 0.7107 6.36 3.2 ≤ θ ≤ 25.0° (h: −28 to 28; k: −13 to 13; l: −21 to 21) 99.8%

17 880 3393 3225

271

0.018 0.050 1.08 0.001

+1.07, −1.00

w = 1/[σ2(Fo2) + (ap)2 + bP], where P = [2Fc2 + Max(Fo2,0)]/3.

completeness to theta no. of data collected no. of unique data no. of data used in refinement no. of params refined Ra wRa goodness-of-fit maximum shift, (Δ/σ)max residual extrema in final diff map, e Å−3

formula Mw T/K a/Å b/Å c/Å α/deg β/deg γ/deg V/Å3 cryst color cryst syst space group Z F(000) Dc/g cm−3 dimensions/mm λ/Å μ/mm−1 collection range

2

1

Table 1. Crystal and Structure Determination Data for 1−3 and 5−7

+2.37, −0.91

0.028 0.077 1.09 0.001

343

7656 4124 4057

C23H15N3O4RePF6 728.56 133(2) 8.1807(9) 11.7759(8) 12.6684(9) 83.666(6) 81.993(7) 75.973(7) 1168.75(17) yellow triclinic P1̅ 2 700 2.070 0.3 × 0.1 × 0.01 1.5418 11.62 3.5 ≤ θ ≤ 67.0° (h: −9 to 7; k: −14 to 13; l: −15 to 15) 99.3%

3

+0.89, −0.89

0.023 0.058 1.05 0.003

753

+5.02, −3.54

0.067 0.158 1.25 0.001

1009

26 237 11 443 10 257

C27H28N3O6PRePF6·0.125C4H10O 861.93 173(2) 13.7655(1) 19.0956(2) 24.8720(3) 90.00 101.053(10) 90.00 6416.59(11) yellow monoclinic P21/c 8 3386 1.784 0.5 × 0.1 × 0.04 1.5418 9.08 3.3 ≤ θ ≤ 67.0° (h: −16 to 16; k: −17 to 22; l: −29 to 29) 100%

C34H26N3O3PRePF6·C3H6O 944.80 133(2) 10.9257(2) 20.0094(3) 17.2111(3) 90.00 100.859(2) 90.00 3695.26(11) orange monoclinic P21/c 4 1864 1.698 0.2 × 0.1 × 0.02 1.5418 7.91 3.4 ≤ θ ≤ 67.0° (h: −13 to 12; k: −21 to 23; l: −20 to 18) 99.9% 14 170 6581 6198

6

5

+0.97, −1.63

0.029 0.079 1.15 0.002

443

16 501 4719 4682

C23H15N3O4ReCF3SO3 732.65 133(2) 8.9977(3) 22.9216(6) 11.8033(3) 90.00 91.10(2) 90.00 2433.88(12) yellow monoclinic P21/c 4 1416 1.999 0.7 × 0.1 × 0.05 1.5418 11.25 3.9 ≤ θ ≤ 71.8° (h: −10 to 7; k: −27 to 28;l: −14 to 14) 98.9%

7

Organometallics Article


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2-Trimethylsiloxyphenyl isocyanide was prepared according to a reported procedure.13 2-Methoxyphenyl isocyanide (2-MeOC6H4NC) was synthesized from 2-methoxyphenyl formamide14 using the synthetic methodology developed by Ugi and co-workers.15 THF and benzene were distilled over sodium prior to use. All other reagents and solvents were of analytical grade and used as received. General Procedure. All reactions were performed under strictly anaerobic and anhydrous conditions in an inert atmosphere of argon using standard Schlenk techniques. Synthesis of cis,trans-[Re(CO)2(phen)(PPh3)(MeCN)](CF3SO3). To a solution of [Re(CO)3(phen)(PPh3)](CF3SO3) (100 mg, 116 μmol) in MeCN (30 mL) was added trimethylamine N-oxide dihydrate (19.3 mg, 174 μmol). The resulting solution was refluxed for 18 h. After removal of the solvent under reduced pressure, the residue was purified by column chromatography on silica gel using dichloromethane− acetone−acetonitrile (20:10:1 v/v/v) as eluent to give analytically pure complex as an orange solid. Yield: 67.4 mg, 77.0 μmol; 66.4%. 1H NMR (400 MHz, CDCl3, 298 K): δ 8.77 (dd, 2H, J = 5.1, 1.4 Hz, 2,9-phen H’s), 8.59 (dd, 2H, J = 8.2, 1.4 Hz, 4,7-phen H’s), 8.14 (s, 2H, 5,6-phen H’s), 7.56 (dd, 2H, J = 8.2, 5.1 Hz, 3,8-phen H’s), 7.23 (m, 3H, phenyl H’s), 7.12 (m, 6H, phenyl H’s), 7.03 (m, 6H, phenyl H’s), 2.14 (s, 3H, methyl H’s). Positive-ion ESI-MS: m/z 726 [M − CF3SO3]+. IR (KBr disk): ν/cm−1 1942, 1871 (CO). Anal. Calcd (%) for C35H26F3N3O5PReS (874.84): C 48.05, H 3.00, N 4.80. Found: C 47.86, H 3.32, N 4.41. Synthesis of cis,trans-[Re(CO) 2(phen)(PPh 2 Me)(MeCN)](CF3SO3). The complex was prepared according to a procedure similar to that for cis,trans-[Re(CO) 2(phen)(PPh3 )(MeCN)](CF3SO3) except [Re(CO)3(phen)(PPh2Me)](CF3SO3) (92.8 mg, 116 μmol) was used in place of [Re(CO)3(phen)(PPh3)](CF3SO3). Yield: 57.9 mg, 71.2 μmol; 61.4%. 1H NMR (400 MHz, CDCl3, 298 K): δ 8.98 (dd, 2H, J = 5.2, 1.3 Hz, 2,9-phen H’s), 8.61 (dd, 2H, J = 8.1, 1.3 Hz, 4,7-phen H’s), 8.12 (s, 2H, 5,6-phen H’s), 7.69 (dd, 2H, J = 8.1, 5.2 Hz, 3,8-phen H’s), 7.21 (td, 2H, J = 7.4, 1.9 Hz, phenyl H’s), 7.09 (td, 4H, J = 7.9, 1.9 Hz, phenyl H’s), 6.90 (m, 4H, phenyl H’s), 2.16 (s, 3H, methyl H’s), 1.57 (d, 3H, JPH = 8.3 Hz, methyl H’s). Positive-ion ESI-MS: m/z 664 [M − CF3SO3]+. IR (KBr disk): ν/cm−1 1934, 1861 (CO). Anal. Calcd (%) for C30H24F3N3O5PReS (821.77): C 44.33, H 2.98, N 5.17. Found: C 44.31, H 2.71, N 4.97. Synthesis of cis,trans-{Re(CO) 2 (phen)[P(OEt) 3 ](MeCN)}(CF3SO3). The complex was prepared according to a procedure similar to that for cis,trans-[Re(CO) 2(phen)(PPh3 )(MeCN)](CF3SO3) except {Re(CO)3(phen)[P(OEt)3]}(CF3SO3) (88.8 mg, 116 μmol) was used in place of [Re(CO)3(phen)(PPh3)](CF3SO3). Yield: 63.3 mg, 81.3 μmol; 70.1%. 1H NMR (400 MHz, CDCl3, 298 K, TMS): δ 9.31 (dd, 2H, J = 5.1, 1.4 Hz, 2,9-phen H’s), 8.74 (dd, 2H, J = 8.3, 1.4 Hz, 4,7-phen H’s), 8.19 (s, 2H, 5,6-phen H’s), 7.95 (dd, 2H, J = 8.3, 5.1 Hz, 3,8-phen H’s), 3.66 (q, 6H, J = 7.0 Hz, ethyl H’s), 0.90 (t, 9H, J = 7.0 Hz, ethyl H’s). Positive-ion ESI-MS: m/z 630 [M − CF3SO3]+. IR (KBr disk): ν/cm−1 1949, 1869 (CO). Anal. Calcd (%) for C23H26F3N3O8PReS (778.71): C 35.47, H 3.37, N 5.40. Found: C 35.52, H 3.61, N 5.34. Synthesis of [Re(CO)3(phen)(CNC6H4-2-O)] (1) and {Re(CO)3(phen)[CN(H)C6H4-2-O]}(PF6) (2). [Re(CO)3(phen)(MeCN)](CF3SO3) (100 mg, 156 μmol) and 2-trimethylsiloxyphenyl isocyanide (35.8 mg, 187 μmol) were dissolved in THF (50 mL). The resulting solution was refluxed overnight. After removal of the solvent under reduced pressure, the residue was purified by column chromatography on alumina using dichloromethane−acetone (9:1 v/v) as eluent to give analytically pure complex 1, which is the N-deprotonated form of 2, as a yellow crystalline solid. 1H NMR (400 MHz, CDCl3, 298 K): δ 9.61 (dd, 2H, J = 5.1, 1.3 Hz, 2,9-phen H’s), 8.50 (dd, 2H, J = 8.2, 1.3 Hz, 4,7-phen H’s), 7.98 (s, 2H, 5,6-phen H’s), 7.86 (dd, 2H, J = 8.2, 5.1 Hz, 3,8-phen H’s), 7.24 (dd, 1H, J = 7.0, 1.9 Hz, phenyl H’s), 7.02 (dd, 1H, J = 7.0, 1.8 Hz, phenyl H’s), 6.86 (m, 2H, phenyl H’s). 13C NMR (100 MHz, CDCl3, 298 K): δ 214.5, 199.6, 198.3, 153.5, 152.7, 147.3, 137.1, 130.4, 127.4, 125.7, 121.4, 121.1, 117.3, 116.3, 108.5. Positive-ion ESI-MS: m/z 570 [M + H]+. IR (KBr disk): ν/cm−1 2009, 1913, 1883 (CO). Anal. Calcd (%) for C22H12N3O4Re (569.04): C 46.47,

Electronic absorption spectra were recorded on a HewlettPackard 8452A diode array spectrophotometer. Steady-state emission and excitation spectra at room temperature and at 77 K were recorded on a SPEX FluoroLog 3-TCSPC spectrofluorometer. Solutions were rigorously degassed on a highvacuum line in a two-compartment cell with no less than four successive freeze−pump−thaw cycles. Measurements of the EtOH−MeOH (4:1, v/v) glass samples at 77 K were carried out with the dilute EtOH−MeOH sample solutions contained in a quartz tube inside a liquid-nitrogen-filled quartz optical dewar flask. The emission lifetimes were measured in the MCS or Fast MCS lifetime mode with NanoLED-375LH (λex = 375 nm; pulse width 400 nm. bLuminescence quantum yield with excitation at 436 nm. cIn acetonitrile at 298 K. dEtOH−MeOH (4:1 v/v).

depending on the position of their MLCT [dπ(Re) → π*(phen)] absorption in the visible region. The electronic absorption data of these complexes are summarized in Table 3. All these complexes in acetonitrile show very intense intraligand (IL) π→π* transitions of phenanthroline, carbene (1−6), and 2-methoxyphenyl isocyanide (7) with molar extinction coefficients on the order of 104 dm3 mol−1 cm−1 in the range 220−350 nm in the UV region (Figure 2). Similar to other related rhenium(I) diimine complexes,8 they all exhibit a moderately intense MLCT [dπ(Re) → π*(phen)] absorption shoulder at ca. 365− 421 nm with molar extinction coefficients on the order of 103 dm3 mol−1 cm−1 and tailing to the visible region down to about 500 nm (Figure 2). The observation of the absorption

also be observed in 4−6 (400−421 nm) compared to that in 2 (373 nm), as a carbonyl ligand in 2 is replaced by weaker π-accepting phosphine ligands. A slightly higher MLCT absorption energy in 6 compared to 4 and 5 is also observed, as P(OEt)3 is a stronger π-accepting ligand compared to PPh3 and PPh2Me.24 Emission Spectroscopy. All complexes in acetonitrile solution displayed long-lived photoluminescence with λem in the range 511−626 nm and lifetime (τ) in the range 0.13−5.59 μs (Figure 3, Table 3). These structureless emission bands show a

Figure 3. Overlaid normalized emission spectra of complexes 2−7 in acetonitrile at 298 K.

significant energy dependence on the electronic properties of the ancillary ligands as reflected by the trends of the λem in the order 7 (511 nm) ≪ 2 (545 nm) ≈ 3 (546 nm) ≪ 6 (617 nm) < 4 (620 nm) < 5 (626 nm), which is in line with the π-accepting ability of the ancillary ligands and the MLCT absorption (see above). Considerable red-shift of emission energy is observed when the ancillary isocyanide ligand is replaced by a carbene ligand or the carbonyl ligand is replaced by a phosphine ligand, as reflected from the emissions of 7, 2, and 6 (Figure 3). Such emission energy trends and the sub-microsecond emission lifetime are suggestive of the 3MLCT [dπ(Re) → π*(phen)] excited-state origin, probably mixed with some LLCT character.25 These complexes also displayed strong photoluminescence in EtOH−MeOH (4:1, v/v) glass at 77 K (Table 3, Figure S2, Supporting Information). With the exception of 7, the emissions of 1−6 in 77 K EtOH−MeOH glassy medium are

Figure 2. Overlaid UV−vis spectra of (a) 2, 3, and 7 and (b) 2, 4, 5, and 6 in acetonitrile at 298 K.

energy in the order 7 (365 nm) > 2 (373 nm) ≈ 3 (375 nm) ≫ 6 (400 nm) > 4 (420 nm) ≈ 5 (421 nm), which is in line with the π-accepting ability of the ancillary ligands, is consistent with the MLCT [dπ(Re) → π*(phen)] transition. As the dπ(Re) orbital could be better stabilized with a stronger π-accepting ligands, complex 7, with the strongest π-accepting ancillary ligands, three carbonyl and one isocyanide ligands, shows the highest MLCT [dπ(Re) → π*(phen)] energy among 1−7. Similarly, significant red-shift of the MLCT absorption could 7081


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also structureless with a few microsecond emission lifetimes. These emissions show similar emission energy trends to their solution emissions but are all blue-shifted. They are also ascribed to the 3MLLCT excited-state origin. The blue-shifted MLLCT emission of related complexes in 77 K EtOH−MeOH glass relative to the solution emission is commonly observed and were coined as “luminescence rigidochromism”.26 In contrast, 7 in 77 K EtOH−MeOH glassy medium displays a highly structured emission band with vibrational progression spacing of ca. 1380 cm−1 and much longer emission lifetime of ca. 304 μs. The significant difference in emission characteristic of 7 in 77 K EtOH−MeOH glass compared to its solution emission as well as those of 1−6 and related MLLCT emission of other rhenium(I) diimine complexes in the same medium is suggestive of a different emission origin. In view of the longlived emission lifetimes and the structured emission bands, the emission of 7 in 77 K glassy medium was ascribed as derived from the triplet ligand-centered (3LC) excited state of the phenanthroline ligand. The change in emissive excited state origin from 3MLLCT at room temperature to 3LC at 77 K glassy medium is probably due to the blue-shift of the 3MLCT state, which becomes higher lying in energy than the 3LC state. Similar observations have also been reported in other related complexes.27 Acidity of N−H in the Carbene Ligand. The formation of neutral complex 1 in our initial attempt to prepare carbene complex 2 suggests a high acidity of the NH group in the carbene ligand, which can be readily deprotonated. To study the acidity of the benzoxazol-2-ylidene ligand in different complexes, spectrophotometric titration studies of 2 and 4−6 with a non-nucleophilic strong base of nBu4NOH were carried out. To avoid UV−vis absorption spectral changes due to alternation of ionic strength during the titration studies, all titrations were performed in 0.1 M n Bu4NPF6 in acetonitrile in order to maintain a fairly constant ionic strength. Upon addition of nBu4NOH up to a stoichiometric amount of 2 in acetonitrile solution (0.1 M nBu4NPF6), a blue-shift of the intense IL ππ* absorptions and a red-shift of the MLCT [dπ(Re) → π*(phen)] absorption with well-defined isosbestic points at 271 and 359 nm were observed (Figure 4a). The observation of well-defined isosbestic points together with the identical UV−vis absorption spectrum compared to that of 1 after addition of one molar equivalent of nBu4NOH suggests a clean N-deprotonation of the benzoxazol-2-ylidene ligand in 2. The bathochromic shift of the MLCT absorption of 2 upon deprotonation can be explained by the destabilization of the dπ(Re) orbital due to a significant decrease of the π-accepting ability of the benzoxazol-2-ylidene ligand upon conversion to an anionic carbene ligand. As MLCT energy decreases, the emission of 2 also shows a red-shift and decrease in intensity upon deprotonation (Figure 4b). For analogous complexes 4−6 with a phosphine ligand, they also exhibit similar UV−vis and emission spectral changes (Figures S3 and S4, Supporting Information) in the titration with nBu4NOH. These changes were also ascribed to the N-deprotonation of the benzoxazol-2ylidene ligand. To quantify the acidity of the NH proton in the benzoxazol-2-ylidene ligand of these complexes, the acid dissociation constants of the NH proton in 2 and 4−6 in acetonitrile−water (9:1, v/v, 0.05 mol dm−3 nBu4NPF6) have also been determined (Table 4). A significantly lower pKa for 2 compared to 4−6 is noted, suggesting a much more acidic NH proton in the benzoxazol-2-ylidene ligand of 2. The much stronger acidity of NH protons in 2 can be rationalized by the

Figure 4. (a) UV−vis absorption and (b) emission spectral changes of 2 (0.0297 mM) in (0.1 M nBu4NPF6) acetonitrile solution upon addition of nBu4NOH. The insets show the plot of (a) absorbance at 310 nm and (b) emission intensity at 547 nm as a function of mole equivalence of nBu4NOH.

Table 4. Summary of pKa Values of the Complexes 2 and 4−6 in MeCN−H2O (9:1, v/v, 0.05 mol dm−3 nBu4NPF6) at 298 K complex

pKa

2 4 5 6

6.3 8.2 8.5 8.1

much stronger electron-withdrawing effect of the trans-carbonyl ligand, which can better stabilize the deprotonated anionic carbene ligand, compared to that of the trans-phosphine ligands in 4−6. The current deprotonation study shows that the N atom of the benzoxazol-2-ylidene ligand in these complexes can be readily functionalized in the presence of base. Electrochemistry. The electrochemical properties of 1−7 in acetonitrile (0.1 M nBu4NPF6) were studied by cyclic voltammetry. The electrochemical data are collected in Table 5, and representative cyclic voltammograms are shown in Figure 5. Except for 1, all other complexes showed one irreversible or quasi-reversible oxidation couple with oxidation potential in the range +1.12 to +1.83 vs SCE, which is ascribed to the typical ReI/II metal-centered oxidation similar to that observed in related tricarbonyl rhenium(I) diimine complexes.28 The observation of the oxidative potential of the oxidation couple in the order 7 (+1.83 V) ≫ 2 (+1.67 V) ≈ 3 (+1.66 V) ≫ 6 (+1.20 V) > 4 (+1.16 V) > 5 (+1.12 V), which is in line with the π-accepting ability of the ligands and hence the electron richness of the rhenium metal center, is supportive of the ReI/II metal-centered oxidation assignment. For 1, the second oxidation wave is also assigned to the ReI/II metal-centered oxidation, whereas the additional oxidation wave observed at 7082


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precursor complexes have been structurally characterized by X-ray crystallography. The photophysical study on these complexes revealed that all these complexes display green to orange-red MLLCT phosphorescence. In short, the conversion of isocyano rhenium(I) phenanthroline complexes into luminescent carbenecontaining rhenium(I) phenanthroline complexes can be achieved. The development of other luminescent rhenium(I) carbene complexes from our previously reported isocyano rhenium(I) diimine complexes and the study of their applications in photocatalysis are currently in progress.

Table 5. Electrochemical Data for 1−7 in Acetonitrile Solution (0.1 mol dm−3 nBu4NPF6) at 298 Ka complex

oxidationb E1/2/V vs SCE (Epa/V vs SCE)

reductionb E1/2/V vs SCE (Epc/V vs SCE)

1 2 3 4 5 6 7

(+1.10), (+1.69) (+1.67) +1.66 +1.16 +1.12 +1.20 +1.83

−1.43 (−1.30) −1.26 (−1.41) (−1.42) (−1.41) −1.22

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Working electrode, glassy carbon; scan rate, 100 mV s−1. bE1/2 is (Epa + Epc)/2; Epa and Epc are peak anodic and peak cathodic potentials, respectively. a

ASSOCIATED CONTENT

S Supporting Information *

Tables, ORTEP drawings, CIF files, and experimental details for the X-ray crystal structures of cis,trans-[Re(CO)2(phen)(PPh3)(MeCN)](CF3SO3), cis,trans-[Re(CO)2(phen)(PPh3Me)(MeCN)](CF3SO3), 1−3, and 5−7, emission spectra of 2−7 in EtOH−MeOH (4:1, v/v) glassy medium at 77 K, UV−vis and emission spectral changes of 4−6 upon addition of nBu4NOH. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work described in this paper was supported by the Hong Kong University Grants Committee Area of Excellence Scheme (AoE/P-03/08) and a grant from City University of Hong Kong (Project No. 7008183). The flash photolysis system was supported by the Special Equipment Grant from the University Grants Committee of Hong Kong (SEG_CityU02). C.-O. Ng acknowledges the receipt of a postgraduate studentship from City University of Hong Kong.

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REFERENCES

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Figure 5. Cyclic voltammograms of (a) the oxidative scan and (b) reductive scan of 3 in acetonitrile (0.1 mol dm−3 nBu4NPF6).

ca. +1.10 V was assigned to the oxidation of the negatively charged N atom on the deprotonated benzoxazol-2-ylidene ligand. In the reductive scan, the first irreversible or quasireversible reduction couple of 1−7 was determined. The relatively low sensitivity to the change of the ancillary ligands and the close similarity of the potentials for phenanthrolinebased reduction observed in related rhenium(I) phenanthroline complexes20f,g,25b,28 are suggestive of the phenanthroline ligandcentered reduction assignment.

■

CONCLUSION A series of luminescent rhenium(I) phenanthroline complexes with benzoxazol-2-ylidene ligands have been synthesized and characterized. A new synthetic methodology for the preparation of dicarbonyl rhenium phenanthroline synthetic precursors was developed. Except 4, all carbene complexes and two of their 7083


Organometallics

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Luminescent Rhenium(I) Phenanthroline Complexes with a

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