Spectroscopic and Excited-State Properties of Luminescent Rhenium(I) N-Heterocyclic Carbene Complexes Containing Aromatic Diimine Ligands

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Spectroscopic and Excited-State Properties of

Luminescent Rhenium(I) N-Heterocyclic Carbene

Complexes Containing Aromatic Diimine Ligands

Wen-Mei Xue, Michael Chi-Wang Chan, Zhong-Min Su, Kung-Kai Cheung,

Shiuh-Tzung Liu,

†

_{and Chi-Ming Che*}

Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong Received October 16, 1997

Rhenium(I) N-heterocyclic carbene complexes of the type [HNCH2CH2NHCRe(L-L)(CO)3]+

have been prepared, where L-L ) 4,4′-dimethoxy-2,2′-bipyridine (2), 4,4′

-bis(tert-butyl)-2,2′-bipyridine (3), 2,2′-bipyridine (4), 4,4′-dichloro-2,2′-bipyridine (5), 4,4′

-bis(carbomethoxy)-2,2′-bipyridine (6), 5-phenyl-1,10-phenanthroline (7), and o-phenylenebis(diphenylphosphine)

(8). The molecular structures of 4, 6, and 8 have been determined by X-ray analyses and show Re-C(carbene) bond distances of 2.171(7), 2.163(4), and 2.199(6) Å, respectively.

HF-SCF and MP2 calculations on the model compound [HNCH2CH2

NHCRe(NHCHCHNH)-(CO)3]+(4m) show that the HOMO is nonbonding d(Re) and the LUMO is mainlyπ*(diimine)

with partial pz(carbene) character. CIS calculations on the excited state of optimized 4m suggest that the lowest energy absorption originates from a HOMO to LUMO spin-forbidden transition. Complexes 2-7 are emissive at room temperature and 77 K. The

room-temperature and 77 K luminescence data of 2-6 are consistent with emission from a 3

-MLCT state. The nature of the emission of 7 at room temperature is also3_{MLCT but changes}

to IL at 77 K. Complex 8 does not emit at room temperature, but at 77 K, the ILπ(pdpp)

fπ*(pdpp) emission is observed. The combination of detailed spectroscopic studies and

theoretical calculations reveal that the emitting state at room temperature is3_{[d(Re) f}

π*-(diimine)], with the latter exhibiting partial σ*(carbene) parentage. The excited-state

energies and redox potentials can be tuned using diimine ligands with varying electron-donating/accepting abilities.

Introduction

Studies on transition-metal carbene complexes have given prominence to their stoichiometric and catalytic reactivities and to their important roles in a number of

organic transformations.1 _{In contrast, little attention}

has been devoted to their photophysical and -chemical

properties,2_{even though photolysis might be expected}

to alter the reactivity of the carbene ligand through population of charge-transfer excited states. The pho-tochemistry of pentacarbonyltungsten and -chromium

carbene complexes have been extensively studied.3

Geoffroy and co-workers reported that the photolysis of

W(CO)5{C(OMe)Ph}resulted in CO elimination as the

only detectable photoreaction.2a _{This reaction was}

proposed to proceed via ligand-field excited states with

the lowest lying W fπ*(carbene) charge-transfer state

being inactive with respect to CO loss. A large body of work describing the photochemistry of chromium car-bene complexes has been reported by Hegedus and

co-workers.4

To gain insight into the photochemical reactions of carbene complexes, it is necessary to characterize the lowest-lying electronic excited states and their photo-physical deactivation mechanisms. Emission spectros-copy often provides a highly sensitive and effective way

to identify and study the lowest-energy excited states,5

but the first example of a luminescent carbene complex

has only appeared recently.6

†_{Department of Chemistry, National Taiwan University, Taipei,} Taiwan.

(1) Do¨tz, K. H.; Fischer, H.; Hofmann, P.; Kreissl, F. R.; Schubert, U.; Weiss, K. Transition Metal Carbene Complexes; Verlag Chemie: Weinheim, Germany, 1983.

(2) (a) Foley, H. C.; Strubinger, L. M.; Targos, T. S.; Geoffroy, G. L.

J. Am. Chem. Soc. 1983, 105, 3064. (b) Bell, S. E. J.; Gordon, K. C.;

McGarvey, J. J. J. Am. Chem. Soc. 1988, 110, 3107. (c) Rooney, A. D.; McGarvey, J. J.; Gordon, K. C.; McNicholl, R. A.; Schubert, U.; Hepp, W. Organometallics 1993, 12, 1277. (d) Rooney, A. D.; McGarvey, J. J.; Gordon, K. C. Organometallics 1995, 14, 107. (e) For recent review, see: Hegedus, L. S. Comprehensive Organometallic Chemistry II.; Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds., Pergamon: Oxford, 1995; Vol. 12, p 549.

(3) (a) Fischer, E. O.; Fischer, H. Chem. Ber. 1974, 107, 657. (b) Casey, C. P.; Shusterman, A. J. J. Mol. Catal. 1970, 8, 1. (c) Casey, C. P.; Shusterman, A. J.; Vollendorf, N. W.; Haller, K. J. J. Am. Chem.

Soc. 1982, 104, 2417. (d) Dahlgren, R. M.; Zink, J. I. Inorg. Chem. 1977, 16, 3154. (e) Edwards, B. H.; Rausch, M. D. J. Organomet. Chem. 1981, 210, 91.

(4) (a) McGuire, M. A.; Hegedus, L. S. J. Am. Chem. Soc. 1982, 104, 5538. (b) Hegedus, L. S.; McGuire, M. A.; Schultze, L. M.; Yujin, C.; Anderson, O. P. J. Am. Chem. Soc. 1984, 106, 2680. (c) Borel, C.; Hegedus, L. S.; Krebs, J.; Satoh, Y. J. Am. Chem. Soc. 1987, 109, 1101. (d) Hegedus, L. S.; deWeck, G.; D’Andrea, S. J. Am. Chem. Soc. 1988,

110, 2122. (e) Lastra, E.; Hegedus, L. S. J. Am. Chem. Soc. 1993, 115,

87. (f) Hegedus, L. S. Acc. Chem. Res. 1995, 28, 299. (g) Hegedus, L. S. Tetrahedron 1997, 53, 4105.

(5) Lees, A. J. Chem. Rev. 1987, 87, 711.

(6) Lai, S. W.; Chan, M. C. W.; Cheung, K. K.; Che, C. M. Angew.

Chem., Int. Ed. Engl. 1998, 37, 182.

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We now report on the luminescence of rhenium(I) N-heterocyclic carbene derivatives with diimine or phosphine ligands, as depicted in Scheme 1. Some of the molecular structures are determined by X-ray crystal analyses. Assignment of the lowest-lying excited state is based on photophysical studies and molecular orbital calculations, and subtle tuning of the excited-state energies and redox potentials is also demon-strated.

Experimental Section

General Procedures. Re(CO)5Cl (Strem), 2,2′-bipyridine

(bpy, Aldrich), 5-phenyl-1,10-phenanthroline (Phphen, GFS), and o-phenylenebis(diphenylphosphine) (pdpp, Strem) were used as received. 4,4′-Dimethoxy-2,2′-bipyridine ((MeO)2

-bpy),7_4,4′_{-bis(tert-butyl)-2,2}′_{-bipyridine (}t_Bu

2-bpy),84,4′

-dichloro-2,2′-bipyridine (Cl2-bpy),9 4,4′-bis(carbomethoxy)-2,2′

-bipyri-dine ((MeO2C)2-bpy),9 and Br(CO)4ReCNHCH2CH2NH (1)10

were prepared by literature methods. The dichloromethane for the photophysical studies was washed with concentrated sulfuric acid, 10% sodium hydrogen carbonate, and water, dried by calcium chloride, and distilled over calcium hydride. The acetonitrile for the photophysics and electrochemistry was distilled over potassium permanganate and calcium hydride. The other solvents used were of analytical grade.

Infrared spectra were recorded with KBr disks on a BIO-RAD FTS165 FT-IR spectrophotometer. Fast atom bombard-ment (FAB) mass spectra were obtained on a Finnigan Mat 95 mass spectrometer. Elemental analyses were performed by Butterworth Laboratory, U.K. 1_H, 13_{C, and} 31_{P NMR}

measurements were performed on a JEOL 270, Bruker DRX 300, or 500 MHz FT-NMR spectrometer with TMS (1_{H and} 13_{C) and H}

3PO4 (31P) as the internal reference. UV-vis

absorption spectra were obtained on a Milton Roy Spectronic 3000 diode-array spectrophotometer.

Cyclic voltammetry was performed with a Princeton Applied Research (PAR) model 175 universal programmer and a model 273 potentiostat. A standard two-compartment cell was used with glassy carbon as the working electrode, Ag-AgNO3

(0.1 mol dm-3 _{in acetonitrile) as the reference electrode,}

and platinum wire as the counter electrode. The support-ing electrolyte was n-tetrabutylammonium hexafluorophos-phate (0.1mol dm-3_{). Cp}

2Fe+/0 was added as an internal

standard.

Emission and Lifetime Measurements. Steady-state

emission spectra were recorded on a SPEX 1681 FLOURO-LOG-2 series F111AI spectrophotometer. Low- temperature (77 K) emission spectra for glasses and solid-state samples were recorded in 5-mm diameter quartz tubes which were placed in a liquid nitrogen Dewar equipped with quartz windows. The emission spectra were corrected for monochro-mator and photomultiplier efficiency and for xenon lamp stability.

The absolute emission quantum yield was measured by the method of Demas and Crosby11_{using quinine sulfate in 0.1 N}

sulfuric acid as the standard. Sample and standard solutions were degassed with at least three freeze-pump-thaw cycles. The quantum yield of the sample was determined by

where the subscripts s and r refer to sample and reference standard solution, respectively; n is the refractive index of the solvents; D is the integrated intensity, andΦ is the lumines-cence quantum yield. The quantity B is calculated by

where A is the absorbance at the excitation wavelength and L is the optical path length.

Emission lifetimes and flash-photolysis measurements were performed with a Quanta Ray DCR-3 pulsed Nd:YAG laser system (pulse output 355 nm, 8 ns). The emission signals were detected by a Hamamatsu R928 photomultiplier tube and recorded on a Tektronix model 2430 digital oscilloscope.

Syntheses. [HNCH2CH2NHCRe(L-L)(CO)3]+ (L-L ) 4,4′-X2-bpy, X ) OMe (2),tBu (3), H (4), Cl (5), CO2Me (6)).

Complex 1 (0.10 g, 0.23 mmol) and L-L (0.27 mmol) in benzene (10 cm3_{) were refluxed for 4 h to give a yellow or}

orange precipitate. Upon cooling, the resultant solid was collected and washed with benzene and diethyl ether. Re-crystallization by diffusion of diethyl ether into an acetonitrile solution afforded crystals of the complexes [HNCH2CH2

-NHCRe(L-L)(CO)3]Br. The perchlorate salt was prepared by

metathesis of the bromide salt in methanol using lithium perchlorate, and crystals were obtained by diffusion of diethyl ether into an acetonitrile solution.

2: Yellow crystals of the ClO4-salt, yield 0.12 g, 79%. IR

(νCO, cm-1): 2022, 1924, 1895. MS (positive FAB): m/z 557

(M+_{), 529 (M}+_{- CO). Anal. Calcd for C}

18H18N4O9ClRe: C, 32.96; H, 2.77; N, 8.54. Found: C, 32.66; H, 2.78; N, 8.20.1_H NMR (CD3CN): δ 3.30 (s, 4, CH2), 4.08 (s, 6, CH3O), 6.80 (s, br, 2, NH), 7.20 (dd, 2, JHH) 6.5, 2.7 Hz, py H), 8.03 (d, 2, JHH ) 2.7 Hz, py H), 8.83 (d, 2, JHH) 6.5 Hz, py H). 13C NMR (CD3CN): δ 45.2 (CH2), 58.1 (CH3O), 112.2, 115.0, 155.8, 158.3,

169.3 (py C), 194.2 (carbene C), 197.6 (ax-CO), 201.6 (eq-CO).

(M+_{), 581 (M}+_{- CO). Anal. Calcd for C}

24H30N4O7ClRe: C, 40.70; H, 4.27; N, 7.91. Found: C, 40.92; H, 4.52; N, 8.15.1_H NMR (CD3CN): δ 1.46 (s, 18, CH3), 3.30 (s, 4, CH2), 6.63 (s, br, 2, NH), 7.67 (dd, 2, JHH) 1.9, 5.9 Hz, py H), 8.43 (d, 2, JHH ) 1.9 Hz, py H), 8.91 (d, 2, JHH) 5.9 Hz, py H). 13C NMR (CD3CN): δ 30.5 (CH3), 36.7 (CMe3), 45.3 (CH2), 122.8, 126.2,

154.3, 156.6, 165.9 (py C), 193.7 (carbene C), 197.6 (ax-CO), 201.1 (eq-CO).

4: Yellow crystals of the Br-salt, yield 0.11 g, 83%. IR (νCO,

cm-1): 2022, 1936, 1905. MS (positive FAB): m/z 497 (M+), 469 (M+- CO). Anal. Calcd for C16H14N4O3BrRe: C, 33.34;

H, 2.45; N, 9.72. Found: C, 33.12; H, 2.53; N, 10.01. 1_{H NMR}

(CD3CN): δ 3.28 (s, 4, CH2), 6.80 (s, br, 2, NH), 7.68 (m, 2, py

(7) Maerker, G.; Case, F. H. J. Am. Chem. Soc. 1958, 80, 2745. (8) Chen, T. Y. Ph.D. Thesis, National Taiwan University, 1993. (9) Cook, M. J.; Lewis, A. P.; McAnliffe, G. S. G.; Skarda, V.; Thomson, A. J.; Glasper, J. L.; Robbins, D. J. J. Chem. Soc., Perkin

Trans. II 1984, 1293.

(10) Liu, C. Y.; Chen, D. Y.; Lee, G. H.; Peng, S. M.; Liu, S. T.

Organometallics 1996, 15, 1055. (11) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991. Scheme 1

Φs) Φr(Br/Bs)(ns/nr) 2_(D

s/Dr)

B ) 1-10-AL

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H), 8.23 (td, 2, JHH,t) 8.0 Hz, JHH,d) 1.5 Hz, py H), 8.51 (d,

2, JHH) 8.0 Hz, py H), 9.09 (m, 2, py H). 13C NMR (CD3CN):

δ 45.2 (CH2), 125.5, 129.2, 140.9, 154.9, 156.6 (py C), 193.5

(carbene C), 197.4 (ax-CO), 200.8 (eq-CO).

(M+), 537 (M+- CO). Anal. Calcd for C16H12N4O7Cl3Re: C,

28.90; H, 1.82; N, 8.43. Found: C, 28.52; H, 1.74; N, 8.81.1_H

NMR (CD3CN): δ 3.29 (s, 4, CH2), 6.71 (s, br, 2, NH), 7.74

(dd, 2, JHH) 6.0, 2.2 Hz, py H), 8.60 (s, 2, py H), 8.98 (d, 2,

JHH) 6.0 Hz, py H). 13C NMR (CD3CN): δ 45.2 (CH2), 126.5,

129.6, 148.7, 155.7, 157.2 (py C), 192.9 (carbene C), 197.0 (ax-CO), 200.2 (eq-CO).

6: Orange crystals of the ClO4-salt, yield 0.13 g, 79%. IR

(M+), 585 (M+- CO). Anal. Calcd for C20H18N4O11ClRe: C,

33.71; H, 2.55; N, 7.87. Found: C, 33.71; H, 2.38; N, 7.73.1_H

NMR (CD3CN): δ 3.26 (s, 4, CH2), 4.04 (s, 6, CH3), 6.53 (s, br,

2, NH), 8.11 (dd, 2, JHH) 5.6, 1.4 Hz, py H), 8.97 (d, 2, JHH)

1.4 Hz, py H), 9.21 (d, 2, JHH) 5.6 Hz, py H). 13C NMR (CD3

-CN): δ 45.3 (CH2), 54.2 (CH3), 125.0, 128.3, 141.8, 155.8, 157.3

(py C), 164.8 (CO2), 192.5 (carbene C), 197.0 (ax-CO), 199.9

(eq-CO).

[HNCH2CH2NHCRe(Phphen)(CO)3]ClO4(7). A mixture

of 1 (0.10 g, 0.23 mmol) and Phphen (0.07 g, 0.27 mmol) in benzene (10 mL) was heated to reflux for 6 h. The resultant orange solution was evaporated to dryness, and the residue was dissolved in CH2Cl2(5 mL). Diethyl ether was added to

precipitate the bromide salt. The yellow crystalline perchlo-rate salt was obtained as for complexes 2-6: yield 0.13 g, 81%. IR (νCO, cm-1): 2023, 1909 (br). MS (positive FAB): m/z 597

(M+), 569 (M+- CO). Anal. Calcd for C24H18N4O7ClRe: C,

41.41; H, 2.61; N, 8.05. Found: C, 41.32; H, 2.36; N, 7.92.1_H NMR (CD3CN): δ 3.19 (s, 4, CH2), 6.98 (s, br, 2, NH), 7.62 (m, 5, aryl H), 7.99 (dq, 2, JHH,d) 13.9 Hz, JHH,q) 5.2 Hz, aryl H), 8.16 (s, 1, aryl H), 8.64 (dd, 1, JHH) 8.6, 1.3 Hz, aryl H), 8.80 (dd, 1, JHH) 7.7, 1.3 Hz, aryl H), 9.53 (qd, 2, JHH,q) 5.2 Hz, JHH,d) 1.3 Hz, aryl H). 13C NMR (CD3CN): δ 45.1 (CH2), 127.5, 128.0, 128.5, 129.9, 130.1, 131.0, 131.5, 137.9, 138.5, 139.9, 141.5, 146.9, 147.9, 155.5 (aryl C), 193.5 (carbene C), 197.4 (ax-CO), 200.6 (eq-CO).

[HNCH2CH2NHCRe(pdpp)(CO)3]Br (8). A benzene

solu-tion (10 mL) of 1 (0.10 g, 0.23 mmol) and pdpp (0.12 g, 0.27 mmol) was refluxed for 4 h to give a white precipitate. Upon cooling to room temperature, the solid was collected and washed with benzene and diethyl ether. Recrystallization by diffusion of diethyl ether into an acetonitrile solution afforded colorless crystals: yield 0.16 g, 78%. IR (νCO, cm-1): 2031,

1958, 1931. MS (positive FAB): m/z 787 (M+), 759 (M+- CO). Anal. Calcd for C36H30N2O3BrP2Re: C, 49.89; H, 3.49; N, 3.23.

Found: C, 50.22; H, 3.41; N, 3.47. 1_{H NMR (CD}

3CN): δ 3.13

(s, 4, CH2), 6.33 (s, 2, NH), 7.16 (m, 4, aryl H), 7.38 (m, 6, aryl

H), 7.46 (m, 4, aryl H), 7.56 (m, 6, aryl H), 7.83 (m, 2, aryl H), 7.90 (m, 2, aryl H). 13_{C NMR (CD} 3CN): δ 45.9 (CH2), 129.3-141.2 (m, aryl C), 189.9 (t,2_J CP) 9.7 Hz, carbene C), 191.5 (t, 2_J CP) 6.6 Hz, ax-CO), 192.2 (d,2JCP) 8.7 Hz, eq-CO), 192.5 (d,2_J CP) 8.6 Hz, eq-CO). 31P NMR (CD3CN): δ 37.6. X-ray Crystallography. Crystals of [HNCH2CH2

NHCRe-(bpy)(CO)3]Br‚CH3CN‚0.5H2O (4), [HNCH2CH2NHCRe(4,4′

-(CO2Me)2-bpy)(CO)3]ClO4‚H2O (6), and [HNCH2CH2

NHCRe-(pdpp)(CO)3]Br (8) were mounted on a Rigaku AFC7R

diffractometer at 301 K with graphite-monochromated Mo KR radiation (λ ) 0.710 73 Å) using ω-2θ scans at a scan speed of 16.0°/min and 2θmax) 50°. Intensity data were corrected

for decay and Lorentz and polarization effects, and empirical absorption corrections were made based on theψ-scan of six strong reflections. The structures were solved by direct

methods (SIR9213_{for 4) or by Patterson methods (for 6 and} 8), expanded by Fourier methods (PATTY14_{), and refined by}

full-matrix least-squares using the software package TeXsan on a Silicon Graphics Indy computer.

For complex 4, two Br atoms and the O atoms of the water molecule were at special positions with an occupation number of1_/

2. All 30 non-H atoms were refined anisotropically. The

hydrogen atoms of the water molecule were not located. Seventeen H atoms were included in the calculation, and these comprised of two H atoms bonded to the N(3) and N(4) atoms located in the difference Fourier synthesis and 15 H atoms at calculated positions with thermal parameters equal to 1.3 times that of the attached C atoms, but their positional parameters were not refined.

For complex 6, all 38 non-H atoms were refined anisotro-pically. The two H atoms bonded to the N(3) and N(4) atoms were located in the difference Fourier synthesis, and their positional parameters were refined. The hydrogen atoms of the water molecule were not located. The other 16 H atoms at calculated positions with thermal parameters equal to 1.3 times that of the attached C atoms were not refined.

For complex 8, all 45 non-H atoms were refined anisotro-pically. The two H atoms bonded to the N(1) and N(2) atoms were located in the difference Fourier synthesis, and their positional parameters were refined. The hydrogen atoms of the water molecule were not located. The other 28 H atoms at calculated positions with thermal parameters equal to 1.3 times that of the attached C atoms were not refined.

The crystal data are summarized in Table 1, and selected bond distances and angles are listed in Table 2.

Hartree-Fock Self-Consistent-Field and Second-Order Moller-Plesset Calculations. HF-SCF15_{and MP}

216

calculations have been performed using the GAUSSIAN 94/ DFT program package17 _{on a Silicon Graphics Indigo 2}

workstation. For the Re atom, the quasirelativistic (QR) pseudopotential (pp) developed by Hay and Wadt18_{with 15}

valence electrons (VE) and the LANL2DZ basis sets associated with the pseudopotential were adopted. The basis sets were taken as Re[8s6p3d]/(3s3p2d), N[10s5p]/(3s2p), O[10s5p]/ (3s2p), C[10s5p]/(3s2p), and H[4s]/(2s). The geometry of the calculation molecule [HNCH2CH2NHCRe(NHCHCHNH)(CO)3]+

-(4m) was adapted from the X-ray structure of 4.

Results and Discussion

Characterization and Crystal Structures. The carbene complexes 2-7 are stable both in the solid and (12) Freni, M.; Giusto, D.; Romiti, P. J. Inorg. Nucl. Chem. 1967,

29, 761.

(13) Altomare, A.; Cascarano, M.; Giacovazzo, C.; Guagliardi, A.; Burla, M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27, 435.

(14) Beurskens, P. T.; Admiraal, G.; Beurskens, G.; Bosman, W. P.; Garcia-Granda, S.; Gould, R. O.; Smits, J. M. M.; Smykalla, C. The

DIRDIF program system. Technical Report of the Crystallography

Laboratory; University of Nijmegen: The Netherlands, 1992. (15) (a) Roothan, C. C. J. Rev. Mod. Phys. 1951, 23, 69. (b) Pople, J. A.; Nesbet, R. K. J. Chem. Phys. 1959, 22, 571. (c) McWeeny, R.; Dierksen, G. J. Chem. Phys. 1968, 49, 4852.

(16) (a) Head-Gorden, M.; Pople, J. A.; Frisch, M. J. Chem. Phys.

Lett. 1988, 153, 503. (b) Frisch, M. J.; Head-Gorden, M.; Pople, J. A. Chem. Phys. Lett. 1990, 166, 275. (c) Frisch, M. J.; Head-Gorden, M.;

Pople, J. A. Chem. Phys. Lett. 1990, 166, 281.

(17) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.; Head-Gordon, M.; Gonzalez, C.; Pople, J. A. Gaussian

94, Revision C.3; Gaussian, Inc.: Pittsburgh, PA, 1995.

(18) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (b) Schwerdtfeger, P.; Dolg, M.; Schwarz, W. H. E.; Bowmaker, G. A.; Boyd, P. D. W. J. Chem. Phys. 1989, 91, 1762.

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solution state. Their infrared spectra show a pattern of three peaks in the carbonyl region (2022-2031,

1924-1941, and 1891-1909 cm-1) which are typical for

fac-Re(L-L)(CO)3X complexes. In the13C NMR spectra, the resonances for the carbene carbon appear in the 189.9-194.2 ppm range, which is comparable to that

of 192.0 ppm in complex 1.10

The molecular structures of 4 (see Supporting Infor-mation), 6, and 8 (Figures 1 and 2, respectively) have been determined by X-ray diffraction methods. The rhenium atom in each case has a pseudo-octahedral geometry (Table 2) with the three carbonyl ligands in a facial configuration and the carbene group trans to one of these (C(3)-Re(1)-C(4) 178.6(3)°, 178.0(2)°, and 175.6(2)° for 4, 6, and 8, respectively). The Re-C(carbene)-N angles approach 120°, in agreement with

sp2_{hybridization at the carbene carbon atoms (e.g., in}

complex 8, Re(1)-C(4)-N(1) 129.3(4)°, Re(1)-C(4)-N(2) 122.8(4)°, N(1)-C(4)-N(2) 107.9(5)°).

Previous studies revealed that the metal-carbene bond distances are perturbed by the organic

substitu-ents attached to the carbene carbon.1 _{In this work, the}

Re-C(carbene) distances (2.171(7), 2.163(4), and 2.199-(6) Å for 4, 6, and 8, respectively) are comparable with those in other heteroatom-stabilized analogues such as fac-ReBr(CO)3(CNHCH2CH2NH)2 (2.14(2) and 2.17(2) Table 1. Crystal Data for Complexes 4, 6, and 8

4 6 8

formula C18H18BrN5O3.5Re C20H20ClN4O12Re C36H30BrN2O3P2Re

fw 626.49 730.07 866.71

color pale yellow yellow colorless

cryst dimens, mm 0.25× 0.20 × 0.30 0.20× 0.20 × 0.35 0.15× 0.05 × 0.20

cryst syst orthorhombic triclinic triclinic

space group Ccca (No. 68) P1h (No. 2) P1h (No. 2)

a, Å 19.512(8) 11.826(6) 12.027(8) b, Å 29.865(8) 12.012(6) 16.75(1) c, Å 14.523(8) 11.694(5) 9.582(3) R, deg 117.45(3) 105.26(4) β, deg 97.33(4) 109.93(4) γ, deg 60.61(3) 73.23(5) V, Å3 _8462(5) _1274(1) _1708(1) Z 16 2 2 F_calcd, g cm-3 1.967 1.903 1.685 abs coeff, cm-1 76.71 49.45 48.62

no. of unique data collected 4088 4485 6008

no. of obsd data with I g 3σ(I) 2584 4036 4776

no. of variables 259 349 412 Ra _0.028 _0.023 _0.030 Rwb 0.039 0.026 0.034 goodness-of-fit, Sc _1.89 _1.45 _1.25 ∆F(max, min, e Å-3₎ _{+1.13, -0.67} _{+0.62, -0.54} _{+0.80, -1.01} a_{R )}_∑₍_||F o| - |Fc||)/∑|Fo|.bRw) [∑w(|Fo| - |Fc|)2/∑w|Fo|2]1/2.cS ) [∑w(|Fo| - |Fc|)2/(n - p)]1/2.

Table 2. Selected Bond Distances (Å) and Angles (deg) for Complexes 4, 6, and 8

Complex 4 Re(1)-N(1) 2.178(6) Re(1)-N(2) 2.175(6) Re(1)-C(1) 1.917(9) Re(1)-C(2) 1.924(9) Re(1)-C(3) 1.965(9) Re(1)-C(4) 2.171(7) C(4)-N(3) 1.33(1) C(4)-N(4) 1.32(1) N(1)-Re(1)-N(2) 74.0(2) N(1)-Re(1)-C(1) 173.2(3) N(1)-Re(1)-C(2) 97.6(3) N(1)-Re(1)-C(3) 92.7(2) N(1)-Re(1)-C(4) 87.5(2) N(2)-Re(1)-C(1) 99.2(3) N(2)-Re(1)-C(2) 171.0(3) N(2)-Re(1)-C(3) 93.2(3) N(2)-Re(1)-C(4) 85.5(2) C(1)-Re(1)-C(2) 89.1(4) C(1)-Re(1)-C(3) 88.4(3) C(1)-Re(1)-C(4) 91.2(3) C(2)-Re(1)-C(3) 90.5(3) C(2)-Re(1)-C(4) 90.9(3) C(3)-Re(1)-C(4) 178.6(3) Re(1)-C(4)-N(3) 125.1(5) Re(1)-C(4)-N(4) 126.7(5) N(3)-C(4)-N(4) 108.2(6) Complex 6 Re(1)-N(1) 2.178(3) Re(1)-N(2) 2.172(3) Re(1)-C(1) 1.908(5) Re(1)-C(2) 1.928(5) Re(1)-C(3) 1.966(5) Re(1)-C(4) 2.163(4) C(4)-N(3) 1.321(5) C(4)-N(4) 1.318(5) N(1)-Re(1)-N(2) 75.0(1) N(1)-Re(1)-C(1) 171.3(2) N(1)-Re(1)-C(2) 98.4(2) N(1)-Re(1)-C(3) 94.5(2) N(1)-Re(1)-C(4) 83.5(1) N(2)-Re(1)-C(1) 97.7(2) N(2)-Re(1)-C(2) 173.2(1) N(2)-Re(1)-C(3) 91.9(2) N(2)-Re(1)-C(4) 87.5(1) C(1)-Re(1)-C(2) 88.8(2) C(1)-Re(1)-C(3) 90.5(2) C(1)-Re(1)-C(4) 91.5(2) C(2)-Re(1)-C(3) 90.1(2) C(2)-Re(1)-C(4) 90.3(2) C(3)-Re(1)-C(4) 178.0(2) Re(1)-C(4)-N(3) 127.1(3) Re(1)-C(4)-N(4) 126.2(3) N(3)-C(4)-N(4) 106.7(4) Complex 8 Re(1)-P(1) 2.437(1) Re(1)-P(2) 2.444(1) Re(1)-C(1) 1.963(6) Re(1)-C(2) 1.950(6) Re(1)-C(3) 1.928(7) Re(1)-C(4) 2.199(6) C(4)-N(1) 1.328(7) C(4)-N(2) 1.318(7) P(1)-Re(1)-P(2) 82.57(5) P(1)-Re(1)-C(1) 175.5(2) P(1)-Re(1)-C(2) 92.7(2) P(1)-Re(1)-C(3) 94.1(2) P(1)-Re(1)-C(4) 89.1(2) P(2)-Re(1)-C(1) 93.3(2) P(2)-Re(1)-C(2) 175.1(2) P(2)-Re(1)-C(3) 93.9(2) P(2)-Re(1)-C(4) 89.5(2) C(1)-Re(1)-C(2) 91.4(2) C(1)-Re(1)-C(3) 87.9(2) C(1)-Re(1)-C(4) 89.1(2) C(2)-Re(1)-C(3) 87.7(2) C(2)-Re(1)-C(4) 89.1(2) C(3)-Re(1)-C(4) 175.6(2) Re(1)-C(4)-N(1) 129.3(4) Re(1)-C(4)-N(2) 122.8(4) N(1)-C(4)-N(2) 107.9(5)

Figure 1. ORTEP plot of the cation in complex 6 (40% probability ellipsoids).

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Å),10 _ReBr(CO)

4(COCH2CH2O) (2.135(13) Å),19

ReBr-(CO)4{C(NHPh)(NHCHMe2)}(2.206(10) Å),20and

ReBr-(CO)3(PPh3){C(NHPh)(NHCHMe2)}(2.215(8) Å).20 They

are longer than the Re-alkylcarbene contact in

CpRe-(NO)(PPh3)(CHPh)+(1.949(6) Å)21but are similar to the

Re-C(sp3_{) bond in the corresponding benzyl derivative}

CpRe(NO)(PPh3)(CH2Ph)+(2.203(8) Å).22 Nevertheless,

the angles around the carbene atoms in 4, 6, and 8 imply

that there are appreciable Re-C(carbene)π interactions

in these complexes.

Electronic Structure and Absorption Spectra. As an aid to interpreting the electronic absorption

spectra of complexes 2-7, HF-SCF15_{and MP}

216

calcula-tions have been performed on the model molecule

[HNCH2CH2NHCRe(NHCHCHNH)(CO)3]+(4m). The

calculated energy and composition of the near frontier orbitals are summarized in Table 3 and reveal that the HOMO has a high d(Re) parentage (69.5%) whereas the

LUMO is dominated by π*(diimine) (78.3%) and the

percentage of p(carbene C) is small (6.5%). Figure 3 shows a simplified molecular orbital diagram of 4m and

its component fragments [(CO)3Re(NHCHCHNH)]+and

CNHCH2CH2NH. The HOMO is essentially the

non-bonding dxz(Re) orbital, and below this are the two

nonbonding dyz(Re) and dx2_-y2_{(Re) orbitals (x, y, z axis}

defined as in Figure 3); these three orbitals are very close in energy. The LUMO is comprised of pz(diimine) and pz(carbene C). Hence, if the lowest energy transi-tion involves the frontier orbitals (see Supporting

Infor-mation), then this is formulated as d(Re) fπ*(diimine)

mixed with a small amount of d(Re) fσ*(carbene C).

The electronic absorption spectral data of complexes 1-8 are summarized in Table 4. We suggest that

(19) Miessler, G. L.; Kim, S.; Jacobson, R. A.; Angelici, R. J. Inorg.

Chem. 1987, 26, 1690.

(20) Chen, L. C.; Chen, M. Y.; Chen, J. H.; Wen, Y. S.; Lu, K. L. J.

Organomet. Chem. 1992, 425, 99.

(21) Kiel, W. A.; Lin, G. Y.; Constable, A. G.; McCormick, F. B.; Strouse, C. E.; Eisenstein, O.; Gladysz, J. A. J. Am. Chem. Soc. 1982,

104, 4865.

(22) Merrifield, J. H.; Strouse, C. E.; Gladysz, J. A. Organometallics

1982, 1, 1204.

Figure 2. ORTEP plot of the cation in complex 8 (40% probability ellipsoids).

Table 3. Calculated Energy (eV) and Composition (%) of the Near Frontier Orbitals of Model Molecule [HNCH2CH2NHCRe(NHCHCHNH)(CO)3]+ (4m) by MP2Method % composition orbital energy, eV Re carbene C HNCH2 -CH2NH 3CO NHCH-CHNH 72 1.265 14.9 5.4 4.6 64.2 11.0 71 0.972 35.2 3.3 9.8 31.3 20.4 70 0.883 39.4 12.5 10.4 22.8 14.9 69 0.465 3.7 0.7 2.4 9.1 84.1 68 0.177 56.6 11.0 5.5 18.6 8.4 67 0.020 51.9 1.0 2.1 32.3 12.8 66 -0.265 73.6 6.5 8.2 6.7 5.0 65 -0.696 66.0 1.7 2.6 9.6 20.2 64 -1.192 69.2 1.9 4.0 11.3 13.6 63 -3.996 6.5 6.5 1.1 7.6 78.3 62 -12.245 69.5 0.3 1.7 19.2 9.2 61 -12.357 68.1 1.1 3.5 18.7 8.7 60 -12.584 73.5 0.2 0.1 23.5 2.7 59 -14.244 0.6 0.8 97.1 0.7 0.8 58 -15.959 8.6 47.3 19.6 13.1 11.4 57 -16.208 5.4 8.9 6.1 4.1 75.4 56 -17.848 1.7 10.2 59.3 4.5 24.3 55 -18.077 5.5 0.1 1.3 19.7 73.4 54 -18.245 2.2 2.7 66.2 7.0 21.9 53 -18.448 4.4 4.0 26.8 9.7 55.1 52 -18.993 4.0 5.6 15.4 8.3 66.7 51 -19.821 1.4 0.7 7.4 89.1 1.5 50 -19.873 1.7 0.1 4.0 90.7 3.5 49 -20.016 0.5 0.2 75.7 22.9 0.7 48 -20.030 1.8 0.3 16.2 78.6 3.1 47 -20.265 1.9 1.5 63.0 29.2 4.4 46 -20.500 5.3 0.2 0.2 86.5 7.8 45 -20.565 3.9 0.3 20.3 71.1 4.4 44 -20.710 5.0 0.5 7.5 83.5 3.5 43 -20.915 10.1 0.6 3.4 81.1 4.9

Figure 3. Simplified orbital mixing diagram for 4m. Orbital energies are derived from MP2calculation.

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complex 1 has a similar electronic structure to that of

W[C(OMe)Ph](CO)5;2a thus, the moderately intense

band centered at 346 nm in the UV-vis absorption spectrum of 1 is assigned as the spin-allowed d(Re) f π*(carbene C) (MLCT) transition, while the very weak shoulder tailing to 560 nm is assigned as a

spin-forbidden d(Re) f π*(carbene C) (3_{MLCT) transition.}

Replacing the CO ligands by diimine evidently affects the electronic structure of the carbene complexes (vide supra). The UV-vis absorption spectrum for complex

4 is shown in Figure 4.

To aid in the interpretation of the absorption spectra of these rhenium(I) carbene complexes, a single

config-uration interaction (CIS) calculation23_{was performed}

on the excited state of the optimized molecule 4m, where all the orbitals and electrons of the ground state were included to take into account the electronic correlation for the excited states. The results and assignments are

listed in Table 5. The calculated lowest energy absorp-tion at 433 nm is a triplet transiabsorp-tion of orbital 62 f 63 (HOMO to LUMO). This can be visualized as a

spin-forbidden charge-transfer transition of dxz(Re) to

π*-(diimine), where the π*(diimine) orbital has a small

percentage of pz(carbene C), and is denoted as3_MLCT.

From the absorption spectrum of 4, a weak shoulder around 450 nm correlates with this transition. The calculated HOMO to LUMO singlet transition is located at 343 nm and correlates with the 363 nm band in the absorption spectrum of 4. The intense absorption band

at 273 nm for 4 (in CH2Cl2) may be initially assigned to

the calculated transition around 293 nm which is an admixture of 60 f 63 and 61 f 63 transitions, namely

dx2_-y2_{(Re) f} π*(diimine) and d_{yz(Re) f} π*(diimine)

accompanied by partial d(Re) f pz(carbene C) character. However, the extinction coefficient for this band (1.54

× 104 _dm3 _mol-1 _cm-1_{) is much larger than that for}

commonly observed MLCT transitions in rhenium(I)

diimine complexes.24 _{Furthermore, free bpy shows an}

intraligand π f π* transition at 282 nm with max of

1.48× 104_dm3_mol-1_cm-1_.25 _{Hence, it is apparent that}

the intense absorption of 4 at 273 nm is dominated by

the intraligandπ f π* transition of the diimine ligand,

which is consistent with previous work on related

systems.24 _{It should be noted that the calculated}

intraligand π f π* transition for the “simplified 2,2′

-bipyridine” ligand [NHCHCHNH] in the optimized model 4m occurs at 232 nm.

The singlet transition energy from HOMO to LUMO

(1_{MLCT), which is assigned to the 363 nm band for}

complex 4, is affected by the electronic properties of the

4,4′-substituents on bpy. The LUMO is composed

mainly ofπ*(diimine) (vide supra), so electron-donating

substituents such as CH3O andtBu will destabilize the

LUMO while electron-withdrawing groups such as Cl

and CO2Me will lower the LUMO energy. Thus, a

correlation between the1_{MLCT transition energy and}

the Hammett parameters (σ) of the substituents on bpy

is anticipated and subsequently demonstrated (see

Supporting Information). The 1_{MLCT transitions of}

(23) Foresman, J. B.; Head-Gorden, M.; Pople, J. A.; Frisch, M. J.

J. Phys. Chem. 1992, 96, 135.

(24) (a) Lees, A. J. Chem. Rev. 1987, 87, 711. (b) Sacksteder, L.; Zipp, A. P.; Brown, E. A.; Streich, J.; Demas, J. N.; DeGraff, B. A. Inorg.

Chem. 1990, 29, 4335. (c) Zipp, A. P.; Sacksteder, L.; Streich, J.; Cook,

A.; Demas, J. N.; DeGraff, B. A. Inorg. Chem. 1993, 32, 5629. (d) Leasure, R. M.; Sacksteder, L.; Nesselrodt, D.; Reitz, G. A.; Demas, J. N.; DeGraff, B. A. Inorg. Chem. 1991, 30, 3722.

Table 4. UV-Vis Absorption Spectral Data of Complexes 1-8 in Dichloromethane and

Acetonitrile at Room Temperature

CH2Cl2 CH3CN complex λ, nma , 10 3_dm3 mol-1cm-1 b λ, nma , 10 3_dm3 mol-1cm-1 b 1 346 tail to 560 3.0 308 7.4 2 350 tail to 490 4.9 342 5.8 294 14.6 294 15.9 277 23.2 252 47.8 255 36.9 3 355 tail to 500 4.5 345 4.7 315 9.6 314 9.2 267 20.0 250 21.4 253 20.7 4 365 tail to 510 3.5 355 4.2 318 8.0 316 11.1 273 15.4 245 25.5 5 380 tail to 530 3.7 375 4.3 283 15.5 278 21.1 255 14.0 253 23.6 6 400 tail to 570 6.2 390 6.1 305 21.9 300 21.5 7 370 tail to 510 4.0 360 4.7 320 6.7 320 8.3 286 18.2 283 24.5 8 350 0.02 300 2.2

a_{Error ) (2 nm.}b_{Error ) (1 in the last digit.}

Figure 4. UV-vis absorption spectra of 4 in dichlo-romethane (s) and acetonitrile (- - -) at room temperature.

Table 5. Electronic Transition Energy Assignment of Optimized Model Molecule 4m by CIS

Calculation no. excitation state λcalc, nm assignmenta 1 triplet-A′′ 433 62 f 63, 57 f 63 2 triplet-A′ 404 51 f 63, 61 f 63 3 singlet-A′′ 343 62 f 63 4 triplet-A′ 328 60 f 63 5 singlet-A′ 323 60 f 63, 61 f 63 6 singlet-A′ 293 60 f 63, 61 f 63 7 triplet-A′ 280 62 f 64, 62 f 66, 62 f 82 8 triplet-A′′ 276 60 f 64, 60 f 66, 60 f 70, 60 f 82 9 triplet-A′′ 270 62 f 63, 56 f 63, 57 f 63 10 singlet-A′′ 248 61 f 64, 61 f 66 11 singlet-A′ 245 62 f 64, 61 f 68, 62 f 66 12 singlet-A′′ 236 60 f 64, 60 f 66, 60 f 82 13 singlet-A′′ 232 57 f 63, 62 f 63, 56 f 63 14 singlet-A′′ 227 61 f 66, 61 f 64, 60 f 82

a_{For the orbitals see Table 3.}

(7)

2-6 (Table 4) red-shift as the electron-withdrawing ability of the substituents increases, and this provides

further support for the d(Re) f π*(diimine) (MLCT)

assignment of the absorption band.

The MLCT absorption maxima are solvent-dependent and blue-shift ca. 10 nm when the solvent is changed from dichloromethane to acetonitrile (Table 4). This solvatochromic effect is a characteristic feature of MLCT transitions. The shift to higher energy in solvents of greater polarity is indicative of a polar ground state and a nonpolar excited state. This is in direct contrast to the effect observed for diimine complexes such as

Ru-(bpy)32+ and related systems, where the MLCT band

maxima shift to lower energy with increasing solvent polarity typifies a nonpolar ground state and a more

polar excited state.26 _{Similar negative solvatochromic}

shifts have previously been observed for the charge-transfer-to-dithiolate and charge-transfer-to-diimine

ab-sorption bands of platinum(II) dithiolate27a_{and diimine}

dithiolate27b_{complexes, respectively.}

Emission Spectra and Nature of the Excited States. Photoluminescence has been recorded for com-plexes 2-7 in fluid solution and frozen-solvent glasses. The room-temperature emission data are listed in Table 6, while Figure 5 depicts the emission spectra of 4 (top) and 7 (bottom), respectively, in dichloromethane and acetonitrile at room temperature and in 4:1 ethanol/ methanol glass at 77 K. In room-temperature fluid solution, the emission bands are broad and unstruc-tured. The emitting excited states are assigned as

dxz-(Re) fπ*(diimine) with partial dxz(Re) f pz(carbene C)

on the basis of a linear relationship between the

emission and1_{MLCT absorption energies (see}

Support-ing Information). The relatively long lifetimes indicate that the transitions involved are spin-forbidden.

The emission energy can be tuned by varying the

diimine ligand. The 4,4′-substituents X on the bpy

chromophore are expected to strongly affect the MLCT

excited-state energy. The carbene complexes with

electron-donating substituents on bpy should, therefore, have a greater energy gap between the excited and ground states than those with electron-withdrawing substituents. Hence, a linear correlation between the emission energy and Hammett parameter for X is observed (Figure 6). The shift of the emission band to

lower energies for more electron-withdrawing substituents X further supports the assignment of a

charge-transfer-to-diimine emitting state. Emission energies are observed to be solvent-dependent, and this is characteristic of MLCT excited states. Emission ener-gies recorded in acetonitrile are red-shifted by ca. 400

cm-1compared to those in dichloromethane.

(25) Xue, W. M.; Che, C. M. Unpublished work.

(26) Ford, W. E.; Calvin, M. Chem. Phys. Lett. 1980, 76, 105. (27) (a) Bevilacqua, J. M.; Zuleta, J. A.; Eisenberg, R. Inorg. Chem.

1993, 32, 3689. (b) Cummings, S. D.; Eisenberg, R. J. Am. Chem. Soc. 1996, 118, 1949.

Table 6. Corrected Emission Maxima, Quantum Yields, and Lifetimes for Complexes 2-7 in Dichloromethane and Acetonitrile at Room

Temperature

CH2Cl2 CH3CN

complex λmax, nma φemb τ, nsb λmax, nma φemb τ, nsb

2 558 0.040 194 570 0.012 64 3 553 0.088 330 565 0.038 150 4 565 0.068 230 577 0.021 97 5 603 0.0075 45 615 0.0020 20 6 620 0.015 97 635 0.0039 40 7 555 0.30 2520 565 0.11 932 a_{Error ) (2 nm.}b_{Error ) (10%.}

Figure 5. (top) Emission spectra of 4 in dichloromethane (s,λex ) 365 nm) and acetonitrile (‚‚‚, λex ) 355 nm) at room temperature and in 4:1 ethanol/methanol glass (- - -,

λex ) 350 nm) at 77 K. The emission intensities are normalized. (bottom) Emission spectra of 7 in dichlo-romethane (s,λex) 370 nm) and acetonitrile (‚‚‚, λex) 370 nm) at room temperature and in 4:1 ethanol/methanol glass (- - -,λex) 350 nm) at 77 K. The emission intensities are normalized.

Figure 6. Room-temperature emission energies vs Ham-mett parameters correlation plots. Complexes are num-bered as indicated in Scheme 1. Open circles indicate data in dichloromethane (correlation coefficient R ) 0.97, slope ) -(2.8 ( 0.4) × 103_{), and closed circles represent data in} acetonitrile (R ) 0.97, slope ) -(2.8 ( 0.4)× 103_).

(8)

The emission spectra of complexes 2-6 measured at 77 K in frozen glasses are also broad and virtually structureless, but the emission maxima are shifted to higher energies than at room temperature. However, complex 7 exhibits a structured emission with peaks at 495 and 528 nm in 4:1 ethanol/methanol glass at 77 K. This is clearly phosphorescence of the Phphen ligand

(3_{LC), as indicated by the vibronic structure and its close}

resemblance to the phosphorescence of the free proto-nated ligand which has peak maxima at 494 and 522

nm.24b_{Similar emission changes with temperature and}

ligand had been observed by Zipp, Demas, and DeGraff

for rhenium(I) diimine complexes.24b,c_{We adhere to their}

explanation using the model shown in Figure 7. The 298 and 77 K emissive behavior of 2-6 belong to the model in Figure 7a, where all emissions are MLCT. The polar MLCT state energy depends strongly on the solvent organization, while the less polar IL state energy is relatively insensitive to solvent properties. At room temperature, the complex and environment can relax to the thermally equilibrated excited state in a shorter time than the emission lifetime, and this significantly lowers the MLCT state energy. However, in a rigid 77 K glass, solvent viscosity prevents thermal equilibration in a time comparable to the emission lifetime. On the other hand, the emissions of 7 can be represented by the model in Figure 7b. In this case, the emission is MLCT at 298 K but the increase in the MLCT state energy upon cooling means that the MLCT-IL state gap is smaller, and the lowest excited state is IL at 77 K.

Apart from the Re(I) derivatives mentioned above,24_the

complexity of multiple luminescences has been encoun-tered for the R-diimine complexes of a number of

transition metals, including Ir(III),28_Cu(I),29_{W(0), and}

Mo(0).30 _{Complex 8, bearing the phosphine ligand pdpp}

instead of diimine, does not emit at room temperature

in the solution or solid state, but luminescence is

observed (λmax460 nm) in a frozen CH2Cl2solution at

77 K (see Supporting Information). This emission is independent of the excitation wavelength and is as-signed to the pdpp IL transition.

Electrochemistry and Excited-State Redox Prop-erties. Cyclic voltammetry is used to determine the ground-state redox potentials for complexes 2-7 (Table 7). A quasireversible oxidation wave in the +1.50 to +1.66 V range (vs SCE) is recorded, which becomes irreversible when the scan speed is decreased from 500

to 100 mV s-1. This couple is assigned to a

metal-centered oxidation, i.e., Re(I) f Re(II).31 _{One or more}

reduction waves are observed between -0.86 and -1.73 V, the first of which is reversible while the second and third are irreversible or quasireversible. With reference to electrochemical studies on related rhenium(I)

deri-vatives,24b,31,32_{this first couple is assigned to a}

ligand-centered L-L0/-_reduction.

Both of the ground-state redox potentials E1/2(Re2+/+)

and E1/2(L-L0/-) are systematically varied with different

diimine ligands. The plots of E1/2(Re2+/+) and E1/2

(L-L0/-_{) of complexes 2-6 vs the Hammett parameters of}

the 4,4′-substituents give linear correlations (see

Sup-porting Information). Since the substituents on bpy are not expected to affect the HOMO, which is predomi-nantly comprised of the Re 5d orbital, the variation of

oxidation potential E1/2(Re2+/+) vs σ is less notable

compared to that for E1/2(L-L0/-) (slope 0.19 ( 0.03 cf.

0.74 ( 0.05 V, respectively).

A comparison of the electrochemical and emission data gives additional support for the assignment of

the Re fπ*(diimine) charge-transfer excited state. A

linear relationship is observed between the energy of the excited state and the difference between the ground-state oxidation and reduction potentials (see Supporting Information). Similar correlations have been reported for a number of transition-metal diimine

complexes.27b,32,33

(28) (a) Watts, R. J.; Brown, M. J.; Griffith, B. G.; Harrington, J. S. J.

Am. Chem. Soc. 1975, 97, 6029. (b) Watts, R. J.; Griffith, B. G.;

Harrington, J. S. J. Am. Chem. Soc. 1976, 98, 674.

(29) (a) Buckner, M. T.; Matthews, T. G.; Lytle, F. E.; McMillin, D. R.

J. Am. Chem. Soc. 1979, 101, 5846. (b) Rader, R. A.; McMillin, D. R.;

Buckner, M. T.; Matthews, T. G.; Casadonte, D. J.; Lengel, R. K.; Whittaker, S. B.; Darmon, L. M.; Lytle, F. E. J. Am. Chem. Soc. 1981,

103, 5906. (c) Casadonte, D. J., Jr.; McMillin, D. R. J. Am. Chem. Soc.

1987, 109, 331.

(30) (a) Manuta, D. M.; Lees, A. J. Inorg. Chem. 1983, 22, 572. (b) Manuta, D. M.; Lees, A. J. Inorg. Chem. 1986, 25, 1354.

(31) Wallace, L.; Rillema, D. P. Inorg. Chem. 1993, 32, 3836. (32) Juris, A.; Campagna, S.; Bidd, I.; Lehn, J. M.; Ziessel, R. Inorg.

Chem. 1988, 27, 4007.

(33) (a) Hino, J. K.; Ciana, L. D.; Dressick, W. J.; Sullivan, B. P.

Inorg. Chem. 1992, 31, 1072. (b) Juris, A.; Balzani, V.; Barigelletti, F.;

Campagna, S.; Belser, P.; Von Zelewsky, A. Coord. Chem. Rev. 1988,

84, 85. (c) Rillema, D. P.; Allen, G.; Meyer, T. J.; Conrad, D. Inorg. Chem. 1983, 22, 1617. (d) Caspar, J. V.; Meyer, T. J. Inorg. Chem.

1983, 22, 2444. (e) Rezvani, A. R.; Crutchley, R. J. Inorg. Chem. 1994,

33, 170.

Figure 7. State diagram for the lowest triplet states of (a) [HNCH2CH2NHCRe(4,4′-X2-bpy)(CO)3]+(2-6) and (b) [HNCH2CH2NHCRe(Phphen)(CO)3]+(7).

Table 7. Electrochemical Data and Excited-State Redox Potentials of Complexes 2-7 in Acetonitrile complex E1/2,ox (∆Ep)a E1/2,red (∆Ep)a Emax, eV E1/2(2+/+*), V E1/2(+*/0), V 2 +1.50 (90) -1.38 (80) 2.48 -0.98 +1.10 3 +1.56 (70) -1.36 (70) 2.51 -0.95 +1.15 4 +1.60 (90) -1.26 (70) 2.41 -0.81 +1.15 5 +1.62 (90) -1.04 (50) 2.28 -0.66 +1.24 -1.62b -1.73b 6 +1.66 (120) -0.86 (60) 2.24 -0.58 +1.38 -1.38 (70) 7 +1.59 (90) -1.31 (50) 2.51c _-0.92 _+1.20 1 +1.52b

a_{Potentials in volts vs SCE, estimated as}1_/

2(Epa+ Epc), where

Epaand Epcare the potentials of peak anodic and cathodic current,

respectively;∆Ep) Epc- Epa.bIrreversible.cE0-0.

(9)

It is also possible to estimate the excited-state redox potentials using the following equations:

where E1/2(2+/+*) and E1/2(+*/0) refer to the following

reactions, respectively:

Due to the broad structureless nature of the 77 K

emissive bands for complexes 2-6, their E0-0values are

difficult to determine and, therefore, their respective emission maxima at 77 K in 4:1 ethanol-methanol glass

are used as approximations for E0-0. In the case of

complex 7, vibronic structure is observed in the 77 K emission, thus the highest energy vibrational peak is

taken as E0-0. It is also important to note that any

effects arising from differences in the solvent used for the electrochemical and spectroscopic experiments are not taken into account. Due to these factors, only the relative trends in the calculated excited-state redox potentials should be considered. The results of the

calculations are presented in Table 7. The redox

potentials for excited states acting as reducing agents,

E1/2(2+/+*), varies from -0.98 V for 2 to -0.58 V for 6.

This reflects the tendency for the ground-state oxidation

potentials E1/2,ox to increase and the E0-0 values to

decrease as the electron-accepting ability of diimine

ligands increases. Variations for E1/2(+*/0) are less

significant because both the ground-state reduction

potential and E0-0decrease as the 4,4′-substituted bpy

becomes more electron-withdrawing. The changes in the excited-state redox potentials for the series generally correlate with the respective Hammett parameters (see Supporting Information). In this investigation, the

excited state of complex 2 containing 4,4′-(MeO)2-bpy

is the most powerful reductant (E1/2(2+/+*) ) -0.98 V)

while the excited state of 6 bearing 4,4′-(CO2Me)2-bpy

is the strongest oxidant (E1/2(+*/0) ) +1.20 V).

Conclusion

A series of luminescent rhenium(I) complexes of the

type [HNCH2CH2NHCRe(L-L)(CO)3]+(L-L ) diimine

and diphosphine) have been prepared and characterized by X-ray analysis. Assignment of their lowest-lying excited states is established by investigating their emissive properties at room temperature and 77 K and by interpretation of molecular-orbital calculations. At room temperature, the excited state of complexes 2-7 with diimine ligands is assigned as being metal-ligand charge-transfer (MLCT) in nature and is proposed to

be predominantly d(Re) fπ*(diimine) with partial d(Re)

f σ*(carbene C). At 77 K, the emitting state of complexes 2-6 remains MLCT but that of 7 bearing the Phphen ligand changes to intraligand (IL). Complex 8 with the pdpp ligand does not emit at room temperature, but at 77 K the pdpp IL emission is observed. Excited-and ground-state redox potentials, excited-state ener-gies, emission lifetimes, and quantum yields can be tuned by changing the electron-donating/accepting abil-ity of the auxiliary diimine ligand.

Acknowledgment. We thank The University of Hong Kong, the Croucher Foundation, and the Hong Kong Research Grants Council for financial support. M.C.-W.C. is grateful for a University Postdoctoral Fellowship from The University of Hong Kong. We are indebted to one of the reviewers for helpful suggestions. Supporting Information Available: ORTEP diagram of 4, HOMO and LUMO of 4m, absorption energy vs Hammett

parameter plots, emission energy vs1_{MLCT absorption energy}

plots, emission spectrum of 8, ground state redox potential vs Hammett parameter plot, correlation of emission energies with ∆E1/2, and excited state redox potential vs Hammett parameter

plots and tables of crystal data, atomic coordinates, calculated hydrogen coordinates, anisotropic displacement parameters, and bond distances and angles for 4, 6, and 8 (36 pages). Ordering information is given on any current masthead page. OM9709042 E_1/2(2+/+*) ) E_1/2(2+/+) - E_max (1) E_1/2(+*/0) ) E_1/2(+/0) + E_max (2) [(carbene)ReII(L-L)-(CO)3] + * f [(carbene)ReII(L-L)(CO)₃]2++ e- (3) [(carbene)ReII(L-L)-(CO)3] + * + e-f [(carbene)ReI(L-L)-(CO)₃] (4)