Organometallics 2009, 28, 5837–5840 DOI: 10.1021/om900552e
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fac-Tricarbonyl Rhenium(I) Azadipyrromethene Complexes David V. Partyka,† Nihal Deligonul,‡ Marlena P. Washington,‡ and Thomas G. Gray*,‡ ‡
Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106, and †Creative Chemistry L.L.C., Cleveland, Ohio 44106 Received June 25, 2009
Summary: Rhenium(I) complexes of azadipyrromethene ligands are reported; three have been characterized crystallographically. The free ligands and their metallo-complexes undergo reductive electrochemistry. Red-light absorption results from optically allowed transitions to a ligand-localized LUMO. Mononuclear rhenium(I) carbonyl complexes draw continuing scrutiny for their ground- and excited-state properties. Tricarbonyl rhenium(I) bipyridine and phenanthroline complexes, all with facial stereochemistry, emit from triplet excited states. Excitation into metal-to-ligand chargetransfer absorptions yields submicrosecond luminescence at room temperature, with longer lifetimes upon cooling.1-11 Emission from these complexes obeys the approximate energy-gap law;12,13 luminescence lifetimes, maxima, and quantum yields respond to changes in the metal-ion coordination sphere or along the diimine perimeter. Facial rhenium(I) carbonyls are valuable apart from their photochemical properties. 186Re and 188Re are beta emitters under clinical consideration, and rhenium complexes are often sought as cold analogues of γ-emitting 99mTc species.14 The fac-M(CO)3þ (M = Tc, Re) core recurs in many complexes having real or potential utility in medical imaging. Zubieta and co-workers have disclosed chelating ligands bearing quinoline, tryptophan, and benzimidazole donors.15 When bound to the fac-[Re(CO)3]þ core, the complexes act
as luminescence probes; when bound to fac-[99mTc(CO)3]þ, as potential radioimaging agents. Technetium and rhenium are strongly homologous, and the design strategy extends to target-selective probes. The substitutional inertness of rhenium(I) assists biological applications. A variety of donor atoms stably bind Re(I) and enable tagging with bioactive moieties. Among such ligands are isonitriles,16 imidazoles,17 and thioethers.18 The diimine carbonyls of rhenium(I) support varied applications.19-23 However, limited visible absorption restricts their photochemical uses. Absorption typically sets in below 450 nm. In a series of 20 rhenium(I) tricarbonyl diimine complexes, MLCT absorption maxima range from 330 nm (ε=4000 M-1 cm-1) to 402 nm (ε=810 M-1 cm-1).24 Combining the triplet photoproperties of rhenium carbonyl diimines with intense uptake of long-wavelength visible light (g600 nm) creates appealing prospects. Azadipyrromethenes are a broad ligand set25-30 with remarkable red-light-absorbing capabilities. Recent work on boron adducts has produced efficient fluorophores and drug candidates for human photodynamic therapy.31 The optically allowed electronic transitions of azadipyrromethene complexes are ligand centered,28,29 and the ligands’ absorption profile is preserved in metallo-complexes.
*To whom correspondence may be addressed. (1) Wrighton, M.; Morse, D. L. J. Am. Chem. Soc. 1974, 96, 998– 1003. (2) Giordano, P. J.; Wrighton, M. S. J. Am. Chem. Soc. 1979, 101, 2888–2897. (3) Giordano, P. J.; Fredericks, S. M.; Wrighton, M. S.; Morse, D. L. J. Am. Chem. Soc. 1978, 100, 2257–2259. (4) Smothers, W. K.; Wrighton, M. S. J. Am. Chem. Soc. 1983, 105, 1067–1069. (5) Caspar, J. V.; Sullivan, B. P.; Meyer, T. J. Inorg. Chem. 1984, 23, 2104–2109. (6) Tapolsky, G.; Duesing, R.; Meyer, T. J. J. Phys. Chem. 1989, 93, 3885–3887. (7) Tapolsky, G.; Duesing, R.; Meyer, T. J. Inorg. Chem. 1990, 29, 2285–2297. (8) Dattelbaum, D. M.; Martin, R. L.; Schoonover, J. R.; Meyer, T. J. J. Phys. Chem. A 2004, 108, 3518–3526. (9) Dattelbaum, D. M.; Omberg, K. M.; Hay, P. J.; Gebhart, N. L.; Martin, R. L.; Schoonover, J. R.; Meyer, T. J. J. Phys. Chem. A 2004, 108, 3527–3536. (10) Wallace, L.; Rillema, D. P. Inorg. Chem. 1993, 32, 3836–3843. (11) Lees, A. J. Chem. Rev. 1987, 87, 711–743. (12) Kober, E. M.; Sullivan, B. P.; Dressick, W. J.; Caspar, J. V.; Meyer, T. J. J. Am. Chem. Soc. 1980, 102, 7383–7385. (13) Kober, E. M.; Marshall, J. L.; Dressick, W. J.; Sullivan, B. P.; Caspar, J. V.; Meyer, T. J. Inorg. Chem. 1985, 24, 2755–2763. (14) Rom~ ao, C. C.; Royo, B. In Comprehensive Organometallic Chemistry III; Crabtree, R.; Mingos, M., Eds.; Elsevier Science: Boston, 2006; Vol. 5, Section 5.13. (15) Wei, L.; Babich, J. W.; Ouellette, W.; Zubieta, J. Inorg. Chem. 2006, 45, 3057–3066.
(16) Garcia, R.; Domingos, A.; Paulo, A.; Santos, I.; Alberto, R. Inorg. Chem. 2002, 41, 2422–2428. (17) Connick, W. B.; Di Bilio, A. J.; Hill, M. G.; Winkler, J. R.; Gray, H. B. Inorg. Chim. Acta 1995, 240, 169–173. (18) Hoepping, A.; Reisgys, M.; Brust, P.; Seifert, S.; Spies, H.; Alberto, R.; Johannsen, B. J. Med. Chem. 1998, 41, 4429–4432. (19) Di Bilio, A. J.; Crane, B. R.; Wehbi, W. A.; Kiser, C. N.; Abu-Omar, M. M.; Carlos, R. M.; Richards, J. H.; Winkler, J. R.; Gray, H. B. J. Am. Chem. Soc. 2001, 123, 3181–3182. (20) Miller, J. E.; Gradinaru, C.; Crane, B. R.; Di Bilio, A. J.; Wehbi, W. A.; Un, S.; Winkler, J. R.; Gray, H. B. J. Am. Chem. Soc. 2003, 125, 14220–14221. (21) Reece, S. Y.; Nocera, D. G. J. Am. Chem. Soc. 2005, 127, 9448– 9458. (22) Stoeffler, H. D.; Thornton, N. B.; Temkin, S. L.; Schanze, K. S. J. Am. Chem. Soc. 1995, 117, 7119–7128. (23) Yam, V. W.-W.; Lo, K. K.-W.; Cheung, K.-K.; Kong, R. Y.-C. J. Chem. Soc., Chem. Commun. 1995, 1191–1193. (24) Sacksteder, L.; Zipp, A. P.; Brown, E. A.; Streich, J.; Demas, J. N.; Degraff, B. A. Inorg. Chem. 1990, 29, 4335–4340. (25) Rogers, M. A. T. J. Chem. Soc. 1943, 596–597. (26) Killoran, J.; Allen, L.; Gallagher, J. F.; Gallagher, W. M.; O’Shea, D. Chem. Commun. 2002, 1862–1863. (27) Zhao, W.; Carreira, E. M. Angew. Chem., Int. Ed. 2005, 44, 1677– 1679. (28) Teets, T. S.; Partyka, D. V.; Esswein, A. J.; Updegraff, J. B. III; Zeller, M.; Hunter, A. D.; Gray, T. G. Inorg. Chem. 2007, 46, 6218–6220. (29) Teets, T. S.; Partyka, D. V.; Updegraff, J. B. III; Gray, T. G. Inorg. Chem. 2008, 47, 2338–2346. (30) Palma, A.; Gallagher, J. F.; M€ uller-Bunz, H.; Wolowska, J.; McInnes, E. J. L.; O’Shea, D. F. Dalton Trans. 2009, 273–279. (31) Gorman, A.; Killoran, J.; O’Shea, C.; Kenna, T.; Gallagher, W. M.; O’Shea, D. F. J. Am. Chem. Soc. 2004, 126, 10619–10631.
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Azadipyrromethenes are chelating nitrogen-donor ligands with superficial resemblances to bipyridines and phenanthrolines. These observations, combined with the favorable excited-state properties of rhenium(I) diimines, prompt investigation of (azadipyrromethene)rhenium(I) chromophores. Boron azadipyrromethenes are fluorescent; triplet emission is not apparent despite oxygen sensitization that must result from a triplet excited state.31,32 Azadipyrromethene complexes of d10 metals fluoresce with quantum yields below 1.5% at room temperature; group 12 bis(azadipyrromethenes) are virtually nonluminescent in the visible region. Thus, emission from d6 complexes of these ligands is a question in itself. Scheme 1 illustrates general preparative methods and a numbering scheme. THF complex 1 is a synthon for azadipyrromethene complexes of other ligands; displacement of the bound THF is facile. Attempts at recrystallizing 1 by vapor diffusion of n-pentane into neat pyridine or tetrahydrothiophene afford the respective solvate complexes in 99% and quantitative yields, respectively. For complex 4 the ligand is reacted with 1 in dichloromethane solution. Reaction of a methylene chloride solution of 1 with neat tert-butylisonitrile yields 5 (73% isolated). Complexes 1-4 are prepared in the open air, and 5 under a nitrogen atmosphere. All new compounds are isolated as air-stable blue or blue-violet solids. Diffraction-quality crystals have been obtained for complexes 2, 3, and 5.33 A thermal ellipsoid projection of 2 appears in Figure 1; those for 3 and 5, in the Supporting Information. Rhenium azadipyrromethene-nitrogen bond lengths range from 2.174(3) A˚ in 2 to 2.199(2) A˚ in 3; that to the pyridine nitrogen of 2 is 2.223(3) A˚. These interatomic distances are normal. For comparison, the crystallographically characterized salt fac-[Re(CO)3(1,10-phenanthroline)(pyridine)][Co(CO)4] shows Re-Nphen distances of 2.177(13) and 2.185(13) A˚ and a Re-Npyridine distance of 2.21(2) A˚.34 (32) Palma, A.; Tasior, M.; Frimannsson, D. O.; Vu, T. T.; MealletRenault, R.; O’Shea, D. F. Org. Lett. 2009, 11, 3638–3641. (33) (a) Crystallographic data for 2: crystal dimensions 0.37 0.09 0.08 mm3, space group P1, a=10.3966(12) A˚, b=11.9293(13) A˚, c= 13.3425(15) A˚; R=67.4560(10)°, β=85.7890(10)°, γ=86.3640(10)°; V= 1523.1(3) A˚3; Z=2; Fcalc=1.740 Mg/m3; μ(Mo KR)=4.038 mm-1; data measured on a Bruker AXS SMART APEX II CCD-based diffractometer (Mo KR, λ=0.710 73 A˚) at 100 ( 2 K; structure solved by direct methods, 17 493 reflections collected, 6637 unique reflections (Rint = 0.0247), data/restraints/parameters 6637/0/433, final R indices (I > 2σ(I)) R1 =0.0210 and wR2 =0.0491, R indices (all data) R1 =0.0238 and wR2=0.0563; largest difference peak and hole 0.876 and -1.190 e A˚-3. (b) Crystallographic data for 3: crystal dimensions 0.47 0.16 0.10 mm3, space group P1, a=11.9231(13) A˚, b=11.9357(13) A˚, c= 12.8357(14) A˚; R=63.9600(10)°, β=74.7280(10)°, γ=83.4900(10)°; V= 1583.3(3) A˚3; Z=2; Fcalc=1.693 Mg/m3; μ(Mo KR)=3.948 mm-1; data measured on a Bruker AXS SMART APEX II CCD-based diffractometer (Mo KR, λ=0.710 73 A˚) at 100 ( 2 K; structure solved by direct methods, 19 031 reflections collected, 7219 unique reflections (Rint = 0.0180), data/restraints/parameters 7219/0/424, final R indices (I > 2σ(I)) R1 =0.0215 and wR2 =0.0550, R indices (all data) R1 =0.0226 and wR2=0.0555; largest difference peak and hole 1.196 and -0.894 e A˚-3. (c) Crystallographic data for 5: crystal dimensions 0.39 0.34 0.32 mm3, space group P21/n; a=15.722(20) A˚, b=12.3980(18) A˚, c= 17.657(3) A˚; β=105.1090(10)°; V=3322.7(8) A˚3; Z=4; Fcalc=1.603 Mg/ m3; μ(Mo KR)=3.702 mm-1; data measured on a Bruker AXS SMART APEX II CCD-based diffractometer (Mo KR, λ=0.710 73 A˚) at 100 ( 2 K; structure solved by direct methods, 28 209 reflections collected, 5806 unique reflections (Rint = 0.0402), data/restraints/parameters 5806/0/ 436, final R indices (I > 2σ(I)) R1=0.0378 and wR2=0.1268, R indices (all data) R1=0.0404 and wR2=0.1324; largest difference peak and hole 0.898 and -1.688 e A˚-3. (34) Lucia, L. A.; Abboud, K.; Schanze, K. S. Inorg. Chem. 1997, 36, 6224–6234.
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Figure 1. (Top) Thermal ellipsoid projection (100 K, 50% probability) of pyridine complex 2. Hydrogen atoms are omitted for clarity. Unlabeled atoms are carbon. (Bottom) Packing diagram of 2 along b. A π-stacking interaction between pyridine ligands is evident. Scheme 1
The mean plane of the azadipyrromethene core tilts relative to the equatorial CCO-Re-CCO plane by 36.7° (CCO represents the two cis carbonyl carbons) in pyridine complex 2. Similar canting occurs for 3 and 5 and in the crystal
Communication
Figure 2. Absorption spectra of rhenium(I) complexes 1-5 (solid) and free HLa (dashed) in CHCl3 at room temperature.
structures of azadipyrromethenes bound to planar trigonal metal sites.28 The rhenium-thioether sulfur bond in THT complex 3 is 2.5415(7) A˚, somewhat longer than the 2.502(2) A˚ Re-S bonds found in fac-[Re(CO)3(THT)3].35 The rhenium-isonitrile carbon bond length in 5 is 2.108(6) A˚. In a mixed-ligand rhenium(I) tris(tert-butylisonitrile) complex, 1.927(8), 1.974(8), and 2.003(8) A˚ Re-C bonds were measured.36 Observed rhenium-carbon and carbon-oxygen bond lengths are unexceptional.37-42 Figure 2 depicts the absorption spectra of rhenium complexes 1-5 in chloroform solution. The spectral signatures of azadipyrromethenes are apparent: an intense (ε = 40 000-55 000 M-1 cm-1) structureless absorption peak at ca. 590 nm and pronounced absorptions (ε > 20 000 M-1 cm-1) near 290 nm. For comparison, the absorption spectrum of the ligand also appears (dashed line). The visible absorption maximum is more intense relative to that of free HLa. Higher-energy absorptions in the ultraviolet also intensify. Absorption spectra of other rhenium(I) azadipyrromethenes are similar. In contrast to diimine analogues, emission has not been detected below 800 nm in the new azadipyrromethene complexes at room temperature or 77 K. This observation echoes earlier findings in studies of d10 complexes of azadipyrromethene ligands. In these cases, emission is minimal, and the low Stokes shift indicates singlet parentage.28 Tricarbonyl rhenium(I) azadipyrromethene complexes are electroactive. Figure 3 depicts cyclic voltammograms of free tetraphenylazadipyrromethene and of 1 in 0.1 M Bu4NPF6 in THF, relative to the saturated calomel electrode. All voltammograms were measured with a 0.001 M concentration of azadipyrromethene or of complex. The ferrocene/ ferrocenium couple appeared as a reversible redox wave at 0.545 V. The free ligand HLa undergoes cleanly reversible (35) Franklin, B. R.; Herrick, R. S.; Ziegler, C. J.; C-etin, A.; Barone, N.; Condon, L. R. Inorg. Chem. 2008, 47, 5902–5909. (36) Heinekey, D. M.; Voges, M. H.; Barnhart, D. M. J. Am. Chem. Soc. 1996, 118, 10792–10802. (37) Winslow, L. N.; Rillema, D. P.; Welch, J. H.; Singh, P. Inorg. Chem. 1989, 28, 1596–1599. (38) Chen, P.; Curry, M.; Meyer, T. J. Inorg. Chem. 1989, 28, 2271– 2280. (39) Guilhem, J.; Pascard, C.; Lehn, J. M.; Ziessel, R. J. Chem. Soc., Dalton Trans. 1989, 1449–1454. (40) Hevia, E.; Perez, J.; Riera, V.; Miguel, D. Organometallics 2002, 21, 1966–1974. (41) Gibson, D. H.; Sleadd, B. A.; Yin, X.; Vij, A. Organometallics 1998, 17, 2689–2691. (42) Guerrero, J.; Piro, O. E.; Wolcan, E.; Feliz, M. R.; Ferraudi, G.; Moya, S. A. Organometallics 2001, 20, 2842–2853.
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Figure 3. Cyclic voltammograms (100 mV s-1) of 1.0 mM 1 and HLa (inset) with 0.10 M Bu4NPF6 supporting electrolyte. Peak potentials vs the saturated calomel electrode are indicated. Table 1. Voltammetric Data for New Complexes in THF (100 mV s-1, 0.1 M Bu4NPF6)
a
compound
E1/2 (V vs SCE)
HLa 1 2 3 4 5
-0.705, -1.325 -0.812, -1.529a -0.831, -1.670a -0.822, -1.541a -0.926, -1.751a -0.893, -1.648a
Irreversible; cathodic peak maxima are indicated.
reductions at -0.705 and -1.325 V. The voltammogram of 1 is more complicated, with the second reduction being irreversible and smaller features tentatively assigned as the redox events of decomposition products. The first reduction shifts cathodically by 107 mV in 1 compared to the free ligand, indicating that binding to the fac-[Re(CO)3(solv)]þ moiety widens the HOMO-LUMO gap, relative to free HLa. Table 1 gathers electrode potentials measured for all new compounds; similar conclusions apply. Density-functional theory calculations find that the azadipyrromethene dominates the frontier orbitals of 2. Figure S3 (Supporting Information) depicts an orbital correlation diagram of optimized 2. A harmonic frequency calculation verifies that the converged structure is a local minimum of the potential energy hypersurface. The highest-occupied Kohn-Sham orbital (HOMO) of 2 is 95% azadipyrromethene localized; the lowest-unoccupied orbital (LUMO) has 98% azadipyrromethene character. Percentage compositions derive from a Mulliken population analysis43 of the electron density. The calculated HOMO-LUMO gap is 1.45 eV, and a 0.95 eV energy gap separates the LUMO from the LUMOþ1, which is a pyridine π* combination. For comparison, the HOMO-LUMO gap of optimized HLa is narrower, 1.32 eV. The low-lying, isolated LUMO suggests electrochemical reversibility, as observed. The HOMOs-1, -2, and -3 have appreciable rhenium density that derives from the metal dxy, dyz, and dxz orbitals. Time-dependent density-functional theory calculations predict three optical transitions with sizable oscillator strengths at wavelengths > 550 nm. Singlet excited states are calculated at 1.86 eV (668 nm), 1.87 eV (662 nm), and 2.21 eV (562 nm). These consist of one-electron promotions that (43) Mulliken, R. S. J. Chem. Phys. 1955, 23, 1833–1840.
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undergo configuration interaction. The major contributors in each are LUMOrHOMO, LUMOrHOMO-1, and LUMOrHOMO-2 excitations. The most intense is the transition calculated at 562 nm, which consists mainly of LUMOrHOMO-2 (38%) and LUMOrHOMO (26%) transitions. Gaussian convolution44 (fwhm 3000 cm-1) of all transitions beyond 500 nm yields a single asymmetric peak at 563 nm, in fair agreement with the absorption maximum of 2 (591 nm) in CHCl3. Figure S3, Supporting Information, plots frontier orbitals of 2 as insets at right. All such orbitals have preponderant (g52%) azadipyrromethene character. The HOMO-1 and HOMO-2 bear 29% and 26% rhenium character, respectively. The visible absorptions of (tricarbonyl)azadipyrromethene complexes are primarily ligand centered with some metal-to-ligand charge-transfer admixture. In summary, we have prepared fac-tricarbonyl rhenium(I) azadipyrromethene complexes by reaction of fac-[Re(CO)3(H2O)3]Cl with the free chromophore in the presence of base. The absorption features of azadipyrromethenes and their boron chelates carry over to rhenium(I) complexes. A fifth ligand completes the coordination sphere: this can be an oxygen, nitrogen, carbon, or sulfur donor. Absorption profiles are insensitive to this fifth ligand. Its relative lability promotes rhenium attachment to Lewis bases in varied environments. Rhenium azadipyrromethenes and the free ligand undergo reductive electrochemistry. A second (44) (a) Gorelsky, S. I. SWizard Program, http://www.sg-chem.net/, University of Ottawa, Ottawa, Canada, 2009. (b) Gorelsky, S. I.; Lever, A. B. P. J. Organomet. Chem. 2001, 635, 187–196.
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reduction event that is reversible for the free ligand is irreversible in metallo-complexes, likely indicating participation of the fac-[Re(CO)3(L)]þ moiety. Time-dependent DFT calculations indicate that transitions to the LUMO account for red-light absorption of azadipyrromethene complexes. The optical and electrochemical properties of rhenium(I) azadipyrromethenes, including their use in light capture, are subjects of continuing investigation.
Acknowledgment. The authors thank the National Science Foundation (grant CHE-0749086 to T.G.G.) for support. M.P.W. was supported by National Science Foundation grant CHE-0748982 to J. D. Protasiewicz. The diffractometer at Case Western Reserve was funded by NSF grant CHE-0541766. T.G.G. is an Alfred P. Sloan Foundation Fellow. N.D. holds a Republic of Turkey Ministry of National Education fellowship. We thank Mr. T. J. Robilotto, CWRU, and Mr. T. S. Teets, Massachusetts Institute of Technology, for experimental assistance; Professor J. D. Protasiewicz, CWRU, for access to instrumentation; and Dr. S. Y. Reece, Sun Catalytix, for valuable discussion. Supporting Information Available: Experimental procedures and characterization data for new compounds. Thermal ellipsoid diagrams of 3 and 5; optimized Cartesian coordinates, partial Kohn-Sham orbital energy-level diagram of 2, and optimized bond lengths; crystallographic data in CIF format. This material is available free of charge via the internet at http:// pubs.acs.org.
