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Highly Emissive Hexanuclear Rhenium(III) Clusters Containing the Cubic Cores [Re6S8]2+ and [Re6Se8]2+ Thomas G. Gray,‡ Christina M. Rudzinski,† Daniel G. Nocera,*,† and R. H. Holm*,‡ Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, and Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
ReceiVed August 4, 1999 Substantial current interest1,2 attends hexanuclear rhenium(III) chalcogenide clusters with cubic [Re6(µ3-Q)8]2+ cores 3,4 (inset, Figure 1), lately available in soluble molecular form by the protocol of dimensional reduction.5,6 Oxidized anions [Re5IIIReIVS8Br6]3and [Re6Se8I6]3- 7 sustain reductive dehalogenation by AgI reagents or phosphines, affording cluster solvates [Re6Se8(solv)6]2+ and phosphine-substituted [Re6Q8(PEt3)6-nXn](2-n)+ (n ) 4-6; Q ) S, X ) Br; Q ) Se, X ) I).8-10 Diversely ligated clusters are now readily accessible, with adjustable microsymmetries secured using nonlabile phosphines as protecting groups. These clusters are isoelectronic with [M6(µ3-X)8X′6]2- (M ) Mo, W; X, X′ ) Cl, Br, I), which phosphoresce with established photoredox and energy-transfer capabilities.11-16 Although their terminal ligand substitution reactions are extensive, spectroscopic investigations have emphasized perhalogenated species.17 Photophysical understanding of cluster-based luminescence hinges upon identifying consequences of altered core compositions, peripheral ligand spheres, and local symmetries, ideally suiting [Re6Q8]2+ clusters for detailed experimentation. We report that these clusters emit vivid red phosphorescence upon UV-visible excitation, with microsecond-scale emissive lifetimes. Luminescence maxima and nonradiative decay constants are correlated through the energy gap law. Collected in Table 1 are luminescence data at 296 K for compounds 1-16.18 Depicted in Figure 1 are absorption and emission spectra of [Re6Se8(Me2SO)6]2+ (9) in dimethyl sulfoxide solution, selected as a representative example. Ill-resolved LMCT †
Massachusetts Institute of Technology. Harvard University. (1) Saito, T. J. Chem. Soc., Dalton Trans. 1999, 97. (2) Arratia-Pe´rez, R.; Herna´ndez-Acevedo, L. J. Chem. Phys. 1999, 110, 2529; 1999, 111, 168. (3) Lee, S. C.; Holm, R. H. Angew. Chem., Int. Ed. Engl. 1990, 29, 1304. (4) Perrin, A.; Sergent, M. New J. Chem. 1988, 12, 337. (5) Long, J. R.; McCarty, L. S.; Holm, R. H. J. Am. Chem. Soc. 1996, 118, 4603. (6) Long, J. R.; Williamson, A. S.; Holm, R. H. Angew. Chem., Int. Ed. Engl. 1995, 34, 226. (7) Zheng, Z.; Gray, T. G.; Holm, R. H. Inorg. Chem. 1999, 38, 4888. (8) Willer, M. W.; Long, J. R.; Holm, R. H. Inorg. Chem. 1998, 37, 328. (9) Zheng, Z.; Long, J. R.; Holm, R. H. J. Am. Chem. Soc. 1997, 119, 2163. (10) Zheng, Z.; Holm, R. H. Inorg. Chem. 1997, 36, 5173. (11) Maverick, A. W.; Gray, H. B. J. Am. Chem. Soc. 1981, 103, 1298. (12) Maverick, A. W.; Najdzionek, J. S.; MacKenzie, D.; Nocera, D. G.; Gray, H. B. J. Am. Chem. Soc. 1983, 105, 1878. (13) Zeitlow, T. C.; Hopkins, M. D.; Gray, H. B. J. Solid State Chem. 1985, 57, 112. (14) Mussell, R. D.; Nocera, D. G. J. Am. Chem. Soc. 1988, 110, 2674. (15) Jackson, J. A.; Turro´, C.; Newsham, M. D.; Nocera, D. G. J. Phys. Chem. 1990, 94, 4500. (16) Jackson, J. A.; Mussell, R. D.; Nocera, D. G. Inorg. Chem. 1993, 32, 4643. (17) Prokopuk, N.; Shriver, D. F. AdV. Inorg. Chem. 1998, 46, 1. (18) Emission spectra were corrected for grating and detector sensitivity using the correction curve generated from an NBS standard lamp using the appropriate red-sensitive Hamamatsu R316-02 photomultiplier tube cooled to -40 °C. Integrated emission quantum yields φem were determined in dry, deoxygenated solvents at 23 ( 2 °C relative to (Bu4N)2[Mo6Cl14] as a standard (φem ) 0.19, 436 nm excitation in acetonitrile). ‡
Figure 1. Absorption and corrected emission spectra (436 nm excitation, 296 K) of 9 in neat dimethyl sulfoxide. Inset: depiction of [Re6Q8L6]z clusters.
Table 1. Excited-State Parameters for Hexanuclear Rhenium(III) Chalcogenide Clustersa 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
clustersb
Eem (103 cm-1)
φemd
τo (µs)
[Re6S8Cl6](Bu4N)4 [Re6S8Br6](Bu4N)4 [Re6S8I6](Bu4N)4c [Re6S8(PEt3)6]Br2 [Re6Se8(PEt3)6]I2 [Re6Se8(MeCN)6](SbF6)2 c [Re6Se8(pyridine)6](SbF6)2 c [Re6Se8(DMF)6](SbF6)2 c [Re6Se8(Me2SO)6](SbF6)2 c trans-[Re6Se8(PEt3)4I2] cis-[Re6Se8(PEt3)4I2] [Re6Se8(PEt3)5I]I [Re6S8(PEt3)5Br]Br trans-[Re6S8(PEt3)4Br2] cis-[Re6S8(PEt3)4Br2] (Bu4N)mer-[Re6S8(PEt3)3Br3]
13.27 12.85 12.50 13.90 13.70 14.40 14.50 14.70 15.10 13.40 13.30 13.50 13.53 13.48 13.33 13.04
0.031 0.012 0.011 0.044 0.068 0.100 0.163 0.203 0.238 0.037 0.029 0.085 0.043 0.008 0.010 0.019
5.1 3.9 2.6 10.0 10.8 14.8 14.0 18.9 22.4 5.4 6.0 6.5 7.0 5.7 4.8 4.2
aE em is the corrected emission energy maximum, kr is the radiative decay rate, knr is the nonradiative decay rate, φem is the quantum yield for emission, and τo is the observed luminescence lifetime. b Parameters were measured in deoxygenated CH2Cl2 at 23 ( 2 °C unless otherwise noted. c Parameters for compounds 3 and 6-9 were measured in neat or excess ligand. d (10%; absorbance of all solutions was e 0.1. e (0.1 µs.
transitions5,19 dominate absorption spectra of [Re6Q8]2+ clusters in the region 210-500 nm. Excitation of any such band elicits broad, structureless emission extending from ca. 600 to 1000 nm. Maximal quantum yields φem occur for 6-9, with oxygen- or nitrogen-based terminal ligands; phosphine ligation in 5 reduces the [Re6Se8]2+ yield by roughly a third. Sulfido clusters exhibit (19) For a discussion of charge-transfer transitions within cyanide-bridged [Re6Q8]2+ (Q ) S, Se, Te) framework solids, cf. Shores, M. P.; Beauvais, L. G.; Long, J. R. J. Am. Chem. Soc. 1999, 121, 775.
10.1021/ic991195i CCC: $18.00 © 1999 American Chemical Society Published on Web 12/09/1999
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Inorganic Chemistry, Vol. 38, No. 26, 1999 5933
ln knr ) (lnβ - S) -
Figure 2. Energy gap plot of compounds 1-16; O, [Re6S8]2+; b, [Re6Se8]2+. 1, 2, 4, 5, 10-16 in neat CH2Cl2; 3 in CH2Cl2 containing 104fold excess of Bu4NI; and 6-9 in neat terminal ligand.
attenuated sensitivity to heavy atom termini, with φem diminishing as 4 ≈ 1 > 2 ≈ 3; progressive phosphine substitution of 2 exerts erratic effects. Luminescence from compounds 1, 2, 4, 5, and 10-16 decays monoexponentially with the lifetimes shown in Table 1. Photophysical properties of five compounds (3 and 6-9) contrast with the remainder and are notably medium-sensitive. In the absence of excess ligand, these clusters display marginal quantum yields (
