Through-Bond versus Through-Space T1 Energy Transfers in

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Organometallics 2010, 29, 317–325 DOI: 10.1021/om900584n

317

Through-Bond versus Through-Space T1 Energy Transfers in Organometallic Compound-Metalloporphyrin Pigments Diana Bellows,† Thomas Goudreault,† Shawkat M. Aly,† Daniel Fortin,† Claude P. Gros,‡ Jean-Michel Barbe,*,‡ and Pierre D. Harvey*,† †

D epartement de Chimie, Universit e de Sherbrooke, Sherbrooke, Qu ebec, Canada and ‡ ICMUB (UMR 5260), Universit e de Bourgogne, Dijon, France Received July 7, 2009

The preparation and characterization of two d9-d9 M2-bonded Pt2(dppm)2(CtCC6H4-M(P))2 complexes (where M = Zn or Pd, and P = diethylhexamethylporphyrin) were achieved. The central [Pt2(dppm)2(CtCC6H4)2] organometallic unit appears to be an independent chromophore and is suspected to be luminescent at 77 K (in 2MeTHF) in the porphyrin-containing complexes, as this is the case for the unfunctionalized Pt2(dppm)2(CtCPh)2 parent compound. However, when this spacer is connected (by a single C-C bond) to either M(P) (M = Zn, Pd), even in the absence of conjugation (as the computed dihedral angle between the C6H4 and porphyrin planes is ∼84.5°), total quenching of the luminescence of the [Pt2(dppm)2(CtCC6H4)2] central unit is observed. Only T1-Tn absorption bands of the metalloporphyrins are observed in the nanosecond transient absorption spectra, indicating the absence of a charge-separated state in these systems, thus indicating the presence of T1 species only. Consequently, efficient T1 energy transfers from [Pt2(dppm)2(CtCC6H4)2] donor to the M(P) acceptor occur. Using a lower limit of measurable value of Φe < 0.0001, the rate for T1 energy transfers is estimated to be >108 s-1. This rate indicates a fast process, which is 4 orders of magnitude larger than as recently reported in electrostatically held dyads between the unsaturated clusters M3(dppm)3(CO)2þ (M = Pd, Pt) and metalloporphyrins (M0 (TPP-CO2-); M0 = Zn, Pd; TPP = tetraphenylporphyrin) as the donor and acceptor, respectively (Aly, S. M.; Ayed, C.; Stern, C.; Guilard, R.; Abd-El-Aziz, A. S.; Harvey, P. D. Inorg. Chem., 2008, 47, 99309940). In the latter cases, the through-space T1 energy transfer rates were on the order of 104 s-1. Introduction Energy transfer is one of the most basic nonradiative processes of the antenna effect in the photosynthetic membrane of plants, algae, and photosynthetic bacteria.1 This process is also important in photovoltaic cells for the same reason of energy migration across a material. Recently, investigations for the potential use of organometallics in photocells and light emitting diodes (LEDs) were made, as many exhibit strong emissive properties and good thermal stability.2 In this respect, our group investigated a series of metal-containing polymers, notably those exhibiting the general structures (-A-B-)n where A is the trans-C6H4Ct CPtL2CtCC6H4 spacer (L = PEt3 or PBu3) and B is a tetrasubstituted quinone diimine (substituent=methyl, methoxy, *To whom correspondence should be addressed. (P.D.H.) Tel: 819-8217092. Fax: 819-821-8017. E-mail: [email protected]. (J.-M.B.) Tel: 33 (0)3 80 39 61 19. E-mail: [email protected]. (1) Light-Harvesting Antennas in Photosynthesis: Advances in Photosynthesis and Respiration, Vol. 13; Green, B. R.; Parson, W. W., Eds.; Kluwer: Boston, 2003. (2) Wong, W.-Y. J. Inorg. Organomet. Polym. Mater. 2005, 15, 197– 219. (3) Gagnon, K.; Berube, J.-F.; Bellows, D.; Caron, L.; M. Aly, S. M.; Wittmeyer, A.; Abd-El-Aziz, A. S.; Fortin, D.; Harvey, P. D. Organometallics 2008, 27, 2201–2214. (4) Fortin, D.; Clement, S.; Gagnon, K.; Berube, J.-F.; Harvey, P. D. Inorg. Chem. 2009, 48, 446–454. r 2009 American Chemical Society

and ethoxy)3,4 and where A is the trans-ArCtCPtL2-Ct CAr spacer (Ar = 3,6-(N-ethyl)carbazoyl) and B is the 2,7-(9,90 -di(n-butyl))fluorene (Chart 1).5 The conclusions of these investigations were, first, that the photophysical properties in the quinone diimine series, notably with respect to the ground-state charge transfer behavior, a nonluminescent state, were strongly dependent on a dihedral angle made by the quinone diimine and C6H4 average plane. Second, the polymers were conjugated in the ground state but localized in the excited states, hence opening the door to the possibility of both electron and energy transfers between the different units A and B. In addition, (-A-B-)n organometallic/coordination polymers6 using NtC assembling ligands and the [Pt2(dppm)2]2þ fragment (dppm = Ph2PCH2PPh2), in place of the trans-PtL2 unit, were made (Chart 2).7 The M2-bonded [Pt2(dppm)2]2þ moiety was chosen on the basis of two (5) Aly, S. M.; Ho, C.-L.; Wong, W.-Y.; Abd-El-Aziz, A. S.; Harvey, P. D. Chem.;Eur. J. 2008, 14, 8341–8352. (6) (a) Harvey, P. D. In Frontiers in Transition Metal-Containing Polymer; Abd-El-Aziz, A. S.; Manners, I., Eds.; Wiley Interscience, John Wiley and Sons Inc.: New York, 2007; pp 321-368. (b) Harvey, P. D. In Inorganic and Organometallic Macromolecules; Design and Applications; Abd-El-Aziz, A. S.; Carraher, C. E., Jr.; Pittman, C. U., Jr.; Zeldin, M., Eds.; Springer: New York, 2008; pp 71-107. (7) Berube, J.-F.; Gagnon, K.; Fortin, D.; Decken, A.; Harvey, P. D. Inorg. Chem. 2006, 45, 2812–2823. Published on Web 12/29/2009

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Organometallics, Vol. 29, No. 2, 2010 Chart 1

Chart 2

electrochemical investigations that demonstrated that the Pt-Pt bond is a good electronic communicator between two redox centers (i.e., ferrocenyls),8 with respect to the trans-PtL2 unit.9 The NtC bridge as electronic communicator was tested in the PCP-NC-[Pt2(dppm)2Cl]þ complex (PCP = paracyclophane) where the PCP-NC residue turned out to be conveniently both fluorescent and phosphorescent, and consequently both S1 and T1 energy transfers (PCP*fPt2) could be tested.10 These emissions were indeed affected by heavy atom effects, but were not completely quenched, indicating that electronic communication across the NtC bridge is modest.10 The through-space T1 energy transfer in electrostatically held dyads between the unsaturated clusters M3(dppm)3(CO)2þ (M = Pd, Pt) and the metalloporphyrins M0 (TPPCO2-) (M0 = Zn, Pd; TPP = tetraphenylporphyrin) as the donor and acceptor, respectively, was also investigated (Chart 3).11 The T1 energy transfer rates are on the order of 104 s-1, which is relatively slow in comparison with other covalently held porphyrin-containing dyads (106-108 s-1).12 Bearing these previous findings in mind, the preparation, characterization, and photophysical properties of the Pt2(dppm)2(CtCC6H4-M(P))2 complexes (M = Zn (4), Pd (5); P = diethylhexamethylporphyrin), where the [Pt2(dppm)2(CtCC6H4)2] central unit acts as a nonconjugated chromophore with the M(P) moieties, are now reported. To our knowledge, complexes 4 and 5 are the first examples of porphyrin units bearing a Pt2-bonded species.13 In complexes 4 and 5, the [Pt2(dppm)2(CtCC6H4)2] luminescence is found totally quenched by T1 energy transfer, demonstrating the efficiency of through-bond processes over through-space.

Experimental Section Materials. Dichlorobis(bis(diphenylphosphino)methane)dichloroplatinum(I) (1),14 5-(4-ethynylbenzene)-13,17-diethyl-2,3,7,8,12, 18-hexamethylpalladium(II) porphyrin (3),15 5-(4-ethynylbenzene)(8) Yip, J. H. K.; Wu, J.; Wong, K.-Y.; Ho, K. P.; Pun, C.S.-N.; Vittal, J. J. Organometallics 2002, 21, 5292. (9) Osella, D.; Gobetto, R.; Nervi, C.; Ravera, M.; D’Amato, R.; Russo, M. V. Inorg. Chem. Commun. 1998, 1, 239–245. (10) Clement, S.; Aly, S. M.; Fortin, D.; Guyard, L.; Knorr, M.; Abd-El-Aziz, A. S.; Harvey, P. D. Inorg. Chem. 2008, 47, 10816–10824. (11) Aly, S. M.; Ayed, C.; Stern, C.; Guilard, R.; Abd-El-Aziz, A. S.; Harvey, P. D. Inorg. Chem. 2008, 47, 9930–9940. (12) See for example: Poulin, J.; Stern, C.; Guilard, R.; Harvey, P. D. Photochem. Photobiol. 2006, 82, 171–176. (13) Suijkerbuijk, B. M. J. M.; Klein Gebbink, R. J. M. Angew. Chem., Int. Ed. 2008, 47, 7396–7421. (14) Grossel, M. C.; Batson, J. R.; Moulding, R. P.; Seddon, K. R. J. Organomet. Chem. 1986, 304, 391–423.

Bellows et al. 13,17-diethyl-2,3,7,8,12,18-hexamethylzinc(II) porphyrin (2),15 tetrakis(4-ethynylbenzene)bis(bis(diphenylphosphino)methane)platinum(II) (7),16 and bis(4-ethynylbenzene)bis(bis(diphenylphosphino)methane)dichloroplatinum(I) (6)17 were prepared according to literature procedures. The following materials were purchased from commercial suppliers: phenylacetylene and sodium methoxide (Aldrich), dichloromethane (EMD), potassium carbonate (Fisher), hexane (ACP), methanol (Commercial Alcohols Inc.). All solvents were purified according to literature procedures.18 All reactions were carried out in an argon atmosphere using Schlenk techniques. All reaction vessels were also flame-dried before use, to eliminate moisture. Bis[5-(4-ethynylbenzene)-13,17-diethyl-2,3,7,8,12,18-hexamethylmetal(II)-porphyrin]bis(bis(diphenylphosphino)methane)diplatinum(I), metal = Zn (4), Pd (5). General procedure using M = Pd as an example: The 5-(4-ethynylbenzene)-13,17-diethyl-2,3,7,8, 12,18-hexamethylpalladium(II) porphyrin (0.0547 g, 0.0836 mmol) was dissolved in a minimum amount of tetrahydrofuran (∼20 mL). To the above mixture was added an excess of NaOMe (0.0226 g, 0.4179 mmol) dissolved in MeOH (∼30 mL), and the reaction was placed under argon and stirred for 30 min, after which complex 1, Pt2(dppm)2Cl2 (0.0514 g, 0.0418 mmol), dissolved in MeOH was added to the reaction vessel. The reaction was left to reflux overnight. A purple-pink precipitate was formed, collected by filtration, and washed twice with cold methanol. The product, [Pt2(μ-dppm)2(CtC(porph(M))2], here M = Pd, was then dried on the vacuum pump overnight. (M=Pd, 5) Yield: 69% (0.0724 g). 1H NMR (CDCl3): δ 10.01 (s, 4H, H-meso), 9.99 (s, 2H, H-meso), 7.80-7.77 (m, 16 H, CHar), 7.51 (d, 4H, JH-H = 7.9 Hz, trans-C6H4), 7.43 (d, 4H, JH-H = 7.4 Hz, trans-C6H4), 7.40 - 7.29 (m, 16H, CHar), 6.61 (d, 8H, JH-H =8.0 Hz, CHar), 5.09 (m, br, 4 H, PCH2P, 3JP-H = 68 Hz), 4.03 (q, 8H, 3JH-H = 7.7 Hz, CH2), 3.51 (s, 12H, CH3), 3.51 (s, 12H, CH3), 2.40 (s, 12H, CH3), 1.89-1.78 (m, 12H, CH2CH3). 31P{1H} NMR (CD2Cl2): δ 2.75 ppm (1JPt-P = 2836 Hz). IR (KBr) δ: 2115 cm-1 (CtC). Anal. Calcd (%) for C126H114N8P4Pt2Pd2: C 61.34, H 4.66, N 4.54. Found: C 61.67, H 4.51, N 4.57. HR-MS (MALDI-TOF): m/z 2464.5429 [M]þ•; 2464.5510 calcd for C126H114N8P4Pt2Pd2. (M=Zn, 4) Yield: 72% (0.0724 g). 1H NMR (CDCl3): δ 10.01 (s, 4H, H-meso), 9.88 (s, 2H, H-meso), 7.80-7.79 (m, 16H, CHar), 7.50 (d, 4H, JH-H = 8.0 Hz, trans-C6H4), 7.43 (d, 4H, JH-H =7.4 Hz, trans-C6H4), 7.39-7.27 (m, 16H, CHar), 6.58 (d, 8H, JH-H =8.0 Hz, CHar), 5.15 (m, br, 4 H, PCH2P, 3JP-H =69 Hz), 4.02 (q, 8H, 3JH-H =7.6 Hz, CH2), 3.58 (s, 12H, CH3), 3.49 (s, 12H, CH3), 2.38 (s, 12H, CH3), 1.6-1.78 (m, 12H, CH2CH3). 31 P{1H} NMR (CD2Cl2): δ 2.94 ppm (1JPt-P = 2860 Hz). IR (KBr) δ: 2084 cm-1 (CtC). Anal. Calcd (%) for C126H114N8P4Pt2Zn2: C 63.45, H 4.82, N 4.70. Found: C 63.81, H 4.67, N 4.47. MS (MALDI-TOF): m/z 2380.42 [M]þ•; 2380.59 calcd for C126H114N8P4Pt2Zn2. Instruments. The mass spectra were obtained on a Bruker Daltonics Ultraflex II spectrometer at the “Plateforme d’Analyse Chimique et de Synthese Moleculaire de l’Universite de Bourgogne (PACSMUB)” in the MALDI/TOF reflectron mode using dithranol as a matrix. High-resolution mass measurements (HR-MS) were carried out in the same conditions as previously using PEG ion series as internal calibrant. The NMR spectra were acquired on a Bruker AC-300 spectrometer (15) Bellows, D.; Aly, S. M.; Gros, C. P.; El Ojaimi, M.; Barbe, J.-M.; Abd-El-Aziz, A. S.; Guilard, R.; Harvey, P. D. Inorg. Chem. 2009, 48, 7613–7629. (16) Yam, V. W.-W.; Yu, K.-L.; Wong, K. M.-C.; Cheung, K.-K. Organometallics 2001, 20, 721–726. (17) Irwin, M. J.; Jia, G.; Vittal, J. J.; Puddephatt, R. J. Organometallics 1996, 15, 5321–5329. (18) (a) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purifications of Laboratory Chemicals; Pergamon Press: Oxford, 1966. (b) Gordon, A. J.; Ford, R. A. The Chemist’s Companion, a Handbook of Practical Data, Techniques, And References; Wiley: New York, 1972; p 436.

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Chart 3

Chart 4

(1H 300.15 MHz and 31P 121.50 MHz) using the solvent as a chemical shift standard, except in 31P NMR, where the chemical shifts are relative to D3PO4 85% in D2O. All chemical shifts (δ) and coupling constants (J) are given in ppm and hertz, respectively. The spectra were measured from freshly prepared samples. The infrared spectra were acquired on a Bomen FT-IR MB series spectrometer equipped with a baseline-diffused reflectance. The UV-visible spectra were recorded on a HewlettPackard diode array model 8452A. The emission spectra were obtained using a double monochromator Fluorolog 2 instrument from Spex. The emission lifetimes were measured on a TimeMaster model TM-3/2003 apparatus from PTI. The source was a nitrogen laser with high-resolution dye laser (fwhm ∼1400 ps), and the fluorescence lifetimes were obtained from deconvolution or distribution lifetimes analysis. The uncertainties were about 10-50 ps based on multiple measurements (3-5). Some of the phosphorescence lifetimes were performed on a PTI LS-100 using a 1 μs tungsten-flash lamp (fwhm ∼1 μs). The flash photolysis spectra and the transient lifetimes were measured with a Luzchem spectrometer using the 355 nm line of a YAG laser from Continuum (Serulite) and the 530 nm line from an OPO module pump by the same laser (fwhm = 13 ns). Quantum yield measurements were performed in 2MeTHF at 77 and 298 K. Three different measurements (i.e., different solutions) were prepared for each photophysical datum (quantum yields and lifetimes). For 298 K measurements samples were prepared under an inert atmosphere (in a glove-

box, PO2 < 50 ppm). For both temperatures, the sample and standard concentrations were adjusted to obtain an absorbance of 0.05 or less. This absorbance was adjusted to be the same as much as possible for the standard and the sample for a measurement. Each absorbance value was measured 10 times for better accuracy in the measurements of the quantum yields. The references used for quantum yield were TPP(Pd) (Φ = 0.14)19 for measuring emission quantum yields of 4 and 5, and 9,10diphenylanthracene (Φ = 1.0)20 for measuring the emission quantum yield of 6. Computations. Calculations were performed with Gaussian 0321 at the Universite de Sherbrooke with Mammouth MP super computer supported by le Reseau Quebecois de Calculs de Haute Performances. The DFT22-25 and TD-DFT26-28 were calculated with the B3LYP29-31 method. 3-21G*32-37 basis sets (19) Bolze, F.; Gros, C. P.; Harvey, P. D.; Guillard, R. J. Porphyrins Phthalocyanines 2001, 5, 569–574. (20) Eaton, D. F. Pure Appl. Chem. 1988, 60, 1107–1114. (21) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, J. T.; Kudin, K. N., Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B., Cossi, M., Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision C.02; Gaussian, Inc.: Wallingford, CT, 2004. (22) Hohenberg, P; W. Kohn, W. Phys. Rev. 1964, 136, B864–B871. (23) Kohn, W.; Sham, L. J. Phys. Rev. 1965, 140, A1133–A1138. (24) The Challenge of d and f Electrons; Salahub, D. R.; Zerner, M. C., Eds.; ACS: Washington, D.C., 1989. (25) Parr, R. G.; Yang, W. Density-Functional Theory of Atoms and Molecules; Oxford Univ. Press: Oxford, 1989. (26) Stratmann, R. E.; Scuseria, G. E.; Frisch, M. J. J. Chem. Phys. 1998, 109, 8218–8224. (27) Bauernschmitt, R.; Ahlrichs, R. Chem. Phys. Lett. 1996, 256, 454–464. (28) Casida, M. E.; Jamorski, C.; Casida, K. C.; Salahub, D. R. J. Chem. Phys. 1998, 108, 4439–4449. (29) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652. (30) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 785–789. (31) Miehlich, B.; Savin, A.; Stoll, H.; Preuss, H. Chem. Phys. Lett. 1989, 157, 200–206. (32) Binkley, J. S.; Pople, J. A.; Hehre, W. J. J. Am. Chem. Soc. 1980, 102, 939–947. (33) Gordon, M. S.; Binkley, J. S.; Pople, J. A.; Pietro, W. J.; Hehre, W. J. J. Am. Chem. Soc. 1982, 104, 2797–2803. (34) Pietro, W. J.; Francl, M. M.; Hehre, W. J.; Defrees, D. J.; Pople, J. A.; Binkley, J. S. J. Am. Chem. Soc. 1982, 104, 5039–5048.

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Figure 1. Ground-state optimized geometry of 4 (DFT; B3LYP). Computed distances (A˚): Pt-Pt = 2.708, Pt-C = 2.051, 2.031; Ct C = 1.228, 1.227; Pt-P = 2.286, 2.289, 2.285, 2.288. Angles (deg): Pt-Pt-C = 177.4, 179.1; Pt-CtC = 179.0, 178.2; P-Pt-P = 179.0, 179.9; Pt-Pt-P = 87.6, 87.1, 90.3, 91.2. Scheme 1a

Table 1. UV-Visible Absorption and Emission Data and Lifetimes of 1-3 at 77 K in 2MeTHF compound Pt2(dppm)2Cl2 (1) Pt2(dppm)2(CtCPh)2 (6) Pt2(dppm)2(CtCPh)4 (7)

a

i = MeONa/THF/MeOH.

were used for Zn, C, H, N, and P, and Stuttgart-Koeln MCDHF RSC ECP38 and VDZ effective core potentials39-42 were used for Pt. The predicted phosphorescence wavelengths were obtained by TD-DFT/Triplets calculation directly.43 The calculated absorption spectra and related MO contributions were obtained from the TD-DFT/Singlets output file and gausssum2.1.44 The methodology was calibrated against the optimized geometry of the related X-ray characterized [Pt2(μdppm)2(CN-tBu)2]2þ cation (Supporting Information).7

Results and Discussion Syntheses and Characterization. The 5-(4-ethynylbenzene)13,17-diethyl-2,3,7,8,12,18-hexamethylzinc(II) porphyrin (2) and -palladium(II) porphyrin (3) ligands react with the starting material 1 in a 2:1 ratio to generate the corresponding adducts 4 and 5, respectively, in the presence of sodium methoxide (Scheme 1). Their unambiguous characterization was performed using 1H (proton integration) and 31P (chemical shift) NMR, MALDI-TOF (molecular ion), chemical analyses, and their comparison with those of the model (35) Dobbs, K. D.; Hehre, W. J. J. Comput. Chem. 1986, 7, 359–378. (36) Dobbs, K. D.; Hehre, W. J. J. Comput. Chem. 1987, 8, 861–879. (37) Dobbs, K. D.; Hehre, W. J. J. Comput. Chem. 1987, 8, 880–893. (38) Figgen, D.; Peterson, K. A.; Dolg, M.; Stoll, H. J. Chem. Phys. 2009, 130, 164108-1-12. (39) Binkley, J. S.; Pople, J. A.; Hehre, W. J. J. Am. Chem. Soc. 1980, 102, 939–947. (40) Stevens, W. J.; Basch, H.; Krauss, M. J. Chem. Phys. 1984, 81, 6026–6033. (41) Stevens, W. J.; Krauss, M.; Basch, H.; Jasien, P. G. Can. J. Chem. 1992, 70, 612–630. (42) Martyna, G. J.; Deng, Z.; Klein, M. L. J. J. Chem. Phys. 1993, 98, 5555–5563. (43) Zhou, X.; Pan, Q.-J.; Li, M.-X.; Zhang, H.-X.; Tang, A.-C. Sci China, Ser. B: Chem. 2007, 50, 599–606. (44) O’Boyle, N. M.; Tenderholt, A. L.; Langner, K. M. J. Comput. Chem. 2008, 29, 839–845.

absorption (λ nm)

emission (λmax)

τe (μs) (at λobs nm)

270, 318sh, 360sh 345sh, 377, 405sh 328, 365

670

2.64 ( 0.05 (640)

645

5.60 ( 0.10 (630)

456

7.04 ( 0.02 (456)

630

1.3 ( 0.5 (630)

compound 6. In order to secure this identification, compound 7 was synthesized and fully characterized (including by X-ray crystallography) in our laboratory. No crystal suitable for X-ray study could be obtained. Instead, compound 4 was optimized by DFT (B3LYP) in order to address the relative orientation of the 4-C6H4 group with respect to the porphyrin ring (Figure 1). The methodology was tested against the related X-ray characterized [Pt2(dppm)2(CN-tBu)2]2þ cation.7 The calculated d(Pt-Pt) distance is 2.65, whereas the experimental value is found at 2.64(1) A˚, hence representing a difference of 0.0-0.2 A˚ based on the X-ray datum uncertainty (0.3% at most). This gas phase distance appears reasonable. A more detailed comparison of selected structural parameters (calculated using different basis sets versus experimental) for this compound is available in the Supporting Information (SI), and again the comparison is reasonable and overall better than other common basis sets such as Lanl2dz and SBKJC. The dihedral angles in compound 4 are calculated to be 84.3° and 84.6°, demonstrating the strong steric effect of the two methyl groups at the β-positions on the relative orientation of the 4-C6H4-aryl. So it is anticipated that conjugation will be minimal all across the CtC-Pt-Pt-CtC bonds. Spectroscopy and Photophysics. The UV-visible and emission signatures of 1 and 6, as well as their respective emission lifetimes, were examined (Table 1, Figure 2). The emission spectrum of 1 in 2MeTHF at 77 K is characterized by a broad red-shifted band at 670 nm (τe = 2.64 ( 0.05 μs). Based on previous DFT computations on the analogue Pd2(dppm)2Cl245 and [Pt2(dppm)2(CN-tBu)2]2þ,7 this emission arises from a dσdσ* triplet state. The broad shape of this emission is an indication of a large excited-state distortion, consistent with the large change in M-M distance in the excited state from bond order one (S0) to zero (T1). The absorption and emission spectra of 6 differ from that of 1. The absorption spectrum of 1 is more red-shifted, and the long-lived emission band of 6 (τe =5.60 ( 0.10 μs) is more (45) Ayed, T.; Guihery, N.; Tangour, B.; Barthelat, J.-C. Theor. Chem. Acc. 2006, 116, 497–504.

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Figure 2. UV-visible (black), emission (blue), and excitation spectra (red) of 1 (up) and 6 (bottom) in 2MeTHF at 77 K.

blue-shifted, indicating that the excited states are different. Indeed, the DFT computations below show the contribution of the π-systems of the CtCPh residues to the frontier MOs. The nature of the lowest energy S1 excited states of compound 6 was addressed by DFT and TDDFT computations. In the absence of an X-ray structure, particularly for the fragment describing the CtC-Pt-Pt-CtC unit, multiple geometry optimization was performed (computed distances (A˚): Pt-Pt = 2.708, Pt-C = 2.054, 2.026; CtC = 1.228, 1.227; Pt-P = 2.292, 2.293, 2.284, 2.284; calculated angles (deg): Pt-Pt-C = 176.5, 176.8; Pt-CtC = 179.0, 178.2; P-Pt-P = 179.6, 179.8; Pt-Pt-P = 86.6, 86,1, 91.2, 91.7). The Pt-Pt is longer than that of the test compound [Pt2(μ-dppm)2(CN-tBu)2]2þ (with a Pt-Pt distance of 2.64 A˚; SI),7 suggesting a weak bonding interaction. This bond distance of 2.708 A˚ or so is not unprecedented since another weak d9-d9 bond (2.72 ( 0.05 A˚) was reported earlier for Pd2(dmb)2Cl2 (dmb=1,8-diisocyano-p-menthane).46 It was deduced that an important ring stress exists within the Pd-Pd-dmb ring since this compound undergoes a facile ring-opening polymerization.47 Moreover, the CtC-PtPt-CtC fragment turns out to be very sensitive toward oxidative additions of HCtCR to produce the corresponding A-frame derivatives.48 This experimental result also witnesses the relative weakness of this Pt-Pt bond. During the course of this study, it was brought to our attention that the presence of solvent molecules around the chromophore could influence the bond length of the Pt-Pt link. We find that the Pt-Pt bond distances for 6 in the gas phase and in the presence of THF as the solvent are 2.708 and 2.713 A˚, respectively, which represent an increase of 0.005 A˚, which is very small. This perturbation has practically no effect on the calculated spectrum discussed below. (46) Harvey, D. P.; Murtaza, Z. Inorg. Chem. 1993, 32, 4721–4729. (47) Sicard, S.; Berube, J.-F.; Samar, D.; Massaoudi, A.; Lebrun, F.; Fortin, J.-F.; Fortin, D.; Pierre, D.; Harvey, D. P. Inorg. Chem. 2004, 43, 5321–5334. (48) Yam, V. W.-V.; Chan, L.-P.; Lai, T.-F. Organometallics 1993, 12, 2197–2202.

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Using the optimized geometry, the MO representations are computed. The first 10 relevant frontier MOs are shown in Figure 3. The HOMO and HOMO-1 include the Pt dxy orbitals along with a π-system of the -CtCPh ligands. The Pt 3 3 3 Pt interactions are antibonding, making this MO a dπ*(Pt2). The HOMO-2 exhibits an expected dσ(Pt2) MO issued from the bonding dx2-y2 interactions.33 The MOs included between the LUMO and LUMOþ6 are primarily π-systems of the dppm-phenyl groups with some minor combinations of σ*(P-CPh) orbitals. On occasions, the Pt dx2-y2 contribution is visible in the MO representation. These seven empty MOs are spread over only 0.005 au. One of the fundamental questions for such a chromophore that contains a d9-d9 Pt-Pt bond is, is there a low-lying dσdσ* electronic transition that is often encountered in such class of compounds? This question is important because there would be the possibility of placing such a state as the lowest energy triplet state. As it is suggested in Figure 3, the lowest energy electronic transitions are all MLL0 CT type (metal/ ligand-to-ligand charge transfer) not dσdσ* nor dπ*dσ*, as the dσ* orbital must be placed at higher energy. The two lowest energy electronic transitions computed by TDDFT turn out to be quasi-degenerated and heavily mixed between the HOMO and HOMO-1 (Table 2). The seven first-computed electronic transitions are again of the same nature as stated above (metal/ligand-to-ligand charge transfer where metal/ligand=Pt2(CtCPh)2 and L0 = dppm). These transitions are spread over 2700 cm-1 (over 50 nm in this region), and the possibility of an upper triplet dσdσ* excited state is essentially nil. By calculating enough transitions (here over 40), a graph of the absorptivity or oscillator strengths as a function of wavelength generates a bar graph (see the blue bars in Figure 4). By assigning a thickness to the bar, a spectrum can be generated (see the red trace in Figure 4), hence generating a calculated spectrum. The comparison of the generated spectrum (red trace in Figure 4) with the experimental one (bottom of Figures 1 and 4, black trace) is good (the peak maxima match reasonably well). The addition of THF solvent molecules into the calculations did not sensitively perturb the spectrum. The triplet state of 6 was also addressed both in the gas phase and in the presence of THF. The comparison of the Pt-Pt distance and Pt-Pt-C angle between the ground and triplet state shows a slight increase in Pt-Pt distance but a major distortion in angle (gas phase: Pt-Pt = 2.708 A˚; Pt-Pt-C = 176.7° in S0; Pt-Pt = 2.743 A˚; Pt-Pt-C = 115.1° in T1; with THF: Pt-Pt=2.713 A˚; Pt-Pt-C=178.4° in S0; Pt-Pt = 2.750 A˚; Pt-Pt-C = 116.3° in T1). The presence of THF does influence greatly the structural parameters. The presence of a large excited distortion is experimentally corroborated by the large bandwidth of the emission spectrum of 6 (Figure 2). The computed angle of about 115° is similar to that observed for parent A-frame compounds.36 The calculated total energy of the S0 and T1 states is respectively -4130.1918 and -4130.1364 au in the gas phase and -4130.2291 and -4130.1631 au in the presence of THF. So the energy differences between the S0 and T1 states are 0.055 and 0.066 au, placing the transition energy approximatively at 822 and 689 nm, respectively. Experimentally, the maximum is observed at 650 nm in 2MeTHF at 77 K. This comparison appears reasonable. Because of the resemblance of the band shape and band positions of the emission in 6 with those of 7,16,34 the

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Figure 3. MO drawings of the first 10 frontier MOs for compound 6. The units are in au. Table 2. Characteristics of the Seven First-Computed Spin-Allowed Electronic Transitions for 6 ν/cm-1

λ /nm

osc. str.

major contributions to the computed electronic transitions

24 439 24 477 24 966 25 495 25 916 26 056 26 270 26 974 27 211 27 259

409.2 408.6 400.5 392.2 385.9 383.8 380.6 370.7 367.5 366.9

0.0529 0.1179 0.0003 0.0012 0.0073 0.1640 0.0116 0.1150 0.0011 0.0016

H-1fLUMO (55%), HOMOfLUMO (30%) H-1fLUMO (27%), HOMOfLUMO (58%) HOMOfLþ1 (68%), HOMOfLþ2 (11%) HOMOfLþ1 (18%), HOMOfLþ2 (61%), HOMOfLþ6 (13%) H-1fLþ1 (35%), HOMOfLþ3 (52%) H-1fLþ1 (51%), HOMOfLþ3 (36%) H-1fLþ3 (81%) H-1fLþ2 (85%) H-2fLþ2 (12%), HOMOfLþ2 (18%), HOMOfLþ4 (15%), HOMOfLþ6 (46%) HOMOfLþ4 (61%), HOMOfLþ6 (20%)

Figure 4. Generated absorption spectrum of compound 6 (in red) based upon the computed electronic transitions calculated by TDDFT (in blue). No S0-S1 absorption is calculated above 450 nm.

emission and photophysical properties of 7, a non Pt2bonded species (Figure 5 and Table 1), were briefly revisited in order to confirm the identity of the emission of 6. These compounds exhibit indeed different spectral absorption and luminescence features and emission lifetimes. Moreover, the excitation spectra superpose well the absorption spectra (Figures 2 and 5). During the course of this study, we also noticed that 7 exhibits an extra structured phosphorescence

Figure 5. UV-visible (black), emission (blue), and excitation spectra (red) of 7 in 2MeTHF at 77 K.

band at ∼450 nm bearing a very good resemblance to the phosphorescence of a compound containing the transPhCtCPt(PR3)2CtCPh unit (R = PEt3, P-nBu3), meaning that this emission is the established mixed MLCT (metal-toligand charge transfer)/LMCT (ligand-to-metal charge transfer) of the PhCtCPt(PR3)2CtCPh center.3,4 The 630 nm band was assigned to a MMLCT (mixed metal-metalto-ligand charge transfer dσ*-pσ/π*(CCPh)) triplet excited state. The excitation spectra of both emissions resemble the

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Figure 6. Emission (blue), excitation (red), and absorption (black) spectra of Zn(P)-C6H4CtCH (top left, 2), Pd(P)-C6H4CtCH (bottom left, 3), 4 (top right), and 5 (bottom right) in 2MeTHF at 77 K.

Figure 7. Emission (blue), excitation (red), and absorption (black) spectra of 4 (top) and 5 (bottom) in 2MeTHF at 298 K.

absorption spectra, except 4 and 13 nm red-shifts are observed for the 450 and 630 nm bands, respectively.16,49 All in all, the assignment of the 650 nm emission in 1 is secured. The absorption and emission spectra of 2-5 (Figures 6 and 7, and Tables 3a and 3b) exhibit the typical features associated with the M(P) chromophores (M = Zn, Pd). This observation was verified by DFT (geometry optimization) and TDDFT (calculations of the lowest energy electronic transitions) for compound 4. The relevant frontier MOs along with the calculated lowest energy electronic transitions are presented in the SI. The only relevant conclusion of these calculations is that the lowest energy excited states are all π,π* excited states localized in the metalloporphyrin unit. Importantly, there is no evidence for a broad emission at 650 nm at 77 K (with λexc = 400 nm) associated with the (49) Yam, V. W.-W.; Hui, C.-K.; Wong, K. M.-C.; Zhu, N.; Cheung, K.-K. Organometallics 2002, 21, 4326–4334.

[Pt2(dppm)2(CtCC6H4)2] chromophore, meaning that this emission is totally quenched in 4 and 5. Both time-resolved spectra and lifetime decays were examined to find it under the M(P) (M = Zn, Pd) emissions but without success, hence reinforcing the total emission quenching of the [Pt2(dppm)2(CtCC6H4)2] spacer emission. Two processes are suspected as possible mechanism for the excited-state quenching of the spacer emission: first the T1 energy transfer, and second rapid singlet-state electron transfer (generating a charge separated state). Usually at 77 K, electron transfer is unlikely. The room-temperature transient absorption spectra of 2-5 (Figure 8) exhibit features associated with triplet-triplet absorption (T1-Tn),50 but stronger evidence comes from the comparison of the emission lifetimes, here corresponding to a triplet emission, with the transient absorption lifetimes (Table 4). These are essentially equal, meaning that the charge-separated state is not present at this temperature (the neutral and charged M(P) most likely do not exhibit identical transient lifetimes). Therefore, only triplet-state species are operating. So, the quenching of the spacer emission in 4 and 5 can only come from a T1 energy transfer from the central [Pt2(dppm)2(CtCC6H4)2] spacer to the M(P) macrocycles. The rate of energy transfer (kET) is quantified by kET = (1/τe) - (1/τeo) where τe is the emission lifetime of the donor in a system where the donor is involved in an intramolecular energy transfer process, and τeo is the emission lifetime of the donor in the absence of energy transfer. In this work, τe would be for the [Pt2(dppm)2(CtCC6H4)2] spacer unit in 4 and 5, and τeo would be for 6. Unfortunately, the emission of the [Pt2(dppm)2(CtCC6H4)2] moiety is totally quenched. Another approach is used where the emission quantum yield is considered:51

ðk ET =k r Þ ¼ ð1=Φe Þ -ð1=Φe o Þ

ð1Þ

where Φe is the emission quantum yield of the donor in a system where the donor is involved in an intramolecular (50) (a) Harriman, A. J. Chem. Soc., Faraday Trans. 2 1981, 77, 1281– 1291. (b) Gust, D.; Moore, T. A.; Luttrull, D. K.; Seely, G. R.; Bittersmann, E.; Bensasson, R. V.; Rougee, M.; Land, E. J.; De Schryver, F. C.; Van der Auweraer, M. Photochem. Photobiol. 1990, 51, 419–426. (51) Harvey, P. D. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego, 2003; Vol. 18, pp 63-250.

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Table 3a. UV-Visible Absorption and Emission Data Lifetimes at 77 K in 2MeTHF Φ %a (λexc= 425 nm) τF (ns) (λobs nm)

absorption (λ nm)

fluo. (λ nm)

phos. (λ nm)

1 2 3 4

270, 318sh, 360sh 334, 412, 538, 572 334, 394, 510, 544 417sh, 538, 570

575, 630 545, 575,597 340, 432, 576sh, 633

670 703, 785, 830 sh 662, 695, 717sh, 738 707sh, 785

5 6

396sh, 508, 543 288sh, 377

546, 621sh 443

664sh, 737 654

compd

a

1.71 ( 0.08 (575) 0.17 ( 0.05 (545) 0.12 ( 0.05 (340) 1.71 ( 0.05 (575) 0.18 ( 0.05 (545) 0.21 ( 0.08 (440)

τP (μs) (λobs nm)

fluo.

phos.

2.64 ( 0.05 (640 nm) 27.8 ( 1.5 ms (780 nm) 1810 ( 5 (663 nm) 28.9 ( 0.08 ms (785 nm)

4.5 0.16 6.0

1.45 2.6 4.0 4.9

1790 ( 10 (665 nm) 5.61 ( 0.10 (630 nm)

0.40

9.38 10.3

Using H2TPP as the reference (Φ = 11%) from ref 37.

Table 3b. UV-Visible Absorption and Emission Data Lifetimes at 298 K in 2MeTHF Φ %a (λex nm) absorption (λ nm)

ε  10-3 (M-1 cm-1)

2

332, 410, 540, 575

3

330, 396, 514, 547

4

330,410sh, 540, 575

5

325,400, 515, 550

24 (332), 342 (410), 18 (540), 14 (575) 19 (330), 237 (396), 20 (514), 51 (547) 61 (330), 510 (410), 42 (540), 28 (575) 30 (325), 322 (400), 29 (515), 63 (550)

a

fluo. (λ nm)

phos. (λ nm)

668, 742sh

τtransient (μs)

fluo.

4.095

4.7 (515)

0.10 ( 0.05 (550)

97 ( 1 (665)

90.73

1.62 ( 0.05 (579)

579sh, 633 552

τP (μs) (λobs nm)

1.45 ( 0.06 (580)

597, 633 549, 600

τF (ns) (λobs nm)

628sh, 697

0.094 ( 0.05 (550)

0.469 20.0 ( 2.5 (667)

20.7b

0.18 (515)

phos.

3.2

2.76 (415) 0.82 (403)

6.06 (403)

Using H2TPP as the reference (Φ = 11%) from ref 37. b A second component is noted with a lifetime of 0.39 μs.

Figure 8. Transient absorption spectra of 2-5 in 2MeTHF at 298 K (λexc = 355 nm delay of 0.02-0.08 μs).

energy transfer process, Φeo is the emission quantum yield of the donor in the absence of energy transfer, and kr is the radiative rate constant of the donor in the absence of energy transfer (kr = Φeo/τeo). In this work, Φe is unknown because of the lack of emission, but the lower limit is addressable. Since the lowest emission quantum yield commonly measurable is 0.0001 on our apparatus, then the lower limit for kET can be estimated. By doing so, eq 1 and the data in Tables 1 and 2 give kET > 1.8  108 s-1. This rate is fast in comparison with other multiporphyrin assemblies,39 which is consistent with the observed total quenching of the [Pt2(dppm)2(Ct CC6H4)2] spacer emission. This T1 energy transfer must proceed via a through-bond mechanism (i.e., Cmeso-Caryl). Indeed, one interesting comparison is that for the kET data between through-space T1 energy transfer in the electrostatically held dyads between the unsaturated clusters M3(dppm)3(CO)2þ (M = Pd, Pt)

and metalloporphyrins (M0 (TPP-CO2-); M0 = Zn, Pd; TPP = tetraphenylporphyrin; the distance between the metals and oxygen atoms are on the order of 2.8-3.2 A˚) as the donor and acceptor, respectively (shown in Chart 3)11 and those estimated for the through-bond systems 4 and 5 (Chart 5). These are on the order of 104 and>108 s-1, respectively. This very large difference in kET values between through-space and through-bond processes was also previously noticed for cofacial bismacrocycles containing M(P) chromophores. Examples are shown in Chart 5. Indeed, the through-space systems including platinum(II) porphyrins exhibit T1 kET’s on the order of 103 s-1,52 whereas the RhSn-bonded cofacial tetraphenylcorrole system exhibits a significantly faster rate, ranging from 1  106 to 2.2  108 s-1.12 (52) Faure, S.; Stern, C.; Espinosa, E.; Douville, J.; Guilard, R.; Harvey, P. D. Chem.;Eur. J. 2005, 11, 3469–3481.

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Table 4. Comparison of the Emission and Transient Absorption Lifetimes for 4-6 and trans-PhCtCPt(PEt3)2CtCPh in 2MeTHF at 77 K (λexc = 355 nm) compound

transient absorption lifetime (μs)

emission lifetime (μs) (at λobs in nm)

trans-PhCtCPt(PEt3)2CtCPha Pt2(dppm)2(CtCPh)2 (6) 4 5

36.0 5.93 22.8  103 1.81  103

35.8 (440) 5.61 ( 0.10 (630) (28.9 ( 0.1)  103 (785, (Zn(P))b (1.79 ( 0.08)  103 (665, (Pd(P))b

a From ref 3. The trans-PhCtCPt(PEt3)2CtCPh species is added for comparison because of its structural relevance to the [Pt2(dppm)2(CtCC6H4)2] spacer of this work. b The chromophore is indicated in brackets.

Chart 5

In these two series, the points of close contacts between the donor and acceptors are approximately the same, and so one cannot ascribe the drastic change in rate to a distance effect. There is a clear increase in energy transfer rates when the process operates through a chemical bond. Moreover, there is no clear relationship (in this work) between the molecular shape (side by side versus cofacial arrangement) of the dyads and the rates of T1 energy transfer.

Conclusion The preparation and characterization of d9-d9 M2bonded Pt2(dppm)2(CtCC6H4-M(P))2 complexes (M = Zn, Pd, and P = diethylhexamethylporphyrin), where the [Pt2(dppm)2(CtCC6H4)2] central organometallic fragment acts as a chromophore that is not conjugated with the M(P) moiety, were reported. The luminescence of the central [Pt2(dppm)2(CtCC6H4)2] unit is found totally quenched in comparison with the parent complex Pt2(dppm)2(CtCPh)2. This quenching is operated by T1 through-bond energy

transfer. Comparisons with other organometallic-containing porphyrin systems clearly demonstrate the efficiency of through-bond processes over through-space (about 4 orders of magnitudes larger). One may ask the question why nature elected to build light-harvesting devices in photosynthetic bacteria (here using bacteriochlorophylls and carotenoids as the chromophores) using supramolecular organizations (via H- and coordination bonds) inside proteins and yet experiencing very efficient (here S1) energy transfer and exciton migration time scales.1 The answer resides in the close contacts in the π-systems of the chromophores and the dissymmetry in the chromophore structures promoting better change in dipole moments upon excitation, favoring better energy transfer processes, hence compensating for the absence of through-bond processes. In order to demonstrate this concept, we recently investigated oligomeric species of the type [(p-C6H4)CtCPt(P(n-Bu)3)2CtC(pC6H4)Zn(P)]n, where Zn(P) is zinc(II)-10,20-di-n-pentylporphyrin (n = 3, 6, 9).53 The rate for triplet energy transfer between the organometallic fragment (p-C6H4)CtCPt(P(nBu)3)2CtC(p-C6H4) and the pigment Zn(P) varies as (5.3 ( 1.5)  104, (32 ( 7)  104, and (130 ( 20)  104 s-1 for n=3, 6, and 9, respectively. In the fastest case (n = 9; kET = 1.3  106 s-1), this value falls short with respect to kET > 108 s-1. This comparison indicates that there is an influence of the nature of the donor chromophore on the rate of transfer. This conclusion also suggests that the incorporation of the [Pt2(dppm)2]2þ chromophore within the backbone of the polymer should increase even more the rate of triplet energy transfer. Future work in this area is in progress.

Acknowledgment. This research was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), le Fonds Quebecois de la Recherche sur la Nature et les Technologies (FQRNT), and the  Centre d’Etudes des Materiaux Optiques et Photoniques de l’Universite de Sherbrooke. The French Ministry of Research (MENRT) and the CNRS (UMR 5260) are also gratefully acknowledged. Supporting Information Available: Molecular structure of [(tBu-NC)Pt(dppm)2Pt(NC-tBu)](BF4)2, comparison of selected bonds and angles obtained by X-ray crystallography with those for optimized geometry for a model compound [(tBuNC)Pt(dppm)2Pt-(NC-tBu)](BF4)2, MO drawings of the first 10 frontier MOs for 4, characteristics of the 10 first-computed S0-Sn electronic transitions for 4, representations of the optimized geometries of compound 6 in the gas phase and in the presence of THF solvent molecules in the S0 and T1 states. This material is available free of charge via the Internet at http:// pubs.acs.org. (53) Liu, L.; Fortin, D.; Harvey, P. D. Inorg. Chem. 2009, 48, 5891– 5900.