Organometallics 2009, 28, 2119–2126
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Synthesis, Characterization, and Electrochemistry of Compounds Containing 1-Diphenylphosphino-1′-(di-tert-butylphosphino)ferrocene (dppdtbpf) Sarah L. Kahn,† Meghan K. Breheney,† Sarah L. Martinak,† Stephanie M. Fosbenner,† Ashley R. Seibert,† W. Scott Kassel,‡ William G. Dougherty,‡ and Chip Nataro*,† Department of Chemistry, Hugel Science Center, Lafayette College, Easton, PennsylVania 18042, and Department of Chemistry, VillanoVa UniVersity, VillanoVa, PennsylVania 19085 ReceiVed September 2, 2008
The anodic electrochemistry of 1-di-tert-butylphosphino-1′-diphenylphosphinoferrocene (dppdtbpf) was performed in methylene chloride with tetrabutlylammonium hexafluorophosphate as the supporting electrolyte. The electrochemistry is complicated by a chemical reaction after the oxidation. Three new transition metal complexes of dppdtbpf ([NiCl2(dppdtbpf)], [PtCl2(dppdtbpf)], and [Au2Cl2(dppdtbpf)]) were prepared and characterized. The electrochemistry of the three new compounds and one previously prepared compound ([PdCl2(dppdtbpf)]) was examined. In addition, the reaction of dppdtbpf with sulfur and/or selenium afforded a series of compounds containing a phosphine sulfide (dppSdtbpSf or dppdtbpSf), a phosphine selenide (dppSedtbpSef or dppdtbpSef), or both a phosphine sulfide and a phosphine selenide (dppSedtbpSf or dppSdtbpSef). These compounds were characterized by NMR, and the structures of three of the compounds were determined. Two of the structures are of the isomeric compounds, dppSdtbpSef and dppSedtbpSf, in which the sulfur and selenium atoms are switched between the two different phosphine groups. The anodic electrochemistry of these sulfur- and/or selenium-containing compounds was examined in methylene chloride. The potentials at which oxidation occurs are more positive than those of the free phosphine. Nearly all of the oxidations are complicated by at least one chemical reaction. Introduction Bis(phosphino)ferrocenes are commonly used ligands in a variety of catalytic applications.1 While the symmetrically substituted 1,1′-bis(phosphino)ferrocenes such as 1,1′-bis(diphenylphosphino)ferrocene (dppf)2 and 1,1′-bis(di-tert-butylphosphino)ferrocene (dtbpf)2j,3 (Chart 1) continue to be actively investigated, closely related asymmetric compounds have not been given equal consideration. Although Cullen first reported 1-diphenylphosphino-1′-di-tert-butylphosphinoferrocene (dppdtbpf) in 1985,4 it has received surprisingly little attention.5 In addition to the synthesis and characterization of dppdtbpf the synthesis of [PdCl2(dppdtbpf)] has been reported.4a The catalytic hydrogenation of a series of olefins using [Rh(NBD)(P-P)]ClO4 (P-P ) dppf, dtbpf, or dppdtbpf) as the catalyst precursor was investigated.4b While no clear trends were observed, the dppf and dppdtbpf complexes tended to give similar results, which were superior to the dtbpf analogue in terms of induction time
* To whom correspondence should be addressed. E-mail: nataroc@ lafayette.edu. † Lafayette College. ‡ Villanova University. (1) (a) Gan, K.-S.; Hor, T. S. A. In Ferrocenes: Homogeneous Catalysis, Organic Synthesis, Materials Science; Togni, A., Hayashi, T., Eds.; VCH: New York, 1995; p 3. (b) Hayashi, T. In Ferrocenes: Homogeneous Catalysis, Organic Synthesis, Materials Science; Togni, A., Hayashi, T., Eds.; VCH: New York, 1995; p 105. (c) Chien, A. W.; Hor, T. S. A. In Ferrocenes: Ligands, Materials and Biomolecules; Sˇte`pnic`ka, P., Eds.; John Wiley & Sons, Ltd.: West Sussex, 2008; p 33. (d) Colacot, T. J.; Parisel, S. In Ferrocenes: Ligands, Materials and Biomolecules; Sˇte`pnic`ka, P., Ed.; John Wiley & Sons, Ltd.: West Sussex, 2008; p 117.
and time for complete uptake of hydrogen. The only other study to employ dppdtbpf examined the catalytic activity of [Ni(COD)dppdtbpf] in the isomerization of 2-methyl-3-butenenitrile using either ZnCl2 or BEt3 as the co-catalyst.6 While not focused exclusively on dppdtbpf, this study does suggest that the (2) Recent examples include the following: (a) Csok, Z.; Vechorkin, O.; Harkins, S. B.; Scopelliti, R.; Hu, X. J. Am. Chem. Soc. 2008, 130, 8156. (b) Jamali, S.; Nabavizadeh, S. M.; Rashidi, M. Inorg. Chem. 2008, 47, 5441. (c) Serra-Muns, A.; Jutand, A.; Moreno-Manas, M.; Pleixats, R. Organometallics 2008, 27, 2421. (d) Kuwano, R.; Kusano, H. Org. Lett. 2008, 10, 1979. (e) Vo, G. D.; Hartwig, J. F. Angew. Chem., Int. Ed. 2008, 47, 2127. (f) Kawatsura, M.; Hirakawa, T.; Tanaka, T.; Ikeda, D.; Hayase, S.; Itoh, T. Tetrahedron Lett. 2008, 49, 2450. (g) Kawatsura, M.; Hirakawa, T.; Tanaka, T.; Ikeda, D.; Hayase, S.; Itoh, T. Tetrahedron Lett. 2008, 49, 2450. (h) Shaw, A. P.; Guan, H.; Norton, J. R. J. Organomet. Chem. 2008, 693, 1382. (i) Song, L.-C.; Wang, H.-T.; Ge, J.-H.; Mei, S.-Z.; Gao, J.; Wang, L.-X.; Gai, B.; Zhao, L.-Q.; Yan, J.; Wang, Y.-Z. Organometallics 2008, 27, 1409. (j) Fey, N.; Harvey, J. N.; Lloyd-Jones, G. C.; Murray, P.; Orpen, A. G.; Osborne, R.; Purdie, M. Organometallics 2008, 27, 1372. (k) Dahl, T.; Tornoee, C. W.; Bang-Andersen, B.; Nielsen, P.; Joergensen, M. Angew. Chem., Int. Ed. 2008, 47, 1726. (l) Guan, B.-T.; Xiang, S.-K.; Wang, B.-Q.; Sun, Z.-P.; Wang, Y.; Zhao, K.-Q.; Shi, Z.-J. J. Am. Chem. Soc. 2008, 130, 3268. (m) Jensen, T.; Pedersen, H.; Bang-Andersen, B.; Madsen, R.; Joergensen, M. Angew. Chem., Int. Ed. 2008, 47, 888. (n) Arias, J.; Bardaji, M.; Espinet, P. Inorg. Chem. 2008, 47, 1597. (3) Clapham, K. M.; Batsanov, A. S.; Greenwood, R. D. R.; Bryce, M. R.; Smith, A. E.; Tarbit, B. J. Org. Chem. 2008, 73, 2176. (4) (a) Cullen, W. R.; Kim, T. J.; Einstein, F. W. B.; Jones, T. Organometallics 1985, 4, 346. (b) Butler, I. R.; Cullen, W. R.; Kim, T. J.; Rettig, S. J.; Trotter, J. Organometallics 1985, 4, 972. (5) The isomeric compound 1,1′-bis(phenyl-tert-butylphosphino)ferrocene has also been prepared and appears in ref 1b and the following. (a) Kim, T.-J. Bull. Korean Chem. Soc. 1990, 11, 134. (b) Butler, I. R.; Coles, S. J.; Hursthouse, M. B.; Roberts, D. J.; Fujimoto, N. Inorg. Chem. Commun. 2003, 6, 760. (6) Acosta-Ramirez, A.; Munoz-Hernandez, M.; Jones, W. D.; Garcia, J. J. Organometallics 2007, 26, 5766.
10.1021/om800850c CCC: $40.75 2009 American Chemical Society Publication on Web 03/04/2009
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Chart 1. Bis(phosphino)ferrocenes and Bis(phosphinechalcogenide)ferrocenes Used in This Report
asymmetric steric and/or electronic properties of dppdtbpf can have a significant impact on reactivity. When BH3 was used as the co-catalyst, [Ni(COD)dppdtbpf] had a higher selectivity for the formation of 3-pentenenitrile than either the dppf or 1,1′bis(diisopropylphosphino)ferrocene (dippf) analogues. In addition, the stoichiometric reaction of [Ni(COD)dppdtbpf], ZnCl2, and 2-methyl-3-butenenitrile yields exclusively a compound in which the -PtBu2 group is bound to Zn and the -PPh2 group remains bound to Ni. While previous studies have focused on the steric and electronic properties of the phosphorus atoms in dppdtbpf, the presumably redox-active iron center of the ferrocenyl backbone has not been examined. Redox activity of the iron center can play an important role in the catalytic activity of compounds containing ferrocenyl phosphine ligands such as redox-switch catalysis using CpRu(dppf)H.7 The closely related dtbpf undergoes a chemically and electrochemically reversible oneelectron oxidation in methylene chloride (CH2Cl2).8 Under similar conditions, the oxidation of dppf is complicated by a follow-up reaction.9 Upon coordination to a transition metal center, the oxidation of dppf and dtbpf occurs at a potential more positive than that of the free phosphine and is typically chemically and electrochemically reversible.8,9 In addition to the transition metal complexes, the anodic electrochemistry of the phosphine sulfides and phosphine selenides of dppf and dtbpf have been investigated (Chart 1). Both dppfS2 and dtbpfS2 undergo chemically and electrochemically reversible oxidations, while dppfSe2 and dtbpfSe2 undergo electrochemically irreversible oxidations.8,10 The oxidation of dtbpfS2 is a one-electron oxidation that presumably occurs at the iron center.8 However, dtbpfSe2 undergoes two one-electron oxidations that result in the intramolecular formation of a Se-Se bond, yielding [dtbpfSe2]2+ (eq 1).8 The reason for the difference in oxidation products between dtbpfS2 and dtbpfSe2 is unclear. Oxidation of Ph3PdS11 and (Me2N)3PdS12 has been shown to
yield [R3P-S-S-PR3]2+ in which intermolecular S-S bond formation has occurred. Similarly, oxidation of (Me2N)3PdSe yields [(Me2N)3P-Se-Se-(NMe2)3]2+.13 To date there are no examples of the oxidation of a phosphine sulfide and a phosphine selenide to yield a Se-S bonded species, [R3P-Se-S-PR3]2+. There are also no examples of the reaction of two different phosphine sulfides or phosphine selenides to yield either an S-S or Se-Se bonded species, [R3P-E-E-PR′3]2+ (E ) S or Se; R * R′). The inherent asymmetry of dppdtbpf provides the opportunity to study the electrochemistry of a dppSdtbpSf and dppSedtbpSef in which the sulfur or selenium atoms are inequivalent. In addition, by taking advantage of the different reactivity of the two phosphines in dppdtbpf, the mixed phosphine sulfidephosphine selenide compounds can be prepared. Besides the synthesis of these compounds, the anodic electrochemistry of these compounds was investigated in CH2Cl2. In addition, the anodic electrochemistry of dppdtbpf was examined in CH2Cl2. Three new transition metal compounds containing dppdtbpf as a ligand were synthesized and characterized. The anodic electrochemistry of these three compounds as well as one previously prepared compound4a was examined. Finally, the mixed phosphine sulfide-phosphine selenide, dppfSSe, was prepared, and the oxidative electrochemistry was studied.
Experimental Section General Procedures. Reactions were carried out under an atmosphere of argon using standard Schlenck techniques. Diacetyl(7) Hembre, R. T.; Moqueen, J. S.; Day, V. W. J. Am. Chem. Soc. 1996, 118, 798. (8) Blanco, F. N.; Hagopian, L. E.; McNamara, W. R.; Golen, J. A.; Rheingold, A. L.; Nataro, C. Organometallics 2006, 25, 4292. (9) Nataro, C.; Campbell, A. N.; Ferguson, M. A.; Incarvito, C. D.; Rheingold, A. L. J. Organomet. Chem. 2003, 673, 47. (10) Swartz, B. D.; Nataro, C. Organometallics 2005, 24, 2447. (11) Blankespoor, R. L.; Doyle, M. P.; Smith, D. J.; Van Dyke, D. A.; Waldyke, M. J. J. Org. Chem. 1983, 48, 1176. (12) (a) Ainscough, E. W.; Brodie, A. M.; Brown, K. L. J. Chem. Soc., Dalton Trans. 1980, 1042. (b) Slinkard, W. E.; Meek, D. W. Inorg. Chem. 1969, 8, 1811. (c) Slinkard, W. E.; Meek, D. W. Chem. Commun. 1969, 361. (13) Willey, G. R.; Barras, J. R.; Rudd, M. D.; Drew, M. G. B. J. Chem. Soc., Dalton Trans. 1994, 3025.
Compounds Containing dppdtbpf ferrocene, decamethylferrocene (Fc*), ferrocene (FcH), dppf, dppdtbpf, 2,2′-thiodiethanol, NiCl2 · 6H2O, and selenium were purchased from Aldrich Chemical Co., Inc. Sulfur was purchased from Fisher Chemicals, Inc. [PdCl2(MeCN)2], [PtCl2(C6H5CN)2], HAuCl4 · H2O, and dtbpf were purchased from Strem Chemicals, Inc. FcH was sublimed prior to use. Lithium tetratkis(pentafluorophenyl)borate was purchased form Boulder Scientific, Inc. and metathesized to the tetrabutylammonium salt ([NBu4][B(C6F5)4]) using the literature procedure.14 Diacetylferrocenium tetrafluoroborate,15 dppfS,16 [(AuCl)2dppf],17 and [PdCl2(dppdtbpf)]4a were prepared according to the literature procedures. Hexanes, CH2Cl2, and diethyl ether (Et2O) were purified under Ar using a Solv-tek purification system similar to one previously described.18 Toluene, methanol, 2-propanol, benzene, and chloroform were degassed prior to use. A JEOL Eclipse 400 FT-NMR spectrometer was used to obtain the 31P{1H } and 1H NMR spectra. The 31P{1H} spectra were referenced to an external sample of 85% H3PO4, while the 1H spectra were referenced to internal TMS. Quantitative Technologies, Inc. performed the elemental analysis. Preparation of [NiCl2(dppdtbpf)]. NiCl2 · 6H2O (0.0663 g, 0.279 mmol) was dissolved in 3.5 mL of a mixture of 2-propanol/ methanol (5:2 v/v). In a separate flask, dtbpdppf (0.1485 g, 0.289 mmol) was dissolved in 4 mL of warm 2-propanol. The dtbpdppf solution was added to the nickel solution, and the reaction was stirred for 30 min at approximately 80 °C. During this time, a dark green precipitate was noted. The solution cooled to room temperature and was then filtered, giving a dark green solid. The solid was washed with a minimal amount of cold methanol, yielding [NiCl2(dtbpf)] (0.0315 g, 20%). Anal. Calcd for C30H36Cl2FeNiP2: C, 55.95; H, 5.63. Found: C, 55.61; H, 5.55. Preparation of [PdCl2(dppdtbpf)]. Elemental analysis and 1H NMR data for this compound have been reported previously.4a 31 P{1H} NMR (CDCl3): δ (ppm) 80.6 (d, 2JP-P ) 20.8 Hz), 38.8 (d, 2JP-P ) 20.8 Hz). Preparation of [PtCl2(dppdtbpf)]. PtCl2(C6H5CN)2 (0.0502 g, 0.098 mmol) and dtbpdppf (0.0541 g, 0.115 mmol) were dissolved in 10 mL of benzene and stirred for 1 h, during which time an orange precipitate was noted. The reaction was then placed in a refrigerator for 2 h, after which time the solution was filtered. The solid residue was washed with 2 × 5 mL of Et2O and dried in Vacuo, giving [PtCl2(dtbpdppf)] as an orange solid (0.0235 g, 31%). Anal. Calcd for C30H36Cl2FeP2Pt: C, 46.17; H, 4.65. Found: C, 46.37; H, 4.70. 31P{1H} NMR (CDCl3): δ (ppm) 47.2 (d, 2JP-P ) 6.94 Hz, 1JP-Pt ) 3790 Hz, -PtBu2), 14.5 (d, 2JP-P ) 6.94 Hz, 1 JP-Pt ) 3950 Hz, -PPh2). 1H NMR (CDCl3): δ (ppm) 8.04 (m, 4H, -C6H5), 7.56 (m, 2H, -C6H5), 7.42 (m, 4H, -C6H5), 4.78 (br s, 2H, -C5H4), 4.45 (br s, 2H, -C5H4), 4.23 (br s, 2H, -C5H4), 3.89 (br s, 2H, -C5H4), 1.57 (d, 18 H, 2JH-P ) 14.3 Hz, -CH3). Preparation of [(AuCl)2dppdtbpf]. HAuCl4 · H2O (0.200 g, 0.589 mmol) was dissolved in 8.0 mL of methanol and cooled to 0 °C. After cooling, 0.70 mL of 2,2′-thiodiethanol in 1.5 mL of methanol was added. The resulting solution was stirred for 15 min at 0 °C, after which dppdtbpf (0.1330 g, 0.2203 mmol) was added. The solution was stirred for 21 h, during which time a precipitate formed. The reaction solution was filtered, and the resulting yellow solid was washed with 3 × 5 mL of methanol and dried in Vacuo, giving (AuCl)2dppdtbpf (0.1003 g, 42.6%). Anal. Calcd for C30H36Au2Cl2FeP2: C, 36.80; H, 3.71. Found: C, 36.74; H, 3.33. (14) LeSuer, R.; Geiger, W. E. Angew. Chem., Int. Ed. 2000, 39, 248. (15) Guillon, C.; Vierling, P. J. Organomet. Chem. 1994, 464, C42. (16) Broussier, R.; Bentabet, E.; Laly, M.; Richard, P.; Kuz’mina, L. G.; Serp, P.; Wheatley, N.; Kalck, P.; Gautheron, B. J. Organomet. Chem. 2000, 613, 77. (17) Hill, D. T.; Girard, G. R.; McCabe, F. L.; Johnson, R. K.; Stupik, P. D.; Zhang, J. H.; Reiff, W. M.; Egglestons, D. S. Inorg. Chem. 1989, 28, 3529. (18) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.; Timmers, F. J. Organometallics 1996, 15, 1518.
Organometallics, Vol. 28, No. 7, 2009 2121 P{1H} NMR (CDCl3): δ (ppm) 69.9 (s, -PtBu2), 27.6 (s, -PPh2). H NMR (CDCl3): δ (ppm) 7.54 (m, 10H, -C6H5), 4.87 (br s, 2H, -C5H4), 4.68 (br s, 2H, -C5H4), 4.42 (br s, 4H, -C5H4), 1.30 (d, 18H, 3JH-P ) 15.4 Hz, -CH3). Preparation of dppSdtbpSf. Chloroform (15.0 mL) was added to a mixture of sulfur (0.125 g, 0.390 mmol) and dppdtbpf (0.1000 g, 0.194 mmol), and the resulting solution was refluxed overnight. Heat was then removed from the reaction, and the solution was filtered. The volume of the filtrate was reduced in Vacuo to approximately 10 mL, methanol (20 mL) was added, and the resulting solution was placed in a refrigerator overnight. The solution was filtered, and the solid was washed with diethyl ether (10 mL) and dried in Vacuo, yielding dppSdtbpSf (0.1015 g, 90.2%) as an orange-yellow solid. Anal. Calcd for C30H36FeP2S2: C, 62.28; H, 6.27. Found: C, 62.06; H, 6.14. 31P NMR (CDCl3): δ (ppm) 77.6 (s, -P(S)tBu2), 41.7 (s, -P(S)Ph2). 1H NMR (CDCl3): δ (ppm) 7.69 (m, 4H, -C6H5), 7.45 (m, 6H, -C6H5), 4.82 (m, 2H, -C5H4), 4.63 (m, 2H, -C5H4), 4.46 (m, 2H, -C5H4), 4.39 (m, 2H, -C5H4), 1.23 (d, 3JH-P ) 15.4 Hz, 18H, -CH3). Preparation of dppSedtbpSef. Chloroform (20.0 mL) was added to a mixture of selenium (0.0767 g, 0.9708 mmol) and dppdtbpf (0.2497 g, 0.4854 mmol). The reaction was refluxed overnight, and upon cooling, the solution was filtered and then the solvent was reduced to approximately 10 mL. Methanol (20 mL) was added, and the solution was placed in a refrigerator overnight. The solution was filtered, leaving the product as a light orange solid. The solid was washed with diethyl ether (10 mL) and dried in Vacuo, yielding dppSedtbpSef (0.2752 g, 84.3%). Anal. Calcd for C30H36FeP2Se2: C, 53.59; H, 5.40. Found: C, 53.25; H, 5.29. 31P NMR (CDCl3): δ (ppm) 74.7 (s, 1JP-Se ) 700 Hz, -P(Se)tBu2), 31.9 (s, 1JP-Se ) 728 Hz, -P(Se)Ph2). 1H NMR (CDCl3): δ (ppm) 7.69 (m, 4H, -C6H5), 7.44 (m, 6H, -C6H5), 4.90 (m, 2H, -C5H4), 4.68 (m, 2H, -C5H4), 4.48 (m, 2H, -C5H4), 4.43 (m, 2H, -C5H4), 1.27 (d, 3JH-P ) 15.4 Hz, 18H, -CH3). Preparation of dppdtbpSf. Sulfur (0.0185 g, 0.577 mmol) and dppdtbpf (0.2999 g, 0.583 mmol) were combined and cooled to 0 °C. Toluene (15.0 mL) was added, and the reaction was stirred for 4 h while maintaining the temperature at 0 °C. Solvent was removed in Vacuo. The residue was purified by column chromatography on silica gel using hexanes/toluene (1:1 v/v) as the eluent. The first band off the column (yellow-orange) was unreacted dppdtbpf. The second band (yellow-orange) gave the desired product (0.0489 g, 15.5% yield) as a yellow solid upon removal of the solvent. The final band from the column (orange-yellow) was dppSdtbpSf. Anal. Calcd for C30H36FeP2S: C, 65.94; H, 6.64. Found: C, 65.57; H, 6.64. 31P NMR (CDCl3): δ (ppm) 77.8 (s, -P(S)tBu2), -17.1 (s, -PPh2). 1H NMR (CDCl3): δ (ppm) 7.67 (m, 2H, -C6H5), 7.45 (m, 8H, -C6H5), 4.86 (m, 2H, -C5H4), 4.66 (m, 2H, -C5H4), 4.48 (m, 2H, -C5H4), 4.40 (m, 2H, -C5H4), 1.23 (d, 3JH-P ) 15.2 Hz, 18H, -CH3). Preparation of dppdtbpSef. A similar synthesis to that of dppdtbpSf was employed using 0.3010 g (0.585 mmol) of dppdtbpf and 0.0455 g (0.576 mmol) of selenium. Purification by column chromatography gave three bands. The first (yellow-orange) was unreacted dppdtbpf. The second (yellow-orange) gave 0.0714 g (20.9% yield) of the product upon removing the solvent. The third band (orange-yellow) was dppSedtbpSef. Anal. Calcd for C30H36FeP2Se: C, 60.73; H, 6.12. Found: C, 60.34; H, 6.11. 31P NMR (CDCl3): δ (ppm) 73.9 (s, 1JP-Se ) 728 Hz, -P(Se)tBu2), -17.1 (s, -PPh2). 1H NMR (CDCl3): δ (ppm) 7.67 (m, 2H, -C6H5), 7.44 (m, 8H, -C6H5), 4.67 (br s, 2H, -C5H4), 4.43 (br s, 2H, -C5H4), 4.37 (br s, 2H, -C5H4), 4.15 (br s, 2H, -C5H4), 1.30 (d, 3JH-P ) 15.8 Hz, 18H, -CH3). Preparation of dppSedtbpSf. Selenium (0.0091 g, 0.12 mmol) and dppdtbpSf (0.0587 g, 0.107 mmol) were combined, and chloroform (10.0 mL) was added. The reaction was allowed to stir overnight. The solution was filtered, and the filtrate was evaporated 31 1
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Table 1. Crystal Data and Structure Analysis Results formula fw cryst syst space group a, Å b, Å c, Å R, deg β, deg γ, deg V, Å3 Z cryst size, mm cryst color radiation; λ, Å temp, K θ range, deg data collected h k l no. of data collected no. of unique data absorp corr final R indices (obs. data) R1 wR2 goodness of fit
dppdtbpSf
dppSdtbpSef
dppSedtbpSf
C30H36FeP2S 546.44 monoclinic P21/c 13.1591(5) 13.9408(6) 14.7454(6) 90 94.767(2) 90 2695.66(19) 4 0.20 × 0.20 × 0.05 orange 1.54178 100(2) 3.37-68.23
C30H36FeP2SSe 625.40 monoclinic P21/n 13.2042(7) 12.4223(6) 17.7166(10) 90 102.650(2) 90 2835.5(3) 4 0.26 × 0.18 × 0.10 yellow 1.54178 100(2) 4.38-68.18
C30H36FeP2SSe 625.40 orthorhombic Pbca 14.0210(2) 15.7391(2) 25.8400(3) 90 90 90 5702.32(13) 8 0.25 × 0.13 × 0.03 yellow 1.54178 100(2) 3.42-68.02
-15 to 13 -15 to 16 -17 to 16 10 814 4658 SADABS
-15 to +15 -14 to +14 -21 to +8 14 384 4985 SADABS
-16 to 16 -18 to 11 -30 to 31 20 071 5124 SADABS
0.0363 0.0891 1.026
0.0553 0.1857 1.077
0.0531 0.1392 1.032
to dryness, yielding 0.0369 g (55.1% yield) of dppSedtbpSf as an orange-yellow solid. Anal. Calcd for C30H36FeP2SSe: C, 57.61; H, 5.80. Found: C, 57.41; H, 6.02. 31P NMR (CDCl3): δ (ppm) 77.6 (s, -P(S)tBu2), 31.9 (s, 1JP-Se ) 728 Hz, -P(Se)Ph2). 1H NMR (CDCl3): δ (ppm) 7.68 (m, 4H, -C6H5), 7.44 (m, 6H, -C6H5), 4.86 (m, 2H, -C5H4), 4.66 (m, 2H, -C5H4), 4.48 (m, 2H, -C5H4), 4.40 (m, 2H, -C5H4), 1.23 (d, 3JH-P ) 15.0 Hz, 18H, -CH3). Preparation of dppSdtbpSef. Sulfur (0.0048 g, 0.15 mmol) and dppdtbpSef (0.0792 g, 0.133 mmol) were combined, and chloroform (10.0 mL) was added. The reaction was stirred overnight and was filtered, and the filtrate was evaporated to dryness, yielding 0.0382 g (45.9% yield) of the product as an orange-yellow solid. Anal. Calcd for C30H36FeP2SSe: C, 57.61; H, 5.80. Found: C, 57.69; H, 5.35. 31P NMR (CDCl3): δ (ppm) 74.7 (s, 1JP-Se ) 723 Hz, -P(Se)tBu2), 41.6 (s, -P(S)Ph2). 1H NMR (CDCl3): δ (ppm) 7.69 (m, 4H, -C6H5), 7.44 (m, 6H, -C6H5), 4.87 (m, 2H, -C5H4), 4.66 (m, 2H, -C5H4), 4.47 (m, 2H, -C5H4), 4.43 (m, 2H, -C5H4), 1.27 (d, 3JH-P ) 15.4 Hz, 18H, -CH3). Preparation of dppfSSe. Chloroform (10.0 mL) was added to a mixture of selenium (0.0091 g, 0.12 mmol) and dppfS (0.0654 g, 0.112 mmol). The reaction was allowed to stir overnight. The solution was filtered, and the filtrate was evaporated to dryness, yielding 0.0180 g (24.2% yield) of dppfSSe as an orange-yellow solid. 31P NMR (CDCl3): δ (ppm) 41.3 (s, -P(S)Ph2, 31.6 (s, 1JP-Se ) 723 Hz, -P(Se)Ph2). 1H NMR (CDCl3): δ (ppm) 7.68 (m, 8H, -C6H5), 7.43 (m, 12H, -C6H5), 4.88 (m, 2H, -C5H4), 4.67 (m, 2H, -C5H4), 4.46 (m, 2H, -C5H4), 4.41 (m, 2H, -C5H4), 1.27 (d, 3 JH-P ) 15.4 Hz, 18H, -CH3). Anal. Calcd for C34H28FeP2SSe: C, 61.37; H, 4.24. Found: C, 61.23; H, 4.08. Electrochemical Procedures. Cyclic voltammetric experiments were conducted at ambient temperature (22 ( 1 °C) using a PAR model 263A potentiostat/galvanostat. Scans were performed under an argon atmosphere using 1.0 mM solutions of the analyte in CH2Cl2 with 0.1 M tetrabutylammonium hexafluorophosphate ([NBu4][PF6]) as the supporting electrolyte. Background subtraction of [NBu4][PF6] in CH2Cl2 was performed for all experiments. In addition, cyclic voltammetric experiments were performed at 20.0, 10.0, 0.0, and -10.0 °C ((0.1 °C) for dppdtbpf with analyte concentrations of 0.5, 1.0, 5.0, and 10 mM. The oxidative electrochemistry of dppdtbpSef was also investigated using 0.05
M [NBu4][B(C6F5)4] as the supporting electrolyte. For all experiments the working electrode was glassy carbon (1.5 mm diameter disk), which was polished with two diamond pastes, first 1.0 µm and then 0.25 µm, and rinsed with CH2Cl2 prior to use. The counter electrode was a platinum wire, while the experimental reference electrode was Ag/AgCl, separated from the solution by a fine frit. FcH or Fc* was used as an internal standard depending on the analyte.19 Cyclic voltammograms were recorded using PowerSuite software and were performed at scan rates of 10, 25, 50, 75, and from 100 to 1000 mV/s in 100 mV/s increments. Bulk electrolysis experiments were performed under argon using a CH Instruments model 630B electrochemical analyzer. The working and auxiliary electrodes were platinum mesh baskets that were in compartments separated by a fine glass frit. The Ag/AgCl reference electrode was in the same compartment as the working electrode, but was also separated by a fine glass frit. A 1.5 mm glassy carbon electrode was used to obtain the cyclic voltammogram after the bulk electrolysis, while a 10 µm glassy carbon electrode was used for linear sweep voltammograms. The solvent was CH2Cl2 and the supporting electrolyte was 0.1 M [NBu4][PF6]. The analyte concentration was 5.0 mM and the temperature was 22 ( 1 °C. X-ray Crystal Structures. Crystals of dppdtbpSf, dppSdtbpSef, and dppSedtbpSf were mounted using Paratone oil onto a glass fiber and cooled to the data collection temperature of 100 K. Data were collected on a Bru¨ker-AXS Kappa APEX II CCD diffractometer (Table 1). Unit cell parameters were obtained from 90 data frames. The systematic absences in the diffraction data were consistent with space groups presented in Table 1. The data sets were treated with SADABS absorption corrections based on redundant multiscan data (Sheldrick, G., Bruker-AXS, 2001). All non-hydrogen atoms were refined with anisotropic displacement parameters. All hydrogen atoms were treated as idealized contribution.
Results and Discussion The anodic electrochemistry of dppdtbpf displays a single wave at 0.11 V vs FcH0/+ (Figure 1). The potential at which (19) When Fc* was added as the standard, the potentials were referenced to FcH by subtracting 0.548 V. Barrie`re, F.; Geiger, W. E. J. Am. Chem. Soc. 2006, 128, 9103.
Compounds Containing dppdtbpf
Organometallics, Vol. 28, No. 7, 2009 2123 Table 2. Formal Potentials (V vs FcH0/+),a Potential Difference, ∆E (defined as E0cmpd - E0phosphine), and Reversibility for dppdtbpf and Transition Metal Compounds Containing a dppdtbpf Ligand (all data reported at a scan rate of 100 mV/s) E0 b
dppf dtbpf c dppdtbpf [NiCl2(dppdtbpf)] [PdCl2(dppdtbpf)] [PtCl2(dppdtbpf)] [(AuCl)2dppf] [(AuCl)2dtbpf]c [(AuCl)2dppdtbpf] a
0.23 0.06 0.11 0.25 0.51 0.51 0.64 0.56 0.59
∆E
ipox/ipred
0.14 0.40 0.40 0.41 0.50 0.48
0.64 0.97 0.88 0.57 1.00 1.00 0.96 0.98 1.00
Determined from the midpoint of Epox and Epred. b Ref 9. c Ref 8.
Figure 1. CV scans for the oxidation of 1.0 mM dppdtbpf in CH2Cl2 at 10.0 °C with 0.1 M [NBu4][PF6] as the supporting electrolyte at two different scan rates: (top) 25 mV/s; (bottom) 400 mV/s.
oxidation of dppdtbpf occurs is approximately the average of the potentials at which oxidation of dppf (0.23 V)9 and dtbpf (0.06 V)8 occur. The reversibility of the oxidation of dppdtbpf is dependent on the analyte concentration, scan rate, and temperature (Figures S.1-S.5). At scan rates greater than 200 mV/s, a reversible couple is observed, while at lower scan rates the oxidation of dtbpdppf is complicated by a follow-up reaction, suggesting an EC mechanism.20 In addition, the oxidation is more reversible at lower concentration and temperatures. This behavior is similar to the oxidation of dippf,21 1,1′-bis(dicyclohexylphosphino)ferrocene (dcpf),22 and dppf.9 The exact nature of this follow-up reaction requires further investigation. The coordination of dppdtbpf to transition metal centers was also examined. In addition to the previously reported [PdCl2(dppdtbpf)],4a the remaining group 10 complexes, [MCl2(dppdtbpf)] (M ) Ni or Pt) and [(AuCl)2dppdtbpf)], were prepared. As with the dippf, dcpf, dppf, and dtbpf analogues, the nickel complex is paramagnetic. Using the Evans method, the magnetic moment was determined to be 3.3 µB (S ) 1, 2 unpaired electrons, high-spin Ni(II) in a tetrahedral environment), which is similar to the related nickel complexes of dippf,21 dcpf,22 dppf,23 and dtbpf.8 Due to the inherent asymmetry of dppdtbpf, the remaining transition metal compounds gave two resonances in the 31P NMR spectra. The -PtBu2 and -PPh2 were assigned by comparison to the dppf23 and dtbpf8 analogues. The anodic electrochemistry of the transition metal complexes of dppdtbpf was examined in CH2Cl2 (Table 2). The reaction (20) Geiger, W. E. In Laboratory Techniques in Electroanalytical Chemistry, 2nd ed.; Kissinger, P. T., Heineman, W. R., Eds.; Marcel Dekker, Inc.: New York, 1996; p 683. (21) Ong, J. H. L.; Nataro, C.; Golen, J. A.; Rheingold, A. L. Organometallics 2003, 22, 5027. (22) Hagopian, L. E.; Campbell, A. N.; Golen, J. A.; Rheingold, A. L.; Nataro, C. J. Organomet. Chem. 2006, 691, 4890. (23) Corain, B.; Longato, B.; Favero, G.; Ajo`, D.; Pilloni, G.; Russo, U.; Kreissl, F. R. Inorg. Chim. Acta 1989, 157, 259.
Figure 2. CV scan for the oxidation of 1.0 mM [(AuCl)2dppdtbpf] in CH2Cl2 with 0.1 M [NBu4][PF6] as the supporting electrolyte at 100 mV/s.
following oxidation of dppdtbpf is likely to involve the lone pair on at least one of the phosphorus atoms. By coordinating dppdtbpf to a transition metal, the anodic electrochemistry generally simplifies (Figure 2). The oxidation of [PdCl2(dppdtbpf)] (Figure S.6), [PtCl2(dppdtbpf)] (Figure S.7), and [(AuCl)2dppdtbpf] is chemically and electrochemically reversible. The potential at which oxidation of these compounds occurs is approximately 0.4 V more positive than that of dppdtbpf, and the potentials are between those for the related dppf23 and dtbpf8 compounds. A previous report of the electrochemistry of [(AuCl)2dppf] suggested that the compound was electrochemically inert in methylene chloride and dimethylformamide;24 however this was not the case using the conditions in this study. The oxidation of [NiCl2(dppdtbpf)] displays a chemically irreversible wave at a potential that is more positive than that of free dppdtbpf (Figure S.8). This behavior is similar to the dppf9 and dtbpf8 analogues. The lack of reversibility in the nickel system has been attributed to weakening of the P-Ni bond upon oxidation of the compound.25 In addition to coordinating to a transition metal, the phosphorus atoms of dppdtbpf react with chalcogenides to form the corresponding phosphine chalcogenides. The disulfide (dppSdtbpSf) and diselenide (dppSedtbpSef) were prepared by reaction of dppdtbpf with 2 equiv of either sulfur or selenium. There are two signals in the 31P NMR for each compound. In dppSdtbpSf the chemical shift for the -P(dS)Ph2 phosphorus (24) Houlton, A.; Roberts, R. M. G.; Silver, J.; Parish, R. V. J. Organomet. Chem. 1991, 418, 269. (25) Bishop, J. J.; Davison, A.; Katcher, M. L.; Lichtenberg, D. W.; Merrill, R. E.; Smart, J. C. J. Organomet. Chem. 1971, 27, 241.
2124 Organometallics, Vol. 28, No. 7, 2009
Figure 3. Perspective view of dppdtbpSf with 30% ellipsoids. H atoms are omitted for clarity.
is similar to that of dppfS2,26 while the chemical shift of the -P(dS)tBu2 phosphorus is similar to that of dtbpfS2.8 The same is true for the chemical shifts and the 31P-77Se coupling constants in dppSedtbpSef.8,26 The inherent asymmetry of dppdtbpf provided the opportunity to prepare the mixed chalcogenide species, dppSdtbpSef and dppSedtbpSf. On the basis of the synthesis of dppfS,16 dppdtbpf and 1 equiv of either S or Se were combined and stirred for 4 h at 0 °C. Column chromatography on silica gel using a 1:1 v/v mixture of hexanes and toluene as the eluent gave in order of elution dppdtbpf, either dppdtbpSf or dppdtbpSef, and either dppSdtbpSf or dppSedtbpSef. Monosubstitution of dppdtbpf occurred exclusively at the more electron-rich -PtBu2 phosphorus. Selective reactivity of dppdtbpf has been noted in the reaction of [Ni(COD)dppdtbpf] with ZnCl2 and 2-methyl-3butenenitrile.7 The product of this reaction is (η3-C4H7)Ni(µCN)(µ-dppdtbpf)ZnCl2, in which the -PtBu2 is bonded to Zn and the -PPh2 group to Ni. It is unclear if this is due to steric and/or electronic factors. Assignment of the 1H and 31P NMR spectra of dppdtbpSf and dppdtbpSef was made by comparison to dppdtbpf, dppSdtbpSf, and dppSedtbpSef. Upon isolation of the monosubstituted compounds, dppfSSe, dppSdtbpSef, and dppSedtbpSf were prepared by adding 1 equiv of the desired chalcogen. The X-ray structures of dppdtbpSf (Figure 3), dppSdtbpSef (Figure 4), and dppSedtbpSf (Figure 5) were determined, and select bond lengths and angles are presented in Table 3. The PdS bond length in dppdtbpSf is similar to the PdS length in dippfS2.27 As measured by δ, the phosphorus atom bonded to sulfur sits significantly out of the plane of the C5 ring and away from the iron center, whereas the phosphorus of the -PPh2 group is only slightly out of the plane of the ring and away from the iron. This is similar to the δ value for the phosphorus atoms in dppf, which is -0.066 Å.28 The PdSe bond length in dppSdtbpSef is similar to that found in dtbpfSe2 (av 2.1197 Å),8 while the PdS bond length is similar to that for dppfS2 (av (26) Pilloni, G.; Longato, B.; Bandoli, G.; Corain, B. J. Chem. Soc., Dalton Trans. 1997, 819. Note: The space group for dppfS2 was initially reported as Cc but later corrected to C2/c by: Clemente, D. A.; Marzotto, A. Acta Crystallogr. Sect. B 2004, 60, 287. (27) Necas, M.; Beran, M.; Woollins, J. D.; Novosad, J. Polyhedron 2001, 20, 741. (28) Casellato, U.; Ajo, D.; Valle, G.; Corain, B.; Longato, B.; Graziani, R. J. Crystallogr. Spectrosc. Res. 1988, 18, 583.
Kahn et al.
Figure 4. Perspective view of dppSdtbpSef with 30% ellipsoids. H atoms are omitted for clarity.
Figure 5. Perspective view of dppSedtbpSf with 30% ellipsoids. H atoms are omitted for clarity. Table 3. Select Bond Lengths (Å) and Angles (deg) for dppdtbpSf, dppSdtbpSef, and dppSedtbpSf dppdtbpSf P(1)-S P(1)-Se P(2)-S P(2)-Se P(1)-C(1) P(2)-C(6) Fe-CCp (av) δPdSa δPa δPdSea XA-Fe-XBb P-Fe-P τc θd C(1)-P(1)-S C(1)-P(1)-Se C(6)-P(2)-S C(6)-P(2)-Se
dppSdtbpSef
Bond Lengths 1.9701(7) 2.1167(11) 1.9687(13) 1.8113(19) 1.813(2) 2.047 -0.251 -0.020
1.809(4) 1.797(4) 2.044 -0.052 -0.271
Bond Angles 179.20 177.74 134.03 155.17 133.24 152.98 1.64 3.29 112.42(7) 111.36(13) 112.94(13)
dppSedtbpSf 1.9867(11) 2.1083(9) 1.811(3) 1.795(3) 2.046 -0.218 -0.095 178.56 145.07 144.40 2.88 111.85(16) 115.24(11)
a
Deviation of the P atom from the C5 plane; a positive value means the P is closer to the Fe. b Centroid-Fe-centroid. c The torsion angle CA-XA-XB-CB where C is the carbon atom bonded to phosphorus and X is the centroid. d The dihedral angle between the two C5 rings.
1.941 Å).26,29 In addition, the δ for the phosphorus atom of the -P(Se)tBu2 group is similar to dtbpfSe2 (-0.268 Å),8 and the δ for the phosphorus of the -P(S)Ph2 group is similar to dppfS2 (29) Fang, Z.-G.; Hor, T. S. A.; Wen, Y.-S.; Liu, L.-K.; Mak, T. C. W. Polyhedron 1995, 14, 2403.
Compounds Containing dppdtbpf
Organometallics, Vol. 28, No. 7, 2009 2125
Table 4. Formal Potentials (V vs FcH0/+),a Potential Difference, ∆E (defined as E0cmpd - E0dppdtbpf), and Reversibility for dppdtbpf and Transition Metal Compounds Containing a dppdtbpf Ligand (all data reported at a scan rate of 100 mV/s) Monosulfides dppfS dppdtbpSf Disulfides dppfS2 dtbpfS2 dppSdtbpSf Monoselenide dppdtbpSef Diselenides dppfSe2 dtbpfSe2 dppSedtbpSef Sulfide-selenides dppfSSe dppSdtbpSef dppSedtbpSf
Epox1
Epox2
Epox3
Epox4
Epred1
0.27a
0.40
0.67
0.72 0.14e
0.60
0.40
0.68
0.95
0.77
Epred2
Epred3
0.55
-0.16
b,c
0.48 0.36b,d 0.40b 0.14a 0.32c 0.24d 0.27
1.10
0.52 0.47 0.44
1.15 1.11 1.14
-0.14 -0.07 0.78
a These waves are likely due to adsorption. b These waves are chemically and electrochemically reversible, so these values are E°. c Ref 10. d Ref 8.
Figure 6. CV scan for the oxidation of 1.0 mM dppdtbpSef in CH2Cl2 with 0.1 M tetrabutylammonium hexafluorophosphate as the supporting electrolyte at 100 mV/s.
(-0.035 Å).26,29 It would seem that the greater steric bulk of the tert-butyl groups seems to require a larger δ value. This phenomenon is also observed in dppSedtbpSf, in which the value of δ for the phosphorus of the -P(S)tBu2 group is significantly larger than that of the phosphorus of the -P(Se)Ph2 group. In addition, the PdSe bond length in dppSedtbpSf is similar to that found in dppfSe2,26 while the PdE bond lengths and the values for δ seem to indicate that the -P(E)tBu2 and -P(E)Ph2 groups do not significantly impact the structure of each other. However, many of the remaining structural parameters are intermediate between dppfS2, dppfSe2, and dtbpfSe2; for dppfS2 and dppfSe2 the angles XA-Fe-XB, P-Fe-P, and τ are all 180° and the angle θ is 0°.26,29 The same angles for dtbpfSe2 are 174.92°, 152.47°, 138.08°, and 6.29°, respectively.8 The anodic electrochemistry of these new compounds was examined in CH2Cl2 (Table 4). The monosubstituted compounds, dppdtbpSf and dppdtbpSef, displayed multiple anodic waves and a single cathodic wave (Figure 6). The potentials at which the first and last anodic waves occur, Epox1 and Epox4, are somewhat different, while the middle two waves, Epox2 and Epox3, are similar. The sharpness of the waves at 0.27 V (dppdtbpSf) and 0.14 V (dppdtbpSef) suggest that these are due to an
Figure 7. CV scan for the oxidation of 1.0 mM dppSedtbpSef in CH2Cl2 with 0.1 M tetrabutylammonium hexafluorophosphate as the supporting electrolyte at 100 mV/s. Scanning both waves (black) and scanning just the first wave (gray).
adsorption process. The cyclic voltammetry of dppdtbpSef was performed in CH2Cl2 solvent using a less reactive supporting electrolyte, [NBu4][B(C6F5)4].30 Under those conditions, three anodic waves (0.28, 0.92, and 1.22 V vs FcH0/+) and one cathodic wave (0.71 V vs FcH0/+) were observed. This suggests that at least one wave in the oxidation of dppdtbpSf and dppdtbpSef with [NBu4][PF6] is due to a species generated by reaction with the supporting electrolyte. Similar to the dppf10 and dtbpf8 analogues, the oxidation of dppSdtbpSf displays one chemically and electrochemically reversible wave. Bulk electrolysis confirms that the process is one-electron (napp ) 0.99 e-, Eapp ) 1.15 V vs Ag/AgCl based on a cyclic voltammogram of the bulk solution, T ) 22 ( 1 °C). Presumably oxidation of this compound occurs at the iron center. Oxidation of dppSedtbpSef displays two anodic waves and three cathodic waves (Figure 7). If the sweep is switched just positive of the first anodic wave, there is no change in the reverse scan. In addition, when the potential is swept from 0.1 to -1.0 V, there is no cathodic wave. The two-electron oxidation of dtbpfSe2 occurs at Se, as opposed to Fe, and results in the formation of an intramolecular Se-Se bond (eq 1).8 The similarity of the oxidation of dppSedtbpSef to dtbpfSe2 suggests that the initial oxidation occurs at the selenium of the -P(Se)tBu2 group. The fate of this selenium radical is unclear. The similarity to the oxidation of dtbpfSe2 suggests interaction with another selenium, either intermolecular with another -P(Se)tBu2 group or intramolecular with the -P(Se)Ph2 group. However, additional reactions, e.g., with the solvent or supporting electrolyte, cannot be ruled out. Initial attempts to perform chemical oxidation of dppSedtbpSef using diacetylferrocenium (E° 0.49 V vs FcH in CH2Cl2)31 did not yield any identifiable products. Additional experimental and computational studies are currently underway in order to gain a better understanding of this process. Oxidation of the mixed chalcogenide compounds, dppfSSe, dppSdtbpSef, and dppSedtbpSf, all exhibited two chemically irreversible anodic waves (Figure 8). The wave at less positive potential is likely due to oxidation of the selenium, while the other wave could be due to oxidation of the iron. (30) Ghent, B. L.; Martinak, S. L.; Sites, L. A.; Golen, J. A.; Rheingold, A. L.; Nataro, C. J. Organomet. Chem. 2007, 692, 2365. (31) Connelly, N. G.; Geiger, W. E. Chem. ReV. 1996, 96, 877.
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irreversible wave. The potentials at which the transition metal compounds undergo oxidation are more positive than that of dppdtbpf. The reaction of dppdtbpf with either sulfur or selenium results in a series of phosphine chalcogenide compounds depending on the reaction stoichiometry that is employed. The disulfide, dppSdtbpSf, undergoes a chemically and electrochemically reversible oxidation. The diselenide, dppSedtbpSef, undergoes two electrochemically irreversible oxidations steps, possibly leading to the formation of Se-Se bonds. Oxidation of the monosulfide, monoselenide, and mixed sulfide-selenides displays multiple irreversible waves.
Figure 8. CV scan for the oxidation of 1.0 mM dppSdtbpSef in CH2Cl2 with 0.1 M tetrabutylammonium hexafluorophosphate as the supporting electrolyte at 100 mV/s.
Conclusion Oxidation of dppdtbpf follows an EC mechanism and occurs at a potential between that of dppf and dtbpf. The reversibility of the oxidation is significantly influenced by the presence of tert-butyl groups on the phosphorus atoms. The synthesis of three different transition metal compounds containing dppdtbpf is straightforward and occurs in reasonable yields. The oxidative electrochemistry of dppdtbpf simplifies to a chemically and electrochemically reversible wave upon coordination with the exception of [NiCl2(dppdtbpf)], which displays a chemically
Acknowledgment. S.L.K., M.K.B., S.L.M., S.M.F., A.R.S., and C.N. thank the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial funding of this research, the Kresge Foundation for the purchase of the JEOL NMR, the Academic Research Committee at Lafayette College for funding EXCEL scholars, and Prof. Tina Huang of Lafayette College for use of the CH Instruments electrochemical analyzer. M.K.B. thanks the donors to the Joseph A. Sherma Chemistry Summer Research Fund. Supporting Information Available: cif files for the structures of dpptbpSf, dppSdtbpSef, and dppSedtbpSf, additional cyclic voltammograms, and a table of 31P NMR data. This material is available free of charge via the Internet at http://pubs.acs.org. OM800850C
