- Homoleptic Tris-Diphosphine Re(I) and Re(II) Complexes and Re(II) Photophysics and Photochemistry
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The ligand-to-metal charge transfer state (LMCT) of [(dmpe)3Re]2+ (dmpe = 1,2-bis(dimethylphosphino)ethane) has been demonstrated to be a potent oxidant (E0(Re2+/Re+) = 2.61 V vs standard calomel electrode). This complex has been traditionally prepared by nontrivial routes in low yields, and very little has been achieved in optimizing the ground state and emission energy properties of the general class of complexes [(PP)3Re]2+ (PP = chelating diphosphine) through phosphine modification. Improved syntheses for Re(I) tris-homoleptic diphosphine complexes [(PP)3Re]+ (PP = 1,2-bis(dimethylphosphino)ethane (dmpe), 1,2-bis(diethylphosphino)ethane (depe), bis(dimethylphosphino)methane (dmpm), bis(diphenylphosphino)methane (dppm), Me2PCH2PPh2, 1,3-bis(dimethylphosphino)propane (dmpp), or 1,2-bis(dimethyl-phosphino)benzene (dmpb)) were achieved by single-pot reactions exploiting the reducing potential of the phosphines when reacted with ReV oxo-complexes in 1,2-dichlorobenzene at 160-180 °C. Single-electron chemical oxidation of [(PP)3Re]+ yields luminescent ReII analogues; appropriate use of Ph3C+, Cp2Fe+, or (4-BrC6H4)3N+ B(C6F5)4- salts produced [(PP)3Re]2+ complexes in good yields. Crystallographic trends for the Re+/Re2+ pairs show significantly lengthened Re2+-P bonds for [(PP)3Re]2+ relative to the corresponding [(PP)3Re]+ system. The redox and luminescence behavior of the complexes indicates the luminescence is from a ligand P(σ)-to-metal (Re(dπ)) charge transfer (2LMCT) state for all the complexes. Structured luminescence at 77 K is postulated to originate from relaxation of the 2LMCT state into two spin-orbit coupled states: the ground state and a state ~3000 cm-1 above the ground state. The excited-state reduction potential (Re(II/I)) for [(depe)3Re]2+ was determined from the free energy dependence of luminescence quenching rate constants. Yields for formation of charge separated ions were determined for three of the complexes with a variety of electron donors. Despite favorable electrostatics, no charge separated ions were observed for radical ion pairs for which the energy of back electron transfer exceeded 1.1 V.
- Adams, Jeramie J.,Arulsamy, Navamoney,Sullivan, B. Patrick,Roddick, Dean M.,Neuberger, Amelia,Schmehl, Russell H.
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- Manipulating the electrolyte medium to favor either one-electron or two-electron oxidation pathways for (fulvalendiyl)dirhodium complexes
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The anodic oxidations of three compounds having two Rh-containing moieties linked by a fulvalendiyl (Fv) dianion have been studied in varying electrolyte media. Following the integrated approach to manipulation of solvents and supporting electrolytes (Barriere, F.; Geiger, W. E. J. Am. Chem. Soc., in press), ΔE1/2 values (Δ1/2 = E 1/2(2) -E1/2(1)) of the successive one-electron oxidations of the neutral compounds to the corresponding dications were altered to either favor or disfavor the disproportionation of the mixed-valent intermediate. In the cases of Rh2Fv(CO)4 (1) and Rh2Fv(CO) 2(μ-dppm) (2) [Fv = C10H8, dppm = bis(diphenylphosphino)methane], replacing the [PF6]- supporting electrolyte anion with [B(C6F5)]- in CH2Cl2 changed the voltammograms from those of a single two-electron wave (having inverted E11/2 potentials) to those of two distinct normal one-electron waves. In the case of Rh2Fv-(COD)2 (3, COD = C8H12), both the solvent and supporting electrolyte were changed to manipulate ΔE 1/2 between normal and inverted values. Changes of up to 330 mV in ΔE1/2 were observed, resulting in modifications of over 105 in the disproportionation equilibrium constant, Kdisp, of the mixed-valent intermediate 3+. The thermodynamic stabilization of 1+ and 2+ allowed for their IR characterization and confirmed the previously postulated formation of Rh-Rh bonds in the cation radicals. An integrated approach to medium effects is shown to be a powerful method for manipulating the equilibrium concentrations of the various redox states that make up a multiple-electron-transfer process.
- Nafady, Ayman,Chin, Teen T.,Geiger, William E.
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- Synthetic, spectroscopic, structural, and electrochemical investigations of ferricenium derivatives with weakly coordinating anions: ion pairing, substituent, and solvent effects
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A facile and effective strategy for the preparation of a series of ferricenium complexes bearing either electron-donating or electron-withdrawing substituents with weakly coordinating anions such as [B(C6F5)4]?or SbF6?is reported. These systems were thoroughly investigated for their ground state electronic structures in both solution and solid states using infrared (IR) and nuclear magnetic resonance (NMR) spectroscopies as well as single crystal X-ray crystallography and electrochemical measurements. The X-ray structures of the six electron-deficient ferricenium derivatives are of particular interest as only a handful (~5) of such derivatives have been structurally characterized to date. Comparison of the structural data for both neutral and oxidized derivatives reveals that the nature of the substituents on the cyclopentadienyl (Cp) ligands displays a more significant impact on the metal-ligand separations (Fe?Ct) in the oxidized species than in their neutral analogs. Our1H-NMR measurements corroborate that in the neutral ferrocene derivatives, electron-donating ring substitutions lead to a greater shielding of the ring protons while electron-withdrawing groupsviainduction deshield the nearby ring protons. However, the data for the paramagnetic ferricenium derivatives reveals that this substitutional behavior is more complex and fundamentally reversed, which is further supported by our structural studies. We ascribe this reversal of behavior in the ferricenium derivatives to theδback-donation from the iron atom into the Cp rings which can lead to the overall shielding of the ring protons. Interestingly, our NMR results for the electron-deficient ferricenium derivatives in solution also indicate a direct correlation between the solvent dielectric constant and the energy barrier for rotation around the metal-ligand bond in these systems, whereas such a correlation is absent or not significant in the case of the electron-rich ferricenium species or the corresponding neutral ferrocene analogs. In this work, we also present the electrochemical behavior of the corresponding ferricenium/ferrocene redox couples including potential values (E1/2), peak-to-peak separation (ΔE1/2), and diffusion coefficients (D) of the redox active species in order to provide a concise outline of these data in one place. Our electrochemical studies involved three different solvents and two supporting electrolytes. Notably, our findings point to the significant effect of ion-pairing in lowering the energy necessary for reduction of the ferricenium ion andE1/2in lower-polarity media. This has significant implications in applications of the ferrocene or ferricenium derivatives as redox agents in low-polarity solvents where an accurate determination of redox potential is critical.
- Carrasco, Maria C.,Hematian, Shabnam,Khan, Firoz Shah Tuglak,Pourhadi, Hadi,Waldbusser, Amy L.
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p. 7433 - 7455
(2021/06/11)
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- The titanium tris-anilide cation [Ti(N[tBu]Ar)3]+ stabilized as its perfluoro-tetra-phenylborate salt: Structural characterization and synthesis in connection with redox activity of 4,4′-bipyridine dititanium complexes
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This work explores the reduction of 4,4′-bipyridine using two equivalents of the titanium(iii) complex Ti(N[tBu]Ar)3 resulting in formation of a black, crystalline complex, (4,4′-bipy){Ti(N[tBu]Ar)3}2, for which an X-ray structure determination is reported. The neutral, black, 4,4′-bipyridine-bridged bimetallic was found to be redox active, with mono- and di-anions being accessible electrochemically, and with the mono- and di-cations also being accessible chemically, and isolable, at least when using the weakly coordinating anion [B(C6F5)4]- as the counter-ion. It proved possible to crystallize the salt [(4,4′-bipy){Ti(N[tBu]Ar)3}2][B(C6F5)4]2 for a single-crystal X-ray structure investigation; in this instance it was revealed that the aromaticity of the 4,4′-bipyridine ligand, that had been disrupted upon reduction, had been regained. A rare cationic d0 metal tris-amide complex, shown by X-ray crystallography to contain an intriguing pyramidal TiN3 core geometry, namely {Ti(N[tBu]Ar)3}+, could also be isolated when using [B(C6F5)4] as the essentially non-interacting counter-ion. This highly reactive cation should be considered as a potential intermediate in the plethora of reactions wherein Ti(N[tBu]Ar)3 has been shown to effect the reduction of substrates including halogenated organic molecules, carbonyl compounds, organic nitriles, and metal complexes. This journal is
- Spinney, Heather A.,Clough, Christopher R.,Cummins, Christopher C.
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p. 6784 - 6796
(2015/04/14)
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