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From gas-phase ionization energies to solution oxidation potentials: Dimolybdenum tetraformamidinate paddlewheel complexes By Van Dorn, Laura O.; Borowski, Susan C.; Lichtenberger, Dennis L.
The gas-phase ionization energies of a series of Mo2(DPhF)4 paddlewheel complexes (DPhF is the N,N'-diphenylformamidinate anion with p-CH3, p-Cl, m-Cl, p-CF3, or m-CF3 Ph substituents) have been measured by UPS and compared with the soln. oxidn. potentials measured by cyclic voltammetry (CV) reported by Ren and coworkers. A linear relationship was found between the gas-phase ionization energies and the soln. oxidn. potentials. D. functional theory (DFT) computations clarify the individual electronic and thermodn. factors that contribute to the correlation. The metal-metal delta bond electron energy is the largest factor in detg. the soln. oxidn. potential. The substituents shift the metal-metal orbital energies by changing the through-space field potential at the metals rather than by an inductive change in charge at the metals or orbital overlap effects. The cation solvation energies det. the extent that the potential shifts are attenuated in soln. The results show that substituent field effects and solvation have major roles in detg. the dimetal redox chem. even when the dimetal unit is protected from direct interaction with the substituent and the solvent.
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Solubilizing the Most Easily Ionized Molecules and Generating Powerful Reducing Agents Gina M. Chiarella, F. A. Cotton, Jason C. Durivage, Dennis L. Lichtenberger and Carlos A. Murillo. J. Am. Chem. Soc., 2013, 135, 17889-17896. (http://dx.doi.org/10.1021/ja408291k)
Electron transfer reactions represent one of the most important and ubiquitous processes in Chemistry and they are also essential to sustain life. Stable, strong oxidizing and reducing agents have many important chemical and material applications. Especially needed are strong redox agents to be utilized in non-aqueous, homogeneous systems and for reactions in which stoichiometric control is critical. Two very soluble compounds having W2(bicyclic guanidinate)4 paddlewheel structures show record low ionization energies and very negative oxidation potentials in THF (–1.84 to –1.90 V vs Ag/AgCl). These compounds are thermally stable and easy to synthesize in high yields and good purity. They are very reactive and potentially useful reducing agents in non-polar, non-protonated solvents.
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The Electronic Structure and Bonding of the First p-Block Paddlewheel Complex, Bi2(trifluoroacetate)4, and Comparison to d-Block Transition Metal Paddlewheel Complexes: A Photoelectron and Density Functional Theory Study Jason C. Durivage, Nadine E. Gruhn, Bo Li, Evgeny V. Dikarev, Dennis L. Lichtenberger J. Clust. Sci., 2008, 19 (1), 275–294.
The photoelectron spectrum and a density functional computational analysis of the first p-block paddlewheel complex, Bi2(tfa)4, where tfa = (O2CCF3)-, are reported. The photoelectron spectrum of Bi2(tfa)4 contains an ionization band between the region of metal-based ionizations and the region of overlapping ligand ionizations that is not seen in the photoelectron spectra of d-block paddlewheel complexes. This additional ionization arises from an a1g symmetry combination of the tfa ligand orbitals that is directed for sigma bonding with the metals, and the unusual energy of this ionization follows from the different interaction of this orbital with the valence s and p orbitals of Bi compared to the valence d orbitals of transition metals. There is significant mixing between the Bi–Bi sigma bond and this a1g M–L sigma orbital. This observation led to a re-examination of the ionization differences between Mo2(tfa)4 and W2(tfa)4, where the metal–metal sigma and pi ionizations are overlapping for the Mo2 molecule but a separate and sharp sigma ionization is observed for the W2 molecule. The coalescing of the sigma and pi bond ionizations of Mo2(tfa)4 is due to greater ligand orbital character in the Mo–Mo sigma bond (~7%) versus the W–W sigma bond (~1%).
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Photoelectron Spectroscopy and DFT Calculations of Easily Ionized Quadruply Bonded Mo24+Compounds and Their Bicyclic Guanidinate Precursors F. Albert Cotton, Jason C. Durivage, Nadine E. Gruhn, Dennis L. Lichtenberger, Carlos A. Murillo, Laura O. Van Dorn, Chad C. Wilkinson. J. Phys. Chem. B, 2006, 110 (40), 19793–19798. http://dx.doi.org/10.1021/jp061820m
A series of five bicyclic guanidinate compounds containing various combinations of five- and six-membered rings and substituted alkyl groups have been shown by photoelectron spectroscopy to be easily ionized, with the one having two six-membered rings and four ethyl groups being the most easily ionized. The corresponding anions are capable of forming paddlewheel compounds having quadruply bonded Mo24+ units which are also easy to ionize. The most easily ionized compound is the ethyl-substituted Mo2(TEhpp)4 complex which has a broad first ionization band centered around 4.27 ± 0.03 eV and an ionization onset at the very low energy of 3.93 ± 0.03 eV. Even the compound with ligands containing two five-membered rings, which favors a long Mo−Mo separation because of the large ligand bite, has an ionization energy (4.78 eV) that is less than those of well-known organometallic reducing agents such as (η5-C9Me7)2Co and (η5-C5Me5)2Cr.
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Facilitating Access to the Most Easily Ionized Molecule:an Improved Synthesis of the Key Intermediate, W2(hpp)4Cl2, and Related Compounds F. Albert Cotton, James P. Donahue, Nadine E. Gruhn, Dennis L. Lichtenberger, Carlos A. Murillo, Daren J. Timmons, Laura O. Van Dorn, Dino Villagrán, Xiaoping Wang. Inorg. Chem., 2006, 45 (1), pp 201–213. http://dx.doi.org/10.1021/ic0515709
A far superior synthesis is reported for W2(hpp)4Cl2, a key intermediate in the synthesis of the most easily ionized closed-shell molecule W2(hpp)4 (hpp ) the anion of the bicyclic guanidine compound 1,3,4,6,7,8-hexahydro-2Hpyrimido[1,2-a]pyrimidine). At 200 °C, the one-pot reaction of the air-stable and commercially available compounds W(CO)6 and Hhpp in o-dichlorobenzene produces W2(hpp)4Cl2 in multigram quantities with isolated yields of over 90%. At lower temperatures, the reaction can lead to other compounds such as W(Hhpp)2(CO)4 or W2(µ-CO)2(µ- hpp)2(η2-hpp)2, which are isolable in good purity depending upon the specific conditions employed. These compounds provide insight into the reaction pathway to W2(hpp)4Cl2 and W2(hpp)4. Two additional derivatives, W2(hpp)4X2 where X is PF6 - or the anion tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (TFPB), have also been synthesized and structurally characterized. A comparison of the electrode potentials of W2(µ-CO)2(µ-hpp)2(η2-hpp)2 and the di-p-anisylformamidinate analogue shows that oxidation of the hpp compound is significantly displaced (1.12 V) and shows that the bicyclic guanidinate ligand is considerably better than the formamidinate anion at stabilizing high oxidation states. A differential pulse voltammogram of W2(hpp)4(TFPB)2 in THF shows two reduction processes with an E1/2 of −0.97 V for the first and −1.81 V (vs Ag/AgCl) for the second. DFT calculations on the W2(hpp)42+ units in W2(hpp)4X2 compounds show that the metal−metal bonding orbitals are destabilized by the axial ligands, which accounts for significant variations in the W−W distances. The low-energy gas-phase ionizations of W2(hpp)4 are also reported and discussed.
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Expeditious Access to the Most Easily Ionized Closed-Shell Molecule, W2(hpp)4 F. Albert Cotton, James P. Donahue, Dennis L. Lichtenberger, Carlos A. Murillo, Dino Villagrán. J. Am. Chem. Soc., 2005, 127 (31), 10808–10809. http://dx.doi.org/10.1021/ja0535458
A new synthetic path, far superior to either of those previously available, to the W2(hpp)4molecule (Hhpp = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine) is reported. The reaction of W(CO)6 with Hhpp in o-dichlorobenzene at 200 °C produces W2(hpp)4Cl2 in a one-pot reaction in over 90% yield. This compound is stable and easily stored for further use, and it can be efficiently reduced in a one-step reaction to the title compound W2(hpp)4.
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Closed-Shell Molecules That Ionize More Readily than Cesium F. Albert Cotton, Nadine E. Gruhn, Jiande Gu, Penglin Huang, Dennis L. Lichtenberger, Carlos A. Murillo, Laura O. Van Dorn, Chad C. Wilkinson. Science, 2002, 298 (5600), 1971-1975. http://www.jstor.org/stable/3833121
We report a class of molecules with extremely low ionization enthalpies, one member of which has been determined to have a gas-phase ionization energy (onset, 3.51 electron volts) lower than that of the cesium atom (which has the lowest gas-phase ionization energy of the elements) or of any other known closed-shell molecule or neutral transient species reported. The molecules are dimetal complexes with the general formula M2(hpp)4 (where M is Cr, Mo, or W, and hpp is the anion of 1,3,4,6,7,8-hexahydro-2Hpyrimido[1,2-a]pyrimidine), structurally characterized in the solid state, spectroscopically characterized in the gas phase, and modeled with theoretical computations. The low-energy ionization of each molecule corresponds to the removal of an electron from the delta bonding orbital of the quadruple metal-metal bond, and a strong interaction of this orbital with a filled orbital on the hpp ligands largely accounts for the low ionization energies. |
