Igartúa-Nieves, Elvin

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    Synthesis, mechanisms, metal-[60]fullerene bond enthalpies and electrochemistry of [60]fullerene transition metal carbonyl complexes
    (2006) Igartúa-Nieves, Elvin; Cortés-Figueroa, José E.; College of Arts and Sciencies - Sciences; Meléndez-Martínez, Enrique; De los A. Joubert, Aidalú; Department of Chemistry; Quintana-Cheeseborough, Anibal
    In this work the ligand exchange reactions on (η²-C₆₀)(η²-dppe)M(CO)₃ (M = Cr, Mo)(dppe = diphenylphosphinoethane) and (η²-C₆₀)M(CO)₅ (M = Cr, Mo, W) with piperidine were investigated as well as the electrochemical profiles of (η²-C₆₀)M(CO)₅. The Lewis base (L) piperidine displace dppe from the complex 𝘧𝘢𝘤-(η²-C₆₀)(η²-dppe)Cr (CO)₃ producing 𝘧𝘢𝘤-(η²-C₆₀)(η¹-pip)₂Cr(CO)₃. The progress of the reactions was followed by observing the decrease of the absorbance values with time at 330 nm and by monitoring the stretching carbonyl region from 1700 to 2100 cm⁻¹. The plots of absorbance vs. time were exponential and first order with respect to [(η²-C₆₀)(η²-dppe)Cr(CO)₃] for reactions under flooding conditions where [pip]>> [(η²-C₆₀)(η²-dppe)Cr(CO)₃]. Curved (upward) plots of k𝘰𝘣𝘴𝘥 vs. [pip] and linear plots of k𝘰𝘣𝘴𝘥 vs. [pip]² for the reactions of 𝘧𝘢𝘤-(η²-C₆₀)(η²-dppe)Cr(CO)₃ indicate that the piperidine-assisted dppe displacement from 𝘧𝘢𝘤-(η²-C₆₀)(η²-dppe)Cr(CO)₃ is second order with respect to the molar concentration of piperidine. The progresses of the reactions of (η²-C₆₀)M(CO)₅ with Lewis bases (L= piperidine and PPh₃) were monitored by observing the decrease of the absorbance values at various wavelength, depending on the complex. The reactions were studied under flooding conditions where the concentrations of L were at least two hundred times greater than the concentrations of (η²-C₆₀)M(CO)₅ (𝘤𝘢. 10⁻⁵ M). The pseudo-first order rate constant values were determined using a non-linear curve fitting of the absorbance versus time plots. Under conditions where [C₆₀]/[L] ≈ 0, the rate constant values were independent of the ligand molar concentration [L]. For L = piperidine, the reactions were biphasic where the plots of absorbance vs. time were biexponentials. These plots consisted of two consecutive segments. In the first segment, the absorbance values decreased with time, whereas in the second segment, the absorbance values increased with time. Displacement of [60]fullerene from (η²-C₆₀)M(CO)₅ producing (η¹-pip)M(CO)₅ was ascribed to the first segment of the plots, while the second segment was assigned to step-wise additions of piperidine to uncoordinated [60]fullerene. Analysis of cyclic voltammetry measurements on [60]fullerene and on (η²-C₆₀)M(CO)₅ complexes suggests a decrease of the C-C double bond character between the carbons in the [6:6] junctions neighboring six-membered rings on [60]fullerene. Each (η²-C₆₀)M(CO)₅ complex exhibits three reversible one-electron reduction potential. The reduction bands of the complexes are [60]fullerene-centered and most of them are shifted to positive values relative to the corresponding reductions of uncoordinated [60]fullerene. This positive shift is ascribed to a Jahn-Teller type distortion of the spherical surface of [60fullerene. This distortion may be a direct consequence of the high electron affinity of [60]fullerene where [60]fullerene accepts electrons from the metal via π-back bonding.
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    Experimental and Ab initio studies of intramolecular Ph/Cl exchange on Wilkinson’s catalyst and ligand-centered electrochemistry of ligand substituted metal carbonyls (M = Cr, Mo, W, Ir)
    (2012) Igartúa-Nieves, Elvin; Cortés-Figueroa, José E.; College of Arts and Sciences - Sciences; Santana-Vargas, Alberto; Meléndez-Martínez, Enrique; Rivera-Pagán, José A.; Cádiz-García, Mayra E.; Department of Chemistry; Martínez-Iñesta, María M.
    Cyclic voltammetry measurements on (η2 -C60)M(CO)5 complexes (M = Cr, Mo, W) and on (C5H10N)3(H)3C60 in dichloromethane showed three C60-centered and reversible reduction/oxidation waves. The E1/2 values of these waves are shifted to positive values relative to the corresponding uncoordinated C60 values. A Jahn -Teller type effect due to C60 spherical surface distortion promoted by C60-metal π-backbonding may explain the observed positive shifts. The complex Ir(CO)(PPh3)2(Cl)(C60) was spectroscopically detected in C60-saturated liquid solutions. Instead, the species Ir(CO)(PPh3)2(Cl)(C60 n- ) (n = 4, 5, 6) were detected electrochemically. Results suggest that electrochemically produced C60 4- and C60 5- displace acetonitrile from Ir(CO)(PPh3)2(Cl)(CH3CN) forming Ir(CO)(PPh3)2(Cl)(C60 4- ) and Ir(CO)(PPh3)2(Cl)(C605- ) followed by one-electron electrochemical reductions forming Ir(CO)(PPh3)2(Cl)(C60 5- ) and Ir(CO)(PPh3)2(Cl)(C606- ), respectively. The Wilkinson’s catalyst when dissolved in benzene or C6D6 under nitrogen or under high vacuum undergoes a series of unreported reactions at room temperature. The species (Ph3P)2Rh(η 2 -(C6H4PPh2))(Ph)(H), Ph-C6D5 and (Ph3P)3Rh(D) were detected in addition to expected formation of the cloro-bridged (Ph3P)4Rh2(μ-Cl)2. The mechanistic description of the reactions based on the behavior of its fluoro-congener, kinetics experiments, and ab initio computational studies, involves reversible intramolecular P-Ph/Rh-Cl exchange on the catalyst producing the non-steady-state intermediate cis-(Ph3P)2Rh(Ph)(Ph2PCl) via metal-phosphido formation. Intermolecular PPh3/PPh2Cl exchange on cis-(Ph3P)2Rh(Ph)(Ph2PCl) produces (Ph3P)3Rh(Ph), that in turn undergoes cyclometalation, solvent oxidative addition and reductive elimination producing Ph-C6D5.