Ligand-Based Reduction and Electron-Transfer-Induced Tub-to-Chair Isomerization of Cyclooctatetraene Ligand in Rh(η⁵-C₅Ph₅)(η⁴-C₈H₈)

Abstract

The electrochemical reduction of Rh(η 5 -C 5 Ph 5 )(η 4 -C 8 H 8 ), 1, has been studied in THF/0.1 M [NBu 4 ]A, where A ) [PF 6 ]or [CF 3 SO 3 ] -. Cyclic, differential pulse, and square-wave voltammetry establish that 1 exists as two isomers having tub-shaped (1,5bonded) or chair-shaped (1,3-bonded) cyclooctatetraene (COT) ligands. The former is the major isomer at room temperature. One-electron reduction of 1,5-1 (E pc ≈ -2.7 V vs ferrocene) results in rapid isomerization to the chair isomer. Electrolysis produces [1,3-1] -(E 1/2 ) -2.37 V), which has ESR spectra consistent with a SOMO that is largely (COT) ligand-based. Equilibrium and rate constants are estimated for a square scheme describing the combined electron-transfer/isomerization sequence. The tub isomer is more highly favored at room temperature for Rh(η 5 -C 5 Ph 5 )(COT) than for the previously studied Co(η 5 -C 5 H 5 )(COT). Because the redox behavior of 1 closely parallels that of the cobalt analogues, the present result strengthens evidence for the decidedly different electronic structures of nominally isoelectronic Co-group versus Ni-group η 4 -COT compounds. Formally d 9 complexes of the former are classified as highly delocalized "18 + δ" complexes, whereas the latter are more traditional 19-electron systems.

Redox-Dependent Isomerization of Organometallic RuII/RuIIICompounds Containing the Hydrotris(methimazolyl)borate Ligand: An Electrochemical Square Scheme Mechanism

“…Conformational changes or ligand substitutions about a metal ion that result in substantial structural changes can occur prior to, during, or after electron-transfer steps. , If the structural rearrangement occurs simultaneously to the electron transfer, the process is termed a “concerted” single-step (E) reaction, whereas a “consecutive” or “gated” process involves the transformation or chemical step (C) occurring before (CE) or after (EC) the electron transfer. , Of particular interest among consecutive reactions are those where both the reduced and oxidized species undergo a chemically reversible structural change so that a “square scheme” mechanism results (Scheme ). ,,− Often in purely synthetic studies only the final stable product(s) are identified out of solution; consequently, unless the square scheme is specifically looked for, some of the species in Scheme may never be detected. In this study a combined synthetic, spectroscopic (NMR), and dynamic electrochemical (cyclic voltammetry) approach is used, enabling characterization of all of the solution-phase species in Scheme and their rates of interconversion.…”

Consequences of the Protonation of the 19-Electron Anion [Co(η5-C5H5)(1,5-C8H12)]−

“…The thermodynamically stable isomer of both systems is the tub-shaped structure in which the metal is 1,5-bonded to the eight-membered ring. One-electron reduction of CoCp(1,5-COT) gave a radical anion, which underwent rapid isomerization to the chair-shaped isomer [CoCp(1,3-COT)] − , a structural change that has been authenticated by a series of electrochemical, ESR, NMR, and theoretical studies. ,− A similar, but much slower, 1,5- to 1,3-type structural change was invoked to account for the new product formed when CoCp(COD) was cathodically electrolyzed at room temperature . The primary experimental evidence for the 1,5- to 1,3-isomerization of the COD ligand was the observation of free 1,3-COD when the product was oxidatively cleaved .…”

“…All experiments were carried out under nitrogen using techniques previously described . Emphasis was given to providing dry THF for electrochemical work by doing the final distillation from dark purple sodium/benzophenone under static vacuum (ca.…”

Electrochemistry of flavins

“…57 In other oxidases, a positively charged residue is present around the flavin N5, 58 and is postulated to stabilize the superoxide anion (formed during the first stepwise electron transfer to dioxygen) via electrostatic interactions. [58][59][60][61][62][63] In vanillyl alcohol oxidase-type flavoproteins, it was observed that a covalent linkage between the flavin cofactor and the protein increases the redox potential of the flavin significantly, limiting the number of feasible electron acceptors. 32,64 It was also observed that bicovalently linked flavin cofactors have a very open active site, while retaining the cofactor in position for catalysis, allowing them to accept bulky substrates.…”

“…When the electron transfer steps and reversible chemical transformations occur in a consecutive manner, the mechanism can be represented as a square-scheme. [50][51][52][53][54][55][56][57][58][59][60][61][62] The complete electrochemical reduction mechanism of riboflavin in DMSO is given in Scheme 2. Fl…”