Synthesis and Characterization of Cationic Rhodium Peroxo Complexes

Exploiting Molecular Symmetry to Quantitatively Map the Excited-State Landscape of Iron-Sulfur Clusters

Cuboidal [Fe4S4] clusters are ubiquitous cofactors in biological redox chemistry. In the [Fe4S4]1+ state, pairwise spin coupling gives rise to six arrangements of the Fe valences ("valence isomers") among the four Fe centers. Because of the magnetic complexity of these systems, it has been challenging to understand how a protein's active site dictates both the arrangement of the valences in the ground state as well as the population of excited-state valence isomers. Here, we show that the ground-state valence isomer landscape can be simplified from a six-level system in an asymmetric protein environment to a two-level system by studying the problem in synthetic [Fe4S4]1+ clusters with solution C3v symmetry. This simplification allows for the energy differences between valence isomers to be quantified (in some cases with a resolution of <0.1 kcal/mol) by simultaneously fitting the VT NMR and solution magnetic moment data. Using this fitting protocol, we map the excited-state landscape for a range of clusters of the form [(SIMes)3Fe4S4-X/L]n, (SIMes = 1,3-dimesityl-imidazol-4,5-dihydro-2-ylidene; n = 0 for anionic, X-type ligands and n = +1 for neutral, L-type ligands) and find that a single ligand substitution can alter the relative ground-state energies of valence isomers by at least 103 cm-1. On this basis, we suggest that one result of "non-canonical" amino acid ligation in Fe-S proteins is the redistribution of the valence electrons in the manifold of thermally populated excited states.

Reactions of POP-pincer rhodium(I)-aryl complexes with small molecules: coordination flexibility of the ether diphosphine

Reactions of the aryl complexes Rh(aryl){κ3-P,O,P-[xant(PiPr2)2]} (1; aryl = 3,5-Me2C6H3 (a), C6H5 (b), 3,5-Cl2C6H3 (c), 3-FC6H4 (d); xant(PiPr2)2 = 9,9-dimethyl-4,5-bis-(diisopropylphosphino)xanthene) with O2, CO, and MeO2CC≡CCO2Me have been performed. Under 1 atm of O2, the pentane solutions of complexes 1 afford the dinuclear peroxide derivatives [Rh(aryl){κ2-P,P-xant(PiPr2)2}]2(μ-O2)2 (2a–2d) as yellow solids. In solution, these species are unstable. In dichloromethane, at room temperature, they are transformed into the dioxygen adducts Rh(aryl)(η2-O2){κ3-P,O,P-[xant(PiPr2)2]} (3a–3d), as a result of the rupture of the double peroxide bridge and the reduction of the metal center. Complex 3b decomposes in benzene, at 50 °C, to give diphosphine oxide, phenol, and biphenyl. Complexes 1 react with CO to give the square-planar mono carbonyl derivatives Rh(aryl)(CO){κ2-P,P-[xant(PiPr2)2]} (4a–4d), which under carbon monoxide atmosphere evolve to benzoyl species Rh{C(O)aryl}(CO){κ2-P,P-[xant(PiPr2)2]} (5a–5d), resulting from the migratory insertion of CO into the Rh-aryl bond and the coordination of a second CO molecule. The transformation is reversible; under vacuum, complexes 5 regenerate the precursors 4. The addition of the activated alkyne to complexes 1b and 1d initially leads to the π-alkyne intermediates Rh(aryl){η2-C(CO2Me)≡C(CO2Me)}{κ3-P,O,P-[xant(PiPr2)2]} (6b, 6d), which evolve to the alkenyl derivatives Rh{(E)-C(CO2Me)=C(CO2Me)aryl}{κ3-P,O,P-[xant(PiPr2)2]} (7b, 7d). The diphosphine adapts its coordination mode to the stability requirements of the different complexes, coordinating cis-κ2-P,P in complexes 2, fac-κ3-P,O,P in compounds 3, trans-κ2-P,P in the mono carbonyl derivatives 4 and 5, and mer-κ3-P,O,P in products 6 and 7.

Inner-Sphere Oxygen Activation Promoting Outer-Sphere Nucleophilic Attack on Olefins

Alkoxylation and hydroxylation reactions of 1,5-cyclooctadiene (cod) in an iridium complex with alcohols and water promoted by the reduction of oxygen to hydrogen peroxide are described. The exo configuration of the OH/OR groups in the products agrees with nucleophilic attack at the external face of the olefin as the key step. The reactions also require the presence of a coordinating protic acid (such as picolinic acid (Hpic)) and involve the participation of a cationic diolefin iridium(III) complex, [Ir(cod)(pic)₂ ]⁺ , which has been isolated. Independently, this cation is also involved in easy alkoxy group exchange reactions, which are very unusual for organic ethers. DFT studies on the mechanism of olefin alkoxylation mediated by oxygen show a low-energy proton-coupled electron-transfer step connecting a superoxide-iridium(II) complex with hydroperoxide-iridium(III) intermediates, rather than peroxide complexes. Accordingly, a more complex reaction, with up to four different products, occurred upon reacting the diolefin-peroxide iridium(III) complex with Hpic. Moreover, such hydroperoxide intermediates are the origin of the regio- and stereoselectivity of the hydroxylation/alkoxylation reactions. If this protocol is applied to the diolefin-rhodium(I) complex [Rh(pic)(cod)], free alkyl ethers ORC₈ H₁₁ (R=Me, Et) resulted, and the reaction is enantioselective if a chiral amino acid, such as l-proline, is used instead of Hpic.

Unbridged Rh(ii)–Rh(ii) complexes of N-heterocyclic carbenes and reactions with O2 to form dirhodium(μ–η1:η1-O2) complexes

Dimeric rhodium(ii) [Rh(L)(CH₃CN)]₂(PF₆)₄ and rhodium(iii) peroxide [Rh(L)(PPh₃)]₂(μ–η¹:η¹-O₂)(PF₆)₄ and [Rh(L)(PCy₃)]₂(μ–η¹:η¹-O₂)(PF₆)₄ (L = bis(N-pyridylimidazolylidenyl)methane) were reported.

Multiple binding modes of an unconjugated bis(pyridine) ligand stabilize low-valent [Cp*Rh] complexes

An unconjugated bis(pyridine) ligand enables sequential one-electron reductions of a [Cp*Rh] complex, revealing the ligand's ability to stabilize low-valent species.

A rhodium(i)–oxygen adduct as a selective catalyst for one-pot sequential alkyne dimerization-hydrothiolation tandem reactions

An air-stable rhodium(i)–oxygen adduct featuring a CNC-pincer ligand, based on 1,2,3-triazol-5-ylidenes, catalyzes the homo-dimerization and hydrothiolation of alkynes, affording the gem-enyne and α-vinyl sulfide isomers, respectively, with excellent selectivity.

RhCl(PPh3)3-mediated C–H oxyfunctionalization of pyrrolido-functionalized bisazoaromatic pincers: a combined experimental and theoretical scrutiny of redox-active and spectroscopic properties

Non-trivial coordination mode of symmetrical NNN ligands with Rh(iii) leads to redox-active NNO-scaffolds via C(sp²)–H oxyfunctionalization at rt, opening an opportunity to juxtapose different redox-active domains.

Pyridine versus acetonitrile coordination in rhodium–N-heterocyclic carbene square-planar complexes

Experimental and theoretical studies on the factors that control the coordination chemistry of N-donor ligands in square-planar complexes of the type RhCl(NHC)L¹L² (NHC = N-heterocyclic carbene) are presented.