Striking Differences in Properties of Geometric Isomers of [Ir(tpy)(ppy)H]⁺: Experimental and Computational Studies of their Hydricities, Interaction with CO₂, and Photochemistry

Abstract

The photochemical reduction of CO 2 to useful chemicals such as CO,formic acid, or methanol has gathered significant attention during the last several decades owing to problems related to the depletion of fossil fuels and global warming. [1] Despite the challenges associated with the high thermodynamic and kinetic stability of CO 2 ,anumber of photocatalytic systems have been investigated with transition metal polypyridyl complexes [2] as photosensitizers and as catalysts/ precatalysts together with sacrificial electron donors.T ypical products of photochemical CO 2 reduction are CO and formate,w hich have been proposed to form through metalcarboxylate and metal-hydride intermediates,r espectively, however in some cases proton reduction to H 2 is acompeting reaction.Sato et al. recently reported an efficient photocatalytic system with [Ir(tpy)(ppy)Cl] + (Ir(tpy)(ppy) = complex 1, tpy = 2,2':6',2''-terpyridine,p py = 2-phenylpyridine) and triethanolamine (TEOA) in CH 3 CN that selectively reduces CO 2 to CO under visible light (410 < l < 750 nm) with aTON of 38 and quantum yield (F CO )o f0 .13.[3] This system is twice as efficient as the well-known [Re(bpy)(CO) 3 Cl] system under the given conditions.[3] Thep roposed catalytic cycle includes the formation of the one-electron-reduced (OER) species, followed by the loss of the coordinated Cl À and the formation of the hydride species with further reduction. While [1-H] + was detected by 1 HNMR during the photocatalytic reaction, it does not react with CO 2 in its ground state.T herefore,t he further reduced species produced by its photoreduction was considered to react with CO 2 to form aCO 2 adduct, followed by the removal of CO.[3] Although the isolation of the hydride complex was mentioned, no synthetic or mechanistic details were provided regarding the reactivity of the hydride.More recently,R eithmeier et al. have developed mononuclear Ir III photocatalysts [Ir(tpy)(mppy)Cl] + (mppy = 4-methyl-2-phenylpyridine) and the dinuclear analogues with bis(2-phenylpyridin-4-yl) bridges for photo-induced CO 2 reduction.[4] They proposed the involvement of five-coordinate [Ir(tpy)(mppy)] +/0 without any hydrides.B ecause the involvement of Ir-hydride intermediates in photochemical CO 2 reduction is not clear, we believe that am ore complete understanding of Ir-hydride intermediates is important for the rational development of new catalysts for CO 2 reduction/CO 2 hydrogenation. Here we have isolated two isomers of [1-H] +

The Trans Effect in Electrocatalytic CO2 Reduction: Mechanistic Studies of Asymmetric Ruthenium Pyridyl-Carbene Catalysts

“…51 In situ isomerization has also been implicated in photochemical CO 2 reduction by Ir polypyridyl catalysts. 52 The reduction-induced isomerization process was further probed through a full scan rate dependence of N-Ru-MeCN 2+ . At slow scan rates, the second reduction of N-Ru-MeCN 2+ (E pc2 ) appears at the same potential as that of C-Ru-MeCN 2+ .…”

“…The notion that geometric isomerization along the catalytic cycle may be a general phenomenon is supported by the observation that photochemical CO 2 reduction catalyzed by [Ir(tpy)(ppy)(Cl)] + (ppy is 2-phenylpyridine) also proceeds through a common five-coordinate intermediate with the strong σ-donor trans to the coordination vacancy. 52,59 Another general design principle is apparent in the pairing of the pyridyl-carbene ligand with the redox-active tpy ligand. The computational and experimental data are consistent with the first and second reductions having a high degree of tpy character.…”

Distinct Mechanisms and Hydricities of Cp*Ir-Based CO2 Hydrogenation Catalysts in Basic Water

“…With the aim of bridging the knowledge gap of thermodynamic vs kinetic hydricities, here we have computed the thermodynamic hydricities for a series of 74 iridium hydrides in aqueous solution. Studied systems include the Cp*Ir complexes with picolinamidate ligands and other previously reported ligands (see the subsection on hydride structures in the Supporting Information for further details). − ,− The thermodynamic hydricities were computed relative to the experimentally measured value of 31.5 kcal/mol for [Cp*Ir­(bpy)­H] + (bpy = 2,2′-bipyridine) by using the isodesmic relation defined by eq where q = −1, 0, +1. …”

Triplet Excited States Modulated by Push–Pull Substituents in Monocyclometalated Iridium(III) Photosensitizers

“…8,9 In contrast, monocyclometalated complexes comprising labile monodentate ligands, the structure of which is symbolized by [Ir(N ∧ N ∧ N)(C ∧ N)X] + (N ∧ N ∧ N. tridentate polypyridyl ligand; X, monodentate anionic ligand), have been explored from a different perspective. Sato and 2,2′:6′,2″-terpyridine (tpy) (Figure 1a), 10 and subsequent related studies were conducted by several research groups, including the Bernhard, 11 Rieger, 12 Fujita, 13 and Bonchio groups. 14 Notably, Ir1 can photocatalyze CO 2 reduction in a single molecular unit with a sacrificial electron donor (i.e., a two-component system), even though such a reaction is typically driven by a three-component system consisting of a photosensitizer, a molecular catalyst, and an electron donor.…”