We prepared two geometric isomers of [Ir(tpy)(ppy)H](+), previously proposed as a key intermediate in the photochemical reduction of CO2 to CO, and characterized their notably different ground- and excited-state interactions with CO2 and their hydricities using experimental and computational methods. Only one isomer, C-trans-[Ir(tpy)(ppy)H](+), reacts with CO2 to generate the formato complex in the ground state, consistent with its calculated hydricity. Under photocatalytic conditions in CH3CN/TEOA, a common reactive C-trans-[Ir(tpy)(ppy)](0) species, irrespective of the starting isomer or monodentate ligand (such as hydride or Cl), reacts with CO2 and produces CO with the same catalytic efficiency.
Photochromic compounds efficiently transduce photonic energy to potential energy for excited-state bond-breaking and bond-forming reactions. A critical feature of this reaction is the nature of the electronic excited-state potential energy surface and how this surface facilitates large nuclear displacements and rearrangements. We have prepared two photochromic ruthenium sulfoxide complexes that feature two isomerization reactions following absorption of a single photon. We show by femtosecond transient absorption spectroscopy that this reaction is complete within a few hundred picoseconds and suggest that isomerization occurs along a conical intersection seam formed by the ground-state and excited-state potential energy surfaces.
Ultrafast isomerization reactions underpin many processes in (bio)chemical systems and molecular materials. Understanding the coupled evolution of atomic and molecular structure during isomerization is paramount for control and rational design in molecular science. Here we report transient X-ray absorption studies of the photo-induced linkage isomerization of a Ru-based photochromic molecule. X-ray spectra reveal the spin and valence charge of the Ru atom and provide experimental evidence that metal-centered excited states mediate isomerization. Complementary X-ray spectra of the functional ligand S atoms probe the nuclear structural rearrangements, highlighting the formation of two metal-centered states with different metal-ligand bonding. These results address an essential open question regarding the relative roles of transient charge-transfer and metal-centered states in mediating photoisomerization. Global temporal and spectral data analysis combined with time-dependent density functional theory reveals a complex mechanism for photoisomerization with atomic details of the transient molecular and electronic structure not accessible by other means.
The complexes [Ru(bpy)2(pyESO)](PF6)2 and [Os(bpy)2(pyESO)](PF6)2, in which bpy is 2,2'-bipyridine and pyESO is 2-((isopropylsulfinyl)ethyl)pyridine, were prepared and studied by (1)H NMR, UV-visible and ultrafast transient absorption spectroscopy, as well as by electrochemical methods. Crystals suitable for X-ray structural analysis were grown for [Ru(bpy)2(pyESO)](PF6)2. Cyclic voltammograms of both complexes provide evidence for S→O and O→S isomerization as these voltammograms are described by an ECEC (electrochemical-chemical electrochemical-chemical) mechanism in which isomerization follows Ru(2+) oxidation and Ru(3+) reduction. The S- and O-bonded Ru(3+/2+) couples appear at 1.30 and 0.76 V versus Ag/AgCl in propylene carbonate. For [Os(bpy)2(pyESO)](PF6)2, these couples appear at 0.97 and 0.32 V versus Ag/AgCl in acetonitrile, respectively. Charge-transfer excitation of [Ru(bpy)2(pyESO)](PF6)2 results in a significant change in the absorption spectrum. The S-bonded isomer of [Ru(bpy)2(pyESO)](2+) features a lowest energy absorption maximum at 390 nm and the O-bonded isomer absorbs at 480 nm. The quantum yield of isomerization in [Ru(bpy)2(pyESO)](2+) was found to be 0.58 in propylene carbonate and 0.86 in dichloroethane solution. Femtosecond transient absorption spectroscopic measurements were collected for both complexes, revealing time constants of isomerizations of 81 ps (propylene carbonate) and 47 ps (dichloroethane) in [Ru(bpy)2(pyESO)](2+). These data and a model for the isomerizing complex are presented. A striking conclusion from this analysis is that expansion of the chelate ring by a single methylene leads to an increase in the isomerization time constant by nearly two orders of magnitude.
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] +
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