Creutz-Taube Ion Research Articles

Cover Picture: Symmetric Electron Transfer Coordinates are Intrinsic to Bridged Systems: An ab Initio Treatment of the Creutz–Taube Ion (Angew. Chem. Int. Ed. 31/2024)

Cover Picture: Symmetric Electron Transfer Coordinates are Intrinsic to Bridged Systems: An ab Initio Treatment of the Creutz–Taube Ion (Angew. Chem. Int. Ed. 31/2024)

Titelbild: Symmetric Electron Transfer Coordinates are Intrinsic to Bridged Systems: An ab Initio Treatment of the Creutz–Taube Ion (Angew. Chem. 31/2024)

Titelbild: Symmetric Electron Transfer Coordinates are Intrinsic to Bridged Systems: An ab Initio Treatment of the Creutz–Taube Ion (Angew. Chem. 31/2024)

Symmetric Electron Transfer Coordinates are Intrinsic to Bridged Systems: An ab Initio Treatment of the Creutz–Taube Ion

AbstractA long‐standing question in electron transfer research concerns the number and identity of collective nuclear motions that drive electron transfer or localisation. It is well established that these nuclear motions are commonly gathered into a so‐called electron transfer coordinate. In this theoretical study, we demonstrate that both anti‐symmetric and symmetric vibrational motions are intrinsic to bridged systems, and that both are required to explain the characteristic shape of their intervalence charge transfer bands. Using the properties of a two‐state Marcus–Hush model, we identify and quantify these two coordinates as linear combinations of normal modes from ab initio calculations. This quantification gives access to the potential coupling, reorganization energy and curvature of the potential energy surfaces involved in electron transfer, independent of any prior assumptions about the system of interest. We showcase these claims with the Creutz–Taube ion, a prototypical Class III mixed valence complex. We find that the symmetric dimension is responsible for the asymmetric band shape, and trace this back to the offset of the ground and excited state potentials in this dimension. The significance of the symmetric dimension originates from geometry dependent coupling, which in turn is a natural consequence of the well‐established superexchange mechanism. The conceptual connection between the symmetric and anti‐symmetric motions and the superexchange mechanism appears as a general result for bridged systems.

Symmetric Electron Transfer Coordinates are Intrinsic to Bridged Systems: An ab Initio Treatment of the Creutz-Taube Ion.

A long-standing question in electron transfer research concerns the number and identity of collective nuclear motions that drive electron transfer or localisation. It is well established that these nuclear motions are commonly gathered into a so-called electron transfer coordinate. In this theoretical study, we demonstrate that both anti-symmetric and symmetric vibrational motions are intrinsic to bridged systems, and that both are required to explain the characteristic shape of their intervalence charge transfer bands. Using the properties of a two-state Marcus-Hush model, we identify and quantify these two coordinates as linear combinations of normal modes from ab initio calculations. This quantification gives access to the potential coupling, reorganization energy and curvature of the potential energy surfaces involved in electron transfer, independent of any prior assumptions about the system of interest. We showcase these claims with the Creutz-Taube ion, a prototypical Class III mixed valence complex. We find that the symmetric dimension is responsible for the asymmetric band shape, and trace this back to the offset of the ground and excited state potentials in this dimension. The significance of the symmetric dimension originates from geometry dependent coupling, which in turn is a natural consequence of the well-established superexchange mechanism. The conceptual connection between the symmetric and anti-symmetric motions and the superexchange mechanism appears as a general result for bridged systems.

Vibrational Coherences of the Photoinduced Mixed-Valent Creutz-Taube Ion Revealed by Excited State Dynamics.

A recent study of photoinduced mixed-valency in the one-electron reduced form (μ-pz)[RuII(NH3)5]24+ of the Creutz-Taube ion used transient absorption spectroscopy with vis-NIR broadband detection to uncover a mixed-valent excited state with a typical intervalence charge transfer band and a nanosecond lifetime [Pieslinger et al. Angew. Chem., Int. Ed. 2022, 61, e202211747]. Herein, we use excited state dynamics simulations with implicit solvation to elucidate the electronic and vibrational evolution in the first 10 ps after the optical excitation. A manifold of excited states with weak interaction between the metal centers is populated already at time zero due to the breakdown of the Condon approximation and dominates the population of electronic states at short time scales (<0.5 ps). A long-lived vibrational wave packet mostly confined to oscillations of the metal center-bridge distances is observed. The oscillations are traced to the electronic structure properties of states with weak metal-metal coupling. The long-lived mixed-valent excited state of the Creutz-Taube ion analogue is formed vibrationally cold and has a more compact geometry. While experimentally, intersystem crossing and vibrational relaxation were deduced to be completed within 1 ps, our analysis indicates that both processes might persist at longer times.

Ru Monoimines with Extended Excited-State Lifetimes and Geometrical Modulation of Photoinduced Mixed-Valence Interactions.

The photophysical properties of monodentate-imine ruthenium complexes do not usually fulfil the requirements for supramolecular solar energy conversion schemes. Their short excited-state lifetimes, like the 5.2 ps metal-to-ligand charge transfer (MLCT) lifetime of [Ru(py)4Cl(L)]+ with L = pz (pyrazine), preclude bimolecular or long-range photoinduced energy or electron transfer reactions. Here, we explore two strategies to extend the excited-state lifetime, based on the chemical modification of the distal N atom of pyrazine. On one hand, we used L = pzH+, where protonation stabilized MLCT states, rendering thermal population of MC states less favorable. On the other hand, we prepared a symmetric bimetallic arrangement in which L = {(μ-pz)Ru(py)4Cl} to enable hole delocalization via photoinduced mixed-valence interactions. A lifetime extension of 2 orders of magnitude is accomplished, with charge transfer excited states living 580 ps and 1.6 ns, respectively, reaching compatibility with bimolecular or long-range photoinduced reactivity. These results are similar to those obtained with Ru pentaammine analogues, suggesting that the strategy employed is of general applicability. In this context, the photoinduced mixed-valence properties of the charge transfer excited states are analyzed and compared with those of different analogues of the Creutz-Taube ion, demonstrating a geometrical modulation of the photoinduced mixed-valence properties.

Excited State Mixed‐Valence Complexes: From the Special Pair to the Creutz–Taube Ion and Beyond

AbstractCompounds and complexes with mixed‐valence electronic ground states, such as the Creutz–Taube ion, have proven to be excellent vehicles through which to study intramolecular electron‐transfer processes. In a recent contribution by Cadranel and co‐workers, time‐resolved pump‐probe spectroscopy reveals photo‐induced metal‐to‐bridge charge transfer within the homovalent analogue of the Creutz–Taube ion, [{(NH3)5Ru}(μ‐pz){Ru(NH3)5}]4+, giving rise to two closely lying excited states with mixed‐valence character, one with a shorter lifetime (τ=136 ps) and weakly‐coupled (Robin‐Day Class II) character, the other a longer‐lived (τ=2.8 ns) configurational isomer with more delocalized electronic structure. Electron transfer reactions from the longer‐lived species demonstrate analogies with the photo‐induced reactions of the photosynthetic special pair, suggesting this state as a reference system for excited state mixed‐valency, and a framework from which to explore photocatalytic reactions.

Excited State Mixed-Valence Complexes: From the Special Pair to the Creutz-Taube Ion and Beyond.

Compounds and complexes with mixed-valence electronic ground states, such as the Creutz-Taube ion, have proven to be excellent vehicles through which to study intramolecular electron-transfer processes. In a recent contribution by Cadranel and co-workers, time-resolved pump-probe spectroscopy reveals photo-induced metal-to-bridge charge transfer within the homovalent analogue of the Creutz-Taube ion, [{(NH3 )5 Ru}(μ-pz){Ru(NH3 )5 }]4+ , giving rise to two closely lying excited states with mixed-valence character, one with a shorter lifetime (τ=136 ps) and weakly-coupled (Robin-Day Class II) character, the other a longer-lived (τ=2.8 ns) configurational isomer with more delocalized electronic structure. Electron transfer reactions from the longer-lived species demonstrate analogies with the photo-induced reactions of the photosynthetic special pair, suggesting this state as a reference system for excited state mixed-valency, and a framework from which to explore photocatalytic reactions.

The Excited-State Creutz-Taube Ion.

The excited-state version of the Creutz-Taube ion was prepared via visible light excitation of [(NH3 )5 RuII (μ-pz)RuII (NH3 )5 ]4+ . The resulting excited state is a mixed valence {RuIII-δ (μ-pz⋅- )RuII+δ } transient species, which was characterized using femtosecond transient absorption spectroscopy with vis-NIR detection. Very intense photoinduced intervalence charge transfers were observed at 7500 cm-1 , revealing an excited-state electronic coupling element HDA =3750 cm-1 . DFT calculations confirm a strongly delocalized excited state. A notable consequence of strong electron delocalization is the nanosecond excited state lifetime, which was exploited in a proof-of-concept intermolecular electron transfer. The excited-state Creutz-Taube ion is established as a reference, and demonstrates that electron delocalization in the excited state can be leveraged for artificial photosynthesis or other photocatalytic schemes based on electron transfer chemistry.

The Excited‐State Creutz–Taube Ion

AbstractThe excited‐state version of the Creutz–Taube ion was prepared via visible light excitation of [(NH3)5RuII(μ‐pz)RuII(NH3)5]4+. The resulting excited state is a mixed valence {RuIII–δ(μ‐pz⋅−)RuII+δ} transient species, which was characterized using femtosecond transient absorption spectroscopy with vis‐NIR detection. Very intense photoinduced intervalence charge transfers were observed at 7500 cm−1, revealing an excited‐state electronic coupling element HDA=3750 cm−1. DFT calculations confirm a strongly delocalized excited state. A notable consequence of strong electron delocalization is the nanosecond excited state lifetime, which was exploited in a proof‐of‐concept intermolecular electron transfer. The excited‐state Creutz–Taube ion is established as a reference, and demonstrates that electron delocalization in the excited state can be leveraged for artificial photosynthesis or other photocatalytic schemes based on electron transfer chemistry.

Mixed-valence ruthenium complexes rotating through a conformational Robin-Day continuum.

The conformational energy landscape and the associated electronic structure and spectroscopic properties (UV/Vis/near-infrared (NIR) and IR) of three formally d(5)/d(6) mixed-valence diruthenium complex cations, [{Ru(dppe)Cp*}2(μ-C≡CC6H4C≡C)](+), [1](+), [trans-{RuCl(dppe)2}2(μ-C≡CC6H4C≡C)](+), [2](+), and the Creutz-Taube ion, [{Ru(NH3)5}2(μ-pz)](5+), [3](5+) (Cp = cyclopentadienyl; dppe = 1,2-bis(diphenylphosphino)ethane; pz = pyrazine), have been studied using a nonstandard hybrid density functional BLYP35 with 35 % exact exchange and continuum solvent models. For the closely related monocations [1](+) and [2](+), the calculations indicated that the lowest-energy conformers exhibited delocalized electronic structures (or class III mixed-valence character). However, these minima alone explained neither the presence of shoulder(s) in the NIR absorption envelope nor the presence of features in the observed vibrational spectra characteristic of both delocalized and valence-trapped electronic structures. A series of computational models have been used to demonstrate that the mutual conformation of the metal fragments--and even more importantly the orientation of the bridging ligand relative to those metal centers--influences the electronic coupling sufficiently to afford valence-trapped conformations, which are of sufficiently low energy to be thermally populated. Areas in the conformational phase space with variable degrees of symmetry breaking of structures and spin-density distributions are shown to be responsible for the characteristic spectroscopic features of these two complexes. The Creutz-Taube ion [3](5+) also exhibits low-lying valence-trapped conformational areas, but the electronic transitions that characterize these conformations with valence-localized electronic structures have low intensities and do not influence the observed spectroscopic characteristics to any notable extent.

Strong metal–metal coupling in mixed-valent intermediates [Cl(L)Ru(μ-tppz)Ru(L)Cl]+, L = β-diketonato ligands, tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine

Five diruthenium(II) complexes [Cl(L)Ru(μ-tppz)Ru(L)Cl] (1-5) containing differently substituted β-diketonato derivatives (1: L = 2,4-pentanedionato; 2: L = 3,5-heptanedionato; 3: L = 2,2,6,6-tetramethyl-3,5-heptanedionato; 4: L = 3-methyl-2,4-pentanedionato; 5: L = 3-ethyl-2,4-pentanedionato) as ancillary ligands (L) were synthesized and studied by spectroelectrochemistry (UV-Vis-NIR, electron paramagnetic resonance (EPR)). X-ray structural characterisation revealed anti (1, 2, 5) or syn (3) configuration as well as non-planarity of the bis-tridentate tppz bridge and strong dπ(Ru(II)) → π*(pyrazine, tppz) back-bonding. The widely separated one-electron oxidation steps, Ru(II)Ru(II)/Ru(II)Ru(III) and Ru(II)Ru(III)/Ru(III)Ru(III), result in large comproportionation constants (K(c)) of ≥10(10) for the mixed-valent intermediates. The syn-configurated (n) exhibits a particularly high K(c) of 10(12) for n = 1+, accompanied by density functional theory (DFT)-calculated minimum Ru-N bond lengths for this Ru(II)Ru(III) intermediate. The electrogenerated mixed-valent states 1(+)-5(+) exhibit anisotropic EPR spectra at 110 K with average values <g> of 2.304-2.234 and g anisotropies Δg = g(1)-g(3) of 0.82-0.99. Metal-to-metal charge transfer (MMCT) absorptions occur for 1(+)-5(+) in the NIR region at 1660 nm-1750 nm (ε ≈ 2700 dm(3) mol(-1) cm(-1), Δν(1/2) ≈ 1800 cm(-1)). DFT calculations of 1(+) and 3(+) yield comparable Mulliken spin densities of about 0.60 for the metal ions, corresponding to valence-delocalised situations (Ru(2.5))(2). Rather large spin densities of about -0.4 were calculated for the tppz bridges in 1(+) and 3(+). The calculated electronic interaction values (V(AB)) for 1(+)-5(+) are about 3000 cm(-1), comparable to that for the Creutz-Taube ion at 3185 cm(-1). The DFT calculations predict that the Ru(III)Ru(III) forms in 1(2+)-5(2+) prefer a triplet (S = 1) ground state with ΔE (S = 0 - S = 1) ∼5000 cm(-1). One-electron reduction takes place at the tppz bridge which results in species [Cl(L)Ru(II)(μ-tppz˙(-))Ru(II)(L)Cl](-) (1˙(-)-3˙(-), 5˙(-)) which exhibit free radical-type EPR signals and NIR transitions typical of the tppz radical anion. The system 4(n) is distinguished by lability of the Ru-Cl bonds.

Quantum entanglement between electronic and vibrational degrees of freedom in molecules

We consider the quantum entanglement of the electronic and vibrational degrees of freedom in molecules with tendencies towards double welled potentials. In these bipartite systems, the von Neumann entropy of the reduced density matrix is used to quantify the electron-vibration entanglement for the lowest two vibronic wavefunctions obtained from a model Hamiltonian based on coupled harmonic diabatic potential-energy surfaces. Significant entanglement is found only in the region in which the ground vibronic state contains a density profile that is bimodal (i.e., contains two separate local maxima). However, in this region two distinct types of density and entanglement profiles are found: one type arises purely from the degeneracy of energy levels in the two potential wells and is destroyed by slight asymmetry, while the other arises through strong interactions between the diabatic levels of each well and is relatively insensitive to asymmetry. These two distinct types are termed fragile degeneracy-induced entanglement and persistent entanglement, respectively. Six classic molecular systems describable by two diabatic states are considered: ammonia, benzene, BNB, pyridine excited triplet states, the Creutz-Taube ion, and the radical cation of the "special pair" of chlorophylls involved in photosynthesis. These chemically diverse systems are all treated using the same general formalism and the nature of the entanglement that they embody is elucidated.

Comparative Study of f-Element Electronic Structure across a Series of Multimetallic Actinide and Lanthanoid-Actinide Complexes Possessing Redox-Active Bridging Ligands

A comparative examination of the electronic interactions across a series of trimetallic actinide and mixed lanthanide-actinide and lanthanum-actinide complexes is presented. Using reduced, radical terpyridyl ligands as conduits in a bridging framework to promote intramolecular metal-metal communication, studies containing structural, electrochemical, and X-ray absorption spectroscopy are reported for (C(5)Me(5))(2)An[-N horizontal lineC(Bn)(tpy-M{C(5)Me(4)R}(2))](2) (where An = Th(IV), U(IV); Bn = CH(2)C(6)H(5); M = La(III), Sm(III), Yb(III), U(III); R = H, Me, Et) to reveal effects dependent on the identities of the metal ions and R-groups. The electrochemical results show differences in redox energetics at the peripheral "M" site between complexes and significant wave splitting of the metal- and ligand-based processes indicating substantial electronic interactions between multiple redox sites across the actinide-containing bridge. Most striking is the appearance of strong electronic coupling for the trimetallic Yb(III)-U(IV)-Yb(III), Sm(III)-U(IV)-Sm(III), and La(III)-U(IV)-La(III) complexes, [8](-), [9b](-), and [10b](-), respectively, whose calculated comproportionation constant K(c) is slightly larger than that reported for the benchmark Creutz-Taube ion. X-ray absorption studies for monometallic metallocene complexes of U(III), U(IV), and U(V) reveal small but detectable energy differences in the "white-line" feature of the uranium L(III)-edges consistent with these variations in nominal oxidation state. The sum of these data provides evidence of 5f/6d-orbital participation in bonding and electronic delocalization in these multimetallic f-element complexes. An improved, high-yielding synthesis of 4'-cyano-2,2':6',2''-terpyridine is also reported.

Selective Binding, Self-Assembly and Nanopatterning of the Creutz-Taube Ion on Surfaces

The surface attachment properties of the Creutz-Taube ion, i.e., [(NH3)5Ru(pyrazine)Ru(NH3)5]5+, on both hydrophilic and hydrophobic types of surfaces were investigated using X-ray photoelectron spectroscopy (XPS). The results indicated that the Creutz-Taube ions only bound to hydrophilic surfaces, such as SiO2 and –OH terminated organic SAMs on gold substrates. No attachment of the ions on hydrophobic surfaces such as –CH3 terminated organic SAMs and poly(methylmethacrylate) (PMMA) thin films covered gold or SiO2 substrates was observed. Further ellipsometric, atomic force microscopy (AFM) and time-dependent XPS studies suggested that the attached cations could form an inorganic analog of the self-assembled monolayer on SiO2 substrate with a “lying-down” orientation. The strong electrostatic interaction between the highly charged cations and the anionic SiO2 surface was believed to account for these observations. Based on its selective binding property, patterning of wide (∼200 nm) and narrow (∼35 nm) lines of the Creutz-Taube ions on SiO2 surface were demonstrated through PMMA electron resist masks written by electron beam lithography (EBL).

Mixed Valence Creutz−Taube Ion Analogues Incorporating Thiacrowns: Synthesis, Structure, Physical Properties, and Computational Studies

A series of nine new complexes incorporating [Ru(II)Cl([n]aneS(3))] (n = 12, 14, 16) metal centers bridged by three ditopic ligands containing two monodentate sites (pyrazine, 4,4'-bipyridine, and 3,6-bis(4-pyridyl)-1,2,4,5-tetrazine) have been synthesized and fully characterized. The solid-state structures of three of the complexes have been further characterized by X-ray crystallography studies. Intermetallic interactions within the new systems have been probed using electrochemistry and optical spectroscopy. Cyclic voltammetry reveals that the three pyrazine bridged systems display two separate Ru(III)/(II) redox couples. Using spectroelectrochemistry, we have investigated the optical properties of these mixed valence, Creutz-Taube ion analogues. An analysis of the intervalence charge transfer bands for the complexes revealed that, despite possessing the same donor sets, the electronic delocalization within these systems is modulated by the nature of the coordinated thiacrown. Computational modeling using density function theory offers further evidence of interaction between metal centers and provides insights into how these interactions are mediated.

Probing the localized-to-delocalized transition

Detailed understanding of the transition between localized and delocalized behaviour in mixed valence compounds has been elusive as evidenced by many interpretations of the Creutz-Taube ion, [(NH3)5Ru(pz)Ru(NH3)5]5+. In a review in 2001, experimental protocols and a systematic model to probe this region were proposed and applied to examples in the literature. The model included: (i) multiple orbital interactions in ligand-bridged transition metal complexes, (ii) inclusion of spin-orbit coupling which, for dpi5-dpi6 complexes, leads to five low-energy bands, two from interconfigurational (dpi-->dpi) transitions at the dpi5 site and three from intervalence transfer transitions, (iii) differences in time scale between coupled vibrations and solvent modes which can result in solvent averaging with continued electronic asymmetry defining 'class II-III', an addition to the Robin-Day classification scheme, and (iv) delineation of coupled vibrations into barrier vibrations and 'spectator' vibrations. The latter provide direct insight into localization or delocalization and time scales for electron transfer. In this paper, the earlier model is applied to a series of mixed-valence molecules.

Towards a comprehensive model for the electronic and vibrational structure of the Creutz–Taube ion

Since the synthesis of the Creutz-Taube ion, the nature of its charge localization has been of immense scientific interest, this molecule providing a model system for the understanding of the operation of biological photosynthetic and electron-transfer processes. However, recent work has shown that its nature remains an open question. Many systems of this type, including photosynthetic reaction centres, are of current research interest, and thereby the Creutz-Taube ion provides an important chemical paradigm: the key point of interest is the details of how such molecules behave. We lay the groundwork for the construction of a comprehensive model for its chemical and spectroscopic properties. Advances are described in some of the required areas including: simulation of electronic absorption spectra; quantitative depiction of the large interaction of the ion's electronic description with solvent motions; and the physics of Ru-NH3 spectator-mode vibrations. We show that details of the solvent electron-phonon coupling are critical in the interpretation of the spectator-mode vibrations, as these strongly mix with solvent motions when 0.75<2J/lambda<1. In this regime, a double-well potential exists which does not support localized zero-point vibration, and many observed properties of the Creutz-Taube ion are shown to be consistent with the hypothesis that the ion has this character.

A priori evaluation of the solvent contribution to the reorganization energy accompanying intramolecular electron transfer: Predicting the nature of the Creutz–Taube ion

The solvent contribution to the reorganization energy is one of the key physical properties controlling electron-transfer processes. A priori computational schemes have been developed that can predict realistic values for all other properties controlling charge transfer such as electronic couplings, vibronic couplings for intramolecular modes, the effects of asymmetry, and the effects of symmetric modes, but robust schemes for treating solvation remain elusive. In recent years, excited-state quantum chemical electronic-structure packages have gained the ability to calculate non-equilibrium solvation effects without the need to treat solvent explicitly, establishing the core of the required technology. While quantum chemical schemes involving excited-states represented as perturbed ground states can readily be applied in such calculations, they in general fail to include sufficient inactive-electron correlation to properly describe the excited-state manifold of charge transfer states, states for which only multi-reference procedures may robustly be applied. We develop computational procedures by which complete-active-space self-consistent-field (CASSCF) and related multi-reference calculations may be applied to electron-transfer systems whose energetics are controlled by the solvent–solute interaction. This is tested through the evaluation of the reorganization energy for electron-transfer in the bis-pentaammineruthenium complexes of α,ω-dipyridyl trans-polyenes in aqueous solution. Predictions are then made for the Creutz–Taube ion that place it clearly in the region of maximum breakdown of the Born–Oppenheimer approximation for which both the localized and delocalized diabatic descriptions of its electronic structure become inappropriate. This result is in qualitative agreement with conclusions drawn over the last four years based on a number of sources.

Contrasting electroabsorbance behavior of two borderline class II/class III mixed-valence systems

Intervalence Stark effect spectroscopy of a symmetrically substituted and an asymmetrically substituted derivative of the Creutz–Taube ion, ( NH 3 ) 5 Ru - pyrazine - Ru ( NH 3 ) 5 5 + , is reported. Like the formally mixed-valent parent ion, both appear to lie near the boundary between valence delocalization and valence localization. Remarkably, despite the close structural similarities of the two compounds investigated, the corresponding electroabsorbance spectra are grossly different. Furthermore, neither spectrum is well fit by a classic Liptay approach. An alternative, qualitative approach that allows for the possibility that the externally applied electric field itself affects the degree of valence delocalization and, therefore, the electroabsorbance spectrum, was employed. The alternative approach relies upon the availability of compounds closely analogous to the investigated compounds, but differing slightly in degree of redox asymmetry. A subtle method for creating such compounds is to partially encapsulate a parent compound with a crown ether, such that a lower symmetry assembly is obtained. Using this approach the unusual Stark responses, including the disparate responses for symmetric vs. asymmetric structures, are reasonably well reproduced. Intervalence-enhanced resonance Raman spectra provide additional support for the proposed interpretation.