Transition Metal Complexes In Solution Research Articles

Enter MnIV-NHC: A Dark Photooxidant with a Long-Lived Charge-Transfer Excited State.

Detailed photophysical investigation of a Mn(IV)-carbene complex has revealed that excitation into its lowest-energy absorption band (∼500 nm) results in the formation of an energetic ligand-to-metal charge-transfer (LMCT) state with a lifetime of 15 ns. To the best of our knowledge, this is the longest lifetime reported for charge-transfer states of first-row-based transition metal complexes in solution, barring those based on Cu, with a d10 configuration. A so-called superoxidant, Mn(IV)-carbene exhibits an excited state potential typically only harnessed via excited states of reactive organic radical species. Furthermore, the long-lived excited state in this case is found to be a dark doublet, with its transition to the quartet ground state being spin-forbidden, a contrast to most first-row literature examples, and a possible cause of the long lifetime. Showcasing excited state properties which in some cases exceed those of complexes based on precious metals, these findings not only advance the library of earth-abundant photosensitizers but also shed general insight into the photophysics of d3 and related Mn complexes.

Resolving Femtosecond Solvent Reorganization Dynamics in an Iron Complex by Nonadiabatic Dynamics Simulations.

The ultrafast dynamical response of solute–solvent interactions plays a key role in transition metal complexes, where charge transfer states are ubiquitous. Nonetheless, there exist very few excited-state simulations of transition metal complexes in solution. Here, we carry out a nonadiabatic dynamics study of the iron complex [Fe(CN)4(bpy)]2– (bpy = 2,2′-bipyridine) in explicit aqueous solution. Implicit solvation models were found inadequate for reproducing the strong solvatochromism in the absorption spectra. Instead, direct solute–solvent interactions, in the form of hydrogen bonds, are responsible for the large observed solvatochromic shift and the general dynamical behavior of the complex in water. The simulations reveal an overall intersystem crossing time scale of 0.21 ± 0.01 ps and a strong reliance of this process on nuclear motion. A charge transfer character analysis shows a branched decay mechanism from the initially excited singlet metal-to-ligand charge transfer (1MLCT) states to triplet states of 3MLCT and metal-centered (3MC) character. We also find that solvent reorganization after excitation is ultrafast, on the order of 50 fs around the cyanides and slower around the bpy ligand. In contrast, the nuclear vibrational dynamics, in the form of Fe–ligand bond changes, takes place on slightly longer time scales. We demonstrate that the surprisingly fast solvent reorganizing should be observable in time-resolved X-ray solution scattering experiments, as simulated signals show strong contributions from the solute–solvent scattering cross term. Altogether, the simulations paint a comprehensive picture of the coupled and concurrent electronic, nuclear, and solvent dynamics and interactions in the first hundreds of femtoseconds after excitation.

A multiscale free energy method reveals an unprecedented photoactivation of a bimetallic Os(II)-Pt(II) dual anticancer agent.

The photoreactivity of relatively large transition metal complexes is often limited to the description of the static potential energy surfaces of the involved electronic states. While useful to grasp some physical grounds of the photoinduced molecular responses, this approach does not statistically sample the multiple molecular degrees of freedom of the systems under investigation, which grow significantly if we consider the explicit coupling with the environment, and does not consider dynamic effects. The problem is even more complex if the reactivity takes place in the excited state. The present work uses state-of-the-art multiscale QM/MM dynamics to describe the photoactivation of a Pt(II)-unit of an in silico designed two-component Os(II)-Pt(II) assembly proposed for a dual anticancer approach, by explicitly accounting for both dynamic and environmental effects. We clearly identify a transition state region with partial metal-to-metal charge transfer (3MMCT) character with no precedents in the scarce Ru(II)-Pt(II) analogues, indicative of a large synergistic effect between the Os(II) and Pt(II) metals and crucial in the photolabilization process of the Pt(II)-Cl bond. This is the first evidence of the ability of Os(II) to promote photoactivation of the Pt(II)-moiety, a contingency that would open new perspectives in this emerging field. The designed complex is therefore able to combine the traditional activity in photodynamic therapy (PDT) with the photoactivated chemotherapy (PCT) exerted by the Pt(II) unit, representing a new paradigm for a combined PDT/PCT anticancer approach while providing an advance in the methodology used to describe the photochemistry of transition-metal complexes in solution.

1,3,5-Triaza-7-Phosphaadamantane (PTA) as a 31P NMR Probe for Organometallic Transition Metal Complexes in Solution

Due to the rigid structure of 1,3,5-triaza-7-phosphaadamantane (PTA), its 31P chemical shift solely depends on non-covalent interactions in which the molecule is involved. The maximum range of change caused by the most common of these, hydrogen bonding, is only 6 ppm, because the active site is one of the PTA nitrogen atoms. In contrast, when the PTA phosphorus atom is coordinated to a metal, the range of change exceeds 100 ppm. This feature can be used to support or reject specific structural models of organometallic transition metal complexes in solution by comparing the experimental and Density Functional Theory (DFT) calculated values of this 31P chemical shift. This approach has been tested on a variety of the metals of groups 8–12 and molecular structures. General recommendations for appropriate basis sets are reported.

Transition Metal Complexes as Catalysts for the Electroconversion of CO2 : An Organometallic Perspective.

The electrocatalytic transformation of carbon dioxide has been a topic of interest in the field of CO2 utilization for a long time. Recently, the area has seen increasing dynamics as an alternative strategy to catalytic hydrogenation for CO2 reduction. While many studies focus on the direct electron transfer to the CO2 molecule at the electrode material, molecular transition metal complexes in solution offer the possibility to act as catalysts for the electron transfer. C1 compounds such as carbon monoxide, formate, and methanol are often targeted as the main products, but more elaborate transformations are also possible within the coordination sphere of the metal center. This perspective article will cover selected examples to illustrate and categorize the currently favored mechanisms for the electrochemically induced transformation of CO2 promoted by homogeneous transition metal complexes. The insights will be corroborated with the concepts and elementary steps of organometallic catalysis to derive potential strategies to broaden the molecular diversity of possible products.

Iron's Wake: The Performance of Quantum Mechanical-Derived Versus General-Purpose Force Fields Tested on a Luminescent Iron Complex.

Recently synthetized iron complexes have achieved long-lived excited states and stabilities which are comparable, or even superior, to their ruthenium analogues, thus representing an eco-friendly and cheaper alternative to those materials based on rare metals. Most of computational tools which could help unravel the origin of this large efficiency rely on ab-initio methods which are not able, however, to capture the nanosecond time scale underlying these photophysical processes and the influence of their realistic environment. Therefore, it exists an urgent need of developing new low-cost, but still accurate enough, computational methodologies capable to deal with the steady-state and transient spectroscopy of transition metal complexes in solution. Following this idea, here we focus on the comparison between general-purpose transferable force-fields (FFs), directly available from existing databases, and specific quantum mechanical derived FFs (QMD-FFs), obtained in this work through the Joyce procedure. We have chosen a recently reported FeIII complex with nanosecond excited-state lifetime as a representative case. Our molecular dynamics (MD) simulations demonstrated that the QMD-FF nicely reproduces the structure and the dynamics of the complex and its chemical environment within the same precision as higher cost QM methods, whereas general-purpose FFs failed in this purpose. Although in this particular case the chemical environment plays a minor role on the photo physics of this system, these results highlight the potential of QMD-FFs to rationalize photophysical phenomena provided an accurate QM method to derive its parameters is chosen.

Resolving structures of transition metal complex reaction intermediates with femtosecond EXAFS

Femtosecond-resolved Extended X-ray Absorption Fine Structure (EXAFS) measurements of solvated transition metal complexes are successfully implemented at the X-ray Free Electron Laser LCLS. Benchmark experiments on [Fe(terpy)2]2+ in solution show a signal-to-noise ratio on the order of 500, comparable to typical 100 ps-resolution synchrotron measurements. In the few femtoseconds after photoexcitation, we observe the EXAFS fingerprints of a short-lived (∼100 fs) intermediate as well as those of a vibrationally hot long-lived (∼ns) excited state.

Symmetry in Multiple Self-Consistent-Field Solutions of Transition-Metal Complexes.

We use a method based on metadynamics to locate multiple low-energy Unrestricted Hartree-Fock (UHF) self-consistent-field (SCF) solutions of two model octahedral d1 and d2 transition-metal complexes, [MF6]3- (M = Ti, V). By giving a group-theoretical definition of symmetry breaking, we classify these solutions in the framework of representation theory and observe that a number of them break spin or spatial symmetry, if not both. These solutions seem unphysical at first, but we show that they can be used as bases for Non-Orthogonal Configuration Interaction (NOCI) to yield multideterminantal wave functions that have the right symmetry to be assigned to electronic terms. Furthermore, by examining the natural orbitals and occupation numbers of these NOCI wave functions, we gain insight into the amount of static correlation that they incorporate. We then investigate the behaviors of the most low-lying UHF and NOCI wave functions when the octahedral symmetry of the complexes is lowered and deduce that the symmetry-broken UHF solutions must first have their symmetry restored by NOCI before they can describe any vibronic stabilization effects dictated by the Jahn-Teller theorem.

X-ray-induced sample damage at the Mn L-edge: a case study for soft X-ray spectroscopy of transition metal complexes in solution.

X-ray induced sample damage can impede electronic and structural investigations of radiation-sensitive samples studied with X-rays. Here we quantify dose-dependent sample damage to the prototypical MnIII(acac)3 complex in solution and at room temperature for the soft X-ray range, using X-ray absorption spectroscopy at the Mn L-edge. We observe the appearance of a reduced MnII species as the X-ray dose is increased. We find a half-damage dose of 1.6 MGy and quantify a spectroscopically tolerable dose on the order of 0.3 MGy (1 Gy = 1 J kg-1), where 90% of MnIII(acac)3 are intact. Our dose-limit is around one order of magnitude lower than the Henderson limit (half-damage dose of 20 MGy) which is commonly employed for protein crystallography with hard X-rays. It is comparable, however, to the dose-limits obtained for collecting un-damaged Mn K-edge spectra of the photosystem II protein, using hard X-rays. The dose-dependent reduction of MnIII observed here for solution samples occurs at a dose limit that is two to four orders of magnitude smaller than the dose limits previously reported for soft X-ray spectroscopy of iron samples in the solid phase. We compare our measured to calculated spectra from ab initio restricted active space (RAS) theory and discuss possible mechanisms for the observed dose-dependent damage of MnIII(acac)3 in solution. On the basis of our results, we assess the influence of sample damage in other experimental studies with soft X-rays from storage-ring synchrotron radiation sources and X-ray free-electron lasers.

Desorption rate of glyphosate from goethite as affected by different entering ligands: hints on the desorption mechanism

Environmental contextGlyphosate is a heavily used herbicide that is mobilised in soil and sediments through adsorption–desorption processes from the surface of mineral particles. We demonstrate that the desorption rate of glyphosate from goethite, a ubiquitous mineral, is nearly independent of the concentration and nature of the substance that is used to desorb it. The results elucidate the desorption mechanism and are relevant to understand and predict the environmental mobility of glyphosate. AbstractThe desorption kinetics of glyphosate (Gly) from goethite was studied in a flow cell using attenuated total reflectance Fourier-transform infrared spectroscopy. Because Gly forms an inner-sphere surface complex by coordinating to Fe atoms at the goethite surface, the desorption process is actually a ligand-exchange reaction, where Gly is the leaving ligand and water molecules or dissolved substances are the entering ligands. A series of possible entering ligands that can be found in nature was tested to evaluate their effect on the desorption kinetics of Gly. Contrarily to expectations, the desorption rate was quite independent of the entering ligand concentration. Moreover, the identity of this ligand (phosphate, citrate, sulfate, oxalate, EDTA, thiocyanate, humic acid, water) had only a small effect on the value of the desorption rate constant. By analogy with the reactivity of transition metal complexes in solution, it is concluded that the rate is mainly controlled by the breaking of the Fe–Gly bond, through a dissociative or a dissociative interchange mechanism. The results are relevant in understanding and predicting the environmental mobility of Gly: irrespective of the identity of the entering ligand, Gly will always desorb from iron (hydr)oxides in nature at nearly the same rate, simplifying calculations and predictions enormously. The importance of studying desorption kinetics using mineral surfaces and environmentally relevant molecules is also highlighted.

Factors That Determine the Mechanism of NO Activation by Metal Complexes of Biological and Environmental Relevance

Invited for this month's cover is the group of Rudi van Eldik at the University of Erlangen-Nürnberg. The image shows aspects that influence the mechanism of interaction of NO with transition-metal complexes in solution.

Comment on: “On the Accuracy of DFT Methods in Reproducing Ligand Substitution Energies for Transition Metal Complexes in Solution: The Role of Dispersive Interactions” by H. Jacobsen and L. Cavallo

ChemPhysChemVolume 13, Issue 6 p. 1407-1409 Correspondence Comment on: “On the Accuracy of DFT Methods in Reproducing Ligand Substitution Energies for Transition Metal Complexes in Solution: The Role of Dispersive Interactions” by H. Jacobsen and L. Cavallo Prof. Stefan Grimme, Corresponding Author Prof. Stefan Grimme grimme@thch.uni-bonn.de Mulliken Center for Theoretical Chemistry, Institut für Physikalische und Theoretische Chemie der Universität Bonn, Beringstr. 4, D-53115 Bonn (Germany)Mulliken Center for Theoretical Chemistry, Institut für Physikalische und Theoretische Chemie der Universität Bonn, Beringstr. 4, D-53115 Bonn (Germany)Search for more papers by this author Prof. Stefan Grimme, Corresponding Author Prof. Stefan Grimme grimme@thch.uni-bonn.de Mulliken Center for Theoretical Chemistry, Institut für Physikalische und Theoretische Chemie der Universität Bonn, Beringstr. 4, D-53115 Bonn (Germany)Mulliken Center for Theoretical Chemistry, Institut für Physikalische und Theoretische Chemie der Universität Bonn, Beringstr. 4, D-53115 Bonn (Germany)Search for more papers by this author First published: 20 March 2012 https://doi.org/10.1002/cphc.201200094Citations: 41Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Citing Literature Volume13, Issue6April 23, 2012Pages 1407-1409 RelatedInformation

Reply to the Comment by Grimme on: “On the Accuracy of DFT Methods in Reproducing Ligand Substitution Energies for Transition Metal Complexes in Solution: The Role of Dispersive Interactions”

ChemPhysChemVolume 13, Issue 6 p. 1405-1406 Correspondence Reply to the Comment by Grimme on: “On the Accuracy of DFT Methods in Reproducing Ligand Substitution Energies for Transition Metal Complexes in Solution: The Role of Dispersive Interactions” Dr. Heiko Jacobsen, Corresponding Author Dr. Heiko Jacobsen jacobsen@kemkom.com KemKom, New Orleans, Louisiana (USA) Heiko Jacobsen, KemKom, New Orleans, Louisiana (USA) Luigi Cavallo, KAUST Catalysis Center, Thuwal, Saudi ArabiaSearch for more papers by this authorProf. Dr. Luigi Cavallo, Corresponding Author Prof. Dr. Luigi Cavallo luigi.cavallo@kaust.edu.sa KAUST Catalysis Center, Thuwal, Saudi Arabia Heiko Jacobsen, KemKom, New Orleans, Louisiana (USA) Luigi Cavallo, KAUST Catalysis Center, Thuwal, Saudi ArabiaSearch for more papers by this author Dr. Heiko Jacobsen, Corresponding Author Dr. Heiko Jacobsen jacobsen@kemkom.com KemKom, New Orleans, Louisiana (USA) Heiko Jacobsen, KemKom, New Orleans, Louisiana (USA) Luigi Cavallo, KAUST Catalysis Center, Thuwal, Saudi ArabiaSearch for more papers by this authorProf. Dr. Luigi Cavallo, Corresponding Author Prof. Dr. Luigi Cavallo luigi.cavallo@kaust.edu.sa KAUST Catalysis Center, Thuwal, Saudi Arabia Heiko Jacobsen, KemKom, New Orleans, Louisiana (USA) Luigi Cavallo, KAUST Catalysis Center, Thuwal, Saudi ArabiaSearch for more papers by this author First published: 16 March 2012 https://doi.org/10.1002/cphc.201200165Citations: 12Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article.Citing Literature Volume13, Issue6April 23, 2012Pages 1405-1406 RelatedInformation

On the Accuracy of DFT Methods in Reproducing Ligand Substitution Energies for Transition Metal Complexes in Solution: The Role of Dispersive Interactions

The performance of a series of density functionals when tested on the prediction of the phosphane substitution energy of transition metal complexes is evaluated. The complexes Fe-BDA and Ru-COD (BDA=benzylideneacetone, COD=cyclooctadiene) serve as reference systems, and calculated values are compared with the experimental values in THF as obtained from calorimetry. Results clearly indicate that functionals specifically developed to include dispersion interactions usually outperform other functionals when BDA or COD substitution is considered. However, when phosphanes of different sizes are compared, functionals including dispersion interactions, at odd with experimental evidence, predict that larger phosphanes bind more strongly than smaller phosphanes, while functionals not including dispersion interaction reproduce the experimental trends with reasonable accuracy. In case of the DFT-D functionals, inclusion of a cut-off distance on the dispersive term resolves this issue, and results in a rather robust behavior whatever ligand substitution reaction is considered.

Probing reaction dynamics of transition-metal complexes in solution via time-resolved X-ray spectroscopy

We report measurements of the photoinduced FeII spin crossover reaction dynamics in solution via time-resolved x-ray absorption spectroscopy. EXAFS measurements reveal that the iron-nitrogen bond lengthens by 0.21±0.03 Å in the high-spin transient excited state relative to the ground state. XANES measurements at the Fe L-edge show directly the influence of the structural change on the ligand-field splitting of the FeII 3d orbitals associated with the spin transition.

Probehead operating at 35GHz for continuous wave and pulse electron paramagnetic resonance applications

The design, construction, numerical modeling, and performance of a probehead for electron paramagnetic resonance (EPR) spectroscopy at 34–36GHz is described. A classical cylindrical cavity operating in the TE011 mode with adjustable frequency and coupling has been found to be well suited for continuous wave and pulse EPR studies of frozen solutions of transition metal complexes at low temperature. The highest attention is given to the probehead performance in the pulse mode. The implemented design has been analyzed in detail using numerical modeling. The distribution of the electromagnetic fields, eigenfrequencies, quality factors, coupling coefficients, and conversion factors are calculated and compared with experimental data. The results demonstrate the effectiveness of the design and can serve as a guide for probehead optimization.

Understanding the reactivity of transition metal complexes involving multiple spin states

In coordination chemistry, many reactions involve several electronic states, in particular states of different spin. This phenomenon of ‘Multiple-State Reactivity’ has been recognized for some time, both for gas-phase reactions of ‘bare’ metal ions, and for transition metal complexes in solution. Until recently, however, much of the discussion of these systems has remained qualitative, because standard computational methods do not allow the location of the critical points for these processes, the Minimum Energy Crossing Points (MECPs) between states of different spin. Increased computational resources and new algorithms now enable MECPs to be located for large, realistic transition metal containing systems, yielding important new insight into the mechanisms of important reactions such as oxidative addition of CH bonds to metal centers and ligand association/dissociation processes. Several examples will be presented for inorganic, organometallic and bioinorganic reactions.

Application of stopped flow techniques and energy dispersive EXAFS for investigation of the reactions of transition metal complexes in solution: Activation of nickel β-diketonates to form homogeneous catalysts, electron transfer reactions involving iron(iii) and oxidative addition to iridium(i)

Stopped-flow techniques of rapid mixing have been combined with energy dispersive X-ray absorption spectroscopy to monitor the reaction of Ni(dpm)2 [dpm = Bu1C(O)CHC(O)Bu1] by aluminium alkyls (AlEt2X, X = OEt and Et) to form the active species for the catalytic di- and tri-merisation of hex-1-ene. Acquisition times down to ca. 30 ms were achieved on Station 9.3 of the SRS using a photodiode array detector. The EXAFS features of the resulting solution complexes are of the form [Ni(O-O)R)(alkene)]. In the presence of PPh3, [Ni(O-O)(R)(PPh3) appears to be the redominant type of species. The reduction of aqueous Fe(III) by hydroquinone was investigated on ID24 at the ESRF by Fe K-edge energy dispersive EXAFS with a CCD camera as detector, spectra were obtained in 1 ms or longer. No intermediate inner sphere complex was detected prior to the formation of aqueous Fe(II). Finally the oxidative addition of CH3SO3CF3 to [IrI2(CO)2]- was monitored on Station 9.3 with a silicon microstrip detector. A single acquisition of 400 micros was feasible, with spectra recorded in multiples of 1.2 ms. In that time, the first stage of the reaction had been completed, with a slower stage thereafter. The results are consistent with the two-stage ionic oxidative addition mechanism.

Gelatinizing of Aqueous Solutions of Transition-Metal Complexes

The gelatinizing of aqueous solutions of transition-metal complexes with hydroxy acids was studied. The mechanism of gelatinizing of these solutions was suggested.

The electron-transfer reaction for Co(H2O)62+/3+

A theoretical scheme is presented to study the rate of the electron-exchange reaction between transition metal complexes in aqueous solution. In the scheme the activation energy is obtained via an activation model and ab initio technique, and the coupling matrix element at the transition state is determined by using perturbation theory and a numerical integral method. The electron-transfer (ET) reactivity of Co(H2O)62+/3+, a prototype system of the ET reactions between transition metal complexes in solution, is determined by employing this scheme. The theoretical results are compared with the corresponding experimental values and good agreement is found. The further extension of the scheme is also discussed.