Calculation of Ligand Dissociation Energies in Large Transition-Metal Complexes

The accurate calculation of ligand dissociation (or equivalently, ligand binding) energies is crucial for computational coordination chemistry. Despite its importance, obtaining accurate <i>ab initio</i> reference data is difficult, and density-functional methods of uncertain reliability are chosen for feasibility reasons. Here, we consider advanced coupled-cluster and multiconfigurational approaches to reinvestigate our WCCR10 set of 10 gas-phase ligand dissociation energies [<i>J. Chem. Theory Comput.</i> <b>2014</b>, <i>10</i>, 3092]. We assess the potential multiconfigurational character of all molecules involved in these reactions with a multireference diagnostic [<i>Mol. Phys.</i> <b>2017</b>, <i>115</i>, 2110] in order to determine where single-reference coupled-cluster approaches can be applied. For some reactions of the WCCR10 set, large deviations of density-functional results including semiclassical dispersion corrections from experimental reference data had been observed. This puzzling observation deserves special attention here, and we tackle the issue (i) by comparing to ab initio data that comprise dispersion effects on a rigorous first-principles footing and (ii) by a comparison of density-functional approaches that model dispersion interactions in various ways. For two reactions, species exhibiting nonnegligible static electron correlation were identified. These two reactions represent hard problems for electronic structure methods and also for multireference perturbation theories. However, most of the ligand dissociation reactions in WCCR10 do not exhibit static electron correlation effects, and hence, we may choose standard single-reference coupled-cluster approaches to compare with density-functional methods. For WCCR10, the Minnesota M06-L functional yielded the smallest mean absolute deviation of 13.2 kJ mol^–1 out of all density functionals considered (PBE, BP86, BLYP, TPSS, M06-L, PBE0, B3LYP, TPSSh, and M06-2X) without additional dispersion corrections in comparison to the coupled-cluster results, and the PBE0-D3 functional produced the overall smallest mean absolute deviation of 4.3 kJ mol^–1. The agreement of density-functional results with coupled-cluster data increases significantly upon inclusion of any type of dispersion correction. It is important to emphasize that different density-functional schemes available for this purpose perform equally well. The coupled-cluster dissociation energies, however, deviate from experimental results on average by 30.3 kJ mol^–1. Possible reasons for these deviations are discussed

Mechanism Elucidation of the <i>cis–trans</i> Isomerization of an Azole Ruthenium–Nitrosyl Complex and Its Osmium Counterpart

Synthesis and X-ray diffraction structures of <i>cis</i> and <i>trans</i> isomers of ruthenium and osmium metal complexes of general formulas (<i>n</i>Bu₄N)­[<i>cis</i>-MCl₄(NO)­(Hind)], where M = Ru (<b>1</b>) and Os (<b>3</b>), and (<i>n</i>Bu₄N)­[<i>trans</i>-MCl₄(NO)­(Hind)], where M = Ru (<b>2</b>) and Os (<b>4</b>) and Hind = 1<i>H</i>-indazole are reported. Interconversion between <i>cis</i> and <i>trans</i> isomers at high temperatures (80–130 °C) has been observed and studied by NMR spectroscopy. Kinetic data indicate that isomerizations correspond to reversible first order reactions. The rates of isomerization reactions even at 110 °C are very low with rate constants of 10^–5 s^–1 and 10^–6 s^–1 for ruthenium and osmium complexes, respectively, and the estimated rate constants of isomerization at room temperature are of ca. 10^–10 s^–1. The activation parameters, which have been obtained from fitting the reaction rates at different temperatures to the Eyring equation for ruthenium [Δ<i>H</i>_{<i>cis‑trans</i>}^‡ <i>=</i> 122.8 ± 1.3; Δ<i><i>H</i></i>_{<i>trans‑cis</i>}^‡ <i>=</i> 138.8 ± 1.0 kJ/mol; Δ<i><i>S</i></i>_{<i>cis‑trans</i>}^‡ <i>=</i> −18.7 ± 3.6; Δ<i><i>S</i></i>_{<i>trans‑cis</i>}^‡ <i>=</i> 31.8 ± 2.7 J/(mol·K)] and osmium [Δ<i>H</i>_{<i>cis‑trans</i>}^‡ <i>=</i> 200.7 ± 0.7; Δ<i><i>H</i></i>_{<i>trans‑cis</i>}^‡ <i>=</i> 168.2 ± 0.6 kJ/mol; Δ<i><i>S</i></i>_{<i>cis‑trans</i>}^‡ <i>=</i> 142.7 ± 8.9; Δ<i><i>S</i></i>_{<i>trans‑cis</i>}^‡ <i>=</i> 85.9 ± 3.9 J/(mol·K)] reflect the inertness of these systems. The entropy of activation for the osmium complexes is highly positive and suggests the dissociative mechanism of isomerization. In the case of ruthenium, the activation entropy for the <i>cis</i> to <i>trans</i> isomerization is negative [−18.6 J/(mol·K)], while being positive [31.0 J/(mol·K)] for the <i>trans</i> to <i>cis</i> conversion. The thermodynamic parameters for <i>cis</i> to <i>trans</i> isomerization of [RuCl₄(NO)­(Hind)]⁻, viz. Δ<i><i>H</i>°</i> = 13.5 ± 1.5 kJ/mol and Δ<i>S</i>° = −5.2 ± 3.4 J/(mol·K) indicate the low difference between the energies of <i>cis</i> and <i>trans</i> isomers. The theoretical calculation has been carried out on isomerization of ruthenium complexes with DFT methods. The dissociative, associative, and intramolecular twist isomerization mechanisms have been considered. The value for the activation energy found for the dissociative mechanism is in good agreement with experimental activation enthalpy. Electrochemical investigation provides further evidence for higher reactivity of ruthenium complexes compared to that of osmium counterparts and shows that intramolecular electron transfer reactions do not affect the isomerization process. A dissociative mechanism of <i>cis</i>↔<i>trans</i> isomerization has been proposed for both ruthenium and osmium complexes

Shedding Light on the Nature of Photoinduced States Formed in a Hydrogen-Generating Supramolecular RuPt Photocatalyst by Ultrafast Spectroscopy

Photoinduced electronic and structural changes of a hydrogen-generating supramolecular RuPt photocatalyst are studied by a combination of time-resolved photoluminescence, optical transient absorption, and X-ray absorption spectroscopy. This work uses the element specificity of X-ray techniques to focus on the interplay between the photophysical and -chemical processes and the associated time scales at the catalytic Pt moiety. We observe very fast (<30 ps) photoreduction of the Pt catalytic site, followed by an ∼600 ps step into a strongly oxidized Pt center. The latter process is likely induced by oxidative addition of reactive iodine species. The oxidized Pt species is long-lived and fully recovers to the original ground state complex on a >10 μs time scale. However, the photosensitizing Ru moiety is fully restored on a much shorter ∼300 ns time scale. This reaction scheme implies that we may withdraw two electrons from a catalyst that is activated by a single photon

OpenMolcas: From source code to insight

In this article we describe the OpenMolcas environment and invite the computational chemistry community to collaborate. The open-source project already includes a large number of new developments realized during the transition from the commercial MOLCAS product to the open-source platform. The paper initially describes the technical details of the new software development platform. This is followed by brief presentations of many new methods, implementations, and features of the OpenMolcas program suite. These developments include novel wave function methods such as stochastic complete active space self-consistent field, density matrix renormalization group (DMRG) methods, and hybrid multiconfigurational wave function and density functional theory models. Some of these implementations include an array of additional options and functionalities. The paper proceeds and describes developments related to explorations of potential energy surfaces. Here we present methods for the optimization of conical intersections, the simulation of adiabatic and nonadiabatic molecular dynamics and interfaces to tools for semiclassical and quantum mechanical nuclear dynamics. Furthermore, the article describes features unique to simulations of spectroscopic and magnetic phenomena such as the exact semiclassical description of the interaction between light and matter, various X-ray processes, magnetic circular dichroism and properties. Finally, the paper describes a number of built-in and add-on features to support the OpenMolcas platform with post calculation analysis and visualization, a multiscale simulation option using frozen-density embedding theory and new electronic and muonic basis sets