Quantification of the Donor-Acceptor Character of Ligands by the Effective Fragment Orbitals

Metal-ligand interactions are at the heart of transition metal complexes. The Dewar-Chat-Duncanson model is often invoked, whereby the metal-ligand bonding is decomposed into the simultaneous ligand→metal electron donation and the metal→ligand back-donation. The separate quantification of both effects is not a trivial task, neither from experimental nor computational exercises. In this work we present the effective fragment orbitals (EFOs) and their occupations as a general procedure beyond the Kohn-Sham density functional theory (KS-DFT) framework for the identification and quantification of donor-acceptor interactions, using solely the wavefunction of the complex. Using a common Fe(II) octahedral complex framework, we quantify the σ-donor, π-donor, and π-acceptor character for a large and chemically diverse set of ligands, by introducing the respective descriptors σd, πd, and πa. We also explore the effect of the metal size, coordination number, and spin state on the donor/acceptor features. The spin-state is shown to be the most critical effect, inducing a systematic decrease of the sigma donation and π-backdonation going from low spin to high spin. Finally, we illustrate the ability of the EFOs to rationalize the Tolman electronic parameter and the trans influence in planar square complexes in terms of these new descriptors.

Expanding horizons in conceptual density functional theory: Novel ensembles and descriptors to decipher reactivity patterns

Conceptual density functional theory (CDFT) and the quantum reactivity descriptors stemming from it have proven to be valuable tools for understanding the chemical behavior of molecules. This article is presented as being intrinsically of dual character. In a first part, it briefly reviews, in a deliberately didactical way, the main ensembles in CDFT, while the second half presents two additional ensembles, where the chemical hardness acts as a natural variable, and their respective reactivity descriptors. The evaluation of these reactivity descriptors on common organic chemical reagents are presented and discussed.

Selective Crystallization of Linkage Isomers, [RhIII(NCS)(SCN)5]3- and [RhIII(SCN)6]3-, to Investigate Structural Trans Influence and Thermal Stability

Linkage isomers of homoleptic complexes, [RhIII(SCN)6]3- and [RhIII(NCS)(SCN)5]3-, formed in aqueous solution were successfully separated by employing methyltriphenylphosphonium (MePPh3+) and 1-ethylquinolinium (EtQu+) ions as countercations, respectively. The single-crystal X-ray analysis of (MePPh3)3[RhIII(SCN)6] (1) indicated that all of the SCN- ligands coordinate to the RhIII ion by S atoms with an octahedral symmetry, where the average bond length of Rh-S is 2.374(7) Å. On the other hand, the RhIII ion of (EtQu)3[RhIII(NCS)(SCN)5]·H2O (2) is coordinated by five S atoms and one N atom of the SCN- ligands with a C4v symmetry. Structural trans influence was observed in the shorter bond length of Rh-S at the trans position of Rh-N. The Rh-S bond length is 2.3398(13) Å significantly shorter than those of 1 by ca. 0.04 Å, although DFT calculations based on the crystal structures indicated that the effective bond order of Rh-N is higher than those of Rh-S. Thermal stability examination by thermogravimetric and differential thermal analyses (TG/DTA) and IR spectroscopy indicated that the linkage isomerization of [RhIII(SCN)6]3- to [RhIII(NCS)(SCN)5]3- proceeded after melting around 174 °C. These results clearly indicate that [RhIII(NCS)(SCN)5]3- is thermodynamically more stable than [RhIII(SCN)6]3- in solid states, although further linkage isomerization hardly occurs.

A novel series of pyrazole derivatives toward biological applications: experimental and conceptual DFT characterization

A new series of 13 pyrazole-derivative compounds with potential antifungal activity were synthetized with good yields. The series have the (E)-2-((1-(R)-3,5-dimethyl-1H-pyrazol-4-yl)diazenyl)phenol general structure and were characterized by means of X-ray diffraction, UV-Vis, FTIR, ¹H-NMR, ¹³C-NMR, and two-dimensional NMR experiments. This experimental characterization was complemented by DFT simulations. A deep insight regarding molecular reactivity was accomplished employing a conceptual DFT approach. In this sense, dual descriptors were calculated at HF and DFT level of theory and GGV spin-density Fukui functions. The main reactive region within the molecules was mapped through isosurface and condensed representations. Finally, chemical descriptors that have previously shown to be close related to biological activity were compared within the series. Thus, higher values of chemical potential ω and electrophilicity χ obtained for compounds 10, 9, 8, 6 and 7, in this order, suggest that these molecules are the better candidates as biological agents.

"MoCl3(dme)" Revisited: Improved Synthesis, Characterization, and X-ray and Electronic Structures

"MoCl₃(dme)" (dme = 1,2-dimethoxyethane) is an important precursor for midvalent molybdenum chemistry, particularly for triply Mo-Mo bonded compounds of the type Mo₂X₆ (X = bulky anionic ligand). However, its exact structural identity has been obscure for more than 50 years. In search of a convenient, large-scale synthesis, we have found that trans-MoCl₄(Et₂O)₂ dissolved in dme can be cleanly reduced with dimethylphenylsilane, Me₂PhSiH, to provide khaki Mo₂Cl₆(dme)₂ in ∼90% yield. If the reduction is performed on a small scale, single crystals suitable for X-ray crystallography can be obtained. Two different crystal morphologies were identified, each belonging to the P2₁/n space group, but with slightly different unit cell constants. The refined structure of each form is an edge-shared bioctahedron with overall C_i symmetry and metal-metal separations on the order of 2.8 Å. The bulk material is diamagnetic as determined by both the Gouy method and SQUID magnetometry. Density functional theory calculations suggest a σ²π²δ*² ground state for the dimer with the diamagnetism arising from a singlet diradical "broken symmetry" electronic configuration. In addition to a definitive structural assignment for "MoCl₃(dme)", this work highlights the utility of organosilanes as easy to handle, alternative reductants for inorganic synthesis.

Novel approach to accurately predict bond strength and ligand lability in platinum-based anticancer drugs

Prompted by the antineoplastic properties of cisplatin, a plethora of platinum(ii)-based complexes have been synthesized in the past decades. At present, their rational design is based on a number of structure-activity relationships involving the nature of the ligands initially coordinated to platinum(ii): either non-labile (acting as a carrier) or labile (undergoing substitution). The coordinate bond strength of the labile ligand plays a key role in the first step of the drug mechanism of action, i.e., the hydrolysis process, which is associated to the retention time of the medicine in the body. Therefore, an accurate determination of the metal-ligand bond strength becomes highly relevant as it will help the rational design of novel chemotherapeutic agents. Herein, we challenge the recently developed intrinsic bond strength index (IBSI) as a rapid and practical tool to assess the ligand lability in Pt(ii) complexes. In a first stage, given the importance of the trans-effect in synthetic strategies of cisplatin-based drugs, the effect of eleven trans-directing ligands T is quantified in two sets of complexes [Pt(NH3)2(H2O)T]n+ and [PtCl2(NH3)T]m+ where T = H2O, F-, NH3, Cl-, Br-, I-, SO32-, CH3-, CN-, CO, and H-. An essential outcome of this work is a novel index IBSItrans = IBSIσ + IBSIπ able to rank the directing ligands by their trans-effect according to their σ-donation and π-backbonding electronic contributions. In a second stage, we apply the IBSI score to a panel of eleven case studies, comprising mostly antineoplastic agents, such as cisplatin, carboplatin, lobaplatin etc., in order to quantify the coordinate bond strength of the ligands, providing insights about the hydrolysis process. The obtained results, in good agreement with experimental data and reported theoretical studies, demonstrate that the IBSI score is able to deliver a rapid and reliable picture of the coordinate bond strength, representing a chemically intuitive tool helpful for the development of novel anticancer agents prior to synthetic efforts.

Understanding Chemical Selectivity through Well Selected Excited States

In this publication, we propose a new set of reactivity/selectivity descriptors, derived within a Rayleigh-Schrödinger perturbation theory framework, for chemical systems undergoing an electrostatic (point-charge) perturbation. From the electron density polarization at first order, qualitative insight on reactivity is retrieved, while more quantitative information (noteworthy selectivity) can be obtained from either the second-order energy response or the number of shifted electrons under perturbation. Noteworthily, only a small number of excitations contribute significantly to the overall responses to perturbation, suggesting chemical reactivity could be foreseen by a careful scrutiny of the electron density reorganization upon excitation.

Computational Studies of Relative Stabilities of Low-Spin d6 cis- and trans-[M(en)2X2]+ Complexes (M = Co, Rh, Ir): Steric and Electronic Effects in the Context of the Structural Trans Influence

Computational studies of low spin d⁶ cis- and trans-[M(en)₂X₂]⁺ complexes (M = Co, Rh, Ir) employing multiple model chemistries find that isomer preferences fall into three categories. Complexes where X is largely a σ-donor (H^-, CH₃^-, CF₃^-) prefer cis geometries, in keeping with predictions associated with the trans influence series. Complexes where this donor characteristic is augmented by π acceptor behavior (B(CF₃)₂^-, BCl₂^-, SiCl₃^-) evince even greater preference for cis geometries. QTAIM charge data suggest this is marked by lower positive charge on the metal in cis complexes. In contrast, complexes where X is a π donor and low in the trans influence series (X = OH^-, F^-, Cl^-, I^-) prefer trans geometries to varying degrees. QTAIM calculations indicate that this arises because the cis complexes are destabilized by distortions of the electron density in the M-X bonds. This can be viewed conceptually as resulting from repulsions between lone pair electrons on the ligands. Complexes where the X ligands are moderately trans-influencing and can interact conjugatively (CN^-, NC^-, NO₂^-, C≡CH^-) prefer trans geometries because they combine destabilization of cis geometries with enhanced stabilization of trans geometries resulting from conjugation.

Building trans-philicity (trans-effect/trans-influence) ladders for octahedral complexes by using an NMR probe

An NMR trans-philicity ladder was built for octahedral [Cr(CO)₅L]^−/0/+ complexes employing a ¹³C NMR probe and quantified by Δσ¹³C NMR descriptors.

Photophysical properties and the NO photorelease mechanism of a ruthenium nitrosyl model complex investigated using the CASSCF-in-DFT embedding approach

A complete state-averaged active space self-consistent field (SA-CASSCF) calculation by means of the SA-CASSCF(18,14)-in-BP86 Miller-Manby embedding approach was performed to explore the ground and excited electronic states of the trans-[RuCl(NO)(NH₃)₄]²⁺ complex. Insights into the NO photodissociation mechanism and Ru-NO bonding properties are provided. In addition, spin-orbit (SO) interactions were taken into account to describe and characterize the spin-forbidden transitions observed at the low-energy regions of the trans-[RuCl(NO)(NH₃)₄]²⁺ UV-Vis spectrum. The SA-CASSCF(18,14)-in-BP86 electronic spectrum is in great agreement with the experimental data of Schreiner [Schreiner et al., Inorg. Chem., 1972, 11, 880].