Stepan Stepanović

A density functional study of the spin state energetics of polypyrazolylborato complexes of first-row transition metals

Density Functional Theory (DFT) was used to analyse and explain spin state energetics of first-row transition metals (Mn(II), Fe(II), Co(II); Cr(III), Mn(III), Fe(III), Co(III); Mn(IV)) in polypyrazolylborato complexes. We explored the effects of substitutions at the 3 and 5 positions of the pyrazolyl rings, as well as the influence of Jahn-Teller (JT) distortions on spin-state switching. Although the stabilizations due to JT distortion are sometimes substantial, this does not lead to switching of the spin ground-state. On the other hand, electron withdrawing or donating substituents do lead to significant changes in the spin-crossover (SCO) properties of the investigated complexes.

Magnetic anisotropy in ‘scorpionate’ first-row transition-metal complexes: a theoretical investigation

In this work we have analyzed in detail the magnetic anisotropy in a series of hydrotris(pyrazolyl)borate (Tp(-)) metal complexes, namely [VTpCl](+), [CrTpCl](+), [MnTpCl](+), [FeTpCl], [CoTpCl], and [NiTpCl], and their substituted methyl and tert-butyl analogues with the goal of observing the effect of the ligand field on the magnetic properties. In the [VTpCl](+), [CrTpCl](+), [CoTpCl], and [NiTpCl] complexes, the magnetic anisotropy arises as a consequence of out-of-state spin-orbit coupling, and covalent changes induced by the substitution of hydrogen atoms on the pyrazolyl rings does not lead to drastic changes in the magnetic anisotropy. On the other hand, much larger magnetic anisotropies were predicted in complexes displaying a degenerate ground state, namely [MnTpCl](+) and [FeTpCl], due to in-state spin-orbit coupling. The anisotropy in these systems was shown to be very sensitive to perturbations, for example, chemical substitution and distortions due to the Jahn-Teller effect. We found that by substituting the hydrogen atoms in [MnTpCl](+) and [FeTpCl] by methyl and tert-butyl groups, certain covalent contributions to the magnetic anisotropy energy (MAE) could be controlled, thereby achieving higher values. Moreover, we showed that the selection of ion has important consequences for the symmetry of the ground spin-orbit term, opening the possibility of achieving zero magnetic tunneling even in non-Kramers ions. We have also shown that substitution may also contribute to a quenching of the Jahn-Teller effect, which could significantly reduce the magnetic anisotropy of the complexes studied.

Role of spin state and ligand charge in coordination patterns in complexes of 2,6-diacetylpyridinebis(semioxamazide) with 3d-block metal ions: A density functional theory study

We report here a systematic computational study on the effect of the spin state and ligand charge on coordination preferences for a number of 3d-block metal complexes with the 2,6-diacetylpyridinebis(semioxamazide) ligand and its mono- and dianionic analogues. Our calculations show excellent agreement for the geometries compared with the available X-ray structures and clarify some intriguing experimental observations. The absence of a nickel complex in seven-coordination is confirmed here, which is easily explained by inspection of the molecular orbitals that involve the central metal ion. Moreover, we find here that changes in the spin state lead to completely different coordination modes, in contrast to the usual situation that different spin states mainly result in changes in the metal-ligand bond lengths. Both effects result from different occupations of a combination of π- and σ-antibonding and nonbonding orbitals.

Spin state relaxation of iron complexes: the case for OPBE and S12g functionals

The structures of nine iron complexes that show a diversity of experimentally observed spin ground states are optimized and analysed with Density Functional Theory (DFT). An extensive validation study of the new S12g functional is performed, with the discussion concerning the influence of the environment, geometry and its overall performance based on the comparison with the well proven OPBE functional. The OPBE and S12g functionals give the correct spin ground state for all investigated iron complexes. Since S12g performs remarkably well it can be considered a reliable tool for studying spin state energetics in complicated transition metal systems.

Benchmarking density functional tight binding models for barrier heights and reaction energetics of organic molecules

Density Functional Tight Binding (DFTB) models are two to three orders of magnitude faster than ab initio and Density Functional Theory (DFT) methods and therefore are particularly attractive in applications to large molecules and condensed phase systems. To establish the applicability of DFTB models to general chemical reactions, we conduct benchmark calculations for barrier heights and reaction energetics of organic molecules using existing databases and several new ones compiled in this study. Structures for the transition states and stable species have been fully optimized at the DFTB level, making it possible to characterize the reliability of DFTB models in a more thorough fashion compared to conducting single point energy calculations as done in previous benchmark studies. The encouraging results for the diverse sets of reactions studied here suggest that DFTB models, especially the most recent third‐order version (DFTB3/3OB augmented with dispersion correction), in most cases provide satisfactory description of organic chemical reactions with accuracy almost comparable to popular DFT methods with large basis sets, although larger errors are also seen for certain cases. Therefore, DFTB models can be effective for mechanistic analysis (e.g., transition state search) of large (bio)molecules, especially when coupled with single point energy calculations at higher levels of theory.

Resolving the origin of the multimode Jahn–Teller effect in metallophthalocyanines

A detailed Density Functional Theory (DFT) analysis was performed in order to study the multimode Jahn-Teller (JT) problem in the electronic ground state of manganese phthalocyanine (MnPc). A comparison with the magnesium phthalocyanine ion (MgPc-) and the phthalocyanine trianion (Pc3-), also prone to the JT effect, is presented. Our results clarify the origin and provide the microscopic insight into the symmetry breaking process. The JT distortion is highly influenced by the coordination of phthalocyanine to the MnII ion, and occurs over the whole system, while the MgPc- complex ion possesses mainly ligand-based instability.

Structural diversity of isothiocyanato Cd(II) and Zn(II) Girard’s T hydrazone complexes in solution and solid state: effect of H-bonding on coordination number and supramolecular assembly of Cd(II) complex in solid state

The isothiocyanato Zn(II) complex (1) and mixed isothiocyanato/thiocyanato Cd(II) complex (2) with the condensation product of 2-acetylpyridine and trimethylammoniumacetohydrazide chloride (Girard’s T reagent) (HLCl) were investigated both experimentally and theoretically. The crystal structures of both complexes showed tridentate N2O coordination of hydrazine ligand. In complex 1 square-pyramidal coordination surrounding of Zn(II) consists of deprotonated hydrazone ligand and two isothiocyanato ligands, while in octahedral Cd(II) complex ligand is coordinated without deprotonation as a positively charged species and coordination geometry is completed with two N-coordinated and one S-coordinated NCS− anions. NMR spectroscopy and molar conductivity results for Cd(II) and Zn(II) complexes indicated their instability in solution. DFT calculations were performed to explain coordination preference and stability of complexes 1 and 2 in solid state and in solution. The obtained Cd(II) complex is the first reported mononuclear pseudohalide/halide Cd(II) complex with quinoline-/pyridine-based hydrazone ligands possessing octahedral geometry in solid state. In this complex, H-bonding has significant impact on coordination number and supramolecular assembly in solid state.

Selective Photo-Induced Oxidation with O-2 of a Non-Heme Iron(III) Complex to a Bis(imine-pyridyl)iron(II) Complex

Non-heme iron(II) complexes of pentadentate N4Py (N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) type ligands undergo visible light-driven oxidation to their iron(III) state in the presence of O2 without ligand degradation. Under mildly basic conditions, however, highly selective base catalyzed ligand degradation with O2, to form a well-defined pyridyl-imine iron(II) complex and an iron(III) picolinate complex, is accelerated photochemically. Specifically, a pyridyl-CH2 moiety is lost from the ligand, yielding a potentially N4 coordinating ligand containing an imine motif. The involvement of reactive oxygen species other than O2 is excluded; instead, deprotonation at the benzylic positions to generate an amine radical is proposed as the rate determining step. The selective nature of the transformation holds implications for efforts to increase catalyst robustness through ligand design.

A Non-Heme Iron Photocatalyst for Light-Driven Aerobic Oxidation of Methanol

Abstract Non‐heme (L)FeIII and (L)FeIII‐O‐FeIII(L) complexes (L=1,1‐di(pyridin‐2‐yl)‐N,N‐bis(pyridin‐2‐ylmethyl)ethan‐1‐amine) underwent reduction under irradiation to the FeII state with concomitant oxidation of methanol to methanal, without the need for a secondary photosensitizer. Spectroscopic and DFT studies support a mechanism in which irradiation results in charge‐transfer excitation of a FeIII−μ‐O−FeIII complex to generate [(L)FeIV=O]2+ (observed transiently during irradiation in acetonitrile), and an equivalent of (L)FeII. Under aerobic conditions, irradiation accelerates reoxidation from the FeII to the FeIII state with O2, thus closing the cycle of methanol oxidation to methanal.

High-yielding method for preparation of carbocyclic or N-containing heterocyclic β-keto esters using in situ activated sodium hydride in dimethyl sulphoxide

ABSTRACT It was found that NaH suspension in DMSO was highly activated when reacted with an alcohol. The in situ generated NaH/alkoxide mixture permitted very rapid and complete deprotonation and acylation of various cyclic ketones with alkyl carbonates at ambient temperature. Activated NaH/alkoxide in DMSO is particularly effective in Dieckmann condensations, where it affords 5- and 6-membered carbocyclic or N-containing heterocyclic β-keto esters in high yields. A heterocyclic Dieckmann condensation was performed on a molar scale, demonstrating the scalability of the procedure. Besides, DMSO is non-toxic, relatively inexpensive and environmentally benign solvent. GRAPHICAL ABSTRACT