Mechanism of Cyclopropanation Reactions Mediated by (5,10,15,20-Tetra-p-tolylporphyrinato)osmium(II) Complexes

Catalytic systems derived from [Os(TTP)]2 or Fe(TTP) (TTP = 5,10,15,20-tetra-p-tolylporphyrinato) are extremely efficient at converting styrenes and diazo reagents to cyclopropanes in high yields and high stereoselectivity. A number of mechanistic studies have been undertaken to elucidate the catalytic pathway. A mono(carbene) complex, (TTP)OsCHCO2Et, has been isolated but is not the catalytically active species. An electron-withdrawing ligand trans to the carbene in (TTP)OsCHCO2Et activates the carbon fragment toward transfer to an olefin. Labeling studies with (TTP)OsCHX and N2CHY and substrate reactivity profiles are consistent with a trans-osmium(II) bis(carbene) species as the active catalyst

Thermochemistry of organometallic reactions in solution: joint ITC and DFT study

The understanding of certain, still unknown, aspects of the chemical bond is made possible by new theoretical tools, particularly static DFT-D or DFT methods corrected for dispersion. These methods allow accounting for, in a physically relevant way, the effects of dispersion at medium and long distance [1]. For the further assessing the accuracy of static DFT-D calculations the providing of referential experimental data was found to be essential. It has been shown that Isothermal titration calorimetry (ITC) techniques can provide reliable thermodynamic parameters of reaction (enthalpy ΔHr, Gibbs free energy ΔGr and entropy ΔSr) [2], while some recent studies showed good agreement between experimental and theoretical results [2]. The study presented here sheds some light on the thermochemistry of reactions in solution by preforming ITC experiments in chlorobenzene and static DFT-D calculations. The study points out that, in cases where solvent molecules can interact significantly with molecules of reactants, an accounting for the explicit solvation is of crucial importance for agreement between experiment and theory. The results of various kinds of organometallic reactions will be presented in some details

Shape and Stereoselective Cyclopropanation of Alkenes Catalyzed by Iron Porphyrins

Iron porphryin complexes are active catalysts for the cyclopropanation of alkenes by ethyl diazoacetate. Fe(TIP) (TIP = meso-tetra-p-tolylporphyrin), an isolated iron(II) porphyrin complex, can be used as the catalyst, or the iron(III) complexes of several porphyrins can be reduced in situ. The reactions produce synthetically useful excesses of the trans cyclopropyl ester products. This stereoselectivity exhibits a modest solvent dependence, with donor solvents giving higher ratios of the trans cyclopropane products. The diastereoselectivity exhibits only a modest dependence on the steric bulk of the porphyrin. The reactions are selective for 1-alkenes and 1, 1-disubstituted alkenes. Conjugated substrates and enol ethers react more rapidly than simple aliphatic alkenes. A mechanistic model for the iron-mediated reactions is proposed which is consistent with the data presented herein

Joint ITC and DFT Study of the Affinity of Some Lewis Bases to HIFP in Solution

HFIP, i.e. 1,1,1,3,3,3-hexafluoropropan-2-ol, was found to be an exceptional medium,[1] either as solvent or co-solvent, that allows many reactions to occur.[2-5] However, the exact role and mode of action of HFIP in various chemical transformations still remains elusive. Despite many reports dealing with water/HFIP complexes, little has been published on other molecular complexes of HFIP as well as on thermochemistry of the formation of such complexes.[6] Within this study the affinity of a series of eight different Lewis bases (3 sulfoxides, 3 Nsp2 pyridine derivatives, 1 aromatic amine, 1 cyclic aliphatic ether) to HFIP (as Lewis acid) is investigated experimentally by Isothermal Titration Calorimetry (ITC) and theoretically using static DFT-D calculations. Measured ITC association enthalpy values ΔHaITC spanned -9.3 kcal/mol - -14 kcal/mol. Computations including a PCM implicit solvation model produced similar exothermicity of association of all studied systems - ΔHa values ranging -8.5 – -12.7 kcal/mol. In general, most of interaction energy is due to the hydrogen bonding and not due to formation of significantly strong halogen bonds. An additional set of calculations combining implicit and explicit solvation by chlorobenzene of the reactants, pointed out the relatively low interference of the solvent with the HFIPbase complexation, which main effect is to slightly enhance the Gibbs energy of the HFIP-Lewis base association. It is speculated that the interactions of bulk HFIP with Lewis bases therefore may significantly intervene in catalytic processes not only via the dynamic miscrostructuration of the medium but also more explicitly by affecting bonds’ polarization at the Lewis bases

The Thermochemistry of London Dispersion-Driven Transition Metal Reactions: Getting the ‘Right Answer for the Right Reason’

Reliable thermochemical measurements and theoretical predictions for reactions involving large transition metal complexes in which long-range intramolecular London dispersion interactions contribute significantly to their stabilization are still a challenge, particularly for reactions in solution. As an illustrative and chemically important example, two reactions are investigated where a large dipalladium complex is quenched by bulky phosphane ligands (triphenylphosphane and tricyclohexylphosphane). Reaction enthalpies and Gibbs free energies were measured by isotherm titration calorimetry (ITC) and theoretically ‘back-corrected’ to yield 0 K gas-phase reaction energies (DE). It is shown that the Gibbs free solvation energy calculated with continuum models represents the largest source of error in theoretical thermochemistry protocols. The (‘backcorrected’) experimental reaction energies were used to benchmark (dispersion-corrected) density functional and wave function theory methods. Particularly, we investigated whether the atom-pairwise D3 dispersion correction is also accurate for transition metal chemistry, and how accurately recently developed local coupled-cluster methods describe the important long-range electron correlation contributions. Both, modern dispersion-corrected density functions (e.g., PW6B95-D3(BJ) or B3LYP-NL), as well as the now possible DLPNO-CCSD(T) calculations, are within the ‘experimental’ gas phase reference value. The remaining uncertainties of 2–3 kcalmol1 can be essentially attributed to the solvation models. Hence, the future for accurate theoretical thermochemistry of large transition metal reactions in solution is very promisin

Crystal structures of a copper(II) and the isotypic nickel(II) and palladium(II) complexes of the ligand (E)-1-[(2,4,6-tri­bromo­phen­yl)diazen­yl]naphthalen-2-ol

In the copper(II) complex, bis­{(E)-1-[(2,4,6-tri­bromo­phen­yl)diazen­yl]naph­thalen-2- olato}copper(II), [Cu(C₁₆H₈Br₃N₂O)₂], (I), the metal cation is coord­inated by two N atoms and two O atoms from two bidentate (E)-1-[(2,4,6-tri­bromo­phen­yl)diazen­yl]naphthalen-2-olate ligands, forming a slightly distorted square-planar environment. In one of the ligands, the tri­bromo­benzene ring is inclined to the naphthalene ring system by 37.4 (5)°, creating a weak intra­molecular Cu...Br inter­action [3.134 (2) Å], while in the other ligand, the tri­bromo­benzene ring is inclined to the naphthalene ring system by 72.1 (6)°. In the isotypic nickel(II) and palladium(II) complexes, namely bis­{(E)-1-[(2,4,6-tri­bromo­phen­yl)diazen­ yl]naphthalen-2-olato}nickel(II), [Ni(C₁₆H₈Br₃N₂O)₂], (II), and bis­{(E)-1- [(2,4,6-tri­bromo­phen­yl)diazen­yl]naphthalen-2-olato}palladium(II), [Pd(C₁₆H₈Br₃N₂O)₂], (III), respectively, the metal atoms are located on centres of inversion, hence the metal coordination spheres have perfect square-planar geometries. The tri­bromo­benzene rings are inclined to the naphthalene ring systems by 80.79 (18)° in (II) and by 80.8 (3)° in (III). In the crystal of (I), mol­ecules are linked by C-H...Br hydrogen bonds, forming chains along [010]. The chains are linked by C-H...Pi inter­actions, forming sheets parallel to (011). In the crystals of (II) and (III), mol­ecules are linked by C-H...Pi inter­actions, forming slabs parallel to (10-1). For the copper(II) complex (I), a region of disordered electron density was corrected for using the SQUEEZE routine in PLATON [Spek (2015). Acta Cryst. C71, 9-18]. The formula mass and unit-cell characteristics of the disordered solvent mol­ecules were not taken into account during refinement

Benchmarking to DFT-d calculations by ITC experimental data

The London forces [1–3], or dispersion, are omnipresent in the nature. It constitutes an important part of the energy contribution to the stabilization of the tertiary structure of peptides, other natural polymers and the spontaneous coalescence of atomic aggregates or apolar molecules. The specifi city of the force of London is that it acts at long distances and it is always attractive, and it is therefore effective intramolecularly and determines in many situations the conformational behaviour of organic molecules and organometallics as well. It plays an essential role in chiral recognition and discrimination processes. The understanding of certain still unknown aspects of the chemical bond is made possible by new theoretical tools, particularly static DFT-D or DFT methods corrected for Dispersion. These allow to account for in a physically relevant way the effects of dispersion at medium and long distance [4]. For the further assessing the accuracy of static DFT-D calculations providing a referential of experimental data was found essential. It has been shown that ITC techniques can provide reliable reaction enthalpy ΔHr, Gibbs free energy of reaction ΔGr and reaction entropy ΔSr as well [5]. Some recent studies showed good agreement between experimental and theoretical results [6–8]. This study will shed some light on the thermochemistry of the reactions in solution by preforming ITC experiments in chlorobenzene, from one side, and static DFT-D calculations at different levels of theory, from another side (Fig. 1). By comparison of obtained results one could conclude on the excellent agreement between experimental and theoretical data, which could be promising for the further development and application of static DFT-D computational methods. As examples, the results of various organometallic reactions will be presented in some details [9]

Cyclopalladated Compounds as Resolving Agents for Racemic Mixtures of Ligands

International audienc

(η5-pentamethylcyclopentadienyl)-(5-methyl-2-[4-(2-methylpropyl)-4,5-dihydro-1,3-oxazol-2-yl]phenyl)-(acetonitrile)-iridium tetrakis(3,5-bis(trifluoromethyl)phenyl)borate

Jean-Pierre Djukic, Mustapha Hamdaoui, CCDC 1845417: Experimental Crystal Structure Determination, 2018, DOI: 10.5517/ccdc.csd.cc1zy9l