Jean-Pierre Djukic

The Crucial Role of Dispersion in the Cohesion of Nonbridged Binuclear Os → Cr and Os → W Adducts

The concept of a dative metal-metal bond is generally used to designate the donor-acceptor (DA) interaction of an electron-saturated metal center with another electron-deficient--or unsaturated--metal center. This type of DA bonding extended to the field of coordination complexes constitutes a borderline case of weak metal-metal interaction, among which the so-called metallophilic interactions occurring with 4d, 5d, and other late-transition-metal complexes are the most documented and representative examples. From a general standpoint, the peculiar position of the so-called dative metal-metal bond in chemical bonding stems from its presumed covalent character, which contrasts with the situation encountered with metallophilic interactions, which are essentially supported by dispersion and electrostatic forces and somewhat sustained by relativistic effects. In this study, the nature of the metal-metal bond in nonbridged 5d-3d Os-Cr and 5d-5d Os-W adducts, i.e., (Me(3)P)(CO)(4)Os-M(CO)(5) (M = Cr, W) and (CO)(5)Os-Cr(CO)(5), was addressed by resorting to state-of-the-art quantum-chemical methods. Semilocal density functional theory (DFT) approximations like Becke-Perdew or TPSS, the double-hybrid B2PLYP functional, as well as the corresponding dispersion, including TPSS-D and B2PLYP-D functionals and the wave-function-based spin-component-scaled second-order perturbative Moller-Plesset theory (SCS-MP2), were used. Energy decomposition analysis combined with the analysis of pairwise interfragment correlation energies from Pipek-Mezey localized molecular orbitals in combination with SCS-MP2 led to a clear demonstration of the significant role of dispersion (London) forces in the stabilization of the title adducts, wherein the Os-metal DA bond bears a rather low covalent character. These results plead in favor of a systematic recourse to dispersion including DFT approximations when addressing organometallic and coordination complexes.

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 (ΔE). It is shown that the Gibbs free solvation energy calculated with continuum models represents the largest source of error in theoretical thermochemistry protocols. The (‘back-corrected’) 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 kcal mol−1 can be essentially attributed to the solvation models. Hence, the future for accurate theoretical thermochemistry of large transition metal reactions in solution is very promising.

Ambipolar organic transistors and near-infrared phototransistors based on a solution-processable squarilium dye†

Implementation of organic transistors in low-end, large-volume microelectronics depends, greatly, on the level of performance that can be achieved, but also on the compatibility of the technology with low-cost processing methodologies. Here we examine the suitability of a family of solution-processable zwitterionic molecules, so-called squarilium dyes, for the fabrication of organic ambipolar transistors and their application in (opto)electronic circuits. Ambipolar organic semiconductors and transistors are interesting because they could deliver performance characteristics (i.e. noise margins and signal gain) similar to that of complementary logic, but with the fabrication simplicity associated with unipolar logic (i.e. single semiconductor material and single type of metal electrodes). By designing squarilium dyes with appropriate electrochemical characteristics we demonstrate single-layer organic transistors that exhibit ambipolar charge transport with balanced electron and hole mobilities. By integrating a number of these ambipolar transistors we are also able to demonstrate complementary-like voltage inverters with wide noise margin and high signal gain. Another interesting feature of the squarilium dyes studied here is their strong absorption in the near-infrared (NIR) region of the electromagnetic spectrum. By exploring this interesting property we are able to demonstrate NIR light-sensing ambipolar organic transistors with promising operating characteristics.

Metalated (η6-arene)tricarbonylchromium complexes in organometallic chemistry

Abstract (η 6 -Arene)Cr(CO) 3 complexes differ from their chromium-free analogs in that they undergo a variety of new transformations as a result of the electron withdrawing effect and the stereocontrol effected by the Cr(CO) 3 moiety. Two classes of application are apparent: (1) those that use the Cr(CO) 3 group as an auxiliary which may eventually be removed upon completion of a sequence of reactions; and (2) those that aim at designing novel molecules incorporating a Cr(CO) 3 moiety and possessing specific stereochemical properties. We present a detailed review of the extensive chemistry of metalated (η 6 -arene)Cr(CO) 3 complexes developed in the past 30 years and illustrate the main methods of preparation as well as the most important structural, physical and chemical properties of this family of compounds.

Efficient hydrosilylation of imines using catalysts based on iridium(III) metallacycles

Ir(III) metallacycles were applied as catalysts for the hydrosilylation of various ketimines and aldimines with sodium tetrakis[(3,5-trifluoromethyl)phenyl]borate, NaBArF24, as an additive. By using a slight excess of the organosilane reagent, the reactions proceeded rapidly and efficiently, at low catalyst loadings and at room temperature. Several examples of cationic Ir(III) catalysts could be synthesised, characterized and tested. In situ-generated catalysts proved to be more active as compared to isolated ones and species with non-coordinating BArF24 counterion gave the highest catalytic activities.

Antiferromagnetic coupling across a tetrametallic unit through noncovalent interactions

Three paramagnetic heterobimetallic lantern complexes of the form [PtM(tba)4(OH2)] (M = Fe, 1; Co, 2; Ni, 3; tba = thiobenzoate) have been prepared in a single-step, bench-top procedure. In all three cases, a lantern structure with Pt–M bonding is observed in solution and in the solid state. Compound 1 is a monomer whereas 3 exists as a dimer in the solid state via a Pt⋯Pt metallophilic interaction. Compound 2 has been characterized in forms with (2a, purple) and without (2b, yellow) Pt⋯Pt metallophilic interactions. The dimers 2a (J = −10 cm−1, based on the spin Hamiltonian Ĥ = −2J(SA·SB)) and 3 (J = −60 cm−1) exhibit antiferromagnetic coupling between the two first-row metal ions in the solid state via a Pt⋯Pt non-covalent metallophilic interaction. The electronic structure of C4v [PtM(tba)4], C2 [PtM(tba)4(OH2)], (M = Fe, Co, Ni) and D2 symmetry [PtM(tba)4(OH2)]2 M = Co, Ni, units have been studied with DFT calculations, confirming the relative spin-state energies observed and the antiferromagnetic exchange pathway through four dz2 orbitals. The compounds 2a and 3 are the first examples of antiferromagnetic coupling through an unbridged M⋯M contact.

Cyclomanganated (η6-arene)tricarbonylchromium complexes: synthesis and reactivity

Abstract The cyclomanganation of ( η 6 -arene)tricarbonylchromium complexes by reaction with benzylpentacarbonylmanganese affords new bimetallic compounds. The mechanism of the manganation as well as other underlying aspects of the reactivity of cyclomanganated complexes are addressed in this report.

Enantiopure Sulfinyl Aniline as a Removable and Recyclable Chiral Auxiliary for Asymmetric C(sp3)−H Bond Activation

An original and recyclable chiral bidentate aniline-sulfoxide-based directing group has been developed. This auxiliary allows challenging stereoselective Pd-catalyzed direct functionalization of small cycloalkanes through C-aryl and C-alkyl bond formation. Although moderate diastereoselectivities are observed, both optically pure enantiomers of the highly functionalized products can be obtained separately by simple silica gel chromatography and cleavage of the chiral auxiliary. This strategy was further applied to the preparation of enantiomerically pure 1,2,3-trisubstituted cyclopropane carboxylic acid derivatives, with three stereogenic centers and bearing both alkyl and aromatic substituents. These molecular scaffolds are not yet reported in the literature. The synthetic utility of this approach is validated by the chiral auxiliary being readily cleaved and recovered posteriori to the C-H activation step, without deterioration of its optical purity. Finally, an unprecedented palladacycle intermediate generated through C-H activation of the cyclopropane moiety has been isolated and fully characterized. Initial DFT calculations shed additional light on the reactivity of this original intermediate.

Hemichelation, a Way To Stabilize Electron-Unsaturated Complexes: The Case of T-Shaped Pd and Pt Metallacycles.

A rational method of synthesis of stable neutral T-shaped 14 electron Pd and Pt complexes is proposed. It takes advantage of the ambiphilic character of the tricarbonyl(η(6)-indenyl)chromium anion, of which the main property is to behave as a hemichelating ligand, that is a nonconventional heteroditopic ligand capable of chelating a metal center by way of covalent and noncovalent bonding, thus preserving its unsaturated valence shell. The reaction of the in situ formed tricarbonyl(η(6)-2-methylindenyl)chromium anion with a series of Pd and Pt metallacycles afforded new air-stable and persistent synfacial heterobimetallic complexes in which the metallacycle binds the indenyl fragment via its metal in an η(1) fashion, leaving the fourth coordination site at the chelated metal virtually vacant. The structures of eight of these novel complexes are disclosed, and their bonding features are investigated by an array of theoretical methods based on the density functional theory (NBO, EDA, ETS-NOCV, AIM, NCI region analysis). Theory shows that the formation of these unusual structures of bimetallic synfacial η(1)-indenyl-Pd/Pt complexes is driven thermodynamically by attractive Coulombic occlusion of the fourth vacant coordination site at Pd/Pt centers by the Cr(CO)3 moiety.

Synthesis of Planar Chiral Iridacycles by Cationic Metal π‐Coordination: Facial Selectivity, and Conformational and Stereochemical Consequences

Facial selectivity during the π-coordination of pseudo-tetrahedral iridacycles by neutral (Cr(CO)(3)), monocationic (Cp*Ru(+)), and biscationic (Cp*Ir(2+)) metal centers was directly influenced by the coulombic imbalance in the coordination sphere of the chelated Ir center. We also showed by using theoretical calculations that the feasibility of the related metallacycles that displayed metallocenic planar chirality was dependent to the presence of an electron-donating group, such as NMe(2), which contributed to the overall stability of the complexes. When the π-bonded moiety was the strongly electron-withdrawing Cp*Ir(2+) group, the electron donation from NMe(2) resulted in major conformational changes, with a barrier to rotation of about 17 kcal  mol(-1) for this group that became spectroscopically diastereotopic (high-field (1)H NMR spectroscopy). This peculiar property is proposed as a means to introduce a new type of constitutional chirality at the nitrogen center: planar chirality at tertiary aromatic amines.