A. A. Markov

On the nature of the chemical bond in heterobimetallic palladium(II) complexes with divalent 3d metals

The complex Pd(μ-OOCMe)4Cu(OH2) · 2Pd3(μ-OOCMe)6 was synthesized and characterized by X-ray crystallography. In the heterometallic moiety of this complex, the PdII and CuII atoms are at an extraordinary short distance (2.521(3) Å). DFT quantum-chemical calculations of the geometric and electronic structure of a series of heterobinuclear paddlewheel complexes PdIIMII(μ-OOCMe)4L (M = ZnII, NiII, CuII, CoII, FeII; L = OH2 and NCH) and their formate analogues PdIIMII(μ-OOCH)4L (M = ZnII, NiII, FeII) showed that the extraordinary short Pd⋯M distance in all these complexes is caused only by the tightening effect of carboxylate bridges rather than by the metal-metal bond. The direct Pd-M interaction becomes possible only after removal of electrons from the antibonding orbitals and formation of oxidized complexes of the [PdIII(μ-OOCMe)4NiIII]2+ type.

Mechanism and energetics of 1,2-addition of dioxygen 1O2(1Δg) to ethylene

The optimal geometry and energy parameters for five electronic states of the {1O2 (1Δg) + C2H4} system that characterize the elementary reactions of two-step 1,2-addition giving the dioxetane molecule were calculated using various quantum chemical methods (RHF, B3LYP, MPn, n = 2–4, QCISD, and CCSD) and basis sets (from 6-31+G(d,p) to 6-311+G(3df,2p) and pVTZ). The first step of the reaction was found to pass through the ethylene perepoxide intermediate. Considering experimental and published calculated data, the dependence of the results on the calculation procedure was exampled. The higher-level methods (QCISD, CCSD, CASSCF) and the standard methods (DFT, MPn) were found to reliably lead to virtually the same description of the energetics of this two-step reaction corresponding to experimental estimates.

Platinum acetate blue: synthesis and characterization.

Platinum acetate blue (PAB) of the empirical formula Pt(OOCMe)2.5±0.25, a byproduct in the synthesis of crystalline platinum(II) acetate Pt4(OOCMe)8, is an X-ray amorphous substance containing platinum in the oxidation state between (II) and (III). Typical PAB samples were studied with X-ray diffraction, differential thermal analysis-thermogravimetric, extended X-ray absorption fine structure, scanning electron microscopy, transmission electron microscopy, magnetochemistry, and combined quantum chemical density functional theory-molecular mechanics modeling to reveal the main structural features of the PAB molecular building blocks. The applicability of PAB to the synthesis of platinum complexes was demonstrated by the preparation of the new homo- and heteronuclear complexes Pt(II)(dipy)(OOCMe)2 (1), Pt(II)(μ-OOCMe)4Co(II)(OH2) (2), and Pt(III)2(OOCMe)4(O3SPhMe)2 (3) with the use of PAB as starting material.

Aquaperoxovanadium(V) complexes: Quantum-chemical study

The addition of a water molecule to mono-, di-, and triperoxovanadium(V) complexes has been studied at the density functional theory (B3LYP/6-31G**) and Møller-Plesset perturbation theory (MP2/6-31G**) levels. It has been demonstrated that the H2O…V donor-acceptor interaction cannot compete with hydrogen bonds and becomes weaker with an increase in the number of peroxo groups in the complex. In the most stable isomers of aquaperoxo complexes, water is mainly held by intermediate hydrogen bonds. The energy of addition of a water molecule to peroxovanadate is lower than or close to the heat of evaporation of water; i.e., the formation of stable aquaperoxovanadium complexes in aqueous solutions is improbable. This conclusion is consistent with the mass spectra of aqueous solutions of peroxovanadates, which show that the concentration of water-free peroxo complexes considerably exceeds the concentration of complexes with a coordinated water molecule. The coordination of the water molecule through the V…OH2 donor-acceptor interaction is prevented by the cis effect of the peroxo group, which has the HOMO orbital of symmetry suitable for interaction with the LUMO orbital of the VO group.

Catalytic peroxide oxidation: The structure of key intermediates in the VV/H2O2 system according to quantum chemical data

The inner-sphere isomerization of the peroxo complexes of vanadium(V) with the general formula [VO6]− was studied using approximations based on the density functional theory (B3LYP/6-31G**) and the Møller-Plesset perturbation theory (MP2/6-31G**). It was found that the complex [V(=O)(ηO2)(O3)]− containing the O3 group as a bidentate ligand was the most stable isomer. The transition state region of a rear-rangement of the triperoxo complex [V(ηO2)3]− into [V(=O)(ηO2)(O3)]− was localized. It was found that the activation barrier (∼30 kcal/mol) was mainly due to O-O bond cleavage in the peroxo ligand. According to calculations, the reaction proceeds through two intermediate complexes whose structure can be interpreted as that containing coordinated singlet dioxygen (especially in the limiting case) because of noticeably shortened O-O bonds in the ηO2 ligand. The calculated reaction scheme of the conversion of [V(ηO2)3]− into [V(=O)(ηO2)(O3)]− is qualitatively consistent with the previously found kinetics of the formation of ozone and the oxidation of alkanes, olefins, arenes, and singlet dioxygen traps.

Heterometallic Palladium(II)–Indium(III) and −Gallium(III) Acetate-Bridged Complexes: Synthesis, Structure, and Catalytic Performance in Homogeneous Alkyne and Alkene Hydrogenation

The reaction of Pd3(OOCMe)6 with indium(III) and gallium(III) acetates was studied to prepare new PdII-based heterometallic carboxylate complexes with group 13 metals. The heterometallic palladium(II)-indium(III) acetate-bridged complexes Pd(OOCMe)4In(OOCMe) (1) and Pd(OOCMe)4In(OOCMe)·MeCOOH (1a) were synthesized and structurally characterized with X-ray crystallography and extended X-ray absorption fine structure in the solid state and solution. A similar Pd-Ga heterometallic complex formed by the reaction of Pd3(OOCMe)6 with gallium(III) acetate in a dilute acetic acid solution, as evidenced by atmospheric pressure chemical ionization mass and UV-vis spectrometry, was unstable at higher concentrations and in the solid state. Complex 1 catalyzes the liquid-phase-selective phenylacetylene and styrene hydrogenation (1 atm of H2 at 20 °C) in acetic acid, ethyl acetate, and N, N-dimethylformamide solutions, while no Pd metal was formed until alkyne and alkene hydrogenation ceased.

Quantum-chemical simulation of the elementary step of the oxidation reactions of styrene and its derivatives involving 1О2 (1Δg)

The results of simulation of the oxidation reaction of styrene and its methyl (two isomers) and phenyl derivatives with molecular oxygen in the excited singlet state (1Δg) have enabled the conclusion that the reaction can proceed through several mechanisms. For styrene and its phenyl derivative, three reaction channels are possible, and for the methyl derivative, there are four possible channels. For the first two substrates, the major channel is 1,2-addition to form dioxetane; for the methyl derivatives, an extra channel to give a hydroperoxide species is possible in addition to the above channel. The multichannel reaction character revealed by calculations makes it possible to qualitatively understand the reason behind the moderate selectivity (no more than 70%) of such reactions in the case of styrene and its derivatives.

Quantum-chemical simulation of unit process for the oxidation reactions of ethylene and its derivatives invoving 1O2 (1Δg)

The results of simulation of oxidation reactions of ethylene derivatives with different substituents (F atoms, CH3O and CH3 groups) and butadiene molecule with participation of 1O2 (1Δg) have shown the possibility to realize different routes for the majority of the considered reactions. The largest product variety is obtained for butadiene and CH3 derivatives of ethylene. For butadiene, along with 1,2-cycloaddition reactions resulting in four-membered dioxetane (which is realized in all cases), the possibility to form six-membered cyclic epidioxides (1,4-addition) and diepoxide products with two three-membered rings (epoxidation) has been found. The formation of hydroperoxide forms along with 1,2-addition reactions is also possible for all CH3 derivatives of ethylene. Formation conditions and relative stability of the noted products have been analyzed for each case and certain features of the revealed reaction pathways with the transfer of two oxygen atoms have been discussed.

Unusual platinum complexes in the gas phase

Previously unknown cationic platinum complexes Pt(C5H4N)(C5H5N)+ and Pt(C5H4N)+, where platinum atom forms an unusual three-membered metallacycle with a deprotonated pyridine molecule, were detected in the gas phase by mass spectrometry and structurally characterized by DFT quantum-chemical calculations.

Unusual sandwich platinum(II) complex: [Pt(phen)2]2+ cation between two Pt(phen)(OOCMe)2 molecules

New carboxylate platinum(II) complexes: syn and anti isomers of Pt(phen)(OOCMe)2 molecular complex, [Pt(phen)(NCMe)2](O3SCF3)2, as well as unusual sandwich complex [Pt(phen)2]2+ · 2syn-[Pt(phen)(OOCMe)2] where [Pt(phen)2]2+ cation is inserted between two syn-Pt(phen)(OOCMe)2 molecules were synthesized and structurally characterized by X-ray diffraction analysis. As distinct from syn- and anti-Pt(phen)(OOCMe)2 and [Pt(phen)(NCMe)2](O3SCF3)2 complexes with flat phenanthroline ligand, the phen ligands in [Pt(phen)2]2+ cation have a curved configuration. Comparative DFT analysis of geometry of model structures phen, phen+, phenH+, and [Ptphen2]n+ (n = 1, 2) showed that electron removal from phen molecule had no effect on its geometry in both free state and platinum(II) complexes.