Vladimir V. Sizov

Aurophilicity in Action: Fine-Tuning the Gold(I)-Gold(I) Distance in the Excited State To Modulate the Emission in a Series of Dinuclear Homoleptic Gold(I)-NHC Complexes.

The solution-state emission profiles of a series of dinuclear Au(I) complexes 4-6 of the general formula Au2(NHC-(CH2)n-NHC)2Br2, where NHC = N-benzylbenzimidazol-2-ylidene and n = 1-3, were found to be markedly different from each other and dependent on the presence of excess bromide. The addition of excess bromide to the solutions of 4 and 6 leads to red shifts of ca. 60 nm, and in the case of 5, which is nonemissive when neat, green luminescence emerges. A detailed computational study undertaken to rationalize the observed behavior revealed the determining role aurophilicity plays in the photophysics of these compounds, and the formation of exciplexes between the complex cations and solvent molecules or counterions was demonstrated to significantly decrease the Au-Au distance in the triplet excited state. A direct dependence of the emission wavelength on the strength of the intracationic aurophilic contact allows for a controlled manipulation of the emission energy by varying the linker length of a diNHC ligand and by judicial choice of counterions or solvent. Such unique stimuli-responsive solution-state behavior is of interest to prospective applications in medical diagnostics, bioimaging, and sensing. In the solid, the investigated complexes are intensely phosphorescent and, notably, 5 and 6 exhibit reversible luminescent mechanochromism arising from amorphization accompanied by the loss of co-crystallized methanol molecules. The mechano-responsive properties are also likely to be related to changes in bromide coordination and the ensuing alterations of intramolecular aurophilic interactions. Somewhat surprisingly, the photophysics of NHC ligand precursors 2 and 3 is related to the formation of ground-state associates with bromide counterions through hydrogen bonding, whereas 1 does not appear to bind its counterions.

Redox transformations in electroactive polymer films derived from complexes of nickel with SalEn-type ligands: computational, EQCM, and spectroelectrochemical study

Polymer complexes of nickel with SalEn-type ligands (SalEn = N,N′-bis (salicylidene) ethylenediamine) possess a number of unique properties, such as high redox conductivity, electrochromic behavior and selective catalytic activity in heterogeneous reactions. However, the mechanism of their redox transformation is still not clear. To understand this mechanism, we have performed a combined study of electrochemical and spectral properties of polymers derived from nickel complexes with various SalEn-type ligands containing methoxy substituents in phenyl rings, and methyl substituents in imino bridges. Experimental data were correlated with the results of density functional theory (DFT) calculations for model chains consisting of one to four monomer units. We found that, in acetonitrile-based supporting electrolyte, oxidation of such complexes, regardless of ligand substituents, proceeds via two routes, leading to formation of two oxidized forms: for the first one, a good correlation between experimental and computation results was observed. It has been demonstrated that positive charge in this form is delocalized in the phenyl moieties of ligand. The second oxidized form is stable only in coordinating solvents at high electrode polarizations and is likely to have the charge localized on the central metal atom, stabilized by axial coordination of solvent molecules. The complicated electrochemical response of each of the polymers that we have studied can be explained in the scope of this model without any additional assumptions by taking into account conversion of one oxidized form into another. Understanding the solvent effect on the oxidation route of the complexes will enable controlling their catalytic properties and stability.

A stimuli-responsive Au(I) complex based on an aminomethylphosphine template: synthesis, crystalline phases and luminescence properties

Herein we report the synthesis of a stimuli-responsive binuclear Au(I) complex based on the 1,5-bis(p-tolyl)-3,7-bis(pyridine-2-yl)-1,5-diaza-3,7-diphosphacyclooctane ligand, which is a novel template for the design of luminescent metal complexes. In the solid state, the complex obtained gives three different crystalline phases, which were characterized by XRD analysis. It was also found that the crystalline phases can be reversibly interconverted by recrystallization or solvent vapour treatment. The emission of these phases varies in the 500–535 nm range. Quite unexpectedly, the emission energy of these phases is mostly determined by the non-covalent interactions of the solvent molecules with the ligand environment, which have nearly no effect on the Au–Au interactions in the chromophoric centre. The complex obtained demonstrates thermo/solvatochromism to display greenish emission in a DCM matrix and blue emission in an acetone matrix at 77 K, in contrast to the blue emission of the phase containing a DCM molecule and greenish-yellow emission of the acetone solvate in a crystal cell at room temperature. The potentially important role of co-crystallized solvent molecules in the ligand-based emission of the complex obtained is supported by DFT calculations.

Valence bond analysis of the bonding in transition metal compounds: the RuNO group in nitrosyl complexes

The ab initio and semi-empirical configuration interaction wave functions of ruthenium complexes [RuL5(XY) q (L & = NH3, Cl−, CN−, XY &= N2, CO) are presented in the form of linear combinations of the valence bond (VB) structures, each structure being referred to some covalent or ionic model of the bonding in the M-XY group. The results of this VB analysis showed that [RuL5(NO)] q complexes can be described as compounds of Ru(III) and neutral NO0 with a covalent π-bond in addition to the usual coordination bond.

Computer simulation of CO2/CH4 mixture adsorption in wet microporous carbons

Adsorption of CH4, CO2 and their mixtures in dry and wet microporous carbons at 298 K is studied using grand canonical Monte Carlo computer simulations. Special attention is paid to the effects of water content and pore size on adsorption capacity and selectivity of the carbonaceous porous material. The presence of pre-adsorbed water may lead to non-monotonic dependence of adsorbed gas capacity on moisture content, which is observed for pure CO2 and CO2/CH4 mixtures at low adsorbate density (i.e., in wider pores, at low gas pressures, for low content of water). This effect is shown to be caused by electrostatic interactions of CO2 molecules with pre-adsorbed water. Under other conditions the presence of water in the pores leads to a decrease of the accessible volume causing a decrease of the adsorption capacity.

Coordination to Imidazole Ring Switches on Phosphorescence of Platinum Cyclometalated Complexes: The Route to Selective Labeling of Peptides and Proteins via Histidine Residues

In this study, we have shown that substitution of chloride ligand for imidazole (Im) ring in the cyclometalated platinum complex Pt(phpy)(PPh3)Cl (1; phpy, 2-phenylpyridine; PPh3, triphenylphosphine), which is nonemissive in solution, switches on phosphorescence of the resulting compound. Crystallographic and nuclear magnetic resonance (NMR) spectroscopic studies of the substitution product showed that the luminescence ignition is a result of Im coordination to give the [Pt(phpy)(Im)(PPh3)]Cl complex. The other imidazole-containing biomolecules, such as histidine and histidine-containing peptides and proteins, also trigger luminescence of the substitution products. The complex 1 proved to be highly selective toward the imidazole ring coordination that allows site-specific labeling of peptides and proteins with 1 using the route, which is orthogonal to the common bioconjugation schemes via lysine, aspartic and glutamic acids, or cysteine and does not require any preliminary modification of a biomolecule. The utility of this approach was demonstrated on (i) site-specific modification of the ubiquitin, a small protein that contains only one His residue in its sequence, and (ii) preparation of nonaggregated HSA-based Pt phosphorescent probe. The latter particles easily internalize into the live HeLa cells and display a high potential for live-cell phosphorescence lifetime imaging (PLIM) as well as for advanced correlation PLIM and FLIM experiments.

DFT study of metastable linkage isomers of six-coordinate ruthenium nitrosyl complexes

Abstract The linkage isomers of the [M(NO)(CN)5]2− (M = Fe, Ru, Os), [Ru(NO)Cl5]2−, trans-[Ru(NO)(NH3)4(L)]q+ (L = NH3, H2O, nicotinamide, imidazole, pyridine, pyrazine, NO2−, OH−, Cl−), and trans-[Ru(NO)L4(OH)]q (L = pyridine, NO2−) ions are studied by density functional theory. DFT calculations show that the electronic ground-state potential surface of these nitrosyl complexes has the local minima corresponding to η2-NO and η1-O linkage isomers, of which the former is characterized by lower energy. The stationary points on the ground state potential energy surface along the GS → MS2 → MS1 path for the [M(NO)(CN)5]2− (M = Fe, Ru, Os) and [Ru(NO)Cl5]2− complexes were found. The calculations of infrared spectra of linkage isomers [Ru(NO)Cl5]2− and [Ru(NO)(CN)5]2− are carried out. The analysis of quantum chemical bond orders indicates a noticeable delocalization of π-electron density along the Ltrans—M—NO axis and a possibility of direct interactions between NO and equatorial ligands in the structures with the bent {MNO} group.

Chemistry of Ruthenium Polypyridine Complexes: IV. Quantum-Chemical Simulation of the Effect of Solvation Shell on the Electronic Structure of Ru(II) Complexes with Nitrogen-containing Ligands

Ab initio quantum-chemical calculations of Ru(II) complexes have been fulfilled with consideration for solvation within the framework of the polarized continuum model. Energy levels of fragments of Ru(II) complexes with organic ligands are shifted relative to each other by electrostatic interactions with the solvation shells.

Electronic structure and spectra of binuclear bridged nitrosyl ruthenium complexes

The B3LYP method in the LanL2DZ basis set was used to carry out geometry optimization for the binuclear bridged complexes [RuCl4(NO)(μ-Pyz)Ru(P)(CO)]−, [Ru(Bipy)2(NO)(μ-Pyz)Ru(NH3)5]5+, and [(NC)Ru(Py)4(μ-CN)Ru(Py)4NO]3+ (Pyz is pyrazine). The electronic spectra of the complexes were calculated by the TDDFT and CINDO-CI methods with allowance for solvation effects. The ground-state electronic configurations of the two ruthenium atoms in these compounds were shown to be different. Among the lower excited states of all complexes, states with essentially weakened Ru-NO bonds were found. The strong absorption in the visible region of the spectrum of [Ru(Py)4NO-CN-Ru(Py)4CN]3+ is due to the interfragment electron transfer RuII → {RuNO} accompanied by weakening of the bond between nitrogen oxide and the complex.

Gold(I) Alkynyls Supported by Mono- and Bidentate NHC Ligands: Luminescence and Isolation of Unprecedented Ionic Complexes

Reactions of NHC·HX (NHC = 1-benzyl-3-methylbenzimidazol-2-ylidene, X = Br-, PF6-) and (AuC≡CR)n (R = Ph, C3H6OH) in the presence of Cs2CO3 initially afford compounds of the general formula [(NHC)2Au]2[(RC2)2Au]X, which can be isolated by crystallization. With increased reaction time, only the expected mononuclear complexes of the type [NHCAuC≡CR] are produced. The crystal structure of [(NHC)2Au]2[(PhC2)2Au]PF6 reveals an unprecedented triple-decker array upheld by a remarkably short (2.9375(7) Å) unsupported Au···Au···Au contact. The mononuclear complex [NHCAuC≡CPh] was found to crystallize as three distinct polymorphs and a pseudopolymorph, which depending on the intermolecular Au···Au distances emit blue, green, or yellow light. Two synthetic approaches were employed for the preparation of a series of dinuclear NHC-ligated Au(I) alkynyl complexes of the general formula [NHC-(CH2)n-NHC(AuC≡CR)2], where NHC = N-benzylbenzimidazol-2-ylidene, R = Ph, C3H6OH, C6H10OH, and n = 1-3. In solution, the complexes with aliphatic substituents on the alkynyl fragment are nonemissive, whereas their phenyl-bearing congeners demonstrate characteristic metal-perturbed 3[IL(C≡CPh)] emission. In the solid state, a clear correlation between intermolecular aurophilic interactions and luminescence was established, including their role in the luminescent thermochromism of the phenylalkynyl complexes. The relationship between the Au···Au distance and emission energy was found to be inverse: i.e., the shorter the aurophilic contact, the higher the emission energy. We tentatively attribute this behavior to a smaller extent of excited-state distortion for a structure with a shorter Au···Au separation.