Detailed energy decomposition of the rotational barrier in 2,2′-bipyridine: a density functional study

On the preferences of five-membered chelate rings in coordination chemistry: insights from the Cambridge Structural Database and theoretical calculations

The purpose of this review is to give an overview of three important N-bidentate ligands: 1,10-phenanthroline (phen), 2,2'-bipyridine (bpy), and ethylenediamine (en). We have not attempted to be comprehensive because of the huge amount of activity being done in coordination chemistry using these ligands. Instead we present a full structural and geometrical study by using the Cambridge Structural Database (CSD) combined with theoretical calculations that allow us to parameterize their coordinating properties and ability to coordinate to transition and non-transition metals. More importantly, we illustrate that upon coordination and formation of the five-membered chelate ring, these ligands are able to adapt themselves to the requirements of the different metals by changing the MN distances and NMN angles. Therefore, a redefinition of the preferences of these ligands to metals with large ionic radii is needed. Finally, we will present some facts about the participation of these ligands in inorganic-organic hybrids (IOHs) based on Keggin polyoxometalates (POMs).

High-resolution electron spectroscopy and molecular structures of Cu–(2,2′-bipyridine) and Cu-(4,4′-bipyridine)

Copper complexes of 2,2′-bipyridine (22BIPY) and 4,4′-bipyridine (44BIPY) were prepared in a laser-vaporization supersonic molecular beam source and identified by laser photoionization time-of-flight mass spectrometry. Electronic spectra and molecular structures were studied with pulsed-field ionization zero electron kinetic energy (ZEKE) electron spectroscopy, density functional theory (DFT) and second-order Møller–Plesset perturbation (MP2) calculations, and spectral simulations. Adiabatic ionization energies and metal–ligand and ligand-based vibrational frequencies of Cu–22BIPY and Cu–44BIPY were measured from the ZEKE spectra. Ground electronic states and molecular structures of the two complexes were determined by comparing the spectroscopic measurements with the theoretical calculations. The ground state of Cu–22BIPY ( 2 B1, C2v) has a planar bidentate structure with Cu binding to two nitrogen atoms and two pyridine molecules in the cis configuration. The ground state of Cu–44BIPY ( 2 A, C2) has a monodentate structure with Cu binding to one nitrogen and two pyridines in a twisted configuration. The ionization energy of Cu–22BIPY is considerably lower and its bond energy is much higher than that of Cu–44BIPY. The different ionization and dissociation energies are attributed to the distinct metal binding modes of the two complexes. It has been found that the DFT calculations yield the correct structures for the Cu–22BIPY complex, whereas the MP2 calculations produce the best structures for the Cu–44BIPY complex.

Conformational Study of an Artificial Metal-Dependent Regulation Site for Use in Designer Proteins

This report describes the dimerisation of glutathione, and by extension, other cysteine-containing peptides or protein fragments, with a 5, 5'-disubstituted-2, 2'-bipyridine or 6, 6"-disubstituted-2, 2':6',2"-terpyridine unit. The resulting bipy-GS2 and terpy-GS2 were investigated as potential metal ion dependent switches in aqueous solution, and were found to predominantly adopt the transoïd conformation at physiological pH. Metal complexation with CuII and ZnII at this pH has been studied by UV/Vis, CD, NMR and ion-mobility mass spectrometry. ZnII titrations are consistent with the formation of a 1:1 ZnII:terpy-GS2 complex at pH 7.4, but bipy-GS2 was shown to form both 1:1 and 1:2 complexes with the former being predominant under dilute micromolar conditions. Formation constants for the resulting 1:1 complexes were determined to be log KM 6.86 (bipy-GS2 ) and 6.22 (terpy-GS2 ), consistent with a higher affinity for the unconstrained bipyridine, compared to the strained terpyridine. CuII coordination involves the initial formation of 1:1 complexes, followed by 1.5Cu:1bipy-GS2 and 2Cu:1terpy-GS2 complexes at micromolar concentrations. Binding constants for formation of the 1:1 complexes (log KM 12.5 (bipy-GS2 ); 8.04 and 7.14 (terpy-GS2 )) indicate a higher affinity for CuII than ZnII. Finally, ion-mobility MS studies detected the free ligands in their protonated form, and were consistent with the formation of two different Cu adducts with different conformations in the gas-phase. We illustrate that the bipyridine and terpyridine dimerisation units can behave like conformational switches in response to Cu/Zn complexation, and propose that in future these can be employed in synthetic biology with larger peptide or protein fragments, to control large scale folding and related biological function.

Electronic Properties of Neutral Dyes in the Channels of Zeolite L: A Combined Spectroscopic and Quantum Chemical Study

Three low molecular weight model compounds for poly(phenylene-ethynylene)s were inserted into the channels of zeolite L by using gas phase insertion. The absorption and emission spectra in solution and dye/zeolite L are reported. Two compounds show emission properties in zeolite L comparable to that in solution. In contrast, the bipyridine containing compound shows a red shift toward longer excitation wavelengths. Matrix rank analysis of the emission spectra gave three contributing species. Quantum chemical calculations provide different conformations depending on the Si/Al distribution of the framework and the extraframework cations and attached protons.

Efficient Regioselective Synthesis of 6-Amino-5-benzoyl-1-Substituted 2(1H)-Pyridinones

[Reaction: see text]. A regioselective and efficient approach toward 6-amino-5-benzoyl-1-substituted 2(1H)-pyridinones by reaction of acyclic ketene aminals with propiolic acid ester was developed. The effect of the solvent and temperature on the regioselectivity of the reaction and the compatibility of the target compounds to functional group manipulations was examined. Substrates with an ortho substituent build atropisomers due to the restricted rotation around the C-N bond. The enantiomers were separated, and the barrier of rotation was determined experimentally. Quantum chemical calculations allowed a ranking of the barrier heights, and a new mechanism of rotation by deformation of the central pyridinone moiety is proposed.

Competitive Complexation of Gaseous MnII by 1,10-Phenanthroline, 2,2‘-Bipyridine, and 4,5-Diazafluorene

Complexes of Mn(II) with 1,10-phenanthroline (phen) and 2,2'-bipyridine (bipy) are investigated by means of electrospray ionization (ESI) mass spectrometry. Under the conditions used, [MnL(n)]2+ with n = 2 and 3, [MnL(n)Cl]+ with n = 0-2, and [Mn2L(n)Cl3]+ with n = 2 and 3 are produced (where L = phen or bipy). The collision-induced dissociation (CID) spectra of the mass-selected ions show various dissociation pathways, most notable among them is the reduction of the ligated Mn(II) to Mn(I) by intracomplex electron transfer. CID experiments of mixed-ligand complexes formed upon ESI from solutions which contain both phen and bipy exhibit preferential eliminations of bipy, indicating that bipy is a significantly weaker ligand for Mn(II) than phen. This effect is mainly attributed to the flexibility of the bipy ligand concomitant with thermodynamic control in ion dissociation. To support this hypothesis, mixed complexes with some methylated derivatives as well as those containing 4,5-diazafluorene (daf) are examined also. Interestingly, the differences between the ligands diminish in charge-separation reactions of dicationic Mn(II) complexes, due to the joined operation of thermodynamic as well as kinetic effects. In addition, the complexes [Mn(bipy)]+, [Mn(phen)]+, [Mn(bipy)]2+, [Mn(phen)]2+, and [Mn(bipy)(phen)]2+ are computed using the mPW1PW91 hybrid density functional along with the Stuttgart-Cologne-type pseudopotential and basis-set suite, and relative energies for charge-separation reactions and losses of neutral ligands are evaluated.

Atropisomeric (R,R)-2,2'-Bi([2]paracyclo[2](5,8)quinolinophane) and (R,R)-1,1'-Bi([2]paracyclo[2](5,8)isoquinolinophane): synthesis, structural analysis, and chiroptical properties

Atropisomeric (R,R)-2,2'-bi([2]paracyclo[2](5,8)quinolinophane) [(R,R)-1] and (R,R)-1,1'-bi([2]paracyclo[2](5,8)isoquinolinophane) [(R,R)-2] have been prepared in moderate overall yield (17 and 9%, respectively) by a four-step sequence starting from (R)-(-)-4-amino[2.2]paracyclophane and (R)-(-)-4-carboxy[2.2]paracyclophane, respectively. The structures have been determined on the basis of NOE (1)H NMR analysis and molecular mechanics (MM) calculations performed with a Spartan02 program, using the MMF94s force field. A preliminary, qualitative analysis of the chiroptical properties of these two compounds has also been attempted. The main spectral data can be interpreted in terms of an almost planar 2,2'-bisquinoline chromophore inserted in a paracyclophane structure in the case of (R,R)-1, while in the case of (R,R)-2, the main role is played by a distorted 1,1'-bisisoquinoline chromophore. On the basis of the above structural results, a hypothesis about the enantioselection capability of these two molecules has also been formulated.

Ab initio calculations and mass spectrometric determination of the gas-phase proton affinities of 4,4'-disubstituted 2,2'-bipyridines

The gas-phase proton affinities of 4,4'-di(R)-2,2'-bipyridines (R: H, Br, Cl, NO(2), Me) were determined by mass spectrometric measurements and by ab initio calculations at the HF/6-31G and MP2/6-31G levels of theory. The energy barriers for rotation about the central C-C bond were also studied computationally. Two minima were found for both unprotonated and protonated species, the global minima being at the trans planar and cis planar conformations, respectively. Local minima for the unprotonated compounds were at the cis nonplanar conformation and for the protonated compounds at the trans nonplanar. Two different proton affinity values were calculated for each compound by employing different conformations for the protonated species. The computational values were in good agreement with the experimental proton affinities. Substituents affect the proton affinity according to their ability to withdraw or to donate electrons, halogen and nitro-substituted bipyridines having a lower proton affinity and methyl-substituted bipyridine having a higher proton affinity than 2,2'-bipyridine itself.