Complexation of trimeric copper(i) and silver(i) 3,5-bis(trifluoromethyl)pyrazolates with amine-borane

A complexation of trimeric copper(i) and silver(i) 3,5-bis(trifluoromethyl)pyrazolates ((ML)₃) with BH₃NEt₃ was studied by IR spectroscopy in hexane and dichloromethane solutions. In hexane, two complexes [(ML)₃][BH₃NEt₃]₂ (1) and [(ML)₃][BH₃NEt₃] (2) were formed depending on the ratio of reagents. In dichloromethane, only one complex [(ML)₃][BH₃NEt₃] (2) was found. The thermodynamic parameters (ΔH ^○, ΔS ^○) of complex 1 in hexane and complex 2 in dichloromethane were obtained. The complex [(AgL)₃][BH₃NEt₃] was isolated in the solid state, its structure was established by X-ray diffraction. The complex is formed due to two bridges B-H-Ag, one BH group is not involved in the complexation. In crystal, molecules 2 form supramolecular dimers due to the intermolecular interactions between metals of two macrocycles.     

Optically Active Tetra‐tert‐butyl‐P5‐deltacyclene Epimers: Preparation,Spectroscopy, Dynamic Equilibriums,H/D Exchange,and Transition‐Metal Complex Chemistry

On the basis of isolated diastereomeric triorganylstannyl‐P₅‐deltacyclenes 7′ and 7′′ , almost pure enantiomers of their destannylation products 8′ and 8′′ are now available. These stereochemically inert cage chiral species contain a configurationally labile P1?H1 group that defines two epimers 8 a and 8 b of each of the enantiomers, which are connected by a rapid equilibrium. Mirror‐symmetric circular dichroism (CD) spectra of the enantiomeric cages are compatible with the identification of epimers. A simulation of the CD spectrum of the major epimer 8′a relates the cage chirality of the system to the observed chiroptical effects. Both cage epimers and two of the phosphorus cage atoms are active as ligands with respect to [M(CO)₅] fragments of Cr, Mo, and W. Four almost isoenergetic regio‐ and stereoisomers of the resulting mononuclear complexes are formed for these metals, but only one of the isomers per metal crystallized in the case of the racemic series of the complexes. The enantiopure versions of cages and cage complexes, however, did not crystallize at all, a well‐known phenomenon for chiral compounds. CD spectra of the optically active complex isomer mixtures are close to identical with the CD spectra of the related free cages and point again to the chiral cages as the dominant source of the CD effects of the complexes. [(Benzene)RuCl₂] complexes of the cage ligand 8 behave totally differently. Only a single species 12 =[(benzene)RuCl₂ ?8 b ] is formed in almost quantitative yield and the minor epimer 8 b plays the role of the ligand exclusively. The reaction works as well for the separated enantiomeric cage versions to yield the highly enriched enantiomers 12′ and 12′′ separately. An efficient kinetic resolution process was identified as the main reason for this finding. It is based on a high stereo‐ and regiochemical flexibility of the P?C cage ligand that is capable of adjusting to the specific requirements of a suitable transition‐metal complex fragment. Such ligand flexibility is regularly observed in metalloenzymes, but is a very rare case in classical and organometallic complex chemistry.     

Reverse Spin‐Crossover and High‐Pressure Kinetics of the Heme Iron Center Relevant for the Operation of Heme Proteins under Deep‐Sea Conditions

By design of a heme model complex with a binding pocket of appropriate size and flexibility, and by elucidating its kinetics and thermodynamics under elevated pressures, some of the pressure effects are demonstrated relevant for operation of heme‐proteins under deep‐sea conditions. Opposite from classical paradigms of the spin‐crossover and reaction kinetics, a pressure increase can cause deceleration of the small‐molecule binding to the vacant coordination site of the heme‐center in a confined space and stabilize a high‐spin state of its Fe center. This reverse high‐pressure behavior can be achieved only if the volume changes related to the conformational transformation of the cavity can offset the volume changes caused by the substrate binding. It is speculated that based on these criteria nature could make a selection of structures of heme pockets that assist in reducing metabolic activity and enzymatic side reactions under extreme pressure conditions.     

Coordination and metalation bifunctionality of Cu with 5,10,15,20-tetra(4-pyridyl)porphyrin: toward a mixed-valence two-dimensional coordination network

We investigated the coordination self-assembly and metalation reaction of Cu with 5,10,15,20-tetra(4-pyridyl)porphyrin (2HTPyP) on a Au(111) surface by means of scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory calculations. 2HTPyP was found to interact with Cu through both the peripheral pyridyl groups and the porphyrin core. Pairs of pyridyl groups from neighboring molecules coordinate Cu(0) atoms, which leads to the formation of a supramolecular metal-organic coordination network. The network formation occurs at room temperature; annealing at 450 K enhances the process. The interaction of Cu with the porphyrin core is more complex. At room temperature, formation of an initial complex Cu(0)-2HTPyP is observed. Annealing at 450 K activates an intramolecular redox reaction, by which the coordinated Cu(0) is oxidized to Cu(II) and the complex Cu(II)TPyP is formed. The coordination network consists then of Cu(II) complexes linked by Cu(0) atoms; that is, it represents a mixed-valence two-dimensional coordination network consisting of an ordered array of Cu(II) and Cu(0) centers. Above 520 K, the network degrades and the Cu atoms in the linking positions diffuse into the substrate, while the Cu(II)TPyP complexes form a close-packed structure that is stabilized by weak intermolecular interactions. Density functional theory investigations show that the reaction with Cu(0) proceeds via formation of an initial complex between metal atom and porphyrin followed by formation of Cu(II) porphyrin within the course of the reaction. The activation barrier of the rate limiting step was found to be 24-37 kcal mol(-1) depending on the method used. In addition, linear coordination of a Cu atom by two CuTPyP molecules is favorable according to gas-phase calculations.     

Chemical characterization of the smallest S-nitrosothiol, HSNO; cellular cross-talk of H2S and S-nitrosothiols

Dihydrogen sulfide recently emerged as a biological signaling molecule with important physiological roles and significant pharmacological potential. Chemically plausible explanations for its mechanisms of action have remained elusive, however. Here, we report that H(2)S reacts with S-nitrosothiols to form thionitrous acid (HSNO), the smallest S-nitrosothiol. These results demonstrate that, at the cellular level, HSNO can be metabolized to afford NO(+), NO, and NO(-) species, all of which have distinct physiological consequences of their own. We further show that HSNO can freely diffuse through membranes, facilitating transnitrosation of proteins such as hemoglobin. The data presented in this study explain some of the physiological effects ascribed to H(2)S, but, more broadly, introduce a new signaling molecule, HSNO, and suggest that it may play a key role in cellular redox regulation.     

Intermolecular hydrogen bonding between neutral transition metal hydrides (eta(5)-C5H5)M(CO)3H (M = Mo, W) and bases

The interaction of CpM(CO)3H (M = Mo, W) hydrides as proton donors with different bases (B = pyridine, (n-Oc)3PO, ((CH3)2N)3PO, H3BNEt3) was studied by variable temperature IR spectroscopy and theoretically by DFT/B3LYP calculations. The data obtained show for the first time the formation of intermolecular hydrogen bonds between the neutral transition metal hydrides and bases in solutions of low polarity. These M-H...B hydrogen bonds are shown to precede the hydrides' deprotonation.     

Proton-transfer and H2-elimination reactions of main-group hydrides EH4- (E = B, Al, Ga) with alcohols

The reaction of the isostructural anions of group 13 hydrides EH4- (E = B, Al, Ga) with proton donors of different strength (CH3OH, CF3CH2OH, and CF3OH) was studied with different theoretical methods [DFT/B3LYP and second-order M?ller-Plesset (MP2) using the 6-311++G(d,p) basis set]. The results show the general mechanism of the reaction: the dihydrogen-bonded (DHB) adduct (EH...HO) formation leads through the activation barrier to the next concerted step of H2 elimination and alkoxo product formation. The structures, interaction energies (calculated by different approaches including the energy decomposition analysis), vibrational E-H modes, and electron-density distributions were analyzed for all of the DHB adducts. The transition state (TS) is the dihydrogen complex stabilized by a hydrogen bond with the anion [EH3(eta2-H2)...OR-]. The single exception is the reaction of BH4- with CF3OH exhibiting two TSs separated by a shallow minimum of the BH3(eta2-H2)...OR- intermediate. The structures and energies of all of the species were calculated, leading to the establishment of the potential energy profiles for the reaction. A comparison is made with the mechanism of the proton-transfer reaction to transition-metal hydrides. The solvent influence on the stability of all of the species along the reaction pathway was accounted for by means of polarizable conductor calculation model calculations in tetrahydrofuran (THF). Although in THF the DHB intermediates, the TSs, and the products are destabilized with respect to the separated reactants, the energy barriers for the proton transfer are only slightly affected by the solvent. The dependence of the energies of the DHB complexes, TSs, and products as well as the energy barriers for the H2 release on the central atom and the proton donor strength is also discussed.