Leonid Konstantinovski

Consecutive thermal H2 and light-induced O2 evolution from water promoted by a metal complex

Discovery of an efficient artificial catalyst for the sunlight-driven splitting of water into dioxygen and dihydrogen is a major goal of renewable energy research. We describe a solution-phase reaction scheme that leads to the stoichiometric liberation of dihydrogen and dioxygen in consecutive thermal- and light-driven steps mediated by mononuclear, well-defined ruthenium complexes. The initial reaction of water at 25°C with a dearomatized ruthenium (II) [Ru(II)] pincer complex yields a monomeric aromatic Ru(II) hydrido-hydroxo complex that, on further reaction with water at 100°C, releases H2 and forms a cis dihydroxo complex. Irradiation of this complex in the 320-to-420–nanometer range liberates oxygen and regenerates the starting hydrido-hydroxo Ru(II) complex, probably by elimination of hydrogen peroxide, which rapidly disproportionates. Isotopic labeling experiments with H217O and H218O show unequivocally that the process of oxygen–oxygen bond formation is intramolecular, establishing a previously elusive fundamental step toward dioxygen-generating homogeneous catalysis.

The mode of action of allicin: trapping of radicals and interaction with thiol containing proteins.

Allicin (thio-2-propene-1-sulfinic acid S-allyl ester) is the main biologically active component of garlic clove extracts. Its biological activity was attributed to either antioxidant activity or thiol disulfide exchange. Antioxidant properties of both allicin and its precursor, alliin (+S-allyl-L-cysteine sulfoxide), were investigated in the Fenton oxygen-radical generating system [H2O2-Fe(II)]. Using the spin trapping technique and ESR, it was found that both compounds possessed significant antioxidant activity. The reaction between allicin and L-cysteine was studied by 1H and 13C-NMR, and a S-thiolation product, S-allylmercaptocysteine, was identified. Allicin irreversibly inhibited SH-protease papain, NADP(+)-dependent alcohol dehydrogenase from Thermoanaerobium brockii (TBAD), and the NAD(+)-dependent alcohol dehydrogenase from horse liver (HLAD). All the three enzymes could be reactivated with thiol containing compounds. Papain could be reactivated with glutathione, TBAD with dithiothreitol or 2-mercaptoethanol (2-ME) but not by glutathione, while HLAD could be reactivated only with 2-ME. This study demonstrates that in addition to its antioxidant activity, the major biological effect of allicin should be attributed to its rapid reaction with thiol containing proteins.

Iron Borohydride Pincer Complexes for the Efficient Hydrogenation of Ketones under Mild, Base‐Free Conditions: Synthesis and Mechanistic Insight

The new, structurally characterized hydrido carbonyl tetrahydridoborate iron pincer complex [(iPr-PNP)Fe(H)(CO)(η(1)-BH(4))] (1) catalyzes the base-free hydrogenation of ketones to their corresponding alcohols employing only 4.1 atm hydrogen pressure. Turnover numbers up to 1980 at complete conversion of ketone were reached with this system. Treatment of 1 with aniline (as a BH(3) scavenger) resulted in a mixture of trans-[(iPr-PNP)Fe(H)(2)(CO)] (4a) and cis-[(iPr-PNP)Fe(H)(2)(CO)] (4b). The dihydrido complexes 4a and 4b do not react with acetophenone or benzaldehyde, indicating that these complexes are not intermediates in the catalytic reduction of ketones. NMR studies indicate that the tetrahydridoborate ligand in 1 dissociates prior to ketone reduction. DFT calculations show that the mechanism of the iron-catalyzed hydrogenation of ketones involves alcohol-assisted aromatization of the dearomatized complex [(iPr-PNP*)Fe(H)(CO)] (7) to initially give the Fe(0) complex [(iPr-PNP)Fe(CO)] (21) and subsequently [(iPr-PNP)Fe(CO)(EtOH)] (38). Concerted coordination of acetophenone and dual hydrogen-atom transfer from the PNP arm and the coordinated ethanol to, respectively, the carbonyl carbon and oxygen atoms, leads to the dearomatized complex [(iPr-PNP*)Fe(CO)(EtO)(MeCH(OH)Ph)] (32). The catalyst is regenerated by release of 1-phenylethanol, followed by dihydrogen coordination and proton transfer to the coordinated ethoxide ligand.

Evidence for a terminal Pt( iv )-oxo complex exhibiting diverse reactivity

Terminal oxo complexes of transition metals have critical roles in various biological and chemical processes. For example, the catalytic oxidation of organic molecules, some oxidative enzymatic transformations, and the activation of dioxygen on metal surfaces are all thought to involve oxo complexes. Moreover, they are believed to be key intermediates in the photocatalytic oxidation of water to give molecular oxygen, a topic of intensive global research aimed at artificial photosynthesis and water splitting. The terminal oxo ligand is a strong π-electron donor, so it readily forms stable complexes with high-valent early transition metals. As the d orbitals are filled up with valence electrons, the terminal oxo ligand becomes destabilized. Here we present evidence for a dn (n > 5) terminal oxo complex that is not stabilized by an electron withdrawing ligand framework. This d6 Pt(iv) complex exhibits reactivity as an inter- and intramolecular oxygen donor and as an electrophile. In addition, it undergoes a water activation process leading to a terminal dihydroxo complex, which may be relevant to the mechanism of catalytic reactions such as water oxidation.

Silanol-Based Pincer Pt(II) Complexes: Synthesis, Structure, and Unusual Reactivity

Aiming at the generation of a silanone intramolecularly bound to platinum, we prepared pincer-type PSiP silanol Pt(II) complexes. While a stable silanone complex was not isolated, unusual reactivity modes, involving its possible intermediacy, were observed. Treatment of the new PSiH 2P-type ligand ( o-IPr 2PC 6H 4) 2SiH 2 ( 7) with (Me 2S) 2Pt(Me)Cl yields the pincer-type hydrosilane complex [{( o- iPr 2PC 6H 4) 2SiH}PtCl] ( 8), which upon Ir(I)-catalyzed hydrolytic oxidation gives the structurally characterized silanol complex [{( o- iPr 2PC 6H 4) 2SiOH}PtCl] ( 3). Complex 3, comprising in its structure the nucleophilic silanol fragment and electrophilic Pt(II)-Cl moiety, exhibits dual reactivity. Its reaction with the non-nucleophilic KB(C 6F 5) 4 in fluorobenzene leads to the ionic complex [{( o- iPr 2PC 6H 4) 2SiOH}Pt] (+) [(C 6F 5) 4B] (-) ( 9), which reacts with CO to yield the structurally characterized [{( o- iPr 2PC 6H 4) 2SiOH}PtCO] (+) [(C 6F 5) 4B] (-) ( 10). Treatment of 3 with non-nucleophilic bases leads to unprecedented rearrangement and coupling, resulting in the structurally characterized, unusual binuclear complex 11. The structure of 11 comprises two different fragments: the original O-Si-Pt(II)-Cl pattern, and the newly formed silanolate Pt(II)-H pattern, which are connected via a disiloxane bridge. Complex 9 undergoes a similar hydrolytic rearrangement in the presence of iPr 2NEt to give the mononuclear silanolate Pt(II)-H complex 17. Both these rearrangement-coupling reactions probably involve the inner-sphere generation of an intermediate silanone 14, which undergoes nucleophilic attack by the starting silanol 3 to yield complex 11, or adds a water molecule to yield complex 17. X-ray diffraction studies of 3, 10, and 11 exhibit a very short Si-Pt bond length (2.27-2.28 A) in the neutral complexes 3 and 11 that elongates to 2.365 A in the carbonyl complex 10. A significantly compressed geometry of the silanolate platinum(II)-hydride fragment B of the binuclear complex 11 features a Pt(2)-O(2)-Si(2) angle of 100.4 (3) degrees and a remarkably short Pt(2)...Si(2) [2.884 (3) A] distance.

An efficient synthesis of bridged-bicyclic peroxides structurally related to antimalarial yingzhaosu A based on radical co-oxygenation of thiols and monoterpenes

Abstract Synthesis of β-sulfenyl endoperoxides 9 was achieved by a four component sequential free radical reaction based on the application of the thiol-olefin-co-oxygenation reaction to monoterpenes, followed by in situ treatment with triphenylphosphine. β-Sulfenyl endoperoxides 9 were oxidized with m-CPBA to β-sulfonyl endoperoxides 10 . This process provides an efficient method for the preparation of peroxides containing the 2,3-dioxabicyclo[3.3.1]nonane system ( 2 ) characteristic of antimalarial agents of the yingzhaosu A ( 3 ) family. A simple NMR diagnostic tool for the identification of stereoisomers is described.

A Pincer‐Type Anionic Platinum(0) Complex

Pincer-type complexes constitute a large family of compounds that have attracted much recent interest. Among these compounds, aryl-anchored, d pincer complexes of the type [M(LCL’)] (M=Ni, Pd, Pt; L= neutral ligand such as phosphine, amine, dialkyl sulfide) are a major group that plays important roles in organometallic reactions and mechanisms, catalysis, and in the design of new materials. In contrast, and to our knowledge, no complexes of this type with the metal in the zero oxidation state have been prepared. Such d [M(LCL’)] complexes with neutral L ligands and an “anionic” aryl anchor would be anionic, and would be expected to possess distinctly different properties to neutral d (M) complexes. We chose to utilize bulky bis-chelating pincer-type ligands in this study as they have been shown to be effective in stabilizing reactive species and have led to unusual complexes. We have recently shown that reduction of PCP-type Pd complexes [Pd(X)C6H3(CH2PiPr2)2] (X=Cl, trifluoroacetate) with sodium metal results in collapse of the pincer system, leading to formation of the diamagnetic binuclear complex [Pd{C6H3(CH2PiPr2)2}2Pd], which contains a 14electron linear Pd moiety and a completely nonplanar “butterfly”-type 16-electron Pd moiety. Oxidation of the binuclear complex, or its reaction with organic halides, regenerates the original mononuclear framework. To avoid collapse of the pincer system upon reduction we decided to use a PCP-Pt complex with the hope that the more diffuse nature of the Pt orbitals (compared with Pd) might stabilize the reduced metal center. In addition, increasing the steric bulk of the pincer phosphine ligand might protect the reduced metal center against intermolecular reactions and might lead to a rare monometallic Pt anionic complex. We are aware of only one monometallic anionic Pt complex, namely [Pt(Me2NCS2)(PEt3)] , which was generated in situ at 78 8C by proton abstraction from the Pt hydride complex [PtH(Me2NCS2)(PEt3)]. [4] This complex was not isolated but was trapped with a variety of electrophiles to give a range of Pt complexes. Herein we report the preparation, characterization, and computational study of the first thermally stable, monometallic anionic Pt complex. The reactivity of this electron-rich, 16-electron PCP-type anionic Pt complex shows that it is a Brønsted base and an effective electron-transfer reagent that is capable of C F activation under exceedingly mild conditions. Reduction of the bulky PCP-Pt complex 1 (Scheme 1) with sodium in dry [D8]THF at room temperature overnight led to a dramatic color change from colorless to dark red. A

Novel derivatives of 6-mercaptopurine: Synthesis, characterization and antiproliferative activities of S-allylthio-mercaptopurines

Biologically active S-allylthio derivatives of 6-mercaptopurine (6-MP) and 6-mercaptopurine riboside (6-MPR) were synthesized. The products, S-allylthio-6-mercaptopurine (SA-6MP) and S-allylthio-6-mercaptopurine riboside (SA-6MPR) were characterized. The antiproliferative activity of the new prodrugs was tested on human leukemia and monolayer cell lines, and compared to that of their parent reactants. The new prodrugs acted by a concentration-dependent mechanism. They inhibited cell proliferation and induced-apoptosis more efficiently than the parent molecules. Leukemia cell lines were more sensitive to the new prodrugs than monolayer cell lines. Higher hydrophobicity of the derivatives improves their penetration into cells, where upon reaction with glutathione, S-allylthioglutathione (GSSA) is formed, and 6-MP or 6-MPR is released for further processing.

Activation of molecular oxygen by a dioxygenase pathway by a ruthenium bis-bipyridine compound with a proximal selenium site.

A ruthenium(II) bipyridine complex with proximal phenylselenium tethers, [Ru](H(2)O)(2), reacted intramolecularly with O(2) in a protic slightly acidic solvent, 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), to yield an O-O bond cleaved product, [Ru](O)(2), with formation of two Ru-O-Se moieties. This stable compound was isolated, and its structure was determined by X-ray diffraction. The identification of the compound in solution was confirmed by ESI-MS and the (1)H NMR with the associated Curie plot that showed that [Ru](O)(2) was paramagnetic. The magnetic susceptibility was 2.8 mu(B) by Evans method suggesting a ground state triplet or biradical. DFT calculations, however, predicted a ground state singlet and an oxidized Se atom. Further it was shown that [Ru](O)(2) is a potent oxygen transfer species of both O(2)-derived atoms to triphenylphosphine and a nucleophilic alkene such as 2,3-dimethyl-2-butene in both HFIP and acetonitrile. UV-vis spectroscopy combined with the measured stoichiometry of PPh(3):O(2) = approximately 2 in a catalytic oxidation of PPh(3) suggests a dioxygenase type activation of O(2) with structural identification of the O-O bond cleavage reaction step, formation of [Ru](O)(2) as an intermediate, and the proof that [Ru](O)(2) is a donor of both oxygen atoms.

Long-range through-bond heteronuclear communication in platinum complexes.

Four analogous platinum stilbene- and stilbazole-based complexes exhibit unusual long-range heteronuclear spin-spin coupling in solution. Single crystal analysis and NMR experiments show that the (19)F, (31)P, and (195)Pt nuclei communicate over large distances (0.9-1.3 nm) through bond rather than through space. Spin-spin couplings between (195)Pt and (19)F over seven bonds and between (31)P and (19)F over eight bonds are observed with (7)J(PtF) = 2.9 Hz and (8)J(PF) = 11.8 Hz. Remarkably, a very large spin coupling between (195)Pt and (19)F over six bonds ((6)J(PtF) = 40.1 Hz) is also observed in a structurally related pyridinium complex. Experimental and gNMR (version 5.0) simulated (19)F{(1)H}, (31)P{(1)H}, and (195)Pt{(1)H} spectra of the complexes reveal a three-spin AMY system (A = (31)P, M = (31)P, Y = (19)F) or a five-spin AMY(3) flanked by a four-spin AMXY or a six-spin AMXY(3) system (X = (195)Pt), respectively. Density functional theory calculations at the PBE0/SDD level of theory show a pi-conjugated metal-ligand network, which may contribute to the experimentally observed spin-spin interactions.