Further observations on water oxidation catalyzed by mononuclear Ru(II) complexes

A family of 28 mononuclear Ru(II) complexes have been prepared and characterized by (1)H NMR, electronic absorption, and cyclic voltammetry. These complexes are studied as catalysts for water oxidation. All the catalysts possess one tridentate ligand, closely related to 2,2';6,2'-terpyridine (tpy) and may be divided into two basic types. In the type-1 catalyst, the three remaining coordination sites are occupied by a bidentate closely related to 2,2'-bipyridine (bpy) and a monodentate halogen (Br, Cl, or I) or water molecule. In the type-2 catalyst, the three remaining coordination sites are occupied by two axial 4-picoline molecules and an equatorial halogen or water. In general the type-2 catalysts are more reactive than the type-1. The type-2 iodo-catalyst shows first-order behavior and, unlike the bromo- and chloro-catalysts, does not require water-halogen exchange to show good activity. The importance of steric strain and hindrance around the metal center is examined. The introduction of three t-butyl groups at the 4, 4', and 4' positions of tpy sometimes improves catalyst activity, but the effect does not appear to be additive.     

Synthetic approaches to polypyridyl bridging ligands with proximal multidentate binding sites

[reactions: see text] A series of 12 bridging ligands was prepared. These ligands include a central linker appended to two 1,8-naphthyrid-2-yl or two 1,10-phenanthrolin-2-yl units. The linkers include pyridazin-3,6-diyl, 1,8-naphthyrid-2,7-diyl, 2,2'-bipyrid-6,6'-diyl, 1,10-phenanthrolin-2,9-diyl, 1,2-di(2'-pyrid-6'-yl)ethyne, and 3,6-di(2'-pyrid-6'-yl)pyridazine. The ligands were synthesized from the diacetyl derivative of the central linker by a Friedl?nder condensation with either 2-aminonicotinaldehyde or 8-amino-7-quinolinecarbaldehyde. The precursor diacetyl derivatives were, in turn, prepared by pathways involving Stille and Sonogashira couplings. Examination of the electronic absorption spectra of the bridging ligands shows the strongest correlation to be between pairs of ligands having the same central linker. Complexation studies will follow.     

Friedländer approach for the incorporation of 6-bromoquinoline into novel chelating ligands

Nitration of 3-bromobenzaldehyde followed by sodium dithionite reduction provides 5-bromo-2-aminobenzaldehyde, which undergoes the Friedl?nder condensation with a variety of enolizable ketones to afford bidentate and tridentate 6-bromoquinoline derivatives. These species may be dimerized with Ni(0) to form biquinolines or treated under Sonogashira conditions to afford 6-alkynyl derivatives. Examination of optical properties indicate an unusually high emission quantum yield for 6,6'-biquinolines. [structure: see text]     

Structure and photophysics of 2-(2'-pyridyl)benzindoles: the role of intermolecular hydrogen bonds

The photophysical properties of two isomeric 2-(2'-pyridyl)benzindoles depend on the environment. Strong fluorescence is detected in nonpolar and polar aprotic solvents. In the presence of alcohols, the emission reveals an unusual behavior. Upon titration of n-hexane solutions with ethanol, the fluorescence intensity goes through a minimum and then increases with rising alcohol concentration. Transient absorption and time-resolved emission studies combined with ground- and excited-state geometry optimizations lead to the conclusion that two rotameric forms, syn and anti, coexist in alcohols, whereas in nonpolar and aprotic polar media, only the syn conformation is present. The latter can form cyclic complexes with alcohols, which are rapidly depopulated in the excited state. In the presence of excess alcohol, syn --> anti rotamerization occurs in the ground state, promoted by the cooperative action of nonspecific and specific effects such as solvent polarity increase and the formation of hydrogen bonds to both donor and acceptor sites of the bifunctional compounds.     

Evaluation of pyridoindoles, pyridylindoles and pyridylpyridoindoles as matrices for ultraviolet matrix-assisted laser desorption/ionization time-of-flight mass spectrometry.

In an effort to gain an understanding of the processes governing ultraviolet matrix-assisted laser desorption/ionization (UV-MALDI), direct comparison was made of the mass spectra of proteins, carbohydrates and synthetic polymers (polyethylene glycol, polyester and polyamide) by using pyridylindoles, pyridoindoles and pyridylpyridoindoles as UV (337 nm)-MALDI-TOFMS matrices in positive and negative ion mode. In order to study the combined effect of the indole N-H and the pyridine nitrogen of the MALDI matrix on the desorption/ionization process in MALDI, compounds were selected that include either or both of these functions in their structure. Within the compounds studied only those that possess simultaneously both functions in a 1,4-relation behave as very good matrices for proteins. These compounds also work as matrices for some carbohydrates and synthetic polymers used as analytes in the present study. Some of the compounds were also found to be useful for the post-source decay (PSD) analysis of cyclodextrins in positive and negative ion mode. In several cases we also examined the matrix behavior of the corresponding N-methylindole derivatives.     

The onset of coulomb explosions in polyatomic molecules

With the development of high intensity femtosecond lasers, the ionisation and dissociation dynamics of molecules has become an area of considerable interest. Using the technique of femtosecond laser mass spectrometry (FLMS), the molecules carbon disulphide, pyrimidine, toluene, cyclohexanone and benzaldehyde are studied with pulse widths of 50 fs in the near infrared (IR) wavelength region (790 nm). Results are presented and contrasted for laser beam intensities around 10(15) and 10(16) W cm(-2). For the lower intensities, the mass spectra yield dominant singly charged parent ions. Additionally, the appearance of doubly charged parent ions is evident for carbon disulphide, toluene and benzaldehyde with envelopes of doubly charged satellite species existing in these local regions. Carbon disulphide also reveals a small triply charged component. Such atomic-like features are thought to be a strong fingerprint of FLMS at these intensities. However, upon increasing the laser intensity to approximately 10(16) W cm(-2), parent ion dominance decreases and the appearance of multiply charged atomic species occurs, particularly carbon. This phenomenon has been attributed to Coulomb explosions in which the fast absorption of many photons may produce transient highly ionised parent species which can subsequently blow apart. Copyright 1999 John Wiley & Sons, Ltd.     

Uniform molecular analysis using femtosecond laser mass spectrometry

The potential of femtosecond laser time-of-flight mass spectrometry (FLMS) for uniform quantitative analysis of molecules has been investigated. Various samples of molecular gases and vapours have been studied, using ultra-fast ( approximately 50 fs) laser pulses with very high intensity (up to 1.6 x 10(16) Wcm(-2)) for non-resonant multiphoton ionisation/tunnel ionisation. Some of these molecules have high ionisation potentials, requiring up to ten photons for non-resonant ionisation. The relative sensitivity factors (RSF) have been determined as a function of the laser intensity and it has been demonstrated that for molecules with very different masses and ionisation potentials, uniform ionisation has been achieved at the highest laser intensities. Quantitative laser mass spectrometry of molecules is therefore a distinct possibility. Copyright 1999 John Wiley & Sons, Ltd.     

Interpreting early land management through compound specific stable isotope analyses of archaeological soils

Compound specific stable isotope analyses of managed soils using isotope ratio mass spectrometry have been undertaken as a means of determining early land use practices. delta (15)N amino acid signals demonstrate differences between manured grassland, unmanured grassland and continuous cereal cultivation under long-term experimental land use control conditions, with delta (15)N in hydrophobic amino acids providing the most distinctive signals. Analysis of early modern/medieval and of Bronze age anthropogenic soils from Orkney demonstrates that such signals are retained in archaeological contexts. delta (13)C analyses of n- alkanoic acid components of the fossil, Bronze Age, anthropogenic soils suggest a major terrestrial input to these soils, with uniform composition of formation materials. Surficial soils demonstrate the assimilation of isotopically lighter carbon, providing a means of assessing the mobility of the n- alkanoic acids within soils and sediments. Copyright 1999 John Wiley & Sons, Ltd.     

trans-[Ru(II)(dpp)Cl2]: a convenient reagent for the preparation of heteroleptic Ru(dpp) complexes, where dpp is 2,9-di(pyrid-2'-yl)-1,10-phenanthroline

The reaction of 2,9-di(pyrid-2'-yl)-1,10-phenanthroline (dpp) with [RuCl(3)·3H(2)O] or [Ru(DMSO)(4)Cl(2)] provides the reagent trans-[Ru(II)(dpp)Cl(2)] in yields of 98 and 89%, respectively. This reagent reacts with monodentate ligands L to replace the two axial chlorides, affording reasonable yields of a ruthenium(II) complex with dpp bound tetradentate in the equatorial plane. The photophysical and electrochemical properties of the tetradentate complexes are strongly influenced by the axial ligands with electron-donating character to stabilize the ruthenium(III) state, shifting the metal-to-ligand charge-transfer absorption to lower energy and decreasing the oxidation potential. When the precursor trans-[Ru(II)(dpp)Cl(2)] reacts with a bidentate (2,2'-bipyridine), tridentate (2,2';6,2'-terpyridine), or tetradentate (itself) ligand, a peripheral pyridine on dpp is displaced such that dpp binds as a tridentate. This situation is illustrated by an X-ray analysis of [Ru(dpp)(bpy)Cl](PF(6)).