Gianfranco Denti

Harvesting sunlight by artificial supramolecular antennae

Abstract We have designed a divergent synthetic strategy, based on the “complexes-as-metals and complexes-as-ligands” procedure, to prepare polynuclear metal compounds of nanometer size and dendritic structure. Such a synthetic strategy is modular, very flexible, efficient, and characterized by a full, step-by-step control of the growing process. It allows us to obtain supramolecular arrays where different metal ions, bridging ligands, and terminal ligands can occupy predetermined sites. In this way, the light absorption, luminescence, and redox properties of these polynuclear compounds can be varied. In particular, it is possible to obtain a synthetic control of the direction(s) of electronic energy transfer after light absorption. This is a step towards the construction of nanometer-sized antennae for harvesting solar energy.

Polynuclear metal complexes of nanometre size. A versatilesynthetic strategy leading to luminescent and redox-active dendrimers madeof an osmium(II)-based core and ruthenium(II)-basedunits in the branches

A docosanuclear metal complex of nanometric size and dendritic shape made of an osmium(ii)-based core and containing 21 ruthenium(ii)-based units in the branches has been prepared. The key building blocks are the [Os(2,3-dpp) 3 ] 2+ ‘complex ligand’, and the [Ru(2,3-Medpp) 2 Cl 2 ] 2+ and [{Ru(bpy) 2 (µ-2,3-dpp)} 2 RuCl 2 ] 4+ ‘complex metals’ {2,3-dpp=2,3-bis(2-pyridyl)pyrazine; 2,3-Medpp + =2-[2-(1-methylpyridiniumyl)]-3-(2-pyridyl)pyrazine; bpy=2,2′-bipyridine}. The first step of the synthesis is the formation of the tetranuclear [Os{(µ-2,3-dpp)Ru(2,3-Medpp) 2 } 3 ] 14+ species in which the peripheral ligands 2,3-Medpp + are 2,3-dpp ligands with the second chelating site inactivated (protected) by methylation. This species is obtained from the reaction of the [Os(2,3-dpp) 3 ] 2+ ‘complex ligand’ core, which contains three open chelating positions, with three equivalents of the [Ru(2,3-Medpp) 2 Cl 2 ] 2+ ‘complex metal’, where the labile Cl - ligands can be replaced by the chelating units of the core. Successive demethylation (deprotection) of the tetranuclear compound opens the six peripheral chelating sites. At this stage, the divergent synthesis can be iterated {reaction with six equivalents of the [Ru(2,3-Medpp) 2 Cl 2 ] 2+ ‘complex metal’} with formation of the protected decanuclear compound [Os{(µ-2,3-dpp)Ru[(µ-2,3-dpp)Ru(2,3-Medpp) 2 ] 2 } 3 ] 32+ . Alternatively, in a convergent approach, the reaction of the deprotected tetranuclear species with six equivalents of the trinuclear [{Ru(bpy) 2 (µ-2,3-dpp)} 2 RuCl 2 ] 4+ ‘complex metal’ leads to the docosanuclear [Os{(µ-2,3-dpp)Ru[(µ-2,3-dpp)Ru{(µ -2,3-dpp)Ru(bpy) 2 } 2 ] 2 } 3 ] 44+ species. The absorption spectra, luminescence properties, and electrochemical behaviour of [Os(2,3-dpp) 3 ] 2+ , [Os(µ-2,3-dpp) 3 {Ru(2,3-Medpp) 2 } 3 ] 14+ , [Os{(µ-2,3-dpp)Ru[(µ-2,3-dpp)Ru(2,3-Medpp) 2 ] 2 } 3 ] 32+ , and [Os{(µ-2,3-dpp)Ru[(µ-2,3-dpp)Ru{(µ -2,3-dpp)Ru(bpy) 2 } 2 ] 2 } 3 ] 44+ have been investigated.

Bottom-up strategy to obtain luminescent and redox-active metal complexes of nanometric dimensions

Abstract By using the “complexes as metals and complexes as ligands” synthetic strategy, it has been possible to obtain oligonuclear metal complexes which contain up to 22 metal ions. Complexes containing two different types of metal ions (Ru and Os; Ru and Rh; Os and Rh; Ru and Ir) have also been prepared. The light absorption, luminescence, and redox properties of these polynuclear compounds can be varied by changing (i) the nuclearity, (ii) the nature of metal ions, bridging ligands and/or terminal ligands, and (iii) the position of the various components in the supramolecular structure. Because of their strong absorption in the visible spectral region and the possibility to predetermine the direction of energy migration, these compounds could be used as photochemical molecular devices for harvesting solar energy.

Synthesis and characterization of dinuclear copper(II) complexes. Crystal structure of aquatrichlorohydroxo-3,6-bis(2′-pyridyl)pyridazinedicopper(II)

Abstract Copper(II) chloride reacts, in appropriate conditions, with the tetradentate ligand 3,6-bis(2′-pyridyl)pyridazine yielding the dinuclear complex [Cu2LCl4(H2O)]·H2O, whose crystal structure has been determined by X-ray methods. It crystallizes in the triclinic space group P 1 , with two molecules in a unit cell of dimensions a = 11.685(6), b = 10.003(6), c = 8.570(5) A, α = 66.94(8), β = 78.86(8), γ = 83.07(8)°. The structure has been solved by Patterson and Fourier methods and refinement by full-matrix least-squares to R = 0.068 for 1513 observed reflections. The structure is strictly related to that reported [1] for the complex [Cu2LCl3(OH)(H2O)], the main difference being the longer Cu⋯Cu separation which is dependent on the different bridging group (Cl instead of OH). The facility of L to undergo large distortions is emphasized. The reactivity of the title complex with water and nitric acid is also discussed.

Characterization of merand fac isomers of [Ru(2,3-dpp)3][PF6]2 (2,3-dpp=2,3-bis(2-pyridyl)pyrazine) by 1H and 99Ru NMR spectroscopy. Proton assignment by 2D techniques

Abstract A careful analysis of the proton 2D-COSY spectrum of the title compound permitted a complete assignment of the signals of the meridional and facial isomers, obtained in a 12:1 ratio. The observed chemical shifts of the pyrazine and pyridine protons are mainly due to ring current effects. The presence of the mer and fac isomers was confirmed by the appearance of two distinct resonances in the 99Ru NMR spectrum.

Hexanuclear polypyridine complexes containing different metals, bridging ligands and/or terminal ligands. Absorption spectra, electrochemical oxidation, luminescence properties and intercomponent energy transfer

Abstract Four novel hexanuclear complexes of general formula [(L) 2 M(μ-BL)] 2 M(μ-BL)M[(μ-BL)M(L) 2 ] 2 12+ , where the metal ions M are Ru 2+ and/or Os 2+ , the bridging ligands BL are 2,3-dpp and/or 2,5-dpp, and the terminal ligands L are bpy and/or biq, have been investigated (dpp  bis(2-pyridyl)pyrazine; bpy  2,2′-bipyridine; biq 2,2′-biquinoline). These polymetallic complexes can be considered as supramolecular species made of six distinct metal-containing units. They display very intense ligand centered absorption bands in the UV region (ϵ max in the order of 2.5×10 5 M −1 cm −1 ) and broad and intense bands in the visible region (ϵ max in the order of 5×10 4 M −1 cm −1 ). On electrochemical oxidation, the metal centers are oxidized at the same or different potentials depending on the nature of the metal ions (Ru 2+ or Os 2+ ) and on their positions (inner or outer) in the supramolecular structure. For all the novel compounds, luminescence can be observed in the red or near-IR spectral region. The luminescence properties, which are characteristic of specific metal-containing units, show that exoergonic electronic energy transfer between adjacent units is 100% efficient, whereas it is much lower when higher energy units are interposed. Various energy migration patterns can be obtained by placing different units in suitable sites of the supramolecular array.

Redox processes of ruthenium (II) polypyridine complexes induced by fast-atom bombardment mass spectrometry

Fast-atom bombardment (FAB) mass spectrometry in the negative ionization mode enables the sputtering into the gas phase of the ruthenium complexes [Ru(2,2′-bipyridine[bpy])2(2,5-bis) (pyrydil)pyrazine[dpp])](PF6)2; [Ru(bpy)2,(2,3dpp)](PF6)2;[Ru(bpy)2,(2,3-dpp-Me)]( PF6)3; and [Ru(bpy)2(ώ-2,3-dpp)]2 RuCl2(PF6)4 as intact radical anions. These data, combined with those avaiiable from the positive FAB spectra allow a full characterization of the analytes.

A decanuclear ruthenium(II)–polypyridine complex: synthesis, absorption spectrum, luminescence and electrochemical behaviour

A novel oligonuclear Ru(II)–polypyridine complex, Ru{(µ-2,3-dpp)Ru[(µ-2,3-dpp)Ru(bpy)2]2}3(PF6)201, where dpp = bis(2-pyridyl)pyrazine and bpy = 2,2′ bipyridine, has been prepared from the reaction of a Ru(2,3-dpp)32+ core 2 with three Ru[(µ-2,3-dpp)Ru(bpy)2]2Cl24+ units; 1 shows a very intense absorption band at 541 nm (Iµ= 1.25 × 105 dm3 mol–1 cm–1), room temperature luminescence (λmax= 809 nm, τ= 55 ns, Φ= 10–3), and independent one-electron oxidation of the six peripheral Ru2+ ions at +1.43 V vs. saturated calomel electrode (SCE).

“Small-Upward” Approach to Nanostructures: Dendritic Polynuclear Metal Complexes For Light Harvesting

A synthetic strategy is described to obtain supramolecular dendritic structures of nanometric dimensions made of metal complex units which absorb light all over the UV and visible spectral region. Specific metal and/or ligands can be placed in designed sites of the supramolecular array by an appropriate choice of the building blocks. A made-to-order control of the patterns of electronic energy transfer can thus be achieved. Supramolecular species containing up to 22 metal atoms have been prepared and their light absorption, luminescence, and energy transfer properties have been investigated

Crystal and molecular structure of [Ru(bpy)2(2,3-dpp)]Cl2·3H2O·CH3CN and 1H and 99Ru NMR spectra of [Ru(bpy)2(2,n-dpp)][PF6]2 (bpy=2,2′-bipyridine, dpp=bis (2-pyridyl)pyrazine, n=3 or 5)

Abstract Dark brown crystals of [Ru(bpy)2(2,3-dpp)]Cl2·3H2O·CH3CN (1) were obtained from [Ru(bpy)2]Cl2 and 2,3-dpp and crystallized in acetonitrile. The X-ray crystal structure (monoclinic, P2 1 /n, a=12.508(2), b=13.905(2), c=21.533(3) A , β=102.20(1)°, Z=4 ) is reported. The six-coordinated ruthenium atom is chelated by two bipyridines (bpy) and two nitrogens of the 2,3-bis(2-pyridyl)pyrazine (dpp). Complete assignment of the proton spectra of [Ru(bpy)2(2,3-dpp)][PFn]2 (2) and [Ru(bpy)2(2,55-dpp)][PFn]2 (3), of which crystals could not be obtained, was performed by one-dimensional and two-dimensional techniques, coordination induced shifts being mainly due to ring current effects. The two complexes give two 99Ru resonances at δ 4535 and 4528 respectively.