Molecular Polygons Self-Assembled from Conjugated 1,1'-Ferrocenediyl Bridged Bis(pyridines), Bis(2,2'-bipyridines), and Bis(1,10-phenanthrolines) and Transition Metals as Building Blocks

Die Bausteine [?5-C5H4-C2-R]2Fe (R = Pyridin-4-yl, Pyridin-3-yl, und 2,2'-Bipyridin-5-yl) und [?5-C5H4-C2-C6H2(OR')2-C2-R]2Fe (R = Pyridin-4-yl, Pyridin-3-yl, 2,2'-Bipyridin-5-yl, 1,10-Phenanthrolin-3-yl und R' = n-C3H7) wurden untersucht. Der erstere Fall war zugänglich durch Reaktionen von 1,1'-Diiodoferrocen mit R-C2-H. Röntgenstrukturanalysen von zwei Bis(pyridinen) ergaben sich ekliptische Konformationen. Letzterer konnte aus dem Precursor [?5-C5H4-C2-C6H2(OR')2-C2-R]2Fe (R = triisopropylsilyl) dargestellt werden. Durch zwei-stufige Reaktion wird R = triisopropylsilyl zum N-Liganden umgesetzt. Die Macrozyklen [(?5-C5H4-C2-4-C5H4N)2Fe]2Ni2(NO3)4, [(?5-C5H4-C2-3- C5H4N)2Fe]2Ag2(ClO4)2, und [(?5-C5H4-C2-3-C5H4N)2Fe]2Pd2Cl4 werden synthetisiert und durch Röntgenstrukturanalysen charakterisiert. Die Macrozuklen erweisen sich als 2 nm-große, molekulare Polygone. CV Experimente zeigen keine Fe-Fe-Wechselwirkung, jedoch schwache Fe-Ni- und Fe-Ag-WW. Schwache antiferromagnetische Ni-Ni-WW konnte bestimmt werden. Die Komplexe der tetradentaten Liganden wurden charakterisiert.The building blocks of [(5-C5H4-C2-R]2Fe (R = pyridine-4-yl, pyridine-3-yl, and 2,2'-bipyridine-5-yl) and [(5-C5H4-C2-C6H2(OR')2-C2-R]2Fe (R = pyridine-4-yl, pyridine-3-yl, 2,2'-bipyridine-5-yl, 1,10-phenanthroline-3-yl and R' = n-C3H7) were investigated. The former was accessible by reactions of 1,1'-diiodoferrocene with R-C2-H. X-ray analyses of the bis(pyridine)s revealed the eclipsed conformer. While the latter was prepared via a pre-organized precursor [(5-C5H4-C2-C6H2(OR')2-C2-R]2Fe (R = triisopropylsilyl). One-pot, two-step reactions furnished the conversion of R from triisopropylsilyl to N-ligands. Macrocycles [((5-C5H4-C2-4-C5H4N)2Fe]2Ni2(NO3)4, [((5-C5H4-C2-3-C5H4N)2Fe]2Ag2(ClO4)2, and [((5-C5H4-C2-3-C5H4N)2Fe]2Pd2Cl4 were obtained and characterized crystallographically. They represent rigid, 2 nm-sized, molecular polygons defined by the centers of Cp rings. No Fe-Fe coupling except weak Fe-Ni and Fe-Ag interactions has been established by CV experiments. Weak antiferromagnetic Ni-Ni interaction was found. Complexes of the tetradentate ligands were demonstrated

<i>trans</i>-[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 [RuCl₃·3H₂O] or [Ru­(DMSO)₄Cl₂] provides the reagent <i>trans</i>-[Ru^II(dpp)­Cl₂] 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 <i>trans</i>-[Ru^II(dpp)­Cl₂] 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]­(PF₆)

Visible Light-Driven Hydrogen Evolution from Water Catalyzed by A Molecular Cobalt Complex

An approximately planar tetradentate polypyridine ligand, 8-(1″,10″-phenanthrol-2″-yl)-2-(pyrid-2′-yl)­quinoline (ppq), has been prepared by two sequential Friedländer condensations. The ligand readily accommodates Co­(II) bearing two axial chlorides, and the resulting complex is reasonably soluble in water. In DMF the complex shows three well-behaved redox waves in the window of 0 to −1.4 V (vs SHE). However in pH 7 buffer the third wave is obscured by a catalytic current at −0.95 V, indicating hydrogen production that appears to involve a proton-coupled electron-transfer event. The complex [Co­(ppq)­Cl₂] (<b>6</b>) in pH 4 aqueous solution, together with [Ru­(bpy)₃]­Cl₂ and ascorbic acid as a sacrificial electron donor, in the presence of blue light (λ_max = 469 nm) produces hydrogen with an initial TOF = 586 h^–1

Eu(III) Complexes of Tetradentate Ligands Related to 2,9-Di(pyrid-20-yl)-1,10-phenanthroline and 2,20-Bi-1,10-phenanthroline

A series of six tetradentate polypyridine-type ligands (L) have been used to prepare the corresponding Eu(III) complexes [Eu(L)2(S)]nþ (n = 2, 3) where S = H2O or CF3SO3 -. Two of the ligands, 2,9-di(pyrid-20-yl)-1,10- phenanthroline (4) and its dipyridophenazine analogue (6) are symmetrical around a central phenanthroline ring. The other four ligands are 2,20-bi-1,10-phenanthroline and its 3,30-di-, tri-, and tetramethylene-bridged analogues (5a-d) whose conformations are governed by the length of the polymethylene bridge. 1H NMR and X-ray analysis indicate that all of the complexes have a C2v symmetry. The biphenanthroline series shows a strong correlation of the conjugation between the two halves of the ligand, as governed by the bridge, with the absorption and emission properties of the Eu(III) complex. The complex having the most distorted, tetramethylene-bridged ligand exhibits a weak, high energy π-π* absorption and low sensitization efficiency. The luminescence decays are monoexponential for complexes of 4 and either monoexponential or biexponential for the complexes of 5 depending on its solution concentration and the length of the bridge. The complexes of 4 exhibit much longer lifetime, higher overall quantum yield, and higher sensitization efficiency than complexes of 5 while the complex of 6 emits very weakly. The Eu(5D0) lifetime for [Eu(4)2(H2O)](ClO4)3 is shorter than for [Eu(4)2(CF3SO3)](CF3SO3)2, reflecting the effect of the coordinated water. The complexes are examined for stability in the presence of water and found to retain most of their luminescent properties even in the presence of a large excess of water

Component Analysis of Dyads Designed for Light-Driven Water Oxidation

A series of seven dyad molecules have been prepared utilizing a [Ru­(tpy)­(NN)­I]⁺ type oxidation catalyst (NN = 2,5-di­(pyrid-2′-yl) pyrazine (<b>1</b>), 2,5-di-(1′,8′-dinaphthyrid-2′-yl) pyrazine (<b>2</b>), or 4,6-di-(1′,8′-dinaphthyrid-2′-yl) pyrimidine (<b>3</b>). The other bidentate site of the bridging ligand was coordinated with 2,2′-bipyridine (bpy), 1,10-phenanthroline (phen), or a substituted derivative. These dinuclear complexes were characterized by their ¹H NMR spectra paying special attention to protons held in the vicinity of the electronegative iodide. In one case, <b>10a</b>, the complex was also analyzed by single crystal X-ray analysis. The electronic absorption spectra of all the complexes were measured and reported as well as emission properties for the sensitizers. Oxidation and reduction potentials were measured and excited state redox properties were calculated from this data. Turnover numbers, initial rates, and induction periods for oxygen production in the presence of a blue LED light and sodium persulfate as a sacrificial oxidant were measured. Similar experiments were run without irradiation. Dyad performance correlated well with the difference between the excited state reduction potential of the photosensitizer and the ground state oxidation potential of the water oxidation dyad. The most active system was one having 5,6-dibromophen as the auxiliary ligand, and the least active system was the one having 4,4′-dimethylbpy as the auxiliary ligand

A Molecular Light-Driven Water Oxidation Catalyst

Two mononuclear Ru­(II) complexes, [Ru­(ttbt)­(pynap)­(I)]­I and [Ru­(tpy)­(Mepy)₂(I)]I (tpy = 2,2′;6,2″-terpyridine; ttbt = 4,4′,4″-tri-<i>tert</i>-butyltpy; pynap = 2-(pyrid-2′-yl)-1,8-naphthyridine; and Mepy = 4-methylpyridine), are effective catalysts for the oxidation of water. This oxidation can be driven by a blue (λ_max = 472 nm) LED light source using [Ru­(bpy)₃]­Cl₂ (bpy = 2,2′-bipyridine) as the photosensitizer. Sodium persulfate acts as a sacrificial electron acceptor to oxidize the photosensitizer that in turn drives the catalysis. The presence of all four components, light, photosensitizer, sodium persulfate, and catalyst, are required for water oxidation. A dyad assembly has been prepared using a pyrazine-based linker to join a photosensitizer and catalyst moiety. Irradiation of this intramolecular system with blue light produces oxygen with a higher turnover number than the analogous intermolecular component system under the same conditions

A Ru(II) Bis-terpyridine-like Complex that Catalyzes Water Oxidation: The Influence of Steric Strain

The complexation of 2,9-dicarboxy-1,10-phenanthroline (DPA) with [Ru­(tpy)­Cl₃] (tpy = 2,2′;6,2″-terpyridine) provides a six-coordinate species in which one carboxyl group of DPA is not bound to the Ru­(II) center. A more soluble tri-<i>t-</i>butyl tpy analogue is also prepared. Upon oxidation, neither species shows evidence for intramolecular trapping of a seven-coordinate intermediate. The role of the tpy ligand is revealed by the preparation of [Ru­(tpy)­(phenq)]²⁺ (phenq = 2-(quinol-8′-yl)-1,10-phenanthroline) that behaves as an active water oxidation catalyst (TON = 334). This activity is explained by the expanded coordination geometry of the phenq ligand that can form a six-membered chelate ring that better accommodates the linear arrangement of axial ligands required for optimal pentagonal bipyramid geometry. When a 1,8-naphthyidine ring is substituted for each of the two peripheral pyridine rings on tpy, increased crowding in the vicinity of the metal center impedes acquisition of the prerequisite reaction geometry

Iron Complexes of Square Planar Tetradentate Polypyridyl-Type Ligands as Catalysts for Water Oxidation

The tetradentate ligand, 2-(pyrid-2′-yl)-8-(1″,10″-phenanthrolin-2″-yl)-quinoline (ppq) embodies a quaterpyridine backbone but with the quinoline C8 providing an additional sp² center separating the two bipyridine-like subunits. Thus, the four pyridine rings of ppq present a neutral, square planar host that is well suited to first-row transition metals. When reacted with FeCl₃, a μ-oxo-bridged dimer is formed having a water bound to an axial metal site. A similar metal-binding environment is presented by a bis-phenanthroline amine (dpa) which forms a 1:1 complex with FeCl₃. Both structures are verified by X-ray analysis. While the Fe^III(dpa) complex shows two reversible one-electron oxidation waves, the Fe^III(ppq) complex shows a clear two-electron oxidation associated with the process H₂O–Fe^IIIFe^III → H₂O–Fe^IVFe^IV → OFe^VFe^III. Subsequent disproportionation to an FeO species is suggested. When the Fe^III(ppq) complex is exposed to a large excess of the sacrificial electron-acceptor ceric ammonium nitrate at pH 1, copious amounts of oxygen are evolved immediately with a turnover frequency (TOF) = 7920 h^–1. Under the same conditions the mononuclear Fe^III(dpa) complex also evolves oxygen with TOF = 842 h⁻¹