First-Generation Chiral Metallodendrimers:  Stereoselective Synthesis of RigidD₃-Symmetric Tetranuclear Ruthenium Complexes

Abstract

Condensation reactions between the enantiopure, substitutionally inert building blocks Δ-[Ru(1,10-phenanthroline-5,6-dione)3][PF6]2 (2) and Δ-[Ru(phen)2(1,10-phenanthroline-5,6-diamine)][PF6]2 (3) gave the D 3-symmetric tetramer [(Δ-(phen)2Ru(tpphz))3-Δ-Ru]8+ (ΔΔ3-1) in 68% isolated yield as the PF6 - salt (where tpphz is tetrapyrido[3,2-a:2‘,3‘-c:3‘‘,2‘‘-h:2‘‘,3‘‘-j]phenazine). Reactions between appropriate enantiomers of 2 and 3 also yielded the remaining D 3 isomers ΛΔ3-1, ΔΛ3-1, and ΛΛ3-1 which collectively represent the highest nuclearity Ru oligomers based on bidentate type ligands prepared in a stereospecific fashion to give diasteromerically and enantiomerically pure products. The complexes were purified via cation-exchange HPLC. 1H and 13C NMR, COSY and HMQC NMR, and UV−visible spectroscopy were employed to characterize these supramolecular assemblies. MALDI-TOF mass spectrometry gave parent molecular ion peaks corresponding to the species m/z 2638 [1 − 8PF6]+, 2783 [1 − 7PF6]+, 2928 [1 − 6PF6]+, 3073 [1 − 5PF6]+, 3218 [1 − 4PF6]+, and 3363 [M − 3PF6]+, proving the macromolecular structure. Circular dichroism spectroscopy was used to determine the absolute stereochemistry and optical purity of each of these stereoisomers.

Linear Multinuclear RuII Photosensitizers

“…One approach to improving the photoactivity is to increase the number of metal atoms in the sensitizer and the probability of light absorption. Dendritic assemblies epitomize this idea, [2] but electrostatic considerations make them unattractive because their globular structures and high charges would impede electron transfers to positively charged acceptors, such as methylviologen (MV 2ϩ ), and/or, in the case of neutral or anionic acceptors, would facilitate the energy-wasting reverse electron transfer that regenerates the initial states. The effect of charge repulsion can be minimized with linear multinuclear arrays, where an end-on approach by an electron acceptor is minimally affected by repulsions from the more distal metal centers.…”

Optimization of Hydrogen‐Evolving Photochemical Molecular Devices

“…This behavior is in agreement with the closely related species 1 and 2 in which as tepwise photoinduced electron transfer from the ruthenium photocenter to the catalytically active palladium or platinum center is observed (Figure 1). [20,32,33] This observation indicates that the ultrafast photoinduced electron-transfer processes are not affected by the exchange of the terminal Pt bound anions.W etherefore considered the potential changes in the catalytic mechanism occurring at the platinum center.The kinetic processes during the light-driven hydrogen production of the photocatalysts 2 and 3 were compared. In spite of the fact that 3 shows as ignificantly increased catalytic activity its underlying photocatalytic behavior is in line with that observed for 2.I mportantly, both catalysts show an instant hydrogen formation and arelatively constant TOFuntil the maximum TONisreached (catalytic activity of 3 is shown in Figure 2).…”

“…As shown in Figure 3, there are no significant differences observed in the steady state UV/Vis spectra of compounds 2 and 3 in acetonitrile.H owever, 3 shows an absorption maximum at 360 nm, which is not present for 2 and can be assigned to p-p* transitions located on the phenazine unit of the tpphz ligand. [31,32] Thephotoinduced processes in the ps range of compound 3 were investigated by using transient absorption spectrosco- . .…”

Tuning of photocatalytic activity by creating a tridentate coordination sphere for palladium

“…This is remarkable, as the unsubstituted tpphz only absorbs at wavelengths below 400 nm. 40,41 Even the bis(4-butylphenyl)tpphz shows an absorption maximum only at 393 nm. These data suggest that the absorption of visible light is associated with the novel substitution pattern, which is supported by TDDFT calculations on the mononuclear ruthenium complex, described in the following paragraph.…”