Spectroelectrochemical properties of homo- and heteroleptic ruthenium and osmium binuclear complexes: intercomponent communication as a function of energy differences between HOMO levels of bridge and metal centres

Abstract

A series of binuclear ruthenium and osmium complexes [(bipy) (5) and [(bipy) 2 Os(bpbt)Os(bipy) 2 ] 2+ (6) {bipy = 2,2¢-bipyridyl; qpy = 2,2¢:5¢,5¢¢:2¢¢,2¢¢¢-quaterpyridyl; pytr-bipy = 3-(2,2¢-bipyrid-6-yl)-5-(pyrid-2-yl)-1,2,4-triazolato, and bpbt = 5,5¢-bis-(pyrid-2¢¢-yl)-3,3¢-bis-1,2,4-triazolato} are reported. Analysis of the electrochemical data focuses on structural factors and on determining the extent of electronic communication between the metal centres in the mixed valence oxidation state. Intervalence charge transfer (IT) bands could be identified in the spectra of the complexes 4 and 6 only. Analysis of their spectroelectrochemical data leads to the conclusion that the IT is superexchange mediated through the HOMO of the bridging ligand. IntroductionMetal complexes based on polypyridyl ligands constitute versatile components for the construction of multifunctional supramolecular systems in molecular photonics and molecular electronics, 1 sensors, 2 photocatalysis, 3 solar energy 5 conversion, 4 artificial photosynthesis, 5 non-linear optics, 6 and electrochemoluminescence 7 amongst others. 8, 9 The myriad of applications are founded on the well defined chemistry of these compounds and take advantage of the versatile photophysical, 10 photochemical 11 and electrochemical 12 properties that are associ-10 ated with the metal-to-ligand charge-transfer (MLCT) excitation in these metal complexes. 13 Amongst the many multinuclear polypyridyl complexes reported, 14 an interesting subclass is represented by those binuclear systems that display additional spectroelectrochemical features 15 due to the electronic interaction between the two metal centres in the mixed valence redox state(s). 15,16 "Communication" between metal centres where direct overlap of atomic orbitals is not possible is achieved when the bridging ligand (BL) has a system of polarisable electrons that are capable of mediating 20 the redistribution of the electronic charge between the two metal centres. Sciences, Dublin City University, Dublin 9, Analysis of these absorption bands 25 can provide detailed information as to the nature and extent of the interaction between the metals centres. 30 Recently, we reported the synthesis and characterisation of a series of homo-and hetero-binuclear ruthenium(II) and osmium(II) complexes of the general formula [M 1 (bipy) 2 -BL-M 2 (bipy) 2 ], where M = Ru or Os, bipy is 2,2¢-bipyridyl (bipy), and BL the bridging ligands 2,2¢:5¢,5¢¢:2¢¢,2¢¢¢-quaterpyridyl (qpy), 3-(2,2¢-35 bipyrid-6-yl)-5-(pyrid-2-yl)-1,2,4-triazolato (pytr-bipy), and 5,5¢-bis-(pyrid-2¢¢-yl)-3,3¢-bis-1,2,4-triazolato (bpbt). 26 In the present contribution, we describe in detail the electrochemical and spectroelectrochemical properties of these compounds. The goal of the present study is to evaluate the extent 40 of electronic communication between the two metal centres as a function of the electronic properties of the bridging ligand and the positioning of the two metal centres. In particular, attention is directed towards the hom...

Photo‐ and Electrochemical Properties of a CO2 Reducing Ruthenium–Rhenium Quaterpyridine‐Based Catalyst

“…This indicates that there is limited communication between the Ru and Re subunits ,,. For 5 the first (and qpy‐based) reduction at approximately −1.4 V vs Fc/Fc + is very similar to that observed for the dinuclear [(bpy) 2 Ru(qpy)Ru(bpy) 2 ] 4+ compound . The qpy‐based reductions of 5 are shifted anodically by 340 mV and 120 mV relative to 1 and 2 respectively.…”

Enhancing Photocatalytic Hydrogen Generation: the Impact of the Peripheral Ligands in Ru/Pd and Ru/Pt Complexes

“…Recently we reported a multi‐step synthesis of ruthenium‐ and osmium‐based bimetallic complexes that contained the bisbpy ligand. The synthesis was based on a Ni 0 ‐catalysed cross‐coupling reaction of brominated ruthenium‐ or osmium‐polypyridine precursors . In the present contribution this ligand was prepared by a two‐step Negishi cross‐coupling.…”

“… Electrochemical measurements have shown that for [Ru(bpy) 2 (μ‐bisbpy)Ru(bpy) 2 ] 4+ the bisbpy ligand is reduced first, and therefore its metal–ligand charge transfer (MLCT) levels are expected to be at lower energy than the peripheral bpy ligand levels The bisbpy ligand conceptually consists of two bpy subunits connected by a freely rotating single bond.…”

Effect of Alkyl Chain Length on the Sensitizing Action of Substituted Non‐Symmetric Squaraines for p‐Type Dye‐Sensitized Solar Cells

“…In the following we will first detail the synthetic procedures of the three unsymmetrical squaraines DS_45, DS_34 and DS_47 (Figure ). The choice of these three squaraines was motivated by the recognition of the vectorial character of electron transfer that is imparted by the presence of the anchoring carboxylic groups only on one side of these dyes. The structures of DS_45, DS_34 and DS_47 are distinguishable for the different extension of the alkyl substituent on the nitrogen atom of the indolenine.…”