PAPER www.rsc.org/dalton | Dalton Transactions Photoinduced ligand transformations in a ruthenium complex of dimethoxytetrapyridotetraazapentacene Shreeyukta Singh, Norma R. de Tacconi, David Boston and Frederick M. MacDonnell* Received 7th August 2010, Accepted 16th September 2010 DOI: 10.1039/c0dt00982b The dinuclear ruthenium(II) complex [(phen) 2 Ru(tatpOMe)Ru(phen) 2 ] 4+ (2 4+ ; phen is 1,10-phenanthroline and tatpOMe is 10,21-dimethoxy-9,10,20,33-tetraazatetrapyrido[3,2-a:2¢3¢- c:3¢¢,2¢¢-l:2¢¢¢,3¢¢¢-n]pentacene) has been synthesized and characterized by 1 H NMR, ESI mass spectroscopy and elemental analysis. Loss of methoxy group from bridging ligand of complex 2 4+ due to irradiation is observed by 1 H NMR and photochemistry. The interrelated electronic properties UV-Vis, electrochemistry, photochemistry and molecular orbital calculation are analyzed and discussed on the bridging ligand of the complex 2 4+ . Introduction Ruthenium(II) polypyridyl complexes continue to enjoy consider- able attention as chromophores for molecular light-to-chemical energy conversion schemes. 1–4 The development of molecular photocatalysts capable of driving conversion of common sub- strates into fuels, i.e. H + into H 2 and CO 2 into CH 3 OH, needs to address the multi-electron requirements of the substrates and still couple this with the single-photon, single-electron excitation common for most chromophores. One strategy has been to build photocatalysts capable of photodriven multi-electron storage, such that they absorb multiple photons over time and build-up reducing (or oxidizing equivalents) within their structure. Brewer and co- workers were first to demonstrate this in 1994 in a trimetallic Ru–Ir–Ru complex which was later extended to Ru–Rh–Ru systems. 5–7 In the latter Rh-based system, photon absorption by the Ru components drives a Rh(III/I) reduction and this center is subsequently able to reduce protons to H 2 . A number of other molecular 8–11 and supramolecular 12–15 Ru–Pt or Ru–Pd bimetallic systems have also been shown to produce H 2 photochemically in the presence of sacrificial donors. We have been exploring the photochemistry of ruthenium complexes, shown in Fig. 1, containing a unique class of pla- Department of Chemistry and Biochemistry, University of Texas at Arling- ton, Arlington, TX, 76019-0065, USA. E-mail: macdonn@uta.edu Fig. 1 Structures of dinuclear ruthenium(II) complexes having central bridging ligand tatpp, tatpOMe and tatpq. nar aromatic acceptor ligands related to the well-known dppz ligand. Mononuclear and dinuclear ruthenium(II) complexes of the tetraazatetrapyridophenazine (tatpp) ligand display unusual photochemical activity in which the tatpp ligand is reduced by up to 2 electrons upon visible light irradiation in the pres- ence of sacrificial donors. 16–19 The dinuclear ruthenium complex, [(phen) 2 Ru II (tatpp)Ru II (phen) 2 ] 4+ , 1 4+ , has also been shown to bind DNA via intercalation and cleave DNA in a process that is enhanced under low oxygen conditions. 20 The closely related complex, [(phen) 2 Ru II (tatpq)Ru II (phen) 2 ] 4+ , 3 4+ , is also photochemically active and can undergo up to four tatpq ligand- based reductions via photon driven processes. 19,21 While both 1 4+ and 3 4+ can photochemically store multiple electrons reversibly on the bridging ligands, neither complex has shown much promise as for solar H 2 generation owing to the modest reduction potentials of these stored electrons. In this work, we explore the substitution of the central hydrogens in tatpp with methoxy electron-donating groups (tatpOMe) in an effort to shift the reduction potentials of the resulting ruthenium complex, [(phen) 2 Ru II (tatpOMe)Ru II (phen) 2 ] 4+ (2 4+ ), to more neg- ative values. If this complex retains the photochemical activity of the parent tatpp complex, then we could aim to photoreduce this new complex by 2-electrons and potentially store them at more negative reduction potentials. In this report, we describe the synthesis and characterization of the tatpOMe ligand and its dinuclear ruthenium(II) complex, as well as an evaluation 11180 | Dalton Trans., 2010, 39, 11180–11187 This journal is © The Royal Society of Chemistry 2010 Downloaded by University of Texas at Arlington on 29 December 2010 Published on 16 November 2010 on http://pubs.rsc.org | doi:10.1039/C0DT00982B View Online