Octahedral Fe(II) and Ru(II) Complexes Based on a New Bis 1,10-Phenanthroline Ligand That Imposes a Well Defined Axis Didier Pomeranc, Vale ´ rie Heitz, Jean-Claude Chambron, and Jean-Pierre Sauvage* Contribution from the Laboratoire de Chimie Organo-Mine ´ rale, UMR 7513 du CNRS, UniVersite ´ Louis Pasteur, Institut Le Bel, 4 rue Blaise Pascal, 67000 Strasbourg, France ReceiVed May 21, 2001 Abstract: A bis-chelating ligand (L1), made of two 7-(p-anisyl)-1,10-phenanthroline (phen) subunits connected with a p-(CH 2 ) 2 C 6 H 4 (CH 2 ) 2 spacer through their 4 positions, has been prepared, using Skraup syntheses and reaction of the anion of 4-methyl-7-anisyl-1,10-phenanthroline with R,R′-dibromo-p-xylene. Its Fe(II) complex, [FeL1(dmbp)](PF 6 ) 2 , was prepared in one step by reaction of L1 with [Fe(dmbp) 3 ](PF 6 ) 2 (dmbp ) 4,4′-dimethyl- 2,2′-bipyridine). On the other hand, its Ru(II) complex, [RuL1(dmbp)](PF 6 ) 2 , was prepared in two steps from Ru(CH 3 CN) 4 Cl 2 and L1, followed by reaction with dmbp. X-ray crystal structure analyses show that in the two octahedral complexes, ligand L1 coils around the metal by coordination of the axial and two equatorial positions. It defines a 21 Å long axis (O‚‚‚O distance) running through the central metal and the terminal anisyl substituents. The complexes were also characterized by 1 H NMR, mass spectrometry, cyclic voltammetry, electronic absorption, and, in the case of Ru(II), fluorescence spectroscopy. Introduction The principles underlying photoinduced charge separation in natural photosynthetic systems (bacterial reaction centers and green plant photosystems I and II) have guided many research groups in designing synthetic compounds able to perform photoinduced charge separation at the molecular level, as described in recent examples. 1 One of these key principles is fractionation of long-range electron transfer in discrete hopping steps. It has led to the concept of triads such as the D-P-A systems, in which D and A are electron-donor or -acceptor components, respectively, with P being a photosensitizer (electron-donor in the excited state). 2-5 The bridge connecting the two units is of crucial importance, because it controls the geometry, the distance, and the electronic communication between the different components. 6 Linear and rigid bridges are particularly well-adapted for obtaining long-lived charge- separated states, because they best physically separate the oxidized donor and reduced acceptor. Among the great variety of chromophores that have been used as photosensitizers in D-P-A triads, porphyrins, 1,2 and ruthenium 3,4 or osmium 5 diimine complexes play a central role. Porphyrins have been easily incorporated in linear systems through trans meso (1, 10) positions, which define a C 2 symmetry axis. 1,2 Ru- or Os- (terpy) 2 2+ complexes (terpy ) 2,2′:6′,2′′-terpyridine) have been used similarly, because 4′-substituted bis-terpy complexes have a well-defined axis running through the 4′-position of each terpy (Figure 1). 4,5 The situation is very different for Ru(bipy) 3 2+ -based systems in which no such axis can be found. As shown in Figure 2, arrangement of two bipyridine ligands with donor and acceptor moieties attached to each 4-position produces four stereoisomers, with only one realizing the ideal situation of a linear arrangement of D, the ruthenium chromophore, and A. In this latter case, the donor and acceptor units are sitting along a coordination axis of the complex (Figure 2c). Synthetic methods were (1) Steinberg-Yfrach, G.; Liddell, P. A.; Hung, S.-C.; Moore, A. L.; Gust, D.; Moore, T. A. Nature 1997, 385, 239-241. (b) Gust, D.; Moore, T. 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