The monometallic complexes [(bpy)2Ru(tpphz)]2+ (4) and [(bpy)2Os(tpphz)]2+ (5), where tpphz is the poorly soluble fully aromatic tetrapyrido[3,2-a:2‘,3‘-c:3‘‘,2‘‘-h:2‘‘‘,3‘‘‘-j]phenazine, have been obtained by reaction of 5,6-diamino-1,10-phenanthroline with [(bpy)2M(phendione)]2+ (M = RuII or OsII). Reaction of 4 and 5 with metallic precursors yielded the homo- and heterobimetallic complexes [(bpy)2Ru(tpphz)Ru(NH3)4]4+ (6), [(bpy)2Ru(tpphz)Ru(bpy)2]4+ (7), [(bpy)2Os(tpphz)Os(bpy)2]4+ (8), and [(bpy)2Ru(tpphz)Os(bpy)2]4+ (9). The mononuclear 4 and 5 aggregate in solution, probably by π−π stacking of the tpphz part as shown from proton NMR. The complexes show one reversible metal-centered oxidation and several reversible (except 6) reductions which add one electron on the tpphz ligand, one electron on one bpy of each metallic end, a second electron on one bpy of each metallic end, and then a second electron on the tpphz ligand. The complexes (except 8) are luminescent in acetonitrile. Quenching of the luminescence by water has been attributed to proton quenching at the phenazine nitrogen atoms.

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The three isomers of diferrocenylbenzenes (ortho, 1o; meta, 1m; para, 1p) as well as 5-substituted derivatives of m-diferrocenylbenzene with R ) NH 2 (2), Cl (3), CH 3 (4), CN (5), NO 2 (6), and N(CH 3 ) 3 3+ (7) have been prepared. Crystal structures of 1o, 3, and 5 have been solved. In 3 and 5, the cyclopentadienyl rings are nearly parallel to the benzene mean planes with angles ranging from 9.99(5)°to 14.74(5)°. One ferrocene group is above and the other below the mean molecular plane. For 1o, there is an important twist between the benzene and cyclopentadiene rings (68.6(8)°and 32.5(8)°) for steric reasons. Controlled potential electrolysis yields the mixed-valence ferrocene/ ferrocenium species in comproportionation equilibrium with homovalent species. Intervalence transitions have been observed and corrected from comproportionation. From the intervalence band parameters, metal-metal couplings (V ab ) are calculated using Hush's equation. The values are much higher for 1o (0.025 eV) and 1p (0.043 eV) than for 1m (0.012 eV) and exhibit little or no variation for the substituted m-diferrocenylbenzenes 2-6. These results are rationalized by extended Hu ¨ckel molecular orbital calculations. The weakness of the interaction in 1m can be ultimately traced to a quantum Interference effect, i.e., a cancellation of the contributions of two electron transfer paths. This cancellation occurs because each path implies a mixing of metal orbitals with a different ligand orbital, and the resulting molecular orbitals exhibit different symmetries.

A series of diferrocenylpolyenes of general formula Fc(CH=CH)(n)()Fc with n = 1-6 (Fc = ferrocenyl group) has been prepared and studied from the point of view of intervalence transitions in the mixed valence state. Well-resolved intervalence transitions are observed in dichloromethane upon partial electrolytic oxidation. Comproportionation constants have been determined from redox titration data and in some cases from electrochemical wave splitting. The corrected spectra of the mixed valence species have been deconvoluted to extract the parameters (position, intensity, width) of the intervalence bands, which allowed the determination of the metal-metal coupling (V(ab)) through the bridging unit using Hush's formula. The decay of V(ab) with distance is close to an exponential law with an exponent of 0.087 Å(-)(1), constituting one of the smallest attenuations reported so far. A small deviation to the exponential law is detected.

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The photophysical properties of mono- and dinuclear complexes based on the bridging ligand tpphz (tpphz = tetrapyrido[3,2-a:2‘,3‘-c:3‘ ‘,2‘ ‘-h:2‘ ‘‘-3‘ ‘‘-j]phenazine) were investigated. The complexes are of general formula [M(bpy)2(tpphz)]2+ [M = Ru(II), Os(II)] and [(bpy)2M1(tpphz)M2(bpy)2] n + [M1= M2 = Ru(II), n = 4; M1= M2 = Os(II), n = 4; M1= Ru(II), M2 = Os(II), n = 4; M1= Ru(II), M2 = Os(III), n = 5]. The tpphz bridging ligand, being aromatic, rigid, and planar, has interesting structural features for the design of covalently linked donor−acceptor systems. In this work particular attention was devoted to the electronic properties of this bridge and their effect on the photophysical behavior. All of the results are consistent with direct involvement of the tpphz bridge in the photophysically active, lowest MLCT excited states. Relevant findings are as follows:  (i) in mononuclear complexes, MLCT excited-state energies are highly sensitive to interactions at the free bpy-like end of the tpphz ligand, such as metalation and protonation; (ii) in the dinuclear complexes, the electronic ground state behaves as a valence-localized, supramolecular system, while a substantial amount of intercomponent electronic coupling is indicated by MLCT excited-state behavior; (iii) in the heterodinuclear complex, fast (k > 109 s-1) energy and/or electron transfer processes take place across the tpphz bridge.

Symmetrical and unsymmetrical ligands containing terpyridyl coordinating units (N, N, N) or a cyclometalating equivalent (N, C, N), connected back-to-back either directly or via a p-terphenylene or 1,3-phenylene spacer, have been used to construct new diruthenium complexes. These compounds incorporate various terdentate chelates as capping ligands, to allow a double control of the electronic properties of each subcomplex and of the ensemble:  via the terminal ligand or through the bridging fragment. Electronic coupling was studied from the intervalence transitions observed in several bimetallic ruthenium complexes of the bis(cyclometalated) type differing by the substitution of a nitrogen atom by carbon in the terminal terpyridyl unit. The largest metal−metal interaction was found in complexes for which the terminal complexing unit is of the 1,3-di-2-pyridylbenzene type, i.e., with the carbon atom located on the metal−metal C 2 axis of the molecule. Investigations of the mechanism of interaction by extended Hückel calculations showed that the replacement of nitrogen by carbon raises the filled ligand levels, increasing the mixing with ligand orbitals and thus the metal−metal coupling. Finally, the intervalence transition was still observed for a bridging ligand containing three phenylene units as spacers, corresponding to a 24-Å metal−metal distance.

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