2,2 -Bipyridyl-3,3 -dicarboxylic acid

The well documented oxidation of 1,10-phenanthroline (4) to 2,2 -bipyridyl-3,3 -dicarboxylic acid (53) by alkaline permanganate has been repeated. It is now found that 4,5-diazafluoren-9-one (54) is consistently a coproduct of the reaction in 20% yield.253 This provides a convenient route to this hitherto difficultly accessible compound. The formation of 4,5-diazafluoren-9-one almost certainly occurs by way of the intermediate, but not isolated, l,10-phenanthroline-5,6-dione, which is known to undergo a benzilic acid type rearrangement in the presence of hydroxide ions to give the diazafluorenone. This rearrangement of l,10-phenanthroline-5,6-dione has recently been studied further.115 Under some conditions 5,6-dihydro-5,6-dihydroxy- 1,10-phenanthroline can be isolated with the diazafluorenone. 

The principal synthetic route to diazafluorenes involves the alkaline oxidation of phenanthrolines, presumed to give initially the rarely isolated 5,6-dione which (i) further oxidizes to a bipyridyl dicarboxylic acid (the main product) or (ii) undergoes a benzilic acid type rearrangement followed by oxidative decarboxylation to give diazafluorenones in moderate yields (Scheme 2) <77JPR959>. The process has been reviewed by Summers <78AHC(22)i>. The yields of 4,5-diazafluorenone have been optimized <73AJC2727>. 

The photochemical and thermal stabilities of Ru complexes have been investigated in detail [8,153-156]. For example, it has been reported that the NCS ligand of the N3 dye, cri-Ru(II)(dcbpy)2(NCS)2 (dcbpy = 2,2 -bipyridyl-4,4 -dicarboxylic acid), is oxidized to produce a cyano group (—CN) under irradiation in methanol solution. It was measured by both ultraviolet-visible (UV-vis) absorption spectroscopy and nuclear magnetic resonance (NMR) [8,153]. In addition, the intensity of the infrared (IR) absorption peak attributed to the NCS ligand starts to decrease at 135°C, and decarboxylation of N3 dyes occurs at temperatures above 180°C [155]. Desorption of the dye from the 2 surface has been observed at temperatures above 200°C. 

Several further examples have appeared of the permanganate oxidation of 4,7-phenanthrolines to 3,3 -bipyridyl-2,2 -dicarboxylic acids.101,227... 

Reactions of 4,7-phenanthroline-5,6-dione have been the subject of considerable study. It is reduced to 5,6-dihydroxy-4,7-phenanthroline by Raney nickel hydrogenation226,249 or by aromatic thiols in benzene,262 and oxidized by permanganate to 3,3 -bipyridyl-2,2 -dicarboxylic acid.263 It forms bishemiketals with alcohols226 and diepoxides with diazomethane.226 The diepoxides by reaction with hydrochloric acid form diols of type 57, R = Cl, which on oxidation with lead tetraacetate give 3,3 -bipyridyl diketones of type 58, R = Cl. Methyl ketones of type 58, R = H, are also obtained by lead(IV) acetate oxidation of the diol 57, R = H, obtained by lithium aluminum hydride reduction of 57, R = Cl. With phenyldiazomethane and diphenyldiazomethane the dione forms 1,3-dioxole derivatives,264,265 which readily hydrolyze back to the dione with concomitant formation of benzaldehyde and benzophenone, respectively. 

Nazeeruddin M. K., Zakeerudin S. M., Humphry-Baker R., Jirousek M., Liska P., Vlachopoulos N., Shklover V., Fischer C.-H. and Gratzel M. (1999), Aeid-base equilibria of (2,2 -bipyridyl-4-4 -dicarboxylic acid)ruthenium(II) eomplexes and the effect of protonation on charge-transfer sensitization of nanoerystalline titania , Inorg. Chem. 38, 6298-6305. 

Phenanthroline is oxidized by alkaline potassium permanganate to 2,4 -bipyridyl-3,3 -dicarboxylic acid and to l,8-phenanthroline-5,6-dione in 28% yield by nitric-sulfuric acid mixtures. The latter reaction also yielded some nitrated products and 2,5-diazafluorenone. 

Several properties of these ruthenium(II) complexes are shown in Table 2. Apparently, the absorption and emission maxima of [Ru((-)-menbpy)3l and [Ru(S( - )-PhEtbpy)3l + exhibit considerably large red shifts, compared to those of [Ru(bpy)3l. A similar red shift was observed in [Ru(dmp)n(dcbpy)3 n] (dcbpy = 2,2 -bipyridyl-4,4 -dicarboxylic acid), as was shown in Table 1. These red shifts are easily understood in terms of the introduction of electron-withdrawing substituents at the 4 and 4 positions of 2,2 -bipyridine. The other important feature is that the lifetime of the MLCT excited state becomes much longer than that of [Ru(bpy)3l +. One of the important reasons is the increase in the energy difference between the triplet d-d ( d-d) and MLCT excited states, as follows [13,29] Since the electron-withdrawing substituent of 2,2 -bipyridine stabilizes the TT orbital of 2,2 -bipyridine, the MLCT excited state becomes lower in energy, but the d-d excited state is little influenced in energy by the substiment. 

Scheme 1. Molecular structure of AR25 [chemical name cis-bis(2,2 -bipyridyl-4,4 -dicarboxylic acid)(5,6-dimethyl-1,10-phen-anthroline)bis(isothiocyanato)ruthenium(II)].

The proven, and thus preferred, general structure for sensitizers is ML2(X>2, where M can be Ru or Os, L is 2,2 -bipyridyl-4,4 -dicarboxylic acid, and X represents a halide, cyanide, thiocyanate, acetyl acetonate, thiocarbamate, or water subsistent group [29]. The stfuctures of metal complexes used as sensitizers can also be mononuclear metal complexes (Figs. 38.2, 38.3, and 38.4a) [6,41, 18], binuclear metal complexes [Ru-Ru (Fig. 38.4b) [30], and Ru-Os (Fig. 38.4c)] complexes [30]. Polynuclear complexes have been employed in order to increase absorption coefficients. However, these bulky sensitizers require more space on the Ti02 surface and penetrate less easily in the small cavities of the nanocrystalline TiOj than the mononuclear complexes [34]. Hence, for polynuclear complexes, the increased absorption coefficients in solution do not necessarily lead to enhanced light absorption on the Ti02 electrode because of the reduced surface concentration of the bulkier sensitizer molecules on the nanoporous Ti02. 

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