Dimethoxybenzene ligand

Dendrimer 1 + is a classical example of a dendrimer containing a luminescent metal complex core. In this dendrimer the 2,2 -bipyridine (bpy) ligands of the [Ru(bpy)3] +-type core carry branches containing 1,2-dimethoxybenzene- and 2-naphthyl-type chromophoric units [15]. 

At the same time, the 3-methoxycyclohexadienyl complex (Scheme 32) can also be prepared, presumably under kinetic conditions that do not allow isomerization of the double bonds. The key technique is acid-induced elimination of an alkoxy group, and the starting material is m-dimethoxybenzene. The addition of the malonate anion is at one of the two equivalent terminal positions, and from a direction anti to the iron. The product is a stable complex that may be purified by chromatography or may be decomposed oxidatively to free the organic ligand. 

Dendrimer 1 (Fig. 5) is a classical example of a dendrimer built around a metal complex core. In this compound, the 2,2 -bipyridine ligands, that constitute the first coordination sphere of the Ru ion, carry branches containing 1,3-dimethoxybenzene and 2-naphthyl chromophoric imits separated by aliphatic connectors (10). Since the interchromophoric interactions are weak, the absorption spectrum of 1 is substantially equal to the summation of the spectra of [Ru(bpy)s], which is characterized by a broad spin-allowed Ru bpy metal-to-ligand (MLCT) band around 450 nm (11), and of the chromophoric groups contained in the branches, which show very intense bands in the near UV region. 

Dendrimer 5 (Fig. 11) consists of a cyclam core appended with 12 dimethoxybenzene and 16 naphthyl units. Cyclam is one of the most extensively investigated ligands in coordination chemistry (33). Both cyclam and its 1,4,8,11-tetramethyl derivative in aqueous solution can be protonated and can coordinate metal ions such as Co(II), Ni(II), Cu(II), Zn(II), Cd(II), and Hg(II) with high stability constants (34). 

A step further in cyclam-based dendritic ligands for metal ions is constituted by dendrimer 6 (Fig. 13), containing two covalently linked cyclam units as a core, appended to six branches, each one of them consisting of a dimethoxybenzene and two naphthyl units 38). Its photophysical properties are qualitatively similar to that observed for 5. For example, the emission spectrum evidences the presence of naphthyl-localized excited states ( max = 337 run), naphthyl excimers Umax ca. 390 nm), and naphthyl-amine exciplexes (lmax = 480 nm). 

The three components of the self-assembled structure have complementary properties so that new functions emerge from their assembly. Dendrimer 5 has a very high molar absorption coefficient in the UV spectral region because of 12 dimethoxybenzene and 16 naphthyl units, but it is unable to sensitize the emission of an Nd ion placed in its cyclam core. The [Ru(bpy)2(CN)2] complex can coordinate (by the cyanide ligands) and sensitize the emission of Nd ions. Self-assembly of the three species leads to a quite unusual Nd complex which exploits a dendrimer and an Ru complex as ligands. Such a system behaves as an antenna that can harvest UV to VIS light absorbed by both the... 

An extension of this work has resulted in the synthesis of related 6,6-disubsti-tuted (bipyridyl) ligand derivatives containing pyromellitimide spacers connected with propynyl, butynyl and propynyloxy linkers.The first of these ligands was shown to yield a chiral di-zinc(II) cationic complex of type [Zn2L2] . This species is stabilised by the presence of an aromatic guest p- or o-dimethoxybenzene) in its cavity. Related dinuclear species were also shown to form with cadmium(II) and nickel(II). 

Aprotic polar solvents thus far employed in metal-catalyzed polymerizations include dimethoxybenzene (DMB), diphenyl ether (DPE), ethylene carbonate, acetonitrile, /V, /V-d imethylformamid e (DMF), and acetone, among others. Most of them are employed for copper catalysts because of their low solubility. For well-solubilized Ru(II), Ni(II), and Fe(II) complexes with phosphine or other ligands, such an additional precaution is not necessary, and toluene or other relatively nonpolar solvents have mostly been used. 

SCHEME 24.31 Effect of anionic ligand structure on CDC between 28 and 1,3-dimethoxybenzene. 

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