Polypyridine ligand

The electroactive units in the dendrimers that we are going to discuss are the metal-based moieties. An important requirement for any kind of application is the chemical redox reversibility of such moieties. The most common metal complexes able to exhibit a chemically reversible redox behavior are ferrocene and its derivatives and the iron, ruthenium and osmium complexes of polypyridine ligands. Therefore it is not surprising that most of the investigated dendrimers contain such metal-based moieties. In the electrochemical window accessible in the usual solvents (around +2/-2V) ferrocene-type complexes undergo only one redox process, whereas iron, ruthenium and osmium polypyridine complexes undergo a metal-based oxidation process and at least three ligand-based reduction processes. 

Because of the presence of many polypyridine ligands, each capable of undergoing several reduction processes [38], the electrochemical reduction of this type of dendritic compound produces very complex electron exchange patterns. 

C.H. Brubaker, Michigan State University How did you deposit the polypyridine ligand on the electrode surface ... 

Let us now consider the N6-coordinated octahedral vanadium complexes with the polypyridine ligands 1,10-phenantroline (phen), 2,2/-bipyridine (bipy) and 2,2/ 6,2//-terpyridine (terpy), Scheme 3. 

It undergoes either two separated reductions or one oxidation, all having features of chemical reversibility in the cyclic voltammetric time scale. Since no detailed studies have been carried out on the underlying electrode mechanism, it is noted that these redox transformations could formally correspond to the sequence V(III)/V(II)/V(I), but, as far as the reduction processes are concerned, it is also likely that they are centred on the polypyridine ligand. 

Cr(III) complexes, in particular those of the above discussed polypyridine ligands phen, bipy, terpy usually possess a considerably distorted octahedral geometry.23,24 As illustrated in Figure 14, [Cr(terpy)2]3 + displays four successive reversible one-electron reduction processes.25... 

The first three steps have been tentatively assigned to the sequence Cr(III)/Cr(II)/Cr(I)/Cr(0), while the fourth has been assigned as ligand centred. However, in judging the remarkable redox ability of these complexes one must bear in mind the comments made previously concerning the reciprocal redox ability of the metal ion and the polypyridine ligands. 

As a further confirmation of the extended redox aptitude of polypyridine ligands, Figure 15 shows the cyclic voltammetric behaviour of the heteroleptic chromium(III) complex with 2,2/-6/,2"-6//,2"/-quaterpyr-idine (qpy), together with its molecular structure.28... 

As usual, in a discussion of coordination, one cannot ignore polypyridine ligands. There are numerous X-ray structures available for iron complexes. We begin with the phenantroline [Fen(phen)3]2 + complex, whose molecular structure is shown in Figure 69.105... 

It is interesting to note that in a further confirmation of the redox-active character of the polypyridine ligands, reductive electrocrystallization of [Mn(bipy)3]2+ and [Mn(terpy)2]2+ complexes (M = Fe, Ru, Os) afforded the corresponding neutral species [M(bipy)3]° and [M(terpy)2]°. Based on the relevant bond lengths, it seems likely that the two added electrons enter the polypyridine ligands according to the formulation [MII(bipy-)2(bipy)]° and [Mn(terpy )2]°.111,112... 

Keywords Sonogashira polycondensation Hybrid polymers Polypyridine ligands Copolymers Macromolecular metal complexes... 

More information from NMR analysis can be obtained when the polypyridine ligands carry aliphatic substituents because the NMR signals of such groups are in a clear region of the spectrum and can be easily distinguished. ... 

Dossing, A. Toftlund, H. Hazell, A. Bourassa, J. Ford, P. C. Crystal structure, luminescence and other properties of some lanthanide complexes of the polypyridine ligand 6,6,-bis[bis(2-pyridylmethyl)aminomethyl]-2,2,-bipyridine. J. Chem. Soc., Dalton Trans. 1997, (3), 335-339. 

Interestingly, metal-metal interaction appears to be dependent on the oxidation state of the system. This is clearly evidenced by the results obtained for Os2 in liquid SO2 at —70°C, in that in these experimental conditions both the first and the second oxidation of each metal center can be seen (Fig. 5.4 Table 5.1).15 Potential splitting for the first oxidation process is 370 mV, whereas potential splitting for the second oxidation process is 150 mV. This result confirms the electron transfer pathway for the superexchange interaction once mono-oxidized, the Os(III)-based orbitals are stabilized and their interaction with the bridge-based LUMO orbitals decreases. A similar result has been recently obtained in molecular grids based on similar polypyridine ligands.25... 

Some metal complexes with polypyridine ligands similar to Dye 2 have also been investigated as sensitizing dyes. Ferrere and Gregg73) synthesized cis-Fen(dcbpy)2(CN)2 complex (Dye 8), and Bignozzi el al.74i75) compared properties of m-Osn(dcbpy)2(CN)2 (Dye 9) with comparable m-Rull(dcbpy)2(CN)2 (Dye 10). The efficiencies are much lower than that of Dye 2, but they proposed the... 

A number of helical and double-helical complexes have been obtained with polypyridine ligands, which bind various metal ions yielding helical and double-helical [9.70-9.74] complexes that present interesting redox and metal-metal interaction properties [9.75]. The graphs of the double-helicates represent braids based on two threads and several crossings [9.1,9.76], that may serve as templates for the synthe-... 

Balzani et al. prepared dendrimers with metal complexes serving both as core [36] and as branching unit The metallodendrimer in Fig. 2.10 is constructed solely from polypyridine ligands and transition metal ions. Such dendritic transition metal complexes can be synthesised both convergently and divergently and different transition metal ions (ruthenium/osmium) can be incorporated. This provides a means of influencing the luminescence properties of the den-drimer. Thus the energy transfer process proceeds from the inside outwards in... 

Polypyridine ligands stabilize complexes of metals of Groups VB and VIB in their low oxidation states forming stable species with interesting photochemical and electrochemical properties. They nearly always form chelates with r 2-coordination, and all the chemical changes occur in the rest of the complex. However, the coordination situations may vary and the structure of the polypyridine ligand may be modified, which opens new areas of application. 

Volume 93 closes with the eleventh installment in the series by A.P. Sadimenko (University of Fort Flare, Republic of South Africa). The present contribution deals with the organometallic chemistry of polypyridine ligands, which has attracted much recent interest, with almost half the references being from the last 10 years. 

Voltammetric measurements indicate that the substitution of cyanide by isocyanide ligands causes a large, progressive anodic shift in the oxidation potential which reflects the decrease in electronic density at the Ru center and is consistent with the observed spectral shifts. On the other hand, the reduction potentials of the isocyanide complexes are only slightly different from those of the cyanides, consistent with the fact that in the Ru(II) polypiridine complexes the reduction occurs at the polypyridine ligands. As the energy of the lowest... 

In conclusion, whereas Co(NH3)63+ is useless because it undergoes a fast photodecomposition reaction, the analogous Co(sep)3+ complex may be employed as an electron transfer photosensitizer because of its intrinsic stability in the excited state and in the reduced form. In the same way, one can think to use cage-type polypyridine ligands for Ru complexes, so as to prevent ligand dissociation reactions. 

---