Dimer species case study

A dozen years ago, the optical properties of a very few cases of Ir(III) complexes were known [1,2]. In the same period, Ru(II)-, Os(II)- and Re(I)-polyimine complexes, for instance, were comparatively much more popular as a consequence of research efforts related to the theme of the interconversion between light and chemical energy [3-5]. In early times, the tricationic Ir(bpy)33+ species proved difficult to prepare, and was clearly identified only after closely related species were studied [ 1,6,7] bpy is 2,2 -bipyridine, Fig. 1. At the same time, the neutral/ac-Ir(ppy)3 species initially appeared as a side-product of dichloro-bridged dimers of the type [Ir(ppy)2Cl]2 [8] ppy is the cyclometalating anion from Hppy, Fig. 1. Today, we witness an impressive expansion of the literature on the luminescence of Ir(III) complexes, as illustrated by Fig. 2. 

Copper is often found in biochemical systems, usually in peptides and proteins. Extensive reviews may be found (66, 106, 557). Semiquantita-tive discussion (112) of the significance of the results, which usually show that the copper environment has axial symmetry and with a distorted octahedral structure are numerous. The use of model compounds has been discussed by Malmstrom and Vanngard (477). In many cases there appears to be dimeric species present (9S). A system of importance which has been extensively studied is the series of complexes with glycine and triglycine (209, 638, 707, 712). The ESR data are tabulated in Table LV. 

Spectroscopic studies under catalytic olefin hydrogenation conditions without added L show the presence of RhClLs (43), H2RhClL3 (44), (RhClL2)2 (45), and H2(RhClL2)2 (46). (In the case of ethylene (C2H4)RhClL2 (47) is also observed.) The amount of the hydrides increases with H2 pressure, while the dimeric species are increased by... 

The most extensively studied of these reactions is the reductive ehmination with the formation of C-N bonds from amido arylpaUadium complexes [397a, 398]. Both monomeric and dimeric species have been studied. In the case of monomeric complexes, some differences occur depending on the nature (mono- or bidentate) of the coordinating phosphines. 

Based on the studies of the tetraureacalix[4]arene dimers [11,17], Ballester and his colleagues postulated and demonstrated, many years later, that tetraureacalix[4] pyrroles (3) also form dimeric capsules [3d, 21]. These capsules have some differences compared to those that are formed by 2 and its analogues. Cavity sizes are larger by about 60 %, but their mode of organization were found to be not very different from that of the tetraureacalix[4]arene-based dimeric capsules. In the case of tetraureacalix[4]pyrroles, however, well-defined dimeric species are observed only after the addition of specific guests to the organic solutions [3d, 21]. 

---