Ligands with extended

An unusual pH dependence has been reported for the Gd111 complex of a tetraamide-based ligand with extended noncoordinating phosphonate side chains (Scheme 12).169,170 The relaxivity increases from pH 4 to 6, followed by a decrease until pH 8.5, then from pH 10.5 it increases again. The system, as well as isostructural lanthanide complexes, was characterized by various techniques such as 31P and 170 NMR and fluorescence measurements. The pH dependence could be attributed to protonation equilibria of the noncoordinating phosphonate groups, which can... 

Ligand-centred (LC) transitions between bonding and antibonding ligand-centred MOs. These transitions are expected for aromatic ligands with extended n- and it -orbitals. 

Macrocyclic ligands with extended 7i-delocalisation were studied in order to improve the conductivity of linear-chain complexes, because extended n delocalisation may reduce the on-site coulombic repulsion. The metal dibenzotetraazaannulene complexes, [M(taa)j (taa = dibenzo-tetraazaannulene M = H2, Ni, Pd, Co, Cu) and [M(tmtaa)] (tmtaa = tetramethyl dibenzotetraazaannulene M = H2, Ni, Pd) (Figure4.13), are... 

A large, macrocyclic ligand such as phthalocyanine is a very stable ligand with extended u-delocalisation and rigid macrocyclic structure. It can form metal complexes, [M(Pc)] (Pc = phthalocyanine), with many metals (M = Ni, Pt, Co, Fe, Cu, etc.) and the planarity of the molecule is retained... 

Compounds supporting bridging alkynyl ligands with extended aromaticity (e.g., 1,4-diethynylbenzene) begin to show characteristics of the low-lying 7t orbital of the alkyne, in much the same manner as the Cu(l)- or Pt(ll)-alkynyl systems do. Here, the metal-based excited-state manifold reduces the alkyne 7t orbital in the excited state leading to a MMLCT configuration (Au-Au da -> Jt (C=C) (226). 

Yan Y-Y, Widhalm M (1998) Synthesis and application of macrocyclic binaphthyl ligands with extended chiral bias. Tetrahedron Asym 9 3607-3610... 

This metathetical route was recently extended to the synthesis of amidinato complexes of Al, Ga, and In containing very bulky amidinate ligands with 2,6-Pr CgHs, adamantyl, m-terphenyl or 9-tiiptycenyl substituents at the central carbon atom. ... 

Pyridine-based N-containing ligands have been tested in order to extend the scope of the copper-catalyzed cyclopropanation reaction of olefins. Chelucci et al. [33] have carefully examined and reviewed [34] the efficiency of a number of chiral pyridine derivatives as bidentate Hgands (mainly 2,2 -bipyridines, 2,2 6, 2 -terpyridines, phenanthrolines and aminopyridine) in the copper-catalyzed cyclopropanation of styrene by ethyl diazoacetate. The corresponding copper complexes proved to be only moderately active and enantios-elective (ee up to 32% for a C2-symmetric bipyridine). The same authors prepared other chiral ligands with nitrogen donors such as 2,2 -bipyridines 21, 5,6-dihydro-1,10-phenanthrolines 22, and 1,10-phenanthrolines 23 (see Scheme 14) [35]. 

In 1994, the scope of this p-hydroxy sulfoximine ligand was extended to the borane reduction of ketimine derivatives by these workers. The corresponding chiral amines were formed with enantioselectivities of up to 72% ee, as shown in Scheme 10.57. It was found that the A -substituent of the ketimine had a major influence on the asymmetric induction, with a ketoxime thioether (SPh) being the most successful substrate. 

Scheme 8 gives species with extended 109 or starbust -type structures, which are strongly luminescent even with the large ligand L = PCy3.85 The diphenylfluorene derivative shows a remarkable heavy atom effect on the intersystem crossing rate.78... 

In the field of porous supramolecular metal complexes, both molecular and extended-solid materials have been extensively studied in recent years. A particularly well-studied class of compounds is the metal-containing molecular squares, that is, square-shaped porous tetrameric structures (30,108). These have been prepared by several approaches, the most common being the reaction of an organic bridging ligand with a metal complex that has available cis-coordination sites (109-113) (Fig. 13). However, the resulting metal centers are usually coordinatively saturated, which makes it difficult for guest molecules to interact directly with the metal atoms. 

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