Ligands with extended 77-systems

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... 

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). 

Synthesis. In attempts to further extend the 7i-system of the metal dithiolene core of 2a, Amb and Rasmussen reported the preparation of a 2,2 -bithiophenedithiolate ligand to synthesize [Bu4N][4a] (Seheme 3) in 2007. The resulting anion 4a represents a funetionalized analogue of 2a, with the %-system extended through the a-thienyl groups coupled to the exterior a-positions of the 2a core. This initial synthetic approach was then optimized and used to produce a series of extended metal thiophenedithiolene eomplexes 4a-e as shown in Scheme 3. ... 

The reaction of hexacyanometalates with metal complexes chelated by penta-dentate ligands may afford polynuclear complexes. The presence of the penta-dentate ligand precludes the polymerization that leads to extended systems. The preparation of a representative heptanuclear, mixed-valance iron complex, [Fe (CNFe° (salmeten))6]Cl2 6H20, is detailed herein. 

Although the vast majority of this review has been concerned with homogeneous systems, supported catalyst and single-crystal studies of rhodium are important topics that have also been considered by some researchers. This topic extends and dovetails nicely with the discussion of the interactions of acyclic and cyclic polyamine ligands and Schiff... 

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