Coordination chemistry bispidine ligands

First transition metal coordination compounds with bidentate bispidine ligands were described in 1957 (30). The initial report with metal complexes of tetradentate bispidine ligands dates back to 1969 (31). Following these early reports, there have been a number of studies on the complexation properties of several bipidine derivatives (32-35). However, extensive, broad, and thorough studies of the bispidine coordination chemistry began only <10 years ago. These studies will be reviewed here. They include structural and theoretical work, spectroscopy, electron-transfer studies, metal ion selective complexation, and applications in biomimetic chemistry, catalysis, and molecular magnetism. 

Another interesting feature for the coordination chemistry of bispidine-type ligands is that diamines may be used in their synthesis. This hnding gives us access to ditopic ligand systems (see Chart 7) (24, 81, 82). 

Aromatic substituents at C2 and C4 are to some extent hindered in rotation. This leads to a third type of isomerism in bispidine chemistry (i.e., atropisomer-ism). The activation energy for the rotation around the C2/C4—aryl bond for bispidones with various meta-substituted phenyl groups was determined by various NMR methods and found to be 70-75 kJ mol (23). For a rotation of 180°, which is usually necessary for the coordination of bispidine ligands to metal ions (e.g., 14, see Scheme 14), two energy barriers have to be overcome. The higher is the result of an interaction of the ortho-disposed proton of the aromatic ring with the proton or the alkyl substituent at the N3 amine nitrogen atom. The... 

While much of the interesting properties of bispidine complexes are based on this difference, it certainly is of interest to develop the coordination chemistry of the isomeric tetradentate ligand. (2.) While 14 and its complexes have Cs symmetry the ligand and complex of the isomer shown in Fig. 2(c) are C2 symmetric, and therefore chiral. If catalytically active (such as complexes based on 14), complexes derived from that shown in Fig. 2(c) should be interesting for enantioselective reactions. 

The copper coordination chemistry of bispidine ligands has been studied extensively, and this has been particularly rewarding (70, 71, 81, 82, 168, 169, 192, 194, 196, 199, 201, 213, 214). The main reasons are that (1.) the bispidine backbone is complementary with respect to copper(ll) and, therefore, complex stabilities may be relatively high, comparable to those with macrocyclic ligands (69, 201), and that modifications of the ligand backbone can be used to tune the stabilities and redox potentials (199) (2.) due to the ligand rigidity, the copperfll)... 

An interesting possible further extension is the functionalization of bispidine ligands with hydrophobic groups, for example, for metal ion selective extractions (69, 339). biopolymers for nuclear medicinal applications (340), solids for heterogeneous catalysis and sensors, or additional coordination sites for the synthesis of heterodinuclear complexes with applications in biomimetic chemistry, catalysis, and as luminescence sensors. There is a variety of possible sites for ligand modification. Of particular interest is the C9 position, which has been selectively and stereospecifically reduced to an alcohol (190), and the two hydrolyzed C1,C5 ester groups (167). 

Many of these new developments are related to the very specific properties of bispidine ligands, others might also have been observed with other ligand systems. New bispidine ligands (Section 11) whose coordination chemistry has not yet been developed in detail, novel bispidine chromophores (Section... 

Borzel, H., Comba, P., Hagen, K. S. et al., Iron coordination chemistry with tetra-, penta- and hexaden-tate bispidine-type ligands, Inorg. Chem. Acta, 337 407-419, 2002. 

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