Borates and Boronates from Tridentate Ligands

Salicylideneamino alcohols are easily prepared ligands that react readily with arylboronic acids to form the corresponding esters, which may be monomeric 45-49 or dimeric 50-59 (Fig. 16) [87-94]. 

The formation of dimeric products is unique for the case of boron, because analogous complexes with other elements are all monomeric [95]. This can be attributed to the small covalent radius of the boron atom and its tetrahedral geometry in four-coordinate boron complexes. Molecular modeling shows that bipyramidal-trigonal and octahedral coordination geometries are more favorable for the formation of monomeric complexes with these ligands. 

Based on the same synthetic strategy compounds 60-67 can be prepared (Fig. 17). Yields are generally high (69-98%) and the products are air-stable, but have only low solubility. 

A further example of a macrocyclic ring system with boron that is related to the structures of the metallocyclophanes mentioned before is compound 68 that can be prepared via condensation between two bis(borazaphenanthrene) molecules. 

The phenyl ether oxygen atoms allow the two borazaphenanthrene rings to pivot with respect to each other, therefore this dimeric boronic acid anhydride can potentially exist in two isomeric forms, either face-to-face or helical (Fig. 18). In the face-to-face form the boron atoms of the bis(borazaphe-nanthrene) moieties have syn-orientation, while they have approximate anti-orientation in the helical form. Compound 68 has been characterized by X-ray crystallography in the helical form [109]. The dimensions of the cavity can be described by the transannular C C contacts between the carbon atoms in 2-position of the phenyl ether units, which have values of 5.12 and 6.21 A. 

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