Boron Heterocycles as Ligands in Transition-Metal Chemistry

This chapter discusses the ligand properties of Lewis acid boron heterocycles and the newest developments in the expanding field of triple- and tetra-decker sandwich compounds. Replacement of carbon atoms by boron atoms in π-carbocyclic and heterocyclic systems leads to a wide variety of planar Lewis acid boron heterocycles. Six-membered Lewis acid ligands can be derived from benzene by substituting boron, nitrogen, and sulfur for carbon. As an alternative to the aromaticity formalism commonly used, compounds containing electron-deficient elements in addition to metal atoms can be regarded as clusters. In clusters composed solely of main-group elements, the following electronic relationship among closo, nido, and arachno structures has been found. In the paramagnetic electron-rich bis(borinato)cobalt complexes, the labilized metal-ligand interaction allows ligand exchange and transfer to other metals. Cyclic voltammetry of (borinato)cyclopentadienyliron and of bis(borinato) complexes of iron, chromium, and vanadium revealed oxidation of the iron complexes with varying degrees of reversibility, depending on the solvent. Between benzene and borazine, several isomeric boron-carbon-nitrogen heterocycles would be expected to exist as potential η6-ligands. In comparison with nitrogen, a sulfur atom is a “soft” electron donor, and therefore sulfur-containing ligands should readily form transition-metal complexes. The complexes decompose slowly in C6H6 and CH2CI2 solution but are stable in dioxane. The most interesting feature of electron-poor boron heterocycles in complex chemistry is their use as bridging ligands in di- and trinuclear complexes.