Boron-Centered Scorpionate-Type NHC-Based Ligands and Their Metal Complexes

The success enjoyed in the last 50 years by use of poly(pyrazolyl)borates as ligands has inspired the development of a great number of scorpionates containing other donor elements. These include polytopic bis- and tris(NHC)borate ligands, featuring N-heterocyclic carbene (NHC) moieties that generally coordinate as bidentate κ2C or tridentate κ3C, respectively. In spite of structural similarities between poly(pyrazolyl)borates and poly(NHC)borates, their methods of synthesis and their reactivity are different and reflect the variations in topology, flexibility, and donor properties. The structural and electronic properties of poly(NHC)borate ligands are compared and correlated with their coordination chemistry, particularly towards transition metals. The advances in the chemistry of scorpionate-type bis- and tris(NHC)borate ligands are reviewed by highlighting and comparing their structural properties and their ability to stabilize low to high metal oxidation states in transition-metal complexes.