STRONGLY DONATING SCORPIONATE LIGANDS

Abstract

The success of tris(pyrazolyl)borates has inspired the development of new scorpionate ligands that are based on other donor groups. These include borate ligands with thioimidazolyl, selenoimidazolyl, phosphine and N-heterocyclic carbene donors, ligands that provide a facial array of [S₃], [Se₃], [P₃], and [C₃] donors, respectively. These ligands are all more strongly donating than the tris(pyrazolyl)borates, with tris(carbene)borate ligands having the greatest donor strength. Despite the design similarities, strongly donating scorpionates differ in their topology and flexibility, donor properties and degradation pathways. The structural and electronic properties of these ligands are contrasted and related to their chemistry, particularly that of the transition metals. Bidentate congeners of all these ligands are also known and are briefly covered. The ability of strongly donating scorpionate ligands to stabilize metal ligand multiple bonds in late transition metal complexes is also briefly discussed.

ABBREVIATION:

• PhBP^R – phenyltris(phosphinoborate)
• PhBC^R – phenyltris(imidazol-2-ylidine)borate
• HBC^R – hydrotris(imidazol-2-ylidine)borate
• HBS^R – hydrotris(thioimidazolyl)borate
• HBSe^R – hydrotris(selenoimidazolyl)borate
• Tp – hydrotris(pyrazolyl)borate
• Tp∗ – hydrotris(3,5-dimethylpyrazolyl)borate
• DFT – Density Functional Theory
• Ad – Adamantyl
• dbabh – 2,3:5,6-dibenzo-7-azabicyclo[2.1.1]hepta-2,5-diene
• NHC – N-heterocyclic carbene
• Fc – Ferrocene

ACKNOWLEDGMENTS

I would like to thank NMSU, the Petroleum Research Foundation and Department of Energy (DE-FG02-08ER15996) for funding my research, as well as graduate and undergraduate students who have contributed to my research program. I also thank Pat Holland (University of Rochester) for providing access to the Cambridge Structural Database.

Notes

^a Two NO stretches observed.

^a Complex was spectroscopically characterized but not isolated.

^a Additional stretches attributed to the presence of two independent molecules in the unit cell.

^a Quasireversible.

^b In C₂H₄Cl₂, NBu₄PF₆ electrolyte.

^a Measured in MeCN, NBu₄PF₆ electrolyte.

^b Measured in liquid SO₂, − 40°C, NBu₄PF₆ electrolyte.

^c Cp₂Fe²⁺ is reported to be unstable under these conditions.

^a Cone angles were calculated from crystallographic data using the program Steric. ^{[ 143 ]}