An investigation of cyanopyrazole ligands and cyanopyrazole scorpionate ligands in the creation of novel transition metal complexes

Abstract

Pyrazole ligands are popular ligands in inorganic and bioinorganic chemistry due to isoelectronic structural isomers of imidazole. Pyrazole ligands can combine with metals to make mononuclear structures, polynuclear structures, all the way up to polynuclear polymers. Pyrazoles are often combined with boron to create larger tridentate ligands called scorpionates. Their uses are numerous. Another reason pyrazole ligands are popular is their ability to be modified. Substitutions at the 3, 4 and 5 positions of the pyrazole ring allow for numerous combinations of both steric and electronic effects that can affect the final structures that form when forming organometallic complexes. This effect is amplified in the scorpionate ligand, sometimes allowing only a small number of metals to bind to the ligand structure. Work with cyanopyrazoles, pyrazoles with a nitrile group at the 4-position, can result in both similar and novel structures compared to their non-cyano analogs due to the electron withdrawing effect of the cyano group and the ability of the cyano group to form an isocyanate dative bond with certain metals. These cyanopyrazoles are the main components in cyanoscorpionate ligands. The three main kinds of scorpionates are the bidentate dihydrobispyrazolylborate (Bp) ligands, tridentate hydrotrispyrazolylborate (Tp) ligands and tetradentate tetrapyrazolylborate (pzTp). These scorpionate ligands can be used to make inorganic complexes such as polymers or structures to measure the property changes of a metal by altering sidechains and secondary ligands. Scorpionates can also be used in bioinorganic complexes to try to emulate the properties of metallo enzymes found throughout biology.