Anionic Pyrazolylborate Ligands

The poly(pyrazolyl)borohydrides 1, 2 and 3 are the anionic pyrazolylborate ligands which form stable complexes with transition metal ions.¹⁾ In particular, 1 forms tetrahedral or square planar complexes with divalent metal ions. The complexes are soluble in organic solvents, and therefore 1 is used for the extraction of divalent metal ions.²⁾ 2 and 3 are utilized as tridentate ligands for homogeneous metal complex catalyst which are used for regioselective hydrogenation of quinoline^3a) and reduction of unactivated ketones.^3b)