Ferrocenyl-substituted tris(pyrazolyl)borates--a new ligand type combining redox activity with resistance to hydrogen atom abstraction.

The low-temperature syntheses of ferrocenyl-substituted tris(pyrazolyl)borate ligands Tp(Fc*) (hydrobis(3-ferrocenylpyrazolyl)mono(5-ferrocenylpyrazolyl)borate), Tp(Fc,Me*) (hydrobis(3-ferrocenyl-5-methylpyrazolyl)mono(5-ferrocenyl-3-methylpyrazolyl)borate), and Tp(Fc,iPr) (hydrotris(3-ferrocenyl-5-isopropylpyrazolyl)borate) are reported. The Tl salts of Tp(Fc*) and Tp(Fc,Me*) can be thermally isomerized to the symmetric Tp(Fc) (hydrotris(3-ferrocenylpyrazolyl)borate) and Tp(Fc,Me) (hydrotris(3-ferrocenyl-5-methylpyrazolyl)borate) species, respectively. Conversely, upon heating, the thermal isomerization of Tp(Fc,iPr) results in the generation of a mixture of regioisomers. These ligands display a reversible three-electron oxidation. The preparations of Tp(CF3,Fc)Tl (hydrotris(3-trifluoromethyl-5-ferrocenylpyrazolyl)borate) and PhTp(Fc) (phenyltris(3-ferrocenylpyrazolyl)borate) are also reported.