Exploring opportunities for tuning phenyltris(pyrazol-1-yl)borate donation by varying the extent of phenyl substituent fluorination

Abstract

The importance of electron deficient Tp ligands motivates the introduction of electron-withdrawing substituents into the scorpionate framework. Since perfluorophenyltris(pyrazol-1-yl)borate affects significant anodic shifts in half-cell potentials in their metal complexes relative those of phenyltris(pyrazol-1-yl)borate analogues, the tuning opportunities achieved using 3,4,5-trifluorophenyl- and 3,5-bis(trifluoromethyl)phenyl(pyrazol-1-yl)borates were explored. Bis(amino)boranes ((3,4,5-F)C₆H₂)B(NMe₂)₂ and ((3,5-CF₃)C₆H₃)B(NMe₂)₂ are precursors to fluorinated tris(pyrazol-1-yl)phenylborates. Thallium salts of these scorpionates exhibit bridging asymmetric κ³-N,N,N coordination modes consistent with the reduced π-basicity of the fluorinated phenyl substituents relative those of other structurally characterized tris(pyrazol-1-yl)phenylborates. While a comparative analysis of the spectral and X-ray crystallographic data for classical Mo(0), Mo(II), Mn(I), Fe(II) and Cu(II) complexes of [((3,4,5-F)C₆H₂)Bpz₃]⁻ and [((3,5-CF₃)C₆H₃)Bpz₃]⁻ could not differentiate these ligands with respect to their metal-based electronic impacts, cyclic voltammetry suggests that 3,4,5-trifluorophenyl- and 3,5-bis(trifluoromethyl)phenyl(pyrazol-1-yl)borates affect similar anodic shifts within their metal complexes, with coordination of [((3,5-CF₃)C₆H₃)Bpz₃]⁻ rendering metal centers more difficult to oxidize, and sometimes even more difficult to oxidize than their [C₆F₅Bpz₃]⁻ analogues. These data suggest that the extent of phenyl substituent fluorination necessary to minimize metal center electron-richness in phenyltris(pyrazol-1-yl)borate complexes cannot be confidently predicted.