Thallium(I) Complexes of Tris(pyridyl)borates and a Comparison to Their Pyrazolyl Analogues

Abstract

Thallium(I) complexes of B-methylated and B-phenylated tris(pyridyl)borates featuring trifluoromethyl groups at the pyridyl ring 6-positions have been synthesized by metathesis using the corresponding potassium salts [MeB(6-(CF3)Py)3]K and [PhB(6-(CF3)Py)3]K with thallium(I) acetate. The closely related tris(pyrazolyl)borate analogue [PhB(3-(CF3)Pz)3]Tl has also been prepared, and comparisons of structural and spectroscopic features between the two scorpionate families are presented. [MeB(6-(CF3)Py)3]Tl displays κ3-coordination of the tris(pyridyl)borate similar to that of tris(pyrazolyl)borate in [MeB(3-(CF3)Pz)3]Tl, while [PhB(6-(CF3)Py)3]Tl and [PhB(3-(CF3)Pz)3]Tl feature κ2-N,N ligand coordination modes with the B-phenyl groups flanking the thallium sites. 19F NMR spectroscopy of [MeB(6-(CF3)Py)3]Tl reveals the presence of a remarkably large 1208 Hz four-bond thallium-fluorine coupling constant in chloroform at room temperature, which is considerably larger than 878 Hz observed for the pyrazolyl borate analogue [MeB(3-(CF3)Pz)3]Tl. Although [PhB(6-(CF3)Py)3]Tl is structurally nonrigid at room temperature in chloroform, at lower temperatures, the ligand arm exchange slows down, revealing 4JTl-F = 1110 Hz. Steric demands of these ligands have been quantified using the buried volume concept. In addition, ligand transfer chemistry from [MeB(6-(CF3)Py)3]Tl and [PhB(6-(CF3)Py)3]Tl to copper(I) under ethylene and computational analyses of the various coordination modes of tris(pyrazolyl)borates and tris(pyridyl)borates are reported.