Unusual Polybrominated Polypyrazolylborates and Their Copper(I) Complexes: Synthesis, Characterization, and Catalytic Activity

Five new homoscorpionate ligands were prepared and structurally characterized as their Tl complexes, three of
which, Tl[TpBr,Ph,Br] (1) (Tp = hydrotris(pyrazolyl)borate, Tl[TpBr,p-Tοl,Br] (2), and Tl[TpBr,p-ClPh,Br] (3), are unique in
being the first examples of an “atypical” B−N bond to the most sterically hindered pyrazole nitrogen. They contain
bromine atoms on the central and outer carbons of the pyrazole ring, with all aryl substituents in the 5-position of
the ligand, forming a protective pocket around the B−H bond. These complexes display a rather high B−H stretch
frequency (above 2 600 cm-1) in the IR region. Two other ligands, Tl[Tpp-ClPh,4Br] (4) and Tl[TpPh,Me,Br] (5), containing
no outer bromine substituents, have normal B−N bonding to the least-hindered nitrogen. These new ligands have
been employed to prepare the series of complexes TpxCu(NCMe) (6−10), for which X-ray studies of two of them
(7 and 10) have shown that the atypical or normal geometry of the ligands is maintained when complexed to the
copper center. The new complexes have also been tested as the catalysts in carbene and nitrene transfer reactions
providing moderate to high yields in the expected products.