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, TlTpBr,Ph,Br] (1) (Tp = hydrotris(pyrazolyl)borate, TlTpBr,p-Tomicronl,Br] (2), and TlTpBr,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, TlTpp-ClPh,4Br] (4) and TlTpPh,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.