Synthesis and spectroscopic investigations of four-coordinate nickel complexes supported by a strongly donating scorpionate ligand

Two four-coordinate nickel complexes, HB((t)BuIm)(3)NiBr and HB((t)BuIm)(3)NiNO, were prepared by reaction of a bulky tris(carbene) borate ligand with NiBr2(PPh3)(2) and NiBr(NO)(PPh3)(2), respectively, and structurally and spectroscopically characterized. In addition to standard techniques, high-frequency and -field electron paramagnetic resonance (HFEPR) was employed to understand the spin triplet (S = 1) ground state of the bromo complex. HFEPR, combined with electronic absorption spectroscopy allows comparison of this novel complex with other paramagnetic four-coordinate Ni(II) species. The tris(carbene) borate ligand is a stronger sigma-donor than corresponding tris(pyrazolyl)borates (traditional "scorpionate" ligands). The tris(carbene) borate ligand may also act as a pi-acceptor, in contrast to tris(pyrazolyl) borates, which show relatively little pi-bonding interactions. The influence of tris(carbene) borate substituents on the donor strength of the ligand have been elucidated from IR spectroscopic investigations of {NiNO}(10) derivatives. HFEPR spectra of HB((t)BuIm)(3)NiBr exhibit hyperfine coupling from Br, which indicates the strong electronic interaction between Ni(II) and this halide ligand, consistent with studies on tris(pyrazolyl) borate Ni(II) complexes. (C) 2009 Elsevier B.V. All rights reserved.