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

Two four-coordinate nickel complexes, HB(tBuIm)3NiBr and HB(tBuIm)3NiNO, 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 σ-donor than corresponding tris(pyrazolyl)borates (traditional “scorpionate” ligands). The tris(carbene)borate ligand may also act as a π-acceptor, in contrast to tris(pyrazolyl)borates, which show relatively little π-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(tBuIm)3NiBr 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.