Structural characterization and electrochemical properties of nickel (II) complexes bearing sterically bulky hydrotris(3-phenyl)- and hydrotris(3-tert-butylpyrazol-1-yl)borato ligands

Abstract

Two series of pseudo-tetrahedral nickel (II) complexes bearing the bulky hydrotris(3-phenyl)- or hydrotris(3-tert-butylpyrazol-1-yl)borato ligands, TpPhNiL and TptBuNiL respectively, have been synthesized and characterized with the intent of investigating the affects of substituent size on the redox properties of the nickel center (L=Cl, Br, I, NCS, NO3). X-ray analysis reveals both Tp ligands bind in a κ3 fashion with the R groups pointing out towards the metal producing a pocket in which the 4th ligand, L, resides. The TpPh ligand results in a larger pocket and produces nickel complexes with distorted solid-state geometries, specifically bent B…Ni…L angles, unlike the corresponding TptBu complexes which show much less variation in structure. Cyclic voltammetry experiments indicate all complexes undergo a Ni2+/1+ reduction at large negative potentials vs. ferrocene with varying levels of reversibility. The NCS complexes are the most reversible and the easiest to reduce with an average reduction potential of −1.51V due to the slightly electron withdrawing nature of the thiocyanate ligand. The strong donor properties of the bidentate nitrate ligand result in complexes that are the most difficult to reduce at an average of −1.80V. Overall, the TpPhNiL complexes are easier to reduce than their corresponding TptBuNiL complexes resulting from the weakened donor ability of the TpPh ligand. While the two ligands do produce different structural motifs, the electrochemical trends are more in line with the electronic properties of the complexes than structural differences.