Synthesis, Structure, and Reactivity of Co(II) and Ni(II) PCP Pincer Borohydride Complexes.

The 15e square-planar complexes [Co(PCP-Pr)Cl] () and [Co(PCP-Bu)Cl] (), respectively, react readily with NaBH to afford complexes [Co(PCP-Pr)(η-BH)] () and [Co(PCP-Bu)(η-BH)] () in high yields, as confirmed by IR spectroscopy, X-ray crystallography, and elemental analysis. The borohydride ligand is symmetrically bound to the cobalt center in η-fashion. These compounds are paramagnetic with effective magnetic moments of 2.0(1) and 2.1(1) μ consistent with a d low-spin system corresponding to one unpaired electron. None of these complexes reacted with CO to give formate complexes. For structural and reactivity comparisons, we prepared the analogous Ni(II) borohydride complex [Ni(PCP-Pr)(η-BH)] () via two different synthetic routes. One utilizes [Ni(PCP-Pr)Cl] () and NaBH, the second one makes use of the hydride complex [Ni(PCP-Pr)H] () and BH·THF. In both cases, is obtained in high yields. In contrast to and , the borohydride ligand is asymmetrically bound to the nickel center but still in an η-mode. [Ni(PCP-Pr)(η-BH)] () loses readily BH at elevated temperatures in the presence of NEt to form . Complexes and are both diamagnetic and were characterized by a combination of H, C{H}, and P{H} NMR, IR spectroscopy, and elemental analysis. Additionally, the structure of these compounds was established by X-ray crystallography. Complexes and react with CO to give the formate complex [Ni(PCP-Pr)(OC(C=O)H] (). The extrusion of BH from [Co(PCP-Pr)(η-BH)] () and [Ni(PCP-Pr)(η-BH)] () with the aid of NH to yield the respective hydride complexes [Co(PCP-Pr)H] and [Ni(PCP-Pr)H] () and BHNH was investigated by DFT calculations showing that formation of the Ni hydride is thermodynamically favorable, whereas the formation of the Co(II) hydride, in agreement with the experiment, is unfavorable. The electronic structures and the bonding of the borohydride ligand in [Co(PCP-Pr)(η-BH)] () and [Ni(PCP-Pr)(η-BH)] () were established by DFT computations.