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

The 15e square-planar complexes [Co­(PCPMe-iPr)­Cl] (2a) and [Co­(PCP-tBu)­Cl] (2b), respectively, react readily with NaBH4 to afford complexes [Co­(PCPMe-iPr)­(η2-BH4)] (4a) and [Co­(PCP-tBu)­(η2-BH4)] (4b) 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 η2-fashion. These compounds are paramagnetic with effective magnetic moments of 2.0(1) and 2.1(1) μB consistent with a d7 low-spin system corresponding to one unpaired electron. None of these complexes reacted with CO2 to give formate complexes. For structural and reactivity comparisons, we prepared the analogous Ni­(II) borohydride complex [Ni­(PCPMe-iPr)­(η2-BH4)] (5) via two different synthetic routes. One utilizes [Ni­(PCPMe-iPr)­Cl] (3) and NaBH4, the second one makes use of the hydride complex [Ni­(PCPMe-iPr)­H] (6) and BH3·THF. In both cases, 5 is obtained in high yields. In contrast to 4a and 4b, the borohydride ligand is asymmetrically bound to the nickel center but still in an η2-mode. [Ni­(PCPMe-iPr)­(η2-BH4)] (5) loses readily BH3 at elevated temperatures in the presence of NEt3 to form 6. Complexes 5 and 6 are both diamagnetic and were characterized by a combination of 1H, 13C­{1H}, and 31P­{1H} NMR, IR spectroscopy, and elemental analysis. Additionally, the structure of these compounds was established by X-ray crystallography. Complexes 5 and 6 react with CO2 to give the formate complex [Ni­(PCPMe-iPr)­(OC­(CO)­H] (7). The extrusion of BH3 from [Co­(PCPMe-iPr)­(η2-BH4)] (4a) and [Ni­(PCPMe-iPr)­(η2-BH4)] (5) with the aid of NH3 to yield the respective hydride complexes [Co­(PCPMe-iPr)­H] and [Ni­(PCPMe-iPr)­H] (6) and BH3NH3 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(PCPMe-iPr)­(η2-BH4)] (4a) and [Ni­(PCPMe-iPr)­(η2-BH4)] (5) were established by DFT computations.