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

Abstract

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.

A Cobalt(I) Pincer Complex with an η2‐Caryl−H Agostic Bond: Facile C−H Bond Cleavage through Deprotonation, Radical Abstraction, and Oxidative Addition

“…If,o nt he other hand, [Co(PCP NMe -iPr)(CNtBu) 2 ]( 2; isocyanides are stronger electron donating ligands than CO) [16,17] is treated with HBF 4 ·Et 2 O, the metal center is now protonated, exclusively affording the octahedral cationic Co III hydride complex [Co(PCP NMe -iPr)(CNtBu) 2 …”

Synthesis and Characterization of Bis(sigma)borate and Bis–zwitterionic Complexes of Rhodium and Iridium

“…Marks and Kolb and Ganem and Osby have very elegantly established the borohydride reductions of various organic molecules as well as the structural features of metal‐borohydrides . Transition metal borohydride complexes exhibit wide coordination chemistry in which the [BH 4 ] − ligand is coordinated via η 1 , η 2 , or η 3 fashion . Moreover, for the isoelectronic nature of [BH 4 ] − and [CH 4 ], borohydrides can be served as structural models for the activation of C−H bonds in saturated hydrocarbons .…”

Variable Borohydride Hapticity in Nickel(II) Scorpionate Complexes [(TpR,Me)Ni(ηn‐BH4)]: TpR,Me = hydrotris{3‐R‐5‐methyl‐1‐pyrazolyl}borate; R = Ph, n = 3 vs. R = Me, n = 4

“…Reactions of borohydride anion with nickel salts and complexes are of interest as routes to metal hydrides and metal borides for catalytic reduction, and hydrogenation,, , as well as H 2 storage , . A related consideration is the biological activity of nickel‐dependent hydrogenases .…”

“…The 3‐center B–H–M bonding and variable hapticity of transition metal–borohydride complexes are also relevant as models for alkane coordination at transition metal centers . Synthesis and characterization of stable nickel borohydride complexes therefore attain some importance; despite some early synthetic and spectroscopic work,, only a handful of nickel borohydride complexes have been structurally characterized , . Attention is also called to structurally characterized complexes of derivative ligands with 3‐center Ni–H–B bonds , …”