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Metal-Ligand Cooperation as Key in Formation of Dearomatized Ni^II-H Pincer Complexes and in Their Reactivity toward CO and CO₂.

ABSTRACT: The unique synthesis and reactivity of [(^RPNP*)NiH] complexes (1a,b), based on metal-ligand cooperation (MLC), are presented (^RPNP* = deprotonated PNP ligand, R = ⁱPr, ^tBu). Unexpectedly, the dearomatized complexes 1a,b were obtained by reduction of the dicationic complexes [(^RPNP)Ni(MeCN)](BF₄)₂ with sodium amalgam or by reaction of the free ligand with Ni⁰(COD)₂. Complex 1b reacts with CO via MLC, to give a rare case of a distorted-octahedral PNP-based pincer complex, the Ni(0) complex 3b. Complexes 1a,b also react with CO₂ via MLC to form a rare example of ?¹ binding of CO₂ to nickel, complexes 4a,b. An unusual CO₂ cleavage process by complex 4b, involving C-O and C-P cleavage and C-C bond formation, led to the Ni-CO complex 3b and to the new complex [(PⁱPr₂NC₂O₂)Ni(P(O)ⁱPr₂)] (5b). All complexes have been fully characterized by NMR and X-ray crystallography.

SUBMITTER: Oren D

PROVIDER: S-EPMC6503609 | biostudies-literature | 2018 Jul

REPOSITORIES: biostudies-literature

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Metal-Ligand Cooperation as Key in Formation of Dearomatized NiII-H Pincer Complexes and in Their Reactivity toward CO and CO2.

Oren Dror D Diskin-Posner Yael Y Avram Liat L Feller Moran M Milstein David D

Organometallics 20180509 14

The unique synthesis and reactivity of [(RPNP*)NiH] complexes (1a,b), based on metal-ligand cooperation (MLC), are presented (RPNP* = deprotonated PNP ligand, R = iPr, tBu). Unexpectedly, the dearomatized complexes 1a,b were obtained by reduction of the dicationic complexes [(RPNP)Ni(MeCN)](BF4)2 with sodium amalgam or by reaction of the free ligand with Ni0(COD)2. Compl ...[more]

PMID: 31080304

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Project description: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.

Dataset Information

Metal-Ligand Cooperation as Key in Formation of Dearomatized Ni^II-H Pincer Complexes and in Their Reactivity toward CO and CO₂.

Publications

Metal-Ligand Cooperation as Key in Formation of Dearomatized Ni<sup>II</sup>-H Pincer Complexes and in Their Reactivity toward CO and CO<sub>2</sub>.

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