$H_2$ and $CO_2$ conversion at a cobalt center supported by an acridane based pincer-type ligand

Abstract

Conversion of $CO_2$ to value-added products has attracted much attention in recent years, because carbon dioxide can be used as an inexpensive and renewable carbon resource. Although significant efforts have been made to find the appropriate catalytic reactions for the $CO_2$ conversion, only few examples are industrially applicable in an economic perspective, which remains as one of the major challenges. Particularly, hydrogenation of $CO_2$ to produce formic acid is a reasonable and realistic candidate, but most of the known reported hydrogenation reactions, unfortunately, require expensive, heavy and earth-rare 2nd- and 3rd-row transition metal catalysts. Thus, the development of efficient catalysts using 1st-row transition metals obviously is timely needed. In general, the cheap and environmentally benign 1st-row transition metal catalysts are prone to reveal low catalytic performance for $CO_2$ hydrogenation. This is due to the weak metal-ligand bonding, which even leads to the catalyst decomposition under catalytic conditions. To solve this particular problem, a series of pincer metal complexes and related systems have been employed and their catalytic hydrogenation of $CO_2$ were studied. Recent advances in the development of the catalytic $CO_2$ hydrogenation and the modification of pincer ligand scaffolds are reviewed in Chapter 1. In Chapter 2, the preparation and characterization of three-coordinate monovalent cobalt complex with a structurally rigidified $^{acri}PNP$ ligand ($^{acri}PNP^–$ = 4,5-bis(diisopropylphosphino)-2,7,9,9-tetramethyl-9H-acridine-10-ide) will be described. A diamagnetic T-shaped cobalt(I) center having the low spin ground state allows the coordination of various σ-donors, such as CO, $PMe_3$, $N_2$, $H_2$ and silane. In the case of $H_2$ coordination, A non-classical dihydrogen species of cobalt(I) was successfully generated and characterized by various spectroscopic methods. In general, a three-coordinate high-spin cobalt(I) species typically reveals oxidative addition of $H_2$ to give cobalt(III) dihydride species, but this low-spin cobalt(I) species, surprisingly, undergoes a hydrogen atom transfer (HAT) reaction to generate a cobalt(II)-hydride species. Although the reactivity of this species clearly different from other high-spin species, the proper assignment of the corresponding cobalt-$H_2$ adduct is not satisfied due to the limitation of X-ray crystallography. As a model study, phenylsilane was utilized to isolate a cobalt(I) sigma complex, which was confidently assigned based on the solid-state structure. The full characterizations of a series of cobalt complexes and the related reactivity study including HAT are discussed. In chapter 3, catalytic $CO_2$ hydrogenation of cobalt catalysts supported by two different diphosphinoamido ligands will be discussed. The corresponding $CO_2$ reaction of two cobalt complexes with dihydrogen reveals that the catalysis undergoes via following three steps

1) insertion of carbon dioxide to a cobalt(II)-hydride bond to generate a cobalt-formate species, 2) heterolytic cleavage of dihydrogen with a cobalt-formate species in the presence of an external base, and 3) the anionic formate extrusion from the cobalt(II) center. Catalytic performance of two cobalt complexes supported by geometrically different diphosphinoamido ligands will be discussed. The resulting product, a [DBUH][formate] salt, leads to catalyst decomposition via protonation of a central amido group in the ligand scaffold followed by dissociative ligand loss. The hydrolytic stability toward an accumulated conjugated acid $[DBUH]^+$ was dramatically improved in the structurally rigidified ligand scaffold and the reduced basicity of the amido group leads to higher catalyst performance relative to the original ligand. In the last chapter, a series of five coordinate cobalt complexes having a redox non-innocent moiety will be presented. By using a ($^{acri}PNP$)Co scaffold, a series of bidentate ligands, such as 2,2’-bipyridine, 1,10-phenanthroline, 2-(pyridin-2-yl)benzene-1-ide, benzo[h]quinolin-10-ide and biphenyl-2,2’-diyl were added. The resulting five coordinate cobalt complexes having the formally 1+, 2+ and 3+ oxidation states were well-characterized by using various spectroscopic techniques and X-ray crystallography. To assign the oxidation state of cobalt, X-ray absorption spectroscopy has been employed and a series of cobalt complexes were carefully evaluated.