1
Introduction to Cobalt Chemistry and Catalysis

Marko Hapke1,2, and Gerhard Hilt3

1Johannes Kepler University Linz, Institute for Catalysis (INCA), Altenberger Strasse 69,, 4040 Linz, Austria

2Leibniz Institute for Catalysis e.V. at the University of Rostock (LIKAT), Albert-Einstein-Strasse 29a,, 18059 Rostock, Germany

3Carl von Ossietzky Universität Oldenburg, Institut für Chemie, Carl-von-Ossietzky-Strasse 9-11,, 26111 Oldenburg, Germany

1.1 Introduction

Cobalt (Co) is the first and lightest element among the group 9 transition metals, further members being rhodium (Rh), iridium (Ir), and meitnerium (Mt). In contrast to their significance in organic synthesis and catalysis, cobalt is by far the most abundant element of the group in the geosphere, compared with rhodium and iridium as its heavier congeners (Co:Rh:Ir = c. 104 : 5 : 1) [1]. While rhodium and iridium complexes have been at the forefront of organotransition metal chemistry with relation to organic syntheses, steadily enabling novel and often unprecedented transformations of simple starting materials to complex products or opening the gate to novel fields of catalysis as has happened with C-H functionalisation reactions, cobalt stood back for a long time. Expression for the different significance of the three transition metals is also found in the literature, as monographs for either rhodium and iridium as catalyst metals for organic synthesis have already been published [2,3]. However, some direct comparisons of the application of group 9 metals for organic synthesis and catalysis can be found in the literature [4]. Next to its membership in the first row of the transition metals, relative abundance, and biorelevance, it is also considered a sustainable metal, among other elements in this nowadays particularly important field [5].

Cobalt (the name is derived from the German word "Kobold" meaning goblin, due to the behaviour and confusion with silver-copper ores in medieval mining) has been isolated for the first time in 1735 by the Swedish chemist Georg Brand, who also recognised its elemental character. It is an essential trace element for humans and animals, and its main purpose is the constitution of vitamin B12 (cobalamin), which has an important role for the regeneration of erythrocytes. Cobalamines are organometallic compounds with cobalt-carbon bonds, possessing cobalt in the oxidation states +1 to +3, and provide the only known cobalt-containing natural products.

Beside the importance for the human physiology, cobalt has evolved from an unwanted and downright abhorred element during silver and copper mining to a metal of strategic industrial importance and in recent years also a rising young star in homogeneous catalysis. How does this chemical version of "rags to riches" come into play? One modern reason is the importance of cobalt as metal used in high-performance alloys (e.g. stellite), permanent magnets, rechargeable batteries, cell phones, and many more technical applications [6]. Requirements of our modern society with respect to the production of chemicals and materials also heavily rely on the late, rare, and rather expensive platinum group metals (PGM). The implementation of sustainability and efficiency thus leading the way to explore the earth-abundant metals for both homogeneous and heterogeneous catalytic purposes [7,8].

From a chemical and catalytical point of view, cobalt already inherits the role of a major player in the awakening of homogeneous organometallic catalysis in the first half of the twentieth century [9]. Otto Roelen at Ruhrchemie (now Oxea) in Oberhausen discovered the "oxo synthesis" in 1938, today named hydroformylation reaction, and introduced HCo(CO)4 as catalyst for this reaction. Still today beside rhodium as metal with higher reactivity cobalt complexes are used as catalysts. Basis for this reaction was work from Walter Hieber on the synthesis of carbonyl metallates via the so-called "Hieber base reaction", affording H2Fe(CO)4 by the reaction of Fe(CO)5 with NaOH. Because for cobalt no mononuclear binary carbonyl compound is known, therefore the related compound HCo(CO)4 was generated from the prominent carbonyl complex Co2(CO)8 by reductive splitting with sodium metal and protonation or even directly by oxidative splitting by molecular hydrogen itself (Scheme 1.1). The resulting cobalt carbonyl hydride is a proton donor, able to protonate water with an acidity comparable to sulfuric acid.

Scheme 1.1 Synthesis of cobalt carbonyl hydride (the reaction with H2 can be reversible).

The mechanism of the hydroformylation process using HCo(CO)4 and related compounds HCo(CO)3L (L = phosphine) has been studied in great detail, first proposed by Breslow and Heck [10]. Scheme 1.2 displays the now generally accepted mechanistic pathway for the cobalt-catalysed process [11]. Starting from the hydridic HCo(CO)4, reversible dissociation of a CO ligand followed by reversible olefin coordination led to migratory insertion, which would pave the way to either the n-aldehyde or iso-aldehyde, depending on the course of the insertion. Following the reaction cycle, alkyl migration led to formation of an acylcobalt species, which after oxidative addition of hydrogen was reductively eliminated as the n-aldehyde. This catalytic cycle combines all the significant elementary steps of homogeneous catalysis with metal complexes and provides a taste on the complexity for studying such reaction mechanisms in detail. Interest and detailed studies in these first molecularly defined catalysts for the purpose of synthesising structurally advanced organic molecules has since filled the knowledge of organometallic chemistry.

Scheme 1.2 Mechanism of the classical cobalt-catalysed hydroformylation reaction of terminal olefins.

1.2 Organometallic Cobalt Chemistry, Reactions, and Connections to Catalysis

Cobalt is a d9-metal and the by far mostly frequently occurring oxidation states in its compounds are -1, 0, +1, +2, and +3. The latter oxidation states also play the major role in stoichiometric/catalytic reactions, while complexes with the oxidation states -1 and 0 are found in some prominent complexes and starting materials. The preference of formal +1/+3 oxidation states in many catalytic transformations is in close relation to the catalytic behaviour of the heavier congeners, rhodium and iridium. In general, the largest number of contemporary catalytic processes include a catalyst generation step, in which, e.g. Co(II) salts are introduced, together with an appropriate ligand and a reducing agent or other additives to lower the oxidation state to +1, from which the species enters the catalytic cycle. On the other hand, a large number of organometallic compounds based on the unsubstituted cyclopentadienyl (Cp), related substituted cyclopentadienyl (Cp´), or pentamethylcyclopentadienyl (Cp*) ligands are reported and well known, beside numerous isolated complexes with P- and N-donor atom-containing ligands. However, the coordination and organometallic chemistry of cobalt is a wide and multifaceted field and has been involved in ground-breaking research in either area [12].

Cobalt is also a widely used catalyst metal for heterogeneously catalysed processes. Especially the famous Fischer-Tropsch process is still relying on cobalt as the principal catalyst metal, as it was already from the initial reports on this large-scale industrial process [13]. Further modern applications in heterogeneous catalysis are often related to the conversion of small molecules in steam-reforming or partial oxidation processes (ethanol, methane) towards the formation of syngas, together with other applications for the allocation of clean energy. A highly current topic is therefore, e.g. the use of cobalt in heterogeneously catalysed electrochemical water splitting [14] or the reduction of CO2 on cobalt-containing surfaces [15]. Analysis of the chemistry and catalytic performance of cobalt on surfaces is still a topic of ongoing investigations [16].

1.2.1 Cobalt Compounds and Complexes of Oxidation States +3 to -1

Cobalt is an electron-rich transition metal, like its latter group congeners; however, it is a first-row transition metal, which inherits also significant differences. Due to its electron richness, it belongs to the so-called "base metals", including the neighbouring first-row transition metals manganese, iron, nickel, and copper. The abundance of low oxidation states (0, -1) is, however, quite unique for cobalt and also rather known for the compounds of neighboring iron than for the heavier metals of group 9. Comparable especially to rhodium catalysis is the oxidation state +3 as usually highest occuring state during catalytic reactions.

1.2.1.1 Co(III) Compounds

Isolated cobalt complexes in the oxidation state +3 are most often found in coordination compounds, because the d6...