# Chapter 1: Introduction

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## Chapter 1: Introduction

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A personal look at the history of aqueous organometallic catalysis “Organometallic chemistry deals with moisture sensitive compounds therefore all manipulations should be carried out under strictly anhydrous conditions” – this was the rule of thumb ever since the preparation of the first organometallic compounds. Not as if there were no isolated examples of water-stable organometallics from the very beginning, in fact Zeises salt, was prepared as early as 1827. Nevertheless, it is true, that compounds having highly polarized M-C, M-H etc. bonds may be easily decomposed in water by protonation. In other cases, oxidative addition of or oxygen abstraction from...

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## Nội dung Text: Chapter 1: Introduction

1. Chapter 1 Introduction 1.1 A personal look at the history of aqueous organometallic catalysis “Organometallic chemistry deals with moisture sensitive compounds therefore all manipulations should be carried out under strictly anhydrous conditions” – this was the rule of thumb ever since the preparation of the first organometallic compounds. Not as if there were no isolated examples of water-stable organometallics from the very beginning, in fact Zeises salt, was prepared as early as 1827. Nevertheless, it is true, that compounds having highly polarized M-C, M-H etc. bonds may be easily decomposed in water by protonation. In other cases, oxidative addition of or oxygen abstraction from water leads to formation of metal hydroxides or oxides, i.e. the redox stability of water may not be sufficient to dissolve without deterioration a compound having a highly reduced metal center. Still, there are the procedures for preparation of important compounds (such as e.g. ) which call for washing the products with water in order to remove inorganics – these compounds cannot be highly sensitive to water. Nowadays we look with other eyes at organometallic compounds the family of which has expanded enormously. Some members of this family are soluble in water due to their ionic nature; the legions of anionic carbonylmetallates (e.g. ) and cationic bisphosphine Rh- chelate complexes (e.g. ) just come to mind. Others obtain their solubility in water from the well soluble ligands they contain; these can be ionic (sulfonate, carboxylate, phosphonate, ammonium, phosphonium etc. derivatives) or neutral, such as the ligands with polyoxyethylene chains or with a modified urotropin structure. 1
2. 2 Chapter 1 One of the most important metal complex catalyzed processes is the hydroformylation of light alkenes. In the early years the catalyst was based on cobalt and this brought about an intense research into the chemistry of cobalt carbonyls. A key intermediate, is well soluble and stable in water and behaves like a strong acid [1] in aqueous solution: For a decade or so was another acclaimed catalyst for the selective hydrogenation of dienes to monoenes [2] and due to the exclusive solubility of this cobalt complex in water the studies were made either in biphasic systems or in homogeneous aqueous solutions using water soluble substrates, such as salts of sorbic acid (2,4-hexadienoic acid). In the late nineteen-sixties olefin-metal and alkyl-metal complexes were observed in hydrogenation and hydration reactions of olefins and acetylenes with simple Rh(III)- and Ru(II)-chloride salts in aqueous hydrochloric acid [3,4]. No significance, however, was attributed to the water-solubility of these catalysts, and a new impetus had to come to trigger research specifically into water soluble organometallic catalysts. New incentives came from two major sources, and it is tempting to categorize these as “academic” and “industrial” ones. In the early fifties the renaissance of inorganic chemistry brought about the need for water soluble, phosphorus-donor ligands in order to establish correlations between metal complex stability and structure and the characteristics of donor atoms in a given ligand set. By that time tertiary phosphines, introduced to organometallic chemistry by F. G. Mann, were widely recognized as capable of coordinating and stabilizing low oxidation state metal ions in organic solvents. For Ahrland, Chatt and co-workers it appeared straightforward to derivatise the well-known and conveniently handled triphenylphosphine by sulfonation in fuming sulfuric acid in order to get the required P- donor ligand for complexation studies in aqueous solution [5]. The monosulfonated derivative, 3-sulfonatophenyldiphenylphosphine, nowadays widely known as TPPMS, was successfully used in complex stability measurements which later led to the categorization of ligands according to their donor atoms (ligands of a and b character and the Ahrland-Chatt triangle, forerunner of the hard and soft characterization). TPPMS was then investigated in extensive details by J. Bjerrum who established stability constants of complexes of a dozen of metal ions with this ligand [6]. In addition to TPPMS, another water soluble tertiary phosphine, 2- hydroxyethyldiethylphosphine (abbreviated that time as dop) was prepared and its complex forming properties studied in Schwarzenbach`s laboratory [7]. All this had nothing to do with catalysis let alone catalysis with
3. Introduction 3 organometallic complexes in aqueous solutions. However, the stage was already set, the ingredients of such catalytic systems were at hand. This was the situation in 1968 when I joined the Institute of Physical Chemistry at the (then) Lajos Kossuth University of Debrecen, Hungary, chaired by Professor M.T. Beck who later became my M.Sc. supervisor. Our work showed convincingly that complexes of ruthenium(II) and rhodium(I) with TPPMS as ligand could be successfully used for hydrogenation of water soluble olefins in aqueous solutions. My Thesis was submitted in 1972 and the first papers [8,9] appeared in 1973 (see also [10] for further recollections). All our catalytic work was carried out in strictly homogeneous aqueous solutions. At about the same time it was already clear that homogeneous catalysis could not be widely practiced in industry without solving the inherent problem of separation of the catalysts from the product mixture applying relatively easy and economic methods. The first written record of the idea of metal complex catalysis in two immiscible liquid phases systems as a viable general solution to this problem can be traced back in the report [11] of a Working Group on Heterogenizing Catalysts, chaired by Manassen (then at the Weizmann Institute, Rehovot, Israel) at a NATO Science Committee Conference in late 1972. The proceedings of the conference were published in 1973 at the same time as our first publications, a clear evidence to that these ideas developed independently. The Group Report did not specifically mentioned aqueous/organic two-phase systems for organometallic catalysis, though later Manassen put this idea into practice [12] using a Rh(I)-TPPMS catalyst for hydrogenation of olefins in water/benzene mixtures (with a correct reference to our related earlier work on homogeneous catalysis). In general, the first papers on catalysis by water soluble phosphine complexes did not draw much enthusiasm from the catalysis society. As one of the most reputed colleagues stated: ”not any of the important processes of organometallic catalysis takes place in aqueous solutions”. It needed the imagination of Kuntz [13-15] to develop the chemistry of (and file patents in 1975-1976 for Rhône-Poulenc on) two-phase hydroformylation, hydrocyanation and telomerization of olefins – three really important processes of organometallic catalysis. Not only the principle of aqueous/organic biphasic procedures was successfully realized for manufacturing important industrial products, but new sulfonated phosphine ligands were also prepared of which the highly water soluble trisulfonated triphenylphosphine (tris(3-sulfonatophenyl)phosphine, TPPTS) was later shown a key component of the rhodium(I) catalyst of large scale hydroformylation. However, even these results did not find their way into immediate industrial utilization.