Organometallic pyrrole complexes of mercury, ruthenium, osmium and iridium

Abstract

Pyrrole, the five-membered nitrogen heterocycle, is fundamentally integrated into the molecules of life. In understanding the chemistry of pyrroles, important applications, ranging from synthetic drugs with a biological mimicry mode of action, to electrical conductors have contributed to modern day advances in science. This thesis explores the syntheses and reactivity of organometallic pyrrolyl complexes. The uniqueness of pyrrole as a heterocycle, is presented by way of an introduction, in Chapter One. A review of the different possible bonding modes of the pyrrolyl ligand, in the literature survey of pyrrolyl complexes in this chapter, places the organometallic pyrrolyl complexes presented in this thesis in context. The pyrrolyl ligand in the reported literature examples, has been shown to bond in a π-(η5-, η3-, η2-) and in a σ-bonding mode. In the σ-bonding mode, bonding to the metal invariably occurs through the nitrogen atom. In the limited literature examples where bonding occurs through a pyrrolic ring carbon, the synthetic routes to these complexes necessarily demand that the reactivity at the nitrogen is blocked, i.e., N-substituted. Emphasis in this thesis is directed at exploring alternative synthetic strategies towards σ-carbon-bound pyrrolyl complexes, where the nitrogen is both substituted and unsubstituted. One strategy which has proved successful, is via the use of pyrrolylmercury compounds. The mercuration of pyrroles has fascinated organic chemists, since the discovery of the ease with which pyrrole undergoes electrophilic aromatic substitution. However, the facile mercuration of pyrroles, attributable to the π-excessive character, makes attempts to selectively isolate the monomercurated pyrroles a non-trivial synthetic problem. With the polymercuration problem outlined in Chapter One, Chapter Two details efforts in exploring synthetic approaches towards the aim of monomercuration. Use of different synthetic strategies has resulted in the successful synthesis of (C4H3NH)HgCl, (C4H3NMe)HgCl, (C4H3NMe)2Hg, (C4H3NCOCH3)HgCl, (C4H3NCOCH3)2Hg, (C4H3NSO2Ph)HgCl and (C4H3NSO2Ph)2Hg. The choice of synthetic strategy is determined by whether the nitrogen is substituted or unsubstituted; and if substituted, whether the N-substituent is electron-withdrawing or electron-donating. Crystal structures of two mercurated pyrroles, (C4H3NCOCH3)HgCl and (C4H3NCOCH3)2Hg, presented in Chapter Two, unequivocally confirm the formulation of monomercurated, 2-C-bound pyrroles. Chapter Three details the successful use of the pyrrolylmercury compounds synthesized as pyrrolyl-transfer reagents to suitable organometallic complexes. The pyrrolyl organometallic complexes synthesized, exhibit different reactivity patterns from reported organometallic aryl complexes. Reduced stability of the five-coordinate pyrrolyl complexes, isolable only as the DMF adducts, MCl(C4H3NMe)(DMF)(CO)(PPh3)2, M= Ru, Os, is contrasted with the remarkable stability of reported analogous aryl complexes, MCl(Ph)(CO)(PPh3)2, M= Ru, Os. When the nitrogen atom of pyrrole caries a substituent capable of forming an η2-chelate, as in -NCOCH3 and -NSO2Ph, stable six-coordinate pyrrolyl complexes, are isolated. Further study of the reactivity exhibited by the easily accessible N-methyl pyrrolyl complexes, showed steric pressure to be also important in determining trends towards migratory insertion in these σ-C-bound pyrrolyl complexes. An investigation of the temperature-dependence of the position of equilibrium between the η2-acyl and dicarbonyl complex for the acyl complex, RuCl(η2-C[O]{C4H3NMe})(CO)(PPh3)2, showed that the latter complex is favoured at lower temperatures. A unique acid-protonation reaction of this acyl complex, gives the hydroxycarbene complex, RuCl2(=C[OH]{C4H3NMe})(CO)(PPh3)2. The influence of the heterocycle in this path of reactivity is implicated, since analogous aryl complexes do not react similarly. The crystal structures of OsCl(2-C4H3NMe)(CN-p-tolyl(CO)(PPh3)2, RuCl(2-C4H3NMe)(η2-NMe2CH2CH2NH2)(CO)PPh3 and Ru(η2-2-C4H3NSO[O]Ph)(η2-S2CNMe2)(CO)PPh3 are reported. These complexes confirm that the position of bonding of the pyrrolyl ring to the metal is via the 2-position, indicating regiospecific transfer of the pyrrolyl-mercury reagents to the ruthenium and osmium complexes. In Chapter Four, the synthetic versatility of the pyrrolyl organometallic complexes is further illustrated by the isolation of the stable dihydropyrrolyl complexes, MCl2(2-C4H4NMe)(CO)(PPh3)2, (M= Ru, Os); OsCl2(2-C4H4NH)(CO)(PPh3)2, and IrCl3(2-C4H4NMe)(PPh3)2. Protonated pyrroles reported in the literature exist only in strong acid solution. The important role of the metal centre in stabilising these crystalline non-aromatic pyrroles, is evident from the crystal structure of RuCl2(2-C4H4NMe)(CO)(PPh3)2, which shows double bond character in the ruthenium-pyrrolyl bond. Detailed NMR analysis of these organometallic pyrrolyl complexes show that the existence of two (3H- and 5H-) isomers are dependent on the steric bulk of the substituent on the nitrogen. Among the reactions that olefin complexes undergo, are migratory insertion reactions and nucleophilic attack at the coordinated ethylene ligand. In both cases, the ethylene ligand is modified into a σ-bonding ligand. With selected pyrrolylmercuric chloride compounds, ((C4H3NR)HgCl, R=H, Me), reactions with the zerovalent complex, Os(η2-C2H4)(CO)2(PPh3)2 results in transfer of the pyrrolyl ligand onto the metal, loss of elemental mercury, together with a migratory insertion of the ethylene ligand, to give the ethylpyrrolyl complexes described in Chapter Five. These singular reactions are prompted only under the conditions of uv irradiation. Thermally, loss of the coordinated ethylene ligand occurs, with isolation of the chloropyrrolyl complexes, OsCl(CO)2(C4H3NR)(PPh3)2, R=H, Me, only. Reactivity in this unique reaction appears to be confined to pyrrolylmercuric chlorides, and does not occur with phenyl mercuric chloride. The nett oxidative addition reactions result in the isolation of stable ethylpyrrolyl complexes for which the β-hydride decomposition pathway is blocked. Attention in Chapter Six shifts from the σ-C-bound pyrrolyl complexes in the previous chapters, to pyrrolylcarbene complexes. A unique intermolecular electrophilic aromatic substitution of pyrrole with the reactive dihalocarbene complex, RuCl2(=CCl2)(CO)(PPh3)2 gives the previously reported 2-C-bound pyrrolylcarbene complex, RuCl2(=CCl[2-C4H3NH])(CO)(PPh3)2. Further investigations on the electrophilic reactivity of this complex reveal the synthetic versatility of this halocarbene complex. In particular, complexes with two different bonding modes of the pyrrolyl ligand (η1-C or η1-N) at the carbene carbon, are isolated. Crystal structures of the pyrrolyl ligand in these two bonding modes in analogous complexes indicate that the pyrrolyl ligand is a better π-donor through the pyrrolic ring carbon than through the pyrrolic nitrogen. Comparison of these crystal structures with an analogous (pyrrolyl)(pyrrolidine)carbene complex confirms the ambidentate nature of the pyrrolyl ligand.