Syntheses and Modification of Iridabenzenes

Abstract

Metallabenzenes are an intriguing class of complexes which occupy a niche in the field of organometallic chemistry. The combination of a metal atom (LnM) with an unsaturated bidentate five-carbon donor (C₅R₅) pushes the boundary for the definition of aromatic compounds and presents challenges for the synthetic and theoretical chemist alike. Two key issues in this area currently require redress: firstly, there are no general synthetic routes to metallabenzenes and secondly, their reaction chemistry has not been widely studied. The work described in this thesis provides straight-forward and robust syntheses of iridabenzenes and a detailed exploration of their reactivity behaviour. The introductory chapter provides a review of metallabenzene chemistry and the general properties, syntheses and reactivity of these complexes. The significance and aims of the current research are subsequently outlined. In Chapter 2, a simple, high-yielding route to the iridabenzene [Ir(C₅H₄{S}-1)(MeCN)(PPh₃)₂][OTf] (92) is described. The reaction chemistry of 92 is rich and varied, and as such most of the new compounds presented in this work are derived from this starting material. One special feature of 92 is its α-thiolate substituent. This highly nucleophilic functional group has the propensity to engage with acids and methylating agents, allowing the preparation of a number of iridabenzenes with different sulfur-containing substituents and ligand sets. Notable examples include the iridabenzene thiol Ir(C₅H₄{SH}-1)Cl₂(PPh₃)₂ (101), its disulfide oxidation product [Ir(C₅H₄{SH}-1)Cl₂(PPh₃)₂]₂ (102) and the dicationic bis(aquo) complex [Ir(C₅H₄{SMe}-1)(H₂O)₂(PPh₃)₂][OTf]₂ (107); the first structurally characterised metallabenzene with aquo ancillary ligands. Chapter 3 focuses on the syntheses and properties of fused-ring iridabenzenoids. Several different bicyclic morphologies were obtained via the unique annulation reactions of 92 or the chloro derivative Ir(C₅H₄{S}-1)Cl(PPh₃)₂ (93) with organonitriles and alkynes. These comprised iridabenzothiazolium dications [Ir(C₅H₄{NH=CR)S}-1)(RCN)(PPh₃)₂][PF₆]2 (109, R = Me; 114, R = p-tolyl), iridabenzothiophenes Ir(C₅H₄{CH=CRS}-1)(C CR)(PPh₃)₂ (117, R = Ph; 129, R = Fc) and novel iridabenzothietes Ir(C₅H₄{C(=CHR)S}-1)(C CR)(PPh₃)₂ (e.g. 119, R = Ph). Significantly, these reactions were adaptive and could incorporate a range of acetylene substrates. Thus it was possible to introduce several different functional groups to the iridabenzene core. Chapter 4 describes the nucleophilic aromatic substitution of iridabenzenes. The cationic complexes [Ir(C₅H₄{SMe}-1)(κ²- S₂CNMe₂)(PPh₃)₂][X] (142a,b; X = OTf, I) were reacted with nucleophiles such as hydride, hydroxide, acetylides and cyclopentadienyl to afford iridacyclohexadienes. The iridacyclohexadienone Ir(C₅H₃{SMe}-1{=O}-5)(κ²- S₂CNMe₂)(PPh₃)₂ (150) formed directly from 142b using hydroxide as the nucleophile under aerobic conditions. The first alkynyl-substituted iridabenzenes, namely [Ir(C₅H₃{SMe}-1{C CR}-3)(κ²- S₂CNMe₂)(PPh₃)₂][Tf₂CPh] (156, R = SiMe₃; 157, R = Ph) were prepared from the corresponding iridacyclohexadienes upon oxidation with DDQ. Thermal treatment of the cyclopentadienyl-substituted iridacyclohexadiene Ir(C₅H₄{SMe}-1{C₅H₅}-3)(κ2-S2CNMe2)(PPh₃)₂ (158) in the presence of palladium on carbon generated the first iridafulvalene Ir(C₅H₃{SMe}-1{=C₅H₄}-3)(κ²- S₂CNMe₂)(PPh₃)₂ (159) after the loss of dihydrogen. This unique, cross-conjugated iridacycle is formally re-aromatised via metal complexation through the cyclopentadienylidene ring, and with [IrCl(COD)]₂ the diiridium π-adduct [Ir(C₅H₃{SMe}-1{η⁵-(C₅H₄)Ir(COD)}-3)(κ²- S₂CNMe₂)(PPh₃)₂][PF₆] (162) was produced. The work described constitutes a solid synthetic platform from which future development of conducting or optically active materials may be explored