Nucleophilic aromatic substitution of metallabenzenes

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dc.contributor.advisor Wright, J en Ferguson, Lauren en 2012-12-11T20:26:42Z en 2012 en
dc.identifier.uri en
dc.description.abstract The research reported in this thesis shows that metallabenzenes can undergo a new benzenelike reaction - nucleophilic aromatic substitution of hydrogen (SN H) (Scheme 0-1). In this process, the nucleophile (Nu) first attacks the cationic metallabenzene (A) at the γ position to the neutral metallacyclohexadiene compound (B). Oxidation with an external oxidant [O] can then result in the new substituted cationic metallabenzene (C). [Scheme 0-1: Nucleophilic aromatic substitution of hydrogen in metallabenzenes. ML4 = metal-ligand fragment ommitted]. The SN H reaction was demonstrated to occur for the three cationic metallabenzenes, [Os(C₅H₄{SMe-3})(CO)₂(PPh₃)₂]CF₃SO₃ (4), [Os(C₅H₄{SMe-1})(CO)(4- MePhNC)(PPh₃)₂]CF₃SO₃ (5) and [Ir(C₅H₄{SMe-1})(κ²-S₂CNEt₂)(PPh₃)2]PF₆ (7). The spectroscopic and structural properties of these complexes indicate that in each case an aromatic description of the metallacyclic rings is appropriate. Treatment of the metallabenzenes 4 or 5 with sodium borohydride produced the metallacyclohexadienes (B) Os(C₅H₃{SMe-1}{H₂-3})(CO)₂(PPh₃)₂(8) or Os(C₅H₃{SMe- 1}{H₂-3})(CO)(4-MePhNC)(PPh₃)₂ (9) respectively. Saturation of the γ-carbon in these complexes results in spectroscopic properties that are distinct from metallabenzenes including upfield signal shifts in the ¹H and ¹³C NMR spectra. In addition, crystal structures of 8 and 9 display clear bond length alternation around the metallacycle rings. 8 and 9 can be oxidised by oxygen or with an external oxidant such as 2,3-dichloro-5,6- dicyanobenzoquinone (DDQ) to reform the starting cation 4 and 5. Employment of nucleophiles other than hydride, provided the potential for forming new substituted metallabenzene cations (C) after oxidation of metallacyclohexadiene intermediates (B). It was found that when 4 is treated with the ethoxide anion the metallacyclohexadiene Os(C⁵H³{SMe-1}{OEt-3}{H-3})(CO)₂(PPh₃) ₂(11) is formed, however on oxidation with 2,3-dichloro-5,6-dicyanobenzoquinone the ethoxide is removed and the initial cation 4 is reformed. In contrast to this, attack by the methyl anion on 4, 5 or 7 yields metallacyclohexadienes Os(C₅H₃{SMe-1}{Me-3}{H-3})(CO)₂(PPh₃)₂ (12), Os(C₅H₃{SMe- 1}{Me-3}{H-3})(CO)(CN-p-tolyl)(PPh₃)₂ (13) or Ir(C₅H₃{SMe-1}{Me-3}{H-3})(κ²- S₂CNEt₂)(PPh₃)₂ (14) respectively. Oxidation of these with 2,3-dichloro-5,6- dicyanobenzoquinone (or copper(II) chloride for 12) produce new methyl-substituted metallabenzenes [Os(C₅H₃{SMe-1}{Me-3})(CO)2(PPh₃)₂]Cl (18), [Os(C₅H₃{SMe-1}{Me- 3})(CO)(CNp-tolyl)(PPh₃)₂]PF₆ (19) or [Ir(C₅H₃{SMe-1}{Me-3})(κ²-S2CNEt₂)(PPh₃)₂]PF₆ (20) respectively. These new substituted cationic metallabenzenes have spectral and structural properties similar to those of the parent cationic metallabenzenes 4, 5 and 7. Oxidation of the buytl-substituted metallacyclohexadiene Os(C₅H₃{SMe-1}{nBu-3}{H- 3})(CO)₂(PPh₃)₂ (15), with 2,3-dichloro-5,6-dicyanobenzoquinone yields the butyl-substituted metallabenzene [Os(C₅H₃{SMe-1}{nBu-3})(CO)₂(PPh₃)₂]Ph₄B (21). This cation undergoes further benzene-like chemistry including benzylic oxidation with N-chlorosuccinamide to produce (C₅H₃{SMe-1}{COCH₂CH₂CH₃-3})(CO)₂(PPh₃)₂]PF₆ (22). The observation of nucleophilic attack occurs exclusively at the γ positions of 4 or 7 was rationalised by a computational study by Johns that involved determination of the Fukui Function of the ring atoms.¹ The calculations predict that the α and γ positions of 4 and 7 are the most likely to undergo attack by a nucleophile. However, to account for the observed exclusive attack at the γ-position, it was observed that steric factors (which are not accounted for by the method) are responsible for the selectivity. Upon employing the nucleophiles such as hydroxide or dicyanomethanide with the metallabenzene 4, the metallacyclohexadienes Os(C₅H₄{SMe-1}{OH-3})(CO)₂(PPh₃)₂ (24) or Os(C₅H₄{SMe-1}{CH(CN)₂-3})(CO)₂(PPh₃)₂ (28) respectively are produced. Oxidation of these complexes with 2,3-dichloro-5,6-dicyanobenzoquinone results in a formal loss of dihydrogen and the metallacyclohexadienone complex Os(C₅H₃{SMe-1}{O-3})(CO)₂(PPh₃)₂ (25) or the metallacyclohexadienylidene complex Os(C₅H₃{SMe-1}{C(CN)₂-3})(CO)₂(PPh₃)₂ (29), respectively are formed. 25 can be protonated with trifluoroacetic acid or alkylated with methyl triflate at the carbonyl oxygen forming the corresponding osmaphenol cation [Os(C₅H₃SMe-1}OH- 3})(CO)₂(PPh₃)₂]CF₃CO₂ (26) or osmaanisole cation [Os(C₅H₃SMe-1}{OCH₃- 3})(CO)₂(PPh₃)₂]CF₃SO₃ (27). A dinculear complex with a dicyanomethylene bridge (Os(C₅H₄{SMe- 1})(CO)₂(PPh₃)₂)₂{C(CN)₂-3} (30) is formed by addition of excess base to a mixture of two equivalents of 4 and one equivalent of malononitrile. The crystal structure of this complex shows the metallacycle ring is much more buckled than the corresponding rings in other metaalcyclohexadiene complexes. On treatment with silver trilfate and triphenylphosphine ,29 forms the coordination dimer [(Os(C₅H₃{SMe-3}{C(CN)₂-3)(CO)₂(PPh₃)₂)₂(Ag(PPh₃))₂][CF₃SO₃]₂ (31) in which two silver-triphenylphosphine groups bridge two metallacyclic units. This dicationic coordination dimer has properties similar to that of 29. en
dc.publisher ResearchSpace@Auckland en
dc.relation.ispartof PhD Thesis - University of Auckland en
dc.rights Items in ResearchSpace are protected by copyright, with all rights reserved, unless otherwise indicated. Previously published items are made available in accordance with the copyright policy of the publisher. en
dc.rights.uri en
dc.rights.uri en
dc.title Nucleophilic aromatic substitution of metallabenzenes en
dc.type Thesis en The University of Auckland en Doctoral en PhD en
dc.rights.holder Copyright: The Author en
pubs.elements-id 369577 en
pubs.record-created-at-source-date 2012-12-12 en

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