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Some reactions of acetylenes with group VIII metal complexes.

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posted on 19.11.2015, 08:45 authored by Martin Edmund. Howden
Chapter 1 presents an outline of the bonding in [special character omitted]-bonded acetylene complexes of metals, and also contains a survey of mono-acetylene complexes of four Group VIII metal systems---rhodium(I), iridium(I), palladium(O) and platinum(O). In chapter 2, some reactions of rhodium(I)-[special character omitted]-ketoenolate complexes with the electrophilic acetylene hexafluorobut-2-yne are described. Compounds of the type (Rh([special character omitted]-ketoenolate)(C2H4)2) as well as (Rh(apeo) (C2H4)2) (Rh(apemino)(C2H4)2), react with C4F6 to give products in which the acetylene adds 1,4 across the rhodium-chelate ring. In addition, the C4F6 is either trimerised to give a hexakis (trifluoromethyl)benzene ring, or dimerised, with incorporation of ethylene, to a 1,2,3,4-tetrakis-(trifluoromethyl)cyclohexa-1,3-diene ligand. These ligands become coordinated to the rhodium, with the [special character omitted]-bonded ethylene ligands being displaced, and it is found that the basicity of the chelating anion is influential in determining which ligand is formed. With (Rh (acac) (C8H14)2), a 9,10,11,12-tetrakis(trifluoromethyl)-bicyclo(6,4,0)dodeca-9,11-diene ligand is formed by combination of two C4F6 molecules and one cyclo-octene molecule. Low temperature (-78 C) reactions of C4F6 with the compounds (Rh ([special character omitted]-ketoenolate) (olefin)2) give a new class of complexes, (Rh ([special character omitted]-ketoenolate) (olefin) (C4F6)), containing both a [special character omitted]-bonded olefin and a [special character omitted]-bonded acetylene ligand. These complexes are probable intermediates in the room temperature formation of cyclohexa-1,3-diene and bicyclododeca-9,11-diene ligands, and may also be involved in the build-up of the benzene-type ligands. The crystal structures of two of these compounds, (Rh (acac) (C2H4) (C4F6)) and (Rh (acac) (C8H14) (C4F6)) are described. Starting from (Rh (dpm) (C2H4) (C4F6)), a series of complexes (Rh (dpm) (Eph3)2(C4F6)) (E = P,As,Sb) can be prepared; it is found that one of the Eph3 ligands is dissociated in chloroform solution. A preliminary investigation of the reactions of (Rh (dpm) (C2H4) (C4F6)) with other acetylenes is also reported. Chapter 3 contains the results of an investigation into the reactions of phenylhaloacetylenes with complexes of platinum(O), palladium(O), rhodium(I) and iridium(I). These results are compared with those obtained for halo-olefins, which have been previously reported. In the case of phenylhaloacetylenes, the products are largely metal-acetylide complexes, many of them previously unknown. Two [special character omitted]-bonded acetylene complexes are obtained, these being (Pt (PPh3)2 (PhC=CCl) ) and (RhCl (PPh3)2 (PhC=CCl) ), and gentle warming of these in hydroxylic solvents results in isomerisation to metal-acetylide compounds. In chapter 4 the kinetics of these two metal-acetylene to metal-acetylide isomerisations are described. In both cases, the reaction is found to exhibit considerably more intramolecular character than is found for metal-olefin to metal-vinyl rearrangements. This result is related to the greater strength of the carbon-halogen bond being broken in acetylene-acetylide isomerisation. Chapter 5 discusses the preparation of some platinum and palladium acetylide and [special character omitted]-allene complexes containing dimethyl- dithiocarbamate as a ligand. In the case of the palladium complex, (Pd (dmdtc) (C=CPh) (PPh3) ), stable products, involving insertion of the acetylene into the palladium-carbon bond, are formed in reactions with hexafluorobut-2-yne and dimethylacetylenedicarboxylate. No reactions occur in the case of the platinum analogue. These results are compared with those for reactions of the acetylenes with (PdX (C=CPh) (PPh3)2) (X = halogen), in which no stable mono-insertion products are formed, although multiple insertion into the palladium-carbon bond may be occurring. Reaction of C4F6 with (Pd (HC=C=CH2) (dmdtc)(PPh3)) is postulated to give a product derived from insertion of the acetylene into the palladium-[special character omitted]-allene bond, followed by rearrangement of the allene moiety to an acetylenic group.


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University of Leicester

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