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Mechanistic Investigation on the Polymerization of Phenylacetylene by 2‑Diphenylphosphinopyridine Rhodium(I) Catalysts: Understanding the Role of the Cocatalyst and Alkynyl Intermediates
journal contribution
posted on 2018-08-15, 21:29 authored by Marta Angoy, M. Victoria Jiménez, F. Javier Modrego, Luis A. Oro, Vincenzo Passarelli, Jesús J. Pérez-TorrenteThe mono- and dinuclear
rhodium(I) complexes featuring 2-(diphenylphosphino)pyridine
ligands, [Rh(cod)(Ph2PPy)]+ and [Rh(nbd)(μ-Ph2PPy)]22+ (cod = 1,5-cyclooctadiene,
nbd = 2,5-norbornadiene), have been prepared in order to be evaluated
as phenylacetylene (PA) polymerization catalysts. In contrast with
compound [Rh(nbd){Ph2P(CH2)2Py}]+, featuring a 2-(2-(diphenylphosphino)ethyl)pyridine ligand,
that showed a moderate catalytic activity, both [Rh(diene)(Ph2PPy)]nn+ (n = 1, cod; n = 2, nbd) complexes
showed no catalytic activity due to the formation of unusual dinuclear
species [Rh2(diene)2(μ-Ph2PPy)(μ-CC-R)]+, supported by a Ph2PPy bridging ligand and an
alkynyl ligand coordinated in a μ-η1:η2 fashion, which are inactive in PA polymerization. However,
compounds [Rh(diene)(Ph2PPy)]nn+ efficiently
polymerize PA in the presence of a cocatalyst as iPrNH2 affording highly stereoregular poly(phenylacetylene)
(PPA) of Mw = 3.42 × 105 (cod) and 2.02 × 105 (nbd) with polydispersities
of 1.39 and initiation efficiencies of 4–7%. NMR studies on
the polymerization reaction have allowed identification of the alkynyl
species [Rh(CCPh)(cod)(Ph2PPy)] as the likely initiating
species involved in the generation of the rhodium-vinyl species responsible
for the propagation step. The iPrNH2 cocatalyst
is possibly involved in the efficient proton transfer from the coordinated
PA to iPrNH2 that allows for a significant
concentration of the key initiating species [Rh(CCPh)(cod)(Ph2PPy)]. The distinct behavior of compounds [Rh(diene)(Ph2PPy)]nn+ as PA polymerization catalysts
is a consequence of the binucleating ability of the Ph2PPy ligand in combination with the low basicity of the pyridine fragment
which allows for the stabilization of the inactive alkynyl-bridge
dinuclear species.
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Ph 2 PPy ligandalkynyl speciesPh 2 PPyproton transferAlkynyl Intermediatespolymerization catalystsPA polymerization catalystsRh 2NMRPA polymerizationinitiation efficienciesalkynyl-bridge dinuclear speciesMechanistic Investigationdinuclear speciespolymerization reactionrhodium-vinyl speciesi PrNH 2alkynyl ligandi PrNH 2 cocatalystbinucleating abilitypolymerize PApropagation steppyridine fragmentPPA
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