Characterization of poly(phenylacetylene) (PPA) samples produced using Rh(I) complexes featuring hemi-labile phosphine ligands by size exclusion chromatography, multi-angle light scattering, (SEC-MALS), or asymmetric field flow fractionation (A4F)-MALS has revealed that some of these PPA samples contain a mixture of linear and branched polymer. The occurrence and extent of branching is dependent on both catalyst structure and polymerization conditions. The levels of branching are consistent with either terminal branching through copolymerization of macro-monomer or chain transfer to polymer, where the branched species are less reactive towards further polymerization than the linear chains. The MM dependence of B, the number of branches per molecule, or λ, the number of branches per repeat unit, suggests that the latter explanation may be correct but further work is needed.
Dinuclear phosphine-amido, [Rh 2 (diene){μ-NH(CH 2 ) 3 PPh 2 } 2 ], and cationic phosphine-amino complexes, [Rh(diene){Ph 2 P(CH 2 ) 3 NHR}] + (diene = cod, nbd, tfb) and [Rh{Ph 2 P(CH 2 ) 3 NHR} 2 ] + , have been prepared from the corresponding amino-functionalized phosphines Ph 2 P-(CH 2 ) 3 NHR (R = H, Me) and suitable rhodium(I) precursors. The dinuclear [Rh 2 (diene){μ-NH(CH 2 ) 3 PPh 2 } 2 ] complexes bearing π-acceptors diene ligands such as nbd or tfb exhibit a remarkable catalytic activity in phenylacetylene (PA) polymerization affording stereoregular polyphenylacetylenes with, unlike the cod precursor, unimodal molar mass distributions of very high molecular weights, M w up to ≈ 1.2 × 10 6 , and moderate polydispersity indexes. These complexes are more active than the mononuclear phosphino-anilido [Rh(diene){Ph 2 P(C 6 H 4 )NMe}] complexes, which are in turn more active than the cationic complexes [Rh(diene){Ph 2 P(CH 2 ) 3 NHMe}] + , [Rh(nbd){Ph 2 P(CH 2 ) 3 NH 2 }] + , and [Rh(nbd){Ph 2 P(C 6 H 4 )NHMe}] + bearing the same diene ligand. In contrast, complexes [Rh{Ph 2 P(CH 2 ) 3 NHR} 2 ] + (R = H, Me) without a diene ligand have been found to be inactive in PA polymerization. The excellent catalytic performance of [Rh 2 (diene){μ-NH(CH 2 ) 3 PPh 2 } 2 ] (diene = nbd, tfb) complexes is a consequence of the mode of activation of PA that likely results in the formation of unsaturated alkynyl species [Rh(diene)(CC-Ph)L] (L = PA, THF), which may be competent for PA polymerization.
The mono- and dinuclear
rhodium(I) complexes featuring 2-(diphenylphosphino)pyridine
ligands, [Rh(cod)(Ph2PPy)]+ and [Rh(nbd)(μ-Ph2PPy)]2
2+ (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)]n
n+ (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)]n
n+ efficiently
polymerize PA in the presence of a cocatalyst as iPrNH2 affording highly stereoregular poly(phenylacetylene)
(PPA) of M
w = 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)]n
n+ 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.
A series of neutral [RhX(nbd)(κC-MeIm∩Z)] and cationic [Rh(nbd)(κ2C,N-MeIm∩Z)]+ (X = Cl, Br; MeIm = 3-methylimidazol-2-yliden-1-yl; ∩Z = N-functionalized wingtip; nbd = 2,5-norbornadiene) complexes featuring NHC ligands functionalized with a 1-aminopropyl,...
The synthesis and structural characterization of zwitterionic [(η(6)-C6H5-BPh3)M(coe)2] (M = Rh, Ir) cyclooctene complexes is described. Both complexes exhibit an unusual exo-endo conformation of both cyclooctene ligands in the solid state. However, an equilibrium between the endo-endo and exo-endo rotational isomers arising from the hindered rotation about the metal-cyclooctene bond is observed in solution. Rotational barriers of around 65 kJ mol(-1) (Rh) and 84 kJ mol(-1) (Ir) have been determined by 2D EXSY NMR spectroscopy. The rotation process has also been studied by DFT calculations that showed that the dynamic behaviour is a consequence of the oscillation of the cyclooctene ligands about the metal-olefin bond instead of completing a full rotation.
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