Mechanistic Investigation on the Polymerization of Phenylacetylene by 2-Diphenylphosphinopyridine Rhodium(I) Catalysts: Understanding the Role of the Cocatalyst and Alkynyl Intermediates

The 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-C≡C-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(C≡CPh)­(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(C≡CPh)­(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.