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

Abstract

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-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)]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(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)]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.

Discovery and Comparison of Homogeneous Catalysts in a Standardized HOT‐CAT Screen with Microwave‐Heating and qNMR Analysis: Exploring Catalytic Hydration of Alkynes

“…The discrepancies are likely caused by alkyne oligomerizations, as known with Rh(I)‐phosphane complexes ( vide infra ) . Complex (TRIPHOS)RhCl 3 revealed preference for Markovnikov hydration (4 turnovers; entry 18). Overall, rhodium shows alkyne hydration activity with a tendency for the anti‐Markovnikov mode, but at low chemoselectivity due to alkyne oligomerization or polymerization.…”

Polymerization of phenylacetylene catalyzed by rhodium(i) complexes with N-functionalized N-heterocyclic carbene ligands

“…Related alkynyl species have been suggested to be the initiating species likely involved in the generation of stable rhodium-vinyl species, responsible for the propagation step, by PA insertion into the Rh-alkynyl bond. 30,35 This activation pattern could be also operative for the neutral complexes [RhX(nbd)(κC-MeIm∩Z)] (X = Cl or Br). After PA coordination, proton transfer to the uncoordinated N-donor function should give rise to a zwitterionic alkynyl complex, likely with a square pyramidal structure having the strong σ-donating alkynyl ligand in the apical position, 36 from which ionization of the chlorido ligand may produce the same cationic species ( pathway i).…”

“…The low PA conversion achieved with 7 points to a possible catalyst deactivation similarly as we found in rhodium catalysts featuring 2-diphenylphosphinopyridine ligands. 30 Finally, the neutral amido complex [Rh(nbd){κ 2 C,Nt-BuIm(CH 2 ) 3 N-t-Bu}] (9) showed an excellent activity reaching a 90% conversion in only 40 min to produce a PPA of M w 8.32 × 10 5 and a Đ of 2.0.…”

“…It is worth mentioning that the hydrogen bond or proton acceptor power of THF is considered to be comparable to that of methanol or monomeric water. 37 In fact, mechanistic studies on PA polymerization by 2-diphenylphosphinopyridine-based rhodium(I) catalysts evidenced the formation of the neutral species [Rh(CuCPh)(cod){κP-Ph 2 PPy}] by proton transfer to the reaction medium from the cationic [Rh(CuCPh) (cod)(κP-Ph 2 PPyH)] + resulting from the activation of PA. 30 The efficiency of the initiation process depends on several factors: (i) the strength of the Rh-N and Rh-Cl bonds, (ii) the strain on the chelate ring that should favor the breaking of the Rh-N bond, and (iii) the basicity of the N-donor function. In general, the initiation efficiency of the cationic catalysts is greater (2-5%) than that of the neutral ones.…”

“…of Ph 2 P (CH 2 ) 3 NMe 2 (see the ESI †). The corresponding cationic compounds 11 and 13, 12 and complexes 14 and 15, based on the 2-(2-(diphenylphosphino)ethyl)pyridine ligand, 30 had been previously described by our research group. The catalytic performance of neutral and cationic Rh(I)-phosphine compounds 10-15 in PA polymerization is shown in Table 2.…”