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.

本研究合成了以2-(二苯基膦基)吡啶（2-(diphenylphosphino)pyridine，缩写为Ph₂PPy）为配体的单核与双核铑(I)配合物：[Rh(cod)(Ph₂PPy)]⁺与[Rh(nbd)(μ-Ph₂PPy)]₂²⁺（其中cod为1,5-环辛二烯（1,5-cyclooctadiene），nbd为2,5-降冰片二烯（2,5-norbornadiene）），旨在将其作为苯乙炔（phenylacetylene, PA）聚合催化剂开展性能评价。与带有2-(2-(二苯基膦基)乙基)吡啶配体的配合物[Rh(nbd){Ph₂P(CH₂)₂Py}]⁺（该配合物展现出中等催化活性）不同，两种[Rh(diene)(Ph₂PPy)]ₙⁿ⁺（n=1对应cod体系，n=2对应nbd体系）配合物均未表现出催化活性，其原因是生成了非活性双核物种[Rh₂(diene)₂(μ-Ph₂PPy)(μ-C≡C-R)]⁺：该物种由Ph₂PPy桥联配体与以μ-η¹:η₂模式配位的炔基配体共同稳定，无法催化苯乙炔聚合。但[Rh(diene)(Ph₂PPy)]ₙⁿ⁺配合物在助催化剂异丙胺（iPrNH₂）存在下可高效催化苯乙炔聚合，所得高度立构规整的聚苯乙炔（poly(phenylacetylene), PPA）的重均分子量（M_w）在cod体系中为3.42×10⁵，nbd体系中为2.02×10⁵，多分散性指数为1.39，引发效率为4%~7%。对聚合反应的核磁共振（NMR）研究证实，炔基物种[Rh(C≡CPh)(cod)(Ph₂PPy)]可能是关键引发物种，其生成的铑-乙烯基物种负责链增长过程。异丙胺（iPrNH₂）助催化剂可能参与了配位态苯乙炔向异丙胺的高效质子转移，这一过程使得关键引发物种[Rh(C≡CPh)(cod)(Ph₂PPy)]的浓度显著提升。[Rh(diene)(Ph₂PPy)]ₙⁿ⁺配合物作为苯乙炔聚合催化剂所表现出的独特行为，源于Ph₂PPy配体的桥联双核配位能力与吡啶片段的低碱性的协同作用，该协同作用可稳定非活性的炔基桥联双核物种。