Dinuclear Phosphine-Amido [Rh2(diene){μ-NH(CH2)3PPh2}2] Complexes as Efficient Catalyst Precursors for Phenylacetylene Polymerization

Dinuclear phosphine-amido, [Rh2(diene)­{μ-NH­(CH2)3PPh2}2], and cationic phosphine-amino complexes, [Rh­(diene)­{Ph2P­(CH2)3NHR}]+ (diene = cod, nbd, tfb) and [Rh­{Ph2P­(CH2)3NHR}2]+, have been prepared from the corresponding amino-functionalized phosphines Ph2P­(CH2)3NHR (R = H, Me) and suitable rhodium­(I) precursors. The dinuclear [Rh2(diene)­{μ-NH­(CH2)3PPh2}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, Mw up to ≈ 1.2 × 106, and moderate polydispersity indexes. These complexes are more active than the mononuclear phosphino-anilido [Rh­(diene)­{Ph2P­(C6H4)­NMe}] complexes, which are in turn more active than the cationic complexes [Rh­(diene)­{Ph2P­(CH2)3NHMe}]+, [Rh­(nbd)­{Ph2P­(CH2)3NH2}]+, and [Rh­(nbd)­{Ph2P­(C6H4)­NHMe}]+ bearing the same diene ligand. In contrast, complexes [Rh­{Ph2P­(CH2)3NHR}2]+ (R = H, Me) without a diene ligand have been found to be inactive in PA polymerization. The excellent catalytic performance of [Rh2(diene)­{μ-NH­(CH2)3PPh2}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)­(C≡C-Ph)­L] (L = PA, THF), which may be competent for PA polymerization.