The Nature of the Active Catalyst in Late Transition Metal-Mediated Ring-Opening Polymerization (ROP) Reactions:  Mechanistic Studies of the Platinum-Catalyzed ROP of Silicon-Bridged [1]Ferrocenophanes

Current mechanisms for the transition metal-catalyzed ring-opening polymerization (ROP) of [1]ferrocenophanes and related strained species such as cyclic carbosilanes invoke a homogeneous mechanism. In this paper we describe experiments which indicate that the Pt(0)-catalyzed ROP of silicon-bridged [1]ferrocenophanes proceeds mainly by a heterogeneous mechanism with colloidal platinum as the catalyst. Mechanistic studies were initiated to elucidate the fate of the ferrocenylsilane component of the precatalyst, fcPt(1,5-cod)SiMe 2 ( 4 ) (fc = Fe(η 5 -C 5 H 4 ) 2 ), a proposed intermediate in the catalytic cycle for fcSiMe 2 ( 1a ) with Pt(1,5-cod) 2 as the initiator. The ring-opened addition product Et 3 SifcSiMe 2 H ( 6 ) was isolated in high yield from the Pt(0)-catalyzed ROP of 1a in the presence of 35-fold excess of Et 3 SiH. Species 6 and Et 3 SiH were subsequently employed as capping agents in the Pt(0)-catalyzed ROP of fcSiMePh ( 1b ) to generate model oligomers, Et 3 SifcSiMe 2 (fcSiMePh) n H ( 10, n ≈ 20) and Et 3 Si(fcSiMePh) n H ( 8, n ≈ 7), respectively. Copolymerization of a mixture of 1a and 1b using Pt(0) catalyst afforded a random copolymer (fcSiMe 2 - r -fcSiMePh) n ( 9 ). Comparative end-group analysis of 8, 9, and 10 was performed with the oligoferrocenylsilane synthesized via the ROP of 1b in the presence of Et 3 SiH initiated by the precatalyst 4 . Significantly, this revealed that the ferrocenyldimethylsilane component of 4 is not incorporated into the resultant polymer backbone which possessed the structure Et 3 Si(fcSiMePh) n H ( 8 ). Similarly, the ROP of 1a in the presence of Et 3 SiH initiated by the precatalyst fcPt(1,5-cod)Sn( t Bu) 2 ( 11 ) gave end-capped oligomers Et 3 Si(fcSiMe 2 ) n H ( 5 ), devoid of Sn( t Bu) 2 groups. The observations that the ferrocenylsilane and ferrocenylstannane components of 4 and 11 are not incorporated into the polyferrocene products and that, in addition, mercury was found to significantly retard ROP, indicates that the previously proposed homogeneous ROP mechanism is incorrect and that colloidal platinum is the main catalyst. A new heterogeneous mechanism for the platinum-mediated ROP of silicon-bridged [1]ferrocenophanes is proposed which is likely to have important implications for the metal-catalyzed ROP mechanisms for related species such as silacyclobutanes.

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