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 polymeri zation (ROP) of [1]-ferrocenophanes and related strained species such as cy clic 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 coll oidal platinum as the catalyst. Mechanistic studies were initiated to eluci date the fate of the ferrocenylsilane component of the precatalyst, fcPt(1, 5-cod)SiMe2 (4) (fc = Fe(eta (5)-C5H4)(2)), a proposed intermediate in the catalytic cycle for fcSiMe(2) (1a) with Pt(1,5-cod)(2) as the initiator. Th e ring-opened addition product Et(3)SifcSiMe(2)H (6) was isolated in high y ield from the Pt(0)-catalyzed ROP of la in the presence of 35-fold excess o f Et3SiH: Species 6 and Et3SiH were subsequently employed as capping agents in the Pt(0)-catalyzed ROP of fcSiMePh (1b) to generate model oligomers, E t(3)SifcSiMe(2)(fcSiMePh)(n)H (10, n approximate to 20) and Et3Si(fcSiMePh) (n)H (8, n approximate to 7), respectively. Copolymerization of a mixture o f la and Ib using Pt(0) catalyst afforded a random copolymer (fcSiMe(2)-r-f cSiMePh)(n) (9). Comparative end-group analysis of 8, 9, and 10 was perform ed with the oligoferrocenylsilane synthesized via the ROP of Ib in the pres ence of Et3SiH 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 Et3Si(fcSiMePh )(n)H (8). Similarly, the ROP of 1a in the presence of Et3SiH initiated by the precatalyst fcPt(1,5-cod)Sn(Bu-t)(2) (11) gave end-capped oligomers Et3 Si(fcSiMe(2))(n)H (5), devoid of Sn(Bu-t)2 groups. The observations that th e ferrocenylsilane and ferrocenylstannane components of 4 and 11 are not in corporated into the polyferrocene products and that, in addition, mercury w as found to significantly retard ROP, indicates that the previously propose d 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 fdr related s pecies such as silacyclobutanes.