Ligand Mobility and Solution Structures of the Metallocenium Ion Pairs [Me 2 C(Cp)(fluorenyl)MCH 2 SiMe 3 + ··· X - ] (M ) Zr, Hf; X ) MeB(C 6 F 5 ) 3 , B(C 6 F 5 ) 4 ) Carlos Alonso-Moreno, †,‡ Simon J. Lancaster, † Joseph A. Wright, † David L. Hughes, † Cristiano Zuccaccia, § Andrea Correa, ⊥ Alceo Macchioni,* ,§ Luigi Cavallo,* ,⊥ and Manfred Bochmann* ,† Wolfson Materials and Catalysis Centre, School of Chemical Sciences and Pharmacy, UniVersity of East Anglia, Norwich NR4 7TJ, U.K., Dipartimento di Chimica, UniVersita ` di Perugia, I-06123 Perugia, Italy, and Dipartimento di Chimica, UniVersita ` di Salerno, I-84084 Fisciano, Italy ReceiVed May 28, 2008 The mixed-alkyl metallocene complexes (IPCF)M(Me)(CH 2 SiMe 3 ) (M ) Zr, Hf; IPCF ) Me 2 C(C 5 H 4 )(C 13 H 8 )) were synthesized by the reaction of (IPCF)M(Me)Cl (M ) Zr, Hf) with Me 3 SiCH 2 MgCl. The crystal structures of (IPCF)Zr(CH 2 SiMe 3 ) 2 , (IPCF)HfMe 2 , and (IPCF)Zr(Me)Cl were determined by X-ray diffraction. The kinetics of site epimerization of the ion pairs (IPCF)M(CH 2 SiMe 3 )(µ- Me)B(C 6 F 5 ) 3 and [(IPCF)MCH 2 SiMe 3 + ··· B(C 6 F 5 ) 4 - ] (M ) Zr, Hf) were studied by variable-temperature NMR spectroscopy, while the solution ground-state structures of the ion pairs [LZrCH 2 Si- Me 3 + ··· B(C 6 F 5 ) 4 - ] (L ) SBI, IPCF; SBI ) rac-Me 2 Si(Ind) 2 ) were probed experimentally by 19 F, 1 H HOESY NMR spectroscopy and theoretically by DFT and molecular dynamics calculations. They reveal differences in the strength of anion interactions between the SBI and IPCF systems which may be significant for their catalytic activity. The tetraarylborate salts are stabilized by agostic interactions to ligand Si-Me moieties, with Hf > Zr. The exchange rates of both the MeB(C 6 F 5 ) 3 - and the B(C 6 F 5 ) 4 - compounds increase with increasing ion pair concentration. This acceleration is also seen on addition of excess [Ph 3 C][B(C 6 F 5 ) 4 ]. Pulsed-field gradient spin-echo (PGSE) NMR measurements indicated that both [(IPCF)ZrCH 2 SiMe 3 + ··· B(C 6 F 5 ) 4 - ] and [(SBI)ZrCH 2 SiMe 3 + ··· B(C 6 F 5 ) 4 - ] were present mainly as ion quadruples in toluene-d 8 /1,2-F 2 C 6 H 4 (8/2 in volume) at millimolar concentrations and, notably, their aggregation increased to a similar extent with the addition of an excess of [Ph 3 C][B(C 6 F 5 ) 4 ]. The results demonstrate the formation of mixed-ion aggregates of the type {[(L)MR + ··· X - ][CPh 3 + ··· X - ] n }. However, whereas the site epimerization rates k ex of the system (SBI)ZrMe(CH 2 SiMe 3 )/[Ph 3 C][B(C 6 F 5 ) 4 ] continue to increase linearly with the total ion concentration, for (IPCF)ZrMe(CH 2 SiMe 3 )/[Ph 3 C][B(C 6 F 5 ) 4 ] mixtures k ex reaches a plateau at ca. 400 s -1 (at 20 °C). Measurement of site epimerization rates as a function of ion pair concentration [(A + ) x (B + ) 1-x X - ] therefore provides evidence for the existence of a rate-limiting barrier in the IPCF system, while it is absent in others. Introduction As a number of kinetic and mechanistic investigations have shown recently, 1-8 the polymerization of alkenes is catalyzed by group 4 metallocenium ion pairs [L 2 M-R + ··· X - ] (M ) Ti, Zr, Hf), or quite possibly by the more fluxional ion quadruples, 3-5 and follows a number of reaction steps: (i) the displacement of the counteranion from its equilibrium position in an associative interchange (I a ) mechanism, 2b (ii) dissymmetric monomer coordination, 6 (iii) migratory chain transfer to the -carbon of the coordinated monomer, and (iv) reassociation of the anion into an equilibrium position that mirrors the starting point of the insertion process (Scheme 1). 2b,5,7c In the polym- erization of small monomers such as ethene or propene, the * To whom correspondence should be addressed. E-mail: m.bochmann@ uea.ac.uk (M.B.). † University of East Anglia. ‡ Present address: Departamento de Quı ´mica Inorga ´nica, Orga ´nica y Bioquı ´mica, Universidad de Castilla-La Mancha, E-13071-Ciudad Real, Spain. § Universita ` di Perugia. ⊥ Universita ` di Salerno. (1) Reviews: (a) Bochmann, M. J. Organomet. Chem. 2004, 689, 3982. (b) Erker, G. Acc. Chem. Res. 2001, 34, 309. (c) Chen, E. Y.-X.; Marks, T. J. Chem. ReV. 2000, 100, 1391. (d) Resconi, L.; Cavallo, L.; Fait, A.; Piemontesi, F. Chem. ReV. 2000, 100, 1253. (e) Bochmann, M. J. Chem. Soc., Dalton Trans. 1996, 255. (f) Brintzinger, H. H.; Fischer, D.; Mu ¨lhaupt, R.; Rieger, B.; Waymouth, R. Angew. Chem., Int. Ed. Engl. 1995, 34, 1143. (g) Jordan, R. F. AdV. Organomet. Chem. 1991, 32, 325. (2) (a) Song, F.; Cannon, R. D.; Bochmann, M. J. Am. Chem. Soc. 2003, 125, 7641. (b) Song, F.; Hannant, M. D.; Cannon, R. D.; Bochmann, M. Macromol. Symp. 2004, 213, 173. (c) Bochmann, M.; Song, F.; Rodriguez, A.; Cannon, R. D. 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