r,r-Bis(trimethylsilyl)-Substituted Benzyl Complexes of Potassium and Calcium Florian Feil and Sjoerd Harder* Universita ¨ t Konstanz, Postfach 5560, M738, 78457 Konstanz, Germany Received July 18, 2000 The first benzylcalcium complex, (R,R-bis(trimethylsilyl)benzyl) 2 calcium‚(THF) 2 (7), has been prepared by reacting CaI 2 with R,R-bis(trimethylsilyl)benzylpotassium (6) in THF. The solid state structure of the K precursor shows an interesting Lewis base-free coordination polymer in which the coordination sphere of K is additionally saturated by agostic Si- Me‚‚‚K interactions. The crystal structure of the Ca product displays a THF-solvated monomeric compound, which shows considerably less delocalization of negative charge into the phenyl ring than the corresponding K compound. NMR investigations as well as ab initio calculations show that the TMS substituents at the benzylic carbon have a charge-localizing influence. Only the more ionic K precursor shows activity in initiating the polymerization of styrene. Introduction Although the organometallic chemistry of magnesium has been developed already in a very early stage, chemistry of the heavier alkaline-earth metals (Ca, Sr, and Ba) attracted increased attention only over the past decade. 1 Research, however, mainly focused on the more volatile and less reactive cyclopentadienides and alkox- ides for use in CVD 2 or on the amides 3 which show potential in syntheses of Ca, Sr, and Ba compounds of intermediate basicity (e.g., fluorenyl 4 or acetylide 5 com- plexes). Only a few reports appeared on well-defined more basic and more reactive compounds, e.g., TMS- substituted alkylcalcium (1, 2) 6,7 or allylcalcium (3) 8 compounds, a tBu-substituted pentadienyl calcium com- pound (4), 9 and a mixed phospholide/alkenide barium compound 5. 10 It is our aim to find an easy route for the preparation of reactive benzylcalcium compounds which could be used as initiators for the anionic polymerization of styrene. The only benzylcalcium compound described in the literature so far has been prepared via the highly undesired mercurial route: 11a (PhCH 2 ) 2 Hg + Ca 0 f (PhCH 2 ) 2 Ca + Hg 0 . We recently developed an easy and efficient method for the synthesis of dibenzylbarium from benzyllithium and a barium amide (see eq 1). 11b This metal exchange reaction is based on the HSAB principle 12 and similarly works with barium alkoxides and dibenzylmagnesium as the starting materials. Hitherto, any attempts to prepare benzylcalcium com- pounds in an analogous manner failed due to incomplete (1) For reviews see: (a) Lindsell, W. E. In Comprehensive Organo- metallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E., Eds.; Pergamon Press: Oxford, 1982; Vol. 1, Chapters 2-4. (b) Hanusa, T. P. Polyhedron 1990, 9, 1345. (c) Hanusa, T. P. Chem. Rev. 1993, 93, 1023. (d) Burkey, D. J.; Hanusa, T. P. Comm. Inorg. Chem. 1995, 17, 41. (2) Hubert-Pfalzgraf, L. G. New J. Chem. 1987, 11, 663. (b) Bradley, D. C. Chem. Rev. 1989, 89, 1317. (c) Hubert-Pfalzgraf, L. G. Appl. Organomet. Chem. 1992, 6, 627. (d) Herrmann, W. A.; Huber, N. W.; Priermeier, T. Angew. Chem. 1994, 106, 102; Angew. Chem., Int. Ed. Engl. 1994, 33, 105. (3) Bradley, D. C.; Hursthouse, M. B.; Ibrahim, A. A.; Abdul Malik, K. M.; Motevalli, M.; Mo ¨seler, R.; Powell, H.; Runnacles, J. D.; Sullivan, A. C. Polyhedron 1990, 9, 2959. (b) Hitchcock, P. B.; Lappert, M. F.; Lawless, G. A.; Royo, B. J. Chem. Soc., Chem. Commun. 1990, 1141. (c) Westerhausen, M. Inorg. Chem. 1991, 30, 96. (d) Vaartstra, B. A.; Huffman, J. C.; Streib, W. E.; Caulton, K. G. Inorg. Chem. 1991, 30, 121. (e) Westerhausen, M.; Schwarz, W. Z. Anorg. Allg. Chem. 1991, 604, 127. (4) Harder, S.; Lutz, M.; Straub, A. W. G. Organometallics 1997, 16, 107. (5) Green, D. C.; Englich, U.; Ruhlandt-Senge, K. Angew. Chem. 1999, 111, 365; Angew. Chem., Int. Ed. 1999, 38, 354. (b) Burkey, D. J.; Hanusa, T. P. Organometallics 1996, 15, 4971. (6) Cloke, F. G. N.; Hitchcock, P. B.; Lappert, M. F.; Lawless, G. A.; Royo, B. J. Chem. Soc., Chem. Commun. 1991, 724. (7) Eaborn, C.; Hawkes, S. A.; Hitchcock, P. B.; Smith, J. D. J. Chem. Soc., Chem. Commun. 1997, 1961. (8) Harvey, M. J.; Hanusa, T. P.; Young, V. G., Jr. Angew. Chem. 1999, 111, 241; Angew. Chem., Int. Ed. 1999, 38, 217. (9) Overby, J. S.; Hanusa, T. P. Angew. Chem. 1994, 106, 2300; Angew. Chem., Int. Ed. Engl. 1994, 33, 2191. (10) Westerhausen, M.; Digeser, M. H.; Gu ¨ ckel, C.; No ¨th, H.; Knizek, J.; Ponikwar, W. Organometallics 1999, 18, 2491. (11) Takahashi, K.; Kondo, Y.; Asami, R. J. Chem. Soc., Perkin Trans 2 1978, 577. (b) Weeber, A.; Harder, S.; Brintzinger, H.-H.; Knoll, K. Organometallics 2000, 19, 1325; (12) Grovenstein, E., Jr. In Recent Advances in Anionic Polymeri- zation; Hogen-Esch, T. E., Smid, J., Eds.; Elsevier: Amsterdam, 1987; p 3. (b) Harder, S.; Streitwieser, A. Angew. Chem. 1993, 105, 1108; Angew. Chem., Int. Ed. Engl. 1993, 32, 1066. (c) Kremer, T.; Harder, S.; Junge, M.; Schleyer, P. v. R. Organometallics 1996, 15, 585. 5010 Organometallics 2000, 19, 5010-5015 10.1021/om0006209 CCC: $19.00 © 2000 American Chemical Society Publication on Web 10/27/2000