More Arrows in the Quiver: New Pathways and Old Problems with Heavy Alkaline Earth Metal Diphenylmethanides Jacob S. Alexander and Karin Ruhlandt-Senge* [a] Introduction The chemistry of s-bonded organometallic compounds of the heavy alkaline earth metals calcium, strontium, and barium has been the subject of considerable recent atten- tion. [1] This is a result of the wide range of applications for these previously ignored compounds, and what was a scarce- ly investigated field even a decade ago is now being ap- proached with significant interest. The number of well-char- acterized compounds is increasing steadily; a particular suc- cess has been the stabilization of these metal centers using cyclopentadienyl ligands. [2, 3] In contrast, s-bonded com- pounds remain scarce, owing largely to the synthetic difficul- ties encountered, most notably their high reactivity and sparse solubility in common aprotic solvents. Because of these challenges, we are interested in broadening the avail- able synthetic pathways to these compounds. Several possible synthetic techniques are available for the preparation of alkaline earth organometallics. Examples in- clude direct metallation, salt elimination, and transamina- tion. Direct metallation necessitates highly active and pure alkaline earth metals [Eq. (1)], in which R = alkyl, aryl; act = activated; and M = Ca, Sr, Ba. 2HR þ M act ! MR 2 þ H 2 ð1Þ Activation methods include the distillation of the metals, [4] the reduction of metal iodides with alkali metals, [5] or by dissolving the metals in anhydrous, deoxygenated liquid ammonia. [6] The difficulty in employing direct metal- lation is not just limited to the activation of the metal; very often the reaction products are insoluble without careful choice of a co-solvent or donor system, and the use of a ster- ically demanding ligand system. While this route has not yet led to structurally authenticated organometallic species, it has been employed with some success. [7, 8] By far the most commonly employed route in the prepa- ration of alkaline earth organometallics is salt elimination. Usually the potassium salt of the desired ligand is utilized in conjunction with an alkaline earth metal iodide to afford the desired product [Eq. (2)], in which R = alkyl, aryl; M = Ca, Sr, Ba; A = Na, K. MI 2 þ 2AR ! MR 2 þ 2AI ð2Þ This method, while powerful, has the drawback that the final product may be potentially contaminated with alkali metal halides. This can be minimized with careful choice of solvent, alkali metal, and halide, but the separation of prod- ucts and consequent purity remains a concern. In addition, the preparation of most potassium salts involves the use of ™superbase∫ chemistry, which employs an nBuLi/KOtBu mixture that may leave residual lithium alkoxide to interfere with later reactions. [9] Also, the high reactivity of the potassi- um salts limits the solvent choice, since ether cleavage chemistry is a frequent occurrence. Transamination chemistry has also been employed suc- cessfully. The ready availability of the alkaline earth metal amides M(N(SiMe 3 ) 2 ) 2 [M = Ca, Sr, Ba], [10±12] their solubility in a host of different solvent systems, and the easy removal [a] Dr. J. S. Alexander, Prof. Dr. K. Ruhlandt-Senge Department of Chemistry 1-014 Center for Science and Technology Syracuse University, Syracuse, NY 13244-4100 (USA) Fax: (+ 1) 315-443-4070 E-mail : kruhland@syr.edu Keywords: alkaline earth metals ¥ arene ligands ¥ ether cleavage ¥ organometallics ¥ structure elucidation Abstract: Progress in the field of s-bonded alkaline earth organometallics has been handicapped by numerous complications, such as high reactivity, low solubili- ty, and the limited availability of suitable starting materials. Here we present two synthetic methods, hydrocarbon elimination and desilylation, as alternative routes into this chemistry. A novel barium diphenylmethanide was prepared using these routes delineating that both methods provide a powerful, versatile synthetic access route to an extended library of organometallic alkaline earth derivatives. ¹ 2004 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim DOI: 10.1002/chem.200305362 Chem. Eur. J. 2004, 10, 1274 ± 1280 1274 FULL PAPER