An Examination of Metal-Ligand Binding Modes in Rubidium Diphenylmethanide Jacob S. Alexander, Damian G. Allis, Bruce S. Hudson, and Karin Ruhlandt-Senge* Department of Chemistry, 1-014 Center for Science and Technology, Syracuse UniVersity, Syracuse, New York 13244-4100 Received July 10, 2003; E-mail: kruhland@syr.edu Alkali organometallics have been utilized for some time in a variety of applications, as summarized in several excellent reviews on the subject. 1-4 In addition to their value in organic synthesis, these compounds (particularly the potassium salts) have shown enormous utility in the preparation of organometallic compounds of heavier group II and III metals and are discussed as reactive intermediates in superbase chemistry. 5 An understanding of the structure-function relations of the reagents requires detailed insights into the metal-ligand binding, and extensive investigations have resulted in theory rationalizing metal-ligand binding trends. 6 The essence of this theory is that smaller cations, due to their higher charge density, are more capable of inducing charge localization in the anion. Structural information on heavy alkali metal benzyl and triphenyl derivatives includes several examples, 6-9 although information on the related diphenyls remains scant. 1,10,11 In our pursuit of heavy alkaline earth organometallics, we identified a recently reported class of silyl-substituted diphenyl- methanes as ideal starting materials. 12 While preparation of organo- metallic lithium compounds is typically facile, 13 difficulties in preparing the heavier alkali metal congeners necessitate alternate access routes. Most commonly, “superbase” chemistry, utilizing nBuLi/MOtBu mixtures (M ) Na, K, Rb, Cs), 5 is employed, although this method can suffer from lack of selectivity and difficulties in separating the lithium side product. Previous work has shown that the alkali metal assisted scission of element-silicon bonds can lead to metalated products through extrusion of silyl ether (eq 1). 14,15 It was expected that the additional drive to form a resonance stabilized anion, as seen here, would make this reaction extremely facile and provide for a powerful new route to these compounds. The reaction of the silylated ligand with heavy alkali metal tert- butoxides (K, Rb, Cs) cleanly affords the diphenylmethanide derivatives under subsequent formation of silyl ether, 16 demonstrat- ing the strong thermodynamic drive toward Si-O bond formation. We here present crown ether encapsulated rubidium diphenyl- methanide, where two different solid-state modifications with different metal coordination modes are observed. Crystallization at -23 °C led to the η 3 contact rubidium diphenylmethanide 1 (Figure 1) in which the metal assumes a geometry with one face capped by the crown ether and the other occupied by the ligand, resulting in a metal coordination number of nine. The rubidium is directly bonded to the deprotonated methylene carbon of the ligand at a distance of 3.063(3) Å with two longer interactions to the phenyl rings at 3.311(3) and 3.393(3) Å. The methine hydrogen position was calculated, and the geometry around the ipso carbon displays a C2-C1-C8 angle of 132.6(3)°. These compare favorably with those in the separated lithium and contact sodium structures. 10,11 The phenyl rings are slightly twisted (4.4° and 7.6°) relative to the plane of the methine, methine hydrogen, and ipso carbons. This is in contrast to the alkali metal triphenyl- methanides, in which these angles regularly exceed 25°. 6 Crystallization of the reaction mixture at 4 °C leads to the formation of the η 6 coordinated rubidium diphenylmethanide 2 where the metal is again encapsulated by crown ether (Figure 2). The metal sits slightly below the center of the crown ether with average Rb-O distances of 2.85(5) Å. THF is located at an axial position, while one η 6 coordinated ligand phenyl group occupies the other with a metal-ring (centroid) distance of 3.076(9) Å, resulting in a formal coordination number of 13. On average, the phenyl bond lengths in the bound ring do not deviate significantly from those in the unbound ring with values of 1.382-1.412(15) Å. With the methine hydrogen atom in a calculated position, the geometry of the central carbon compares well with 1, displaying a C2-C1-C8 angle of 133.0(9)°. Figure 1. Crystal structure of 1. Non-carbon atoms are displayed as thermal ellipsoids with 30% probability. Hydrogen atoms have been removed for clarity. Figure 2. Crystal structure of 2. Non-carbon atoms are displayed as thermal ellipsoids at 30% probability. Hydrogen atoms have been removed for clarity. Si(SiMe 3 ) 4 + KOtBu f KSi(SiMe 3 ) 3 + Me 3 Si-OtBu P(SiMe 3 ) 3 + KOtBu f KP(SiMe 3 ) 2 + Me 3 Si-OtBu (1) HCPh 2 SiMe 3 + MOtBu 9 8 M ) K, Rb, Cs MCHPh 2 + Me 3 Si-OtBu (2) Published on Web 11/15/2003 15002 9 J. AM. CHEM. SOC. 2003, 125, 15002-15003 10.1021/ja037201y CCC: $25.00 © 2003 American Chemical Society