Karin Ruhlandt-Senge

Not Just Heavy “Grignards”: Recent Advances in the Organometallic Chemistry of the Alkaline Earth Metals Calcium, Strontium and Barium

Since their initial development in the early 1900’s, Grignard reagents have proven to be immensely useful and are among the most common organometallic reagents. The nature of the reagents in solution is complex and depends on substituents, solvent, concentration and temperature. Despite continuing questions about their solid state and solution composition and conformation, organomagnesium reagents find new applications continually. In contrast, little information about the heavier alkaline earth organometallic compounds RMX and R2M (R = alkyl, aryl; M = Ca, Sr, Ba, X = halide) exists. High reactivity due to the predominantly ionic character of the metal-ligand bond and increased lability complicates synthetic access. Recent interest in the organometallic chemistry of the alkaline earth metals calcium, strontium and barium has been sparked by the realization that heavy alkaline earth metal organometallics play unique roles in synthetic applications, such as polymerization initiators and as reagents to modify polymers. As a result, the organometallic chemistry of calcium, strontium and barium has received significant attention over the last few years. This article summarizes recent results in this emerging area of chemistry. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Synthesis and stabilization—advances in organoalkaline earth metal chemistry

The last decade has seen an impressive growth in alkaline earth metal chemistry, with applications ranging from synthetic organic and polymer chemistry to materials science. As a consequence, alkaline earth metal chemistry has made a leap from obscurity into the spotlight of modern organometallic chemistry. Much of this rapid development was made possible by the establishment of novel synthetic procedures that allowed facile access to the target compounds, as many conventional synthetic routes posed and continue to pose significant limitations. Novel approaches have allowed the preparation of a multitude of compounds, initiating progress not thought possible just five years ago. Examination of the new compounds delineates several factors responsible for their structure and function. Key elements in the coordination, aggregation behavior, and reactivity of these systems have been linked to secondary interactions, including M-Cpi, M-Npi, M-F, and M-H(agostic) interactions. This feature article will provide a very brief overview of established synthetic procedures, including a brief discussion on specific shortcomings. This will be followed by a detailed presentation of novel methodologies that are the core of the rapid development of alkaline earth metal chemistry. The second part of the article will be concerned with the analysis of various secondary interactions and their role in the further development of this rapidly emerging field of chemistry.

CALCIUM, STRONTIUM, AND BARIUM ACETYLIDES : NEW EVIDENCE FOR BENDING IN THE STRUCTURES OF HEAVY ALKALINE EARTH METAL DERIVATIVES

An unprecedented ligand bending mode is displayed by the acetylide ligands in the first structurally characterized σ-bound organometallic strontium and barium complexes [M([18]crown-6)(CCSiPh3 )2 ] (M=Sr, Ba). Furthermore, the observed decrease of the angle at the sp-hybridized C atom on descending Group 2 (see structures depicted) affords new information that will lead to a better understanding of the bonding in alkaline earth metal compounds.

The X-ray crystal structures of the mononuclear silyllithium species Li(THF)3SiPh3 and Li(THF)3Si(SiMe3)3

Abstract The organosilyllithium tetrahydrofuranate complexes Li(THF) 3 SiPh 3 ( 1 ) and Li(THF) 3 Si(SiMe 3 ) 2 ( 2 ) were crystallized and structurally characterized by X-ray crystallography. Both compounds possess monomeric structures with relatively long ( ca . 2.67 A) LiSi bonds. In addition, the lithium-bound silicons have distorted tetrahedral geometries with low CSiC and SiSiSi angles of 101.3° (av.) ( 1 ) and 102.6° (av.) ( 2 ). Compound 1 , which has not been reported as an isolated species, has been characterized by 1 H, 7 Li, 29 Si NMR and IR spectroscopy. These studies indicate that the LiSi bond remains intact in THF solution at −80°C.

Thiolates, selenolates, and tellurolates of the s-block elements

Abstract The potential of alkali and alkaline earth metal chalcogenolates in synthetic chemistry and various technical applications has sparked the recent interest in the chemistry of alkali and alkaline earth metal thiolates, selenolates and tellurolates. As a result, an increasing body of work concerned with exploring synthetic routes to the target compounds, analyzing the influence of metal, ligand and donor on the structural chemistry, and correlating structure and function has appeared in the literature, most of which during the last few years. This article describes recent trends in this area of alkali and alkaline earth chemistry, by discussing synthetic access routes, analyzing structure determining factors such as metal, donor and ligand influence, comparing structural similarities and disparities in alkali and alkaline earth chemistry, and discussing structure–function relationships.

Reaction pathways towards 1,1,2,2-tetratbutyldistannanes

Abstract 1,2-Dichlorotetratbutyldistannane (1) is synthesized as a byproduct in the reaction of tbutylmagnesium chloride with tin tetrachloride or by the reaction of ditbutyltindihydride with ditbutyltin dichloride in the presence of amines. Reaction of 1 with lithium aluminumhydride yielded the dihydrido derivative 2. The treatment of 2 with bromoform gave the dibromo distannane 3 nearly quantitatively. Reactions of 2 with one equivalent of LDA or KH yielded unsymmetrically substituted alkali metal distannanes of the type MtBu2SnSn(H)tBu2 (4, M=Li; 5, M=K). Reaction of either two equivalents of KH with 2, or one equivalent of KH with 5 gave the dipotassium distannide 6. The molecular structure of the dichloro derivative 1 was determined by X-ray diffraction.

Barium Triphenylmethanide: An Examination of Anion Basicity

Differences in anion basicity seem to be key for the formation of the first charge-separated barium triphenylmethanide versus a novel heteroleptic vinyl ether which results from cleavage of the attendant [18]crown-6. Ba: green; O: red; P: yellow; N: blue.

s-Block organometallics: analysis of ion-association and noncovalent interactions on structure and function in benzyl-based compounds.

The organometallic chemistry of alkali and alkaline-earth metals has been marred by synthetic setbacks because of their high reactivity. Advances in their synthesis and a better understanding of the stabilization effects of ligands and coligands have resulted in the revolution of s-block organometallics. Among those, benzyl-based derivatives have played a key role in developing this chemistry because factors such as the ligand size, charge delocalization, and introduction of electronic parameters along with metal effects can be analyzed. This article will focus on s-block benzylates and di- and triphenylmethanide derivatives with specific emphasis on the factors that stabilize the highly reactive metal species.

Novel triphenyltin substituted derivatives of heavier alkaline earth metals

Abstract The synthesis and characterization of a family of novel alkaline earth metal stannides, in addition to tri- and pentastannanes are described. [Ba(18-crown-6)(HMPA) 2 ][SnPh 3 ] 2 ( 4 ), [Ca(18-crown-6)(HMPA) 2 ][Sn(SnPh 3 ) 3 ] 2 ( 5 ), and [Sr(18-crown-6)(HMPA) 2 ][Sn(SnPh 3 ) 3 ] 2 ( 6 ) were synthesized by insertion of the corresponding alkaline earth metals into the tintin bond of hexaphenyldistannane. Ph 3 SnSnPh 2 SnPh 3 ( 7 ), and Sn(SnPh 3 ) 4 ( 8 ) became available by treating 4 with diphenyldichlorostannane. All compounds were studied by NMR spectroscopy, and X-ray crystallography. The stannanes 7 and 8 were also characterized by elemental analysis.

Developments in heterobimetallic s-block systems: synthesis and structural survey of molecular M/Ae (M=Li, Na, K, Cs; Ae=Ca, Sr) aryloxo complexes.

A series of novel heterobimetallic group 1/strontium and group 1/calcium aryloxo complexes having the composition [MAe(Odpp)3] [Ae=Sr and M=Na (1), K (2, 3), Cs (4); Ae=Ca and M=Na (5), K (6), Cs (7)] or [M2Ae(Odpp)4] [M=Li and Ae=Sr (9), Ca (10)] have been prepared using 2,6-diphenylphenol (HOdpp) as the ligand. Through the use of solid-state direct metalation, these compounds were obtained either directly from the reaction vessel or after workup in toluene. The Lewis base adduct [KCa(Odpp)3(thf)] (8) was obtained by treatment of [KCa(Odpp)3] (6) with tetrahydrofuran (thf). All of the compounds displayed extensive metal-pi-arene interactions, which provide significant stabilization in these reactive species. The thermal stabilities and volatilities of representative heterobimetallic strontium and calcium complexes were investigated using thermogravimetric analysis.