Synthetic, Structural And Theoretical Investigations Of Alkali Metal Germanium Hydrides – Contact Molecules And Separated Ions

In press, available from Chemistry – A European Journal. This is a paper a year or so in the making that, had I started it a year from now, would have taken a very different route. Much of the work I've done in neutron and terahertz spectroscopy has demonstrated that the inclusion of the crystal environment in quantum chemical treatments of solid-state systems is the key to interpreting the data (makes sense). This paper examines the unusual orientation of the [GeH3] anion in two crown ether complexes with potassium (K+) and rubidium (Rb+) cations. The crystal cells of these two complexes are far larger than computational resources would handle now (and definitely when the project started), but they'd be easily handled on better equipment (such as an 8-processor box with a terabyte or so of scratch space). The isolated molecule calculations (Dr. Alex Granovsky's PC-GAMESS version with basis sets from EMSL) demonstrate that the potential energy surfaces corresponding to anion orientations in the vicinity of only the solvated cations is shallow (at best) and that any moderate collection of electrostatic interactions (such as those in the crystal cell) may be enough to stabilize the unexpected anion orientation. It is also of interest to note that the [GeH3] anion prefers to bind to the K/Rb cations by the hydrogens (which we refer to as "inverted") and NOT the germanium anionic lone pair (the traditional, van't Hoff arrangement, all of those calculations being performed at B3LYP/6-311G(d,p) and MP2/6-311G(d,p) levels of theory with LANL2DZ ECPs for the K and Rb). This "oddity" was considered previously by the great Paul v. R. Schleyer and coworkers for a similar Na-SiH3 system some time back (Angew. Chem. 1994, 106, 221-223). This project will hopefully be revisited with solid-state density functional theory to see just how the crystal interactions combine to impose the non-traditional [GeH3] binding orientation.

karin geh3

W. Teng, D. G. Allis and K. Ruhlandt-Senge

Abstract: The preparation of a series of crown ether-ligated alkali metal (M = K, Rb, Cs) germyl derivatives M(crown ether)GeH3 via hydrolysis of the respective tris(trimethylsilyl)germanides is reported. Depending on the alkali metal and the crown ether diameter, the hydrides display either contact molecules or separated ions in the solid state, providing a unique structural insight into the geometry of the obscure GeH3 anion.

Germyl derivatives displaying M-Ge bonds in the solid state are of the general formula M([18]crown-6)(thf)GeH3 with M = K, 1; M = Rb, 4. Interestingly, the lone pair at germanium is not pointed towards the alkali metal, rather two of the three hydrides are approaching the alkali metal center to display M-H interactions.

Separated ions display alkali metal cations bound to two crown ethers in a sandwich-type arrangement and non-coordinated GeH3 anions to afford complexes of the type [M(crown ether)2][GeH3] with M = K, crown ether = [15]crown-5, 2; M = K, crown ether = [12]crown-4, 3 and M = Cs, crown ether = [18]crown-6, 5.

The highly reactive germyl derivatives were characterized using X-ray crystallography, 1H and 13C NMR, and IR spectroscopy. Density functional theory (DFT) and Second-Order Moeller-Plesset perturbation theory (MP2) calculations were performed to analyze the geometry of the GeH3 anion in the contact molecules 1 and 4.'t_Hoff

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