Solid-state 73Ge NMR characterization of organogermanium compounds

Although NMR spectroscopy is a routine characterization technique for other group 14 elements, it is very rarely used in the case of germanium. This is due to the unfavourable NMR properties of 73Ge including a moderate quadrupole moment (-0.196 Barn), low natural abundance (7.76%) and extremely low gyromagnetic ratio (=-0.9332x107 rad T-1 s-1, corresponding to a Larmor frequency of only 31.6 MHz at 21.1 T). We are interested in developing 73Ge solid-state NMR spectroscopy as a method of structural characterization due to the availability of ultrahigh magnetic fields. We were initially interested in exploring the NMR properties of a germanium dication trapped in a molecular cage. Although attempts to obtain an NMR spectrum of this compound were ultimately unsuccessful, we were able to obtain a spectrum of GeCl2·dioxane, a valuable germanium(II) precursor. This relatively low symmetry compound yielded the largest germanium quadrupolar coupling constant CQ=44 MHz recorded to date [1]. We also investigated a series of organogermanium compounds. While tetraphenylgermane exhibited the expected single sharp line under MAS conditions, the static spectrum had a surprising line shape (Figure 1). This was attributed to the first observation of the chemical shielding anisotropy (CSA) in a 73Ge NMR spectrum. Examination of two other tetraarylgermanes under MAS condition revealed appreciable quadrupolar lineshape. We attributed this to slight deviations from ideal tetrahedral Figure 1: Static 73Ge NMR spectrum of tetraphenylgermane at 21.1 T