Synthesis and structures of crystalline bis(trimethylsilyl)methanido complexes of potassium, calcium and ytterbium

Crystalline [K(μ-R)(thf)]∞ (1) was obtained from equivalent portions of n-butyllithium in hexane, bis(trimethylsilyl)methane (RH) and potassium t-butoxide in thf, removal of volatiles and extraction with hexane. Desolvation of 1 in a vacuum led to KR. The first three-coordinate metallate(II) alkyls [K(MR3)]∞ [M=Ca (2), M=Yb (3)] of calcium and ytterbium(II) were prepared from the appropriate metal(II) iodide and three equivalents of KR in benzene. Mixing LiR, YbI2 and two equivalents of KR in a mixture of diethyl ether and small amount of thf yielded the red (like 3) [Li(thf)4][YbR3] (4). Each of 1–4 was obtained in good yield and was characterised by multinuclear NMR spectra in C6D6 and single crystal X-ray diffraction. The central metal is in a trigonal planar 1 or pyramidal 2–4 environment and the average MC bond lengths are 2.98 (1), 2.50 (2), 2.52 (3 and 4) Å. Crystalline 2 and 3 are isomorphous and consist of double chains of [MR3]− anions linked by K+ cations along the a axis, whereas complex 4 has an ionic structure with isolated [Li(thf)4]+ cation and [YbR3]− anion.