Novel metal-[metal oxide]-nonmetal sandwich-like superalkali compounds Li_3OMC_5H_5 (M = Be, Mg, and Ca): How to increase the aromaticity of Li_3~+ ring?

摘要

The structures of novel metal-[metal oxide]-nonmetal sandwich-like superalkali compounds, that is, H- and T-shaped Li_3OMC _5H_5 (M = Be, Mg, and Ca), with all the real frequencies are obtained for the first time at the MP2/6-311+G(2d, p) level. For M = Be, the T-shaped isomer is more stable than the H-shaped one, but the H-shaped isomer is more stable than the T-shaped one for M = Mg and Ca. The natural bond orbital analysis indicates that these compounds are complex zwitterions and can be denoted as (Li_3)~+(O~(2-)M~(2+))(C _5H_5)~- with two aromatic rings (Li_3~+ and C_5H_5~-). Four factors to increase the nucleus-independent chemical shift (NICS) of Li_3~+ ring in Li_3OMC_5H_5 systems are found. (1) Replacing the T-shaped structure with parallel Li_3~+ and C_5H_5~- ring by the corresponding H-shaped structure with perpendicular Li_3~+ and C_5H_5~- ring, the NICS value considerably increases from -7.8 ～ -8.2 to -22.2 ～ -43.4 ppm. (2) The existence of the neighboring alkaline earth metal oxide subunit evidently increases the NICS in H-shaped structures from about -11.1 to -16.1 ～ -37.0 ppm. (3) Based on the finding that the alkaline earth atomic number dependence of the aromaticity of Li_3~+ ring, the larger atomic number increases the NICS value. (4) The end C_5H_5~- subunit increases the NICS value. For example, the increase is -6.4 ppm for H-shaped Li _3OCaC_5H_5. In addition, the expected out-of-plane σ-aromaticity of Li_3~+ ring is not exhibited, in contrast to that in the sandwichlike structure of Li _3OLi_3 (Chen et al., J Chem Phys, 2005, 123, 164306), but the in-plane σ- aromaticity of it is increased. Why? This is because (1) the size of the OM subunit near the Li_3~+ ring is small, and (2) the large-sized C_5H_5~- subunit is far from the Li_3~+ ring. For these Li_3OMC _5H_5, the H-shaped structure exhibits electride characteristics, and the T-shaped structure with lithium anion exhibits alkalide characteristics.