Novel metal‐[metal oxide]‐nonmetal sandwich‐like superalkali compounds Li3OMC5H5 (M = Be, Mg, and Ca): How to increase the aromaticity of Li ring?

Abstract

AbstractThe structures of novel metal‐[metal oxide]‐nonmetal sandwich‐like superalkali compounds, that is, H‐ and T‐shaped Li3OMC5H5 (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 (Li3)+(O2−M2+)(C5H5)− with two aromatic rings (Li and C5H). Four factors to increase the nucleus‐independent chemical shift (NICS) of Li ring in Li3OMC5H5 systems are found. (1) Replacing the T‐shaped structure with parallel Li and C5H ring by the corresponding H‐shaped structure with perpendicular Li and C5H 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 ring, the larger atomic number increases the NICS value. (4) The end C5H subunit increases the NICS value. For example, the increase is −6.4 ppm for H‐shaped Li3OCaC5H5. In addition, the expected out‐of‐plane σ‐aromaticity of Li ring is not exhibited, in contrast to that in the sandwich‐like structure of Li3OLi3 (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 ring is small, and (2) the large‐sized C5H subunit is far from the Li ring. For these Li3OMC5H5, the H‐shaped structure exhibits electride characteristics, and the T‐shaped structure with lithium anion exhibits alkalide characteristics. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010