Novel metal‐[metal oxide]‐nonmetal sandwich‐like superalkali compounds Li 3 OMC 5 H 5 (M = Be, Mg, and Ca): How to increase the aromaticity of Li 3 + ring?

The structures of novel metal‐[metal oxide]‐nonmetal sandwich‐like superalkali compounds, that is, H‐ and T‐shaped Li 3 OMC 5 H 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 5 H 5 ) − with two aromatic rings (Li   3 +and C 5 H   5 − ). Four factors to increase the nucleus‐independent chemical shift (NICS) of Li   3 +ring in Li 3 OMC 5 H 5 systems are found. (1) Replacing the T‐shaped structure with parallel Li   3 +and C 5 H   5 −ring by the corresponding H‐shaped structure with perpendicular Li   3 +and C 5 H   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 5 H   5 −subunit increases the NICS value. For example, the increase is −6.4 ppm for H‐shaped Li 3 OCaC 5 H 5 . In addition, the expected out‐of‐plane σ‐aromaticity of Li   3 +ring is not exhibited, in contrast to that in the sandwich‐like structure of Li 3 OLi 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 5 H   5 −subunit is far from the Li   3 +ring. For these Li 3 OMC 5 H 5 , 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