Aromaticity of 5- and 7‑Membered Metallacyclocumulenes of Group 4

Using the recent aromatic criteria,
the conjugation and aromatic
properties of group 4 metallacyclocumulenes Cp2M-η4–(R)­CCCC–R (1) and Cp2M-η3(R–CCCC­(R)–R­(C)
CR) (2) (M = Ti, Zr, Hf) were studied. To these
nontrivial bonded metallacycles, the most recent universal criteria
of aromaticity were applied that do not require any parametrisation
such as isomerisation stabilization energy, electron density of delocalized
bonds (EDDB), nuclear independent chemical shift, and gauge-included
magnetically induced currents (GIMIC). All methods used indicated
the absence of the common π-type aromaticity. The most efficient
EDDB and GIMIC methods showed in-plane aromatic pathway in 5-membered
cycle 1 against 7-membered 2. The distribution
of the induced currents (ICs) for such unique aromaticity type was
demonstrated. It shows that the diatropic ICs of 1 are
located both inside and outside the cycle. The population analysis
of molecular orbitals and natural orbitals for delocalized bonds revealed
the abidance to the 4n + 2 electron rule. According
to EDDB and GIMIC data, the degree of aromaticity for 1 tends to decrease from Ti to Zr and Hf.