Are Metallocene−Acetylene (M = Ti, Zr, Hf) Complexes Aromatic Metallacyclopropenes?

Abstract

The bonding nature of metallocene acetylene complexes Cp 2 M(η 2 -H 3 SiC 2 SiH 3 ) 1M and Cp 2 M(η 2 -HC 2 H) 1M 0 (M = Ti, Zr, Hf) was studied by density functional theory method. It is found that this acetylene complex has indeed a metallacyclopropene moiety with two in-plane M-C σ-bonds and one out-of-plane π-bond interacting with the metal center, resulting in the formation of a delocalized three-center and two-electron (3c-2e) system. Along with its delocalized out-of-plane bonding, this complex has been characterized as aromatic on the basis of the computed stabilizing energy and negative nucleus-independent chemical shifts (NICS). The aromatic stabilization increases from Ti to Zr and Hf, and this is because of the increased charge separation between the Cp 2 M fragment and the H 3 SiC 2 SiH 3 (also HC 2 H) unit. The decrease of the M-C bond length from Zr to Hf is attributed to the increased s character of both M and C hybridization of the M-C σ-bonds.

Peculiarities of Vibrational Spectra and Electronic Structure of the Five‐Membered Metallacyclocumulenes of the Group 4 Metals

“…These molecules contain three-membered metallacycles that are considered [10,22a] to be aromatic metallacyclopropene moieties with a delocalized 3c-2e π-bond system. This treatment is in accord with the negative NICS values [10] and experimental ν(C=C) frequencies in the IR and Raman spectra (Table 3), where the shift of the initial acetylenic ν(CϵC) frequency (Δν = 500-650 cm -1 ) is typical for strong complexation. [15,22] However, the AIM critical point search for Cp* 2 Ti(η 2 -Me 3 SiC 2 SiMe 3 ) (6) ( Figure 6) and a model complex, Cp 2 Ti(η 2 -tBuC 2 tBu), resulted in an ordinary pattern (Figure S4).…”

“…Several theoretical DFT studies have been devoted to this matter. [9][10][11] Some studies [9] confirm the strong metal-π interaction in 1 and 2, which results in substantial cyclic electron delocalization (in-plane aromaticity) and (η 2 -σ,σ) + (η 2 -π,π) coordination of the ligand. Aromaticity is also evidenced by high negative nucleus-independent chemical shift (NICS) values.…”

On the nature of the dative bond: Coordination to metals and beyond. The carbon case

“…More recently, a detailed DFT study on the nature of the dative vs purely covalent bonding in titanocene, zirconocene and hafnocene complexes 3a-c, indicated a delocalized three-center and two-electron metallacyclopropene form involving a d 0 configuration of M, two in-plane M-C -bonds, and one out-of-plane M-(C 2 ) -bond (Fig. 5) [115]. The metallacyclopropene ring was ascertained to be magnetically aromatic on the basis of negative values of the nucleus independent chemical shift 1 Å above the center of the ring (NICS(1)…”

“…5). metallocenes (left), and aromatic character data for the corresponding two-π-electron metallacyclopropene ring (right box) [115].…”

Imidozirconocene‐Mediated Ring Cleavage of Epoxides – Evidence for Bifunctional Reactivity from DFT

“…[21] All calculations have been performed at B3LYP/LANL2DZ level [22] as this has been found to perform reasonably well by earlier studies on organometallic chemistry. [23][24][25][26] Zero point energies and thermal contributions to thermodynamic functions and activation parameters, as well as harmonic frequencies have been computed at the same level of theory on the optimized structures. Frequency calculations have been performed to confirm the nature of the stationary points, transition states (TSs) have one imaginary frequency and minima have real frequencies.…”