Short‐Lived Orthobenzyne Complexes with Early Transition Metals of Group IV. First Direct Characterization and Electronic Cartography by Coupling FVT/UV‐PES with Calculations

Abstract

Short-lived η 2 -benzyne metallocene complexes [Cp 2 M(η 2 -c-C 6 H 4 )] with group IV metals are important intermediates in organic chemistry, their olefin insertion products being building blocks for aromatic and heterocyclic compounds. For the first time, an electronic cartography of these short-lived species has been obtained through the determination of their ionization potentials. They have been directly characterized in the gas phase by UV photoelectron spectroscopy, by monitoring, in situ, the thermal -H abstraction of benzene from [a]

Group‐IV‐based selective CH bond activation of a diamondoid—A density functional theory study

“…A similar benzyne formation from Cp 2 MPh 2 was studied computationally by Miqueu and co‐workers [34]. The reported free energy changes and barriers for formation of Cp 2 M(benzyne) from Cp 2 MPh 2 were 0.80/4.30/−12.3 kcal/mol (Δ G for Zr/Hf/Ti) and 32.1/36.2/31.4 (Δ G ‡ for Zr/Hf/Ti) [34].…”

“…A similar benzyne formation from Cp 2 MPh 2 was studied computationally by Miqueu and co‐workers [34]. The reported free energy changes and barriers for formation of Cp 2 M(benzyne) from Cp 2 MPh 2 were 0.80/4.30/−12.3 kcal/mol (Δ G for Zr/Hf/Ti) and 32.1/36.2/31.4 (Δ G ‡ for Zr/Hf/Ti) [34]. These numbers, obtained with CAM‐B3LYP and the fairly large basis set SDD+f(M),6‐31++G**(C,H), are very similar to the numbers we found for the substituted systems, shown in Tables 1 and 2.…”

“…Bond distances are shown in Figure 8. This assignment is supported by the recent computational analysis, which used advanced methods for the analysis of ground state wave functions and densities (such as NBO, ELF, and AIM), concluding for Cp 2 Zr(C 6 H 4 ) and the Hf analogue (but to a lesser degree for the Ti analogue), that in this resonance description the metallacyclic structure seems to have the larger weight [34]. This implies that the oxidation state of the metal centre remains the same throughout, with a value of +IV (d 0 ).…”

“…The CH bond activations of tethered groups that were reported in 1987 by Erker and Mühlenbernd [22] have not been studied by DFT before, although the type of complex proposed as an intermediate (a bis‐cyclopentadienyl group IV metal benzyne complex) has been studied [34]. Figures 4 and 5 show the mechanism that was proposed by Erker et al, for the zirconium cyclometallation of a tert ‐butyl group and of a phenyl group (in C(Me) 2 ‐Ph), respectively, but for the general group IV case (M = Zr, Hf, or Ti).…”

“…Being highly reactive is perfectly compatible with thermodynamic stability on a similar level as the reactant. The best experimental evidence that Cp 2 M(benzyne) species are not significantly uphill from their Cp 2 MPh 2 precursors comes from the Miqueu group as well [34]. They were able to spectroscopically observe all three Cp 2 M(benzyne) species (M = Zr, Hf, Ti) in the gas phase, after thermolysis of their precursors [34].…”