Theoretical DFT studies of chromium tricarbonyl complexes with polycyclic aromatic ligands

This paper presents a theoretical analysis of the structures of tricarbonyl chromium complexes of carbo- and heterocyclic polyaromatic ligands (PAL) and the mechanisms of interring haptotropic rearrangement in such complexes performed by using density functional theory (DFT) with the nonempirically constructed PBE functional and extended split basis sets. The reaction paths were calculated for interring haptotropic rearrangements and rotations of the metalcarbonyl fragment in the regioisomeric complexes. The structures and energy characteristics of stationary points of the systems were determined. The migration of the Cr(CO)3 group was shown to occur at the periphery of the ligand via transition states with the structure of η3-allylic or η4-trimethylmethane complexes. Calculated geometries of the complexes and the activation barriers were in a close agreement with the experimental data. The reaction of η6-tricarbonylchromium complexes of PAL with n-BuLi (lithiation) was also studied by the DFT. The kinetic and thermodynamic factors that control the direction and selectivity of metallation were calculated for the model η6-biphenylenetricarbonylchromium complex. Both approaches indicate that lithiation occurs exclusively at the aromatic ring bonded to the transition metal, which agrees with the experimental data. The selectivity inside this ring is governed by a thermodynamic factor. The solvation effects were simulated for the lithium salt of the model η6-naphthalenechromium tricarbonyl complex in which lithium is localized at the α(1)-position of coordinated ring. The simulation showed the most stable coordination of the lithium atom with two THF molecules. Addition of extra THF molecules is thermodynamically unfavorable. The tricarbonylchromium complexes of naphthalene, biphenyl, biphenylene and dibenzothiophene calculated relative energies for all solvated by two THF molecules lithium salts indicate that the difference in energies ΔE ⩽ 1 kcal mol−1 corresponds to the experimentally observed absence of selectivity, while the difference more than 2.5 kcal mol−1 corresponds to the selectivity of the reaction. No additional coordination of lithium to heteroatom was observed for the sulfur-containing dibenzothiophene complex. Similar calculation shows that double metallation in the dibenzothiophene complex occurs at positions 1 and 4. The developed approach enables one to predict the direction and selectivity of metallation reactions of transition metal complexes with different arenes and thus to synthesize labeled complexes for the investigation of degenerate IRHR.