Direct Ortho-Metallation of Aryl Substituted Heteroaromatic Ligands with TiCl 4 and ZrCl 4 - An Experimental and DFT Study with 2-Phenyl Indole as the Ligand.

21 August 2019, Version 1
This content is a preprint and has not undergone peer review at the time of posting.


TiCl 4 and ZrCl 4 each react with aryl substituted heteroaromatic ligands such as 2-Phenyl-1H-indole, to thermally undergo one-pot direct orthometallations, and yield new types of cyclometallated complexes. TiCl 4 coordinates at ambient temperature to form the indole complex ( 1 ), which undergoes isomerization to the indolenine ( 2 ). DFT calculations indicate that complex ( 2 ) is more stable than ( 1 ) by 6.4 kcal/mol. Upon warming to about 105°C, extrusion of HCl takes place with simultaneous orthometallation ( 3 ), yielding a metallacyclic complex ( 4 ). The mechanism of the orthometallation has been investigated by DFT, and the transition state c onfir med by IRC. At the elevated temperature the transition state ( 3 ) involves the synchronous transformation of four atoms, Ti, ortho C, H, and the apical Cl. The ortho C of the phenyl group acquires a partial positive charge through conjugation, forming a (C-H)δ + ...Clδ - interaction, with a simultaneous elongation and breaking of the Ti-Cl bond, resulting in the formation of a Ti-C bond. The latter bond is created at the same time a Ti-Cl bond is breaking, and an HCl is being formed, as illustrated in transition state ( 3 ). This HCl is retained in the crystal structure of the final product ( 4 ), by electrostatic interaction with one of the chloride ligands. The reaction sequence may be repeated with ZrCl 4 in place of TiCl 4 . Complex ( 4 ) has been isolated and characterized by solid state 13 C NMR CPMAS/DDMAS spectra, X-ray photoelectron spectroscopy (XPS), infrared and analytical data. The intermediate structures ( 1 through 4 ), as well as the sequence of ligand transformations to produce the ortho-metallated complex are supported by DFT calculations. The new cyclometallated complexes are thermally stable, unlike several other complexes featuring a Ti-C bond. They may have important applications, such as in α-olefin polymerization catalysis, and as building blocks in metalodrugs for cancer therapy.


heteroaromatic ligands
2-phenyl indole
reaction mechanism
polymerization catalysis
Ti/Zr organometallics


Comments are not moderated before they are posted, but they can be removed by the site moderators if they are found to be in contravention of our Commenting Policy [opens in a new tab] - please read this policy before you post. Comments should be used for scholarly discussion of the content in question. You can find more information about how to use the commenting feature here [opens in a new tab] .
This site is protected by reCAPTCHA and the Google Privacy Policy [opens in a new tab] and Terms of Service [opens in a new tab] apply.