Mechanism of Redox‐Active Ligand‐Assisted Nitrene‐Group Transfer in a Zr^IV Complex: Direct Ligand‐to‐Ligand Charge Transfer Preferred

Abstract

The mechanism of the nitrene-group transfer reaction from an organic azide to isonitrile catalyzed by a Zr(IV) d(0) complex carrying a redox-active ligand was studied by using quantum chemical molecular-modeling methods. The key step of the reaction involves the two-electron reduction of the azide moiety to release dinitrogen and provide the nitrene fragment, which is subsequently transferred to the isonitrile substrate. The reducing equivalents are supplied by the redox-active bis(2-iso-propylamido-4-methoxyphenyl)-amide ligand. The main focus of this work is on the mechanism of this redox reaction, in particular, two plausible mechanistic scenarios are considered: 1) the metal center may actively participate in the electron-transfer process by first recruiting the electrons from the redox-active ligand and becoming formally reduced in the process, followed by a classical metal-based reduction of the azide reactant. 2) Alternatively, a non-classical, direct ligand-to-ligand charge-transfer process can be envisioned, in which no appreciable amount of electron density is accumulated at the metal center during the course of the reaction. Our calculations indicate that the non-classical ligand-to-ligand charge-transfer mechanism is much more favorable energetically. Utilizing a series of carefully constructed putative intermediates, both mechanistic scenarios were compared and contrasted to rationalize the preference for ligand-to-ligand charge-transfer mechanism.

New avenues for ligand-mediated processes – expanding metal reactivity by the use of redox-active catechol, o-aminophenol and o-phenylenediamine ligands

“…Recently, a detailed DFT computational study regarding the mechanism of this nitrogen group-transfer reaction was reported by the Baik group. 31 This study nicely supports the mechanism proposed by Heyduk, as electron transfer from the NNN ligand to the azido substrate was found to be energetically favoured over the classic metal-to-substrate electron transfer pathway. However, the calculated pathway to convert 16 into imido species 17 involves several intermediates.…”

Revealing the thermodynamic driving force for ligand-based reductions in quinoids; conceptual rules for designing redox active and non-innocent ligands

“…5 and more recent scaffolds. [73][74][75] It is even potentially applicable to ligands with longer unsaturated tethering chains, such nacnac or acac (Fig. 5).…”

An Isolable Azide Adduct of Titanium(II) Follows Bifurcated Deazotation Pathways to an Imide

“…Contrarily, Cummins et al used kinetic measurements and crossover experiments to propose that their V III azide adduct extrudes N 2 in a second-order process. More recently, Hall et al used DFT methods to demonstrate that Abu-Omar et al’s Re V oxo complexes traverse {Re­(α-N 3 R)} adducts when converting to the corresponding Re VII oxoimide products. , Finally, Baik and Ghosh identified a {Zr­(α,β-N 3 R)} intermediate in nitrene transfer catalysis . In view of these distinct pathways, we studied the conversion of 3 to 4 by 1 H NMR spectroscopy, using Si­(SiMe 3 ) 4 as an internal reference.…”

“…Although a transition state was not identified, the microscopic reverse of this process has a thermodynamic driving force of 33.7 kcal mol –1 ( vide infra , Figure ), which well surpasses the experimental barrier for deazotation. Second, Hillhouse et al’s isolation of {Ni­(β,γ-N 3 R)} adducts − coupled with the viability of Baik and Ghosh’s {Zr­(α,β-N 3 R)} computed intermediate suggests that N 2 extrusion could occur via initial slippage of the azide ligand to generate η 2 -N 3 Ad isomers of 3 (Figure , red dashed line). In fact, this leads to a [(Tp t Bu,Me )­TiCl­(β,γ-N 3 Ad)] intermediate ( 3 β,γ ) at slightly higher energy than 3 (7.0 kcal mol –1 ).…”

“…Interestingly, group 5 transition metals continue to define the border for isolable organoazide adducts of mononuclear d 2 fragments. ,, Given the decreasing d-orbital energies within transition series, this poses the question of whether divalent group 4 metals are too reducing to stabilize organoazides as ligands. As for the conversion of organoazides into imide complexes, a mechanism involving the formation and capture of free nitrene has sporadically been suggested. ,, But in general, mononuclear imide complexes (MNR) are assumed to form via simple mono- or binuclear pathways through intermediates where the metal/azide ratio is unity (examples in Chart B). − ,− Deviation from this ratio has been suggested for multimetallic systems such as Bergman’s [Cp 2 M­(μ-N t Bu)­(γ,γ;μ-N 3 Ph)­IrCp*] complexes (M = Zr, Hf), , but in this case, it is the binuclear nature of the complexes that dictates a 2:1 metal/azide ratio. Herein, we report how a tetrahedral, high-spin Ti II complex binds an organic azide to afford mixtures of the corresponding azide and imide products (Chart C), the ratio of which is determined by the relative concentrations of the reactants.…”