Mechanistic Insight into the Reactivity of Oxotransferases by Novel Asymmetric Dioxomolybdenum(VI) Model Complexes

Abstract

The asymmetric molybdenum(VI) dioxo complexes of the bis(phenolate) ligands 1,4-bis(2-hydroxybenzyl)-1,4-diazepane, 1,4-bis(2-hydroxy-4-methylbenzyl)-1,4-diazepane, 1,4-bis(2-hydroxy-3,5-dimethylbenzyl)-1,4-diazepane, 1,4-bis(2-hydroxy-3,5-di-tert-butylbenzyl)-1,4-diazepane, 1,4-bis(2-hydroxy-4-flurobenzyl)-1,4-diazepane, and 1,4-bis(2-hydroxy-4-chlorobenzyl)-1,4-diazepane (H(2)(L1)-H(2)(L6), respectively) have been isolated and studied as functional models for molybdenum oxotransferase enzymes. These complexes have been characterized as asymmetric complexes of type [MoO(2)(L)] 1-6 by using NMR spectroscopy, mass spectrometry, elemental analysis, and electrochemical methods. The molecular structures of [MoO(2)(L)] 1-4 have been successfully determined by single-crystal X-ray diffraction analyses, which show them to exhibit a distorted octahedral coordination geometry around molybdenum(VI) in an asymmetrical cis-β configuration. The Mo-O(oxo) bond lengths differ only by ≈0.01 Å. Complexes 1, 2, 5, and 6 exhibit two successive Mo(VI)/Mo(V) (E(1/2), -1.141 to -1.848 V) and Mo(V)/Mo(IV) (E(1/2), -1.531 to -2.114 V) redox processes. However, only the Mo(VI)/Mo(V) redox couple was observed for 3 and 4, suggesting that the subsequent reduction of the molybdenum(V) species is difficult. Complexes 1, 2, 5, and 6 elicit efficient catalytic oxygen-atom transfer (OAT) from dimethylsulfoxide (DMSO) to PMe(3) at 65 °C at a significantly faster rate than the symmetric molybdenum(VI) complexes of the analogous linear bis(phenolate) ligands known so far to exhibit OAT reactions at a higher temperature (130 °C). However, complexes 3 and 4 fail to perform the OAT reaction from DMSO to PMe(3) at 65 °C. DFT/B3LYP calculations on the OAT mechanism reveal a strong trans effect.

Heptacoordinated Molybdenum(VI) Complexes of Phenylenediamine Bis(phenolate): A Stable Molybdenum Amidophenoxide Radical

“…5,6,7 In the current contribution, we although similar reactivity with MoO2Cl2 derivatives has been observed earlier. 8,9 In general, the cleavage of both metal-oxo bonds and the formation of a Mo(VI) compound without any multiply bonded terminal ligands is rare.…”

Electronic Fine‐Tuning of Oxygen Atom Transfer Reactivity of cis‐Dioxomolybdenum(VI) Complexes with Thiosemicarbazone Ligands

“…Similar observations have previously been made with other ligands, including other tridentate Schiff base ligands, such as those obtained by condensation of substituted salicylaldehydes with ortho-aminobenzenethiol, [17] substituted thiophenolate, [44] substituted dithiolenes, [45] and salan-type ligands. [21,41] A particularly pronounced effect of the NO 2 substituent was also observed by Britovsek and Gibson et al with dioxomolybdenum complexes with para-substituted salan-type ligands. [21] To determine the corresponding second-order rate constants, analogous OAT transfer experiments were carried out by using a range of catalyst concentrations (2, 3.5, 5, 7.5, and 10 mm).…”

“…This may be relevant because it was recently reported that a trans-effect that weakens the Mo-oxo bond of the catalytically active group can give rise to enhanced OAT activity. [41] In the [MoO 2 (L R )MeOH] complexes, the Mo-O1 bonds trans to the imine N donor tend to be slightly longer than the Mo-O2 bonds in the trans-position of the weakly coordinated methanol.…”

Oxidomolybdenum(VI) complexes with aminoalcohol bis(phenolate) [O,N,O,O′] ligands: Synthesis and catalytic studies

“…The Mo(IV) complexes are then rapidly oxidised back to the initial oxocompounds with DMSO to complete the catalytic cycle [13].…”