MnIII-Peroxo adduct supported by a new tetradentate ligand shows acid-sensitive aldehyde deformylation reactivity

Abstract

The new tetradentate L⁷BQ ligand (L⁷BQ = 1,4-di(quinoline-8-yl)-1,4-diazepane) has been synthesized and shown to support Mn^II and Mn^III-peroxo complexes. X-ray crystallography of the [Mn^II(L⁷BQ)(OTf)₂] complex shows a monomeric Mn^II center with the L⁷BQ ligand providing four donor nitrogen atoms in the equatorial field, with two triflate ions bound in the axial positions. When this species is treated with H₂O₂ and Et₃N at −40 °C, a Mn^III-peroxo adduct, [Mn^III(O₂)(L⁷BQ)]⁺ is formed. The formation of this new intermediate is supported by a variety of spectroscopic techniques, including electronic absorption, Mn K-edge X-ray absorption and electron paramagnetic resonance methods. Evaluation of extended X-ray absorption fine structure data for [Mn^III(O₂)(L⁷BQ)]⁺ resolved Mn–O bond distances of 1.85 Å, which are on the short end of those previously reported for crystallographically characterized Mn^III-peroxo adducts. An analysis of the X-ray pre-edge region of [Mn^III(O₂)(L⁷BQ)]⁺ revealed a large pre-edge area of 20.8 units. Time-dependent density functional theory computations indicate that the pre-edge intensity is due to Mn 4p–3d mixing caused by geometric distortions from centrosymmetry induced by both the peroxo and L⁷BQ ligands. The reactivity of [Mn^III(O₂)(L⁷BQ)]⁺ towards aldehydes was assessed through reaction with cyclohexanecarboxaldehyde and 2-phenylpropionaldehyde. From these experiments, it was determined that [Mn^III(O₂)(L⁷BQ)]⁺ only reacts with aldehydes in the presence of acid. Specifically, the addition of cyclohexanecarboxylic acid to [Mn^III(O₂)(L⁷BQ)]⁺ converts the Mn^III-peroxo adduct to a new intermediate that could be responsible for the observed aldehyde deformylation activity. These observations underscore the challenges in identifying the reactive metal species in aldehyde deformylation reactions.