Copper(ii) complex with a redox-noninnocent Schiff base bearing a tetraphenyldisiloxane unit: synthesis, structure and catalytic oxidation of cyclohexane

Abstract

An organic–inorganic diamine, 1,3-bis(aminopropyl)tetraphenyldisiloxane, was prepared and introduced as a flexible spacer into the structure of a salen-type Schiff base (H₂L⁷) extending the available small library of similar compounds derived from 1,3-bis(aminopropyl)tetramethyldisiloxane and substituted 2-hydroxybenzaldehydes (H₂L¹–H₂L⁶). Like the previously reported mononuclear copper(II) complexes [CuL¹]–[CuL⁶], the new copper(II) complex [CuL⁷], obtained by reaction of Cu(OAc)₂·H₂O with H₂L⁷ in a mixture of organic solvents, has a tetrahedrally distorted square-planar (N₂O₂) coordination geometry. X-ray crystallography has shown that compared to [CuL¹]–[CuL⁶] the Si–O–Si angle in [CuL⁷] is even closer to linear due to stronger intramolecular interactions between Ph groups than between Me groups in the central–R₂Si–O–SiR₂– fragment (R = Ph and Me, respectively). [CuL⁷] can be electrochemically reversibly oxidised by two successive one-electron processes, generating stable phenoxyl mono- and diradicals. Both oxidations are ligand-centred, leading to the formation of coordinated phenoxyl radicals. The UV spectrum of [CuL⁷] consists of π → π* and LMCT σ → d transitions. The low-energy d–d absorption is well described by AILFT CAS(9,5)/NEVPT2 calculations. The one-electron oxidised compound [CuL⁷]⁺ should exist in the triplet ground state as ³[CuL⁷]⁺ with one unpaired electron located on the d_x²−y² orbital of copper(II) (d⁹, S_Cu = ½) and another electron on the molecular orbital (MO) comprising p_z oxygen and carbon atoms of the phenoxyl radical (S_rad = ½). The broad absorption in the vis-NIR region of the optical spectrum of the one-electron oxidised complex is due to intervalence charge transfer in the triplet species ³[CuL⁷]⁺, but not in the [CuL⁷]²⁺ one. The doubly oxidised [CuL⁷] species shows very close doublet and quartet states, where the doublet state has an unpaired electron located on the Cu(II) d-orbital, while the quartet state has one unpaired electron on the Cu(II) d-orbital and two unpaired electrons on π-bonding orbitals. In all state-averaged CASSCF cases, the occupation of the Cu(II) d-orbital is nearly 1.0, indicating its limited involvement in the excited states. Catalytic studies showed that [CuL⁷] acts as a catalyst for the oxidation of alkanes with peroxides under very unusual solvent-free conditions, converting cyclohexane into cyclohexanol and cyclohexanone (with hydrogen peroxide or tert-butyl hydroperoxide as the oxidant) or into cyclohexanol and ε-caprolactone (with m-chloroperoxybenzoic acid as the oxidant). Theoretical investigations of the catalytic reaction mechanisms disclosed the principal intermediates.