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

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_{<i>x</i>²-y²} orbital of copper(II) (d⁹, <i>S</i>_Cu = ½) and another electron on the molecular orbital (MO) comprising p_<i>z</i> oxygen and carbon atoms of the phenoxyl radical (<i>S</i>_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 <i>tert</i>-butyl hydroperoxide as the oxidant) or into cyclohexanol and ε-caprolactone (with <i>m</i>-chloroperoxybenzoic acid as the oxidant). Theoretical investigations of the catalytic reaction mechanisms disclosed the principal intermediates.