Flexibility and lability of a phenyl ligand in hetero-organometallic 3d metal-Sn(iv) compounds and their catalytic activity in Baeyer-Villiger oxidation of cyclohexanone.

The single compartmental Schiff base N,N'-ethylenebis(salicylaldimine) (H2L) and [SnPh2Cl2] were utilized to synthesize heterobimetallic 3d metal-Sn complexes, the CoIIISnIV compound [{SnPhCl2}(1κO2N2,2κO2-μ-L)(μ-OMe){CoPh}] (1), the NiIISnIV compound [{SnPh2Cl2}(1κO2N2,2κO2-μ-L)Ni] (2) and the CuIISnIV compound [{SnPh2Cl2}(1κO2N2,2κO2-μ-L)Cu] (3). Attempting to prepare the ethoxido bridged compound analogous to 1 (in ethanol) gives the phenylcobalt(iii) complex [Co(κO2N2)Ph(H2O)] (1A). Single crystal X-ray structure analyses reveal that 1 is derived from an intermetallic (Sn to Co) phenyl shift and that 1A is a transmetallated product; in compounds 2 and 3, the phenyl groups remain coordinated to SnIV but one of the π rings interacts with the 3d-metal. Thus, while systems 1 and 1A show the lability of the phenyl ligand, 2 and 3 reveal its flexible nature. Theoretical DFT calculations demonstrate that the conceivable Ph group shift occurs in the oxidized CoIII intermediate [{SnIVPh2Cl2}(κO2N2-μ-L){CoIII(MeO)}] (5) rather than in the corresponding CoII species [{SnIVPh2Cl2}(κO2N2-μ-L){CoII(MeOH)}] (4). Their catalytic studies in the Baeyer-Villiger oxidation of cyclohexanone into ε-caprolactone with two different oxidants reveal that the sacrificial aldehyde method (with dioxygen/benzaldehyde) is better than that with aqueous H2O2 (30%). The effects of various reaction parameters such as solvent, catalyst amount, temperature, time and heating method were studied allowing the achievement of yields up to 83% with 89% selectivity.