Air oxygenation chemistry of 4-TBC catalyzed by chloro bridged dinuclear copper(ii) complexes of pyrazole based tridentate ligands: synthesis, structure, magnetic and computational studies

Abstract

Four dinuclear bis(μ-Cl) bridged copper(II) complexes, [Cu₂(μ-Cl)₂(L^X)₂](ClO₄)₂ (L^X = N,N-bis[(3,5-dimethylpyrazole-1-yl)-methyl]benzylamine with X = H(1), OMe(2), Me(3) and Cl(4)), have been synthesized and characterized by the single crystal X-ray diffraction method. In these complexes, each copper(II) center is penta-coordinated with square-pyramidal geometry. In addition to the tridentate L^X ligand, a chloride ion occupies the last position of the square plane. This chloride ion is also bonded to the neighboring Cu(II) site in its axial position forming an SP-I dinuclear Cu(II) unit that exhibits small intramolecular ferromagnetic interactions and supported by DFT calculations. The complexes 1–3 exhibit methylmonooxygenase (pMMO) behaviour and oxidise 4-tert-butylcatechol (4-TBCH₂) with molecular oxygen in MeOH or MeCN to 4-tert-butyl-benzoquinone (4-TBQ), 5-methoxy-4-tert-butyl-benzoquinone (5-MeO-4-TBQ) as the major products along with 6,6′-Bu^t-biphenyl-3,4,3′,4′-tetraol and others as minor products. These are further confirmed by ESI- and FAB-mass analyses. A tentative catalytic cycle has been framed based on the mass spectral analysis of the products and DFT calculations on individual intermediates that are energetically feasible.