707 J. Indian Chem. Soc., Vol. 95, July 2018, pp. 707-712 Oxidation of diphenylmethane catalyzed by complexes of the type [Cu 2 (-O 2 CR) 4 (4-CNpy) 2 ] Zinnatara Islam, Prity K. Kouli and Birinchi K. Das* Department of Chemistry, Gauhati University, Guwahati-781 014, Assam, India E- mail: birinchi.das@gmail.com Manuscript received 01 July 2018, accepted 10 July 2018 Three dicopper( II) tetracarboxylate complexes, viz. [Cu 2 ( -O 2 CC 6 H 5 ) 4 (4-CNpy) 2 ], [Cu 2 ( -O 2 CCH 2 Cl) 4 (4-CNpy) 2 ] and [Cu 2 ( - O 2 CCH 3 ) 4 (4-CNpy) 2 ], where 4-CNpy stands for 4-cyanopyridine, have been examined as catalysts in the oxidation of diphenylmethane using 70% aqueous TBHP as the terminal oxidant under solvent-free condition. 100% conversion of diphenylmethane is achieved in 8 h to obtain benzophenone at 100% selectivity with [Cu 2 ( -O 2 CCH 2 Cl) 4 (4-CNpy) 2 ] as the catalyst. Keywords: Dicopper( II) tetracarboxylates, catalytic oxidation, diphenylmethane oxidation, TBHP, copper-catalyzed oxidation. Introduction The aromatic ketone benzophenone is an important in- termediate for drugs, pharmaceuticals, flavoring agents, agro- chemicals, etc. 1 . It can be obtained from the selective oxida- tion of diphenylmethane (DPM). However, the catalytic oxi- dation of the aliphatic C-H bonds in this hydrocarbon com- pound is quite difficult because of their inert nature making DPM a challenging target 2 . Earlier, the oxidation of the me- thylene group attached to the aromatic rings in DPM and related compounds was performed 3 using stoichiometric quantities of oxidizing agents like KMnO 4 or K 2 Cr 2 O 7 . Since the use of these stoichiometric reagents is expensive and, at the same time, lead to the release of heavy metal ions as part of industrial wastes, attention has been given in recent times to develop more efficient and environmentally friendly catalysts for this type of oxidation processes 4,5 . It was dem- onstrated earlier that by making use of air (i.e. O 2 ) as the oxidant, it is possible to oxidize ethylbenzene to acetophe- none using a supported catalyst based on a cobalt( III)- oxocubane cluster 6 . For such purposes, the use of environ- mentally benign oxidizing agents like hydrogen peroxide (H 2 O 2 ) and tert-butylhydroperoxide (TBHP) which produce water and t-butanol as by-products is also viewed as prefer- able 7 . In an earlier study published 8 from this laboratory, a heterogeneous catalyst prepared by supporting a tetranuclear cubane-like Co III -oxo cluster supported on porous silica was found to catalyze the 100% conversion of DPM with TBHP as the oxidant in acetonitrile (MeCN). However, in this case the turnover number (TON) was low and a small amount of benzhydrol was found to form as a co-product. A Cu II complex was earlier shown to act as a catalyst in the oxidation of DPM by 30% aq. H 2 O 2 at 353 K in MeCN to afford benzophenone in 87% yield in 5 h 9 . A molybdenum(VI) oxo-diperoxo complex was reported 10 to catalyze the oxida- tion of DPM with H 2 O 2 in MeCN at 353 K to yield a 3:2 mixture of diphenylmethanol and benzophenone in 12 h. When Co-doped mesoporous titania-silica was used to cata- lyze the oxidation of DPM in acetic acid using 30% aq. H 2 O 2 , benzophenone, 2-hydroxybenzophenone (2-HOBP), 2- benzylphenol, and diphenylmethanol were obtained as pro- ducts 1 . It is thus clear that although many catalytic systems have been studied for the oxidation of DPM, most of them suffer from various drawbacks. Some systems exhibited low to moderate catalytic activity under severe reaction conditions such as long reaction time or relatively high temperature. Moreover, in many of them the preparation of catalysts in- volved tedious procedures as well as high cost. In view of this, discovery of new and effective catalysts for these reac- tions constitutes an important goal. Herein we shall describe the effectiveness of dimeric tetracarboxylates of copper(II) as catalysts in the homogeneous oxidation of DPM using 70% aq. TBHP as the oxidant. The dicopper(II) tetracarboxylates, Cu 2 (-O 2 CR) 4 L 2 , typi-