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Dalton Transactions

Unveiling the effects of in situ generated arene anion radical and imine radical on catecholase like activity: A DFT supported experimental investigation

Authors: Debasis Das; Sanchari Dasgupta; Jaydeep Adhikary; Sanjib Giri; Antonio Bauza; Antonio Frontera

Publication Date: -0001-11-30  Article ASAP

Two new dinuclear nickel(II) complexes namely [Ni2(L1)2(OAc)2(H2O)2].CH3CN (1) and [Ni2(L2)2(SCN)2(CH3OH)2].CH3OH (2) have been synthesized from designed Schiff-base ligand 4-Bromo-2-[(2-hydroxy-1,1-dimethyl-ethylimino)-methyl]-phenol (HL1) and its reduced analogue 4-Bromo-2-[(2-hydroxy-1,1-dimethyl-ethylamino)-methyl]-phenol (HL2), respectively. Both 1 and 2 have been characterised by usual physicochemical techniques (UV-Vis, FT-IR, ESI-MS study, Single crystal XRD) and their variable temperature magnetic study has been performed. The nickel(II) centres in the dinuclear complexes 1 and 2 are antiferromagnetically coupled through participation of bridging phenoxyl oxygen. In acetonitrile solution both 1 and 2 retain their dinuclear structural integrity as is evident from ESI-MS study. Catecholase like activity of 1 and 2 have been performed in acetonitrile medium using 3, 5 -di-tert-butylcatechol (3, 5-DTBC) as model substrate. Complex 1 shows higher catalytic activity than that of complex 2. ESI-MS study suggests that dinuclear species undergo cleaving into mononuclear entity in presence of 3,5-DTBC and that mononuclear species supposed to act as active catalyst in the catalytic cycle. EPR study of catalytic reactions confirms that organic radicals have been generated during catalysis in both cases. However, in case of complex 1 catalyzed reaction a single isotropic signal at g = 1.97 is obtained which is most likely to be due to imine radical formation. On the other hand, for complex 2 catalyzed reaction the spectrum shows signal with hyperfine splitting (g = 2.11, 2.05, 1.9) and thereby suggesting the generation of a new radical i.e. arene anion radical in catecholase activity study. Extensive DFT calculations have been performed to support the experimental observation and thus to put forward the most probable mechanistic pathways operating in the two cases. Higher efficiency of imine radical pathway over arene anion radical has been rationalized by DFT calculations.  Read more