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Exploring solvent dependent catecholase activity in transition metal complexes: An experimental and theoretical approach


In contrast to the polynuclear complex models for biomimetic activities, a series of mononuclear complexes is designed herein this work in order to assess their catecholase activities and kinetics. Four new complexes namely, [Mn(pdmH2)(Phen)Cl]Cl∙H2O (1), [Mn(pmmH)2(SCN)2] (2), [Ni(pmmH)2(SCN)2] (3) and [Zn(phen)3][(pdm)]∙11H2O (4) utilizing aminoalcohol ligand, pyridine-2,6-dimethanol (pdmH2) or 2-pyridinemonomethanol (pmmH) and the auxiliary ligand, 1,10-phenanthroline (phen) or thiocynate (SCN-) are synthesized. The complexes are characterized by elemental analysis, FTIR, UV-visible, EPR, fluorescence (solution and solid state), hirshfeld surface analysis, magnetic, single crystal X-ray and DFT/TD-DFT studies. X-ray structures confirm the geometry around the M(II) ions to be octahedral in all the complexes. In 1─3, the primary amonoalcohol ligand binds to metal ion in a neutral (pdmH2 or pmmH) mode while 4 exists as a cation-anion type complex where deprotonated ligand (pdm2‒) is present in outer sphere having no coordination to metal. Zn(II) complex also shows a remarkable luminescence in solid state photoluminescence spectrum. Temperature variable magnetic studies show the presence of antiferromagnetic exchange in 1─3 ( = -2.8, -1.7 and -5.2, respectively)with observation of anisotropy (D = 4.0 and E = 3.4) in 3. DFT/TD-DFT results provide ample information regarding the structures, spin densities, charge distribution, and electronic spectra alongwith the transitions. The spin density values,ρ = 4.793, 4.792 and 1.676 confirm the presence of five, five and two unpaired electrons on the metal d-orbitals of 1, 2 and 3, respectively. Interestingly, the solvent dependent catecholase activity has been observed for the first time in monunuclear Mn(II) complexes (1 and 2) with Kcat = 2602.8 h─1 (acetonitrile), 1490.4 h─1 (methanol) for 1, and1083.6 h─1 (acetonitrile), 806.4 h─1 (methanol) for 2. This can be rationalized in terms of the coordinating powers of the solvent i.e, DMSO > MeOH > MeCN. Further, the order of activity in 1 and 2 (i.e, 1 > 2) and inactivity of 3 and 4 is beautifully corroborated by DFT which ascertains the highest charge contribution on metal in 1 [33% on HOMO and 2% on LUMO of Mn(II)] resulting in the formation of the most stable metal-substrate adduct thus enhancing the activity.

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Publication details

The article was received on 23 Aug 2019, accepted on 20 Nov 2019 and first published on 20 Nov 2019

Article type: Paper
DOI: 10.1039/C9NJ04374H
New J. Chem., 2019, Accepted Manuscript

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    Exploring solvent dependent catecholase activity in transition metal complexes: An experimental and theoretical approach

    M. I., M. Shahid, A. Ansari, M. Kumar, M. N. Akhtar, M. A. AlDamen, Y. Song, M. Ahmad and I. M. Khan, New J. Chem., 2019, Accepted Manuscript , DOI: 10.1039/C9NJ04374H

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