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.