Bio-relevant manganese(II) compartmental ligand complexes: Syntheses, crystal structures and studies of catalytic activities

Three new mono-manganese(II) complexes of a compartmental ligand, namely [Mn(HL)(H2O)3](NO3)2·H2O (1), [Mn(HL)(SCN)2(H2O)]·H2O (2), and [Mn(HL){N(CN)2}(H2O)2](NO3)·H2O (3), where L = 2,6-bis{2-(N-ethyl)pyridineiminomethyl}-4-methylphenolato, have been synthesized and characterised by routine physicochemical techniques and complexes 1, 2 also by X-ray single crystal structure analysis. All the mono nuclear complexes contain MnII high spin species at octahedral core as evidenced by magnetic moment (measured at 300 K) and EPR study at 77 K. As revealed by crystal structure analyses, the protonation of one imine nitrogen atom of the potential dinucleating ligand L hampers to form the expected dinuclear MnII complex. However, complexes 1–3 show excellent catecholase-like activity with both 3,5-di-tert-butylcatechol and tetrachlorocatechol as substrates. In addition complexes 1 and 2 also exhibit phosphatase activity, while 3 forms an adduct with p-nitrophenyl phosphate as substrate. To the best of our knowledge this is the first report of MnII complexes being able to catalyze the oxidation of TCC to TCQ. Catecholase and phosphatase activities have been monitored by UV–vis spectrophotometer and Michaelis–Menten equation has been applied to rationalize all the kinetic parameters where complex 1 shows maximum kcat value followed by 2 and 3 (where for phosphatase activity 3 only forms an adduct).

The “end-off” dinucleating compartmental ligand 2,6-bis{2-(N-ethyl)pyridineiminomethyl}-4-methylphenolato generates mono-manganese(II) complexes (1–3) instead of forming the expected di-manganese(II) species most probably due to the protonation of one of the imine nitrogen atoms. All the three complexes exhibit excellent catecholase like activity not only with easily oxidizable substrate, 3,5-DTBC but also with TCC, a substrate which is hard to oxidize. Complexes 1 and 2 also show phosphatase like activity with substrate PNPP while 3 forms adduct.Figure optionsDownload high-quality image (111 K)Download as PowerPoint slideResearch highlights
► Syntheses, characterisation and catalytic efficiency of three new MnII complexes.
► Inspite of the ligand being dinucleating the complexes were mononuclear.
► Catecholase and phosphatase activities shown by mononuclear MnII complexes.
► Protonation of one imine N results in unusual nuclearity and catalytic efficiencies.