In situ transformation of a tridentate to a tetradentate unsymmetric Schiff base ligand via deaminative coupling in Ni( ii ) complexes: crystal structures, magnetic properties and catecholase activity study

A new dimeric Ni( ii ) complex, [Ni 2 L 1 2 (CH 3 CN) 4 ](ClO 4 ) 2 ·2CH 3 CN ( 1 ), was synthesized using an N 2 O donor reduced Schiff base [(HL 1 ) = 2-[(3-methylamino-propylamino)-methyl]-4-phenol]. Surprisingly, during an attempt to replace its ClO 4 − ion with SCN − , the N 2 O donor ligand in situ converted to a tetradentate N 2 O 2 donor ligand and formed a metal complex, [Ni(HL 2 )(NCS)(CH 3 CN)] ( 2 ). A probable mechanism via deaminative coupling for this conversion is proposed. Using 2 as a metalloligand under basic conditions, a trinuclear metal complex, [Ni 3 (L 2 ) 2 (NCS) 2 (H 2 O) 4 ]·H 2 O ( 3 ), was prepared. Single crystal structural characterization revealed that in all three metal complexes, the Ni( ii ) atoms were in an octahedral environment with coordinated solvent molecules (CH 3 CN in 1 and 2 and H 2 O in 3 ). Among the three metal complexes, 1 and 3 showed catecholase-like biomimicking activity. The calculation of the turnover numbers ( K cat = 7.9 for 1 , 14.5 for 3 ) reveals that 3 is a better catalyst than 1 . Mechanistic cycles are proposed for this biomimicking activity on the basis of ESI-MS spectrometry and iodometric measurements. Temperature-dependent magnetic susceptibility measurements suggest that the Ni( ii ) ions in metal complexes 1 and 3 are antiferromagnetically coupled ( J = −32.22 cm −1 for 1 , J = −10.4 cm −1 for 3 ), consistent with their geometries and bridging angles. Theoretically calculated J values ( J = −40.15 cm −1 for 1 , J = −14.53 cm −1 for 3 ) by the DFT method corroborate well with the experimental values.