Ligand-Flexibility Controlled and Solvent-Induced Nuclearity Conversion in CuII-Based Catecholase Models: A Deep Insight Through Combined Experimental and Theoretical Investigations

A Schiff-base ligand, 4-bromo-2-[(2-hydroxy-1,1-dimethylethylimino)methyl]phenol (HL1), and its reduced analogue 4-bromo-2-[(2-hydroxy-1,1-dimethylethylamino)methyl]phenol (HL2) have been synthesized. Treatment of HL1 and HL2 with Cu(ClO4)2·6 H2O in the presence of sodium dicyanamide in MeOH and MeCN has been investigated. HL1 generates [Cu(L1)(MeOH)(dca)] (1) in MeOH (dca = dicyanamide) but [Cu2(L1)2(dca)2] (2) in MeCN. HL2 in MeOH produces [Cu4(L2)4(MeOH)2(ClO4)2] (3). On the other hand, in MeCN an interesting redox reaction is observed. CuII undergoes reduction, and [CuI(MeCN)4](ClO4) (4) is formed with concomitant oxidation of HL2 to HL1. The solvent-dependent nuclearity change (i.e, conversion of mononuclear species 1 into dinuclear species 2) has been investigated by ESI-MS, through the addition of MeCN to a methanolic solution of 1, and the origin of the conversion has been explained by means of DFT calculations. The catecholase activity of 1, 2 and 3 in MeCN and DMF has been investigated with the model substrates 3,5-di-tert-butylcatechol (3,5-DTBC), tetrachlorocatechol (TCC) and pyrocatechol (PRC). Compound 1 is inactive with all substrates, whereas 2 is active only with 3,5-DTBC in DMF. In contrast, 3 – which exists as a dinuclear species in solution, as is evident from ESI-MS – is highly active with all three substrates, especially in MeCN. The exceptionally high catalytic activity of 3 over 2 is likely to be due to the higher flexibility of the reduced Schiff-base ligand in comparison with its Schiff base analogue.