Ligand‐Flexibility Controlled and Solvent‐Induced Nuclearity Conversion in Cu^II‐Based Catecholase Models: A Deep Insight Through Combined Experimental and Theoretical Investigations

Abstract

A Schiff‐base ligand, 4‐bromo‐2‐[(2‐hydroxy‐1,1‐dimethylethylimino)methyl]phenol (HL 1 ), and its reduced analogue 4‐bromo‐2‐[(2‐hydroxy‐1,1‐dimethylethylamino)methyl]phenol (HL 2 ) have been synthesized. Treatment of HL 1 and HL 2 with Cu(ClO 4 ) 2 · 6 H 2 O in the presence of sodium dicyanamide in MeOH and MeCN has been investigated. HL 1 generates [Cu(L 1 )(MeOH)(dca)] ( 1 ) in MeOH (dca = dicyanamide) but [Cu 2 (L 1 ) 2 (dca) 2 ] ( 2 ) in MeCN. HL 2 in MeOH produces [Cu 4 (L 2 ) 4 (MeOH) 2 (ClO 4 ) 2 ] ( 3 ). On the other hand, in MeCN an interesting redox reaction is observed. Cu II undergoes reduction, and [Cu I (MeCN) 4 ](ClO 4 ) ( 4 ) is formed with concomitant oxidation of HL 2 to HL 1 . 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.