Synthesis of a new N-substituted bis-benzimidazolyl diamide ligand and its trinuclear copper(II) complex: structural and fluorescence studies.

The synthesis of a new N-substituted fluorescent probe based on a bis-benzimidazole diamide N(2),N(2')-bis[(1-(4-methylbenzyl)-benzimidazol-2-yl)methyl]biphenyl-2,2'-dicarboxamide (L1) with a biphenyl spacer group and its trinuclear copper(II) complex [Cu3(L1)3Cl3]·3Cl·3H2O] has been described. X-ray studies shows that the trinuclear complex crystallizes as [{Cu3(L1)3Cl3}2·6Cl·13CH3CN·2H2O] in triclinic space group P-1 with two independent molecules in the asymmetric unit. Each copper(II) adopts a distorted penta-coordinated geometry in each unit. The fluorescence spectra of L1 in methanol show an emission band centered at 300 nm. This band arises due to benzimidazolyl moiety in the ligating system. The diamide L1 in the presence of Fe(3+) show the simultaneous 'quenching' of (300nm) and 'enhancement' of (375 nm) emission band. The new emission band at 375 nm is attributed to intra ligand π-π(*) transition of the biphenyl moiety. While Cu(2+) and Ag(+) show only the quenching of the 300 nm band. No such behavior was observed with other metal ions like Ni(2+), Co(2+), Mn(2+), Mg(2+), Zn(2+) and Pb(2+). The quenching constant with Fe(3+), Ag(+) and Cu(2+) are calculated by the Stern-Volmer plots.