PAPER www.rsc.org/dalton | Dalton Transactions Basicity and coordination properties of a new phenanthroline-based bis-macrocyclic receptor† Carla Bazzicalupi, a Andrea Bencini,* a Antonio Bianchi,* a Lucia Borsari, a Andrea Danesi, a Claudia Giorgi, a Carlos Lodeiro, b Palma Mariani, a Fernando Pina,* b Samuele Santarelli, a Abel Tamayo b and Barbara Valtancoli a Received 8th March 2006, Accepted 27th April 2006 First published as an Advance Article on the web 12th May 2006 DOI: 10.1039/b603505a The synthesis and characterisation of the new macrocyclic ligand 6-methyl-2,6,10-triaza-[11]-12,25-phenathrolinophane (L1), which contains a triamine aliphatic chain linking the 2,9 positions of 1,10-phenanthroline and of its derivative L2, composed by two L1 moieties connected by an ethylenic bridge, are reported. Their basicity and coordination properties toward Cu(II), Zn(II), Cd(II), Pb(II) and Hg(II) have been studied by means of potentiometric and spectroscopic (UV-Vis, fluorescence emission) measurements in aqueous solutions. L1 forms 1 : 1 metal complexes in aqueous solutions, while L2 can give both mono- and dinuclear complexes. In the mononuclear L2 complexes the metal is sandwiched between the two cyclic moieties. The metal complexes with L1 and L2 do not display fluorescence emission, due to the presence of amine groups not involved in metal coordination. These amine groups can quench the excited fluorophore through an electron transfer process. The ability of the Zn(II) complexes with L1 and L2 to cleave the phosphate ester bond in the presence has been investigated by using bis(p-nitrophenyl)phosphate (BNPP) as substrate. The dinuclear complex with L2 shows a remarkable hydrolytic activity, due to the simultaneous presence within this complex of two metals and two hydrophobic units. In fact, the two Zn(II) act cooperatively in substrate binding, probably through a bridging interaction of the phosphate ester; the interaction is further reinforced by p-stacking pairing and hydrophobic interactions between the phenanthroline unit(s) and the p-nitrophenyl groups of BNPP. Introduction Macrocycles have been often selected as ligands for the synthesis of polynuclear metal complexes, because, being more rigid than open chain ligands, they may allow to fix the position and, therefore, the distances between the metals in an easier way. 1–26 It is known, in fact, that the intermetallic distance can strongly influence the magnetic, photophysical or reactivity properties of the complexes. Basically, two main approaches have been used to obtain macrocyclic structures able to bind two or more metal cations: (i) macrocycles or cryptands containing a cavity large enough to accommodate at least two metal ions 1–13 or (ii) ligands containing two or more macrocyclic binding units separated by appropriate spacers. 14–26 Earlier we reported the synthesis and metal complexation features of a variety of macrocyclic ligands of the first type (i), 27 containing a polyamine chain of different length linking the 6,6 ′ positions of a 1,10-phenathroline units. Depending on the number of nitrogen donors these ligands can form stable mono- and/or dinuclear metal complexes in aqueous solutions. It has a Dipartimento di Chimica, Polo Scientifico, Universit` a di Firenze, Via della Lastruccia 3, 50019, Sesto Fiorentino, Firenze, Italy. E-mail: andrea.bencini@unifi.it b REQUIMTE/CQFB, Departamento de Qu´ ımica, Universidade Nova de Lisboa, 2829-516, Monte de Caparica, Portugal. E-mail: fjp@dq.fct.unl.pt †Electronic supplementary information (ESI) available: Fig. S1: pH dependence of the 1 H NMR signals of L1 and distribution diagram of the L1 protonated species. Fig. S2: pH dependence of the absorption spectrum of L1 aqueous solution. See DOI: 10.1039/b603505a been shown that these ligands can be used as fluorescence sensors for metal cations. These ligands, in fact, contains within their cyclic framework both a polyamine chain, which can be used essentially for metal binding, and a phenanthroline moiety, which can act simultaneously for metal binding and signalling. 27c,e Furthermore, the dinuclear Cu(II) and Zn(II) complexes with these phenanthroline-containing macrocycles have been shown to be effective functional model systems for hydrolytic metallo-enzymes, displaying a high hydrolytic ability toward the phosphate ester bond of simple substrates, such as bis(p- nitrophenyl)phosphate (BNPP), 28,29 as well as a marked affinity for nucleic acids and nuclease activity toward DNA. 28 We have now firstly synthesized a phenanthroline-containing ligand of the type (ii) (L2 in Scheme 1), composed by two macrocyclic units linked by a short ethylenic linkage. In this paper we report on its binding features toward Cu(II), Zn(II), Cd(II), Pb(II) and Hg(II) in aqueous solutions. The photophysical properties of the complexes and the ability of the Zn(II) complexes in BNPP hydrolysis have been also analysed and compared with those of its synthetic precursor L1. Results and discussion Protonation of L1 and L2 The metal binding features of polyamine ligands can be strongly influenced by their basicity; at the same time, it has been shown that the photophysical properties of ligands containing fluoro- genic units depends on their protonation state. 27c,e,30 We decided, 4000 | Dalton Trans., 2006, 4000–4010 This journal is © The Royal Society of Chemistry 2006