Synthesis, Structure, Spectroscopic Characterization, and Protein Binding Affinity of New Water-Soluble Hetero- and Homometallic Tetranuclear [Cu II 2 Zn II 2 ] and [Cu II 4 ] Clusters Ayan Patra, † Tamal K. Sen, ‡ Atanu Ghorai, § Ghezai T. Musie,* ,⊥ Swadhin K. Mandal, ‡ Utpal Ghosh, § and Manindranath Bera* ,† † Department of Chemistry, University of Kalyani, Kalyani, West Bengal 741235, India ‡ Department of Chemical Sciences, Indian Institute of Science Education & Research Kolkata, Mohanpur, West Bengal 741252, India § Department of Biochemistry and Biophysics, University of Kalyani, Kalyani, Nadia, West Bengal 741235, India ⊥ Department of Chemistry, The University of Texas at San Antonio, San Antonio, Texas 78249, United States * S Supporting Information ABSTRACT: Two new water-soluble hetero- and homome- tallic tetranuclear clusters, Na 4 [Cu 2 Zn 2 (ccdp) 2 (μ-OH) 2 ]· CH 3 OH·6H 2 O (1) and K 3 [Cu 4 (ccdp) 2 (μ-OH)(μ-OH 2 )]· 14H 2 O(2), have been synthesized in methanol-water at room temperature by exploiting the flexibility, chelating ability, and bridging potential of a carboxylate-rich dinucleating ligand, N, N′-bis(2-carboxybenzomethyl)-N,N′-bis(carboxymethyl)- 1,3 diaminopropan-2-ol (H 5 ccdp). Complex 1 is obtained through the self-assembly of two monoanionic [CuZn(ccdp)] - fragments, which are, in turn, exclusively bridged by two μ-OH - groups. Similarly, complex 2 is formed through the self-assembly of two monoanionic [Cu 2 (ccdp)] - species exclusively bridged by one μ-OH - and one μ-OH 2 groups. Complexes 1 and 2 are fully characterized in the solid state as well as in solution using various analytical techniques including a single-crystal X-ray diffraction study. The X-ray crystal structure of 1 reveals that two Cu II centers are in a distorted square-pyramidal geometry, whereas two Zn II centers are in a distorted trigonal-bipyramidal geometry. The solid- state structure of 2 contains two dinuclear [Cu 2 (ccdp)] - units having one Cu II center in a distorted square-pyramidal geometry and another Cu II center in a distorted trigonal-bipyramidal geometry within each dinuclear unit. In the powder state, the high-field EPR spectrum of complex 1 indicates that two Cu II ions are not spin-coupled, whereas that of complex 2 exhibits at least one noninteracting Cu II center coordinated to a nitrogen atom of the ligand. Both complexes are investigated for their binding affinity with the protein bovine serum albumin (BSA) in an aqueous medium at pH ∼7.2 using fluorescence spectroscopy. Synchronous fluorescence spectra clearly reveal that complexes 1 and 2 bind to the active sites in the protein, indicating that the effect is more pronounced toward tyrosine than tryptophan. Density functional theory calculations have been carried to find the Fukui functions at the metal sites in complexes 1 and 2 to predict the possible metal centers involved in the binding process with BSA protein. ■ INTRODUCTION Properly designed hetero- and homometallic multinuclear complex systems not only present synthetic challenges but also can provide novel reactivity patterns as well as physical properties. The neighboring metal centers in multimetallic systems are expected to cooperate in promoting reactions, and new electronic interactions might lead to distinct physical properties. Cooperative interactions have commonly been observed in biological systems, and nature has constructed numerous multimetallic protein complexes that perform an extraordinary array of catalytic transformations. 1-4 This biochemistry has encouraged inorganic chemists to develop the area from an inorganic viewpoint. Furthermore, from the materials viewpoint, the multimetallic complexes can exhibit a full spectrum of new magnetic, optical, and redox properties as a result of a synergetic effect between the di fferent metals. 5-8 Cooperative interactions have been applied in the fields of supramolecular chemistry, enzyme-like catalytic systems, and functional molecular sensors. 9 Furthermore, heterodimetallic complexes offer an enormous potential in the fields of homogeneous catalytic processes. 10 Recently, progress has been documented in the application of M-O-M1-based heterometallic complexes in homogeneous catalytic processes with the concept of dual catalysis. 11 Similarly, the field of molecular-based magnetic materials has shown spectacular advances over the last 2 decades. 12 The advances have rekindled interest in the fields of synthesis and characterization of hetero- and homometallic dinuclear complexes. As a result, a number of hetero- and homometallic dinuclear complexes of copper and zinc are well documented in the literature using both symmetrical and unsymmetrical dinucleating ligands. 13-18 However, Received: September 27, 2012 Published: March 1, 2013 Article pubs.acs.org/IC © 2013 American Chemical Society 2880 dx.doi.org/10.1021/ic302099y | Inorg. Chem. 2013, 52, 2880-2890