Ancillary-Ligand-Assisted Variation in Nuclearities Leading to the Formation of Di-, Tri-, and Tetranuclear Copper(II) Complexes with Multifaceted Carboxylate Coordination Chemistry.

The self-assembly of a carboxylate-based dinucleating ligand, N,N'-bis[2-carboxybenzomethyl]-N,N'-bis[2-pyridylmethyl]-1,3-diaminopropan-2-ol (H(3)cpdp), and copper(II) ions in the presence of various exogenous ancillary ligands results in the formation of the new dinuclear complex [Cu(2)(cpdp)(μ-Hisophth)](4)·2H(2)isophth·21H(2)O (1), trinuclear complex [Cu(3)(Hcpdp)(Cl)(4)] (2), and tetranuclear complex [Cu(4)(cpdp)(μ-Hphth)(μ(4)-phth)(piconol)(Cl)(2)]·3H(2)O (3) (H(2)phth = phthalic acid; H(2)isophth = isophthalic acid; piconol = 2-pyridinemethanol; Cl(-) = chloride). In methanol-water, the reaction of H(3)cpdp with CuCl(2)·2H(2)O at room temperature leads to the formation of 2. On the other hand, 1 and 3 have been obtained by carrying out the reaction of H(3)cpdp with CuCl(2)·2H(2)O/m-C(6)H(4)(CO(2)Na)(2) and CuCl(2)·2H(2)O/o-C(6)H(4)(CO(2)Na)(2)/piconol, respectively, in methanol-water in the presence of NaOH at ambient temperature. All three complexes have been characterized by elemental analysis, molar electrical conductivity and magnetic moment measurements, FTIR, UV-vis spectroscopy, and PXRD, including single-crystal X-ray structural analyses. The molecular structure of 1 is based on a μ-alkoxide and μ-isophthalate-bridged dimeric [Cu(2)] core; the structure of 2 represents a trimeric [Cu(3)] core in which a μ-alcohol-bridged dinuclear [Cu(2)] unit is exclusively coupled with a [CuCl(2)] species by two μ:η(1):η(1)-syn-anti carboxylate groups forming a triangular motif; the structure of 3 embodies a tetrameric [Cu(4)] core, with two copper(II) ions in a distorted-octahedral coordination environment, one copper(II) ion in a distorted-trigonal-bipyramidal coordination environment, and the other copper(II) ion in a square-planar coordination environment. In fact, 2 and 3 represent rare examples of copper(II)-based multinuclear complexes showing outstanding features of rich coordination chemistry: (i) using a symmetrical dinucleating ligand, trinuclear complex 2 is generated with four- and five-coordination environments around copper(II) ions; (ii) the unsymmetrical tetranuclear complex 3 is obtained by using the same ligand with four-, five- and six-coordination environments around copper(II) ions; (iii) tetracopper(II) complex 3 shows four different bridging modes of carboxylate groups simultaneously such as μ:η(2), μ:η(1):η(1), μ(3):η(2):η(1):η(1), and μ(4):η(1):η(1):η(1):η(1), the μ(4):η(1):η(1):η(1):η(1) mode of phthalate being unprecedented. The formation of these [Cu(2)], [Cu(3)], and [Cu(4)] complexes can be controlled by changing the exogenous ancillary ligands and pH of the reaction solutions, thus allowing an effective tuning of the self-assembly. The magnetic susceptibility measurements suggest that the copper centers in all three complexes are antiferromagnetically coupled. The thermal properties of 1-3 have been investigated by thermogravimetric and differential thermal analytical (TGA and DTA) techniques, indicating that the decomposition of all three complexes proceeds via multistep processes.