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.