The self-assembly
of a carboxylate-based dinucleating ligand, N,N′-bis[2-carboxybenzomethyl]-N,N′-bis[2-pyridylmethyl]-1,3-diaminopropan-2-ol
(H3cpdp), and copper(II) ions in the presence of various
exogenous ancillary ligands results in the formation of the new dinuclear
complex [Cu2(cpdp)(μ-Hisophth)]4·2H2isophth·21H2O (1), trinuclear
complex [Cu3(Hcpdp)(Cl)4] (2),
and tetranuclear complex [Cu4(cpdp)(μ-Hphth)(μ4-phth)(piconol)(Cl)2]·3H2O (3) (H2phth = phthalic acid; H2isophth
= isophthalic acid; piconol = 2-pyridinemethanol; Cl– = chloride). In methanol–water, the reaction of H3cpdp with CuCl2·2H2O 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 H3cpdp with CuCl2·2H2O/m-C6H4(CO2Na)2 and CuCl2·2H2O/o-C6H4(CO2Na)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 [Cu2] core; the structure of 2 represents
a trimeric [Cu3] core in which a μ-alcohol-bridged
dinuclear [Cu2] unit is exclusively coupled with a [CuCl2] species by two μ:η1:η1-syn-anti carboxylate groups forming a triangular
motif; the structure of 3 embodies a tetrameric [Cu4] 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:η2, μ:η1:η1, μ3:η2:η1:η1, and μ4:η1:η1:η1:η1, the μ4:η1:η1:η1:η1 mode of phthalate
being unprecedented. The formation of these [Cu2], [Cu3], and [Cu4] 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.