Unusual complexation of Cu(I) by pyrimidine/pyridine-pyrazole derived ligands exploiting the molecular function of 2-mercapto-4,6-dimethylpyrimidine – Syntheses, crystal structures and electrochemistry

Reaction of 2 equiv. amount of copper(II) chloride dihydrate with 2 equiv. of methyl-5-methyl-1-(4,6-dimethyl-2-pyrimidyl)pyrazole-3-carboxylate (DpymPzC) in presence of 1 equiv. of 2-mercapto-4,6-dimethylpyrimidine (DpymtH) at pH ∼ 6 afforded the tricoordinated copper(I) complex [Cu(DpymPzC)Cl] (1). The same reaction with copper(II) perchlorate hexahydrate, as the metal salt under the same equivalent ratio at pH ∼ 6 formed the tetracoordinated copper(I) complex [Cu(DpymPzC)2]ClO4 (2). In both the cases, the role of DpymtH is nothing but only to reduce the copper(II) salt in situ finally forming the copper(I) complex. On the other hand, the direct reaction between the copper(I) thiocyanate and DpymPzC in 2:2 equiv. ratio produced a tricoordinated copper(I) complex [Cu(DpymPzC)SCN] (3). In a similar reaction of 2 equiv. amount of copper(II) chloride dihydrate with 2 equiv. amount of ethyl-5-methyl-1-(2-pyridyl)pyrazole-3-carboxylate (PyPzC) in presence of 1 equiv. of DpymtH at pH ∼ 6, an intense red coloured microcrystalline compound (4) was obtained. In contrast, 1 equiv. of PyPzC and 2 equiv. of DpymtH on reaction with 1 equiv. of copper(II) chloride dihydrate at pH ∼ 6 produced a novel tetranuclear mixed coordinated [Cu4(DpymtH)4Cl4] complex (5). Here DpymtH plays dual role – a reducing agent for the copper(II) salt followed by a chelating ligand towards copper(I) so formed in situ. Among the above species, 1, 2 and 5 are crystallographically characterized. In 1, the central copper atom is in distorted triangular planar geometry with N2Cl chromophore whereas in 2, the same is in distorted tetrahedral geometry with N4 chromophore. Notably, the extent of distortion from the ideal geometry is more in 2. In 5, which is in chair conformation, out of four copper atoms, two being in S2Cl chromophore are tricoordinated and the remaining two are tetracoordinated with NS2Cl chromophore. The metal centers are bridged through DpymtH in its ‘thione’ form. Interestingly, the chelation (in part) results in formation of the highly stable four-membered two chelate rings around the two tetracoordinated copper atoms in 5. The two copper centers along the long arm of the chair are separated through a distance of 5.190 Å while those in the short arm are at a length of 3.629 Å. The electronic, IR spectra and electrochemistry of the complexes 1, 2 and 5 have also been investigated.

Methyl-5-methyl-1-(4,6-dimethyl-2-pyrimidyl)pyrazole-3-carboxylate (DpymPzC) upon reaction with CuX2 · xH2O (X = Cl, ClO4, x = 2 for Cl, x = 6 for ClO4) in presence of 2-mercapto-4,6-dimethylpyrimidine (DpymtH) forms tricoordinated copper(I) complex (1) with Cl− and tetracoordinated copper(I) complex (2) with ClO4-. In contrast, a tetranuclear mixed coordinated copper(I) complex (5) of DpymtH is afforded whenever ethyl-5-methyl-1-(2-pyridyl)pyrazole-3-carboxylate (PyPzC) is used in lieu of DpymPzC under the same reaction procedure. Here PyPzC remains as an idle ligand. In 5, the chelation, in part, results in formation of a highly stable unprecedented four-membered chelate rings around the tetracoordinated copper centers.Figure optionsDownload as PowerPoint slide