A tridentate bis(pyrazolyl) ligand binds to Cu(II), without using the pyrazole group: a very unusual coordination mode of the ligand Hbdmpb, 1,3-bis(3,5-dimethylpyrazol-1-yl)-2-butanoic acid

A new pyrazole-based ligand, namely 1,3-bis(3,5-dimethylpyrazol-1-yl)-2-butanoic acid (Hbdmpb) was synthesised together with its copper complex Na[Cu(bdmpb)2(OOCCH3)H2O] · 4H2O. Both the free ligand and its Cu compound were fully characterised and their crystal structures were determined by X-ray analysis. The free-ligand molecular structure is uneventful. The Cu compound is highly unusual, as the pyrazole nitrogen atoms do not bind to the Cu ion. The copper(II) ion is coordinated by four nearly coplanar oxygen atoms from two dehydronated ligands bdmpb (Cusingle bondO(1a) 1.942(4), Cusingle bondO(1b) 1.933(4) Å), a monodentate acetate group (CuO(1) 1.927(3) Å) and a water molecule (Cusingle bondO(1w) 1.937(4) Å). The nitrogen atoms of the pyrazole rings do not coordinate to the metal center, but instead are involved in strong intramolecular hydrogen bonds. The coordinated water molecule is strongly H-bonded to two pyrazole N atoms from two bdmpb ligands (N(12a) ⋯ Hsingle bondO(1w) 2.762(7), N(12b) ⋯ Hsingle bondO(1w) 2.774(7) Å). The other two pyrazole N atoms with a lone pair are hydrogen-bonded to water molecules in the lattice (N(22a) ⋯ Hsingle bondO(2w) 2.763(7), N(22b) ⋯ Hsingle bondO(6w) 2.892(7) Å). The sodium ion is six-coordinated by the oxygen atom O(2) of the acetato ligand and by five water molecules. The EPR spectrum recorded in the solid state shows a characteristic signal for an axial anisotropic S = 1/2 species. The spectrum recorded in methanol glass confirms the absence of the coordination of pyrazole nitrogen atoms to the copper centers.