Synthesis, crystal structures and magnetic properties of Cu(II) compounds bridged by the terephthalate ligand and hydrogen bonds

Two tp-bridged Cu(II) compounds, [Cu(tmen)(tp)(H₂O) ₂]_n (1) (tmen = N,N,N′,N′- tetramethylethylenediamine; tp = terephthalate) and [Cu(pyrazole) ₂(tp)]_n (2), showing coordinative zigzag and linear structures, respectively, were characterized by means of X-ray crystallography and magnetic measurements. Both coordination polymers, with a 3D extended network array (1) and a 2D sheet structure (2), are formed by incorporating hydrogen bonds into 1D chains. Magnetic data indicate that they exhibit weak ferromagnetic interactions through hydrogen bonds and weak antiferromagnetic couplings via tp ligands. A 3D network [Cu(tmen)(tp)(H₂O) ₂]_n (1) (tmen = N,N,N′,N′- tetramethylethylenediamine; tp = terephthalate) and a 2D sheet [Cu(pyrazole)₂(tp)]_n (2), featuring 1D chains interwoven by hydrogen bonds, have been prepared and characterized by means of X-ray analyses and magnetic measurements. For 1, coordinative zigzag chains contain Cu(II) centers capped by the chelate ligand tmen, in which the tetragonal structure is elongated due to Jahn-Teller distortion. Coordinated water molecules are hydrogen-bonded to two free carboxylate oxygens of tp bridges, leading to the observed 3D structure. The use of the non-chelating capping ligand pyrazole produced the covalent-bonded 1D linear compound 2 with hydrogen bonds. A severe octahedral distortion of the Cu(II) center arises from a small bite angle (52.3(1)°) of two carboxylate oxygen atoms of tp, which are in turn hydrogen-bonded to the N-H groups of pyrazole ligands coordinated to Cu(II) atoms in neighboring chains. Magnetic data were fitted with the high-temperature series expansion for the Heisenberg chain spin Hamiltonian H = -JΣ_iS_i • S_{i + 1} together with consideration of the molecular field approximation (zJ′). Both compounds interestingly exhibit ferromagnetic interactions with g = 2.17, J = 4.08 cm ^-1, zJ′ = -0.28 cm^-1 for 1 and g = 2.09, J = 1.47 cm^-1, zJ′ = -0.04 cm^-1 for 2. By taking into account structural parameters of distances between Cu atoms, it is reasonably assigned that the ferromagnetic couplings (J > 0) in these systems originate from the hydrogen bonds. The spin density of the dx2-y2 orbital on a Cu(II) atom in a chain is propagated and induced over the dz2 orbital of another Cu(II) atom in an adjacent chain. This orbital orthogonality gives rise to such interactions. The negative zJ′ term suggests that the tp bridges communicate only tiny antiferromagnetic interactions.