Synthesis, structural, magnetic and thermal characterization of {[Cu(bipy)]₂(μ-HP₂O₇)(μ-Cl)} ·H₂O

Copper(II) hydroxide reacts with 2,2′-bipyridine (bipy) and sodium pyrophosphate in a 2:2:1 stoichiometric ratio under ambient conditions at pH 1.6 in water. The resulting neutral dinuclear Cu^II complex features a bridging set containing bridging monoprotonated pyrophosphate and a monoatomic chloro bridge (making this the first pyrophosphate bridged coordination complex containing an alternate, additional halide bridge between the metal centers). Single-crystal X-ray diffraction studies revealed the complex to be {[Cu(bipy)]₂(μ-HP₂O₇)(μ-Cl)} ·H₂O. The structure consists of a dimeric copper(II) system with each metal ion in a square pyramidal geometry. The asymmetric [Cu(bipy)]²⁺ units are bridged by bis-bidentate pyrophosphate and an axially bridging chlorine atom. The incorporation of an additional bridge (halide) and the presence of protonated pyrophosphate (HP₂O ₇^3-) in the structure are both rare for this family of structures. Inter-molecular π-π interactions between the bipy ligands from adjacent dinuclear complexes and hydrogen bonding between the lattice water molecule and the bridging chloro atom create a 1D network. Variable-temperature magnetic susceptibility measurements show the occurrence of a weak antiferromagnetic interaction with a maximum at 3.0 K. The analysis of the magnetic data in the whole temperature range (1.9-300 K) allows the determination of the value of the intramolecular magnetic coupling (J = -3.19 cm^-1, the Hamiltonian being defined as Ĥ = - JŜ_A.Ŝ_B). This value of J is significantly lower than that observed for the analogous dicopper(II) system devoid of the bridging chloro group. Reasons for this difference and its importance are discussed.