Single-crystal ESR and X-Ray Crystallographical Studies of a Binuclear Copper(II) Complex with a Coaxially Stacked Square-pyramidal Geometry

Abstract

Abstract A binuclear complex of bis(μ-N,N′-dipicolinoyl-1,3-propanediamine)-bis(sulfato)dicopper(II) has been isolated as a blue single crystal tridecahydrate, [{Cu(SO4)(ppda)}2]·13H2O, from an aqueous solution. The crystals are orthorhombic, C2221, with a=15.42(1), b=24.92(2), c=25.73(2) Å, and Z=8. Each copper(II) ion is in a square-pyramidal coordination with two pyridine nitrogen atoms, two carbonyl oxygen atoms, and a sulfate oxygen atom at the apex. The two equivalent copper(II) ions, about 3.9 Å apart from each other, are aligned coaxially. The eight Cu–Cu vectors are approximately parallel to one another along the c axis. The most important feature of the crystal structure is the presence of the bulky ligands and of many water molecules, which magnetically isolate the Cu–Cu coupled pair from one another. This facilitates the ESR study of the molecular anisotropies by using the single crystal, even at room temperature. The ESR parameters determined are as follows: g⁄⁄=2.280(2), g⊥=2.067(1), |D|=422.4(8), |A⁄⁄|=170(2), and |A⊥|=14(1) in 10−4 cm−1. Bleaney and Bowers treatments of the exchange coupling contribution in the D parameter indicated a ground singlet state about 7 cm−1 below a triplet state for the coupled copper(II) ions. A comparison of the ESR parameters measured for the single crystal with those in the aqueous solution enabled us to conclude that the molecular structure in the solution was very similar to that in the crystalline phase, apart from the coordination of sulfate ions.