A Combined Experimental and Theoretical Charge Density Study of the Chemical Bonding and Magnetism in 3-Amino-propanolato Cu(II) Complexes Containing Weakly Coordinated Anions

The experimental (100 K) and theoretical charge densities in the binuclear complexes [Cu2(ap)2(L)2] (ap = 3-aminopropanolate) 1 (L = nitrite), 2 (L = nitrate), and 3 (L = formate) have been examined. These complexes contain the same centrosymmetric alkoxy-bridged motif, where each strongly Jahn-Teller distorted Cu(II) ion is ligated to three O atoms and one N atom in a square-planar arrangement. This primary coordination sphere is augmented by a long contact with the O atom of a pendant L anion from an adjacent molecule in the crystal lattice. Topological analyses of the experimental and theoretical densities according to the quantum theory of atoms in molecules (QTAIM) are in excellent agreement. Consideration of a number of topological indicators including rho(r), vector differential(2)rho(r), the delocalization indices delta(A,B), and the contour integral(A intersection B) rho(r) of the density over the zero flux surface shared by the two atoms confirms that the Cu-O and Cu-N bonding in the primary coordination sphere has a strong covalent component, but the weak Cu...O interactions are primarily electrostatic in nature. In this first investigation of the source function in a coordination complex, it is shown to provide an insight into the differing electrostatic and covalent contributions to the chemical bonds. The two Cu(II) centers are strongly antiferromagnetically coupled, but the topological analyses indicates the lack of any direct Cu...Cu interaction. The molecular graph suggests an exchange pathway via the bridging O-atoms, thus providing experimental support of the classical superexchange mechanism. Periodic DFT calculations on 2 and 3 show that the intradimer coupling proceeds via spin-delocalization and provide values of the magnetic coupling constants -2 J of 324.5 and 244.8 cm(-1), respectively, which compare well with the previously determined experimental values.