Coexistence of Intramolecular Ligand-Mediated and Through Hydrogen-Bond Magnetic Interactions in a Chain of Dicopper(II) Units

The reaction of 2,8-dimethyl-5,11-bis(pyridin-2-ylmethyl)-1,4,5,6,7,10,11,12-octahydroimidazo[4,5-h]imidazo[4,5-c][1,6]-diazecine (dimp) with copper(II) nitrate in water produces the compound [Cu2(dimp)(H2O)2(NO3)2](NO3)2. The single-crystal X-ray structure shows the formation of hydrogen-bonded chains in the lattice that are formed by dicopper(II) units doubly connected by nitrate/water bridges. Within the one-dimensional chains, the Cu ions are separated by either intramolecular or intermolecular distances of 7.309(2) Å or 6.255(2) Å, respectively. The magnetic susceptibility data revealing weak antiferromagnetic exchange interactions between the copper(II) ions were interpreted by considering two possible models, namely, an isolated dinuclear and a 1-D chain picture. The latter leads to an alternation J1 = −11.6 and J2 = −3.0 cm–1 along the chain. In order to clarify the relative strengths of the exchange couplings through hydrogen bonds and via the bridging dimp ligand, solution EPR studies and quantum chemical calculations were carried out. EPR studies unambiguously conclude on the existence of an exchange interaction Ja mediated by the dinucleating dimp ligand, while the through-H coupling Jb is physically absent in solution. On the basis of dinuclear units extracted from the X-ray data, Ja was estimated around −5.0 cm–1 from DFT-based calculations (M06 functional), whereas Jb is negligible. In contrast, wave function configuration interaction calculations (DDCI) support a description where both inter- and intramolecular pathways coexist with a preeminent role of H bonds with Ja = −2.8 and Jb = −10.4 cm–1. Not only are these values very consistent with the extracted set of parameters (J1, J2 = −11.6, −3.0 cm–1) but the possibility to generate leading exchange coupling through weak bonds is evidenced by means of wave function-based calculations.