Ferromagnetic Interaction in an Asymmetric End-to-End Azido Double-Bridged Copper(II) Dinuclear Complex: A Combined Structure, Magnetic, Polarized Neutron Diffraction and Theoretical Study

A new end‐to‐end azido double‐bridged copper(II) complex [Cu2L2(N3)2] (1) was synthesized and characterized (L=1,1,1‐trifluoro‐7‐(dimethylamino)‐4‐methyl‐5‐aza‐3‐hepten‐2‐onato). Despite the rather long CuCu distance (5.105(1) Å), the magnetic interaction is ferromagnetic with J= +16 cm−1 (H=−JS1S2), a value that has been confirmed by DFT and high‐level correlated ab initio calculations. The spin distribution was studied by using the results from polarized neutron diffraction. This is the first such study on an end‐to‐end system. The experimental spin density was found to be localized mainly on the copper(II) ions, with a small degree of delocalization on the ligand (L) and terminal azido nitrogens. There was zero delocalization on the central nitrogen, in agreement with DFT calculations. Such a picture corresponds to an important contribution of the d x 2−y 2 orbital and a small population of the d z 2 orbital, in agreement with our calculations. Based on a correlated wavefunction analysis, the ferromagnetic behavior results from a dominant double spin polarization contribution and vanishingly small ionic forms. Where's the spin? The experimental spin density in a ferromagnetic, end‐to‐end azido‐bridged, dinuclear copper(II) complex has been clarified by using polarized neutron diffraction experiments. The spin densities were also calculated with a DFT formalism to obtain a clearer interpretation of the role of N3 as a magnetic coupler.