Asymmetric azido-copper(II) bridges:: Ferro- or antiferromagnetic?: Experimental and theoretical magneto-structural studies

Reaction of NaN3 with the [Cu-II(tn)](2+) ion (tn = 1,3-diaminopropane) in basic aqueous solution yields the azidobridged complex of formula [Cu-2(tn)(2)(N-3)(4)] (1), which is characterized by X-ray crystallography. The structure of 1 is made up of dinuclear neutral complexes, of formula [Cu-2(tn)(2)(N-3)(4)], resulting from the assembling of two mononuclear units through two equivalent end-on azide bridges connecting asymmetrically two Cu(tn)(N-3)(2) entities. These dinuclear units are connected through two asymmetric end-to-end N-3 bridges to form a chain of dimers. Magnetic measurements for compound 1 show weak antiferromagnetic exchange interactions between the Cu(II) ions. The magnetic data were modeled using the susceptibility expression derived for an alternating AF S = 1/2 chain. A very satisfactory fit over the whole temperature range was obtained with g = 2.1438(4), J(1) = -3.71(2) cm(-1), and J(2) = -3.10(2) cm(-1) (J(1) and J(2) are the singlet-triplet separations). This magnetic behavior differs from those observed for similar examples which were reported as having alternating ferro- and antiferromagnetic exchange interactions; thus, DFT calculations were done to understand the nature of the magnetic coupling in such asymmetric end-on and end-to-end N-3 bridges. Theoretical results show that the double asymmetric end-on bridges produce antiferromagnetic coupling while the end-to-end ones can present ferro- or antiferromagnetic coupling depending on the copper coordination sphere.