Theoretical and experimental study of the effectiveness of the 5-pyrimidyl-tetrazolate bridging ligand in mediating magnetic exchange interactions.

The 5-pyrimidyl-tetrazolate anion (pmtz) has six basic nitrogen atoms and, consequently, can exhibit a variety of bridging coordination modes. This ligand reacts with either copper(II) and nickel(II) ions in the presence of bi- and tridentate amines to afford the following pmtz-bridged complexes: Cu(4)(μ-pmtz)(4)(tmda)(4)(4) (1) (tmda = N,N,N',N'-tetramethylethylenediamine), Cu(2)(μ-pmtz)(tren)(2)(3) (2) (tren = tris(2-aminoethyl)amine), Ni(2)(μ-pmtz)(tren)(2)(3) (3), and Ni(2)(μ-pmtz)(2)(ampa)(2)(SCN)(2n) (ampa = bis(3-aminopropyl)amine) (4). The structure and bridging coordination mode of these complexes depend on the stereoelectronic preferences of the metal ion and the coordination properties of the polyamine (denticity and relative disposition of the donor atoms). Thus, complex 1 is a square tetranuclear compound where the ligand adopts an asymmetric k(2)N(1),N(7):k(2)N(4),N(11) bis(chelating)/bridging mode. Complexes 2 and 3 are dinuclear species. In the former the pmtz exhibits a novel k-N(1):k-N(4) imidazolyl/bridging mode whereas the latter shows a symmetric k(2)N(1),N(7):k(2)N(4),N(11) bis(chelating)/bridging mode. Complex 4 exhibits, however, a linear chain structure where the pmtz ligand connects neighboring nickel(II) atoms by using a tridentate k(2)N(1),N(7):kN(3) chelating/bridging mode. Variable-temperature magnetic susceptibility studies reveal that complexes 1-4 show weak to moderate antiferromagnetic (AF) coupling between the metal ions through the pmtz ligand with J = -14.1 cm(-1) for 1, J = -31.1 cm(-1) for 2, J = -4.09 cm(-1) for 3, and J = -8.7 cm(-1) for 4. The magneto-structural results, as well as DFT theoretical calculations carried out on the experimental geometries and model complexes, demonstrate, first, that the magnitude of the AF interaction observed for imidazolyl pmtz-bridged complexes mainly depends on the Cu-N-C angle (the greater the Cu-N-C angle, the stronger is the AF interaction) and, second, that the tridentate chelating/bridging mode is more effective in mediating AF exchange interactions than the bis(chelating)/bridging or imidazolyl/bridging modes in pmtz-bridged complexes.