Magneto-structural correlations: synthesis of a family of end-on azido-bridged manganese(II) dinuclear compounds with S = 5 spin ground state.

The preparation of a series of multidentate pyridyl-imine ligands, L1-L3, and their reactivity with the Mn(II)/N3- system is described (L1 = [N,N-bis(pyridine-2-yl)benzylidene]ethane-1,2-diamine; L2 = [N,N-bis(pyridine-2-yl)benzylidene]propane-1,3-diamine, and L3 = [N,N-bis(pyridine-2-yl)benzylidene]butane-1,4-diamine). Complexes comprising dinuclear end-on bis(mu-azido)-bridged manganese(II) units of formulas [Mn2(L1)2(N3)4][Mn2(L1)2(N3)2(CH3OH)2](ClO4)2 (two cocrystallized dinuclear units, 1.2), [Mn2(L2)2(N3)2](ClO4)2 (3), and [Mn2(L3)2(N3)2](ClO4)2 (4) have been synthesized. The crystal structures of complexes 1-4 as well as their magnetic properties are presented. Each manganese atom of cocrystallized complexes in compound 1.2 is heptacoordinated, displaying Mn-N-Mn angles, theta, of 102.53(12) and 101.70(12) degrees and Mn...Mn distances of 3.5091(7) and 3.4680(7) A. On the other hand, each manganese center in compounds 3 and 4 is located within an octahedral coordination environment, the complexes displaying theta angles of 104.29(11) and 103.60(18) degrees , respectively, and Mn...Mn vectors of 3.5371(7) and 3.5338(10) A, respectively. Magnetic susceptibility studies revealed the presence of intramolecular ferromagnetic superexchange, yielding an S = 5 spin ground state in all complexes. Fitting of the experimental data led to coupling constants, intermolecular exchange values, and g factors (in the J/zJ'/g format) of 0.77 cm(-1)/0.01 cm(-1)/2.20 (1.2), 2.04 cm(-1)/0.01 cm(-1)/1.99 (3), and 1.75 cm(-1)/-0.05 cm(-1)/2.04 (4), respectively (using H = -2JS1S2 as the convention for the Heisenberg spin-Hamiltonian). These results are consistent with predictions from recent DFT calculations performed on end-on bis(mu-N3-)-bridged Mn(II) dinuclear complexes. A plot of experimental J vs theta, including data from the only preexisting compound of this kind, reveals a linear relationship, which could be the first evidence of a possible magneto-structural correlation between these two parameters.