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 MnII/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(μ-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, θ, of 102.53(12) and 101.70(12)° and Mn···Mn distances of 3.5091(7) and 3.4680(7) Å. On the other hand, each manganese center in compounds 3 and 4 is located within an octahedral coordination environment, the complexes displaying θ angles of 104.29(11) and 103.60(18)°, respectively, and Mn···Mn vectors of 3.5371(7) and 3.5338(10) Å, 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 = −2JS 1 S 2 as the convention for the Heisenberg spin-Hamiltonian). These results are consistent with predictions from recent DFT calculations performed on end-on bis(μ-N3 -)-bridged MnII dinuclear complexes. A plot of experimental J vs θ, 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.