Rare Example of μ-Nitrito-1κ2O,O′:2κO Coordinating Mode in Copper(II) Nitrite Complexes with Monoanionic Tridentate Schiff Base Ligands: Structure, Magnetic, and Electrochemical Properties

Three new copper(II) complexes, [CuL1(NO2)](n) (1), [CuL2(NO2)] (2), and [CuL3(NO2)] (3), with three similar tridentate Schiff base ligands [HL1 = 6-amino-3-methyl-1-phenyl-4-azahept-2-en-1-one, HL2 = 6-amino-3-methyl-1-phenyl-4-azahex-2-en-1-one, and HL3 = 6-diethylamino-3-methyl-1-phenyl-4-azahex-2-en-1-one] have been synthesized and characterized structurally and magnetically. In all three complexes, the tridentate Schiff base ligand and one oxygen atom of the nitrite ion constitute the equatorial plane around Cu(II), whereas the second oxygen atom of the nitrite ligand coordinates to one of the axial positions. In 1, this axially coordinated oxygen atom of the nitrite ligand also coordinates weakly to the other axial position of a Cu(II) ion of another unit to form a one-dimensional chain with the mu-nitrito-1 kappa O-2,O':2KO bridging mode. Complexes 2 and 3 are discrete monomers that are joined together by intermolecular H bonds and C-H center dot center dot center dot center dot pi interactions in 2 and by only C-H center dot center dot center dot center dot pi interactions in 3. A weak antiferromagnetism (J = -1.96(2) cm(-1)) is observed in complex 1 due to its asymmetric nitrite bridging. Complexes 2 and 3 show very weak antiferromagnetic interactions (J = -0.089 and -0.096 cm(-1), respectively) attributed to the presence of intermolecular H-bonding and C-H center dot center dot center dot center dot pi interactions. The corresponding Cu(I) species produced by the electrochemical reduction of complexes 1 and 2 disproportionate to Cu-0 and Cu2+, whereas the reduced Cu(I) species of complex 3 seems to be stable presumably due to a higher tetrahedral distortion of the equatorial plane in 3 compared to that in 1 and 2.