Copper(II) Mediated Anion Dependent Formation of Schiff Base Complexes^†

A tetranuclear mixed ligand copper(II) complex of a pyrazole containing Schiff base and a hydroxyhexahydropyrimidylpyrazole and copper(II) and nickel(II) complexes of the Schiff base having N-donor atoms have been investigated. A 2 equiv amount of 5-methyl-3-formylpyrazole (MPA) and 2 equiv of 1,3-diamino-2-propanol (1,3-DAP) on reaction with 1 equiv of copper(II) nitrate produce an unusual tetranuclear mixed ligand complex [Cu₄(L₁)₂(L₂)₂(NO₃)₂] (1), where H₂L₁ = 1,3-bis(5-methyl-3-formylpyrazolylmethinimino)propane-2-ol and HL₂ = 5-methyl-3-(5-hydroxyhexahydro-2-pyrimidyl)pyrazole. In contrast, a similar reaction with nickel(II) nitrate leads to the formation of a hygroscopic intractable material. On the other hand, the reaction involving 2 equiv of MPA and 1 equiv each of 1,3-DAP and various copper(II) salts gives rise to two types of products, viz. [Cu(T3-porphyrinogen)(H₂O)]X₂ (X = ClO₄, NO₃, BF₄ (2)) (T₃-porphyrinogen = 1,6,11,16- tetraza-5,10,15,20- tetrahydroxy-2,7,12,17-tetramethylporphyrinogen) and [Cu(H₂L₁)X]X·H₂O (X = Cl (3), Br (4)). The same reaction carried out with nickel(II) salts also produces two types of compounds [Ni(H₂L₁)(H₂O)₂]X₂ [X = ClO₄ (5), NO₃ (6), BF₄ (7)] and [Ni(H₂L₁)X₂]·H₂O [X = Cl (8), Br (9)]. Among the above species 1, 3, and 5 are crystallographically characterized. In 1, all four copper atoms are in distorted square pyramidal geometry with N₄O chromophore around two terminal copper atoms and N₅ chromophore around two inner copper atoms. In 3, the copper atom is also in distorted square pyramidal geometry with N₄Cl chromophore. The nickel atom in 5 is in a distorted octahedral geometry with N₄O₂ chromophore, where the metal atom is slightly pulled toward one of the axial coordinated water molecules. Variable-temperature (300 to 2 K) magnetic susceptibility measurements have been carried out for complex 1. The separations between the metal centers, viz., Cu(1)···Cu(2), Cu(2)···Cu(2A), and Cu(2A)···Cu(1A) are 3.858, 3.89, and 3.858 Å, respectively. The overall magnetic behavior is consistent with strong antiferromagnetic interactions between the spin centers. The exchange coupling constants between Cu(1)···Cu(2) and Cu(2)···Cu(2A) centers have turned out to be −305.3 and −400.7 cm^-1, respectively, resulting in a S = 1/2 ground state. The complexes are further characterized by UV−vis, IR, electron paramagnetic resonance, and electrochemical studies.