Structure, stereochemistry, and physico-chemical properties of trinuclear and dinuclear metal(II) complexes of a phenol-based tetrapodal schiff base ligand

Abstract

The tetrapodal ligand 1,1,1,1-tetrakis[(salicylaldimino)methyl]methane (H₄L) has been used to synthesize a number of divalent metal complexes, which include (i) the trinuclear compounds [Mg₃(HL)₂]·nH₂O (1), [Ni₃(HL)₂]·2C₇H₈ (3), [Ni₃L'₂]·0.5C₇H₈ (4), [Co₃(HL)₂] (5), and [Co₃L'₂]·C₆H₆ (6); (ii) the dinuclear compounds [Ni₂L] (2), [Cu₂L]·CH₃CN (8), and [Pd₂L] (9); (iii) an unusual dimeric compound [{Ni(H_2.5L)}₂](ClO₄)·2H₂O (7); and (iv) the inclusion compounds [Ni₂LNaClO₄]·CH₃CN (10) and [Cu₂L_cNaClO₄] (11). The molecular structures of compounds 1, 3, 4, 6, 7, and 10 have been determined. In [M₃(HL)₂] complexes, one of the salicylaldimine chelating units remains uncoordinated, which on being hydrolyzed is transformed to the amine-ending complex [M₃L'₂]. All of the trinuclear complexes have the same core coordination sphere [N₃M(μ-O_phenolate)₃M(μ-O_phenolate)₃MN₃] where the terminal metals are connected to the central metal via face-shared phenolate oxygens. In the trinuclear compounds, the terminal metals are distorted from octahedral to trigonal prismatic to different extents in 1 and 6, while in 3 and 4 they are trigonal antiprismatically distorted. The stereochemical configurations obtained by the terminal metals in 3 and 6 are homochiral (Δ···Δ), but heterochiral (Δ···Δ) in 1 and 4. In compound 7, the two mononuclear complex units are held together by three equivalent O····H···O bridges, indicating 50% deprotonation of all the metal-coordinated phenols. The temperature-dependent magnetic behavior of 7 has indicated the presence of very weak antiferromagnetic exchange coupling (J =-0.2 cm^-1) between the two nickel(II) centers. Very similar magnetic behavior observed for the trinuclear nickel(II) compounds 3 and 4 is attributed to a ferromagnetic exchange interaction between the adjacent metals (J = 7.6 cm^-1), although an interaction between the terminal metals is absent; in contrast, the adjacent cobalt(II) centers in 6, however, are involved in an antiferromagnetic exchange interaction (J =-5.7 cm^-1). The dinuclear complexes [M₂L], in which each of the metal centers are chelated with a pair of salicylaldimines, act as hosts (when M = Ni and Cu) for alkali metals (Li⁺, Na⁺, and K⁺). The host-guest binding constants (K) have been determined in (CH₃)₂SO solution, and the results show that [Ni₂L] is a better host compared to [Cu₂L]. The decreasing order of K values for both hosts is Na⁺ > Li⁺ > K⁺. [Ni₂L_cNaClO₄]·CH₃CN (10) has been shown to have a polymeric structure in which sodium is octahedrally surrounded by four nickel-coordinated phenolate and two perchlorate oxygens. The magnesium(II) complex 1 exhibits strong fluorescence in CH₂Cl₂ at room temperature with λ_em = 425 nm, and the lifetime for fluorescent decay is 18.5 ns. The thermal behaviors of 3 and 6 with regard to their loss of aromatic solvent molecules have been studied. The evolution of the toluene molecules from 3 takes place between 140° and 230 °C, while the benzene is evolved between 100° and 180 °C in 6. The enthalpy of desolvation of 3 is 43.4 kJ mol^-1.