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 (H(4)L) has been used to synthesize a number of divalent metal complexes, which include (i) the trinuclear compounds [Mg(3)(HL)(2)].nH(2)O (1), [Ni(3)(HL)(2)].2C(7)H(8) (3), [Ni(3)L'(2)].0.5C(7)H(8) (4), [Co(3)(HL)(2)] (5), and [Co(3)L'(2)].C(6)H(6) (6); (ii) the dinuclear compounds [Ni(2)L] (2), [Cu(2)L].CH(3)CN (8), and [Pd(2)L] (9); (iii) an unusual dimeric compound [{Ni(H(2.5)L)}(2)](ClO(4)).2H(2)O (7); and (iv) the inclusion compounds [Ni(2)L subset NaClO(4)].CH(3)CN (10) and [Cu(2)L subset NaClO(4)] (11). The molecular structures of compounds 1, 3, 4, 6, 7, and 10 have been determined. In [M(3)(HL)(2)] complexes, one of the salicylaldimine chelating units remains uncoordinated, which on being hydrolyzed is transformed to the amine-ending complex [M(3)L'(2)]. All of the trinuclear complexes have the same core coordination sphere [N(3)M(mu-O(phenolate))(3)M(mu-O(phenolate))(3)MN(3)] 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 (Delta...Delta), but heterochiral (Lambda...Delta) 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 [M(2)L], 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 (CH(3))(2)SO solution, and the results show that [Ni(2)L] is a better host compared to [Cu(2)L]. The decreasing order of K values for both hosts is Na(+) > Li(+) > K(+). [Ni(2)L subsetNaClO(4)].CH(3)CN (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 CH(2)Cl(2) at room temperature with lambda(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 degrees and 230 degrees C, while the benzene is evolved between 100 degrees and 180 degrees C in 6. The enthalpy of desolvation of 3 is 43.4 kJ mol(-1).