Structure, Stereochemistry, and Physico-Chemical Properties of Trinuclear and Dinuclear Metal(II) Complexes of a Phenol-Based Tetrapodal Schiff Base Ligand

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 [Mg-3(HL)(2)]center dot nH(2)O (1), [Ni-3(HL)(2)center dot 2C(7)H(8) (3), [Ni3L2']center dot 0.5C(7)H(8) (4), [Co-3(HL)(2)] (5), and [Co3L2']center dot C6H6 (6); (ii) the dinuclear compounds [Ni2L] (2), [Cu2L]center dot CH3CN (8), and [Pd2L] (9); (iii) an unusual dimeric compound [{Ni(H2.5L)}(2)](ClO4)center dot 2H(2)O (7); and (iv) the inclusion compounds [Ni2L subset of NaClO4]center dot CH3CN (10) and [Cu2L subset of NaClO4] (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 [M3L2']. All of the trinuclear complexes have the same core coordination sphere [N3M(mu-O-phenolate)(3)M(mu-O-phenolate)(3)MN3] 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 center dot center dot center dot Delta), but heterochiral (Lambda center dot center dot center dot Delta) in 1 and 4. In compound 7, the two mononuclear complex units are held together by three equivalent O center dot center dot center dot H center dot center dot center dot 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(11) 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)(2)SO 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+ > L+. [Ni2L subset of NaClO4]center dot 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 lambda(em) = 425 nm, and the lifetime for fluorescent decay is 18.5 ns. The thermal beaviors 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 degrees and 180 degrees C in 6. The enthalpy of desolvation of 3 is 43.4 kJ mol(-1).