Metal Site Segregation in Chair-Shaped MII4 Cluster: Crystallography, Mass Spectrometry, and Magnetic and Optical Properties - zeng's paper

The flexible ligand, 2-[(2-hydroxy-3-methoxy-benzylidene)-amino]-ethanesulfonate (L2-), uses five of its six donor atoms in coordination through meridian, chelate and terminal modes with MIIX2 to form chair-shaped tetranuclear clusters Zn4L2(OCH3)2(CH3OH)2X2 [X = Cl (Zn4-Cl) and Br (Zn4-Br)], and solid solutions Zn4-xNix-Br or Zn4-xNix-Cl (x = 0-2). The structures contain butterfly tetramers which consist of segregated six-coordinated metals (Zn or Ni) at the center but only five-coordinated zinc at the ends. Syntheses using different proportions of metal salts (Zn and Ni) gave solid solutions with a substitution rate of 2/3 times the starting quantity of NiBr2 up to a saturation content of 50 %, Zn2Ni2L2(OCH3)2(CH3OH)2Br2 (Zn2Ni2-Br). The elemental and powder X-ray diffraction follow Vegard’s law. The ferromagnetic coupling between nearest neighbors of 3.7 cm-1 and the structural refinement for (Zn2Ni2-Br) confirm the Ni atoms are at the center. This segregation is assumed to be related to the size and coordination number and the softness difference of the metals. The ESI-MS display a series of positive ions ABn or CBn (A = {ZnBr} and B = {ZnL}, C = {Zn(OCH3)}, n = 0~4) and negative ions AxBy (x = 0~1, y = 1~2), but none corresponds to the parent cluster. The luminescence is enhanced from that of Li2L upon coordination to the metals. Major dependence of the emission and the excitation profile intensity on the concentration in the range 5-500 μM has been observed for Li2L and its metal complexes.