The preparation, characterization, and magnetism of copper 15-metallacrown-5 lanthanide complexes

The preparation and characterization of a series of encapsulated-lanthanide 15-metallacrown-5 complexes are reported. Planar ligands such as picoline hydroxamic acid (picha) or nonplanar cr-amino hydroxamic acids (e.g., glyci ne hydroxamic acid (glyha)) led to one-step syntheses of metallacrowns in y ields as high as 85%. The reaction of the appropriate hydroxamic acid with copper acetate and 1/5 equiv of gadolinium(III) or europium(III) nitrates i n DMF or water yielded crystals of GcB(NO3)(3)[15-M-Cu(II)N(picha)-5], 1, E u(NO3)(3)[15-MCCu(II)N(picha)-5], 2, and Eu(NO3)(3)[15-MCCu(II)N(glyha)-5], 3. Several other 15-metallacrown-5 complexes were synthesized with (1) Cu( II) or Ni(II) in the metallacrown ring metal position, (2) various lanthani des (La(III), Nd(III), Sm(III), Eu(III), Gd(III), Dy(III), Ho(III), Er(III) , and Yb(III)) encapsulated in the center of the ring, and (3) chiral cl-am ino hydroxamic acids (e.g., phenylalanine hydroxamic acid (H(2)pheha), leuc ine hydroxamic acid (H(2)leuha), and tyrosine hydroxamic acid (H(2)tyrha)). It is believed that all of the complexes containing Cu(II) ions have the r ing metals either in four-coordinate, square-planar environments, bound to two tetradentate hydroximate ligands, or in five-coordinate, square-pyramid al geometries if solvent is bound. Spectroscopic and magnetic characterizat ion of the Ni(II) complexes suggests that they are either five- or six-coor dinate. The encapsulated lanthanides are generally pentagonal bipyramidal, with five oxygen donors from the metallacrown ring and solvent or bidentate nitrate ions in the axial positions. The circular arrangement of ions resu lts in interesting magnetic behavior. With Dy(III) encapsulated in the cent er of the ring, a magnetic moment as high as 10.9 mu(B) is achieved. Analys is of the variable-temperature susceptibility of La(NO3)(3)[15-MCCu(II)N(pi cha)-5] indicates that the five Cu(II) ions are antiferromagnetically coupl ed, forming an S = 1/2 ground spin state with a moment of 1.7 mu(B) at liqu id helium temperatures. Complex 1 shows ferromagnetic coupling of the Gd(II I) ion to the five Cu ions at temperatures below 15 K. Studies of the metal lacrown complexes in solution show that they are stable and soluble in DMF and water. A proton relaxation study on complex 1 has revealed. a relaxivit y of 9.8 mM(-1) s(-1) (20 degrees C and 30 MHz), a value that is comparable to those of clinically useful MRI contrast enhancement agents. Complex 1 c rystallizes in the triclinic space group P (1) over bar, with a = 12.657(3) Angstrom, b = 14.833(3) Angstrom, c 17.707(3) Angstrom, alpha = 79.65(2)de grees, beta = 86.06(2)degrees, gamma = 68.69(2)degrees, V = 3046.6(12) Angs trom, and Z = 2 (R1 = 0.0534, wR2 = 0.1289). Complex 2 crystallizes in the monoclinic space group P2(1)/n, with a 16.319(2) Angstrom, b = 21.863(2) An gstrom, c 18.410(3) Angstrom, beta 96.85(1)0, V = 6522(2) A(3), and Z = 4 ( R1 = 0.0463, wR2 = 0.0750). Complex 3 crystallizes in the triclinic space g roup P (1) over bar, with a = 11. 173(6) Angstrom, b = 11.534(6) Angstrom, c = 13.311(5) Angstrom, alpha 93.81(3)degrees, beta = 94.82(4)degrees, gamm a = 107.20(4)degrees= 1625(2) Angstrom(3), and Z = 2 (R1 0.2979).