The Preparation, Characterization, and Magnetism of Copper 15-Metallacrown-5 Lanthanide Complexes

Abstract

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 alpha-amino hydroxamic acids (e.g., glycine hydroxamic acid (glyha)) led to one-step syntheses of metallacrowns in yields 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 in DMF or water yielded crystals of Gd(NO(3))(3)[15-MC(Cu(II)N(picha))-5], 1, Eu(NO(3))(3)[15-MC(Cu(II)N(picha))-5], 2, and Eu(NO(3))(3)[15-MC(Cu(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 lanthanides (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 alpha-amino hydroxamic acids (e.g., phenylalanine hydroxamic acid (H(2)pheha), leucine 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 ring metals either in four-coordinate, square-planar environments, bound to two tetradentate hydroximate ligands, or in five-coordinate, square-pyramidal geometries if solvent is bound. Spectroscopic and magnetic characterization of the Ni(II) complexes suggests that they are either five- or six-coordinate. 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 results in interesting magnetic behavior. With Dy(III) encapsulated in the center of the ring, a magnetic moment as high as 10.9 &mgr;(B) is achieved. Analysis of the variable-temperature susceptibility of La(NO(3))(3)[15-MC(Cu(II)N(picha))-5] indicates that the five Cu(II) ions are antiferromagnetically coupled, forming an S = (1)/(2) ground spin state with a moment of 1.7 &mgr;(B) at liquid helium temperatures. Complex 1 shows ferromagnetic coupling of the Gd(III) ion to the five Cu ions at temperatures below 15 K. Studies of the metallacrown complexes in solution show that they are stable and soluble in DMF and water. A proton relaxation study on complex 1 has revealed a relaxivity 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 crystallizes in the triclinic space group P&onemacr;, with a = 12.657(3) Å, b = 14.833(3) Å, c = 17.707(3) Å, alpha = 79.65(2) degrees, beta = 86.06(2) degrees, gamma = 68.69(2) degrees, V = 3046.6(12) Å, 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) Å, b = 21.863(2) Å, c = 18.410(3) Å, beta = 96.85(1) degrees, V = 6522(2) Å(3), and Z = 4 (R1 = 0.0463, wR2 = 0.0750). Complex 3 crystallizes in the triclinic space group P&onemacr;, with a = 11. 173(6) Å, b = 11.534(6) Å, c = 13.3...

A classification of spin frustration in molecular magnets from a physical study of large odd-numbered-metal, odd electron rings

“…There are a few reports of five-metal rings but no detailed magnetic studies of such compounds (8), with the exception of a {Cu 5 } cage (9). The only significant examples of larger frustrated molecules are studies of the {M 30 } Keplerates (10) and a recent report of a heptametallic {VO} 7 ring (11), where the studies are limited to low temperature magnetization.…”

“…We have reported studies of a {Cr 8 Ni} ring that we described as a magnetic Möbius strip, because the magnetic studies suggest a spin ground state that could not be described using classical spins (14). Using the strictest definition of frustration (13), this compound cannot be frustrated because the {Cr 8 Ni} ring contains an even number of electrons and has a singly degenerate spin ground state; however, the magnetic behavior cannot be described using a picture of classical spins.…”

“…Using the strictest definition of frustration (13), this compound cannot be frustrated because the {Cr 8 Ni} ring contains an even number of electrons and has a singly degenerate spin ground state; however, the magnetic behavior cannot be described using a picture of classical spins. The molecule has also been described as a valence bond solid (15).…”

Crystal structure of N-hydroxyquinoline-2-carboxamide monohydrate

“…N-Hydroxyquinoline-2-carboxamide, also known as quinoline-2-hydroxamic acid (QuinHA), has been used for the preparation of various metallacrown complexes (Stemmler et al, 1999;Trivedi et al, 2014;Jankolovits et al, 2013). Presently, the Cambridge Structural Database (Groom et al, 2016) contains ten entries on coordination compounds based on N-hydroxyquinoline-2-carboxamide, nine of which have been reported within the past four years.…”

Copper (II)-aminohydroxamate ternary complexes evidenced by mass spectrometry

“…12-MC-4 complexes usually contain Cu(II) in the core of the four-oxygen centre, 13,14 while 15-MC-5 complexes prefer to encapsulate a bigger ion in the five-oxygen cavity (uranyl(II), 15 lanthanide(III) 16 or calcium(II)). The latter are therefore good candidates for applications in the fields of identification and removal of chemical deposits of the cold war or as MRI contrasting agents.…”