Heptanuclear Heterometallic [Cu₆Ln] Clusters: Trapping Lanthanides into Copper Cages with Artificial Amino Acids

Abstract

Employment of the artificial amino acid 2-amino-isobutyric acid, aibH, in Cu(II) and Cu(II)/Ln(III) chemistry led to the isolation and characterization of 12 new heterometallic heptanuclear [Cu(6)Ln(aib)(6)(OH)(3)(OAc)(3)(NO(3))(3)] complexes consisting of trivalent lanthanide centers within a hexanuclear copper trigonal prism (aibH = 2-amino-butyric acid; Ln = Ce (1), Pr (2), Nd (3), Sm (4), Eu (5), Gd (6), Tb (7), Dy (8), Ho (9), Er (10), Tm (11), and Yb (12)). Direct curent magnetic susceptibility studies have been carried out in the 5-300 K range for all complexes, revealing the different nature of the magnetic interactions between the 3d-4f metallic pairs: dominant antiferromagnetic interactions for the majority of the pairs and dominant ferromagnetic interactions for when the lanthanide center is Gd(III) and Dy(III). Furthermore, alternating current magnetic susceptibility studies reveal the possibility of single-molecule magnetism behavior for complexes 7 and 8. Finally, complexes 2, 5-8, 10, and 12 were analyzed using positive ion electrospray mass spectrometry (ES-MS), establishing the structural integrity of the heterometallic heptanuclear cage structure in acetonitrile.

Synthesis, Crystal Structure, and Magnetic Properties of a Family of Undecanuclear [CuII9LnIII2] Nanoclusters

“…For 2 , the antiferromagnetic and rather small strengths of J 1 and J 2 may be rationalized as follows: 1) The average Cu–O–Cu bond angle is 112.3° for J 1 and 102.9° for J 2 , respectively, and 2) there is only one bridging OH – between the Cu ··· Cu pairs in 2 with one additional Cl – ion for J 1 and two additional bridging ligands (a η 1 :η 1 :μ 2 carboxylate and a Cl – ion) for J 2 , which lowers the strength of J 1 and J 2 . Similar behavior has been observed in the reported compound [Cu 6 Gd],7k in which the Cu–O–Cu average bond angle is 109° and the Cu ··· Cu pair is mediated by a OH – , a η 1 :η 1 :μ 2 OAc – ligand, and a monoatomic NO 3 – bridge, leading to a J value of –3.4 cm –1 . The weak antiferromagnetic interaction is probably caused by the counter‐complementarity effect between the bridging ligands 24.…”

“…In addition, based on the reported experimental and theoretical explorations, the dihedral angles ( α ) between the planes of O–Cu–O and O–Gd–O in the [CuO 2 Gd] units can significantly affect the electron transfer from the singly occupied 3d orbital of Cu 2+ to an empty 5d orbital of Gd 3+ , and the result reveals that the smaller the α value, the stronger the ferromagnetic coupling 25. For 2 , the J 3 value of the Cu ··· Gd interaction (1.8 cm –1 ) is stronger than those of the reported compounds [Cu 8 Gd 4 ]7j and [Cu 6 Gd]7k (0.5 and 1.0 cm –1 , respectively), which can be attributed to the relatively smaller average value of α (3.45°) in 2 compared with those in [Cu 8 Gd 4 ] and [Cu 6 Gd] (13.97 and 9.15°, respectively).…”

Structural Trends in a 3 d‐4 f System: Ln(NO3)3−Cu(2,2‐bipyridine)2(NO3)2 (Ln=La−Nd, Sm−Yb)

“…However, the optical properties of the 4 f ions are modified to a much lesser extent by complexation, the most notable effect being changes in luminescence intensity through energy transfer proceses . The preparation of a whole series of lanthanide complexes under similar conditions is expected to give useful insights into structure‐property trends caused by the gradual change in ionic size, electron count and spin‐orbit coupling along the series …”

“…There have been several reports of Cu(II)‐ Ln (III) heteronuclear complexes ,. Since Ln (III) nitrate hydrates are the usual starting materials for the assembly of such complexes, they generally contain [ Ln (NO 3 ) x (OH 2 ) y ] ( x –3)– which may be linked to transition metal complex cations either through coordination bridges, or via weaker non covalent interactions . Single molecule magnetic behavior is seen for certain 3 d –4 f complexes such as ligand bridged Cu 6 Ln clusters which maintain the cluster integrity even in solution as well as in Cu Ln circular and chain like compounds .…”

“…Since Ln (III) nitrate hydrates are the usual starting materials for the assembly of such complexes, they generally contain [ Ln (NO 3 ) x (OH 2 ) y ] ( x –3)– which may be linked to transition metal complex cations either through coordination bridges, or via weaker non covalent interactions . Single molecule magnetic behavior is seen for certain 3 d –4 f complexes such as ligand bridged Cu 6 Ln clusters which maintain the cluster integrity even in solution as well as in Cu Ln circular and chain like compounds . There are also isolated lanthanide ions such as [Dy(NO 3 ) 5 (H 2 O)] 2–[21] and [Dy/Tb(phthalocyaninato) 2 ] –[24] exhibiting slow relaxation of magnetization at low temperature leading to their designation as single ion magnets.…”