Structural Diversity due to Amino Alcohol Ligands Leading to Rare μ₄-Hydroxo-Bridged Tetranuclear and “Bicapped Cubane” Cores in Copper(II) Complexes: A Theoretical and Experimental Magnetostructural Study

Abstract

The μ4-hydroxo- and alkoxo-bridged tetranuclear copper(II) complexes, [Cu4(μ4-OH)(dmae)4][Ag(NO3)4] (1), [Cu4(μ4-OH)(dmae)4][Na(NO3)4] (2), [Cu4(μ4-OH)(dmae)4][K(NO3)4] (3), and hexanuclear alkoxo-bridged "bicapped cubane" copper(II) complex [Cu6(ae)8(ClO4)2](ClO4)2·MeOH (8) (dmae = N,N-dimethylaminoethanolato and ae = 2-aminoethanolato) were synthesized via self-assembly from chelating amino alcohols and copper(II), silver (1), sodium (2), and potassium (3) nitrates or copper(II) perchlorate (8). The complexes are characterized by elemental analyses, single-crystal X-ray diffraction, and variable temperature magnetic measurements. The crystal structures of complexes 1-3 consist of almost planar tetranuclear [Cu4(μ4-OH)(dmae)4](3+) units, in which Cu(II) ions are also weakly bonded to nitrate anions. The adjacent tetranuclear units of 1-3 are connected by ionic interactions between nitrate anions and sodium, potassium, or silver cations resulting in the formation of 1D polymers. The crystal structure of 8 consists of hexanuclear [Cu6(ae)8](4+) "bicapped cubane" units, in which the capping Cu(II) ions are weakly bonded to perchlorate anions. The adjacent hexanuclear units of 8 are connected by hydrogen bonds resulting in the formation of 3D hydrogen bonded networks. Also the results from the synthetic studies leading to the formation of new copper(II) and silver(I) complexes (4-7) using Hae, 3-aminopropanol (Hap), and N,N-dimethylaminopropanol (Hdmap) in similar reactions, which gave tetra- and hexanuclear complexes, are presented. Experimental magnetic studies showed that complexes 1 and 2 exhibit dominant antiferromagnetic coupling leading to an S = 0 ground state, whereas 8 exhibits a large dominant antiferromagnetic interaction between the capping copper atoms and the copper atoms of the top and bottom faces of the cubane unit that define two isosceles triangles. This interaction leads to an S = 1/2 ground state for each triangle. The weak ferromagnetic interaction between these doublet states through the cubane unit leads to a triplet S = 1 ground state for 8. The values of the magnetic exchange coupling constant were the following: J = +1.8 (1) and +2.9 cm(-1) (2) between adjacent copper atoms; J' = -29.2 (1) and -32.2 cm(-1) (2) between opposite copper atoms; J = -297.6 cm(-1) and zJ' = +0.07 cm(-1) (8). Magnetic coupling constants calculated for 2 and 8 by DFT methods are in general of the same nature and magnitude as the experimental ones.

Syntheses, Characterization, and Magneto–Structural Analyses in μ1,3‐Acetato‐Bridged Tetracopper(II) and μ1,3‐ and μ1,1,3‐Acetato‐Bridged Pentanickel(II) Clusters

“…As for the magnetic behavior of complex 3 , the observation of a relatively strong, albeit not quantitatively determined, antiferromagnetic‐type exchange interaction is in good agreement with previously reported data for similar systems 33,34,70–72. A comparable structural motif is present in [Cu 4 (L n ) 2 (O)(O 2 CPh) 4 ]70 and [Cu 4 (O)(L n ) 2 (Ac) 4 ]34 (L n = N 2 O‐donor Schiff base ligand), in which a strong antiferromagnetic exchange coupling of –289(4) cm –1 was measured in the former, and in the range from –210.1 to –271.3 cm –1 in the second.…”

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“…As for the magnetic behavior of complex 3 , the observation of a relatively strong, albeit not quantitatively determined, antiferromagnetic‐type exchange interaction is in good agreement with previously reported data for similar systems 33,34,70–72. A comparable structural motif is present in [Cu 4 (L n ) 2 (O)(O 2 CPh) 4 ]70 and [Cu 4 (O)(L n ) 2 (Ac) 4 ]34 (L n = N 2 O‐donor Schiff base ligand), in which a strong antiferromagnetic exchange coupling of –289(4) cm –1 was measured in the former, and in the range from –210.1 to –271.3 cm –1 in the second.…”

Tetracarboxylate Ligands as New Chelates Supporting Copper(II) Paddlewheel-Like Structures

“…Magnetic studies have been made on two Cu 4 −μ 4 -hydroxo complexes. 42,43 Both show overall antiferromagnetism, but the two systems are significantly different from each other in their χT against T curves. They are not directly comparable to our complex since the magnetic orbitals (d x 2 −y 2 ) lie in the Cu 4 (OH) plane and overlap at the OH bridge.…”