title: Designing binuclear transition metal complexes: a new example of the versatility of N,N'-bis(2-aminobenzyl)-4,13-diaza-18-crown-6.
abstract: N,N'-Bis(2-aminobenzyl)-4,13-diaza-18-crown-6 (L) is a versatile receptor able to adapt to the coordinative preferences of different metal cation guests. With first-row transition metal ions, L tends to form binuclear complexes but, depending on the nature of the particular metal ion, the structure of the binuclear complex may be very different. Herein we report a study of the structure and magnetic properties of the corresponding nickel(II) and cobalt(II) complexes. The X-ray crystal structure of the nickel complex (1), with formula [Ni2(L)(CH3CN)4](ClO4)4.CH3CN, shows that this compound presents a symmetric coordination environment with L adopting an anti arrangement. Each Ni(II) ion is six-coordinate in a distorted octahedral environment, and both metal ions are quite far from each other. On the other hand, the X-ray crystal structure of the cobalt complex (2), with formula [Co(L)(mu-OH)Co(CH3CN)](ClO4)3, reveals a rather different structure. Coordination number asymmetry is found: one of the Co(II) is five-coordinate in a distorted trigonal-bipyramidal coordination environment, while the second Co(II) ion is six-coordinate in a distorted octahedral arrangement. Now L adopts a syn arrangement and a hydroxide group acts as a bridge between both cobalt ions. This hydroxo-bridged Co(II) binuclear complex shows structural features that mimic the active site of methionine aminopeptidases. The magnetic properties of 1 and 2 have been investigated in the temperature range 2.0-300 K. Whereas 1 displays a Curie law except for temperatures below 50 K where zero-field splitting of the S= 1 ground state is observed, antiferromagnetic exchange in the singular asymmetric binuclear Co(II) complex 2 has been observed. This magnetic behaviour has been fitted considering first-order spin-orbit coupling in the assumed axially distorted octahedral site and totally quenched orbital contribution in the five-coordinate site in which zero-field splitting of the S= 3/2 ground state is operative.
authors.affiliation: Departamento de Quimica Fundamental, Universidade da Coruña, Campus da Zapateira s/n, 15071 A Coruña, Spain.
authors.forename: Lea
authors.initials: L
authors.lastname: Vaiana
authors.forename: Carlos
authors.initials: C
authors.lastname: Platas-Iglesias
authors.forename: David
authors.initials: D
authors.lastname: Esteban-Gómez
authors.forename: Fernando
authors.initials: F
authors.lastname: Avecilla
authors.forename: Juan Modesto
authors.initials: JM
authors.lastname: Clemente-Juan
authors.forename: José Antonio
authors.initials: JA
authors.lastname: Real
authors.forename: Andrés
authors.initials: A
authors.lastname: de Blas
authors.forename: Teresa
authors.initials: T
authors.lastname: Rodríguez-Blas
mesh_terms: D001372:Aza Compounds / Q000737:chemistry*; D002412:Cations; D003035:Cobalt / Q000737:chemistry*; D043844:Crown Ethers / Q000138:chemical synthesis* / Q000737:chemistry*; D018360:Crystallography, X-Ray; D008280:Magnetics; D008958:Models, Molecular; D009532:Nickel / Q000737:chemistry*; D009942:Organometallic Compounds / Q000737:chemistry*; D013696:Temperature
chemical_list: D001372:Aza Compounds; D002412:Cations; D043844:Crown Ethers; C516882:N,N'-bis(2-aminobenzyl)-4,13-diaza-18-crown-6; D009942:Organometallic Compounds; D003035:Cobalt; C015762:18-crown-6; D009532:Nickel