Ferromagnetic coupling and magnetic anisotropy in oxalato-bridged trinuclear chromium(III)-cobalt(II) complexes with aromatic diimine ligands.
Two novel heterotrinuclear chromium(III)-cobalt(II) complexes of formula {[Cr(III)(bpy)(ox)(2)](2)Co(II)(Me(2)bpy)}.2H(2)O (1) and {[Cr(III)(phen)(ox)(2)](2)Co(II)(Me(2)bpy)}.1.5H(2)O (2) [ox = oxalato, bpy = 2,2'-bipyridine, Me(2)bpy = 6,6'-dimethyl-2,2'-bipyridine, and phen = 1,10-phenanthroline] have been synthesized using the "complex-as-ligand/complex-as-metal" strategy. The X-ray crystal structure of 2 consists of neutral oxalato-bridged Cr(III)(2)Co(II) bent entities formed by the coordination of two anionic [Cr(III)(phen)(ox)(2)](-) complexes through one of their oxalato groups toward a cationic cis-[Co(II)(Me(2)bpy)](2+) complex. The three tris(chelated), six-coordinated metal atoms possess alternating propeller chiralities leading thus to a racemic mixture of heterochiral (Lambda,Delta,Lambda)- and (Delta,Lambda,Delta)-Cr(III)Co(II)Cr(III) triads, whereby the two peripheral chromium(III) ions adopt a trigonal distorted trapezoidal bipyramidal geometry and the central high-spin cobalt(II) ion exhibits a compressed rectangular bipyramidal one. The intermolecular pi-pi stacking interactions between the enantiomeric pairs of heterochiral Cr(III)(2)Co(II) entities through the aromatic diimine terminal ligands lead to a unique two-dimensional supramolecular network. Variable temperature (2.0-300 K) magnetic susceptibility and variable-field (0-5.0 T) magnetization measurements for 1 and 2 reveal the presence of weak but non-negligible intermolecular antiferromagnetic interactions [zj = -0.012 (2a) and -0.08 cm(-1) (2b)] between the Cr(III)(2)Co(II) molecules possessing a moderately anisotropic S = 9/2 ground state. This results from the moderately weak intramolecular ferromagnetic coupling [J = +2.43 (1) and +2.34 cm(-1) (2)] between the two peripheral Cr(III) (S(Cr) = 3/2) and the central high-spin Co(II) (S(Co) = 3/2) ions across the oxalato bridge as well as the appreciable single-ion axial magnetic anisotropy of the central high-spin Co(II) (S(Co) = 3/2) ion [D(Co) = -2.29 (1) and -2.15 cm(-1) (2)]. A simple molecular orbital analysis of the exchange interaction in 1 and 2 identifies the sigma- and pi-type pathways involving the d(x(2)-y(2))(Cr)/d(xy)(Co) and d(xz)(Cr)/d(yz)(Co) pairs of orthogonal magnetic orbitals, respectively, as the two main individual contributions responsible for the overall ferromagnetic coupling observed.
Vallejo
Julia
J
Departament de Química Inorgànica/Instituto de Ciencia Molecular (ICMol), Universitat de València, Polígono La Coma s/n, E-46980, Paterna, València, Spain.
Castro
Isabel
I
Cañadillas-Delgado
Laura
L
Ruiz-Pérez
Catalina
C
Ferrando-Soria
Jesús
J
Ruiz-García
Rafael
R
Cano
Joan
J
Lloret
Francesc
F
Julve
Miguel
M
D016880:Anisotropy; D002857:Chromium / Q000737:chemistry*; D003035:Cobalt / Q000737:chemistry*; D018360:Crystallography, X-Ray; D007097:Imines / Q000737:chemistry*; D008024:Ligands; D008280:Magnetics; D008958:Models, Molecular; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D010070:Oxalates / Q000737:chemistry*; D013237:Stereoisomerism
D007097:Imines; D008024:Ligands; D009942:Organometallic Compounds; D010070:Oxalates; D002857:Chromium; D003035:Cobalt