title: Dinuclear Metallacycles with Single M-X-M Bridges (X = Cl-, Br-; M = Fe(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II)): Strong Antiferromagnetic Superexchange Interactions.
abstract: A series of monochloride-bridged, dinuclear metallacycles of the general formula [M2(μ-Cl)(μ-L)2](ClO4)3have been prepared using the third-generation, ditopic bis(pyrazolyl)methane ligands L = m-bis[bis(1-pyrazolyl)methyl]benzene (Lm), M = Cu(II), Zn(II), and L = m-bis[bis(3,5-dimethyl-1-pyrazolyl)methyl]benzene (Lm*), M = Fe(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II). These complexes were synthesized from the direct reactions of M(ClO4)2·6H2O, MCl2, and the ligand, Lmor Lm*, in the appropriate stoichiometric amounts. Three analogous complexes of the formula [M2(μ-Cl)(μ-L)2](BF4)3, L = Lm, M = Cu(II), and L = Lm*, M = Co(II), Cu(II), were prepared from the reaction of [M2(μ-F)(μ-L)2](BF4)3and (CH3)3SiCl. The bromide-bridged complex [Cu2(μ-Br)(μ-Lm*)2](ClO4)3was prepared by the first method. Three acyclic complexes, [Co2(μ-Lm)μ-Cl4], [Co2(μ-Lm*)Cl4], and [Co2(μ-Lm*)Br4], were also prepared. The structures of all [M2(μ-X)(μ-L)2]3+(X = Cl-, Br-) complexes have two ditopic bis(pyrazolyl)methane ligands bridging two metals in a metallacyclic arrangement. The fifth coordination site of the distorted trigonal bipyramidal metal centers is filled by a bridging halide ligand that has an unusual linear or nearly linear M-X-M angle. The NMR spectra of [Zn2(μ-Cl)(μ-Lm*)2](ClO4)3and especially [Cd2(μ-Cl)(μ-Lm*)2](ClO4)3demonstrate that the metallacycle structure is maintained in solution. Solid state magnetic susceptibility data for the copper(II) compounds show very strong antiferromagnetic exchange interactions, with -J values of 536 cm-1for [Cu2(μ-Cl)(μ-Lm)2](ClO4)3·xCH3CN, 720 cm-1for [Cu2(μ-Cl)(μ-Lm*)2](ClO4)3, and 945 cm-1for [Cu2(μ-Br)(μ-Lm*)2](ClO4)3·2CH3CN. Smaller but still substantial antiferromagnetic interactions are observed with other first row transition metals, with -J values of 98 cm-1for [Ni2(μ-Cl)(μ-Lm*)2](ClO4)3, 55 cm-1for [Co2(μ-Cl)(μ-Lm*)2](ClO4)3, and 34 cm-1for [Fe2(μ-Cl)(μ-Lm*)2](ClO4)3. EPR spectra of [Cu2(μ-Cl)(μ-Lm*)2](BF4)3confirm the dz2ground state of copper(II). In addition, the sign of the zero-field splitting parameter D was determined to be positive for [Cu2(μ-F)(μ-Lm*)2](BF4)3. Electronic spectra of the copper(II) complexes as well as Mössbauer spectra of the iron(II) complexes were also studied in relation with the EPR spectra and magnetic properties, respectively. Density functional theory calculations were performed using ORCA, and exchange integral values were obtained that parallel but are slightly higher than the experimental values by about 30%.
authors.affiliation: Department of Chemistry and Biochemistry, University of South Carolina , Columbia, South Carolina 29208, United States.
authors.forename: Daniel L
authors.identifier: http://orcid.org/0000-0002-1525-7066
authors.initials: DL
authors.lastname: Reger
authors.affiliation: Department of Chemistry and Biochemistry, University of South Carolina , Columbia, South Carolina 29208, United States.
authors.forename: Andrea E
authors.initials: AE
authors.lastname: Pascui
authors.affiliation: Department of Chemistry and Biochemistry, University of South Carolina , Columbia, South Carolina 29208, United States.
authors.forename: Elizabeth A
authors.initials: EA
authors.lastname: Foley
authors.affiliation: Department of Chemistry and Biochemistry, University of South Carolina , Columbia, South Carolina 29208, United States.
authors.forename: Mark D
authors.initials: MD
authors.lastname: Smith
authors.affiliation: Faculty of Chemistry, University of Wrocław , 50-383 Wrocław, Poland.
authors.forename: Julia
authors.initials: J
authors.lastname: Jezierska
authors.affiliation: Faculty of Chemistry, Wrocław University of Technology , 50-370 Wrocław, Poland.
authors.forename: Agnieszka
authors.initials: A
authors.lastname: Wojciechowska
authors.affiliation: National High Magnetic Field Laboratory, Florida State University , Tallahassee, Florida 32310, United States.
authors.forename: Sebastian A
authors.initials: SA
authors.lastname: Stoian
authors.affiliation: National High Magnetic Field Laboratory, Florida State University , Tallahassee, Florida 32310, United States.
authors.forename: Andrew
authors.identifier: http://orcid.org/0000-0001-6225-9796
authors.initials: A
authors.lastname: Ozarowski