Tetranuclear Hetero-Metal [Co II 2 Ln III 2 ] (Ln = Gd, Tb, Dy, Ho, La) Complexes Involving Carboxylato Bridges in a Rare μ 4 -η 2 :η 2 Mode: Synthesis, Crystal Structures, and Magnetic Properties Sk Md Towsif Abtab, † Mithun Chandra Majee, † Manoranjan Maity, † Ja ́ n Titis ̌ , ‡ Roman Boc ̌ a,* ,‡ and Muktimoy Chaudhury* ,† † Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India ‡ Department of Chemistry, FPV, University of SS Cyril and Methodius, 917 01 Trnava, Slovakia * S Supporting Information ABSTRACT: A new family of 3d-4f heterometal 2 × 2 complexes [Co II 2 (L) 2 (PhCOO) 2 Ln III 2 (hfac) 4 ](1-5) (Ln = Gd (compound 1), Tb (compound 2), Dy (compound 3), Ho (compound 4), and La (compound 5)) have been synthesized in moderate yields (48-63%) following a single-pot protocol using stoichiometric amounts (1:1 mol ratio) of [Co II (H 2 L)- (PhCOO) 2 ] (H 2 L= N,N′-dimethyl-N,N′-bis(2-hydroxy-3,5- dimethylbenzyl)ethylenediamine) as a metalloligand and [Ln III (hfac) 3 (H 2 O) 2 ] (Hhfac = hexafluoroacetylacetone) as a lanthanide precursor compound. Also reported with this series is the Zn-Dy analog [Zn II 2 (L) 2 (PhCOO) 2 Dy III 2 (hfac) 4 ] 6 to help us in understanding the magnetic properties of these compounds. The compounds 1-6 are isostructural. Both hexafluoroacetylacetonate and benzoate play crucial roles in these structures as coligands in generating a tetranuclear core of high thermodynamic stability through a self-assembly process. The metal centers are arranged alternately at the four corners of this rhombic core, and the carboxylato oxygen atoms of each benzoate moiety bind all of the four metal centers of this core in a rare μ 4 -η 2 :η 2 bridging mode as confirmed by X-ray crystallography. The magnetic susceptibility and magnetization data confirm a paramagnetic behavior, and no remnant magnetization exists in any of these compounds at vanishing magnetic field. The metal centers are coupled in an antiferromagnetic manner in these compounds. The [Co II 2 Dy III 2 ] compound exhibits a slow magnetic relaxation below 6 K, as proven by the AC susceptibility measurements; the activation energy reads U/k B = 8.8 K (τ 0 = 2.0 × 10 -7 s) at B DC = 0, and U/k B = 7.8 K (τ 0 = 3.9 × 10 -7 s) at B DC = 0.1 T. The [Zn II 2 Dy III 2 ] compound also behaves as a single- molecule magnet with U/k B = 47.9 K and τ 0 = 2.75 × 10 -7 s. ■ INTRODUCTION The study of heterometal complexes containing 3d-4f metal ion combinations is an active area of research in contemporary coordination chemistry. 1 The initial interest in this field originated from a classic paper reported by Gatteschi et al. 2 on the ferromagnetic interaction between copper(II) and isotropic gadolinium(III) ion within a discrete trinuclear complex. The discovery of lanthanide-based single-molecule magnets (SMMs) is an important step forward 3 that includes 3d-4f compounds of certain lanthanide ions (Tb 3+ , Dy 3+ , Ho 3+ , etc.) with large spin ground states (S T ) and exhibit strong magnetic anisotropy. Some of the resulting compounds find possible applications in the areas of information storage, molecular spintronics, quantum computation, magnetically addressable liquid crystals, magnetic alloys for refrigeration, etc. 4 Interestingly, only a handful of such complexes with interesting magnetic properties have been reported thus far because of the inherent difficulties encountered during their preparation, mainly due to product scrambling. Most of these reported complexes are based on copper(II), 5 manganese(II), 6 and iron(III) 7 ions which can bring in, along with the lanthanide ions, larger ground state and stronger anisotropy in the resulting compounds. Surprisingly, however, there are very few examples of 3d-4f compounds involving cobalt(II) ion 8 in spite of their Ising-type magnetic anisotropy that supports SMM behavior. We have recently reported 9a the syntheses and structural characterization of carboxylato complexes [M(H 2 L)- (PhCOO) 2 ] (M = Ni II and Co II ) involving a tetradentate phenol-based pro-ligand H 2 L. The nickel analogue of this pair has been used as a metalloligand for the successful synthesis of a family of tetra- and dinuclear nickel(II)-vanadium(IV/V) heterometal complexes. 9b Herein, we report the synthesis of a new series of tetranuclear 2 × 2 heterometal complexes [Co II 2 (L) 2 (PhCOO) 2 Ln III 2 (hfac) 4 ](1-5) of cobalt(II) and lanthanide(III) ions (Ln = Gd, Tb, Dy, Ho, La), using Received: June 12, 2013 Published: January 17, 2014 Article pubs.acs.org/IC © 2014 American Chemical Society 1295 dx.doi.org/10.1021/ic401484d | Inorg. Chem. 2014, 53, 1295-1306