Tetranuclear Lanthanide(III) Complexes in a Seesaw Geometry: Synthesis, Structure, and Magnetism Joydeb Goura, † James P. S. Walsh, ‡ Floriana Tuna,* ,‡ and Vadapalli Chandrasekhar* ,†,§ † Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur-208016, India ‡ School of Chemistry and Photon Science Institute, University of Manchester, Oxford Road, Manchester, M13 9PL, United Kingdom § Tata Institute of Fundamental Research, Centre for Interdisciplinary Sciences, 21 Brundavan Colony, Narsingi, Hyderabad-500075, India, National Institute of Science Education and Research, Institute of Physics Campus, Bhubaneswar-751005, India *S Supporting Information ABSTRACT: The reaction of 2-methoxy-6-(pyridin-2- ylhydrazonomethyl)phenol (LH) with Ln(III) (Ln = Gd, Tb, Dy, Ho) salts in the presence of an excess of triethylamine a ff orded [Gd 4 (L) 4 ( μ 4 -OH)( μ 3 -OH) 2 (NO 3 ) 4 ] · (NO 3 ) · 4CH 3 CN · CH 3 OH · 2H 2 O ( 1 ), [Tb 4 (L) 4 ( μ 4 -OH)( μ 3 - OH) 2 (NO 3 ) 4 ]·(NO 3 )·4CH 3 CN·3H 2 O(2), [Dy 4 (L) 4 (μ 4 -OH)- ( μ 3 -OH) 2 (NO 3 ) 4 ] · (NO 3 ) · 6CH 3 CN · H 2 O ( 3 ), and [Ho 4 (L) 4 ( μ 4 -OH)( μ -OH) 2 (NO 3 ) 4 ] · (NO 3 ) · 8CH 3 CN · 3CH 3 OH·2H 2 O(4). All four complexes contain a mono- cationic tetranuclear core with a unique seesaw topology. The tetranuclear assembly is formed through the coordination of four [L] − , one μ 4 -OH, two μ 3 -OH, and four chelating nitrate ligands, with a charge-balancing nitrate counteranion. Magnetic studies reveal a weak antiferromagnetic coupling throughout the series. Compound 1 can be modeled well with an isotropic exchange between all centers parametrized by J = −0.09 cm −1 . Compound 3 exhibits slow magnetic relaxation at low temperatures. ■ INTRODUCTION Homometallic lanthanide complexes of varying nuclearities and topologies have attracted a lot of interest in recent years, 1 with potential applications in areas as diverse as catalysis, 2 luminescence, 3 imaging, 4 and molecular magnetism. 5 The discovery of single-molecule magnet (SMM) behavior 6 (and the qualitative understanding that a high ground-state spin combined with a large magnetic anisotropy are vital prerequisites 1,7 ) has led to the routine examination of lanthanide complexes in search of this property. 1 As a result, many lanthanide complexes ranging over mono-, 8 di-, 9 tri-, 10 and polynuclear, 11 have become attractive synthetic targets for chemists. A large number of polymetallic Dy(III) systems have been structurally and magnetically characterized, and some of them have been found to exhibit SMM behavior. Although the specific orientations of the anisotropy axes at the individual ions is the principal factor in determining whether or not the metal cluster exhibits SMM behavior, exchange coupling between metals has also been found to affect the relaxation dynamics in some cases. 1 Unfortunately, control of these factors remains a big challenge. Recently, two oxo-bridged polymetallic lantha- nide cages, [Dy 5 O(O i Pr) 13 ] 12 and [Dy 4 K 2 O(O t Bu) 12 ], 13 studied by one of us have been found to possess the highest energy barriers to magnetization reversal. We have recently been involved in the use of polydentate Schiff base chelating ligands for the assembly of heterometallic 3d-4f 14 and homometallic 4f 15 complexes of di fferent nuclearities and core topologies. Among these, a tetranuclear Dy(III) complex with a rhombus-shaped core topology was studied by ac susceptibility and was found to possess two distinct relaxation dynamics correlated to the two crystallo- graphically different Dy(III) ions present in the structure. Spurred by this, we have been examining other polydentate ligands that could potentially allow the assembly of homometallic lanthanide complexes with different topologies. 16 A search of the literature revealed that the hydrazone Schiff base ligand, 2-methoxy-6-(pyridin-2-ylhydrazonomethyl)phenol (LH) has been used for the preparation of 3d 17 and 3d-4f 18 polynuclear metal complexes. The contiguous coordination units present within this ligand, namely a methoxy group, a phenolic unit, an imine, and a pyridyl nitrogen, make this ligand very suitable for the preparation of homometallic tetranuclear lanthanide complexes. Accordingly, herein, we report the synthesis and the structural/magnetic characterization of the first family of tetranuclear Ln(III) complexes assembled from the ligand LH, consisting of [Gd 4 (L) 4 ( μ 4 -OH)( μ 3 - OH) 2 (NO 3 ) 4 ] · (NO 3 ) · 4CH 3 CN · CH 3 OH · 2H 2 O ( 1 ), [Tb 4 (L) 4 (μ 4 -OH)(μ 3 -OH) 2 (NO 3 ) 4 ]·(NO 3 )·4CH 3 CN·3H 2 O (2), [Dy 4 (L) 4 (μ 4 -OH)(μ 3 -OH) 2 (NO 3 ) 4 ]·(NO 3 )· 6CH 3 CN· Received: November 7, 2013 Published: March 10, 2014 Article pubs.acs.org/IC © 2014 American Chemical Society 3385 dx.doi.org/10.1021/ic4027915 | Inorg. Chem. 2014, 53, 3385−3391