Effects of the Exchange Coupling on Dynamic Properties in a Series of CoGdCo Complexes Jean-Pierre Costes,* ,† Ghenadie Novitchi, ‡ Veacheslav Vieru, § Liviu F. Chibotaru,* ,§ Carine Duhayon, † Laure Vendier, † Jean-Pierre Majoral, † and Wolfgang Wernsdorfer* ,∥,⊥ † Laboratoire de Chimie de Coordination (LCC)-CNRS, Université de Toulouse, CNRS, 31077 Toulouse, France ‡ Laboratoire National des Champs Magné tiques Intenses, UPR CNRS 3228, 25 rue des Martyrs, B.P. 166, 38042 Grenoble cedex 9, France § Theory of Nanomaterials Group, Katolieke Universiteit Leuven, Celestijnenlaan 200F B-3001 Heverlee, Belgium ∥ Institut Né el, UPR CNRS 2940, Université Grenoble-Alpes, B.P. 166, 38042 Grenoble cedex 9, France ⊥ Physikalisches Institut and Institute of Nanotechnology, Karlsruhe Institute of Technology, Wolfgang-Gaede-Str. 1, 76131 Karlsruhe, Germany * S Supporting Information ABSTRACT: Reaction of 2-hydroxy3-methoxybenzaldehyde (o-vanillin) with 1,1,1-tris(aminomethyl)ethane, Me-C- (CH 2 NH 2 ) 3 , or with N,N′,N′′-trimethylphosphorothioic trihydrazide, P(S)[NMe-NH 2 ] 3 , yields two tripodal LH 3 and L 1 H 3 ligands which are able to give cationic heterotrinuclear [LCoGdCoL] + or [L 1 CoGdCoL 1 ] + complexes. The Co II ions are coordinated to these deprotonated ligands in the inner N 3 O 3 site, while the Gd III ion is linked to three deprotonated phenoxo oxygen atoms of two anionic [LCo] − or [L 1 Co] − units. Air oxidation of these trinuclear complexes does not yield complexes associating Co III and Gd III ions. With the first ligand, the structurally characterized resulting complex is the neutral mononuclear LCo III compound, while in the second case, oxidation of the Co II ions turned out to be impossible. The [L 1 CoLnCoL 1 ] + complexes behave as single-molecule magnets with effective energy barriers for the reversal of magnetization varying from U eff = 51.3 K, τ o =2 × 10 −6 s for the yttrium complex to U eff = 29.5, 29.4, 27.4 K and τ o = 1.3 × 10 −7 , 1.47 × 10 −7 , 1.50 × 10 −7 s for the gadolinium ones, depending on the used counteranions. The energy decrease is compensated by the suppression of quantum tunneling of magnetization in absence of applied field, thanks to the introduction of a ferromagnetic Co−Gd interaction. Current work also shows that uncritical use of conventional spin Hamiltonians, based on quenched orbital momenta, can be misleading and that ab initio calculations are indispensable for establishing the picture of real magnetic interaction. Ab initio calculations show that the Co II sites in the investigated compounds possess large unquenched orbital moments due to the first-order spin−orbit coupling resulting in strongly axial magnetic anisotropy. Although the Co II ions are not axial enough for showing slow relaxation of magnetization by themselves, blocking barriers of exchange type are obtained thanks to the exchange interaction with Gd III ions. ■ INTRODUCTION A lot of complexes associating 3d transition-metal ions and 4f lanthanide ions have been first prepared and characterized by structural determinations in order to study their magnetic properties. 1−6 Such complexes are interesting for their magnetic 3d−4f interactions are very often ferromagnetic. It has been recently shown that the 3d-Gd magnetic interactions are dominated by spin polarization effects, an odd number of bridging atoms in between the ions bearing the spins favoring a parallel alignment of the spins, and a ferromagnetic interaction, while an even number of bridging atoms induces an antiparallel alignment and an antiferromagnetic interaction. 7 If the role of the d xy orbital is decisive in switching the exchange from ferro- to antiferromagnetic in complexes associating vanadyl and Gd ions, 8 it seems that the singly occupied σ d x 2 −y 2 orbitals play a preponderant role in a lot of 3d−4f complexes. 9−13 Although several 3d−4f complexes behave as single-molecule magnets, 14 it is very difficult to suppress quantum tunneling of the magnetization in zero external applied field. A way to success consists of adding a metal ion that is exchange coupled to the lanthanide ion. 15,16 Then the small effective field created near the lanthanide ion should be able to suppress quantum tunneling. There is also another problem to circumvent. Putting an anisotropic ion in the vicinity of the first one complicates matters, the easy axis of magnetization of each ion having very often a subtractive effect, more than an additive effect on the resulting magnetization. 17 Furthermore, the magnetic interaction between two Ln ions is weak, but it becomes larger if a Ln ion is replaced by a 3d ion. Keeping in Received: October 15, 2018 Article pubs.acs.org/IC Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX © XXXX American Chemical Society A DOI: 10.1021/acs.inorgchem.8b02921 Inorg. Chem. XXXX, XXX, XXX−XXX Downloaded via UNIV OF SOUTH DAKOTA on December 20, 2018 at 17:01:32 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.