The search for 3d–4f single-molecule magnets: synthesis, structure and magnetic properties of a [Mn III 2 Dy III 2 ] cluster Abhudaya Mishra, a Wolfgang Wernsdorfer, b Simon Parsons, c George Christou a and Euan K. Brechin* c Received (in Cambridge, UK) 1st February 2005, Accepted 1st March 2005 First published as an Advance Article on the web 15th March 2005 DOI: 10.1039/b501508a Tetranuclear [Mn III 2 Ln III 2 ] complexes formed using tripodal ligands display frequency dependent out-of-phase ac suscept- ibility signals and temperature and sweep rate dependent hysteresis loops. The synthesis and characterization of polynuclear clusters of paramagnetic metal ions have attracted intense study since the discovery that such molecules can display the phenomenon of single-molecule magnetism (SMM). 1 In these molecules there exists an energy barrier to the relaxation of the magnetization due to the combination of a large ground state spin multiplicity and a significant negative zero-field splitting of that ground state. This imparts a molecular magnetic memory effect that can be observed as temperature and sweep rate dependent hysteresis loops in single crystal M vs. H studies. 2 There are now several species displaying such behaviour—the majority of which are transition metal clusters containing Mn ions, since Mn clusters often display large spin ground states and large and negative magneto-anisotropies associated with the presence of Jahn–Teller distorted Mn III ions. In contrast there are only three reports of 3d–4f complexes behaving as SMMs: the tetranuclear complex [Cu II LTb III (hfac) 2 ] 2 (where H 3 L 5 1-(2-hydroxybenzamido)-2-(2-hydroxy-3-methoxy- benzylideneamino)-ethane); the dodecanuclear complex [Mn IV 2 - Mn III 4 Dy III 6 (H 2 shi) 4 (Hshi) 2 (shi) 10 (CH 3 OH) 10 (H 2 O) 2 ] (where H 3 shi 5 salicylhydroxamic acid); and the pentadecanuclear compound [Mn 11 Dy 4 O 8 (OH) 6 (OMe) 2 (O 2 CPh) 16 (NO 3 ) 5 (H 2 O) 3 ]. 3–5 The first two complexes display frequency dependent out-of- phase ac susceptibility signals at low temperature, but these in themselves are not definitive evidence of SMM behaviour. Only the [Mn 11 Dy 4 ] complex has been shown to display temperature and sweep rate dependent hysteresis loops in low temperature magnetization studies. These loops are unambiguous evidence of SMM behaviour. The advantages of using lanthanide ions in the synthesis of new SMMs is that they will provide both large spin and considerable single ion anisotropy. It therefore seemed logical to extend our previous work on the synthesis of Mn III SMMs using tripodal organic bridging ligands 6 to the synthesis of new heterometallic Mn III –Ln III clusters. Herein we describe the magnetic properties of [Mn III 2 Ln III 2 ] complexes that not only show frequency dependent out-of-phase ac susceptibility signals but also temperature and sweep rate dependent hysteresis loops. These loops also display the step-like features associated with quantum tunnelling of the magnetization. Combination of the trimetallic species [Mn 3 O(O 2 CR) 6 (py) 3 ] (R 5 CH 3 , CMe 3 , Ph) with H 3 tripod (H 3 tmp, H 3 thme, H 4 peol, Fig. 1), Ln(NO 3 ) 3 ?xH 2 O, (Ln 5 Gd, Dy, Tb) and NMe 4 OH in MeCN produces the tetranuclear species [NMe 4 ] 2 [Mn III 2 Ln III 2 (tripod) 2 (O 2 CR) 4 (NO 3 ) 4 ] in good yield after 2 days. In this communication we will limit our discussion to the complex [NMe 4 ] 2 [Mn III 2 Dy III 2 (tmp) 2 (O 2 CMe 3 ) 4 (NO 3 ) 4 ]?2MeCN? 0.5H 2 O, 1?2MeCN?0.5H 2 O. Complex 1{{ (Fig. 1) crystallizes in the monoclinic space group P2 1 /c. The central metallic core has a planar diamond-like topology with the Mn III ions in the centre and the Dy III ions in *ebrechin@staffmail.ed.ac.uk Fig. 1 The structure of the anion of complex 1 viewed from above the [Mn 2 Dy 2 ] plane (top), from the side of the [Mn 2 Dy 2 ] plane (middle) and the structures of H 3 thme, H 3 tmp and H 4 peol, respectively (bottom). COMMUNICATION www.rsc.org/chemcomm | ChemComm 2086 | Chem. Commun., 2005, 2086–2088 This journal is ß The Royal Society of Chemistry 2005