Dalton Transactions PAPER Cite this: Dalton Trans., 2014, 43, 6752 Received 16th January 2014, Accepted 14th February 2014 DOI: 10.1039/c4dt00168k www.rsc.org/dalton Effect of the capping ligand on luminescent erbium(III) β-diketonate single-ion magnets† M. Ramos Silva,* a P. Martín-Ramos, a,b J. T. Coutinho, c L. C. J. Pereira c and J. Martín-Gil d Erbium complexes featuring β-diketonate ligand 2,4-hexanedione (Hh) and N,N-donor-ligands 2,2’-bi- pyridine (bipy), 5-nitro-1,10-phenanthroline (5NO 2 phen) and bathophenanthroline (bath) have been syn- thesized. The structures of the ternary complexes [Er(h) 3 (bipy)], [Er(h) 3 (5NO 2 phen)] and [Er(h) 3 (bath)] have been determined by single crystal X-ray diffraction. Excitation of the complexes in the ultraviolet region (337 nm) led to near infrared (NIR) luminescence at 1532 nm characteristic of the trivalent erbium ion in the three compounds, with an improved antenna effect in the 5-nitro-1,10-phenanthroline complex. The AC susceptibilitystudies conducted at frequencies ranging from 33 to 9995 Hz and at temperatures in the 1.7 to 10 K range revealed that the application of a static magnetic field induces a slow magnetic relaxation in all three compounds. The complex with the bulkier capping ligand (bathophenanthroline) exhibits the highest energy barrier U/k B = 23 K. Introduction The discovery of single molecule magnets (SMMs), 1 molecules that display slow magnetization relaxation below a character- istic blocking temperature, has triggered in the last twenty years intensive research on transition metal clusters. The SMM behaviour results from a large ground spin state combined with a large and negative easy-axis type magnetoanisotropy. Ground spin states as large as S = 83/2 were observed in a cluster of 19 manganese ions. 2 However, although the metallic centres in the cluster show a high degree of Jahn–Teller distor- tion, their arrangement and their ferromagnetic interactions create a system with very low anisotropy. Pursuing anisotropy, scientists turned their attention to clusters that included lanthanide ions. f-elements have a large unquenched orbital moment and a strong spin–orbit coupling; the interaction of the ground J state with the crystal field gen- erates the magnetic anisotropy barrier separating opposite orientations of the magnetic moment ground state. However, lanthanide ions usually show weak magnetic exchange coup- lings. The first approach was to make heteroclusters contain- ing both f and d-elements. 3 Nevertheless, since the discovery that terbium(III) bis(phthalocyaninato), a single lanthanide ion, shows slow magnetic relaxation in 2003, 4 lanthanide single-ion magnets (SIMs) have become a hot topic in mole- cular magnetism. In this approach, anisotropic barriers of several hundred wavelengths have been found. 5 Most studies to date have been published on Dy 3+ complexes. 5,6 This is not fortuitous: Dy 3+ has an odd number of electrons; it is a Kramer ion, with a ground state doubly degenerate. Dy 3+ has a J = 15/2, the highest for lanthanides, so the lowest sub-state can have m J = ±15/2. Another requirement for a strong single-ion anisotropy is a large separation between the ground ±m J state and the first excited ±m J state. This can be achieved by manipulating the crystal field (that is, by manipulating the ligands). For an oblate ion like Dy 3+ , that is, for an ion with an equatorially expanded quadrupole moment of the f-electron charge cloud, the sandwich type ligand geometry is the most favourable. 7 For different ligands like β-diketonate and N,N donors, a few studies have been reported. 8 In two of them, the effect of changing the capping ligand is inspected. The first one 9 reports the increase in the anisotropy barrier by changing † Electronic supplementary information (ESI) available: Differential scanning calorimetry (DSC), FTIR, Raman, additional PL emission intensities compari- sons, lifetime measurements, DC and AC magnetic data are provided. Atomic coordinates, thermal parameters and bond lengths and angles have been de- posited at the Cambridge Crystallographic Data Centre (CCDC). Any request to the CCDC for this material should quote the full literature citation. CCDC 973455–973457. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt00168k a CEMDRX, Physics Department, Universidade de Coimbra, Rua Larga, P-3004-516 Coimbra, Portugal. E-mail: manuela@pollux.fis.uc.pt; http://pollux.fis.uc.pt/; Fax: +351 239 829158; Tel: +351 239 410648 b Signal Theory and Communications Department, Higher Technical School of Telecommunications Engineering, Universidad de Valladolid, Campus Miguel Delibes, Paseo Belén 15, 47011 Valladolid, Spain c Solid State Group, UCQR, IST/CTN, Instituto Superior Técnico, UL, Estrada Nacional 10, km 139.7, 2695-066 Bobadela LRS, Portugal d Advanced Materials Laboratory, ETSIIAA, Universidad de Valladolid, Avenida de Madrid 44, 34004 Palencia, Spain 6752 | Dalton Trans. , 2014, 43, 6752–6761 This journal is © The Royal Society of Chemistry 2014