inorganics Review Future Directions for Transuranic Single Molecule Magnets Nicola Magnani and Roberto Caciuffo * ID European Commission, Joint Research Centre (JRC), Directorate for Nuclear Safety and Security, P.O. Box 2340, 76125 Karlsruhe, Germany; [email protected] * Correspondence: [email protected]; Tel.: +49-7247-951-382 Received: 18 January 2018; Accepted: 8 February 2018; Published: 13 February 2018 Abstract: Single Molecule Magnets (SMMs) based on transition metals and rare earths have been the object of considerable attention for the past 25 years. These systems exhibit slow relaxation of the magnetization, arising from a sizeable anisotropy barrier, and magnetic hysteresis of purely molecular origin below a given blocking temperature. Despite initial predictions that SMMs based on 5f-block elements could outperform most others, the results obtained so far have not met expectations. Exploiting the versatile chemistry of actinides and their favorable intrinsic magnetic properties proved, indeed, to be more difficult than assumed. However, the large majority of studies reported so far have been dedicated to uranium molecules, thus leaving the largest part of the 5f-block practically unexplored. Here, we present a short review of the progress achieved up to now and discuss some options for a possible way forward. Keywords: actinides; slow magnetic relaxation; single molecule magnets 1. Introduction Mononuclear or multiple-spin centre complexes displaying large single-ion magnetic anisotropy and, for the latter, strong intramolecular exchange coupling have been intensively studied as potentially useful systems for quantum information processing, magnetic refrigeration, ultrahigh density memory components, or spintronic applications [1]. To be considered a single molecule magnet (SMM), a complex should display a large effective anisotropy barrier against magnetization reversal, Ueff, and −1 a small relaxation rate, t0 , resulting in slow magnetic dynamics and in the opening of a hysteresis cycle below a blocking temperature TB. The aim of extensive research in the past 25 years has been to raise TB and Ueff as much as possible, ideally to synthesize systems with SMM behaviour above room temperature. In order to improve the SMM figures of merit, initial studies focused on increasing the total spin of the molecule Stot using a large number of coupled magnetic centers. Synthetic efforts were therefore concentrated on 3d block elements (mainly Mn, Fe and Cr), which afford strong magnetic exchange thanks to the large radial extension of their unfilled electronic shell [2–6]. This class of compounds suffers however from the small single-ion anisotropy typical of transition metals, whose angular momentum is quenched by the ligand field. Moving to the 4f block allowed one to partially overcome the impasse. Rare earths, with unquenched orbital degrees of freedom, exhibit a much larger single-ion anisotropy than transition metals, resulting in Ueff values above 1800 K [7] and remanent magnetization up to about 60 K [8]. However, the exchange interaction between rare earths is usually smaller than in transition metals, so that the advantage offered by a coupled multi-centre structure is lost. In the above scenario, actinides appeared as promising substitutes for the synthesis of high-TB SMMs. Indeed, elements of the 5f block potentially combine single-ion anisotropy as large as in Inorganics 2018, 6, 26; doi:10.3390/inorganics6010026 www.mdpi.com/journal/inorganics Inorganics 2018, 6, 26 2 of 9 Inorganics 2018, 6, x FOR PEER REVIEW 2 of 8 earthsrare earths with withexchange exchange interactions interactions strong strong enough enough to tocouple couple magnetic magnetic centres centres in in polynuclear complexes.complexes. Moreover, Moreover, contrary contrary to to rare rare earths, earths, actini actinidesdes can can be be stabilized stabilized in in different different oxidation oxidation states states andand areare proneprone to to form form covalent covalent bonds, bonds, thus offeringthus offering an unparalleled an unparalleled chemical chemical versatility versatility in designing in designingthe molecular the molecular structure [structure9–11]. However, [9–11]. However, all actinide-based all actinide-based SMMs reported SMMs reported up to now up performto now performsignificantly significantly worse than worse the than best rare-earththe best rare-earth ones, in termsones, in of terms both relaxationof both relaxation barrier and barrier blocking and blockingtemperature temperature [12]. The [12]. slow The progress slow inprogress the field in is the in partfield due is in to part the relativedue to the smallness relative of smallness the actinide of thescientific actinide community, scientific community, and to the and fact to that the only fact athat handful only a ofhandful laboratories of laboratories are equipped are equipped to handle to handletransuranic transuranic elements, elements, so that so most that of most the reportedof the reported work concernswork concerns uranium uranium complexes complexes only. SMMonly. SMMbehaviour behaviour has actually has actually been observedbeen observed in several in several monometallic monometallic UIII [ 13UIII] and[13] UandV [ 14UV] [14] complexes complexes and andin heterometallic in heterometallic coordination coordination compounds compounds containing containing UV UandV and either either Mn MnIIIIor or DyDyIIIIII cationscations [15–17]. [15–17]. The results obtained on these systems have been the subject of recent reviews [12,18–20]. [12,18–20]. Here, Here, we we will critically summarizesummarize thethe attempts attempts to to obtain obtain SMMs SMMs based based on on transuranic transuranic elements, elements, mainly mainly Np andNp andPu, focusingPu, focusing on the on reasons the reasons for the for limited the limited success su obtainedccess obtained so far andso far trying and totrying indicate to indicate a possible a possibledirection direction for future for research. future research. 2.2. A A Survey Survey of of the the Existing Existing Transuranic Transuranic Single Molecule Magnets InIn thisthis article,article, following following a conventiona convention used used in a largein a large part of part the literature,of the literature, we extend we the extend definition the definitionof SMMs toof includeSMMs to molecular include molecular complexes complexes for which for clear which evidence clear evidence of a low of temperature a low temperature slowing slowingdown of down the magnetization of the magnetization dynamics dynamics exists, even exis thoughts, eventheir though blocking their blocking temperature temperature has not been has notexperimentally been experimentally determined. determined. TheThe three three SMMs SMMs based based on on transuranic transuranic elements elements which which have have been been reported reported in the in theliterature literature so far so are far VI VI V V picturedare pictured in Figure in Figure 1: heterovalent1: heterovalent trimetallic trimetallic neptunyl neptunyl complex complex {(Np {(NpO2)ClO22}{(Np)Cl2}{(NpO2)Cl(thf)O2)Cl(thf)3}2 (thf3 }=2 IV IV tetrahydrofuran),(thf = tetrahydrofuran), hereafter hereafter referred referred to as Np to3 (a) as [21]; Np3 the(a) Np [21];-based the Np sandwich-based complex sandwich Np(COT) complex2 III III (COTNp(COT) = cyclooctatetraenide),2 (COT = cyclooctatetraenide), commonly kn commonlyown as neptunocene known as neptunocene(b) [22]; and the (b) Pu [22]; monometallic and the Pu PuTpmonometallic3 (Tp = trispyrazolylborate) PuTp3 (Tp = trispyrazolylborate) (c) [23]. (c) [23]. FigureFigure 1. Ball-and-stickBall-and-stick structural structural representation representation of of the the three three transuranic transuranic SMMs SMMs reported reported in in the literatureliterature so so far: far: ( (a)) Np 3;;( (bb)) Np(COT) Np(COT)22; ;and and ( (cc)) PuTp PuTp33. .The The color color scheme scheme used used for for labeling labeling atoms atoms is is thethe following: following: light light gray gray = H, = H, pink pink = B, = dark B, dark gray gray = C, =blue C, blue= N, red = N, = redO, green = O, green= Cl, orange = Cl, orange= Np, yellow = Np, =yellow Pu. = Pu. 3 2.1.2.1. Np Np3 Np3 was the first studied example of a polymetallic transuranium complex displaying both Np3 was the first studied example of a polymetallic transuranium complex displaying both slow slowrelaxation relaxation of the of magnetization the magnetization and effective and effectiv superexchangee superexchange interactions interactions between between the actinide the actinide centers. centers. Two of the three Np ions are pentavalent, with O and Cl ligands arranged around them in a distorted pentagonal bipyramidal arrangement; in turn, one neptunyl oxygen from each NpV Inorganics 2018, 6, 26 3 of 9 Inorganics 2018, 6, x FOR PEER REVIEW 3 of 8 Two of the three Np ions are pentavalent, with O and Cl ligands arranged around them in a distorted coordinatespentagonal bipyramidalinto the equatorial arrangement; plane inof turn, the onehexavalent neptunyl Np oxygen center, from whose each NpdistortedV coordinates tetragonal into bipyramidalthe equatorial ligand plane geometry of the hexavalent is completed Np by center, two Cl whose anions distorted [24]. Measurements tetragonal bipyramidal of the dc magnetic ligand susceptibilitygeometry is completedχ as a function by two of temperature Cl anions [24 T]., shown Measurements in Figure 2a, of thedisplay dc magnetic a clear upturn