inorganics Editorial RareEditorial Earth and Actinide Complexes StephenRare M. Earth Mansell and1,* and Stephen Actinide T. Liddle Complexes 2,* 1 Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University, 1, 2, StephenEdinburgh, M. Mansell EH14 4AS,* UK and Stephen T. Liddle * 21 SchoolInstitute of Chemistry, of Chemical The Sciences, University School of Manc of Engineeringhester, Oxford and PhysicalRoad, Manchester, Sciences, Heriot-Watt M13 9PL, UK University, * Correspondences:Edinburgh, EH14 [email protected] 4AS, UK (S.M.M.); [email protected] (S.T.L.); 2 Tel.:School +44-131-451-4299 of Chemistry, (S.M.M.); The University +44-161-275-4612 of Manchester, (S.T.L.) Oxford Road, Manchester, M13 9PL, UK Academic* Correspondences: Editor: Duncan [email protected] H. Gregory (S.M.M.); [email protected] (S.T.L.); Received:Tel.: +44-131-451-4299 4 October 2016; Accepted: (S.M.M.); +44-161-275-461212 October 2016; Published: (S.T.L.) date Academic Editor: Duncan H. Gregory Received: 4 October 2016; Accepted: 12 October 2016; Published: 14 October 2016 The rare earth metals (scandium, yttrium, lanthanum and the subsequent 4f elements) and actinidesThe (actinium rare earth and metals the 5f (scandium, elements) are yttrium, vital components lanthanum andof our the techno subsequentlogy-dominated 4f elements) society. and Examplesactinides (actiniuminclude the and fluorescent-red the 5f elements) europium are vital ions components used in euro ofour banknotes technology-dominated to deter counterfeiting society. [1],Examples the radioactive include theamericium fluorescent-red used in europium smoke detectors ions used [2] inthat euro save banknotes countless to lives deter every counterfeiting year as well [1], asthe neodymium radioactive used americium in the strongest used in smoke permanent detectors magn [2ets] that [3]. save However, countless the livesrare earth every and year actinide as well elementsas neodymium remain used poorly in the recognised strongest permanentby non-scie magnetsntists, and [3]. However,even by many the rare undergraduates earth and actinide in chemistry.elements remain poorly recognised by non-scientists, and even by many undergraduates in chemistry. TheThe similar similar radii of the respectiverespective +3+3 cationscations (Figure (Figur1e )1) belies belies their their individually individually unique unique spectral spectral [ 4] [4]and and magnetic magnetic [5 ][5] properties properties that that contribute contribute to to their their fascinating fascinating chemistry.chemistry. In In this this Special Special Issue, Issue, devoteddevoted to to molecular molecular rare rare earth earth an andd actinide actinide complexes, complexes, work work from from Natrajan Natrajan and andco-workers co-workers [6] has [6] exploredhas explored how howfluorinated fluorinated ligands ligands improve improve the lumine the luminescencescence of 4f ofcomplexes, 4f complexes, while while Baker Baker and andco- workersco-workers [7] [investigated7] investigated the theoptical optical properties, properties, as aswell well as asstructure, structure, of of a anew new class class of of uranyl uranyl selenocyanate.selenocyanate. Pointillart Pointillart and and co-workers’ co-workers’ article article [8] [8 ]bridges bridges the the areas areas of of lanthanide lanthanide optical optical and and magneticmagnetic properties—literally—by properties—literally—by using using bridging bridging tetrathiafulvalene tetrathiafulvalene derivatives. derivatives. The The growing growing field field ofof Single Single Molecule Molecule Magnetism Magnetism originates originates in the in the d-block, d-block, but butrecent recent interest interest in the in f-elements the f-elements has been has growing.been growing. Powell Powell and co-workers and co-workers [9] explore [9] explore the theuse useof dimeric of dimeric dysprosium dysprosium (which (which has has a ahighly highly anisotropicanisotropic f-electron f-electron distribution) distribution) compounds compounds with with a a “hula-hoop” “hula-hoop” geometry, geometry, defined defined by by the the ligand ligand thatthat sitssits inin an an equatorial equatorial plane plane around around both both Dy atoms. Dy atoms. The main The current main medicalcurrent usemedical for the use lanthanides, for the lanthanides,as Magnetic Resonanceas Magnetic Imaging Resonance (MRI) Imaging contrast agents,(MRI) alsocontrast relies agents, on the uniquealso relies electronic on the properties unique electronicof the lanthanides. properties The of the article lanthanides. by Parac-Vogt The arti andcle co-workersby Parac-Vogt [10 ]and demonstrates co-workers the[10] combination demonstrates of thegadolinium combination for MRIof gadolinium imaging (thanksfor MRI to imaging its seven (thanks unpaired to its electrons) seven unpaired connected electrons) to a luminescent connected toBODIPY a luminescent fragment BODIPY in order tofragment explore combinedin order MRIto explore and optical combined imaging, MRI addressing and optical thedrawbacks imaging, addressingof both techniques the drawbacks through of their both complementary techniques through properties. their complementary properties. 3+ FigureFigure 1. 1. TheThe ionic ionic radii of thethe 6-coordinate6-coordinate M M3+ cationscationsof of the the rare rare earth earth and and actinide actinide metals metals (except (except for 4+ forTh Th which which isTh is Th)[4+11) [11–13].–13]. Another growing application of molecular lanthanide complexes is in catalysis, and in this issue, InorganicsKostakis 2016 and, 4, 31; co-workers doi:10.3390/inorganics4040031 [14] report the use of lanthanide coordinationwww.mdpi.com/journ polymers as catalystsal/inorganics in Inorganics 2016, 4, 31; doi:10.3390/inorganics4040031 www.mdpi.com/journal/inorganics Inorganics 2016, 4, 31 2 of 3 a domino reaction. As with the d-block, organometallic lanthanide chemistry has proven to be of vital use in the development of homogeneous catalysis. This issue reflects this growing interest with papers demonstrating the synthesis of organometallic lanthanide complexes using imide (Anwander and co-workers) [15], amidinate (Edelman and co-workers) [16], reduced bipyridine (Mills and co-workers) [17] and metallocene (Ce4+ complexes by Gordon and co-workers [18], the reactivity of Sm2+ by Maron and co-workers [19] and U3+/U4+ bromides by Kiplinger and co-workers [20]) ligand frameworks. A review from Eisen and co-workers [21] is devoted to actinide catalysis and an article from Visseaux and co-workers [22] details the extension of organometallic Nd catalysis into the solid state demonstrating the numerous current applications of these interesting species. The review by Turner [23] highlights N2 and P4 activation chemistry of the f-block, an area of great future catalytic potential. We hope you will enjoy the breadth of chemistry offered in this open access Special Issue that highlights the many differences between complexes of the rare earths and actinides. However, the similarity of ionic radii is inescapable for the +3 oxidation state, which gives rise to one of the most challenging remaining problems for f-block chemists and the potential renaissance of nuclear power (as well as tackling historical problems). The separation of highly radioactive and frustratingly long-lived heavier actinides from shorter-lived radioactive isotopes of the lanthanides would greatly aid planning for the long-term storage of nuclear waste. It is promising that the current resurgence of interest in the fundamental chemistry of the f-block can feed into the goal of discriminating between the actinides and lanthanides based on differences in bonding and reactivity thereby boosting efforts in the area of nuclear waste separation and storage. In fact, the article by Beekmeyer and Kerridge n− n− compares covalency in [CeCl6] and [UCl6] as a means of shedding light on this very problem [24]. We look forward to many more academic and practical advances in all of the above fields of research in the near future. References 1. Rare Earths: Neither Rare, Nor Earths. Available online: http://www.bbc.co.uk/news/magazine-26687605 (accessed on 30 September 2016). 2. Smoke Detectors and Americium. Available online: http://www.world-nuclear.org/information- library/non-power-nuclear-applications/radioisotopes-research/smoke-detectors-and-americium.aspx (accessed on 30 September 2016). 3. Brown, D.N. Fabrication, processing technologies, and new advances for RE-Fe-B magnets. IEEE Trans. Magn. 2016, 52, 1–9. [CrossRef] 4. Bünzli, J.-C.G.; Piguet, C. Taking advantage of luminescent lanthanide ions. Chem. Soc. Rev. 2005, 34, 1048–1077. [CrossRef][PubMed] 5. Sessoli, R.; Powell, A.K. Strategies towards single molecule magnets based on lanthanide ions. Coord. Chem. Rev. 2009, 253, 2328–2341. [CrossRef] 6. Swinburne, A.; Langford Paden, M.; Chan, T.; Randall, S.; Ortu, F.; Kenwright, A.; Natrajan, L. Optical properties of heavily fluorinated lanthanide tris β-diketonate phosphine oxide adducts. Inorganics 2016, 4, 27. [CrossRef] 7. Nuzzo, S.; Browne, M.; Twamley, B.; Lyons, M.; Baker, R. A structural and spectroscopic study of the first uranyl selenocyanate, [Et4N]3[UO2(NCSe)5]. Inorganics 2016, 4, 4. [CrossRef] 8. Pointillart, F.; Speed, S.; Lefeuvre, B.; Riobé, F.; Golhen, S.; Le Guennic, B.; Cador, O.; Maury, O.; Ouahab, L. Magnetic and photo-physical properties of lanthanide dinuclear complexes involving the 4,5-bis(2-pyridyl-N-oxidemethylthio)-40,50-dicarboxylic