Modern coordination chemistry

On Tuesday, the 13^th of November 2007, after a sleepless 24-hour trip and twelve-months of service in the Greek army, I reached the 2^nd floor of the Institut für Anorganische Chemie at the University of Karlsruhe (now known as Karlsruhe Institute of Technology, KIT) and while trying to find her office, I heard “You must be George”. Lost in space, I saw Prof. Dr Annie K. Powell, Annie, smiling and waiting to shake hands, thus removing any sense of anxiety, fear and stress. She introduced me to all of the group members and then insisted I go and rest, before meeting her again the following day. This warm welcome was followed by numerous group seminars and outings, scientific discussions, conferences, badminton games and wine tastings, during which Annie always reminded the participants to tackle a scientific problem following a holistic and pure approach.

Professor Sally Brooker and myself are very proud to serve as guest editors for this special issue of Dalton Transactions celebrating Annie's 60^th birthday, with contributions from past and current members of her group, as well as from her colleagues and friends. Annie did her PhD (1985) with Mike Ware at the University of Manchester, where she became familiar with the coordination chemistry of Fe^III complexes formed in aqueous media, aiming to produce bimetallic systems for photo-reduction purposes. Then, Annie worked as a postdoctoral research associate in the group of Heinrich Vahrenkamp at the University of Freiburg, Germany, synthesising carbonyl cluster compounds and becoming familiar with single crystal X-ray structure analysis and the field of the bioinorganic chemistry of Zn^II. In 1988, Annie started her independent career as a Lecturer at the University of Kent, Canterbury, and the following year moved to the University of East Anglia (UEA), Norwich, where she became a Professor in 1998. In 1992, she reported the synthesis and structure of Fe₁₉ in Angewandte Chemie,¹ which attracted the interest of Dante Gatteschi (University of Florence). Magnetic studies on this Fe₁₉ sample revealed the largest spin ground state, at the time, for a molecular species, whilst subsequent studies established its single molecule magnetic (SMM) behaviour. These high impact findings led to funding from several sources: BBSRC (excellent miniature model for loaded ferritin), NERC (model for anaerobic iron reducing bacteria), and SERC/EPSRC/Wellcome Trust (magnetic studies). In 1999, Annie moved to Karlsruhe, Germany, as a Full Professor of Inorganic Chemistry. Over the subsequent two decades she and her team have focussed on developing the concept of “coordination clusters” as building blocks for the production of functional nanostructured materials, supported by numerous EU and DFG grants. Since 2007, Annie has also held a position, and has a group, at the Institute for Nanotechnology on the North Campus of KIT. Her groups have used the coordination cluster approach to produce species containing mixed metal, 3d and/or 4f, ions and also mixed oxidation states. They have extensively studied the magnetic interactions of these mixed metal and mixed valence compounds, expanded their synthetic approaches to permit deposition on surfaces, introduced chirality into the systems, and explored spintronic and relaxivity phenomena. The ferromagnetically coupled Mn₁₉ Aggregate,² the bell-shaped Mn₁₁Gd₂ Single-Molecule Magnet,³ the enhanced slow magnetic relaxation in coupled Dy₃ triangles,⁴ ringing the changes in Fe^III/Yb^III cyclic coordination clusters⁵ and the importance of the transition metal ions in magnetisation relaxation of {M^III₂Dy^III₂} (M = Cr, Mn, Fe, Al) butterfly complexes⁶ are a few key highlights of her work. Her long standing and productive collaboration with Prof. Sally Brooker (Otago University, NZ) has also led to high impact papers, most notably a macrocyclic tetrametallic 3d–4f complex showing the key role of axiality for Ln SMMs,⁷ which has been cited over 175 times.

Annie has received numerous fellowships and distinctions (Julius von Haast Award Fellowship, Wilsmore Fellow, Seaborg Lectureship, to name a few) and has supervised numerous undergraduate and visiting scholars, as well as 52 PhD and 30 postdoctoral researchers, whilst several members of her group have established their own groups in England, Ireland, Canada, India, China and Japan.

Researchers from all over the world have contributed 42 research articles to this special issue. Indeed, it is particularly notable that these contributions come from 13 countries; Germany, UK, Ireland, USA, Canada, Romania, Greece, Spain, Poland, Japan, Thailand, Australia, New Zealand, France and Ethiopia – as this highlights the international impact of Annie's research, as well as how widely she is known, respected and liked. The research aspects of this special issue vary from molecular magnetism to supramolecular, nanoscience, and main group chemistry.

Weber reported two isostructural Fe^III spin crossover complexes in which an abrupt spin transition above 100 K is observed with the transition temperature depending on the size of the anion. (DOI: 10.1039/c9dt00846b).

Serpell and Kostakis defined the heterometallic Zn₂Dy₂ entity that covalently decorates a highly ordered amyloid fibril core and the functionalised assembly exhibits catalytic Lewis acid behaviour. (DOI: 10.1039/c9dt01134j).

Roesky reported a new family of Phosphine-substituted 1,2,3-triazoles as P,C- and P,N-ligands for photoluminescent Au metal complexes (DOI: 10.1039/c9dt01239g).

Zheng subtly varied the coordination geometry in a series of Co^II-sulformamide complexes and studied its influence on the magnetic anisotropy ((DOI: 10.1039/c9dt01296f).

Boskovic reported Tetraoxolene-bridged rare-earth complexes: a radical-bridged dinuclear Dy single-molecule magnet (DOI: 10.1039/c9dt01320b).

Keene investigated the role of ligand basicity in pyridyl complexes derived from a Cu^II-Schiff base coordination polymer (DOI: 10.1039/c9dt01527b).

Marinescu, Clérac and Andruh investigated the magnetic properties of a series of heterometallic 3d–4d coordination polymers assembled from trans-[Ru^III(L)(CN)₂]⁻ tectons and 3d cations (DOI: 10.1039/c9dt01593k).

Brooker reported the assembly of dinuclear helicate and tetranuclear cages using appropriately designed ditopic triazole-azine ligands (DOI: 10.1039/c9dt01890e).

Feldmann by using ionic-liquid based synthesis prepared four different Te–Re carbonyls in the system TeI₄/Re₂(CO)₁₀ with good control due to specific experimental conditions. (DOI: 10.1039/c9dt01897b).

Zafar and Wright reported the synthesis and characterization of new water-soluble Pd^II complexes which may become useful in the future for treating a variety of platinum-resistant cancers (DOI: 10.1039/c9dt01923e).

Fei, Qiao and Chen evaluated the biological and anticancer activity of optically pure chiral dinuclear Cu^I complexes derived from rosin (DOI: 10.1039/c9dt01942a).

Hana and Elias designed the synthesis of mixed metal Ir^III–Co^III dyads and studied their photophysical properties and their application for dihydrogen photo-evolution (DOI: 10.1039/c9dt01989h).

Bochmann reported a family of Cu^I cyclic (alkyl)(amino)carbene complexes with potentially bidentate N^N, N^S and S^S ligands and studied their efficiency as white photoluminescents (DOI: 10.1039/c9dt02036e).

Bartolomé reported the synthesis and characterisation of two new Nd-based complexes, coordinated by furoate ligands, whilst their magnetic studies display field-induced relaxation behaviour. (DOI: 10.1039/c9dt02047k).

Guthausen investigated the use of a high-spin Polyoxometalate-based cluster systems as a contrast agent and studied its NMR relaxivity up to 1.4 GHz/33 T (DOI: 10.1039/c9dt02052g).

Breher introduced a new class of phosphine-functionalised ligands. Due to the presence of various heteroatoms these ligands proved to be very flexible and this notion has been observed for several transition-metal complexes. (DOI: 10.1039/c9dt02057h).

Lewis reported a family of gallium alkyl–xanthato based complexes with low breakdown temperatures, concentrating on the balance of the precursor breakdown characteristics versus the temperature required for the formation of the target phase (DOI: 10.1039/c9dt02061f).

Peng and Li studied the Field-induced slow magnetic relaxation in two-dimensional and three-dimensional Co^II coordination polymers (DOI: 10.1039/c9dt02070e).

Bernhardt reported the synthesis and characterisation of trivalent copper compounds stabilised by acetylacetone dithiocarbazate Schiff base ligands, highlighted their difference in behaviour when compared with bis(thiosemicarbazone) relatives in complexes with copper and studied their spectroscopic and electrochemical properties (DOI: 10.1039/c9dt02071c).

Perry showed how azamacrocycles and tertiary amines can be used to form size tuneable hollow structures or monodisperse oxide nanoparticles depending on the azamacrocycle identity source (DOI: 10.1039/c9dt02080b).

Kumar, Thomas and Ruben attempted to elucidate structure–SCO property relationships that govern SCO in selected mono-, bi-, and multi-nuclear Fe^II-Schiff base complexes. (DOI: 10.1039/c9dt02085c).

Giménez-Marqués and Coronado developed a chemical protocol for the synthesis of robust hybrid [Fe(Htrz)₂(trz)](BF₄)@SiO₂ nanoparticles (NPs) with sizes as small as 28 nm and ultrathin silica shells below 3 nm. (DOI: 10.1039/c9dt02086a).

Schnepf and Unterreiner investigated the influence of the FeCp(CO)₂⁺ moiety on the dynamics of the metalloid [Ge₉(Si(SiMe₃)₃)₃]⁻ cluster in thf and monitored this with time-resolved absorption spectroscopy (DOI: 10.1039/c9dt02091h).

Giordani reported preliminary investigations of the supramolecular assembly of acetylsalicylic acid to a merocyanine–Zn complex which translates into a photo-controllable drug delivery system, thus paving the way for the use of such smart platforms for medicinal chemistry purposes (DOI: 10.1039/c9dt02092f).

Tang provided an up-to-date overview of the emerging field of Single-molecule toroics (SMTs) architectures that have been reported in recent years. (DOI: 10.1039/c9dt02113b).

Schmitt presented a highly augmented, (12,3)-connected porous Zr based metal organic framework containing hydrated coordination sites for the catalytic transformation of gaseous CO₂ to cyclic carbonates (DOI: 10.1039/c9dt02117e).

Schunemann and Wolny presented the vibrational properties and cooperativity of the 3D spin crossover network [Fe(pyrazine)][Pt(CN)₄] (DOI: 10.1039/c9dt02139f).

Dunbar and Pinkowicz reported a family of new 3-D coordination networks based on Mn^II units and [Mo^III(CN)₇]⁴⁻ whilst their magnetic ordering temperatures and coercivities were correlated with the connectivity patterns, metric parameters and the geometry of the heptacyanomolybdate(III) anion. (DOI: 10.1039/c9dt02164g).

Rajaraman and Murrie investigated the magnetic anisotropy in a series of trigonal bipyramidal Mn^II complexes (DOI: 10.1039/c9dt02187f).

Massi investigated the photophysical and biological use of phenol substituted Re tetrazolato complexes and noted that variations of the electron density on the conjugated species bound to the tetrazolato ring have little influence on the ground or excited states of the rhenium centre. (DOI: 10.1039/c9dt02198a).

Labuta and Hill reported the knock-on synthesis of a tritopic calix[4]pyrrole host that represents a promising new family of chromophores for estimation of biologically relevant anions or other species. (DOI: 10.1039/c9dt02365h).

Harding and Harding identified the influence of aromatic anions in the spin crossover behaviour of Fe^III complexes (DOI: 10.1039/c9dt02373a).

Rentschler presented the synthesis, characterisation and magnetic properties of a nonanuclear Ga^III₈Dy^III compound. The compound possesses a unique chemical structure and exhibits field-induced SMM behaviour with an effective energy barrier (U_eff) of 39 K (27.1 cm⁻¹). (DOI: 10.1039/c9dt02432h).

Klopper, Gerhards and Bräse presented a new class of halide bearing Cu^I AlkylPyrPhos complexes, which exhibit a remarkably high solubility, especially in halogenated solvents, thus addressing the key requirement for the production of organic light emitting diodes based on solution processing. (DOI: 10.1039/c9dt02447f).

Ibrahim demonstrated that an unprecedented isopolyanion (W₄O₁₆) can be embedded within the cavity of the archetypal superlacunary POM {P₈W₄₈} by the direct reaction of Mn^II ions with [H₇P₈W₄₈O₁₈₄]³³⁻ in basic medium using a simple, one-pot procedure (DOI: 10.1039/c9dt02478f).

Kühne and Morgan reported two air-stable isostructural Er^III compounds that behave as Single Ion Magnets, whilst theoretical calculations reveal a significant tunnelling in the ground doublet state, which is suppressed by the application of a weak magnetic field, thus opening new routes in the field of Er(III) SIMs. (DOI: 10.1039/c9dt02434d).

Langley, Rajaramanand Murray reported new examples of triangular Tb^III and Ho^III and hexagonal Dy^II single molecule toroics and highlighted that the combination of experimental studies and ab initio calculations is key in determining the toroidal behaviour in molecular complexes. (DOI: 10.1039c9dt02419k).

Bekiari, Psycharis and Perlepes reported the synthesis and characterisation of two tetranuclear thorium Th^IV₄O based compounds bearing a rare core and studied their properties (DOI: 10.1039/c9dt03189h).

Bodenstein and Eichhöfer studied the influence of the type of neutral ligand, L, on the electronic and magnetic properties of trigonal planar Fe^II bis(trimethylsilyl)amido complexes [Fe(N(SiMe₃)₂L] (DOI: 10.1039/c9dt01702j).

Gordon and Crowley reported the synthesis and spectroscopic properties of three ethynyl-ferrocene substituted bipy complexes of Ru^II, identifying that the bipy framework is more resilient to deactivation, compared to the terpy analogue, and concluded that bipy may be useful for the design of further photo- and redox-active materials (DOI: 10.1039/c9dt02025j).

This collection of articles is representative of Annie's wide interests and contributions to the field of chemistry over almost three decades, during which time she has opened up new directions in coordination chemistry. In addition to acknowledging her many scientific achievements, we also want to take this opportunity to thank Annie for always being so kind and generous, and for being such an interested and caring supervisor, colleague and friend.

Finally, we would like to acknowledge Andrew Shore and Mike Andrews, and especially Paige Boxhall, for their efforts and support in producing this special issue.

References

1. S. L. Heath and A. K. Powell, Angew. Chem., Int. Ed. Engl., 1992, 31, 191–193.
2. A. M. Ako, I. J. Hewitt, V. Mereacre, R. Clérac, W. Wernsdorfer, C. E. Anson and A. K. Powell, Angew. Chem., Int. Ed., 2006, 45, 4926–4929.
3. V. M. Mereacre, A. M. Ako, R. Clérac, W. Wernsdorfer, G. Filoti, J. Bartolomé, C. E. Anson and A. K. Powell, J. Am. Chem. Soc., 2007, 129, 9248–9249.
4. I. J. Hewitt, J. Tang, N. T. Madhu, C. E. Anson, Y. Lan, J. Luzon, M. Etienne, R. Sessoli and A. K. Powell, Angew. Chem., Int. Ed., 2010, 49, 6352–6356.
5. A. Baniodeh, C. E. Anson and A. K. Powell, Chem. Sci., 2013, 4, 4354–4361.
6. Y. Peng, M. K. Singh, V. Mereacre, C. E. Anson, G. Rajaraman and A. K. Powell, Chem. Sci., 2019, 10, 5528–5538.
7. H. L. C. Feltham, Y. Lan, F. Klöwer, L. Ungur, L. F. Chibotaru, A. K. Powell and S. Brooker, Chem. – Eur. J., 2011, 17, 4362–4365.

---