Organometallic and Coordination Chemistry with Rare Earth Elements. We are an organometallic research group, with an expertise in synthetic inorganic chemistry, specifically with the f-block elements. We are particularly interested in cyclopentadienyl (Cp), cyclooctatetraenyl (COT), and guanidinate ligands, which have been widely employed in inorganic chemistry. Cp- and COT-based complexes have led to landmark advances across various chemistry disciplines, including catalysis, small molecule activation, and single-molecule magnetism. Through reactivity and structure/property relationship studies, we can design and synthesize complexes to address problems in several exciting areas.
Our investigations have led us to explore bulky COT-derivates, such as dibenzocyclooctatetraene (dbCOT), which exhibits annulated benzo rings to the central COT ring. We use dbCOT to isolate and characterize highly reactive metallocene compexes. We also devised synthetic routes to access divalent-like reactivity of rare earth metals through the isolation of guanidinate-based complexes, featuring a weakly coordinated tetraphenylborate anion. These guanidinate scaffolds are highly tunable, and allow us to finely control the steric bulk and electronic properties of metal complexes by modulation of the amine and imino functional groups. We have also introduced tunable and redox-active multidentate ligands, such as salophen, into the realm of metallocene chemistry to generate heterobimetallic complexes with applications in single-molecule magnetism. Collectively, we synthesize and characterize new lanthanide metallocene complexes, and investigate the electronic structure through serval techniques, such as magnetometry, spectroscopy, and computations.
Heavy p-Block Elements. Bismuth is the most stable heavy p-block element, with a long half-life of 1.9×1019 years and reveals a wide array of oxidations states ranging from –3 to +5. Bismuth is a soft gray metal that forms an oxide layer which varies in thickness causing different wavelengths of light to interfere upon reflection, giving bismuth metal its classic rainbow look. When elemental bismuth is crystallized from molten salt fluxes, it forms intricate crystals showing step dislocations. As bismuth is non-toxic it is often used in medicinal chemistry and can be found in the heartburn medication Pepto-Bismol in the form of bismuth subsalicylate.
Bismuth possesses diffuse valance orbitals and strong spin-orbit coupling stemming from relativistic effects. These characteristics influence the crystal field and magnetic exchange in complexes differently compared to the lighter group 15 elements. The combination of bismuth with rare earth metals poses a new innovative path to SMM design. Here, bismuth allows strong superexchange by penetrating the contracted 4f-orbitals of the lanthanides. In the Demir group, the synthesis and isolation of rare earth (RE) metal bismuth complexes, ranging from the diatomic {Bi2} bridge to polynuclear {Bi6} systems and beyond, are of current interest, as well as the development of synthetic methods to gain access to related RE-heavy main group elements.
Uranium Chemistry. Uranium chemistry is unique with respect to chemical bonding and reactivity of other metals of the periodic table. The chemistry of uranium lies at the interface between transition metal (TM) and lanthanide chemistry. Similar to TMs, uranium exists in a broad range of oxidation states (+1-+6), however the ‘hard’ Lewis acidic metal center, is comparable to that of the lanthanides. Owing to more pronounced relativistic effects, the 5f- and 6d- orbitals are more extended in relation to the closely related lanthanide ions, resulting in an increased level of covalency. These factors result in an intricate electronic structure, where inter-electron repulsion, spin-orbit coupling, and crystal field affects are relatively equal in terms of their influence on orbital population.
Uranium-ligand multiply bonded compounds have been recognized as promising systems for uranium-based homogeneous catalysis and actinide/lanthanide separation by shedding light on the extent of covalency in bonding of 5f-orbitals. Covalency is expected to be more prominent when softer ligands are used such as carbon-based ligands (i.e., divalent carbon compounds, so-called carbenes) rather than N or O ligands. Hence, investigating the electronic and reactivity properties of molecular uranium carbenes can improve fundamental knowledge that is crucial to the development of selective catalysts for bond activation in small molecules and new generation nuclear fuel materials.
Organometallic Chemistry with Neptunium. To understand 5f–orbital participation, and covalency of the actinides, additionally to the uranium chemistry outlined above, new and exciting complexes with neptunium (Np) will be pursued. The chemistry of neptunium is more challenging than that of the lanthanide metals or uranium. Compared to uranium’s rich organometallic chemistry, analogous chemical studies with Np are still in its infancy. Reasons for the rarity of this chemistry are: First, 237Np occurs only in trace amounts in the Earth’s crust. Therefore, the only 237Np available for synthetic purposes is man-made limiting the availability of the metal to researchers globally. Second, 237Np is a radioactive metal (t1/2 = 2.1 x 109 yr), proceeding through the Np decay pathway where the primary decay mode is alpha. Third, there are very few facilities worldwide that have the infrastructure in place to enable organometallic chemistry with this radioactive isotope. We are in the process of constructing a laboratory space that is solely designated for safe handling of 237Np and which will allow us to advance its coordination and organometallic chemistry. The Demir group has acquired a Bruker Apex II single-crystal X-ray diffractometer and NMR spectrometer that is specifically placed to be solely used for this isotope. The lab will have further equipment that is needed to characterize air-sensitive compounds. We are also grateful for Michigan State University’s Department of Environmental Health and Safety to assist us in the construction of this Np laboratory.
Projects
Single Molecule Magnets (SMMs). The concept of magnetism describes the macroscopic attraction of materials featuring unpaired electrons to an external magnetic field. This effect is utilized in commercial magnetic hard drives, where the smallest information storage unit, also referred to as “bit”, comprises multiple tiny domains arising from magnetic coupling among the atomic magnetic moments which results in a collective orientation along a crystallographic axis. These bits are switched from an off state “0” to an on state “1” upon the application of a magnetic field, which microscopically induces a magnetic moment through rotating all magnetic domains along the external field. For permanent magnets, a fraction of these domains remains orientated even after the removal of the external magnetic field. This results in a remnant magnetic moment which can be used to store information, decoded as combinations of multiple bits in 1 and 0 states.
The Demir group aims at transferring this bulk property to the molecular realm by exploiting the enormous magnetic moments innate to lanthanide (Ln) ions. Their high magnetic anisotropy originates from strong spin orbit coupling and large unquenched orbital angular momentum. The Ln ion employed frequently in SMM design is dysprosium(3+) (DyIII), which exhibits not only a large magnetic moment but is also a Kramer’s ion, giving rise to an inherent doubly degenerate ground state (or bistability). The stabilization of the maximum angular momentum state (mJ = ±15/2) through an axial ligand sphere engenders striking SMM properties. For example, sandwiching Dy3+ between two substituted cyclopentadienyl ligands yielded SMM behavior that was observed, beyond the boiling point of liquid nitrogen (77 K).
One alternative route to achieve high temperature SMMs targets at strong magnetic coupling of multiple Ln3+, which would give rise to a larger spin ground state (S) and therefore, higher operating temperatures. In transition metal (TM) complexes, coupling between paramagnetic ions can be promoted via orbital overlap of the valence d-orbitals with the orbitals of a given bridging ligand. In doing so, spin density is transferred from the TM ion onto the ligand framework and can promote communication between multiple metal ions. By contrast, for Ln3+ ions the generation of strong magnetic coupling is challenging as their valence 4f-orbitals are deeply contracted beneath the 5d and 6s orbitals. Hence, with diamagnetic ligands no significant orbital overlap or spin density transfer can occur.
To overcome the Ln-inherent inertness of the spin-active 4f orbitals, the Demir group aims at exploiting bridging ligands with diffuse spin orbitals able to penetrate the deeply buried 4f orbitals. The first class of bridging ligands includes species of heavy group 15 molecules. Strong magnetic coupling between lanthanide ions can be engendered through the diffuse valence orbitals of the heavy main group elements, such as bismuth (Bi). Recently, the Demir group isolated an unprecedented series of heterobimetallic cubane complexes, [Cp*2Ln2Bi6]2– (Ln = Tb, Dy), which exhibit strong ferromagnetic coupling, promoted through the bridging chair-like Bi66– entity. Additionally, the elusive and highly reactive Bi2 dimer has been introduced into the realm of rare earth metallocenes. Both the diamagnetic Bi22– and super reduced Bi23–• radical were isolated as bridging motifs, where the latter gave rise to the first examples of heavy p-block radicals bridging any d- or f-metal ions, [(Cp*2RE)2(μ-η2:η2-Bi2•)]–. Here, the nonzero orbital angular momentum and spin-orbit coupling gives rise to a complicated electronic structure necessitating a in-depth exploration of electronic structure. Both the Tb3+ and Dy3+ complexes are single-molecule magnets, and represent the first SMMs with magnetic exchange mediated by purely p-block radicals beneath the second row. As the first example of radicals of this class, these molecules have significant ramifications for not only single-molecule magnetism, main-group chemistry, and coordination chemistry in general.
The second class of these ligands are aromatic radical ligands. In particular, the Demir group investigates underexplored polydentate aromatic ligands for their suitability to coordinate to Ln3+ ions, and their subsequent redox chemistry to access unprecedented radical bridging ligands. As a surrogate, we deploy the diamagnetic Y3+ ion, which mimics the electrostatic properties of its heavy Ln analogs due to its comparable ionic radius and electronic configuration. The latter facilitates spectroscopic and computational analysis considerably.
Recently, the Demir group has successfully introduced azobispyridine (abpy) into RE chemistry in a unique η3-coordinated radical Y3+ complex Cptet2Y(η3-abpy⦁), and the first bisbenzimidazole (Bbim) radical, isolated as a bridged homodinuclear Y3+ complex, [(Cp*Y)2(μ-Bbim⦁)]–. The combination of the bbim ligand with paramagnetic Ln3+ ions led to the first bisbenzimidazole radical-bridged dilanthanide complexes, where the dysprosium congener exhibits magnetic hysteresis to 5.5 K and a coercive field of 0.54 T.
The Demir group also explores sterically demanding, polydentate ligands to probe their suitability to induce beneficial highly axial or equatorial ligand fields for Dy3+ or Er3+ ions, respectively. As a first venture, we introduced the dibenzo-substituted COT ligand dbCOT to the realm of rare earth (RE) chemistry, where it could be coordinated as a rare η2 ligated dianion in [Cptet2RE(η2-dbCOT)]– (RE = Y, Dy). Current research focuses on a widespread application and analysis of this ligand across the Ln series.
Qubits. Classical bits are the basic functions of a classical computer. Qubits (short for Quantum-bits) are the quantum counterpart and unlike the distinct ‘0’ or ‘1’ state of a classical bit, a qubit functions on a superposition between these two states. This superposition grants qubits the ability to process more data compared to classical bits. Thus, quantum computers are superior over classical computers in performing complex tasks. The lifetime of the superposition, coined as coherence time, is an important measure of qubit functionality. Current research interest in realizing physical qubits is spread across several fields such as superconducting circuits, ion traps, polarized photons and molecular spins. The Demir group concentrates on the exploration of organic and lanthanide electronic spins as potential qubits by studying their qubit behavior in different molecular environments.
Small Molecule Activation Towards Homogeneous Catalysis. Dinitrogen gas is the major component of air (78%) and consists of an extremely strong N≡N triple bond (945 kJ/mol). Being a non–polar molecule, with very low proton and electron affinities, N2 proves to be extremely chemically inert under a wide array of conditions. Due to the inertness inherent to dinitrogen, fixation,activation and conversion to value-added nitrogen compounds proves to be difficult.
Nitrogen fixation is of utmost importance in nature and in industrial processes to form reactive nitrogen compounds such as ammonia, urea, nitrates or nitrites. The two main types of nitrogen fixation are biological nitrogen fixation (BNF) and industrial nitrogen fixation (INF). Biological nitrogen fixation happens through the nitrogenase enzyme, found in prokaryotes, which contains the FeMo cofactor as an active site for N2 chemisorprion to form ammonia (NH3). Industrial nitrogen fixation (INF), done by the Haber–Bosch process, was discovered in 1909 and commercialized in 1913. The latter heterogeneously catalyzed process revolutionized the production of fertilizer and explosives. In comparison to BNF, INF requires high temperature and pressure on an iron-based catalyst. This calls for alternative pathways to activate dinitrogen and produce ammonia under milder reaction conditions.
A promising starting point is to activate dinitrogen with other molecular metal-based systems comprising s–, d–, and f–block metals, respectively. Activation of N2 using rare-earth (RE) metals is particularly exciting and only possible if at least divalent, reactive oxidation state of a RE metal is accessible. Since the latter is challenging, new routes that generate reactive divalent compound in situ by combining a trivalent RE metal species with a reducing agent, such as an alkali metal are introduced successfully. Since lower oxidation states than II are unlikely to exist in solutions for these elements, a controlled activation of N2 is facilitated. The most common reduced N2 species that is formed are complexes that contain the N22– bridging ligand, along with N23–• and N24– species that are hitherto less investigated. While the topic of N2 activation and fixation is especially relevant for environmental and industrial science, the N23–• anion is also particularly exciting for the design of strongly coupled single–molecule magnets (SMMs). Here, strong magnetic exchange coupling arises through the radical nature of the N23−• ion, since it possesses diffuse spin orbitals which can penetrate the deeply buried 4f orbitals of the lanthanides, as seen through the isolation of
The N23–• radical is championing the collection of open-shell ligands for radical–bridged SMMs, however, the ancillary ligands bear importance as well. Rigorous tailoring of the ligand field allows for improved performance in key characteristics of SMM behavior. Enhanced axiality was enforced by two tetramethyl-substituted Cp ligands, Cptet, which afforded a higher blocking temperature and coercive field than the pioneering N23–• radical–bridging compounds with (Me3Si)2N– as an ancillary ligand. The class of dinitrogen radical–bridged SMMs is further advanced in the Demir group through additional adjustments of the ligand field. As seen in the mononuclear SMM with record blocking temperature, constructing an axial ligand field that gives rise to short RE–Cp distances and wide Cp–RE–Cp angles is beneficial.
Divalent Lanthanide Reactivity. Owing to the low ionization energy of the outermost d- and s- electrons, chemistry of the rare earth metals is primarily composed of metal ions where the formal oxidation state is 3+. While the majority of the rare earth metals are most stable in the trivalent oxidation state, the oxidation states of many of the rare earth metals has thus far been expanded. The highly reducing +2 oxidation state has been realized through the implementation of large, sterically hindering ancillary ligands. Where the electronic structure of the divalent lanthanide series has been shown to have a strong dependence on the ancillary crystal field, an attribute not observed with the trivalent counterparts.
The Demir group is actively collaborating with the Odom group at MSU to explore the influence of the ancillary ligand set on the electronic structure of these highly reducing, divalent rare earth metal ions. Metals of this class have also been shown to possess highly unusual reactivity, affording the isolation of compounds which otherwise would be unobtainable. Recently, a room temperature stable bis(amide) Y2+ complex, Y(NHAr*)2 (where Ar* = 2,6-(2,4,6-(iPr)3C6H2)C6H3), was isolated and characterized. The divalent character of the Y2+ ion is reported to exhibit a [Kr]4d1 electron configuration.
MOFs and PAFs. Metal-Organic Frameworks (MOFs), and Porous Aromatic Frameworks (PAFs) are classes of porous materials that feature two- or three- dimensional architectures, respectively. The pores are large enough (~ 2 nm) to accommodate small molecules and thus, such solids have been proposed for a multitude of applications ranging from gas storage, gas separation, catalysis, but also for metal ion uptake that is relevant for purification processes of solutions.
Laying at the interface of organic and inorganic chemistry, MOFs consist of metal nodes which are ligated by organic linkers forming extended coordination polymers. The wide array of both metal ions and organic ligands affords a broad platform which can be meticulously designed to be suitable for multiple applications. The Demir group devises extended conductive and magnetic materials where in particular lanthanides and uranium are employed. A special emphasis is additionally placed on the use of redox-active linkers.
By contrast, PAFs are comprised of carbon nodes linked by aromatic π-systems and feature exceptionally high surface areas. The strong carbon-carbon bonds result in extremely stable materials, capable of withstanding harsh conditions such as high temperatures and acidic media. These materials can be functionalized to insert coordination sites on the linkers that can interact with molecules and metal ions paving the way for gas storage (such as hydrogen and methane), and the purification of metal ion mixtures. We are exploring the potential PAF use for efficient separation of lanthanide ions from one another.