A High-Performance Single-Molecule Magnet Utilizing Dianionic Aminoborolide Ligands.

The first example of a homoleptic f-block borolide sandwich complex is presented and shown to be a high-performance single-molecule magnet (SMM). The bis(borolide) complex [K(2.2.2)][[1-(piperidino)-2,3,4,5-tetraphenylborolyl]2Dy] (1) features an unusual example of an anionic Ln3+ metallocene that supports short metal-ligand bonds and a high degree of linearity around the central Dy3+ ion, resulting in comparatively large barriers to magnetization reversal (Ueff = 1600 cm-1 for the most linear orientation) and, importantly, a high blocking temperature (TB, defined as T(τ100s)) of 66 K. These metrics put complex 1 among the very best performing SMMs reported to date and highlight the potential of dianionic borolide ligands to increase ligand field axiality, compared to monoanionic cyclic ligands, to ultimately maximize magnetic anisotropy in f-block-based SMMs.

Nickel-Borolide Complexes and Their Complex Electronic Structure.

Borolides (BC42-) can be considered as dianionic heterocyclic analogues of monoanionic cyclopentadienides. Although both are formally six-π-electron donors, we herein demonstrate that the electronic structure of their corresponding transition metal complexes differs significantly, leading to altered properties. Specifically, the 18-electron sandwich complex Ni(iPr2NBC4Ph2)2 (1) features an ∼90° angle between the Ni-B-N planes and is best described as a combination of three limiting resonance structures with the major contribution stemming from a formally Ni2+ species bound to two monoanionic radical (BC4•-) ligands. Compound 1 displays two sequential one-electron oxidation events over a small potential range of <0.2 V, which strikingly contrasts the large potential separations between redox partners in the family of metallocenes, and the potential reasons for this unusual observation are discussed.

Efficient Redox-Neutral Photocatalytic Formate to Carbon Monoxide Conversion Enabled by Long-Range Hot Electron Transfer from Mn-Doped Quantum Dots.

Energetic hot electrons generated in Mn-doped quantum dots (QDs) via exciton-to-hot-electron upconversion possess long-range transfer capability. The long-range hot electron transfer allowed for superior efficiency in various photocatalytic reduction reactions compared to conventional QDs, which solely rely on the transfer of band edge electrons. Here we show that the synergistic action of the interfacial hole transfer to the initial reactant and subsequent long-range hot electron transfer to an intermediate species enables highly efficient redox-neutral photocatalytic reactions, thereby extending the benefits of Mn-doped QDs beyond reduction reactions. The photocatalytic conversion of formate (HCOO-) to carbon monoxide (CO), which is an important route to obtain a key component of syngas from an abundant source, is an exemplary redox-neutral reaction that exhibits a drastic enhancement of catalytic efficiency by Mn-doped QDs. Mn-doped QDs increased the formate to CO conversion rate by 2 orders of magnitude compared to conventional QDs with high selectivity. Spectroscopic study of charge transfer processes and the computational study of reaction intermediates revealed the critical role of long-range hot electron transfer to an intermediate species lacking binding affinity to the QD surface for efficient CO production. Specifically, we find that the formate radical (HCOO)•, formed after the initial hole transfer from the QD to HCOO-, undergoes isomerization to the (HOCO)• radical that subsequently is reduced to yield CO and OH-. Long-range hot electron transfer is particularly effective for reducing the nonbinding (HOCO)• radical, resulting in the large enhancement of CO production by overcoming the limitation of interfacial electron transfer.

Unsupported Lanthanide-Transition Metal Bonds: Ionic vs Polar Covalent?

Lanthanide-transition metal complexes continue to be of interest, not only because of their synthetic challenge but also of their promising magnetic properties. Computational work examining the chemical bonding between lanthanides and transition metals in PyCp2Ln-TMCp(CO)2 (DyPyCp22- = [2,6-(CH2C5H3)2C5H3N]2-) reveals strong Ln-TM dative bonds. Gas-phase optimized geometries are in good agreement with experimental structures at the density functional theory (DFT) level with large-core pseudopotentials. From La to Lu, there is a small increase in the bond dissociation energy, as well as a decrease in Ln-Fe bond lengths. Energy decomposition analyses attribute this trend to an increase in the electrostatic contribution from the decreasing bond length and a modest increase in the orbital contribution. The natural bond orbital analysis clearly indicates that 3d6 "lone pairs" in the [FeCp(CO)2]- fragment act as a Lewis bases donating nearly 0.5 electron to Ln virtual orbitals of mainly d character. The interfragment bonding was also quantified by the quantum theory of atoms in molecules, which indicates that the Ln-Fe bond is more covalent than the Ca-Fe bond in the hypothetical CpCa-FeCp(CO)2 but less covalent than the Zn-Fe bond in the hypothetical CpZn-FeCp(CO)2. Further comparisons suggest that to the [PyCp2Ln]+ cation the [FeCp(CO)2]- anion appears much like a halide. Overall, these Ln-TM dative bonds appear to have strong electrostatic contributions as well as significant orbital mixing and dispersion contributions.

Synthesis of Magnetite Nanorods from the Reduction of Iron Oxy-Hydroxide with Hydrazine.

Nanowires and nanorods of magnetite (Fe3O4) are of interest due to their varied biological applications but most importantly for their use as magnetic resonance imaging contrast agents. One-dimensional (1D) structures of magnetite, however, are more challenging to synthesize because the surface energy favors the formation of isotropic structures. Synthetic protocols can be dichotomous, producing either the 1D structure or the magnetite phase but not both. Here, superparamagnetic Fe3O4 nanorods were prepared in solution by the reduction of iron oxy-hydroxide (ß-FeOOH) nanoneedles with hydrazine (N2H4). The amount of hydrazine and the reaction time affected the phase and morphology of the resulting iron oxide nanoparticles. One-dimensional nanostructures of Fe3O4 could be produced consistently from various aspect ratios of ß-FeOOH nanoneedles, although the length of the template was not retained. Fe3O4 nanorods were characterized by transmission electron microscopy, X-ray powder diffraction, X-ray photoelectron spectroscopy, and SQUID magnetometry.

Axial Elongation of Mononuclear Lanthanide Metallocenophanes: Magnetic Properties of Dysprosium- and Terbium-[1]Ruthenocenophane Complexes.

We report the first f-block-ruthenocenophane complexes 1 (Dy) and 2 (Tb) and provide a comparative discussion of their magnetic structure with respect to earlier reported ferrocenophane analogues. While axial elongation of the rare trigonal-prismatic geometry stabilizes the magnetic ground state in the case of Dy3+ and results in a larger barrier to magnetization reversal (U), a decrease in U is observed for the case of Tb3+ .

Kinetic versus thermodynamic metalation enables synthesis of isostructural homo- and heterometallic trinuclear clusters.

Temperature-dependent metalation of the new hexadentate ligand (tris(5-(pyridin-2-yl)-1H-pyrrol-2-yl)methane; H3TPM) enables the selective synthesis of both mononuclear (i.e. Na(THF)4[Fe(TPM)], kinetic product) and trinuclear (i.e. Fe3(TPM)2, thermodynamic product) complexes. Exposure of Na(THF)4[Fe(TPM)] to FeCl2 or ZnCl2 triggers cluster expansion to generate homo- or heterometallic trinuclear complexes, respectively. The developed approach enables systematic variation of ion content in isostructural metal clusters via programmed assembly.

Tuning Second Coordination Sphere Interactions in Polypyridyl-Iron Complexes to Achieve Selective Electrocatalytic Reduction of Carbon Dioxide to Carbon Monoxide.

The development of noble-metal-free catalysts capable of electrochemically converting carbon dioxide (CO2) selectively into value-added compounds remains one of the central challenges in catalysis research. Here, we present a systematic study of Fe(II) complexes of the functionalized ligands bpyRPY2Me (bpyPY2Me = 6-(1,1-bis(pyridin-2-yl)ethyl)-2,2'-bipyridine) in the pursuit of water-stable molecular Fe complexes that are selective for the catalytic formation of CO from CO2. Taking advantage of the inherently high degree of tunability of this ligand manifold, we followed a bioinspired approach by installing protic functional groups of varying acidities (-H, -OH, -OMe, -NHEt, and -NEt2) into the ligand framework to systematically modify the second coordination sphere of the Fe center. This family of [(bpyRPY2Me)FeII] complexes was characterized using single-crystal X-ray analysis, 1H NMR spectroscopy, and mass spectrometry. Comparative catalytic evaluation of this set of compounds via voltammetry and electrolysis experiments identified [(bpyNHEtPY2Me)Fe]2+ in particular as an efficient, iron-based, non-heme CO2 electroreduction catalyst that displays significant selectivity for the conversion of CO2 to CO in acetonitrile solution with 11 M H2O. We propose that the NH group acts as a local proton source for cleaving the C-O bond in CO2 to form CO. Interestingly, the complex with the most acidic functional group in the second coordination sphere, [(bpyOHPY2Me)Fe]2+, favors formation of H2 over CO. Our results correlate the selectivity of water versus carbon dioxide reduction to the acidity of the second coordination sphere functional group and emphasize the continued untapped potential that synthetic molecular chemistry offers in the pursuit of next-generation CO2 reduction electrocatalysts.

Trends in trigonal prismatic Ln-[1]ferrocenophane complexes and discovery of a Ho3+ single-molecule magnet.

Lanthanide metallocenophanes are an intriguing class of organometallic complexes that feature rare six-coordinate trigonal prismatic coordination environments of 4f elements with close intramolecular proximity to transition metal ions. Herein, we present a systematic study of the structural and magnetic properties of the ferrocenophanes, [LnFc3(THF)2Li2]-, of the late trivalent lanthanide ions (Ln = Gd (1), Ho (2), Er (3), Tm (4), Yb (5), Lu (6)). One major structural trend within this class of complexes is the increasing diferrocenyl (Fc2-) average twist angle with decreasing ionic radius (r ion) of the central Ln ion, resulting in the largest average Fc2- twist angles for the Lu3+ compound 6. Such high sensitivity of the twist angle to changes in r ion is unique to the here presented ferrocenophane complexes and likely due to the large trigonal plane separation enforced by the ligand (>3.2 Å). This geometry also allows the non-Kramers ion Ho3+ to exhibit slow magnetic relaxation in the absence of applied dc fields, rendering compound 2 a rare example of a Ho-based single-molecule magnet (SMM) with barriers to magnetization reversal (U) of 110-131 cm-1. In contrast, compounds featuring Ln ions with prolate electron density (3-5) don't show slow magnetization dynamics under the same conditions. The observed trends in magnetic properties of 2-5 are supported by state-of-the-art ab initio calculations. Finally, the magneto-structural relationship of the trigonal prismatic Ho-[1]ferrocenophane motif was further investigated by axial ligand (THF in 2) exchange to yield [HoFc3(THF*)2Li2]- (2-THF*) and [HoFc3(py)2Li2]- (2-py) motifs. We find that larger average Fc2- twist angles (in 2-THF* and 2-py as compared to in 2) result in faster magnetic relaxation times at a given temperature.

Synergistic Effects of Imidazolium-Functionalization on fac-Mn(CO)3 Bipyridine Catalyst Platforms for Electrocatalytic Carbon Dioxide Reduction.

The electrocatalytic reduction of carbon dioxide (CO2) could be a powerful tool for generating chemical fuels and feedstock molecules relevant to the chemical industry. One of the major challenges for molecular catalysts remains the necessity of high overpotentials, which can be overcome by identifying novel routes that improve the energetic reaction trajectory of critical intermediates during catalysis. In this combined experimental and computational study, we show that imidazolium functionalization of molecular fac-Mn(CO)3 bipyridine complexes results in CO2 reduction at mild electrochemical potentials in the presence of H2O. Importantly, our studies suggest that imidazolium groups in the secondary coordination sphere promote the formation of a local hydration shell that facilitates the protonation of CO2 reduction intermediates. As such, we propose a synergistic relationship between the functionalized catalyst and H2O, which stands in contrast to other systems in which the presence of H2O frequently has detrimental effects on catalysis.

Towards understanding of lanthanide-transition metal bonding: investigations of the first Ce-Fe bonded complex.

The syntheses, structural, and magnetic characterization of three new organometallic Ce complexes stabilized by PyCp22- (PyCp22- = [2,6-(CH2C5H3)2C5H3N]2-) are reported. Complex 1 provides the first example of a crystallographically characterized unsupported Ce-Fe bond in a molecular compound. Results from IR spectroscopy and computational analyses suggest weaker Fe → Ce electron-donation than in a previously reported Dy-Fe bonded species.

Magnetic Properties of a Terbium-[1]Ferrocenophane Complex: Analogies between Lanthanide-Ferrocenophane and Lanthanide-Bis-phthalocyanine Complexes.

A rare example of an organometallic terbium single-ion magnet is reported. A Tb3+ -[1]ferrocenophane complex displays a larger barrier to magnetization reversal than its isostructural Dy3+ analogue, which is reminiscent of trends observed for lanthanide-bis-phthalocyanine complexes. Detailed ab initio calculations support the experimental observations and suggest a significantly larger ground-state stabilization for the non-Kramers ion Tb3+ in the Tb complex than for the Kramers-ion Dy3+ in the Dy complex.

Structure and Magnetization Dynamics of Dy-Fe and Dy-Ru Bonded Complexes.

We present an investigation of isostructural complexes that feature unsupported direct bonds between a formally trivalent lanthanide ion (Dy3+ ) and either a first-row (Fe) or a second-row (Ru) transition metal (TM) ion. The sterically rigid, yet not too bulky ligand PyCp22- (PyCp22- =[2,6-(CH2 C5 H3 )2 C5 H3 N]2- ) facilitates the isolation and characterization of PyCp2 Dy-FeCp(CO)2 (1; d(Dy-Fe)=2.884(2) Å) and PyCp2 Dy-RuCp(CO)2 (2; d(Dy-Ru)=2.9508(5) Å). Computational and spectroscopic studies suggest strong TM→Dy bonding interactions. Both complexes exhibit field-induced slow magnetic relaxation with effectively identical energy barriers to magnetization reversal. However, in going from Dy-Fe to Dy-Ru bonding, we observed faster magnetic relaxation at a given temperature and larger direct and Raman coefficients, which could be due to differences in the bonding and/or spin-phonon coupling contributions to magnetic relaxation.

Hard Single-Molecule Magnet Behavior by a Linear Trinuclear Lanthanide-[1]Metallocenophane Complex.

A synthetic protocol was developed that involves the transmetalation of a mono-dysprosium-[1]ferrocenophane complex with DyX3 (X = Cl- or I-) to afford [Dy3Fc6Li2(THF)2]-, featuring a rare linear arrangement of magnetically anisotropic Dy3+ ions. The close spatial inter-lanthanide proximity, in combination with µ2-bridging sp2-hybridized CCp groups, enforces significant magnetic coupling and results in hard single-molecule magnet (SMM) behavior, with an effective barrier to magnetization reversal of up to 268 cm-1. Our results highlight the versatility of lanthanide metallocenophane architectures toward the development of novel multinuclear SMM frameworks.

Electrocatalytic CO2 Reduction by Imidazolium-Functionalized Molecular Catalysts.

We present the first examples of CO2 electro-reduction catalysts that feature charged imidazolium groups in the secondary coordination sphere. The functionalized Lehn-type catalysts display significant differences in their redox properties and improved catalytic activities as compared to the conventional reference catalyst. Our results suggest that the incorporated imidazolium moieties do not solely function as a charged tag but also alter mechanistic aspects of catalysis.

A comparative study of magnetization dynamics in dinuclear dysprosium complexes featuring bridging chloride or trifluoromethanesulfonate ligands.

We utilized a rigid ligand platform PyCp22- (PyCp22- = [2,6-(CH2C5H3)2C5H3N]2-) to isolate dinuclear Dy3+ complexes [(PyCp2)Dy-(µ-O2SOCF3)]2 (1) and [(PyCp2)Dy-(µ-Cl)]2 (3) as well as the mononuclear complex (PyCp2)Dy(OSO2CF3)(thf) (2). Compounds 1 and 2 are the first examples of organometallic Dy3+ complexes featuring triflate binding. The isolation of compounds 1 and 3 allows us to comparatively evaluate the effects of the bridging anions on the magnetization dynamics of the dinuclear systems. Our investigations show that although the exchange coupling interactions differ for 1 and 3, the dynamic magnetic properties are dominated by relaxation via the first excited state Kramers doublet of the individual Dy sites. Compounds 1 and 3 exhibit barriers to magnetization reversal (Ueff = 49 cm-1) that can be favorably compared to those of the previously reported examples of [Cp2Dy(µ-Cl)]2 (Ueff = 26 cm-1) and [Cp2Dy(thf)(µ-Cl)]2 (Ueff = 34 cm-1).

Slow Magnetic Relaxation in a Lanthanide-[1]Metallocenophane Complex.

The first example of a lanthanide metallocenophane complex has been isolated as [Li(THF)4][DyFc3Li2(THF)2] (1). The molecular structure of complex 1 differs dramatically from those of main group and transition metal ferrocenophane complexes and features a distorted trigonal prismatic geometry around the Dy(III) ion and close intramolecular Dy···Fe distances. Furthermore, complex 1 exhibits all characteristics of a soft single-molecule magnet.

Transition metal redox switches for reversible "on/off" and "slow/fast" single-molecule magnet behaviour in dysprosium and erbium bis-diamidoferrocene complexes.

Single-molecule magnets (SMMs) are considered viable candidates for next-generation data storage and quantum computing. Systems featuring switchability of their magnetization dynamics are particularly interesting with respect to accessing more complex logic gates and device architectures. Here we show that transition metal based redox events can be exploited to enable reversible switchability of slow magnetic relaxation of magnetically anisotropic lanthanide ions. Specifically, we report anionic homoleptic bis-diamidoferrocene complexes of Dy3+ (oblate) and Er3+ (prolate) which can be reversibly oxidized by one electron to yield their respective charge neutral redox partners (Dy: [1]- , 1; Er: [2]- , 2). Importantly, compounds 1 and 2 are thermally stable which allowed for detailed studies of their magnetization dynamics. We show that the Dy3+[1]- /1 system can function as an "on"/"off" or a "slow"/"fast" redox switchable SMM system in the absence or presence of applied dc fields, respectively. The Er3+ based [2]- /2 system features "on"/"off" switchability of SMM properties in the presence of applied fields. Results from electrochemical investigations, UV-vis-NIR spectroscopy, and 57Fe Mössbauer spectroscopy indicate the presence of significant electronic communication between the mixed-valent Fe ions in 1 and 2 in both solution and solid state. This comparative evaluation of redox-switchable magnetization dynamics in low coordinate lanthanide complexes may be used as a potential blueprint toward the development of future switchable magnetic materials.

Dinuclear Cobalt Complexes with a Decadentate Ligand Scaffold: Hydrogen Evolution and Oxygen Reduction Catalysis.

A new decadentate dinucleating ligand containing a pyridazine bridging group and pyridylic arms has been synthesized and characterized by analytical and spectroscopic techniques. Four new dinuclear cobalt complexes featuring this ligand have been prepared and thoroughly characterized both in the solid state (X-ray diffraction) and in solution (1D and 2D NMR spectroscopy, ESI-MS, and electrochemical techniques). The flexible but stable coordination environment provided by the ligand scaffold when coordinating Co in different oxidation states is shown to play a crucial role in the performance of the set of complexes when tested as catalysts for the photochemical hydrogen evolution reaction (HER) and chemical oxygen reduction reaction (ORR).

A well-defined terminal vanadium(III) oxo complex.

The ubiquity of vanadium oxo complexes in the V+ and IV+ oxidation states has contributed to a comprehensive understanding of their electronic structure and reactivity. However, despite being predicted to be stable by ligand-field theory, the isolation and characterization of a well-defined terminal mononuclear vanadium(III) oxo complex has remained elusive. We present the synthesis and characterization of a unique terminal mononuclear vanadium(III) oxo species supported by the pentadentate polypyridyl ligand 2,6-bis[1,1-bis(2-pyridyl)ethyl]pyridine (PY5Me2). Exposure of [V(II)(NCCH3)(PY5Me2)](2+) (1) to either dioxygen or selected O-atom-transfer reagents yields [V(IV)(O)(PY5Me2)](2+) (2). The metal-centered one-electron reduction of this vanadium(IV) oxo complex furnishes a stable, diamagnetic [V(III)(O)(PY5Me2)](+) (3) species. The vanadium(III) oxo species is unreactive toward H- and O-atom transfer but readily reacts with protons to form a putative vanadium hydroxo complex. Computational results predict that further one-electron reduction of the vanadium(III) oxo species will result in ligand-based reduction, even though pyridine is generally considered to be a poor π-accepting ligand. These results have implications for future efforts toward low-valent vanadyl chemistry, particularly with regard to the isolation and study of formal vanadium(II) oxo species.