Structural determination of oleanane-28,13ß-olide and taraxerane-28,14ß-olide fluoro-lactonization products from the reaction of oleanolic acid with SelectfluorTM.

The X-ray crystal structure data of 12-α-fluoro-3ß-hy-droxy-olean-28,13ß-olide methanol hemisolvate, 2C30H47FO3·CH3OH, (1), and 12-α-fluoro-3ß-hy-droxy-taraxer-28,14ß-olide methanol hemisolvate, 2C30H47FO3·CH3OH, (2), are described. The fluoro-lactonization of oleanolic acid using SelectfluorTM yielded a mixture of the six-membered δ-lactone (1) and the unusual seven-membered γ-lactone (2) following a 1,2-shift of methyl C-27 from C-14 to C-13.

Folding a Molecular Strand into a Trefoil Knot of Single Handedness with Co(II)/Co(III) Chaperones.

We report the synthesis of a right-handed (Δ-stereochemistry of strand crossings) trefoil knot from a single molecular strand containing three pyrazine-2,5-dicarboxamide units adjacent to point-chiral centers and six pyridine moieties. The oligomeric ligand strand folds into an overhand (open-trefoil) knot through the assistance of coordinatively dynamic Co(II) "chaperones" that drive the formation of a three-metal-ion circular helicate. The entangled structure is kinetically locked by oxidation to Co(III) and covalently captured by ring-closing olefin metathesis to generate a trefoil knot of single topological handedness. The stereochemistry of the strand crossings in the metal-coordinated overhand knot is governed by the stereochemistry of the point-chiral carbon centers in the ligand strand. The overhand and trefoil knots were characterized by NMR spectroscopy, mass spectrometry, and X-ray crystallography. Removal of the metal ions from the knot, followed by hydrogenation of the alkene, yielded the wholly organic trefoil knot. The metal-free knot and parent ligand were investigated by circular dichroism (CD) spectroscopy. The CD spectra indicate that the topological stereochemistry of the knot has a greater effect on the asymmetry of the chromophore environment than do the point-chiral centers of the strand.

Two- and Three-Spin Hybrid Inorganic-Organic [2]Rotaxanes Containing Metallated Salen Groups.

Mono- and bis-salen functionalised [2]rotaxanes have been synthesised from the esterification of [2]rotaxanes containing phenol-terminated threads (salen = N,N'-bis(salicylidene)ethylenediamine). The [2]rotaxanes have general formula [RH][Cr7NiF8(O2CtBu)16], where [RH]+ is a thread with a central secondary ammonium site that templates a [Cr7NiF8(O2CtBu)16]- ring. The threads are terminated at one or both ends by carboxylic acid functionalised salen groups. The {M(salen)} groups can be free-base [M = (H+)2] or metallated [M = Cu2+, Ni2+, (VO)2+]. The [2]rotaxanes have been characterised by single crystal XRD and solid- and solution-state EPR spectroscopy. Where two paramagnetic M ions are involved [M = Cu2+ and/or (VO)2+] the [2]rotaxanes contain three electron spin S = ½ centres, since the {Cr7Ni} ring has an S = ½ ground state which is well isolated at low temperatures. These three-spin [2]rotaxanes have been characterised in solution by pulsed dipolar EPR spectroscopies (DEER, also known as PELDOR, and RIDME). The M···M and M···{Cr7Ni} interactions measured are consistent with dipolar interactions and also with the distances from single crystal XRD.

Weak Exchange Interactions in Multispin Systems: EPR Studies of Metalloporphyrins Decorated with {Cr7Ni} Rings.

Both metalloporphyrins and heterometallic {Cr7Ni} rings are of significant research interest due to their proposed roles in quantum information processing devices. In this study, we present a series of complexes in which [Cr7NiF3(Etglu)(O2CtBu)15] (N-EtgluH5 = N-ethyl-d-glucamine) heterometallic rings are coordinated to metalloporphyrin linkers: the symmetric [M(TPyP)] for M = Cu2+, VO2+, and H2TPyP = 5,10,15,20-tetra(4-pyridyl)porphyrin; and the asymmetric [{VO}(TrPPyP)] for H2(TrPPyP) = 5,10,15-(triphenyl)-20-(4-pyridyl)porphyrin. The magnetic interactions present in these complexes are unraveled using the continuous wave (CW) electron paramagnetic resonance (EPR) technique. The nature of the coupling between the {Cr7Ni} rings and the central metalloporphyrin is assessed by numerical simulations of CW EPR spectra and determined to be on the order of 0.01 cm-1, larger than the dipolar ones and suitable for individual spin addressability in multiqubit architectures.

Zintl Clusters as a Platform for Lewis Acid Catalysis.

Clusters of the main group elements phosphorus and arsenic, commonly categorized as Zintl clusters, have been known for over a century. And, only now is the application of these systems as catalysts for organic synthesis being investigated. In this work, boranes are tethered via an aliphatic linker to Zintl-based clusters and their Lewis acidity is examined experimentally, by the Gutmann-Beckett test and competency in the hydroborative reduction of six organic substrates, as well as computationally, by fluoride ion affinity and hydride ion affinity methods. The effects of tuning the aliphatic linker length, substituents at the boron, and changing the cluster from a seven-atom phosphorus system to a seven-atom arsenic system on reactivity are studied.

Amino Functionality Enables Aqueous Synthesis of Carboxylic Acid-Based MOFs at Room Temperature by Biomimetic Crystallization.

Enzyme immobilization within metal-organic frameworks (MOFs) is a promising solution to avoid denaturation and thereby utilize the desirable properties of enzymes outside of their native environments. The biomimetic mineralization strategy employs biomacromolecules as nucleation agents to promote the crystallization of MOFs in water at room temperature, thus overcoming pore size limitations presented by traditional postassembly encapsulation. Most biomimetic crystallization studies reported to date have employed zeolitic imidazole frameworks (ZIFs). Herein, we expand the library of MOFs suitable for biomimetic mineralization to include zinc(II) MOFs incorporating functionalized terephthalic acid linkers and study the catalytic performance of the enzyme@MOFs. Amine functionalization of terephthalic acids is shown to accelerate the formation of crystalline MOFs enabling new enzyme@MOFs to be synthesized. The structure and morphology of the enzyme@MOFs were characterized by PXRD, FTIR, and SEM-EDX, and the catalytic potential was evaluated. Increasing the linker length while retaining the amino moiety gave rise to a family of linkers; however, MOFs generated with the 2,2'-aminoterephthalic acid linker displayed the best catalytic performance. Our data also illustrate that the pH of the reaction mixture affects the crystal structure of the MOF and that this structural transformation impacts the catalytic performance of the enzyme@MOF.

N-Heterocyclic Carbene to Actinide d-Based π-bonding Correlates with Observed Metal-Carbene Bond Length Shortening Versus Lanthanide Congeners.

Comparison of bonding and electronic structural features between trivalent lanthanide (Ln) and actinide (An) complexes across homologous series' of molecules can provide insights into subtle and overt periodic trends. Of keen interest and debate is the extent to which the valence f- and d-orbitals of trivalent Ln/An ions engage in covalent interactions with different ligand donor functionalities and, crucially, how bonding differences change as both the Ln and An series are traversed. Synthesis and characterization (SC-XRD, NMR, UV-vis-NIR, and computational modeling) of the homologous lanthanide and actinide N-heterocyclic carbene (NHC) complexes [M(C5Me5)2(X)(IMe4)] {X = I, M = La, Ce, Pr, Nd, U, Np, Pu; X = Cl, M = Nd; X = I/Cl, M = Nd, Am; and IMe4 = [C(NMeCMe)2]} reveals consistently shorter An-C vs Ln-C distances that do not substantially converge upon reaching Am3+/Nd3+ comparison. Specifically, the difference of 0.064(6) Å observed in the La/U pair is comparable to the 0.062(4) Å difference observed in the Nd/Am pair. Computational analyses suggest that the cause of this unusual observation is rooted in the presence of π-bonding with the valence d-orbital manifold in actinide complexes that is not present in the lanthanide congeners. This is in contrast to other documented cases of shorter An-ligand vs Ln-ligand distances, which are often attributed to increased 5f vs 4f radial diffusivity leading to differences in 4f and 5f orbital bonding involvement. Moreover, in these traditional observations, as the 5f series is traversed, the 5f manifold contracts such that by americium structural studies often find no statistically significant Am3+vs Nd3+ metal-ligand bond length differences.

Studying Cation Exchange in {Cr7Co} Pseudorotaxanes: Preparatory Studies for Making Hybrid Molecular Machines.

In the design of dynamic supramolecular systems used in molecular machines, it is important to understand the binding preferences between the macrocycle and stations along the thread. Here, we apply 1H NMR spectroscopy to investigate the relative stabilities of a series of linear alkylammonium templated pseudorotaxanes with the general formula [H2NRR'][Cr7CoF8(O2CCH2 tBu)16] by exchanging the cation in solution. Our results show that the pseudorotaxanes are able to exchange threads via a dissociative mechanism. The position of equilibrium is dependent upon the ammonium cation and solvent used. Short chain primary ammonium cations are shown to be far less favourable macrocycle stations than secondary ammonium cations. Collision-induced dissociation mass spectrometry (CID-MS) has been used to look at disassembly of the pseudorotaxanes in a solvent-free environment and stability trends compared to those in acetone-d6. The energy needed to induce 50 % of the precursor ion loss (E50) is used and shows a similar trend to the equilibria measured by NMR. The relative stabilities of these hybrid inorganic-organic pseudo-rotaxanes are different to those of host-guest compounds involving crown ethers and this may be valuable for the design of molecular machines.

Disorder and Sorption Preferences in a Highly Stable Fluoride-Containing Rare-Earth fcu-Type Metal-Organic Framework.

Rare-earth (RE) metal-organic frameworks (MOFs) synthesized in the presence of fluorine-donating modulators or linkers are an important new subset of functional MOFs. However, the exact nature of the REaXb core of the molecular building block (MBB) of the MOF, where X is a µ2 or 3-bridging group, remains unclear. Investigation of one of the archetypal members of this family with the stable fcu framework topology, Y-fum-fcu-MOF (1), using a combination of experimental techniques, including high-field (20 T) solid-state nuclear magnetic resonance spectroscopy, has determined two sources of framework disorder involving the µ3-X face-capping group of the MBB and the fumarate (fum) linker. The core of the MBB of 1 is shown to contain a mixture of µ3-F- and (OH)- groups with preferential occupation at the crystallographically different face-capping sites that result in different internally lined framework tetrahedral cages. The fum linker is also found to display a disordered arrangement involving bridging- or chelating-bridging bis-bidentate modes over the fum linker positions without influencing the MBB orientation. This linker disorder will, upon activation, result in the creation of Y3+ ions with potentially one or two additional uncoordinated sites possessing differing degrees of Lewis acidity. Crystallographically determined host-guest relationships for simple sorbates demonstrate the favored sorption sites for N2, CO2, and CS2 molecules that reflect the chemical nature of both the framework and the sorbate species with the structural partitioning of the µ3-groups apparent in determining the favored sorption site of CS2. The two types of disorder found within 1 demonstrate the complexity of fluoride-containing RE-MOFs and highlight the possibility to tune this and other frameworks to contain different proportions and segregations of µ3-face-capping groups and degrees of linker disorder for specifically tailored applications.

Halobenzene Adducts of a Dysprosocenium Single-Molecule Magnet.

Dysprosium complexes with strong axial crystal fields are promising candidates for single-molecule magnets (SMMs), which could be used for high-density data storage. Isolated dysprosocenium cations, [Dy(CpR)2]+ (CpR = substituted cyclopentadienyl), have recently shown magnetic hysteresis (a memory effect) above the temperature of liquid nitrogen. Synthetic efforts have focused on reducing strong transverse ligand fields in these systems as they are known to enhance magnetic relaxation by spin-phonon mechanisms. Here we show that equatorial coordination of the halobenzenes PhX (X = F, Cl, Br) and o-C6H4F2 to the cation of a recently reported dysprosocenium complex [Dy(Cpttt)(Cp*)][Al{OC(CF3)3}4] (Cpttt = C5H2tBu3-1,2,4; Cp* = C5Me5) reduces magnetic hysteresis temperatures compared to that of the parent cation. We find that this is due to increased effectiveness of both one- (Orbach) and two-phonon (Raman) relaxation mechanisms, which correlate with the electronegativity and number of interactions with the halide despite κ1-coordination of a single halobenzene having a minimal effect on the metrical parameters of [Dy(Cpttt)(Cp*)(PhX-κ1-X)]+ cations vs the isolated [Dy(Cpttt)(Cp*)]+ cation. We observe unusual divergent behavior of relaxation rates at low temperatures in [Dy(Cpttt)(Cp*)(PhX)][Al{OC(CF3)3}4], which we attribute to a phonon bottleneck effect. We find that, despite the transverse fields introduced by the monohalobenzenes in these cations, the interactions are sufficiently weak that the effective barriers to magnetization reversal remain above 1000 cm-1, being only ca. 100 cm-1 lower than for the parent complex, [Dy(Cpttt)(Cp*)][Al{OC(CF3)3}4].

Synthesis and Magnetic Properties of Bis-Halobenzene Decamethyldysprosocenium Cations.

The decamethyldysprosocenium cation, [Dy(Cp*)2]+ (Cp* = {C5Me5}), was a target single-molecule magnet (SMM) prior to the isolation of larger dysprosocenium cations, which have recently shown magnetic memory effects up to 80 K. However, the relatively short Dy···Cp*centroid distances of [Dy(Cp*)2]+, together with the reduced resonance of its vibrational modes with electronic states compared to larger dysprosocenium cations, could lead to more favorable SMM behavior. Here, we report the synthesis and magnetic properties of a series of solvated adducts containing bis-halobenzene decamethyldysprosocenium cations, namely [Dy(Cp*)2(PhX-κ-X)2][Al{OC(CF3)3}4] (X = F or Cl) and [Dy(Cp*)2(C6H4F2-κ2-F,F)(C6H4F2-κ-F)][Al{OC(CF3)3}4]. These complexes were prepared by the sequential reaction of [Dy(Cp*)2(µ-BH4)]∞ with allylmagnesium chloride and [NEt3H][Al{OC(CF3)3}4], followed by recrystallization from parent halobenzenes. The complexes were characterized by powder and single crystal X-ray diffraction, NMR and ATR-IR spectroscopy, elemental analysis, and SQUID magnetometry; experimental data were rationalized by a combination of density functional theory and ab initio calculations. We find that bis-halobenzene adducts of the [Dy(Cp*)2]+ cation exhibit highly bent Cp*···Dy···Cp* angles; these cations are also susceptible to decomposition by C-X (X = F, Cl, Br) activation and displacement of halobenzenes by O-donor ligands. The effective energy barrier to reversal of magnetization measured for [Dy(Cp*)2(PhF-κ-F)2][Al{OC(CF3)3}4] (930(6) cm-1) sets a new record for SMMs containing {Dy(Cp*)2} fragments, though all SMM parameters are lower than would be predicted for an isolated [Dy(Cp*)2]+ cation, as expected due to transverse ligand fields introduced by halobenzenes and the large deviation of the Cp*···Dy···Cp* angle from linearity promoting magnetic relaxation.

Isolation of a Bent Dysprosium Bis(amide) Single-Molecule Magnet.

The isolation of formally two-coordinate lanthanide (Ln) complexes is synthetically challenging, due to predominantly ionic Ln bonding regimes favoring high coordination numbers. In 2015, it was predicted that a near-linear dysprosium bis(amide) cation [Dy{N(SiiPr3)2}2]+ could provide a single-molecule magnet (SMM) with an energy barrier to magnetic reversal (Ueff) of up to 2600 K, a 3-fold increase of the record Ueff for a Dy SMM at the time; this work showed a potential route to SMMs that can provide high-density data storage at higher temperatures. However, synthetic routes to a Dy complex containing only two monodentate ligands have not previously been realized. Here, we report the synthesis of the target bent dysprosium bis(amide) complex, [Dy{N(SiiPr3)2}2][Al{OC(CF3)3}4] (1-Dy), together with the diamagnetic yttrium analogue. We find Ueff = 950 ± 30 K for 1-Dy, which is much lower than the predicted values for idealized linear two-coordinate Dy(III) cations. Ab initio calculations of the static electronic structure disagree with the experimentally determined height of the Ueff barrier, thus magnetic relaxation is faster than expected based on magnetic anisotropy alone. We propose that this is due to enhanced spin-phonon coupling arising from the flexibility of the Dy coordination sphere, in accord with ligand vibrations being of equal importance to magnetic anisotropy in the design of high-temperature SMMs.