Variable-temperature NMR Research Articles

Spin-Crossover in Iron(II) Complexes of N,N′-Disubstituted 2,6-Bis(Pyrazol-3-yl)Pyridines: An Effect of a Distal Substituent in the 2,6-Dibromophenyl Group

A series of new bis(pyrazol-3-yl)pyridines (LR) N,N′-disubstituted by 4-functionalized 2,6-dibromophenyl groups have been synthesized to study the effect of a distal substituent on the spin-crossover (SCO) behaviour of the iron(II) complexes [Fe(LR)2](ClO4)2 by variable-temperature magnetometry, NMR spectroscopy, and X-ray diffraction. The SCO-assisting tendency of the substituents with different electronic and steric properties (i.e., the bromine atom and the methyl group) in the para-position of the 2,6-dibromophenyl group is discussed. Together with earlier reported SCO-active iron(II) complexes with N,N′-disubstituted bis(pyrazol-3-yl)pyridines, these new complexes open the way for this family of SCO compounds to emerge as an effective ‘tool’ in revealing structure–function relations, a prerequisite for successful molecular design of switchable materials for future breakthrough applications in sensing, switching, and memory devices.

Rhodium and Iridium Complexes Bearing “Capping Arene” Ligands: Synthesis and Characterization

A series of olefin-coordinated RhI and IrI complexes bearing “capping arene” ligands (5-XFP and 6-XFP, see below) of the general formulas (FP)M(olefin)X, [(FP)M(olefin)2][M(olefin)2X2], and [(FP)M(olefin)2]BF4 (FP = “capping arene” ligands, X = halide or pseudohalide, olefin = ethylene, cyclooctene, (olefin)2 = (C2H4)2 or cyclooctadiene) were synthesized and characterized. Single-crystal X-ray diffraction studies revealed structural differences that are a function of the identity of the capping arene ligand and the metal. For 5-XFP ligands (5-XFP = 1,2-bis(N-7-azaindolyl)-benzene and derivatives with substituents on the arene moiety), the coordination to both Rh and Ir gives rise to complexes that are best described as 16-electron and square planar. For 6-XFP ligands (6-XFP = 8,8′-(1,2-phenylene)diquinoline and derivatives with substituents on the arene moiety), the structures of Rh and Ir complexes are better considered as 18-electron and trigonal bipyramidal due to an η2-C,C interaction between the metal center and the arene group of the capping arene ligand. Variable-temperature 1H NMR spectroscopy studies of ethylene rotation demonstrated that the Ir complexes possess higher activation barriers to rotation in comparison to Rh complexes and the 6-XFP complexes tend to give ethylene higher rotational barriers in comparison to 5-XFP complexes for complexes of the type (FP)Rh(η2-C2H4)Cl. DFT calculations are consistent with enhanced Rh to ethylene π-back-donation for Rh complexes ligated by the 6-XFP ligands in comparison to the 5-XFP ligands.

Influence of Configuration at the 4- and 6-Positions on the Conformation and Anomeric Reactivity and Selectivity of 7-Deoxyheptopyranosyl Donors: Discovery of a Highly Equatorially Selective l-glycero-d-gluco-Heptopyranosyl Donor.

The preparation of four per-O-benzyl-d- or l-glycero-d-galacto and d- or l-glycero-d-gluco heptopyranosyl sulfoxides and the influence of their side-chain conformations on reactivity and stereoselectivity in glycosylation reactions are described. The side-chain conformation in these donors is determined by the relative configuration of its point of attachment to the pyranoside ring and the two flanking centers in agreement with a recent model. In the d- and l-glycero-d-galacto glycosyl donors, the d-glycero-d-galacto isomer with the more electron-withdrawing trans,gauche conformation of its side chain was the more equatorially selective isomer. In the d- and l-glycero-d-gluco glycosyl donors, the l-glycero-d-gluco isomer with the least disarming gauche,gauche side-chain conformation was the most equatorially selective donor. Variable temperature NMR studies, while supporting the formation of intermediate glycosyl triflates at -80 °C in all cases, were inconclusive owing to a change in the decomposition mechanism with the change in configuration. It is suggested that the equatorial selectivity of the l-glycero-d-gluco isomer arises from H-bonding between the glycosyl acceptor and O6 of the donor, which is poised to deliver the acceptor antiperiplanar to the glycosyl triflate, resulting in a high degree of SN2 character in the displacement reaction.

Structurally Strained Cyclic Xanthene Dimers: A Model for the Rigid Crown Ether Moiety in the Reduced Graphene Oxide Framework

Abstract Highly strained cyclic xanthene dimers, which serve as models for reduced graphene oxide (RGO), were synthesized by Ni(0) mediated homo-coupling reactions. The dimers are obtained as highly stable colorless solids. X-ray crystallographic analysis of a tetra-tert-butyl derivative demonstrates that it possesses a highly strained structure with an extremely short non-bonded O⋯O distance (2.5662(44) Å). The results of variable temperature NMR experiments show that no signal broadening occurs even at 398 K, indicating that conformational flipping requires considerably high energy.

Self-Assembly-Directed Organization of a Fullerene-Bisporphyrin into Supramolecular Giant Donut Structures for Excited-State Charge Stabilization.

Functional materials composed of spontaneously self-assembled electron donor and acceptor entities capable of generating long-lived charge-separated states upon photoillumination are in great demand as they are key in building the next generation of light energy harvesting devices. However, creating such well-defined architectures is challenging due to the intricate molecular design, multistep synthesis, and issues associated in demonstrating long-lived electron transfer. In this study, we have accomplished these tasks and report the synthesis of a new fullerene–bis-Zn-porphyrin e-bisadduct by tether-directed functionalization of C60 via a multistep synthetic protocol. Supramolecular oligomers were subsequently formed involving the two porphyrin-bearing arms embracing a fullerene cage of the vicinal molecule as confirmed by MALDI-TOF spectrometry and variable temperature NMR. In addition, the initially formed worm-like oligomers are shown to evolve to generate donut-like aggregates by AFM monitoring that was also supported by theoretical calculations. The final supramolecular donuts revealed an inner cavity size estimated as 23 nm, close to that observed in photosynthetic antenna systems. Upon systematic spectral, computational, and electrochemical studies, an energy level diagram was established to visualize the thermodynamic feasibility of electron transfer in these donor–acceptor constructs. Subsequently, transient pump–probe spectral studies covering the wide femtosecond-to-millisecond time scale were performed to confirm the formation of long-lived charge-separated states. The lifetime of the final charge-separated state was about 40 μs, thus highlighting the significance of the current approach of building giant self-organized donor–acceptor assemblies for light energy harvesting applications.

A Mechanistic Probe into 1,2-cis Glycoside Formation Catalyzed by Phenanthroline and Further Expansion of Scope.

Phenanthroline, a rigid and planar compound with two fused pyridine rings, has been used as a powerful ligand for metals and a binding agent for DNA/RNA. We discovered that phenanthroline could be used as a nucleophilic catalyst to efficiently access high yielding and diastereoselective α-1,2-cis glycosides through the coupling of hydroxyl acceptors with α-glycosyl bromide donors. We have conducted an extensive investigation into the reaction mechanism, wherein the two glycosyl phenanthrolinium ion intermediates, a 4C1 chair-liked β-conformer and a B2,5 boat-like α-conformer, have been detected in a ratio of 2:1 (β:α) using variable temperature NMR experiments. Furthermore, NMR studies illustrate that a hydrogen bonding is formed between the second nitrogen atom of phenanthroline and the C1-anomeric hydrogen of sugar moiety to stabilize the phenanthrolinium ion intermediates. To obtain high α-1,2-cis stereoselectivity, a Curtin-Hammett scenario was proposed wherein interconversion of the 4C1 chair-like β-conformer and B2,5 boat-like α-conformer is more rapid than nucleophilic addition. Hydroxyl attack takes place from the α-face of the more reactive 4C1 β-phenanthrolinium intermediate to give an α-anomeric product. The utility of the phenanthroline catalysis is expanded to sterically hindered hydroxyl nucleophiles and chemoselective coupling of an alkyl hydroxyl group in the presence of a free C1-hemiacetal. In addition, the phenanthroline-based catalyst has a pronounced effect on site-selective couplings of triol motifs and orthogonally activates the anomeric bromide leaving group over the anomeric fluoride and sulfide counterparts.

Asymmetric Synthesis of 2-Arylindolines and 2,2-Disubstituted Indolines by Kinetic Resolution.

Kinetic resolution of 2‐arylindolines (2,3‐dihydroindoles) was achieved by treatment of their N‐tert‐butoxycarbonyl (Boc) derivatives with n‐butyllithium and sparteine in toluene at −78 °C followed by electrophilic quench. The unreacted starting materials together with the 2,2‐disubstituted products could be isolated with high enantiomer ratios. Variable temperature NMR spectroscopy showed that the rate of Boc rotation was fast (ΔG ≠≈57 kJ/mol at 195 K). This was corroborated by DFT studies and by in situ ReactIR spectroscopy. The enantioenriched N‐Boc‐2‐arylindolines were converted to 2,2‐disubstituted products without significant loss in enantiopurity. Hence, either enantiomer of the 2,2‐disubstituted products could be obtained with high selectivity from the same enantiomer of the chiral ligand sparteine (one from the kinetic resolution and the other from subsequent lithiation‐trapping of the recovered starting material). Secondary amine products were prepared by removing the Boc group with acid to provide a way to access highly enantioenriched 2‐aryl and 2,2‐disubstituted indolines.

Dynamics in Flexible Pillar[n]arenes Probed by Solid-State NMR.

Pillar[n]arenes are supramolecular assemblies that can perform a range of technologically important molecular separations which are enabled by their molecular flexibility. Here, we probe dynamical behavior by performing a range of variable-temperature solid-state NMR experiments on microcrystalline perethylated pillar[n]arene (n = 5, 6) and the corresponding three pillar[6]arene xylene adducts in the 100–350 K range. This was achieved either by measuring site-selective motional averaged 13C 1H heteronuclear dipolar couplings and subsequently accessing order parameters or by determining 1H and 13C spin–lattice relaxation times and extracting correlation times based on dipolar and/or chemical shift anisotropy relaxation mechanisms. We demonstrate fast motional regimes at room temperature and highlight a significant difference in dynamics between the core of the pillar[n]arenes, the protruding flexible ethoxy groups, and the adsorbed xylene guest. Additionally, unexpected and sizable 13C 1H heteronuclear dipolar couplings for a quaternary carbon were observed for p-xylene adsorbed in pillar[6]arene only, indicating a strong host–guest interaction and establishing the p-xylene location inside the host, confirming structural refinements.

Dinuclear Fe(III) Hydroxypropyl-Appended Macrocyclic Complexes as MRI Probes.

Four high-spin Fe(III) macrocyclic complexes, including three dinuclear and one mononuclear complex, were prepared toward the development of more effective iron-based magnetic resonance imaging (MRI) contrast agents. All four complexes contain a 1,4,7-triazacyclononane macrocyclic backbone with two hydroxypropyl pendant groups, an ancillary aryl or biphenyl group, and a coordination site for a water ligand. The pH potentiometric titrations support one or two deprotonations of the complexes, most likely deprotonation of hydroxypropyl groups at near-neutral pH. Variable-temperature 17O NMR studies suggest that the inner-sphere water ligand is slow to exchange with bulk water on the NMR time scale. Water proton T1 relaxation times measured for solutions of the Fe(III) complexes at pH 7.2 showed that the dinuclear complexes have a 2- to 3-fold increase in r1 relaxivity in comparison to the mononuclear complex per molecule at field strengths ranging from 1.4 T to 9.4 T. The most effective agent, a dinuclear complex with macrocycles linked through para-substitution of an aryl group (Fe2(PARA)), has an r1 of 6.7 mM-1 s-1 at 37 °C and 4.7 T or 3.3 mM-1 s-1 per iron center in the presence of serum albumin and shows enhanced blood pool and kidney contrast in mice MRI studies.

(Invited) Synthesis of Disilane-Bridged Aromatic Compounds for the Luminescent Materials

π-Conjugated molecules with electron-donating (D) and -accepting (A) groups such as D-π-A compounds have been focused on strong photo/electro-luminescent materials by intermolecular charge transfer (ICT). However, π-conjugated molecules cause concentration quenching due to π-π stacking in the aggregated state, and these compounds display the low solubility in organic solvents. To develop functional organic luminophores, we synthesized the disilane-modified aromatic compounds. Conformational studies were investigated by X-ray diffraction and variable-temperature NMR experiments. These molecules display high solubility derived from the suppression of intermolecular interaction by large SiMe2 moieties and stimuli-responsive emission properties from σ-π conjugation, displaying blue-green ICT emission in the solid state with higher quantum yield.

Enhanced Gearing Fidelity Achieved Through Macrocyclization of a Solvated Molecular Spur Gear.

Molecular spur gear dynamics with high gearing fidelity can be achieved through a careful selection of constituent molecular components that favorably position and maintain the two gears in a meshed configuration. Here, we report the synthesis of a new macrocyclic molecular spur gear with a bibenzimidazole stator combined with a second naphthyl bis-gold-phosphine gold complex stator to place two 3-fold symmetric 9,10-diethynyl triptycene cogs at the optimal distance of 8.1 Å for gearing. Micro electron diffraction (μED) analysis confirmed the formation of the macrocyclic structure and the proper alignment of the triptycene cogs. Gearing dynamics in solution are predicted to be extremely fast and, in fact, were too fast to be observed with variable-temperature 1H NMR using CD2Cl2 as the solvent. A combination of molecular dynamics and metadynamics simulations predict that the barriers for gearing and slippage are ca. 4 kcal mol-1 and ca. 9 kcal mol-1, respectively. This system is characterized by enhanced gearing fidelity compared to the acyclic analog. This is achieved by rigidification of the structure, locking the two triptycenes in the preferred gearing distance and orientation.

Two‐step Conformational Control of a Dibenzo Diazacyclooctane Derivative by Stepwise Protonation

AbstractThe conformational control of a dibenzo‐1,5‐diazacyclooctane (DACO) derivative with tertiary and secondary amino groups by stepwise protonation is investigated by single crystal X‐ray diffraction analysis, computational simulation, and variable temperature NMR measurements. The DACO derivative undergoes two‐step protonation, to afford bent‐shape monoprotonated and diprotonated species, whose conformations are different from each other. The monoprotonated form is characterized by a Tröger's‐base‐like proton‐bridged pseudo bicyclic structure, while the diprotonated form features a deformed boat‐shape eight membered ring structure. The degree of bending and conformational rigidity of the DACO skeleton become higher as the degree of the protonation is increased. Such a multistep conformational control of the DACO derivative would be useful for developing new stimuli‐responsive supramolecular systems that exhibit a multistep change in properties.

Characterizing Solid–Liquid Interactions in a Mesoporous Catalyst Support Using Variable-Temperature Fast Field Cycling NMR

Variable-temperature fast field cycling NMR measurements are used to probe the surface dynamics of different liquids imbibed within a γ-alumina catalyst support material. The imbibed liquids were g...

Au-Containing Coordination Polymers Based on Polyphosphorus Ligand Complexes.

Whereas the self-assembly of pentaphosphaferrocenes [CpRFe(η5-P5)] (CpR = Cp*, Cp×, and CpBn) with Cu and Ag salts has been well-studied in the past, the coordination chemistry toward Au complexes has been left untouched so far. Herein, the results of the self-assembly processes of [CpRFe(η5-P5)] with Au salts of different anions (GaCl4-, SbF6-, and Al(OC(CF3)3)4 (TEF-)) are reported. Next to a variety of molecular coordination products, the first coordination polymers based on polyphosphorus ligand complexes and Au salts are also obtained. Thereby, a 2D coordination polymer comprising metal vacancies is isolated. In all products, the Au centers are coordinated in a linear or a trigonal planar environment. In solution, highly dynamic processes are observed. Variable-temperature NMR spectroscopy, solid-state NMR spectroscopy, and X-ray powder diffraction were applied to gain further insight into selected coordination compounds.

Pagoda[5]arene with Large and Rigid Cavity for the Formation of 1∶2 Host–Guest Complexes and Acid/Base-Responsive Crystalline Vapochromic Properties

Pagoda[5]arene (P5), which is composed of five 2,6-dimethoxylanthracene (2,6-DMA) subunits bridged by methylene groups at 1,5 positions, was conveniently synthesized in 43% by trifluoroacetic acid ...

Reactivity of Cu(I) Nacnac Complexes Toward Polypnictogen Compounds.

The nacnac Cu(I) compound [LCu(MeCN)] (2) (L = [{N(C6H3Me2-2,6)C(Me)}2CH]-) was reacted with complexes containing aromatic cyclo-E5 ([Cp*Fe(η5-E5)], E = P (1a), As (1b), Cp* = η5-C5Me5), cyclo-P4 ([Cp‴Co(η4-P4)] (3), Cp‴ = η5-C5H2tBu3) and cyclo-E3 ligands ([Cp‴Ni(η3-E3)], E = P (4a), As (4b)) yielding the heterometallic complexes [(Cp*Fe)(μ,η5:2-E5)(LCu)] (E = P (5a), As (5b)), [(Cp*Fe)(μ3,η5:2:1-E5)(LCu)2] (E = P (6a), As (6b)), [(Cp‴Co)(μ,η4:2-P4)(LCu)] (7), [(Cp‴Co)(μ3,η4:2:1-P4)(LCu)2] (8), and [(Cp‴Ni)(μ,η3:2-E3)(LCu)] (E = P (9a), As (9b)). These complexes are rare examples of the coordination of a group 11 metal to aromatic cyclo-En (E = P, As; n = 3-5) ligands. All products were comprehensively characterized by crystallographic and spectroscopic methods. Their dynamic behavior in solution was studied by VT (variable-temperature) NMR spectroscopy, and their electronic structures were elucidated by DFT calculations.

Fused Quinoidal Dithiophene-Based Helicenes: Synthesis by Intramolecular Radical-Radical Coupling Reactions and Dynamics of Interconversion of Enantiomers.

A series of fused quinoidal dithiophene-based double and triple helicenes (1-M, 2-M, 2-M-Cl, 3-M, 3-M-Cl) were synthesized by intramolecular radical-radical coupling followed by oxidative dehydrogenation reaction. These helical molecules show dynamic interconversion of enantiomers in solution as revealed by variable-temperature NMR measurements, and the energy barriers are correlated to the substituents and topological structures. Notably, dynamic high performance liquid chromatography was used to quantitatively investigate the room-temperature racemization process between the (P,P,M)- and (P,M,M)- enantiomers of the triple helical 3-M-Cl, which gave an interconversion energy barrier in consistent with density functional theory calculations. Their optical and electrochemical properties are dependent on the fusion mode. Our studies provide both new synthetic strategy and new dynamic analytical method for helicenes with unique electronic structure.

Reactions of Cycloplatinated Guanidine Complexes with Hg(OC(O)CF3)2: Formation of a One-Dimensional Coordination Polymer Containing a Pt2Hg(μ2-S(O)Me2-S,O) Repeating Unit versus a Discrete Pt2Hg2 Complex.

Separate reactions of cycloplatinated 2-tolyl- and 2-anisylguanidine complexes, [Pt{κ2(C,N)}(OC(O)CF3)(S(O)Me2)] (1 and 2), with Hg(OC(O)CF3)2 in 1:0.5 and 1:1 molar ratios afforded the one-dimensional coordination polymer (1D CP) {[PtIII{κ2(C,N)}(OC(O)CF3)2]2Hg0}(μ2-S(O)Me2-S,O)·C7H8 (3·C7H8) as bright red crystals and the discrete tetrametallic complex [PtII{κ2(C,N)}(μ2-OC(O)CF3)2HgI-]2 (4) as yellow crystals in good yields. The two different products obtained in the aforementioned reactions are ascribed to the subtle differences in the N substituent of the guanidinate(1-) ligands in 1 and 2. The plausible mechanisms of formation of 3 and 4 are outlined. Complexes 3 and 4 were characterized by elemental analyses and IR and multinuclear NMR (1H, 13C{1H}, 19F, and 195Pt) spectroscopy. Complex 4 was also characterized by 199Hg NMR spectroscopy. The molecular structures of 3·C7H8 and 4 were determined by single-crystal X-ray diffraction studies. 1D CP 3·C7H8 contains a Pt(III)-Hg(0)-Pt(III)(μ2-S(O)Me2-S,O) repeating unit with a pair of unsupported Pt-Hg covalent bonds, while 4 contains a Pt(II)-Hg(I)-Hg(I)-Pt(II) chain with a pair of trifluoroacetate ligand supported Pt→Hg coordinate bonds. 1D CP 3·C7H8 falls apart into a mixture of three species, namely 6-8 and 9-11 in C6D6 and CDCl3, respectively, as revealed by multinuclear NMR spectroscopy. In CDCl3, 4 partially isomerizes to [PtII(OC(O)CF3)(μ2-OC(O)CF3){κ3μ2(C,N,O)}HgI-]2 (12), wherein each Pt→Hg coordinate bond is supported by one μ2-bridging trifluoroacetate ligand and one chelating bridging guanidinate(1-) ligand, as inferred from variable-temperature 1H and 19F NMR spectroscopy. Complex 12 is the major species and 4 is the minor species in CDCl3, while opposite situation prevails in C6D6. The observance of a mixture of two solution species for 4 is ascribed to a rapid "carboxylate shift" process induced by the oxygen atom of the ═N(C6H4(OMe)-2) unit of the guanidinate(1-) ligand through neighboring-group participation. UV-visible absorption and emission spectra of 4 were measured in CHCl3, and from the outcome of the investigation, the possible existence of [Cl2(H)C-Cl···PtII(OC(O)CF3)(μ2-OC(O)CF3){κ3μ2(C,N,O)}HgI-]2 (12″) was suggested, which is likely to have a pair of Pt-Hg covalent bonds made possible by CHCl3 coordination on the sixth site of the Pt(II) atom.

Similarities and Differences among Protein Dynamics Studied by Variable Temperature Nuclear Magnetic Resonance Relaxation.

Understanding and describing the dynamics of proteins is one of the major challenges in biology. Here, we use multifield variable-temperature NMR longitudinal relaxation (R1) measurements to determine the hierarchical activation energies of motions of four different proteins: two small globular proteins (GB1 and the SH3 domain of α-spectrin), an intrinsically disordered protein (the C-terminus of the nucleoprotein of the Sendai virus, Sendai Ntail), and an outer membrane protein (OmpG). The activation energies map the motions occurring in the side chains, in the backbone, and in the hydration shells of the proteins. We were able to identify similarities and differences in the average motions of the proteins. We find that the NMR relaxation properties of the four proteins do share similar features. The data characterizing average backbone motions are found to be very similar, the same for methyl group rotations, and similar activation energies are measured. The main observed difference occurs for the intrinsically disordered Sendai Ntail, where we observe much lower energy of activation for motions of protons associated with the protein-solvent interface as compared to the others. We also observe variability between the proteins regarding side chain 15N relaxation of lysine residues, with a higher activation energy observed in OmpG. This hints at strong interactions with negatively charged lipids in the bilayer and provides a possible mechanistic clue for the "positive-inside" rule for helical membrane proteins. Overall, these observations refine the understanding of the similarities and differences between hierarchical dynamics in proteins.

Molecular Rotor Functionalized with a Photoresponsive Brake.

A molecular motor that has been previously shown to rotate when fueled by electrons through a scanning tunneling microscope tip has been functionalized with a terarylene photochrome fragment on its rotating subunit. Photoisomerization has been performed under UV irradiation. Variable-temperature 1H NMR and UV-vis studies demonstrate the rotational motion and its braking action after photoisomerization. The braking action can be reversed by thermal heating. Once the rigid and planar closed form is obtained, the rotation is effectively slowed at lower temperature, making this new rotor a potential motor with an independent response to electrons and light.