Variable-temperature 1H NMR Studies Research Articles

Dinitrogen Functionalization with Terminal Alkynes, Amines, and Hydrazines Promoted by [(η5-C5Me4H)2Zr]2(μ2,η2,η2-N2): Observation of Side-On and End-On Diazenido Complexes in the Reduction of N2 to Hydrazine

Functionalization of the N2 ligand in the side-on bound dinitrogen complex, [(eta5-C5Me4H)2Zr]2(mu2,eta2,eta2-N2), has been accomplished by addition of terminal alkynes to furnish acetylide zirconocene diazenido complexes, [(eta5-C5Me4H)2Zr(C[triple bond]CR)]2(mu2,eta2,eta2-N2H2) (R = nBu, tBu, Ph). Characterization of [(eta5-C5Me4H)2Zr(C[triple bond]CCMe3)]2(mu2,eta2,eta2-N2H2) by X-ray diffraction revealed a side-on bound diazenido ligand in the solid state, while variable-temperature 1H and 15N NMR studies established rapid interconversion between eta1,eta1 and eta2,eta2 hapticity of the [N2H2]2- ligand in solution. Synthesis of alkyl, halide, and triflato zirconocene diazenido complexes, [(eta5-C5Me4H)2ZrX]2(mu2,eta1,eta1-N2H2) (X = Cl, I, OTf, CH2Ph, CH2SiMe3), afforded eta1,eta1 coordination of the [N2H2]2- fragment both in the solid state and in solution, demonstrating that sterically demanding, in some cases pi-donating, ligands can overcome the electronically preferred side-on bonding mode. Unlike [(eta5-C5Me4H)2ZrH]2(mu2,eta2,eta2-N2H2), the acetylide and alkyl zirconocene diazenido complexes are thermally robust, resisting alpha-migration and N2 cleavage up to temperatures of 115 degrees C. Dinitrogen functionalization with [(eta5-C5Me4H)2Zr]2(mu2,eta2,eta2-N2) was also accomplished by addition of proton donors. Weak Brønsted acids such as water and ethanol yield hydrazine and (eta5-C5Me4H)2Zr(OH)2 and (eta5-C5Me4H)2Zr(OEt)2, respectively. Treatment of [(eta5-C5Me4H)2Zr]2(mu2,eta2,eta2-N2) with HNMe2 or H2NNMe2 furnished amido or hydrazido zirconocene diazenido complexes that ultimately produce hydrazine upon protonation with ethanol. These results contrast previous observations with [(eta5-C5Me5)2Zr(eta1-N2)]2(mu2,eta1,eta1-N2) where loss of free dinitrogen is observed upon treatment with weak acids. These studies highlight the importance of cyclopentadienyl substituents on transformations involving coordinated dinitrogen.

Conformational behavior of dithia[n.3.3](1,3,5)cyclophanes and dithia[n.3.3](1,2,6)cyclophanes

The conformational behavior of a series of crown-fused dithia[n.3.3](1,2,6)cyclophanes ( 126- CPs ) and dithia[n.3.3](1,3,5)cyclophanes ( 135- CPs ) was investigated by variable-temperature 1H and 13C NMR spectroscopy, X-ray crystallography and density functional theory (DFT) calculations. Single crystal X-ray structure analysis showed that two thia-bridges in 126- CPs adopted a pseudochair– pseudochair ( cc) conformation and the cyclophane decks underwent a ring-tilting motion in the case of [10.3.3](1,2,6)cyclophane ( 1a ). In contrast, the thia-bridges in 135- CPs took both cc and pseudoboat– pseudochair ( bc) conformations, and the ring-tilting process was also found in [10.3.3](1,3,5)cyclophane ( 2a ). Variable temperature 1H NMR study revealed that there was no wobbling-motion for two thia-bridges in 126- CPs while thia-bridges in 135- CPs experienced a wobbling-process with a conformational barrier of 9.21 and 8.80 kcal mol −1, respectively, for 2a and [13.3.3](1,3,5)cyclophane ( 2b ). DFT calculations for the two cyclophanes series revealed that 126- CPs preferred a cc conformation which was consistent with the experimental observation; similarly, 135- CPs took a preferential cc conformation, agreeing with 2a having a predominant cc conformer ( cc: bc ratio=70:30), but not 2b having a predominant bc conformer ( cc: bc ratio=15:85) in the solid state.

(1-Naphthyl)phenylamino functionalized three-coordinate organoboron compounds: syntheses, structures, and applications in OLEDs

New three-coordinate organoboron compounds functionalized by a (1-naphthyl)phenylamino group, B(mes)2(dbp-NPB) (1), B(db-NPB)3 (2), and B(dbp-NPB)3 (3), have been synthesized. A variable temperature 1H NMR study showed that the aryl groups around the boron center in these compounds have a rotation barrier ∼70 kJ mol−1. The new boron compounds are amorphous solids with Tg being 110 °C, 171 °C and 173 °C, respectively. The electronic properties of the new boron compounds were investigated by cyclic voltammetry and UV–visible spectroscopy. All three boron compounds are blue emitters in the solid state. In solution the emission spectra of the boron compounds shift toward a longer wavelength with increasing solvent polarity. In CH2Cl2, the emission quantum efficiency of the three compounds was determined to be 0.22, 0.27 and 0.23, respectively. Several series of electroluminescent (EL) devices where compounds 1–3 are used as either an emitter/electron transport material, a hole transport material, or a hole injection material have been fabricated and their performance has been compared to the corresponding devices of BNPB, a previously investigated molecule, NPB, a commonly used hole transport material, and CuPc, a commonly used hole injection material. The EL results indicate that the new boron compounds are not suitable as emitters/electron transport materials, but they are promising as hole transport and hole injection materials in EL devices.

Proton exchange kinetics from the bound waters on the oxo-centered rhodium(iii) trimer [Rh3(µ3-O)(µ-O2CCH3)6(OH2)3]+: a variable pH and temperature 1H NMR study

Proton exchange from the bound to the bulk waters on the oxo-centered rhodium(III) trimer, [Rh(3)(micro(3)-O)(micro-O(2)CCH(3))(6)(OH(2))(3)](+)(abbreviated as Rh(3)(+)), was investigated over the temperature range of 219.1-313.9 K using a (1)H NMR line-broadening technique. By solving the modified Bloch equations for a two-site chemical exchange, lifetimes (tau) for proton transfer at pH = 2.7, 3.6, and 7.0 ([Rh(3)(+)]= 26 mM, T= 298 K) were determined to be 0.3 (+/-.08) ms, 2 (+/-0.3) ms, and 0.2 (+/-0.2) ms, respectively. From the temperature dependence of the rate, the activation parameters were determined to be DeltaH(++)= 16.2 (+/-0.5) kJ mol(-1) and DeltaS(++)=- 123 (+/-2) J mol(-1) K(-1), DeltaH(++)= 14.9 (+/-0.5) kJ mol(-1) and DeltaS(++)=- 141 (+/-2) J mol(-1) K(-1), and DeltaH(++)= 45 (+/-2) kJ mol(-1) and DeltaS(++)=- 22 (+/-5) J mol(-1) K(-1) for pH = 2.7, 3.6 and 7.0, respectively. All results are reported for a mixed solvent system [acetone : 250 mM NaClO(4)(aq)(3:1)], which was necessary to depress the freezing point of the solution so that the (1)H NMR signal due to bound water could be observed. The pK(a) of Rh(3)(+) was measured to be 8.9 (+/-0.2) in the mixed solvent, which is near the pK(a) for an aqueous solution (8.3 (+/-0.2)). Surprisingly, the lifetimes for protons on Rh(3)(+) are close to those observed for the Rh(OH(2))(6)(3+) ion, in spite of the considerable difference in structure, Brønsted acidity of the bound waters and average charge on the metal ion.

Evidence of a self-inclusion phenomenon for a new class of mono-substituted alkylammonium-β-cyclodextrins

A new class of mono-substituted N-alkyl-N,N-dimethylammonium-beta-cyclodextrins has been synthesized in a three step procedure from the native beta-cyclodextrin. The structural analysis of these compounds undertaken by combined use of 1D and 2D NMR spectra indicate that the two methyl groups bound on the nitrogen are magnetically inequivalent due to a self-inclusion phenomenon of the alkyl chain inside the CD cavity. A variable-temperature 1H NMR study showed that these mono-substituted CD derivatives formed temperature-independent intramolecular complexes with their own alkylammonium substituent. The strength of the interaction between the alkyl moiety and the cyclodextrin cavity has been evaluated by a competitive method using an adamantane derivative. Finally, surface tension measurements demonstrated the surface active character of these compounds and confirmed their self-inclusion ability.

Diphosphines with expandable bite angles: highly active ethylene dimerisation catalysts based on upper rim, distally diphosphinated calix[4]arenes.

The binding properties of two large diphosphines, cone-5,17-dibromo-11,23-bis(diphenylphosphino)-25,26,27,28-tetrapropoxycalix[4]arene (1) and cone-5,17-bis(diphenylphosphino)-25,26,27,28-tetrapropoxycalix[4]arene (2) toward Ni(II) centres have been investigated. Whatever the starting complex, NiBr2 or [NiCp]BF4, quantitative formation of a chelate complex was observed, illustrating the preorganisation of the ligands. An X-ray structure determination was carried out for [NiCp1]BF4 which revealed that the nickel atom is positioned to one side of the calixarene axis, the PNiP plane being roughly parallel to the calixarene reference plane. The molecule has C(1) symmetry in the solid state, a feature which is also observed in solution at low temperature. As shown by variable-temperature 1H and 31P NMR studies, the complex undergoes two distinct motions: 1) a fan-like swinging of the coordination plane which displaces the metal from one side of the calixarene axis to the other, a motion during which the PNiP angle is likely to undergo a significant enlargement; 2) a rapid oscillation of each PPh2 unit about the corresponding Ni--P bond. In the latter dynamics the two endo-oriented PPh rings alternately occupy the calixarene entry. The two flexible ligands were assessed in ethylene oligomerisation. Activation with methylaluminoxane of the paramagnetic complexes [NiBr2.(1 or 2)] afforded highly active ethylene dimerisation catalysts, with turnover frequencies up to 10(6) (mol C2H4) (mol Ni)(-1) h(-1). The selective formation of 1-butene can be rationally controlled by using low catalyst concentrations.

Intramolecular 9-membered hydrogen bonding of 2-arylmethylphenols having carbonyl groups at 2′-position

Thermodynamic parameters of nine-membered intramolecular hydrogen bonding between carbonyl groups and phenolic hydroxyl groups of 2-arylmethylphenols having methoxycarbonyl, dimethylcarbamoyl, and formyl groups were determined by variable temperature 1H NMR studies and van't Hoff analysis. The enthalpy of the hydrogen bonding was related to the electron-withdrawing ability of the substituents on the phenol and the basicity of the carbonyl group. The entropy loss of the hydrogen bonding was dependent on the rotation freedom of the phenol group.

One-component gels based on peptidic dendrimers: dendritic effects on materials properties.

This paper describes the gelation of symmetric dendrimers based on building blocks constructed from L-lysine. These dendrimers form gel-phase materials in nonpolar organic solvents. The thermal properties and concentration dependence of the gelation were investigated, and it was found that there was a clear dendritic effect on the behavior of the soft materials formed, with higher generation dendrimers giving rise to more thermally stable gels. Variable temperature 1H NMR studies indicated that this behavior was probably a consequence of more extensive interdendrimer hydrogen bonding occurring between the peptidic groups in the higher generation dendrimers. The supramolecular aggregates were found to have a fibrillar structure, with the dimensions and alignment of the fibers being dependent on dendritic generation. Circular dichroism measurements confirmed that these fibers possessed chiral organization of the peptidic groups on the supramolecular (nano) scale, assigned as helicity. This paper indicates that dendritic functionalization provides a useful way of tuning gel-phase materials properties, with clear dendritic effects on gel formation being quantified for the first time, hence illustrating the way dendritic functionalization can play a positive role in the formation of highly functional organic materials with desirable properties.

Synthesis, characterization and spectroscopy of alkyl substituted edge-bridged open ruthenocenes

The edge-bridged open ruthenocenes bis(η5-6,6-dimethylcyclohexadienyl)ruthenium (1), bis(η5-2,6,6-trimethylcyclohexadienyl) ruthenium (2), bis(η5-3,6,6-trimethylcyclohexadienyl)ruthenium (3), bis(η5-2,4,6,6-tetramethylcyclohexadienyl)ruthenium (4) are prepared by reactions between di-μ-chlorodichlorobis[(1-3η:6-8η)-2,7-dimethyloctadienyl] diruthenium and the corresponding dienes. Compound 4 can also be prepared in lower yield by reduction of RuCl3·nH2O with zinc in the presence of 2,4,6,6-tetramethylcyclohexadiene. Variable temperature 1H NMR studies of 1–4 reveal rapid rotation about the Ru–C bonds. The barrier to rotation in 4 is 7.7 kcal/mol with a gauche–eclipsed ground state conformation. Even lower barriers to rotation in 1–3 contribute to the inability to accurately measure their activation energies in toluene.

Synthesis, Characterization, and Variable-Temperature1H NMR Behavior of Organo-Bridged Dicobaloximes

Organo-bridged dicobaloximes with four different dioximes Py(L)2CoCH2-R-CH2Co(L)2Py (L = dmgH, dpgH, chgH, and gH) have been synthesized and characterized by 1H and 13C NMR and FAB mass spectroscopy. The cis influencing order observed in dicobaloximes is similar to the previously observed order in monocobaloximes. The cyclic voltammetric results show that an irreversible single-step two-electron reduction of CoIII to CoI takes place. The Co−C bond in 4a cleaves during crystallization and results in the formation of o-vinylbenzyl cobaloxime. The variable-temperature 1H NMR study suggests that the Co−C bond rotation is restricted and its magnitude depends on both the nature of the bridging ligand and the dioxime.

Heterotopic assemblage of two different disk-shaped ligands through trinuclear silver(I) complexation: ligand exchange-driven molecular motion.

The sandwich-shaped heterotopic trinuclear Ag+ complex Ag(3)1.2 was exclusively formed from two different tris(thiazolyl) and hexa(thiazolyl) disk-shaped ligands, 1 and 2, with the aid of three Ag+ ions. The variable-temperature 1H NMR study on its complexation behavior revealed that metal-ligand exchanges between the two neighboring thiazolyl nitrogen donors of 2 take place at the three Ag+ centers in concert. DeltaH++ and DeltaS++ for the exchange process were calculated to be 50.5 kJ mol(-1) and -26.7 J mol(-1) K(-1), respectively, and its energy barrier at 298 K was estimated to be 58.5 kJ mol(-1). Each concerted metal-ligand exchange leads to an intramolecular 60 degrees-rotational motion ((P) <==>(M) conversion) between the two disk-shaped ligands.

Synthesis and structural studies of heteroleptic complexes of ytterbium(III) involving aryloxy- or alkoxy- and cyclopentadienyl ligands

Treatment of tris-cyclopentadienyl-ytterbium in thf with one equivalent of 2,6-di( tert-butyl)phenol, N, N-dimethyl-2-aminoethanol or N, N-diethyl-2-aminoethanol resulted in substitution of one cyclopentadienyl ligand and formation of [YbCp 2(O-C 6H 3 t Bu-2,6)(thf)] ( 1), [{YbCp 2(μ-OCH 2CH 2NMe 2)} 2] ( 2) or [{YbCp 2(μ-OCH 2CH 2NEt 2)} 2] · (thf) 2 ( 3), respectively. All compounds were characterised by spectroscopic and X-ray crystallographic techniques, the latter two also being studied by variable temperature 1H NMR spectroscopy. Compound ( 1) is mononuclear with the Yb centre bound by two η 5-cyclopentadienyl ligands one O-bound thf and an O-bound phenoxy ligand. Compounds ( 2) and ( 3) are centrosymmetric dimers with the Yb centre bound by two η 5-cyclopentadienyl ligands, while the bidentate ligands chelate the metal centre and also bridge to the adjacent Yb through the alkoxy oxygen atom. Variable temperature 1H NMR studies on compounds ( 2) and ( 3) show a solution-state equilibrium between the dimeric solid-state structure and one with the nitrogen atoms non-bound to Yb.

Synthesis, Structures and Fluxional Behavior of Ruthenium(II) Complexes Bearing a Bidentate 1,8-Naphthyridine Ligand

Abstract The ruthenium complexes bearing 1,8-naphthyridine (napy) and terpyridine analogous (N,C,N)-tridentate ligands were synthesized and characterized. The reaction of [RuCl2(napy-κ2N,N′)(dmso)2] with 2 equiv of AgPF6 and subsequent addition of LH and CO gave [RuL(napy-κ2N,N′)(CO)](PF6)n (6a: L = N-methyl-3,5-di(2-pyridyl)-4-pyridyl, n = 2; 6b: L = 2,6-di(2-pyridyl)phenyl, n = 1) via [RuL(napy-κ2N,N′)(dmso)](PF6)n (5a: L = N-methyl-3,5-di(2-pyridyl)-4-pyridyl, n = 2; 5b: L = 2,6-di(2-pyridyl)phenyl, n = 1). The crystal structures of 5a and 6a show distorted octahedral coordination with the tridentate (N,C,N)-ligand as mer-fashion, two nitrogens of bidentate napy and the sulfur of DMSO (5a) or the carbon of the CO ligand (6a). Detailed experiments for irradiation and variable-temperature 1H NMR studies reveal a fluxional process of the chelated napy ligand in solution.

Synthesis and structures of di(2-pyridyl)amine diruthenium(I) complexes, including an example of monodentate coordination

<p>Addition of di(2-pyridyl)amine (dpa) to a solution formed by reaction of [Ru(CO) 2Cl 2] n with NaOAc resulted in the precipitation of [Ru(OAc)(CO) 2(dpa)] 2, a complex containing a rare example of monodentate dpa coordination. X-ray crystallography revealed a dinuclear complex with a Ru–Ru bond, having two cis-carbonyls on each ruthenium trans to oxygen donors of two μ-acetate ligands and a monodentate dpa approximately colinear with the Ru–Ru bond. A variable temperature 1H NMR study of this complex indicated fluxional behaviour of the dpa ligand in solution at room temperature, with the spectrum at 243 K corresponding to that predicted from the solid state structure. Heating of [Ru(OAc)(CO) 2(dpa)] 2 in ethanol formed [Ru 2(OAc)(CO) 4(dpa) 2] +, which was isolated as the acetate salt. The X-ray structure of this complex and the analogous benzoate bridged [Ru 2(PhCO 2)(CO) 4(dpa)]Cl confirmed the presence of a diruthenium(I) core with terminal chelating dpa ligands and bridged by one carboxylate and two carbonyl ligands.

X-ray structural and dynamic behaviour study of allyl palladium compounds with fluorinated benzenethiolate bridges

In solution, allyl palladium homobimetallic complexes bridged by fluorinated benzenethiolates, [{Pd(μ-SR)(η 3-C 3H 5)} 2] where R=C 6F 5, 1; C 6HF 4-4, 2; C 6H 4F-2, 3; C 6H 4F-3, 4 and C 6H 4F-4, 5, are found as a mixture of syn/ anti and cis/ trans isomers. The variable temperature 1H and 19F NMR study of these compounds show that the four isomers undergo interconversion through two probable mechanisms, allyl rotation assisted by the solvent and inversion of the configuration at the sulphur atoms. The X-ray crystal structure determination of [{Pd(μ-SC 6F 5)(η 3-C 3H 5)} 2] and [{Pd(μ-S C 6HF 4-4)(η 3-C 3H 5)} 2] shown both complexes to be bimetallic with the metal centres found in a slightly distorted square planar environments.

Multimetallic Complex Composed of Redox-Active Bridging Quinonediimine Ligand

Abstract A π-conjugated quinonediimine ligand possessing two terminal redox-active ferrocenyl groups, N,N′-bis{(4′-ferrocenylcarboxamide)phenyl}-1,4-benzoquinonediimine (Lox), was synthesized. The quinonediimine ligand Lox was reduced to Lred composed of the phenylenediamine moiety on treatment with hydrazine hydrate. The reaction of Lox with the palladium(II) complex 1 bearing the N,N′-bis(2-phenylethyl)-2,6-pyridinedicarboxamide ligand afforded the multimetallic complex 2. Variable temperature 1H NMR studies of 2 indicated the existence of syn and anti isomers, in which the anti isomer is enthalpically more favorable than the syn isomer in CD2Cl2 by 0.9 kcal mol−1, but entropically less favorable by 2.6 cal mol−1 K−1 from the van’t Hoff plot. The redox function of the quinonediimine moiety is modulated by complexation with 1. The complex 2 showed three separate redox waves, which are assignable to the successive one-electron reduction of the quinonediimine moiety and oxidation of the two terminal ferrocenyl groups. Chemical reduction of 2 in CH2Cl2 with [Co(Cp)2] resulted in the appearance of ESR signals with weak 105Pd coupling centered around g = 2.004. The unpaired electron appears to locate mostly on the quinonediimine moiety with some delocalization onto the palladium center.

Isomerization by ligand shuffling along a Cr24+unit: further reactions leading to cleavage of a quadruple bond

The paddlewheel complex Cr2(DpyF)4 (1), where DpyF = N,N′-di(2-pyridyl)formamidinate, has a short Cr–Cr distance of 1.938(2) A for the CrCr quadruple bond. In solution, there is a series of interconverting isomers, as determined by variable temperature 1H NMR studies. Compound 1 can be readily oxidized by CuCl2 to produce [Cr2(DpyF)4][CuCl2]2 (2) and [Cr2(DpyF)4CuCl3][CuCl2] (3). In both cases, the metal–metal quadruple bond is cleaved in favor of six-coordinate Cr(III) ions. Magnetic measurements for the oxidized products show that the chromium ions in the dichromium units exhibit strong antiferromagnetic interactions.

Synthesis and Characterization of Nickel(II) Schiff Base Complexes with Methoxy or Methyl Groups at 2,6-Positions of the Pendant Phenyl Ring: the Control of cis and trans Geometries

Abstract Two nickel(II) complexes with N,S chelates were prepared by the reaction of nickel(II) acetate tetrahydrate with 2-substituted benzothiazolines. Bis[2-(2,6-dimethoxyphenylmethyleneamino)benzenethiolato]nickel(II) 1, in which the 2,6-positions of the pendant phenyl ring are occupied by methoxy groups, has an approximately square-planar geometry around a nickel atom with a cis-N2S2 donor atom arrangement. X-ray and variable-temperature 1H NMR studies indicated the existence of an apical Ni···OMe interaction in 1. On the other hand, bis[2-(mesitmethyleneamino)benzenethiolato]nickel(II) 2 with methyl groups at the 2,4,6-positions of the pendant phenyl ring exhibits a perfect planar trans-N2S2 coordination around the nickel atom. An X-ray study revealed the existence of an intraligand CH/π interaction in 2. A difference in the geometry around –CH=N– (E and Z) was also observed between 1 and 2 (Z for 1 and E for 2).

Cyclic Organohydroborate Complexes of Metallocenes. VII. Synthesis, Structure, and Fluxional Behavior of Cp2ZrX{(μ-H)2BC8H14} (X = H, D, Cl)

The reaction of K[H2BC8H14] with Cp2ZrCl2 in a 1:1 molar ratio afforded Cp2ZrCl{(μ-H)2BC8H14}, 1, which when reacted with KH yielded Cp2ZrH{(μ-H)2BC8H14}, 2. These complexes were structurally characterized by single-crystal X-ray analysis. Partially deuterated 1 and 2, complexes Cp2ZrCl{(μ-D)( μ-H)BC8H14}, 3, and Cp2ZrD{(μ-H)2BC8H14}, 4, were prepared. 2H NMR spectra revealed that in complex 3, which contains a terminal chloride on zirconium, no exchange of bridge and Cp hydrogens occurred. However, in complex 4, which includes a terminal hydrogen on zirconium, there was facile mixing of terminal, bridge, and Cp-hydrogens. Furthermore, exchange of terminal zirconium−hydrogen with bridge hydrogen was more rapid than exchange with the Cp-hydrogen. Additionally, this fluxional behavior occurred even at room temperature and in the solid state. Variable-temperature 1H NMR studies are consistent with these results of dynamic hydrogen exchange in complexes 1 and 2. The complexes Cp2Nb{(μ-H)2BC8H14}, 7, and Cp2Nb...

Syntheses of large-membered macrocycles having multiple hydrogen bonding moieties

New macrocyclic compounds have been synthesized by Schiff-base condensation reaction with methylenebis(4,4′-methyl-6,6′-salicylaldehyde) and 1,2-bis(2-aminoethoxy)ethane based on a high dilution method. [2+2], [3+3], and [4+4]-Cyclocondenced products were effectively isolated and characterized by 1H NMR and HR mass (FAB) spectroscopies as well as X-ray analyses. Reduction of the macrocycles with NaBH 4 afforded the corresponding multi-amino, multi-phenolic macrocyclic compounds. The reduced molecules have low energy barriers for conformation change, which are estimated by variable-temperature (VT) 1H NMR study.