Temperature-dependent 1H Research Articles

Gas-Phase NMR Studies of N,N-Dimethylthioamides. Influence of the Thiocarbonyl Substituent on the Internal Rotation Activation Energies

Temperature-dependent gas-phase 1H NMR spectra of seven thiocarbonyl-substituted N,N-dimethylthioamides (YCSN(CH3)2) obtained at 300 MHz are consistent with the following free activation energies ΔG⧧298 (kcal mol-1): Y = H, 22.5 (0.1); CH3, 18.0 (0.1); F, 18.3 (0.1); Cl, 16.9 (0.2); CF3, 17.2 (0.1); CH2CH3, 17.6 (0.1); CH(CH3)2, 16.3 (0.1). The results are compared to condensed-phase values and to the corresponding gas-phase oxoamides.

Metal(II) N2S2 Schiff-base complexes incorporating pyrazole or isoxazole (M = Ni, Cu or Zn). Spin states, racemization kinetics and electrochemistry

A. la Cour, M. Findeisen, A. Hazell, R. Hazell and G. Zdobinsky, J. Chem. Soc., Dalton Trans., 1997, 121 DOI: 10.1039/A602281B

(Allyl)zirconium(IV) Complexes with the N,N′ ‐bis(trimethylsilyl)benzamidinato (“siam”) Ligand — Molecular Structures and η1–η3 Interconversions of the Allyl Groups

AbstractComplexes of the type (siam)2Zr(allyl)2 (1a) and (siam)Zr(allyl)nClm are described (2a: n = 1, m = 2; 2b: n = 2, m = 1; 2c: n = 3, m = 0; siam: N,N′ ‐bis(trimethylsilyl)benzamidinato group). The molecular structure of 1a shows that both allyl groups are differently bonded. One allyl ligand is η3‐, the other η1‐coordinated. The X‐ray crystal structure of the tris‐(allyl) complex 2c shows a chiral molecule with two η3‐allyl ligands. The third allyl ligand is coordinated in a mode intermediate between η3 and η1, thus serving as a model for an intermediate in the η3‐η1 intramolecular rearrangement of the allyl ligands observed in solution. Temperature‐dependent 1H‐ and 13C‐NMR spectra of 1a, 2b and 2c indicate a rapid η1‐η3 rearrangement of the allyl groups at 60°C. At −50°C two different coordination modes of the allyl groups can be observed in the case of 2b and 2c.

Methyl-, acetyl- and allyl-palladium and -piatinum complexes containing the novel chiral phosphorus-imine 2-(diphenylphosphino)-benzylidene- S( − )- α-methyl-benzylamine ligand

Neutral compounds of the type [MX 2(L)] and [MX(Me)(L)] and ionic complexes of the type [M(Me)(L)](O 3SCF 3), in which X = Cl, Br, I; M = Pd, Pt; L = 2-(diphenylphosphino)-benzylidene- S- − )- α-methyl-benzylamine, have been prepared and characterized. Single crystal X-ray determinations of [PdCl 2(L)] ( 1a) and [PtI 2(L)] ( 3b) showed, in both cases, a chelate coordination of the PN ligand thereby forming a six-membered ring. The square planar surrounding is completed by the two halide atoms. The single crystal X-ray determination of [PdCl(Me)Cl(L)] ( 4a) shows an analogous geometry with a chelating PN ligand, a chloride atom and a methyl group, which is positioned cis to the phosphorus atom, completes the square planar surrounding. The methylpalladium and -platinum complexes reacted with CO to give the corresponding acetyl complexes. The insertion rates increased in the order Cl<Br<O 3SCFP 3 − while the reaction is first order in metal complex and first order in CO concentration. Complexes [Pd( η 3-allyl)(PN)] +Y − (Y = Cl, O 3SCF 3) with symmetric allyl groups 2-RC 3H 5 (R = Me, C(O)Me), 2-MeC 3Me 4 and asymmetrically substituted allyl groups 2-R-C 3H 2Me 2 (R = H, Me) have been prepared. Temperature dependent 1H, 31P{ 1H} and 13C{ 1H} NMR has been used to determine the influence of the chiral ligand on the structural aspects and dynamic features. It is shown that a delicate balance between counteracting steric and electronic factors determines the type of isomer, i.e. with the P atom cis or trans to the CMe 2 moiety of the asymmetric allyl group.

Syntheses and Properties of Extremely Stable Di(1-azulenyl)phenylmethyl and (1-Azulenyl)diphenylmethyl Cations Having Dimethylamino Substituents on Their Phenyl Groups

Abstract A series of extremely stable di(1-azulenyl)phenylmethyl and (1-azulenyl)diphenylmethyl cations having dimethylamino substituents on their phenyl groups, i.e., di(1-azulenyl)[4-(dimethylamino)phenyl]methyl (5a) and (1-azulenyl)bis[4-(dimethylamino)phenyl]methyl (6a) cations and their 3-methyl-1-azulenyl (5b and 6b) and 3,6-di-t-butyl-1-azulenyl (5c and 6c) homologs, were synthesized by hydride abstraction from the corresponding hydrides. Their properties were fully characterized. As expected, the pKR+ values of these cations dramatically increased with the dimethylamino substituents on their phenyl groups. The values of 5a—c (pKR+ 13.2—13.8) and 6a—c (pKR+ 12.6—13.3) are higher by 1.4—2.7 and 8.7—9.6 pK units than those of the corresponding analogous phenyl- and diphenylmethyl cations. The redox behavior of each cation was also affected by the substituents. The oxidation of 5a—c in acetonitrile exhibited a barely separated two-step oxidation wave at potential ranges of +0.75 — +0.87 and +0.89 — +1.01 V vs. Ag/Ag+ upon cyclic voltammetry (CV), as occurs in the oxidation of tri(1-azulenyl)methyl cations. The wave is ascribed to the oxidation of the two azulene rings to generate a trication species. The oxidation of 6a—c also showed two waves at a narrow potential range at +0.74 — +0.92 V. Although the reduction of 6a—c exhibited an irreversible wave at −0.94 V, that of 5a—c showed a reversible wave at −0.87 — −0.95 V upon CV. The temperature-dependent 1H NMR spectra of 5b and 6b were also examined to clarify their stereochemistries. Stereoisomerizations due to the pyramidal dimethylamino substituents in addition to the propeller conformations of three aromatic rings were observed by low-temperature NMR studies. At higher temperature the NMR reflects the rapid isomerization of these stereoisomerisms.

Formation of stable organometallic planar-tetracoordinate carbon compounds containing a cationic (μ-R 1CCR 2) [μ-chloro(ZrCp 2) 2] framework

The methylzirconocene cation [Cp 2ZrCH 3(THF) +BPh 4 −] reacts with Cp 2Zr(Cl)(CCR) reagents ( 5a–c, R  −CH 2Ph,−CH 2CH 2CH 3,−CH 3) to yield the dinuclear metallocene actions [(Cp 2Zr) 2( μ-Cl)( μ- η 1: η 2-RCCCH 3] + ( 6a–c) (with BPh 4 − anion) that contain a planar-tetracoordinate carbon atom. Complex 6b was characterized by X-ray diffraction. The activation energy of the degenerate rearrangement of 6c was determined by temperature dependent dynamic 1H NMR spectroscopy ( ΔG ‡(240 K) = 11.3 ± 0.5 kcal mol −1 ).

Molecular reorientation and phase transition in pyridinium hexafluoroantimonate

Abstract The temperature dependence of 1H and 19F NMR second moment and spin lattice relaxation times in high (v L = 60 MHz and low (B 1 = 2 mT) magnetic fields allow one to determine both ion dynamics in polycrystalline pyridinium hexafluoroantimonate. The solid-solid phase transition discovered at 268 K appears to be connected with symmetrization of energy barriers for pseudohexagonal cation reorientation. The energy difference Δ characterizing the inequivalence of the potential wells can be treated as an order parameter. The effect of coupling of the rotational modes of cations and anions is found at the phase transition.

Synthesis and NMR conformational studies ofp-tert-butyldihomooxacalix[4]-arene derivatives

Abstract The tetramethyl (2) and tetraethyl (3) ethers, and the tetraacetate (4) derivatives of the p-tert-butyldihomooxacalix[4]arene were prepared. The mobility of these compounds studied by temperature-dependent 1H NMR spectroscopy. For the tetraacetate derivative, at room temperature, 1,2- or 1,3-alternate conformations are suggested. Those conformations were confirmed by NOE difference and COSY spectra for the tetraethyl ether derivative in CDCl3 at -20°C.

An NMR Investigation of the Effect of Hydrogen Bonding on the Rates of Rotation about the C−N Bonds in Urea and Thiourea

The interaction between urea and tetrabutylammonium acetate was investigated in dimethylformamide/dimethyl sulfoxide solutions using 1H and 15N NMR. The chemical-shift behavior of the urea protons is consistent with a urea-acetate hydrogen-bonded complex involving both carboxylate oxygens and the urea hydrogens trans to the carbonyl oxygen with Kassoc = 120 ± 10. Line shape analysis of the temperature-dependent 1H NMR spectra show that ΔG⧧ for rotation about the C−N bond of urea changes only slightly from 11.0 ± 0.1 to 11.2 ± 0.1 kcal/mol on 1:1 molar addition of tetrabutylammonium acetate to a dilute solution of urea. A parallel investigation of the interaction of thiourea with tetrabutylammonium acetate gave a binding constant, Kassoc = 90 ± 10. The ΔG⧧ for rotation about the C−N bond of thiourea was found to increase from 13.5 ± 0.1 to 14.0 ± 0.1 kcal/mol on 1:1 addition of tetrabutylammonium acetate to a dilute solution of thiourea in dimethylformamide/dimethyl sulfoxide. Measurements were also made of the self-association of several ureas and of ΔG⧧ for rotation about both C(O)−N bonds of 1,1-dimethylurea.

Synthesis and Dynamic Stereochemistry of Di-1-azulenyl(1- and 2-naphthyl)methyl Hexafluorophosphates. Clear Evidence of the One-Ring Flip Mechanism of a Molecular Propeller by Controlling the Flipping Ring

Abstract In order to control the flipping ring of a molecular propeller and to obtain clear evidence of a one-ring flip mechanism, stable carbocations, di-1-azulenyl(1-naphthyl and 2-naphthyl)methyl cations, bis(3-methyl-1-azulenyl)(1-naphthyl (5b) and 2-naphthyl (6b))methyl cations, and bis(3,6-di-t-butyl-1-azulenyl)(1-naphthyl and 2-naphthyl)methyl cations were synthesized by hydride abstraction of the corresponding hydrocarbons. These cations showed extreme stabilities with high pKR+ values (10.3—12.7). The dynamic stereochemistries of 5b and 6b were studied by temperature-dependent 1H NMR spectra, which were analyzed by a flip mechanism. While the 1H NMR signals of 6b did not resolve satisfactorily, the low-temperature 1H NMR spectra of 5b showed that the flipping ring of the molecular propeller was controlled by a one-ring flip of the 1-naphthyl group. These analyses afforded clear evidence of the one-ring flip mechanisms of (1-azulenyl)methyl cations.

Synthesis and characterisation of the six-membered cyclopalladated complexes of 2-benzylbenzothiazole

The six-membered cyclopalladated complex di- μ-acetato-bis[2-(2′-thiazolylmethyl)phenyl- C 1, N]dipalladium(II), [{Pd( μ-O 2CMe) ( η 2-bbt)} 2] ( η 2-bbt = 2-(2′-thiazolylmethyl) phenyl- C 1, N), has been prepared by the reaction of palladium(II) acetate and 2-benzylbenzothiazole. Its di-μ-halogeno analogues [{Pd( μ-K) ( η 2-bbt)} 2], synthesised by the metathetical reaction with LiCL or NaX, undergo bridge-splitting reactions with some pyridine derivatives and thallium(I) acetylacetonate to yield corresponding mononuclear cyclopalladated complexes. The cyclopalladated feature and the six-membered structure have been confirmed by the 1H NMR spectra of the mononuclear pyridine complexes. Temperature-dependent 1H NMR spectra have been observed for [{Pd( μ-O 2CMe) ( η 2-bbt)} 2] and [Pd(acac) ( η 2-bbt)] and are attributed to inversions of acetato bridges and the boat-formed six-membered ring, respectively.

Synthesis and Dynamic Stereochemistry of Tris(2-methyl-1-azulenyl)methyl Cation and the Corresponding Methane Derivative. Evidence That the Conjugative Effect Largely Contributes to the Transition State of the Ring Flipping

Abstract The stable carbocation, tris(2-methyl-1-azulenyl)methyl hexafluorophosphate (4·PF6−) was prepared by a hydride-abstraction reaction of the corresponding methane derivative, tris(2-methyl-1-azulenyl)methane (5). The dynamic stereochemistry of 4 and 5 was studied based on the temperature-dependent 1H NMR spectra, which were analyzed by a flip mechanism; the steric effect of the three 2-methyl groups was also investigated by comparing it with that of the 3,3′,3″-trimethyl analogue, tris(3-methyl-1-azulenyl)methyl hexafluorophosphate (1b·PF6−). The threshold rotation mechanism for 4 was a two-ring flip, in contrast to a one-ring flip for 1b, and the activation energies for 4 (78.0 and 73.4 kJ mol−1) were higher than those for 1b due to increased crowding in the transition state for the rotation. Although the activation energies for 5 (49.9 and 43.2 kJ mol−1) were lower than those for 1b, the mechanism for 5 was also a two-ring flip. The mechanism was variable between one- and two-ring flip processes due to a steric effect of the three 2-methyl groups. These results indicate that a conjugative interaction between the central cation and the three azulene rings largely contribute to the transition state of the ring flipping as well as to the ground state.

Synthesis and Dynamic Stereochemistry of a Tri(1-azulenyl)methyl Cation Containing a Different Substituent on Each Azulene Ring

Abstract The first example of a tri(1-azulenyl)methyl cation (2) containing a different substituent on each azulene ring was synthesized, and the dynamic stereochemistry was studied by temperature-dependent 1H NMR spectra. The dynamic behavior of 2 supported a one-ring flip mechanism and the validity of the flip mechanism for the analysis of the conformational change of the tri(1-azulenyl)methyl cations.

Syntheses of Azulene Analogues of Triphenylmethyl Cation: Extremely Stable Hydrocarbon Carbocations and the First Example of a One-Ring Flip as the Threshold Rotation Mechanism for Molecular Propellers

Abstract A series of azulene analogues of triphenylmethyl cation (tri(1-azulenyl)methyl, di(1-azulenyl)phenylmethyl, and (1-azulenyl)diphenylmethyl hexafluorophosphates) were synthesized by hydride abstraction from the corresponding methane derivatives with DDQ. In order to examine the effect of substituents on the cations, and to enhance their stabilities, a series of cations bearing 3-methyl, 3-methoxycarbonyl, 3,6-di-t-butyl, 6-t-butyl, or 3-t-butyl groups on each of the azulene rings were also synthesized. Their pKR+ values showed that the stabilities of these cations dramatically increase with the number of azulene rings. Tris(3,6-di-t-butyl-1-azulenyl)methyl cation showed the highest pKR+ value (14.3) ever observed. The high stabilities of these cations were attributed to a large conjugative effect between the central cation and the azulene ring(s). The dynamic stereochemistry of these cations was also studied based on the temperature-dependent 1H NMR spectra, which were analyzed by a flip mechanism. Low-temperature NMR studies indicated that tri(1-azulenyl)methyl cations exist in two types of propeller conformations (symmetrical and unsymmetrical propellers, which have C3 (A) and C1(B) symmetries, respectively), and at higher temperature the NMR reflect the rapid isomerization. The lower activation energy of the process B → (or → A), compared with that of B → , indicates that the threshold rotation mechanism for the cation is a one-ring flip. This is the first example of a molecular propeller with a threshold rotation mechanism comprising a one-ring flip.

1H NMR study of self-association of actinomycin D in an aqueous solution

This paper presents the results of a proton magnetic resonance study (500 MHz) of self-association of actinomycin D (AMD) antibiotic in an aqueous solution. The equailibrium constants and thermo-dynamic parameters (ΔH, ΔS) of molecular association as well as the limiting values of proton chemical shifts of associate molecules were determined from the concentration and temperature dependences of1H NMR chemical shifts of AMD. The results were analyzed using dimeric and infinite-dimensional cooperative models of molecular self-association. The value of the cooperativity parameter indicates that AMD self-association is anticooperative, i.e., formation of aggregates larger than dimers is energetically unfavorable. The values of induced proton chemical shifts were used to determine the most probable mutual orientation of chromophores in AMD structure.

環状スルフランの構造，結合エネルギーおよび反応性

A series of 10-S-3 type sulfuranes, tetraazathiapentalenes (10a-i, 11a-f, 12, and 17) fused with pyrimidine ring and/or pyridine ring, were prepared by oxidation of the corresponding thioureas. The restricted internal rotation of the pyrimidine ring was observed by temperature dependent 1H NMR spectrum which gave the kinetic parameters of ΔG298_??_=16.6kcal/mol, ΔH=15.9kcal/mol, and ΔS_??_=-2.4 eu at 25°C for the rotational barrier in 10a. The substituent effect to the kinetic data can be reasonably explained in terms of electronic balance of the N-S-N hypervalent bond or the different degree of contribution of resonance canonical structures. Comparison of the rotational barrier of 10a with that of model compounds (18 and 24) suggests that hypervalent N-S-N stabilization in 10a is more than 6kcal/mol. Methylation of these 10-S-3 type sulfuranes (10a, 10d, and 11a) was investigated to clarify the electronic effect in these molecules. The crystal structure of the neutral symmetric sulfurane 10a reveals to be a planar molecule in which the sulfur atom was found to be in the same plane of the two pyrimidine rings. The S-N bond lengths 1.948(3) and 1.938(3) A are longer than the sum of the covalent bond radii, consistent with a bond order less than unity. The crystal structures of the dicationic symmetric sulfurane 17 and the unsymmetrical sulfurane 11a also show to be planar molecules. The S-N distances in 11a were different each other due to the electronic imbalance between the apical ligands. Semi-empirical calculation (AM 1) of these systems (10a and 17) indicate the expected electronic features of the hypervalent molecules (10-S-3 sulfuranes) and the small contribution of π-overlapping in the N-S-N bonds.

Syntheses of and metal cation oscillation in ionophoric biscalix [4] arenes

A series of biscalix[4]arenes (2n) doubly-bridged with oxyethylene chains at the lower rim have been synthesized. As in conventional ionophoric monocalix[4]arene derivatives, these compounds showed ion selectivity towards Na+ . The 1H NMR spectroscopic studies established that Na+ and K+ ions complexed with 2n oscillate intramolecularly between two calix[4]arene ionophoric sites. Two coalescence temperatures were found in the temperature-dependent 1H NMR spectra in the presence of these metal cations: the coalescence for the intermolecular metal exchange appears at a high temperature region whereas the coalescence for the intramolecular metal oscillation appears at a low temperature region. These two processes could be discriminated by the difference in the concentration dependence: the intermolecular process is concentration-dependent whereas the intramolecular process is concentration-independent. This is a unique dynamic system in which metal cations jump from one ionophoric site to another ionophoric site in biscalix[4]arenes in the NMR time-scale.

Synthesis of azophenolic crown ethers of C s symmetry incorporating cis-1-phenylcyclohexane-1,2-diol residues as a steric barrier and diastereotopic face selectivity in complexation of amines by their diastereotopic faces

Azophenolic crown ethers 1 and 2 of Cs symmetry incorporating cis-1-phenylcyclohexane-1,2-diol residues as a steric barrier have been prepared. Diastereotopic face selectivity in complexation with 2-methoxyethylamine, n-propylamine and ethanolamine was examined using temperature-dependent 1H NMR spectroscopy. Both bind ethanolamine stereoselectively to one of their diastereotopic faces; the prediction of which diastereoisomeric complex was preferentially formed is made on the basis of a CPK molecular-model examination of the complexes.

Dynamic stereochemistry of tri(2-methyl-1-azulenyl)methyl cation; steric effect of 2-methyl groups on rotational barriers and mechanism

The stable carbocation, tri(2-methyl-1-azulenyl)methyl hexafluorophosphate ( 2) was prepared. Steric effect of three 2-methyl groups was investigated by comparison with the 3,3′,3″-trimethyl analogue, tri(3-methyl-1-azulenyl)methyl cation ( 1). Dynamic stereochemistry of 2 was studied by temperature dependent 1H NMR spectra, which were analyzed by flip mechanism. The threshold rotational mechanism for 2 was a two-ring flip in contrast to a one-ring flip for 1, and the activation energies for 2 (18.6 and 17.5 kcal/mol) were higher than for 1 due to increased crowding in the transition state for the rotation.

Homoleptische 1,4-Diazadienverbindungen elektronenarmer Übergangsmetalle. Synthese, Eigenschaften und Strukturen von (DAD)2Ti und (DAD)3Ti [1] / Homoleptic 1,4-Diazadiene Compounds of Early Transition Metals. Syntheses, Properties and Structures of (DAD)2Ti and (DAD)3Ti [1

Syntheses and structures of homoleptic 1,4-diaza-1,3-diene (DAD) complexes of titanium are described. The spiranoid DAD complexes of formula (DAD)2Ti 3a-c are formed by reaction of TiCl4·2THF with the disodium salts Na2DAD 2a-c or by reduction of TiCl4·2TH F with magnesium in presence of two equivalents of DAD 1a-c . The endiaide complexes 3a-c are thermally stable, very sensitive to oxygen and water, and soluble in all com monorganic solvents. An X-ray crystal structure determination of 3 c reveals the titanium atom to be in a strongly distorted tetrahedral environment. The two five-membered metallacyclic rings are folded along the N,N′ axis. Temperature-dependent 1H NMR spectra of 3a-c indicate a rapid intramolecular inversion of this non-planar ring system in solution even at room temperature. Reduction of TiCl4·2THF with magnesium in presence of excess DAD 1d-f leads to the formation of the blue-violet complexes (DAD)3Ti 4d-f. At low temperatures three sterically crowded DAD ligands cause a rigid conformation as demonstrated by their unique lowtemperature 1H NMR spectra.