Variable-temperature 13C Research Articles

Solid-state magnetic resonance study of NaC 60(THF) 3

NaC 60(THF) 3 has been prepared via a new solution-phase route. The solid-state variable temperature ESR and 13C NMR studies on NaC 60(THF) 3 (in comparison with NaC 60(THF) 5) suggest that the unusual down-field chemical shift in C 60 − ( relative to C 60) can be understood partially on the basis of Pasquarello et al 's model.

Aryldiazenido complexes: structure, fluxionality, and properties of the iridium ethylene aryldiazenido complex [(η5-C5Me5)Ir(C2H4)(p-N2C6H4OMe)][BF4] and a comparison with the analogous nitrosyl complex [(η5-C5Me5)Ir(C2H4)(NO)][BF4

[Cp*Ir(C2H4)(N2Ar)][BF4] (1BF4; Ar = p-N2C6H4OMe) has been synthesized by reacting [ArN2] [BF4] with Cp*Ir(C2H4)2 at low temperature. An initial electrophilic attack of the incoming diazonium ion at iridium, followed by expulsion of C2H4, is postulated to account for the mild reaction conditions that are in sharp contrast to the usual inertness of the bis(ethylene) compound toward ligand substitution. The IR and nitrogen NMR data for 1BF4 and its 15Nα derivative unambiguously establish that the ArN2 ligand has the singly bent geometry in this complex in solution. The X-ray crystal structure confirms this for the solid state, and establishes that the plane of the aryldiazenido ligand is approximately perpendicular to the plane defined by the Ir atom and the centers of mass of the Cp* and ethylene ligands. An extended Hückel molecular orbital analysis of the singly bent aryldiazenido ligand has been carried out and satisfactorily accounts for the observed orientation of the ArN2 ligand. An analysis of the variable-temperature 1H and 13C NMR of 1BF4 indicates that both restricted rotation of the C2H4 ligand and a conformational isomerization of the aryldiazenido ligand are occurring, and ΔG≠270 for the ethylene rotation barrier is estimated at ≤ 51.5 ± 0.4 kJ mol−1. This is lower than the barrier of ΔG≠353 = 68.7 ± 0.2 kJ mol−1 determined previously for the analogous nitrosyl complex [Cp*Ir(C2H4)(NO)] [BF4] (2BF4) and it is suggested that in these half-sandwich complexes both NO and ArN2 function as single-faced π-acceptors, and in these circumstances ArN2 is the better π-acceptor. The ethylene in 1BF4 is readily displaced by PPh3 to give [Cp*Ir(PPh3)(N2Ar)][BF4] (3BF4). This reacts with NaBH4 to yield Cp*IrH(PPh3)(N2Ar) (4) in which the ArN2 ligand has switched to the doubly bent geometry, on the basis of the 15Nα NMR chemical shift data. Attempts to synthesize the corresponding chloro analogue 5 resulted in only the chloride salt of the singly bent ArN2 cation 3. For example, reaction of 3BF4 with HCl yields the aryldiazene complex [Cp*IrCl(PPh3)(NHNAr)] [BF4] (6), but deprotonation of this with Et3N yields 3Cl, not 5. Compound 1BF4 crystallized in the space group P21/n with a = 8.5780(10) Å, b = 20.5310(23) Å, c = 12.0310(15) Å, β = 93.500(10)°, and Z = 4. The structure was refined to Rf = 0.0281 on the basis of 2611 observed reflections with I°> 2.5σ(Io) in the range 2θ = 0–50° (Mo-Kα). Keywords: iridium, cyclopentadienyl, aryldiazenido, nitrosyl, ethylene.

Empirical correlation of 13C NMR chemical shift with orientational ordering in solid K nC 60(THF) m(n = 1–3)

K nC 60(THF) m (n=1–3, m=n) have been prepared via solution-phase route. Variable temperature 13C NMR data reveal that there exists a strong correlation of 13C NMR chemical shift with the degree of orientational order of C 60 n in these materials. The implications of this observation to the origin of down-field chemical shifts (relative to neutral C 60) in fullerene anions are discussed briefly.

Variable-temperature 13C magic-angle spinning nuclear magnetic resonance investigations on the interaction between Lewis acid sites and carbon monoxide in H-ZSM-5 zeolites

13C Magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopic measurements were carried out in order to investigate the interaction between Lewis acid sites in H-ZSM-5 and adsorbed carbon monoxide. A new measure for the “overall” Lewis acidity of zeolites could be derived from the 13C MAS NMR spectroscopic data. Further, the chemical shift of CO adsorbed on Brønsted acid sites and physisorbed CO could be determined to amount to δ B = 178 ppm and δ P = 186 ppm, respectively.

CONFORMATIONAL PREFERENCES OF SPIROCYCLIC PENTAOXYPHOSPHORANES VARYING IN RING SIZE

Abstract New bicyclic pentaoxyphosphoranes 1–3 containing ring sizes varying from five to seven membered were synthesized by oxidative addition of a quinone or a diol to a cyclic phosphite. Variable-temperature solution 1H and 13C NMR studies revealed the presence of dynamic intramolecular ligand exchange processes, one in which apical-equatorial ring interchange occurred between trigonal bipyramidal ground states and a higher temperature process supporting an exchange intermediate with the ring located diequatorially in a trigonal bipyramid. Activation energies for the latter process were determined. X-ray analysis supported the interpretation of the solution-state behavior and showed that saturated six-membered rings prefer a boat conformation occupying apical-equatorial positions in trigonal bipyramidal structures. The stability of the six-membered ring in this conformation is supported by the shorter P-O bond lengths found for this ring size compared to that for phosphoranes having five- and seven-mem...

Effect of ortho substituents on the internal rotation processes and conformational preferences of 1,2-diaryl-1,1,2,2-tetrachloroethanes: a 1H and 13C NMR variable temperature and X-ray structural study

The conformational behaviour of 1,2-diaryl-1,1,2,2-tetrachloroethanes, ortho,ortho′-disubstituted with CN (1), F (2) and CO2CH3(3), and of the mesityl derivative (4), has been studied in solution by variable temperature 1H and 13C NMR spectroscopy. The internal rotation processes around the exocyclic C–C and central C–C bonds appear to have similar energy barriers and thus behave differently from the compounds without ortho substituents, studied previously, which have a higher barrier for the interconversion of anti/gauche conformers. At low temperature, compounds 1–3 exist as mixtures of gauche and anti conformers with the former prevailing; compound 4 is almost exclusively in the gauche form. The solid state crystal and molecular structure has been determined for compounds 2 and 3. The molecule of compound 2 has point symmetry , which defines perfect staggering around the central C–C bond with the aromatic rings anti. The structure is affected by statistical disorder due to two alternate orientations of the phenyl rings. The crystal structure of compound 3 indicates one asymmetrical molecule in a gauche conformation. Statistical disorder involves both the methoxycarbonyl substituents at the phenyl rings; these latter display almost the same orientation with respect to the C(sp3) carbons, in both compounds.

Reactions of di- and poly-nuclear complexes 13. Synthesis, structure and fluxional behaviour of hydrido- and thiolato- or selenato-bridged complexes [Mo 2Cp 2(μ-H)(μ-ER)(CO) 4] (E  S, R  Me, tBu, Bz, or Ph; E  Se, R  Ph)

Oxidative addition of thiols or selenoles across the Mo 2 centre of [Mo 2Cp 2(CO) 4] gives complexes with bridging thiolate or selenate and hydride ligands [Mo 2Cp 2(μ-H)(μ-ER)(CO) 4](E  S, R  Me ( 4), tBu ( 5), Bz ( 6), Ph ( 7); E  Se, R  Ph ( 8)). The complex 4 is also formed on reaction of [Mo 2Cp 2(μ-SMe) 2(CO) 4] with vanadocene [VCp 2] with the possible involvement of a thioaldehyde, hydride derivative of the vanadocene. Crystals of 4 are monoclinic, space group C2/ c, with a = 17.225 (1), b = 8.418 (1) c = 22.960 (2) A ̊ , β = 90.346 (5)°, R = 0.023 for 4213 observed reflections. In the solid state, molecules of 4 contain a planar Mo 2(μ-H)(μ-S) core and the Cp ligands are cis with respect to the MoMo bond, the two CpMo(CO) 2 unites being related by an approximate plane of symmetry which contains the bridging H and S donor atoms and which is normal to the MoMo bond. The fluxional nature of complexes 4–8 in solution is shown from variable-temperature 1H and 13C NMR spectra to arise from cis-trans isomerisation.

Synthesis and171Yb-{1H} nuclear magnetic resonance spectra of ytterbium(II) aryloxides [Yb(OR′)2(L)n][(L)n=(OEt2)2, (thf)2, (thf)3, (pyridine)2or Me2PCH2CH2PMe2] and [{Yb(µ-OR′)(X)}2](X = OR′ or NR2)(R′= C6H2But2-2,6-Me-4, R = SiMe3, thf = tetrahydrofuran)

Crystalline homo- and hetero-leptic ytterbium(II) aryloxides derived from bulky 2,6-di-tert-butyl-4-methylphenol (R′OH) were prepared as follows: (a)[Yb(OR′)2(OEt2)2]1 from 2 R′OH and [Yb(NR2)2(OEt2)2]I(R = SiMe3); (b)[{Yb(OR′)(µ-OR′)}2]2 by desolvating 1 or from [{Yb(NR2)(µ-NR2)}2]II and 4 R′OH; and (c)[{Yb(NR2)(µ-OR′)}2]3 from II, and 2 R′OH or II and 2. Single-crystal X-ray diffraction studies established complex 1 to be monomeric with a distorted tetrahedral configuration and 2 to be dimeric with asymmetrically bridging –OR′ ligands. Complexes 1–3 were characterised by multinuclear (1H, 13C, 29Si and 171Yb) NMR spectroscopy and elemental (C, H, N) analysis. Variable-temperature 1H and 13C NMR spectral data for 2 led to ΔG‡ for the terminal/bridge –OR′ site-exchange process. The known aryloxide [Yb(OR′)2(thf)3]4 was obtained from 1 and an excess of tetrahydrofuran (thf); reacting 1 with the appropriate stoichiometric amount of thf, pyridine (py) or 1,2-bis(dimethylphosphino)ethane (dmpe) yielded [Yb(OR′)2(thf)2]5, [Yb(OR′)2(py)2]6 or [Yb(OR′)2(dmpe)]7, respectively. The δ[171Yb-{1H}] NMR spectral data for complexes 4–7 showed that displacement of diethyl ether from 1(δ 238 at 193 K) by another neutral coligand had a marked effect on the magnitude of the chemical shift, ranging from δ 286 at 233 K for 4 to δ 999 at 296 K for 7.

NMR and Molecular Modeling-Based Conformational Analysis of Some N-Alkyl 1- and 2-Benzazepinones: Useful Central Nervous System Agent Design Motifs

Variable temperature 1H NMR lineshape analyses and 13C NMR spin-lattice relaxation time studies were undertaken to characterise the conformationally dynamic N-alkyl 1- and 2-benzazepinones 1 and 2. For 1, dynamic interchange between two minor-image puckered forms of the azepine ring occurs with a barrier of 56 kJ mol−1. There is a significantly greater degree of ring flexibility in the case of 2. Molecular modelling studies were used to examine the degree of sidechain flexibility in these compounds.

Chain Segmental Motion and Side-Chain Internal Rotations of Poly(1-naphthylalkyl acrylate)s in Solution Studied by 13C Nuclear Magnetic Relaxation

C longitudinal relaxation times and nuclear Overhauser enhancements were measured as a function of temperature in three magnetic fields for a series of poly(1-naphthylalkyl acry1ate)s in 1,1,2,2- tetrachloroethane-dz. Variable-temperature 13C transverse relaxation time measurements at 75.4 and 125.7 MHz also were undertaken. The relaxation data have been described by using various models for main-chain segmental motion and side-chain internal rotations. Among these, the Dejean-Laupretre-Monnerie (DLM) model offered the best description of segmental motion for the poly(1-naphthylalkyl acrylate) chains. Internal rotations about the side-chain bonds, including naphthyl internal motion, were described by hindered rotations superimposed on segmental motions, the latter being described by the DLM model. An attempt was made to correlate rates and activation energies associated with the various modes of reorientation in the aforementioned polymers with the rates of excimer formation between the side-chain naphthalene chromophores.

Complexes of the platinum metalsXLV. Synthesis and x-ray crystal structure of hexacarbonyl bis (μ- N,N′-diphenylacetamidinato)diruthenium (I/I)

The ruthenium(I) amidinato complex [Ru{PhNC(Me)NPh}(CO) 3] 2 has been prepared by heating [Ru 3(CO) 12] with the free amidine, PhNC(Me)NHPh, in boiling benzene under a carbon monoxide atmosphere, and by heating [{Ru(O 2CMe)(CO) 2} 2] n with free amidine and triethylamine in acetonitrile under carbon monoxide. The structure of [Ru{PhNC(Me)NPh}(CO) 3] 2 has been determined by X-ray diffraction methods. The binuclear complex has two Ru(CO) 3 units linked by an RuRu bond [2.714(1) Å] and a cis pair of bridging amidinato ligands. Variable temperature 1H and 13C { 1H} NMR spectra show evidence of hindered rotation about the Nphenyl bonds of the amidinate ligands. An attempt to prepare the corresponding propionamidinate derivative gave a product which is tentatively formulated as [Ru{PhNC(Et)NPh}(CO) 2(HO 2CMe)] 2 on the basis of analytical and spectroscopic evidence.

Indolizine studies. Part 3. Synthesis and dynamic NMR analysis of indolizine-2-carboxamides

Reaction of indolizine-2-carboxylic acids with primary and secondary amines in the presence of 1,1′-carbonyldiimidazole provides convenient and efficient access to the corresponding secondary and tertiary amides and offers a significant improvement on previously reported methodology. The N–CO rotational barriers in the symmetrically substituted tertiary amides have also been explored using variable-temperature 1H and 13C NMR spectroscopy.

Evidence for π bonding in the boron–thiolate compounds (2,4,6-Me3C6H2)2B(SPh) and (2,4,6-Pri3C6H2)B(SPh)2

The use of the bulky aryl substituent groups 2,4,6-Me3C6H2 and 2,4,6-Pri3C6H2 has allowed the structural characterization of two monomeric boron-sulfur compounds (2,4,6-Me3C6H2)2B(SPh)1 and (2,4,6-Pri3C6H2)B(SPh)22. The crystal structures of 1 and 2 show a close alignment between the boron and sulfur p orbitals that is consistent with a π interaction. Furthermore, the B–S distances in 1[1.790(6)] and 2[1.801(6)A] are slightly shorter than the sum of the covalent radii (with allowance made for ionic effects) of boron and sulfur. Variable-temperature 1H and 13C NMR data for 2 indicate an average barrier to rotation around the B–S bond of ca. 12 kcal mol–1. This value is significantly less than the 18.4 kcal mol–1 reported earlier for 1. The lower rotational barrier observed in 2 is consistent with the delocalization of the B–S π bond over the three atom BS2π system which has a similar electronic arrangement to that of an allyl anion. Crystal data at 130 K: 1, triclinic, space group P, a= 7.851(5), b= 11.685(6), c= 13.096(7)A, α= 63.52(2), β= 73.93(2), γ= 74.54(2)°, Z= 2, R= 0.073; 2, monoclinic, space group P21/c, a= 18.373(3), b= 12.713(2), c= 10.844(2)A, β= 99.96(1)°, Z= 4, R= 0.072.

Preparation and reactions of [MoH(η3-C4H7)(Ph2PCH2CH2PPh2-κ2P)(Ph2PCH2CH2PPh2-κP)]: the intramolecular interconversion between η3-2-methylallyl and η4-trimethylenemethane ligands

The complex [MoH(η3-C4H7)(dppe-κ3P)(dppe-κP)](dppe = Ph2PCH2CH2PPh2) has been prepared. Variable-temperature 1H, 13C and 31P NMR spectroscopy demonstrates that this 16-electron, η3-2-methylallyl species (the dominant form at 25 °C) rapidly interconverts, intramolecularly, to the 18-electron, η4-trimethylenemethane species, [MoH2(η4-C4H6)(dppe-κ2P)(dppe-κP)](the dominant form at –80 °C). Protonation of [MoH(η3-C4H7)(dppe-κ2P)(dppe-κP)] with HCl at 25 °C gives stoichiometric yields of [MoH2,Cl2(dppe-κ2P)2] and ButCH2C(Me)CH2. Stopped-flow spectrophotometric studies of the protonation reaction indicate that the alkene formation involves rapid protonation of the molybdenum to give [MoH2(η3-C4H7)(dppe-κ2P)(dppe-κP)]+, which undergoes rate-limiting intramolecular hydrogen migration to form [MoH(η2-Me2CCH2)(dppe-κ2P)2]+. Subsequent formation of ButCH2C(Me)CH2 occurs rapidly and is not amenable to kinetic analysis. However, the dimerisation is too fast to be catalysed by the excess of HCl alone, and a mechanism involving formation of ButCH2C(Me)CH2 at the molybdenum centre is proposed.

A detailed study defining the extent of preorganization of an aza macrocycle containing the phenyldinaphthomethane subunit (a three bladed propeller) using dynamic NMR and molecular dynamics

The aza-propeller crown 4,7,10-tritosyl-4,7,10-triaza-1,13-dioxacyclooctadecane 1 has been examined by variable temperature 13C NMR studies, and the movement of each segment of the molecule (ArOCCN, NCCN, propeller) was determined kinetically. Several sets of coalescences were found to occur at the same activation energy, which was also that found for the flip of the propeller segment. This movement of the macrocycle as a whole is a relatively slow process (ms scale) compared to the torsional movements round the crown ether ring (ps scale). These torsional movements were explored by molecular dynamics (MD) modelling; the macrocyclic ring is mobile when simulated at 200 K. The tosyl groups are in permanent motion although none twists by more than a few degrees round the N–S torsion. The segments next to the propeller (which are both gauche in the crystal structure) converted from gauche to anti, while the TsNCCNTS segments remained anti throughout.

General Synthesis, Spectroscopic Properties, and Dipole Moments of 2-Substituted Tropothiones

Abstract Several 2-substituted tropothiones (2,4,6-cycloheptatriene-1-thiones) having an electron-donating group (Me, Ph, NH2, NHMe, OH, OMe, or SMe) were synthesized by direct sulfurization from the corresponding tropones in a one-pot reaction. In contrast to the instability of the parent tropothione (1a), most of these 2-substituted derivatives were insensitive to air and were thermally stable compounds which can be handled at room temperature. These tropothione derivatives were characterized by spectroscopic and dipole moment measurements. The IR spectra showed ν(C=S) vibrations at around 1100—1040 cm−1. The UV-vis spectra displayed three π–π* transitions accompanied by n–π* transitions. The 400-MHz 1H NMR spectra of the thiones showed that only protons at the 7 position resonate well-separated downfield from the other ring protons due to the strong C=S anisotropy. In the 13C NMR (100.6 MHz) spectra, the C=S carbon signals appeared at < δ = 183 and the other ring carbons resonated at δ = 154—112. Variable-temperature 13C as well as 14N NMR spectroscopic studies clarified that 2-amino- and 2-(methylamino)tropothiones exist as an equilibrium mixture of the thione and enethiol structures in an approximate ratio of 64 : 36. This is different from the results of a previous investigation showing that the structures exist as the thione form exclusively. Dipole moments of some derivatives of 1a were measured and compared with those of the corresponding tropones. These tropothione derivatives obtained were more ionic than both the parent compound 1a and the corresponding tropones.

Ring puckering of the pyrrolidine ring of poly ( l-proline) form I as studied by variable-temperature high-resolution 13C NMR spectroscopy

A number of studies have been made on the structural role of proline residues in proteins and naturally occurring cyclic peptides because the flexibility of the proline ring influences the degree of backbone order in proteins [l]. Hence, information on the nature of the side-chain conformation and the mobility in poly(r_-proline) is of much interest. It is well known that poly(L-proline) takes two kinds of specific main-chain forms, form I and form II, in the crystalline state [2-81. In form I the imide group is in the cis configuration and in form II it is in the trans conformation. In our previous work 191, we measured high-resolution 13C NMR spectra of poly(L-proline) form II in the solid state over a wide range of temperatures and found the existence of two kinds of energetically stable conformations for the pyrrolidine ring undergoing no chemical exchange. The aim of this work is to determine the sidechain conformation and the molecular motions of poly(L-proline) form I in the crystalline state

Infrared, Raman and NMR spectra, conformational stability, and ab initio calculations of divinylmethoxyborane

The infrared spectra (4000–50 cm −1) of gaseous and solid divinylmethoxyborane, (CH 2=CH) 2BOCH 3, as well as the Raman spectra (3500–20 cm −1) of the liquid and solid have been recorded. Qualitative depolarization values have been obtained from the Raman spectrum of the liquid. All normal modes, except the torsions, have been assigned based on infrared band contours, depolarization values, group frequencies, and normal coordinate calculations. From a comparison of the spectra in the fluid and solid states, it is concluded that the molecule exists predominantly in a single conformation in all physical states. Frequencies and potential energy distributions for the normal modes have been calculated with the 3–21G basis set. A comparison of these calculated frequencies to the observed spectra is consistent with the predominant form having a “planar” heavy atom skeleton with C s , symmetry. From the variable low temperature 13C NMR data, a barrier to rotation about the B-O bond of 10.1 ± 0.1 kcal mol −1 has been determined, which is in excellent agreement with a barrier of 8.5 kcal mol"1 obtained from ab initio calculations. Structural parameters, conformational stability, and barriers to internal rotation have been obtained from ab initio Hartree-Fock gradient calculations employing both the 3–21G and 6–31G * basis sets. The results are compared to the corresponding data for some similar organoboranes.

Order–disorder transitions in adamantane derivatives: vibrational spectroscopic and 13C NMR studies of 1-chloroadamantane

The order–disorder behaviour of 1-chloroadamantane (1-C10H15Cl) has been investigated by differential scanning calorimetry, and variable-temperature vibrational and 13C NMR spectroscopy. The factor group splittings in the vibrational spectra are in accord with the known crystal structures of the two phases. The 13C spin-lattice relaxation times and dipolar dephasing measurements have been analysed to give the barriers to rotation in both phases and to determine the nature of the rotations in each phase. In the ordered phase, the motion is limited to rotation about the molecular axis. In the disordered phase, additional motions occur about axes through the tertiary carbon atoms.

NMR EXCHANGE STUDIES ON THE COMPLEXES cis-[Rh(CO)2(pyridine N-oxide)(X)] (X = Cl, Br)

Abstract 13C nmr spectra of the complexes cis-[Rh(CO))2(pyO)(X)] (X = Cl, Br; pyO = pyridine N-oxide) are discussed. Variable temperature 13C nmr spectroscopy has been used to monitor the exchange process occurring in acetone-d 6 solution. Rate constants for the exchange reaction have been determined by complete band shape analysis. The enthalpy of activation for the chloro- and bromo-complexes was found to be 53.3 ± 1.0 kJ mol−1 and 48.3 ± 0.8 kJ mol−1 respectively. The entropy of activation was found to be 5.5 ± 0.6 J mol−1 K−1 and -11.0 ± 0.3 J mol−1 K−1, for the chloro- and bromo-complexes, respectively. It is suggested that the fluxionality of the molecules results from an intermolecular exchange of pyridine N-oxide rather than from loss of carbon monoxide. A mechanism for the exchange process is proposed.