NMR Bandshape Research Articles

ChemInform Abstract: Syntheses and Dynamic Nuclear Magnetic Resonance Studies of Complexes of Trimethylplatinum(IV) Iodide with Ditelluroether Chelate Ligands.

Trimethylplatinum(IV) iodide complexes with ditelluroether ligands, namely [PtlMe3(L–L)][L–L = MeTe(CH2)3TeMe, PhTe(CH2)3TePh or o-C6H4(TeMe)2] have been isolated as pure solids. 195Pt-{1H} and 125Te-{1H} NMR studies show that ambient-temperature solutions of the complexes with aliphatic ligand backbones consist predominantly of DL isomers. The complex [PtlMe3{MeTe(CH2)3TeMe}] also exists in significant amounts of both meso forms. In contrast, the meso-1 form is present in only very low abundance in [PtlMe3{PhTe(CH2)3TePh}] and [PtlMe3{o-C6H4(TeMe)2}]. Pyramidal tellurium inversion interconverts these isomeric species and precise rates of this process were measured at near-ambient temperatures by 195Pt-{1H} and 125Te-{1H} NMR two-dimensional exchange spectroscopy experiments, or by 1H one-dimensional NMR bandshape analysis at above-ambient temperatures in the case of [PtlMe3{o-C6H4(TeMe)2}]. Tellurium pyramidal inversion-energy barriers [ΔG‡(298.15 K)] were in the range 70–84 kJ mol–1, reflecting the nature of the the substituent at Te and the ligand backbone. Comparison of the tellurium inversion energy in [PtlMe3{MeTe(CH2)3TeMe}] with data for Se and S in analogous complexes of chalcogen ligands establishes the trend in inversion energies, namely Te > Se > S, on a firm quantitative basis.

Molecular modelling studies of side-chain rotation in substituted triazine rings

Molecular modelling computation using the gaussian 94 package of programs was carried out to estimate four different barriers to internal rotation about amine nitrogen to sp 2 carbon bonds for two compounds with triazine rings. Whereas two of the processes appear to retain a planar side-chain nitrogen environment throughout the internal rotation, the other two involve pyramidalisation. In the latter two cases, after the barrier is passed, the process may or may not include a transient inversion at nitrogen. The internal rotation for cyclotriazine systems is discussed in relation to those for aniline and formamide. The four barrier magnitudes calculated for the triazines are in the same order as (though somewhat larger than) those measured by variable-temperature NMR bandshape analysis for a relevant compound.

Stereochemically non-rigid transition metal complexes of 2,6-bis(phenyliminomethyl)pyridine and derivatives. Crystal structure of fac-[ReBr(CO) 3(PMTFA)] (PMTFA=2,6-pyridylene-bis-(methylene-2-trifluoromethylaniline)

The Schiff bases 2,6-bis(phenyliminomethyl)pyridine and its derivatives (L) form bidentate chelate complexes with transition metals Re I, Pt IV and Pd II of general formulation fac-[ReBr(CO) 3L], fac-[PtXMe 3L] (X=Cl, Br or I), and cis-[Pd( p-CF 3C 6F 4) 2L]. In solution these complexes are stereochemically non-rigid, undergoing 1,4-metallotropic shifts and restricted rotations of the substituted aromatic rings attached to the metal-coordinated imino nitrogen. Rates and activation energies of these internal motions were measured by one-dimensional NMR bandshape analysis and two-dimensional EXSY NMR experiments over a range of solution temperatures. Free energy values, Δ G ‡ (298.15 K), for the fluxional shifts were in the range 76–97 kJ mol −1 with the Pd II complexes having the lowest energies. An X-ray crystal structure of fac-[ReBr(CO) 3PMTFA] (PMTFA=2,6-pyridylene-bis(methylene-2-trifluoroaniline)) showed the pendant imine function to be in an E-conformation with its nitrogen trans to the pyridyl nitrogen.

Solution fluxionality of some pyridine-2,6-dialdehydes, -diketones and -diesters when acting as bidentate ligands in rhenium(I) and platinum(IV) complexes. Crystal structure of [ReBr(CO) 3L] (L=methylethyldipicolinate)

2,6-Disubstituted pyridines, where the substituents are aldehyde, ketone or ester functions, form bidentate chelate complexes with the transition metal moieties fac-Re IX(CO) 3 (X=halogen). 2-Substituted pyridines, where the substituents are aldehyde or ester functions, form similar types of complexes with the isoelectronic transition metal moieties fac-Re IX(CO) 3 and Pt IVXMe 3. The fac-Re IX(CO) 3 complexes of the 2,6-disubstituted pyridine ligands were shown by 1H NMR spectra to undergo metallotropic shifts whereby the Re coordination switches between adjacent ON pairs of the ONO ligand donor set. Rates and activation energies of these fluxional shifts were measured by dynamic NMR bandshape analysis. Magnitudes of Δ G ‡ (298.15 K) were in the range 59–64 kJ mol −1 for the diketone and diester ligands. The dialdehyde ligand, 2,6-pyridinedicarboxaldehyde, formed an appreciably less-stable Re I complex that was highly fluxional and showed a tendency to dissociation at ambient solution temperatures. The unsymmetrical diester ligand, methylethyldipicolinate, formed two distinct Re I complex species in solution, in the approximate abundance ratio of 2:1, the more abundant structure involving coordination to the carbonyl of the ethyl ester function. This particular complex forms exclusively in the solid state and an X-ray crystal structure of [ReBr(CO) 3L] (L=methylethyldipicolinate) is reported.

The solid- and solution-state structures of 2-nitrosopyridine and its 3- and 4-methyl derivatives

2-Nitrosopyridine, 1, 3-methyl-2-nitrosopyridine, 2, and 4-methyl-2-nitrosopyridine, 3, exist in organic solvents as monomer–azodioxy dimer equilibria with the dimers predominating at ambient temperatures. In the case of compounds 1 and 3 only the Z-dimers co-exist with the monomers, whereas for compound 2 both Z- (major) and E- (minor) dimers are present with the monomer. Variable temperature 1H NMR bandshape analysis and 2D-EXSY spectra of 1–3 provided thermodynamic and kinetic data for the dissociation equilibria Z- (or E-) dimer ⇌ 2 monomer, ΔH values being in the range 53–58 kJ mol−1 and ΔG‡ (298.15 K) values in the range 70–82 kJ mol−1. An X-ray crystal structure of 1 identified the compound as a Z-dimeric species with the two pyridine rings twisted by 62.7° and their heterocyclic N atoms pointing towards each other.

Synthesis and dynamic NMR studies of stereochemical non-rigidity in rhenium(I) complexes of 2,6-bis[1-(phenylimino)ethyl]pyridine derivatives

The complexes fac-[ReCl(CO) 3L] [L= ortho or para substituted methyl or tbutyl aromatic derivatives of 2,6-bis[1-(phenylimino)ethyl]pyridine (BIP)] were synthesised. In solution all complexes exist as bis-imine E, E and E,Z isomers, with the former greatly predominating (>85% in most cases). 1,4-Metallotropic shifts of the ReCl(CO) 3 moiety between the ligand N donor pairs occur in all cases. In the 1H NMR spectra of the methyl- and the para- tBu aromatic derivatives of BIP these shifts were detected as exchange between equivalent E, E forms of the complexes. In the case of the ortho- tBu derivative, where the E, Z-isomer was more in evidence (21% at 303 K), E, E⇌ E, Z isomerisation occurs rather more readily than the accompanying metallotropic shifts. Rates and activation energies for the detected dynamic processes were based on NMR bandshape analysis. The results indicated that Me and tBu substitution of the phenyl rings in BIP had a negligible effect on the Re IN 1,4-metallotropic shifts, whereas ortho substitution of the rings, particularly by tBu had a considerable influence on the relative proportions of E,E and E,Z conformers, presumably because of steric effects.

13C, 15N and 31P NMR study of the disproportionation of cyanogold(I) complexes: [R 3PAu 13C 15N

The disproportionation of cyanogold(I) complexes of general formula [R 3PAu 13C 15N] forming [(R 3P) 2Au] + and [Au( 13C 15N) 2] − ions has been investigated using 13C, 15N and 31P NMR spectroscopy for a series of phosphines with R=cyclohexyl, i-propyl, Et, Me, cyclohexyl/diphenyl, o-tolyl, p-tolyl, m-tolyl, p-tolyl/diphenyl, allyl/diphenyl, phenyl, tri(cyanoethyl) CEP, and 1-naphthyl. The 13C NMR of the 13C 15N group in these complexes exhibited two distinct resonances, one due 13C in the starting [R 3PAu 13C 15N] complex and the second in the [Au( 13C 15N) 2] − anion. The 31P NMR spectra revealed two 31P resonances due to [R 3PAu 13C 15N] complex, and the [(R 3P) 2Au] + cation. The 15N NMR revealed only an averaged resonance due to [R 3PAu 13C 15N] and [Au( 13C 15N) 2] − anion, except in the cases of [Me 3PAu 13C 15N] and [Et 3PAu 13C 15N] where two resonances were observed. The coupling constants, 1J( 13C– 15N), 2J( 31P– 13C) and 3J( 31P– 15N) were obtained for all complexes and the free energies of activation for ligand disproportionation were determined using 31P–{ 1H} NMR band shape analysis.

A dynamic NMR study of sulphur inversion in transition metal complexes of 2,6-dithiaspiro[3.3]heptane (2,6-DTSH), 2,6-DTSH-2-oxide and 2,6-DTSH-2,2-dioxide

The following complexes were synthesised [W(CO) 5L 1], [{W(CO) 5} 2L 1], [W(CO) 5L 2], [W(CO) 5L 3], [{PdCl 2(PPh 3)} 2L 1], [PdCl 2(PPh 3)L 2] and [PdCl 2(PPh 3)L 3] where L 1=2,6-dithiaspiro[3.3]heptane (2,6-DTSH), L 2=2,6-DTSH-2-oxide and L 3=2,6-DTSH-2,2′-dioxide. In solution these complexes exhibit pyramidal inversion of the metal-coordinated sulphur atom(s), rates and activation energies of which were evaluated by total NMR bandshape analysis. Δ G ‡ values were in the range 45–50 kJ mol −1. These magnitudes are discussed in terms of ring geometry distortions and sulphur lone pair interactions.

Trimethylplatinum (IV) complexes of anionic O\\O and N\\O donor ligands : synthesis, NMR and fluxional behaviour

Reaction of pentane-2,4-dione, pyridine-2-carboxylic acid or pyridine-2,6-dicarboxylic acid with trimethylplatinum(IV) gives dimeric complexes of general formulae fac-[PtMe 3L] 2, in which the ionised ligand acts in a chelating and a bridging fashion. High-resolution solid-state 19;Pt NMR data shows that the two platinum atoms are equivalent ; the chemical shielding anisotropy and the principal components of the shielding tensor are reported. The complexes are soluble in co-ordinating solvents, yielding monomeric species of general formulae fac-[PtMe 3L(solvent)], which are fluxional. The pyridine adducts, fac-[PtMe 3L(py)] (L = pentane-2,4-dionato or pyridine-2-carboxylato), are also stereochemically non-rigid. The energetics of the dynamic processes have been studied by standard 1H NMR band shape analysis techniques ; Δ G ‡ (298 K) is in the range 69–86 kJ mol −1. Solid-state 13C, and solution-state 13C and 19;Pt NMR data are also reported.

Structural investigations of C-nitrosobenzenes. Part 2.1 NMR studies of monomer–dimer equilibria including restricted nitroso group rotation in monomers

Energy barriers associated with rotation of nitroso groups in monomeric nitrosobenzene and eleven monomeric substituted nitrosobenzenes in solution have been measured by total 1H NMR bandshape analysis. ΔG ‡ (298.15 K) values for the rotations are in the range 31–41 kJ mol–1. The values for 4-substituted compounds correlate well with the Hammett σp+ parameter for the substituent. The experimental energy barrier for nitrosobenzene is compared with theoretical calculations. Monomer–dimer equilibria of these compounds in solution have been investigated, with ΔH, ΔS and ΔG data calculated for the dissociation of both Z- and E- azodioxy dimers to monomers. Kinetic data, based on time-dependent and two-dimensional exchange spectroscopy (2D-EXSY) NMR measurements, have been obtained for the dissociation of 3-methylnitrosobenzene and 3,5-dimethylnitrosobenzene dimers. Energy profiles for the ground and transition states of both dimers and monomers (including exchange between both their rotameric forms) are discussed.

1,2-Metallotropic shifts in tricarbonylrhenium(I) complexes of pyridazine (pydz): a dynamic NMR investigation of the effects of cis-chelating ligands

Complexes of general formula fac-[Re(CO) 3(L–L)(pydz)][BF 4] (L–L=neutral bidentate chelate ligand; pydz=pyridazine) were prepared in high yield from fac-[ReBr(CO) 3(L–L)]. The pyridazine ligand coordinates to the metal moiety in a monodentate fashion, and undergoes a facile intramolecular 1,2-Re–N shift. The kinetics of the 1,2-metallotropic shift were measured by either one-dimensional NMR bandshape analysis or two-dimensional exchange spectroscopy. The free energies of activation, Δ G ≠ (298 K), were found to be dependent on the nature of the chelate ligand, and were in the range 77–87 kJ mol −1. The origins of the chelate ligand effects are discussed.

Studies in nitrosopyrazoles. Part 2.1 Solution and solid-state NMR studies of 3,5-disubstituted and 1,3,5-trisubstituted-4-nitrosopyrazoles. Crystal structures of 1,3,5-trimethyl-4-nitrosopyrazole and 3,5-di-tert-butyl-4-nitrosopyrazole

Variable temperature NMR studies of a range of 3,5-disubstituted (R = R′ = Me, But) and 1,3,5-trisubstituted (R, R′, R″ = Me, CF3, Ph or Bui)-4-nitrosopyrazoles have led to the identification of individual rotational isomers at low solution temperatures arising from the slowing down of the rotation of the nitroso function with respect to the pyrazole ring. Rotational energy barriers [ΔG‡ (298.15 K)], calculated by total NMR bandshape analysis, were in the range 38–65 kJ mol-1. The –NO rotation process also occurs in the solid state and leads to disorder in the crystal structures of 1,3,5-trimethyl-4-nitrosopyrazole and 3,5-di-tert-butyl-4-nitrosopyrazole. The 1-H-3,5-disubstituted-4-nitrosopyrazoles exhibit rapid annular tautomerism in solution.

Nitrogen inversion and N–O bond rotation processes in di-and tri-substituted hydroxylamines. A dynamic NMR study

The barriers to inversion in several acyclic di- and tri-substituted hydroxylamines are determined by 1H NMR band shape analysis. The barriers range from 49.1 to 66.8 kJ mol–1 and are discussed in terms of a conformational process which involves nitrogen inversion and rotation around the N–O bond. The N–benzyl group with an ortho hydroxy substituent increases the nitrogen inversion barrier by 10 kJ mol–1, which indicates the requirement of breaking of the intramolecular hydrogen bond prior to inversion.

15N AND31P NMR STUDIES OF CYANO(TRIALKYL/TRIARYL)PHOSPHINE GOLD(I) COMPLEXES

Abstract The ligand scrambling reaction of R3PAuC15N to form (R3P)2Au+ and Au(C15N)2 − has been studied (by 15N and 31P NMR spectroscopy) for R = Me, Et, i-Pr, and Ph. 31P NMR showed two resonances due to R3PAuCN and (R3P)2Au+ species, while 15N NMR showed only an averaged resonance due to R3PAuC15N and Au(C15N)2 − species, except for Et3PAuC15N, for which two separate resonances were detected. R3PAu13C15N (where R = Me, Et and Ph) complexes were also prepared and 2J(31P-13C) as well as 3J(31P-15N) constants were measured. The free activation energy for ligand scrambling in Ph3PAuCN was determined by 31P NMR band shape analysis to be 39.7 kJ/mol−1.

Conformational studies of transition metal carbonyl complexes of 1,5-dithiacyclooctane (1,5-DTCO) and crystal structure of [ReCl(CO) 3(1,5-DTCO)

A new synthesis of the mesocyclic ligand 1,5-dithiacyclooctane (1,5-DTCO) is reported. Complexes of type [{W(CO) 5} n (1,5-DTCO)] ( n= 1,2) were isolated and their conformational and exchange dynamics in solution examined by NMR. The complex [W(CO) 4(1,5-DTCO)], where the ligand acts as a bidentate chelate, undergoes facile boat-chair⇌chair-boat ring conformational exchange, NMR bandshape analysis of the motion yielding an activation energy, ΔG ‡ (298.15 K), of 25.1 ± 0.2 kJ mol −1 . The complexes [ReX(CO) 3(1,5-DTCO)] (X = Cl, Br) were isolated. An X-ray crystal structure of the X = Cl complex showed that the ligand adopts a boat-chair conformation, with a trans-annular S…S contact distance of 3.277 Å. This ring conformation is retained in solution.

The structure of trans-1,4-diisocyanocyclohexane in solution, in the solid state, and as a ligand bridging bulky tungsten(II) complexes

The conformational structures of the bridging ligand trans-1,4-diisocyanocyclohexane were determined in solution, as a solid, and as a ligand bridging bulky seven-coordinate tungsten(II) complexes. In solution CNC6H10NC exists as an equilibrium mixture of diaxial and diequatorial conformers, with the diequatorial conformer preferred in a 2 : 1 ratio. Infrared data indicate that the bulk solid does not consist of the diequatorial conformer, and a single-crystal study verifies that the diaxial conformer is preferred in the solid state. The crystal data found for CNC6H10NC are: space group P21/n, Z = 2, a = 5.996(1) Å, b = 7.203(2) Å, c = 8.972(3) Å, β = 96.89(2)°, and V = 384.7(2) Å3. 13C NMR bandshape analysis provides a low activation barrier (40.7(4) kJ mol−1) in CH2Cl2 solution for diaxial—diequatorial conversion. In spite of the accessibility of both conformers in solution, the ligand unexpectedly adopts the diaxial conformation in the bimetallic complex (Br2(PEt3)2(CO)2W)2CNC6H10NC. The crystal data found for (Br2(PEt3)2(CO)2W)2CNC6H10NC are: space group Pca21, Z = 4, a = 17.162(5) Å, b = 16.226(6) Å, c = 18.233(3) Å, and V = 5077.3(25) Å3.

1,2-Metallotropic shifts in trimethylplatinum(IV) and tricarbonylrhenium(I) halide complexes of pyridazine and 4-methylpyridazine

Pyridazine (pydz) and 4-methylpyridazine (4Me-pydz) form stable bis(monodentate) complexes of general formulae fac-[PtXMe3L2](X = Cl, Br or I) and fac-[ReX(CO)3L2](X = Cl, Br or I). These complexes in above-ambient temperature solutions exhibit 1,2-fluxional shifts between the nitrogen donor pairs of each ring. All linkage isomers of the pydz complexes are equivalent whereas three NMR-distinct linkage isomers, namely (1,1), (1,2)/(2,1) and (2,2) species, occur in solutions of the 4Me-pydz complexes. Fluxional exchange kinetics have been measured by variable-temperature 1H NMR bandshape analysis and two-dimensional exchange spectroscopy (EXSY). Platinum-195 NMR data have been obtained for the complexes [PtXMe3(4Me-pydz)2](X = Br or I) and their fluxional dynamics investigated by 195Pt two-dimensional EXSY and total bandshape analysis. Energy barriers, ΔG‡(298.15 K) for the 1,2-M–N fluxion are in the ranges 69–74 kJ mol–1 for the PtIV complexes and 84–90 kJ mol–1 for the ReI complexes. The crystal structure of [PtClMe3(pydz)2] shows the cis pydz rings oriented in a propeller arrangement with their unco-ordinated nitrogens oriented away from the cis positioned chlorine.

Dynamic NMR studies of ring rotation in substituted ferrocenes and ruthenocenes

Variable temperature 1H and 13C{ 1H} NMR studies on 1,1′,3,3′-tetra(alkyl)-ferrocenes and -ruthenocenes (alkyl = t-pentyl,t-butyl) have provided accurate barrier energies for restricted rotations of the substituted cyclopentadienyl rings. Energies (Δ G† (298.15 K)) are in the range 40–57 kJ mol −1 and are dependent on the ‘sandwich’ metal (Fe ⋙ Ru) and alkyl substituent (t-pentyl < t-butyl). Ring rotation in 1,1′,3.3′-tetra(phenyl)ferrocene was too rapid for measurement even at 173 K. Spectral changes were analysed primarily on the basis of exchange between a mirror pair of structures having both 5-membered rings eclipsed and the alkyl substituents staggered. In the case of 1,1′,3,3′-tetra(t-butyl)ferrocene a pair of staggered ring rotamers also contributed to the observed NMR bandshape changes.

Syntheses and dynamic nuclear magnetic resonance studies of complexes of trimethylplatinum(IV) iodide with ditelluroether chelate ligands

Trimethylplatinum(IV) iodide complexes with ditelluroether ligands, namely [PtlMe3(L–L)][L–L = MeTe(CH2)3TeMe, PhTe(CH2)3TePh or o-C6H4(TeMe)2] have been isolated as pure solids. 195Pt-{1H} and 125Te-{1H} NMR studies show that ambient-temperature solutions of the complexes with aliphatic ligand backbones consist predominantly of DL isomers. The complex [PtlMe3{MeTe(CH2)3TeMe}] also exists in significant amounts of both meso forms. In contrast, the meso-1 form is present in only very low abundance in [PtlMe3{PhTe(CH2)3TePh}] and [PtlMe3{o-C6H4(TeMe)2}]. Pyramidal tellurium inversion interconverts these isomeric species and precise rates of this process were measured at near-ambient temperatures by 195Pt-{1H} and 125Te-{1H} NMR two-dimensional exchange spectroscopy experiments, or by 1H one-dimensional NMR bandshape analysis at above-ambient temperatures in the case of [PtlMe3{o-C6H4(TeMe)2}]. Tellurium pyramidal inversion-energy barriers [ΔG‡(298.15 K)] were in the range 70–84 kJ mol–1, reflecting the nature of the the substituent at Te and the ligand backbone. Comparison of the tellurium inversion energy in [PtlMe3{MeTe(CH2)3TeMe}] with data for Se and S in analogous complexes of chalcogen ligands establishes the trend in inversion energies, namely Te > Se > S, on a firm quantitative basis.

Conformational analysis of some four-carbon 2,2′-bridged biphenyls

1,2,3,4-Dibenzocyclo-octa-1,3-diene (systematic name: 5,6,7,8-tetrahydrodibenzo[a,c]cyclo-octene) and a number of its 6-monosubstituted and cis- and trans-6,7-disubstituted derivatives have been resolved into enantiomers and diastereoisomers by chiral chromatography. 3JHH-values and empirical force-field calculations indicate that the preferred conformation of the cyclo-octadiene ring is a twist boat-chair (TBC) form with twist boat (TB) forms 4–12 kJ mol–1 higher in energy. In the TBC form, the C-6 (-7) substituents can be oriented in equatorial (e) and axial (a) directions. The monosubstituted compounds appear as e(major) and a forms, the trans-disubstituted as e,e(major) and a,a forms, and the cis-disubstituted as enantiomeric e,a and a,e forms. The e–a, e,e–a,a and e,a–a,e interchanges involve inversion of the biphenyl moiety with barriers in the range 95–102 kJ mol–1, determined by 1H NMR band shape analysis and by following the thermal racemization or epimerization by CD spectroscopy. The free-energy differences between e and a or e,e and a,a forms are reproduced by standard MMP2-85 calculations, except for a few compounds, among them the trans-disubstituted compounds with halogen substituents. For these, replacement of bond dipoles by atomic charges in the calculations led to more realistic results. The e,e–a,a equilibrium of the trans-dibromo compound is remarkably insensitive to solvent polarity.