Low-temperature 13C NMR Spectra Research Articles

Doubly Diastereoconvergent Preparation and Microsolvation-Controlled Properties of (Z)- and (E)-1'-Lithio-1'-(2,6-dimethylphenyl)propenes.

A doubly diastereoconvergent reaction can ad libitum generate either one or the other of two diastereomeric products with complete consumption of the diastereomeric precorsors or their mixtures. Thus, the preparation of configurationally pure (Z)-1'-lithio-1'-(2,6-dimethylphenyl)propene [(Z)-1] from any Z,E mixture of the corresponding bromoalkenes with n-butyllithium succeeded by means of a user-friendly (E)-1 → (Z)-1 configurational interconversion. The subsequent treatment of (Z)-1 with a minimum amount of THF afforded exclusively (E)-1 as the other diastereomeric product and was mediated by a beneficial (Z)-1 → (E)-1 interconversion. This behavior provided microsolvation-controlled choices of highly diastereoselective derivatizations of 1. Low-temperature 13C NMR spectra established that (Z)-1 was dissolved as a trisolvated monomer in THF but as a disolvated dimer in monodentate, ethereal, non-THF solvents, whereas (E)-1 was always monomeric. Backed by such knowledge, kinetic experiments indicated that the electrophiles 1-bromobutane or ClSiMe3 in Et2O reacted at 32 °C with the tiny (NMR-invisible) population of monomeric (Z)-1 that was formed in a mobile equilibrium from the inactive, predominantly dimeric (Z)-1. The equilibration of monomeric (Z)-1 and (E)-1 in THF as the solvent was fast (seconds on the 1H NMR time scale), whereas the corresponding stereoinversion of both solvated and unsolvated (E)-1 → (Z)-1 in non-THF solvents occurred on the laboratory time scale (minutes at ambient temperatures). Dicyclopropyl ketone added rapidly to the monomers (Z)-1&3THF and (E)-1&3THF with a rate ratio of at least 14:1 in THF at -78 °C. Di-tert-butyl ketone added less rapidly to the less shielded (Z)-1 [but never to (E)-1]; this singly diastereoconvergent process was much more slowly reversible in THF.

Studies on the s-cis–trans isomerism for some furan derivatives through IR and NMR spectroscopies and theoretical calculations

The s-cis–trans isomerism of two furan derivatives [2-acetyl- (AF) and 2-acetyl-5-methylfuran, (AMF)] was analyzed, using data from the deconvolution of their carbonyl absorption band in two solvents (CH2Cl2 and CH3CN). These infrared data showed that the O,O-trans conformers predominate in the less polar solvent (CH2Cl2), but these equilibria change in a more polar solvent (CH3CN) leading to a slight predominance of the O,O-cis conformers, in agreement with the theoretical calculations. The later results were obtained using B3LYP-IEFPCM/6-31++g(3df,3p) level of theory, which taking into account the solvent effects at IEFPCM (Integral Equation Formalism Polarizable Continuum Model). Low temperature 13CNMR spectra in CD2Cl2 (ca. −75°C) showed pairs of signals for each carbon, due to the known high energy barrier for the cis–trans interconversion leading to a large predominance of the trans isomers, which decreases in acetone-d6. This was confirmed by their 1HNMR spectra at the same temperatures. Moreover, despite the larger hyperconjugative interactions for the O,O-cis isomers, obtained from NBO data, these isomers are destabilized by the their Lewis energy.

Conformational preferences for some 5-substituted 2-acetylthiophenes through infrared spectroscopy and theoretical calculations

The s- cis– trans isomerisms of some derivatives of thiophene (2-acetyl, AT; 2-acetyl-5-bromo, ABT and 2-acetyl-5-chloro, ACT) were analyzed, using data from deconvolution of their carbonyl absorption bands in two solvents (CCl 4 and CHCl 3). These infrared data showed that the O,S- cis conformer largely predominates in the studied solvents and that the same occurs in the gas phase, as observed from theoretical calculations. The latter results were obtained using B3LYP/6-311++G(3df,3p) and MP2/6-311++G(3df,3p) levels of theory, with zero-point energy correction. Moreover, the use of the IEFPCM (Integral Equation Formalism Polarizable Continuum Model) to take into account the solvent effects, using the same levels of theory, confirmed the results observed from infrared data. Low temperature 13C NMR spectra in CS 2/CD 2Cl 2 (−90 °C) and in acetone- d 6 (−80 °C) did not show pairs of signals for each carbon, due to the known low energy barrier (∼8 kcal mol −1) for the cis– trans interconversion. Data from NBO calculations show that the n O ( 2 ) → σ S-C 5 * and n O ( 2 ) → σ C 2 – C 3 * interactions occur only in the O,S -cis isomer and can explain its conformational preference.

Conformational analysis of N -phenyl- and N -trifyl-4,4-dimethyl-4-silathiane 1-sulfimides

N-Substituted 4,4-dimethyl-4-silathiane 1-sulfimides [R = Ph (1), CF3 (2)] were studied experimentally by variable temperature dynamic NMR spectroscopy. Low temperature 13C NMR spectra of the two compounds revealed the frozen ring inversion process and approximately equal content of the axial and equatorial conformers. Calculations of the 4-silathiane derivatives 1, 2 and the model compound [R = Me (3)] as well as their carbon analogs, the similarly N-substituted thiane 1-sulfimides [R = Ph (4), CF3 (5), Me (6)] at the DFT/B3LYP/6–311G(d,p) level in the gas phase and in chloroform solution using the PCM model at the same level of theory showed a strong dependence of the relative stability of the conformer on the solvent. The electronegative trifluoromethyl group increases the relative stability of the axial conformer. Copyright © 2010 John Wiley & Sons, Ltd.

Conformational Consequences of the Dynamic Processes in the Stereolabile Atropisomers of Acyl-Substitutedm-Terphenyl Derivatives

By means of low-temperature NMR spectroscopy, conformers (stereolabile atropisomers) due to the restricted rotation about the Ar-Ar and Ar-C(O)R bonds were detected in a number of acylphenyl derivatives, substituted in positions 2 and 6 by the 3-isopropylphenyl moiety (compounds 1-3, R=H, Me, and t-Bu, respectively). The conformational assignment was accomplished on the basis of the symmetry of the low-temperature 13C NMR spectra with the added support of ab initio calculations. The interconversion barriers were also determined by complete line shape simulation of the NMR spectra, and the experimental values were satisfactorily reproduced by ab initio calculations. In the case of the asymmetric derivative 4, two enantiomers, generated by the restricted t-BuC(O)-Ar rotation, were found sufficiently stable to allow their separation by means of the enantioselective HPLC technique at ambient temperature and to obtain the corresponding CD spectra.

Conformational Equilibria in Formic Acid and the Adduct of Formic Acid and Hexafluoroacetone, HCO2C(CF3)2OH

Low-temperature 1H and 13C NMR spectra of formic acid (1) showed separate signals for the E and Z conformations in solvents containing a hydrogen bond acceptor, dimethyl ether. The population of E-1 (6.2% in 3:1:1 CHClF2/CHCl2F/(CH3)2O) was larger than that for 13C-labeled methyl formate in the same solvent (0.2%), which indicated that the relative populations are not determined by steric effects. The free-energy difference between the E and Z conformations of 1 was 0.9 kcal/mol. In a 1:3 CD2Cl2/(CH3)2O solvent mixture, peaks for E and Z conformations were found at low temperatures by 1H and 13C NMR for both formic acid and an adduct with hexafluoroacetone, HCO2C(CF3)2OH (2). The population of E-1 in this solvent mixture was 4.3% by 13C NMR. The carbon spectrum showed two peaks in the carbonyl carbon region of nearly equal intensities at -151.6 degrees C, with E-2 (48%) absorbing downfield of the major Z-2 (52%). The large population of E-2 confirms that electron-withdrawing groups R' in RCO2R' enhance the populations of the E-isomers. The free-energy barriers for 2 of 6.24 (E-to-Z) and 6.26 kcal/mol (Z-to-E) were determined from rate constants obtained by line shape analysis at -143.2 degrees C.

Generation of trans-spanning diphosphine ligands via alkene metathesis: Synthesis, structure, and dynamic behavior of a missing link in a series of square-planar platinum complexes

Reaction of KPPh 2 and Br(CH 2) 5CH CH 2 gives the phosphine PPh 2(CH 2) 5CH CH 2 (89%), which is treated with the platinum tetrahydrothiophene complex [Pt(μ-Cl)(C 6F 5)(S(CH 2CH 2–) 2)] 2 to yield trans-(Cl)(C 6F 5)Pt(PPh 2(CH 2) 5CH CH 2) 2 ( 4b, 80%). Ring-closing alkene metathesis (Grubbs’ catalyst) gives ▪ ( 5b, 84%), which features a trans-spanning diphosphine ligand. The Z/ E C C mixture is hydrogenated (1 atm, 10% Pd/C) to give ▪ ( 6b, 99%). The crystal structures of 4b and ( Z)- 5b are determined. In the former, both (CH 2) 5CH CH 2 moieties are directed on the same side of the platinum square plane. Low temperature 13C NMR spectra of 6b show two sets of signals for the diastereotopic P Ph 2 groups. These coalesce upon warming, which requires the passage of the chloride ligand through the macrocycle. Analysis by the complete bandwidth method gives Δ H ‡ and Δ S ‡ values of 6.0 ± 0.4 kcal mol −1 and −13.9 ± 2.6 eu. The 31P, 13C, and 2H CP/MAS NMR spectra of polycrystalline 6b and 6b- d 2 are studied, and indicate appreciable conformational mobility of the methylene chain in the solid state.

Conformational Study of cis-1,4-Di-tert-butylcyclohexane by Dynamic NMR Spectroscopy and Computational Methods. Observation of Chair and Twist-Boat Conformations

[reaction: see text] Low-temperature 13C NMR spectra of cis-1,4-di-tert-butylcyclohexane (1) showed signals for the twist-boat (1a) and chair (1b) conformations. 13C NMR signals were assigned to specific carbons based on the different populations, different symmetries (time-averaged C(2v) for 1a and time-averaged C(s) for 1b), and calculated chemical shifts (GIAO, HF/6-311+G*). In addition to slow ring inversion and interconversion of the chair and twist-boat conformations, slow rotation of the tert-butyl groups was found. Most of the expected 13C peaks were observed. Free-energy barriers of 6.83 and 6.35 kcal/mol were found for interconversion of 1a (major) and 1b (minor) at -148.1 degrees C. Conformational space was searched with Allinger's MM3 and MM4 programs, and free energies were obtained for several low-energy conformations 1a-c. Calculations were repeated with ab initio methods up to the HF/6-311+G* level. Molecular symmetries, relative free energies, relative enthalpies and entropies, frequencies, and NMR chemical shifts were obtained. A boat conformation (1d; C(2v) symmetry) was generated and optimized as a transition state by ab initio, MM3, and MM4 calculations.

Organoboron water, part I: Synthesis and multinuclear magnetic resonance studies on the structure of tetramethyldialuminoxane

AbstractThe structure of tetramethyldialuminoxane was studied. The low‐temperature 1H and 13C NMR spectra suggest that the methyl groups bridge the aluminum atoms in the dialuminoxane trimer. A new method of synthesis of methylaluminoxanes and tetramethyldialuminoxanes in the reaction of tetraethyldiboroxane (organoboron water) with trimethylaluminum (1:1 and 1:2 respectively) was applied to synthesize the investigated system. Copyright © 2004 John Wiley & Sons, Ltd.

The first 1,3,2-diazabora-[3]ferrocenophanes – molecular structures and dynamic behaviour in solution

Dilithiated 1,1 ′-bis(trimethylsilylamino)ferrocene ( 1) reacts with aminoboron dihalides ( 2) X 2B-N(R ′)R [X=Br, R ′=R=Et ( 2a); X=Cl, R ′=Me, R=CH 2Ph ( 2b), X=Cl, R ′=Et, R=Ph ( 2c)] to give 2-amino-1,3,2-diazabora-[3]ferrocenophanes ( 3a– c) for the first time. The steric constraints exerted by the [3]ferrocenophane unit and the presence of the N-SiMe 3 groups cause rather different B–N bonding situations in these tri(amino)boranes. The boron atom has the choice between three nitrogen atoms for BN(pp)π bonding: in the cases of 3a and 3b, it prefers the NEt 2 and the N(Me)CH 2Ph group, respectively, over the N-SiMe 3 groups, whereas in 3c the N(Et)Ph group appears to be the weaker π-donor. This can be concluded from the X-ray structural analyses carried out for 3a and 3c, and from the low temperature 1H, 13C, and 29Si NMR spectra of 3a– 3c.

Investigation on uranyl interaction with bioactive ligands. Synthesis and structural studies of the uranyl complexes with glycine and N-(2-mercaptopropionyl)glycine

Summary Three new uranium(VI) compounds have been prepared with glycine (Hgly), N-(2-mercaptopropionyl)glycine (Hmpg) and 2,2′-dithiobis(N-propionylglycine) (tpg). The crystal structure of [UO2(Hgly)2Cl2] has been solved, revealing the uranium atom to be seven-co-ordinated. The two zwitterionic glycines are coordinated to uranium atom by the carboxylate groups, one in a monodentate and the other in a bidentate mode. Solid state studies and low temperature 1H, 13C, 2D COSY and HMQC solution NMR spectra of the [UO2(mpg)(CH3COO)], show that mpg coordinates uranium atom by the carboxylate group.

Eight-membered-ring solid-state conformational interconversion via the atom-flip mechanism, a CPMAS 13C NMR and crystallographic stereochemical study.

Nefopam methohalide (chloride, bromide, and iodide) medium-ring quaternary ammonium salts of the non-narcotic analgesic tertiary amine drug give crystals belonging to the identical monoclinic P2(1)/c space group, and all of these pseudopolymorphs exhibit the same packing motif. A singular boat-boat (BB) more compact conformation is observed in the nefopam methochloride crystal. Larger halide anions (bromide and iodide) increase the void distance between the 2(1)-screw axis related adjacent ammonium cations to accommodate void-size dependent equilibrium quantities of the twist-chair-chair (TCC) more extended conformation. The BB:TCC occupancy factors are 0.961(5):0.039(5) [193 K], 0.780(5):0.220(5) [293 K], and 0.755(6):0.245(6) [343 K] for the methobromide crystal, while values of 0.657(5):0.343(5) [193 K] and 0.592(7):0.408(7) [293 K] were measured for the methiodide. Above a minimum of ca. 2.53 A, the occupancy factors were found to be linearly correlated to the intermolecular (TCC)Me(eq)-H...H-Me(ax)(TCC) distance between abutting methyl group protons in 2(1)-screw axis related neighbors. Temperature-dependent occupancy factors for the two conformers are interpreted in terms of a medium ring atom-flip facile interconversion between the two low energy conformations in crystals containing the appropriate size intercation void. A BB/TCC atom-flip interconversion in the methochloride unit cell would result in van der Waals interactions due to an estimated 2.31 A close intermolecular (TCC)Me(eq)-H...H-Me(ax)(TCC) distance between adjacent 2(1)-screw symmetry ammonium cations. The 203 K low-temperature CPMAS 13C NMR spectrum of the methiodide salt showed two slow exchange limit (SEL) delta 57.91 [BB] and delta 63.10 [TCC] OCH2CH2N peaks. A variable low-temperature CPMAS NMR investigation of the solid methiodide showed complex dynamic behavior that cannot be interpreted solely on the basis of an atom-flip conformational interconversion. Local magnetic fields from the gem-dimethyl rapidly rotating proton magnetic dipoles provide a distance-dependent T1 relaxation mechanism for neighboring carbons in the solid-state.

Conformational Study of Cyclodecane and Substituted Cyclodecanes by Dynamic NMR Spectroscopy and Computational Methods

Low-temperature 13C NMR spectra of cyclodecane (1) showed the presence of a minor conformation, assigned to the twist-boat-chair-chair (TBCC), in addition to the expected boat-chair-boat (BCB). If only the TBCC and BCB conformations were assumed to be appreciably populated, then a free-energy difference between the two conformations of 0.73 ± 0.3 kcal/mol could be obtained from the five area measurements over a temperature range of −148.6 to −131.0 °C, with populations of 5.2 and 94.8% for the TBCC and BCB conformations at −146.1 °C. However, an alternative description of the conformations of 1 was suggested by the ab initio calculations, which predicted that the twist-boat-chair (TBC) and TBCC conformations have comparable free energies and populations. Equal amounts of TBCC and TBC would give populations of 5.2, 5.2, and 89.6% and relative free energies of 0.72, 0.72, and 0.00 kcal/mol for the TBCC, TBC, and BCB conformations at −146.1 °C, based on the experimental areas at this temperature. The experimental spectra could neither confirm nor disprove the presence of the TBC. Saunders' calculations of the strain energies of 1 using Allinger's MM3 program were reproduced to obtain a complete set of these parameters and drawings of the conformations, and free energies and populations were obtained at +25 and −171.1 °C. Free energies were also calculated at the HF/6-31G* and HF/6-311G* levels, and chemical shifts were obtained for three conformations at the HF/6-311G* level by the GIAO method. Chlorocyclodecane (2) was shown by 13C and 1H NMR spectroscopy to have three conformations at −165.5 °C. To aid in conformational assignments, the 13C chemical shifts were calculated for all of the BCB and TBCC conformations of 2 using the GIAO method at the HF/6-311G* level. The free energies for each of the possible BCB, TBCC, and TBC conformations were also calculated using Allinger's MM3 program. From the line shape changes in the experimental 13C NMR spectra, the free-energy barriers, a consideration of the X-ray structures of substituted cyclodecanes, and these calculated chemical shifts and free energies, the three conformations of 2 at −165.5 °C were suggested to be 2e BCB (31.2%), 2a BCB (14.9%), and a TBCC conformation (53.9%) (numbering as in Figure 1); the 2e and 2a BCB assignments could be reversed. Free-energy barriers for interconversion of BCB conformations of 2 at −159.8 °C were 5.4 ± 0.2 and 5.5 ± 0.2 kcal/mol, and the free-energy barriers at −120.9 °C for equilibration of the TBCC conformation with the rapidly interconverting BCB conformations were 7.07 ± 0.2 and 7.08 ± 0.2 kcal/mol. The 13C NMR spectrum of cyclodecyl acetate (3) at −160.0 °C showed a similar pattern of chemical shifts and intensities for the substituted ring carbon.

The Crystal Structure and Low Temperature 13C NMR Spectrum of the Seven-Coordinate Complex of Tungsten(II), [WI2(CO)3(NCPh)2

Abstract The crystal structure of [WI2(CO)3(NCPh)2] (1) has been determined by X-ray methods. The crystals are orthorhombic, space group Pnam, a = 7.815(3), b = 13.839(5), c = 18.475(3) Å, Z = 8, R = 0.0346 for 1497 observed data. The complex [WI2(CO)3(NCPh)2] has a seven-coordinate geometry, which may be described as a distorted capped octahedron, with the two iodo-ligands mutually trans, while each nitrile is trans to a carbonyl group. The capped octahedral geometry consists of a capping carbonyl (C(10)} on the face defined by I(1), C(20) and C(20)#. However, the geom etry may also be described as a capped trigonal prism, which has one triangular face occupied by three carbonyl groups and the second triangular face occupied by N(1) , N (1)# and I(1) atoms, respectively. The iodide I(2) occupies the capping position on the rectangular face defined by the atoms C(20), C(20)#, N (1), and N (1)# . The low temperature 13C NMR spectrum (203 K, CD2Cl2) of 1 shows a single carbonyl resonance at 219.27 ppm which indicates that the complex is undergoing a rapid fluxional process in solution at 203 K.

Low-temperature single-crystal X-Ray diffraction,1H and13C solid-state NMR and14N NQR studies of 1,8-bis(dimethyl-amino)naphthalene

Low-temperature 13C and 1H MAS NMR and 14N NQR spectra of solid 1,8-bis(dimethylamino)naphthalene (DMAN) have been measured. X-Ray diffraction shows that the molecule is more symmetric than at room temperature. The low-temperature geometry of DMAN is very similar to the ab initio optimized geometry. Solid-state NMR and nuclear quadrupole resonance (NQR) confirm the symmetry of the molecule at low temperature despite the asymmetry of intermolecular interactions in the crystal. Experimental values of the principal components of electric field gradients at 14N, which characterize the properties of the lone pair electrons of the nitrogen atoms, are in good agreement with the ab initio calculated values.

Low-temperature 13C NMR investigations on carbon monoxide hydrogen bonded to Brønsted acid sites in H [sbnd]Y zeolites

13C NMR investigations were carried out at 4.5 K in order to analyze the lineshape of the solid-state spectrum of 13CO adsorbed on Brønsted acid sites in H Y zeolites. The spectrum was found to be dominated by the chemical shift anisotropy and by the heteronuclear magnetic dipolar interaction between the 13C nucleus of the 13CO molecule and the proton of the Brønsted acid site where the molecule is adsorbed. Therefore, an analysis of the lineshape of the low-temperature 13C NMR spectrum allows a precise determination of the hydrogen bond distance r H C between these nuclei. A value of r H C =0.21±0.01 nm has been obtained by this method which is in excellent agreement with recent quantum chemical calculations.

Seven-co-ordinate dibromo complexes of molybdenum(II) and tungsten(II) derived from [MBr2(CO)3(NCMe)2]. Crystal structures of the isostructural complexes [WX2(CO)3(NCMe)(SBPh3)]·Ch2Cl2(X = Br or I)

Reaction of the complexes fac-[M(CO)3(NCMe)3](M = Mo or W)(prepared in situ) with an equimolar quantity of Br2 at –78 °C afforded high yields of the seven-co-ordinate complexes [MBr2(CO)3(NCMe)2]. The reaction of these complexes with an equimolar amount of L (L = PPh3, AsPh3 or SbPh3) in CH2Cl2 at room temperature gave the new mixed-ligand complexes [MBr2(CO)3(NCMe)L]. The molecular structure of [WBr2(CO)3(NCMe)(SbPh3)]·CH2Cl2 and its diiodo analogue [WI2(CO)3(NCMe)(SbPh3)]·CH2Cl2 have been crystallographically determined. Crystals of the former are monoclinic of space group P21/c, a= 8.302(3), b= 26.199(3), c= 13.020(4)A, β= 97.53(2)°, Z= 4. Crystals of the latter are also monoclinic of space group P21/c, a= 8.450(2), b= 25.941 (6), c= 13.042(4)A, β= 96.47(2)°, Z= 4. The two complexes are isostructural and can best be described as distorted capped trigonal prismatic with one triangular face having three carbonyl ligands and the other triangular face having X(2), Sb and N. The X(1 ) atom caps the rectangular face defined by C(1), X(2), N and C(3). The low-temperature (–6O °C, CDCl3)13C NMR spectrum of [WI2(CO)3(NCMe)(SbPh3)]·CH2Cl2 was obtained and correlated with the solid-state structure. Reaction of [WBr2(CO)3(NCMe)2] with 2 equivalents of AsPh3 in CH2Cl2 at room temperature gave [WBr2(CO)3(AsPh3)2].

Cis-Cyclohexano-9-crown-3 ether. Solid state and low-temperature solution stereochemistry as determined by X-ray crystallography and nuclear magnetic resonance spectroscopy

The X-ray crystal structure of the title material has been determined at −130 °C. Low-temperature 1H1H COSY, 13C1H HETCOR, and DEPT 13C NMR spectra have been recorded, which permit unambiguous assignments of all carbon resonances when ring inversion is slow on the NMR timescale. The limiting low-temperature solution phase 13C spectrum has many common features with the solid phase 13C CPMAS spectrum recorded at 300 K. Spectra for the 7,10-tetra-deuterio derivative have also been obtained and substituent influences on 13C shieldings are discussed in detail.

Tricyclohexylphosphinecarbondisulphide seven-coordinate complexes of molybdenum(II) and tungsten(II)

The complexes [MI 2(CO) 3(NCMe) 2] (M = Mo or W) react with one equivalent of S 2CPCy 3 in CH 2Cl 2 to afford the new seven-coordinate complexes [MI 2(CO) 3(S 2CPCy 3)]. Reaction of [MI 2(CO) 3(NCMe) 2] (M = Mo or W), with two equivalents of S 2CPCy 3 afford the new dicationic complexes [M(CO) 2(S 2CPCy 3) 2]2I in good yield. The dicationic nature [W(CO) 3(S 2CPCy 3) 2]2I was confirmed by the formation of the complex [W(CO) 3(S 2CPCy 3) 2][BPh 4] 2 by iodide exchange with Na[BPh 4]. Further reaction in situ of [M(CO) 3(S 2CPCy 3) 2]2I with one equivalent of S 2CPCy 3 afforded the tris ligand complex [W(CO) 3(S 2CPCy 3) 3]2I. The reaction of [W(CO) 3(S 2CPCy 3) 3]2I with Na[BPh 4] in acetonitrile gave [W(CO) 3(S 2CPCy 3) 3][BPh 4] 2, which confirmed the dicationic nature of [W(CO) 3(S 2CPCy 3) 3]2I. The mixed ligand complexes [MI(CO) 3(PPh 3)(S 2CPCy 3)]I (M = Mo or W) and [WI(CO) 3L(S 2CPCy 3)]I (L = P(OPh) 3, AsPh 3 or SbPh 3) were synthesised by reaction of [MI 2(CO) 3-(NCMe) 2] with an equimolar quantity of L in CH 2Cl 2 followed by an in situ reaction with one equivalent of S 2CPCy 3. The cationic nature of [MI(CO) 3(PPh 3)(S 2CPCy 3)]I was confirmed by iodide exchange with Na[BF 4] to afford the complexes [MI(CO) 3(PPh 3)(S 2CPCy 3)][BF 4]. The reaction of two equivalents of [WI 2(CO) 3(NCMe)(PPh 3)] with S 2CPCy 3 afforded the ligand-bridged complex [W 2I 4(CO) 6(PPh 3) 2(μ-S 2CPCy 3)]. Treatment of [WI 2(CO) 3(NCMe) 2] with two equivalents of PPh 3 followed by an in situ reaction with one equivalent of S 2CPCy 3 afforded the dicationic complex [W(CO) 3(PPh 3) 2(S 2CPCy 3)]2I. The ionic nature of the complex was confirmed by iodide exchange with Na[BPh 4] to give [W(CO) 3(PPh 3) 2(S 2CPCy 3)][BPh 4] 2. Treatment of [W(CO) 3(PPh 3) 2(S 2CPCy 3)]2I with one further equivalent of S 2CPCy 3 afforded the complex [W(CO) 3(PPh 3) 2(S 2CPCy 3)]2I. The low temperature 13C NMR spectra (carbonyl region) of several-coordinate complexes are interpreted to suggest likely structures for these compounds.

NMR spectra and conformation of cembrene in solution

Using dimeric NMR spectroscopy, a complete interpretation of the1H and13C NMR spectra of the diterpene hydrocarbon cembrene has been made. The experimental values of the SSCs for the1H atoms in the PMR spectra of cembrene agree well with those calculated for the lowest-energy (“crystal”) conformation. In the light of the observation of intramolecular NOEs and of the low-temperature13C NMR spectra, it has been concluded that the cembrene molecule retains the “crystal” conformation in solution.