Variable-temperature H NMR

X-ray crystallography and variable temperature H NMR studies show that the conformation of the coordinated imidazolidin-2-ylidene, in both the neutral and cationic complexes 70, is anti, anti with respect to the Ph of the backbone of the NHC, exclusively in the solid state and predominantly in solution at lower temperatures (-75°C). At room temperature in solution, possible conformer interconversion by the rotation around the phenyl-N bond of the NHC substituent is apparent from the broadness of the peaks in the NMR spectra. Hydrosilylation of acetophenone by Ph SiH catalysed by 70 at room temperature or at -20°C results in maximum ee of 58%. However, at lower temperatures the reaction rates are much slower [55]. 

FIG. 36. Variable-temperature H NMR spectra of [TpBul]BeH. Reprinted with permission from Ref. (56). Copyright 1992 American Chemical Society. 

Variable-temperature H NMR studies of MesP Ga(Trip)2 278 and t-Bu2-GaP(Mes )SiPh379,80 show barriers to rotation around the Ga—P bond of... 

In a recent report [141] Stoddart et al. reported a new class of rotaxanes with dendritic stoppers by using a so-called threading approach (Fig. 25). Alkylation of bipyridinium based units with Frechet s third tier branched aryl ethereal dendron, in the presence of BPP34C10 afforded 58 as one of the products. Variable temperature H-NMR spectroscopy in different NMR solvents helped determine the novel shuttling process of BPP34C10 from one bipyridinium unit to the other in 58. The dendritic framework of 58 assists in its solubility in a wide range of solvents. 

P NMR data have been reported for Ir4(CO)8(PPh2Me)4 [6 -8.4(1), —40.6(2), 61.6(1) p.p.m.] together with a variable temperature H NMR study, which indicates that phosphine intra-exchange occurs at high temperatures whereas the low temperature spectra are consistent with a number of alternative stereochemical arrangements46. ... 

Reactions of c -[Ru(bpy)2Cl2] with ligands (86) or (87) (X = CH2) in EtOH(aq) lead to [Ru(bpy)2(86)] + and [Ru(bpy)2(87, X = CH2)] respectively. When X = 0 in ligand (87), the product is the pyridine carboxylate complex [Ru(bpy)2(pyC02)], the structure of which is confirmed by X-ray crystallography. Complexes of the type [Ru(bpy)2L] " in which L represents a series of mono- and dihydrazones have been prepared and characterized by spectroscopic methods (including variable temperature H NMR) and a structure determination for L = biacetyl di(phenylhydrazone). When L is 2-acetylpyridine hydrazone or 2-acetylpyridine phenylhydrazone, [Ru(bpy)2L] + shows an emission, but none is observed for the dihydrazone complexes. The pyrazoline complex [Ru(bpy)2L] (L = 5-(4-nitrophenyl)-l-phenyl-3-(2-pyridyl)-2-pyrazoline) can be isolated in two diastereoisomeric forms. At 298 K, these exhibit similar MLCT absorptions, but at 77 K, their emission maxima and lifetimes are significantly different. ... 

In order to acquire a more quantitative understanding of the flipping process, Timothy Wong carried out a series of variable-temperature H NMR studies on 89 and 85, whose pertinent data, together with those reported previously for 92, " and 94, ° < 2 gj.g summarized in Table 1. As can be seen, the... 

Figure 1.21 Variable temperature H NMR spectra in CD2CI2, showing the passage from a slow to fast exchange for the two halves of the bipyridine ring in 92.

Figure 1.25 Experimental (solid line) and calculated (dotted line) variable temperature H NMR spectrum of the N-methyl resonances of 97, in d -DMF. This series of spectra shows the exchange of the methyl group A with C, and B with D. The resonances for A and B, and for C and D overlap at all temperatures considered.

Figure 2.3 Variable-temperature H NMR spectra recorded in a hexamethyl phosphoric triamide solution of 2,6-dihydroxybenzoic acid at 135, 163, 177, and 240 K. (Reproduced with permission from ref. 26.)...

Campbell, J. L. E., Johnson, K. E., and Torkelson, J. R., Infrared and variable-temperature H-NMR investigations of ambient-temperature ionic liquids prepared by reaction of HCl with l-ethyl-3-methyl-lH-imidazolium chloride, Inorg. Chem., 33, 3340,1994. 

The conformational interconversion of diazabicyclo [k.l.m] alkanes (Figure 6 Section 5.21.3.1.2) has been investigated by variable temperature H NMR (68JA2428). The bis-deuterochloride salt of l,10-diazabicyclo[8.8.8]hexacosane showed a slow isomerization from exo-exo to endo-endo forms during which the NH proton resonance shifted from 8.0 p.p.m. (exo-exo) to 4.6 p.p.m. (endo-endo). The equilibrium constant for the process Ke —endo-endo / exo-exo) was greater than 100. Similar measurements on differently lengthed diazabicyclo[k, l. m ]alkanes revealed that even chain Iengthed species prefer the... 

Synthesis and Variable-Temperature H NMR Conformational Analysis of Bis(ri5-cyclopentadienyl)titanium Pentasulfide 151... 

The synthesis of Fe N(SiMe3)2 2 completed the divalent M(ll) series M = Mn—Ni. It was shown to have a two-coordinate monomeric structure in the vapor phase by gas electron diffraction. Its crystal structure showed that it was a symmetrically bridged dimer like its manganese and cobalt counterparts. Variable temperature H NMR spectroscopy indicated an association energy of only 3 kcal mol. The divalent series was also expanded to Cr(II) with use of the — NPr and —NCy2 (Cy = cyclohexyl) groups to afford the bridged... 

Figure 3.10 Variable-temperature H NMR spectra of complex 107 in acetone-dg solution.

Figure 3.21 Variable-temperature H NMR spectra of 124 in acetone-de solution. The resonances assigned to the new derivative are indicated. The inset corresponds to the spectrum at -80"C for a solution with an AgBF4 Phdat ratio of 2. 

Variable temperature H-NMR spectra of 5-nitroso-2,4,6-trimethyl-5,6-dihydro-4H-1,3,5-dithiazine (464) (all three methyl groups shown to be cis by X-ray crystallography) gave an unusually low barrier to rotation about the N—N bond (17.2 kcal mol-1) attributed to the electron-withdrawing sulfur substituents and steric crowding.359... 

The facility of 1//-azepines to form transition metal carbonyl complexes was realized soon after they were first synthesized. Variable temperature H NMR studies on the tricarbonyliron complex formed either by photolysis of l-ethoxycarbonyl-l//-azepine with tricarbonyliron in THF, or by heating the azepine with nonacarbonyldiiron in hexane, demonstrated that it undergoes rapid reversible valence tautomerism and that there is considerable restricted rotation about the N—CO bond (B-69MI51600). The molecular geometry of the complex has been determined by X-ray analysis (see Section 5.16.2.2). 

Variable temperature H NMR has been employed by several groups to determine inversions and rotation barriers. Bolinger and Rauchfuss83 examined the inversion (Scheme 5) for the series (Cp)2Ti(E2R) (E = S, Se R = C2(C02Me)2, C2(CF3)2, C6H4Me). 

Variable-temperature H NMR spectra were measured for 177 and 178 in CD2CI2 solutions205. In the former the N-methyl groups and the methylene protons appeared as sharp singlets down to the lowest attainable temperature, while in 178, with two chiral centers, signals due to only one diastereomer could be observed, and the initially diastereotopic N-methyls (due to the chiral carbon center) coalesced with an activation... 

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