Carbon 13 chemical shifts carbenes

The carbon chemical shifts of the azolium salts can be found at the downheld end of the aromatic range at 5 = 140-160 ppm and the carbenes themselves about A5 = 100 ppm downheld of the imidazolium salts. Coordinahon to transihon metals brings the carbon chemical shift upheld from the value of the free carbene. Whereas the resonance in [Cp Ru(NHC)Cl] complexes are typically around 8 = 200 ppm [116], the same signal in [Ag(NHC)Cl] complexes can be found at 8 = 170-190 ppm [50] (see Figure 1.23). 

Todd et al. (23) have studied the NMR spectra of complexes of the type (CO)sMCRR, M = Cr, W R = CHs, Ph, m- andp-Ph R = NH2, OR (Tables XXXVI and XXXVII). They have also found (cf Ref. 123) that substitution of a methyl for a phenyl group (R) causes an 8 to 11 ppm downfield shift. A unique solvent effect was observed for this system the carbene carbon chemical shift was 6 to 7 ppm upfield in THF from the value in CHCI3. This effect was attributed to the formation of a solvent-solute complex in THF solution. 

Carbon-13 NMR spectra of tungsten derivatives (CO)sWC(CH3)(SC6H4Y) were obtained by Ward et al. (127) (Table XXXVII). Only small variations of cis and trans carbonyl and carbene carbon chemical shift were detected as the nature of the para group, Y, was varied. The authors concluded that the lack of any conjugative effect indicated that the phenyl ring was probably perpendicular to the carbene plane. 

Figure 12.29 Why carbene carbon is more deshielded than carbenium ion carbon (a) comparison of carbenium ion and carbene carbon chemical shifts, (b) anisotropic contributions to carbene carbon chemical shift...

Once coordinated to a metal centre, the signal corresponding to the carbene carbon atom is usually shifted upfield. The chemical shift of the carbene carbon atom (C ) for a given metal in a given oxidation state is usually characteristic (Table 1.1). 

Table 1.1 C H NMR chemical shifts of the carbenic carbon atom...

Table II lists all pertinent chemical shifts and coupling constants for the known phosphinocarbenes and their respective diazo precursors. The (phosphino)(silyl)carbenes are all characterized by high field chemical shifts for phosphorus ( 24 to 50 ppm) and silicon (-3 to -21 ppm), and low field chemical shifts for carbon (120 to 143 ppm) with large couplings to phosphorus (147 to 203 Hz).

Classical shielding arguments indicate an electron-rich phosphorus atom, or equally, an increase in coordination number. The silicon atom seems also to be electron-rich, while the carbon has a chemical shift in the range expected for a multiply bonded species. The coupling constant data are difficult to rationalize, as it is not possible to predict the influence of orbital, spin-dipolar, Fermi contact, or higher-order quantum mechanical contributions to the magnitude of the coupling constants. However, classical interpretation of the NMR data indicates that the (phosphino)(silyl)carbenes have a P-C multiple bond character. 

Table 1.1. Chemical shifts for carbon atoms (C ) and protons (H ) in representative heteroatom-substituted carbene complexes L M=Cot(R)H(j.

The chemical shifts of the carbene carbons of identically substituted iron and ruthenium complexes have been compared. In general, the ruthenium complexes appear about 20 ppm upfield from their iron analogues.144... 

Analysis of the 13C NMR spectra of a series of N-heterocyclic carbenes (NHCs) with general structures 144, 145 and 146 suggests that the chemical shift of the carbene carbon (S = 211-244) correlates with the N(l)-C(2)-N(3) bond angle (101-106°) in the solid state <2003AGE5243>. The 13C carbene shifts of the unsaturated systems 144 (<5 = 205-220) are 15-25ppm upfield from the saturated systems 145 (see also Table 40, Section 2.4.4.2.5). 

The authors pointed out that the C-NMR chemical shifts of the carbene carbon atoms could not be used as indicator for the electronic properties of the carbene ligands, but that the vCO stretching frequency of the corresponding rhodiumfl) carbonyl complexes is a valid indicator. Both observations are in line with the recommendations of a recent review article [103]. 

Analyses of NMR of A -heterocyclic carbenes (Figure 15) seem to suggest that the chemical shift of the carbene carbon correlates to the NGN bond angle in the solid state (Table 4). 

Additional useful information the signal of I at 8 224.31 is similar to the chemical shift of carbene carbons in similar compounds the peaks between 8 184 and 202 correspond to carbonyls and the peak at 8 73.33 is typical of CH2CH2 bridges in dioxycar-bene complexes. 

A similar reaction with bis(dimediylamino) chloroiminium chloride was performed at temperatures below -20 °C and led to the fonnation of the bis(dimethylamino) carbene with a conversion of about 60%. For the first time, the bis(dimethylamino) carbene was formed without complexation with metal cations, which usually occurs in the deprotonation method. The C chemical shift of the carbene carbon atom was significantly shifted toward low field compared to the complexed version. To our surprise, the carbene was not found to dimerize at higher temperatures, but instead a complex mixture was obtained with no traces of dimer being detectable. This raises the question whether the cations are necessary for the dimerization process, as already discussed in literature [7]. 

In the experiment with the naphthoxy substituted iminium salt, the corresponding carbene was formed at -80 °C. The NMR spectrum clearly showed a signal at 258.1 ppm, which was assigned to the carbene carbon. Alder reported similar chemical shifts for other amino oxy carbenes [16]. 

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