Carbon 13 chemical shifts carbene complexes

Carbene complex 138 has been isolated under considerably less extreme conditions/" The air and water stable complex 138 is isolated from the reaction of potassium tetrachloroplatinate with a free pyridine in ethanoic acid, even though a cyclometallated complex coordinated to the platinum through a nitrogen might have been anticipated. In principle, the carbene structure of 138 could be redrawn as the ionic structure 139 (Scheme 34) however, a short Pt-G distance of 1.952(7) A in the X-ray structure and a solution G chemical shift of 324 ppm for the carbon bonded to platinum confirms that carbene form 138 more accurately represents the molecular form present. 

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 

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 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]. 

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). 

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. 

The C nmr spectra of carbene complexes are characterized by marked deshielding of the carbene carbon atom (Formacek and Kreiter. 1972). The chemical shift of the carbene carbon atom in (CO)sCrC(OCH3)CH3 occurs 360 ppm downfield from tetramethylsilane (TMS). The enormous downfield shifts of the carbene carbon atom are useful in characterizing the complexes, but are not readily interpreted. For example, downfield C chemical shifts are thought to reflect the electropositive character of the carbon atoms. Other more important effects must be involved for metal-carbene complexes since (C0)5W C(QH5)2 has a chemical shift of 358 ppm (T. J. Burkhardt and C. P. Casey, unpublished observations, 1974), whereas the related carbonium 

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]. 

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. 

Many examples of vinylidene complexes have been isolated, and the two tautomers of alk5me and vinylidene complexes shown in Equations 2.10a and 2.10b are sitnilar in stability. As shown by these two equations, either can be more stable, depending on the number and identity of the ligands on the metal fragment. Wiylidene complexes have also been invoked as intermediates in both stoichiometric and catalytic reactions. Vinyhdenes are typically electrophilic at the carbene carbon CJ and nucleophilic at the metal and C. The HI NMR chemical shift of the carbene carbon typically lies downfield, between 250 and 380 ppm, while the NMR chemical shift of the P-carbon typically lies between 87 and 143 ppm. 

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