Concepts in the NMR structural analysis of perfluoroalkyl groups: characterization of the bis(n-perfluoroalkyl)zinc compounds: Zn(n-CmF2m+1)2·2THF (m = 4, 6, 7, 8) and Zn(n-C6F13)2·2CH3CN

Two-dimensional NMR spectroscopy was used in the structural characterization of the bis(n-perfluoroalkyl)zinc compounds Zn(Rf)2·2D with Rf=n-C4F9, n-C6F13, n-C7F15, n-C8F17; D=tetrahydrofuran and Rf=n-C6F13; D=acetonitrile. Since 4J(19F,19F) is the strongest coupling occurring in the 19F spin system of n-perfluoroalkyl ligands, CFx groups in 1,3-position could be identified by 19F,19F correlation spectroscopy (COSY). In this way the complete information on neighborhood relations in the n-CmF2m+1 chain is cut into two series of 4J connected CFx groups. Important information how these two series must be joined together was obtained from the assignment of the CF3 group signal (relative intensity), from weak 19F,19F couplings (resulting from interactions via three, five, or even six bonds), and from 1J-13C, 19F correlations, which characterize the α-CF2 group by the typical downfield shift of the metal substituted carbon atom. When this information was not sufficient, 13C,19F correlations via one and two bonds were carried out to identify directly neighboring CFx groups. It was demonstrated that this type of two-dimensional correlation spectroscopy provides an independent and unambiguous possibility to characterize the constitution of perfluoroalkyl groups. From the obtained chemical shift values the following characteristic trends were deduced. The CF3 group exhibits the most downfield and the neighboring CF2 group the most highfield 19F chemical shift values. The subsequent CF2 groups up to the CF2 group in β-position to the Zn atom show increasing δ(19F) values. The resulting sequence of signals is only interrupted by the resonance of the α-CF2 group appearing at a not well-defined position. In the 19F decoupled 13C spectrum the peak of the α-CF2 group constitutes the most downfield shift value. The 13C resonances of the remaining CF2 groups, although falling into a very narrow range, are shifted to higher field with increasing distance from the Zn atom. The signal of the CF3 group, appearing between those of the α- and β-CF2 groups, is well characterized by its chemical shift (very close to 118.8 ppm) and by its different phase in the 13C{19F} DEPT 135 spectrum. Thus, a well-defined relation between 13C chemical shifts and structure was established.