NMR Spectra of Clusters

At low fields, signals due to other inner atoms (nitrogen, phosphorus) are also observed. As in carbide complexes, in clusters of the type Rh (N)L and Rh (P)L, there is an interaction of all metal atoms with nonmetal atoms. The ratio Rh)//( N— Rh) agrees well with the gyromagnetic ratio for 

IR spectra of polynuclear and cluster carbonyls were discussed in Chapter 2. The v(M—C) frequencies in carbide clusters occur in the 500-850 cmrange, and therefore are usually higher than the v(M —CO) frequencies in carbonyls. The A (Co —C) force constant for the Co —C bond in [Co6C(CO)i2S2] is 155 N and is lower than 

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Also rather few solid state metal NMR spectra of clusters have been reported. This type of experiment is very promising for the metal NMR of clusters, basically for two reasons. First, solid state NMR gives tensorial values of the chemical shifts, and the anistropy of coupling constants reflecting more precisely the electronic structure. The measurement of dipole-dipole interactions, where possible, should allow the determination of distances involving light nuclei such as H, Li, Be which are not easily accessible by X-ray diffraction. The second reason for interest in solid state studies is that the fluxional behaviour is often frozen out and that consequently NMR results are more amenable to correlation with X-ray studies. 

Obtain STD NMR spectra of clusters of 5-10 fragment compounds in the presence and absence of target protein. 

Related phosphine-substituted derivatives were also obtained, and the chirality of the clusters demonstrated by the NMR spectra of the compounds (246). Molecules of this type have obvious utility in establishing the potential intermediacy of the polynuclear adduct in a catalytic reaction by the formation of optically active products. 

Figure 11. Li MAS NMR spectra of the spinel LiMn204 (structure shown on the left) as a function of synthesis temperature (adapted from ref 44). The cluster of Mn + ions formed as a result of Li substitution on the octahedral site is shown, and the resonances due to Li ions near this cluster are assigned.

The NMR spectra of Pbs and Sn5 have not been reported even though the clusters were first isolated in 1975 [40]. Preliminary data for Pb5 ion show that it is static on the NMR time scale (Fig. 4) [41]. The Dsh structure of the trigonal bipyramidal cluster is expected to give rise to two NMR resonances. While only one resonance has been located (5 ° Pb = -3,591 ppm), it clearly shows coupling to a second inequivalent set of Pb atoms with j( Pb- ° Pb = 2,344 Hz). The calculated intensity distribution for the three equatorial Pb nuclei is expected to be 0.004 0.28 1 0.28 0.004, whereas the signal for the two axial Pb atoms is expected to be 0.054 0.387 1 0.387 0.054. Although it is difficult to compare the observed line intensities for the assignment, experimental data are closer to the... 

The fluorine nmr spectra of modified polyethylenimine with trifluorodecanoyl groups attached to 9.5% of the amines, designated (C2H4N)m(CF3(CH2)8CO)0 095m are shown in Fig. 10. In each spectrum there is only one broad resonance (40 to 60 Hz width at half-height) with chemical shifts from 0.8 ppm at low pH to 1.0 ppm at high pH. In this polymer all the trifluorodecanoyl side chains spend most of the time in apolar clusters. Furthermore, the clusters, as far as can be determined... 

The carbide atom in 1 is located in the center of the square face such that it is partially exposed whereas the carbide atom in 2 is completely encapsulated by the six ruthenium atoms. From a spectroscopic viewpoint, carbide atoms are very distinctive and the earlier reviews have dealt with these aspects in detail.7 8 The IR spectrum of 1 contains peaks at 701 (s) and 670(m) cm 1, and 2 contains peaks at 717(sh), 703(s), 680(m), and 669(m) cm-1.22 I3C-NMR spectra of 1 and 2 do not appear to have been reported. This is probably due to the low yields in which these compounds were initially obtained at a time when, 3C-NMR was still not in widespread use in cluster chemistry. In general, the 13C-NMR resonance of carbide atoms ranges from 8 250 to 500. The high frequency resonances exhibited in 13C-NMR spectra reflect the different diamagnetic and paramagnetic effects experienced by a nucleus in such an unusual chemical environment.23... 

The 31P H NMR spectra of a number of heteronuclear gold cluster compounds are found to be deceptively simple and NMR studies have been used as a probe of the behavior of these species in solution. This is especially true of the higher nuclearity clusters, which often exhibit spectra that are much simpler than would be predicted on the basis of their solid-state structures. For example, [Pt(H)(PPh3)(AuPPh3)7]2+, which adopts the solid-state structure illustrated in Fig. 9 (137) in which the phosphine ligands occupy several different chemical environments within the molecule, shows only two resonances in the 31P 1H NMR spectrum. These are in a ratio of 7 1 and exhibit satellites due to coupling to the central platinum nucleus as Fig. 10 illustrates. 

Similar observations have been made for the 31P 1H NMR spectra of homonuclear gold clusters, which, with a few exceptions, are characterized by singlets in their 31P 1H NMR spectra. This has been attributed... 

Intermolecular dynamic processes are, in general, less common for gold heteronuclear cluster compounds. An example of such behavior in solution is provided by the compounds [(Ph3PAu)(p.-H)M(CO)5] (M = Cr, Mo, W) and [(Ph3PAu)(/ti-H)W(CO)4 P(OMe3) ] (32). The 31P 1H NMR spectra of these species are sharp singlets that shift appreciably... 

The NMR spectra of the cluster suggest that two skeletal isomers are present in solution at ambient temperature. It seems likely that these two isomers exhibit metal core structures similar to those which the two isomeric forms of the very closely related cluster [Au2Re2H6(PMe2Ph)4(PPh3)2] are thought to adopt (Fig. 11) (52). 

Fig. 29. Solid-state 31P- tH NMR spectra of the clusters [M2Ru4H2(CO)12(PPli3)2] [M = Ag (a) or Au (b)]. The two resonances in (a) are each split into doublets by 107>109Ag-31P coupling. Groups of peaks marked by x are spinning sidebands.

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