Fluxional processes

In another fluxional process involving ruthenium instead of rhodium, it has been shown that the rate-controlling step is the complex dissociation and that the ligand exchanges between the two annular nitrogen atoms by an intermolecular process. 

In solution, the phenyl derivative 12.29 (R = Ph) is fluxional. The mechanism of the fluxional process has been shown by an N NMR investigation of a partially N-labelled sample, Le., PhCN2 N3S3 ( N = 99% N), to involve a series of 1,3-nitrogen shifts (Section 4.8.4). Thermolysis or photolysis of 12.29 generates the corresponding 1,2,3,5-dithiadiazolyl radicals [RCNaSa]". ... 

Reactions leading to a series of remarkable dizirconium alkyls and hydrides are outlined in Scheme 213. In solution, the tetrahydride undergoes a rapid fluxional process (observed by NMR spectroscopy) that exchanges the four hydrides. At low temperature three distinct hydrides can be observed, which is consistent with either a dibridged structure, or a tri-bridged species undergoing... 

The spiro dimer of a-tocopherol (9, see also Fig. 6.4) is formed as mixture of two diastereomers by dimerization of the o-QM 3 in a hetero-Diels-Alder reaction with inverse electron demand. Both isomers are linked by a fluxion process (Fig. 6.22), which was proven by NMR spectroscopy.53 The detailed mechanism of the interconversion, which is catalyzed by acids, was proposed to be either stepwise or concerted.53-55... 

The spectral line widths are related to the rate of the rotational motions, which average anisotropies in the g- and hyperfine matrices (Chapter 5), and to the rates of fluxional processes, which average nuclear positions in a radical. 

Monomeric sulfur diimides have an extensive coordination chemistry, as might be anticipated from the availability of three potential donor sites and two re-bonds.131 Under mild conditions with suitable coordinatively unsaturated metal complexes, sulfur diimides may coordinate without rupture of the -N=S=N- unit. Four modes of coordination have been identified or invoked as intermediates in fluxional processes (Scheme 8). 

Dynamic intramolecular rearrangements are observed for a variety of diene-metal complexes at, or near, ambient temperature. This stereochemical non-rigidity may be detected by variable temperature NMR experiments40 in which the signals observed for a static structure coalesce into time averaged signals for the fluxional process. For purposes of this section, processes with activation energies > ca 25 kcal mol 1 or which are irreversible will be considered to be isomerization phenomena and will be discussed in Section IV. 

In solution, certain complexes are observed to undergo an envelope-flip from one face of the diene ligand to the other. The XH NMR spectra of Cp2Zr(s-cis diene) complexes (11 R = R" = H) at ambient temperatures indicate a fluxional process which equilibrates the Cp signals as well as the terminal protons (equation 4). At lower temperature, signals for a... 

Rh (172)]. These species merit comparison to the rhenium- and manganese-dicopper species 158 and 159, which have molecular mirror symmetry and a V-shaped trimetal unit that lacks a Cu-Cu bond. Although 171 and 172 appear symmetric in solution on the NMR time scale due to fluxional processes, in the solid state the two copper centers are clearly inequivalent and a Cu-Cu bond is present. The metal triangle is supported by two B-H Cu linkages, one to each Cu center, involving p- and y-B H vertexes in the M -bound CBBBBEi belt. 

Fig. 13. The fluxional processes present in some fullerene complexes.

Treatment of the stannylene Sn[ N(CH2Bu ) 2CioH6-l,8] with Si(NN) (3) gave the crystalline bis(silyl)stannylene 30 which underwent fluxional processes in thf-dg or PhMe-dg and in the latter or CgDg it finally fragmented yielding 31, 3 and Sn metal The crystalline complexes 32 and 33 were obtained as shown in Scheme 9.13. °... 

Rh, Ir) (188, 400) and H2M3(CO) (RC CR) (M = Ru, Os) (7dJ). The latter complexes were also the subject of a detailed study (163) which showed four fluxional processes to be present (1) hydrogen exchange, (2) local CO scrambling, (3) total CO scrambling, (4) rotation of the acetylene ligand. 

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