Nicht‐Berry Pseudorotation beim Carbonylgruppenaustausch von Tetracarbonyl(η‐(Z)‐cycloalken)eisen‐Komplexen

Abstract

Exchange of Carbonyl Group Sites in Tetracarbonyl(q-(2)-cycloa1kene)iron Complexesvia Non-Berry Pseudorotation SummaryThe tetracarbonyliron complexes of cyclobutene, cyclopentene, 4,4-dimethylcyclopentene, 2,5-dihydrofurane, cyclohexen, (Z)-cyclohepten, -octene, -nonene, and -decene were prepared by thermal or photochemical reaction of the corresponding olefins with nonacarbonyldiiron and pentacarbonyliron, respectively. The low-temperature behaviour of the mostly new complexes which can be stored over a longer period only below 250 K and which exhibit four C, 0-stretching frequencies in the IR. spectra (cJ Table 2), indicative for a trigonal bipyramidal structure with the olefin ligand in an equatorial position, was studied in CC12F2 by I3C-NMR. spectroscopy between 200 and 115 K. In this temperature range all complexed olefin ligands with the exception of (Z)-cyclooctene (cf. [ 1 11) show an averaged C,-symmetry on the NMR. time scale. About 115 K the tetracarbonyliron group gives rise to three l3C-signa1s in a ratio of 1 : 1 : 2 for the complexes of (a-cycloheptene, (2)-cyclodecene and 2,5-dihydrofurane (CJ Table 3). This is an agreement with the fixed equatorial position of the non-rotating olefin ligands. The complexes of cyclooctene and cyclononene give only two I3C-signals in a ratio of 1 : 1 for the carbonyl groups. The temperature dependence of the signals indicates that in these cases the two axial carbonyl groups exhibit accidentally the same chemical shift. In all cases a complete line shape analysis of the 13C-signals of the carbonyl groups could only be accomplished by using two exchange constants ( c ! Tables 4 and 5 as well as Fig. 2-5 and Fig. 8). ,The same is true for the cyclobutene complex, but only one exchange constant could be determined (at 120 K: two I3C-signals in a ratio of 1 : 3 with the beginning of a further coalescence). The cyclopentene and cyclohexene complexes showed only one 13C-signal even at 115 K. The observed temperature-dependent line shapes of the 13C-signals can be interpreted in terms of a Non-Berry pseudorotation mechanism involving a three site exchange with each of the two diastereotopic axial carbonyl groups and the two equatorial carbonyl ') 2,