ChemInform Abstract: INTRAMOLECULAR CARBONYL GROUP SITE EXCHANGE IN SYMMETRICALLY SUBSTITUTED (η‐1,3‐DIENE)TRICARBONYLIRON COMPLEXES. SHORT COMMUNICATION

Abstract

SummaryActivation parameters have been determined for the intramolecular site exchange of the carbonyl groups in tricarbonyliron complexes of 1,4-and 2,3-disubstituted 1,3-butadienes, of 1,3-~ycloalkadienes and of 1,2-dimethylidenecycloalkanes by temperature-dependent 13C-NMR. spectroscopy (see Table 1). The variations of the activation parameters are discussed in terms of a turnstile-type mechanism for the exchange process and of the differences in the ground state and transition state stability of the complexes. A VB-model of the complexes is used to explain the difference in the stabilities.X-ray analyses of (1 -4-rp1,3-diene)tricarbonyliron complexes show the diene moiety and two carbonyl groups to occupy the basal positions in a square-based pyramidal arrangement of the ligands (cJ: e.g. [l] [2]). In solution, at room temperature, the ligands in these and related complexes undergo a rapid cyclic and concerted site exchange (LIG&~ about 38 to 53 kJmol-I) as can be seen from several recent I3C-NMR. studies (cf: [3-51 and literature cited therein). The exchange process is strongly retarded with decreasing LUMO energy of the 1,3-diene while other factors such as steric effects and the non-planarity of the diene seem to exert only minor influences. The exchange process can proceed, mechanistically, by a type of Berry pseudorotation or a type of turnstile rotation of the ligands. The two processes are indistinguishable by means of dynamic I3C-NMR. spectroscopy. The turnstile type mechanism which consists in successive 120" rotations of the tricarbonyliron fragment relative to the diene moiety, is assumed to be favoured over the Berry-type mechanism since the latter one would involve a trigonal bipyramidal intermediate with the diene ligand in one of the apical and equatorial positions -an arrangement which seems to be unrealizable for steric reasons. I ) 2,