HAPTOTROPIC REARRANGEMENT BETWEEN 2 ISOMERS OF (MU(2)-ETA(3)-ETA(5)-GUAIAZULENE)FE-2(CO)(2) REVISITED - BOTH THERMAL AND PHOTOCHEMICAL PROCESSES INDUCE THE HAPTOTROPIC INTERCONVERSION

Interconversion between two haptotropic isomers of (mu(2):eta(3):eta(5 )-guaiazulene)Fe-2(CO)(5) (1a,b) occurred at 70-105 degrees C. The the rmal equilibrium ratios between 1a and 1b are around 55: 45 in this te mperature range, indicating that 1a is more stable than 1b by 0.1 kcal /mol. The activation energies estimated from kinetic studies are Delta G(373)(double dagger) = 28 +/- 1 kcal/mol, Delta H-373(double dagger) = 23 +/- 2 kcal/mol, and Delta S-373(double dagger) = -15 +/- 5 cal/( mol deg) either from 1a to 1b or from 1b to 1a. UV or visible photolys is also induced the interconversion to give a mixture of 1a,b in a rat io of 35:65. The haptotropic rearrangement of the ruthenium analogues, (mu(2):eta(3):eta(5)-guaiazulene)Ru-2(CO)(5) (2a,b), also occurred ei ther thermally or photochemically to give a mixture of isomers in a ra tio of 55:45. Photochemical replacement of CO ligands in 1a,b by PEt(3 ) gave a mixture of mono- and diphosphine compounds, u(2):eta(3):eta(5 )-guaiazulene)Fe-2(PEt(3))(CO)(4) (4) and ):eta(3):eta(5)-guaiazulene) Fe-2(PEt(3))(2)(CO)(3) (5), the ratio of which was controlled by the a mount of added PEt(3). Mechanistic aspects are discussed on the basis of the reaction profile of the haptotropic interconversion and the lig and substitution.