Diruthenium carbonyl complexes bound to guaiazulene: Preparation and thermally reversible photoisomerization studies of phosphine and phosphite derivatives of (μ₂,η³:η⁵-guaiazulene)Ru₂ (CO)₅ and iron homologues

Preparation of a series of (μ₂,η³:η⁵-guaiazulene)M₂( CO)_5-n(L)_n (M = Fe, Ru; n = 1, 2; L = phosphines and phosphites) was achieved by photochemical or thermal replacement of CO ligands in (μ₂,η³:η⁵-guaiazulene)M₂( CO)₅ [M = Fe (1) or Ru (2)], which exist as mixture of two haptotropic isomers (1-A and 1-B, 2-A and 2-B), by L, and the products were subjected to thermally reversible photoisomerization studies. Phosphine and phosphite derivatives of diiron complexes (μ₂,η³:η⁵-guaiazulene)Fe₂ (CO)₄(L) (3) and (μ₂,η³:η⁵-guaiazulene)Fe₂ (CO)₃-(L)₂ (4) were prepared in high yields by UV irradiation of 1 in the presence of L. In sharp contrast, (μ₂,η³:η⁵-guaiazulene)Ru₂ (CO)₄(L) (5) was successfully prepared by the thermal replacement of a CO ligand in 2 by L. Although these derivatives may be formed as a mixture of two possible haptotropic isomers, only one isomer (3-A, 4-A, and 5-A) was isolated in each reaction. Detailed studies on the thermal substitution reaction of 2 revealed that formation of 5-A from a mixture of 2-A, and 2-B involved four elementary reactions: (1) the haptotropic rearrangement of 2-A to 2-B, (2) addition of L to 2-B to form (μ₂,η¹:η⁵-guaiazulene)Ru₂ (CO)₅-(L) (6), (3) η¹ to η³ haptotropic shift involving dissociation of CO to form a thermodynamically less stable haptotropic isomer of 5-B, and (4) the haptotropic rearrangement of 5-B to 5-A. The intermediate 6 was isolated and completely characterized, whereas 5-B was detected by spectroscopic methods. Isolation of 6 was accomplished by reaction of 2 with L at room temperature, which furnished selective conversion of 2-B to 6 with 2-A remaining intact. This reaction actually led to kinetic separation of 2-A from a mixture of 2-A and 2-B, and complete characterization of 2-A and 2-B was made. Studies on the interconversion of haptotropic isomers of 2, 3, 4, and 5 revealed the following: (1) the diiron complexes 3 and 4 were generally inactive toward the rearrangement; (2) in contrast to facile interconversion between 2-A and 2-B, the rearrangement of diruthenium compounds, 5, was detectable only in a complex bearing a small phosphite ligand.