Preparation, structures, and haptotropic rearrangement of novel dinuclear ruthenium complexes, (μ2,η3:η5-guaiazulene)Ru2(CO)4(CNR)

Novel isonitrile derivatives of a diruthenium carbonyl complex, (μ2,η3:η5-guaiazulene)Ru2(CO)5 (2), were synthesized by substitution of a CO ligand by an isonitrile, and were subjected to studies on thermal and photochemical haptotropic interconversion. Treatment of 2 (a 45:55 mixture of two haptotropic isomers, 2-A and 2-B) with RNC at room temperature resulted in coordination of RNC and alternation of the coordination mode of the guaiazulene ligand to form (μ2,η1:η5-guaiazulene)Ru2(CO)5(CNR), 5d–5f, [5d; R=tBu, 5e; 2,4,6-Me3C6H2, or 5f; 2,6-iPr2C6H3] in moderate to good yields. Thermal dissociation of a CO ligand from 5 at 60 °C resulted in quantitative formation of a desirable isonitrile analogue of 2, (μ2,η3:η5-guaiazulene)Ru2(CO)4(CNR), 4d–4f, [4d; R=tBu, 4e; 2,4,6-Me3C6H2, or 4f; 2,6-iPr2C6H3], as a 1:1 mixture of the two haptotropic isomers. A direct synthetic route from 2 to 4d–4f was alternatively discovered; treatment of 2 with one equivalent of RNC at 60 °C gave 4d–4f in moderate yields. All of the new compounds were characterized by spectroscopy, and structures of 5d (R=tBu) and 4d-A (R=tBu) were determined by crystallography. Thermal and photochemical interconversion between the two haptotropic isomers of 4d–4f revealed that the isomer ratios in the thermal equilibrium and in the photostatic state were in the range of 48:52–54:46.