Photochromic Behavior and Diastereomeric Isomerism in [(η6-spirobenzopyran)RuCp]PF6

• Published in: Organometallics Vol. 24; no. 26; pp. 6382 - 6392
• Main Authors: Moriuchi, Akihiro, Uchida, Kazunori, Inagaki, Akiko, Akita, Munetaka
• Format: Journal Article
• Language: English
• Published: American Chemical Society 19.12.2005
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/om0506771

Complexation of photochromic spirobenzopyran 1 (SP-X) with the labile cationic ruthenium complex [Cp*Ru(NCMe)3]PF6 (2) furnishes two isomers of the η6-arene adducts, [Cp*Ru(η6-SP-X)]PF6 (3 and 4), i.e., one of the two diastereomeric adducts of the indoline part in the closed form of 1 (3) and the adduct of the dihydrobenzopyran part in the open trans-merocyanine form of 1 (4). UV irradiation of the pale yellow adduct 3 does not cause an apparent color change, but 1H NMR monitoring reveals formation of the diastereomer 3‘ with the inverted configuration of the chiral spiro carbon atom, which gradually reverts to 3 when left at ambient temperature in the dark over the course of a few days. The two diastereomers 3/3‘ are interconverted with each other through the open, merocyanine form III, which should result from photochemical C−O bond heterolysis of 3/3‘. In the case of the SP-H complex 3b, photochromic behavior (3b ↔ III‘b (trans isomer of IIIb)) is noted but the amount of the colored species III‘b formed is limited. On the other hand, irradiation of the deep red isomer 4 with visible light brings about trans to cis isomerization of the olefinic part to result in complete conversion to an equilibrated mixture of the two diastereomeric closed species 5 and 5‘, which are interconverted with each other at ambient temperature via the transient cis-open intermediate II. UV irradiation of the equilibrated mixture 5/5‘ reverses the ring-opening process to give a photoequilibrated mixture containing 4 in addition to 5/5‘. Thus, 3 and 4 undergo interconversion of the diastereomers via photochemical C−O bond heterolysis and photochromic behavior is noted for 4.