Visible-Light-Triggered Photoswitching of Diphosphene Complexes.

Although diphosphene transition metal complexes are known to undergo E to Z isomerization upon irradiation with UV light, their potential for photoswitching has remained poorly explored. In this study, we present diphosphene complexes capable of reversible photoisomerizations through haptotropic rearrangements. The compounds [(2-κ P,κ C)Mo(CO) ][OTf] (3 a[OTf]), [(2-κ P,κ C)Fe(CO)][OTf] (3 b[OTf]), and [(2-κ P)Fe(CO) ][OTf] (4[OTf]) were prepared using the triflate salt [(L )P=P(Dipp)][OTf] (2[OTf) as a precursor (L =4,5-dichloro-1,3-bis(2,6-diisiopropylphenyl)-imidazolin-2-yl; Dipp=2,6-diisiopropylphenyl, OTf=triflate). Upon exposure to blue or UV light (λ=400 nm, 470 nm), the initially red-colored η -diphosphene complexes 3 a,b[OTf] readily undergo isomerization to form blue-colored η -complexes [(2-κ P,κ C)M(CO) ][OTf] (5 a,b[OTf]; a: M=Mo, n=2; b: M=Fe, n=1). This haptotropic rearrangement is reversible, and the (κ P,κ C)-coordination mode gradually reverts back upon dissolution in coordinating solvents or more rapidly upon exposure to yellow or red irradiation (λ=590 nm, 630 nm). The electronic reasons for the reversible visible-light-induced photoswitching observed for 3 a,b[OTf] are elucidated by DFT calculations. These calculations indicate that the photochromic isomerization originates from the S excited state and proceeds through a conical intersection.