Visible-Light-Triggered Photoswitching of Diphosphene Complexes.

Abstract

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-κ2P,κ6C)Mo(CO)2][OTf] (3a[OTf]), [(2-κ2P,κ6C)Fe(CO)][OTf] (3b[OTf]), and [(2-κ2P)Fe(CO)4][OTf] (4[OTf]) were prepared using the triflate salt [(LC)P=P(Dipp)]OTf (2[OTf) as a precursor (LC = 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 ƞ2-diphosphene complexes 3a,b[OTf] readily undergo isomerization to form blue-colored ƞ1-complexes [(2-κ1P,κ6C)M(CO)n][OTf] (5a,b[OTf]; a: M = Mo, n = 2; b: M = Fe, n = 1). This haptotropic rearrangement is reversible, and the (κ2P,κ6C)-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 3a,b[OTf] are elucidated by DFT calculations. These calculations indicate that the photochromic isomerization originates from the S1 excited state and proceeds through a conical intersection.