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-κ2 P,κ6 C)Mo(CO)2 ][OTf] (3 a[OTf]), [(2-κ2 P,κ6 C)Fe(CO)][OTf] (3 b[OTf]), and [(2-κ2 P)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 η2 -diphosphene complexes 3 a,b[OTf] readily undergo isomerization to form blue-colored η1 -complexes [(2-κ1 P,κ6 C)M(CO)n ][OTf] (5 a,b[OTf]; a: M=Mo, n=2; b: M=Fe, n=1). This haptotropic rearrangement is reversible, and the (κ2 P,κ6 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 S1 excited state and proceeds through a conical intersection.

Cationic Phosphinidene as a Versatile P1 Building Block: [LC-P]+ Transfer from Phosphonio-Phosphanides [LC-P-PR3]+ and Subsequent LC Replacement Reactions (LC = N-Heterocyclic Carbene).

Cationic imidazoliumyl(phosphonio)-phosphanides [LC-P-PR3]+ (1a-e+, LC = 4,5-dimethyl-1,3-diisopropylimidazolium-2-yl; R = alkyl, aryl) are obtained via the nucleophilic fragmentation of tetracationic tetraphosphetane [(LC-P)4][OTf]4 (2[OTf]4) with tertiary phosphanes. They act as [LC-P]+ transfer reagents in phospha-Wittig-type reactions, when converted with various thiocarbonyls, giving unprecedented cationic phosphaalkenes [LC-P═CR2]+ (5a-f[OTf]) or phosphanides [LC-P-CR(NR2')]+ (6a-d[OTf]). Theoretical calculations suggest that three-membered cyclic thiophosphiranes are crucial intermediates of this reaction. To test this hypothesis, treatment of [LC-P-PPh3]+ with phosphaalkenes, that are isolobal to thioketones, permits the isolation of diphosphirane salts 11a,b[OTf]. Furthermore, preliminary studies suggest that the cationic phosphaalkene [LC-P═CPh2]+ may be employed to access rare examples of η2-P═C π-complexes with Pd0 and Pt0 when treated with [Pd(PPh3)4] and [Pt(PPh3)3] for which analogous complexes of neutral phosphaalkenes are scarce. The versatility of [LC-P]+ as a valuable P1 building block was showcased in substitution reactions of the transferred LC-substituent using nucleophiles. This is demonstrated through the reactions of 5a[OTf] and 6c[OTf] with Grignard reagents and KNPh2, providing a convenient, high-yielding access to MesP═CPh2 (16) and otherwise difficult-to-synthesize 1,3-diphosphetane 17 and P-aminophosphaalkenes.

Bifunctional Fluorophosphonium Triflates as Intramolecular Frustrated Lewis Pairs: Reversible CO2 Sequestration and Binding of Carbonyls, Nitriles and Acetylenes.

Electrophilic fluorophosphonium triflates bearing pyridyl (3[OTf]) or imidazolyl (4[OTf])-substituents act as intramolecular frustrated Lewis pairs (FLPs) and reversibly form 1 : 1 adducts with CO2 (5+ and 6+ ). An unusual and labile spirocyclic tetrahedral intermediate (72+ ) is observed in CO2 -pressurized (0.5-2.0 bar) solutions of cation 4+ at low temperatures, as demonstrated by variable-temperature NMR studies, which were confirmed crystallographically. In addition, cations 3+ and 4+ actively bind carbonyls, nitriles and acetylenes by 1,3-dipolar cycloaddition, as shown by selected examples.