Highly Enhanced Bisignate Circular Dichroism of Ferrocene-Bridged Zn(II) Bisporphyrin Tweezer with Extended Chiral Substrates due to Well-Matched Host–Guest System

Abstract

Four new chiral tweezer-diamine complexes, consisting of an achiral ferrocene-bridged Zn(II)bisporhyrin host (1) and two small diamines (1R,2R)-1,2-diphenylethylene diamine {(1R,2R)-DPEA} and (1S,2S)-1,2-cyclohexane diamine {(1S,2S)-CHDA} and two extended diamines (1R,2R)-N,N′-bis-(isonicotinoyl)-1,2-diphenylethylene diamine {(1R,2R)-DPEApy} and (1S,2S)-N,N′-bis-(isonicotinoyl)-1,2-cyclohexane diamine {(1S,2S)-CHDApy} chiral guests, are reported. Additions of (1R,2R)-DPEA and (1S,2S)-CHDA separately to 1 in dichloromethane result in the formation of 1:1 sandwich complexes 1·DPEA(R,R) and 1·CHDA(S,S), respectively, at low guest concentration and 1:2 anti complexes 1·(DPEA(R,R))2 and 1·(CHDA(S,S))2, respectively, at higher guest concentration. In contrast, separate additions of (1R,2R)-DPEApy and (1S,2S)-CHDApy to 1 produce only 1:1 sandwich complexes of 1·DPEApy(R,R) and 1·CHDApy(S,S), respectively. The binding constants at 295 K between 1 and (1R,2R)-DPEA are observed to be (4.7 ± 0.2) × 104 M–1 and (4.3 ± 0.3) × 103 M–1 for 1:1 sandwich and 1:2 anti form, respectively, while the respective values with (1S,2S)-CHDA are (1.5 ± 0.2) × 105 M–1 and (5.9 ± 0.3) × 103 M–1. However, much larger values of (2.5 ± 0.3) × 105 M–1 and (1.3 ± 0.3) × 106 M–1 have been observed with DPEApy(R,R) and CHDApy(S,S), respectively, to produce the corresponding 1:1 sandwich complexes. 1·DPEApy(R,R) (Acal, −1759 cm–1 M–1) (Acal = Δε1 – Δε2, representing the total amplitude of the calculated circular dichroism (CD) couplets) shows ∼10-fold increase in CD amplitude compared to the values observed for 1·DPEA(R,R) (Acal, +187 cm–1 M–1), while 1·CHDApy(S,S) (Acal, +1886 cm–1 M–1) shows nearly 3-fold increase of the CD amplitude compared to the value observed for 1·CHDA(S,S) (Acal, −785 cm–1 M–1) at 295 K. The Acal values of −1759 cm–1 M–1 and +1886 cm–1 M–1 observed for the 1·DPEApy(R,R) and 1·CHDApy(S,S), respectively, are extremely high. To the best of our knowledge, these are some of the largest values reported for a chirality induction process involving bisporphyrin tweezer receptors. The large enhancement in the CD signal intensity is due to the well complementarity size between Zn(II)bisporphyrin host and the extended chiral diamines guest, which results large unidirectional twisting of two porphyrin units to accommodate the guests having preorganized binding sites with minimum host–guest steric interactions. It is interesting to note that 1·DPEA(R,R) and 1·DPEApy(R,R) show CD signal opposite in sign to each other, which happens to be the case between 1·CHDA(S,S) and 1·CHDApy(S,S) also.