Highly
Enhanced Bisignate Circular Dichroism of Ferrocene-Bridged Zn(II)
Bisporphyrin <i>Tweezer</i> with Extended Chiral Substrates
due to Well-Matched Host–Guest System
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Abstract
Four
new chiral <i>tweezer-</i>diamine complexes, consisting
of an achiral ferrocene-bridged Zn(II)bisporhyrin host (<b>1</b>) and two small diamines (1<i>R</i>,2<i>R</i>)-1,2-diphenylethylene diamine {(1<i>R</i>,2<i>R</i>)-DPEA} and (1<i>S</i>,2<i>S</i>)-1,2-cyclohexane
diamine {(1<i>S</i>,2<i>S</i>)-CHDA} and two extended
diamines (1<i>R</i>,2<i>R</i>)-<i>N</i>,<i>N</i>′-bis-(isonicotinoyl)-1,2-diphenylethylene
diamine {(1<i>R</i>,2<i>R</i>)-DPEApy} and (1<i>S</i>,2<i>S</i>)-<i>N</i>,<i>N</i>′-bis-(isonicotinoyl)-1,2-cyclohexane diamine {(1<i>S</i>,2<i>S</i>)-CHDApy} chiral guests, are reported. Additions
of (1<i>R</i>,2<i>R</i>)-DPEA and (1<i>S</i>,2<i>S</i>)-CHDA separately to <b>1</b> in dichloromethane
result in the formation of 1:1 sandwich complexes <b>1·</b>DPEA<sub>(<i>R</i>,<i>R</i>)</sub> and <b>1·</b>CHDA<sub>(<i>S</i>,<i>S</i>)</sub>, respectively, at low guest concentration and 1:2 anti complexes <b>1·</b>(DPEA<sub>(<i>R</i>,<i>R</i>)</sub>)<sub>2</sub> and <b>1·</b>(CHDA<sub>(<i>S</i>,<i>S</i>)</sub>)<sub>2</sub>, respectively, at higher
guest concentration. In contrast, separate additions of (1<i>R</i>,2<i>R</i>)-DPEApy and (1<i>S</i>,2<i>S</i>)-CHDApy to <b>1</b> produce only 1:1 sandwich complexes
of <b>1·</b>DPEApy<sub>(<i>R</i>,<i>R</i>)</sub> and <b>1·</b>CHDApy<sub>(<i>S</i>,<i>S</i>)</sub>, respectively. The binding constants at 295 K between <b>1</b> and (1<i>R</i>,2<i>R</i>)-DPEA are observed
to be (4.7 ± 0.2) × 10<sup>4</sup> M<sup>–1</sup> and (4.3 ± 0.3) × 10<sup>3</sup> M<sup>–1</sup> for 1:1 sandwich and 1:2 anti form, respectively, while the respective
values with (1<i>S</i>,2<i>S</i>)-CHDA are (1.5
± 0.2) × 10<sup>5</sup> M<sup>–1</sup> and (5.9 ±
0.3) × 10<sup>3</sup> M<sup>–1</sup>. However, much larger
values of (2.5 ± 0.3) × 10<sup>5</sup> M<sup>–1</sup> and (1.3 ± 0.3) × 10<sup>6</sup> M<sup>–1</sup> have been observed with DPEApy<sub>(<i>R</i>,<i>R</i>)</sub> and CHDApy<sub>(<i>S</i>,<i>S</i>)</sub>, respectively, to produce the corresponding 1:1 sandwich complexes. <b>1·</b>DPEApy<sub>(<i>R</i>,<i>R</i>)</sub> (<i>A</i><sub>cal</sub>, −1759 cm<sup>–1</sup> M<sup>–1</sup>) (<i>A</i><sub>cal</sub> = Δε<sub>1</sub> – Δε<sub>2</sub>, representing the total
amplitude of the calculated circular dichroism (CD) couplets) shows
∼10-fold increase in CD amplitude compared to the values observed
for <b>1·</b>DPEA<sub>(<i>R</i>,<i>R</i>)</sub> (<i>A</i><sub>cal</sub>, +187 cm<sup>–1</sup> M<sup>–1</sup>), while <b>1·</b>CHDApy<sub>(<i>S</i>,<i>S</i>)</sub> (<i>A</i><sub>cal</sub>, +1886 cm<sup>–1</sup> M<sup>–1</sup>) shows nearly
3-fold increase of the CD amplitude compared to the value observed
for <b>1·</b>CHDA<sub>(<i>S</i>,<i>S</i>)</sub> (<i>A</i><sub>cal</sub>, −785 cm<sup>–1</sup> M<sup>–1</sup>) at 295 K. The <i>A</i><sub>cal</sub> values of −1759 cm<sup>–1</sup> M<sup>–1</sup> and +1886 cm<sup>–1</sup> M<sup>–1</sup> observed
for the <b>1·</b>DPEApy<sub>(<i>R</i>,<i>R</i>)</sub> and <b>1·</b>CHDApy<sub>(<i>S</i>,<i>S</i>)</sub>, 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 <i>tweezer</i> 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 <b>1·</b>DPEA<sub>(<i>R</i>,<i>R</i>)</sub> and <b>1·</b>DPEApy<sub>(<i>R</i>,<i>R</i>)</sub> show CD signal opposite
in sign to each other, which happens to be the case between <b>1·</b>CHDA<sub>(<i>S</i>,<i>S</i>)</sub> and <b>1·</b>CHDApy<sub>(<i>S</i>,<i>S</i>)</sub> also