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Influence of the Solvent and Metal Center on Supramolecular Chirality Induction with Bisporphyrin Tweezer Receptors. Strong Metal Modulation of Effective Molarity Values
No full textWe describe the synthesis of a bisporphyrin tweezer receptor <b>1</b>路H<sub>4</sub> and its metalation with Zn颅(II) and Rh颅(III)
cations. We report the thermodynamic characterization of the supramolecular
chirality induction process that takes place when the metalated bisporphyrin
receptors coordinate to enantiopure 1,2-diaminocyclohexane in two
different solvents, toluene and dichloromethane. We also performed
a thorough study of several simpler systems that were used as models
for the thermodynamic characterization of the more complex bisporphyrin
systems. The initial complexation of the chiral diamine with the bisporphyrins
produces a 1:1 sandwich complex that opens up to yield a simple 1:2
complex in the presence of excess diamine. The CD spectra associated
with the 1:1 and 1:2 complexes of both metalloporphyrins, <b>1</b>路Zn<sub>2</sub> and <b>1</b>路Rh<sub>2</sub>, display
bisignate Cotton effects when the chirogenesis process is studied
in toluene solutions. On the contrary, in dichloromethane solutions,
only <b>1</b>路Zn<sub>2</sub> yields CD-active 1:1 and 1:2
complexes, while the 1:2 complex of <b>1</b>路Rh<sub>2</sub> is CD-silent. In both solvents, porphyrin <b>1</b>路Zn<sub>2</sub> features a stoichiometrically controlled chirality inversion
process, which is the sign of the Cotton effect of the 1:1 complex
is opposite to that of the 1:2 complex. In contrast, porphyrin <b>1</b>路Rh<sub>2</sub> affords 1:1 and 1:2 complexes in toluene
solutions with the same sign for their CD couplets. Interestingly,
in both solvents, the signs of the CD couplets associated with the
1:1 sandwich complexes of <b>1</b>路Zn<sub>2</sub> and <b>1</b>路Rh<sub>2</sub> are opposite. The amplitudes of the
CD couplets are higher for <b>1</b>路Zn<sub>2</sub> than
for <b>1</b>路Rh<sub>2</sub>. This observation is in agreement
with <b>1</b>路Rh<sub>2</sub> having a smaller extinction
coefficient than <b>1</b>路Zn<sub>2</sub>. We performed
DFT-based calculations and assigned molecular structures to the 1:1
and 1:2 complexes that explain the observed signs for their CD couplets.
Unexpectedly, the quantification of the thermodynamic stability of
the two metallobisporphyrin/diamine 1:1 sandwich complexes revealed
the existence of interplay between effective molarity values (EM)
and the strength of the intermolecular interaction (<i>K</i><sub>m</sub>; N路路路Zn or N路路路Rh) used
in their assembly. The EM for the N路路路Rh颅(III) intramolecular
interaction is 3 orders of magnitude smaller than that for the N路路路Zn颅(II)
interaction, both of which are embedded in the same scaffold of the <b>1</b>路M<sub>2</sub> bisporphyrin receptor
