Synthesis of Five‐Porphyrin Nanorings by Using Ferrocene and Corannulene Templates

Abstract The smallest and most strained member of a family of π‐conjugated cyclic porphyrin oligomers was synthesized by using pentapyridyl templates based on ferrocene and corannulene. Both templates are effective for directing the synthesis of the butadiyne‐linked cyclic pentamer, despite the fact that the radii of their N5 donor sets are too small by 0.5 Å and 0.9 Å, respectively (from DFT calculations). The five‐porphyrin nanoring exhibits a structured absorption spectrum and its fluorescence extends to 1200 nm, reflecting strong π conjugation and Herzberg–Teller vibronic coupling.

Strained p systems,s uch as picotubes, [1] nanohoops, [2] bowls, [3] cages, [4] and helicences, [5] have attracted increasing attention because of their remarkable electronic structures and properties.Previously,wehave investigated the synthesis of butadiyne-linked nanorings consisting of 6-50 porphyrin units. [6] Herein, we present the synthesis of the smallest and most strained macrocycle in this family,t he five-porphyrin nanoring c-P5.I nt his work, we compared the ability of two pentadentate templates to direct the formation of this cyclic pentamer: T5 Fc and T5 cor ,w hich are based on ferrocene and corannulene cores,r espectively ( Figure 1, Scheme 1, and Scheme 2).
Thedesign of these templates started with acomputational study.D ensity functional theory (DFT) geometry optimizations using Gaussian09/D.01 at the B3LYP/6-31G* level [7] with Grimmes D3 dispersion correction [8] indicate that both templates are too small for the cavity of c-P5.The radii of the N5 donor sets (measured to the centroid of the five Natoms) are 7.73 and 7.37 f or T5 Fc and T5 cor ,r espectively.T he optimal N5 radius for binding c-P5,c omputed by multiple methods,i s8 .27 AE 0.07 . [9] Thec orannulene core of T5 cor Figure 1. Twoorthogonalviews of the DFT-calculated geometries of a) c-P5·T5 Fc and b) c-P5·T5 cor ,s howing the deviations of the Zn atoms from the Zn5 mean planes in . (B3LYP/6-31G* with D3 dispersion correction; meso-aryl groups and the PO(t-Bu) 2 were omitted to simplify the calculations.) Scheme 1. Synthesis of the templates T5 Fc and T5 cor ,w ith overall yields.
adopts the usual bowl conformation, but upon complexation in c-P5·T5 cor ,the bowl becomes flatter,thereby extending the radius of the N5 donor set by 0.17 t o7 .54 . All five zinc centers are in the same plane in the ligand-free c-P5 nanoring, whereas they distort into an envelope conformation reminiscent of cyclopentane when c-P5 binds T5 Fc and T5 cor ( Figure 1). Theb etter fit of T5 Fc for c-P5,c ompared with T5 cor ,isreflected in the deviations from planarity of the Zn5 acceptor set:t he root-mean-square deviation from the mean plane is 0.43 i nc-P5·T5 Fc versus 0.67 i nc-P5·T5 cor . Although these calculations demonstrated that the geometries of the templates are not ideal, we decided to test whether they could direct the synthesis of c-P5,a nd this approach turned out to be successful.
Both templates were prepared through transition-metalcatalyzed CÀHa ctivation (Scheme 1). Thef errocene-based template T5 Fc was synthesized by phosphine-activated palladium-catalyzed aryl-aryl coupling, [10] while T5 cor was synthesized from corannulene by iridium-catalyzed borylation, [3a,11] followed by Suzuki coupling.B oth templates are effective in directing the palladium-catalyzed oxidative coupling of porphyrin monomer P1 to give the five-porphyrin nanoring in yields of 4.0 %f or c-P5·T5 Fc and 6.1 %f or c-P5·T5 cor (Scheme 2). We also synthesized av ersion of c-P5 with different solubilizing aryl groups (OC 8 H 17 rather than t-Bu; see the Supporting Information). GPC analysis shows that the main byproducts in these reactions are larger linear and cyclic porphyrin oligomers (see the Supporting Information). The yield of c-P5 is consistently higher when using T5 cor rather than T5 Fc as the template,f or both porphyrin monomers. Addition of excess pyridine quantitatively displaces both templates from their complexes,y ielding the template-free nanorings.T he template complexes can be regenerated immediately by adding T5 Fc or T5 cor to as olution of c-P5.
The 1 HNMR spectra of nanoring complexes c-P5·T5 Fc and c-P5·T5 cor (Figure 2) were fully assigned by using 2D correlation techniques (see the Supporting Information). As expected, the template protons are shielded by the porphyrin ring current;f or example the a-pyridine protons are shifted by Dd(= d H,T5 Àd H,c-P5·T5 ) = 6.45 ppm in both complexes (see list of Dd values in the Supporting Information).
Thed istortions in the DFT-calculated geometries ( Figure 1) are not reflected in the 1 HNMR spectra, presum-ably because there is rapid interconversion between five degenerate envelope conformations for each complex. The symmetry of the ferrocene-based template T5 Fc is effectively C 5v on the NMR timescale,o wing to fast rotation of the isoquinoline substituents and of the phosphine oxide.T his symmetry is retained in c-P5·T5 Fc and the rims of the nanoring become non-equivalent, thereby resulting in four b-pyrrole doublets (a, a',band b';S cheme 2a nd Figure 2) and six aromatic aryl signals because each porphyrin has two nonequivalent faces.
Thecorannulene template T5 cor is chiral, but racemization through bowl-to-bowl inversion is expected to be fast at room temperature [12] and the c-P5·T5 cor complex has C 5h symmetry on the 1 HNMR timescale,which explains why there are four (rather than eight) b-pyrrole doublets (a, a',ba nd b')a nd  three (rather than six) signals for the aryl protons.A s mentioned above,D FT calculations (Figure 1b)i ndicate that the T5 cor template is stretched when it binds c-P5·T5 cor , flattening the bowl and reducing the barrier to bowl-to-bowl inversion, but we were unable to test this prediction because the complex is not sufficiently soluble for alow-temperature NMR study.
TheN IR absorption spectra of c-P5·T5 Fc , c-P5·T5 cor ,a nd template-free c-P5 all exhibit sharp finger patterns (Figure 3). This behavior is similar to that of the six-porphyrin ring c-P6, [6b] whereas larger macrocycles of this type do not have structured Qbands. [13] Thea bsorption spectrum of templatefree c-P5 is similar to those of c-P5·T5 Fc and c-P5·T5 cor ,t hus indicating that c-P5 is shape-persistent and that its conformation is not strongly perturbed by the templates;o nly as light broadening arises from the increased flexibility of c-P5 in the absence of template.T he fluorescence spectra of the three compounds extend far into the NIR region (Figure 3), like that of c-P6. [6b] Thefluorescence quantum yields,decay times, and radiative rate constants are compared with those of c-P6 and c-P6·T6 in Table 1. [13] Thel ow fluorescence quantum yields of all these compounds result from the fact that S 1 -S 0 transitions are only weakly allowed in circular p systems. [6b,13] Binding of either T5 Fc or T5 cor to c-P5 reduces the radiative rate and the fluorescence quantum yield. Taken in isolation, the low fluorescence quantum yield of c-P5·T5 Fc might be viewed as evidence for photoinduced electron transfer involving the redox-active ferrocene core.H owever,t he fact that c-P5·T5 Fc and c-P5·T5 cor have similar fluorescence quantum yields implies that this is ac onsequence of the regular circular geometry of the complexes,which suppresses the symmetry-breaking vibrations required for Herzberg-Te ller coupling. [13] Thef ormation constants (K f )o ft he nanoring-template complexes reflect how well the templates fit the cavity of the five-porphyrin nanoring.T he nanoring-template complexes c-P5·T5 Fc and c-P5·T5 cor are too stable for their formation constants to be determined by direct titration, so we measured K f by displacing the templates with pyridine, giving log K f values of 29.3 AE 0.2 and 28.5 AE 0.1 for c-P5·T5 Fc and c-P5·T5 cor ,respectively (see the Supporting Information). Thechelate cooperativity of complex formation is quantified by the effective molarity (EM), calculated from the formation constant (K f ), the statistical factor (K s )o ft he complex, and the corresponding microscopic binding constant (K 1 )f or the ligand site (isoquinoline for T5 Fc and pyridine for T5 cor ). The geometric average of the four effective molarities (EM) of the five-coordinate complex can be calculated from Equation (1).
Thee ffective molarities for c-P5·T5 Fc and c-P5·T5 cor are EM = 41 AE 9 m and EM = 36 AE 5 m,r espectively.W hile being higher than the values in many supramolecular systems, [14] these effective molarities are lower than those for the corresponding six-porphyrin ring c-P6,e ither with ar igid T6 template (EM = 126 AE 5 m) [15] or with af lexible cyclodextrinbased template (EM = 74 AE 20 m), [6d] which reflects the poor size complementarity of T5 Fc and T5 cor for c-P5.
In conclusion, templates based on ferrocene and corannulene can be used to direct the synthesis of the fiveporphyrin nanoring c-P5,which has adiameter of 2.1 nm. The corannulene-based template gives ah igher yield of c-P5, despite being too small and having al ower affinity for c-P5. Thelack of correlation between the size of the template and its ability to direct the formation of c-P5 may indicate that the transition state for template-directed coupling is smaller than the final product. Thefive-porphyrin nanoring exhibits highly structured absorption and fluorescence spectra and al ow radiative rate,t hus indicating that emission is strongly suppressed due to the high rotational symmetry of the lowest excited state,w ith the majority of the fluorescence arising from dynamic symmetry breaking through Herzberg-Te ller coupling.T his work demonstrates that perfect sizecomplementarity is not essential in template-directed synthesis,a nd it illustrates how templates can be used to synthesize strained p-conjugated macrocycles. [16] Figure 3. NIR absorption (e,solid lines) and fluorescence spectra (dashed lines) of c-P5 (black), c-P5·T5 Fc (blue), and c-P5·T5 cor (red) in toluene containing 1% pyridine at 298 K. The fluorescence intensity is normalizeds uch that the areas of the peaks are proportional to their quantum yields. Data at 1116-1148 nm are not shown due to overlap with solvent signals. [a] Solvent:toluene with 1% pyridine, 298 K. [b] Quantum yields measured as described in Ref. [13] using linear porphyrin hexamer l-P6 as astandard (F F = 28 %).

Angewandte Chemie
Communications