A Dipyrrin Programmed for Covalent Loading with Fullerenes

Abstract We describe here a di‐(β,β′‐sulfoleno)pyrrin programmed for efficient and specific β,β′‐functionalization via [4+2] cycloaddition reactions. At 120 °C and in the presence of an excess of C60‐fullerene the di‐(β,β′‐sulfoleno)pyrrin decomposed cleanly, furnishing a di‐(β,β′‐fullereno)pyrrin and the corresponding monofullereno‐dipyrrin in an overall yield of 96 %. Hence, relatively mild thermolysis of the di‐(β,β′‐sulfoleno)pyrrin induced stepwise extrusion of two equivalents of SO2, producing highly reactive dipyrrin‐β,β′‐diene intermediates readily, providing a very effective path to [4+2]‐cycloadducts. As presented here by the example of the covalent attachment of C60‐fullerene units, a convenient general methodology for the efficient synthesis of covalent dipyrrin β,β′‐cycloadducts is made available.

Single crystalso ft he diene-maskedd ipyrrin di-(b,b'-sulfoleno)pyrrin 1 [15] (monoclinic space group P2 1 /c)w ere obtained by mixingo fn-hexane into the solution of 1 in CH 2 Cl 2 .T he unit-cellc ontained four molecules of 1.I nt he conjugated psystem of 1 the individual bonds showed significant bondlength alteration (Figure 1), as found in other dipyrrins, [17] and consistentw ith the formula shown in Scheme 2. The methine bridge of 1 exhibits Z,cis-geometry and an H-bond is observed in the crystal between H2N to N1. The Z,cis-geometry is a common structural feature of 2,2'-dipyrrins,w hen not doubly protonated or attached to metal-ions with specific binding modes. [18] The best planest hrough the two pyrrolic rings in the conjugateds ystem were roughlyc o-planar( 3.58 dihedral angle). The mean planeso ft he aryl group (at the dipyrrin meso-position) ando ft he dipyrrin core were at ad ihedral angle of 74.18,s imilart ot he situation in other meso-phenyl substituted dipyrrins. [17, 18b, 19] The angle C4-C5-C6 (124.28,s ee Figure S1) of the di-(b,b'-sulfoleno)pyrrin 1 is only slightly smaller than that (127.28)i nt he (2:1)-complex of Zn II -ions with 1, [15] and similar to that of ar elatedd ipyrrinw ith an aromatic meso-substituent. [20] The two Sa toms are positioned on opposite sides with respectt ot he plane of the dipyrrin core, with out of plane distances of 0.16 and 0.28 for S1 and S2, respectively.I na ddition, the crystal structure revealed ar emarkable p-stacking arrangement of the 2,2'-dipyrrin 1.I nn eighbors, tight p-p packing of the dipyrrinc ores of two molecules of 1 oriented the planes of the conjugated p-system in parallel, and at am utual distance of 3.75 .H owever,t he two molecules of 1 are relatedb yacentero fi nversion in ad imer,s ot hat their dipyrrins-units are pointingi nto opposite directions (see Figure 1).
The UV/Vis spectrumo ft he yellow 2.2'-dipyrrin 1 exhibits the typical absorption maximum at 435 nm, [15] as is showni n Figure 2. Upon attachment of C 60 -fullerene units, the absorption maximum shifted by 18 nm to longerw avelength for the monoadduct 2,and by 20 nm in the spectrumo fthe bisadduct Scheme1.Structuralformulae of the b,b'-sulfolenopyrrole based dipyrrin 1, [15] of at etrasulfolenocorrole [13] and as ymmetrical tetrasulfolenoporphyrin. [12a, b] . Figure 1. ORTEP plot of the crystal structureo ft he di-(b,b'-sulfoleno)pyrrin 1.T op left:front view with numbering system;H -atoms at carbons were omitted for the sake of clarity.T op right:front view and bond lengths of the dipyrrin core of 1;H-atoms at carbons and the 3,5-di-tert-butyl-phenylg roup were omitted for sake of clarity.B ottom left:top view of p-stacked dimer, highlighting the dipyrrin cores. Bottom right:side view of two p-stackedd ipyrrins 1,highlightingthe distance between the two dipyrrin planes. In both figures at the bottom H-atoms at carbons and the tert-butyl groups were again omitted.
Scheme2.Thermolysis of dipyrrin 1 in the presence of C 60 resulted in the loss of SO 2 and the covalent attachmentofC 60 -fullerene units to the b-and b'-positions of the pyrrole moieties.
3.T he dihydrofullerene addends cause additional absorptions, with very characteristicw eak maximaa t7 02 nm [21] and broad bands with increasingi ntensity at shorter wavelengths,c onsistent with one or two C 60 -addends in 2 and 3,respectively.Similar effects on the UV/Vis-spectral characteristics weref ound in covalentporphyrin b,b'-conjugates of C 60 ,which wereobtained by thermolysis of tetrasulfoleno-porphyrin. [12b, 13] Fluorescence, as reported for 1 and its Zn-complex, [15] is absent in the fullerene adducts 2 and 3,w here the dipyrrin luminescence is quenched effectively by the closely positioned C 60 -addends.
The molecular formulas of fullereno-dipyrrins 2 and 3 were deducedf rom MALDI-TOF mass spectra. The spectrum of 2 featured ap seudo-molecular ion at m/z 1169.0 [M+ +H] + ,c orresponding to C 87 H 32 N 2 O 2 S. As trong fragment at m/z 1105.2 [MÀ SO 2 + H] + indicated the loss of aS O 2 group. The mass spec-trum of 3 confirmed its molecular formula as C 147 H 32 N 2 by showingamolecular ion at m/z = 1824. 6 [M] + and as trong fragment at m/z 1105.2, from loss of one fullereneu nit.
The structures of the fullereno-dipyrrins were established by detaileda nalysisw ith one and two-dimensional NMRs pectra ( 1 H, 1 H-COSY and ROESY, 1 H, 13 C-HSQC andH MBC spectra). The monoadduct 2 is less symmetric than its precursor 1,a nd its NMR spectra showed,c orrespondingly, more signals (see Figures 3a nd 4). In the 1 HNMR spectrum of 2,f or example, four singletso fm ethylene groups were seen at intermediate field, and two singlets for a-pyrrolic protons at low field (see Figure 4). In the spectrum of 2,t he two methylene singletsa t 3.14 ppm and4 .10 ppm were present at ap osition quite similar to that in the 1 HNMR spectrum of 1.T wo further methylene singletsi nt he spectrum of 2 appeared at 3.60 ppm (broad singlet) and 4.46 ppm, shiftedt ol ower field by de-shieldingb y the close-by fullereneu nit. Each of these latter methylene signals coupled to C-atomsw ith chemical shift values of 39.2 ppm and 38.3 ppm (as seen in HSQC-spectra), as is typical for pyrrole b-methylene groupsl inked to af ullerene( Ta ble S1 and Figure S5). [13] In contrast, the other two methylene signals (3.14 ppm and 4.10 ppm) correlated to carbonsa t5 5.5 ppm and 53.4 ppm in the HSQC spectra of 2,i ndicating their attachment to the remaining sulfolene unit ( Figure S5). The deduced location of methylene groups was confirmed in a 1 H, 1 H-ROESY spectrum.T here, the signals at 3.60 and 3.14 ppm showed correlations with H-atoms at the ortho-position of the mesophenylg roup and were assigned to H 2 C3 1 and H 2 C7 1 ,r espectively ( Figure 3). Such NOE correlations were not seen for the two signals at 4.46 and 4.10 ppm, consistent with assignment as H 2 C2 1 and H 2 C8 1 ,r espectively.T he methylene groups exhibited as ingle resonance,e ach, in the 1 HNMR spectrum of 2 at room temperature, due to rapid conformational inversion of   the six-membered ring connecting the fullerene and pyrrole units of 2,a sh as been observedi na nalogous fullereno-porphyrins. [12b] As inglet at 7.80 ppm was assigned to HC1,a si tc orrelated with the fullerene-linked methylene group at 4.46 ppm in the ROESY spectrum of 2 ( Figure 3). The other low field singlet (at 7.78 ppm) was assigned to HC9, as it correlated with the methylene group at4 .10 ppm (Figure 3). The attachment of aC 60fullerenew as further securedb ya 1 H, 13 C-HMBC spectrum, in which the methylene signals at 4.46 ppm and 3.60 ppm correlated with 13 C-signalsa td = 133.4 ppm (pyrrole C1), as well as at d = 66.0 ppm (sp 3 -C of the fullerene unit) and at 156.7 ppm (adjacent sp 2 -C of the fullerene), that is, the typical chemical shift values of C-atoms at the fullerene C [6,6]-bond that has undergone [4+ +2] cycloaddition ( Figure S6). [12b, 13, 21, 22] The pyrrole NH gave ab road weak signal at slightly lower field, when compared to the spectrum of its precursor 1.
The di-(b,b'-fullereno)pyrrin 3 exhibited as impler pattern of the signals in its 1 HNMR spectrum than the monoadduct 2, due to its more symmetric effective structure. Only two (broad)s inglets of the eight diastereotopic methylene protons appeared at 4.48 ppm and 3.60 ppm as ac onsequence of fast conformational equilibration. The pyrrole a-H was shifted to 8.02 ppm (Figure 3a nd Ta ble S1 in the Supporting Information). In the 1 H, 13 C-HSQC spectrum of 3,t he two methylene group singlets at 3.60 ppm and 4.48 ppm of the correlated to carbon resonances at 38.6 and 40.0 ppm of the neighboring fullerenea ddend ( Figure S7), and,i na 1 H, 13 C-HMBC spectrum, to the typical quaternary carbono ft he fullerene moiety at 66.4 ppm ( Figure S8). These correlations secured the attachment of the two fullerene units in 3.T he shielding effect of the phenyl group at the dipyrrin meso-position assisted the assignment of the signal at 3.60 ppm to H 2 C3 1 and H 2 C7 1 .W hen the 1 HNMR data of 1, 2 and 3 are compared, the signals of the pyrrole NH and a-H's, of the protons of the methylene group next to fullerene and at the ortho-positiono ft he mesophenyls ubstituent shift to lower field, upon consecutive loading with the C 60 -fullerene, whereas the signal of the protoni n the para-position of the meso-phenyl group shifts to higher field (Figure 3a nd Figure S2).
Single crystals of the di-(b,b'-fullereno)pyrrin 3 grew from a solutiono f3 in CS 2 when EtOH was mixed in slowly at 23 8C. Bis-adduct 3 crystalized in the monoclinic space group C2/c. The unit-cellcontained four molecules of 3 and half amolecule in the asymmetricu nit, which was completed by ac rystallographic twofold rotation axis. The NH in the dipyrrinu nit was disordered due to the effective symmetry of the structure of the dipyrrin core.
In 3 the two fullerene units are positioned on opposite faces with respectt ot he reference plane of the dipyrrin core (see Figure5). The two pyrrolic rings in the dipyrrincore are twisted with ad ihedral angle of 9.58 in the bisadduct 3 (increased from 3.58 in 1). The angle between the mean planes of the aryl group and of the dipyrrin core has increased to 82.88,ac onsequenceo ft he steric bulk of the C 60 substituent. In the crystal, the mean planes of the dipyrrin cores of neighboring molecules are positioned in parallel and at ad istance of 10.03 . Twon eighbor molecules relate to each other by ac enter of inversion, so that their dipyrrinc ores are oriented in opposite directions.T he fullereneu nits of 3 prevent intermolecular p-p stacking interactions of the dipyrrin core. They make remarkably short intermolecularc ontacts with closestd istances between C7ÀC49c of 3.28 and C27ÀC36b of 3.36 (edge-toedge interactions in the same row or column) (see Figure 5 and FigureS3).