Synthesis, spectroscopic and crystallographic analysis of the Zn-complex of a di(β,β′-sulfoleno)pyrrin: model for Zn-complexes of bilirubin and of phylloxanthobilins

Abstract

A high yield preparation, spectroscopic and crystallographic investigation of the crystalline Zn-complex of a di(β,β′-sulfoleno)pyrrin are reported here. In the brightly green fluorescent Zn-complex of the hardly luminescent di(β,β′-sulfoleno)pyrrin, the metal ion is bound by two di(β,β′-sulfoleno)pyrrin ligands, as revealed first by its mass spectra. The crystal structure of this Zn-complex of the di(β,β′-sulfoleno)pyrrin confirmed a regular 2:1 composition of the bidentate di(β,β′-sulfoleno)pyrrin ligand and the metal ion. The latter was coordinated in a distorted tetrahedral fashion, as found in other dipyrrin Zn-complexes. The here studied Zn-complex of a designed di(β,β′-sulfoleno)pyrrin ligand provides insights into the coordination properties of the proposed (2:1)- and (2:2)-complexes of phylloxanthobilin and bilirubin, respectively, which are two abundant natural bilin-type tetrapyrroles.Graphical abstract

A Dipyrrin Programmed for Covalent Loading with Fullerenes

“…As reported here, heating of the di‐(β,β′‐sulfoleno)‐dipyrrin 1 liberated SO 2 very readily (in contrast to the analogous β,β′‐sulfolenopyrroles), providing a mild and efficient path for twofold [4+2]‐cycloaddition of the dienophile C 60 .…”

“…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.…”

“… Structural formulae of the β,β′‐sulfolenopyrrole based dipyrrin 1 , of a tetrasulfolenocorrole and a symmetrical tetrasulfolenoporphyrin …”

“…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).…”

“…Indeed, the di-(b,b'-sulfoleno)pyrrin 1 is 'programmed" for heat-inducedl oss of SO 2 and subsequenti ntroduction of a variety of covalentm odifications at the pyrrole b-positions by [4+ +2]-cycloaddition chemistry.A sw as similarly explored with tetra-(b,b'-sulfoleno)porphyrins [12b-e] and at etra-(b,b'-sulfoleno)corrole, [13] the reactive dipyrrin 1 proofedt ob eac onvenient 2.2'-dipyrrin building block. 2,2'-Dipyrrins are versatile bidentate ligandsf or coordination of metal ions [1,15] and for higher order supramolecular architectures, [1] useful in imaging and energy conversion. Indeed, the di-(b,b'-sulfoleno)dipyrrin 1 was first studied as am odel component in zinc chelates, [15] as modelso ft he structurally more complex transition metal complexes of naturaly ellow bilins, such as phylloxanthobilins [25] and bilirubin.…”