Metal Complexes with Tetrapyrrole Ligands, XLIV. Configuration and Conformation of the anti‐5,15‐Dialkyl‐2,3,7,8,12,13,17,18‐octaethyl‐5H,15H‐porphyrins (Decaalkylporphodimethenes)

Abstract

The configurations and conformations of 5.1 S-dialkyloctaethylporphodimethenes HdOEPRd are described for R = Me. Et, iPr, tBu. The "normal" compounds (1 a to ld) have the syn-axial ("au") configuration, the %nomalous" isomers (2b to 4b) the anti configuration at CS.15. This is proved by a crystal structure analysis of the anomalous quasiplanar diisopropyl derivative 3b (monoclinic, p?&). 'H-NMR analyses corroborate these assignments and give dctails of the conformations in solution. By heating in glacial acetic acid, 3b is quantitatively isomerized to give 3a.In our studies on metal porphodimethenes" we have described a set of metal-free 5,15-decaalkylporphodimethenes, H?(OEPR,) (1 -4)3). They have been obtained by reductive alkylation of octaethylporphyrinatozinc, Zn(OEP), and subsequent demetalation with dilute hydrochloric acid. The dimethyl derivative, H2(0EPMe2) (1 a), occurred as a single species which, by analogy to its crystallographically well-characterized nickeI(II), osmium(II), oxotitanium(lV), and nitridomanganese(V) derivatives4), is assigned the syn-axial configuration (see Figure 1). The diethyl-, diisopropyl-, and di-tert-butylporphodimethenes 2 -4, however, appeared each as sets of two isomers: the "normal" series 2a-4a (yields between 5 and 58%), and the "anomalous" series 2b-4b (yields between 2 and 8%). Each series has similar spectroscopic data which display small, but distinct differences between the series.Inspection of molecular models and comparison of NMR data of l a , 4a, and their metal complexes have led to a tentative configuration assignment: the normal series was given the syn-axial "aa" configuration which is schematically depicted in Figure 1, and the anomalous series the syn equatorial configuration "ee"3'. These isomers are actually just conformers of the same syn configuration, and as the ee configuration suffers from steric congestion resulting from an overcrowding of the molecular plane with alkyl substituents, it should somehow be convertible to the more stable aa configuration. However, despite several efforts, no interconversion between the anomalous and normal series of isomers has been observed. Therefore, the previous tentative assignment must be regarded as doubtful.This paper describes a revision of the tentative assignment based on a determination of the crystal structure of the anomalous diisopropyl isomer 3b. This molecule turned out to exist in the crystal as the quasiplanar conformer of the anti configuration schematically depicted as "dd" in Figure 1. The keys "dd" are meant to point out the "diagonal" conformation of the two isopropyl groups; if the porphodimethene core is planar, the two substituent positions at C5,15 can no longer be termed axial or equatorial because both have intermediate positions between the extremes mentioned ("diagonal"). An examination of molecular models never suggested the existence of this symmetrical conformation of the anti isomers. Therefore, the symmetrical appearance of the 'H-NMR spectra of the anomalous series 2b-4b was err...

Calix[4]phyrins. Effect of Peripheral Substituents on Conformational Mobility and Structure within a Series of Related Systems

“…A similar structure was observed earlier in the case of a less substituted calix[4]phyrin analogue (cf. Scheme , general structure 1 , R 3 = R 5 = H, R 4 = R 6 = i- Pr, Ar = H) . Unfortunately, all efforts to obtain diffraction grade crystals of the other configurational isomer, namely cis - 1e , met with failure.…”

Cation and Anion Radicals of (5,15-Dimethyl-2,3,7,8,12,13,17,18-octaethyl-5H,15H- porphinato)nickel(II): Comparison of the Nickel Complexes of Porphodimethene and Chlorin Chromophores

“…Stable metalloporphodimethenes have been synthesized by placing alkyl groups at the saturated meso positions, thereby preventing oxidation to the porphyrin. [19][20][21][22][23][24][25][26][27][28][29] There are three possible stereoisomers for Ni(II) porphodimethenes which are related by the stereochemistry of the alkyl groups at the C5 and C15 positions. These isomers are denoted syn-axial, syn-equatorial, and anti, of which the synaxial isomer is the most stable.…”

“…Unlike Ni(II) porphyrins and chlorins, which range from planar to S4 ruffled structures, a common feature of porphodimethenes is folding of the macrocycle about the C5-C15 axis and ruffling of the pyrroles. [21][22][23][24]27,28 For the Ni(II) complex this affords a "rooflike" structure with the apex along the C5-Ni-C15 axis, which allows the lowspin nickel to achieve short Ni-N distances, av = 1.908 Á. 24 We present here the electrochemical, optical, and EPR properties of the one-electron oxidation and reduction products of (5,15dimethyl-2,3,7,8,12,13,17,18-octaethyl-57f, 15£-porphinato)nickel(II)24 [Ni(OEPMe2), 1 refers to the syn-axial configuration] to investigate how the electronic and structural differences between Ni(II) chlorins and porphodimethenese alter the physical properties of these dihydroporphyrins.…”

Hoch reduzierte Porphyrine

“…Chrm.S i t . Die Isolierung gelang durch Blitzchromatographie mit Petrolether/Essigester an SiO,(4)(5)(6)(7)(8)(13)(14)(15) bzw. hasischem AI,O,, Aktivititsstufe 111(17- 19), praparative Gaschromatographie an Carbowax 20M auf Volaspher'" A 2 (2, 3) oder an Silicon SE 30 auf Chromosorh" W (16 und Diastereomer).…”