Flavosemiquinone model systems. Part 2. Methyl-substituted quinoxaline radical ions
01 Jan 1984-Journal of The Chemical Society-perkin Transactions 1 (The Royal Society of Chemistry)-Vol. 16, Iss: 11, pp 1767-1769
TL;DR: In this paper, 2, 3, 6, 7, and 2,3,6, 7-tetramethyl-quinoxaline were reduced in aprotic (THF) and acidic media (DMF-HClO4) to yield the corresponding quinoxaline radical anions and 1,4-dihydroquinoxalin radical cations.
Abstract: 2,3-Dimethyl-, 6,7-dimethyl-, and 2,3,6,7-tetramethyl-quinoxaline were reduced in aprotic (THF) and acidic media (DMF–HClO4) to yield the corresponding quinoxaline radical anions and 1,4-dihydroquinoxaline radical cations. Analysis of their e.s.r. spectra was accomplished by computer simulation; a consistent assignment of coupling constants in quinoxaline radical ions could be made on the basis of the methyl substitution pattern. The hyperfine splitting is in agreement with Huckel MO correlations and may be used to explain the spin distribution in flavosemiquinones.
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TL;DR: In an attempt to model possible metal-pterin or metal-flavin ligand interactions the title compounds have been prepared and their structure, spectroscopy and spectroelectrochemistry (EPR, IR, UV-VIS) compared with results for the corresponding rhodium analogues.
Abstract: In an attempt to model possible metal–pterin or metal–flavin ligand interactions the title compounds have been prepared and their structure, spectroscopy and spectroelectrochemistry (EPR, IR, UV–VIS) compared with results for the corresponding rhodium analogues. The X-ray crystal structure of [(C5Me5)IrCl(DML)](PF6), DML = 1,3-dimethyllumazine, shows O4/N5 chelate coordination of iridium with a shorter bond to the nitrogen centre [2.103(4)vs. 2.186(3)A], a slightly more unsymmetrical situation than that found in the rhodium analogue. Electrochemical studies (cyclic voltammetry, polarography) showed largely reversible one-electron reduction processes to radical species [(C5Me5)IrCl(L)]˙ which allowed us to investigate the EPR, IR and UV–VIS response to the reduction. Although the EPR spectra of the iridium compounds remained unresolved, the low g factors (g < 2) and particular line shapes revealed small but significant contributions from the heavy-metal centre. Long-wavelength absorptions at ca. 600 nm and low-energy shifts of the carbonyl stretching bands confirm the formulation [(C5Me5)IrIIICl(L–1)]˙ for the electronic situation.
22 citations
TL;DR: In this article, the structure of 1,4-diethyl-1, 4-dihydropyrazinum radical cation was determined and interpreted with the help of molecular orbital calculations.
Abstract: -1,4-Dialkyl-1,4-dihydro-1,4-diazine radical cations 1–3 have been established in recent years as unusually stable intermediates of corresponding two-step redox systems. The stability is evident from large comproportionation constants Kc > 1012 and from the isolability of persistent radical cation salts with counter anions such as Br-, I-, I3
-, PF6
-, BPh4-, or (TCNQ2)-. The structures of several crystalline derivatives have been determined, showing planar π systems and, in one instance, an anion-dependent tendency to form π/π dimers. Effects of dimerization are also evident from comparative magnetic susceptibility measurements of 1,4-diethyl-1,4-dihydroquinoxalinium iodide and tetraphenylborate. UV/Vis absorption spectra of the radical cations have been determined and interpreted with the help of molecular orbital calculations. The most simple member of the series, 1,4-diethyl-1,4-dihydropyrazinum radical cation 1, exhibits a long wavelength forbidden band (2B1u → 2Au) with a conspicuous vibrational fine structure. The results obtained for the small but very stable new radical cations 1 and 2 provide clues to the stability of flavosemiquinone oxidation states in pertinent oxidoreductase enzymes and show ways to new components for the design of materials with anisotropic physical properties.
15 citations
TL;DR: The molecular structures of two pyrazine derivatives show a typical non-parallel stacking of the aromatic rings and the encapsulation of the central pyrazyl fragment with interplanar centroid-to-centroid distances of ca.
Abstract: The thermolytic coupling of Ph2CN2 and (t-Bu)(Ph)CN2 with doubly cyclomanganated 2,5-diphenylpyrazine and 4,6-diphenylpyrimidine afforded substantial amounts of new triple decker compounds of either Ci and C2 symmetry respectively containing, in both series, two η3-bonded Mn(CO)3 fragments which intervene as scaffolds sustaining the helical non-conjugated triaryl backbone. The molecular structures of two pyrazine derivatives show a typical non-parallel stacking of the aromatic rings and the encapsulation of the central pyrazyl fragment with interplanar centroid-to-centroid distances of ca. 3.5 A. The stacking of the aromatics in the triple-decker pyrimidine derivatives has been assessed by 1H NMR experiments at low temperature. All the triple-decker-type compounds are electroactive. Pyrimidine triple-deckers can reversibly be electrochemically reduced to the corresponding anions.
13 citations
TL;DR: In this paper, a structural database search was performed to identify the first N-heterocyclic radical anions in crystals, and the largest structural changes were observed in the vicinity of the N-centers bearing the highest effective nuclear charge.
Abstract: Structures and Molecular Properties of Charge-Pertubed Molecules. 2, 3-Diphenylquinoxaline Radical Anions in Solution and in Crystals
The Na⊕ and K⊕ radical-ion salts of 2, 3-diphenylquinoxaline seem to be (according to a structural database search) among the first ones of N-heterocyclic radical anions in crystals. The one-electron reduction in aprotic 1, 2-dimethoxyethan (DME) solution at metal mirrors and the crystallization under Ar have been preceded by cyclovoltammetric (CV) and ESR/ENDOR measurement. The first electron insertion at −1.63 V proves to be reversible, whereas the irreversible second step, which is accompanied by an overcrossing of the CV line, can be rationalized by an ‘ECE-DISP’ mechanism via a dianion redox disproportionation. The ENDOR spectrum resolves four 1H couplings and allows to simulate the ESR spectrum including the 14N hyperfine splittings. Both dark-blue single crystals of the radical ion salts show unexpected similarities for Met⊕ = Na⊕, K⊕ despite the 36-pm difference in their ionic radii. The largest structural changes inflicted by the one-electron reduction of the N-heterocyclic molecule are observed in the vicinity of the N-centers bearing the highest effective nuclear charge. The DME-chelated metal cations coordinate at the N electron pairs and form Met⊕(DME)-bridged polymer chains of the radical anion, which are differently ondulated in the Na⊕ and K⊕ radical anion salts. The take-home lesson suggests that many more N-heterocyclic molecules might be analogously reduced under optimized conditions and isolated as single crystals.
12 citations