Electrochemical, Spectroscopic, and DFT Study of C₆₀(CF₃)_n Frontier Orbitals (n = 2−18):  The Link between Double Bonds in Pentagons and Reduction Potentials

Abstract

The frontier orbitals of 22 isolated and characterized C(60)(CF(3))(n) derivatives, including seven reported here for the first time, have been investigated by electronic spectroscopy (n = 2 [1], 4 [1], 6 [2], 8 [5], 10 [6], 12 [3]; the number of isomers for each composition is shown in square brackets) fluorescence spectroscopy (n = 10 [4]), cyclic voltammetry under air-free conditions (all compounds with n <or= 12), ESR spectroscopy of C(60)(CF(3))(n)- radical anions at 25 degrees C (n = 4 [1] and 10 [1]), and quantum chemical calculations at the DFT level of theory (all compounds including n = 16 [3] and 18 [2]). DFT calculations are also reported for several hypothetical C(60)(CF(3))(n) derivatives. The X-ray structure of one of the compounds, 1,6,11,16,18,26,36,41,44,57-C(60)(CF(3))(10), is reported here for the first time. Most of the compounds with n <or= 12 exhibit two or three quasi-reversible reductions at scan rates from 20 mV s(-1) up to 5.0 V s(-1), respectively. The 18 experimental 0/- E(1/2) values (vs C(60)(0/-)) are a linear function of the DFT-predicted LUMO energies (average E1/2 deviation from the least-squares line is 0.02 V). This linear relationship was used to predict the 0/- E(1/2) values for the n = 16 and 18 derivatives, and none of the predicted values is more positive than the 0/- E(1/2) value for one of the isomers of C(60)(CF(3))(10). In general, reduction potentials for the 0/- couple are shifted anodically relative to the C(60)(0/-) couple. However, the 0/- E(1/2) values for a given composition are strongly dependent on the addition pattern of the CF3 groups. In addition, LUMO energies for isomers of C(60)(X)(n) (n = 2, 4, 6, 8, 10, and 12) that are structurally related to many of the CF(3) derivatives were calculated and compared for X = CH(3), H, Ph, NH(2), CH(2)F, CHF(2), F, NO(2), and CN. The experimental and computational results for the C(60)(CF(3))(n) compounds and the computational results for more than 50 additional C(60)(X)(n) compounds provide new insights about the frontier orbitals of C(60)(X)(n) derivatives. For a given substituent, X, the addition pattern is as important, if not more important in many cases, than the number of substituents, n, in determining E(1/2) values. Those addition patterns with double bonds in pentagons having two C(sp(2)) nearest neighbors result in the strongest electron acceptors.

Nitrogen Directs Multiple Radical Additions to the 9,9′‐Bi‐1‐aza(C60‐Ih)[5,6]fullerene: X‐ray Structure of 6,9,12,15,18‐C59N(CF3)5

“…In C 1 -C 59 N(CF 3 ) 7 , two CF 3 groups form an isolated p-C 6 (CF 3 ) 2 hexagon, while in C 1 -C 59 N(CF 3 ) 9 four CF 3 groups are arranged in a pmp ribbon as in the most abundant isomer of C 1 -C 60 (CF 3 ) 4 . [5] From the combined analysis of the spectroscopic data and the DFT-calculated relative stabilities, we proposed that both isolated isomers of C 59 N(CF 3 ) 11 have a pyrrole moiety as part of their addition patterns, while the remaining six CF 3 groups are arranged either in pmpmp-(isomer 1) or SPP-(isomer 2) addition pattern (Figure 2). It …”

Optische Stabilität axial‐chiraler push‐pull‐substituierter Buta‐1,3‐diene: Effekt einer einzelnen Methylgruppe auf der Oberfläche von C60

“…[9] Hier berichten wir über Synthese und Kristallstruktur von Kupplungsprodukten aus C 60 und Push-Pull-Chromophoren, die über eine Cycloadditions-Cycloreversions-Sequenz mit TCNE und TCNQ erhalten wurden. Das Hauptaugenmerk gilt dabei den herausragenden stereochemischen Eigenschaften der axial-chiralen Buta-1,3-diene, bei denen eine einzelne Methylgruppe am Fullerenkäfig die Barriere der Rotation um die Chiralitätsachse so stark erhöht, dass Trennung, Isolierung und chiroptische Charakterisierung der Enantiomere nichtplanarer Charge-Transfer(CT)-Chromophore des Buta-1,3-dien-Typs erstmals möglich wurden.…”

Nitrogen Directs Multiple Radical Additions to the 9,9′‐Bi‐1‐aza(C60‐Ih)[5,6]fullerene: X‐ray Structure of 6,9,12,15,18‐C59N(CF3)5

“…The absence of terminal CF 3 groups (seen as quartets in the 19 F NMR spectra) implies that five CF 3 groups should be arranged in a para 5 (or p 5 ) loop rather than as a ribbon of edge-sharing p-C 6 (CF 3 ) 2 hexagons as most commonly observed in the C 60 (CF 3 ) n compounds. [5] The absorption spectrum (see the Supporting Information) is similar to that of C s -C 60 (CF 3 ) 6 . [6] The most probable addition pattern that agrees with the spectroscopic data and was previously observed for azafullerenes features an isolated pyrrole moiety on the fullerene core ( Figure 1).…”

“…However, SPP-C s -C 60 (CF 3 ) 6 was found only as a minor isomer in the high-temperature synthesis, whereas C 1 -C 60 -(CF 3 ) 6 with a ribbon addition pattern (para 3 -meta-para; p 3 mp) was at least ten times more abundant. [5,6] The C s isomer is also 14.4 kJ mol À1 less stable than p 3 mp-C 1 -C 60 (CF 3 ) 6 at the DFT Figure 1. Negative-ion APCI mass spectra of the C 59 N(CF 3 ) n products obtained in a sealed glass ampoule at 530 8C for 24 h (top) and a hot flow tube at 500 8C for 3 h (bottom).…”

“…When trifluoromethyl groups were added to C 60 under similar conditions, the closed-shell species with even n values up to 22 were formed (see the Supporting Information). The product in the flow tube was separated by chromatography into four main fractions: I) C 59 N(CF 3 ) [11][12][13][14][15] , II) C 59 N(CF 3 ) 9 , III) C 59 N-(CF 3 ) 7 , and IV) C 59 N(CF 3 ) 5 (inset in Figure 1). This one-step HPLC process yielded a 98 % compositionally pure sample of C 59 N(CF 3 ) 5 as proven by NI APCI mass spectrometry, which detects also a single isomer with C s symmetry (see the 19 F NMR spectrum in Figure 2, top).…”