Photoreactions of Endohedral Metallofullerene with Siliranes: Electronic Properties of Carbosilylated Lu3N@Ih-C80 †

Photochemical carbosilylation of Lu3N@Ih-C80 was performed using siliranes (silacyclopropanes) to afford the corresponding [5,6]- and [6,6]-adducts. Electrochemical studies indicated that the redox potentials of the carbosilylated derivatives were shifted cathodically in comparison with those of the [5,6]-pyrrolidino adducts. The electronic effect of the silirane addends on Lu3N@Ih-C80 was verified on the basis of density functional theory calculations.

Meanwhile, the exohedral functionalization of EMFs has been studied extensively as an effective method for modifying the properties of EMFs for many applications [1][2][3][4][5][6][7][8][9][10][11][12]. As a part of our study of the chemical derivatization of EMFs, we reported the addition reactions of Lu 3 N@I h -C 80 with disilirane, silylene, and digermirane to afford the corresponding silylated and germylated adducts [18][19][20]. The redox potentials of these derivatives were shifted cathodically relative to that of pristine Lu 3 N@I h -C 80 , as expected from the electron-donating effect of silyl and germyl groups. These results prompted us to apply alternative silylation methods to Lu 3 N@I h -C 80 , which will enable fine-tuning of its electronic properties. More recently, we reported the photochemical addition of Sc 3 N@I h -C 80 and silirane (silacyclopropane) 1 to afford three isomeric carbosilylated derivatives of Sc 3 N@I h -C 80 [21]. Spectroscopic measurements indicated that the addition of 1 occurred at the [5,6] (a pentagon and a hexagon), and the [6,6] (two hexagons) ring junctions. In addition, the oxidation potentials of the carbosilylated adducts were shifted moderately compared to that of pristine Sc 3 N@I h -C 80 . Herein, we report the carbosilylation of Lu 3 N@I h -C 80 via photoreactions using siliranes 1 [21] and 2 [22] to afford the corresponding [5,6]-and [6,6]-adducts. The electronic effect of silirane addition on Lu 3 N@I h -C 80 is discussed in comparison with that of the 1,3-dipolar addition of azomethine ylides [23,24].

Preparation and Structural Analysis of the Carbosilylated Derivatives of Lu 3 N@I h -C 80
A toluene solution of Lu 3 N@I h -C 80 and 1 was photo-irradiated using two 500 W halogen lamps (cutoff < 400 nm) (Scheme 1). After photolysis for 60 h, preparative high performance liquid chromatography (HPLC) of the reaction mixture was performed to separate three compounds 3a, 3b, and 3c (Figures S1 and S2, in the Supporting Information). Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry of 3a, 3b, and 3c exhibited molecular ion peaks at m/z 2009 (M − ), as expected for 1:1 adducts of Lu 3 N@I h -C 80 and 1 ( Figure S3). The spectra also show base peaks at m/z 1499 for the fragment ion Lu 3 N@I h -C 80 − . Similarly, photoreaction of Lu 3 N@I h -C 80 and 2 provided three isomeric adducts 4a, 4b, and 4c. The yields of 3a, 3b, 4a, and 4b were calculated to be 30%, 7%, 26% and 16%, respectively, whereas those of 3c and 4c were not determined because they contained inseparable impurities. Meanwhile, the exohedral functionalization of EMFs has been studied extensively as an effective method for modifying the properties of EMFs for many applications [1][2][3][4][5][6][7][8][9][10][11][12]. As a part of our study of the chemical derivatization of EMFs, we reported the addition reactions of Lu3N@Ih-C80 with disilirane, silylene, and digermirane to afford the corresponding silylated and germylated adducts [18][19][20]. The redox potentials of these derivatives were shifted cathodically relative to that of pristine Lu3N@Ih-C80, as expected from the electron-donating effect of silyl and germyl groups. These results prompted us to apply alternative silylation methods to Lu3N@Ih-C80, which will enable fine-tuning of its electronic properties. More recently, we reported the photochemical addition of Sc3N@Ih-C80 and silirane (silacyclopropane) 1 to afford three isomeric carbosilylated derivatives of Sc3N@Ih-C80 [21]. Spectroscopic measurements indicated that the addition of 1 occurred at the [5,6] (a pentagon and a hexagon), and the [6,6] (two hexagons) ring junctions. In addition, the oxidation potentials of the carbosilylated adducts were shifted moderately compared to that of pristine Sc3N@Ih-C80. Herein, we report the carbosilylation of Lu3N@Ih-C80 via photoreactions using siliranes 1 [21] and 2 [22] to afford the corresponding [5,6]-and [6,6]-adducts. The electronic effect of silirane addition on Lu3N@Ih-C80 is discussed in comparison with that of the 1,3-dipolar addition of azomethine ylides [23,24].

Preparation and Structural Analysis of the Carbosilylated Derivatives of Lu3N@Ih-C80
A toluene solution of Lu3N@Ih-C80 and 1 was photo-irradiated using two 500 W halogen lamps (cutoff < 400 nm) (Scheme 1). After photolysis for 60 h, preparative high performance liquid chromatography (HPLC) of the reaction mixture was performed to separate three compounds 3a, 3b, and 3c (Figures S1 and S2, in the Supporting Information). Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry of 3a, 3b, and 3c exhibited molecular ion peaks at m/z 2009 (M − ), as expected for 1:1 adducts of Lu3N@Ih-C80 and 1 ( Figure S3). The spectra also show base peaks at m/z 1499 for the fragment ion Lu3N@Ih-C80 − . Similarly, photoreaction of Lu3N@Ih-C80 and 2 provided three isomeric adducts 4a, 4b, and 4c. The yields of 3a, 3b, 4a, and 4b were calculated to be 30%, 7%, 26% and 16%, respectively, whereas those of 3c and 4c were not determined because they contained inseparable impurities. Assuming 1,2-addition of 1 and 2 to Lu3N@Ih-C80, four structures A-D are possible for the addition sites ( Figure 1). To obtain an insight into their addition patterns, visible-near-infrared (vis-NIR) spectroscopy was conducted for the adducts (Figure 2). Previous studies indicate that the vis-NIR spectra of the [5,6]-and [6,6]-pyrrolidino derivatives of Lu3N@Ih-C80 exhibit absorption maxima at approximately 870 and 820 nm, respectively [24,25]. The vis-NIR spectra of 3a, 3b, 4a, and 4b show absorption maxima around 880 nm, which are similar to those of the [5,6]-pyrrolidino derivatives [24,25] (Figure 2). Meanwhile, the vis-NIR spectra of 3c and 4c show broad absorption maxima at 811 and 803 nm, which resemble those of the [6,6]-pyrrolidino derivatives [24,25]. Therefore, 3a, 3b, 4a, Assuming 1,2-addition of 1 and 2 to Lu 3 N@I h -C 80 , four structures A-D are possible for the addition sites ( Figure 1). To obtain an insight into their addition patterns, visible-near-infrared (vis-NIR) spectroscopy was conducted for the adducts (Figure 2). Previous studies indicate that the vis-NIR spectra of the [5,6]-and [6,6]-pyrrolidino derivatives of Lu 3 N@I h -C 80 exhibit absorption maxima at approximately 870 and 820 nm, respectively [24,25]. The vis-NIR spectra of 3a, 3b, 4a, and 4b show absorption maxima around 880 nm, which are similar to those of the [5,6]-pyrrolidino derivatives [24,25] (Figure 2). Meanwhile, the vis-NIR spectra of 3c and 4c show broad absorption maxima at 811 and 803 nm, which resemble those of the [6,6]-pyrrolidino derivatives [24,25]. Therefore, 3a, 3b, 4a, and 4b are assignable to the [5,6]-adducts A and B. On the other hand, 3c and 4c are probably the [6,6]-adducts C and/or D (See also Figure 3). and 4b are assignable to the [5,6]-adducts A and B. On the other hand, 3c and 4c are probably the [6,6]-adducts C and/or D (See also Figure 3).   In the 1 H-NMR spectrum of 3a, signals for four methyl groups and three tert-butyl groups, as well as aromatic protons were observed ( Figure S5). The ring protons of the silacyclopentane addend of 3a constitute an ABX spin system at 3.66, 2.85, and 1.84 ppm as double doublets. Meanwhile, the 13 C-NMR spectrum of 3a exhibited 86 quaternary and six tertiary sp 2 carbon signals, as well as two sp 3 carbon signals of the Ih-C80 cage ( Figure S7). The carbon signals of the methylene and methine of the silacyclopentane ring, the three tert-butyl and four methyl groups were also shown in the spectrum. These spectral data are consistent with the structure of 3a as the [5,6]-adduct with C1 symmetry. The 1 H-NMR spectrum of 3b is very similar to that of 3a ( Figure S6). On the basis of the vis-NIR and 1 H-NMR spectra, 3a and 3b were determined to be a pair of diastereomers A and B, although the absolute configurations of 4-tert-butylphenyl (denoted as tBp) groups remain unknown.
The 13 C-NMR spectrum of 4a, the main product obtained from 2, is similar to that of 3a, showing 86 sp 2 carbon atom signals ( Figure S10). The carbon signals of the silirane addend and two sp 3 carbon signals of the Ih-C80 cage were also observed. The 1 H-NMR spectra of 4a and 4b also indicate the structures of the silirane addends as for the case of 3a and 3b ( Figures S8 and S9). As shown in the vis-NIR spectra, it is suggested that 4a and 4b are a pair of diastereomers of the [5,6]-adducts. On the other hand, spectroscopic studies of 3c and 4c have hitherto been unsuccessful because of their low yields and impurities.    In the 1 H-NMR spectrum of 3a, signals for four methyl groups and three tert-butyl groups, as well as aromatic protons were observed ( Figure S5). The ring protons of the silacyclopentane addend of 3a constitute an ABX spin system at 3.66, 2.85, and 1.84 ppm as double doublets. Meanwhile, the 13 C-NMR spectrum of 3a exhibited 86 quaternary and six tertiary sp 2 carbon signals, as well as two sp 3 carbon signals of the Ih-C80 cage ( Figure S7). The carbon signals of the methylene and methine of the silacyclopentane ring, the three tert-butyl and four methyl groups were also shown in the spectrum. These spectral data are consistent with the structure of 3a as the [5,6]-adduct with C1 symmetry. The 1 H-NMR spectrum of 3b is very similar to that of 3a ( Figure S6). On the basis of the vis-NIR and 1 H-NMR spectra, 3a and 3b were determined to be a pair of diastereomers A and B, although the absolute configurations of 4-tert-butylphenyl (denoted as tBp) groups remain unknown.
The 13 C-NMR spectrum of 4a, the main product obtained from 2, is similar to that of 3a, showing 86 sp 2 carbon atom signals ( Figure S10). The carbon signals of the silirane addend and two sp 3 carbon signals of the Ih-C80 cage were also observed. The 1 H-NMR spectra of 4a and 4b also indicate the structures of the silirane addends as for the case of 3a and 3b (Figures S8 and S9). As shown in the vis-NIR spectra, it is suggested that 4a and 4b are a pair of diastereomers of the [5,6]-adducts. On the other hand, spectroscopic studies of 3c and 4c have hitherto been unsuccessful because of their low yields and impurities.    In the 1 H-NMR spectrum of 3a, signals for four methyl groups and three tert-butyl groups, as well as aromatic protons were observed ( Figure S5). The ring protons of the silacyclopentane addend of 3a constitute an ABX spin system at 3.66, 2.85, and 1.84 ppm as double doublets. Meanwhile, the 13 C-NMR spectrum of 3a exhibited 86 quaternary and six tertiary sp 2 carbon signals, as well as two sp 3 carbon signals of the Ih-C80 cage ( Figure S7). The carbon signals of the methylene and methine of the silacyclopentane ring, the three tert-butyl and four methyl groups were also shown in the spectrum. These spectral data are consistent with the structure of 3a as the [5,6]-adduct with C1 symmetry. The 1 H-NMR spectrum of 3b is very similar to that of 3a ( Figure S6). On the basis of the vis-NIR and 1 H-NMR spectra, 3a and 3b were determined to be a pair of diastereomers A and B, although the absolute configurations of 4-tert-butylphenyl (denoted as tBp) groups remain unknown.
The 13 C-NMR spectrum of 4a, the main product obtained from 2, is similar to that of 3a, showing 86 sp 2 carbon atom signals ( Figure S10). The carbon signals of the silirane addend and two sp 3 carbon signals of the Ih-C80 cage were also observed. The 1 H-NMR spectra of 4a and 4b also indicate the structures of the silirane addends as for the case of 3a and 3b (Figures S8 and S9). As shown in the vis-NIR spectra, it is suggested that 4a and 4b are a pair of diastereomers of the [5,6]-adducts. On the other hand, spectroscopic studies of 3c and 4c have hitherto been unsuccessful because of their low yields and impurities. In the 1 H-NMR spectrum of 3a, signals for four methyl groups and three tert-butyl groups, as well as aromatic protons were observed ( Figure S5). The ring protons of the silacyclopentane addend of 3a constitute an ABX spin system at 3.66, 2.85, and 1.84 ppm as double doublets. Meanwhile, the 13 C-NMR spectrum of 3a exhibited 86 quaternary and six tertiary sp 2 carbon signals, as well as two sp 3 carbon signals of the I h -C 80 cage ( Figure S7). The carbon signals of the methylene and methine of the silacyclopentane ring, the three tert-butyl and four methyl groups were also shown in the spectrum. These spectral data are consistent with the structure of 3a as the [5,6]-adduct with C 1 symmetry. The 1 H-NMR spectrum of 3b is very similar to that of 3a ( Figure S6). On the basis of the vis-NIR and 1 H-NMR spectra, 3a and 3b were determined to be a pair of diastereomers A and B, although the absolute configurations of 4-tert-butylphenyl (denoted as tBp) groups remain unknown.
The 13 C-NMR spectrum of 4a, the main product obtained from 2, is similar to that of 3a, showing 86 sp 2 carbon atom signals ( Figure S10). The carbon signals of the silirane addend and two sp 3 carbon signals of the I h -C 80 cage were also observed. The 1 H-NMR spectra of 4a and 4b also indicate the structures of the silirane addends as for the case of 3a and 3b (Figures S8 and S9). As shown in the vis-NIR spectra, it is suggested that 4a and 4b are a pair of diastereomers of the [5,6]-adducts. On the other hand, spectroscopic studies of 3c and 4c have hitherto been unsuccessful because of their low yields and impurities.

Electrochemical Studies
The electrochemical properties of 3a, 3b, 4a, and 4b were measured using cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The redox potentials of the related compounds are summarized in Table 1. As shown in Figures S11 and S12, the oxidation of 3a, 3b, 4a, and 4b resulted in the removal of the silirane addends during the electrochemical analyses, affording pristine Lu 3 N@I h -C 80 . The first oxidation (E ox 1 ) potentials of 3a, 3b, 4a, and 4b are lower than that of Lu 3 N@I h -C 80 within the range of 360-400 mV. These remarkable cathodic shifts of the oxidation potentials are commonly observed for silylated fullerenes because of the electron-donating properties of the silyl groups [18][19][20][21]. Meanwhile, the values of the first reduction (E red 1 ) potentials of 3a, 3b, 4a, and 4b are close to that of Lu 3 N@I h -C 80 . These redox data are compared to those of 5 [23] and 6 [24] as the [5,6]-pyrrolidino adducts of Lu 3 N@I h -C 80 . The first reduction potentials of 5 and 6 are −1.13 and −1.14 V, respectively. According to the reported voltammograms, the E ox 1 potentials of 5 and 6 are both about +0.34 V, respectively, although the exact E ox 1 values are not given in the literature [23,24]. As a result, the redox potentials of 3a, 3b, 4a, and 4b shift cathodically compared to those of 5 and 6. These effects of silyl functional groups are also observed for carbosilylated derivatives of Sc 3 N@I h -C 80 [21]. The E red 1 potentials of 3a, 3b, 4a, and 4b are interesting from the viewpoints of Lu 3 N@I h -C 80 -based OPV acceptors, for which optimization of the lowest unoccupied molecular orbital (LUMO) levels is an important factor in improving the open circuit voltages of the corresponding solar cells [15,16]. The E red 1 potentials of the [5,6]-pyrrolidino adducts 5 [23] and 6 [24] are shifted positively relative to that of Lu 3 N@I h -C 80 . In contrast, 3a, 3b, 4a, and 4b maintain the low reduction potential of pristine Lu 3 N@I h -C 80 . Although the functions of 3a, 3b, 4a, and 4b as OPV acceptors are unknown, carbosilylation would be an effective method to adjust the electronic properties of fullerenes for electronic functional materials.

Theoretical Calculations
To obtain an insight into the structural and electronic properties of 3a, 3b, 4a, and 4b, isomers of [5,6]-carbosilylated Lu 3 N@I h -C 80 were calculated by the density functional theory using the B3LYP method [26][27][28]. The partial structures of the calculated molecules are shown in Figure 4 with the relative energies. The optimized structures of these molecules are also shown in Figures S13-S16. These structures were calculated with a few different orientations of the Lu 3 Figure 4. As expected, the relative energies of the adducts vary depending on the orientations of the Lu 3 N cluster and the configurations of the silirane addends. Among these isomers, 3A-I, 4A-I, 3B-I, and 4B-I, in which the Y-shaped Lu 3 N clusters straddle the addition sites, have relatively lower energies. These calculations indicate that the orientations of the Lu 3 N clusters in 3a, 3b, 4a, and 4b should be somewhat restricted. As a result, we regard 3A-I, 3B-I, 4A-I, and 4B-I as the most preferable optimized structures for 3a, 3b, 4a, and 4b, although the absolute configurations of the tBp groups remain unclear.  In addition, the redox properties of carbosilylated Lu3N@Ih-C80 are consistent with the calculated energies of the highest occupied molecular orbital (HOMO) and LUMO levels of the optimized structures. As shown in Table 2 properties of 3a, 3b, 4a, and 4b. Therefore, the carbosilylated structures were verified given the electron-donating properties of the silyl groups. In addition, the redox properties of carbosilylated Lu 3 N@I h -C 80 are consistent with the calculated energies of the highest occupied molecular orbital (HOMO) and LUMO levels of the optimized structures. As shown in Table 2 properties of 3a, 3b, 4a, and 4b. Therefore, the carbosilylated structures were verified given the electron-donating properties of the silyl groups.
MALDI-TOF mass experiments were performed (Autoflex III Smartbeam, Bruker Daltonics, Billerica, MA, USA) with 1,1,4,4-tetraphenyl-1,3-butadiene (TPB) as the matrix in both positive and negative ion modes. Absorption spectra were measured using a UV spectrophotometer (UV-3150, Shimadzu Corp., Kyoto, Japan). Cyclic voltammograms and differential pulse voltammograms were recorded on an electrochemical analyzer (BAS CV50W, BAS Inc., Tokyo, Japan). The reference electrode was a saturated calomel reference electrode (SCE). The glassy carbon electrode was used as the working electrode, and a platinum wire was used as the counter electrode. All potentials are referenced to the ferrocene/ferrocenium couple (Fc/Fc + ) as the standard. (n-Bu) 4 NPF 6 (0.1 M) in ODCB was used as the supporting electrolyte solution.

Conclusions
In summary, the photochemical addition of siliranes 1 and 2 to Lu 3 N@I h -C 80 afforded the corresponding [5,6]-and [6,6]-adducts. The [5,6]-adducts 3a, 3b, 4a, and 4b were characterized on the basis of spectroscopic and electrochemical studies and theoretical calculations. The electron-donating effect of carbosilylation on Lu 3 N@I h -C 80 was confirmed by the redox properties of 3a, 3b, 4a, and 4b, which showed remarkably low first oxidation potentials. The carbosilylation also resulted in cathodic shifts of the first reduction potentials compared to those of the [5,6]-pyrrolidino adducts. Such functional groups with various electronic effects will contribute to the utilization of EMFs for future applications. Further studies of novel functionalizing methods based on organosilanes are now underway.
Supplementary Materials: Supplementary materials are available online.