Trifluoromethyl Derivatives of a Monometallic Cyanide Cluster

Dec 6, 2016 - Recently discovered monometallic cyanide cluster fullerenes are a novel family of endohedral molecules, whose chemical reactivity has no...
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Trifluoromethyl Derivatives of a Monometallic Cyanide Cluster Fullerene, YCN@C82(6)(CF3)16/18 Fei Jin,† Song Wang,† Shangfeng Yang,*,† Nadezhda B. Tamm,‡ Ilya N. Ioffe,† and Sergey I. Troyanov*,‡ †

Hefei National Laboratory for Physical Sciences at Microscale, CAS Key Laboratory of Materials for Energy Conversion, and Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, China ‡ Chemistry Department, Moscow State University, 119991 Leninskie Gory, Moscow, Russia S Supporting Information *

of YCN@Cs-C82(6) (ca. 3 mg) with gaseous CF3I was carried out in a quartz two-section ampule at 430 °C for 2 h, following the routine described elsewhere for pristine fullerenes.5 (Caution! The pressure of CF3I reaches ca. 6 bar under the experimental conditions.) The trifluoromethylated YCN@C82(CF3)14−20 mixture was dissolved in n-hexane and subjected to HPLC separation using a semipreparative Buckyprep column (10 × 250 mm, Nacalai Tesque Inc.) and n-hexane (1.5 mL min−1). Of a total of 24 fractions, only the one with a retention time of 64.9 min gave small orange-colored crystals upon recrystallization from CS2. Additional HPLC separation of the fraction collected between 13.4 and 13.8 min using an analytical Buckyprep column (4.6 × 250 mm, Nacalai Tesque Inc.) and n-hexane (0.5 mL min−1) resulted in two subfractions, the second of which gave small redcolored crystals after recrystallization from o-dichlorobenzene. An XRD study using synchrotron radiation enabled identification of the crystals as YCN@C82(CF3)16·0.45CS2 and YCN@ C82(CF3)18·0.35C6H4Cl2.6 Other HPLC fractions did not give crystals because of low purity and insufficiency of their amount for any additional HPLC separation [see the Supporting Information (SI) for more details]. A considerable number of HPLC fractions for a mixture of only four compositions (mainly 16 or 18 CF3 groups and also 14 and 20) manifests extensive position isomerism in the trifluoromethylated molecules. As indicated above, a limited amount of material prevented the structural characterization of most of those products. Still, the two molecular structures determined by X-ray crystallography already provide interesting information about exohedral addition patterns and their interplay with the position of the monometallic cyanide cluster inside the cage. In contrast to the considerable disorder of the Cs-C82(6) cage and the Y atom in the previously reported YCN@C82· NiII(OEP)·1.73C6H6·1.27CHCl3 (OEP = octaethylporphyrin) crystals of the pristine clusterfullerene,4a the present crystals of the trifluoromethylated YCN@C82(CF3)16/18 molecules show full ordering of both Y and the cage. However, in the YCN@ C82(CF3)16 case, the CN group is disordered over two distinct positions. The YCN@C82(CF3)16/18 molecules are presented in

ABSTRACT: Recently discovered monometallic cyanide cluster fullerenes are a novel family of endohedral molecules, whose chemical reactivity has not yet been probed. High-temperature trifluoromethylation of the yttrium cyanide cluster fullerene YCN@C82(6), followed by high-performance liquid chromatography separation and an X-ray diffraction study of the two crystallized fractions, resulted in the structural characterization of YCN@C82(6)(CF3)16/18. In both molecules, exohedrally attached CF3 groups delimit the spherical π system into localized double bonds, benzenoid rings, larger aromatic assemblies, and a conjugated fragment with the only intact pentagon that is involved in coordination to the interior Y atom. We also present theoretical results on charge distributions in the compounds reported.

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ndohedral fullerenes have attracted much attention as promising materials for biomedical applications, energy storage, etc.1 The encapsulation of metal atoms or metal− nonmetal clusters within a fullerene cage has remarkable consequences with regard to higher production yield and diversity of structures.1,2 In the past decade, several types of cluster fullerenes have been reported that feature metal nitride, carbide, oxide, sulfide, hydrocarbide, and carbonitride clusters.2 Recently, monometallic cyanide cluster fullerenes YCN@Cs(6)C82 and TbCN@C2(5)-C82 (the IPR isomer numbering in parentheses is according to the spiral algorithm3) emerged as a new family of endohedral molecules.4 With the formal transfer of only two electrons to the fullerene cage, monometallic cyanide compounds can be expected to behave differently from many other types of cluster fullerenes, and their chemical properties have not yet been studied. Here, we present the first chemical functionalization of a monometallic cyanide cluster fullerene: high-temperature trifluoromethylation of YCN@Cs-C82(6). After high-performance liquid chromatography (HPLC) separation of the products, two isolated compounds were studied by X-ray diffraction (XRD) and thus characterized as YCN@C82(6)(CF3)16/18. Density functional theory (DFT) calculations were employed to investigate the details of their electronic structure. The starting YCN@Cs-C82(6) material was synthesized via a modified Krätschmer−Huffman direct-current arc discharge synthesis, as described in ref 4a. Subsequent trifluoromethylation © XXXX American Chemical Society

Received: October 20, 2016

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DOI: 10.1021/acs.inorgchem.6b02556 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

In the π system of the molecule, there are one isolated and one nearly isolated double CC bond (average C−C bond length of 1.33 Å) and two almost isolated benzenoid rings (average C−C bond length of 1.404 Å). It is worthy noting that two of the CF3 groups are attached to triple-hexagon junctions (THJs), which are generally less reactive in pristine fullerenes.5,7 In endohedral molecules, however, THJs prove to be less inert. For example, all characterized Sc3N@C80(CF3)14−18 molecules show 2−8 CF3 additions to THJs, whereas 2−5 cage pentagons remain free from CF3 additions because of their coordination to Sc atoms of the Sc3N cluster.8 The CN group in YCN@C82(6)(CF3)16 is disordered over two orientations characterized by Y−N distances of 2.27(1) and 2.32(1) Å and Y−N−C angles of 125(1) and 156(3)°, respectively. However, far too short N−C distances (1.00 and 0.77 Å) suggest limited reliability of the determined CN positions and possible mixing of the Y−N−C and Y−C−N configurations. Indeed, our DFT calculations of the YCN@ C82(6)(CF3)16 molecule9 show a quite smooth energy dependence on the Y−N−C angle. While the optimized Y−N and N−C distances are predicted quite consistently with different classes of exchange-correlation functionals (2.28 and 1.18 Å respectively with both PBE and PBE0), the Y−N−C angle depends even on the basis set used: 118° with PBE0/Sapporo-DZP versus 139° with PBE0/Def2-SVP and 120° at the PBE/TZ2p level. We suppose that the PBE0/Sapporo-DZP results are the most qualitatively correct because they also predict an Y−C−N minimum with an angle of 152°, which likely corresponds to the second of the disordered configurations. However, the calculated energy difference of 16 kJ mol−1 is much higher than the observed disorder would suggest. In YCN@C82(CF3)18, 14 of 18 CF3 groups form 7 pairs coupled by the mirror plane of the Cs-C82(6) cage (Figure 2). There are also three isolated CC bonds and five isolated or almost isolated benzenoid rings. Again, one of the 12 pentagons within a larger 14-membered conjugated fragment coordinates the endohedral Y atom and does not bear CF3 groups, but that is a different pentagon compared to YCN@C82(CF3)16, and the whole fragment is different too and is not fully isolated. One can hypothesize that the developing trifluoromethylation pattern can readily shift an endohedral YCN moiety to the available neighboring positions associated with different pentagons, but, ultimately, Y coordination protects the last remaining pentagon from exohedral addition anyway. It is worth noting that the enantiomers of YCN@C82(6)(CF3)18 appear to cocrystallize with statistical distribution over equivalent sites. Interestingly, their orientations are coupled not by the genuine mirror plane of the Cs-C82 cage but by a pseudo mirror symmetry of the molecular shapes shown in Figure 1d (see more details in the SI). Thus, apparently disordered in the XRD data are only one CF3 group, two cage C atoms, and the endohedral YCN moiety. To assign the disordered YCN positions to the respective cage orientations, four possible YCN orientations were compared at the RI-PBE/TZ2p level.9 In the most favorable positional conformer thus selected, Y atom was found to be coordinated with five C atoms of the empty pentagon (Y−C distances in the range of 2.42−2.71 Å) and six more C atoms from two neighboring hexagons (2.43−2.87 Å). The determined Y−N, Y−C, and N−C distances were 2.40(1), 2.57(5), and 1.00(6) Å, respectively, with a Y−N−C angle of 88(3)°, versus the calculated values of 2.343 Å, 2.679 Å, 1.185 Å, and 92.9°. Again, the determined N−C distance turned out to be considerably shortened, but one must note good agreement with

two projections in Figure 1, with the CN group shown in its most probable position.

Figure 1. Two views of the YCN@C82(6)(CF3)16 (a and b) and YCN@ C82(6)(CF3)18 (c and d) molecules. The fragments involved in Y coordination are highlighted with red; N and C atoms of the CN groups are shown in blue and black, respectively. Although both molecules are asymmetric, the arrangement of 17 CF3 groups in the YCN@ C82(6)(CF3)18 molecule is nearly mirror-symmetric (see part d).

In the YCN@C82(6)(CF3)16 molecule, 16 CF3 groups are distributed nonuniformly over the Cs-C82(6) cage, and their addition pattern completely disregards the mirror symmetry of the pristine C82(6) cage. Interestingly, 11 of the cage pentagons bear one or two CF3 groups each, while the remaining pentagon that coordinates the endohedral Y atom (Y−C distances in the range of 2.36−2.71 Å) is not affected by exohedral functionalization. More precisely, however, the said pentagon (highlighted with red in Figure 1a,b and in the Schlegel diagram of Figure 2) is part of a larger 14-membered 6-vinyl-6-phenylfulvenic conjugated fragment, which includes five more C atoms with Y−C distances in the range of 2.46−2.87 Å.

Figure 2. Schlegel diagrams of YCN@C82(6)(CF3)16 and YCN@ C82(6)(CF3)18. In both molecules, 11 pentagons that bear CF3 groups are filled with orange, and the remaining one that coordinates Y is filled with red. Black circles denote the positions of the attached CF3 groups. The occupied THJs in YCN@C82(6)(CF3)16 are indicated with arrows. Also shown are the localized double bonds and benzenoid rings. B

DOI: 10.1021/acs.inorgchem.6b02556 Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

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theory regarding CN orientation with respect to the Y atom. Like in the pristine YCN@C82(6) molecule4a and in contrast to YCN@C82(CF3)16, CN coordinates Y in an η2 fashion. In order to gain a better understanding of the electronic structure of the new metal cyanide cluster fullerenes, we carried out DFT calculations of the charge distribution in YCN@C82(6) and YCN@C82(6)(CF3)16 at the PBE0/Sapporo-DZP level (see the SI for more details).9 It was found that both molecules have a singlet ground state separated from the first triplet state by at least 0.5 eV in YCN@C82 and even more in the trifluoromethylated derivative. This singlet nature of the molecules suggests that the formal oxidation state of Y in both compounds is +3, with one electron transferred to CN and two more to the carbon cage. However, examination of the Hirschfeld atomic charges provided rather different results such as Y atomic charges of +0.5 to +0.6 and a CN charge of only −0.3. We came to a conclusion that the Hirschfeld procedure is largely inadequate for the endohedral systems in question and carried out density analysis in terms of Bader’s QTAIM theory with the use of Multiwf n software.10 The QTAIM approach determines a Y charge of +1.97 in the parent YCN@C82(6) and a virtually similar +1.87 in YCN@C82(6)(CF3)16. This is an agreement with the previous QTAIM survey of a number of endohedral fullerenes, where the actual charge of the endohedral atoms was typically lower than the formal oxidation state.11 The CN charge is −0.77 in YCN@C82 and −0.80 in YCN@C82(CF3)16, which leaves −1.20 and −1.07, respectively, to the fullerene cage. The distribution of the transferred charge over the fullerene cage is, as one would expect, rather uneven. In YCN@C82(6), the six C atoms nearest to the Y atom (2.4−2.6 Å) already accommodate −0.59, and inclusion of five more atoms within 2.9 Å from Y increases that value to −0.97. In YCN@C82(6)(CF3)16, the 10 C sp2 atoms that lie within 2.9 Å from Y bear −1.05, i.e., almost the entire charge transferred to the carbon cage. Thus, the unusual 14-electron 6-vinyl-6-phenylfulvenic unsaturated fragment responsible for coordinating Y in YCN@C82(6)(CF3)16 is, perhaps, favored by its enhanced electron-withdrawing properties. In summary, trifluoromethylation of YCN@C82(6) results in a complex mixture of isomers containing 14−20 attached CF3 groups. Crystal structure determination of two compounds, YCN@C82(6)(CF3)16/18, reveals asymmetric addition patterns that involve 11 cage pentagons and leave the remaining one free for coordination of the Y atom. Similar structural features were found earlier in a series of Sc3N@C80(CF3)n compounds.8 While metal fullerene charge-transfer interactions in the endohedral molecules are largely localized to the vicinity of the metal atoms, the CF3 groups appear to localize them even further within the conjugated fragment that directly coordinates the Y atom. Those coordinating fragments are even-membered and valence-saturated, but they are very different from the benzenoid or larger polyaromatic substructures that provide energetic stabilization to various exohedrally derivatized fullerenes including the rest of the present YCN@C82(6)(CF3)16/18 molecules. Perhaps, further studies of those unconventional metal-coordinating substructures could shed more light on the interplay between the exohedral addition and endohedral coordination and on the ensuing effects on the development of the addition patterns.

Communication

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02556. Data on the HPLC separation of YCN@C82(CF3)n, MS spectra, disordering in the crystal structure of YCN@ C82(CF3)18, and DFT results (PDF) X-ray crystallographic data in CIF format (CIF) X-ray crystallographic data in CIF format (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*Fax/Tel: +86 551 63601750. E-mail: [email protected]. *Tel: +007 495 9395396. Fax: +007 495 9391240. E-mail: [email protected]. ORCID

Shangfeng Yang: 0000-0002-6931-9613 Sergey I. Troyanov: 0000-0003-1663-0341 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 21132007, 21371164, and 2151101074) and the Russian Foundation for Basic Research (Grants 15-03-04464, 15-03-05083, and 16-53-53012). We are grateful to the supercomputer center of Moscow State University for computational support.12

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REFERENCES

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DOI: 10.1021/acs.inorgchem.6b02556 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.6b02556 Inorg. Chem. XXXX, XXX, XXX−XXX