Hedgehog Buckyball: A High-Symmetry Complete Polyhedral Oligomeric Silsesquioxane (POSS)

Abstract

In this study, we report UV-MALDI-TOF MS evidence of a fullerene-like silsesquioxane, a high-symmetry polyhedral oligomeric silsesquioxane (POSS or SSO) formulated as R₆₀-Si₆₀O₉₀ or T₆₀ (T = RSiO_1.5). The T₆₀ preparation can be performed using a normal hydrolytic condensation of [(3-methacryloxy)propyl]trimethoxysilane (MPMS) as an example. Theoretically, four 3sp³ hybrid orbitals (each containing an unpaired electron) of a Si atom are generated before the bond formation. Then it bonds to another four atom electrons using the four generated hybrid orbitals which produced a stable configuration. This fullerene-like silsesquioxane should exhibit much more functionality, activity and selectivity and is easier to assemble than the double bonds in a fullerene.

The fullerene buckyball (FBB), C₆₀, was named “Molecule of the Year” for 1991 by Science [1]. Since the discovery of the C₆₀ buckyball, fullerene science has continued to accelerate, investigating both the basic science and its potential applications [2,3]. One investigation involves a major focus on its analogue, the Si₆₀ cluster [4,5]. For Si₆₀ clusters, the cages should not be very stable due to the use of three (3sp³) orbitals to bond to other Si atoms. Thus, the cages need some other atoms for the fourth bond whereas the C₆₀ fullerene uses the second period sp² orbitals along with a π bond to bond exclusively with other C atoms. Most investigations focus on endohedral Si₆₀ isomers (using the fourth bond of the 3sp³ orbital) which are unstable [6]. To produce a stable Si₆₀ configuration, Wang and Yang conducted ab initio calculations based on density functional theory on a Si₆₀ fullerene-like cage passivated with F or Cl atoms [7]; however, this research is limited by a complex experimental synthesis.

In this study, we report UV-MALDI-TOF MS evidence of a fullerene-like silsesquioxane, a high-symmetry polyhedral oligomeric silsesquioxane (POSS or SSO) formulated as R₆₀-Si₆₀O₉₀ or T₆₀ (T = RSiO_1.5) [8]. Theoretically, four 3sp³ hybrid orbitals (each containing an unpaired electron) of a Si atom are generated before the bond formation. Then it bonds to another four atom electrons using the four generated hybrid orbitals which produced a stable configuration. A significant difference between a FBB and T₆₀ POSS is that the cage in the former is the four-bond (three 2sp² hybrid orbits and one original 2p orbital) connection of each C atom on the FBB surface, while it is the three-bond (three of four 3sp³ hybrid orbits) connection of each Si atom on the POSS surface. The remaining bond to each silicon connects to a pendant organic group adorning the surface of the T₆₀ cage, showing a hedgehog buckyball (HBB, see Figure 1). These organic groups exhibit much more functionality, activity and selectivity and are easier to assemble than the double bonds in a fullerene, which facilitates the synthesis of POSS-based materials possessing unique properties and the ability to set up applications [9,10,11,12,13,14].

The HBB T₆₀ preparation can be performed using a normal hydrolytic condensation of [(3-methacryloxy)propyl]trimethoxysilane (MPMS) that we have used to prepare silsesquioxane coatings [15,16]. Hydrolysis and condensation normally give rise to smaller oligomers with a dozen or fewer monomers interconnected into rings [17]. By extending the condensation time we are able to produce more viscous products (M-POSS or MSSO) with molecular weights in the range calculated for methacryloxypropy-T₆₀ (10,755 Daltons).

Generally the groups and the location of the groups are determined by FTIR and NMR (¹H, ¹³C and ²⁹Si); then all possible predicted structures are established by the molecular weights assigned from the peaks of UVMALDI-TOF MS and the general formula [8,18,19]. Figure S1 schemes the structural formula of MSSO, facilitating analysis and assignment of FTIR and ¹H- and ¹³C-NMR spectra. The typical FTIR MSSO spectrum (Figure S2b) bands at 417–478 and 1122–1129 cm⁻¹ are primarily ascribed to the stretching of O–Si–O and Si–O–Si; the obvious bands at about 1298, 1410 and 1724 cm⁻¹ derive from stretches of CH=CH₂ and C=O groups in methacrylate chains; a decrease in the intensity of the Si–OCH₃ group band at 2938 cm⁻¹ is observed relative to the lower molecular weight MSSO, evidenced together with the generation of a broad band at 3423 cm⁻¹, assigned to –OH groups from Si–OH. Except for bands at 2938 and 3423 cm⁻¹, these data are in good agreement with those of the MPMS spectrum in Figure S2a and give complementary information for the characterization of the structure.

The following peaks in the ¹H-NMR spectrum (Figure S3) were assigned: 0.698 ppm (1); 1.784 ppm (2); 1.925 ppm (7); 3.520, 3.580 ppm (CH₃–OH, Si–OH); 4.111, 4.103 ppm (3); 5.545, 6.091 ppm (5, 6); 7.280 ppm (CHCl₃); 8.004, 8.049 ppm (HCOOCH₃, HCOOH). The following peaks in the ¹³C-NMR spectrum (Figure S4) were assigned: 8.534 ppm (1); 17.865 ppm (7); 21.954 ppm (2); 66.121, 65.894 ppm (3); 76.674, 77.000, 77.319 ppm (CHCl₃, CH₃-OH, Si–OH); 125.006 ppm (5); 135.983 ppm (6); 162.785 ppm (HCOOH); 167.019 ppm (4, HCOOCH₃). Besides ¹H- and ¹³C-NMR spectroscopy, ²⁹Si NMR spectroscopy permits quantitative measurement of the degree of condensation by the relative abundance of the T³ silicon nuclei, Si–(O–Si)₃ [20,21,22,23]: −65.681 ppm is characteristic of the T³ species, and –56.577 ppm can be assigned to T² structures, Si–(O–Si)₂(OH), from incompletely condensed species in the product mixture; no T⁰ or T¹ structures were present in the ²⁹Si-NMR spectrum which is consistent with higher molecular weight and cyclic MSSO.

Figure 2 shows the UV-MALDI-TOF MS in the m/z = 0–11,000 Da range corresponding to MSSO molecules. Three high-symmetry complete MSSOs, T₈, T₂₀ and T₆₀ (see Figure 3), were assigned (see

Table 1. Three high-symmetry complete MSSOs assigned by the mass spectrum (m/z = 0–11,000 Da).

Experiment (m/z)	Assigned structure	Calculation (m/z) (+H+, K+)	Symmetry
1,472.73	R8Si8O12 (+K+)	1,472.98	Oh
3,579.23	R20Si20O30 (+H+)	3,585.96	Ih
10,763.81	R60Si60O90 (+H+)	10,755.88	Ih

). These predicted structures have a compliance between the experimental measurement value and the calculated molecular weight according to ion adducts (M_w + H⁺ or M_w + Na⁺ or M_w + K⁺) [24], however, there are still few differences, probably from ion adducts selected, solvent used for the measurement operation, calculations and so on. T₆₀, one of three high-symmetry complete MSSOs, is a fullerene-like HBB and is denoted as MP-HBB as shown in Figure 3c. The difference between HBB and FBB (as mentioned above) provides an ideal vehicle for exciting research which will be timely, rapidly evolving, multidisciplinary and even appealing on an aesthetic level.

Additional corroborating evidence for the proposed cyclic structures can be observed in the gel permeation chromatography (GPC) chromatograms obtained for the MSSO samples. Figure 4 shows the mass distribution of the MSSOs measured by a GPC device which provides refractive index data. The distribution profiles indicate that the oligomers are formed in three successive groups with average molecular weights of 1413.8, 4538.8 and 16,260.0 (see Figure 4b3, b2 and b1, respectively) corresponding to fractions containing the T₆₀, T₂₀ and T₈ cyclic compounds [25].

Further research will be conducted on this MP-HBB to separate it from multiple MSSO structures using a gel permeation column. The intensity of the MP-T₆₀ present can be determined by GPC or size exclusion chromatography (SEC), and the mass of the elutant is again measured by a mass spectrometer [25,26].