Squeezing formaldehyde into C60 fullerene

Abstract

The cavity inside fullerene C₆₀ provides a highly symmetric and inert environment for housing atoms and small molecules. Here we report the encapsulation of formaldehyde inside C₆₀ by molecular surgery, yielding the supermolecular complex CH₂O@C₆₀, despite the 4.4 Å van der Waals length of CH₂O exceeding the 3.7 Å internal diameter of C₆₀. The presence of CH₂O significantly reduces the cage HOMO-LUMO gap. Nuclear spin-spin couplings are observed between the fullerene host and the formaldehyde guest. The rapid spin-lattice relaxation of the formaldehyde ¹³C nuclei is attributed to a dominant spin-rotation mechanism. Despite being squeezed so tightly, the encapsulated formaldehyde molecules rotate freely about their long axes even at cryogenic temperatures, allowing observation of the ortho-to-para spin isomer conversion by infrared spectroscopy. The particle in a box nature of the system is demonstrated by the observation of two quantised translational modes in the cryogenic THz spectra.

Fig. 1. Synthesis of CH₂O@C₆₀.

Sulfide 1 is 70% filled with formaldehyde (CH₂O) using a solution of the monomer, then oxidised to the sulfoxide 2 before photochemically induced loss of sulfur monoxide (SO) gave the orifice contracted bis-hemiacetal 3. Phosphine and phosphite induced deoxygenative ring closures to give CH₂O@5 followed by a thermal extrusion reaction gave CH₂O@C₆₀. The route for labelled materials CD₂O@C₆₀ and ¹³CH₂O@C₆₀ were identical except that the initial filling was carried out by heating 1 with paraformaldehyde in a sealed tube which gave only 25% incorporation of formaldehyde.

Fig. 2. Single crystal x-ray structure of the nickel(II) octaethylporphyrin/benzene solvate of CH₂O@C₆₀.

Recorded at 100 K with CCDC deposition number 2126579 (R1 = 0.050). a Thermal ellipsoids drawn at 50% probability, hydrogens, except on the CH₂O, are omitted for clarity. b Representation showing the observed electron density at the CH₂O location. Electron density surface drawn at the 2.1 e ų level. c and d Orthogonal views of difference electron density at the centre of the C₆₀ cage (contour levels drawn at approximately 0.9 e ų level). The centroid of the cage carbons is shown as a green sphere.

Fig. 3. Ultraviolet spectra and voltammetry of CH₂O@C₆₀.

a Long wavelength part of UV-vis spectra of C₆₀ and CH₂O@C₆₀ in toluene. b Differential Pulse Voltametry of C₆₀ and CH₂O@C₆₀ showing first four reductions relative to ferrocene (Fc/Fc⁺) at 0 V. The fullerenes were dissolved in a 4:1 mixture of toluene and acetonitrile containing 0.1 M Bu₄N.BF₄ as electrolyte. The cell contained a 3 mm diameter glassy carbon working electrode, a 1 cm² sheet of platinum as the counter electrode and a silver wire pseudo-reference electrode. Ferrocene was added as an internal standard.

Fig. 4. ¹³C NMR of CH₂O@C₆₀.

Taken prior to complete removal of H₂O@C₆₀ and C₆₀ and sublimation, 23 mM in ODCB-d₄ at 16.45 T and 298 K (a–e). a showing the CH₂O triplet ¹³C spectrum, a proton decoupled ¹³C{¹H} spectrum and a non-refocused INEPT spectrum with inter-pulse delay of 1.44 ms. b expansion of the ¹³C spectrum around 143ppm, showing the ¹³C signals for CH₂O@C₆₀, H₂O@C₆₀ and empty C₆₀. c ¹³C (CH₂O@C₆₀) NMR signal amplitude modulation following the J-modulated spin-echo sequence shown in the figure [90(¹³C) – delay – 180(¹³C, ¹H) – delay – Acquire (¹³C)], acquired with 16 transients. Fitting the modulation gives |⁰J_HC | = 70.6 ± 0.3 mHz. d and e Inversion-recovery curves for the T₁ spin-lattice relaxation time constant of CH₂O nuclei in CH₂O@C₆₀ for ¹H and ¹³C (central line) respectively. f ¹³C T₁ of the central line of ¹³C-labelled CH₂O in ¹³CH₂O@C₆₀, measured at 16.45 T by inversion recovery as a function of sample temperature, in a 1 mM solution in toluene-d₈. The error bars represent Standard Error estimates in the fitted T₁ values.

Fig. 5. Interconversion of ortho- and paraformaldehyde observed by infra-red spectroscopy.

Change of para and ortho species signal amplitude, Δs, measured at the 1255 cm⁻¹ ro-vibrational band of CH₂O@C₆₀ after the temperature jump from 20 K to 5 K. The data points are the normalised line areas integrated between 1235 and 1255 cm⁻¹ for the ortho spin isomers (black dots) and between 1255 and 1280 cm⁻¹ for the para spin isomers (blue squares) of CH₂O@C₆₀. The para signal grows with a time constant of 12.4 ± 0.6 min, while the ortho signal decays with a time constant of 13.3 ± 0.4 min at 5 K.

Fig. 6. Temperature dependence of the CH₂O@C₆₀ (f = 1.0) far-infra-red (THz) absorption spectra.

Absorption coefficient as a function of wavenumber, between 5 K and 80 K. The translational modes are observed at 166.8 cm⁻¹ and 231.1 cm⁻¹. Inset: temperature dependence of the 167 cm⁻¹ (crosses) and 231 cm⁻¹ (open dots) integrated absorption peak area s, normalised to the peak area at 5 K, after subtracting the 80 K spectrum.

Fig. 7. Molecular orbitals of CH₂O@C_60.

Shown at the minimum energy conformation calculated using DFT with B3LYP-D3 functional and cc-pVTZ basis set.