C₁₂₀O_n from C₆₀Br₂₄

Abstract

Microcrystalline mixture of fullerene dimer oxides ($\text{C}{}_{120}\text{O}{}_{\mathrm{n}}\text{,}\text{n=1,2,3,…}$) was extracted from decomposed C₆₀Br₂₄. Low-temperature FTIR studies of the product showed the absence of orientational ordering transitions indicating freezing of C₆₀ rotations due to inter-cage bonding and epoxidation. Characteristic features due to inter-cage vibrations are seen in the far IR spectrum. Bond cleavages in MS/MS leave all the oxygen atoms of the dimer oxide with one fullerene molecule. We propose that dimers are formed by [3+2] cycloaddition of C₆₀ with its oxides, C₆₀O_n rather than cycloaddition of the monomeric oxides. Results are discussed based on the possible reaction sequences.

Introduction

Fullerenes can be manipulated in a variety of ways to yield new materials and chemical reactivity of C₆₀ is an area of intense research [1]. Fullerene oxides were first identified in the carbon soot generated by the resistive heating of graphite during the bulk preparation of fullerenes [2]. Oxidation of C₆₀ by photochemical [3] and electrochemical [4] methods and with strong oxidising agents such as ozone [5], m-chloroperoxybenzoic acid [6], etc. results in C₆₀ epoxides in which up to six oxygen atoms can be bridged to the C₆₀ sphere. Recently fullerene dimer oxides [7] have caught the attention, being the precursors for odd numbered all carbon fullerene dimers such as C₁₁₉[8], [9]. C₁₂₀O is formed in thermal conditions by the [3+2] cycloaddition of C₆₀ with C₆₀O [10], [11]. Gromov et al. [12] proposed structures for two isomers of the dimeric fullerene C₁₂₀O₂, one with fullerene cages bis-linked by furanoid rings and one with a single furanoid bridge. Deng et al. [13] have prepared V-shaped fullerene trimer oxides, C₁₈₀O_n, along with dimer oxides C₁₂₀O_n by simple heating of a mixture of C₆₀ and C₆₀ oxides and characterised them by electrospray ionisation mass spectrometry (ESI-MS). There are reports on the matrix-assisted laser-induced aggregation of C₆₀[14] and C₇₀[15] oxides giving rise to dimeric species. Fullerene dimer oxides have been extracted from fullerene soot by a special method called `hydrothermally initiated dynamic extraction' or HIDE technique by Takahashi et al. [16]. C₁₂₀O undergoes thermal reactions with sulphur forming C₁₂₀OS [17]. The electrochemical and EPR studies on C₁₂₀O throw more light into the electronic structures of fullerene dimer oxides [18]. Theoretical calculations on the structure and energetics of fullerene dimer oxides also exist in the literature [19], [20]. C₁₂₀O upon decarbonylation produces C₁₁₉, which has been isolated and characterised [8]. Infrared spectroscopic studies by Taylor et al. [21] confirm CO and CO₂ losses from fullerene dimer oxides by thermal means. Because fullerene oxides can easily liberate attached oxygen atoms as CO and CO₂, the established techniques for their mass spectrometric studies are laser desorption mass spectrometry (LD-MS) and electrospray techniques (ESI-MS), often in the negative ion mode [13]. During matrix-assisted laser desorption ionisation (MALDI), laser-induced aggregation of C₆₀[14] and C₇₀ oxides [15] occurs giving dimeric species. Recently, it was shown that the narrow line-width signal appearing in the electron paramagnetic resonance (EPR) spectrum of C₆₀O⁻ is due to C₁₂₀O⁻ formed from the unavoidable C₁₂₀O impurity in air exposed C₆₀[22].

Halogen compounds are well known as important synthetic intermediates due to their reactivity towards various nucleophiles. Chlorinated [23] and fluorinated [24] fullerenes have been used in the preparation of various derivatives.

In this Letter we show that C₆₀ formed by the solid state thermal decomposition of C₆₀Br₂₄ readily undergoes oxidation reactions with atmospheric oxygen which otherwise happens only with strong oxidising agents like ozone or by using photochemical or electrochemical methods. The fullerene epoxides thus formed undergo dimerisation reactions forming fullerene dimer oxides. Based on the observations and the available literature reports, possible reaction sequences during fullerene bromide decomposition have been proposed.